Fluoropolymers High Purity Acid Handling

Fluoropolymers survive acidic environments, but extractables must be examined.
For several years many companies that process or utilize high purity acids in the semiconductor industry have settled upon fluoropolymers as a material of construction to ensure that limited amounts of impurities could be leached into the acids.¹
Fluoropolymers offer an advantage over other polymers since they are often manufactured in such a manner that no foreign additives that later could become extractables are needed for processing stability.² Fluoropolymers have advantages over metal in that they are not subject to chemical change (oxidation, rouging, etc.) and hence, will not have greater particle generation over time. In other words, a fluoropolymer’s greatest period of leaching is the initial week of installation whereas a metal will become a greater source of leachate over time.^3,4
A large number of case histories (ten of thousands) exist in the general chemical industry for the use of polyvinylidene fluoride (PVDF, PVF2), polytetrafluoroethylene (PTFE), and perfluoroalkoxy (PFA) resins. These materials of construction have many applications in strong acids even at high temperatures, and as long as they are used within the manufacturer’s recommended temperature ranges for the named chemical, these resins have proven success for over 25 years.^5,6,7,8,9
With all of this history, answering the question as to whether the polymer will stand up to a chemical and temperature environment is easy. The less obvious part of the equation is whether the fluoropolymer could be adding extractables to high purity versions of semiconductor-grade acids? For example, it is well proven in deionized water testing that a polymer simply maintaining its physical properties in exposure to water does not necessarily qualify it as a fluid handling component for high purity application due to extractables that can be emitted as high as part per million (ppm) levels.¹⁰
High Purity Acid Testing:
HNO₃, HF, H2SO₄, HCl
Data generated on extractables in this study involved high purity water (ionic extractions ), HNO₃ (70%), HF (49%), H₂SO₄ (96% and 20%), and HCl (37% and 30%). Other extractable data in acid has been generated on fluoropolymers and published in earlier works.¹¹ Due to the permeation effect of polarity of the polymer and chemical being tested for extraction, the aggressiveness of a particular acid for certain elements, and the molecular size of an acid molecule plus additional water in its composition, it was thought to test four different acids to best develop a cross section of data.
The fluoropolymers selected were emulsion type PVDF (hereafter referred to as E-PVDF) and a designated specific high purity (HP) version of PFA (hereafter referred to as HP-PFA). Both of these polymers are commonly used by professionals in the above acids in general chemical containment as well as in high purity applications.

Test Methods: IC, ICP-MS, TOC
The scope of this data generation was to compare resins in their raw form to negate the effects of processing. Since processing could be largely dependent on the quality and cleanliness of a manufacturer, pellets were used in the corresponding method of extraction. The authors thought it could be unfair to represent sticks of tubing from one PVDF processor and one PFA processor when there are several established processors of each resin type commercially promoting the product lines. Designers should be cautioned that the same resin molded or extruded by different processors could have widely different results depending on handling techniques utilized by the processor.¹²
The test methods used to provide data were:
* Ion Chromatography (IC): leachable ions and leachable anions in high purity water
* Inductively Coupled Plasma/Mass Spectroscopy (ICP-MS): leachable elements in high purity acids.
* Total Oxidizable Carbon (TOC): TOC measured from acid exposure.
Extraction Results and Discussion
* Water Testing. More than 20 years of data exists on PVDF and PFA in water.^{13, 14} The water industry has settled, that for rigid piping and components, PVDF is more than suitable to handle 18 megohm/cm water and is typically chosen over PFA due to strength considerations and overall costs.^{15, 16, 17} Tables 1 and 2 list IC results for leachable anions and leachable cations from 100 ml of ultrapure water at ambient conditions in exposure of 1 gram of pre-cleaned HP-PFA and E-PVDF for 7 days. The results are given in ppb (ug/L) and in neither case were extractables even near 1 ppb for any ion tested.
* Acid Testing. TOC extractions are a consideration for high purity acid in that while bacteria typically will not survive in strong acidic environments, TOCs themselves added to the acid from fluid handling components act as system contaminants. TOC measurements were generated after 7 days exposure to 49% HF, 96% H₂S0₄ and 20% H₂S0₄ at ambient temperature. The results are listed in Table 3. Since the control blank measurement in HF was above the detection limit, it is appropriate to consider actual extractions from the tested resin to be less than the measured value. PFA and PVDF TOC extractable results were in the part per billion range.
ICP-MS was used to determine the extractable contributions of HP-PFA and the emulsion PVDF resin in contact with strong acids for 50 elements for 7 days at room temperature. The data was generated by using 1 gram of polymer to 150 grams of chemical. Extraction results are presented in Tables 4, 5, 6, 7. No data is listed for the control blanks for each chemical because the laboratory reported only a few measured leachables attributed to the blanks (0.1 ppb Al in the 37% HCl blank; 1.0 ppb Al and 0.2 Na 96% H₂SO₄ blank).
Discussion of Results
The testing in acids by IC, ICP-MS, and TOC confirm, in a test with a high ratio of contact surface to liquid volume, that both the PFA grade tested and the PVDF grade tested yield extractables below the part per million level in total cumulative leachate. TOC was the highest contributor of measured extractions for the three acids tested.
PFA does not change color in long term acid exposure and has gained wide use based on the above performance and the aesthetic nature of the color stability attributed to this resin. The E-PVDF (ASTM D3222, Type I, Class 1 and 2) used in this test does not substantially change color over time when exposed to this set of acids. Some commercial PVDF resins do change color substantially when exposed to concentrated hydrochloric acid, concentrated sulfuric acid and even water over time and this has created industry caution in specifying the less expensive PVDF fluoropolymer for use in some high purity facilities. The scope of this study did not include extraction tests on such color sensitive suspension type PVDF (ASTM D3222 Type II) resins.

Throughout the testing, the elements detected from each resin were consistent. In no case were the following elements detected in this test: Sb, As, Be, Bi, B, Li, Hg, Nb, Pd, Pt, Rh, Rb, Ru, Sc, Se, Si, Ag, Ta, Tl, Th, U, V, and Zr. In most cases, calcium was the largest leachate contributor from HP-PFA, followed by sodium and potassium. In all cases, sodium was the largest leachate contributor from E-PVDF and usually calcium was the second highest contributor.

Nitric acid seemed to be the most effective extractive media for both resins, but none of the acids were able to extract above 150 ppb elemental leachate from either fluoropolymer in any test. In analyzing this data at such a minute level, the reviewer must understand that these extractions are typically non-exact. For example, at this level of sensitivity, 0.3 ppb and 0.1 ppb are essentially the same number and within limits of error. One would not say that one product is three times better than the other, but with enough data, trends can be determined as to the frequency expectations of finding various elements in any tested polymer. In other words, the data developed in this study does not appear to suggest that either resin is largely superior to the other in any of the test acids, but perhaps the data will help in understanding what can be looked for when attributing low level extractables to these two fluoropolymers.
Historical testing and conventional wisdom support that continued testing on the same samples would lead to lower extractions each time a new rinse is performed. This is a great advantage of the use of plastics for high purity fluid handling. The plastic materials only contain a finite amount of entrapped extractables and because the materials are not subjected to a corrosion rate as would be typical for a metallic product, the extractions do not increase or have unpredictable changes over time.¹⁸
References
1 Parker, Kevin. “Containers Prevent Product Contamination, Allow Safe Handling of Hazardous Materials,” Chemical Processing, pp. 43-48 (May 1991).
2 Holton, James; Seiler, David; Fulford, Kenneth; Cargo, James T. “Extractable Analysis of Modified PVDF Polymers Utilized in DI Water Applications,” Ultrapure Water, pp. 47-52 (May/June 1993).
3 Banes, Patrick H. “Fundamentals of Passivation in Water Systems,” Ultrapure Water, pp. 60-69 (April 1998).
4 Burkhart, M.; Klaiber; F. Wermelinger, J. “Is Polyvinylidene Fluoride Piping Safe for Hot Ultrapure-Water Applications?,” Microcontamination, pp. 27-31 (February 1995).
5 McCallion, J. “PVDF Exchangers Thrive in Pickling Acids,” Chemical Processing, pp. 59-61 (September 1995).
6 Fusco, Joseph C. “Plastics Developments Advance Pipe Performance and Safety,” Chemical Processing, pp. 48-52 (December 1990).
7 Sixsmith, Tom. “Selecting the Right Plastic Piping System,” Plant Services, pp. 16-18 (September 1991).
8 Dennis, Gary. “Picking the Best Thermoplastic Lining,” Chemical Engineering, pp. 122-124 (October 1998).
9 Glein, Gary A. “Dual-Laminate Tanks and Piping,” Chemical Processing, pp. 82-85 (February 1996).
10 Hanselka, R. Williams, R,; Bukay, M. “Materials of Construction of Water Systems - Part 1: Physical and Chemical Properties of Plastics,” Ultrapure Water, 4 (5), pp. 46-50 (July/August 1987).
11 Mikkelsen, Kirk, J. Alberg, Michele J.; Prestidge, Janice K. "Comparing the Purity of Commercially Available Fluoropolymers,” MICRO, pp. 37-48 (June 1998).
12 Patrick, Frank N. “High Purity Product Qualification Using Electron Spectroscopy for Chemical Analysis (ESCA) Methodology,” Ultrapure Water, pp. 54-58 (October 2000).
13 Goodman, J.; Andrews, S. “Fluoride Contamination from Fluoropolymers in Semiconductor Manufacture,” Solid State Technology, pp. 65-68 (July 1990).
14 Balazs, Majorie K. “A Five Year Study Using PVDF Pipes in an Ultrapure Water System,” ISPEAK Newsletter (July 1997).
15 Henley, M. "PVDF Remains Favorite Piping in Semiconductor Plants,” Ultrapure Water 14 (10), pp. 16-22 (December 1997).
16 Governal, Robert A. “Ultrapure Water: A Battle Every Step of the Way,” Semiconductor International, pp. 176-180 (July 1994).
17 Wulf, Brian "Pristine Processing - Designing Sanitary Systems,” Chemical Engineering, pp. 76-79 (November 1996).
18 Meltzer, Theodore H. “Extractables from PVDF Piping Systems Conveying High Purity Waters,” Pharmaceutical Technology (March 1997)

BENEFICIAL CONTAMINATION: PART 2

BENEFICIAL CONTAMINATION: PART 2
Last month we discussed how, under certain circumstances, a little contamination can be good for you. Commercially-produced heart valves of an alloy primarily of cobalt, chromium, molybdenum, and tungsten, Stellite 21, were nonthrombogenic; they did not induce potentially deadly clots because an easy-release hydrocarbon coating had been introduced as a manufacturing artifact. Two techniques, internal reflection infrared spectroscopy (Multiple Attenuated Internal Reflection, or MAIR) and critical surface tension, were used to study and monitor essential surface characteristics and, in effect, detect beneficial contamination.
MAIR allows analysts to obtain a fingerprint of films as thin as 10 A. Internal reflection spectroscopy involves placing the surface of interest in contact with an internal reflection plate. A beam of light is directed toward the plate in such a manner that it repeatedly reflects inside the plate where it contacts the sample surface; an augmented IR scan is obtained. Some clinicians prefer IR measurements over high vacuum techniques such as Electron Spectroscopy for Chemical Analysis (ESCA), which may provide more definitive identification of molecular species at a specific location on the sample than does IR spectroscopy; but because ESCA involves placing the sample in a vacuum chamber, there is the nagging concern that certain materials on the surface may “turn away” from the surface toward the bulk or even vaporize. While MAIR does not provide complete molecular identification, the infrared scans indicate functional groups, like methyl groups. In the case of the metal heart valves, identifying part of a molecule was sufficient. Methyl groups were found on the surface of vigorously polished, non-thrombogenic metal implant material; and, while vigorous aqueous cleaning had little effect, heavy mechanical scrubbing eroded the surface.
The second technique, critical surface tension (CST), is related to contact angle measurement, which is a refinement of the water-drop test. Contact angle measurement provides an indication of non-specific organic contamination. The contact angle between a liquid droplet and the surface is determined by the nature of the gas/liquid/surface interface. Looking beyond water, the contact angle is also influenced by the quality of the liquid with solutions being less accurate than pure compounds and actual material solutions being worst of all because they attack the surface. Surface quality of a variety of materials from paper to metal have been grossly characterized by marking the surface with dyne pens, which look at bit like magic markers and contain various solvents.
In the study of surface quality of heart valves, researchers determined the critical surface tension (CST) of a solid as measured in dynes/cm. CST, the highest surface tension any liquid (actual or theoretical) can have and still completely wet the surface of the solid, involves measuring contact angles for as many as 16 liquids of known surface tension. The surface tension of each liquid in dynes/cm is plotted on the x axis and the cosine of the contact angle on the y-axis (a Zisman plot). The CST of the solid is surface tension where the cosine of the contact angle equals one.
CST characterized the desirable surface quality of the heart valve material. The CST of the properly contaminated alloy, the alloy polished with organic-based compounds, was 20 to 25 dynes/cm. Alloy polished with organic-free abrasives had a CST of over 35 dynes/cm. Alloy polished with organics but then detergent-scrubbed also showed an increased CST of 27 dynes/cm. Contact angle measurements predicted performance of the implanted device. Lower contact angles, for example, were obtained if the coating was applied with rigorous polishing but with incomplete coverage or if the coating was applied without rigorous polishing and then water-washed. In both instances, devices were thrombogenic.
What can we extrapolate from these studies? First, the mere presence of material on a surface does not necessarily mean that it is a contaminant; it may be essential to proper performance. Further, surface characterization techniques should be selected because they provide the most useful information, not necessarily complete identification. While MAIR-IR does not provide complete molecular identification, it gives enough information about the immediate surfaces of implantable clinical devices, without the concern of altering the surface during sample preparation. Similarly, the overall indication of contamination using CST was predictive of surface performance. Finally, analytical and surface testing were used not dogmatically but rather pragmatically in conjunction with actual performance studies, in this case, in living systems.

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Making Informed Choices in Wet Bench Fire Safety

Making Informed Choices in Wet Bench Fire Safety
As the costs and consequences of failure grow, so does the list of fire-safe, approved materials.
In the precision manufacturing and process industries, the term “wet bench” generally refers to cleanroom process equipment that contains, dispenses, rinses, or in some manner processes or utilizes corrosive chemicals. The sheet plastics traditionally used in the construction of wet benches, polypropylene or polyvinyl chloride (PVC), provided good construction properties and corrosion resistance along with relative ease of fabrication and welding. These early materials, however, did not offer a high level of fire or flame resistance. The introduction of fire retardant polypropylene in the mid-1990s improved the flame resistance of wet bench materials.

Changing Standards
Until recently, the industry relied on UL94-V0 and V5a and ASTM E-84 standards to evaluate the performance of plastics when exposed to flame. UL94 measures the flammability of plastics used as components in appliances and other devices. E-84, also known as the Steiner Tunnel Test, measures the tendency of building materials to spread flame and produce smoke. None of these standards addressed specific concerns about plastics used in cleanrooms.
Risk underwriters and insurers, facing financial losses from major fires in wafer fabrication plants, demanded a reduction in fire risk from their insureds. Wet bench plastics were identified as culprits in accelerating flame spread and worsening damage to work in process and equipment from excessive smoke. In 1997 Factory Mutual Research Corporation (FMRC) released a new standard to measure the performance of plastics in these areas. This standard, titled Cleanroom Material Flammability Test Protocol 4910, references a fire propagation index (FPI) to measure a plastic’s fire propagation potential and cites a smoke damage index (SDI). Together, these indices are used to evaluate a plastic’s suitability for cleanroom use.
Using the FM4910 standard, plastic sheet manufacturers began to develop and modify sheet formulations to meet it. Submitted products were tested by FMRC. Successful candidates were given official listings as compliant with FM4910. In 2000 Underwriters Laboratories (UL) released |L2360, Test Methods for Determining the Combustibility Characteristics of Plastics Used in Semi-Conductor Tool Construction. This protocol also measures the fire propagation and smoke generation qualities of wet bench plastics. In addition, it provides a class rating of 1, 2, or 3, depending on the material’s FPI and SDI levels. Manufacturers have submitted materials to UL for testing and several sheets and resins have been UL2360 listed.

Spreading the News
With more options for wet bench construction, material selection has become problematic. For those buyers/specifiers whose insurers favored a protocol (FM or UL), their choices are limited to the listed fire-safe materials. In other cases, the end user has mandated fire-safe plastic use. In some municipalities the authority having jurisdiction (AHJ) has stipulated fire-safe plastic use or compliance with a protocol or industry standard. Yet there are also those situations in which no clear-cut directives are in place for the buyer/specifier that would limit their choices. Wet benches made from polypropylene and FRPP are still being built and installed in retrofitted and newly constructed fabs.
Dissemination of information about fire-safe plastics has been a slow process. Industry associations such as SEMI, Semiconductor Industry Association (SIA), Semiconductor Safety Association, National Fire Protection Association and others have reviewed the FM4910 and UL2360 protocols. In some cases they have endorsed them or included them in their own standards.
Adoption of FM4910 and UL2360 into building codes is slowly under way. Fire and building approval professionals learn about new materials through official documentation, their membership in relevant organizations, their industry contacts, the reading of published data and sales materials, and through their personal research.
The promotion by FM and UL of their own standards has become a primary source of information. The publishing and announcement of their protocols are important methods for tooling buyers, specifiers, builders, AHJs and interested third parties.

Choosing the Right Material
At best this objective approach provides the designer/builder with merely a list of approved materials. Still to be resolved is how to determine which listed materials are best suited for the wet bench in question. Three significant factors must be considered to help narrow down the choices:
* Functionality. The types of process chemistries which will come in contact with the wet bench must be examined to ensure that the appropriate fire-safe plastics are used. In some cases, tooling models offered by manufacturers may be used with a variety of chemistries. Thorough research will ensure that the material selected is compatible and rugged enough for all intended applications. The average workload of the installed bench may also affect the decision of which sheet material to choose. The conditions in which the bench will be located are a key factor. A hostile manufacturing environment may require a more durable, corrosion-resistant cabinet than if the only process chemistries contact were in the wetted areas. A trend toward hybrid tools has developed among equipment designers. In such units the shell is constructed from a fire-safe but less expensive cabinet-grade material.
* Budget. Opinions vary about the relative cost of wet bench plastics. In the overall cost of a tool the plastic sheet content may represent only 15-30% of the total expense. This is not to say that the comparative increase from one fire-safe material to another is not relevant or important, however. When multiple benches are constructed, significant savings may be realized when materials are selected according to performance requirements. The hybrid bench built from cabinet-grade materials (outer shell) and process chemical-grade materials (wetted areas) can be both cost-effective and well suited to the cleanroom environment.
* Builder Selection. The importance of which tool builder is selected cannot be minimized. Experience, technical expertise, reputation, and quality are important considerations. Judgment based on hands-on knowledge of fire-safe materials is invaluable. A savvy fabricator can save money, time and labor and build in tool longevity by helping select the best plastics for the job.

Communication Channels
There are many ways specifiers, designers, builders, AHJs, or third parties with approval responsibilities, can find up-to-date fire-safe plastics information in trade journals, at trade shows, and on vendor websites. If one were to ask wet bench fabricators, specifiers, buyers, and especially plastic sheet manufacturers, which information source has the greatest impact on fire-safe product selection, it would be the FM and UL websites. Proof that a resin, compound or sheet is indeed fire-safe must be documented by a listing on the respective websites. Until the product is posted on the FM or UL site, even with the written test and listing results in the manufacturer’s hands, the industry is reluctant to accept it.
The FM Global website, www.fmglobal.com, encompasses more than 1000 pages with information about its standards, insurance services, and its resource center. While its main function is to help its clients prevent and control property loss, it also provides information for customers, including manufacturers of listed products. A downloadable listing of FM4910 materials (40, as of this writing) is included the Sidebar.
The UL website, www.ul.com, unlike the FM Global site, does not offer a unified posting of all UL2360-listed products, but instead offers search capability by company name and product number.
It is clear that the web has become a viable, vital research, sales, and teaching tool. An internet presence is now a necessity; since 1995, when many companies were either starting their websites or merely thinking about the idea, the number of sites has exploded exponentially. The ways we use the web now to make decisions about fire-safe plastics, for example, have changed how we do business and on which experts and data sources users rely.

For Further Reading
FM4910: Cleanroom Materials Flammability Test Protocol, September 1997, Factory Mutual Research, 1151 Boston Providence Turnpike, Norwood MA.
NFPA318, Standard for Protection of Cleanrooms, 2000 Editions, National Fire Protection Association, 1 Batterymarch Park, Quincy MA.
“Process Compatibility Parameters for Wet Bench Plastic Materials,” International Sematech, 2706 Montopolis Drive, Austin, TX.
SEMI S2-0200, Environmental, Health & Safety Guideline for Semiconductor Manufacturing Equipment, Semiconductor Equipment and Materials International, 805 E. Middlefield Road, Mountain View, CA.
UL2360: Test Methods for Determining the Combustibility of Plastics Used in Semi-Conductor Tool Construction, Underwriters Laboratories, Inc., 333 Pfingsten Road, Northbrook IL.
“Wet Bench Fire Safety: Update,” A2C2, February 2000, pp 17-19.
“Wet Bench Fire Safety: One Issue Fizzles, New Ones Ignite,” A2C2 February 2001, pp 19-26.

Fundamentals of Cleaning: Drying, Part 2

Evaporation rates are slow, we learned, because the rate of heat transfer from air to parts is quite low because gas-solid heat transfer coefficients are so poor. In last month’s column, a figure showed how these coefficients have little dependence on temperature, but de-pend greatly on air velocity.
Is heat transfer coefficient the whole story? No, indeed. It is the rate at which heat can be transferred from the moving air stream to the water which controls drying rate. What’s the difference be-tween coefficient and rate? The defining equation is:
heat transfer rate = (heat transfer coefficient) x (part surface area) x (temperature difference)
The units are:
BTU/hr = (BTU/hr - ft2 - °F) x (ft2) x (°F)
Temperature difference is that between the free stream air and the water on the parts.
A key point is that heat transfer coefficient has little dependence upon air temperature, but heat transfer rate is highly dependent upon air temperature, and that rate limits drying rate. Let’s examine some practical situations. We’ll use 10 ft2 for surface area and ambient temperature (75°F) for water temperature on parts. Remember, that the heat of evaporation of water is about 1000 BTU/lb. Figure 1 of this column will control.
The same format is shown above for heat transfer rate. Suppose we need to evaporate ~10 lb/hr of water. The heat transfer rate is ~10,000 BTU/hr.

Which should we prefer?
From the standpoint of utility costs, the one with the highest free air velocity will be significantly cheaper. It costs much more to heat air than to make it flow. How about part integrity? Some parts can’t stand temperatures of up to 350°F. Plastic components may warp. Metal components may be damaged by unwanted chemical reactions. From the standpoint of particle contamination, the least air flow is the better choice.
There are two aspects to this point. First, suppose this operation is done in a cleanroom. Then particle loading is probably light, and it is of sub-micron particles. The velocities required for drying of parts are at least 10X higher than would be observed at the face of a filter. A higher air velocity, or volumetric flow, will contact the presumably cleaned part surface with more sub-micron particles. Thus the good cleaning work may be negated.
Second, suppose the operation is not done in a cleanroom. These air velocities, 25 to 50 ft/sec, are large and will convey large, small, and sub-micron particles onto the cleaned part surface. The velocity in the center column of Table 1 is equal to the calculated terminal velocity for a particle whose size is given in the right column of that table. Here, the cleaned part surfaces become infected with particles of all sizes.
There are valid reasons to chose both high and low air velocities, depending upon what will happen to your parts after processing.

Vacuum drying, another method of evaporating water, can resolve the dilemma. Vapor pressure and vaporization temperature are correspondingly reduced. Hence evaporation rate is enhanced without having to use high air velocities.

Drying can be done by converting the liquid water to a solid, liquid, or a gas. One saves time and money by using impact-based methods to remove liquid water before using evaporation. The latter is expensive and can make clean parts dirty.

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Sampling for Airborne Biological Contaminants: A RATIONAL APPROACH

Sampling for Airborne Biological Contaminants:
A RATIONAL APPROACH
Nothing we do raises as many questions as microbial air sampling.
While the science of microbial air sampling is fairly straightforward, somewhere along the line our approach to it has become much like that of a backward soothsayer who divines the question after first being given the answer. For example, we often first buy the sampler and then try to fit it into our operation
Sampling strategies differ when applied to quality control, validation, or research. Each requires a differing degree of sampling sophistication and a different approach to data analysis. All microbial aerosol samplers must be judged in terms of their capability to collect microbes under different operating conditions while minimizing the environmental stress on the organisms collected. There is, therefore, no single sampling method that is suitable to all occasions.
Another point to remember: rarely is microbial air sampling conducted as a quality control point of a product. More often than not, air sampling is done to determine the cleanliness, or absence thereof, of the environment in which a product is manufactured. Or in this instance, to answer the familiar question: how clean is clean?
Sampling Theory
Microbial air samplers are characterized by:
* Mode of capture,
* Flow rate and flow characteristics, and
* Collection efficiency as a function of particle size and shape.
As a rule, aerosol collection devices that exhibit the lowest shear forces collect samples where microorganisms have the highest viability. Conversely, these samplers usually have the lowest physical efficiencies in terms of numbers of airborne particles collected. The microbiological collection efficiency, therefore, depends largely upon the sampling method used.
The primary objective of any sampling program is to produce a set of samples that are representative of the source under investigation, and, that these samples are suitable for subsequent analysis. Because the air we sample in any environment is not homogeneous even in a clean room that exhibits good laminar airflow, there can be no duplicate samples. We therefore need to consider sampling conditions, sampling time, and sample size as limitations in our data collection scheme.
Collecting a representative sample of airborne microbes is probably the most difficult to achieve. Apart from the inherent absence of microbial uniformity in air, is the problem of ensuring that particles of all sizes have an equal probability of entering the sampler. This can be partially remedied when the sampling rate is chosen so that the velocity of air entering the sampler inlet equals the velocity of the air being sampled.
Achieving or approaching isokinetic sampling conditions is particularly important in an area in which air sampling is done under dynamic conditions. If the velocity of the air entering the sampler is greater than that of the room’s air movement, small particles will predominate because they move more easily across the streamlines. Conversely, if the velocity is slower, larger particles predominate because, unlike smaller particles, they do not follow the curvature of the streamlines around the sampler inlet.
Anisokinetic sampling may result in sampling errors that range from 20% to 300%, depending upon particle size and varying environmental conditions. Ideally, to overcome the effect of dynamic air movement on the capture of microbial aerosols, stagnation point sampling can be used. In the absence of any air movement, smaller particles are efficiently captured and a particle size profile estimated.
To further complicate this issue, consider the following: air is not a natural environment for most microbes. Survival of microorganisms in air is affected by a large number of environmental factors, the most important of which are temperature and humidity. Under natural conditions these numerous factors operate simultaneously. Consider also that force is required to generate an aerosol, and likewise, to capture particles within that aerosol. These forces can damage or even fracture fragile structures such as microbes in their vegetative state.
The fragile nature of airborne microorganisms is largely species dependent, and determined by its physiological condition. Once airborne, microbes become stressed through desiccation or hydration depending upon the condition of their natural growth site. Radiation, oxygen, ozone and various other gaseous and particulate pollutants, if not lethal, may further stress these organisms. Some stressed and injured microorganisms may, however, fully recover when given a suitable environment. This property of reversible injury or repair in microorganisms is widespread and the implications of it are important in developing the testing protocol.
The agar medium selected for use in all microbiological sampling devices should therefore be fresh and pre-screened for sterility by placing it in an incubator at 36°C for 24 hours. In conducting the initial microbial assays, malt extract agar is recommended for the general detection of fungi while agar containing casein peptone, soy peptone, and sodium chloride is used for bacterial sampling. Trypticase soy agar has probably the most universal applicability for the collection of aerobic bacteria and fungal species, whereas, for the detection of anaerobic species, a thioglycollate medium is recommended. Once initial bioburden estimates are established and a greater specificity is required to target certain organisms, specialized agars containing antibiotics and/or other inhibitory and growth regulating compounds are available from commercial sources. However, when these selective media are used in an air sampling devices, the collection efficiency may be severely hindered. These media are generally inhibitory to small inocula; even of the organisms for which they are “selective” by retarding recovery of those that are injured or stressed.
Because the organisms found in air come from different environments, the temperatures used to enhance their growth on an artificial medium should approach that of their normal habitat. Most of the organisms found in the air fall within two distinct temperature preferences. The psychrophilic organisms, or those that prefer cold, thrive at low temperatures between 0° and 30°C, while the mesophyllic organisms prefer moderate temperatures between 15° and 43°C. Therefore, to recover the maximum number of organisms in any air sample, consider incubating the medium at 22°C for 24 hours (or 48 and 72 hours as necessary) to recover psychrophilic organisms, immediately followed by incubation at 36°C to recover the mesophyllic organisms.
The outcome of a well-planned sampling strategy depends on good science, logic and to a lesser degree, a measure of good luck. Taking the time to estimate the types of organisms that may be present; describing the static, dynamic and physical characteristics of the clean room and its air, and, hopefully conducting a viable/nonviable particulate profile of the space to be sampled, will yield data that becomes the basis for the entire microbiological sampling scheme.
Since there is no single agar medium on which all microorganisms will grow, no single incubation temperature that will encourage growth, and no single assay procedure that can completely characterize the microbial contamination in all environments. Likewise, there is no universal sampling device.
Sampler Selection
Because of the different modes of collection used by the various microbiological air sampling devices, it is both difficult to compare and contrast their sampling efficiencies. Sieve samplers, for example, provide particle size distribution, while slit-to-agar samplers are used to determine the airborne bioburden as a function of time. Settling plates are suitable to locate a point source where larger particles are generated; centrifugal samplers provide an easy, rapid means of taking numerous samples of the gross airborne bioburden where size and temporal considerations are unimportant; low shear-force liquid impingers are ideal for the recovery of stressed organisms and therefore provide the best recovery of the widest range of airborne microorganisms.
The following is a brief review of the types of microbial air samplers and their use.
* Gravitational Samplers. Settling plates are the simplest forms of collection of airborne biological particles. A Petri dish containing agar medium will collect particles that settle by gravity. It is passive, non-volumetric, and imprecise by over-representing larger particles due to their rapid settling rate. Collection in turbulent air is seriously affected by shadowing or turbulent deposition. Settling plates, however, can be used as an adjunct to other sampling methods or as a pre-screening tool in areas suspected of containing a significant bioburden where deposition is most likely. For example, placing settling plates around water purification equipment during servicing may indicate the need for improved aseptic technique if viable droplets are deposited in decreasing concentric concentrations from the aerosol source. Settling plates can be useful to initially evaluate the airborne microbial release of cooling towers and condenser units.
* Impingers (AGI-30). Most impingers are designed to operate by drawing aerosols through an inlet tube that is curved to simulate the nasal passage. The air is subsequently passed through a jet into a liquid medium. The jet, which is positioned 30 mm above the bottom of the glass impinger, consists of a short piece of capillary tube. When the pressure-drop across this capillary tube attains a minimum of half an atmosphere, the flow through it becomes sonic and may therefore be rate-limiting. The most efficient sampling rate for the AGI-30 to capture particles in the 0.8µm to 15.0 µm range is 12.5 l/min. Larger particles are collected on the curved inlet and are recovered by pipetting a known volume of collecting fluid into the impinger inlet; which then flows slowly through the jet into the impinger.
The usual volume of collecting fluid is 20 ml, but depending on application may be reduced to as little as 2 ml to increase the concentration of collected microorganisms. The collecting fluid can be plated, filtered or diluted as necessary. Sampling time should be adjusted to prevent evaporation of the collecting fluid and cooling of the sample. The collection medium used with an impinging type sampler should prevent osmotic shock of vegetative organisms. Because deionized water may promote lyophilization, a dilute poly peptone-peptone broth or a physiological saline transport medium may demonstrate greater recovery efficiency.
The AGI-30 was designated as the reference standard sampler and the confidence in this device is largely historical. For this reason, the all-glass impinger is the best unit to determine the initial bioburden of an area.
* Impactors. Inertial forces are responsible for impaction action. The impactor is basically a jet, under which is an agar impaction surface. Particle-laden air sucked through the jet is directed at the collection agar surface so that particles impact onto it. In a cascade impactor, particles are captured by inertial force. Particles impact in cascading order: larger particles are captured first while those smaller remain airborne and follow the streamlines; each successive stage having increased air velocity through the corresponding jet. This type of sampler is the most commonly used in cleanroom air sampling. Several types are designed to serve various applications.
* Stacked Sieve Samplers consist of a series of six stages each composed of a plate having 200 holes. Each plate is held above a Petri dish containing nutrient agar with successive plates having smaller holes. At constant flow (28.3 l/min), largest particles impact on the first stage whereas smallest ones impact on the last impaction stage. Unless sampling times are short or the relative humidity high, the areas of nutrient agar directly under each hole of a stage can rapidly dry.
Fastidious microorganisms consequently may fail to grow on the upper stages due to the desiccation of the agar, but grow on the lower stages because the relative humidity of air increases as it passes through the sampler. Covering the agar with a water-evaporation retardant, or increasing the water content of the agar can reduce this problem. The precision with which airborne microbes are captured depends on the collection surface. The agar containing Petri dishes need to be level and filled precisely with the recommended amount of agar to give the correct plate to agar surface distance. Using plastic Petri dishes may give rise to an electrostatic effect, but it can be argued that the small problems of wall losses can be offset by the practicalities of use. The major advantage of multiple stage cascade impactor, is that it can provide particle size data.
There is a two-stage version of the impact sampler that separates viable particles into “respirable” and “non-respirable” fractions. The upper stage corresponds roughly to deposition on stages 1 and 2 of the six-stage version and the lower stage, to deposition on stages 3 to 6. However, side-by-side comparisons with the six-stage version revealed an approximately 40% lower recovery rate of bacteria with the two-stage sampling device, except when sampling dilute concentrations of particles larger than 1 µm. Using the correction factors provided by the manufacturer when completing the count can minimize this difference in efficiency.
There are several variants of the sieve sampler, including one-stage devices. While these units will not provide particle size distribution, they are efficient collectors across the entire size spectrum. These units have the advantage of being rapid and consistent but should be used only as a validation tool, once the scope of the bioburden is known.
* Slit-to-Agar Samplers. In slit-to-agar samplers, air is drawn through a narrow slit, then accelerated and directed toward the surface of a Petri dish containing agar media. The collection surface is often placed on a rotating turntable. Airborne microorganisms impact onto the agar surface and are separated spatially by the plate’s rotation; thereby providing a stochastic analysis. The sampler is operated at a fixed sampling rate of 50 l/min; rotational speeds can be varied and the collection efficiency is for particles 0.5 µm and higher. This sampler is best used to monitor the effect of materiel movement or operational variations of support and production equipment.
* Centrifugal Samplers. The principle of collection of these sampling devices is centrifugation, which involves the creation of a vortex in which particles with sufficient inertia leave the airstreams to be impacted upon a collection surface like a semi-solid medium. The most frequent example of this type is the Reuter centrifugal air sampler. Air is drawn into the sampler by an impeller housed inside an open shallow drum. The air is then accelerated by centrifugal force toward the inner wall of the drum. Lining the inner wall is a plastic strip supporting a thin layer or agar medium, onto which airborne particles are impacted. These strips are subsequently removed and incubated. The motor for the impeller is battery operated and the whole sampler unit is small enough to be hand-held.
There has been some concern expressed about its effective flow rate and collection efficiency. Air enters and leaves the sampler through a single opening; therefore flow rate quantification is theoretical. The sampling rate is calculated from the impeller revolution of 4092 rpm; yielding an effective sampling rate of 40 l/min. Studies have shown, however, that the effective sampling rate may be nearer 100 l/min under certain conditions. While these devices collect larger particles (15 µm) quite efficiently, less than 10% of particles under 2 µm in diameter are deposited. Therefore comparisons to the stacked sieve, slit-to-agar, and liquid impinger samplers cannot be made. A newly redesigned Reuter sampler allows separate entry and exit airflow that may improve its performance, but provides an actual flow rate and can more closely approximate isokinetic conditions.
Conclusion
If different groups of organisms are to be studied, which necessitates the inoculation of different media, repeat samples must be taken at each sample point. This increases the sampling time and may result in errors if rapid and large variations occur in the air. This sampler however has the distinct advantage of ease of use. Additionally, if the sampling protocol is properly defined, it can be used as its own control and the need for volumetric sampling becomes moot. Rapid comparisons of qualitative microorganism profiles in the intra- and extramural air can be made to determine the proper sampling strategy via quantitative instruments. This makes the Reuter centrifugal sampler the most versatile instrument as a pre-screening tool.

Fundamentals of Cleaning: Fundamentals of Cleaning: Drying, Part 1

Critical cleaning professionals know that drying is one of the latter stages of the work called cleaning. Simply put, drying is typically the step of cleaning water from parts.
Many believe that "evaporation" and "drying" are the same. As a method of liquid removal, however, evaporation often presents serious concerns. There are other methods of removing water from parts.
In this and the following columns, we'll look at drying of parts from a mechanistic point of view.
Water can be removed from parts in three ways. This is because water can exist in three distinct phases: solid, liquid, and vapor. Drying can be thought of as being a process of phase transformation (and removal of the newly formed phase). For example, evaporative drying is removal of liquid water by converting it to a vapor.
Freeze drying plays an important role in food processing. It is also some-times used in cleaning some piping systems or small complex structures. In freeze drying, the temperature is dropped to <32 °F. Liquid water is transformed to solid water. But the drying isn't complete until the "soil water" (ice) is removed from the food (or parts). And there's the rub. If ice particles can be removed, usually via entrainment in high velocity air, the structures can be made totally free of water. But this can be difficult.
Liquid drying, an infrequently used technique, involves force--often, gravity. Because liquid droplets have more mass than the same volume of air, liquid droplets drain water from parts. Water, which has a very high surface tension, tends to form in larger droplets, films, or sheets. If these agglomerates of water are not trapped by some convex characteristic of the parts, large amount of water will drain wonderfully well. Some have harnessed surface forces to literally extrude films of water, in "sheets," from parts.
The force of gravity can be enhanced by other forces. If the parts are vibrated (and possibly rotated), sheets of water can be quickly dislodged. Usually, about half of all the water on parts can be removed through accelerated gravity-based drying.
The most common force used in drying is impact by high velocity gas, usually air. One firm makes a nozzle called a "Wind Jet" that accelerates compressed air to a velocity around 1000 ft/ sec. Pointing this nozzle at a wet part can dry it in less than one second! My data show this method of drying evaporates only ~10% of the water removed. The quality of drying achieved, called "dry to the touch," may be perfectly adequate, or a starting point for a second phase transformation.
Conversion of Liquid to Vapor (Evaporation) To most of us, evaporation is drying. Evaporation can be time-consuming. It can make the numbers on your electrical bill look like the national debt. Furthermore, soluble minerals in the water wont evaporate, but will remain as deposits (water spots) on your part surfaces.
The rate of drying parts is limited by the rate at which heat can be transferred to the water, causing it to evaporate. Slow heat transfer from gas to wet parts is normally the limiting process step. Though slow, evaporative drying is the best way to produce total dryness.
Said another way, gas-to-solid or gas-to-liquid heat transfer coefficients are quite low. They are a function of the physical properties of the air, temperature, and the air velocity. Most users expect that temperature has more effect on heat transfer coefficients than does air velocity. But the reverse is true. As shown in Figure 1, temperature has only a lightly negative effect on heat transfer coefficient. It is increased air velocity, not temperature, that can make the difference in drying rate. Air moving at 5 to 50 ft/ sec removes little water by impingement--just the opposite of air moving at 1000 ft/ sec.

Fig. 1. Drying coefficient varies little with temperature.

Preventing and Measuring Contamination In And Out of The Cleanroom

BENEFICIAL CONTAMINATION

 Sometimes, a little contamination can be good for you--even life-saving. This may be counterintuitive to those involved in contamination control or analytical chemistry; there is, after all, the tendency to assume the fewer contaminants, the better.

To understand the value of strategically placed contamination, we need to take an historical perspective. When biologists, for example, attempted to control growth conditions by using high-purity water, they found that many plants and microbes grew poorly in deionized water (DI) because plants required trace impurities which came to be known as micronutrients. This phenomenon is not restricted to biological systems. Ultrapure, high-ohm water (UPW) can remove trace metals and produce corrosion or other undesirable surface modifications. Small amounts of oils remaining on a metal surface after solvent cleaning may prevent corrosion. If a process change, for example from a solvent to a surfactant, eliminates the trace beneficial contaminant from the surface, the component may corrode. If an anticorrosion additive is introduced, it may be necessary to redefine the appropriate surface cleanliness to ensure that the additive is not contributing to contamination.
We all intuitively support the concept of contaminant-free biomedical implants, but sometimes a specific contaminant is needed to produce the appropriate surface. Back in the 1960s, researchers began to make progress in developing nonthrombogenic biomedical implants, implants that don't tend to produce blood clots, which can be life-threatening. As any contact lens wearer knows, when foreign materials are introduced into the body, there is a tendency toward protein build-up. Where implants are exposed to the bloodstream, as in heart valves, it is important to avoid the kind of protein which attracts platelets and forms a clot. If a clot were to remain in place in a heart valve, it could cause mechanical blockage; if a clot were to grow and then break free, a variety of vascular problems, including stroke, could result. After much trial and error (fortunately, predominantly in animal, rather than human experiments), commercial implants made of Stellite 21 were found to be successful.
No one knew quite why Stellite 21, an alloy primarily of cobalt, chromium, molybdenum, and tungsten, was nonthrombogenic, but a young physicist, Robert Baier, Ph. D., had became involved in the analytical detective work. It wasn't obvious why this particular alloy would be superior to some other alloy, pure metal, or metal oxide. Bob Baier obtained and examined some of the existing implants; they were highly polished (surgeons prefer a very smooth, shiny surface). Freshly made Stellite investment castings, however, were dull gray.
Surface smoothness does not in and of itself confer nonthrombogenic qualities. When devices made of the same alloy were polished metallographically and then implanted, they produced the same undesirable, dangerous blood clots as other materials. In contrast, when the commercial devices were removed and examined, in some cases years after implantation, there was the expected buildup of protein, but there was no clot formation. The reason was that the device manufacturer had fortuitously chosen a diamond-in-tallow polish. Residue of this polish, the presence of which on the surface might be considered undesirable contamination, was the key to achieving nonthrombogenic properties. Protein adhered, but clots and scar tissue didn't form.
One might reasonably ask why we don't observe spontaneous formation of clots in uninjured blood vessels. Researchers analyzed the interior surface of excised jugular veins and determined that the innermost layer behaved as if it were primarily hydrocarbon composition, a low-surface-energy layer, in spite of its hydrophilic (" water-loving") character. Not surprisingly, the surface of the commercially polished alloy was found to be comprised not of metal but of methyl groups (CH 3 ), similar to normal blood vessels. The waxes used in polishing the devices were mixtures of long chain fatty compounds (primarily stearic and palmitic acids). Simply put, coating the metal with the polishing compound (the contaminant) didn't result in a clot-resistant product; the energy associated with final polish facilitated a reaction between the chromium and the waxes to form an adherent metallic soap which is covalent-ly bound to the underlying surface.

Related Topics:

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PARTICLE TESTING FOR CLEANROOM FORMS AND LABELS

PARTICLE TESTING FOR CLEANROOM FORMS AND LABELS
Whether your company is in the semiconductor, disk drive, pharmaceutical, or biotechnology industry, you have invested considerable time and money to ensure the yields and efficiency of your production processes. Every day, you face the substantial challenge of protecting the product you are manufacturing by choosing the correct materials to be used within your manufacturing environment, as well as the materials used to identify and track your product and process during and after its conversion. Many of these latter materials are forms, labels, and tags.
The cleanroom form, label, and tags industry draws most of its materials specifications from Federal or Military standards. Cleanliness testing procedures are drawn from yet another array of specifications ranging from NASA KSC-C-123H to IEST-RP-CC020.2.
Traditionally, these items are tested for sub-visible particle levels using wet method testing, followed by particle enumeration using filtration, electronic laser, or microscopy. Wet method testing is the most accurate and detailed method of testing for particle counts on materials. Most, however, would argue that these methods do not represent the overall average particulate cleanliness of the items upon delivery and do not offer a cost effective means to accurately monitor particle counts over a broad statistical sampling for a specific production run. Most manufacturers, and their customers, will use this "worst case" data, derived from either wet or dry method testing, and will presume that the product will consistently meet this criterion. Put another way, the data sheet says it is a Class 10 cleanroom product, so it must be a Class 10 product. Unfortunately, Federal Standard 209 E deals with a cubic foot of air, not a surface of a label or its liner. Therefore, there is no such thing as a Class 10 or Class 100 labels. To make matters worse, the majority of label and tag vendors do not know what they are producing and will make the subject even cloudier.
How do we deal with this issue? Make no mistake: producing precision surface cleaned products is difficult. Precision surface cleaning of forms, labels and tags (down to 0.3 µm particles) requires a combination of different processes to achieve the cleanliness levels needed in today's critical cleanroom environments.
 Precision surface cleaning com-bines electrostatic ionization to neutralize the triboelectric surface charge that develops during the manufacturing process, air knife and contact cleaning of the material during and after the manufacturing process. Pre-cleaning (with an air knife) of the materials will focus on the 1 µm and larger particles and precision surface cleaning (contact) of the materials will break the boundary layer where 0.3 micron particles lay. In addition, they must deal with ionic contamination and nonvolativle residue (NVR) contamination from not only the materials themselves but also the processes they use to manufacture and convert the products. Finally they must be able to monitor particle counts on two surfaces (top and bottom) of the material and keep their cleaning process within specification.
To accomplish this some, manufacturers have turned to a laser based surface particle measuring instrument (example: Pentagon's QIII^TM) to ensure statistical process and material particle cleanliness. The measurement device, and its probe, are stationary and the material surface is drawn under the probe at a constant rate of speed in a Class 10 minienvironment. Particles on that surface are disturbed and fluidized by high pressure ULPA air discharged through openings in the bottom of the probe head. These particles are then pulled through the vacuum port in the probe head to the QIII^TM detector, which counts and analyzes the particles. The machine will then display the particle counts in 0.3 µm to 5 µm readings.
In today's critical environments, it is mandatory to be able to clean and measure surface particles at 0.3 µm levels. It is essential that techniques and standards be developed to aid in the detection and measurement of these surface particles in a cost effective and repeatable method. Although no current industry standard exists for this method, it does offer statistical and repeatable measuring for both the manufacturer and the user.

Cu Integrated with Low-k Dielectrics: The Future Is NOW

Speed no longer depends on feature size, but on interconnect distance.
To allow the continuation of Moore's Law, IC manufacturers have increased the power of semiconductor devices by decreasing feature size. However, the limiting factor the industry is facing when reaching 0.13 µm technology nodes and beyond is an increase in signal delays at the interconnect level. Smaller feature sizes mean increased density, closer proximity of circuit interconnects and bigger line-to-line capacitance, which results in an even greater signal delay. Interconnect delays increase with the square of the reduction in feature size whereas gate delays generally decrease linearly with the same reduction in feature size. When reaching the 0.18 µm node, the speed performance of a device no longer depends on its feature size but on interconnect distance.
The conventional approach to compensate for this increased delay is to add more layers of metal, but this increases production costs and generates more heat in the device, affecting its performance and reliability. To avoid these cost and performance problems and still allow the continuation of Moore's Law, the industry is migrating to copper instead of conventional aluminum as the interconnect metal. Copper has much greater conductivity than aluminum and is less susceptible to electro-migration, allowing the copper lines to be thinner under current load.
However, to significantly increase speed performance in next generation devices, copper must be integrated with ultra-low-k dielectrics (k< 2.5). The transition to copper alone only improves speed performance by 30% but when silicon oxide (k= 4) is replaced with ultra-low-k dielectrics an increase in speed performance can be as high as 266%. Today's low-k dielectrics strategy is a gradual migration from oxide (k= 4) to fluorinated oxide (k= 3.5) to low-k dielectrics (k< 3) and finally to ultra-low-k materials below k= 2. And since each generation of dielectric materials has different mechanical properties and characteristics, device manufacturers need to develop CMP and other related processes for each generation of dielectric material. Such a multi-step strategy is, however, very costly and high risk because of the uncertainty for the success of device manufacturability, tool and process extendibility, manufacturing yield and device reliability. An alternative to the migration strategy described above is to leap directly to ultra-low-k dielectrics below 2.2. The risk and cost of migrating to tighter design rules will be substantially lower because of the elimination of expensive development cycles for each generation of dielectric and its associated process integration challenges. Ultra-low-k dielectrics are generally porous and their k value can easily be changed by increasing the porosity without changing the actual material and process tool. However, the ultra low k dielectrics used in copper structures have insufficient adhesion and mechanical strength to survive the stress placed on them by a conventional CMP process.

Fig 1a Rupture in the line Fig 1b Delamination of low k Fig 1c Stress free polishing
The physical limitations of CMP especially for 0.13 µm manufacturing nodes and beyond have created a challenge for the IC industry in adopting low k material and aggressive design rules. Developing a viable solution for these processes integrated with ultra low k dielectrics without compromising device performance reliability yield and overall cost of ownership has become most challenging. Due to the mechanical force applied in conventional CMP, copper lines are moved back and forth during the polishing process. This results in critical damage to the interconnect structures Figure 1a, delamination of the ultra low k dielectric Figure 1b, and eventually yield loss in the current 0.13 µm process technologies making it highly unextendable for future technology nodes.
A stress free polishing SFP technology developed by ACM Research is the semiconductor industry's first to provide a copper low k integration capability that enables the leap from conventional oxide to ultra low k dielectrics. The new process called Ultra SFP^TM induces no mechanical stress on the wafer because it is a non contact electric current controlled process Demonstrated results Figure 1c confirm the absence of mechanical damage to copper interconnect lines or to the ultra low k dielectrics. Since mechanical strength is no longer an issue for the low k dielectric process integration these results allow for an accelerated implementation time frame of ultra low k dielectrics into copper interconnect structures. Migration to smaller design rules is made easier without the impact of the costly development of each generation of low k dielectric slurries pads qualifications characterizations of the process and related integration issues
The Process The concept was developed to remove copper by using a stress free process based on electropolishing as in reverse electro plating In the past experiments with electropolishing were abandoned because of unsuccessful results on the wafer. When the technique was applied to polishing copper on a wafer difficulties were encountered in controlling thickness uniformity from the center of the wafer to its edge. The profile of a copper film polished with the conventional electropolishing method showed that the thickness of the copper film at the edge of the wafer was close to zero. However the center of the wafer showed over 3000 Å thickness forcing the polishing process to stop otherwise no current would pass through the center of the wafer to the edge where the electrode is located. These problems were resolved by applying the new patented technology employing localized control ensuring a highly uniform removal rate from the center to the edge of the wafer independent of wafer size. The SFP system uses localized current and voltage control dividing the wafer into zones that are polished in sequence starting at the center. Think of it as the circular ripple of a raindrop when it hits the water.

Figure 2 The Ultra SFP can achieve a throughput of 30 wafers per hour using multiple stacked chambers accessed by dual robot end effectors
The precise control of power supply allows the SFP process to control the removal of copper at the atomic layer level meaning its copper removal rate is  proportional to the current density. In addition, ACM has demonstrated on customer wafers a thickness removal within wafer nonuniformity of 0.8 to 1.2% (1 sigma) and corresponding thickness removal wafer-to-wafer nonuniformity of 0.59% (1 sigma).
Because only the copper is removed during the Ultra SFP process, no dielectric loss or erosion has been observed using the technique. This will significantly improve the global planarity of the interconnect layer as the number of stacked interconnect layers increases, particularly for most logic devices such as CPUs or ASICs, which may have up to ten layers in the future.
The stand-alone, single-wafer processing system can achieve a throughput of 30 wafers per hour using multiple stacked chambers accessed by dual robot end-effectors (Figure 2). The use of multiple stacked modules, three polishing and three cleaning modules including bevel and backside cleaning, reduces the system foot-print dramatically to 6 x 10 x 8 ft, about half the size of a typical CMP tool, enhancing the system's COO. The stress free polishing system also achieves a significantly lower cost-per-wafer than CMP tools because it does not use slurries, pads, and pad conditioning. In addition, capital productivity is higher due to elimination of process-induced defects such as erosion, delamination, dielectric loss, lithography de-focusing and scratches.

Removing Particles With A Foam Medium

To achieve acceptable yields, synthesis sequences must approach perfection.
The conversion of silicon wafers into useful chips is a complex, multi-step, chemical process. Unlike conventional chemical synthesis, in which intermediate isolation and purification can improve overall purity and yields, as the feature size of chips diminish, semiconductor synthesis sequences must approach perfection in order to achieve acceptable yields.
When viewed as a chemical process, semiconductor chip production exhibits some characteristics not often found in conventional chemical manufacturing, such as:

·        Chemistry is on the wafer surface, not in the bulk medium. Chemical consumption is low. The number of moles consumed during treatment is small, even though the total moles present in the tank volume may be large.

·        Reactants and solvents must be extremely pure. The low molar consumption means low levels of chemical impurities can compete easily.

·        The equipment produces significant particulate impurities.

Why foam? Foams are metastable, created by adding mixing energy to a liquid and gas, yielding foam, such as shaving cream. The foam volume is approximately 15 times larger than the original liquid volume, or has an expansion ratio of E/ R= 15. Immediately following production, foam starts to decay, reverting to the expansion gas and the original liquid state. This is called draining and the rate of draining is the drain time. Advantages of foam include:

·        Energy to create foam can be added to the gas/ liquid mixture and then transported to the substrate so that the substrate will not be subjected to the energy input.

·        Draining starts immediately, forming the original liquid phase. The substrate is treated with the same composition whether delivered via foam or condensed phase liquid.

·        Drainage rate can be controlled.

·        E/ R of the foam is generally between 10 and 20, reducing the volume of reactants and solvents by a factor of 1.0/( E/ R). This E/ R effect will also reduce the system particulate exposure, as less material will pass through the treatment vessel. Foams can be a delivery medium for other ingredients.

·        Foams exhibit thixotropic flow properties, flowing best under shear.

·        Foam bubble walls provide surface tension gradients.

Megasonic cleaning is an accepted technique for the removal of particulates from wafers. This process involves subjecting the contents of the liquid bath to a beam of sonic energy of appropriate frequency produced by a transducer assembly attached to the vessel wall. The higher frequency of mega-sonic systems produces smaller bubbles/ waves and is less damaging to the substrate, while also capable of removing smaller diameter particles. Shwartzman recognized a possible particle size limitation for megasonic cleaning and suggested effective removal down to a size of 0.3 microns. ¹
Shortly after Shwartzman's megasonic disclosure, there were two other important cleaning concepts described. The first, Dussault ² , defined that semiconductor wafers could be effectively cleaned by treating the wafer surface with a thin film of flowing liquid while at the same time exposing the wafer to ultrasonic energy. The second was Banks ³ , who described a deposit cleaning technique, primarily for boilers and heat exchangers, in which the chemical cleaning solution was agitated by allowing a dissolved gas to "boil" as the pressure was reduced, followed by re-pressurization, and the cycle repeating.

Leenaars ⁴ apparently recognized the link between megasonics and the Dussault technique. He stated that in the megasonics method, the force by which the particles are removed from the surface of the substrate depends upon the cross-section of the particle to be removed and hence is proportional to the square of its radius. The force by which the particle adheres to the substrate, on the contrary, is directly proportional to the radius of the particle.

Experimental work has shown that the removing force, which can be exerted by an "interface of a liquid" on a particle, is a force caused by the surface tension of the interface and is directly proportional to the radius of the particle. Leenaars specifically identifies the term "interface of a liquid" as the surface of a liquid, the phase boundary between a liquid and a gas, and the phase boundary between two liquids.

When this information was coupled to the wafer/ interface contact rate concept, identified by Dussault ² , the result was the commonly accepted idea that wafers exiting solution tanks will be more particle free if the rate of exit is relatively slow, preferably at a speed lower than 10 cm/ sec. ⁴ The first application of this particle removal concept was wafer drying, particularly emphasizing watermarks, which occur during the drying process on clean silicon wafer surfaces. These watermarks are the result of hydrolysis of the very pure water, producing small amounts of hydroxide ion, which, in the presence of oxygen, allow the silicon substrate to oxidize, creating an oxide deposit upon final drying:

H 2 O = H+ + OH-[K = 10 -¹⁴ ] Si + 6OH-= SiO 3-2 + 3H 2 O + 4e-[E 0 = +1.73 v]
This conclusion is supported by the fact that watermarks are eliminated if the clean wafer has the surface water displaced with a hydrophobic liquid prior to final drying and that the oxidation may be prevented by eliminating oxygen during the drying process. In addition, the semiconductor industry recognizes the corrosive characteristics of ultrapure water.
The generally accepted remedy for watermarks is drying with a water-soluble organic solvent, such as isopropyl alcohol, utilizing Marangoni, or surface tension gradient drying, where, in each case, the surface tension gradient is slowly tracked across the substrate surface being dried. Process equipment using this drying technique includes boiling isopropyl alcohol units, spin rinse units with low levels of isopropyl alcohol, and systems which can reduce the isopropyl alcohol level to zero, a concept apparently first suggested by Leenaars. The combination of these results, and the fact that a variety of other organic liquids can be used, suggests the organic liquid may not be required.
If these three drying schemes are compared on the basis of surface tension, which is known to be the important parameter, the data definitely define that isopropyl alcohol, or its equivalent, is unnecessary. Figure A displays the surface tension of mixtures of isopropyl alcohol and water, from 100% water to 100% isopropyl alcohol in the temperature range from 20-50° C. 5 If the results from the three drying schemes are compared on the surface tension scale, the 100% boiling alcohol system corresponds to about 15-20 dynes/ cm, while the lower level alcohol system corresponds to perhaps 60-65 dynes/ cm, although the exact concentration is not specified. These systems are known to produce satisfactory results and they are both used commercially. They are accepted as Marangoni or surface tension gradient wafer dryers.

The "no alcohol" system is also used commercially and known to produce satisfactory drying results. The operating directions define that in order to achieve proper drying without alcohol but using warm nitrogen gas only, the minimum operating gas temperature is 70° C. Figure B displays the surface tension of water as a function of temperature, which is approximately linear, from 72 dynes/ cm at 0° C to 58 dynes/ cm at 100 °C. The surface tension of water at 70° C is approximately 64 dynes/ cm, a value similar to the lower level alcohol system.

The concept of interface particle removal as outlined by Leenaars ⁴ , specified a surface tension gradient, which has almost always been generated by adding a variety of soluble organic liquids (often defined as polar organic liquids, noting that non-polar organic liquids are not normally water soluble) to the general drying system. This technique has become standardized using isopropyl alcohol at continuously decreasing concentrations. The "no alcohol" system indeed satisfies the original concept in that the surface tension gradient is generated with warmer water providing a lower surface tension medium, identical to the organic liquid systems. No experimental work has demonstrated a requirement for the polar organic liquid; only a requirement for a surface tension gradient, while the no alcohol system confirms the same final result can be achieved when the polar organic liquid is omitted.

Leenaars and Marra found that "no drying could be induced with pure room temperature N2 gas or with, for example, alkane vapors." This observation reinforced the soluble organic liquid concept and eliminated the effect of nitrogen from the "no alcohol" system, leaving only the 70° C temperature as the factor responsible the surface tension gradient.
They also identified another important feature. Depending upon the physical properties of the organic compound being used, they found that the liquid bath had to be operated at an overflow condition. The reason for this is that in certain instances the organic compound concentration in the bulk fluid increased, independently, via surface transport, while the surface tension gradient was also operating, drying the substrate. This resulted in the surface tension gradient vanishing because the concentration of the organic compound in the bulk fluid was approaching the concentration in the drying fluid film. In the "no alcohol" system, this process liability would not occur as the surface tension gradient is generated thermally. In this case the drying fluid film temperature would equilibrate to the bulk fluid temperature, thereby maintaining the thermal gradient as well as the surface tension gradient.
The developments from Leenaars and his coworkers have produced a comprehensive and uniform concept for cleaning and drying semiconductor wafers confirmed commercially through a variety of devices and applications. A significant feature of this surface tension gradient concept has, however, been overlooked, or, at least, not developed. Although all the current commercial applications involve the liquid surface as the "interface of a liquid," the phase boundary between a liquid and a gas was also identified and experimentally confirmed in the original work. ⁴
Leenaars outlines three preferred embodiments of the method:

·        The "interface of a liquid" is moved over the surface of the substrate, by immersing the substrate into the liquid -- an advancing liquid;

·        The "interface of a liquid" is moved over the surface of the substrate, by withdrawing the substrate from the liquid -- a retracting liquid; and,

·        The "interface of a liquid" is its phase boundary with a gas bubble which is moved over the surface of the substrate, the substrate being immersed in the liquid -- both an advancing and a retracting liquid.

The gas bubble embodiment provides two advantages. Since the liquid is both advancing and retracting, the particle removal efficiency is independent of the wetting characteristics of the particle and the substrate. Also, the efficiency of the sys-tem can be easily increased by moving several gas bubbles at a time over the surface of the substrate. ⁴
Example 6 of the patent ⁴ describes this bubble cleaning: the whole wafer was immersed into a water containing beaker, after which a beam of monochromatic laser radiation having a wavelength of 514 nm and a cross section of about 20 microns was directed by means of an argon laser onto the surface of the substrate, as a result of which vapor bubbles were formed in the proximity of the beam on the surface of the substrate. The beam was moved in a lateral direction over the substrate at a speed of 16 microns/ sec. This resulted in the removal of about 95% of the particles.
The conversion of this laser-generated bubble technique into a commercial system requires development of a medium with a very large number of phase boundaries between a liquid and a gas, so the multiple bubble efficiency can be utilized. Fortunately, the required system already exists; it is aqueous foam. The technical definition of foam -- agglomerations of gas bubbles separated from each other by thin liquid films -- is equivalent to one of Leenaars' definitions (the "interface of a liquid" is its phase boundary with a gas bubble.) ⁴
This technology is complete and clearly presents a uniform and consistent understanding of particle removal as well as wafer drying, starting with surface tension gradients, simple liquid/ gas interfaces, graduating to interfaces at bubble walls, multiple bubble wall interfaces, and then, by extrapolation, extending to aqueous foam, a medium containing millions of bubble wall interfaces.
The Banks cleaning technique ³ was followed by a similar gas agitation cleaning of magnetic separators involving the addition of the compressed gas from an external source while the unit was submerged in the cleaning fluid ⁶ -- same result, different procedure. These cleaning techniques would not be directly applicable to wafer cleaning, but the concept does have merit, especially in the case of wafers with complex surface patterns. This "gas agitation cleaning" is accomplished because of the bubble collapse.

Ogaya ⁷ and Liu ⁸ advanced the concept of wafer cleaning by internal gas generation. Ogaya used carbonated water under pressure, which was slowly discharged into a vessel containing submerged wafers. The cleaning mechanism suggested involves the particulates acting as a nuclei for the bubble formation caused by depressurization. The approach used by Liu is similar to the Banks technique. The wafers are submerged under pres-sure in a cleaning fluid containing a soluble expansion gas. The pressure is quickly reduced to ambient pressure causing vigorous effervescence, resulting in cleaning.

Storing energy in a compressed system, external to the cleaning vessel, followed by energy discharge in the vessel in order to clean the substrate, is a positive feature. The negative feature is not accommodating the problem of particle redeposition, as the substrate is not progressively removed from the cleaning medium containing the particulates as the cleaning process proceeds.

Conclusions Leenaars ⁴ has shown that a surface tension gradient can produce positive drying results, and, in the form of an "interface of a liquid," can remove particles on wafer surfaces. Particle removal is improved if the "interface of a liquid" is produced by the phase boundary with a gas bubble, producing both an advancing and retracting interface. Multiple bubbles provide more efficient performance. Banks ³ has shown that decompression of a solution containing a soluble gas will transfer energy to the surroundings as the effervescing bubbles collapse. Beery ⁹ has shown that fluids with low surface tension and low viscosity can penetrate vias and trenches producing positive cleaning results when the proper chemistry is chosen.

Aqueous foam compositions can pro-vide these results because foam is an agglomeration of gas bubbles separated from each other by thin liquid films, and because foam can be produced by simple mixing of non-soluble expansion gases, such as air, or by decompression of solutions containing soluble expansion gases.

References

1. Shwartzman, S., U. S. Patent 4,118,649, October 3, 1978.

2.      Dussault, J. G. M., U. S. Patent 4,178,188, December 11, 1979.

3.      Banks, W. P., U. S. Patent 4,238,244, December 9, 1980.

4.      Leenaars, A. F. M., U. S. Patent 4,781,764, November 1, 1988.

5.      Vazquez, G., J. Chem. Eng. Data, 40, 611-614 (1995).

6.      Bender, H., U. S. Patent 4,266,982, May 12, 1981.

7.      Ogaya, K., Japanese Patent 63-239982-A2, October 5, 1988.

8.      Liu, B. Y. H., U. S. Patent 4,817,652, April 4, 1989.

9.      Beery, D., et al, "Post Etch Residue Removal: Novel Dry Clean Technology Using Densified Fluid Cleaning (DFC)," presented at IITC, Burlingame, CA June, 1999.

Banned Materials List in clean rooms

• Cleanroom approved notebooks
• Computers and approved peripherals. (Subject to conditions,laptop computers are the preferred choice)
• Specialized/dedicated tooling (Subject to conditions)
• Dycem cleaning detergent
• Spectro grade IPA such as UVASOL or supplied from Fisher Scientific
• Spectro grade Acetone in small quantities (<100 ml)
• Kapton tape with Y966 acrylic adhesive
• Blue PCX polyethylene tape with Y966 acrylic adhesive
• ScotchWeld 2216 in small quantities (degassed preferred)
• ScotchWeld 1838 in small quantities (degassed preferred)
• Cleanroom approved pens
• 3M Velostat plastic light proofing material
• • Food & drink.
  • Wood products
  • Non cleanroom paper (cleanroom approved paper is available in the dispensers provided).
  • Non cleanroom notebooks (cleanroom approved notebooks are available).
  • Bare aluminium, it is preferred that aluminium is alocromed before being brought into cleanroom.
  • Cardboard/cardboard boxes of any type.
  • Pens and Pencils. The cleanroom has ample supply of approved pens.
  • Any type of grease or oil excepting space qualified Apiezon 100.
  • Books and book bags.
  • Make up.
  • Continuous non cleanroom approved printer paper.
  • Wash bottles etc. ( the cleanroom has adequate provision of Teflon wash bottles filled with Spectro grade IPA and Critical Neutral detergent).
  • Solvents not listed in the approved materials list above.
  • Compressed gasses e.g. Nitrogen, Argon and Helium etc. (the cleanroom has dedicated Nitrogen and Argon supply lines available equivalent to zero grade).
  • Vacuum pumps, unless first discussed with by the CCE and appropriate location allocated. ( the cleanroom has been fitted with a dedicated vacuum exhaust manifold and can serve up to five vacuum pumps at any one time. Preferred rotary pumps are of the scroll type.
  • Silica Gel/bagged silica gel (particulate contamination).
  • Masking tape,insulation tape and sellotape.
  • Silicone adhesive backed Kapton tape.
  • Silicone sealing compounds/greases.
  • Silicone based mastics.
  • Leather
  • Oils of any type.
  • Mobile phones (interference with test equipment and general transfer contamination)
  • Bubble wrap (slip agents and silicone contamination)
  • Black ESD bags (slip agents and silicone contamination)
  • Plastic bags of any kind (slip agents and silicone contamination)
  • Pink poly cleanroom bags (slip agents and antistatic additive contamination)
  • Any type of adhesive not listed in the Approved materials list above
  • Open cell foam materials
  • Smoking materials
  • Powders, aerosols, DOP
  • Commercial vacuum cleaners (non HEPA filter cleaners)
  • Plastic containers which are not approved by the CCE
  • Velcro
  
   

Ground support equipment (GSE) cleaning procedures

General cleaning Guidelines

It is recommended you read the list of approved and banned materials first. For computers and associated equipment, please let the cleanroom manager know you plan to bring these items in and if you require LAN access from within the cleanroom. Before entering any part of the cleanroom, pre-clean the items with IPA and fibre free wipes. Detailed cleaning instructions will follow this general guideline.
For equipment such as computers, keyboards etc, vacuum clean using a general purpose vacuum cleaner. Particular attention should be paid to the ventilation ports.This pre clean should be done external to the cleanroom and anteroom and items should be covered with clean bagging material, but not of cleanroom quality. As a guide, notebook computers are preferred to desktop computers (although this is not always practical) not only due to size but also from a cleanliness aspect. Clean all equipment at an external location to the cleanroom. This equipment can then be bagged to minimise contamination.
Small metallic items (screws, nuts, washers) and non metallic items that are safe to use with solvents must be cleaned using the ultra sonic cleaner located in the chemical lab. Upon entering the cleanroom anteroom remove from packaging and visually inspect for any obvious signs of contamination that may have been missed and re-clean if necessary. The best way to visually inspect any item is to view the item surface at a five degree angle with good background lighting
When cleaning items using wipes provided in the anteroom do the following:

• Fold the wiper in half and then fold in half again.
• Now apply IPA to the folded wiper
• The maximum number of wiping actions per fold should be no more than four times.
• Wipe the surface in a single direction
• Once four wiping actions have been done, use the other folds to continue cleaning as above procedure.

If the person who performs the cleaning operation then goes into the cleanroom, a new pair of cleanroom gloves must be fitted. The clean item can then be taken into the cleanroom and any necessary final cleanup can be done within the cleanroom as described above using wipers and IPA provided. (see note 1 and note 2)
NOTE1: There are two sets of solvent carriers and solvent wash bottles in the cleanroom. The first set is located in the anteroom which is used for item cleanup and the other is located within the cleanroom. The second set located within the cleanroom is ONLY for use on optics and flight instruments/hardware. The solvent used in the second set is UVASOL and is spectroscopic grade propan-2-ol (IPA). Do not use this second set for general/surface cleaning. When using the spectroscopic grade IPA ensure that the nozzle of the wash bottles do not come into contact with the wiper or any other surface as cross contamination will occur if this happens
NOTE2: There are two grades of wipers used in the cleanroom. General purpose grade wipers (non woven polyester) are available in the Anteroom. These wipers should be used for item and general cleanup and should be recycled by disposing of them in the cleanroom laundry basket and NOT in the refuse bins. The second wiper is of a better quality (sealed edge non woven polyester with low particulate and extractable's). These wipers are available within the cleanroom in dispensers and should be used on flight quality instruments and hardware.

Monitoring of processes and equipment

When bringing equipment into the cleanroom, ensure that you observe the following:
All cleanroom users should ensure that they consult the cleanroom manager before they bring any equipment and processes into the cleanroom.This cannot be stressed enough! The monitoring of equipment and processes going into the cleanroom is an important part of the general contamination control of the cleanroom. If bringing equipment or processes into the cleanroom ensure that you follow the detailed procedures listed below.
General tooling such as screwdrivers, nut drivers, rulers, vernier calipers etc. do not need to be bought into the cleanroom as there is a fully stocked cleanroom tool cabinet for all cleanroom users benefit. The tooling in the cabinet is for cleanroom use only and must never be removed from cleanroom. When using cleanroom tooling ensure that they are put back into the cabinet after use. Do not leave tooling lying about as this means that other users will not be able to use them. If you are a visitor and you need specialised tooling ensure that  you contact the cleanroom manager to confirm whether the tooling is available.
If you are bringing a process into the cleanroom ensure you have done a risk assessment on the process and you have discussed this with the cleanroom manager. Under no circumstances may any process go into the cleanroom unless it has been discussed with the cleanroom manager, again this is a monitoring activity to aid in cleanroom contamination control and for Health and Safety reasons.

Detailed Cleaning procedures

Introduction

In order to meet project contamination cleanliness requirements, ground support equipment (GSE) used in clean areas at MSSL for flight hardware integration and testing needs to be cleaned at regular intervals. This is to ensure that surface contamination levels are minimised and the possibility of contamination redistribution from the GSE to the flight hardware is significantly reduced.

Required equipment

• Isopropyl Alcohol (IPA): Analar grade for course and intermediate cleaning and spectro grade for precision cleaning.
• Cleanroom Wipes: Lint-free Polyester
• Vacuum Cleaner: HEPA-filtered for precision cleaning
• Acetone: Analar grade for course and intermediate cleaning and spectro grade for precision cleaning
• Cleanroom Swabs: Woven Polyester
• Compressed Air or Nitrogen, filtered for particles to 0.5 micron
• White light source (100W)

Before use in a controlled clean area, GSE must be cleaned and inspected for visible cleanliness. GSE cleaning should be performed to visibly clean � sensitive level (VC-S) as defined in MSSL/PA/PS/Q012 �Procedure for verifying surfaces to a visibly clean level�. To achieve this surface cleanliness standard, GSE cleaning should be performed in three stages as defined below:

Course cleaning - General

Course cleaning can be performed in non clean areas (workshop, office etc). The idea is to remove the bulk of contaminants from the hardware. Nitrile gloves should be worn during course cleaning

Intermediate cleaning - General

Intermediate cleaning should be performed in the anteroom. Particular attention should be paid to surfaces, crevices, corners, vents etc and these should be cleaned using appropriate solvents or detergents. A more thorough inspection of all surfaces is required at this stage. Cleanroom wipes and nitrile gloves should be worn while performing intermediate cleaning.

Precision cleaning - General

Precision cleaning should be performed inside the cleanroom but away from flight hardware (ideally at entrance to cleanroom). Spectro grade solvents should be used and polyester cleanroom wipes. Cleanroom dress code of mask, hood, cleanroom bunny suit, overshoes and nitrile gloves should be observed. A methodical and thorough surface inspection should be carried out as defined in MSSL/PA/PS/Q012 �Procedure for verifying surfaces to a visibly clean level� to a VC-S level after cleaning.

Cleaning procedures
 Course cleaning

• Determine the surface sensitivity to handling and solvents. If the surface is sensitive to IPA or acetone, use filtered critical neutral detergent. If the surface is sensitive to handling, limit or eliminate vacuum use and handling as necessary to prevent GSE damage.
• If computing equipment is to be brought into clean areas laptop devices are preferred. If this is not possible desktop computers should be thoroughly cleaned inside and outside the casing paying particular attention to exhaust fans from the power supply.
• Computer monitors should be thoroughly vacuumed paying particular attention to the monitor vents prior to cleaning procedure below.
• Computer keyboards should be turned upside down and a jet of clean filtered compressed air or nitrogen directed between the keys prior to cleaning procedure listed below.
• Remove loose particles from the GSE by thoroughly vacuuming all surfaces, including holes, crevices, and corners, with a vacuum cleaner. This step should not be performed for items sensitive to handling. Minimise direct nozzle contact with GSE surfaces and continue until no particles are visible on GSE surfaces.

Intermediate cleaning

• Lightly dampen a cleanroom wipe with IPA (Analar grade or equivalent). Do not saturate the wipe or dip the wipe in the solvent container.
• Clean the GSE surface with the IPA dampened wipe, wiping the surfaces in a unidirectional manner. Do not overlook crevices, corners, or holes. If necessary, lightly dampen a swab with IPA and use the swab to clean recessed areas. Clean recessed surfaces with the swab by rotating the swab over the surface. During cleaning, fold the wipe to expose a clean surface or replace with a new wipe if the wiping surfaces become contaminated. Replace swabs when visibly contaminated. Lightly dampen each new wipe surface or swab with IPA and continue cleaning. Repeat until GSE surfaces do not display visible contamination.
• If solvent remains on any GSE surfaces, particularly blind holes or other recesses, dry the surfaces completely with filtered, oil-free GN 2 or air. Regulate the gas flow as necessary to prevent solvent splashing and damage to GSE surfaces. After solvent wiping, inspect surfaces for visible cleanliness as defined in MSSL/PA/PS/Q012.01 �Procedure for Verifying Surfaces to a Visibly Clean Level�
• If visual inspection reveals surface contaminants, repeat the solvent wiping procedure above and re-inspect. If contamination still remains visible after solvent wiping, repeat the solvent wiping procedure with acetone substituted for IPA. Before using acetone, however, verify that it will not degrade the materials undergoing cleaning. If contaminants remain visible after using acetone, contact the contamination control manager (CCM).

Precision cleaning

• Lightly dampen a cleanroom wipe with spectro grade IPA. Do not saturate the wipe or dip the wipe in the solvent container.

• Clean the GSE surface with the spectro grade IPA dampened wipe, wiping the surfaces in a unidirectional manner. Do not overlook crevices, corners, or holes. If necessary, lightly dampen a swab with IPA and use the swab to clean recessed areas. Clean recessed surfaces with the swab by rotating the swab over the surface. During cleaning, fold the wipe to expose a clean surface or replace with a new wipe if the wiping surfaces become contaminated. Replace swabs when visibly contaminated. Lightly dampen each new wipe surface or swab with IPA and continue cleaning.
• If solvent remains on any GSE surfaces, particularly blind holes or other recesses, dry the surfaces completely with filtered, oil-free GN 2 or air. Regulate the gas flow as necessary to prevent solvent splashing and damage to GSE surfaces.
• After solvent wiping, inspect surfaces for visible cleanliness as defined in MSSL/PA/PS/Q012 �Procedure for Verifying Surfaces to a Visibly Clean Level�. The surface cleanliness level should be VC-S. A white light source (100W) can be used to aid inspection. Repeat above cleaning procedure until GSE surfaces do not display visible contamination.
• All cleaned GSE cables can then be sleeved in Llumalloy bagging. Sleeve ends should be sealed with clean cable ties or Kapton tape.

Note: Depending on project contamination control requirements certain GSE may require cleaning to a more stringent level such as VC-HS. The above procedure can be adapted to suit this requirement by substituting the appropriate inspection procedure as defined in MSSL/PA/PS/Q012. Cleaning methods will remain the same.