High full-well capacity pixel with graded photodetector implant

Embodiments of a process for forming a photodetector region in a CMOS pixel by dopant implantation, the process comprising masking a photodetector area of a surface of a substrate for formation of the photodetector region, positioning the substrate at a plurality of twist angles, and at each of the plurality of twist angles, directing dopants at the photodetector area at a selected tilt angle. Embodiments of a CMOS pixel comprising a photodetector region formed in a substrate, the photodetector region comprising overlapping first and second dopant implants, wherein the overlap region has a different dopant concentration than the non-overlapping parts of the first and second implants, a floating diffusion formed in the substrate, and a transfer gate formed on the substrate between the photodetector and the transfer gate. Other embodiments are disclosed and claimed.

TECHNICAL FIELD

The present invention relates to semiconductor pixels and in particular, but not exclusively, to a pixel with a graded photodetector implant having a high full-well capacity.

BACKGROUND

As the pixel size of complementary metal oxide semiconductor (CMOS) image sensors becomes smaller for higher pixel density and lower cost, the active area of the photodetector also becomes smaller. For pinned photodetectors that are commonly used in CMOS image sensors, the smaller photodetector area leads to reduced full-well-capacity, meaning that the maximum number of charges that can be held in the photodetector is reduced. The reduced full-well-capacity in turn results in a pixel with lower dynamic range and lower signal-to-noise ratio. Therefore, methods to increase the full-well-capacity of the pinned photodetector are highly desired.

In the p-n-p pinned photodetector most commonly used for CMOS image sensors, the most straightforward way to increase the pixel's full well capacity is to increase the doping level (i.e., the concentration of dopants) in the n-type layer, for example by increasing the implantation dosage. For small pixel sizes, however, the increased n-type doping can lead to significant increase in dark current and in defective pixels commonly referred to as white pixels. One reason for this is because of the increased electrical field along shallow trench isolation (STI) sidewalls due to the high n-type doping and the shrinking distance between n-type implant and STI edge.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Embodiments of apparatus, system and method for a pixel with a graded implant having a high full-well capacity are described herein. In the following description, numerous specific details are described to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the 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 but are nonetheless encompassed within the scope of the invention.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

FIGS. 1A-1Ctogether illustrate an embodiment of a complementary metal oxide semiconductor (CMOS) pixel100, such as those found in a pixel array within an image sensor.FIG. 1Aillustrates a planar view of pixel100.FIG. 1Billustrates a cross-sectional view of a portion of pixel100, taken substantially along section line B-B inFIG. 1A.FIG. 1Cillustrates a cross sectional view of a portion of pixel100taken substantially along section line C-C inFIG. 1A. Illustrated pixel100is an active four-transistor pixel, commonly known as a “4T active pixel,” but in other embodiments pixel100could include more or less transistors. As shown inFIG. 1Bpixel100is formed in an epitaxial (epi) layer104formed on substrate102, and includes a photodetector106, a floating node114, and a transfer gate112that, when switched on, transfers charge accumulated in photodetector106to floating node114. Shallow trench isolations (STIs)116,124and126can be used to physically separate and electrically isolate pixel100from adjacent pixels in a pixel array.FIG. 1Ashows a plan view of pixel100with STI regions shown as regions116,124and126, floating node114and transfer gate112.

As shown inFIG. 1Bphotodetector106is formed in epi layer104and includes a pinning region110and an implant108abutting and at least partially surrounding pinning region110. In the illustrated embodiment, pinning region110is positioned at or near the surface of epi layer104, but in other embodiments the pinning region can be positioned elsewhere or can be omitted entirely. In the illustrated embodiment pinning region110is a P-type region, implant108forms an N-type region and epi layer104is a P-type region, making photodetector106a p-n-p photodetector. In other embodiments of photodetector106the charge types (e.g, positive or negative charge carriers) of these elements can be reversed—that is, in other embodiments pinning region110can be N-type, implant region108can be P-type and epi layer104can be N-type, forming an n-p-n photodetector. In still other embodiments epi layer104can be undoped, whatever the charge types of the pinning region and the implant.

As shown inFIGS. 1A and 1C, in the illustrated embodiment of photodetector106implant region108includes three different component regions: an overlap region108cand a pair of non-overlap regions108aand108b. Overlap region108c, so called because it results from the overlap of two or more implanted regions, has a relatively higher dopant concentration than non-overlap regions108aand108b. As a result implant region108can have a larger lateral extent (i.e., it occupies more of the space between STIs124and126or, put differently, has a smaller distance d1and d3) and is graded (i.e., it has a spatial dopant concentration gradient). Combining larger lateral extent with grading results in an implant with high full-well capacity but less of the problems associated with high electric fields at the lateral edges of the implant. As explained below, implant region108has three different component regions because it is made by overlapping two implant regions.

In a p-n-p embodiment of pixel100, during an integration period (also referred to as an exposure period or accumulation period) photodetector106receives incident light, as shown by the arrow inFIG. 1B, and generates charge at the interface between pinning region110and implant region108. After the charge is generated it is held as free electrons in implant region108. At the end of the integration period, the electrons held in N-type implant region108(i.e., the signal) are transferred into floating node114by applying a voltage pulse to turn on transfer gate112. When the signal has been transferred to floating node114, transfer gate112is turned off again for the start of another integration period of photodetector106. After the signal has been transferred from N-type implant region108to floating node114, the signal held in floating node114is used to modulate amplification transistor124, which is also known as a source-follower transistor. Finally, address transistor122is used to address the pixel and to selectively read out the signal onto the signal line. After readout through the signal line, a reset transistor120resets floating node114to a reference voltage, which in one embodiment is Vdd.

FIG. 1Dillustrates a cross-section of an alternative embodiment of a pixel150. Pixel150is in most respects similar to pixel100, the primary difference being the structure of the implanted region. In pixel150, the photodetector also includes a graded implant region152with three component regions: an overlap region152cwith a relatively higher dopant concentration and two non-overlap regions152aand152bwith relatively lower dopant concentrations. Unlike implant region108, in implant region152non-overlap regions152aand152binclude notches154and156on the sides of the implanted region. Notches154and156can result from the tilt angle at which dopants are implanted into epi layer104on substrate102.

FIG. 2illustrates a spherical coordinate system and its superposition onto a Cartesian coordinate system. A spherical coordinate system is defined by (i) a reference plane containing an origin and an azimuth reference direction, and (ii) a zenith, which is a line that passes through the origin and is normal to the reference plane. InFIG. 2, the origin O is formed at the intersection of the x, y and z axes, Cartesian x-y plane forms the reference plane, the x axis forms the azimuth reference direction, and the z axis forms the zenith. The spherical coordinates of a point P are given by its radius R; its inclination angle θ, which is the angle between the zenith and the line segment OP; and its azimuth ψ, which is the angle measured from the azimuth reference direction to the orthogonal projection of the line segment OP on the reference plane (the x-y plane in this case). InFIG. 2, angle α is the angle relative to the zenith (the z axis in this case) of the orthogonal projection of OP onto the x-z plane, and angle β is the angle relative to the zenith (the z axis in this case) of the orthogonal projection of OP onto the y-z plane.

FIGS. 3A-3Btogether illustrate an initial part of an embodiment of a process for forming the pixel embodiments shown inFIGS. 1A-1D;FIG. 3Bis a plan view, whileFIG. 3Aillustrates a cross-sectional view taken substantially along section line3A-3A inFIG. 3B. In the illustrated embodiment, shown inFIG. 3A, elements of pixel100other than photodetector106(i.e., STIs116,124and126, floating diffusion114, transfer gate112, and so forth) are first formed in epi layer104on substrate102, after which a mask layer302is applied to the front side of epi layer104. Mask layer302is designed to prevent dopants from impinging on and penetrating epi layer104during dopant implantation. In one embodiment, mask layer302can be made of conventional photoresist, but in other embodiments it can be of a different substance. Mask layer302can be applied by various known methods.

After mask layer302is in place, it is patterned and etched by known methods (such as photolithography and wet or dry chemical etching) to create an opening304of width W in the mask layer, exposing the front side of a region of epi layer104in which photodetector106will be formed. With opening304in mask layer302, dopants can be implanted in the desired region without also implanting them in other regions where other components are or will be formed. The illustrated embodiment only shows that part of mask layer302that surrounds opening304and shows other elements of pixel100exposed. While such an arrangement can be used in one embodiment, in other embodiments the mask layer can cover all or some of the other illustrated pixel elements, such as floating node114, gate112, etc., during dopant implantation.

Substrate102is positioned such that is can be twisted about an axis306that is substantially normal to the front side of substrate102. In terms of the spherical coordinate system shown inFIG. 2, substrate102and the elements formed on it are positioned substantially in the reference plane (the x-y plane inFIG. 2), and axis306corresponds to the zenith (the z axis inFIG. 2). In the illustrated embodiment, axis306coincides approximately with the center of opening304, but in other embodiments axis306can be offset from opening304. For example, in a production environment there are usually many image sensors, each with a pixel array having a large number of pixels, formed on a semiconductor wafer. In such a production environment, axis306can coincide with the center of the semiconductor wafer, which may or may not coincide with the center of any pixel or pixel array on the wafer.

As shown inFIG. 3B, substrate102can be twisted about axis306to any arbitrary twist angle ψ relative to a reference direction. In the illustrated embodiment, twist angle ψ is defined as the angle between a fixed reference line310and a line308that rotates with the substrate and substantially bisects opening304and floating diffusion114. With reference again toFIG. 2, reference line310is analogous to the azimuth reference (the x axis inFIG. 2), and twist angle ψ r is analogous to the azimuth angle. In other embodiments twist angle ψ can be defined differently, so long as it can be used to characterize a rotation of substrate102about an axis such as axis306.

FIGS. 4A-4Cillustrate another part of an embodiment of a process for forming the photodetector implant region in the pixel embodiments shown inFIGS. 1A-1D.FIG. 4Ais a plan view, whileFIGS. 4B and 4Cillustrate cross-sectional views taken substantially along section lines4B-4B and4C-4C inFIG. 4A. With ψ as shown inFIG. 3B, the initial twist position corresponds to ψ=a°.

As shown inFIG. 4B, after positioning the substrate at the initial twist position, dopants410are directed at the exposed surface of epi layer104. In addition to being directed at the substrate at a selected dosage and energy, dopants410are directed at the substrate at a non-zero tilt angle θ relative to a line substantially normal to the front side of epi layer104, in this case axis306. Again referring toFIG. 2, with substrate102and the elements formed on it positioned substantially in the reference plane (the x-y plane inFIG. 2), tilt angle θ corresponds to inclination angle θ shown inFIG. 2. Because of the presence of mask layer302, dopants410are only able to reach the exposed front side of epi layer104in opening304. As further discussed below, tilt angle θ is selected based on various factors including the lateral extent desired for implant region406under mask302.

As dopants410bombard the exposed part of the front side of epi layer104, they penetrate into the interior of epi layer104and form a first implant region406within epi layer104. As a result of the non-zero tilt angle θ, a portion408of first implant406ends up being formed in the part of epi layer104that is underneath mask layer302(see alsoFIG. 4C). Another portion409of first implant region406is formed due to twist angle ψ=a° in the part of epi layer104that is underneath transfer gate112. Use of tilt angle θ thus increases the lateral extent of the implant and reduces the distance between implant region406and STI124(i.e., distance d1inFIG. 1C). Generally, the larger the tilt angle θ, the more first implant region406will extend laterally, making portion409larger and also making region408under mask layer302larger and distance d1between the edge of implant region406and STI124smaller.

FIG. 4Cillustrates how portion408of implant406extends under mask layer302. Since the plane along which section4C-4C is at an angle relative to the plane along which section4B-4B is taken, the angle α shown inFIG. 4Cis not tilt angle θ, but instead is the orthogonal projection of tilt angle θ onto the plane of section4C-4C.

FIGS. 5A-5Ctogether illustrate another part of an embodiment of a process for forming the photodetector implant region in the pixel embodiments shown inFIGS. 1A-1D.FIG. 5Ais a plan view, whileFIGS. 5B and 5Cillustrate cross-sectional views taken substantially along section line5B-5B and5C-5C inFIG. 5B. Starting with the state shown inFIGS. 4A-4B, substrate102is rotated about axis306, as illustrated by arrow402, to an additional twist position different than the initial twist position. With ψ defined as shown inFIG. 3B, the additional twist position illustrated inFIG. 5Bcorresponds to ψ=−a°, which could be interpreted as a reflection of line308about fixed line310. Although only one additional twist position is illustrated, in other embodiments there can be more than one additional twist position after the initial twist position, depending on the desired final structure of photodetector implant region108.

After positioning substrate102at the additional twist position, dopants410are again directed at the front surface of epi layer104. In addition to being directed at the substrate with a selected dosage and energy, dopants410are again directed at the substrate at a non-zero tilt angle θ relative to a line substantially normal to the front side of epi layer104, in this case axis306. As before, tilt angle θ is selected based on various factors including the desired lateral extent of the implant region. In one embodiment, the tilt angle θ used at the additional twist position can be the same as the tilt angle used at the initial twist position, but in other embodiments the tilt angle used at an additional twist position need not be the same as the tilt angle used at the initial twist position or the tilt angle used at any other additional twist position.

Because of the presence of mask layer302, dopants410are only able to reach the exposed front side of epi layer104in opening304. As dopants410bombard the exposed part of the front side of epi layer104, they penetrate the surface and form a second implant502within the epi layer. Second implant region502overlaps in part with first implant region406to form an overlap region503with relatively higher dopant concentration. The combination of first implant region406with second implant region502thus creates photodetector implant region108ofFIGS. 1A to 1D, in which overlap region108ccorresponds to overlap region503, non-overlapping region108bcorresponds to the region of first implant406that does not overlap with second implant region502, and non-overlapping region108aofFIGS. 1A and 1Ccorresponds to the region of second implant502that does not overlap with first implant region406. As a result of the overlap of first implant region406and second implant region502, overlap region108chas a relatively higher dopant concentration than non-overlapping regions108aand108b, making implant108a graded implant.

As with first implant region406, the non-zero tilt angle θ at which dopants are implanted results in a portion504of second implant region502being formed in the part of epi layer104that is underneath mask layer302. Another portion505of second implant region502is formed due to twist angle ψ=−°, in the part of epi layer104that is underneath transfer gate112. Use of tilt angle θ thus increases the lateral extent of second implant region502and reduces the distance between implant region502and STI126(i.e., distance d3inFIG. 1C). Generally, the larger the tilt angle θ, the more second implant region502will extend laterally, making region504under mask layer302larger and the distance d3between the edge of implant region502and STI126smaller. As seen inFIG. 5A, the overlap of portion409of first implant region406and portion505of second implant region502forms overlap portion506under transfer gate112.

FIG. 5Cillustrates a cross-section of implant406taken along line5C-5C inFIG. 5A, and shows how portions408and504extend under mask layer302. Since the plane along which section5C-5C is at an angle relative to the plane along which section5B-5B is taken, the angle α shown inFIG. 5Cis not tilt angle θ, but instead is the orthogonal projection of tilt angle θ onto the plane of section5C-5C.

FIGS. 6A-6Btogether illustrate another part of an embodiment of a process for forming the photodetector implant region in the pixel embodiments shown inFIGS. 1A-1D. InFIG. 6A, photodetector implant region108has been formed in epi layer104and mask layer302remains in place on the front side of epi layer104. InFIG. 6B, mask layer302is removed from the front side of substrate102, leaving graded implant region108with one lateral side at a distance d1from STI124and the opposite lateral side at a distance d3from STI126. Because implant region108is graded, distances d1and d3can be smaller than they would otherwise need to be to avoid creating problems associated with high electric fields at the edges of the implant, such as dark current and white pixels.

In other embodiments, mask layer302can be left in place while other elements of pixel100, such as pinning layer110, are formed; mask layer302can then be removed later to leave the final pixel100. Alternatively, if any more pixel elements remain to be formed, mask layer302can be removed and additional elements added to pixel100with or without the use of additional mask layers.

FIG. 7illustrates an embodiment of an imaging system700. Optics701, which can include refractive, diffractive or reflective optics or combinations of these, are coupled to image sensor702to focus an image onto the pixels in pixel array704of the image sensor. Pixel array704captures the image and the remainder of imaging system700processes the pixel data from the image.

Image sensor702comprises a pixel array704and a signal reading and processing circuit710. In one embodiment, image sensor702is a backside-illuminated image sensor including a pixel array704that is two-dimensional and includes a plurality of pixels arranged in rows706and columns708. One or more of the pixels in pixel array704can be one of the pixel embodiments shown inFIGS. 1A-1D. During operation of pixel array704to capture an image, each pixel in pixel array704captures incident light (i.e., photons) during a certain exposure period and converts the collected photons into an electrical charge. The electrical charge generated by each pixel can be read out as an analog signal, and a characteristic of the analog signal such as its charge, voltage or current will be representative of the intensity of light that was incident on the pixel during the exposure period.

Illustrated pixel array704is regularly shaped, but in other embodiments the array can have a regular or irregular arrangement different than shown and can include more or less pixels, rows, and columns than shown. Moreover, in different embodiments pixel array704can be a color image sensor including red, green, and blue pixels designed to capture images in the visible portion of the spectrum, or can be a black-and-white image sensor and/or an image sensor designed to capture images in the invisible portion of the spectrum, such as infra-red or ultraviolet.

Image sensor702includes signal reading and processing circuit710. Among other things, circuit710can include circuitry and logic that methodically reads analog signals from each pixel, filters these signals, corrects for defective pixels, and so forth. In an embodiment where circuit710performs only some reading and processing functions, the remainder of the functions can be performed by one or more other components such as signal conditioner712or DSP716. Although shown in the drawing as an element separate from pixel array704, in some embodiments reading and processing circuit710can be integrated with pixel array704on the same substrate or can comprise circuitry and logic embedded within the pixel array. In other embodiments, however, reading and processing circuit710can be an element external to pixel array704as shown in the drawing. In still other embodiments, reading and processing circuit710can be an element not only external to pixel array704, but also external to image sensor702.

Signal conditioner712is coupled to image sensor702to receive and condition analog signals from pixel array704and reading and processing circuit710. In different embodiments, signal conditioner712can include various components for conditioning analog signals. Examples of components that can be found in the signal conditioner include filters, amplifiers, offset circuits, automatic gain control, etc. In an embodiment where signal conditioner712includes only some of these elements and performs only some conditioning functions, the remaining functions can be performed by one or more other components such as circuit710or DSP716. Analog-to-digital converter (ADC)714is coupled to signal conditioner712to receive conditioned analog signals corresponding to each pixel in pixel array704from signal conditioner712and convert these analog signals into digital values.

Digital signal processor (DSP)716is coupled to analog-to-digital converter714to receive digitized pixel data from ADC714and process the digital data to produce a final digital image. DSP716can include a processor and an internal memory in which it can store and retrieve data. After the image is processed by DSP716, it can be output to one or both of a storage unit718such as a flash memory or an optical or magnetic storage unit and a display unit720such as an LCD screen.

The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. These modifications can be made to the invention in light of the above detailed description.