Sunday, May 17, 2009

Purification of Ultra Pure Water

Point of Use Purification of Ultra Pure Water: Reduce Water Level Metal Contamination for Next Generation Semiconductors


Ultrapure water is a critical fluid in the front end of line (FEOL) wet cleaning process. The ultra pure water (UPW) used for dilute chemical blending and post chemistry rinse must meet stringent requirements to control yield-diminishing contaminants on the wafer surface. Although it is difficult to directly correlate contaminants in UPW to wafer device defects, there is experimental data describing both the mechanisms of metal adsorption on wafer surfaces and the impact of surface metals on wafer contamination. Research suggests that the adsorption of metals on the wafer surface is correlated to the rinse solution concentration for hydrophilic wafer surfaces to a point of equilibrium and is favored by higher pH solutions. Wafer surface studies demonstrate that Cu, Co and Ni deposited on the wafer surface prior to the diffusion process can precipitate as silicide particles during the post diffusion quenching process. These can behave electrically as metallic inclusions for majority and minority carriers in silicon.

These studies support the theory that metal contaminants in process materials such as UPW and chemicals lead to wafer surface contamination and may ultimately impact the electrical integrity of the device. The level of impact is influenced by many variables including: the type of device; technology node; the type of clean; processing step; and the specific metal concentration. With the equivalent oxide thickness decreasing and the number of cleaning steps increasing, it is likely that metal contaminants will continue to be a significant factor in the performance of the clean process and the final wafer surface quality.

Metal contamination specifications are included in both the Yield Enhancement and the Front End Processing (FEP) sections of the International Technology Roadmap for Semiconductors (ITRS). The 2003 ITRS update specifies wafer surface metal levels at 5E9 for gate oxide integrity (GOI) killers (Ca, Ba, Sr and Fe) and 1E10 for critical metals (Ni, Cu, Cr, Co, Hf and Pt,). The Yield Enhancement Wafer Environmental Contamination Control (WECC) section (table 114a in ITRS update) specifies critical metals and gate oxide integrity killers in UPW at 1ppt each for the 2003 update.

The pre-gate clean process at International SEMATECH’s (ISMT) Advanced Technology Development Facility, Inc. has historically tolerated a wafer surface nickel concentration of 5 E10 atoms/cm2. To meet the specifications put forth in the ITRS roadmap the wafer surface critical metal specifications were lowered to 5E9 atoms/cm2. Although the central UPW supply to the Advanced Technology Development Facility was relatively low in nickel (<10ppt) it was shown to have a significant impact on wafer surface metal levels. Additional metal removal treatment was required at the point of use (POU) to lower the UPW nickel concentration and achieve an acceptable wafer surface level with the standard HF-SC1-SC2 process. Several dissolved ion purifier/filters were evaluated at ISMT for metal removal efficiency, rinse-up characteristics, and cost of ownership. Based on the preliminary results from this evaluation a purifier/filter was selected for further testing in the pre-gate clean process.

ISMT partnered with Mykrolis Corp. to install and evaluate a POU filter-purifier (Protego) for nickel removal at the pre-gate wet deck.

Key attributes of the Protego purifier are:

a. High ion removal rate and overall capacity; the ion exchange groups cover the entire surface, maximizing exposure and removal efficiency
b. Multi-element cation removal, including key metals:Al, Ca, Cr, Cu, Ni, Fe, Pb, Mn, and Zn.
c. Optional 0.05┬Ám filtration capability with a hydrophilic ultra high density polyethylene membrane

Samples were collected throughout the ISMT UPW system and at POU. POU samples were collected at the Dianippon Screen DNS – FC-821L cleaning tool, which was used for all critical pre-gate clean and project work. The wet deck is unique in that the rinse tanks are “all-in-one” tanks that perform both dilute chemical cleans and rinsing in the same tank. POU samples were collected after each component in the polish system and at the individual once through baths.

The filter performed well, reducing the UPW nickel concentration in the rinse tank from 7 ppt to <0.5ppt with correlating reductions in wafer surface levels from >1E+11 to < 5E+9 (see Figure 1). Additional filters were installed at the UPW supply to the low pressure dryer (LPD), POU heater and ozone generator to provide metal removal for all common UPW sources to the tool.

The primary objectives of reduced maintenance and tool downtime and consistent nickel removal have been achieved. Nickel levels in the filtered UPW product are less than 1ppt and equivalent to levels achieved with the regenerable filter. Corresponding wafer surface nickel levels are less than the specified 5E09 atoms/cm2. The quality of the filtered product is equivalent to the UPW supply with respect to inorganic ions, TOC, particles and reactive silica, and was reached after an acceptable 2- hour rinse up. The operating conditionsof the tool were maintained with the desired flow-rate achieved at a minimal pressure drop.

Summary

The metal purifier has been shown to achieve consistently low nickel levels at the POU. The use of POU components to achieve an additional level of quality at key critical applications is an effective strategy that is increasingly employed as UPW suppliers strive to meet the diverse needs of end-users in a cost effective manner.

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