JP2016127157A - Laser annealing device and method for manufacturing semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser annealing device capable of reducing damage to an element structure on the side opposite to a laser irradiation surface even when a laser beam of a wavelength region transmitting an SiC substrate is used.SOLUTION: A laser light source outputs a pulse laser beam. A stage holds a substrate. The pulse laser beam output from the laser light source is guided to the substrate held by a stage. A moving mechanism moves one of a path of the pulse laser beam and the substrate held by the stage to the other so that an incident position of the pulse laser beam moves on a surface of the substrate. A photodetector detects the reflected light of a pulse laser beam from the substrate. A control device adjusts at least one of the number of shots of the pulse laser beam incident on the same position of the substrate and the intensity of the pulse laser beam on the basis of the intensity of the reflected light.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a laser annealing apparatus suitable for forming an ohmic electrode on a silicon carbide (SiC) substrate and a method for manufacturing a semiconductor element having an ohmic electrode provided on the SiC substrate.

  As a semiconductor material for semiconductor power devices, SiC having an energy band gap wider than that of silicon attracts attention. Power semiconductor devices such as Schottky barrier diodes, MOSFETs, and JFETs using SiC have been put into practical use. SiC makes it difficult to produce a wafer with fewer defects than Si. For this reason, the epitaxial layer with few defects formed on the SiC wafer is used as the drift layer. The thickness of the epitaxial layer is set according to the required breakdown voltage. In the case of using SiC as the drift layer, an equivalent breakdown voltage can be ensured with a thickness of about 1/10 compared to the case of using Si. For example, an epitaxial layer made of SiC having a thickness of 10 μm can ensure a breakdown voltage equivalent to that of a Si wafer having a thickness of 100 μm.

  In the Schottky barrier diode, an anode electrode is formed on the surface of the epitaxial layer. In the switching element, an element structure having a switching function is formed on the surface of the epitaxial layer. The SiC wafer that is the base of the epitaxial layer serves as a support substrate for the epitaxial layer. In order to reduce energization loss, it is preferable to make the SiC wafer thinner. If the SiC wafer is thinned before the element structure is formed on the surface of the epitaxial layer, it becomes difficult to form the element structure due to damage or warpage during the process. Accordingly, it is preferable to thin the SiC wafer after forming the element structure on the surface of the epitaxial layer.

  An ohmic electrode is formed on the back surface of the thinned SiC wafer. When laser annealing is applied during the formation of the ohmic electrode, the thermal effect on the element structure formed on the surface on the front side can be reduced as compared with the case where annealing is performed in an electric furnace. A metal silicide such as nickel silicide is used as the ohmic electrode.

  Patent Document 1 listed below discloses a method of forming an ohmic electrode by laser annealing using a pulse laser beam in the ultraviolet region. As a pulse laser in the ultraviolet region, a third harmonic of a solid-state laser, an excimer laser, or the like is used.

JP 2014-123589 A

  A gas laser such as an excimer laser has an unstable laser beam quality as compared with a solid-state laser such as a YAG laser, and it is difficult to perform high-quality annealing with high reproducibility. In order to perform high-quality annealing with high reproducibility, it is preferable to use a solid-state laser. However, in order to obtain an ultraviolet pulse laser using a solid-state laser, the fundamental wave must be converted into a third harmonic. Since it is difficult to increase the conversion efficiency from the fundamental wave to the third harmonic, the output of the third harmonic is reduced. As a result, it is difficult to ensure high productivity.

  In laser annealing using the fundamental wave or the second harmonic of a solid-state laser, a higher laser output can be obtained than when using the third harmonic. However, light in the wavelength range of the fundamental wave or the second harmonic passes through the SiC substrate. When the metal film formed on the surface of the SiC substrate absorbs the fundamental wave or the second harmonic and is heated, the silicide reaction between the metal film and the SiC substrate proceeds. When the silicide reaction proceeds and the metal film becomes thin, the amount of laser light absorbed by the metal film decreases.

  The laser light that has not been absorbed by the metal film passes through the SiC substrate and reaches the opposite surface on which the element structure is formed. If an element structure such as an aluminum wiring absorbs laser light and is heated excessively, the wiring may burn out.

  An object of the present invention is to provide a laser annealing apparatus that can reduce damage to an element structure on the side opposite to a laser irradiation surface even when using laser light in a wavelength region that passes through a SiC substrate. Another object of the present invention is to provide a method for manufacturing a semiconductor device that can reduce damage to the device structure on the side opposite to the laser irradiation surface even when using laser light in a wavelength region that passes through a SiC substrate. That is.

According to one aspect of the invention,
A laser light source that outputs a pulsed laser beam;
A stage for holding a substrate;
A propagation optical system for guiding the pulse laser beam output from the laser light source to the substrate held on the stage;
A moving mechanism for moving one of the path of the pulse laser beam and the substrate relative to the other so that the incident position of the pulse laser beam moves on the surface of the substrate held on the stage;
A photodetector for detecting reflected light of the pulsed laser beam from the substrate;
A control device that adjusts at least one of the number of shots of the pulsed laser beam and the intensity of the pulsed laser beam incident on the same portion of the substrate based on the intensity of the reflected light detected by the photodetector; It is to provide a laser annealing apparatus having the same.

According to another aspect of the invention,
Preparing a substrate in which a metal film made of nickel, titanium or tungsten is formed on a first surface of a base substrate made of silicon carbide;
A step of causing the base film and the metal film to undergo a silicidation reaction by injecting a plurality of shots of a pulsed laser beam in a wavelength region that is absorbed by the metal film and transmitted through the base substrate into the metal film. When,
Measuring the intensity of the reflected light of the pulsed laser beam from the substrate;
There is provided a method for manufacturing a semiconductor device, comprising adjusting at least one of the number of shots of the pulse laser beam incident on the same location and the intensity of the pulse laser beam based on the intensity of the reflected light.

  As the silicidation of the metal film proceeds, the metal film becomes thinner and the intensity of reflected light becomes weaker. When a pulsed laser beam is incident with the metal film being thin, many components of the laser light reach the opposite side of the substrate. By adjusting at least one of the number of shots of the pulse laser beam and the intensity of the pulse laser beam based on the intensity of the reflected light, the laser energy reaching the opposite surface of the substrate is reduced and formed on the opposite surface. It is possible to reduce damage to the element structure.

FIG. 1A to FIG. 1D are cross-sectional views of a substrate in the course of manufacturing a semiconductor device manufacturing method according to an embodiment. FIG. 1E to FIG. 1F are cross-sectional views of a substrate in the course of manufacturing a semiconductor device manufacturing method according to an embodiment. FIG. 2 is a schematic diagram of a laser annealing apparatus according to an embodiment. FIG. 3A is a plan view showing the incident region of the pulse laser beam on the surface of the substrate and the movement history of the incident region, and FIG. 3B is a plan view showing the locus of the center of the incident region of the pulse laser beam. FIG. 4 is a block diagram of a control device according to the embodiment. FIG. 5 is a flowchart of an example of processing executed by the control device. FIG. 6 is a flowchart of another example of processing executed by the control device.

  With reference to FIGS. 1A to 1F, a method of manufacturing a semiconductor device according to an embodiment will be described.

  As shown in FIG. 1A, an underlying substrate 10 made of SiC is formed by epitaxially growing n-type SiC on the surface of a substrate made of n-type silicon carbide (SiC). For the base substrate 10, for example, 4H—SiC, 6H—SiC, or 3C—SiC can be used. A p-type guard ring 11 is formed in the surface layer portion of the epitaxial layer by ion implantation. The surface opposite to the surface on which the guard ring 11 is formed is referred to as a “first surface” 10A, and the surface on which the guard ring 11 is formed is referred to as a “second surface” 10B. As shown in FIG. 1B, an insulating film 12 made of silicon oxide is formed on the second surface 10B of the base substrate 10. The insulating film 12 has an opening that exposes a region surrounded by the guard ring 11.

  As shown in FIG. 1C, a Schottky electrode 13 is formed on the surface of the base substrate 10 exposed at the bottom surface of the opening formed in the insulating film 12. As an example, Schottky contact is realized by performing a heat treatment after forming a titanium film. A surface electrode 14 is formed on the Schottky electrode 13. For example, aluminum is used for the surface electrode 14. The guard ring 11, the Schottky electrode 13, and the surface electrode 14 are collectively referred to as an element structure 15.

  As shown in FIG. 1D, the base substrate 10 is thinned by grinding the base substrate 10 from the first surface 10A. As shown in FIG. 1E, a metal film 16 is formed on the first surface 10A of the base substrate 10. For the metal film 16, a metal that undergoes a silicide reaction with the base substrate 10, for example, nickel (Ni), titanium (Ti), or tungsten (W) is used. The metal film 16 and the base substrate 10 constitute a substrate 18 to be annealed. The substrate 18 on which the metal film 16 is formed is prepared by the steps so far.

  As shown in FIG. 1F, laser annealing is performed by irradiating the metal film 16 (FIG. 1E) with a pulsed laser beam 19. The wavelength of the pulsed laser beam 19 is included in a wavelength range that is absorbed by the metal film 16 and transmitted through the base substrate 10. The beam profile of the pulsed laser beam 19 is top flat. This laser annealing is performed while moving (scanning) the incident region of the pulse laser beam 19 within the surface of the metal film 16. The overlapping rate (overlap rate) of the incident region is, for example, 50% to 99%. By this laser annealing, the metal film 16 (FIG. 1E) is silicided, and a metal silicide film 17 is formed.

FIG. 2 shows a schematic diagram of a laser annealing apparatus according to the embodiment. A pulsed laser beam in the green wavelength region is output from the laser light source 20. The laser light source 20 includes, for example, two solid state laser oscillators 201 and 202. The two solid-state laser oscillators 201 and 202 may oscillate simultaneously, or may oscillate with a time difference. As the solid-state laser oscillators 201 and 202, for example, an Nd: YAG laser oscillator, an Nd: YLF laser oscillator, an Nd: YVO ₄ laser oscillator, or the like that outputs a fundamental wave or a second harmonic is used. Two pulse laser beams output from the solid-state laser oscillators 201 and 202 are integrated into one beam path by the beam combiner 21.

  The integrated pulse laser beam is guided to the substrate 18 held on the stage 30 by the propagation optical system 25. The propagation optical system 25 includes an attenuator 251, a telescope 252, a homogenizer 253, and a partial reflection mirror 254. The attenuator 251 attenuates the power of the pulse laser beam. The telescope 252 expands the beam diameter of the pulse laser beam that has passed through the attenuator 251. The homogenizer 253 elongates the beam cross-sectional shape on the surface of the substrate 18 and makes the light intensity uniform in the beam cross-section. The partial reflection mirror 254 reflects most components of the pulse laser beam that has passed through the homogenizer 253 downward.

  The stage 30 is supported on the substrate 32 via the moving mechanism 31. The moving mechanism 31 can move the substrate 18 held on the stage 30 in two directions parallel to the surface thereof, for example, the major axis direction of the beam cross section and a direction orthogonal thereto. By moving the substrate 18, the incident position of the pulse laser beam can be moved in the two-dimensional direction on the surface of the substrate 18. Instead of moving the substrate 18 with respect to the path of the pulse laser beam, the path of the pulse laser beam may be moved with respect to the substrate 18. That is, one of the path of the pulse laser beam and the substrate 18 may be moved with respect to the other.

  A part of the reflected light of the pulse laser beam from the substrate 18 passes through the partial reflection mirror 254 and enters the photodetector 26. The reflected light intensity signal detected by the photodetector 26 is input to the control device 40. The control device 40 controls the laser light source 20, the attenuator 251, and the moving mechanism 31 based on the intensity of the reflected light.

  FIG. 3A shows a plan view of the incident region of the pulse laser beam on the surface of the substrate 18 (FIG. 2) and a history of movement of the incident region. The planar shape of the one-shot incident region 35 is a rectangle having a length L and a width W. For example, the length L is about 2.5 mm and the width W is about 0.3 mm. The size of the incident region 35 is adjusted so as to obtain an optimum power density according to the power of the pulse laser beam.

  By making the pulse laser beam incident while moving the substrate 18, the incident region 35 of the pulse laser beam moves on the surface of the substrate 18 in the main scanning direction 37 (width direction) and the sub-scanning direction 38 (length direction). Can be made. The overlapping rate in the main scanning direction 37 between the incident area 351 of a certain shot and the incident area 352 of the next shot moved in the main scanning direction 37 is, for example, 50% to 99%. The overlapping ratio in the sub-scanning direction 38 between the incident area 353 of a certain shot and the incident area 354 of the next shot moved in the sub-scanning direction 38 is, for example, 50% to 99%.

  FIG. 3B shows a locus 36 at the center of the incident region 35 of the pulse laser beam. In one scanning process in which the main scanning and the sub scanning are repeated, the pulse laser beam is incident on the entire region to be annealed on the surface of the substrate 18. When both the overlapping rate in the main scanning direction 37 and the overlapping rate in the sub-scanning direction 38 are 50%, a total of four shots of the pulse laser beam are incident on the same portion of the surface of the substrate 18 in one scanning process. It will be. When the overlapping rate in the main scanning direction 37 and the overlapping rate in the sub-scanning direction 38 are both 66%, a total of nine shot pulse laser beams are incident on the same portion of the surface of the substrate 18 in one scanning step. It will be. In this way, by changing the overlap rate, the number of shots incident on the same location can be changed in one scanning process. The overlapping rate can be changed by adjusting at least one of the repetition frequency of the pulse laser beam and the moving speed of the substrate 18.

  FIG. 4 is a block diagram of the control device 40 according to the embodiment. The control device 40 includes a reflected light intensity detection unit 401, a laser irradiation condition adjustment unit 402, a laser drive unit 403, an intensity adjustment unit 404, and a moving mechanism drive unit 405. The function of each unit is realized by, for example, a central processing unit (CPU) executing a computer program, or realized by an electronic circuit such as an A / D conversion circuit, a D / A conversion circuit, and a logic circuit. A reflected light intensity-irradiation condition correspondence table 408 is stored in the storage device of the control device 40.

  The reflected light intensity signal detected by the photodetector 26 is input to the reflected light intensity detector 401. The reflected light intensity detector 401 outputs a reflected light intensity measurement value RM based on the intensity signal input from the photodetector 26.

  The laser irradiation condition adjustment unit 402 determines the laser irradiation condition based on the reflected light intensity measurement value RM and the reflected light intensity-irradiation condition correspondence table 408. Laser irradiation conditions include the overlap rate, the number of shots, and the laser power. These irradiation conditions are adjusted by adjusting the repetition frequency RF of the pulse laser beam output from the laser light source 20, the attenuation factor AF of the attenuator 251, the moving speed HSV of the stage 30 (FIG. 2) by the moving mechanism 31 and the sub-scanning. This is done by adjusting the direction moving distance VSL.

  The laser irradiation condition adjustment unit 402 gives the repetition frequency target value RFT to the laser driving unit 403 and the attenuation rate target value AFT to the intensity adjustment unit 404 so that laser irradiation is performed under the determined laser irradiation conditions. A target value HSVT for the scanning direction moving speed and a target value VSLT for the sub-scanning direction moving distance are given to the moving mechanism driving unit 405. Specific processing for adjusting the laser irradiation condition will be described later with reference to FIG. 5 or FIG.

  The laser driving unit 403 controls the laser light source 20 based on the repetition frequency target value RFT. Specifically, a trigger signal is transmitted to the solid-state laser oscillators 201 and 202 according to the repetition frequency target value RFT. The solid-state laser oscillators 201 and 202 output a pulse laser beam in synchronization with the trigger signal. For example, the timings of the trigger signals given to the solid laser oscillators 201 and 202 are adjusted so that the pulse laser beams are output from the two solid laser oscillators 201 and 202 at the same timing.

  By outputting a pulse laser beam from the two solid-state laser oscillators 201 and 202 at the same timing, the peak power of the laser pulse can be doubled. Note that the laser pulse waveforms may partially overlap on the time axis, or after the laser pulse from one solid-state laser oscillator 201 falls, the laser pulse from the other solid-state laser oscillator 202 rises. It may be.

  The intensity adjusting unit 404 controls the attenuator 251 based on the attenuation rate target value AFT. Thereby, the attenuation factor AF of the pulse laser beam by the attenuator 251 matches the attenuation factor target value AFT.

  The movement mechanism drive unit 405 controls the movement mechanism 31 based on the target value HSVT of the moving speed in the main scanning direction and the target value VSLT of the moving distance in the sub scanning direction. Thereby, the moving mechanism 31 is driven so that the main scanning direction moving speed HSV and the sub-scanning direction moving distance VSL of the stage 30 (FIG. 2) are equal to the target value HSVT and the target value VSLT, respectively.

  FIG. 5 shows a flowchart of an example of processing executed by the control device 40 (FIG. 4). In a state where the substrate 18 is held on the stage 30, the first scanning process is executed in step SA1. Specifically, the pulse laser beam output from the laser light source 20 is incident on the substrate 18 while moving the stage 30 in the main scanning direction 37 and the sub-scanning direction 38 (FIGS. 3A and 3B). Predetermined values are adopted as the laser light intensity and the overlapping rate in the main scanning direction 37 and the sub-scanning direction 38 in the first scanning process. Further, during scanning, the photodetector 26 detects the intensity of the reflected light.

  In step SA2, it is determined whether or not the intensity of the reflected light measured in the immediately preceding scanning process is equal to or greater than a determination threshold value. As the intensity of the reflected light to be determined, for example, an average value of the intensity of the reflected light for each shot of the pulse laser beam is employed. In addition, as the intensity of the reflected light, an average value may be obtained by excluding a value extremely deviating from the average, or a median value, a mode value, or the like may be adopted.

  If the intensity of the reflected light is greater than or equal to the determination threshold, the process returns to step SA1 and the scanning process is performed again. If the intensity of the reflected light is less than the determination threshold, the annealing process is terminated.

  When the silicide reaction proceeds by laser annealing, the metal film 16 (FIG. 1E) becomes thin, and the intensity of reflected light decreases. In the embodiment, when the silicide reaction proceeds and the intensity of the reflected light becomes less than the determination threshold, the annealing treatment is finished.

  If excessive laser irradiation is performed in a state where the intensity of the reflected light is reduced, many components of the laser light incident on the substrate 18 reach the second surface 10B (FIG. 1F) of the base substrate 10. For this reason, the element structure 15 (FIG. 1F) formed on the second surface 10B is damaged. In the method according to the embodiment, when the intensity of the reflected light becomes less than the determination threshold, the annealing process is finished. For this reason, the damage received by the element structure 15 due to excessive laser irradiation can be reduced.

  In the example shown in FIG. 5, it is determined based on the intensity of the reflected light whether the scanning process is executed again or is finished. By adjusting the number of executions of the scanning process, the number of shots on the same portion of the substrate 18 is adjusted.

  FIG. 6 shows a flowchart of another example of processing executed by the control device 40 (FIG. 4). In step SA1, as in the example shown in FIG. 5, the first scanning step is executed and the intensity of the reflected light is measured.

  In step SA3, it is determined whether or not the intensity of the reflected light is greater than or equal to a first determination threshold value. As the intensity of the reflected light, a statistic similar to the intensity of the reflected light determined as the determination target in step SA2 shown in FIG. 5 is employed. If the intensity of the reflected light is greater than or equal to the first determination threshold, the process returns to step SA1 to execute the next scanning process. If the intensity of the reflected light is less than the first determination threshold, it is determined in step SA4 whether or not the intensity of the reflected light is greater than or equal to the second determination threshold. The second determination threshold is smaller than the first determination threshold, and is the same as the determination threshold used in step SA2 shown in FIG. 5, for example.

  If the intensity of the reflected light is less than the second determination threshold, the annealing process is terminated. If the intensity of the reflected light is greater than or equal to the second determination threshold, the laser irradiation condition is changed in step SA5. The correspondence relationship between the intensity of the reflected light and the laser irradiation condition is defined in the reflected light intensity-irradiation condition correspondence table 408 (FIG. 4). Specifically, at least one of the intensity of the laser beam and the overlapping rate is reduced. Changing the overlap rate is equivalent to changing the number of shots to the same location on the substrate 18. Thereafter, the next scanning process is executed in step SA1 under the corrected laser irradiation conditions.

  In the state where the intensity of the reflected light is less than the first determination threshold and not less than the second determination threshold, sufficient silicidation is not performed, but if the next scanning process is performed under the initial laser irradiation conditions, Corresponds to a state in which the element structure 15 (FIG. 1F) may be damaged. In the example shown in FIG. 6, in such a state, the next scanning process is executed by changing the laser irradiation condition. In the next scanning step, laser irradiation is performed under a condition in which at least one of the intensity of the laser light and the overlapping rate is reduced. Therefore, damage to the element structure 15 can be reduced and sufficient silicidation can be performed.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 10 Ground substrate 10A 1st surface 10B 2nd surface 11 Guard ring 12 Insulating film 13 Schottky electrode 14 Surface electrode 15 Element structure 16 Metal film 17 Metal silicide film 18 Substrate 19 Pulse laser beam 20 Laser light source 21 Beam combiner 25 Propagation optics System 26 Photodetector 30 Stage 31 Moving mechanism 32 Base 35 Incidence area 36 Trajectory 37 Main scanning direction 38 Sub scanning direction 40 Controller 201, 202 Solid state laser oscillator 251 Attenuator 252 Telescope 253 Homogenizer 254 Partial reflectors 351, 352, 353, 354 Incident area 401 Reflection intensity detection unit 402 Laser irradiation condition adjustment unit 403 Laser drive unit 404 Intensity adjustment unit 405 Movement mechanism drive unit 408 Reflected light intensity-irradiation condition correspondence table AF Attenuation rate AFT Attenuation rate target value HSV Main scanning direction Transfer Speed HSVT main target RF repetition frequency RFT repetition frequency target value RM reflected light intensity measurements VSL sub-scanning direction in the scanning direction travel speed movement distance VSLT subscanning direction movement distance target values of

Claims (6)

A laser light source that outputs a pulsed laser beam;
A stage for holding a substrate;
A propagation optical system for guiding the pulse laser beam output from the laser light source to the substrate held on the stage;
A moving mechanism for moving one of the path of the pulse laser beam and the substrate relative to the other so that the incident position of the pulse laser beam moves on the surface of the substrate held on the stage;
A photodetector for detecting reflected light of the pulsed laser beam from the substrate;
A control device that adjusts at least one of the number of shots of the pulsed laser beam and the intensity of the pulsed laser beam incident on the same portion of the substrate based on the intensity of the reflected light detected by the photodetector; Laser annealing apparatus having.   The laser annealing apparatus according to claim 1, wherein the laser light source includes a solid-state laser oscillator that outputs a fundamental wave or a second harmonic. The control device moves the incident position of the pulse laser beam on the surface of the substrate at a certain overlap rate so that a scanning step of scanning the entire region to be annealed of the substrate is repeated. Controlling the light source and the moving mechanism;
3. The laser annealing according to claim 1, wherein at least one of the intensity of the pulse laser beam and the overlap rate in the next scanning step is adjusted based on the intensity of the reflected light measured in one of the scanning steps. apparatus. Preparing a substrate in which a metal film made of nickel, titanium or tungsten is formed on a first surface of a base substrate made of silicon carbide;
A step of causing the base film and the metal film to undergo a silicidation reaction by injecting a plurality of shots of a pulsed laser beam in a wavelength region that is absorbed by the metal film and transmitted through the base substrate into the metal film. When,
Measuring the intensity of the reflected light of the pulsed laser beam from the substrate;
Adjusting the at least one of the number of shots of the pulse laser beam incident on the same location and the intensity of the pulse laser beam based on the intensity of the reflected light.   The method of manufacturing a semiconductor device according to claim 4, wherein the pulse laser beam is a fundamental wave or a second harmonic of a solid-state laser. In the step of causing the base substrate and the metal film to undergo a silicidation reaction, the entire area of the substrate to be annealed is scanned by moving the incident position of the pulse laser beam at a certain overlap rate on the surface of the substrate. Repeat the scanning process,
6. The semiconductor device according to claim 4, wherein at least one of the intensity of the pulse laser beam and the overlap rate in the next scanning step is adjusted based on the intensity of the reflected light measured in one scanning step. Manufacturing method.

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