JP2007088193A - Sapphire substrate and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a sapphire substrate with a nitride semiconductor film which is free from dispersion in warp among substrates before epitaxial growth, during epitaxial growth and after epitaxial growth, and has desired warp after epitaxial growth. <P>SOLUTION: When a single crystal sapphire substrate is obtained by polishing a surface of a wafer obtained by cutting a sapphire single crystal; the rear surface of the wafer is made a mirror surface by performing chemical mechanical polishing for it to a convex form to obtain a concave degree of a desired value in the range of 8 to 50 μm after the substrate is completed, the rear surface is thereafter fixed in its plane state and is subjected to chemical mechanical polishing, so that its major surface is parallel to the rear surface and is made a 300 to 600 μm-thick mirror surface. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a sapphire substrate and a method for manufacturing the same, and more particularly to a method for manufacturing a sapphire substrate suitable for crystal growth for a nitride semiconductor light emitting device.

  The sapphire substrate is used as a substrate for epitaxial growth of a nitride semiconductor light emitting device. When epitaxially growing a nitride semiconductor light emitting device using a sapphire substrate, if the sapphire substrate has variations in warpage or thickness, the in-plane temperature distribution on the main surface of the sapphire substrate will not be affected by the warpage shape, amount of warpage, or variations in thickness. Produces uniformity. Further, if there are warping or thickness variations between the substrates, the temperature between the main surfaces of the sapphire substrates varies as described above.

  Such non-uniformity of the in-plane temperature of the main surface of the sapphire substrate causes variations in the composition of the epitaxially grown nitride semiconductor film. In the light-emitting element using the nitride semiconductor film having a non-uniform composition obtained in this manner, the output wavelength and electrical characteristics of the light-emitting element vary.

  As for warping mitigation, a method of heat-treating a sapphire substrate at a temperature of 1150 to 1400 ° C. has already been disclosed, thereby making it possible to almost completely alleviate warping of the sapphire substrate ( Patent Document 1). This method has been implemented and its effectiveness has been confirmed.

  However, when a GaN film is grown on the surface of the sapphire substrate with the warpage alleviated by the above method, there is a warp that usually has a convex shape with the main surface protruding 30 to 50 μm, although there is a difference depending on the diameter of the sapphire substrate. In the worst case, it causes cracking of the sapphire substrate. This is because, as the thickness of the epitaxially grown GaN film increases, warpage in which the epitaxial growth surface side is convex occurs due to the difference in lattice constant between the sapphire crystal and the GaN crystal.

  In order to prevent this, heat treatment of the sapphire substrate is performed under conditions that are weaker than the conditions in which the warp of the sapphire substrate is completely relaxed, for example, a temperature lower than the temperature at which the warp is completely relaxed, and the warp that is intentionally caused by epitaxial growth. Attempts have been made to produce a sapphire substrate having warpage in the opposite direction (see Patent Document 2).

  In order to produce a sapphire substrate having a warped epitaxial growth surface according to the method described in Patent Document 2, it is necessary to measure the strain of the sapphire substrate before the heat treatment and to change the heat treatment temperature according to the obtained strain amount. There is. Therefore, this method is poor in mass productivity and causes an increase in substrate manufacturing cost. In addition, the temperature during epitaxial growth is usually higher than the heat treatment temperature, and the strain left intentionally is relaxed during the epitaxial growth, so that a sapphire substrate with a nitride semiconductor film that warps by a desired amount of warpage after epitaxial growth is obtained. That is a difficult technique.

  In order to solve such a problem, the present inventor is a sapphire substrate having a diameter of 3 inches or more, wherein the main surface is warped in a concave shape within a concave degree range of 8 to 50 μm. It was shown that a sapphire substrate is effective (see Patent Document 3).

When manufacturing this sapphire substrate, a sapphire single crystal is cut to obtain a wafer, and the back surface of the wafer is polished into a convex shape so that the concave degree after completion of the substrate is 8 to 50 μm. The polished wafer is heat-treated at a temperature of 1300 to 1700 ° C. to eliminate distortion, and the main surface is mechanochemically polished to make a mirror surface using the back surface of the obtained wafer as a flat reference.
JP-A-57-095899 JP 2004-168622 A Japanese Patent Application No. 2005-1718392

The manufacturing method of the sapphire substrate includes a step of removing the processing strain by heat-treating the polished wafer at a temperature of 1300 to 1700 ° C. If the heat treatment step is not necessary, the sapphire substrate is manufactured more inexpensively and safely. It becomes possible.
An object of the present invention is to provide a sapphire substrate that does not require a process of removing a processing strain by heat-treating a polished wafer at a temperature of 1300 to 1700 ° C. and enables stable mass production of a nitride semiconductor light emitting device having excellent characteristics. Is provided at a lower cost.

  That is, the manufacturing method of the present invention is a method for manufacturing a sapphire substrate having a diameter of 3 inches or more, having no distortion on the main surface and the back surface, and having a concave degree of the main surface in the range of 8 to 50 μm. In this method, when a single crystal sapphire substrate is obtained by polishing the surface of a wafer obtained by cutting a sapphire single crystal, the back surface of the wafer has a desired value in the range of 8 to 50 μm in concave degree after completion of the substrate. A process of mechanochemical polishing to a convex shape to form a concave shape and a mirror surface, and then fixing the back surface to be in a flat state and mechanochemical polishing to make the main surface parallel to the back surface and mirror surface A sapphire substrate manufacturing method including a step of performing as a main step.

  In another aspect of the present invention, the thickness of the sapphire substrate obtained in addition to the above invention is 300 to 600 μm.

  According to the present invention, there is no need for a heat treatment step to remove the processing strain by heat-treating the polished wafer at a temperature of 1300 to 1700 ° C., there is no residual processing strain on the back surface and the main surface, and the main surface is It becomes possible to stably, inexpensively and safely manufacture a sapphire substrate having a diameter of 3 inches or more that warps in a concave shape with a desired value in the range of 8 to 50 μm. If a nitride semiconductor light emitting device is manufactured using such a sapphire substrate, it is possible to reduce variations in wavelength and electrical characteristics among the obtained nitride semiconductor devices.

  Further, by setting the warpage amount of the sapphire substrate to a desired value within the range of 8 to 50 μm, it is possible to control the substrate warpage amount after the epitaxial growth and to stabilize the device process.

  In the present invention, the degree of concave means the difference in height between the lowest part of the concave surface and the highest part of the concave circumference when the convex surface side is placed on a flat surface.

  In the sapphire substrate obtained by the method of the present invention, no strain remains on the main surface and the back surface, so that a situation in which the strain of the sapphire substrate is alleviated by the temperature during epitaxial growth does not occur. Therefore, the difference between the crystal lattice constant of the sapphire substrate and the crystal lattice constant of the nitride semiconductor to be epitaxially grown, and the relationship with the thickness of the nitride semiconductor film can be simplified so that the cause of warpage occurring during epitaxial growth can be analyzed. This makes it easy to estimate the amount of warpage generated. As a result, it is possible to easily estimate in advance how much concave shape the main surface side should have as a concave shape, and the shape after film growth does not deviate from a desired amount of warp to the extent that a problem occurs.

  When the concavo-convexity of the main surface of the sapphire substrate is less than 8 μm, the warp that occurs during epitaxial growth cannot be alleviated, and the substrate may be destroyed during epitaxial growth. On the other hand, when the concave degree of the main surface of the sapphire substrate exceeds 50 μm, the temperature distribution in the substrate surface varies at the initial stage of epitaxial growth, and the composition of the resulting nitride semiconductor film is affected and finally obtained. The variation in the emission wavelength and the variation in the electrical characteristics of the light emitting element increase. As a result, it is impossible to stably mass-produce good light emitting elements.

  In the present invention, in order to obtain the sapphire substrate described above, for example, the main surface of a wafer obtained by cutting a single crystal is fixed to a block using wax, and the back surface has a concave degree of 8 to 8 after completion of the substrate. Polishing into a convex shape so as to obtain a desired value in the range of 50 μm, followed by mechanochemical polishing to obtain a mirror surface. By mechanochemical polishing, the strain remaining on the back surface can be removed as in the case of heat treatment at 1300 to 1700 ° C.

  Thereafter, the back surface is fixed to the block in a flat state with wax or the like, the main surface is polished so as to be parallel to the back surface, and finally the mechanochemical polishing is performed to make the main surface a mirror surface.

Further, the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a central cross-sectional view of a fire substrate manufactured by the method of the present invention. Concaveness means a shown in FIG. 1, and in the present invention, a having a of 8 to 50 μm is produced.
The thicker the sapphire substrate, the less the change in warpage during epitaxial growth, so the concave degree can be reduced, and the variation in temperature distribution in the substrate surface during epitaxial growth is reduced. This is preferable, but the substrate cost is increased to 600 μm or less. It is preferable.

  On the other hand, when the substrate thickness is less than 300 μm, the warpage change during epitaxial growth becomes large, so that the degree of concaveness is inevitably increased, and temperature variation in the substrate surface during epitaxial growth is likely to occur, and the composition of the nitride semiconductor layer is increased. It is easily affected, and the variation in wavelength and electrical characteristics of the light-emitting element manufactured from the obtained film becomes large, making it difficult to obtain a good light-emitting element stably and in high yield. Therefore, the thickness of the sapphire substrate is preferably 300 to 600 μm.

  FIG. 2 is a process diagram illustrating the production method of the present invention. In the cutting step, for example, a cylindrical sapphire single crystal is cut with a wire saw or the like to produce a wafer. In the surface attaching step, the wafer surface as the main surface in the future is attached to the block with wax without deforming the surface. In the back surface polishing step, the back surface of the wafer is polished using loose abrasive grains, and then mechanochemically polished to obtain a mirror surface. Processing is performed so that the back surface after the mechanochemical polishing has a convex shape with a desired value in the range of 8 to 50 μm. By this mechanochemical polishing, the distortion of the processed surface is removed. In order to reduce the processing cost, the main surface of the wafer may be polished with diamond or the like before mechanochemical polishing, and then mechanochemical polishing may be performed. In the case of polishing using a commercially available wafer, the cutting step in FIG. 2 is not necessary.

  Next, the wafer is peeled off from the block, and the wax adhering to the wafer is removed.

  In the next back surface attaching step, the back surface of the mirror-polished wafer is attached to the block with wax so that the back surface becomes flat. In the main surface polishing step, the main surface is polished in the same manner as described above so as to be parallel to the back surface, and mechanochemical polishing is performed. Thereafter, the wafer is removed from the block, and the wax adhering to the wafer is removed.

  When the flatness of the main surface by mechanochemical polishing is 5 μm or less, it is preferable because thickness variation during epitaxial growth is reduced. Since the mechanochemical polishing removes the distortion of the processed surface, no processing distortion is observed on the main surface. In order to reduce the processing cost, the main surface of the wafer may be polished with diamond or the like before mechanochemical polishing, and then mechanochemical polishing may be performed.

  The sapphire substrate obtained by such a method has a concave warp shape on the main surface, and the degree of concavity is determined by the amount of protrusion on the back surface in the first step, and there is no processing distortion on either the main surface or the back surface.

  Further, the present invention will be described based on examples.

Example 1
A sapphire single crystal ingot having a diameter of 3 inches was cut with a wire saw to obtain a wafer having a c-plane main surface and a thickness of about 750 μm. At this time, there were approximately 20 μm concave and convex grooves parallel to each other due to the use of a wire saw on the main surface and the back surface.

Next, the main surface was affixed to the surface of the ceramic block with wax. At this time, the wafer was not deformed by pressurization or the like. Thereafter, the back surface was lapped with a 75 μm single side with a GC # 320 abrasive grain using a concave surface plate, and the back surface was mirror-polished by 75 μm mechanochemical polishing. The polishing amount at this time was 5 μm or more. The obtained back surface had a surface roughness of about 1 angstrom.
In addition, the back surface was made into the convex shape in which the concave degree after completion is 10 μm in a state of being stuck to the ceramic block.
Next, the wafer was peeled off from the ceramic block to remove the wax, and then the outer periphery of the substrate was chamfered. Thereafter, wax was applied to the back surface of the wafer, and pressed firmly against the ceramic block, and fixed to the ceramic block so that the wafer back surface was flat and parallel to the ceramic block. And it grind | polished like the above so that a main surface might become parallel with a back surface, and it was set as the mirror surface by 75 micrometers mechanochemical grinding | polishing. The polishing amount at this time was 5 μm or more. The surface roughness of the obtained main surface was about 1 angstrom.
Thereafter, the wafer was peeled off from the ceramic block to remove the wax, and a sapphire substrate having a mirror surface of the main surface and a back surface of 5 μm or less, a concave degree of 10 μm, and a thickness of 600 μm was obtained.

Next, a GaN single crystal film having a thickness of 3 μm was grown on the main surface of the sapphire substrate using a vapor phase epitaxial method.
The warp of the obtained substrate with a GaN single crystal film was a shape protruding to the main surface side, and the concave degree was 25 μm, but the substrate was not damaged.

  When a nitride semiconductor light emitting device is produced using the sapphire substrate obtained by the present invention, the variation in wavelength and electrical characteristics of the obtained light emitting device can be reduced, and the adverse effect due to the substrate warpage in the device process can be reduced. It is.

(Example 2)
Next, a sapphire substrate was prepared in the same manner as in Example 1 except that the thickness of the obtained substrate was 300 μm.
The obtained sapphire substrate has a main surface and a back surface with a mirror surface with a surface roughness of about 1 angstrom, a flatness of 5 μm or less, a concaveness of 10 μm, and a thickness of 300 μm. There was no difference from the one obtained.

  Next, a GaN single crystal film was grown by vapor phase epitaxy in the same manner as in Example 1, but the warpage of the obtained substrate with the GaN single crystal film was a shape protruding to the main surface side, and the concave degree was 30 μm. However, the substrate was not damaged.

  When a nitride semiconductor light emitting device is produced using the sapphire substrate obtained by the present invention, the variation in wavelength and electrical characteristics of the obtained light emitting device can be reduced, and the adverse effect due to the substrate warpage in the device process can be reduced. It is.

(Example 3)
A sapphire substrate was prepared in the same manner as in Example 1 except that the thickness of the obtained substrate was 450 μm.
The obtained sapphire substrate has a main surface and a back surface with a mirror surface having a surface roughness of about 1 angstrom, a flatness of 5 μm or less, a concaveness of 10 μm, and a thickness of 300 μm. There was no difference from what was obtained in.

  Next, a GaN single crystal film was grown by vapor phase epitaxy in the same manner as in Example 1, but the warpage of the obtained substrate with the GaN single crystal film was a shape protruding to the main surface side, and the concave degree was 27 μm. However, the substrate was not damaged.

  When a nitride semiconductor light emitting device is produced using the sapphire substrate obtained by the present invention, the variation in wavelength and electrical characteristics of the obtained light emitting device can be reduced, and the adverse effect due to the substrate warpage in the device process can be reduced. It is.

(Comparative Example 1)
A sapphire substrate was prepared in the same manner as in Example 1 except that the finally obtained thickness was 200 μm.
The obtained sapphire substrate has a main surface and a back surface with a mirror surface having a surface roughness of about 1 angstrom, a flatness of 5 μm or less, a concaveness of 10 μm, and a thickness of 200 μm. There was no difference from what was obtained in.

  Next, a GaN single crystal film was grown by vapor phase epitaxy as in Example 1. Although the obtained substrate with a GaN single crystal film was not damaged, the warpage was 50 μm, which was larger than each example.

  When a nitride semiconductor light emitting device is manufactured using the sapphire substrate obtained in this example, it is considered difficult to reduce adverse effects due to substrate warpage in the device process due to large variations in wavelength and electrical characteristics of the obtained light emitting device.

(Conventional example)
In the same manner as in Example 1, five wafers having a diameter of 3 inches and a thickness of about 750 μm, in which concave and convex grooves of about 20 μm exist parallel to each other on the main surface and the back surface, were obtained.
Next, both surfaces of these five wafers were polished. As the free abrasive grains at this time, abrasive grains having a particle diameter of about 60 μm, which is a mixture of SiC abrasive grains having relatively high hardness and B4C abrasive grains, were used. Further, as the surface plate, a cast iron plate having a surface plate flatness of about 10 μm with respect to the surface plate outer diameter of 630 mm was used, and the processing pressure was set to 60 g / cm ² or less.
The warpage amounts of the five wafers thus obtained were all 5 μm or less, but the shape was not necessarily unified to a concave or convex shape, and was about 1.0 × 10 ⁹ dyn / cm ^{2 on} both surfaces of the wafer. It was stressed.
Next, the five wafers were held at 800 ° C. for about 10 hours and subjected to heat treatment to alleviate processing strain and stress. At this time, the stress on the wafer surface was alleviated to about 7.0 × 10 ⁸ dyn / cm 2 by heat treatment.
Thereafter, the main surface of each wafer was mechanochemically polished to 75 μm to obtain a sapphire substrate as a mirror surface. The polishing amount at this time was 5 μm or more. The surface roughness of the obtained main surface was about 1 angstrom. Moreover, the obtained sapphire substrate had the main surface mirror-polished so that the stress on the main surface disappeared and the degree of depression varied from 35 to 45 μm due to the stress remaining on the back surface.

  Even if each sapphire substrate obtained in this way is used to produce a nitride semiconductor light emitting device, it is possible to reduce variations in wavelength and electrical characteristics of the resulting light emitting device, and at the same time, adverse effects due to substrate warpage in the device process. Although it is possible to reduce, in order to make the final substrate concaveness constant, the stress remaining on the surface after polishing on both sides is measured, and the heat treatment conditions according to the obtained stress value The disadvantage of having to adjust is inevitable.

In addition, when the temperature and time are different during epitaxial growth during the fabrication of a nitride semiconductor light emitting device using the substrate of this example, a difference occurs in the amount of strain relaxation on the back surface of the substrate during epitaxial growth. Variations and variations in electrical characteristics increase.
Furthermore, the method of this example requires a heat treatment for 10 hours as described above, and is extremely disadvantageous in that the sapphire substrate is mass-produced stably and inexpensively compared with the method of the present invention that does not require heat treatment. I understand that.

It is a figure which shows the 3 inch diameter sapphire substrate of this invention. It is a figure which shows the process of manufacturing the sapphire substrate of this invention.

Explanation of symbols

a: Concaveness

Claims (2)

A method for producing a sapphire substrate having a diameter of 3 inches or more, in which the main surface and the back surface are not distorted, and the main surface has a concave degree in the range of 8 to 50 μm. When the surface of the obtained wafer is polished to obtain a single crystal sapphire substrate, the back surface of the wafer is convex so that the concave degree after completion of the substrate is a desired concave value in the range of 8 to 50 μm. The main process includes a process of mirror polishing by mechanochemical polishing and a process of mirror polishing by fixing the back surface in a flat state and then making the main surface parallel to the back surface. A method for manufacturing a sapphire substrate. The manufacturing method according to claim 1, wherein the thickness of the obtained sapphire substrate is 300 to 600 μm.

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