Touching microlens structure for a pixel sensor and method of fabrication

• US 20060261427A1
• Filed: 03/17/2006
• Published: 11/23/2006
• Est. Priority Date: 05/18/2005
• Status: Active Grant

• Current Assignee: GlobalFoundries, Inc.
• Original Assignee: International Business Machines Corporation
• Inventors: Hoague, Timothy J., Dillon, John E., Leidy, Robert K.
• Granted Patent: US 7,898,049 B2
• Time in Patent Office: Days
• US Class Current: 257/432
• CPC Class Codes: H01L 27/14627 Microlenses
• CPC Class Codes: H01L 27/14685 Process for coatings or optical elements
• CPC Class Codes: H01L 31/02327 the optical elements being integrated or being directly associated to the device, e.g. back reflectors optical coatings H01L31/0216

Abstract

A structure and method for increasing the sensitivity of pixel sensors by eliminating a gap space formed between adjacent microlens structures in a pixel sensor array. Advantageously, exposure and flowing conditions are such that adjacent microlens structures touch (are webbed) at a horizontal cross-section, yet have a round lens shape in all directions. Particularly, exposure and flowing conditions are such that each touching microlens structure is formed to have a matched uniform radius of curvature at a horizontal cross-section and at a 45 degree cross-sections. To improve quality of mircrolens structure uniformity exhibited at all pixel locations including those near a pixel array edge or corner, a top anti-reflective coating layer is applied on top of a photoresist layer prior to the exposure and flowing steps.

Claims

1. A sensor including an array of pixels, each pixel comprising:

a layer comprising a plurality of microlens structures for receiving light;

a semiconductor substrate including a light sensitive element formed therein for receiving light incident to said pixel and focused by a respective microlens structure; and

,

one or more dielectric material and embedded metallization layers provided between said substrate and a top layer,

wherein each microlens structure of a pixel is webbed in a direction such that said structure touches a microlens structure of an adjacent pixel in said array, and such that a curvature of said mircrolens structure is uniform in all directions to thereby maximize collection of light incident to the microlens structure from all directions, said mircrolens structure uniformity exhibited at all pixel locations including those near a pixel array edge or corner.

2. The sensor as claimed in claim 1, wherein each said touching microlens structure is of a circular plan shape.

3. The sensor as claimed in claim 2, wherein each said touching microlens structure is of circular plan shape formed to have a matched uniform radius of curvature at a horizontal cross-section and at a 45 degree cross-section.

4. The sensor as claimed in claim 3, wherein said touching microlens, structure at said 45 degree cross-section is formed at a lower vertical distance below a horizontal where the webbed edges touch at said horizontal cross-section so that the radius of curvature of the microlens can be matched to the radius of curvature of the microlens at the horizontal cross-section.

5. A method for fabricating a microlens structure in a pixel sensor array comprising the steps of:

a. providing a substrate including a plurality of light sensitive elements adapted to receive light incident to a respective pixel microlens;

b. forming a photoresist material layer over said substrate;

c. applying an anti-relfective coating layer on top of said photoresist material layer;

d. patterning said photoresist layer using a first sub-resolution condition to form microlens structure images, said first sub-resolution condition applied in a manner sufficient to form partially connected lens portions at gaps between adjacent microlens structures;

e. developing said photoresist layer to partially form the patterned microlens structures having partially connected lens portions at said gaps;

f. blanket applying a second sub-resolution condition to the partially formed microlens structures; and

,

g. flowing said partially formed microlens structures, wherein each microlens structure is webbed such that said microlens structure touches a microlens structure of an adjacent pixel in said array, and such that a curvature of said microlens structure is uniform in all directions to thereby maximize collection of light incident to the microlens structure from all directions, said mircrolens structure uniformity exhibited at all pixel locations including those near a pixel array edge or corner.

6. The method as claimed in claim 5, wherein said first and second sub-resolution conditions comprise a light exposure dose applied to said patterned photoresist layer to initiate cross-linking of polymers of said photoresist material.

7. The method as claimed in claim 6, wherein said patterning step c), comprises applying said first sub-resolution condition to a mask image that ensures polymer cross-linking at gaps located between adjacent cells at a horizontal cross section.

8. The method as claimed in claim 6, wherein said first and second sub-resolution conditions comprises an exposure dose of UV light at energies ranging from about 0.01 to about 2 Joules/cm².

9. The method as claimed in claim 5, wherein said flowing step f) comprises a post-exposure bake process for applying heat to said microlens structures at a temperature ranging from about 180°

C., to about 220°

C.

10. The method as claimed in claim 5, wherein each said touching microlens structure is of circular plan shape formed to have a matched uniform radius of curvature at a horizontal cross-section and at a 45 degree cross-section.

11. The method as claimed in claim 10, wherein said touching microlens structure at said 45 degree cross-section is formed at a lower vertical distance below a horizontal where the webbed edges touch at said horizontal cross-section so that the radius of curvature of the microlens can be matched to the radius of curvature of the microlens at the horizontal cross-section.

12. The method as claimed in claim 5, wherein said anti-reflective coating layer applied on top of said photoresist material layer is of sufficient thickness to tighten process control conditions at or near said pixel array edge or corner.

13. A method of controlling dimensions of a formed microlens structure of a sensor array, said dimensions including a lens size, height and radius of curvature, said method comprising:

applying first exposure conditions to a photoresist layer having an anti-reflective coating layer applied thereon, said photoresist layer patterned to partially form touching microlens structures and developing said layer after first dose exposure;

blanket applying second exposure conditions to said partially formed touching microlens structures of said array; and

,

flowing said partially formed touching microlens structures of said array at temperatures sufficient to form adjacent microlens structures having a round lens shape in all directions.

14. The method as claimed in claim 13, wherein each said touching microlens structure is formed to have a matched uniform radius of curvature at a horizontal cross-section and at a 45 degree cross-section.

15. The method as claimed in claim 13, wherein said flowing step is conducted at a temperature sufficient to ensure shrinkage of each said touching microlens structure at said 45 degree cross-section such that the radius of curvature of the microlens at the 45 degree cross-section can be matched to the radius of curvature of the microlens at the horizontal cross-section.

16. The method as claimed in claim 15, wherein said blanket applying second exposure conditions and said flowing steps are conducted to ensure shrinkage of each said touching microlens structure at said 45 degree cross-section at a lower vertical distance below a horizontal where the lens edges touch at said horizontal cross-section.

17. The method as claimed in claim 13, wherein each said touching microlens structure is formed to have a lens width “

w” and

a height “

h” and

a radius of curvature “

R”

at any cross-section governed according to R²=h²+(w/2)²/2h.