JP2003068644A - Method for crystallizing silicon and laser annealing system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for crystallizing silicon which enables to form a silicon film having large grains from an amorphous silicon film. SOLUTION: The method for crystallizing silicon for crystallizing a silicon film of an amorphous layer comprises a process (a) of irradiating the silicon film with a first pulse laser beam having first intensity and first duration for melting it, a process (b) after melting the silicon film, irradiating the silicon film in the middle of a cooling process with second pulse laser light having second intensity and second duration, and growing generated silicon crystal grains without completely melting them, and a process (c) of repeating the processes (a) and (b), partially shifting a pulse laser irradiated region, to enlarge a crystallized region. Each of the first and the second pulse laser light is an integrated laser beam, having a wavelength in the range of 400 to 900 nm, from a plurality of laser diode-pumped solid-state lasers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon crystallization method and a laser annealing apparatus, and particularly to a silicon crystallization method capable of forming large crystal grains and having a simple maintenance of the apparatus used, and a laser annealing apparatus used in this method. Regarding

[0002]

2. Description of the Related Art Amorphous silicon films are used for solar cells,
Widely used in liquid crystal display devices, semiconductor devices, and the like. Amorphous silicon has a lower mobility than crystalline silicon. By crystallizing the amorphous silicon, an electronic device having more excellent characteristics can be created.

Various annealing techniques for polycrystallizing amorphous silicon have been developed. Currently, a technique of irradiating amorphous silicon with XeCl excimer laser light to polycrystallize it is widely used. The XeCl excimer laser requires time and cost for maintenance. The electrical characteristics of the obtained polycrystalline silicon film are not sufficiently satisfactory. There is a need for better quality, lower cost polycrystalline silicon films.

[0004]

Polycrystalline silicon contains an aggregate of single-crystallized crystal grains (grains). The grain boundary, which is the boundary between crystal grains, affects the electrical characteristics of polycrystalline silicon. The electrical characteristics of polycrystalline silicon having a large amount of grain boundary are generally more likely to deteriorate than those of polycrystalline silicon having a small amount of grain boundary.

As the size of each grain (grain size) increases, the grain boundary generally decreases. In order to obtain polycrystalline silicon having excellent electrical characteristics, a technique for forming polycrystalline silicon having a large grain size is required.

Further, if a region for forming a semiconductor element, at least a region that influences the characteristics of the semiconductor element (for example, a channel region of a thin film transistor (TFT)) can be formed in one grain, the characteristics of the semiconductor element can be greatly improved. .

It is an object of the present invention to provide a silicon crystallization method capable of forming a silicon film having a large grain from an amorphous silicon film.

Another object of the present invention is to provide a laser annealing apparatus capable of performing such silicon crystallization.

[0009]

According to one aspect of the present invention, there is provided a silicon crystallization method for crystallizing a silicon film of an amorphous layer, comprising: (a) a silicon film having a first strength and a first time; Irradiating a first pulsed laser beam having a length to melt the silicon film, and (b) after melting,
The silicon film in the course of cooling is irradiated with a second pulsed laser beam having a second intensity lower than the first intensity and a second time length, and the generated silicon crystal grains are not completely melted, The steps of growing crystal grains, and (c) repeating the steps (a) and (b) by partially shifting the irradiation region to expand the crystallization region, and the first and the first There is provided a silicon crystallization method in which the two pulsed laser lights are the sum of laser lights having wavelengths of 400 nm to 900 nm from a plurality of laser diode pumped solid state lasers.

According to another aspect of the present invention, a plurality of laser diode pumped solid state lasers are included, and the wavelength is 400 nm to 9 nm.
00 nm, a first intensity, a first pulse laser light source group capable of generating a first pulse laser light having a first time length, and a plurality of laser diode pumped solid state laser, wavelength 400nm ~ 900nm, the first. A second pulse laser light source group capable of generating a second pulse laser light having a second intensity and a second time length lower than the intensity, and laser diode pumped solids of the first and second pulse laser light source groups Timing control means for controlling the pulsed laser light generation timing of the laser, a stage on which an object to be processed is placed and can be driven in a two-dimensional plane, stage control means for controlling the driving of the stage, and stage mounted on the stage. A laser ani having an optical system for irradiating the processing target region of the processing target to be placed with the first pulse laser light or the second pulse laser light. Le device is provided.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A laser annealing apparatus according to an embodiment of the present invention will be described with reference to FIG. The pulse laser light sources 11 and 12 can generate a pulse laser beam having a predetermined intensity and a predetermined time length, respectively. The configurations of the pulse laser light sources 11 and 12 will be described later.

The laser beam from the pulsed laser light source 11 is, for example, an s-wave, and the λ / 2 phase plate 13 causes the laser beam to emit light.
Changed to waves, mirrors M11, M12, attenuator A
It is incident on the polarization demultiplexer (polarization beam splitter) PBS via 1. PBS transmits, for example, p-polarized light.

The laser beam from the pulse laser light source 12 is, for example, an s wave, and is incident on the polarization demultiplexer (polarization beam splitter) PBS via the mirrors M21 and M22, the attenuator A2, and the mirror M23. P
The BS reflects, for example, s-polarized light. In this way
The two laser beams are emitted on the same optical path.

The light emitted from the PBS is mirrors M31 and M32.
It is incident on the beam expander 16 via the and the beam diameter is expanded. The laser beam having the expanded beam diameter is relieved of coherence by the spatial coherence canceling element 17, and is converted into a beam having a uniform intensity distribution by the beam homogenizer 18. The spatial coherence canceling element 17 and the beam homogenizer 18 will also be described later.

The laser beam emitted from the beam homogenizer 18 is reflected downward by the mirror M40 and enters the entrance window 22 of the gas curtain device 20. The gas curtain device 20 includes an upper plate 2 having an entrance window 22 and a gas inlet 23.
4 and the side wall member 25 connected to the lower surface of the peripheral portion thereof define a semi-enclosed space. The gas recovery port 2 is provided below the side wall member 26.
6 is provided. Here, the gas atmosphere forming mechanism is not limited to the gas curtain. For example, a vacuum chamber capable of gas exchange may be used.

The stage 30 has an X stage 33 and a Y stage 32 each equipped with a driving means such as a stepping motor, and is movable in a two-dimensional plane. A surface plate 31 is installed on the Y stage 32. An object 19 such as a glass substrate having an amorphous silicon film formed thereon is placed on the job board 31. The order of the X stage and the Y stage may be reversed.

The gas curtain device 20 is arranged at a predetermined distance above the Joban 31. When the inert gas is introduced from the gas introduction port 23, the processing space defined by the upper plate 24, the side wall 25, and the processing object 19 on the Joban 31 is filled with the inert gas and is shielded from the outside air. When the laser beam is irradiated in this state, the amorphous silicon film of the processing object 19 can be melted by the laser beam and solidified (crystallized) in the inert gas atmosphere.

The host controller 35 is composed of a computer or the like, generates desired pulse laser light from the pulse laser light sources 11 and 12 through the timing controller 36 and power supplies 37 and 38, and the stage controller 39 through the stage controller 39. The 30 can be driven at a desired speed and in a desired direction.

FIG. 2A shows the pulse laser light sources 11 and 1.
2, the configuration of the optical system including the PBS and the beam expander 16 will be described in more detail. A plurality of laser light sources 11a, 11
b ,. ． 12a, 12b ,. ． Each includes a laser diode pumped solid state laser, and has a wavelength of 400 nm to 900 n.
The s-polarized laser light of m is emitted. For example, Nd: YA
G laser and second harmonic generator, Nd: YLF laser and 2
A second harmonic generator, or Nd: YVO ₄ laser and secondary high frequency generator. Each pulsed laser light source 11, 1
Although the case where 2 is composed of two laser light sources is illustrated, three or more laser light sources may be used.

Light emitted from the laser light sources 11a and 11b is converted into p-waves by the λ / 2 phase plates 13a and 13b, and the mirrors M11a and M11b and the mirror M12.
The light is reflected by a and M12b and enters the PBS.

Light emitted from the laser light sources 12a and 12b is reflected by the mirrors M23a and M23b, and enters the PBS as s-polarized light. PBS transmits p-polarized light, and
By reflecting the polarized light, the s-polarized light and the p-polarized light can be emitted on the same optical path. The laser light emitted from the PBS is converted into circularly polarized light by the λ / 4 phase plate 14. The beam expander 16 includes, for example, a combination of a concave lens and a convex lens, and once diverges a parallel light beam to form a parallel light beam having an enlarged diameter.

When the pulsed laser light source 11 emits an s-polarized laser beam, if the reflected light returns from the optical system ahead (downstream) of the λ / 4 phase plate, it is s-polarized by the λ / 4 phase plate. It is reflected by PBS. The reflected light of the s-polarized laser beam from the pulse laser light source 12 is reflected by the λ / 4 phase plate,
It is converted to polarized light and transmitted through PBS. In this way, the reflected light does not return to the lasing laser light source.

Pulse laser light having the following two types of characteristics is generated by the two groups of pulse laser light sources shown in FIG. Each of the two groups of pulsed laser light sources operates under the control of the timing control device 36 of FIG. 1, and the pulsed laser light generation timing is controlled.

FIG. 2B is a graph showing the change in intensity of the double pulse laser light generated. The horizontal axis represents time and the vertical axis represents intensity. The first pulse laser beam P1 has high intensity and a narrow pulse width (short time length) so that the amorphous silicon film can be sufficiently melted. The second pulsed laser light P2 has a low intensity and a long pulse width so that the cooling rate of the once melted silicon is delayed and the crystallization rate is slowed to grow crystal grains. Alternatively, both the first pulse laser light and the second pulse laser light have long pulse widths, the intensity of the first pulse laser light is high, and the intensity of the second pulse laser light is low.

For example, as a first example, the first pulse laser light P1 has a time length of 14 nsec, the second pulse laser light P2 has a time length of 120 nsec, and the intensity ratio thereof is 6.9.
Is. As a second example, the time lengths of the first pulse laser beam P1 and the second pulse laser beam P2 are both 120 nse.
c, the intensity ratio is 2.2.

The time length of the first pulsed laser light is 5 nsec.
As described above, it is preferable that the time length is less than 50 nsec and the second pulsed laser light time length is 50 nsec or more and less than 300 nsec. Alternatively, the time lengths of the first pulse laser light and the second pulse laser light are both 50 nsec or more and 300
It is preferably less than nsec.

When the amorphous silicon film is actually crystallized, it is preferable to obtain optimum conditions by irradiating the target amorphous silicon film with pulsed laser light under various conditions.

Generating a laser pulse having such pulse intensity and time length with a single solid-state laser is as follows.
Now it's difficult. By using a plurality of solid-state lasers and simultaneously generating pulsed lights from the plurality of solid-state lasers and combining them, the first pulsed laser light P1 having sufficient intensity can be generated. Further, the second pulsed laser light P2 having a sufficient time length can be generated by oscillating and multiplexing the plurality of solid-state lasers at different timings.

The interval between the first pulsed laser light P1 and the second pulsed laser light P2 depends on the time length and is, for example, (14
nsec, 120nsec) or (120nsec, 12
(0 nsec) is 230 nse each.
c, 450 nsec. The repetition cycle of the double pulse is, for example, on the order of msec. Note that the first and second pulsed laser lights are a combination of a plurality of laser pulses, and thus are strictly pseudo double pulses.

Laser light has coherence by nature. In order to efficiently crystallize the amorphous silicon film, it is desirable to form a long beam and scan while irradiating a wide area. It is desirable that the intensity distribution within the long beam be as constant as possible.

When the laser beams interfere in the irradiation area,
An unintended intensity distribution is generated, which prevents a uniform annealing process. The spatial coherence canceling element is an element that divides the laser beam into a plurality of beam regions and reduces the spatial coherence between the beam regions.

The laser beam usually has an intensity distribution within the beam area. The homogenizer is an element that divides the laser beam into a plurality of regions and redistributes the light flux in each region into a certain common region to uniformize the intensity within the irradiation area.

FIGS. 3A and 3B show examples of the configurations of the spatial coherence canceling element 17 and the homogenizer 18. When the optical axis is in the Z direction, FIG. 3A is a side view seen from the X direction, and FIG. 3B is a side view seen from the Y direction.

The spatial coherence canceling element 17 is an element in which plates of optical materials having different lengths in the Z direction are laminated in the Y direction, and an element in the X direction.
It has a configuration in which elements stacked in the direction are combined. A difference in optical path length is generated, and spatial coherence is reduced. Although the configuration is shown in which the optical path length gradually changes in the areas adjacent to each other, the optical path difference can be set arbitrarily. For example, a random optical path difference may be set.

The structure of the beam is not limited to the illustrated structure as long as it can be divided into a plurality of regions within the beam cross section and different optical path differences are generated in the respective regions. For example, in the case of a configuration such as a fiber bundle, two elements are not required, and the same effect can be achieved with a single element.

The homogenizer 18 forms, for example, a plurality of light fluxes arranged in the Y direction by arranging the X direction cylindrical lenses 18a in the Y direction, and redistributes each light flux in the other X direction cylindrical lenses 18b. The cylindrical lenses 18c are arranged in the X direction to form a plurality of light beams arranged in the X direction, and the other Y-direction cylindrical lens 1 is formed.
Each light flux is redistributed at 8d. In the configuration shown in the figure, each of the light fluxes is imaged on a common region by the imaging lenses 18p and 18q.

In this way, a long beam having a constant intensity is formed within the irradiation area. This long beam is irradiated onto the processing object 19 on which the amorphous silicon film shown in FIG. 1 is deposited.

FIGS. 4A and 4B are a plan view and a partial sectional view showing the structure of the object to be processed 19. As shown in FIG. 4B, two layers of silicon oxide films 41 and 43 are laminated on the glass substrate 40. Lower silicon oxide film 4
1 has an opening 42 formed therein. The upper silicon oxide film 43 forms a sharp cusp 44 on the opening 42.
An amorphous silicon film 46 is deposited on the entire surface of the silicon oxide film 43 having such cusps.

As shown in FIG. 4A, the object to be processed 19
The cusps 44 are formed at regular intervals along the pair of opposite sides. For example, the laser beam is scanned from the right side to the left side, folded back at the left side, scanned from the left side to the right side, and the scanning is repeated zigzag.

As shown in FIG. 4C, a plurality of cusps 4 are formed in the laser beam LB irradiation area at the beginning of each scan.
4 is included. The cusp 44 induces crystal nucleus generation when the amorphous silicon film 46 is melted and solidified. Crystal nuclei are generated at the bottom of the cusp 44 and one large seed crystal grows due to the filter effect. By scanning the laser beam LB, the single crystal region expands in the scanning direction based on each seed crystal.

Note that, as shown in FIG. 4D, the substrate may be divided into a plurality of sections S, and the cusps 44 may be formed in the peripheral portion of each section. By forming a cusp at a portion where crystal growth starts, a seed crystal is grown from the cusp and a wide single crystal region is formed.

Laser irradiation is repeated several times at the same position by overlapping irradiation. The part which could not be crystallized by one irradiation is also crystallized by a plurality of irradiations.

FIG. 4E schematically shows the light absorption coefficients of amorphous silicon and crystalline silicon. The horizontal axis indicates the photon energy E, and the vertical axis indicates the absorption coefficient α. The curve Qc is the absorption coefficient of crystalline silicon, and the curve Qα is the absorption coefficient of amorphous silicon. At the wavelength of the laser light used in this example, the absorption coefficient Qα of amorphous silicon is larger than that of crystalline silicon by one digit or more. In the wavelength range of 400 to 900 nm, the absorption coefficient of amorphous silicon is larger than that of crystalline silicon.

As shown in FIG. 4 (F), when there are various crystallized regions and grain boundaries (amorphous regions) in the laser irradiation region, the grain boundary absorbs more laser light and preferentially. To melt.

As shown in FIG. 4 (G), when the molten region expands, only a relatively large crystallization region remains in the solid phase. If crystallization resumes from such a state, a larger grain size than before will be obtained.

As shown in FIG. 4A, when the stripe-shaped regions are scanned with their ends overlapped, the overlapping portions are affected by the upper and lower crystal regions, and distortion is likely to accumulate. In the case of this embodiment, the region where strain is accumulated and the crystallinity is deteriorated becomes amorphous and preferentially melts. When crystallized again, the strain is relaxed.

As a result of the experiment, a larger grain size was obtained than when the XeCl excimer laser beam was used. At the wavelength of XeCl excimer laser light, amorphous silicon and crystalline silicon have almost the same absorption coefficient. Therefore, it may be difficult to realize the state shown in FIG.

FIGS. 4H and 4I schematically show the structure of a thin film transistor using a crystallized silicon film. Figure 4
As shown in (H), the island-shaped active region AR is formed by patterning the crystallized silicon. The insulated gate electrode G is arranged so as to cross the silicon region.

As shown in FIG. 4 (I), the insulated gate electrode is formed by forming an insulating film such as a silicon oxide film or a silicon nitride film on a silicon region, and forming a gate of a conductive material such as Cr or Al on the insulating film. It is formed by arranging electrodes.

Although the present invention has been described above with reference to the embodiments, the present invention is not limited to these. For example, various changes, improvements, combinations, etc. are possible.
It will be obvious to those skilled in the art.

[0051]

A crystallized silicon film having a large grain size can be obtained.

[Brief description of drawings]

FIG. 1 shows a laser annealing apparatus according to an embodiment of the present invention.

FIG. 2 is an enlarged view of a portion from a light source unit to a beam expander of FIG. 1 and a graph showing a pulse waveform.

FIG. 3 is an enlarged view of a portion of a coherence canceling element and a homogenizer of FIG.

4A and 4B are a plan view, a cross-sectional view, a graph for explaining a crystallization process, a plan view, a plan view of a thin film transistor, and a cross-sectional view showing a structure of a processed object.

[Explanation of symbols]

11, 12 pulse laser light source 13, 14 Phase plate M mirror PBS polarization demultiplexer A Attenuator 16 beam expander 17 Spatial coherence canceling element 18 Homogenizer 20 gas curtain device 30 stages

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 5F052 AA02 BA01 BA07 BA14 BB02                       BB03 BB07 CA04 DA01 EA15                       HA01 JA01 JA09                 5F110 AA16 CC01 DD02 DD13 DD17                       DD30 EE03 EE04 FF02 FF03                       GG02 GG13 PP03 PP04 PP05                       PP07 PP23 PP36 PP40

Claims (14)

[Claims] 1. A method for crystallizing a silicon film of an amorphous layer, comprising: (a) irradiating the silicon film with a first pulse laser beam having a first intensity and a first time length. And (b) after the melting, a second pulsed laser beam having a second intensity lower than the first intensity and a second time length is applied to the silicon film being cooled. Irradiation does not completely melt the generated silicon crystal grains, but grows the crystal grains, and (c) the irradiation region is partially shifted, and the steps (a) and (b) are repeated. And expanding the crystallization region, wherein the first and second pulsed laser lights are both wavelengths 4 from a plurality of laser diode pumped solid state lasers.
A silicon crystallization method in which laser light of 00 nm to 900 nm is added. 2. The method of crystallizing silicon according to claim 1, wherein the silicon film which is irradiated with the first and second pulsed laser beams is shielded from the outside air by an inert gas atmosphere mechanism. 3. The first and second pulsed laser lights are:
The second harmonic of Nd: YAG, the second harmonic of Nd: YLF, or the second harmonic of Nd: YVO ₄ .
Alternatively, the method for crystallizing silicon according to 2. 4. The first time length is 5 nsec or more 50.
4. The method according to claim 1, wherein the second time length is less than nsec and the second time length is 50 nsec or more and less than 300 nsec, or the first and second time lengths are both 50 nsec or more and less than 300 nsec. Silicon crystallization method. 5. The silicon crystallization method according to claim 1, wherein the first and second pulsed laser lights are irradiated through a common spatial coherence canceling element and a homogenizer. . 6. The scanning according to the irradiation of the first and second pulsed laser beams is performed in a stripe shape, and the stripe-shaped crystallization regions contacting each other are formed by folding back at the stripe end portions to form stripe-shaped crystallization regions. The described silicon crystallization method. 7. The amorphous-phase silicon film is deposited on a silicon oxide film formed on a glass substrate, and the silicon oxide film forms a cusp or crystal seed to form a seed crystal. Claim 1 having a step
7. The method for crystallizing silicon according to any one of 6 to 6. 8. A first pulse laser light source group including a plurality of laser diode pumped solid-state lasers, capable of generating a first pulse laser light having a wavelength of 400 nm to 900 nm, a first intensity, and a first time length, Includes multiple laser diode pumped solid state lasers with wavelength 4
A second pulse laser light source group capable of generating a second pulse laser light having a second intensity lower than the first intensity and a second time length of 00 nm to 900 nm; and the first and second pulse laser light sources Timing control means for controlling the pulsed laser light generation timing of each laser diode pumped solid-state laser of the group, a stage on which an object to be processed is placed and driven in a two-dimensional plane, and stage control means for controlling the drive of the stage And a laser annealing device having an optical system for irradiating the processing target area of the processing target placed on the stage with the first pulse laser light or the second pulse laser light. 9. The laser annealing apparatus according to claim 8, further comprising a mechanism capable of forming an inert gas atmosphere that shields the processing target region from the outside air around the processing target region. 10. The first and second pulse laser light source groups respectively include a plurality of Nd: YAG laser light sources and second harmonic generation means, a plurality of Nd: YLF laser light sources and second harmonic generation means, or The laser annealing apparatus according to claim 8 or 9, comprising a plurality of Nd: YVO ₄ laser light sources and second harmonic generation means. 11. The first time length is 5 nsec or more 5
Less than 0 nsec, and the second time length is 50 nsec
Or more and less than 300 nsec, or the first and second time lengths are both 50 nsec or more and 300 nsec
The laser annealing device according to any one of claims 8 to 10, which is less than. 12. The laser annealing apparatus according to claim 8, wherein the optical system includes a spatial coherence canceling element and a homogenizer which are common to the first and second pulsed laser beams. . 13. The laser annealing apparatus according to claim 12, wherein the spatial coherence canceling element divides the laser beam into a plurality of regions and forms an optical path that gives different retardations to the respective regions. 14. The stage control means can scan the processing target area on the processing target in a folded stripe shape.
The laser annealing apparatus according to the item.