JPH05216081A - Production of wavelength conversion element - Google Patents

Abstract

PURPOSE:To provide the process for production of the wavelength conversion element suitable for mass production with a smaller number of production stages than heretofore. CONSTITUTION:This process for production consists of a stage for forming a three-dimensional waveguide 7 extending on the main surface of a substrate consisting of a ferroelectric substance, a stage for forming a grating 10 consisting of plural metallic bands respectively extending in the direction orthogonal with the three-dimensional waveguide 7 on the main surface and a metallic film 9 on the opposite surface of the main surface and a stage for irradiating the metallic bands with electron beams. Plural polarization inversion layers 4 are formed along the extension direction of the three-dimensional waveguide 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a wavelength conversion element having a waveguide and using second harmonic generation by quasi-phase matching.

[0002]

BACKGROUND OF THE INVENTION An element using quasi-phase matching (QPM) is known as a wavelength conversion element using second harmonic generation. Quasi-phase matching utilizes, for example, that the output of the second harmonic periodically repeats the maximum and minimum at each interference distance (coherence length) as it propagates, and the sign of the polarization wave generated at each coherence length is determined. This is a matching method in which the outputs are increased alternately by the inversion without causing the second harmonic output to cancel. If quasi-phase matching is used, even if an optically isotropic material or a material with a large tensor component that cannot be phase-matched as a bulk material due to its dispersion characteristics is used, the polarization of the polarization wave is alternately inverted. Apparent phase matching can be achieved by adjusting the period of the inverted portion. Utilizing this quasi-phase matching method, a wavelength conversion element has been developed in which a domain-inverted layer is periodically formed along the extension direction in a channel type waveguide made of a ferroelectric material.

In order to periodically invert the sign of the polarization wave, the sign of the nonlinear coefficient may be inverted, and in the ferroelectric substance, the inversion characteristic of the domain can be utilized. For example, in a LiNbO ₃ crystal, the ferroelectric domain structure has a 180 ° inversion domain in the c-axis direction, and in the + c plane, polarization domain inversion easily occurs due to external factors such as impurities, strain stress, heat, and electric field.

Methods for forming a periodic domain inversion structure on the main surface of a ferroelectric block such as LiNbO ₃ crystal include heat treatment near the Curie point, high voltage treatment with a comb-shaped electrode, and electron beam drawing treatment.

For example, FIG. 1 shows a specific method of manufacturing a wavelength conversion element using heat treatment near the Curie point. First, a polarization inversion layer is formed by a heat treatment method near the Curie point. After coating a photoresist film on the Ta film 2 deposited on the LiTaO ₃ crystal substrate 1, patterning of the periodic holes is performed by electron beam drawing or the like, and dry etching is performed to form the domain inversion layer forming apertures of a predetermined period. To form.
Then, the photoresist film is removed to form a grating for the Ta film 2 (FIG. 1a). Next, pyrophosphoric acid is coated on the grating of the Ta film and heat-treated to perform proton exchange, and the proton exchange layer 4a having a predetermined period is formed into LiTaO ₃
It is formed on the crystal substrate 1 (FIG. 1b). The grating of the Ta film 2 is removed, and the polarization inversion of the proton exchange layer 4a is performed by heat treatment to form the polarization inversion layer 4 having a predetermined period (FIG. 1c).

Next, a three-dimensional waveguide is created. A Ta film 5 having an opening pattern orthogonal to the domain-inverted layer 4 having a predetermined period is formed on the substrate 1 (FIG. 1d). After coating pyrophosphoric acid on the Ta film 5, heat treatment is performed to perform proton exchange to form the waveguide 7 (FIG. 1e). The Ta film 5 is removed and the polarization inversion layer 4 is removed.
A waveguide-type wavelength conversion element composed of the waveguide 7 orthogonal to is formed (FIG. 1f).

However, in the heat treatment near the Curie point, a periodic pattern (grating) such as Ta.
Is performed, and the heat treatment is performed at a temperature near the Curie temperature, followed by cooling. Therefore, the cross-sectional shape of the inversion domain layer becomes triangular or semicircular, and a fine pattern of the inversion domain cannot be formed. Further, in such a wavelength conversion element manufacturing method, a periodic polarization inversion layer is formed by using proton exchange by applying pyrophosphoric acid, and then a three-dimensional waveguide is formed by further proton exchange, so that the manufacturing process is complicated. take time.

On the other hand, in the method of manufacturing a wavelength conversion element using the above-mentioned high voltage treatment with the comb-shaped electrode, the crystal substrate must be treated at a high temperature, and the crystal is damaged, and the electrode material is thermally diffused in the crystal. There are drawbacks such as
187735). Further, in the wavelength conversion element manufacturing method using the electron beam drawing process, when the electron beam diameter is made small on the crystal substrate to periodically scan to form the inversion domain layer of the periodic pattern, the irradiation is performed before. Due to the effect of the inverted polarization by the electron beam, the subsequent electron beam is bent and there is a drawback that an inverted domain layer having an accurate pattern cannot be formed. On the contrary, when the inversion domain layer is formed by scanning with an increased electron beam diameter, the inversion domain layer cannot be formed with high accuracy because the intensity of the electron beam has a Gaussian distribution on its radius.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a wavelength conversion element manufacturing method which has fewer manufacturing steps than conventional ones and is suitable for mass production.

[0010]

A method of manufacturing a wavelength conversion element according to the present invention comprises a step of forming a three-dimensional waveguide extending on a main surface of a substrate made of a ferroelectric material, and the three-dimensional waveguide on the main surface. A grating consisting of a plurality of metal strips each extending in a direction orthogonal to the waveguide, a step of forming a metal film on the facing surface of the main surface, and a step of irradiating the metal strip with an electron beam, It is characterized in that a plurality of polarization inversion layers are formed along the extending direction of the three-dimensional waveguide.

[0011]

According to the present invention, the manufacturing process time can be shortened and the wavelength conversion element manufacturing method can be simplified.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows a specific method of manufacturing a waveguide type wavelength conversion element made of LiTaO ₃ crystal using quasi phase matching. First, using LiTaO ₃ as a substrate (thickness 0.5 mm), a Ta film 2 is vapor-deposited to a predetermined thickness on the entire −z surface of the LiTaO ₃ crystal substrate 1 in an electron beam vapor deposition machine. next,
A photoresist film is applied on the Ta film 2 with a spin coater to a predetermined thickness, and patterning is performed with a hole pattern for a waveguide by a photo process using a contact exposure device or an electron beam drawing device. Next, the RIE device (Reactiv
dry etching is performed in CF ₄ gas by means of an ion etching apparatus, and a Ta film 2 having elongated openings for removing the photoresist film is formed on the substrate 1 by acetone or remover. (Fig. 2a).

Next, pyrophosphoric acid is applied to the Ta film 2 and the exposed portion of the substrate and then heat-treated in an electric furnace to exchange protons to form a three-dimensional waveguide 7 (FIG. 2b). After washing with water, the Ta film 2 is removed to obtain a substrate carrying the extending three-dimensional waveguide 7 (FIG. 2c). Next, using a sputtering apparatus, a metal film 9 of chromium Cr or the like is formed on the entire + Z surface of the substrate with a film thickness of 1000 angstroms, and a Cr film is similarly formed on the entire opposite -Z surface. Then, a photoresist film having a film thickness of about 1 μm is formed on the metal film on the −Z surface by spin coating. After that, a 3.5 μm pitch (1.7
(5 μm line and space) patterning for the inversion domain layer is performed so as to be orthogonal to the three-dimensional waveguide 7. Next, dry etching is performed with Cl ₂ + O ₂ gas by an RIE device, and then the photoresist film portion is removed, A grating 10 consisting of a plurality of parallel Cr metal bands at periodic intervals, that is, a Cr film periodic pattern is formed on the -Z plane (Fig. 2d). Each Cr metal band extends in a direction orthogonal to the three-dimensional waveguide. A registration pattern (positioning pattern) is also formed at the same time as the patterning for the inversion domain layer. At this time, even if the substrate is hit with an electron beam, the inversion domain layer is not formed because the Cr film is present on the entire surface of the -Z plane and the toe amount is insufficient.

Next, using an electron beam drawing device, after positioning by the registration mark, the electron beam is irradiated once on each metal strip of the Cr film periodic pattern 10 having a pitch of 3.5 μm along it ( Figure 2e). At this time, electrons are uniformly accumulated right under each metal band of the Cr film periodic pattern 10 to form a highly accurate inversion domain layer. The electron beam is irradiated under the conditions of an accelerating voltage of 30 KV, an electron beam diameter of 0.1 μm and a dose amount of 100 to 2000 μC / cm ² .

Finally, the Cr film periodic patterns on both surfaces are removed by wet etching to form a plurality of domain-inverted domain layers 4 along the extending direction of the three-dimensional waveguide 7 (FIG. 2).
f). In this way, a metal such as Cr is formed on the entire + Z surface of the LiTaO ₃ crystal and Cr is formed on the −Z surface at a desired pitch.
Since the inversion domain layer can be formed at a low temperature by a method including a step of forming a metal grating such as the above and a step of irradiating an electron beam toward the metal grating on the -Z plane, crystal damage is reduced. Moreover, since the electron beam is irradiated only once toward the metal grating, the number of steps can be reduced.

As the ferroelectric substance, LiNbO ₃ , KNbO ₃ , KTiPO _4, etc. can be used in addition to LiTaO ₃ . Although Cr is used as the metal periodic pattern, Au, Al, Ag, or the like is used as the other metal. The electron beam irradiation condition and the length and width of each metal band of the Cr film periodic pattern are set according to the type of ferroelectric substance and the length and width of the three-dimensional waveguide.

Before the electron beam irradiation step on the metal grating, a three-dimensional waveguide is formed by proton exchange in the direction perpendicular to the inversion domain layer. To form a ferroelectric waveguide, this three-dimensional waveguide is formed. Alternatively, the ferroelectric waveguide may be formed by a vapor deposition method, an epitaxial growth method, or the like. Further, the shape of the waveguide may be, in addition to the buried type in the above-described embodiment, a microstrip type in which a ferroelectric waveguide is formed to project on the main surface of the substrate, or a ridge type in which the thickness of the ferroelectric is changed. .. It is also possible to form a three-dimensional waveguide after the electron beam irradiation step.

[0018]

As described above, according to the method of manufacturing a wavelength conversion element of the present invention, a step of forming a three-dimensional waveguide extending on the main surface of a substrate made of a ferroelectric material and three steps on the main surface.
A three-dimensional three-dimensional process includes a step of forming a metal film on a surface opposite to the main surface, and a step of irradiating the metal band with an electron beam, the grating including a plurality of metal bands each extending in a direction orthogonal to the three-dimensional waveguide. Since a plurality of polarization inversion layers are formed along the extension direction of the waveguide, a wavelength conversion element having an inversion domain layer with a fine period can be manufactured, and the period of the polarization inversion layer to be set corresponding to the ferroelectric substance to be used. Can be easily set by changing the metal periodic pattern, and the conventional manufacturing process can be simplified.

[Brief description of drawings]

FIG. 1 is a schematic perspective view of a member in each step of a wavelength conversion element manufacturing method.

FIG. 2 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 4 polarization inversion layer 7 waveguide 9 metal film 10 metal grating

Claims (1)

[Claims] 1. A step of forming a three-dimensional waveguide extending on a main surface of a substrate made of a ferroelectric material, and a plurality of metal strips extending on the main surface in a direction orthogonal to the three-dimensional waveguide. A step of forming a metal film on the opposite surface of the main surface and a step of irradiating the metal band with an electron beam, and a plurality of polarization inversions along the extending direction of the three-dimensional waveguide. A method for manufacturing a wavelength conversion element, which comprises forming a layer.