Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2002-04-24
2004-09-07
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S235000
Reexamination Certificate
active
06787755
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique capable of illuminating a large-area illumination surface with laser light that is high in uniformity. The invention is particularly suitable for annealing of a semiconductor film.
2. Description of the Related Art
In recent years, extensive studies have been made of techniques of crystallizing or improving the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (i.e., a semiconductor film that is not a single crystal but is crystalline, for example, polycrystalline or microcrystalline) by subjecting it to laser annealing. A silicon film is widely used as such a semiconductor film.
Glass substrates have an advantage that they are less expensive and higher in workability and enable formation of a large-area substrate more easily than quartz substrates that have widely been used conventionally. This is the reason for the above-mentioned studies. The reason why lasers are used for crystallization is because of low melting points of glass substrates. Lasers can crystallize a non-single-crystal film by applying high energy without changing the substrate temperature to a large extent.
Since crystalline silicon films formed by laser annealing have high mobility, they are widely used in monolithic liquid crystal electro-optical devices in which, for example, both of pixel driving TFTs (thin-film transistors) and driver circuit TFTs on a single glass substrate by using such a crystalline silicon film. Having a number of crystal grains, such a crystalline silicon film is called a polysilicon film or a polycrystalline semiconductor film.
On the other hand, because of high mass-productivity and advantages in industrial applicability, a laser annealing method is used by preference in which a laser beam emitted from an excimer laser or the like having large output power is processed by an optical system so as to form a several centimeter square spot or a line of several millimeters in width and tens of centimeters in length on an illumination surface and the illumination surface is scanned with the laser beam (i.e., the laser beam illumination position is moved relative to the illumination surface).
In particular, in contrast to the case of using a spot-like laser beam that requires two-dimensional scanning, the use of a linear laser beam allows the entire illumination surface to be illuminated by one-dimensional scanning in the direction perpendicular to the longitudinal direction of the linear laser beam, whereby high mass-productivity is obtained. The scanning in the direction perpendicular to the longitudinal direction is employed because it is most efficient. Because of the high mass-productivity, the use of a linear laser beam is now becoming the mainstream in the laser annealing technology.
Laser-annealing a non-single crystal semiconductor film by scanning it with pulse laser beams that have been processed into a linear shape have several problems.
For example, there is a problem that, in general, when a laser beam is applied to the surface of a semiconductor coating formed on a substrate that has differences in height due to undulation or the like of the substrate, the laser beam does not focus on the surface locally.
This problem causes a case that laser annealing is not performed uniformly over the entire film surface. For example, when a linear laser beam is used in laser annealing, there occurs a marked phenomenon that stripes are formed at overlapping portions of beams. The semiconductor characteristics of the film vary very much from one stripe to another.
This problem is particularly serious in a case where laser light is applied to large-area substrates, because differences in height are relatively large in large-area substrates. For example, a substrate of 600 mm×720 mm has undulation of about 100 &mgr;m, which is very large for a laser beam used having a certain kind of feature.
A specific state of a laser beam in the vicinity of the focal point will be described below. The energy profile, at and in the vicinity of the focal point, of a laser beam depends on the form of an optical system that produces the laser beam.
For example, in the case of a beam produced by simply converging a laser beam into a linear shape, a deviation from the focal point affects the beam width and the energy density.
FIG. 1A
shows an optical system that simply converge a laser beam into a linear shape. Reference numeral
100
denotes a laser beam,
101
and
102
denote cylindrical lenses for expanding the laser
100
, and
103
denotes a cylindrical lens for converging it in the width direction.
In general, in this type of optical system, the energy uniformity on an illumination surface
104
is poor because the laser beam
100
is simply converged into a linear shape. When this type of optical system is used, it is required that the laser beam
100
before being processed into a linear shape be very high in energy uniformity. Since a deviation from the focal point varies the energy density on the illumination surface
104
, it is not desirable to form a laser beam by an optical system having the above type of configuration.
FIG. 1B
shows an optical system in which a concave cylindrical lens
105
is added to the optical system of FIG.
1
A. In a case where a linear beam is formed in the manner shown in
FIG. 1B
, the concept “focal point of a laser beam” itself is meaningless because the laser beam is parallel in the vicinity of an illumination surface
106
. Therefore, the problem of a deviation from the focal point does not occur either. However, the laser beam energy density is high at the lens
105
from which a linear laser beam is output, and the lens
105
is not so durable as to sustain such a high energy density. Therefore, at present, this type of optical system is not practical. Further, when this type of optical system is used, it is required that an original laser beam (i.e., a laser beam before being processed into a linear shape) be very high in energy uniformity.
In the above two examples, it is required that a laser beam before being processed into a linear shape be very high in energy uniformity. At present, no laser beam generating device is available that generates a laser beam having sufficiently high uniformity for the purpose of annealing a semiconductor film. The above configurations thus require development of a new technology.
Because of low uniformity in the energy profile of a linear laser beam, at present the above two examples are not suitable for annealing of a semiconductor film. Next, a description will be made of examples of optical systems that are currently in actual use.
An optical system having a configuration shown in
FIG. 2A
forms a linear laser beam. The configuration of this optical system is such that a laser beam is divided vertically and horizontally and divisional beams are combined into a single beam on an illumination surface while being processed into a linear shape individually. This configuration makes it possible to uniformize the energy profile of a linear laser beam.
FIGS. 3A-3C
show energy profiles, in the width direction, of a linear laser beam formed by the lens group of
FIG. 2A
at the focal point (combined focal point) and positions slightly deviated from the focal point. In the cross-sections at the positions slightly deviated from the combined focal point, the energy profile assumes a step-like shape because divisional beams are not completely combined into a single beam.
FIGS. 3A-3C
show energy profiles at a position immediately upstream of the focal point, at the focal point, and at a position immediately downstream of the focal point, respectively, and correspond to broken lines a-c in
FIG. 2B
, respectively. The term “focal point of a linear laser beam” as used here means a plane where divisional beams are substantially combined into a single beam. In many cases, the width of a linear laser beam is set at 1 mm or less, because it is generally required to have high energy density. There
Tanaka Koichiro
Yamazaki Shunpei
Le Que T.
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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