Abstract:
A blind hole ( 3 ) is formed on a substrate ( 1 ) from a first side of the substrate toward a second side of the substrate ( 1 ). A conductor ( 11 ) is filled in the blind hole ( 3 ). The substrate ( 1 ) is removed from the opposite side to expose the conductor ( 13 ) filled in the blind hole ( 3 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of Japanese Patent Application P 2002-324135, filed Nov. 7, 2002 in the Japanese Patent Office, the disclosure of which is incorporated herein in its entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for fabricating a through-hole interconnection substrate and a through-hole interconnection substrate. More specifically, the present invention is adapted for a high-density three-dimensional packaging of stacking a silicon IC chip and the like or to a contact thereof. The present invention is also adapted for a silicon optical bench for implementing an optical device such as a laser diode, a photodiode and an optical waveguide. 
   According to the present invention, a metal for a conductor is filled in micro-holes for through-hole electrodes. The through hole electrodes are utilized for interconnecting wiring patterns formed on front and back surfaces of a silicon substrate, to be employed as electrodes or contacts, and to form bumps. 
   2. Description of the Related Art 
   An example of a related art technology for filling metal in micro-holes is a molten-metal suction method disclosed in Japanese Patent Laid-Open No. 2002-158191. According to this method, a molten metal is filled in the holes by means of a pressure difference. An example of a method for forming bumps on one surface of a substrate simultaneously with this filling work, is one in which metal layers are formed in the peripheries of openings of the micro-holes, followed by the metal filling by the molten-metal suction method. 
   In the molten-metal suction method, heat sometimes deteriorates adhesion of a heat-resistant sheet, thus making it impossible to fill the metal fully in the ends of the micro-holes. 
   Specifically, when the melting temperature of the metal material in use exceeds 350° C. (degrees centigrade) during the filling work, such high temperature is beyond a tolerance of the heat-resistant sheet. 
   SUMMARY OF THE INVENTION 
   In order to solve the above problems, a first aspect of the invention is directed to a method for fabricating a through-hole interconnection substrate. The method includes forming a blind hole in a substrate from a first side of the substrate toward a second side of the substrate, forming a conductor in the blind hole, and removing a portion of the substrate from the second side of the substrate to expose an end of the conductor. 
   The conductor may be molten and pressurized into the blind hole. 
   The method may include the step of forming an insulated layer on a surface of the substrate and an inner wall of the blind hole. 
   The substrate may be etched from the opposite side. 
   A second aspect of the invention is directed to a through-hole interconnection substrate. The through-hole interconnection substrate includes a substrate having a through-hole and a conductor protruding through the through-hole. The substrate is formed with a blind hole extending from a first side of the substrate toward a second side of the substrate, the conductor is formed in the blind hole by pressurizing molten conductor material, and a portion of the substrate and an end portion of the conductor are removed from the second side of the substrate, exposing the conductor filled in the blind hole. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects and advantages of the invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is an exploded perspective view of a three-dimensional multilayer device. 
       FIGS. 2A to 2E  are cross-sectional views of an exemplary embodiment of an insulator substrate according to the invention, showing steps of forming through-hole interconnections. 
       FIGS. 3A to 3D  are cross-sectional views of an exemplary embodiment of a semiconductor substrate according to the invention, showing steps of forming through-hole interconnections. 
       FIGS. 4A to 4E  are cross-sectional views of the semiconductor substrate, showing steps following  FIG. 3D . 
       FIGS. 5A to 5C  are schematic views showing steps of a molten-metal suction method. 
       FIG. 6A  is a schematic view of an apparatus for use in photo assisted electro-chemical etching. 
       FIG. 6B  is a schematic view showing a principle of the photo assisted electrochemical etching. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Exemplary embodiments of the invention will now be described with reference to the accompanying drawings. The described exemplary embodiments are intended to assist the understanding of the invention, and are not intended to limit the scope of the invention in any way. 
   Referring to  FIG. 1 , a multilayer device  100  includes IC (Integrated Circuit) chips  101 ,  102 , and  103  as three stacked layers. Multilayer device  100  includes a sensor chip  104  on IC chip  103 . IC chips  101 ,  102 , and  103  include through-hole interconnections  101   a ,  102   a , and  103   a  in peripheral edges thereof, respectively. Through-hole interconnections  101   a ,  102   a  and  103   a  electrically connect IC chips  101 ,  102  and  103  with each other. Sensor chip  104  includes gas sensor  104   a , pressure sensor  104   b , and IR sensor  104   c  on a surface thereof. 
   A method for fabricating multilayer device  100  includes the steps of processing a work, forming a circuit pattern, and bonding a wire. The work is processed as below. 
   Fabrication of a work or a through-hole interconnection substrate of an insulated material will be described with reference to  FIGS. 2A to 2E  (the case where the work is a substrate is assumed in the description below). 
   The work is fabricated by the steps of forming blind holes (refer to  FIG. 2A ), forming metal layers (refer to  FIG. 2B ), and filling molten metal ( FIGS. 2C and 2D ). 
   Referring to  FIG. 2A , a plurality of micro-holes  3  are formed on one surface  5  of a glass substrate  1 . Micro-holes  3  are made blind. Thickness T of glass substrate  1  is larger than depth D of each micro-hole  3  from one surface  5 . 
   For example, a DRIE (Deep Reactive Ion Etching) method, a laser method, a micro drill method or a sandblast method may be applied to form micro-holes  3 . The DRIE is an ICP-RIE (Inductively Coupled Plasma-Reactive Ion Etching) method. The laser method employs a laser for drilling. The micro drill method employs a micro drill (micro diameter drill) for drilling. In the sandblast method, micropowder is sprayed. 
   Additionally, the substrate is not limited to glass substrate  1 . For example, a ceramic, a resin or a composite material thereof is also applicable as long as it has heat resistance higher than a melting temperature of a metal to be filled therein. The thickness of the substrate is on the order of several ten micrometers (μm) to several centimeters (cm). The diameter and depth of each micro-hole are on the order of several nanometers (nm) to several millimeters (mm). There are no limitations in the number of micro-holes to be formed on the substrate. 
   Referring to  FIG. 2B , metal layers  7  are formed in the peripheries of openings of the plurality of micro-holes  3 , for example, by sputtering, and are patterned into a predetermined shape. The shape of metal layers  7  is predetermined to assist in the formation of a bump shape (described below). An example of the metal layer (underlayer) is a layer of Cr and then Au sputtered with thicknesses of 30 nm and 500 nm, respectively. After coating photoresist thereon, the resist is patterned by photolithography. The Au and then the Cr are etched by use of the patterned resist as a mask. 
   Referring to  FIGS. 2C and 5A , a molten-metal bath  67  and substrate  1  are disposed in a chamber  51 . Substrate  1  is supported by substrate holder  55 . A molten metal  11  is stored in bath  67 . Molten metal  11  is a gold-tin eutectic solder (Au—20 wt % Sn). Molten metal  11  is heated up, for example, to 330° C. to be molten by a heater  65 . The atmospheric pressure in chamber  51  is reduced to vacuum. Next, referring to  FIG. 5B , substrate  1  is immersed in molten metal  63  in bath  67 . At this stage, molten metal  63  is not filled in micro-holes  3 . Next, referring to  FIG. 5C , after substrate  1  reaches a temperature substantially equal to that of molten metal  63 , chamber  51  is pressurized, for example, to the atmospheric pressure or higher. This pressurization fills molten metal  63  into micro-holes  3 . Subsequently, substrate  1  is raised from bath  67 . At this time, bumps are formed on micro-holes  3 . 
   Glass substrate  1  formed by the above process corresponds to  FIG. 2D . Molten metal  11  has been filled and is solidified inside the plurality of micro-holes  3  of substrate  1 , forming blind contacts  13 . The formation of metal layers  7  also forms bumps  15 . 
   Referring to  FIG. 2E , the opposite surface (bottom surface)  17  of glass substrate  1  is then ground and polished off for flattening. The grinding and polishing allow the bottom surfaces of the filled metal to appear. Thus, contacts  13  are exposed from glass material Ma. Specifically, glass substrate  1  including through-hole interconnections  13  and bumps  15  is completed. 
   Next, the steps of forming micro-holes in a substrate made of a material other than the insulated material will be described. 
   Referring to  FIG. 3A , a plurality of micro-holes  23  are formed on one surface  25  of a silicon substrate  21 . In this case, micro-holes  23  are made blind. A thickness T 2  of silicon substrate  21  is larger than a depth D 2  of each micro-hole  23  from one surface  25 . 
   To the formation of holes  23 , for example, the Photo Assisted Electro-Chemical Etching (hereinafter, referred to as a PAECE method) is applied. In the PAECE, an aqueous hydrofluoric acid (HF) solution is brought into contact with the front surface of an n-type silicon substrate, and lights of a xenon lamp are irradiated onto the back surface thereof. The silicon substrate functions as an anode. A platinum plate in the aqueous hydrofluoric acid solution functions as a cathode. A voltage is applied between the silicon substrate and the platinum plate. 
   Specifically, referring to  FIG. 6A , an apparatus  70  includes electrolytic bath  71  storing electrolyte  72  of the HF solution. Apparatus  70  includes a cathode electrode  73  immersed in the electrolyte, and silicon substrate  21 . Apparatus  70  includes a DC power  74  between silicon substrate  21  and cathode electrode  73 . Apparatus  70  includes a light source  75  placed outside an electrolytic bath  71 . Apparatus  70  includes an infrared filter  76  between electrolytic bath  71  and light source  75 . 
   On surface  21   b  of the silicon substrate, a V-groove  21   a  is formed by use of KOH in advance. Lights are radiated from light source  75 , pass through filter  76 , and are irradiated onto back surface  21   c  of the silicon substrate, which coincides with V-groove  21   a . During this irradiation, current flows between substrate  21  and electrode  73 . 
   Referring to  FIG. 6B , V-groove  21   a  is selectively etched to form a hole. Specifically, by the irradiation of lights  75   a  onto back surface  21   a  of the silicon substrate, carriers (positive holes) are produced on back surface  21   c . These carriers concentrate on the tip end of the bottom of V-groove  21   a , and the tip end is intensively etched. 
   The substrate is not limited to silicon substrate  21 . The substrate may be made of, for example, a chemical compound, a semiconductor or a metal, as long as it has heat resistance greater than the melting temperature of the metal to be filled therein. The thickness of the substrate is the order of several ten micrometers to several centimeters. The diameter and depth of each micro-hole are the orders of several nanometers to several millimeters. There are no limitations in the number of micro-holes to be formed on the substrate. 
   A DRIE method, a laser method, a micro drill method or a sandblast method may be applied to a substrate of a non-insulated material in place of the PAECE method. 
   Referring to  FIG. 3B , an insulated layer  27  is formed on the inner walls of micro-holes  23  and the surface of the substrate. For example, a SiO2 film, a SiN film or the like is formed by use of a method such as thermal oxidization, CVD or coating of a spin-on-glass film. The thickness of insulated layer  27  is the order of several ten nanometers to several millimeters. 
   Next, referring to  FIG. 3C , metal layers  29  are formed by sputtering in the peripheries of openings of micro-holes  23 , and patterned into a predetermined shape. The shape of metal layers  29  is predetermined to assist in the formation of a bump shape (described below). An example of the metal layer (underlayer) is a layer of Cr and then Au sputtered with thicknesses of 30 nm and 500 nm, respectively. After coating photoresist thereon, the resist is patterned by photolithography. The Au and then the Cr are etched by use of the patterned resist as a mask. 
   Referring to  FIG. 3D , a molten metal  33  is filled in micro-holes  23  of silicon substrate  21  by the molten-metal suction method. Subsequently, substrate  21  is raised from the molten metal bath  67 . At this time, bumps  37  (refer to  FIG. 4A ) are formed on micro-holes  23 . 
   Silicon substrate  21  after the process will be described with reference to  FIG. 4A . Molten metal  33  has been filled in the plurality of micro-holes  23 , and formed the plurality of contacts  35 . Bumps  37  are formed on metal layers  29 . As described above, the surface of silicon substrate  21  is covered with insulated layer  27 . 
   Referring to  FIG. 4B , the opposite surface (bottom surface)  39  of silicon substrate  21  is ground and polished. The grinding and polishing are stopped back from insulated layer  27  formed in micro-holes  23 . Thickness T 3  of silicon substrate  21  is larger than depth D 3  of each micro-hole  23  on which insulated layer  27  is provided and in which molten metal  33  is filled. 
   Referring to  FIG. 4C , only the substrate material is etched, for example, by chemical etching. This etching allows the bottom portions of the micro-holes (that is, contacts  35  as filled metal covered with the insulated layer) to appear in the order of several micrometers. Plate thickness T 4  of silicon substrate  21  is made smaller than length D 4  of each contact  35 . The bottom portions of the micro-holes may be exposed from the start only by, for example, the chemical etching, without grinding and polishing, other than in the method described above. 
   Referring to  FIG. 4D , an insulated layer  41  is formed on the surface of the exposed substrate material. A process temperature during the formation of the insulated layer  41  is set at a temperature lower than a melting point of the filled metal. This set temperature prevents the filled metal from melting and falling out of micro-holes  23  during the operation. There are no limitations on the material of insulated layer  41 , except that it must be possible to form insulated layer  41  at a process temperature lower than the melting point. The thickness of insulated layer  41  is the order of several micrometers to several ten micrometers. 
   Specifically, if the filled metal is gold-tin eutectic solder (Au—20 wt % Sn) with the melting point of 280° C., a SiO 2  film with a thickness of 5 μm is deposited at 200° C. by plasma CVD. Again, the opposite surface (bottom surface)  39  of the substrate is ground and polished, exposing the bottoms of the metal-filled portions. Thus, the through-hole interconnections are completed. 
   Silicon substrate  21  after the process will be described with reference to  FIG. 4E . The surfaces of material Mb of silicon substrate  21  is covered with insulated layer  27  and insulated layer  41 . Contacts  35  with bumps  37  made of metal layers  29  are formed. The surface of the substrate material is covered with the insulated layer, and there is no potential risk that the filled metal would contaminate the substrate material. 
   Thus, according to an aspect of the invention, a substrate is formed with a blind hole. Next, the inner wall of the hole and the surface of the substrate are formed with an insulated material, except in the case of a substrate formed of an insulated material. A metal layer is formed around an opening of the hole. A molten-metal suction method is employed to fill a metal in the hole and to form a bump. 
   Thus, according to the method, the sealing of the hole on one side by the substrate itself requires no heat-resistant sheet, and allows sealing not to be broken by heat. 
   In a case of a substrate of an insulated material, after filling a metal, the bottom surface is ground and polished to expose a filled metal. This completes a through-hole interconnection. 
   Next, in a case of a substrate without an insulated material, after filling a metal, the bottom surface is ground and polished in the same manner. However, the grinding and polishing are stopped back from the insulated layer formed in a micro-hole. Thereafter, only a substrate material is etched, using, for example, chemical etching, exposing the bottom of the micro-hole. The insulated layer at the bottom of the micro-hole is employed as a protection layer against etching. The reason not to grind and polish the filled metal in the micro-hole is to prevent the attaching or dispersing of a metal powder to or in the substrate material and the resulting contamination of the substrate. In a case of a substrate of a single crystal, for example, chemical etching after grinding and polishing can remove a fractured layer on the polished surface that is produced by grinding and polishing. This effectively removes defects such as micro-cracks on the surface of the substrate. 
   Next, an insulated layer is formed on the exposed surface of the substrate. A process temperature during the formation of the insulated layer is set at a temperature less than melting point of the filled metal. This prevents the filled metal from melting and flowing out during the operation. Thereafter, again, the bottom of the substrate is ground and polished to expose a metal filled portion. This completes a through-hole interconnection. The surface of the substrate is covered with an insulated layer, and no contamination due to the filled metal occurs. 
   Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.