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Analysis of surface contaminants by plasma chromatography-mass spectroscopy
Thin Solid Films, 45 (1977) 115-122 © ElsevierSequoia S.A., Lausanne--Printed in the Netherlands
I 15
A N A L Y S I S OF S U R F A C E C O N T A M I N A N T S BY P L A S M A CHROMATOGRAPHY-MASS SPECTROSCOPY* TIMOTHYW. CARR IBM Corporation, Department F06, Building 052, P.O. Box 390, Poughkeepsie, N. Y. 12602 ( U. S.,4. )
(Received March 31, 1977; accepted April 8, 1977)
Plasma c h r o m a t o g r a p h y - m a s s spectroscopy is a relatively new ultra-sensitive analytical technique which permits the characterization of trace contaminants of the order of parts per billion and less. The tecflnique is capable of detecting both positive and negative ions which are formed at atmospheric pressure as a result of ion-molecule reactions. The capabilities of the plasma c h r o m a t o g r a p h y - m a s s spectroscopy technique and its application to the analysis of trace levels of organic contaminants such as ethyl cellosolve acetate and trihydroxybenzophenone on wafer surfaces are discussed.
1. INTRODUCTION The development of highly sensitive microanalytical techniques is essential to the semiconductor industry inasmuch as semiconductors are impurity-sensitive systems. The presence of surface contaminants will have a deleterious effect upon both yield and reliability of semiconductor devices and therefore must be detected and controlled at extremely low levels. Plasma c h r o m a t o g r a p h y - m a s s spectroscopy (PC-MS) is a relatively new ultra-sensitive analytical technique which permits the characterization of trace contaminants of the order of parts per billion and less 1-3. The plasma chromatograph is a coupled ion-molecule reactor and ion-drift spectrometer which operates at atmospheric pressure. The basis of the technique is the formation of both positive and.negative ions which ate generated as a result of ion-molecule reactions initiated by 60 keV ]~- rays emitted from a 63Ni foil. The ions generated are separated in the drift spectrometer because of their difference in ionic mobility in an inert drift gas under the influence of an applied electric field. As the individual ions reach an electrometer detector, their mobility spectra are recorded. The primary advantage of P C - M S is its phenomenal lower limit of detection to a large class of volatile chemicals. Typically, the instrument can detect the presence of picogram (10- 12 g) quantities of organic surface contaminants, and it is not unusual to measure some compounds at the femtogram (10 -15 g) level. Most analytical systems such as gas chromatography, mass spectroscopy, Auger Paper presented at the International Conferenceon Metallurgical Coatings, San Francisco, California, U.S.A., March 28-April I. 1977.
116
T . W . CARR
analysis, I R - U V , high pressure liquid chromatography etc. operate in the microgram (10 6 g) to nanogram (10 -9 g) range. Another significant advantage of PC MS surface analysis is its capability of compound characterization. Most of the surface analytical techniques such as electron spectroscopy for chemical analysis (ESCA), Auger, ion scattering spectrometry and X-ray fluorescence perform elemental analysis. In addition, PC is a non-vacuum technique, operating at atmospheric pressure. Therefore the loss of volatile surface contaminants associated with the high vacuum in the sample housing of the other surface techniques is not experienced with PC-MS. This paper discusses the capabilities of the PC MS technique and its application to the analysis of trace levels of organic contaminants such as ethyl cellosolve acetate and trihydroxybenzophenone on wafer surfaces. 2. EXPERIMENTAL A schematic diagram of the Alpha I! P C - M S combined system is shown in Fig. 1: this system is manufactured by the Franklin G N O Corp., West Palm Beach, Florida, and consists of a Beta VII plasma chromatograph coupled to a specially modified quadrupole mass spectrometer. The mass spectrometer used is a modified Extranuclear Laboratories Spectr-EL quadrupole. The combined PC MS instrument is described in detail elsewhere'* 6. PC
CARRIER GAS
f Io
SAMPLE INLET [__J
REACTOR
MS
~
lib In I
m =1=
GAT,NO , / GRID SCAN~ / GRID
I
"°, MOLT,PL,ERANODE ~ APERTURE PC DETECTOR
MULTIPLIER CATHODE -
Fig. 1. A schematicdiagram of the Alpha 11PC-MS combined system. In operation, the sample is placed in a quartz sample chamber where it is flushed with nitrogen for several minutes. The temperature of the sample fixture is then increased at a rate of 2 C m i n - 1 and the volatile components on the sample are vaporized and mixed with a heated carrier gas such as nitrogen or zero-grade air. The sample-carrier gas then enters the ionization region that contains the 63Ni [3ray source. Initially, most of the incident energy is absorbed by the carrier gas molecules because of their greater concentration. However, as the flow co:,tinues into the reaction region, sample molecules are ionized. The sample ions are formed
ANALYSIS OF SURFACE CONTAMINANTS BY P C - M S
1 17
through a series of charge transfer and energy transfer reactions occurring between the carrier ions, the electronically excited species and the neutral sample molecules. The ions formed are caused to drift down the reaction region by a small applied field of 200 V c m - I A grid between the reaction region and the drift region is normally biased to block all ions from reaching the drift area. Periodically, the grid is opened for a short time, typically 0.2 ms, to let a "burst" of ions into the drift region. Separation of the ion-molecule complexes occurs in the drift region because of their difference in ionic mobility in an inert drift gas under the influence of an applied electric field. The mobility of an ion in the drift region is a function of its mass, its structure and its interactive forces with the neutral drift gas molecules. The lighter and more compact the ion, the greater its mobility will be. An electrometer detector is located at the end of the drift region, and it records the ion current as a function of time. The ions generated in the plasma chromatograph are allowed to enter the quadrupole mass spectrometer by passage through an 8 x 10- 3 in aperture, and the ions are then focused into the quadrupole mass filter. A Channeltron electron multiplier affords the detection of both positive and negative ions. The operating parameters of the plasma chromatograph used in this work are summarized in Table I. TABLE I OPERATION PARAMETERSOF THE PLASMACHROMATOGRAPH Drift gas (cm 3 m i n - l) Carrier gas (cm 3 min- 1) Voltage (V) Gate width (ms) Repetition rate (ms) Temperature (°C)
500 N 2 100 N 2 or zero-grade air + 2800 0.20 27 210
A Nicolet model SD-72IA integrating analog-to-digital converter mounted in a Nicolet model 1074 4096-channel signal averager was used to digitize the accumulated plasmagrams. Usually 5 i 2 scans of 27 ns duration were collected and stored on magnetic tape with a Ni.colet model NIC-283A magnetic tape coupler and a Kennedy model 9700 tape deck. The data stored on the magnetic tape could be analyzed by reading the tape back through the signal analyzer and by displaying the data on a Tektronix model D10 oscilloscope. Hard copies of the data could be obtained by recording the data from the signal averager memory on a Hewlett-Packard model 7035B X - Y recorder. 3.
RESULTS AND DISCUSSION
The detection and characterization of trace levels of organic surface contaminants has presented a most difficult problem to the materials analyst. The challenge to solve this problem has been stimulated in the semiconductor industry by the growing technological importance of such phenomena as adhesion, corrosion and embrittlement. Of the commonly used surface technologies, ESCA is
118
T.W. CARR
the only technique which provides direct information on the chemical nature of the atoms in an unknown samplC" 8. However, since the ESCA technique is a vacuum technique the volatile contaminants on the surface will be pumped away while the sample chamber is being evacuated. P C - M S has demonstrated a potential solution to these problems. Figure 2 shows the positive-mode ion mobility and mass spectra obtained from the analysis of a clean surface. The peaks observed in both spectra are associated with the nitrogen carrier gas system in the absence of a sample. Carroll e t al. 9 have identified the positive-mode reactant ions generated in a nitrogen carrier gas as NH4 +, N O + and H(H/O)2 +. These ions are referred to as reactant ions because they are involved in the ionization reactions with the neutral sample molecules. These reactant ions may be involved in several classes of ion-molecule reactions depending upon the chemical characteristics of the sample molecules. The NH4 + and H(H20)2 + ions are generally involved in proton transfer reactions which can be described as simple acid-base reactions governed by the relative basicity of the sample molecules: H(H20)2 + + M + M H + + 2 H 2 0
(l)
The NO+ reactant ion may be involved in several reactions which can be described as either a charge transfer reaction NO + + M ~ M + + N O
(2)
a hydride extraction reaction NO + +HM ~ HNO+M +
(3)
or an addition reaction NO++M
--. M N O +
(4)
In reaction (2) the ionization potential of the sample molecule must be lower than the ionization potential of N O (9.5 eV). Reaction (3) depends upon the acidity of the sample molecules, and reaction (4) is dependent upon the chemical nature of the sample molecules. Figure 3 shows the positive-mode ion mobility and mass spectra obtained from the analysis of a thermal oxide SiO2 surface that is contaminated with ethyl cellosolve acetate, a commonly used organic solvent. The mobility spectrum contains two large ionic peaks with drift times of 10.26 ms and 12.32 ms. Ethyl cellosolve acetate has a molecular weight of 132 amu, and the ion of drift time 12.32 ms has an ionic weight of 133 amu which would correspond with the sample molecules being protonated. The protonation of the ethyl cellosolve acetate molecules probably occurs at the carbonyl oxygen. The ion with a drift time of 10.26 ms has an ionic weight of 87 amu and is formed by fragmentation of the parent molecule. This fragment has an empirical formula of C4H702. As can be seen from this spectrum, a minimum of fragmentation accompanies the ionization process which results in a simple spectrum to analyze and interpret.
A N A L Y S I S O F S U R F A C E C O N T A M I N A N T S BY
PC-MS
119
H(H20)2 +
NO+ NH4+
A
A
18 10AMU
H (H20)2 + • N2
30
37
--
~--
110 AMU
(b) H {H20)2 +
NO+
5 ms
2 5 ms
(a) Fig, 2. The positive-mode spectra of a clean thermal oxide surface: (a) ion mobility spectrum; (b) mass spectrum. ethyl cellosolve acetate
IC4H702] +
H3C - C - O - CH2 - CH 2 - O - C2H 6 O
(M+I) +
133
60 AMU
160 AMU
(b) [C4H702 ]+
-
~
10.26
(M+I) +
12.32 ~
5 ms
.
.
.
.
.
. 25 ms
(a) Fig. 3. The positive-mode spectra spectrum; (b) mass spectrum.
ofSiO 2
contaminated with ethyl cellosolve acetate: (a) ion mobility
120
T.W. CARR
Figure 4 shows the positive-mode mobility and mass spectra which result from the analysis of a surface contaminated with trihydroxybenzophenone. Similar to the example of ethyl cellosolve acetate a relatively simple spectrum is observed with the most predominant species in the mobility spectrum having a drift time of 16.00 ms. This ion has a corresponding weight of 231 amu and is produced as a product of a proton transfer reaction since trihydroxybenzophenone has a molecular weight of 230 amu.
M+I
Trihydrox¥ Benzophenone OH
\ OH
(b) 200 AMU
231
M+I
~
300 A M U
(a) 5 ms
Fig. 4. The positive-modespectra of SiO2contaminated with trihydroxybenzophenone: (a) ion mobility spectrum; (b) mass spectrum.
The PC technique has demonstrated the capability of resolving isomers of organic compounds. Figure 5 is a composite positive-mode plasmagram for the para, ortho and meta isomers of dichlorobenzene. From these spectra it can readily be seen that the NO + reactant ion is involved in the formation of the product ions of dichlorobenzene. Analysis of the mass spectra data indicates that the product ions are the molecular ion. Therefore the mechanism involved in their production is a charge transfer reaction NO + + C6H4C12 --* C6H4C12 + + N O
(5)
The negative-mode ion mobility and mass spectra obtained from ortho-dichlorobenzene is shown in Fig. 6. In the negative mobility spectrum only a single peak is observed which has been identified as the chloride ion CI-. The most probable mechanism responsible for the formation of the chloride ion is a dissociative
A N A L Y S I S OF S U R F A C E C O N T A M I N A N T S BY
PC-MS
121
CI
1
I I
P
(d)
(c)
ortho
(b)
para
(a)
no sample NH4 +
5 ms
NO +
H (H20)2 +
Fig. 5. The positive-mode ion mobility spectra of the isomers of dichlorobenzene : (a) carrier gas in the absence of the sample ; (b) para-dichlorobenzene: (c) ortho-dichlorobenzene; (d) meta-dichlorobenzene.
(c)
Neg. Mass Spec. 110 AMU
35 37
10 AMU
I-
(b) 5 ms (a)
I
N ~ . PC o
5 ms
25 ms
Fig. 6. The negative-mode mobility and mass spectra ofdichlorobenzene : (a) carrier gas with no sample; (b) negative mobility spectrum of ortho-dichlorobenzene; (c) negative mass spectrum of orthodichlorobenzene. e l e c t r o n c a p t u r e reaction. T h e n e g a t i v e mass s p e c t r u m c o n t a i n s two peaks o f mass 35 a n d 37 which c o r r e s p o n d to the n a t u r a l isotopic d i s t r i b u t i o n for chlorine. T h e P C - M S t e c h n i q u e is in its early stage o f d e v e l o p m e n t , a n d c o n s e q u e n t l y has b e e n little d e s c r i b e d in the literature. F o r e x a m p l e this is the first r e p o r t e d
122
T.W.
('ARR
a p p l i c a t i o n o f the t e c h n i q u e for the a n a l y s i s o f o r g a n i c c o n t a m i n a n t s o n semic o n d u c t o r devices. T h i s i n s t r u m e n t o p e n s m a n y n e w a r e a s o f r e s e a r c h a n d a n a l y t i c a l a p p l i c a t i o n s . T h e d i r e c t m e a s u r e m e n t o f c o m p o u n d s such as ethyl c e l l o s o l v e a c e t a t e a n d t r i h y d r o x y b e n z o p h e n o n e o n w a f e r surfaces, w i t h o u t c o m p l i c a t e d a n d e r r o r - p r o d u c i n g c o n c e n t r a t i o n s a n d t r e a t m e n t o f the s a m p l e , is a g r e a t a d v a n t a g e . T h e s e a p p l i c a t i o n s are largely u n e x p l o r e d , but the p r o m i s e o f s o l u t i o n s to difficult a n a l y t i c a l p r o b l e m s is e n c o u r a g i n g . REFERENCES l 2 3 4 5 6 7
F.W. Karasek, Anal. ('hem.. 46 (1974) 710A. R.A. Keller and M. M. Metro, Sep. Pur~/Z Methods~ 3 (1974) 207. S.A. Benezra, J. Chromato~r. Sci.. 14(1976) 122. F.W. Karasek, Res/Dev., 21 (1970) 34. G.W. Griffin, I. Dzidic, D. I. Carroll, R. N. Stillwell and E. C. Horning. Anal. Chem., 45 (1973) 1204. F.W. Karasek, S. H. Kim and H. H. Hill, Anal. Chem., 48 (1976) 1133. D.A. Shirley (ed.), Electron Spectroscopy, North-Holland, Amsterdam, 1972.
8
C.A. Evans, Anal. Chem..47(1975) 818A.
9
D.I. Carroll, I. Dzidic, R. N. Stillwell and E. C. Horning, Anal. Chem., 47(1975) 1956.
I 15
A N A L Y S I S OF S U R F A C E C O N T A M I N A N T S BY P L A S M A CHROMATOGRAPHY-MASS SPECTROSCOPY* TIMOTHYW. CARR IBM Corporation, Department F06, Building 052, P.O. Box 390, Poughkeepsie, N. Y. 12602 ( U. S.,4. )
(Received March 31, 1977; accepted April 8, 1977)
Plasma c h r o m a t o g r a p h y - m a s s spectroscopy is a relatively new ultra-sensitive analytical technique which permits the characterization of trace contaminants of the order of parts per billion and less. The tecflnique is capable of detecting both positive and negative ions which are formed at atmospheric pressure as a result of ion-molecule reactions. The capabilities of the plasma c h r o m a t o g r a p h y - m a s s spectroscopy technique and its application to the analysis of trace levels of organic contaminants such as ethyl cellosolve acetate and trihydroxybenzophenone on wafer surfaces are discussed.
1. INTRODUCTION The development of highly sensitive microanalytical techniques is essential to the semiconductor industry inasmuch as semiconductors are impurity-sensitive systems. The presence of surface contaminants will have a deleterious effect upon both yield and reliability of semiconductor devices and therefore must be detected and controlled at extremely low levels. Plasma c h r o m a t o g r a p h y - m a s s spectroscopy (PC-MS) is a relatively new ultra-sensitive analytical technique which permits the characterization of trace contaminants of the order of parts per billion and less 1-3. The plasma chromatograph is a coupled ion-molecule reactor and ion-drift spectrometer which operates at atmospheric pressure. The basis of the technique is the formation of both positive and.negative ions which ate generated as a result of ion-molecule reactions initiated by 60 keV ]~- rays emitted from a 63Ni foil. The ions generated are separated in the drift spectrometer because of their difference in ionic mobility in an inert drift gas under the influence of an applied electric field. As the individual ions reach an electrometer detector, their mobility spectra are recorded. The primary advantage of P C - M S is its phenomenal lower limit of detection to a large class of volatile chemicals. Typically, the instrument can detect the presence of picogram (10- 12 g) quantities of organic surface contaminants, and it is not unusual to measure some compounds at the femtogram (10 -15 g) level. Most analytical systems such as gas chromatography, mass spectroscopy, Auger Paper presented at the International Conferenceon Metallurgical Coatings, San Francisco, California, U.S.A., March 28-April I. 1977.
116
T . W . CARR
analysis, I R - U V , high pressure liquid chromatography etc. operate in the microgram (10 6 g) to nanogram (10 -9 g) range. Another significant advantage of PC MS surface analysis is its capability of compound characterization. Most of the surface analytical techniques such as electron spectroscopy for chemical analysis (ESCA), Auger, ion scattering spectrometry and X-ray fluorescence perform elemental analysis. In addition, PC is a non-vacuum technique, operating at atmospheric pressure. Therefore the loss of volatile surface contaminants associated with the high vacuum in the sample housing of the other surface techniques is not experienced with PC-MS. This paper discusses the capabilities of the PC MS technique and its application to the analysis of trace levels of organic contaminants such as ethyl cellosolve acetate and trihydroxybenzophenone on wafer surfaces. 2. EXPERIMENTAL A schematic diagram of the Alpha I! P C - M S combined system is shown in Fig. 1: this system is manufactured by the Franklin G N O Corp., West Palm Beach, Florida, and consists of a Beta VII plasma chromatograph coupled to a specially modified quadrupole mass spectrometer. The mass spectrometer used is a modified Extranuclear Laboratories Spectr-EL quadrupole. The combined PC MS instrument is described in detail elsewhere'* 6. PC
CARRIER GAS
f Io
SAMPLE INLET [__J
REACTOR
MS
~
lib In I
m =1=
GAT,NO , / GRID SCAN~ / GRID
I
"°, MOLT,PL,ERANODE ~ APERTURE PC DETECTOR
MULTIPLIER CATHODE -
Fig. 1. A schematicdiagram of the Alpha 11PC-MS combined system. In operation, the sample is placed in a quartz sample chamber where it is flushed with nitrogen for several minutes. The temperature of the sample fixture is then increased at a rate of 2 C m i n - 1 and the volatile components on the sample are vaporized and mixed with a heated carrier gas such as nitrogen or zero-grade air. The sample-carrier gas then enters the ionization region that contains the 63Ni [3ray source. Initially, most of the incident energy is absorbed by the carrier gas molecules because of their greater concentration. However, as the flow co:,tinues into the reaction region, sample molecules are ionized. The sample ions are formed
ANALYSIS OF SURFACE CONTAMINANTS BY P C - M S
1 17
through a series of charge transfer and energy transfer reactions occurring between the carrier ions, the electronically excited species and the neutral sample molecules. The ions formed are caused to drift down the reaction region by a small applied field of 200 V c m - I A grid between the reaction region and the drift region is normally biased to block all ions from reaching the drift area. Periodically, the grid is opened for a short time, typically 0.2 ms, to let a "burst" of ions into the drift region. Separation of the ion-molecule complexes occurs in the drift region because of their difference in ionic mobility in an inert drift gas under the influence of an applied electric field. The mobility of an ion in the drift region is a function of its mass, its structure and its interactive forces with the neutral drift gas molecules. The lighter and more compact the ion, the greater its mobility will be. An electrometer detector is located at the end of the drift region, and it records the ion current as a function of time. The ions generated in the plasma chromatograph are allowed to enter the quadrupole mass spectrometer by passage through an 8 x 10- 3 in aperture, and the ions are then focused into the quadrupole mass filter. A Channeltron electron multiplier affords the detection of both positive and negative ions. The operating parameters of the plasma chromatograph used in this work are summarized in Table I. TABLE I OPERATION PARAMETERSOF THE PLASMACHROMATOGRAPH Drift gas (cm 3 m i n - l) Carrier gas (cm 3 min- 1) Voltage (V) Gate width (ms) Repetition rate (ms) Temperature (°C)
500 N 2 100 N 2 or zero-grade air + 2800 0.20 27 210
A Nicolet model SD-72IA integrating analog-to-digital converter mounted in a Nicolet model 1074 4096-channel signal averager was used to digitize the accumulated plasmagrams. Usually 5 i 2 scans of 27 ns duration were collected and stored on magnetic tape with a Ni.colet model NIC-283A magnetic tape coupler and a Kennedy model 9700 tape deck. The data stored on the magnetic tape could be analyzed by reading the tape back through the signal analyzer and by displaying the data on a Tektronix model D10 oscilloscope. Hard copies of the data could be obtained by recording the data from the signal averager memory on a Hewlett-Packard model 7035B X - Y recorder. 3.
RESULTS AND DISCUSSION
The detection and characterization of trace levels of organic surface contaminants has presented a most difficult problem to the materials analyst. The challenge to solve this problem has been stimulated in the semiconductor industry by the growing technological importance of such phenomena as adhesion, corrosion and embrittlement. Of the commonly used surface technologies, ESCA is
118
T.W. CARR
the only technique which provides direct information on the chemical nature of the atoms in an unknown samplC" 8. However, since the ESCA technique is a vacuum technique the volatile contaminants on the surface will be pumped away while the sample chamber is being evacuated. P C - M S has demonstrated a potential solution to these problems. Figure 2 shows the positive-mode ion mobility and mass spectra obtained from the analysis of a clean surface. The peaks observed in both spectra are associated with the nitrogen carrier gas system in the absence of a sample. Carroll e t al. 9 have identified the positive-mode reactant ions generated in a nitrogen carrier gas as NH4 +, N O + and H(H/O)2 +. These ions are referred to as reactant ions because they are involved in the ionization reactions with the neutral sample molecules. These reactant ions may be involved in several classes of ion-molecule reactions depending upon the chemical characteristics of the sample molecules. The NH4 + and H(H20)2 + ions are generally involved in proton transfer reactions which can be described as simple acid-base reactions governed by the relative basicity of the sample molecules: H(H20)2 + + M + M H + + 2 H 2 0
(l)
The NO+ reactant ion may be involved in several reactions which can be described as either a charge transfer reaction NO + + M ~ M + + N O
(2)
a hydride extraction reaction NO + +HM ~ HNO+M +
(3)
or an addition reaction NO++M
--. M N O +
(4)
In reaction (2) the ionization potential of the sample molecule must be lower than the ionization potential of N O (9.5 eV). Reaction (3) depends upon the acidity of the sample molecules, and reaction (4) is dependent upon the chemical nature of the sample molecules. Figure 3 shows the positive-mode ion mobility and mass spectra obtained from the analysis of a thermal oxide SiO2 surface that is contaminated with ethyl cellosolve acetate, a commonly used organic solvent. The mobility spectrum contains two large ionic peaks with drift times of 10.26 ms and 12.32 ms. Ethyl cellosolve acetate has a molecular weight of 132 amu, and the ion of drift time 12.32 ms has an ionic weight of 133 amu which would correspond with the sample molecules being protonated. The protonation of the ethyl cellosolve acetate molecules probably occurs at the carbonyl oxygen. The ion with a drift time of 10.26 ms has an ionic weight of 87 amu and is formed by fragmentation of the parent molecule. This fragment has an empirical formula of C4H702. As can be seen from this spectrum, a minimum of fragmentation accompanies the ionization process which results in a simple spectrum to analyze and interpret.
A N A L Y S I S O F S U R F A C E C O N T A M I N A N T S BY
PC-MS
119
H(H20)2 +
NO+ NH4+
A
A
18 10AMU
H (H20)2 + • N2
30
37
--
~--
110 AMU
(b) H {H20)2 +
NO+
5 ms
2 5 ms
(a) Fig, 2. The positive-mode spectra of a clean thermal oxide surface: (a) ion mobility spectrum; (b) mass spectrum. ethyl cellosolve acetate
IC4H702] +
H3C - C - O - CH2 - CH 2 - O - C2H 6 O
(M+I) +
133
60 AMU
160 AMU
(b) [C4H702 ]+
-
~
10.26
(M+I) +
12.32 ~
5 ms
.
.
.
.
.
. 25 ms
(a) Fig. 3. The positive-mode spectra spectrum; (b) mass spectrum.
ofSiO 2
contaminated with ethyl cellosolve acetate: (a) ion mobility
120
T.W. CARR
Figure 4 shows the positive-mode mobility and mass spectra which result from the analysis of a surface contaminated with trihydroxybenzophenone. Similar to the example of ethyl cellosolve acetate a relatively simple spectrum is observed with the most predominant species in the mobility spectrum having a drift time of 16.00 ms. This ion has a corresponding weight of 231 amu and is produced as a product of a proton transfer reaction since trihydroxybenzophenone has a molecular weight of 230 amu.
M+I
Trihydrox¥ Benzophenone OH
\ OH
(b) 200 AMU
231
M+I
~
300 A M U
(a) 5 ms
Fig. 4. The positive-modespectra of SiO2contaminated with trihydroxybenzophenone: (a) ion mobility spectrum; (b) mass spectrum.
The PC technique has demonstrated the capability of resolving isomers of organic compounds. Figure 5 is a composite positive-mode plasmagram for the para, ortho and meta isomers of dichlorobenzene. From these spectra it can readily be seen that the NO + reactant ion is involved in the formation of the product ions of dichlorobenzene. Analysis of the mass spectra data indicates that the product ions are the molecular ion. Therefore the mechanism involved in their production is a charge transfer reaction NO + + C6H4C12 --* C6H4C12 + + N O
(5)
The negative-mode ion mobility and mass spectra obtained from ortho-dichlorobenzene is shown in Fig. 6. In the negative mobility spectrum only a single peak is observed which has been identified as the chloride ion CI-. The most probable mechanism responsible for the formation of the chloride ion is a dissociative
A N A L Y S I S OF S U R F A C E C O N T A M I N A N T S BY
PC-MS
121
CI
1
I I
P
(d)
(c)
ortho
(b)
para
(a)
no sample NH4 +
5 ms
NO +
H (H20)2 +
Fig. 5. The positive-mode ion mobility spectra of the isomers of dichlorobenzene : (a) carrier gas in the absence of the sample ; (b) para-dichlorobenzene: (c) ortho-dichlorobenzene; (d) meta-dichlorobenzene.
(c)
Neg. Mass Spec. 110 AMU
35 37
10 AMU
I-
(b) 5 ms (a)
I
N ~ . PC o
5 ms
25 ms
Fig. 6. The negative-mode mobility and mass spectra ofdichlorobenzene : (a) carrier gas with no sample; (b) negative mobility spectrum of ortho-dichlorobenzene; (c) negative mass spectrum of orthodichlorobenzene. e l e c t r o n c a p t u r e reaction. T h e n e g a t i v e mass s p e c t r u m c o n t a i n s two peaks o f mass 35 a n d 37 which c o r r e s p o n d to the n a t u r a l isotopic d i s t r i b u t i o n for chlorine. T h e P C - M S t e c h n i q u e is in its early stage o f d e v e l o p m e n t , a n d c o n s e q u e n t l y has b e e n little d e s c r i b e d in the literature. F o r e x a m p l e this is the first r e p o r t e d
122
T.W.
('ARR
a p p l i c a t i o n o f the t e c h n i q u e for the a n a l y s i s o f o r g a n i c c o n t a m i n a n t s o n semic o n d u c t o r devices. T h i s i n s t r u m e n t o p e n s m a n y n e w a r e a s o f r e s e a r c h a n d a n a l y t i c a l a p p l i c a t i o n s . T h e d i r e c t m e a s u r e m e n t o f c o m p o u n d s such as ethyl c e l l o s o l v e a c e t a t e a n d t r i h y d r o x y b e n z o p h e n o n e o n w a f e r surfaces, w i t h o u t c o m p l i c a t e d a n d e r r o r - p r o d u c i n g c o n c e n t r a t i o n s a n d t r e a t m e n t o f the s a m p l e , is a g r e a t a d v a n t a g e . T h e s e a p p l i c a t i o n s are largely u n e x p l o r e d , but the p r o m i s e o f s o l u t i o n s to difficult a n a l y t i c a l p r o b l e m s is e n c o u r a g i n g . REFERENCES l 2 3 4 5 6 7
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C.A. Evans, Anal. Chem..47(1975) 818A.
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