Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2001-12-05
2003-04-01
Elms, Richard (Department: 2824)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S072000, C438S151000, C438S166000
Reexamination Certificate
active
06541795
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the structure of a thin film insulated gate type semiconductor device (thin film transistor, or TFT) formed on an insulating surface and a production method for the same. A semiconductor device according to the present invention is used for active matrices of liquid crystal displays or the like, driving circuits of image sensors or the like, SOI integrated circuits and conventional semiconductor integrated circuits (microprocessors, microcontrollers, microcomputors or semiconductor memories, etc).
BACKGROUND OF THE INVENTION
In recent years, much research has been being carried out into forming an insulated gate type semiconductor device (MISFET) on an insulating substrate or on a surface separated from a semiconductor substrate by a thick insulating film (an insulating surface). In particular, a semiconductor device in which a semiconductor layer (active layer) is of a thin film form is called a thin film transistor (TFT). In such a semiconductor device, it is difficult to obtain an element having such good crystallinity as that of a single crystal semiconductor, and usually a non-single crystal semiconductor which is not single crystal but has crystallinity has been employed.
Such a non-single crystal semiconductor has inferior characteristics as compared with a single crystal semiconductor. In particular, there has been the problem that when a reverse voltage (that is, a negative voltage in case of an N channel type TFT and a positive voltage in case of a P channel type TFT) is applied to gate electrodes, a leak current between a source and a drain is increased. This problem has been fatal particularly when a TFT is used for a switching transistor of an active matrix circuit.
It has been reported that this problem can be solved by thinning a semiconductor layer (active layer) in which channels are formed in a TFT. For example, Hisao Hayashi et al report in Jpn. J. Appl. Phys. vol. 23 (1984) L819 that they studied how the characteristics of a TFT were affected when the thickness of an active layer of crystalline silicon was changed from 100 Å to 1000 Å and obtained the desirable characteristic that as the active layer gets thinner, electric field effect mobility increases, and threshold voltage and leak current decrease.
According to this report, however, the electric field effect mobility was very low, 10 cm
2
/Vs at maximum, and accordingly while the above TFT could be used for a switching transistor for an active matrix circuit, it was impossible to use the TFT for a circuit for driving the active matrix circuit. According to this report, a film obtained in an as-depo state was utilized for a crystalline silicon film, and it did not have preferred crystallinity.
On the other hand, a method in which crystal growth is effected by heat annealing (solid phase growth method, SPC) and a method in which crystallization is carried out through a liquid phase state or in a solid phase by irradiating with a laser or strong light equivalent to a laser (photo annealing) has been known as methods for obtaining a semiconductor film having good crystallinity from a non-single crystal semiconductor. For example, to obtain a silicon film from non-single crystal silicon by heat annealing it has been necessary to heat an amorphous silicon film at 500 to 650° C.
However, because of influences exerted by the substrate (including a base), it has not been possible to obtain good crystallinity by these methods without using a silicon film having a thickness of at least 500 Å.
SUMMARY OF THE INVENTION
The present invention has been made in view of these kinds of problem, and an object thereof is to provide a TFT with which better characteristics can be obtained using a good crystalline silicon film.
Another object is to provide a preferable constitution of a semiconductor integrated circuit produced using a TFT having such good characteristics.
The present invention is characterized in that after an amorphous semiconductor film having a thickness of 400 Å or more is crystallized by heat annealing or photo annealing or the combined use thereof, this is wholly or selectively etched to prepare a thin crystalline semiconductor film having a thickness of 300 Å or less and this is used as an active layer (a part where channel-forming regions are formed, that is, a part on which gate electrodes are formed) in a TFT.
The present invention is characterized by the thickness of an active layer, and hereinafter the thickness means the average thickness of the prescribed region unless otherwise indicated. In a polycrystalline material, irregularities are formed thereon by the presence of grain boundaries or the like and the film thickness is sometimes abnormally small or large in places for some reason. However, since such abnormal parts will not exert influences on elements and the whole circuit, they may be ignored. It is for such reasons that attention is paid to the average thickness of the specific parts in the present invention.
The present invention is characterized as well in that the crystallinity of the semiconductor film of the active layer is excellent, and it is different from a conventional TFT in that sense. However, it is very difficult to discuss objectively crystallinity. Accordingly, excellence in crystallinity of a semiconductor film will be evaluated by the electric field effect mobility of a TFT produced therewith. While the electric field effect mobility varies depending on the gate voltage and other conditions, the maximum value thereof is considered to reflect objectively the crystallinity of the active layer in the TFT, and therefore it is suited for the evaluation. In the present invention, there can be obtained a silicon film having crystallinity sufficient to obtain the characteristic of a maximum electric field effect mobility of typically 50 cm
2
/Vs or more, preferably 100 cm
2
/Vs or more, and having a thickness of 300 Å or less.
In the present invention, two methods can be employed for the etching process described above when silicon is used as a semiconductor. The first method is characterized in that a process in which a silicon film is slightly oxidized to form a silicon oxide film and this is etched is repeated as many times as necessary. This method is excellent in controllability of etching depth as compared with a method in which a silicon film is directly dissolved by etching.
Oxidation can be carried out by head oxidation, anodic oxidation or an oxidizing agent in order to carry out oxidation in the above process. Since the thickness of the silicon film oxidized is determined by temperature or voltage and time in heat oxidation or anodic oxidation, it can very uniformly be controlled even when a large substrate is processed. The case where an oxidizing agent is used is the same. When an oxidizing agent is used, solutions of nitric acid, hydrogen peroxide, perchlorate and permanganate can be used as the oxidizing agent. For example, a mixed solution of hydrogen peroxide and ammonia can carry out oxidation very stably.
After forming a thin silicon oxide film by the above method, the silicon oxide is etched, the silicon oxide film formed on the surface is etched by exposing the silicon film to an etchant which does not etch silicon (for example a solution of a hydrogen fluoride such as 1% hydrofluoric acid or the like). This results in causing the silicon film to get thin only by the oxidized part. The problem with this method is that the necessity to repeat the process means that a longer time is taken as the depth to be etched becomes greater.
The second method is a method in which etching is carried out using a solution containing a component also positively etching silicon oxide in addition to an oxidizing agent. It is different from the first method in that the process is finished in one stage, and accordingly it is excellent in terms of it suitability for mass production. Solutions prepared by adding hydrofluoric acid to an oxidizing agent such as hydrogen peroxide or nitri
Kusumoto Naoto
Ohtani Hisashi
Takemura Yasuhiko
Elms Richard
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Wilson Christian D.
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