Image sensor pixel cell with non-destructive readout

A pixel cell includes a photodiode coupled to photogenerate image charge in response to incident light. A deep trench isolation structure is disposed proximate to the photodiode to provide a capacitive coupling to the photodiode through the deep trench isolation structure. An amplifier transistor is coupled to the deep trench isolation structure to generate amplified image data in response to the image charge read out from the photodiode through the capacitive coupling provided by the deep trench isolation structure. A row select transistor is coupled to an output of the amplifier transistor to selectively output the amplified image data to a column bitline coupled to the row select transistor.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to image sensors. More specifically, examples of the present invention are related to circuits that readout image data from image sensor pixel cells.

Image sensors have become ubiquitous. They are widely used in digital cameras, cellular phones, security cameras, as well as medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular complementary metal-oxide-semiconductor (CMOS) image sensors has continued to advance at great pace. Furthermore, the increasing demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.

The demands on the image sensor to perform over a large range of lighting conditions, varying from low light conditions to bright light conditions are becoming more difficult to achieve as pixel cells become smaller. This performance capability is generally referred to as having high dynamic range (HDR). In conventional image capture devices with small photosensitive devices, the pixel cells often require multiple successive samples or exposures of the photodiodes with long and short integration times to achieve HDR.

In a conventional CMOS pixel cell, image charge is transferred from a photosensitive device (e.g., a photodiode) and is converted to a voltage signal inside the pixel cell on a floating diffusion node. A challenge with this approach is that each readout of the conventional pixel cell is destructive. In particular, the charges in a photodiode disappear after each readout, which reduces light sensitivity compared to pixel cells that can accumulate light during an entire frame time.

DETAILED DESCRIPTION

Examples in accordance with the teaching of the present invention describe examples of image sensor pixel cells having non-destructive readouts in accordance with the teachings of the present invention. As will be shown, in various examples, a capacitive coupling is provided to the photodiode in each pixel cell through which non-destructive readouts of the pixel cell can be performed. For instance, in various examples, deep trench isolation structures are disposed proximate to the photodiodes of each pixel cell in a pixel array to sense voltages in the photodiodes without affecting the accumulated charges in the photodiode. As a result, techniques such as automatic exposure control (AEC) may be provided with the non-destructive readouts of the pixel cell, which therefore improves overall light sensitivity of the pixel cell compared to conventional pixel cells since light can be accumulated during an entire frame time of the pixel cell in accordance with the teachings of the present invention.

To illustrate,FIG. 1is a schematic illustrating one example of an image sensor pixel cell102including a non-destructive readout in accordance with the teachings of the present invention. In the depicted example, the pixel cell102includes a photodiode PD104coupled to photogenerate image charge in response to incident light136. A deep trench isolation structure CDTI116is disposed proximate to the photodiode PD104to provide a capacitive coupling to the photodiode PD104through the deep trench isolation structure CDTI116. In the example schematic depicted inFIG. 1, deep trench isolation structure CDTI116is illustrated as a capacitor that is coupled to provide the capacitive coupling to photodiode PD104.

An amplifier transistor112is coupled to the deep trench isolation structure CDTI116to generate amplified image data in response to the image charge read out from the photodiode PD104through the capacitive coupling provided by the deep trench isolation structure CDTI116. In the example schematic depicted inFIG. 1, a switch transistor120is coupled between the deep trench isolation structure CDTI116and a floating diffusion FD110, which is coupled to an amplifier transistor112as shown. In the depicted example, the amplifier transistor112is a source follower coupled transistor having its gate terminal selectively coupled to the deep trench isolation structure CDTI116through a switch transistor120. A row select transistor114is coupled to an output of the amplifier transistor112to selectively output the amplified image data to a column bitline118coupled to the row select transistor114.

As shown in the depicted example, a floating diffusion FD110is coupled to the amplifier transistor112, and a transfer transistor106is coupled between the photodiode PD104and the floating diffusion FD110to selectively couple the floating diffusion FD110to the photodiode PD104. A reset transistor108is coupled to the floating diffusion FD110to selectively reset charge in the floating diffusion FD110and the photodiode PD104. For instance, in one example, the charge in floating diffusion FD110may be reset to a reset voltage through reset transistor108, and the charge in photodiode PD104may be reset to a reset voltage through reset transistor108and transfer transistor106. In the example schematic illustrated inFIG. 1, the charge in the capacitive coupling provided by deep trench isolation structure CDTI116may be reset to a reset voltage through reset transistor108and switch transistor120. In one example, it is appreciated that the reset voltage that is coupled to the deep trench isolation structure CDTI116to reset the charge in deep trench isolation structure CDTI116may be a different reset voltage than the reset voltage that is coupled to reset the charge in the photodiode PD104and/or the floating diffusion PD110. In one example, the different reset voltages may be switched on the reset supply side before the reset transistor108is activated.

Therefore, in one example, sensing of the pixel cell102can be performed by first resetting the deep trench isolation structure CDTI116to a known potential during the photodiode PD104reset just before the start of integration. Next, integration may begin with photodiode PD104photogenerating charge in response to incident light136. In one example, correlated double sampling (CDS) may be performed by first resetting the floating diffusion FD110to a reset voltage through reset transistor108. The charge on floating diffusion FD110after the reset may then be sampled (e.g., SHR) from column bitline118through amplifier transistor112and row select transistor114. Next, a non-destructive read of the charge that is photogenerated in photodiode PD104can then be sampled by closing (i.e., turning ON) the switch transistor120to short the floating diffusion FD110to the deep trench isolation structure CDTI116, which will non-destructively sample (e.g., SHS) the image charge that is photogenerated in photodiode PD104in response to incident light136through the capacitive coupling provided by deep trench isolation structure CDTI116in accordance with the teachings of the present invention.

Assuming the sample value of the floating diffusion FD110after the reset is SHR, and assuming that the sample of the floating diffusion FD110after the switch transistor120shorts the floating diffusion FD110to the deep trench isolation structure CDTI116is SHS, the correlated double sampling (CDS) signal value is SHS-SHR. In one example, it is appreciated that automatic exposure control (AEC) may be realized by monitoring the CDS signal value until a threshold value of charge is photogenerated in photodiode PD104, at which time the photogenerated image charge in photodiode PD104can then be read out through the transfer transistor106, floating diffusion FD110, amplifier transistor112, row select transistor114, and column bitline118in accordance with the teachings of the present invention.

FIG. 2is a diagram illustrating a layout of an example image sensor pixel cell202included in a semiconductor chip, with a non-destructive readout in accordance with the teachings of the present invention. It should be appreciated that image sensor pixel cell202illustrated inFIG. 2may be one example of pixel cell102shown inFIG. 1, and that similarly named and numbered elements below are coupled and function as described above. As shown in the example, pixel cell202includes a photodiode204coupled to photogenerate image charge in response to incident light236. A deep trench isolation structure216is disposed proximate to the photodiode204to provide a capacitive coupling to the photodiode204through the deep trench isolation structure216.

An amplifier transistor212is coupled to the deep trench isolation structure216to generate amplified image data in response to the image charge read out from the photodiode204through the capacitive coupling provided by the deep trench isolation structure216. In the example schematic depicted inFIG. 2, a switch transistor220is coupled between the deep trench isolation structure216and a floating diffusion FD210, which is coupled to amplifier transistor212as shown. In the depicted example, the amplifier transistor212is a source follower coupled transistor having its gate terminal selectively coupled to the deep trench isolation structure216through the switch transistor220. A row select transistor214is coupled to an output of the amplifier transistor212to selectively output the amplified image data to a column bitline218coupled to the row select transistor214.

As shown in the depicted example, floating diffusion FD210is coupled to the amplifier transistor212, and a transfer transistor206is coupled between the photodiode204and the floating diffusion FD210to selectively couple the floating diffusion FD210to the photodiode204. A reset transistor208coupled to the floating diffusion FD210to selectively reset charge in the floating diffusion FD210and the photodiode204. For instance, in one example, the charge in floating diffusion FD210may be reset to a reset voltage through reset transistor208, and the charge in photodiode204may be reset to a reset voltage through reset transistor208and transfer transistor206. In the example schematic illustrated inFIG. 2, the charge in the capacitive coupling provided by deep trench isolation structure216may be reset to a reset voltage through reset transistor208and switch transistor220.

In the example depicted inFIG. 2, a conductive material238, such as for example polysilicon or the like, is disposed within the deep trench isolation structure216. In the example, an oxide material228lines an interior of the deep trench isolation structure216. In one example, the oxide material228lining the interior of the deep trench isolation structure216is a charged oxide trench liner. For instance, in one example, oxide material228may be a negatively charged oxide trench liner, and in another example, oxide material228may be a positively charged oxide liner. As such, it is appreciated that in the illustrated example the capacitance of the capacitive coupling to photodiode204provided with deep trench isolation structure216is suitably matched to sense the image charge that is accumulated in photodiode204in accordance with the teachings of the present invention.

FIG. 3is a schematic illustrating another example of an image sensor pixel cell302including a non-destructive readout in accordance with the teachings of the present invention. It should be appreciated that image sensor pixel cell302illustrated inFIG. 3shares similarities with pixel cell102shown inFIG. 1and/or pixel cell202shown inFIG. 2, and that similarly named and numbered elements below are coupled and function as described above. In the depicted example, the pixel cell302includes a photodiode PD304coupled to photogenerate image charge in response to incident light336. A deep trench isolation structure CDTI316is disposed proximate to the photodiode PD304to provide a capacitive coupling to the photodiode PD304through the deep trench isolation structure CDTI316. In the example schematic depicted inFIG. 3, deep trench isolation structure CDTI316is illustrated as a capacitor that is coupled to provide the capacitive coupling to photodiode PD304.

As shown in the illustrated example, a first amplifier transistor312A is coupled to the deep trench isolation structure CDTI316to generate first amplified image data in response to the image charge read out from the photodiode PD304through the capacitive coupling provided by the deep trench isolation structure CDTI316. A first row select transistor314A is coupled to an output of the first amplifier transistor312A to selectively output the first amplified image data to a column bitline318coupled to the first row select transistor314A.

In the depicted example, the first amplifier transistor312A is a source follower coupled transistor having its gate terminal coupled to the deep trench isolation structure CDTI316to provide non-destructive readouts of the image charge in photodiode PD304. In the example, the first row select transistor314A is coupled to selectively output the non-destructive readouts from the first amplifier transistor312A to the column bitline318in accordance with the teachings of the present invention. Thus, it is appreciated that first amplifier transistor312A and first row select transistor314A are used for non-destructive readouts of pixel cell302.

In one example, pixel cell302further includes a second amplifier transistor312B and a floating diffusion FD310, which are coupled to generate second amplified image data in response to the image charge read out from the photodiode PD304through the floating diffusion FD310. In the example, a second row select transistor314B is coupled to an output of the second amplifier transistor312B to selectively output the second amplified image data to the column bitline318coupled to the second row select transistor314B in accordance with the teachings of the present invention.

As shown in the example depicted inFIG. 3, floating diffusion FD310is coupled to the second amplifier transistor312B, and a transfer transistor306is coupled between the photodiode PD304and the floating diffusion FD310to selectively couple the floating diffusion FD310to the photodiode PD304. A reset transistor308is coupled to the floating diffusion FD310to selectively reset charge in the floating diffusion FD310and the photodiode PD304. For instance, in one example, the charge in floating diffusion FD310may be reset to a reset voltage through reset transistor308, and the charge in photodiode PD304may be reset to a reset voltage through reset transistor308and transfer transistor306. In one example, pixel cell302further includes an optional reset connection309through which the charge in the capacitive coupling provided by deep trench isolation structure CDTI316may also be reset to a reset voltage through reset transistor308. In one example, it is appreciated that the reset voltage that is coupled to the deep trench isolation structure CDTI316through the optional reset connection309to reset the charge in deep trench isolation structure CDTI316may be a different reset voltage than the reset voltage that is coupled to reset the charge in the photodiode PD304and/or the floating diffusion PD310. In one example, the different reset voltages may be switched on the reset supply side before the reset transistor308is activated.

Similar to the examples described in detail above, the image charge accumulated in photodiode PD304can be monitored with non-destructive readouts through the capacitive coupling provided with deep trench isolation structure CDTI316, first amplifier transistor312A, first row select transistor314A, and column bitline318. In one example, it is appreciated that automatic exposure control (AEC) may be realized by monitoring signal values sampled from the photodiode PD304through the non-destructive readouts until a threshold value of charge is photogenerated in photodiode PD304, at which time the photogenerated image charge may then be read out through the transfer transistor306, floating diffusion FD310, second amplifier transistor312B, second row select transistor314B, and column bitline318in accordance with the teachings of the present invention.

FIG. 4is a diagram illustrating a layout of another example image sensor pixel cell402included in a semiconductor chip, with a non-destructive readout in accordance with the teachings of the present invention. It should be appreciated that image sensor pixel cell402illustrated inFIG. 4shares similarities with pixel cell102shown inFIG. 1, pixel cell202shown inFIG. 2, and/or pixel cell302shown inFIG. 3, and that similarly named and numbered elements below are coupled and function as described above. As shown in the example, pixel cell402includes a photodiode404coupled to photogenerate image charge in response to incident light436. A deep trench isolation structure416is disposed proximate to the photodiode404to provide a capacitive coupling to the photodiode404through the deep trench isolation structure416.

A first amplifier transistor412A is coupled to the deep trench isolation structure416to generate first amplified image data in response to the image charge read out from the photodiode404through the capacitive coupling provided by the deep trench isolation structure416. In the depicted example, the first amplifier transistor412A is a source follower coupled transistor having its gate terminal coupled to the deep trench isolation structure416. A first row select transistor414A is coupled to an output of the first amplifier transistor412A to selectively output the first amplified image data to a column bitline418coupled to the first row select transistor414A.

As shown in the depicted example, a floating diffusion FD410is coupled to a second amplifier transistor412B, and a transfer transistor406is coupled between the photodiode404and the floating diffusion FD410to selectively couple the floating diffusion FD410to the photodiode404. A reset transistor408is coupled to the floating diffusion FD410to selectively reset charge in the floating diffusion FD410and the photodiode404. For instance, in one example, the charge in floating diffusion FD410may be reset to a reset voltage through reset transistor408, and the charge in photodiode404may be reset to a reset voltage through reset transistor408and transfer transistor406. In one example, pixel cell402further includes an optional reset connection409through which the charge in the capacitive coupling provided by deep trench isolation structure416may also be reset to a reset voltage through reset transistor408.

In the example depicted inFIG. 4, a conductive material438, such as for example polysilicon or the like, is disposed within the deep trench isolation structure416. In the example, an oxide material428lines an interior of the deep trench isolation structure416. In one example, the oxide material428lining the interior of the deep trench isolation structure416is a charged oxide trench liner. For instance, in one example, oxide material228may be a negatively charged oxide trench liner, and in another example, oxide material228may be a positively charged oxide liner. As such, it is appreciated that in the illustrated example, the capacitance of the capacitive coupling to photodiode404provided with deep trench isolation structure416is suitably matched to sense the image charge that is accumulated in photodiode404in accordance with the teachings of the present invention.

FIG. 5is a block diagram illustrating an example imaging system500including a pixel array having pixel cells with non-destructive readouts in accordance with the teachings of the present invention. As shown in the depicted example, imaging system500includes pixel array502coupled to control circuitry532and readout circuitry530, which is coupled to function logic534.

In one example, pixel array502is a two-dimensional (2D) array of imaging sensors or pixel cells (e.g., pixel cells P1, P2, . . . , Pn). In one example, each pixel cell is a CMOS imaging pixel. It is noted that the pixel cells P1, P2, . . . Pn in the pixel array502may be examples of pixel cell102ofFIG. 1, pixel cell202ofFIG. 2, pixel cell302ofFIG. 3, or pixel cell402ofFIG. 4, and that similarly named and numbered elements referenced below are coupled and function similar to as described above. As illustrated, each pixel cell is arranged into a row (e.g., rows R1to Ry) and a column (e.g., column C1to Cx) to acquire image data of a person, place, object, etc., which can then be used to render a 2D image of the person, place, object, etc.

In one example, after each pixel cell has accumulated its image data or image charge, the image data is readout by readout circuitry530through column bitlines518and then transferred to function logic534. In various examples, readout circuitry530may also include additional amplification circuitry, additional analog-to-digital (ADC) conversion circuitry, or otherwise. Function logic534may simply store the image data or even manipulate the image data by applying post image effects (e.g., crop, rotate, remove red eye, adjust brightness, adjust contrast, or otherwise). In one example, readout circuitry530may readout a row of image data at a time along readout column bitlines518(illustrated) or may readout the image data using a variety of other techniques (not illustrated), such as a serial readout or a full parallel readout of all pixels simultaneously.

In one example, control circuitry532is coupled to pixel array502to control operational characteristics of pixel array502. For example, control circuitry532may generate a shutter signal for controlling image acquisition. In one example, the shutter signal is a global shutter signal for simultaneously enabling all pixels within pixel array502to simultaneously capture their respective image data during a single acquisition window. In another example, the shutter signal is a rolling shutter signal such that each row, column, or group of pixels is sequentially enabled during consecutive acquisition windows.