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SEMI F33-0998 © SEMI 1998 1 SEMI F33-0998 METHOD FOR C A LIBR ATION OF ATMO SPHERIC PRESSURE IONIZA TION M A SS SPECTROM ETER (A PIM S) 1 Purpose 1.1 T his test method may provide g u id elines for the calibration of th …

SEMI F32-0998 © SEMI 1998 6

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SEMI F33-0998 © SEMI 19981

SEMI F33-0998

METHOD FOR CALIBRATION OF ATMOSPHERIC PRESSURE

IONIZATION MASS SPECTROMETER (APIMS)

1 Purpose

1.1 This test method may provide guidelines for the

calibration of the APIMS for measurement of

impurities in nitrogen, argon, helium and hydrogen.

APIMS is currently the technique of choice for

measurements of low level impurities in gas

distribution systems and components because it is

essentially the only commercially available method

capable of ppt impurity analysis and it has a superior

response time. This method may provide guidelines for

application of other techniques with similar detection

limits and response time to APIMS which are not

commercially available at this time.

2 Scope

2.1 This method applies to the analyte calibration of

the APIMS for a target impurity range of 100 ppt to as

high as 100 ppb impurity range. The actual calibration

range should bracket the impurity measurement range

of interest, dependent upon the measurement to be

conducted. Anything else is outside the range of the

calibration.

3 Limitations

3.1 The actual range of calibration will depend upon

the type of APIMS used. Counting detection electronics

will saturate in the 200,000 CPS to 1,000,000 CPS

range, depending upon the make and model. Counting

detection will have to use different ions in different

concentration regions, depending upon the impurity.

The actual calibration procedure will be different for

APIMS using analog detection versus counting

detection. Interference between impurities can cause

limitation in measurements in the presence of multiple

impurity species.

4 Referenced Documents

4.1 SEMI Standards

SEMI C9.1 — Guide for Analysis of Uncertainties in

Gravimetrically Prepared Gas Mixtures

SEMI C15 — Test Method for ppm and ppb Humidity

Standards

5 Terminology

5.1 Acronyms

5.1.1 APIMS — Atmospheric Pressure Ionization

Mass Spectrometer

5.1.2 CPS — Counts per second

5.1.3 m/z — m in atomic mass units and z in

elementary charge units

5.1.4 NIST — National Institute of Standards and

Technology

5.1.5 ppb — Molar parts per billion (nmole/mole).

The same as ppbv.

5.1.6 ppm — Molar parts per million (µmole/mole).

The same as ppmv.

5.1.7 ppt — Molar parts per trillion (pimole/mole).

The same as pptv.

5.1.8 R

— The statistic described b y the ratio of the

sum of squares of the regression divided by the total

sum of the squares.

5.2 Definitions

5.2.1 zero gas — Nitrogen, argon, helium or hydrogen

with an estimated level an order of magnitude, or more,

lower than the lowest calibration point for each

impurity of interest.

6 Summary of Method

6.1 The calibration of the APIMS is conducted by

adding known concentrations of impurities to a zero gas

and measuring the corresponding ion intensities. A

calibration response factor can be determined by

regression analysis.

7 Interferences

7.1 It is essential to confirm that the mass chosen is

indeed representative of the species of interest and that

other commonly present impurities do not contribute at

the same mass. For example: the intensity of the peak at

m/z = 29 correlates with the concentration of H

in N

because of the N

formation. However, spurious

signal at this peak can arise in two different ways:

(i) Contributions of other ions of the same mass, such

as N

15+

or C

(ii) Contributions of the same ion from different parent

species, i.e., N

due to recombination of matrix

gas nitrogen with fragments of H

O, CH

, etc.

Before using m/z = 29 as a measure of H

in N

interference such as these must be understood. This

also may apply to other ions.

SEMI F33-0998 © SEMI 1998 2

7.2 High impurity levels can cause clustering with, or

quenching of, the other impurity ions and affect the

sensitivity of the APIMS for those impurities. This is

especially true for impurity levels of 10 ppb and above.

8 Apparatus

8.1 APIMS — The APIMS used for impurity detection

can be of any type.

8.2 Impurity Standard — Any device (either standard

cylinder or permeation tube based or of any other type)

that can reliably deliver impurity concentrations usually

in the range of 100 ppb to 10 ppm with an accuracy of

± 5%. The standards should be traceable to an

applicable national standard, such as NIST.

8.3 Dynamic Dilution System — A dilution system

that can dilute the output gas of the impurity standard

with a zero gas to produce a gas with impurity

concentrations in the range of interest.

9 Reagents and Materials

9.1 Zero gas

9.2 Impurity standard

10 Safety Precautions

10.1 This method applies to calibration and

measurement of impurities in nitrogen, argon, helium

and hydrogen. This method does not address the

additional safety precautions for APIMS calibration and

impurity measurement in hydrogen gas.

11 Test Specimen

11.1 Not applicable.

12 Preparation of Apparatus

12.1 The impurity standard and zero gas should be

connected to the dynamic dilution system, which is in

turn connected to the inlet of the APIMS. The entire

dilution system should be purged until all impurities

have stabilized at a constant dilution system flow, for

all components in the dilution system. The relative

standard deviation for each analyte should be less than

10% for the past 12 hours before the calibration.

Constant pressure and flow should be maintained to the

APIMS ionization region at all time.

13 Calibration and Standardization

13.1 Definition of an Impurity Standard — An

impurity “standard” is defined to be any device capable

of delivering a flow of known impurity level gas at a

controlled pressure. (See Section 8.2.)

13.2 Examples of Standards — Some examples of

impurity standards and the principles upon which they

are based are as follows:

13.2.1 Cylinder Standards — The “standard” will

include a regulator specified for use with the cylinder.

The impurity concentration can be calculated based on

that added to the cylinder in preparation. A minimum

use pressure must be specified.

13.2.2 Permeation and effusion tube standards. These

must include purification means and some means of

regulating the outlet pressure. The impurity delivery

rate can be calculated based on the weight loss of the

tube as a function of time.

13.2.3 Moisture standard methods based on saturation

of gas with water vapor at a fixed temperature and

pressure. The moisture concentration in the gas may

then be calculated from a knowledge of the saturation

vapor pressure of water over a plane of the pure phase

of ice at the saturation temperature and of the

interaction virial coefficients of the gas-vapor mixture.

The two-pressure and two temperature methods are

refinements of this approach requiring additional

chambers whose temperature and pressure must be

known.

13.3 Dilution of Standards — For all impurity

generation methods, lower concentrations can be

generated by dynamic dilution, i.e., by combining a

known flow of the standard gas with a known flow of

zero gas. A diluted standard is acceptable provided:

13.3.1 The zero gas should have an impurity level at

least an order of magnitude below the stated level of the

analysis (to be verified using the same flow path as

during the subsequent analysis, and at the lowest flow

rate actually used in the calibration).

13.3.2 The absolute accuracy of the dilution system

components are verified by comparison with a reliable

flow standard. The frequency of verification must take

into account the transport of the instrument and other

changes that will affect the dilution system.

13.3.3 The linearity of the dilution system can be

demonstrated over the entire range of operation. This

last criterion is particularly important whenever some

portion of the combined flow is discarded, as mixing

problems can easily arise at large dilution factors.

13.4 Mass Calibration — The mass scale of the mass

spectrometer should be calibrated according to the

manufacturer’s instructions over the mass range of

interest.