Abstract:
Formation of metal pipes resulting from formation of metal silicide contacts are reduced or avoided. To reduce formation of metal pipes, an epitaxial layer is formed over the diffusion region on which the metal silicide contact is formed. The epitaxial layer reduces defects which enhances diffusion of metal atoms or molecules.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fabrication of semiconductor devices, and more particularly, to reducing formation of metal silicide bridging. 
     DESCRIPTION OF THE RELATED ART 
     Transistors are commonly used in the integrated circuits.  FIG. 1  a shows a conventional n-type metal oxide semiconductor (MOS) transistor  110  formed on an active region of a substrate  105 . A p-well  108  is located in active region of the substrate. Shallow trench isolations  180  are used to isolate the active region from other device regions. The transistor has source/drain diffusion regions  125   a - b  adjacent to a gate  130 , which includes a gate electrode  131  over gate dielectric layer  132 . Located on the gate sidewalls are dielectric sidewall spacers  160 . Metal silicide contacts  140  and  141  are provided on the surface of the substrate in the source/drain regions and gate electrodes. 
     Nickel silicide or nickel alloy silicides have been extensively used as contacts due to low sheet resistance properties. Ni atoms, however, are highly diffusive. During processing, Ni atoms diffuse beneath the spacers, causing formation of Ni pipes  148 .  FIG. 1   b  shows a TEM image of transistors. As shown, nickel silicide contacts  140  and  141  are disposed adjacent to gate conductors  130  with sidewall spacers  160 . Tungsten plugs  165  are provided over the nickel silicide contacts. Nickel pipes  148  exist beneath the spacers. Ni pipes can lead to junction leakage, negatively affecting device performance or functionality. 
     In view of the foregoing, it is desirable to provide transistors with metal silicide contacts which reduce or minimize formation of metal pipes. 
     SUMMARY OF THE INVENTION 
     The present invention relates to fabrication of semiconductor devices. In accordance with one aspect of the invention, a method of fabricating an integrated circuit (IC) is disclosed. The method includes providing a substrate prepared with a transistor having a gate and source/drain diffusion regions adjacent to the gate and forming an epitaxial layer on the substrate over the source/drain diffusion regions. The method further comprises the step of depositing a metal or alloy layer on the substrate covering the source/drain diffusion regions and the substrate is processed to form metal silicide contacts over the source/drain diffusion regions. The epitaxial layer formed reduces formation of metal pipes. 
     An IC is disclosed in another aspect of the invention. The IC comprises a transistor disposed on a substrate, the transistor includes a gate and source/drain diffusion regions in the substrate adjacent thereto. The IC further comprises metal silicide contacts on the source/drain diffusion regions, wherein the silicide contacts comprise metal or alloy layer reacted with epitaxial layer over the source/drain diffusion regions. 
     In yet another aspect of the invention, a method of forming metal silicide contacts is presented. The method includes providing a substrate prepared with a diffusion region and forming an epitaxial layer over the diffusion region. The method further comprises depositing a metal or alloy layer over the diffusion region and the substrate is processed to form metal silicide contacts over the diffusion region. 
     These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description and the accompanying drawings. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIGS. 1   a - b  show a conventional transistor with metal pipes; 
         FIG. 2  shows a transistor in accordance with one embodiment of the invention; and 
         FIGS. 3   a - g  show a process for forming a transistor in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, details are set forth such as specific materials, parameters, etc. in order to provide a thorough understanding of the present invention. It will be evident, however, that the present invention may be practised without these details. In other instances, well-known process steps, equipment, etc. have not been described in particular detail so as not to obscure the present invention. 
     The present invention relates to transistors with metal silicide contacts. The transistors can be incorporated into ICs. The ICs can be any type of IC, for example dynamic or static random access memories, signal processors, or system on chip devices, mixed signal or analog devices such as A/D converters and switched capacitor filters. Other types of ICs are also useful. Such ICs are incorporated in, for example, communication systems and various types of consumer products. 
       FIG. 2  shows a transistor  220  in accordance with one embodiment of the invention. The transistor is formed in an active region  206  in the substrate  201 . In one embodiment, the substrate comprises a silicon on insulator (SOI) substrate. Other types of substrates, such as silicon or silicon germanium, are also useful. The SOI substrate comprises an insulator or buried oxide layer  205  between top silicon layer  201   b  and bulk silicon  201   a . The active region is isolated from other device regions by isolation regions  280 . The isolation regions, for example, comprise shallow trench isolations (STIs). The bottom of the STIs preferably extends below the top surface of the buried oxide layer. 
     The active region is heavily doped with dopants of a second polarity type. The transistor comprises a gate  230  on the substrate. The gate includes a gate electrode  231  over a gate dielectric  232 . Typically, the gate electrode is formed from polysilicon and the gate dielectric is formed from thermal oxide. Other types of materials are also useful. Source/drain diffusion regions  225   a - b  are located adjacent to the gate beneath the substrate surface. The source/drain diffusion regions are heavily doped with dopants of a first polarity type. The first polarity type, for example, comprises n-type, forming a n-type transistor. Forming p-type transistors with p-type dapants as the first polarity type is also useful. Dielectric spacers  260  are disposed on the gate sidewalls. The dielectric spacers, for example, are formed from silicon oxide. Other types of dielectric materials, such as silicon nitride, are also useful. 
     Contacts  240  and  241  are provided over the diffusion regions and gate electrode. In one embodiment, the contacts comprise metal silicide. Preferably, the metal silicide comprises nickel or a nickel alloy, such as nickel-platinum or nickel tantalum. Other types of metal silicides can also be useful. In accordance with one embodiment of the invention, the metal silicide contacts are derived by processing a metal layer with an epitaxial silicon (epi) layer on the surface of the substrate over the diffusion regions. The epi layers, for example, can be provided on the surface of the diffusion regions selectively. Other techniques for providing epi layers are also useful. Preferably, the epi layers are fully consumed by the metal layer during processing to form the metal silicide contacts. For example, the thickness of the epi layer is about 200 Å for a typical nickel layer of about 100 Å. Other thickness is also useful. By providing an epi layer on the surface of the diffusion regions, formation of nickel pipes are reduced or minimized. 
       FIGS. 3   a - g  show a process for forming a transistor in accordance with one embodiment of the invention. Referring to  FIG. 3   a , a semiconductor substrate  301  is provided. The substrate, in one embodiment, comprises a SOI substrate. Other types of substrates, such as a p-type silicon substrate, may also be useful. The SOI substrate includes a buried oxide layer  305  below the surface of the substrate. This results in a top silicon layer  301   b  and bulk silicon  301   a  sandwiching the buried oxide layer. Typically, the top silicon layer is from a few hundred to a few thousand angstroms thick while the buried oxide is about a few thousand angstroms thick. Other thicknesses for the top silicon and buried oxide layers are also useful. The substrate is prepared with an active region  306 . The active region comprises a heavily doped region with dopants of second polarity type. To form the active region, conventional ion implantation techniques, such as implantation with a mask can be used. Other techniques for forming the active region are also useful. 
     Isolating the active regions from other device regions on the substrate are isolation regions  380 . The isolation regions, for example, comprise STI regions. In one embodiment, the STI regions extend below the top surface of the buried oxide layer. Various conventional processes can be employed to form the STI regions. For example, the substrate can be etched using conventional etch and mask techniques to form trenches which are then filled with dielectric material such as silicon oxide. Chemical mechanical polishing (CMP) can be performed to remove excess oxide and provide a planar substrate top surface. The STI regions can be formed, for example, prior to or after the formation of the doped wells. 
     Referring to  FIG. 3   b , the process continues to form the gate layers on the substrate. For example, forming the gate layers comprises sequentially forming a gate dielectric layer  332  and a gate electrode layer  331  on the substrate surface. In one embodiment, the gate dielectric layer comprises thermal oxide. Other types of gate dielectrics are also useful. The gate electrode layer, for example, comprises polysilicon. Other types of gate electrode materials, such as metal, are also useful. The gate layer, in one embodiment, can comprise a heavily doped polysilicon layer of the first polarity type. The polysilicon can be in-situ doped or doped by ion implantation after deposition, such as during formation of the source/drain diffusion regions. Providing undoped polysilicon is also useful. 
     In one embodiment of the invention, a buffer layer  335  is formed over the gate electrode layer. The purpose of the buffer layer is to prevent the formation of an epi layer over the gate electrode during a subsequent selective epi process. The buffer layer, for example, comprises nitride. Other types of buffer materials, such as oxide, are also useful. Preferably, the buffer material comprises a material different from the spacer material of the gate. 
     Referring to  FIG. 3   c , the layers are then patterned using conventional mask and etch techniques to form a gate  330 . Spacers  360  are formed on the gate sidewalls, as shown in  FIG. 3   d . The spacers comprise, for example, a dielectric material such as oxide and/or nitride. In one embodiment, the spacers comprise silicon oxide. The spacers can be formed using conventional spacer processes, such as depositing a blanket dielectric layer on the substrate and gate. The dielectric layer is then patterned to remove the horizontal portions, leaving the spacers on sidewalls of the gate. 
     Source/drain diffusion regions  325   a - b  are formed in the active region of the substrate adjacent to the gate. The diffusion regions, for example, comprise heavily doped regions of the first polarity type. The doped regions can be formed by ion implantation. The implant can be self-aligned or formed using an implant mask. Other techniques for forming the diffusion regions are also useful. 
     In one embodiment, the diffusion regions comprise lightly doped (LDD) and heavily doped (HDD) regions via, for example, a two step ion implantation process. For example, the LDD regions are formed after the gate is patterned and the HDD regions are formed after spacer formation. After the formation of diffusion regions, the dopants are activated by, for example, a thermal annealing process. The anneal also serves to facilitate recovery from any deformation to the crystal structure incurred during the process of ion implantation. Typically, the anneal is carried out at about 1050-1100° C. In the case of a soak or spike anneal, a flash or laser anneal is performed thereafter at about 1200-1300° C. 
     Referring to  FIG. 3   e , an epi layer  370  is formed on the substrate over the diffusion regions. Prior to forming the epi layer, the surface of the substrate is cleaned by, for example, DHF clean. The DHF clean removes native oxide from the surface of the substrate. The epi layer, for example, is formed by selective deposition techniques. Preferably, the epi is formed by low temperature selective epi deposition techniques. Selective techniques deposit the epi layer only on the silicon surface. This advantageously creates a self-aligned process which avoids a masking and etching to remove unwanted epi. Preferably, the thickness of the epi is selected to ensure being fully consumed during a subsequent process to form metal silicide. The thickness of the epi, for example, is about 200 Å. 
     After formation of the epi layer over the diffusion regions, the surface of the substrate is cleaned. For a nitride buffer layer, the cleaning is achieved by, for example, a H 3 PO 4  clean chemistry. The buffer layer is removed. As shown in  FIG. 3   f , a metal layer  345  is deposited over the substrate, covering the diffusion regions and gate. Preferably, the metal layer comprises nickel or nickel alloy, such as nickel platinum or nickel tantalum. Other types of materials may also be useful. Typical thickness of the metal layer is about 80-150 Å. A cap layer (not shown) can be formed over the metal layer. The cap layer, for example, comprises titanium nitride (TiN). The cap layer is about, for example, 50 Å thick. Other types of cap layer or thicknesses are also useful. 
     Referring to  FIG. 3   g , a salicide process is preformed to form metal silicide contacts  340  and  341  over the diffusion regions ( 325   a - b ) and the gate electrode ( 331 ). In one embodiment, the salidide process forms nickel or nickel alloy silicide contacts. The salicide process, for example, comprises annealing the substrate, causing a reaction between the metal and silicon (e.g., substrate, epi and polysilicon). The parameters of the salicide anneal should be selected to avoid salicide over-growth into the diffusion regions. In one embodiment, the substrate is annealed at a temperature of about 400° C. in a nitrogen ambient for about 5 seconds. Other process parameters may also be useful. The cap layer as well as excess metal layer on the substrate, such as over the STIs and gate sidewalls, are removed. Removal can be achieved using a selective wet etch clean to the metal material, such as nickel or nickel alloy. 
     In accordance with one embodiment of the invention, the thickness of the epi and metal layers are selected to ensure full consumption of the epi during the salicide process with the desired amount of silicidation. For example, about 100 Å of Ni or NiPt reacts with about 200 Å of epi silicon to form about 200 Å of nickel silicide. The actual amount of silicidation depends on design requirements. 
     The process continues by forming interconnections to the contacts of the transistors. For example, a dielectric is deposited and patterned to create vias and trenches, which are then filled with conductive material, such as copper, to form interconnects. Additional processes are performed to complete the IC, for example, additional interconnect levels, final passivation, dicing, and packaging. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments, therefore, are to be considered in all respects illustrative rather than limiting the invention described herein. Scope of the invention is thus indicated by the appended claims, rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.