Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2000-08-31
2003-10-07
Zarabian, Amir (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S059000, C359S199200
Reexamination Certificate
active
06630687
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wiring structure of an integrated circuit that is formed on a substrate having an insulating surface by using thin film transistors (hereinafter called TFTs). The invention also relates to a wiring structure of a liquid crystal display device of a peripheral circuits integration type that is formed on a substrate having an insulating surface by using TFTs.
2. Description of the Related Art
A technique is known in which a silicon film having crystallinity is formed on a glass substrate or a quartz substrate and a TFT is formed by using the silicon film thus formed. This type of TFT is called “high-temperature polysilicon TFT” or “low-temperature polysilicon TFT.”
The high-temperature polysilicon TFT is formed on a highly heat-resistant substrate such as a quartz substrate because in order to obtain a crystalline silicon film with an active layer heating at 800° C. to 900° C. is required. On the other hand, the low-temperature polysilicon TFT is formed on a substrate that is relatively low in heat resistance such as a glass substrate by a process requiring less than 600° C.
A high-temperature polysilicon TFT has the advantage that TFTs similar in characteristics can easily be integrated *i
25
on a substrate, and that it can be manufactured by utilizing the various process conditions and manufacturing apparatuses of conventional IC processes. On the other hand, a low-temperature polysilicon TFT has the advantage that a glass substrate may be used that is inexpensive and can easily be increased in size (large-area substrate).
According to current technologies, there are no large differences in characteristics between a high temperature polysilicon TFT and the low-temperature polysilicon TFT. Both types of TFTs provide mobility values of about 50 to 100 cm
2
/Vs and S-values of about 200 to 400 mV/dec (V
D
=1 V).
Techniques of producing a liquid crystal display device where integrated circuits, an active matrix circuit, and peripheral circuits for driving the active matrix circuit are formed on the same substrate (also known as a peripheral circuits integration type liquid crystal display device) are now being studied.
However, the characteristics of conventional high-temperature polysilicon and low-temperature polysilicon TFT are much poorer than those of a MOS transistor formed on a single crystal silicon wafer. Typically, a MOS transistor formed on a single crystal silicon wafer yields an S-value of 60 to 70 mV/dec.
Furthermore, in both high-temperature polysilicon and low-temperature polysilicon TFTs according to the current technologies, because of low mobility, the driving frequency of such TFTs is obliged to be less than several megahertz.
For example, where peripheral circuits of a liquid crystal display device are formed by using high-temperature or low-temperature polysilicon TFTs, it is impossible to directly input, (to drive the TFTs), a clock signal or a video signal of more than tens of megahertz necessary for display.
For the above reason, a plurality of wiring lines (interconnections) are used to transmit clock signals or video signals and the clock signals or video signals are supplied to the TFTs in such a manner as to be reduced in frequency (called divisional driving). For example, a 10-MHz frequency of an original clock signal is divided into 2.5 MHz by using four wiring lines. The respective TFTs are driven at this low frequency. This increases the number of wiring lines and the number of TFTs, resulting in problems such as increased installation area.
The present inventors have developed a TFT which exhibits performance equivalent to that of a MOS transistor formed on a single crystal silicon wafer though it uses a crystalline silicon film.
Such a TFT uses a crystalline silicon film having a crystal structure that is continuous in a predetermined direction, for instance, in the source-drain direction as well as having grain boundaries extending in the same, predetermined direction.
This type of crystalline silicon film is obtained by introducing a very small amount of a metal element (for instance, nickel) for accelerating crystallization into an amorphous silicon film, then heating the amorphous silicon film at 500° to 630° C. (for instance, 600° C.) to cause lateral crystal growth, and thereafter forming a thermal oxidation film.
Having much superior characteristics such as an S-value of smaller than 100 mV/dec and mobility of higher than 200 cm
2
/Vs, this type of TFT, in itself, can be driven at tens to hundreds of megahertz or even higher frequencies. By using this type of TFT, TFTs capable of being driven at high speed can be integrated on a large-area substrate.
As a result, not only can circuits having much superior performance be obtained but also the numbers of thin-film transistors and wiring lines necessary or driving can be reduced to a large extent from the conventional cases, thereby greatly contributing to miniaturization and increase in the degree of integration of devices.
However, where an integrated circuit is formed by using TFTs over such a large area as a several centimeter square to a tens of centimeter square as in the case of the peripheral circuits integration type active matrix liquid crystal display device, the rounding of high-frequency signals that are transmitted by wiring lines becomes a very serious problem when such integrated circuit is driven at a high frequency such as tens to hundreds of megahertz or higher.
This problem will be described below for peripheral circuits of a liquid crystal display device.
FIG. 5
is a top view of a peripheral circuits integration type active matrix liquid crystal display device.
As shown in
FIG. 5
, an opposed substrate
902
having an opposed electrode (not shown) on its inside surface is opposed to a substrate
901
with liquid crystal material (not shown) interposed in between.
A data lines (source lines) driving peripheral circuit
903
, a scanning lines (gate lines) driving peripheral circuit
904
, and an active matrix display section
905
in which respective pixels are provided with pixel electrodes and switching TFTs that are connected to the respective pixel electrodes are provided on the substrate
901
.
A flat cable
906
, which extends from external circuits to supply signals to the liquid crystal display device, is electrically connected to peripheral wiring lines
907
at an end portion of the substrate
901
. The peripheral wiring lines
907
are connected to wiring lines
9
and
909
in the peripheral circuits
903
and
904
. The peripheral wiring lines
907
and the wiring lines
908
and
909
in the peripheral circuits
903
and
904
are arranged parallel or approximately parallel with each other.
The wiring lines
907
to
909
are formed as thin films of a conductive material such as aluminum, and at the same time as the TFTs of the peripheral circuits
903
and
904
and the active matrix circuit of the display section
905
.
Part of the wiring lines
907
to
909
are used for transmitting a signal of a very high frequency, for instance, more than 10 MHz. Typical examples of those wiring lines are a video signal line for transmitting a video signal and a clock signal line for supplying a clock signal.
In general, the clock signal frequency amounts to about 12.5 MHz in the case of VGA (640×480×3 (three colors of RGB) pixels), and the video signal frequency increases with the clock signal frequency, such as when the image resolution becomes higher.
In particular, in the peripheral circuits integration type liquid crystal display device, the peripheral circuits
903
and
904
which drive the display section
905
which might be several centimeter square to a tens of centimeter square are usually provided alongside display section
905
and hence have similar length.
Each of the peripheral circuits
903
and
904
has wiring lines that extend from one end to the other within the circuit. The clock signal line and the video signal line are examples of such
Koyama Jun
Ogata Yasushi
Ohtani Hisashi
Yamazaki Shunpei
Fish & Richardson P.C.
Rose Kiesha
Semiconductor Energy Laboratory Co,. Ltd.
Zarabian Amir
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