Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2000-07-13
2004-04-20
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S797000
Reexamination Certificate
active
06723590
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for laser light irradiation (so called laser annealing) in fabrication of a semiconductor device, and to a method for fabricating a semiconductor device by laser light irradiation, wherein mass production is high, characteristics are little different among the devices, and a production yield is high. More particularly, the invention relates to a method for improving or recovering (repairing) crystallinity of a semiconductor material by irradiating the material with laser light. The semiconductor material includes a semiconductor material having wholly or partially amorphous components, a substantially intrinsic polycrystalline semiconductor material, and a semiconductor material whose crystallinity has been severely deteriorated by damage due to ion irradiation, ion implantation, or ion doping.
Recently, researches have been conducted concerning low temperature semiconductor device fabrication processes mainly because it has become necessary to form semiconductor devices on an insulating substrate made of glass or the like. Also, miniaturization of devices and making of a multilayer structure have required.
In semiconductor fabrication processes, it may be necessary to crystallize an amorphous component contained in a semiconductor material or an amorphous semiconductor material. Also, it may be necessary to recover (repair) the crystallinity of a semiconductor material deteriorated by ion irradiation. Furthermore, when there exists crystallinity, it may be required to be enhanced. In general, thermal annealing is used for these purposes. When silicon is used as a semiconductor material, it is annealed at 600 to 1100° C. for 0.1 to 48 hours or longer. As a result, the amorphous component is crystallized, the crystallinity is recovered, or the crystallinity is improved.
In thermal annealing. as higher temperature is used, the processing time can be shortened. However, at 500° C. or lower, almost no effect produces. At about 600° C., a long processing time is needed. Accordingly, it has been required that the thermal annealing be replaced by other means in order to lower the process temperature. Hence, a laser irradiation technique has attracted attention as an ultimate low temperature process. Since laser light can be irradiated only onto a region that needs high energy corresponding to the energy of thermal annealing, it is not necessary that the whole substrate be processed to a high temperature.
Generally, two methods have been proposed to irradiate laser light. First method uses a continuous oscillating laser such as an argon ion laser. The laser beam having a spot shape is irradiated to a semiconductor material. In this method, the semiconductor material is melted by variations in. the energy distribution inside the beam and by movement of the beam, and then the material is solidified mildly. As a result, the semiconductor material is crystallized. Second method employs a pulse oscillating laser such as an excimer laser. The pulse laser having high energy is irradiated to a semiconductor material. In this method, the material is momentarily melted and solidified, whereby the material is crystallized.
The first method has the problem that it takes a long time to perform the processing, for the following reason. Since the maximum energy of a continuous oscillating laser is limited, the maximum beam spot size is several millimeters. On the other hand, in the second method, a large spot of several cm
2
or more can be used because the maximum energy is very large. Hence, the productivity can be improved.
However, in order to process one large area substrate with a normally used beam having a square or rectangle form, it is necessary to scan (move) the beam vertically a horizontally. This produces limitations on the productivity. This problem can be solved by modifying the cross section of the beam into a linear form, making the width of the beam larger than the size of the substrate to be processed, and scanning this beam.
The remaining problem for improvement is uniformity of the effect of laser irradiation. A pulse laser somewhat varies in energy from pulse to pulse and so it is difficult to uniformly process the whole substrate. Especially, it is important to obtain uniform the characteristics of regions where adjacent laser spots overlap each other.
Also, when a pulse laser is irradiated, even if the uniformity of the energy inside the beam of one shot pulse can be accomplished by improvements in the optical system, it is difficult to reduce variations in the characteristics of devices due to overlap of pulse laser. Especially, where devices are located just at ends of the beam of laser light, the characteristics (especially the threshold voltages of MOS transistors) vary considerably from device to device.
In semiconductor devices, considerable variations in the threshold voltages of digital circuits are admitted. In analog circuits, the difference between the threshold voltages of adjacent transistors is required to be 0.02 V or less.
It has been reported that if weak pulse laser light is preliminarily irradiated before irradiation of strong pulse laser light, the nonuniformity is lowered and the uniformity is improved. However, overlap of laser spots has not been discussed sufficiently.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the above problem by using a linear laser beam (linear laser light). That is, a relatively weak, first pulse linear laser light is irradiated to a substrate. Then, a second linear laser light having higher energy is irradiated at right angles to the first laser light to process the substrate. Absolute outputs of the first laser light and the second laser light may be determined by the. required uniformity and characteristics.
In order that the first linear laser light make substantially right angles with respect to the second laser light, the direction of either laser light is varied, or the substrate is rotated at about a ¼ revolution (approximately 90°), generally n/2+¼ (n is 0, 1, 2, . . . ) revolution, i.e., (n/2+¼)×360°. A basic embodiment is shown in
FIGS. 1A-1C
. A substrate of a rectangle ABCD is disposed as shown in
FIG. 1A. A
linear laser light
1
is scanned in the direction indicated by the arrow, i.e., from top to bottom, to process the substrate. It is assumed that the laser light output has a relatively weak energy. In the region
2
(indicated by the broken lines) irradiated with the laser light, nonuniformity by variations in pulse intensity of the laser light and overlap of laser spots may be observed. A region
3
is not yet irradiated with the laser light.
Then, the substrate is rotated at a ¼ revolution, i.e., 90° (FIG.
1
B). The laser light
4
is scanned again in the direction indicated by the arrow, or from top to bottom, to process the substrate. At this time, the laser light output is larger than the laser light output irradiated first (FIG.
1
C).
From the above description, in the present invention, the direction of the nonuniformity due to second laser irradiation is perpendicular to that the nonuniformity due to the first laser irradiation. Therefore, since these two kinds of nonuniformities cancel out, semiconductor devices having high uniformity can be obtained.
The present invention can be applied to, as an object irradiated with a laser light, a film having no pattern, or substantially a completed device. Since the present invention is characterized in that two linear laser beams are used substantially in an orthogonal relation to each other and irradiated to the object at least twice, the laser light can be used less wastefully for square and rectangular substrates than for circular substrates in the present invention, a circular substrate can be processed. In the invention, some patterns are available, depending on the configurations of circuits formed on a substrate to be processed.
Also, it is desired to irradiate a laser light which is large enough to cover the
Takemura Yasuhiko
Yamaguchi Naoaki
Zhang Hongyong
Costellia Jeffrey L.
Niebling John F.
Nixon & Peabody LLP
Semiconductor Energy Laboratory Co,. Ltd.
Simkovic Viktor
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