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SEMI F33-0998 © SEMI 1998 3 13.4.1 The v alidity of the m a ss select e d for each impu rity mus t be confirme d (i) by de monstrat ing that the intensity at the mass i ncreases m o notomically with added concen tration …

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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.
SEMI F33-0998 © SEMI 19984
Table 4 Common Masses in Helium Bulk Gas
m/z Ions Source
4He
+
Bulk Gas
5HeH
+
Analyte
8He
2
+
Bulk Gas
9He
2
H
+
Analyte
12 He
3
+
Bulk Gas
12 C
+
Analyte
14 N
+
Analyte
15 CH
3
+
Analyte
16 He
4
+
Bulk Gas
16 CH
4
+
Analyte
18 H
2
O
+
Analyte
20 Ne
+
Bulk Gas Isotope
22 HeH
2
O
+
Analyte
24 NeHe
+
Bulk Gas Impurity
26 He
2
H
2
O
+
Analyte
28 N
2
+
Analyte
28 CO
+
Analyte
28 He
7
+
Bulk Gas
32 O
2
+
Analyte
40 Ar
+
Bulk Gas Impurity
44 CO
2
+
Analyte
44 ArHe
+
Bulk Gas Impurity
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.
14 Procedure
14.1 Zero gas should be passed through the dilution
system and the APIMS. Based on prior calibrations, the
system should be below 100 ppt, or at least an order of
magnitude lower than the lowest calibration point, for
the impurities to be calibrated.
14.2 Prior to beginning the calibration, record all
relevant parameters, such as discharge voltage, lens
voltages, ionization chamber pressure and/or flow, gas
type, etc.
14.3 The user should use the zero gas only, plus a
minimum of 5 challenge concentrations, with a
minimum of 5 data points, with the blended data points
spanning the range of interest of impurity measurement,
excluding the zero point, while maintaining constant
flow to the APIMS. Allow each challenge concentration
to stabilize for at least 15 minutes, and the signal
variation should be less than 3% before taking data
points at that concentration. Record the relevant ion
intensities as a function of time, for each stabilized
concentration.
14.4 Once the linearity of the calibration has been
established, a single point calibration check is
permitted, as long as the accuracy of the dilution system
is confirmed.
15 Calculations or Interpretation of Results
15.1 Perform a regression analysis for the individual
ion intensities against the impurity concentration. Be
sure to use only ion intensities below the saturation
region of your APIMS ion counting electronics, if using
counting detection. (Ask the manufacturer for the
saturation region of the units counting electronics.) The
R
2
from the fit of all the regression coefficients should
be 0.98 or above. If this is not the case, the calibration
will have to be repeated as described in the previous
section.
16 Reporting Results
16.1 As a minimum, the calibration will include the
following information for each impurity:
y = c
0
+ c
1
* x
(
1)
where,
y is the ion intensity, c
0
is ion intensity
background, c
1
is the linear response factor,
and x is the impurity level.
16.2 As an option, a quadratic regression may be
reported for the calibration, and will include the
following information for each impurity:
y = c
0
+ c
1
* x + c
2
* x
2
(2)
where,
y is the ion intensity, c
0
is ion intensity
background, c
1
is the first order response
factor, c
2
is the second order response factor
and x is the impurity level.
The c
0
/c
1
ratio or negative x - intercept is
useful to define the background concentration
of each particular ion. All regression coefficients
should be reported.
17 Precision and Accuracy
17.1 This method does not explicitly consider the
estimation of the accuracy of the impurity standard in
detail, because such a calculation can be made by
applying procedures described elsewhere. The precision
and accuracy required in validation will vary with the
proposed application and may be left to the discretion