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SEMI F33-0998 © SEMI 1998 2 7.2 Hi gh imp urit y level s can c ause c l u steri ng with, or quenching of, the other im purity ions an d affect the sensitivity of the APIMS for those i mpurities. This is especially tr ue …

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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
2
— 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
2
in N
2
,
because of the N
2
H
+
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
14
N
15+
or C
2
H
5
+
.
(ii) Contributions of the same ion from different parent
species, i.e., N
2
H
+
due to recombination of matrix
gas nitrogen with fragments of H
2
O, CH
4
, etc.
Before using m/z = 29 as a measure of H
2
in N
2
,
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.
SEMI F33-0998 © SEMI 19983
13.4.1 The validity of the mass selected for each
impurity must be confirmed (i) by demonstrating that
the intensity at the mass increases monotomically with
added concentration of the impurity species of interest
and (ii) that other usually present impurity species do
not make significant contributions to the intensity at
that mass. Tables 1–4 list the typical masses for various
species in argon, nitrogen, hydrogen and helium.
Table 1 Common Masses in Argon Bulk Gas
m/z Ions Source
14 N
+
Analyte
15 CH
3
+
Analyte
16 CH
4
+
Analyte
17 OH
+
Analyte
18 H
2
O
+
Analyte
19 H
3
O
+
Analyte
20 Ne
+
Bulk Gas Impurity
28 CO
+
Analyte
32 O
2
+
Analyte
36 Ar
+
Bulk Gas Isotope
37 (H
2
O)
2
H
+
Analyte
38 Ar
+
Bulk Gas Isotope
40 Ar
+
Bulk Gas
41 ArH
+
Analyte
44 CO
2
+
Analyte
68 ArCO
+
Analyte
68 ArN
2
+
Analyte
76 Ar
2
+
Bulk Gas Isotope
78 Ar
2
+
Bulk Gas Isotope
80 Ar
2
+
Bulk Gas
NOTE: In APIMS, the same ion may be formed even though different
parent molecules are introduced into the ionization region. Thus, it is
necessary to be sure that the chosen m/z represents the ion of interest
and that the ion of interest originates from the molecule of interest.
Table 2 Common Masses in Nitrogen Bulk Gas
m/z Ions Source
12 C
+
Analyte
14 N
+
Bulk Gas
15 CH
3
+
Analyte
16 CH
4
+
Analyte
17 OH
+
Analyte
18 H
2
O
+
Analyte
19 H
3
O
+
Analyte
28 N
2
+
Bulk Gas
28 CO
+
Analyte
29 N
2
H
+
Analyte
29 COH
+
Analyte
29 C
2
H
5
+
Analyte
29 N
2
+
Bulk Gas Isotope
32 O
2
+
Analyte
37 (H
2
O)
2
H
+
Analyte
42 N
3
+
Bulk Gas
43 N
3
H
+
Analyte
44 CO
2
+
Analyte
44 N
2
O
+
Analyte
46 N
2
H
2
O
+
Analyte
55 (H
2
O)
3
H
+
Analyte
56 N
4
+
Bulk Gas
70 N
5
+
Bulk Gas
NOTE: In APIMS, the same ion may be formed even though different
parent molecules are introduced into the ionization region. Thus, it is
necessary to be sure that the chosen m/z represents the ion of interest
and that the ion of interest originates from the molecule of interest.
Table 3 Common Masses in Hydrogen Bulk Gas
m/z Ions Source
2H
2
+
Bulk Gas
17 CH
5
+
Analyte
18 H
2
O
+
Analyte
19 H
3
O
+
Analyte
29 N
2
H
+
Analyte
29 COH
+
Analyte
33 O
2
H
+
Analyte
34 O
2
H
2
+
Analyte
35 O
2
H
3
+
Analyte
37 (H
2
O)
2
H
+
Analyte
45 CO
2
H
+
Analyte
NOTE: In APIMS, the same ion may be formed even though different
parent molecules are introduced into the ionization region. Thus, it is
necessary to be sure that the chosen m/z represents the ion of interest
and that the ion of interest originates from the molecule of interest.