Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1996-10-23
2002-09-03
Sherry, Michael J. (Department: 2829)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S487000
Reexamination Certificate
active
06444506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a thin-film transistor using a crystalline silicon film.
The invention also relates to a process to crystallize or improve the crystallinity of an amorphous silicon film or a crystalline silicon film formed on an insulating substrate such as a glass substrate by laser annealing.
The invention also relates to threshold voltage control on a thin film transistor that is formed by using a crystalline silicon film.
2. Description of the Related Art
In recent years, extensive studies have been made of the techniques of crystallizing or improving the crystallinity of an amorphous silicon film or a crystalline silicon film (a silicon film that is not a single crystal but is polycrystalline, microcrystalline, or in some other form of crystallinity), formed on an insulating substrate such as a glass substrate by laser annealing.
Having a large mobility, a crystalline silicon film formed through laser annealing is widely used in a monolithic liquid crystal electrooptical device etc. in which thin-film transistors (TFTs) for a pixel region (pixel driving) and driver circuits are formed by using the crystalline silicon film on a single glass substrate, for instance.
On the other hand, because it can provide high productivity and hence is superior from the industrial viewpoint, a laser annealing method is used by preference. In the laser annealing method, a high-power pulse laser beam emitted from an excimer laser or the like is processed by an optical system to assume a several centimeter square spot or a linear shape of several millimeters in width and several tens of centimeters in length on an irradiation surface and the irradiation surface is scanned with the laser beam (the laser beam illuminating position is moved relatively to the irradiation surface).
In particular, the use of a linear laser beam is advantageous in obtaining high productivity, because in many cases the entire irradiation surface can be subjected to laser light irradiation by scanning in only one direction that is perpendicular to the beam longitudinal direction in l o contrast to the case of using a spot-like laser beam which requires two dimensional scanning.
A TFT using a crystalline silicon film, if a crystalline silicon film constituting the channel forming region is intrinsic, has a general tendency that the threshold voltage is slightly shifted to the negative side of 0 V and the current-rising voltage is about −2 to −4 V in the case of an n-channel transistor. As a result, there is a marked tendency that the TFT has a normally-on state (it is in an on state even if the gate voltage is 0 V).
When a TFT having a normally-on state is used as a switching element, for instance, current flows through it even if the gate voltage is 0 V. The gate voltage needs to be always biased to the positive side to render the switch in an off state. Therefore, a circuit using such TFTs has various problems such as high current consumption and the necessity of a circuit for applying a bias voltage.
To solve the above problems, threshold voltage control is conventionally performed in which even in the case of producing an n-channel TFT, a crystalline silicon film constituting a channel forming region is doped with a p-type impurity, for instance, boron, to shift the threshold voltage to the positive side. A resulting TFT has a normally-off state (it is off when the gate voltage is 0 V). However, causing an increase in the number of manufacturing steps, the threshold voltage control is a factor of preventing reduction in manufacturing cost.
SUMMARY OF THE INVENTION
An object of the present invention is to shift the threshold voltage of a TFT using a crystalline silicon film to the positive side, thereby causing an n-channel TFT to exhibit a normally-off state.
Another object of the invention is to reduce the S-value and increase the mobility.
To attain the above objects, according to a first aspect of the invention, there is provided a manufacturing method of a semiconductor device comprising a first step of laser-annealing a non-single-crystal silicon film that is formed on a substrate having an insulating surface in a hydrogen-inclusive atmosphere; and a second step of forming an insulating film to become a gate insulating film on the non-single-crystal silicon film, the first and second steps being performed consecutively.
In the above manufacturing method, it is preferred that the non-single-crystal silicon film not be exposed to the air between the first and second steps.
According to a second aspect of the invention, there is provided a manufacturing method of a semiconductor device comprising the steps of preparing a consecutive processing apparatus having a laser irradiation chamber, a substrate transfer chamber, and a processing chamber, the respective chambers being airtight; laser-annealing a non-single-crystal silicon film that is formed on a substrate having an insulating surface in a hydrogen-inclusive atmosphere in the laser processing chamber; transferring the substrate to the processing chamber via the substrate transfer chamber; and forming an insulating film to become a gate insulating film on the non-single-crystal silicon film in the processing chamber.
In the above manufacturing method, it is preferred that the insulating film be a silicon nitride film or a multi-layer film including a silicon nitride film.
It is preferred that the multi-layer film include a silicon oxynitride film formed on the non-single-crystal silicon film and a silicon nitride film formed on the silicon oxynitride film.
It is preferred that the multi-layer film include a silicon oxide film formed on the non-single-crystal silicon film and a silicon nitride film formed on the silicon oxide film.
It is preferred that the multi-layer film include a first silicon nitride film formed by nitrifying a surface of the non-single-crystal silicon film and a second silicon nitride film formed on the first silicon nitride film.
In this specification, the term “consecutive” means that there is no step that changes the composition, film quality, shape, or structure of a non-single-crystal silicon film that has just been subjected to the first step, between the first and second steps.
Therefore, providing, between the first and second steps, a substrate transfer step, an alignment step, a slow cooling step, a step of heating a substrate to a temperature suitable for the second step, or a like step is within the scope of the term “consecutive” as used in this specification.
On the other hand, providing, between the first and second steps, a step of exposing a non-single-crystal silicon film to a particular atmosphere (for instance, an oxidizing atmosphere) that changes its film quality, a heating step (for instance, a heating step intended for hydrogen removal, or a heating step performed in an oxidizing atmosphere or the like) to intentionally change the film quality of a non-single-crystal silicon film, an ion doping step, a film formation step, an etching step, a plasma processing step, a coating application step, or a like step is not included in the scope of the term “consecutive” as used in this specification.
In the invention, in crystallizing or improving the crystallinity of a non-single-crystal silicon film by laser annealing, the non-single crystal silicon film is irradiated with laser light in a hydrogen-inclusive atmosphere.
A TFT formed by using a crystalline silicon film that has been obtained by laser-annealing a non-single-crystal silicon film in a hydrogen-inclusive atmosphere exhibits a positive threshold voltage shift of about 2 to 4 V and a current-rising voltage of about 0 V or larger, in both cases of an n-channel and p-channel TFTs. The reason for these phenomena is unknown.
Therefore, the conventional step of controlling the threshold voltage by boron doping can be eliminated.
There is a tendency that the positive shift of the threshold voltage becomes larger as the hydrogen concentration of an atmosphere
Kusumoto Naoto
Takayama Toru
Yonezawa Masato
Pert Evan
Semiconductor Energy Laboratory Co,. Ltd.
Sherry Michael J.
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