JP5291895B2 - Laser annealing apparatus and laser annealing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain isotropic crystal grains by growing a crystal in many directions even if using a laser oscillator which emits a linearly polarized laser beam. <P>SOLUTION: The polarization direction of a pulse laser beam 1 emitted in a linearly polarized state is changed at each pulse, and a semiconductor film 2 is irradiated with the pulse laser beam 1 having three or more different polarization directions so that the number of times of irradiation per unit area is three or more. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a laser annealing apparatus and a laser annealing method for crystallizing a semiconductor film by irradiating the semiconductor film with pulsed laser light.

Laser annealing is performed by irradiating an amorphous semiconductor film such as an amorphous silicon film (hereinafter referred to as an a-Si film) formed on a substrate made of low-melting glass (usually alkali-free glass) with a laser beam. This is a technique for forming a polycrystalline semiconductor film by solidification and recrystallization. Since the crystallized silicon film has better electrical characteristics than the a-Si film, a thin film transistor (:: a liquid crystal display or an organic EL display that requires high-definition display such as a mobile phone or a digital still camera). It is adopted in Thin Film Transistor (TFT).
In this laser annealing, a pulse laser beam emitted from a laser light source is usually processed into a beam having a cross section (hereinafter referred to as a linear beam) using an optical system, and this linear beam is converted into a-Si. The film is scanned in the minor axis direction of the linear beam.

Conventionally, an excimer laser, which is mainly a gas laser, has been used as a laser light source for laser annealing. However, in recent years, solid materials such as YAG, YLF, and YVO ₄ that are advantageous in terms of cost and maintenance and can provide stable laser output. A laser annealing apparatus that uses the fundamental wave of a laser by converting the wavelength into a visible region having a good absorption rate with respect to the a-Si film has attracted attention (for example, Patent Document 1 below).

  FIG. 5 is a diagram for explaining the relationship between the crystal grain length and the transistor performance. The transistor performance of polycrystalline silicon is determined by the number of crystal grain boundaries in the region (channel) in which electrons move. As shown in FIG. 5, the more the number of crystal grain boundaries, the more difficult the electrons move. Performance is degraded. Therefore, the longer the crystal grains are (ie, the larger the crystal grains), the better the transistor performance. On the other hand, in a transistor that is actually manufactured, the channel direction (the direction in which current flows) may be the same as the beam scanning direction described above or in the vertical direction. In order to align the performance of each transistor, it is necessary to form polycrystalline silicon having the same number of crystal grain boundaries regardless of the direction in which the channel is formed. For that purpose, the length of each crystal grain of the polycrystalline silicon needs to be the same in the beam scanning direction and the vertical direction. In particular, liquid crystal displays that require a high level of uniformity in operation performance and light emission performance between pixels, and transistors for forming pixels of organic EL displays have crystals with a uniform crystal grain length in any direction. Grain is required.

In the laser annealing using the excimer laser described above, the fundamental wavelength of the laser light oscillated from the excimer laser is 308 nm (XeCl) or 248 nm (KrF), so that it can be used without wavelength conversion and is randomly polarized. , Crystal grains having a uniform crystal grain length can be obtained in any direction.
On the other hand, in laser annealing using a solid-state laser, it is necessary to use a wavelength conversion element in order to convert the fundamental wave of the solid-state laser into a harmonic, and some wavelength conversion elements corresponding to high output have a specific polarization. Since the light having the direction is selectively transmitted, when the wavelength conversion element is used, the wavelength-converted harmonic is inevitably converted into a linearly polarized laser beam. As a result, since the a-Si film is irradiated with linearly polarized laser light, an energy distribution is formed in the polarization direction on the irradiated surface, and silicon grows in the direction of the distribution (see Non-Patent Document 1 below). ). As a result, crystal grains having different crystal lengths in the polarization direction and other directions are formed. Therefore, there is a problem that the performance of the transistors is not uniform depending on the direction of the channel.

Japanese Patent Application Laid-Open No. 2004-34295 IEEE JOURNAL OF QUANTUM ELECTONICS, VOL. QE-22, NO. 8, AUGUST, 1986, pp. 1384-1403

  The present invention has been made in view of the above-described problems, and a laser capable of obtaining isotropic crystal grains by growing crystals in a number of directions even when using a laser oscillator that emits linearly polarized laser light. An object is to provide an annealing apparatus and a laser annealing method.

In order to solve the above problems, the laser annealing apparatus and the laser annealing method of the present invention employ the following means.
The present invention relates to a laser annealing apparatus for irradiating a semiconductor film with a pulsed laser beam to crystallize the semiconductor film, a laser oscillator that emits a linearly polarized pulsed laser beam, and a pulsed laser beam from the laser oscillator. Polarization direction changing means for generating a pulse laser beam having three or more different polarization directions by changing the polarization direction for each pulse, and the pulse laser beam having three or more different polarization directions is The semiconductor film is irradiated so that the number of irradiations per region is 3 or more.
The present invention also relates to a laser annealing method for crystallizing a semiconductor film by irradiating the semiconductor film with a pulsed laser beam, the linearly polarized pulsed laser beam is emitted, and the polarization direction of the pulsed laser beam is changed for each pulse. In order to generate pulsed laser beams having three or more different polarization directions, and to irradiate the pulsed laser beams having three or more different polarization directions at least three times per unit region. The semiconductor film is irradiated.

  According to the above apparatus and method, the polarization direction of the pulsed laser light emitted as linearly polarized light is changed for each pulse, and pulsed laser light having three or more different polarization directions is irradiated at a number of times per unit region. Since the semiconductor film is irradiated so as to be three times or more, an energy distribution in at least three directions is formed on the irradiated surface. If the number of different polarization directions is increased and the number of irradiations per unit region is increased, an energy distribution dispersed in more directions can be formed on the irradiation surface. For this reason, crystal growth occurs in a large number of directions compared to the case of irradiating pulsed laser light having linearly polarized light in a single direction, whereby isotropic crystal grains can be obtained, and transistor performance is improved. Can be aligned.

In the above laser annealing apparatus, the polarization direction changing means includes a half-wave plate disposed on the optical path of the pulsed laser beam, and the half-wave plate centered on the optical axis of the pulsed laser beam. And a drive unit that rotates the motor.
Further, in the laser annealing method described above, a half-wave plate is disposed on the optical path of the pulsed laser light, and the pulse is obtained by rotating the half-wave plate around the optical axis of the pulsed laser light. The polarization direction of the laser beam is changed for each pulse.

  The half-wave plate has a function of rotating the direction of linearly polarized light by twice the angle formed with the optical axis, so that each pulse of the pulsed laser light passes according to the angle to be rotated. By setting the angle of the optical axis at that time, the polarization direction of each pulse can be easily changed. In addition, the polarization direction can be easily changed for each pulse by rotating the half-wave plate so that the angle formed with the optical axis changes for each pulse.

In the laser annealing apparatus, the polarization direction changing unit includes a magneto-optical element disposed on the optical path of the pulsed laser light, and a magnetic field generator that applies a magnetic field to the magneto-optical element.
In the laser annealing method described above, a magneto-optical element is disposed on the optical path of the pulse laser beam, and a magnetic field is applied to the magneto-optical element, whereby the polarization direction of the pulse laser beam is changed for each pulse.

  The magneto-optical element has a function of rotating the direction of linearly polarized light by a rotation angle proportional to the thickness of the element and the magnitude of the magnetic field to be applied. Therefore, the magnetic field when each pulse passes according to the angle to be rotated. By setting the magnitude of, the polarization direction of each pulse can be easily changed to an arbitrary direction.

  According to the present invention, even if a laser oscillator that emits linearly polarized laser light is used, an excellent effect is obtained in that isotropic crystal grains can be obtained by crystal growth in many directions.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the common part in each figure, and the overlapping description is abbreviate | omitted.

FIG. 1 is a diagram showing a schematic configuration of a laser annealing apparatus 10 according to an embodiment of the present invention.
The laser annealing apparatus 10 is an apparatus that irradiates the semiconductor film 2 with the pulsed laser light 1 to crystallize the semiconductor film 2. As with the laser annealing apparatus 10, in laser annealing, the pulsed laser light 1 shaped into a linear beam is usually scanned in the beam short axis direction with respect to the semiconductor film 2 to melt and solidify the semiconductor film 2. To crystallize.
The semiconductor film 2 is an amorphous semiconductor film (for example, an a-Si film). The semiconductor film 2 is formed on a substrate 3 (for example, non-alkali glass). This film formation can be performed using, for example, PVD (physical vapor deposition) or CVD (chemical vapor deposition).

  As shown in FIG. 1, the laser annealing apparatus 10 includes a laser oscillator 12, a polarization direction changing unit 14, a beam homogenizer 16, a condenser lens 18, a control device 20, and a substrate stage 5. Hereinafter, each component will be described.

The laser oscillator 12 is a solid-state laser device that oscillates the pulse laser beam 1 from the solid-state laser element 12a. The pulse timing of the laser oscillator 12 is controlled by the control device 20.
A YAG crystal, a YLF crystal, a YVO ₄ crystal, or the like can be applied to the solid-state laser element 12a. The laser oscillator 12 converts the laser beam from the solid-state laser element 12a from a fundamental wave to a harmonic (second or third harmonic) in order to improve the absorption rate of the laser beam with respect to the semiconductor film 2. It has the element 12b. The wavelength of the second harmonic of the YAG laser and the YVO ₄ laser is 532 nm, and the wavelength of the third harmonic is 355 nm. The wavelength of the second harmonic of YLF is 527 nm, and the wavelength of the third harmonic is 351 nm.

  As described above, since the laser light that has passed through the wavelength conversion element 12b becomes linearly polarized light, the pulsed laser light 1 emitted from the laser oscillator 12 becomes linearly polarized light. A bidirectional arrow on the optical path of the pulsed laser beam 1 in FIG. 1 schematically shows the polarization direction at that position.

  The polarization direction changing unit 14 changes the polarization direction of the pulse laser beam 1 from the laser oscillator 12 for each pulse, and generates the pulse laser beam 1 having three or more different polarization directions. The operation of the polarization direction changing means 14 is controlled by the control device 20. Here, the “three directions” is the minimum number of directions, and the larger the number of different polarization directions, the better. For this reason, it is preferable that the number of directions is not less than 4 directions, but it is preferable that the directions are 5 directions, 10 directions, 15 directions, or more. It is preferable to irradiate the film 2 several times (for example, 10 times, 20 times or more). In other words, it is preferable to irradiate the semiconductor film 2 with the pulsed laser light 1 so that each angular interval in the polarization direction of each incident pulse exceeds 0 and is 45 degrees or less. Moreover, it is more preferable that each of the angle intervals is more than 0 and 30 degrees or less, and more preferably, all the angles are more than 0 and 15 degrees or less. By irradiating the pulse laser beam 1 so as to satisfy such conditions, the same effect as that obtained when the semiconductor film 2 is irradiated with the pulse laser beam having substantially random polarization can be obtained. A specific configuration example of the polarization direction changing unit 14 will be described later.

The beam homogenizer 16 has a function of shaping the incident pulse laser beam 1 into a linear beam. The beam homogenizer 16 having such a function is well known in the art and will not be described in detail. However, the beam homogenizer 16 is preferably composed of a beam expander, a cylindrical lens, a cylindrical lens array, and the like.
The condensing lens 18 condenses the pulsed laser light 1 from the beam homogenizer 16 on the surface of the semiconductor film 2.

The substrate stage 5 supports the substrate 3 from the back side. The substrate stage 5 is configured to be movable in a predetermined feeding direction. When the short-axis direction of the linear beam is, for example, the vertical direction in FIG. 1, the substrate stage 5 moves in the vertical direction in FIG. By moving the substrate stage 5, the pulse laser beam 1 can be scanned in the surface direction of the substrate 3. The operation (position and speed) of the substrate stage 5 is controlled by the control device 20.
The position of the substrate stage 5 may be fixed and the irradiation position of the pulse laser beam 1 may be moved to scan the pulse laser beam 1 in the surface direction of the substrate 3.

  The laser annealing apparatus 10 configured as described above differs in that the laser oscillator 12 emits linearly polarized pulsed laser light 1 and the polarization direction changing means 14 changes the polarization direction of the pulsed laser light 1 for each pulse. A pulsed laser beam 1 having three or more polarization directions is generated. Then, the semiconductor film 2 is irradiated with pulsed laser light 1 having three or more different polarization directions so that the number of irradiations per unit region is three or more. The control device 20 controls the oscillation timing of the pulse laser from the laser oscillator 12 in this case, the change of the polarization direction for each pulse by the polarization direction changing means 14 and the moving speed of the substrate stage 5.

  According to the above-described embodiment, the polarization direction of the pulsed laser light 1 emitted as linearly polarized light is changed for each pulse, and the pulsed laser light 1 having three or more different polarization directions is irradiated per unit region. Since the semiconductor film 2 is irradiated so that the number of times is three or more, an energy distribution in at least three directions is formed on the irradiation surface. If the number of different polarization directions is increased and the number of irradiations per unit region is increased, an energy distribution dispersed in more directions can be formed on the irradiation surface. For this reason, crystal growth occurs in many directions as compared with the case of irradiating the pulsed laser beam 1 having linear polarization in a single direction, whereby isotropic crystal grains can be obtained. Can be aligned.

FIG. 2 is a diagram illustrating a first configuration example of the polarization direction changing unit 14. The polarization direction changing means 14 of the first configuration example rotates the half-wave plate 22 disposed on the optical path of the pulse laser beam 1 and the half-wave plate 22 around the optical axis of the pulse laser beam 1. And a drive unit 23.
The half-wave plate 22 has a function of rotating the direction of linearly polarized light by twice the angle formed with the optical axis. For example, when linearly polarized light having an angle of 10 degrees with the optical axis is incident on the half-wave plate 22, the linearly polarized light is rotated by 20 degrees.
The drive unit 23 is, for example, a drive motor, and the rotation speed thereof is controlled by the control device 20 described above.

  According to the first configuration example configured as described above, each angle can be easily set by setting the angle of the optical axis when each pulse of the pulse laser beam 1 passes according to the angle to be rotated. The polarization direction of the pulse can be changed. In addition, the polarization direction can be easily changed for each pulse by rotating the half-wave plate 22 by the drive unit 23 so that the angle formed with the optical axis changes for each pulse.

FIG. 3 is a diagram illustrating a second configuration example of the polarization direction changing unit 14. The polarization direction changing means 14 of the second configuration example includes a magneto-optical element 25 disposed on the optical path of the pulsed laser light 1 and a magnetic field generator 26 that applies a magnetic field to the magneto-optical element 25.
The phenomenon that the polarization plane rotates when a magnetic field is applied to a transparent non-magnetic material and linearly polarized light is transmitted in a direction parallel to the magnetic field is called the Faraday effect. The transparent non-magnetic material is called a magneto-optical element. This configuration example uses this Faraday effect.

The rotation angle α of linearly polarized light by the Faraday effect is expressed as α = V · H · L, where H is the strength of the magnetic field and L is the length of the magneto-optical element 25 through which the polarized light passes. V is a constant that depends on the type of substance constituting the magneto-optical element 25, the wavelength of polarized light, and temperature, and is called the Verde constant. As described above, the magneto-optical element 25 has a function of rotating the direction of linearly polarized light at a rotation angle proportional to the thickness of the element (dimension in the light transmission direction) and the magnitude of the applied magnetic field.
The magnetic field generator 26 is composed of, for example, a coil and a power source for supplying a current to the coil, and the intensity of the magnetic field applied to the magneto-optical element 25 is controlled by controlling the amount of current flowing to the coil by the control device 20. Is controlled.

  According to the second configuration example configured as described above, by changing the magnitude of the magnetic field applied to the magneto-optical element 25 when each pulse passes according to the angle to be rotated, The polarization direction of each pulse can be easily changed to an arbitrary direction.

FIG. 4 is a diagram illustrating a third configuration example of the polarization direction changing unit 14. The polarization direction changing means 14 of the third configuration example rotates the depolarization element 28 and the glass plate 29 facing each other on the optical path of the pulsed laser light 1 and the optical axis of the pulsed laser light 1. Drive units 30 and 31 are provided.
The depolarizer 28 has a function of rotating the polarization direction in proportion to the optical path length of the light passing therethrough. One surface of the depolarization element 28 is a tapered surface inclined with respect to a plane perpendicular to the optical axis, and this inclination causes a difference in the optical path length passing through the element depending on the position of incident light. The light that has passed through the element 28 becomes light having a different polarization direction depending on the position. In this configuration example, the function of the depolarizer 28 is used.
According to this configuration example, the polarization direction can be changed for each pulse by rotating the depolarization element 28 by the driving unit 30.

However, since the outgoing light from the depolarization element 28 has a predetermined refraction angle with respect to the incident angle, only the depolarization element 28 changes the traveling direction of the pulsed laser light 1. Therefore, a glass plate 29 having a taper surface parallel to the taper surface of the depolarization element 28 is disposed so as to face the depolarization element 28, and the traveling direction of the pulsed laser light 1 is refracted by the glass plate 29 in advance. The glass plate 29 is rotated by the drive unit 31 in synchronization with the depolarization element 28. As a result, the traveling direction of the pulse laser beam 1 that has passed through the depolarizing element 28 is returned to the traveling direction before being refracted by the glass plate 29. As a result, the traveling direction of the pulsed laser light 1 can be matched before entering the glass plate 29 and after passing through the depolarizing element 28. In this case, the optical axes of the two do not match but are parallel.
The drive units 30 and 31 are, for example, drive motors, and the rotation speed is controlled by the control device 20.

  Although the embodiments of the present invention have been described above, the embodiments of the present invention disclosed above are merely examples, and the scope of the present invention is not limited to these embodiments. For example, in the above embodiment, an a-Si film is targeted as an amorphous semiconductor film, but other amorphous semiconductor films (for example, compound semiconductor films having an amorphous structure such as an amorphous silicon germanium film). May be targeted. The scope of the present invention is indicated by the description of the scope of claims, and further includes meanings equivalent to the description of the scope of claims and all modifications within the scope.

It is a figure which shows schematic structure of the laser annealing apparatus concerning embodiment of this invention. It is a figure which shows the 1st structural example of a polarization direction change means. It is a figure which shows the 2nd structural example of a polarization direction change means. It is a figure which shows the 3rd structural example of a polarization direction change means. It is a figure explaining the relationship between crystal grain length and transistor performance.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Pulse laser beam 2 Semiconductor film 3 Substrate 5 Substrate stage 10 Laser annealing apparatus 12 Laser oscillator 12a Solid-state laser element 12b Wavelength conversion element 14 Polarization direction change means 16 Beam homogenizer 18 Condensing lens 20 Control apparatus 22 1/2 wavelength plate 23, 30, 31 Drive unit 25 Magneto-optical element 26 Magnetic field generator 28 Depolarization element 29 Glass plate

Claims (2)

A laser oscillator that emits linearly polarized pulsed laser light;
Polarization direction changing means for changing the polarization direction of the pulsed laser light from the laser oscillator for each pulse to generate pulsed laser light having three or more different polarization directions,
A laser annealing apparatus that irradiates the semiconductor film with the pulsed laser light having a polarization direction of three or more directions and irradiates the semiconductor film so that the number of irradiation times per unit region is three or more ,
The polarization direction changing means has a half-wave plate disposed on the optical path of the pulsed laser light, and a drive unit that rotates the half-wave plate about the optical axis of the pulsed laser light. A laser annealing apparatus characterized by the above. A linearly polarized pulsed laser beam is emitted, and the polarization direction of the pulsed laser beam is changed for each pulse to generate a pulsed laser beam having three or more different polarization directions, and having three or more different polarization directions. A laser annealing method for crystallization by irradiating the semiconductor film with the pulsed laser light so that the number of irradiation times per unit region is 3 or more ,
A half-wave plate is disposed on the optical path of the pulse laser beam, and the polarization direction of the pulse laser beam is changed for each pulse by rotating the half-wave plate about the optical axis of the pulse laser beam. A laser annealing method characterized by changing.

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