HK1222485B - Blue enhanced image sensor - Google Patents

Description

Blue-enhanced image sensor

Technical Field

The present invention relates generally to semiconductor devices. More particularly, examples of the present invention relate to image sensors with enhanced blue light absorption.

Background Art

Image sensors have become ubiquitous. They are widely used in digital cameras, cell phones, security cameras, and in medicine, automobiles, and other applications. The technology used to manufacture image sensors continues to advance rapidly. For example, demands for higher resolution and lower power consumption have driven further miniaturization and integration of these devices.

One type of image sensor, the complementary metal oxide semiconductor (CMOS) image sensor, is very popular in commercial electronics. However, as these semiconductor devices have been scaled down, the photodiode area has also been reduced, resulting in fewer photons incident on each photodiode. Several challenges for scaling down CMOS image sensors are maintaining low light sensitivity and reducing image noise, both of which worsen with decreasing incident photon counts.

A commonly accepted approach to improving CMOS image sensor performance is to enhance quantum efficiency and reduce crosstalk, as this results in a more favorable signal-to-noise ratio. One approach to improving quantum efficiency is to increase semiconductor thickness so that the photodiode can absorb more light before it can penetrate the sensing volume. However, blue pixel quantum efficiency does not improve with increasing semiconductor thickness because this high-energy (short-wavelength) light is absorbed right at the semiconductor's surface. Furthermore, typical color filter options (e.g., red, green, and blue) block much of the blue light incident on the surface of the CMOS device, further limiting blue photon absorption.

Summary of the Invention

In one aspect, the present invention provides a backside illuminated image sensor comprising: a semiconductor material having a front side and a back side; an image sensor circuit and a filter array, wherein the semiconductor material is disposed between the image sensor circuit and the filter array, and wherein the image sensor circuit is disposed on the front side of the semiconductor material, and the filter array is disposed proximate to the back side of the semiconductor material; a first pixel comprising: a first doped region disposed in the semiconductor material, wherein the first doped region extends a first depth from the image sensor circuit into the semiconductor material; and a second doped region disposed in the semiconductor material, wherein the second doped region is disposed between the back side of the semiconductor material and the first doped region, and wherein the second doped region is electrically isolated from the first doped region; and a second pixel comprising a third doped region disposed in the semiconductor material, wherein the third doped region extends a second depth from the image sensor circuit into the semiconductor material.

On the other hand, the present invention provides an imaging system, comprising: a pixel array comprising a semiconductor material, wherein the semiconductor material has a front side and a back side; an image sensor circuit and a filter array, wherein the semiconductor material is disposed between the image sensor circuit and the filter array, and wherein the image sensor circuit is disposed on the front side of the semiconductor material and the filter array is disposed proximate to the back side of the semiconductor material; a first pixel comprising: a first doped region disposed in the semiconductor material, wherein the first doped region extends a first depth from the image sensor into the semiconductor material; and a second doped region disposed in the semiconductor material, wherein the second doped region is disposed between the back side of the semiconductor material and the first doped region, and wherein the second doped region is electrically isolated from the first doped region; and a second pixel comprising a third doped region disposed in the semiconductor material, wherein the third doped region extends a second depth from the image sensor circuit into the semiconductor material, and wherein the third doped region is electrically coupled to the second doped region.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive examples of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

1 is a cross-sectional view illustrating one example of a backside illuminated image sensor in accordance with the teachings of the present invention.

2A is a diagram illustrating one example of a top view of a backside illuminated image sensor in accordance with the teachings of the present invention.

2B is a cross-sectional view illustrating the example image sensor in FIG. 2A as cut along line A-A' in accordance with the teachings of the present invention.

2C is a cross-sectional view illustrating the example image sensor of FIG. 2A as cut along line B-B' in accordance with the teachings of the present invention.

3 is a schematic diagram illustrating one example of an image sensor circuit in accordance with the teachings of the present invention.

4 is a diagram illustrating one example of an imaging system in accordance with the teachings of the present invention.

Those skilled in the art will appreciate that the elements in the figures are illustrated for simplicity and clarity and are not necessarily drawn to scale. For example, the dimensions of some elements in the figures may be exaggerated relative to other elements to help improve understanding of the various embodiments of the present invention. Furthermore, to facilitate understanding of these various embodiments of the present invention, common but well-understood elements that are useful or necessary in commercially feasible embodiments are generally not depicted.

DETAILED DESCRIPTION

As will be shown, methods and apparatus for image sensors with enhanced blue light absorption are disclosed. In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, one skilled in the art will recognize that the techniques described herein can be practiced without employing one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to "one embodiment," "an embodiment," "an example," or "an example" means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. Thus, the appearances of phrases such as "in one embodiment" or "in an example" in multiple places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

1 is a cross-sectional view illustrating one example of a backside illuminated image sensor 100. Backside illuminated image sensor 100 includes a semiconductor material 125 having a front side 123 and a back side 121, image sensor circuitry 131, and an optical filter array 109. Semiconductor material 125 is disposed between image sensor circuitry 131 and optical filter array 109. Image sensor circuitry 131 may be disposed on front side 123 of semiconductor material 125, and optical filter array 109 may be disposed proximate back side 121 of semiconductor material 125.

A first pixel is disposed in backside illuminated image sensor 100. The first pixel includes a first doped region 111 disposed in semiconductor material 125. First doped region 111 extends from image sensor circuitry 131 to a first depth 141 into semiconductor material 125. The first pixel also includes a second doped region 113 disposed between backside 121 of semiconductor material 125 and first doped region 111. Second doped region 113 is disposed in semiconductor material 125 such that second doped region 113 absorbs blue light. In one example, second doped region 113 extends 0.75 μm or less from backside 121 into semiconductor material 125 to avoid absorbing significant amounts of green and red light. In one example, second doped region 113 is electrically isolated from first doped region 111.

Backside illuminated image sensor 100 further includes a second pixel including a third doped region 115 disposed in semiconductor material 125. Third doped region 115 extends from image sensor circuitry 131 to a second depth 143 into semiconductor material 125. In one example, second depth 143 is greater than first depth 141. In one example, second doped region 113 is electrically coupled to third doped region 115.

In one or more examples, backside illuminated image sensor 100 may further include a plurality of first doped regions 111, second doped regions 113, and third doped regions 115. In these examples, filter array 109 may include several unconventional filters. First filter 103 in filter array 109 may allow blue and red light to pass through and be positioned proximate to at least one of second doped regions 113. Second filter 105 in filter array 109 may allow blue and green light to pass through and be positioned proximate to at least another of second doped regions 113. Third filter 107 in filter array 109 may allow blue light to pass through and be positioned proximate to at least one of third doped regions 115. In another or the same example, backside illuminated image sensor 100 may include a third pixel that includes at least another of third doped regions 115 and a fourth filter in the filter array (for example, one of filters 107). A fourth filter may be positioned proximate to a third pixel and allow blue, red, and green light to pass through. This third pixel may serve as a "white pixel" and output image charges generated by the incident blue, green, and red light.

It is worth noting that the semiconductor material 125, the first doped region 111, the second doped region 113, and the third doped region 115 can be made of a wide variety of semiconductor elements and compounds. In one example, the semiconductor material 125 can include silicon; however, in the same or different examples, the semiconductor material 125 can include germanium, gallium, arsenic, or the like. In one example, the semiconductor material 125 is p-type, and the first doped region 111, the second doped region 113, and the third doped region 115 are n-type. However, in different examples, the semiconductor material 125 is n-type, and the first doped region 111, the second doped region 113, and the third doped region 115 are p-type.

In operation, the backside illuminated image sensor 100 utilizes different doped regions to receive light of different wavelengths. Using unconventional filters (for example, filters that allow blue light, blue light and red light, and blue light and green light to pass through) and doped regions located at different depths in the semiconductor material 125 can improve blue light absorption. For example, in one embodiment, the first filter 103 can allow blue light and red light to pass through. Once blue and red photons pass through the first filter 103, the blue light is absorbed by the second doped region 113. Conversely, red light passes through the second doped region 113 and is substantially absorbed by the first doped region 111. The second doped region 113 is positioned close to the back side 121 of the semiconductor layer 125 due to the shorter extinction length of blue light relative to red light in commonly utilized semiconductor materials. Since red light has a longer extinction length, it can freely pass through the second doped region 113 and enter the first doped region 111. This allows the backside illuminated image sensor 100 to absorb more blue light than would be the case if the surface area of the semiconductor material 125 were allocated to a standard red, green, and blue filter array. This same process can also be used to increase blue light absorption while measuring green light. In one example, the second filter 105 allows blue light to pass through with green light. Once the blue and green photons pass through the second filter 105, the blue light is absorbed by the second doped region 113. The green light passes through the second doped region 113 and is substantially absorbed by the first doped region 111. Since green light has a longer extinction length than blue light in commonly utilized semiconductors, it freely passes through the second doped region 113 and into the first doped region 111. The third filter 107 allows blue light to pass through. The blue light passes through the third filter 107 and is absorbed by the third doped region 115.

It should be noted that in the depicted example, the second doped regions 113 extend over all of the first doped regions 111. This improves blue light collection because nearly the entire surface area on the backside illuminated image sensor 100 is used to collect blue light. Furthermore, both green and red signals can still be readily measured, thanks to the first doped regions 111 absorbing substantially all green or red light. In one example, several second doped regions 113 can be electrically coupled together and also coupled to the third doped region 115. In another or the same example, three second doped regions are electrically coupled together and coupled to the third doped region 115. To prevent image charge from flowing between uncoupled doped regions, in one example, the first doped region 111 is electrically isolated from the second doped regions 113 and the third doped region 115 by pinning wells in the semiconductor layer 125 located between all or some of the doped regions. Furthermore, in another example, the first doped region 111 is disposed in the semiconductor material 125 at least 0.75 μm away from the backside 121 of the semiconductor material 125. This helps prevent unintended absorption of blue light and subsequent optical crosstalk as the extinction length of blue light is less than 0.75 μm in many commonly utilized semiconductors.

FIG2A is a diagram illustrating one example of a top view of a backside illuminated image sensor (for example, backside illuminated image sensor 100). In the depicted example, first and second pixels are arranged in a 2 by 2 array. The first row of the array may include both of the first doped regions. The second row of the array may include the first doped region and the third doped region. The second doped region may be disposed between the filter array and all of the first doped regions. In another example, a third pixel may be present that includes a fourth filter in the filter array. This filter allows blue, green, and red light to pass. Thus, the third pixel may function as a "white pixel" and generate image charge in response to incident blue, green, and red light.

FIG2B illustrates a cross-sectional view of the example image sensor of FIG2A taken along dashed line A-A'. In the depicted example, the second doped region 213 extends above the first doped region 211. As a result of the second doped region 213 extending above the first doped region 211, blue light is blocked from reaching the first doped region 211. In one example, the lateral boundaries of the second doped region 213 are aligned with the lateral boundaries of the first doped region 211 disposed below the second doped region 213. However, in alternative examples, the lateral boundaries of the second doped region 213 may extend beyond the lateral boundaries of the underlying first doped region 211, or the lateral boundaries of the underlying first doped region 211 may extend beyond the lateral boundaries of the second doped region 213.

FIG2C illustrates a cross-sectional view of the example image sensor of FIG2A taken along dashed line B-B′. In the depicted example, a first doped region 211 is disposed below a second doped region 213. Additionally, the second doped region 213 extends above the first doped region 211, and the lateral boundaries of the first doped region 211 and the second doped region 213 are aligned. However, in alternative examples, the lateral boundaries of the second doped region 213 may extend beyond the lateral boundaries of the underlying first doped region 211, or the lateral boundaries of the underlying first doped region 211 may extend beyond the lateral boundaries of the second doped region 213. A third doped region 215 is disposed proximate to the first doped region 211 and electrically coupled to the second doped region 213. In the depicted example, the second doped region 213 and the third doped region 215 are electrically coupled by virtue of the overlapping doped regions. However, in alternative examples, the second doped region 213 and the third doped region 215 are electrically coupled by means of wiring and/or other features of the circuit (not depicted).

3 is a schematic diagram illustrating one example of an image sensor circuit 300. In the depicted example, image sensor circuit 300 includes a plurality of first photodiodes 311, a plurality of second photodiodes 313, a first transfer transistor 333, a second transfer transistor 343, a floating diffusion 329, a reset transistor 322, an amplifier transistor 324, and a row select transistor 326 coupled to a readout column 312.

In operation, image charge is accumulated in photodiode 311 and photodiode 313 (which may include first doped region 111, second doped region 113, and third doped region 115). When incident light enters photodiode 311/photodiode 313 and is converted into hole-electron pairs, the image charge can be transferred to floating diffusion 329 for readout as image data. A first transfer transistor 333 and a second transfer transistor 343 can be coupled between photodiode 311/photodiode 313 and floating diffusion 329 to selectively transfer image charge from photodiode 311/photodiode 313 to floating diffusion 329. In one example, first transfer transistor 333 is electrically coupled between the first doped region (included in photodiode 311) and floating diffusion 329, and second transfer transistor 343 is coupled between the third doped region (included in photodiode 313) and floating diffusion 329.

3 also illustrates a reset transistor 322 coupled between a reset voltage _VDD and a floating diffusion 329 to selectively reset the charge in the floating diffusion 329 in response to a reset signal RST. In the depicted example, an amplifier transistor 324 includes an amplifier gate coupled to the floating diffusion 329 to amplify the signal on the floating diffusion 329 to output image data. A row select transistor 326 is coupled between the readout column 312 and the amplifier transistor 324 to output image data to the readout column 312.

In the depicted example, four photodiodes (two of which are identical) share the same floating diffusion 329. In this example, each photodiode has its own transfer transistor. Charge can be transferred from the four photodiodes to the floating diffusion 329 in series or simultaneously by applying a voltage to each transfer transistor. Although the example depicted in FIG3 shows four photodiodes connected to the floating diffusion 329, in different examples, any number of photodiodes can be connected to the floating diffusion 329. For example, in an alternative example, each photodiode can be coupled to its own floating diffusion and reset transistor.

FIG4 is a diagram illustrating one example of an imaging system 400. Imaging system 400 includes a pixel array 405, readout circuitry 410, function logic 415, and control circuitry 420. Each pixel in pixel array 405 (for example, pixels P1, P2, ..., Pn) includes at least one doped region (for example, first doped region 111, second doped region 113, and/or third doped region 115) disposed in a semiconductor material (for example, semiconductor material 125). In one example, pixel array 405 is a two-dimensional (2D) array of individual pixels (for example, pixels P1, P2, ..., Pn) including rows (for example, rows R1 to Ry) and columns (for example, columns C1 to Cx). In one example, it should be understood that pixel array 405 can be composed of a repeating 2 by 2 array including first and second pixels (as shown in FIG2). In the same example, the second doped regions (e.g., second doped regions 113) within individual 2 by 2 arrays are disposed above the first doped regions (e.g., first doped regions 111) in the individual 2 by 2 arrays, but are decoupled from the second doped regions in the other 2 by 2 arrays. Pixel array 405 can be used to acquire image data of a person, location, object, etc., which can then be used to reproduce a 2D image of the person, location, object, etc. In one example, after each image sensor pixel in pixel array 405 has acquired its image data, or image charge, the image charge is then read out by readout circuitry 410 and transferred to function logic 415. In one example, readout circuitry 410 is coupled to read out the image charge from a floating diffusion (e.g., floating diffusion 329), and function logic 415 is coupled to readout circuitry 410 to perform logic operations on the image charge.

In various examples, readout circuitry 410 may include amplification circuitry, analog-to-digital (ADC) conversion circuitry, or other. Function logic 415 may simply store the image data or even manipulate the image data by applying post-image effects (e.g., cropping, rotation, red-eye removal, brightness adjustment, contrast adjustment, or other). In one example, readout circuitry 410 may read out image data one row at a time along a readout column line (illustrated), or may use a variety of other techniques (not illustrated), such as serial readout or simultaneous full parallel readout of all pixels.

In one example, control circuitry 420 is coupled to control the operation of pixels (e.g., P1, P2, P3, etc.) in pixel array 405. For example, control circuitry 420 can generate a shutter signal for controlling image acquisition. In one example, the shutter signal is a global shutter signal that simultaneously enables all pixels within pixel array 405 to simultaneously capture their corresponding image data during a single acquisition window. In another example, the shutter signal is a rolling shutter signal that sequentially enables each row, column, or group of pixels during successive acquisition windows. In another example, image acquisition is synchronized with a lighting effect, such as a flash.

In one example, imaging system 400 may be included in a digital camera, a cell phone, a laptop computer, or the like. Additionally, imaging system 400 may be coupled to other hardware components, such as a processor, memory components, outputs (USB ports, wireless transmitters, HDMI ports, etc.), lighting/flash, electronic inputs (keyboard, touch display, trackpad, mouse, microphone, etc.), and/or a display. The other hardware components may transmit instructions to imaging system 400, extract image data from imaging system 400, or manipulate image data supplied by imaging system 400.

The above description of the examples of the present invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the precise forms disclosed. Although specific embodiments and examples of the present invention are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present invention. Indeed, it should be understood that specific example structures, materials, and use cases are provided for illustrative purposes and that substitutions may also be utilized in other embodiments and examples according to the teachings of the present invention.

These modifications may be made to examples of the present invention in light of the above detailed description. The terms used in the appended claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and claims. Rather, the scope is to be determined entirely by the appended claims, which are to be construed in accordance with established principles of claim interpretation. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive.

Claims (19)

1. A backside illuminated image sensor, comprising: a semiconductor material having a front side and a back side; an image sensor circuit and an optical filter array, wherein the semiconductor material is disposed between the image sensor circuit and the optical filter array, and wherein the image sensor circuit is disposed on the front side of the semiconductor material and the optical filter array is disposed proximate the back side of the semiconductor material; A first pixel comprising: a first doped region disposed in the semiconductor material, wherein the first doped region extends a first depth into the semiconductor material from the image sensor circuitry; and a second doped region disposed in the semiconductor material, wherein the second doped region is disposed between the back side of the semiconductor material and the first doped region, and wherein the second doped region is electrically isolated from the first doped region; a second pixel comprising a third doped region disposed in the semiconductor material, wherein the third doped region extends a second depth from the image sensor circuitry into the semiconductor material; a plurality of the first doping regions, the second doping regions and the third doping regions; a first filter in the filter array that allows blue light and red light to pass therethrough and is photoaligned with at least one of the second doped regions of the plurality of second doped regions; a second filter in the filter array that allows blue and green light to pass and excludes red light and is photoaligned with at least another one of the plurality of second doped regions; and A third filter in the filter array allows blue light to pass therethrough and is photoaligned with at least one of the third doped regions in the plurality of third doped regions. 2 . The backside illuminated image sensor of claim 1 , wherein the second doped region is electrically coupled to the third doped region. 3. The backside illuminated image sensor of claim 2 , wherein the first pixels and the second pixels are arranged in a 2-by-2 array, wherein a first row of the array includes two first doped regions, and wherein a second row of the array includes the first doped region and the third doped region, and wherein the second doped region is disposed between the filter array and the first doped region. 4. The backside illuminated image sensor of claim 3, wherein the 2 by 2 array repeats itself, and wherein the second doped regions within the individual 2 by 2 arrays are decoupled from the second doped regions in other 2 by 2 arrays. 5. The backside illuminated image sensor of claim 1 , further comprising: a third pixel including at least another of the third doped regions; and A fourth filter in the filter array, wherein the fourth filter is disposed proximate the third pixel and allows blue, red, and green light to pass therethrough. 6 . The backside illuminated image sensor of claim 1 , wherein the second doped region is disposed in the semiconductor material such that the second doped region absorbs blue light. 7 . The backside illuminated image sensor of claim 6 , wherein the second doped region extends 0.75 μm or less from the backside of the semiconductor material into the semiconductor material. 8 . The backside illuminated image sensor of claim 1 , wherein the first doped region is disposed in the semiconductor material at least 0.75 μm away from the backside of the semiconductor material. 9. The backside illuminated image sensor of claim 1, wherein the semiconductor material is p-type, and wherein the first doped region, the second doped region, and the third doped region are n-type. 10. The backside illuminated image sensor of claim 1, wherein the semiconductor material is n-type, and wherein the first doped region, the second doped region, and the third doped region are p-type. 11 . The backside illuminated image sensor of claim 1 , wherein the first doped region is electrically isolated from the second doped region and the third doped region by a pinning well. 12. The backside illuminated image sensor of claim 1, wherein the second depth is greater than the first depth. 13. An imaging system, comprising: A pixel array comprising a semiconductor material, wherein the semiconductor material has a front side and a back side; an image sensor circuit and an optical filter array, wherein the semiconductor material is disposed between the image sensor circuit and the optical filter array, and wherein the image sensor circuit is disposed on the front side of the semiconductor material and the optical filter array is disposed proximate the back side of the semiconductor material; A first pixel comprising: a first doped region disposed in the semiconductor material, wherein the first doped region extends a first depth into the semiconductor material from the image sensor circuitry; and a second doped region disposed in the semiconductor material, wherein the second doped region is disposed between the back side of the semiconductor material and the first doped region, and wherein the second doped region is electrically isolated from the first doped region; a second pixel comprising a third doped region disposed in the semiconductor material, wherein the third doped region extends a second depth into the semiconductor material from the image sensor circuitry, and wherein the third doped region is electrically coupled to the second doped region; a plurality of the first doping regions, the second doping regions and the third doping regions; a first filter in the filter array that allows blue light and red light to pass therethrough and is photoaligned with at least one of the second doped regions of the plurality of second doped regions; a second filter in the filter array that allows blue and green light to pass and excludes red light and is photoaligned with at least another one of the plurality of second doped regions; and A third filter in the filter array allows blue light to pass therethrough and is photoaligned with at least one of the third doped regions in the plurality of third doped regions. 14. The imaging system of claim 13 , further comprising a third pixel, wherein the third pixel includes a fourth filter in the filter array, the fourth filter allowing blue light, green light, and red light to pass therethrough, and wherein the third pixel generates an image charge in response to incident blue light, green light, and red light. 15 . The imaging system of claim 13 , wherein the first doped region is electrically isolated from the second doped region and the third doped region by a pinning well. 16. The imaging system of claim 13, further comprising a floating diffusion, wherein the first pixel and the second pixel are electrically coupled to the floating diffusion. 17 . The imaging system of claim 16 , further comprising a first transfer transistor electrically coupled between the first doped region and the floating diffusion, and a second transfer transistor electrically coupled between the third doped region and the floating diffusion. 18. The imaging system of claim 13, wherein the pixel array comprises rows and columns of pixels. 19. The imaging system of claim 13, wherein the image sensor circuit comprises: readout circuitry coupled to the pixel array, wherein the readout circuitry is coupled to read out image charge from the pixel array; function logic coupled to the readout circuitry to manipulate the image charge read out from the pixel array; and A control circuit controls the operation of the pixel array.