Contains reports on two research projects.U. S. Navy Bureau of Ships under Contract NObsr 7760

Baker, A. G.

Ellis, James R., Jr.

Loeb, A. L.

Robinson, C. C.

DSpace@MIT

XVII. COMPUTER COMPONENTS AND SYSTEMSDr. E. W. Fletcher J. M. Ballantyne A. H. NelsonProf. A. L. Loeb R. Casale S. B. RussellProf. C. C. Robinson C. K. Crawford F. W. Sarles, Jr.T. More, Jr.A. FUNDAMENTAL PHYSICS OF THE THIN-FILM STATE1. MAGNETO-OPTIC EFFECTS IN FERROMAGNETIC THIN FILMSIf a ferromagnetic thin film is magnetized in a direction longitudinal to the plane ofincidence, there is a magnetically induced change in the polarization of the reflected andthe transmitted light. When the incident light is plane polarized in the plane of incidence,a second component appears in the reflected and the transmitted beams perpendicular tothe plane of incidence. Thus, the incident plane-polarized light is reflected and trans-mitted elliptically polarized, with the azimuth of the major axis of the ellipse rotatedslightly away from the plane of incidence. By a reversal of the magnetization of thefilm, the sign of the ellipticity is changed and the major axis is rotated symmetricallyto the other side of the plane of incidence. The polarization change in the reflected beamis called the "Longitudinal Kerr Effect," and, for lack of a better name, the change inthe transmitted beam will be called the "Longitudinal Faraday Effect." The former effecthas been discussed at length in previous reports; in the present report only the theoryfor the longitudinal Faraday effect will be covered.The film configuration for which the derivation was made is shown in Fig. XVII-1.DIELECTRIC FERROMAGNETIC DIELECTRIC2 3X7MAGNETIZATIONFig. XVII-1. A ferromagnetic thin film between two dielectrics.The amplitude of the electric field obliquely incident on the ferromagnetic from medium1 is I, and the amplitude of the electric field transmitted into medium 3 is T, and sub-scripts p and s refer to a component plane polarized in the p or s direction. The formThis work was supported in part by the U. S. Navy Bureau of Ships under ContractNObsr 77603.291(XVII. COMPUTER COMPONENTS AND SYSTEMS)of the transmission coefficients is, then,T =t I +t I (1)p pp p ps sT =t I +t I (2)s sp p ss sThe coefficients t and t are the ordinary transmission coefficients associatedpp sswith an unmagnetized metallic film and are evaluated in the conventional manner. Theirform is shown in the following equations.-jp/212 23 ep pt r (3)PP - r23 r21 ep st12 t23 ets s (4)ss -JP-23 r21 es swhere4TTd cos 2(5)P=oIn Eqs. 3, 4, and 5, the t's and r's are the transmission and reflections coefficientsat the interfaces designated by the subscripts under the conditions that both media areinfinitely thick. The light is incident from the medium given by the first subscript.(Any optics textbook will supply these coefficients. )By following a similar procedure, the Faraday transmission coefficients are foundto be:t12 23 - j p /s pst= (6)l-r r eps - r23 21 ejp st12 t23 e-j p / 2p spt = (7)sp 1 - r23 r21 ep pIn Eqs. 6 and 7 the transmission coefficients with the double-letter subscriptsprovide the change in polarization with magnetization and are the source of the magneto-optic effect. Expressions for the magneto-optic coefficients t 2 3  and t 2 3  areps spgiven by292(XVII. COMPUTER COMPONENTS AND SYSTEMS)Q sin o2 s 2  z1 (z x3 zx2)t =i +ps cos c Z + Z Z+Z1 z 2 x 3 x 2Zz 3(Zz3 z2)Z +Zz z+ 2 1(Z+Z) (ZZ)sin 2 cos 1t = -iQsp Lcos 2(Z -Z) Zz z 3 Zz 2  zx 2(Z+Z) Z + Zzwhe3  z2 re x2  x3rewherez2Trrd z3+ Z +zz3 z2z z +Zxl x1  x3K1 +Z (Z +Z (Z +Z)1 23d x2cos 2 2  Z +xZcos #2 x2 x31/21 ox EN cos (o0cos uooz N EIn these expressions, N with the proper subscript is the index of refraction of themedium in question (N 2 will be complex). The angle is the angle between the directionof propagation and the y-axis and is assigned a subscript according to the mediumtraversed. The letter Q is the complex magneto-optic amplitude that was first intro-duced by Voigt and applies only to the ferromagnetic film.This derivation is based on the assumption that only the first few internal reflectionsof a light ray affect the transmission through the film. For films thicker than 1/8 of awavelength, this approximation should be quite valid.C. C. Robinson2. DIELECTRIC FILMS FORMED BY DECOMPOSITIONThe electron gun previously used for metallic deposition (1, 2) has been used to formdielectric layers by electron-impact decomposition of tetraethoxysilane. This depositionmethod has been used by others (3,4, 5) to make thin insulating films for etchingprocesses and tunneling devices.Three-layer (gold-dielectric-gold) capacitors were made in order to determinesome of the dielectric properties of these insulating films. In this experiment, 1200 ev293(XVII. COMPUTER COMPONENTS AND SYSTEMS)electrons with a substrate current density of approximately 1/10 ma/cc2 were used todecompose the absorbed layers of the silane vapor. A 20-minute exposure at 1 1 Hgtotal pressure, as measured by a conventional ionization gauge, was used. Dissipationfactors of approximately 50 per cent which were found on 5 runs suggest that a highcarbon deposition is taking place along with the desired silicon-oxygen film deposition.It is still not known whether the carbon is coming from the ethyl radicals, or from thedecomposition of organic contaminents present in the system.Under the most ideal conditions, it should be possible to break the ethyl bonds andleave only Si, O combinations on the substrate. Such a film should have excellent dielec-tric properties. To do this, an energy source that is above the ethyl radical bond energylevel, but below the bond energies holding the radical together, is needed. With thehigh-energy sources that are being used it is thought that a polymerization process (3, 4)occurs and gives films composed of C, H, O, Si. Other experiments show, however, thatthe use of very low-energy beams, (energies below the first secondary emission cross-over of the substrate-dielectric film combination) is, perhaps, impractical because ofcharging effects that cut off the beam.From this work and from the work of Christy (4), it is concluded that dielectric filmscan be made that will be useful for field emission devices and other applications.A. G. Baker, J. R. Ellis, Jr.(Mr. Allen G. Baker is now at Lincoln Laboratory, M. I. T. )References1. A. G. Baker and W. C. Morris, Deposition of metallic films by electron impactdecomposition of organometallic vapors, Rev. Sci. Jnstr. 32, 458 (1961).2. E. W. Fletcher, A. G. Baker, and W. C. Morris, Electron decompositionplating, First Joint Progress Report of the Laboratories for Molecular Science andMolecular Engineering, M. I. T., January 1961, p. 159.3. D. A. Buck and K. R. Shoulders, An approach to microminiature printed systems,a paper presented at the Eastern Joint Computer Conference, Philadelphia, December1958.4. R. W. Christy, J. Appl. Phys. 31, 1680-1683 (1960).5. It has come to our attention that I. Giaever, of General Electric ResearchLaboratories, Schenectady, New York, has done some work in this field.3. METALLIC FILMS FORMED BY DECOMPOSITIONMetallic films of Pb and Sn made by the electron-beam decomposition method des-cribed by Fletcher, Baker, and Morris (1) are thought to have a high carbon content. Someof the films had a brown appearance, while others had bright metallic appearances. Itis thought that the carbon deposition is due to the decomposition of residual organic vaporin the vacuum system, and/or from the actual decomposition of the ethyl, methyl or294(XVII. COMPUTER COMPONENTS AND SYSTEMS)butyl radicals (1). The electron energies used in the initial experiments were approx-imately 1 KCV; however, the radical bonding energies are only a few volts.In order to have an operation that is such that only the radical-to-metal bond wouldbe broken, a secondary emission source was tried. A 1-KV electron beam was allowedto strike a copper plate at a 600 angle to the normal. A glass substrate with a stainlesssteel mask was placed parallel to the copper plate, approximately 2 cm above the strikingpoint on the plate.Nearly 90 per cent of the secondary electrons are usually found to be in the region0-10 volts with only approximately 3 per cent near the primary beam energy, and anearly cosine distribution is found to exist (2). For tetra-butyl tin, with a 20-minuteorun at 1 t , a metallic film, estimated to be approximately 400 A thick, was depositedon the metal mask, whereas no observable deposit was formed on the glass substrate.The glass surface probably attained a negative charge that enfeebled the striking beamcurrent.It can be seen that additional experiments should be made with low-energy electronsand metallic substrates.A. G. BakerReferences1. E. W. Fletcher, A. G. Baker, and W. C. Morris, Electron decompositionplating, First Joint Progress Report of the Laboratories for Molecular Science andMolecular Engineering, M. I. T., January 1961, p. 159.2. K. R. Spangenberg, Vacuum Tubes (McGraw-Hill Book Company, New York,1948), p. 52.B. CRYSTAL ALGEBRAThe structures analyzed thus far appear to fall into the following classes:A. Structures derived from stacked hexagonal nets.Al. Cubically stacked nets.A2. Hexagonally stacked nets.A3. Hexagonal nets stacked otherwise.B. Structures not derived from stacked hexagonal nets.For each of these classes, algorithms have been derived for locating all possiblesites; and for each structure analyzed, a distribution pattern has been derived to des-cribe the location or absence of ions at all sites.Examples of structures previously reported on are:Al: Rocksalt, Sphalerite, Spinel, Cesium Chloride, Perovskite;A2: Nickel Arsenide, Wurtzite, Olivine;A3: Quartz; and295(XVII. COMPUTER COMPONENTS AND SYSTEMS)B: Rutile.In the Quarterly Progress Report No. 61 (pages 270-274) the calcium silicide struc-ture was discussed. This structure, which belongs to class A3, will be dealt withfurther in the present report; other silicides belonging to A3 are also included. Finally,two structures of class Al will be discussed.a. The Calcium Silicide StructureIn the previous quarterly report, all possible ways were considered of stackingclose-packed calcium nets and silicon nets conforming to the Ca- and Si-environments.It was found that there are only two distinct stacking modes: one that wouldcorrespond to a polar structure and a small period along the hexagonal axis,and another that corresponds to a nonpolar structure, having a larger period.Comparison of these structures with experimental observations indicates thatthe latter stacking mode is the one that corresponds to the naturally occurringcalcium silicide structure.The following algorithm has accordingly been derived for the description of thecalcium silicide structure:Ca at h = 5k + 2h = 5kSi ath = 5k + 4where k is an integer.The location of the ions in each h-plane is given byv = 3L + [f(h, k)]mod 3w = 3M + [f(h, k)]mod 31 1f(h, k) = h- 2k - k2 mod 6 2 mod 2b. The Titanium, Chromium, and Molybdenum Silicide StructuresIn these structures each hexagonal net is occupied by both elements. As is the casewith quartz, the hexagonal nets are stacked directly above each other (f(h) = constant),but the description of the occupied sites requires subdivision into twelve subarrays (1,2).In terms of the notation described in these reports, the three structures are coded asshown in Table XVII-1. It is observed that these three structures differ from each otherin the variety of combinations represented, in a manner similar to the distinctionbetween hexagonal and cubic close packing.296(XVII. COMPUTER COMPONENTS AND SYSTEMS)Table XVII-1. Distribution patterns for three silicides.TiSi 2  CrSi 2  MoSi 2Ti: Si: Cr: Si: Mo: Si:ap a(qDr) ap a(qEr) ap a(qDr)cp c(qDr) cp c(q@r) bp b(qEr)dp d(q$r) bp b(q@r)bp b(q@r)c. Stannite and ChalcopyriteThe structures just discussed can all be built with the aid of the hexagonal-net modu-ledra (3). We have also returned to the study of class Al, and are considering twostructures that can be represented by proper stacking of tetrahedral and octahedralinterstitial moduledra. Since these models are now available in a variety of colors, itis possible to use them for structures in which anions are (cubically or hexagonally)close packed, and in which several different cations occupy the same h-plane. Two suchstructures are stannite (Cu 2 FeSnS4 ) and chalcopyrite (CuFeS 2 ). Both of these structuresare derivatives of sphalerite (ZnS), with zinc appropriately replaced by copper, iron,and tin ions. In sphalerite, one-half of all of the tetrahedral sites is occupied by zinc;in chalcopyrite, one-quarter is occupied by copper, and one-quarter by iron, while instannite one-quarter of all of the tetrahedral sites is occupied by copper, one-eighth bytin, and one-eighth by iron. Both of these structures are now being built with the aid ofmoduledra, and their distribution patterns are being derived.A. L. LoebReferences1. A. L. Loeb, Subdivision of hexagonal arrays, High Speed Computer SystemResearch, Quarterly Progress Report No. 10, Computer Components and SystemsGroup, M. I. T. , 30 April 1960, pp. 26-31.2. A. L. Loeb, Topology of Structures, High Speed Computer System Research,Quarterly Progress Report No. 11, op. cit., 31 July 1960, pp. 18-19.3. Ibid., loc. cit.297

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