Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
1999-07-22
2001-06-19
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C438S487000
Reexamination Certificate
active
06249385
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus to be used for an annealing or exposure process including laser light irradiation. For example, the invention relates to an apparatus that provides a uniform irradiation effect in a laser annealing apparatus in which irradiation with a large-area beam is performed. This type of laser annealing apparatus is used for semiconductor manufacturing processes.
2. Description of the Related Art
The technique of crystallizing an amorphous silicon film by irradiation with laser light is known. Further, the technique of irradiating a silicon film that has been damaged by implantation of impurity ions to recover its crystallinity and to activate implanted impurity ions is also known. They are called laser annealing techniques.
A typical example of the latter technique is a technique for annealing the source and drain regions of a thin-film transistor. In this technique, the source and drain regions are annealed by laser light irradiation after implanting impurity ions such as phosphorus or boron ions into those regions.
Such a process with laser light irradiation has a feature that a substrate receives almost no thermal damage.
This feature decreases limitations on materials to be processed and provides an advantage in, for instance, forming a semiconductor device on a substrate such a glass substrate that is low in heat resistance. In particular, this feature is important in manufacturing an active matrix liquid crystal display device whose application range has expanded in recent years.
In the active matrix liquid crystal display device, it is desired to use a glass substrate due to requirements of cost reduction and increase in area.
The glass substrate cannot withstand a heat treatment at as high a temperature as more than 600° C. or even more than 700° C. The above-mentioned technique of crystallizing an amorphous silicon film or performing annealing after implantation of impurity ions by laser light irradiation is effective in avoiding this problem.
In the method of using laser light irradiation, even a glass substrate receives almost no thermal damage. As a result, a thin-film transistor using a crystalline silicon film can be formed even with the use of a glass substrate.
However, in general, laser light as generated from a lasing device (hereinafter referred to as “original beam”) is small in beam area. Therefore, a common method of processing a large subject area is to scan it with laser light, which however has such problems as a long processing time and low uniformity in the effect of process in the subject area. In particular, because of a non-uniform light intensity profile, an ordinary original beam causes very poor uniformity in the effect of processing if it is used as it is.
In view of the above, various techniques have been proposed which modify an original beam so as to obtain a beam that is as uniform as possible and even a beam that is changed in size and shape so as to conform to the size, shape, etc. of a surface or region to be irradiated. The common shapes of a resulting beam are a rectangular shape and a linear shape. According to these techniques, uniform laser annealing can be performed over a large area.
FIG. 1
shows an example of a laser irradiation apparatus for modifying an original beam. In
FIG. 1
, a laser oscillator
1
is an excimer laser, for instance. The excimer laser
1
oscillates to emit laser light by establishing an excited state called an excimer state by decomposing a predetermined gas by high-frequency discharge.
For example, the KrF excimer laser produces excited states KrF* by high-voltage discharge by using material gases of Kr and F. Although this excited state has a life of several nanoseconds to several microseconds and hence is not stable, corresponding ground states KrF is even less stable. An inverted distribution is thereby produced in which the density of excited states is higher than that of ground states. This causes stimulated emission and laser light can be produced at relatively high efficiency.
It goes without saying that the laser oscillator
1
is not limited to an excimer laser and may be a pulsed laser or a CW laser. In general, the pulsed laser is suitable for the purpose of obtaining a high energy density.
An original beam emitted from the laser oscillator
1
is modified into a proper size by a concave lens
2
and a convex lens
3
.
FIG. 1
shows a case where the original beam is enlarged in the vertical direction. The beam at this stage is still called an original beam because its light intensity is equivalent to that of the state immediately after the emission from the laser oscillator
1
.
The original beam then enters an optical device called a homogenizer, which includes at least two lens devices (each called a multi-cylindrical lens, a lenticular lens or a flyeye lens)
4
and
5
each having a large number of cylindrical lenses. In general, the multi-cylindrical lenses
4
and
5
are disposed orthogonally as shown in the insert view in FIG.
1
.
The number of multi-cylindrical lenses may be one or three or more. Where only one multi-cylindrical lens is provided, the non-uniformity of the original beam in one direction is dispersed. Where two or more multi-cylindrical lenses are disposed so as to be orientated in the same direction, the same effect as would be obtained by increasing the number of constituent cylindrical lenses can be obtained.
By causing the original beam to pass through the homogenizer, a highly uniform beams with dispersed energy density can be obtained. The principle and the problems of the homogenizer will be described later. The beam is then modified into an intended shape by various kinds of lenses
6
,
7
, and
9
, changed in direction by a mirror
8
, and finally applied to a sample (see FIG.
1
).
Next, the principle and the problems, which is to be solved by the invention, of the homogenizer will be described. To avoid complexity, in the following an optical discussion will be given to only one surface. Laser light that has passed through a multi-cylindrical lens is as shown in FIG.
2
A.
In
FIG. 2A
, L is a multi-cylindrical lens including three constituent cylindrical lenses, and a laser light (an original beam) entering each cylindrical lens is refracted by it. The beams diffuse after being converged at focuses F
1
-F
3
. There occurs a mixing region where all of the light beams that have passed through the respective cylindrical lenses are mixed with each other.
Now assume that the original beam has a deviation in its light intensity profile and hence the beams entering the respective cylindrical lenses have different light intensities. However, the deviation is dispersed in the mixing region because the beams that have passed through the respective cylindrical lenses are mixed with each other there. The light intensity is thus uniformized and a beam having a less varied light intensity profile can be obtained (see FIG.
2
A).
Incidentally, if attention is paid to the optical paths after the passage through the multi-cylindrical lens, it is understood that the beams are regarded as being emitted from point light sources F
1
-F
3
that are arranged at regular intervals. Further, since the original beam is coherent, the beams emitted from the respective point light sources also have equal phases and hence interfere with each other. That is, portions where the beams cancel out each other (nodes) and portions where the beams intensify each other (antinodes) occur depending on the distance x between the irradiation surface and the point light sources F
1
-F
3
and the interval
2
a
between the point light sources F
1
-F
3
(see FIG.
2
B).
Contrary to the intended purpose, this means that the multi-cylindrical lens introduce a new version of non-uniformity to the light intensity profile. The positions of nodes and antinodes can be determined strictly in a case where the number of point light sources is as small as two or three. However, in an ordinary homogenizer the number of co
Kusumoto Naoto
Tanaka Koichiro
Yamazaki Shunpei
Fish & Richardson P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Sugarman Scott J.
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