U.S. patent application number 08/986361 was filed with the patent office on 2003-07-31 for formation of protective coatings for color filters.
Invention is credited to WESTER, NEIL.
Application Number | 20030142226 08/986361 |
Document ID | / |
Family ID | 25532337 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030142226 |
Kind Code |
A1 |
WESTER, NEIL |
July 31, 2003 |
FORMATION OF PROTECTIVE COATINGS FOR COLOR FILTERS
Abstract
A structure and method for producing color filters with a
protective silation layer is described. In one embodiment, each
filter is coated with a silation layer to prevent bleeding of
material between closely spaced filters during the fabrication
process. In a second embodiment, the silation layer is used to
protect an array of filters from physical damage during detaping
operations. In a third embodiment, the silation layer is used
before fabrication later filters in a color filter array to prevent
damage to previous filter layers.
Inventors: |
WESTER, NEIL; (TEMPE,
AZ) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD
7TH FLOOR
LOS ANGELES
CA
90025
|
Family ID: |
25532337 |
Appl. No.: |
08/986361 |
Filed: |
December 8, 1997 |
Current U.S.
Class: |
348/273 ;
257/440; 257/E31.121 |
Current CPC
Class: |
H01L 31/02162
20130101 |
Class at
Publication: |
348/273 ;
257/440 |
International
Class: |
H04N 003/14 |
Claims
What is claimed:
1. A structure for detecting light comprising: a light sensor; a
first color filter deposited over the light sensor; and a
protective polymer silation layer deposited over the first color
filter.
2. The structure of claim 1 further comprising: a tape applied to
the polymer silation layer for protecting of the first color filter
during a grind/gold process.
3. The structure of claim 1 further comprising: a second color
filter, said second color filter blocking a different colored light
than light blocked by the first color filter, the second color
filter adjacent to the protective polymer silation layer which
surrounds the first color filter.
4. The structure of claim 1 wherein the protective polymer silation
layer is formed from a hexomethyl di-silizane gas.
5. A method of fabricating a light detecting structure comprising:
fabricating a first color filter over a first photodiode on a
substrate to form a filter-photodiode combination; placing the
substrate including the filter-photodiode combination in a chamber
and exposing the filter-photodiode combination to a silane compound
in vapor form to form a silane coating over the filter-photodiode
combination; and removing the filter-photodiode combination from
the chamber.
6. The method of claim 5 wherein the silane compound in vapor form
is hexamethyl di-silizane in vapor form.
7. The method of claim 5 further comprising the steps of: applying
a tape over the silane coating; grinding a surface of the light
detecting structure; and removing the tape from the silane
coating.
8. The method of claim 5 further comprising the steps of:
fabricating a second color filter over a second photodiode on the
substrate, said second color filer designed to absorb light of a
different wavelength than said first color filter; exposing the
second color filter to a silane compound in vapor form.
9. The method of claim 5 further comprising the steps of: exposing
the first color filter to a developer solution to remove excess
material before exposing the filter photodiode combination to the
silane compound.
10. A structure for generating a digital image comprising: a
substrate; a plurality of photo-detectors, formed on said
substrate; a plurality of color filters, each photo-detector in
said plurality of photo-detectors having an input covered by a
filter in said plurality of color filters, a first filter in said
plurality of color filters designed to block a first frequency of
light and a second filter in said plurality of color filters
designed to block a second frequency of light; support electronics
which generates a color digital image by combining data from at
least two photo-detectors in the plurality of photo-detectors; and
a silane protective layer around said plurality of
photo-detectors.
11. The structure of claim 10 wherein said support electronics
combines data by interpolating to a point between the plurality of
photo-detectors.
12. The structure of claim 10 wherein the silane layer is between 1
and 15 angstroms thick.
13. The structure of claim 10 wherein the silane layer is formed
from a hexamethyl di-silizane vapor.
14. The structure of claim 10 wherein the silane layer covers each
individual filter such that said silane layer is sandwiched between
adjacent filters.
15. A structure for detecting light comprising: a means for sensing
light; a mean for filtering the color of light received by said
means for sensing light; a means for protecting the means for
filtering the color of light, said means for protecting including a
silation material.
16. The structure of claim 15 further comprising: an adhesive tape
taped to the means for protecting the means for filtering.
17. The structure of claim 15 wherein the means for protecting is a
polymer silation.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The present invention relates generally to the fabrication
of color light detectors. More particularly, the present invention
relates to the formation of protective coatings on filters used in
color light detectors.
[0003] (2) Description of Related Art
[0004] Color light detectors are becoming increasingly important in
digital imaging applications. Digital imaging systems often use
arrays of photo-detectors to generate an image of a subject. In
order to generate a color digital image, color filters are
fabricated over light sensors such as photo-detectors. Each filter
allows only a predetermined color of light to reach a corresponding
photo-detector thus determining what color light will be sensed by
the photo-detector. By grouping sets of light sensors together, the
intensity and color of light reaching an area can be
determined.
[0005] Each color pixel in a digital image is typically generated
by combining the output of a group or set containing several
photo-detectors. In one implementation, the output of three
corresponding photo-detectors are combined to generate one color
pixel. Each corresponding photo-detector is located in close
proximity to the other two corresponding photo-detectors. Each
corresponding photo-detector has a different color filter filtering
received light. In one example, a blue color filter, a red color
filter and a green color filter may each be used over a
corresponding photo-detector. By determining the intensity of light
passing through each color filter, the intensity of light of a
particular color or wavelength can be determined. An electronic
processor interpolates the data from the three photo-detectors and
combines them to determine the color of light received by the
photo-detectors in the general region of the pixel. This
information is processed electronically and combined with other
sets of photo-detectors to generate a digital color image.
[0006] Photo-detectors and color filters are typically formed in a
complimentary metal oxide semiconductor (CMOS) fabrication process.
A number of effects occur during the fabrication process which
reduce the filtering capability or damage the color filters. In
particular, three problems faced by the fabrication process are
described in the following three paragraphs.
[0007] A first problem which results from the fabrication of color
filters is color bleeding of compounds from adjacent color filters.
Color filter arrays which are placed over the photo-detectors or
imaging sensors are often generated by depositing pigment dispersed
polymer films. The type of pigment determines the filtering
capability of the filter. In a typical color detection system,
adjacent filters thus have different pigments. The performance of
the system is optimized when each photo diode is covered with a
single color filter, whether it be red, blue or green. The filter
blocks other colors from passing through the filter. Ideally, it is
desirable to fabricate filters which transmit one hundred percent
of the light at a predetermined frequency range, and completely
block light transmission outside of the predetermined range of
frequencies. Thus ideally, pigments which determine what color of
light will pass through a filter is preferably completely confined
to a filter and does not "bleed" into adjacent filters. In
practice, the contact between the various different color filters
(red, blue, green) results in a slight intermixing of pigments
("bleed") during the fabrication process. The bleed results in a
broadening of each individual filter response reducing the color
delineation capabilities of each filter. This bleed degrades the
overall performance of the system.
[0008] A second problem with current fabrication techniques is that
during a grinding and gold deposition process (grind/gold process),
color filters are often damaged. After the final deposition of CFA
(filter) layer materials, wafers or substrates containing the color
filters are transferred to a grind/gold process where a protective
front side tape is applied to the wafer while the backside of the
wafer is thinned and coated with gold. After completion of the
grind/gold process, the tape is removed in a detaping operation.
Due to the polymeric nature of the filter material (CFA material),
the filters are vulnerable to physical damage during the detaping
operation. Damage to the color filters jeopardizes the
functionality of the fabricated light detector.
[0009] A third problem with current methods of fabricating filters
in CMOS processes results from repeating processing steps on the
entire filter set each time a filter of a different color is added.
After generation of a first color filter, each subsequent color in
the filter set is produced by a subsequent deposition and photo
definition of pigment dispersed in polymer films. Thus a three
color set (red, green and blue) of color filters involves three
depositions of pigments. Each filter is composed of a CFA layer.
Each CFA layer is manufactured from the same starting materials. In
the prior art, no physical or chemical resistant barriers are used
between the layers. Thus misprocessing or process excursion in the
working or processing of a layer may damage previously created
filters and require reworking of previously deposited filter
layers.
[0010] Thus a method of protecting each filter as it is generated
is desirable. In particular, the method would preferably utilize a
barrier to protect each individual filter in a multicolor filter
array such that the barrier would prevent bleeding between adjacent
color filters. The barrier would preferably be non polymeric to
help prevent damage during the detape process. The procedure would
also preferably be performed after each CFA layer is deposited
creating a barrier between layers. The barrier allows accidental
misprocessing or minor processing variations to occur without
damage to previously fabricated layers reducing the probability
that a rework of previously deposited layers is necessary. Such a
technique for generating a protective barrier is described in the
following application.
BRIEF SUMMARY OF THE INVENTION
[0011] A structure for detecting light is described. The structure
includes first color filter deposited over a light sensor. A
protective polymer silation layer is deposited over the first color
filter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1A-1E illustrates a cross-section of a color filter
array structure at various stages of the fabrication process.
[0013] FIGS. 2A-2E illustrate a cross-section of a color filter
array structure at various stages of an alternative fabrication
process. The illustrated structure which results uses a silation
layer appropriate for protecting against surface damage in a detape
process.
[0014] FIG. 3 illustrates a cross-section of a device using a color
filter.
[0015] FIGS. 4A and 4B illustrate a flow diagram illustrating the
steps used in fabricating color filters and protective layers
around color filters.
DETAILED DESCRIPTION OF THE INVENTION
[0016] In the following description, a protective silation layer
will be formed over color filters used in semiconductor light
sensing devices. The silation layer will protect and reduce the
probability of damage to the color filters. For example, by using a
silation layer, the detaping processes in grind/gold deposition
processes will damage fewer color filters. The formation of a
polymer silation layer between CFA layers also reduces the need for
reworking of previously deposited layers when a subsequent CFA
layer is misprocessed. Finally, the use of the silation layer
between adjacent color filters reduces bleeding of pigments or CFA
polymer material between adjacent different colored filters.
Reducing bleeding improves the color selectivity of each
filter.
[0017] In the accompanying description, certain details will be
provided to facilitate understanding of the invention. For example,
the specification will recite processing times as well as
processing materials such as hexamethyl di-silizane. However, it is
recognized that other silation materials may be used and different
processing times 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, for example, describing the steps used to create a
photo-detector will be omitted, because such details would obscure
the invention and are already well understood by those of ordinary
skill in the art.
[0018] The advantages and uses of the present invention may be
understood by examining simplified cross-sectional representations
of the color filter and protective layer. FIG. 1 and FIG. 2
illustrates a simplified cross-sectional view of the color filter
and protective layer or coating at various stages in processing.
FIG. 1 shows a simplified cross-sectional representation of a
filter and protective layer when using the protective layer to
reduce bleeding of pigment between adjacent filters. The embodiment
shown in FIG. 1 also minimize the probability of having to "rework"
previously created filters when a misprocessing step occurs.
[0019] FIG. 1a illustrates a CFA polymer 104 which forms the color
filter deposited over a substrate 108. Typically the substrate 108
may be a silicon wafer. A light sensor such as a photo-detector
device may be formed underneath the CFA polymer 104. The
photo-detector may be formed between the substrate 108 and CFA
polymer 104 or the photo-detector may be embedded into the
substrate 108. The CFA polymer 104 is preferably deposited in a
spin coating process. CFA polymer 104 typically includes a base
polymer resin such as a polyacrylate containing single strands of
polymer. A solvent typically puts the resin in a solution form and
a photo sensitizer is used to cause cross linking of the polymer
strands. Organic metallic pigments are mixed with the resin. The
type of pigment added determines the wavelengths of light filtered
by the CFA polymer.
[0020] FIG. 1b illustrates the formation of a silation layer 112
over the CFA polymer 104. In a preferred embodiment, a hexamethyl
di-silizane (HMDS) silation process is used to form the silation
layer 112. The silation layer may be made up of a silicon oxide. In
the preferred embodiment, the silation layer 112 is only two to
three angstroms thick and is transparent to light.
[0021] In order to form adjacent color filters, a second or
subsequent layer of CFA polymer 116 is deposited over substrate 108
in a "subsequent" deposition process. The subsequent layer of CFA
polymer 116 may surround the silation layer 112 as illustrated in
FIG. 2c. The subsequent layer of CFA polymer 116 contains different
organo-metallic pigments than the initial CFA polymer 104. A photo
definition process removes excess CFA polymer material in the
subsequent layer resulting in a second filter 120 in close
proximity to the first filter formed by CFA polymer 104 as
illustrated in FIG. 1d. The second filter 120 and the first filter
have different light transmission characteristics because of the
different organo-metallic pigment incorporated into the polymer.
The silation layer 112 surrounding the first filter minimizes
bleeding or intermixing of material between the second filter 120
and the first filter.
[0022] In the event of misprocessing, silation layer 112 allows
removal of the second filter 120 without damaging the first filter
formed by CFA polymer 104. When a misprocessing step occurs during
the formation of the second filter 120, the second filter 120 is
removed leaving the first filter with its protective silation layer
112 intact as illustrated in FIG. 1e. In a typical set of
photo-detectors, second and third color filters are also fabricated
around the original filter.
[0023] FIG. 2 illustrates a simplified cross-section of a filter
array structure including silation layers appropriate for scratch
protection during package assembly. FIG. 2A illustrates a CFA
polymer or first filter 204 formed over a photo-detector and a
substrate 208. Exposing the first filter 204 to a Hexomethyl
di-silizane (HMDS) gas results in the formation of a silation layer
212 over first filter 204 as illustrated in FIG. 2b.
[0024] In FIG. 2c, a subsequent CFA polymer layer 216 is deposited
over the first filter 204 and silation layer 212. A photo
definition process is performed to remove excess material resulting
in a second filter 220 on top of the first filter 104 and silation
layer 112. The first silation layer 112 prevents bleeding of
pigment between the first filter 204 and the second filter 220.
First silation layer 112 also protects the first filter 204 from
damage in the event of processing errors during fabrication of the
second filter 220. To form a second silation layer 224 over the
second filter 220 as illustrated in FIG. 2e, the second filter 220
is exposed to a HMDS gas. After exposure to the HMDS gas, both the
first color filter 204 and the second color filter 220 are
protected by corresponding silation layers 212 and 224. Each
silation layer 212 and 224 is typically a few angstroms thick and
minimizes the chance of damage to the formed color filters 204, 220
during taping and subsequential detaping of the filter surface.
[0025] FIG. 3 illustrates a cross-section of a device using the
formed color filters. Oxide layer 304 acts as a substrate
supporting the detection apparatus. A light sensing device such as
a semiconductor photo-detector is typically incorporated into the
oxide or fabricated in a layer on the surface of the oxide 304. In
the illustrated embodiment of FIG. 3, a silicon nitride layer 308
is grown on top of the oxide 304. A color filter 312 is formed in a
well between metal lines 324, 328. A silicon nitride layer
surrounds metal lines 324, 328. The color filter 312 is typically
made of a polymer with a pigmented acrylate filled in. The filter
allows light of a predetermined frequency range to pass through.
The color filter 312 blocks out other colors of light. A protective
silane layer 320 covers the color filter layer 312. It should be
noted that the illustration shown is not to scale because the
silane layer 320 is typically only two to three angstroms thick
while the color filter 312 is typically a height of approximately
15,000 angstroms. Thus in a scaled drawing, the silane layer 320
would barely be visible.
[0026] Silane layer 320 serves as a protective coating and is
preferably optically transparent. Thus silane layer 320 preferably
does not affect the light transitivity of the color filter 312.
Metal lines 324, 328 are typically composed of
aluminum-silicon-copper alloy and in the illustrated embodiment are
surrounded by silicon nitride material. In one embodiment of the
invention the metal lines 324, 328 are used as contacts for
connection to photo-detectors under the filter 312.
[0027] In a preferred embodiment of the invention, three different
color filters 312 corresponding to three photo-detectors will be
used to generate a color image. Each photo-detector will detect
light in a corresponding frequency range. The three filters and
detectors (forming a set) are coupled to processing electronics
(not shown) which determine the approximate intensity of light in
each frequency range in the general vicinity of the three
photo-detectors. In one embodiment, the processing electronics may
be part of a graphics card in a personal computer. The information
is interpolated by the electronics to determine the color and
intensity of light striking the particular region or pixel.
[0028] FIG. 4 is a flow diagram illustrating the steps used in
fabricating color filters and protective layers around color
filters for use in light detecting devices. In step 404, a photo
diode designed to detect light is formed on a substrate. Other
non-organic structures such as metal lines may also be formed. The
substrate typically includes a silicon oxide material. One example
of a suitable substrate is a semiconductor wafer used in
semiconductor processing. In step 408, a polymer coating for
forming a filter is deposited on the substrate, preferably in a
spin coating process. The polymer coating is typically a resin
impregnated with an organic metallic pigment to determine the light
transmissively characteristics of the polymer. In step 412, the
material is baked at approximately 90.degree. Celsius for
approximately 90 seconds to cure the polymer coat.
[0029] To form device structures, a mask is placed over the polymer
coating and the entire surface is exposed to light of approximately
365 nanometers wavelength for approximately 200 milliseconds in
step 410. The resulting material is developed in a developer
solution to remove excess material in step 420. In one embodiment,
the developer is a dilute ammonium hydroxide alkaline in photo
resist solution. The resulting structure is baked again in step 424
at approximately 180.degree. centigrade for three minutes to cure
the material. The formation of a first filter for filtering one
color of light is thus completed.
[0030] In steps 428-460, a silane protective layer is formed over
the color filter. The filter and accompanying substrate is placed
in a second chamber and heated to approximately 130.degree.
centigrade in step 428. The second chamber is evacuated of gases in
step 432 to create an approximate vacuum. In step 436, the chamber
is back-filled with nitrogen or another inert gas. In step 440, the
chamber is evacuated to a sub-atmospheric pressure. Hexomethyl
di-silizane (HMDS) in liquid form is introduced into the second
chamber in step 444. Although HMDS is in a liquid form outside of
the chamber, the sub-atmospheric pressure results in the formation
of a HMDS vapor within the chamber. The polymer filter material is
exposed to the HMDS vapor for approximately 15 seconds in step 448
resulting in the formation of a silation layer. The second chamber
is evacuated in step 452 and again backfilled with nitrogen in step
456 to approximately atmospheric pressure. The completed filter
with silation layer is removed in step 460.
[0031] When prevention of bleeding between different colored
filters is desired, the steps described in steps 408 through 460
must be repeated for each different color of filter implemented to
form a silation layer around each filter type. For example, a red,
green, blue filter scheme would require three repetitions of steps
408 through 460, each repetition modifying step 408 to introduce a
polymer material containing a different mix of pigment. The
different pigments result in different light transmission
characteristics.
[0032] When the purpose of the silation layer is merely to avoid
damage during a detaping process, steps 408 through 460 need only
be repeated once to cover all organic surfaces with a silane layer
for the taping/detaping process. Performing step 428 through 452
once after all filters have been formed results in one silation
layer over all of the filters (the HMDS automatically bonds to
exposed polymer layers). A tape could then be applied to the entire
surface to protect the surface while grinding other surfaces of the
wafer and applying gold contacts. After completion of the grinding
and gold contact application procedure, the tape, typically a
cellophane tape, could be removed. The silation layer reduces the
probability of damage to the underlying color filters during the
de-tape process.
[0033] Once completed, each filter typically allows only
predetermined wavelengths or colors of light to reach the photo
diode. Using sets of several photo diodes in close proximity, each
photo diode having a different color filter allows a processor to
determine the color and intensity of light striking a small region.
By using multiple sets of photo diodes in an array structure, a
color image can be reconstructed.
[0034] While certain exemplary embodiments have been described in
detail 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 is not
to be limited to the specific arrangements and constructions shown
and described, since various other modifications may occur to those
with ordinary skill in the art.
* * * * *