- Email: [email protected]
Mass spectrometric contamination control of process gases
Vacuum/volume Printed
in Great
38/numbers
8-l
O/pages
777 to 781 /I 988
Mass spectrometric of process gases E Hasler
0042-207X/88$3.00+.00 Pergamon Press plc
Britain
and G Rettinghaus,
contamination
Balzers AG, FL-9496
Balzers, Fiirstentum,
control Liechtenstein
The characteristics of a quadrupole-based trace gas analysis system for routine quality control of inert as well as of corrosive gases are presented. Some problems concerning the quantification of sub-ppm impurities will be discussed. The performance of the system will be demonstrated with several examples including compounds like SiH, and SiF,.
1. Introduction In modern electronic device fabrication the use of high purity gases is mandatory. Therefore, the requirements on the so-called ‘electronic gases’ have been steadily raised in recent years by the microelectronics industry. Today the accepted level of contaminants in high purity gases is typically below 1 ppm. Until now one of the most important analytical tools for the quality control of high purity gases has been gas chromatographic techniques using different kinds of detectors. These methods were supplemented with specific sensors. However, this approach often fails if gases like silane, HCl, etc., have to be analysed. Furthermore a multisensor arrangement usually causes problems of accuracy. In this paper a quadrupole-based analysis system will be presented as a versatile and very sensitive instrument for the quantification of contaminants in inert as well as reactive and corrosive gases. Determinations of impurities in the ppm and sub-ppm range with a quadrupole mass spectrometer demand some specific precautions in order to obtain precise and reproducible results. This is especially true for reactive and corrosive gases. The following problems and artifacts often impair the successful detection and quantification of traces : (1) memory-effects in the gas supply system ; (2) non-stable or too high background in chamber ;
the
analysis
(3) surface interactions and reactions with the inlet, the ion source, the vacuum chamber of the pumping system: and (4) ion molecule reactions and clustering.
2. The quadrupole-based trace gas analysis system To overcome those problems a dedicated quadrupole-based analysis system has been designed (Figure 1). It comprises a continuous molecular beam inlet, a beam chopper and a lock-in detector together with a dedicated pumping station. A crucial part of a trace analysis system is the inlet. The molecular beam inlet technique applied reduces the sample to surface contacts to a minimum. This is of particular importance in the inlet because of adsorption/desorption effects and also in the ion source region because of reactions with hot surfaces. A two stage continuous inlet has the additional advantages of short conditioning and sample switching times. This is very important for routine analyses and outweighs the higher sample consumption. The sample consumption under normal working conditions is 15 ml min- ’ and can be decreased down to 1 ml min- ’ if there is only a small sample volume available. To demonstrate the performance of our inlet 2 ppm nitrogen and 0.4 ppm oxygen present as impurities in helium were determined just after an analysis of ambient air (Figure 2). The conditioning time necessary for sub-ppm range analyses is about 3 min. This is extremely fast.
I Molecular beam inLet system 2 Capillary 3 First-stoge pump (rotary vane pump) 4 Second-stage pump (turbopump 4 or cryopump 9) 5 Orifice 6 Chopper 7 Ion source, mass analyzer and detector
Figure
1. Quadrupole-based
trace analysis
8 Moss spectrometer controller 9 Cryopump IO LN2 trap
11 Turbomoleculor pump
system. 777
E Hasler
and G Rettinghaus;
Mass spectrometric
contamination
control
of process
gases
r 60 % -
-Z.Ovpm 20%
N, (m/e=261
0
0%
I
I
I
I
I
I
I
I
I
I
-0.4
vpm 02 (m/e=32)
-2.0
vpm NZ (m/e=
26)
vpm O2 (m/e=32) 0
1
2
3
4
5
6
7
6
9
10
( min)
Conditioning
time
TAS
307
L
Figure 2. Determination
of low levels of Nz and O2 immediately
after analysis
In order to discriminate between background and sample, the molecular beam is modulated with a mechanical chopper and the signal is recorded with a lock-in-amplifier. The interfering background signal is therby eliminated and an overall suppressing-factor of > 100 between sample and the background could be achieved. The analysis chamber is pumped by a combination of a turbomolecular and a two-stage He refrigerator pump with the I4 K surface enclosing the ion source. Due to the high efficiency of this pump combination the background in the ion source region is reduced drastically. A further advantage of the cryopump is its capability to handle very corrosive and reactive gases. To remove the compounds which are trapped on the cold head a special decontamination feature was designed allowing the bellows mounted cryopump to be retracted from the vacuum chamber through a gatevalve. The Fomblin oil of the fore-pumps and the ball bearings of the turbopumps are purged with dry nitrogen. The exhaust gases are neutralized in an external wet scrubber that contains a 10% KOH solution. Hazards accompanying the handling of reactive gases have been minimized by the installation of a series of active and passive security measures including auto-shut down procedures in case of power failures and other incidents. The analysis system is equipped with a software package allowing semi-automated analyses of gas samples by predefined and compound specific procedures and quantification routines. In an analysis up to 24 masses can be selected. Following the measurements the various concentrations are calculated with an optimal error propagation taking into account overlaps of individual spectra. To calibrate the mass spectrometer a set of calibration gas 778
at ambient
air
mixtures have to be used. Because of the wide dynamic range (linear response) of the quadrupole mass spectrometer calibration data acquired with higher concentration gas mixtures remain valid down to trace levels. Gas mixtures in the vpm range often suffer from instabilities.
3. Results Figure 3 shows the mass spectrum of a calibration gas mixture that should contain, according to the producer’s certificate, the components N,, O2 and CO, in a concentration of 30 vpm each. The true concentrations were determined to be 35 vpm Nz, 22 vpm O2 and 28 vpm CO*. Our results were further confirmed by a gas chromatographic analysis that was carried out by a second independent group. An increased noise level on mass I9 is due to a background peak eliminated by the applied lock-in detection technique (Figure 4). Contaminants in the gas mixture are Hz0 (1.4 vpm), HCI (0.2 vpm) and Ar (1 vpm). The background noise is 3 ppb. In an illustrative example the different reaction steps in a silane production plant were monitored. Starting material is the compound silane-tetrafluoride. The mass spectra were recorded with 40 eV ionization energy in order to suppress the formation of doubly ionized species. With an amplification of lo5 various impurities like disiloxane, CO,, H,O, HCI and Ar can be detected and identified (Figure 5). The next reaction step, the reduction of silane-tetrafluoride with a metal hydride yielded the crude silane (Figure 6) where the presence of several by-products SiF& (x = 4 -y) indicates that the reduction is not complete’.
Mass spectrometric
E Hasler and G Rettinghaus:
contamination
1
N:/CO’
co: I
35
“pm
NI
22
“pm
oz
28
vpm cot
control of process gases
35 “urn Np
in He
J
22
“pm OP
28
“PI-n coz
1
400 prh
;d Pr‘
Ar’ 2.
L
-
/
40
20
I
I
m/e
-T
I
NZ/OP/C02 in He
I Figure 3. Analysis at 30 vpm.
80
60
I
of calibrated
mixture
containing
1
I
N,/OZ/COz in He I
Figure 4. Application
N,, 0, and CO, each
of lock-in
Fragment
1
technique.
peaks of SiF,
SiF,
Figure 5. Monitoring
of silane production
from silane tetrafluoride
impurity
peaks 779
E Hasler
and G ReUinghaus:
Mass spectrometric
contamination
control
of process
gases
,.__
Silane
SiFH:
CH:
Silone
Figure 6. Mass spectrum
of crude silane indicating
presence
(crude)
x 100
of Si,H, by-products.
Disilane “.f:_&,;
I
0.5
20
40
60
80
Figure 7. Mass spectrum 780
of pure silane
ppm
100
Silane
Silane
120
x lo5
140
160
180
m/e
E Hasler and G Rettinghaus:
Mass spectrometric
contamination
The crude silane is finally submitted to purification. shows the mass spectrum of the final product.
control of process gases
Figure 7
system are ppb-detection limits, semi-automatic cedures and fast sample switching times.
analyses
pro-
4. Conclusions The characteristics trace gas analysis
and applications of an optimized quadrupole system were shown. The advantage of the
Reference
’ Bakers. Technical Report, BG 800 006 DE (8407).
781
in Great
38/numbers
8-l
O/pages
777 to 781 /I 988
Mass spectrometric of process gases E Hasler
0042-207X/88$3.00+.00 Pergamon Press plc
Britain
and G Rettinghaus,
contamination
Balzers AG, FL-9496
Balzers, Fiirstentum,
control Liechtenstein
The characteristics of a quadrupole-based trace gas analysis system for routine quality control of inert as well as of corrosive gases are presented. Some problems concerning the quantification of sub-ppm impurities will be discussed. The performance of the system will be demonstrated with several examples including compounds like SiH, and SiF,.
1. Introduction In modern electronic device fabrication the use of high purity gases is mandatory. Therefore, the requirements on the so-called ‘electronic gases’ have been steadily raised in recent years by the microelectronics industry. Today the accepted level of contaminants in high purity gases is typically below 1 ppm. Until now one of the most important analytical tools for the quality control of high purity gases has been gas chromatographic techniques using different kinds of detectors. These methods were supplemented with specific sensors. However, this approach often fails if gases like silane, HCl, etc., have to be analysed. Furthermore a multisensor arrangement usually causes problems of accuracy. In this paper a quadrupole-based analysis system will be presented as a versatile and very sensitive instrument for the quantification of contaminants in inert as well as reactive and corrosive gases. Determinations of impurities in the ppm and sub-ppm range with a quadrupole mass spectrometer demand some specific precautions in order to obtain precise and reproducible results. This is especially true for reactive and corrosive gases. The following problems and artifacts often impair the successful detection and quantification of traces : (1) memory-effects in the gas supply system ; (2) non-stable or too high background in chamber ;
the
analysis
(3) surface interactions and reactions with the inlet, the ion source, the vacuum chamber of the pumping system: and (4) ion molecule reactions and clustering.
2. The quadrupole-based trace gas analysis system To overcome those problems a dedicated quadrupole-based analysis system has been designed (Figure 1). It comprises a continuous molecular beam inlet, a beam chopper and a lock-in detector together with a dedicated pumping station. A crucial part of a trace analysis system is the inlet. The molecular beam inlet technique applied reduces the sample to surface contacts to a minimum. This is of particular importance in the inlet because of adsorption/desorption effects and also in the ion source region because of reactions with hot surfaces. A two stage continuous inlet has the additional advantages of short conditioning and sample switching times. This is very important for routine analyses and outweighs the higher sample consumption. The sample consumption under normal working conditions is 15 ml min- ’ and can be decreased down to 1 ml min- ’ if there is only a small sample volume available. To demonstrate the performance of our inlet 2 ppm nitrogen and 0.4 ppm oxygen present as impurities in helium were determined just after an analysis of ambient air (Figure 2). The conditioning time necessary for sub-ppm range analyses is about 3 min. This is extremely fast.
I Molecular beam inLet system 2 Capillary 3 First-stoge pump (rotary vane pump) 4 Second-stage pump (turbopump 4 or cryopump 9) 5 Orifice 6 Chopper 7 Ion source, mass analyzer and detector
Figure
1. Quadrupole-based
trace analysis
8 Moss spectrometer controller 9 Cryopump IO LN2 trap
11 Turbomoleculor pump
system. 777
E Hasler
and G Rettinghaus;
Mass spectrometric
contamination
control
of process
gases
r 60 % -
-Z.Ovpm 20%
N, (m/e=261
0
0%
I
I
I
I
I
I
I
I
I
I
-0.4
vpm 02 (m/e=32)
-2.0
vpm NZ (m/e=
26)
vpm O2 (m/e=32) 0
1
2
3
4
5
6
7
6
9
10
( min)
Conditioning
time
TAS
307
L
Figure 2. Determination
of low levels of Nz and O2 immediately
after analysis
In order to discriminate between background and sample, the molecular beam is modulated with a mechanical chopper and the signal is recorded with a lock-in-amplifier. The interfering background signal is therby eliminated and an overall suppressing-factor of > 100 between sample and the background could be achieved. The analysis chamber is pumped by a combination of a turbomolecular and a two-stage He refrigerator pump with the I4 K surface enclosing the ion source. Due to the high efficiency of this pump combination the background in the ion source region is reduced drastically. A further advantage of the cryopump is its capability to handle very corrosive and reactive gases. To remove the compounds which are trapped on the cold head a special decontamination feature was designed allowing the bellows mounted cryopump to be retracted from the vacuum chamber through a gatevalve. The Fomblin oil of the fore-pumps and the ball bearings of the turbopumps are purged with dry nitrogen. The exhaust gases are neutralized in an external wet scrubber that contains a 10% KOH solution. Hazards accompanying the handling of reactive gases have been minimized by the installation of a series of active and passive security measures including auto-shut down procedures in case of power failures and other incidents. The analysis system is equipped with a software package allowing semi-automated analyses of gas samples by predefined and compound specific procedures and quantification routines. In an analysis up to 24 masses can be selected. Following the measurements the various concentrations are calculated with an optimal error propagation taking into account overlaps of individual spectra. To calibrate the mass spectrometer a set of calibration gas 778
at ambient
air
mixtures have to be used. Because of the wide dynamic range (linear response) of the quadrupole mass spectrometer calibration data acquired with higher concentration gas mixtures remain valid down to trace levels. Gas mixtures in the vpm range often suffer from instabilities.
3. Results Figure 3 shows the mass spectrum of a calibration gas mixture that should contain, according to the producer’s certificate, the components N,, O2 and CO, in a concentration of 30 vpm each. The true concentrations were determined to be 35 vpm Nz, 22 vpm O2 and 28 vpm CO*. Our results were further confirmed by a gas chromatographic analysis that was carried out by a second independent group. An increased noise level on mass I9 is due to a background peak eliminated by the applied lock-in detection technique (Figure 4). Contaminants in the gas mixture are Hz0 (1.4 vpm), HCI (0.2 vpm) and Ar (1 vpm). The background noise is 3 ppb. In an illustrative example the different reaction steps in a silane production plant were monitored. Starting material is the compound silane-tetrafluoride. The mass spectra were recorded with 40 eV ionization energy in order to suppress the formation of doubly ionized species. With an amplification of lo5 various impurities like disiloxane, CO,, H,O, HCI and Ar can be detected and identified (Figure 5). The next reaction step, the reduction of silane-tetrafluoride with a metal hydride yielded the crude silane (Figure 6) where the presence of several by-products SiF& (x = 4 -y) indicates that the reduction is not complete’.
Mass spectrometric
E Hasler and G Rettinghaus:
contamination
1
N:/CO’
co: I
35
“pm
NI
22
“pm
oz
28
vpm cot
control of process gases
35 “urn Np
in He
J
22
“pm OP
28
“PI-n coz
1
400 prh
;d Pr‘
Ar’ 2.
L
-
/
40
20
I
I
m/e
-T
I
NZ/OP/C02 in He
I Figure 3. Analysis at 30 vpm.
80
60
I
of calibrated
mixture
containing
1
I
N,/OZ/COz in He I
Figure 4. Application
N,, 0, and CO, each
of lock-in
Fragment
1
technique.
peaks of SiF,
SiF,
Figure 5. Monitoring
of silane production
from silane tetrafluoride
impurity
peaks 779
E Hasler
and G ReUinghaus:
Mass spectrometric
contamination
control
of process
gases
,.__
Silane
SiFH:
CH:
Silone
Figure 6. Mass spectrum
of crude silane indicating
presence
(crude)
x 100
of Si,H, by-products.
Disilane “.f:_&,;
I
0.5
20
40
60
80
Figure 7. Mass spectrum 780
of pure silane
ppm
100
Silane
Silane
120
x lo5
140
160
180
m/e
E Hasler and G Rettinghaus:
Mass spectrometric
contamination
The crude silane is finally submitted to purification. shows the mass spectrum of the final product.
control of process gases
Figure 7
system are ppb-detection limits, semi-automatic cedures and fast sample switching times.
analyses
pro-
4. Conclusions The characteristics trace gas analysis
and applications of an optimized quadrupole system were shown. The advantage of the
Reference
’ Bakers. Technical Report, BG 800 006 DE (8407).
781