JP4859868B2 - Crystal thin film - Google Patents

Description

  The present invention relates to a crystal thin film used for an oscillator, a vibrator, a surface acoustic wave device for a high frequency filter, an optical waveguide, a semiconductor substrate, and the like.

  Crystal thin films are widely used in oscillators, vibrators, surface acoustic wave elements for high frequency filters, optical waveguides, and the like, and are very important materials in industry. As a method for producing them, a crystal single crystal obtained by a hydrothermal synthesis method is polished and thinned, or a crystal thin film is directly formed by a sol-gel method, a plasma chemical vapor deposition (CVD) method, a sputtering method, or a laser ablation method. There is a method of manufacturing.

In recent years, many reports have been made on the following documents and the like as techniques for thinning single crystal quartz.
JP-A-8-157297 JP-A-8-225398 JP-A-8-268718 JP-A-8-91977 Japanese Patent Laid-Open No. 9-315897

  In these studies, a quartz single crystal thin film is produced by a sol-gel method using a silicon alkoxide as a raw material, a plasma CVD method, a sputtering method, or the like. The sol-gel method requires many complicated processes such as addition of alcohol, water and amine to the raw material solution, reflux, coating on the substrate, drying, and heat treatment. Further, the plasma CVD method, the sputtering method, and the laser ablation method require a large-scale apparatus. For example, the laser ablation method is a method in which a sintered compact target placed under an ultra-high vacuum is irradiated with ultraviolet laser pulse light to evaporate into a plasma state, and a thin film is grown on a substrate crystal. Possible vacuum devices, excimer lasers and sintered bodies are needed. Further, the growth rate of the quartz crystal thin film formed on the substrate was also as small as 0.25 μm / h.

  The sol-gel method has many production steps and is complicated, and is not suitable for industrial production. The laser ablation method requires expensive equipment such as vacuum equipment and expensive raw materials such as high-purity silicon dioxide as a target, and has problems such as a slow crystal growth rate. Is unsuitable.

  On the other hand, the inventors use atmospheric pressure vapor phase epitaxy (AP-VPE), in which epitaxial growth is performed on a substrate by reaction of ethyl silicate and oxygen under atmospheric pressure without using a vacuum device, An object of the present invention is to provide a useful crystal thin film that can be used for an oscillator, a vibrator, a surface acoustic wave device for a high frequency filter, an optical waveguide, a semiconductor substrate, and the like.

  The inventors have used an atmospheric pressure vapor phase epitaxial growth method (AP-VPE) in which epitaxial growth is performed on a substrate by a reaction between an inexpensive silicon alkoxide raw material and oxygen under atmospheric pressure without using a vacuum apparatus, We have found a method for producing quartz thin films.

  The above-described method for producing a crystal epitaxial thin film includes a step of vaporizing and introducing silicon alkoxide onto a substrate under atmospheric pressure, and reacting the introduced silicon alkoxide with oxygen to deposit crystal on the substrate. Including a process.

  Furthermore, the above-mentioned quartz epitaxial thin film manufacturing method is characterized in that a reaction accelerator for silicon alkoxide and oxygen, for example, hydrogen chloride is supplied. In addition, a step of providing a buffer layer on the substrate is performed, and then a crystal epitaxial thin film is grown on the buffer layer.

  Furthermore, the quartz epitaxial thin film manufacturing method described above is characterized in that the deposition thickness per unit time on the substrate has a high growth rate of 3 μm.

  In the above method, examples of silicon alkoxides that can react with oxygen and can be used to produce quartz include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane.

  In addition to oxygen, ozone, dinitrogen monoxide, water, and the like can be given as oxygen gas for reacting with silicon alkoxide to generate crystal.

The quartz crystal thin film according to claim 1 according to the present invention includes a step of depositing a buffer layer containing an amorphous material of hexagonal crystal on the substrate surface;
On the buffer layer, one or more silicon alkoxides selected from the group of tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane and tetrabutoxysilane are used as a silicon source, and the silicon source is vaporized under atmospheric pressure. The process of introducing,
A quartz thin film produced by a production method comprising: reacting an introduced silicon source with oxygen to deposit a quartz epitaxial thin film on the buffer layer,
The buffer layer has a thickness of 25 nm or more and 80 nm or less,
The full width at half maximum of the X-ray diffraction peak indicating the degree of crystal of the crystal thin film epitaxially grown thereon is smaller than 10.0 minutes.

The quartz crystal thin film according to claim 2 according to the present invention includes a step of depositing a buffer layer containing an amorphous material of hexagonal crystal on the substrate surface;
On the buffer layer, one or more silicon alkoxides selected from the group of tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane and tetrabutoxysilane are used as a silicon source, and the silicon source is vaporized under atmospheric pressure. The process of introducing,
A quartz thin film produced by a production method comprising: reacting an introduced silicon source with oxygen to deposit a quartz epitaxial thin film on the buffer layer,
The half width of the X-ray diffraction peak indicating the degree of crystal of the crystal thin film is within 1.0 minute when the thickness of the buffer layer is 50 nm.

Crystal thin film according to claim 3 of the present invention, in a crystal thin film according to claim 1 or 2, wherein the buffer layer is GaN same Ku crystal form and crystal is hexagonal, the use of either ZnO Features.

  According to the present invention, a silicon alkoxide (tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, etc.) is used as a silicon source, and an oscillator, a vibrator, and a high-frequency filter are used by a simple method without using a vacuum device. It was possible to provide a crystal epitaxial thin film useful for surface acoustic wave devices and optical waveguides.

  Further, the method for producing the above-mentioned quartz epitaxial thin film is an atmospheric pressure vapor phase epitaxial growth method using silicon alkoxide as a raw material, and this method is more effective than the laser ablation method and the sputtering method in which a conventional thin film has been obtained. (1) It can be carried out under atmospheric pressure, does not require an expensive vacuum device, and does not require a large-scale device. (2) The growth rate of the thin film is extremely high and the mass productivity is excellent. (3) It does not require a high-purity raw material, can be easily and inexpensively (4), and exhibits various characteristics such as a thin film having excellent crystallinity, high purity, and good optical properties. Therefore, this method is extremely useful for producing a quartz single crystal thin film. Therefore, the crystal epitaxial thin film according to the present invention exhibits extremely excellent vibration characteristics.

  The epitaxial growth under atmospheric pressure of the present invention uses a silicon alkoxide as a raw material, which is evaporated by heating, transported onto a substrate with a carrier gas such as nitrogen, and reacted with oxygen gas on the substrate to produce a quartz thin film. It is what. As a heat source for the silicon alkoxide, a heater such as a high-frequency induction heater or a resistance heater can be used.

  In the present invention, by heating a silicon alkoxide as a raw material, a part of the alkoxide is supplied as a vapor phase to the growth section. The supply amount can be adjusted by the heating temperature and the carrier gas flow rate. Generally, the heating temperature is about 25 to 120 ° C. in consideration of the vapor pressure of silicon alkoxide.

  The carrier gas for supplying a part of the raw material to the growth part may be an inert gas, and nitrogen, argon, helium, etc. can be used, but nitrogen is preferable from the viewpoint of low cost. is there.

  The silicon raw material does not need to have a high purity (for example, 3N or 5N) used as a target for laser ablation, and a purity of about 99.5% is sufficient, but the higher the purity, the more the crystal And crystal quality can be improved.

  On the other hand, oxygen for reacting with silicon alkoxide to form crystal is supplied as a gas to the growth section together with a carrier gas. When the amount of oxygen to be supplied is large, the growth rate of the obtained crystal thin film is high and the crystallinity tends to be improved. Conversely, when the amount of oxygen is small, the reverse tendency is exhibited. This is thought to be due to the small equilibrium constant of the reaction for obtaining crystals from silicon alkoxide and oxygen. Accordingly, the amount of oxygen to be supplied is preferably excessive with respect to the alkoxide of silicon. If this amount is expressed as an oxygen partial pressure, it varies depending on the amount of silicon alkoxide to be evaporated, but generally 0.1 to 0. The amount is about 3 atm. Further, by supplying hydrogen chloride (HCL) to the reaction between silicon alkoxide and oxygen, the growth rate of crystal increases. This is considered that hydrogen chloride has a catalytic effect in the reaction of silicon alkoxide with oxygen. If this amount is expressed as an oxygen partial pressure, the amount is generally about 0.001 to 0.003 atm although it depends on the amount of silicon alkoxide to be evaporated.

  Examples of the substrate used in the present invention include sapphire, silicon, and GaAs. These substrates are heated and kept at a constant growth temperature. Further, the orientation of the substrate with respect to the flow of the source gas may be parallel to or perpendicular to the flow of the source gas, and may be arranged at a certain angle.

  Crystal growth on the substrate may be performed directly on the substrate. However, by providing a buffer layer on the substrate and further growing a crystal epitaxial layer on the substrate, the crystallinity of the epitaxial layer is improved. Become.

  Such a buffer layer can be obtained, for example, by growing amorphous quartz on a substrate at a relatively low temperature and then annealing. It can also be obtained by previously forming another substance on the sapphire substrate, such as GaN or ZnO which is the same hexagonal crystal as quartz. The buffer layer contributes to the relaxation of the lattice mismatch difference.

  Actually, when a 50 nm amorphous quartz buffer layer obtained by depositing and annealing quartz at 550 ° C. is provided on a sapphire substrate, it is obtained from an X-ray diffraction peak indicating the degree of crystallinity of the epitaxial layer. The full width at half maximum (FWHM) was 1.0 minute, and the FWHM was about 10.0 minutes when grown under the same conditions when the buffer layer was not provided. Therefore, the buffer layer was provided by comparing the two. It can be seen that the crystallinity of the crystal epitaxial layer is improved. In addition, FWHM is a sharp peak, so that this value is small, and it shows that crystallinity is good.

  As described above, the reason why the crystallinity is further improved by the presence of the buffer layer is as follows: (1) relaxation of lattice mismatch, (2) suppression of cracks due to difference in thermal expansion coefficient, and (3) lateral direction in the initial stage of growth. (Substrate direction) Although various reasons such as promotion of growth are conceivable, it has been found from the experimental results that the thickness of the buffer layer is preferably about 25 to 80 nm.

  Next, as an example of epitaxial growth under atmospheric pressure of the present invention, a case where nitrogen is used as a carrier gas will be described below. The vapor phase epitaxial growth apparatus used in the present invention is a vertical quartz reactor. As shown in FIG. 1, the reactor 1 includes a raw material supply unit 2 and a growth unit 3, and is maintained at respective temperatures. The raw material supply section is heated to 25 to 120 ° C (70 ° C is shown), where a silicon alkoxide as a silicon raw material is partially vaporized from the bubbler 4 (vaporizer). It is carried to the growth section by nitrogen supplied from On the other hand, oxygen for forming crystal and hydrogen chloride (catalyst) for promoting the reaction are introduced into the growth section in the apparatus together with nitrogen at a predetermined partial pressure.

  The growth portion is usually heated between 550 and 850 ° C., which is the crystal growth temperature, and the sapphire serving as the substrate 5 is also held at the same temperature. Then, the ethyl silicate and oxygen supplied to the growth part react efficiently by the catalytic effect of hydrogen chloride, generate crystals, and adsorb and grow on the sapphire (0001) surface to form an epitaxial layer. Nitrogen introduced from the bottom of the apparatus is (1) stagnated in the growth section to promote the reaction, and (2) to induce gas to a predetermined exhaust port, and the total pressure in the apparatus is atmospheric pressure. To be kept. Ethane, carbon dioxide, water, unreacted tetraethoxysilane, and oxygen generated after the reaction are discharged from the exhaust port together with nitrogen as a carrier gas. Note that the rod 6 in the figure is a quartz rod for placing and removing a sapphire substrate thereon. Typical reaction conditions are shown in Table 1. “SCCM” in the table means “Standard cubic centimeter per minute”.

エピタキシャル層の成長速度は、温度とともに増加し、最大３μｍ／ｈの成長速度が得られた。この成長速度は通常のレーザアブレーション法などによる成長速度である０．２５μｍ／ｈ程度に比べて、１２倍ほど大きな成長速度が得られている。

The growth rate of the epitaxial layer increased with temperature, and a maximum growth rate of 3 μm / h was obtained. This growth rate is about 12 times larger than the growth rate of about 0.25 μm / h, which is a growth rate by a normal laser ablation method or the like.

  Moreover, the quartz crystal thin film epitaxial layer of the present invention thus obtained did not contain impurities and had high crystallinity. Therefore, the obtained thin film can be used for an oscillator, a vibrator, a surface acoustic wave device for a high frequency filter, an optical waveguide, a semiconductor substrate, and the like.

    The present invention will be described in detail with reference to examples.

In the apparatus shown in FIG. 1, oxygen (purity 99.9% or more) is flowed at a flow rate of 200 sccm and nitrogen gas (99.9% or more) is flowed at a flow rate of 20 sccm from the upper center of FIG. Silane (purity 99.5%) was heated to 70 ° C., and nitrogen gas was allowed to flow from the left side of FIG. 1 at a flow rate of 95 sccm. Further, hydrogen chloride gas diluted to 5% with nitrogen was flowed at 20 sccm from the right side of FIG. Further, nitrogen gas was allowed to flow at 280 sccm from below in FIG. These flow rates and diluted nitrogen gas were added to adjust the total amount to 800 sccm and the total pressure to 1 atm. At this time, the partial pressure of oxygen was 3.3 × 10 ⁻¹ atm, and the partial pressure of tetraethoxysilane was 3.3 × 10 ⁻³ atm. Sapphire (0001) (lattice mismatch degree 3.3%) was used for the substrate, the substrate temperature was set to 550 to 850 ° C., and a quartz epitaxy thin film having a thickness of 3 μm was obtained. The obtained thin film was evaluated by measuring infrared absorption characteristics using a double crystal X-ray diffraction (DC-XRD), a scanning electron microscope (SEM), and a reflective infrared spectrometer.

  Next, the crystal epitaxial layer obtained in Example 1 will be described. First, as an example of the crystal epitaxial layer obtained in the present invention, the results of X-ray diffraction (XRD) when the growth temperature is 800 ° C. are shown in FIG. The measurement is performed using “RIGAKU RINT 2000”, and the measurement conditions are as shown in Table 2.

図２に示すＸ線回折プロフィールによれば、六方晶水晶相（0003）に対する強い回折ピークが２θ＝５０．６°にあり、成長した層は六方晶系の構造を有していることがわかる。２θ＝４１．８°に見られるピークは、サファイア基板（0006）の回折ピークである。

According to the X-ray diffraction profile shown in FIG. 2, a strong diffraction peak for the hexagonal crystal phase (0003) is at 2θ = 50.6 °, and the grown layer has a hexagonal structure. . The peak seen at 2θ = 41.8 ° is the diffraction peak of the sapphire substrate (0006).

  In the epitaxial film, a specific growth surface is oriented on the substrate surface, so that a peak only on a specific surface appears strongly. In the present invention, since a crystal having a hexagonal crystal structure is grown on a sapphire (0001) substrate, it is expected that the peak of the crystal (000x) appears strongly, and the peak of the crystal (0003) actually appears strongly. This shows that the obtained quartz crystal thin film exhibits c-axis oriented epitaxial growth.

  Next, the full width at half maximum (FWHM) was determined from the peak of two-crystal X-ray diffraction of the crystal phase (0003) grown at each temperature, and the results plotted against each growth temperature are shown in FIG. According to FIG. 3, the full width at half maximum (FWHM) indicating the degree of crystallinity decreased as the growth temperature increased. In Example 1, the value is 10.0 minutes at 850 ° C., and it can be seen that a crystal thin film having excellent crystallinity was produced.

Further, the growth rate at each growth temperature was calculated from the film thickness of a cross-sectional photograph of a scanning electron microscope (SEM) and is shown in FIG. “SHIMADZU SUPERSCAN-220” was used for the measurement. According to FIG. 4, crystallization started from a growth temperature of 600 ° C., and showed a value of 3 μm at 850 ° C. The reciprocal of temperature and the growth rate were plotted, and when the activation energy was determined from the slope of this graph, it was 16.7 kcal / mol. In general, the production of silicon dioxide thin films by thermal decomposition or reaction of tetraethoxysilane and oxygen only tetraethoxysilane (insulating film) is only possible at a high temperature of at 1000 ° C or more, SiO ₂ is on the substrate at that temperature below It has been reported that it does not accumulate. Moreover, the activation energy is as large as 190 kcal / mol (thermal decomposition of tetraethoxysilane only) and 230 kcal / mol (reaction of ethyl silicate with oxygen). From these results, it is considered that HCL has a catalytic effect for promoting the reaction in the production of a crystal thin film by a tetraethoxysilane-oxygen-hydrogen chloride-nitrogen system.

  From the above, it has been found that a crystal epitaxial thin film having excellent crystallinity can be obtained at a lower temperature than the conventional method by using tetraethoxysilane-oxygen-hydrogen chloride-nitrogen as a silicon raw material.

Next, in order to investigate the absorption measurement of the obtained crystal epitaxial layer, infrared rays (2000 to 4000 cm ⁻¹ ) were irradiated and the infrared absorption characteristics were evaluated. For the measurement, “SHIMADZU FTIR-8700” was used. FIG. 5 shows the IR absorption spectrum of the produced quartz thin film. Although absorption at a wave number of 3585 cm ⁻¹ indicating a hydroxyl group (OH group) is observed in the quartz thin film, this spectrum is almost the same as the infrared absorption spectrum of single crystals of quartz reported so far. It was found that a quartz epitaxial thin film having excellent optical characteristics can be obtained.

Before forming the crystal epitaxial growth layer, buffer layers of various thicknesses were provided on the substrate, and the effect of the buffer layer was examined. The buffer layer was prepared by depositing quartz at 550 ° C. and annealing, and a quartz epitaxial layer was grown thereon. DC-XRD was measured about the obtained epitaxial layer, and FWHM was calculated | required. The results are shown in FIG. According to FIG. 6, it can be seen that the crystallinity of the crystal epitaxial thin film varies depending on the thickness of the buffer layer, and the crystallinity is best when it is about 50 nm. In addition, the thing with good crystallinity showed the reduction | decrease of absorption of wave number 3585cm < ^-1 > which shows a hydroxyl group (OH group) in IR absorption spectrum.

1 is a schematic diagram of an apparatus for performing epitaxial growth of a crystal thin film according to the present invention. It is a spectrum figure of two-crystal X-ray diffraction (DC-XRD) of the crystal epitaxial growth layer produced by the tetraethoxysilane-oxygen-hydrogen chloride-nitrogen system. It is a graph which shows the relationship between epitaxial growth temperature and a half value width (FWHM). It is a graph which shows the relationship between epitaxial growth temperature and the growth rate of an epitaxial layer. It is a figure which shows the infrared-light absorption spectrum of the crystal thin film produced by the tetraethoxysilane-oxygen-hydrogen chloride-nitrogen system. It is a graph which shows the relationship between the thickness of a buffer layer, and a half value width (FWHM).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Reaction apparatus 2 Raw material supply part 3 Growth part 4 Vaporizer (bubbler)
5 Substrate 6 Rod 7 Furnace

Claims (3)

Depositing a buffer layer containing an amorphous material of hexagonal crystal on the substrate surface;
On the buffer layer, one or more silicon alkoxides selected from the group of tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane and tetrabutoxysilane are used as a silicon source, and the silicon source is vaporized under atmospheric pressure. The process of introducing,
A quartz thin film produced by a production method comprising: reacting an introduced silicon source with oxygen to deposit a quartz epitaxial thin film on the buffer layer,
The buffer layer has a thickness of 25 nm or more and 80 nm or less,
A quartz thin film, wherein the half width of an X-ray diffraction peak indicating the degree of crystal of the quartz thin film epitaxially grown thereon is smaller than 10.0 minutes. Depositing a buffer layer containing an amorphous material of hexagonal crystal on the substrate surface;
On the buffer layer, one or more silicon alkoxides selected from the group of tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane and tetrabutoxysilane are used as a silicon source, and the silicon source is vaporized under atmospheric pressure. The process of introducing,
A quartz thin film produced by a production method comprising: reacting an introduced silicon source with oxygen to deposit a quartz epitaxial thin film on the buffer layer,
A quartz thin film, wherein a half width of an X-ray diffraction peak indicating a degree of crystal of the quartz thin film is within 1.0 minute when the thickness of the buffer layer is 50 nm. In crystal thin film according to claim 1 or 2, wherein the buffer layer, crystal thin film characterized by using either the same Ku crystal form and crystal is GaN is hexagonal, of ZnO.

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