I t may be that the water is not evenly distributed between the two phases. This would be consistent with the known high affinity of water for sulfonated copolymer (9). The kinetic effects are in qualitative agreement with the known tendency for the rate of sulfonation of mono- and dialkyl substituted benzenes to vary with the water content (6). The present efforts have been devoted to the finding of conditions for preparing homogeneous fractionally sulfonated cation exchange materials. The experimental efforts include the development of a procedure which preserves the bead form of styrene divinylbenzene copolymer. Tests for homogeneity have been satisfied, and the results show that secondorder rate equations are obeyed in the region of less than 60% of full sulfonation, and that a rate slowing occurs at higher degrees of sulfonation. (9) G. E. Boyd and B. Z . Soldano, Z . Electrochern., 57,162 (1953).
The results of this study include necessary tests of th; conditions that must be explored if one is to establish whether a n ion exchanger is sufficiently well defined to justify the assumption that measurements of its properties can be reproduced. The present measurement of swelling ratios of the fractionally sulfonated materials and the good agreement by extrapolation to independent measurements by Wiley on corresponding full sulfonated materials show that this important physical property, at least, can be related among corresponding materials studied in different laboratories. RECEIVED for review January 30, 1967. Accepted June 14, 1967. The present studies were begun with the support of the United States Atomic Energy Commission under Contract AT (45-1) 1544 with Washington State University. Division of Analytical Chemistry, 153rd Meeting, ACS, Miami Beach, Fla., April 10, 1967.
Determination of Sodium in Ultrapure Silicon and Silicon Dioxide Films by Activation Analysis James F. Osborne, Graydon B. Larrabee, and Victor Harrap Texas Instruments Inc., Dallas, Texas
The technique of neutron activation coupled with sensitive counting techniques has been used to determine sodium in sub-nanogram amounts in very thin layers of silicon dioxide films and at the sub-ppb level in silicon. Techniques are described for incrementally 0 removing thin layers of the oxide (50-200 A), measuring the thickness of the residual oxide with an ellipsometer, and analyzing the etch solution for sodium-24 content. The production of sodium-24 from other impurities was evaluated and was found to be minor in samples used in this work. Preirradiation handling and wrapping was found to be an important factor when analyzing for sodium at these levels. ANALYSIS FOR SODIUM in ultrapure single crystal silicon and in very thin silicon dioxide films grown on this silicon has become vitally important for the new and advanced metal-oxidesemiconductor (MOS) technology. This MOS technology provides a significant departure from the usual semiconductor practice in that the silicon dioxide film becomes a n active part of the device. Figure 1 shows a schematic of a n MOS field effect transistor. The oxide film supports a thin metal film which in turn is used to supply a charge through the oxide to the semiconductor substrate. I n this manner the metal-oxide can act as a gate in the same way as a grid in a vacuum tube. The silicon dioxide film must be virtually free of any mobile ions which can drift through the oxide when a bias is applied to the metal film and thereby cause device instability. Sodium ion has been identified as the principal source of mobile ions ( I , 2) in these films. The electrical effects and distribution of sodium in oxide films have been described by Carlson et at. (3, 4) and Yon et ai. (5). Various authors have reported the determination of sodium in silicon by neutron activation (6, 7 ) but none describe the exact manner in which the quantitative measurement was made. James and Richards (8) and Erokhina et al. (9) list detection limits of 2.5-5 X 10-'0 gram of sodium a t neutron fluxes of 1-9 X 1OI2 n/sec/cm2. While this detection limit is satisfactory for the analysis of 0.1- t o 1-gram samples of 1144
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Figure 1. Schematic of a metal-oxidesemiconductor (MOS) field effect transistor silicon, it is necessary t o use higher fluxes and more sensitive counting techniques for the analysis of oxide films of a few thousand angstroms in thickness. This detection limit is even less acceptable when it is necessary to determine the :odium concentration in incrementally removed 50- to 1000-A layers of the silicon dioxide film. The total volume of each etch lap sample being analyzed is only 1.5 X 10-6 to 3 X lop5 cc and total sample weight is 3.5 to 70 pg. The production of 15-hour sodium-24 by the (n,r) reaction is controlled only by the neutron flux and time of irradiation. In this work, a flux of 1 X 10'3 n/sec/cm2 for 24 hours was (1) E. H. Snow, A. S . Grove, B. E. Deal, and C . T. Sah, J. Appl. Pkys., 36, 1664 (1965). (2) A. G. Reverz, IEEE Trans. Electron Decices, ED-12, 97 (1965). (3) H. G. Carlson, G. A. Brown, C. R. Fuller, and J. Osborne, Phys. Failure Electronics, 4, 390 (1966). (4) H. G. Carlson, C. R. Fuller, D. E. Meyer, J. F. Osborne, V. Harrap, and G. A. Brown, Ibid., 5, in press. ( 5 ) E. Yon, W. H. KO, and A. B. Kuper, IEEE Trms. Electron Deuices, ED-13, 276 (1966). (6) B. A. Thompson, B. M. Strause, and M. B. Lebowf, ANAL. CHEM., 30, 1023 (1958). (7) G. H. Morrison and J. F. Cosgrove, Zbid., 27, 810 (1955). (8) J. A. James and D. H. Richards, J . Electron. Corrrrol, 3, 500 (1957) (9) K. I Erokhina, I. Kh. Lernberg, I. E. Makasheva, I. A. Maslow. and A. P. Obukhov, Zacodsk. Lab., 26, 821 (1960).
EXPERIMENTAL
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Figure sodium-24 and silicon sample containing 0.7 ppb sodium used for the irradiation (Union Carbide Reactor, Tuxedo, N. Y.). This should yield a theoretical sensitivity of 5.63 X 106 dpm/pg of sodium. The decay scheme for sodium-24 is shown in Figure 2. The two gamma transitions(2.75 and 1.36 MeV) follow the 1.36-MeV beta emission and are in prompt coincidence because the life time of the lower state is second (10). The method of detection o r counting the sodium-24 provides the key for the successful analysis for very small amounts of sodium. Beta counting the sodium activity is not attractive primarily because of the many other beta activities present in the irradiated sample. Elaborate radiochemical separations of sodium are long (silicon is slow dissolving), difficult t o perform, and ultimately yield only mediocre radiochemical purity. This approach becomes particularly unattractive when large numbers of samples are to be analyzed for sodium24, as in analyzing multiple incremental etches through a n oxide film on a silicon substrate. The gamma spectrum of sodium-24 is shown in Figure 2. Each disintegration of sodium-24 yields two gammas in equal yield which makes gamma counting the better detection method. Direct gamma counting without respect t o energy is not possible because of the many other activities often present in the irradiated samples (31Si, I98Au, TU, 76As, lz2Sb, 124Sb,140La, etc.). The gamma spectrum of a n irradiated high purity silicon sample containing 0.7 ppb sodium is also shown in Figure 2. There are three approaches to the problem of gamma counting irradiated silicon and silicon dioxide samples, multichannel gamma ray spectrometry, gamma counting with energy discrimination, and gamma-gamma coincidence counting. The present investigation was undertaken t o develop neutron activation analytical methods t o determine the sodium content of high purity silicon where large samples were available (0.1 t o 3 grams) and of very thin silicon dioxide films where many small samples were t o be analyzed. (10) National Academy of Sciences, National Research Council, “Nuclear Data Sheets,” Washington, 1958-62.
Sample Preparation and Irradiation. All bulk samples were in the form of large pieces o r slices (0.3 to 3 grams) of silicon which were to be analyzed for sodium content. All slices were cut with a diamond saw and chemically etched to remove all saw work damage. Although no information o n the surface concentration was desired, great care was taken t o minimize sample handling and to prevent sodium surface contamination prior t o irradiation. The reason for this circumspection before irradiation was the complete lack of data on the diffusion coefficient of sodium into silicon coupled with the lack of assurance that post irradiation etching would remove all traces of sodium from cracks o r fissures in pieces of polycrystalline silicon. The bulk pieces of silicon were carefully wrapped in high purity reactor grade zirconium foil and sealed in a quartz tube along with the flux monitor. The flux monitor used was sodium chloride sealed in a small quartz tube. The oxides for analysis were films 1000 t o 10,000 angstroms thick formed on 1-inch diameter single crystal silicon slices. Unlike the bulk silicon samples, surface sodium concentrations were most important, both in understanding of device characteristics and in determining the type and source of sodium contamination. Since surface concentration was considered so important, preirradiation handling of the sample was rigorously controlled. The sample slices were wrapped in zirconium foil or placed in quartz holders and sealed in a large quartz tube with the sodium flux monitor. All irradiations were carried out a t the Union Carbide reactor for 24 hours a t a flux of 1 X 1013n/sec/cm2. Samples were returned to the counting facility within 6 t o 7 hours after removal from the reactor. Post Irradiation Processing. All bulk samples were unwrapped and etched in planar etch (15 parts HN03-5 parts CH3COOH-2 parts H F by volume) for 1 minute, rinsed in deionized water, dried, weighed, and counted by either multichannel gamma ray spectrometry or gamma-gamma fast coincidence techniques. The silicon slices and the oxide films were unwrapped and rinsed in 6 N hydrochloric acid t o determine the amount of soluble sodium o n the surface of the oxide. Since there is generally oxide on both the front and back faces of the silicon slice, it is necessary to remove the back oxide completely. This is accomplished by masking the front face with polypropylene tape and etching off the back face with 5 hydrofluoric acid. This solution containing the back oxide is retained and counted. The masking is removed from the front face and the surface cleaned with trichloroethylene. The total oxide thickness was then measured with a Gaertner Model L-119 e1lipsom:ter. The oxide film was incrementally removed in 50-200 A steps by etching in 5 z by volume hydrofluorjc acid. These oxides etch a t a very reproducible rate (350 A/min for HF) and more or less oxide can be removed by changing the etching time. After each etch, the total thickness of oxide remaining was determined by a n ellipsometer measurement. The etch solutions were counted in a NaI well counter. Counting Procedures and Equipment. MULTICHANNEL GAMMA RAYSPECTROMETRY. A 512-channel Nuclear Data Model 130AT gamma spectrometer with a 3 X 3 inch sodium iodide crystal was used for the gamma ray spectrometry. Each spectrum was analyzed for sodium using a linear least squares fitting computer program as described by Helmer et al. (11) and Heinen et al. (12). GAMMA COUNTING USINGENERGYDISCRIMINATION. Etch solutions from the incremental etching of a silicon dioxide
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(11) R. G. Helmer, R. L. Heath, D. D. Metcalf, and G. A. Cazier, ZDo, 17015 (1964). (12) K. G . Heinen and G. B. Larrabee, ANAL.CHEM., 38, 1853, (1966). VOL. 39, NO. 10, AUGUST 1967
1145
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Figure 4. Schematic of gamma-gamma fast coincidence counting system
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Figure 3. Typical sodium concentration profile through a silicon dioxide film (7000
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film were counted in a system where the base line of the amplifier was set to pass only those gammas with energies greater than 1.5 MeV. Interference from other radioactive isotopes with gamma energies greater than this value was checked by analyzing the back oxide using gamma-ray spectrometry. A typical profile of the sodium distribution through a n oxide, determined by this technique, is shown in Figure 3. The U-shaped profile agrees qualitatively with the model proposed by Snow ( I ) and with the results of Yon et al. (5). GAMMA-GAMMA FASTCOINCIDENCE COUNTING.The use of both energy and time constraints in gamma counting is a technique very common t o the nuclear physicist but used only infrequently by the analytical chemist. In this technique, Figure 4, two detectors are used to count the sample, where one detector looks only a t 2.75 MeV gamma and the other looks only a t 1.36 MeV gamma. Since these two gamma are in coincidence (IO), a coincidence unit is introduced which requires that the gamma be detected within a given time period, 54 nanoseconds in this work. The probability of occurrence of accidental coincidence counts is controlled by the count rate in each channel and the resolving time (27) of the coincidence system. This resolving time is determined by taking a “cable curve” where 27 is defined as the measured full width a t half maximum. The 27 cable curve obtained, using sodium-24, on the system used in this work is shown in Figure 5 and is determined by introducing delays from a Hamner Model 30 variable delay line. A Hamner AEC modular fast-fast gamma-gamma coincidence system was used in this work. RESULTS
Evaluation of Interfering Nuclear Reactions. The neutron activation analysis for sodium using the thermal neutron n, y reaction may be complicated by the production of sodium-24 by the fast neutron reactions with aluminum [27Al(n,a)24Na] and magnesium [ 24Mg(n,p)24Na].The capture cross sections for the competing fast neutron reactions are significantly lower than the primary reaction (13). The neutron threshold energies required are much higher and in a nuclear reactor the neutron flux with energies greater than 3.1 MeV is significantly lower than the thermal flux. (13) C . N. Hogg and L. D. Weber, ASTM Special Technical Publication No. 341, 134 (1963).
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While all these factors are in the analyst’s favor, there was a probability that the levels of sodium found could have come from these low yield interfering reactions. This is particularly true for silicon slices which are lapped with a n alumina polishing compound. After lapping, the slices are meticulously cleaned, then chemically etched to a high mirror polish prior t o oxidation t o grow the 1000- t o 10,000-A silicon dioxide surface films. The level of interference from these interfering reactions was evaluated by irradiating duplicate samples wrapped in cadmium foil (with a similarly wrapped monitor) and unwrapped or bare samples with a monitor. Care was taken to ensure that oxidized silicon slices that had been processed together were used for this study. The cadmium-wrapped samples were shielded from thermal neutrons and only sodium-24 from fast neutron reactions was produced. On the other hand the bare sample had sodium-24 produced from both thermal [23Na(n,y)24Na] and fast neutrons [24Mg(n,p)24Na], [27Al(n,a) 4Na]. An evaluation of the irradiation was carried out and the results of the true sodium us. total sodium from sodium, aluminum, and magnesium are shown in Table I. The agreement is excellent which shows that the interference from aluminum and magnesium in this type of samples is very small. Preirradiation Packaging. The usual method of wrapping samples for neutron activation in aluminum foil is particularly unsuited for the samples under investigation in this work because of the sodium-24 produced by the 27Al(n,a)24Nareaction. To evaluate the overall problems of handling and preirradiation packaging, a group of identical unoxidized silicon slices was packaged in three different ways under closely controlled environmental conditions. ALUMINUM FOIL. The silicon slices were separated with aluminum foil spacers, and the entire package was wrapped in aluminum foil. ZIRCONIUM. The silicon slices were separated with 0.007inch reactor grade zirconium foil spacers, wrapped in zirconium foil, then in aluminum foil. QUARTZ.Special quartz irradiation containers were fabricated approximately 1 cm in height and 2.6 cm i.d. The container was only slightly larger than the silicon slices and had a quartz cover that fitted inside the container resting on the inside slices. Using silicon slices identical to those for analysis as spacers, the slices were stacked into the quartz container. The quartz cover was placed o n the top, and the container was wrapped in aluminum foil and submitted with the flux monitor for irradiation. The top and bottom slices which served as spacers were discarded, and the remaining silicon slices were analyzed.
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DELAY (NANOSECONDS) Figure 5. Cable curve to determine 2 T resolving time of coincidence system The results of the different wrapping techniques are shown in Table 11, where the slices were first analyzed as received, for sodium, using gamma ray spectrometry. The aluminum wrapped slices showed significantly higher apparent sodium content. This result is perhaps not too surprising since considerable sodium-24 would be produced by the 27Al(n,~)24Na reaction and some would be transferred t o the silicon slices, The slices were then rinsed in 6N hydrochloric acid and twice in deionized water to remove the free sodium on the surface, and the apparent sodium concentration in the HC1 rinse was determined by gamma counting. There was only 3 t o 5 times more sodium-24 in the HC1 rinse from the aluminum wrapped slices, even though there was 100 times more sodium in the slice as received and unwrapped. It is assumed that the sodium-24 either diffuses into the silicon during irradiation, or is imbedded in the silicon by some recoil mechanism from the 27Al(n,~)24Na reaction. We are unable t o explain how sodium-24 from the aluminum foil becomes so intimately attached t o the silicon slice. However, since it is not removed by standard washing techniques, very misleading conclusions could be drawn from this aluminum-wrapped irradiation. As a further illustration of how the results from this irradiation can be misinterpreted, the silicon slices were etched with a planar etch (15 "03-5HAc-2 H F by volume) to remove 7-8 microns of the outer surface of the silicon. The sodium-24 content in this etch was determined by energy discrimination gamma counting, and the concentration of sodium calculated for this small volume of silicon removed. As can be seen, the results are expressed in part per million and indicate serious sodium Contamination even though in reality the sodium-24 came from the aluminum foil wrapping. Finally, the sodium content of the bulk silicon slice after etching was determined using gamma ray spectrometry. All the sodium values after etching off the outer layer are in reasonable agreement with those from the unoxidized slice which came from the same silicon single crystal.
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