JP2006347776A - Sapphire substrate and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide sapphire substrates with which the fluctuation in warpage between the substrates can be avoided before epitaxial growth, during epitaxial growth and after epitaxial growth and further, the sapphire substrates each having a nitride semiconductor film and a desired warpage can be obtained after epitaxial growth. <P>SOLUTION: The sapphire substrate has a diameter of ≥3 in and a main surface (used for growth) and a back surface (of the opposite side to the main surface) which are free from strain. Further, the main surface is warped into a recessed form having a recessed degree within a range of 8-50 μm so that the warpage of the sapphire substrate having a nitride semiconductor film becomes a desired warpage after epitaxial growth. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  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. Also, the temperature of the main surface of the sapphire substrate varies between the substrates.

  Such non-uniformity of the main surface temperature of the sapphire substrate causes a variation 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.

  By the way, when a GaN film is grown on a sapphire substrate whose warpage has been relaxed by the above-described method, although there is a difference in the diameter of the sapphire substrate, a warp that usually has a convex of 30 to 50 μm occurs on the main surface side. Cause cracks.

  This is because the variation in the surface temperature of the sapphire substrate that occurs at the initial stage of epitaxial growth and the variation in temperature distribution between the substrates used can be reduced by using a substrate with relaxed warpage, but with the increase in the thickness of the GaN film to be epitaxially grown, Due to the difference in the lattice constant between the sapphire crystal and the GaN crystal, a warp in which the epitaxial growth surface side is convex occurs, and the variation in the substrate surface temperature increases.

In order to prevent this, the heat treatment of the sapphire substrate is intentionally caused by epitaxial growth by performing the heat treatment under a condition that is weaker than the condition in which the warpage of the sapphire substrate is completely relaxed, for example, at a temperature lower than that in which the warpage is completely relaxed. Attempts have been made to produce a sapphire substrate having a warp opposite to the warp (see Patent Document 2).
JP-A-57-095899 JP 2004-168622 A

  In accordance with the above method, heat treatment is performed at a temperature lower than that in which the warpage is completely relaxed in advance, and warpage that cancels out warpage that occurs during epitaxial growth, that is, warpage in the opposite direction to warpage that occurs during growth, that is, the growth surface is concave. In order to manufacture a sapphire substrate having a warp, it is necessary to measure the strain of the sapphire substrate before the heat treatment and change the heat treatment temperature according to the strain, which is not suitable for mass production and causes an increase in substrate cost. In addition, the temperature at the time of epitaxial growth is usually higher than the heat treatment temperature, and the strain left intentionally is relaxed during the epitaxial growth, so that it is possible to obtain a sapphire substrate with a nitride semiconductor film having a desired warping amount after epitaxial growth. It is a difficult technique.

  An object of the present invention is to provide a sapphire substrate capable of stably mass-producing nitride semiconductor light-emitting devices with excellent characteristics. Specifically, there is no variation in warpage between substrates before, during and after epitaxial growth. The present invention also provides a sapphire substrate that makes it possible to obtain a sapphire substrate with a nitride semiconductor film having a desired warp after epitaxial growth, and a method for stably obtaining such a sapphire substrate at low cost.

  As a result of diligent research to solve the above-mentioned problems, the inventor of the present invention is excellent in mass productivity, has no warpage variation between substrates before, during and after epitaxial growth, and has a desired anti-epitaxial property after epitaxial growth. The attached sapphire substrate and its manufacturing method have been found and the present invention has been achieved.

  In other words, the sapphire substrate of the present invention described in claim 1 is a sapphire substrate having a diameter of 3 inches or more, and has a main surface (a growth surface is a main surface and an opposite surface is a back surface) and a back surface. There is no distortion, and the main surface is warped in a concave shape within the range of 8 to 50 μm so that the warpage amount of the sapphire substrate having the nitride semiconductor film after epitaxial growth becomes a desired warpage amount. It is the board | substrate characterized by these.

The invention according to claim 2 is the sapphire substrate whose back surface is either a lap surface or a blast surface in addition to the invention according to claim 1.
The invention according to claim 3 is a method for producing the sapphire substrate according to claim 1 or 2, wherein a single crystal is cut to obtain a wafer, and the surface of the wafer is polished to obtain a single crystal. When obtaining the sapphire substrate, 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, and the polished wafer is processed at a temperature of 1300 to 1700 ° C. A method for producing a single crystal sapphire substrate, comprising: a step of heat-treating; and a step of subjecting the heat-treated wafer to a mirror surface by mechanochemical polishing of a main surface with the back surface of the wafer as a flat reference.

  According to the present invention, it is possible to stably and inexpensively manufacture a sapphire substrate having a diameter of 3 inches or more that has no residual processing distortion on the back surface and the main surface is warped in a concave shape in the range of 8 to 50 μm. This makes it possible to reduce variations in wavelength and electrical characteristics in a nitride semiconductor light emitting device manufactured using such a sapphire substrate. In addition, by arbitrarily setting the amount of warpage of the sapphire substrate at 8 to 50 μm, the amount of substrate warpage after epitaxial growth can be controlled, leading to stabilization of 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 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 cause of warpage occurring during epitaxial growth can be simplified so that it can be almost analyzed by the relationship between the crystal lattice constant of the sapphire substrate and the crystal lattice constant of the nitride semiconductor to be epitaxially grown, and the thickness of the nitride semiconductor film, It is easy to estimate the amount of warpage generated by epitaxial growth. As a result, it is possible to 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 so as to cause a problem.

  Of course, if the warp of the main surface of the sapphire substrate is in the same direction as the warp caused by epitaxial growth, the warp caused by epitaxial growth can be mitigated if the concave degree is less than 8 μm even in the opposite direction. In other words, the substrate may be destroyed. On the other hand, when the curvature of the main surface of the sapphire substrate is more than 50 μm in the direction opposite to the warp caused by epitaxial growth, the temperature distribution in the substrate surface varies, particularly in the initial stage of epitaxial growth, and the composition of the nitride semiconductor film obtained As a result, the variation in the emission wavelength of the light emitting element and the variation in the electrical characteristics become large, and a good light emitting element cannot be stably mass-produced.

  In order to obtain the above-described sapphire substrate of the present invention, a single crystal is cut to obtain a wafer, and the main surface of this wafer is fixed to a block using, for example, wax, and the concave degree after completion of the substrate is 8 to 50 μm. Polish to a convex shape. This surface is the back surface. Next, the wafer is removed from the block, the wax is removed, and heat treatment is performed at 1300 to 1700 ° C. This heat treatment can remove strain remaining on the back surface. Thereafter, the back surface is fixed to the block in a flat state with wax or the like, the main surface is polished, and finally the main surface is mechanochemically polished to be a mirror surface.

  By using a mirror finish, processing distortion due to polishing is usually removed. Therefore, after the main surface is mirror-polished, the wafer is removed from the block, and the wax or the like is removed from the wafer surface, whereby a sapphire substrate having no distortion on the main surface and the back surface of the present invention can be obtained.

  Further, the present invention will be described with reference to the drawings. FIG. 1 is a central sectional view of an example of a fire substrate according to the present invention. The concave degree refers to a shown in FIG. 1, and in the present invention, a is 8 to 50 μm. The thicker the sapphire substrate, the smaller the change in warpage during epitaxial growth, and the less the variation in the temperature distribution in the substrate surface during epitaxial growth. This is preferable, but it is preferably 600 μm or less in order to increase the substrate cost.

  On the other hand, when the substrate thickness is less than 300 μm, the warp change during epitaxial growth becomes large, temperature variation within the substrate surface is likely to occur, the composition of the nitride semiconductor layer is easily affected, and the film is manufactured from the obtained film. The variation in wavelength and electrical characteristics of the light-emitting element increases, and it becomes difficult to obtain a good light-emitting element in a stable and high yield. Therefore, the thickness of the sapphire substrate is preferably 300 to 600 μm. When the difference in strain between the front surface and the back surface of the wafer obtained by cutting is large, it is preferable to perform the following steps after heat treatment to remove the wafer strain.

  FIG. 2 is a process diagram illustrating the production method of the present invention. In the cutting step, for example, a cylindrical single crystal is cut with a wire saw or the like to produce a wafer. In the surface attaching step, the wafer surface, which will be the main surface in the future, is attached to the block with wax without deforming the surface. And in a back surface single-sided lapping process, the surface of a wafer is grind | polished using a loose abrasive grain. At this time, the degree of curvature of the surface is set so that the concave degree of the finished substrate becomes a desired value within the range of 8 to 50 μm.

  Next, the wafer is peeled off from the block, the wax adhering to the wafer is removed, and the wafer is held in the temperature range of 1300 to 1700 ° C. for 1 hour or longer to remove the distortion of the back surface caused by polishing. Whether the distortion has been removed can be confirmed by confirming that the back surface shape of the wafer attached to the block after the back surface single-sided lapping process is the same as the wafer back surface shape after the heat treatment. There is no problem if the temperature of the heat treatment is 1300 ° C. or higher at which the strain is removed, but if it exceeds 1700 ° C., metal attached to the surface inside the substrate diffuses into the wafer and becomes an impurity, This is not preferable because the holding jig and the wafer are fused to cause damage to the wafer.

  If desired, the back surface is blasted after the back surface polishing. Such a method has the advantage that, for example, the roughness of the back surface can be made uniform, and variations in temperature distribution during epitaxial growth can be reduced. In order to make the state of the back surface more uniform, the back surface may be mechanochemically polished before blasting.

  In the next back surface attaching step, the back surface of the heat-treated wafer is attached to a block so that the back surface becomes flat with wax, and mechanochemical polishing is performed so that the main surface becomes flat. 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. Normally, the mechanochemical polishing removes the distortion of the processed surface, so that no processing distortion is seen 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.

  In the sapphire substrate obtained by such a method, the warpage shape of the main surface is concave, the warpage amount is the convex amount of the back surface in the first step, and there is no residual processing distortion on 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, the main surface and the back surface had unevenness of about 20 μm parallel to the wire due to the use of a wire saw.
Next, the main surface was affixed on the surface of the ceramic block with wax as a first step. At this time, the wafer was not deformed by pressurization or the like. Then, the back surface was lapped with 75 μm single-sided with a GC # 320 abrasive grain using a concave surface plate to form a convex shape with a concave degree of 10 μm when attached to the ceramic block.
Next, after the wafer was peeled off from the ceramic block to remove the wax, as a second step, heat treatment was performed by holding at 1500 ° C. for 1 hour. In the state where the substrate before the heat treatment was peeled off from the ceramic block, the warpage of the back surface was approximately 50 μm due to processing distortion, but after the heat treatment, the warp was 10 μm as planned. As a result, it was found that the processing distortion was completely eliminated.
Next, after chamfering the outer peripheral portion of the substrate, as a third step, wax was applied to the back surface of the wafer after the heat treatment and pressed firmly against the ceramic block, and fixed to the ceramic block so that the back surface of the wafer was flat. Then, the main surface was mechanochemically polished to 75 μm to make the main surface 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.
Thereafter, the wafer was peeled off from the ceramic block to remove the wax, and a single-sided sapphire substrate having a main surface as a mirror surface, a flatness of 5 μm or less, a warp of 10 μm in warpage, 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 by using the following vapor phase epitaxial method.
The warpage of the obtained substrate with a GaN single crystal film was 25 μm, and the substrate was not damaged.
When a nitride semiconductor light emitting device is manufactured using the sapphire substrate obtained according to the present invention, it is possible to reduce variations in wavelength and electrical characteristics of the light emitting device, and at the same time, reduce adverse effects due to substrate warpage in the device process.

(Example 2)
A sapphire substrate was prepared in the same manner as in Example 1 except that the heat treatment temperature was 1300 degrees.
The obtained sapphire substrate has a mirror surface with a surface roughness of about 1 angstrom, a flatness of 5 μm or less, a warp of 10 μm, and a thickness of 600 μm. There was no difference.
In the state where the substrate before the heat treatment was peeled off from the ceramic block, the warpage of the back surface had a concave degree of about 49 μm due to processing distortion, but after the heat treatment, as in Example 1, the planned concave degree of 10 μm was obtained. It turned out to be warped, and it was found at a glance that the processing distortion was completely eliminated.
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 almost the same as in Example 1, and the substrate was damaged. There wasn't.
When a nitride semiconductor light emitting device is manufactured using the sapphire substrate obtained according to the present invention, it is possible to reduce variations in wavelength and electrical characteristics of the light emitting device, and at the same time, reduce adverse effects due to substrate warpage in the device process.

(Example 3)
A sapphire substrate was prepared in the same manner as in Example 1 except that the heat treatment temperature was 1700 degrees.
The obtained sapphire substrate has a mirror surface with a surface roughness of about 1 angstrom, a flatness of 5 μm or less, a warp of 10 μm, and a thickness of 600 μm. There was no difference.
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 almost the same as in Example 1, and the substrate was damaged. There wasn't.
In the state where the substrate before the heat treatment was peeled off from the ceramic block, the warpage of the back surface had a concave degree of about 49 μm due to processing distortion, but after the heat treatment, as in Example 1, the planned concave degree of 10 μm was obtained. It turned out to be warped, and it was found at a glance that the processing distortion was completely eliminated.
Moreover, the situation that the impurity of the surface diffuses inside the sapphire substrate by the heat treatment did not occur.
When a nitride semiconductor light emitting device is manufactured using the sapphire substrate obtained according to the present invention, it is possible to reduce variations in wavelength and electrical characteristics of the light emitting device, and at the same time, reduce adverse effects due to substrate warpage in the device process.

(Comparative Example 1)
A sapphire substrate was prepared in the same manner as in Example 1 except that the heat treatment temperature was 1800 degrees. However, the holder used at the time of heat treatment and the sapphire wafer were partially fused, and a sapphire substrate could not be obtained.

(Conventional example)
In the same manner as in the example, a sapphire single crystal wafer having a diameter of 3 inches and a thickness of about 750 μm was obtained in which the main surface and the back surface had irregularities of about 20 μm parallel to the wire.
Next, this wafer was subjected to double-sided lapping. 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.
Although the amount of warpage of the obtained single crystal sapphire wafer was 5 μm or less, the shape was not necessarily unified to a concave or convex shape, and a stress of about 1.0 × 10 ⁹ dyn / cm ^{2 on} both sides of the substrate. It was a thing with.
Next, the obtained wafer was 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 both surfaces of the wafer was relaxed to about 7.0 × 10 ⁸ dyn / cm ² by heat treatment.
Thereafter, the main surface of the single crystal sapphire substrate was mechanochemically polished 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. Further, the obtained sapphire group had a main surface with mirror polishing, so that the stress on the main surface disappeared and the concave surface had a concave shape of about 40 μm due to the stress remaining on the back surface.
Even if the sapphire substrate obtained is used to produce a nitride semiconductor light emitting device, it is possible to reduce the variation in wavelength and electrical characteristics of the light emitting device and at the same time reduce the adverse effects due to substrate warpage in the device process. Since the degree of recession varies depending on the stress remaining on the back surface, the disadvantage that the heat treatment conditions must be changed according to the stress value so that the stress is measured after double-sided lapping processing and the desired recession degree after heat treatment is inevitable. . In addition, if the epitaxial growth temperature and time at the time of fabricating the nitride semiconductor light emitting device are different, there is a difference in the strain relaxation amount on the back surface of the substrate at the time of epitaxial growth, resulting in a larger variation in wavelength and electrical characteristics of the light emitting device than in the examples. .
In addition, the method of this example requires a heat treatment of 10 hours, and is extremely disadvantageous in terms of mass production at a stable and inexpensive sapphire substrate as compared with the method of the present invention that requires a heat treatment time of 1 hour. I understand.

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 (3)

The main surface is a sapphire substrate having a diameter of 3 inches or more, the main surface has no distortion on the main surface and the back surface, and the warpage amount of the sapphire substrate having a nitride semiconductor film after epitaxial growth becomes a desired warpage amount, A sapphire substrate having a concave degree of 8 to 50 μm and warping in a concave shape. The sapphire substrate according to claim 1, wherein the back surface is either a lapping surface or a blast surface. When a sapphire single crystal is cut to obtain a wafer, and the surface of the 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 8 to 50 μm. A process of polishing into a shape, a process of heat-treating the polished wafer at a temperature of 1300 to 1700 ° C., and a main surface of the heat-treated wafer as a mirror surface by using the back surface of the wafer as a flat reference to make a mirror surface A process for producing a sapphire substrate.

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