Surface treatment method for sic substrates

Abstract

A surface treatment method for a SiC substrate, including the following processes or steps: anodizing a workpiece surface (W1) of the SiC substrate by passing a current having a current density of 15 mA / cm 2 or more through the SiC substrate as an anode in the presence of an electrolyte (S); arranging an abrasive wheel layer (32) of a surface treatment pad (3) to face the workpiece surface, and selectively removing oxides formed on the workpiece surface by the anodizing with the abrasive wheel layer; and simultaneously or sequentially performing the anodizing of the workpiece surface and the selective removal of the oxides formed on the workpiece surface with the abrasive wheel layer.

Description

TECHNICAL FIELD

[0001] The present application relates to a surface treatment method for a SiC substrate. BACKGROUND

[0002] JP 2021-27359 A discloses a polishing method utilizing anodization. The polishing method is a method in which, by combining oxidation with polishing, a difficult-to-process material that can be anodized can be formed into a desired shape with high efficiency and high accuracy. Specifically, in the polishing method, an anodization process and a polishing process are simultaneously or alternately performed under a condition that a polishing rate of the polishing process is higher than an oxidation rate of the anodization process to perform polishing. This polishing method utilizing anodization is called ECMP. ECMP stands for electrochemical mechanical polishing.

[0003] As described in JP 2021-27359 A, it is generally known that when the current density is increased in ECMP, the processing speed increases while the surface roughness intensifies. Further, JP 2021-27359 A discloses the following: since the oxidation rate saturates with an increase in the current density, if the current density is increased to a certain value, the polishing rate cannot be increased any more even if the current density is further increased.

[0004] Accordingly, according to the conventional knowledge, no significant advantage of increasing the current density in ECMP has been obtained. For example, according to JP 2021-27359 A, the upper limit of the actual current density is at most about 10 mA / cm 2 and it is used in a rough polishing process before a fine polishing process.

[0005] Therefore, in this type of surface treatment method using the principle of formation of an anodized film and selective removal of an oxide film, in order to achieve more superior processing characteristics (e.g., processing speed or surface roughness) than those of conventional methods, there is still room for further study. The present application is made in view of the above-described circumstances. That is, the present application provides a technology that can achieve more superior processing characteristics than those of the past, for example, in a surface treatment method for a SiC substrate utilizing anodization. SUMMARY

[0006] A surface treatment method for a SiC substrate (W) according to one aspect of the present application includes: in the presence of an electrolyte (S), forming an anodized film on a surface of the SiC substrate (W) by applying a current density of 15 mA / cm 2Anodizing of the workpiece surface (W1) of the SiC substrate is performed by passing a larger current through the SiC substrate as the anode; and the grinding wheel layer (32) of the surface treatment pad (3) is arranged to face the workpiece surface, and the grinding wheel layer is used to selectively remove the oxide (W3) generated on the workpiece surface by anodizing.

[0007] In some cases, parenthesized reference numerals are attached to elements in various parts of this application. In such cases, the reference numerals are merely examples of the correspondence between the same elements and the specific configurations described in the embodiments described below. Therefore, the invention is not limited in any way by the reference numerals. Attached Figure Description

[0008] Figure 1 This is a schematic configuration diagram of a surface treatment apparatus for implementing a surface treatment method according to an embodiment of the present invention;

[0009] Figure 2 It shows that it can be used Figure 1 An overview of the SiC wafer manufacturing process implemented by the surface treatment equipment shown;

[0010] Figure 3 Is using Figure 1 Scanning electron microscope image of the workpiece surface on an anodized SiC substrate in the surface treatment equipment shown;

[0011] Figure 4 yes Figure 3 A schematic cross-sectional view of a portion of the structure near the workpiece surface of the SiC substrate shown;

[0012] Figure 5 This is after the oxide film is removed by solution. Figure 4 A schematic cross-sectional view of the structure of the SiC substrate shown;

[0013] Figure 6 This shows the effect of current density change on Figure 5 A graph showing the effect of pit depth;

[0014] Figure 7 This shows the effect of current density change on Figure 5 A graph showing the effect of pit depth;

[0015] Figure 8 This shows the effect of current density change on Figure 5 A graph showing the effect of pit depth;

[0016] Figure 9 These are scanning electron microscope images of the workpiece surface of a SiC substrate when the current density is changed by adjusting the electrolyte concentration.

[0017] Figure 10 This shows the removal by solution. Figure 9 A graph showing the measurement results of the surface roughness after oxide film in each SiC substrate;

[0018] Figure 11 These are scanning electron microscope images of the surface of a SiC substrate when the electrolyte concentration is changed while the current density remains constant.

[0019] Figure 12 These are scanning electron microscope images of a section near the surface of a workpiece on a SiC substrate before and after anodizing.

[0020] Figure 13 It is a graph showing the relationship between current density and surface roughness after polishing;

[0021] Figure 14A This is a schematic cross-sectional view showing the difference in oxide film formation modes caused by changes in current density;

[0022] Figure 14B This is a schematic cross-sectional view showing the difference in oxide film formation modes caused by changes in current density;

[0023] Figure 14C This is a schematic cross-sectional view showing the difference in oxide film formation modes caused by changes in current density;

[0024] Figure 15 This is a graph showing the effect of pulsed applied current on the recovery of OH- concentration during anodizing in the region near the workpiece surface of the SiC wafer to be processed;

[0025] Figure 16 It shows based on Figure 15 The graph shows the experimental calculation results of the total amount of OH- reaction per unit time, representing the simulation results.

[0026] Figure 17 This is a graph showing the experimental results confirming that pulsed applied current causes an increase in the oxidation rate during anodic oxidation;

[0027] Figure 18 It is a graph showing the relationship between current density and processing speed when a pulsed current is applied;

[0028] Figure 19 It is a graph showing the relationship between the period and the oxidation rate when a pulsed current is applied; and

[0029] Figure 20 This is a graph showing the relationship between duty cycle and oxidation rate when a pulsed current is applied. Detailed Implementation

[0030] Implementation

[0031] Embodiments of the present invention will now be described with reference to the accompanying drawings. It should be noted that if various modifications applicable to an embodiment are inserted midway through a series of descriptions of an embodiment, these modifications may interfere with the understanding of the embodiment. Therefore, modifications will not be inserted midway through a series of descriptions of embodiments, but will be described collectively after the embodiments.

[0032] Surface treatment equipment

[0033] Reference Figure 1 According to this embodiment, the surface treatment apparatus 1 is a processing apparatus that processes a single-crystal SiC wafer or a SiC substrate W as a workpiece, and is capable of polishing or grinding the workpiece surface W1 of the SiC substrate W using anodizing. That is, the surface treatment apparatus 1 is configured with an ECMP apparatus or an ECMG apparatus. ECMG stands for electrochemical mechanical polishing.

[0034] The surface treatment apparatus 1 includes a container 2, a surface processing pad 3, a drive unit 4, and a power supply unit 5. In this embodiment, when the SiC substrate W is immersed in an electrolyte S that does not contain etchant components, the container 2 can hold the SiC substrate W. The etchant components are components constituting a solution (e.g., hydrofluoric acid) that has the ability to dissolve the oxide film (i.e., the SiC oxide film formed on the surface W1 of the workpiece by anodizing). The electrolyte S is, for example, an aqueous solution of sodium chloride, potassium chloride, or sodium nitrate.

[0035] The surface treatment pad 3 includes an electrode 31 and a grinding wheel layer 32. The electrode 31 is a plate-shaped member made of a satisfactory conductor (e.g., metal), and is formed, for example, from a copper plate. The grinding wheel layer 32 is bonded to the electrode 31. That is, the surface treatment pad 3 has a configuration in which the electrode 31 and the grinding wheel layer 32 are bonded along the thickness direction of the surface treatment pad 3. The grinding wheel layer 32 has a polishing material with a Mohs hardness that is intermediate between the Mohs hardness of single-crystal SiC and the Mohs hardness of the oxide film of single-crystal SiC. That is, the grinding wheel layer 32 is rotated by the drive device 4 while being arranged to face the workpiece surface W1 of the SiC substrate W, such that the oxide film formed on the workpiece surface W1 by anodizing can be selectively removed by polishing or grinding. In this embodiment, the surface treatment pad 3 is configured such that the grinding wheel layer 32 passes through the electrolyte S and faces the workpiece surface W1 of the SiC substrate W.

[0036] The driving device 4 rotates the surface treatment pad 3 about a predetermined rotation axis parallel to the thickness direction, and displaces the SiC substrate W and the surface treatment pad 3 relative to each other in a plane orthogonal to the rotation axis. In the presence of the electrolyte S, the power supply device 5 applies a voltage by using the SiC substrate W, which is the workpiece, as the anode and the electrode 31 in the surface treatment pad 3 as the cathode, to pass a current for anodizing the workpiece surface W1 to be processed by the grinding wheel layer 32.

[0037] In this embodiment, the power supply device 5 is configured to achieve a current density of 15 mA / cm² in the presence of electrolyte S. 2 A larger current flows through the SiC substrate W, which serves as the anode. The power supply device 5 can output a pulsed current with on-time and off-time as the current for anodizing the workpiece surface W1. Here, the term "on-time" refers to the allowable current density of 15 mA / cm². 2 Or the time during which a larger current flow is permitted. The term "off time" refers to the period during which the current density is not allowed to be 15 mA / cm². 2 Or a longer current flow time, specifically, the time during which the current is essentially zero. That is, the power supply device 5 can selectively output a DC current and a rectangular pulse current with a predetermined frequency of periodic current density variation that is essentially unaffected by time variation. In this way, the surface treatment apparatus 1 according to this embodiment has the following configuration: wherein, it has 15mA / cm 2 Or a higher current density DC current or pulsed current can be used as the current for anodizing the workpiece surface W1 to perform high-speed and high-precision ECMP or ECMG.

[0038] Summary of the surface treatment method of the embodiments

[0039] The surface treatment apparatus 1 according to this embodiment can perform a surface treatment method (i.e., a polishing method or a grinding method) on a SiC substrate W having the following processes (1) to (3) in order to achieve processing characteristics superior to those of conventional ECMP (e.g., processing speed or flatness). Processes (2) and (3) can be performed simultaneously or sequentially.

[0040] (1) The surface treatment pad 3 is arranged to face the workpiece surface W1 of the SiC substrate W across the electrolyte S.

[0041] (2) In the presence of electrolyte S, the current density is 15 mA / cm². 2 A larger DC current or pulsed current is passed through the SiC substrate W, which serves as the anode. This anodizes the workpiece surface W1, which will be processed by the grinding wheel layer 32.

[0042] (3) Use the grinding wheel layer 32 to grind or polish the workpiece surface W1. That is, use the grinding wheel layer 32 to selectively remove the oxide film (i.e., the oxide film formed on the workpiece surface W1 by anodizing).

[0043] Figure 2 A through C in the text are used Figure 1 A schematic diagram of the manufacturing process of the SiC wafer or SiC substrate W in the surface treatment equipment 1 shown. Figure 2 The conventional method P shown as a comparative example indicates that a known CMP is used instead of the one described above. Figure 1 This is a summary of the ECMP or ECMG manufacturing process of SiC wafers performed in the surface treatment equipment 1 shown. CMP stands for Chemical Mechanical Polishing.

[0044] The general outline of conventional manufacturing method P will now be described. Conventional manufacturing method P comprises, in sequence, an ingot forming process, a slicing process, a wafer polishing process, a rough CMP process, and a fine CMP process. The ingot forming process is used to grow a block of single-crystal SiC and form it into a cylindrical ingot. The slicing process obtains a thin, disk-shaped SiC substrate W or SiC wafer from the ingot through wire slicing. The wafer polishing process is used to planarize the SiC substrate W by removing the "undulations" that occurred in the SiC substrate W during the slicing process. The rough CMP and fine CMP processes are used to process the workpiece surface W1 of the SiC substrate W to a mirror finish, which is a preferred surface condition for semiconductor device manufacturing processes.

[0045] Typically, during wafer polishing, a "damage layer" with a certain degree of "subsurface damage" is formed on and near the workpiece surface W1 of the SiC substrate W. Subsurface damage includes, for example, cracks and residual stress. First, the workpiece surface W1 is mirror-finished during a rough CMP process. The damage layer is then removed through a subsequent fine CMP process.

[0046] As disclosed in JP 2017-92497 A, ECMP is a non-destructive polishing process that achieves a higher processing speed than CMP. Therefore, by replacing the CMP process with ECMP, high-speed and non-destructive polishing of the workpiece surface W1 can be achieved. In the ECMP process, a soft grinding wheel containing relatively soft abrasive grains (e.g., cerium dioxide abrasive grains) is used as the grinding wheel layer 32.

[0047] Therefore, in this embodiment, for example, as in Figure 2In manufacturing method A shown, the rough CMP process in conventional manufacturing method P can be replaced by a rough ECMP process, and the fine CMP process in conventional manufacturing method P can be replaced by a fine ECMP process. In this way, the manufacturing cost can be reduced by approximately 40% compared to the manufacturing cost of conventional manufacturing method P.

[0048] The ECMG process can be accomplished by using a hard grinding wheel containing relatively hard abrasive grains (e.g., diamond grains) as the grinding wheel layer 32. That is, as in Figure 2 In manufacturing method B shown, the wafer polishing process in conventional manufacturing method P can be replaced by the ECMG process. In this way, the manufacturing cost can be reduced by approximately 20% compared to the manufacturing cost of conventional manufacturing method P.

[0049] Compared to the wafer polishing process in conventional manufacturing method P, the ECMG process, as a low-damage polishing process, reduces the frequency of subsurface damage. Therefore, manufacturing method B can be changed to manufacturing method C when the wafer polishing process in conventional manufacturing method P is replaced by the ECMG process. In manufacturing method C, the coarse CMP process in conventional manufacturing method P is omitted, and the fine CMP process in conventional manufacturing method P is replaced by a fine ECMP process. In this way, the manufacturing cost can be reduced to about half of the manufacturing cost of conventional manufacturing method P.

[0050] High current density of anodic oxidation current

[0051] As described in JP 2021-27359 A, it is known that in ECMP using SiC substrate W as the workpiece, an increase in current density leads to an increase in processing speed but an increase in surface roughness. Therefore, conventionally, when using ECMP in the fine polishing process, the current density is reduced by lowering the applied voltage below the passive potential in order to achieve a satisfactory surface roughness after processing while accepting the disadvantage of reduced processing speed.

[0052] However, as a result of in-depth research, the inventors discovered that, contrary to conventional knowledge, by using a voltage sufficiently higher than the passivation potential of 15 mA / cm... 2 Or, higher current density regions can improve surface roughness. Specifically, such as... Figure 3 and Figure 4 As shown, the oxide film W2 formed on the workpiece surface W1 by anodizing has an undulating structure composed of many oxide W3 particles. Figure 5As shown, when the oxide film W2 dissolves in a solution containing etchant components (e.g., hydrofluoric acid), numerous pits W4 are formed on the workpiece surface W1. The depth of the pits W4 is measured based on conventional knowledge under conditions of DC current and low current density with constant current, corresponding to the fine polishing process, as the current density varies. The results are as follows... Figure 6 As shown. Figure 6 The pit depth Sz on the vertical axis is the maximum value of the pit depth W4 in a predetermined region (1μm × 1μm) on the wafer used as the sample, and was measured using an atomic force microscope SPM 9700 manufactured by Shimadzu Corporation. Figure 6 As shown, the pit depth Sz decreases with increasing current density.

[0053] Under high current density conditions, the pit depth Sz was found to vary with current density for both DC current and pulsed current. The results show that... Figure 7 and Figure 8 As shown in the image. Figure 7 The case of direct current (i.e., non-pulse current) is shown, and Figure 8 The case of pulsed current is illustrated. The pit depth Sz was measured under each current condition at five measurement points with approximately equal spacing, comprising points near the center of a straight line passing through the surface of the wafer, which serves as the sample. Figure 7 and Figure 8 As shown, in both DC current and pulse current, a value of 5mA / cm was observed. 2 Up to 30mA / cm 2 Within a certain range, the pit depth Sz decreases due to the increase in current density, meaning the pit shape becomes more uniform.

[0054] Figure 9 These are scanning electron microscope images of the surface of a SiC substrate when the current density is varied by adjusting the electrolyte concentration while using direct current. The application conditions are a voltage of 25V and an application time of 30 seconds. Figure 9 The concentration of the sodium chloride solution or electrolyte and the corresponding current density are shown. Figure 10 This shows the removal through solution Figure 9 The graph shows the measurement results of the surface roughness after the oxide film W2 in each SiC substrate. Figure 10 The maximum height Sz and root mean square height Sq on the central vertical axis were measured using a scanning white light interferometer New View 8300 manufactured by ZYGO.

[0055] like Figure 9 and Figure 10As shown, increasing the electrolyte concentration has been found to increase the current density, reduce the protrusion structure in the oxide film W2, and improve the uniformity of the oxide film W2. To verify whether this result is due to the change in electrolyte concentration rather than the change in current density, results were obtained by changing only the electrolyte concentration while keeping the current density constant. The results are as follows. Figure 11 As shown. The application condition is 10 mA / cm. 2 The current density and the application time of 30 seconds. Figure 11 The concentration of the sodium chloride solution or electrolyte is shown. For example... Figure 11 As shown, it has been found that even with different electrolyte concentrations, the surface morphology is almost identical as long as the current density is the same. Therefore, from Figure 9 to Figure 11 The results clearly show that increasing the current density improved the uniformity of the oxide film W2.

[0056] Figure 12 The results of observing the cross-section of oxide film W2 when it is anodized at different current densities are shown. Figure 12 The top left corner shows the state before anodizing. From Figure 12 The results clearly show that increasing the current density resulted in the formation of a dense and uniform oxide film W2 with low undulations.

[0057] Figure 13 The results show the surface roughness measurements after polishing by ECMP while varying the current density, when anodizing was performed using direct current instead of pulsed current. Figure 13 The results clearly show that by using 15mA / cm 2 Or a higher current density can achieve excellent surface properties.

[0058] Figure 14A to Figure 14C The diagram schematically illustrates a possible mechanism by which high current density reduces surface roughness. In the figure, the dashed arrows indicate the anodic oxidation current. Figure 14A to Figure 14C As shown, it is believed that the higher the current density, the more uniform the current flow in the plane (i.e., the left and right directions in the figure), and the denser the structure of the oxide film W2.

[0059] In this way, the oxide film W2 becomes satisfactorily dense and uniform through high current density. High current density can lead to an increase in processing speed. Therefore, this embodiment can provide a technique that can achieve processing characteristics superior to those of conventional methods (e.g., in surface treatment methods utilizing anodized SiC substrates W). In other words, according to this embodiment, high-speed and high-precision ECMP or ECMG can be provided.

[0060] Processing conditions

[0061] Various processing conditions in the surface treatment method according to this embodiment will now be described.

[0062] (1) The surface treatment method according to this embodiment includes the following process: setting anodizing conditions such that the current density in the anodizing of the workpiece surface W1 is 15 mA / cm². 2 Or even greater. The anodizing conditions to be set include at least one of the following parameters: the temperature and concentration of the electrolyte S, the output current value from the power supply device 5, the output current waveform, the output voltage value, the output voltage waveform, etc. Specifically, for example, the resistance value of the electrolyte S can be adjusted by the temperature and / or concentration of the electrolyte S. This allows the desired anodizing current to be maintained in a satisfactory manner. Here, the term "anodic oxidation current" refers to the current applied for anodizing, that is, the portion of the current flowing from the power supply device 5 into the SiC substrate W, which is actually used for the anodizing of the workpiece surface W1, rather than for the electrolysis of the electrolyte S. The pulsed current will be described below.

[0063] (2) As described above, the surface treatment method according to this embodiment can be applied to both ECMP and ECMG. That is, the grinding wheel layer 32 grinds or polishes the workpiece surface W1, which is anodized by applying an electric current. The grinding rate or polishing rate can be adjusted by the type and number of grinding wheels in the grinding wheel layer 32. Specifically, for example, a grinding rate of 5 μm / min or greater can be achieved by using diamond grinding wheels of #8000 to #30,000. Alternatively, for example, a polishing rate of 5 μm / h or greater can be achieved by using cerium dioxide grinding wheels of approximately #8000.

[0064] (3) Preferably, the removal rate (i.e., polishing rate or grinding rate) of the oxide W3 (i.e., oxide film W2) by the grinding wheel layer 32 is substantially equal to the oxidation rate of the workpiece surface W1 during anodizing. In this way, the surface roughness of the treated workpiece surface W1 can be satisfactorily reduced.

[0065] (4) As in Figure 2 In manufacturing method A shown, the rough ECMP and fine ECMP processes can be performed sequentially at a polishing rate lower than that of the rough ECMP process. In this case, anodizing can be performed in both the rough and fine ECMP processes under the same current density conditions. Similarly, as in... Figure 2In the manufacturing method C shown, the ECMG and ECMP processes can be performed in this order. In this case, anodizing can be performed under the same current density conditions in both the ECMG and ECMP processes. In this way, by applying high current density conditions to the grinding, rough polishing, and fine polishing processes, the SiC planarization process can be made as consistent as possible.

[0066] Enhancing oxidation rate through current pulsed generation

[0067] In ECMP where the SiC substrate is used as the workpiece, the inventors have discovered that even when increasing the current density to increase the processing speed, there is a problem that limits the increase in processing speed due to the saturation of the oxidation rate in anodic oxidation. According to the inventors' research, the saturation of the increase in oxidation rate accompanying the increase in current density is considered to be due to insufficient supply of reactive species (i.e., OH-) in the electrolyte S near the workpiece surface W1. The region near the workpiece surface W1 is referred to below as the "near-surface region".

[0068] Specifically, the consumption of OH- through anodic oxidation in the near-surface region leads to a decrease in OH- concentration. Then, according to the principle of material diffusion, OH- is supplied from the bulk region of the electrolyte S (i.e., the region farther from the workpiece surface W1 compared to the near-surface region) to the near-surface region. Therefore, according to Fick's law, the OH- concentration in the near-surface region decreases as it approaches the workpiece surface W1.

[0069] Whether the supply of OH- in the near-surface region is sufficient depends on the relationship between the oxidation rate (i.e., the rate of OH- consumption) and the rate of OH- supply from the bulk region. In this regard, when the current conditions in anodizing are direct current and constant current, there is a concern that an insufficient OH- supply rate leads to a lack of OH- in the near-surface region, and thus makes it difficult to maintain a stable anodizing state.

[0070] The inventors have discovered that by providing the applied current for anodizing as a pulsed current with on-time and off-time, and by supplying OH- from the body region to the near-surface region during the off-time to restore the OH- concentration in the near-surface region, the oxidation rate is improved. The inventors have also discovered that by setting the off-time to be relatively short (specifically, for example, about 0.01 seconds to 10 seconds), a satisfactory oxidation rate can be maintained during the on-time while satisfactory increases in the total processing time can be avoided.

[0071] Figure 15This is the result of a computer simulation, showing the effect of providing a turn-off time for the anodic oxidation current on the recovery of OH- concentration in the near-surface region. In the figure, "Toff" on the horizontal axis indicates the length of the turn-off time. The vertical axis indicates the OH- concentration at a location 10 μm away from the workpiece surface W1, which is assumed to be a flat surface. The simulation was based on the following assumptions:

[0072] Diffusion coefficient D = 1.9 × 10 -9 m 2 / s

[0073] The OH- concentration in the main region is C = 6.02 × 10⁻⁶. 17 cm -3

[0074] Figure 16 It shows the basis Figure 15 The simulation results shown are experimental calculations of the total amount of OH- reaction per unit time. The horizontal solid line in the figure indicates the case where Toff = 0 (i.e., DC current and constant current). The terms "DC current and constant current" are referred to as "constant current" below.

[0075] from Figure 15 and Figure 16 It is evident that by using an anodizing current as a pulsed current to provide a turn-off time of 0.01 seconds or longer, the OH- supply shortage in the near-surface region can be satisfactorily eliminated. Specifically, as... Figure 15 and Figure 16 As shown, the inventors have discovered through simulations that OH- can be satisfactorily supplied to the near-surface region within a shutdown time of 0.01 seconds or longer and 10 seconds or shorter. However, the effect of restoring the OH- concentration by providing a shutdown time tends to saturate in regions with relatively long Toff intervals. Therefore, a shutdown time of approximately 0.1 to 1 second is considered more preferable.

[0076] The inventors conducted experiments using a 30 square millimeter wafer to determine the difference in oxidation rate between constant current application and pulsed current application during anodizing. The pulsed current had a rectangular waveform with an on-time of 1 second and an off-time of 1 second (i.e., a 2-second cycle and a duty cycle of 0.5). After anodizing, the oxide film W2 was removed with hydrofluoric acid, and the oxidation rate was calculated based on the amount removed. The experimental results are as follows: Figure 17 As shown. In Figure 17 In the diagram, the dots represent pulsed currents, and the diamonds represent constant currents. The current density values ​​for the pulsed current case are the values ​​during the on-time.

[0077] from Figure 17It is evident that applying pulsed current achieves a high oxidation rate across the range from high to low current density. The oxidation rate increases with increasing current density. Therefore, when applying pulsed current, the processing speed can be satisfactorily increased by increasing the current density. In particular, even if a large amount of OH- in the near-surface region is consumed by applying a current with a high current density, the OH- concentration in the near-surface region can be satisfactorily recovered during the off-time. Therefore, by applying pulsed current, high-speed processing is expected to be achieved by applying a current with a high current density.

[0078] Improving processing speed by pulsed current

[0079] The processing speed is affected not only by the oxidation rate but also by the properties of the oxide film W2. Specifically, in conventional ECMP, an oxide film W2 with relatively high density and relatively high hardness is formed by applying a constant current with a relatively low current density. When the composition of the oxide film W2 formed by applying a constant current corresponding to that of conventional ECMP is analyzed using an XPS instrument, the oxide film contains approximately 40% SiOC, approximately 30% SiO, and approximately 10% Si2O3. XPS stands for X-ray photoelectron spectroscopy.

[0080] In contrast, when analyzing the composition of the oxide film W2 formed by applying a pulsed current using XPS, the SiOC content was significantly lower than that of the oxide film W2 formed by applying a constant and low current, while the SiO content was significantly higher. When observing the cross-section using transmission electron microscopy, the formation of an internal void layer in the oxide film W2 formed by applying a pulsed current was more pronounced than in the oxide film formed by applying a constant and low current. There was a trend that increasing the pulse period led to an increase in the number of voids in the void layer. Specifically, under a duty cycle of 0.5, even with a period of 0.02 seconds, the formation of the void layer was more significant than with a constant and low current, and the number of voids increased with increasing period (e.g., from 0.1 seconds to 1 second). That is, the inventors found that a relatively low-density oxide film W2, easily polishable or abrasive, was formed within a period of 0.02 to 1 second.

[0081] Considering the above results, the following issue is considered to be the effect of the applied pulsed current. Since OH- is satisfactorily supplied to the near-surface region and promotes the anodic oxidation of SiC, more SiO with a higher degree of oxidation than SiOC is generated. As the degree of oxidation increases in the order of SiOC and SiO, the coefficient of thermal expansion becomes larger, promoting the formation of voids in the oxide film W2 due to internal stress. Therefore, the formation of the void layer due to the difference in the coefficient of thermal expansion before and after oxidation becomes more significant due to the generation of more SiO. In this way, the oxide film W2, containing a large amount of SiO and having a greater number of voids in the void layer, has low hardness, and the grinding or polishing rate increases. When many cracks appear on the surface of the oxide film W2 due to the formation of voids, OH-, as a reactive substance, enters the interior of the film through the cracks and may further promote anodic oxidation. As mentioned above, the effect of increasing the polishing rate can be expected as an effect of the pulsed current applied.

[0082] Figure 18 The relationship between current density and polishing rate is shown when the current density changes during the application of a pulsed current. The pulsed current period is 2 seconds and the duty cycle is 0.75. A 4-inch wafer was used as the polishing target. The polishing rate was calculated as follows: Before and after polishing, the thickness change of the 4-inch wafer, used as a sample, was measured at nine equally spaced measurement points along a straight line passing through the center of the wafer surface, and the average thickness change per unit time at the nine measurement points was calculated as the polishing rate of the wafer. Figure 18 As shown, under pulsed current application, the polishing rate was found to increase with increasing current density, even at 40 mA / cm². 2 The same applies to larger high-current regions.

[0083] Pulse period

[0084] Figure 19 The graph shows the change in oxidation rate as the period of the pulsed current varies. In the graph, the vertical axis indicates the oxidation rate, and the horizontal axis T indicates the period. The horizontal solid line in the graph indicates a reference value for the constant current condition.

[0085] The evaluation conditions are as follows: A 30 mm² wafer was used as the sample. The duty cycle, current density, and application time were kept constant to match the charge amount across the various current application conditions. The duty cycle was 0.5, and the current density was 20 mA / cm². 2 .

[0086] from Figure 19It is evident that a higher oxidation rate than that under constant current was achieved within a period range of at least 0.01 to 20 seconds. Here, as described above, a turn-off time of 0.01 seconds or longer is required to restore the OH- concentration in the near-surface region. Therefore, a period of 0.01 seconds or more and 20 seconds or less is suitable for achieving a satisfactory oxidation rate. In particular, when the period is 0.01 seconds, the turn-off time is 0.005 seconds, resulting in an oxidation rate slightly higher than that under constant current conditions. Thus, for example, if the duty cycle is 0.25 to 0.75 at the minimum turn-off time of 0.01 seconds as described below, the minimum suitable period is approximately 0.02 seconds.

[0087] As described above, considering the satisfactory processability resulting from the formation of a low-density oxide film, the cycle time is preferably in the range of 0.02 seconds to 1 second. Figure 19 As shown, in the region where the cycle time exceeds 20 seconds, the oxidation rate is slightly higher than that under constant current conditions. This is believed to be because, although the effect of increasing the oxidation rate by providing a turn-off time, as described above, has saturated, the overall cycle time is prolonged. Therefore, considering... Figure 15 and Figure 16 The simulation results shown Figure 19 The evaluation results of the actual oxidation rate shown and the cycle time in the actual manufacturing process are preferably 0.1 seconds or longer and 2 seconds or shorter.

[0088] Taking all the above into consideration, the preferred period is 0.02 seconds to 2 seconds, more preferably 0.02 seconds to 2 seconds, even more preferably 0.02 seconds to 1 second or 0.1 seconds to 2 seconds, and most preferably 0.1 seconds to 1 second.

[0089] Duty cycle

[0090] Figure 20 The graph shows the change in oxidation rate as the duty cycle of the pulsed current changes. In the graph, the vertical axis indicates the oxidation rate, and the horizontal axis T indicates the duty cycle. The horizontal solid line in the graph indicates a reference value for the constant current condition.

[0091] The evaluation conditions are as follows: A 30 mm² wafer was used as the sample. To match the charge amount across different current application conditions, the current density was set to be the same (i.e., 20 mA / cm²). 2 And adjust the application time so that the product of the duty cycle and the application time remains constant. For example... Figure 20 As shown, by setting the duty cycle in the range of 0.25 to 0.75, a satisfactory oxidation rate higher than that of a constant current was achieved.

[0092] Summary of the effects of current pulsed

[0093] As described above, pulsed current applied during anodizing not only leads to increased oxidation but also improves polishability and grindability associated with reduced density and hardness of the oxide film W2, as well as flatness through improved uniformity of the oxide film W2. Therefore, the polishing or grinding rate can be increased, and wafer manufacturing costs can be reduced.

[0094] That is, for example, contrary to the common sense that increasing current density increases processing speed while exacerbating surface roughness, increasing the applied pulse current to a high level can achieve high-speed processing while maintaining satisfactory flatness. As mentioned above, the inventors have discovered that at up to approximately 150 mA / cm 2 A satisfactory oxide film W2 suitable for ECMP or ECMG is formed at a current density.

[0095] Regarding wafer size, similar polishing rates can be achieved even when the diameter is further increased from 4 inches as in the described embodiment. Specifically, the surface treatment method according to this embodiment is satisfactorily applicable to wafer sizes, for example, in the range of 1 inch to 8 inches.

[0096] Revise

[0097] This invention is not limited to the embodiments described above. Therefore, the embodiments described above can be appropriately modified. Typical modifications will be described below. In the following description of modifications, the differences from the embodiments described above will be mainly described. In the embodiments and modifications described above, the same reference numerals are assigned to the same or equivalent parts. Therefore, in the following description of modifications, parts having the same reference numerals as those in the embodiments described above can be appropriately incorporated into the description of the embodiments described above, unless there is a technical inconsistency or special additional description.

[0098] This invention is not limited to the specific device configurations described in the above embodiments. That is, Figure 1 This is a simplified schematic diagram used to simply illustrate the outline of the surface treatment apparatus 1 according to the present invention and the surface treatment method that can be performed by the apparatus 1. Therefore, the configuration of the surface treatment apparatus 1 actually manufactured and sold may not be the same as... Figure 1 The exemplary configuration shown matches. It can be based on... Figure 1 The exemplary configuration shown is appropriately modified to the configuration of the surface treatment equipment 1 actually manufactured and sold.

[0099] For example, the configuration of the surface treatment pad 3 is not limited to the specific device configuration described in the above embodiments. Specifically, the electrode 31 and the grinding wheel layer 32 do not need to be joined along the thickness direction of the surface treatment pad 3. More specifically, for example, the electrode 31 and the grinding wheel layer 32 can be arranged adjacent to each other in a plane orthogonal to the thickness direction of the surface treatment pad 3. That is, the surface treatment apparatus 1 can be configured such that the surface treatment pad 3 rotates or moves such that the electrode 31 and the surface treatment pad 3 alternately face a specific portion of the workpiece surface W1 in time. Alternatively, the electrode 31 can be a separate body from the surface treatment pad 3. That is, the surface treatment apparatus 1 can be configured such that the electrode 31 and the surface treatment pad 3 are arranged to alternately face the entire or specific portion of the workpiece surface W1 in time. The type of abrasive grains contained in the grinding wheel layer 32 is not particularly limited.

[0100] The electrolyte S may contain etchant components. That is, the surface treatment apparatus 1 according to the present invention and the surface treatment method that can be performed by the surface treatment apparatus 1 can be the following apparatus and method: wherein the workpiece surface W1 is polished or ground by selectively removing the oxide film W2 generated by anodizing using both an etchant and a surface treatment pad 3.

[0101] The surface treatment apparatus 1 according to the present invention and the surface treatment method that can be performed by the surface treatment apparatus 1 are generally applied to Figure 2 Any one of the ECMG, rough ECMP, and fine ECMP processes in manufacturing methods A to C shown. However, for example, in manufacturing method A, it is expected that the workpiece surface W1 after the rough ECMP process will be satisfactorily mirror-finished, with slight subsurface damage caused by the rough ECMP process. Therefore, ECMP can be used in conjunction with conventional constant current application for the fine ECMP process.

[0102] Obviously, the elements constituting the above embodiments are not necessarily necessary unless explicitly stated that they are necessary or they are obviously necessary in principle. The invention is not limited to specific numerical values ​​unless specific values ​​such as the number, quantity, and range of components are explicitly indicated as necessary, or specific values ​​are obviously limited in principle. Similarly, the invention is not limited to the shape, orientation, positional relationship, etc., of components unless the shape, orientation, positional relationship, etc., of components are explicitly indicated as necessary, or when they are in principle limited to a specific shape, orientation, positional relationship, etc.

[0103] Modifications are not limited to the examples above. That is, for example, multiple implementations may be combined with each other in addition to those shown above, as long as they are not technically contradictory. Similarly, multiple modifications may be combined with each other, as long as they are not technically contradictory.

Claims

1. A method for treating the surface of a SiC substrate, comprising: In the presence of an electrolyte, by setting the current density to 15 mA / cm² 2 Or a larger periodic pulsed current is used to anodize the workpiece surface of the SiC substrate by passing the SiC substrate as the anode, the periodic pulsed current having a current density of 15 mA / cm². 2 Or a larger current on-time and a turn-off time for not passing that current to maintain a stable anodizing state; as well as The grinding wheel layer of the surface treatment pad is arranged to face the workpiece surface, and the grinding wheel layer is used to selectively remove oxides formed on the workpiece surface by anodizing.

2. The method according to claim 1, wherein, The removal rate of the oxide by the grinding wheel layer is equal to the oxidation rate during the anodizing of the workpiece surface.

3. The method according to claim 1 or 2, wherein, The grinding wheel layer grinds or polishes the surface of the workpiece that has been anodized by applying the current.

4. The method according to claim 3, wherein, The selective removal of the oxides using the grinding wheel layer includes: The workpiece surface is ground at a grinding rate of 5 μm / min or higher; and Polish the workpiece surface at a polishing rate of 5 μm / h or higher.

5. The method according to claim 4, further comprising: Anodizing corresponding to grinding the surface of the workpiece and anodizing corresponding to polishing the surface of the workpiece are performed under the same current density conditions.

6. The method according to claim 4, further comprising: Anodizing corresponding to rough polishing and anodizing corresponding to fine polishing are performed under the same current density conditions. The selective removal of the oxide using the grinding wheel layer includes: The rough polishing of the workpiece surface; and Fine polishing of the workpiece surface is performed at a polishing rate lower than that of coarse polishing.

7. The method according to claim 5, further comprising: Anodizing corresponding to rough polishing and anodizing corresponding to fine polishing are performed under the same current density conditions. The selective removal of the oxide using the grinding wheel layer includes: The rough polishing of the workpiece surface; and Fine polishing of the workpiece surface is performed at a polishing rate lower than that of coarse polishing.

8. The method according to claim 1 or 2, further comprising: The anodizing of the workpiece surface and the selective removal of the oxides using the grinding wheel layer are performed simultaneously or sequentially.

9. The method according to claim 3, further comprising: The anodizing of the workpiece surface and the selective removal of the oxides using the grinding wheel layer are performed simultaneously or sequentially.

10. The method according to claim 1 or 2, wherein, The anodizing process includes setting the current density to 15 mA / cm². 2 With 150mA / cm 2 between.

11. The method according to claim 3, wherein, The anodizing process includes setting the current density to 15 mA / cm². 2 With 150mA / cm 2 between.

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