Abstract:
A method for producing an electrical feedthrough in a substrate includes: forming a first printed conductor on a first side of a substrate which electrically connects a first contact area of the substrate on the first side; forming a second printed conductor on a second side of a substrate which electrically connects a second contact area of the substrate on the second side; forming an annular trench in the substrate, a substrate punch being formed which extends from the first contact area to the second contact area; and selectively depositing an electrically conductive layer on an inner surface of the annular trench, the substrate punch being coated with an electrically conductive layer and remaining electrically insulated from the surrounding substrate due to the annular trench.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing an electrical feedthrough in a substrate, and a substrate having an electrical feedthrough. 
     2. Description of the Related Art 
     Electrical feedthroughs in a substrate or in a subarea of a substrate, such as a wafer, for example, exist in numerous specific embodiments. The objective is always to achieve the smallest possible feedthrough at a low electrical volume resistance. To achieve this, frequently a narrow through hole having practically vertical walls is produced in the substrate in question, the wall is electrically insulated, and the through hole is then completely or partially filled with a metal or a metal alloy in order to obtain the desired low volume resistance. 
     Depending on the application, this known approach has limitations. On the one hand, there are applications in which the presence of metal results in interference. The micromechanical pressure sensor is named here as one example of numerous MEMS applications. 
       FIG. 3  shows a schematic cross-sectional illustration for explaining the object on which the present invention is based, with reference to a substrate having an electrical feedthrough and a pressure sensor as an example. 
     In  FIG. 3 , reference numeral  10  denotes a silicon semiconductor substrate. A first area  1  having an electrical feedthrough  6   a  and a second area  11  having a micromechanical component in the form of a pressure sensor are provided in silicon semiconductor substrate  2 . Feedthrough  6   a  is connected to a first electrical contact terminal DK 1  of pressure sensor  11  via a printed conductor  15   a  on front side V of substrate  10 . Pressure sensor  11  has a diaphragm  3  which is provided above a cavity  3   a . A piezoresistive resistor  4  and an insulation trough  4   a  situated therebeneath are diffused into diaphragm  3 . First electrical contact terminal DK 1  as well as a second electrical contact terminal DK 2  contact piezoresistive resistor  4  in such a way that the piezoelectric resistance therebetween is detectable. 
     A first insulating layer Ii is provided between electrical metal printed conductor  15   a  and front side V of substrate  10 . A second insulating layer  12  is provided between an electrical metal printed conductor  15   b  on the back side, and back side R of substrate  2 . Insulating layers I 1  and  12  may be oxide layers, for example. Feedthrough  6   a  connects printed conductor  15   a  on the front side to printed conductor  15   b  on the back side. A wall insulating layer  7   a , which is likewise made of an oxide, for example, insulates feedthrough  6   a  from surrounding substrate  10 . Lastly, reference numeral  9  denotes a so-called seed layer for applying the metal of feedthrough  6   a , which at the same time may be used as a diffusion barrier. 
     In such classical micromechanical pressure sensors  11 , deformation of silicon diaphragm  3 , which is situated on silicon substrate  10 , is measured via the piezoresistive resistor. The deformation of diaphragm  3 , and thus the resistance signal of piezoresistive resistor  4 , changes when the pressure changes. As a result of the different material parameters of silicon and metal, narrow metal printed conductors  15   a  located at the surface and in the vicinity of diaphragm  3  result in voltages which are transmitted via substrate  10  to diaphragm  3 . The temperature-dependent portion of the voltages may be compensated for, with some effort. However, the inelastic properties of many metals also result in hysteresis in the characteristic curve of the pressure sensor. It is not possible to compensate for this effect. When metallic areas are provided not only at the surface but also at a depth in substrate  2 , even greater adverse effects on voltage-sensitive components, for example such as pressure sensors, are expected. 
     On the other hand, there are a number of applications in which primarily also high voltages or also only high voltage peaks (ESD, for example) are to be conducted by a substrate or a subarea of the substrate via an electrical feedthrough. This is difficult using the approach described above. The etched through holes are usually insulated by oxide deposition. The achievable oxide thicknesses are greatly limited by the process control and the specific geometry. Therefore, the maximum electric strength is also greatly limited. In addition, the surface of the through holes, which are produced using a trench etching process or a laser process, is rather rough. This roughness causes electrical field peaks which likewise reduce the electric strength. 
     Alternative approaches without metals are not feasible in many applications, since the extremely low volume resistances which are often necessary are achievable only using metals. 
     A micromechanical component having wafer through-contacting as well as a corresponding manufacturing method are known from published German patent application document DE 10 2006 018 027 A1. A blind hole is introduced into the front side of a semiconductor substrate using a trench etching process, and the side wall of the blind hole is porously etched using an electrochemical etching process. The blind hole is filled with a metal plating and subsequently opened by thinning the semiconductor substrate on the back side. 
     A micromechanical component having wafer through-contacting as well as a corresponding manufacturing method are known from published German patent application document DE 10 2006 042 366 A1, in which metallic material is initially applied to a first area on the surface of the top side of a semiconductor substrate. The first area is designed in such a way that it leaves open a second area on the top side of the semiconductor substrate, which does not have the metallic material, and completely encloses this second area. A thermal step is then carried out which produces a first volume area within the semiconductor substrate having P+ or P++ doping. The thermal step results in a diffusion process in which metallic material diffuses from the top side to the bottom side of the semiconductor substrate. As a result of the diffusion process, the first volume area thus produced encloses a second volume area, which is preferably composed of the unaffected P-doped semiconductor material. To provide electrical insulation between the second volume area and the P-doped semiconductor material enclosing the first volume area, the first volume area is porosified using a suitable etching process. 
     German patent application DE 10 2010 039 339.4, which is not deemed a prior art, discloses the combination of a metallic punch as the feedthrough having a wide insulation ring. It is characteristic for this system that low resistances and a high electric strength as well as an efficient strain decoupling may be achieved. However, this approach allows only for relatively large TSVs (through silicon vias) and the manufacturing process is complex and expensive. 
     BRIEF SUMMARY OF THE INVENTION 
     The idea underlying the present invention is that an annular trench is formed in the substrate which is formed from a first side of the substrate to an opposing second side of the substrate, the annular trench being coated by an electrically conductive layer, but at the same time remaining electrically insulated from the rest of the surrounding substrate due to the annular trench. 
     The coated substrate punch thus created connects one contact area, which is connected to a printed conductor, on the first side of the substrate to a contact area, which is connected to a printed conductor, on the second side of the substrate. Preferably, the annular trench is subsequently filled completely or partially with an insulating material. The substrate punch is used as a low-resistance feedthrough due to its conductive coating. 
     This type of feedthrough has a high electric strength, low leakage currents, low parasitic capacitances, as well as low electrical resistance. The resistance of the feedthrough is not a function of the substrate doping. With the aid of the method according to the present invention, very small feedthroughs having a high aspect ratio may be implemented. Also, feedthroughs may be implemented in very thick layers which have a planar surface. 
     The manufacturing process is very cost-effective and requires only one-time trenching of the substrate. Diffusion barriers are not required. 
     A robust process control is possible, it being possible for the process to take place as a via-last process. The maximum temperature may be lower than 400° C. and the process is CMOS compatible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  to  1   j  show schematic cross-sectional illustrations for explaining various process stages of a method for producing an electrical feedthrough in a substrate according to a first specific embodiment of the present invention. 
         FIG. 2  shows a schematic cross-sectional illustration for explaining an electrical feedthrough in a substrate according to a second specific embodiment of the present invention. 
         FIG. 3  shows a schematic cross-sectional illustration for explaining the object on which the present invention is based, with reference to a substrate having an electrical feedthrough and a pressure sensor as an example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Identical or functionally equivalent components are denoted by the same reference numerals in the figures. 
       FIGS. 1   a  through  1   j  show schematic cross-sectional illustrations for explaining various process stages of a method for producing an electrical feedthrough in a substrate according to a first specific embodiment of the present invention. 
     According to  FIG. 1   a , a micromechanical component  11  in the form of a pressure sensor, which has already been explained in detail with reference to  FIG. 3 , is provided in a silicon semiconductor substrate  10 . 
     After forming a first insulating layer  21   a , made of an oxide, for example, on front side V of substrate  10 , through holes corresponding to electrical contact terminals DK 1 , DK 2  for piezoresistive resistor  4  as well as a through hole are initially formed in first insulating layer  21   a , the through hole defining a contact area  22  of substrate  10  of a feedthrough through substrate  10  which is to be subsequently produced. 
     Electrical contact terminals DK 1 , DK 2  of piezoresistive resistor  24  and a metallic printed conductor  23  which connects contact area  22  to electrical contact terminal DK 1  are subsequently formed by deposition and appropriate structuring of a metal layer. 
     Another insulating layer  21 , made of an oxide, for example, is finally deposited on front side V on top of the electrical circuit configuration. 
     For printed conductor  23 , one or multiple metal layer(s) with or without diffusion barriers or adhesive layers may be deposited. In the exemplary specific embodiment, printed conductor  23  is formed from an aluminum layer. 
     Furthermore, with regard to  FIG. 1   b , the substrate is sanded on back side R, an area  25  being removed to reduce the overall thickness of the substrate stack and the height of the feedthrough to be formed. Back side R may be conditioned using a back-etch process in a plasma process or in a liquid etching medium or in a CMP process (chemical mechanical polishing). 
     As shown in  FIG. 1   c , another insulating layer  26 , made of an oxide, for example, is deposited on back side R. 
     As shown in  FIG. 1   d , a contact hole  27   a ) is formed in insulating layer  26  to define an additional contact area  27  on back side R which is opposite contact area  22  on the front side. 
     Similarly to how the front side is processed, a metal plating layer  28   a  is structured by one or more sublayer(s) with or without diffusion barriers or adhesive layers on top of insulating layer  26  and contact area  27 . Thus, metal plating layer  28   a  electrically connects contact area  27  as shown in  FIG. 1   e.    
     Furthermore, with reference to  FIG. 1   f , metal plating layer  28   a  is structured in a printed conductor  28  on the back side, and a lattice G is provided in the process in the area where an annular trench R is to be subsequently formed in substrate  10 , the lattice exposing substrate  10  in the area where annular trench R is to be formed. Lattice G in printed conductor  28  is preferably also used in this case as a mask for the perforation of insulating layer  26  lying underneath it. 
     As shown in  FIG. 1   g , annular trench R in silicon substrate  10  is formed using a trench etching process during which an etching medium is guided through lattice G to substrate  10 , lattice G being completely undercut and a substrate punch  17  being formed which electrically connects contact area  22  on the front side to contact area  27  on the back side. In this case, insulating layer  21   a  on the front side is used together with printed conductor  23  on the front side as an etch stopping layer. 
     In a subsequent process step, which is illustrated in  FIG. 1   h , a conductive layer  16 , made of tungsten, for example, is deposited in a redox reaction with silicon through lattice G on the vertical trench surfaces of annular trench R in order to make substrate punch  17  low-resistance. Substrate punch  17 , which is coated with tungsten layer  16 , remains electrically insulated from surrounding substrate  10 , since no or hardly any tungsten is deposited on the front side on insulating layer  21   a  during this selective deposition. 
     Furthermore, with reference to  FIG. 1   i , another insulating layer  26   a , made of a nitride or an oxide, is subsequently deposited on back side R in order to close lattice G and to passivate the walls of annular trench R. 
     Finally, with reference to  FIG. 1   j , a contacting hole  20  is formed in insulating layer  26   a  in order to expose in this area printed conductor  28  on the back side, thus making a subsequent electrical contacting (not shown) of printed conductor  28  possible. 
     Optionally, the electrical contact between the different metal layers and silicon substrate  10  may be improved with the aid of a temperature step. This step may be carried out multiple times or even earlier in the process. 
     On back side R, additional process steps (not illustrated) may finally take place to produce additional components. 
     The described and illustrated specific embodiment allows a simple production of a low-resistance feedthrough having a high electric strength which is formed using substrate punch  17  coated with the conductive tungsten layer. 
     The boundary of the electrical connection of coated substrate punch  17  to lower contact area  27  lies completely or partially in the area of annular trench R, whereby it is achieved that the tungsten deposition easily reaches the transition area to the lower metal layer, thus making a very low transfer resistance possible. 
     The electrical connection of the coated substrate punch to contact area  22  on the front side also lies completely or partially in the area of annular trench R, whereby here, too, the tungsten deposition reaches the transition area to the upper metal layer, thus making a very low transfer resistance possible. 
     It is advantageous to use aluminum as the material for printed conductors  23  and  28  and to carry out a temperature step above 350° C. prior to the tungsten deposition, thus achieving a dissolution of aluminum in silicon. In particular, an aluminum layer may optionally be used which has a low silicon content or none at all to achieve the creation of a strong and deep alloy phase between aluminum and silicon during the temperature step. In this way, a low-resistance direct contact is made possible between the alloy phase and tungsten layer  16  following the tungsten reaction. 
       FIG. 2  shows a schematic cross-sectional illustration for explaining an electrical feedthrough in a substrate according to a second specific embodiment of the present invention. 
     In the specific embodiment illustrated in  FIG. 2 , the feedthrough described with reference to  FIGS. 1   a  through  1   j  is applied to a micromechanical sensor component  35  having a movable structure. The feedthrough is located in a first substrate  10   a  on which sensor structure  35  is provided on front side V. Sensor structure  35  is capped using a cap wafer  10   b  which is glued on via an adhesive layer  50 . A printed conductor  33  of sensor structure  35  is connected to a contact area  22  on front side V which is electrically connected to contact area  27  on the back side via coated substrate punch  17 . 
     The feedthrough is produced using substrate punch  17  coated with tungsten layer  16  similarly to the specific embodiment according to  FIGS. 1   a  through  1   j , it being advantageous to initially produce sensor structure  35  on the front side and to cap it with cap wafer  10   b  for protection, in order to subsequently carry out the described trenching and deposition process to produce the feedthrough. 
     Although the present invention has been described with reference to multiple exemplary embodiments which may be arbitrarily combined with one another, the present invention is not limited thereto, and may be further modified in various ways. 
     In particular, the above-mentioned materials are only examples, and are not to be construed as being limiting. In addition, the micromechanical components such as the pressure sensor, the printed conductors, and further electrical components, for example, may be produced in or on the substrate, either before or after producing the feedthroughs. 
     Of course, any arbitrary additional protective, insulating, passivation, and diffusion barrier layers may be deposited to further increase the reliability. 
     The method according to the present invention is not limited to the described micromechanical components but is applicable, in principle, to any electrical circuit configurations which require a low-resistance feedthrough of high electric strength. 
     Also, the present invention is not limited to the described materials but is applicable to any material combinations made of conductive and non-conductive materials.