Process for manufacturing a magnetic bubble memory chip
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for manufacturing a magnetic bubble memory chip, and more particularly, to a process for manufacturing a dual spacing type magnetic bubble memory chip having a thin bubble film, on which a first area is provided with minor loop transmission lines for memorizing bubble information and a second area is provided with major transmission lines for recording or reading out the bubble information with respect to the minor loop transmission lines. 
     Recent developments in the design of magnetic bubble memory chips has led to a demand that the bubble information accommodating sections of these chips, comprising the minor loop transmission lines, be made more and more compact to enable a higher density installation of these devices. For instance, in a 4 megabit chip the pattern periods should be 4 μm. On the other hand, in the function sections, such as swap gates, replicate gates, and generators, and major transmission lines (major lines) for recording or reading out the bubble information accommodated in the minor loop transmission line organization, relatively large patterns, such as 16 μm periods, are used to enable a higher degree of positioning accuracy during the manufacturing process of the bubble memory chip. However, where different size transmission patterns are used in a single memory chip, as mentioned above, it has been found that problems concerning the characteristics will occur due to differences in the driving forces. 
     FIG. 1 illustrates a cross-section of a magnetic bubble memory chip having a dual spacing layer structure manufactured in accordance with a prior art technique, and FIG. 2 illustrates an enlarged cross-section of the bubble memory chip of FIG. 1 during the manufacturing process. According to the known art, a layer of SiO 2  2 is formed on a garnet film 1 by sputtering and gold (Au) conductive patterns 3 for controlling function gates are formed thereon. In order to level the steps of the conductive pattern 3, a high temperature tolerant resin 4, such as polyladder-organosiloxane (PLOS), is coated thereon and thermoset, and then only the PLOS layer 4 in an area 5 of the chip corresponding to the small-sized transmission patterns for the information accommodating section is etched, with a plasma etching agent including CF 4  etchant gas. The permalloy patterns 6 for the transmission patterns are formed thereafter. 
     The above-mentioned prior art method has several problems. A first is the unevenness of the rate of etching in each batch when manufacturing the magnetic bubble memory chips. Conventionally, a planner plate type etching machine is generally used for etching such a high temperature tolerant resin 4, and although this type of etching machine is superior to a cylindrical type etching machine in etching evenness, usually there is still a variation in the rate of etching of within ±5% per batch. For instance, if the film thickness of the high temperature tolerant resin layer 4 is 3800 Å and the thickness is to be reduced by etching by 3000 Å, to obtain a spacer having a thickness of 1300 Å including the SiO 2  layer 2 of 500 Å, a variation of etching within ±150 Å will occur per batch when the thickness of the high temperature tolerant resin is reduced by 3000 Å. In addition, when variation between the batches are taken into account, there may be a variation of at least ±300 Å. Such a large variation of ±300 Å will cause a serious problem, i.e., degradation of performance of the bubble memory chip, since 1300 Å is the most favorable thickness for a spacer, as will be described hereinafter. 
     A second problem is that when an uneven face 7 having a thickness of 200 Å to 300 Å is formed on the etched surface, as seen from FIG. 2, the permalloy film formed thereon may have a local high spot Hc, which will affect the characteristics of the bubble memory. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a process for manufacturing a magnetic bubble memory chip, which process eliminates the disadvantages as discussed above with reference to the known conventional manufacturing process. 
     It is another object of the present invention to provide a process for manufacturing a magnetic bubble memory chip, wherein there is less variation in the thickness among the spacers and the surfaces thereof are smooth, thus, attaining stable characteristics. 
     According to the present invention, there is provided a process for manufacturing a magnetic bubble memory chip having a thin bubble film, on which a first area is provided with minor loop transmission lines for memorizing bubble information and a second area is provided with major transmission lines for recording or reading out the bubble information with respect to the minor loop transmission lines. This process comprises the steps of: (1) forming a first insulative layer on the thin bubble film over the first and second areas; (2) forming conductive patterns on the first insulative layer for controlling the function gates; (3) coating a second insulative layer of resin over the first insulative layer and conductive patterns for controlling function gates, and thermosetting the coated second insulative resin layer, (4) removing by etching the part of the first insulative layer and the second insulative resin layer on the first area; (5) forming a third insulative layer over the whole surface including the first and second areas; and (6) forming minor loop transmission lines and major transmission lines on the third insulative layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a magnetic bubble memory chip manufactured in accordance with a known conventional process; 
     FIG. 2 is a cross-sectional view of one of the steps of the known conventional process of manufacturing the bubble memory chip; 
     FIG. 3 is a diagram illustrating the relationship between the thickness of the spacer of a magnetic bubble memory chip and the characteristics thereof, i.e., the bias margin; 
     FIGS. 4A to 4F are a series of cross-sectional views of a process for manufacturing a magnetic bubble memory chip according to the present invention; 
     FIG. 5 is a plan view of an embodiment of a magnetic bubble memory chip manufactured in accordance with the present invention; 
     FIG. 6 is a plan view of a minor loop organization and areas adjacent to the gates thereof in the embodiment of FIG. 5; and, 
     FIGS. 7 and 8 are plan views of the shapes of soft magnetic patterns used in the first and second areas, respectively, in the embodiment of FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 shows the relationship between the thickness of the spacer and the bubble propagation performance when using a soft magnetic pattern having 4 μm pattern periods. As can be seen the figure, a thickness of 1300 Å ensures the most favorable relationship. 
     FIGS. 4A to 4F sequentially illustrate the respective steps in a process of manufacturing a magnetic bubble memory chip according to the present invention. In FIGS. 4A to 4F, reference numeral 10 denotes a thin garnet film of a magnetic bubble; 11, a first insulative layer; 12, a conductive pattern for controlling function gates; 13, a second insulative layer; 14, a first area; 15, a second area; 16, a photoresist layer; 17, a third insulative layer; and 18 and 18&#39;, soft magnetic patterns for bubble transmission. 
     An embodiment of a process for manufacturing a magnetic bubble memory chip according to the present invention will now be described with reference to FIGS. 4A to 4F. 
     In a first step, as shown in FIG. 4A, a first insulative layer 11 is formed by sputtering SiO 2  to a 500 Å thickness on a magnetic bubble thin film 10 formed on a substrate. 
     In a second step, as shown in FIG. 4B, a function gate controlling conductor consisting of Au, at a thickness about 3500 Å, is formed on the first insulative layer 11 by vapor deposition, and then a function gate controlling conductive pattern 12 is formed by sputtering with a photolithographic method. In this case, TaMo having a thickness 100 Å to 150 Å also can be used to improve the adhesiveness of the Au conductor 12 to the SiO 2  insulative layer 11. 
     In a third step, as shown in FIG. 4C, a polyladder-organosiloxane resin (PLOS) having a thickness of 2600 Å, is coated over the whole surface and thermoset to form a second insulative layer 13. 
     In fourth step, as shown in FIG. 4D, the second area 15 of this chip is covered with a resist 16. On the other hand, in the first area 14 of this chip which will serve as an information accommodating section, the first and second insulative layers 11 and 13 comprising SiO 2  and PLOS resin, respectively, are plasma etched with an etching agent including CF 4  etchant gas in a planner plate type dry etching machine, to expose the surface of the thin film 10 in the first area 14 of the magnetic bubble memory chip. In this case, both the first and second insulative layers of SiO 2  and PLOS resin can be etched with the same etchant, since a main component of PLOS is Si. 
     In a fifth step, as shown in FIG. 4E, a third insulative layer 17 of SiO 2 , having a thickness of 1300 Å, is formed by sputtering over the first 14 and second 15 areas. As seen from FIG. 3, the thickness of the insulative layer 17 is favorably 1200 Å to 1500 Å, most preferably 1300 Å. 
     At the sixth and final step, as shown in FIG. 4F, after a soft magnetic material, such as permalloy, is formed by vapor deposition on the third insulative layer 17, soft magnetic patterns 18 and 18&#39; for bubble transmission, having different pattern periods, are formed thereon by photolithography or ion-etching. In this case, the soft magnetic pattern 18 constitutes transmission lines in the minor-loop organization in the first area 14 and the pattern period thereof is, for example, 4 μm to 6 μm. On the other hand, the soft magnetic pattern 18&#39; constitutes major transmission lines and a pattern for connecting the major transmission lines to the minor-loop organization, and the pattern period thereof is, for example, 8 μm to 16 μm. 
     According to the illustrated embodiment, since the magnetic bubble thin film 10 is not etched at all with the plasma etching agent using CF 4 , a slight overetching is effective to prevent any nonuniformity of the etching and variations of the etched layer thickness. Therefore, magnetic bubble memory chips having a stable propagation performance can be obtained. 
     FIG. 5 is a plan view of an embodiment of a magnetic bubble memory chip manufactured in accordance with the present invention. In FIG. 5, reference numerals 20-1 to 20-n and 21-1 to 21-n denote minor-loops organization; 22 and 23, boot loops for memorizing information concerning faulty loops in the minor-loop organization; 24, detectors; 25, recording major lines; 26, read out major lines; 27, generators; 28, swap-gates; and 29, replicate-gates. 
     The embodiment shown in FIG. 5 is an odd-and-even number type major-minor organization, in which the first area comprises the minor-loop organization 20-1 to 20-n and 21-1 to 21-n, which serve as an information accommodating section, and the boot loops 22 and 23. The first area 14 uses small soft magnetic patterns 18 (FIG. 4F) having 4 μm periods and the thickness of the spacer thereof is 1300 Å, as seen from FIG. 4F. 
     The area other than the first area 14 is the second area, comprising, for instance, the generators 27, the recording major lines 25, the read out major lines 26, the swap-gates 28, the replicate gates 29, the detectors 24, and some small portions of the minor loop organization adjacent to the gates. The second area is constituted by soft magnetic patterns of 8 μm to 16 μm periods and the thickness of the spacer thereof is 4400 Å, as seen from FIG. 4F. 
     FIG. 6 is a plan view of the minor-loop organization and the areas adjacent to the gates thereof, in which reference numeral 30 denotes a minor-loop organization; 25, recording major lines; 26, read out major lines; 28, swap-gates; and 29, replicate gates. In FIG. 6, the area illustrated by hatching in the minor loop organization 30 is the first area. On the other hand, the area of a few bits adjacent to the gates using large soft magnetic patterns 35 for connecting is included in the second area. Also, as will be understood from FIG. 6, the boundary portions between the first and second areas are defined by bubble transmission gaps between adjacent patterns of the respective areas. 
     FIGS. 7 and 8 illustrate soft magnetic patterns used for the transmission lines in the minor loop organization 30 (FIG. 6) in the first area, and for the patterns 35 (FIG. 6) in the second area, respectively. The pattern illustrated in either FIGS. 7 or 8 is a so-called &#34;Wide Gap&#34; pattern, which has more advantages in size and in the possibility of high density loading than the half disk pattern or asymmetrical chevron pattern conventionally used in the prior art, since the gap spacing of the former is larger than that of the latter. 
     As discussed hereinbefore in detail, a method for manufacturing a magnetic bubble memory chip according to the present invention is particularly advantageously adopted when manufacturing a dual-spacing layer structure or dual spacing type bubble chip consisting of thin and thick layers. Consequently, according to the present invention, when the thin layer is formed, the unevenness of etching and variations in thickness thereof are prevented, so that magnetic bubble memory chips having stable bubble propagation performance and characteristics with less variations can be obtained.