Vacuum Technique in Analytical Chemistry Peter F. Váradi Communications Satellite Corporation Clarksburg, Md. 2 0 7 3 4
Interaction between vacuum science and analytical chemical i n s t r u m e n t development seems essential t o t h e continued progress in both fields
REPORT FOR ANALYTICAL CHEMISTS
INTERACTION BETWEEN TWO S C I E N TIFIC DISCIPLINES is u s u a l l y dif
ficult. I t is common t h a t even two or more scientific fields housed in t h e same laboratory building have no communication or ex change of ideas among each other. Most scientific meetings, for effi ciency, usually a r e separated into sessions covering narrow fields. However, t h e big achievements in science a r e derived from interac tions of various disciplines. This paper will review t h e appli cations of v a c u u m techniques t o analytical chemistry a n d show t h a t a direct communication between these t w o disciplines should result in highly improved analytical chemical instrumentation a n d in new tools for t h e v a c u u m scientist. Prof. H . A. Laitinen, in an edito rial in t h e October 1969 issue of ANALYTICAL
CHEMISTRY
(1)
sum
marized : " I t h a s long been t r u e t h a t m a n y nonanalytical chemists, a n d even nonchemists, m a k e important contributions t o analytical chem istry. I t is characteristic of this t y p e of p a r t - t i m e activity t h a t the measurement technique is in cidental t o t h e main problem. T h e r e is little incentive for fur ther development of t h e method by t h e original investigator, a n d it is here t h a t analytical special ists should be alert t o new mea surement principles t h a t have n o t been fully developed."
entists, however, h a d t o develop their own analytical tools t o ex plore t h e minuscule residues in v a c u u m devices. T h u s , t h e utilization of t h e developments in vacuum techniques took hold very slowly in analytical chemistry, usually filter ing through physics. I n many cases, when analytical chemical methods were developed in vacuum techniques t h e y never, or very hesi t a n t l y , infiltrated t h e circles of a n alytical chemists, primarily b e cause of lack of direct communica tions.
• ANALYTICAL DISTILLATION (MICRO, MOLECULAR) • GAS ANALYSIS • GAS CHROMATOGRAPHY • GRAVIMETRY • THERMAL METHODS: DTA, DTG, PYROLYSIS, FUSION • MASS SPECTROSCOPY • INFRARED SPECTROSCOPY • FLUOROMETRIC ANALYSIS • VACUUM UV SPECTROSCOPY • X-RAY SPECTROSCOPY • ELECTRON MICROPROBE • MICROSCOPY (ELECTRON, SEM, OPTICAL) •AUGER-LEED • ESCA •ACTIVATION ANALYSIS
Analytical Methods Utilizing Vacuum Techniques—The State of the Art
A survey of analytical chemical methods indicate t h a t quite a few employ vacuum. These methods are t a b u l a t e d in T a b l e I . Vacuum is used for uninhibited t r a n s p o r t a tion of materials, for u n a t t e n u a t e d transmission of radiation, or to pro vide a protecting environment. Some of t h e methods depend on vacuum, others use vacuum only in certain applications. T h e vacuum conditions utilized in, or necessary for, these analytical methods a n d iWi
their vacuum requirements relative to t h e available vacuum levels of the same period are plotted in Fig ure 1. This figure compares t h e de velopment of t h e "commercially available vacuum levels" a n d t h e various vacuum levels utilized in analytical chemical methods. T h e lower line indicates t h e vacuum ob-
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X-ray Spectroscopy
10 Years
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1940
T h e t i m e when t h e only v a c u u m equipment used b y t h e analytical chemist was t h e water aspirator h a s long been gone. So is t h e time when v a c u u m scientists used tedi ous classical analytical chemical methods to discover atomic hydrogen (2). B o t h t h e v a c u u m technique a n d analytical chemistry h a v e gone through m a n y decades of development. Analytical chemistry adapted m a n y measuring techniques devel oped in physics. Physics depended very much on t h e progress m a d e in v a c u u m techniques. V a c u u m sci
Table I. Analytical Chemical Methods Utilizing Vacuum
Vacuum UV Spectroscopy
•1
Analytical Distillation
20 Years 1920
1910
1900
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tainable in commercial applications during t h e p a s t 70 years. This a r bitrary line is an estimate based on milestones achieved in vacuum technology b y Gaede in 1905, Gaede and Langmuir's development of t h e diffusion p u m p in 1916, a n d the late development of oil diffu sion pumps, Daniel Alpert's ultra high vacuum work in t h e early 1950's, a n d t h e development of commercial ion pumps in t h e late 1950's. ( F o r reviews see 3-5.) An exact date cannot be specified when these developments were used t o produce vacuum in commercial equipment a t those levels a n d therefore t h e lower line indicates only an estimate. Interestingly, the estimated commercially achiev able vacuum levels (on a log scale) follow a straight line vs. t h e time in this 70-year period. On t h e same figure, t h e various analytical chemical methods are in dicated showing t h e approximate date of introduction t o analytical chemistry and t h e level of vacuum utilized in t h e method. T h e area above t h e upper line represents t h e vacuum levels presently utilized in analytical chemistry. T h e following conclusions can b e drawn from this figure: a) T h e utilization of v a c u u m in analytical chemistry trailed behind commer cial utilization of vacuum b y one or two decades during t h e p a s t 40 y e a r s ; a n d b) Application of vac u u m in analytical chemistry has in creased substantially in t h e past 20 years.
The Case of Mass Spectrometry
every other year since then, and de scribed t h e use of the method in a n alytical chemistry (8). Until 1954 it was claimed t h a t all publications in mass spectrometry were listed, the 1956 review gave u p this effort and only t h e most meaningful p u b lications were mentioned causing the drop in publications during t h a t period. T h e publications listed in
I t is important t o study t h e growth of analytical chemical tech niques, as well as to know t h e n u m ber of techniques utilizing vacuum. Among t h e analytical chemical methods utilizing vacuum, mass spectroscopy is one of t h e oldest. ANALYTICAL C H E M I S T R Y included a
review of this method for the first time in 1949. W r i t t e n by Hippie and Shepherd (6) the review stated :
ANALYTICAL C H E M I S T R Y were for a
two-year period. Here, t h e number of publications in ANALYTICAL C H E M I S T R Y were divided b y two t o
" T h e statistically inclined might d r a w inferences from t h e fact t h a t Chemical Abstracts re ported 11 references to mass spectrometry in 1943, 15 in 1944, 17 in 1945, 26 in 1946, and 40 in 1947. Actually, there is no doubt of t h e sharply increasing interest a n d importance of t h e mass spectrometer as a n analyti cal tool."
bring them to t h e same base em ployed in Chemical Abstracts. These two curves show t h a t t h e incubation period for t h e method was 10 to 15 years and w a s fol lowed by a sharp increase in publi cations. T h e number of publica tions reviewed b y Chemical Ab stracts follows a n exponential rise, while t h e analytical chemical utili zation shows a leveling off period from 1959 t o 1965, probably be cause of t h e impact of t h e competi tive gas chromatographic tech niques. T h e 1966 and 1968 reviews show t h a t mass spectrometry r e covered significantly a n d displayed an exponential increase in publica tions. This was caused p a r t i a l l y by t h e combination of t h e mass spectrometer with other analytical techniques, b u t more significantly, due t o t h e marketing of low cost and versatile units, a contribution of vacuum science t o analytical chemistry.
I t is interesting t o continue the statistical considerations of Hippie and Shepherd with t h e use of d a t a t a k e n from two different sources and plotted in Figure 2 ( 7 ) . One source, Chemical Abstracts, started to reference mass spectrometry in 1938. T h e increase of t h e number of references (listed under three headings—Mass Spectra ; Mass Spectroscopy; and Mass Spectrom etry) with time are clearly evi dent. T h e second source was t h e biyearly reviews published in ANALYTICAL C H E M I S T R Y .
began
in
T h e extent to which mass spec trometry is used today in analyti-
Reviews
1949, were
published
MASS SPECTROSCOPY APPLICATIONS
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Figure 2. Literature explosion in mass spectrometry 30A
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Figure 3.
Application of mass spectroscopy in analytical chemistry
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
NOW a plug-in, finetune, and forget liquid helium transfer and cooling system.
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cal chemistry can be seen from Fig ure 3. A large number of the appli cations of mass spectrometry utilize the method in combination with other analytical techniques such as gas chromatography, thermal anal ysis, and also with computers. Prior to 1960 the work horse of an alytical chemistry was the high res olution (3000-10,000 AMU) mass spectrometer. The new trend of combination of mass spectroscopy with other methods resulted in the wide utilization of the medium (500-1000 AMU) resolution mass spectrometer. These medium reso lution instruments were developed by vacuum scientists and made it possible for the various vacuum companies to enter the analytical chemical instrumentation business. The availability of small com puters and the application of mod ern vacuum technology will, how ever, result in a trend to use low resolution (50-150 AMU) and rela tively inexpensive mass spectrom eters in analytical chemistry. These instruments are well known to vac uum scientists as residual gas ana lyzers. Combination Instrumentation
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Another instrument in which vac uum science contributed to analyti cal chemistry is the vacuum microbalance. The microbalance origi nated in chemistry (9), but was also extensively used and perfected by vacuum scientists (10). Its uti lization in vacuum surface studies is well known. Its utilization in analytical chemistry does not re quire vacuum, but lately it is used more often when a sample is ana lyzed by its changes in a vacuum environment. The vacuum re quirements for such work are not high, analytical chemists use vac uum down to 10~5 torr. If vacuum conditions were improved in ana lytical vacuum microbalance work, it would lead to a combination in strument with greater capabilities. Figure 4 shows a combination in strument which is planned for com pletion at COMSAT Labs. In this analytical system the decomposi tion products of a sample such as a plastic can be analyzed. The anal ysis includes the determination of weight loss by a microbalance, the measurement of decomposition
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970 · 31 A
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Table II. Analytical Chemistry and Surface Sciences • • • • • • • • •
CHEMICAL ABSORPTION SPECTROSCOPY INFRARED (ATR) EMISSION SPECTROSCOPY X-RAY SPECTROSCOPY MASS SPECTROSCOPY ELECTRON MICROPROBE ESCA OLFACTORY SENSE
BULK BULK MONOLAYER BULK 50 n FEW MONOL. 5-10 M MONOLAYER —
D N N D N D N N D
10-»g 10-9g 10-9g 10-"g 10"" g 10-" g lO-^-lO-^g 10-" g 10-isg
D-DESTRUCTIVE N-NONDESTRUCTIVE
products such as noncondensables with a total and partial pressure gauge (Topatron) and identification of the condensable decomposition products by infrared spectroscopy. Infrared analytical techniques have sometimes been used in v a c u u m surface studies, but the application of the ir-atr (attenuated total reflection) for vacuum environment is relatively new {11). I n this technique, the ir rays from a monochromator are passed through a window into the vacuum chamber. There it is directed into a multiple internal reflection crystal. T h e ir radiation passing through the ciystal is directed to a detector through another window. When a layer having absorption bands in the ir resides on t h e crystal surface, it extracts energy from the beam and selectively attenuates the emerging radiation according to its absorption spectrum. The sensitivity of this method was reported
Figure 4 . materials 32 A
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Analysis of decomposition of
(11) to be as low as Vio of a monolayer. T h e present method utilizes a chamber of the t y p e shown in Figure 5 {12). This shows the vacuum chamber with the heating element for temperature control and mirrors mounted in a standard ir spectrograph. T h e potential of this ir-atr method to analyze monolayers is quite formidable.
Surface Sciences and Analytical Chemistry
The trend in analytical chemist r y is to analyze smaller and smaller quantities of material. Table I I summarizes the various analytical chemical techniques, giving their ultimate sensitivity (13). As indicated, several methods are available for surface studies from monolayers to layers 50/A thick. Vacuum and surface scientists have great interest in these methods. The surface scientist in vacuum technology developed their
Figure 5. problems
Application of ir t o v a c u u m
ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 1 1 , SEPTEMBER 1970
own methods, such as Auger spectroscopy and the L E E D technique. While Auger spectroscopy or L E E D (14) m a y have only a limited application in analytical chemistry, the analytical chemical methods, e.g. ir-atr, X - r a y spectroscopy, and especially E M - S E M (electron microprobe-scanning electron m i croscope) and ESCA (electron spectroscopy for chemical analysis) (15} 16) are extremely important for surface sciences. The vacuum level now utilized in these instruments is in the lO - 3 to 10~7 torr range providing an inadequate, " d i r t y " environment for surface studies. T h e speed of deposition of a monolayer contaminant in a lO""* torr vacuum is a millisecond. If ultra-high v a c uum techniques were utilized in these methods, a 10 - 1 0 t o r r vacuum would keep a sample clean for hours. One of the most powerful analytical instruments is the electron microprobe-scanning electron microscope combination. This system consists of an electron beam, which scans the surface to be analyzed. T h e resulting primary X - r a y s are then analyzed in dispersive or nondispersive X-ray spectrometer. The secondary electron emission, back scattered electrons, and the sample current are separately and simultaneously detected, and when displayed, produce images of the surface making it possible to study the topography on a microscopic scale and also to provide information about its elemental composition. I n the following examples the capabilities of this instrument are demonstrated: Figure 6 shows the E M - S E M picture of a special t y p e M O S Si transistor (17,18) containing chromium gates under 250X
Correction: In the report "Radioactive Inert Gases" by Vladimir Balek, August issue, page 17 A, t h e first sentence under the heading Inert Gas Release f r o m Solids should read: Inert gas incorporated into a solid can be liberated as a result of chemical reactions, physical t r a n s f o r m a t i o n , o r different damages of its crystalline lattice. In the t h i r d colu m n , t h e t h i r d paragraph should read: For large grains of solids, 10* c m or greater, in which the diffusion coefficient of t h e inert gas is s m a l l , the release rate of inert gas emanation is given by Equation 2 .
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Figure 6.
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magnification. T h e photograph on the left shows the specimen current image of a part of the transistor, while t h e other shows the chromium distribution on the same surface of the transistor. T h e quantity of t h e Cr is obtained in a line scan mode of operation, which leads to a dis play which almost resembles a three-dimensional image. This pic ture may then be processed through a computer to produce an X-Y-intensity display; also contour m a p ping of the various abundance levels of the measured elements is instan taneously possible. Other more conventional displays are shown in Figure 7. T h e amount of Si, left, and t h e amount of oxygen, right, on the same sur face are shown. B y electronic means this information m a y be dis played in a number of ways. Su perposition of pictures electroni cally m a y define elemental concen trations as well as t h e composition of surfaces. The shocking limitation in all commercially available E M - S E M instruments at present resides in t h e design of t h e vacuum system. At present these instruments operate at vacuums not better than 10~° torr, typically in the 10~4 to ΙΟ"5
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Ο Κα-X-rays Magnification
χ250
Figure 8. Effect of v a c u u m on surface studies in an EM-SEM. Magnification XlOO
torr range. This limits its use for surface studies. The effect of t h e " d i r t y " vacuum of the E M - S E M instrument on sur face studies conducted on t h e t r a n sistor sample shown in Figures 6 and 7 is seen in Figure 8. This picture was taken a t a magnifica tion of 100X and therefore the en tire device is displayed. T h e mea surements shown in Figures 6 and 7 were made a t the place where the darker square can be seen. This darker square is the area of t h e ras ter scanned by the electron beam. The various dark areas and lines were produced by t h e electron beam's decomposition or evapora tion of contaminants arising from
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the " d i r t y " 10~5 torr vacuum. T h e deposition of carbon in t h e electron microprobe due to t h e cracking of organic contaminants of the "dirty v a c u u m " has been known for only a few years (19, 20, 21). Microprobes today use Λ-arious means of decontaminating systems (22), e.g. cold fingers, or He, air or argon jets ; but if t h e vacuum level of the elec tron microprobe wore in t h e 10~10 torr range, the sample would remain clean for hours. T h e electron m i croprobe could be designed with modern vacuum technology to op erate a t least in t h e 10~8 or 10 9 ton· range and m a k e t h e instrument useful for vacuum technologists studying surface science and also improve t h e analytical capability and versatility of the instrument. Extrapolations
I n this review and through se lected examples we have attempted to show t h a t vacuum technology can improve analytical chemical methods as in mass spectroscopy. Also, several analytical chemical methods exist which have potential applications in vacuum technology if properly adapted. Figure 9 attempts a prediction for the decade ahead. I t shows the presently used vacuum levels in various analytical instruments and also an extrapolation for future re quirements. T h e future vacuum needs of these measuring techniques differ. Those with potentials for surface studies or higher sensitivi-
ties will require better vacuum lev els. F o r some methods, better vac u u m levels would not yield much improvement. T h e most promising and newest analytical methods are connected with X - r a y analysis, a n d electron spectroscopy. These analytical methods, with their great potential are not, or are very rarely, used in vacuum sciences. Electrically charged particles were harnessed long ago by vacuum scientists and p u t to use for heat ing, evaporating, pumping, and measuring. I o n gauges were rare 20 years a g o ; t o d a y they can be found everywhere, except on ana lytical chemical equipment. R e sidual gas analysers were a novelty 10 years ago; now they are com monplace. Analytical exploitation of electrons was started a few years ago by v a c u u m scientists with Auger spectroscopy; today ESCA and E M - S E M techniques are hold ing much promise for vacuum in strumentation. X - r a y s arc specifically interest ing for vacuum scientists. I n t h e past they were considered a nui sance and caused much research in connection with ion gauges and l'ga's. X - r a y s were never used in the vacuum sciences in spite of their great potential. Improvement in analytical chem ical instrumentation is needed and new vacuum instrumentation is re quired as well. T o this end, inter action between vacuum science and
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970 · 35 A
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much research in connection with analytical chemistry is strongly indicated. References (1) H. A. Laitinen, ANAL. C H E M . 41,
1521 (1969). (2) I. Langmuir, J. Amer. Chem. Soc. 37, 417 (1915) and subsequent papers. (3) S. Dushman and J. M. Lafferty, "Scientific Foundations of Vacuum Technique," J. Wiley, New York, Ν. Υ., 1962. (4) H . A. Steinherz, "Handbook of High Vacuum Engineering," Reinhold, New York, Ν . Υ., 1963. (5) A. E . Barrington, "High Vacuum En gineering," Prentice Hall, Englewood Cliffs, N . J., 1963. (6) J. A. Hippie and Martin Shepherd, ANAL. C H E M . , 21, 32 (1949).
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(7) P. F . Vâradi, "Industrial Applications of Mass Spectrometry/' 26th Annual Research Conference on Instrumentation Science, Geneva, Ν . Υ... 1967. (S) ANALYTICAL CHEMISTRY, Mass Spec
troscopy Reviews: 21, 32 (1949); 22, 23 (1950); 24, 27 (1952); 26, 58 (1954); 28, 610 (1956); 30, 604 (1958); 32, 211 (1960); 34, 243 (1962); 36, 278 (1964); 38, 350R (1966); 40, 273R (1968)·. (9) W. W. Wendlandt, "Thermal Meth ods of Analysis," Intcrscience, New York, Ν . Υ., 1964. (10) S. P . Wolsky and E . J. Zdanuk, "Ultra Micro Weight Determination in Controlled Environments," Intersci ence, New York, Ν. Υ., 1969. (11) G. L. Haller and A. C. Gilby, "High Temperature, High Vacuum Cell for Ir Surface Studies," Pittsburgh Confer ence on Analytical Chemistry, Cleve land, Ohio, 1970. (12) Courtesy of Wilks Scientific Corp., Norwalk, Conn. (13) Part of this table was obtained from : L. Marton, "Electron Probe Mi croanalysis," Academic Press, Now York, Ν . Υ., 1969. (14) I,. A. Harris, ANAL. CHEM., 40, (14)
24A (1968). (15) K. Siegbahn, C. Nordling, and A. Fahlman, "Electron Spectroscopy for Chemical Analysis," Air Force Mat. Lab. AFML-TR-68-189, 1968. (16) D. M. Hercules, ANAL. CHEM. 42, (1)
20A (1970) ; and D. Bettcridge and A. D. Baker, ANAL. C H E M . 42, (1) 43A
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(1970) (17) J. Lindmayer and W. P . Noble, Jr., IEEE Trans. Electron Devices, 15, (9) 637 (1968). (18) J. Lindmayer, "Thermal Stability and Radiation Effects in M-I-S Sys tems," European Meeting, Semicon ductor Device Research, Munich, W. Germany, 1969. (19) A. J. Tousimis, Biomed. Sci. lu strum. 1, 249 (1963). (20) A. J. Tousimis, "X-Ray and Elec tron Probe Analysis in Biomedical Re search," p. 87, Plenum Press, New York, Ν. Υ. 1969. (21) S. H. Moll and G. W. Bruno, "2nd Natl. Conf. on Electron Microprobe Analysis," p. 57, Boston, Mass., 1967. (22) A. J. Tousimis and L. Marton, "Ad vances in Electronics and Electron Physics," Supplement 6, p. 144, Aca demic Press, New York, Ν . Υ., 1969. This paper was presented at the German Vacuum Society's meeting in Bochum, West Germany in March 1970.
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Peter F. Vâradi studied chemistry at the University of Szeged, Hungary, and was granted a Ph.D. in 1949. Since that time his work has been oriented toward chemical research and development and materials characterization in electronics, in the Research Institute for Telecommunications in Budapest, Hungary, and then in 1957 in Telefunken GmbH Development Laboratory in Ulm/Donau, West Germany. In 1958 he joined Machlett Labs., Division of Raytheon Co. in Stamford, Conn., where he became Section Head of Advanced Development. In 1968 he became Manager of the Materials Technology Branch of COMSAT Laboratories in Clai'ksburg, Md. He is the author and coauthor of numerous scientific papers and 10 U.S. and foreign patents. The scientific papers cover a broad range in the fields of electronic materials, vacuum technology, and in the various branches of analytical chemistry. In this latter field he pioneered pyrolysis-gas chromatography ^ANALYTICAL· CHEMISTRY, 1962-63) and the GC-MS techniques
^ANALYTICAL
CHEMISTRY,
1962, and U. S. Patent No. 3,318,149). His present interest in analytical chemistry is directed towards X-ray and electron analytical methods and their adaptations to computer technology. Dr. Vâradi is a fellow of the American Institute of Chemists; member of ASTM, and of the Society for Applied Spectroscopy; Senior Member of the American Vacuum Society, and member of the German and French Vacuum Societies. He is a Fellow of the British Interplanetary Society.
