TWO-DIMENSIONAL TIME DELAY INTEGRATION VISIBLE CMOS IMAGE SENSOR
A two dimensional time delay integration CMOS image sensor having a plurality of pinned photodiodes, each pinned photodiode collects a charge when light strikes the pinned photodiode, a plurality of electrodes separating the plurality of pinned photodiodes, the plurality of electrodes are configured for two dimensional charge transport between two adjacent pinned photodiodes, and a plurality of readout nodes connected to the plurality of pinned photodiodes via address lines.
This application is a continuation of and claims the benefit and priority of U.S. application Ser. No. 11/683,811, entitled “TWO-DIMENSIONAL TIME DELAY INTEGRATION VISIBLE CMOS IMAGE SENSOR,” filed on Aug. 3, 2007, which is assigned to the assignee hereof and hereby expressly incorporated by reference herein.
BACKGROUND OF THE INVENTION1. Field of the Invention
The invention relates generally to a Complementary Metal Oxide Semiconductor (CMOS) image sensor. More particularly, the invention relates to two-dimensional time delay integration visible CMOS image sensor.
2. Description of Related Art
Unmanned Aerial Vehicles (UAVs) are remotely piloted or self-piloted aircrafts that can carry cameras, sensors, and other communication equipment. UAVs may be remotely controlled (e.g. flown by a pilot at a ground control station) or fly autonomously based on pre-programmed flight plans or more complex dynamic automation systems. UAVs are typically used for reconnaissance and intelligence-gathering, and for more challenging roles, including combat missions.
Ideally, an image taken from a camera onboard the UAV should be clear to provide accurate intelligence-gathering and determine appropriate targets. However, since UAVs shake from wind gusts during their flight operation, the image received from UAV is not clear enough to accurately identify targets on the ground. Consequently, there is a low signal to noise ratio due to wind and mechanical vibrations of the camera. This problem is compounded with moving scene imagery.
To improve signal to noise ratio, prior art stabilizers were integrated with the gimbal assembly of high speed cameras onboard the UAVs. The stabilizers reduce interferences caused by wind or mechanical vibrations. Additionally, the signal to noise ratio may be improved using Charge-Coupled Devices (CCDs) with Time Delay Integration (TDI). CCDs with TDI technology allow an image in a charge domain to move at about the same speed as the moving scene or target. However, CCDs with TDI are one dimensional and require multiple chip systems.
Conventional CMOS integrated circuits can achieve TDI in one dimension. The CMOS integrated circuits provide TDI using a switch matrix or a transistor chain CCD equivalent. The switch matrix typically accumulates additional noise and the signal to noise ratio improvement is less than proportional to the square root of the number of TDI channels. The transistor chain CCD equivalent cannot have high QE photodiode and is not a mainstream CMOS or CMOS Image Sensor (CIS) process.
With an ever increasing demand for improved imaging sensors, there remains a need for a two dimensional TDI visible CMOS image sensor that allow a charge to move at the same speed and follow a similar path in the charge domain as the moving image so that more charge from the scene can be integrated resulting in an improved signal to noise ratio. If readout noise is dominant, the signal to noise ratio improvement is proportional to the number of TDI channels.
SUMMARY OF THE INVENTIONThe present invention fills this need by providing a time delay integration CMOS image sensor having a first pinned photodiode and a second pinned photodiode, the first pinned photodiode collects a charge when light strikes the first pinned photodiode, the second pinned photodiode receives the charge from the first pinned photodiode, and a plurality of electrodes in series located between the first and the second pinned photodiodes, the plurality of electrodes are configured to transfer the charge from the first pinned photodiode to the second pinned photodiode. The plurality of electrodes may be activated consecutively at different cycles.
In one embodiment, the time delay integration CMOS image sensor may include a plurality of readout nodes coupled to the second pinned photodiode via address lines. The number of readout nodes may be equal to the number of pinned photodiodes. The plurality of electrodes, the plurality of readout nodes and the address lines may form an orthogonal or hexagonal grid around the perimeter of each pinned photodiode.
The exact nature of this invention, as well as the objects and advantages thereof, will become readily apparent from consideration of the following specification in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:
Photodiodes are widely used in digital imaging devices to convert optical signals into electrical signals. Photodiodes may be arranged in linear or planar arrays with a plurality of photosensitive sensors, generally designated as pixels, on a semiconductor chip. Each pixel generates an output signal representing the amount of light incident on the pixel.
A pinned photodiode (PPD) is used to produce and integrate photoelectric charges generated in CCD or CMOS image sensors.
Using the pinned photodiode 11 with transfer gate 13 allows for complete charge removal from light sensing area to the floating diffusion 15.
Combining two transfer gates or electrodes in series provides charge transport from one pixel to the next.
In operation, the CMOS image sensor 27 allows charge(s) 36 to travel from one pinned photo photodiode 31 to another. Initially, in
Next, in
In
In
In
According to an embodiment of the invention, the lateral charge 36 transport occurs over a 4 cycle period, as shown in
As shown in
With moving scene imagery, pinned photodiode 44 of the time delay integration visible CMOS image sensor 40 generates a charge that moves in two dimensions at about the same speed and follows a similar path as the moving image. Similarly, mechanical vibrations of a camera cause random walk of any image point on the sensor 40.
To better approximate the curved random walk of a scene, the sensor may be configured to allow for charge transport in three or more directions.
With moving scene imagery, pinned photodiode 70 of the time delay integration visible CMOS image sensor 60 generates a charge that moves in two dimensions at about the same speed and follows a similar path as the moving image. Similarly, mechanical vibrations of a camera cause random walk of any image point on the sensor 40.
A person skilled in the art would appreciate the potential applications of the two dimensional time delay integration visible CMOS image sensor of the present invention. The sensor may be used for translational image stabilization during single frame integration time. For example, very high bandwidth of translational vibrations can be stabilized from about 30 Hz to about 1 MHz. The maximum translational vibration amplitude may be limited by imager resolution. The sensor may also be used for rotational image stabilization during single frame integration time. For example, very high bandwidth of rotational movement can be stabilized from about 30 Hz to about 1 MHz. The maximum rotational vibration amplitude may be limited by pixel size and tolerable distortions.
Other applications of the sensor include residue light photography without tripod or flash, TDI camera with increased alignment tolerance and flow cytometry for capturing images of moving cells in fluids. The sensor may also be used, in combination with a stabilized gimbal, to enhance pointing accuracy to a few tens of grads. Additionally, the sensor may be used with Inertial Measurement Unit (IMU) to suppress random motion. Depending on frame rate, IMU may be replaced with processing algorithm.
While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.
Claims
1. An image sensor comprising:
- first and second regions, the first region located adjacent to the second region;
- a first photodiode located adjacent to the first region;
- a second photodiode located adjacent to the second region;
- a first transfer gate positioned above the first region, and configured to transfer a charge from the first photodiode to the first region; and
- a second transfer gate positioned above the second region, configured to receive the charge in the second region, and configured to transfer the charge from the second region to the second photodiode.
2. The image sensor of claim 1, wherein the first and second regions are substantially free of any floating diffusion region or drain region.
3. The image sensor of claim 1, further comprising:
- a first readout node located outside the first and second regions, and configured to collect a first charge from the first photodiode.
4. The image sensor of claim 3, further comprising:
- a third transfer gate coupled between the first readout node and the first photodiode, and configured to facilitate the first node to collect the first charge from the first photodiode.
5. The image sensor of claim 1, further comprising:
- a second readout node located outside the first and second regions, and configured to collect a second charge from the second photodiode.
6. The image sensor of claim 5, further comprising:
- a fourth transfer gate coupled between the second readout node and the second photodiode, and configured to facilitate the second node to collect the second charge from the second photodiode.
7. The image sensor of claim 1, wherein the first transfer gate is located directly adjacent to the second transfer gate.
8. The image sensor of claim 1, wherein:
- the first photodiode has a first Fermi level,
- the second photodiode has a second Fermi level substantially the same as the first Fermi level,
- the first region has a first region Fermi level substantially higher than the first Fermi level, and
- the second region has a second region Fermi level substantially the same as the first region Fermi level.
9. The image sensor of claim 8, wherein:
- the first transfer gate creates a first well in the first region, such that the first well has a first quasi-Fermi level substantially lower than the first Fermi level when the charge is transferred from the first photodiode to the first well, and
- the second transfer gate creates a second well in the first region, such that the second well has a second quasi-Fermi level substantially similar to the first quasi-Fermi level when the charge is transferred from the first well to the second well.
10. The image sensor of claim 9, wherein:
- the first transfer gate restores the first region Fermi level in the first region after the charge is transferred from the first region to the second region, and
- the second transfer gate restores the second region Fermi level in the second region to transfer the charge from the second region to the second photodiode.
11. A method for transferring a charge between a first photodiode and a second photodiode, comprising the steps of:
- creating a first well adjacent to the first photodiode during a first time period; and
- creating a second well adjacent to the first well and the second photodiode during a second time period partially but not entirely overlapping with the first time period.
12. The method of claim 11, wherein the creating the first well step includes:
- adjusting, in a first region adjacent to the first photodiode, a first quasi-Fermi level substantially lower than a first Fermi level of the first photodiode.
13. The method of claim 12, wherein the creating the second well step includes:
- adjusting, in a second region adjacent to the second photodiode and the first region, a second quasi-Fermi level substantially the same as the first quasi-Fermi level and substantially lower than a second Fermi level of the second photodiode.
14. The method of claim 11, further comprising the steps of:
- collapsing the first well after the first time period; and
- collapsing the second well after the second time period.
15. The method of claim 14, wherein:
- the collapsing the first well step includes restoring, in a first region adjacent to the first photodiode, a first region Fermi level substantially higher than a first Fermi level of the first photodiode, and
- the collapsing the second well step includes restoring, in a second region adjacent to the second photodiode and the first region, a second region Fermi level substantially higher than a second Fermi level of the second photodiode.
16. The method of claim 11, wherein:
- the first time period has a first portion distinct from the second time period, and
- the second time period has a second portion distinct from the first time period.
17. A method for transferring a charge between a first photodiode and a second photodiode, comprising the steps of:
- applying a first voltage to a first electrode to adjust a first quasi-Fermi level of a first region located adjacent to the first photodiode during first and second cycles; and
- applying a second voltage to a second electrode to adjust a second quasi-Fermi level of a second region located adjacent to the first region and the second photodiode electrode during the second cycle and a third cycle.
18. The method of claim 17, wherein:
- the first quasi-Fermi level is substantially lower than a first Fermi level of the first photodiode, and
- the second quasi-Fermi level is substantially the same as the first quasi-Fermi level and substantially lower than a second Fermi level of the second photodiode.
19. The method of claim 17, further comprising the steps of:
- floating the first electrode to restore a first region Fermi level of the first region after the second cycle; and
- floating the second electrode to restore a second region Fermi level of the second region after the third cycle.
20. The method of claim 19, wherein:
- the first region Fermi level of the first region is substantially higher than the first Fermi level of the first photodiode, and
- the second region Fermi level of the second region is substantially higher than the second Fermi level of the second photodiode.
Type: Application
Filed: Dec 2, 2010
Publication Date: Mar 24, 2011
Inventor: Stefan Lauxtermann (Camarillo, CA)
Application Number: 12/959,327
International Classification: H01L 31/113 (20060101); H01L 31/18 (20060101);