U.S. patent application number 09/803346 was filed with the patent office on 2001-08-02 for color image sensor with embedded microlens array.
Invention is credited to Abramovich, Irit.
Application Number | 20010010952 09/803346 |
Document ID | / |
Family ID | 23868085 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010010952 |
Kind Code |
A1 |
Abramovich, Irit |
August 2, 2001 |
Color image sensor with embedded microlens array
Abstract
A method for producing a color CMOS image sensor including a
matrix of pixels (e.g., CMOS APS cells) that are fabricated on a
semiconductor substrate. A silicon-nitride layer is deposited on
the upper surface of the pixels, and is etched using a reactive ion
etching (RIE) process to form microlenses. A protective layer
including a lower color transparent layer formed from a polymeric
material, a color filter layer and an upper color transparent layer
are then formed over the microlenses. Standard packaging techniques
are then used to secure the upper color transparent layer to a
glass substrate.
Inventors: |
Abramovich, Irit; (Alon
Hagalil, IL) |
Correspondence
Address: |
BEVER HOFFMAN & HARMS, LLP
2099 GATEWAY PLACE
SUITE 320
SAN JOSE
CA
951101017
|
Family ID: |
23868085 |
Appl. No.: |
09/803346 |
Filed: |
March 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09803346 |
Mar 9, 2001 |
|
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09470558 |
Dec 23, 1999 |
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6221687 |
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Current U.S.
Class: |
438/151 ;
438/156 |
Current CPC
Class: |
H01L 27/14601 20130101;
H01L 27/14685 20130101; H01L 27/14627 20130101 |
Class at
Publication: |
438/151 ;
438/156 |
International
Class: |
H01L 021/00; H01L
021/84 |
Claims
What is claimed is:
1. A method for forming a microlens over an image sensing element
in an image sensor, the method comprising: depositing a dielectric
layer over the image sensing element, the dielectric layer having a
first index of refraction; reactive ion etching the dielectric
layer to form a microlens; and forming a protective layer on the
microlens, the protective layer having a second index of
refraction; wherein the first index of refraction of the dielectric
layer is different from the second index of refraction of the
protective layer.
2. The method according to claim 1, wherein depositing the
dielectric layer comprises depositing silicon nitride.
3. The method according to claim 1, wherein reactive ion etching
comprises: depositing photoresist layer on the dielectric layer;
forming the photoresist layer into a lens-shaped photoresist
portion; and performing an anisotropic reactive ion etching process
such that the lens-shaped photoresist portion is copied into the
dielectric layer, thereby forming the microlens.
4. The method according to claim 1, wherein forming the protective
layer comprises depositing packaging adhesive directly onto the
microlens.
5. The method according to claim 1, wherein forming the protective
layer comprises: depositing a lower color transparent layer over
the microlens; planarizing the lower color transparent layer;
forming a color filter layer on the lower color transparent layer;
and depositing an upper color transparent layer on the color filter
layer.
6. The method according to claim 5, further comprising: applying a
cement layer to the upper color transparent layer; and attaching a
packaging substrate to the cement layer.
7. The method according to claim 1, further comprising attaching a
packaging substrate to the protective layer.
8. The method according to claim 1, further comprising: depositing
a passivation layer over the image sensing element; planarizing the
passivation layer; and depositing an oxi-nitride layer on an upper
surface of the passivation layer, wherein depositing the dielectric
layer over the image sensing element comprises depositing a
silicon-nitride layer on an upper surface of the oxi-nitride
layer.
9. A method for forming a microlens over an image sensing element
in a color image sensor, the method comprising: depositing a
silicon-nitride layer over the image sensing element; etching the
silicon-nitride layer to form a microlens; forming a first color
transparent layer on the microlens; and forming a color filter on
the first color transparent layer.
10. The method according to claim 9, further comprising forming a
second color transparent layer on the color filter.
11. The method according to claim 9, wherein etching comprises:
depositing photoresist layer on the silicon-nitride layer; forming
the photoresist layer into a lens-shaped photoresist portion; and
performing an anisotropic reactive ion etching process such that
the lens-shaped photoresist portion is copied into the
silicon-nitride layer, thereby forming the microlens.
12. The method according to claim 9, further comprising: applying a
cement layer to the upper color transparent layer; and attaching a
packaging substrate to the cement layer.
13. The method according to claim 9, further comprising: depositing
a passivation layer over the image sensing element; planarizing the
passivation layer; and depositing an oxi-nitride layer on an upper
surface of the passivation layer, wherein the silicon-nitride layer
is deposited on an upper surface of the oxi-nitride layer.
14. An image sensor comprising: an image sensing element formed in
a semiconductor substrate; a microlens located over the image
sensing element, the microlens being formed from a dielectric
material having a first index of refraction; and a protective layer
formed on the microlens, the protective layer having a second index
of refraction, wherein a first index of refraction of the
dielectric material is different from the second index of
refraction of the protective layer.
15. The image sensor according to claim 14, wherein the dielectric
material is silicon-nitride.
16. The image sensor according to claim 14, wherein the protective
layer comprises packaging cement.
17. The image sensor according to claim 14, wherein the protective
layer comprises: a lower color transparent layer formed on the
microlens; a color filter layer formed on the lower color
transparent layer; and an upper color transparent layer formed on
the color filter layer.
18. The image sensor according to claim 17, wherein the lower color
transparent layer comprises an acrylic polymer.
19. The image sensor according to claim 17, further comprising a
packaging substrate attached to an upper surface of the upper color
transparent layer.
20. The image sensor according to claim 14, further comprising: a
passivation layer formed over the image sensing element; and an
oxi-nitride layer formed on an upper surface of the passivation
layer, wherein the silicon-nitride layer Is deposited on an upper
surface of the oxi-nitride layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to solid state image sensors.
More specifically, the present invention relates to a method for
fabricating color image sensors and to a color image sensor
fabricated by the method.
RELATED ART
[0002] Solid state color image sensors are used, for example, in
video cameras, and are presently realized in a number of forms
including charge-coupled devices (CCDs) and CMOS image sensors.
These image sensors are based on a two dimensional array of pixels.
Each pixel includes color filter located over a sensing element. An
array of microlenses located over the color filter focuses light
from an optical image through the color filter into the image
sensing elements. Each image sensing element is capable of
converting a portion of the optical image passed by the color
filter into an electronic signal. The electronic signals from all
of the image sensing elements are then used to regenerate the
optical image on, for example, a video monitor.
[0003] FIG. 1(A) is a cross-sectional view showing a portion of a
conventional color image sensor 10. Color image sensor 10 is formed
on an n-type semiconductor substrate 11 having a p-well layer 15.
An array of photodiodes 20 and charge transfer regions 25 are
formed in p-well layer 15, and are covered by a silicon oxide or
nitride film 30. A polysilicon electrode 35 is located over charge
transfer region 25 such that it is surrounded by film 30. A
photo-shielding metal layer 40 is formed over electrode 35, and a
surface protective coating 45 and a planarization layer 50 are
formed over metal layer 40. A color filter layer 60 is formed on
planarization layer 50, and an intermediate transparent film 70 is
formed over color filter layer 60. A microlens 80 for focusing
light beams 85 is formed from silicon dioxide (SiO.sub.2) or a
resin material on intermediate transparent film 70. An air gap 90
is provided over microlens 80, and a glass packaging substrate 95
is located over air gap 90.
[0004] In operation, light beams 85 are focused by microlens 80
through color filter layer 60 such that they converge along the
focal axis F of microlens 80 to strike photodiode 20, wherein
photoenergy from light beams 85 frees electrons in photodiode 20.
When a select voltage is applied to polysilicon electrode 35, these
freed electrons generate a current in charge transfer region 25. A
sensor circuit (not shown) of color image sensor 10 then determines
the amount of light received by photodiode 20 by measuring the
amount of current generated in charge transfer region 25.
[0005] Conventional solid-state imaging device 10 is designed for
light beams 85 whose incident angle is perpendicular to substrate
11, as shown in FIG. 1(A), before being focused by microlens 80
onto photodiode 20. However, during actual operation of color image
sensor 10, light beams can strike microlens 80 at oblique incident
angles. A consequence of these oblique light beams is shown in FIG.
1(B). In particular, light beams 87 enter microlens 80 at an
oblique angle, which directs light beams 87 away from focal axis F
such that they converge at the edge of photodiode 20. Because the
photoenergy of light beams 87 is not fully transferred to
photodiode 20, color image sensor 10 is unable to generate an
accurate image.
[0006] Another problem associated with conventional solid-state
imaging device 10 is that non-standard packaging methods are
required due to the formation of microlenses 80 over color filter
layer 60 and intermediate transfer layer 70. Standard packaging
methods typically include securing a glass substrate to an IC
device using a layer of cement (e.g., epoxy). This cement typically
has an index of refraction that is the same as silicon-dioxide and
other resins typically used to form microlens 80 and other layers
of conventional solid-state imaging device 10. Therefore, to
facilitate proper focusing of the light beams, air gap 90 must be
provided between glass packaging substrate 95 and microlens 80.
Because air gap 90 is used in place of cement, the packaging method
used to produce conventional solid-state imaging device 10 is
non-standard.
[0007] It would be possible to avoid the oblique light beam problem
(discussed above) by moving microlens 80 closer to photodiode 20,
thereby shortening the distance traveled by the light beams between
microlens 80 and photodiode 20. This shortened distance would
reduce the displacement of focused oblique light beams 87 (see FIG.
1(B)) relative to the center of photodiode 20, thereby transferring
more photoenergy from these oblique light beams to photodiode
20.
[0008] One possible method of moving microlens 80 closer to
photodiode 20 would be to reduce the thickness of the various
layers located below microlens 80. A problem with this method is
that the thicknesses of these underlying layers are not easily
reduced. First, photo-shielding layer 40 is typically formed during
the formation of aluminum wiring utilized to transmit signals to
and from each pixel of conventional solid-state imaging device 10.
Therefore, the thickness of photo-shielding layer 40 is limited by
the wiring specifications. Repositioning microlens 80 closer to
photodiode 20 is further restricted by planarization layer 50,
which is required to provide a flat surface for forming color
filter layer 60 and microlens 80. Therefore, it is not possible to
significantly reduce the distance between a surface-mounted
microlens 80 and photodiode 20 in conventional solid-state imaging
device 10 by reducing the thickness of the layers underlying
microlens 80.
[0009] Another possible method of moving microlens 80 closer to
photodiode 20 would be to form microlens So under color filter
layer 60 (i.e., between photodiode 20 and color filter layer 60).
This arrangement would also address the non-standard packaging
problem because, with color filter layer 70-located above microlens
80, it would be possible to use cement to secure glass packaging
substrate 95 according to standard packaging methods. However,
forming microlens 80 under color filter layer 60 is not practical
because, as discussed above, the index of refraction of
conventional microlens materials (i.e., resin) is the same as that
of other materials typically used to produce conventional
solid-state imaging device 10. Therefore, because air gap 90 must
be provided over conventional microlens 80, it would be very
difficult to produce conventional solid-state imaging device 10
with microlens 80 located under color filter layer 60 using
conventional microlens materials.
[0010] What is needed is a method for fabricating a color image
sensor that minimizes the distance between the microlens and
photodiode, and minimizes the fabrication and production costs of
the color image sensor.
SUMMARY
[0011] The present invention is directed to a method for producing
a color CMOS image sensor in which the microlens structure is
embedded (i.e., located between the photodiode array and the color
filter layer), thereby avoiding the oblique light beam problem,
discussed above, because each microlens is located closer to its
associated photodiode than in conventional image sensor structures.
In addition, because the color filter layer is located above the
microlenses and sandwiched between two color transparent layers,
conventional image sensor packaging techniques (i.e., applying
cement to the upper color transparent layer, then applying a glass
substrate) may be utilized to produce color CMOS image sensors.
[0012] In accordance with a first embodiment of the present
invention, an image sensor is produced by depositing a dielectric
(e.g., silicon-nitride) layer over an image sensing element (e.g.,
a photodiode), etching the dielectric layer to form a microlens,
and then depositing a protective layer on the microlens, wherein
the protective layer has an index of refraction that is different
from that of the dielectric. When silicon-nitride is utilized as
the dielectric, conventional protective layer materials may be
formed on the microlens because the refractive index of
silicon-nitride is different from silicon-dioxide and other
materials utilized as conventional protective layer materials.
Therefore, the silicon-nitride microlenses of the present invention
may be embedded under conventional protective materials without
eliminating the optical performance of the microlenses. In
alternative embodiments, other dielectrics may be used to form the
microlens, provided the protective materials formed on the
microlens have an index of refraction that is different from that
of the dielectric. Because the microlens surface is located below a
protective layer, conventional packaging techniques may be used
that attach the protective layer to a substrate using cement,
thereby reducing manufacturing costs and complexity.
[0013] In accordance with another embodiment of the present
invention, a color image sensor is produced by depositing a
silicon-nitride layer over an image sensing element (e.g., a
photodiode), etching the silicon-nitride layer to form a microlens,
depositing a color transparent layer on the microlens, and then
forming a color filter on the color transparent layer. The
silicon-nitride microlens has an index of refraction that is
different from the color transparent layer, thereby forming an
effective microlens structure that is embedded below the color
filter. By forming the microlens below the color filter, the
microlens is positioned closer to the image sensing element,
thereby minimizing the oblique light beam problems, described
above. In addition, by forming a second color transparent layer
over the color filter, conventional packaging techniques may be
used that attach the second color transparent layer to a substrate
using cement, thereby reducing manufacturing costs and
complexity.
[0014] The novel aspects of the present invention will be more
fully understood in view of the following description and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIGS. 1(A) and 1(B) are cross-sectional side views showing a
conventional solid-state imaging device in which normal and oblique
light beams are focused by a microlens;
[0016] FIG. 2 is a schematic diagram of a solid-state imaging
device according to a first embodiment of the present
invention;
[0017] FIG. 3 is a flow diagram showing the basic steps for
fabricating the solid-state imaging device shown in FIG. 2;
[0018] FIG. 4(A) is a schematic diagram of a color image sensor
device according to a second embodiment of the present
invention;
[0019] FIG. 4(B) is a flow diagram showing the basic steps for
fabricating the color image sensor device shown in FIG. 4(A);
and
[0020] FIGS. 5(A) through 5(K) are cross-sectional views showing
process steps associated with the production of a color imaging
device in accordance with another embodiment of the present
invention.
DETAILED DESCRIPTION
[0021] The present invention is described below with reference to
color CMOS active-pixel sensors (APSs), and in particular to color
CMOS APSs utilizing photodiode light sensitive regions. The
fabrication and operation of CMOS active-pixel sensors (APSs) are
described in co-owned and co-pending U.S. application Ser. No.
09/315,893, entitled "Method And Structure For Minimizing White
Spots In CMOS Image Sensors", invented by Yossi Netzer [Docket No.
TSL-031], which is incorporated herein by reference. However, the
methods and structures described below may also be used to produce
passive CMOS image sensors and CMOS APSs utilizing photogate light
sensitive regions. In addition, the methods and structures may be
used to produce CMOS APSs having any number of transistors (e.g.,
one, four or five). Moreover, the present inventors believe the
methods and structures of the present invention may also be used to
produce image sensors including MOS pixel arrays. As used herein,
the general phrase "image sensor" is intended to cover all of these
sensor array types.
[0022] FIG. 2 is a cross-sectional view showing a portion of an
image sensor 100 in accordance with an embodiment of the present
invention. Image sensor 100 includes an image sensing element 110,
a dielectric layer 140 formed over image sensing element 110 that
is etched to include a microlens 145, and a protective layer 150
formed on microlens 145. Image sensing element 110 includes a
photodiode region 114 that is diffused into a silicon substrate
112, and a passivation layer 118 formed on substrate 112. In one
embodiment, dielectric layer 140 is formed on passivation layer
118, and has an index of refraction that is different from that of
protective layer 150, thereby allowing microlens 145 to focus light
beams passing through protective layer 150 onto photodiode region
114. In another embodiment, one or more intermediate layers (e.g.,
oxi-nitride, not shown) are formed between passivation layer 118
and dielectric layer 140.
[0023] FIG. 3 is a flow diagram showing the basic steps associated
with the formation of image sensor 100 in accordance with the
present invention. The process shown in FIG. 3 is performed after
image sensing element 110 (FIG. 2) is fabricated using known
techniques. At the end of this initial fabrication process, image
sensing element 110 includes passivation layer 118 formed over
photodiode region 114.
[0024] Referring to FIG. 3, the process begins with the deposition
of dielectric layer 140 over passivation layer 118 (Step 310). The
term "rover" is intended to cover both the deposition of dielectric
material directly on passivation layer 118, and the deposition of
dielectric material on an intermediate layer(s) formed on
passivation layer 118. In a presently preferred embodiment,
dielectric material is silicon-nitride, which has an index of
refraction that is higher than silicon-dioxide and other materials
typically utilized in CMOS fabrication processes to form protective
layer 150.
[0025] Next, dielectric layer 140 is etched to form microlenses 145
(Step 320). In one embodiment, this step is performed using a
reactive-ion etching process according to known techniques. As
indicated in FIG. 2, the etching process is controlled such that a
portion of dielectric layer 140 remains over passivation layer
118.
[0026] Finally, protective layer 150 is formed over microlens 145
and other residual portions of dielectric layer 140 (Step 330). In
black-and-white image sensors, protective layer 150 may be
polyimide, resin, or may be packaging adhesive (e.g., epoxy cement)
that is applied directly to the upper surface of microlens 145. As
discussed in additional detail below, in color image sensor
applications protective layer 150 may include one or more color
transparent layers and color filter layers. In either of these
applications, at least the portion of protective layer 150 that
contacts microlens 145 is formed using a material having an index
of refraction that is different from (i.e., lower than) that of
dielectric layer 140. By forming protective layer in this manner,
microlens 145 is able to effectively focus light beams onto
photodiode region 114. Further, because microlens 145 is formed
either directly on or immediately over passivation layer 118, the
distance between microlens 145 and photodiode 114 is minimized,
thereby minimizing the problems caused by oblique light beams
(discussed above).
[0027] While Steps 310, 320 and 330 include the basic process steps
for forming an image sensor in accordance with the present
invention, another benefit of image sensor 100 is that conventional
packaging techniques may be utilized. In particular, a packaging
substrate may be attached to protective layer 150 using a packaging
adhesive, such as epoxy cement (Step 340). Alternatively, when
protective layer 150 is formed from packaging adhesive, the
packaging substrate is attached directly to protective layer 150.
Unlike prior art image sensors that require air gaps between the
microlens and the packaging substrate, the present invention
facilitates the use of conventional packaging techniques (i.e.,
applying cement directly onto protective layer 150 or microlens
145, and attaching the packaging substrate directly to the cement),
thereby reducing packaging costs.
[0028] FIG. 4(A) is a cross-sectional view showing a portion of a
color image sensor 200 in accordance with a second aspect of the
present invention. Color image sensor 200 includes an image sensing
element 210, a silicon-nitride layer 240 formed over image sensing
element 210 that is etched to include a microlens 245, a lower
(first) color transparent (CT) layer 252 formed on microlens 245, a
color filter layer 255 formed on lower CT layer 252, and an upper
CT layer 257 formed on color filter layer 255. Similar to image
sensor device 100 (discussed above), image sensing element 210
includes a photodiode region 214 that is formed in substrate 212,
and a passivation region including silicon-dioxide (SiO.sub.2)
layer 218 that is formed on substrate 212.
[0029] Lower CT layer 252, color filter layer 255 and upper CT
layer 257 form a color filter structure (protective layer) 250 over
microlens 245 that functions, in part, to protect microlens 245. In
one embodiment, lower CT layer 252 is formed from a polymeric
material (e.g., negative photoresist based on an acrylic polymer)
having an index of refraction that is lower than that of
silicon-nitride, thereby allowing 8 microlens 245 to focus light
beams passing through lower CT layer 252 onto photodiode region
214. Lower CT layer 252 provides both a planar surface and adhesion
for color filter layer 255. Color filter layer 255 is formed from
known materials (e.g., negative photoresist based on an acrylic
polymer including color pigments) using known techniques. Finally,
upper CT layer 257 is formed from a polymeric material-(e.g.,
negative photoresist based on an acrylic polymer), and serves both
to seal and protect color filter layer 255.
[0030] FIG. 4(B) is a flow diagram showing the basic steps
associated with the formation of color image sensor 200 in
accordance with the present invention. The process shown in FIG.
4(B) is performed after image sensing element 210 (FIG. 4(A)) is
fabricated using known techniques.
[0031] Referring to FIG. 4(B), silicon-nitride layer 240 is
deposited over silicon-dioxide layer 218 (Step 410), and
silicon-nitride layer 240 is etched to form microlenses 245 (Step
420). Next, lower CT layer 252 is formed over microlens 245 and
other residual portions of silicon-nitride layer 240, and is then
planarized using known techniques (Step 430). Color filter layer
255 is then formed on lower CT layer 252 using known techniques
(Step 440). Finally, upper CT layer 257 is formed on color filter
layer 255. Although not shown in FIG. 4(B), a subsequent step of
attaching a packaging substrate to upper CT layer 257 using
conventional packaging techniques is made possible by embedding
microlens 245 below color filter structure 250.
[0032] FIGS. 5(A) through 5(K) illustrate the method of producing
color image sensor 200 in additional detail.
[0033] FIG. 5(A) is a cross-sectional view showing an initial
structure that includes image sensing element 210. Image sensing
element 210 includes photodiode region 214 and a charge transfer
region 215 that are diffused into semiconductor (e.g., silicon)
substrate 212, and base silicon-dioxide (SiO.sub.2) layer 218
formed on substrate 212. Metal wires 220 are located in base
SiO.sub.2 layer 218 that connect to a polysilicon gate region 222
and to charge transfer region 215, thereby forming select
transistor 116. These structures are fabricated using known
techniques.
[0034] FIG. 5(B) illustrates an optional step of depositing a
supplemental passivation (SiO.sub.2) layer 219 on base SiO.sub.2
layer 218, and planarizing supplemental SiO.sub.2 layer 219 to
provide a flat surface for the dielectric material used to form the
embedded microlens. The planarized surface provided by supplemental
SiO.sub.2 layer 219 is not always required (in some cases, base
SiO.sub.2 layer 218 has a sufficiently planar surface). When used,
the thickness of supplemental SiO.sub.2 layer 219 is determined by
the surface features of base SiO.sub.2 layer 118 (e.g., by exposed
wires 220), but made as thin as possible so that the
subsequently-formed microlens structures are as close to photodiode
region 214 as possible.
[0035] FIG. 5(C) illustrates another optional step of depositing an
oxi-nitride layer 230 on planarized supplemental SiO.sub.2 layer
219. Alternatively, oxi-nitride layer 230 may be formed directly on
base SiO.sub.2 layer 219 (i.e., when planarized supplemental
SiO.sub.2 layer 219 is not used). In one embodiment, oxi-nitride
layer 230 has a thickness in the range of 2.5 to 3.5 microns, and
functions as a stress relief layer.
[0036] FIG. 5(D) illustrates a subsequent step of depositing
silicon-nitride layer 240 over image sensing element 210. When both
steps shown in FIGS. 5(B) and 5(C) are used, 8 silicon-nitride
layer 240 is formed on oxi-nitride layer 230. Note that
silicon-nitride layer 240 may be formed on planarized supplemental
SiO.sub.2 layer 219 or base SiO.sub.2 layer 218 if these steps are
respectively omitted. In the present example, silicon-nitride layer
240 has a thickness in the range of 3 to 5 microns.
[0037] FIG. 5(E) is a cross-sectional view showing the formation of
a photoresist portion 510 on silicon-nitride layer 240 and
subsequent application of etchant 520. Photoresist portion 510 is
formed by depositing a layer of photoresist on silicon-nitride
layer 240, exposing the photoresist layer through a mask (either
"halftone" or sharp geometry), developing the photoresist layer,
and removing portions of the photoresist layer that were exposed.
This process is performed using well-known techniques. When a sharp
geometry mask is used, photoresist portion 510 is heated to create
the required lens-shaped geometry using known techniques. This
heating process is not needed when a "halftone" mask is used to
form photoresist portion 510. The resulting photoresist portion 510
has a shape that essentially mirrors than of the desired microlens
and is located directly over the portion of silicon-nitride layer
240 used to form the microlens. The actual shape of photoresist
portion 510 depends upon the selectivity of the photoresist
material versus that of silicon-nitride layer 240. Etching is
subsequently performed using an anisotropic reactive ion etching
(RIE) process that "copies" the lens-like shape of photoresist
portion 510 into silicon-nitride layer 240. That is, the thinner
peripheral portions of photoresist portion 510 are removed before
the thicker central portions, thereby causing more etching of
silicon-nitride layer 240 under the periphery of photoresist
portion 510 than under the central portion. Consequently, the
lens-like shape of photoresist portion 510 is "copied" into
silicon-nitride layer 240.
[0038] FIG. 5(F) is a cross-sectional view showing silicon-nitride
layer 240 after the etching process. The resulting shape of
microlens 240 is essentially the same as that of photoresist
portion 510. In one embodiment, microlens 245 has a peak thickness
T1 in the range of 3 to 5 microns. The remaining portions of
silicon-nitride layer 240 located adjacent to microlens 245 have a
thickness T2 in the range of 0.65 to 1 micron. Residual photoresist
material 515 and other polymeric residues are then removed using a
solvent 530.
[0039] FIG. 5(G) is a cross-sectional view showing the subsequent
deposition and planarization of lower CT layer 252 on microlens 245
and the remaining portions of silicon-nitride layer 240. After
planarization, lower CT layer preferably has a thickness T3 in the
range of 1.1 to 1.3 microns.
[0040] FIG. 5(H) is a cross-sectional view showing the subsequent
formation of color filter layer 255 on lower CT layer 252. Color
filter layer 255 is formed using known techniques and has a
resulting thickness T4 in the range of 1.0 to 1.4 microns.
[0041] FIG. 5(I) is a cross-sectional view showing the subsequent
formation of upper CT layer 257 on color filter layer 255. Upper CT
layer 257 is formed from polymeric material or resin, and has a
resulting thickness T5 in the range of 0.8 to 1.1 microns.
[0042] A benefit provided by the fabrication process illustrated in
FIGS. 5(A) through 5(I) is that standard packaging techniques can
be used, thereby reducing overall production costs. A simplified
representation of these standard packaging techniques is depicted
in FIGS. 5(J) and 5(K). As shown in FIG. 5(J), a transparent cement
540 (e.g., novolac epoxy resin) is applied to an upper surface of
upper color transparent layer 257. Next, as shown in FIG. 5(K), a
packaging substrate 550 (e.g., glass) is mounted onto cement 540,
thereby attaching packaging substrate 550 to color transparent
layer 257. Note that, unlike the prior art structure shown in FIG.
1(A), microlens 245 is embedded between color filter structure 250
and image sensing element 210. Therefore, the present invention
facilitates the use of standard packaging (i.e., attaching
packaging substrate 550 using cement), thereby providing such color
CMOS image sensor devices at a lower cost than conventional
devices.
[0043] Although the invention has been described in connection with
several embodiments, it is understood that this invention is not
limited to the embodiments disclosed, but is capable of various
modifications which would be apparent to a person skilled in the
art. For example, the particular parameters set forth in the above
examples are exemplary, and may be altered to meet the requirements
of particular fabrication processes. Thus, the invention is limited
only by the following claims.
* * * * *