22
FILTRATION
system. The Miller-Nelson controlled the flow rate
as well as humidity and temperature of the air
supplied to the test bench. A contaminant vapour
was added at a specific rate and diluted with the
air stream to achieve the desired concentration.
The air then passed through a media sample holder.
Most of the air was then directed to a fume hood
but a portion was passed to an On-Line
Technologies model 2010 Fourier transform
infrared spectrophotometer (FTIR).
The temperature and humidity in semiconductor
manufacturing facilities is tightly controlled at
about 25ºC and 45% relative humidity. We added
Figure 3: TMS and Challenges of removing HMDS vapour to an air stream in our test bench
NH
3
, from HMDS, ammonia and siloxanes and found that under these conditions HMDS can
removal with acid Historically, filtration in the lithography area has hydrolyse in air to form TMS and ammonia as
impregnated concentrated on ammonia removal via acid shown in Figure 1.
activated carbon impregnated carbon or cation exchange resins.
These methods, however, are not effective in Figure 2 shows the concentration of TMS, NH
3
removing siloxanes (organics) such as HMDS or HMDSO downstream of a sample of activated
HMDSO. Donaldson enlisted its industry expertise carbon when exposed to HMDS as described above.
in organics removal to take a more critical look at As expected, activated carbon alone removes TMS
this issue. Typically, activated carbon would be very well, despite its volatility, and does little to
used to remove organic compounds like HMDS and remove ammonia.
HMDSO, however, ammonia is not removed without
acid impregnation. Also as expected, Figure 3 shows that acid
impregnated carbon alone removes ammonia very
To better understand the most effective removal well, but does little to remove TMS. It is also
strategy, Donaldson worked collaboratively with apparent that HMDSO is generated through the
semiconductor customers who provided filter media acid catalysed dimerisation of TMS.
for testing from their fabs after use. Donaldson
studied the mechanism of reactions of HMDS in The lens hazing solution
the air and on the filter media. The analysis Through testing and analysis, Donaldson proves
showed that these fabs’ filter media contain both that layering acid impregnated carbon with a non
TMS and HMDSO. However none of these filters impregnated activated carbon or base impregnated
were found to contain HMDS indicating the carbon effectively removes ammonia, TMS and
instability of HMDS in air and on the surfaces of HMDSO. The benefits of this combined adsorbent
these filter adsorbents. system are evident in Figure 4. This shows that the
acidic adsorbent, such as acid impregnated carbon
Filtration knowledge reveals the can effectively remove NH
3
, and activated carbon
answer is effective at removing TMS and HMDSO. These
Donaldson conducted experiments using a typical observations show that the optimised filtration
breakthrough test bench design. The test bench solution, BSMmax filters with layering technology,
consisted of an ultrahigh purity air line connected is most effective at removing the two primary
to a Miller-Nelson model HCS-401 controller culprits of lens hazing, ammonia and siloxanes.
www.euroasiasemiconductor.com September 2008
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