Abstract:
Trench MIS devices including a thick insulative layer at the bottom of the trench are disclosed, along with methods of fabricating such devices. An exemplary trench MOSFET embodiment includes a thick oxide layer at the bottom of the trench, with no appreciable change in stress in the substrate along the trench bottom. The thick insulative layer separates the trench gate from the drain region at the bottom of the trench yielding a reduced gate-to-drain capacitance making such MOSFETs suitable for high frequency applications. In an exemplary fabrication process embodiment, the thick insulative layer is deposited on the bottom of the trench. A thin insulative gate dielectric is formed on the exposed sidewall and is coupled to the thick insulative layer. A gate is formed in the remaining trench volume. The process is completed with body and source implants, passivation, and metallization.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No.: 09/927,320, filed Aug. 10, 2001, incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to trench metal-insulator-semiconductor (MIS) devices and in particular to trench MOSFETs that are suitable for high frequency operation. 
     BACKGROUND 
     Some metal-insulator-semiconductor (MIS) devices include a gate located in a trench that extends downward from the surface of a semiconductor substrate (e.g., silicon). The current flow in such devices is primarily vertical and, as a result, the cells can be more densely packed. All else being equal, this increases the current carrying capability and reduces the on-resistance of the device. Devices included in the general category of MIS devices include metal-oxide-semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), and MOS-gated thyristors. 
     Trench MOSFETs, for example, can be fabricated with a high transconductance (g m,max ) and low specific on resistance (R on ), which are important for optimal linear signal amplification and switching. One of the most important issues for high frequency operation, however, is reduction of the MOSFET internal capacitances. The internal capacitances include the gate-to-drain capacitance (C gd ), which is also called the feedback capacitance (C rss ), the input capacitance (C iss ), and the output capacitance (C oss ). 
       FIG. 1  is a cross-sectional view of a conventional n-type trench MOSFET  10 . In MOSFET  10 , an n-type epitaxial (“N-epi”) layer  13 , which is usually grown on an N +  substrate (not shown), is the drain. N-epi layer  13  may be a lightly doped layer, that is, an N −  layer. A p-type body region  12  separates N-epi layer  13  from N +  source regions  11 . Current flows vertically through a channel (denoted by the dashed lines) along the sidewall of a trench  19 . The sidewall and bottom of trench  19  are lined with a thin gate insulator  15  (e.g., silicon dioxide). Trench  19  is filled with a conductive material, such as doped polysilicon, which forms a gate  14 . Trench  19 , including gate  14  therein, is covered with an insulative layer  16 , which may be borophosphosilicate glass (BPSG). Electrical contact to source regions  11  and body region  12  is made with a conductor  17 , which is typically a metal or metal alloy. Gate  14  is contacted in the third dimension, outside of the plane of FIG.  1 . 
     A significant disadvantage of MOSFET  10  is a large overlap region  18  formed between gate  14  and N-epi layer  13 , which subjects a portion of thin gate insulator  15  to the drain operating voltage. The large overlap limits the drain voltage rating of MOSFET  10 , presents long term reliability issues for thin gate insulator  15 , and greatly increases the gate-to-drain capacitance, C gd , of MOSFET  10 . In a trench structure, C gd  is larger than in conventional lateral devices, limiting the switching speed of MOSFET  10  and thus its use in high frequency applications. 
     One possible method to address this disadvantage is described in the above-referenced application Ser. No. 09/591,179 and is illustrated in FIG.  2 .  FIG. 2  is a cross-sectional view of a trench MOSFET  20  with an undoped polysilicon plug  22  near the bottom of trench  19 . MOSFET  20  is similar to MOSFET  10  of  FIG. 1 , except for polysilicon plug  22 , which is isolated from the bottom of trench  19  by oxide layer  21  and from gate  14  by oxide layer  23 . The sandwich of oxide layer  21 , polysilicon plug  22 , and oxide layer  23  serves to increase the distance between gate  14  and N-epi layer  13 , thereby decreasing C gd . 
     In some situations, however, it may be preferable to have a material even more insulative than undoped polysilicon in the bottom of trench  19  to minimize C gd  for high frequency applications. Accordingly, a trench MOSFET with decreased gate-to-drain capacitance, C gd , and better high frequency performance is desirable. 
     SUMMARY 
     In accordance with the present invention, a metal-insulator-semiconductor (MIS) device includes a semiconductor substrate including a trench extending into the substrate from a surface of the substrate. A source region of a first conductivity type is adjacent to a sidewall of the trench and to the surface of the substrate. A body region of a second conductivity type opposite to the first conductivity type is adjacent to the source region and to the sidewall. A drain region of the first conductivity type is adjacent to the body region and to the sidewall. The trench is lined with a first insulative layer along a portion of the sidewall that abuts the body region. The trench is also lined with a second insulative layer along a bottom portion of the trench. The second insulative layer is coupled to the first insulative layer, and the second insulative layer is thicker than the first insulative layer. The stress in the substrate along the bottom portion of the trench does not change appreciably. 
     In an exemplary embodiment of a fabrication process for such an MIS device, a trench including a sidewall and a bottom is formed in a substrate. A thick insulative layer is deposited on the bottom of the trench. A thin insulative layer is formed on the sidewall, and is coupled to the thick insulative layer. A gate is formed above the portion of the thick insulative layer and adjacent to the thin insulative layer in the trench. 
     The thick insulative layer separates the trench gate from the drain conductive region at the bottom of the trench resulting in a reduced gate-to-drain capacitance. This makes MIS devices in accordance with the present invention, such as trench MOSFETs, suitable for high frequency applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention will be better understood by reference to the following description and drawings. In the drawings, like or similar features are typically labeled with the same reference numbers. 
         FIG. 1  is a cross-sectional view of a conventional trench MOSFET. 
         FIG. 2  is a cross-sectional view of a trench MOSFET with a polysilicon plug at the bottom of the trench. 
         FIG. 3  is a cross-sectional view of one embodiment of a trench MOSFET in accordance with the present invention. 
         FIGS. 4A-4K  are cross-sectional views illustrating one embodiment of a process for fabricating a trench MOSFET in accordance with the present invention. 
         FIG. 5  is a cross-sectional view of an alternative embodiment of a trench MOSFET in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  is a cross-sectional view of one embodiment of a trench MOSFET  30  in accordance with the present invention. MOSFET  30  has some similarities to MOSFET  10  of FIG.  1 . The elements of MOSFET  30  outside of trench  19  can be the same as those of MOSFET  10  of FIG.  1 . In MOSFET  30 , however, only the sidewall of trench  19  is lined with thin gate insulator  15  (e.g., silicon dioxide). Unlike MOSFET  10  of  FIG. 1 , a thick insulative layer  31  (e.g., silicon dioxide) lines the bottom of trench  19  of MOSFET  30  of FIG.  3 . Thick insulative layer  31  separates gate  14  from N-epi layer  13  (which may be an N −  layer). This circumvents the problems that occur when only thin gate insulator  15  separates gate  14  from N-epi layer  13  (the drain) as in FIG.  1 . Thick insulative layer  31  also provides a more effective insulator than is achievable with polysilicon plug  22  as shown in FIG.  2 . Thus, thick insulative layer  31  minimizes the gate-to-drain capacitance, C gd , and yields a trench MOSFET  30  useful for high frequency applications. 
       FIGS. 4A-4K  are cross-sectional views illustrating one embodiment of a process for fabricating a trench MOSFET, such as MOSFET  30  of  FIG. 3 , in accordance with the present invention. As shown in  FIG. 4A , the process begins with a lightly-doped N-epi layer  413  (typically about 8 μm thick) grown on a heavily doped N +  substrate (not shown). A trench mask  450 , which may be photoresist or an oxide, is deposited on N-epi layer  413  and patterned to form an opening  452  where a trench  419  is to be located. Trench  419  is etched through opening  452 , typically using a dry plasma etch, for example, a reactive ion etch (RIE). Trench  419  may be about 0.5-1.2 μm wide and about 1-2 μm deep. 
     Mask  450  is removed, and a thick insulative layer  431  (e.g., about 0.1-0.3 μm) is deposited on N-epi layer  413 , as shown in FIG.  4 B. The deposition process is chosen, according to conventional deposition techniques such as chemical vapor deposition (CVD), to yield conformal deposition of insulative layer  431  on the sidewall and bottom of trench  419 , as well as on the top surface of N-epi layer  413 . Thick insulative layer  431  may be, for example, a low temperature oxide (LTO), a phosphosilicate glass (PSG), a BPSG, or another insulative material. In some embodiments, a thin insulative layer (e.g., 100-200 Å of silicon dioxide) could be thermally grown, for example, using a well known dry oxidation process at 950 ° C. for 10 minutes, prior to deposition of thick insulative layer  431 . 
     As shown in  FIG. 4C , a barrier layer  454  is then deposited by CVD. This deposition can be non-conformal, filling trench  419  and overflowing past the topmost surface of thick insulative layer  431 . Barrier layer  454  may be, for example, silicon nitride (Si 3 N 4 ), and may be 2-4 μm thick. Barrier layer  454  is etched back, typically by performing a dry etch followed by a wet etch, using etchants that have high selectivity for barrier layer  454  over thick insulative layer  431 . Barrier layer  454  is etched back into trench  419  until only about 0.1-0.2 μm remains in trench  419 , as shown in FIG.  4 D. 
     Thick insulative layer  431  is then etched, typically by a wet etch technique, using an etchant that has high selectivity for insulative layer  431  over barrier layer  454  and over N-epi layer  413 . Insulative layer  431  is etched from the top of N-epi layer  413  and from the sidewall of trench  419  until insulative layer  431  remains only in the bottom of trench  431 . The remainder of barrier layer  454  is removed, leaving the structure shown in FIG.  4 E. 
     As shown in  FIG. 4F , a thin gate insulator  415  (e.g., about 100-1000 Å thick) is then formed on the top surface of N-epi layer  413  and on the sidewall of trench  419 . Thin gate insulator  415  may be, for example, a silicon dioxide layer that is thermally grown using a dry oxidation technique at 1050° C. for 20 minutes. In some embodiments, a sacrificial gate oxide (not shown) can be thermally grown and removed by a wet etch to clean the sidewall of trench  419  prior to growing thin gate insulator  415 . The wet etch of such a sacrificial gate oxide is kept short to minimize etching of thick insulative layer  431 . 
     As shown in  FIG. 4G , a conductive material  456  is deposited by CVD, possibly by low pressure CVD (LPCVD), to fill trench  419  and overflow past the topmost surface of thin gate insulator  415 . Conductive material  456  may be, for example, an in-situ doped polysilicon, or an undoped polysilicon layer that is subsequently implanted and annealed, or an alternative conductive material. Conductive material  456  is etched, typically using a reactive ion etch, until the top surface of material  456  is approximately level with the top of N-epi layer  413 , thereby forming gate  414 , as shown in FIG.  4 H. In an n-type MOSFET, gate  414  may be, for example, a polysilicon layer with a doping concentration of 10 20  cm −3 . In some embodiments, conductive material  456  may be etched past the top of trench  419 , thereby recessing gate  414  to minimize the gate-to-source overlap capacitance. 
     Using known implantation and diffusion processes, p-type body regions  412  and N +  source regions  411  are formed in N-epi layer  413  as shown in FIG.  41 . The PN junctions between p-type body regions  412  and the remainder of N-epi layer  413  are located at a depth above the interface between thick insulative layer  431  and thin gate insulator  415 . 
     As shown in  FIG. 4J , an insulative layer  416 , which may be borophosphosilicate glass (BPSG), is deposited by CVD on the surfaces of N-epi layer  413  and gate  414 . Insulative layer  416  is etched, typically using a dry etch, to expose portions of p-type body regions  412  and N +  source regions  411 , as shown in FIG.  4 K. Electrical contact to body regions  412  and source regions  411  is made with a conductor  417 , which is typically a deposited (e.g., by physical vapor deposition, plating, sputtering, or evaporation) metal or metal alloy. Electrical contact to gate  414  is made in the third dimension, outside of the plane of FIG.  4 K. Electrical contact to the drain (not shown) is made to the opposite surface of the N +  substrate (not shown) on which N-epi layer  413  is grown. 
     This method thus allows incorporation of thick insulative layer  431  at the bottom of trench  419  to minimize C gd  with minimal undesirable effects or manufacturing concerns, which may be caused by thermally growing thick insulative layer  431 . For example, stress effects from growing a thick oxide in the concave bottom of trench  419  are avoided by depositing the oxide rather than thermally growing it. Thinning of the insulative layers at the juncture of thick insulative layer  431  and thin gate insulator  415 , possibly caused by formation of a “bird&#39;s beak” during a thermal growth of thick insulative layer  431 , are avoided by depositing thick insulative layer  431 . In addition, shifts in the etched sidewall profile of trench  419  are also avoided by depositing thick insulative layer  431 . Growing thick insulative layer  431  could cause such shifts, resulting in a “bulb” effect at the bottom of trench  419  that is not compensated by subsequent growth of thin gate insulator  415  on the sidewall of trench  419 . 
       FIG. 5  is a cross-sectional view of an alternative embodiment of a trench MOSFET  50  in accordance with the present invention. MOSFET  50  has many similarities to MOSFET  30  of FIG.  3 . In particular, only the sidewall of trench  19  is lined with thin gate insulator  15 , while thick insulative layer  31  lines the bottom of trench  19 . In MOSFET  30  of  FIG. 3 , thick insulative layer  31  may increase the on-resistance (R on ) of MOSFET  30  due to an increase in the spreading resistance in the accumulation layer at the bottom of trench  19 . MOSFET  50  of  FIG. 5 , however, includes a high doping region  53  at the bottom of trench  19  to help spread current more effectively. High doping region  53  is formed in N-epi layer  13 , which overlies an N +  substrate  55 . High doping region  53  may be created by implanting an n-type dopant, such as arsenic or phosphorous, before mask  450  is removed after the trench etch shown in FIG.  4 A. Thus, thick insulative layer  31  minimizes gate-to-drain capacitance, C gd , and high doped region  53  minimizes on-resistance, R on , yielding a trench MOSFET  50  well-suited for high frequency applications. 
     The foregoing embodiments are intended to be illustrative and not limiting of the broad principles of this invention. Many additional embodiments will be apparent to persons skilled in the art. For example, the structures and methods of this invention can be used with any type of metal-insulator-semiconductor (MIS) device in which it is desirable to form an insulating layer between a trench gate and a region outside the trench. Also, various insulative or conductive materials can be used where appropriate, and the invention is also applicable to p-type MOSFETs. The invention is limited only by the following claims.