Dual EPI active pixel cell design and method of making the same

Details Dual EPI active pixel cell design and method of making the same Dual EPI active pixel cell design and method of making the same

1999-01-21

2001-12-25

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

C348S014160, C250S21400C

Reexamination Certificate

active

06333204

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to active pixel sensors and in particular to an active pixel sensor which provides improved red absorption while providing improved latch-up and noise reduction.
2. Description of Related Art
A conventional Active Pixel Sensor (APS) cell consists of an np diode in the p-type substrate, an NFET transfer device, a source-follower amplifying transistor, a pre-charge transistor, and a bit switch transistor. Electron-hole pairs are produced in the diode by impinging photons. The electrons are collected in the pre-charged n region of the diode, and eventually transferred to the source-follower gate for amplification. A p-type pinning layer is often included at the surface of the diode.
FIG. 1
a
illustrates a schematic cross-section of the photodiode and the transfer gate.
FIG. 1
b
illustrates the schematic of the circuit embodied in each Active Pixel Sensor device
10
. The number of electrons collected determines the level of gate drive on the source-follower transistor
12
. The source potential is transferred down the column through the bit switch
14
, where it can be processed appropriately for video imaging.
Photons are absorbed by the silicon in the pre-charged n region of the diode at various depths, depending on the photon energy. Long wavelength photons have an absorption depth of 5 or more micrometers. Electron-hole pairs are created when the photons are absorbed, and provided that this creation occurs within a diffusion length of the photodiode depletion region, they will be collected by the photodiode and contribute to the optical signal.
Heavily-doped silicon has a short diffusion length, so a deep region of lightly-doped material is preferable to enable the collection of the electron-hole pairs generated by long wavelength red photons. However, high performance CMOS technologies of the type preferred for the logic areas outside of the photodiode use a shallow lightly-doped region for latch-up immunity and to provide a quiet substrate. This causes difficulties when integrating the manufacturing steps needed for the APS device with the manufacturing steps needed for the high performance CMOS logic areas.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an active pixel cell device which optimizes red response and latch-up simultaneously.
It is another object of the present invention to provide an active pixel sensor which allows easy integration with existing embedded microprocessor and analog designs necessary for the high level of integration required by products using APS technology.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, an active pixel sensor cell having a substrate doped at a first concentration, a plurality of photosensitive devices and logic devices, a layer doped at a second concentration extending a first distance from the top surface of the substrate into the substrate beneath at least some of the logic devices, and said layer doped at a second concentration extending a second distance, which is greater than the first distance, from said top surface into said substrate beneath at least some of the photosensitive devices.
This invention also includes a method of making an active pixel sensor device in which the p− layer has a varying thickness. In the logic region the p− layer has a first thickness and in an adjacent pixel region the p− layer has a second thickness which is greater than the thickness of the p− layer in the logic region.
The method of making the active pixel sensor device includes the steps of:
(a) providing a p type wafer substrate having a p+ region and a p− epi region wherein a portion of the p+ and p− region defines a logic region and another portion of the p+ and p− regions defines a pixel region.
(b) implanting a p+ region in the p− epitaxy region positioned in the logic region;
(c) creating a p− epitaxy layer on the substrate above the logic and pixel regions.
In another aspect, the method of the invention includes the steps of:
(a) providing a p+ wafer substrate wherein a portion of the p+ region defines a logic region and another portion of the p+ region defines a pixel region;
(b) etching the pixel region and growing p− silicon in the pixel region;
(c) creating a p− epitaxy layer on the substrate above the logic and pixel regions.


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