Thin Film Analytical Technique Based on Sputtering Gary
E. Thomas, Erik E. de Kluizenaar, Ludo W. J. van Kollenburg, and Leo C. Bastings
Philips Research Laboratories, Eindhoven, The Netherlands
A thin film analytlcal technique has been investigated In which a beam of rare gas ions or neutral atoms Is used to sputter a layer of material from the film. The sputtered material is collected and subsequently analyzed uslng varlous spectroscopic techniques. Results are given for the analyses of three metal alloys, yttrium Iron garnet, galllum arsenide, and monolayers of tin and silver.
The analysis of thin solid films, which are increasingly being used in experimental and production electronic, magnetic, and electrooptical devices, presents often challenging problems to the analytical chemist, particularly when quantitative analyses are desired. In certain cases as, for example, the determination of impurities in a metal film on a reasonably inert substrate, chemical etchants can be used to dissolve the film selectively and the solution can be analyzed using the most suitable technique from the entire spectrum of wet-chemical, electrochemical, spectroscopic, and radiochemical techniques (1). However, in a great many cases, selective etchants are simply not available. Obvious examples are cases requiring the determination of the doping and/or impurity levels of a doped film on a virtually identical substrate, or the determination of the elemental ratios in films of the type A,B,C, . . . on a substrate of AdBeCf. . . . For such problems, two approaches are being extensively investigated. One involves probing the surface regions of the film with a technique having an inherently limited information depth less than the thickness of the film. Auger electron spectroscopy (2) (which has an information depth of 2-5 nm) and electron microprobe analysis ( 3 ) (with an information depth of 1-5 wm) are examples of such techniques. The second approach involves using energetic ions to sputter material from the surface of the film, and detecting and identifying either those particles sputtered directly as secondary ions (secondary ion mass spectroscopy ( 4 , 5 ) or excited neutrals (bombardment-induced light emission (6, 7)), or those particles sputtered as neutrals but which are excited or ionized in a plasma maintained in a volume above the surface of the sample (8, 9). Although there has recently been considerable progress in achieving quantitative results with the techniques mentioned above, the capabilities in this regard are still limited. In this article, we present initial results of a technique which combines the advantages of being able to use sputtering as a universal etchant with the freedom to use conventional solution analytical techniques. Basically, an ion beam is used to sputter material from the surface of the film in a high vacuum Torr) environment. The sputtered material is collected on an inert substrate in the vacuum system and the deposit is subsequently dissolved and analyzed. In fact, the principle involved is nothing more than the transfer of the film from its original (chemically troublesome) sublayer to a substrate where chemical etching becomes feasible. Once this has been done, an analytical technique where the quantitative aspects are well understood, and where possible sources of error are recognized, can be used to determine the composition. The main uncertainty to be evaluated is whether the film
can be transferred representatively. The transfer involves two physically distinct steps: sputtering and collection. Regarding the former, both model calculations (10) and several experimental studies on metal alloys (11-13) indicate that, on homogeneous samples, the composition of sputtered material should be identical to that of the original substrate. This is not expected to be the case when two or more distinct macroscopic phases are present a t the sputtered surface, since each would be expected to be removed at a different rate, thus falsifying the sampling step. In the collection step, two factors can influence the results. The first is simply the experimental geometry. Particles are sputtered from a surface with an angular distribution which is not always predictable (11, 12) and which may differ for various components of the film. If the collector substrate does not geometrically intercept the total flux from the surface, then sampling problems can arise. Second, it is known from secondary ion mass spectroscopy studies that not only single atoms but also large molecular aggregates are sputtered from the surface, and that all particles have a wide distribution of kinetic energies ( 4 ) . This complex flux could lead to composition distortions because of differences in sticking probabilities on the surface of the collector. In this initial study, we have attempted to choose trial materials in which some of these potential problems might be expected to adversely affect the total accuracy of the analysis. The samples are listed in Table I. Although the goal of the present work was to develop a thin layer analytical technique, there are few thin layer systems that are sufficiently well characterized to serve as standards in this initial stage of evaluation of the method. Therefore, bulk samples were chosen from which a thin layer was sputtered. For each sample, the total amount of each element of interest present on the collector substrate was determined and, from the results, either the elemental ratio or the composition based on the sum of the amounts was derived. In virtually all cases, a fragment of the sample was also removed mechanically and analyzed concurrently for comparison.
EXPERIMENTAL The apparatus used in the sputtering step of the analyses has been described previously (7). Focussed, mass-analyzed beams of Kr+ , Xe+ or neutral argon atoms a t an energy of 10 keV with current density of -100 pA/cm2 on a 5-mm diameter spot were used for the bombardments. The sputtering and collection geometry is shown in Figure 1. The angle between the beam and the collector plate was fixed a t 55'. The angle of incidence of the beam on the target was selected for each type of sample by optimizing the deposition rate on a quartz crystal film thickness monitor placed in the collector position in a preliminary experiment. For the samples measured in this work, the angle 0 varied between 30 and 5 5 O . Unless otherwise specified, the collector substrates had a diameter of 30 mm. In the analysis step, three techniques were used. These were flame atomic absorption spectroscopy using a Pye-Unicam SP1900 Atomic Absorption Spectrophotometer, flameless atomic absorption using a Perkin-Elmer 403 Spectrophotometer equipped with an HGA 70 graphite furnace, and emission spectroscopy using an argon plasma torch as the excitation source (14-16). In all cases, blank analyses were done on fresh collector substrates to check for contamination, and possible spectral interferences were checked with synthetic solutions. The relative precision in the solution analyses was 5%.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
2357
~~
~
Table I. Samples Compositions
Sample
Fe-Ni
Disc cast from homogeneous melt Polished discs
Cu-Ag alloys
NBS standard 1 2 6 B used
as starting material Eutectic composition,
28% Cu, 7 2 % Ag 3 0 % Cu, 70% Ag 10% Cu, 90% Ag
1 2 3
possibly inhomogeneous Rolled sheet
Fe-Ni-Cr alloys 69.1% Fe, 12.9% Ni, 18.0% C r 47.7% Fe, 47.2% Ni, 5.1% Cr
1 2
Yttrium-iron garnet
Sintered polycrys talline plates with highly polished sur faces Single crystal grown from liquid phase Deposited on sodalime glass plates
stoichiometric Y3Fe5Ol2
GaAs Ag-Sn monolayers a
Comments
Form
6 4 % Fe, 3 6 % N i
Sn: 0.11 pg/cm2 Ag: 0.16 pg/cm2
Good insulator
See ref. 1 7
70= percent by weight.
Flgure 1. The sputtering and collection geometry
RESULTS Fe-Ni Alloy. The sample was sputtered with a 10-keV Kr+ ion beam. The collector substrate was a gold-covered quartz crystal from a Kronos Model GM-330 film thickness monitor. The crystal had a surface area of 0.86 cm2. A layer of 40 A was collected on the crystal (assuming a specific gravity of 8.4 for the deposit). The deposit was dissolved in hydrochloric acid and the Ni and Fe were analyzed using flameless atomic absorption spectroscopy. The results are shown in Table 11. Table 11. Fe-Ni Alloy Results Bulk composition, ivt 56
Amounts found, ug Derived composition, wt %
Ni
Fe
Xi
Fe
Si
Fe
36
64
1.1
1.9
37
63
Table 111. Cu-Ag Alloy Results Sample
1 1 2 2 3 3
Amounts found ug
Bulk composition, IVt ii
Ion
Cu
cu
Ag
Kr+ Xe+ Kr+ Xe+
28
72
30
70
Kr'
10
90
5.5 5.4 6.0 6.0 1.4 1.4
Xe'
Derived composition, Wt 94 Ag
11.5 14.5 15.0 14.5 19.0 21.0
Cu
Ag
32 27 29 29
68 73 71 71 93 94
7 6
Cu-Ag Alloys. Three alloy compositions were tried. Sample 1 had the eutectic composition (28 wt % Cu), sample 2 was close to the eutectic (30 wt % Cu) and sample 3 fairly far removed (10 wt % Cu). All samples were sputtered with both Kr+ and Xe+ beams a t an energy of 10 keV. Again, the collector substrate was the film thickness monitor crystal. Approximately 200 A of deposit was collected in all cases. The crystals were etched with nitric acid and the analyses carried out using flameless atomic absorption. The results are listed in Table 111. Fe-Ni-Cr Alloys. Two samples of differing composition were tried, both being sputtered with Kr+ ions. The collector substrate was a platinum foil. The deposit was dissolved in hydrochloric acid. Ni and Cr were analyzed using flameless atomic absorption spectroscopy and Fe using atomic absorption in an air-acetylene flame. The results are listed in Table IV. Yttrium-Iron Garnet. The choice of yttrium-iron garnet as a test material was based on the expectat,ion that the magnetic garnet materials would be typical candidates for the practical application of this method. They are technologically often used in the form of a thin layer of one garnet material on a similar garnet substrate, which makes chemical stripping difficult. They are also excellent insulators, which increases the experimental difficulties when using standard surface techniques involving probing with or detection of charged particles. The particular samples used in this work were polycrystalline, densely sintered plates with a highly polished surface. They were bombarded with a 10-keV neutral Ar beam to avoid problems associated with the deflection of the primary beam due to surface charging. The surface was smooth enough that the profile of the sputter crater could be measured mechanically after the sputter step with a stylus apparatus (Talysurf 4). On two of the samples, a number of Talysurf scans was made in a grid pattern ove- the crater and, from these measurements, the total volume of the crater and, hence, the total amount of sputtered material, was calculated and compared with the amount of material collected.
Table IV. Fe-Ni-Cr Alloy Results Bulk composition, ivt (C
Sample
1 2
2358
Amounts found, u g
Fe
91
Cr
69.1 47.7
12.9 47.2
18.0 5.1
Fe
78.6 52.3
Derived composition, wt 96
Xi
Cr
19.1 62.6
22.3 5.0
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
Fe
SI
Cr
65.5 43.6
15.9 52.2
18.6 4.2
5
Table VI. GaAs Results
Table V. Yttrium-Iron Garnet Results Amount Sample
1 2 3 4 5 6 7
Amounts found,
os
Mole ratioa collected,
Y
Fe
Fe/Y
ug*
24.0 36.7 38.6 14.9 14.9 14.9
26.0 38.5 34.6 14.9
1.7
68 102
1.7
Amount
"8 200
Amounts found, ug
Sample
sputtered,
1 2 3
20 2 5 0 i 25 i
1.4 1.6 15.5 1.7 15.2 1.6 17.2 18.3 1.7 a The stoichiometric Fe/Y'ratio is 1.67. Based on YSFe5012.
From Talysurf measurements.
The material was collected on a fused quartz plate and the deposit dissolved in hydrochloric acid. Yttrium was determined using emission spectroscopy with an inductively coupled rf plasma as excitation source, and iron was determined using atomic absorption in an air-acetylene flame. In all cases, only the Y-Fe ratio was measured. The results are shown in Table V. For samples 1 and 2 on which the sputter crater was measured, 35-40% of the sputtered material was collected. If it is assumed that the material is ejected with a cosine angular distribution, then, with the geometry illustrated in Figure 1, it can be calculated that roughly 70% of the flux of ejected material should have been intercepted by the collector. GaAs. The GaAs sample was sputtered with a 10-keV Krf beam onto a fused quartz collector substrate. The deposit was dissolved in nitric acid. The Ga and As were analyzed using atomic absorption in an air-acetylene flame. Bulk analysis of the GaAs sample yielded a Ga-As mole ratio of 1.03. The results from three sputtering trials are listed in Table VI. Sn-Ag Monolayers. Taking into account the high sensitivity of analytical methods such as atomic absorption spectroscopy, which was used extensively in this work, it should be possible to analyze submonolayer quantities of many elements sputtered from surfaces. As a test of the feasibility of such analyses using the sputter-collect method, analysis of the material sputtered from a glass plate covered with approximately one monolayer of both Sn and Ag was attempted. The procedure for producing a well-defined coverage of these e!ements on glass has been described (27). Radioactive analysis of the samples used here indicated coverages of 0.11 pg/cm2 and 0.16 pg/cm2 for Sn and Ag, respectively (1;'). The samples were sputtered with the neutral Ar beam onto a fused quartz collector substrate. The deposit was dissolved in nitric acid. Both Sn and Ag were detected in the solution, the amount of the latter being 0.025 pg from an analysis using flameless atomic absorption. From the crater measurements on the garnet samples, the area of the neutral beam was expected to be approximately 0.2 cm2, which means that the total amount of sputtered silver would have been 0.03 pg.
DISCUSSION A large number of factors can play a role in the transfer of the film from the target to the collector. Among these are the energy and nature of the incoming ion, and the angle of incidence of the beam on the target in the sputtering step, and the nature and surface properties (roughness and temperature, for example) of the substrate in the collection step. None of these factors has been investigated here; the ion energy and the angle of incidence used were simply convenient for the existing sputter apparatus, and the col-
Ga
As
89 68 110
87 67 95
Mole ratio, Ga/As
1.1 1.1 1.25
lector materials (the gold layer on the film thickness monitor cyrstal, the Pt foil, and the fused quartz discs) were arbitrarily chosen, the only criterion being that they should not introduce contamination during the chemical treatment. Nevertheless, without any attempts a t refinement being made, the results are encouraging. As can be seen from Tables 11-VI, only in a few cases does the absolute error in the analyses exceed 20% (Cu in Cu-Ag sample 3, Ni in Fe-Ni-Cr sample 1, and the mole ratio in GaAs sample 3),while, in the majority of the analyses, it is less than 10%. There are indications that the collection step is not ideal. From the experiments with yttrium-iron garnet, where the absolute amounts of sputtered and collected material were measured, it appears that only roughly 50% of the incident flux remained. In this case, no serious discrepancy in the Fe-Y ratio resulted but, in general, incomplete collection of the flux might be expected to falsify the measured elemental ratio. This may have been the case for GaAs (Table VI) where the deposits were consistently deficient in As, which is the more volatile of the two components. It is to be expected that if the collector, which was at room temperature in these experiments, were cooled to a much lower temperature, the arriving flux would accommodate more completely on its surface. Certainly some of the material on the collector will be resputtered by high energy reflected primary particles and energetic sputtered species emerging from the target. Since the intensity, composition, and energy distribution of this secondary flux is difficult to estimate, the importance of this effect cannot be assessed. However, even if a high collection efficiency is achieved, the resputtering effect will remain as a source of error in the analyses (10). Regarding the sputtering step, all that can be said is that the composition of the sputtered material is not grossly different from that of the bulk solid. In the case of the 10% Cu-90% Ag alloy, where poor results were obtained, the problem might lie in the sputtering step. From the phase diagram of the Cu-Ag system (18), two macrophases are expected to be present in the solid, one being the eutectic mixture, and the other being a solid solution of varying composition of copper in silver. If areas of both composition were present at the surface, then, as mentioned previously, a distortion of the composition of the sputtered flux might result due to differing sputter rates. It should be stressed that unless a sample is known to be microscopically homogeneous over the area to be sputtered, this, or any other analytical technique based on sputtering, cannot be expected to give reliable results. The experiment on the Sn-Ag monolayers shows that small quantities of material can be transferred by sputtering. The detection limit for quantitative analysis with many modern analytical techniques lies in the range lo-'* g for most metals. With the present method, assuming 100% collection efficiency and an ion beam area of 0.1 cm2, quantitative analyses of elements in the concentration range 0.1-100 ppm should be feasible if 1000 A of material is sputtered from the sample. For an analysis where a component is present only on the surface of the target (as in the Sn-Ag system described here), coverages in the range 0.01-10 monolayers are detectable. The suitability of this method for analyzing surface layers requires further exami-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
2359
nation since, in such thin layers, phenomena such as recoil implantation (19), mixing, and initial selectivity (IO), which accompany the sputtering process, could be troublesome. ACKNOWLEDGMENT We thank F. J. de Boer for performing several analyses using the plasma torch, W. Tolksdorf and J. Kelly for providing the garnet and tin-silver samples, respectively, and C. van de Stolpe for his stimulating interest in this investigation. LITERATURE CITED (1) W. A. Pliskin and S. J. Zanin in "Handbook of Thin Film Technology", L. I. Maissel and R . Glang, Ed., McGraw-Hill, New York. 1970, Chap. 11. (2) C. C.Chang. Surf. Sci., 25, 53 (1971). (3) W. Reuter, Surf. Sci.,25, 80 (1971). (4) H. W. Werner, Vacuum, 24, 493 (1974). (5) C. A. Andersen and J. R . Hinthorne, Anal. Chem., 45, 1421 (1973). (6) C. W. White, D. L. Simms, and N. H. Tolk, Science, 177, 481 (1972).
(7) G. E . Thomas and E. E. de Kluizenaar. Acta Nectron.. 18, 63 (1975). (8) H. Oechsner and W. Gerhard, Phys. Lett. A, 40, 21 1 (1972). (9) J. W. Coburn and E. Kay, Appl. Phys. Lett., 19, 350 (1971). IO) H. W. Werner, to be published. 11) M. Kaminsky, "Atomic and Ionic Impact Phenomena on Metal Sur-
faces", Springer-Verlag. Berlin, 1965. 12) G. Carter and J. S. Colligon, "Ion Bombardment of Solids", Heinemann Educational Books, Ltd., London, 1968. 13) M. L. Tarng and G. K. Wehner, J. Appi. Phys., 42, 2449 (1971). 14) P. W. J. M. Boumans and F. J. de Boer, Spectrochim. Acta, Part B, 27, 391 (1972). (15) P. W. J. M. Boumans and F. J. de Boer, Proc. Anal. Div. Chem. SOC. (London). 12, 140 (1975). (16) P. W. J. M. Boumans and F. J. de Boer, Spectrochim. Acta, Part 6, 30, in press. (1975). (17) C. H. de Minjer and P. F. J. v.d. Boom, J. Electrochem. SOC., 120, 1644 (1973) (18) M. Hansen, "Constitution of Binary Alloys" McGraw-Hill, New York,
1958.
(19) C. 8. Kerkdijk and R . Kelly, Surf. Sci,, 47, 294 (1975).
RECEIVED
for review May 29, 1975. Accepted August 5 ,
1975.
Determination of Silicon in Glasses and Minerals by Atomic Absorption Spectrometry R. A. Burdo and W. M. Wise Research and Development Laboratories, Corning Glass Works, Corning, N. Y. 14830
Silicon in glasses and minerals is determined by atomic absorption spectrometry. Samples are fused with a sodlum carbonate-sodium borate flux and then dlssolved in an acidic molybdate solution to form a silico-molybdate complex. The presence of molybdate prevents the polymerization of slllca, buffers possible flame interferences, and provides an accuracy of 0.2 to 0.3 % absolute for samples containing 14 to 94% silica. The method considerably reduces the time required for silica analysis by classical gravimetric procedures.
There are a number of reported literature methods for the atomic absorption determination of silicon in high silica materials. Nearly all of the methods rely on a lithium metaborate fusion (1-4), a lithium carbonate-boric acid fusion (5, 6), a sodium peroxide fusion (7), or an HF-Teflon bomb attack (8, 9) for sample decomposition. However, most of these methods either have not been applied to glasses or lack sufficient accuracy for our purposes. In this study, a fusion technique employing a sodium carbonate-sodium borate flux is chosen for its speed, simplicity, and universality in glass decomposition. The problem of chemical interferences on the silicon absorption signal is investigated. The formation of a silico-molybdate complex is proposed as an effective means of eliminating such interferences, preventing the polymerization of silica, and providing excellent accuracy for silica analysis in comparison to classical gravimetric procedures.
EXPERIMENTAL Apparatus. A Varian Techtron AA-5 atomic absorption spectrometer was employed under the analytical conditions stated in Table I. Reagents. All chemicals are analytical reagent grade. Aqueous reagents are prepared in distilled, deionized water. All solutions are stored in plastic containers. Flux. Mix sodium carbonate (NaZC03) and sodium borate 2360
(NanBdOi) in equal proportions by weight. Both reagents are -200 mesh. Ammonium Molybdate Solution (39 g/l.). Dissolve 78 g of ammonium molybdate ((NH&jM07024*4H20) in about 700 ml of water, filter through a Whatman No. 41 fast filter into a 2-1. volumetric and dilute to volume with water. Diluting Solution. Dissolve 1 g of flux in 100 ml of 0.394 N nitric acid (1:39). Add 100 ml of molybdate solution and dilute to 500 ml with 0.126 N nitric acid (1:124). Discard solution after several days if a hard scale of hydrated molybdenum oxide forms on the container walls. Procedure. Sample Fusion. Hand grind the sample t o -100 mesh using a corundum mortar and pestle. Desiccate as required. Weigh 100 mg of sample (to the nearest 0.1 mg) and 1.0 g of flux into a 30-ml platinum crucible. Mix powders well, cap crucible with a platinum cover, and place directly in a muffle furnace a t 1000 "C for 30 to 40 minutes. Melt splattering is avoided by the use of anhydrous flux reagents. On removal of the crucible from the furnace, swirl the liquid melt onto the inner sides of the crucible and solidify the melt by half-immersing the crucible in an ice bath such that no water enters the crucible. The platinum cover should remain intact until the melt solidifies in order to prevent the loss of melt due to the possible ejection of small particles on rapid cooling. Melt Dissolution. Transfer 100 ml of molybdate solution and 100 ml of 0.394 N nitric acid to a 250-ml Teflon beaker containing a half-inch magnetic stir bar (Teflon-coated). Place a similar stir bar in the platinum crucible, immerse the crucible into the beaker, and begin magnetic stirring. No heat is required. The melt dissolves in 10 to 30 min, forming a yellow silico-molybdate solution. Immerse the platinum cover near the end of dissolution only if it shows evidence of melt splattering. Quantitatively transfer the yellow solution to a 500-ml volumetric (glass with plastic screw cap) and dilute to volume with 0.126 N nitric acid. Standard Solutions. Substitute 0.1000 g of pure silica powder (99.9+ %, -100 mesh) in the fusion procedure to yield a standard stock solution of 93.5 ,ug/ml silicon. Dilutions of the stock solution are made with the diluting solution already described. Solutions containing less than 40 wg/ml silicon may form 4 hard and nonadsorbing scale of hydrated molybdenum oxide an the container walls after 10 days depending on the silicon concentration and the fullness of the container. The scale is easily cleaned with ammonium hydroxide solution.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
