JPH04260689A - Method of forming thin film of single crystal on substrate - Google Patents

Abstract

PURPOSE:To produce a thin film of single crystal of lithium niobate on a substrate, mainly usable as a second higher harmonic generating element (SHG), by liquid-phase epitaxial growth in the invention. CONSTITUTION:In growing a thin film of single crystal of lithium niobate on a substrate by liquid-phase epitaxial growth, contact face between a holder supporting the substrate and the substrate is chamfered and liquid-phase epitaxial growth is carried out to form the thin film of single crystal of lithium niobate on the substrate.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a lithium niobate single crystal thin film on a substrate, which is mainly used as a thin film waveguide type second harmonic generating element, by a liquid phase epitaxial growth method.

0002

[Background Art] With the progress of optical application technology in recent years,
There is a demand for shorter wavelength light sources. This is because the recording density and photosensitivity can be improved by shortening the wavelength, and applications can be considered in the field of optical equipment such as optical disks and laser printers.

For this reason, the wavelength of the incident laser light is reduced by 1/
Research has been carried out on second harmonic generation (SHG) devices that can convert As such a second harmonic generating element, a bulk single crystal of a nonlinear optical crystal has conventionally been used using a high-output gas laser as a light source. However, there is a strong demand for miniaturization of devices such as optical disk devices and laser printers, and while gas lasers require an external modulator for optical modulation, semiconductor lasers can be directly modulated. Semiconductor lasers have come to be mainly used in place of gas lasers due to their high efficiency and low cost.

[0004] For this reason, thin film waveguide type SHG elements have become necessary because it is necessary to obtain high conversion efficiency with a low light source output of several mW to several tens of mW. Such thin film waveguide type SHG
Conventional nonlinear optical materials for devices include those that use a waveguide made of a layer with a changed refractive index by diffusing Ti etc. into a lithium niobate bulk single crystal, and those that use a waveguide made of a layer with a changed refractive index by diffusing Ti etc. into a lithium niobate bulk single crystal, and those that are made by using a waveguide on a lithium tantalate substrate using a high frequency sputtering method. Waveguides using formed lithium niobate are known, but none of them have been able to achieve high conversion efficiency.

By the way, when the present inventor first formed a waveguide made of a lithium niobate single crystal thin film on a substrate,
We have discovered that a second harmonic generating element with high conversion efficiency can be obtained by adding a different element to the substrate and lattice matching the substrate and lithium niobate.

[0006] As a method for manufacturing such a second harmonic generation element, a foreign element is added to a part of the substrate surface, thermally diffused, and then a lattice-matched lithium niobate single crystal thin film is formed on this surface. Usually, when performing liquid phase epitaxial growth, a desired crystal plane is cut out from a single crystal bulk and used as a substrate, this crystal plane is optically polished to become a liquid phase epitaxial growth surface, and this is held with a holder. The substrate is then immersed in a melt of lithium niobate, and the substrate is gradually pulled up to form a lithium niobate single crystal thin film on the substrate surface.

However, in this conventional method, the edge of the surface parallel to the axial direction of the substrate often has a fine structure formed on the edge of the surface parallel to the normal direction of the crystal growth surface of the substrate, which is the contact point between the substrate and the holder. This resulted in severe scratches, cracks caused by thermal shock, and in extreme cases, the board broke.

This state is illustrated in FIG. 1. That is, when the substrate 1 is held by the holder 2 and immersed in the melt, minute scratches are likely to occur at the contact point 3 between the substrate and the holder.

[0009]

[Problem to be solved by the invention] Therefore, the inventors of the present invention
As a result of various studies to improve this point, it was discovered that the occurrence of scratches could be prevented by chamfering the edges seen on the board, and the present invention was completed.The purpose of the present invention is to prevent any damage to the board. An object of the present invention is to provide a method for manufacturing a lithium niobate single crystal thin film that is lattice-matched to a substrate without any process.

0010

[Means for Solving the Problems] The gist of the present invention is a method for growing a single crystal thin film on a substrate by liquid phase epitaxial growth, which is characterized in that at least the edge of the substrate and the portion that contacts the substrate holder are chamfered. This is a method of forming a single crystal thin film on a substrate. By chamfering,
Chips caused by contact between the holder and the edge of the board,
It can prevent chipping of the edges caused by handling.

That is, in the present invention, if minute scratches are generated on the edges by chamfering the edges of the substrate, large cracks will occur from the scratches during heating, and the substrate will break. This prevents the occurrence of minute scratches that tend to occur on the edges. This state is shown in FIG. In the drawing, the edge 3 of the surface parallel to the axial direction of the substrate, which is the contact point between the substrate 1 and the holder 2, is chamfered to form the holder.
It is installed on the

The present invention will be described in detail below. The substrate may be made of ZnO, Gd3Ga5012, MgO, alumina, etc., but lithium tantalate is the best. The reason for using lithium tantalate is that the crystal system of the lithium tantalate substrate is similar to that of lithium niobate single crystal, making it easy to grow epitaxially, and furthermore, since the lithium tantalate substrate is commercially available, it is possible to obtain a high-quality one. This is because it can be obtained stably.

The present invention is particularly advantageous when obtaining a lithium niobate single crystal thin film that is lattice matched to a lithium tantalate substrate. When the substrate is made of lithium tantalate, the manufacturing method for this substrate is CZ (Czochralski).
It is desirable to manufacture it by the method. As raw materials, for example, tantalum pentoxide, lithium carbonate, titanium oxide, vanadium pentoxide, etc. are used. These raw materials are placed in an iridium crucible,
Alternatively, it is advantageous to pull the lithium tantalate single crystal by heating and melting it in a platinum-rhodium crucible.

A plane (0001) perpendicular to the c-axis is cut out from the obtained lithium tantalate bulk single crystal and used as a substrate. Lithium tantalate single crystal is hexagonal,
The (0001) plane is shown in FIG. As a substrate (0001
) plane is used because the (0001) plane consists of only the a-axis, and as will be described later, it is easy to adjust the lattice matching with the lithium niobate single crystal.

In the present invention, it is preferable to chamfer the edges of the (0001) plane. The preferred range for the R surface of the chamfer is JIS B0701 R0.1 to R2.0, and the preferred range for the C surface is JIS B0701 C0.
．． 1 to C2.0. These R side and C side are JIS
It is defined by B 0701, and the preferred range for the R plane is the value expressed by the radius of a circle (mm), and the preferred range for the C plane is by cutting at an angle of 45° and determining the length of the cut. The value is expressed in mm.

In the present invention, a lithium niobate single crystal thin film is formed on this substrate, but in this case, the substrate and the lithium niobate single crystal thin film must be lattice matched. The lattice matching will be explained below. Lattice matching means that the lattice constant of the thin film is 99.81 to 100.07% of that of the substrate, preferably 99.93 to 100.03%.

The means for lattice matching the lithium niobate single crystal thin film and the lithium tantalate substrate is not particularly limited, but the a-axis lattice constant of lithium tantalate is the a-axis lattice constant of ordinary lithium niobate. (5.148
Å), the lattice constant of lithium niobate formed on lithium tantalate is increased. As a means for this purpose, it is preferable to mix a different element into lithium niobate, and it is advantageous to include Na and Mg in the lithium niobate single crystal thin film. The reason for this is that N
The ions or atoms of a and Mg have the effect of increasing the lattice constant (a-axis) of lithium niobate by substitution or doping in the crystal lattice of lithium niobate. Na
By adjusting the composition of Mg and Mg, lattice matching between the lithium tantalate substrate and the lithium niobate single crystal can be easily obtained. In particular, Na can greatly increase the lattice constant of lithium niobate. Although Mg can also increase the lattice constant, it is not as effective as Na. However, it has the important effect of preventing photodamage. Moreover, when the above-mentioned Na and Mg are contained, the content is,
The amount of Na is preferably 0.1 to 14.3 mol%, preferably 0.3 to 4.8 mol%, and the amount of Mg is preferably 0.8 to 10.8 mol%, respectively, based on the lithium niobate single crystal. teeth,
The content is preferably 3.5 to 8.6 mol%. The reason is that when the Na content is less than 0.1 mol%,
Regardless of the amount of Mg added, the lattice constant is not large enough to match the lattice constant of the lithium tantalate substrate, and
If it exceeds 14.3 mol %, the lattice constant becomes too large, and in either case, lattice matching between the lithium tantalate substrate and the lithium niobate single crystal cannot be obtained.

[0018] Na, Mg in single crystal of lithium niobate
FIG. 6 shows the relationship between the content of lithium niobate and the a-axis lattice constant of the lithium niobate single crystal. Another way to increase the lattice constant of lithium niobate is to use Li/N of lithium niobate.
There is a way to change the ratio of b. The state of the change is shown in FIG. From this figure, the molar ratio of Li/Nb is 41/59 ~
56/44 is preferred.

As another means, when lattice matching lithium niobate with a single crystal substrate whose a-axis lattice constant is smaller than 5.148 Å, a different element is mixed with lithium niobate to reduce the lattice constant of lithium niobate. to match the lattice. When Ti is added as a different element, the a-axis lattice constant of lithium niobate changes as shown in FIG. The appropriate amount of Ti to be added is 0.2 to 30 moles.

As a means of lattice matching, a different element can be added to the lithium tantalate substrate to match the a-axis lattice constant of lithium niobate. in this case,
Examples of the addition means include a diffusion means or a method of adding the material to the raw material and pulling it up using the CZ method and molding it into a substrate.

In the present invention, lithium tantalate used as a substrate can be used if it has a hexagonal crystal structure and an a-axis lattice constant of 5.128 to 5.173 Å.
The shape of the substrate is not limited to a flat plate shape, and may be either a fiber shape or a bulk shape. In the present invention, a lithium tantalate substrate (
0001) is preferable.

[0022] The lithium tantalate substrate (0001
) plane refers to a plane perpendicular to the c-axis of lithium tantalate. The reason why it is desirable to use the (0001) plane of the lithium tantalate substrate as the growth plane of the lithium niobate single crystal thin film is that the lithium tantalate has a hexagonal crystal structure (see Figure 3), and the This is because the (0001) plane is composed of only the a-axis, so it can be lattice-matched with the lithium niobate single crystal thin film simply by changing the lattice constant of the a-axis.

The surface roughness of the thin film forming surface of the lithium tantalate substrate greatly influences the crystallinity of the lithium niobate single crystal. The surface roughness of the lithium niobate single crystal thin film forming surface of the lithium tantalate substrate is JIS B0601, R.
It is desirable that max=10 to 1000 Å. The reason for this is that it is extremely difficult to make the value of Rmax smaller than 10 Å, and if the value of Rmax becomes larger than 1000 Å, the crystallinity of the lithium niobate single crystal thin film decreases.

In the present invention, the lattice constant is measured by ordinary powder X-ray diffraction. The lattice constant is Cu
It is calculated by the least squares method using the 2θ values of 15 peaks of lithium niobate detected at −2θ=45 to 90° and the surface index thereof. Note that in the measurement, Si is used as an internal standard. As a specific method for lattice matching, a liquid phase epitaxial growth method is used, and Li2O, Nb2O5
, V2O5, Na2O, MgO, etc., and the a-axis lattice constant of the lithium niobate single crystal thin film is matched to the a-axis lattice constant of the lithium tantalate substrate.

In the present invention, the composition range of Li2O, V2O5, and Nb2O5 excluding Na2O and MgO is A (88.90, 2.22, 8.88 ), B
(55.00, 43.00, 2.00), C (46.5
0,51.50,2.00), D(11.11,80.
00, 8.89), E (37.50, 5.00, 57.
50) is advantageously within the composition range indicated by the region surrounded by the five composition points. Note that the composition point is (L
mol% of i2O, mol% of V2O5, mol% of Nb2O5).

[0026] Furthermore, Li excluding the Na2O and MgO
The composition range of 2O, V2O5, and Nb2O5 is Li
In the triangular diagram of the three-component system 2O-V2O5-Nb2O5, F (49.49, 45.46, 5.05), G
(11.11, 80.00, 8.89), H(42
．． 81, 22.94, 34.25), and the Li
The composition range of 2O-V2O5-Nb2O5 is I(47.64, 46.12, 6.
24), J (27.01, 64.69, 8.30)
, K(36.71, 37.97, 25.32), L(
The range surrounded by the four composition points M(44.05, 32.97, 22.98) is suitable, and further M(45.36,
46.45, 8.19), N (32.89, 57.05
, 10.06), O(36.71, 44.30, 18.
99), P(44.95, 40.54, 14.51) is the optimum range (see FIG. 7).

The reason why it is advantageous to use such a composition range is that lattice matching between the lithium niobate single crystal thin film made of Na and Mg and the lithium tantalate substrate is facilitated, and the resultant lithium niobate single crystal thin film is This is because it has excellent optical properties, particularly low optical propagation loss, and can yield a high-quality lithium niobate single crystal thin film. In addition, as the composition ratio of Na2O in the present invention, the molar ratio is Na2O.
2O/Li2O is 2.0/98.0 to 93.5/6.
It is desirable that the range satisfies 5.

The reason for this is that Na2
This is because if the proportion of O deviates, it is difficult to lattice match the lithium tantalate substrate and the lithium niobate single crystal thin film. By the epitaxial growth method of the present invention, at least K2O and V2 are added to the melt as a flux.
It is advantageous to add O5.

The reason for this will be explained below. By using K2O, V2O5 as a melting agent, L from the melting agent can be reduced.
Since the supply of i can be prevented, Li2O and Nb in the raw materials can be prevented.
By changing the composition ratio of 2O5, the Li/Nb ratio in the precipitated lithium niobate single crystal thin film can be changed. When the Li/Nb value changes, the a-axis lattice constant also changes, so the a-axis lattice constant of the lithium niobate single crystal thin film can be controlled by controlling the composition ratio of Li2O and Nb2O5 in the raw materials, A lithium niobate single crystal thin film and a lithium tantalate substrate can be lattice matched.

[0030] Furthermore, the above-mentioned K2O, V2O5, Li2O,
Na2O or MgO may be added to the melt made of Nb2O5. The reason for this is that by adding Na2O or MgO, the a-axis lattice constant of the lithium niobate single crystal thin film can be increased. Also, MgO
can prevent photodamage. The melt composition has a molar ratio of L
It is desirable that i2O/Nb2O is 43/57 to 56/44, preferably 43/57 to 50/50. The reason for this is that when the content is outside the above range, the crystal structure of the precipitated lithium niobate single crystal changes and the optical properties deteriorate. Further, it is preferable that the theoretical amount of MgO that can be precipitated from the MgO/raw material composition of lithium niobate single crystals does not exceed a molar ratio of 30/100. The reason for this is that if the above ratio is exceeded, Mg niobate crystals will precipitate.

[0031] Furthermore, the composition of K2O and V2O5 is K2O
2. It is desirable that the theoretical amount of lithium niobate single crystal that can be precipitated from the melting agent (KVO3 equivalent)/raw material composition consisting of V2O5 is in a range that satisfies a molar ratio of 25/75 to 75/25. The reason for this is that if it is outside the above range, the crystal structure of the precipitated lithium niobate single crystal will change and the optical properties will deteriorate. Also, K2
It is advantageous that the composition ratio of O and V2O5 is 1:1 in terms of molar ratio.

Furthermore, in the present invention, lattice matching is achieved by adjusting the a-axis lattice constant of the lithium tantalate substrate by adding a different element to match the a-axis lattice constant of the lithium niobate crystal. can be done. Next, after bringing the melt having the above composition into a supercooled state, the lithium tantalate substrate is brought into contact with the melt. The cooling rate for bringing the melt into a supercooled state is 0.5 to 30
Preferably, the rate is 0°C/hour.

[0033] Also, the temperature for epitaxial growth is 6
The temperature is preferably 00 to 1250°C. The reason for this is that the melting point of lithium niobate is 1250°C, and crystals cannot form at temperatures higher than this, and 600°C is the melting point of the melting agent, so at lower temperatures the raw material becomes a melt. This is because they are unable to do so. During the epitaxial growth, it is desirable to rotate the lithium tantalate substrate. This is because crystals with uniform properties and film thickness can be formed by rotating the lithium tantalate substrate. Preferably, the substrate is rotated in a horizontal state. The rotation speed is preferably 50 to 150 rpm.

Further, it is preferable that at least the growth surface of the lithium tantalate substrate is optically polished and then chemically etched. The thickness of the lithium tantalate substrate is preferably 0.5 to 2.0 mm. The reason for this is that substrates thinner than 0.5 mm are prone to cracking, and substrates thicker than 2.0 mm are prone to pyroelectric effects (discharge effects due to heating), and are charged due to heating and polishing, so polishing debris, etc. This is because scratches are likely to occur due to adhesion. The thickness of the lithium niobate single crystal thin film of the present invention crystallized on the lithium tantalate substrate is:
It can be controlled by appropriately selecting the contact time between the lithium tantalate substrate and the melt and the temperature of the melt.

The growth rate of the lithium niobate single crystal thin film of the present invention is preferably 0.01 to 1.0 μm/min. If it is faster than this, waviness will occur in the lithium niobate single crystal thin film, and if it is slower than this, it will take too much time to grow the thin film. In the present invention, it is desirable to remove flux from the surfaces of the lithium tantalate substrate and the lithium niobate single crystal thin film after liquid phase epitaxial growth. This is because if the flux remains, the film thickness will become non-uniform. The removal of the flux includes:
It is preferable to perform this by rotating a lithium niobate single crystal thin film formed on a lithium tantalate substrate at 100 to 10,000 rpm.

The time required for the rotation is preferably 5 to 60 minutes. The stirring time of the melt is preferably 6 to 48 hours in a suitable stirring operation. The reason for this is that if the stirring time is short, undissolved crystal nuclei are generated in the melt, and crystal growth occurs around these crystal nuclei, resulting in unevenness on the surface of the lithium niobate single crystal thin film. be. The surface roughness of the lithium niobate single crystal thin film is JIS BO601 Rmax=0.001~0.
1 μm is suitable.

As a method for manufacturing the lithium tantalate substrate, a CZ (Czochralski) method is preferable. Moreover, examples of raw materials include lithium carbonate, tantalum pentoxide, titanium oxide, and vanadium pentoxide. Also,
The a-axis lattice constant of the lithium tantalate substrate can be increased by adding a different element such as sodium. By the way, in order to use lithium niobate single crystal as a constituent material of optical devices such as SHG elements, the lithium niobate and lithium tantalate single crystals have optically useful properties such as electro-optic effects and nonlinear optical effects. It is necessary to have the characteristics.

In general, in order for lithium niobate or lithium tantalate single crystals to have various optically useful properties such as electro-optic effects and nonlinear optical effects, they must be prepared in the manufacturing process.
It is necessary to heat the crystal to a temperature above the Curie point and apply an electric field to polarize the crystal. Furthermore, it is known that single crystals of lithium niobate, lithium tantalate, and the like containing different elements cannot be easily poled. However, the lithium niobate single crystal thin film of the present invention is always in a polarized state even if the lithium tantalate substrate is in a polarized state or even if it is electrically neutralized by polarization reversal. It exhibits various properties such as excellent electro-optic effect and nonlinear optical effect.

Therefore, the lithium niobate single crystal thin film and the lithium tantalate substrate of the present invention do not require a poling process, so the manufacturing process is simple, and it is possible to use different elements, which were difficult to use in the past. It has the advantage that a lithium tantalate substrate containing lithium tantalate can be used. As a different element,
At least one selected from Mg, Ti, V, Cr, Fe, Ni, Nd, etc. is desirable. A method for increasing the difference in refractive index between the thin film waveguide layer and the substrate is to decrease the refractive index of the lithium tantalate substrate or to increase the refractive index of the lithium niobate thin film.

For this purpose, M on the lithium tantalate substrate
g or V to a lithium niobate thin film of Ti, Cr, Fe.
It is preferable to contain different elements such as , Ni, and Nd. In particular, when performing SHG oscillation, it is necessary to increase the difference in refractive index between the lithium tantalate substrate and the lithium niobate thin film. When the lithium tantalate substrate contains the different element, the different element does not need to be present uniformly over the entire substrate. In the present invention, niobium is added to the lithium tantalate substrate by adding a different element to a specific location of the lithium tantalate substrate to form a pattern in which the refractive index of the waveguide forming portion is relatively lower than that of the non-forming portion. By simply forming a lithium oxide single crystal thin film in the form of a slab, the lithium niobate single crystal thin film formed in the pattern portion becomes a waveguide, and the processing steps for forming the waveguide can be omitted.

The method of making the substrate refractive index of the waveguide forming portion relatively lower than that of the non-forming portion is to lower the substrate refractive index of the waveguide forming portion or increasing the substrate refractive index of the non-forming portion. This is desirable. When different elements exist non-uniformly, a portion that meets the waveguide conditions becomes a waveguide.

[0042] The content of the different element in the surface portion of the lithium tantalate substrate is preferably within the composition range shown below. Ti; 0.2 to 30 mol% Cr;
0.02-20 mol%Fe; 0.02-20 mol%
Ni; 0.02 to 20 mol% Nd;
0.02-10 mol% Mg; 0.1-
20 mol % V; 0.05 to 30 mol % The above content is calculated by: different element/(LiTaO3 + different element) x 100.

The lithium niobate single crystal thin film of the present invention also contains chromium (Cr), neodymium (Nd), rhodium (Rh), and zinc (Zn) in order to change optical properties such as refractive index as necessary. ), nickel (Ni), cobalt (Co), titanium (Ti), and vanadium (V). The content of each of these elements is Rh; 0.05 to 20 mol% Zn; 0.02 to 30 mol% Ni;
0.1 to 20 mol% Co; 0.0
5 to 20 mol% Cr; 0.02 to 20 mol%
Ti: 0.2 to 30 mol% Nd: 0.
02 to 10 mol% V; 0.05 to 3
It is desirable that it be 0 mol%.

[0044] Next, the present invention will be explained in more detail with reference to Examples.

[Example] Example 1 (1) Na2CO3 20 mol%, Li2CO3 30
Mol%, V2O5 39 mol%, Nb2O5 11 mol%
A mixture in which 2 mol% of MgO was added based on the theoretical amount of LiNbO3 that can be precipitated from the melt composition was placed in a platinum crucible, and in an epitaxial growth growth apparatus under an air atmosphere.
The contents of the crucible were dissolved by heating to 1100°C. Further, the melt was stirred using a propeller at a rotation speed of 100 rpm for 12 hours.

(2) The (0001) plane of a 2 mm thick lithium tantalate single crystal was optically polished and then chemically etched. Then, chamfering (R 0.5) was performed using waterproof abrasive paper (#2000). The surface roughness of this substrate was JIS B0601 Rmax=100 Å. After the melt was gradually cooled to 915° C. at a cooling rate of 60° C. per hour, the substrate was preheated at 915° C. for 30 minutes, and then immersed in the melt for 8 minutes while rotating at 100 rpm. The growth rate of lithium niobate was 1 μm/min.

(3) Pull up the substrate material from the melt, shake it off on the melt for 30 seconds at a rotational speed of 1000 rpm, and then slowly cool it to room temperature at a rate of 1° C./min, leaving a layer of about 8 μm on the substrate material. A single crystal thin film of lithium niobate containing sodium and magnesium was obtained. (4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 3 mol% and 2 mol%, respectively.
Met. Also, the lattice constant (a-axis) of the thin film is 5.156 Å
, the refractive index measured at an incident light wavelength of 1.15 μm is 2.23.
It was 5±0.001. The surface roughness of lithium niobate was JIS BO601 Rmax=1000 Å.

Example 2 (1) 51 mol% of Li2CO3, 39 mol% of V2O5, 10 mol% of Nb2O5, 45% of the theoretical amount of LiNbO3 that can precipitate Na2CO3 from the melt composition.
Mol% addition, LiN that can precipitate MgO from the melt composition
A mixture containing 6 mol% of bO3 relative to the theoretical amount was placed in a platinum crucible and heated to 1100°C in an air atmosphere in an epitaxial growth growth apparatus to melt the contents of the crucible. Then, the melt was heated at 100 rpm using a propeller.
Stirring was continued for 6 hours at a rotational speed of m.

(2) After optically polishing the (0001) plane of a lithium tantalate single crystal with a thickness of 1.5 mm, chamfering (C 1.0) was performed. The surface roughness of the substrate is JIS BO601, Rmax
=170 Å. After slowly cooling the melt to 922°C at a cooling rate of 60°C per hour, the substrate was cooled to 922°C.
The sample was preheated for 10 minutes and immersed in the melt for 23 minutes while rotating at 100 rpm. The growth rate of lithium niobate was 1.65 μm/min.

(3) Pull up the substrate material from the melt, shake it off on the melt for 30 seconds at a rotation speed of 1000 rpm, and then heat the Curie of lithium tantalate single crystal at a cooling rate of 300°C per hour. After cooling slowly to room temperature and keeping it at that temperature for 1 hour, it was slowly cooled to room temperature at a cooling rate of 60°C per hour, and a layer of sodium and magnesium-containing lithium niobate with a thickness of about 38 μm was deposited on the substrate material. A crystal thin film was obtained.

(4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 1.5 mol% and 5.5 mol%, respectively. Also, the lattice constant (a-axis) is 5.155 Å, and the incident light wavelength is 1.15 μ.
The refractive index measured in m was 2.231±0.001. The surface roughness of lithium niobate single crystal thin film is JIS
BO601 Rmax=1000 Å.

Example 3 (1) Na2CO3 12.0 mol%, V2O5 39
mol%, Nb2O5 11 mol%, Li2CO3 38
．． 0 mol%, LiN that can precipitate MgO from the melt composition
A mixture containing 5 mol% of bO3 relative to the theoretical amount was placed in a platinum crucible and heated to 1100° C. in an air atmosphere in an epitaxial growth growth apparatus to melt the contents of the crucible. The melt was then stirred for 20 hours using a propeller at a rotation speed of 150 rpm.

(2) Using water-resistant abrasive paper (#2000), chamfer (R1) a 1 mm thick lithium tantalate substrate.
．． 0), a MgO layer with a thickness of 250 Å was formed by sputtering and thermally diffused at 940° C. to form a diffusion layer with a thickness of 250 Å. The refractive index was 0.015 lower than when MgO was not diffused. Further, the surface roughness of the substrate surface was JIS BO601 Rmax=170 Å. The amount of MgO diffused was 10 mol%.

(3) After slowly cooling the melt to 938°C at a cooling rate of 60°C per hour, the substrate was cooled at 938°C for 5 minutes.
It was preheated for 0 minutes and immersed in the melt for 20 minutes while rotating at 100 rpm. The growth rate of lithium niobate was 0.7 μm/min. (4) Pull up the substrate material from the melt and rotate at 100 r.
pm for 30 seconds on the melt, after shaking off the melt, 1
The mixture was slowly cooled to room temperature at a rate of .degree. C./min to obtain a lithium niobate single crystal thin film containing sodium and magnesium with a thickness of about 14 .mu.m on the substrate material.

(5) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 1 mol % and 6 mol %, respectively. Also, the lattice constant (
a-axis) was 5.153 Å, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.231±0.001. The surface roughness of lithium niobate single crystal thin film is JIS BO6
01 Rmax=1400 Å. When the Bockels constant of this thin film was measured using a Mach-Zehnder interferometer, r=22.0×10-12 m/V, r13=
6.6×10 −12 m/V was obtained. AppliedPh
ysics Letters, Vol. 24, No.
9, 1974, r33=12×10−12 m/
V, r13 = 2.3 x 10-12 m/V, which is 2-3 times the known value.

Example 4 (1) Li2CO3 50 mol%, V2O 540 mol%,
Nb2O5 10 mol%, 43 mol% added to the theoretical amount of LiNbO3 that can precipitate Na2CO3 from the melt composition, LiNbO that can precipitate MgO from the melt composition
A mixture containing 7 mol% of the theoretical amount of No. 3 was placed in an iridium crucible and heated to 1100° C. in an air atmosphere in an epitaxial growth growth apparatus to melt the contents of the crucible. Then, the melt was heated at 200 r using a propeller.
Stirring was continued for 12 hours at a rotational speed of pm.

(2) The melt was slowly cooled to 915°C at a cooling rate of 60°C per hour. The (0001) plane of the lithium tantalate single crystal was optically polished to a thickness of 1.8 mm. Next, chamfering (C0.5) was performed using water-resistant abrasive paper (#2000) and wrapping film (abrasive grain 1 μm).
I did it. A MgO film with a thickness of 1000 Å and a width of 5 μm was formed by photolithography and RF sputtering, and then thermally diffused at 920° C. to obtain a substrate material having an MgO diffusion channel of 1000 Å and a width of 5 μm. The thickness of the diffusion layer was 1000 Å. The ordinary refractive index of this channel portion was reduced by 15×10 −3 compared to the portion where MgO was not diffused. In addition, the surface roughness is JI
SBO601 Rmax=300 Å. This substrate material was preheated at 915° C. for 30 minutes and then immersed in the melt for 17 minutes while rotating at 100 rpm. The growth rate was 1.94 μm/min.

(3) Pull up the substrate material from the melt, shake it off on the melt for 30 seconds at a rotational speed of 1000 rpm, and then cool the Curie temperature of the lithium tantalate single crystal at a cooling rate of 300°C per hour. (650℃), kept at that temperature for 1 hour, and then slowly cooled to room temperature at a cooling rate of 60℃ per hour. A lithium single crystal thin film was obtained.

(4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 2 mol % and 6 mol %, respectively. Also, the lattice constant (
a-axis) was 5.155 Å, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.231±0.001. (5) The obtained lithium niobate single crystal thin film was
When the end face was polished perpendicularly to the MgO diffusion channel with a width of 5 μm and the laser beam was incident on the end face, the near-field pattern of the emitted light was observed.
It was confirmed that gO was well confined on the diffusion channel. The surface roughness of the obtained lithium niobate single crystal thin film was JIS BO601 Rmax=1000 Å.

Example 5 (1) Li2CO3 26.4 mol%, Nb2O5 2
8.6 mol%, K2CO3 22.5 mol%, V2O5
A mixture containing 22.5 mol% of MgO and 5 mol% of the theoretical amount of LiNbO that can be precipitated from the melt composition was placed in a platinum crucible and heated to 1100°C in an air atmosphere in an epitaxial growth growth apparatus. The contents of the crucible were dissolved.

(2) The melt was slowly cooled to 893°C at a cooling rate of 60°C per hour. After optically polishing the (0001) plane of the lithium tantalate single crystal to a thickness of 1.5 mm, photolithography and RF sputtering were used to
After forming an MgO film with a thickness of 800 Å and a width of 5 μm, and a Cu film with a thickness of 400 Å on a portion other than the MgO film with a width of 5 μm, thermal diffusion was performed at 1000°C. Diffusion layer is MgO, C
Both uO was 500 Å. Then, MgO with a width of 5 μm
The material with diffusion channels was chemically etched and used as a substrate material. Chamfering was performed using 1 μm diamond slurry. (R 0.5)

The channel portion in which MgO is diffused and the portion in which Cu is diffused other than the channel portion each have an ordinary refractive index of 1 compared to the substrate material in which nothing is diffused.
There was a decrease of 0x10-3 and an increase of 1x10-3. The surface roughness was JIS BO601 Rmax=400 Å. This substrate material was placed at a position of 15 mm above the melt.
It was preheated at 3° C. and then immersed in the melt for 11 minutes while rotating at 100 rpm. The growth rate of the lithium niobate single crystal thin film was 0.54 μm/min.

(3) Pull up the substrate material from the melt, shake it off on the melt for 30 seconds at a rotation speed of 1000 rpm, slowly cool it to room temperature at a cooling rate of 1° C./min, and place it on the substrate material. A MgO-containing lithium niobate single crystal thin film with a thickness of about 6 μm was obtained. The lattice constant (a-axis) is 5.153 Å
, the refractive index measured at an incident light wavelength of 1.15 μm is 2.2
It was 30±0.001. The surface roughness of the obtained lithium niobate single crystal thin film was JIS BO601 R.
max=1400 Å. An SHG element was manufactured by ion beam etching the thickness of this thin film to a phase matching film thickness (2.50±0.05 μm). A laser with a wavelength of 830 nm is incident on this SHG element, and the second harmonic (415 nm) is
nm) was confirmed.

Example 6 (1) 48 mol% of Li2CO3, 42 mol% of V2O5, 10 mol% of Nb2O5, 43% of the theoretical amount of LiNbO3 that can precipitate Na2CO3 from the melt composition.
Mol% addition, LiN that can precipitate MgO from the melt composition
A mixture containing 7 mol% of bO3 relative to the theoretical amount was placed in an iridium crucible and heated to 1100° C. in an air atmosphere in an epitaxial growth apparatus to melt the contents of the crucible. Next, the molten material is heated using a propeller for 2
The mixture was stirred for 12 hours at a rotation speed of 0.00 rpm.

(2) The melt was slowly cooled to 915°C at a cooling rate of 60°C per hour. The (0001) plane of the lithium tantalate single crystal was optically polished to a thickness of 1.8 mm. Then, chamfering (C 0.2) was performed using water-resistant abrasive paper (#2000) and 1 μm diamond slurry. A MgO film with a thickness of 1000 Å and a width of 5 μm was formed by photolithography and RF sputtering. Next, this substrate was brought close to the melt in the epitaxial growth apparatus at 2.67 cm/min, and preheating and thermal diffusion were performed simultaneously. The thickness of the diffusion layer was 1000 Å.

The substrate material was immersed in the melt for 17 minutes while rotating at 100 rpm. Growth rate is 1.94
It was μm/min. (3) Pull up the substrate material from the melt and rotate at 1000 rotations.
After shaking off the melt on the melt for 30 seconds at rpm,
Slowly cool the lithium tantalate single crystal to the Curie temperature at a cooling rate of 300°C per hour, maintain it at that temperature for 1 hour, and then slowly cool it to room temperature at a cooling rate of 60°C per hour,
A lithium niobate single crystal thin film containing sodium and magnesium with a thickness of about 37 μm was obtained on the substrate material. (4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were each 2
The mol% was 6 mol%. Also, the lattice constant (a-axis) is 5.
The refractive index measured at 155 Å and an incident light wavelength of 1.15 μm was 2.231±0.001.

(5) The end face of the obtained lithium niobate single crystal thin film was polished perpendicularly to the MgO diffusion channel with a width of 5 μm, and a laser beam was incident on the end face to form a near-field pattern of the emitted light. When the laser beam was observed, it was confirmed that the laser light was well confined on the MgO diffusion channel with a width of 5 μm. The surface roughness of the obtained lithium niobate single crystal thin film was determined according to JIS BO601.
Rmax=900 Å.

Comparative Example 1 (1) Na2CO3 20 mol%, Li2CO3 30
Mol%, V2O5 39 mol%, Nb2O5 11 mol%
A mixture in which 2 mol% of MgO was added based on the theoretical amount of LiNbO3 that can be precipitated from the melt composition was placed in a platinum crucible, and in an epitaxial growth growth apparatus under an air atmosphere.
The contents of the crucible were dissolved by heating to 1100°C. Further, the melt was stirred using a propeller at a rotation speed of 100 rpm for 12 hours.

(2) The (0001) plane of a 2 mm thick lithium tantalate single crystal was optically polished and then chemically etched. The surface roughness of this board is JIS BO601 R
max=100 Å. 60 melt per hour
After slowly cooling the substrate to 915°C at a cooling rate of 915°C,
After preheating at 15°C for 30 minutes, the melt was heated at 100 rpm.
After shaking the melt over the melt for 30 seconds at m
Although the substrate was slowly cooled down to room temperature at a rate of 1 minute, large cracks had formed in the substrate.

The results of Examples 1 to 6 and Comparative Example 1 are listed in Table 1 regarding the condition of the substrate and the propagation loss of the thin film.

[0070]

[Table 1]

Example 7 This example is basically the same as Example 1, but the thin film and substrate are made of Ti, Cr, Ni, Nd, V, Fe, Rh, and C.
Zn was diffused as a different element (to a thickness of 500 Å), and the propagation loss was measured. The results are shown in Table 2. All showed good propagation loss.

[0072]

[Table 2]

[0073] The numbers in the table are propagation losses (dB/cm), and the content of foreign elements is the average value near the surface.

[0074]

[Effects of the Invention] As described above, in the present invention,
A method in which a lithium niobate single crystal thin film lattice-matched to the substrate is grown by liquid phase epitaxial growth on a surface obtained by doping and thermally diffusing a different element to at least a portion of the substrate, parallel to the a-axis direction of the substrate. By chamfering the edge of the surface (0001), it was possible to prevent scratches that are likely to occur at the contact points between the substrate and the holder.

[0075]

[Brief explanation of the drawing]

[Figure 1] Explanatory diagram of a state in which a conventional board is held in a holder

[Fig. 2] An explanatory diagram of a state in which a chamfered substrate according to the present invention is held in a holder.

[Figure 3] Schematic diagram showing the (0001) plane of a lithium tantalate substrate, which is the growth plane of a niobium lithium single crystal thin film

[Figure 4] Relationship diagram between the li/Nb molar ratio of lithium niobate and the a-axis lattice constant of lithium niobate single crystal

[Figure 5] T
Relationship diagram between i content and a-axis lattice constant of lithium niobate single crystal

[Figure 6] Relationship diagram between the contents of Na and Mg and the a-axis lattice constant of lithium niobate single crystal

[Figure 7] Triangular composition diagram of LiO-VO-Nb ternary components

[Explanation of symbols]

1 Board 2 Holder 3 Chamfered surface

Claims (2)

[Claims] 1. A method for forming a single crystal thin film on a substrate by liquid phase epitaxial growth, the method comprising chamfering at least the contact area between the edge of the substrate and the substrate holder. . 2. The method for forming a lithium niobate single crystal thin film on a substrate according to claim 1, wherein a lithium niobate single crystal thin film is formed on the substrate.