Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-02-29
2002-05-21
Font, Frank G. (Department: 2877)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S024000, C372S025000, C372S031000, C372S057000, C438S030000, C438S166000, C359S624000, C359S668000
Reexamination Certificate
active
06393042
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for uniformly irradiating a laser beam to a large area. In addition, the invention relates to its application.
2. Description of the Related Art
In recent years, a wide research has been made on a technique for carrying out a laser annealing to an amorphous semiconductor film or a crystalline semiconductor film (semiconductor film having crystallinity, such as polycrystal or microcrystal, not single crystal), that is, a non-single crystal semiconductor film formed on an insulating substrate of glass or the like to crystallize it or to improve its crystallinity. A silicon film is often used as the above semiconductor film.
As compared with a quartz substrate which has been hitherto often used, a glass substrate has such merits that it is inexpensive and is rich in workability, and a large substrate can be easily formed. This is the reason why the foregoing research has been carried out. Besides, the reason why a laser is preferably used for crystallization is that the melting point of the glass substrate is low. The laser can give high energy to only a non-single crystal film without greatly changing the temperature of the substrate.
Since a crystalline silicon film formed by the laser annealing has a high mobility, a thin film transistor (TFT) is formed by using this crystalline silicon film. For example, it is actively used for a monolithic liquid crystal electro-optical device in which TFTs for pixel driving and for driver circuits are formed on one glass substrate. Since the crystalline silicon film is made of a number of crystal grains, it is called a polycrystal silicon film or polycrystal semiconductor film.
A method in which a pulse laser beam of an excimer laser or the like having high output is processed by an optical system so that a rectangular spot of several cm or a linear beam of several mm in width x not less than
10
cm in length is formed on a surface to be irradiated, and the laser beam is scanned (irradiation position of the laser beam is relatively moved with respect to the irradiated surface) to carry out the laser annealing, is superior in mass productivity and is excellent in technology. Thus, this method is used by choice.
Particularly, when a linear laser beam is used, unlike the case of using a spot-like laser beam which requires back-and-forth and right-and-left scanning, laser irradiation to the whole surface to be irradiated can be made by scanning in only the direction perpendicular to the line direction of the linear laser. Thus, high mass productivity can be obtained. The reason why scanning is made in the direction perpendicular to the line direction is that it is the most effective scanning direction. Because of this high mass productivity, at present, in the laser annealing, it has become the mainstream to use the linear laser beam obtained by processing the excimer laser beam through a suitable optical system.
Recently, a continuous-wave laser, such as an Ar laser, having a higher output has been developed. There is also a report that an Ar laser was used for annealing of a semiconductor film and an excellent result was obtained. In this case, since the output of the Ar laser is not sufficient, an irradiation surface has a spot shape.
As a laser widely used for crystallization, an excimer laser is known as a gas laser, and a ND:YAG laser, Nd:YVO
4
laser, or argon laser is known as a solid laser.
Since the continuous-wave argon laser has a wavelength of about 500 nm, the absorption coefficient of the argon laser to a silicon film is about 10
5
/cm. On the other hand, since the excimer laser is ultraviolet light of 400 nm or less, the absorption coefficient is about 10
6
/cm which is higher than the argon laser by one digit. Thus, in the argon laser, the intensity is decreased to 1/e (e is a natural logarithm) at the point when the light travels 100 nm through the silicon film, while in the excimer laser, the intensity is decreased to 1/e at the point when the light travels 10 nm through the silicon film.
In general, in the field of a TFT, it is considered to be suitable that the thickness of a polycrystal silicon film is about 50 nm. If the silicon film has a thickness more than 50 nm, there is a tendency that off characteristics become deteriorated, and if less than 50 nm, the reliability is influenced. In the case where the argon laser light is irradiated to the silicon film having a thickness of 50 nm, more than half of the laser light transmits the silicon film and is absorbed by the glass substrate, so that the glass substrate is heated more than needed. Actually, when a silicon oxide film having a thickness of 200 nm and a silicon film having a thickness of 50 nm were sequentially formed on a Corning 1737 substrate and crystallization was attempted, the glass was deformed before the silicon film was sufficiently crystallized.
On the other hand, in the case of irradiation of the excimer laser, almost all energy is absorbed by the silicon film having a thickness of 50 nm, so that almost all laser light can be used for crystallization of the silicon film. Like this, the merit of using the excimer laser for crystallization of the silicon film is that the absorption coefficient of the excimer laser to the silicon film is high.
FIG. 24A
is a top view of a silicon film, which is irradiated with a conventional pulse oscillation excimer laser while being scanned, seen from the above.
FIG. 24B
is a sectional view of the silicon film cut along a section (surface perpendicular to the silicon film containing a line E-F) parallel to the scanning direction of the pulse oscillation excimer laser.
FIG. 24C
is a sectional view of the silicon film cut along a surface (surface perpendicular to the silicon film containing a line G-H) perpendicular to the section and perpendicular to the silicon film.
As is understood from
FIG. 24B
, irradiation traces of the pulse laser produce undulations of the same order as the thickness of the silicon film. On the other hand, although the undulations shown in
FIG. 24C
are vary small as compared with the undulations of
FIG. 24B
, periodic undulations appear. These are due to the interference of linear beams shaped by a beam homogenizer as described later.
An optical system serving to homogenize an energy distribution (light intensity) in a linear beam is called a beam homogenizer.
FIGS. 25A and 25B
show an example of the beam homogenizer.
On an optical path, cylindrical lens groups (also called a multi-cylindrical lens or cylindrical lens array)
12
and
13
, a cylindrical lens
14
, a slit
15
, a cylindrical lens
16
, and a mirror
17
are sequentially disposed from an outgoing side of a laser apparatus
11
as an optical source of an excimer laser. A cylindrical lens
18
is disposed on an optical path in the reflecting direction of the mirror
17
.
The cylindrical lens
12
divides a beam into plural beams in a predetermined one direction (direction perpendicular to the paper surface of the side view), and the beams divided in this direction are synthesized in the cylindrical lens
16
. On the other hand, the cylindrical lens group
13
also divides a beam into plural beams in a predetermined one direction (direction parallel to the paper surface of the side view), and the beams separated in the dividing direction of the cylindrical lens group
13
are synthesized in the cylindrical lens
14
.
Thus, the laser beam emitted from the oscillator is divided two-dimensionally by the cylindrical lens groups
12
and
13
, and is inputted on the cylindrical lens
14
. Some of the plural beams are synthesized in the predetermined direction (direction perpendicular to the paper surface of the side view) so that plural beams divided in the one direction (direction parallel to the paper surface) are formed and pass through the slit
15
. The beams are condensed by the cylindrical lens
16
so that they become again one beam. The condensed beam is reflected by the mirror
17
, is condensed by the cylindrical lens
18
Font Frank G.
Nixon & Peabody LLP
Robinson Eric J.
Rodriguez Armando
Semiconductor Energy Laboratory Co,. Ltd.
LandOfFree
Beam homogenizer and laser irradiation apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Beam homogenizer and laser irradiation apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Beam homogenizer and laser irradiation apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2855257