OPTICAL HEATING METHOD, AND OPTICAL HEATING DEVICE FOR n-TYPE SiC SEMICONDUCTOR

Abstract

Provided is an optical heating method by which it is possible to efficiently heat a to-be-processed body including an n-type SiC semiconductor.　The method includes a step (a) in which a to-be-processed body including a n-type SiC semiconductor is irradiated with heating light, which has a color temperature in the range of 2,200K to 2,600K and is emitted from a light source unit provided with an infrared light lamp, through a window member to heat the to-be-processed body.

Description

Optical heating method and optical heating device for n-type SiC semiconductor

[0001] The present invention relates to an optical heating method, and more particularly to an optical heating method for an object to be treated including an n-type SiC semiconductor.The present invention also relates to an optical heating device for an n-type SiC semiconductor.

[0002] In semiconductor manufacturing processes, various heat treatments such as film formation, oxidation / diffusion, modification, and annealing are performed on objects to be treated, including semiconductor wafers. Light is often used in these heat treatments. Heating an object to be treated using light in this way is called "light heating." Furthermore, the light used for heating is called "heating light."

[0003] A known example of a semiconductor heating device that uses optical heating is the technology disclosed in Patent Document 1. The device in Patent Document 1 uses an LED lamp as a light source that emits heating light with a wavelength of 810 nm to 980 nm.

[0004] Japanese Patent Application Laid-Open No. 2020-009927

[0005] "Polymer Materials Technology for Photoresist Materials" by Akimasa Soyano, Journal of the Society of Rubber Science and Technology of Japan, Vol. 85, No. 2 (2012), pp. 33-39

[0006] In recent years, the development of power semiconductor devices capable of handling higher voltages and currents than conventional devices has been progressing. While conventional devices generally use Si, the use of SiC has been considered for recent devices in order to realize compact devices with high breakdown voltage characteristics. SiC is classified into n-type SiC, which has a relatively low resistivity, and semi-insulating SiC (sometimes referred to as "semi-insulating SiC"), which has a relatively high resistivity. N-type SiC, in particular, has attracted attention due to its wide range of applications.

[0007] Generally known n-type SiC is SiC doped with nitrogen (N) as a dopant, and generally known semi-insulating SiC is SiC doped with vanadium (V) as a dopant or SiC in which lattice defects are intentionally formed.

[0008] SiC has a larger band gap than conventional Si, a high breakdown field strength, and other properties, and is expected to reduce environmental impact through low loss, miniaturization, and weight reduction. However, SiC transmits most of the light emitted from LED lamps such as those described in Patent Document 1, making it difficult to heat efficiently. Patent Document 1 does not specifically consider the case where a SiC wafer is used as the heating object. For this reason, Patent Document 1 does not mention at all that there is a wavelength range of light that is suitable for heat-treating SiC wafers.

[0009] In view of the above-mentioned problems, the present invention aims to provide an optical heating method capable of efficiently heating a workpiece containing an n-type SiC semiconductor, and also aims to provide an optical heating device suitable for heating a workpiece containing an n-type SiC semiconductor.

[0010] The light heating method according to the present invention is characterized by comprising a step (a) of irradiating a workpiece including an n-type SiC semiconductor with heating light having a color temperature in the range of 2,200 K to 2,600 K emitted from a light source unit equipped with an infrared lamp through a window member to heat the workpiece.

[0011] In the light heating method, the heating light preferably has a color temperature in the range of 2,400K to 2,600K.

[0012] The present inventors have made the following discovery while conducting extensive research into methods for heating SiC-containing workpieces. SiC comes in a variety of crystal structures, including 4H-SiC (band gap 3.3 eV), 6H-SiC (band gap 3 eV), and 3C-SiC (band gap 2.2 eV). In the case of 6H-SiC, which has a band gap of 3 eV, the transmittance of light gradually increases in the wavelength range longer than about 420 nm, and almost all light in the wavelength range longer than 500 nm is transmitted.

[0013] As mentioned above, SiC is classified into n-type, semi-insulating, and other types depending on the dopant used. The absorption rate of SiC can change simply by doping it with a dopant. For this reason, there is no wavelength range that is recognized as being suitable for photothermal treatment of SiC.

[0014] In light of the above background, the present inventors investigated the possibility of deriving the absorptance for each wavelength of n-type SiC, which is particularly expected to be put to practical use, and finding a wavelength band of light suitable for optically heating n-type SiC. At the time of filing this application, it was difficult to directly measure the absorption spectrum of n-type SiC, so the present inventors adopted a method of measuring the reflectance R and transmittance S of n-type SiC and deriving the absorptance A (= 1 - R - S), thereby deriving the absorptance for each wavelength of n-type SiC.

[0015] Specifically, the reflectance R is measured and derived using an n-type SiC wafer, a light source whose intensity spectrum of emitted light is known, and a spectrophotometer arranged on the same side of the n-type SiC wafer as the light source, while the transmittance S is measured and derived using an n-type SiC wafer, a light source whose intensity spectrum of emitted light is known, and a spectrophotometer arranged on the opposite side of the n-type SiC wafer from the light source.

[0016] FIG. 1 is a graph showing the relationship between wavelength and absorptance in n-type SiC. The graph shown in FIG. 1 is a graph calculated based on the reflectance characteristic data and transmittance characteristic data of n-type SiC measured by the above-described method. FIG. 1 shows that the absorptance is 45% or more for light with wavelengths longer than 800 nm, and the absorptance increases beyond the maximum point near 600 nm. In other words, even when the workpiece includes n-type SiC, high heating efficiency can be achieved by irradiating the workpiece with light from the light source having a main emission wavelength longer than 800 nm.

[0017] Figure 2 is a graph showing the radiant intensity spectrum of light for each color temperature when the power is constant. In the graph shown in Figure 2, the vertical axis represents the radiant intensity relative to the peak intensity of the spectrum when the color temperature is 2,600 K, and the horizontal axis represents the wavelength. The relationship between color temperature T, wavelength λ, and radiant intensity E(λ) is derived from Planck's radiation formula, which is expressed as the following equation (1):

[0018] (h is Planck's constant, c is the speed of light in a vacuum, and k is Boltzmann's constant.)

[0019] As shown in Fig. 2, light with a color temperature of 2,600 K or less has a radiant intensity spectrum with a peak wavelength in a wavelength band of 1,100 nm or more. If light exhibiting such a radiant intensity spectrum is used as heating light, it corresponds to light in a wavelength band in which the absorption rate of n-type SiC is relatively high, as can be seen from Fig. 1, and therefore n-type SiC can be efficiently heated.

[0020] 2, the lower the color temperature, the longer the wavelength at which the intensity peak shifts, and the lower the radiation intensity (spectral radiance) of the light. For this reason, with light having a color temperature lower than 2,200 K, the rapid drop in radiation intensity makes it difficult to supply the energy required to heat the n-type SiC substrate.

[0021] Therefore, according to the graphs shown in FIGS. 1 and 2 and the circumstances described above, the color temperature of the light used to heat n-type SiC is preferably 2,200 K to 2,600 K, and more preferably 2,400 K to 2,600 K.

[0022] In the above-described light heating method, by utilizing heating light with a color temperature in the range of 2,200 K to 2,600 K, the heating light can be absorbed by the object to be treated to an extent that a heating effect can be exerted, even when the object to be treated includes an n-type SiC semiconductor. This allows the object to be heated efficiently and without contact.

[0023] The light heating method may include a step (b) of measuring the temperature of the object to be treated by using a radiation thermometer having a wavelength sensitivity range of a predetermined wavelength range of 2.5 μm to 14 μm during the step (a) to receive light emitted from the object to be treated.

[0024] A radiation thermometer with a sensitivity wavelength range of 2.5 μm to 14 μm can detect heating light in a wavelength range with a relatively low radiation intensity and measure the temperature of the object to be treated. In other words, the above-mentioned optical heating method allows the temperature of the object to be measured using a radiation thermometer that is not easily affected by heating light, making it possible to accurately monitor the temperature of the object to be treated during heating.

[0025] The upper limit of the wavelength range of sensitivity of the radiation thermometer may be set appropriately depending on the melting point of SiC contained in the object to be treated, but this does not exclude the use of a radiation thermometer capable of measuring a temperature range higher than the melting point to measure the temperature of the object to be treated.

[0026] The optical heating device of the present invention is an optical heating device for an n-type SiC semiconductor, and is characterized by comprising: a chamber that accommodates a workpiece containing an n-type SiC semiconductor; a support member that supports the workpiece within the chamber; a light source unit having an infrared lamp that emits heating light with a color temperature in the range of 2,200 K to 2,600 K; and a window member that passes the heating light emitted from the light source unit and guides it to the workpiece.

[0027] In the optical heating device, the infrared lamp may be a halogen lamp.

[0028] The optical heating device described above can efficiently heat n-type SiC semiconductors, which are used in power semiconductor devices and are expected to have a particularly wide range of uses, without contact.

[0029] The optical heating device may further include a reflecting member that reflects light emitted from the infrared lamp and traveling in a direction opposite to the object to be treated toward the object to be treated.

[0030] The optical heating device having the above configuration can utilize the heating light emitted from the infrared lamp and traveling in the direction opposite to the object to be treated to heat the object to be treated, thereby enabling the object to be heated more efficiently.

[0031] According to the present invention, an optical heating method capable of efficiently heating a workpiece containing an n-type SiC semiconductor is realized. Also, according to the present invention, an optical heating device suitable for heating a workpiece containing an n-type SiC semiconductor is realized.

[0032] 1 is a graph showing the relationship between wavelength and absorptance in n-type SiC; 2 is a graph showing the radiation intensity spectrum of light for each color temperature; 3 is a cross-sectional view schematically showing the configuration of an embodiment of a light heating device; 4 is a graph showing the tendency of the absorbance characteristics of resins used in semiconductor photoresists with respect to light; and 5 is a schematic plan view of a light source unit as viewed from the -Z side.

[0033] The optical heating method according to the present invention includes a step (a) of heating an object to be treated, the object including an n-type SiC semiconductor, by irradiating the object through a window member with heating light emitted from a light source and having a color temperature in the range of 2,200 K to 2,600 K. This optical heating method will be described below with reference to drawings of an optical heating device that is one embodiment of the method.

[0034] Note that the drawings below are all schematic illustrations, and the dimensional ratios and numbers in the drawings do not necessarily correspond to the actual dimensional ratios and numbers. Furthermore, in the following description, the workpiece is described assuming an n-type 4H—SiC semiconductor doped with nitrogen (N) as a dopant. However, as described above, the workpieces targeted by the present invention are not limited to workpieces containing a nitrogen (N)-doped n-type 4H—SiC semiconductor, but also include workpieces containing materials generally recognized in the semiconductor industry as "n-type SiC" or those that can be classified as n-type SiC. This concept is inferred based on the fact that, although it is assumed that the light absorption rate varies somewhat depending on the type of dopant and the crystal structure, it is unlikely that extreme spectral differences will appear between materials that are treated as similar products classified in the same category based on SiC.

[0035] Fig. 3 is a cross-sectional view schematically illustrating the configuration of one embodiment of the optical heating device 1. The optical heating device 1 illustrated in Fig. 3 includes a chamber 10 that accommodates a workpiece W1 including an n-type SiC semiconductor, a light source unit 2, and a radiation thermometer 14. The light source unit 2 includes a plurality of infrared lamps 11 and a reflecting member 12.

[0036] In the following description, an X-Y-Z coordinate system will be referred to where appropriate, in which a plane parallel to the main surfaces (W1a, W1b) of the workpiece W1 is defined as the X-Y plane, and the normal direction of this X-Y plane is defined as the Z direction, as shown in Fig. 3. As shown in Fig. 3, the light source unit 2 and the workpiece W1 face each other in the Z direction.

[0037] In the following, when expressing a direction, if it is necessary to distinguish between positive and negative directions, the direction will be written with a positive or negative sign, such as "+Z direction" or "-Z direction," and when expressing a direction without distinguishing between positive and negative directions, it will be simply written as "Z direction."

[0038] The light source unit 2 emits heating light L1 having a color temperature in the range of 2,200 K to 2,600 K. Several examples of the radiation intensity spectrum of heating light L1 having a color temperature in the range of 2,200 K to 2,600 K are shown in Figure 2. Note that the radiation intensity spectrum shown in Figure 2 is merely a waveform theoretically derived based on (1) above, and the radiation intensity spectrum of actual heating light L1 need not be the same as the radiation intensity spectrum shown in Figure 2 as long as the measured color temperature is within the above range.

[0039] The color temperature of the heating light L1 emitted by the light source unit 2 is measured using a color temperature meter (color meter) placed at a position where the light emitted from the light source unit 2 can be directly observed.

[0040] As shown in FIG. 3 , the chamber 10 includes a support member 13 inside. The support member 13 supports the object W1 to be processed so that the main surfaces W1a and W1b of the object W1 are arranged on the X-Y plane. In FIG. 3 , the main surface W1b of the object W1 is arranged to face the light source unit 2. That is, circuit elements, wiring, etc. are formed on the main surface W1a or W1b, and the main surface W1b is the surface that is irradiated with the heating light L1 emitted from the light source unit 2. However, the present invention does not exclude the case where the object W1 is a bare substrate without wiring, etc., and is arranged so that the main surface W1a of the object W1 faces the light source unit 2.

[0041] The support member 13 may support the workpiece W1 in any manner as long as the main surface W1a of the workpiece W1 is positioned on the X-Y plane. For example, the support member 13 may have a plurality of pin-shaped protrusions that support the workpiece W1 at points.

[0042] As shown in FIG. 3, the chamber 10 includes a first window 10a facing the main surface W1a of the workpiece W1 supported by the support member 13, and a second window 10b facing the main surface W1b.

[0043] The first window 10a is a window used by the radiation thermometer 14 to measure the temperature of the main surface W1a of the workpiece W1. The radiation thermometer 14 is a thermometer that measures the surface temperature of an object to be measured by receiving light emitted from the object to be measured. In this embodiment, the wavelength range of sensitivity of the radiation thermometer 14 is a predetermined wavelength range ranging from 2.5 μm to 14 μm. In other words, the first window 10a is made of a material that transmits light that falls within the wavelength range of sensitivity of the radiation thermometer 14. As an example, the first window 10a is made of general quartz glass, calcium fluoride, or the like.

[0044] The second window 10b is a window member for guiding the heating light L1 emitted from the light source unit 2 to the main surface W1b of the workpiece W1. As described above, the color temperature of the heating light L1 is in the range of 2,200 K to 2,600 K. The second window 10b is made of a material that has a transmittance of 50% or more for at least the wavelength band in which the radiation intensity of the heating light L1 is 50% or more of its peak. As an example, the second window 10b is made of synthetic quartz. However, the material of the second window 10b may be selected appropriately depending on the color temperature of the heating light L1.

[0045] 4 is a graph showing the tendency of the light absorbance characteristics of resins used in semiconductor photoresists, as shown in the above-mentioned Non-Patent Document 1. Resins used in semiconductor photoresists include novolac resin, methacrylic resin, and PHS resin. As shown in FIG. 2, these resins are characterized by exhibiting relatively high absorbance for light with a wavelength of 300 nm or less, and novolac resin and PHS resin in particular exhibit a sudden increase in absorbance for light with a wavelength of 300 nm or less.

[0046] For example, when the workpiece is a SiC substrate, a semiconductor photoresist is used as a photoresist mask when ion implantation is performed on the SiC substrate. For this reason, important indices for a resin used in a semiconductor photoresist include optical transparency, chemical resistance, and solubility in a developer, and optical transparency is adjusted according to the spectrum of light emitted from a light source.

[0047] However, as mentioned above, many of the resins used in semiconductor photoresists exhibit high absorbance for light with wavelengths of 300 nm or less. In contrast, the radiation intensity of the heating light L1 emitted from the light source unit 2 is extremely low at wavelengths around 300 nm, as shown in Figure 2. In other words, using light with a color temperature in the range of 2,200 K to 2,600 K as the heating light L1 has the advantage of reducing the risk of absorption by the semiconductor photoresist.

[0048] 5 is a schematic plan view of the light source unit 2 as viewed from the -Z side. As shown in Figures 3 and 5, the light source unit 2 is composed of a plurality of infrared lamps 11 and a reflecting member 12 that reflects the heating light L1 emitted from the infrared lamps 11 and traveling toward the +Z side toward the -Z side.

[0049] Each of the infrared lamps 11 emits heating light L1 with a color temperature in the range of 2,200 K to 2,600 K. The color temperatures of the heating light L1 emitted from the infrared lamps 11 are preferably substantially the same. Here, "substantially the same" means that deviations in wavelength due to variations in elements during the manufacturing process are allowed. Typically, deviations in color temperature may be within ±50 K.

[0050] Specifically, the infrared lamp 11 in this embodiment is a halogen lamp, but other types of heaters, such as sheathed heaters and carbon heaters, may also be used. From the perspective of achieving higher light output, it is preferable that the infrared lamp 11 be a halogen lamp. It is known that the color temperature emitted from the infrared lamp 11 depends on the input power, and in the case of a halogen lamp, for example, this can be controlled by adjusting the applied voltage.

[0051] 3, the reflecting member 12 is made of a frame body arranged to cover the infrared lamp 11, and a reflecting surface 12a is formed on the inner wall surface thereof to reflect the heating light L1 emitted from the infrared lamp 11. The reflecting member 12 is made of, for example, an aluminum plate.

[0052] According to the optical heating device 1, the color temperature of the heating light L1 emitted from the light source 2 is within the range of 2,200 K to 2,600 K, so that the heating light L1 is absorbed by the workpiece W1 even if the workpiece W1 includes an n-type SiC semiconductor. This allows for non-contact heating of the workpiece W1.

[0053] Furthermore, during the light irradiation step, the temperature of the workpiece W1 can be detected by receiving the light emitted from the workpiece W1 with the radiation thermometer 14. That is, by feeding back the detection result by the radiation thermometer 14 to a controller (not shown) that controls the light output of the light source unit 2, it is possible to heat the workpiece W1, which includes an n-type SiC semiconductor, with high precision. Note that, as shown in Figures 1 and 2, the main emission wavelength range including the peak wavelength of the heating light L1 is clearly outside the sensitivity wavelength range of the radiation thermometer 14.

[0054] For reference, a verification experiment was conducted to compare the heating efficiency of an LED element and a halogen lamp, and the details of the experiment will be described below.

[0055] Example 1 A halogen lamp was used that emitted light with a color temperature of 2,600 K. The peak wavelength of the light emitted from the halogen lamp with a color temperature of 2,600 K was 1,115 nm.

[0056] Example 2 A halogen lamp was used that emitted light with a color temperature of 2,400 K. The peak wavelength of the light emitted from the halogen lamp with a color temperature of 2,400 K was 1,207 nm.

[0057] Example 3 A halogen lamp was used that emitted light with a color temperature of 2,200 K. The peak wavelength of the light emitted from the halogen lamp with a color temperature of 2,200 K was 1,317 nm.

[0058] Comparative Example 1 An LED element was used that emitted light with a peak wavelength of 395 nm.

[0059] An n-type 4H—SiC substrate doped with nitrogen (N) as a dopant was irradiated with light from each light source to achieve the same illuminance. As described above, the reflectance R and transmittance S of the n-type SiC were measured, and the absorptance A (= 1 − R − S) was calculated. Heating efficiencies were then compared based on the value derived from the wavelength integral of the product of the absorptance A and the radiant intensity (hereinafter referred to as "light absorption efficiency"). The comparison was performed by setting the light absorption efficiency when an LED element with a peak wavelength of 395 nm was used as the light source to 1, and determining that the light absorption efficiency for other light sources was greater than 1 to be substantially high.

[0060] The results are shown in Table 1 below.

[0061]

[0062] As can be seen from the results in Table 1 above, it can be confirmed that Examples 1 to 3 all have significantly higher heating efficiency than Comparative Example 1.

[0063] From the above, it can be seen that heating light L1 having a color temperature in the range of 2,200 K to 2,600 K can heat the n-type SiC semiconductor more efficiently.

[0064] From the viewpoint of increasing the heating efficiency, it is more preferable that the color temperature of the heating light L1 be within the range of 2,400K to 2,600K.

[0065] According to Table 1 above, the heating efficiency of the halogen lamp can be improved if the color temperature is further reduced from 2,200 K. However, if the color temperature is further reduced below 2,200 K, it becomes difficult to irradiate heating light L1 with sufficient radiation intensity to heat n-type SiC.

[0066] In the above-described embodiment, a configuration is described in which a reflective member 12 is mounted on the light source unit 2, but for example, in cases where the workpiece W1 can be sufficiently heated only by the heating light L1 emitted from the infrared lamp 11 to the -Z side, the light source unit 2 does not need to be equipped with a reflective member 12.

[0067] The reflective member 12 may also be a reflective film provided on the wall surface of the infrared lamp 11. The reflective film provided on the wall surface of the infrared lamp 11 may be, for example, a metal film made of aluminum (Al), or a reflective film formed by agglomerating particles whose main component is a metal oxide such as silica (SiO2) or alumina (Al2O3).

[0068] Furthermore, in the above-described embodiment, the infrared lamp 11 included in the light source unit 2 has a straight tube shape as shown in Fig. 5, but is not limited to this shape. The infrared lamp 11 may have, for example, a light bulb shape or a circular heater having a ring shape.

[0069] REFERENCE SIGNS LIST 1: Light heating device 2: Light source 10: Chamber 10a: First window 10b: Second window 11: Infrared lamp 12: Reflecting member 12a: Reflecting surface 13: Supporting member 14: Radiation thermometer L1: Heating light W1: Object to be treated W1a, W1b: Main surfaces of object to be treated

Claims

1. A light heating method comprising the steps of: (a) irradiating a workpiece, including an n-type SiC semiconductor, with heating light having a color temperature in the range of 2,200 K to 2,600 K emitted from a light source unit equipped with an infrared lamp through a window member to heat the workpiece.

2. The light heating method according to claim 1, wherein the heating light has a color temperature within the range of 2,400K to 2,600K.

3. The optical heating method according to claim 1 or 2, further comprising a step (b) of measuring the temperature of the object to be treated by a radiation thermometer having a sensitivity wavelength range within a predetermined wavelength range of 2.5 μm to 14 μm during the execution of the step (a) by receiving the light radiated from the object to be treated.

4. An optical heating device for n-type SiC semiconductors, comprising: a chamber for accommodating an object to be treated that includes an n-type SiC semiconductor; a support member for supporting the object to be treated within the chamber; a light source unit having an infrared lamp that emits heating light having a color temperature in the range of 2,200 K to 2,600 K; and a window member that passes the heating light emitted from the light source unit and guides it to the object to be treated.

5. The optical heating device for n-type SiC semiconductor according to claim 4, wherein the infrared lamp is a halogen lamp.

6. An optical heating device for n-type SiC semiconductor as described in claim 4 or 5, characterized in that it is provided with a reflective member that reflects light emitted from the infrared lamp and traveling in a direction opposite to the workpiece toward the workpiece.

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