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SEMI F38-1104 © SEMI 1999, 2004 2 a background l evel as specified in Section 7.1.2. 6.3 Test Environment — Particle challenge testing does not require a clean area. Testing in a Class 100 area is recommended but not req…
SEMI F38-1104 © SEMI 1999, 2004 1
SEMI F38-0699 (Reapproved 1104)
TEST METHOD FOR EFFICIENCY QUALIFICATION OF POINT-OF-USE
GAS FILTERS
This test method was technically reapproved by the Global Gases Committee and is the direct responsibility
of the North American Gases Committee. Current edition approved by the North American Regional
Standards Committee on August 16, 2004. Initially available on www.semi.org September 2004; to be
published November 2004. Originally published June 1999.
1 Purpose
1.1 The purpose of this document is to define a
comprehensive standard test sequence to qualify the
particle filtration efficiency achievable using Point-of-
Use (POU) gas filters.
2 Scope
2.1 This test method defines an evaluation method for
Point-of-Use filters of various media (e.g., metallic,
ceramic, and polymeric) typically used for filtering
inert and process gases in semiconductor applications.
Point-of-Use filters are designed to handle relatively
low flow rates (0.5 – 50 slm.) and moderately high
pressure drops. The filter housing and filtration
element are combined into one sealed and inseparable
unit.
2.2 This test method is intended to demonstrate the
ability of a Point-of-Use gas filter to equal or exceed a
specific particle filtration efficiency class when
challenged with a monodispersed aerosol in the size
range described in Section 6.6.
2.2.1 The efficiency class of the test method is defined
as the log reduction value (LRV), where LRV is the
Log [Input Concentration/System Background Level].
NOTICE: This standard does not purport to address
safety issues, if any, associated with its use. It is the
responsibility of the users of this standard to establish
appropriate safety and health practices and determine
the applicability of regulatory or other limitations prior
to use.
3 Limitations
3.1 These test methods do not apply to gas filter
cartridges.
3.2 These test methods do not apply to bulk gas filters.
3.3 The various media (e.g., metallic, ceramic, and
polymeric) may have slightly different critical particle
sizes (most penetrating particle sizes). Therefore, many
challenge tests have to be performed over a range of
particle sizes to assure that the “worst case” challenge
has been performed.
3.4 The efficiency test is for steady state flow and
room temperature only. Comparison to efficiency in
pulsed flow, elevated temperature etc., will have to be
developed separately.
3.5 Statistical evaluation on the variability in efficiency
of a media type should be done with a valid sample
size. This evaluation is not within the scope of this test
method.
3.6 The upper concentration limit of the CNC as
specified by the manufacturer should not be exceeded.
Exceeding the CNC concentration will understate the
challenge concentration due to coincidence counting,
thus affecting filter efficiency calculations.
3.7 This is not intended as a method for precise
determination of filter efficiency. It is intended as a
means for determining if a filter has an efficiency equal
to or better than an approximate nominal value.
3.8 Commercially available radioactive neutralizers
have pressure limitations. Users should be aware of
these limits.
4 Referenced Standards
4.1 None.
5 Summary of Method
5.1 The test sequence consists of three steps:
5.1.1 Static background with filter.
5.1.2 Filter challenge with first monodispersed particle
size (see Section 6.6 for particle size test sequence).
5.1.3 Filter challenge with subsequent monodispersed
particle sizes.
6 Apparatus and Facility
6.1 Gas Source — Clean, dry gas (nitrogen or air) with
less than 0.5 ppm moisture and less than 0.5 ppm total
hydrocarbons.
6.2 Particle Detector — A condensation nucleus
counter (CNC) with a counting efficiency of 50% at
0.01 micron, as reported by the manufacturer, is
recommended to measure total particle concentration of
particles greater than 0.01 micron. The CNC must meet
SEMI F38-1104 © SEMI 1999, 2004 2
a background level as specified in Section 7.1.2.
6.3 Test Environment — Particle challenge testing does
not require a clean area. Testing in a Class 100 area is
recommended but not required. Refer to Section 6.7 for
installation precautions.
6.4 Configuration for Efficiency Test — One example
of the configuration is shown in Figure 1. The setup
shown in Figure 1 is intended only as a guideline. This
system consists of:
an aerosol generation system,
aerosol concentration detectors,
an air flow measuring system,
a filter pressure drop sensor, and
the test filters.
6.5 Test Flow Rate — Maximum manufacturer’s rated
flow rate.
6.6 Challenge Particle Sizes — Experimentation of
various media (e.g., metallic, ceramic, and polymeric)
have resulted in most penetrating particle sizes ranging
from 0.05–0.11 micron.
1
2
3
4
6.6.1 Several monodispersed particle challenge tests
shall be done over a range of particle sizes so that
testing at the most penetrating particle size is
performed. An electrostatic classifier must be used to
produce a monodispersed challenge.
6.6.2 Due to the effects of particle loading in relation
to filtration efficiency, a clean filter should be used for
each particle size challenge. The differential pressure
across the test filter should be continuously monitored
to eliminate the possibility of particle loading. The
following particle increments are a guideline. Smaller
increments are achievable and are desirable.
6.6.2.1 Challenge Particle Size In Microns:
1 Rubow, K.L., C.B. Davis (1991) "Particle Penetration
Characteristics of Porous Metal Filter Media for High Purity Gas
Filtration", Proceedings of Institute of Environmental Sciences
Annual Technical Meeting
2 Rubow, K.L., and Liu, B.Y.H., Evaluation of Ultra-High Efficiency
Membrane Filters, Proceedings of the 30th Annual Technical
Meeting, Institute of Environmental Sciences, 1984.
3 Rubow, K.L., Liu, B.Y.H., and Grant, D.C., Characteristics of
Ultra-High Efficiency Membrane Filters in Gas Applications; Journal
of Environmental Sciences, May/June, 1988.
4 B. Gotlinsky, P. Conner, D. Capitanio, L. Johnson, and S. Tousi,
Testing of All-Metal Filters for High Purity Semiconductor Process
Gases, Proceedings of the 37th Annual Technical Meeting, Institute
of Environmentak Sciences, 1991.
0.05
0.07
0.10
6.7 Sample Installation — Reasonable precautions
shall be taken when installing the test filter to avoid
particle contamination of the system from ambient.
These precautions may include, but are not limited to:
installation in a Class 100 laminar flow area,
installation in a purged glove bag or other controlled
ambient enclosure, use of a purge flow downstream of
the sample point so that all parts of the system are
under purge, isolation, or termination of the CNC
sample flow, and isolation of the isokinetic sampler
exhaust from ambient.
6.8 Customary practices shall be employed for the
design of the sampling system. This includes any
specialized equipment or procedures recommended by
the CNC manufacturer. Every effort should be taken to
minimize differences in the particle concentrations of
the sampling volume relative to the actual test volume.
Care should be taken to provide an adequate exhaust
length so that back diffusion of particles does not affect
background at 2 times the sampling flow rate. This
length will depend upon the particle concentration in
the ambient environment and on other factors.
6.9 Test Particle — Solid non-volatile particles.
Sodium chloride is recommended, but other particles
could be used such as polystyrene latex beads (PSL) if
sufficiently high concentrations can be generated.
7 Procedure
7.1 Static Background Test
7.1.1 Purge the system at a high velocity flush of 100
slm or the maximum flow rate that the system
components will allow.
7.1.2 Purge the system at test flow rate (Section 6.5).
Measure the particle concentration with clean gas to
determine the background concentration. The
background count is established when the counter has
sampled a minimum of 3 scf (85 sl), and the arithmetic
average during the last 3 scf (85 sl) of gas sampled is
<2 particles/scf (<0.07 particles/sl). The sample
interval shall be 1 minute in duration. Ensure that the
background counts are stable or decreasing. If
background cannot be achieved after 6 scf (170 sl) have
been sampled, there may be a problem with the counter
or test apparatus. Past experience has shown this to be
an adequate volume of gas to give a satisfactory low
level background particle counts.
7.2 Penetration Test
SEMI F38-1104 © SEMI 1999, 2004 3
7.2.1 An example of the apparatus is shown in Figure
1. The input particle concentration is measured by
CNC-B, while the output particle concentration is
measured by CNC-A. The pressure and flow rates in
both legs should be carefully balanced (by maintaining
an identical differential pressure in both legs) to insure
that the challenge level is the same.
7.2.2 Set the first and subsequent challenge particle
sizes per Section 6.6.
7.2.3 Particle challenge concentrations should be set to
the maximum output of the electrostatic classifier for
each of the particle size tests (Section 6.6). This will
determine the test volume and the duration of the test.
Generally this requires generating a polydispersed
aerosol having a mean size close to the desired
monodispersed aerosol size. Any difference between
the flow rate exiting the electrostatic classifier and the
test flow rate specified in Section 6.5 will be made up
by dilution gas as shown in Figure 1.
7.2.4 The total particle challenge must be limited to
1 10
9
particles per liter of filter rated flow in slm.
This is done to limit the effects of a particle cake build-
up which would enhance the efficiency of the test filter.
The test should be concluded upon exposure to 1 10
9
particles per liter of rated flow or after 10 downstream
counts are observed, whichever occurs first.
7.2.4.1 As an example, a 10 liter per minute filter
would be challenged in one minute with (10 slm) (1
minute) (1 10
9
particles/liter) or 1 10
10
particles.
If the challenge concentration is 1 10
8
particles per
liter and the flow rate is 10 slm, then the test would
need to last 10 minutes or when 10 downstream
particles were observed.
7.2.4.2 The sample interval shall be 1 minute in
duration. The sample interval in the penetration test is
equivalent to the sample interval in the system
background test (Section 7.1.2).
7.2.5 Measure the input and output particle
concentration.
7.2.6 Calculate the ratio of output to input particle
concentration.
8 Reporting of Test Results
8.1 Raw data shall be reported for each test in the
format of a table, including the number of sample
intervals, the sampling volume of each interval, the
sampling time of each interval, and the total number of
particles registered in each interval. In addition, the
relevant parameters for each test described in Sections
8.2 and 8.3 should be identified.
8.2 Static Background Test — Identify the period of
time required to obtain 45 consecutive sampling
intervals without a particle being detected for the spool
piece and test filter.
8.3 Penetration Test — Identify the input
concentration, the output concentration, the ratio of
output to input concentration (fractional penetration),
and the challenge particle size. The most penetrating
particle size is the challenge particle at the largest
fractional penetration. Care must be taken to distinguish
between true penetration and system background counts
(see Section 8.5) as measured in Section 7.1.2.
8.4 Efficiency Class
8.4.1 If no penetration is observed then report the POU
Gas Filter passed a X LRV efficiency test, where X =
the maximum detectable efficiency for the test (as
defined in Section 2.2). No penetration is defined as
when the filter output is not statistically different from
the system background (as defined in Section 8.5).
8.4.2 If penetration is observed , then report that the
POU Gas Filter failed a X LRV efficiency test, where X
is the maximum detectable efficiency for the test (as
defined in Section 2.2). Penetration is defined as when
the filter output is statistically different from the system
background (as defined in Section 8.5).
8.4.2.1 The LRV defines the number of nines in the
efficiency.
8.4.2.2 Example: A filter with a fractional penetration
of 0.00001 has a LRV of 5 and an efficiency of
99.999%
8.5 Data Reduction
8.5.1 The statistical analysis is based on the
assumption that the particles generated are randomly
distributed and are statistically independent of each
other. The background counts are independent of the
particle performance of the component. The particle
counts observed from the test include the counts from
the component and the background count.
Let:
X
B
= average background particle count
X
t
= average total particle count from test
X
c
= average particle count generated by the test
component
Therefore,
X
t
X
c
X
B
8.5.2 A statistical analysis of the data is performed to