JP4610476B2 - Aluminum nitride single crystal multilayer substrate and method of manufacturing aluminum nitride single crystal multilayer substrate - Google Patents

Description

  The present invention relates to an aluminum nitride single crystal multilayer substrate having an aluminum nitride single crystal film on the surface, and a method for manufacturing an aluminum nitride single crystal multilayer substrate having an aluminum nitride single crystal film on the surface.

Along with the development of multimedia communication in recent years, to the surface acoustic wave (GHz) surface acoustic wave (SAW) element for communication systems such as wireless base station devices and portable information devices that support high-speed communication services. Needs are growing. The center frequency f ₀ of the surface acoustic wave element is a relationship between the sound velocity (surface acoustic wave propagation velocity) v on the surface of the piezoelectric substrate and the pitch of the interdigital electrode (1/4 of the wavelength λ of the surface acoustic wave). Represented by

  Therefore, in order to cope with the increase in the frequency of the surface acoustic wave device, there are a method of miniaturizing the electrode pitch using a high-precision photolithography technique and a method of employing a piezoelectric material having a high sound speed. Aluminum nitride (AlN) is expected to be about twice as fast as existing piezoelectric materials such as quartz (sound speed 3150 m / s) and lithium niobate (sound speed 3900 m / s). It is considered as a key material.

  The piezoelectric characteristics of the piezoelectric material such as the propagation speed of the elastic wave and the electromechanical coupling coefficient strongly depend on the crystal properties such as crystallinity and cut-out plane orientation. The form of the piezoelectric material is preferably a single crystal, and the orientation direction of the piezoelectric material crystal needs to be aligned with respect to at least one axis. AlN is considered to have a stable liquid phase only under ultra-high pressure conditions, and there is no report that a liquid phase was obtained under pressure conditions of about vacuum to several atmospheres. Therefore, unlike an existing piezoelectric material capable of growing a single crystal from a melt, a method of forming a thin film that satisfies the above-described conditions using a vacuum deposition technique such as a DC magnetron sputtering method is common.

  In these vacuum deposition techniques, it is possible to form a thin film in which the c-axis of AlN is inclined with respect to the substrate normal with respect to the substrate surface. It is known that in a surface acoustic wave device made of an AlN thin film whose c-axis is inclined by approximately 15 ° or more, the propagation speed of Rayleigh waves and the electromechanical coupling coefficient are improved (Non-Patent Document 1).

Y. Kobayashi, N .; Tanaka, H .; Okano, K .; Takeuchi, T .; Usuki and K.K. Shibata, Japan Journal of Applied Physics Part 1, 1995, Volume 34, 2668.

  The above-described technique has an excellent advantage that it can be applied to various base substrates such as metals and ceramics, but has a problem including a large variation in the crystal orientation direction. The variation in the crystal orientation direction is generally evaluated by the rocking curve half-width (FWHM) of the X-ray diffraction peak. In the {0 0 0 2} plane corresponding to the c-axis of AlN, this value is 1 ° or more, and in the {1 −1 0 0} plane orthogonal to the {0 0 0 2} plane, the value is 5 ° or more. . For this reason, the piezoelectric characteristics vary, and the performance such as frequency selectivity indicated by the Q value is greatly inferior to the single crystal of other substances produced by melt growth.

  The four-digit number including curly braces or parentheses used in the claims and the specification of the present application indicates the Miller index used in the hexagonal crystal, and the negative index is the original index. Instead of notation, it is expressed as follows.

Prior to the present invention, the present inventors developed a method for producing a highly crystalline AlN film using a reductive nitridation reaction using alumina, carbon (C), nitrogen (N ₂ ) and carbon monoxide (CO) as reaction raw materials. And it has already been proposed in Japanese Patent Application Laid-Open No. 2004-137142. According to this method, the target thin film is not deposited on the substrate as in the conventional means, but the alumina is converted into AlN from the α-alumina (Al ₂ O ₃ ) substrate surface toward the inside. A good AlN crystal can be formed.

  In this proposed invention, an α-alumina substrate composed of a-plane: {1 1 −2 0} plane is used, a crystal having a relationship in which the c-axis of AlN is perpendicular to the substrate plane, so-called c-axis oriented crystal A thin film is obtained.

  An object of the present invention is to provide a c-axis tilted AlN single crystal multilayer substrate having improved crystal orientation as compared with the prior art by applying the previously proposed technique.

  The present inventors have intensively studied to achieve the above object, and the {0 0 0 2} plane of AlN produced by the above reaction has an α-alumina crystal having a plane spacing close to the plane spacing (0.249 nm). It was found that it was easy to take a parallel relationship with the crystal plane inside. That is, the {1 0 -14} plane (plane spacing 0.255 nm), the {1 1 -2 0} plane (plane spacing 0.238 nm), the {0 0 0 6} plane (plane spacing 0. 0 nm) of α-alumina. 217 nm) and {1 1 −2 3} plane (plane spacing 0.209 nm).

  Furthermore, among these, the {1 1 -2 0} face of α-alumina has a tendency to preferentially grow so that the {0 0 0 2} face of AlN is parallel, and these The present invention has been completed based on the findings.

That is, the aluminum nitride single crystal multilayer substrate of the present invention is cut out at a crystal plane orthogonal to the {1 -1 0 0} plane and having an interplane angle of 15 ° to 40 ° with the {1 1 −2 0} plane. and α- alumina single crystal aluminum nitride on the substrate single crystal film is formed, 15 ° with respect to <0 0 0 1> axis is the normal line of the α- alumina single crystal substrate of the aluminum nitride single crystal film It is characterized by tilting by 40 ° .

  In another aluminum nitride single crystal multilayer substrate of the present invention, the half-width (FWHM) of the ω mode rocking curve of the {0 0 0 2} diffraction peak of the aluminum nitride single crystal film is 1080 arcsec or less in the above-described configuration, The rocking curve half-width (FWHM) of the {1 −1 0 0} diffraction peak is 7200 arcsec or less.

Moreover, the manufacturing method of the aluminum nitride single crystal multilayer substrate of the present invention is a crystal that is orthogonal to the {1 -1 0 0} plane and has a face-to-face angle of 15 ° to 40 ° with the {1 1 −2 0} plane. An α-alumina single crystal substrate having a surface as a substrate surface is heated at a temperature of 1500 to 1650 ° C. in the presence of carbon, nitrogen, and carbon monoxide, and an aluminum nitride single crystal is formed on the α-alumina single crystal substrate. A film is formed.

  According to the present invention, the crystal plane of the single crystal α-alumina orthogonal to the {1 −1 0 0} plane, that is, the crystal zone having the crystal axis of <1 −1 0 0> is {1 1 − 20}, a so-called inter-plane angle of 15 ° to 40 ° is used as a raw material, and the reaction proposed in Japanese Patent Application Laid-Open No. 2004-137142 or Japanese Patent Application No. 2005-31086 is used. AlN is formed on the outermost surface. When [1 −1 0 0] is taken as a zone axis, single crystal α-alumina and single crystal AlN satisfying the following orientation relationship are generated.

  That is, since the angle formed by the cut surface of the single crystal α-alumina and the {1 1 −2 0} plane is an inclination angle with respect to the substrate normal line of the c-axis of the generated AlN, any c of 15 ° to 40 ° An AlN single crystal film having an axis inclination angle is obtained. This AlN single crystal film has less variation in the orientation direction as compared with the conventional method in which an AlN raw material is supplied and deposited from the outside, and the defect density generated accordingly is greatly reduced.

  The obtained AlN single crystal multilayer substrate can be expected to further improve device characteristics by using it as a substrate material for a surface acoustic wave device that achieves a high electromechanical coupling coefficient and a high propagation speed.

  Also, a seed for producing an AlN single crystal free-standing substrate having a crystal orientation having a face-to-face angle of 15 ° to 40 ° with respect to AlN {0 0 0 2} by using an epitaxial growth technique such as a vapor phase growth method. It can be suitably used as a crystal. These substrates are designed to improve light-emitting efficiency in the design of light-emitting devices using Group III nitrides such as AlN, while reducing the defect density and controlling the polarization phenomenon that occurs in Group III nitrides. Connected.

  Hereinafter, the present invention will be described in detail according to the embodiments of the invention.

In the present invention, an AlN crystal film is directly formed on a single crystal α-alumina substrate by nitriding the surface of the single crystal α-alumina substrate. Specifically, a single crystal α-alumina substrate and graphite are placed in the soaking section of the heat treatment apparatus, and the composition of the N ₂ —CO mixed gas is adjusted to control the oxygen potential and nitrogen potential. The single crystal α-alumina substrate is nitrided according to the following reaction formula.

alon refers to aluminum oxynitride represented by Al _{(64 + x) / 3} □ _{(8-x) / 3} O _32-x N _x (where □ is a cation vacancy). Al ₂ O ₃ sat.) Means alon saturated with Al ₂ O ₃ . Moreover, alon (AlNsat.) In Reaction Formula (3) means alon saturated with AlN. Further, x is a variable caused by alon's non-stoichiometry and takes a value in the range of 2 <x <6.

Under the condition that the sum of the partial pressure of nitrogen and carbon monoxide constituting the furnace atmosphere P _CO + P _N2 is 1 bar, and the carbon activity a _c is 1, the reaction formula (1) is higher than 1630 ° C. and below 1630 ° C. Then, it is supposed that reaction shown by Reaction Formula (2) and Reaction Formula (3) occurs.

  Whether these reactions proceed can be estimated from the Gibbs energy change shown by the following general formula.

Here, R is a gas constant, T is an absolute temperature, and _{Kr (x)} is an equilibrium constant calculated from the activity of each phase.

FIG. 1 is a graph with the Celsius temperature on the horizontal axis and the energy amount on the vertical axis. The lower right lines indicate the standard Gibbs energy change terms Δr ₍₁₎ G °, Δr ₍₂₎ G °, Δr ₍₃₎ G ° when 1 mol of N ₂ is reacted for each of the above three reaction formulas. The function of is subtracted. The horizontal line shows an equal nitrogen partial pressure curve when the total atmospheric pressure is 1 bar.

When the energy amount of the so-called RT term composed of the equilibrium constant and the absolute temperature exceeds the standard Gibbs energy change amount, the Gibbs energy change Δr ₍₁₎ G, Δr ₍₂₎ G, Δr _{( 3)} G becomes negative and nitriding proceeds. That is, the region above the straight line in FIG. 1 is an AlN stable region, and the region below the straight line is an Al ₂ O ₃ stable region. Above 1630 ° C., there is an alon stable region at the boundary between the two. The nitriding reaction proceeds by selecting the temperature condition and the atmospheric composition corresponding to the AlN stable region in FIG.

  There is no restriction | limiting in particular in the heating apparatus used in the method of nitriding the surface of the alpha alumina substrate mentioned above, The thing of arbitrary structures can be used. However, it must be capable of exposing the α-alumina substrate to the temperature conditions shown in FIG. 1 in a mixed gas composed of nitrogen and carbon monoxide. Further, it is desirable that the design be such that the temperature difference in the α-alumina substrate can be kept within 5 ° C.

  It is desirable that the heating furnace material is composed only of graphite, α-alumina, AlN and alon, which are substances involved in the reaction, from the viewpoint of avoiding contamination of the resulting AlN crystal with impurities. Further, it is more desirable to use only the technique disclosed in Japanese Patent Application No. 2005-227165 by providing only a graphite and providing a mechanism for controlling the aluminum concentration in the furnace.

  Water vapor or organic matter adsorbed on the furnace material increases the oxygen concentration in the furnace during heating, causing fluctuations in the atmosphere composition and affecting the reaction behavior. It is desirable to introduce a removal process such as cleaning by vacuuming in a low temperature region.

  The α-alumina substrate to be used preferably has a smooth surface in order to obtain an AlN crystal with good quality and controlled orientation. For this reason, a general sapphire substrate for epitaxial growth is preferably used.

  In order to obtain the aluminum nitride single crystal film of the present invention, the cut surface of the α-alumina single crystal is orthogonal to the {1 −1 0 0} plane and the inter-plane angle with the {1 1 −2 0} plane is A substrate having a crystal plane of 15 ° to 40 ° is used. When the surface is not orthogonal to the {1 −1 0 0} plane, in addition to a plurality of {1 1 −2 0} planes, for example, the (1 1 −2 0) plane and the (−1 2 −1 0) plane Thus, crystals having a parallel relationship between the AlN {0 0 0 2} planes are easily formed. Moreover, even if it is a plane orthogonal to {1 -1 0 0}, if the angle formed with the {1 1 -2 0} plane exceeds 40 °, in addition to the (1 1 -2 0) plane, (0 0 The 0 1) plane and the AlN {0 0 0 2} plane are easily generated in parallel. That is, in any case, reproducibility is insufficient to obtain the aluminum nitride single crystal film of the present invention.

  As the carbon coexisting in the reaction system, various commercial products including furnace materials can be used. The purity of carbon is preferably 99.9% or more, and more preferably 99.999% or more.

  The gas species to be introduced are not limited to nitrogen and carbon monoxide. For example, even if oxygen or carbon dioxide is introduced instead of carbon monoxide, and carbon monoxide generated by reacting with graphite in the furnace is subjected to the reaction, the same effect as in the present invention can be obtained.

  Nitrogen and carbon monoxide are usually in the form of a gas, but those having a purity as high as possible are preferable, and generally 99.9999% or more of nitrogen and 99.9% or more of carbon monoxide are applicable. . Among impurities such as carbon dioxide and water vapor, oxygen atoms generated by decomposition change the chemical potential of the nitriding reaction and cause an error with respect to the control parameter.

The total pressure of the reaction system is not particularly limited, but is preferably about 1 bar from the viewpoint of ease of production and operation of the reaction apparatus. During the reaction, a mixed gas having a predetermined partial pressure is supplied at a predetermined flow rate. The mixing ratio of nitrogen and carbon monoxide is determined from the range of (P _N2 / P _CO ³ ) that falls within the AlN stable region according to the reaction temperature, according to the phase stability diagram shown in FIG.

The temperature condition for obtaining the AlN single crystal substrate of the present invention with good reproducibility is 1500 to 1650 ° C. If the temperature is less than 1500 ° C., it is relatively difficult to control the atmosphere composition for appropriately obtaining the nitriding driving force described later, and a sufficient rate of diffusion of nitrogen into the substrate cannot be obtained. Further, at a high temperature exceeding 1650 ° C., an AlN crystal having a relationship other than the crystal orientation relationship of the present invention is likely to be generated, and it becomes difficult to obtain a single crystal. That is, a crystal plane other than the α-alumina {1 1 −2 0} plane, for example, a crystal having a parallel relationship between the {0 0 0 6} plane and the AlN {0 0 0 2} plane tends to occur.

RT term value determined by selection of reaction temperature T and (P _N2 / P _CO ³ ) and standard Gibbs energy change terms Δ _{r (1)} G °, Δr ₍₂₎ G °, Δr ₍₃₎ The difference in G ° is a measure of the substantial nitriding driving force. This corresponds to the distance between the vertical axis coordinates of the straight line indicating the standard Gibbs energy change amount in FIG. 1 and the coordinates uniquely determined by the selected condition.

In order to obtain the maximum effect of the nitriding reaction used in the present invention, it is necessary to appropriately set the nitriding driving force. It is desirable to select the ratio of the mixed gas so that the ideal nitriding driving force when 1 mol of N ₂ reacts with each of the above three reaction formulas falls within the range of 0 to 150 kJ at 1500 to 1650 ° C. When the nitriding driving force is too high exceeding the above-mentioned range, the same phenomenon occurs as when it is placed at a high temperature exceeding 1650 ° C., and as a result, it becomes difficult to produce a single crystal.

Since the flow rate of the mixed gas needs to always allow nitrogen atoms to reach the substrate surface, a gas at 25 ° C. and 1 atm is 5 mL / min with respect to a cross-sectional area of 1 cm ² of the reactor in a plane perpendicular to the gas flow. It is preferable to introduce the above. More preferably, an apparatus for preheating the gas introduced before reaching the α-alumina substrate is installed. A commercially available flow meter can be used without limitation to control the partial pressure of carbon monoxide and nitrogen to be introduced.

  Although the rate of temperature increase can be arbitrarily determined, 5 ° C. or more per minute is preferably employed.

The heating time is appropriately determined depending on the desired AlN film thickness. For example, in a heating apparatus using graphite for all furnace materials (hereinafter referred to as a graphite furnace), at 1600 ° C. under conditions of P _CO = 0.10 bar and P _N2 = 0.90 bar, {1 1 −2 3} When the α-alumina substrate cut out on the surface is nitrided, the growth rate of the AlN film is 10 to 20 nm in a reaction for 12 hours.

After the completion of the reaction, AlN is oxidized if it enters the Al ₂ O ₃ stable region of FIG. 1 during cooling. Therefore, in a temperature range exceeding 1000 ° C., the substrate is always oxidized so that the AlN is always in a stable state. Take measures to gradually reduce the carbon concentration. Examples of this method include an operation of stopping the supply of carbon monoxide before the start of cooling and continuing the supply of only nitrogen to reduce the carbon monoxide concentration in the reaction system to less than 0.1%. Even at 1000 ° C. or lower, it is desirable to continue the supply of only nitrogen and dilute the carbon monoxide concentration as much as possible.

  Although the cooling rate can be arbitrarily determined, it is desirable to control the cooling rate to be 20 ° C. or less per minute in order to suppress residual strain caused by the difference in thermal expansion between the α-alumina substrate and the AlN single crystal film.

  By the above method, an AlN single crystal multilayer substrate that sufficiently exhibits the effects of the present invention can be produced.

  The orientation direction of the produced AlN crystal is X-ray (CuKα ray) diffraction angle 2θ = 36.02 ° (corresponding to AlN {0 0 0 2} plane) and 33.20 ° (AlN {1 −1 0 0} plane). Corresponding)) and make sure that AlN is a single crystal. In addition, a 2θ-ω scan is performed in the direction in which AlN {0 0 0 2} appears to identify a crystal plane in the single crystal α-alumina that is in a parallel relationship.

  The crystal quality is evaluated by the full width at half maximum of the X-ray diffraction ω mode rocking curve. As described in the background art section, the quality of crystal quality can be judged by the full width at half maximum of this measurement. In order to confirm that the crystals are aligned in all three dimensions, measurement is performed on the {0 0 0 2} plane and the {1 −1 0 0} plane orthogonal to each other.

[Example 1]
A disc-shaped single crystal α-alumina substrate having a diameter of 50.8 mm cut out on the (1 1 −2 3) plane with a N ₂ —CO mixed gas at 1600 ° C. using a reaction system having the configuration shown in FIG. By nitriding, an AlN single crystal film was formed.

  First, the inside of the furnace was once evacuated by a rotary pump and replaced with a mixed gas having a partial pressure ratio of carbon monoxide and nitrogen of 1: 9, and an atmosphere having the same composition was allowed to flow at a constant flow rate. The exhaust valve was set to open at 1.02 bar, and was maintained at approximately 1 bar during heating.

  During heating, the temperature of the substrate was controlled by measuring the infrared light emitted from the graphite block immediately below the single crystal α-alumina substrate with a radiation thermometer. After 12 hours have passed since reaching 1600 ° C., carbon monoxide charging was stopped while the temperature was maintained at 1600 ° C., and pure nitrogen of twice the volume of the furnace was allowed to flow for 30 minutes to change the atmosphere in the furnace to nitrogen. Replaced. Then, it cooled so that a cooling rate might always be 20 degrees C or less per minute.

  A pole figure about 2θ = 36.02 ° was created for the obtained substrate, and the figure of FIG. 3 consisting of peaks on the AlN {0 0 0 2} plane was obtained. It was confirmed that the c-axis of AlN has an angle of 29.4 ° with respect to the substrate normal and is tilted in one direction.

  Furthermore, a pole figure about 2θ = 33.20 ° was created to obtain the figure of FIG. 4 consisting of peaks of the AlN {1 −1 0 0} plane. As a result of analysis, these three peaks form an angle of 90 ° with the peak coordinates of AlN {0 0 0 2} shown in FIG. 3, and the angle between the respective peak coordinates is 60 °. It was. That is, this indicates that all six-fold symmetric AlN {1 −1 0 0} planes are aligned in the film. From the above information, it is derived that single crystal AlN is generated in the substrate.

  Further, a 2θ-ω scan is performed on the AlN {0 0 0 2} plane, and the AlN {0 0 0 2} plane and the α-alumina (1 1 -2 0) plane are in a parallel relationship as shown in FIG. confirmed.

As a result of evaluating the X-ray diffraction ω mode rocking curve full width at half maximum (FWHM) of the obtained single crystal AlN, it was 925 arcsec for the {0 0 0 2} plane and 6577 arcsec for the {1 −1 0 0} plane.
[Comparative Example 1]
Using a reaction system similar to that of Example 1, a disk-shaped single crystal α-alumina substrate having a diameter of 50.8 mm cut out at (1 1 -2 3) plane was nitrided at 1750 ° C. with a N ₂ —CO mixed gas. As a result, an AlN film having a plurality of orientation relationships was formed.

  The reaction was completed by performing the same operation as in Example 1 except that the partial pressure ratio of carbon monoxide and nitrogen in the introduction atmosphere after evacuation was changed to 6: 4 and the holding temperature was changed to 1750 ° C.

  A pole figure about 2θ = 36.02 ° was created for the obtained substrate, and the figure of FIG. 6 consisting of peaks of at least six AlN {0 0 0 2} planes was obtained.

These peaks were analyzed in the same manner as in Example 1, and each of the AlN {0 0 0 2} planes was (1 1 -2 0) plane, (-1 2 -1 0) plane, (2 It was found that the -1 -1 0) plane, (0 0 0 1) plane, (1 1 -2 3) plane, and (1 1 -2 -3) plane were generated in parallel.
[Comparative Example 2]
Using the same reaction system as in Example 1, and under the same reaction conditions as in Example 1, (0 0 0 1) plane, (0 1 -1 0) plane, (0 1 -1 1) plane, (0 1 −1 2) plane, (0 1 −1 3) plane, (0 −1 1 2) plane, a disk-shaped single-crystal α-alumina substrate having a diameter of 50.8 mm, respectively, is nitrided to obtain a plurality of An AlN film having the orientation relationship was formed.

  Using the same method as in Comparative Example 1 for the obtained substrate, the number of orientation directions of the AlN {0 0 0 2} plane and the α-alumina crystal plane in a parallel relationship were specified. The results are shown in Table 1. As a result, in these crystal planes, it was not possible to obtain a single crystal film in which the c-axis of AlN is inclined with respect to the substrate normal.

FIG. 1 is an Al—O—N—C phase stability diagram showing an AlN stability region. FIG. 2 is a schematic diagram of the heating apparatus used in the examples. FIG. 3 is a pole figure showing the orientation direction of the AlN {0 0 0 1} plane in the substrate obtained in Example 1. FIG. 4 is a pole figure showing the orientation direction of the AlN {1 −1 0 0} plane in the substrate obtained in Example 1. FIG. 5 is an X-ray diffraction 2θ scan chart showing a plane parallel to the AlN {0 0 0 1} plane in the substrate obtained in Example 1. FIG. 6 is a pole figure showing the orientation direction of the AlN {0 0 0 1} plane in the substrate obtained in Comparative Example 1.

Explanation of symbols

21 annular electrode 22 graphite heating element 23 heat insulating material 24 graphite support base 25 single crystal α-alumina substrate

Claims (2)

Aluminum nitride single crystal on an α-alumina single crystal substrate cut out with a crystal plane orthogonal to the {1 -1 0 0} plane and having a face-to-face angle of 15 ° to 40 ° with the {1 1 -2 0} plane film is formed, and being inclined 15 ° to 40 ° relative to the <0 0 0 1> axis is the normal line of the α- alumina single crystal substrate of the aluminum nitride single crystal film, Aluminum nitride single crystal laminated substrate. An α-alumina single crystal substrate having a crystal plane perpendicular to the {1 -1 0 0} plane and having an interplane angle of 15 ° to 40 ° with the {1 1 −2 0} plane, carbon, The aluminum nitride single crystal film is formed on the α-alumina single crystal substrate by heating at a temperature of 1500 to 1650 ° C in the presence of nitrogen and carbon monoxide . Manufacturing method of aluminum nitride single crystal multilayer substrate.

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