JP4936838B2 - Light control device - Google Patents

Description

  The present invention relates to a light control device, and more particularly to a light control device in which a control electrode is formed on a crystalline substrate having an electro-optic effect.

2. Description of the Related Art Conventionally, in the optical communication field and the optical measurement field, a light control device such as a waveguide type light modulator in which an optical waveguide or a modulation electrode is formed on a substrate having an electro-optic effect has been widely used.
In order to realize a wider optical modulation frequency, it is important to match the speed of the modulation signal microwave and the light wave, and various methods have been devised so far. Specific examples include thicker buffer layers, higher aspect ratios of electrodes, and ridge structures.

In Patent Document 1 or 2 below, an optical waveguide and a modulation electrode are incorporated in an extremely thin substrate (hereinafter referred to as “first substrate”) having a thickness of 30 μm or less, and has a lower dielectric constant than the first substrate. Other substrates are bonded to reduce the effective refractive index with respect to the microwave, thereby achieving speed matching between the microwave and the light wave.
JP-A 64-18121 JP 2003-215519 A

As described above, the use of a thin first substrate greatly increases the degree of freedom in designing the light control device. For example, a broadband optical modulator having a low driving voltage can be manufactured without using a buffer layer. It becomes possible. Furthermore, from the viewpoint of reducing the propagation speed of microwaves, it is synonymous with the use of a material having a low dielectric constant for the substrate. Specifically, the first substrate is specifically set to 150 μm or less, and particularly in the region of 26 GHz or more. The following non-patent document 1 discloses that the influence of the dielectric loss (tan δ) of the wave itself can be reduced, and is applied to widening the bandwidth of the optical modulator.
Y. Yamaneet.al., “Investigation of sandblast machining techniques for broadband LNmodulators”, Sumitomo Osaka Cement Technical report 2002, pp49-54 (2003)

As shown in FIG. 1A, when a crystalline substrate 1 such as a conventional lithium niobate (hereinafter referred to as “LN”) having a thickness of several hundred μm or more is used, the optical waveguide 4 is placed on the substrate. Then, a buffer layer 3 made of SiO ₂ or the like is formed thereon as required for light wave absorption suppression, speed matching conditions, etc., and then a control electrode 2 composed of a signal electrode, a ground electrode, etc. is formed. The FIG. 1A shows an example of an X-cut type optical modulator, but when a Z-cut type substrate is used, a control electrode is disposed immediately above the optical waveguide, so that a buffer layer is always formed. ing.

FIG. 1B is an enlarged view of a portion indicated by reference symbol A in FIG. 1A. On the buffer layer 3, Ti (2-1) and Au (2-2) are formed. A base layer for forming a modulation electrode is formed by vapor deposition, and an Au electrode (2-3) is formed thereon by electrolytic plating.
Since the electrode underlayer is usually shaped according to the electrode pattern, it needs to be easily removable by etching or the like. For this reason, the electrode base layer is set to a minimum necessary film thickness so that the same potential can be formed during plating in consideration of etching time and maintaining pattern accuracy.

  However, when using a thinned substrate as described above or using an X-cut type substrate, the buffer layer is unnecessary or only a buffer layer thinner than usual is formed. In the case where the Au electrode is formed by electrolytic plating, the surface of the electrode is enlarged due to abnormal growth of crystal grains, and when creating a broadband light control device, it is considered that the present invention becomes a microwave scattering source. It was elucidated by the research of.

  In particular, it was found that when a thinned LN crystalline substrate is used, stress due to abnormal grain growth affects the optical waveguide, resulting in deterioration of temperature characteristics and substrate damage. In addition, the abnormal grain growth part has a locally high plating thickness (large in-plane distribution) and cannot satisfy the speed matching between the microwave and the light wave, so the yield is reduced when creating a broadband light control device. It was also a cause.

  The problem to be solved by the present invention is to solve the above-mentioned problems, and in the case where there is no buffer layer, such as a thin crystalline substrate, the control electrode formed by electrolytic plating is abnormal. An object of the present invention is to provide an optical control device that suppresses grain growth, has broadband characteristics, and has improved temperature characteristics and substrate damage.

In order to solve the above problems, in the invention according to claim 1, in the light control device in which the control electrode is formed by plating on the crystalline substrate having the electro-optic effect, the crystalline substrate is made of lithium niobate or tantalum. X-cut crystal containing any of lithium acid, the thickness of the crystalline substrate is 50 μm or less, and at least one layer of electrode orientation and the like is suppressed from growing between the crystalline substrate and the control electrode It has a layer and does not have a buffer layer , and the thickness of the growth suppressing layer for electrode orientation and the like is 75 nm or more.
In the present invention, the “electrode orientation and other growth suppression layer” refers to the structure of the electrode on which the crystal orientation of the crystalline substrate and the surface roughness due to the orientation exhibited by Au of the underlying layer are formed on the substrate. It means a layer having a function of affecting the growth and suppressing the growth of abnormal grains on the electrode.

  The invention according to claim 2 is the light control device according to claim 1, wherein the thickness of the control electrode is 2 μm or more.

  According to a third aspect of the present invention, in the light control device according to the first or second aspect, the electrode-alignment-growth suppression layer includes Ga, Mo, W, Ta, Si, Ti, Cr, Ni-Cr, and these materials. It is characterized in that at least one material selected from nitrides or oxynitrides of the above is used.

  According to a fourth aspect of the present invention, in the light control device according to any one of the first to third aspects, the growth suppressing layer for electrode orientation and the like is amorphous or polycrystalline.

  According to a fifth aspect of the present invention, in the light control device according to any one of the first to fourth aspects, the electrode orientation and other growth suppression layer is formed of two or more layer structures.

According to the invention of claim 1, the crystalline substrate is an X-cut type crystal containing either lithium niobate or lithium tantalate, and the thickness of the crystalline substrate is 50 μm or less, and is controlled with the crystalline substrate. The electrode has a growth suppression layer having at least one layer, such as an electrode orientation, and does not have a buffer layer , and the thickness of the growth layer having the electrode orientation is 75 nm or more. Light control device that can suppress the influence of the crystal orientation of the substrate on the electrode structure, prevent abnormal grain growth of the electrode, form a control electrode with excellent high frequency characteristics, and improve temperature characteristics and substrate damage Can be provided.
In particular, even when a crystal of lithium niobate or lithium tantalate, in which abnormal grain growth of the electrode is likely to occur, the growth suppressing layer for electrode orientation and the like can effectively suppress abnormal grain growth.
In addition, the thickness of the growth suppressing layer for electrode orientation or the like is preferably 75 nm or more, more preferably the thickness of the growth suppressing layer for electrode orientation or the like is 100 nm or more.

Furthermore, since the crystalline substrate is an X-cut crystal containing either lithium niobate or lithium tantalate, and the thickness of the crystalline substrate is 50 μm or less, abnormal grain growth of the electrode occurs in particular. Even in the case of an X-cut substrate that uses lithium niobate crystals and does not necessarily require a buffer layer, the growth suppressing layer such as electrode orientation can effectively suppress abnormal grain growth. .
  In addition, when the thickness of the crystalline substrate is 50 μm or less, it is possible to design a broadband light control device without providing a buffer layer as in the prior art, but abnormal grain growth of the electrode is likely to occur. The abnormal grain growth can be effectively suppressed by the growth suppressing layer for electrode orientation and the like according to the present invention.
  In addition, in order to achieve speed matching between the light wave propagating through the crystalline substrate and the microwave propagating through the control electrode, when the thickness of the control electrode is increased, the thermal stress applied from the electrode to the substrate is increased. Even in such a case, the growth suppressing layer such as the electrode orientation contributes to the relaxation of the stress, and particularly when the thickness of the substrate is thinner than the thickness of the electrode, the stress relaxation is highly effective, and the temperature of the light control device is high. Characteristics and substrate damage can be improved.

  According to the second aspect of the present invention, since the thickness of the control electrode is 2 μm or more, it is easy to perform speed matching between the microwave and the propagating light wave, and the electrode orientation growth growth suppressing layer of the present invention enables Abnormal grain growth can be effectively suppressed. Moreover, even when the thermal stress applied from the electrode to the substrate is increased by increasing the thickness of the control electrode, the electrode orientation growth suppressing layer contributes to the relaxation of the stress.

  According to the invention of claim 3, the electrode orientation and other growth suppression layer is at least one material selected from Ga, Mo, W, Ta, Si, Ti, Cr, Ni—Cr and nitrides or oxynitrides of these materials. Therefore, the influence of the crystal orientation of the crystalline substrate on the electrode structure can be effectively suppressed, and abnormal grain growth of the electrode can be prevented. Further, depending on the selected material, other functions such as improving the adhesion of the electrode to the substrate, suppressing the pyroelectric effect, and suppressing impurity contamination of the buffer layer can also be provided.

  According to the invention of claim 4, since the growth suppressing layer for electrode orientation is amorphous or polycrystalline, the crystal structure of the growth suppressing layer for electrode orientation is more random than the crystal of the crystalline substrate. Thus, the influence of the orientation of the crystalline substrate on the electrode structure can be more effectively suppressed, and abnormal grain growth of the electrode can be prevented.

  According to the invention of claim 5, since the growth suppressing layer for electrode orientation and the like is formed from two or more layer configurations, the orientation of the crystal of the crystalline substrate can be improved by adopting different materials and different structures for each layer. The influence on the electrode structure can be more effectively suppressed, and abnormal grain growth of the electrode can be prevented.

Hereinafter, the present invention will be described in detail using preferred examples.
As shown in FIG. 2, the present invention provides a light control device in which a control electrode 11 is formed on a crystalline substrate 1 having an electro-optic effect by plating, and includes the crystalline substrate 1 and the control electrode 11. It is characterized by having at least one layer for preventing growth of electrode orientation equality 10 between them.

  As a crystalline substrate having an electro-optic effect, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used. Further, when an X-cut type substrate is used as the crystalline substrate, abnormal grain growth of the electrode is more likely to occur than in the case of the Z-cut type. It can be used.

  The control electrode includes various electrodes formed on the crystalline substrate in order to drive a light control device such as a signal electrode, a ground electrode, or a DC electrode. In order to form the control electrode, an underlayer such as Ti or Au is formed by a vapor deposition method, a predetermined electrode pattern is left by a photolithography method, the underlayer is masked, and an electroplating method or an electroless plating method is used for Au. An electrode is formed. Thereafter, the photoresist film and a part of the base layer are removed by wet etching.

In the present invention, it is possible to form the electrode by the above process by forming the underlayer on the surface of the substrate 1 after the formation of the electrode orientation and the like growth suppressing layer 10 on the surface of the substrate 1. is there. Naturally, when the growth suppressing layer for electrode orientation and the like has conductivity, it is possible to form a base layer or a bonding layer on the surface of the substrate and to form a growth suppressing layer for electrode orientation or the like on the substrate layer. In order to avoid complication, it is preferable to use part or all of the underlayer and the growth suppressing layer such as electrode orientation.
Note that, by setting the thickness of the control electrode to 2 μm or more, and more preferably 10 μm or more, it is possible to realize an advantage that it becomes easy to perform speed matching between the microwave that is the modulation signal and the propagating light wave.

As the growth suppressing layer such as electrode orientation, at least one material selected from Ga, Mo, W, Ta, Si, Ti, Cr, Ni—Cr and nitrides or oxynitrides of these materials can be used. For example, when using Ti, NI-Cr, Ta, Mo, Cr, Si, SiN, or SiON, it is possible to improve the adhesion of the electrode to the substrate, and when using Si or Ga. Can suppress the pyroelectric effect, and when Ta, TaN, W, SiN, or SiON is used, it is also possible to suppress impurity diffusion into the buffer layer.
In addition, when the growth suppressing layer such as the electrode orientation has conductivity, it needs to be shaped into a shape corresponding to the electrode pattern by etching or photolithography, but it does not become an obstacle when applying an electrode such as a modulation signal. When the resistance value is high, it is also possible to form a growth suppressing layer such as an electrode orientation on the entire substrate surface. However, when it is formed on the entire surface of the substrate, it is necessary to select a material that does not absorb light waves propagating through the optical waveguide.

In addition, by making the crystal structure of the growth suppressing layer for electrode orientation amorphous or polycrystalline, the crystal structure of the growth suppressing layer for electrode orientation becomes more random than the crystal of the crystalline substrate, and the crystalline substrate It is possible to more effectively suppress the influence of the orientation of the substrate, the orientation exhibited by Au of the underlayer, etc. on the electrode structure.
Furthermore, by setting the thickness of the growth suppressing layer for electrode orientation or the like to 75 nm or more, preferably 100 nm or more, it is possible to further suppress the influence of the crystal orientation of the crystalline substrate on the electrode structure. However, if the thickness of the growth suppressing layer such as the electrode orientation is made too thick, it takes time to etch the layer corresponding to the electrode pattern, etc., and pattern accuracy accompanying a decrease in manufacturing efficiency and an increase in the amount of side etching Causes a drop. Even when etching is not performed, the distance between the optical waveguide in the substrate and the control electrode is increased, which may cause a decrease in modulation efficiency. Is preferred.

  Moreover, it is preferable to set the electrode orientation and other growth suppression layer so that the lattice constant of the constituent element has a difference of 10% or more with respect to the lattice constant of the crystalline substrate. As a result, the growth suppressing layer for electrode orientation and the like obtained by the difference in these lattice constants can more easily suppress the influence of the orientation of the crystal of the crystalline substrate on the electrode structure, which is easy to form an amorphous or polycrystalline, It is possible to prevent abnormal grain growth of the electrode.

  As shown in FIG. 3, it is also possible to form the electrode orientation uniform growth suppression layer from two or more layer (10-1, 10-2) configurations. In this way, by forming a growth suppressing layer such as an electrode orientation with a plurality of layers, a synergistic effect can be obtained by using different materials or adopting different crystal structures in each layer, and the crystal of the crystalline substrate can be obtained. It is possible to further suppress the influence of the orientation or the like on the electrode structure. Furthermore, by forming a growth suppression layer with electrode orientation and the like in a plurality of layers, the influence of the growth suppression layer with electrode orientation on the crystal substrate having the electro-optic effect, for example, by canceling each film stress to each other, more temperature characteristics It is possible to provide an excellent device.

Further, when the thickness of the crystalline substrate is 50 μm or less, it is not necessary to provide a buffer layer between the crystalline substrate and the control electrode. Even if a buffer layer of 0.3 μm or less is formed, If it is normal, there is a concern that the crystal orientation of the crystalline substrate may affect the electrode structure, but this influence can be effectively suppressed by using the electrode orientation growth suppressing layer of the present invention. It becomes possible.
On the other hand, in order to achieve speed matching between the light wave propagating through the crystalline substrate and the microwave propagating through the control electrode, it is preferable to increase the thickness of the control electrode. The thickness of the substrate becomes larger, and the influence of thermal stress applied from the electrode to the substrate also increases. However, the growth suppressing layer for electrode orientation and the like according to the present invention contributes to the relaxation of the stress, and in particular, even when the thickness of the substrate is thinner than the thickness of the electrode, the stress relaxation effect of the growth suppressing layer for electrode orientation and It is possible to provide a light control device with improved substrate damage.
When a crystalline substrate (thin plate) having a thickness of 50 μm or less is used, since the mechanical strength of the thin plate is low, the reinforcing plate is bonded to the back surface of the thin plate using an adhesive or a direct bonding method.

(Example)
An X-cut LN substrate having a thickness of 500 μm was used as a crystalline substrate, an optical waveguide was formed by a Ti diffusion method, and a Ti film having a thickness of 100 nm was formed on the substrate as a growth suppressing layer for electrode orientation and the like. . Next, an Au film having a thickness of 35 nm was formed on the Ti film as an underlayer by the same vapor deposition method.
Through a photolithography process, a photoresist mask corresponding to the electrode pattern was formed, and an Au electrode having a height of 30 μm was formed by electrolytic plating. Thereafter, the entire photoresist mask and a part of the underlayer and a growth suppressing layer such as an electrode orientation were removed by wet etching.

  A thermoplastic resin is applied to the surface of the substrate on which the electrodes are formed, and the substrate is attached and fixed to a polishing jig. 20 wt% aq), and rough polishing is performed until the thickness of the substrate reaches approximately 50 μm under the conditions of a rotation speed of 35 rpm and a lapping pressure of 12.75 to 9.81 kPa. Thereafter, precision mirror polishing was performed to a set thickness by mechanochemical polishing (CMP) using a nonwoven fabric as the pad material and colloidal silica as the processing liquid. This time, the set thickness of the substrate was 10 μm, and a broadband light control device was manufactured.

(Comparative example)
A light control device was produced in the same manner as in the example except that the thickness of the Ti film formed on the substrate was 50 nm.

The state of the electrode surface in Examples and Comparative Examples is shown in FIG. 4 (Example) and FIG. 5 (Comparative Example).
As can be understood from FIGS. 4 and 5, when the thickness of the Ti film is 100 nm and the role of a growth suppressing layer such as an electrode orientation is provided, abnormal growth of crystal grains is not observed on the electrode surface, and the conventional Ti film When the thickness is 50 nm, abnormal growth of crystal grains appears.

As a result, in the light control device according to the present invention, by using a growth suppressing layer such as an electrode orientation, it is possible to suppress the abnormal growth of crystal grains from appearing on the electrode surface, and to scatter microwaves at the control electrode. It becomes possible to prevent. In addition, due to the generation of crystal grains, a difference in thermal expansion coefficient occurs between the crystal grain region and other regions (for example, an amorphous part), and the temperature characteristics deteriorate due to the stress inside the electrode due to temperature change. It is possible to effectively suppress the phenomenon that the thin plate substrate is damaged or the phenomenon that the thin plate substrate is damaged by the present invention.
In this embodiment, the Au electrode was formed by the electroplating method. However, the present invention is not limited to this, but various platings using electroless Au, electric field Cu, electric field Ag, electroless Cu, electroless Ag, and electric field Ni. It is also useful in the law.

  The light control device according to the present invention suppresses the growth of abnormal grains on the control electrode formed by plating even when there is no buffer layer or a thin layer, such as a thin crystalline substrate. It is possible to provide a light control device having broadband characteristics and improved temperature characteristics and substrate damage.

A sectional view of a conventional light control device is shown. It is a figure which shows a part of light control device which concerns on this invention. It is a figure which shows a part of other Example of the light-control device based on this invention. It is a photograph which shows the state of the electrode surface of an Example. It is a photograph which shows the state of the electrode surface of a comparative example.

DESCRIPTION OF SYMBOLS 1 Crystalline substrate 2,11 Control electrode 3 Buffer layer 4 Optical waveguide 10 Electrode orientation etc. growth suppression layer

Claims (5)

In a light control device in which a control electrode is formed by plating on a crystalline substrate having an electro-optic effect,
The crystalline substrate is an X-cut type crystal containing either lithium niobate or lithium tantalate, and the thickness of the crystalline substrate is 50 μm or less,
Between the crystalline substrate and the control electrode, has at least one electrode orientation growth suppression layer, and does not have a buffer layer,
The light control device characterized in that the thickness of the electrode orientation and other growth suppression layer is 75 nm or more.   The light control device according to claim 1, wherein the control electrode has a thickness of 2 μm or more.   3. The light control device according to claim 1, wherein the electrode orientation growth control layer is made of Ga, Mo, W, Ta, Si, Ti, Cr, Ni—Cr, and nitrides or oxynitrides of these materials. A light control device using at least one selected material.   4. The light control device according to claim 1, wherein the growth suppressing layer for electrode orientation and the like forms an amorphous or polycrystalline material. 5.   5. The light control device according to claim 1, wherein the electrode orientation and other growth suppression layer is formed of two or more layer structures. 6.

Images