U.S. patent application number 12/810998 was filed with the patent office on 2010-11-11 for light guide array for an image sensor.
Invention is credited to Hiok-Nam TAY.
Application Number | 20100283112 12/810998 |
Document ID | / |
Family ID | 40796956 |
Filed Date | 2010-11-11 |
United States Patent
Application |
20100283112 |
Kind Code |
A1 |
TAY; Hiok-Nam |
November 11, 2010 |
Light Guide Array for An Image Sensor
Abstract
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.
Inventors: |
TAY; Hiok-Nam; (Singapore,
SG) |
Correspondence
Address: |
Hiok Nam Tay
Blk 409 Woodlands Street 41#13-109
Singapore
730409
SG
|
Family ID: |
40796956 |
Appl. No.: |
12/810998 |
Filed: |
December 22, 2008 |
PCT Filed: |
December 22, 2008 |
PCT NO: |
PCT/US08/88077 |
371 Date: |
June 28, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12218749 |
Jul 16, 2008 |
|
|
|
12810998 |
|
|
|
|
61009454 |
Dec 28, 2007 |
|
|
|
61069344 |
Mar 14, 2008 |
|
|
|
61063301 |
Feb 1, 2008 |
|
|
|
61062773 |
Jan 28, 2008 |
|
|
|
61069344 |
Mar 14, 2008 |
|
|
|
61063301 |
Feb 1, 2008 |
|
|
|
61062773 |
Jan 28, 2008 |
|
|
|
61009454 |
Dec 28, 2007 |
|
|
|
Current U.S.
Class: |
257/432 ;
257/E31.127; 438/70 |
Current CPC
Class: |
H01L 27/14685 20130101;
H01L 27/14625 20130101; H01L 27/14621 20130101; H01L 27/14687
20130101; H01L 27/14623 20130101; H01L 27/14643 20130101; H01L
27/14629 20130101 |
Class at
Publication: |
257/432 ; 438/70;
257/E31.127 |
International
Class: |
H01L 31/0232 20060101
H01L031/0232; H01L 31/18 20060101 H01L031/18 |
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.
Description
REFERENCE TO CROSS-RELATED APPLICATIONS
[0001] 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 INVENTION
[0002] 1. Field of the Invention
[0003] The subject matter disclosed, generally relates to
structures and methods for fabricating solid state image
sensors.
[0004] 2. Background Information
[0005] 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.
[0006] 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.
[0007] FIG. 1A shows a sectional view of a prior art solid-state
image sensor 1 showing adjacent pixels in a CMOS type sensor,
reproduced from U.S. Pat. No. 7,119,319. Each pixel has a
photoelectric conversion unit 2. Each conversion unit 2 is located
adjacent to a transfer electrode 3 that transfers charges to a
floating diffusion unit (not shown). The structure includes wires 4
embedded in an insulating layer 5. The sensor typically includes a
flattening layer 6 below the color filter 8 to compensates for top
surface irregularities due to the wires 4, since a flat surface is
essential for conventional color filter formation by lithography. A
second flattening layer 10 is provided above the color filter 8 to
provide a flat surface for the formation of microlens 9. The total
thickness of flattening layers 6 and 10 plus the color filter 8 is
approximately 2.0 um.
[0008] 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.
[0009] The microlens 9 focuses light onto the photo photoelectric
conversion units 2. As shown in FIG. 1B because of optical
diffraction, the microlens 9 can cause diffracted light that
propagates to nearby photoelectric conversion units and create
optical crosstalk and light loss. The amount of cross-talk
increases when there is a flattening layer above or below the color
filter, positioning the microlens farther away from the light
guide. Light can crosstalk into adjacent pixels by passing through
either flattening layer (above or below color filter) or the color
filter's sidewall. Metal shields are sometimes integrated into the
pixels to block cross-talking light. In addition, alignment errors
between microlens, color filter, and light guide also contribute to
crosstalk. The formation, size, and shape of the microlens can be
varied to reduce crosstalk. However, extra cost must be added to
the precise microlens forming process, and crosstalk still cannot
be eliminated.
[0010] Backward reflection from the image sensor at the substrate
interface is another issue causing loss of light reception. As
shown in FIG. 1A, the light guide is in direct contact with the
silicon. This interface can cause undesirable backward reflection
away from the sensor. Conventional anti-reflection structures for
image sensors include the insertion of a oxide-plus-nitride
dual-layer film stack directly above the silicon substrate, or a
oxynitride layer having variation of nitrogen-to-oxygen ratio
there, but only reduces reflection between the silicon substrate
and a tall oxide insulator. This approach is not applicable when
the interface is silicon substrate and a nitride light guide.
BRIEF SUMMARY OF THE INVENTION
[0011] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1A is an illustration showing a cross-section of two
image sensor pixels of the prior art;
[0013] FIG. 1B is an illustration showing light cross-talk between
adjacent pixels of the prior art;
[0014] FIG. 2 is an illustration showing a cross-section of two
image sensor pixels of the present invention;
[0015] FIG. 3A is an illustration showing light traveling along an
air gap between two color filters;
[0016] FIG. 3B is an illustration showing the redirection of light
from the air gap into the color filters;
[0017] FIG. 3C is a graph of light power versus the distance along
the air gap;
[0018] FIG. 3D is a graph of gap power loss versus gap width versus
distance along the air gap of widths 0.6 um and 1.0 um for three
different colors;
[0019] FIG. 3E is a graph of maximal gap power loss versus gap
width at a depth of 1.0 um;
[0020] FIG. 3F is a table of maximal gap power loss for different
gap widths at a depth of 1.0 um;
[0021] FIG. 3G is a table of gap area as percentage of pixel area
for different gap widths and different pixel pitches;
[0022] FIG. 3H is a table of pixel power loss for different gap
widths and different pixel pitches;
[0023] FIG. 3I is a graph of pixel power loss versus pixel pitch
for different gap widths;
[0024] FIGS. 4A-L are illustrations showing a process used to
fabricate the pixels shown in FIG. 3;
[0025] FIG. 5 is an illustration showing ray traces within the
pixel of FIG. 2;
[0026] FIG. 6A is an illustration showing a pixel at a corner of
the array;
[0027] FIG. 6B is an illustration showing light ray traces within
the pixel of FIG. 6A;
[0028] FIG. 7 is an illustration showing a top view of four pixels
within an array;
[0029] FIG. 8 is an alternate embodiment of the sensor pixels with
ray tracing;
[0030] FIGS. 9A-M are illustrations showing a process used to
fabricate the pixels shown in FIG. 8;
[0031] FIGS. 10A-H are illustrations showing a process to expose a
bond pad;
[0032] FIG. 11 is an illustration showing an anti-reflection stack
within the sensor;
[0033] FIGS. 12A-E are illustrations showing an alternate process
to form an anti-reflection stack within the sensor;
[0034] FIG. 13A is a graph of transmission coefficient versus light
wavelength for an anti-reflection stack;
[0035] FIG. 13B is a graph of transmission coefficient versus light
wavelength for the anti-reflection stack;
[0036] FIG. 13C is a graph of transmission coefficient versus light
wavelength for the anti-reflection stack;
[0037] FIGS. 14A-G are illustrations showing an alternate process
to form two anti-reflection stacks within the sensor;
[0038] FIG. 15A is a graph of transmission coefficient versus light
wavelength for a first anti-reflection stack on a left hand portion
of FIG. 14G;
[0039] FIG. 15B is a graph of transmission coefficient versus light
wavelength for a second anti-reflection stack shown on a right hand
portion of FIG. 14G.
DETAILED DESCRIPTION
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] Referring to the drawings more particularly by reference
numbers, FIGS. 2, 4A-L, 5 and 6A-B show embodiments of two adjacent
pixels in an image sensor 100. Each pixel includes a photoelectric
conversion unit 102 that converts photonic energy into electrical
charges. In a conventional 4T pixel, electrode 104 may be a
transfer electrode to transfer the charges to a separate sense node
(not shown). Alternately, in a conventional 3T pixel, electrode 104
may be a reset electrode to reset the photoelectric conversion unit
102. The electrodes 104 and conversion units 102 are formed on a
substrate 106. The sensor 100 also includes wires 108 that are
embedded in an insulating layer 110.
[0045] 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 FIG. 4B, each first light guide 116 may have a sidewall 118 that
slopes at an angle .alpha. relative to a vertical axis. The angle
.alpha. is selected to be less than 90-asin(n.sub.insulating
layer/n.sub.light guide), preferably 0, so that there is total
internal reflection of light within the guide, wherein
n.sub.insulating layer and n.sub.light guide are the indices of
refraction for the insulating layer material and light guide
material, respectively. The light guides 116 internally reflect
light from the second light guide 130 to the conversion units
102.
[0046] 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
FIG. 6A, wherein the centerline C2 of the second light guide 130 is
offset from the centerline C1 of the first light guide 116 or of
the photoelectric conversion unit 102. The offset may vary
depending upon the pixel position within an array. For example, the
offset may be greater for pixels located at the outer portion of
the array. The offset may be in the same lateral direction as the
incident light to optimize reception of light by the first light
guide. For incident light arriving at a nonzero angle relative to
the vertical axis, offset second light guides 130 pass on more
light to the first light guides 116. Effectively second light guide
130 and first light guide 116 together constitute a light guide
that takes different vertical cross-section shapes at different
pixels. The shape is optimized to the incident light ray angle at
each pixel.
[0047] FIGS. 5 and 6B illustrate ray tracing for a pixel at the
center of an array and at a corner of the array, respectively. In
FIG. 5, incident light rays come in vertically. The second light
guides 130 are centered to the first light guides 116. Both light
rays a and b reflect once within the second light guide 130 then
enter the first light guide 116, reflects once (ray a) or twice
(ray b) and then enter conversion units 102. In FIG. 6B, the second
light guides 130 are offset to the right, away from the center of
the array, which is towards the left.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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. FIG. 3A shows a
vertical gap between two color filter regions filled with a lower
refractive index medium, e.g. air. Incident light rays entering the
gap and nearer one sidewall than the other is diverted towards and
into the former, whereas the rest are diverted towards and into the
latter. FIG. 3B shows wavefronts spaced one wavelength apart.
Wavefronts travel at slower speed in higher refractive index
medium, in this example the color filter having an index n of
approximately 1.6. Thus the spacing between wavefronts in the gap,
assuming air filled, is 1.6 times that of the color filter,
resulting in the bending of wavefronts at the interface between the
color filter and air gap and causing the light rays to divert into
the color filter. FIG. 3C is a graph of propagated light power P(z)
along a vertical axis z of the air gap divided by the incident
light power P(0) versus a distance z. As shown by FIG. 3C, light
power decreases deeper into the gap for different gap widths, more
rapidly for lesser gap widths on the order of one wavelength and
converges to be essentially negligible for a gap width of 0.4 times
wavelength or less at a depth of 1.5 times wavelength. From FIG.
3C, it is preferable to have a depth equal to at least 1 times the
wavelength of the longest wavelength of interest, which is 650 nm
in this embodiment for a visible light image sensor. At this depth,
the percentage of light power incident upon the gap and lost to the
space further below (henceforth "gap loss") is less than 15%. The
color filter thus needs to have thickness at least 1 time the
wavelength in order to filter incident light entering the gap to
prevent unfiltered light from passing on to light guides 130, 114
and eventually to the conversion unit 102. If the gap is filled
with a transparent medium other than air, with refractive index
n.sub.gap>1.0, then presumably the gap would need to narrow to
0.45 um/n.sub.gap or less, since effectively distances in terms of
wavelength remains the same but absolute distances are scaled by
1/n.sub.gap.
[0053] Referring to FIG. 3C, for red light of wavelength in air of
650 nm, at a depth of 0.65 um (i.e. 1.0 time wavelength in air) the
gap power flux attenuates to 0.15 (15%) for a gap width of 0.6 time
wavelength in air, i.e. 0.39 um. Attenuation reaches maximum at
around 1 um of depth. Attenuation is steeper with depth for shorter
wavelengths.
[0054] FIG. 3D shows the gap loss versus gap width W for 3
colors--blue at 450 nm wavelength, green at 550 nm, and red at 650
nm--at depths of 0.6 um and 1.0 um, respectively. For a depth of
1.0 um, the highest gap loss among the 3 colors and the maximal gap
loss for gap widths of 0.2 um to 0.5 um are plotted in FIG. 3E. Gap
loss against gap width is tabulated in FIG. 3F. In FIG. 3G, gap
area as percentages of pixel areas is tabulated against pixel pitch
and gap width. Each entry (percentage gap area) in the table of
FIG. 3G is multiplied with the corresponding column entry (i.e. gap
loss) to give pixel loss as tabulated in FIG. 3H. FIG. 3I plots
pixel loss versus pixel pitch for different gap widths ranging from
0.2 um to 0.5 um.
[0055] FIG. 3I shows that keeping gap width below 0.45 um would
result in less than 8% pixel loss for pixel pitch between 1.8 um
and 2.8 um--the range of pixel sizes for compact cameras and camera
phones--for color filter thickness of 1.0 um. For less than 3%, a
gap width below 0.35 um is needed; for less than 1.5%, a gap width
below 0.3 um; and for less than 0.5%, a gap width below 0.25 um.
FIG. 3I also shows that pixel loss is less for bigger pixels given
the same gap width. Thus for pixels larger than 5 um, the above
guidelines result in at least halving the pixel loss.
[0056] Referring to FIGS. 2 and 5 again, it is clear that the first
air-gap 422 prevents crosstalk from the color filter of one pixel
to an adjacent pixel by internal reflection. Thus the color filters
114B, 114G each functions like a light guide. Together, the color
filter, the second light guide, and the first light guide along ray
a in FIG. 5 are cascaded together to capture incident light and
convey to the photoelectric conversion unit 102 while minimizing
loss and crosstalk. Unlike prior art which uses metal walls or
light absorbing walls between color filters to reduce crosstalk, at
the expense of losing light that impinging on such walls, the first
air-gap 422 achieves negligible gap loss by diverting light to the
nearest color filter. And since there is no underlying flattening
layer below the color filters that bridges between adjacent light
guides like in prior art (see FIG. 1B), the associated crosstalk is
also eliminated.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] FIGS. 4A-L show a process for forming the image sensor 100.
The sensor may be processed to a point wherein the conversion units
102 and electrodes 104 are formed on the silicon substrate 106 and
the wires 108 are embedded in the insulator material 110 as shown
in FIG. 4A. The insulator 110 may be constructed from a low
refractive index ("RI") material such as silicon dioxide (RI=1.46).
The top of the insulator 110 can be flattened with a chemical
mechanical polishing process ("CMP").
[0066] As shown in FIG. 4B, insulating material may be removed to
form light guide openings 120. The openings 120 have sloping
sidewalls at an angle .alpha.. The openings 120 can be formed, by
example, using a reactive ion etching ("RIE") process. For silicon
oxide as the insulating material, a suitable etchant is
CF.sub.4+CHF.sub.3 in a 1:2 flow ratio, carried in Argon gas under
125 mTorr, 45.degree. C. The sidewall angle may be adjusted by
adjusting the RF power between 300 W and 800 W at 13.56 MHz.
[0067] FIG. 4C shows the addition of light guide material 122. By
way of example, the light guide material 122 can be a silicon
nitride that has an index of refraction of 2.0, greater than the
refractive index of the insulating material 110 (e.g. silicon
oxide, RI=1.46). Additionally, silicon nitride provides a diffusion
barrier against H.sub.2O and alkali metal ions. The light guide
material can be added for example by plasma enhanced chemical vapor
deposition ("PECVD").
[0068] 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 H.sub.2O 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
H.sub.2O and alkali metal ions. The protection film 410 may be
between 10,000 Angstroms and 4,000 Angstroms thick, preferably
7,000 Angstroms.
[0069] A shown in FIG. 4D a support film 134 is formed on top of
the silicon nitride. The support film 134 may be silicon oxide
deposited by High Density Plasma ("HDP").
[0070] In FIG. 4E, the support film is etched to form openings. The
openings may include sidewalls 136 that slope at an angle .beta..
The angle .beta. is selected so that
.beta.<90-asin(1/n2.sub.light guide), where n2.sub.light guide
is the index of refraction of the second light guide material 130,
such that there is a total internal reflection within the second
light guides 130. Incorporating two separate lights guides reduces
the etching depth for each light guide. Consequently, slope side
wall etching is easier to achieve with higher accuracy. The support
film 134 and second light guides 130 can be made from the same
materials and with the same processes as the insulating layer 110
and first light guides 116, respectively.
[0071] As shown in FIG. 4E the sidewall may have a vertical portion
and a sloped portion. The vertical portion and sloped portion can
be achieved by changing the etching chemistry or plasma conditions
during the etching process. The etch recipe during the vertical
portion etch is selected to be favorable for forming the vertical
sidewall 162, then switched to a recipe favorable for forming the
sloped sidewall.
[0072] FIG. 4F shows the addition of light guide material. By way
of example, the light guide material can be a silicon nitride
deposited for example by plasma enhanced chemical vapor deposition
("PECVD").
[0073] FIG. 4G shows each second light guide 130 has a pocket 210.
The pockets 210 are separated by a support wall 212, being part of
the support film 134. Pocket 210 is form by etching down light
guide material to expose the wall 212 and further till the top
surface of light guide is below the top surface of the wall 212 by
between 0.6 um to 1.2 um.
[0074] As shown in FIG. 4H, a color film material 114B having a dye
of a particular color is applied to fill the pockets 210 and
extends above the support film 134. In this example, the color
material may contain blue dye. Color filter material is typically
made of negative photoresist, which forms polymers that when
exposed to light becomes insoluble to a photoresist developer. A
mask (not shown) is placed over the material 114B with openings to
expose areas that are to remain while the rest is etched away.
[0075] FIG. 4I shows the sensor after the etching step. The process
can be repeated with a different color material such as green or
red to create color filters for each pixel as shown in FIG. 4J. The
last color material applied fills the remaining pockets 210, thus
requires no mask step. In other words, exposure light is applied
everywhere on the image sensor wafer to exposure the last color
filter film everywhere. During the bake step, the last color filter
forms a film that overlaps all pixels, include pixels of other
colors. The overlap of the last color filter on other pixels is
removed during a subsequent color filter etch-down process shown in
FIG. 4K.
[0076] Referring to FIG. 4G, the pockets 210 provide an
self-alignment feature to self-align the color filter material with
the second light guide 130. The pockets 210 may be wider than the
corresponding mask openings. To reduce the thickness of the support
wall 212 for an desired second light guide opening for a given
pixel pitch, the pressure in the plasma chamber may be increased to
enhance sideway (i.e. isotropic) etch (by increasing ion
scattering) to undercut the mask.
[0077] As shown in FIG. 4K the color filters 114B, 114G are etched
down to expose the support wall 212, being part of the support film
134. A portion of the support film 134 is then removed as shown in
FIG. 4L so that there is an air/material interface for the color
filters 114B, 114G. A further portion of the support film 134 may
be removed as shown in FIG. 4L so that there is an air/material
interface for the second light guide 130 to further aid internal
reflection by allowing light rays closer to the perpendiculars to
the interface to undergo total internal reflection. The first gap
422 has a width sufficiently small, 0.45 um or less, so that
incident red light and light of lesser wavelengths impinging into
the first gap 422 is diverted to either color filter 114B or 114G,
thus improving light reception. Light internally reflects along the
color filters 114B, 114G and light guides 130 and 116. The color
filters 114B, 114G have a higher refractive index than air so that
the color filters 114B, 114G provides internal reflection.
Likewise, the second light guide 130 has an air interface which
improves the internal reflection properties of the guide. If the
support film 134 is not completely removed, as long as the support
film has a lower refractive index (e.g. silicon oxide, 1.46) than
the light guide material (e.g. silicon nitride, 2.0), the interface
between the second light guide 130 and the support film 134 has
good internal reflection. Likewise, the interface between the first
light guide 116 and the first insulator film 110 enjoys good
internal reflection. FIG. 7 is a top view showing four pixels 200
of a pixel array. For embodiments that include both first and
second light guides the area B may be the area of the second light
guide top surface and the area C represents the area of the first
light guide bottom surface. The area A minus the area B may be the
area of the first air gap 422 between color filters.
[0078] FIG. 8 shows an alternate embodiment wherein the second and
first light guides are both etched using the same mask after the
support film 134 is formed, and both filled with light guide
material in one step. A process for fabricating this alternate
embodiment is shown in FIGS. 9A-M. The process is similar to the
process shown in FIGS. 4A-L, except the opening for the first light
guide is formed after the opening for the second light as shown in
FIG. 9F, where no additional mask is needed because the protective
film 410 and the support film 134 above act as hard masks to block
etchants. Both light guides are filled in the same step shown in
FIG. 9G.
[0079] FIGS. 10A-H show a process to expose bond pads 214 of the
image sensor. An opening 216 is formed in a first insulator
material 110 that covers a bond pad 214 as shown in FIGS. 10A-B. As
shown in FIGS. 10C-D the first light guide material 116 is applied
and a substantial portion of the material 116 is removed, leaving a
thinner layer to seal the first insulator material 110 below. The
support film material 134 is applied and a corresponding opening
218 is formed therein as shown in FIGS. 10E-F. The second light
guide material 130 is applied as shown in FIG. 10G. As shown in
FIG. 10H a maskless etch step is used to form an opening 220 that
exposes the bond pad 214. The etchant preferably has a
characteristic that attacks light guide material 116 and 130 (e.g.
silicon nitride) faster than the insulator material 110 and 134
(e.g. silicon oxide) and color filter 114 (photoresist). Dry etch
in CH.sub.3F/O.sub.2 has 5x.about.10x greater etch rate on silicon
nitride than on color filter or silicon oxide.
[0080] FIG. 11 shows an embodiment wherein an anti-reflection (AR)
stack comprising a top AR film 236, a second AR film 234, and a
third AR film 236 covers the conversion units 102. The
anti-reflection stack improves the transmission of light from the
first light guide 116 to the conversion units 102. Members of the
AR stack together may constitute layer 230 that also blanket the
substrate 106, conversion units 102 and electrodes 104 to protect
the elements from chemical pollutants and moisture. For example,
the second AR film 234 may be a contact etch-stop nitride film
common in CMOS wafer fabrication for stopping the oxide etching of
contact holes to prevent over-etch of polysilicon contacts whose
contact holes are shallower than source/drain contacts by typically
2,000 Angstroms. The third AR film 232 may be silicon oxide. This
silicon oxide film may be a gate insulating film under the gate
electrode 114, or the spacer liner oxide film that runs down the
side of the gate electrode 114 between the gate and the spacer (not
shown) in common deep submicron CMOS processes, a silicide-blocking
oxide film deposited before contact silicidation to block contact
siliciding, or a combination thereof, or a blanket oxide film
deposited after salicide-block oxide etch that etches away all
oxide in areas coinciding with the bottom of light guides 116.
Using an existing silicon nitride contact etch-stop film as part of
the AR stack provides cost savings. The same contact etch-stop film
also functions to stop the etch of the opening in insulator 110 for
fabrication of the light guide. Finally, the top AR film 236 is
formed in the opening in the insulator 110 prior to filling the
opening with light guide material.
[0081] 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.
[0082] 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 (Si.sub.3N.sub.4),
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 FIG. 2, as shown
in FIG. 3 of U.S. Application 61/009,454. The third AR film 232 may
further comprise gate liner oxide as shown in FIG. 3 of the same.
Alternately, the third AR film 232 may be formed by a blanket
silicon oxide deposition everywhere on the wafer after a
salicide-block etch removes salicide-block oxide 64, gate-liner
oxide 55, and gate-oxide 54 shown in FIG. 2 of U.S. Application
61/009,454 by using a salicide-block-etch mask having a mask
opening coinciding with the bottom of light guide 116.
[0083] The anti-reflection structure shown in FIG. 11 can be
fabricated by first forming the third AR film 232 and the second AR
film 234 over the substrate, respectively. The insulator 110 is
then formed on the second AR film 234. Silicon nitride film is
deposited by PECVD on the first insulator 110 in a manner that
covers and seals the insulator and underlying layers to form a
protection film 410 with a thickness between 10,000 Angstrom and
4,000 Angstrom, preferably 7,000 Angstrom. The support film 134 is
formed on the protection film 410 by, for example, HDP silicon
oxide deposition.
[0084] 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 CHF.sub.3, 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 CH.sub.3F/O.sub.2. 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 FIG. 5.
[0085] FIGS. 12A-E show a process for fabricating another
embodiment of anti-reflection between the light guide 116 and
substrate 202. Referring to FIG. 12E, in this embodiment an
etch-stop film 238 is interposed between the light guide 116 and
the anti-reflection (AR) stack comprising the top AR film 236,
second AR film 234, and third AR film 232. The light guide
etch-stop film 238 may be formed of the same material as the light
guide 116, and may be silicon nitride, with a thickness between 100
Angstrom and 300 Angstrom, preferably 150 Angstrom. Forming the AR
stack in this embodiment has an advantage of more precise control
of the thickness of the second AR film, at the expense of one more
deposition step and the slight added complexity of etching through
a oxide-nitride-oxide-nitride-oxide stack instead of
oxide-nitride-oxide stack for contact hole openings (not shown).
The previous embodiment uses the second AR film 234 as a light
guide etch stop and loses some of thickness to the final step of
insulator pit etch over-etch.
[0086] As shown in FIGS. 12A-B, the third 232 and second 234 AR
films are applied to the substrate 106 and then a top AR film 236
is applied onto the second AR film 234, followed by a light guide
etch-stop film 238 made of silicon nitride. As shown in FIG. 12C,
the insulator layer 110 and wiring electrodes 108 are formed above
the AR films 232, 234, and 236, and light guide etch-stop film 238.
FIG. 12D shows an opening etched into insulator 110, stopping at
the top of the light guide etch-stop film 238. FIG. 12E shows the
opening filled with light guide material.
[0087] FIG. 13A is a graph of transmission coefficient versus light
wavelength for the anti-reflection stack of FIG. 11 and FIG. 12E,
for top AR film 236 (oxide) nominal thickness of 800 Angstroms, and
varied +/-10%, whereas second AR film 234 (nitride) thickness is
500 Angstroms and third AR film 232 (oxide) thickness is 75
Angstroms. The transmission curves exhibit steep decline in the
violet color region (400 nm to 450 nm). The nominal thicknesses of
the AR films 232, 234, and 236 constituting the AR stack are chosen
to position the maximum of the transmission curve in the blue color
region (450 nm to 490 nm) instead of green color region (490 nm to
560 nm) so that any shift in film thicknesses due to manufacturing
tolerance would not result in transmission coefficient fall-off
much more in violet than in red color region (630 nm to 700
nm).
[0088] FIG. 13B is a graph of transmission coefficient versus light
wavelength for the anti-reflection stack of FIG. 11 and FIG. 12E,
for nominal second AR film (nitride) of 500 Angstroms thick, and
varied +/-10%.
[0089] FIG. 13C is a graph of transmission coefficient versus light
wavelength for the anti-reflection stack of FIG. 11 and FIG. 12E,
for third AR film 232 (nitride) nominal thickness of 75 Angstroms,
and varied +/-10%.
[0090] FIGS. 14A-G show a process for fabricating another
embodiment of anti-reflection stack between the light guides 116
and substrate 202 to provide two different AR stacks at two
different pixels that each optimizes for a different color region.
Third and second AR film 232 and 234 are provided over the
photoelectric conversion unit 201 in FIG. 14A, similar to the
embodiment shown in FIG. 12A. In FIG. 14A, the top AR film 236 is
deposited to the thickness of thicker top AR film 236b shown in
FIG. 14B. Subsequently a lithography mask (not shown) is applied to
create mask openings over the pixels that use the thinner top AR
film 236a. An etch step is applied to thin the top AR film 236
under the mask opening to the smaller thickness of top AR film 236a
in FIG. 14B. Subsequent steps, shown in FIGS. 14C to 14G, are
similar to FIGS. 12B-E. Green color filters 114G is applied on the
pixels having the thinner top AR film 236a, whereas Blue and Red
color filters on the pixels having the thicker top AR film
236b.
[0091] FIG. 15A is a graph of transmission coefficient versus light
wavelength for the anti-reflection stack of FIG. 14G for a nominal
thinner top AR film 236a of nominal thickness 0.12 um, a second AR
film 234 of nominal thickness 500 Angstroms, and a third AR film
232 of nominal thickness 75 Angstroms. This graph peaks in the
green color region at approximately 99%, and drops gently to
approximately 93% at the center of the red color region. This graph
shows that the top AR film 236a can be used at red pixels as well
as green pixels.
[0092] FIG. 15B is a graph of transmission coefficient versus light
wavelength for the anti-reflection stack of FIG. 14G for a top AR
film 236b of nominal thickness 0.20 um, a second AR film 234 of
nominal thickness 500 Angstroms, and a third AR film 232 of nominal
thickness 75 Angstroms. This graph peaks in two separate color
regions, viz. purple and red. This graph shows that the top AR film
236b can be used at blue pixels and red pixels.
[0093] 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.
[0094] 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 FIGS. 12B-E.
[0095] 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.
[0096] The subject technology is further described in Annex 1,
which is incorporated herein.
* * * * *