ASTM standard to keep wipers clean
09/01/2000
Hank Hogan
WEST CONSHOHOCKEN, PA—A soon-to-be released standard may help cleanrooms stay clean. E2090 from the American Society for Testing and Materials (ASTM), which has been approved by the ASTM E21 committee, specifies new standards for wiper testing.
 A lab worker examines a wiper on an SEM workstation. Photo courtesy of Texwipe. |
According to Robert Moss, chairman of E21, the standard is only awaiting official publication.
This new approach joins an existing wiper non-volatile residue test; however, the new standard looks at particle instead of chemical contaminants, an expansion of testing that ASTM has long sought.
"What we've been trying to do, literally for years, is to put together a whole set of test methods and documents to look at everything," remarks Moss.
The standard features three key innovations. One is the use of an orbital shaker filled with a low-surface-tension cleaning liquid to maximize particle release. The second is that a broad range of particles is measured, with all sizes of particles released from the wiper captured by the new testing method. The third is the use of microscopy for particle counting.
The new wiper testing standard was developed with the assistance of the Texwipe Company LLC of Upper Saddle River, NJ. Texwipe's work in this area was motivated by more than a desire to write standards. According to Steven Paley, chief technical officer for Texwipe, the effort actually has its roots in the company's drive to develop newer and cleaner products.
"In order to develop a process to make something that next order of magnitude cleaner, we needed a more sensitive measurement technique," he says.
In E2090, wipers are immersed in an application-specific low-surface-tension liquid and then agitated for five minutes or so in an orbital shaker. This mimics the actual wiper use.
After being shaken, the liquid is filtered and particles are counted. The standard calls for three classifications: small particles less than five microns in size, large particles ranging from five to 100 microns and fibers and particles greater than 100 microns. Any of these sizes can cause a defect in a cleanroom manufacturing process.
This breakdown categorizes contaminants by origin as well as size. For wiper manufacturers, such information can be critical in product development. Historically, large fibers coming from the edge of a wiper have been a significant source of particles.
The final step in the testing process is to count each class of particles using optical and scanning electron microscopy. This calls for quite a bit more work than is required using an automated counting device. There's sample preparation involved, as well as some required instrumentation skill. As a result, this testing method takes longer. However, the microscopy approach does eliminate false readings due to artifacts, particle geometry or other causes.
"If you really want an accurate measurement, you have to go to microscopy," remarks Paley.
Although the standard is only just being published, Moss of ASTM is already considering improvements. For instance, the standard doesn't yet incorporate a statement about accuracy and precision. The E21 committee reviewed Texwipe's methods and data but didn't replicate the method at different testing labs. However, Moss thinks that running the standard at many different facilities will correct this.
"I would like to see as many other organizations as possible take up this test method, and use it and report how successful they are in replicating the data," says Moss. Because the critical part of any test method is that it's reproducible, and reproducible everywhere."