Process for forming thin films of metastable binary compounds

Abstract: A process for forming chromium dioxide thin films which are receptive to high density magnetic re­ cording. The process comprises depositing both chromium and oxygen on a substrate by evaporative techniques and concurrently bombarding the substrate with high energy ions of at least one of the film constituents to form a latent CrO x film forming layer. The process is carried out at approximately room temperature. The as-grown latent film forming layer is subsequently heat treated by a rapid thermal anneal step which raises the temperature of the as-grown fillm to about 500°C. The rapid thermal anneal step preferably comprises a series of at least five separate pulses over a 10-second time span. After the rapid thermal anneal, the sample is rapidly quenched to room tem­ perature.

Description: The present invention relates to a process for forming thin films of metastable binary compounds, such as CrO₂, and more particularly to a new process for forming a thin-film CrO₂ high density recording medium for computer data storage. Chromium dioxide (CrO₂) is well known for its use as a recording medium in high grade magnetic recording tapes. It is used in particulate form. This particulate form of the material prevents its application to high density (gigabit) storage. It has not been possible previously to make acceptable smooth, continuous thin films of CrO₂. This results from the metastable nature of CrO₂ at room temperature. The only previously known method of depositing thin films of CrO₂ have been by chemical transport methods ("Epitaxial Growth of CrO₂ on sapphire in air", Japan J. Appl. Phys., Vol. 17, (1978), 249.). This transport method used flowing O₂ at atmospheric pressure and high temperatures. The substrates are also heated. Due to the process control for the required film thickness, morphology, e.g., roughness, this method leads to additional processing requirements in order to obtain acceptable disc recording material. Attempts to deposit CrO₂ by conventional reactive vapor deposition, diode sputtering, magnetron sputtering, and ion beam sputtering have been unsuccessful. Particulate CrO₂ is usually made using equilibrium processing at high pressures and elevated temperatures. The particulate CrO₂ method is not compatible with the highest densities possible with disk recording. Theoretical calculations have indicated that CrO₂ is also a useful recording material for optical recording ("CrO₂ predicted as a half-metallic ferromagnet", K. Schwarz, J. Phys. F. Met. Phys. Vol. 16 (1986) Op. 211-215.). The Faraday rotation is -5.2X10⁴deg/cm for light of 2.5 eV photon energy. Rapid thermal annealing is a known method step in semiconductor device processing which has been in use over the past few years. This method has been used in silicon device technology to: a) activate ion implanted impurities, b) grow thin films of SiO₂ on silicon, c) generally to crystallize disordered or amorphous layers (see for example "Handbook of Thin Film Technology" edited by L.I. Maissel and G. Glang, McGraw-Hill Book Company, New York i.a. 1970, pp. 13-36). In this method, the sample is brought up to a very high temperature (700 - 1000°C) in very short time (order of few milliseconds and shorter). Commercial equipment is available to do this flash annealing process. When CrO x (1.6 < x < 3) coated samples produced by conventional reactive vapor deposition processes and post-heat treated under equilibrium or near equilibrium conditions in oxygen, for example, the film converts from an amorphous mixture to a crystalline Cr₂O₃ phase. Also, when these vapor deposited films are subjected to rapid thermal annealing, a non-equilibrium heat treatment technique, the same crystalline Cr₂O₃ phase is produced. This is because Cr₂O₃ is the thermodynamically stable oxide state in the Cr-O system. It is a primary object of the present invention to provide an improved process for forming smooth, continuous thin films of metastable binary compounds. It is a further object of the present invention to provide an improved method for forming chromium dioxide (CrO₂) films particularly useful for high density magnetic recording. The objects of the present invention are accomplished by a process as disclosed in claim 1. Preferably the thin films of metastable binary compounds consist of chromium dioxide, which films are receptive to high density magnetic recording. A method for manufacturing magnetic recording mediums consisting of Co and at least one kind out of P, As, Sb and Bi is disclosed in Patent Abstract of Japan, vol. 8, no. 63, (E-233) (1500), 24th March 1984; from JP-A-58 212 118 (Hitachi Condenser), where Co is evaporated and the at least one other constituent is irradiated to be adhered on the surface of the substrate from an ion source. The method is not used for forming a smooth continuous layer of a metastable binary compound. A preferred embodiment of the inventive process comprises depositing a film forming layer on the substrate by vapor depositing chromium atoms from a metallic source onto the substrate. Concurrently forming a high energy beam of O₂+ ions and impinging same at approximately room temperature upon said substrate utilizing evacuated high energy ion beam appparatus, said film forming layer comprising an amporphous layer of CrO x (where x varies form 1.6 to 3) having a plurality of metastable nuclei therein. The steps are carried out at approximately room temperature. The process is enhanced if the substrate is maintained at a temperature of between 10 and 40°C. The as-grown film forming layer is subsequently treated by a rapid thermal anneal step which raises the temperature of the as-grown film to about 500°C within a time period of about 10 seconds after preheating the film to about 400°C for about 90 seconds. The rapid thermal anneal step preferably comprises a series of a least five separate pulses over said 10-second time span. After the rapid thermal anneal the sample is rapidly quenched to room temperature. The film forming chromium dioxide mixture may be deposited by other energetic particle bombardment means than ion bombardment during deposition prior to the rapid thermal anneal. In another preferred embodiment of the inventive process suitable anisotropic dopants are introduced into the process during the concurrent vapor deposition and ion bombardment step in order to tailor the magnetic properties. Other objects, features and advantages of the present invention will become apparent from the subclaims, the subsequent description of the invention and the accompanying drawings. Fig. 1 comprises a cross-sectional view of typical apparatus which may be utilized in carrying out the present process for fabricating chromium dioxide thin films by evaporation with concurrent reactive ion bombardment. Fig. 2 comprises a series of curves with stoichiometry plotted versus the number of O₂ ions per chromium ion arriving at the target surface for a number of different ion beam energy levels. Fig. 3 comprises a curve plotting the stoichiometry of chromium and oxygen versus the temperature effect on said stoichiometry, as the produced film varies between Cr₂O₃ and CrO₂, the desired constituency. Fig. 4 comprises a curve with the stoichiometry in the film plotted versus the background oxygen pressure. Fig. 5 comprises two temperature versus time curves for the rapid thermal anneal step of the present process. Fig. 6 comprises a cross-sectional view of an alternative apparatus for providing energetic particle bombardment of the sample surface onto which the film mixture is deposited. Fig. 7 comprises a cross-sectional view of a flash-lamp activated rapid thermal anneal apparatus. The present invention consists of a novel CrO₂ thin film structure and a process for making that structure. A plurality of steps are involved in the process. These are generally described below. First the structure (device) will be described, then the processing steps, and then some more structural and process considerations will be presented. The structure is a CrO₂ thin film recording medium containing suitable anisotropic dopants with a thickness that is acceptable for the required magnetic recording bit density. This film is deposited onto a suitable substrate. For a magnetic recording medium, a substrate such as an aluminum alloy disc or a silicon wafer is used. The recording medium is then protected with a Cr₂O₃ film whose thickness is consistent with read-write head and recording medium separation. The thickness of typical films would be 100 nm for the CrO₂ thickness and 15 nm for the Cr₂O₃. To optimize the CrO₂ magnetic properties, anisotropic dopant materials such as Fe, Co, Sb, In, etc. (or their compounds) may be added to the CrO₂. These techniques are well-known to those working in the magnetic recording field, e.g., "Advances in properties and manufacturing of chromium dioxide", H.Y. Chen, D. M. Hiller, J. E. Hudson, and C.J.A. Westembroek, IEEE Transactions on Magnetics , Vol. MAG-20, (1984) 25; and "Synthesis and properties of ferromagnetic chromium oxide", T.J. Sowboda, P. Arthur, N.I. Cox, N. Ingraham, A.L. Oppegard, and M.S. Sadler, J. of Applied Physics, Sup. to vol. 32, No.3, pg. 374S. The process for depositing one or both of the films involves simultaneous vapor deposition of chromium and ion bombardment of the growing film with energetic oxygen particle ions, atoms or molecules. The relative metal atom, ion, and neutral background molecule fluxes and ion energies are determined by material requirements known to those skilled in the art. Both the CrO₂ and Cr₂O₃ films (if the latter is desired as an insulation or protective layer) can be deposited in one evaporation process cycle. The transition from one compound to the other, which is determined by the relative fluxes and energies of the particles arriving at the growing film surfaces, will be described subsequently. The ion bombardment concurrent with film deposition produces an initial latent CrO x film mixture having active centers/sites that direct the path for nucleating the metastable nonequilibrium phase CrO₂. This reaction path requires that the chromium atom to oxygen ratio be in a range close to the CrO₂ composition, that the active centers/sites be induced by energetic particle bombardment (preferably oxygen), and that the thermal anneal be rapid and highly nonequilibrium. The as-deposited mixture films are amorphous and have poor magnetic properties. The magnetic properties of the amorphous CrO₂ are improved by nonequilibrium heating (e.g., rapid thermal annealing). This rapid thermal processing, to the knowledge of the inventors, has never been applied to nucleate or grow particular chromium oxide binary compound films. It has been used in silicon processing. The nonequilibrium nature of the annealing is used to prevent the non-ferromagnetic Cr₂O₃ (the lowest energy phase) from growing. If the films have not been bombarded with ions during growth, the ferromagnetic CrO₂ state will not grow using this process. Accordingly, the ion bombardment, of one type or another, is a critical process parameter. The CrO₂ is deposited by vapor depositing the Cr at a rate corresponding to 1.0-1.2 nm/sec while simultaneously bombarding the film with O 2 + ions with an energy of 200-600 eV and beam current density of 0.5-2.5mA/cm² with the substrate held at between 10-40°C. These depositions should be done with an O₂ background pressure of between 6.67x10⁻³ and 10.66.x10⁻³ Pa (5X10⁻⁵ and 8X10⁻⁵ Torr). Cr₂O₃ may be deposited as part of the same overall process using procedures similar to the CrO₂ except that the deposition parameters are somewhat different. This insulating Cr₂O₃ material is deposited with substrate temperatures between 40°-80°C using lower energy O 2 + ions or mixtures of Ar⁻ and O 2 + ions. For these films O 2 + ion energies of 200-600 eV and beam current densities of 0.5-2.5 mA/cm² in an C₂ background pressure of 10.66x10⁻³ Pa (8.X10⁻⁵ Torr) are required. When mixtures of Ar and O₂ are used in the ion source, e.g., 50% of each, Cr₂O₃ films are always obtained. FIG. 1 shows a simplified apparatus suitable for practicing the present invention. A chromium source 10 is placed on a suitable surface 12 for heating (not shown). Oxygen is introduced through a suitable ionization device 14 from which it is directed onto a sample substrate 16. Substrate heating and temperature control is also provided. The process is carried out in a suitable containment chamber 18 which is evacuate by connection to a vacuum pump. Suitable energies are imparted to the Cr atoms and O₂ ions by biasing the various elements within the ranges taught herein. The magnetic properties of the amorphous CrO₂ are improved by substrate heating. It has been found that utilizing a rapid thermal anneal as the heating process produces greatly improved results. As far as is known this rapid thermal processing anneal has not been applied to nucleate and grow binary compound films such as the oxides of chromium where the oxide is grown from an essentially amorphous mixture with nucleating sites introduced by energetic bombardment producing a film mixture. EXAMPLES As described above, oxides of chromium, like CrO₂ or Cr₂O₃, can be readily grown by this method. Thin films of CrO x , (where x varies from 1.6 to 3) have been obtained by evaporation with simultaneous ion bombardment in an oxygen background pressure of 5.33x10⁻³ to 2.67x10⁻² Pa (4x10⁻⁶ to 2x10⁻⁴ Torr). The films obtained were 1500Å thick. Silicon substrates and insulating substrates such as quartz, sapphire, amorphous SiO₂ on silicon, etc. were used. By X-ray diffraction the films were amorphous. These films were subjected to rapid thermal annealing in a setup heated with four 1kW flash lamps 70 (as shown in Fig. 7). The peak temperature was controlled by an automatic power controller (not shown). The samples 76 were placed on a quartz platform 74. The whole annealing step is carried out in a suitable container in an air or oxygen atmosphere. Reflectors 78 perform the function of concentrating the energy produced by the flash lamps 70 into the samples 76. Other rapid thermal processes such as electron bombardment, as is well known in the art, laser flash annealing or other energetic proton beam processes such as microwaves. A time versus temperature profile is shown in Fig. 5. Soak temperatures of 400, and 430°C were used. These were reached with a very steep temperature-time ramp. The samples reached the desired temperature (as measured by a thermocouple placed near the sample) within 40 seconds. At this temperature, the samples were soaked (kept at the elevated temperature) for periods of time ranging from 30 seconds to 2 minutes. After this soak period, the specimens were given uniformly 5 flashes over 10 seconds. The peak temperatures reached were 490°C. As soon as the peak was reached the controller was switched off and the samples quenched. After each flash anneal, the samples were examined and a) their conductivity (resistivity) measured and b) x-ray diffraction pattern taken. The resistivity varied from very high values (1000 Ohm/cm) to fractions of an Ohm-cm after annealing (CrO₂ is a conducting oxide). In general, the x-ray patterns showed no structure to start with but crystallites grew to a large size as successive anneals were carried out. Films which were deposited with simultaneous ion bombardment, O 2 + ion energies of 150 eV, 270 eV, and 400 eV, showed excellent conversion of the film mixture to polycrystalline CrO₂ after pulse annealing. With O 2 + energies both higher and lower than these values (<150 eV and > 500 eV) the films tend to crystallize into the Cr₂O₃ structure. This could be seen in x-ray diffraction patterns. No pattern was discernible at 490°C; however, the CrO₂ pattern became clearly visible with a peak temperature of 502°C. Similar diffraction patterns for Cr₂O₃ could clearly be seen. As stated previously heat sources other than radiant energy (e.g., flash tube) can be used for the rapid thermal annealing. For example, electron beams can be used. In situ observations of electron beam rapid thermal annealing into the CrO₂ phase have been made in a Transmission Electron Microscope (TEM). The structure is self-evident from what is stated above. For in-plane magnetic recording the shape anisotropy will force the magnetization into the plane of the film. From methods that have been developed in the laboratory, anisotropy directions can be controlled. For example, ion bombardment with an ion velocity component parallel to the film surface has been used to create an anisotropy direction in the plane of the film. In studying the formation of metastable compounds such as CrO₂, films have been deposited by thermal evaporation from a metallic Cr evaporation source while bombarding the growing film with either O 2 + ions or a mixture of Ar⁺ and O 2 + ions from a multiple aperture Kaufman ion source. Due to the operation of the ion source, there is also a neutral molecular bombardment of the growing film from gas that escapes from the ion source. The composition of films produced were measured in situ using Rutherford backscattering. The chemical bonding of the oxygen and chromium is measured using Electron Spectroscopy for Chemical Analysis (ESCA). When using mixtures of Ar and O₂ gases in the ion source, it was not possible to incorporate more oxygen in the film than that corresponding to the Cr₂O₃ stoichiometry. Films with a higher oxygen content than the CrO₂ composition have been formed by depositing the films at a temperature of -190°C. Deposited latent mixture films are amorphous. This has been established by X-ray diffraction. ESCA measurements show that films deposited with a CrO₂ composition do have CrO₂ chemical bonding along with other phases of Cr-O bonding. Although the films are amorphous, films that are ferromagnetic can be deposited at room temperature. Vibrating sample magnetometer measurements of the CrO₂ films show ferromagnetism in these films. By rapidly thermally annealing amorphous CrO₂ films, it has been possible to optimize or enhance the magnetic properties of the as deposited films to that expected for crystalline CrO₂. Vibrating sample magnetometer measurements of such films treated by the rapid thermal anneal step of the present invention have clearly established this. The methods described above are believed to be extendable to other energetic particle bombardment processes such as magnetron sputtering [1, 6, 7], ion beam deposition [3], diode sputtering [2], arc evaporation [5] and ion plating [4]. Literature references describing these processes are set forth below. What these other deposition processes have in common with the previously described O₂ ion beam bombardment/Cr evaporation deposition process are energetic ion (particle) bombardment while growing the film. In these other processes the following process parameters need to be controlled: (1) ion energy, (2) relative arrival rates of the Cr atoms and the O₂ ions, and (3) the substrate temperature. Fig. 6 is a schematic cross-section of a magnetron sputtering set-up in which energetic Cr ions impinge on the sample substrates 60. A chromium cathode 61 is connected to a suitable dc or rf supply 62 with a matching network, as in the lower sample support 63. The magnetron produces a magnetic field 64 via pole pieces 65. This field in turn produces intense glow regions 66 and the less intense glow region 67. Energetic chromium ions (Cr) are shown moving toward the sample substrates 60. The O 2 + ions are also shown. The Ar⁺ ions are shown also. These would be present when it is desired to control the stoichiometry of the film being deposited to form a Cr₂O₃ layer. The Ar flow would be stopped when it is desired to form the latent film mixture favorable to CrO₂ formation. This entire apparatus would of course be placed in a containment chamber (not shown) as would be understood by those skilled in the art. It is known that the only conducting state in the Cr-O x system is the CrO₂ state. CrO₂ films produced by the present process are conducting. Heat treating using the herein disclosed rapid thermal annealing techniques significantly reduce the resistivity in the CrO₂ films. This is further evidence of the conversion of other chrome oxide states which may exist in the initial film mixture into the CrO₂ state. REFERENCES [1] "Thin Film Processes," J. L. Vossen and W. Kern, Academic Press (1978), pp. 76-173. [2] Op . Cit ., pp. 12-73. [3] Op . Cit ., pp. 175-206. [4] "Deposition Technologies for Films and Coatings," R. F. Bunshah et al., Noyes Publications (1982), pp. 244-287. [5] Op . Cit ., p. 108 and references therein. [6] "Plastic Flow in Ion-Assisted Deposition of Refractory Metals," B. Window, F. Sharples and N. Savvides, J. Vac. Sci. Technol ., A6 (1988), pp. 2333-2340. [7] "Hollow-Cathode-Enhanced Magnetron Sputtering," J. J. Cuomo and S. M. Rossnagel, J. Vac. Sci. Technol ., A4 (1986), pp. 393-396.

Claims: Process for forming thin films of metastable binary compounds, such as CrO₂, which comprises: depositing on a substrate the constituent materials by a process selected from the group of evaporation, where the substrate is bombarded with high energy ions, such as O 2 + and - in the case of CrO₂ - chromium atoms or ions, RF where necessary sputtering, magnetron sputtering, diode sputtering, ion plating, ion beam sputtering or arc evaporation - in the case of CrO₂, of metallic chromium in an O₂ atmosphere wherein chromium atoms or ions provide the energetic particle bombardment -, subsequently treating the as-grown film forming layer with a rapid thermal anneal heat treatment which causes the film forming layer to form the desired crystalline metastable binary compound. Process as set forth in claim 1 wherein the binary metastable compound is chromium dioxide and wherein the rapid thermal anneal step includes heating the film forming layer to about 500°C after first soaking the film to raise its temperature to about 400°C. Process as set forth in claim 1 for forming a chromium dioxide (CrO₂) thin film on a substrate suitable for high density magnetic recording comprising the steps of: depositing a film forming layer on the substrate by vapor depositing chromium atoms from a metallic source onto the substrate; and concurrently forming a high energy beam of O₂+ ions and impinging same at approximately room temperature upon said substrate utilizing evacuated high energy ion beam apparatus, said film forming layer comprising an amorphous layer of CrO x (where x varies from 1.6 to 3) having a plurality of metastable nuclei therein; subsequently performing a rapid thermal anneal heating operation on said latent film at approximately atmospheric pressure, said heating step comprising first heating the substrate to a first level and then rapidly heating same to approximately 500°C for a period of between about 5 and 10 seconds; and after the rapid thermal anneal, rapidly quenching the sample to room temperature. Process for forming thin films as set forth in claim 3 including maintaining said substrate at a temperature between approximately 10 and 40°C during the deposition step of said CrO x film forming layer. Process for forming thin chromium dioxide films as set forth in claim 3 wherein the step of providing high energy beam of O₂+ ions further includes impinging said O₂+ ions on said substrate in a direction substantially parallel to the surface to enhance the magnetic anisotropy of the film. Process for forming chromium dioxide thin films as set forth in claim 3 further including the step of vapor depositing at least one anisotropic doping agent on the substrate together with the chromium and oxygen chosen from the group including Fe, Co, Sb and In to enhance the magnetic anisotropy characteristics of the resulting CrO₂ film. Process as set forth in claim 3 wherein metallic chromium atoms/ions are deposited onto the substrate via magnetron sputtering in an oxygen atmosphere. Process for forming a chromium dioxide thin film as set forth in claim 3 wherein the rapid thermal anneal step includes: first heating the substrate having the film forming layer thereon to approximately 400°C; and then rapidly raising the temperature to approximately 500°C as a series of at least 5 pulses within a 10-second interval; and then rapidly quenching the substrate to room temperature. Process for forming chromium dioxide thin films as set forth in claim 3 including the step of forming a second layer comprising Cr₂O₃ on top of the CrO x film forming layer, said last step comprising discontinuing the ion bombardment of the substrate and increasing the arrival ratio of chromium to oxygen to approach the stoichiometry of Cr₂O₃ and raising the substrate temperature to between about 40-80°C.

Inventors: BREZOCZKY BLASIUS, CUOMO JEROME J, GUARNIERI C RICHARD, RAMANATHAN KUMBAKONAM V, BREZOCZKY, BLASIUS, CUOMO, JEROME J., GUARNIERI, C. RICHARD, RAMANATHAN, KUMBAKONAM V.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: G11B5
IPC codes: G11B5, C23C14