Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1997-07-21
2004-03-30
Wilczewski, Mary (Department: 2822)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S486000, C438S487000
Reexamination Certificate
active
06713330
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a semiconductor device having thin-film transistors (TFTs) formed on an insulating substrate made of glass or the like and also to a method of fabricating such a semiconductor device.
BACKGROUND OF THE INVENTION
Known semiconductor devices having TFTs on an insulating substrate made of glass or the like include active-matrix liquid-crystal displays and image sensors which use such TFTs to activate pixels.
Generally, TFTs used in these devices are made of a silicon semiconductor in the form of a thin film. Silicon semi-conductors in the form of a thin film are roughly classified into amorphous silicon semiconductors (a-Si) and crystalline silicon semiconductors. Amorphous silicon semiconductors are fabricated at low temperatures. In addition, they are relatively easy to manufacture by chemical vapor deposition. Furthermore, they can be easily mass-produced. Therefore, amorphous silicon semiconductors have enjoyed the widest acceptance. However, their physical properties such as conductivity are inferior to those of crystalline silicon semiconductors. In order to obtain higher-speed characteristics from amorphous silicon semiconductors, a method of fabricating TFTs consisting of a crystalline silicon semiconductor must be established and has been keenly sought for. It is known that crystalline silicon semiconductors include polysilicon, silicon crystallites, amorphous silicon containing crystalline components, and semi-amorphous silicon that is midway in nature between crystalline state and amorphous state.
Known methods of obtaining these crystalline thin-film silicon semiconductors include:
(1) During fabrication, a crystalline film is directly created.
(2) An amorphous semiconductor film is once formed. Then, the film is irradiated with laser light so that the energy of the laser light imparts crystallinity to the film.
(3) An amorphous semiconductor film is once formed. Thermal energy is applied to the film to crystallize it.
Where the method (1) above is utilized, it is technically difficult to form a semiconductor film having good physical properties over the whole surface uniformly. Also, the film is formed at a high temperature of over 600° C. and so cheap glass substrates cannot be used. Hence, this method presents problems regarding costs.
In the method (2), an excimer laser is used most commonly today. If this excimer laser is employed, the laser light illuminates only a small area and hence the throughput is low. Furthermore, the stability of the laser is not stable enough to uniformly process the whole surface of a large-area substrate. Therefore, we feel that this method is a technique of the next generation.
The method (3) above can process substrates of larger areas compared with the methods (1) and (2). However, a high temperature exceeding 600° C. is also necessary. It is necessary to lower the heating temperature where cheap glass substrates are used. Especially, liquid crystal displays having larger areas have tended to be manufactured today. With this trend, larger glass substrates have to be employed. Where larger glass substrates are used in this way, shrinkage and stress produced during a heating step that is essential for semiconductor fabrication deteriorate the accuracies of mask alignment and other steps. This presents serious problems. Especially, in the case of Corning 7059 which is most commonly used today, the strain point is 593° C. Therefore, if the prior art heating-and-crystallization step is effected, a large distortion is induced. Besides the problem of temperature, the heating time, i.e., the time required for crystallization, poses problems. In particular, the heating time necessary for crystallization is as long as tens of hours or longer in the present process. Therefore, it is necessary to shorten the heating time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide means for solving the foregoing problems.
It is a more specific object of the invention to provide a method of fabricating a thin film of crystalline silicon semiconductor by forming a thin film of amorphous silicon and heating this film at a lower temperature and in a shorter time than heretofore to crystallize it.
Of course, a crystalline silicon semiconductor fabricated by the manufacturing process according to the invention has physical properties comparable or superior to the physical properties of crystalline silicon semiconductor devices fabricated by the prior art techniques and can be used in active layer regions of TFTs.
We formed amorphous silicon semiconductor films as described above by CVD processes and sputtering processes. These films were heated to crystallize them. We conducted experiments on this method of heating amorphous silicon semiconductor films and discussed the method as follows.
As an experiment, an amorphous silicon film was formed on a glass substrate. This film was crystallized by heating. We discussed the mechanism by which the film was heated and crystallized. Crystals began to grow at the interface between the glass substrate and the amorphous silicon. We observed that where a given film thickness was exceeded, the crystals grew like columns vertical to the substrate surface.
We understand the above-described phenomenon as follows. Crystal nuclei, or seed crystals, exist at the interface between the glass substrate and the amorphous silicon film, and crystals grow from these nuclei. We consider that these crystal nuclei are trace amounts of impurity metal elements existing on the surface of the substrate and the crystalline component of the glass surface. It is considered that crystalline component of silicon oxide (known as crystallized glass) is present on the surface of the glass surface.
Accordingly, we have thought that the crystallization temperature might be lowered by introducing crystal nuclei more positively. To confirm the effects of this temperature drop, we conducted an experiment. That is, a trace amount of other metal was deposited on a substrate. A thin film of amorphous silicon was formed on the metal layer. Then, the amorphous silicon was heated and crystallized. Where some metals are deposited on the substrate, crystallization temperature drop was confirmed. We imagined that crystals were growing from crystal nuclei of foreign substances. We further investigated the mechanism on plural impurity metals which permitted temperature decreases.
A crystallization process can be classified into two phases, i.e., creation of nuclei at the initial stage and crystal growth from the nuclei. The speed of the creation of nuclei at the initial stage can be known by measuring the time taken until microscopic dot-like crystals are created at a constant temperature. Where any of the above-described impurity metals was deposited as a thin film, the time was shortened. This demonstrates that introduction of crystal nuclei lowers the crystallization temperature. We discovered an unforeseen fact. Specifically, the growth of crystal grains subsequent to nucleation was investigated while varying the heating time. Where some metal was deposited as a film and then a thin film of amorphous silicon formed on the metal film was crystallized, crystals grew at an amazing rate after the nucleation. The mechanism of this phenomenon will be described in greater detail later.
In any case, we have discovered that if a trace amount of some metal is deposited as a film, a thin film of amorphous silicon is formed on the metal film, and then the amorphous silicon film is heated and crystallized, then sufficient crystallization is caused by the above-described two effects at a temperature lower than 580° C. in a time of about 4 hours, which would have never been conceived heretofore. The material which showed the most conspicuous effects and we have selected out of impurity metals exhibiting such effects is nickel.
We now give examples of structure, illustrating the effect of nickel. A substrate made of Corning 7059 was not treated at all. That is, a thin film consisting of a trace
Teramoto Satoshi
Zhang Hongyong
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Wilczewski Mary
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