semi合集-English.pdf - 第3874页

SEMI F24-0697 © SEMI 1997, 2003 4 RELATED INFORMATION 1 NOTES ON ASPIRATION EFFICIE NCIES OF WALL TAP SAMPLE PORTS NOTICE: This related information is not an offi cial part of SEMI F24 and is n ot intende d to modify or …

SEMI F24-0697 © SEMI 1997, 2003 3

counter for at least 5 minutes at the instrument

manufacturer’s specified flow rate. The exact purge

time should be sufficient to purge the entire exhaust

line. The purging must be performed before sample 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.

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

X and S

, as defined in Section 4.

NOTE 5: 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 F24-0697 © 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 F24 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

U, Co, p

Figure R1-1

Schematic Diagram of Process Line and Wall Tap

Sample Port

R1-2 Variables

= Diameter of particle (cm)

= Intrinsic density of particle (g/cm

)

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)

= Atmospheric pressure (= 1 × 10

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

= 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

0. 84

−

435pd

/ p

Calculate the velocity ratio and Stokes number. The

intrinsic particle density may assume a worst case value

of 10 g/cm

=U/v

UC*

A wall tap sample port can be approximated as a

sampling probe oriented 90° to the flow. Calculate the

aspiration efficiency:

1 + 8S

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.