CMOS IMAGE SENSOR, A PHOTODIODE THEREOF AND A METHOD OF FORMING THE SAME
A photodiode (PD) of a complementary metal-oxide-semiconductor (CMOS) image sensor includes a top PD of a second type disposed in a first-type layer; and a bottom PD of the second type disposed in the first-type layer and below the top PD, the bottom PD including at least one sub-photodiode (sub-PD) of the second type connected to the top PD and at least one sub-well of the first type surrounded by the at least one sub-PD.
The present invention generally relates to a CMOS image sensor, and more particularly to a CMOS image sensor with vertical-sub-photodiodes.
2. Description of Related ArtA complementary metal-oxide-semiconductor (CMOS) image sensor has been widely applied to mobile applications. The CMOS image sensor may be applied to other applications such as automotive and security applications. Requirements for the automotive and security applications are quite different from that for the mobile applications. For example, one strong request is high dynamic range (HDR), which is capable of capturing ultra dim and bright scenes at the same frame and with good quality.
Dynamic range (DR) of the CMOS image sensor may be classified into two types: one is intrinsic dynamic range (DR) that is determined by read noise and full well capacity (FWC), related to physical device and circuit capabilities; the other one is extended dynamic range (EDR) that may be achieved, for example, by multiple exposures and gains, etc. The intrinsic DR not only contributes to the final DR achieved, but is also important for reducing the side effects of EDR schemes. To obtain more intrinsic DR, there is a need to largely improve full well capacity (FWC) that defines the amount of charges an individual pixel can hold. A need has thus arisen to propose a novel CMOS image sensor with higher FWC pixel.
SUMMARY OF THE INVENTIONIn view of the foregoing, it is an object of the embodiment of the present invention to provide a scalable (in terms of pixel size and process technology) solution for high full well capacity (FWC) pixel of a CMOS image sensor. The proposed CMOS image sensor may be adaptable to a front-side illuminated (FSI) CMOS image sensor or a backside illuminated (BSI) CMOS image sensor.
According to one embodiment, a photodiode (PD) of a CMOS image sensor includes a top PD of a second type and a bottom PD of the second type. The top PD is disposed in a first-type layer. The bottom PD is disposed in the first-type layer and below the top PD, and the bottom PD includes at least one sub-photodiode (sub-PD) of the second type connected to the top PD and at least one sub-well of the first type surrounded by the at least one sub-PD.
According to the embodiment, a CMOS image sensor includes a substrate; a bottom PD of a second type disposed above the substrate, the bottom PD including at least one sub-photodiode (sub-PD) of the second type; at least one sub-well of a first type disposed above the substrate and surrounded by the at least one sub-PD; a top PD of the second type disposed above the bottom PD, the top PD connecting the at least one sub-PD; a transfer-gate channel of the first type disposed above the top PD; a plurality of pixel wells of the first type disposed above the substrate, adjacent pixel wells defining a PD region composed of the top PD and the bottom PD; and an isolation region disposed above the substrate and between adjacent pixels.
According to another embodiment, a method of forming a CMOS image sensor is disclosed. A crystalline layer of a first type is formed on a substrate. An isolation region is formed in the crystalline layer, the isolation region being disposed between adjacent pixels. A transfer-gate channel of the first type is formed in an upper portion of the crystalline layer. A bottom photodiode (PD) layer of a second type is formed in a bottom portion of the crystalline layer. A plurality of pixel wells of the first type are formed in the crystalline layer, between adjacent pixel wells defining a PD region. At least one sub-well of the first type is formed in the bottom PD layer, the bottom PD layer not occupied by the at least one sub-well thereby resulting in at least one sub-photodiode (sub-PD) acting as a bottom PD of the PD region. A top PD of the second type is formed in a top portion of the crystalline layer, the top PD and the bottom PD constituting the PD region.
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Afterwards, a transfer gate 19 is formed above the crystalline layer 11 (e.g., on a top surface of the oxide layer 13). In the embodiment, the transfer gate 19 partially overlaps the transfer-gate channel 14. One edge (i.e., a first edge) of the transfer gate 19 partially overlaps the top PD 18.
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Generally speaking, a photodiode (PD) of a CMOS image sensor 100 according to the embodiment may include a top PD 18 of a second type disposed in a first-type layer (e.g., the crystalline layer 11); and a bottom PD of the second type disposed in the first-type layer and below the top PD 18, the bottom PD including at least one sub-photodiode (sub-PD) 15B of the second type connected to the top PD 18.
According to the embodiment discussed above, as area (junction capacitance) of the bottom PD increases due to the plural vertical side walls of the at least one sub-PD 15B, charge storage density of the CMOS image sensor 100 increases accordingly for a given Vpin as defined in
The area of vertical side walls may be scaled with depth of the bottom PD 15B and/or the amount of the sub-PDs 15B. Therefore, more full well capacity (FWC) can be achieved by more advanced process technology that is capable of handling deeper PD or forming more sub-PDs 15B (or sub-wells 17).
Moreover, as the outer depletion side wall (as denoted by 131 in
It is further noted that if deep trench isolation (DTI) is adopted for the isolation region 12, the impact on the quantum efficiency (QE), crosstalk and symmetry of the bottom PD 15 becomes less stringent.
Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.
Claims
1. A photodiode (PD) of a complementary metal-oxide-semiconductor (CMOS) image sensor, comprising:
- a top PD of a second type disposed in a first-type layer; and
- a bottom PD of the second type disposed in the first-type layer and below the top PD,
- wherein the bottom PD includes at least one sub-photodiode (sub-PD) of the second type, the sub-PD being directly connected to the top PD physically and electrically; and the bottom PD includes at least one sub-well of the first type, the sub-well being surrounded by the at least one sub-PD.
2. The photodiode of claim 1, wherein the at least one sub-PD is ring-shaped.
3. The photodiode of claim 1, wherein the at least one sub-well is enclosed by a ring-shaped sub-PD.
4. The photodiode of claim 1, wherein the at least one sub-PD is symmetrical relative to a center of the bottom PD.
5. A complementary metal-oxide-semiconductor (CMOS) image sensor, comprising:
- a substrate;
- a bottom PD of a second type disposed above the substrate, the bottom PD including at least one sub-photodiode (sub-PD) of the second type;
- at least one sub-well of a first type disposed above the substrate and the sub-well surrounded by the at least one sub-PD;
- a top PD of the second type disposed above the bottom PD, the top PD directly connecting the at least one sub-PD physically and electrically;
- a transfer-gate channel of the first type disposed above the top PD;
- a plurality of pixel wells of the first type disposed above the substrate, adjacent pixel wells defining a PD region composed of the top PD and the bottom PD; and
- an isolation region disposed above the substrate and between adjacent pixels.
6. The CMOS image sensor of claim 5, further comprising:
- a transfer gate disposed above and partially overlapping the transfer-gate channel; and
- a floating diffusion (FD) node disposed in an upper portion of the pixel well, the FD node abutting the transfer-gate channel.
7. The CMOS image sensor of claim 6, wherein a first edge of the transfer gate partially overlaps the top PD, and an opposite second edge of the transfer gate partially overlaps the FD node.
8. The CMOS image sensor of claim 5, wherein the at least one sub-PD is ring-shaped.
9. The CMOS image sensor of claim 5, wherein the at least one sub-well is enclosed by a ring-shaped sub-PD.
10. The CMOS image sensor of claim 5, wherein the at least one sub-PD is symmetrical relative to a center of the bottom PD.
11-17. (canceled)
Type: Application
Filed: Jun 21, 2017
Publication Date: Dec 27, 2018
Inventors: Yang Wu (Tainan City), Inna Patrick (Tainan City)
Application Number: 15/629,461