U.S. patent application number 12/619296 was filed with the patent office on 2010-04-01 for cmos imager and system with selectively silicided gates.
Invention is credited to Howard E. Rhodes.
Application Number | 20100079645 12/619296 |
Document ID | / |
Family ID | 23479056 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100079645 |
Kind Code |
A1 |
Rhodes; Howard E. |
April 1, 2010 |
CMOS IMAGER AND SYSTEM WITH SELECTIVELY SILICIDED GATES
Abstract
The invention also relates to an apparatus and method for
selectively providing a silicide coating over the transistor gates
of a CMOS imager to improve the speed of the transistor gates. The
method further includes an apparatus and method for forming a self
aligned photo shield over the CMOS imager.
Inventors: |
Rhodes; Howard E.; (Boise,
ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
23479056 |
Appl. No.: |
12/619296 |
Filed: |
November 16, 2009 |
Related U.S. Patent Documents
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Filing Date |
Patent Number |
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11585199 |
Oct 24, 2006 |
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12619296 |
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11078709 |
Mar 14, 2005 |
7348613 |
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11585199 |
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10617706 |
Jul 14, 2003 |
6930337 |
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11078709 |
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09777890 |
Feb 7, 2001 |
6611013 |
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10617706 |
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09374990 |
Aug 16, 1999 |
6333205 |
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09777890 |
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Current U.S.
Class: |
348/302 ;
348/E5.091 |
Current CPC
Class: |
H01L 27/14643 20130101;
H01L 27/14609 20130101; H01L 27/14603 20130101; H01L 27/14689
20130101; H01L 27/1462 20130101; H01L 27/14623 20130101; H01L
27/14687 20130101 |
Class at
Publication: |
348/302 ;
348/E05.091 |
International
Class: |
H04N 5/335 20060101
H04N005/335 |
Claims
1-86. (canceled)
87. A CMOS imager, comprising: an array of pixels, at least one
pixel comprising: a photo-collection region to accumulate
photo-generated charge, wherein silicide is substantially absent
from a surface of the photo-collection region; and a transfer
transistor to transfer the charge from the photo-collection region
to a storage node, wherein the transfer transistor includes a
polysilicon transfer gate comprising silicide and the storage node
is substantially free of silicide.
88. The CMOS imager of claim 87, further comprising a readout
circuit coupled to the array to receive an output signal
corresponding to the charge on the node.
89. The CMOS imager of claim 87, wherein the at least one pixel
further comprises a reset transistor coupled to the node, wherein
the reset transistor includes a polysilicon reset gate comprising
opaque silicide.
90. The CMOS imager of claim 89, wherein the at least one pixel
further comprises a row select transistor coupled in series with a
source follower transistor, wherein the source follower transistor
includes a polysilicon source follower gate comprising opaque
silicide coupled to the node.
91. The CMOS imager of claim 89, wherein the transfer gate and the
reset gate further comprise a barrier metal portion.
92. The CMOS imager of claim 87, wherein the silicide comprises a
refractory metal.
93. The CMOS imager of claim 87, wherein the transfer gate
comprises a barrier metal layer.
94. The CMOS imager of claim 87, wherein a photogate is formed over
the photo-collection region and silicide is substantially absent
from a surface of the photogate.
95. The CMOS imager of claim 88, wherein the readout circuit is
configured to provide correlated sampling of signals from the
pixel.
96. The CMOS imager of claim 95, wherein the silicide comprises a
refractory metal.
97. The CMOS imager of claim 96, further comprising an
analog-to-digital converter for producing digital signals from
analog signals from pixels of the array, and image processing
circuitry for processing said digital signals.
98. The CMOS imager of claim 87, further comprising a load
transistor coupled to a ground source, wherein the at least one
pixel further comprises a source follower transistor coupled in
series with the load transistor, wherein the source follower
transistor includes a polysilicon source follower gate comprising
opaque silicide coupled to the node.
99. The CMOS imager of claim 87, further comprising: a readout
circuit configured to provide correlated sampling of the pixel; an
analog-to-digital converter; and image processing circuitry,
wherein the storage node is a floating diffusion node, the
polysilicon transfer gate is adjacent to the photo-collection
region and to the floating diffusion node, and the at least one
pixel of the array, further comprises: a reset transistor,
including a polysilicon reset gate, coupled to the floating
diffusion node and to an n+ diffusion region configured to be
coupled to a voltage source, the reset transistor configured to
transfer voltage from the voltage source to the floating diffusion
node; a source follower transistor, including a polysilicon source
follower gate, coupled to the floating diffusion node; and silicide
located on a surface of each one of the polysilicon transfer gate,
the polysilicon reset gate, and the polysilicon source follower
gate, and being substantially absent from a surface of the floating
diffusion node.
100. The CMOS imager of claim 99, further comprising a load
transistor coupled to a row select transistor and a ground
source.
101. The CMOS imager of claim 87, further comprising: a readout
circuit configured to provide correlated sampling of each pixel; an
analog-to-digital converter; and image processing circuitry,
wherein the transfer transistor has a surface of a source/drain
region substantially free of silicide, and the at least one pixel
of the array, further comprises: a shallow trench isolation region;
a reset transistor coupled between the storage node and a voltage
source, the reset transistor having a silicided gate and having a
surface of a source/drain region substantially free of silicide;
and a source follower transistor having a silicided gate coupled to
the storage node and having a surface of a source/drain region
substantially free of silicide.
102. The CMOS imager of claim 101, further comprising a load
transistor coupled to a row select transistor.
103. A camera system, comprising: a bus; a processor; random access
memory coupled to the processor via the bus; a CMOS imager coupled
to the processor via the bus, the imager comprising an array of
pixels, each pixel containing silicided transistor gates,
transistor source/drain regions free from silicide, a photodiode,
and shallow trench isolation, the imager configured to perform
correlated sampling of each pixel, analog-to-digital conversion of
the output of each pixel, and image processing.
104. A camera system, comprising: a processor; random access memory
coupled to the processor; a non-volatile memory subsystem coupled
to the processor and configured to enable use of removable storage
media; and a CMOS imager, coupled to the processor, comprising a
pixel array having silicided gates of transistors, unsilicided
source/drain regions of the transistors, and an analog-to-digital
converter.
105. The system of claim 104, wherein the silicided transistors
include a plurality of silicided transfer transistors and the CMOS
imager further comprises a readout circuit configured to provide
correlated sampling.
106. The system of claim 105, wherein the silicided transistors
include a refractory metal.
107. The system of claim 106, wherein the silicided transistors
include a barrier metal layer.
108. The system of claim 104, wherein the silicided transistors
include a refractory metal.
109. The system of claim 104, wherein the CMOS imager further
comprises image processing circuitry for processing signals derived
from the pixel array.
110. The system of claim 104, wherein the removable storage media
includes optical storage media.
111. The system of claim 110, wherein the silicided transistors
include a plurality of silicided transfer transistors and the CMOS
imager further comprises a readout circuit configured to provide
correlated sampling of pixels of the pixel array, and image
processing circuitry for processing signals derived from said
readout circuit.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to improved semiconductor
imaging devices and in particular to an imaging device which can be
fabricated using a standard CMOS process. Particularly, the
invention relates to a method for providing a silicide coating over
the transistor gates used in a CMOS imager to improve the operating
speed of the transistors.
DISCUSSION OF RELATED ART
[0002] There are a number of different types of semiconductor-based
imagers, including charge coupled devices (CCDs), photodiode
arrays, charge injection devices and hybrid focal plane arrays.
CCDs are often employed for image acquisition and enjoy a number of
advantages which makes it the incumbent technology, particularly
for small size imaging applications. CCDs are also capable of large
formats with small pixel size and they employ low noise charge
domain processing techniques. However, CCD imagers also suffer from
a number of disadvantages. For example, they are susceptible to
radiation damage, they exhibit destructive read out over time, they
require good light shielding to avoid image smear and they have a
high power dissipation for large arrays. Additionally, while
offering high performance, CCD arrays are difficult to integrate
with CMOS processing in part due to a different processing
technology and to their high capacitances, complicating the
integration of on-chip drive and signal processing electronics with
the CCD array. While there has been some attempts to integrate
on-chip signal processing with the CCD array, these attempts have
not been entirely successful. CCDs also must transfer an image by
line charge transfers from pixel to pixel, requiring that the
entire array be read out into a memory before individual pixels or
groups of pixels can be accessed and processed. This takes time.
CCDs may also suffer from incomplete charge transfer from pixel to
pixel during charge transfer which also results in image smear.
[0003] Because of the inherent limitations in CCD technology, there
is an interest in CMOS imagers for possible use as low cost imaging
devices. A fully compatible CMOS sensor technology enabling a
higher level of integration of an image array with associated
processing circuits would be beneficial to many digital
applications such as, for example, in cameras, scanners, machine
vision systems, vehicle navigation systems, video telephones,
computer input devices, surveillance systems, auto focus systems,
star trackers, motion detection systems, image stabilization
systems and data compression systems for high-definition
television.
[0004] The advantages of CMOS imagers over CCD imagers are that
CMOS imagers have a low voltage operation and low power
consumption; CMOS imagers are compatible with integrated on-chip
electronics (control logic and timing, image processing, and signal
conditioning such as A/D conversion); CMOS imagers allow random
access to the image data; and CMOS imagers have lower fabrication
costs as compared with the conventional CCD since standard CMOS
processing techniques can be used. Additionally, low power
consumption is achieved for CMOS imagers because only one row of
pixels at a time needs to be active during the readout and there is
no charge transfer (and associated switching) from pixel to pixel
during image acquisition. On-chip integration of electronics is
particularly advantageous because of the potential to perform many
signal conditioning functions in the digital domain (versus analog
signal processing) as well as to achieve a reduction in system size
and cost.
[0005] A CMOS imager circuit includes a focal plane array of pixel
cells, each one of the cells including either a photogate,
photoconductor or a photodiode overlying a substrate for
accumulating photo-generated charge in the underlying portion of
the substrate. A readout circuit is connected to each pixel cell
and includes at least an output field effect transistor formed in
the substrate and a charge transfer section formed on the substrate
adjacent the photogate, photoconductor or photodiode having a
sensing node, typically a floating diffusion node, connected to the
gate of an output transistor. The imager may include at least one
electronic device such as a transistor for transferring charge from
the underlying portion of the substrate to the floating diffusion
node and one device, also typically a transistor, for resetting the
node to a predetermined charge level prior to charge
transference.
[0006] In a CMOS imager, the active elements of a pixel cell
perform the necessary functions of (1) photon to charge conversion;
(2) accumulation of image charge; (3) transfer of charge to the
floating diffusion node accompanied by charge amplification; (4)
resetting the floating diffusion node to a known state before the
transfer of charge to it; (5) selection of a pixel for readout; and
(6) output and amplification of a signal representing pixel charge.
Photo charge may be amplified when it moves from the initial charge
accumulation region to the floating diffusion node. The charge at
the floating diffusion node is typically converted to a pixel
output voltage by a source follower output transistor. The
photosensitive element of a CMOS imager pixel is typically either a
depleted p-n junction photodiode or a field induced depletion
region beneath a photogate, or a photoconductor. For photodiodes,
image lag can be eliminated by completely depleting the photodiode
upon readout.
[0007] CMOS imagers of the type discussed above are generally known
as discussed, for example, in Nixon et al., "256.times.256 CMOS
Active Pixel Sensor Camera-on-a-Chip," IEEE Journal of Solid-State
Circuits, Vol. 31(12) pp. 2046-2050, 1996; Mendis et al, "CMOS
Active Pixel Image Sensors," IEEE Transactions on Electron Devices,
Vol. 41(3) pp. 452-453, 1994 as well as U.S. Pat. No. 5,708,263 and
U.S. Pat. No. 5,471,515, which are herein incorporated by
reference.
[0008] To provide context for the invention, an exemplary CMOS
imaging circuit is described below with reference to FIG. 1. The
circuit described below, for example, includes a photogate for
accumulating photo-generated charge in an underlying portion of the
substrate. It should be understood that the CMOS imager may include
a photodiode or other image to charge converting device, in lieu of
a photogate, as the initial accumulator for photo-generated
charge.
[0009] Reference is now made to FIG. 1 which shows a simplified
circuit for a pixel of an exemplary CMOS imager using a photogate
and having a pixel photodetector circuit 14 and a readout circuit
60. It should be understood that while FIG. 1 shows the circuitry
for operation of a single pixel, that in practical use there will
be an M.times.N array of pixels arranged in rows and columns with
the pixels of the array accessed using row and column select
circuitry, as described in more detail below.
[0010] The photodetector circuit 14 is shown in part as a
cross-sectional view of a semiconductor substrate 16 typically a
p-type silicon, having a surface well of p-type material 20. An
optional layer 18 of p-type material may be used if desired, but is
not required. Substrate 16 may be formed of, for example, Si, SiGe,
Ge, and GaAs. Typically the entire substrate 16 is p-type doped
silicon substrate and may contain a surface p-well 20 (with layer
18 omitted), but many other options are possible, such as, for
example p on p-substrates, p on p+substrates, p-wells in n-type
substrates or the like. The terms wafer or substrate used in the
description includes any semiconductor-based structure having an
exposed surface in which to form the circuit structure used in the
invention. Wafer and substrate are to be understood as including,
silicon-on-insulator (SOI) technology, silicon-on-sapphire (SOS)
technology, doped and undoped semiconductors, epitaxial layers of
silicon supported by a base semiconductor foundation, and other
semiconductor structures. Furthermore, when reference is made to a
wafer or substrate in the following description, previous process
steps may have been utilized to form regions/junctions in the base
semiconductor structure or foundation.
[0011] An insulating layer 22 such as, for example, silicon dioxide
is formed on the upper surface of p-well 20. The p-type layer may
be a p-well formed in substrate 16. A photogate 24 thin enough to
pass radiant energy or of a material which passes radiant energy is
formed on the insulating layer 22. The photogate 24 receives an
applied control signal PG which causes the initial accumulation of
pixel charges in n+ region 26. The n+ type region 26, adjacent one
side of photogate 24, is formed in the upper surface of p-well 20.
A transfer gate 28 is formed on insulating layer 22 between n+ type
region 26 and a second n+ type region 30 formed in p-well 20. The
n+ regions 26 and 30 and transfer gate 28 form a charge transfer
transistor 29 which is controlled by a transfer signal TX. The n+
region 30 is typically called a floating diffusion region. It is
also a node for passing charge accumulated thereat to the gate of
an amplifying transistor, such as source follower transistor 36
described below. A reset gate 32 is also formed on insulating layer
22 adjacent and between n+ type region 30 and another n+ region 34
which is also formed in p-well 20. The reset gate 32 and n+ regions
30 and 34 form a reset transistor 31 which is controlled by a reset
signal RST. The n+ type region 34 is coupled to voltage source VDD,
e.g., 5 volts. The transfer and reset transistors 29, 31 are
n-channel transistors as described in this implementation of a CMOS
imager circuit in a p-well. It should be understood that it is
possible to implement a CMOS imager in an n-well in which case each
of the transistors would be p-channel transistors. It should also
be noted that while FIG. 1 shows the use of a transfer gate 28 and
associated transistor 29, this structure provides advantages, but
is not required.
[0012] Photodetector circuit 14 also includes two additional
n-channel transistors, source follower transistor 36 and row select
transistor 38. Transistors 36, 38 are coupled in series, source to
drain, with the source of transistor 36 also coupled over lead 40
to voltage source VDD and the drain of transistor 38 coupled to a
lead 42. The drain of row select transistor 38 is connected via
conductor 42 to the drains of similar row select transistors for
other pixels in a given pixel row. A load transistor 39 is also
coupled between the drain of transistor 38 and a voltage source
VSS, e.g. 0 volts. Transistor 39 is kept on by a signal VLN applied
to its gate.
[0013] The imager includes a readout circuit 60 which includes a
signal sample and hold (S/H) circuit including a S/H n-channel
field effect transistor 62 and a signal storage capacitor 64
connected to the source follower transistor 36 through row
transistor 38. The other side of the capacitor 64 is connected to a
source voltage VSS. The upper side of the capacitor 64 is also
connected to the gate of a p-channel output transistor 66. The
drain of the output transistor 66 is connected through a column
select transistor 68 to a signal sample output node VOUTS and
through a load transistor 70 to the voltage supply VDD. A signal
called "signal sample and hold" (SHS) briefly turns on the S/H
transistor 62 after the charge accumulated beneath the photogate
electrode 24 has been transferred to the floating diffusion node 30
and from there to the source follower transistor 36 and through row
select transistor 38 to line 42, so that the capacitor 64 stores a
voltage representing the amount of charge previously accumulated
beneath the photogate electrode 24.
[0014] The readout circuit 60 also includes a reset sample and hold
(S/H) circuit including a S/H transistor 72 and a signal storage
capacitor 74 connected through the S/H transistor 72 and through
the row select transistor 38 to the source of the source follower
transistor 36. The other side of the capacitor 74 is connected to
the source voltage VSS. The upper side of the capacitor 74 is also
connected to the gate of a p-channel output transistor 76. The
drain of the output transistor 76 is connected through a p-channel
column select transistor 78 to a reset sample output node VOUTR and
through a load transistor 80 to the supply voltage VDD. A signal
called "reset sample and hold" (SHR) briefly turns on the S/H
transistor 72 immediately after the reset signal RST has caused
reset transistor 31 to turn on and reset the potential of the
floating diffusion node 30, so that the capacitor 74 stores the
voltage to which the floating diffusion node 30 has been reset.
[0015] The readout circuit 60 provides correlated sampling of the
potential of the floating diffusion node 30, first of the reset
charge applied to node 30 by reset transistor 31 and then of the
stored charge from the photogate 24. The two samplings of the
diffusion node 30 charges produce respective output voltages VOUTR
and VOUTS of the readout circuit 60. These voltages are then
subtracted (VOUTS-VOUTR) by subtractor 82 to provide an output
signal terminal 81 which is an image signal independent of pixel to
pixel variations caused by fabrication variations in the reset
voltage transistor 31 which might cause pixel to pixel variations
in the output signal.
[0016] FIG. 2 illustrates a block diagram for a CMOS imager having
a pixel array 200 with each pixel cell being constructed in the
manner shown by element 14 of FIG. 1. Pixel array 200 comprises a
plurality of pixels arranged in a predetermined number of columns
and rows. The pixels of each row in array 200 are all turned on at
the same time by a row select line, e.g., line 86, and the pixels
of each column are selectively output by a column select line,
e.g., line 42. A plurality of rows and column lines are provided
for the entire way 200. The row lines are selectively activated by
the row driver 210 in response to row address decoder 220 and the
column select lines are selectively activated by the column driver
260 in response to column address decoder 270. Thus, a row and
column address is provided for each pixel. The CMOS imager is
operated by the control circuit 250 which controls address decoders
220, 270 for selecting the appropriate row and column lines for
pixel readout, and row and column driver circuitry 210, 260 which
apply driving voltage to the drive transistors of the selected row
and column lines.
[0017] FIG. 3 shows a simplified timing diagram for the signals
used to transfer charge out of photodetector circuit 14 of the FIG.
1 CMOS imager. The photogate signal PG is nominally set to 5V and
pulsed from 5V to 0V during integration. The reset signal RST is
nominally set at 2.5V. As can be seen from the figure, the process
is begun at time to by briefly pulsing reset voltage RST to 5V. The
RST voltage, which is applied to the gate 32 of reset transistor
31, causes transistor 31 to turn on and the floating diffusion node
30 to charge to the VDD voltage present at n+ region 34 (less the
voltage drop Vth of transistor 31). This resets the floating
diffusion node 30 to a predetermined voltage (VDD-Vth). The charge
on floating diffusion node 30 is applied to the gate of the source
follower transistor 36 to control the current passing through
transistor 38, which has been turned on by a row select (ROW)
signal, and load transistor 39. This current is translated into a
voltage on line 42 which is next sampled by providing a SHR signal
to the S/H transistor 72 which charges capacitor 74 with the source
follower transistor output voltage on line 42 representing the
reset charge present at floating diffusion node 30. The PG signal
is next pulsed to 0 volts, causing charge to be collected in n+
region 26. A transfer gate voltage TX, similar to the reset pulse
RST, is then applied to transfer gate 28 of transistor 29 to cause
the charge in n+ region 26 to transfer to floating diffusion node
30. It should be understood that for the case of a photogate, the
transfer gate voltage TX may be pulsed or held to a fixed DC
potential. For the implementation of a photodiode with a transfer
gate, the transfer gate voltage TX must be pulsed. The new output
voltage on line 42 generated by source follower transistor 36
current is then sampled onto capacitor 64 by enabling the sample
and hold switch 62 by signal SHS. The column select signal is next
applied to transistors 68 and 70 and the respective charges stored
in capacitors 64 and 74 are subtracted in subtractor 82 to provide
a pixel output signal at terminal 81. It should also be noted that
CMOS imagers may dispense with the transfer gate 28 and associated
transistor 29, or retain these structures while biasing the
transfer transistor 29 to an always "on" state.
[0018] The operation of the charge collection of the CMOS imager is
known in the art and is described in several publications such as
Mendis et al., "Progress in CMOS Active Pixel Image Sensors," SPIE
Vol. 2172, pp. 19-29 1994; Mendis et al., "CMOS Active Pixel Image
Sensors for Highly Integrated Imaging Systems," IEEE Journal of
Solid State Circuits, Vol. 32(2), 1997; and Eric R., Fossum, "CMOS
Image Sensors: Electronic Camera on a Chip," IEDM Vol. 95 pages
17-25 (1995) as well as other publications. These references are
incorporated herein by reference.
[0019] In the prior art, the desire to incorporate a silicide over
the gate stack to improve speed was hampered by the undesirable
effect the silicide layer had on the photogate. If the photogate is
covered by a silicide layer, the collection of charge is inhibited
by the blocking of light by the silicide layer. It is for this
reason that photogate type devices have not been able to use a
silicide gate stack. Since the size of the pixel electrical signal
is very small due to the collection of photons in the photo array,
the signal to noise ratio of the pixel should be as high as
possible within a pixel. Accordingly, all possible charge should be
collected by the photocollection device.
SUMMARY OF THE INVENTION
[0020] The present invention provides an imaging device formed as a
CMOS integrated circuit using a standard CMOS process. The
invention relates to a method for providing a more conductive
layer, such as a silicide or a barrier/metal layer, incorporated
into the transistor gates of a CMOS imager to improve the speed of
the transistor gates, but selectively removing the silicide or
barrier/metal from a photogate to prevent blockage of the
photogate.
[0021] The above and other advantages and features of the invention
will be more clearly understood from the following detailed
description which is provided in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a representative circuit of a CMOS imager.
[0023] FIG. 2 is a block diagram of a CMOS active pixel sensor
chip.
[0024] FIG. 3 is a representative timing diagram for the CMOS
imager.
[0025] FIG. 4 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer in an interim stage of
processing.
[0026] FIG. 5 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
4.
[0027] FIG. 6 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
5.
[0028] FIG. 7 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer in an interim stage of
processing according to a further embodiment of the present
invention.
[0029] FIG. 8 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
7.
[0030] FIG. 9 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
8.
[0031] FIG. 10 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
9.
[0032] FIG. 11 illustrates a partially cut away side view of a
portion of a semiconductor CMOS, imager wafer subsequent to FIG.
10.
[0033] FIG. 12 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer in an interim stage of
processing according to a second embodiment of the present
invention.
[0034] FIG. 13 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
12.
[0035] FIG. 14 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
13.
[0036] FIG. 15 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer in an interim stage of
processing according to a third embodiment of the present
invention.
[0037] FIG. 16 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
15.
[0038] FIG. 17 illustrates a partially cut away side view of a
portion of a semiconductor CMOS imager wafer subsequent to FIG.
16.
[0039] FIG. 18 is an illustration of a computer system having a
CMOS imager according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] In the following detailed description, reference is made to
the accompanying drawings which form a part hereof, and in which is
shown by way of illustration specific embodiments in which the
invention may be practiced. It should be understood that like
reference numerals represent like elements. These embodiments are
described in sufficient detail to enable those skilled in the art
to practice the invention, and it is to be understood that other
embodiments may be utilized, and that structural, logical and
electrical changes may be made without departing from the spirit
and scope of the present invention.
[0041] The terms "wafer" and "substrate" are to be understood as
including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS)
technology, doped and undoped semiconductors, epitaxial layers of
silicon supported by a base semiconductor foundation, and other
semiconductor structures. Furthermore, when reference is made to a
"wafer" or "substrate" in the following description, previous
process steps may have been utilized to form regions or junctions
in the base semiconductor structure or foundation. In addition, the
semiconductor need not be silicon-based, but could be based on
silicon-germanium, germanium, or gallium arsenide.
[0042] The term "pixel" refers to a picture element unit cell
containing a photosensor and transistors for converting
electromagnetic radiation to an electrical signal. For purposes of
illustration, a representative pixel is illustrated in the figures
and description herein, and typically fabrication of all pixels in
an imager will proceed simultaneously in a similar fashion. The
following detailed description is, therefore, not to be taken in a
limiting sense, and the scope of the present invention is defined
by the appended claims.
[0043] Reference is now made to FIG. 4. This figure shows a
partially cut away side view of a portion of a semiconductor CMOS
imager wafer in an interim stage of processing according to a first
aspect of the present invention. The imager includes a substrate
310 preferably doped to a first conductivity type. For exemplary
purposes, it is assumed that the substrate 310 is a well doped to a
p-type conductivity, i.e., a p-well. Substrate 310 has an n-doped
region 316 therein for photocollection. An insulating layer 315 is
formed over the substrate 310. The insulating layer is preferably a
silicon dioxide grown on the substrate 310 by conventional means
such as thermal oxidation of silicon. The substrate 310 has field
oxide regions 341 formed using the Local Oxidation of Silicon
(LOCOS) process to surround and isolate the cells which may be
formed by thermal oxidation. While the invention is described with
reference to LOCOS formed field oxide regions 341, it should be
understood that the field oxide regions may be formed with shallow
trench isolation (STI).
[0044] A photogate 340, a transfer gate 350 and a reset gate 360
have been fabricated over the insulating layer 315. The gates 340,
350, 360 include a doped polysilicon layer 320 covered by a more
conductive layer such as a barrier/metal layer or silicide layer
325 or refractory metal silicide or barrier metal, if desired,
according to conventional methods. Preferably the silicide is a
tungsten, titanium, tantalum, molybdenum or cobalt silicide. The
barrier metal may be those such as titanium nitride, tungsten
nitride or the like. Preferably the barrier metal is formed of a
TiN/W, WN.sub.x/W or WN.sub.x.
[0045] The doped polysilicon layers 320 may be formed by
conventional methods, such as chemical vapor deposition (CVD).
Conductive layer 325 of titanium, tantalum, cobalt or tungsten is
then deposited using a chemical vapor deposition (CVD), sputtering
or a physical vapor deposition (PVD) of the silicide or a CVD or
PVD deposition of the metal followed by a thermal step to cause the
metal to react with the underlying polysilicon to form the metal
silicide. The wafer is then annealed at approximately 600.degree.
C. to about 800.degree. C. for approximately 30 seconds in a
nitrogen environment to react with a portion of the polysilicon
layer 320 to form conductive layer 325. The excess metal is then
removed to arrive at the structure shown in FIG. 4. Preferably the
conductive layer 325 is formed by depositing WSi.sub.x over the
doped polysilicon layers 320. The WSi.sub.x may be deposited onto
the doped polysilicon layers 320 by conventional methods such as
CVD. Photoresist is then used to define features 340, 350, 360 and
the silicide and polysilicon layers and etched, preferably using a
dry etch that stops in the underlying gate oxide. The resist is
stripped and the wafer is shown in FIG. 4.
[0046] The substrate is then patterned, exposing the photogate, and
the conductive layer 325 is removed from the photogate 340 by a wet
or dry etch to arrive at the device as shown in FIG. 5. The
conductive layer 325 remains over both the transfer gate 350 and
the reset gate 360 after the pattern mask is removed. This process
improves the speed of the fabricated transistor gates by depositing
a conductive layer on these gates while the process removes the
conductive layer from the photogate 340 to prevent blockage of the
photo-generated charge. Thus, the transistor gates 350, 360 have
the desired speed due to the presence of the silicide but the area
of the photogate 340 is not shielded by the silicide.
[0047] Spacers 324 are formed along the sides of the gate stacks
340, 350, 360 as shown in FIG. 6. The spacers 324 may be formed of
TEOS (tetraethyloxysilicate) or silicon nitride using conventional
deposition and etch back technique. A resist and mask (not shown)
are used to shield areas of the substrate 310 that are not to be
doped. The doped regions 312, 314, 318 are then formed in the
substrate 310. The doped regions 312, 314, 318 are doped to a
second conductivity type, which for exemplary purposes will be
considered to be n-type. The doping level of the doped regions 312,
314, 318 may be different but for process simplicity could all be
heavily n+ doped with arsenic, antimony of phosphorous at an
implant dose of from about 1.times.10.sup.15 ions/cm.sup.2 to about
1.times.10.sup.16 ions/cm.sup.2. There may be other implants (not
shown) to set transistor threshold voltages, provide short channel
punch-through protection, provide improved field isolation, etc. as
is known in the art.
[0048] For the pixel cell of the first embodiment, the photosensor
cell is essentially complete at this stage, and conventional
processing methods may then be used to form contacts and wiring to
connect gate lines and other connections in the pixel cell. For
example, the entire surface may then be covered with a passivation
layer of, e.g., silicon dioxide, BPSG, PSG, BSG or the like which
is CMP planarized and etched to provide contact holes, which are
then metallized to provide contacts to the photogate, reset gate,
and transfer gate. Conventional multiple layers of conductors and
insulators may also be used to interconnect the structures in the
manner shown in FIG. 1.
[0049] Reference is now made to FIG. 7. This figure shows a
partially cut away side view of a portion of a semiconductor CMOS
imager wafer in an interim stage of processing according to a
second embodiment of the present invention. The imager includes a
p-well substrate 310 having n-doped region 316 therein for
photocollection. An insulating layer 315 is formed over the
substrate 310. The insulating layer is preferably a silicon dioxide
grown on the substrate 310 by conventional means such as thermal
oxidation of silicon. The substrate 310 has field oxide regions 341
formed to surround and isolate the cells which may be formed by
thermal oxidation of silicon using the LOCOS process. While the
invention is described with reference to field oxide regions 341,
it should be understood that the field oxide regions may be
replaced with shallow trench isolation (STI). A doped polysilicon
layer 320 may be formed by conventional methods, such as chemical
vapor deposition (CVD) over the insulating layer 315. A photogate
insulator 342 grown or deposited over layer 320 and is patterned
over the polysilicon layer 320 above n-doped region 316 as shown in
FIG. 7.
[0050] Referring now to FIG. 8, a metal layer 326 of titanium or
cobalt is then deposited using CVD or PVD technique, preferably
sputtering. The wafer is then annealed at approximately 600.degree.
C. to about 800.degree. C. for approximately 30 seconds in a
nitrogen environment to react with a portion of the polysilicon
layer 320 to form conductive layer 325. The unreacted metal layer
326 over insulating regions such as 342 is then removed to arrive
at the structure shown in FIG. 9.
[0051] A resist and mask (not shown) is then applied to the
substrate 310 and the wafer is patterned and the silicide and
polysilicon layers are etched to form transfer gate 350 and reset
gate 360 over the substrate 310 as shown in FIG. 10. While the
photogate insulation 342 does not have to be removed, it may be
removed if desired. FIG. 10 shows the insulator 342 left in place.
The gates 350 and 360 include the doped polysilicon layer 320
covered by conductive layer 325. The conductive layer 325 is
selectively removed from the substrate 310 as shown in FIG. 10 by a
wet or dry etch or other chemical and/or mechanical methods in
regions not protected by the patterned photoresist. The conductive
layer 325 remains over both the transfer gate 350 and the reset
gate 360 after the pattern mask is removed. This process improves
the speed of the transistor gates by depositing a silicide layer on
these gates while the process selectively prevents silicide from
forming over the photogate region 340 by using a patterned
insulating layer 342 to prevent blockage of the photo-generated
charge. Thus, the transistor gates 350, 360 have the desired speed
due to the presence of the silicide but the area of the photogate
340 is not shielded by the silicide.
[0052] Spacers 324 are formed along the sides of the gate stacks
340, 350, 360 as shown in FIG. 11. The spacers 324 may be formed of
any insulator such as oxide or nitride using conventional
deposition and anisotropic etch back technique. A resist and mask
(not shown) is further used to shield areas of the substrate 310
that are not to be doped. The doped regions 312, 314, 318 are then
formed in the substrate 310. The doped regions 312, 314, 318 are
doped to a second conductivity type, which for exemplary purposes
will be considered to be n-type. The doping level of the doped
regions 312 may vary but preferably are heavily n+ doped with
arsenic, antimony of phosphorous at a dopant concentration level of
from about 1.times.10.sup.15 ions/cm.sup.2 to about
1.times.10.sup.16 ions/cm.sup.2. Separate masking photoresist
layers may be used to implant regions 312, 314, 318 to differing
dopant concentrations or a single mask may be used to implant them
all the same concentration.
[0053] For the pixel cell of the second embodiment, the photosensor
cell is essentially complete at this stage, and conventional
processing methods may then be used to form contacts and wiring to
connect gate lines and other connections in the pixel cell. For
example, the entire surface may then be covered with a passivation
layer of, e.g., silicon dioxide, BPSG, PSG, BSG or the like which
is CMP planarized and etched to provide contact holes, which are
then metallized to provide contacts to the photogate, reset gate,
and transfer gate. Conventional multiple layers of conductors and
insulators may also be used to interconnect the structures in the
manner shown in FIG. 1.
[0054] Reference is now made to FIG. 12. This figure shows a
partially cut away side view of a portion of a semiconductor CMOS
imager wafer in an interim stage of processing according to a
second embodiment of the present invention. The imager includes a
substrate 310 preferably doped to a first conductivity type. For
exemplary purposes, it is assumed that the substrate 310 is a well
doped to a p-type conductivity; i.e., a p-well. Substrate 310 has
an n-doped region 316 therein for photocollection. An insulating
layer 315 is formed over the substrate 310. The insulating layer is
preferably a silicon dioxide grown on the substrate 310 by
conventional means such as thermal oxidation of silicon. The
substrate 310 has field oxide regions 341 formed using the LOCOS
process to surround and isolate the cells which may be formed by
thermal oxidation. While the invention is described with reference
to LOCOS formed field oxide regions 341, it should be understood
that the field oxide regions may be formed using replaced with
shallow trench isolation (STI).
[0055] A photogate 340, a transfer gate 350 and a reset gate 360
have been fabricated over the insulating layer 315. The gates 340,
350, 360 include a doped polysilicon layer 320 covered by a more
conductive layer such as a barrier/metal layer or silicide layer
325. Preferably the silicide is a tungsten, titanium, tantalum,
molybdenum or cobalt silicide. The barrier metal may be those such
as titanium nitride, tungsten nitride or the like. Preferably the
barrier metal is formed of a TiN/W, WN.sub.x/W or WN.sub.x. The
doped polysilicon layers 320 may be formed by conventional methods
as described above. Conductive layer 325 of titanium, tantalum,
cobalt or tungsten is then deposited using a chemical vapor
deposition (CVD) or a physical vapor deposition (PVD) of the
silicide or a CVD or PVD deposition of the metal followed by a
thermal step to cause the metal to react with the underlying
polysilicon to form the metal silicide. The wafer is then annealed
at approximately 600.degree. C. to about 800.degree. C. for
approximately 30 seconds in a nitrogen environment to react with a
portion of the polysilicon layer 320 to form conductive layer 325.
The excess metal is then removed. Preferably the conductive layer
325 is formed by depositing WSi.sub.x over the doped polysilicon
layers 320. The WSi.sub.x may be deposited onto the doped
polysilicon layers 320 by conventional methods such as CVD. A
photoresist layer 351 is formed and patterned over photogate
340.
[0056] The conductive layer 325 is removed from the photogate 340
by a wet or dry etch to arrive at the device as shown in FIG. 13.
The conductive layer ring 325 remaining after removal of conductive
layer 325 over photogate 340 allows a light shield to be aligned
over the array while allowing light to pass to the photogate
340.
[0057] Spacers 324 are formed along the sides of the gate stacks
340, 350, 360 and the conductive layer ring 325 remaining after
etching over the photogate 340' as shown in FIG. 14. The spacers
324 may be formed of any insulator such as oxide or nitride using
conventional deposition and anisotropic etch back technique. A
resist and mask (not shown) is further used to shield areas of the
substrate 310 that are not to be doped. The doped regions 312, 314,
318 are then formed in the substrate 310. The doped regions 312,
314, 318 are doped to a second conductivity type, which for
exemplary purposes will be considered to be n-type. The doping
level of the doped regions 312 may vary but preferably are heavily
n+ doped with arsenic, antimony of phosphorous at a dopant
concentration level of from about 1.times.10.sup.15 ions/cm.sup.2
to about 1.times.10.sup.16 ions/cm.sup.2. Separate masking
photoresist layers may be used to implant regions 312, 314, 318 to
differing dopant concentrations or a single mask may be used to
implant them all the same concentration.
[0058] For the pixel cell of the third embodiment, the photosensor
cell is essentially complete at this stage, and conventional
processing methods may then be used to form contacts and wiring to
connect gate lines and other connections in the pixel cell. For
example, the entire surface may then be covered with a passivation
layer of, e.g., silicon dioxide, BPSG, PSG, BSG or the like which
is CMP planarized and etched to provide contact holes, which are
then metallized to provide contacts to the photogate, reset gate,
and transfer gate. Conventional multiple layers of conductors and
insulators may also be used to interconnect the structures in the
manner shown in FIG. 1.
[0059] Reference is now made to FIG. 15. This figure shows a
partially cut away side view of a portion of a semiconductor CMOS
imager wafer in an interim stage of processing according to a third
embodiment of the present invention. The imager includes a
substrate 310 preferably doped to a first conductivity type. For
exemplary purposes, it is assumed that the substrate 310 is a well
doped to a p-type conductivity, i.e., a p-well. Substrate 310 has
an n-doped region 316 therein for photocollection. An insulating
layer 315 is formed over the substrate 310. The insulating layer is
preferably a silicon dioxide grown on the substrate 310 by
conventional means such as thermal oxidation of silicon. The
substrate 310 has field oxide regions 341 formed using the LOCOS
process to surround and isolate the cells which may be formed by
thermal oxidation. While the invention is described with reference
to LOCOS formed field oxide regions 341, it should be understood
that the field oxide regions may be formed using replaced with
shallow trench isolation (STI).
[0060] A photogate 340, a transfer gate 350 and a reset gate 360
have been fabricated over the insulating layer 315. The gates 340,
350, 360 include a doped polysilicon layer 320 covered by a more
conductive layer such as a barrier/metal layer or silicide layer
325. Preferably the silicide is a tungsten, titanium, tantalum,
molybdenum or cobalt silicide. The barrier metal may be those such
as titanium nitride, tungsten nitride or the like. Preferably the
barrier metal is formed of a TiN/W, WN.sub.x/W or WN.sub.x. The
doped polysilicon layers 320 may be formed by conventional methods
as described above. Conductive layer 325 of titanium, tantalum,
cobalt or tungsten is then deposited using a chemical vapor
deposition (CVD) or a physical vapor deposition (PVD) of the
silicide or a CVD or PVD deposition of the metal followed by a
thermal step to cause the metal to react with the underlying
polysilicon to form the metal silicide. The wafer is then annealed
at approximately 600.degree. C. to about 800.degree. C. for
approximately 30 seconds in a nitrogen environment to react with a
portion of the polysilicon layer 320 to form conductive layer 325.
The excess metal is then removed. Preferably the conductive layer
325 is formed by depositing WSi.sub.x over the doped polysilicon
layers 320. The WSi.sub.x may be deposited onto the doped
polysilicon layers 320 by conventional methods such as CVD.
[0061] Reference is made to FIG. 16. Spacers 324 are formed along
the sides of the gate stacks 340, 350, 360 and the conductive layer
ring 325 remaining after etching over the photogate 340, transfer
gate 350 and reset gate 360. The spacers 324 may be formed of any
insulator such as oxide or nitride using conventional deposition
and anisotropic etch back technique. A resist and mask (not shown)
is further used to shield areas of the substrate 310 that are not
to be doped. The doped regions 312, 314, 318 are then formed in the
substrate 310. The doped regions 312, 314, 318 are doped to a
second conductivity type, which for exemplary purposes will be
considered to be n-type. The doping level of the doped regions 312
may vary but preferably are heavily n+ doped with arsenic, antimony
of phosphorous at a dopant concentration level of from about
1.times.10.sup.15 ions/cm.sup.2 to about 1.times.10.sup.16
ions/cm.sup.2. Separate masking photoresist layers may be used to
implant regions 312, 314, 318 to differing dopant concentrations or
a single mask may be used to implant them all the same
concentration. A resist and mask (not shown) is used to form
insulating layer 370 over substrate 310. The insulating layer 370
is formed such that the insulating layer aligns with the remaining
conductive layer 325 as shown in FIG. 16.
[0062] The insulating layer 370 may be formed of any type of
insulating material, such as an oxide or nitride. A light shield
374 is then deposited over insulating layer 374. The light shield
layer may be formed of any conventionally known light blocking
material. The wafer is then patterned with resist to clear resist
over the photogate 340 and wherever a subsequent contact is
desired. The light shield 374, insulating layer 370 and conductor
325 are all etched sequentially with a single resist patterning.
The resist is stripped and the wafer is as shown in FIG. 16.
[0063] A translucent or transparent insulating layer 380 is then
deposited over the substrate. The substrate is optionally
planarized using CMP or spin-on-glass (SOG). Contact holes 382 are
formed in insulating layer 380 to arrive at the structure shown in
FIG. 17. Insulating layer 380 may be formed of, for example,
silicon dioxide, BPSG, PSG, BSG, SOG or the like which is CMP
planarized and etched to provide contact holes 382, which are then
metallized to provide contacts to the photogate 340, reset gate
350, and transfer gate 360. Conventional multiple layers of
conductors and insulators may also be used to interconnect the
structures in the manner shown in FIG. 1.
[0064] A typical processor based system which includes a CMOS
imager device according to the present invention is illustrated
generally at 400 in FIG. 18. A processor based system is exemplary
of a system having digital circuits which could include CMOS imager
devices. Without being limiting, such a system could include a
computer system, camera system, scanner, machine vision, vehicle
navigation, video phone, surveillance system, auto focus system,
star tracker system, motion detection system, image stabilization
system and data compression system for high-definition television,
all of which can utilize the present invention.
[0065] A processor system, such as a computer system, for example
generally comprises a central processing unit (CPU) 444 that
communicates with an input/output (I/O) device 446 over a bus 452.
The CMOS imager 442 also communicates with the system over bus 452.
The computer system 400 also includes random access memory (RAM)
448, and, in the case of a computer system may include peripheral
devices such as a floppy disk drive 454 and a compact disk (CD) ROM
drive 456 which also communicate with CPU 444 over the bus 452.
CMOS imager 442 is preferably constructed as an integrated circuit
as previously described with respect to FIGS. 4-17.
[0066] The above description and accompanying drawings are only
illustrative of preferred embodiments which can achieve the
features and advantages of the present invention. For example, the
CMOS imager array can be formed on a single chip together with the
logic or the logic and array may be formed on separate IC chips. It
is not intended that the invention be limited to the embodiments
shown and described in detail herein. Accordingly, the invention is
not limited by the forgoing descriptions, but is only limited by
the scope of the following claims.
* * * * *