JP6469455B2 - Laser annealing equipment - Google Patents

Description

  The present invention relates to a laser annealing apparatus that performs laser annealing by causing a laser beam transmitted through an aperture to enter an object to be annealed.

  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

  In order to perform uniform annealing, it is preferable to make the beam profile uniform on the processed surface. However, the laser beam in the ultraviolet region is diffracted when passing through the aperture, thereby generating interference fringes on the processed surface. This interference fringe may interfere with homogeneous annealing.

  An object of the present invention is to provide a laser annealing apparatus capable of suppressing generation of interference fringes and performing uniform annealing.

According to one aspect of the invention,
A laser light source that outputs a pulsed laser beam;
A first aperture arranged on a path of the pulse laser beam output from the laser light source, and shaping a beam cross section of the pulse laser beam into a rectangle;
A stage for holding the workpiece,
An imaging optical system that forms an image of the transmission region of the first aperture on the surface of the workpiece;
A second aperture disposed on a path of the pulse laser beam from the first aperture to the workpiece and having a rectangular transmission region;
The transmission region of the second aperture includes an inner edge of an annular bright portion formed by one higher-order diffracted light other than the zeroth order diffracted by the first aperture, and is disposed inside the outer edge. Thus, a laser annealing apparatus in which a part of the bright part is shielded from light is provided.

  By shielding a part of the annular bright portion, it is possible to suppress the generation of interference fringes and perform uniform annealing.

FIG. 1 is a schematic diagram of a laser annealing apparatus according to an embodiment. FIG. 2A is a schematic diagram showing a beam cross section of a pulsed laser beam after being collimated by a collimating lens, and FIG. 2B is a schematic diagram showing a positional relationship between a transmission region of a second aperture and a bright part. . FIG. 3A is a sketch of a surface photograph of a polyimide plate after a pulsed laser beam is incident on a polyimide plate placed at the position of the substrate, and FIG. 3B is a graph showing the depth of the dotted line 3B-3B in FIG. 3A. FIG. 3C is a graph showing the depth distribution of the recesses formed by removing the second aperture and irradiating the polyimide plate with laser. FIG. 4 is a plan view of the incident region of the pulse laser beam on the surface of the substrate and a diagram showing the movement history of the incident region. FIG. 5 is a schematic diagram illustrating another example of the positional relationship between the transmission region of the second aperture and the bright portion. FIG. 6A is a chart showing the relationship between the size of the transmission region of the first aperture and the size of the transmission region of the second aperture, and FIG. 6B is a diagram of the first aperture according to another configuration example of the embodiment. It is a top view of the transmissive area and the transmissive area of the second aperture. 7A to 7D are cross-sectional views of a semiconductor element manufactured using the laser annealing apparatus according to the embodiment in the course of manufacturing. FIG. 7E to FIG. 7F are cross-sectional views in the process of manufacturing a semiconductor device manufactured using the laser annealing apparatus according to the embodiment.

FIG. 1 shows a schematic diagram of a laser annealing apparatus according to an embodiment. The laser light source 20 outputs an ultraviolet pulse laser beam. As the laser light source 20, for example, an Nd: YAG laser oscillator that generates third harmonics is used. The wavelength of the third harmonic of the Nd: YAG laser is about 355 nm. Instead of the Nd: YAG laser oscillator, other solid-state laser oscillators such as an Nd: YLF laser oscillator and an Nd: YVO ₄ laser oscillator can be used.

  The pulsed laser beam output from the laser light source 20 is incident on the substrate 18 that is a processing target via the power adjuster 22, the first aperture 25, and the imaging optical system 26. The imaging optical system 26 includes a collimating lens 261 and an objective lens 262. On the path of the pulse laser beam between the collimating lens 261 and the objective lens 262, the second aperture 27 and the folding mirror 28 are arranged.

  The power adjuster 22 includes a half-wave plate 221, a beam splitter 222, a quarter-wave plate 223, and beam dampers 224 and 225. By changing the fast axis direction of the half-wave plate 221, the ratio of the p-wave component and the s-wave component with respect to the beam splitter 222 can be changed. The s-wave component is reflected by the beam splitter 222 and enters the beam damper 224. The p-wave component goes straight through the beam splitter 222, is converted into circularly polarized light by the quarter-wave plate 223, and then enters the first aperture 25. By changing the fast axis direction of the half-wave plate 221, the power of the pulse laser beam after passing through the power adjuster 22 can be adjusted. The pulsed laser beam incident on the first aperture 25 is a Gaussian beam.

  The first aperture 25 has a rectangular transmission region, and shapes the beam cross section of the pulse laser beam into a rectangle. In this embodiment, the transmission area is a square. The center point of the transmission region coincides with the center point of the beam cross section of the pulse laser beam incident on the first aperture 25. Since the transmission region of the first aperture 25 is arranged in the center of the Gaussian beam and the region near the center, the beam profile in the transmission region of the first aperture 25 can be considered to be nearly uniform.

  The pulsed laser beam that has passed through the first aperture 25 is collimated by the collimating lens 261. The collimated pulsed laser beam is incident on the second aperture 27.

  The second aperture 27 has a rectangular transmission region. The center point of the transmission region coincides with the center point of the beam cross section of the pulse laser beam. The pulse laser beam transmitted through the second aperture 27 is reflected downward by the folding mirror 28 and converged by the objective lens 262. The imaging optical system 26 including the collimating lens 261 and the objective lens 262 images the transmission region of the first aperture 25 on the surface (processed surface) of the substrate 18.

  The reflected light of the pulse laser beam reflected by the substrate 18 follows the incident path in reverse, and is converted into linearly polarized light by the quarter-wave plate 223. The reflected light converted into linearly polarized light is reflected by the beam splitter 222 and enters the beam damper 225.

  The substrate 18 is held on the stage 30. The stage 30 is supported on the base 32 via the moving mechanism 31. The moving mechanism 31 can move the stage 30 in two directions parallel to the surface of the substrate 18 with respect to the path of the pulse laser beam.

  The control device 40 controls the output timing of the pulse laser beam from the laser light source 20, the direction of the fast axis of the half-wave plate 221, and the movement of the stage 30 by the moving mechanism 31. Instead of moving the stage 30, the path of the pulse laser beam may be moved with respect to the stage 30. That is, one of the path of the pulse laser beam and the stage 30 may be moved with respect to the other so that the incident region of the pulse laser beam moves on the surface of the substrate 18.

  FIG. 2A shows a beam cross section of the pulse laser beam after being collimated by the collimating lens 261 (FIG. 1). The 0th-order diffracted light, the 1st-order diffracted light, and the second-order diffracted light by the first aperture 25 form bright portions 50, 51, and 52, respectively. In FIG. 2A, the bright portions 50, 51, 52 are hatched with relatively sparse intervals. A relatively dense hatch is attached to the dark part.

  The bright portion 50 by the 0th-order diffracted light is located at the center of the beam cross section, and has a substantially square planar shape reflecting the shape of the transmission region of the first aperture 25. The bright portion 51 by the first-order diffracted light has an annular shape along a square outer peripheral line that encloses the bright portion 50. The bright part 52 by the second-order diffracted light has an annular shape along a square outer peripheral line including the bright part 51 by the first-order diffracted light. FIG. 2A shows the bright portions 50, 51, and 52 by the 0th-order to second-order diffracted light, but the bright portion by the third-order or higher-order diffracted light may be observed.

  FIG. 2B shows a positional relationship between the transmission region 271 of the second aperture 27 and the bright portions 50, 51, 52. The transmission region 271 of the second aperture 27 includes the inner edge of the bright portion 51 by the first-order diffracted light, and is disposed on the inner side of the outer edge. For this reason, some components of the pulse laser beam passing through the bright part 51 are transmitted through the transmission region 271, and other components are shielded by the second aperture 27. All components that pass through the bright portion 52 due to the second-order diffracted light are shielded by the second aperture 27.

  FIG. 3A shows a sketch of a surface photograph of a polyimide plate after a pulse laser beam is incident on the polyimide plate placed at the position of the substrate 18 (FIG. 1). A recess 55 is formed by ablating the polyimide plate. The recess 55 reflects the shape of the transmission region of the first aperture 25 and has a planar shape with rounded square corners. A region with relatively dense hatching corresponds to an inclined side surface, and a region with relatively sparse hatching corresponds to a substantially flat bottom surface.

  FIG. 3B shows an actual measurement result of the depth distribution along the one-dot chain line 3B-3B in FIG. 3A. The horizontal axis represents position, and the vertical axis represents depth. It can be seen that the portion corresponding to the region with relatively dense hatching in FIG. 3A is a slope in FIG. 3B. It can be seen that the relatively sparse hatched region is almost flat.

  For comparison, FIG. 3C shows the depth distribution of the recesses formed by removing the second aperture 27 (FIG. 1) and irradiating the polyimide plate with laser. It can be seen that the bottom surface of the recess is wavy. This is because interference fringes are generated on the surface of the substrate 18 (FIG. 1) due to interference between the diffracted lights diffracted by the first aperture 25.

  As in the above-described embodiment, the influence of interference can be reduced by arranging the second aperture 27 and shielding a part of the outer peripheral side of the bright portion 51 (FIG. 2B). The actual measurement result shown in FIG. 3B is obtained under the condition where about 50% of the energy of the pulse laser beam passing through the bright part 51 is shielded. By shielding light energy of 30% or more and 70% or less of the light energy of the pulse laser beam passing through the bright part 51, it is possible to reduce the influence of interference fringes.

  FIG. 4 shows a plan view of the incident region 35 of the pulse laser beam on the surface of the substrate 18 (FIG. 1) and a history of movement of the incident region 35. An image of the transmission area of the first aperture 25 corresponds to the incident area 35. By reflecting the shape of the transmission region of the first aperture 25 in this image, the incident region 35 has a planar shape in which square corners are rounded. 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 (FIG. 1), the incident region 35 of the pulse laser beam is on the surface of the substrate 18 in the main scanning direction 37 (width direction) and the sub-scanning direction 38 (length). Direction). The overlapping rate (the overlapping rate with respect to the main scanning direction 37) of the incident region 351 of a certain shot and the incident region 352 of the next shot moved in the main scanning direction 37 is, for example, 50% to 99%. The overlapping rate (the overlapping rate with respect to the sub-scanning direction 38) between the incident region 353 of a certain shot and the incident region 354 of the next shot moved in the sub-scanning direction 38 is, for example, 50% to 99%. By repeating the main scanning and the sub-scanning, the entire region to be annealed of the substrate 18 is scanned.

  The main scanning direction 37 is parallel to one side of the image of the first aperture formed on the surface of the substrate 18, and the sub-scanning direction 38 is parallel to the other side. When the control device 40 controls the moving mechanism 31, the incident area 35 moves in the main scanning direction 37 and the sub scanning direction 38. When the overlapping ratio of the incident regions 35 in the main scanning direction 37 and the sub-scanning direction 38 is 50%, a pulse laser that is incident on the same portion of the surface of the substrate 18 in the step of scanning the entire region of the annealing target region of the substrate 18 once. The number of beam shots is four. When the overlap ratio is 66%, the number of shots of the pulse laser beam incident on the same portion of the surface of the substrate 18 is nine in the step of scanning the entire region to be annealed of the substrate 18 once.

  In this way, by making the planar shape of the incident region 35 substantially rectangular and making the main scanning direction 37 and the sub-scanning direction 38 parallel to the sides of the incident region 35, the incident pulses over the entire region to be annealed. The number of shots of the laser beam can be made the same. As a result, uniform laser annealing can be performed. On the other hand, when the incident area 35 is circular, the number of shots of the incident pulse laser beam cannot be made the same over the entire area to be annealed.

  FIG. 5 shows another example of the positional relationship between the transmission region 271 of the second aperture 27 and the bright portions 50, 51, 52. In FIG. 2A, a part of the bright part 51 due to the first-order diffracted light is shielded by the second aperture 27. In the example shown in FIG. 5, a part of the bright part 52 by the second-order diffracted light is shielded by the second aperture 27. In this case, the transmission region of the second aperture 27 includes the inner edge of the bright portion 52 by the second-order diffracted light, and is disposed on the inner side of the outer edge. Further, a part of the bright part by the third or higher order diffracted light may be shielded.

  Thus, in the embodiment, the transmission region 271 of the second aperture 27 includes the inner edges of the annular bright portions 51 and 52 formed by one higher-order diffracted light other than the zeroth order, and the outer edges. As a result, the light portions 51 and 52 are partially shielded from light. Thereby, generation | occurrence | production of the interference fringe in the surface of the board | substrate 18 (FIG. 1) can be suppressed.

  FIG. 6A shows the relationship between the size of the transmissive region of the first aperture 25 (FIG. 1) and the size of the transmissive region 271 of the second aperture 27 (FIG. 1). In the example shown in FIG. 6A, the wavelength of the pulse laser beam is 355 nm, and the focal length of the collimating lens 261 is 2500 mm.

  When the length of one side of the transmission region of the first aperture 25 is 0.5 mm, the lengths of the outer sides of the bright portions 51 and 52 (FIG. 2A) by the first-order diffracted light and the second-order diffracted light are 8.66 mm, respectively. And 12.99 mm. When the length of one side of the transmission region of the first aperture 25 is 1 mm, the lengths of the outer sides of the bright portions 51 and 52 (FIG. 2A) by the first-order diffracted light and the second-order diffracted light are 4.33 mm and 6 respectively. .50 mm.

  In the example shown in FIG. 2B, when the length of one side of the transmission region of the first aperture 25 is 0.5 mm, the length of one side of the transmission region of the second aperture 27 is slightly shorter than 8.66 mm. do it. In the example shown in FIG. 5, when the length of one side of the transmission region of the first aperture 25 is 0.5 mm, the length of one side of the transmission region of the second aperture 27 is slightly shorter than 12.99 mm. do it.

  FIG. 6B is a plan view of the transmission region 251 of the first aperture 25 and the transmission region 271 of the second aperture 27 according to another configuration example of the embodiment. In the above-described embodiment, the planar shape of the transmission region 251 of the first aperture 25 and the transmission region 271 of the second aperture 27 is square, but in the configuration example illustrated in FIG. The planar shapes of the transmission region 251 and the transmission region 271 of the second aperture 27 are rectangular.

  When the horizontal and vertical lengths of the transmission region 251 of the first aperture 25 are 1 mm and 0.5 mm, respectively, the horizontal and vertical lengths of the outer edge of the bright portion 51 by the first-order diffracted light Are 4.33 mm and 8.66 mm, respectively. If the dimension of the transmission region 271 of the second aperture 27 is slightly smaller than the dimension of the outer edge of the bright part 51, a part of the pulse laser beam that passes through the bright part 51 can be shielded. Thus, when the transmission region 251 of the first aperture 25 is a horizontally long rectangle, the transmission region 271 of the second aperture 27 is a vertically long rectangle.

  Next, an example of a method for manufacturing a semiconductor element using the laser annealing apparatus according to the embodiment will be described with reference to FIGS. 7A to 7F. In the following example, a Schottky diode using a SiC substrate is manufactured.

  As shown in FIG. 7A, the base substrate 10 made of SiC is formed by epitaxially growing n-type SiC on the surface of the 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. 7B, 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. 7C, 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. 7D, the base substrate 10 is thinned by grinding the base substrate 10 from the first surface 10A. As shown in FIG. 7E, 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. 7F, laser annealing is performed by irradiating the metal film 16 (FIG. 7E) 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. 7E) is silicided, and the metal silicide film 17 is formed.

  In this laser annealing step, the laser annealing apparatus according to the above embodiment is used. Thereby, uniform laser annealing 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 Base 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 22 Power adjuster 25 1 aperture 26 imaging optical system 27 second aperture 28 folding mirror 30 stage 31 moving mechanism 32 base 35 incident area 37 main scanning direction 38 sub-scanning direction 40 controller 50 bright part 51 by 0th order diffracted light by first order diffracted light Bright part 52 Bright part 55 by second-order diffracted light Recess 221 Half-wave plate 222 Beam splitter 223 Quarter-wave plate 224, 225 Beam damper 251 Transmission area 261 of first aperture Collimator lens 262 Objective lens 271 Transmission of second aperture Regions 351, 352, 35 3, 354 Incident area

Claims (5)

A laser light source that outputs a pulsed laser beam;
A first aperture arranged on a path of the pulse laser beam output from the laser light source, and shaping a beam cross section of the pulse laser beam into a rectangle;
A stage for holding the workpiece,
An imaging optical system that forms an image of the transmission region of the first aperture on the surface of the workpiece;
A second aperture disposed on a path of the pulse laser beam from the first aperture to the workpiece and having a rectangular transmission region;
The transmission region of the second aperture includes an inner edge of an annular bright portion formed by one higher-order diffracted light other than the zeroth order diffracted by the first aperture, and is disposed inside the outer edge. Thus, a laser annealing apparatus in which a part of the bright part is shielded from light.   2. The laser annealing apparatus according to claim 1, wherein the bright portion, which is partially shielded from light, is formed by first-order diffracted light or second-order diffracted light.   3. The laser annealing apparatus according to claim 1, wherein light energy of 30% or more and 70% or less of light energy of the pulse laser beam passing through the bright portion is shielded by the second aperture. The imaging optical system is
A collimating lens for collimating the pulsed laser beam transmitted through the first aperture;
An objective lens for converging the pulse laser beam collimated by the collimating lens,
4. The laser annealing apparatus according to claim 1, wherein the second aperture is disposed on a path of the pulse laser beam between the collimating lens and the objective lens. 5. further,
A moving mechanism that moves one of the path of the pulse laser beam and the stage relative to the other so that the incident region of the pulse laser beam moves on the surface of the workpiece;
A control device for controlling the laser light source and the moving mechanism;
The control device moves the processing object in a direction parallel to one side of the rectangular image of the first aperture formed on the surface of the processing object, while the pulse laser beam is emitted from the laser light source. The laser annealing apparatus according to any one of claims 1 to 4, wherein laser annealing is performed by outputting.

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