Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
1998-04-16
2001-01-23
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S103000, C117S108000, C117S904000
Reexamination Certificate
active
06176926
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure in which a laser beam which has been processed into a linear beam is scanned and irradiated. The present invention can be used in a process for manufacturing a semiconductor device using the emission of a laser beam, an exposing process using the irradiation of a laser beam, and so on.
2. Description of the Related Art
In recent years, there has been studied a technique of forming an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having crystallinity such as polycrystal or microcrystal of non-monocrystal) on an insulating substrate made of glass or the like.
In this technical field, a technique has been widely researched in which laser annealing is conducted on an amorphous silicon film or a low-crystalline silicon film to crystalize the film or to improve crystallinity.
The glass substrate is inexpensive and rich in processing property, in comparison with a quartz substrate which has been conventionally frequently used, as a result of which it is advantageous in that a large-area substrate can be readily fabricated. This is a reason why the above study has been made. Also, the reason why the layer is frequently used for crystallization is that a melting point of the glass substrate is low. The laser can give a high energy to only the non-monocrystal film without largely changing the temperature.
Because a crystalline silicone film formed by laser annealing is high in mobility, a thin film transistor (TFT) is formed using the crystalline silicon film
The above technique enables a monolithic liquid crystal electro-optic device where TFTs for pixels and drive circuits are disposed on a single glass substrate to be obtained.
A crystalline silicon film obtained by laser annealing are formed of a large number of crystal grains, and therefore is called “polycrystal silicon film” or “polycrystal semiconductor film”.
In the above-described laser annealing technique, since a laser beam needs to be irradiated onto an area 10 cm square or more, the irradiating method must be devised.
There have been proposed some methods of irradiating a laser beam, for example:
(1) A laser beam is converted into a square spot of several cm square on a plane to be irradiated and irradiated thereon while it is scanned.
(2) A laser beam is processed through an optical system so as to be converted into a linear beam of several mm width x several tens cm length, and such a linear laser beam is irradiated on the plane while it is scanned.
Of those methods, in the method (1), portions where laser beams as irradiated are overlapped are increased, thereby being liable to make the irradiation effect uneven. Also, the productivity is low.
On the other hand, in the method (2), the irradiation unevenness is difficult to exhibit in comparison with the method (1), and the productivity is also high.
Particularly in the method (2), the use of the linear laser beam is different from the use of a spot-like laser beam requiring scanning in the front and rear direction and in the right and left direction in that the laser can be irradiated on the entire plane to be irradiated by scanning only in a direction (width direction) perpendicular to the linear direction (longitudinal direction) of the linear laser, the high productivity can be obtained. The reason that scanning is made in the direction perpendicular to the linear direction is that it is the scanning direction where the coefficient is the highest. Because of the high productivity, the use of the linear laser beam is a main stream for laser annealing at present.
However, there arise some problems when laser annealing is conducted on the non-monocrystal semiconductor film while scanning a pulse laser beam which has been processed into a line.
In particular, one of the severe problems is that laser annealing is not uniformly conducted on the entire film surface.
When the linear laser started to be used, a phenomenon that stripes are produced on portions where the adjacent beams are overlapped with each other was remarkable, and the semiconductor characteristic of the film was largely different depending on each of the stripes.
What is shown in
FIG. 1A
is a picture that photographs a surface state of a crystalline silicon film obtained by scanning a linear laser beam, which is longitudinal along a lateral direction of a paper surface, in its width direction (a vertical direction of the paper surface) and irradiating it on the crystalline silicon film.
As is apparent from
FIG. 1A
, the degree of overlapping the linear laser beam is reflected on the crystallinity, to thereby exhibit a striped pattern.
In the case of fabricating a liquid crystal display unit using a silicon film exhibiting the stripped pattern, for example, there occurs a disadvantage that the stripes appear as they are.
It is presumed that this results from reflecting a difference in crystallinity of the striped pattern on the characteristic of a TFT array.
The above problem can be greatly improved by improving a non-monocrystal semiconductor film on which a laser beam is to be irradiated, thinning the scanning pitches (intervals between the respective adjacent linear laser beams) of the linear laser beam, optimizing a condition under which the linear laser beam is scanned, or other manners. More specifically, in the application of a liquid crystal display device, the degree of overlapping the linear laser beams on each other can be restrained from directly adversely affecting an image quality.
However, subsequent to the solving of the problem caused by the overlapped pulse laser shots, the nonuniformity of the energy distribution of the beam per se has been remarkable.
In general, in the case of forming the linear laser beam, an original rectangular (or square, or circle) beam is processed into a line through an appropriate optical system.
The original rectangular beam is about 2 to 5 in aspect ratio. That original beam is deformed into a linear beam
100
or more in aspect ratio through an optical system. For example, it is deformed in a linear laser beam 1 mm in width and 200 mm in length.
The formation of the laser beam is devised such that an energy distribution within the laser beam gets uniform in quality. In particular, employing an optical system called “beam homogenizer,” the energy density within the laser beam is made uniform.
An outline of the structure of a device for irradiating a linear laser beam is shown in FIG.
2
. In
FIG. 2
, there are shown a laser oscillator indicated by reference numeral
201
, a beam homogenizer consisting of lenses
202
,
203
,
204
and
205
, a mirror
206
and an objective lens
207
.
In this example, the combination of the lenses
203
and
205
is a beam homogenizer for improving the energy distribution in a longitudinal direction of the linear laser beam.
Also, the combination of the lenses
202
and
204
is a beam homogenizer for improving the energy distribution in a width direction of the linear laser beam.
The action of the beam homogenizer is that the original longitudinal beam is divided into a plurality of beams which are then enlarged, respectively, and re-superimposed on each other.
Seemingly, the beam divided and re-constructed by the beam homogenizer becomes more uniform in the distribution of energy as the division becomes more fine.
However, in fact, when the beam is irradiated onto the semiconductor film, the stripped pattern shown in
FIG. 1B
appears regardless of the fineness of the division.
The stripped patterns are innumberably formed so as to be orthogonal to the longitudinal direction of the linear laser beam. The formation of such stripped pattern is caused by the stripped distribution of the energy of the original rectangular beam or optical system.
The present inventor has conducted a simple experiment to make sure of the cause why the above-described stripes are formed.
This experiment has been made to investigate how the above striped pattern is changed by rotating a laser beam in a stat
Fish & Richardson P.C.
Hiteshew Felisa
Semiconductor Energy Laboratory Co,. Ltd.
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