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SEMI F54-1000 © SEMI 2000 3 narrow band width. The mobility band width of the extracted aerosol mus t be no greater than 20% of the mean particle m obility. The method for calcu lati ng electrical mobility a nd mobility …

SEMI F54-1000 © SEMI 2000 2

6 Summary of Method

6.1 The measurement is performed using aerosol

standards consisting of NaCl particles suspended in air.

Aerosols are produced by nebulizing a solution of NaCl

in water. After drying and charge neutralization,

particles of the required size are extracted from the

aerosol using an electrostatic classifier (EC). The

concentration of particles in the resulting monodisperse

aerosol is then recorded by the CNC and compared to

that measured simultaneously by a reference

instrument. Examples of suitable reference instruments

are described in Section 8.8.

NOTE 4: This method has been shown to provide repeatable

results in tests performed in separate laboratories on similar

CNCs. Examples of these tests are found in Agarwal and

Sem

and Bartz, et al

7 Requirements

7.1 The reference instrument must have a proven

counting efficiency

of not less than 95% over the entire

range of particle sizes tested. Low counting efficiency

in the reference instrument results in an error in the

measured particle concentration. The counting

efficiency for the reference instrument must be checked

by the manufacturer against a calibrated standard, such

as a calibrated aerosol electrometer.

7.2 Coincidence error in either the CNC or the

reference instrument causes inaccuracy in the measured

particle concentration. The concentration limit for each

instrument must be obtained from the manufacturer’s

specifications. The particle concentration in the aerosol

standard must be kept below both concentration limits.

7.3 When an aerosol electrometer is used as the

reference instrument, the minimum detectable particle

concentration must be obtained from the manufacturer’s

specifications. A particle concentration below the

minimum detectable value will cause inaccurate results.

The particle concentration in the aerosol standard must

be kept above the minimum detectable particle

concentration.

7.4 The transport loss of particles from the EC to the

CNC, and from the EC to the reference instrument must

be checked. Any disparity in particle transport loss to

the two instruments will cause erroneous results. The

method for calculating the transport loss of particles in

3 Agarwal, J. K. and Sem, G. J. Continuous Flow, Single-Particle-

Counting Condensation Nucleus Counter. Journal of Aerosol

Science, 11: 343-357 (1980)

4 Bartz, H., Fissan, H., Helsper, C., Kousaka, Y., Okuyama, K.,

Fukushima, N., Keady, P. B., Kerrigan, S., Fruin, S. A., McMurry, P.

H., Pui, D. Y. H. and Stolzenburg, M. R. Response Characteristics for

Four Different Condensation Nucleus Counters to Particles in the 3-

50 nm Diameter Range. Journal of Aerosol Science, 16 (5): 443-456

(1985)

sample lines is found in Pui, et al

. The transport loss

must be calculated for each particle size tested. Record

all transport loss calculations.

7.5 When an aerosol electrometer is used as the

reference instrument, a correction must be made for

multiple charging of particles in the aerosol standard.

Multiple charging can cause substantial error when

measuring particles larger than 0.06 micrometer. The

correction factor for multiple charging must be obtained

from the manufacturer of the EC. An example of the

method for calculating the correction factor is found in

Liu and Pui

8 Apparatus

8.1 Atomizer — A sub-micrometer aerosol generator

capable of nebulizing a liquid solution. The atomizer

must be capable of producing a stable aerosol

containing particles as small as 0.01 micrometer after

drying.

8.2 Liquid Trap — A device which allows free

passage of an aerosol stream containing sub-micrometer

particles, but which removes the entrained liquid phase

by gravitational settling into a reservoir.

8.3 Diffusion Drier — A device which allows free

passage of an aerosol stream containing sub-micrometer

particles, but which dries the aerosol by diffusion of

water vapor into a surrounding desiccant medium. The

design flow rate of the diffusion drier must at least

match the output flow rate of the atomizer.

8.4 Aerosol Neutralizer — A bipolar charging device

which neutralizes aerosols by producing both positive

and negative air ions. The neutralizer must reduce the

charge on the particles to the Boltzmann equilibrium

level. The design flow rate of the neutralizer must at

least match the output flow rate of the atomizer.

8.5 Bypass Filter — A high efficie ncy point of use gas

filter. The design flow rate of the bypass filter must at

least match the output flow rate of the atomizer.

8.6 Mixing Vessel — A device which disperses

particles in a flowing aerosol by inducing turbulence.

8.7 Electrostatic Classifier (EC) — A device which

extracts particles from a polydisperse aerosol according

to their electrical mobility. The electrical mobility of a

particle depends upon its size and charge. The

EC must

extract a monodisperse aerosol standard having a

5 Pui, D. Y. H, Ye, Y. and Liu, B. Y. H. Sampling, Transport, and

Deposition of Particles in High Purity Gas Supply System.

Proceedings 9th ICCCS: 287-293, (1988)

6 Liu, B. Y. H. and Pui, D. Y. H. A Submicron Aerosol Standard and

the Primary, Absolute Calibration of the Condensation Nuclei

Counter. Journal of Colloid and Interface Science, 47 (1): 155-171

(1974)

SEMI F54-1000 © SEMI 20003

narrow band width. The mobility band width of the

extracted aerosol must be no greater than 20% of the

mean particle mobility. The method for calculating

electrical mobility and mobility band width are found in

Liu and Pui

8.8 Reference Instrument — An instrument which can

measure the concentration of particles in an aerosol.

The calibration of the reference instrument must have

been checked against a calibrated standard within the

past 12 months. Calibration and maintenance records

for the reference instrument must be maintained.

Suitable reference instruments include aerosol

electrometers and CNCs having lower size sensitivities

below 0.01 micrometer.

8.9 Condensation Nucleus Counter (CNC) or

Condensation Particle Counter (CPC) — The particle

counter to be used in process gas particle

measurements, and for which the calibration check is

required. The CNC detects sub-micrometer particles

using nucleation and droplet growth of a super-

saturated working fluid.

8.10 Tubing — In order to minimize particle losses, all

tubing used to transport charged aerosol particles must

be constructed from electrically conductive materials

(e.g., stainless steel) and grounded.

9 Reagents and Materials

9.1 De-ionized or distilled water in accordance with

ASTM D1193, Type 1 is required to dissolve NaCl for

aerosol generation.

9.2 NaCl is required for aerosol generation.

9.3 A supply of dry, oil-free compressed air is

required to generate and transport aerosols in the

apparatus.

10 Preparation of Apparatus

10.1 The test apparatus is shown in Figure 1. The

particle concentration in the aerosol is controlled using

a bypass filter. If the flow rate of the monodisperse

aerosol from the EC is insufficient to supply the CNC

and reference instrument simultaneously, filtered air

must be added to the stream. The air must be added to

the aerosol upstream of a mixing vessel. If the flow

rate of the monodisperse aerosol from the EC is greater

than that required to supply the CNC and reference

instrument simultaneously, the excess aerosol must be

vented.

Compressed

Air

Liquid

Trap

Diffusion

Drier

Bypass Filter

for Dilution

erosol

Neutralizer

tomizer

Sheath Air

Excess Air Vent

Electrostatic

Classifier

Monodisperse

Reference

Instrument

Nucleus

Counter

Filtered

Mixing

Vessel

Filtered Make-Up Air

or Excess Aerosol Vent

erosol Standard

Condensation

Vent

ent

(CNC)

(EC)

Figure 1

Schematic Diagram of Test Apparatus

SEMI F54-1000 © SEMI 2000 4

11 Calibration and Standardization

11.1 All flow meters and voltage indicators used in the

apparatus, including instruments incorporated into the

EC and particle counters, must have been checked for

proper calibration against calibrated standards within

the past 12 months. Record all calibration data.

12 Procedure

12.1 Fill the atomizer reservoir with a solution of NaCl

in water. The required NaCl concentration depends

upon the range of particle sizes to be tested, and must

be determined from the atomizer manufacturer’s

specifications. Record the concentration of NaCl in the

solution.

12.2 Determine the required aerosol, sheath air and

excess air flow rates for the EC. The flow rates depend

upon the range of particle sizes to be tested and the

required mobility bandwidth, and must be determined

from the EC manufacturer’s specifications.

12.3 Close the bypass filter valve. Adjust the flow

rates of the atomizer, CNC, and reference instrument to

their manufacturer’s specified values. Adjust the flow

rates of the

EC to their required values as described in

Section 12.2. Adjust the flow rate of the make-up air or

aerosol vent to the required value as described in

Section 10.

12.4 Set the EC voltage to obtain the selected particle

size for testing. Record the voltage setting and the

selected particle size.

12.5 Observe the particle concentration indicated by

the reference instrument. If the particle concentration

exceeds the concentration limit of the reference

instrument or the CNC, adjust the flow rate through the

bypass filter to lower the aerosol particle concentration.

After adjusting the bypass flow rate, check all flow

rates to ensure that they remain at the required values.

Record all flow rates.

12.6 Wait for the particle concentrations indicated by

the CNC and reference instrument to become steady;

the concentrations indicated by each instrument must

not change by more than 5% over a five minute

interval. In order to minimize the error in the recorded

particle concentrations, a minimum of 500 particles

must be counted in a sample. Record the steady

particle concentrations indicated by the CNC and

reference instrument.

12.7 If another particle size is to be tested, return to

Section 12.4.

NOTE 5: At least eight different particle sizes must be tested.

The particle sizes must be selected such that the measured

CNC counting efficiency at least spans the range 50% to 95%

(0.50 to 0.95). In order to eliminate systematic error, the

particles sizes must be tested in random order.

13 Calculation and Interpretation of Results

13.1 When an aerosol electrometer is used as the

reference instrument, correct the measured particle

concentrations for multiple charging, as described in

Section 7.5.

13.2 For each particle size tested, calculate the ratio of

the concentration of particles reported by the CNC to

that reported by the reference instrument. This ratio

represents the counting efficiency of the CNC.

13.3 Plot the calculated counting efficiency against the

particle size as shown in Figure 2. This plot shows the

measured counting efficiency curve of the CNC.

Particle Size

Counting Efficienc

Figure 2

Schematic Plot of Measured Counting Efficiency

13.4 Determine the lower size sensitivity of the CNC,

as defined in Section 5. (The counting efficiency at any

particle size can be estimated using linear interpolation

between the data points.)

14 Reporting Results

14.1 The test record must include the following items:

a. test date;

b. test operator;

c. serial numbers of the reference instrument, CNC

and EC;

d. all particle transport loss calculations as described

in Section 7.4;

e. all flow meter and voltage indicator calibration

data as described in Section 11;

f. all raw data and calculations utilized in the test

method as listed in Sections 12 and 13;