CMOS three-dimensional image sensor detectors with assured non collection of late arriving charge, more rapid collection of other charge, and with improved modulation contrast

• US 8,314,924 B2
• Filed: 02/16/2010
• Issued: 11/20/2012
• Est. Priority Date: 02/17/2009
• Status: Expired due to Fees

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Microsoft Corporation
• Inventors: Bamji, Cyrus, Mehta, Swati, Elkhatib, Tamer Ahmed Taha
• Primary Examiner(s): RATCLIFFE, LUKE D
• Application Number:
  US12/658,833
• Publication Number:
  US 20110291164A1
• Time in Patent Office: 1,008 Days
• Field of Search: 356 301- 285, 35613901-1391
• US Class Current: 356/5.01
• CPC Class Codes: G01S 17/894 3D imaging with simultaneous measurement of time-of-flight at a 2D array of receiver pixels, e.g. time-of-flight cameras or flash lidar
• CPC Class Codes: G01S 7/4816 of receivers alone
• CPC Class Codes: H01L 27/14609 Pixel-elements with integrated switching, control, storage or amplification elements scanning details of imagers (circuitry of solid-state image sensors H04N25/00); circuitry of imagers H04N25/70
• CPC Class Codes: H04N 25/771 comprising storage means other than floating diffusion

Abstract

A CMOS-implementable TOF detector promptly collects charge whose creation time can be precisely known, while rejecting collection of potentially late arriving charge whose creation time may not be precisely known. Charges created in upper regions of the detector structure are ensured to be rapidly collected, while charges created in the lower regions of the detector structure, potentially late arriving charges, are inhibiting from being collected.

Claims

1. A method ensuring rapid collection of photonically generated charges created in an upper region of a CMOS sensor, while impeding collection of photonically generated charges created in a lower region of said CMOS sensor, the method comprising the following steps:

(a) from time of generation of said photonically generated charges to time of collection of said photonically generated charges, moving said photonically generated charges created in said upper region upward toward a collection region of said sensor, said moving resulting from at least one of (i) drift field resulting from an electric field of at least 1 kV/m to urge said photonically generated charges upward toward a collection region of said CMOS sensor, and (ii) diffusion over a diffusion length of less than about 1 μ

m; and

(b) from time of generation of said photonically generated charges, delaying upward movement of substantially all said photonically generated charges created in said lower region for a time period exceeding recombination lifetime of said photonically generated charges.

2. The method of claim 1, wherein at step (a)(i), said drift field results from at least one of (1) coupling voltage to a surface region of said CMOS sensor, and (2) creating a doping gradient in said upper region.

3. The method of claim 1, wherein at step (a)(ii) said diffusion results from at least one of (1) thermal energy, and (2) mutual repulsion among said charges.

4. The method of claim 1, wherein step (b) includes subjecting said photonically generated charges created in said lower region to at least one of (i) a high doping region formed within said CMOS sensor, said high doping region having dopant concentration sufficiently high to shorten charge carrier lifetime to a value less than time required for said photonically generated charges to reach said upper surface, and (ii) a voltage barrier created to inhibit upward movement of said photonically generated charges toward said upper surface.

5. The method of claim 4, wherein at step (b)(ii) said CMOS sensor includes a substrate and an epitaxial layer, and said voltage barrier results from forming said epitaxial layer with a higher dopant concentration than a dopant concentration of said substrate.

6. The method of claim 2, wherein said CMOS sensor includes at least one layer of epitaxial material, and said doping gradient results at least in part from difference in doping concentration between said epitaxial material and a layer in said sensor underlying said epitaxial material.

7. The method of claim 2, wherein said CMOS sensor includes at least a first layer of epitaxial material and a second layer of epitaxial material, and said doping gradient results at least in part from different in doping concentration between said first epitaxial material and said second epitaxial material.

8. The method of claim 1, wherein said CMOS sensor is a time-of-flight (TOF) sensor.

9. The method of claim 8, wherein at step (a)(i), said drift field results from at least one of (1) coupling voltage to a surface region of said CMOS sensor, and (2) creating a doping gradient in said upper region.

10. The method of claim 8, wherein at step (a)(ii) said diffusion results from at least one of (1) thermal energy, and (2) mutual repulsion among said photonically generated charges.

11. The method of claim 8, wherein step (b) includes subjecting said photonically generated charges created in said lower region to at least one of (i) a high doping region formed within said CMOS sensor, said high doping region having dopant concentration sufficiently high to shorten charge carrier lifetime to a value less than time required for said photonically generated charges to reach said upper surface, and (ii) a voltage barrier created to inhibit upward movement of said photonically generated charges toward said upper surface.

12. The method of claim 11, wherein at step (b)(ii) said CMOS sensor includes a substrate and an epitaxial layer, and said voltage barrier results from forming said epitaxial layer with a higher dopant concentration than a dopant concentration of said substrate.

13. The method of claim 9, wherein said CMOS sensor includes at least one layer of epitaxial material, and said doping gradient results at least in part from difference in doping concentration between said epitaxial material and a layer of said CMOS sensor underlying said epitaxial material.

14. The method of claim 9, wherein said CMOS sensor includes at least a first layer of epitaxial material and a second layer of epitaxial material, and said doping gradient results at least in part from difference in doping concentration between said first epitaxial material and said second epitaxial material.

15. A CMOS detector that generates photocharge in response to detection of incoming photonic energy, the detector including:

a substrate having a substrate dopant concentration N_s and including an upper substrate surface, and a substrate thickness that includes at least an upper substrate region and a lower substrate region,

a layer of epitaxial material overlying at least a region of said upper substrate surface, and having a epitaxial dopant concentration N_E; and

at least one gate structure fabricated on said upper substrate surface, coupleable to a source of bias voltage, and disposed so as to create a first electric field in at least a portion of said upper substrate region;

wherein magnitude of said N_S and magnitude of said N_E are selected to create a second electric field within said substrate that in combination with said first electric field rapidly conveys photocharges created within said upper substrate region to said upper substrate surface for collection, while substantial numbers of photocharges generated at a depth lower than said upper substrate region recombine without being collected; and

wherein photocharges generated at a depth not exceeding depth of said upper substrate region are photocharges created more recently than photocharges generated within said lower substrate region.

16. The sensor of claim 15, wherein N_E>

N_S.

17. The sensor of claim 15, wherein said first electric field extends downward into at least a portion of said upper substrate region to further hasten collection of more recently generated said photocharge.

18. The sensor of claim 17, wherein photocharges generated in said upper substrate region move upward within said sensor to be collected as a result of at least one characteristic of said sensor selected from (i) coupling a bias voltage to said gate structure creates an electric field exceeding about 1 kV/m, which field creates a drift field that moves photocharges created in said upper substrate region upward for collection, and (ii) presence of diffusion over a diffusion length less than about 1 μ

m.

19. The sensor of claim 15, further including at least one additional layer of semiconductor material having a layer thickness and having a layer dopant concentration N_L, where N_E>

N_L>

N_S;

wherein magnitude of N_L relative to magnitude of N_S results in a low drift field within said layer thickness such that charges generated within said lower substrate region that reach said additional layer recombine and perish and will not be collected.

20. The sensor of claim 15, wherein said sensor is a sensor in a time-of-flight (TOF) system, and sensed said photocharges provide a measure of depth distance to a target object imaged by said system.