JP2006113506A - Optical functional device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve stability and long-term reliability by eliminating birefringence induced by warpage of a silicon substrate in a process of a quartz-based optical wave circuit and by reducing the inner stress to zero when a thin film constituting a thin film heater of an optical function device is fabricated. <P>SOLUTION: After a quartz optical waveguide 101 is fabricated on a silicon substrate 102, an insulating thin film comprising a Ta<SB>2</SB>O<SB>5</SB>thin film 100 having compressive stress depending on the difference in coefficient of expansion between the silicon substrate 102 and the quartz optical waveguide 101 is deposited to form a thin film having tensile stress. The optical functional device is an optical switch having a Mach-Zehnder interferometer structure including 3-dB directional couplers and thermo-optical phase shifters in input output ports. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an optical waveguide propagating in a waveguide in a silica-based lightwave circuit comprising a plane and a three-dimensional optical waveguide formed on a silicon substrate constituting an optical functional device used in the fields of optical communication and optical information processing. Especially in optical functional devices using thermo-optic effect that can control the phase of the mode with high accuracy, eliminating the birefringence caused by the warpage of the silicon substrate that occurs in the silica-based lightwave circuit process, and the thin film heater of the optical functional device The present invention relates to an optical functional device having optical waveguide characteristics that are excellent in stability and long-term reliability by eliminating the generation of internal stress during the formation of a thin film constituting the.

The thermo-optic phenomenon is reported in the 3rd Ferroelectric Application Conference 20-O-6: Takaya Watanabe “New Electro-Optical Optical Deflector” on May 25, 1981, with various transparent materials. ing. Since then, research using quartz glass or LiNbO ₃ as a substrate material has been carried out in various places. In recent years, research and development on high performance, miniaturization, and cost reduction of various optical functional devices has been actively conducted for optical communication networks, and silica-based optical waveguides with excellent reliability and stability have recently been Progress is being made. In particular, silica-based optical waveguides fabricated on silicon substrates have the characteristics of low loss, excellent stability and excellent compatibility with optical fibers, and are commercially available and function as practical optical circuits. It has become the most powerful means of wave circuits. Among the optical functional devices to be configured, from the viewpoint of device stability and reliability, symmetrical Mach-Zehnder type optical switches consisting of two 3 dB directional couplers made of silicon and arm optical waveguides of the same length are the mainstream of development. It has become. This optical switch uses a thermo-optic effect of quartz glass as an optical waveguide by turning on or off a thin film heater that is a thermo-optic phase shifter shifter just above a core embedded in a clad on a silicon substrate. A refractive index distribution is generated to give a phase difference to the guided light in the optical waveguide to generate a bar state or a cross state, thereby realizing optical switching. These silica-based thermo-optic optical switches are indispensable for wavelength-division multiplexing (WDM) silica-based planar lightwave circuits by applying ultra-high Δ waveguides and heat insulation structures to reduce the size and power consumption of switch elements. Growing into a module.
Takaya Watanabe “New Electro-Optical Light Deflector”, Third Ferroelectric Application Conference 20-O-6: May 25, 1981

The problem to be solved relates to the film structure of the optical functional device.
As a conventional method for manufacturing a silica-based optical waveguide circuit formed on a silicon substrate, there is a flame hydrolysis deposition (FHD) method. (For example, refer to Patent Document 1.)
The FHD method currently used as a method for producing a silica-based optical waveguide is a method in which a raw material gas such as SiCl ₄ is introduced into an oxyhydrogen burner to cause an oxidation reaction in a flame, and SiO ₂ fine particles are formed on the silicon substrate with an underclad layer. A method of depositing a core layer. The silicon substrate on which the SiO ₂ fine particle layer is deposited is heated to a few hundreds of hundreds of degrees Celsius in an electric furnace, and high temperature heat work is required to make the glass fine particles form a transparent glass layer. After processing the core layer into a ridge shape so as to become an optical waveguide by photolithography, glass fine particles to be an overcladding layer are again deposited on the silicon substrate including the optical waveguide of this ridge structure by the flame hydrolysis deposition method, and again Make glass transparent in an electric furnace. Basically, these FHD processes are based on ceramic technology. The manufacturing method of the optical waveguide is a high-temperature process, but it has been fixed to the old FHD method. Since this FHD method requires a high temperature treatment of at least 1000 ° C., compressive stress after annealing remains on the silicon substrate. As a result, a phenomenon that the silicon substrate is warped is seen. For example, as shown in FIG. 8, in the waveguide in which the silica-based optical waveguide 801 on the silicon substrate 800 having a diameter of 3 inches is formed, the residual stress is 1.7 × 10 ⁹ dyn from the observation of the X-ray topograph of the cross section. / Cm ² has been reported. For this reason, a birefringence phenomenon occurs in the guided light, and it is essential to make the device wavelength-independent for the device used in the wavelength division multiplexing communication system. For this reason, a birefringence controller composed of a stress applying film (α-Si film) for compensating the birefringence of the silica-based optical waveguide has been developed. However, with the progress of multiplexing of WDM communication systems with 64 channels or more, a large-diameter silicon substrate of 6 inches or more is widely used, and the birefringence of each silica-based optical waveguide is compensated by a birefringence controller. The method cannot be applied technically or production-technically.
Japanese Patent Application No. 58-147378 (7th page, FIG. 4) Yasuho et al., 310 “Silica-Based Optical Waveguide on Silicon Substrate”, National Convention on Optical and Radio Waves, IEICE (1st page, Fig. 4)

The problem to be solved is that the silica-based optical system on the silicon substrate is used even when forming a thin film heater as a thermo-optic phase shifter constituting a thermo-optic optical switch or an optical filter. Waveguide warpage is a serious problem. The quartz-based planar lightwave circuit on the silicon substrate is a bimetal having a different temperature coefficient. Furthermore, the thin film of the thin film heater formed on the quartz-based planar lightwave circuit of the silicon substrate is also bimetal. Moreover, in the thin film formation process, the kinetic energy of the thin film atoms, the condensation energy, the radiation energy from the evaporation source, and the electromagnetic energy of the discharge gas, the chemical energy of the reaction gas, etc., depending on the thin film formation process. The entire substrate system is heated. As a result, the temperature rise of the system is 50 to 100K when using the vacuum deposition method, 100 to 250K using the sputtering method, and 400 to 1000K using the CVD method. When thin films are used, the temperature rise of the system is often at room temperature, so bimetals made at high temperatures are used at low temperatures, and the Young's modulus of the thin film and the substrate is within the thin film. An internal stress that is proportional to the difference between the difference in thermal expansion coefficient, the difference in thermal expansion coefficient, and the lowered temperature is always generated. The most essential part of the cause of internal stress inevitably appears when two substances with different surface free energies come into contact. However, since a single object generates the same interface free energy, that is, internal stress, on the front surface and the back surface, the substrate does not deform. However, when a thin film is formed on the front surface, the surface free energy differs between the front surface and the back surface, so that the substrate is bent. List the obvious causes of internal stress in the thin film,
(1) Bimetal effect due to temperature change of thin film / substrate system (2) Volume change due to temperature change of thin film (3) Volume change due to liquid / solid phase change during thin film formation process (4) Interface between thin film and substrate Free energy (5) Surface free energy of thin film (6) Free energy between thin film grains.
Internal stress occurs in the thin film due to the causes (1) to (6). (3) is essential due to the nature of the thin film itself, and the stress due to this cause cannot be controlled. (4) and (5) are also causes determined by the properties of the thin film and the substrate surface.
(1) and (2) are causes that depend on the conditions of thin film formation, and the magnitude of the stress due to these depends on the conditions, so it can be said that control is possible to some extent, but the temperature is unavoidable at the time of thin film formation. It cannot be made zero because of the rise. It is well known that many thin films are in a polycrystalline state. Since the size of the crystal grains depends on the conditions at the time of forming the thin film and the state of the substrate surface, the stress according to (6) should be changed considerably. The cause of internal stress is not necessarily due to (1) to (6). This is because the state of point defects such as vacancies and interstitial atoms and the state of structural defects such as voids that are aggregates of vacancies depend on the formation process. The internal stresses in vacuum deposited films and sputtered films are often approximated for the reasons listed in (1) to (6), and at least whether the stress is pulled or compressed is the material of the thin film and the substrate. It's almost decided, and it won't change unless you have very special conditions. It is a preferable amount that the internal stress of the thin film is actually zero rather than being controlled. This can be reduced in part by the choice of the thin film formation process, but is generally not always possible. In such a case, it may be possible to bring the stress close to 0 as a whole by stacking a thin film exhibiting tensile stress and a thin film exhibiting compressive stress.

In the optical functional device of the present invention,
1. Quartz formed on a silicon substrate in an interferometer-type or branch-type optical functional device using a thermo-optic effect in which a silica-based optical waveguide circuit comprising a lower clad layer, a core, and an upper clad layer is disposed on a silicon substrate The thin film that forms the thin film heater on the surface of the optical waveguide circuit and the thin film heater provided on the tensile stress thin film located directly above the predetermined area of the core of the optical waveguide overlaps the compressive stress thin film and the tensile stress thin film. An optical functional device.
2. This is an optical functional device in which a compressive stress thin film and a tensile stress thin film constituting a thin film heater are composed of the same material made of different film forming processes such as vapor deposition and sputtering.
3. This is an optical functional device in which a compressive stress thin film and a tensile stress thin film constituting the thin film heater are made of different materials.
4). It is an optical functional device composed of the same material produced by sputtering, in which the compressive stress thin film and the tensile stress thin film constituting the thin film heater are composed of different film forming process conditions.
In the optical functional device of the present invention, an insulator thin film is deposited on the surface of a silica-based optical waveguide circuit, a tensile stress thin film is formed, and the warp generated in the silica-based optical waveguide circuit on the silicon substrate is eliminated. Since an optical functional device composed of a thin film heater without distortion is realized on the system optical waveguide circuit, there is no wavelength dependency.

For example, a combination of Si (substrate) -quartz PLC (SiO 2) -Si thin film is ideal as a solution to the birefringence phenomenon generated in a quartz-based lightwave circuit formed on a silicon substrate, not limited to the FDH method. However, since α-Si, which is a Si-based thin film, has a semiconductor property, if a thin film heater is formed on the α-Si, leakage current may occur. As a solution to this, it is possible to realize a strain-free thin film heater by depositing an insulating thin film on an α-Si thin film and forming a thin film heater thereon. Of course, it goes without saying that the tensile stress of the underlying insulator thin film on the α-Si thin film is canceled by the compressive stress thin film constituting the thin film heater.

As described above, in the optical functional device obtained by the present invention, an insulator thin film is deposited on the surface of the silica-based optical waveguide circuit to form a tensile stress thin film, thereby eliminating the warp generated in the silica-based optical waveguide circuit on the silicon substrate. ing. Since the optical functional device of the present invention cancels the warpage inherent in the silicon substrate, the birefringence phenomenon does not occur. Furthermore, since an optical functional device composed of a thermo-optic phase shifter composed of a thin film heater without distortion is realized on the quartz optical waveguide circuit without distortion, wavelength dependence is unlikely to occur at all. The optical functional device obtained by the present invention eliminates the birefringence generated by the warpage of the silicon substrate that occurs during the quartz-based lightwave circuit process and eliminates the generation of internal stress during the formation of the thin film constituting the thin film heater. It is industrially completed as an optical functional device having optical waveguide characteristics with excellent stability and long-term reliability, and can greatly contribute to the construction of many backbone optical networks and subscriber optical networks.

In the optical functional device of the present invention, an insulator thin film is deposited on the surface of a silica-based optical waveguide circuit, a tensile stress thin film is formed, and the warp generated in the silica-based optical waveguide circuit on the silicon substrate is eliminated. Since an optical functional device composed of a thin film heater without distortion is realized on the system optical waveguide circuit, there is no wavelength dependence. Since there is no warpage of the silica-based optical waveguide on the silicon substrate, there is no residual stress, so the reliability is also stable, and as a wavelength-independent device, a thermo-optic optical switch or optical filter is configured. Even when a thin film heater as a thermo-optic phase shifter is formed, the thin film structure is characterized in that residual stress is zero.

FIG. 1 is a plan view of an optical functional device as a basis of the present invention formed on a wafer. After forming the silica-based optical waveguide 101 on the silicon substrate 102, the insulator thin film made of the Ta ₂ O ₅ thin film 100 is caused by warping due to compressive stress in the difference in expansion coefficient between the silicon substrate 102 and the silica-based optical waveguide 101. The optical switch chips 103, 104, 105, 106, 107, 108 having a Mach-Zehnder type interferometer having a 3 dB directional coupler and a thermo-optic phase shifter at the input / output ports are formed. Is formed. Ta ₂ O ₅ is a stable film in which moisture and the like are unlikely to enter the interface because the thin film is less likely to be crystalline in the ion-assisted film formation and crystal grain boundaries are not easily generated. Here, the configuration of the optical switch chip 103 among the optical switch chips 103, 104, 105, 106, 107, 108 will be described. The optical switch chip 103 having a Mach-Zehnder interferometer configuration includes optical waveguides 109 and 110 on the input port (output port) side, optical waveguides 112 and 113 on the output port (input port) side, and a thermo-optic phase shifter 111. An insulator thin film made of the Ta ₂ O ₅ thin film 100 is deposited, a tensile stress thin film is formed, and the warp applied to the quartz optical waveguide circuit 102 on the silicon substrate 102 is eliminated. An optical functional device composed of a thin film heater without distortion is realized. At the cutting line 114 of the optical switch chip 103 formed on the silicon substrate 102, the optical switch chip (103, 104, 105, 106, 107, 108) is divided into individual optical switch chips (103, 104, 105, 106, 107, 108) by a dicing saw, etc. It becomes a switch.

FIG. 2 is a cross-sectional view of the optical switch which is the optical functional device of the present invention shown in FIG. A representative one of a plurality of optical switches which are the optical functional device of the present invention in FIG. 1 will be described with reference to the drawings. Core portions 202 and 203 are formed in a silica-based optical waveguide circuit 206 manufactured on the silicon substrate 200 by the FDH method. The warpage caused by the compressive stress (-1.7 × 10 ⁹ dyn / cm ² ) generated in the FDH process is caused by Ar ₂ O ₅ thin film 201 (thickness 5000 mm) formed by the ion beam assisted deposition method of FIG. A Ta ₂ O ₅ thin film having a tensile stress of + 1.7 × 10 ⁹ dyn / cm ² is formed with an assist (ion output current of 300 mA / cm ² ). By controlling the ion output current in this manner, the insulator thin film is deposited on the quartz optical waveguide circuit 206, and the tensile stress thin film is formed, thereby eliminating the warp. As a thermo-optic phase shifter, a Cr film was selected as a thin film heater electrode on a Ta ₂ O ₅ thin film as an insulator film.
FIG. 4 shows the film thickness and compressive stress dependence of the Cr film formed by magnetron sputtering. The internal stress of the Cr film at a sputtering rate of 0.15 Pa and a sputtering rate of 10 Å / sec is constant at −10.0 × 10 ⁹ dyn / cm ² . Here, the Cr film 204 was 1000 mm. On this sputtered Cr film 204, 500 mm of Cr film 205 was formed by vapor deposition. The internal stress of the Cr film 205 is + 10.0 × 10 ⁹ dyn / cm ² . By applying the present invention in this way, the total stress of the two-layer Cr thin film heater (204 + 205) made of the same material becomes zero. Distortion caused by a Cr thin film heater in a conventional single film can be reduced to zero.

In the case of a Ta ₂ N film often used as a thin film heater electrode, since the compressive stress is (1 to 3 × 10 ⁹ dyn / cm ² ) with a Ta ₂ N composition, Cu shown in FIG. By sputtering a Ta ₂ N film 1000 後 after vapor deposition on a 100 to 200 Å base, the total stress of the thin film heater having a Cu + Ta ₂ N composition, which is a combination of materials having different compositions, could be made zero. FIG. 6 shows a tensile stress of +150 N / m for a base Cr film of 04 Pa and a thickness of 1500 mm by changing the Ar gas pressure as the sputtering condition of the Cr film made of the same material. On top of this, under a condition of 0.15 Pa, the film thickness is 1500 mm, the compressive stress is -150 N / m, and the total stress of the thin film heater becomes zero.

FIG. 7 shows a basic configuration of a lattice type optical filter which is an optical functional device of the present invention. An optical filter that uses multiple interference of light waves in the time domain, and includes a phase shifter, a coupling rate variable coupler, a 3 dB coupler, and a delay arm. Since the light wave is always confined in the two waveguide arms, there is no principle loss and loss variation, and various filter characteristics can be obtained. The insulator thin film 701 is deposited on the silica-based optical waveguide circuit on the optical waveguide circuit formed on the silicon substrate 700 to form a tensile stress thin film, thereby eliminating the warp. The insulator thin film 701 is formed by applying a tensile stress of + 1.7 × 10 ⁹ dyn / cm ^{2 to} the Ta ₂ O ₅ thin film 701 (thickness: 5000 mm) by Ar ion (ion output current 300 mA / cm ² ) by ion beam assisted deposition. A Ta ₂ O ₅ thin film is formed. By controlling the ion output current in this manner, the insulator thin film is deposited on the quartz optical waveguide circuit, and the tensile stress thin film is formed, so that the tensile stress (+ 1.7 × 10 ⁹ dyn / cm ² ) is applied to the silicon. Compressive stress generated in the substrate of −1.7 × 10 ⁹ dyn / cm ² is canceled out, and the warp of the substrate is zero. Next, a Cr film is selected as a thin film heater electrode on the Ta ₂ O ₅ thin film that is the insulator film 701. The internal stress of the Cr film at a sputtering rate of 0.15 Pa and a sputtering rate of 10 Å / sec is constant at −10.0 × 10 ⁹ dyn / cm ² . Here, the Cr film was 1000 mm. On this sputtered Cr film, 500 nm of Cr film was formed by vapor deposition. The internal stress of this Cr film is + 10.0 × 10 ⁹ dyn / cm ² . The total internal stress of Cr thin film heaters made of the same material is zero. An input port 702 and an output port 716. Reference numerals 703, 704, 705, 706, 707, 708, 709, 710, 711, 712, 713, 714, 715 are thermo-optic phase shifters. Thus, an optical filter composed of a quartz-based planar optical circuit is an indispensable device for a WDM transmission system.

It is a top view of the wafer by which the optical functional device is arrange | positioned. It is sectional drawing of the optical switch with which the optical functional device is arrange | positioned. An ion current dependence of Ta ₂ O ₅ thin film internal stress by the ion beam assisted deposition. It is the relationship between Cr film thickness by sputtering and internal stress. It is the relationship between the Cr film thickness by the vapor deposition method, Cu film thickness and internal stress. It is the relationship between Cr film thickness and internal stress when the Ar gas pressure of sputtering is changed. FIG. 7 shows the basic configuration of the lattice type optical filter of the present invention. It is sectional drawing which shows the curvature | sledge of the silicon substrate after a FHD process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Insulating film 101 Optical waveguide circuit 103 Optical switch chip 104 Optical switch chip 105 Optical switch chip 106 Optical switch chip 107 Optical switch chip 108 Optical switch chip 109 Optical waveguide 110 Optical waveguide 111 Optical waveguide 113 Optical waveguide 114 Cutting line 200 Silicon substrate 201 Insulating film 202 Core 203 Core 204 Heater thin film 205 Heater thin film 206 Silica-based optical waveguide circuit (cladding)
700 Silicon substrate 701 Insulating film 702 Input port 703 Thermo-optic phase shifter shifter 704 Thermo-optic phase shifter shifter 705 Thermo-optic phase shifter shifter 706 Thermo-optic phase shifter shifter 707 Thermo-optic phase shifter shifter 708 Thermo-optic phase shifter Shifter 709 Thermo-optic phase shifter shifter 710 Thermo-optic phase shifter shifter 711 Thermo-optic phase shifter shifter 712 Thermo-optic phase shifter shifter 713 Thermo-optic phase shifter shifter 714 Thermo-optic phase shifter shifter 715 Thermo-optic phase shifter shifter 716 Output port 800 Silicon substrate 801 Silica-based optical waveguide circuit

Claims (4)

Quartz formed on a silicon substrate in an interferometer-type or branch-type optical functional device using a thermo-optic effect in which a silica-based optical waveguide circuit comprising a lower clad layer, a core, and an upper clad layer is disposed on a silicon substrate The thin film constituting the thin film heater provided on the tensile stress thin film positioned directly above the predetermined region of the core of the optical waveguide is formed of the compressive stress thin film and the tensile stress thin film. An optical functional device characterized by being stacked. 2. The optical functional device according to claim 1, wherein the compressive stress thin film and the tensile stress thin film constituting the thin film heater are made of the same material comprising different film forming processes such as vapor deposition and sputtering. The optical functional device according to claim 1, wherein the compressive stress thin film and the tensile stress thin film constituting the thin film heater are made of different materials. 2. The optical functional device according to claim 1, wherein the compressive stress thin film and the tensile stress thin film constituting the thin film heater are made of the same material produced by a sputtering method having different film forming process conditions.

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