Conductive guard rings for elevated active pixel sensors

Details Conductive guard rings for elevated active pixel sensors Conductive guard rings for elevated active pixel sensors

1999-11-19

2001-05-08

Munson, Gene M. (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S053000, C257S226000, C257S448000, C257S458000

Reexamination Certificate

active

06229191

ABSTRACT:

FIELD OF INVENTION
This invention relates generally to photo diode active pixel sensors. In particular, it relates to voltage biased conductive guard rings which surround the photo diode active pixel sensors.
BACKGROUND
An array of image sensors or active pixel sensors detect the intensity of light received by the image sensors. The image sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the image sensors. The image sensors can convert an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the image sensors. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allow s electronic imaging systems to be low cost, compact and require low power consumption.
Historically, image sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems. An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1
shows a cross-section of a prior art array of image sensors. This array of image sensors includes elevated photo diode sensors located over a substrate
100
. An interconnection structure
110
electrically connects an N-layer
112
of the photo diode sensors to the substrate
100
. An I-layer
114
is formed over the N-layer
112
. A P-layer
116
is formed over the I-layer
114
. The P-layer
116
, the I-layer
114
and the N-layer
112
form the array of photo diode sensors. Conductive vias
120
,
122
,
124
,
126
electrically connect anodes of a first photo diode sensor, a second photo diode sensor, a third photo diode sensor and a fourth photo diode sensor to the substrate
100
. A transparent conductive layer
118
is located over the array of photo diode sensors, and electrically connects cathodes of the first photo diode sensor, the second photo diode sensor, the third photo diode sensor and the fourth photo diode sensor to the substrate
100
.
The photo diode sensors conduct charge when the photo diode sensors receive light. The substrate
100
generally includes sense circuitry and signal processing circuitry. The sense circuitry senses how much charge the photo diode sensors have conducted. The amount of charge conducted represents the intensity of light received by the pixel sensors.
FIG. 2
is a schematic of a typical circuit included on the substrate
100
which is electrically connected to each photo diode sensor
136
. The circuit includes a switch
132
which drives the cathode of each photo diode sensor
136
to an initial cathode voltage and charges a cathode capacitor
134
. The switch
132
is subsequently opened and the cathode capacitor
134
discharges as the photo diode sensor
136
connected to the cathode capacitor
134
conducts charge. The rate in which the cathode capacitor
134
discharges is dependent upon the intensity of light received by the photo diode sensor
136
connected to the cathode capacitor
134
. Therefore, the intensity of light received by the photo diode sensor
136
can be determined by sampling the voltage on the cathode capacitor
134
a period of time after the switch
132
has been opened.
The charge conducted by a reverse biased diode is generated in a space charge region and neutral region of the diode. Photo diode sensors are configured so that the charge conducted by a reverse biased photo diode sensor is generated in the space charged region. The magnitude of the charge conducted is directly dependent on the volume of the space charged region. In an array of photo diode sensors, the space charged region of each photo diode sensor extends beyond the physical boundaries of the photo diode sensor. The space charged region is defined by the electric field between the anode and the cathode of the photo diode sensor.
FIG. 3
is a plot which approximately represents the cathode voltage of each of the photo diode sensors
136
which are driven by an electronic circuit similar to the circuit shown in FIG.
2
. The switch
132
of the circuit is opened at time
160
. The cathode voltage of each photo diode sensor then decreases as the cathode capacitor
134
discharges as the photo diode sensor
136
connected to the cathode capacitor
134
conducts charge. The cathode voltage is sampled at time
162
. The greater the intensity of received light, the lower the cathode voltage is at the time
162
that the cathode voltage is sampled. The cathode voltage stops decreasing at time
164
because the photo diode sensor has saturated.
If the intensity of the light received by the photo diode sensors
136
is too great, the photo diode sensors will saturate. That is, the cathode capacitor
134
will fully discharge before the cathode voltage is sampled. Saturation of the photo diode sensors can be avoided by either limiting the intensity of the received light or limiting the time the photo diode sensors are allowed to conduct charge due to exposure to light.
When the photo diode sensors saturate, the sensors no longer collect charge, and the electric fields across the photo diode sensors collapse. When the electric fields across the sensor collapse, charge is collected or trapped in defects within the I-layer of the sensors. The charge collected within the defects of the I-layer can cause the photo diode sensors to suffer from image lag.
Image lag generally occurs when photo diode sensors sense a series of progressive images. That is, the photo diode sensors detect several images, one after another. For example, an array of photo diode sensor can used to generate a video stream of images. Image lag occurs when an image being sensed affects an image sensed in the future. The future image is typically the next subsequent image.
Upon detection of a “next image” the charge collected within the defects will be released. The effect is that the inherent capacitance of the photo diode sensor discharges more quickly than the capacitance of the photo diode sensor would have discharged had charge not been collected within the defects. That is, the charge collected within the defects affects the sensing of the next subsequent image, causing image lag.
It is desirable to have an active pixel sensor array formed adjacent to a substrate which can detect a series of images without suffering from as much image lag as prior art active pixel sensor arrays. It is desirable that the active pixel sensor array include a structure which does not require extra formation processing steps.
SUMMARY OF THE INVENTION
The present invention is an elevated active pixel sensor array formed adjacent to a substrate in which the active pixel sensors are surrounded by guard rings. The guard rings are biased to a voltage potential which reduces the image lag of the active pixel sensor array when detecting a series of images. The active pixel sensor array does not require extra formation processing steps.
A first embodiment of this invention includes an array of active pixel sensors. The array of active pixel sensors includes a substrate that includes electronic circuitry. An interconnect structure is formed adjacent to the substrate. The interconnect structure includes a plurality of conductive vias. A plurality of conductive guard rings are formed adjacent to the interconnect structure. Each conductive guard ring is electrically connected to the electronic circuitry through at

Affiliated with

Also associated with

Rating

Conductive guard rings for elevated active pixel sensors does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Conductive guard rings for elevated active pixel sensors, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Conductive guard rings for elevated active pixel sensors will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2504545