Continuous wave semiconductor laser crystallization method for wide bandgap semiconductors

Abstract

To provide a continuous wave semiconductor laser crystallization method for a wide bandgap semiconductor film having a bandgap energy of 2.8 eV or more, and improving the mobility of the film by growing a single crystal band.SOLUTION: A method for crystallizing a film and a method for manufacturing the crystallized film includes a step of irradiating an indium oxide wide bandgap semiconductor film with a semiconductor laser whose absorption coefficient of the wide bandgap semiconductor film is 3000 cm-1 to 50000 cm-1 using a microchelobron laser beam scanning method.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This invention relates to a method for continuous-wave semiconductor laser crystallization of wide-bandgap semiconductor films. [Background technology]

[0002] In recent years, semiconductor materials with large bandgap energies of 2 eV or more have attracted attention because they exhibit low off-currents when applied to metal-oxide-semiconductor field-effect transistors (MOSFETs), making them effective for energy saving. Amorphous IGZO is a representative example of such a material, and it has been demonstrated that it can achieve excellent energy-saving effects whether used as a TFT in displays or as a 3D LSI.

[0003] A drawback of amorphous IGZO is that its mobility, a performance indicator when used as a MOSFET, is only 10 cm. 2 Its mobility is low at / Vs. 2 / Vs, approximately 100 cm² of polycrystalline Si 2 It is significantly lower compared to / Vs. Due to its low mobility, next-generation ultra-high-resolution displays cannot increase their frame rates, and applications to 3D LSIs face the challenge of not being able to increase their frequencies.

[0004] As a solution to this problem, a method has been devised in which an amorphous In2O3 film, which has been hydrogenated during deposition, is grown as a solid-state polycrystalline In2O3 with a grain size of about 0.1 μm by thermal annealing at about 250°C (Non-Patent Literature 1). As a result, the mobility of the MOSFET is 140 cm². 2 It achieves a high / Vs and low off-current, which is an advantage of wide bandgap materials (Non-Patent Document 1).

[0005] Furthermore, just as research on polycrystalline Si films with TFT applications in mind initially began with thermal annealing solid-phase polycrystallization of a-Si films, and was eventually put into practical use with laser crystallization that could form high-quality polycrystalline Si films, a method for crystallizing oxide films with lasers has also been proposed (Non-Patent Literature 2). In Non-Patent Literature 2, an InZnO film was crystallized with an excimer laser at a wavelength of 308 nm and applied to TFTs.

[0006] However, because excimer lasers are pulsed lasers, the heating time is short, at most 20 ns, resulting in only small crystal grain sizes of a few tens of nanometers, and thus inferior TFT characteristics compared to the thermal annealing method.

[0007] Furthermore, if polycrystalline films are used in MOSFETs, an insurmountable dilemma arises (Figure 1). Specifically, while amorphous materials are homogeneous and therefore do not have uniformity issues, they have low mobility and slow response. To improve mobility, polycrystalline materials are used, but while increasing the size of the grain boundaries in the polycrystalline material increases mobility, the number of grain boundaries spanning multiple MOSFETs decreases. As a result, variations in mobility between MOSFETs become pronounced due to differences in the number of grain boundaries spanning multiple MOSFETs. Therefore, it is not possible to satisfy both mobility and uniformity simultaneously with polycrystalline materials. To solve this problem, it is necessary to crystallize the semiconductor film into a single crystal. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 6544090 [Non-patent literature]

[0009] [Non-Patent Document 1] DOI: https: / / doi.org / 10.1038 / s41467-022-28480-9 [Non-Patent Document 2] Appl. Phys. Lett. 102, 122107 (2013); https: / / doi.org / 10.1063 / 1.4798519

Summary of the Invention

Problems to be Solved by the Invention

[0010] Under the above background, laser crystallization of a wide-bandgap semiconductor film having a bandgap energy of 2.8 eV or more and growth of a single crystal region of the film have been demanded.

Means for Solving the Problems

[0011] As a result of intensive studies to solve the above problems, the present inventors have succeeded in crystallizing an In2O3 film as a wide-bandgap semiconductor film with a continuous-wave semiconductor laser and also succeeded in growing a single crystal region by the μCLBS method, thus completing the present invention.

[0012] That is, the present invention is as follows. [1] A method for crystallizing a wide-bandgap semiconductor film formed on a substrate, the method including a step of irradiating the film with a semiconductor laser. [2] A method for manufacturing a crystallized film, the method including a step of irradiating a wide-bandgap semiconductor film formed on a substrate with a semiconductor laser. [3] The method according to [1] or [2], wherein the crystal is a single crystal. [4] The method according to [1] or [2], wherein the absorption coefficient of the wide-bandgap semiconductor film with respect to the irradiated laser is 3000 cm -1 [[ID=3...]]~50000 cm -1 The method according to [1] or [2], wherein the absorption coefficient is. [5] The method according to [1] or [2], wherein the wide-bandgap semiconductor film is an indium oxide film. [6] The method according to [1] or [2], wherein the laser irradiation is by a micro-shellobron laser beam scanning method. [7] A thin film transistor material including a film manufactured by the method according to [2]. [8] A method for forming a pattern using a wide bandgap semiconductor film, comprising a step of irradiating a laser to an arbitrary partial region of a wide bandgap semiconductor film formed on a substrate to crystallize the region, and etching and removing the film in the region not irradiated with the laser.

Advantages of the Invention

[0013] According to the present invention, a method for forming a polycrystalline film using a semiconductor laser of a wide bandgap semiconductor film and a method for forming a single crystal zone using a μCLBS method are provided. The single crystal zone obtained according to the present invention has high device uniformity. Also, it has become possible to achieve both high mobility and low off-current.

Brief Description of the Drawings

[0014] [Figure 1] A diagram for explaining the polycrystalline dilemma. [Figure 2] A Tauc plot showing light absorption due to defects. [Figure 3] An α-λ plot of In2O3 films deposited under various conditions. [Figure 4] A diagram showing an outline of a laser irradiation method by a micro-schellobron laser beam scanning method. [Figure 5] An EBSD crystal orientation map of the In2O3 film obtained in Embodiment 1 of the present invention and an inverse pole figure orientation map representing the crystal orientation in color. [Figure 6] An EBSD crystal orientation map of an In2O3 film obtained by a conventional thermal annealing method. [Figure 7] An EBSD crystal orientation map of an In2O3 film in which only the laser irradiation region of the In2O3 film was selectively crystallized by selective irradiation of a laser in Embodiment 1 of the present invention. [Figure 8] An EBSD crystal orientation map of an In2O3 film single-crystallized by the μCLBS method in Embodiment 2 of the present invention.

Modes for Carrying Out the Invention

[0015] The present invention relates to a method for crystallizing a wide bandgap semiconductor film formed on a substrate, which includes a step of irradiating a semiconductor laser onto the film, and a method for manufacturing a crystallized film.

[0016] When performing laser annealing, if the target film material cannot absorb the irradiated laser light, the target object cannot be melted and crystal growth cannot occur. Here, the light absorption of the target material depends on the relationship between the optical bandgap energy E g of the target material and the photon energy E L of the laser. Light absorption does not occur unless the relationship E L > E g is satisfied. For example, since the E g of the In2O3 film is about 3.7 eV, the wavelength of the laser needs to be 335 nm or less. Therefore, the available laser light is an excimer laser with a wavelength of 248 nm or 308 nm.

[0017] However, since the excimer laser is a pulsed laser, the heating time of the film is short, and a large particle size cannot be obtained. Recently, an ultraviolet continuous-wave semiconductor laser has been expected as a low-cost laser light source to replace the excimer laser. However, since the shortest wavelength is 378 nm, the oxide semiconductor film cannot absorb the semiconductor laser. In addition, since the semiconductor laser, which is a continuous-wave laser, could not be used as a laser annealing light source, single-crystal zone growth could not be achieved either.

[0018] Therefore, the inventor of the present invention invented a chevron laser beam scanning method (μCLBS) as a method for crystallizing a Si film or a Cu2O film (Patent No. 6544090). This method is a method of forming a single-crystal zone in the scanning region by scanning a chevron-type continuous-wave laser beam onto these films. When a thin-film transistor (TFT), which is a type of MOSFET, was formed in the Si single-crystal zone, the mobility was 540 cm 2 / Vs, and performance comparable to that of a single crystal was obtained. If this technology is used, there is a possibility that the In2O3 film can also achieve single-crystal zone growth. Furthermore, the inventors focused on light absorption due to defects such as oxygen vacancies, in addition to interband light absorption, in semiconductor materials.

[0019] The Tauc plot is a well-known method for determining the optical band gap of semiconductor materials. Here, the present invention will be explained using the Tauc plot of the In2O3 film shown in Figure 2. In a Tauc plot, the horizontal axis represents photon energy E (eV), and the vertical axis represents (αE). 2 Here, α is the absorption coefficient. E is obtained as hv, and the relationship v = c / λ holds between v and the wavelength λ. Here, h is Planck's constant, v is the frequency of light, and c is the speed of light. In Figure 2, (αE) 2 The point where the tangent line (shown as a dotted line in the figure) in the region where α increases linearly intersects the horizontal axis represents the bandgap energy of this material, which in this example is 3.65 eV. Below 3.65 eV, light absorption occurs due to defects, so α cannot be 0. Therefore, αE cannot be 0. In other words, by increasing the defect density in this region, light with E below 3.65 eV can also be absorbed. This means that continuous-wave semiconductor lasers with λ of 450 nm or less can be used for laser annealing.

[0020] The curve (ii) in Figure 3 is an α-λ plot representing the Tauc plot in Figure 2. In Figure 3, the absorption coefficient is changed by controlling the defect density by changing the O2 / Ar flow rate ratio during In2O3 film deposition. Absorption coefficients where λ is approximately 350 nm or higher are due to light absorption by defects. When the O2 flow rate is increased to saturate the oxygen in In2O3, the absorption coefficient becomes 1000 cm⁻¹. -1 The following (not shown): The defect density changes depending on the film deposition conditions, as shown by curves (i) to (iv), and the absorption coefficient can be seen to fluctuate by nearly two orders of magnitude at a wavelength of 405 nm, for example. Based on the above, it has become possible to crystallize wide-bandgap semiconductor films even with continuous-wave semiconductor lasers with a λ of 450 nm or less.

[0021] The type and material of the substrate used in this invention are not particularly limited as long as they are used as thin-film transistor materials, and any such substrate can be used. Examples include glass or quartz substrates, and plastic substrates (such as polyethylene, polystyrene, or acrylic resin).

[0022] The wide-bandgap semiconductor film used in this invention is not particularly limited, as long as its bandgap energy is 2.8 eV or higher. For example, In2O3 films, ZnO films, SnO films, SnO2 films, GaN films, GaInN films, etc., can be used. Furthermore, methods that have been used for a long time, such as sputtering, vacuum deposition, and chemical vapor deposition, can be employed to form the thin film. The thickness to which wide-bandgap semiconductor films are deposited on a substrate is 5 to 100 nm, for example, 50 nm.

[0023] In this invention, a thin film formed on a substrate is irradiated with a continuous-wave semiconductor laser or a pulsed continuous-wave semiconductor laser while the substrate is moved at a predetermined speed, thereby recrystallizing an amorphous or polycrystalline thin film. A typical line beam can be used for the laser beam shape. In this case, the crystal nuclei are not controlled, resulting in a polycrystalline film.

[0024] The laser beam shape can also be a microchevron beam as described in Japanese Patent Publication No. 6544090, as shown in Figure 4. In this case, it is called the microchevron laser beam scanning (μCLBS) method. Specifically, the laser light (linear output light) from a semiconductor laser device is reshaped into a roughly V-shaped (chevron-shaped) beam spot when viewed from the direction of light propagation by inverting half of the laser light using a dove prism. The beam spot is then oriented so that the V-shaped apex is facing forward in the direction of propagation and moved (scanned) relative to a planar amorphous thin film, causing the irradiation trajectory to become a single crystal through lateral growth. In Figure 4, by making the beam spot 41 roughly V-shaped, a roughly V-shaped melting region 42 is formed in the film 40, the solid-liquid interface becomes convex with respect to the direction of propagation, crystal growth in the molten region due to nucleation at both ends of the laser scan path is prevented, and single-crystallized regions 43 are continuously formed as the laser progresses.

[0025] The absorption coefficient of the wide-bandgap semiconductor film targeted by laser irradiation is 3000 cm². -1 ~50,000cm -1 That is the case. Laser irradiation conditions vary depending on the scanning speed, which also affects the laser power density. However, under typical conditions where the scanning speed is fixed at 10 mm / s, the preferred laser power density is 2.5 × 10⁻⁶. 6 W / cm 2 That is the case.

[0026] The wide-bandgap semiconductor film crystallized in this way can be obtained as a polycrystalline film with a grain size of 0.2 μm or larger, or as a single crystal with a grain size of 4 μm or larger. The wide-bandgap semiconductor film of the present invention can be used as a thin-film transistor material for transparent displays, 3D LSIs, and other applications.

[0027] Furthermore, in this invention, a laser may be irradiated onto any portion of the wide-bandgap semiconductor film formed on the substrate, i.e., the portion of the film that you wish to retain, thereby causing crystallization of that portion. Then, by etching away the film in the portion that was not irradiated with the laser (amorphous or microcrystalline region), a pattern of any shape can be formed using the wide-bandgap semiconductor film. The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples. [Examples]

[0028] An InOx film of 50 nm thickness was deposited using a DC sputtering method with argon and oxygen, targeting In. The oxygen flow rate was set to 12% of the total flow rate, and the substrate temperature was 100°C. As a result, as shown in curve (iv) of Figure 3, the absorption coefficient at a wavelength of 405 nm was 8000 cm². -1 An InOx film with the following properties was obtained. Subsequently, by laser scanning at a wavelength of 405 nm, the scanning speed was 10 mm / s and the laser power was 0.43 W (estimated power density 2.7 × 10⁻¹⁶). 6 W / cm 2 The In2O3 film was crystallized under the following conditions. In this case, the absorption coefficient is relatively small, so the laser power absorbed by the film is substantially small, and the film is simply heated to achieve polycrystallization.

[0029] Figure 5 shows the results of evaluating the crystallinity of the crystallized In2O3 film by electron backscatter diffraction (EBSD), along with an inverse pole figure orientation map where crystal orientations are represented by color. For comparison, Figure 6 shows the results for an In2O3 film obtained by thermal annealing. The crystal grain size obtained by thermal annealing was approximately 0.2 μm, while that obtained by the laser annealing method of the present invention was approximately 0.8 μm. The method of the present invention made it possible to obtain a polycrystalline In2O3 film with a large grain size and relatively uniform crystal orientation, resulting in high grain size uniformity, by laser irradiation without thermal annealing.

[0030] Furthermore, as shown in Figure 7, the present invention allows for the selective formation of polycrystalline regions by selectively irradiating them with a laser. The non-irradiated regions are amorphous or microcrystalline. In oxide semiconductors, amorphous or microcrystalline regions are susceptible to acid, so by selectively etching these regions, only the regions that have been polycrystalline by laser irradiation can be left. In other words, In2O3 films can be patterned without photolithography techniques. [Examples]

[0031] An InOx film of 50 nm thickness was deposited using DC sputtering with argon and oxygen, targeting In. The oxygen flow rate was set to 9.5% of the total flow rate, and the substrate temperature was 250°C. As a result, as shown in Figures 2 and 3(ii), an absorption coefficient of 36,000 cm² was obtained at a wavelength of 405 nm. -1 An InOx film with the following properties was obtained. Subsequently, a micro-chevron laser beam scanning (μCLBS) method using a 405 nm wavelength laser was performed with a scanning speed of 1 mm / s and a laser power of 0.4 W (estimated power density 2.5 × 10⁻¹⁶). 6 W / cm 2 The In2O3 film was crystallized under the following conditions. Figure 8 shows the results of evaluating the crystallinity of an In2O3 film crystallized by the method of the present invention using electron backscatter diffraction (EBSD). The grain size is 5 μm or larger, indicating that it has grown to a size sufficient for forming a single crystal TFT.

[0032] The wide-bandgap semiconductor film obtained by the method of the present invention is a single crystal, thus exhibiting high device uniformity and high stability against environmental humidity and oxygen. Furthermore, it is possible to achieve both high mobility and low off-current. Furthermore, this method, which allows for the formation of single-crystal oxide semiconductor thin films without substrate constraints, can contribute to the realization of innovative devices such as TFTs for transparent displays and TFTs for 3D LSIs.

[0033] In contrast, a solid-phase crystallized In2O3 film, used as a comparative example, exhibited a polycrystalline film with a grain size of approximately 140 nm (Figure 6). Furthermore, the crystal orientation of each grain was random. This suggests that a large number of crystal nuclei were formed during sputter deposition, and nucleation growth occurred during thermal annealing. Grain boundaries can cause performance degradation and characteristic variations when applied to devices. [Explanation of symbols]

[0034] 40: Membrane 41: Beam Spot 42: Melting region 43: Single crystallization region

Claims

1. A method for single-crystallizing a wide-bandgap semiconductor film formed on a substrate and having defects introduced into it, comprising the step of irradiating the film with a laser in one scanning direction.

2. A method for manufacturing a single-crystal film, comprising the step of irradiating a wide-bandgap semiconductor film formed on a substrate and into which defects have been introduced with a laser in one scanning direction.

3. The absorption coefficient of the wide-bandgap semiconductor film with respect to the irradiating laser is 3000 cm². －１ ~50,000 cm －１ The method according to claim 1 or 2.

4. The method according to claim 1 or 2, wherein the wide-bandgap semiconductor film is an indium oxide film.

5. The method according to claim 1 or 2, wherein the laser irradiation is performed by a microchevron laser beam scanning method.

6. A thin-film transistor material comprising a film manufactured by the method described in claim 2.

7. A method for forming a pattern using a wide-bandgap semiconductor film, comprising the steps of irradiating an arbitrary portion of a wide-bandgap semiconductor film formed on a substrate and having defects introduced with a laser in one scanning direction to single-crystallize the said region, and etching away the film in the areas not irradiated with the laser.

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