Patent Publication Number: US-2007117328-A1

Title: Vertical transistor with field region structure

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to semiconductor devices, and more particularly, to a vertical transistor with field region.  
      2. Description of Related Art  
      Widely applied on vertical transistor manufacturing, a field region with floating ring structure can provide a breakdown voltage. A depletion region is formed between the field region and an epi layer to increase the breakdown voltage of the vertical transistor. In order to achieve a higher breakdown voltage, the length of the field region with floating ring structure is generally long, and thus a larger device area is required. This increases the manufacturing cost. Meanwhile, due to the floating ring structure of the field region, the electric field distribution is not uniform, which renders the breakdown voltage thereof unstable.  
      Therefore, an improved structure for vertical transistors is desired.  
     SUMMARY OF THE INVENTION  
      One object of the present invention is to provide an improved structure for a vertical transistor.  
      According to the present invention, an improved structure comprises a field region surrounding the vertical transistor. The vertical transistor is composed of an array of core regions. The field region of the present invention is connected to respective well of the rim core regions of the vertical transistor, to provide a desired breakdown voltage with a shorter length, compared with the field region in floating ring structure.  
      Another object of the present invention is to provide an improved structure with uniform electric field distribution for vertical transistors.  
      According to the present invention, the field region connected to the respective well of the rim core regions is conducted to a voltage, i.e. source voltage. The doping density of the field region is adjustable. By two means thereof, it is able to uniform the electric field distribution across the field region and the epi layer, and thus provide a stable breakdown voltage. 
    
    
     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 embodiments of the present invention taken in conjunction with the accompanying drawings.  
       FIG. 1A  shows a top view of a traditional structure of a vertical transistor.  
       FIG. 1B  shows a top view of an improved structure of the vertical transistor according to one embodiment of the present invention.  
       FIG. 2  shows a cross-sectional view of the traditional structure of the vertical transistor.  
       FIG. 3  shows a cross-sectional view of the improved structure of the vertical transistor according to one embodiment of the present invention.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention provides an improved structure capable of ensuring a stable breakdown voltage and a desired breakdown voltage with shorter length of a field region, compared with the field region with floating ring structure.  
       FIG. 1A  shows a top view of a traditional structure of a vertical transistor  50 . A field region  106  is applied to determine a breakdown voltage range of the vertical transistor  50 .  
       FIG. 1B  illustrates a top view of an improved structure of a vertical transistor  100  according to one embodiment of the present invention. The structure of the vertical transistor  100  comprises core regions  102 , a gate pad  104  connected to respective gate of each core region  102 , and a field region  106   a.  The field region  106   a  formed surrounding the core regions  102  is capable of increasing the breakdown voltage of the vertical transistor  100  by the depletion region formed between the field region  106   a  and an epi layer  206 .  
       FIG. 2  shows the cross-sectional view of the traditional structure of the vertical transistor  50 . Each core region  102  comprises a drain metal  202 , a substrate  204 , the epi layer  206 , a well  208 , a gate oxide  210 , a gate layer  212 , a first heavy doping region  214 , a covered shell  216 , a second heavy doping region  218 , and a source metal  220 .  
       FIG. 3  shows the cross-sectional view of the improved structure of the vertical transistor  100  according to one embodiment of the present invention. The field oxide  224  is formed on the field region  106   a.  Unlike traditional structure of the vertical transistor  50 , the field region  106   a  of the present invention is connected to a well  208 , to thereby be conducted to a voltage, i.e. source voltage. By the applied voltage and adjustment of the doping density, the depletion region is fully depleted, and the across voltage is uniform, therefore a stable breakdown voltage can be provided. Meanwhile, the length of the field region  106   a  is shorter than the field region  106  in traditional structure of the vertical transistor  50 , therefore the device area is reduced.  
      The present invention realizes a stable breakdown voltage and reduced device area by modulating the doping density, length, and geometrical pattern. of the field region  106   a,  and by connecting the field region  106   a  to the well  208 .