Abstract:
A semiconductor device having a trench type gate and a fabrication method therefor is provided. The semiconductor device includes a trench formed in a semiconductor substrate and a gate insulating layer formed on the inner walls of the trench. A gate fills the trench and is insulated from the semiconductor substrate by the gate insulating layer. A barrier layer is formed between the gate insulating layer and the gate for preventing migration of impurities from the gate to the gate insulating layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a trench type gate and a fabrication method therefor. 
     2. Description of the Related Art 
     As the size and capacity of integrated semiconductor devices increases, the need for a power controlling semiconductor device having a high breakdown voltage, a high current, and high switching characteristics also increases. Such a power controlling semiconductor device should consume less power in a normal operating state and be small. 
     One commonly used power controlling semiconductor device is a dynamic metal oxide semiconductor field effect transistor (DMOSFET) adopting a general planar diffusion technology. More recently, however, MOSFET devices having a trench type gate in which a semiconductor substrate is etched to a predetermined depth to form a trench have attracted the attention of the industry. The trench is filled with a gate polysilicon. 
     FIG. 1 is a sectional view of a conventional power MOSFET having a trench type gate. In FIG. 1, an N +  semiconductor substrate  10  is doped with a first conductive type impurity at a high concentration. An N −  epitaxial layer  12  is formed on the substrate  10 . A P −  body region  14  is doped with a second conductive type impurity at a low concentration is formed on the N −  epitaxial layer  12 . An N +  source region  16  is formed on the P −  body region  14 . A gate insulating layer  18  is formed on the N +  source region  16 . A gate  20  fills a trench (not shown). An interlayer dielectric (ILD) film  22  is formed on the gate insulating layer  18 . A source electrode  24  is connected to the N +  source region  16 . A gate electrode  26  is connected to the trench type gate  20 . 
     In a conventional device, to reduce signal delay in the gate, the trench is filled with highly doped polysilicon after forming a trench in a semiconductor substrate. Alternatively, after filling the trench with undoped polysilicon, the polysilicon is doped by soaking the device in phosphoryl chloride (POCl 3 ) solution or by implanting phosphorous (P) ions into the trench. 
     According to the conventional method for forming a gate, a large amount of ionized impurities e.g. phosphorous ions, working as positive charges, concentrate on the interface between a gate oxide layer and a polysilicon layer or in the gate oxide layer during its fabrication process. If a negative bias is applied under these conditions, leakage current in the gate oxide layer increases at a low voltage due to the ionized positive ions. This phenomenon occurs mostly in a power MOSFET adopting a thick gate oxide layer having a thickness greater than 300 A. This phenomenon becomes severe as the amount of ions accumulated in the gate oxide layer increases. 
     In particular, the oxide layer of a power MOSFET adopting a trench type gate thins at the corners of the trench. As a result, leakage current at low voltages increases when a negative bias is applied to a gate electrode thereby considerably lowering the reliability of the gate oxide layer. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a semiconductor device that overcomes the problems associated with prior art semiconductor devices. 
     It is another object of the present invention to provide a semiconductor device and a fabrication method therefor in which accumulation of ionized impurities in the interface between a gate insulating layer and a gate or in the gate insulating layer is suppressed thereby minimizing leakage current and improving the operating characteristics of the gate insulating layer. 
     It is yet another object of the present invention to provide a power semiconductor device and a fabrication method therefor capable of withstanding a high voltage. 
     The semiconductor device having a trench type gate includes a trench formed on a semiconductor substrate and a gate insulating layer formed on the trench. A gate is formed to fill the trench, the gate being insulated from the semiconductor substrate by the gate insulating layer. A barrier layer is formed between the gate insulating layer and the gate for preventing migration of impurities from the gate to the gate insulating layer. The gate is formed of polysilicon doped with impurities and the barrier layer is formed of a refractory metal. The refractory metal is titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), or tungsten (W). 
     A silicide layer is formed between the gate and the barrier layer for reducing gate resistance. The silicide layer may be formed of a refractory metal, for example, titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), or tungsten (W). 
     Another embodiment of the present invention is a power semiconductor device having a trench type gate. The power semiconductor device comprises a first conductivity type semiconductor substrate, a second conductivity type body region formed on the substrate, and a first conductivity type source region formed on the body region. A trench is formed through the source and body region. A gate is formed in the trench, the gate being insulated from the substrate by a gate insulating layer. A barrier layer is formed between the gate insulating layer and the gate for preventing migration of impurities from the gate to the gate insulating layer. An interlayer dielectric film is formed on the barrier layer. A gate electrode is connected to the gate via a contact hole formed in the interlayer dielectric film and a source electrode is connected to the source region via a contact hole formed in the interlayer dielectric film. 
     The gate is formed of polysilicon doped with impurities and the barrier layer is formed of a refractory metal. The refractory metal is titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), or tungsten (W). 
     A silicide layer is formed between the gate and the barrier layer for reducing gate resistance. The silicide layer may also be formed of a refractory metal, for example, titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), or tungsten (W). 
     A method for fabricating a semiconductor device having a trench type gate is provided. The method comprises forming a trench in a semiconductor substrate and forming a gate insulating layer on inner walls of the trench and on the semiconductor substrate. The method further comprises forming a barrier layer on the gate insulating layer and forming a gate electrode by filling the trench after forming the barrier layer. Forming the barrier layer includes forming the barrier layer of a refractory metal and wherein forming the gate includes forming the gate of polysilicon doped with impurities. Forming the barrier layer of a refractory metal includes forming the barrier layer of titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), or tungsten (W). 
     Forming the gate comprises forming a polysilicon layer doped with impurities after forming the barrier layer and forming a silicide layer on the barrier layer by annealing the device after forming the polysilicon layer. 
     A method for fabricating a power semiconductor device having a trench type gate is provided. The method comprises forming a second conductivity type body region on a first conductivity type semiconductor substrate, forming a first conductivity type source region on the body region, and forming a trench in the semiconductor substrate. The method further comprises forming a gate insulating layer on the semiconductor substrate after forming the trench, forming a barrier layer on the gate insulating layer, and forming a gate by filling the trench after forming the barrier layer. The method also comprises forming an interlayer dielectric film after forming the gate, patterning the interlayer dielectric film thereby exposing the source region and the gate, and forming a source electrode connected to the source region and a gate electrode connected to the gate. 
     Forming the barrier layer includes forming the barrier layer of a refractory metal and wherein forming the gate includes forming the gate of a polysilicon doped with impurities. Forming the barrier layer of a refractory metal includes forming the barrier layer of titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), or tungsten (W). 
     Forming the gate electrode comprises forming a polysilicon layer doped with impurities after forming the barrier layer and forming a silicide layer on the barrier layer by annealing the device after forming the polysilicon layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment that proceeds with reference to the following drawings. 
     FIG. 1 is a section view of a conventional power metal oxide semiconductor field effect transistor (MOSFET) having a trench type gate; 
     FIG. 2 is a section view of a semiconductor device having a trench type gate according to a preferred embodiment of the present invention; and 
     FIGS. 3 through 6 are section views illustrating a method for fabricating the semiconductor device having a trench type gate shown in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following fully describes the present invention with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, the embodiments set forth herein are provided to ensure a thorough disclosure and to fully convey the concepts of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity and the same reference numerals in different drawings represent the same elements. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be formed directly on the other layer or substrate or intervening layers may also be present therebetween. 
     FIG. 2 is a section view of a semiconductor device having a trench type gate according to a preferred embodiment of the present invention. The device shown is a power MOSFET. In this embodiment, first and second conductive types are defined N-type and P-type, respectively. 
     Referring to FIG. 2, an N +  semiconductor substrate  30  and an N −  epitaxial layer  32  are used as a base substrate. The semiconductor substrate  30  is highly doped with a first conductivity type impurity and the epitaxial layer  32  is lightly doped with a first conductivity type impurity. The epitaxial layer  32  is formed on the substrate  30 . A second conductive type P −  body region  34  is formed on the base substrate (substrate  30  and epilayer  32 ). An N +  source region  36  highly doped with the first conductive type impurity is formed on the P −  body region  34 . 
     A trench is formed extending to a portion of the N −  epitaxial layer  32  passing through the N +  source region  36  and the conductive P −  body region  34 . A gate insulating film  38  is formed to cover the trench and the surface of the N +  source region  36 . A barrier layer  40 , a silicide layer  42 , and a gate  44  are stacked in sequence on the gate insulating layer  38 . 
     The barrier layer  40  is formed of a refractory metal such as titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), or tungsten (W). The barrier layer  40  prevents concentration of migrating positive ions from the gate  44  to the gate insulating layer  38  within the gate insulating layer  38 . The gate  44  is formed, e.g., of an impurity-doped polysilicon. The silicide layer  42  formed between the barrier layer  40  and the gate  44  reduces resistance of the gate  44  improving the operating speed of the device. A gate electrode  48  and a source electrode  50  are connected to the gate  44  and the source region  36 , respectively, via contact holes formed in an interlayer dielectric (ILD) film  46 . 
     FIGS. 3 through 6 are section views illustrating a method for fabricating the power MOSFET having a trench type gate shown in FIG.  2 . Referring to FIG. 3, an N −  epitaxial layer  32  is formed on a semiconductor substrate  30 . The epitaxial layer  32  is doped with a low concentration of a first impurity type, e.g., an N −  type. The substrate  30  is doped with a high concentration of a first impurity type, e.g., an N −  type. A thin oxide layer (not shown) is formed on the N −  epitaxial layer  32 . The oxide layer serves as a buffer layer for reducing stress caused by different thermal expansion coefficients between a nitride layer acting as a mask, formed in a subsequent step, and the semiconductor substrate  30 . In some cases, however, forming the oxide layer may be omitted. 
     An insulating layer (not shown), e.g. nitride layer, having an etching selectivity with respect to the oxide layer is formed on the entire surface of the oxide layer. The nitride layer and the oxide layer are patterned in sequence by general photolithography to form a nitride layer pattern (not shown) and an oxide layer pattern (not shown) each acting as a mask. Impurity ions having the opposite conductive type to the semiconductor substrate  30 , e.g. P-type impurity, are implanted at a low concentration on the N −  epitaxial layer  32  using the nitride layer pattern as an ion implantation mask. The implanted impurity ions are diffused using a predetermined annealing process forming a P −  body region  34  on the N −  epitaxial layer  32 . 
     Referring to FIG. 4, an N +  source region  36  is formed on the P −  body region  34  by general photolithography and ion implantation processes. A photoresist pattern (not shown) that opens a trench region is formed by general photolithography. Using the photoresist pattern, etching is performed to a predetermined depth of the N −  epitaxial layer  32  passing through the N +  source region  36  and the P −  body region  34 . This etching process forms a trench. 
     Referring to FIG. 5, a thin oxide layer is deposited on the entire surface of the resultant structure including the trench thereby forming in a gate insulating layer  38 . A refractory metal, e.g., Ti, is deposited on the gate insulating layer  38  to a thickness of approximately 300 to 600 A. The refractory metal is deposited by general deposition such as sputtering, physical vapor deposition (PVD), or chemical vapor deposition (CVD) resulting in a barrier layer  40  that prevents migration of ions from a gate to be formed in a subsequent step to the gate insulating layer  38 . 
     Then, a polysilicon layer  44 ′ is formed on the barrier layer  40  filling the trench. The resultant structure is then annealed at a temperature higher than 800° C. in order to reduce resistance of the gate. The polysilicon layer  44 , is highly doped with an N −  type impurity. Hence, a silicide layer  42 , which is a compound between the refractory metal and silicon, is formed in the interface between the barrier layer  40  formed of the refractory metal and the polysilicon layer  44 ′. Where the trench is filled with undoped polysilicon, the resultant structure obtained after deposition of the polysilicon layer  44 ′ is soaked in a phosphoryl chloride (POCl 3 ) solution to form the silicide layer. The polysilicon layer  44 ′, the silicide layer  42 , and the barrier layer  40  are then sequentially patterned. 
     Referring to FIG. 6, an insulating layer such as phosphorous silicate glass (PSG) or boro-phosphorus silicate glass (BPSG) layer is deposited on the resultant structure having a gate  44 , thereby forming an ILD film  46  for insulating a transistor from other conductive layers. The ILD film  46  is patterned by general photolithography forming contact holes exposing the source region  36  and the gate  44 . The resultant structure is thermally treated to planarize the ILD film  46 . The thermal treatment may be performed before the contact holes are formed. 
     A metal layer is deposited on the entire surface of the structure having the contact holes. The metal layer is then patterned by general photolithography forming a gate electrode  48  and a source electrode  50 . The subsequent steps are the same as the steps of fabricating a general MOSFET. 
     The above embodiments have been described only for MOSFETS having an N +  source region. However, the present invention can be applied to MOSFETS having a P +  source region as well. 
     As described above, in the semiconductor device having a trench type gate and the fabrication method therefor, the barrier layer is formed between the gate insulating layer and the gate. Doing so suppresses accumulation of ionized impurities in the interface between the gate insulating layer and the gate or in the gate insulating layer thereby minimizing leakage current. Also, breakage of the gate insulating layer is prevented improving the operating characteristics of the gate insulating layer. Additionally, resistance of the gate by forming the silicide layer between the gate barrier and the gate is thereby reduced considerably decreasing delay of the gate signal. 
     Having illustrated and described the principles of our invention in a preferred embodiment thereof, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the accompanying claims.