Patent Publication Number: US-7718350-B2

Title: Method of metal plating by using frame

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a frame plating process, and more particularly to a frame plating process that enables a fairly good film thickness distribution to be obtained at a specific site needing film thickness precision in a simple way yet without depending on the film thickness distribution of a plated film based on plating conditions. 
     2. Explanation of the Prior Art 
     One technique of forming a micropatterned thin film (micropattern), for instance, includes a frame plating process. According to that process, for instance, an electrode film is formed on a substrate, and a resist is formed by coating on the electrode film. Then, that resist layer is patterned by photolithography so that a frame for forming a plated film is formed by the remnants of the patterned resist (mask pattern). Thereafter, that frame is used for electroplating using the previously formed electrode layer as a seed so that a patterned thin film comprising an electroconductive material is formed. 
     Micropattern thin films formed by the frame plating process are used for, for instance, microparts, microlayer components, interconnecting patterns or the like that constitute, for instance, a microdevice. Exemplary micro-devices are thin-film inductors, thin-film magnetic heads, semiconductor devices, sensors using thin films, and actuators using thin films. 
     When a given shape of frame plated film is formed for the purpose of fabricating such microdevices, there is often a demand for only a specific area of the formed frame plated film to have an increased film thickness precision (that means that there must be a decreased fluctuation of the thickness distribution of the plated film). One possible approach to increasing the plated film thickness precision at such a specific site is to polish or otherwise smoothen that site alone; however, the operation of partial polishing or the like taking aim at a microarea alone involves much difficulty, and is impractically poor in efficiency as well. 
     The present invention has been made with such situations in mind, and has for the object the provision of a frame plating process that enables a good film thickness distribution to be obtained at a specific site needing film thickness precision in a simple way yet without depending on the thickness distribution of a plated film based on plating conditions. 
     SUMMARY OF THE INVENTION 
     According to the present invention, such problems as described above are solved by the provision of a frame plating process formation of a plated film having a site needing film thickness precision at which the plated film is partly required to have thickness precision, which comprises a step of providing a substrate, a step of forming an electrode film on said substrate, a resist pattern formation step of forming a resist pattern on said electrode film in such a way as to provide a given pattern of opening concavity, a dry film resist pattern formation step of locating a part of a dry film resist in such a way as to cap an upper position of said given pattern of opening concavity corresponding to the site needing film thickness precision, and a plated film formation step of growing a plated film in such a way as to fill up said given pattern of opening concavity. 
     In a preferable embodiment of the invention, said dry film resist pattern formation step involves laminating the dry film resist on the resist pattern including the given pattern of opening concavity, and patterning said dry film resist by photolithography, so that the upper position of said given pattern of opening concavity corresponding to the site needing film thickness precision is capped by a part of said patterned dry film resist. 
     In another preferable embodiment of the invention, said dry film resist pattern formation step involves using a patterned dry film resist to which patterning has previously been applied in a given pattern to apply said patterned dry film resist onto the resist pattern including the given pattern of opening concavity, so that the upper position of said given pattern of opening concavity corresponding to the site needing film thickness precision is capped by a part of said patterned dry film resist. 
     In yet another embodiment of the invention, there are plural such sites needing film thickness precision, and there are as many parts of the dry film resist as said plural such sites. 
     In a further preferable embodiment of the invention, said resist pattern formation step involves a photoresist film on one surface of the substrate, after which said photoresist film is selectively exposed to light and developed to form the resist pattern including the given pattern of opening concavity. 
     In a further preferable embodiment of the invention, a resist removal step of removing the resist pattern and dry film resist pattern is further added after said plated film formation step. 
     In a further preferable embodiment of the invention, an etching step of using the plated film as a mask to remove an unnecessary portion of the electrode film is further added after said resist removal step. 
     In a further preferable embodiment of the invention, there is a plated film of composite shape formed, which is made up of a combined pattern comprising the resist pattern formed on said electrode film and a pattern of the dry film resist formed at said dry film resist pattern formation step. 
     The frame plating process of the invention comprises the dry film resist pattern formation step at which a part of the dry film resist is located in such a way as to cap the upper position of the given pattern of opening concavity corresponding to the site needing film thickness precision. It is thus possible to obtain a fairly good film thickness distribution at the specific site needing film thickness precision in a simple manner yet without depending on the film thickness distribution of the plated film based on plating conditions. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIGS. 1A to 1I  are illustrative, in section and over time, of the frame plating process steps according to the invention; they are sectional views of the site corresponding to section A-A in  FIG. 4 . 
         FIGS. 2A to 2I  are illustrative, in section and over time, of the frame plating process steps according to the invention; they are sectional views of the site corresponding to section B-B in  FIG. 4 . 
         FIG. 3  is a plan view of how the frame is formed on the way through the frame plating process of the invention. 
         FIG. 4  is a plan view of how the pattern of the dry film resist pattern is formed on the frame after the formation of the frame in the frame plating process of the invention. 
         FIG. 5  is a view in which the area α marked off by a broken line in  FIG. 4  is excerpted and redrawn in perspective. 
     
    
    
     DETAILED EXPLANATION OF THE INVENTION 
     The present invention is now explained with reference to the best mode for carrying out it. 
     The frame plating process of the invention is provided to form a plated film, a part of which has a site needing film thickness precision. 
     The “site needing film thickness precision” here refers to a special site (specific site) of the configuration of a given form of frame plated film formed by the process of the invention, which site is required to have improved plated film thickness precision and an extremely limited fluctuation of the plated film thickness distribution. 
     Referring to the morphology of the frame plated film formed by the process of the invention, there is the mention of a plated film that grows in the direction coming out of the drawing paper with a solid-white area  102  (electrode film  102 ) as a seed, as shown typically in the plan view of  FIG. 3 . 
       FIG. 3  is a plan view illustrating that the pattern of a resist frame  103  is being formed on the electrode film  102 . The electrode film  102  exposed at the bottom is configured in such a way as to have a straight portion  102   a  of decreased width, a fanning portion  102   b  of increasing width, which is joined to that portion  102   a  of decreased width, and a portion  102   c  of increased yet constant width, which is joined to the fanning portion  102   b . And, as frame plating is implemented using as an electrode the electrode film  102  configured as shown in  FIG. 3 , there is a substantially battledore form of frame plated film formed. Taking the morphology of this substantially battledore form of frame plated film as one preferable example of the embodiment here, the steps of the frame plating process are now explained in greater details with reference to the drawings. 
     In the embodiment here, the “site needing film thickness precision” of the frame plated film, which is required to have plated film thickness precision, is corresponding to a substantial middle of the straight portion  102   a  of decreased width, shown in  FIG. 3 . One would have a better understanding of this corresponding site by reference to the following description. 
       FIGS. 1A to 1I  are illustrative, in section and over time, of the frame plating process steps according to the invention; they are sectional views of the site corresponding to section A-A in  FIG. 4 .  FIGS. 2A to 2I  are illustrative, in section and over time, of the frame plating process steps according to the invention; they are sectional views of the site corresponding to section B-B in  FIG. 4 .  FIG. 3  is a plan view of how the frame is formed on the way through the frame plating process of the invention, with reference numeral  106  standing for an opening concavity.  FIG. 4  is a plan view of how the patterned dry film resist  110  is formed on the frame  103  and a part of the opening concavity  106  after the formation of the frame  103  in the frame plating process of the invention (the state of  FIG. 3 ).  FIG. 5  is a view in which the area α marked off by a broken line in  FIG. 4  is excerpted and redrawn in perspective. 
     (1) Step of Providing a Substrate 
     For the process of forming a plated film according to the invention, a substrate  101  for forming a plated film on it is first provided, as shown in  FIGS. 1A and 2A . For the substrate  101 , there is the mention of a silicon substrate, an AlTiC substrate, a glass substrate or the like. 
     It is noted that for the substrate  101 , various substrates or films may be used provided that they are capable of supporting the photoresist film to be described later. For instance, the plane of a multilayer structure formed on the way through the fabrication of microdevices such as thin-film inductors, thin-film magnetic heads, semiconductor devices, sensors using a thin film and actuators using a thin film may just as well be used as the substrate  101 , so that the frame plating process of the invention is applied onto the multilayer structure. 
     (2) Step of Forming a Resist Pattern 
     Then, the electrode film formation step is carried out to form on the substrate  101  an electroconductive electrode film  102  that provides a plating seed, as depicted in  FIGS. 1B and 2B . 
     The electrode film  102  is formed using techniques such as sputtering or CVD, and its composition should preferably be the same as that of the plated film to be formed later. It is noted that prior to the formation of the electrode film  102 , an adhesion enhancement layer such as a Cr or Ti layer may just as well be formed in advance. The electrode film  102  has a thickness of usually about 30 to 50 nm. 
     (3) Resist Pattern Formation Step 
     Then, there is a step implemented for forming on the electrode film  102  a resist pattern in such a way as to provide a given pattern of opening concavity. To be more specific, as depicted in  FIGS. 1C and 2C , a photoresist is coated on the surface of the electrode film  102  using a coating technique such as spin coating. Thereafter, the photoresist is heated (baked), if required, into a photo-resist film  103   a.    
     Then, photolithography is used for the patterning of the photoresist film  103   a  (selective exposure via a mask  120  and then development for patterning) to form a resist pattern  103  (frame  103 ), as depicted in  FIGS. 1D and 2D . 
     The thus formed resist pattern  103  (frame  103 ) provides an opening concavity  106  that is generally configured into a substantially battledore form as shown in  FIG. 3 . 
     It is noted that the resist pattern  103  (frame  103 ) may just as well be formed using a dry film resist. 
     (4) Step of Forming the Dry Film Resist Pattern 
     Then, at the step of forming the dry film resist pattern, a part of the resist of the resist dry film pattern is located in such a way as to cap an upper position of the given pattern of opening concavity  106  corresponding to the site needing film thickness precision, as depicted in  FIGS. 1E and 2E . 
     That is, a dry film resist  109  is roll coated in such a way as to cap the whole of the resist pattern  103  and the opening concavity  106  delimited by that resist pattern  103 . 
     Thereafter, photolithography is used to pattern the drying film resist  109  (selective exposure via the mask  130  and development for patterning) into a dry resist pattern  110  (dry resist  110 ), as depicted in  FIGS. 1F and 2F . 
     The general configuration of the formed dry resist pattern  110  (dry resist  110 ) is indicated by a dotted area in the plan view of  FIG. 4 , and an area a delimited by a broken line in  FIG. 4  is excerpted and redrawn as a perspective view in  FIG. 5 . In  FIGS. 4 and 5 , a part  110   a  of the dry resist pattern  110  plays a role as a cap that is formed at the upper position of the opening concavity corresponding to the site needing film thickness precision. In the embodiment shown in  FIG. 4 , the part  110   a  of the dry resist pattern  110  is shown as lying at only one site, because there is only one site needing film thickness precision involved. When there are plural such sites needing film thickness precision, however, the dry resist pattern is modified in such a way that as many parts  110   a  as those sites are present. 
     In the embodiment here, after the roll coating of the dry film resist  109 , photolithography is used to pattern the dry film resist  109 . Instead of using this method, however, a previously patterned dry film resist  109  may just as well be roll coated in such a way as to cap the predetermined sites of the resist pattern  103  and opening concavity  106 . 
     It is also contemplated that the dry film resist  109  may be previously patterned in conformity with only the morphology of the part  110   a  having a function of capping the site needing film thickness precision (in  FIG. 4 , for instance, suppose a morphology defined only by a bridged rectangular piece shape), and the dry film resist  109  in a rectangular piece shape may then be fixed to the site needing film thickness precision. 
     In the roll coating of the dry film resist  109 , it is contemplated that the temperature of the substrate  101  is set at about 20 to 100° C.; the roll temperature is at a about 80 to 150%; the roll pressure is at about 0.1 to 1 MPa; and the roll speed is at about 0.5 to 3 m/min. 
     When exposure and development are carried out harnessing photolithography after the roll coating of the dry film resist  109 , it is particularly preferable to control the exposure dose involved, because only the overlying dry film resist  109  can be printed into a given shape without having influences on the underlying resist pattern  103 . 
     It is also noted that both types, negative and positive, may be used for the resist for the formation of the resist pattern  103  (frame  103 ) and the dry film resist; however, it would be preferable to use the negative type for both in consideration of the fact that both are stacked one upon another for exposure to light and development. 
     (5) Step of Forming the Plated Film 
     Then, at the step of forming the plated film, the plated film is grown in such a way as to fill up the given pattern of opening concavity  106 . 
     To this end, at the step of forming the plated film, the electrode film  102  positioned at the bottom of the opening concavity  106  is used as a seed to grow plated films  107 ,  107   a  (both films are the same; in the present disclosure, however, the plated film at the site needing film thickness precision is tentatively indicated by  107   a  for a better understanding of the invention) in such a way as to fill up the opening  106 . 
     Upon the completion of formation of the plated films, the state depicted in  FIG. 1G , and  FIG. 2G  is reached. In the invention, even after the opening concavity  106  is thoroughly filled up, the plated films still continue to grow, as depicted in  FIG. 2G , and over-plating is going to be delimited by the shape of the dry resist pattern  110 . Although that over-plating takes effect in such a range that the effectiveness of the invention is ensured, it is acceptable to increase the amount of over-plating intentionally, because there may be a composite shape of plated films obtained through a combination of the resist pattern formed on the electrode film and the pattern of the dry film resist. 
     In the invention, the part  110   a  of the dry film resist  110  is located at the upper position of the opening concavity  106  corresponding to the site needing film thickness precision in such a way as to cap it. After the opening concavity  106  is fully filled up with the plated film  107   a , therefore, the thickness direction growth of plating at the position of the opening concavity  106  corresponding to the site needing film thickness precision is restricted by the part  110   a  of the capping dry film resist  110 , with the result that the plated film  107   a  ( FIG. 1G ) at the site needing film thickness precision is much improved in terms of film thickness precision and much reduced in terms of a fluctuation of plated film thickness distribution. 
     As noted previously, at the step of forming the plated film according to the invention, even after the plated film  107   a  at the site needing film thickness precision has its thickness direction growth restricted by abutting thoroughly on the plane defined by the part  110   a  of the capping dry film resist  110 , plating carries on a little while longer. This is to make sure the precision of the thickness of the plated film  107   a  at the site needing film thickness precision. For this reason, over-plating carries on at a site (an uncapped site of the dry film resist) with no restriction on the thickness direction growth of plating, so that an excessive plated film is formed along the pattern of the dry film resist pattern. 
     The composition of the plating bath used may be properly determined with the plated film to be formed in consideration, and plating conditions such as current density and bath temperature may just as well be properly determined, too. 
     (6) Step of Removing the Resists 
     Following the plated film-formation step, there is the resist removal step of removing the resist pattern  103  and dry film resist pattern  110 , as depicted in  FIGS. 1H and 2H . 
     At the resist removal step, the resist pattern  103  and dry film resist pattern  110  are removed with the use of an organic solvent such as isopropyl alcohol (IPA), N-methyl-2-pyrrolidone (NMP) or acetone. As a result, there are the electrode film  102  and plated film  105  remaining on the substrate  101 . 
     (7) Etching Step 
     Following the resist removal step, there is the etching step carried out for removal of an unnecessary portion of the electrode film  102  using the plated films  107 ,  107   a  as a mask, as depicted in  FIGS. 1I and 2I . 
     The etching used may be any of wet etching using an acidic or alkaline etchant, and dry etching such as sputter etching, active ion beam etching (RIE), and plasma etching. Which etching is used may be optionally determined in consideration of the physical properties of the plated film used as the mask, and the physical properties of the electrode film  102  to be etched. 
     The present invention is now explained in further details with reference to specific experiments. 
     EXAMPLES 
     Preparation of Example 1 Sample 
     There was an experiment carried out, in which the inventive frame plating process was used to form a plated film having the site needing film thickness precision, at which the thickness precision of the plated film is partly needed. 
     The frame plating process was carried out according to the following steps. 
     First of all, there was the silicon substrate  101  provided, having a size of 6 inches φ and a thickness of 1.2 mm. 
     Then, a 50 nm thick Cu film was formed by sputtering on the silicon substrate  101  into the electrode film  102 . 
     Then, a liquid resist (AZ4000 Series (of the positive type) made by AZ Electo-Material Co., Ltd.) was spin coated on the surface of the Cu electrode film  102 , after which it was prebaked at a temperature of 100° C. for 60 seconds into the resist film  103   a  having a thickness of 5 μm. 
     Then, exposure and development were performed under the following conditions: 
     [Exposure] 
     
         
         
           
             Aligner: NSR-i12TFH made by Nikon Cooperation 
             The mask used: Binary mask 
             Exposure conditions: NA=0.4 Sigma=0.6
           Dose=1,000 mJ/cm 2      Focus=0 μm
 
[Development]
   
         
           
         
       
    
     A developer comprising a 2.38% TMAH (tetramethyl anhydrite) aqueous solution was used for a 60-second development at five paddles. 
     The resist pattern was formed in such morphology as shown in  FIG. 3 . That is, the opening concavity  106  delimited by the resist pattern  103  had the morphology comprising the straight portion  102   a  having a decreased width, the fanning portion  102   b  joining to that decreased width portion  102   a  and having an increasing width and the portion  102   c  joining to the increasing width portion  102   b  and having an increased yet constant width. The width Wn of the straight portion  102   a  having a decreased width was set at 5 μm, and the site needing film thickness precision was set at substantially the middle of the straight portion  102   a  having a decreased width. 
     Then, the dry film resist  109  was roll coated (laminated) in such a way as to cover the whole of the resist pattern  103  and opening concavity  106 . 
     The dry film resist  109  used was the Photec Series of Resist (of the negative type) made by Hitachi Kasei Co., Ltd. and having a thickness of 10 μm. For the roll coating of the dry film resist  109 , the substrate  101  was heated at a temperature of 80° C. for 10 minutes, with a roll temperature of 110° C., a roll pressure of 0.4 Mpa, and a roll speed of 1.0 m/min. 
     Then, photolithography was used to pattern the dry film resist  109  into the dry resist pattern  110 . 
     Exposure and development were performed under the following conditions: 
     [Exposure] 
     
         
         
           
             Aligner: Mask Aligner PLA-501 made by Canon Co., Ltd. 
             The mask used: Binary mask having the shape dotted in  FIG. 4   
             Exposure conditions: NA=0.4 Sigma=0.6
           Dose=15 mJ/cm 2      Focus=0 μm
 
[Development]
   
         
           
         
       
    
     Development was implemented by a 20-second dipping in a developer comprising a 1% Na 2 CO 3  aqueous solution. 
     The formed dry resist pattern  110  had such a general shape as shown by a dotted area in the plan view of  FIG. 4 . In  FIG. 4 , the part  110   a  of the dry resist pattern  110  played a role as the cap formed at the upper position of the opening concavity corresponding to the site needing film thickness precision, and had a width Wc of 3 μm. 
     Then, while the electrode film  102  was used as a seed, the plated films  107 ,  107   a  were formed on the electrode film  102 . 
     That is, the plated film  107  having a Cu composition was grown to a thickness of 5.5 μm in such a way as to fill up the opening concavity  106 . The plating system used was an opposite parallel anode paddle plating one with a copper sulfate plating bath. It is noted that at the upper position of the opening concavity  106  corresponding to the site needing film thickness precision, there was the part  110   a  of the dry resist pattern  110  present, which functioned as the cap: the thickness of the plated film  107   a  at that site was going to be limited to 5.0 μm. 
     Then, isopropyl alcohol (IPA) was used for removal of the resist pattern  103  and dry film resist pattern  110 . 
     Then, the plated films  107 ,  107   a  were used as a mask and an aqueous solution containing 1% ferric chloride was used to dissolve off an unnecessary portion of the electrode film  102 . 
     The thickness film distribution of the plated film  107   a  (thickness: 5 μm) corresponding to the site needing film thickness precision—the site that had to have the maximum precision throughout the plated film—was measured in the following way. It has thus been found that the film thickness distribution across the plane of the film was 1%, indicating that a fairly good film thickness distribution is obtained at the specific site  107   a  needing film thickness precision. 
     (How to Measure the film Thickness Distribution) 
     The film thicknesses of the 40 sites for measurement were measured at random by observation of sections by FIB (focused ion beam) made by FEI Co., Ltd. 
     From these measurement data, the thickness distribution is then found by
 
Thickness distribution=((Maximum thickness value−Minimum thickness value)/Average thickness value)×100(%)
 
     Preparation of Comparative Example 1 Sample 
     Out of the steps of preparing the aforesaid sample of Example 1, there was the dry film resist pattern formation step omitted, at which the part  110   a  of the dry film resist  110  was located in such a way as to cap the upper position of the opening concavity  106  corresponding to the site needing film thickness precision. That is, without recourse to the dry film resist having a capping function, the frame pattern  103  shown in  FIG. 3  was formed, after which frame plating was initiated to form a plated film of 5 μm in thickness. 
     As a result of measurement of the film thickness distribution of the thus formed sample of Comparative Example 1 at its site similar to that of the aforesaid sample of Example 1, the film thickness distribution across the plane of the film was 5%. 
     From the aforesaid results, the effectiveness of the invention would be appreciated. That is, the frame plating process of the invention comprises the dry film resist pattern formation step at which a part of the dry film resist is located in such a way as to cap the upper position of the given pattern of opening concavity corresponding to the site needing film thickness precision. With that frame plating process, it is thus possible to obtain a fairly good film thickness distribution at the specific site needing film thickness precision in a simple manner yet without depending on the film thickness distribution of the plated film based on plating conditions.