U.S. patent application number 16/414669 was filed with the patent office on 2020-07-30 for cmos image sensor with compact pixel layout.
The applicant listed for this patent is SmartSens Technology (CAYMAN) Co., Ltd.. Invention is credited to Zexu Shao, Xin Wang, Chen Xu.
Application Number | 20200243593 16/414669 |
Document ID | 20200243593 / US20200243593 |
Family ID | 1000004954573 |
Filed Date | 2020-07-30 |
Patent Application | download [pdf] |
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United States Patent
Application |
20200243593 |
Kind Code |
A1 |
Xu; Chen ; et al. |
July 30, 2020 |
CMOS IMAGE SENSOR WITH COMPACT PIXEL LAYOUT
Abstract
An image sensor array of shared pixel units fabricated by a CMOS
technology, wherein each pixel unit includes a plurality of
photodiodes and respective transfer transistors and floating drains
whose layout constitutes mirror images. The plurality of
photodiodes each share a single reset transistor and source
follower amplifier transistor wherein the shared floating diode is
spaced at the minimum distance from a gate electrode of the source
follower transistor as is allowed by the CMOS fabrication
technology chosen to manufacture the image sensor array.
Inventors: |
Xu; Chen; (Shanghai, CN)
; Shao; Zexu; (Shanghai, CN) ; Wang; Xin;
(Shanghai, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SmartSens Technology (CAYMAN) Co., Ltd. |
Grand Cayman |
|
KY |
|
|
Family ID: |
1000004954573 |
Appl. No.: |
16/414669 |
Filed: |
May 16, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04N 5/378 20130101;
H01L 27/14641 20130101; H04N 5/3745 20130101; H01L 27/14643
20130101; H01L 27/14603 20130101 |
International
Class: |
H01L 27/146 20060101
H01L027/146; H04N 5/3745 20060101 H04N005/3745 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2019 |
CN |
201910072333.8 |
Mar 25, 2019 |
CN |
201910226135.2 |
Claims
1. An image sensor array of shared pixel units fabricated by a CMOS
technology providing improved compactness and performance, each
pixel unit comprising: a first and second photodiode, each having
associated adjacent first and second transfer transistors; a first
portion of a floating diode coupled to and adjacent to the first
transfer transistor and a second portion of a floating diode
coupled to and adjacent to the second transfer transistor; a first
portion of a reset transistor and a second portion of a reset
transistor; a first portion of a source follower amplifier
transistor and a second portion of a source follower transistor;
wherein the layout of the first photodiode, the first transfer
transistor, the first portion of the floating diode, the first
portion of the reset transistor and the first portion of the source
follower transistor is a mirror image of the layout of the second
photodiode, the second transfer transistor, the second portion of
the floating diode, the second portion of the reset transistor and
the second portion of the source follower transistor; and wherein
the first and second portions of the reset transistor form a single
reset transistor and the first and second portions of the source
follower transistor form a single source follower transistor; and
wherein the first and second portions of the floating diode form a
single shared floating diode which is spaced at the minimum
distance from a gate electrode of the source follower transistor as
is allowed by the CMOS fabrication technology chosen to manufacture
the image sensor array.
2. The pixel unit of the image sensor array of shared pixel units
of claim 1, wherein the spacing between the shared floating diode
and the gate electrode of the source follower transistor is chosen
from the range 0.05 micrometers to 0.25 micrometers.
3. The pixel unit of the image sensor array of shared pixel units
of claim 1, wherein the single reset transistor, the single shared
floating drain, and the single source follower transistor are
arranged along a horizontal line which constitutes the mirror image
axis; and wherein the floating drain is coupled through a doped
Silicon pathway on one side to the reset transistor and is spaced
at the minimum distance from the gate electrode of the source
follower transistor on its opposing side.
4. The pixel unit of the image sensor array of shared pixel units
of claim 3, additionally including a row select transistor
electrically coupled to the source follower transistor and
occupying a layout location at one of the two pixel unit corner
locations lying along a vertical line running perpendicular to the
horizontal mirror axis and through the source follower
transistor.
5. The image sensor array of shared pixel units of claim 1,
comprising n rows and m columns of photodiodes wherein each pixel
unit spans two rows and one column resulting in the use of n/2 rows
and m columns of pixel units to layout the array of shared pixel
units.
6. An image sensor array of shared pixel units fabricated by a CMOS
technology providing improved compactness and performance, each
pixel unit comprising: a first, second, third and fourth
photodiode, each having associated adjacent first, second, third,
and fourth transfer transistors; a first portion of a first
floating diode coupled to and adjacent to the first transfer
transistor, and a second portion of a first floating diode coupled
to and adjacent to the second transfer transistor; a third portion
of a second floating diode coupled to and adjacent to the third
transfer transistor, and a fourth portion of a second floating
diode coupled to and adjacent to the fourth transfer transistor; a
first portion of a reset transistor and a second portion of a reset
transistor; a first portion of a source follower amplifier
transistor and a second portion of a source follower transistor;
wherein the layout of the first photodiode, the first transfer
transistor, the first portion of the first floating diode, the
first portion of the reset transistor, and the first portion of the
source follower transistor is a mirror image of the layout of the
second photodiode, the second transfer transistor, the second
portion of the first floating diode, the second portion of the
reset transistor, and the second portion of the source follower
transistor; and wherein the layout of the third photodiode, the
third transfer transistor, and the third portion of the second
floating diode is a mirror image of the layout of the fourth
photodiode, the fourth transfer transistor, and the fourth portion
of the second floating diode; and wherein the first and second
portions of the reset transistor form a single reset transistor,
and the first and second portions of the source follower transistor
form a single source follower transistor; and wherein the first and
second portions of the first floating diode form a single shared
floating diode which is spaced at the minimum distance from a first
side of a gate electrode of the source follower transistor as is
allowed by the CMOS fabrication technology chosen to manufacture
the image sensor array; and wherein the third and fourth portions
of the second floating diode form a single shared photodiode which
is spaced at a minimum distance from a second side of the gate of
the source follower transistor as is allowed by the CMOS
fabrication technology chosen to manufacture the image sensor
array.
7. The pixel unit of the image sensor array of shared pixel units
of claim 6, wherein the spacing between the shared floating diodes
and the gate electrode of the source follower transistor is chosen
from the range 0.05 micrometers to 0.25 micrometers.
8. The pixel unit of the image sensor array of shared pixel units
of claim 6, wherein the single reset transistor, the first and
second shared floating drains, and the single source follower
transistor are arranged along a horizontal line which constitutes
the mirror image axis.
9. The pixel unit of the image sensor array of shared pixel units
of claim 8, wherein the layout of the first and second photodiodes
is a mirror image of the layout of the third and fourth photodiodes
wherein the mirror axis is the central vertical median line running
perpendicular to the horizontal mirror axis and through the source
follower transistor.
10. The pixel unit of the image sensor array of shared pixel units
of claim 6, additionally including a row select transistor
electrically coupled to the source follower transistor and
occupying a layout location at one of the two pixel perimeter
locations lying along the central vertical median line running
perpendicular to the horizontal mirror axis and through the source
follower transistor.
11. The image sensor array of shared pixel units of claim 6,
comprising n rows and m columns of photodiodes wherein each pixel
unit spans two rows and two columns resulting in the use of n/2
rows and m/2 columns of pixel units to layout the array of shared
pixel units.
12. The pixel unit of the image sensor array of shared pixel units
of claim 1, wherein the single reset transistor, the shared
floating drain, and the single source follower transistor are
arranged along a horizontal line which constitutes the mirror image
axis; and wherein a dual conversion gain transistor lies between
the reset transistor and the first floating drain along the
horizontal mirror axis.
13. The pixel unit of the image sensor array of shared pixel units
of claim 6, wherein the single reset transistor, the first and
second shared floating drains, and the single source follower
transistor are arranged along a horizontal line which constitutes
the mirror image axis; and wherein a dual conversion gain
transistor lies between the reset transistor and the first floating
drain along the horizontal mirror axis.
14. The pixel unit of the image sensor array of shared pixel units
of claim 4, wherein the row select transistor and the source
follower transistor are arranged such that their source to drain
channel direction lies along the vertical line.
15. The pixel unit of the image sensor array of shared pixel units
of claim 1, wherein the row select transistor and the source
follower transistor are arranged such that their source to drain
channel direction lies along the central vertical median line.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates generally to image sensors, and more
particularly to CMOS image sensors. The present invention provides
an image sensor array and circuit design employing a compact pixel
unit layout to enable very large pixel arrays. An image sensor
comprising the invented circuit design may be incorporated within a
digital camera.
Description of Related Art
[0002] An image capture device includes an image sensor and an
imaging lens. The imaging lens focuses light onto the image sensor
to form an image, and the image sensor converts the light into
electrical signals. The electric signals are output from the image
capture device to other components of a host electronic system. The
image capture device and the other components of a host electronic
system form an imaging system. Image sensors have become ubiquitous
and may be found in a variety of electronic systems, for example a
mobile device, a digital camera, a medical device, or a
computer.
[0003] A typical image sensor comprises a number of light sensitive
picture elements ("pixels") arranged in a two-dimensional array.
Such an image sensor may be configured to produce a color image by
forming a color filter array (CFA) over the pixels. The technology
used to manufacture image sensors, and in particular, complementary
metal-oxide-semiconductor ("CMOS") image sensors, has continued to
advance at great pace. For example, the demands of higher
resolution and lower power consumption have encouraged the further
miniaturization and integration of these image sensors. However,
miniaturization has led to pixel arrays becoming much larger in the
number of pixels, but not much larger in overall area due to the
use of narrower interconnect lines and smaller electronic
components in the pixels and in the readout and control circuits.
Miniaturization has also come with the loss of pixel
photosensitivity and dynamic range which require new approaches in
order to mitigate. Pixel units in which two or more photodiodes may
share a common readout circuit allowing the net area per pixel to
be reduced. New approaches to image sensor circuit design and
layout are required to further enable the employment of large
imaging arrays.
[0004] An advanced dynamic range enhancement technique may employ a
well capacity adjusting scheme. In this scheme, the well capacity
is increased one or more times during the time period in which the
photodiode charge is being transferred to the floating diode. The
employment of such a dynamic range enhancement scheme in the
context of an image sensor pixel is included in an alternative
embodiment of the invention. The dynamic range enhancement scheme
employed in the alternate embodiments of the invention involves
constructing a well capacity adjusting circuit by adding a
capacitor and a capacitor control transistor in an electrically
parallel configuration with the floating drain on the pixel cell.
By coupling the additional capacitor to the floating drain the
photodiode charge is transferred to a larger capacitance resulting
in a lower conversion gain and the ability to accommodate more
charge before saturation. This ability is employed while scenes
with high light intensity are being imaged and not while scenes
with normal and low light intensity are being imaged.
[0005] An additional way to affect conversion gain is to reduce the
net floating drain capacitance by reducing the diode capacitance by
minimizing the area of the floating drain and also reducing the
interconnect capacitance associated with the connection between the
floating drain and the gate electrode of the source follower
amplifier transistor by choosing a layout design of the pixel that
minimizes the separation between the floating drain and the gate
electrode of the SF transistor.
[0006] The present invention fulfills these needs and provides
further advantages as described in the following summary.
SUMMARY OF THE INVENTION
[0007] The present invention teaches certain benefits in
construction and use which give rise to the objectives described
below.
[0008] An image sensor comprises a pixel cell array comprising a
plurality of rows and columns of pixels wherein the readout of the
data placed on each column is converted by an analog to data (ADC)
circuit element into a binary coded datum held in a column memory
circuit element. The invented pixel cell array employs a compact
layout allowing increased compactness and conversion gain
performance.
[0009] A primary objective of the present invention is to provide
an image sensor pixel having advantages not taught by the prior
art.
[0010] Other features and advantages of the present invention will
become apparent from the following more detailed description, taken
in conjunction with the accompanying drawings, which illustrate, by
way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings illustrate the present invention.
In such drawings:
[0012] FIG. 1 is a diagram illustrating an imaging system including
a pixel cell array having image sensor pixel cells included in an
integrated circuit system, according to one embodiment of the
present invention;
[0013] FIG. 2 is an electrical schematic that illustrates a prior
art image sensor pixel cell with rolling shutter readout;
[0014] FIG. 3 is an electrical schematic diagram of pixel unit in
accordance with a first embodiment of the invention;
[0015] FIG. 4 is an electrical schematic diagram of a pixel unit in
accordance with a second embodiment of the invention;
[0016] FIG. 5 illustrates a portion of an image sensor array and
pixel units whose layout is in accordance with the first embodiment
of the invention;
[0017] FIG. 6 illustrates a portion of an image sensor array and
pixel units whose layout is in accordance with the second
embodiment of the invention;
[0018] FIG. 7 is an electrical schematic diagram of pixel unit in
accordance with a third embodiment of the invention;
[0019] FIG. 8 is an electrical schematic diagram of a pixel unit in
accordance with a fourth embodiment of the invention;
[0020] FIG. 9 illustrates a portion of an image sensor array and
pixel units whose layout is in accordance with the third embodiment
of the invention; and
[0021] FIG. 10 illustrates a portion of an image sensor array and
pixel units whose layout is in accordance with the fourth
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The above-described drawing figures illustrate the
invention, an image sensor pixel cell array employing a compact
layout allowing increased compactness and performance.
[0023] Various embodiments of the image sensor pixel cell are
disclosed herein. In the following description, numerous specific
details are set forth in order to provide a thorough understanding
of the present invention. One skilled in the relevant art will
recognize, however, that the techniques described herein can be
practiced without one or more of the specific details, or with
other methods, components, materials, etc. In other instances,
well-known structures, materials, or operations are not shown or
described in detail to avoid obscuring certain aspects. A substrate
may have a front side and a back side. Any fabrication process that
is performed from the front side may be referred to as a frontside
process while any fabrication process that is performed from the
back side may be referred to as a backside process. Structures and
devices such as photodiodes and associated transistors may be
formed in a front surface of a substrate. A dielectric stack that
includes alternating layers of metal routing layers and conductive
via layers may be formed on the front surface of a substrate.
[0024] Throughout the specification and claims, the following terms
take the meanings explicitly associated herein, unless the context
clearly dictates otherwise. The terms "coupled" and "connected,"
which are utilized herein, are defined as follows. The term
"connected" is used to describe a direct connection between two
circuit elements, for example, by way of a metal line formed in
accordance with normal integrated circuit fabrication techniques.
In contrast, the term "coupled" is used to describe either a direct
connection or an indirect connection between two circuit elements.
For example, two coupled elements may be directly coupled by way of
a metal line, or indirectly connected by way of an intervening
circuit element (e.g., a capacitor, resistor, or by way of the
source/drain terminals of a transistor). The term "circuit" means
either a single component or a multiplicity of components, either
active or passive, that are coupled together to provide a desired
function. The term "signal" means at least one current, voltage, or
data signal. Should the invention involve a stacked chip
arrangement, the front sides of two chips may be directly connected
since the electrical interconnects on each chip will most commonly
be formed on the front sides of each chip, or the front side of one
chip may be directly connected to the back side of the second,
which may employ through chip interconnects. Although circuit
elements may be fabricated on the back side, when reference is made
to certain circuit elements residing within or formed in a
substrate, this is generally accepted to mean the circuits reside
on the front side of the substrate.
[0025] FIG. 1 is a diagram illustrating one example of an imaging
system 100 including an example pixel array 102 having a plurality
of image sensor pixels included in an example integrated circuit
system with features in accordance with the teachings of the
present invention. As shown in the depicted example, imaging system
100 includes pixel array 102 coupled to control circuitry 108 and
readout circuitry 104, which is coupled to function logic 106.
Control circuitry 108 and readout circuitry 104 are in addition
coupled to state register 112. In one example, pixel array 102 is a
two-dimensional (2D) array of image sensor pixels (e.g., pixels P1,
P2 . . . , Pn). As illustrated, each pixel is arranged into a row
(e.g., rows R1 to Ry) and a column (e.g., column C1 to Cx) to
acquire image data of a person, place, object, etc., which can then
be used to render a 2D image of the person, place, object, etc. In
one example, after each pixel has acquired its image data or image
charge, the image data is readout by readout circuitry 104 using a
readout mode specified by state register 112 and then transferred
to function logic 106. In various examples, readout circuitry 104
may include amplification circuitry, analog-to-digital (ADC)
conversion circuitry, or otherwise. State register 112 may include
a digitally programmed selection system to determine whether
readout mode is by rolling shutter or global shutter. Function
logic 106 may simply store the image data or even manipulate the
image data by applying post image effects (e.g., crop, rotate,
remove red eye, adjust brightness, adjust contrast, or otherwise).
In one example, readout circuitry 104 may readout a row of image
data at a time along readout column lines (illustrated), or may
readout the image data using a variety of other techniques (not
illustrated), such as a serial readout or a full parallel readout
of all pixels simultaneously. In one example, control circuitry 108
is coupled to pixel array 102 to control operational
characteristics of pixel array 102. Some aspects of the operation
of control circuitry 108 may be determined by settings present in
state register 112. For example, control circuitry 108 may generate
a shutter signal for controlling image acquisition. In one example,
the shutter signal is a global shutter signal for simultaneously
enabling all pixels within pixel array 102 to simultaneously
capture their respective image data during a single acquisition
window. In another example, the shutter signal is a rolling shutter
signal such that each row, column, or group of pixels is
sequentially enabled during consecutive acquisition windows.
[0026] FIG. 2 is an electrical schematic that illustrates one
example of an image sensor pixel cell 200 with rolling shutter
readout found in the prior art. This figure and example pixel are
provided to simplify explanation of pixel operation in anticipation
of a description of an example of the present invention. Each
sensor pixel 200 includes a photodiode 210 (e.g., photosensitive
element) and pixel support circuitry 211 as shown. Photodiode 210
may be a "pinned" photodiode as is commonly present in CMOS image
sensors. Photodiode 210 may be disposed on a sensor chip of a
stacked die system, while pixel support circuitry 211 may be
disposed on a separate circuit chip. In the example of FIG. 2,
pixel support circuitry 211 includes a reset transistor 220, source
follower (SF) amplifier transistor 225, and row select transistor
230 on a circuit chip coupled to a transfer transistor 215 and
photodiode 210 on a sensor chip of a stacked die system as shown.
An amplifier transistor in a source follower configuration is one
in which the signal is input on the gate electrode and taken out on
the source electrode. In another example, not shown, pixel support
circuitry includes row select transistor 230 on a circuit chip
coupled to a reset transistor 220, source follower (SF) transistor
225, transfer transistor 215 and photodiode 210 on a sensor chip of
a stacked die system. During operation, photosensitive element 210
photo-generates charge in response to incident light during an
exposure period. Transfer transistor 215 is coupled to receive a
transfer signal TX, which causes transfer transistor 215 to
transfer the charge accumulated in photodiode 210 to floating
diffusion (FD) node 217. Floating diffusion 217 is in effect the
drain of the transfer transistor 215 while the photodiode 210 is
the source of transfer transistor 215. In one embodiment, transfer
transistor 215 is a metal-oxide semiconductor field-effect
transistor (MOSFET). Reset transistor 220 is coupled between power
rail VDD and floating diffusion node 217 to reset sensor pixel 200
(e.g., discharge or charge floating diffusion node 217 and
photodiode 210 to a preset voltage) in response to a reset signal
RST. Floating diffusion node 217 is coupled to control the gate
terminal of source-follower transistor 225. Source-follower
transistor 225 is coupled between power rail VDD and row select
transistor 230 to amplify a signal responsive to the charge on the
floating diffusion FD node 217. Row select transistor 230 couples
the output of pixel circuitry from the source-follower transistor
225 to the readout column, or bit line 235, in response to a row
select signal RS. Photodiode 210 and floating diffusion node 217
are reset by temporarily asserting or enabling the reset signal RST
and transfer signal TX. The accumulation period or accumulating
window (e.g., exposure period) begins when the transfer signal TX
is disabled, which permits incident light to photo-generate charge
in photodiode 210. As photo-generated electrons accumulate in
photodiode 210, its voltage decreases (electrons are negative
charge carriers). The voltage or charge on photodiode 210 is
representative of the intensity of the light incident on photodiode
210 during the exposure period. At the end of the exposure period,
the reset signal RST is disabled, which turns off the reset
transistor 220 and isolates floating diffusion FD node 217 from
VDD. The transfer signal TX is then enabled to couple photodiode
210 to floating diffusion node 217. The charge is transferred from
photodiode 210 to the floating diffusion FD node 217 through
transfer transistor 215 which causes the voltage of floating
diffusion FD node 217 to drop by an amount proportional to
photo-generated electrons accumulated on photodiode 210 during the
exposure period. The accumulation period or exposure window
actually ends when the transfer transistor 215 is disabled, since
the photodiode 210 is actually still accumulating charge while the
transfer transistor 215 is enabled and transferring charge to the
floating diffusion 217.
[0027] An important design metric in image sensors is dynamic
range, which is defined as the logarithmic ratio between the
largest non-saturating photocurrent and the smallest detectable
photocurrent. For a sensor with a fixed saturation charge, also
referred to as full well capacity, saturation limits the highest
signal. Generally, the smallest detectable photocurrent is
dominated by reset sampling noise of the photodiode 210 and the
floating diffusion 217. Efforts to reduce the impact of reset
sampling noise on dynamic range have relied on correlated double
sampling (CDS). CDS is a technique of taking two samples of a
signal out of the pixel and subtracting the first from the second
to remove reset sampling noise. Generally, the sampling is
performed once immediately following reset of the photodiode 210
and floating diffusion 217 and once after the photodiode 210 has
been allowed to accumulate charge and transfer it to the floating
diffusion 217. The subtraction is typically performed in peripheral
circuitry outside of the pixel 200 and may increase conventional
image sensor area although it may not increase pixel area. An image
sensor utilizing a rolling shutter readout mode may incorporate CDS
with only added peripheral circuit elements and no additional
circuit elements in the pixel 200. An image sensor utilizing global
shutter, however, may require multiple capacitors and transistors
inside the pixel 200 which decreases the fill factor. It is
advantageous to maintain reduced fill factor by partitioning the
additional components required for CDS onto a circuit chip separate
from and stacked on top of a sensor chip.
[0028] FIG. 3 is an electrical schematic diagram of dual pixel unit
300 whose layout is illustrated in FIG. 5 in accordance with a
first embodiment of the invention. The electrical schematic diagram
shown in FIG. 3 more clearly illustrates the electrical connections
between the electrical components depicted in the layout
illustrated in FIG. 5. The component names are common in both
figures and will be used in the description of the operation of the
dual pixel unit 300. FIG. 3 depicts a set of two transfer
transistors (TXa, TXb), each coupled to respective photodiodes
(PDa, PDb) and coupled to and sharing a floating drain (FD) for
accumulating and transferring an image charge in response to light
incident upon the photodiodes. Also residing on pixel unit 300 are
a reset transistor (RST) and an amplifier transistor (SF) for
converting the image charge to an image signal (PIXO) for coupling
out of pixel unit 300. A power supply provides voltage VDD to both
reset transistor RST and source follower transistor SF. In order to
read out image signal PIXO in rolling shutter mode only row select
transistor RS is required to transfer read signal rs_pix to
off-pixel readout circuits.
[0029] FIG. 4 is an electrical schematic diagram of dual pixel unit
400 whose layout is illustrated in FIG. 6 in accordance with a
second embodiment of the invention. The electrical schematic
diagram shown in FIG. 4 more clearly illustrates the electrical
connections between the electrical components depicted in the
layout illustrated in FIG. 6. The component names are common in
FIGS. 3, 4, 5, and 6, and will be used in the description of the
operation of the pixel unit. FIG. 4 illustrates a pixel unit 400
like that illustrated in FIG. 3 with the addition of a signal
dynamic range enhancing feature based on modifying conversion gain.
Pixel unit 400 includes Double Conversion Gain capability as
provided by dynamic range enhancement capacitor Cdcg and capacitor
control transistor DCG. By switching in capacitor Cdcg through the
action of capacitor control transistor DCG the pixel unit
conversion gain is modified to accommodate higher or lower
illumination incident of the photodiodes leading to increased
dynamic range.
[0030] FIG. 5 illustrates a layout of a portion of an image sensor
pixel array 500 comprising dual pixel units 510 whose layout is in
accordance with a first embodiment of the invention. The layout of
pixel unit 510 shows an arrangement of two photodiodes PDa and PDb
with their respective transfer transistors TXa and TXb. Both
transfer transistors couple to and share a common floating drain
FD. The two photodiodes and their respective transfer transistors
and their respective portions of their shared floating drain FD are
shown to be mirror images of each other about the horizontal line
axis H H'. In addition, along the horizontal line H H' are located
a reset transistor RST and a source follower amplifier transistor
SF. Horizontal line axis H H' bisects transistor RST and the gate
electrode SFg of transistor SF so that their upper portions and
lower portions are mirror images. Reset transistor RST and floating
drain FD are connected by a doped Silicon pathway. FIG. 5 also
illustrates a drain region SFd of transistor SF and a source region
SFs of transistor SF, as well as a drain region RSd of transistor
RS and a source region RSs of transistor RS. Transistors RS and SF
are shown to have their respective source and drain regions lie
along a vertical line V V' wherein the vertical line is
perpendicular to horizontal line H H'. Row select transistor RS is
shown to occupy a layout location at the upper of the two pixel
unit corner locations lying along a vertical line running
perpendicular to the horizontal mirror axis and through the source
follower transistor.
[0031] A key element of the first embodiment of the invention is
that floating drain FD is separated from gate electrode SFg of
transistor SF by the minimum distance allowed by the design rules
of the CMOS fabrication technology employed to fabricate the image
sensor array. This minimum separation allows for the minimization
of interconnect capacitance due to an interconnect lead that may
connect FD to gate electrode SFg and in so doing also minimize the
conversion gain to enhance the dynamic range performance of the
pixel unit. The spacing between the shared floating diode and the
gate electrode of the source follower transistor may be chosen from
the range 0.05 micrometers to 0.25 micrometers. The layout
illustrated in FIG. also improves array compactness by sharing
floating drain FD and transistors RST, SF and RS among two
photodiodes.
[0032] Typically the sizes and placements of the photodiodes within
pixel cell array 500 are chosen such that an array of dual pixel
units 510 will result in all of the photodiodes of the assembled
array falling on a uniform grid. In the instance dual pixel units
510 are employed to form a color image sensor, light filters of
various colors may be placed at each photodiode location within the
incident light path. A commonly known two-by-two arrangement of
light filters is a Bayer filter pattern which consists of a red, a
blue, and two green filters (RGrGbB).
[0033] FIG. 6 illustrates a layout of a portion of an image sensor
pixel array 600 comprising dual pixel units 610 whose layout is in
accordance with the second embodiment of the invention. The layout
of pixel unit 610 shows an arrangement of two photodiodes PDa and
PDb with their respective transfer transistors TXa and TXb. Both
transfer transistors couple to and share a common floating drain
FD. The two photodiodes and their respective transfer transistors
and their respective portions of their shared floating drain FD are
shown to be mirror images of each other about the horizontal line
axis H H'. In addition, along the horizontal line H H' is located a
reset transistor RST, a capacitor control transistor DCG, and a
source follower amplifier transistor SF. Horizontal line axis H H'
bisects transistors RST, DCG and the gate electrode SFg of
transistor SF so that their upper portions and lower portions are
mirror images. Reset transistor RST and capacitor control
transistor DCG are connected by a doped Silicon pathway. Capacitor
control transistor DCG and floating drain FD may also be connected
by a doped Silicon pathway. FIG. 6 also illustrates a drain region
SFd of transistor SF and a source region of transistor SF as well
as a drain region RSd of transistor RS and a source region RSs of
transistor RS. Transistors RS and SF are shown to have their
respective source and drain regions lie along a vertical line V V'
wherein the vertical line is perpendicular to horizontal line H
H'.
[0034] A key element of the second embodiment of the invention is
that floating drain FD is separated from gate electrode SFg of
transistor SF by the minimum distance allowed by the design rules
of the CMOS fabrication technology employed to fabricate the image
sensor array. This minimum separation allows for the minimization
of interconnect capacitance due to an interconnect lead that may
connect FD to gate electrode SFg and in so doing also minimize the
conversion gain to enhance the dynamic range performance of the
pixel unit. The spacing between the shared floating diode and the
gate electrode of the source follower transistor may be chosen from
the range 0.05 micrometers to 0.25 micrometers. The layout
illustrated in FIG. 6 also improves array compactness by sharing
floating drain FD and transistors RST, DCG, SF and RS among two
photodiodes.
[0035] FIG. 7 is an electrical schematic diagram of dual pixel unit
700 whose layout is illustrated in FIG. 9 in accordance with a
third embodiment of the invention. The electrical schematic diagram
shown in FIG. 7 more clearly illustrates the electrical connections
between the electrical components depicted in the layout
illustrated in FIG. 9. The component names are common in both
figures and will be used in the description of the operation of the
pixel unit. FIG. 7 depicts a set of two pairs of transfer
transistors (TXa and TXb, TXc and TXd), each pair of transistors
coupled to respective photodiodes (PDa and PDb, PDc and PDd). Each
pair of transfer transistors are also coupled to and sharing a
floating drain (FD1 and FD2) for accumulating and transferring an
image charge in response to light incident upon the photodiodes.
Also residing on pixel unit 700 are a reset transistor (RST) and an
amplifier transistor (SF) for converting the image charge to an
image signal (PIXO) for coupling out of pixel unit 700. A power
supply provides voltage VDD to both reset transistor RST and source
follower transistor SF. In order to read out image signal PIXO in
rolling shutter mode, only row select transistor RS is required to
transfer read signal rs_pix to off-pixel readout circuits.
[0036] FIG. 8 is an electrical schematic diagram of dual pixel unit
800 whose layout is illustrated in FIG. 10 in accordance with a
fourth embodiment of the invention. The electrical schematic
diagram shown in FIG. 8 more clearly illustrates the electrical
connections between the electrical components depicted in the
layout illustrated in FIG. 10. The component names are common in
FIGS. 7, 8, 9, and 10, and will be used in the description of the
operation of the pixel unit. FIG. 8 illustrates a pixel unit like
that illustrated in FIG. 7 with the addition of a signal dynamic
range enhancing feature based on modifying conversion gain. Pixel
unit 800 includes Double Conversion Gain capability as provided by
dynamic range enhancement capacitor Cdcg and capacitor control
transistor DCG. By switching in capacitor Cdcg through the action
of capacitor control transistor DCG the pixel unit conversion gain
is modified to accommodate higher or lower illumination incident of
the photodiodes leading to increased dynamic range.
[0037] FIG. 9 illustrates a layout of a portion of an image sensor
pixel array 900 comprising quad (four) pixel units 910 whose layout
is in accordance with the third embodiment of the invention. The
layout of pixel unit 910 shows an arrangement of a first pair of
transfer transistors (TXa and TXb) and a second pair of transfer
transistors (TXc and TXd), with each pair of transistors coupled to
respective photodiodes (PDa and PDb, PDc and PDd). The first pair
of transfer transistors are coupled to and share a floating drain
FD1 and the second pair of transfer transistors are coupled to and
share a floating drain FD2, both for accumulating and transferring
an image charge in response to light incident upon the photodiodes.
Also residing on pixel unit 910 are a reset transistor (RST) and an
amplifier transistor (SF) for converting the image charge to an
image signal (PIXO) for coupling out of pixel unit 910. The two
photodiodes PDa and PDb and their respective transfer transistors
TXa and TXb and their respective portions of their shared floating
drain FD1 are shown to be mirror images of each other about the
horizontal line axis H H'. The two photodiodes PDc and PDd and
their respective transfer transistors TXc and TXd and their
respective portions of their shared floating drain FD2 are shown to
be mirror images of each other about the horizontal line axis H H'.
In addition, along the horizontal line H H' is located the reset
transistor RST and the source follower amplifier transistor SF.
Horizontal line axis H H' bisects transistor RST and the gate
electrode SFg of transistor SF so that their upper portions and
lower portions are mirror images. Although not illustrated, reset
transistor RST and floating drain FD1 may be connected by a doped
Silicon pathway. FIG. 9 also illustrates a drain region SFd of
transistor SF and a source region SFg of transistor SF as well as a
drain region RSd of transistor RS and a source region RSs of
transistor RS. Transistors RS and SF are shown to have their
respective source and drain regions lie along a vertical line V V'
wherein the vertical line is perpendicular to horizontal line H H'.
FIG. 9 additionally illustrates that the layout of the first pair
of photodiodes and transfer transistors is a mirror image of the
layout of the second pair of photodiodes wherein their mirror axis
is the central vertical median line (of the four photodiodes)
running perpendicular to the horizontal mirror axis and through the
source follower transistor.
[0038] A key element of the third embodiment of the invention is
that floating drains FD1 and FD2 are separated from gate electrode
SFg of transistor SF by the minimum distance allowed by the design
rules of the CMOS fabrication technology employed to fabricate the
image sensor array. This minimum separation allows for the
minimization of interconnect capacitance due to an interconnect
lead that may connect FD1 and FD2 to gate electrode SFg and in so
doing also minimize the conversion gain to enhance the dynamic
range performance of the pixel unit. The spacing between the shared
floating diode and the gate electrode of the source follower
transistor may be chosen from the range 0.05 micrometers to 0.25
micrometers. The layout illustrated in FIG. 9 also improves array
compactness by sharing transistors RST, SF, and RS among four
photodiodes.
[0039] Typically, the sizes and placements of the photodiodes
within pixel cell array 900 are chosen such that an array of quad
pixel units 910 will result in all of the quad pixel units of the
assembled array falling on a uniform grid. In the instance quad
pixel units 910 are employed to form a color image sensor, light
filters of various colors may be placed at each photodiode location
within the incident light path. A commonly known two-by-two
arrangement of light filters is a Bayer filter pattern which
consists of a red, a blue, and two green filters (RGrGbB). The quad
pixel unit arrangement illustrated in FIG. 9 may assign color
filter R to photodiode PDa, color filter Gb to photodiode PDb,
color filter Gr to photodiode PDc, and color filter B to photodiode
PDd. An advantage of the fourth embodiment of the invention as
illustrated in FIG. 9 is that the sensitivity of the two green
filtered photodiodes is more closely matched as a result of the
invented layout.
[0040] FIG. 10 illustrates a layout of a portion of an image sensor
pixel array 1000 comprising quad (four) pixel units 1010 whose
layout is in accordance with a fourth embodiment of the invention.
The layout of pixel unit 1010 shows an arrangement of a first pair
of transfer transistors (TXa and TXb) and a second pair of transfer
transistors (TXc and TXd), with each pair of transistors coupled to
respective photodiodes (PDa and PDb, PDc and PDd). The first pair
of transfer transistors are coupled to and share a floating drain
FD1 and the second pair of transfer transistors are coupled to and
share a floating drain FD2, both for accumulating and transferring
an image charge in response to light incident upon the photodiodes.
Also residing on pixel unit 910 are a reset transistor (RST) and an
amplifier transistor (SF) for converting the image charge to an
image signal (PIXO) for coupling out of pixel unit 910. The two
photodiodes PDa and PDb and their respective transfer transistors
TXa and TXb and their respective portions of their shared floating
drain FD1 are shown to be mirror images of each other about the
horizontal line axis H H'. The two photodiodes PDc and PDd and
their respective transfer transistors TXc and TXd and their
respective portions of their shared floating drain FD2 are shown to
be mirror images of each other about the horizontal line axis H H'.
In addition, along the horizontal line H H' is located the reset
transistor RST, a capacitor control transistor DCG, and the source
follower amplifier transistor SF. Horizontal line axis H H' bisects
transistor RST, DCG, and the gate electrode SFg of transistor SF so
that their upper portions and lower portions are mirror images.
Reset transistor RST and capacitor control transistor DCG are
connected by a doped Silicon pathway. Capacitor control transistor
DCG and floating drain FD may also be connected by a doped Silicon
pathway. FIG. 10 also illustrates a drain region SFd of transistor
SF and a source region SFg of transistor SF as well as a drain
region RSd of transistor RS and a source region RSs of transistor
RS. Transistors RS and SF are shown to have their respective source
and drain regions lie along a vertical line V V' wherein the
vertical line is perpendicular to horizontal line H H'. FIG. 10
additionally illustrates that the layout of the first pair of
photodiodes and transfer transistors is a mirror image of the
layout of the second pair photodiodes wherein their mirror axis is
the central vertical median line (of the four photodiodes) running
perpendicular to the horizontal mirror axis and through the source
follower transistor.
[0041] A key element of the fourth embodiment of the invention is
that floating drains FD1 and FD2 are separated from gate electrode
SFg of transistor SF by the minimum distance allowed by the design
rules of the CMOS fabrication technology employed to fabricate the
image sensor array. This minimum separation allows for the
minimization of interconnect capacitance due to an interconnect
lead that may connect FD1 and FD2 to gate electrode SFg and in so
doing also minimize the conversion gain to enhance the dynamic
range performance of the pixel unit. The spacing between the shared
floating diode and the gate electrode of the source follower
transistor may be chosen from the range 0.05 micrometers to 0.25
micrometers. The layout illustrated in FIG. 10 also improves array
compactness by sharing transistors RST, SF, and RS among four
photodiodes.
[0042] Reference throughout this specification to "one embodiment,"
"an embodiment," "one example," or "an example" means that a
particular feature, structure, or characteristic described in
connection with the embodiment or example is included in at least
one embodiment or example of the present invention. Thus, the
appearances of the phrases such as "in one embodiment" or "in one
example" in various places throughout this specification are not
necessarily all referring to the same embodiment or example.
Furthermore, the particular features, structures, or
characteristics may be combined in any suitable manner in one or
more embodiments or examples. Directional terminology such as
"top", "down", "above", "below" are used with reference to the
orientation of the figure(s) being described. Also, the terms
"have," "include," "contain," and similar terms are defined to mean
"comprising" unless specifically stated otherwise. Particular
features, structures or characteristics may be included in an
integrated circuit, an electronic circuit, a combinational logic
circuit, or other suitable components that provide the described
functionality. In addition, it is appreciated that the figures
provided herewith are for explanation purposes to persons
ordinarily skilled in the art and that the drawings are not
necessarily drawn to scale.
[0043] The above description of illustrated examples of the present
invention, including what is described in the Abstract, are not
intended to be exhaustive or to be limited to the precise forms
disclosed. While specific embodiments of, and examples for, the
invention are described herein for illustrative purposes, various
equivalent modifications are possible without departing from the
broader spirit and scope of the present invention. Indeed, it is
appreciated that the specific example structures and materials are
provided for explanation purposes and that other structures and
materials may also be employed in other embodiments and examples in
accordance with the teachings of the present invention. These
modifications can be made to examples of the invention in light of
the above detailed description. The terms used in the following
claims should not be construed to limit the invention to the
specific embodiments disclosed in the specification and the claims.
Rather, the scope is to be determined entirely by the following
claims, which are to be construed in accordance with established
doctrines of claim interpretation.
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