Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-11-05
2003-05-13
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S159000
Reexamination Certificate
active
06562669
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device including a circuit constituted by thin film transistors (hereinafter referred to as TFTs) and a method of manufacturing the same. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel and an electronic instrument incorporating such an electro-optical device as a part.
Note that, the “semiconductor device” in the present specification indicates any device which can function by using semiconductor characteristics, and any of an electro-optical device, a semiconductor circuit and an electronic instrument is a semiconductor device.
2. Description of the Related Art
In recent years, attention has been paid to a technique for constructing a thin film transistor (TFT) by using a semiconductor thin film (thickness of about several nm to several hundreds of nm) formed on a substrate having an insulating surface. The thin film transistor is widely used for an electric device such as an IC or an electro-optical device, and particularly its development as a switching element of an image display device has been hastened. The thin film transistor includes a top gate type TFT and a bottom gate type TFT.
Since the bottom gate type TFT is little influenced by impurity diffusion from a substrate to a semiconductor layer as compared with the top gate type TFT, its reliability is high. A general structure thereof is such that an impurity region overlaps with a gate electrode.
The development of a semiconductor device including a large area integrated circuit formed of these TFTs has been advanced.
An active matrix type liquid crystal display device, an EL display device, and a direct-contact image sensor are known as its typical examples. Especially, since a TFT (hereinafter referred to as a polysilicon TFT) using a crystalline silicon film (typically a polysilicon film) as an active layer has a high field effect mobility, it is also possible to form circuits having various functions.
For example, in the active matrix type liquid crystal display device, in every function block, a pixel circuit for carrying out an image display, and a driving circuit for controlling the pixel circuit, such as a shift register circuit using a CMOS circuit as a base, a level shifter circuit, a buffer circuit, and a sampling circuit, are formed on one substrate.
In the pixel circuit of the active matrix type liquid crystal display device, a TFT (pixel TFT) is disposed in each of several tens to several millions of pixels, and a pixel electrode is provided at each of the pixel TFTs. A counter electrode is provided on a counter substrate side with a liquid crystal sandwiched therebetween, and forms a kind of capacitor using the liquid crystal as a dielectric. A voltage applied to each pixel is controlled by a switching function of the TFT, and an electric charge to the capacitor is controlled, so that the liquid crystal is driven and the amount of transmitted light is controlled to display an image.
The pixel TFT is made of an n-channel type TFT, and, as a switching element, applies the voltage to the liquid crystal to drive it. Since the liquid crystal is driven by an alternating current, a system called frame inversion driving is often adopted. In this system, in order to suppress consumed electric power to be low, it is important that characteristics required for the pixel TFT are such that an off current value (drain current flowing when the TFT is in an off operation) is made sufficiently low.
SUMMARY OF THE INVENTION
The present invention provides a technique for solving such problems, and an object of the present invention is to improve operation characteristics of a semiconductor device and to reduce consumed electric power in an electro-optical device typified by an active matrix type liquid crystal display device manufactured by using TFTs and in a semiconductor device.
Particularly, an object of the present invention is to obtain a structure of a pixel TFT (n-channel type TFT) in which an off current value is sufficiently low, and the ratio of an on current value to the off current value is high.
As shown in
FIG. 1A
, the present invention is characterized by including a region
102
a
having such a concentration gradient in an impurity region
102
that as a distance (distance in a channel length direction) from a channel formation region becomes larger, a concentration of an impurity element imparting one conductivity type is increased.
That is, the invention is characterized by including an impurity region in which as a distance from an end portion of a gate electrode
105
becomes larger toward a peripheral portion (peripheral portion in a section cut in the channel length direction) of a semiconductor layer in the channel length direction, the concentration of the impurity element (phosphorus) is gradually increased. Accordingly, in the impurity region, its electric resistance is large at the side of the channel formation region, and is low at the side of the peripheral portion of the semiconductor layer.
Further, in the present invention, since the concentration is gradually increased in the impurity region, there is no definite boundary, and in the present specification, a region in the impurity region
102
in which its impurity concentration is 1×10
20
/cm
3
or higher is called a drain region
102
b.
Further, although the drain side has been explained in the above, it is desirable that the source side also has the same structure. In the impurity region at the source side, there is formed a region
103
a
having such a concentration gradient that as the distance from the channel formation region becomes larger, the concentration of the impurity element imparting one conductivity type is increased. Further, in the present specification, in the source side impurity region, a region having an impurity concentration of 1×10
20
/cm
3
or higher is called a source region
103
b.
The present invention is characterized in that the regions
102
a
and
103
a
having such concentration gradients are intentionally formed to realize a TFT in which an off current value is considerably low and the ratio of an on current value to the off current value is high. The gate electrode
105
overlaps with the channel formation region
101
and the impurity region
102
a
through a gate insulating film
104
. Note that, in
FIG. 1A
, reference numeral
100
designates a substrate having an insulating surface;
108
, an interlayer insulating film;
109
, a source electrode; and
110
, a drain electrode. Further, as shown in
FIG. 7
, a structure may be adopted in which a gate electrode does not overlap with an impurity region.
In a conventional TFT structure, as shown in
FIG. 13
, there is a boundary due to a definite concentration difference, and the concentration is different like a staircase among a channel formation region
1
, a low concentration impurity region
2
, and a high concentration impurity region
3
. That is, there is a discontinuous concentration distribution. In addition, the concentration in each region is almost constant. Accordingly, in the prior art, since the concentration difference at the boundary between the high concentration impurity region
3
and the low concentration impurity region
2
, and the concentration difference at the boundary between the low concentration impurity region
2
and the channel formation region
1
are relatively large, high electric field concentration is generated in the vicinity of each of the boundaries.
Since the off current flows by quantum mechanical effects such as tunneling between bands, it is conceivable that the off current is mainly effected by an electric field. Accordingly, the electric field concentration generated at the boundary causes the off current value of the TFT to increase. Especially, in the conventional TFT structure, there has been a problem in that a high electric field is concentrated by the large concentration difference at the boundary between the chan
Suzawa Hideomi
Tsunoda Akira
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Isaac Stanetta
Niebling John F.
Semiconductor Energy Laboratory Co,. Ltd.
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