Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2000-12-15
2003-05-06
Coleman, William David (Department: 2823)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S906000
Reexamination Certificate
active
06558987
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1999-59464, filed on Dec. 20, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thin film transistors (TFT) of the type used in liquid crystal display (LCD) devices. More particularly, it relates to the insulation layers of such TFTs.
2. Discussion of the Related Art
A liquid crystal display (LCD) device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce a predetermined image. Liquid crystal molecules have a definite orientation that results from their peculiar characteristics. The specific orientation can be modified by an electric field that is applied across the liquid crystal molecules. In other words, electric fields applied across the liquid crystal molecules can change the orientation of the liquid crystal molecules. Due to optical anisotropy, incident light is refracted according to the orientation of the liquid crystal molecules.
In general, liquid crystal display (LCD) devices use thin film transistors (TFTs) as switching elements that control the electric fields applied across the liquid crystal molecules.
An active matrix LCD (AM-LCD) incorporates a matrix of thin film transistors (TFTs) and pixel electrodes. Active matrix LCD (AM-LCD) are beneficial in that they can have high resolution and can be superior to alternative types when displaying moving images.
FIG. 1
is a cross-sectional view illustrating a conventional active matrix liquid crystal display (LCD) panel. As shown in
FIG. 1
, the LCD panel
20
has a lower substrate
2
, an upper substrate
4
, and an interposed liquid crystal layer
10
. The lower substrate
2
, which is referred to as an array substrate, has a TFT “S” that acts as a switching element to change the orientation of the liquid crystal molecules in the liquid crystal layer
10
. As shown, the TFT “S” contacts a pixel electrode
14
. The upper substrate
4
includes a color filter
8
that produces a color pixel image, and a common electrode
12
on the color filter
8
. The common electrode
12
serves as a corresponding electrode for the pixel electrode
14
. Together, voltages applied to those electrodes produce an electric field across the liquid crystal layer
10
. The pixel electrode
14
is arranged over a pixel region, i.e., a display area. Furthermore, to prevent leakage of the liquid crystal layer, interposed between the substrates
2
and
4
is a sealant
6
.
The principle of operating the AM-LCD device will now be explained. When a gate electrode
30
receives a gate signal that causes the TFT to turn ON, the data signals on a data line are applied to the pixel electrode
14
. This induces an electric field across the liquid crystal molecules and causes those molecules to attain an orientation that depends on the data signals. On the other hand, when the gate electrode
30
receives a gate signal that causes the TFT to turn OFF, data signals are not applied to the pixel electrode
14
. This removes the electric field and causes the liquid crystal molecules to attain a “relaxed” orientation.
In general, the manufacturing process for an active matrix liquid crystal display (LCD) panel depends on the materials used and on the design goal. For example, the resistivity of the materials used for the gate and data lines are important factors in the picture quality of large (say over 18 inch) LCD panels and of high resolution (for example SXGA or UXGA) LCD panels. With such LCD panels it is beneficial to use Aluminum (Al) or an Al-alloy for the gate lines and data lines.
LCD devices usually use inverted staggered type TFTs because such TFTs have a relatively simple structure and a superior efficiency. Inverted staggered type TFTs can be classified as either back channel etched (EB) or etch stopped (ES), depending on the fabrication method used. The manufacturing method of a back channel etched type TFT will now be explained.
FIG. 2
is a cross-sectional view of a back channel etched type TFT “S” as used in a conventional LCD device. As shown in
FIG. 2
, the TFT “S” includes: a substrate
1
; a gate electrode
30
on the substrate
1
; a gate insulation layer
32
over the substrate and over the gate electrode
30
; an active layer
34
on the gate insulation layer
32
; a source electrode
38
and a drain electrode
40
that are spaced apart from each other and that overlap sides of the active layer
34
; and an ohmic contact layer
36
that is disposed between the active layer
34
and the source and drain electrodes
38
and
40
.
The gate insulation layer
32
can be formed at relatively low temperature (below 350°). A material having superior insulation properties, such as Silicon Nitride (SiN
x
) or Silicon Oxide (SiO
2
), is beneficially used for the gate insulation layer
32
. Moreover, pure amorphous silicon (a-Si:H) is beneficially used to form the active layer
34
. Such silicon can also be formed at the relatively low temperatures used to form the gate insulation layer
32
. After forming the active layer, the ohmic contact layer
36
is formed using a doped amorphous silicon (such as n
+
a-Si:H). To dope the amorphous silicon, a doping gas having Boron or Phosphorous can be used. In a conventional LCD device, Phosphine (PH
3
), which includes Phosphorous, is generally used. The source and drain electrodes
38
and
40
are beneficially comprised of Chrome or Molybdenum.
FIG. 3
is a flow-chart illustrating the manufacturing process steps of the conventional TFT illustrated in FIG.
2
.
In the first step, ST
200
, a glass substrate is provided and cleaned. That cleaning removes debris particles, organic materials, and other alien substances from the substrate. This reduces defects and enhances the overall properties of the resulting TFT. Furthermore, it improves adhesion between the substrate and subsequent layers, include a subsequent metal layer.
In the second step, ST
210
, a metal such as Aluminum or Molybdenum is deposited. Then, using a lithography process the gate electrode and a first capacitor electrode, which are portions of a gate line, are formed.
In the third step, ST
220
, the gate insulation layer and the semiconductor layers (the active layer and the ohmic contact layer) are sequentially formed. The gate insulation layer is beneficially comprised of Silicon Oxide or Silicon Nitride, and has a thickness of about 3000 Å.
In the fourth step, ST
230
, the source and drain electrodes are formed by depositing and patterning a metallic material such as Chrome (Cr) or Cr-alloy.
In the fifth step, ST
240
, a channel region is formed by removing the portion of the doped amorphous silicon (ohmic contact layer) between the source and drain electrodes. In this step, the source and drain electrodes are used as masks. If the portion of the ohmic contact layer between the source and drain electrodes is not removed, serious problems of deteriorated electrical characteristics and lower efficiencies can result in the final TFT.
Generally, the process steps of fabricating the TFT include a step of forming the gate insulation layer, a step of forming the semiconductor layers, and a step of forming the electrodes. The forming of the gate insulation layer and the forming of the semiconductor layers are beneficially performed in the same apparatus.
A CVD (chemical vapor deposition) apparatus is generally used to form the gate insulation layer. A mixture of various gases is injected into the CVD apparatus, which is kept in a vacuum. The gate insulation layer is then formed by chemical reactions of the injected gases. For example, an inert gas (Helium: He), a silicon compound gas (Silane: SiH
4
), and a nitrogen compound gas (Ammonia: NH
3
) can be reacted together in the CVD apparatus to form a Silicon Nitride (SiN
x
) gate insulation layer. However, polymer materials are inevitably created in the CVD apparatus. When forming the insulation layer, those polymer materials can act
Coleman William David
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
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