Thin film magnetic storage medium and method for the manufacture thereof

Abstract: A magnetic storage medium is composed of a non-wett­ able substrate (10) upon which a transient liquid metal layer (18) is deposited and maintained as a distribution of discontinuous liquid features. A magnetic film layer (14) is deposited on the transient liquid metal layer (18) resulting in a reaction of the liquid metal with the magnetic film. The topology of the magnetic film is controllable by adjusting the thickness of the transient liquid metal layer (18).

Description: The present invention relates to a magnetic thin film storage medium with controlled grain morphology and topology and to a method for the manufacture thereof. The demand for increased capacity of storage media has resulted in the development of the magnetic thin film disk. The magnetic thin film disk should have a high and controllable coercivity, preferably in the range between 0.48 x 10 5 and 1.59 x 10 5 Am -1 (600 and 2000 Oe), and a high remanent magnetization. Binary or ternary cobalt-based alloys, such as CoCr, CoRe, CoPt, CoNi, CoNiCr, CoPtCr and the like are commonly used as the magnetic material in thin-film magnetic disk technology. Platinum is one of the key elements in achieving high coercivity, of greater than 0.72 x 10 5 Am -1 (900 Oe), required for disks. Depending upon the required range of coercivity, up to 20 percent of Pt can be added to Co based alloy. The magentic film is deposited on a non-magnetic substrate, typically NiP-coated Al-Mg or glass disks. The recording density of a medium is inversely proportional to the distance (fly height) between the disk and the magnetic heads with which the information is being recorded. Thus the surface of the disk substrate should be extremely smooth to permit a lower fly height. However the extreme smoothness of the disk generally results in a high contact area between the disk and head which, in turn, results in a high value of stiction or friction. The high stiction or friction can cause damage to the disk, recording head and its assembly as well as disk drive motors. In order to alleviate the problem, a lubricant, typically carbon, is applied to the outermost film layer. However, extremely smooth disks even with a lubricant coating may still exhibit unacceptably high stiction and friction levels. Moreover, over a period of time the lubricant is removed from the disk surface. Therefore, a controlled surface topology or texture is required to enhance flyability and lubricant retention. In order to overcome the problems of high stiction and friction, prior to the deposition of the magnetic layer the surface of the disk substrate is roughened by one of the common mechanical abrasive techniques known as texturing. The mechanical texturing usually is associated with the formation of weldments and asperities along the texture lines. These weldments can result in an increase in fly height as well as severe wear of the magnetic layer during operation of the disk. Therefore, it is desireable to texture or roughen the substrate surface by means other than mechanical abrasion while not adversely affecting the disk magnetic properties. It is also desireable to enhance coercivity of the magnetic disks while reducing the quantity and expense of platinum element additions. In U.S. Patent No. 4,539,264 entitled "Magnetic Recording Medium" a magnetic recording medium is described as having a non-magnetic substrate, a bismuth layer of less than 10 nm (100Å) thick and a magnetic metal thin film formed on the bismuth layer. While the described medium achieves increased coercivity levels, the bismuth layer remains as a discrete underlayer and the topological effects resulting from a transient liquid layer are not achieved. The patent also fails to suggest the application of a layer of metal film between the bismuth layer and the magnetic layer which is an alternative embodiment of the present invention. The present invention overcomes the above difficulties by applying an underlayer of a transient liquid film between a non-wetting or poorly wetting substrate and the magnetic thin film. The transient liquid metal layer is applied while the substrate, which is not wetted by the liquid metal, is maintained at a temperature above the melting point of the transient liquid metal. The result is that the liquid metal is caused to "ball-up" and form a layer of disconnected liquid features. Instead of forming a distinct underlayer, a distribution of the concentration of the transient liquid metal across the grain of the magnetic layer is manifest. The transient liquid metal elements include, but are not limited to, gallium, indium, tin, bismuth, lead, cadmium, mercury, selenium, tellurium and their alloys with other metals including silver, palladium, platinum, or gold as well as binary or tertiary compounds of the transient liquids themselves. The preferred transient liquid metals are gallium, indium and tin. The substrate can be, but is not limited to, silicon dioxide, glass, polymers, or metal substrates treated in such a manner to be rendered non-wetting to the transient liquid metal, with glass being the preferred substrate material. The magnetic thin film may be, but is not limited to, Co-Pt-Cr, Co-Cr, Co-Ni-Cr, Co-Re. In order to manufacture the medium in accordance with the present invention, the substrate is held at a temperature above or close to the melting point of the transient liquid metal during deposition of the transient liquid layer onto the substrate by conventional sputtering, evaporation, plating or other deposition techniques as are known in the art. The outer magnetic film layer is then deposited onto the transient liquid at either an elevated temperature above the melting point of the transient liquid metal or alternatively at a more conventional lower temperature at which the underlayer features, while metastably undercooled, are nevertheless liquid. The transient liquid will become alloyed with the magnetic layer thereby imparting to the magnetic layer a controlled topology which provides a disk surface with improved tribology. The magnetic medium will not include a pure transient liquid metal underlayer. The advantages of the present invention include the elimination of the variability and dependence upon direction of the magnetic properties, control of the magnetic film surface roughness and control of the coercivity of the medium. It will be apparent to those skilled in the art that the area of the medium surface subject to roughening can be limited by the use of known masking techniques. When the described interaction between the magnetic film layer and the transient liquid underlayer is undesirable, it is possible to apply a metal film layer between the underlayer and a magnetic film outerlayer which will not adversely affect the surface topology. Moreover, the application of such an additional layer permits control of the surface topology over a wider range of roughness to improve the tribology while maintaining the coercivity of the film constant. Preferred metal films for the interlayer are, but not limited to, chromium, molybdenum, vanadium, palladium and platinum. A principal object of the present invention is therefore the provision of a magnetic medium with controlled surface morphology and topology, and magnetic characteristics. Another object of the invention is the provision of a method of manufacturing a magnetic medium composed of a non-wetting substrate, a transient liquid underlayer, and a magnetic film outer layer. A further object of the invention is the provision of a magnetic medium composed of a non-wetting substrate, a transient liquid metal underlayer and a magnetic film layer where the surface roughness of the magnetic film layer is controllable by adjusting the transient liquid metal underlayer thickness and deposition temperature. Further and still other objects of the present invention will become more clearly apparent when the following description is read in conjunction with the accompanying drawings, wherein: Fig. 1A is a cross-sectional representation of a transient liquid metal layer deposited on a substrate in accordance with the present invention; Fig. 1B is a cross-sectional representation of a magnetic medium manufactured in accordance with the present invention; Figs. 2A to 2D are SEM images of the surface topology of evaporated gallium film of 2.5, 5, 10 and 20 nm average thickness respectively applied to a silicon dioxide substrate; Figs. 3A to 3D are SEM images of the surface topology of the structures obtained by sputter depositing 75 nm of Co-Pt-Cr alloy film over 2.5, 5, 10 and 20 nm thick films of Ga respectively on a silicon dioxide substrate; Fig. 4 is a SEM image of the surface topology of a sputtered Co-Pt-Cr film without any transient liquid underlayer; Fig. 5 is a graphical representation of the magnetic properties of 75 nm thick Co-Pt-Cr film without any underlayer; Fig. 6 is a graphical representation of the magnetic properties of 75 nm thick Co-Pt-Cr film sputtered on a 2.5 nm thick layer of gallium deposited onto a clean silicon dioxide substrate; Fig. 7 is a graphical representation of the magnetic properties of 75 nm thick Co-Pt-Cr film sputtered onto a 5 nm thick layer of gallium deposited on a clean silicon dioxide substrate; Fig. 8 is a graphical representation of the magnetic properties of 75 nm thick Co-Pt-Cr film sputtered onto a 10 nm thick layer of gallium deposited on a clean silicon dioxide substrate; Fig. 9 is a graphical representation of the magnetic properties of 75 nm thick Co-Pt-Cr film sputtered onto a 20 nm thick layer of gallium deposited on a clean silicon dioxide substrate; Fig. 10 is a cross-sectional representation of an alternative preferred magnetic medium manufactured in accordance with the present invention; Fig. 11 is a SEM image of the surface topology of a 75 nm thick layer of Co-Pt-Cr alloy film applied onto an interlayer of 10 nm thick Cr film which is applied on a 10 nm thick layer of gallium deposited on a silicon dioxide substrate; and Fig. 12 is a graphical representation of the coercivity of Co-Pt-Cr alloy film deposited onto a Cr interlayer film deposited on a gallium underlayer of different thicknesses. Referring now to Fig. 1A in particular, there is shown in accordance with the present invention a substrate 10 such as silicon dioxide, glass, polymer or thin coating thereof on any suitable non-magnetic storage disk material. The substrate is selected to be poorly wettable by liquid metals. As used in the context of this invention, the term non-wettable refers to the break up of a liquid uniformly deposited film to form a distribution of disconnected liquid metal features due to the lack of wetting or interaction between a substrate and the liquid metal. Deposited upon the substrate is an underlayer of a transient liquid metal 12 such as gallium, indium, tin, bismuth, lead, cadmium, mercury, selenium, tellurium and their alloys with other elements, including silver, palladium, platinum or gold as well as binary or tertiary compounds of the transient liquids themselves. The preferred transient liquid metals are gallium, indium and tin. The average film 12 thickness is the range between approximately 2.5 nm and 30 nm, preferably in the range between 2.5 nm and 10 nm. The term transient liquid layer refers to the temporary existence of the liquid metal layer, until a deposition of a sufficient quantity of a magnetic alloy material results in the dissolution and reaction of the liquid metal in the magnetic alloy, with the substantially complete disappearance of the liquid phase from the magnetic medium structure. As a result, in the final magnetic medium there is no distinct transient liquid metal film layer, but rather there is an intergranular segregation of such metal, as shown in Fig. 1B. The transient liquid metal layer is applied to the substrate while the substrate is maintained at a temperature in excess of the melting point of the transient liquid metal. In the example of a gallium film, the substrate is held at a temperature above approximately 30°C during transient liquid metal deposition. Due to the poor wetting of the substrate by the liquid metal, the liquid metal forms spherical features as shown in Fig. 1A. The transient liquid film underlayer is applied onto the substrate by conventional means such as vapor deposition or sputtering and the like while the substrate is maintained at a temperature above the melting point of the transient liquid metal. A magnetic film 14 is deposited onto the transient liquid metal film layer in a conventional manner with the substrate and transient liquid being held either at a temperature above the melting point of the transient liquid metal or at any lower conventionally used temperature at which the transient liquid metal underlayer still exists in a liquid state. If the underlayer is allowed to solidify prior to deposition of the magnetic layer, it will exist in the resulting magnetic medium structure and will adversely offset the mechanical and adhesion properties of the magnetic disk. The magnetic film preferably is Co-Pt-Cr but any other magnetic thin film material may also be used in practicing the invention. The thickness of the magnetic film is in the range between 10 nm and 150 nm, preferably in the range between 20 nm and 100 nm. The surface of magnetic film 14 generally follows the topology of the spherical structures of the transient liquid metal layer and exhibits regions of increased concentration of the transient liquid metal 18, e.g. gallium, in the area between where the spherical structures were located and in the area where the spherical structures were located. As is well known in the art, a top coat, typically carbon, and a lubricant can be applied to the magnetic medium and is required for many applications. Such a treatment is fully compatible with the magnetic medium described herein. While in the prior art the substrate has been mechanically roughened to control the topology of the subsequently deposited magnetic film layer, problems have occurred as mentioned above. In accordance with the teachings of the present invention, the magnetic film layer topology is controlled by adjusting the thickness of the transient liquid layer thereby obviating the problems associated with mechanical roughening while maintaining, and even improving, the magnetic properties of the medium. Figs. 2A to 2D are scanning electron microscope images of evaporated gallium films deposited over a silicon dioxide substrate at a temperature above the melting point of gallium. By virtue of the poor wetability of the substrate, rather than the gallium forming a smooth uniform film layer, the gallium forms many spherical structures or features. The quantity of the spherical structures and the size distribution of the structures are dependent upon the average thickness of the gallium layer, temperature of the substrate during deposition, and the degree of wetting of the substrate by the transient liquid metal. The degree of wetting of the substrate can be affected by the quantity of addition of alloying elements to the gallium which alloying elements strongly interact with the substrate material. The thickness dependency in the case of pure gallium deposited onto a silicon dioxide substrate maintained at 32°C to 35°C during deposition is shown in Figs. 2A to 2D. Fig. 2A is a scanning electron microscope image of a 2.5 nm thick layer of evaporated gallium film on a silicon dioxide substrate. Fig. 2B is a scanning electron microscope image of a 5 nm thick layer of evaporated gallium film on a silicon dioxide substrate. Fig. 2C is a scanning electron microscope image of a 10 nm thick layer of evaporated gallium film on a silicon dioxide substrate. Fig. 2D is a scanning electron microscope image of a 20 nm thick layerr of evaporated gallium film on a silicon dioxide substrate. It will be apparent from the Figs. 2A to 2D that the quantity of the spherical structures per unit surface area decreases with increasing gallium film layer thickness and the size of the spherical structures increases with increasing gallium film layer thickness. In manufacturing a preferred embodiment of the medium a 75 nm thick Co-Pt-Cr alloy magnetic film layer is deposited over a layer of gallium which was previously deposited on a silicon dioxide substrate. The gallium will dissolve or alloy with the Co and the gallium layer will in effect disappear as a distinct film layer. The surface topology of the magnetic film layer is shown in the scanning electron microscope images of Figs. 3A to 3D. In Fig. 3A a 75 nm thick magnetic film layer was deposited over a 2.5 nm average thickness layer of gallium. In Fig. 3B a 75 nm thick magnetic film layer was deposited over a 5 nm average thickness layer of gallium. In Fig. 3C a 75 nm thick magnetic film layer was deposited over a 10 nm thick magnetic film layer. In Fig. 3D a 75 nm thick magnetic film layer was deposited over a 20 nm average thickness layer of gallium. Examination of the Figs. 3A to 3D shows that with increasing underlayer film thickness, there is manifest in the magnetic film layer surface fewer and larger spherical structures. It will be apparent to those skilled in the art that the surface topology and morphology of the outer magnetic film layer is controllable by the selection of the average thickness of the transient liquid film underlayer. As a comparison and to further demonstrate the effect of the transient liquid metal underlayer, Fig. 4 is a scanning electron microscope image of the surface of a layer of Co-Pt-Cr alloy magnetic film deposited directly onto the substrate without an intervening gallium layer. The image in Fig. 4 does not show any clearly defined spherical structures. The effect of the gallium underlayer on the magnetic film layer is believed to be due to the disparity of conditions for film nucleation and growth on the surface of the gallium spheres and on the silicon oxide, and by possible shadowing effects due to the spherical shape of the gallium surface features. An important consideration in the manufacture of a magnetic medium is the affect each of the layers has with regard to the coercive force of the magnetic material layer and the coercive squareness ratio. Figs. 5 to 9 are graphical representations of the magnetic properties of a 75 nm thick layer of Co-Pt-Cr alloy film deposited on an underlayer of gallium of different average thickness deposited on a silicon dioxide substrate. In Fig. 5 there is no gallium underlayer, the magnetic layer is deposited directly onto the substrate. In Fig. 6 there is a 2.5 nm average thickness of the gallium layer. In Fig. 7 there is a 5 nm average thickness gallium layer. In Fig. 8 there is a 10 nm average thickness of the gallium layer. In Fig. 9 there is a 20 nm average thickness of the gallium layer. The increase of coercivity H due to the gallium underlayer becomes apparent when comparing the value of 0.39 x 10 5 Am -1 (485 Oe) for the sample without a gallium underlayer in Fig. 5 with the values of 1.10 x 10 5 Am -1 (1384 Oe), 1.03 x 10 5 Am -1 (1292 Oe), 1.46 x 10 5 Am -1 (1829 Oe) and 3.07 x 10 5 Am -1 (1680 Oe) for the samples in Figs. 6 through 9 where the average gallium layer thickness is 2.5 nm, 5 nm, 10 nm and 20 nm respectively. Referring to the graphical representations, there is also manifest a decrease in the B-H loop squareness as the gallium underlayer thickness is increased as indicated by the lower squareness ratio (SR) and coercive squareness ratio (S*) values with increasing average gallium layer thickness as shown in the following Table I. Ga Thickness (nm) Hc Am -1 x 10 5 (Oe) SR S* 0 0.39 (485) .8237 .8283 2.5 1.10 (1384) .7110 .8251 5 1.03 (1292) .7470 .6996 10 1.46 (1829) .6752 .6676 20 3.07 (1680) .6808 .5839 While increased thickness of the gallium underlayer positively affects topology and coercivity of the magnetic thin films, excessive underlayer thicknesses may drastically reduce remanent magnetization, and therefore the read-write characteristics of the magnetic medium. The effect is due to excessive reactions between the transient liquid metal and the magnetic alloy, resulting in the overall reduction of the magnetic phase. In such instances an interlayer 16 of chromium, palladium, tantalum, molybdenum, vanadium, with chromium being the preferred material, is deposited on the transient liquid film 12 deposited on substrate 10 as shown schematically in Fig. 10. The magnetic film layer is then deposited onto the metal film interlayer. While not limited to the elements mentioned above, such interlayer is required to form an intermetallic compound with the gallium which is rich in gallium, with the preferred atomic composition being Ga 6 B, where B is the barrier element, such as chromium. The result is early solidification of the liquid underlayer even with only a thin barrier layer, since 1 atom of Cr combines with 6 atoms of Ga in a solid compound. Fig. 11 is a scanning electron microscope image of the magnetic film layer of a medium composed of a silicon dioxide substrate on which a 10 nm average thickness underlayer of gallium has been deposited, followed by a barrier interlayer of 10 nm thick chromium layer on which a 75 nm thick Co-Pt-Cr alloy magnetic film layer has been deposited. A comparison of Fig. 3C and Fig. 11 shows that the morphology is substantially the same as the film deposited on a 10 nm thick underlayer of gallium without a chromium interlayer. Moreover, the addition of the chromium interlayer permits control of the surface topology over a wide range of surface roughness to improve the tribology while still maintaining high coercivity of the film. Curve 20 shows the coercivity (Hc) as a function of gallium underlayer thickness with a 30 nm thick Co-Pt-Cr alloy magnetic film layer. Curve 22 shows the coercivity (Hc) as a function of gallium underlayer thickness with a 10 nm thick layer of chromium deposited on the gallium underlayer and a 30 nm thick Co-Pt-Cr alloy magnetic film layer deposited or the chromium layer. Curve 22 shows that high coercivity is also achieved with the chromium barrier interlayer. Additional measurements have shown that the chromium layer seals the gallium underlayer without adversely affecting the magnetization characteristics of the medium. The tribological performance of the magnetic films with the topology controlled according to the present invention is superior to the performance achieved with conventional magnetic films. Results of tribology tests on a smooth glass substrate are shown in the table below. Start/Stop Test measurements, after 3000 start and stop cycles. Ga(nm) Cr(nm) CoPtCr (nm) Stiction (g) 0 0 60 >20 5 20 60 4 5 0 60 4.2 10 0 60 3.8 20 20 60 2.05 As shown in the table above, the use of a transient liquid metal underlayer during manufacture of a magnetic medium, with or without a chromium or other metal interlayer, reduces the stiction as compared to a magnetic medium manufactured without the use of a transient liquid metal underlayer. It will be apparent to those skilled in the art that it is possible by masking of the substrate to limit the deposits of the transient liquid underlayer to predetermined locations on the substrate. As a result, the surface roughness of the magnetic medium can be varied at different locations on the medium.

Claims: A magnetic storage medium comprising: a non-wettable substrate (10); and a magnetic film layer (14) with a surface substantially following a topology of spherical structures formed by a transient liquid metal layer (18) of predetermined thickness deposited upon said substrate (10) to form a distribution of disconnected molten metal features. The magnetic storage medium as set forth in claim 1, wherein said magnetic film layer (14) comprises regions of increased concentration of said transient liquid metal (18) in the area between where the spherical structures were located and in the area where the spherical structures were located. The magnetic storage medium as set forth in claim 1 or 2, further comprising an intermediate metal film layer deposited between said transient liquid metal layer (18) and said magnetic film layer for inhibiting reaction of said transient liquid metal and said magnetic film layer (14). The magnetic storage medium as set forth in claim 3, wherein said intermediate metal film layer is selected from the group consisting of chromium, palladium, tantalum, molybdenum and vanadium. The magnetic storage medium according to any one of the above claims, wherein said transient liquid metal layer (18) comprises a compound selected from the group consisting of gallium, indium, tin, bismuth, lead, cadmium, mercury, selenium, tellurium and their alloys with other metals including silver, palladium, platinum and gold, and binary and ternary compounds of the liquid metals. The magnetic storage medium according to any one of the above claims, wherein said magnetic film layer (14) is an alloy of which cobalt is a major constituent. The magnetic storage medium according to any one of the above claims, wherein said magnetic film layer (14) is selected from the group consisting of Co-Pt-Cr, Co-Cr, Co-Ni-Cr, and Co-Re. The magnetic storage medium according to any one of the above claims, wherein said substrate (10) is selected from the group consisting of silicon dioxide, glass, polymers and metal substrates treated in such a manner as to be rendered substantially non-wettable to said transient liquid metal (18). A method of manufacturing a magnetic storage medium comprising the steps of: providing a non-wettable substrate (10); depositing a transient liquid metal layer (18) of a predetermined thickness on said substrate (10) while said substrate is maintained at a temperature in excess of the melting point of said liquid metal to form a distribution of disconnected molten metal spherical structures; and maintaining said liquid metal layer (18) in a liquid state while depositing a magnetic film layer (14) on said transient liquid metal layer (18) whereby the transient liquid metal reacts with the magnetic film layer. The method of claim 9, further comprising a step of depositing an intermediate metal film layer upon said transient liquid metal layer. The method according to claims 9 or 10, wherein said substrate (10) during deposition is at a temperature for maintaining said transient liquid metal layer (18) in a liquid state. The method as set forth in claim 11, wherein said temperature is in excess of the melting point of said liquid metal. The method according to any one of claims 9 to 12, wherein said predetermined thickness is selected for controlling the topology of said magnetic film layer (14). The method according to any one of the above claims 9 to 13, wherein said transient liquid metal (18) is deposited at a preselected region of said substrate (10).

Inventors: BASEMAN ROBERT J, JAHNES CHRISTOPHER V, KHANDROS IGOR Y, MIRZAMAANI SEYYED M, RUSSAK MICHAEL A, BASEMAN, ROBERT J., JAHNES, CHRISTOPHER V., KHANDROS, IGOR Y., MIRZAMAANI, SEYYED M., RUSSAK, MICHAEL A.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B5
IPC codes: G11B5