Method for forming a pattern having a fine gap.

Abstract: A method is disclosed of forming a fine pattern on a substrate (1), in which an etching mask pattern (B) is formed on a layer of material (A) of a pattern to be formed, an overlying layer (C) is deposited on a pattern material layer (A) and the mask pattern (B), and thereafter, the overlying layer and the pattern material layer are etched by ion etching. This method makes it possible, due to the pattern-­ widening effect caused by the deposition of the overlying layer and by ion etching, to form a pattern having a gap smaller than 0.5 um or a continuous-disk pattern having a period of 2 um or less by photolithography having a 1 um resolution. It is also possible to form a pattern adapted to enable an easy planing process, by utilizing the difference in etching rate between the mask pattern and the overlying layer. An apparatus used for carrying out the pattern forming method comprises film layer forming means and ion etching means, whereby the overlying layer forming step and the ion etching step can be successively performed by using one and the same apparatus.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a micro-fabrication technique which may be advantageously used for manufacturing various kinds of devices, such as a magnetic bubble memory device, a semiconductor device, a surface acoustic wave device, or a Josephson device. More particularly, the present invention relates to a method for forming a pattern of a material having a fine gap, such as a conductive material or magnetic material, for example, on a substrate of the device. 2. Description of the Related Art In the forming of patterns in the devices as mentioned above, several important problems arise. One of these problems concerns making a fine pattern for improving the characteristics and reducing the size or increasing the density of the device. For example, in a permalloy bubble memory device in which a bubble propagation path is defined by a permalloy pattern constituted by permalloy elements arranged with gaps therebetween, the width of the gap between the adjacent permalloy elements is one of the factors deciding the bubble propagation characteristics, and it is advantageous to make the gap width as small as possible. However, as described hereinafter, it is very difficult to form a gap of less than 1 µm (submicron gap), in particular, of 0.5 µm or less, by using a conventional technique. Accordingly, under the existing circumstances, it is necessary to pursue any improvement in the propagation characteristics and memory density of the permalloy bubble memory device by using newly-designed patterns having gaps of 1 µm or more. In this connection, the T-I pattern, chevron pattern, or the like, were used at the early stages. At the present stage, however, the so-called gap-tolerant patterns, e.g., half-disk pattern and asymmetric chevron pattern, are used, whereby a bubble memory having a 2 µm bubble diameter, 8 µm bit period, and 1 Mbit memory density, is achieved. Furthermore, recently, the so-called wide-gap pattern has been proposed as an approach to the realization of a bubble memory having a 1 µm bubble diameter, 4 µm bit period, and 4 Mbit memory density. However, to achieve a further improvement in operating characteristic and memory density, the gaps in the permalloy pattern must be made smaller than 1 µm, in particular, smaller than 0.5 µm. Further, there is a well-known ion-implanted bubble memory device in which, in order to overcome the above-mentioned gap problem, the bubble propagation path is defined by a contiguous-disk pattern formed by using an ion-implantation technique. The contiguous-disk pattern is constituted by contiguously arranged or overlapped patterns having a relatively simple shape, such as round, oval, or square. Therefore, the contiguous-disk pattern basically requires no gap for separation of the adjacent patterns and is simple in shape, thus making it possible to form a pattern having a smaller bit period than the permalloy pattern, by using a conventional technique. At present, a 4 µm period pattern for the propagation of 1 µm diameter bubble has been achieved. However, in achieving a contiguous-disk pattern with a period smaller than 4 µm, in particular, a period of 2 µm or less, the problem described below occurs. In the contiguous-disk pattern, the shape of the cusps defined by the contiguous or overlapped disk patterns is one of the factors deciding the bubble propagation characteristics. However, a 2 µm period pattern has cusps too minute to be precisely formed by using a conventional technique. It should be noted that, in the pattern forming art, it can be deemed that the cusp is a kind of gap and, accordingly, the gap forming technique described hereinafter includes the cusp forming technique. Another problem in the forming of a pattern in the devices as mentioned above concerns a sectional configuration of the pattern. In many of these devices, a plurality of patterns are deposited one above the other. In these circumstances, to prevent deficiencies such as disconnection of the overlying pattern, a planing process must be performed by coating a resin over the underlying pattern. However, according to a conventional pattern forming method, it is impossible to control the sectional configuration of a pattern in such a manner that the sides of the pattern have a gentle slope. Accordingly, the pattern planing process cannot be easily performed. The fine pattern, such as the permalloy pattern and contiguous-disk pattern as described above, is conventionally formed by using lithographic techniques employing light, X-rays, or electron beam (hereinafter, generically referred to as "photolithography"). According to a conventional pattern forming method, first, a layer of material from which a pattern is formed (i.e., pattern material layer) is deposited on a surface of a substrate of a device. Next, a mask pattern used for the etching is formed of a photoresist material on the pattern material layer by photolithography. Thereafter, the pattern material layer is etched to form the pattern. According to this method, the smallest size of the formed pattern attainable basically depends on the limit of the mask pattern forming technique, i.e., of the photolithography. Recently, photolithography techniques have remarkably advanced, but the limit of their practical resolution is 1 µm. Accordingly, it is very difficult to form a submicron gap by using the conventional pattern forming method mentioned above. On the other hand, there are some known methods for forming a submicron gap by employing a photolithography having a 1 µm resolution. One of these methods uses the technique of shortening the time for optical exposure of the pattern onto the photoresist material in the mask pattern forming process. That is, a positive photoresist material is used and the exposure time is shorter than the normal time, whereby it is possible to form a mask pattern having a submicron gap and, thus, a submicron pattern gap, under the condition of a 1 µm resolution. However, the shortening of the exposure time is naturally limited, since an inferior patterning of the mask pattern is apt to occur, due to variations in the amount of light and the size of the exposed pattern. Another known method uses an etching technique employing energized particles, for example, an ion etching technique. An ion etching process presents the phenomenon wherein a part of the material removed from the pattern material layer is redeposited onto the mask pattern and the pattern material layer and, as a result, the effect is obtained that the pattern is widened and, thus, the pattern gap is correspondingly narrowed. Therefore, it is possible to form a submicron gap pattern on the basis of a 1 µm gap mask pattern. However, according to the conventional pattern forming method, an effective pattern-widening effect (i.e., gap-narrowing effect) cannot be obtained, even if the ion etching technique is used, as described hereinafter. Moreover, even if both pattern-widening effects caused by the shortening of the exposure time and the ion etching technique are combined, the smallest gap width which can be obtained by the conventional pattern forming method is 0.5 µm. In addition, a very high level of process control technique is required for the above, and this is not suitable to practical use. A further known method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 54(1979)-155771 . This method basically utilizes the above-mentioned pattern-widening effect caused by ion etching. That is, a pattern gap control layer is deposited on the pattern material layer, and a mask pattern is formed on the pattern gap control layer. Thereafter, the pattern gap control layer and the pattern material layer are ion-etched, so that a remarkable pattern-widening effect is obtained and, accordingly, a pattern gap which is remarkably smaller than the mask pattern gap is formed. However, this method has problems in that the processes of forming the pattern gap control layer and the etching process are expensive and the processing takes a long time, and that a process of removing the mask pattern and the pattern gap control layer after the etching process has been completed is complicated, as described hereinafter with reference to the drawings. Therefore, this method is disadvantageous from the viewpoint of production efficiency. There is also disclosed in European Patent Application EP-A-0 043 942 a method for forming narrow dimensioned dielectric regions within an integrated circuit. According to this method, the fine gap pattern is obtained by using a material deposited on the vertical surfaces of the pattern, thereby narrowing the original pattern gap. An etching process is then conducted, with the deposited material selectively maintained to define the gaps at the appropriate locations. Furthermore, according to the conventional and known pattern forming methods, the sectional configuration of the formed pattern depends, in principle, on the etching tchnique and also cannot be controlled. Therefore, a pattern formed by the conventional and known methods has a sectional configuration with the sides thereof being steeply sloped, thereby making it difficult to perform the planing process. SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a pattern forming method by which a pattern having submicron gaps or cusps exceeding the limit of the photolithography can be easily and efficiently formed. According to one method of the present invention as claimed in claim 1, there is provided a method for forming a pattern having a fine gap on a substrate, comprising the steps of : depositing a layer of a pattern material on the surface of the substrate; forming an etching mask pattern having a first gap on the pattern material layer; depositing a pattern-widening overlying layer which substantially maintains the topography of the mask pattern on the exposed surfaces of the pattern material layer and the mask pattern, thereby forming a second gap which is smaller than said first gap; and etching the overlying layer and the pattern material layer by ion etching employing energized particles of an inert gas, thereby forming a pattern having a fine gap smaller than the second gap from the pattern material layer. According to this method, a large pattern-widening effect is caused by both the deposition of the pattern-widening overlying layer and the ion etching, and accordingly, it is possible to form a pattern having a fine gap of 0.5 µm or less, on the basis of an etching mask pattern having a gap of 1 µm or more formed by photolithography. Moreover, according to an embodiment of this method, it is possible to form a contiguous-disk pattern having a fine cusp on the basis of an etching mask pattern having a gap of 1 µm or more. The present invention is now described in detail based on the preferred embodiment and in comparison with the prior art, with reference to the accompanying drawings. BRIEF EXPLANATION OF THE DRAWINGS Figures 1A through 1C illustrate a basic process of a conventional pattern forming method; Fig. 2 is a graph illustrating the pattern-widening effects caused by the shortering of the pattern exposure time and the ion etching in the conventional method; Figs. 3A through 3D illustrate a basic process of another conventional pattern-forming method; Figs. 4A through 4E illustrate a basic process of a first embodiment of a pattern forming method according to the present invention; Figs. 5A through 5C are graphs illustrating the gap-narrowing effect (i.e., pattern-widening effect) in the method of the present invention; Figs. 6A through 6J illustrate a process of manufacturing a permalloy magnetic bubble memory device according to the present invention; Fig. 7 is a graph illustrating the characteristics of the bubble memory devices manufactured by the present invention and the conventional method in comparison with each other; Fig. 8 is a view illustrating a disadvantage of the first embodiment of the present invention; Figs. 9A through 9G illustrate a second embodiment of the present invention; Figs. 10A through 10I illustrate a third embodiment of the present invention; Figs. 11A through 11H illustrate a fourth embodiment of the present invention; Figs. 12A through 12C illustrate a known ion-implanted magnetic bubble memory device, wherein Fig. 12A is a partial plan view, and Figs. 12B and 12C are sectional views taken along lines B-B and C-C, respectively, in Fig. 12A; Figs. 13A through 13D illustrate a basic process of a method for forming a mask pattern for the ion-implantation, which is a fifth embodiment of the present invention; Figs. 14A and 14B illustrate two variations of the contiguous-disk pattern. DESCRIPTION OF THE PREFERRED EMBODIMENTS First, pattern forming methods according to the prior art will be described. A basic process of a conventional method is illustrated in Figs. 1A through 1C. Referring first to Fig. 1A, a pattern material layer 2 is deposited on a substrate 1 by vapor deposition. On the pattern material layer 2, an etching mask pattern 3 having a thickness of t₂ is formed of a photoresist by photolithography. The reference character g₁ designates the width (at the bottom) of a gap of the mask pattern 3. Next, the pattern material layer 2 is etched as illustrated in Fig. 1B, and, finally, the mask pattern 3 is removed, so that a desired pattern 2P is formed on the substrate 1 as illustrated in Fig. 1C. The reference character g₂ designates the gap width (at the bottom) of a gap of the pattern 2P. When forming a permalloy propagation pattern of a permalloy bubble memory device, for example, according to the above-mentioned method, the pattern material layer 2 is made of permalloy with the thickness t₁ of 300nm (3,000 Å), and the mask pattern 3 is formed of a positive photoresist material, for example, "AZ1350J" (RTM, Shipley company) with the thickness t₂ of 675nm (6,750 Å). In this case, if the exposure time for the photoresist material is set at the normal time, the gap width g₁ of the mask pattern 3 has the limit of 1 µm. Moreover, if a chemical etching is used to etch the permalloy layer 2, the gap width g₂ of the permalloy pattern 2P also has the limit of 1 µm. Therefore, a submicron gap cannot be formed. On the other hand, as described hereinbefore, if the pattern exposure time of the photoresist is shorter than normal, the mask pattern gap width g₁ can be made smaller than 1 µm. As also described hereinbefore, by employing ion etching, the pattern width of the permalloy pattern 2P can be made wider than that of the mask pattern 3, with the result that the permalloy pattern gap width g₂ can be made narrower than the mask pattern gap width g₁. Referring to Fig. 2, there is illustrated the mask pattern width (at the bottom) w₁ and the permalloy pattern width (at the bottom) w₂ , in relation with the exposure time T, in the example in which the mask pattern 3 is formed by exposing a pattern having a pattern width and gap width which are both 1 µm, and ion etching is used to form the permalloy pattern 2P. It can be understood from Fig. 2 that, if the exposure time is shortened, the mask pattern width w₁ is made wider than 1 µm (i.e., the mask pattern gap width g₁ is narrower than 1 µm), and also that the permalloy pattern width w₂ is wider than the mask pattern width w₁ , due to the pattern-widening effect caused by ion etching. For example, in the case of an exposure time of T = 0.20 sec, the mask pattern width w₁ is 1.46 µm and the permalloy pattern width w₂ is 1.50 µm, whereby the permalloy pattern gap width g₂ is 0.5 µm. However, as described hereinbefore, the shortening of the exposure time is naturally limited. Moreover, although an increased mask pattern thickness t₂ contributes to the increase of the pattern-widening effect caused by the ion etching, a too large thickness t₂ causes the permalloy redeposited onto the mask pattern to form a wall at the edge of the permalloy pattern 2P, thereby resulting in an inferior patterning. Consequently, according to the conventional methods, if pattern-widening effects (i.e., gap-narrowing effect) caused by both the shortening of the exposure time and the ion etching are combined, the smallest pattern gap width which can be formed by photolithography having a 1 µm resolution, is approximately 0.5 µm. A process of the pattern forming method disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 54(1979)-155771 is illustrated in Figs. 3A through 3D. Referring first to Fig. 3A, on a substrate 4, a pattern material layer 5 and a pattern gap control layer 6 are successively deposited. Further, on the pattern gap control layer 6, an etching mask pattern 7 having a gap width S₁ is formed. Thereafter, as illustrated in Fig. 3B, the pattern gap control layer 6 is etched, and, as illustrated in Fig. 3C, the pattern material layer 5 is then etched, and, finally, as illustrated in Fig. 3D, the mask pattern 7 and the pattern gap control layer 6 are removed, so that a pattern 5P is formed. In this process, ion etching is used, so that a part of the etched material is redeposited, as designated by the reference numeral 8, on the sides of the mask pattern 7, the pattern gap control layer 6 and the pattern material layer 5, whereby the pattern is widened and, thus, the gap width S₂ of the pattern 5P after the etching has been finished is narrower than the original gap width S₁ of the mask pattern 7. The above-mentioned publication discloses an embodiment of this method used to form a permalloy bubble propagation pattern, in which the pattern material layer 5 is made of permalloy and has a 0.4 µm thickness, the pattern gap control layer 6 is made of gold (Au) and has a 1.0 µm thickness, the mask pattern 7 is formed from the photoresist material "AZ1350J" and has a 1.2 µm thickness, the initial gap width S₁ is 1.7 µm, and the pattern gap width S₂ is 0.5 µm. This publication also teaches that it is possible to control the pattern gap width S₂ by varying the thickness of the pattern gap control layer 6. Moreover, it is easy to extrapolate that, if the initial gap width S₁ is 1 µm, the pattern gap width S₂ would be narrower than 0.5 µm. However, this method has the problems mentioned below. One of these problems is that the pattern-widening effect depends on the ion etching only. Accordingly, in order to obtain a large pattern-widening effect, it is required to relatively increase the thickness of the pattern gap control layer 6, and thus the layer depositing time and the etching time are increased. Another problem is that the mask pattern 7 and the pattern gap control layer 6 must be removed after the etching process has been finished, and this removing process is expensive and requires much labour and time. Therefore, the production efficiency is low. Next, the pattern forming methods according to the present invention will be described. A first embodiment of the present invention method is illustrated in Figs. 4A through 4E and comprises the following steps. (1) First, referring to Fig. 4A, on a substrate 1, a pattern material layer A having a thickness t A is deposited. Moreover, on the layer A, an etching mask pattern B of photoresist with the thickness t B is formed by photolithography. The reference character G₁ designates the initial gap width (at the bottom) of the mask pattern B. (2) Next, referring to Fig. 4B, a pattern-widening overlying layer C having a thickness t C is deposited on the exposed surfaces of the pattern material layer A and the mask pattern B, so that the undulation of the mask pattern B is substantially maintained. This widens the mask pattern B and, thus, the gap width (at the bottom) G₂ is narrower than the initial gap width G₁. (3) Thereafter, referring to Fig. 4C, the overlying layer C is etched by using an ion etching technique employing energized particles of an inert gas, such as Ne,Ar,Xe. In this step, a part C' of the etched material of the overlying layer C is redeposited on the sides of the mask pattern B. Accordingly, the bottom portion of the mask pattern B is widened on each side by the thickness of the redeposited part C' and, thus, the gap width (at the bottom) G₃ is narrower than the gap width G₂ after the overlying layer has been formed. (4) The ion etching is continued, to etch the pattern material layer A as illustrated in Fig. 4D, and finally the mask pattern B is removed as illustrated in Fig. 4E, whereby a desired pattern AP is formed. The gap width (at the bottom) G₄ of the pattern AP is narrower than the mask pattern gap width G₃ after the etching of the overlying layer C, due to the pattern-widening effect caused by ion etching. When forming a permalloy propagation pattern of a permalloy bubble memory device according to the above-mentioned present invention method, the pattern material layer A is made of a permalloy with a thickness t A of 300nm (3000 Å), and the mask pattern is formed of the above-mentioned photoresist "AZ1350J". However, the thickness t B of the mask pattern is desirably larger than that in the conventional methods described above with respect to Figs. 1a to 1c; most appropriately 1 µm (10000 Å). This is because if the same thickness t B = 675nm (6,750 Å) as that in the conventional methods is used, it is impossible to obtain a satisfactory pattern-widening effect. The overlying layer C can be made of various materials, such as Cu, NiFe (permalloy), Au, Ti, Cr₂O₃, SiO₂, Cr or Si, which, in the main, may be chosen according to the adhesion to the mask pattern B and the etching rate. For example, where the pattern material layer A is made of permalloy, it is advantageous that the overlying layer be made of Cu or NiFe. The overlying layer C may be formed by vapor deposition or sputtering. In this process, a planetary substrate holder, for example, is used to cause the material of the overlying layer to be deposited on the substrate at an angle of 0° through 45° with respect to the normal line of the substrate, so that the thickness of the overlying layer C on the side surfaces of the mask pattern B is uniform and as thick as possible. Moreover, when forming the overlying layer C by vapor deposition, it is desirable to keep the substrate 1 at a low temperature (for example, 150°C or less) in the vapor deposition process, thereby preventing any change in the quality and shape of the mask pattern B. The above-described gap widths G₂ , G₃ , and G₄ depend on the thickness t C of the overlying layer C. Figures 5A through 5C illustrate the increase in pattern width, i.e., the decrease in gap width, of the mask pattern B and the permalloy pattern AP, in relation to the thickness t C of the permalloy overlying layer C, wherein the initial gap width G₁ of the mask pattern B is 1 µm. Figure 5A shows the decrease (G₁ - G₂) of the gap width G₂ after the overlying layer has been formed, compared with the original gap width G₁ of the mask pattern B. Figure 5B shows the decrease (G₂ - G₄) of the permalloy pattern gap width G₄ , compared with the mask pattern gap width G₂ after the deposition of the overlying layer C. It should be noted that a dotted line g₀ in Fig. 5B shows the decrease in gap width achieved without the deposition of the overlying layer (i.e., t C = 0), which corresponds to the decrease of the pattern gap width g₂ , compared with the mask pattern gap width g₁ of 1 µm, in the above-mentioned conventional method illustrated in Figs. 1A through 1C. Therefore, the difference (G₂ - G₄ - g₀) between (G₂ - G₄) and g₀ defines an increase in the pattern-widening effect (i.e., gap-narrowing effect) in the etching process caused by the deposition of the overlying layer C. Figure 5C shows the sum of the gap-width decrease (G₁ - G₂) shown in Fig. 5A and the gap-width decrease (G₂ - G₄) shown in Fig. 5B, i.e., the total decrease (G₁ - G₄) of the permalloy pattern gap width G₄ , compared with the initial gap width G₁ of the mask pattern, as well as the difference between the total, gap-width decrease (G₁ - G₄) and the gap-width decrease g₀ achieved without the deposition of the permalloy overlying layer C, i.e., the gap-width decrease (G₁ - G₄ - g₀) caused by the deposition of the permalloy overlying layer C. As can be seen from the figures, according to the conventional method illustrated in Figs. 1A through 1C, without the deposition of the permalloy overlying layer the gap-width decrease g₀ is 0.29 µm. Therefore, where the mask pattern gap width g₁ = 1 µm, the permalloy pattern gap width g₂ is 0.71 µm. Contrary to this, according to the present invention, if the thickness t C of the permalloy overlying layer C is assumed as 0.24 µm, the decrease (G₁ - G₂) of the mask pattern gap width caused by the deposition of the overlying layer C is 0.20 µm, and the decrease (G₂ - G₄) of the permalloy pattern gap width caused by the etching is 0.55 µm, and accordingly, the total gap-width decrease (G₁ - G₄) is 0.75 µm. Therefore, where the mask pattern initial gap width G₁ = 1 µm, the permalloy pattern gap width G₄ is 0.25 µm. In this case, the gap-width decrease caused by the deposition of the permalloy overlying layer C is g₂ - G₄ = 0.71 - 0.25 = 0.46 µm. As described above, according to the present invention, the deposition of the overlying layer C makes it possible to form a pattern gap which is remarkably smaller than an initial gap of the mask pattern. Moreover, according to the present invention, the initial gap width of the mask pattern can be decreased, prior to the etching process, by the deposition of the overlying layer C, and, in addition, a large gap-narrowing effect can be achieved in the etching process. Accordingly, as compared with the conventional method illustrated in Figs. 3A through 3D, which depends only on the gap-narrowing effect caused by the etching, the rate of dependence on the gap-narrowing effect caused by the etching in the present invention is smaller, and thus the overlying layer C can be made thinner, whereby the processing time for layer deposition and etching can be reduced. Additionally, according to the present invention, since the overlying layer C deposited on the mask pattern is completely etched in the etching process, it is required merely to remove the mask pattern after the etching has been finished. This removing process is very simple, compared with that of removing both the mask pattern and the pattern gap control layer in the conventional method illustrated in Figs. 3A through 3D. Consequently, the present invention makes it possible to realize a higher efficiency of production, as compared with the conventional method illustrated in Figs. 3A through 3D. As described above, according to the present invention, it is possible to form stably, very easily, and efficiently a pattern having submicron gaps, in particular fine gaps having a width of 0.5 µm or less, by photolithography having a resolution limit of 1 µm for practical use. Accordingly, the pattern forming method of the present invention can be applied to the various devices as mentioned above for achieving an improvement in the characteristics and a reduction in the size or an increment in the density. Referring to Figs. 6A through 6J, an embodiment of the present invention, which is used to manufacture a permalloy bubble memory device, will now be described. (1) First, referring to Fig. 6A, on a substrate (not shown) of non-magnetic garnet, e.g., gadolinium gallium garnet (G.G.G.), a thin layer 10 of a magnetic garnet, in which magnetic bubbles can be propagated, is deposited by liquid-phase epitaxial growth. On this magnetic layer, a 50nm (500 Å) thick SiO₂ layer 11 is deposited by sputtering. Further, a 20nm (200 Å) thick Ta-Mo alloy layer 12 and a 380nm (3800 Å) thick Au layer 13, from which a conductor pattern is formed, are successively deposited by vapor deposition. (2) Next, referring to Fig. 6B, on the Au layer 13, a 500nm (5000 Å)thick etching mask pattern 14 corresponding to the conductor pattern is formed of, for example, a positive photoresist (e.g., AZ1350J) by photolithography. (3) Thereafter, ion etching is used to pattern the Au layer 13 and the Ta-Mo alloy layer 12, thereby forming a conductor pattern CP(12, 13), and the mask pattern 14 is then removed. (4) Next, referring to Fig. 6D, a heat-resisting resin (PLOS) layer 15 is deposited over the entire surface to plane the steps defined by the conductor pattern CP. (5) Next, referring to Fig. 6E, a 300nm (3000 Å) thick permalloy (NiFe) layer 16 (corresponding to the pattern material layer A illustrated in Fig. 4A), from which a permalloy propagation pattern is formed, and a 40nm (400 Å) thick Cr₂O₃ layer 17, which serves as an antireflection coating in the pattern exposure process, are successively deposited by vapor deposition. (6) Thereafter, referring to Fig. 6F, on the Cr₂O₃ layer 17, a 1 µm (10000 Å) thick etching mask pattern 18 (corresponding to the mask pattern B illustrated in Fig. 4A), which substantially corresponds to the permalloy pattern to be formed, is formed of a positive photoresist (e.g., AZ1350J) by photolithography. The smallest gap width of the mask pattern 18 is 1 µm. (7) Further, referring to Fig. 6G, over the mask pattern 8, a 200nm (2000 Å) thick permalloy (Nife) overlying layer 19 (corresponding to the overlying layer C illustrated in Fig. 4B) is deposited by vapor deposition. In this process, the temperature of the mask pattern is maintained at 150°C or less to avoid changes in quality and deformation in the mask pattern 18. (8) Thereafter, referring to Fig. 6H, an ion etching is used to etch the permalloy overlying layer 19, the Cr₂O₃ layer 17, and the permalloy layer 16, successively. (9) After the etching process, the mask pattern 18 and the Cr₂O₃ layer 17 are removed, whereby a permalloy pattern PP (corresponding to the pattern AP illustrated in Fig. 4E) is formed as illustrated in Fig. 6I. The gap width of the pattern PP is 0.3 µm. (10) Finally, referring to Fig. 6J, a passivation layer is formed by depositing a 200nm (2000 Å) thick heat-resisting resin (PLOS) layer 20 and a 600nm (6000Å) thick SiO₂ layer 21, successively. Referring to Fig. 7, solid lines L₁ and dotted lines L₂ show the bubble propagation characteristics in the magnetic bubble memory devices manufactured by a method according to the present invention and by a conventional method, respectively. In Fig. 7, the abscissa shows the magnetic drive field H D and the ordinate shows the magnetic bias field H B . These bubble memory devices have permalloy half-disk propagation patterns, both having an 8 µm bit period, and a 0.3 µm and 1 µm gap width, respectively. From this figure, it can be understood that the bias margins are approximately equal to each other, but the minimum drive field in the device according to the present invention is lower by 15 Oe than that in the conventional device. It should be noted that, in the above-described embodiment, the present invention can be employed not only for forming the permalloy pattern PP, but also for forming the conductor pattern CP. On the other hand, the above-described first embodiment of the present invention has a disadvantage in that the formed pattern is widened over the entire periphery thereof, as compared with the mask pattern. This is particularly disadvantageous when forming the permalloy propagation pattern in the permalloy bubble memory device as described above. For example, in the case of the half-disk pattern illustrated in Fig. 8, the formed pattern AP shown by a two-dotted chain line is widened by a size "d" over the entire periphery, as compared with the mask pattern B shown by a solid line. Where the gap width G₁ and the leg width W of the mask pattern B are both 1 µm and the pattern-width increase "d" is 0.2 µm, the gap width G₄ of the permalloy pattern AP is decreased as G₁ - 2d = 0.6 µm, while the pattern leg width W₁ is increased as W + 2d = 1.4 µm. Although the decrease in gap width contributes to an improvement in bubble propagation characteristics, the increase in pattern leg width gives rise to the problem of the occurrence of the phenomenon wherein a bubble is turned around the pattern leg (i.e., the bubble is not propagated across the pattern gap, but is propagated along the periphery of the pattern leg). This problem can be solved in alternative embodiments of the present invention, which will be described below by taking, as an example, the forming of the half-disk permalloy pattern illustrated in Fig. 8. Hereinafter, the same or similar portions as those illustrated in Figs. 4A through 4E are designated by the same reference characters. Figures 9A through 9G illustrate a process of a second embodiment of the present invention method, which comprises the following steps. (1) First, referring to Fig. 9A and Fig. 9B, which is a sectional view taken along the line 9B - 9B in Fig. 9A, on a substrate 1 an underlying mask pattern D is formed prior to forming a permalloy layer, that is, the pattern material layer. The underlying mask pattern D can be formed of any non-magnetic and heat-resisting material which has adhesive characteristics and selective etching characteristics, in relation to the pattern forming material. For example, where the pattern forming material is permalloy, an organic matter (e.g., photoresist, polyimide), Al, Al-Cu alloy, or SiO₂ can be used. The underlying mask pattern D is thicker than the pattern material layer and has edges which are patterned to define at least a part of the edges, except for the edges opposite to each other with the pattern gap therebetween, of the pattern to be formed; in particular, a part which is to be prevented from being widened. In the illustrated embodiment, the underlying mask pattern D is an SiO₂ layer having a 500nm (5000 Å) thickness and has an edge DE patterned to correspond to the edge on the concaved side of the mask pattern B illustrated in Fig. 8. In the figures, the reference character D₁ designates a region in which the legs of the permalloy pattern are formed. (2) Next, referring to Fig. 9C and Fig. 9D, which is a sectional view taken along the line 9D-9D in Fig. 9C, the exposed surfaces of the substrate 1 and the underlying mask pattern D are covered with a 300nm (3000 Å) thick permalloy layer A deposited thereon by vapor deposition, and successively a 700nm (7000 Å) thick mask pattern B is formed. The edge on the concaved side of the mask pattern B is aligned with the edge DE of the underlying mask pattern D. (3) Next, referring to Fig. 9E, over the entire surfaces of the permalloy, layer A and the mask pattern B, a 200nm (2000 Å) thick permalloy overlying layer C is deposited by vapor deposition. (4) Thereafter, an ion etching process is used to etch the permalloy overlying layer C and the permalloy layer A, thereby forming a permalloy pattern AP, as illustrated in Fig. 9F, and then the mask pattern B is removed as illustrated in Fig. 9G. The underlying mask pattern D can be removed by any etching technique having selective etching characteristics, in relation to the permalloy pattern AP. According to the above-described method, as illustrated particularly in Fig. 9C, the permalloy pattern AP is merely widened at the edges with the gap therebetween and at the convexed-side edge, which are not defined by the underlying mask pattern D, but not at the concaved-side edge defined by the underlying mask pattern D. Therefore, where G₁ = W = 1 µm and d = 0.2 µm, the gap width G₄ (Fig. 9G) is a G₁ - 2d = 0.6 µm, while the pattern leg width W₂ (Fig. 9G) is W + d = 1.2 µm, with the result that the increase is halved. In this embodiment, only the underlying mask pattern defining the concaved-side edge of the permalloy pattern AP is formed. However, if necessary, an additional underlying mask pattern defining the convexed-side edge of the permalloy pattern AP can be formed to prevent that edge from being widened. According to the above-described second embodiment, however, due to the limitations of photolithography, neither the gap width G₁ nor the leg width W of the mask pattern B can be made smaller than 1 µm. Therefore, the width of the region D₁ of the underlying mask pattern D must be made larger than 3 µm and, accordingly, the leg width W₂ of the permalloy pattern AP must be made larger than 1 µm. This problem can be solved by a third embodiment of the present invention illustrated in Figs. 10A through 10I, which comprises the following steps: (1) First, referring to Fig. 10A, on the substrate 1 is formed an underlying mask pattern D, which is substantially the same as that in the second embodiment described above, except that the width of a region D₂ in which the legs of the half-disk pattern are formed is different, as described hereinafter. (2) Next, referring to Fig. 10B and Fig. 10C, which is a sectional view taken along the line 10C-10C in Fig. 10B, a 300nm (3000 Å) thick permalloy layer A is deposited and, successively, a mask pattern B' is formed thereon, as in the second embodiment. However, the mask pattern B' is different from the mask pattern B of the second embodiment, in that the thickness of the mask pattern B' is 1µm (10000 Å) and it is patterned to overlap the underlying mask pattern D. (3) Next, referring to Fig. 10D, a 200nm (2000 Å) thick permalloy overlying layer C is deposited. (4) Thereafter, ion etching is performed, and then the mask pattern B' is removed, thereby forming a permalloy pattern AP, as illustrated in Fig. 10E. (5) Next, if necessary, the useless portion of the permalloy layer A remaining on the underlying mask pattern D is removed. For this purpose, the following two methods may be used. In the first method, referring to Fig. 10F, a selective etching technique is used to remove the underlying mask pattern D only, and then, referring to Fig. 10G, a mechanical technique, such as scraping, for example, is used to remove the useless portion of the permalloy layer A. The selective etching of the underlying mask pattern D can be achieved as follows; where the pattern D is made of an organic matter, by O₂ gas plasma etching; in the case of Al, by using an alkali etchant or an etchant containing phosphoric acid as the chief ingredient; and, in the case of SiO₂, by CF₄ gas plasma etching. In the second method, referring to Fig. 10H, a resin E is deposited over the entire surface of the article being in the condition illustrated in Fig. 10E for planing, and then, referring to Fig. 10I, ion etching is performed to etch the entire surface, thereby removing the useless portion of the permalloy layer A, together with the resin E. According to the method described above, the edges of the legs of the permalloy pattern AP, which are not defined by the underlying mask pattern D, are widened, thereby forming a submicron gap, as in the above-described second embodiment. Moreover, the mask pattern B' is formed, with the gap width G limited merely to 1 µm, due to the limitation of the photolithography, and accordingly, the width of the region D₂ of the underlying mask pattern D illustrated in Fig. 10A can be made smaller than 3 µm. Therefore, it is possible to obtain a prior reduction in the width of the region D₂ by a size corresponding to the increase in pattern width, thereby preventing the pattern leg width from being increased. For example, if the width of the region D₂ is made as G₁ + 2(W - d) = 2.6 µm, the pattern gap width G₄ illustrated in Fig. 10E is made as G₁ - 2d = 0.6 µm, and the pattern leg width W₃ is made as (2.6 - 0.6)/2 = 1 µm. Namely, an increase does not occur. It is also possible to reduce the pattern leg width W₃ to less than 1 µm, if the width of the region is further reduced. Furthermore, in this third embodiment, if necessary, it is also possible to form an additional underlying mask pattern defining the convexed-side edge of the half-disk pattern, to prevent that edge from being widened. A fourth embodiment of the present invention will now be described. This embodiment makes it possible to form a half-disk permalloy pattern AP as illustrated in Fig. 11A, in which the gap width is G₄ = 0.3 µm and the pattern leg widths are W₄ = 0.5 µm and W₅ = 0.7 µm, without using an underlying mask pattern as described above, and comprises the following steps: (1) First, referring to Fig. 11B and Fig. 11C, which is a sectional view taken along the line 11C-11C in Fig. 11B, on a substrate 1 is formed a 300nm (3000 Å) thick permalloy layer A, and successively a 1 µm (10000 Å) thick mask pattern B1 is formed. However, unlike the mask pattern B or B' in the aforementioned embodiments, the mask pattern B1 is patterned to cover a region other than the regions corresponding to the gaps of the pattern AP to be formed (phantom illustrated by the two-dotted chain line). In other words, the mask pattern B1 is merely provided with gaps substantially corresponding to the gaps of the pattern AP. The gap width G₁ of the mask pattern is 1 µm. (2) Next, referring to Fig. 11D, a 300nm (3000 Å) thick permalloy overlying layer C is deposited. (3) Thereafter, the permalloy overlying layer C and the permalloy layer A are etched by ion etching, and then the mask pattern B1 is removed, whereby, as illustrated in Fig. 11E and Fig. 11F, which is a sectional view taken along the line 11F-11F in Fig. 11E, gaps G having the width G₄ = 0.3 µm are formed in the permalloy layer A. (4) Subsequently, referring to Fig. 11G and Fig. 11H, which is a sectional view taken along the line 11H-11H in Fig. 11G, on the permalloy layer A formed with the gaps G is formed a 700nm (7000 Å) thick mask pattern B2 which covers the regions corresponding to the gaps G and the pattern AP. In other words, the mask pattern B2 has a configuration such that the pattern AP is continued at the gaps. The width W6 of the pattern-connecting portion of the mask pattern B2 is made as W₄ + W₅ + G₄ = 1.5 µm. (5) Thereafter, the permalloy pattern A is etched and the mask pattern B2 is removed, whereby the permalloy pattern AP as illustrated in Fig. 11A is formed. According to the above-described method, the mask patterns in which the smallest size is 1 µm or more, and which can be formed by photolithography, can be employed for forming a fine pattern in which all of the gap width G₄ and the leg widths G₄ and G₅ are smaller than 1 µm. Next, an embodiment of the present invention which is used to manufacture a contiguous-disk bubble propagation pattern in an ion-implanted bubble memory device will be described. First, the ion-implanted bubble memory device is briefly described with reference to Figs. 12A through 12C, wherein Fig. 12A is a partial plan view thereof, and Figs. 12B and 12C are sectional views taken along the lines 12B-12B and 12C-12C in Fig. 12A, respectively. In these figures, the reference numeral 31 designates a substrate (G.G.G.) of the bubble memory device, 32 a magnetic garnet thin film in which magnetic bubbles can be propagated, and 32A a contiguous-disk bubble propagation pattern. In the outer region 32B surrounding the pattern 32A, ions are implanted on the surface of the magnetic film 32 to a depth of about 30% of the thickness thereof, so that the direction of magnetization is held to be normal to the plane in the region of the pattern 32A, while falls down in plane in the outer region 32B. When an external in-plane reorienting magnetic field is applied to this pattern, a bubble Bu is propagated along the periphery of the contiguous-disk pattern 32A, due to the movement of the charged wall. The contiguous-disk pattern 32A has cusps G which define bubble trap positions. The distance p between the adjacent cusps defines the pattern period or bit period. For ion-implantation in the region 32B, an ion-implantation mask pattern 33P is employed, which is made of, for example, Au, to correspond to the contiguous-disk pattern 32A. A conventional process for forming the mask pattern 33P comprises steps of, first, forming on an Au layer an etching mask pattern corresponding to the pattern 33P by photolithography, and, next, etching the Au layer. However, where p is less than 2 µm, the cusp G has a very fine configuration and it is difficult to form it precisely or sharply. Such a fine cusp can be formed according to the present invention which will be described below with reference to Figs. 13A through 13D, wherein Figs. 13A through 13D illustrate sectional views in the successive steps of the method and Figs. 13B through 13D also illustrate plan views of the pattern. (1) First, referring to Fig. 13A, on the surface of the magnetic garnet thin film 32, a 600nm (6000 Å) thick Au layer 33 (corresponding to the pattern material layer a illustrated in Fig. 4A), from which the ion-implantation mask pattern 33P is formed, and a 600nm (6000 Å) thick photoresist layer 34 are successively deposited. (2) Next, referring to Fig. 13B, photolithography is used to pattern the photoresist layer 34, thereby forming an etching mask pattern 34P (corresponding to the mask pattern B illustrated in Fig. 4A) which is constituted of independent disk patterns. Where p = 2 µm and the photolithographic resolution is 1 µm, the mask pattern 43P can be formed to have a disk diameter D of 1 to 1.25 µm and a gap width G₁ of 1 to 0.75 µm. (3) Next, referring to Fig. 13C, a 600nm (6000 Å) thick Cu or NiFe overlying layer 35 (corresponding to the overlying layer C illustrated in Fig. 4B), that is a pattern-widening layer, is deposited by vapor deposition. The overlying layer 35 contributes to the widening of the mask pattern 34P, thereby reducing a gap width G₂ to less than G₁. (4) Thereafter, ion etching is performed to etch the overlying layer 35 and the Au layer 33, thereby forming a contiguous-disk mask pattern 33P, as illustrated in Fig. 13D, in which the disk patterns are overlapped or joined to one another, due to the pattern-widening effect as described hereinbefore. (5) Subsequently, the mask pattern 34P is removed, and, thereafter, ions are implanted in the magnetic garnet thin film 32, using the mask pattern 33P as a mask, whereby the contiguous-disc bubble propagation pattern 32A as illustrated in Figs. 12A through 12C is formed. As described above, according to the present invention, a discontiguous-disc pattern, which can be easily formed by photolithography having a 1 µm resolution, is used as a basis, and the pattern-widening effect caused by deposition of the overlying layer and ion etching is utilized to widen the pattern and form a contiguous-disc pattern. Accordingly, the cusp G having a fine configuration can be precisely formed, whereby it is possible to realize a high-density ion-implanted contiguous-disk bubble propagation pattern having a period of 2 µm or less. The contiguous-disk pattern 32A in the above-described embodiment is a typical example of a contiguous-disk pattern, in which the disk patterns overlap each other, However, the "contiguous-disk pattern" is defined to include a pattern 32A₁ , illustrated in Fig. 14A, in which the disk patterns are in contact with each other at their circumferences, and a pattern 32A₂ , illustrated in Fig. 14B, in which the disk patterns are spaced with fine gaps therebetween. However, the permitted limit of the gap width of the pattern 32A₂ is approximately half that of the diameter of a bubble to be propagated. Moreover, the above-mentioned embodiment is that of a typical disk pattern which can be most easily formed. However, the present invention can be applied to a different kind of pattern, such as an oval pattern or square pattern.

Claims: A method for forming a pattern having a fine gap on a substrate, comprising the steps of : (a) depositing a layer (A) of material, from which said pattern (AP) is formed, on the surface of said substrate; (b) forming an etching mask pattern (B) having a first gap on said pattern material layer (A); (c) depositing a pattern-widening overlying layer (C) which substantially maintains the topography of the etching mask pattern (B) on the exposed surfaces of said pattern material layer (A) and the etching mask pattern (B), thereby forming a second gap which is smaller than said first gap, and (d) etching said overlying layer (C) and said pattern material layer (A) by ion-etching employing energized particles of an inert gas, thereby forming said pattern (AP) having said fine gap which is smaller than said second gap from said pattern material layer (A). A method according to claim 1, wherein step (b) includes forming said etching mask pattern (B) from a photoresist material by photolithography. A method according to claim 1, wherein step (c) includes depositing said overlying layer (C) by vapor deposition. A method according to claim 3, wherein step (c) includes performing the vapor deposition of said overlying layer (C) in a direction which forms an oblique angle with a line normal to the surface of said substrate. A method according to claim 1, wherein step (c) includes depositing said overlying layer (C) by sputtering. A method according to claim 1, wherein step (c) includes forming said overlying layer (C) of a material which is different from the material of said pattern material layer (A). A method according to claim 1, wherein step (c) includes forming said overlying layer (C) of a material which is the same as the material of said pattern material layer (A). A method according to claim 1, wherein step (a) includes depositing a permalloy (NiFe) pattern material layer, and wherein step (d) includes forming a permalloy pattern for magnetic bubble propagation in a magnetic bubble memory device. A method according to claim 8, wherein step (c) includes depositing a copper (Cu) overlying layer (C). A method according to claim 8, wherein step (c) includes depositing a permalloy (NiFe) overlying layer (C). A method according to claim 1, wherein step (b) includes forming a first gap of the etching mask patern (B) with a width of approximately 1 µm or more, and step (d) includes forming the fine gap of said pattern (AP) with a width of less than approximately 0.5 µm. A method for forming a pattern having a fine gap on a substrate, comprising the steps of : (a) forming an underlying mask pattern (D) having a thickness on the surface of said substrate, said underlying mask pattern (D) having an edge defining a part of an edge portion of said pattern (AP), but not the edge portions of said pattern which define said gap therebetween; (b) depositing a layer (A) of a pattern material having a thickness which is less than the thickness of the underlying mask pattern (D) on the exposed surfaces of said substrate and said underlying mask pattern (D); (c) forming an etching mask pattern (B) having a first gap on said pattern material layer (A); (d) depositing a pattern-widening overlying layer (C) which substantially maintains the topography of the mask pattern on the exposed surfaces of said pattern material layer (A) and the etching mask pattern (B), thereby forming a second gap which is smaller than the first gap; (e) etching said overlying layer (C) and said pattern material layer (A) by ion etching employing energized particles of an inert gas, thereby forming said pattern (AP) having said fine gap from said pattern material layer (A) said fine gap being smaller than said second gap. A method according to claim 12, wherein step (c) includes forming said etching mask pattern (B') on a portion of said underlying mask pattern (D) on which is formed said pattern material layer (A), further comprising the step (f) of removing a useless portion of the pattern material layer (A) on the underlying mask pattern (D) after step (e). A method according to claim 13, wherein step (a) includes forming said underlying mask pattern (D) of a material having selective etching characteristics with respect to the etching characteristics of the pattern material layer (A), further comprising the sequential steps of removing the underlying mask pattern layer (D) by etching after step (f), and thereafter removing, by a mechanical technique, the useless portion of the pattern material layer (A) remaining on the underlying mask pattern. A method according to claim 13, further comprising the sequential steps of depositing a covering material (E) on the exposed surfaces of the substrate to create a planar exposed surface and removing a useless portion of the pattern material layer (A) remaining on the underlying mask pattern (D), and said covering material (E) by etching. A method according to claim 1, characterized in that after the step of etching said overlying layer (C) and said pattern material layer (A), it further includes the steps of : (e) removing said etching mask pattern (B); (f) forming a second mask pattern (B2) on the pattern material layer (A) formed with said fine gap, so as to cover said fine gap and the region corresponding to the pattern (AP) to be formed; and (g) etching the exposed region of said pattern material layer (A) so as to form said pattern (AP); and in that said pattern material layer (A) is a permalloy layer. A method according to claim 1, characterized in that the pattern (AP) to be formed is a contiguous-element pattern, constituted of pattern elements arranged with adjacent elements being substantially interconnected at portions thereof. A method according to claim 17 wherein said contiguous-element pattern is a contiguous-disk pattern (32A), and wherein step (b) includes forming a discontiguous-disk etching mask pattern. A method according to claim 17, characterized in that the contiguous-element pattern is an ion implantation-mask pattern (33P), the pattern material layer (A) is a magnetic thin-film (32) and said contiguous-element pattern serves for magnetic bubble propagation, the ion implantation mask being formed on a surface of the magnetic thin-film (32) in which bubbles can be propagated, and ions being implanted on the exposed surface of said magnetic thin-film. A method according to claim 19, wherein said etching mask pattern (32A) is a discontiguous-disk pattern, and said ion-implantation mask pattern (33P) is a contiguous-disk pattern. A method according to claim 19, wherein said ion-implantation mask pattern (33P) has a pattern period of 2 µm or less. A method according to claim 19, wherein said ion-implantation mask pattern layer (33P) is made of gold (Au). A method according to claim 22, wherein said overlying layer (C) is made of copper (Cu). A method according to claim 22, wherein said overlying layer (C) is made of permalloy (NiFe).

Inventors: MAJIMA, TEIJI, WATANABE, HIROMICHI
Applicants: FUJITSU LIMITED
Application year: 1984
Main classification: H01L21
IPC codes: H01L21, H01F41