JP6136666B2 - Optical waveguide and electro-optic device - Google Patents

Description

  The present invention relates to an optical waveguide and an electro-optical device using a lithium niobate film.

  Lithium niobate (LN) has a large electro-optic constant and has been applied to devices such as optical modulators, optical gyros, optical switches, and electric field sensors.

  At present, LN is mainly used to fabricate devices using a bulk single crystal, but as disclosed in Patent Document 1, a technique for fabricating a device by forming an LN film on a sapphire substrate or the like is also disclosed.

  When a device is manufactured using a film in this way, it is possible to reduce the size and power consumption compared to the case of processing a bulk single crystal.

  However, when the present inventor made a trial manufacture of an optical waveguide having a ridge shape structure using an LN film, the light propagation loss was very large, and there was a problem that it could not be used as a device.

  Patent Document 2 discloses a technique for repairing Li deficiency generated on the surface during processing of bulk LN.

  However, this technique requires heat treatment at a high temperature of 900 ° C. or higher. However, in the case of an LN film, if it is heated to a high temperature, stress is generated due to a difference in thermal expansion from the substrate and cracks are generated in the LN film. Further, in the above document, an optical element that suppresses an increase in DC drift and optical loss can be provided by repairing the deficiency of Li.

JP 2006-195383 A JP 2006-284964 A

  An object of the present invention is to provide a low-loss optical waveguide having a ridge-shaped portion using a lithium niobate film.

  In the optical waveguide having a ridge-shaped portion using a lithium niobate film, the present invention has a Li / Nb element ratio of 0.4 to 0.9 and an Nb oxidation number of 4.8 to 5.0. It is an optical waveguide characterized by having a work-affected region.

  The present invention may be an epitaxial film having a lithium niobate film thickness of 2 μm or less.

  The present invention may be an electro-optical device using an optical waveguide or an electro-optical device having a buffer layer on a lithium niobate film.

  According to the present invention, it is possible to provide a low-loss optical waveguide having a ridge-shaped portion using a lithium niobate film.

It is a sectional side view of the film | membrane on the single crystal substrate which concerns on embodiment. It is a sectional side view which shows the optical waveguide which concerns on embodiment. 1 is a side sectional view showing an electro-optical device according to an embodiment. It is a schematic diagram of the propagation loss measurement by the scattering detection method of a prism coupler. It is the heat processing temperature which concerns on embodiment, a light propagation loss, a Li / Nb element ratio, the relationship figure of Nb oxidation number, and the relationship figure of Nb oxidation number and light propagation loss.

  FIG. 2 is a side sectional view showing an optical waveguide according to an embodiment of the present invention. This optical waveguide is formed by processing the single crystal substrate 1, the buffer layer 2, the lithium niobate film 3 (hereinafter referred to as the LN film 3), the ridge shape portion 4 obtained by processing the shape of the LN film 3, and the processing. And an altered region 5. Hereinafter, preferred embodiments of the present invention will be described in the order of processes.

  First, as shown in FIG. 1, a single crystal substrate 1 having an LN film 3 formed thereon is prepared.

  As the single crystal substrate 1, a substrate on which a high quality LN film 3 can be formed is preferable, and a sapphire single crystal substrate or a silicon single crystal substrate is preferable. The crystal orientation of single crystal substrate 1 is not particularly limited. When a silicon single crystal substrate is used, since the refractive index is larger than that of the LN film 3, a buffer layer 2 having a lower refractive index than that of the LN film 3 is formed between the single crystal substrate 1 and the LN film 3 in order to form an LN optical waveguide. It is necessary to devise such as pinching. The low refractive index buffer layer 2 is a transparent material that does not deteriorate the propagation loss, and the thickness needs to be optimized so that the device characteristics are improved. When a sapphire substrate is used, the buffer layer 2 is not necessary because sapphire has a lower refractive index than the LN film 3.

  The LN film 3 is preferably an epitaxial film. By using an epitaxial film close to a single crystal, desired electro-optical characteristics can be obtained. For the orientation of the LN film 3, a suitable one is used depending on the form of the device. When the LN film 3 is formed as a c-axis oriented epitaxial film, the c-axis oriented LN film 3 has a three-fold symmetry, so that the underlying single crystal substrate 1 has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane is preferable, and in the case of a silicon single crystal substrate, a (111) plane substrate is preferable.

  When the composition of the LN film 3 is expressed by LixNbOz, x is 0.9 to 1.05, and z is usually 2.8 to 3.2. Li and Nb may replace 10% or less with another element. Examples of replacement include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, and the like. There may be a combination of two or more.

  The film thickness of the LN film 3 is preferably 0.5 to 2.5 μm. More preferably, it is 1.0-2.0 micrometers. If it is too thin, light cannot be confined in the LN film 3, and light leaks out of the film and is guided, which may reduce the change in effective refractive index and increase propagation loss. If it is thick, a higher voltage is required to apply an electric field, and problems such as cracking due to internal stress occur during or after film growth.

  The LN film 3 may be formed by any method such as a sputtering method, a CVD method, and a sol-gel method, but a high quality epitaxial film is preferable. When sapphire is used for the single crystal substrate 1, the LN film 3 can be epitaxially grown directly on the sapphire single crystal substrate.

  It is desirable that the LN film 3 is polarized for manufacturing a device, and there is a case where polarization processing is required after the film formation.

  The ridge-shaped portion 4 is a portion that becomes the center of the optical waveguide. A mask such as a resist is patterned on the LN film 3, and the ridge-shaped portion 4 is formed by etching. Here, the ridge shape part 4 points out the location protruded on the convex shape part. Since the LN film 3 is thicker at the locations protruding above the left and right locations, the effective refractive index is higher. Therefore, light can be confined also in the left-right direction, and it functions as a three-dimensional optical waveguide. The shape of the ridge-shaped portion 4 may be any shape that allows light to be guided. An upward convex dome shape, a triangular shape, or the like may be used. The width, height, shape, and the like of the ridge shape portion 4 need to be optimized so as to improve the device characteristics.

  The work-affected region 5 is a region damaged by etching when the ridge-shaped portion 4 is formed by dry etching. The LN film 3 is difficult to perform wet etching, and dry etching is suitable for fine processing. Examples of dry etching include reactive ion etching and milling. The processed alteration region 5 after processing has an Nb oxidation number of 4.5 or less and an Li / Nb element ratio of 0.4 to 0.9 when analyzed by X-ray electron spectroscopy. The light propagation loss in the altered region 5 is very large. Since light oozes out not only inside the ridge-shaped portion 4 but also outside the ridge-shaped portion 4 and propagates, the side surface of the ridge-shaped portion 4 and the work-affected region 5 in the vicinity of the ridge-shaped portion 4 cause the optical waveguide. Propagation loss increases, making it difficult to use as a device.

By heat-treating this optical waveguide, a low-loss optical waveguide can be provided when the Nb oxidation number becomes 4.8 or more and 5.0 or less. Specifically, heat treatment is performed at 140 to 300 ° C. in the atmosphere. In the experiment by the inventor, when heat treatment is performed at 650 ° C. or higher, the internal stress of the LN film 3 increases and cracks are generated. Further, when the temperature exceeds 300 ° C., an altered phase such as Li ₃ NbO ₄ or LiNb ₃ O ₈ is deposited on the surface of the LN film 3, so that the temperature is preferably 300 ° C. or lower. If it is less than 140 ° C., the work-affected region 5 does not change, or the change is too small. When heat treatment is performed at 140 to 300 ° C., the Nb oxidation number in the work-affected region 5 becomes 4.8 or more and 5.0 or less, and the light propagation loss is greatly improved. On the other hand, the Li / Nb element ratio hardly changes and remains at 0.4 or more and 0.9 or less.

  In the optical waveguide of this embodiment, the surface of the LN film 3 remains Li-deficient. In the prior art, a decrease in insulation due to Li deficiency is one of the causes of a DC drift problem in an optical modulator, and an LN crystal having the highest quality with as few defects as possible has been demanded. DC drift is a change with time of the operating point of the light output. The operating point is normally adjusted by a DC bias so that the light output becomes an average value of the maximum light output and the minimum light output. However, since the resistance of the LN film 3 is not sufficiently high, the bias effect gradually varies. It is. However, for example, when the following optical modulator is manufactured, the problem of DC drift does not occur and Li deficiency does not become a problem. As shown in FIG. 3, the buffer layer 6 and the first and second electrodes for applying an electric field to the optical waveguide are formed on the LN film 3 on which the optical waveguide is formed. At this time, when the volume resistivity of the LN film 3 is sufficiently low compared with the volume resistivity of the buffer layer 6, for example, in the case of 1/100 or less, the DC voltage is almost applied to the buffer layer 6, The effect of DC is reduced. That is, no DC drift occurs. In this case, the operating point correction may be performed by a known method other than applying a DC voltage.

  The low-loss optical waveguide of this embodiment can be applied to electro-optic devices such as an optical modulator, an optical fiber gyro element, an optical switch, and an electric field sensor.

Example 1
First, the LN film 3 was formed on the c-plane sapphire substrate. An LN target was mounted on an RF sputtering apparatus equipped with a substrate heating apparatus, and a sputtering gas in which O 2 and Ar were mixed was introduced to form a 1 μm c-axis oriented LN epitaxial film.

  On the LN film 3, the ridge-shaped portion 4 was patterned with a resist by photolithography, and the LN film 3 was dry-etched with a milling apparatus. The milling apparatus was RF-350 manufactured by Veeco, and the conditions of a beam voltage of 300 to 700 V and a beam current of 300 to 800 mA were used. Then, the resist portion was peeled off with an organic solvent to form a ridge type optical waveguide having a height of 0.4 μm, a width of 2 μm, and a length of 20 mm.

  In order to improve the work-affected region 5 when the ridge-shaped portion 4 was formed, heat treatment was performed at 200 ° C. for 1 hour.

  Both ends of the formed optical waveguide were cut and polished, and the optical fiber was aligned and fixed with a UV adhesive. Using a semiconductor laser with a wavelength of 1550 nm, light propagation loss was measured. The result was 2 dB / cm.

(Example 2)
In order to see the effect of dry etching and heat treatment, the light propagation loss is reduced by the prism coupler scattering detection method schematically shown in FIG. 4 without forming the ridge-shaped portion 4 in the 1.0 μm c-axis oriented LN film 3. It was measured. The measurement was performed using a semiconductor laser with a wavelength of 1550 nm and a rutile (TiO ₂ ) single crystal prism, and the change in the amount of scattered light in the TM mode was measured by changing the probe distance, and the propagation loss was determined from the slope of the graph. Further, the Li / Nb element ratio and the Nb oxidation number on the surface of the LN film 3 were determined by X-ray electron spectroscopy. The X-ray electron spectrometer used in the measurement is Quantera 2 manufactured by ULVAC-PHI, the X-ray source is a monochromatic Al-Kα ray, the X-ray output is 15 kV 25 mW, the beam diameter is 100 μm, and the path energy is 55 eV. Went. The oxidation number was estimated from the area ratio of each bond peak obtained.

  First, Table 1 shows the effect of dry etching. After dry etching, the work-affected region 5 in which the Li / Nb element ratio decreases and the Nb oxidation number decreases is generated on the outermost surface, and the light propagation loss increases. The degree of alteration varies depending on dry etching conditions. Sample 2 is dry-etched at a higher power and higher rate than sample 1, and has a lower Li / Nb element ratio and Nb oxidation number. Sample 1 is shown in Table 1 and Sample 2 is shown in Table 2.

FIG. 5A shows the change in the light propagation loss and FIG. 5B shows the change in the Li / Nb element ratio when the sample in which the light propagation loss is increased by the dry etching is subjected to heat treatment for 1 hour at each temperature. The change in the Nb oxidation number is shown in FIG. FIG. 5D shows the relationship between the Nb oxidation number and the light propagation loss. FIG. 5 (C) shows that the Nb oxidation number increases from around 100 ° C. for both Sample 1 and Sample 2. Further, FIG. 5D shows that the light propagation loss decreases as the Nb oxidation number increases. In particular, when the Nb oxidation number was 4.8 or more and 5.0 or less, the light propagation loss was a small value of less than 2 dB / cm. Although the Nb oxidation number on the surface of the LN film 3 is lowered due to oxygen deficiency due to damage caused by dry etching, it is considered that the oxygen deficiency is compensated by heat treatment at 140 ° C. or higher, the Nb oxidation number is increased, and light propagation loss is reduced. . On the other hand, Li deficiency due to dry etching hardly changes the Li / Nb element ratio in the heat treatment at 140 to 300 ° C., and the Li / Nb element ratio is 0.4 to 0.9 from FIG. It can be seen that the altered region 5 remains. Note that the Li / Nb element ratios at 25 ° C., 60 ° C., 80 ° C., 100 ° C., 120 ° C., 140 ° C., 200 ° C., 300 ° C., 400 ° C., 500 ° C., and 600 ° C. in FIG. At 0.71, 0.79, 0.81, 0.76, 0.75, 0.78, 0.82, 0.81, 1.17, 1.23, 1.28, and Sample 2, respectively. 0.48, 0.45, 0.46, 0.51, 0.47, 0.48, 0.45, 0.46, 0.94, 1.08, and 1.14. From the comparison of the same heat treatment temperature, it can be seen from FIG. 5A and FIG. 5B that the work-affected region 5 having a Li / Nb element ratio of 0.4 or more and 0.9 or less remains. The light propagation loss is small, and it is considered that the problem of DC drift can be avoided depending on the device as described above. When the heat treatment was performed at 400 ° C. or higher, the Li / Nb element ratio exceeded 1, but precipitation of Li ₃ NbO ₄ was observed on the surface of the LN film 3 by SEM or XRD observation. In order for this precipitation to make the device characteristics unstable, the heat treatment temperature is preferably 300 ° C. or lower. The above results were the same when the 2.0 μm c-axis oriented LN film 3 was formed.

  The present invention can be applied to electrical devices such as an optical modulator, an optical fiber gyro element, an optical switch, and an electric field sensor.

1 Single crystal substrate 2 Buffer layer 2
3 LN film 4 Ridge-shaped portion 5 Work-affected region 6 Buffer layer 6
7 First electrode 8 Second electrode 9 Prism 10 Probe 11 Detector 12 Light 13 Scattered light

Claims (3)

In an optical waveguide having a ridge-shaped portion using a lithium niobate film that is an epitaxial film having a thickness of 2 μm or less formed on a sapphire substrate , the Li / Nb element ratio is 0.4 or more and 0.9 or less, and Nb An optical waveguide characterized by having a work-affected region having an oxidation number of 4.8 to 5.0. An electro-optical device using the optical waveguide according to claim 1 . The electro-optical device according to claim 2 , further comprising a buffer layer on the lithium niobate film.

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