JP4752072B2 - Polishing method and polishing apparatus - Google Patents

Description

  The present invention relates to a polishing method for mirror-polishing the surface of a workpiece, and a polishing apparatus for performing the method. It is possible to polish to high quality.

2. Description of the Related Art Conventionally, in the semiconductor manufacturing field, chemical mechanical polishing (CMP) using a solid-liquid reaction between a workpiece and a polishing liquid has been used for mirror polishing of a Si wafer.
The present inventor has previously developed a bell jar type CMP apparatus capable of controlling the environment in the polishing process, and considers the influence of the processing conditions such as the processing atmosphere and the applied pressure on the polishing result (the following patent document). 1).

FIG. 15 shows changes in the processing rate when air, oxygen gas, nitrogen gas or argon gas is sealed in the bell jar and the Si substrate is polished in the bell jar while changing their gas pressure. The vertical axis of FIG. 15 shows the Si processing rate (nm / min), and the horizontal axis shows the difference between the pressure (kPa) of the sealed gas in the bell jar and the atmospheric pressure (0 is the atmospheric pressure, and the + side is the atmospheric pressure. The increase from the atmospheric pressure, and the minus side is the decrease from the atmospheric pressure). In addition, a rhombus is displayed when the sealed gas is air, a triangle is displayed when oxygen gas is used, a square is displayed when nitrogen gas is used, and a circle is displayed when argon gas is used. Here, the trade name IC1000 / SUBA400 (φ200 mm) is used as the polishing cloth (pad), and the colloidal silica for Si (trade name Compol-80) is used as the slurry (polishing liquid).
As is apparent from FIG. 15, when air or oxygen gas is selected as the polishing atmosphere and the Si substrate is polished under these high pressures, the polishing is performed at a speed twice to 2.5 times that when polishing under atmospheric pressure. It becomes possible to do.
JP 2003-225859 A

In recent years, silicon carbide (SiC) and sapphire have attracted attention as next-generation device substrates. In SiC, a C atom is surrounded by four Si atoms, a Si atom is surrounded by four C atoms, and the Si atom and the C atom are connected by a hard covalent bond. This SiC has hardness next to diamond and is excellent in chemical resistance. Therefore, it can be used as a semiconductor substrate that can operate even under severe conditions such as high-temperature environments and radiation that cannot be used with ordinary silicon substrate devices.
However, SiC is extremely difficult to polish due to its physical properties. When mirror polishing is performed by conventional CMP, polishing can be performed only at a rate of several percent or less of the processing rate for polishing Si. Further, when polishing is performed using diamond abrasive grains in order to increase the polishing rate, scratches and work-affected layers remain on the surface, and a high-quality polished surface cannot be obtained.

In addition, single crystal sapphire (Al ₂ O ₃ ) is excellent in light transmittance and is indispensable as a material for lenses for optical devices, substrates for LEDs, and the like. It has the next modified Mohs hardness, is highly resistant to chemicals, and is extremely difficult to process.

The present invention was devised to improve such a situation, and provides a polishing method capable of polishing difficult-to-work materials such as SiC and sapphire to a high-quality and high-quality surface. It is aimed.

The present invention is a method for polishing a workpiece, wherein a silicon carbide, sapphire, silicon nitride, or gallium nitride crystal is the workpiece, the pressure of the processing atmosphere containing oxygen is set to be higher than atmospheric pressure , In the processing atmosphere, the workpiece is polished using a slurry containing titania particles while being irradiated with ultraviolet rays .
The photocatalyst titania (TiO ₂ ) is irradiated with ultraviolet rays in an oxygen-containing atmosphere to generate active oxygen . This active oxygen weakens (or breaks) the strong atomic bonds of the work piece. It becomes easy to scrape the surface of the workpiece by a general polishing process. Also, the high pressure oxygen facilitates the generation of active oxygen in the container. Together, these actions greatly improve the polishing processing rate. With the polishing method of the present invention, it is possible to polish a difficult-to-work material such as silicon carbide, sapphire, silicon nitride, or gallium nitride at a high processing rate.

Further, in the polishing method of the present invention, it is set the content of TiO ₂ in the slurry in the range of 0.1 wt% of 10.0 wt%.
By using a slurry containing TiO ₂ in this range, a smooth processed polished surface can be obtained with high efficiency.

Further, in the polishing method of the present invention, the intensity of ultraviolet rays is set to 5 mW / cm ² or more.
When irradiating ultraviolet rays having this intensity or more, TiO2 in the slurry generates sufficient active oxygen for realizing a high processing rate.

In the polishing method of the present invention, even when a difficult-to-work material such as SiC or sapphire is targeted, high-efficiency and high-quality polishing is possible.

The perspective view which shows the grinding | polishing apparatus in embodiment of this invention in the state fractured | ruptured partially The schematic diagram which shows the principal part of the grinding | polishing apparatus in embodiment of this invention. The graph which shows the processing rate when grind | polishing SiC with the grinding | polishing method in embodiment of this invention, and the processing rate of a comparative example Graph showing the processing rate of SiC when no photocatalyst is used The figure which contrasted the processing rate when grind | polishing SiC with the grinding | polishing method in embodiment of this invention, and the processing rate when grind | polishing without performing UV irradiation. The figure which shows the line profile of SiC grind | polished with the grinding | polishing method in embodiment of this invention. The figure which shows the line position of the line profile displayed in FIG. The table | surface which shows the relationship between the amount of TiO2 addition to a slurry when grinding | polishing SiC with the grinding | polishing method of embodiment of this invention, a processing rate, and surface roughness. AFM image of SiC polished by polishing method in embodiment of the present invention (Part 1) AFM image of SiC polished by polishing method in embodiment of the present invention (No. 2) The table | surface which shows the relationship between the ultraviolet-ray intensity | strength when grind | polishing SiC with the grinding | polishing method of embodiment of this invention, a processing rate, and surface roughness. The figure which contrasted the processing rate when grind | polishing without performing UV irradiation and the processing rate when grind | polishing sapphire with the grinding | polishing method in embodiment of this invention. The figure which shows the AFM image (a) of sapphire ground by the grinding | polishing method in embodiment of this invention, and the AFM image (b) of sapphire ground by the normal method AFM image of sample (SiC) surface polished with diamond abrasive Graph showing processing rate with conventional polishing method

Explanation of symbols

20 Bell jar 201 Lid 202 Container body 22 Polishing cloth (pad)
23 Surface plate 24 Slurry supply part 25 Sample 26 Press plate 27 Arm 28 Roller 29 Viewing window 31 Slurry supply pump 32 Vacuum pump 33 Gas cylinder 34 Motor 35 Ultraviolet light source 41 Tightening bolt 42 Valve 43 Valve 44 Pipe 45 Pipe 46 Gauge 47 Packing material

FIG. 1 is a perspective view of a polishing apparatus according to an embodiment of the present invention, showing a part of a bell jar cut out. FIG. 2 schematically shows the main part of the polishing apparatus.
This apparatus includes a bell jar 20 including a lid 201 and a container main body 202, a vacuum pump 32 that exhausts the inside of the bell jar 20, a gas cylinder 33 that supplies atmospheric gas into the bell jar 20, and a slurry that is supplied into the bell jar 20. A slurry supply pump 31 is provided. In the bell jar 20, a polishing cloth (pad) 22, a surface plate 23 on which the pad 22 is pasted, and a slurry supply unit 24 that drops the supplied slurry onto the pad 22. A pressing plate 26 that presses the sample 25 to be polished against the pad 22 by its own weight, and a roller 28 supported by an arc-shaped arm 27, and the surface plate 23 is rotationally driven below the bell jar 20. It has a motor 34 and also has an ultraviolet light source 35 that irradiates ultraviolet light from the viewing window 29 of the bell jar 20.

The bell jar 20 has a pressure container structure that can withstand pressurization and decompression, and the lid 201 is airtightly fixed to the container main body 202 by the fastening bolt 41 when closed.
The pressure reduction of the bell jar 20 by the vacuum pump 32 and the pressurization of the bell jar 20 by the gas cylinder 33 can be adjusted by valves 42 and 43. The pressure in the bell jar 20 can be measured by a gauge 46.
The slurry supply pump 31 is connected to a pipe 44 for feeding the slurry to the bell jar 20 and a pipe 45 for collecting the slurry from the bell jar 20. The slurry circulates between the slurry supply pump 31 and the bell jar 20. The slurry sent from the slurry supply pump 31 is supplied onto the pad 22 through the slurry supply unit 24, and the slurry accumulated in the bell jar 20 is supplied to the slurry. Suction is performed by the pump 31.

A sample 25 to be polished is placed on a pad 22 on which a pressing plate 26 is placed. A packing material 47 (FIG. 2) is provided on the surface of the pressing plate 26 in contact with the sample 25 in order to uniformly process the sample surface to be processed. The arc-shaped arm 27 holds a roller 28 positioned on the central axis of the surface plate 23 and a roller (not shown) positioned in the vicinity of the peripheral edge of the surface plate 23. Is supported rotatably.
When the surface plate 23 to which the pad 22 is fixed is driven and rotated by the motor 34, the pressing plate 26 comes into contact with the two rollers 28 and starts to rotate. The sample 25 rotates on the pad 22 together with the pressing plate 26 while being pressed toward the pad 22 by the pressing plate 26. Therefore, the surface of the sample 25 that contacts the pad 22 is polished.

Here, a single crystal SiC substrate is used as the sample 25. Further, oxygen gas or air is supplied from the gas cylinder 33 into the bell jar 20. Moreover, colloidal silica is used for the slurry.
In the polishing method of the present invention, titania (TiO 2) particles as a photocatalyst are mixed in the slurry.
When irradiated with ultraviolet rays from an ultraviolet light source 35 in oxygen, the TiO2 particles generate active oxygen having strong oxidizing power by a photocatalytic reaction. The SiC processing rate is dramatically improved by the action of the active oxygen. At this time, the active oxygen is combined with Si atoms and C atoms on the surface of the SiC substrate, so that the strong covalent bond between Si and C is weakened (or cut), and the mechanical properties of the pad 22 that contacts the SiC substrate are reduced. It is considered that Si and C on the surface of the substrate are easily scraped off by a strong force.

Next, details of the polishing method and the results will be described.
For the single crystal SiC substrate as sample 25, a single crystal SiC generated by an improved Rayleigh method or the like is sliced with a diamond outer peripheral blade or the like, and further subjected to pre-processing polishing with diamond abrasive grains. . The sample 25 is 4H-SiC type, the polished surface is (0001 surface), and the sample diameter is 2 inches. FIG. 14 shows an AFM (atomic force microscope) image of the sample surface polished with diamond abrasive grains.

The sample 25 was polished with a smooth mirror and the polished polishing for removing the damaged layer was performed with the apparatus shown in FIG. The implementation conditions are as follows.
The pad 22 used was a trade name IC1000 (K-GRY) / SUBA400 (made by Nitta Haas) (φ200 mm).
As the slurry, a product name Horizon-SiC (manufactured by D-process) in which 5 wt% of soft silicon dioxide (SiO 2) particles (colloidal silica) having a particle diameter of about 10 nm are dispersed in an aqueous solution is used. TiO2 particles having a diameter of 0.8 μm were added.
The rotation speed of the surface plate 23 was set to 90 (rpm) (the relative speed between the sample 25 and the pad 22 was around 40 m / min).
The pressing force of the pressing plate 26 was set to 180 g / cm ² .
The processing time for each sample was 2 hours.

Finish polishing was carried out under the above-mentioned conditions while adding 0.5 wt% of TiO2 to the slurry and irradiating from the viewing window 29 of the bell jar 20 with ultraviolet light with an ultraviolet intensity set to 100 mW / cm ² .
FIG. 3 shows the processing rate of the SiC substrate when the pressure of the oxygen gas or air sealed in the bell jar 20 from the gas cylinder 33 is changed variously, and, for comparison, nitrogen nitrogen in the bell jar 20 for comparison. The processing rate of the SiC substrate in the case where polishing is performed with the gas and argon gas sealed and the other conditions all the same is also shown.

The vertical axis of FIG. 3 shows the SiC processing rate (μm / h), and the horizontal axis shows the pressure (kPa) of the sealed gas in the bell jar as a difference from atmospheric pressure (0 is atmospheric pressure). In addition, the case where the sealed gas is oxygen gas is indicated by a rectangle, the case of air is indicated by a diamond, the case of argon gas is indicated by a triangle, and the case of nitrogen gas is indicated by a circle.
For comparison, FIG. 4 shows the processing rate when SiC is polished with the addition of TiO 2 to the slurry and the irradiation of ultraviolet rays and other conditions set to the same as in FIG. Yes.

The following can be understood from FIGS. That is, when SiC is polished in an atmosphere of oxygen gas and air (that is, in a processing atmosphere containing oxygen), the processing rate increases by increasing the pressure of the processing atmosphere, but in addition, the photocatalyst is added to the slurry. Is added and polished while irradiating with ultraviolet rays, the processing rate is improved about twice. Moreover, the result of measuring the surface roughness Ra (arithmetic mean roughness) of this SiC processed surface is 0.3 nm or less, and a smooth mirror surface is obtained.
On the other hand, when a photocatalyst is added to the slurry in a nitrogen gas or argon gas atmosphere that does not contain oxygen, the processing rate is rather lowered.

FIG. 5 shows the influence of the photocatalytic reaction on the polishing processing rate for each processing atmosphere. Here, the result of polishing under the same conditions as in the atmosphere is expressed as “filled gas: none”, and the pressure of each processing atmosphere at the time of polishing is set to 500 KPa otherwise. Also, the addition of TiO2 to the slurry and the processing rate when polishing without UV irradiation was shown as "No UV irradiation", while polishing was performed while adding TiO2 to the slurry and UV irradiation The processing rate at that time is shown as “with UV irradiation”. As the photocatalyst, anatase TiO2 having an average particle diameter of 0.5 μm is used, and 0.5 wt% of TiO2 is added to the slurry containing colloidal silica described above. The pressing force of the pressing plate 26 is set to 500 g / cm ² . Other polishing conditions are the same as those in FIG.
These measurement results show that when the pressure of the processing atmosphere containing oxygen is increased and TiO2 is added to the slurry and polishing is performed while irradiating with ultraviolet rays, the processing rate is significantly increased. Is selected, the increase in the processing rate is remarkable.
This is because oxygen existing in the processing atmosphere promotes the generation of active oxygen due to the photocatalytic reaction of TiO2, and the Si-C bond is easily broken by the action of the active oxygen, so that the processing rate is drastically improved. .

The photocatalytic reaction is also effective in smoothing the polished surface. When comparing the Ra of the processed surface polished by “without UV irradiation” in the processing atmosphere of oxygen 500 KPa and the processed surface polished by “with UV irradiation” in the same processing atmosphere shown in FIG. Ra is 0.304 nm, but Ra with “UV irradiation” is 0.281 nm, and “With UV irradiation” has a smaller surface roughness.
FIG. 6 shows a line profile of a processed surface polished by “with UV irradiation” in a processing atmosphere of oxygen 500 KPa. This is the result of measuring the unevenness on three parallel straight lines crossing the polished surface of 2.35 μm square shown in FIG. 7 using AFM, and graphing the average value of the unevenness. Represents from the beginning to the end of the straight line in μm, and the vertical axis represents the height unevenness in pm (picometer). It can be seen from this line profile that the polished surface is extremely smooth.

The table shown as FIG. 8 shows the relationship between the processing rate RR (μm / h) and the surface roughness Ra of the SiC processed surface when the SiC substrate is polished while changing the amount of TiO 2 added to the slurry. Here, oxygen gas is selected as the processing atmosphere gas, and the pressure in the bell jar 20 is set to 500 KPa. Further, the intensity of the ultraviolet rays irradiated from the viewing window 29 of the bell jar 20 is set to 100 mW / cm ² . Other implementation conditions are the same as in FIG.
As is apparent from the table of FIG. 8, when the amount of TiO2 added increases, the amount of active oxygen generated increases, and the processing rate increases accordingly. However, if the amount of TiO2 particles is too large, the polishing quality may be deteriorated. From the table of FIG. 8, if the amount of TiO2 added to the slurry is within the range of at least 0.1 wt% to 10.0 wt%, the SiC substrate can be polished at a high processing rate and in a good state. I understand.
FIG. 9 shows No. in the table of FIG. 2 shows an AFM image of the SiC processed surface of the sample of No. 2, and FIG. 4 shows an AFM image of the SiC processed surface of Sample No. 4.

In addition, the table shown in FIG. 11 shows the relationship between the processing rate RR (μm / h) and the surface roughness Ra of the SiC processed surface when the SiC substrate is polished by changing the intensity of ultraviolet light irradiated to the slurry containing TiO 2. Show. Here, oxygen gas is selected as the processing atmosphere gas, and the pressure in the bell jar 20 is set to 500 KPa. The amount of TiO2 added to the slurry is set to 0.5 wt%. Other polishing conditions are the same as those in FIG.
As is apparent from the table of FIG. 11, when the ultraviolet intensity increases, the amount of active oxygen generated by TiO 2 increases, and the processing rate increases accordingly. However, when the generation amount of active oxygen is saturated, the processing rate cannot be increased even if the ultraviolet intensity is further increased. From the table of FIG. 11, it can be seen that if the ultraviolet intensity is 5 mW / cm ² or more, the SiC substrate can be polished at a high processing rate and in a good state. The AFM image of FIG. 3 shows the processed state of the SiC processed surface in the sample No. 3.

  Thus, when polishing the workpiece, the pressure of the processing atmosphere containing oxygen is set to be higher than atmospheric pressure, and the polishing process is performed using the slurry in which the photocatalyst is dispersed while irradiating ultraviolet rays, so that the SiC Even such difficult-to-process materials can be finished with high efficiency and high quality.

Next, polishing of sapphire will be described.
Also for sapphire, like SiC, the pressure of the processing atmosphere containing oxygen is set higher than the atmospheric pressure, and polishing is performed using a slurry in which a photocatalyst is dispersed while irradiating ultraviolet rays, thereby achieving a high-quality polished surface. Can be obtained with high efficiency.

  FIG. 12 shows the processing rate when the single crystal sapphire substrate is polished while changing the processing atmosphere and processing conditions. In FIG. 12, the processing rate when polishing with colloidal silica slurry in the atmosphere is shown as “under air: SiO 2”, and the processing rate when polishing with a slurry of TiO 2 particles (0.5 wt%) mixed with pure water. Is expressed as “under air: TiO 2”, and 0.5 wt% of TiO 2 particles are added to the colloidal silica slurry, and the processing rate when polished with “with UV irradiation” is “under air: TiO 2 (0.5 wt%). ) + UV ”. Further, the processing rate when the inside of the bell jar is filled with oxygen of 500 KPa and polished with a colloidal silica slurry is shown as “oxygen 500 KPa: SiO 2”, and 0.5 wt% TiO 2 particles are added to the colloidal silica slurry. The processing rate when polishing with “irradiation” is shown as “oxygen 500 KPa: SiO 2 + TiO 2 + UV”.

Here, the C surface of the sapphire substrate is the polished surface. The pad used is the same as that for SiC polishing, the pressing force of the pressing plate is set to 500 g / cm ² , the rotation speed is set to 90 rpm, and the processing time is set to 1 hour. The colloidal silica slurry uses “D-process, Horizonor-Al ₂ O ₃ ” having an abrasive concentration of 5 wt%, an abrasive particle size of 70 nm, and a pH of 7.5, and the photocatalyst has an average particle size of Uses anatase TiO2 of 140 nm.
As is apparent from FIG. 12, when TiO2 is added to the slurry under an oxygen atmosphere with increased pressure and the sapphire single crystal is polished while being irradiated with ultraviolet rays, the processing rate is significantly increased.

  FIG. 13A shows an AFM image of the polished surface by “oxygen 500 KPa: SiO 2 + TiO 2 + UV”, and Ra of the polished surface is 0.374 nm. FIG. 13B shows an AFM image of the polished surface by “under air: SiO 2”, and Ra of the polished surface is 0.324 nm. Therefore, with “oxygen 500 KPa: SiO 2 + TiO 2 + UV”, a smooth mirror-polished polished surface having the same surface roughness as “under air: SiO 2” and inferior to the surface roughness of “under air: SiO 2” can be obtained at a processing rate twice that of “under air: SiO 2”. Yes.

Here, the polishing of SiC and sapphire has been described. However, it has been confirmed that the present invention can be applied to polishing difficult materials such as SiN and GaN as well as other materials.
Moreover, when the abrasive grain of the slurry used for grinding | polishing is colloidal silica, it is desirable that the particle diameter is 10 nm-several hundred nm. The abrasive grains of the slurry may be fumed silica other than colloidal silica, ceria, alumina, or the like.

Moreover, although the example which included TiO2 with an average particle diameter of 0.8 micrometer or 0.5 micrometer here was demonstrated, since generation | occurrence | production of active oxygen depends on the surface area of TiO2, it is not necessarily directly related to a particle size, In the present invention, even TiO2 having an average particle diameter of several nm to several microns can be used.
The pad may be soft or hard, and there is no problem even if a nonwoven fabric or artificial leather is used.

  The present invention enables high-efficiency and high-quality polishing of difficult-to-process materials such as SiC and sapphire. Can be used.

Claims (3)

A method for polishing a workpiece,
A silicon carbide, sapphire, silicon nitride, or gallium nitride crystal is a workpiece,
The pressure of the processing atmosphere containing oxygen is set to be higher than the atmospheric pressure, and the workpiece is polished while irradiating with ultraviolet rays using a slurry containing titania particles in the processing atmosphere. Polishing method.   2. The polishing method according to claim 1, wherein the titania content in the slurry is set in a range of 0.1 wt% to 10.0 wt%. 2. The polishing method according to claim 1, wherein the intensity of the ultraviolet ray is set to 5 mW / cm ² or more.

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