METHOD FOR POLISHING A SiC SUBSTRATE

Abstract

Method for polishing a SiC substrate (11) by supplying a polishing fluid to a polishing pad (18) containing abrasive grains or to a polishing pad (18) not containing abrasive grains and bringing the polishing pad into contact with the SiC substrate, the method comprising: a first polishing step to polish the SiC substrate (11) using a first polishing fluid containing a permanganate, inorganic salts with an oxidizing power and water, and a second polishing step to perform a final polishing of the SiC substrate (11) using a second polishing fluid containing a permanganate, a pH adjusting agent and water, after the first polishing step, wherein the second polishing fluid further does not contain any inorganic salts with an oxidizing capacity.

Description

BACKGROUND OF THE INVENTION AREA OF THE INVENTION

[0001] The present invention relates to a method for polishing a SiC substrate using a polishing fluid. DESCRIPTION OF THE STATE OF THE ART

[0002] Power electronic devices such as inverters incorporate semiconductor devices known as power components, which are suitable for regulating electrical energy. Conventional power components were mainly manufactured using a silicon (Si) single crystal, and improvements in their performance were achieved through enhancements in the component structure.

[0003] In recent years, improvements in device performance through enhancements to the device structure have reached their peak. For this reason, attention has been paid to SiC (silicon carbide) single crystals, which offer advantages over Si single crystals in the realization of power devices with higher dielectric strength and lower losses.

[0004] Prior to the fabrication of power devices in a substrate composed of a single SiC crystal, the substrate is planarized by chemical-mechanical polishing (CMP). To increase the polishing efficiency of CMP, a polishing technique using a polishing pad containing abrasive grains and a polishing fluid with oxidizing properties was developed (see, e.g., JP 2008-68390 A). Further prior art is disclosed in WO 2013 / 150822 A1, US 2014 / 0057438 A1, US 2002 / 0194789 A1, and US 2008 / 0153292 A1. SUMMARY OF THE INVENTION

[0005] However, if a SiC single-crystal substrate is polished using a polishing fluid with an oxidizing power as mentioned previously, disorder would occur in the crystal lattice of the SiC single crystal, leading to a significant reduction in the performance of the power device.

[0006] Therefore, an objective of the invention is to provide a polishing fluid with which the disorder in the crystal lattice can be limited, and a method for polishing a SiC substrate using the polishing fluid.

[0007] The present invention is defined by the method for polishing a SiC substrate according to the features of independent claim 1. A preferred embodiment is set out in dependent claim 1.

[0008] According to one aspect of the invention, a polishing fluid is provided which is to be used for polishing a SiC substrate, wherein the polishing fluid contains a permanganate, a pH adjusting agent and water.

[0009] According to a further aspect of the present invention, a method for polishing a SiC substrate is provided by supplying a polishing fluid to a polishing pad containing abrasive grains or to a polishing pad not containing abrasive grains and bringing the polishing pad into contact with the SiC substrate, wherein the method includes: a first polishing step for polishing the SiC substrate using a first polishing fluid containing a permanganate, inorganic salts with oxidizing power and water, and a second polishing step for performing a final polishing of the SiC substrate using a second polishing fluid containing a permanganate, a pH adjusting agent and water, after the first polishing step.

[0010] Preferably, the above method for polishing a SiC substrate is carried out using a polishing device comprising: a clamping table holding the SiC substrate, the polishing pad, tanks storing the first polishing fluid and the second polishing fluid separately, and a feeding device that selectively supplies one of the first polishing fluid and the second polishing fluid stored in the tanks, with the first polishing fluid being supplied in the first polishing step, while the second polishing fluid is supplied instead of the first polishing fluid in the second polishing step.

[0011] Since the polishing fluid according to the invention contains permanganate, pH adjuster, and water, any disorder that might occur in the crystal lattice during the polishing of a SiC substrate can be limited compared to using a polishing fluid containing permanganate, inorganic salts with oxidizing properties, and water. Furthermore, in the inventive method for polishing a SiC substrate, the final polishing of the SiC substrate is carried out using the polishing fluid containing permanganate, pH adjuster, and water after the substrate has been polished using the polishing fluid containing permanganate, inorganic salts with oxidizing properties, and water. This allows the disorder in the crystal lattice to be limited while maintaining high polishing efficiency.

[0012] By studying the following description and the attached claims with reference to the accompanying drawings, which show a preferred embodiment of the invention, the above and further objectives, features and advantages of the present invention and the manner of its realization will become clearer and the invention will be best understood. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 schematically illustrates a configuration example of a polishing device used in a method according to the invention for polishing a SiC substrate, and Fig. Figure 2 is a graphical representation showing the relationship between inorganic salts with oxidizing power and polishing rate. DETAILED DESCRIPTION OF THE PREFERRED EXECUTION FORM

[0013] When a SiC substrate is polished using a polishing fluid with oxidizing properties, it is possible to increase the polishing efficiency; however, this introduces disorder into the crystal lattice, leading to a significantly reduced performance of the resulting power devices. The inventor has conducted intensive and detailed studies on this phenomenon. These studies revealed that the cause of the disorder in the crystal lattice lies in the inorganic salts contained in the polishing fluid, which impart oxidizing properties. Based on this finding, the present invention was developed. An embodiment of the invention is described below with reference to the accompanying drawings.

[0014] All conventional polishing fluids contain, for example, permanganate, inorganic salts with oxidizing properties (oxidizing inorganic salts), and water. The permanganate is an oxoacid salt of manganese, represented by potassium permanganate and sodium permanganate. The inorganic salts with oxidizing properties refer to oxidizing solids (corresponding to Category I hazardous materials according to the Japanese Fire Service Law) such as chlorine-containing compounds, sulfates, nitrates, and chromates.

[0015] In contrast, the polishing fluid according to the invention contains a pH-adjusting agent instead of the inorganic salts with oxidizing properties that were conventionally used. Specifically, the polishing fluid according to the invention contains a permanganate, a pH-adjusting agent, and water. The pH-adjusting agent refers to a reagent for adjusting the pH value towards the acidic side, such as hydrochloric acid (hydrogen chloride), sulfuric acid, nitric acid, and phosphoric acid. Using such a polishing fluid, the disorder in the crystal lattice that might occur during the polishing of a SiC substrate can be limited.

[0016] The following describes a method for polishing a SiC substrate using the polishing fluid according to an embodiment of the invention. The method for polishing a SiC substrate according to the present embodiment includes at least a first polishing step and a second polishing step. In the first polishing step, the SiC substrate is polished using a polishing fluid (first polishing fluid) containing a permanganate, inorganic salts with oxidizing properties, and water. In the second polishing step, the SiC substrate undergoes final polishing using a polishing fluid (second polishing fluid) containing a permanganate, a pH adjuster, and water.

[0017] First, the polishing device to be used in the method for polishing a SiC substrate according to the present embodiment is described. Fig. Figure 1 schematically illustrates a configuration example of the polishing device to be used in the method for polishing a SiC substrate of the present embodiment. As shown in Fig. As shown in Figure 1, a polishing device 2 in the present embodiment includes a clamping table 4 for holding a SiC substrate 11 by suction. The clamping table 4 is connected to a rotary drive source (not shown), such as a motor, and rotates about an axis of rotation parallel to the vertical direction. The upper surface of the clamping table 4 is formed with a recess in which a retaining plate 6, made of a porous material, is installed. An upper surface of the retaining plate 6 forms a holding surface 6a on which a disk-shaped SiC substrate 11, composed of a single SiC crystal, is held by suction. A negative pressure from a suction source (not shown) acts on the holding surface 6a via a passage 4a formed inside the clamping table 4. As shown in Figure 1, the clamping table 4 is connected to a rotary drive source (not shown) and rotates about an axis of rotation parallel to the vertical direction. Fig. As shown in Figure 1, a film 13, which has a larger diameter than the holding surface 6a, is attached to the underside of the SiC substrate 11. When the negative pressure of the suction source acts on the holding surface 6a, with the film 13 in contact with the holding surface 6a, the SiC substrate 11 is held against the holding table 4 by suction through the intervening film 13.

[0018] Above the clamping table 4, a polishing unit 8 is mounted for polishing the SiC substrate 11. The polishing unit 8 includes a spindle 10, which forms a rotary shaft. A disc-shaped wheel holder 12 is provided at a lower end (tip) of the spindle 10. A polishing wheel 14, which has essentially the same diameter as the wheel holder 12, is mounted on a lower surface of the wheel holder 12. The polishing wheel 14 includes a wheel base 16, which is made of a metallic material such as stainless steel or aluminum. A disc-shaped polishing pad 18 is attached to the lower surface of the wheel base 16. The polishing pad 18 is formed, for example, by mixing abrasive grains with a material such as polyurethane. It should be noted that the polishing pad 18 does not necessarily have to contain abrasive grains. A rotary drive source (not shown), such as a motor, is connected to an upper end (base end) of the spindle 10.The polishing wheel 14 rotates around an axis of rotation parallel to the vertical direction by means of a rotational force transmitted by the rotary drive source.

[0019] The spindle 10, the wheel holder 12, the wheel base 16, and the polishing pad 18 are each provided with longitudinally extending holes 10a, 12a, 16a, and 18a, respectively, which pierce them vertically. A lower end of longitudinal hole 10a and an upper end of longitudinal hole 12a are connected to each other, as are a lower end of longitudinal hole 12a and an upper end of longitudinal hole 16a, and a lower end of longitudinal hole 16a and an upper end of longitudinal hole 18a. An upper end of longitudinal hole 10a is connected to a feed control unit (feeding device) 20 via a pipe or the like. The feed control unit 20 is further connected to a first tank 22 and a second tank 24 via a pipeline or the like.The first tank 22 contains a polishing fluid containing permanganate, inorganic salts with oxidizing properties, and water. The second tank 24 contains a polishing fluid containing permanganate, a pH adjuster, and water. The feed control unit 20 supplies one of the polishing fluids stored separately in the first tank 22 and the second tank 24. A polishing fluid 15, fed from the feed control unit 20 into the longitudinal hole 10a, is supplied via an opening in the longitudinal hole 18a, which is located in the center of the lower surface of the polishing pad 18, to a contact surface between the SiC substrate 11 and the polishing pad 18.While the clamping table 4 and the spindle 10 continue to rotate, the polishing wheel 14 is lowered to bring the lower side of the polishing pad 18 into contact with the upper side of the SiC substrate 11, while the polishing fluid 15 is supplied, allowing the SiC substrate 11 to be polished.

[0020] In the method for polishing a SiC substrate according to the present embodiment, the first polishing step is carried out in which the SiC substrate 11 is polished using the polishing fluid, which contains permanganate, inorganic salts with oxidizing properties, and water. Specifically, the SiC substrate 11 is polished under the control of the feed control unit 20, so that the polishing fluid (containing permanganate, inorganic salts with oxidizing properties, and water), which is stored in the first tank 22, is supplied to the contact surface between the SiC substrate 11 and the polishing pad 18. The first polishing step is completed when the SiC substrate 11 has been polished to a predetermined polishing level. It should be noted that the polishing fluid used in the first polishing step may contain abrasive particles.

[0021] After the first polishing step, the second polishing step is performed, in which the SiC substrate 11 undergoes final polishing using the polishing fluid containing permanganate, a pH adjuster, and water. Specifically, the SiC substrate 11 is polished under the control of the feed control unit 20, so that the polishing fluid (containing the permanganate, pH adjuster, and water), stored in the second tank 24, is supplied to the contact surface between the SiC substrate 11 and the polishing pad 18. In this way, the polishing fluid supplied to the contact surface between the SiC substrate 11 and the polishing pad 18 is alternated, resulting in a difference between the first and second polishing steps. Once the SiC substrate has been polished to a predetermined polishing level, the second polishing step is completed.It should be noted that the liquid used in the second polishing step may contain added abrasive particles.

[0022] In the aforementioned first polishing step, a polishing solution containing permanganate, inorganic salts with oxidizing properties, and water is used to maintain high polishing efficiency. In contrast, the second polishing step uses a polishing solution containing permanganate, pH adjuster, and water, thus suppressing any disorder that might occur in the crystal lattice. It should be noted that the polishing amounts in both the first and second steps are preferably regulated to maintain a high level of efficiency and polishing quality.

[0023] Since the polishing fluid in the present embodiment contains permanganate, pH adjuster, and water, the disorder that could appear in the crystal lattice during polishing of the SiC substrate 11, as described above, can be limited compared to the polishing fluid containing permanganate, inorganic salts with oxidizing power, and water. Furthermore, in the method for polishing a SiC substrate according to the present embodiment, the final polishing of the SiC substrate 11 is carried out using the polishing fluid containing permanganate, pH adjuster, and water after the SiC substrate 11 has been polished using the polishing fluid containing permanganate, inorganic salts with oxidizing power, and water, thus suppressing the disorder in the crystal lattice while maintaining high polishing efficiency.

[0024] The following describes an experiment conducted to verify the advantageous effect of the invention. In this experiment, the relationships between the concentrations of permanganate and pH-adjusting agent in the polishing fluid (second polishing fluid) used in the second polishing step and the quality of the SiC substrate obtained after polishing were investigated. Specifically, a SiC substrate that had undergone a first polishing step was subjected to a second polishing step using polishing fluids that differed in their concentrations of permanganate and pH-adjusting agent. It should be noted that the first polishing step was performed using a polishing fluid containing 0.5 wt% potassium permanganate and 0.2 wt% inorganic salts with oxidizing properties. Fig.Figure 2 is a graphical representation showing the relationships between the inorganic salts with oxidizing power contained in the polishing fluid used in the first polishing step and the polishing rate. In each case, a sufficient polishing rate was obtained.

[0025] In the second polishing step, polishing fluid A, prepared by mixing potassium permanganate (permanganate), phosphoric acid (pH adjuster), and water, and polishing fluid B, prepared by mixing sodium permanganate (permanganate), phosphoric acid (pH adjuster), and water, were used. It was found that good polishing quality could be obtained when polishing fluid A was prepared by mixing 0.1 wt.% to 5.0 wt.% potassium permanganate and 0.01 wt.% to 2.0 wt.% phosphoric acid with water, adjusting the molar ratio of potassium permanganate to phosphoric acid within the range of 0.1 to 2.5. Similarly, it was found that good polishing quality could be obtained when polishing fluid B was prepared by mixing 0.1 wt.% to 5.0 wt.% sodium permanganate and 0.01 wt.% to 2.0 wt.% phosphoric acid with water.% phosphoric acid is produced with water, whereby a molar ratio of sodium permanganate to phosphoric acid is adjusted within the range of 0.1 to 2.5.

Claims

[1] Method for polishing a SiC substrate (11) by supplying a polishing fluid to a polishing pad (18) containing abrasive grains or to a polishing pad (18) not containing abrasive grains and bringing the polishing pad into contact with the SiC substrate, the method comprising: a first polishing step to polish the SiC substrate (11) using a first polishing fluid containing a permanganate, inorganic salts with an oxidizing power and water, and a second polishing step to perform a final polishing of the SiC substrate (11) using a second polishing fluid containing a permanganate, a pH adjusting agent and water, after the first polishing step, wherein the second polishing fluid further does not contain any inorganic salts with an oxidizing capacity. [2] Method for polishing a SiC substrate (11) according to claim 1, wherein the method uses a polishing device comprising: a clamping table (4) that holds the SiC substrate (11), the polishing pad (18), Tanks (22, 24) which store the first polishing fluid and the second polishing fluid separately, and supply means (20) which selectively supplies one of the first polishing fluid and the second polishing fluid which are stored in the tanks (22, 24), wherein the first polishing fluid is supplied in the first polishing step and The second polishing fluid is added instead of the first polishing fluid in the second polishing step.

Images