Curved image sensor

An image sensor includes a plurality of photodiodes arranged in an array and disposed in a semiconductor material with pinning wells disposed between individual photodiodes in the plurality of photodiodes. The image sensor also includes a microlens layer. The microlens layer is disposed proximate to the semiconductor material and is optically aligned with the plurality of photodiodes. A spacer layer disposed between the semiconductor material and the microlens layer. The spacer layer has a concave cross-sectional profile across the array, and the microlens layer is conformal with the concave cross-sectional profile of the spacer layer.

TECHNICAL FIELD

This disclosure relates generally to image sensor fabrication, and in particular but not exclusively, relates to curved image sensors.

BACKGROUND INFORMATION

Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors has continued to advance at a great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these devices. While advances in pixel design have dramatically improved image sensor performance, several optical limitations have proved difficult to overcome by optimizing pixel circuitry alone.

Chemical mechanical polishing (CMP) is integral to semiconductor device fabrication. It can be used to thin wafers, remove excess deposition flux, and planarize surfaces. Further, the polish rate is selective to the composition of the films on the surface. Since CMP may be less precise than other fabrication techniques and is generally rougher on the semiconductor wafers, CMP may induce defects into electronic devices if not properly controlled. Defects may include pitting of the wafer, scratches on the wafer surface, and the destruction of layers of device architecture. Accordingly, optimizing the CMP process is desirable in electrical device fabrication.

DETAILED DESCRIPTION

FIG. 1Ashows one example of a cross section of partially completed curved image sensor100. Partially completed curved image sensor100includes: semiconductor material101, plurality of photodiodes103, pinning wells105, spacer layer121, and dye-edge structures171. Plurality of photodiodes103are arranged in an array and disposed in semiconductor material101and pinning wells105are disposed between individual photodiodes103. Spacer layer121is disposed between semiconductor material101and dye-edge structures171. In one example, spacer layer121is an oxide such as silicon oxide or the like, and dye-edge structures171are a nitride such as silicon nitride or the like, and are disposed on spacer layer121on opposite ends of the array of photodiodes103as shown. In the depicted example, partially completed curved image sensor100has just undergone a CMP process. Since, in the depicted example, dye-edge structures171are harder (or more resistant to the CMP process) than spacer layer121, spacer layer121was removed faster than dye-edge structures171. As a result, spacer layer121was dished (thinner towards the middle of the array of photodiodes103, and thicker towards the sides of the array of photodiodes103). In one or more examples, the dye-edge structures may be thinner or rounded after CMP.

FIG. 1Bshows an illustration of a top down view of a semiconductor wafer including the partially completed curved image sensor100ofFIG. 1A. It is worth noting that dye-edge structures171are disposed along scribe lines on semiconductor material101. In the depicted example, the surface of each area lacking dye-edge structures171may be dished. This results in the center of spacer layer121being thinner than the areas of spacer layer121closer to dye-edge structures171. Although the depicted example uses chemical mechanical polishing to achieve curving/thinning of the wafer, other suitable processes such as etching may be employed to form the same or similar curved image sensor structures.

FIG. 2shows an illustration of one example of curved image sensor200. In the depicted example, curved image sensor200includes: semiconductor material201, plurality of photodiodes203, pinning wells205, spacer layer221, optical grid layer231(with optical grid233), color filter layer241(with red color filters243, green color filters245, and blue color filters247), microlens layer261, and optical lens281. As shown in the example illustration, plurality of photodiodes203is disposed in semiconductor material201and pinning wells205are disposed between individual photodiodes203in plurality of photodiodes203to electrically isolate individual photodiodes203. In one example, pinning wells205may include doped semiconductor material; however, in another other same example, pinning wells may include a metal/semiconductor oxide, metal/semiconductor nitride, polymer or the like. Color filter layer241is disposed between microlens layer261and semiconductor material201. Color filter layer241and microlens layer261are optically aligned with plurality of photodiodes203to direct incident light into plurality of photodiodes203. Spacer layer221is disposed between semiconductor material201and color filter layer241, and spacer layer221has a concave cross-sectional profile across the array of photodiodes203. In the depicted example, color filter layer241and microlens layer261are conformal with the concave cross-sectional profile of spacer layer221. This may help minimize optical defects on the edge of curved image sensor200. Optical grid layer231is disposed between color filter layer241and spacer layer221, and optical grid layer231is optically aligned with plurality of photodiodes203such that optical grid layer231directs light into plurality of photodiodes203via an internal reflection process. In the depicted example, optical grid layer231is conformal with the concave cross-sectional profile of spacer layer221. In one example, optical grid233may include a metal mesh. In another example, optical grid233may include metal, oxide, or semiconductor structure fabricated through processing techniques such as thermal evaporation, chemical vapor deposition, or the like.

In one example, plurality of photodiodes203is arranged into an array including rows and columns (see infraFIG. 4) and the vertex of the concave cross-sectional profile of spacer layer221is located at a center of the array of plurality of photodiodes203. In the depicted example, optical lens281is disposed between a source of image light and semiconductor material201, and optical lens281is positioned to direct image light into semiconductor material201. To optimize device performance, in one example, a radius of curvature of the concave cross-sectional profile of spacer layer221approximates a radius of curvature of optical lens281. In one example, a shutter may be disposed between image light and curved image sensor200to block image light from reaching curved image sensor200between frames or during calibration measurements.

In operation, plurality of photodiodes203will absorb image light to generate image charge. Image light is focused onto semiconductor material201(and corresponding optical structures, e.g., microlens layer261, color filter layer241, and optical grid layer231) via optical lens281. In conventional image sensors, the lens focal plane is curved but the semiconductor device stack is flat, resulting in image sensor edges that are blurred because they are out of focus (nonconforming with the curvature of the lens). Here, curved image sensor200may receive focused image light from optical lens281along the edges of curved image sensor200because the radius of curvature of curved image sensor200now approximates that of optical lens281. Image light received along the edges of curved image sensor200may then be efficiently passed through microlens layer261and color filter layer241into plurality of photodiodes203.

FIGS. 3A-3Fshow an example process for forming a curved image sensor (e.g., curved image sensor200). The order in which some or all ofFIGS. 3A-3Fappear in process300should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process300may be executed in a variety of orders not illustrated, or even in parallel.

FIG. 3Adepicts an illustration of providing semiconductor material301including plurality of photodiodes303arranged in an array. In the depicted example, providing semiconductor material301may include forming a plurality of photodiodes303in a backside of semiconductor material301. However, in a different example, plurality of photodiodes303may be formed in a frontside of semiconductor material301. These two configurations may be used to form either a backside illuminated image sensor or a frontside illuminated image sensor, respectively. Additionally, in the depicted example, pinning wells305are disposed between individual photodiodes303in plurality of photodiodes303. Pinning wells305may include doped semiconductor regions or may include metals, metal oxides, semiconductor oxides, and/or semiconductor nitrides. Pinning wells305are disposed between individual photodiodes303to prevent unwanted charge transfer between individual photodiodes303and associated circuitry (e.g., readout circuitry411, see infraFIG. 4).

FIG. 3Bdepicts an illustration of depositing spacer layer321proximate to a surface of semiconductor material301. In the depicted example, spacer layer321is disposed on semiconductor material301. In one example, spacer layer321includes silicon oxide; however, in a different example, spacer layer includes other oxides, nitrides, polymers, or the like. Although not depicted, in one example, transfer gates may be formed on the surface of semiconductor material301prior to the deposition of spacer layer321and a high-k dielectric material may be disposed between the transfer gates and the surface of semiconductor material301. Individual transfer gates may be electrically coupled to individual photodiodes303to transfer image charge accumulated in in photodiodes303to floating diffusions (not depicted) also disposed in semiconductor material301. In one example, an isolation trench is etched in semiconductor material301, and spacer layer321is deposited in the isolation trench and on the surface of semiconductor material301.

FIG. 3Ddepicts forming dye-edge structures371on a surface of spacer layer321proximate to opposite ends of the array of photodiodes403as shown. In the depicted example, dye-edge structures371are formed by etching the dye-edge layer371. This etching process may include a wet and/or dry etch depending on considerations such as the desired etch rate, presence of other layer of device architecture, etc. In the depicted example, dye-edge structures371encircle the array including the plurality of photodiodes303, and dye-edge structures371are elevated on the surface of spacer layer321. However, in another example, dye-edge structures371may not encircle plurality of photodiodes303, and may only be disposed around some edges of the array of plurality of photodiodes303. The thickness, material composition, and location of dye-edge structures371may be tuned to optimize the architecture/curvature of spacer layer321and the associated CMP process.

In the depicted example, dye-edge structures371are disposed above the edges of scribe lines. While forming dye-edge structures371, semiconductor material301may be still part of a larger wafer including multiple image sensors (see e.g.,FIG. 1B). To form individual image sensors, semiconductor material301is diced into multiple semiconductor dyes along the scribe lines.

FIG. 3Eshows an example illustration of polishing spacer layer321and dye-edge structures371. Polishing results in a concave cross-sectional profile of spacer layer321across the array of photodiodes403. Polishing may include chemical mechanical polishing (CMP) where a wafer with many image sensors is loaded onto a wafer polisher. In the depicted example, because dye-edge structures371include a harder material than spacer layer321, spacer layer321polishes faster than dye-edge structures371, this results in the concave structure of spacer layer321which therefore provides an image sensor with increased edge resolution, in accordance with the teachings of the present invention. In one example, dye-edge structures371include silicon nitride and spacer layer321includes silicon oxide.

FIG. 3Fshows an illustration of an example of removing dye-edge structures371from the surface of spacer layer321and forming the remainder of the curved image sensor optical architecture. In one example, dye-edge structures371are removed by an etching process. In the depicted example, optical grid layer331is disposed between spacer layer321and color filter layer341. Optical grid layer331, may include a metal optical grid333which directs light into plurality of photodiodes303. Color filter layer341and microlens layer361are also formed. Color filter layer341is disposed between microlens layer361and spacer layer321, and color filter layer341, microlens layer361, and optical grid layer331are optically aligned with plurality of photodiodes303. It is worth noting that, in one example, color filter layer341includes red343, green345, and blue347color filters which may be arranged into a Bayer pattern, EXR pattern, X-trans pattern, or the like. It is worth noting that although the location of individual colors filters is specified in the depicted example, the placement, location, and order of color filters can take a number of configurations. For instance, in a different or the same example, color filter layer341may include infrared filters, ultraviolet filters, or other light filters that isolate invisible portions of the EM spectrum. In the same or a different example, microlens layer361may be fabricated from a photo-active polymer that is patterned on the surface of color filter layer341. Once rectangular blocks of polymer are patterned on the surface of color filter layer341, the blocks may be melted (or reflowed) to form the dome-like structure characteristic of microlenses. However, it should be noted that in one example, optical grid layer331, color filter layer341, and microlens layer361may not be present in the curved image sensor, or may be replaced by equivalent or substantially similar components (e.g., color filter layer may be replaced with a metal mesh to block specific wavelengths of light).

It is worth noting that for all semiconductor, oxide, and/or metal formation processes depicted inFIGS. 3A-3F, any suitable processing methods may be used. Thus, for any patterning, photolithography (utilizing negative or positive resists) may be employed to template the structure. Additionally, for material depositions, any suitable deposition technique may be used including: thermal evaporation, chemical vapor deposition, atomic layer deposition, molecular beam epitaxy, self-assembly, or the like.

FIG. 4is a block diagram illustrating one example of an imaging system including the curved image sensor ofFIG. 2(e.g., curved image sensor200). Imaging system400includes pixel array405, control circuitry421, readout circuitry411, and function logic415. In one example, pixel array405is a two-dimensional (2D) array of photodiodes, or image sensor pixels (e.g., pixels P1, P2. . . , Pn). As illustrated, photodiodes are arranged into rows (e.g., rows R1to Ry) and columns (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. However, the rows and columns do not necessarily have to be linear and may take other shapes depending on use case.

In one example, after each image sensor photodiode/pixel in pixel array405has acquired its image data or image charge, the image data is readout by readout circuitry411and then transferred to function logic415. Readout circuitry411may be coupled to readout image data from the plurality of photodiodes in pixel array405. In various examples, readout circuitry411may include amplification circuitry, analog-to-digital (ADC) conversion circuitry, or otherwise. Function logic415may simply store the image data or even alter/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 circuitry411may readout a row of image data at a time along readout column lines (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 circuitry421is coupled to pixel array405to control operation of the plurality of photodiodes in pixel array405. Control circuitry421may be configured to control operation of the pixel array405. For example, control circuitry421may 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 array405to 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. In another example, image acquisition is synchronized with lighting effects such as a flash.

In one example, imaging system400may be included in a digital camera, cell phone, laptop computer, or the like. Additionally, imaging system400may be coupled to other pieces of hardware such as a processor, memory elements, output (USB port, wireless transmitter, HDMI port, etc.), lighting/flash, electrical input (keyboard, touch display, track pad, mouse, microphone, etc.), and/or display. Other pieces of hardware/software may deliver instructions to imaging system400, extract image data from imaging system400, or manipulate image data supplied by imaging system400.