Abstract:
A method for fabricating a metallic pattern on a substrate is disclosed. The method consists of depositing an adhesive and/or plating base material on a substrate upon which a narrow self-supporting border of photoresist is applied. An anodic layer is then deposited and the photoresist removed. The adhesive and plating base material is etched in those areas previously covered by the photoresist and fresh photoresist applied to encapsulate those areas of the anodic layer which form the final pattern of interest. The unwanted anodic material is then etched and the photoresist removed.

Description:
This is a continuation of application Ser. No. 212,359, filed Dec. 3, 1980. 
    
    
     BACKGROUND OF THE INVENTION 
     When fabricating the pole pieces of thin film magnetic recording heads or other high tolerance elements, new techniques have developed in electroplating and etching sheet materials whose composition and structure must be accurately controlled in order to achieve uniformity of performance. As taught in U.S. Pat. No. 3,853,715, conventional masking techniques are quite ineffective in producing these elements. 
     What has been done in the prior art, in dealing with the plating of an alloy, such as Permalloy, a mixture of nickel and iron, was to plate the alloy in sheet form and then etch the sheet into desired patterns. However, when depositing films by electroplating, it is necessary to employ an adhesive layer between the alloy and the substrate that will support the alloy pattern. Since on certain adhesive layers it is not possible to electroplate, it is at times necessary to deposit a thin layer of fairly noble metal, such as Au, Pt, Pd, Cu, Ni, etc. on the adhesion layer. 
     Unfortunately, many adhesive and plating base layers that are compatible with the magnetic alloy in the substrate become cathodic to the alloy during etching, producing severe undercutting. For example, nickel-iron alloy is made adhesive to glass or silicon by interposing a thin layer of chromium or titanium between the nickel iron alloy and its associated substrate. As taught in U.S. Pat. No. 3,853,715, when such plural layers are etched, a severe undercut is observed in the etched material. Such undercutting is due to several separate effects taking place during etching and is neither reproducible nor controllable. The undercutting is due to the fact that the chemical etching is an accelerated form of corrosion. The corrosion is isotropic in principle; it should take place at equal rates both normal to the thickness of the etched metal and parallel to the thickness. This results in uniform undercutting of the metal. But due to the extremely small film thicknesses and pattern dimensions, the dimensions of the metal crystallites and grain cannot be ignored. The grain boundaries and grains etch at different rates, resulting in ragged edges. 
     During the terminal stages of etching, when the adhesive and/or the plating base metal layers are exposed, the dissimilar metals form a galvanic cell which results in extremely rapid etching of the anodic metal. In case of titanium and chromium, each of these metals passivate extremely quickly and becomes cathodic to nickel, nickel-iron and to the metals of the iron group. When the metals such as platinum, palladium, gold or copper are present in the sandwich with the iron group metals, they act cathodically and the etching of the nickel, nickel-iron alloy, etc. is impossible to control. 
     Such undercutting is detrimental to the making of batch-fabricated arrays, such as the pole pieces of thin film magnetic heads. 
     U.S. Pat. No. 3,853,715 recognizes the above-described difficulties and teaches a solution. In order to achieve uniform etching of multi-layered electroplated metals without undercutting, the patent teaches placement of a very narrow border of photoresist on top of the cathodic adhesive metal layer prior to electroplating the anodic metal. The narrow border closes upon itself to serve as a frame, while a second photoresist layer is deposited and developed so as to be present only over the anodic material to be retained after etching. The second photoresist overlaps the first photoresist to completely encapsulate the anodic layer. It is taught that subsequent etching of the surplus anodic material not needed in the ultimate pattern leaves the desired portions of the pattern free from attack, avoiding the undercutting that occurs when two or more dissimilar metals are subject to a common etchant. 
     It has now been found that the prior art solution to the problem of undercutting is not entirely adequate. More specifically, it has been determined that after the electroplated anodic material, such as Ni--Fe alloy, has been etched away, the etchant then has access to the cathodic metal/adhesive layers and lateral etching occurs below the border photoresist. Once this begins to occur, the adhesive maintaining the photoresist in place loses its structural integrity and the photoresist begins to remove from the substrate further compounding the undercutting problem. 
     DESCRIPTION OF THE INVENTION 
     In order to substantially eliminate the problem of undercutting, the present invention places a very narrow border of photoresist to define the later formed electrodeposited anodic material, such as Permalloy. Although the prior art, namely, U.S. Pat. No. 3,853,715, also uses a border of photoresist, the present invention differs therefrom in completely encapsulating the anodic material by photoresist which extends all the way to the inert substrate. Like the prior art, the present invention places the narrow border of photoresist on the cathodic metal/adhesive layer in a very narrow border region of width and dimension intended to define the final metal product being produced. After the photoresist is formed, the anodic material is deposited on the cathodic metal/adhesive underlayer and the photoresist then removed to expose said cathodic metal/adhesive underlayer only in those regions previously covered by the narrow self-supporting border of photoresist. The cathodic metal/adhesive underlayer is then removed in these exposed regions and photoresist is then re-applied extending all the way to the inert support and covering and thus encapsulating the anodic material intended as the final product. 
     By practicing the present invention, the photoresist acts as a substantially impermeable barrier to lateral etching, thus substantially eliminating the problem of undercutting. The corrosive etching is allowed to travel along the cathodic metal/adhesive underlayer, but when the etchant confronts the photoresist, a barrier is encountered terminating the etching process. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 show prior art processing steps as taught in U.S. Pat. No. 3,853,715. 
     FIGS. 3-7 show the sequential processing steps for the elimination of undercutting during chemical etching according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an electroplated sheet being processed whereby substrate 1 of silicon dioxide, glass or other similar self-supporting insulating material is caused to support a thin layer of adhesion metal 2, such as chromium, titanium, tantalum, tungsten, niobium, vanadium or zirconium. Such adhesive metal 2 is used primarily to make the main metal of interest such as Ni--Fe alloy, referred to as the anodic metal, adherant to the substrate. Since one cannot readily electroplate or electrolessly plate on such adhesion layer, it is desirable to sequentially metallize the adhesion layer 2 with readily plateable metal 3, such as Au, Pt, Pd, Cu, Ni, Ni--Fe or with a metal alloy. Such adhesion layer 2 and conductive layer 3 can be applied by sputtering, evaporating, or in any other manner. 
     According to the prior art, at this stage of the processing sequence, photoresist 7 and 17 is deposited by conventional lithographic techniques and the anodic material 6 such as Permalloy is deposited. 
     After the plating of the Permalloy layer 6, another photoresist layer 8 is applied by conventional photolithographic techniques to the top of anodic metal 6. The excess anodic material shown as areas 4 and 5 of FIG. 1 is then etched away, FeCl 3  being a suitable etchant for Ni--Fe, the photoresist borders 7, 17 and 8 encapsulating the anodic material. 
     The prior art teaches that the photoresist prevents the active metal such as Ni--Fe from being etched, while in the presence of cathodic metal, such as chromium, titanium, gold, etc. After the more active metal 6 was etched with FeCl 3 , it is taught that the plating base metal 3 and adhesion layer 2 are etched with suitable chemical etchants. As stated previously, however, it has been discovered that etchants used to remove non-wanted anodic areas 4 and 5 together with plating base metal 3 and adhesion layer 2 laterally etch layers 2 and 3 below photoresist boundary areas 7 and 17 causing serious undercutting and loss of sharp edge etching as shown in FIG. 2 at 9 and 10. Once lateral etching begins, photoresist areas 7 and 17 lose their adhesion to supporting layers 2 and 3, which further aggrevates the problem. Furthermore, because of the difference in electrochemical potentials between the adhesion metal layer 2, such as titanium, and the anodic metal, such as Ni--Fe, a battery is formed as soon as the adhesive metal layer is exposed to the etchant, and the etch rate of the Ni--Fe is increased to such an extent that it makes control of the etching process very difficult if not impossible. 
     In order to overcome these difficulties, reference is made to FIGS. 3-7. FIG. 3 shows the processing which would be substantially equivalent to the prior art at an equivalent point in its processing cycle. More specifically, upon adhesive metal layer 2 and plateable metal layer 3 has been applied a very narrow border of photoresist shown as elements 15, 16. The anodic layer is then deposited upon metal layer 3 as a uniform coating shown as regions 4, 5 and 6. It is region 6 which is intended to be the pattern of interest as the final product and its configuration is defined by the very narrow self-supporting border of photoresist 15, 16. 
     Unlike the prior art, the border of photoresist 15, 16 is now removed by any well known technique, thus exposing adhesive metal layer 2 and plateable metal layer 3 selectively in those areas which were defined by the very narrow self-supporting border of photoresist. As shown in FIG. 4, adhesive metal layer 2 and plateable metal layer 3 are selectively removed by any well known technique, such as by sputter etching or ion milling, resulting in voids 11 and 12 extending all the way to base 1 at 13, 14. These voids, preferably approximately 0.1 to 0.2 mils wide, define the final pattern or patterns of anodic material, which will become, for example, the pole pieces of thin film magnetic heads. 
     The voids 11, 12 are filled with photoresist 8 which is caused to completely encapsulate anodic material 6 on three sides with the fourth side of this region being encapsulated by inert base 1. 
     As shown in FIG. 5, photoresist 8 can extend slightly beyond filled voids 11, 12. The excess anodic material 4, 5, such as electrodeposited Permalloy, is then etched away, FeCl 3  being a suitable etchant for Fe--Ni alloy. Plating base metal 3 and adhesive layer 2 can be etched with suitable chemical etchants. Because photoresist 8 is in physical contact with support base 1, the etching of areas 4 and 5, as well as layers 2 and 3, does not in any way adversely effect the sharp edge integrity of the anodic metal or support layers 2 and 3 located within the boundaries defined by photoresist 8 (FIG. 6). The photoresist material 8 is lastly removed by any well known technique resulting in the final sharp-edge pattern, unattainable in the prior art (FIG. 7). 
     The teachings of U.S. Pat. No. 3,853,715 including preferred photoresist materials as well as etchants are hereby incorporated by reference herein. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.