JPH0695191A - Formation of proton exchange layer - Google Patents

Abstract

PURPOSE:To provide the method for formation of the proton exchange layer which can form a channel type proton exchange layer on the Y-cut face or X-cut face of a dielectric crystal. CONSTITUTION:A substrate 1 is an X-or Y-cut substrate consisting of a lithium tantalate or lithium niobate crystal. This method includes a stage for forming a pattern mask having the slit width at which the generation of the corrosion by a proton exchange source is obviated, i.e., slits of over 0mum and <=10mum width on the main surface of the X-cut face inclusive of Y-Z axes or Y-cut face inclusive of X-Z axes and a stage for executing the proton exchange by bringing the proton exchange source into contact with the main surface via the slits.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming a proton-exchanged layer, in which an extended core or dielectric waveguide made of a dielectric material surrounded by a clad having a low refractive index, particularly a three-dimensional proton-exchanged layer is formed. .

[0002]

2. Description of the Related Art Three-dimensional waveguides are used for wavelength conversion elements using second harmonic generation or quasi-phase matching (QPM). The second harmonic generation element (QPM-SHG element) utilizing this quasi phase matching is a wavelength conversion element having a periodic domain inversion structure along the extension direction of the channel type waveguide. Quasi-phase matching uses, for example, the fact that the output of the second harmonic periodically repeats the maximum and minimum for each coherence length (coherence length) as it propagates, and the sign of the polarization wave generated for each coherence length is used. Is alternately inverted (periodic domain inversion structure), and the output is increased by adding the outputs of the second harmonics. As shown in FIG. 1, as the wavelength conversion element, a Z-cut substrate 1 of LiNbO ₃ crystal having a domain inversion characteristic (a Z-cut surface 2 including the Z axis of the crystal as a normal and including an XY axis as a main surface Substrate) is mainly used. Thus, the three-dimensional waveguide is
A channel type waveguide 2 extending in a dielectric crystal substrate,
It is used for the proton exchange layer of the periodic domain inversion structure 3 along the extension direction of the channel type waveguide (dotted arrow indicates the polarization direction).

Lithium tantalate (LiTaO ₃ ) which is resistant to light damage
Is also used as a crystal substrate, but since the Curie point of LiTaO ₃ is as low as about 600 ° C., lithium niobate crystal (LiNbO 3
_The well-known method of forming a proton exchange layer by diffusion of titanium (Ti) cannot be applied to LiTaO ₃ crystals. Further, even if the proton exchange layer is formed on the LiTaO ₃ crystal substrate by Ti diffusion, the polarization direction is random, so in order to effectively use this proton exchange layer, the substrate needs to be subjected to a poling treatment at a Curie point or higher. is there. Therefore, for a lithium tantalate crystal substrate, a proton exchange method capable of producing a proton exchange layer in a low temperature process of 300 ° C. or lower is widely used.
Z-X with the Y-axis of the substrate or the crystal having the X-axis of the crystal shown in FIG.
It has been said that the substrate having the Y-cut surface 5 including the axis as the main surface is not suitable for forming a proton exchange layer because of corrosion due to proton exchange (Kazuhisa YAMAMOTO, Kiminori MI
ZUUCHI and Tetsuo TANIUCHI “Low-Loss Channel Waveg
uides in MgO: LiNbO ₃ ans LiTaO ₃ by Pyrophosphoric A
cid Proton Exchange ”Jpn. J. Appl. Phys. Vol.31 (1
992) pp. 1059-1064 Part 1, No. 4, April 1992).

In addition, X or Y of lithium niobate crystal
When the proton exchange is directly performed on the cut plate, corrosion occurs similarly to the lithium tantalate crystal, which is not suitable for the production of the proton exchange layer. However, in the case of a lithium niobate crystal, proton exchange after diffusion of Ti or direct proton exchange to a lithium niobate crystal after MgO doping is known. However, in this case, the former requires a Ti diffusion process, and both can only perform proton exchange with lithium niobate crystals containing Ti and MgO.

[0005]

As described above, it is impossible to directly carry out the proton exchange with the X or Y cut plate of the lithium niobate crystal or the lithium tantalate crystal.

[0006]

The method for forming a proton exchange layer according to the present invention is a method for forming a three-dimensional proton exchange layer on the surface of a substrate made of a dielectric crystal by proton exchange, the substrate comprising: Is a X- or Y-cut substrate made of lithium tantalate crystal or lithium niobate crystal, and a pattern mask having a slit width not corroded by a proton exchange source, that is, a slit having a width of more than 0 μm and not more than 10 μm, is used in the YZ axis Forming on the main surface of the X-cut surface including X or the Y-cut surface including X-Z axis, and a step of contacting the main surface with the proton exchange source through the slit to perform proton exchange. Characterize.

[0007]

According to the present invention, the channel type proton exchange layer can be easily formed on the Y-cut surface or the X-cut surface of the crystal.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows a method for forming a proton exchange layer in which a three-dimensional proton exchange layer is formed on a LiTaO ₃ crystal substrate.
First, a Y-cut plate of LiTaO ₃ crystal was used as a substrate (thickness 0.5 m
m), a Ta film 20 having a predetermined film thickness was vapor-deposited on the entire Y-cut surface 5 of the substrate 1. Next, a photoresist film was applied to the Ta film 20 with a spin coater to a predetermined thickness, and a slit opening pattern was patterned using a photoprocess using a contact exposure device or an electron beam drawing device.
Next, dry etching was performed in CF ₄ or SF ₆ gas, the photoresist film was removed by acetone or remover, and a Ta film 20 having slits with elongated openings was formed on the substrate 1 (FIG. 2A). ).

Next, using pyrophosphoric acid as a proton source, a proton exchange temperature of 230 ° C. to 265 ° C., a proton exchange time of 15 minutes to
Proton exchange is performed for 2 hours to remove the Ta film 20 and Y
The proton exchange waveguide 3 for the three-dimensional waveguide propagating in the X-axis direction was formed on the cut plate 5 (FIG. 2B). Here, the slit extension direction was the Z-axis direction. Further, FIG. 2 (c)
As shown in FIG. 5, the proton exchange 3 having the slit extension direction as the X-axis direction was also formed. The broken line arrow indicates the direction of polarization.

In forming the proton exchange layer, the inventor has determined that the slit width W is 1, 2, 3, 4, 5, 6, 8,
10, 12, 14, 16, 18, 20, 25, 30, 4
0,50,60,70,80,90,100,200,
When various kinds of proton exchange waveguides 3 having different widths of 300, 400, 500 and 1000 μm are formed, even if the slit width is 10 μm or more, proton exchange may be performed without corrosion under relatively low temperature and short time conditions. It was found that the probability of corrosion increases as the value becomes wider. This corrosion was a plurality of cracks extending substantially in the X-axis direction.

FIG. 3 shows the rate of corrosion of the proton exchange waveguide 3 with respect to the slit width. FIG. 3A shows a case where the slit exchange direction is 230 ° C. and the proton exchange time is 1 hour, and the slit extension direction is the Z-axis direction, and almost no corrosion occurs in the slit width of 80 μm or less. FIG. 3B shows the case where the proton exchange temperature was 260 ° C. and the proton exchange time was 1 hour, and the slit extension direction was the Z-axis direction, and almost no corrosion occurred in the slit width of 20 μm or less. FIG. 3 (c) shows the case where the proton exchange temperature was 260 ° C. and the proton exchange time was 30 minutes, and the slit extension direction was the Z-axis direction, and almost no corrosion occurred in the slit width of 70 μm or less.

When the corrosion rate of the proton exchange waveguide with respect to these slit widths is averaged, the slit width is 0.
It has become clear that it is preferable that the width exceeds the crack width of corrosion and is 10 μm or less. A width of 10 μm is sufficient for the three-dimensional waveguide. It is possible to obtain a three-dimensional waveguide capable of propagating light under any of the conditions, but by performing proton exchange at low temperature or for a short time, it is possible to obtain low loss.
It is possible to more stably create the three-dimensional proton exchange layer.

Further, as another embodiment, even when an X-cut substrate of lithium tantalate crystal is used under the same conditions as the above Y-cut substrate, a method using a slit-shaped mask with a width of 10 μm or less has the same corrosion reduction effect. It was possible to exchange protons, and a proton exchange waveguide 3 could be formed on the X-cut surface 4 as shown in FIG. 4 (dotted arrow indicates the direction of polarization). Furthermore, since the X-cut lithium tantalate crystal has a relatively higher corrosion resistance than the Y-cut plate, under the same conditions as the Y-cut plate, protons can be exchanged without corrosion even with a wider slit.

Further, although these two examples are for the case of a lithium tantalate crystal substrate, the same effect is exhibited for an X or Y cut plate of a lithium niobate crystal.
When the proton exchange is performed under the same conditions as the lithium tantalate crystal, the lithium niobate crystal is more susceptible to corrosion, but in the case of the lithium niobate crystal, the proton exchange rate is faster than the lithium tantalate crystal, Since the change in refractive index required for the waveguide achieves the proton exchange at a low temperature in a short time, a practical three-dimensional waveguide can be obtained by the direct proton exchange method using this slit.

The proton exchange was carried out with pyrophosphoric acid, but a proton exchange source such as benzoic acid may be used. The mask can be used as long as it can withstand proton exchange of pyrophosphoric acid, such as gold (Au) other than tantalum (Ta) and platinum (Pt). If benzoic acid is proton exchanged, aluminum (A
l), chromium (Cr), titanium (Ti) can also be used.
Further, if an annealing step at 340 ° C. for about 30 minutes is added after the proton exchange, the loss can be further reduced.

As described above, a Ta mask having slits with a width of 10 μm or less is formed on the X or Y cut surface of an X or Y cut substrate made of lithium tantalate crystal or lithium niobate crystal, and this is subjected to proton exchange. By doing so, a channel type proton exchange layer can be produced, and a slit longitudinal direction orthogonal to the Z axis is suitable for confining TE mode light.
It can be used as a dimensional waveguide.

Further, as shown in FIG. 5A, both ends of the slit of the Ta mask are closed without being opened, and the Y-cut peripheral edge of the substrate is covered with the mask (arrow A). By removing the mask after the exchange and forming the proton exchange layer 3 shown in FIG. 5 (b) in the Y-cut plane 5, the proton exchange layer could be produced more stably (the broken line arrow indicates polarization). Indicates the direction). For example, when proton exchange is performed at 265 ° C. for 30 minutes with the mask edge portion in the proton exchange plane, the ratio of obtaining a good three-dimensional proton exchange layer with a low corrosion ratio to the slit width is tripled or more. . Although the substrate is also a Y-cut lithium tantalate crystal, the X-cut plate can also be used. Alternatively, a Y or X cut plate of lithium niobate crystal may be used.

As shown in FIG. 6, the periodic domain inversion layer of the second harmonic generating element (QPM-SHG element) using quasi phase matching was formed specifically. For example, when a fundamental wave is set to λ = 860 nm to form a primary QPM element, first, a Ta film is formed on the entire Y-cut surface with a thickness of 300 Å crystal. This is subjected to 1.8 μm by means such as photolithography and reactive ion etching (RIE).
A plurality of slits of m line and space (3.6 μm pitch) were processed into a Ta mask. Where the width is 1.8
The longitudinal direction of the μm slit was the Z direction. In addition, the third
In the case of QPM, the line and space is 5.4 μm. That is, T deposited on the LiTaO ₃ crystal substrate 1
After applying a photoresist film on the a film 20, patterning of the periodic openings was performed by electron beam drawing or the like, and dry etching was performed to form the polarization inversion layer forming apertures of a specific period. Then, the photoresist film is removed to remove the Ta film 20.
Was formed (FIG. 6A).

Next, this substrate was heated in pyrophosphoric acid 260
Proton exchange was performed under the conditions of 30 ° C. and 30 minutes (a line-and-space proton exchange layer of 1.8 μm was formed). That is, pyrophosphoric acid was applied onto the grating of the Ta film and heat-treated for proton exchange to form a proton exchange layer 3a having a predetermined period on the Y-cut surface 5 (FIG. 6 (b)). After that, the grating of the Ta film 20 was removed, and the polarization inversion of the proton exchange layer 3a was performed by heat treatment in the vicinity of the Curie point to form the polarization inversion layer 3 having a predetermined period (FIG. 6).
(C)). That is, in an argon (Ar) atmosphere, 550
Annealing was performed at 2 ° C. for 2 minutes to use the above proton exchange layer as a periodic domain inversion layer.

Next, a three-dimensional waveguide was prepared. A Ta film 20 having an opening pattern orthogonal to the domain-inverted layer 3 having a predetermined period was formed on the substrate 1 (FIG. 6D). That is, Y again
A Ta film having a film thickness of 300 Å was formed on the entire cut surface, and the Ta film was processed into a slit having a width of 4 μm by a method such as a photolithography method and a reactive ion etching method. Here, the longitudinal direction of the slits is the X direction, and the number of slits may be one or more.

Next, this substrate was placed in pyrophosphoric acid at 260 ° C.
Proton exchange was performed under the condition of 30 minutes to form the waveguide 7 (FIG. 6 (e)). Then, annealing was performed in an O ₂ atmosphere at 340 ° C. for 30 minutes, and the latter proton exchange layer was used as a three-dimensional waveguide. Further, the proton-exchanged Ta film 20 was removed to prepare a waveguide type wavelength conversion element composed of the waveguide 7 orthogonal to the polarization inversion layer 3 (FIG. 6 (f)). Both ends of the waveguide 7 were optically polished, and the fundamental wave of TE mode was made incident on the waveguide to extract the second harmonic (SHG) light from the opposite end face.

[0022]

As described above, according to the method of forming a three-dimensional proton exchange layer on the surface of a substrate made of a dielectric crystal by proton exchange according to the present invention, the substrate is a lithium tantalate crystal or niobium. An X- or Y-cut substrate made of a lithium oxide crystal, which has a pattern mask having a slit width that does not cause corrosion by a proton exchange source, that is, a slit having a width of more than 0 μm and 10 μm or less, and an X-cut surface including the Y-Z axis -Y including Z axis
Since it includes a step of forming on the main surface of the cut surface and a step of performing proton exchange by bringing a proton exchange source into contact with the main surface via a slit, a channel-type proton exchange layer is formed on the Y-cut surface or the X-cut surface of the crystal. Can be formed.

Furthermore, since the three-dimensional waveguide can also be formed on the Y-cut surface or the X-cut surface of the crystal, the wavelength conversion element can be easily manufactured by a series of manufacturing steps.

[Brief description of drawings]

FIG. 1 is a schematic perspective view of a wavelength conversion element.

FIG. 2 is a schematic perspective view of a member in each step of the method for forming a proton exchange layer of the example.

FIG. 3 is a graph showing the corrosion rate of the proton exchange waveguide with respect to the slit width of the tantalum mask obtained by the method of forming the proton exchange layer of the example.

FIG. 4 is a schematic perspective view of a member in each step of a method of forming a proton exchange layer of another embodiment.

FIG. 5 is a schematic perspective view of a member in each step of a method of forming a proton exchange layer according to another embodiment.

FIG. 6 is a schematic perspective view of a member in each step of the method for manufacturing the wavelength conversion element according to the embodiment.

[Explanation of symbols for main parts]

 1 substrate 2 Z cut surface 3 three-dimensional proton exchange layer 4 X cut surface 5 Y cut surface 20 tantalum mask

Claims (5)

[Claims] 1. A method for forming a three-dimensional proton exchange layer on the surface of a substrate made of a dielectric crystal by proton exchange, the substrate comprising a lithium tantalate crystal or a lithium niobate crystal, X or Y
The pattern mask, which is a cut substrate and has slits with a slit width that does not cause corrosion due to the proton exchange source, is
A step of forming on the main surface of an X-cut surface including the -Z axis or a Y-cut surface including the X-Z axis, and a step of bringing a proton exchange source into contact with the main surface through the slit to perform proton exchange. A method for forming a proton exchange layer, comprising: 2. The slit width exceeds 0 μm and is 10 μm
The method for forming a proton exchange layer according to claim 1, wherein: 3. The method for forming a proton exchange layer according to claim 1, wherein the pattern mask is made of tantalum. 4. The method of forming a proton exchange layer according to claim 1, wherein the proton exchange source is composed of pyrophosphoric acid. 5. The method of forming a proton exchange layer according to claim 1, wherein the slit of the pattern mask is closed on the main surface of the X-cut surface or the Y-cut surface.