Patent Publication Number: US-2007114601-A1

Title: Gate contact structure for a power device

Description:
FIELD OF THE INVENTION  
      The present invention is related generally to a semiconductor device and, more particularly, to a gate contact structure for a power device.  
     BACKGROUND OF THE INVENTION  
      In a power device, a gate polysilicon within a trench in a source region is connected to a gate metal on the periphery by a gate contact structure and thus a gate current flows from the gate metal into the gate polysilicon within the trench to active the power device.  FIG. 1  shows a typical layout of a conventional power device, and  FIG. 2  shows a cross-sectional view of a gate contact structure of the conventional power device shown in  FIG. 1  along the AA′ direction. As shown in  FIG. 1  and  FIG. 2 , there is a gate region  110  located outside of a source region  120 , a trench  130  is located within the source region  120 , and a contact window  140  is located within the gate region  110 . When a gate current  180  flows from a gate metal  160  to a gate polysilicon  150  through the contact window  140 , it must flow through the region  170  before reaching the portion of the gate polysilicon  150  within the trench  130 . Due to the region  170  of the gate polysilicon  150 , limiting the gate current  180  to flow within specific height and width, and the resistance of the gate polysilicon  150  greater than that of the gate metal  160  to cause the gate current  180  flowing through a quite large series resistor, the turn on speed of the power device degrades. Moreover, the height and width of the region  170  are reduced with the decreasing scale of the device, thereby increasing the series resistor resulted from thereof, which also has the region  170  to withstand higher voltage and be damaged easily.  
      To improve the power device, U.S. Pat. No. 5,597,765 to Yilmaz et al. discloses a termination structure located along a transistor peripheral or a die edge for a power MOSFET as shown in  FIG. 3  to reduce or eliminate the channeling phenomena, U.S. Pat. No. 5,904,525 discloses a fabrication of high-density trench double-diffused MOS using sidewall spacers as shown in  FIG. 4 , U.S. Pat. No. 6,620,691 discloses a semiconductor trench device with enhanced gate oxide integrity structure as shown in  FIG. 5  to improve the breakdown voltage of the oxide layer, and U.S. Pat. No. 6,861,701 discloses a trench power MOSFET with planarized gate bus as shown in  FIG. 6 . However, these arts all have a region  210  that causes a large series resistor and thereby cannot enhance the performance of the power device. For resolving the problem of the large series resistor, it is proposed a structure without the region  210  as shown in  FIG. 7 , which comprises a substrate  310  having a trench  320 , a gate polysilicon  340  in the trench  320 , an insulator  330  covering the gate polysilicon  340 , a contact window  360  in the insulator  330  above the trench  320 , and a gate metal  350  electrically contacting the gate polysilicon  340  through the contact window  360 . Although such structure can resolve the problem of the large series resistor, it is hard to align the contact window  360  to the trench  320  in the lithographic process since the trench  320  is not large, and it is easy to damage the gate polysilicon  340  in the trench  320  in the etching process, which cause the manufacturing processes hard to control.  
      Therefore, it is desired a gate contact structure for a power device to resolve the problem of the large series resistor and the manufacturing process control.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a gate contact structure for a power device.  
      According to the present invention, a gate contact structure for a power device comprises a substrate having a trench, a gate conductor in the trench and striding over a side of the trench, a first insulator between the gate conductor and the trench, a second insulator covering the gate conductor, a contact window in the second insulator for exposing a surface of the gate conductor which strides over the side of the trench, and a gate metal electrically contacting the gate conductor through the contact window.  
      By using the gate conductor and the contact window both striding over the side of the trench, a gate current can vertically flow from the gate metal in the contact window to the gate conductor in the trench to avoid the gate current to laterally flow along the gate conductor in order to reduce the series resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  shows a typical layout of a conventional power device;  
       FIG. 2  shows a cross-sectional view of a gate contact structure of the conventional power device shown in  FIG. 1  along the AA′ direction;  
       FIG. 3  shows a cross-sectional view of a conventional power device;  
       FIG. 4  shows a cross-sectional view of a conventional power device;  
       FIG. 5  shows a cross-sectional view of a conventional power device;  
       FIG. 6  shows a cross-sectional view of a conventional power device;  
       FIG. 7  shows a cross-sectional view of a gate contact structure of a conventional power device;  
       FIG. 8  shows a layout of a power device according to the present invention; and  
       FIG. 9  shows a cross-sectional view of a gate contact structure of the power device shown in  FIG. 8  along the AA′ direction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 8  shows a layout of a power device according to the present invention, in which a gate region  410  is located outside of a source region  420 , and in the gate region  410 , a trench  430  is overlapped by a portion of a contact window  450  and a portion of a gate conductor  440 . The gate conductor  440  and the contact window  450  both stride over one side of the trench  430  that is far away from the source region  420 . When a gate current flows from a gate metal  460  to the gate conductor  440  through the contact window  450 , it will vertically flow into the gate conductor  440  within the trench  430  and enter the source region  420 , thereby reducing the series resistor resulted if the current laterally flows through the gate conductor  440 . In addition, the gate conductor  440  has a portion striding over the side of the trench  430  far away from the source region  420 , and therefore it is reduced the size of the power device and increased the density of the power devices in manufacture.  
       FIG. 9  is a cross-sectional view of the gate contact structure of the power device shown in  FIG. 8  along the AA′ direction. A substrate  470  includes the gate region  410  and the source region  420 , and in the gate region  410 , an insulator  480  (such as a silicon dioxide) is located on the bottom and sidewalls of the trench  430 . The gate conductor  440  (such as a polysilicon) is located in the trench  430  and strides over the side of the trench  430  that is far away from the source region  420 . The width C of the gate conductor  440  at the opening of the trench  430  is smaller than the width D of the trench  430 . An insulator  490  (such as a silicon dioxide) covers the gate conductor  440 , and the contact window  450  is formed in the insulator  490  such that it has a portion above the trench  430  and strides over the one side of the trench  430  that is far away from the source region  420 . The width E of the portion of the contact window  450  above the trench  430  is smaller than the width C of the gate conductor  440  at the opening of the trench  430 , and the width F of the portion of the contact window  450  above the gate conductor  440  striding over the side of the trench  430  that is far away from the source region  420  is smaller than the length B of the gate conductor  440  striding over the side of the trench  430  that is far away from the source region  420 . The gate metal  460  electrically connects to the gate conductor  440  through the contact window  450 . When a gate current  510  flows from the gate metal  460  into the gate conductor  440  through the contact window  450 , it will vertically flow into the gate conductor  440  within the trench  430 , thereby reducing the series resistor resulted if the gate current  510  laterally flows through the gate conductor  440 . As a result, the performance of the power device is improved. In addition, the length B of the gate conductor  440  striding over the side of the trench  430  that is far away from the source region  420  is so short that the ion implant can diffuse under the gate conductor  440  to reach the edge of the trench  430 , as shown in region  500 . Thus, this structure of the present invention can be implemented by the current semiconductor processes with the same number of masks by altering the layout design without adding more process steps or altering the processes. Preferable, the length B is 0.5 μm, the width D is 1.1 μm, and the width G (G=E+F) of the contact window  450  is 0.7 μm.  
      In the situation of without altering the processes, the present invention achieves the goals of reducing the series resistor of the power device, making the processes control easier, and being applicable to all types of trench power devices, for example, trench power MOSFET and insulated gate bipolar transistor (IGBT).  
      The present invention is thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.