U.S. patent application number 09/863851 was filed with the patent office on 2001-09-27 for method and structure for reducing leakage currents of active area diodes and source/drain diffusions.
Invention is credited to Dungan, Thomas, Kopley, Thomas Edward, Vook, Dietrich W..
Application Number | 20010024864 09/863851 |
Document ID | / |
Family ID | 24060592 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024864 |
Kind Code |
A1 |
Kopley, Thomas Edward ; et
al. |
September 27, 2001 |
Method and structure for reducing leakage currents of active area
diodes and source/drain diffusions
Abstract
A fabrication method for providing isolation between adjacent
regions of an integrated circuit includes providing a guard layer
over field edges that are the interfaces between field oxide
regions and diffusion regions in which dopant is introduced. The
guard layer will inhibit introduction of dopant along the
field-edge, so that a substantially dopant-free transition strip is
formed. The transition strip inhibits current leakage from the
active region to the field oxide region. In one embodiment, the
active region is an active area diode, such as used to form an
Active Pixel Sensor (APS) pixel. The guard layer is biased so as to
further inhibit current leakage during circuit operation. In
another embodiment, the method is used in the fabrication of
transistors for APS pixels having an overlay photodiode
structure.
Inventors: |
Kopley, Thomas Edward;
(Menlo Park, CA) ; Vook, Dietrich W.; (Menlo Park,
CA) ; Dungan, Thomas; (Fort Collins, CO) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC
Legal Department, DL429
Intellectual Property Administration
P. O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
24060592 |
Appl. No.: |
09/863851 |
Filed: |
May 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09863851 |
May 22, 2001 |
|
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|
09517635 |
Mar 3, 2000 |
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Current U.S.
Class: |
438/448 ;
257/E21.544; 257/E27.131; 257/E27.132 |
Current CPC
Class: |
H01L 27/14609 20130101;
H01L 27/14689 20130101; H01L 27/14603 20130101; H01L 21/761
20130101 |
Class at
Publication: |
438/448 |
International
Class: |
H01L 021/76 |
Claims
What is claimed is:
1. A method of providing isolation between adjacent regions of an
integrated circuit comprising the steps of: selecting a circuit
layout for fabrication of said integrated circuit on a substrate,
including defining adjacent substrate locations for a first region
and a second region for which leakage current from said second
region to said substrate is undesired during operation of said
integrated circuit, at least said second region being an active
region within which a dopant is to be introduced; forming a guard
layer on said substrate such that said guard layer resides on a
peripheral portion of said second region, said peripheral portion
of said second region extending along an edge of said first region;
and introducing said dopant into said second region to establish
predetermined electrical characteristics within said second region,
said guard layer inhibiting introduction of said dopant into said
peripheral portion, thereby leaving a transition strip within said
second region and along said edge of said first region.
2. The method of claim 1 wherein said step of forming said guard
layer includes patterning a conductive material to reside on said
peripheral portion of said second region, said method further
comprising a step of forming an electrical connection to said guard
layer such that said guard layer is coupled to a source of a fixed
voltage.
3. The method of claim 1 further comprising a step of forming field
oxide within said first region, thereby providing a field oxide
region, said guard layer being formed along a plurality of sides of
said second region such that said guard layer resides along an
entire boundary between said field oxide region and said second
region.
4. The method of claim 3 wherein said second region is a
source/drain region of a transistor.
5. The method of claim 1 wherein said step of selecting said
circuit layout includes designing a circuit layout for a
photodetection circuit that utilizes CMOS technology, said first
region being a field oxide region.
6. The method of claim 1 further comprising a step of forming an
active layer on said substrate to provide a photodiode region that
overlays said substrate.
7. The method of claim 5 wherein said step of forming said guard
layer includes depositing electrically conductive material to
surround a photodiode region, said photodiode region being said
second region.
8. The method of claim 7 further comprising a step of connecting
said guard layer to a source of a fixed voltage, thereby forming a
steady-state guard layer.
9. The method of claim 1 wherein said step of forming said guard
layer includes overlapping said guard layer onto a peripheral
portion of said first region.
10. The method of claim 1 wherein said step of forming said guard
layer includes patterning conductive material to define a gate of a
MOSFET and to provide said guard layer along an interface between a
source/drain region of said MOSFET and a field oxide region, said
transition strip thereby isolating said source/drain region from
said field oxide region.
11. The method of claim 10 wherein said step of patterning
conductive material includes electrically connecting said guard
layer to said gate.
12. A method of forming an active area diode for an imaging circuit
comprising steps of: forming a field oxide region on a substrate,
said field oxide region having an edge that at least partially
defines a geometry of an active area diode region; forming a
conductive layer on said substrate such that said conductive layer
generally follows a contour of said edge of said field oxide
region, while extending onto a peripheral portion of said active
area diode region; and modifying electrical characteristics of said
active area diode region such that said active area diode region is
responsive to light energy, said conductive layer inhibiting said
modification of said electrical characteristics within said
peripheral region.
13. The method of claim 12 further comprising a step of connecting
said conductive layer to a constant source of voltage.
14. The method of claim 12 wherein said step of modifying said
electrical characteristics includes diffusing a dopant into said
active area diode region.
15. The method of claim 12 wherein said step of forming said
conductive layer includes patterning polysilicon to extend onto
peripheral portions of each of said field oxide region and said
active area diode region.
16. A method of forming an overlay photodiode structure of an
imaging circuit comprising steps of: forming a field oxide region
on a substrate, said field oxide region having an edge that at
least partially defines a geometry of an active region of a
transistor; forming a conductive layer on said substrate such that
said conductive layer generally follows a contour of said edge of
said field oxide region, while extending onto a peripheral portion
of said active region; forming said transistor on said substrate,
including doping said active region; and forming at least one
overlayer to provide a photodiode arrangement that overlays said
substrate, said photodiode arrangement being responsive to light
energy and being electrically coupled to said transistor.
17. The method of claim 16 wherein said step of forming said
transistor includes fabricating said transistor with at least one
other transistor to provide a multi-transistor imaging pixel within
an array of adjacent pixels having said overlay photodiode
structure.
18. An integrated circuit having CMOS transistors comprising: a
substrate; a first region along a surface of said substrate, said
first region having an edge, said first region being a field oxide
region; a second region along said surface, said second region
having a periphery which is adjacent to said edge of said first
region; a guard layer residing on said surface and following
adjacency of said first and second regions, said guard layer
extending onto only a peripheral portion of said second region;
wherein said second layer includes said peripheral transition
portion that is substantially free of a selected dopant and
includes a remaining active portion in which said selected dopant
is implanted, said peripheral transition portion being defined by
said guard layer extending onto said second region.
19. The integrated circuit of claim 18 wherein said guard layer has
a fixed connection to a source of a constant voltage.
20. The integrated circuit of claim 19 wherein said second region
is a photodiode region.
21. The integrated circuit of claim 18 wherein said second region
is a source/drain region of a MOSFET, said guard layer being
contiguous with a gate of said MOSFET.
Description
TECHNICAL FIELD
[0001] The invention relates generally to methods for providing
isolation between adjacent regions of an integrated circuit and
more particularly to methods of reducing current leakage from an
active region to a field oxide region in a circuit, such as an
image sensor circuit.
BACKGROUND ART
[0002] Complementary metal-oxide-semiconductor (CMOS) technology is
used in the design and fabrication of integrated circuits for many
types of applications. CMOS technology uses n-type transistors
(NMOS) and p-type transistors (PMOS) that are formed by doping
selected regions of a substrate and by forming layers on the
substrate. A p-type material, such as boron, may be introduced to a
bulk silicon substrate in a blanket ion implantation step. Field
oxide regions and n-type regions may then be formed using well
known integrated circuit fabrication techniques. Similarly, the
processes for depositing conductive and dielectric layers on the
substrate to complete the circuit are known.
[0003] One general area for applying CMOS technology that has
received significant attention is image capture and processing.
Imaging applications include video, still photography, and
navigation that is based upon optical detection. Linear or
two-dimensional arrays of pixels are formed along the surface of
the substrate, with each pixel periodically generating a signal
having a current or voltage level that is indicative of the
intensity of light incident to that pixel. A typical
three-transistor pixel 10 that is used in current CMOS image
sensors is shown in FIG. 1. Sensors that use this technology are
often referred to as CMOS active pixel sensors (APS). A timing
diagram for the operation of the three-transistor pixel 10 is shown
in FIG. 2. In typical operation, a node N1 is set to a
predetermined voltage V.sub.dd') (which may be different than the
circuit operating voltage V.sub.dd) by turning on an n-channel
reset transistor 12. The state of the reset transistor is
determined by controlling a reset voltage (V.sub.reset). In FIG. 2,
V.sub.reset goes high at time T0, causing the node N1 to ramp to
V.sub.dd'. At time T1, the reset transistor 12 is turned off and
photoelectrons are generated by the incident light on a photodiode
14. The photoelectrons are injected into node N1, reducing the
voltage on that node by a value of
V.sub.sense=V.sub.dd'-(I.sub.photo.times.T.sub.il-
luminate/C.sub.N1). In this equation, I.sub.photo is the
photocurrent induced by the incident light, T.sub.illuminate is the
illumination time period and C.sub.N1 is the capacitance on node
N1. Both V.sub.dd' and V.sub.sense can in principle be read out of
the pixel through a source-follower 16 by activating a row-select
transistor 18. In a two-dimensional array of pixels, there
typically are row-select transistors and column-select transistors
that allow the pixels to be sequentially sampled. The row-select
transistor 18 is activated by manipulating a row-select (RS)
signal. The illumination on the pixel is then proportional to
V.sub.dd'-V.sub.sense=I.sub.photo.times.T.sub.illumi-
nate/C.sub.N1. Persons skilled in the art refer to this operation
as Correlated Double Sampling (CDS). Sampling occurs at time T2
before T.sub.illuminate and time T3 during T.sub.illuminate. The
pixel is reset at time T4, since V.sub.reset is caused to go
high.
[0004] One of the major problems of using CMOS technology in
imaging sensors is the relatively large dark current intrinsic to
the CMOS process. A significant cause of the large dark current is
the reverse-bias diode leakage in the photodiode 14 of a pixel, as
well as in the source diffusion of the MOS field effect transistor
(MOSFET) 12 connected to the photodiode. The diode leakage is often
dominated by the edge leakage currents. Furthermore, in
deep-submicron generations of CMOS technology, this leakage current
will only increase and take major engineering efforts to
suppress.
[0005] The physical layout of the CMOS APS pixel 10 of FIG. 1 will
be described with reference to FIGS. 3, 4 and 5. FIG. 3 is a top
view of the circuit layout of the APS pixel of FIG. 1, showing the
various layers and diffusion regions. FIG. 4 is a top view that
isolates the active area diode of FIG. 3, while FIG. 5 is a side
sectional view of FIG. 4. The active area diode is illustrated as
being an n+/p diode fabricated in a p-substrate or p-well. However,
the descriptions of the operations and problems apply equally to a
p+/n diode in an n-substrate or n-well. The pn-junction of the
diode 14 is defined by the p-substrate or p-well 20, which will be
referred to as the p-layer. Electrical connections 22 and 24 to the
diode are formed by depositing layers that are in contact with an
n+ region 26 and a p+ region 28, respectively. The n+ region 26 may
be formed by ion implantation or other doping techniques into the
active area that is identified as the photodiode 14 in FIG. 3. The
active area is delineated by a field oxide (FOX) region 30.
Typically, the FOX region is a thick layer of silicon dioxide
(SiO.sub.2) that electrically isolates the active area from other
regions of the substrate, which is typically a silicon substrate.
There are several well known processes for forming the FOX. Any of
the processes may be used to form the FOX of FIGS. 3-5. However,
each of the known processes is susceptible to the formation of a
high density of defects at the edges of the FOX. The defects are
primarily due to mechanical stress effects and contamination. The
high density of defects located within the pn-junction diode's
depletion region contributes to the high reverse-bias leakage
current found at the field-edge of the diode. There has been much
research and development regarding providing process steps (such as
oxide deposition, etching and annealing) that minimize the edge
leakage. However, the edge leakage problem is expected to become
worse as the CMOS process is applied at the deep-submicron
level.
[0006] Referring specifically to FIGS. 1 and 3, the gates of the
three transistors 12, 16 and 18 are formed by a patterned
polysilicon layer. The polysilicon layer is identified by hatching
in FIG. 3. The reset transistor 12 has a gate 32, the row-select
transistor 18 has a gate 34, while the transistor 16 has a gate 36
that is electrically coupled to the N1 node 22. The source/drain
regions of the three transistors are formed by diffusions using the
appropriate dopants. As can be seen in FIGS. 1 and 3, the
transistors 16 and 18 have source/drain regions that are formed by
a common diffusion region 38.
[0007] Dark current in the CMOS APS pixel 10 with an active area
photodiode 14 is caused mainly by the photodiode leakage, which
bleeds charge from the node 22 (N1). This reduces the voltage on
the node, even when the reset transistor 12 is turned off during
the illumination time T.sub.illuminate. Therefore, the diode
leakage produces an offset in the differential voltage produced by
the illumination, given by
V.sub.dd'-(I.sub.photo+I.sub.dark).times.T.sub.illuminate/C.sub.N1.
For low light illumination, it is possible for I.sub.dark to be
approximately the same as I.sub.photo. Thus, the dark current
limits the dynamic range of the image sensor. Dark current
reduction has usually been addressed by attempting to lower the
intrinsic diode leakage of the CMOS technology via processing
steps. This minimization of the diode leakage characteristics is
very difficult in advanced deep-submicron CMOS technologies that
use advanced field oxide formation techniques and have much higher
doping concentrations in the diode.
[0008] Field-edge leakage can also be a significant problem at the
transistor level of the pixel 10. Each of the three MOSFETs 12, 16
and 18 is formed by growing a thin gate oxide over the active area
of the transistor and then forming the gates 32, 34 and 36 by
patterning strips of polysilicon over the thin gate oxide. The n+
dopant is implanted after the gates have been formed. For each
transistor, two separate n+/p diodes are then simply the source and
drain diffusions in combination with the p+ contact. Because the
source and drain diffusions are delineated by field oxides on three
sides, they have the same field-edge leakage problem as the
isolated n+/p diode.
[0009] There have been attempts to isolate active components of two
complementary transistors. One such attempt is described in U.S.
Pat. No. 5,847,433 to Kerber, which is not related to imaging
applications. In order to produce isolated active regions of a CMOS
circuit, a field plate is formed and doped jointly with wells
located between the field plate. Therefore, the field plate
includes an n-doped region and a p-doped region, as well as a
boundary region between the two doped regions. Each doped region is
electrically connected to the well that is located beneath it. As a
result, a flat band condition prevails at the substrate surface.
While the Kerber method provides an improvement, dark current
leakage is still a problem, particularly in imaging
applications.
[0010] What is needed is an integrated circuit fabrication method
that is applicable to imaging applications and that provides
further reductions in the edge leakage component of diodes and
source/drain diffusions of MOSFETs.
SUMMARY OF THE INVENTION
[0011] A fabrication method for providing isolation between regions
of an integrated circuit includes forming a guard layer on a
peripheral portion of at least one of two adjacent regions of a
substrate, so that when dopant is introduced into the substrate, an
intermediate transition strip is left substantially dopant-free.
The transition strip inhibits current leakage from an active region
to an adjacent region, which is preferably a field oxide region. In
the preferred embodiment, the active region is an active area
diode, such as the one used in an Active Pixel Sensor (APS) pixel.
Also in the preferred embodiment, the guard layer is a conductive
material that is connected to a source of a bias voltage. For an
APS pixel, the biased guard layer blocks the doping of the active
area diode during fabrication and effectively blocks edge leakage
current during circuit operation, as long as the guard layer is
biased below the threshold voltage of the MOS system in which it is
formed. The use of some process technologies will require bias in
accumulation.
[0012] In one embodiment, the guard layer is formed of a patterned
conductive material (such as polysilicon) on the peripheral portion
of the active region. The guard layer extends across the peripheral
portion to at least the edge of the adjacent field oxide region.
Optionally, the guard layer extends onto a peripheral portion of
the field oxide region, thereby relaxing the alignment tolerances.
The overlap of the guard layer onto the active area will depend
upon factors such as the alignment specifications of the
photolithography tools used in the fabrication technology, but will
typically be about the same as the overlap of the guard layer onto
the field oxide region, if such an overlap is utilized. Since
depositing polysilicon is a standard part of a CMOS circuit, such a
modification of the existing active area diode is relatively simple
to accomplish.
[0013] In an alternative application, the guard layer is patterned
conductive material that extends along the interface between a
source/drain region of a MOSFET and a field oxide region. The
dopant-free transition strip that is formed by using the guard
layer as a shield in a subsequent step of introducing dopant into
the source/drain region isolates the doped region from the field
oxide region. In this embodiment, the guard layer is preferably
formed of polysilicon and is simultaneously formed with and
connected to the gate of the MOSFET. When the gate is biased to a
high voltage (i.e., the MOSFET is turned on), there will be an
inversion layer generated under the guard layer, thereby extending
along the interface of the source/drain region and the field oxide
edge. Optionally, the guard layer may be patterned about both of
the source/drain regions. Connecting the polysilicon guard layer to
the gate of the MOSFET has advantages with regard to fabrication
simplicity. However, the practice may result in an unacceptable
leakage level (due to the gate-edge leakage) and/or an unacceptable
increase in source/drain capacitance. In addition, because of the
standard overlap of the gate region onto the source/drain regions,
gate-to-source/drain capacitance will increase. If the extra
leakage or capacitance is problematic, it is possible to separate
the gate polysilicon from the polysilicon guard layer. In this
case, the polysilicon guard layer should be biased by another
contact.
[0014] In a third embodiment, the guard layer is used for MOSFETs
that share diffusions. MOSFETs that share a common diffusion are
often used in CMOS circuits to lower the total layout area. By
depositing the guard layer along the interface between an adjacent
field oxide region and a common diffusion region, edge leakage is
reduced. In this embodiment, the guard layer may be connected to
one of the gates of the two MOSFETs, but not both, unless the two
gates are tied to the same circuit node. For applications in which
the two transistors are operated separately, the guard layer may be
connected to one of the two gates or may be connected to a separate
contact.
[0015] An advantage of the invention is that depositing the guard
layer prior to introducing the dopant into an active region
provides control over edge leakage without significantly diverging
from existing fabrication techniques. In the preferred embodiment,
the guard layer is formed of polysilicon that is formed at the same
time as the gates of the transistors. The guard layer has the same
thickness as the gates and preferably has the same width, but this
is not critical.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic of a three-transistor CMOS Active
Pixel Sensor (APS) pixel for use in imaging applications.
[0017] FIG. 2 is a timing diagram for the three signals controlled
in the operation of the APS pixel of FIG. 1.
[0018] FIG. 3 is a top view of a prior art circuit layout of layers
and diffusion regions for forming the APS pixel of FIG. 1.
[0019] FIG. 4 is a top view of a simplification of the photodiode
of the APS pixel of FIGS. 1 and 3.
[0020] FIG. 5 is a side sectional view of the photodiode of FIG. 4,
taken along lines 5-5.
[0021] FIG. 6 is a top view of a circuit layout of layers and
diffusion regions for forming the APS pixel of FIG. 1, but with a
leakage-control guard layer in accordance with one embodiment of
the invention.
[0022] FIG. 7 is a top view of the photodiode of FIG. 6.
[0023] FIG. 8 is a side sectional view of the photodiode of FIG. 7,
taken along lines 8-8.
[0024] FIG. 9 is a top view of a simplification of a transistor in
accordance with the prior art.
[0025] FIG. 10 is a side sectional view of the prior art transistor
of FIG. 9, taken along lines 10-10.
[0026] FIG. 11 is a top view of the transistor of FIG. 9, but with
a guard layer formed in accordance with an embodiment of the
invention.
[0027] FIG. 12 is a side sectional view of the transistor of FIG.
11, taken along lines 12-12.
[0028] FIG. 13 is a top view of the transistor of FIG. 9, but with
a guard layer in accordance with another embodiment of the
invention.
[0029] FIG. 14 is a top view of a simplification of adjacent
MOSFETs that share a common diffusion in accordance with the prior
art.
[0030] FIG. 15 is a top view of one embodiment of forming a guard
region along the common diffusion of FIG. 14 in accordance with the
invention.
[0031] FIG. 16 is a top view of another embodiment of forming a
guard layer for the common diffusion of FIG. 14 in accordance with
the invention.
[0032] FIG. 17 is a top view of a circuit layout of layers and
diffusion regions for forming an overlay photodiode in accordance
with the prior art.
[0033] FIG. 18 is a side sectional view of the prior art device of
FIG. 17, taken along lines 18-18.
[0034] FIG. 19 is a top view of a circuit layout of the layers and
diffusion regions of FIG. 17, but with a guard layer in accordance
with the invention.
[0035] FIG. 20 is a side sectional view of the device of FIG. 19,
taken along lines 20-20.
[0036] FIG. 21 is a process flow of steps for forming the circuits
in accordance with the invention.
DETAILED DESCRIPTION
[0037] As previously noted, one of the major problems of using CMOS
technology in imaging sensors, such as those used in video, still
photography and optical navigation, is that the CMOS process has a
relatively large inherent dark current. One of the major causes of
the large dark current is the reverse-bias diode leakage in the
photodiode. This diode leakage is dominated in many cases by edge
leakage currents. The invention that will be described with
reference to FIGS. 6-8 will address this cause. Another cause is
the reverse-bias diode leakage in the source/drain diffusions of
MOSFETs connected to the photodiode. The means of addressing this
cause of the large dark current will then follow.
[0038] With reference to FIG. 6, the circuit layout that is shown
is consistent with the circuit layout of FIG. 3. However, a guard
layer 40 (shown as being hatched) has been incorporated into the
fabrication of the photodiode 14. The guard layer is preferably
formed of polysilicon at the same time that the three polysilicon
gates 32, 34 and 36 are formed. Since the incorporation of the
guard layer does not affect the schematic representation of the
electrical circuit, the reference numerals that were used in FIGS.
1 and 3 are also used in FIG. 6. The polysilicon guard layer 40 is
formed before the dopant is introduced into the active area of the
photodiode. Therefore, the polysilicon guard layer will block the
n+ implant, leaving a transition strip below the portion of the
guard layer that overlaps the region surrounded by the field oxide
30.
[0039] FIGS. 7 and 8 correspond to FIGS. 4 and 5, but include the
polysilicon guard layer 40. The transition strip 42 that is
substantially dopant free can be seen in FIG. 8. The polysilicon
guard layer 40 is illustrated as being deposited on a portion of
the gate oxide layer that is used in forming the gates of the three
transistors. This provides further compatibility between the
conventional steps of forming the transistor gates and the steps
for forming the guard layer. However, other approaches may be
utilized. For example, if the guard layer 40 is to be allowed to
have a "floating" potential, rather than being biased, the portion
of the gate oxide layer 41 below the guard layer 40 may be
eliminated. This would short the guard layer to the n+ region
26.
[0040] As seen in FIG. 6, the guard layer 40 connects to the gate
32 of the reset transistor 12. During the illumination period, the
gate of the reset transistor is at .0. volts (as indicated in the
timing diagram of FIG. 2). The bias of the guard layer provides a
reduction in dark current. For CMOS processes that do not exhibit
the expected leakage current reduction at .0. volts, it is possible
to bias the guard layer 40 at a lower voltage, so as to bring the
underlying silicon into accumulation. This may be performed by
separating the guard layer from the gate 32 and providing a
separate electrical connection to a fixed source of a negative
voltage. Alternatively, the reset transistor 12 may be driven to a
negative gate voltage, rather than a .0. voltage. In either case,
the simple circuit layout technique mitigates the need of extensive
and expensive technology development to ensure low dark current in
CMOS APS pixels 10.
[0041] During fabrication, the polysilicon guard layer 40 blocks
the implantation of dopant, as indicated by the transition strip 42
in FIG. 8. During operation, the guard layer is biased below the
threshold voltage of the MOS system in which it is formed.
Consequently, the n+ region 26 will be electrically and
structurally isolated from the field oxide edge 30. It follows that
the edge component of diode leakage will be reduced. How much of a
reduction of leakage current that is obtained will depend on the
exact processing steps used to fabricate the diode 14. This is
because at a typical guard layer bias of .0. volts, there is a
depletion region under the polysilicon that can still allow leakage
current as a result of field-edge defects. However, as noted above,
if leakage currents are not sufficiently reduced, it is possible to
bias the polysilicon so as to bring the underlying substrate region
into accumulation. In this case, there should be no contribution to
leakage from the field-edge defects.
[0042] The overlap of the guard layer onto the active area diode 14
will depend upon the alignment specifications of the
photolithography tools used in the technology. Typically, the
overlap will be approximately the same as the overlap length of the
guard layer onto the field oxide region 30.
[0043] A comparison of FIGS. 5 and 8 shows that the use of the
guard layer 40 reduces the area of the n+ region 26. Consequently,
the sensitivity to light will be reduced. If one wants to retain
the same active diode area, the structure of the diode can be
increased in size.
[0044] While the guard layer has been described as being formed of
polysilicon, this is not critical. Other materials may be
substituted. Preferably, the material is conductive, so that it can
be biased below the threshold voltage of the MOS system. However,
the use of a dielectric guard layer provides some of the advantages
of the polysilicon guard layer, if it blocks the dopant
implantation during the fabrication process. Also in the preferred
embodiment, the guard layer is not removed after the dopant
implantation step, but applications in which the guard layer is
removed have been contemplated.
[0045] Similar to the active area diode, the MOSFET diffusions that
form the source/drain regions of the transistors can benefit from
the use of a guard layer over the field-edge that forms their
boundaries. The conventional MOSFET transistor is shown in FIGS. 9
and 10. A MOSFET 44 is formed by growing a thin gate oxide 46 over
the active area of the transistor. A layer of polysilicon is then
formed over the gate oxide 46 by material deposition and pattern
etching. This provides the gate 48 of the MOSFET. The source/drain
regions 50 and 52 are then formed by implantation of an n+ dopant
that extends to a field oxide region 54. Contacts 56 and 58 may be
patterned to reside on the source/drain regions. A p+ layer 60 is
also shown in FIGS. 9 and 10.
[0046] As a result of the fabrication process, two separate n+/p
diodes 62 and 64 are formed, with the source/drain regions
providing the n+ contact and the substrate 66 providing the p
contact. Because the source and drain diffusions are delineated on
three sides by the FOX 54, they have the same field-edge leakage
problem as the active area diode described above. Referring now to
FIGS. 11 and 12, the formation of the gate oxide 46 and the
polysilicon that forms the gate 48 can be extended to provide a
guard layer 68 around the drain region 52 of the transistor 44.
While the illustration shows the guard layer as being applied only
to the drain region, it is possible to achieve the same benefits
for the source region 50 if the guard layer is patterned over the
edge of the source region along the FOX 54. Optionally, the guard
layer can be formed about both of the source/drain regions.
[0047] By forming the guard layer 68 prior to introducing the
dopant into the source/drain regions 50 and 52, a transition strip
70 that is substantially free of dopant is formed at the edge of
the FOX 54. This provides the same advantages described with
reference to FIGS. 4-8. Specifically, the edge component of the
diode leakage is significantly reduced. However, when the guard
layer 68 is connected to the gate 48 in the manner shown in FIG.
11, the bias of the gate to a high voltage (FET turned on) will
create an inversion layer below the entire polysilicon to extend
the n+ region 26 to the FOX 54. The result may be an increased
leakage (due to gate-edge leakage) and extra source/drain
capacitance. In addition, because of the standard overlap of the
gate onto the source/drain regions, gate-to-source/drain
capacitance is increased. If the extra leakage or capacitance is
problematic, it is possible to separate the gate polysilicon 48
from the polysilicon material that forms the guard layer. This
embodiment is shown in FIG. 13. Preferably, the guard layer 72 of
FIG. 13 is biased by connection to a fixed source of voltage (e.g.,
.0. volts). The drain region 52 will have small links 74 and 76 in
which the diffusion region contacts the FOX 54, so as to contribute
to leakage current. The dimensions of the links 74 and 76 will
depend upon the spacing rule of the technology's polysilicon. The
extra leakage will be negligible for large MOSFETs.
[0048] The invention is also applicable to MOSFETs that share
diffusions. The conventional structure is shown in FIG. 14. A pair
of MOSFETs are shown in a side-by-side arrangement. The first
transistor includes a polysilicon gate 78 and a source region 80,
while the second transistor includes a polysilicon gate 82 and a
diffused source region 84. The two transistors share a drain 86
diffusion. MOSFETs that share a common diffusion are often used in
CMOS circuits to lower the total layout area.
[0049] Referring now to FIG. 15, the common drain 86 has reduced
leakage as a result of a polysilicon guard 88 that is formed on
opposite sides of the common drain. In the embodiment of FIG. 15,
the guard 88 is connected to the second gate 82 in order to provide
a bias. The guard is separated from the first gate 78, so that the
two gates are not shorted. Of course, if the gates are tied to a
single circuit node, the polysilicon guard 88 can be connected to
both gates. This gate-to-gate coupling is particularly useful for
MOSFETs with multiple gate fingers.
[0050] FIG. 16 shows a modification of the embodiment of FIG. 15.
In this embodiment, the guard layer 90 is electrically separated
from both of the gates 78 and 82. As previously noted, this
approach may be used to reduce drain capacitance. The guard 90 of
FIG. 16 is preferably connected to a source of a fixed voltage,
such as electrical ground.
[0051] There are advantages to implementing a CMOS APS pixel 10 of
the type shown in FIG. 1 using a photodiode 14 that overlays the
underlying CMOS chip. One advantage is that the bulk photodiode is
removed from the substrate, so that the only diode that can
contribute to dark current is the one formed by the source
diffusion of the reset transistor, which has a much smaller area
than the substrate embedded photodiode 14 of FIG. 3. An exemplary
circuit layout of an overlay photodiode arrangement is shown in the
top view of FIG. 17 and the side sectional view of FIG. 18. In the
conventional approach, the reset transistor 12 is formed using
conventional techniques. As a result, a FOX region 92 and
source/drain regions 94 and 96 are formed in a substrate 98. A gate
oxide layer 100 and the polysilicon gate 32 are patterned before
ion implantation forms the source/drain regions.
[0052] After the transistors 12, 16 and 18 are formed at the
substrate level, a dielectric layer 102 is deposited at the same
level as formation of a pair of contacts 104 and 106 to the
source/drain regions 94 and 96. Other dielectric layers 108, 110
and 112 are formed, with each level of dielectric material
including a patterned metal 114, 116 and 118, as well as a
patterned contact 120, 122 and 124. The patterned metal layers and
contact layers form a via from the transistor 12 to an overlay
photodiode 126. Correlating FIGS. 17 and 18, the portion of the
metal layer 114 that resides above the contact 106 (Cl) in FIG. 18
is the node 22 (N1) in FIG. 17.
[0053] Although the source diffusion 96 of the reset transistor 12
is relatively small, it may still induce a relatively large dark
current, especially in advanced deep-submicron CMOS technologies
and in fact may be the largest contributor to dark current in the
pixel. The approach of providing a polysilicon guard layer may be
applied to the overlay photodiode structure in order to address
this possibility. Specifically, a guard layer of polysilicon may be
formed over the field-edge of the source diffusion in order to
reduce dark current. Because the source diffusion 96 is a small
portion of the total pixel area, adding the guard layer will result
in only a small increase in pixel area. FIGS. 19 and 20 illustrate
the overlay photodiode with the diffusion-inhibiting guard layer
128 about the source diffusion 96. The guard layer is formed before
the dopant is introduced into the source region. Therefore, a
transition strip that is relatively free of dopant is formed
between the FOX 92 and the source region 96. The transition strip
130 is illustrated in FIG. 20.
[0054] The steps for carrying out the invention will be described
briefly with reference to FIG. 21. In step 132, a circuit layout is
selected. In the preferred embodiment, the circuit layout defines
layers of a CMOS imaging sensor having an array of APS pixels. The
APS pixel 10 of FIG. 1 is merely one example. The array of pixels
may be linear or may be two-dimensional.
[0055] In step 134, the field oxide regions are formed.
Conventional techniques may be used to form these regions. The
approach is not critical to the invention.
[0056] A diffusion-inhibiting guard layer is formed over selected
field edges, as shown at step 136. Some of the alternatives were
described and illustrated above. Preferably, the guard layer is
formed of polysilicon and is formed simultaneously with the gates
of transistors.
[0057] The dopant is introduced into the appropriate regions at
step 138. The regions that are guarded by the guard layer that was
formed in step 136 may be active area diode regions, source/drain
diffusion regions, or both. The remainder of the circuit
fabrication is then completed at step 140. The guard layer or guard
layers are biased at step 142 in order to inhibit edge leakage.
* * * * *