WO2019040950A1

WO2019040950A1 - Single-mask lateral doping profiles

Info

Publication number: WO2019040950A1
Application number: PCT/US2018/048207
Authority: WO
Inventors: Donald B. HONDONGWA; Eric R. Fossum
Original assignee: Trustees Of Dartmouth College; Varex Imaging
Priority date: 2017-08-25
Filing date: 2018-08-27
Publication date: 2019-02-28

Abstract

Included in the present disclosure are methods for forming a predetermined non-uniform lateral doping profile within a region of a semiconductor substrate, as well as devices incorporating non-uniform lateral doping profiles. The method may include (i) providing an implant mask on the semiconductor substrate, the implant mask including a portion that is disposed over the region and that includes a plurality of openings within masked portions configured to prevent ions incident thereon during ion implantation from impinging on the underlying semiconductor substrate, and (ii) performing each of at least one tilted ion implantation step with the implant mask, wherein the geometry of the openings and the masked portions of the implant mask is configured in combination with implant conditions for each tilted ion implantation step, including selection of a variable non-zero tilt angle to provide window modulation of the ion implantation dose.

Description

SINGLE-MASK LATERAL DOPING PROFILES

RELATED APPLICATIONS

[0001] This application claims the benefit of US Provisional Application

No. 62/550,446, filed August 25, 2017, which is hereby incorporated herein by reference in its entirety for purposes of each PCT member state and region in which such incorporation by reference is permitted or otherwise not prohibited.

BACKGROUND

[0002] Tailored non-uniform lateral doping profiles in semiconductor substrates have myriad applications and benefits for bulk and thin film semiconductor device design, such as for enabling desired functionality, optimizing device parameter tradeoffs, and/or providing enhanced performance. For instance, non-uniform lateral doping profiles may be designed to increase and/or optimize junction breakdown voltage in diodes or transistors (e.g., lateral double-diffused metal-oxide-semiconductor (LDMOS) transistors), and provide built-in drift field assisted lateral charge transport such as in transistor channels or large photodiodes.

[0003] More specifically, for example, large photodiodes are typically implemented in complementary metal-oxide-semiconductor (CMOS) image sensors for scientific and medical imaging applications in which there may be limited signal, so large diodes are required to improve the signal to noise performance. One of the design challenges in large photodiodes is achieving fast and complete charge transfer from the diode during readout, which is essential for providing high-speed, low noise imaging. As the photodiode area increases, the charge transfer time during readout increases primarily because of the increased distance that the charge must travel. The addition of an electric field to the photodiode will improve the speed of charge transfer. [0004] Several approaches have been proposed for creating drift-field inducing doping profiles to improve the charge transfer speed in large photodiodes. One such approach creates staircase doping profiles by using multiple masking steps, wherein a respective additional mask step is required to define a respective masking material layer for providing each additional doping level in the staircase. Grading of doping implants has also been achieved by using multiple mask steps to vary the thickness of a masking material. The number of masking steps required for these approaches (one for each doping level) makes them not only costly but also may render them impractical, particularly for providing a doping profile having many doping levels (e.g., to provide a smoother dopant (and hence potential) profile and/or provide a non-uniform dopant profile over a relatively lengthy extent).

[0005] Another approach is providing arbitrary doping profiles with a single implant step using a mask defined in a single masking step, which can be used in photodiodes. The method solely relies on, and is thus limited by, the geometric properties of the mask to define the profile.

SUMMARY OF SOME EMBODIMENTS

[0006] Some embodiments of the present disclosure relate to providing a non-uniform lateral doping profile in a region of a semiconductor based on a tilted dopant implant using a single implant mask having spatially varying mask opening sizes and/or inter-opening masked portion sizes over the region. Use of the method includes, but is not limited to, implementing the doping profile in large pinned photodiodes to induce an electric field to aid the charge transfer process.

[0007] It will be appreciated by those skilled in the art that the foregoing brief description and the following description with respect to the drawings are illustrative and explanatory of some embodiments of the present invention, and are neither representative nor inclusive of all subject matter and embodiments within the scope of the present invention, nor intended to be restrictive or characterizing of the present invention or limiting of the advantages which can be achieved by embodiments of the present invention, nor intended to require that the present invention necessarily provide one or more of the advantages described herein with respect to some embodiments. Thus, the accompanying drawings, referred to herein and constituting a part hereof, illustrate some embodiments of the invention, and, together with the detailed description, serve to explain principles of some embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Aspects, features, and advantages of some embodiments of the invention, both as to structure and operation, will be understood and will become more readily apparent in view of the following description of non-limiting and non-exclusive embodiments in conjunction with the accompanying drawings, in which like reference numerals designate the same or similar parts throughout the various figures, and wherein: [0009] FIG. 1 is an illustrative cross-sectional view of shadowing or windowing effects associated with dopant ions being implanted at a nonzero tilt angle through an opening in mask into a substrate, in accordance with some embodiments of the present disclosure;

[0010] FIG. 2 shows doping difference between to openings as a function of changing width of the openings for different tilt angles, in accordance with principles of some embodiments of the present disclosure;

[0011] FIG. 3 is an illustrative oblique perspective view of two-dimensional shadowing or windowing effects associated with dopant ions being implanted at a nonzero tilt angle and an through an opening in mask into a substrate, in accordance with some embodiments of the present disclosure; [0012] FIGS. 4A-E schematically depict an illustrative process for forming a non-uniform lateral doping profile in accordance with some embodiments of the present disclosure;

[0013] FIG. 5 depicts an illustrative implant process in accordance with some embodiments of the present disclosure;

[0014] FIG. 6 depicts an illustrative implant process in accordance with some embodiments of the present disclosure;

[0015] FIG. 7 depicts an illustrative mask that may be used to generate a 2D non-uniform lateral doping profile in accordance with some embodiments of the present disclosure;

[0016] FIG. 8 schematically depicts a cross-sectional view of a mask scheme in accordance with some embodiments of the present disclosure;

[0017] FIG. 9 shows non-uniform lateral doping profiles that were created in the pinned photodiodes in accordance a CMOS image sensor process simulation performed in accordance with some embodiments according to the present disclosure;

[0018] FIG. 10 shows the potential profiles corresponding to the doping profiles in

FIG. 9; and

[0019] FIG. 11 shows and example of a doping profile created using a mask with variations in two dimensions, in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0020] Throughout the description and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. [0021] The phrase "an embodiment" as used herein does not necessarily refer to the same embodiment, though it may. Similarly, the phrase "some embodiments" as used herein at different instances does not necessarily refer to the same "some embodiments," though it may. In addition, the meaning of "a," "an," and "the" include plural references; thus, for example, "an embodiment" is not limited to a single embodiment but refers to one or more embodiments. Similarly, the phrase "one embodiment" does not necessarily refer the same embodiment and is not limited to a single embodiment. As used herein, the term "or" is an inclusive "or" operator, and is equivalent to the term "and/or," unless the context clearly dictates otherwise. The term "based on" is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise.

[0022] Also, as used herein, "n" and "p" designations (e.g., as in "n-type," "p-type,"

"n-well," etc.) are used in ordinary and customary manner to designate donor and acceptor type impurities that promote electron and hole carriers, respectively, as majority carriers. The term "substrate" is to be understood as a semiconductor-based material such as silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a "substrate" in the present disclosure, previous process steps may have been utilized to form regions, layers, or junctions in or on the base semiconductor structure or foundation.

[0023] Similarly, it will be understood that embodiments in accordance with the present disclosure are not limited to being elemental-silicon-based, but may be based on, for example, silicon compounds (e.g., silicon-germanium (SiGe), silicon carbide (SiC)), germanium, gallium arsenide (GaAs), gallium nitride (GaN), as well as other elemental, binary, ternary, quaternary, or other semiconductor materials, which further are not limited to being monocrystalline but may be polycrystalline and/or amorphous. [0024] Further, as used herein with respect to a semiconductor substrate, the term

"lateral" refers to one or more directions in a plane that is parallel (or substantially parallel) to a surface plane of the semiconductor substrate. In addition, it will be understood that simply for ease of reference and clarity of exposition with respect to describing devices formed in a semiconductor substrate or, similarly, with respect to describing the fabrication of such devices, terms such as "upper," "top," "lower," "bottom," "overlying,"

"underlying," "above," "below," "frontside," and "backside," and the like, with reference to a layer, junction, doped region, or other structure refers to a relative spatial position with respect to a cross-sectional perspective of perpendicular to the semiconductor substrate and does not denote a preferred or required orientation. In this regard, the "top" surface of the substrate as used herein typically refers to a surface on, in, or through which devices are primarily formed (e.g., surface on which transistor gate stacks are formed), unless the context dictates otherwise. It is understood, therefore, as noted, that terminology such as "top," "upper," "bottom," "lower," and the like, as used herein is a convention simply for convenience and ease of reference with respect to referring to different layers, and does not otherwise impart any limitation on the overall design and/or orientation of an image sensor or pixel in accordance with the present disclosure.

[0025] In this regard, for ease of reference, as used herein, two layers, regions, or other structures/elements may be referred to as being "adjacent" if they do not include one or more intervening layers, regions (e.g., doped regions), or other structures/elements. In other words, two layers, regions, or other structures/elements referred to spatially (e.g., "on," "above," "overlying," "below," "underlying," "laterally," etc.) with respect to each other may have one or more intervening layers, regions, or other structures/elements; however, use of the term "adjacent" (or, similarly, "directly," such as "directly on," "directly overlying," and the like) denotes that no intervening layers, regions, or other structures/elements are present.

[0026] Also for ease of reference, in accordance with common convention, a "tilt angle" or "tilt" refers to the angle between the substrate surface normal and an ion beam incident on the substrate, while a "twist angle" or "twist" refers to the angle between (i) the plane containing the ion beam and the substrate surface normal and (ii) the plane that is perpendicular to the primary flat of the wafer corresponding to the substrate and that contains the substrate surface normal. For clarity of exposition in describing the orientation of an ion beam relative to a rectilinear mask opening, since a rectilinear mask opening may be oriented arbitrarily relative to the wafer flat, the term "rotation," as used herein to describe the orientation of an ion beam, refers to the angle between (i) the plane containing the ion beam and the wafer normal and (ii) the plane that contains the substrate surface normal and is perpendicular to the direction of a specified side of the rectilinear mask opening. For a one dimensional non-uniform doping profile, the specified side, as used herein, is the side perpendicular to the direction along which the doping profile is nonuniform. For a two dimensional non-uniform lateral doping profile, the specified side may be arbitrary. It will be understood that a "rotation" may be readily transformed to a "twist" if the orientation of the mask relative to the flat is known or specified.

[0027] Briefly, as will be further understood in view of the present disclosure, methods according to some embodiments provide for establishing a desired particular nonuniform lateral doping profile in a semiconductor substrate based on performing one or more tilted dopant implants at selected variable tilt angles with a single mask having mask openings of spatially-varying size over a region in the substrate in which the non-uniform lateral doping profile is to be established. The variable tilt angles of the one or more tilted implants are used in addition to and in combination with the geometry of the spatially -varying openings to generate and control the variations in the lateral doping concentration. Each tilted implant may be performed at a selected variable rotation angle about the substrate surface normal. Using the implant tilt and rotation as additional adjustable parameters each adds an extra degree of freedom for designing and providing the desired non-uniform lateral doping profile, thereby improving the controllability of non-uniform lateral doping profiles. As such, methods for establishing non-uniform lateral doping profile in accordance with some embodiments of the present disclosure may also enable the capability of generating various desired particular non-uniform lateral doping profiles otherwise not achievable in practice if, for example, relying only on the single mask geometry to control the doping profile. Following the non-uniform doping profile implant, a dedicated diffusion step and/or thermal processes of subsequent fabrication steps may be used to smooth the doping profile.

[0028] Use of methods according to some embodiments include, but are not limited to, establishing a desired particular non-uniform doping profile in large pinned photodiodes (e.g., in CMOS image sensors) to induce an electric field to aid the charge transfer process. In various embodiments, such as for CMOS photodiodes, monotonic, approximately linear doping profiles that will create approximately linear potential gradients (i.e., approximately linear drift fields) to improve the charge transfer characteristics is often desired. It will be understood, however, that methods according to embodiments in accordance with the present disclosure embrace establishing one or two-dimensional non-uniform lateral doping profiles that may include both positive and negative dopant concentration gradients, and may also include extended portions having an approximately uniform dopant concentration (i.e., approximately zero dopant concentration). For clarity of exposition, it will be understood by those skilled in the art that implementing methods in accordance with embodiments of the present disclosure may also intentionally or unintentionally provide doping variations in the depth dimension. In other words, it will be understood that methods for providing lateral doping profiles in one or two dimensions in accordance with some embodiments of the present disclosure neither requires nor precludes uniform doping profiles in the depth dimension and, similarly, neither requires nor precludes non-uniform doping profiles (intentionally or unintentionally) in the depth dimesion.

[0029] Referring now to FIG. 1, an illustrative cross-sectional view of dopant ions

18 being implanted at a tilt angle Θ through an opening in mask 12 into a substrate 10 (e.g., Si) to provide implanted dopants 16 is depicted to elucidate, among other things, how the implant tilt angle may be exploited as a parameter in addition to, and in combination with, the mask geometry to generate a desired non-uniform doping profile, in accordance with some embodiments of the present disclosure. As will be understood by those skilled in the art, for clarity of exposition, FIG. 1 depicts only a portion of mask 12 and substrate 10, which is shown as including a typically used but optional thin oxide 14 (e.g., a so-called scattering oxide, which may be a thermal oxide) formed thereon.

[0030] Briefly, as discussed, methods according to some embodiments of the present disclosure rely on the implant conditions and mask geometry to create a desired doping profile. The angle and energy of the implants are used for precise placement of the dopants and, as further described below in connection with FIG. 1, the

shadowing/windowing of angled implants may be exploited to generate a desired non-uniform doping profile. If the substrate region over which the non-uniform doping profile is to be provided (e.g., a diode region) is viewed as being divided into a series of small segments (e.g., FIG. 1 depicting one segment), then the doping profile is given by the local average doping in each segment. This means, for example, that to provide a linearly increasing doping profile, the local average concentration would be designed to be linearly increasing as well. [0031] The average number of dopant particles implanted through a mask opening depends on the angle of the implant, an effect that can be described as window modulation or windowing. Due to the angle of the implant, the effective window size is geometrically altered as shown in FIG. 1.

[0032] For an implant at angle Θ from the vertical (the tilt angle) through an opening window w, the effective window size can be shown to be given by Equation 1, where w is the geometric window width, h is the thickness of the mask, and w' is the effective window size.

w' = w - h - |tan(0) | (1) [0033] The local average doping concentration (the local density, "LD") in the segment can be described by Equation 2, assuming that the doping is proportional to the window opening. local density (LD) = C_v— ^w' (2)

s

where w' is as described in Equation 1, s is the segment size, and C_p is a proportionality constant dependent on the implant dose, energy, and thermal diffusion. The local density can thus be controlled by adjusting the ratio of the effective window size (w ') to the segment size (s), which ratio is a function of the implant angle and the mask geometry. It should be noted that both Equation 1 and 2 are first order approximations implicitly assuming that the mask completely blocks the implants.

[0034] Analysis of Equation 1 shows that for some values of Θ the effective window size can be zero, and in this case, none of the implant particles make it to the wafer. An acceptance angle, 0_max, above which the effective window goes to zero may be defined. For a mask of thickness h and a window opening of width w, an aspect ratio r₀ = ^ may be defined, and the acceptance angle is given by Equation 3. θτηαχ = tan o) (3) [0035] Equation 3 can be used to determine the range of usable implant angles for a given set of geometric limits which are typically a function of the chosen fabrication process. It also shows that larger angles contribute more to wider openings than to narrower ones, a fact that can be used to create more complex doping profiles. [0036] From the foregoing analysis, various strategies for implementing arbitrary doping profiles can be developed. For instance, an illustrative implementation is to choose one fixed tilt angle and vary the mask openings to vary the local density as desired. In this case, the segment size s is described by Equation 4. s = w + f (4) [0037] Here /is the feature size and w is the window opening size. The Local Density, LD, is thus described by Equation 5.

w' w - \h^■ tan(0) |

LD = C„ — = Γ ^■ (5) w + f ^{~ p} w + f

[0038] Equation 5 is only valid for Θ < 9_max, otherwise LD goes to zero. The equation shows that the LD, and thus the resulting profile, depends on both the mask geometry and the implant angle. For a fixed opening, LD can be varied by changing the angle or mask thickness (e.g., LD can go to zero as the angle or mask thickness is increased), a feature that is not present in the method proposed in the single mask method discussed in the hereinabove Background, in which method the lower limit to the doping is set by the minimum achievable mask opening.

[0039] Two segments with openings of width w₀ and w₁ may also be considered.

The doping difference between the two can be calculated with equation 6.

Z - LD₀ = C_p — - ) (6) [0040] For 0 < 0_max Equation 6 and Equation 1 result in equation 7 where

AID = LD₁ - LD₀.

ALD w₁ - h^■ |tan(0) | w₀ - h^■ |tan(0) |

C_p w_± + A w₀ + f₀ ^

[0041] For fi =fi =f = Equation 7 simplifies to Equation 8 below.

ALD , , „ „ Wi — w₀ \

—= ( ÷„. η₍₈)ΐ) . _{ι + /)}^ _{+ )} ) _(S) [0042] Equation 8 shows that the doping difference between segments with different opening sizes depends on the angle, feature size, and the mask thickness. Larger angles result in a steeper implant profile (i.e., larger doping gradient) for the same mask geometry. For a fixed segment size, s, (i.e., w_} + fj = wi + fj = s), Equation 7 shows that the doping difference is only dependent on the mask geometry and not the implant angle. FIG. 2 shows how the doping difference changes as the window opening is scaled for various implant angles with f = 0.22 μιτι, wo = 0.22 μιτι, h = 1.0 μιτι.

[0043] The analysis presented hereinabove describes and informs the generation of one-dimensional non-uniform lateral doping profiles based on the combination of the implant tilt angle and the geometry of an implant mask comprising multiple variable-sized openings disposed over the region in which the one-dimensional doping profile is to be generated. This analysis can be extended to describe and inform the generation of two- dimensional (2D) non-uniform lateral doping profiles. The extension to 2D is based on the same principles of using mask geometry and implant angles to create the initial local density profile, followed by diffusion to smooth the implant. In the case of two-dimensional profiles, the shadowing effect of both the tilt from the vertical and the rotation of the implant relative to the mask opening must be considered in implementing the doping profile. [0044] FIG. 3 schematically depicts an illustrative oblique perspective of dopant species 38 being implanted at a tilt angle Θ and rotation angle φ through an opening in mask 32 into an underlying substrate (not shown) to provide implanted dopants 36 in the substrate, to elucidate, among other things, how the implant tilt and rotation angles may be exploited as parameters in addition to, and in combination with, the mask geometry to generate a desired two-dimensional non-uniform doping profile, in accordance with some embodiments of the present disclosure. As will be understood by those skilled in the art, for clarity of exposition, FIG. 3 depicts only a portion of mask 32, and that mask 32 is disposed on a substrate (the upper surface of which is in the X-Y plane), which is not shown for clarity of exposition. Also for clarity, the implant ion beam 18, which is incident over the entire mask (including the masked portions and the mask opening), is schematically represented by a single arrow. In addition, to illustrate the tilt and rotation angles, a vector guideline 38' parallel to the implant ion beam 18 is projected onto the origin of the reference coordinate frame.

[0045] As shown, in this case, both the length and the width of the implanted dopants 36 can be modulated by changing the tilt and the rotation. The effective window dimensions of the implant are given by Equations 9 and 10 where Θ is the tilt from vertical tilt and φ is the azimuthal angle as measured from the x-axis (which is parallel to the edges of the mask openings along the w_x).

wx' ^{= w}x ^~ h ' tan(0)^■ cosOp) (9) wy' = Wy — h^■ tan(#)^■ sin(^?) (10) [0046] The local density as in Equation 2 transforms to an area average given by

wherein S_x and S_y are the segment sizes along the x and y directions, respectively. As is the case with ID analysis, the local density can be controlled by the mask geometry and the implant vertical tilt angle with horizontal rotation as an additional degree of freedom.

[0047] In view of the present disclosure, including the guidance and principles provided by the foregoing first order analytical models of local doping concentration as a function of implant tilt and rotation angles as well as local mask geometry, those skilled in the art may define implant parameters (including the tilt and rotation angles) and mask geometry to generate desired one-dimensional and two-dimensional non-uniform local density profiles so as to provide desired non-uniform lateral doping profiles following diffusion. Technology computer-aided design (TCAD, CAD for semiconductor

manufacturing technology) process and device simulation tools (e.g., Synopsis TCAD) may further be used to develop and optimize implant parameters (including tilt and rotation) and implant mask geometry to provide a particular non-linear lateral doping profile that optimizes device performance (e.g., to reduce the charge transfer transit time in CMOS photodiodes), while also accounting for three-dimensional effects.

[0048] FIGS. 4A-E schematically depict an illustrative process for forming a non-uniform lateral doping profile in accordance with some embodiments of the present disclosure. For ease of reference, one or more coordinate axes are included in some of these figures. More specifically, FIG. 4A schematically depicts a cross-sectional view following patterning of an implant mask 12 on a substrate 10 on which a thin oxide layer 14 was formed (e.g., by thermal oxidation). As may be appreciated, FIG. 4A is similar to that of FIG. 1A but schematically represents, instead of only one segment as in FIG. 1A, the full extent of all segments (n segments) of the variable-sized opening implant mask 12 and underlying substrate 10 over the region in which a non-uniform lateral doping profile is to be established. It will be understood, however, that because in this illustrative embodiment the length of this region (along the x direction) is greater (and possibly much greater) than the sum of the depicted segment lengths, FIG. 4A is depicted in break view, showing only the segments near the end portions of this region. While in accordance with some embodiments the number of segments (n) may be as few as two, providing a smooth doping profile for many implementations may require more than two segments, such as at least ten, or at least twenty, or at least fifty, which may also depend on the length over which a non-uniform lateral doping profile is being designed.

[0049] As will be understood by those skilled in the art, the thin oxide layer 14 (e.g., a so-called scattering or screen / protection oxide), though commonly used in implant processes, is not required according to some embodiments of the present disclosure. The implant mask 12 may be provided on the semiconductor substrate using a single lithography masking step, and may be implemented as patterned photoresist. In some embodiments, however, the implant mask may comprise one or more patterned layers of materials other than photoresist, such as oxide, silicon nitride, and/or polysilicon.

[0050] The illustrative embodiment illustrated in FIG. 4A, by way of non-limiting example, is depicted as using a p-type substrate 10, and using a constant mask feature size f with the window opening size (wi) increasing monotonically in the x-direction; thus, the segment size (s) also increases monotonically with increasing x. Also by way of non-limiting example, the feature width f in this illustrative embodiment may be set at or near the minimum feature size for the technology node, and the minimum window opening size (wi) may also be set near the minimum feature size (e.g., depending on the mask height (h) and the tilt angle) with each successive window opening size increasing by a constant offset, which may be much less than the minimum feature size as determined by the design rules. [0051] It will be understood, however, that in various alternative embodiments, to provide a desired doping profile, f may not be constant and/or the window opening size (wi) may not be configured to change monotonically over all n segments.

[0052] FIG. 4B shows a plan view of mask 12 with oxide layer 14 exposed through the mask openings, corresponding to the cross-sectional view of FIG. 4A. As can be seen in FIG. 4B, the width (wi) of each window opening in implant mask 12 is substantially constant along the y-direction over the region in which the non-uniform doping profile is to be generated.

[0053] FIG. 4C illustrates performing a tilted implantation step to implant dopant ions 18 at a selected variable tilt angle Θ relative to a substrate surface normal (with zero rotation angle about the x-axis) through the variable sized openings in mask 12 into a substrate 10 to provide implanted dopants 16 with the desired local dopant density profile provided by configuring the variable tilt angle in combination with the mask geometry (e.g., f, Wi). By way of non-limiting example for purposes of illustration, h = 1 μιτι, f = 0.25 μιτι, wi = 0.25 μιτι, Wi+i = Wi + 0.20 nm, n = 10, corresponding to a non-linear profile length of about 6.85 μιτι, and the tilt angle Θ may be selected as not more than about 14.0 degrees, and in some such implementations would be selected to be no less than 9 degrees, or not less than 11 degrees. FIG. 4D illustrates the substrate 10 with implanted dopants 16 providing the desired local dopant density profile following completion of ion implantation.

[0054] In various embodiments, one or more additional dopant implants may be performed using the same mask 12, possibly with different implant parameters (e.g., dopant species, tilt angle, rotation, dose, energy, etc.). The resulting non-uniform local density profile will be a superposition of the respective profiles for the respective implants.

[0055] Following implantation, mask 12 and oxide 14 are removed, and through a dedicated post-implant anneal, or through thermal processes of subsequent fabrication steps, or through both, the implanted dopants 16 are subject to diffusion to smooth out the doping profile. FIG. 4E schematically depicts an n-type dopant region 13 resulting from the diffusion of implanted dopants 16, while FIG. 4F schematically depicts an illustrative dopant concentration along the x-axis in n-type dopant region 13 (not to scale, and representing x-axis continuously).

[0056] In this illustrative embodiment, the implanted dopants 16 are n-type (e.g., phosphorus) with monotonically increasing local dopant concentration for increasing x. It will be understood, however, that various embodiments of providing a non-uniform doping profile according to the present disclosure may be implemented with any combination of substrate and implant doping types, and with any non-uniform doping profile.

[0057] For instance, in some embodiments, the non-uniform doping process may implant dopants of the same dopant type as the substrate region in which the non-uniform doping profile is to be provided, so as to non-uniformly increase the doping concentration in that substrate region. By way of non-limiting example, FIG. 5 depicts an implant process corresponding to the implant and subsequent processing described hereinabove in connection with FIGS. 4A-E, except the substrate 10 in FIG. 5 includes an n-type well 15 into which the non-uniform lateral local density profile of n-type dopants 16 are implanted. It will be understood that in various alternative embodiments, the implant for forming n-type well 15 may instead be performed after implantation of n-type dopants 16.

[0058] In some alternative embodiments, the non-uniform doping process may implant dopants having opposite type to, and lower concentration than, the dopants of a substrate region in which the non-uniform doping profile is to be provided, so as to provide the non-uniform doping profile by dopant compensation. For instance, by way of non-limiting example, FIG. 6 depicts an illustrative implant process corresponding to the implant and subsequent processing described hereinabove in connection with FIG. 5, except the implant dopant species 18 in FIG. 6 is p-type, and the mask orientation is reversed such that the variable-sized mask openings decrease in size in the x-direction. Accordingly, the p-type implant results in a lateral local density profile of implanted p-type dopants 17 that monotonically decreases in the x-direction. Thus, following implant dopant diffusion and activation, the p-type implant partially and non-uniformly compensates n-type dopants of n-well 15, thereby resulting in a non-uniform monotonically increasing n-type dopant concentration profile along the x-direction in n-well 15. It will be understood that in various alternative embodiments, the implant for forming n-type well 15 may instead be performed after implantation of p-type dopants 17.

[0059] As described above, 2D non-uniform lateral doping profiles may also be implemented based on the same principles of using mask geometry and implant angles to create the initial local density profile, followed by diffusion to smooth the implant. FIG. 7 depicts an illustrative mask 70 that may be used to generate a 2D non-uniform lateral doping profile. In various embodiments, a tilted dopant implantation (e.g., in accordance with the implant processes and principles described hereinabove in connection with one- dimensional embodiments) and possibly one or more additional dopant implant steps may be performed using mask 70, with different implant steps possibly being performed with different implant parameters (e.g., dopant species, tilt angle, rotation, dose, energy, etc.). For instance, in some embodiments, a single tilted implant may be performed at, for example, a 45 degree rotation, whereas in some embodiments, two tilted implants may be performed at, for example, zero and 90 degree rotation, respectively (e.g., at the same or different tilt angles, energy, dose, etc.).

[0060] As shown, mask 70 is configured such that the opening widths vary in two dimensions, and, in this particular illustrative embodiment W; > Wi i for all i > 1, this illustrative configuration being directed to providing a non-uniform lateral doping profile along the 45 degree (diagonal) direction. Such a configuration may be well-suited, for example, for fabricating the photodiode of a CMOS image sensor pixel that is configured to transfer charge out of the photodiode at or near a comer along the 45 degree diagonal of the photodiode (e.g., a transfer gate and floating diffusion may be disposed at or near the position corresponding to either the upper right corner or lower right corner of mask 70, from the perspective of FIG. 7, the corner depending on whether the implant species through mask 70 into the n-type well of the photodiode is p-type (transfer gate and floating diffusion would be disposed at lower left corner) or n-type (transfer gate and floating diffusion would be disposed at lower left corner).

[0061] In view of the present disclosure, those skilled in the art will understand that the steps of forming on the substrate an implant mask having variable-sized openings and performing a tilted ion implantation through the implant mask to provide a desired local density doping profile that will be subsequently smoothed by thermal diffusion, as for example described above in connection with FIGS. 4A through 4D as well as in connection with FIGS. 5-7, may be performed at any of various points in a particular semiconductor device fabrication process, depending on various considerations, such as thermal budget, etc. Thus, for example, those skilled in the art will understand how to incorporate methods for providing non-uniform doping profiles in accordance with some embodiments of the present disclosure (e.g., such as those described in connection with FIGS. 4A-E, 5, and 6) into a CMOS image sensor process so as to provide a non-uniform lateral doping profile in the pinned photodiode to enhance charge transfer, even though specific CMOS image sensor processing steps and resulting structural features (e.g., photodiode pinning layer implant, transfer gate stack, floating diffusion, etc.) are not explicitly shown in the illustrative embodiments of the non-uniform lateral doping profile processes described herein (e.g., with reference to FIGS. 4A-E, 5, and 6). In this regard, those skilled in the art will understand in view of the present disclosure that methods for providing non-uniform doping profiles in accordance with some embodiments of the present disclosure (e.g., such as those described in connection with FIGS. 4A-E, 5, and 6) possibly may be incorporated in a CMOS image sensor fabrication process, for example, prior to or following shallow trench isolation (STI), or prior to or after diode pinning layer (e.g., p+) implantation, and/or prior to or after forming an n-well in the photodiode region, etc., to provide a non-uniform lateral doping profile in the photodiode to provide drift-field assisted charge transfer.

Illustrative Examples

[0062] The following examples are provided to illustrate some embodiments of the present disclosure as well as various features and advantages that may be associated with some embodiments, and is not intended to limit the present invention.

[0063] Simulations were performed in TCAD to check the implementation of various embodiments using a realistic fabrication process for CMOS imaging devices as the baseline. In these simulations, doping profiles were able to be created that varied monotonically from one end of the diode to another and showed that the shape of the profile was a function of the implant angle.

[0064] Simulations were done for diodes 50 μιτι long and implants at ±12.5, ±15, and ±17.5 degrees, and with no added implant for reference. Simulations of the photodiode without an implant profile were simulated as well for use as a bench mark. The mask for the doping was designed to meet the minimum specifications for a 0.18 μιτι fabrication process. A fixed window spacing, f, of 0.22 μιτι, and a mask thickness (h) of 1 μιτι was used. The window width was varied by steps of 20 nm starting from 220 nm up to 380 nm to create ten doping levels. To cover the longer length diodes, the diode was divided into roughly equal sections and in each section the window opening was kept constant, while the opening increased from section to section resulting in a staircase doping profile. FIG. 8 schematically depicts the mask creation scheme used, showing three sections having different window opening sizes but with the window opening size in each section being constant.

[0065] Similar to the mask and compensation doping processes described hereinabove with respect to FIG. 6, the mask was used to create a non-uniform doping profile of implanted p-type dopants that increased from right to left (the floating diffusion being to the right), and a constant n-type doping was added to create an n-type diode with a net doping that is increasing left to right. We chose to create such an inverse profile in boron instead of creating the profile using the donor dopants because boron atoms are relatively small and thus have a high diffusivity constant, therefore, requiring smaller thermal budgets. This method also has the advantage that it does not affect the carefully crafted doping profile around the gate because the boron doping goes to zero. This provides a lot of freedom in the placement of the additional doping step in the processing lineup allowing it to be added to sections of the fabrication process that have minimal impact on the thermal budget.

[0066] FIG. 9 shows non-uniform lateral doping profiles that were created in the pinned photodiodes in accordance with the hereinabove described CMOS image sensor process simulation performed in accordance with some embodiments according to the present disclosure. Also shown for reference is the profile for the pinned photodiode in which no non-uniform implant was added (identified as "No prof). FIG. 10 shows the potential profiles corresponding to the doping profiles in FIG. 9.

[0067] As disclosed hereinabove, principles and methods disclosed herein regarding one-dimensional lateral doping profiles may be extended to two dimensions. To test the 2D extension of the process, a 3D implant and diffusion simulation was performed in TCAD. The mask used is illustrated in FIG. 7, having rectangular openings whose length varies in the x-direction and whose width was varied in the y-direction, with, in this particular example, Wi > Wi-1 for all i > 1. W-l was set to be larger than WO. The aim was to see if a doping profile in the two dimensions could be generated. The implant was performed at ±17.5° tilt, and 45° rotation measured counter-clockwise from the x-axis. 3D simulations were attempted with 50 μιτι pixels, however the required computational resources overloaded the computer systems, so the device was limited to 10 μιτι. Although rectangular openings are shown, the openings can have any 2D geometry, such as a ellipse or other polygon.

[0068] An example of a doping profile created using a mask with variations in two dimensions is shown in FIG. 11. The doping concentration increases outward from the bottom-left comer, as expected based on the theory presented. The boron concentration is reduced as the area of the mask opening is reduced. This kind of profile can be used to funnel charge to the lower left corner, where the transfer gate can be placed. It will be understood, however, that in the case where the implant species through the mask shown in FIG. 7 is n-type (i.e., matches the n-type well of the photodiode), then the resulting doping profile can be used to funnel charge to the upper right comer, where the transfer gate of the CMOS image sensor pixel can be placed to selectively transfer charge from the photodiode to a floating diffusion.

[0069] In view of the present disclosure, including the illustrative embodiments and examples, it will be understood that various embodiments provide for generating nonuniform doping profiles using a single mask by using the implant angles as additional design parameters that can increase design flexibility and provide for accurately controlling the non-uniform doping profile. [0070] As described hereinabove, non-uniform doping methods according to some embodiments of the present disclosure may be used for providing drift fields in photodiodes to improve the charge transfer speed, which is particular applicable to large pinned photodiodes.

[0071] An additional illustrative application of non-uniform doping methods according to some embodiments of the present disclosure is to provide voltage-dependent capacitors. A potential profile like the one shown in FIG. 10 will result in voltage dependent capacitance. This is another useful feature of this method. The capacitance's voltage dependence can be controlled by the doping profile. In the case of the profile shown in FIG. 10, the capacitance will increase as the voltage reduces.

[0072] Drift fields can also be used in other charge storage or transfer devices. In charge storage devices, like diffusion capacitance, the drift field can be used to direct charge to a particular location in the device. This is essentially what it is doing in the photodiode. In transfer devices, the drift field can be arranged to direct charge along a particular path, to avoid shallow trench isolation (STI) perimeters for example as a means of noise mitigation.

[0073] An additional feature of this method is that one mask design can be used to create many different profiles by using multiple implants at different angles (tilt and/or rotation). The resulting profile, in this case, will be a superposition of the profiles from the individual angles. It will be understood that while the method requires the process to provide for sufficient diffusion to adequately smooth the implanted dopant to form the required doping profile (thus possibly requiring an additional annealing step), an additional annealing step may not be required (or may be limited in terms of thermal energy required) if, for example, can be mitigated by using appropriate positioning of the non-uniform doping implant step in a fabrication process to take advantage of any pre-existing diffusion steps. [0074] Accordingly, it will be understood in view of the present disclosure that in some embodiments, a method for forming a predetermined non-uniform lateral doping profile within a region of a semiconductor substrate comprises (i) providing an implant mask on the semiconductor substrate, the implant mask including a portion that is disposed over the region and that includes variable sized openings within masked portions configured to prevent ions incident thereon during ion implantation from impinging on the underlying semiconductor substrate, and (ii) performing each of at least one tilted ion implantation step with the implant mask, wherein the geometry of the variable sized openings and the masked portions of the implant mask is configured in combination with implant conditions for each tilted ion implantation step, including selection of a variable non-zero tilt angle to provide window modulation of the ion implantation dose, to provide the predetermined non-uniform lateral doping profile.

[0075] Implant conditions may also include implant energy, dose, and twist or rotation angle. In some embodiments, the variable sized openings are rectilinear having sides oriented substantially parallel and perpendicular to a plane that contains the substrate surface normal and the tilted ion implantation beam.

[0076] A dedicated post-implant anneal may be performed following the tilted implantation to cause diffusion of the implanted dopant species, though in some embodiments thermal processes of subsequent fabrication steps may be sufficient so that such a dedicated post-implant anneal is not required.

[0077] The implant mask may be provided on the semiconductor substrate using a single lithography masking step, and may be implemented as patterned photoresist. In some embodiments, the implant mask may comprise one or more patterned layers of materials other than photoresist, such as oxide, silicon nitride, and/or poly silicon. [0078] In some embodiments, the predetermined non-uniform lateral doping profile is provided along a first direction of the region, wherein a lateral doping profile over the region along a lateral direction perpendicular to the first direction at every position in the region is substantially uniform, the lateral doping profile thereby being one-dimensional.

[0079] In some embodiments, the predetermined non-uniform lateral doping profile is provided such that for each arbitrary first lateral direction over the region along which the doping profile is non-uniform, there is a doping profile over the region along a direction perpendicular to the first lateral direction that is non-uniform, the lateral doping profile thereby being two-dimensional.

[0080] In some embodiments, the implant mask is configured as contiguous segments extending along a first lateral direction, wherein each segment includes a respective one of the variable sized openings within a portion of the masked portion, wherein the variable sized opening for each segment is rectangular with a respective width dimension along the first lateral direction that is determined based on the predetermined non-uniform doping profile and on the implant parameters, wherein the respective width dimension is constant over the region along a direction perpendicular to the first lateral direction, the implant mask thereby being configured to provide the predetermined non-uniform doping profile as a one-dimensional non-uniform lateral doping profile.

[0081] In some embodiments, the variable sized openings are rectangular and configured as a two-dimensional array of rows and columns, wherein each variable sized opening has a width along the row direction and a length along the column direction. In some embodiments, variable sized openings in the same column have equal widths and variable lengths, and variable sized openings in the same row have equal lengths and variable widths. [0082] The non-uniform lateral doping profile may be configured to change monotonically along one dimension and, optionally, may be substantially linear.

[0083] The present invention has been illustrated and described with respect to some specific illustrative embodiments thereof, which embodiments are merely illustrative of some of the principles of some embodiments of the invention and are not intended to be exclusive or otherwise limiting embodiments. Accordingly, although the above description of illustrative embodiments of the present invention, as well as various illustrative modifications and features thereof, provides many specificities, these enabling details should not be construed as limiting the scope of the invention, and it will be readily understood by those persons skilled in the art that the present invention is susceptible to many modifications, adaptations, variations, omissions, additions, and equivalent implementations without departing from this scope and without diminishing its attendant advantages. For instance, except to the extent necessary or inherent in the processes themselves, no particular order to steps or stages of methods or processes described in this disclosure, including the figures, is implied. In many cases the order of process steps may be varied, and various illustrative steps may be combined, altered, or omitted, without changing the purpose, effect or import of the methods described. Similarly, the structure and/or function of a component may be combined into a single component or divided among two or more components. It is further noted that the terms and expressions have been used as terms of description and not terms of limitation. There is no intention to use the terms or expressions to exclude any equivalents of features shown and described or portions thereof. Additionally, the present invention may be practiced without necessarily providing one or more of the advantages described herein or otherwise understood in view of the disclosure and/or that may be realized in some embodiments thereof. It is therefore intended that the present invention is not limited to the disclosed embodiments but should be defined in accordance with claims that are based on the present disclosure, as such claims may be presented herein and/or in any patent applications claiming priority to, based on, and/or corresponding to the present disclosure.

Claims

What is claimed is:

1. A method for forming a predetermined non-uniform lateral doping profile within a region of a semiconductor substrate, the method comprising:

providing an implant mask on the semiconductor substrate, the implant mask including a portion that is disposed over the region and that includes a plurality of openings within masked portions configured to prevent ions incident thereon during ion implantation from impinging on the underlying semiconductor substrate, and

performing each of at least one tilted ion implantation step with the implant mask, wherein the geometry of the openings and the masked portions of the implant mask is configured in combination with implant conditions for each tilted ion implantation step, including selection of a variable non-zero tilt angle to provide window modulation of the ion implantation dose.

2. The method according to claim 1, wherein the plurality of openings have at least two different areas.

3. The method according to claim 1, wherein dopants implanted by the at least one tilted ion implantation step uses at least two different non-zero tilt angles.

4. The method according to claim 1, further comprising annealing the substrate in a process step dedicated to causing diffusion of dopants implanted by the at least one tilted ion implantation step.

5. The method according to claim 1, wherein configuration of the implant conditions to provide the predetermined non-uniform lateral doping profile includes selection of a variable rotation angle.

6. The method according to claim 1, wherein the lateral doping profile is one- dimensional.

7. The method according to claim 1, wherein the lateral doping profile is two- dimensional.

8. The method according to claim 1, wherein the implant mask is configured as contiguous segments extending along a first lateral direction, wherein each segment includes a respective one of the variable sized openings within a portion of the masked portion, wherein the variable sized opening for each segment is rectangular with a respective width dimension along the first lateral direction that is determined based on the predetermined non-uniform doping profile and on the implant parameters, wherein the respective width dimension is constant over the region along a direction perpendicular to the first lateral direction, the implant mask thereby being configured to provide the

predetermined non-uniform doping profile as a one-dimensional non-uniform lateral doping profile.

9. The method according to claim 1, wherein the variable sized openings are rectangular and configured as a two-dimensional array of rows and columns, wherein each variable sized opening has a width along the row direction and a length along the column direction.

10. The method according to claim 6, wherein the variable sized openings in the same column have equal widths and variable lengths, and variable sized openings in the same row have equal lengths and variable widths.

11. The method according to claim 1, wherein the non-uniform lateral doping profile is configured to change monotonically along one axis over the region.

12. The method according to claim 1, wherein a plurality of tilted ion implantations are performed with the implant mask, each of the tilted ion implantations being performed at a different at least one of tilt angle and rotation angle.

13. The method according to claim 1, wherein the region is a region in which a charge storage device is fabricated.

14. The method according to claim 10, wherein the charge storage device is a photodiode of a an image sensor.

15. A diode formed in a semiconductor substrate having a surface, the diode comprising a lateral doping gradient that is non-uniform along two-dimensions in a plane substantially parallel to said surface.

16. The diode according to claim 15, wherein the lateral doping gradient is formed using a single mask.

17. The diode according to claim 15, wherein the lateral doping gradient is configured to provide a drift field that selectively directs charge stored in the diode laterally along non-parallel directions towards an edge portion of the diode.

18. An image sensor pixel comprising:

a photosensitive element formed in a semiconductor substrate;

a floating diffusion region formed in the semiconductor substrate;

a transfer gate configured to selectively cause photocharge stored in the

photosensitive element to be transferred to the floating diffusion; and

wherein the photosensitive element includes a lateral doping gradient that is configured to provide a drift field that selectively directs charge stored in the diode laterally along non-parallel directions towards the floating diffusion.

19. The image sensor pixel according to claim 18, wherein the lateral doping gradient is formed using a single mask.

20. The image sensor pixel according to claim 18, wherein the photosensitive element is substantially rectangular, and the transfer gate is disposed at a position intersecting two edges of the rectangular photosensitive element.