Surface Acoustic Wave (SAW) Detectors for Real Time AMC Detection
Barbara & Ed Kanegsberg, November 2003
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.
Principal
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
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
Specificity
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.
Applications
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.
Additional Considerations
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
Next month: What is TOF/SIMS?
References
1 B. Kanegsberg and M. Chawla, A2C2 Magazine, (Feb-June, 2001).2 Rodier, D. “Monitoring Molecular Contamination of Critical Surfaces in Semiconductor Manufacturing Clean Technology,” vol. 12, no. 8, (2002) p. 71-73 and http://www.pmeasuring.com/education/appNotes/molecular.3 Kanegsberg and Kanegsberg, A2C2 Magazine, (March, 2003).4 Kanegsberg and Kanegsberg, A2C2 Magazine, (Jan-Feb, 2002).
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
