Patent Publication Number: US-6709936-B1

Title: Narrow high performance MOSFET device design

Description:
This application is a division of U.S. application Ser. No. 10/054,282, filed Nov. 13, 2001 and issued as U.S. Pat. No. 6,597,043. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor integrated circuit structures and, in particular, to a novel MOSFET gate design that includes narrow and short gate cells that are connected both in parallel and in series in order to increase the device current derive, while maintaining the overall device performance characteristics of larger geometry devices, and while minimizing short channel effects (SCE) and narrow channel effects (NCE). 
     2. Discussion of the Related Art 
     It has been shown that MOSFET transistors show degraded performance due to enhanced short channel effects (SCE) as the transistors are made shorter while maintaining large channel width. For example, the threshold voltage of an N-channel MOSFET transistor gets smaller as the channel length of the transistor is made shorter. In general, the overall performance characteristics of a MOSFET device become impaired as a result of severe short channel effects if the device is made shorter than that provided by currently available technology. 
     The channel width of a MOSFET device has a similar effect on the transistor characteristics, but in the opposite sense. That is, the threshold voltage of an N-channel MOSFET transistor becomes larger when the channel width is made narrower. 
     H. S. Lee, “An Analysis of the Threshold Voltage for Short Channel IGFETs”, Solid State Electronics, 1973, vol. 15, pgs. 1407-1417, teaches that when the MOSFET transistor channel gate is made short as well as narrow, the two threshold voltage shift effects, i.e. SCE and NCE, have a compensating tendency. Also, the threshold voltage of such a short/narrow device can actually improve and may approach that of a larger geometry device. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel MOSFET gate design. The new gate design includes short/narrow gate cells that are connected in parallel and in series in order to increase the device current derive. The overall device performance characteristics would look like those of a larger geometry device, with minimal short channel effects and narrow channel effects. 
     One embodiment of the present invention provides a narrow high performance MOS device structure that includes a rectangular-shaped semiconductor substrate well region having a first conductivity type. The well region is surrounded by shallow trench isolation (STI). A region of STI dielectric material is also formed at the center of the substrate region. Four substrate diffusion regions, each having a second conductivity type opposite the first conductivity type, are formed in the substrate diffusion region in a respective comer of the substrate well region. The four diffusion regions are spaced-apart such that a substrate channel region is defined between each adjacent pair of substrate diffusion regions. A common conductive gate electrode is formed to have four fingers, each one of the fingers extending over a corresponding substrate channel region. The fingers of the common conductive gate electrode are spaced-apart from the underlying substrate channel regions by dielectric material formed therebetween. 
    
    
     Further features and advantages of the present invention will become apparent from the following detailed description and accompanying drawings that set forth an illustrated embodiment in which the principals of the invention are utilized. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view illustrating a conventional MOSFET device design 
     FIG. 2A is a plan view illustrating device well formation for a MOSFET device design in accordance with the present invention. 
     FIG. 2B is a plan view illustrating a MOSFET device design in accordance with the present invention. 
     FIG. 2C is a partial cross-section view taken along line  2 C— 2 C in FIG.  2 B. 
     FIG. 2D is a partial cross-sectional view taken along line  2 D— 2 D in FIG.  2 B. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a conventional MOSFET device design structure  10  that includes a gate electrode  12  and a device well that includes a source region  14  and a drain region  16 . The device well is surrounded by shallow trench isolation (STI) in the conventional manner. Those skilled in the art will appreciate that a substrate channel region underlies the gate electrode  12  and that gate dielectric material separates the gate  12  from the substrate channel region. The conventional device  10  illustrated in FIG. 1 has a channel width/Length ratio w/L=0.6/0.15=4. The total device well area in the FIG. 1 design is 0.6×0.45 um=0.27 um2. 
     FIG. 2A shows a rectangular-shaped device well having an island of oxide trench isolation formed in the middle of the device well. 
     FIG. 2B shows a narrow MOSFET device structure  200  in accordance with the present invention. The MOSFET device  200  has the same current derive as the conventional device design  10  shown in FIG.  1 . The total device well for the FIG. 2B design is 0.5×0.5=0.25 um2. 
     FIG. 2C shows a cross-section of the FIG. 2B device structure  200  taken along line  2 C— 2 C in FIG.  2 B. 
     FIG. 2D shows a cross-section of the FIG. 2B device structure  200  taken along line  2 D— 2 D in FIG.  2 B. 
     Referring to FIGS. 2A-2D, a narrow high performance transistor structure  200  in accordance with the present invention is formed in a rectangular-shaped device well  202  of semiconductor substrate material, typically silicon, having a first conductivity type. A region of shallow trench isolation (STI) dielectric material  204 , typically silicon dioxide, is formed at the center of the device well region  202 . Four substrate diffusion regions, having a second conductivity type opposite the first conductivity type, are formed in respective comers of the substrate device well region  202 . The diffusion regions are spaced-apart to define a substrate channel region  209  between each adjacent pair of diffusion regions. Diagonally opposite pairs of diffusion regions are connected to drain and source contacts to provide the drain regions  206   a  and source regions  206   b , respectively, of the MOSFET transistor structure. A common conductive gate electrode  208 , typically polysilicon, has four gate electrode fingers  208   a . Each gate electrode finger  208   a  extends over a corresponding substrate channel region  209  and is spaced-apart therefrom by gate dielectric material  210 , typically silicon dioxide. 
     Thus, the present invention provides a narrow/short high performance MOSFET device design that, in comparison with conventional device designs, has a smaller device well area, provides similar or higher current derive, reduces both short channel effects and narrow channel effects in the overall device performance, and saves additional expensive processing steps which would be required to minimize short channel effects or narrow channel effects separately in conventional device designs. 
     Those skilled in the art will appreciate that the structure of the narrow/short high performance MOSFET device design provided by the present invention can be fabricated utilizing conventional integrated circuit manufacturing techniques. For example, the fabrication process could proceed as follows: First, shallow trench isolation (STI) oxide is formed in the silicon substrate to device a substrate device well  202  having the first conductivity type; an STI region  204  is simultaneously formed at the center of the well region. A layer of gate oxide is then formed over the well region  202  and a layer of polysilicon is formed over the gate oxide. A patterned photoresist mask is then formed over the poly layer and utilized to etch the poly layer to define the conductive gate electrode  208 . The poly etch can be designed to stop on the gate oxide to protect the underlying well region, or optionally, as shown in FIGS. 2A-2D, be designed to etch through the gate oxide as well. Following the poly etch step, the gate electrode structure is utilized in a self-aligned ion implantation step to define the source and drain regions of the MOSFET device; the poly gate electrode is simultaneously doped to the desire conductivity level. Although these conventional fabrication steps, considered individually, are not considered to be part of the invention, the process module resulting in the novel device structure of the present invention is considered to be within the scope of the present invention. 
     It should be understood that various alternatives to the embodiments of the invention described above can be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.