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SEMI F25-0697 © SEMI 1997, 2003 3 counter for at least 5 m i nutes at the instrum ent manufacturer’s specifie d flow rate. The exact purge time should take into account the reactivity of the gas and should be sufficient …

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SEMI F25-0697 © SEMI 1997, 2003 2
S
M
=
( X
Mi
− X
M
)
2
∑
(N
M
−1)
S
B
=
( X
Bi
− X
B
)
2
∑
(N
B
− 1)
NOTE 1: The third is obtained from the first two, i.e.:
S
C
= S
M
2
+ S
B
2
5 Requirements
5.1 Maximum Permissible Particle Concentration —
10 particles per 25 standard liters as determined by the
instrument specified in Section 6.
5.2 The specification will be considered met if the
calculated concentration of particles plus 2 times the
standard deviation does not exceed 10 particles per 25
standard liters, i.e.:
X
C
+ 2 × S
C
≤10 particles/25 standard liters
6 Apparatus
6.1 Particle Counter — An instrument suitable for
counting particles in compressed oxidant gases at a
pressure of up to 8 × 10
5
Pa (8 atmospheres) with a
50% counting efficiency at 0.2 micrometer or smaller.
The counting efficiency should reach 90% at 0.3
micrometer or smaller. The resolving power of the
instrument near 0.2 micrometer should be no worse
than 10%. The counting efficiency is determined by a
calibration at 1 × 10
5
Pa (1 atmosphere) pressure using
polystyrene latex spheres in an inert gas and a reference
particle counter with a proven counting efficiency of
not less than 95% at 0.2 micrometer.
NOTE 2: Suitable test methods for determining counting
efficiency and resolving power are contained in Japanese
Industrial Standard JIS B 9921 (1989), “Light Scattering
Automatic Particle Counter”. More sensitive particle counters
result in a higher measured particle concentration.
7 Test Method
NOTE 3: The details of the sampling configuration,
measurement procedure, and instrument calibration procedure
and frequency must be agreed upon by the user and supplier,
taking into account good engineering practice. Material
safety data sheets should be referred to for safe handling of
specialty gases.
7.1 Determine the average observed concentration of
counts in the background (
B
X ) by passing nitrogen,
believed to be free of particles 0.2 micrometer or more
in diameter, through the instrument and recording the
total number of counts. The nitrogen purge assembly
for performing this test, using a filter which removes
particles in this size range, is shown in Figure 1. Count
a minimum of 3 equal intervals, each of at least 25
standard liters (0.95 SCF), or 30 minutes, whichever is
greater. Calculate
B
X as defined in Section 4.
B
X
must not exceed 2 counts per 25 standard liters.
8
Figure 1
Schematic Diagram of Configuration for
Obtaining Particle Samples from
Oxidant Specialty Gas Systems
7.2 The sampling point should be near the point of use,
and sampling lines should be clean, free of combustible
substances, and as short as possible.
7.3 The sampling system configuration is shown in
Figure 1. The specialty gas system should be connected
through a sample valve directly to the particle counter,
a pressure gauge, and a flow control device (FCD).
When sampling is performed using wall tap sample
ports, the aspiration efficiency should be checked using
the method described in Related Information 1. The
sampling system should contain minimum dead volume
and sample tube length. All components in the
sampling system should be leak tight and cleaned in
accordance with good engineering practice. A leak
check of the system should be performed in accordance
with normally accepted practice. The FCD may be part
of the particle counter. The particle counter should be
maintained at the pressure of the gas system. The FCD
can be a metering valve and a flow meter or a critical
orifice. The exhaust line should be leak tight and piped
to an appropriate exhaust system. The exhaust line
diameter should be large enough to produce no more
than 1 × 10
4
Pa (1.5 psi) pressure drop during purge and
sample flow.
NOTE 4: The alternative sampling configurations described
in SEMI C6.2 can also be used for oxygen.
7.4 Using the high purity nitrogen system shown in
Figure 1, purge the sampling system and particle
SEMI F25-0697 © SEMI 1997, 2003 3
counter for at least 5 minutes at the instrument
manufacturer’s specified flow rate. The exact purge
time should take into account the reactivity of the gas
and should be sufficient to purge the entire exhaust line.
The purging must be performed before oxidant gas is
introduced into the sampling system and after
completion of the measurement. The valve leading
from the purge nitrogen system should be closed when
sampling specialty gases and/or a back flow prevention
device should be included in the purge nitrogen system.
NOTE 5: It is known that oxidant gases, although non-
flammable, may support combustion. Cleaning in accordance
with good engineering practice and nitrogen purging of the
sampling system reduce the risk of reaction. The purity of the
purge nitrogen should be sufficient to remove reactive
substances from the sampling system.
7.5 Count the particles in each of at least 3 equal
intervals. Each sample interval must be at least 25
standard liters (0.95 SCF), or 30 minutes, whichever is
greater. Data obtained during the first 5 minutes after
the sample valve is opened may be discarded. Record
the number of counts in the sample volume for each
interval. Calculate
C
X and S
C
, as defined in Section 4.
NOTE 6: For small volume specialty gas systems, the user
and supplier may agree to a smaller sample gas volume than
that stated above. The sample point location and the process
line pressure and flow rate during the test should be recorded.
8 Report
8.1 The report shall contain the values of all the
variables defined in Section 4.
9 Related Documents
Hart, J. J., W. T. McDermott, A. E. Holmer, and J. P.
Natwora, Jr. Particle Measurement in Specialty Gases.
Solid State Technol., 38(9):111-116, September 1995.
Wang, H. C. and R. Udischas. Counting Particles in
High Pressure Electronic Specialty Gases. Solid State
Technol., 37(6):97-107, June 1994.
NOTICE: SEMI makes no warranties or
representations as to the suitability of the standards set
forth herein for any particular application. The
determination of the suitability of the standard is solely
the responsibility of the user. Users are cautioned to
refer to manufacturer's instructions, product labels,
product data sheets, and other relevant literature,
respecting any materials or equipment mentioned
herein. These standards are subject to change without
notice.
By publication of this standard, Semiconductor
Equipment and Materials International (SEMI) takes no
position respecting the validity of any patent rights or
copyrights asserted in connection with any items
mentioned in this standard. Users of this standard are
expressly advised that determination of any such patent
rights or copyrights, and the risk of infringement of
such rights are entirely their own responsibility.
SEMI F25-0697 © SEMI 1997, 2003 4
RELATED INFORMATION 1
NOTES ON ASPIRATION EFFICIENCIES OF WALL TAP SAMPLE
PORTS
NOTICE: This related information is not an official part of SEMI F25 and is not intended to modify or supercede the official
standard. Determination of the suitability of the material is solely the responsibility of the user.
R1-1 Introduction
R1-1.1 Isokinetic sample probes are usually not
provided in speciality gas systems. Particle sampling is
usually performed using wall tap sample ports. Wall
tap sample ports are oriented 90° to the process line
flow as shown in Figure R1-1. Non-isokinetic flow into
the sample tube results from the 90° change in flow
direction. Additional deviation from isokinesis can
result from stream tube contraction or expansion when
the velocities U and v are not equal. The effects of
non-isokinetic flow on the measurement should be
checked. The particle sampling process should be
performed with an aspiration efficiency close to 1.
Process Line
v, C
To Sampling System
Wall Tap
D
U, Co, p
Figure R1-1
Schematic Diagram of Process Line and Wall Tap
Sample Port
R1-2 Variables
d
p
= Diameter of particle (cm)
ρ
p
= Intrinsic density of particle (g/cm
3
)
η = Dynamic viscosity of the gas (g/s-cm)
λo = Mean free path of the gas at atmospheric pressure
(cm)
p = Pressure of the gas (Pa)
P
O
= Atmospheric pressure (= 1 × 10
5
Pa)
D = Diameter of wall tap sample tube (cm)
U = Average velocity of the process line flow (cm/s)
v = Average velocity of the sample line flow (cm/s)
C* = Stokes-Cunningham slip correction factor
S
tk
= Stokes number
R = Velocity ratio
Co = Particle concentration in the process line (cm
–3
)
C = Particle concentration in the sample line (cm
–3
)
R1-3 Calculations
R1-3.1 The Stokes-Cunningham slip correction factor
is calculated for particles in the size range of interest:
C* =1 + 2.492
p
o
λ
o
pd
p
+ 0.84
p
o
λ
o
pd
p
e
−0.435pd
p
/p
o
λ
o
Calculate the velocity ratio and Stokes number. The
intrinsic particle density may assume a worst case value
of 10 g/cm
3
:
R = U/v
S
tk
=
d
p
2
ρ
p UC*
18
η
D
A wall tap sample port can be approximated as a
sampling probe oriented 90° to the flow. Calculate the
aspiration efficiency:
C
Co
=
1
1 + 8S
tk
R
1/2
Repeat the calculations for particle sizes in the range of
interest. When the aspiration efficiency is significantly
different from 1, the calculated efficiency should be
used to correct the measured particle concentration, C.
R1-4 References
Stevens, D. C. Review of Aspiration Coefficients of
Thin-Walled Sampling Nozzles. J. Aerosol Sci.,
17(4):729-743, 1986.
Vincent, J. H., D. C. Stevens, D. Mark, M. Marshall,
and T. A. Smith. On the Aspiration Characteristics of
Large-Diameter, Thin-Walled Aerosol Sampling Probes
at Yaw Orientations With Respect to the Wind. J.
Aerosol Sci., 17(2):211-224, 1986.