U.S. patent application number 09/749611 was filed with the patent office on 2001-12-13 for method for fabricating an image sensor.
Invention is credited to Jo, Wan-Hee.
Application Number | 20010051390 09/749611 |
Document ID | / |
Family ID | 19631152 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010051390 |
Kind Code |
A1 |
Jo, Wan-Hee |
December 13, 2001 |
Method for fabricating an image sensor
Abstract
A method capable of enhancing the uniformity of the color filter
array and removing scum deposits from the surface of the color
filter array to improve the yield ratio of the device is disclosed.
The method includes the steps of (a) formulating devices on a
substrate and forming a first and a second interlayer insulating
layer and a first and a second metal line, wherein the devices
include a photodiode; (b) forming an optical shielding layer on the
second interlayer insulating layer; (c) using the optical shielding
layer as a mask to perform a dry etching for the second inter layer
insulating layer by a predetermined thickness, thereby producing a
plurality of grooves; (d) utilizing the plurality of grooves to
form a color filter array on the second inter layer insulating
layer; (e) utilizing the optical shielding layer as a polishing
stop layer to perform a chemical mechanical polishing, thereby
planarizing the color filter array; and (f) sequentially depositing
a low temperature layer and a device protection layer on the color
filter array.
Inventors: |
Jo, Wan-Hee; (Ichon-shi,
KR) |
Correspondence
Address: |
Pillsbury Madison & Sutro LLP
Intellectual Property Group
Ninth Floor, East Tower
1100 New York Avenue, N.W.
Washington
DC
20005-3918
US
|
Family ID: |
19631152 |
Appl. No.: |
09/749611 |
Filed: |
December 28, 2000 |
Current U.S.
Class: |
438/70 ;
257/E27.132 |
Current CPC
Class: |
H01L 27/14609 20130101;
H01L 27/14685 20130101; H01L 27/14623 20130101; H01L 27/14627
20130101; H01L 27/14621 20130101 |
Class at
Publication: |
438/70 |
International
Class: |
H01L 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1999 |
KR |
1999-63832 |
Claims
What is claimed is:
1. A method for fabricating an image sensor, comprising the steps
of: (a) formulating devices on a substrate and forming a first and
a second interlayer insulating layer and a first and a second metal
line, wherein the devices include a photodiode; (b) forming an
optical shielding layer on the second interlayer insulating layer;
(c) using the optical shielding layer as a mask to perform a dry
etching for the second inter layer insulating layer by a
predetermined thickness, thereby producing a plurality of grooves;
(d) filling the plurality of grooves to generate a color filter
array on the second inter layer insulating layer; (e) utilizing the
optical shielding layer as a polishing stop layer to perform a
chemical mechanical polishing, thereby planarizing the color filter
array; and (f) depositing a low temperature layer and a device
protection layer sequentially.
2. The method as recited in claim 1, wherein the step (a) further
includes the step of, before forming the first interlayer
insulating layer, shaping a pre-metal dielectric layer on the
photodiode.
3. The method as recited in claim 1, wherein the optical shielding
layer is used as a third metal line at the step (b).
4. The method as recited in claim 1, wherein the color filter array
is formed as a sequence of a green color filter, a blue color
filter and a red color filter at the step (d).
5. The method as recited in claim 1, wherein the device protection
layer is formulated by a low temperature process and the step (f)
further includes the step of creating a plurality of micro-lenses
on the device protection layer.
6. The method as recited in claim 1, wherein the separation
distance between the photodiode and the color filter array is
considered to perform a dry etching for the second inter layer
insulating layer.
7. A method for forming a color filter array for a color image
sensor comprising the steps of: forming isolation regions on a
semiconductor substrate, the isolation regions defining a plurality
of active regions; forming photo detectors in the active regions of
the semiconductor substrate; forming an insulating layer; forming a
metal layer on the insulating layer; removing portions of the metal
layer to form a metal pattern that exposes portions of the
insulating layer above the photodetectors; etching the exposed
portions of the insulating layer to remove a predetermined
thickness of the insulating layer to form a plurality of grooves in
the insulating layer; sequentially forming a first color filter in
a first set of grooves, a second color filter in a second set of
grooves, and a third color filter in a third set of grooves;
planarizing the color filters to form a color filter array using
the metal pattern as a planarization endpoint; and depositing a
protective layer on the color filter array.
8. A method according to claim 7, wherein the step of sequentially
forming each of the first, second, and third color filters further
comprises coating the metal pattern and the grooves with a layer of
photosensitive material; selectively exposing predetermined
portions of the layer of photosensitive material that will form a
color filter; and removing the remaining portion of the layer of
photosensitive material.
9. A method according to claim 8, wherein the first color filter is
a green color filter.
10. A method according to claim 7, wherein the step of planarizing
the color filters further comprises a chemical mechanical polishing
process.
11. A method according to claim 10, wherein the step of etching the
exposed portions of the insulating layer further comprises a plasma
etch process.
12. A method according to claim 11, wherein the plasma etch process
is substantially anisotropic.
13. A method according to claim 7, wherein the step of depositing a
protective layer on the color filter array further comprises
forming a low temperature oxide layer.
14. A method according to claim 13, wherein the step of depositing
a protective layer on the color filter array further comprises
forming a passivation layer on the low temperature oxide layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for fabricating an
image sensor; and, more particularly, to a method for fabricating
an image sensor that will effectively prevent contamination of the
surface of a color filter array.
DESCRIPTION OF THE PRIOR ART
[0002] Conventional image sensor include both charge coupled device
(CCD) image sensors and complementary metal oxide semiconductor
(CMOS) image sensors. The basic technology used to form the CMOS
image sensor is common to both types of sensors.
[0003] The CMOS image sensor comprises a photo detector for
detecting light and a logic circuit for converting the detected
light into an electric signal representing data regarding the
detected light. Although efforts have been made to increase the
fill factor of the image sensor and thereby increase the sensor
sensitivity, further increases in the fill factor are limited
because the associated logic circuitry cannot be completely
removed. The fill factor, sometimes referred to as the aperture
efficiency, is the ratio of the size of the light-sensitive area to
the size of the total pixel size. Accordingly, in order to increase
the sensitivity of the light, a micro lens formation technology has
been used to converge and focus the incident light onto the photo
detector by changing the path of the light that reaches the lens on
the outside of the photo detector. In order for the image sensor to
detect and provide a color image, it typically must include both a
photo detector for receiving the light and generating and
accumulating charge carriers and a color filter array (CFA), i.e.,
a plurality of color filters sequentially arranged above the photo
detector. The CFA typically uses one of two alternative three-color
primary configurations, either red R, green G and blue B (RGB) or
yellow Y, magenta M and cyan C (CMY). A plurality of micro-lenses
may be positioned above the color filter array to increase the
photo-sensitivity of the image sensor.
[0004] Although one inner polysilicon and three outer metal layers
have been used to form the interconnector structures in the
conventional image sensor, the third metal layer is typically
completely removed from above each detector pixel, a passivation
layer is deposited on each pixel and then the color filter array is
fabricated.
[0005] Accordingly, in a conventional image sensor, increased
thickness between a photodiode of each pixel and the corresponding
color filter array results in a larger loss of the available light
signal. Further, scum resulting from the under-development of the
organic films used in forming the color filter array will tend to
degrade the features of the resulting pixels
[0006] Referring to FIG. 1, there is illustrated a conventional
method for fabricating an image sensor. A field insulating layer 12
is formed on a silicon substrate 11 in order to isolate two
neighboring pixels; a photodiode 13 is formed by implanting and/or
diffusing impurity; and an inner connection polysilicon 14 is then
formed on the field insulating layer 12. After a pre-metal
dielectric (PMD) layer 15 for transmitting the light is deposited
on the inner connection polysilicon 14, a first interlayer
insulating layer 16 for insulating two neighboring metal lines is
deposited. After a first metal line 17 and a second interlayer
insulating layer 18 are sequentially deposited on the first
interlayer insulating layer 16, a second metal line 19 is
constructed on the second interlayer insulating layer 18 so as to
be opposite to the first metal line 17.
[0007] After a third interlayer insulating layer 20 is formed to
insulate neighboring metal lines is formed on the second metal line
19, a third metal layer is deposited. This third metal layer will
be only used to formulate metal lines for the peripheral circuits
and will typically be removed completely from the light sensing
areas of the unit pixel. After the third metal layer is removed, a
high temperature process is used to form an oxide layer, a nitride
layer or a multi-layer film of oxide and nitride layers, on the
third interlayer insulating layer 20 to provide a device protection
layer 21 that will protect the device from moisture and mechanical
damage.
[0008] After a color photoresist layer for providing color
sensitivity is deposited on the device protection layer 21,
developing processes are performed to generate the color filter
array, i.e., an array of a blue color filter 22, a red color filter
23 and a green color filter 24. The red color filter 23 is thicker
than the blue color filter 22 by a predetermined thickness and the
green color filter 24 is, in turn, thicker than the red color
filter 23 by another predetermined thickness. The differences in
thickness between the various filters produces a color filter array
having a stepped structure that tends to be contaminated by scum
deposits (A, B, C). The scum is not primarily deposited on the
oxide device protection layer 21, but is more typically found on
photoresist layers that form the color filter array. For example,
in the process of sequentially forming the color filter array of
the blue color filter 22, the red color filter 23 and the green
color filter 24, the scum deposits A and B produced during
formation of the red color filter 23 or the green color filter 24
may remain on the blue color filter 22. Similarly, the scum
deposits produced during the formation of the blue color filter 22
may remain under the red color filter 23, while scum originating
from the green color filter 24 may remain on the red color filter
23. However, although the scum originating from the blue color
filter 22 may remain on the device protection layer 21, because the
device protection film 21 and the blue color filter 22 are formed
from dissimilar materials, scum is not typically found on the oxide
layer. Further, because the scum remaining on the red color filter
23 originates primarily from the green color filter 24, the upper
regions of the color filter array may frequently suffer from scum
deposits.
[0009] Three color filters in the color filter array are
necessarily stepped as a result of the three fabricating processes
of the color filter array.
[0010] If the stepped color filter array is planarized, the last
formed color filter in the CFA is thicker than the previously
formed color filters. As a result, the final thicknesses typically
increase in sequence from the blue color filter 22, to the red
color filter 23, and finally to the green color filter 24.
[0011] An over coating material (OCM) 25 is formed on the color
filter array, wherein the over coating material 25 is used to
perform a flattening or planarization process with respect to the
stepped color filter array. A plurality of micro-lenses 26 is then
formed on the over coating material 25 above the color filter
array.
[0012] As a result of this process, the conventional method for
fabricating a color filter array typically results in scum from the
photoresist material being found on the color filter array.
SUMMARY OF THE INVENTION
[0013] It is, therefore, an object of the present invention to
provide a method capable of enhancing the uniformity of the color
filter array and removing the scum on the surface of the color
filter array to improve the yield ratio of the device.
[0014] In accordance with a preferred embodiment of the present
invention, there is provided a method for fabricating an image
sensor, comprising the steps of: (a) formulating devices on a
substrate and forming a first and a second interlayer insulating
layer and a first and a second metal pattern, wherein the devices
include a photodiode; (b) forming an optical shielding layer on the
second interlayer insulating layer; (c) using the optical shielding
layer as a mask to perform a dry etch of the second inter layer
insulating layer to remove a predetermined thickness, thereby
producing a plurality of grooves; (d) filling the plurality of
grooves to form a color filter array on the second inter layer
insulating layer; (e) utilizing the optical shielding layer as a
polishing stop layer to perform a chemical mechanical polishing
(CMP), thereby planarizing the color filter array; and (f)
sequentially depositing both a low temperature layer and a device
protection layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The above and other objects and features of the present
invention will become apparent from the following description of
preferred embodiments given in conjunction with the accompanying
drawings, in which:
[0016] FIG. 1 represents a schematic method for fabricating a
conventional image sensor; and
[0017] FIGS. 2A to 2G show a method for fabricating an image sensor
in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Referring to FIGS. 2A to 2G, there is illustrated a method
for fabricating an image sensor in accordance with the present
invention by significantly modifying the conventional image sensor
fabrication process with the inner connection polysilicon and the
three-layer metal line structure.
[0019] Referring to FIG. 2A, a field insulating layer 32 is formed
on a silicon substrate 31 to separate adjacent pixels electrically
from each other. A photodiode 33 is then formed by a conventional
impurity diffusion or an ion implantation technique. A layer of
polysilicon is then deposited on the field insulating layer 32, and
conventional selective patterning and etch techniques are used to
form an inner connection polysilicon pattern 34.
[0020] After a light transmitting insulating layer, e.g., a
pre-metal dielectric layer 35, is deposited and planarized, a first
metal layer is deposited on the pre-metal dielectric layer 35 and
selectively patterned to form a first metal line 36 on the
pre-metal dielectric layer 35 except above the photodiode 33. After
a first interlayer insulating layer 37, for insulating adjacent
metal lines from each other, is deposited and planarized on the
first metal line 36, a second metal layer is deposited on the first
interlayer insulating layer 37 and selectively patterned and etched
to generate a second metal line 38 positioned generally above the
first metal line 36.
[0021] After a second interlayer insulating layer 39 for insulating
the adjacent metal lines 38 from each other is deposited and
planarized, a third metal layer for generating an optical shielding
layer is deposited on the second interlayer insulating layer 39.
After a photoresist layer is deposited on the third metal layer and
patterned utilizing conventional photolithography exposure and
developing processes, the patterned photoresist layer is used as a
mask for etching the third metal layer, thereby forming an optical
shielding layer 40. While the metal layer within the unit pixel in
the prior art is completely etched except with regard to peripheral
circuits, the optical shielding layer 40 is formed in accordance
with the present invention in order to prevent light being from
transmitted into the circuits neighboring the unit pixel while
allowing transmission to the region of the color filter array and
the underlying photodiode. Further, the optical shielding layer 40
has an electrically shielded structure and may be used as a third
metal line for connecting neighboring circuits with the exception
of the photodiode 33.
[0022] Referring to FIG. 2B, the optical shielding layer 40 is used
as a mask to etch a portion of the second interlayer insulating
layer 39 using a dry etching technique, thereby generating first
and a second groove regions D and E, each having a predetermined
thickness. The thickness of the conventional color filter array is
determined by the thickness of the photosensitive layers deposited
to form each of the color filters. Controlling the thickness of the
color filter array, therefore, requires controlling the thickness
of the various photosensitive layers. In contrast, the present
invention permits the thickness of the color filter array to be
controlled by adjusting the dry etch process to remove a portion of
the second interlayer material to set the thickness (d1) of the
resulting homogeneous color filter array. The second interlayer
insulating layer 39 under the optical shielding layer 40 is not
etched and remains intact while a portion of the second interlayer
insulating layer 39 is removed in the first and second groove
regions D and E. These groove regions will, in turn, be used in
forming the green and blue color filters.
[0023] Referring to FIG. 2C, a color material, e.g., a negative
photoresist, is deposited on the dry-etched first groove region D
of the second interlayer insulating layer 39; a green color filter
41 is then formed by selective exposure and developing processes
that remove all but the illuminated regions of the photoresist
layer. It is preferable that the green color filter 41 be formed
first, simply because the number of green color filters in the
final array 41 are double that of either the red color filters or
the blue color filters. In other words, the minimum amount of scum
may accumulate under the green color filter while the maximum
amount of scum may be accumulated on the green color filter so that
the next process, e.g., the planarization process, may be used to
remove substantially all of the deposited scum.
[0024] Referring to FIG. 2D, a color material, e.g., a negative
photoresist, is deposited in the second groove region E of the
second interlayer insulating layer 39; and exposure and developing
processes are used to form the blue color filter 42. The thickness
of the blue color filter 42 is generally about 0.2 .mu.m greater
than that of the green color filter 41.
[0025] Referring to FIG. 2E, a negative photoresist is then
deposited in another groove region (not shown) and the developing
process is used to form the red color filter 43. The thickness of
the negative photoresist for the red color filter 43 is generally
about 0.2 .mu.m thicker than that of the negative photoresist for
the blue color filter 42 and the green color filter 41 is followed
by the red color filter 41. Accordingly, the color filter array is
formed as described above, wherein the color filters in the color
filter array have different thicknesses so that the color filter
array is stepped and the scum deposits F and G remain primarily on
the surface of the color filters.
[0026] Referring to FIG. 2F, the optical shielding layer 40 is used
as a polishing stop layer to perform a chemical mechanical
polishing (CMP). Accordingly, the green color filter 41, the blue
color filter 42 and the red color filter 43 are polished to be
planarized so that the red color filter 43 is removed during the
formation of the green filter 41. The scum F and G on the color
filter array is completely removed and the thickness in the color
filter array is standardized.
[0027] Referring to FIG. 2G, a low temperature oxide (LTO) layer 44
is deposited on the color filter array. The low temperature oxide
layer 44 protects the color filter array during the next
passivation process. The low temperature oxide layer 44 is used to
separate the photoresist from the oxygen gas, thereby preventing
oxidation of the color filter array. A passivation layer 45 is then
formed on the low temperature oxide layer 44. The passivation
process may be the same low temperature process that was also used
to form the low temperature oxide layer 44. The passivation process
is intended to further protect the image sensor by preventing
damage to the color filter array.
[0028] Micro lenses 46 are then formed on the passivation layer 45
in locations corresponding to the color filter array. Since the
color filter array according to the present invention was
previously planarized, the over coating material (OCM), used in the
prior art process as a planarizing layer, may be omitted from the
present process.
[0029] Because, as described above, the color filter array is
incorporated within the second interlayer insulating layer 39 and
the metal layer 40, the color filter array elements will suffer
less damage during the formation of the color filter array.
[0030] Further, because the metal layer 40 is used as a polishing
stop layer, the deposited scum may be removed from the surface of
the color filter array.
[0031] Because the etching thickness of the interlayer insulating
layer under the color filter array may be controlled, the resulting
thickness of the color filter array may also be controlled.
[0032] Because the color filters 41, 42 and 43 have the same
thickness, uniformity between two neighboring pixels may be
guaranteed. And because the distance between the photodiode 33 and
the color filter array may be reduced, the optical signal strength
may be increased.
[0033] While the invention has been shown and described with
respect to the preferred embodiments, it will be understood by
those skilled in the art that various changes and modifications may
be made without departing from the spirit and scope of the
invention as defined in the following claims.
* * * * *