Patent Publication Number: US-2011057312-A1

Title: Contact structure and method for producing a contact structure

Description:
The present invention relates to a contact structure and to a method for producing a contact structure for semiconductor substrates, in particular for terminal faces of semiconductor substrates, comprising a base contact part arranged on a terminal face of the semiconductor substrate and at least one connecting contact part arranged on the base contact part, wherein the connecting contact part is formed from a connecting contact material which has a lower melting point than a base contact material of the base contact part. 
     Contact structures for semiconductor substrates are frequently also referred to as “bumps”, which are essentially composed of a metallization which is applied to a terminal face of a semiconductor substrate and which forms a raised contact with respect to the circumjacent surface of the semiconductor substrate. These raised contacts, which for instance are also formed on terminal faces of printed circuit boards (PCB), essentially fulfill the function of establishing an electrically conductive and mechanically firm connection between the semiconductor substrate and the contact substrate subsequent to the formation of a contact with the terminal face of a substrate to be contacted by at least partially melting the metallization. Moreover, due to the raised contacts formed by the bumps, a gap is formed between the semiconductor substrate and the contact substrate and can be filled with an electrically insulating filler material in order to mechanically stabilize the substrate arrangements formed by the contacting between the semiconductor substrate and the contact substrate and to realize a sealing covering of the contact points. 
     In particular in the case of relatively large substrates to be connected, in practice it has frequently proven laborious to realize a contacting of the substrates in all opposing contact points while simultaneously realizing a plane-parallel arrangement of the substrates. The capillary forces generated in a reflow process for melting the contact metallizations frequently give rise to warpage of the substrates with the consequence that the gap between the substrates is not formed uniformly. 
     Thus, it is an object of the invention to suggest a contact structure that enables the formation of a largely uniform gap between the substrates when two substrates are contacted to produce a substrate arrangement. 
     To attain this object, the inventive contact structure has the features of claim  1 . 
     The inventive contact structure for semiconductor substrates or the like comprises a base contact part arranged on a terminal face of the semiconductor substrate and at least one connecting contact part arranged on the base contact part, wherein the connecting contact part is formed from a connecting contact material which has a lower melting point than a base contact material of the base contact part. 
     Due to the differentiation in terms of the base contact material, respectively the connecting contact material used for the base contact part and the connecting contact part, the inventive contact structure enables a functional differentiation between the parts of the contact structure in such a manner that the base contact material, which compared with the connecting contact material has a relatively high melting point, enables the setting of a defined contact height, which can be retained also subsequent to the melting of the connecting contact part to produce the mechanical, electrically conductive joined connection with a terminal face of the contact substrate. 
     Preferably, the base contact part is formed from the base contact material applied to the terminal face in an electroless deposition process. In this process, the generally known good controllability of the electroless deposition process is advantageously used by setting the suitable process parameters, in order to subsequently arrange the metallization to be melted, which is essential for the actual connecting process and which is formed by the connecting contact part, at a precisely defined height, respectively distance above the surface of the substrate. In particular, the electroless deposition process makes it possible for a plurality of terminal faces arranged in a terminal face arrangement on the surface of the semiconductor substrate to carry out a selective formation of arbitrary individual base contact parts or else groups of base contact parts, for instance by temporary covering a part of the terminal faces, in order to form base contact parts only on the freely accessible terminal faces of the semiconductor substrate. In this manner, base contact parts as a matter of fact can also be formed at various heights on a terminal face arrangement of a semiconductor substrate. 
     It is also particularly advantageous if the connecting contact part is formed by the connecting contact material which is applied to the base contact part by means of a sputtering process or else by means of a vapor-phase deposition process. Connecting contact parts formed using such methods can be produced with a particularly small layer thickness, so that connecting contact parts can be realized which, due to their low mass, are only relatively slightly modified also in their height in a melting process. As a result, by means of this measure, contact structures can be realized which have a height that is substantially fixedly determined by the height of the base contact parts and which, due to the low mass of the connecting contact parts, are likewise only slightly modified in their height in a melting process. Consequently, the gap width between two substrates of a substrate arrangement is essentially defined from the outset by the height of the contact structures. 
     In particular for enhancing the adhesive force between the base contact part and the connecting contact part, the base contact part may have a layer structure which comprises a base layer and at least one built-up layer in the transition zone towards the connecting contact part. 
     Preferably, the at least one built-up layer is formed by electroless deposition of the coating material onto the base layer of the base contact part in order to use the previously described advantageous options and effects equally for the built-up layer. 
     Depending on the texture of the material of the terminal faces of the semiconductor substrate to be provided with the contact structure, the base contact part may have a base layer made of nickel or a base layer made of copper, wherein the former is utilized in the formation of a semiconductor substrate terminal face made of aluminum and the latter is utilized in the formation of a semiconductor substrate terminal face made of copper. In particular for enabling good adhesion of the connecting contact part applied to the base contact part, a built-up layer made of gold or palladium may be formed on the base layer made of nickel or else the base layer made of nickel may be provided with a first built-up layer made of palladium and a second built-up layer arranged thereon made of gold. Moreover, it may also be advantageous if a base layer made of nickel is provided with a built-up layer made of copper. In any case, it is advantageous if all layers of the base contact part, i.e. both the base layer and the one or more built-up layers are applied to the terminal face in a wireless deposition process. 
     The use of tin or an alloy containing tin as a connecting contact material proves to be particularly advantageous for the production of the connecting contact part, in order to realize a good mechanical adhesion to the base contact part and a lowest possible contact resistance on the one hand and, on the other hand, in order to realize a mechanically resistant and reliable connection between the base contact part and the terminal face of the contact substrate to be contacted due to the good wettability of tin also in the case of a thinly applied tin coating, i.e. a small layer thickness of the connecting contact part, by means of melting the connecting contact part. 
     The properties specified above and created by the differentiated configuration of the contact structure, which comprises a base contact part and a connecting contact part, produce a particularly advantageous effect if the height of the base contact part is several times the height of the connecting contact part, in particular in the instance where the height of the connecting contact part is ten to twenty percent of the height of the base contact part. 
     The inventive method has the features of claim  16 . 
     In the inventive method, a base contact part is formed by electroless deposition of a base contact material onto the terminal face, and subsequently a connecting contact part is formed by means of sputtering or else by means of vapor-phase deposition of a connecting contact material onto the base contact part, wherein the connecting contact part has a lower melting point than the base contact material. 
     In a first advantageous variant of the method, prior to the formation of the connecting contact part, at least one resist layer which covers the base contact part is applied to a contact side of the semiconductor substrate, and subsequent to the formation of a coating opening which exposes a coating surface of the base contact part and which widens downwardly, in the resist layer, the connecting contact material is deposited onto the surface of the resist layer and the base contact part. 
     In a second advantageous variant of the method, prior to the formation of the connecting contact part, a resist structure comprising at least one lower resist layer and an upper resist layer arranged thereon is applied to a contact side of the semiconductor substrate, a staggered coating opening which widens downwardly is subsequently formed in the resist structure in such a manner that, firstly, a window opening exposing the lower resist layer is formed in the upper resist layer, the lower resist layer covering the base contact part is then removed through the window opening, and subsequent to the formation of the coating opening exposing the base contact part, the connecting contact material is deposited onto the surface of the upper resist layer and the base contact part. 
     Preferably, in both method variants, a coating surface, which is defined by projecting an opening cross-section onto the contact side of the semiconductor substrate, is smaller than a base surface of the coating opening. 
     It is especially advantageous to configure the coating surface, which is defined by projecting the opening cross-section onto the contact side of the semiconductor substrate, so as to be smaller than the surface of the base contact part in order to be able to apply a particularly small defined amount of the connecting contact material onto the base contact part. 
     Hereinafter, preferred embodiments of the invention will be described in more detail with reference to the drawings. 
    
    
     
       In the drawings: 
         FIG. 1 : shows a base contact part formed on a contact side of a semiconductor substrate; 
         FIG. 2 : shows a resist layer applied to the contact side of the semiconductor substrate according to a first variant of a method for producing a contact structure; 
         FIG. 3 : shows a coating opening formed in the resist layer; 
         FIG. 4 : shows the deposition of a connecting contact material onto the contact side of the semiconductor substrate; 
         FIG. 5 : shows the removal of the resist coating subsequent to the deposition of the connecting contact material; 
         FIG. 6 : shows a contact structure subsequent to the removal of the resist coating; 
         FIG. 7 : shows a contact structure subsequent to the remelting of the connecting contact part arranged on the base contact part; 
         FIG. 8 : shows a substrate arrangement comprising a contact arrangement formed from a plurality of contact structures; 
         FIG. 9 : shows a resist structure on a contact side of a semiconductor substrate according to a second variant of a method for producing a contact structure; 
         FIG. 10 : shows a window opening formed in an upper resist layer of the resist structure; 
         FIG. 11 : shows a coating opening formed in the resist structure; 
         FIG. 12 : shows the deposition of a connecting contact material onto the contact side of the semiconductor substrate; 
         FIG. 13 : shows the removal of the resist structure from the contact side of the semiconductor substrate. 
     
    
    
       FIG. 1  shows the circumjacent region of a base contact part  22  of a contact structure  24  illustrated in  FIGS. 6 and 7  and composed of the base contact part  22  and a connecting contact part  23 , the base contact part  22  being formed on a terminal face  20  of a semiconductor substrate  21  by means of an electroless deposition method known per se. 
     The production of the base contact part  22  on the terminal face  20 , as for instance illustrated in  FIG. 1 , is performed in such a manner that, due to the isotropic growth of the base contact part  22  produced in the deposition process, a region of a passivation layer  26  covering a contact side  25  of the semiconductor substrate  21 , which region surrounds the terminal face  20 , is covered as well. 
     The generation of the base contact part  22 , in particular in the instance where the terminal face  20  is formed of copper, may be performed heterogeneously on the basis of the deposition of one single metal, for instance copper, or else in particular also in the case of an aluminum terminal face, by sequential configuration with a base layer made of nickel and one or more built-up layers arranged thereon, wherein for instance a combination with (immersion) gold, copper or palladium as a built-up layer and, where appropriate, an arrangement of two built-up layers made of palladium or gold on a nickel base layer has proven to be advantageous. 
     The semiconductor substrate  21  for instance may be formed of materials such as Si, GaAs, SiGe or InP. Instead of a semiconductor substrate, a printed circuit board or the like which comprises a corresponding terminal face may equally be provided. The terminal face may also be formed by a carrier surface of an otherwise electrically non-conductive surface of a substrate and which has been prepared for electroless deposition, in order to produce electrically inactive contact structures, which merely fulfill a mechanical function, in particular a spacer function. 
       FIGS. 2 to 5  show a first variant for producing the contact structure  24  illustrated in  FIGS. 6 and 7 , wherein, firstly, a resist layer  27  made of a photoresist is applied to the contact side  25  of the semiconductor substrate  21  and subsequently, the resist layer  27  is lithographically structured, the result thereof being illustrated in  FIG. 3 , whereby a coating opening  29  which exposes the base contact part  22  is formed on the contact side  25 . 
     When the coating opening  29  is formed by the application of a suitable developer onto the exposed resist layer  27 , slanted recess walls  32  are created which form an undercut  31  in such a manner that a coating surface  31  defined by projecting an opening cross-section  30  of the coating opening  29  onto the contact side  25  of the semiconductor substrate  21  is smaller than a base surface  33  of the coating opening  29 . 
     In the instance where, as shown in  FIG. 3  using dash-dotted lines, the resist layer  27  covers a periphery of the surface of the base contact part  22  a coating surface  31  is realized which is clearly smaller than the surface of the base contact part  22 . 
     As shown in  FIG. 4 , the connecting contact part  23  is formed on the resist layer  27  by means of sputtering or else by means of vapor-phase deposition of a connecting contact material  34 , preferably containing tin, wherein the connecting contact part merely partially covers the base contact part  22  due to a coating shadow  44  created by the undercut  31 . Moreover, due to the coating shadow, the recess walls  32  are free from a coating and thus are freely accessible for a subsequent rinsing process or the like to remove the resist layer  27 , for instance with the aid of a solvent  37 , as shown in  FIG. 5 , such that subsequent to the rinsing process, the contact structure  24  still remains on the contact side  25  of the semiconductor substrate  21 , as shown in  FIG. 6 . 
     Subsequently, the connecting contact part  23  of the contact structure  24 , as shown in  FIG. 7 , can be melted or else the connecting contact part  23  of the contact structure  24  illustrated in  FIG. 6  is melted in a reflow process only in the production of the contact arrangement  35  illustrated in  FIG. 8 , which connects the semiconductor substrate  21  with a contact substrate  36 , respectively a further semiconductor component, so that a joined connection between the base contact parts  22  of the semiconductor component  21  with contact metallizations  36  of the contact substrate  36  is created by the molten connecting contact parts  23 . 
     The contact arrangement  35  illustrated in  FIG. 8  can be formed at a wafer level, i.e. may serve for connecting two wafers, or else chips or substrates in chip size can be connected with one another or can each be contacted with a wafer. 
     Dimensional relations in which the height of the connecting contact part in the range of 10 to 20 percent of the height of the base contact part  22  have proven to be particularly advantageous in terms of the dimensions, respectively heights, of the base contact part  22  and the connecting contact part  23 . In the case of a height of 3 to 20 μm of the base contact part  22 , a height of approximately 0.3 to 2 μm is obtained for the connecting contact part  23 . 
       FIGS. 9 to 13  show a second variant for producing the contact structure  24  illustrated in  FIGS. 6 to 7 , wherein in contrast to the first variant illustrated in  FIGS. 2 to 5 , starting from the semiconductor substrate  21  illustrated in  FIG. 1 , which has the base contact part  22  arranged on the contact side  25  thereof, a resist structure  38  is applied which has a first resist layer  39  directly applied to the contact side  25  and which has a second resist layer  40  applied to the resist layer  39 . 
     The illustrated exemplary embodiment of the upper resist layer  40  shows a photoresist layer, similar to the variant illustrated in  FIGS. 2 to 5 , which can be structured with the aid of an exposure process and a development process. The lower resist layer  39  of the illustrated exemplary embodiment is a chemically activatable resist layer, for instance a so-called PMMA resist layer. 
     As shown in  FIG. 10 , for forming the coating opening  41  illustrated in  FIG. 11 , firstly, a window opening  42  is formed in the upper resist layer  40  and a suitable developer or solvent is then applied to the lower resist layer  39 , in order to form the coating opening  41  which exposes the base contact part  22  in the lower resist layer  39  for instance by polymerization. 
     As is in particular shown in  FIG. 11 , a coating opening  41  is obtained which has a staggered recess wall  43  and which widens downwardly towards the base contact part  22 . 
     As already described in view of the first variant, in particular with respect to  FIG. 3 , the coating opening  41  which widens downwardly towards the base contact part  22  has an opening cross-section  30  which is smaller compared with a base surface  33  of the coating opening  41  with the resultant coating shadow. 
     In the instance that, as illustrated in  FIG. 11  using dash-dotted lines, the upper resist layer  40  covers a periphery of the surface of the base contact part  22 , a coating surface  31  is obtained which is clearly smaller than the surface of the base contact part  22 . 
     As shown in  FIG. 12 , the connecting contact part  23  is formed on the upper resist layer  40  by means of sputtering or vapor-phase deposition of the connecting contact material  34 , wherein the connecting contact part only partially covers the base contact part  22  due to a coating shadow  45  created by the staggered recess wall  43 , so that the previously described advantages are realized. 
     Following a rinsing process, which enables the removal of the resist arrangement  38 , respectively the dissolution of the lower resist layer  39 , for instance with the aid of a solvent  46 , as illustrated in  FIG. 13 , the contact structure illustrated in  FIG. 6  still remains on the contact side  25  of the semiconductor substrate  21 .