U.S. patent application number 10/185777 was filed with the patent office on 2002-11-28 for method for creating a color microlens.
Invention is credited to Li, Zong-Fu.
Application Number | 20020176037 10/185777 |
Document ID | / |
Family ID | 21978725 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020176037 |
Kind Code |
A1 |
Li, Zong-Fu |
November 28, 2002 |
Method for creating a color microlens
Abstract
The making and use of color microlenses in color image sensors
and color display devices is described and claimed. The color
microlenses combine the function of a colorless microlens and a
color filter into a single structure simplifying the fabrication
of, and increasing the reliability of display devices and image
sensors using the described color microlenses.
Inventors: |
Li, Zong-Fu; (Gilbert,
AZ) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD, SEVENTH FLOOR
LOS ANGELES
CA
90025
US
|
Family ID: |
21978725 |
Appl. No.: |
10/185777 |
Filed: |
June 27, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10185777 |
Jun 27, 2002 |
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09902012 |
Jul 9, 2001 |
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6433844 |
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09902012 |
Jul 9, 2001 |
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09052609 |
Mar 31, 1998 |
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6271900 |
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Current U.S.
Class: |
349/95 ;
257/E31.128 |
Current CPC
Class: |
G01J 3/51 20130101; G02B
3/0012 20130101; G02F 1/133526 20130101; G02B 3/0056 20130101; H01L
27/14627 20130101; G01J 3/14 20130101; G02B 3/0018 20130101; H01L
31/02327 20130101; G01J 3/513 20130101; H01L 27/14621 20130101;
G02B 5/201 20130101 |
Class at
Publication: |
349/95 |
International
Class: |
G02F 001/1335 |
Claims
1. A method for manufacturing a color microlens array comprising:
forming a first colored microlens resist layer on a surface, over a
plurality of photodetecting regions that will define an image
sensing area of an integrated circuit die; patterning the first
colored microlens resist layer; and baking the patterned first
colored microlens resist layer to cause flowing of the patterned
layer resulting in a microlens, with a curved top surface, being
formed above each of the plurality of photodetecting regions.
2. The method of claim 1 further comprising: repeating the forming,
patterning, and baking operations using a second colored microlens
resist layer, the second resist layer to filter a different color
than the first resist layer.
3. The method of claim 1 further comprising: fixing the patterned
first colored microlens resist layer by deep ultraviolet exposure
to cause cross-linking, prior to baking.
4. The method of claim 1 further comprising: stabilizing a surface
of the microlens using silylation.
5. The method of claim 1 further comprising: exposing the microlens
to deep ultra-violet bleaching to improve transparency.
6. The method of claim 1 wherein the depositing, patterning, and
baking operations are repeated three times, each time on a
different colored polymer resist layer.
7. The method of claim 1 wherein the patterning operation is
performed using a photolithographic process.
8. The method of claim 1 further comprising planarizing the surface
prior to forming the first colored microlens resist layer.
9. The method of claim 8 wherein the surface is planarized by spin
coating a planarization layer and then baking the spin coated
planarization layer.
10. The method of claim 9 wherein the planarization layer is of a
photo-definable material.
11. The method of claim 1 wherein the first microlens resist layer
is soft baked prior to being patterned.
12. The method of claim 1 wherein the patterning includes exposing
the resist layer to ultraviolet light, developing the exposed
resist layer, and removing excess resist material from the exposed
resist layer.
13. A method for manufacturing a color microlens array comprising:
forming a first colored microlens resist layer on a surface, over a
plurality of pixel regions in a color display layer; patterning the
first colored microlens resist layer; and baking the patterned
first colored microlens resist layer to cause flowing of the
patterned layer resulting in a microlens, with a curved top
surface, being formed above each of the plurality of pixel
regions.
14. The method of claim 13 further comprising: repeating the
forming, patterning, and baking operations using a second colored
microlens resist layer, the second resist layer to filter a
different color than the first resist layer.
15. The method of claim 13 further comprising: fixing the patterned
first colored microlens resist layer by deep ultraviolet exposure
to cause cross-linking, prior to baking.
16. The method of claim 13 further comprising: stabilizing a
surface of the microlens using silylation.
17. The method of claim 13 further comprising: exposing the
microlens to deep ultra-violet bleaching to improve
transparency.
18. The method of claim 13 wherein the depositing, patterning, and
baking operations are repeated three times, each time on a
different colored polymer resist layer.
19. The method of claim 13 wherein the patterning operation is
performed using a photolithographic process.
20. The method of claim 13 further comprising planarizing the
surface prior to forming the first colored microlens resist
layer.
21. The method of claim 20 wherein the surface is planarized by
spin coating a planarization layer and then baking the spin coated
planarization layer.
22. The method of claim 21 wherein the planarization layer is of a
photo-definable material.
23. The method of claim 13 wherein the first microlens resist layer
is soft baked prior to being patterned.
24. The method of claim 13 wherein the patterning includes exposing
the resist layer to ultraviolet light, developing the exposed
resist layer, and removing excess resist material from the exposed
resist layer.
Description
[0001] This Application is a Divisional of Ser. No. 09/902,012,
filed on Jul. 9, 2001; which is a Divisional of Ser. No.
09/052,609, filed on Mar. 31, 1998 and which issued as U.S. Pat.
No. 6,271,900 on Nov. 27, 2001.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of image sensors
and display devices.
BACKGROUND
[0003] Microlenses have long been used in imaging devices to focus
light on sensors including charge couple device (CCD) sensors and
complementary metal oxide semiconductor (CMOS) sensors. The
microlenses significantly improve the light sensitivity of the
imaging device by collecting light from a large light collecting
area and focusing it on a small light sensitive area of the sensor.
The ratio of the overall light collecting area of a sensor to the
light sensitive area of the sensor is defined to be a fill factor.
Typical fill factors in prior art designs are less than 50%.
[0004] One prior art method of generating a color image signal is
shown in FIG. 1A. Light from a subject to be imaged comes in as
light rays 104 and passes through a set of microlenses 108, 112,
116. The microlenses are formed on a planarization layer 120. After
passing through the planarization layer 120, the light 104 is
filtered by color filters 124, 128, 132 which together form a color
filter array. Each color filter 124, 128, 132 in the color filter
array only allows light of a specific color to pass through. A
"color" is defined to be light having a specific range of
frequencies. Typical color filters 124, 128, 132 used in the color
filter array are red, green and blue filters (RGB) or cyan, magenta
and yellow (CMY) filters. Each microlens and color filter
combination corresponds to a sensor 136, 140, 144. Each sensor is a
light sensitive device capable of converting the intensity of light
striking the sensor 136, 140, 144 into an electrical signal. A
microlens, color filter, and sensors such as sensors 136, 140, 144
correspond to a pixel of an image. The sensors 136, 140, 144 are in
close proximity to each other, and each sensor receives filtered
light from a corresponding color filter 124, 128, 132. By combining
the output of the sensors 136, 140, 144, a processor, such as a
graphics processor, can determine the approximate intensity and
color of light striking the area in the proximity of sensor 136,
140, 144. By creating an array of such sensors (red sensor 160,
blue sensor 164, green sensor 168) as shown in FIG. 1B, an overall
color image can be reconstructed.
[0005] The fabrication of separate microlenses, color filters, and
image sensors in the structure illustrated in FIGS. 1A and 1B has
several disadvantages. For example, one disadvantage of the
traditional structure is that many process steps are needed to form
a first layer 148 including the sensors 136, 140, 144; a second
layer 152 including the color filters 124, 128, 132, and a third
planarization layer 156 to support microlenses 108, 112, 116.
[0006] Another disadvantage of the current structure is that the
microlenses 108, 112, 116 are separated from the corresponding
image sensors 136, 140, 144 by the planarization layer 156 and the
color filter layer 152. The separation reduces the light reaching
the sensors 136, 140, 144 because some light is absorbed passing
through the multiple layers 152, 156. Furthermore, the separation
results in increased crosstalk between pixels. "Crosstalk" results
when off axis light strikes a microlens such as microlens 112 at an
obtuse angle of incidence. The off-axis light passes through
planarization layers 156 and a color filter 128 missing the sensor
140 which corresponds to the color filter 128 and instead striking
an adjacent sensor 136. Alternately, the off-axis light coming in
through microlens 112 may pass between filters 124 and 128 and
reach adjacent sensor 136 resulting in an erroneous readings from
the image sensor 136.
[0007] Additional disadvantages of the currect micro-lens filter
combinations include the additional process steps being used to
fabricate the multi-level structure of FIG. 1, the decreased
reliability resulting from separation of layers 148, 152, 156 and
the increased material costs used to fabricate separate transparent
microlenses 108, 112, 116, color filters 124, 128, 132, and
associated planarization layer 156.
[0008] A second use of the microlens, color filter layer, structure
is in color display devices. FIG. 2 illustrates an example of using
the microlens color filter structure in a thin film transfer (TFT)
liquid crystal display device. In FIG. 2, light from a backlight or
other light source 204 passes through a color filter layer 208
containing color filters 212, 216 and 220. The color filters 212,
216, 220 are typically different colors allowing only one color of
light to pass through each filter. Microlenses 224, 228 and 232 in
microlens layer 236 focuses the light from corresponding color
filters 212, 216, 220 through a substrate 240 and a liquid crystal
display (LCD) layer 244 to a TFT substrate 248. Each TFT switch
252, 256, 260 corresponds to a corresponding color filter 212, 216,
220. By controlling the amount of light passing through each switch
252, 256, 260, the output of each color filter 212, 216, 220 can be
controlled. Combining the outputs of the color filters and TFT
switches generates the output of a pixel of the color display
device.
[0009] Display devices formed using the described techniques suffer
from the previously described disadvantages including (1)
difficulty in fabrication; (2) crosstalk between filters and
switches caused by the increased separation generated by the
microlens layer; and (3) problems with device reliability resulting
from adhesion between multiple layers and increased material costs
resulting from the necessity for multiple layers.
[0010] Thus an improved method for forming microlens and color
filter structures is desired.
[0011] The present invention describes a method of forming a color
microlens array on a semiconductor substrate. The method involves
depositing a colored microlens resist on a semiconductor surface.
The colored microlens resist is patterned and then baked to cause
flowing of the colored microlens resist resulting in a color
microlens with a curved surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1A is a cross section drawing of a conventional color
filter array structure for acquiring color images.
[0013] FIG. 1B illustrates an example of an arrangement of color
filters in a detection device.
[0014] FIG. 2 is a cross section of a thin film transfer (TFT)
based liquid crystal display utilizing a microlens system.
[0015] FIG. 3 illustrates a cross section of a color imaging device
for acquiring color images utilizing a colored microlens array
which combines microlenses and color filters.
[0016] FIG. 4 is a cross section of a TFT liquid crystal display
utilizing the color microlenses of the present invention.
[0017] FIG. 5 shows the processes used in fabricating a colored
microlens.
[0018] FIGS. 6A through 6E show the cross-section of a microlens
system after key processing operations.
DETAILED DESCRIPTION
[0019] In the following description, an array of colored
microlenses will be described. In the embodiment, the colored
microlenses are formed over a planar substrate using semiconductor
processing techniques, including photolithography and baking of a
microlens resist. Combining the function of a microlens and a color
filter into a single colored microlens reduces the number of
components and number of operations used to fabricate color display
and image acquisition devices. Reducing the number of components
also increases device reliability. Examples of devices which
utilize color microlenses include, but are not limited to, colored
imaging displays, such as TFT displays, for example, and image
acquisition devices such as charge coupled device (CCD) digital
cameras.
[0020] In the accompanying description, certain details will be
provided to facilitate understanding of the invention. For example,
certain processes used to form the color microlenses are described.
However it is recognized that other methods of fabricating a color
microlens may be appropriate. The included details are provided to
facilitate understanding of the invention and should not be
interpreted to limit the scope of the invention. Certain details
will be omitted because such detail would obscure the invention and
are already well understood by those of ordinary skill in the
art.
[0021] FIG. 3 illustrates a set of microlenses 304, 308, 312 for
use in a color imaging device. Light rays 316 from an external
source passes through the colored microlenses 304, 308, 312 and are
incident upon a set of sensors 320, 324 and 328. Each microlens
304, 308, 312 in the set typically allows a different color of
light to pass through. Thus, in one embodiment, one microlens may
be red, another blue and a third microlens green. Together, the set
of three microlenses detect light corresponding to a pixel of an
image. In this embodiment, the three microlenses 304, 308, 312 are
located in close proximity to each other (typically within one
micron), each microlens 304, 205, 312 positioned to allow one color
of light to reach the sensor 320, 324, 328 corresponding to the
microlens 304, 308, 342 respectively. A processor or other
appropriate graphic circuitry can combine the output of the three
sensors 320, 324, 328 to determine a color and intensity of light
striking the general region around the three sensors 320, 324, 328.
In an image, the general region corresponds to a pixel. To improve
resolution, pixels should be small and, thus, the microlenses
should be small. The diameter of microlens ranges in size from 8
microns to 15 microns for different devices.
[0022] FIG. 4 illustrates the use of color microlens 404, 408, 412
in a color display device. A light source 416 provides illumination
which passes through microlenses 404, 408, 412 through a counter
substrate layer 416 and to a liquid crystal display (LCD) layer
420. Each microlens of a set filters a different color, as well as
focuses light from the light source 416 to a particular region of
the LCD crystal layer 420 being switched. The crystals in the LCD
under each color microlens act as a switch and filters that light.
An applied electric potential determines when light can pass
through the liquid crystal in the region underneath the microlens
or when light is blocked from passing through the LCD layer 420.
Electrodes residing on both sides of the LCD layer 420 are used to
apply the electric potential.
[0023] In another embodiment, a thin film transfer (TFT) switch
428, 432, 436 may be used to switch the crystals in the LCD layer
420. The three microlenses 404, 408, 412 form a set corresponding
to a color display device pixel. Thus, it is desired, but not
required, that the microlenses 404, 408 and 412 have small
dimensions, each microlens typically less than 10 microns in
diameter by 3 microns in height so that they can be placed in close
proximity. A human eye receives the output of the display device
and merges the microlens outputs for a pixel to generate the actual
color which is intended to be displayed.
[0024] FIG. 5 is a flow diagram illustrating a lithographic method
of fabricating color microlenses. In block 504 the surface of the
semiconductor substrate upon which the microlenses will be formed
is planarized. Planarization provides a flat and smooth substrate
surface upon which a microlens resist can be deposited. In some
embodiments, when a surface is already polished and smooth,
planarization may be unnecessary. One method of planarization
involves spin coating a planarization layer which is subsequently
baked. The materials used in the spin coated planarization layer
can be classified into either non-photo-definable and
photo-definable materials. "Non-photo-definable" materials
(non-photo-sensitive materials) include acrylics and
polyorganosiloxiane, for example. Examples of "photo-definable"
materials (photo-sensitive materials) include acrylic based resists
and epoxy based resists. In this embodiment, photo-definable
planarization is used because non-photo-definable planarization
often requires an extra photolithographic patterning operation to
open areas of bond pads, while photo-definable planarization layers
can be patterned directly and etched.
[0025] In block 508, a color microlens resist material is deposited
on the planarized surface. In one embodiment, deposition of the
color microlens resist is achieved by spin coating a planarized
layer with the color microlens resist. The thickness of the coating
is determined by the required thickness of the microlens. The
thickness of the microlens resist is a function of the focal length
requirements of the microlens, a shorter focal length requires a
thicker lens, and thus, a thicker microlens resist layer. The focal
length of the microlens should be designed to effectively focus
light on the corresponding sensor. The microlens thickness (t) vs.
focus length (f) may be estimated according to the following
relationship: 1 f = ( 3 At ( 2 - cos + cos 3 ) ) 1 3 ( n 1 n 1 - n
o )
[0026] where:
[0027] A: Area of microlens
[0028] .THETA.: Contact angle between microlens and supporting
substrate
[0029] n.sub.1: refractive index of microlens
[0030] n.sub.0: refractive index of air
[0031] The "contact angle" is a function of the microlens curvature
and can be computed as the angle between a first line tangent to
the microlens surface at a point on the microlens near the
interface between the microlens and the support substrate and a
second line parallel to the support substrate surface. The contact
angle is illustrated as angle .THETA. 450 of FIG. 4.
[0032] The thickness and shape of the color microlens may be
computed using ray tracing programs and is also dependent on the
index of refraction of the microlens resist material. Different
colored microlenses may contain different pigments having different
indexes of refraction. Thus different microlenses in a set may have
different dimensions. In typical sensor applications for which the
pixel sizes are around 10 microns by 10 microns, the thickness of
the microlens can vary from 2 to 4 microns depending on the index
of refraction of the microlens material, the distance of the
microlens from the sensor, and the area of the sensor. The
determination of lens shapes is well understood in the art and can
be computed via commercially available rate tracing programs.
[0033] In block 512, the microlens resist is baked at a relatively
low temperature known as a "soft bake". In a positive resist, the
soft bake process involves baking the microlens resist at a
temperature of about 100.degree. Celsius (C) for a time of
approximately one minute. After the soft bake, a patterning process
is performed in which the microlens resist is typically exposed to
ultraviolet (UV) light in a photolithographic process in block 516.
In one embodiment the UV light has a wavelength or I-line of
approximately 365 nanometers and dose of 100 Millijoules/cm.sup.2.
After exposure to the UV light, the microlens resist is developed
in a developer solution.
[0034] After the patterning block 516, the excess microlens resist
material is removed leaving the appropriate amount of microlens
resist to form a microlens. Typically, the structures remaining
have an approximately square form. The square form is fixed using
deep ultraviolet exposure, otherwise known as post-patterning flood
exposure in block 520. The deep UV exposure causes cross-linking in
the resist improving the transparency of the microlens resist
material.
[0035] The shape of the microlens after post-patterning flood
exposure is still a square form. In block 524, the microlens array
is baked at a high temperature to cause the microlens resist to
flow and form the desired curved shape. In one implementation of
the invention, the microlens array is heated to a temperature of
approximately 150.degree. C. for a predetermined period of time
(e.g., approximately two minutes).
[0036] Blocks 508 through 524 are repeated for each different
colored microlens to be deposited on a planarized surface. Thus if
a red, green and blue microlens are to be formed on the planarized
surface, three iterations of the operations set forth in blocks 508
through 524 are typically required, one iteration for the red
microlens, a second iteration for the green microlens and a third
iteration for the blue microlens. When in block 528, it is
determined that the last microlens has been formed on the
planarized surface, an optional silylating layer is formed over the
microlenses in block 532.
[0037] Typically, the microlens array, formed in accordance with
block 504 through 524, is a polymeric lens array and is formed from
photoresists. However, prior to silylation, these polymeric
microlenses formed from photo resists lack the mechanical, thermal
and environmental stability required for most devices. Thus, in
this embodiment, the surface of the microlens array is silicated
through silylation of the microlens resist. This silicated process
is known to stabilize the resist and is described in literature
such as Introduction to Microlithography edited by L. Thompson, C.
Grant Wilson, and M. J. Bowden published by The American Chemical
Society copyrighted 1994. On pages 243 to 244.
[0038] In one embodiment of the invention, the silylated microlens
are further subject to deep ultraviolet bleaching. In the bleaching
process, the microlens array is exposed to Deep (DUV) radiation of
approximately 200-300 nanometers and intensity of 500
milliWatts/centimeter.sup.2 wavelength for a period of one minute
time such as that which occurs in Fusion DUV systems. The UV
bleaching changes the light transmittance characteristics of the
color microlenses. Bleaching reduces the tendency of the
microlenses to have a yellowish tint.
[0039] In block 540, the silylated color microlens surface is
converted to a silicated surface using an oxygen reactive ion
treatment (RIE). A silicated surface is preferred to the salyated
surface because the silicated surface is stiffer, more stable and
resistant to deformation. In order to convert the salyated surface
to a silicated surface, the salyated microlens surface is exposed
to an oxygen reactive ion etch for approximately 30 seconds. The
RIE etch power should be low enough such that it does not cause
significant etching. A typical RIE etch power may be approximately
60 watts.
[0040] FIGS. 6A through 6D illustrate cross sections of the
microlens structure at various stages in the processing described
in FIG. 5. In FIG. 6A, the planarized surface 604 is shown with a
deposited layer of color microlens resist 608. FIG. 6B illustrates
the "square" form of the remaining microlens resist 608 after the
patterning block described in block 516. FIG. 6C illustrates the
patterned microlens resist during exposure to DUV radiation 612. In
order to round angular edges, the color microlens resist is subject
to a thermal flow or cross link baking process described in block
524 of FIG. 5 to produce a curved microlens 608 as illustrated in
FIG. 6D. The color microlens resist, which now forms a color
microlens of FIG. 6D, is subject to silylated, DUV bleaching and
RIE to produce the coated microlens structure 612 illustrated in
FIG. 6E.
[0041] While certain exemplary embodiments have been described in
detail and shown in the accompanying drawings and description, it
is to be understood that such embodiments are merely illustrative
and not restrictive on the broad invention. This invention is not
to be limited to the specific arrangement and constructions shown
and described; since various other modifications may occur to those
of ordinary skill in the art.
* * * * *