Abstract:
A semiconductor device comprising a semiconductor substrate having first and second surfaces opposing each other, the substrate including a plurality of cells sharing a common drain region, each of the cells having source and gate regions, a surface source electrode connected to the source region of each of the cells and provided on the first surface, a strap member coupled with the surface source electrode by ultrasonic waves, a gate polysilicon wiring layer connecting the gate region of each of the cells and having a silicide layer in at least a portion of a surface thereof, a surface gate electrode connected to the gate polysilicon wiring layer and provided on the first surface, and a drain electrode provided on the second surface and shared by the cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-141495, filed May 16, 2002, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and, more particularly, to a semiconductor device in which a surface source electrode of a MOSFET (insulated gate field-effect transistor) is electrically connected to a lead frame via a strap member. For example, the present invention is used in a trench gate type power MOSFET. 
     2. Description of the Related Art 
       FIG. 5  is a plan view schematically showing a portion of a conventional trench gate type power MOSFET using a wire bonding structure.  FIG. 6  is a sectional view schematically showing a structure taken along a line VI—VI in FIG.  5 . 
     This power MOSFET is comprised of a plurality of MOSFET cells, and includes a surface source electrode (Source-AL)  13  connecting to source diffusion layers  4  and a surface gate electrode (Gate-AL)  14  connecting to a gate polysilicon wiring layer  7   a . The MOSFET cells are formed in a semiconductor chip  40  which is mounted on a lead frame (not shown). A plurality of wires (Source-Wire)  41  are connected by bonding so as to electrically connect the surface source electrode  13  to the lead frame. A wire (Gate-Wire)  15  is connected by bonding so as to electrically connect the surface gate electrode  14  to a lead portion of the lead frame. 
     As each of the surface source electrode  13  and surface gate electrode  14 , an Al electrode mainly composed of Al is used. A portion of the gate polysilicon wiring layer  7   a  is formed into a planar stripe in the array of the source diffusion layers  4 . To reduce the internal resistance (to be referred to as rg hereinafter) of the gate, a gate Al (Gate-Al) wiring layer  42  mainly composed of Al is formed in contact with the upper surface of the gate polysilicon wiring layer  7   a . The gate Al wiring layer  42  and the surface source electrode  13  are two-dimensionally interdigitated. 
     Recently, it is strongly demanded to reduce the ON resistance of a power MOSFET. As a package for achieving this demand, a strap member structure which connects the surface source electrode by a strap member is attracting attention. A representative example is Jpn. Pat. Appln. KOKAI Publication No. 2000-114445. 
     The conventional power MOSFET strap member structure connects a Cu strap member to the surface source electrode via an adhesive such as Ag paste. 
     Unfortunately, in reliability tests such as a temperature cycle test for a power MOSFET, this strap member structure shortens the life of the power MOSFET because the thermal expansion coefficients of an Al electrode on the chip surface, the Ag paste, the Cu strap member, and a Cu lead frame are different. 
     To solve this problem, a method of connecting a strap member by using ultrasonic waves (to be referred to as US hereinafter) has been proposed. 
       FIG. 7  is a plan view schematically showing a portion of a conventional power MOSFET using a strap member.  FIG. 8  is a sectional view taken along a line VIII—VIII in FIG.  7 . In  FIGS. 7 and 8 , the same reference numerals as in  FIGS. 5 and 6  denote the same parts. 
     The reliability of this power MOSFET strap member is greatly improved by connecting a Cu or Al strap member (Source-Strap)  16  onto a surface source electrode  13  by US. 
     In this structure, the strap member is connected onto the surface source electrode  13 . Therefore, if a gate Al wiring layer  42  exists as in the prior art shown in  FIGS. 5 and 6 , the gate Al wiring layer  42  is broken and short-circuited to the surface source electrode  13  when the strap member is connected by US near the gate Al wiring layer  42 . This often causes a short circuit between a gate G and source S. 
     Accordingly, the gate Al wiring layer is omitted as shown in  FIG. 7 , so the gate Al wiring layer  42  and the surface source electrode  13  are not two-dimensionally interdigitated. 
     The existence, however, of the gate Al wiring layer  42  has a large effect on the internal resistance rg of the gate. If this gate Al wiring layer is omitted as shown in  FIG. 7 , rg will be increased about twice (approximately 3Ω) as compared with about 1.5Ω when the gate Al wiring layer  42  exists as in the prior art. This rg rise lowers the conversion efficiency when the power MOSFET is used for synchronous rectification. Therefore, this power MOSFET cannot be used for synchronous rectification. 
     In the conventional power MOSFET as described above, no gate Al wiring layer can be formed when a strap member is connected onto the surface source electrode by US in order to improve the reliability. This will result in the increase of the internal resistance rg of the gate. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a semiconductor device comprises a semiconductor substrate having first and second surfaces opposing each other, the substrate including a plurality of cells sharing a common drain region, each of the cells having source and gate regions; a surface source electrode connected to the source region of each of the cells and provided on the first surface; a strap member coupled with the surface source electrode by ultrasonic waves; a gate polysilicon wiring layer connecting the gate region of each of the cells and having a silicide layer in at least a portion of a surface thereof; a surface gate electrode connected to the gate polysilicon wiring layer and provided on the first surface; and a drain electrode provided on the second surface and shared by the cells. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a plan view schematically showing a portion of a trench gate type power NMOSFET using a strap member according to the first embodiment; 
         FIG. 2  is a sectional view schematically showing a structure taken along a line II—II in  FIG. 1 ; 
         FIG. 3  is a sectional view schematically showing a semiconductor chip mounted on a lead frame; 
         FIG. 4  is a sectional view schematically showing a portion of a trench gate type power NMOSFET using a strap member according to the second embodiment; 
         FIG. 5  is a plan view schematically showing a portion of a trench gate type power MOSFET using a conventional wire bonding structure; 
         FIG. 6  is a sectional view schematically showing a structure taken along a line VI—VI in  FIG. 5 ; 
         FIG. 7  is a plan view schematically showing a portion of a power MOSFET using a conventionally proposed strap member structure; and 
         FIG. 8  is a sectional view schematically showing a structure taken along a line VIII-VII in FIG.  7 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments will be described in detail below with reference to the accompanying drawings. 
       FIG. 1  is a plan view schematically showing a portion of a trench gate type power N(-channel) MOSFET using a strap member according to the first embodiment.  FIG. 2  is a sectional view schematically showing a structure taken along a line II—II in FIG.  1 . 
     In this power MOSFET shown in  FIGS. 1 and 2 , a semiconductor chip  20 , which includes a MOSFET having a surface source electrode  13  connected to source diffusion layers  4  and a surface gate electrode  14  connected to a gate polysilicon wiring layer  7   a , is mounted on a lead frame as shown in FIG.  3 . 
     A wire (Gate-Wire)  15  is connected by bonding so as to electrically connect the surface gate electrode  14  to a lead portion of the lead frame. 
     In addition, to electrically connect the surface source electrode  13  to the lead frame, a Cu or Al strap member  16  is connected onto the surface source electrode  13  by US. 
     In the semiconductor chip  20 , reference numeral  1  denotes an N + -type silicon substrate;  2 , an N-type epitaxial layer formed on one major surface of the N + -type silicon substrate  1 ;  3 , a P-type base region formed on the surface of the epitaxial layer  2 ;  4 , the N + -type source regions selectively formed on the surface of the P-type base region  3 ;  5 , gate trenches extending from the surface of the P-type base region  3  to the epitaxial layer  2  through portions of the N + -type source regions  4 ; and  6 , a gate oxide film (silicon oxide film) formed on the surfaces of the N + -type source regions  4  and on the inner surfaces of the gate trenches  5 . 
     Reference numeral  7   a  denotes the gate polysilicon wiring layer formed by patterning a heavily doped polysilicon film deposited on the silicon oxide film  6 ; and  7   b , trench gates which are formed by burying the polysilicon film in the gate trenches  5  and connect to the gate polysilicon wiring layer  7   a.    
     Reference numeral  8  denotes an insulating film which covers the surface such that the upper surface of the gate polysilicon wiring layer  7   a  is partially exposed; and  9 , a Ti/TiN silicide layer, for example, formed on the exposed portion of the upper surface of the gate polysilicon wiring layer  7   a.    
     Reference numeral  10  denotes an interlayer dielectric film which covers the gate polysilicon wiring layer  7   a  and Ti/TiN silicide layer  9 ;  11 , contact holes selectively formed to reach the P-type base region  3  through the interlayer dielectric film  10 , the underlying silicon oxide film  6 , and the N + -type source regions  4 ; and  12 , a diffusion layer formed on the bottom and its vicinity of each contact hole  11  to obtain an ohmic contact with the N + -type source regions  4 . In addition, a contact hole (not shown) is formed in the interlayer dielectric film  10  on the silicide layer  9  on the gate polysilicon wiring layer  7   a  in a chip peripheral portion. 
     Reference numerals  13 ,  14 , and  14   a  denote metal layers (e.g., Al layers) deposited by sputtering and patterned on the interlayer dielectric film  10 . More specifically, reference numeral  13  denotes the surface source electrode connecting to the N + -type source regions  4  through the contact holes  11 ;  14   a , a wiring layer in contact with the silicide layer  9  on the upper surface of the gate polysilicon wiring layer  7   a  through the contact hole (not shown) in the chip peripheral portion; and  14 , the surface gate electrode (pad) connecting to the wiring layer  14   a . On the other major surface (rear surface) of the N + -type silicon substrate  1 , a drain electrode (not shown) which is a metal layer (e.g., an Al layer) is formed. 
     An example of a fabrication method of the MOSFET device comprised of the semiconductor chip  20  shown in  FIGS. 1 and 2  will be explained below. 
     On the major surface of the semiconductor substrate  1  made of N + -type silicon, the N-type epitaxial layer  2  is formed by epitaxial growth. Then, using ion implantation, a P-type impurity, e.g., boron, is implanted into the surface of the epitaxial layer  2  and then diffused thereinto to provide the P-type base region  3 . 
     Subsequently, an N-type impurity, e.g., arsenic, is selectively implanted into the P-type base region  3 , thereby forming source regions  4  therein. 
     Thereafter, gate trenches  5  are selectively formed by, e.g., reactive ion etching (RIE) so as to extend from the surface of the P-type base region  3  to the epitaxial layer  2  through portions of the N + -type layers  4 . 
     The substrate surface is then oxidized to form the gate oxide film (gate oxide film)  6 . 
     Thereafter, a polysilicon film for the gate electrode, which is heavily doped with the N-type impurity, is deposited until the gate trenches  5  are completely filled. This polysilicon film is selectively etched to the silicon surface except for a gate wiring portion, thereby forming a gate polysilicon wiring layer  7   a  and trench gates  7   b.    
     An insulating film  8  is deposited on the substrate surface by CVD and patterned to expose a portion of the gate polysilicon wiring layer  7   a . Ti/TiN, for example, is deposited on the exposed wiring layer  7   a  by sputtering, and rapid thermal annealing (RTA) is performed to form a silicide layer  9  therein. 
     Thereafter, the interlayer dielectric film  10  is deposited by CVD, and contact holes  11  for the contact with the source regions  4  are formed by RIE. A P-type impurity, e.g., boron, is implanted and diffused in the contact holes  11  to form a diffusion layer  12  for obtaining an ohmic contact with each of the source regions  4 . 
     A metal for forming the surface source electrode  13 , the surface gate electrode  14  and the wiring layer  14   a  is deposited by sputtering. Finally, the drain electrode is provided on the opposite surface of the substrate  1  to obtain the trench gate type power NMOSFET as described above. 
     The reliability of the strap member of the trench gate type power NMOSFET according to the above embodiment can be improved by connecting the strap member  16  onto the surface source electrode  13  by US. 
     Also, the silicide layer  9  is formed by silicidation in a portion of the upper surface of the gate polysilicon wiring layer  7   a , and the surface gate electrode  14  is connected via the wiring layer  14   a  in contact with the silicide layer  9 . This realizes a low internal resistance rg (about 1.5Ω) similar to that in the case where the gate Al wiring layer ( 42  in  FIG. 5 ) is provided. Accordingly, the gate Al wiring layer ( 42  in  FIG. 5 ) used in the prior art can be omitted. 
     Consequently, a trench gate type power NMOSFET having a low ON resistance can be implemented although a strap member is used. 
       FIG. 3  shows a state in which the power NMOSFET chip  20  is mounted on a lead frame  22  via a conductive coupling layer  21 . That is, the drain electrode of the MOSFET is coupled with a lead portion  22   a  of the lead frame  22 , and the strap member  16  connected to the surface source electrode is coupled with a lead portion  22   b . Likewise, the gate wire  15  is also connected to a lead portion (not shown). 
       FIG. 4  is a sectional view schematically showing a portion of a trench gate type NMOSFET using a strap member according to the second embodiment. 
     This power NMOSFET of the second embodiment has the same planar pattern as the power NMOSFET of the first embodiment described above. However, the sectional structure and formation steps of a gate polysilicon wiring layer are different as will be described below. Since the rest is the same, the same reference numerals as in  FIG. 2  denote the same parts in FIG.  4 . 
     That is, the polysilicon film for the gate electrode, which is heavily doped with the N-type impurity, is deposited until gate trenches  5  are completely filled, and this polysilicon film is selectively etched to the silicon surface except for the gate wiring layer portion, thereby forming the gate polysilicon wiring layer  7   a  and trench gates  7   b . These processes are the same as in the first embodiment. Thereafter, Ti/TiN, for example, is deposited by sputtering without depositing any insulating film  8  on the substrate surface, and subjected to RTA to form the silicide layer  9 . At the same time, the upper portion of the polysilicon layer in each trench gate  7   b  is subjected to the silicidation to form the silicide layer  9 . The interlayer dielectric film  10  is then formed. 
     In the second embodiment described above, the insulating film  8  which covers the upper surface of the gate polysilicon wiring layer  7   a  partially exposed is omitted, and the Ti/TiN silicide layer  9  is formed on both the entire upper surface of the gate polysilicon wiring layer  7   a  and the polysilicon surface of each trench gate  7   b.    
     This structure has the same advantages as the power NMOSFET of the first embodiment described previously. 
     Note that although each of the above embodiments takes a trench gate type power NMOSFET as an example, the present invention is of course also applicable to a trench gate type power PMOSFET or an IGBT in accordance with the above embodiments. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit and scope of the general inventive concept as defined by the appended claims and their equivalents.