JP7614707B2 - Method for reclaiming SiC substrate - Google Patents

Description

The present invention relates to a method for regenerating a SiC substrate that has a thin nitride film formed on its surface.

In recent years, gallium nitride (GaN) has been used as a semiconductor material for light-emitting diodes (LEDs), laser diodes (LDs), power devices used in power conversion, and the like. A GaN layer, which is a single crystal layer of GaN, is formed by epitaxial growth on a substrate made of single crystal silicon carbide (SiC), whose crystal lattice constant is relatively close to that of GaN.

This GaN layer is then used to manufacture various devices such as LEDs. Device manufacturers may mass-produce various devices using a GaN layer on a SiC substrate under multiple different manufacturing conditions to extract favorable manufacturing conditions, and then incorporate these favorable manufacturing conditions into the mass production process.

Conventionally, SiC substrates used to extract favorable manufacturing conditions have been discarded, but because SiC substrates are expensive, used SiC substrates are now being regenerated and reused (see, for example, Patent Document 1).

For example, to regenerate a SiC substrate, first, the GaN layer formed on the surface side of the SiC substrate is removed by blasting or lapping (GaN layer removal process). Next, the damaged layer formed on the surface side of the SiC substrate by blasting or the like is removed by polishing (surface polishing process). After that, the surface of the SiC substrate is cleaned (cleaning process).

JP 2011-86672 A

When the GaN layer is removed by blasting or the like in this way, a relatively deep damage layer is formed in the SiC substrate. For example, a damage layer is formed that is about 10 μm deep from the surface of the SiC substrate. Therefore, a separate surface polishing process is required to remove the damage layer.

However, to remove about 10 μm of the surface side of a SiC substrate by polishing, it takes, for example, about 10 hours. Furthermore, since 10 μm of the SiC substrate is removed in one regeneration, the number of times it can be regenerated and reused is limited.

The present invention was made in consideration of these problems, and aims to reduce the number of steps required to regenerate SiC substrates and to increase the number of times that SiC substrates can be regenerated.

According to one aspect of the present invention, there is provided a method for regenerating a SiC substrate having a nitride layer formed on its front surface, the method comprising: a holding step of holding the back surface of the SiC substrate on a holding surface of a chuck table of a polishing apparatus with the nitride layer exposed upward ; a removal step of polishing the front surface of the SiC substrate using a polishing pad to which abrasive grains are fixed in the polishing apparatus while supplying an acidic polishing liquid that is free of abrasive grains to the nitride layer from the polishing pad , thereby removing the nitride layer; and a cleaning step of cleaning the SiC substrate, wherein the acidic polishing liquid contains an aqueous solution of sodium permanganate or an aqueous solution of potassium permanganate, the nitride layer is a GaN layer, and in the removal step, the GaN layer is polished and removed with the abrasive grains made of silica .

Preferably, the nitride layer is a GaN layer.

In one aspect of the present invention, a method for producing a SiC substrate involves polishing the surface side of a SiC substrate using a polishing pad of a polishing device while supplying an acidic polishing liquid from the polishing pad, thereby removing the nitride layer formed on the surface side of the SiC substrate.

In this way, the nitride thin film is removed by polishing only, without blasting or other processes. This reduces the number of steps required to regenerate the SiC substrate. Furthermore, because blasting or other processes are not performed, no damaged layer is formed on the SiC substrate. This reduces the thickness of the SiC substrate that is removed compared to when blasting or other processes are performed. This increases the number of times the SiC substrate can be regenerated.

FIG. 1A is a cross-sectional view of a SiC substrate and a GaN layer, and FIG. 1B is a perspective view of the SiC substrate and the like when a protective tape is attached to the SiC substrate. FIG. FIG. 2 is a cross-sectional view of a polishing unit, a polishing pad, etc. FIG. 2 is a flow diagram of a method for reclaiming a SiC substrate.

An embodiment according to one aspect of the present invention will be described with reference to the attached drawings. First, the SiC substrate 1 to be regenerated in this embodiment will be described with reference to FIG. 1(A). FIG. 1(A) is a cross-sectional view of the SiC substrate 1 and the GaN layer 3.

The SiC substrate 1 is a disk-shaped substrate having a circular front surface 1a and a rear surface 1b. The thickness of the SiC substrate 1 from the front surface 1a to the rear surface 1b is, for example, 350 μm. On the front surface 1a side of the SiC substrate 1, a GaN layer (nitride layer) 3 having a thickness of about 0.5 μm to several μm is formed by epitaxial growth.

Devices such as LEDs are formed on the GaN layer 3. In this embodiment, the GaN layer 3 on the front surface 1a side is removed by polishing to regenerate the SiC substrate 1, but before polishing the GaN layer 3, a protective tape 5 is attached to the rear surface 1b of the SiC substrate 1 to form a substrate with tape 7.

The protective tape 5 has a flexible film-like substrate layer. The substrate layer is made of PO (polyolefin), PET (polyethylene terephthalate), polyvinyl chloride, polystyrene, etc.

An adhesive layer (glue layer) is provided on one side of the base layer. The adhesive layer is made of ultraviolet-curing silicone rubber, acrylic material, epoxy material, etc. This adhesive layer is attached to the back surface 1b of the SiC substrate 1.

Figure 1 (B) is a perspective view of the SiC substrate 1 when the protective tape 5 is attached to the SiC substrate 1. By attaching the protective tape 5 to the SiC substrate 1, it is possible to mitigate the impact on the SiC substrate 1 during polishing, transportation, etc., and reduce damage to the SiC substrate 1.

In addition, FIG. 1(B) shows a state in which a circular protective tape 5 is attached to the back surface 1b, but after a rectangular protective tape 5 having one side larger than the diameter of the back surface 1b is attached to the back surface 1b, the protective tape 5 may be cut so that it has a circular shape having approximately the same diameter as the outer shape of the back surface 1b.

The substrate 7 with tape is transported to the polishing device 2, which polishes the GaN layer 3. The configuration of the polishing device 2 will now be described with reference to FIG. 2. FIG. 2 is a perspective view of the polishing device 2.

The polishing device 2 has a base 4 that supports each of the components. Cassette mounting tables 6a and 6b are provided on the upper surface of the front side (one side in the X-axis direction) of the base 4. For example, a cassette 8a containing a tape-attached substrate 7 before polishing is placed on the cassette mounting table 6a.

For example, a cassette 8b for storing the tape-attached substrate 7 after polishing is placed on the cassette placement stage 6b. A transport robot 10 is provided between the cassette placement stages 6a and 6b to transport the tape-attached substrate 7 to the cassette placement stages 6a and 6b.

A positioning table 12 is provided behind the cassette mounting table 6a (the other side in the X-axis direction) to determine the position of the tape-attached substrate 7 using multiple positioning pins. The tape-attached substrate 7 is transported from the cassette 8a to the positioning table 12 by the transport robot 10.

A carry-in mechanism (loading arm) 14 and an unloading mechanism (unloading arm) 16 are provided behind the positioning table 12. The carry-in mechanism 14 carries out the tape-attached substrate 7 from the positioning table 12 to a carry-in/out area described below.

An opening 4a is provided on the top surface of the base 4, behind the loading mechanism 14 and the unloading mechanism 16. An X-axis moving table 18, which is rectangular when viewed from above, is provided within this opening 4a.

A disk-shaped chuck table 20 is provided on the X-axis moving table 18. The chuck table 20 has a porous plate 22 (see FIG. 3) on the top. One end of a flow path 20b is connected to the opposite side of the surface (holding surface 20a) of the porous plate 22 (see FIG. 3).

The other end of the flow path 20b is connected to a suction source (not shown) such as an ejector. When the suction source is operated, a negative pressure acts on the surface of the porous plate 22. Therefore, the surface of the porous plate 22 functions as a holding surface 20a that holds the tape-attached substrate 7 by suction.

A rotation mechanism (not shown) is provided below the chuck table 20, and the chuck table 20 is connected to this rotation mechanism. When the rotation mechanism is operated, the chuck table 20 rotates around a predetermined rotation axis.

An X-axis direction movement mechanism (not shown) is provided below the rotation mechanism. By operating the X-axis direction movement mechanism, the chuck table 20 and other components move along the X-axis direction. For example, the chuck table 20 moves along the X-axis direction between the loading/unloading area 24 located on the front side of the opening 4a and the polishing area 26 located on the rear side of the opening 4a.

A wall-shaped support 4b is provided at the rear end of the base 4 of the polishing device 2, protruding above the top surface of the base 4. A pair of Z-axis guide rails 28, each parallel to the Z-axis direction, are provided on the front surface of the support 4b.

A Z-axis moving plate 30 is attached to each Z-axis guide rail 28 in a slidable manner. A nut portion (not shown) is provided on the back side (rear side) of the Z-axis moving plate 30, and a Z-axis ball screw 32 parallel to the Z-axis guide rail 28 is rotatably connected to this nut portion.

A Z-axis pulse motor 34 is connected to one end of the Z-axis ball screw 32. When the Z-axis pulse motor 34 rotates the Z-axis ball screw 32, the Z-axis moving plate 30 moves in the Z-axis direction along the Z-axis guide rail 28.

A polishing unit 36 is fixed to the front surface of the Z-axis moving plate 30. The polishing unit 36 has a cylindrical spindle housing 38. A spindle 40 is rotatably provided within the spindle housing 38.

A motor (not shown) that rotates the spindle 40 is connected to the upper end of the spindle 40. The upper surface of a disk-shaped mount 42 is connected to the lower end of the spindle 40. A disk-shaped polishing wheel 44 having a diameter larger than the diameter of the SiC substrate 1 is provided on the lower surface of the mount 42.

The polishing wheel 44 has a disk-shaped base 46 (see FIG. 3) connected to the underside of the mount 42. The base 46 is made of a metal such as aluminum or stainless steel. A polishing pad 48 (see FIG. 3) made of urethane foam (i.e., polyurethane foam) is fixed to the underside of the base 46. However, the polishing pad 48 is not limited to urethane foam, and may be made of other resin foams or nonwoven fabric.

The polishing pad 48 has a disk shape with approximately the same diameter as the base 46. A plurality of abrasive grains (not shown) are fixed in a dispersed state to the polishing pad 48. The abrasive grains in this embodiment are made of silica and have an average grain size of about 0.1 μm to 10 μm. However, the abrasive grains are not limited to silica, and may be made of alumina, diamond, cBN (cubic boron nitride), etc.

One end of a flow path 50 through which the polishing liquid is supplied is connected to the top of the polishing unit 36. The other end of the flow path 50 is connected to a polishing liquid supply source 52. An acidic polishing liquid is stored in the polishing liquid supply source 52. The acidic polishing liquid is supplied from the polishing liquid supply source 52 to the polishing unit 36 via the flow path 50.

In this embodiment, the acidic polishing liquid is a sodium permanganate aqueous solution. However, the acidic polishing liquid is not limited to a sodium permanganate aqueous solution, and may be a potassium permanganate aqueous solution or another acidic aqueous solution.

Next, the internal structure of the polishing unit 36 and the polishing wheel 44 will be described. FIG. 3 is a cross-sectional view of the polishing unit 36 and the polishing wheel 44. The acidic polishing liquid is supplied to the polishing wheel 44 via a first polishing liquid supply passage 54 that passes through the spindle 40 and the mount 42 in the Z-axis direction.

The first polishing liquid supply passage 54 is a cylindrical cavity having a diameter smaller than the outer diameter of the spindle 40, and is formed in the spindle 40 and the mount 42. The lower end of the first polishing liquid supply passage 54 is connected to the upper end of the second polishing liquid supply passage 56 formed in the polishing wheel 44.

The second polishing liquid supply path 56 is a cylindrical cavity formed with the same diameter as the first polishing liquid supply path 54. The second polishing liquid supply path 56 is formed so as to penetrate the polishing wheel 44 in the thickness direction of the polishing wheel 44.

The second polishing liquid supply path 56 and the first polishing liquid supply path 54 are arranged to form a straight line parallel to the Z-axis direction. The acidic polishing liquid 58 supplied from the polishing liquid supply source 52 is supplied below the polishing wheel 44 via the first polishing liquid supply path 54 and the second polishing liquid supply path 56.

The GaN layer 3 on the surface 1a side is polished using the polishing wheel 44 while supplying the acidic polishing liquid 58. After the polishing process, the chuck table 20 moves from the polishing area 26 to the loading/unloading area 24, and the substrate 7 with tape is unloaded from the chuck table 20 by the unloading mechanism 16.

A spinner cleaning unit 60 is provided between the cassette mounting table 6b and the unloading mechanism 16 (see FIG. 2). The spinner cleaning unit 60 cleans and spin-dries the SiC substrate 1. After cleaning, the tape-attached substrate 7 is transported from the spinner cleaning unit 60 to the cassette 8b by the transport robot 10.

Next, a regeneration method for removing the GaN layer 3 on the SiC substrate 1 using the polishing device 2 and regenerating the SiC substrate 1 will be described. FIG. 4 is a flow diagram of the regeneration method for the SiC substrate 1. First, as described with reference to FIG. 1(B), a protective tape 5 is attached to the back side of the SiC substrate 1 to form a substrate with tape 7 (protective tape attachment step (S10)).

After the protective tape application step (S10), the substrate 7 with tape is placed on the chuck table 20 in such a manner that the GaN layer 3 is exposed. Then, the suction source is operated to hold the substrate 7 with tape on the holding surface 20a. As a result, the back side of the SiC substrate 1 is held on the holding surface 20a (holding step (S20)).

After the holding step (S20), the chuck table 20 and the polishing wheel 44 are rotated in a predetermined direction, for example, at 750 rpm. Then, the polishing wheel 44 is gradually lowered along the Z-axis direction until the bottom surface of the polishing pad 48 comes into contact with the GaN layer 3 on the surface 1a side. When the bottom surface of the polishing pad 48 comes into contact with the GaN layer 3 on the surface 1a side, the GaN layer 3 on the surface 1a side is polished.

2 and 3, during polishing, a part of the lower surface of the polishing pad 48 comes into contact with the entire surface of the GaN layer 3 on the front surface 1a side. At this time, the polishing wheel 44 presses the GaN layer 3 toward the holding surface 20a side. For example, the polishing wheel 44 presses the GaN layer 3 toward the holding surface 20a side with a pressure of 500 g/ ^cm2 .

During polishing, the polishing wheel 44 is pressed against the holding surface 20a while supplying an acidic polishing liquid 58 from the polishing wheel 44 to the GaN layer 3. The amount of polishing liquid 58 supplied per unit time is, for example, 100 ml/min to 200 ml/min.

The polishing rate of the GaN layer 3 (i.e., the thickness removed per unit time) can be adjusted by adjusting the predetermined conditions. The predetermined conditions are determined according to, for example, the number of rotations per unit time of the chuck table 20 and the polishing wheel 44, the amount of acidic polishing liquid 58 supplied per unit time, the pressure from the polishing wheel 44 to the SiC substrate 1, etc.

Therefore, the time required for polishing to remove the GaN layer 3 can be calculated in advance based on the polishing rate of the GaN layer 3 and the thickness of the GaN layer 3. For example, when the pressure is 500 g/ ^cm2 , the supply rate of the polishing liquid 58 is 100 ml/min to 200 ml/min, and a GaN layer 3 of 0.5 μm is polished, the time required for removing the GaN layer 3 is calculated to be, for example, 10 minutes to 15 minutes.

By polishing the GaN layer 3 for the calculated time, the GaN layer 3 is removed from the surface 1a side (removal step (S30)). In the removal step (S30), an acidic polishing liquid 58 that functions as an oxidizing agent is supplied to the GaN layer 3, so that the surface of the GaN layer 3 is oxidized and altered.

The abrasive grains of the polishing pad 48 polish the surface of the altered GaN layer 3. As a result of the alteration of the GaN layer 3, it becomes easier to polish (i.e., the polishing rate of the GaN layer 3 is improved) compared to when the GaN layer 3 is not altered.

The removal step (S30) mainly removes the GaN layer 3, but in order to more reliably remove the GaN layer 3, the polishing time may be extended from the calculated time described above by, for example, several seconds to one minute. In this case, the surface 1a of the SiC substrate 1 is polished slightly.

After the removal step (S30), the SiC substrate 1 is cleaned using the spinner cleaning unit 60 (cleaning step (S40)). For example, the spinner table (not shown) is rotated while the back surface 1b of the SiC substrate 1 is held by the spinner table.

While the spinner table is rotating, a fluid such as pure water or two fluids consisting of pure water and air are sprayed onto the surface 1a of the SiC substrate 1, thereby removing the acidic polishing liquid 58, debris, etc. from the SiC substrate 1.

In this embodiment, the GaN layer 3 on the front surface 1a side is polished using a polishing wheel 44 to remove the GaN layer 3 without performing blasting or lapping. This reduces the number of steps required to regenerate the SiC substrate 1.

In addition, since blasting or lapping is not performed, a damaged layer is not formed on the SiC substrate 1 when the GaN layer 3 is removed. Therefore, the thickness of the SiC substrate 1 that is removed can be reduced compared to when blasting or lapping is performed. Therefore, the number of times the SiC substrate 1 can be regenerated can be increased.

However, when polishing the surface 1a side with a polishing pad 48 that does not have abrasive grains while supplying a polishing liquid 58 containing abrasive grains to the surface 1a side, the polishing rate becomes relatively low. In contrast, in this embodiment, a polishing pad 48 with abrasive grains fixed thereto (i.e., fixed abrasive grains) is used. Therefore, the polishing rate can be improved compared to the case where the GaN layer 3 on the surface 1a side is polished with a polishing pad 48 that does not have abrasive grains while supplying a polishing liquid 58 containing free abrasive grains.

Furthermore, in this embodiment, the GaN layer 3 is polished while altering the surface of the GaN layer 3 by supplying an acidic polishing liquid 58. In this way, a relatively high polishing rate can be achieved by the synergistic effect of the fixed abrasive grains of the polishing pad 48 and the acidic polishing liquid 58.

The structures, methods, etc. according to the above embodiments can be modified as appropriate without departing from the scope of the present invention. For example, the epitaxial growth layer formed on the surface 1a is not limited to the GaN layer 3.

The _epitaxially grown layer may be aluminum nitride ( _AlN ), indium nitride (InN), etc., each of which is represented by _InxAlyGazN (0≦x<1, 0≦y<1, 0≦z<1, x+y+z=1). Also, an epitaxially grown layer in which nitride layers of different compositions are stacked may be formed on the surface 1a.

Incidentally, if an acidic polishing liquid 58 is used, it is not necessary to use a polishing wheel 44 with abrasive grains fixed to a polishing pad 48. Although the polishing rate will be lower than when a polishing pad 48 with abrasive grains fixed thereto is used, abrasive grains may be added to the acidic polishing liquid 58, and the GaN layer 3 may be polished with a polishing pad 48 that does not have abrasive grains. In this case, too, the number of steps required to regenerate the SiC substrate 1 can be reduced, and the number of times the SiC substrate 1 can be regenerated can be increased.

1 SiC substrate 1a front surface 1b rear surface 2 polishing device 3 GaN layer (nitride layer)
4 Base 4a Opening 4b Support 5 Protective tape 6a, 6b Cassette placement table 7 Tape-attached substrate 8a, 8b Cassette 10 Transport robot 12 Positioning table 14 Carry-in mechanism (loading arm)
16 Unloading mechanism (unloading arm)
18 X-axis moving table 20 Chuck table 20a Holding surface 20b Flow path 22 Porous plate 24 Loading/unloading area 26 Polishing area 28 Z-axis guide rail 30 Z-axis moving plate 32 Z-axis ball screw 34 Z-axis pulse motor 36 Polishing unit 38 Spindle housing 40 Spindle 42 Mount 44 Polishing wheel 46 Base 48 Polishing pad 50 Flow path 52 Polishing liquid supply source 54 First polishing liquid supply path 56 Second polishing liquid supply path 58 Polishing liquid 60 Spinner cleaning unit

Claims (1)

A method for regenerating a SiC substrate having a nitride layer formed on a front surface side, comprising the steps of:
a holding step of holding the back side of the SiC substrate on a holding surface of a chuck table of a polishing apparatus with the nitride layer exposed upward ;
a removing step of polishing the front surface side of the SiC substrate using a polishing pad having abrasive grains fixed thereto in the polishing apparatus while supplying an acidic polishing liquid not having the abrasive grains to the nitride layer , thereby removing the nitride layer;
a cleaning step of cleaning the SiC substrate;
The acidic polishing liquid includes an aqueous solution of sodium permanganate or an aqueous solution of potassium permanganate,
the nitride layer is a GaN layer;
The method for regenerating a SiC substrate , wherein the removing step comprises polishing and removing the GaN layer with abrasive grains made of silica .