JP3252403B2 - Laser irradiation apparatus and method for forming silicon thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser irradiation apparatus used in a semiconductor manufacturing process or the like. Although the laser irradiation apparatus of the present invention is used in various laser apparatuses such as laser annealing and introduction of gettering distortion, crystallization which is often used as an example of laser irradiation will be described below.

[0002]

2. Description of the Related Art In recent years, as the density of semiconductor integrated circuits has increased, the dimensions of each element of the semiconductor integrated circuit have been reduced to improve the degree of integration in the horizontal direction. A three-dimensional structure in which an insulating film is formed over the entire surface, a semiconductor thin film is provided on the insulating film, and an element is formed using the semiconductor thin film has been actively researched and developed. In particular, a method for recrystallizing a polycrystalline silicon film formed on an insulating film by irradiating the polycrystalline silicon film with a laser beam has been studied.

In the field of semiconductor integrated circuits, it has become an important issue to reduce the electric capacitance between each element or wiring portion of the semiconductor integrated circuit and the substrate silicon as the speed of the circuit increases. P which is often used so far
Compared with the n-junction isolation, the parasitic capacitance can be reduced by using a silicon film formed on an insulating film.

[0004] In this sense, a recrystallization technique using a laser beam, that is, a laser recrystallization technique has attracted attention.

In particular, among the flat-panel image display devices, research on an active matrix type liquid crystal display device has been advanced, and an image quality equal to or higher than that of a CRT type image display device has been obtained. In order to achieve high-definition image quality and reduce manufacturing costs, it is necessary to configure a driving circuit for driving the thin film transistor of the pixel on the same insulating substrate as the pixel. When the silicon thin film is irradiated with a laser beam and crystallized, the driving circuit can be formed on the same insulating substrate as the pixels.

A method of irradiating a continuous wave argon laser beam has been studied so far, and a conventional integrated circuit fabrication is performed by optimizing a silicon substrate structure, optimizing a laser beam structure, or optimizing a laser beam scanning method. Crystalline materials that can be used for the purpose described above have been obtained. There is a problem that the processing time required for annealing with an argon laser is still long for the purpose of forming a semiconductor integrated circuit. Therefore, laser recrystallization using an excimer pulse laser having a large beam size has been studied in order to shorten the laser annealing time.

In the field of active matrix type liquid crystal displays, crystallization of a silicon thin film using an excimer laser has been studied.

[0008]

However, in crystallization using a pulsed laser, the pulse width of the laser to be oscillated is determined by the performance of the laser oscillating device. There is a problem that the size of the polycrystalline particles constituting the silicon thin film is restricted by the laser oscillator, and a crystalline silicon thin film having sufficient properties cannot be obtained. In the crystallization of a silicon thin film by irradiation with a pulse laser, it is necessary to raise the temperature of the silicon thin film to a temperature equal to or higher than the melting point, and crystallize the silicon thin film.
As reported in PP55 "Transient temperature distribution in silicon film during pulsed laser annealing", a pulse width of a certain value or more is required to melt a silicon thin film. Therefore, when the pulse width of the pulse laser is small due to the performance of the laser oscillator, it is necessary to increase the pulse width by some method.

In laser annealing, when the energy of a laser electromagnetic wave is incident on a silicon thin film, the energy changes to thermal energy, the temperature of the silicon thin film rises, and crystallization is achieved by rearrangement of silicon atoms. Therefore, in crystallization by a laser beam, there is a problem in the efficiency of converting electromagnetic wave energy of laser into heat energy.

In the case of a XeCl excimer laser having a wavelength of 308 nm, the laser penetration length in a silicon thin film is about 10 n.
m, and the pulse width is about 30 to 50 ns, which is much smaller than the time required for heat to be conducted from the surface of the silicon thin film to the inside of the silicon thin film. Due to the low thermal conductivity of the silicon thin film, only a part of the electromagnetic wave energy of the laser reaching the silicon thin film surface is converted into the heat energy required for crystallization of the silicon thin film, and the remaining energy is converted to silicon energy on the silicon thin film surface. It is consumed for atomization, ionization, plasmaization, etc. of atoms.

When the pulse width is short and the thickness of the silicon thin film is 100 nm or more, the IEEE TRANSACTIONS ON
As reported in DEVICES. VOL.36, NO.12, PP2868-2872, the silicon thin film crystallizes only up to about 50 nm thick from the surface, and no thermal energy can be obtained on the insulating film side deep from the surface. There was a problem that crystallization did not proceed. Conventional annealing using a pulse beam has been performed using an optical system as shown in FIG. In FIG. 11, the laser beam emitted from the laser oscillator LS passes through an attenuator AT that attenuates the laser intensity as necessary, passes through a laser beam forming unit HM that improves the spatial intensity distribution of the beam, and Reached the surface of the sample through a lens system that can be varied as needed. In this optical system, the pulse width of the laser beam depends on the laser oscillator,
The disadvantage was that the beam could not be improved to the required pulse width.

An object of the present invention is to change two lasers having different energy distributions by changing the optical path length and spatial energy distribution of each beam split by a beam partial split mirror in laser annealing using a pulse laser. An object of the present invention is to provide a laser irradiation apparatus which can irradiate an object to be processed with changing a timing of a beam, and to provide a method of forming an excellent crystalline silicon thin film.

[0013]

According to the present invention, there is provided a laser irradiation apparatus comprising: a laser light source for oscillating a laser beam; a first mirror for partially splitting the laser beam; A laser mirror for irradiating the object to be processed with the laser beam condensed by the second mirror, wherein the first mirror and the second mirror are provided.
Change the length of the optical path of the laser beam split by the first mirror between the mirrors and the length of the optical path of the laser beam not split, and change the spatial intensity distribution of the laser beam to at least one of the optical paths. And a laser beam forming unit for providing the laser beam. Further, in the method for forming a silicon thin film of the present invention, the laser beam focused by the second mirror in the laser irradiation device according to claim 1 is irradiated on the silicon thin film formed on the insulating substrate,
The method is characterized in that the silicon thin film is crystallized.

[0014]

According to the present invention, the energy of the pulse laser beam is divided by the beam splitting partial transmission mirror, and the optical path length of each split beam from the beam splitting partial transmission mirror to the beam focusing partial transmission mirror is changed. By doing so, a laser beam having a larger pulse width than the laser beam oscillated by the laser oscillator is obtained on the surface of the sample to be irradiated with the laser, so that the energy of the laser can be converted to thermal energy much more efficiently. A high-quality crystalline silicon thin film can be manufactured.

Further, according to the present invention, since the spatial energy distribution of the laser beam can be controlled by the action of the laser beam forming section and the attenuator provided in the optical path, a crystalline silicon thin film having necessary characteristics can be freely obtained. It becomes possible.

[0016]

Next, an embodiment of the present invention will be described with reference to the drawings.

The present invention is basically configured as shown in FIG. The pulse laser beam LB emitted from the laser oscillation section LS passes through an attenuator AT that varies the energy intensity of the beam, and passes through an optical path BE and an optical path BCDE by a beam partial split mirror PM. The beam is split into M1 and M2 are total reflection mirrors. Beams that have passed through a plurality of optical paths are collected on the same optical path by a beam condensing partially transmitting mirror CM at a point E. The spatial energy intensity distribution of the pulse beam leaving the laser oscillator LS is not constant. Therefore, the spatial energy intensity distribution of the beam is changed by the laser beam forming unit HM as needed. Further, the laser beam is passed through a convex lens or a concave lens LE to change the energy intensity as needed, and the laser beam is irradiated. Depending on the purpose, the laser beam is sampled without using an attenuator, laser beam forming unit, or lens.
May be irradiated.

The pulse width of a currently typical XeCl excimer laser is 30 to 50 ns. Pulse width is 50 ns
Then, if the distance of the path BCDE is 15 m longer than the path BE, the speed of the light is 300,000 kms ^-1 and the beam passing through the path BCDE is
The beam reaches the beam condensing partially transmitting mirror CM with a delay of 50 ns from that after passing through -E. Then, if the beam emitted from the laser oscillator is a pulse as shown in FIG.
The beam via CDE and the beam via path BE are:
As shown in FIG. 2B, the beam reaches a sample surface SB as a temporally continuous beam. In this case, the pulse width obtained on the sample surface is 100 ns. Generally, the pulse width is τ (s)
In order to make the pulse laser beam having a pulse width of 2τ (s), the optical system in FIG.
Assuming that the speed of light is c (ms ^-1 ), it can be obtained by making the route BCDE longer cτ (m) than the route BE.

If the spread of the beam is the same in the plane perpendicular to the traveling direction of the beam, the energy per unit time of the beam reaching the sample surface is half that of the beam leaving the laser oscillator. However, the energy reaching the sample surface is the same as the energy of the laser beam emitted from the laser oscillator.

The penetration length of an electromagnetic wave having a wavelength of 308 nm into a silicon thin film is 10 nm. However, since the time length of the laser pulse is about 50 ns, when the pulse laser is irradiated on the sample surface, the energy of the laser becomes
Only the portion 10 nm from the surface of the silicon thin film becomes instantaneously hot, but since the heat transfer coefficient of silicon is small, the surface of the silicon thin film is changed before the energy of the pulse laser is changed to the thermal energy for melting the silicon thin film. It is consumed for the vaporization and plasma conversion of silicon atoms. That is,
If the time width of the pulse laser is short, only the surface of the silicon thin film is dissolved and crystallized. Therefore, the thickness is 100 n
In the case of a silicon thin film having a thickness of m or more, the portion not smaller than about 50 nm from the surface is not sufficiently crystallized. Therefore, even if the heat transfer coefficient of silicon is small, it is possible to sufficiently crystallize a silicon thin film having a thickness of about 150 nm by increasing the pulse width by devising the optical system as described above.

In the above example, an example is shown in which the pulse beam divided by the beam subdivision unit is transformed into a temporally continuous pulse. However, by freely adjusting the length of the path BCDE, the temporal An effective effect for crystallization of a silicon thin film can be obtained by deforming not only a continuous pulse but also a discontinuous pulse. For example, pulse width 50n
s laser beam into path BE and path BCDE
When the light passes through an optical system having a distance difference of 30 m, a pulse laser as shown in FIG. 3 can be obtained.

FIG. 4 shows a method of applying the method of FIG. 1 to make the pulse width of the laser beam on the irradiation surface of the sample four times the pulse width of the pulse beam emitted from the laser oscillator. If the pulse width of the laser pulse oscillated by the laser as in the example described above is 50 ns, the path HIJ
The path of −K is 15 nm longer than the path HK, and the path of L−
If the route of MNP is 30m longer than route LP,
A pulse beam having a pulse width of 200 ns as shown in FIG. 5 is obtained.

FIG. 6 shows a method of making the pulse width of the laser beam on the irradiation surface of the sample three times the pulse width of the pulse beam emitted from the laser oscillator. Pulse width of laser beam in laser oscillator is 50 ns
In the case of, the energy division ratio of the beam on the path BI and the beam on the path BC by the pulse division partial transmission mirror PM1 is 2: 1 and the beam on the CG path
-Partial transmission mirror PM2 for beam splitting of beam in D path
The beam splitting ratio is 1: 1 and the path C
-The path of GHD is 15 m longer than the path CD, and the path of the path BIJE is 3 m longer than the path BCDE.
If the length is 0 m, the pulse width is 150 ns as shown in FIG.
Laser beam can be obtained.

When the pulse width is increased as in the above example, the energy per unit time decreases, but the amount of energy applied to the sample surface is the same as before the pulse is processed by the above optical system. By processing the laser beam by the above optical system, the energy of the electromagnetic wave of the laser beam is efficiently converted into thermal energy in the silicon thin film, so that a good crystalline silicon thin film can be obtained.

In order to obtain a silicon crystal having uniform characteristics in a range where the silicon thin film is irradiated with the beam, it is necessary to make the spatial energy intensity distribution of the pulsed laser beam uniform. For example, in the example of FIG. 1, the laser beam forming unit HM may be installed between the beam condensing partially transmitting mirror and the sample. The laser beam forming unit can be provided not only in FIG. 1 but also in FIGS. 4 and 6.

Alternatively, depending on the purpose, if necessary, as shown in FIG. 8, the laser beam forming unit HM may be provided with a path BE and a path B-B between the beam partial dividing unit and the beam condensing partially transmitting mirror. It can be installed at any one or both between C-D-E. Although the energy distribution of the laser beam emitted from the laser oscillator is spatially Gaussian distributed, for example, as shown in FIG. 9, a laser beam forming unit HM for making the energy distribution of the laser beam uniform in the path BE is provided. When installed, as shown in FIG.
The sample can be continuously irradiated with pulsed laser beams having different energy distributions in time. FIG.
, The normal direction of the optical axis of the laser beam is represented as “X direction”.

Furthermore, the energy intensity of the laser beam can be controlled by installing an attenuator for appropriately attenuating the intensity of the laser beam before and after the beam splitting partial transmission mirror and before and after the beam focusing transmission mirror.

As described above, spatially and temporally can be achieved by appropriately arranging the partial transmission mirror for beam splitting, the mirror, the attenuator, the partial transmission mirror for beam focusing, and the laser beam forming section at an appropriate distance. Pulse laser beams having various energy distributions can be obtained.

Thus, a recrystallized silicon thin film or a crystalline silicon thin film having good characteristics required for a semiconductor integrated circuit or an active matrix type thin film transistor can be obtained.

In the above embodiments, a semiconductor integrated circuit and a silicon thin film of a thin film transistor of an active matrix type display band have been described. However, the present invention is not limited to the above embodiments. It can be applied to fields such as property modification of metals, formation and decomposition of polymers, chemical reactions, and biological reactions.

In the above embodiment, the XeCl excimer laser is used as an example. However, the pulse laser is not limited to this, and excimer lasers such as ArF and KrF, YAG
The present invention can be applied to a pulse laser such as a laser and a ruby laser.

[0032]

As described above, according to the present invention,
In a laser irradiation apparatus that recrystallizes or crystallizes a silicon thin film using a pulse laser, the laser beam is split by a partially split mirror for beam splitting, the path of the split beam is changed, and the beam is collected. By making the optical path of the laser beam the same again with the partially transmitting mirror for light and increasing the pulse width of the beam, the electromagnetic wave energy of the laser beam is efficiently converted into thermal energy in the silicon thin film, and the crystal grains are converted. This has the effect of obtaining a high-quality recrystallized or crystallized silicon thin film with a large diameter and few crystal defects.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a laser irradiation apparatus according to the present invention.

FIG. 2 is a diagram illustrating a pulse of a laser beam improved according to the present invention.

FIG. 3 is a diagram illustrating a pulse improved by the second example of the present invention.

FIG. 4 is a diagram illustrating a laser irradiation apparatus according to a third example of the present invention.

FIG. 5 is a diagram illustrating a pulse of a laser beam improved by using the laser irradiation apparatus according to the third example of the present invention.

FIG. 6 is a diagram illustrating a laser irradiation apparatus according to a fourth example of the present invention.

FIG. 7 is a diagram illustrating a pulse of a laser beam improved by using the laser irradiation apparatus according to the fourth example of the present invention.

FIG. 8 is a diagram illustrating a laser irradiation apparatus according to a fifth example of the present invention.

FIG. 9 is a diagram illustrating an application example of a laser irradiation apparatus according to a fifth example of the present invention.

FIG. 10 is a view for explaining pulses of a laser beam improved by the laser irradiation apparatus of FIG. 9 of the present invention.

FIG. 11 illustrates a conventional laser irradiation apparatus.

[Explanation of symbols]

 LS Laser oscillator AT Attenuator PM, PM1 Beam splitting partially transmitting mirror CM, CM1 Beam condensing partially transmitting mirror M1, M2, M3, M4 Total reflection mirror HM, HM1, HM2 Laser beam forming unit LE Composite optical system of convex lens and concave lens SA sample

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI H01S 3/101 H01S 3/101

Claims (2)

(57) [Claims] 1. A laser light source for oscillating a laser beam, a first mirror for partially splitting the laser beam, and a second mirror for collecting the split laser beam in an optical path of an undivided laser beam. A laser irradiation device for irradiating the object to be processed with the laser beam condensed by the second mirror, wherein the optical path of the laser beam split by the first mirror between the first mirror and the second mirror is split Change the length of the optical path of the laser beam, and at least one of
A laser irradiation device comprising a laser beam forming unit for changing a spatial intensity distribution of a laser beam. 2. A laser irradiation apparatus according to claim 1, wherein the laser beam focused by the second mirror is irradiated on a silicon thin film formed on an insulating substrate to crystallize the silicon thin film. Method for forming a silicon thin film.