Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2002-09-26
2004-08-17
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S030000, C438S034000, C438S058000, C438S066000, C438S096000, C438S097000, C438S143000, C438S149000, C438S166000, C438S310000, C438S378000, C438S471000, C438S482000, C438S488000, C438S585000, C438S778000
Reexamination Certificate
active
06777272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix display device among electronic circuits constituted by using crystalline thin film semiconductors.
2. Description of the Related Art
Since a technique of manufacturing a thin film transistor (hereinafter referred to as TFT) by using an amorphous or crystalline semiconductor film formed on a glass substrate or a quartz substrate has been known, an attempt of applying this technique to an active matrix display circuit has been made. The simplest structure is such that only an active matrix circuit is constituted by the TFTs, and circuits for driving the active matrix circuit, such as a data driver (source driver) and a scan driver (gate driver), are constituted by integrated circuits using single crystal semiconductor.
However, this method requires a technique for connecting a large number of terminals between the active matrix circuit and the respective driver circuits, so that it is disadvantageous in enhancing the integration. On the other hand, there is a proposal in which a driver circuit is also constituted by TFTs in addition to an active matrix circuit (Japanese Patent Publication No. Hei 5-9794 and Japanese Patent Publication No. Hei 2-61032, etc.).
Like the above described structure, an active matrix display device in which a driver circuit and an active matrix circuit are formed on the same substrate is called a monolithic type active matrix display device. When the active matrix display device is the monolithic type, wiring lines required to be connected from the outside of the substrate are limited to those for only power supply, video signals, and synchronous signals, so that the monolithic type is advantageous in integration.
For driving a driver circuit, when using a silicon film as an active layer, a TFT is limited to those which include an active layer of crystalline silicon (polysilicon), and such a TFT is referred to as a high temperature polysilicon TFT or a low temperature polysilicon TFT according to process temperatures of a silicon film.
The high temperature polysilicon TFT is formed by a technique using a heat treatment at a relatively high temperature such as 800° C., 900° C. or more, as means for forming a crystalline silicon film. This technique may be called a derivative technique of IC manufacturing processes using a single crystal silicon wafer. As a substrate on which the high temperature polysilicon TFT is formed, a quartz substrate capable of withstanding the temperature at the heat treatment is naturally used.
On the other hand, the low temperature polysilicon TFT is formed by using an inexpensive glass substrate (its heat-resisting property is naturally inferior to the quartz substrate) as a substrate. In the production of a crystalline silicon film constituting the low temperature polysilicon TFT, there is used a heat treatment at a temperature of not higher than 600° C. against which the glass substrate is able to withstand, or a laser annealing technique which hardly gives thermal damage to the glass substrate.
The technique of manufacturing the high temperature polysilicon TFT has a feature that TFTs having uniform characteristics can be integrated on the substrate. On the other hand, the technique of manufacturing the low temperature polysilicon TFT has a feature that the glass substrate, which is inexpensive and is easily formed to have a large area, can be used as the substrate.
Incidentally, in the technique under the present circumstances, there is no large difference in characteristics between the high temperature polysilicon TFT and the low temperature polysilicon TFT. As a subtle difference, the high temperature polysilicon is superior in uniformity of the production yield and the characteristics in the surface of the substrate, and the low temperature polysilicon is superior in the productivity and the production cost.
In both of the high temperature and low temperature polysilicon TFTs, there have been obtained such characteristics that the mobility is about 50 to 100 (cm2/Vs), and S-value is about 200 to 400 (mV/dec)(VD=IV). The characteristics are such that it is possible to realize a high speed operation higher than a TFT using amorphous silicon by a factor of about double figures. However, the characteristics are largely inferior to those of a MOS transistor using a single crystal silicon wafer. In general, the S-value of the MOS transistor using the single crystal silicon wafer is about 60 to 70 (mV/dec), and the operation frequency thereof is higher than that of the high temperature polysilicon TFT or the low temperature polysilicon TFT by a factor of about single figure to double figures.
Since a data driver circuit using the high temperature or low temperature polysilicon TFT having such characteristics has the limit in signal processing capacity, it is necessary to make a specific design for constituting a large scale matrix. For example, if the matrix is a small scale matrix such that the number of pixels (the number of pixel electrodes of an active matrix circuit) is less than fifty thousands, the basic structure shown in
FIGS. 1A and 1B
is sufficient.
FIG. 1A
shows an active matrix circuit
3
, and a scan driver
2
and a data driver
1
for driving the active matrix circuit
3
. The active matrix circuit
3
is connected to the scan driver
2
and the data driver
1
by a large number of wiring lines
5
and
4
. Since these wiring lines are formed at the same time when the above circuits are formed, there is no difficulty in the production. A large number of pixels
6
are disposed in the active matrix circuit
3
, and each of the pixels includes a switching transistor
7
and a pixel electrode
8
. A plurality of switching transistors may be used (FIG.
1
A).
FIG. 1B
shows the details of the data driver circuit. That is, the data driver circuit has such a structure that in accordance with pulses sequentially generated from a shift register, a video signal is sampled by sampling transistors, and the signals are stored by analog memories (capacitors), and when sampling for all rows is ended, analog switches (and analog buffers) are concurrently driven by a latch pulse, and the signals are sent to the active matrix (FIG.
1
B).
For example, if the number of pixels are less than fifty thousands, in order to process the picture image information of thirty frames per one second, the processing speed of the data driver is sufficient when it is fifty thousands (pixels)×thirty (frames/second )=1.5 MHz.
This is a speed which can be handled by the conventional high temperature or low temperature polysilicon TFT. However, if the number of pixels is increased, TFTs cannot follow operation speed. A first method of solving this problem is to provide plural lines of shift registers. For example, two lines of shift registers are provided in parallel to each other, and the respective registers are made to transmit pulses the phases of which are shifted by half a period.
A second method is to provide plural lines of video signals. For example, four lines of video signals are provided, and these are sampled by one shift register, so that an operation speed can be reduced to ¼. An example will be explained with reference to FIG.
9
. When a pulse is generated from a shift register of n-th stage, sampling is carried out by four sampling transistors connected to respective signal lines of video signals
1
to
4
. The subsequent operation is the same as the case of FIG.
1
B. In this way, since one stage of shift register can drive four columns of data lines, when the number of data lines is 4N, it is sufficient that the number of stages of the shift register is N. Thus, as compared with the case of
FIG. 1B
, the operation speed can be reduced to ¼ (FIG.
9
).
In order to adopt such a system, it is necessary to divide the video signal to ¼.
FIG. 10
shows such a circuit which is constituted by four stages of shift registers
1
to
4
. A sampling transistor and an analog memory similar
Koyama Jun
Ogata Yasushi
Yamazaki Shunpei
Isaac Stanetta
Niebling John F.
Semiconductor Energy Laboratory Co,. Ltd.
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