JP5126108B2 - Nitride semiconductor substrate - Google Patents

Description

  The present invention relates to a nitride semiconductor substrate.

  In recent years, a low-dislocation free-standing gallium nitride single crystal grown by the HVPE (Hydride Vapor Phase Epitaxy) method or the like as a gallium nitride single crystal substrate used for a long-life blue laser, a high-intensity blue LED, or a high-performance electronic device The substrate is manufactured. The gallium nitride single crystal substrate is generally processed with a double-sided mirror, and becomes a transparent substrate when processed to have no processing strain. Therefore, it is necessary to distinguish between the front surface on which the semiconductor element is formed and the back surface that is not. Conventionally, various methods have been adopted for discriminating the front and back of a substrate.

  For example, there is a method of forming an index flat (IF). Generally, the front and back of the substrate cannot be distinguished only by the orientation flat (OF) indicating the crystal orientation. In addition, for a gallium nitride substrate that is transparent and difficult to see, simply chamfering the front side and the back side of the substrate makes the contour clear even if the chamfered shape has an inclined angle or an arc cross section. Although it can be seen, the front and back of the substrate cannot be distinguished. Therefore, an IF processed to a length different from that of the OF is formed on the outer periphery of the substrate (for example, see Patent Document 1).

  Further, there is a method of chamfering only one side of the end surface of the substrate. Of the front and back surfaces of the substrate, only the front surface side is chamfered, and the back surface side is left as it is without chamfering, so that the front and back surfaces are processed asymmetrically (for example, see Patent Document 2).

  Furthermore, there is a method of chamfering the notch portion. A notch is formed instead of the orientation flat (OF) indicating the crystal orientation. Since it is difficult to discriminate between the front and the back with a notch, the amount of chamfering between the front and back of the notch is further varied (see, for example, Patent Document 3).

  Although it does not discriminate between the front and back of the substrate, when grinding and thinning the backside of the wafer, the chamfering amount on the backside is reduced as a method of reducing stress on the wafer and suppressing cracks and cracks. There is a method (for example, refer to Patent Document 4) in which chamfering is performed so as to be larger than the surface side.

JP 2002-356398 A ([0008] to [0009], FIG. 6) JP-A-58-71616 (2nd page, FIG. 12, FIG. 13) JP 2000-331898 ([0010], FIG. 1) Japanese Patent Laying-Open No. 2005-251961 ([0011], FIG. 1 and FIG. 2)

However, when an IF is attached as in Patent Document 1, it is possible to visually discriminate the front and back, but nitride semiconductor substrates such as gallium nitride single crystal substrates are very expensive materials, and the substrate area is small. Since the size of the chip becomes smaller, the number of chips that can be acquired is reduced, which is a cause of high cost.
Further, in Patent Document 2, it is effective in the case of an opaque substrate, but in the case of a transparent substrate such as a nitride semiconductor substrate, it is not always effective to discriminate between the front and back surfaces when chamfering is performed on only one side. It was. Further, in the method of forming a chamfer at the notch portion as in Patent Document 3, unlike the case of Si or GaAs, it is difficult to form the notch itself in the case of a hard nitride semiconductor substrate such as gallium nitride. It was difficult to apply.

  Although it is conceivable to apply the technique of Patent Document 4 to a nitride semiconductor substrate so as to increase the amount of chamfering on the back surface side, dry etching is performed to remove processing distortion during chamfering processing. In this case, it is difficult for the gas to flow to the back surface, and the processing strain generated during the chamfering process remains, and there is a risk that cracking is likely to occur in the subsequent process due to the residual stress.

  Therefore, in the past, IF processing has been most effective for visually distinguishing the front and back surfaces of a transparent substrate.

  An object of the present invention is to provide a nitride semiconductor substrate that can be visually discriminated even if IF processing is omitted, and that can remove processing strain at an end face.

According to a main aspect of the present invention, in a transparent nitride semiconductor substrate in which a surface on which a semiconductor element is formed is a front surface, the opposite side of the front surface is a back surface, and the front surface and the surface connected to the back surface are end surfaces. The end surface is inclined from the front surface to the back surface, and in a cross section in the substrate thickness direction passing through the center of the front surface and the back surface of the nitride semiconductor substrate, an angle formed by the front surface and the end surface is θ1, Provided is a nitride semiconductor substrate characterized in that θ1> θ2 when the angle between the back surface and the end surface is θ2, and the difference between the angle θ1 and the angle θ2 is 10 ° or more. In this case, it is preferable that the end surface has a linear inclination or a shape having an arc. In the latter, in particular, it is preferable to have a shape having an arc that curves in a concave shape toward the center side of the nitride semiconductor substrate . The difference between the angle θ1 and the angle θ2 is preferably 60 ° or less.

  According to the present invention, it is possible to visually discriminate between the front and the back even if the IF processing is omitted, and it is possible to remove the processing distortion of the end face portion.

FIG. 5 is a cross-sectional view passing through the center of the substrate when an angle θ1 formed with the surface of the nitride semiconductor substrate according to an embodiment of the present invention is larger than an angle θ2 formed with the back surface. It is explanatory drawing which shows an example processed with the grindstone which has an inclination, in order to process into the cross-sectional shape of the nitride semiconductor substrate of one Embodiment of this invention. It is explanatory drawing of the dry etching which compared the nitride semiconductor substrate of one embodiment of this invention, and the nitride semiconductor substrate whose inclination of an end surface is (theta) 1 <(theta) 2. It is explanatory drawing which shows an example which processes a grindstone center axis | shaft upwards rather than substrate center thickness, in order to process into the cross-sectional shape of the nitride semiconductor substrate of one Embodiment of this invention. It is explanatory drawing which shows the definition of (theta) 1 and (theta) 2 with respect to the inclination processed into the shape with the arc of the nitride semiconductor substrate of one Embodiment of this invention. For an LED in which an epitaxially grown layer having an LED structure including a layer represented by AlxGa (1-x) N (1 ≧ x> 0) is stacked on a self-standing GaN substrate of a nitride semiconductor substrate according to an embodiment of the present invention. It is a schematic diagram which shows the cross-section of an epitaxial wafer.

  An embodiment of the present invention will be described.

FIG. 1 shows a cross-sectional view of the nitride semiconductor substrate of the embodiment. The nitride semiconductor substrate 10 is transparent, the surface on which the semiconductor element is formed is the front surface 11, the opposite side of the front surface 11 is the back surface 12, and the outer peripheral surface connected to the front surface 11 and the back surface 12 is the end surface 13. Has a surface. The front surface 11 and the back surface 12 form a pair of opposing surfaces. In such a nitride semiconductor substrate 10, the end surface 13 is inclined from the front surface 11 to the back surface 12. In addition, in the cross section in the substrate thickness direction passing through the centers of the front surface 11 and the back surface 12 of the nitride semiconductor substrate 10, the angle formed by the inclination of the front surface 11 and the end surface 13 is θ1, and the angle formed by the inclination of the end surface 13 of the back surface 12. When θ2 is θ2, the end face is processed into a shape satisfying θ1> θ2.

  Here, the end surface 13 is inclined from the front surface 11 to the back surface 12 because there is no vertical surface or uncut surface orthogonal to the front surface 11 and the back surface 12 and the entire end surface is processed and inclined. is there. Therefore, only the edge portion on the front surface 11 side is chamfered, the edge portion on the back surface 12 side is not chamfered, and a vertical surface remains on the end surface, and as a result, θ1> θ2 is satisfied. Is not included.

  By inclining the end face 13 from the front surface 11 to the back surface 12 and θ1> θ2 and setting the difference between θ1 and θ2 to 10 ° or more, it was found that the front and back sides can be discriminated even if IF processing is omitted. The nitride semiconductor is generally gallium nitride, for example, but may be other hexagonal systems such as AlN and AlGaN.

  In the present embodiment, the difference between the angle θ1 formed by the front surface 11 and the inclined surface and the angle θ2 formed by the rear surface 12 and the inclined surface is preferably 60 ° or less. The reason why the difference between the angle θ1 and the angle θ2 is 10 ° or more and 60 ° or less is that when the angle θ is smaller than 10 °, it is difficult to visually determine the inclination direction. This is because defects such as cracks and cracks during epitaxial growth begin to occur.

In the embodiment in which the inclination of the shape of the end face is a straight straight line, the end face processing is performed on the entire end face from the edge of the nitride semiconductor substrate 10 on the side of the surface 11 by a total grinding wheel 15 as shown in FIG. This can be done by grinding.
The overall grinding wheel 15 is provided such that its rotating shaft 16 is parallel to the rotating shaft of the suction stage 17 that sucks and rotates the nitride semiconductor substrate 10. The overall grinding wheel 15 is a grindstone having an inclined surface such that a grinding surface (end surface) 18 that is a contact surface with the nitride semiconductor substrate 10 is an inclined straight line that matches the inclined shape of the end surface of the substrate. used. The overall grinding wheel 15 is rotated at a high speed, and the nitride semiconductor substrate 10 is moved at right angles to the axial direction thereof, so that the grinding surface 18 of the overall grinding wheel 15 is brought into contact with the ridgeline on the front end surface of the nitride semiconductor substrate 10. Make contact. The nitride semiconductor substrate 10 is rotated slowly to complete the end face processing in one rotation. At this time, the coolant 20 is discharged from the coolant nozzle 19 toward the overall grinding wheel 15 to cool the overall grinding wheel 15 and remove grinding heat and chips generated during processing. As a result, the end face 13 of the nitride semiconductor substrate 10 can be processed into an inclined linear shape.
In the case of forming OF on the nitride semiconductor substrate 10, the nitride semiconductor substrate 10 is formed into the total grinding wheel 15 when the rotational position of the nitride semiconductor substrate 10 reaches a position where the OF is to be formed during the end face processing. The nitride semiconductor substrate 10 starts to move closer to (the nitride semiconductor substrate 10 is always rotating slowly at a constant speed), and half of the OF length is formed. The movement of the nitride semiconductor substrate 10 is started so that the substrate 10 moves away from the overall grinding wheel 15, and the formation of the OF is completed by stopping the movement of the nitride semiconductor substrate 10. After the movement of the nitride semiconductor substrate 10 is stopped, the end surface processing of the outer circumferential arc portion is resumed. As described above, the end surface processing of the outer circumferential arc portion, the OF formation, and the end surface processing of the OF can be realized while the nitride semiconductor substrate 10 is rotated once, that is, in one sequence.

In the case of the processing method shown in FIG. 2, the inclination angle of the end face 13 of the nitride semiconductor substrate 10 can be freely changed by preparing a general grinding wheel 15 corresponding to the inclination angle to be processed.

  As described above, the end surface 13 is inclined from the front surface 11 to the back surface 12 and θ1> θ2 is established, so that the end surface processing can be performed in one sequence. In the case of the conventional example in which both the front surface side and the back surface side of the nitride semiconductor substrate 10 are chamfered, the nitride semiconductor substrate is circularly processed (also called outer diameter processing) and OF / IF processing is performed in the first sequence. In the second sequence, chamfering is performed on the outer periphery of the arc and the front (or back) side of the OF, IF, and in the third sequence, chamfering is performed on the outer periphery of the arc and the back (or front) side of the OF, IF. It will be. For this reason, processing time becomes very long and apparatus capability is very low. In this respect, if the end face is processed into a linearly inclined shape while cutting the outer periphery as in the present embodiment, the end face processing can be performed in one sequence simultaneously with the outer diameter processing, so the processing time can be shortened, Processing device capability can also be increased.

  Further, when the substrate is chamfered, the grinding amount increases, so the life of the grindstone is shortened, which causes a high cost.However, according to the present embodiment, the grinding amount is reduced. Can improve.

As described above, nitride semiconductor substrate 10 whose end face is processed is usually subjected to dry etching in order to remove the processing distortion on the substrate surface and to remove the processing distortion generated during the end face processing. As shown in FIG. 3, the principle of dry etching is that an etching gas, for example, a chlorine-based gas is converted into plasma in a chamber 20a under a high vacuum, and ion species 21 of the etching gas is generated by plasma formation. The generated ion species 21 of the etching gas are accelerated by applying a high-frequency voltage to the cathode 22 on which the substrate is placed, and linearly collide with the substrate to cause a chemical reaction, and the substrate surface is etched. The ionized species 21 of the plasma-ized etching gas has a very high linearity and is difficult to go around the back surface of the substrate.
Therefore, in dry etching, the ion species 21 cannot efficiently wrap around the back surface in the cross-sectional shape of the substrate 14 processed so that the inclination of the end face of the nitride semiconductor substrate satisfies θ1 <θ2. For this reason, the processing distortion of the end face that occurred during the outer diameter and end face processing cannot be removed, and cracks and cracks are likely to occur in the subsequent process. However, when the end face according to the present embodiment is inclined from the front surface to the back face and the shape of the nitride semiconductor substrate 10 is θ1> θ2, the gas is reliably supplied to the entire end face, so that the processing strain can be removed, Generation of cracks and cracks can be effectively suppressed in the subsequent process.

Moreover, when implementing this invention, the inclination of an end surface does not necessarily need to be a straight straight line, but it can also be set as the shape where the inclination of an end surface has an arc. In this case, the end surface processing can be performed by grinding the end surface from the surface side edge end portion of the nitride semiconductor substrate 10 with a cylindrical grinding wheel 28 as shown in FIG. The cylindrical grinding wheel 28 is provided so that the rotation axis 26 is orthogonal to the rotation axis of the adsorption stage 17 that adsorbs and rotates the nitride semiconductor substrate 10. The rotation shaft 26 is positioned above the height of the substrate center thickness. The cylindrical grinding wheel 28 is rotated so as to be orthogonal to the substrate rotation direction. As the cylindrical grinding wheel 28, a grinding wheel having a cylindrical surface at an end surface to be in contact with the nitride semiconductor substrate 10 is used. The cylindrical grinding wheel 28 is rotated to move the nitride semiconductor substrate 10 at a right angle to the axial direction thereof, so that the grinding surface of the cylindrical grinding wheel 28 is brought into contact with the ridgeline of the front surface side end surface of the nitride semiconductor substrate 10. At this time, the coolant 20 is discharged from the coolant nozzle 19 toward the grindstone to cool the cylindrical grindstone 28 and remove grinding heat and chips generated during processing. Thereby, the end surface of the nitride semiconductor substrate 10 can be processed into an inclined shape having an arc.
In addition, when forming OF by this method, after finishing the end face processing, the rotation of the nitride semiconductor substrate 10 is stopped, and the portion of the nitride semiconductor substrate 10 where the OF is to be formed faces the cylindrical grinding wheel 28. Thus, the nitride semiconductor substrate 10 is positioned, the cylindrical grinding wheel 28 is rotated, and the nitride semiconductor substrate 10 is moved at right angles to the axial direction thereof to contact the cylindrical grinding wheel 28 to form the OF. In this way, it can be realized by two sequences.
In the case of the conventional example in which both the front surface side and the back surface side of the nitride semiconductor substrate are chamfered, for example, the outer diameter of the nitride semiconductor substrate is processed in the first sequence, and the outer peripheral portion is processed in the second sequence. Chamfering is performed on the front surface (or back surface) side, chamfering processing is performed on the back surface (or front surface) side of the outer peripheral portion in the third sequence, OF processing is performed in the fourth sequence, and OF portion is processed in the fifth sequence. Chamfering on the front surface (or back surface) side, chamfering on the back surface (or front surface) side of the OF section in the sixth sequence, IF processing in the seventh sequence, and IF section in the eighth sequence The front surface (or back surface) side is chamfered, and the back surface (or front surface) side of the IF portion is chamfered in the ninth sequence.

  In the case of the processing method shown in FIG. 4, by setting how much the rotating shaft 26 of the cylindrical grinding wheel 28 is processed at the upper or lower position from the center position of the substrate thickness, the inclination angle can be arbitrarily set. Can change.

  In this way, when the inclination of the end face is an arcuate shape, the substrate thickness d is generally about 250 to 450 μm and is not a large arc. However, unlike the case of a straight line, the definition of θ1 and θ2 is different. Becomes a problem. Here, as shown in FIG. 5, when the arc is drawn, the tangent 34 of the arc 32 at the intersection 33 between the straight line 31 passing through the center of the substrate thickness d and the arc 32 and the arc 32 is the surface 11. The angle formed is defined as θ1 and the angle θ2 formed with the back surface 12.

  The bond material of the grindstone used for chamfering of Si or GaAs is generally metal, but in the case of a nitride semiconductor substrate such as GaN, it is a hard material compared to Si or GaAs, and the abrasive grains are clogged. Since clogging is likely to occur, generally, a bond material using a resin or a ceramic material that is easy to spontaneously cut is used. At this time, since the shape of the grindstone is easily deformed, the cylindrical grindstone shown in FIG. 4 is used instead of the method using the general grinding grindstone shown in FIG. In contrast, it is preferable to adopt a method of processing the end face by rotating it vertically and tracing it.

  In this way, when the end surface is processed to be inclined while the outer periphery of the substrate is being cut, since the inclined shape is formed at the same time as the outer diameter processing, it is not necessary to operate the chamfering sequence, so the processing time can be greatly reduced. .

According to embodiments of the present invention, one or more of the following effects are provided.
(1) By tilting the end surface of the substrate from the front surface to the back surface and satisfying θ1> θ2, it is possible to visually distinguish the front and back surfaces of the nitride semiconductor transparent substrate even if IF processing is omitted. (2) Since the sequence for end face processing can be reduced as compared with the conventional case, it is possible to reduce the chamfering processing cost which is one factor of high cost.
(3) By setting the relationship between the chamfering angles of the front and back surfaces to θ1> θ2, the entire end surface of the substrate can be exposed to an etching gas during dry etching, so that the processing distortion of the end surface portion can be effectively removed. As a result, cracks and crack defects during epitaxial growth do not occur.
(4) Since the outer diameter machining and the end face machining need only be performed once, the throughput during the outer diameter machining is improved, so that the cost can be reduced. In addition, since the IF is not required for the transparent substrate, it is possible to eliminate the deterioration of the grindstone caused by IF processing and the reduction of the chip acquisition area, increase the number of chip acquisition, and reduce the cost.

  Next, examples will be described.

Example 1
In Example 1, a transparent gallium nitride substrate was produced, and the front and back surfaces were discriminated.
A gallium nitride thick film substrate was obtained by HVPE growth on a sapphire substrate with the C-plane as the surface. The front surface (Ga polar surface) of the thick film substrate was attached to a ceramic plate having a flat surface using wax so that the back surface (N polar surface) was a processed surface. Then, the back surface of the substrate was ground using a diamond abrasive wheel # 325 to obtain a flat surface. Next, while supplying the diamond slurry to the grooved tin surface plate, the surface plate and the pressed work were rotated to process the back surface of the substrate. Furthermore, the back surface of the substrate was polished using a general-purpose commercially available colloidal silica slurry. Then, the ceramic plate was heated to make the wax liquid, the workpiece was removed, and organic cleaning was performed.

  Next, it was attached to a ceramic plate having a flat surface using wax so that the surface of the substrate (Ga polar surface) was a processed surface. Then, the substrate surface was ground to a flat surface using a diamond abrasive wheel # 325. Next, the surface of the substrate was processed by rotating the surface plate and the pressed workpiece while supplying diamond slurry to the grooved tin surface plate. The diamond abrasive grain size was gradually changed to a fine one, and finally the substrate surface was finished to a flat mirror surface using 1 μm diamond abrasive grains. Furthermore, the ceramic plate was heated to make the wax liquid, the workpiece was removed, and organic cleaning was performed. Then, dry etching was performed to remove the processing strain on the surface (Ga polar surface), and a transparent gallium nitride substrate having a thickness of 300 μm was obtained.

  Next, the end surface was processed to be inclined from the front surface to the back surface while the outer periphery was cut by the processing method shown in FIG. With this method, an inclined shape is formed at the same time as the outer diameter machining, and it is not necessary to operate a chamfering sequence, so that the machining time can be greatly reduced. The substrate was divided so as to pass through the center of the substrate, and the cross section was observed with a microscope to determine the tilt angle. In this way, samples having various angles θ1 and θ2 shown in Table 1 were prepared. Next, 30 test subjects were collected, and a discrimination experiment was performed to determine which direction the sample was inclined without teaching the front and back. As a result, when the angle of the end surface where the end surface was inclined from the front surface to the back surface was θ1> θ2 and the difference in inclination was 10 ° or more, all 30 people who performed visual determination could be determined accurately.

(Example 2)
In Example 2, an LED epitaxial layer was further grown on the gallium nitride substrate obtained in Example 1, and cracks and cracks generated on the outer periphery of the substrate were examined.
A transparent gallium nitride substrate having a thickness of 300 μm was obtained in the same manner as in Example 1. Next, it was processed into an inclined shape while cutting the outer periphery by the processing method shown in FIG. The substrate was divided so as to pass through the center of the substrate, and the cross section was observed with a microscope to determine the tilt angle. In the same manner, five samples shown in Table 2 were prepared. Each sample was subjected to an etching process, and after removing the processing distortion of the inclined portion, organic cleaning was performed.

Since the substrate end face is inclined from the front surface to the back surface and is processed into a shape satisfying θ1> θ2, the etching gas is sufficiently supplied, so that the processing strain generated during the outer periphery processing is removed. An epitaxial layer for LED having a structure shown in FIG. 6 was grown on these substrates by using a reduced pressure MOVPE method. The grown layers are, in order from the gallium nitride substrate 1, the Si-doped n-type GaN buffer layer 2, the Si-doped n-type Al _0.15 GaN cladding layer 3, the three-period InGaN-MQW layer 4, and the Mg-doped p-type Al _{0. .15} GaN cladding layer 5, Mg-doped p-type Al _0.10 GaN cladding layer 6 and Mg-doped p-type GaN contact layer 7.

  At that time, the number of cracks and cracks on the outer periphery of the substrate was examined. As a result, no cracks or cracks occurred in the five substrates for the difference between θ1 and θ2 up to 60 °, but one or two cracks or cracks in the samples of 70 ° to 90 ° occurred.

DESCRIPTION OF SYMBOLS 10 Nitride semiconductor substrate 11 Front surface 12 Back surface 13 End surface 15 Total grinding wheel 17 Adsorption stage 21 Ion species

Claims (3)

In a transparent nitride semiconductor substrate having a surface on which a semiconductor element is formed as a front surface, a side opposite to the surface as a back surface, and a surface connected to the front surface and the back surface as an end surface,
The end surface is inclined linearly from the front surface to the back surface;
In the cross section in the substrate thickness direction passing through the center of the front and back surfaces of the nitride semiconductor substrate,
When the angle between the front surface and the end surface is θ1, and the angle between the back surface and the end surface is θ2, θ1> θ2, and the difference between the angle θ1 and the angle θ2 is 10 ° or more and 60 ° or less. A nitride semiconductor substrate, characterized in that In a transparent nitride semiconductor substrate having a surface on which a semiconductor element is formed as a front surface, a side opposite to the surface as a back surface, and a surface connected to the front surface and the back surface as an end surface,
The end surface is inclined in a shape having an arc from the front surface to the back surface ;
In the cross section in the substrate thickness direction passing through the center of the front and back surfaces of the nitride semiconductor substrate,
When the angle between the front surface and the end surface is θ1, and the angle between the back surface and the end surface is θ2, θ1> θ2, and the difference between the angle θ1 and the angle θ2 is 10 ° or more and 60 ° or less. nitride semiconductor substrate according to claim <br/> that is. 3. The nitride semiconductor substrate according to claim 2, wherein the slope of the end face has a shape having an arc that is concavely curved toward the center side of the nitride semiconductor substrate.

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