Method of forming an angled pinned photodiode for high quantum efficiency
A pinned photodiode with a surface layer of a first conductivity type laterally displaced from an electrically active area of a gate structure and a charge collection region of a second conductivity type formed by an angled implant is disclosed. The angle of the charge collection region implant may be tailored so that the charge collection region contacts an adjacent edge of the transfer gate of the pixel sensor cell and minimizes, therefore, the gate overlap region and an undesirable barrier potential.
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The present invention relates to the field of semiconductor devices and, in particular, to improved photodiodes for high quantum efficiency.
BACKGROUND OF THE INVENTIONThe semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others.
Because of the inherent limitations and expense of CCD technology, CMOS imagers have been increasingly used as low cost imaging devices. A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photodiode, a photogate or a photoconductor overlying a doped region of a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes a charge transfer section formed on the substrate adjacent the photodiode, photogate or photoconductor having a charge sensing node, typically a floating diffusion node, connected to the gate of a source follower output transistor. The imager may include at least one transistor for transferring charge from the charge accumulation region of the substrate to the floating diffusion node and also has a transistor for resetting the diffusion node to a predetermined charge level prior to charge transfer.
In a conventional CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. The charge at the floating diffusion node is converted to a pixel output voltage by the source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate.
Exemplary CMOS imaging circuits as well as detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,204,524 to Rhodes, U.S. Pat. No. 6,310,366 to Rhodes et al. and U.S. Pat. No. 6,326,652 to Rhodes, the disclosures of which are incorporated by reference herein.
A schematic top view of a semiconductor wafer fragment of an exemplary CMOS sensor pixel four-transistor (4T) cell 10 is illustrated in
The CMOS image sensor 10 of
The charge accumulating region 21 is formed as a pinned photodiode 11 which has a p-type layer 24, an n-type region 26 and the p-type substrate 20. The pinned photodiode 11 includes two p-type regions 20, 24 and the n-type photodiode region 26 which is fully depleted at a pinning voltage. Impurity doped source/drain regions 22 (
Generally, in CMOS image sensors such as the CMOS image sensor cell 10 of
Minimizing dark current in the photodiode is important in CMOS image sensor fabrication. Dark current is generally attributed to leakage in the charge collection region 21 of the pinned photodiode 11, which is strongly dependent on the doping implantation conditions of the CMOS image sensor. High dopant concentrations in electrical connection region 23 (
A common problem associated with the pinned photodiode 11 of
CMOS imagers also typically suffer from poor signal to noise ratios and poor dynamic range as a result of the inability to fully collect and store the electric charge collected in the region 26. Since the size of the pixel electrical signal is very small due to the collection of electrons in the region 26 produced by photons, the signal to noise ratio and dynamic range of the pixel should be as high as possible.
There is needed, therefore, an improved active pixel photosensor for use in a CMOS imager that exhibits reduced dark current and reduces the undesirable barrier potential occurring in an overlap region below a gate structure adjacent a photodiode. A method of fabricating an active pixel photosensor exhibiting these improvements is also needed.
BRIEF SUMMARY OF THE INVENTIONIn one aspect, the invention provides a pinned photodiode with a pinned layer laterally displaced from an electrically active area of a transfer gate of a pixel sensor cell by a predetermined distance. The pinned layer is in contact with a charge collection region formed by an angled implant. The angle of the charge collection region implant may be tailored so that the charge collection region contacts an adjacent edge of the transfer gate of the pixel sensor cell and minimizes, therefore, the gate overlap region and the undesirable barrier potential.
In another aspect, the invention provides a method of forming a pinned surface layer of a first conductivity type of a pinned photodiode by implanting desired dopants in an area of a substrate laterally displaced from an electrically active portion of a transfer gate of a pixel sensor cell by a predetermined distance. A doped region of a second conductivity type is formed by an angled implant below and in contact with the laterally displaced pinned layer. The desired dopants of the second conductivity type are implanted at angles other than 0 degrees, where 0 degrees is defined to be perpendicular to the silicon substrate.
These and other features and advantages of the invention will be more apparent from the following detailed description that is provided in connection with the accompanying drawings and illustrated exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention.
The terms “wafer” and “substrate” are to be understood as a semiconductor-based material including 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 “wafer” or “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, silicon-on-insulator, silicon-on-saphire, germanium, or gallium arsenide, among others.
The term “pixel” refers to a picture element unit cell containing a photosensor and transistors for converting electromagnetic radiation to an electrical signal. For purposes of illustration, a representative pixel is illustrated in the figures and description herein and, typically, fabrication of all pixels in an imager will proceed simultaneously in a similar fashion.
Referring now to the drawings, where like elements are designated by like reference numerals,
The process for making the structures illustrated in
Also illustrated in
Although the embodiments of the present invention will be described below with reference to the transfer gate stack 130 without sidewall spacers formed on its sides, it must be understood that the invention is not limited to this embodiment. Accordingly, the invention also contemplates the formation of a gate stack with insulating sidewall spacers formed on the sides of the transfer gate. If desired, the sidewall spacers may be formed, for example, of silicon dioxide, silicon nitride, silicon oxynitride, ON, NO, ONO or TEOS, among others.
In addition and if desired, a silicide layer (not shown) may be also formed in the multi-layered gate stacks 130, between the conductive layer 132 and the second insulating layer 133. Advantageously, the gate structures of all other transistors in the imager circuit design may have this additionally formed silicide layer. This silicide layer may be titanium silicide, tungsten silicide, cobalt silicide, molybdenum silicide, or tantalum silicide. This added conductive layer could also be a barrier layer/refractory metal such as TiN/W or WNX/W or it could be entirely formed of WNX.
An insulating layer 121 may be formed over the substrate 110 including the STI regions 155 and the transfer gate 130, as also shown in
A doped layer or well 120 of a first conductivity type, which for exemplary purposes is p-type, is also illustrated in
Subsequent to the formation of the STI regions 155 and of the transfer gate 130, a first photoresist layer 177 is next formed over the structure of
As illustrated in
A first angled dopant implantation 179 (
For the purposes of the present invention, the term “angled implantation” is defined as implantation conducted at incidence angles with the substrate 110 other than 0 degree angles, where 0 degrees is perpendicular to the silicon substrate. Thus, the term “angled implantation” refers to implantation conducted at incidence angles with the substrate greater than 0 degrees to less than 90 degrees.
The first angled ion implantation 179 (
The angle of the first dopant implantation 179 may be tailored so that the n-type region 126 is approximately coincident with the edge of the gate structure 130 and is spaced apart from the STI region 155 by a second offset distance D2 (
The angle of the first angled dopant implantation 179 is function of the implant energy as well as the first offset distance D1 (
Subsequent to the first angled implant 179 (
A second photoresist layer 167 (
A second angled dopant implantation 189 (
As shown in
Ion implantation may be conducted by placing the substrate 110 in an ion implanter, and implanting appropriate p-type dopant ions through the second opening 168 (
Subsequent to the second angled implant 189 of
As a result of the angled implant for the formation of the charge collection region 126 and of the pinned surface layer 188, ion-implant channeling is reduced in the photodiode 199 with the angle implanted and laterally displaced pinned surface layer 188, and the angle implanted charge collection region 126, as compared to a conventional 0 degree implant. In addition, the n-type doped region 126 formed by an angled implant is aligned with the edge of the transfer gate 130 and eliminates the transfer gate overlap region which, as explained above, typically occurs below the transfer gate 130. Thus, any undesirable barrier potential that affects the transfer of charge from the n-type charge collection region 126 to the floating diffusion region 125 is eliminated.
The devices of the pixel sensor cell 100 including the reset transistor, the source follower transistor and row select transistor are then formed by well-known methods. Conventional processing steps may be also employed to form contacts and wiring to connect gate lines and other connections in the pixel cell 100. For example, the entire surface may be covered with a passivation layer of, e.g., silicon dioxide, BSG, PSG, or BPSG, which is CMP planarized and etched to provide contact holes, which are then metallized to provide contacts to the reset gate, transfer gate and other pixel gate structures, as needed. Conventional multiple layers of conductors and insulators to other circuit structures may also be used to interconnect the structures of the pixel sensor cell.
The straight implantation 169 (
The angled dopant implantation 179a (
As in the previous embodiment, subsequent to the angled dopant implantation 179a, the photoresist layer 177 is removed by conventional techniques to complete the formation of p-n-p photodiode 299 formed by regions 288 and 226, as illustrated in
Although the above embodiments have been described with reference to the formation of a p-n-p photodiode, such as the p-n-p photodiodes 199 (
In addition, although the present invention has been described above with reference to a 4T pixel cell, such as the pixel sensor cells 100 (
A typical processor based system 600, which has a connected CMOS imager having pixels constructed according to the invention is illustrated in
A processor based system, such as a computer system, for example generally comprises a central processing unit (CPU) 644, for example, a microprocessor, that communicates with an input/output (I/O) device 646 over a bus 652. The CMOS imager 642 communicates with the system over bus 652. The computer system 600 also includes random access memory (RAM) 648, and may include peripheral devices such as a floppy disk drive 654, and a compact disk (CD) ROM drive 656 or a flash memory card 657 which also communicate with CPU 644 over the bus 652. It may also be desirable to integrate the processor 654, CMOS image sensor 642 and memory 648 on a single IC chip.
Although the present invention has been described above with reference to a 4T pixel cell as part of a CMOS imager, the invention has equal applicability to a photodiode, such as the p-n-p photodiodes 199 (
The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.
Claims
1-31. (canceled)
32. A method of forming a photodiode for a pixel sensor cell, said method comprising:
- forming a gate of a transistor over a substrate;
- forming a first doped layer of a first conductivity type in said substrate, said first doped layer being displaced laterally from an electrically active portion of said gate by a predetermined distance; and
- forming a doped region of a second conductivity type in said substrate and below said first doped layer by implanting ions of said second conductivity type in a first direction and at an incidence angle with said substrate different than a zero degree angle in a first area of said substrate below said first doped layer.
33. The method of claim 32, wherein said first doped layer is formed by implanting ions of said first conductivity type at an incidence angle with said substrate different than a zero degree angle.
34. The method of claim 32, wherein said first doped layer is formed by implanting ions of said first conductivity type at an incidence angle with said substrate of about zero degree angle.
35. The method of claim 32, wherein said first direction is a right-to-left direction relative to said gate and into said substrate.
36. The method of claim 32, wherein said first doped layer has an implant dose within the range of from about 1×1012 to about 1×1014 atoms per cm2.
37. The method of claim 32, wherein said first doped layer is formed to be laterally displaced from said electrically active portion of said gate by about 0 Angstroms to about 5,000 Angstroms.
38. The method of claim 37, wherein said first doped layer is formed to be laterally displaced from said electrically active portion of said gate by about 300 Angstroms to about 3,000 Angstroms.
39. The method of claim 32, wherein said act of forming said first doped layer further comprises forming a photoresist layer over said substrate and said gate, and patterning and etching said photoresist layer to expose a second area of said substrate, said second area being located between said gate and said at least one isolation region, said second area being spaced from said gate by said predetermined distance.
40. The method of claim 32, wherein said act of forming said doped region of said second conductivity type further comprises forming a photoresist layer over said substrate and said gate, and patterning and etching said photoresist layer to expose said first area of said substrate located between a sidewall of said gate and said at least one isolation region.
41. The method of claim 32, wherein said act of implanting ions of said second conductivity type further comprises directing a dopant at said incidence angle which is different than a zero degree angle in said first area of said substrate located between said gate and said at least one isolation region.
42. The method of claim 32, wherein said doped region has an implant dose within the range of from about 1×1011 to about 1×1014 atoms per cm2.
43. The method of claim 32, wherein said first conductivity type is p-type and said second conductivity type is n-type.
44. The method of claim 32, wherein said photodiode is a p-n-p photodiode.
45. The method of claim 32, wherein said photodiode is part of a CMOS imager.
46. The method of claim 32, wherein said photodiode is part of a CCD imager.
47. A method of forming a photodiode, said method comprising:
- forming at least one shallow trench isolation region in a silicon substrate;
- forming a transistor gate over said silicon substrate and spaced apart from said at least one shallow trench isolation region;
- forming a first doped layer of a first conductivity type in said silicon substrate;
- forming a second doped layer of said first conductivity type in said first doped layer by implanting ions in a first direction and at an incidence angle with said silicon substrate other than zero degrees, said second doped layer being in contact with said isolation region and being displaced laterally from an electrically active area of said transistor gate by a predetermined distance; and
- forming a doped region of a second conductivity type in said first doped layer by implanting ions in a second direction and at an incidence angle with said silicon substrate other than zero degrees.
48. The method of claim 47, wherein said second doped layer has an implant dose within the range of from about 1×1012 to about 1×1014 atoms per cm2.
49. The method of claim 47, wherein said second doped layer is laterally displaced from said electrically active area of said transistor gate by about 0 Angstroms to about 5,000 Angstroms.
50. The method of claim 49, wherein said second doped layer is laterally displaced from said electrically active area of said transistor gate by about 300 Angstroms to about 3,000 Angstroms.
51. The method of claim 47, wherein said act of forming said doped region further comprises forming at least a portion of said doped region between said second doped layer and said transfer gate.
52. The method of claim 47, wherein said doped region has an implant dose within the range of from about 1×1011 to about 1×1014 atoms per cm2.
53. The method of claim 47, wherein said first direction is opposite said second direction.
54. The method of claim 47, wherein said photodiode is part of a CMOS imager.
55. The method of claim 47, wherein said photodiode is part of a CCD imager.
56. A method of forming a p-n-p photodiode, said method comprising:
- forming at least one field oxide region in a substrate;
- forming a transistor gate over said substrate and spaced apart from said at least one field oxide region;
- forming a first p-type doped layer in said substrate;
- forming a photoresist layer over said transistor gate and said field oxide region;
- patterning said photoresist layer to form a first opening extending between a first location and a second location, said first location corresponding to a first point over a photodiode area and said second location corresponding to a second point over said field oxide region;
- conducting a first angled implant through said first opening to form a p-type surface layer in said first p-type doped layer, said p-type surface layer being laterally displaced from an electrically active area of a gate structure formed over said substrate; and
- conducting a second angled implant to form an n-type doped region in said first p-type doped layer, said n-type doped region being located below said p-type surface layer.
57. The method of claim 56, wherein said p-type surface layer is displaced laterally from said electrically active area of said transistor gate by a predetermined distance.
58. The method of claim 56, wherein said predetermined distance is of about 0 Angstroms to about 5,000 Angstroms.
59. The method of claim 56, wherein said p-type surface layer has an implant dose within the range of from about 1×1012 to about 1×1014 atoms per cm2.
60. The method of claim 56, wherein said n-type doped region has an implant dose within the range of from about 1×1011 to about 1×1014 atoms per cm2.
61. The method of claim 56, wherein said p-n-p photodiode is part of a CMOS imager.
62. The method of claim 56, wherein said p-n-p photodiode is part of a CCD imager.
Type: Application
Filed: Dec 13, 2004
Publication Date: May 12, 2005
Applicant:
Inventor: Howard Rhodes (Boise, ID)
Application Number: 11/008,990