JPH0651238A - Laser - Google Patents

Abstract

PURPOSE:To obtain a laser beam having pulses which are exponentially attenuated by splitting each pulse into two, and delaying one pulse with respect to the other, and recombining the pulses together using a half mirror. CONSTITUTION:One pulse of a pulse laser beam being oscillated by an eximer laser oscillator 1 is split by a half mirror 2 into two, one directed to a surface 4 to be illuminated, and the other directed to one of mirrors 3; the half mirror 2 having transmittance and reflectance in the ratio of 1:1. The pulse directed to one of the mirrors 3 is allowed to pass through an optical path as indicated by the arrow and impinge again on the half mirror 2, where it is split into two, one transmitted through the mirror 2 and the other directed to the surface 4 to be illuminated. The surface to be illuminated is illuminated with the laser beam the energy density of which is attenuated by 1/2 each time one of its pulses circulates through a loop formed by the four mirrors. Therefore, the pulses sequentially arrive at the surface to be illuminated, each with a delay of a time corresponding to the pulse width, so that a constant time distribution is given.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser device capable of irradiating a laser in a pulse shape having a pulse waveform that decays exponentially.

[0002]

2. Description of the Related Art A pulse oscillation laser represented by an excimer laser is known as a laser for intermittently irradiating a momentary laser beam. This pulsed laser emits hundreds (mJ / mJ) in an extremely short time of tens of nanoseconds.
It has a feature that it can be irradiated with high energy of cm ² ).

As a known application of an excimer laser, a method is known in which an amorphous silicon semiconductor provided on a glass substrate is crystallized by irradiation with an excimer laser.

The feature of this method is that sufficient energy can be applied to crystallize amorphous silicon, and at the same time, the underlying glass substrate is not damaged by heat. This is because the laser is actually irradiated for a very short time of several tens of nanoseconds, and almost all of the energy is absorbed in the amorphous silicon semiconductor.

Further, it is well known to form a beam of laser light by an optical system, not limited to a pulse oscillation type laser. That is, the beam of laser light can be spatially controlled in the conventional technique. On the other hand, in the case of crystallizing an amorphous silicon semiconductor film by laser irradiation, a method of preventing surface roughness due to abrupt solidification by irradiating a laser beam having a high energy density with a weak laser beam is known. Has been.

However, no method has been put into practical use as a method of gradually roughening the energy density of the laser beam over time to prevent the surface of the irradiated surface from becoming rough or to extend the irradiation time of the irradiated surface. .

[0007]

A conventional pulse irradiation type laser device is a type in which a laser beam having a temporally constant energy density is intermittently irradiated, and the spatial energy density distribution is an optical system. However, the temporal energy density distribution could not be changed.

[0008]

The present invention is directed to a laser oscillating device which performs pulse oscillation, a half mirror which divides a laser beam oscillated from the laser oscillating device into another direction from its traveling direction, and A laser device having an optical system for making a laser beam divided into directions enter the half mirror in the same direction as the other direction.

In the structure of the above invention, an excimer laser oscillator can be typically used as the laser oscillator for pulse oscillation. The half mirror that splits the laser light is preferably one that splits the laser light at a ratio of 1: 1.

According to the configuration of the present invention, the laser light once divided by the half mirror is made incident again on the same half mirror in the same direction as the first divided direction, and the divided surface is first irradiated to the divided surface. Immediately after the end of the irradiation with the one laser beam, the other divided laser beam is irradiated to obtain a pulsed laser beam having a time distribution in which the light amount, that is, the energy density is exponentially attenuated. There is also a method of irradiating two pulses divided by a half mirror to the same place from different angles through different paths, but this method only has a first pulse and a second pulse. However, it has the effect of extending the laser irradiation time.

For example, when a 1: 1 split half mirror is used, the amount of pulsed laser light that first reaches the surface to be illuminated is about 50% of that during oscillation. And about 5 remaining
By passing through the half mirror again, 0% of the laser light becomes about 25% of the half, and is directed to the irradiation surface. Then, by repeating the same process, about 12.5% of the laser light, which is half of about 25%, is irradiated again to the surface to be irradiated. Of course, in the case of this example, the loss of the laser beam in the middle is not taken into consideration, but the above effect can be obtained by minimizing the loss of the laser beam.

The structure of the present invention will be described below with reference to examples.

[0013]

【Example】

[Embodiment 1] In this embodiment, in a laser device that divides a pulsed laser light into a plurality of pieces and irradiates the laser light divided into a plurality of pieces onto the same irradiation location, L is an optical path difference and c is a speed of light. , T is the time width of the pulse of the pulse laser, that is, the pulse width, and the optical path difference of each laser light satisfies the following condition, L = ct. In this embodiment, the half mirror has a transmissivity / reflectance ratio of 1: 1. The half mirror used in this embodiment is formed by depositing a metal thin film having a high reflectance on a glass plate or a quartz plate having a high flatness.

The structure of this embodiment is shown in FIG. In FIG. 1, 1 is an excimer laser oscillator (KrF wavelength 24
Although the oscillation frequency is variable, it was fixed at 20 Hz and used. The time width of the pulse is 2 × 10 ^-8 se
c. 2 is a half mirror, 3 is a mirror,
Reference numeral 4 is a surface to be irradiated, and a material to be laser-irradiated is selected as necessary. In this embodiment, one pulse of the pulsed laser light oscillated by the excimer laser oscillator 1 has a half mirror 2 having a transmittance and a reflectance of 1: 1.
Is divided into a pulse directed to the irradiated surface 4 and a pulse directed to one of the mirrors 3. The pulse directed to one of the mirrors 3 is again incident on the half mirror 2 through the optical path indicated by the arrow, and is split into a pulse that is transmitted and a pulse that is directed to the irradiated surface 4. At this time, the light quantity of the laser pulse directed to the irradiated surface 4 is half the light quantity of the laser pulse initially directed to the irradiated surface 4. Since it is considered that the light quantity of the laser pulse and the energy density of the laser pulse are proportional to each other, in this case, the second laser pulse has half the energy density of the first laser pulse toward the irradiation surface. . Then, each time the laser pulse goes around the loop constituted by the four mirrors shown in FIG. 1, the laser pulse whose energy density is attenuated by ½ is applied to the irradiation surface. Of course, the four mirrors and the half mirror of the apparatus shown in FIG. 1 are provided with a fine movement mechanism and a fine movement rotation mechanism, and it is needless to say that fine adjustment can be performed according to the pulse width and oscillation frequency of the laser. Absent.

In this embodiment, since the optical path difference L of each laser beam satisfies L = ct, each pulse is as shown in FIG. That is, in this embodiment, since L = 2 (x + y) = ct is provided, the pulses arriving at the surface to be irradiated are shifted by a time corresponding to the width of the pulse. It has a time distribution as shown in.

In this embodiment, it is assumed that the speed of light c is 3 × 10 ⁸ m / s, which is in vacuum, and the time width t of the laser pulse is 2 × 10 ^-8 s. L = 3
It becomes ^{x10 8} x2 ^{x10 -8} = 6 m, and in the present embodiment, the condition of x + y = 3 m should be satisfied.
In this embodiment, if 2 (x + y) <ct, the laser pulses overlap each other and 2 (x +).
If y)> ct, the laser pulse intervals are widened. In the configuration of the present invention, 2 (x + y)
The case of <ct, that is, L <ct is excluded because overlapping of the laser pulses does not provide a useful temporal distribution of energy density.

In the case of the present embodiment, FIG. 2 is a graph showing the relationship between the time T at the irradiated point and the energy density E of the irradiated laser light obtained by setting L = ct. In FIG. 2, it is assumed that there is no loss of laser light in the optical system. In FIG. 2, t is the pulse width of the laser, which is the time during which the laser is actually irradiated with one pulse. The value of this pulse width is 10-50
On the other hand, the time interval from pulse to pulse is 0.5 sec when the oscillation frequency is 20 Hz.
Therefore, the time width t of the pulse is sufficiently larger than the time interval from pulse to pulse. Although only one pulse is shown in FIG. 2, the next pulse emitted from the excimer laser oscillator exists on the left side of FIG. 2 and the previous pulse exists on the right side. become. That is, when the excimer laser oscillator 1 oscillates, a pulse beam, which is one pulse beam, is temporally divided and applied to the surface to be irradiated, and a continuous beam whose laser power density is changed temporally is generated. FIG. 2 shows how an effect similar to irradiation is obtained.

FIG. 2 shows a characteristic of an exponentially attenuated pulse beam, which is one of the characteristics obtained by adopting the configuration of the present invention. That is, it is shown that the relative value of the energy density of the first pulse (in this case, the light quantity is considered to be proportional to the energy density) gradually becomes 1/2.

In this case, the intensity of the second beam pulse is 1/2 of that of the beam pulse reaching the first, and the intensity of the third beam pulse is more than that of the second beam pulse. Halves. That is, as shown in FIG. 2, the intensity of the nth beam pulse is (1/2) ^{n -1} times as high as the intensity of the first beam pulse (where n is the value in FIG.
The number is from the first pulse of one rectangular pulse shown in (1). As a result, the laser beam, which is exponentially attenuated in time, is applied to the irradiation surface.

The feature of the present invention is that one laser pulse is temporally (1/2) ^n- as shown in FIG. 2 depending on the arrangement of the optical system as shown in FIG. 1 showing the outline of this embodiment. ¹
This is where a laser pulse waveform that is attenuated by a factor of two (n is the number from the first of the rectangular pulses shown in FIG. 2) is obtained.

In this embodiment, the ratio of the transmissivity and the reflectivity of the half mirror is set to 1: 1, but if it is deviated from 1: 1, it becomes impossible to obtain an exponentially attenuated pulse.

Further, as apparent from the above description, the structure of the present invention is applied to a laser oscillating device for irradiating pulse oscillation or one shot pulse to obtain laser light which is exponentially attenuated. . That is, in the configuration of the present invention, a part of the laser light is divided by a half mirror, and delayed by making one optical path of the divided laser light, and the delayed laser light is directed toward the irradiated surface. The CW laser (continuous laser) is used when the CW laser (continuous laser) is used. It is not possible to obtain the above-described effect of obtaining the laser beam that is exponentially attenuated.

Although the KrF excimer laser (wavelength 248 nm) is used in this embodiment, other pulse oscillation type laser devices such as XeF excimer laser (wavelength 351 nm) and ArF excimer laser (wavelength 193 n) are used.
It goes without saying that the effects of the present invention can be obtained even if m) or the like is used.

Further, when the configuration of the first embodiment as shown in FIG. 1 is adopted, the pulse having the exponentially attenuated pulse as described above is obtained by using one half mirror and four mirrors. Another feature is that laser light can be obtained, and the interval of pulsed laser light that attenuates exponentially can be controlled by changing the position of the mirror.

[Embodiment 2] It is also useful to use the laser device described in Embodiment 1 for crystallizing an amorphous silicon semiconductor film formed on a glass substrate.

That is, by adopting the structure shown in the first embodiment, it is possible to prolong the melting time of the surface of the amorphous silicon semiconductor film which is the surface to be irradiated with the pulsed laser beam, and to improve the surface of the semiconductor film. This is because the crystallinity can be prevented and the crystallinity can be remarkably improved.

In the above, an example in which the amorphous silicon semiconductor film is crystallized by irradiation with a pulsed laser has been described, but the surface to be irradiated is not limited to amorphous silicon,
The present invention is used when other non-single-crystal semiconductors, carbon films, other annealing, or when it is desired to melt the surface of a substance, or when it is intended to irradiate energy by using a generally pulsed laser beam, that is, a pulsed laser beam. The configuration of can be applied to them.

[0028]

According to the present invention, one pulse of pulsed laser light is divided into two by a half mirror, and one of the two divided pulses is delayed with respect to another pulse. By traveling a long optical path and then merging the two pulses again by means of the half mirror, it was possible to obtain laser light with exponentially decaying pulses.

[Brief description of drawings]

FIG. 1 shows a laser device according to a first embodiment using the configuration of the present invention.

FIG. 2 shows one pulse of a pulsed laser that the laser device according to the first embodiment irradiates an irradiation surface with
The relationship between the energy density and time when reaching the irradiated surface is shown.

[Explanation of symbols]

 1 Laser oscillator for pulse oscillation 2 Half mirror 3 Mirror 4 Irradiated surface

Claims (4)

[Claims] 1. A pulse laser beam is divided into a plurality of beams, and the divided pulse laser beam is irradiated onto a surface to be irradiated with a time difference equal to or more than a pulse width. A laser device characterized by having a difference. 2. A laser device in which a pulsed laser beam is divided into a plurality of two, and the plurality of divided laser beams are applied to the same irradiation location, L is an optical path difference, c is a speed of light,
A laser device characterized in that the optical path difference of each laser light satisfies the following condition, where L ≧ ct, where t is a pulse time width of a pulse laser. 3. A laser oscillator that performs pulse oscillation, a half mirror that splits a laser beam oscillated from the laser oscillator in another direction from its traveling direction, and a laser beam split in the another direction. A laser device having an optical system for making the light incident on the half mirror in the same direction as above, wherein the optical path difference L between the two divided laser beams is the speed of light c and the time interval t of pulse oscillation.
A laser device satisfying L ≧ ct. 4. A laser device in which a pulsed laser beam having a pulse time width t is divided into two by a half mirror and merged again in the same irradiation direction.
One of the divided pulsed laser beams is provided so as to have an optical path longer than the other pulsed laser beam by an optical path L, and the optical path L, the time width t of the pulse, and the speed of light. A laser device characterized in that the pulse of the pulsed laser light applied to the surface to be irradiated is attenuated exponentially by configuring so that the relationship with the height c satisfies L = ct.