Pixel cells and pixel cell arrays having low leakage and improved performance characteristics

• US 5,982,012 A
• Filed: 01/14/1998
• Issued: 11/09/1999
• Est. Priority Date: 01/14/1998
• Status: Expired due to Fees

• Current Assignee: Foveonics, Inc.
• Original Assignee: Foveon, Inc. (Avis Budget Group Incorporated)
• Inventors: Merrill, Richard B.
• Primary Examiner(s): Monin, Jr., Donald L.
• Application Number:
  US09/006,779
• Time in Patent Office: 664 Days
• Field of Search: 257/461, 257/462, 257/463, 257/464, 257/466, 257/566, 257/561, 257/562
• US Class Current: 257/463
• CPC Class Codes: H01L 27/14681 Bipolar transistor imagers

Abstract

The present invention relates to a pixel cell and pixel cell array modified to improve performance. One improvement taught by the present invention is implantation of dopant into the silicon to form the base region after formation of polysilicon, resulting in highest base dopant concentrations lying at the thin oxide and emitter interfaces. A second improvement taught by the present invention is a reduction in the size of the heavily doped portion of the emitter to extend no further than the footprint of the emitter contact, thereby inhibiting leakage between the emitter and adjacent polysilicon. A third improvement taught by the present invention is electronic isolation of pixel cells by inter-pixel regions doped with conductivity-altering impurity of a type opposite that of the base rather than by field oxides, thereby eliminating leakage at the field oxide edge. A fourth improvement disclosed by the present invention is the introduction of silicides at the emitter contact and the base capacitor contact in order to reduce contact resistance.

Claims

1. A bipolar phototransistor pixel cell comprising:

a collector region of a first conductivity type having a collector surface;

a base region of a second conductivity type opposite the first conductivity type, having a base surface and bordered on all sides except the base surface by the collector region;

an emitter region of the first conductivity type having an emitter surface and bordered on all sides except the emitter surface by the base region;

a thin oxide layer formed on top of the collector surface and the base surface; and

a polysilicon layer formed on top of the thin oxide layer, wherein implantation of dopant of the second conductivity type through the thin oxide layer and the polysilicon to form the base region produces a first band of concentrated dopant located directly beneath the thin oxide layer and a second band of concentrated dopant located at a depth below the base surface and adjacent to the emitter region.

2. The pixel cell according to claim 1 wherein the emitter region further comprises a heavily doped emitter region having a heavily doped emitter surface bordered on all sides except for the heavily doped emitter surface by a lightly doped emitter region, and wherein the bipolar phototransistor pixel cell further comprises an emitter contact formed on top of the heavily doped emitter surface.

3. The pixel cell according to claim 1 further comprising a silicide layer on top of at least one of the emitter surface and the polysilicon.

4. A bipolar phototransistor pixel cell comprising:

a collector region of a first conductivity type having a collector surface;

a base region of a second conductivity type opposite the first conductivity type, having a base surface and bordered on all sides except the base surface by the collector region;

an emitter region including a heavily doped emitter region having a heavily doped emitter surface bordered on all sides except for the heavily doped emitter surface by a lightly doped emitter region;

an emitter contact formed on top of the heavily doped emitter surface;

a thin oxide layer formed on top of the collector surface and the base surface; and

a polysilicon layer formed on top of the thin oxide layer.

5. A bipolar phototransistor pixel cell comprising:

a collector region of a first conductivity type having a collector surface;

a base region of a second conductivity type opposite the first conductivity type, having a base surface and bordered on all sides except the base surface by the collector region;

an emitter region of the first conductivity type having an emitter surface and bordered on all sides except the emitter surface by the base region;

a thin oxide layer formed on top of the collector surface and the base surface;

a polysilicon layer formed on top of the thin oxide layer; and

a silicide layer formed on top of at least one of the emitter surface and the polysilicon layer.

6. A pixel cell array comprising a plurality of bipolar phototransistor pixel cells configured in rows and columns within a common collector region of a first conductivity type having a collector surface, the array including inter-pixel isolation regions of the first conductivity type formed within the common collector region between pixel cells, the pixel cells including,

a base region of a second conductivity type opposite the first conductivity type, having a base surface and bordered on all sides except the base surface by the collector region,

an emitter region of the first conductivity type having an emitter surface and bordered on all sides except the emitter surface by the base region,

a thin oxide layer formed on top of the collector surface and the base surface, and

a polysilicon layer formed on top of the thin oxide layer, wherein implantation of dopant of the second conductivity type through the thin oxide layer and the polysilicon to form the base region produces a first band of concentrated dopant located directly beneath the thin oxide layer and a second band of concentrated dopant located at a depth below the base surface and adjacent to the emitter region.

7. A pixel cell array according to claim 6 wherein:

the emitter region of the pixel cells further comprises a heavily doped emitter region having a heavily doped emitter surface bordered on all sides except for the heavily doped emitter surface by a lightly doped emitter region; and

wherein the pixel cells further comprise an emitter contact formed on top of the heavily doped emitter surface.

8. A pixel cell array according to claim 6 wherein the pixel cells further include a silicide layer on top of at least one of the emitter surface and the polysilicon.

9. A pixel cell array comprising a plurality of bipolar phototransistor pixel cells configured in rows and columns within a common collector region of a first conductivity type having a collector surface, the array including inter-pixel isolation regions of the first conductivity type formed within the common collector region between pixel cells, the pixel cells including,

a base region of a second conductivity type opposite the first conductivity type, having a base surface and bordered on all sides except the base surface by the collector region,

an emitter region including a heavily doped emitter region having a heavily doped emitter surface bordered on all sides except for the heavily doped emitter surface by a lightly doped emitter region,

an emitter contact formed on top of the heavily doped emitter surface,

a thin oxide layer formed on top of the collector surface and the base surface, and

a polysilicon layer formed on top of the thin oxide layer.

10. A pixel cell array comprising a plurality of bipolar phototransistor pixel cells configured in rows and columns within a common collector region of a first conductivity type having a collector surface, the array including inter-pixel isolation regions of the first conductivity type formed within the common collector region between pixel cells, the pixel cells including,

a base region of a second conductivity type opposite the first conductivity type, having a base surface and bordered on all sides except the base surface by the collector region;

an emitter region of the first conductivity type having an emitter surface and bordered on all sides except the emitter surface by the base region;

a thin oxide layer formed on top of the collector surface and the base surface;

a polysilicon layer formed on top of the thin oxide layer; and

a silicide layer formed on top of at least one of the emitter surface and the polysilicon layer.