Fingerprinting particles
10/01/2001
Mark A. DeSorbo
 Researchers at Advanced Particle Systems test the "ParticleExplorer," a device that puts a name on contamination. |
BERLIN, GERMANY—The maniacal, malevolent masses of anonymous microcosms plague even the cleanest of cleanrooms and leads to symptomatic airborne molecular contamination (AMC)—a constant quagmire that forces engineers and end-users to become particle police on a case to determine how or what infests batches of drugs or renders wafer lots useless.
Classifying the particle many no longer be a laborious process, for Advanced Particle Systems GmbH (APSys) has launched its ParticleExplorer, a device for cleanroom applications that not only detects and counts, but also automatically puts a name on perpetrating particles.
"It is the first device of its kind to automatically identify the particle," says Markus Lankers, head of research and development for APSys. "Most of the time, people do not know anything about their particles, but if you know it's human dust, you can better think about how best to deal with the problem."
Sir Raman's spectroscopy
Fourier transfer infrared has been the medium of choice for detection and measurement, preparation of particles with liquid or gas is required, while other methods used to identify contaminant sources can be as erratic as the origin, Lankers says.
The ParticleExplorer, however, identifies particles using laser Raman-spectroscopy, which stems from the Raman Effect that was discovered by physicist Sir Chandrasekara Venkata Raman.
According to research conducted by Raman, who won the 1930 Nobel Prize in Physics for his discovery, energy and the frequency and wavelength of scattered light is changed as the light either imparts rotational or vibrational energy to scatter molecules or take energy away. The spectrum created from this phenomenon characterizes transmitting substances. Raman spectrometry is a useful technique in physical and chemical research, particularly for the characterization of materials.
Detection by the ParticleExplorer is based on this same scattered light analysis. A powerfully focused laser scans the surface from where particles are emitted, and scattered light characterizes particle sizes and records properties.
The laser beam focuses on the particle to the micrometer, while the integrated Raman-spectrometer automatically provides high-resolution particle images. Software, developed by APSys, uses an extensive database to identify materials.
"It has a very simple user-interface," Lankers adds. "The Raman-spectroscopy gives a molecular fingerprint of the substance. You can focus the light beam to 0.5 microns so you can identify very small particles. With infrared you get similar results, but only within 20 to 30 microns."
CleanRooms Hall of Famer Al Lieberman says that using the Raman Effect to pinpoint AMC is a possibility, but the types of gas and how vapors potentially react to spectroscopy wavelengths, in terms of measurement difficulty, should also be taken into consideration.
"It looks as though this could be very useful device," says Lieberman, a technical specialist for Particle Measure Systems Inc. (Alameda, CA). "The question is how broad are the capabilities of this device with present day solid state or gas lasers, and what are the limitations and sensitivity ranges you can expect."
Scientific shortcut
Because most particle counters determine the number of particles in different size categories, manufacturers must send collected samples to laboratories for analysis. This process can eat up a significant period of valuable time and require a considerable amount of equipment, says Ken Goldstein, principal of Cleanroom Consultants Inc. (Scottsdale, AZ).
"A device that does it all at once, is a very interesting," says Goldstein, a member of the CleanRooms Editorial Advisory Board. "In the old days, you had to capture the source. If the contamination is on the surface, you're good to go. But if it's airborne you have to use devices like cascade impactors, which clumsily put particles on the surface of a plate."
After that, he says the plate must be examined under electron microscope with additional hardware that allows particles to be identified.
Another method of identifying particles is with an absolute filter, which is made up of a polycarbonate with fixed openings. The filter resembles a window screen with fixed sized openings. Such filters can be controlled, but the medium is not very reliable, says Goldstein, adding that absolute filters work well to trap particles, but they are mostly used for liquid analysis.
"The ones that get trapped are the ones you can analyze, but the ones that get through, you cannot," he adds. "Before this new device, it could take hours, days or weeks to get a sample that you could look at under a microscope. Basically, this device eliminates one or more of the steps."
Testing...1,2,3
At the time of this report, Lankers says researchers continued to test the ParticleExplorer in ISO Class 5 (Class 100) and ISO Class 6 (Class 1000) cleanrooms.
"The machine is hermetically closed so we found no particulate contamination originating from the machine," Lankers says.
The major focus of the test, he explained, was to identify particles, 80 to 85 percent of which could be identified.
"In our environments, the most common particles we found were human dust, hair and skin flakes," Lankers says. "We also found polymers that we believe originated from cleanroom gloves and coveralls. We found some oily substances, lubricants, that we believe originated from cleanroom equipment."
Potential customers, he adds, want to know the identities of particles, especially when contamination hinders or halts production.
"Disk drive manufacturers, for example, have a hard time with skin flakes and hair," Lankers says. "If you can identify the contamination source, then it can be minimized or eliminated at the source."