Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-01-11
2003-03-18
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S292000
Reexamination Certificate
active
06534808
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to the field of photocells and photo-imagers, and more particularly to multi-color photocells and imagers.
BACKGROUND OF THE INVENTION
Photocells and photo-imagers (also called “image sensors”) made of an array of photocells are used in a wide variety of imaging applications. One type of photo-imager is a CCD (charge-coupled device), in which each photocell is normally sensitive to only one band of wavelengths (i.e., one color). A conventional CCD imager can be complex and require highly accurate timing mechanisms and is therefore expensive to manufacture. Conventional CCDs also require 5-10 volt supplies and are limited in detecting lower-wavelength (e.g., blue light) signals.
One type of imager that operates at a lower voltage, is simpler, and is less expensive to manufacture than the CCD imager is a metal-insulator-semiconductor (“M-I-S,” also called “metal-insulator-silicon”) imager. An M-I-S imager is made of an array of M-I-S photocells, one of which is shown in a schematic diagram in FIG.
1
A. The M-I-S photocell shown in
FIG. 1A
is derived from the applicants' prior unexamined Japanese patent application JP 10-27896, published Jan. 27, 1998, the disclosure of which is herein incorporated by reference. As the name implies, the M-I-S photocell comprises a metal contact or electrode
120
(also called a gate), an insulator
130
, and a semiconductor diffusion layer
140
. When a negative drive voltage is applied to diffusion layer
140
, a depletion region
150
is formed within diffusion layer
140
in the area below contact
120
. This M-I-S structure
100
is disposed on top of a substrate
10
, which is typically made from silicon. Insulator
130
is typically made of a thermally grown oxide such as SiO
2
. When thin, between 20 and 50 Å, typically 30 Å, insulator
130
acts as a dielectric material and is light-transparent. Contact
120
is also light-transparent, typically made of a metal, such as SnO
2
, or heavily doped polysilicon, and has a typical thickness of 2000 Å, when made of polysilicon. The resistivity of contact
120
varies depending on the material: for polysilicon, it has a resistivity of 100&OHgr;-200&OHgr; per square; for SnO
2
, it has a resistivity of approximately 10&OHgr;-20&OHgr; per square. When substrate
10
comprises n-Si, then diffusion layer
140
comprises single-crystal p-Si, and contact
120
is an n-contact or a p-contact. The photocell generates current when light transmits through contact
120
and insulator
130
and is incident on depletion region
150
creating electron-hole pairs.
The M-I-S photocell can be arranged in an array to form an M-I-S photo-imager, a portion of which is shown in FIG.
1
B. Semiconductor diffusion layers
140
are arranged in strips over substrate
10
. Insulator
130
is disposed over diffusion layers
140
. Between the diffusion layers are isolation strips
160
made by depositing more thickly the insulator material directly on substrate
10
. (These strips
160
may also be made using a LOCOS (local oxidization of silicon) process.) Perpendicular to the diffusion layers and isolation strips are strips of contact
120
. The depletion regions
150
are formed at the intersections of the contact
120
and the diffusion layers
140
.
The quantum efficiency of each photocell with a 2000 Å polysilicon contact layer is shown in TABLE 1 as a function of color and wavelength.
TABLE 1
Color
Wavelength (Å)
Quantum Efficiency
Blue
4000
0.014
Green
5400
0.36
Red
7000
0.60
Near Infrared
9000
0.75
Infrared
12000
0.026
Although providing a significant improvement over CCD imagers with respect to power dissipation and the detection of some colors, this M-I-S photo-imager with a 2000 Å polysilicon contact does not detect blue light very well, as demonstrated in TABLE 1, and is not able to separate out more than one color.
SUMMARY OF THE INVENTION
Therefore, a need has arisen for an improved photo-imager which is capable of detecting several colors, including blue light. In accordance with the present invention, a device, such as a photocell, for detecting light includes at least two structures or tiers, one disposed over the other, each detecting a different wavelength of light. Each structure could be an M-I-S structure or a semiconductor-insulator-metal (S-I-M) structure.
Preferably, each M-I-S structure includes a semiconductor diffusion layer capable of developing a depletion region, a thin insulator layer disposed on the diffusion layer, and a contact layer disposed on the thin insulator layer. Each S-I-M structure includes a contact layer, a thin insulator layer disposed on the contact layer, and a semiconductor diffusion layer disposed on the thin insulator layer, the diffusion layer capable of developing a depletion region. When light is incident on each M-I-S or S-I-M depletion region, a current indicative of the light detected in each M-I-S or S-I-M structure flows through the respective contact layer.
A photo-imager according to the present invention includes an array of photocells each photocell including the two M-I-S or S-I-M structures.
Preferably, the wavelength detected by the bottom tier is longer than the wavelength detected by the top tier.
Preferably, a third M-I-S or S-I-M structure (or tier) is disposed over the first two tiers, and all three tiers detect different wavelengths. Preferably, the third tier has a metal-insulator-semiconductor or semiconductor-insulator-metal structure analogous to those of the first two tiers.
Preferably, the top, middle, and bottom tiers of the three-tiered device detect light having progressively longer wavelengths. Preferably, the top tier detects mainly blue light, the middle tier mainly green light, and the bottom tier mainly red light.
Preferred embodiments of this three-tiered device include all M-I-S structures, all S-I-M structures, an M-I-S/S-I-M/M-I-S (in order from top to bottom) structure, an S-I-M/M-I-S/S-I-M structure, and an S-I-M/S-I-M/M-I-S structure.
Also in accordance with the present invention is a device, such as a photocell, for detecting light that includes at least one M-I-S structure. The M-I-S structure includes a semiconductor diffusion layer, a thin insulator layer disposed on the diffusion layer, and a contact layer disposed on the thin insulator layer. The diffusion layer is made of polysilicon or amorphous silicon and is capable of developing a depletion region. When light is incident on the depletion region, a current indicative of the light detected in the depletion region flows through the contact layer. Preferably, a photo-imager according to the present invention includes an array of photocells each having such an M-I-S structure.
Also in accordance with the present invention is a device, such as a photocell, for detecting light that includes an S-I-M structure. The S-I-M structure includes a contact layer, a thin insulator layer disposed on the contact layer, and a semiconductor diffusion layer disposed on the thin insulator layer. The diffusion layer is capable of developing a depletion region. When light is incident on the depletion region, a current indicative of the light detected in the depletion region flows through the contact layer. Preferably, a photo-imager according to the present invention includes an array of photocells each having an S-I-M structure. The diffusion layer can be made of polysilicon or amorphous silicon.
Also in accordance with the present invention are methods for fabricating a device, such as a photocell, for detecting light and a photo-imager comprising an array of those photocells. One method includes forming on a substrate three tiers or structures, one disposed over the next, each detecting a different wavelength of light. Each tier could be an M-I-S or an S-I-M structure. Preferably, the top, middle, and bottom tiers detect light having progressively longer wavelengths. Preferably, the top tier detects mainly blue light, the middle tier mainly gr
Fujii Ichiro
Nishimura Akitoshi
Brady W. James
Hoel Carlton H.
Meier Stephen D.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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