Process for manufacturing thin film transistors

Details Process for manufacturing thin film transistors Process for manufacturing thin film transistors

1999-05-24

2001-07-17

Fourson, George (Department: 2823)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

On insulating substrate or layer

C438S158000

Reexamination Certificate

active

06261880

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for manufacturing thin film transistors and, more specifically, to a process for manufacturing thin film transistors by using four photomasks.
BACKGROUND OF THE INVENTION
The liquid crystal displays (LCD) are widely applied in electrical products, such as digital watches, calculator, etc. for a long time. Moreover, with the advance of techniques for manufacture and design, the thin film transistors-liquid crystal display (TFT-LCD) is introduced into the portable computers, personal digital assistants, color televisions, and replaced gradually the kinescopes that are used for conventional displays. However, following the design rules of TFT-LCD tend to large scale, there are a lot of problems and challenges, such as low yields and low throughput, in manufacturing and developing TFT-LCD apparatus.
In general, the TFT-LCD comprises a bottom plate on which formed with thin film transistors and pixel electrodes, and a top plate on which constructed with color filters. The liquid crystal molecules are filled between the top plate and the bottom plate. In the operation, a signal voltage is applied to the TFT that is the switching element of each unit pixel. The TFT receives the signal voltage, it turns on so that data voltage carrying image information can be applied to the corresponding pixel electrode and the liquid crystal via the TFT. When the data voltage is applied to the TFT, the arrangement of the liquid crystal molecules is changed, thereby changing the optical properties and displaying the image.
There is a requirement to reduce the photolithography processes in manufacturing TFT devices for decreasing the process cycle time and cost at all times. Namely, it is better to reduce the number of the photomasks used in forming the TFT devices. According to the prior art of manufacturing inverted gate TFTs device for a TFT-LCD, six or more photomasks are needed. As shown in
FIG. 1
, a first metal layer is defined to serve as the gate structure
24
, and an insulating layer
28
is formed on the substrate
22
to cover the gate structure
24
. An a-silicon layer
30
is formed above the insulating layer
28
and the gate structure
24
. An n+ a-silicon layer
32
is formed above the a-silicon layer
30
. Then, an ITO layer
34
is deposited on the insulating layer
28
to form the pixel electrode
26
and connect to the S/D structures formed in latter steps. Then, the S/D structures
38
are formed on the n+ a-silicon layer
32
by patterning the second metal layer, the connection structure
40
are formed on insulating layer
28
and filled into the contact hole
36
simultaneously. Moreover, a passivation layer
42
is formed on the substrate
22
to cover the S/D structures
38
and the a-silicon layer
32
.
It is noted that the TFT device as illustrated in
FIG. 1
is manufactured by using six photomasks. Wherein the first photomask is used to define the gate structure
24
, the second photomask is used to define the a-silicon layer
30
and n+ a-silicon layer
32
, the third photomask is used to define the pattern of the ITO layer
34
, the fourth photomask is used to define the contact hole
36
on the insulating layer
28
, the fifth photomask is used to define the S/D structures
38
and the connection structure
40
, the sixth photomask is used to pattern the passivation layer
42
. However, much process cycle time and cost are required for using six photomasks to manufacture the TFT device.
SUMMARY OF THE INVENTION
The first objective of the present invention is to provide a method for manufacturing a TFT device by using four lithography steps.
The second objective of the present invention is to provide a structure of TFT device with two insulating layer for promoting the yield of the TFT device.
The third objective of the present invention is to provide a method of manufacturing the TFT devices for decreasing the cost and cycle time of the process.
A method for forming a TFT device comprises the following steps. First, a first metal layer is formed on the substrate. Then, a first insulating layer is formed on the first metal layer. A silicon layer is formed above the first insulator layer. A doped silicon layer is formed above the silicon layer. Next, a first photomask is used to define the doped silicon layer, the silicon layer, the first insulating layer, and the first metal layer. Then, a second insulating layer is formed on the doped silicon layer and the substrate, and a transparent conducting layer is formed on the second insulating layer. a second photomask is used to define the second insulating layer and the transparent conducting layer to expose a surface of the doped silicon layer. A second metal layer is formed on the transparent conducting layer and doped silicon layer, and a third photomask is used to define the second metal layer to form the S/D structures. Then, the S/D structures are used to serve as a mask for etching the doped silicon layer. Next, a passivation layer is formed on the second metal layer, the transparent conducting layer and the substrate, and a four photomask is used to define the passivation layer.


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patent: 5824572 (1998-10-01), Fukui et al.
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patent: 6140668 (2000-10-01), Mei et al.
patent: 6156583 (2000-12-01), Hwang
patent: 4344897 (1994-07-01), None

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