Surface treatment method

Abstract

To provide a surface treatment method capable of improving strength of a surface of an object, while suppressing the change of physical properties of the object.SOLUTION: A surface treatment method includes irradiating an object part st of a surface 1s of an object 1 as an object for surface treatment, with light (treatment light) for surface treatment from a light irradiation device 10, according to a predetermined irradiation condition. The treatment light is ultraviolet light having a wavelength which is 1 nm or longer and shorter than 308 nm. The irradiation condition includes a condition of regulating a range of fluence. The fluence represents a height of the energy of the treatment light applied per unit area of an irradiated portion of the surface 1s of the object 1. The lowest value of the fluence when the irradiated portion of the surface 1s of the object 1 is melted is represented by a melting threshold. In this case, the range of the fluence regulated under the irradiation condition is a range which is higher than 0 and lower than the melting threshold.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a surface treatment method for performing a predetermined treatment on the surface of an object.

Background Art

[0002] Various surface treatment methods have been proposed to impart functions such as an optical function, a water-repellent function, or a hydrophilic function to the surface of an object. As an example of such a surface treatment method, Patent Document 1 describes a roughening method for a silicon substrate that forms uniform irregularities on the surface of the silicon substrate used for a solar cell or the like.

[0003] According to the roughening method, an uneven structure in which a large number of protrusions on the submicron order with a diameter and height of about 0.1 μm to 1 μm are arranged is formed on the surface of the silicon substrate. This uneven structure reduces the reflectance of light incident on the surface of the silicon substrate and enhances the light confinement effect, thereby improving the energy conversion efficiency from light to electricity in the solar cell.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] A solar cell is provided outdoors, for example. Therefore, it is desirable that the surface of the silicon substrate described above has not only the function of enhancing the light confinement effect but also higher strength. Shot peening is known as a surface treatment method for improving the strength of the surface of an object. However, when a process involving a physical impact is performed on a semiconductor element such as a silicon substrate, the crystal structure of the semiconductor element may be destroyed, and the original function of the semiconductor element may not be exhibited.

[0006] An object of the present invention is to provide a surface treatment method capable of improving the strength of the surface of an object while suppressing changes in the physical properties of the object.

Means for Solving the Problems

[0007] A surface treatment method according to an aspect of the present invention includes a step of irradiating a target portion on the surface of an object with treatment light having a wavelength of 1 nm or more and shorter than 308 nm according to predetermined irradiation conditions, and the irradiation conditions include setting the fluence to be higher than 0 and lower than the melting threshold of the target portion.

Effects of the Invention

[0008] According to the present invention, it becomes possible to improve the strength of the surface of an object while suppressing changes in the physical properties of the object.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0010] Hereinafter, a surface treatment method according to an embodiment of the present invention will be described with reference to the drawings.

[0011] 1. Surface Treatment Method FIG. 1 is a schematic diagram for explaining a surface treatment method according to an embodiment of the present invention. As shown in FIG. 1, the surface treatment method according to the present embodiment includes irradiating a target portion st on the surface 1s of an object 1 to be surface-treated with light for surface treatment from a light irradiation device 10 according to predetermined irradiation conditions. In the following description, the light for surface treatment is referred to as processing light. The processing light according to the present embodiment is light in the X-ray region to the ultraviolet region having a wavelength of 1 nm or more and shorter than 308 nm. As ultraviolet light in such a wavelength region, for example, pulsed laser light of a KrF excimer laser can be used. That is, the light irradiation device 10 may include a KrF excimer laser as a light source for the processing light. The oscillation wavelength of the KrF excimer laser is 248 nm.

[0012] The above irradiation conditions include conditions that define the range of fluence. In the present embodiment, fluence is the height of the energy of the processing light given per unit area of the irradiated portion on the surface of the object, and can be expressed in units such as [J / cm 2 . When the above pulsed laser light is used as the processing light, the fluence can also be defined as the height of the energy density of the processing light given to the irradiated portion when the surface of the object is irradiated with one pulse of the laser.

[0013] Assume a case where a surface of an object is irradiated with high-energy light. In this case, depending on the relationship between the material and state of the surface of the object and the amount of energy of the light, the irradiated portion of the surface of the object scatters, peels off, and melts due to the energy of the light. By utilizing such characteristics, high-energy light is used for various processes such as drilling and cutting.

[0014] Here, the minimum value of fluence when the irradiated portion of the surface of the object melts is called the melting threshold F th In this case, the range of fluence defined by the irradiation conditions according to the present embodiment is higher than 0 and lower than the melting threshold F th and is preferably in a range closer to the melting threshold F th For example, a value closer to the melting threshold F th from the melting threshold F th to half of the melting threshold F th [J / cm 2 ).

[0015] According to the above surface treatment method, protrusions having a height on the order of nanometers are formed at a high density on the irradiated portion of the surface of the object irradiated with the treatment light. Along with the formation of the high-density protrusions, compressive residual stress is applied to the irradiated portion of the surface of the object. Thereby, the strength of the surface of the object is improved. This mechanism was derived from the experiments and considerations of the present inventor. Details will be described later.

[0016] Also, according to the above surface treatment method, since the fluence at the time of irradiating the object with the treatment light is made lower than the melting threshold F th the irradiated portion of the surface of the object does not melt due to the treatment light. Therefore, the crystal structure near the surface of the object does not change significantly. As a result, it becomes possible to improve the strength of the surface of the object while suppressing changes in the physical properties of the object.

[0017] 2. Melting threshold F th Melting threshold F this determined based on several factors such as the wavelength and pulse width of the processing light, the material constituting the surface of the object, the state of the surface of the object, and the temperature of the object. The inventor irradiated the surface of each of a plurality of objects made of a plurality of types of materials with pulsed laser light while changing the fluence under predetermined common experimental conditions (such as temperature environment). In addition, the degree of damage to the surface of the object corresponding to a plurality of fluences was evaluated. This experiment is called a melting confirmation experiment.

[0018] FIG. 2 is a diagram showing the tendency between the change in fluence confirmed by the melting confirmation experiment and the change in the degree of melting of the object. In FIG. 2, the vertical axis represents the degree of deformation of the surface of the object (the depth or diameter of the dent formed by melting), and the horizontal axis represents the height of the fluence. As shown in FIG. 2, according to the melting confirmation experiment, the degree of deformation of the surface of the object gradually decreases on the nanometer order as the value of the fluence decreases.

[0019] As described above, the melting threshold F th is the minimum value of the fluence when the irradiated portion of the surface of the object melts. To obtain the exact melting threshold F th , it is necessary to conduct the experiment as described above. In contrast, the melting threshold F th can also be estimated to some extent from the one-dimensional heat diffusion model. For example, assume a case where pulsed laser light with a pulse width sufficiently longer than the heat relaxation time of the irradiated portion of the surface of the object (specifically, pulsed laser light with a pulse width of about 10 -10 seconds or more) is used as the processing light. At this time, when F th is taken as the melting threshold, T m is taken as the melting point of the material of the object, T0 is taken as the initial temperature of the material of the object, C i is taken as the heat capacity of the object, k i is taken as the thermal conductivity of the object, τ L is taken as the pulse width of the pulsed laser light, and A is taken as the absorption rate of the pulsed laser light with respect to the target portion of the object, the melting threshold F th can be expressed by the following formula (1) according to the one-dimensional heat diffusion model.

[0020] [Number]

[0021] In addition, assume a case where pulsed laser light with a pulse width shorter than the heat relaxation time of the irradiated portion of the surface of the object is used as the processing light (specifically, pulsed laser light with a pulse width of about 10 -13 seconds or less). At this time, th let F m be the melting threshold, T i be the melting point of the material of the object, T0 be the initial temperature of the material of the object, C i be the heat capacity of the object, A be the absorption rate of the pulsed laser light with respect to the target portion of the object, and a be the penetration coefficient of the processing light with respect to the target portion of the object. In this case, the melting threshold F th can be expressed by the following formula (2) according to the one-dimensional heat diffusion model.

[0022] [Number]

[0023] Note that the absorption rate A in the above formulas (1) and (2) can also be expressed by the reflectance of the pulsed laser light with respect to the target portion of the object. That is, when the reflectance is R, the absorption rate A can be expressed as (1 - R). The reflectance R also varies depending on the incident angle of the pulsed laser light with respect to the object and the polarization direction (p-polarization and s-polarization) of the pulsed laser light irradiated on the object.

[0024] 3. Structure obtained by irradiating the processing light with a fluence lower than the melting threshold F th The inventor further performed, on the surface of the object, a process with a fluence lower than the melting threshold F thAn experiment was conducted in which the processing light (pulsed laser light) was irradiated at a fluence slightly lower than [fluence value]. As a result, the inventor confirmed that a plurality of dot-like protrusions on the order of nanometers were formed dispersedly at intervals of about the wavelength of the processing light on the irradiated portion of the surface of the object. Specifically, each of the plurality of protrusions had a height of about 10 nm to 100 nm with respect to other portions of the irradiated portion. As described above, the structure in which a plurality of dot-like protrusions on the order of nanometers are formed dispersedly is called a nanodot structure.

[0025] In addition, the inventor prepared two silicon substrates as two objects, and irradiated the surfaces of those with two types of pulsed laser light having different wavelengths at a fluence lower than the melting threshold value F th An experiment was conducted.

[0026] Specifically, the inventor irradiated the surface of one silicon substrate (hereinafter referred to as the first experimental substrate) with the pulsed laser light of a XeCl excimer laser at a fluence lower than the melting threshold value F th The oscillation wavelength of the XeCl excimer laser is 308 nm. In addition, the inventor observed the surface of the first experimental substrate after irradiation with the pulsed laser light using a scanning electron microscope (SEM). FIG. 3 is an image showing the state of the surface of the first experimental substrate irradiated with the pulsed laser light of the XeCl excimer laser.

[0027] In addition, the inventor irradiated the surface of the other silicon substrate (hereinafter referred to as the second experimental substrate) with the pulsed laser light of a KrF excimer laser at a fluence lower than the melting threshold value F th As described above, the oscillation wavelength of the KrF excimer laser is 248 nm. In addition, the inventor observed the surface of the second experimental substrate after irradiation with the pulsed laser light using a scanning electron microscope (SEM). FIG. 4 is an image showing the state of the surface of the second experimental substrate irradiated with the pulsed laser light of the KrF excimer laser. Note that the display magnifications of the images in FIGS. 3 and 4 are equal so that the scales included in the respective images match.

[0028] As shown by the arrows in FIGS. 3 and 4, on each of the two silicon substrates, a plurality of dot-like protrusions are formed by irradiating with pulsed laser light at a fluence lower than the melting threshold F th That is, according to the images in FIGS. 3 and 4, it can be seen that a nanodot structure is formed on each of the first experimental substrate and the second experimental substrate.

[0029] Here, while observing the surface state of the first experimental substrate, the inventor counted the number of protrusions formed per unit area. As a result, on the irradiated portion of the surface of the first experimental substrate after the experiment, a plurality of protrusions were formed at a density of 20.2 [pieces / μm 2 .

[0030] Also, while observing the surface state of the second experimental substrate, the inventor counted the number of protrusions formed per unit area. As a result, on the irradiated portion of the surface of the second experimental substrate after the experiment, a plurality of protrusions were formed at a density of 28.5 [pieces / μm 2 .

[0031] From the above results, the inventor grasped that in the nanodot structure obtained by irradiating the processing light at a fluence lower than the melting threshold F th , a plurality of dot-like protrusions tend to be arranged at intervals corresponding generally to the wavelength of the processing light. That is, the inventor grasped that the shorter the wavelength of the processing light, the shorter the adjacent interval (dot pitch) between the plurality of protrusions in the nanodot structure, and the longer the wavelength of the processing light, the longer the adjacent interval between the plurality of protrusions in the nanodot structure. Further, in addition to the above experiments, the inventor confirmed based on further experiments and the findings reported so far that the density of the nanodot structure is inversely proportional to the square of the wavelength of the processing light.

[0032] 4. Compressive residual stress (a) As described in the summary of the invention, shot peening is known as a surface treatment method for improving the strength of the surface of an object. Shot peening is a process in which an object formed of a metal material or the like is forcibly impacted on its surface to cause deformation, thereby leaving a compressive stress that repels the impact. Thus, if a compressive stress can be left on the surface of the object, the strength of the surface of the object can be improved.

[0033] Considering this point, if a compressive stress remains in the nanodot structure formed by irradiating the surface of the object with pulsed laser light, it can be said that the strength of the surface of the object has been improved. Therefore, the inventor of the present invention confirmed what kind of stress remains in each of the nanodot structure formed on the first experimental substrate and the nanodot structure formed on the second experimental substrate.

[0034] (b) Specifically, the inventor of the present invention evaluated by microscopic Raman spectroscopy which of compressive stress and tensile stress remains in the first experimental substrate after being irradiated with pulsed laser light. In the evaluation by microscopic Raman spectroscopy, light having a predetermined wavelength is irradiated onto the irradiated portion of the surface of the first experimental substrate after being irradiated with pulsed laser light, and the light scattered by the irradiated portion (Raman scattered light) is measured. Then, the difference in wave number between the incident light and the Raman scattered light is defined as the Raman shift, and the relationship between the intensity of the Raman scattered light and the Raman shift is obtained. The unit of the Raman shift is, for example, [cm -1 .

[0035] FIG. 5 is a diagram showing the evaluation result by microscopic Raman spectroscopy for the first experimental substrate. In FIG. 5, the evaluation result by microscopic Raman spectroscopy is shown by a graph having the vertical axis as the Raman scattering intensity (intensity of Raman scattered light) and the horizontal axis as the Raman shift.

[0036] In the graph of FIG. 5, the evaluation result (Raman spectrum) of the microscopic Raman spectroscopy for the first experimental substrate before the pulsed laser light is irradiated is shown by a dotted line. The Raman shift (peak wave number) when the Raman spectrum of the surface of the first experimental substrate before the pulsed laser light is irradiated shows a peak is defined as the reference wave number α. The reference wave number α is determined by the material constituting the surface to be evaluated (in this example, the surface of the first experimental substrate). When the material is Si, it is approximately 520 [cm -1 .

[0037] Also, in the graph of FIG. 5, the Raman spectrum corresponding to the first experimental substrate after the pulsed laser light is irradiated is shown by a thick solid line. As shown in FIG. 5, the peak wave number corresponding to the first experimental substrate after the pulsed laser light is irradiated is deviated from the reference wave number α. Such a deviation of the peak wave number from the reference wave number α indicates that a strain has occurred in the crystal structure of the surface to be evaluated.

[0038] Regarding the deviation of the peak with respect to the reference wave number α, it is known that when the peak wave number is deviated from the reference wave number α to the lower wave number side, the strain occurring in the crystal structure of the evaluation object is a strain caused by tensile stress. Also, it is known that when the peak wave number is deviated from the reference wave number α to the higher wave number side, the strain occurring in the crystal structure of the evaluation object is a strain caused by compressive stress.

[0039] From these points, according to the results of FIG. 5, it can be seen that a strain caused by tensile stress has occurred in the first experimental substrate after the pulsed laser light is irradiated, in other words, tensile stress remains.

[0040] (c) The present inventor evaluated by microscopic Raman spectroscopy which of compressive stress and tensile stress remains in the second experimental substrate after the pulsed laser light is irradiated, in the same manner as the first experimental substrate.

[0041] Fig. 6 is a diagram showing the evaluation results of the second experimental substrate by microscopic Raman spectroscopy. In Fig. 6, similar to the example of Fig. 5, the evaluation results by microscopic Raman spectroscopy are shown in a graph with the vertical axis representing the Raman scattering intensity (the intensity of the Raman scattered light) and the horizontal axis representing the Raman shift.

[0042] In the graph of Fig. 6, the Raman spectrum of the second experimental substrate before being irradiated with the pulsed laser beam is indicated by a dotted line. The peak wave number of the Raman spectrum of the surface of the second experimental substrate before being irradiated with the pulsed laser beam is defined as the reference wave number α.

[0043] In addition, the Raman spectrum corresponding to the second experimental substrate after being irradiated with the pulsed laser beam is shown by a thick solid line in the graph of Fig. 6. As shown in Fig. 6, the peak wavenumber corresponding to the first experimental substrate after being irradiated with the pulsed laser beam is shifted to the higher wavenumber side from the reference wavenumber α when there is no distortion in the crystal structure.

[0044] From this point of view, the results in Figure 6 show that distortion due to compressive stress occurs in the second experimental substrate after it is irradiated with the pulsed laser light, in other words, compressive stress remains.

[0045] (d) As described above, tensile stress remains in the first experimental substrate after being irradiated with the pulsed laser beam, whereas compressive stress remains in the second experimental substrate after being irradiated with the pulsed laser beam. The inventors have hypothesized the following mechanism for the difference in the stress remaining on the surfaces of the experimental substrates.

[0046] Fig. 7 is a diagram for explaining the mechanism presumed for the stress remaining on the surfaces of the first and second experimental substrates. The upper part of Fig. 7 shows a schematic enlarged partial cross-sectional view of a part of the surface of the first experimental substrate. The lower part of Fig. 7 shows a schematic enlarged partial cross-sectional view of a part of the surface of the second experimental substrate.

[0047] As described above, the first experimental substrate and the second experimental substrate have different wavelengths of the pulsed laser light to be irradiated. The first experimental substrate is irradiated with pulsed laser light having a wavelength of 308 nm, and the second experimental substrate is irradiated with pulsed laser light having a wavelength of 248 nm. As a result, there is also a difference between the density of the nanodot structure formed on the surface of the first experimental substrate after the irradiation with the pulsed laser light and the density of the nanodot structure formed on the surface of the second experimental substrate after the irradiation with the pulsed laser light.

[0048] For example, on the surface sf1 of the first experimental substrate, as shown in the upper part of FIG. 7, when irradiated with pulsed laser light having a wavelength of 308 nm, a plurality of protrusions pr are formed at a low density. On the other hand, on the surface sf2 of the second experimental substrate, as shown in the lower part of FIG. 7, when irradiated with pulsed laser light having a wavelength of 248 nm, which is shorter than 308 nm, a plurality of protrusions pr are formed at a higher density than on the first experimental substrate.

[0049] Here, on the surfaces sf1 and sf2 of each experimental substrate, it is considered that a compressive stress acts on the formation portion of each protrusion pr so as to go toward the tip of the protrusion pr, as indicated by the thick solid arrow in FIG. 7. On the other hand, on the peripheral portion of each protrusion pr, a tensile stress acts so as to move away from the center of each protrusion pr in the planar direction, as indicated by the thick dotted arrow in FIG. 7.

[0050] Therefore, the inventor considered that whether the stress remaining on the surfaces sf1 and sf2 of each experimental substrate appears as a tensile stress or whether the stress remaining on the surfaces sf1 and sf2 of each experimental substrate appears as a compressive stress is determined according to the density of the plurality of protrusions pr.

[0051] That is, in the first experimental substrate, due to the low density of the protrusions pr formed on its surface sf1, the compressive stress generated in the plurality of protrusions pr is smaller than the tensile stress generated in the other parts. As a result, it is presumed that tensile stress remains on the surface of the first experimental substrate as a whole. On the other hand, in the second experimental substrate, due to the higher density of the protrusions pr formed on its surface sf2 than that of the first experimental substrate, the compressive stress generated in the plurality of protrusions pr is sufficiently larger than the tensile stress generated in the other parts. As a result, it is presumed that compressive stress remains on the surface of the second experimental substrate as a whole.

[0052] 5. Effects (a) According to the above surface treatment method, protrusions pr having a height on the order of nanometers are formed at a high density on the target part of the object irradiated with the processing light. Along with the formation of such high-density protrusions pr, compressive residual stress is applied to a wide range of the target part. Thereby, the strength of the target part is improved. Also, according to the above surface treatment method, since the fluence during irradiation of the processing light on the target part is made lower than the melting threshold value, the target part is not melted by the processing light. Therefore, the crystal structure of the target part does not change significantly. As a result, it becomes possible to improve the strength of the surface of the object while suppressing changes in the physical properties of the object.

[0053] (b) As described above, when the processing light having a wavelength of 248 nm is irradiated onto the target part of the object, protrusions pr having a height on the order of nanometers are formed at a high density. The compressive residual stress is considered to exist at the tip parts of the plurality of protrusions pr formed on the surface of the object by the irradiation of the processing light. Therefore, by increasing the density of the protrusions pr, the strength of the surface of the object is further improved.

[0054] (c) The single-crystalline silicon solar cell includes a light-receiving surface that receives sunlight, and the light-receiving surface is constituted by, for example, the surface of a silicon substrate. The above surface treatment method may be used to improve the strength of the light-receiving surface of the silicon substrate of the single-crystalline silicon solar cell.

[0055] FIG. 8 is a diagram showing an example of irradiating a processing light on a light-receiving surface of a silicon substrate of a single-crystalline silicon solar cell. As shown in the upper part of FIG. 8, on the light-receiving surface rf of the silicon substrate of the single-crystalline silicon solar cell, an uneven structure in which a large number of square pyramid-shaped protrusions (pyramid-shaped protrusions) pr1 are arranged on the sub-micron order is formed. This uneven structure confines light having a wavelength mainly longer than 500 nm in the single-crystalline silicon solar cell among sunlight.

[0056] The processing light is irradiated on the light-receiving surface rf of the silicon substrate by the above surface treatment method. In this case, when the processing light is irradiated on the light-receiving surface rf of the silicon substrate, as shown in the lower part of FIG. 8, a large number of protrusions pr on the nanometer order are formed at a high density on the light-receiving surface rf. Such a high-density protrusion pr confines light having a specific wavelength (for example, a wavelength of 500 nm or less) among the light incident on the light-receiving surface rf in the single-crystalline silicon solar cell. As a result, the energy conversion efficiency from light to electricity in the single-crystalline silicon solar cell is improved.

[0057] (d) The present inventor examined a further effect obtained by irradiating a processing light on a silicon substrate of a single-crystalline silicon solar cell. Specifically, the present inventor confirmed whether there is a difference in the band gap between a single-crystalline silicon solar cell irradiated with the pulsed laser light of a KrF excimer laser as the processing light and a single-crystalline silicon solar cell not irradiated with the processing light. As a result, it was confirmed that the band gap of the single-crystalline silicon solar cell irradiated with the processing light is higher than the band gap of the single-crystalline silicon solar cell not irradiated with the processing light.

[0058] As information regarding changes in the band gap of a substance, it is known that when compressive residual stress is applied to a semiconductor material, the lattice constant (interatomic distance) of the semiconductor material decreases, resulting in an increase in the band gap. In addition, it has been reported that for a substance having a nanodot structure, the band gap changes depending on the size of the nanodot structure. According to this information, it is considered that if processing light in the X-ray region having a wavelength shorter than 248 nm used in the experiment is used, it becomes possible to make the size of the nanodot structure smaller and to obtain a higher band gap.

[0059] The peak of the spectral sensitivity of a single-crystalline silicon solar cell is inherently shifted to the longer wavelength side with respect to the peak of the solar spectrum. Therefore, it is desirable to shift the peak of the spectral sensitivity of the single-crystalline silicon solar cell to the shorter wavelength side so as to approach the peak of the solar spectrum. As the degree of coincidence between the peak of the spectral sensitivity of the single-crystalline silicon solar cell and the peak of the solar spectrum increases, the energy conversion efficiency from light to electricity in the single-crystalline silicon solar cell is further improved.

[0060] Here, it is known that for a single-crystalline silicon solar cell, the peak of the spectral sensitivity of the single-crystalline silicon solar cell moves to the shorter wavelength side as the band gap increases. As described above, when the single-crystalline silicon solar cell is irradiated with processing light, the band gap of the single-crystalline silicon solar cell increases. Thereby, the single-crystalline silicon solar cell irradiated with the processing light has a higher energy conversion efficiency from light to electricity than the single-crystalline silicon solar cell not irradiated with the processing light. In this case, by using light having a shorter wavelength as the processing light, the energy conversion efficiency from light to electricity in the single-crystalline silicon solar cell is further improved.

[0061] 6. Other Embodiments (a) In the above embodiment, the use of pulsed laser light of a KrF excimer laser as the processing light has been described, but the present invention is not limited to this. As the processing light, a continuously oscillating laser light (CW laser light) may be used. Further, as the processing light, the fourth harmonic of a YAG laser having a wavelength of 266 nm may be used, or an ArF excimer laser having a wavelength of 193 nm may be used. Alternatively, as the processing light, light in the X-ray region having a wavelength within the range of 1 nm to 10 nm may be used.

[0062] (b) In the above embodiment, an example of irradiating a silicon substrate with the processing light as the object has been described, but the surface treatment method according to the present invention can also be applied to the surface treatment of semiconductor elements other than the silicon substrate as the object. Further, the surface treatment method according to the present invention can also be applied to metal products other than semiconductor elements (for example, metal products made of Ti, Mo, Pt, Au, Al, and alloys containing any of them).

[0063] 7. Summary of the embodiment (Item 1) The surface treatment method according to Item 1 includes a step of irradiating a target portion on the surface of an object with processing light having a wavelength of 1 nm or more and a wavelength shorter than 308 nm according to predetermined irradiation conditions, wherein the irradiation conditions include making the fluence higher than 0 and lower than the melting threshold of the target portion.

[0064] According to the surface treatment method, protrusions having a height on the order of nanometers are formed at a high density in the target portion of the object irradiated with the processing light. Along with the formation of such high-density protrusions, compressive residual stress is applied to a wide range of the target portion. Thereby, the strength of the target portion is improved. Further, according to the above surface treatment method, since the fluence at the time of irradiating the target portion with the processing light is made lower than the melting threshold, the target portion is not melted by the processing light. Therefore, the crystal structure of the target portion does not change significantly. As a result, it is possible to improve the strength of the surface of the object while suppressing changes in the physical properties of the object.

[0065] (Item 2) In the surface treatment method according to Item 1, the treatment light may have a wavelength of 248 nm or less.

[0066] In this case, the treatment light having a wavelength of 248 nm or less is irradiated onto the target portion of the object. Thereby, protrusions having a height on the nanometer order are formed at a higher density on the target portion of the object irradiated with the treatment light. Therefore, by increasing the density of the protrusions, the strength of the surface of the object is further improved.

[0067] (Item 3) In the surface treatment method according to Item 1 or Item 2, the treatment light is pulsed laser light having a pulse width sufficiently longer than the heat relaxation time of the target portion of the object, and the melting threshold is F th is taken as the melting threshold, T m is taken as the melting point of the material of the object, T0 is taken as the initial temperature of the material of the object, C i is taken as the heat capacity of the object, k i is taken as the thermal conductivity of the object, τ L is taken as the pulse width of the pulsed laser light, and A is taken as the absorption rate of the pulsed laser light with respect to the target portion of the object, it may be represented by the following formula (1).

[0068]

Equation

[0069] By using the above formula (1), an appropriate melting threshold corresponding to the target portion of the object can be easily obtained. Thereby, the efficiency and reliability of the surface treatment are improved.

[0070] (Item 4) In the surface treatment method according to Item 1 or Item 2, the treatment light is pulsed laser light having a pulse width shorter than the heat relaxation time of the target portion of the object, and the melting threshold is F th is taken as the melting threshold, T mLet \(T_m\) be the melting point of the material of the object, \(T_0\) be the initial temperature of the material of the object, and \(C\) i be the heat capacity of the object, \(A\) be the absorption rate of the pulsed laser light with respect to the target portion of the object, and \(a\) be the penetration coefficient of the processing light with respect to the target portion of the object, it may be represented by the following formula (2).

[0071]

Equation

[0072] By using the above formula (2), an appropriate melting threshold corresponding to the target portion of the object can be easily obtained. Thereby, the efficiency and reliability of the surface treatment are improved.

[0073] (Item 5) In the surface treatment method according to any one of Items 1 to 4, the object is a semiconductor device having a light-receiving surface that receives light, and the target portion may be set on the light-receiving surface.

[0074] In this case, when the processing light is irradiated onto the light-receiving surface of the semiconductor device, a large number of protrusions on the order of nanometers are formed at high density on the light-receiving surface. Such high-density protrusions confine light of a specific wavelength (for example, a wavelength of 500 nm or less) among the light incident on the light-receiving surface within the semiconductor device. As a result, the energy conversion efficiency from light to electricity in the semiconductor device is improved.

Explanation of Reference Signs

[0075] 1... Object, 1s, sf1, sf2... Surfaces, 10... Light irradiation device, pr, pr1... Protrusions, rf... Light-receiving surface, st... Target portion

Claims

1. including a step of irradiating a target portion on the surface of an object with a processing light having a wavelength of 1 nm or more and shorter than 308 nm according to predetermined irradiation conditions; the irradiation conditions include making the fluence higher than 0 and lower than the melting threshold of the target portion, a surface treatment method.

2. The surface treatment method according to claim 1, wherein the processing light has a wavelength of 248 nm or less.

3. The processing light is a pulsed laser light having a pulse width sufficiently longer than the heat relaxation time of the target portion of the object, The melting threshold is F ｔｈ is used as the melting threshold, T ｍ is used as the melting point of the material of the object, T ０ is used as the initial temperature of the material of the object, C ｉ is used as the heat capacity of the object, k ｉ is used as the thermal conductivity of the object, τ Ｌ is used as the pulse width of the pulsed laser light, and A is the absorption rate of the pulsed laser light with respect to the target portion of the object. The surface treatment method according to claim 1 or 2, which is represented by the following formula (1). 【Number 1】

4. The processing light is a pulsed laser light having a pulse width shorter than the heat relaxation time of the target portion of the object, The melting threshold value is F ｔｈ is used as the melting threshold value, T ｍ is used as the melting point of the material of the object, T ０ is used as the initial temperature of the material of the object, C ｉ is used as the heat capacity of the object, A is the absorption rate of the pulsed laser light with respect to the target portion of the object, and a is the penetration coefficient of the processing light with respect to the target portion of the object, the surface treatment method according to claim 1 or 2, which is represented by the following formula (2). 【Number 2】

5. The object is a semiconductor device having a light receiving surface that receives light, The surface treatment method according to claim 1 or 2, wherein the target portion is set on the light receiving surface.

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