Vertical Transistor Device Structure with Cylindrical-Shaped Field Plates

Abstract

A vertical power transistor device includes a semiconductor layer of a first conductivity type, with a plurality of dielectric regions disposed in the semiconductor layer. The dielectric regions extend in a vertical direction from a top surface of the semiconductor layer downward. Each dielectric region has a rounded-square cross-section in a horizontal plane perpendicular to the vertical direction. Adjacent ones of the dielectric regions are laterally separated by a narrow region of the semiconductor layer. Each dielectric region has a cylindrical field plate member centrally disposed therein. The cylindrical field plate member extends in the vertical direction from the top surface downward to near a bottom of the dielectric region. The dielectric region laterally separates the cylindrical field plate member from the narrow region. A source region is disposed at the top surface, and a drain region is disposed at the bottom, of the semiconductor layer.

We claim: 
     
         1 . A method comprising:
 forming, in a semiconductor substrate of a first conductivity type, first and second trenches laterally separated by a narrow region of the semiconductor substrate, each of the trenches extending in a vertical direction from a top surface of the semiconductor substrate downward, each of the trenches having a rounded-square shaped cross-section in a horizontal plane perpendicular to the vertical direction;   filling at least a portion of each of the trenches with a dielectric material;   forming first and second cylindrical field plates of a conductive material in the dielectric material of the first and second trenches, respectively, the first and second cylindrical field plates each being centrally located in the respective first and second trenches, the first and second cylindrical field plates each extending vertically from near a top surface of the semiconductor substrate downward to near a bottom of the respective first and second cylindrically-shaped trenches;   forming source and body regions in an upper portion of the narrow region, the source region being of the first conductivity type and the body region being of a second conductivity type opposite to the first conductivity type, the body region separating the source region from a lower portion of the narrow region, the lower portion of the narrow region comprising a drift region; and   forming a ring-shaped gate member embedded within the dielectric material adjacent the body region, the gate member being insulated from the body region and the first and second cylindrical field plates.   
     
     
         2 . The method of  claim 1 , further comprising:
 forming a drain region of the first conductivity type at the bottom of the narrow region, the drain region being connected to the drift region;   forming a source electrode connected to the source region; and   forming a drain electrode connected to the substrate.   
     
     
         3 . The method according to  claim 1 , wherein the dielectric material comprises an oxide. 
     
     
         4 . The method according to  claim 1 , wherein the first conductivity type is n-type. 
     
     
         5 . The method according to  claim 1 , wherein the ring-shaped gate member is a trench gate member. 
     
     
         6 . The method according to  claim 1 , wherein the narrow region has a doping concentration in a range of about 1×1015/cm3 to about 1×1017/cm3. 
     
     
         7 . The method according to  claim 1 , wherein the drift region comprises an epitaxial layer having a graded doping profile. 
     
     
         8 . The method according to  claim 1 , wherein the drift region has a doping concentration that varies from near the body region down to near a bottom of the drift region. 
     
     
         9 . The method according to  claim 8 , wherein the doping concentration is highest near the bottom of the drift region.