JP2009186634A - Method for manufacturing periodic polarization reversal structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of preventing connection of adjacent polarization reversal part and forming periodic polarization reversal structure in short time when forming the periodic polarization reversal structure by a voltage application method. <P>SOLUTION: There are provided an insulating film 7 with a plurality of clearances provided on a main face 1a of a ferroelectric single crystal substrate 1, and a conductive film 20 provided to coat the clearances 8 of the insulating film 7 and the insulating film. The conductive film 20 includes an insulating film coating part 6 for coating the insulating film, and electrode piece parts 5 provided in the clearances. The electrode piece parts 5 are arrayed under the condition separated from each other toward a polarization reversal direction x. The conductive film 20 includes a notched part 21 extending toward the polarization reversal direction x. The notched part 21 passes through the plurality of clearances 8. <P>COPYRIGHT: (C)2009,JPO&INPIT

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, "Manufacture of polarization inversion grating for LiNbO3 SHG devices by applying voltage" Kenji Kindaka, Masatoshi Fujimura, Toshiaki Sugawara, 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 is a method of manufacturing a periodically poled structure by a voltage application method using an electrode structure provided on the main surface of a single domain ferroelectric single crystal substrate,
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 portions are arranged in a state of being separated from each other in the polarization inversion direction. The conductive film is provided with a cutout portion extending in the polarization inversion direction, and the cutout portion passes through a plurality of gaps.

  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 increased the contact area where the edge of each electrode piece and the ferroelectric substrate are in contact with each other by slitting the conductive film covering the insulating layer and the gap. Usually, for example, when an electric field is applied to a rectangular electrode piece, the electric field distribution has a shape in which the intensity is maximized at the edge of the electrode piece. For this reason, polarization reversal also tends to progress from the edge of the electrode piece, and the formation distribution of polarization reversal is biased inside the rectangular electrode piece. And the formation rate of polarization inversion is low. However, as in the present invention, by forming a notch extending in the direction of polarization inversion in the conductive film, an electrode edge can be formed inside the electrode piece, and the electric field distribution is It becomes more uniform inside. Thereby, the formation distribution of the polarization inversion is likely to be more uniform, and the inversion time may be shortened.

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 planar shape of the patterned insulating film 7 is schematically shown in FIG. 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. A planar shape of the conductive film 20 is shown in FIG. 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.

  Here, each electrode piece portion 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 the polarization inversion direction x, and a gap 4 is formed between the adjacent electrode pieces 5 as seen in the direction x. A plurality of electrode pieces 5 are arranged in the longitudinal direction (direction perpendicular to the polarization inversion direction x) y of the electrode pieces 5, and between the adjacent electrode pieces 5 as viewed in the direction y. A gap 9 is formed.

  Here, as shown in FIG. 3, the conductive film 20 is provided with a notch 21 extending in the polarization inversion direction x. In the region where the conductive film 20 exists, as shown in FIG. 1B, the insulating film 7 is buried under the conductive film, and therefore the outline of the electrode piece 5 is shown by a dotted line in FIG. On the other hand, in the notch 21, the insulating film 7 and the gap 8 are exposed on the surface, and the electrode piece 5 is not formed in the gap 8. This state is schematically shown as a partial cross-sectional view in FIG.

  Thereafter, a predetermined voltage is applied between the conductive film 20 and the uniform electrode 2 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.

  For example, when an electric field is applied to the quadrangular electrode piece 5, the electric field distribution has a shape in which the intensity is maximum at the edge of the electrode piece. For this reason, the polarization inversion is also likely to progress from the edge of the electrode piece 5, and the polarization inversion formation distribution is biased inside the square electrode piece. And the formation rate of polarization inversion is low. However, as in this example, the electrode edge E can be formed also inside the electrode piece portion 5 by forming the cutout portion 21 elongated in the polarization inversion direction x in the conductive film 20. The electric field distribution becomes more uniform inside the electrode piece. As a result, the formation distribution of polarization inversion tends to be more uniform, and the inversion time is shortened.

  There is no particular limitation on the location where the cutout portion of the conductive film is formed. In the example of FIG. 3, the notches 21 are formed inward from the opposing sides 20a and 20b of the conductive film, respectively. However, in the example shown in FIG. 6, the notch 21A is formed from one side 20a of the conductive film 20 toward the opposite side 20b. In the example shown in FIG. 7, a cutout portion 21B extending from one side 20a of the conductive film 20 toward the opposite side 20b is formed, and also extending from the opposite side 20b of the conductive film 20 toward the one side 20a. The cutout portion 21C was formed at a different height.

  The width b of each notch is not particularly limited. However, b is preferably 2 μm or more, and more preferably 10 μm or more, from the viewpoint of effectively using the charge generated at the edge of the electrode piece for polarization inversion formation. On the other hand, if the width b of the notch is too large, the region that can be used for polarization inversion becomes small. From this point of view, the width b of the notch is preferably 50 μm or less, and more preferably 20 μm or less.

  The interval a between adjacent notches is not particularly limited. However, from the viewpoint of efficiently forming the domain-inverted portion in a short time, a is preferably 200 to 1000 μm, and more preferably 200 to 400 μm.

  Further, b / a is not particularly limited. However, from the viewpoint of efficiently forming the domain-inverted part in a short time, b / a is preferably 20 to 200, and more preferably 20 to 40.

  The length c of the notches 21, 21A, 21B, and 21C is not particularly limited, and may be designed according to the size of the polarization inversion portion to be formed.

  In a preferred embodiment, as shown in FIG. 8, 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 wires 15 and 19 are 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. 4, 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 arrangement direction x of the polarization inversion portion and the non-polarization inversion portion. 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, and 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 the 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 period Γx (see FIG. 4) 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.

  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 polarization inversion. The gap Gy between the gaps of the insulating films is preferably 2.0 μm or less, and 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)
In accordance with the method described with reference to FIGS. 1, 2, 4 and 8, a periodically poled structure was formed. 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. By performing wet etching using this resist pattern as a mask, an insulating film pattern 7 as shown in FIGS. 1A and 2 was formed. The period Γx was about 7 μm, Gy was 1.2 μm, and Γy was 10 μm.

Subsequently, conductive films 20 and 2 were formed by sputtering. These film thicknesses were 1000 angstroms, and the material was tantalum. However, the conductive film 20 was not provided with a notch. A periodic domain-inverted structure could be obtained by applying the method described with reference to FIG. 8 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 3.3 mm was 2.3 minutes.

Example 1
A periodically poled structure was formed in the same manner as the control example. However, the form of the conductive film 20 was as shown in FIG. 3, and the notch 21 was formed. The width b of the notch 21 was 5 μm, the length c of the notch 21 was 2.4 mm, and the interval a between adjacent notches was 200 μm. As a result, the time required for polarization reversal over a length of 3.3 mm was 1.8 minutes. That is, the time required per unit polarization inversion area was shortened by about 20%.

(Example 2)
A periodically poled structure was formed in the same manner as the control example. However, the form of the conductive film 20 was as shown in FIG. 6, and the notch 21A was formed. The width b of the notch 21A was 5 μm, the length c of the notch 21 was 4.8 mm, and the distance a between adjacent notches was 200 μm. As a result, the time required for polarization reversal over a length of 3.3 mm was 1.8 minutes. That is, the time required per unit polarization inversion area was shortened by about 20%.

(Example 3)
A periodically poled structure was formed in the same manner as the control example. However, the form of the conductive film 20 was as shown in FIG. 7, and the notches 21B and 21C were formed. The width b of the notches 21B and 21C was 5 μm, the length c of the notches 21 was 4.8 mm, and the interval a between adjacent notches was 200 μm. As a result, the time required for polarization reversal over a length of 3.3 mm was 1.8 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 showing a state in which a patterned insulating film 7 is formed on a substrate 1. FIG. 2 is a plan view schematically showing an electrode structure 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 sectional drawing of the electrode structure in the vicinity of a notch part. It is a top view of the electrode structure concerning other embodiments. It is a top view of the electrode structure concerning other embodiments. 2 is a diagram schematically showing an example of a method for applying a voltage to a substrate 1. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Board | substrate 1a, 1b Main surface 2 Uniform electrode 5 Electrode piece part 6 Insulating film coating | coated part 7 Insulating film 20 Conductive film 21, 21A, 21B, 21C Notch part E Edge of electrode piece a Space | interval of adjacent notch part b Notch width x Polarization inversion direction y Longitudinal direction of electrode piece Gy Spacing of electrode piece viewed in direction y Γx Period in x direction Γy Period in 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 that is provided in the gap, and the electrode piece portion faces in a polarization inversion direction. Are arranged in a state of being separated from each other, the conductive film is provided with a cutout portion extending in the polarization reversal direction, and the cutout portion passes through the plurality of gaps. A method for producing a periodic domain-inverted structure.   The method according to claim 1, wherein a plurality of the notches are formed in the conductive film.   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.

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