JP5339322B2 - Laser crystal growth method by laser - Google Patents

Description

  The present invention relates to a silicon crystal growth method in which polycrystalline silicon is irradiated with laser light to grow a silicon crystal.

  Amorphous silicon has been used for a thin film transistor used for switching pixels in a liquid crystal display. FIG. 6 is a diagram illustrating a thin film transistor in a liquid crystal display. In recent years, polycrystalline silicon (polysilicon) having higher electron mobility and switching characteristics than amorphous silicon has been used as a material for thin film transistors. By using polycrystalline silicon, the transistor can be reduced, and the liquid crystal display can be reduced in size and power consumption. By adopting polycrystalline silicon, it is possible to reduce the size and weight of the display and reduce the price, for example, by making it possible to build a circuit for driving an external display in the display. It becomes. In order to produce high-quality polycrystalline silicon, it is necessary to irradiate the polycrystalline silicon with laser light and perform laser annealing to promote crystal growth. Usually, the crystal is grown by an ultraviolet laser, but the ultraviolet laser is absorbed not only by the crystal grain boundary part but also by the already crystallized part. Accordingly, the amount of heat absorbed increases, and particularly when a thin film transistor for liquid crystal is manufactured, the surroundings are adversely affected by heat generation. In particular, it has been difficult to manufacture a polycrystalline silicon thin film transistor on a plastic substrate which is currently desired from the viewpoint of cost reduction and weight reduction of a display. Therefore, there is a demand for a crystal growth method for polycrystalline silicon that generates less heat.

As conventional techniques, there are Patent Documents 1 and 2.
Patent Document 1 describes a technique for performing melt polycrystallization of amorphous silicon using visible and ultraviolet lasers. However, Patent Document 1 irradiates a visible light pulse laser and an ultraviolet light pulse laser at the same time, and does not consider the low temperature of crystal growth of polycrystalline silicon.
Patent Document 2 describes annealing in which laser beams having different wavelengths are irradiated at different timings. However, the purpose of annealing is not the crystal growth of polycrystalline silicon, and the wavelengths are different accordingly. Further, no consideration is given to the low temperature of the crystal growth of polycrystalline silicon.
JP 2000-12484 A JP 56-29323 A

Since conventional polycrystalline silicon crystal growth methods are mainly based on ultraviolet lasers, not only the crystal grain boundaries but also the already crystallized parts absorb the heat from the laser, increasing the overall heat generation. There are many disadvantages in the production of thin film transistors used for switching liquid crystal displays. On the other hand, if a visible light laser is irradiated, energy can be intensively applied to the crystal grain boundary portion and heat generation can be suppressed, but the energy is not sufficient for crystal growth.
The present invention complements the disadvantages of ultraviolet laser irradiation and visible light laser irradiation in a silicon crystal growth method in which polycrystalline silicon is irradiated with laser light to grow silicon crystals. The purpose is to grow.

In order to achieve the above object, the present invention has the following configuration.
That is, a method of growing a silicon crystal by irradiating polycrystalline silicon with a laser beam, wherein the laser beam includes a first pulsed laser beam having a wavelength in the ultraviolet region and a first pulse having a wavelength in the visible region. It consists of a two-pulse laser beam, the second pulse laser beam, and the first pulse laser beam irradiation before and after the first pulse laser beam irradiated is irradiated at different timings, selectively heat only the grain boundaries A silicon crystal growth method characterized by:
The polycrystalline silicon may be generated by irradiating an amorphous silicon with an ultraviolet laser.
Preferably, the irradiation timing interval between the first pulse laser beam and the second pulse laser beam is irradiated with an interval equal to or greater than the pulse width in the full width at half maximum of the first pulse laser beam or the second pulse laser beam. It is to be.
It is also preferable to repeat the laser beam irradiation a plurality of times.

  By adopting the above-described configuration, the present invention complementarily compensates for the disadvantages of ultraviolet laser irradiation and visible light laser irradiation, and can grow a silicon crystal at a lower temperature than before. The present inventor has experimented to grow a silicon crystal more efficiently with less energy (less calorific value) than simultaneous irradiation by irradiating ultraviolet light pulse laser and visible light pulse laser at different timings instead of simultaneous irradiation. I found out that This is because by giving a time difference between both pulse laser irradiations, it is possible to earn enough time for the annealing effect of one laser to penetrate and synergistically enhance the annealing effect of the other laser. it is conceivable that. Therefore, if the method of the present invention is used, silicon crystals can be grown at a lower temperature than before, which is advantageous for manufacturing a thin film transistor or the like.

The principle of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing an absorption state when a polycrystalline silicon is irradiated with an ultraviolet laser (wavelength 355 nm) and a visible laser (wavelength 532 nm) is irradiated. In the figure, grain boundary is a grain boundary, a-Si is amorphous silicon, c-Si is crystalline silicon, and poly-Si is polycrystalline silicon. FIG. 2 is a graph showing the wavelength dependence of the absorption rate of laser light of polycrystalline silicon, amorphous silicon, and crystalline silicon. As can be seen from FIG. 2, amorphous silicon (a-Si) has high absorption characteristics in ultraviolet light and visible light, whereas crystalline silicon (c-Si) has high absorption characteristics in ultraviolet light. Visible light is hardly absorbed.
The upper diagram of FIG. 1 is a conceptual diagram when the polycrystalline silicon is irradiated with an ultraviolet laser (355 nm). The crystal grown part has the characteristics of crystalline silicon (c-Si) and the crystal grain boundary part has the characteristics of amorphous silicon (a-Si), but the ultraviolet light is crystalline silicon (c-Si) as shown in FIG. Since it is absorbed by both Si) and amorphous silicon (a-Si), the entire laser light energy is absorbed and crystal growth is promoted, but the calorific value is large.
The lower diagram of FIG. 1 is a conceptual diagram when a visible light laser (532 nm) is irradiated onto polycrystalline silicon. As shown in FIG. 2, visible light is hardly absorbed in the crystalline silicon (c-Si) portion, but is absorbed much in the amorphous silicon (a-Si). The laser is efficiently absorbed only in the crystal grain boundary portion having the above characteristics, and the heat generation amount is small. However, since the overall heating amount is small, it is insufficient for crystal growth. In order to reduce the temperature of the polycrystalline silicon thin film growth process by ultraviolet laser light irradiation, visible laser light is irradiated before and after the ultraviolet laser light. Visible laser light is selectively absorbed only in a part of the amorphous Si-like portion such as a crystal grain boundary, so that only the vicinity of the crystal grain boundary is heated without increasing the temperature of the entire polycrystalline silicon thin film. That is, since only the crystal grain boundary is activated by the visible laser light irradiation, the lateral growth of the crystal by the ultraviolet laser light irradiation is promoted.

Hereinafter, the principle of crystal growth will be described with reference to FIG. The increase in grain size of polycrystalline silicon by laser irradiation with a relatively low energy density (secondary grain growth) is due to the non-preferred orientation of silicon grains passing through the grain boundaries. Caused by moving to the preferred orientation. That is, the grain size of the preferentially oriented crystal grains is increased by moving the grain boundary to the right. (The preferred orientation portion increases and the non-priority orientation portion decreases). By selectively absorbing laser light to the grain boundary, a temperature rise near the grain boundary and a heating time near the grain boundary are prolonged. This is considered to increase the particle size, that is, to move the grain boundary for the following two reasons.
1. The moving speed of the grain boundary is proportional to exp (−Q / kT). Here, Q, k, and T are the grain boundary activation energy, Boltzmann constant, and temperature. Irradiation with a visible light laser is expected to improve the moving speed of grain boundaries by increasing T.
2. Since the amount of movement of the grain boundary is the movement speed of the crystal grain boundary × time, the amount of movement of the grain boundary increases as the heating time is extended.

The experimental conditions and experimental results are shown below.
<Experimental conditions>
・ Amorphous silicon (a-Si)
Substrate: Quartz Deposition method: LP-CVD
Film thickness: 50 [nm]
・ Nd: YAG laser Visible light: 532nm (2nd harmonic, 2ω)
Ultraviolet light: 355nm (3rd harmonic, 3ω)
Repeat cycle: 11 [Hz]
Pulse duration: 5 [nsec] (full width at half maximum)
・ Irradiation conditions Substrate heating temperature: Room temperature Atmosphere: In air Energy density: 2ω 66 [mJ / cm ² ]
3ω 133 [mJ / cm ² ] (lower limit of crystallization)
Delay time of visible light with respect to ultraviolet light: -15ns to + 15ns
Precursor: 3ω, 133mJ / cm ² , 11shot

<Experimental result>
4 and 5 show the experimental results. FIG. 4 is a result of observing each crystal growth state with a scanning electron microscope (SEM) when the delay time of visible light with respect to ultraviolet light is changed from −15 ns to +15 ns. Note that “Precursor” is the result when only ultraviolet light is irradiated. As can be seen from this figure, compared to irradiation with ultraviolet light only (Precursor) and simultaneous irradiation with ultraviolet light and visible light (± 0 ns), the irradiation timing of ultraviolet light and visible light was changed (-15 ns, -10 ns -5 ns, +5 ns, +10 ns, and +15 ns) have a larger crystal grain size, indicating that crystal growth is promoted.
FIG. 5 shows a Raman spectroscopic analysis result and dissolution time graph of polycrystalline silicon after irradiation with ultraviolet light and visible light laser. The Raman peak shift position of 521 cm ^{−1 of} single crystal silicon and the difference (Dw) between the respective Raman peak shifts are proportional to the internal stress (F = −kDw). The full width at half maximum of Raman peak is related to crystal defects. Incidentally, the full width at half maximum of the Raman peak when measuring single crystal silicon (= crystal defects are almost zero) was 4.2 cm ⁻¹ . As the number of crystal defects increases, the full width at half maximum of the Raman peak increases.
From the results of the Raman peak shift in FIG. 5 (a) and the full width at half maximum of Raman peak in FIG. It can be seen that it is close to Si (the crystallinity is improved).
FIG. 5 (c) shows the melting time after laser irradiation. In view of the energy density of the irradiated laser, the melting part is in the vicinity of the grain boundary having a low melting point. It can be seen that the melting time is longer for 2ω delayed irradiation than for the reverse case. This result agrees with the Raman result shown in FIGS. 5 (a) and 5 (b).
It is considered that the extension of the melting time caused a temperature increase in the vicinity of the grain boundary due to 2ω delayed irradiation, and accordingly, the heating time in the vicinity of the grain boundary near the grain boundary was extended. The improvement in crystallinity is considered to be due to (1) acceleration of the crystal growth rate due to the temperature rise in the vicinity of the grain boundary and (2) progress of crystal growth due to the extension of the heating time in the vicinity of the grain boundary.

  Although an example of the embodiment of the present invention has been described above, the present invention is not limited to this, and it goes without saying that various modifications can be made within the scope of the technical idea described in the claims. Yes.

Diagram showing absorption of ultraviolet and visible light into polycrystalline silicon Graph showing the absorption characteristics of amorphous silicon, polycrystalline silicon, and crystalline silicon Illustration of the principle of crystal growth Diagram showing the results of crystal growth by ultraviolet and visible laser irradiation Analysis results of polycrystalline silicon after ultraviolet and visible laser irradiation (a) Raman peak shift (b) Raman peak full width at half maximum (c) Dissolution time after laser irradiation Thin film transistor for liquid crystal display

Claims (4)

A silicon crystal growth method for growing a silicon crystal by irradiating a polycrystalline silicon with a laser beam, wherein the laser beam includes a first pulse laser beam having a wavelength in an ultraviolet region and a second pulse having a wavelength in a visible region. It consists of a laser beam, the second pulse laser beam, it is with the first pulse laser beam irradiation before and after the first pulse laser beam irradiated is irradiated at different timings, to selectively heat only the grain boundaries A silicon crystal growth method characterized by the above.   The silicon crystal growth method according to claim 1, wherein the polycrystalline silicon is produced by irradiating amorphous silicon with an ultraviolet laser. The irradiation timing interval between the first pulse laser beam and the second pulse laser beam is irradiated with an interval equal to or greater than the pulse width in the full width at half maximum of the first pulse laser beam or the second pulse laser beam. The silicon crystal growth method according to claim 1, wherein the silicon crystal growth method is characterized. The silicon crystal growth method according to claim 1, wherein the laser beam irradiation is repeated a plurality of times.