Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1999-07-02
2003-04-29
Wilczewski, Mary (Department: 2822)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S487000
Reexamination Certificate
active
06555422
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of polycrystallizing an amorphous silicon film by using a laser light, and to a method of improving crystallinity of polycrystalline silicon film by using a laser light. Also, the present invention relates to a thin film transistor using as an active layer the polycrystalline silicon film obtained by these methods, and to a semiconductor device using the thin film transistor.
2. Description of Related Art
In recent years, researches have been enthusiastically promoted on lowering temperature in manufacturing process of a semiconductor device, in particular, a thin film transistor (hereinafter, referred to as TFT). The main reason for this is that a need for forming a TFT on an insulating substrate such as glass which is inexpensive and is rich in processibility has been arisen. Also from the view point of aiming at a further minute device and a further multi-layered device, the lowering temperature in manufacturing process of a TFT is required.
In a manufacturing process of a high-performance TFT, a step is necessary of crystallizing an amorphous component in a semiconductor material, or an amorphous semiconductor material. For such a purpose, thermal annealing has conventionally been employed. When silicon is used as a semiconductor material, an amorphous component is crystallized by annealing at a temperature of 600° C. to 1100° C. for 0.1 to 48 hours, or for more than 48 hours.
Such thermal annealing as the above takes shorter processing time as the temperature rises higher. However, it is almost utterly ineffective when the temperature is 500° C. or lower. Accordingly, from the view point of lowering temperature in manufacturing process, it is necessary to replace the thermal annealing step with other measures. When a glass substrate is used as a substrate in particular, since the heat resistant temperature of the glass substrate is about 600° C., measures comparable to the above thermal annealing has been required when the temperature is lower than the heat resistant temperature.
As a way to fulfill the requirement mentioned above, polycrystallization of an amorphous component through irradiation of a semiconductor material with a laser light has recently attracted attention. Thermal annealing by irradiation of laser light can apply high energy comparable to the thermal annealing restrictedly to a desired portion, and hence has an advantage that not an entire substrate needs to be subjected to the heat of high temperature.
As to irradiation of laser light, approximately two methods have been proposed.
The first method uses continuous-wave laser and is a method of irradiating a beam like a spotlight to a semiconductor material. This is a method of polycrystallizing the semiconductor material utilizing the fact that the semiconductor material are slowly solidified after it is melted owing to difference in energy distribution within a beam and moving of the beam.
The second method is one that utilizes the fact that a crystal growth proceeds when a semiconductor material is instantaneously melted and is solidified by irradiating a laser pulse of large energy onto the semiconductor substrate using a pulse-generating laser apparatus such as an excimer laser apparatus.
A problem in the first method is that the process takes much time. This is because the size of the beam spot is several mm square at most, for having limitations in the maximum energy of the continuous-wave laser.
The second method makes a trial for employing a way to “scan” relatively to the substrate with a laser light the shape of which is changed into a linear one and which has a length longer than the substrate to be processed. With employment of such a way, the throughput can be significantly improved. The term “scan” here means to irradiate linear laser lights so as to overlap a little with one another.
However, the above technique of irradiating linear pulse laser lights so as to overlap a little with one another will generate linear stripes on the surface of the semiconductor material irradiated with laser. These stripes considerably affects a device formed on the semiconductor material, or a device to be formed later. Particularly, when a plurality of elements are formed on the substrate and every element has to have a uniform characteristic, the stripes become a serious problem. In such a case, the characteristic is uniform in each stripe pattern, but varies between the stripes that are different from each other.
In this way, uniformity in irradiation effect matters even in the annealing method using linear laser lights. High uniformity here designates that similar device characteristics is observed in a device formed in any portion on the substrate. To enhance uniformity means that crystallinity of a semiconductor material is unified.
Then, an excimer laser of large output has been developed lately, which is capable of annealing a large area with a single shot. When using this excimer laser of large output, amorphous silicon in a large area may be polycrystallized all at once. It has been proved that also the film quality of the polycrystallized silicon film is uniform to a certain degree within the plane.
Here, as a conventional example, a schematic top view illustrating a case where this excimer laser of large output is used in manufacturing an active matrix type liquid crystal display device is shown in FIG.
39
.
In
FIG. 39
, reference numeral
3500
denotes a substrate;
3501
and
3505
, active matrix circuits;
3502
and
3506
, source driver circuits;
3503
,
3504
,
3507
and
3508
, gate driver circuits. Reference numerals
3509
to
3512
denote irradiated regions with excimer laser light of large output, and an amorphous silicon film in the respective regions is polycrystallized with one shot or plural shots of the laser light. Thus, in this conventional example, the laser light is irradiated onto all the amorphous silicon films on the entire substrate by three times shifting, relatively to the substrate, of the laser light. It should be noted that although laser light irradiation regions
3509
to
3512
are shown in a pattern different from each other, for convenience's sake in description, the same laser light is irradiated onto these regions.
It is readily understood here that irradiation of the laser light is carried out plural times onto repeatedly laser light irradiation regions, which are denoted by reference numerals
3513
to
3517
. For instance, laser light is irradiated twice or more onto the region
3513
, and four or more times onto the region
3517
. It has been found that characteristic of polycrystalline silicon film is different when the number of irradiation time of the laser light is different, and therefore in such a conventional example, variation in characteristic of the polycrystalline silicon film is generated within the substrate surface. In this conventional example, the uniformity within the plane of the polycrystalline silicon film is thus cannot be obtained even when using the excimer laser of large output. As a result, though the throughput may be increased as compared to the case using a linear laser, problems are still remained as to the uniformity within the plane of polycrystalline silicon.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above, and therefore an object of the invention is to provide a method of manufacturing a thin film transistor, in which when polycrystallizing an amorphous semiconductor film using a laser light, or when crystallinity of a semiconductor film is improved using a laser light, uniformity in the polycrystalline silicon film used for a thin film transistor in the substrate surface is realized to prevent variations in characteristic of the thin film transistor that uses the polycrystalline silicon film as an active layer and to enhance the throughput. Also, it is an object of the present invention to provide a high-performance semiconductor device using a thin film transistor fabricated by the man
Hayashi Keisuke
Koyama Jun
Yamazaki Shunpei
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Wilczewski Mary
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