Semiconductor device manufacturing: process – Chemical etching
Reexamination Certificate
2001-11-08
2003-02-25
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Chemical etching
Reexamination Certificate
active
06524958
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to bottom gate thin film transistors (TFTs) for use in liquid crystal display (LCD) devices and organic electroluminescent display devices. The invention also relates to methods of producing the TFTs.
2. Description of the Prior Art
Hydrogenated amorphous silicon (a-Si:H) TFTs are widely utilized for switching elements in liquid crystal display devices since they can be accurately fabricated on inexpensive glass substrates by a low temperature process.
There are two types of structures for a-Si TFTs, a top gate structure and a bottom gate structure. The top gate structure has a drawback that the interface between an a-Si:H thin film, which is later formed into the channel, and a gate insulating film is often contaminated during the fabrication. In contrast, the bottom gate structure is advantageous in that, since the a-Si:H thin film and the gate insulating film are successively produced without being exposed to atmosphere, the performance of the TFTs is not degraded by the contamination and the electron mobility is larger than the top gate TFTs. Thus, for switching elements in liquid crystal display devices or the like, bottom gate TFTs are advantageous. Bottom gate TFTs have two major types, etch stopped type and channel etch type (also referred to as back channel etch type). Channel etch type TFTs require less photomasks in the fabrication process than etch stopped type TFTs. This makes channel etch type bottom gate TFTs advantageous in terms of manufacturing cost, and therefore, channel etch type bottom gate TFTs have increasingly been favored recently.
FIGS. 7A
to
7
F show the production process of a channel etch type bottom gate TFT, each of the cross sectional views illustrating a step of the production process. With reference to these figures, a fabrication procedure of a channel etch type bottom gate TFT is described below.
A gate electrode
62
is deposited on an insulating substrate
61
to a thickness of 200 nm by sputtering and thereafter patterned into an island by photolithography and etching (FIG.
7
A). Typically, the gate electrode
62
is made of an aluminum film, or a layered film made of an aluminum film and a film of a metal having a high melting point, such as titanium.
Subsequently, an SiN
x
film, which serves as a gate insulating film
63
, is deposited to a thickness of 300 nm by plasma enhanced chemical vapor deposition (PECVD), and thereafter, without exposing the surface to atmosphere, an a-Si:H film, which serves as a high resistivity semiconductor film
64
, is deposited to a thickness of 200 nm by PECVD. Then, an n+ a-Si:H film, which serves as a low resistivity semiconductor film
65
, is deposited to a thickness of 20 nm by PECVD. Thereafter, stacked layers of the high resistivity semiconductor film
64
and the low resistivity semiconductor film
65
are processed into an island by photolithography and etching (FIG.
7
B).
Subsequently, a source/drain electrode metal
66
is deposited by sputtering (FIG.
7
C). Thereafter, a resist
67
is coated (FIG.
7
D), and an opening is formed by photolithography and etching in a portion thereof which is located above the channel region. Thereafter, the low resistivity semiconductor film
65
is etched with the use of the same resist pattern to form a back channel (FIG.
7
E).
This step is generally referred to as a channel etch step.
Subsequently, in order to protect the back channel exposed by the etching, a silicon nitride film serving as a passivation film
68
is deposited to a thickness of 300 nm by CVD. Finally, an opening
69
for connecting a pixel electrode is opened in a predetermined position in the passivation film
68
by photolithography and etching. Thus, a TFT is completed.
SUMMARY OF THE INVENTION
However, the foregoing prior art method has at least the following problems. In the prior art method, due to the fact that the etching selective ratio between the low resistivity semiconductor film
65
and the high resistivity semiconductor film
64
is small, overetch is caused in the channel etch step and thereby the high resistivity semiconductor film
64
is considerably etched in addition to the low resistivity semiconductor film
65
(FIG.
7
E). When such overetch occurs, hydrogen in the back channel, which is composed of an a-Si:H film, is lost, and etching damage is caused by which the film uniformity with respect to the vertical orientation of the film is degraded. The etching damage deteriorates various TFT characteristics. For example, the field effect mobility of the TFT decreases to about half.
To reduce the etching damage to the back channel, if the film thickness of the high resistivity semiconductor film
64
is in the time required for the film deposition correspondingly increase reducing efficiency in production. On the other hand, if the deposition rate is increased and the deposition time is thereby reduced, film quality is degraded. In other words, both of the approaches have problems; the former increases fabrication cost due to an increase in production tact time, whereas the latter degrades production yields and TFT characteristics. Therefore, the prior art method cannot achieve efficient production of bottom gate TFTs which sufficiently function in high resolution display devices in which moving pictures are displayed.
Accordingly, it is a first object of the present invention to provide a method of producing a bottom gate TFT that solves the foregoing and other problems in the prior art. It is a second object of the invention to provide a liquid crystal display device and an organic electroluminescent display device to which the manufacturing method of the invention is applied. These and other objects are accomplished, in accordance with the present invention, by the following embodiments which include a range of aspects.
Embodiment 1
According to a first aspect of the invention, there is provided a method of producing a bottom gate thin film transistor, comprising the steps of:
forming a gate electrode on an insulating substrate;
forming a gate insulating film over the gate electrode;
forming a first semiconductor thin film for a channel over the gate insulating film;
forming a second semiconductor thin film for a source and a drain over the first semiconductor thin film;
processing stacked layers of the first semiconductor thin film and the second semiconductor thin film so as to be formed into an island;
subsequent to the step of processing stacked layers, depositing a source/drain electrode metal over the stacked layers of the first semiconductor thin film and the second semiconductor thin film;
etching a region of the deposited source/drain electrode metal, the region being located above the channel, in the depth direction to expose the second semiconductor thin film, whereby a source electrode and a drain electrode are formed; and
etching away the exposed portion of the second semiconductor thin film in the depth direction with the use of a non-ionic excited species to form a channel.
In this fabrication method, non-ionic excited species are used in the etching of the second semiconductor thin film for the source and drain (so-called channel etch). The use of non-ionic excited species reduces etching damage to the back channel because the excited species are not accelerated by electric field. Therefore, the production yield increases, and reliability in product quality of the produced channel etch bottom gate TFTs remarkably improves.
According to a second aspect of the invention, the fabrication method of the first aspect may be such that the non-ionic excited species is generated by bringing molecules of a chemical substance into contact with a metal heated by electric resistance heating to decompose the molecules of the chemical substance.
This fabrication method utilizes a catalytic CVD technique and makes it possible to produce a large amount of non-ionic excited species with simple equipment. In addition, in this method (contact-decomposition reaction method),
Sakai Masahiro
Terauchi Masaharu
Le Thao P
Nelms David
Parkhurst & Wendel L.L.P.
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