Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
1999-10-01
2002-05-21
Ben, Loha (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S618000, C359S626000, C372S024000, C372S025000, C372S033000, C219S121800, C438S487000, C438S166000
Reexamination Certificate
active
06392810
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure for carrying out irradiation while scanning a linear laser beam, and to a structure for annealing a non-single crystal semiconductor film through irradiation of a linear laser beam while scanning the laser beam in a beam width direction. The present invention also relates to a semiconductor device, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, extensive studies have been made on techniques for obtaining a crystalline semiconductor film (semiconductor film having crystallinity of single crystal, or polycrystal, microcrystal, etc.) by annealing a non-single crystal semiconductor film (not a single crystal semiconductor film such as an amorphous, polycrystalline, or microcrystalline semiconductor film) formed on an insulating substrate of glass etc. to crystalize the film or to improve its crystallinity. A silicon film is often used for the above semiconductor film.
As compared with a quartz substrate that has been conventionally frequently used, the glass substrate has such advantages that it is inexpensive, it is superior in workability, and a large substrate can be easily formed. This is the reason why the above-mentioned researches are carried out. Further, the reason why a laser is preferably used for crystallization is that the melting point of the glass substrate is low. The laser is capable of giving high energy to only the semiconductor film without varying the temperature of the substrate very much.
Since a crystalline silicon film formed by performing a laser annealing to a silicon film has high mobility, it is extensively used in such a manner that thin film transistors (TFTs) are formed with this crystalline silicon film, and are employed for, for example, a monolithic liquid crystal electrooptical device in which a TFT for driving pixels and a TFT for driver circuits are formed on one glass substrate. Since the crystalline silicon film is made of a large number of crystal grains, it is also called a polycrystalline silicon film or a polycrystalline semiconductor film.
A method in which a pulse laser beam of an excimer laser etc. with a high power output is optically converted into a square spot of several cm or a linear shape of several mm in width x several tens cm in length on a surface to be irradiated and the laser beam is scanned (irradiation position of the laser beam is moved relatively to the surface to be irradiated) to perform a laser annealing, is superior in mass production and is excellent in industry, so that the method is used by preference.
Particularly, if the linear laser beam is used, contrary to the case where a spot-like laser beam requiring to perform scanning in lengthwise and crosswise directions is used, laser irradiation to all the surface to be irradiated can be performed by scanning in only a direction normal to a line direction of the linear laser, so that a high mass production property can be obtained. The reason why scanning is performed in the direction normal to the line direction is that it is the scanning direction with the highest efficiency. Because of this high mass production property, usage of a linear laser beam obtained by converting an excimer laser beam through a suitable optical system has come to be the mainstream in the laser annealing nowadays.
Generally, in the case where the linear laser beam is formed, an originally rectangular beam is converted into a linear shape through a suitable lens group. The aspect ratio of the rectangular beam is about 2 to 5, and the rectangular beam is deformed into the linear beam with an aspect ratio of 100 or more through, for example, a lens group (this is referred to as a beam homogenizer) shown in
FIGS. 2A and 2B
.
FIGS. 2A and 2B
are an upper view and a sectional view, respectively, which show a conventional optical system for forming a linear laser beam. The foregoing lens group is designed such that the distribution of energy in the beam is also uniformed at the same time as the deformation. The method of uniforming the energy distribution is such that the original rectangular beam is divided into parts, and then, the divided parts are respectively enlarged and are overlapped to perform uniforming.
The apparatus shown in
FIGS. 2A and 2B
has a function of irradiating, as a linear beam, a laser light from an oscillator
201
(in this state, the light has a substantially rectangular shape) through an optical system designated by
202
,
203
,
204
,
205
, and
207
. Reference numeral
206
designates a mirror.
The cylindrical lens array
202
has a function of dividing a beam into many parts. The divided many beams are synthesized by a cylindrical lens
205
into one.
This structure is needed to uniform the strength distribution in the beam. The combination of the cylindrical lens array
203
and the cylindrical lens
204
has a function similar to the combination of the cylindrical lens array
202
and the cylindrical lens
205
.
That is, the combination of the cylindrical lens array
202
and the cylindrical lens
205
has a function of uniforming the energy (strength) distribution of the linear laser beam in the longitudinal direction, and the combination of the cylindrical lens array
203
and the cylindrical lens
204
has a function of uniforming the energy (strength) distribution of the linear laser beam in the width direction. When the cylindrical lens
207
is disposed through the mirror
206
, a narrower linear laser beam can be obtained.
An optical system functioning to uniform an energy distribution in a beam is called a beam homogenizer. The optical system shown in
FIGS. 2A and 2B
is also one of beam homogenizers. A method of uniforming the energy distribution is such that an original rectangular beam is divided into parts, and then, the divided parts are respectively enlarged and are overlapped to perform uniforming.
Some problems have occurred when a laser annealing is applied to a non-single crystal semiconductor film by scanning a pulse laser beam converted into a linear shape. One of the problems is that the laser annealing can not be performed uniformly over the whole film surface according to conditions of the non-single crystal semiconductor film, for example, a film thickness.
In fabrication of a semiconductor device using a semiconductor film, there is a case where the thickness of the semiconductor film is made to be changed in accordance with the properties of a semiconductor component or device to be fabricated. For example, for obtaining high performance, a thin film with a thickness of, for example, about 25 to 55 nm is necessary. Alternatively, for obtaining high reliability, a thick film with a thickness of, for example, about 55 nm to 100 nm is required. Thus, according to the characteristics required for the semiconductor component to be formed, the film thickness of the semiconductor film is made to be changed.
For example, in the case where a non-single crystal semiconductor film with a thickness of 50 nm or less, rather than a non-single crystal semiconductor film with a thickness in the range of 50 nm to 60 nm, is irradiated with a laser beam, a phenomenon where stripes are formed at beam overlapping portions becomes conspicuous, and there is a case where semiconductor characteristics of the film become extremely different among the respective stripes (see FIG.
1
).
Even in the case where a non-single crystal semiconductor film having a thickness of 60 nm or more is similarly subjected to a laser annealing, a phenomenon where stripes are formed at overlap portions between a beam and a beam can occur.
For example, in the case where a semiconductor device, for example, a thin film transistor is fabricated by using a crystalline semiconductor film in which the stripes are formed, and a liquid crystal display having such thin film transistors is fabricated, there occurs a disadvantage that the stripes directly appear on a screen display. Although this problem has been remedied by
Ben Loha
Nixon & Peabody LLP
Robinson Eric J.
Semiconductor Energy Laboratory Co,. Ltd.
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