JP2007103427A - Method of manufacturing zinc oxide single crystal substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a step terrace structure on the surface of a zinc oxide single crystal substrate (ZnO wafer). <P>SOLUTION: The method of manufacturing a zinc oxide single crystal substrate includes a cutting step to cut a vertical plane to the C axis of a zinc oxide single crystal at a specified thickness so as to form it into an element formation plane, a shaping step to shape the element formation plane to a specified shape, a rough polishing step to polish the element formation plane, a heating step to apply a heat at least to the element formation plane, and a fine polishing step to polish the element formation plane by the chemical mechanical polishing method thereafter. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a zinc oxide semiconductor material, and more particularly to a method for manufacturing a zinc oxide single crystal substrate (wafer) for forming an electronic device.

  In recent years, zinc oxide (ZnO) single crystal wafers are attracting attention as substrates for forming electronic elements including semiconductor elements. Fields of particular interest include, for example, blue light emitting diode (LED) and blue laser diode substrates. In other words, the gallium nitride (GaN) formed on this substrate has the same crystal structure and close lattice constant (lattice mismatch is about 2%), and can form a high quality epitaxial layer and has been conventionally used. Unlike a sapphire substrate, an electrode can be directly drawn out from a zinc oxide single crystal substrate. Furthermore, since zinc oxide can be easily dissolved, it has excellent characteristics as a film formation substrate. For this reason, the expectation as a film-forming board | substrate of gallium nitride etc. replacing the sapphire and silicon carbide (SiC) currently used is increasing (refer nonpatent literature 1).

In the interest of such a zinc oxide single crystal substrate, various researches and developments have been made in order to improve the properties as a substrate (wafer), for example, to improve both the electrical conductivity and the flatness of the substrate. In addition, a technique for performing a heat treatment in two stages (see Patent Document 1) and a technique for performing a heat treatment after polishing the C-plane of a zinc oxide single crystal to order the surface atomic arrangement (see Patent Document 2) are disclosed. Yes.
JP 2005-39131 A JP 2005-67988 A Maeda, Sato, Niikura, “Growing ZnO single crystals by hydrothermal synthesis” Text of the 120th meeting of the Japan Society of Applied Physics, Crystal Engineering, April 23, 2004

  Although various attempts have been made to flatten the electronic element formation surface of the zinc oxide single crystal substrate as described above, the performance of the electronic element is improved when a high-quality epitaxy layer is formed on the substrate. In order to stabilize the performance and improve the production yield, surface flatness at the thickness level of the monoatomic layer of the zinc oxide single crystal substrate is required. That is, it must have a so-called step terrace structure, have a wide terrace, and the steps must be managed at the atomic layer thickness level. However, sufficient flatness has not been obtained by the conventional technology for manufacturing a zinc oxide single crystal substrate disclosed in the above cited references.

  In view of the above-described problems, the present invention provides a zinc oxide single crystal substrate having a step-and-terrace structure on the electronic element formation surface and a method for producing such a zinc oxide single crystal substrate. For the purpose.

  The method for producing a zinc oxide single crystal substrate of the present invention is a method for producing a zinc oxide single crystal substrate for forming an electronic device using a zinc oxide single crystal, wherein a plane perpendicular to the C axis is defined as an element formation surface. After the cutting step of cutting the zinc oxide single crystal at a predetermined thickness, the shaping step of shaping the element forming surface to have a predetermined shape, the rough polishing step of polishing the element forming surface, and the rough polishing step And a heat treatment step for applying heat to the element formation surface, and a fine polishing step for polishing the element formation surface by a chemical mechanical polishing method after the heat treatment step.

  That is, this method for producing a zinc oxide single crystal substrate has a heat treatment step through a cutting step, a shaping step, and a rough polishing step. In the heat treatment step, heat is applied to the element formation surface. Thereby, the element formation surface has a step-and-terrace structure. And it has a fine polishing process after that. In the fine polishing step, the element forming surface is polished by a chemical mechanical polishing method. Thereby, a step level difference becomes small.

  According to the present invention, a flattened zinc oxide single crystal substrate having a step-and-terrace structure and a method for manufacturing such a zinc oxide single crystal substrate can be provided.

  An embodiment of the present invention will be described.

  A zinc oxide (ZnO) single crystal used for a zinc oxide single crystal substrate can be manufactured by a plurality of methods such as a hydrothermal synthesis method, a chemical vapor transport method, and a flux method, but the zinc oxide used in this embodiment is used. The single crystal was grown by a hydrothermal synthesis method using a single crystal growing apparatus. Of course, the zinc oxide single crystal used in the present embodiment is not limited to one manufactured by a hydrothermal synthesis method.

  In the hydrothermal synthesis method of this embodiment, the solution for dissolving the raw material is a mixed solution having a concentration of lithium hydroxide (LiOH) mol / l and potassium hydroxide (KOH) 3 mol / l. It was carried out in a supercritical state at high pressure (300 ° C. to 400 ° C., 800 to 1000 atm).

  The inside of the single crystal growth device is divided by a baffle plate into a growth zone where the upper seed crystal is placed and a melting zone where the lower raw material is placed. Crystals began to grow. Thus, a zinc oxide single crystal having a size of about 2 inches was obtained.

  The zinc oxide single crystal thus obtained is thinly sliced to form a zinc oxide single crystal substrate. The element formation surface on which the semiconductor is grown is a plane perpendicular to the C-axis of the zinc oxide single crystal. It is known that a so-called zinc termination surface (+ C surface) or a so-called oxygen termination surface (-C surface) is suitable. For this purpose, the zinc oxide single crystal is cut along a plane perpendicular to the C axis and then polished. Here, the general knowledge that the oxygen termination surface is superior to the zinc termination surface in terms of flatness, but the inventors attempted to planarize the zinc termination surface, which is said to be inferior in planarity. Therefore, if the manufacturing method of this embodiment is adopted, it is considered that surface characteristics equivalent to or better than those of the zinc termination surface can be obtained at the oxygen termination surface.

  In other words, the characteristics of a zinc oxide single crystal substrate are greatly related to the performance and manufacturing yield of an electronic device formed on the substrate, for example, a semiconductor device in which a compound semiconductor material or an intrinsic semiconductor material is formed as an epitaxy layer. .

  For example, the greater the difference between the lattice constant of the material substance forming the substrate and the lattice constant of the epitaxy layer, that is, the greater the lattice mismatch, the more dislocation defects, and the performance as a semiconductor device deteriorates. In the case of a laser diode, the light intensity of emitted light decreases as the number of dislocation defects increases. Therefore, the lattice constant is one of the important items for evaluating the performance of the substrate. In this respect, the lattice mismatch between the zinc oxide single crystal substrate and the lattice constant of gallium nitride is about 2%. The zinc oxide single crystal substrate is used as a substrate for blue laser diodes, blue LEDs, and white LEDs. Very suitable.

  Further, when forming the epitaxy layer, the surface state of the element formation surface of the zinc oxide single crystal substrate becomes a problem. In order to form a good epitaxy layer, it is desirable that the element formation surface has a so-called step terrace structure having a step portion and a terrace portion (flat portion). That is, since a semiconductor element is formed on this step-and-terrace structure, in order to form a high-quality epitaxy layer, it is necessary to have a terrace portion and a step having a terrace portion that is flat but has atomic layers aligned on the surface. It becomes. Therefore, the content of the step / terrace structure is one of the other important items for evaluating the performance of the substrate. If the step difference is zero, it is not suitable for forming an epitaxy layer.

  The step portion has a step of 5 atomic layers or less, and the surface roughness of the terrace portion is preferably 2 atomic layers or less as the surface state of the element formation surface. In this range, a good epitaxy layer is formed. It is clear from past experience with other substrates that it can be formed. Further, from a theoretical aspect, it is most desirable that the step difference of the step portion is 2 atomic layers or less, and the surface roughness of the terrace portion is 1 atomic layer or less. Here, in the case of zinc oxide, since the thickness of one atomic layer is approximately 0.5 nm (nanometer), the desired step difference is 2.5 nm or less, and the surface roughness of the terrace portion is 1 nm or less. Further, the most desirable step portion step is 1 nm or less, and the surface roughness of the terrace portion is 0.5 nm or less.

  In the description of the present embodiment, a manufacturing method that realizes a step difference of 1 nm or less on a zinc termination surface and a surface roughness of a terrace part of 0.5 nm or less on a zinc termination surface, which has been conventionally difficult, is clarified. To do.

  That is, conventionally, it has been considered that having a heat treatment step as a final step is an essential condition for obtaining a flat zinc oxide single crystal substrate. However, the inventor of the present application accumulates various experiments and concludes that good surface properties can be obtained by introducing a polishing process by chemical mechanical polishing (CMP) after the heat treatment process. It came to.

  Conventionally, heat treatment should have been performed in the final step for planarization of the substrate surface. According to the results of experiments conducted by the inventors of the present application, the zinc termination surface (+ C surface after the heat treatment was performed) ) Was observed, a step difference of several nm corresponding to about 10 atomic layers was generated.

  The inventors of the present application have carefully analyzed this phenomenon. Then, the following knowledge about the cause of the step difference of several nm was obtained. That is, the inventor induces surface diffusion of the zinc oxide single crystal substrate by performing heat treatment, and induces rearrangement of the surface atomic arrangement to reduce the surface energy, which is the thermodynamic property inherent to the crystal. Although the terrace structure is formed, the moving speed at the upper part of the step (on the surface space side) is faster, so the front surface advances toward the upper part of the step, causing step bunching (bundling) and creating a step. I guess it is.

  Therefore, the inventor of the present application, after the formation of the terrace portion in the heat treatment step, further peels off the atomic layer where the step bunching has occurred to lower the step difference in the step. I thought. Then, as one of the methods for peeling off the atomic layer, it has been considered to introduce a fine polishing step. In order to prove this idea, the fine polishing performed after the heat treatment step was performed by chemical mechanical polishing, and repeated experiments with various polishing conditions were repeated to find appropriate polishing conditions. The result was obtained.

  A specific method for manufacturing a zinc oxide single crystal substrate having a step-and-terrace structure from a zinc oxide single crystal will be described below in comparison with a conventional method.

  The zinc oxide single crystal is a 2 inch size produced by the hydrothermal synthesis method described above. This was cut to a thickness of about 1 mm (cutting step), and the element formation surface was shaped into a circle for use as a substrate (shaping step). Next, as a rough polishing step, the + C surface, which is a zinc terminal surface, was polished by sand finishing (rough polishing step). Polishing is performed twice, and the grain roughness of the abrasive for the first polishing is about 1500, and the grain roughness of the abrasive for the second polishing is about 2500. The process so far is not significantly different from the conventional process. The reason why the polishing process is divided into a plurality of stages (twice in the present embodiment) is to achieve both the polishing speed and the polishing accuracy, and does not necessarily need to be divided into a plurality of stages.

  Conventionally, chemical mechanical polishing is performed next, followed by heat treatment in the final process. In this embodiment, in addition to this, chemical mechanical polishing is further performed after heat treatment. As another processing flow, if the substrate is flattened to about 10 nm in the rough polishing step, heat treatment is performed without performing chemical mechanical polishing, and the final step is chemical. -Mechanical polishing is performed.

  Here, the thermal process was performed under conditions of temperature and time sufficient to obtain a terrace structure in the atmosphere by placing a zinc oxide single crystal substrate on platinum. For example, the temperature was in the range of 1000 ° C. to 1200 ° C., and the time was in the range of 0.5H to 3H. FIG. 1 shows an image obtained by an atomic force microscope (AFM) of the cross section of the zinc termination surface at this stage. As shown in FIG. 1, a terrace structure having a width of a little less than 200 nm is formed on the zinc termination surface, but it can be seen that the step difference is 3 nm or more. FIG. 1 will be described later in more detail.

  Chemical mechanical polishing, which is a fine polishing step after the heat treatment step, was performed using a conventionally known apparatus. Here, the type of polishing liquid (slurry) and the polishing rate are important in order to separate the atomic layer one layer at a time by chemical / mechanical polishing and finally make the step difference to one atomic layer or two atomic layers. For example, the polishing liquid used is a mixture of an alkali solution such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) mixed with colloidal silica having an average particle diameter of about 16 nm to 60 nm, and the polishing rate is set to 0.00. A range of 04 μm / min (micrometer / min) to 0.17 μm / min was suitable. It was also effective to perform the fine polishing step in a plurality of stages by changing the conditions of the polishing liquid and the polishing rate. FIG. 2 shows an image obtained by the atomic force microscope (AFM) of the cross section of the zinc termination surface after the final stage. As shown in FIG. 2, the step difference of the zinc termination surface was smaller than the target 1 nm, and a value of about 0.5 nm was obtained. Note that FIG. 2 will be described later in more detail.

  The polishing rate and polishing time of chemical mechanical polishing can be determined in advance for each lot of zinc oxide single crystal substrates by monitoring the step terrace structure with an atomic force microscope. .

  By the method for manufacturing a zinc oxide single crystal substrate of the above-described embodiment, a highly accurate step-and-terrace structure that cannot be obtained conventionally can be obtained. Specifically, a zinc oxide single crystal substrate having a terrace structure with a step difference of about 0.5 nm (corresponding to a monoatomic layer on the C plane of ZnO) could be obtained on the zinc termination surface. The step difference of 0.5 nm is a value that is better than 1 nm, which is the most desirable value of the step difference, for the zinc oxide single crystal substrate, and is a theoretical limit value.

<Example>
A zinc oxide substrate (wafer) having a thickness of 1 mm is cut out from a zinc oxide single crystal having a size of about 2 inches obtained by the hydrothermal synthesis method by a general cutting machine. And this cut-out zinc oxide wafer is shape | molded circularly.
And it grind | polishes in the 1st sand finishing process which is a rough grinding | polishing process. The polished surface was a surface perpendicular to the C-axis of the zinc oxide crystal and was a + C surface that was a zinc termination surface. The average grain size of the abrasive grains used here was 3.6 μm (micrometer) to 10.1 μm and the polishing rate was 0.85 μm / min to 2.55 μm / min. Results were obtained.
And it grind | polishes further in the 2nd sand finishing process which is a rough grinding | polishing process. The average grain size of the abrasive grains used here was 2.3 μm (micrometer) to 7.1 μm, and the polishing rate was tested in the range of 0.41 μm / min to 1.25 μm / min. Obtained.
Then, heat treatment is performed. The heat treatment was performed in the atmosphere with a zinc oxide wafer placed on a platinum platform. The temperature was controlled in the range of 1000 ° C. to 1200 ° C. and the heat treatment time was in the range of 1H (hour) to 3H, and good results were obtained.
As a final treatment, chemical mechanical polishing (CMP), which is a fine polishing process, was performed. Chemical mechanical polishing is performed in two steps. The polishing conditions in the first CMP step are colloidal silica having an average particle size of 20 nm (nanometer) to 60 nm, and sodium hydroxide (NaOH) or potassium hydroxide. Polishing was performed using an alkaline solution of (KOH) at a polishing rate of 0.05 μm / min to 0.17 μm / min.
The polishing conditions in the second CMP step were 0.04 μm / min using colloidal silica having an average particle diameter of 16 nm (nanometer) to 48 nm and an alkali solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH). Polishing was performed at a polishing rate of 0.13 μm / min.
As described above, fine polishing can be performed by two CMP processes to achieve both polishing speed and polishing accuracy.

  FIG. 2 described above shows a display screen image obtained by observing a cross section of the + C plane of the zinc oxide wafer finally obtained by the process shown in this example with an atomic force microscope. Here, the horizontal axis direction of the cross-sectional view shown in the upper part of FIG. 2 indicates one direction of the + C plane, and the vertical axis direction indicates a direction perpendicular to the + C plane. The boundary with the light-colored portion indicates the surface of the zinc oxide wafer. The numerical values shown in the lower part of FIG. 2 indicate that each of the four cursors in the cross-sectional view is placed on a different terrace, and the first and second from the left are a set, and the third and fourth from the left Are set as a set, and the separation distance in the horizontal axis direction and the step / step difference on the surface of the zinc oxide wafer in the vertical axis direction in each set are shown.

  The vertical position indicated by the first cursor is 2.963802 nm, the vertical position indicated by the second cursor is 3.480270 nm, and the vertical position indicated by the third cursor is 3.951635 nm. The position in the vertical axis direction indicated by the fourth cursor was 4.467289 nm. That is, the step difference between the terrace corresponding to the first cursor and the terrace corresponding to the second cursor is 0.516468 nm, and the terrace corresponding to the third cursor and the terrace corresponding to the fourth cursor are The step difference is 0.515654 nm, which corresponds to the thickness of the monoatomic layer. The separation distance in the horizontal axis direction indicated by the first cursor and the second cursor is 119.7042 nm, and the separation distance in the horizontal axis direction indicated by the third cursor and the fourth cursor is 123.1572 nm. is there.

  A comparative example is shown below. The comparative example ends the processing with the heat treatment step as the final treatment, and as shown in the embodiment, the chemical mechanical polishing is not performed after the heat treatment step. In the following comparative examples, in order to clarify the difference from the examples, a zinc oxide single crystal production step, a zinc oxide substrate (wafer) cut out, a molding step, a rough polishing step, a fine polishing step, Each step of the heat treatment step has the same contents as the above-described embodiment as a single step. Also,

<Comparative example>
A zinc oxide substrate (wafer) having a thickness of 1 mm is cut out from a zinc oxide single crystal having a size of about 2 inches obtained by the hydrothermal synthesis method by a general cutting machine. And this cut-out zinc oxide wafer is shape | molded circularly.
And it grind | polishes in the 1st sand finishing process which is a rough grinding | polishing process. The polished surface was a surface perpendicular to the C-axis of the zinc oxide crystal and was a + C surface that was a zinc termination surface. The average grain size of the abrasive grains used here was 3.6 μm (micrometer) to 10.1 μm, and the polishing rate was 0.85 μm / min to 2.55 μm / min.
And it grind | polishes further in the 2nd sand finishing process which is a rough grinding | polishing process. The average grain size of the abrasive grains used here was in the range of 2.3 μm (micrometer) to 7.1 μm, and the polishing rate was in the range of 0.41 μm / min to 1.25 μm / min.
Then, chemical mechanical polishing (CMP), which is a fine polishing process, was performed. Chemical mechanical polishing is performed in two steps. The polishing conditions in the first CMP step are colloidal silica having an average particle size of 20 nm (nanometer) to 60 nm, and sodium hydroxide (NaOH) or potassium hydroxide. Polishing was performed using an alkaline solution of (KOH) at a polishing rate of 0.05 μm / min to 0.17 μm / min.
The polishing conditions in the second CMP step were 0.04 μm / min using colloidal silica having an average particle diameter of 16 nm (nanometer) to 48 nm and an alkali solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH). Polishing was performed at a polishing rate of 0.13 μm / min.
Although the cross section of the + C plane of the zinc oxide wafer after the processing up to this stage was observed with an atomic force microscope, the quality is good even if the polishing conditions in the rough polishing process and the fine polishing process are changed within the above-mentioned range. The formation of a step terrace structure was not confirmed.
And as a final process, heat processing was performed. The heat treatment was performed in the atmosphere with a zinc oxide wafer placed on a platinum platform. The temperature was controlled in the range of 1000 ° C. to 1200 ° C., and the heat treatment time was tested in the range of 1H (hour) to 3H.

  FIG. 1 described above shows a display screen image when the cross section of the + C plane of the zinc oxide wafer finally obtained by the process shown in this comparative example is observed with an atomic force microscope. Here, the horizontal axis direction of the cross-sectional view shown in the upper part of FIG. 1 indicates one direction of the + C plane, and the vertical axis direction indicates a direction perpendicular to the + C plane. The boundary with the light-colored portion indicates the surface of the zinc oxide wafer. In addition, the numerical values shown in the lower part of FIG. 1 indicate that the two cursors in the cross-sectional view are arranged on different terraces, and the separation distance in the horizontal axis direction corresponding to the first and second cursors from the left side and It shows the steps and steps on the surface of the zinc oxide wafer in the vertical axis direction.

  The position in the vertical axis direction indicated by the first cursor was 4.763111 nm, and the position in the vertical axis direction indicated by the second cursor was 7.63392 nm. That is, the step difference between the terrace corresponding to the first cursor and the terrace corresponding to the second cursor is 2.868281 nm, which corresponds to the thickness of five monoatomic layers. The separation distance in the horizontal axis direction indicated by the first cursor and the second cursor is 165.7800 nm.

The display screen image of the atomic force microscope (AFM) of the surface of the zinc termination | terminus surface of the final zinc oxide board | substrate obtained by the manufacturing method shown to a comparative example is shown. The display screen image of the atomic force microscope (AFM) of the surface of the zinc termination | terminus surface of the final zinc oxide board | substrate obtained by the manufacturing method shown in an Example is shown.

Claims (2)

A method for producing a zinc oxide single crystal substrate for forming an electronic device using a zinc oxide single crystal,
A cutting step of cutting the zinc oxide single crystal with a predetermined thickness so that a plane perpendicular to the C-axis is an element forming surface; a shaping step of shaping the element forming surface into a predetermined shape; and the element formation A rough polishing step for polishing a surface, a heat treatment step for applying heat to the element formation surface after the rough polishing step, and a fine polishing for polishing the element formation surface by a chemical mechanical polishing method after the heat treatment step. And a method for producing a zinc oxide single crystal substrate. The method for producing a zinc oxide single crystal substrate according to claim 1, wherein the element formation surface is a zinc termination surface.

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