METHOD OF FORMING RECESSED PROFILE ON FERRITE SINGLE CRYSTAL BY CHEMICAL ETCHING

Abstract: A method of forming, on a surface of a Mn-Zn ferrite single crystal, a recessed portion which has a predetermined cross sectional profile and at least one straight ridge (15) which defines a boundary in the profile. An etching mask (14) is formed on the surface of the ferrite single crystal, so as to provide the recessed portion by etching such that the straight ridge (15) of the profile lies in a (100) or (110) plane of the ferrite single crystal which is perpendicular to the surface on which the etching mask is formed. The ferrite single crystal with the etching mask is exposed to an aqueous solution which substantially consists of water and the balance consisting principally of phosphoric acid, whereby the surface of the ferrite single crystal is chemically etched, so as to produce the recessed portion such that an inclined surface is formed so as to extend parallel to the straight ridge, and such that the straight ridge defines an edge of the inclined surface.

Description: The present invention relates in general to a method of forming a desired recessed portion on a surface of a ferrite single crystal, and more particularly to such a method adapted to chemically etch a surface of the ferrite single crystal, so as to produce a magnetic head core having an inclined surface or surfaces, such as a head core for a rigid magnetic disk drive (RDD), or a composite type magnetic head core wherein only a portion adjacent to the magnetic gap of the head core is formed of a magnetic metal. A Mn-Zn ferrite single crystal has been used as a material for magnetic head cores. For the ferrite single crystal to serve its intended purpose, the crystal should be precisely shaped as required. For instance, a monolithic type head core 1 as illustrated in Fig. 1 is known as a typical magnetic head core for a rigid magnetic disk drive (RDD). In this head core 1, a sliding surface for sliding contact with a recording medium includes air-bearing surfaces 3 and a signal recording/reproducing (writing/reading) track 4, which surfaces 3 and track 4 are defined by grooves 2 formed in the ferrite single crystal. For preventing the edges of the air-bearing surfaces 3 and track 4 from being damaged due to contact with the surface of the recording medium, the edges are chamferred as indicated at 5, 6, 7, 8, 9 and 10. These chamferred or inclined surfaces 5-10 define the nominal widths of the air-bearing surfaces 3 and track 4. Commonly, the grooves 2 and the chamferred or inclined surfaces 5-10 of the magnetic head core 1 are formed by grinding with a diamond wheel, or by other machining operations. The grooving and chamferring by grinding require a total of eight grinding passes or paths for each head core, and are the most time-consuming step of the process for forming the desired profile. Further, it is inevitable that the widths of the air-bearing surfaces 3 and track 4 have an error of about ± 10 µm from the nominal values, due to a positioning error of the grinding wheel, and a variation in the thicknes or height of the workpiece. Moreover, the surfaces finished by the diamond wheel inevitably suffers from chipping of one micrometer or more. Recently, there has been an increasing requirement for improved accuracy of the widths of the air-bearing surfaces 3 and track 4 of the head core, say, to within ± 1 µm, keeping pace with an increasing demand for improved recording density per unit area of the recording medium, and improved accuracy of air-bearing surfaces 3 and magnetic characteristics of the RDD head core 1. It is also noted that the chipping or cracking of the air-bearing surfaces 3 and track 4 may lead to chipping or removal of the ferrite material, causing damages of the magnetic head core and the recording medium, during an operation of the head core. Thus, the conventional magnetic head core suffers from the problems of reliability associated with its floating action and magnetic characteristics. On the other hand, methods of producing such magnetic head cores by means of photolithography or chemical etching process have been proposed, as disclosed in JP-A-62-234214. Described more specifically, monolithic RDD magnetic head cores are prepared from a so-called ferrite bar 12 formed of a ferrite single crystal, as illustrated in Fig. 2(a). The ferrite bar 12 has a predetermined magnetic gap 11, and an annular magnetic path formed so as to include the gap. The ferrite bar 12 has a surface 13 which corresponds to a sliding surface of each magnetic head core produced. The sliding surface 13 is mirror-ground, and is subjected to a process for removing residual strain due to the grinding operation. Then, a resist 14 is applied to the processed surface of the ferrite bar 12, so as to form an appropriate etching mask in a pattern corresponding to a recessed portion to be formed on the surface 13. The ferrite bar 12 with the etching mask 14 is subjected to a chemical etching operation, whereby the recessed portion having a predetermined cross sectional profile partly defined by the air-bearing surfaces 3 and track 4 is formed on the ferrite bar 12. It is desirable that the recessed portion to be formed by etching on the surface of the ferrite single crystal have a high degree of dimensional accuracy. Namely, consistent dimensional accuracy is required of side etching widths W and straightness of straight ridges 15 which define the dimensions of the track 4 and chamferred surfaces 7, 8, for example, as indicated in Fig. 2(b). As indicated above, the dimensional accuracy to within ± 1 µm is important to assure improved recording density, magnetic characteristics and reliability of the magnetic head core. However, the ferrite bar 12 of a Mn-Zn ferrite single crystal for a magnetic head core has a glass filler at the magnetic gap 11. Since the ferrite bar 12 with the glass filler cannot be heated to a temperature higher than a melting point of the glass, a strain on the surface of the ferrite single crystal caused in the grinding process cannot be eliminated. As a result, the side etching widths W tend to be excessive with respect to the nominal values, and the straightness of the straight edges 15 tends to be deteriorated. Thus, the chemical etching process suffers from difficulty in obtaining acceptable dimensional accuracy of the recessed profile formed on the ferrite bar 12. EP-A-148014 describes a process in which a ferrite core element, e.g. of Mn-Zn-ferrite, is subjected to chemical etching to remove a top portion of a projection adjacent a previously formed recess. One side surface of the recess is inclined. See figure 5 of EP-A-148014 Also, dry etching methods such as sputtering and ion-beam etching rather than chemical etching are proposed to process the ferrite bar 12 to form thereon a recessed portion having a desired cross sectional shape. Examples of such dry etching methods are disclosed in JP-A-51-114111 and JP-A-54-12819 of unexamined Japanese Patent Applications. However, the dry etching methods suffer from a low etching rate, and are difficult to practice in an industrially justifiable manner. The present invention was developed to solve the problems of the prior art discused above. It is therefore an object of the present invention to provide a method suitable for forming a desired recessed portion on a surface of a Mn-Zn ferrite single crystal, to economically produce a highly reliable magnetic head core which has an improved degree of dimensional accuracy. The invention is set out in claim 1. According to the method of the present invention, the desired recessed portion is formed by etching on the surface of the Mn-Zn ferrite single crystal such that each straight ridge as indicated at 15 in Fig. 2(b) defining a boundary between two adjacent surfaces (4; 7, 8) of the recessed portion lies in a (100) or (110) plane of the ferrite single crystal which is perpendicular to the surface on which the etching mask is formed. This arrangement permits an improvement in the linearity or straightness of each straight ridge which defines an edge of the inclined surface formed as a result of the chemical etching. Accordingly, the dimensional accuracy of the recessed portion formed on the surface of the ferrite single crystal can be improved. Namely, the dimensional error may be held to within ± 1 µm. Therefore, the present invention makes it possible to enjoy the inherent advantages of a chemical etching process over a machining process, i.e., absence of chipping and/or cracking of the ferrite single crystal, and a comparatively large area of processing at one time. The instant method employing a chemical etching is also advantageous over a dry etching method, in terms of etching rate. For example, an ion-beam etching method using Ar ions is capable of etching at a rate of only 3 µm per hours, while the etching rate of the present method may be 20 µm per hour, that is, as large as more than six times that of the ion-beam etching. Consequently, the present method provides a significant increase in the productivity of magnetic head cores using a Mn-Zn single ferrite crystal. In one form of the present invention, the method further comprises a step of exposing the surface of the Mn-Zn ferrite single crystal to a solution which contains an amine compound, prior to forming the etching mask on the surface of the ferrite single crystal. Preferably, this preliminary treatment of the ferrite crystal surface with the solution containing the amine compound is effected after the surface is cleaned with an organic solvent or pure water. It is also preferable that the preliminary treatment be effected after the Mn-Zn ferrite single crystal is annealed. In an alternative form of the invention, the method further comprises a step of exposing the surface of the Mn-Zn ferrite single crystal to an aqueous solution containing phosphoric acid, so as to etch the surface to a depth of at least 1 nm. This preliminary etching treatment with the aqueous solution of phosphoric acid is also preferably effected after the ferrite crystal surface is cleaned with an organic solvent or pure water. The above forms of the invention assure increased adhesion between the etching mask and the relevant surface of the Mn-Zn ferrite single crystal, thereby considerably improving the straightness of the straight ridge or ridges of the recessed portion, and accordingly enhancing the dimensional accuracy of the recessed portion. The Mn-Zn ferrite single crystal to which the present method is applicable may be a single crystal of ferrite, or a monocrystalline portion of a Mn-Zn ferrite material. Where such a monocrystalline portion of a Mn-Zn ferrite is used, an exposed monocrystalline surface of the monocrystalline portion is usually mirror-ground to desired smoothness with a diamond abrasive grain, in a known manner. This preliminary surface treatment of the monocrystalline surface is desired since the condition of the monocrystalline surface prior to the subsequent chemical etching has an influence on the chemically etched surface, even if the chemical etching process occurs uniformly on the entire surface area. The mirror-grinding operation is preferably conducted by using a diamond abrasive whose grain size is four micrometer or smaller. A strain adjacent to the surface of the ferrite single crystal lowers the etching rate of the surface, and degrades the dimensional accuracy of the etched crystal. Therefore, it is recommended to remove such a strain adjacent to the surface to be chemically etched, before the chemical etching process according to the principle of the invention. This strain removal may be accomplished by preliminary chemical or ion-beam etching, or annealing (heat treatment) in an inert atmosphere such as N₂. Preferably, both of the preliminary etching and the annealing are performed to remove the strain. The annealing operation is generally effected at a temperature between 200°C and 600°C, preferably between 250°C and 550°C, for at least 10 minutes, preferably at least 30 minutes. Where the Mn-Zn ferrite bar has a glass filler at a magnetic gap or other portions, the upper limit of the annealing temperature is certainly below the melting point of the glass filler. After the above-indicated surface treatments are completed, and before the etching mask is applied to the monocrystalline surface of the Mn-Zn ferrite according to the invention, the monocrystalline surface of the Mn-Zn ferrite is preferably subjected to a treatment with a solution containing an amine compound, as described above, in order to suitably reduce the side etching widths W of the recessed portion and improve the straightness of the straight ridges. Namely, the treatment with the amine compound solution greatly contributes to improvement in the dimensional and configurational accuracy of the profile of the recessed portion to be formed on the finally chemically etched surface of the ferrite. The amine compounds are preferably selected from alkanol amines such as ethanol amine, diethanol amine and triethanol amine. However, it is possible to use the other amine compounds such as: aliphatic primary amines such as ethyl amine, propyl amine and butyl amine; aliphatic secondary amines such as diethyl amine and dipropyl amine; aliphatic tertiary amines such as triethyl amine; aliphatic unsaturated amines such as allyl amine; alicyclic amines; diamines; triamines; and aromatic amines such as aniline, toluidine and benzyl amine. The selected amine compound is generally used as an aqueous solution, in which the monocrystalline surface of the Mn-Zn ferrite is immersed, for an intended cleaning. The cleaning conditions such as the concentration of the amine compound and the temperature and time are determined depending upon the specific amine compound and the desired cleaning result. Since there is a recognition that the concentration of the amine compound in the treating solution has some relationship with the side etching width W, the amine compound concentration is generally held not higher than 50%, preferably within 5-25%, so that the treatment with the amine compound solution provides an intended result. As described above, the preliminary treatment with the amine compound solution may be replaced by a preliminary etching with a solution of phosphoric acid, which is also effected prior to the application of the etching mask to the surface of the Mn-Zn ferrite single crystal. By this preliminary etching, the Mn-Zn ferrite surface on which the etching mask is formed is etched to a depth of at least 1 nm. The instant preliminary treatment also effectively reduces the side etching widths W of the recessed portion to be eventually formed on the relevant surface, considerably improves the straightness of the straight ridges of the recessed portion, and greatly enhances the dimensional and configurational accuracy of the recessed portion. Usually, the relevant surface of the ferrite is immersed in the phosphoric acid solution, and the etching is effected similarly as the subsequently effected etching operation to form the desired recessed portion, as described below. The phosphoric acid concentration and the etching temperature and time are determined depending upon the desired etching depth. After the monocrystalline surface of the Mn-Zn ferrite is subjected to a preliminary treatment with a solution containing an amine compound or phosphoric acid, the appropriate etching mask is formed on the treated surface, e.g. by a suitable known method such as screen printing, which method is selected according to the required patterning accuracy and economy. From the standpoint of the patterning accuracy and ease of formation of the mask pattern, it is recommended to form the etching mask by exposing a layer of a photo resist to a radiation through an appropriate photomask. The photo resist may be either a positive type or a negative type. The etching mask may be formed of a suitable metal such as Cr, or SiO or SiO₂, by vacuum deposition, sputtering or CVD. The material of the etching mask and the method to form the mask are suitably selected, from the standpoint of adequacy to meet the desired degree of adhesion of the mask to the ferrite surface, as well as taking the operational ease and economy into consideration. The above and optional objects, features and advantages of the present invention will be better understood by reading the following detailed description of the invention, when considered in connection with the accompanying drawings, in which: Fig. 1 is a perspective view showing one example of a known mangetic head core for a rigid magnetic disk drive; Fig. 2(a) is a perspective view illustrating one embodiment of a method of the present invention adapted to produce a magnetic head core for a rigid magnetic disk drive; Fig. 2(b) is an illustration indicating a chemical etching step of the method of Fig. 2(a); Fig. 3 is a graph showing a relationship between the concentration of a phosphoric acid solution and the etching rate; Fig. 4 is a graph showing a relationship between the rate of etching with the phosphoric acid and the etching temperature; Fig. 5 is a graph showing a relationship between the side etching width and the concentration of ethanol amine solution used in Example 1; Figs. 6 and 7 are graphs showing relationships between the side etching width and the cleaning time, obtained in Example 2, where the temperature of the ethanol amine solution was 25°C and 80°C, respectively; Fig. 8 is a graph showing a relationship between the preliminary etching depth of a ferrite bar prior to application of a masking resist to the ferrite surface, and the side etching width, obtained in Example 4; and Fig. 9 is a graph showing a relationship between the preliminary etching depth and the side etching depth, obtained in Example 5. Referring to Figs. 2(a) and 2(b), there are indicated in dashed lines in Fig. 2(b) two parallel straight ridges 15, 15 which are formed as a result of chemical etching of the ferrite bar 12 of a ferrite single crystal through the etching mask 14. These straight ridges 15 define the width of the track 4. Similar straight ridges 15, 15 are formed so as to define the width of each air-bearing surface 3 as indicated in Fig. 2(a). Therefore, it is desired that the straightness of the ridges 15 be held to within 1 micrometer. To meet this desire according to the principle of the present invention, the ferrite bar 12 is chemically etched so as to form the desired recessed portion having a predetermined cross sectional shape, so that the straight ridges 15 lie in a (100) or (110) plane or face of the crystal which is perpendicular to the surface on which the etching mask is formed. Where the recessed portion to be formed by etching according to the present invention includes the air-bearing surfaces 3 each defined by the two straight ridges 15, it is desired that the etched portion has a symmetrical cross sectional shape, for easier subsequent processing and improved flying characteristics of the magnetic head core produced. Generally, the cross sectional shape of the recessed portion formed by etching is influenced by a vertical etching rate Vv in the direction of depth perpendicular to the surface being etched, and horizontal etching rates Vh1 and Vh2 in the direction of width between the straight ridges 15, 15, as indicated in Fig. 2(b). It will be understood that the recessed or etched portion is given a symmetrical cross sectional shape where the horizontal etching rates Vh1 and Vh2 are equal to each other. According to the present invention, one of the planes (100), (110), (211), (311), (332), (611) and (331) of the ferrite single crystal is preferably selected as the surface to be etched, for assuring a symmetrical cross sectional shape and improved magnetic characteristics of the magnetic head core. Further, in order to assure high level of straightness of the straight ridges (15) to within ± 1 micrometer and the symmetrical cross sectional shape of the etched portion, the etched surface is selected from among the crystal planes indicted above, and the straight ridges are formed so as to extend in the (100) or (110) plane of the crystal perpendicular to the etched surface. Therefore, if the (100) plane of the crystal is the surface to be etched, the ridges should extend in the direction 〈100〉 or 〈110〉. If the (311) plane is selected as the surface to be etched, the ridges should extend in the direction 〈332〉. The other combinations of the plane of the etched surface of the ferrite single crystal and the direction in which the straight ridge or ridges extend, include: (110) and 〈100〉; (110) and 〈110〉; (211) and 〈111〉; (332) and 〈311〉; (611) and 〈331〉; and (331) and 〈611〉. An angle ϑ of the inclined surfaces 7, 8 relative to the etched surface of the ferrite single crystal is determined by a ratio of Vh1 or Vh2 to Vv. Further, the angle may be controlled depending upon the crystal plane selected as the surface to be etched. The monocrystalline surface of the Mn-Zn ferrite with the appropriate etching mask is then subjected to a chemical etching, whereby the desired recessed portion having the predetermined cross sectional shape is formed on the Mn-Zn ferrite. The chemical etching to form the recessed pattern is effected, most preferably by using an aqueous solution consisting of water and the balance being consisting principally of phosphoric acid. This aqueous solution may contain a small amount of other acids such as sulfuric acid in addition to phosphoric acid. The above-indicated balance is considered to consist principally of phosphoric acid if the content of phosphoric acid is 80% or more with respect to the entire amount of the acids contained in the aqueous solution. Referring to the graph of Fig. 3, there is shown a relationship between the concentration of phosphoric acid in the aqueous solution, and the etching rate. It follows from the graph that the etching rate is extremely high at the phosphoric acid concentration exceeding 80%. It is also noted that the rate of increase in the etching rate with the phosphoric acid concentration becomes high when the concentration exceeds the 80% level. Accordingly, with the phosphoric acid concnetration exceeding 80%, it is difficult to accurately control the etching amount, and therefore difficult to control the widths of the track 4 and the air-bearing surfaces 3 as shown in Fig. 2(a). The etching rate is also influenced by the etching temperature, as indicated in the graph of Fig. 4. For improved magnetic characteristics of the magnetic head core produced, it is desirable that the top surface of the track 4 be spaced apart at least 10 µm from the bottom of the recess formed by etching. Therefore, the etching depth indicated at H in Fig. 2(b) is desirably 10 µm or more. To this end, the etching operation requires more than about five hours, if the etching temperature is 50°C or lower, as is understood from the graph of Fig. 4. This etching time is too long to be industrially justified. The reduction of the etching time is desirable for another reason. That is, where the etching mask is formed of a photo resist, the adhesion between the etching mask and the ferrite surface is decreased due to expansion of the photo resist during a long period of immersion in the etching solution. For improved dimensional accuracy of the etched portion, it is desirable to minimize the etching time. If the etching temperature exceeds 90°C, the amount of an aqueous component of the etching solution containing phosphoric acid changes to an appreciable extent, and the temperature distribution within the etching solution becomes uneven, whereby the etching amount considerably differs at the local areas of the ferrite surface. For this reason, it is recommended that the etching temperature be 90°C or lower, preferably within a range between 50°C and 90°C. Since the etching rate is extremely low with the phosphoric acid concentration of 5% by weight or lower as indicated in Fig. 3, the etching solution preferably has the phosphoric acid concentration of 5-80% by weight. However, the etching temperature and the phosphoric acid concentration of the etching solution are not limited to those indicated above, particularly when the required etching amount is comparatively small and where the etching mask has a high degree of adhesion to the ferrite surface. In this case, the optimum etching conditions are also determined by the other factors. While the method of the present invention has been described in connection with a magnetic head core for a rigid magnetic disk drive, the principle of the invention is equally applicable to the formation of any recessed patterns having desired cross sectional shapes, on a surface of a ferrite single crystal, for example, for forming air-bearing surfaces, tracks, magnetic gaps and other portions of magnetic head cores for a floppy disk drive or a video cassette recorder, or a composite type magnetic head core having a magnetic metal layer adjacent to the magnetic gap. The instant method makes it possible to form minute recesses, grooves or holes that cannot be formed by a conventional machining operation. EXAMPLES To further clarify the present invention, some examples of the invention will be described together with some comparative examples. It is to be understood that the invention is not limited to the illustrated examples. It is noted that the percentage values that will appear in the following examples are on the basis of weight, unless otherwise specified. Example 1 Ferrite bars each consisting of a Mn-Zn ferrite single crystal were prepared. The (110) surface of each ferrite single crystal bar was mirror-ground in an ordinary manner by a diamond abrasive having a grain size of 1/4 µm. The Mn-Zn ferrite bars were subjected to a preliminary treatment using aqueous solutions having different concentrations of ethanol amine from 0% to 100%. That is, the ferrite bars were immersed for 30 minutes, in the respective different solutions kept at 25°C. An etching resist mask was formed by a photolithographic method, on the mirror-ground surface of each of the thus treated ferrite bars, under the conditions indicated in Table 1. Then, the masked ferrite bars were chemically etched in an aqueous solution containing 77.8% of phosphoric acid (H₃PO₄) kept at 80°C. Thus, a recessed portion corresponding to the etching mask pattern was formed on the mirror-ground surface of each Mn-Zn ferrite bar, with a vertical etching amount of 20 µm. Resist OFPR800 Resist thickness 1.5 µm Pre-baking 85°C x 30 min. Exposure 200 mJ/cm² Development 23°C x 60 sec. Post-baking 140°C x 30 min. The horizontal etching amount or side etching width of the recessed portion formed on each Mn-Zn ferrite bar was measured, and indicated in the graph of Fig. 5, in relation to the concentration of ethanol amine of the corresponding preliminary treatment solution. The upper and lower curves in the graph show the maximum and minimum values of the horizontal etching amount. It follows from the graph of Fig. 5 that the side etching width or horizontal etching amount has dependence upon the ethanol amine concentration of the treating preliminary treatment solution. It is noted that the preliminary treatments with the ethanol amine concentration of 50% or lower, preferably 5-25% provided desirable results in terms of the dimensional accuracy and the straightness of the straight ridges of the chemically etched recessed portion on the ferrite bars. Comparative Example 1 The (110) plane of each of Mn-Zn ferrite single crystal bars was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm. Without the preliminary treatment with the ethanol amine solution as used in Example 1, the etching resist mask was formed on the mirror-ground surface of each ferrite bar, under the same condition as indicated in Table 1. Then, the masked ferrite bars were chemically etched in an aqueous solution containing 77.8% of phosphoric acid (H₃PO₄) kept at 80°C. Thus, a recessed portion corresponding to the etching mask pattern was formed on the mirror-ground surface of each Mn-Zn ferrite bar, with a vertical etching amount of 20 µm. The side etching width and the straightness of the straight ridges of the recessed portion formed on each ferrite bar of Comparative Example 1 were measured and observed. The measurements of the side etching widths ranged from 90 µm to 135 µm, which are not desirable to obtain the intended recessed portion. Further, the straight ridges of the recessed portion showed poor straightness. Thus, the absence of preliminary treatment of the mirror-ground surface of the Mn-Zn ferrite bar resulted in excessively large side etching widths and poor straightness of the straight ridges. EXAMPLE 2 Ferrite bars each consisting of a Mn-Zn ferrite single crystal were prepared. The (110) surface of each ferrite single crystal bar was mirror-ground in an ordinary manner by a diamond abrasive having a grain size of 1/4 µm. The Mn-Zn ferrite bars were subjected to a preliminary treatment using an aqueous solution containing 20% of ethanol amine kept at 25°C or 80°C. The ferrite bars were immersed in the solutions, for different time periods from 25 minutes to 100 minutes. Then, an etching resist mask was formed by photolithography on the mirror-ground surface of each of the thus treated ferrite bars, under the conditions indicated in Table 1. Then, the masked ferrite bars were chemically etched in an aqueous solution containing 77.8% of phosphoric acid (H₃PO₄) kept at 80°C. Thus, a recessed portion corresponding to the etching mask pattern was formed on the mirror-ground surface of each Mn-Zn ferrite bar, with a vertical etching amount of 20 µm. The graphs of Figs. 6 and 7 show relationships between the side etching width of the recessed portion formed on the ferrite bars and the time of immersion of the bars in the ethanol amine solution. The ferrite bars whose side etching width is shown in Fig. 6 were immersed in the ethanol amine solution kept at 25°C, while the ferrite bars whose side etching width is shown in Fig. 7 were immersed in the ethanol amine solution kept at 80°C. In these graphs, the maximum value of the side etching width is indicated in dashed curves, while the minimum value is indicated in solid curves. The dashed curves mean comparatively poor straightness of the straight ridges of the recessed portions formed on the ferrite bars. It follows from the graphs of Figs. 6 and 7 that the side etching width of the recessed portions formed on the Mn-Zn ferrite single crystal bars decreases with an increase in the immersing time of the bars in the ethanol amine solution. The side etching width was 21 µm where the ferrite bar was immersed for 100 minutes in the ethanol amine solution kept at 80°C. Consequently, this preliminary treatment condition was found to be most preferable to assure a high degree of straightness of the straight ridges of the recessed portions formed by the chemical etching. Table 2 shows the different preliminary treatment conditions, and the corresponding straightness and side etching width of the recessed portions formed on the ferrite bars. Preliminary Treatment Straightness Side Etching Width (µm) Ethanol Amine Concentration Temp. (°C) Time (min.) 20% 25 30 Good 27-76 20% 25 100 Fair 26 20% 80 30 Good 20-40 20% 80 100 Excellent 21 Comparative Example 2 The (110) surface of a Mn-Zn ferrite single crystal bar was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm. The ferrite bar was then annealed at 500°C for 2 hours in an atmosphere of N₂. An etching resist mask was formed on the annealed surface of the ferrite bar, under the photolithographic condition indicated in Table 1. Then, the masked ferrite bar was chemically etched in an aqueous solution containing 77.8% of phosphoric acid (H₃PO₄) kept at 80°C. Thus, a recessed portion corresponding to the etching mask pattern was formed on the mirror-ground surface of each Mn-Zn ferrite bar, with a vertical etching amount of 20 µm. Although the straightness of the straight ridges of the recessed portion was found to be excellent, the side etching width was as large as 105 µm. This indicates that the mere preliminary annealing of the mirror-ground surface of the ferrite bar at 500°C for two hours is not effective enough to assure sufficient reduction in the side etching width of the recessed portion formed on the surface of the ferrite single crystal. EXAMPLE 3 The (110) surface of a Mn-Zn ferrite single crystal bar was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm. The ferrite bar was then annealed at 500°C for 2 hours in an atmosphere of N₂, and was subjected to a preliminary surface treatment for 30 minutes in an aqueous solution containing 1.5% of ethanol amine kept at 25°C. An etching resist mask was then formed on the annealed surface of the ferrite bar, under the photolithographic condition indicated in Table 1. Then, the masked ferrite bar was chemically etched in an aqueous solution containing 77.8% of phosphoric acid (H₃PO₄) kept at 80°C. Thus, a recessed portion corresponding to the etching mask pattern was formed on the mirror-ground surface of each Mn-Zn ferrite bar, with a vertical etching amount of 20 µm. The side etching width and the straightness of the ridges of the recessed portion thus formed by chemical etching on the Mn-Zn ferrite surface were measured and observed. The straightness was found to be excellent, and the side etching width was as small as 18 µm. Thus, the instant example indicates that the annealing of the Mn-Zn ferrite surface prior to the immersion in the ethanol amine solution permits a sufficiently small side etching width and excellent ridge straightness of the recessed portion, even where the concentration of ethanol amine in the treatment solution is relatively low and the time of immersion in the solution is relatively short. Example 4 The (110) surface of each of prepared Mn-Zn ferrite single crystal bars was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm. The ferrite bars were then immersed for different periods of time in an aqueous solution containing 77.8% of phosphoric acid kept at 50°C, to etch their mirror-ground surfaces to different preliminary etching depths. The preliminary etching time per 15 nm of preliminary etching depth was 30 seconds. Etching resist masks were formed under the photolithographic condition of Table 1, on the mirror-ground surfaces of the ferrite bars which were given the different preliminary etching depths. The masked surfaces were then chemically etched in an aqueous solution containing 77.8% of phosphoric acid kept at 80°C, whereby the appropriate recessed portions were formed on the mirror-ground surfaces of the ferrite bars, with a vertical etching amount of 20 µm. The graph of Fig. 8 shows the measured side etching widths of the recessed portions of the Mn-Zn ferrite single crystal bars, in relation to the preliminary etching depth obtained in the preliminary etching step. As indicated in Fig. 8, the minimum side etching width was reduced from 95 µm to 35 µm by subjecting the mirror-ground surface of the ferrite bar to a preliminary etching by 1.6 nm. Further, the difference between the maximum and minimum side etching widths (variation of the side etching width) was reduced from 37 µm to 7 µm. Thus, it was found that the preliminary etching step provided a considerable improvement in the dimensional and configurational accuracy of the formed recessed portions of the ferrite bars. It follows from the graph of Fig. 8 and Table 3 below that the depth of the preliminary etching prior to the formation of the etching mask is preferably at least 100 nm, for further improvement in the dimensional and configuration accuracy of the formed recessed portion of the ferrite bar. Namely, the side etching width of the ferrite bars was consistently held at about 20 µm, with a minimum variation from one ferrite bar to another. Preliminary Etching Depth Straightness Side Etching Width µm Total Evaluation 1.3 nm Fair 35-40 Good 48 nm Fair 22-24 Fair 170 nm Excellent 20 Excellent COMPARATIVE EXAMPLE 3 The (110) plane of each of Mn-Zn ferrite single crystal bars was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm. Without the preliminary treatment with the phosphoric acid aqueous solution as used in Example 4, the etching resist mask was formed on the mirror-ground surface of each ferrite bar, under the same condition as indicated in Table 1. Then, the masked ferrite bars were chemically etched in an aqueous solution containing 77.8% of phosphoric acid (H₃PO₄) kept at 80°C. Thus, a recessed portion corresponding to the etching mask pattern was formed on the mirror-ground surface of each Mn-Zn ferrite bar, with a vertical etching amount of 20 µm. The side etching width and the straightness of the straight ridges of the recessed portion formed on the ferrite bar of Comparative Example 3 were measured and observed. The measurements of the side etching widths ranged from 90 µm to 135 µm, which are not desirable to obtain the intended recessed portion. Further, the straight ridges of the recessed portion showed poor straightness. Thus, the absence of preliminary treatment of the mirror-ground surface of the Mn-Zn ferrite bar with the phosphoric acid aqueous solution resulted in excessively large side etching widths and poor straightness of the straight ridges. EXAMPLE 5 The (110) surface of each of Mn-Zn ferrite single crystal bars was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm. The ferrite bars were then annealed at 500°C for 2 hours in an atmosphere of N₂. The ferrite bars were then immersed for different periods of time in an aqueous solution containing 77.8% of phosphoric acid kept at 50°C, under the same condition as in Example 1, whereby the mirror-ground surfaces of the ferrite bars were etched to different preliminary etching depths. Etching resist masks were formed under the photolithographic condition of Table 1, on the mirror-ground surfaces of the ferrite bars which were given the different preliminary etching depths. The masked surfaces were then chemically etched in an aqueous solution containing 77.8% of phosphoric acid kept at 80°C, whereby the appropriate recessed portions were formed on the mirror-ground surfaces of the ferrite bars, with a vertical etching amount of 20 µm. The graph of Fig. 9 shows the measured side etching widths of the recessed portions of the Mn-Zn ferrite single crystal bars of Example 5, in relation to the preliminary etching depth obtained in the preliminary etching step. A comparison of the result of Fig. 9 with the result of Fig. 8 of Example 4 reveals that the annealing of the mirror-ground surfaces of the ferrite bars prior to the formation of the etching resist masks permits the side etching width to be consistently held at 20 µm, even with the preliminary etching depth of 1.3 nm. It was found that only the annealing treatment without the preliminary etching resulted in a considerably large variation of 22-105 µm in the side etching width, from one ferrite bar to another. That is, the absence of the preliminary etching causes insufficient consistency in the side etching width of the products obtained from the same ferrite blank. EXAMPLE 6 A selected surface of each of Mn-Zn ferrite single crystal bars having different crystal orientations was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm. The ferrite bars were then annealed at 500°C for 2 hours in an atmosphere of N₂. The ferrite bars were then immersed in an aqueous solution containing 77.8% of phosphoric acid kept at 50°C, under the same condition as in Example 1, whereby the mirror-ground surfaces of the ferrite bars were etched to a preliminary etching depth of 10 nm. Etching resist masks were formed under the photolithographic condition of Table 1, on the preliminarily etched mirror-ground surfaces of the ferrite bars. The masked surfaces were then chemically etched in an aqueous solution containing 77.8% of phosphoric acid kept at 80°C, whereby the appropriate recessed portions were formed on the mirror-ground surfaces of the ferrite bars, with a vertical etching amount of 10 µm. Table 4 shows the dimensional accuracy and cross sectional shape of the recessed portions formed on the ferrite single crystal bars which have the different crystal orientations. It will be understood from Table 4 that the dimensional accuracy of the recessed portions is comparatively low and the cross sectional shape of the recessed portions is asymmetrical, where the straight ridges each defining a boundary between two adjacent surfaces do not lie in a (100) or (110) plane of the ferrite single crystal which is perpendicular to the etched surface on which the recessed portion is formed by chemical etching through the etching mask. More specifically, the following combinations of the etched surface and the direction of the ridges are relatively poor in the dimensional accuracy and symmetry of the recessed portion formed on the ferrite single crystal, and the recessed portion cannot suitably be used as a track of a magenetic head core: (211) and 〈110〉; (311) and 〈110〉; and (332) and 〈110〉. EXAMPLE 7 A ferrite bar 12 as shown in Fig. 2(a) for monolithic RDD magnetic head cores was prepared from two ferrite single crystals which were bonded together so as to form an annular magnetic path, such that the surfaces defining the magnetic gap 11 were defined by (100) planes of the two crystals while the sliding surface 13 and the side surfaces of the bar 12 were defined by (110) planes of the crystals. The sliding surface 13 was mirror-ground in an ordinary manner with a diamond abrasive having a grain size of 1/4 µm, and then annealed for two hours at 500°C in an atmosphere of N₂. The annealed surface 13 was subjected to a preliminary etching in an aqueous solution containing 77.8% of phosphoric acid kept at 50°C, to etch the surface to a preliminary etching depth of 10 nm. Subsequently, the etching resist mask 14 was formed on the thus treated surface 13, by the photolithographic method whose condition is indicated in Table 1. The masked surface was chemically etched in an aqueous solution containing 77.8% of phosphoric acid kept at 80°C. Thus, there was formed a plurality of recessed portions each including two air-bearing surfaces 3 and one track 4 which are defined by the 70°-inclined chamferred surfaces 5-10 and the groove having a depth of 20 µm formed between the air-bearing surfaces 3. In the instant example, the side etching width was also 20 µm. Therefore, the width of the portions of the etching mask 14 which correspond to the air-bearing surfaces 3 and track 4 should be larger by 40 µm than the nominal width of the air-bearing surfaces and track 3, 4. The chamferred surfaces 16 were then formed at the end portions of the air-bearing surfaces and tracks 3, 4 of the recessed portions, as shown in Fig. 2(a). Then, the ferrite bar 12 was subjected to a cutting operation so that the yoke portion 17 including the track 4 was left for each recessed portion which corresponds to one monolithic RDD magnetic head core. Finally, the ferrite bar 12 was cut into the individual magnetic head cores. According to the instant method of forming the magnetic head cores in a chemical etching process, the air-bearing surfaces 3 and the track 4, and the chamferred surfaces 5-10 adjacent to the surfaces 3 and track 4 were concurrently formed, such that the widths of the surfaces 3 and track 4 were determined by the parallel edges (straight ridges 15) of the chamferred surfaces 5-10. Accordingly, the number of process steps of the present method to form the magnetic head cores is considerably reduced, as compared with the conventional method by grinding with a diamond wheel. Further, the present method assures extremely high dimensional accuracy of the air-bearing surfaces 3 and track 4 to within ± 1 µm, which is determined by the dimensional accuracy of the photolithographically formed etching mask and by the side etching width accuracy in the chemical etching step. This dimensional accuracy of the air-bearing surfaces and track 3, 4 is a significant improvement over the highest possible dimensional accuracy of about ± 10 µm which is available in the conventional method wherein the chamferred surfaces defining the widths of the air-bearing surfaces and track 3, 4 are formed by grinding.

Claims: A method of forming an recessed portion on a surface of a Mn-Zn ferrite single crystal, said recessed portion having an inclined surface (7,8) and at least one straight ridge (15) which defines an edge of said inclined surface (7,8), said inclined surface being parallel to said ridge, the method comprising the steps of forming an etching mask (14) on said ferrite single crystal, and exposing said ferrite single crystal with said etching mask thereon to etching liquid thereby chemically etching said ferrite single crystal, to produce said straight ridge (15), characterised in that said inclined surface (7,8) is formed by said etching step with said mask (14) thereon, and in that said at least one straight ridge lies in a (100) or (110) plane of said ferrite single crystal which is perpendicular to said surface. A method according to claim 1, wherein the combination of the plane of said surface of said ferrite single crystal and the direction of extension of said at least one straight ridge is selected from the following combinations:- (100) and 〈100〉; (100) and 〈110〉; (110) and 〈110〉; (211) and 〈111〉; (311) and 〈332〉; (332) and 〈311〉; (611) and 〈331〉; and (331) and 〈611〉. A method according to claim 1 or 2, further comprising the step of exposing said surface of the Mn-Zn ferrite single crystal to a solution containing an amine compound, prior to forming said etching mask on said surface. A method according to claim 1 or claim 2, further comprising a step of removing a strain from said Mn-Zn ferrite single crystal, prior to forming said etching mask (14) on said surface. A method according to claim 4, comprising the step of exposing said surface of the Mn-Zn ferrite single crystal to a solution containing an amine compound, prior to forming said etching mask on said surface. A method according to claim 5, further comprising performing said step of removing strain by annealing said Mn-Zn ferrite single crystal, before the treatment with said solution containing an amine compound. A method according to claim 6, wherein said step of annealing said Mn-Zn ferrite single crystal is effected at a temperature of 200-600°C for a period of at least 10 minutes in an inert atmosphere. A method according to any one of claims 3 and 5 to 7, wherein said amine compound is selected from: alkanol amines; aliphatic primary amines; aliphatic secondary amines; aliphatic tertiary amines; aliphatic unsaturated amines; alicyclic amines; diamines; triamines; and aromatic amines. A method according to claim 8, wherein said amine compound is selected from alkanol amines. A method according to any one of claims 3 and 5 to 9 wherein the concentration of said amine compound in said solution is held within a range not exceeding 50% by weight. A method according to claim 1 or 2, further comprising a step of exposing said surface of the Mn-Zn ferrite single crystal to an aqueous solution containing phosphoric acid, so as to etch said surface to a depth of at least 1 nm (10 angstroms), prior to forming said etching mask on said surface. A method according to any one of claims 1 to 11, further comprising a step of mirror-grinding said surface of said Mn-Zn ferrite single crystal, prior to forming said etching mask on said surface. A method according to claim 12, wherein said step of mirror-grinding said surface of said Mn-Zn ferrite single crystal is effected with a diamond abrasive having a grain size of 1/4 µm. A method according to any one of claims 1 to 13, wherein said surface of the Mn-Zn ferrite single crystal is formed is a (110) plane of said crystal, while said at least one straight ridge extends in a 〈100〉 direction of the crystal. A method according to any one of claims 1 to 14, wherein said etching liquid is an aqueous solution of phosphoric acid. A method according to claim 15, wherein said aqueous solution contains 5-80% by weight of phosphoric acid. A method according to claim 16, wherein said aqueous solution is kept at a temperature of 50-90°C.

Inventors: FUKUDA, NAOYA, SUZUKI, MIKI, TAKEYA, FUMINORI, Takeya, Fuminori, c/o NGK Insulators Ltd.
Applicants: NGK INSULATORS, LTD.
Application year: 1989
Main classification: C30B33
IPC codes: G11B5, C30B33, H01F1