Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2001-08-28
2003-05-27
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
C257S072000, C257S225000, C257S233000, C257S234000, C257S059000
Reexamination Certificate
active
06570197
ABSTRACT:
This application claims the benefit of Korean patent application No. 2000-51295, filed Aug. 31, 2000 in Korea, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical detecting sensor, and more particularly, to a thin film transistor (TFT) type optical detecting sensor.
2. Discussion of the Prior Art
In general, optical detecting sensors are used in facsimile and digital copying machines, and in fingerprint recognition systems as image readers. The optical detecting sensor stores electric charges according to an intensity of light reflected from a detecting subject, and then outputs the electric charges via a drive circuit. Recently, TFT type optical detecting sensors have been suggested in optical detecting systems such that the TFT changes its electrical characteristics in response to incident light.
An inverted staggered type TFT has been selected for typical TFT type optical detecting sensors because of its simple structure and superior quality. The inverted staggered type TFT has been classified into at least two categories: a back channel etch type TFT and an etch stopper type TFT.
A typical TFT type optical sensor will include a light source for generating light, a window for introducing the light to a subject for detection, a sensor TFT, a storage capacitor, and a switch TFT. The sensor TFT generates an optical current according to an intensity of the light reflected from the subject for detection, and the storage capacitor receives the optical current and stores electric charges of the optical current as data. Then, the switch TFT transmits the electric charges according to a control signal generated from an exterior circuit, to transfer the data to a main system.
FIG. 1
shows a conventional TFT type optical sensor including an array substrate
1
and a back light unit
2
disposed under the array substrate
1
. The array substrate
1
detects a subject, stores data relating to the subject, and transmits the data to a main system (not shown) such as a fingerprint recognition system, for example. The back light unit
2
generates light for the array substrate
1
. As shown in
FIG. 2
, the array substrate
1
includes a plurality of unit pixels “P” each including a sensor TFT “T
1
,” a storage capacitor “C,” and a switch TFT “T
2
.” The sensor TFT “T
1
” and the switch TFT “T
2
” are both conventionally formed of the back channel etch type TFT, for example.
FIGS. 2 and 3
show the unit pixel “P” to include a sensor gate line
21
, a sensor data line
61
, a switch gate line
25
, and a switch data line
65
. The sensor gate line
21
and the sensor data line
61
cross with each other, and the switch gate line
25
and the switch data line
65
are spaced apart from the sensor gate line
21
and the sensor data line
61
, respectively. The unit pixel “P” is divided into a photo-sensing region “A,” a storing region “B,” and a switching region “C,” all of which are formed on a transparent substrate
10
. A sensor gate electrode
22
, a first capacitor electrode
24
, a switch gate electrode
26
are formed in the photo-sensing region “A,” the storing region “B.” and the switching region “C,” respectively. The sensor gate electrode
22
and the switch gate electrode
26
integrally protrude from the sensor gate line
21
and the switch gate line
25
, respectively. Alternatively, parts of the sensor gate line
21
and the switch gate line
25
may not protrude, but may be used as the sensor gate electrode
22
and the switch gate electrode
26
, respectively. The first capacitor electrode
24
integrally protrudes from the sensor gate line
21
.
In
FIG. 3
, a first insulating layer
30
covers the sensor electrode
22
, the first capacitor electrode
24
, and the switch gate electrode
26
. On the first insulating layer
30
, a sensor silicon layer
41
and a switch silicon layer
42
are formed in the sensing region “A” and the switching region “B,” respectively. A sensor ohmic contact layer
52
and a switch ohmic contact layer
54
are formed on the sensor silicon layer
41
and the switch silicon layer
42
, respectively.
A sensor source electrode
62
and a sensor drain electrode
63
are formed over the sensor silicon layer
41
, and a switch source electrode
66
and a switch drain electrode
67
are formed over the switch silicon layer
42
. A first capacitor electrode
24
integrally protrudes from the sensor gate line
21
toward the unit pixel region “P.” The sensor source electrode
62
is connected with the sensor data line
61
, and the sensor drain electrode
63
is spaced apart from the sensor source electrode
62
with the sensor gate electrode
22
centered therebetween. The switch source electrode
66
is connected with the switch data line
65
, and the switch drain electrode
67
is spaced apart from the switch source electrode
65
with the switch gate electrode
26
centered therebetween. A second capacitor electrode
64
is formed between the switch drain electrode
67
and the sensor drain electrode
63
and is interconnecting therewith. The second capacitor electrode
64
overlaps the first capacitor electrode
24
.
A second insulating layer
70
covers the sensor source electrode
62
, the sensor drain electrode
63
, the second capacitor electrode
64
, the switch source electrode
66
, and the switch drain electrode
67
. On the second insulating layer
70
, a shielding pattern
80
made of an opaque material is formed over the switch silicon layer
42
.
For the above-described optical detecting sensor according to the prior art, the sensor silicon layer
41
preferably has a thickness larger than 3000 Å (angstrom) to provide high efficiency. Accordingly, since the switch TFT “T
2
” is formed by the same fabrication process of forming the sensor TFT “T
1
,” the thickness of the switch silicon layer
42
is also preferably larger than 3000 Å. Although the preferably large thickness of the silicon layer provides for high efficiency of the sensor TFT “T
1
,” the large thickness increases off current of the switch TFT “T
2
”, thereby causing noise.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a TFT type optical detecting sensor that substantially obviates one or more of problems due to limitations and disadvantages of the prior art.
An object of the present invention is to provide an improved TFT type optical sensor wherein silicon layers of sensor TFT and switch TFT have different thicknesses to achieve high efficiency of the sensor TFT and to decrease off current of the switch TFT.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a thin film transistor optical detecting sensor including an array substrate comprising a transparent substrate, a plurality of sensor thin film transistors disposed on the transparent substrate, each having a first silicon layer of a first thickness, a plurality of storage capacitors, each connected with a corresponding one of the plurality of sensor thin film transistors, storing charges of an optical current, and a plurality of switch thin film transistors, each having a second silicon layer of a second thickness less than the first thickness.
In another aspect, a method of fabricating a thin film transistor optical sensor includes steps of forming a first metal layer on a substrate, the first metal layer includes a sensor gate electrode, a switch gate electrode, and a first capacitor electrode, forming a first insulating layer on the first metal layer, forming an amorphous silicon layer and an etch stop la
Abraham Fetsum
LG. Philips LCD Co. Ltd.
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