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Low frequency CMUT with thick oxide

  • US 8,455,289 B1
  • Filed: 12/02/2011
  • Issued: 06/04/2013
  • Est. Priority Date: 12/02/2011
  • Status: Active Grant
First Claim
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1. A method of forming a semiconductor transducer comprising:

  • providing semiconductor substrate having top and bottom surfaces wherein the top surface lies in a plane;

    growing approximately a 5.15 micrometer thick oxide layer on both the top and bottom surfaces of the semiconductor wafer using a high-pressure oxidation (HIPOX) process;

    wherein the 5.15 micrometer thick oxide layer is grown at a temperature in range of 900°

    C. to 1200°

    C. with an operating pressure of 10 to 20 atmospheres;

    forming a patterned photoresist layer on the thick oxide layer on the top surface of the semiconductor substrate;

    etching the thick oxide layer, exposing the top surface of the semiconductor substrate and forming a post structure on the top surface of the semiconductor substrate from the thick oxide layer, wherein the post structure having a top surface, being non-conductive, and laterally surrounding a cell region of the top surface of the substrate has no portion of the post structure lying below the plane of the top surface of the semiconductor substrate;

    wherein etching the thick oxide layer comprises;

    etching approximately 4.65 micrometers of the approximately 5.15 micrometer thick layer utilizing a non-polymerizing plasma etch, wherein the slope of the side wall at the end of the non-polymerizing etch is approximately 80°

    , making the opening slightly larger at the top than at the bottom of the thick oxide layer; and

    removing the remaining approximately 0.5 micrometers of the thick oxide layer with a wet etchant that is highly or completely selective to silicon, thus leaving the cell region on the top surface of the semiconductor substrate without damage;

    forming a non-conductive structure that touches the cell region of the top surface of the semiconductor substrate; and

    forming a conductive structure that touches the top surface of the post structure and lies directly vertically over the non-conductive structure to form a cavity that lies vertically between the non-conductive structure and the conductive structure.

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