JP4974872B2 - Method for manufacturing periodically poled structure - Google Patents

Description

  The present invention relates to a method for manufacturing a periodically poled structure.

  Quasi-phase-matching second harmonic generation (Second-Harmonic) in which a periodically poled structure is formed in a ferroelectric single crystal such as lithium niobate single crystal or lithium tantalate single crystal. -Generation) devices can generate light of a relatively arbitrary wavelength from ultraviolet to infrared. This device can be used in a wide range of applications such as optical disk memory, medical use, photochemistry use, and various optical measurement applications.

In the method described in Non-Patent Document 1, an insulating film is provided on the surface of a lithium niobate Z substrate, a strip-like elongated gap is provided in the insulating film, and the conductive film is formed so as to cover the insulating film and the gap. Provided. Then, by applying a pulse voltage to the conductive film, a periodically poled structure is formed on the substrate.
IEICE Transactions CI, Vol. J78-C-1, No.5 pp.238-245, "Preparation of polarization inversion grating for LiNbO3 SHG devices by applying voltage" Kenji Kindaka, Masatoshi Fujimura, Toshiaki Hagiwara, Nishihara Hiroshi

In Patent Document 1, a metal comb electrode is formed on the surface of a lithium niobate substrate. This comb-shaped electrode includes a long and narrow power supply electrode and a long and thin electrode piece extending from the edge of the power supply electrode. Each electrode piece is separated into predetermined lengths in the longitudinal direction.
WO 2005 124447

In the voltage application method of Patent Document 2, each electrode piece of the comb-shaped electrode on the substrate surface is divided into a plurality of pieces, and a gap is provided between adjacent electrode pieces. An attempt is made to form a deep domain-inverted structure by changing the length of the electrode pieces or defining the size of the gap between the electrode pieces.
JP2006-003488

  In the methods described in Patent Documents 1 and 2, the electrode piece is divided into a plurality of elongated pieces in the longitudinal direction, and a gap is provided between the pieces. As a result, the polarization inversion is started and progressed starting from each edge of each piece of the electrode piece, thereby shortening the time required for the polarization inversion and increasing the polarization inversion depth. . However, there is a tendency that adjacent polarization inversion portions tend to be connected, which may lead to a decrease in fundamental wave energy conversion efficiency.

  In the method described in Non-Patent Document 1, adjacent polarization inversion portions are not easily connected. However, when the electrode piece becomes long, the time required for polarization inversion tends to be long, and the polarization inversion width tends to be uneven. In particular, since it takes time to reverse the polarization, productivity is lowered.

  An object of the present invention is to provide a method capable of forming a periodic polarization reversal structure in a short time while preventing the connection of adjacent polarization reversal portions when forming a periodic polarization reversal structure by a voltage application method. .

The present invention uses the electrode structure provided on the main surface of the ferroelectric single crystal substrate is a single-poling, Ru method der to produce a periodically poled by the voltage application method.

  The electrode structure includes an insulating film having a plurality of gaps provided on the main surface of the ferroelectric single crystal substrate, and a conductive film provided to cover the gaps of the insulating films and the insulating film. The conductive film includes an insulating film covering portion that covers the insulating film, and an electrode piece portion provided in the gap, and the electrode piece portion is perpendicular to the longitudinal direction of the electrode piece portion on the main surface. The electrodes are arranged so as to be spaced apart from each other, and the width of the electrode piece at the edge of the conductive film in the longitudinal direction of the electrode piece is on the inner side when viewed from the electrode piece at the edge of the conductive film It is characterized by being larger than the width of one part.

According to the present invention, the electrode structure is provided so as to cover the insulating film having a plurality of gaps provided on the main surface of the ferroelectric single crystal substrate and the gaps and the insulating films of the insulating films. The conductive film includes an insulating film covering portion that covers the insulating film, and an electrode piece portion provided in the gap. This makes it difficult for adjacent polarization inversion portions to be connected. In addition, the present inventor increases the electrode width at the edge of the conductive film in the longitudinal direction of the electrode piece, and increases the area where the electrode edge (edge) where charges are concentrated contacts the substrate. It was found that a large amount of charge contributing to inversion can be taken in, and the inversion time is shorter than before.

The present invention will be further described below with reference to the drawings as appropriate.
First, as shown in FIG. 1A, an insulating film is formed on one main surface 1a of the ferroelectric crystal substrate 1. 1b is the other main surface. Next, an elongated gap 8 is formed in the insulating film, and the patterned insulating film 7 is left. A large number of gaps 8 are formed in the insulating film 7, and the main surface 1 a of the substrate 1 is exposed in each gap 8. Next, as illustrated in FIG. 1B, a conductive film 20 is formed on the substrate 1. The conductive film 20 covers the substrate main surface 1a, and includes an insulating film covering portion 6 that covers the insulating film 7 and an electrode piece portion 5 that directly covers the main surface 1a. A uniform electrode 2 is formed on the other main surface 1 b of the substrate 1.

  FIG. 2 is a plan view schematically showing the main surface 1a of the substrate 1, and FIG. 3 is an enlarged plan view showing the form of the electrode pieces.

  Each electrode piece 5 is formed on the substrate main surface 1 a, and the insulating film covering portion 6 is formed on the insulating film 7. Each electrode piece portion 5 is connected to the insulating film covering portion 6 without a break. When viewed in a plan view, each of the electrode pieces 5 has an elongated stripe shape. A plurality of electrode pieces 5 are arranged in a direction x perpendicular to the longitudinal direction of the electrode pieces, and a gap 4 is formed between adjacent electrode pieces 5 as viewed in the direction x. . A plurality of electrode pieces 5 are arranged in the longitudinal direction y of the electrode pieces 5, and a gap 9 is formed between the adjacent electrode pieces 5 as viewed in the direction y.

  Thereafter, a predetermined voltage is applied between the conductive film 5 and the uniform electrode 7 to form a large number of domain-inverted portions in the substrate 1, thereby forming a periodic domain-inverted structure. This voltage application method is not particularly limited. For example, a substrate may be placed in an inert atmosphere and a voltage may be applied, or a substrate may be placed in an insulating liquid and a voltage may be applied.

  Here, the present inventor paid attention to the form of the edge portion (edge portion) of the electrode as indicated by A in FIG. That is, at the edge of the electrode structure, the terminal electrode piece 5 extends in the longitudinal direction y, but the edge 20a of the conductive film 20 crosses the middle of the terminal electrode piece 5. As a result, the electrode piece 5 ends at the edge 20a, and the gap 8 is not filled with the conductive film, and the substrate surface 1a is exposed.

  Here, conventionally, the width T of the electrode piece 5 is not particularly changed even at the edge 20 a of the conductive film 20. That is, T was equal to the width t of the electrode piece 5 at a location away from the edge 20a.

  However, the inventor pays attention to the edge portion 20a, and as shown in FIG. 5, the width T of the electrode piece portion 22 at the edge portion 20a of the conductive film 20 in the longitudinal direction y is viewed from the inside of the electrode piece portion 22. It was made larger than the width t of the electrode piece portion 5 in FIG. The inner electrode piece 5 indicates an electrode piece where the conductive film fills all the gaps in the insulating film. The inner electrode piece 5 is preferably an electrode piece 5 adjacent to the edge electrode piece 22 as viewed from the edge. That is, the electrode piece was widened in the vicinity of the edge 20 a of the conductive film 20. As a result, it has been found that charges generated near the edge of the conductive film can be concentrated at the end of the electrode piece and the amount of polarization inversion per unit time can be significantly increased.

  The period Γx (see FIG. 3) in the X direction when patterning the insulating film is designed to a value suitable for the wavelength of the wavelength converted light to be generated. For example, when generating a green second harmonic, Γx is about 7 μm. Since the width t of the electrode piece 5 is determined by the influence of the period Γx, it does not define a particularly preferable range. However, for practical purposes, t can be set to 0.8 to 1.5 μm.

  From the viewpoint of the present invention, by increasing (T−t) and (T / t), it is possible to promote charge recovery at the terminal electrode piece, and the amount of polarization inversion portion generated per unit time. Can be increased. From this viewpoint, (Tt) is preferably 18 μm or more, and more preferably 30 μm or more. Further, (T / t) is preferably 10 or more, more preferably 15 or more.

  However, the upper limit of T is not particularly limited, and may be less than the period Γx.

  Further, there is no particular upper limit for (T / t), and it may be less than (Γx / t).

In the present invention, preferably, the electrode piece portion continuous from the edge portion includes a widened portion having a constant width extending toward the longitudinal direction of the electrode piece portion, and a tapered portion having a width narrowed from the widened portion. . For example, in the example of FIG. 5, the electrode piece portion 22 continuous from the edge portion 20a includes a widened portion 22b having a constant width T extending in the longitudinal direction of the electrode piece portion, and a taper whose width is narrowed from the widened portion 22b. Part 22a. Thereby, the influence on the polarization inversion efficiency can be minimized.

In a preferred embodiment, as shown in FIG. 6 , a separate support substrate 11 is laminated under the substrate 1, and at least the uniform electrode 2 is interposed between the substrates 1 and 11. The electrode 12 is also formed on the main surface of the support substrate 11 opposite to the substrate 1. And the board | substrates 1 and 11 are immersed in the insulating gas 13, and the electric wire 19 is connected to the electrically conductive film 20 and the electrode 12 on the board | substrate 1, respectively. Then, a voltage is applied from the power source 18 to the conductive film 20 and the electrode 12 to form a periodically poled structure.

In the present invention, the electrode pieces are preferably arranged in a state of being separated from each other in the longitudinal direction of the electrode pieces.

  In the example of FIG. 3, the conductive film 20 formed in the gap between the insulating films is divided into a plurality of electrode pieces 5 by the insulating film in the direction y. As a result, since the polarization inversion progresses starting from each edge of each electrode piece portion 5, a deep polarization inversion can be formed in a short time.

  In addition, in this example, it is possible to prevent the polarization inversion portion from being connected in the direction x. At the same time, when viewed in the longitudinal direction y of each electrode piece, the polarization inversion portions generated below the adjacent electrode pieces 5 are easily connected to each other, and a series of elongated polarization inversion portions are easily formed.

The kind of the ferroelectric material that constitutes the substrate on which the periodically poled structure is to be formed is not limited. However, single crystals of lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), lithium niobate-lithium tantalate solid solution, and K ₃ Li ₂ Nb ₅ O ₁₅ are particularly preferable.

  The ferroelectric single crystal is selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc), and indium (In) in order to further improve the light damage resistance of the three-dimensional optical waveguide. More than one metal element can be contained, and magnesium is particularly preferred.

  The ferroelectric single crystal can contain a rare earth element as a doping component. This rare earth element acts as an additive element for laser oscillation. As this rare earth element, Nd, Er, Tm, Ho, Dy, and Pr are particularly preferable.

  In the present invention, a Z-cut substrate is used as the substrate, but an off-cut Z substrate may be used. This off-cut angle is preferably 10 ° or less, more preferably 5 ° or less, from the viewpoint of the effect of the present invention.

  In addition, if the off-cut angle of the off-cut Z substrate is 10 ° or less, the deterioration of wavelength conversion efficiency can be ignored without adjusting the optical axis with the semiconductor laser and correcting the tilt, realizing a highly efficient wavelength conversion element. can do.

The material of the insulating film is not limited, but may be an oxide such as SiO ₂ or Ta ₂ O ₅ or a nitride such as silicon nitride. As a method for forming the insulating film, an evaporation method, a sputtering method, or a spin coating method may be used. The thickness of the insulating film is not particularly limited, but is preferably 500 Å or more and 3000 Å or less. In the case where the thickness of the insulating film is small, the insulating property is lowered and it is difficult to form periodic polarization inversion. When the insulating film is too thick, the patterning accuracy is deteriorated.

  A method for forming the gap by patterning the insulating film is not particularly limited. For example, a gap can be formed by spin-coating a photoresist on an insulating film, forming a resist pattern through mask exposure and development, and performing an etching process using the resist pattern as a mask. The etching process may be wet etching or dry etching, but ideally wet etching is preferable because it hardly damages the substrate surface.

  The material of the electrode piece and uniform electrode used in the voltage application method is not limited, but a laminated film of Al, Au, Ag, Cr, Cu, Ni, Ni-Cr, Pd, Ta, Mo, W, Ta, AuCr Etc. are preferable.

  The material of the support substrate 11 needs to be highly insulating, have a uniform volume resistivity within the material, and have a predetermined structural strength. Examples of this material include silicon, sapphire, crystal, and glass.

  When the voltage application method is performed by immersing the substrate in an insulating liquid, insulating oil (for example, silicon oil) or a fluorine-based inert liquid can be exemplified.

  The gap Gy between the gaps of the insulating film is not particularly limited, but is preferably 0.5 μm or more, and more preferably 0.8 μm or more, from the viewpoint of promoting the connection of the polarization inversion portions adjacent in the y direction. The gap Gy between the gaps of the insulating films is preferably 2.0 μm or less, more preferably 1.5 μm or less, from the viewpoint of promoting the connection of the polarization inversion portions adjacent in the y direction. The period Γy in the y direction is not particularly limited, but may be 5 to 100 μm.

  The method for forming the uniform electrode on the back side of the substrate is not particularly limited, and may be a vapor deposition method or a sputtering method. The film thickness of the uniform electrode can be, for example, 500 to 3000 angstroms.

(Control example)
Following the procedure described with reference to FIGS. 1-4 and 6, to form a periodic domain inversion structure. However, as the substrate 1, a MgO-added LiNbO ₃ (MgOLN) Z-cut substrate was used. An SiO ₂ film was formed as an insulating film on the + z surface 1 a of the substrate 1. The thickness of the insulating film was about 2000 angstroms. Next, a photoresist was spin-coated on the insulating film, and a resist pattern was formed through mask exposure and development. An insulating film pattern 7 as shown in FIG. 1A was formed by performing a wet etching process using this resist pattern as a mask. The period Γx was about 7 μm, Gy was 1.2 μm, and Γy was 10 μm. The width t of the electrode piece was 0.8 μm.

Subsequently, conductive films 20 and 2 were formed by sputtering. These film thicknesses were 1000 angstroms, and the material was tantalum. The electrode shape at the edge of the conductive film was as shown in FIG. A periodic domain-inverted structure could be obtained by applying the method described with reference to FIG. 6 to the substrate 1 thus manufactured. However, insulating oil was used as the insulating liquid, and the temperature was set to 150 ° C. Further, the voltage application condition was set to about 3 kV / mm, which is the electric field strength serving as the coercive electric field of the wafer, and a rectangular pulse having a width of about 1 msec was applied. The number of times of pulse application depends on the pattern area. For example, when it is 20 mm ² , 20000 pulses are suitable.

  The surface of the substrate thus obtained was wet-etched with nitric acid and then observed with a microscope. When viewed in the x direction (lateral direction), the adjacent domain-inverted portions were not connected to each other. When viewed in the y direction (longitudinal direction), adjacent polarization inversion portions are connected to each other, and one connected polarization inversion portion is formed. The time required for polarization reversal over a length of 14.5 mm was 9 minutes.

(Example)
In the control example, the electrode shape in the vicinity of the edge of the conductive film was changed according to the present invention as shown in FIG. However, the width t of the electrode piece 5 adjacent inside from the electrode piece of the edge 20a was 0.8 μm, and the width T of the electrode piece 22 in the edge 10a of the conductive film was 3.2 μm.

  The surface of the substrate thus obtained was wet-etched with nitric acid and then observed with a microscope. When viewed in the x direction (lateral direction), the adjacent domain-inverted portions were not connected to each other. When viewed in the y direction (longitudinal direction), adjacent polarization inversion portions are connected to each other, and one connected polarization inversion portion is formed. The time required for polarization reversal over a length of 14.5 mm was 7 minutes. That is, the time required per unit polarization inversion area was shortened by about 20%.

  Although specific embodiments of the present invention have been described, the present invention is not limited to these specific embodiments and can be implemented with various changes and modifications without departing from the scope of the claims.

(A) is a schematic sectional drawing which shows the state which formed the insulating film 7 on the board | substrate 1, (b) is a cross section which shows the state which formed the electrically conductive film 20 which coat | covers the insulating film 7 and the main surface 1a. FIG. 2 is a plan view schematically showing electrodes on a main surface 1a of a substrate 1. FIG. 2 is an enlarged plan view showing an electrode shape on a main surface 1a of a substrate 1. FIG. It is a top view which shows the electrode shape of the vicinity of the electrically conductive film edge in a comparative example. It is a top view which shows the electrode shape of the vicinity of the electrically conductive film edge part in embodiment of this invention. 2 is a diagram schematically showing an example of a method for applying a voltage to a substrate 1. FIG.

DESCRIPTION OF SYMBOLS 1 Board | substrate 1a, 1b Main surface 2 Uniform electrode 5, 22 Electrode piece part 6 Insulating film coating | coated part 7 Insulating film 20 Conductive film 20a Edge part of electrically conductive film 22a Tapered part t Width of electrode piece part T In edge part of electrically conductive film Width of the electrode piece x Direction perpendicular to the longitudinal direction of the electrode piece y Y Longitudinal direction of the electrode piece Gy Spacing of the electrode piece viewed in the direction y Γx Period in the x direction Γy Period in the y direction

Claims (3)

A method of manufacturing a periodically poled structure by a voltage application method using an electrode structure provided on a main surface of a single-domain ferroelectric single crystal substrate,
The electrode structure is provided with an insulating film having a plurality of gaps provided on the main surface of the ferroelectric single crystal substrate, and a conductive film provided so as to cover the gaps and the insulating film of the insulating film. The conductive film includes an insulating film covering portion that covers the insulating film, and an electrode piece portion provided in the gap, and the electrode piece portion is formed on the main surface. The electrode pieces are arranged in a state of being separated from each other in a direction perpendicular to the longitudinal direction of the electrode pieces, and the width of the electrode pieces at the edge of the conductive film in the longitudinal direction of the electrode pieces is A method for producing a periodic domain-inverted structure, characterized in that the width is larger than the width of an electrode piece on the inside as viewed from the electrode piece on the edge.   The method according to claim 1, wherein the plurality of electrode pieces are arranged in a state of being separated from each other in the longitudinal direction of the electrode pieces. The electrode piece portion continuous from the edge portion includes a widened portion having a constant width extending toward the longitudinal direction of the electrode piece portion, and a tapered portion having a width narrowed from the widened portion. The method according to claim 1 or 2.

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