Anodizing method and apparatus and semiconductor substrate manufacturing method

Abstract

A porous layer having a multilayered structure is formed. An Si substrate ( 102 ) to be processed is anodized in a first electrolytic solution ( 141, 151 ) while being held between an anode ( 106 ) and a cathode ( 104 ) in an anodizing bath ( 101 ). The first electrolytic solution ( 141, 151 ) is exchanged with a second electrolytic solution ( 142, 152 ). The Si substrate ( 102 ) is anodized again, thereby forming a porous layer having a multilayered structure on the Si substrate ( 102 ).

What is claimed is:  
     
         1 . An anodizing method of forming a porous layer on a substrate, comprising the steps of: 
 preparing an anodizing bath used to anodize a substrate,    anodizing said substrate to be processed in a first electrolytic solution while holding said substrate between an anode and a cathode in said anodizing bath,    exchanging the first electrolytic solution with a second electrolytic solution, and    anodizing said substrate in the second electrolytic solution, thereby forming a porous layer having a multilayered structure on said substrate.    
     
     
         2 . The method according to  claim 1 , wherein a current density of a current to be flowed across said anode and said cathode is changed between anodizing using the first electrolytic solution and anodizing using the second electrolytic solution.  
     
     
         3 . The method according to  claim 1 , wherein a conductive diaphragm is inserted between said anode and said substrate to be processed to prevent contamination of said substrate by said anode.  
     
     
         4 . The method according to  claim 3 , wherein said conductive diaphragm is arranged to flow the whole current from said anode to said substrate to be processed through said conductive diaphragm.  
     
     
         5 . The method according to  claim 3 , wherein said conductive diaphragm is arranged to cover a surface of said anode that opposes a backside surface of said substrate to be processed.  
     
     
         6 . The method according to  claim 3 , wherein said conductive diaphragm is arranged to isolate an electrolytic solution in contact with a surface of said substrate to be processed, that is on said anode side, from an electrolytic solution in contact with said anode.  
     
     
         7 . The method according to  claim 4 , wherein at least a surface of said conductive diaphragm that opposes said substrate to be processed is formed from a silicon material.  
     
     
         8 . The method according to  claim 4 , further comprising the step of changing said conductive diaphragm every time an anodizing condition is changed.  
     
     
         9 . The method according to  claim 4 , further comprising the step of preparing a conductive diaphragm corresponding to each anodizing condition, and every time the anodizing condition is changed, using a conductive diaphragm corresponding to the condition.  
     
     
         10 . The method according to  claim 1 , further comprising the step of forming a porous layer having a relatively low porosity as a surface layer of said substrate to be processed, and forming a porous layer having a relatively high porosity as an underlayer of said surface layer.  
     
     
         11 . The method according to  claim 10 , further comprising the step of anodizing to form a porous layer having a porosity of not more than 30% as said surface layer.  
     
     
         12 . The method according to  claim 10 , further comprising the step of anodizing to form a porous layer having a porosity of not less than 30% as said underlayer of said surface layer.  
     
     
         13 . The method according to  claim 10 , further comprising the step of anodizing to form a porous layer having a thickness of not more than 5 nm as said underlayer of said surface layer.  
     
     
         14 . An anodizing method of forming a porous layer on a substrate, comprising the step of: 
 preparing at least two anodizing baths used to anodize a substrate, anodizing said substrate to be processed while holding said substrate between an anode and a cathode in one anodizing bath, and anodizing said substrate while holding said substrate between an anode and a cathode in the next anodizing bath, thereby forming a porous layer having a multilayered structure on said substrate.    
     
     
         15 . The method according to  claim 14 , wherein different electrolytic solutions are used as electrolytic solutions used for anodizing in all or some of said at least two anodizing baths.  
     
     
         16 . The method according to  claim 14 , wherein a current density of a current to be flowed across said anode and said cathode is changed in anodizing in all or some of said at least two anodizing baths.  
     
     
         17 . The method according to  claim 14 , wherein a conductive diaphragm is inserted between said anode and said substrate to be processed to prevent contamination of said substrate by said anode.  
     
     
         18 . The method according to  claim 17 , wherein said conductive diaphragm is arranged to flow the whole current from said anode to said substrate to be processed through said conductive diaphragm.  
     
     
         19 . The method according to  claim 17 , wherein said conductive diaphragm is arranged to cover a surface of said anode that opposes a backside surface of said substrate to be processed.  
     
     
         20 . The method according to  claim 17 , wherein said conductive diaphragm is arranged to isolate an electrolytic solution in contact with a surface of said substrate to be processed, that is on said anode side, from an electrolytic solution in contact with said anode.  
     
     
         21 . The method according to  claim 17 , wherein at least a surface of said conductive diaphragm that opposes said substrate to be processed is formed from a silicon material.  
     
     
         22 . The method according to  claim 21 , further comprising the step of changing said conductive diaphragm every time an anodizing condition is changed.  
     
     
         23 . The method according to  claim 21 , further comprising the step of preparing a conductive diaphragm corresponding to each anodizing condition, and every time the anodizing condition is changed, using a conductive diaphragm corresponding to the condition.  
     
     
         24 . The method according to  claim 14 , further comprising the step of forming a porous layer having a relatively low porosity as a surface layer of said substrate to be processed, and forming a porous layer having a relatively high porosity as an underlayer of said surface layer.  
     
     
         25 . The method according to  claim 24 , further comprising the step of anodizing to form a porous layer having a porosity of not more than 30% as said surface layer.  
     
     
         26 . The method according to  claim 24 , further comprising the step of anodizing to form a porous layer having a porosity of not less than 30% as said underlayer of said surface layer.  
     
     
         27 . The method according to  claim 24 , further comprising the step of anodizing to form a porous layer having a thickness of not more than 5 μm as said underlayer of said surface layer.  
     
     
         28 . A semiconductor substrate manufacturing method comprising the steps of: 
 processing a first substrate using the anodizing method of  claim 1  to form a porous layer having a multilayered structure on said first substrate;    forming at least one non-porous layer on said porous layer;    bonding a second substrate to a surface of said non-porous layer of said first substrate;    separating a bonded substrate stack to a side of said first substrate and a side of said second substrate at a portion of said porous layer; and    removing said porous layer left on the side of said separated second substrate.    
     
     
         29 . The method according to  claim 28 , further comprising the step of removing said porous layer left on the side of said separated first substrate to allow reuse of said first substrate.  
     
     
         30 . The method according to  claim 28 , wherein the separation step comprises separating said bonded substrate stack at a portion having a high porosity in said porous layer having the multilayered structure.  
     
     
         31 . The method according to  claim 28 , wherein said non-porous layer contains a single-crystal silicon layer.  
     
     
         32 . The method according to  claim 28 , wherein said non-porous layer comprises a single-crystal silicon layer and a silicon oxide layer which are sequentially formed on said porous layer.  
     
     
         33 . The method according to  claim 28 , wherein said non-porous layer contains a compound semiconductor layer.  
     
     
         34 . The method according to  claim 28 , wherein said second substrate comprises a silicon substrate.  
     
     
         35 . The method according to  claim 28 , wherein said second substrate comprises a substrate having a silicon oxide layer formed on a surface to be bonded to said first substrate.  
     
     
         36 . The method according to  claim 28 , wherein said second substrate comprises a transparent substrate.  
     
     
         37 . The method according to  claim 28 , further comprising, after the step of removing said porous layer, the step of planarizing a surface layer on the side of said second substrate.  
     
     
         38 . The method according to  claim 37 , wherein the planarization step comprises annealing in an atmosphere containing hydrogen.  
     
     
         39 . The method according to  claim 28 , wherein the step of removing said porous layer comprises selectively etching said porous layer using, as an etchant, any one of 
 a) hydrofluoric acid,    b) a mixed solution prepared by adding at least one of an alcohol and hydrogen peroxide to hydrofluoric acid,    c) buffered hydrofluoric acid, and    d) a mixed solution prepared by adding at least one of an alcohol and hydrogen peroxide to buffered hydrofluoric acid.    
     
     
         40 . The method according to  claim 33 , wherein the step of removing said porous layer comprises selectively etching said porous layer using an etchant whose etching rate for said porous layer is higher than that for a compound semiconductor.  
     
     
         41 . The method according to  claim 28 , wherein the step of removing said porous layer comprises selectively polishing said porous layer using said non-porous layer as a stopper.  
     
     
         42 . The method according to claims  28 , wherein the bonding step comprises the step of bringing said first substrate having said non-porous layer into tight contact with said second substrate.  
     
     
         43 . The method according to  claim 28 , wherein the bonding step comprises, after the step of bringing said first substrate having said non-porous layer into tight contact with said second substrate, the step of performing a process selected from the group consisting of anode bonding, pressing, heating, and a combination thereof.  
     
     
         44 . A semiconductor substrate capable of being manufactured by the manufacturing method of  claim 28 .  
     
     
         45 . A semiconductor substrate in a process of executing the manufacturing method of claims  28 .  
     
     
         46 . A substrate having a porous layer formed by the anodizing method of claims  1 .  
     
     
         47 . An anodizing apparatus for executing the anodizing method of  claim 1 .  
     
     
         48 . An anodizing apparatus for forming a porous layer on a substrate, comprising: 
 an anodizing bath having an anode and a cathode;    a plurality of tanks for storing electrolytic solutions to be supplied to said anodizing bath;    a supply mechanism for selectively supplying the electrolytic solution stored in any one of said plurality of tanks to said anodizing bath; and    a drain mechanism for draining off the electrolytic solution from said anodizing bath back into said tank which supplied the electrolytic solution.    
     
     
         49 . The apparatus according to  claim 48 , wherein said apparatus further comprises a holding mechanism for holding a conductive diaphragm between said anode and a substrate to be processed, and said conductive diaphragm prevents contamination of said substrate by said anode.  
     
     
         50 . The apparatus according to  claim 49 , wherein said holding mechanism holds said conductive diaphragm to flow a whole current from said anode to said substrate to be processed through said conductive diaphragm.  
     
     
         51 . The apparatus according to  claim 49 , wherein said holding mechanism holds said conductive diaphragm to cover a surface of said anode that opposes a backside surface of said substrate to be processed.  
     
     
         52 . The apparatus according to  claim 49 , wherein said holding mechanism holds said conductive diaphragm to isolate an electrolytic solution in contact with a surface of said substrate to be processed, that is on said anode side, from an electrolytic solution in contact with said anode.  
     
     
         53 . The apparatus according to  claim 49 , wherein at least a surface of said conductive diaphragm that opposes said substrate to be processed is formed from a silicon material.  
     
     
         54 . The apparatus according to  claim 49 , wherein said holding mechanism detachably holds said conductive diaphragm.  
     
     
         55 . An anodizing apparatus for forming a porous layer on a substrate, comprising: 
 at least two anodizing baths each having an anode and a cathode; and    a conveyor mechanism for conveying a substrate anodized in one anodizing bath into the next anodizing bath.    
     
     
         56 . The apparatus according to  claim 55 , wherein said substrate is anodized in all or some of said at least two anodizing baths under different conditions.  
     
     
         57 . The apparatus according to  claim 55 , further comprising 
 a cleaning unit for cleaning said substrate processed in said final anodizing bath of said at least two anodizing baths, and    a drier unit for drying said substrate cleaned by said cleaning unit.    
     
     
         58 . The apparatus according to  claim 55 , wherein each of said anodizing baths comprises a holding mechanism for holding a conductive diaphragm between said anode and said substrate to be processed, and said conductive diaphragm prevents contamination of said substrate by said anode.  
     
     
         59 . The apparatus according to  claim 58 , wherein said holding mechanism holds said conductive diaphragm to flow a whole current from said anode to said substrate to be processed through said conductive diaphragm.  
     
     
         60 . The apparatus according to  claim 58 , wherein said holding mechanism holds said conductive diaphragm to cover a surface of said anode that opposes said substrate to be processed.  
     
     
         61 . The apparatus according to  claim 58 , wherein said holding mechanism holds said conductive diaphragm to isolate an electrolytic solution in contact with a surface of said substrate to be processed, that is on said anode side, from an electrolytic solution in contact with said anode.  
     
     
         62 . The apparatus according to  claim 59 , wherein at least a surface of said conductive diaphragm that opposes said substrate to be processed is formed from a silicon material.  
     
     
         63 . The apparatus according to  claim 55 , wherein said holding mechanism detachably holds said conductive diaphragm.