Mass Spectrographic Analysis of N. B. HANNAY and A. J. AHEARN Bell Telephone Laboratories, Murray
Hill, N. J.
T
HE application of mass spectroscopy to the analysis of solids has become of great interest within the last few years, and several different approaches to the problem have been made. The choice of technique and instrumentation depends greatly upon the nature of the analytical problem. Dempster ( 4 ) was the first to dipcuss the analysis of traces of impurities in solids with t,he mass spectrograph. A4 doublefocusing mass spectrograph, designed for mass measurements ( 2 ) , a spark source (3),and photographic detection of the ions, was used for this work. Shall- and Rall ( I S ) constructed a double-focusing mass spectrograph of the Nattauch type, with a spark source and photographic detection, for the analysis of solids. S o applications to analysis using this instrument have been reported. Gorman, Jones, and Hipple ( 5 , 9 ) , using a modified Denipster instrument with a spark source and electrical detection, have demonstrated the quantitative possibilities of mass spectrometric analysis of solids. Their work \vas done with stainless steel samples, n-ith determinations of chromium and nickel in concentrations ranging from 0.3 to 25%. For specialized problems, ion sources may be used to simplify the instrumentation. Hickam ( 7 , 8) studied impurities in copper by vaporizing t’he metal into an electron bombardmenttype ionization chamber, with time integration of the ion currents. Inghrani and his coworkers (IO, 1 2 ) have obtained very high sensitivities using isotopir dilution techniques and both thermal ionization and electron-bombardment ion sourcw For a general analysis of impurities in solids, the vacuum spark source has several great advantages. It is reasonably independent of such factors as relative volatility of the various impurities, differences in ionization efficiencies and nature of the material being analyzed, so that sensitivities remain high regardless of the element being determined. Contamination nnd background difficulties are less than for mine other types of sources, as the spark is confined only to the material under analysis, and no other source parts are heated. Disadvantages of the spark source, such as the large spread in init,ial energies of the ions and the erratic fluctuations of current from the source, must he allowed for in the instrumentation. The use of the spark source creates problems in the ion-detection method, The electrical detection scheme of Gorman, Jones, and Hipple used a ratio measurement t,o avoid the problem created by the current fluctuations due to the erratic nature of the spark. I n this work they measured the ratio of the resolved ion current emerging from the magnetic analyzer to the current arriving a t an electrode xhich served to monitor the total cur-
Ge4+ Ge3’
rent from the source. I n the studies of high concentration samples, this method gave good quantitative results: but in samples of very low concentrations very high sensitivity is required in the detection system for the resolved ion current. For a typical sample, in the instrument used for the work described below, the average ion current to the final collector is of the order of 5 X IO-” ampere for the singly charged peaks of the principal component. A4I p.p.m, impurity would give a current of less than 10-]6 ampere, requiring a device such as an electron multiplier for detection. While this is feasible, the instrumentation to achieve maximum sensitivity in the vicinity of the large radio-frequency spark voltages becomes difficult. Photographic detection provides high sensitivity, integration of currents despite source fluctuations, and the simultaneous recording of a wide mass range: but it is less quantitative, requires the introduction of photographic plates into the vacuum system, and does not provide visual information while the source is in operation. The choice between electrical detection and photographic methods is not clear, and would depend considerably on the problems to be investigated. it hen quantitative results are desired, the electrical method appears preferable; when semiquantitative identifications of trace impurities are required, the photographic method has advantages. INSTRUM ENTATION
The instrumental aspects of the work have been described in detail (6). A Mattauch-type instrument with double focusing and a spark source was constructed. The instrument was designed for either electrical recording or photographic detection: however, the photographic detection method was used in most of this work, because of the nature of the problems. Ilford Q plates have been used for all work. The instrument is designed with a demountable source and access to the photographic plate region. The sample and plate can be changed in about 0.5 hour, by using cutoff devices in the pump lines to permit continuous operation of the pumps during this procedure, and pumping times can be as low as an hour. For electrical recording, a blank metal plate carrying an exit slit is substituted for the photographic plate and the current through this aperture strikes a collector mounted on a glass press. A “monitor” electrode between the electric and magnetic fields permits ratio measurements of the type made by Gorman, Jones, and Hipple. SAMPLES
Samples for this instrument are handled in the form of rods, 0.030 to 0.040 inch in diameter and 0.25 to 0.50 inch long. These are held in pin vises, so the cross-sectional shape of the rods is
Sb2+ (Gel
Ge“
GeU a
I
/4
I
I
35
23.3
60.5 70-76
121
150
b I
/P
3z5 40.3
16
75
cu
cu3+
C
21
63 98.5 l i 7 Figure 1. Typical Mass Spectra
28 a.
S b i n Ge.
b.
As in Sb.
c.
1056
Au monolayer on Cu
/20
V O L U M E 26, NO. 6 , J U N E 1 9 5 4 unimportant. Only the tips of the rods are analyzed, as t’he spark is confined to this area and the amount of wearing away of the electrodes in any reasonable exposure time is small. Because the rods form part of the electrical circuit in the spark source a modification of the technique is required for insulat,ors. >lost metals, and semiconductors like silicon and germanium, can be sawed or formed readily into rods of the desired shape. I n some cases only small chips of material may be available, which are too small to be held directly in the pin vises; such material can often be forced into the ends of metal tubes with small diameters and then held in pin vises. In the limited amount of work that has been done with insulators, the materials were ground up into a powder and packed into a metal tube 0.040 inch in diameter-gold, for example. The metal tube not only serves as a container for the potT-der, but also provides the necessary conducting path. With this arrangement the mass spectrum of the powder and the metal with its impurit,ies are observed ; therefore, high purity of the metal tube is desirable. Since all metals are likely to show some impurity, it probably will be necessary to choose a tube material that is free of the impurities one expects to encounter in the particular sample under study. By having several different metal tubes av:tilable, it should be possible to investigate any impurity. A run made on the empty metal t,uhe serves as a blank. The same technique also should permit the handling of metal powders. Contaminations on surface. of caonductors have been studied. There appears to be no simple way to study surface contaminations on insulators, or on conductors of a shape t h a t does not permit handling in the pin J k e . Because of the high sensitivity of the instrument, surface contaminations and bulk impurities appear in the mass spectra. Careful etching of the sample is desirable to avoid unnecessary background. Surface contaminations are distinguished easily from bulk impurities as t,hey appear strongly only in the first exposure, so there ie usually no difficult,y in determining impurity lines. .\14SS S P E C T R A
Typical mass spectra are shown in Figure 1. Positions on the plate and line widths vary as the square root of the mass. I t may be seen that nearly a 15 t o 1 mass range can be covered in a single exposure, Each plate ordinarily permits six exposures, so several exposure times and magnetic fields (mass ranges) can be covered for each sample. For a 0.002-inch principal slit, the resolution of the instrument is 1500-that is, m/Am = 1500, where &n is the least distinguishable mass difference at’ mass in. This high resolution is useful in certain analytical identifications as it usually is sufficient to distinguiph between and identify two lines of the same nominal mass, differing only by t,he small amount arising from their different packing fractions. I n Figure 1, the spectra show: i n ) germanium sample with added antimony a t the concentration of 6 p.p.m. (atom fraction); ( b )arsenicimpurity in antimony at 100 p.p.m.; ( c ) copper sample with I monolayer of gold deposited on it,s surface. Exposure bimes were 3 minutes, 30 rcconds, and 1 minute, respectively. I n th(b original plates, thc antimony in germanium lines showed clearly with an exposure time of 5 seconds. ASALYTIC4L RESULTS
A considerable variety of met ale and semiconductors has been run in the machine with no important differences in the ease of handling. Many elemente have appeared as impurities with no evidence of “blind spots” in the method. From these results, it appears qualitatively that, 15ithiii a factor of about ten, the sensitivities for the diRerent impurity elements in any metal will be equal. The differences arise because of different degrees of efficiency of ionization, and differences in photographic plate sensitivities for various ions. Larger sensitivity differences hold for the emission spectrograph. One of the reasons for the relatively small spread in senpitivities in the mass spectrograph is the nature of the spark (high voltage, very short “on” periods) which minimizes difference8 in volatility, ionization efficiencies, etc
1057 I n order to obtain semiquantitative anall ses, rather than qualitative identifications or comparisons of concentrations of impurities, calibrations with standard samples are necessary as in the case of the emission spectrograph. If further work indicates that mass spectrographic sensitivities vary less than emission spectrographic sensitivities, the calibration problem for the mass spectrograph will be somewhat simplified. For many systems, standard samples could be prepared using radioactive tracers as the added impurity. 0.5 0.4
0.3 0.2 >
J,
0.1
UI
0.06
=,4 0.08 zJ
0.04
0.03
0.02
0.01
2
Figure 2.
3
4
5 6 8 IO 20 30 40 80 80100 EXPOSURE TIME IN SECONDS
200
4hsorhance of Lines for Varying Exposure Times
Calculations have been carried out for two systems, horon in silicon and antimony in germanium. These M ere chosen because samples were available covering a wide range in concentrations. Concentrations u ere determined from electrical resistivity measurements, mith u = nev, where u is the electrical conductivity, n is the carrier concentration, e is the electronic charge, and v is the mobility. From this equation, n can be calculated from measurements of u and published values of v . Mobilities for germanium were taken from Conwell ( I ) , and for silicon from data obtained by hlorin (12). Assuming complete ionization of the impurity centers a t room temperature, the impurity concentration is equal to the carrier concentration n. I n optical spectroscopy, an internal standard is required to allow for variations in the source and in photographic plate characteristics. The most convenient internal standard in the present uork has been to take a line originating from the principal component-Le., germanium or silicon-and to refer the impurity intcniit\. to this line. The instrumental arrangement permits six exposures on each plate, so that each sample was run for exposure times ranging from 1 second to 15 minutes. Spark conditions were kept nearly constant throughout the exposures and from one sample to another. Absorbances for the impurity line and the standard line were measured with a recording densitometer, and these absorbances were plotted as a function of exposure time. A typical plot, Figure 2, shous the expected “absorbance LIS. intensity” character. If it is assumed that the source remained constant during the exposures, then the relative values of the ion currents for the t x o lines can be determined by the factor necessary to shift one curve into the othcr along the intensit., (exposure) axis. This is simply the ratio of exposure times for a given absorbance in the linear portion of the curves. This procedure was folloiied for each sample and the same reference line was used in each case. The relative intensity values give the mass spectrographic determination of concentration (in arbitrary units). The results for boron in silicon and antimony in germanium are shown in Figures 3 and 4. Five different samples were available for each system and in most cases each sample u a s run tuice, but not in consecutive
1058
ANALYTICAL CHEMISTRY
runs. In view of the experimental uncertainties in maintaining constancy of the spark during the exposures between samples, in determining relative intensities from the absorbance measurements, and in the mobilities used to calculate impurity concentrations, the results appear to be satisfactory. These results indicate that after calibration analyses are accurate within an uncertainty factor of 2 or 3. The absorbance represented by the 10%-est concentration antimony lines was as low as could be measured on the densitometer; hence the indicated uncertainty for this sample is large. Exposure times of less than 1 second or a change to a secondary line of the impurity element are necessary to keep absorbance low enough for quantitative measurement for samples above a concentration of about I n the latter case a weak isotopic line would be most satisfactory, but where this is not available a multiple-charge line could be used. Surface contaminations are distinguished readily from bulk impurities, as they show up strongly only in the first exposureLe., in the initial sparking. Thereafter, they appear weakly as the spark gradually wears the electrode away and reaches new areas of the surface. Bulk impurities remain constant throughout the exposures. IMPURITY ATOMS PER C C
this method is usable, but no effort was made to learn if senritivities are as high as for metals.
10'8
COYCLUSIONS
Figure 3.
Boron in Silicon Determinations
Samples for surface contamination studies were prepared by evaporation of knoa n quantities of appropriate metals onto the surface of clean copper wires or germanium rods. Mass spectra were obtained from samples having 0.1 and 1.0 monolayer of gold on copper, 0.1 and 1.0 monolayer of indium on copper, and a sample of germanium on which one monolayer each of indium, silver, aluminum, chromium, iron, nickel, copper, and palladium was deposited. In all these cases the lines of the deposited material n ere readily visible. The number of atoms of impurity necessary for detection as a surface contamination Ti'as compared with the number necessary as a bulk impurity to give a line of the same absorbance. I n the case of surface contamination, roughly lo-* sq. cm. of surface is sparked initially and for a one-monolayer sample this corresponds to approximately 10'2 atoms of the impurity. By comparison, to obtain a line of the same absorbance but originating from a bulk impurity, an exposure of 20 seconds for an impurity concentration of 10-6 and a total weight loss of about 0.01 mg. are required, corresponding to 10" atoms of the impurity. Very roughly the same number of impurity atoms are required in either case. To check the feasibility of the method for insulators and for powdered samples, samples of lithium carbonate and the same lithium carbonate with 5 p.p.m. of zirconium nitrate were packed into gold tubes and run. The zirconium was detectable; BO
An instrument with photographic detection is capable of semiquantitative analyses a t concentrations a t least as low a t for most elements. The presence of background lines prevents determination of such elements as carbon, oxygen, nitrogen, and hydrogen a t low concentrations. Surface contaminations as lo^ as 0.1 monolayer are detectable. These sensitivitiesIhave been reached with photographic detection; the method undoubtedly can be made more quantitative with electrical recording, as shown by Gorman, Jones, and Hipple, but with a probable loss in sensitivity. The present limitation on sensitivity is set by faint background lines originating from traces of organic contaminants in the source chamber, and by diffuse background due to ion scattering and change of ion mass or charge in transit. The primary usefulness of this method is for problems not suited for emission spectrographic analysis. These are for the most part cases where the emission spectrographic sensitivity is inadequate, and where general, semiquantitative results are needed. For this type of work the present method has given good results, and provides information not available by any other method. ACKNOW LEDGMENT
The authors are indebted to TV. L. Bond and E. 11. Kelly for preparing the samples for the surface contamination studies. LITERATURE CITED
Conwell, E. AI., Proc. I.R.E., 40, 1327 (1952). Dempster, A. J., Proc. Am. Phil. SOC.,75, 755 (1935). Dempster, A. J., Re%.Sci. Znstr., 7, 46 (1936). Dempster, A. J., U. 8. Dept. of Commerce, Bibliography of Scientific and Industrial Reports, MDDC 370 (1946). Gorman, J. G., Jones, E. J., and Hipple, J. A , , AN.AI,.CHEX., 23,438 (1951). Hannay, N. B., Re?. Sci.Znstr. (to be published). Hickam, W.A1.- Phys. Rev., 74, 1222 A (1948). on Chemical Analysis of InorHickam. W. hl.. "Svmoosium - ganic 'Solids by Neans of the Mass Spectrometer", AST-11 Bull. 149, 17 (1953). Hipple, J. A., and Gorman, J. G., Ibid., A S T M Bull. 149, 10 (1953). Inghram, 31. G., Ibid.,A S T N Bull. 149, 1 (1953). (11) Inghram, 31. G., J . Phys. Chern., 57, 809 (1953). (12) hlorin, F. J., private communication. (13) Shaw, A. E., and Rall, W., ReL. Sci. Instr., 18, 278 (1947). ~~
RECEIVED for review January 5, 1954. Accepted March 23, 1964.
