|Publication number||US20070187734 A1|
|Application number||US 11/276,085|
|Publication date||Aug 16, 2007|
|Filing date||Feb 14, 2006|
|Priority date||Feb 14, 2006|
|Also published as||CN101022118A, CN101022118B, US7659564, US8106432, US8440490, US20100084690, US20120122261|
|Publication number||11276085, 276085, US 2007/0187734 A1, US 2007/187734 A1, US 20070187734 A1, US 20070187734A1, US 2007187734 A1, US 2007187734A1, US-A1-20070187734, US-A1-2007187734, US2007/0187734A1, US2007/187734A1, US20070187734 A1, US20070187734A1, US2007187734 A1, US2007187734A1|
|Inventors||James Adkisson, John Ellis-Monaghan, Mark Jaffe, Dale Pearson, Dennis Rogers|
|Original Assignee||Adkisson James W, Ellis-Monaghan John J, Jaffe Mark D, Pearson Dale J, Rogers Dennis L|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (16), Classifications (11), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to semiconductor optical image sensors, and particularly, to a novel CMOS (complementary metal oxide semiconductor) image sensor photodiode structure having a deep trench with a diode all along its sidewall to increase the capacitance of the structure without increasing the area of the cell. This trench structure can be designed in such a way that the trench will be fully depleted during the reset phase of operation and can hold a larger charge due to its larger capacitance than a conventional photodiode.
CMOS image sensors are now replacing conventional CCD sensors for applications requiring image pick-up such as digital cameras, cellular phones, PDA (personal digital assistant), personal computers, and the like. Advantageously, CMOS image sensors are fabricated by applying present CMOS fabricating process for semiconductor devices such as photodiodes or the like, at low costs. Furthermore, CMOS image sensors can be operated by a single power supply so that the power consumption for that can be restrained lower than that of CCD sensors, and further, CMOS logic circuits and like logic processing devices are easily integrated in the sensor chip and therefore the CMOS image sensors can be miniaturized.
As the pixel size in CMOS imagers continue to scale down, to reduce costs, several problems arise: First, the smaller area reduces the total amount of electrons that can be stored in a given cell due to the reduced cell capacitance. Second, the smaller area increases the crosstalk between cells because electron diffusion is primarily responsible for the transport of the electrons from deep into the substrate to the surface photodiode. Third, the signal to noise ratio degrades because of a smaller number of electrons that are being measured and the noise sources (which primarily come from the periphery of the photodiode and its support circuits) that have not scaled. Current trench-type CMOS imager photosensor devices are described in U.S. Pat. Nos. 6,232,626; 6,500,692; 2004/0195600 and additionally, U.S. Pat. Nos. 6,611,037; 6,767,759; 6,730,980 and 6,838,742.
The charge transfer transistor gate 25 is shown surrounded by thin spacer structures 23 a,b. An STI region 40 is formed proximate the pixel imager cell for isolating the cell 10 from an adjacent pixel cell. In operation, light coming from the pixel is focused onto the photodiode where electrons collect at the n-type region 20. When the transfer gate 25 is operated, i.e., turned on by applying a voltage to the transfer gate comprising, for example, an n-type doped polysilicon layer or conducting layer 70, the photo-generated charge 24 is transferred from the charge accumulating n-type doped region 20 via a transfer device surface channel 16 to the floating diffusion region 30, e.g., doped n+ type, as shown by arrow A.
While touted as providing increased surface area as compared to a flat photosensor element occupying a comparable area on a substrate, thus exhibiting a higher charge capacity and improved dynamic range, there are drawbacks, particularly, in the fact that this prior art photosensor cell of
Thus, a characteristic of these trench type CMOS imager photosensor devices is the existence of large dark current, i.e., leakage current, which discharges the pixel capacitance when there is no light over the pixel. The dark current measured at the pixel output depends on the photodiode, the transistors, and the interconnectivity in the pixel. None of the above-indicated prior art references teach isolating the charge collection region thus, and do not address dark current performance.
It would be highly desirable to provide a CMOS imager having the photodetector diode formed on the sidewall of a deep trench, resulting in collector isolation, and, a simplified process therefore.
It would be highly desirable to provide a CMOS imager having a photodetector diode formed on the sidewall of a deep trench, the photodiode thus exhibiting increased electron capacity by increasing the photodiode capacitance without adding to the cell size, or to the cell leakage.
It thus an object of the invention to provide a CMOS image sensor having a photodetector diode formed on the sidewall of a deep trench, resulting in collector isolation, and, a simplified process therefore.
It is a further object of the invention to provide a CMOS imager having a photodetector diode formed on the sidewall of a deep trench, the photodiode having increased electron capacity by increasing the photodiode capacitance without adding to the cell size, or to the cell leakage.
According to one aspect of the invention, there is provided a pixel sensor cell comprising a non-lateral (e.g., vertical) collection region which is isolated from a physical boundary (e.g., substrate surfaces such as top surface or sidewalls/bottom of trench). In an embodiment of the invention, a trench is formed in a substrate of a first conductivity type (p type); a first doped layer of a second conductivity type (n type) is formed surrounding the trench walls to form a collection region; a second doped layer of the first conductivity type (p type) is formed between the first doped layer and the trench walls, and a third doped layer of the first conductivity type is formed on a surface of the substrate coupled to the second doped layer, wherein the second and third doped layers form a “pinning layer” for the sensor cell and isolate the collection region (e.g. first doped layer) from the trench walls and substrate surface.
Advantageously, the isolating of the deep trench photodiode collection region (e.g. first doped layer) from the trench walls and substrate surface of the pixel sensor cell enables improved dark current performance with the same or smaller pinning voltage.
A number of embodiments are described that include a pixel sensor cell structure having a semiconductor substrate having a surface; a photosensitive element formed in a substrate having a non-laterally disposed charge collection region, the non-lateral charge collection region being entirely isolated from a physical boundary including the substrate surface. The photosensitive element comprises a trench having sidewalls formed in the substrate of a first conductivity type material; a first doped layer of a second conductivity type material formed adjacent to at least one of the sidewalls; and a second doped layer of the first conductivity type material formed between the first doped layer and the at least one trench sidewall and formed at a surface of the substrate, the second doped layer isolating the first doped layer from the at least one trench sidewall and said substrate surface.
In a further embodiment, an additional photosensitive element is provided that includes a laterally disposed charge collection region including a layer of second conductivity type material that contacts the first layer of the second conductivity type material of the non-laterally disposed charge collection region of the first photosensitive element. This layer of second conductivity type material underlies the second doped layer of the first conductivity type material formed at the substrate surface.
In accordance with this further embodiment, the additional photosensitive element is formed adjacent a transfer gate device enabled for transferring charge carriers from both the laterally disposed charge collection region of the additional photosensitive element and charge carriers from the non-laterally disposed charge collection region of the photosensitive element across a gate channel to a formed diffusion region. The second conductivity type material of the first layer of the non-laterally disposed charge collection region of the photosensitive element is of a concentration such that the photosensitive element is fully depleted of accumulated charge carriers prior to depletion of charge carriers accumulated at the laterally disposed charge collection region of the additional photosensitive element.
According to another aspect of the invention, there is provided a method for fabricating a pixel sensor cell including a photosensitive element having a non-laterally disposed charge collection region. The method comprises:
forming a trench recess in a substrate of a first conductivity type material, the trench having sidewall and bottom portions;
filling the trench recess with a material having second conductivity type material;
outdiffuse second conductivity type material out of the filled trench material to the substrate region surrounding the trench sidewalls and bottom to form the non-laterally disposed charge collection region;
removing the filled trench material to provide the trench recess;
filling the trench recess with a material having a first conductivity type material;
forming a surface implant layer having first conductivity type material, the surface implant layer formed at either side of the trench,
wherein a collection region of the trench-type photosensitive element is formed of the outdiffused second conductivity type material and is isolated from the substrate surface.
A further step is performed whereby first conductivity type material is diffused out of the filled trench material to form a layer of first conductvity type material in a substrate region surrounding the trench sidewalls and bottom and between the non-laterally disposed charge collection region of the photosensitive element
In a further embodiment, a second photosensitive element is formed having a charge collection region that is laterally disposed and abuts the non-laterally disposed charge collection region of the first photosensitive element. This step entails implanting second conductivity type material underneath the surface implant layer having the first conductivity type material, wherein the implanted second conductivity type material forms a charge collection region that is isolated from the substrate surface.
The objects, features and advantages of the present invention will become apparent to one skilled in the art, in view of the following detailed description taken in combination with the attached drawings, in which:
As shown in
The photosensor device itself, e.g., photodiode 120, comprises a trench structure 121 filled with first conductivity type material, e.g., p-type polysilicon 115, and includes a p-type dopant material outdiffused past the trench boundary forming a p-type boundary layer 118 surrounding the trench 121. A non-lateral (e.g. vertical) collection region, typically of a second conductivity type, e.g., n-type doped layer 112 is formed immediately adjacent and surrounding the p-type region 118. In operation, photocarriers are generated in a photoactive collection area 106 of the p-type substrate and are collected in the n-type region 112 surrounding the p-type polysilicon filled trench 121. The n-type doped region 112 surrounding the trench couples in to the transfer gate directly.That is, as shown in
While only a single CMOS image sensor cell 200 is depicted for exemplary purposes, it is understood that the pixel cell structure depicted is contemplated for inclusion in an array of pixels arranged in rows and columns with rows and columns of pixels addressable by pixel select circuitry not shown.
As shown in
The second photosensor device itself, e.g., photodiode 220, comprises a trench structure 221 filled with first conductivity type material, e.g., p-type polysilicon 215, and includes a p-type dopant material outdiffused past the trench boundary forming a p-type boundary layer 218 surrounding the trench 221. A non-lateral (e.g. vertical) collection region, typically of a second conductivity type, e.g., n-type doped layer 212 is formed immediately adjacent and surrounding the p-type region 218.
In operation, photocarriers are generated in a photoactive collection area 206 of the p-type substrate and are collected both in the n-type doped region forming photodiode 250 and in the n-type region 212 surrounding the p-type polysilicon filled trench 221. The n-type doped region forming photodiode 250 couples its collected charge and charge collected from the n-type doped region 212 surrounding the trench to the transfer gate directly. That is, as shown in
As in the first embodiment of the invention, characteristic of the photosensor (photodiode) element 220 of the second embodiment is that the charge collection regions 212 and 250 are isolated from a physical boundary such as, for example, the top substrate surface or sidewalls/bottom of trench surface. For instance, in the second embodiment of the invention, a p-type surface layer 230 a,b is formed that functions as a pinning layer for the sensor cell and isolates the collection region 212, 250 from surface boundaries. As shown in
The method 300 to create the trench-type photodiode structures 120, 220 of the first and second embodiments of the invention is now described with respect to
With respect to the first embodiment of the invention with just a vertical photodiode as depicted in
Referring now to
Referring now to
With respect to the second embodiment of the invention (depicted in
It should be understood that in both first and second embodiments, the photodiode device is exposed to a suitable amount of temperature as a result of the thermal budgets employed during the subsequent CMOS imager processing flow at durations suitable to enable a second outdiffusion of the in-situ doped first conductivity type material, e.g., p-type polysilicon 315, to form an outdiffused p-type layer 118 (
It is understood that the additional step of forming the second out-diffused layers 118, 218 of the respective photodiodes 100, 200 is such that the concentration of outdiffused first conductivity type (p-type) material in layers 118, 218 is greater than the concentration of the respective outdiffused second conductivity type (n-type) material forming the respective photocarrier collection regions 112, 212.
The benefits of the CMOS photosensor cell of the invention include: 1) increased cell capacity; 2) the ability to be fully depleted; 3) a pinned structure (no oxide interfaces with n-type dopant abutting); 4) reduced crosstalk (That is, the deep trench structure creates a lateral field deep in the silicon which preferentially collects charge generated close to this cell (and adjacent cells will collect their charge preferentially as well)); 5) no increase in cell area needed to get higher capacity (which would occur if capacity was designed to be increased by making the photodiode larger); 6) small increase in dark current (That is, dark current increase would be very large if capacity was designed to be increased by increasing dopant concentrations; and, 7) no change in pinning potential (which would occur if capacity was designed to be increased by increasing dopant concentration).
As mentioned, the total capacitance of the structure is increased without increasing the area of the photodiode. The pinning potential is still determined by the current photodiode parameters. In connection with the second embodiment of the invention, as depicted in
While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.
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|U.S. Classification||257/293, 257/E31.038, 257/E27.133|
|Cooperative Classification||H01L27/14689, H01L27/14643, H01L31/035281, H01L27/1463|
|European Classification||H01L27/146F, H01L27/146V6, H01L27/146A12|
|Feb 14, 2006||AS||Assignment|
Owner name: INTERNATIONAL BUSINESS MACHINES CORPORATION,NEW YO
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:ADKISSON, JAMES W.;ELLIS-MONAGHAN, JOHN J.;JAFFE, MARK D.;AND OTHERS;SIGNING DATES FROM 20060120 TO 20060210;REEL/FRAME:017162/0204
|Nov 30, 2010||CC||Certificate of correction|
|Sep 20, 2013||REMI||Maintenance fee reminder mailed|
|Feb 9, 2014||LAPS||Lapse for failure to pay maintenance fees|
|Apr 1, 2014||FP||Expired due to failure to pay maintenance fee|
Effective date: 20140209