Surface Acoustic Wave (SAW) Detectors for Real Time AMC Detection
A&rborne Molecular Contamination (AMC),1 arises from unwanted, gas-phase materials deposited on a surface through the process of molecular migration. As cleanliness standards become more stringent, it becomes increasingly important to rapidly detect unanticipated contamination. AMC may be monitored through witness samples, through direct or extractive measurement of the product itself, or through air sampling. This month we discuss Surface Acoustic Wave (SAW) detectors, which have proven useful in monitoring AMC by real-time air monitoring.
A SAW device operates similarly to the QCM-D thin film monitor.2 A vibratory resonance wave is excited in a piezoelectric crystal, usually quartz. The resonant frequency decreases as mass is deposited on the surface. In the SAW the wave travels along the surface as contrasted to traveling through the crystal bulk in QCM. In SAW detectors, contamination is typically determined by the differential between a sealed and exposed crystal.
SAW detectors developed using Gallium Arsenide (GaAs) have some potential advantages over quartz. Because GaAs is a semiconductor, excitation and detection circuitry can be integrated into the same device. This allows smaller, more easily packaged devices and lower power consumption. The integrated circuit needs no high frequency interconnects; this in turn allows higher resonant frequencies and resulting higher sensitivity.
Advantages of SAW detectors include real time detection, portability, and sensitivity. SAW detectors enable frequent measurements (typically, seconds to minutes per measurement). Real-time monitoring allows AMC to be detected before much product has been contaminated. By having detectors at different locations, the source may be pin-pointed.
Another feature of SAW devices is compactness. Some are self-contained, battery operated units that can be moved within the clean room. They can be used also in remote or inaccessible areas.
SAW detectors have high sensitivity relative to QCM. Surface waves have much higher frequency; the sensitivity of the device is proportional to the frequency squared. This makes the SAW detector about one hundred times as sensitive as QCM. Detection of contamination with mass less than 20 picograms/cm2 (2x10-11 g), less than one percent of a monolayer, are claimed.3
A basic SAW detector provides trends of mass accumulation, not identification of specific chemical species. However, contaminants on the sensor chip may be identified through such analytical techniques as Time-of-Flight/Secondary Ion Mass Spectroscopy (TOF/SIMS). Alternatively, detection of contamination by SAW may prompt analytical testing of witness samples.
Coatings can enhance the sensitivity of SAW detectors to specific chemicals. An array of several SAW detectors with different coatings can be analyzed to obtain a chemical fingerprint.
SAW detectors are most commonly used in critical processes such as semiconductor wafer or hard disk manufacturing or other high-value manufacturing processes. Such detectors may be used to monitor the effectiveness of chemical filters. Other applications include preparing space payloads, chemical or biological weapon detection, airport security, and environmental monitoring.
SAW detectors indicate small changes of the detector surface due to interactions with airborne components. Changes in water content may be periodic and reversible. Other changes may be indicative of contaminants which remain on the surface due to low volatility or by adsorption to upper layers of the surface. Particles could alight on the surface of the detector. Because of their larger mass, the patterns should be readily distinguishable from typical AMC. In fact, SAW detectors have been used for detection of non-volatile residue (NVR) which would include both particulate and dissolved contaminants.
Where chemical reactivity with the surface is a concern, a SAW coating may need to be customized to mimic the surface of the product. Further, if there is chemical reactivity, one must decide if this is a good thing or a bad thing. One would generally assume the fewer contaminants the better; but, occasionally, the opposite is true.4