U.S. patent application number 13/027898 was filed with the patent office on 2012-08-16 for image sensors with stacked photo-diodes.
Invention is credited to Jaroslav Hynecek.
Application Number | 20120205765 13/027898 |
Document ID | / |
Family ID | 46636251 |
Filed Date | 2012-08-16 |
United States Patent
Application |
20120205765 |
Kind Code |
A1 |
Hynecek; Jaroslav |
August 16, 2012 |
IMAGE SENSORS WITH STACKED PHOTO-DIODES
Abstract
This describes color filter arrangements for image sensor arrays
that are formed using image sensor pixels with stacked
photo-diodes. The stacked photo-diodes may include first and second
photo-diodes and may have the ability to separate color signal
according to the depth of carrier generation in a silicon
substrate. A single color filter may be formed over the stacked
photo-diodes to provide full red-green-blue sensing capability.
Charge drain regions may also be formed at different depths in the
silicon substrate. If the charge drain regions are formed beneath
the stacked photo-diodes in the substrate, full red-green-blue
color sensing may be achieved without the use of color filters.
Inventors: |
Hynecek; Jaroslav; (Allen,
TX) |
Family ID: |
46636251 |
Appl. No.: |
13/027898 |
Filed: |
February 15, 2011 |
Current U.S.
Class: |
257/432 ;
257/E31.127 |
Current CPC
Class: |
H01L 27/14621 20130101;
H01L 27/14632 20130101; H01L 27/1461 20130101; H01L 27/14627
20130101; H01L 27/14645 20130101 |
Class at
Publication: |
257/432 ;
257/E31.127 |
International
Class: |
H01L 31/0232 20060101
H01L031/0232 |
Claims
1. An image sensor comprising: a substrate; a first plurality of
image sensor pixels formed in the substrate; and a second plurality
of image sensor pixels formed in the substrate, wherein the first
plurality of image sensor pixels each comprise a first charge drain
region formed at a first depth within the semiconductor substrate
and wherein the first charge drain regions comprise n+ doped
regions.
2. The image sensor defined in claim 1, wherein the second
plurality of image sensor pixels each comprise a second charge
drain region formed at a second depth that is different than the
first depth within the semiconductor substrate.
3. The image sensor defined in claim 2, wherein the image sensor
lacks a color filter array so that image light is received by the
first and second plurality of image sensor pixels without passing
through any intervening color filter array element.
4. The image sensor defined in claim 1, wherein the first and
second plurality of image sensor pixels comprise stacked
photo-diodes.
5. The image sensor defined in claim 2, wherein the first and
second plurality of image sensor pixels comprise stacked
photo-diodes.
6. An image sensor comprising: a substrate; a first plurality of
image sensor pixels formed in the substrate; and a second plurality
of image sensor pixels formed in the substrate, wherein the first
plurality of image sensor pixels each comprise a first charge drain
region formed at a first depth within the semiconductor substrate,
wherein the second plurality of image sensor pixels each comprise a
second charge drain region formed at a second depth that is
different than the first depth within the semiconductor substrate,
wherein the first and second plurality of image sensor pixels
comprise stacked photo-diodes, wherein each of the stacked
photo-diodes comprises: a first photo-diode formed using a first
p-type doped region and a first n-type doped region; and a second
photo-diode formed using a second p-type doped region and a second
n-type doped region, wherein the first photodiode is stacked
vertically below the second photodiode.
7. An image sensor comprising: a substrate; a first plurality of
image sensor pixels formed in the substrate; and a second plurality
of image sensor pixels formed in the substrate, wherein the first
plurality of image sensor pixels each comprise a first charge drain
region formed at a first depth within the semiconductor substrate,
wherein the second plurality of image sensor pixels each comprise a
second charge drain region formed at a second depth that is
different than the first depth within the semiconductor substrate,
wherein the first and second plurality of image sensor pixels
comprise stacked photo-diodes, wherein each of the stacked
photo-diodes comprises: a first photo-diode formed using a first
p-type doped region and a first n-type doped region; and a second
photo-diode formed using a second p-type doped region and a second
n-type doped region, wherein the first photodiode is stacked
vertically below the second photodiode, wherein the first
photo-diode collects charge generated by blue light and wherein the
second photo-diode collects charge generated by light selected from
a group consisting of: green light and yellow light.
8. The image sensor defined in claim 7, wherein each of the stacked
photo-diodes further comprises: a p+ doped potential barrier region
formed between the first photo-diode and the second
photo-diode.
9. The image sensor defined in claim 2, wherein the second charge
drain regions comprise n+ doped regions.
10. The image sensor defined in claim 2, wherein the image sensor
comprises a front-side-illuminated image sensor, wherein the
substrate comprises a surface in which the first and second
plurality of image sensors are formed, and wherein light enters the
image sensor through the surface.
11. The image sensor defined in claim 2, wherein the image sensor
comprises a back-side-illuminated image sensor, wherein the
substrate comprises a top surface in which the first and second
plurality of image sensors are formed and a bottom surface, and
wherein light enters the image sensor through the bottom
surface.
12-28. (canceled)
Description
BACKGROUND
[0001] Typical complementary metal-oxide-semiconductor (CMOS) image
sensors sense light by converting impinging photons into electrons
that are integrated (collected) in sensor pixels. Once the
integration cycle is complete, collected charge is converted into a
voltage signal, which is supplied to output terminals of an image
sensor. This charge to voltage conversion is performed within each
sensor pixel. The pixel output voltage (i.e., an analog voltage
signal) is transferred to the output terminals using various pixel
addressing and scanning schemes. The analog voltage signal can also
be converted on-chip to a digital equivalent before reaching the
chip output.
[0002] The sensor pixels include buffer amplifiers (i.e., source
followers) that drive sensing lines connected to the pixels through
address transistors. After the charge to voltage conversion and
after the resulting voltage signal has been read out from the
pixels, the pixels are reset in preparation for a successive charge
accumulation cycle. In pixels that include floating diffusions (FD)
serving as charge detection nodes, the reset operation is performed
by turning on a reset transistor that connects the floating
diffusion node to a voltage reference.
[0003] Removing charge from the floating diffusion node using the
reset transistor, however, generates kTC-reset noise as is well
known in the art. The kTC noise must be removed using correlated
double sampling (CDS) signal processing technique in order to
achieve desired low noise performance. Typical CMOS image sensors
that utilize CDS typically require four transistors (4T) per pixel.
An example of the 4T pixel circuit with a pinned photo-diode can be
found in Guidash (U.S. Pat. No. 5,991,184), incorporated herein as
a reference.
[0004] Color sensing in typical single-chip CMOS and charge-coupled
device (CCD) image sensors is accomplished by placing
light-absorbing/color-transmitting filters over the sensor pixels
in a predetermined pattern. The different pixels in a given pixel
sub-group or sub-array is therefore sensitive to a certain
wavelength band of the electromagnetic spectrum. Signals gathered
using the different pixel subgroups that have different color
sensitivity are then used to construct super-pixel signals using
various interpolating and signal processing schemes in an attempt
to recover the resolution that has been lost as a result of using
color filters.
[0005] Examples of conventional color filter array patterns are
found in Bayer (U.S. Pat. No. 3,971,065) and Kasano ("A 2.0 um
Pixel Pitch MOS Image Sensor with an Amorphous Si Film Color
Filter," Digest of Technical Papers ISCC, vol. 48, February 2005,
pp. 348349), incorporated herein as references. The color filtering
schemes as described in Bayer and Kasano may undesirably sacrifice
resolution and sensitivity as a result of light absorption when
color filters are used.
[0006] In an effort to counteract this reduction in resolution and
sensitivity, stacked image sensor pixels that have three
photo-diodes placed above each other have been developed by Foveon
(see, Merrill U.S. Pat. No. 6,894,265, incorporated herein as a
reference). No color filter is placed over the image pixels of
Merrill. The three photo-diodes are formed in a silicon substrate
at different depths. Impinging photons entering the image sensor
may generate carriers in the silicon substrate. The generated
carriers are collected at the different depths by the corresponding
photo-diodes. A voltage signal is then obtained by connecting the
three buried photo-diodes (i.e., the photo-diodes are "buried" in
the substrate) to circuitry formed over the surface of the silicon
substrate. The voltage signal is sensed, processed, and reset using
conventional image pixel readout/reset operations.
[0007] Forming image sensors using this approach provides improved
resolution and sensitivity because no color filters are used (i.e.,
no photons are absorbed in the color filters). It may, however, be
difficult and costly to form three photo-diodes that are buried
deeply in the silicon substrate. It may also be challenging to
sense charge collected by all three buried photo-diodes without
adding noise.
[0008] It would therefore be desirable to be able to provide image
sensors that exhibit an improved resolution, sensitivity, and
noise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a diagram of a conventional Bayer color filter
arrangement.
[0010] FIG. 2A is a diagram of an illustrative
green-clear-green-clear color filter arrangement that is used for
image sensor arrays having pixels with stacked photo-diodes in
accordance with an embodiment of the present invention.
[0011] FIG. 2B is a diagram of an illustrative
green-magenta-green-magenta color filter arrangement that is used
for image sensor arrays in accordance with an embodiment of the
present invention.
[0012] FIG. 3 is a diagram of an illustrative color signal
processing scheme that can be used in conjunction with the color
filter arrangement of FIG. 2 in accordance with an embodiment of
the present invention.
[0013] FIG. 4 is a diagram of an illustrative cyan-clear-cyan-clear
color filter arrangement that is used for image sensor arrays
having pixels with stacked and non-stacked photo-diodes in
accordance with an embodiment of the present invention.
[0014] FIG. 5 is a simplified cross-sectional diagram of an image
sensor array having charge drains at different depths in accordance
with an embodiment of the present invention.
[0015] FIG. 6 is a diagram of an illustrate
clear-blue/green-clear-blue/green color filter arrangement that is
used for image sensor arrays having pixels with stacked
photo-diodes in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION
[0016] FIG. 1 represents a conventional filter arrangement often
referred to as the Bayer color filter array (CFA). As shown in FIG.
1, pixel array 101 consists of individual pixels 106 arranged in a
regular grid pattern. The pattern of four by four pixels forms
sub-array 104 that is repeated throughout a larger array that can
contain millions of individual pixels. Sub-array 104 includes eight
green filters 102, four blue filters 105, and four red filters 103
formed over pixels 106. Filters 102, 105, and 103 are formed from
an organic material that is saturated by a suitable dye that
absorbs and transmits light having a selected wavelength band.
Micro-lenses are typically formed on top of the color filters to
help focus light towards a desired region in each image sensor
pixel 106 (i.e., to direct the incoming light away from the
insensitive regions such as wires and transistors).
[0017] While the Bayer filter arrangement is functional, it may,
however, exhibit several drawbacks. For example, filters 102, 105,
and 103 absorb a large portion of the light spectrum so that only
the wavelength associated with the desired color is passed through.
Absorbing the large portion of incoming light in this way may
significantly reduce image sensor sensitivity.
[0018] The 4.times.4 sub-array 104 also reduces resolution and
produces a Moire effect in the captured image. The Moire effect is
wavy pattern that can manifest itself when a captured image
contains areas with repetitive detail that exceeds the resolution
of the image sensor. The missing color information (i.e., portions
of the incoming light that have been absorbed by the filters) must
be recovered using a suitable interpolation technique, which
further reduces image resolution and increases noise. Moreover, the
color filters must have a certain minimum thickness in order to
properly absorb light from the undesired wavelength bands. This
thickness requirement makes it harder for the micro-lenses to
properly focus light on the sensitive regions in pixels 106.
[0019] It may therefore be desirably to use different color filter
arrays and pixels that are formed using stacked photo-diodes (e.g.,
photo-diodes that have potential energy wells formed at different
depths within a semiconductor substrate). The use of stacked
photo-diodes enables the detection of carriers (e.g., electrons)
generated at different depths in a silicon substrate.
[0020] Incoming light that penetrates the silicon substrate of the
image sensor may include various wavelength bands, each of which
generates electrons at the different depths. Because the electrons
generated a the different depths can be temporarily stored in
separate potential energy wells, it is therefore possible for
pixels with stacked photo-diodes to separate the color signal
within the silicon substrate into multiple color bands, thereby
reducing the number (or thickness) of color filters that must be
formed over the image sensor pixels.
[0021] FIG. 2A shows one suitable filter layout arrangement for
pixels that are formed using stacked photo-diodes. A stacked
photo-diode may include a buried bottom photo-diode formed below a
top photo-diode. The top photo-diode and the buried photo-diode may
be separated by a p+ energy barrier (as an example). The top
photo-diode may be capable of collecting charge generated from
mostly blue light, whereas the bottom photo-diode may be capable of
collecting charge generated from green light and/or red light
(e.g., the bottom photo-diode may be capable of collecting charge
generated from green light, red light, yellow light, etc.).
[0022] As shown in FIG. 2A, pixel array 201 may include individual
pixels 205 that are organized into 3.times.3 sub-arrays 204. Green
filters may be formed over pixels 202, whereas no filter may be
formed over pixels 203 (e.g., clear filters may be formed over
pixels 203). The 3.times.3 sub-array 204 may include four green
filters and five clear filters (e.g., filters that pass through
yellow and blue light) arranged in a checkerboard pattern. Pixels
203 may be capable of sensing blue and yellow light because pixels
203 are formed using stacked photo-diodes. For example, pixel 203
may include a top photo-diode configured to detect predominantly
blue light and a bottom photo-diode configured to detect yellow
light. Pixels 202 may be formed using non-stacked photo-diodes to
detect green light or cyan (sometimes referred to as blue-green)
light. If desired, pixels 202 may also be formed using stacked
photo-diodes to increase dynamic range, to reduce pixel crosstalk,
etc.
[0023] Another suitable color filter arrangement is shown in FIG.
2B. As shown in FIG. 2B, pixel array 251 is grouped into 3.times.3
sub-arrays 254. Sub-array 254 may include four green pixels 252 and
five magenta pixels 253 arranged in a checkerboard pattern. A green
filter may be formed over each green pixel 252, whereas a magenta
filter may be formed over each magenta pixel 253. The green pixels
may be formed using non-stacked photo-diodes, whereas the magenta
pixels may be formed using non-stacked or stacked photo-diodes. As
a result image sensor arrays that use magenta filters may exhibit
higher light sensitivity relative to those with red filters. FIG. 3
shows a simplified schematic diagram illustrating steps involved in
a signal processing scheme that can be used to process signals
generated by pixel array 201 of FIG. 2A. As shown in FIG. 3, pixel
signals 301 (e.g., signals that include blue signal B' gathered
using the top photo-diode in pixel 203, yellow signal Y gathered
using the bottom photo-diode in pixel 203, and green signal G
gathered using the bottom photo-diode in pixel 202) are first
processed using signal interpolation circuit 302. Circuit 302
replaces missing pixel color by interpolating signals from
neighboring pixels. This interpolation step may be necessary,
because the 3.times.3 pixel matrix has reduced resolution similar
to the 4.times.4 Bayer pixel array.
[0024] Interpolated signals B', Y, and G may be fed to circuit 303
for color adjustments. During color adjustment operations, circuit
303 may subtract a small portion of yellow signal Y from blue
signal B' to compensate for the fact that yellow signal Y also
passes through the top photo-diode where blue signal B' is
generated, thereby producing adjusted blue signal B. Red signal R
may be obtained by subtracting green signal G from yellow signal Y.
Circuit 303 does not adjust green signal G. No pre-processing is
performed on signal G, which may be desirable because the green
signal is the dominant signal used for constructing luminance.
[0025] Signals B, R, and G are then fed to final color correction
circuit 304. Final color correction circuit 304 may generate final
RGB colors 305 (including signals B.sub.o, R.sub.o, and G.sub.o).
If desired, other color processing and color correction schemes
that are suitable for use with stacked photo-diodes and that are
well known in the art may also be implemented.
[0026] Another suitable color filter arrangement for pixels with
stacked photo-diodes is shown in FIG. 4. As shown in FIG. 4, pixel
array 401 is grouped into 3.times.3 sub-arrays 404. Sub-array 404
may include four cyan pixels 402 and five yellow-blue pixels 403
arranged in a checkerboard pattern. A cyan filter may be formed
over each cyan pixel 402, whereas a clear filter may be formed over
each yellow-blue pixel 403. The yellow-blue pixels may be formed
using stacked photodiodes, whereas the cyan pixels may be formed
using non-stacked photo-diodes. In general, cyan filters absorb
less light in comparison to green filters. As a result, image
sensor arrays that use cyan filters may exhibit higher light
sensitivity relative to those with green filters.
[0027] The blue signal gathered using this type of image sensor has
full sensor resolution, because every pixel may be used to deliver
an independent blue signal. For example, the top photo-diode of
pixels 402 may deliver a first set of blue signals, whereas the top
photo-diode of pixels 403 may deliver a second set of blue signals.
As a result, no interpolation is necessary. This feature may be
helpful in minimizing the Moire effect and in increasing the
overall sensor resolution.
[0028] FIG. 5 is a simplified cross-sectional diagram for image
sensor array 501 having pixels formed with stacked photo-diodes. As
shown in FIG. 5, array 501 may include barrier 504 (e.g., a p+
doped region) formed at a depth Xb in the silicon substrate.
Barrier 504 may serve to provide color separation between the blue
and yellow (green and red) signals. For example, charge signals
generated above barrier 504 is sensed separately from charge
signals generated below barrier 504.
[0029] Array 501 may, for example, include yellow-blue pixels 510
and blue-green pixels 512 arranged in an alternating checkerboard
pattern (see, e.g., FIG. 5). Yellow-blue pixel 510 may include
charge drain region 505 formed at a depth Xm, whereas blue-green
pixel 512 may include charge drain region 506 formed at a depth Xr.
Charge drain regions 505 and 506 may include n+ doped regions that
drain away (remove) carriers generated at their respective depths.
Pixel 510 may include a top photo-diode (e.g., a first photodiode
formed using a first p-type doped region and a first n-type doped
region) formed in region 514 and a buried photo-diode (e.g., a
second photodiode formed using a second p-type doped region and a
second n-type doped region, where the first photodiode is stacked
vertically above the second photodiode) formed in region 518. Pixel
512 may include a top photo-diode formed in region 516 and a buried
photo-diode formed in region 520. Charge (e.g., carriers such as
electrons) generated in the regions below Xm and Xr are not
collected in the top nor bottom photo-diodes. Instead, the charge
is drained away from the pixels. If desired, charge drain region
505 need not be formed for pixel 510, because any charge generated
below Xm can be collect by region 518 without affecting colors in
the visible spectrum.
[0030] Sensor array 501 may also include passivation oxide layer
502 (or other suitable material) deposited over pixels 510 and 512.
An array of micro-lenses may be formed over layer 502. No color
filters are deposited on top of sensor array 501, thereby reducing
the height of the image sensor (e.g., sensor array 501 lacks a
color filter array so that image light is received by the
photo-diodes in pixels 510 and 512 without passing through any
intervening color filter array element). As a result, the
sensitivity and focusing performance of the micro-lenses are
significantly improved over arrays that use color filters.
[0031] The arrangement of sensor array 501 in FIG. 5 is merely
illustrative. As shown in FIG. 5, sensory array 501 receives light
from an upper surface of the semiconductor substrate in which the
stacked photo-diodes are formed. This type of sensor illumination
technique is commonly known as front-side-illumination. If desired,
sensory array 501 may be illuminated from the lower (back) surface
of the semiconductor substrate (e.g., using
back-side-illumination). If the back-side-illumination arrangement
is implemented, distances Xr, Xm, and Xb will be measured from the
lower surface of the substrate. For example, regions 514 and 516
will be closer to the lower surface compared to regions 518 and
520, respectively, and charge drain regions 505 and 506 will be
even further away from the lower surface compared to regions 518
and 520, respectively.
[0032] A pixel arrangement that can be used with the image sensor
array described in connection with FIG. 5 is shown in FIG. 6. As
shown in FIG. 6, array 601 may include blue-green pixels 602 and
blue-yellow pixels 603 arranged in a checkerboard pattern.
Blue-green pixels 602 may have charge drains formed at depth Xr,
whereas blue-yellow pixels 603 may have charge drains formed at
depth Xm. For example, blue-green pixel 602 may include a top
photo-diode operable to detect blue light and a bottom photo-diode
operable to detect green light, whereas blue-yellow pixel 603 may
include a top photo-diode operable to detect blue light and a
bottom photo-diode operable to detect yellow light. Sensor array
601 configured in this way exhibits improved sensitivity and
resolution because no color filters is needed.
[0033] Various embodiments have been described illustrating image
sensors that have stacked photo-diodes. The image sensors with
stacked photo-diodes may be used in any electronic device.
[0034] Image sensors may have pixels that are formed using stacked
photo-diodes. The stacked photo-diodes may sense color by
vertically separating generated photo-carriers. Each pixel may
therefore provide at least one color-coded signal without requiring
the use of conventional light absorbing color filters.
[0035] In one suitable embodiment, an image sensor array may be
formed using green filters and clear filters arranged in a
checkerboard pattern. The pixels formed under the green filters may
include stacked or non-stacked photo-diodes, whereas the pixels
formed under the clear filters may include stacked
photo-diodes.
[0036] In another suitable embodiment, an image sensor array may be
formed using cyan filters and clear filters arranged in a
checkerboard pattern. The pixels formed under the green cyan
filters may include non-stacked photo-diodes, whereas the pixels
formed under the clear filters may include stacked
photo-diodes.
[0037] In another suitable embodiment, an image sensor array may be
formed without any color filter arrays. This image sensor array may
include blue-yellow pixels and blue-green pixels. The blue-yellow
pixels may each include a charge drain formed at a first depth
within a silicon substrate, whereas the blue-green pixels may each
include a charge drain formed at a second depth within the silicon
substrate. The second depth may be less than the first depth. The
charge drains formed at the different depths may serve to drain the
charge generated below their respective depths. The blue-yellow
pixels and the blue-green pixels may be stacked photo-diodes.
[0038] The foregoing is merely illustrative of the principles of
this invention which can be practiced in other embodiments.
* * * * *