Method of fabricating semiconductor device containing dielectrically isolated PN junction for enhanced breakdown characteristics

• US 7,323,386 B2
• Filed: 03/14/2006
• Issued: 01/29/2008
• Est. Priority Date: 02/02/2004
• Status: Expired due to Fees

• Current Assignee: Alpha & Omega Semiconductor, Ltd.
• Original Assignee: Hamza Yilmaz
• Inventors: Yilmaz, Hamza
• Primary Examiner(s): Baumeister; B. William
• Assistant Examiner(s): Wagner; Jenny L.
• Application Number:
  US11/375,975
• Publication Number:
  US 20060170036A1
• Time in Patent Office: 686 Days
• Field of Search: 257/139, 257/256, 438/270, 438/268, 438/589, 438/140
• US Class Current: 438/268
• CPC Class Codes: H01L 29/0619 with a supplementary region doped oppositely to or in rectifying contact with the semiconductor containing or contacting region, e.g. guard rings with PN or Schottky junction
• CPC Class Codes: H01L 29/0623 Buried supplementary region, e.g. buried guard ring multi-RESURF H01L29/0634
• CPC Class Codes: H01L 29/0634 Multiple reduced surface field (multi-RESURF) structures, e.g. double RESURF, charge compensation, cool, superjunction (SJ), 3D-RESURF, composite buffer (CB) structures
• CPC Class Codes: H01L 29/0649 Dielectric regions, e.g. SiO2 regions, air gaps
• CPC Class Codes: H01L 29/0653 adjoining the input or output region of a field-effect device, e.g. the source or drain region
• CPC Class Codes: H01L 29/1095 Body region, i.e. base region, of DMOS transistors or IGBTs cell layout H01L29/0696
• CPC Class Codes: H01L 29/407 Recessed field plates, e.g. trench field plates, buried field plates
• CPC Class Codes: H01L 29/41766 with at least part of the source or drain electrode having contact below the semiconductor surface, e.g. the source or drain electrode formed at least partially in a groove or with inclusions of conductor inside the semiconductor H01L29/41733 - H01L29/41758 take precedence
• CPC Class Codes: H01L 29/456 on silicon
• CPC Class Codes: H01L 29/66348 with a recessed gate
• CPC Class Codes: H01L 29/66719 With a step of forming an insulating sidewall spacer
• CPC Class Codes: H01L 29/66727 with a step of recessing the source electrode
• CPC Class Codes: H01L 29/66734 with a step of recessing the gate electrode, e.g. to form a trench gate electrode
• CPC Class Codes: H01L 29/7397 and a gate structure lying on a slanted or vertical surface or formed in a groove, e.g. trench gate IGBT
• CPC Class Codes: H01L 29/7802 Vertical DMOS transistors, i.e. VDMOS transistors
• CPC Class Codes: H01L 29/7811 with an edge termination structure guard regions per se H01L29/0619; field plates per se H01L29/402
• CPC Class Codes: H01L 29/7813 with trench gate electrode, e.g. UMOS transistors trench gate electrodes per se H01L29/4236
• CPC Class Codes: H01L 29/808 with a PN junction gate , e.g. PN homojunction gate H01L29/7725, H01L29/775, H01L29/778, H01L29/806 take precedence
• CPC Class Codes: H01L 29/8083 Vertical transistors SIT H01L29/7722
• CPC Class Codes: H01L 29/8605 Resistors with PN junctions
• CPC Class Codes: H01L 29/872 : Schottky diodes

Abstract

A semiconductor device includes a field shield region that is doped opposite to the conductivity of the substrate and is bounded laterally by dielectric sidewall spacers and from below by a PN junction. For example, in a trench-gated MOSFET the field shield region may be located beneath the trench and may be electrically connected to the source region. When the MOSFET is reverse-biased, depletion regions extend from the dielectric sidewall spacers into the “drift” region, shielding the gate oxide from high electric fields and increasing the avalanche breakdown voltage of the device. This permits the drift region to be more heavily doped and reduces the on-resistance of the device. It also allows the use of a thin, 20 Å gate oxide for a power MOSFET that is to be switched with a 1V signal applied to its gate while being able to block over 30V applied across its drain and source electrodes, for example.

Claims

1. A method of fabricating a trench-gate semiconductor device comprising:

providing a semiconductor substrate of a first conductivity type;

forming an epitaxial layer of the first conductivity type on the substrate;

forming first and second trenches in the epitaxial layer, the first and second trenches being separated by a mesa;

forming dielectric spacers on the sidewalls of the trenches while leaving the bottoms of the trenches exposed;

filling bottom portions of the trenches with a semiconductor material of a second conductivity type, the semiconductor material of the second conductivity type forming PN junctions with the epitaxial layer of the first conductivity type at the bottoms of the trenches;

removing portions of the dielectric spacers above the semiconductor material of the second conductivity type;

forming a dielectric layer on the walls of the trenches above the semiconductor material of the second conductivity type and on the top surface of the semiconductor material of the second conductivity type; and

filling an upper portion of the trenches with a conductive gate material.

2. The method of claim 1 comprising forming a body region of the second conductivity type in the mesa and forming a source region of the first conductivity type in the mesa, both the body region and the source region abutting a wall of the first trench.

3. The method of claim 1 wherein forming the source region comprises implanting a dopant of the first conductivity type through an opening in a first mask layer formed above a surface of the epitaxial layer.

4. The method of claim 2 comprising:

depositing a second dielectric layer over the surface of the epitaxial layer and the conductive gate material; and

removing a first portion of the second dielectric layer, leaving a second portion of the dielectric layer above the conductive gate material and a portion of the source region and forming an opening in said second dielectric layer above a portion of the mesa.

5. The method of claim 4 comprising implanting a dopant of the second conductivity type through the opening in the second dielectric layer to form a body contact region in the mesa.

10. The method of claim 1 wherein, after filling a bottom portion of the trenches with a semiconductor material of a second conductivity type, an upper portion of each trench above the bottom portion remains empty such that portions of the dielectric spacers above the semiconductor material remain exposed.

11. The method of claim 1 wherein forming dielectric spacers on the sidewalls of the trenches while leaving the bottoms of the trenches exposed comprises forming dielectric layers on the sidewalls and bottoms of the trenches and removing the portions of the dielectric layers that cover the bottoms of the trenches.

12. The method of claim 11 wherein forming dielectric layers comprises thermally growing oxide layers and wherein removing the portions of the dielectric layers that cover the bottoms of the trenches comprises removing the portions of the oxide layers that cover the bottoms of the trenches using a directional reactive ion etch (RIE) process.

6. A method of fabricating a trench-gate semiconductor device comprising:

providing a semiconductor substrate of a first conductivity type;

forming an epitaxial layer of the first conductivity type on the substrate;

forming first and second trenches in the epitaxial layer, the first and second trenches being separated by a mesa;

forming dielectric spacers on the sidewalls of the trenches;

filling a bottom portion of the trenches with a semiconductor material of a second conductivity type;

removing portions of the dielectric spacers above the semiconductor material of the second conductivity type;

forming a dielectric layer on the walls of the trenches above the semiconductor material of the second conductivity type and on the top surface of the semiconductor material of the second conductivity type;

filling an upper portion of the trenches with a conductive gate material; and

a body region of the second conductivity type in the mesa and forming a source region of the first conductivity type in the mesa, both the body region and the source region abutting a wall of the first trench;

wherein forming the source region comprises;

forming a second dielectric layer above the conductive gate material;

depositing a polysilicon layer doped with a dopant of the first conductivity type over the second dielectric layer and over the mesa;

etching the polysilicon layer directionally so as to leave a polysilicon spacer on a sidewall of the second dielectric layer; and

heating the polysilicon spacer to cause the dopant of the first conductivity type in the polysilicon spacer to diffuse into the epitaxial layer.

7. A method of fabricating a trench-gate semiconductor device comprising:

providing a semiconductor substrate of a first conductivity type;

forming an epitaxial layer of the first conductivity type on the substrate;

forming first and second trenches in the epitaxial layer, the first and second trenches being separated by a mesa;

forming dielectric spacers on the sidewalls of the trenches;

filling a bottom portion of the trenches with a semiconductor material of a second conductivity type;

removing portions of the dielectric spacers above the semiconductor material of the second conductivity type;

forming a dielectric layer on the walls of the trenches above the semiconductor material of the second conductivity type and on the top surface of the semiconductor material of the second conductivity type;

filling an upper portion of the trenches with a conductive gate material; and

forming a body region of the second conductivity type in the mesa and forming a source region of the first conductivity type in the mesa, both the body region and the source region abutting a wall of the first trench;

wherein forming the source region comprises;

forming a second dielectric layer above the conductive gate material;

introducing a dopant of the first conductivity type through the top surface of the mesa to form a source layer;

depositing a polysilicon layer doped with a dopant of the first conductivity type over the second dielectric layer and over the mesa;

etching the polysilicon layer directionally so as to leave a polysilicon spacer on a vertical surface of the second dielectric layer; and

removing a portion of the source layer while leaving in place a remaining portion of the source layer, at least part of the remaining portion of the source layer being located under the polysilicon spacer.

8. The method of claim 7 comprising depositing a third dielectric layer over the second dielectric layer and the polysilicon spacer and directionally etching the third dielectric layer on a vertical surface of the polysilicon spacer.

9. A method of fabricating a power MOSFET comprising:

growing an epitaxial layer on a semiconductor substrate, both said epitaxial layer and said substrate being doped with material of a first conductivity type;

forming a first mask on a surface of said epitaxial layer, said first mask having an opening where a trench is to be formed;

etching said epitaxial layer through said opening in said first mask to form a trench in said epitaxial layer;

forming a first dielectric layer on the sidewalls and a bottom of said trench;

removing a portion of said first dielectric layer on the bottom of said trench;

depositing a second epitaxial layer doped with material of a second conductivity type in a lower portion of said trench so as to form a field shield region;

removing a second portion of said first dielectric layer on the sidewalls of said trench above said second epitaxial layer, thereby forming a dielectric spacers on the sides of said field shield region;

forming a second dielectric layer on the exposed portions of the sidewalls of the trench and on a top surface of said field shield region;

filling said trench with a first polysilicon layer;

removing a portion of said first polysilicon layer such that a surface of said first polysilicon layer is located at a level below a top surface of said first mask, thereby forming a polysilicon gate;

depositing a glass layer on said first mask and said polysilicon gate;

planarizing said glass layer such that a surface of said glass layer is coplanar with the top surface of the first mask, thereby forming a glass plug directly above said polysilicon gate;

removing at least a portion of the first mask;

implanting dopant of the second conductivity type to form a body region in said epitaxial layer;

depositing a second polysilicon layer over a top surface of said glass plug and said epitaxial layer, said second polysilicon layer being doped with material of said first conductivity type;

etching said second polysilicon layer directionally so as to form a polysilicon spacer on a sidewall of said glass plug;

heating said polysilicon spacer so as to cause dopant of said first conductivity type to diffuse from said polysilicon spacer into said epitaxial layer, thereby creating a source region;

depositing a metal layer over said glass plug and said first epitaxial layer;

forming a second mask over said metal layer; and

etching said metal layer through an opening in said second mask to form a source metal section of said metal layer.