Light Guide Array for An Image Sensor
An image sensor pixel that includes a photoelectric conversion unit (102) supported by a substrate (106) and an insulator (110) adjacent to the substrate. The pixel includes a cascaded light guide (116,130) that is located within an opening of the insulator and extends above the insulator such that a portion (130) of the cascaded light guide has an air interface (424). The air interface improves the internal reflection of the cascaded light guide. The cascaded light guide may include a self-aligned color filter (114B,114G) having, air-gaps (.422). between adjacent color filters. These characteristics of the light guide eliminate the need for a microlens. Additionally, an anti-reflection stack (230) is interposed between the substrate (106) and the light guide (116) to reduce backward reflection from the image sensor. Two pixels of having different color filters may have a difference in the thickness of an anti-reflection film within the anti-reflection stack.
This application claims priority to U.S. Patent Application 61/009,454 filed on Dec. 28, 2007; Application 61/062,773 filed on Jan. 28, 2008; Application 61/063,301 filed on Feb. 1, 2008; Application 61/069,344 filed on Mar. 14, 2008; and application Ser. No. 12/218,749 filed on Jul. 16, 2008.
BACKGROUND OF THE INVENTION1. Field of the Invention
The subject matter disclosed, generally relates to structures and methods for fabricating solid state image sensors.
2. Background Information
Photographic equipment such as digital cameras and digital camcorders may contain electronic image sensors that capture light for processing into still or video images. Electronic image sensors typically contain millions of light capturing elements such as photodiodes.
Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, photo sensors are formed in a substrate and arranged in a two-dimensional array. Image sensors typically contain millions of pixels to provide a high-resolution image.
Light guides 7 are integrated into the sensor to guide light onto the conversion units 2. The light guides 7 are formed of a material such as silicon nitride that has a higher index of refraction than the insulating layer 5. Each light guide 7 has an entrance that is wider than the area adjacent to the conversion units 2. The sensor 1 may also have a color filter 8 and a microlens 9.
The microlens 9 focuses light onto the photo photoelectric conversion units 2. As shown in
Backward reflection from the image sensor at the substrate interface is another issue causing loss of light reception. As shown in
An image sensor pixel that includes a photoelectric conversion unit supported by a substrate and an insulator adjacent to the substrate. The pixel may have a cascaded light guide, wherein a portion of the cascaded light guide is within the insulator and another portion extends above the insulator. The cascaded light guide may include a self-aligned color filter. The pixel may have an anti-reflection stack between the substrate and the cascaded light guide.
Disclosed is an image sensor pixel that includes a photoelectric conversion unit supported by a substrate and an insulator adjacent to the substrate. The pixel includes a light guide that is located within an opening of the insulator and extends above the insulator such that a portion of the light guide has an air interface. The air interface improves the internal reflection of the light guide. Additionally, the light guide and an adjacent color filter are constructed with a process that optimizes the upper aperture of the light guide and reduces crosstalk. These characteristics of the light guide eliminate the need for a microlens. Additionally, an anti-reflection stack is constructed above the photoelectric conversion unit and below the light guide to reduce light loss through backward reflection from the image sensor. Two pixels of different color may be individually optimized for anti-reflection by modifying the thickness of one film within the anti-reflection stack.
The pixel may include two light guides, one above the other. The first light guide is located within a first opening of the insulator adjacent to the substrate. The second light guide is located within a second opening in a support film, which is eventually removed during fabrication of the pixel. A color filter is located within the same opening and thus self-aligns with the second light guide. The second light guide may be offset from the first light guide at the outer corners of the pixel array to capture light incident at a non-zero angle relative to the vertical axis.
An air gap is created between neighboring color filters by removing the support film material adjacent to the filter. Air has a lower refractive index than the support film and enhances internal reflection within the color filter and the light guide. In addition, the air gap is configured to “bend” light incident on the gap into the color filter and increase the amount of light provided to the sensor.
Reflection at the silicon-light-guide interface is reduced by creating a nitride film and a first oxide film below the first light guide. A second oxide film may be additionally inserted below the nitride film to broaden the range of light frequencies for effective anti-reflection. The first oxide film can be deposited into an etched pit before application of the light-guide material. In an alternate embodiment, all anti-reflection films are formed before a pit is etched, and an additional light-guide etch-stop film covers the anti-reflection films to protect them from the pit etchant.
Referring to the drawings more particularly by reference numbers,
Each pixel has a first light guide 116. The first light guides 116 are constructed with a refractive material that has a higher index of refraction than the insulating layer 110. As shown in
The second light guides 130 are located above first light guides 116 and may be made from the same material as the first light guide 116. The top end of the second light guide 130 is wider than the bottom end, where the second light guide 130 meets the first light guide 116. Thus the gap between adjacent second light guides 130 at the bottom (henceforth “second gap”) is larger than at the top, as well as larger than the air gap 422 between the color filters 114B, 114G above the second light guides 130. The second light guides 130 may be offset laterally with respect to the first light guides 116 and/or the conversion unit 102, as shown in
Light ray c, which comes in from the left at an angle up to 25 degrees relative to the vertical axis, reflects off the right sidewall of the second light guide 130, hits and penetrates the lower-left sidewall of the same, enters the first light guide 116, and finally reaches conversion unit 102. The offset is such that the first light guide 116 recaptures the light ray that exits lower-left sidewall of second light guide 130. At each crossing of light guide sidewall, whether exiting the second light guide or entering the first light guide, light ray c refracts in a way that the refracted ray's angle to the vertical axis becomes less each time, enhancing propagation towards the photoelectric conversion unit. Thus, having a light guide built from a first light guide 116 and a second light guide 130 allows the vertical cross-section shape of the light guide to vary from pixel to pixel to optimize for passing light to the photoelectric conversion unit 102.
Building a light guide from two separate light guides 116, 130 has a second advantage of reducing the etch depth for each light guide 116, 130. Consequently, side wall slope angle control can achieve higher accuracies. It also makes deposition of lightguide material less likely to create unwanted keyholes, which often happen when depositing thin film into deep cavities, causing light to scatter from the light guide upon encountering the keyholes.
Color filters 114B, 114G are located above the second light guides 130. The sidewall upper portion at and adjacent to the color filters is more vertical than the rest of second lightguide. Viewing it another way, sidewalls of adjacent color filters facing each other are essentially parallel.
First air-gap 422 between the color filters has a width of 0.45 um or less, and a depth of 0.6 um or greater. An air gap with the dimensional limitations cited above causes the light within the gap to be diverted into the color filters and eventually to the sensors. Thus the percentage loss of light impinging on the pixel due to passing through the gap (henceforth “pixel loss”) is substantially reduced.
Light incident upon a gap between two translucent regions of higher refractive indices become diverted to one or the other when the gap is sufficiently narrow. In particular, light incident upon an air gap between two color filters diverts to one color filter or the other when the gap width is sufficiently small.
Referring to
Referring to
Air interface may continue from the color filter sidewall along the second light guide sidewall and end above protection film 410, creating a second air gap 424. The air interface between second air gap 424 and the second light guide 130 enhances internal reflection for the second light guide 130.
A protection film 410 may be formed above insulating layer 110 of silicon nitride to prevent alkali metal ions from getting into the silicon. Alkali metal ions, commonly found in color filter materials, can cause instability in MOS transistors. Protection film 410 also keeps out moisture. The protection film 410 may be made of silicon nitride (Si3N4) of thickness between 10,000 Angstroms and 4,000 Angstroms, preferably 7,000 Angstroms. If either first light guide 116 or second light guide 130 is made of silicon nitride, the protection film 410 which is formed of silicon nitride is continuous across and above the insulating layer 110 to seal the transistors from alkali metal ions and moisture. If both first 116 and second 130 light guides are not made of silicon nitride, the protection film 110 may cover the top surface of the first light guide 116 to provide similar sealing or, alternatively, cover the sidewalls and bottom of first light guide 116.
First 422 and second 424 air gaps together form a connected opening to air above the top surface of the image sensor. Viewing this in another way, there exists a continuous air interface from the protection film 410 to the top surfaces of the color filters 114B, 114G. In particular, there is an air-gap between the top surfaces 430 of the pixels. The existence of this opening during manufacture allows waste materials formed during the forming of first air gap 422 and second air gap 424 to be removed during the manufacture of the image sensor. If for some reason the first air-gap 422 is sealed subsequently using some plug material, this plug material should have a refractive index lower than the color filter material so that (i) there is internal reflection within the color filter, and (ii) light incident within the air-gap 422 is diverted into the color filters 114B, 114G. Likewise if some fill material fills the second air gap 424, this fill material needs to have a lower refractive index than the second light guide 130.
Together, the color filter 114 and light guides 130 and 116 constitute a “cascaded light guide” that guides light to the photoelectric conversion unit 102 by utilizing total internal reflection at the interfaces with external media such as the insulator 110 and air gaps 422 and 424. Unlike prior art constructions, light that enters the color filter does not cross over to the color filter of the next pixel but can only propagate down to the second light guide 130. This makes it unnecessary to have a microlens above to focus light to the center of the pixel area to prevent light ray passing out from a color filter of a pixel to an adjacent pixel. Doing away with microlens has a benefit of eliminating the aforementioned problem of alignment error between microlens and color filter that can cause crosstalk, in addition to lowering manufacturing costs.
As mentioned before, a cascaded light guide further holds an advantage over prior art that uses opaque wall material between color filters in that incident light falling into the first air gap 422 between color filters 114B and 114G is diverted to either one, thus no light is lost, unlike prior art pixels where light is lost to the opaque walls between the filters.
An advantage of this color filter forming method over prior art methods is that the color filter sidewall is not defined by the photoresist and dye materials constituting the color filters. In prior art color filter forming methods, the color filter formed must produce straight sidewalls after developing. This requirement places a limit on the selection of photoresist and dye material because the dye must not absorb light to which the photoresist is sensitive, otherwise the bottom of the color filter will receive less light, resulting in color filter that is narrower at its bottom than its top. The present color filter forming method forms the color filter sidewall by the pocket 210 etched into the support film 134 and not relying on the characteristics of the color filter material or the accuracy of lithography, resulting in a cheaper process.
Another advantage over prior art color filter forming methods is that gap spacing control is uniform between all pixels, and highly accurate at low cost. Here, the gap spacing is a combination of the line-width in the single lithography step that etches the openings in the support film, plus the control of sideway etching during dry etch, both easily controlled uniformed and highly accurately without adding cost. If such gaps were to be created by placing 3 color filters of different colors at 3 different lithography steps as in the prior arts, uniformity of gap widths is impossible, the lithography steps become expensive, and sidewall profile control becomes even more stringent.
A cascaded light guide wherein a color filter 114 and a light guide 130 are formed in the same opening in the support film 134 (henceforth “self-aligned cascaded light guide”) has an advantage over prior art in that there is no misalignment between the color filter 114 and the light guide 130. The color filter 114 has sidewalls that self-align to sidewalls of the light guide 130.
As shown in
The light guide material may be etched down to leave a thinner and flatter protection film 410 to cover the insulator. This seals the conversion unit 102, gate 104, and electrodes 108 against H2O and alkali metal ions during the subsequent processes. Alternatively, if the first light guide material 122 is not silicon nitride, a silicon nitride film may be deposited on top of light guide material 122 after an etch-down of the latter to flatten the top surface, to form a protection film 410 that seals the conversion unit 102, gate 104, and electrodes 108 against H2O and alkali metal ions. The protection film 410 may be between 10,000 Angstroms and 4,000 Angstroms thick, preferably 7,000 Angstroms.
A shown in
In
As shown in
As shown in
Referring to
As shown in
The top AR film 236 has a lower refractive index than the light guide 116. The second AR film 234 has a higher refractive index than the top AR film 236. The third AR film 232 has a lower refractive index than the second AR film 234.
The top AR film 236 may be silicon oxide or silicon oxynitride, having refractive index about 1.46, with a thickness between 750 Angstrom and 2000 Angstrom, preferably 800 Angstrom. The second AR film 234 may be silicon nitride (Si3N4), having refractive index about 2.0, with a thickness between 300 Angstrom and 900 Angstrom, preferably 500 Angstrom. The third AR film 232 may be silicon oxide or silicon oxynitride (SiOxNy, where 0<x<2 and 0<y<4/3), having refractive index about 1.46, with a thickness between 25 Angstrom and 170 Angstrom, preferably 75 Angstrom. The third AR film 232 may comprise the gate oxide under the gate 104 and above the substrate 106 of
The anti-reflection structure shown in
The support film 134 is masked and a first etchant is applied to etch openings in the support film 134. The first etchant is chosen to have high selectivity towards the protection film material. For example, if the support film 134 comprises HDP silicon oxide and the protection film 410 comprises silicon nitride, the first etchant may be CHF3, which etches HDP silicon oxide 5 times as fast as silicon nitride. A second etchant is then applied to etch through the silicon nitride protection film 410. The second etchant may be CH3F/O2. The first etchant is then applied again to etch the first insulator 110 and to stop on the contact etch-stop film 234 which comprises silicon nitride. The contact etch-stop film 234 acts as an etchant stop to define the bottom of the opening. The top AR film 236 is then formed in the opening by anisotropic deposition methods, for example, PECVD or HDP silicon oxide deposition, that deposits predominantly to the bottom of the opening than to the sidewalls. An etchant can be applied to etch away any residual top AR film material that extends along the sidewalls of the opening, for example by dry etch using the first etchant and holding the wafer substrate at a tilt angle and rotated about the axis parallel to the incoming ion beam. Light guide material is then formed in the openings, for example by silicon nitride PECVD. Color filters may be formed over the light guide and a portion of the support film between adjacent color filters and a further portion between adjacent light guides may be etched to create the structure shown in
As shown in
A pixel array may use the thinner top AR film 236a for green pixels only while the thicker top AR film 236b for both blue and red pixels. Alternately, the pixel array may use the thinner top AR film 236a for both green and red pixels while the thicker top AR film 236b for blue pixels only.
Another embodiment to provide two different AR stacks that each optimizes for a different color region can be provided by creating different second AR film thicknesses while keeping the same top AR film thickness. Two different thicknesses are determined, one for each color region. The second AR film is first deposited to the larger thickness. Subsequently a lithography mask is applied to create a mask opening over the pixels that uses the smaller second AR film thickness. An etching step is applied to thin the second AR film under the mask opening to the smaller thickness. Subsequent steps are identical to
While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.
The subject technology is further described in Annex 1, which is incorporated herein.
Claims
1. An image sensor pixel, comprising:
- a substrate;
- a photoelectric conversion unit supported by the substrate;
- an protection film extending over and across the substrate; and,
- a cascaded light guide wherein a first portion of said cascaded light guide is between the protection film and the substrate and a second portion extends above the protection film.
2. The pixel of claim 1, wherein each cascaded light guide includes a transparent portion and a color filter that is contiguous with said transparent portion and extends above said insulator.
3. The pixel of claim 1, wherein the second portion includes a color filter.
4. The pixel of claim 3, wherein the color filters from cascaded light guides of adjacent pixels are separated by a first air gap that has a width no more than 0.45 um.
5. The pixel of claim 4, wherein said first air gap has a depth at least 1.0 times a wavelength of light.
6. The pixel of claim 5, where said wavelength of light is 450 nm.
7. The pixel of claim 3, wherein the color filter self-aligns within the cascaded light guide.
8. The pixel of claim 3, wherein the top surface of the color filter is an air interface.
9. (canceled)
10. The pixel of claim 1, wherein all optical interfaces along the vertical axis of the cascaded light guide and above the protection film are flat and parallel.
11. (canceled)
12. The pixel of claim 1, wherein at least two cascaded light guides from two different pixels have different cross-sectional profiles.
13. The pixel of claim 1, wherein the vertical centerlines of the first and second portions are mutually offset.
14. The pixel of claim 13, wherein the cascaded light guide is configured so that light exits and re-enters said cascaded light guide.
15. A method for fabricating an image sensor pixel, comprising:
- forming a support film with an opening and over a substrate that supports a photoelectric conversion unit; and,
- forming a color filter in the opening of the support film.
16. The method of claim 15, further comprising forming a protection film between the color filter and the substrate.
17. The method of claim 15, further comprising removing at least a portion of the support film between two adjacent color filters.
18. The method of claim 15, further comprising forming a transparent light guide in the opening of the support film.
19. The method of claim 18, further comprising removing a portion of the support film adjacent the transparent light guide.
20. The method of claim 18, further comprising forming a lower transparent light guide between the transparent light guide and the substrate.
21. The method of claim 20, where the vertical centerline of the lower transparent light guide is offset from the vertical centerline of the opening of the support film.
22. The method of claim 18, wherein the forming of the color filter creates a flat air interface on a top surface of the color filter.
23. A method for fabricating an image sensor pixel array, comprising:
- forming an insulator over a substrate that supports a photoelectric conversion unit;
- forming a plurality of walls adjacent to the insulator;
- forming a plurality of light guides between the walls;
- forming a plurality of color filters adjacent to the light guides; and,
- removing at least a portion of the walls so that there is an air gap between adjacent color filters.
24. The method of claim 23, wherein the walls are formed by forming a support film and creating openings within the support film.
25. The method of claim 23, further comprising forming a protection film over the insulator.
26. The method of claim 23, wherein a portion of the support film is removed so that a portion of each light guide has an air interface.
27. An image sensor pixel, comprising:
- a substrate;
- a photoelectric conversion unit supported by said substrate;
- a light guide coupled to said photoelectric conversion unit;
- anti-reflection means for reducing reflection between said light guide and said photoelectric conversion unit.
28. The pixel of claim 27, wherein said anti-reflection means includes a first anti-reflection film and a second anti-reflection film, said first anti-reflection film having an index of refraction lower than an index of refraction of said second anti-reflection film and an index of refraction of said light guide, and said first anti-reflection film located between the second anti-reflection film and the light guide.
29. The pixel of claim 28, wherein said anti-reflection means includes a third anti-reflection film which has an index of refraction lower than an index of refraction of said second anti-reflection film and wherein said second anti-reflection film is between said first anti-reflection film and said third anti-reflection film.
30. The pixel of claim 28, wherein a first pixel has a thinner anti-reflection film than a corresponding anti-reflection film of a second pixel having a color filter of a different color.
31. (canceled)
32. (canceled)
33. The pixel of claim 28, wherein said second anti-reflection film is a contact etch stop.
34. The pixel of claim 28, wherein said second anti-reflection film includes silicon nitride.
35. The pixel of claim 27, further comprising a light-guide etch-stop layer between said light guide and said anti-reflection means.
36. A method for fabricating an image sensor pixel, comprising:
- forming an anti-reflection stack on a photoelectric conversion unit supported by a substrate; and,
- forming a light guide adjacent to the photoelectric conversion unit.
37. (canceled)
38. (canceled)
39. A method for forming a portion of an image sensor pixel, comprising:
- forming a first anti-reflection film over a substrate that supports a photoelectric conversion unit;
- forming an insulator over said first anti-reflection film;
- etching an opening in the insulator with an etchant that etches the insulator faster than the first anti-reflection film;
- forming a second anti-reflection film within the opening; and
- forming light guide material within the opening.
40. The method of claim 39, further comprising etching a vertical sidewall portion of the second anti-reflection film.
41. (canceled)
42. A method for fabricating a color filter for an image sensor pixel, comprising:
- forming at least one wall;
- forming a color filter within the wall;
- removing at least a portion of the wall.
Type: Application
Filed: Dec 22, 2008
Publication Date: Nov 11, 2010
Inventor: Hiok-Nam TAY (Singapore)
Application Number: 12/810,998
International Classification: H01L 31/0232 (20060101); H01L 31/18 (20060101);