Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1998-11-09
2001-08-21
Pham, Long (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S294000
Reexamination Certificate
active
06278169
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to image sensors, and more particularly, to a technique for shielding portions of pixel arrays from light within image sensors.
BACKGROUND OF THE INVENTION
Image sensors are solid state devices used in image acquisition applications. As examples, digital cameras, camcorders and video conferencing machines employ image sensors. Image sensors typically are implemented in an integrated circuit (IC) and include an array of pixels, each pixel being sensitive to received light. A pixel typically produces a voltage which is linearly dependent on the intensity of light it receives. A pixel may be formed of a semiconductor device and particularly from a p
junction portion of the device. A photocharge generator, within the depletion region of the p
junction, generates current in response to light received, which current is converted to a voltage.
Shielding light from a portion of the pixels in the array is required for a number of reasons. Typically, several rows and several columns of the array of pixels require shielding. One reason for requiring the shielding is that each pixel from which an output voltage is read requires a dark reference pixel, an output voltage of which is used a dark pixel reference voltage. The signal produced by the dark reference pixel is that ideally produced in the absence of light. Thus, for each row of pixels, there is at least one pixel which requires light shielding so as to act as the dark reference pixel for that row of reference pixels.
Another reason why shielding may be required is that the sensor may be covered by glass and subsequently exposed to light. Circuits peripheral (located adjacent) to the image sensor may be sensitive to light and require operation in the absence of light. As such, those peripheral circuits also require shielding. Further, the semiconductor substrate, on which the pixels are formed, should be shielded from light in order to avoid a photocharge generated on the substrate being collected by the pixels.
Prior art image sensors, often formed as charge-coupled devices (CCDs) using a different process from the standard Complementary Metal-Oxide Semiconductor (“CMOS”) processes, typically use a single layer of metal as the light shield. The metal layer is formed to cover the particular rows and columns of the pixel array to be shielded. The quality of the metal shield is important for accurate performance of the image sensor. If the output produced by dark reference pixels is not accurate, which may occur if light reaches the substrate on which the pixel is formed, then the entire row or column, the pixel outputs of which are compared to the dark reference pixel, may produce inaccurate results.
FIG. 1
is a conceptual diagram illustrating a portion of an image sensor including an array
10
of pixels P
n
. The array includes twenty-three rows R
1
-R
23
and twenty-three columns C
1
-C
23
of pixels, the pixels being numbered from P
1
to P
529
. Each row Rn of pixels is controlled by row control logic
12
. Similarly, each column Cn of pixels is controlled by column control logic
14
. Column control logic
14
provides an output to buffer
16
. Row control logic
12
and column control logic
14
also may be shielded (not shown).
As shown by shading, the left-most three columns C
1
-C
3
and the upper-most three rows R
1
-R
3
require shielding to provide dark reference pixels for the remaining pixels in those columns and rows.
SUMMARY OF THE INVENTION
The Applicants herein have recognized drawbacks with the prior art shielding approach. For one, the metal layer used for shielding, which is deposited on the top of an unplanarized surface of pixels or circuits, may crack, especially at the steep steps of the surface. Light then may travel through the crack formed and cause faulty image sensor performance.
Applicants also have recognized that, for submicron CMOS processes, i.e., 0.5 &mgr;m and smaller CMOS processes, the width of a single layer of metal required to shield the rows and columns of the pixel array and adjacent circuitry, would violate the CMOS process “design rule”. For a particular CMOS process, the design rule requires that each metal layer within the IC should never be wider then a specified amount without an opening or slot. The width is reduced as the process size is reduced.
As an example, for a 0.5 &mgr;m CMOS process, the design rule may require that the maximum solid width of any one layer of metal be less than 30 &mgr;m. Otherwise reliability problems such as metal lift-off and metal-metal dielectric breakdown may occur. If each pixel in the array is approximately 8 &mgr;m×8 &mgr;m, and 20 columns are to be shielded, then the entire width of columns to be shielded is 160 &mgr;m. A single continuous metal layer, as was used in prior art CCD image sensors, would not be available as it would violate the process design rule for such a submicron process.
Most prior art CCD image sensors use different but more expensive processes than CMOS process with a minimum feature size ranging from 0.8 &mgr;m to 2 &mgr;m, most commonly 1 &mgr;m or 1.2 &mgr;m modified CMOS. For such CCD modified CMOS processes, the metal maximum width rule may not be applicable.
A general metalization problem in the industry is that of delarnination, i.e., the various layers of the dielectrics and metals peel away causing reliability problems. There are many possible causes for this delamination, some of which are process-related and some of which are package-related.
CMOS process design rules attempt to combat the problem, requiring a slot or opening at spaced intervals in each metal layer. The presence of the opening allows the overlying dielectric to anchor the underlying dielectric, giving it greater stability.
The present invention proposes using at least two layers of metal shielding, each of which would not violate the particular CMOS process design rule, in combination to accomplish the shielding required.
One embodiment of the present invention is directed to a method of shielding light from rows and columns of an array of pixels and adjacent circuitry of a CMOS image sensor comprising the steps of: providing a first layer of metal shielding above the rows and columns of the array; and providing a second layer of metal shielding above the first layer.
In an embodiment, the method further includes the steps of forming slots within each of the first and second layers of metal shielding.
In an embodiment, the steps of forming includes the steps of forming such that slots within the first layer are misaligned with slots within the second layer.
In an embodiment, the method further includes the step of providing vias interconnecting the first and second layers.
In an embodiment, the method further includes the step of providing an antireflective coating on the top surface of the first layer. In an embodiment, the method further includes the step of providing an antireflective coating on the bottom surface of the second layer.
Another embodiment of the invention is directed to a light shield for shielding columns and rows of an array of pixels and adjacent circuitry in a CMOS image sensor. A first metal layer is located above the columns and rows of the array. A second metal layer is located above the first metal layer.
In an embodiment, each of the first and second metal layers includes slots.
In an embodiment, the slots are located such that the slots of the first layer are misaligned with the slots of the second layer.
In an embodiment, the shield further includes a plurality of vias interconnecting the first and second layers.
In an embodiment, the shield includes an antireflective coating on the top surface of the first layer. In an embodiment, the shield further includes an antireflective coating on the bottom surface of the second layer. In an embodiment, the antireflective coating includes Titanium Nitride.
Another embodiment is directed to a CMOS image sensor including an array of pixels. A first metal shield layer is located over certain columns and rows
Bain Dave
Decker Steve
Deignan Anne
Qian Dahong
Sayuk Mark
Analog Devices Inc.
Coleman William David
Pham Long
Wolf Greenfield & Sacks P.C.
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