Abstract:
A process for the formation of a wiring pattern, which includes the steps of: exposing a resist through a photomask, the photomask having a pattern whose line width is equal to or less than a resolution limit; and developing the exposed resist to form a resist pattern having groove depressions on the surface thereof, the depressions not reaching the back of the resist pattern. The resist may be a positive resist in which case the resist pattern is formed on an underplate feed film; a plating metal is precipitated on the feed film in a region not covered by the resist pattern; the resist pattern is stripped after the precipitation; and the feed film is selectively removed in a region not covered by the plating metal. Alternatively, the resist may be a negative resist in which case the resist pattern is formed on a substrate; a metallic material is deposited on the resist pattern and the substrate; and the resist is stripped from the substrate to remove the overlying metallic material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a resist pattern, to a process for the formation thereof, and to a process for the formation of a wiring pattern. More particularly, it relates to a technique for the formation of a resist pattern which is stable in shape, dimensions, accuracy and other characteristics. Furthermore, it relates to a process for the formation of a fine wiring pattern by plating or by a lift off process. The resist pattern, the process for the formation of the resist pattern, and the process for the formation of a wiring pattern can be suitably employed for a semiconductor device manufacturing process or electronic parts manufacturing process which require fine patterning. 
     2. Description of the Related Art 
     As processes for the formation of a thick fine wiring having a thickness exceeding 1 μm, there may be mentioned a semi-additive (plating) process as shown in FIGS. 9A to  9 G, and a lift off process as illustrated in FIGS. 10A to  10 F. 
     Initially, the semi-additive process will now be described. According to this process, a feed film  32  (or plating base) of a metallic material is formed on a substrate  31  (FIG.  9 A), and then a positive resist  33  is applied onto the feed film  32  (FIG.  9 B). Subsequently, the resist  33  is exposed to ultraviolet radiation through an aperture  34   a  of a photomask  34  (FIG.  9 C), and then is subjected to development (FIG.  9 D). A region of the resist  33  exposed to ultraviolet radiation becomes soluble, and the exposed region is hence dissolved by development to give a resist pattern  35  rectangular in cross section. 
     After this step, a voltage is applied to the feed film  32  to conduct electroplating, and a plating metal is precipitated on the feed film  32  in a region not covered by the resist pattern  35  (FIG.  9 E), to form a plated film  36 . After the completion of plating, the resist pattern  35  is stripped (FIG. 9 F), and the feed film  32  is removed by etching in a region not covered by the plated film  36  to give a target wiring pattern  37  on the substrate  31  (FIG.  9 G). 
     Next, the lift off process will be described. According to this process, a negative resist  42  is applied (FIG. 10B) onto the surface of such a substrate  41  as shown in FIG. 10A, and the resist  42  is then exposed to ultraviolet radiation through an aperture  43   a  of a photomask  43  (FIG.  10 C), and the exposed resist is subjected to development (FIG.  10 D). The resist  42  in a region exposed to ultraviolet radiation becomes insoluble, and the exposed region remains even after the development to give a resist pattern  44  which is of a reversed taper shape in cross section. 
     Next, an electrode material  45  is deposited all over the substrate  41  from above the resist pattern  44  (FIG.  10 E), and the resist pattern  44  and the electrode material  45  deposited on the resist pattern  44  are stripped off to give a target wiring pattern  46  on the substrate  41  (FIG.  10 F). 
     As is apparent from the aforementioned explanation, both the semi-additive process and lift off process require forming a resist pattern having a thickness greater than the thickness of a target wiring, as their operations demonstrate, and thus require forming a comparatively thick resist pattern. In addition, the resist pattern formed by the semi-additive process must be rectangular in cross section, and that formed by the lift off process must be of a reverse taper shape in cross section. 
     Furthermore, both of the semi-additive process and lift off process are characterized in that a film of a wiring material is formed after the formation of a resist pattern to give a wiring pattern. The dimensions, shape and accuracy of the wiring pattern therefore reflect the dimensions, shape and accuracy of the resist pattern. Consequently, it is important to retain the shape of the resist pattern until the formation of a film of the wiring material is completed in order to provide a fine wiring pattern sufficiently having target dimensions, shape and accuracy. 
     According to conventional processes for the formation of a resist pattern, however, the following results, for example, occur when the thickness of a resist film to be formed is increased: 
     (1) volumetric shrinkage of the resist pattern due to degassing (gas emission) when the resist is baked in a photolithography step, 
     (2) defective film of the wiring material because of a collision between flying particles of the wiring material and gas particles, which gas particles are derived from degassing with increasing temperature in the formation of a film of the wiring material, and 
     (3) stress of the wiring material. The resist sags or deforms because of these results, and an ideal shape of the resist cannot be maintained, and in consequence, a target fine wiring pattern cannot be obtained when a film of the wiring material is formed. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished to solve the aforementioned technical problems, and its object is to provide a process for the formation of a wiring pattern, which can yield a fine wiring pattern having target dimensions, shape and accuracy while suppressing the deformation of a resist pattern due to heat or stress. 
     The resist pattern according to the present invention includes a plurality of depressions formed on its surface, the depressions not reaching the back of the resist pattern. 
     As the present resist pattern has a plurality of depressions formed on its surface, which depressions neither penetrate the pattern nor reach the back of the resist pattern, the volume of the resist pattern can be smaller and the surface area thereof can be greater than normal. Since the volume of the resist pattern can be decreased by forming depressions on the resist pattern as thus described, the volume of degassing upon baking of the resist pattern can be reduced, and the volumetric shrinkage of the resist pattern can therefore be decreased. Furthermore, by making the resist pattern pectinate or comb-like in cross section, a stress applied from the wiring pattern can be decreased and hence deformation due to the stress in the resist pattern can be mitigated. 
     In addition, as the surface area of the resist pattern is increased, gas can sufficiently be emitted from the resist pattern when the resist pattern is baked. The volume of emitted gas in the film formation step can therefore be reduced to avoid scattering of flying particles of the wiring material by gas particles emitted from the resist pattern and to ensure attachment of a film on the substrate. Consequently, the deformation of the resist pattern due to, for instance, volumetric shrinkage of the resist pattern can be suppressed, and the formation step of a film of the wiring material is not hindered by gas emission from the resist pattern, resulting in the formation of a precise and satisfactory wiring pattern. In this connection, as the depressions do not reach the back of the resist pattern, they do not affect the pattern shape of the wiring pattern. 
     In particular, a thick resist having a thickness of 2 μm or more often suffers volumetric shrinkage and/or deformation, and the application of the configuration to a resist having a thickness of 2 μm or more yields marked advantages. 
     A process for the formation of a resist pattern according to the present invention includes the steps of: exposing a resist through a photomask and developing the exposed resist, the photomask having a pattern whose line width is equal to or less than a resolution limit, to form depressions on the surface of a resist pattern, the depressions not reaching the back of the resist pattern. 
     According to the present process for the formation of a resist pattern, depressions can be formed on the surface of a resist pattern in a conventional manner, which depressions do not reach the back of the resist pattern, only by the use of a photomask having a pattern whose line width is equal to or less than the resolution limit. Consequently, conventional exposure equipment or the like can be used as intact, and resist patterns can be formed with facility at low costs. 
     A process for the formation of metalization according to the present invention includes the steps of: applying a resist onto an underplate feed film, exposing the resist through a photomask, the photomask having a pattern whose line width is equal to or less than a resolution limit, developing the exposed resist to form groove depressions on the surface of a resist pattern, the depressions not reaching the back of the resist pattern, precipitating a plating metal on the feed film in a region not covered by the resist pattern, stripping the resist pattern after the precipitation, and selectively removing the feed film in a region not covered by the plating metal. 
     The aforementioned process is a process for the formation of a wiring pattern according to a so-called semi-additive process. The application of the process explained above to this process can reduce the volume of, and can increase the surface area of, the resist pattern, and therefore can decrease the volumetric shrinkage of the resist pattern upon baking to give a target shape of the resist pattern. Consequently, a target fine wiring pattern can be formed. 
     A process for the formation of a wiring according to another aspect of the present invention includes the steps of: applying a resist onto a substrate, exposing the resist through a photomask, the photomask having a pattern whose line width is equal to or less than a resolution limit, developing the exposed resist to form groove depressions on the surface of a resist pattern, the depressions not reaching the back of the resist pattern, depositing a metallic material on the resist pattern and on the substrate, and subsequently stripping the resist from the substrate to remove the metallic material on the resist. 
     The process just mentioned above is a process for the formation of a wiring pattern according to a so-called lift off process. When the process explained above is applied to this process, the volume of the resist pattern can be smaller and its surface area can be larger than normal, and therefore the volume of degassing with an increasing temperature in the deposition of a metallic material can be decreased, and the volumetric shrinkage and/or deformation of the resist pattern can significantly be suppressed. In addition, by forming projections and depressions on the surface of the resist pattern, the stress of the metallic material can be dispersed in direction, and a stress applied on the edges of the resist pattern can markedly be decreased. By these effects, the deformation of the resist pattern caused by the film formation of the metallic material can be prevented. Moreover, the shape of the resist pattern can be maintained even after the film formation, and a target wiring pattern can be formed by lift-off, and a resist pattern having an exactly intended shape, dimensions, accuracy and the like can be obtained. 
     For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 F are cross sectional views showing a process for the formation of a wiring pattern according to an embodiment of the present invention. 
     FIG. 2 is a diagram illustrating a photomask used in the above process. 
     FIGS. 3A to  3 G are cross sectional views showing a process for the formation of a wiring pattern according to another embodiment of the present invention. 
     FIG. 4 is a diagram illustrating a photomask used in the above process. 
     FIG. 5A is a top view showing an example of a photomask, and FIG. 5B is a cross sectional view of a resist pattern obtained by exposure and development using this photomask. 
     FIG. 6A is a top view showing another example of a photomask, and FIG. 6B is a cross sectional view of a resist pattern obtained by exposure and development using the present photomask. 
     FIG. 7A is a top view showing a further example of a photomask, and FIG. 7B is a cross sectional view of a resist pattern obtained by exposure and development using the present photomask. 
     FIG. 8A is a top view showing yet another example of a photomask, and FIG. 8B is a cross sectional view of a resist pattern obtained by exposure and development using the present photomask. 
     FIGS. 9A to  9 G are cross sectional views showing a process for the formation of a wiring pattern by a semi-additive (plating) process. 
     FIGS. 10A to  10 F are cross sectional views showing a process for the formation of a wiring pattern by a lift off process. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention are explained in detail with reference to the drawings. 
     (First Embodiment) 
     Referring to FIGS. 1A to  1 F, a photosensitive chemically sensitized negative resist  2  was rotationally applied to a thickness of 5 μm on such a glass substrate  1  as shown in FIG. 1A, and the coated resist was prebaked on a hot plate at 90° C. for 90 seconds (FIG.  1 B). 
     The resist  2  formed on the substrate  1  was exposed to ultraviolet radiation (i-ray) using a photomask  3  (FIG.  1 C). In this procedure, an exposure dose was set to overexposure greater than normal. The photomask used herein has a masking pattern  3   a  and a masking pattern  3   b  to form grooves  5  described below. The masking pattern  3   a  is formed in a region where the resist is to be removed and has a line width (5 to 200 μm) equal to or more than the resolution limit. The masking pattern  3   b  has a line width (1.5 μm) equal to or less than the resolution limit, as shown in FIG. 2 (the regions of the masking patterns  3   a ,  3   b  of the photomask are diagonally shaded in FIG.  2 ). A 5:1 reduction projection aligner (step-and-repeat system) was used for exposure, and the exposure dose was set to 80 mJ/cm 2 . 
     The aforementioned substrate  1  was placed on a hot plate at 110° C. and was post-exposure baked (PEB; Post Exposure Bake) for 60 seconds, and was subjected to developing in an alkaline developer, was cleaned with water, and was dried through an N 2  blow to give a resist pattern  4  on the substrate  1  (FIG.  1 D). At this stage, the resist pattern  4  on the substrate  1  comprises three types of regions, i.e., a region  4   a  “where the resist  2  is completely dissolved” (corresponding to the masking pattern  3   a ), a region  4   b  “where no resist  2  is dissolved” (corresponding to a transmission region  3   c  of the photomask  3 ), and a region  4   c  “where the resist  2  is not completely but partially dissolved” (corresponding to the masking pattern  3   b ). 
     The region  4   a  where the resist  2  was completely dissolved and removed is a region where the ultraviolet radiation was completely cut off by the masking pattern  3   a  in the photomask  3 , which masking pattern  3   a  has a line width equal to or more than the resolution limit. The region  4   b  where no resist  2  was dissolved is a region which was exposed through the transmission region  3   c  in the photomask  3 . The region  4   c  where the resist  2  was not completely but partially dissolved is a region where the ultraviolet radiation was cut off by the masking pattern  3   b  having a line width equal to or less than the resolution limit, and is formed pectinate or comb-like and composed of narrow grooves  5  each having such a depth as not to reach the substrate  1 , and has a corrugated surface. Furthermore, the edges of the resist pattern  4  are of a reverse taper shape. 
     Next, a Ti/Cu film  6  composed of an upper layer of Cu and a lower layer of Ti was formed on the substrate  1  by vapor deposition (FIG.  1 E), and the substrate  1  was then dipped in acetone, and the resist pattern  4  and the Ti/Cu film  6  formed on the resist pattern  4  were stripped by lift-off to give a target fine wiring pattern  7  (FIG.  1 F). The substrate was not heated during the vacuum deposition, and the thickness of the film was set to Ti: 50 nm and Cu: 4 μm. 
     When the masking pattern  3   b  having a line width equal to or less than the resolution limit is formed on a region to be exposed and ultraviolet rays are irradiated to the substrate to dissolve the resist partially and thereby to make the exposed regions pectinate or comb-like in cross section as in the present embodiment, the volume of the resist pattern  4  can be smaller than normal. The volume of gas emitted from the resist pattern  4  with an increasing temperature upon the formation of the Ti/Cu film can therefore be decreased. Consequently, the volumetric shrinkage and deformation of the resist pattern  4  caused by degassing can significantly be suppressed. 
     In addition, by reducing the volume of degassing upon the formation of the Ti/Cu film, the film-forming particles including Cu and Ti can be protected from a collision with the gas particles, and hence are allowed to deposit on the substrate without hindrance. A wiring pattern can therefore be obtained with high accuracy. 
     Furthermore, by rendering the resist pattern  4  pectinate or comb-like in cross section, the stress (moment) of the Ti/Cu film is decreased, and the stress applied on the edges of the resist pattern  4  can be substantially reduced. By these effects, the deformation of the resist pattern  4  caused by the formation of the Ti/Cu film can be prevented. In addition, the shape of the resist pattern can be maintained even after the film formation, a target wiring pattern can be formed by lift-off, and the resist pattern  4  having, for instance, an intended shape, dimensions and accuracy can be prepared. 
     The resist to be used in the present embodiment is not limited to chemically sensitized negative resists, and any kind of resists which can provide shapes capable of lifting off can be employed. The exposure system is as well not limited to a reducing-type projection exposure, and any type of exposure systems that can yield a target resolution can serve to obtain similar advantages. The process for the film formation is also not limited to the vapor deposition process, and any technique in which lift-off can be performed is voluntarily employed. The materials for the substrate and wiring are not limited to those mentioned above, and the embodiment can be applied to other different materials. 
     (Second Embodiment) 
     FIGS. 3A to  3 G are cross sectional views showing a process for the formation of a wiring pattern according to another embodiment of the present invention. In the present embodiment, a fine wiring is formed by the plating process (semi-additive process). Initially, as shown in FIG. 3A, an underplate feed film (Ti/Au film)  12  composed of an upper layer of Au (200 nm in thickness) and an under layer of Ti (50 nm in thickness) was formed on a sapphire substrate  11 ; onto the feed film  12 , a photosensitive positive resist  13  was rotationally applied to a thickness of 7 m, and the product was prebaked on a hot plate at 100° C. for 90 seconds (FIG.  3 B). 
     Subsequently, the positive resist  13  formed on the substrate  11  was exposed using a photomask  14  (FIG.  3 C). The exposure dose in this step should be set to around a normal dose or somewhat underexpose than normal. The photomask  14  used in this step includes a transmission pattern  14   a  and a transmission pattern  14   b  as shown in FIG. 4 (the region of the masking pattern  14   c  in the photomask  14  is diagonally shaded in FIG.  4 ). The transmission pattern  14   a  is formed in a region where the resist  13  is to be removed and has a line width (10 to 400 μm) of equal to or more than the resolution limit, and the transmission pattern  14   b  has a line width (1 μm) of equal to or less than the resolution limit. A 1:1 projection aligner (mirror projection system) was used in the exposure, and the exposure dose was set to 230 mJ/cm 2 . 
     The resultant substrate was subjected to development with an alkaline developer and the exposed region of the resist  13  was dissolved and removed to form a resist pattern  15  (FIG.  3 D). Subsequently, the substrate  11  was cleaned with water and was then dried through an N 2  blow. The resist was dissolved insufficiently in the exposed region exposed through the transmission pattern  14   b  having a line width of equal to or less than the resolution limit, and the resist pattern  15  has, in this stage, corrugated projections and depressions (or grooves)  15   a  as shown in FIG. 3D formed on its surface. 
     Next, the substrate  11  was baked on a hot plate at 120° C. for 10 minutes, and a film of Au 5 μm in thickness was precipitated by electroplating to form a plated film  16  of Au (FIG.  3 E). Thereafter, the substrate  11  was dipped in an organic solvent to strip the resist pattern  15  (FIG.  3 F), and the feed film  12  in a region not covered by the plated film  16  was etched and was removed by ion-milling to give a target fine wiring pattern  17  (FIG.  3 G). 
     As in the present embodiment, when the transmission pattern  14   b  having a line width equal to or less than the resolution limit is also formed in a region to be unexposed, and ultraviolet rays are irradiated thereto, and the resist is partially dissolved and the surface of the unexposed region is made corrugated, the volume of the resist pattern  15  can be decreased and its surface area can be increased. Thus, the volumetric shrinkage of the resist pattern  15  upon baking can be reduced and the volume of degassing in the film-formation of the wiring pattern can be reduced. Consequently, a target shape of the resist pattern can be obtained to give the target fine wiring pattern  17  with high accuracy. 
     In the present embodiment, as well, the resist to be used is not limited to positive resists, and includes any type of resists that can provide a target shape of the plated film; and the exposure system is as well not limited to a 1:1 projection aligner exposure, and any type of exposure systems that can yield a target resolution can serve to obtain similar advantages. The process for the film formation is also not limited to the electroplating process, and includes electroless plating processes, as well. The materials for the substrate and wiring are not limited to those mentioned above, and any type of materials that can be used for plating processes can be employed. 
     (Mask Pattern) 
     The masking pattern or transmission pattern to be formed on a mask and having a light width of equal to or less than the resolution limit may be in any shape. FIGS. 5A,  5 B through FIGS. 8A,  8 B respectively illustrate a transmission pattern formed on a photomask and having a line width equal to or less than the resolution limit, and a cross section of a resist pattern formed by the used of the photomask. To be more specific, FIGS. 5A,  5 B show the cross section of a resist pattern  22  obtained by exposure and development using a photomask  21  having a linear transmission pattern  21   a , which transmission pattern  21   a  has a line width of equal to or less than the resolution limit. FIGS. 6A,  6 B illustrate the cross section of a resist pattern  24  obtained by exposure and development using a lattice-type pattern  23   a  having a line width of equal to or less than the resolution limit. FIGS. 7A,  7 B show a resist pattern  26  obtained by exposure and development using a photomask  25  having a dot pattern  25   a , where the dot pattern  25   a  has a line width of equal to or less than the resolution limit. FIGS. 8A,  8 B show a resist pattern  28  obtained by exposure and development using a photomask  27  having a concentric pattern  27   a , where the pattern  27   a  is equal to or less than the resolution limit in line width. Furthermore, the pattern may be in a curved, polygonal, oval or another shape and have a line width of equal to or less than the resolution limit. It may also be of a shape as a combination of these shapes. 
     While preferred embodiments of the invention have been disclosed, various modes of carrying out the principles disclosed herein are contemplated as being within the scope of the following claims. Therefore, it is understood that the scope of the invention is not to be limited except as otherwise set forth in the claims.