JP2008145506A - Optical element using piezoelectric element, and method of forming the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical branching filter comprising a piezoelectric element of a cantilever beam structure formed with zinc oxide (ZnO). <P>SOLUTION: A Ge layer 120 is used as a sacrificing layer, oxygenated water is used as an etching solution to make a cantilever beam structure (a, b). An n type ZnO layer (low resistance layer) 140- a ZnO layer (piezoelectric conductive film) 150- an n type ZnO layer (low resistance layer) 160 is formed as a piezoelectric element (c, d). The piezoelectric element portion so formed is deformed by an applied voltage, to provide wavelength selectivity through interference by multiple reflection in the portion where the sacrificing layer is removed; thus, the optical branching filter outputting reflected light is manufactured. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical element using a cantilever-structure zinc oxide piezoelectric element and a method for producing the same.

Devices using MEMS (Micro Electro Mechanical System) or MST (Microsystem technology) are attracting attention from many researchers. In constructing a very small electromechanical system such as MEMS or MST, a driving element for mechanically moving the system is required.
Now, it has been proposed to use a piezoelectric element having a cantilever structure as a configuration of the drive element. PZT is mainly used for the piezoelectric material used for this, but since this material contains lead, it has caused environmental pollution. Zinc oxide has been proposed as a piezoelectric material that can replace this material. It is advantageous to produce this drive element using zinc oxide (ZnO) which is an inexpensive and safe material.
Recently, research reports on the application of ZnO piezoelectric elements have been made by overseas research institutions [see Non-Patent Documents 1 and 2]. As described above, those having a free standing (cantilever) structure using a ZnO thin film grown on a Si substrate by using the piezo effect for driving are known.
However, there are no successful cases in Japan. Since zinc oxide is completely melted by the etching agent used for device fabrication, selective etching, which is an essential technique for device fabrication, is difficult.
B. Rogers, L. Manning, T. Sulchek, DJAdams, Ultramicroscopy 100 (2004) 267. Don L. DeVoe, Albert P. Pisano, Modeling and Optimal Design of Piezoelectric Cantilever Microactuators, J. of Microelectoromechanical Systems Vol.6, No.3 1997 p266-270 Republished Patent Publication WO02 / 017368

  An object of the present invention is to create a cantilever structure piezoelectric element using zinc oxide (ZnO), and to construct an optical branching filter and an optical switch using the created cantilever structure piezoelectric element.

In order to achieve the above object, according to the present invention, a germanium (Ge) layer is formed on a silicon (Si) substrate, a silicon oxide (SiO ₂ ) film is formed on the Ge layer, and the SiO ₂ film is formed. After the piezoelectric element layer of zinc oxide (ZnO) is formed, the Ge layer is etched with hydrogen peroxide to leave a part of the Ge layer to form a cantilever structure, and electrodes are formed on the piezoelectric element layer. An optical element forming method using a cantilever structure zinc oxide piezoelectric element, characterized in that an optical element is formed.

An optical element formed by the above-described optical element forming method is also the present invention, and the optical element is a portion where the piezoelectric element layer of ZnO is driven by a voltage applied to the electrode and the Ge layer is removed. The reflected light that is reflected after receiving interference with the incident light may be output.
A light detection element using a pn junction formed on a part of the Si substrate may be configured to detect light that is not reflected by the duplexer.
The optical element may be an optical switch in which the conduction path of output light is switched when the piezoelectric element layer of ZnO is driven by a voltage applied to the electrode and the optical path with respect to incident light is changed. At this time, it is desirable that a part of the conduction path is formed on the substrate.

In the optical element forming method described above, the substrate is a transparent substrate in the visible light region, a semi-transparent metal film is formed on the substrate, and then a germanium (Ge) layer is formed. Thereafter, a translucent metal film similar to the translucent metal film is formed, and the silicon oxide (SiO ₂ ) film is formed on the translucent metal film to form an optical element. This optical element can also be a branching filter that outputs light transmitted through the substrate after the incident light on the piezoelectric element layer receives interference at the portion where the Ge layer is removed.

  A piezoelectric element having a cantilever structure made of ZnO having the above-described structure is configured by taking advantage of the highly transparent characteristics of ZnO, and is a demultiplexer for extracting a specific frequency from frequency-multiplexed light or light conduction. As an optical switch that switches the path, it has a high response speed and can be used for optical communication by high-density wavelength division multiplexing (DWDM), which can expand the transmission capacity by multiplexing multiple optical signals with different wavelengths on one fiber. Is optimal.

In the present invention, as described above, since zinc oxide (ZnO) is completely melted by an etching agent used for device fabrication, germanium (Ge) is used as a sacrificial layer, and this Ge sacrificial layer is used as a hydrogen peroxide solution. A cantilever piezoelectric element is manufactured by etching with an etchant.
Another feature of the piezoelectric element fabrication of the present invention is that an oxide film such as SiO ₂ is inserted between the Ge sacrificial layer and the ZnO layer to protect the ZnO layer during etching.
The piezoelectric element of the cantilever structure of ZnO having such a structure is an optical switch that switches a light demultiplexer or a light conduction path that takes out a specific frequency from frequency-multiplexed light by making use of the highly transparent characteristics of ZnO. Can be used as These demultiplexers and optical switches have a high response speed, and can multiplex optical signals with different wavelengths on a single fiber to increase the transmission capacity. Ideal for communication.
Embodiments of the present invention will be described below in detail with reference to the drawings.

<Thin film deposition process of ZnO>
FIG. 1 is a schematic diagram for explaining a ZnO thin film deposition process having a Ge layer sacrificial layer to be etched.
(1) The silicon (Si) substrate 110 is cleaned before the sputtering process (see FIG. 1A). Here, after washing with hydrofluoric acid, washing with acetone and ethanol in that order, washing with pure water and drying were performed.

(2) A Ge thin film 120 as a sacrificial layer is deposited on the Si substrate 110 by using an RF sputtering apparatus (see FIG. 1B).
Example conditions for depositing Ge;
RF power: 50-200 W, substrate temperature: normal temperature, deposition pressure: 1-50 Pa,
Ar gas flow rate: 5-30 sccm, film thickness: 0.1-0.6 μm

(3) The SiO ₂ thin film 130 is deposited on the Ge thin film 120 by using an RF sputtering apparatus (see FIG. 1C).
Example of deposition conditions for SiO ₂ thin film;
RF power: 50-200 W, substrate temperature: normal temperature, deposition pressure: 1-50 Pa
Ar gas flow rate: 5-30 sccm, film thickness: 0.1-0.5 μm

(4) Using a ZnO target doped with 10% Ga as an impurity, an n-type ZnO thin film 140 is deposited using an RF sputtering apparatus (see FIG. 1D). For the formation of a zinc oxide (ZnO) layer for forming the following piezoelectric element, see, for example, the republished patent publication WO02 / 017368.
Example of deposition conditions for n-type ZnO;
RF power: 50-200 W, substrate temperature: normal temperature, deposition pressure: 1-50 Pa
Ar gas: flow rate 5-30 sccm, film thickness: 0.05-0.1 μm,
Resistivity: 1 × 10 ⁻² Ω · cm or less

(5) Using a ZnO target having no impurities, a ZnO thin film 150 oriented in the c-axis with respect to the substrate is deposited using an RF sputtering apparatus (see FIG. 1E).
Example of ZnO deposition conditions;
RF power: 50-200 W, substrate temperature: normal temperature, deposition pressure: 1-50 Pa
Ar gas: flow rate 5-30 sccm, film thickness: 0.2-1.0 μm
Resistivity: 1x10 ² Ω · cm or more

(6) An n-type ZnO thin film 160 is deposited using an RF sputtering apparatus using a ZnO target doped with 10% Ga as an impurity (see FIG. 1F).
Example of deposition conditions for n-type ZnO;
RF power: 50-200 W, substrate temperature: normal temperature, deposition pressure: 1-50 Pa
Ar gas: flow rate 5-30 sccm, film thickness: 0.05-0.1 μm
Resistivity: 1 × 10 ⁻² Ω · cm or less

  Through the processes (1) to (6) described above, a multilayer film is formed in which a germanium (Ge) film is formed on a substrate and a ZnO thin film is deposited thereon via an oxide film. This can be processed to create a cantilever piezoelectric element. Below, the manufacturing process of the duplexer and optical switch by the cantilever structure piezoelectric element which utilized the characteristic of ZnO with high transparency is demonstrated using drawing FIGS.

<Production of optical demultiplexer>
The multilayer film on which the ZnO thin film produced in the above process is deposited is processed to form a cantilever structure using the Ge layer 120 as a sacrificial layer and hydrogen peroxide as an etchant. An n-type ZnO layer (low resistance layer) 140-ZnO layer (piezoelectric conductive film) 150-n type ZnO layer (low resistance layer) 160 is formed as a piezoelectric element. The formed piezoelectric element portion is deformed by an applied voltage, and based on the same principle as the Fabry-Perot interferometer, wavelength selectivity is provided by interference due to multiple reflection of the portion from which the sacrificial layer has been removed. 210 is produced. This process will be described with reference to the schematic diagram of FIG.

(1) A substrate after completion of the thin film forming process described above is prepared. (See Fig. 2 (a))
(2) A predetermined portion of the Ge thin film 120 which is a sacrificial layer is removed by etching to form a cantilever structure (see FIG. 2B).
In the etching method used here, a part of the Ge thin film was removed by vertically immersing the substrate in a 30% hydrogen peroxide solution heated to 60-70 ° C. and immersing the substrate up to the height of the etched portion.

(3) In order to form a piezoelectric element, after applying the resist 170, the resist 170 is removed by a photolithography process (see FIG. 2C).
(4) The etching time is controlled by dry etching to stop the etching just before the lower n-type ZnO thin film 140 (see FIG. 2D).

(5) The Al electrodes 181 and 182 are formed, and the n-type ZnO layer (low resistance layer) 140-ZnO layer (piezoelectric conductive film) 151-n-type ZnO layer (low resistance layer) 161 constitutes the piezoelectric element. (See FIG. 2 (e)).
As shown in FIG. 2E, when an electric field is applied to the Al electrodes 181 and 182, the piezoelectric element portion of the ZnO thin film is deformed around the cantilever portion. Depending on the amount of deformation, it has wavelength selectivity based on the same principle as the Fabry-Perot interferometer. Therefore, if the reflected light is guided to the optical sensor, wavelength-selected detection is possible, and the duplexer 200 using the reflected light is configured. Is done.

<Conditions for reflection type duplexer>
The conditions for using the reflection type duplexer shown in FIG. 2 (e) will be described in detail below.
The conditions for total reflection at the interface will be described with reference to FIG. FIG. 3A shows an interface between substances having refractive indexes n ₁ and n ₂ . FIG. 3B shows an interface between air (refractive index 1) and a material having a refractive index n. The critical angle φ for total reflection is sin φ = n ₁ / n ₂ for FIG. 3A and sin φ = 1 / n for FIG. 3B. Accordingly, the conditions for total reflection are incident angle θ> critical angle φ, n ₂ sin θ> n ₁ for FIG. 3A, and n sin θ> 1 for FIG. 3B.

The portion where the total reflection condition occurs in the layer configuration shown in FIG. 2E is the interface where the incident light reaches from the high refractive index to the low refractive index as described above. and the interface between the SiO ₂ layer 130, the interface between the SiO ₂ layer 130 and the air layer. The refractive index of the constituent materials is shown in Table 1.

The total reflection condition at the interface between the n-type ZnO and SiO ₂ is φ = 47.5 ° because sin φ = 0.74 if the critical angle φ of total reflection is assumed. Since light should not be reflected at this portion, the incident angle θ needs to be approximately 48 ° or more. Next, the total reflection condition at the interface between the SiO ₂ layer and the air layer is sin φ = 0.71 assuming that the critical angle φ of total reflection is, and in this case, the total reflection condition is φ = 45.23 °. It is necessary to select the incident angle so that the light transmitted through the SiO ₂ layer is not totally reflected at this portion and is transmitted at least about 20 to 60%.
If the Si ₃ N ₄ layer is used instead of the SiO ₂ layer, there is almost no difference in refractive index between n-type ZnO and Si ₃ N ₄ , so that the reflection at this portion is extremely small.
Regarding the reflection of incident light on the Si substrate surface, if light having a wavelength of 1000 nm or less is used as the light source, most of the light transmitted through the SiO ₂ layer is efficiently reflected on the Si substrate surface having a metallic luster. In order to further increase the reflection efficiency or use a light source having a longer wavelength, a high reflectivity of 90% or more can be expected if the Si surface is coated with a metal having a very high reflectivity such as Al.

In order to use the same interference as the Fabry-Perot interferometer, the interference between the reflected light at the interface between the SiO ₂ layer and the air layer and the reflected light from the Si substrate surface may be used.
The interference will be described with reference to FIG. 4A shows the interference caused by the transmitted light, and FIG. 4B shows the interference caused by the reflected light. D shown in FIG. 4 corresponds to the thickness of the Ge sacrificial layer 121 in FIG. Therefore, the reflection demultiplexer of FIG. 2 (e) is brightened by adding the interference between the reflected lights. If the refractive index is 1 for air, it is 2d = (2m + 1) × λ / 2.
It becomes. Since m = 0, 1, 2, 3 ----, considering zero order interference, m = 0. Therefore, the thickness d of the sacrificial layer from which Ge is removed is d = λ / 4. Further, considering first-order interference, since m = 1, d = λ × 3/4. If the wavelength of incident light is 0.5 μm, d = 0.125 μm when m = 0, and 0.375 μm when m = 1. As the wavelength of the incident light is changed, the thickness d of the sacrificial layer is changed according to the interference equation.
From the above results, the thickness d of the sacrificial layer can be obtained by substituting the wavelength of the incident light into the interference equation.
Further, if an optical sensor is built in the surface of the Si substrate, it is possible to control the amount of reflected light quantitatively by checking the amount of light reaching the surface of the Si substrate. This configuration will be described with reference to FIG.

<Production of optical sensor built-in duplexer>
The schematic diagram of FIG. 5 shows a method and a structure of a duplexer that also incorporates an optical sensor.
(1) In FIGS. 1 and 2, a thin film deposited material made of a silicon (Si) substrate is used, but here, a predetermined location inside an n-type Si substrate as shown in FIG. A substrate 111 on which a p-type Si region 112 is formed is used.
(2) FIG. 5B shows the thin film fabrication process completed on the substrate 111 in the same process as in FIG. Here, the same reference numerals as in FIG. 1 denote the same parts.

(3) A predetermined portion of the Ge thin film 120 which is a sacrificial layer is removed by etching to form a cantilever structure (see FIG. 5C).
Here, a part of the Ge thin film 120 is removed by vertically immersing the substrate in a 30% hydrogen peroxide solution heated to 60-70 ° C. to the height of the etched portion.
(3) After applying the resist 171, the unnecessary resist 171 is removed by a photolithography process (see FIG. 5D).
(4) A predetermined structure is created by controlling the etching time by a dry etching method (see FIG. 5E).

(5) As shown in FIG. 5 (f), Al electrodes 181, 182, 183, and 184 are formed, an n-type ZnO layer (low resistance layer) 140-ZnO layer (piezoelectric conductive film) 152-n-type ZnO layer (Low resistance layer) The piezoelectric element part by 162 and the pn light receiving element part by the p-type Si112-n-type Si substrate 111 are comprised. In FIG. 5F, by applying an electric field to the Al electrodes 181 and 182, the piezoelectric element portion is deformed around the cantilever portion. It has wavelength selectivity depending on the amount of deformation. Non-reflected light can be detected by a pn light receiving element formed on the Si substrate 111.

<Transmission type duplexer>
As can be understood from the explanation of interference in FIG. 4, the duplexer due to interference can also be configured using transmitted light. This will be described with reference to FIG. FIG. 6 shows a method of manufacturing a transmission type duplexer. 1 and 2 have the same reference numerals.
(1) As shown in FIG. 6A, in the thin film forming process described in FIG. 1, the substrate is changed to a sapphire substrate 115 having high transparency in the visible light region, and a semitransparent metal film 132 is formed thereon. To do. After the germanium (Ge) layer 120 is formed on the semitransparent metal film 132, a semitransparent metal film 134 is formed, and the substrate on which the silicon oxide (SiO ₂ ) layer 130 is formed on the semitransparent metal film 134 is formed. prepare. The sapphire substrate 115 may be a glass substrate (refractive index of 1.4) having high transparency in the visible light region.
The translucent metal films 132 and 134 can be created by, for example, a sputtering method.
(2) A predetermined portion of the Ge thin film 120, which is a sacrificial layer, is removed by etching to form a cantilever structure (see FIG. 6B).
In the etching method used here, a part of the Ge thin film was removed by vertically immersing the substrate in a 30% hydrogen peroxide solution heated to 60-70 ° C. and immersing it to the height of the etched portion.

(3) In order to form a piezoelectric element, after applying the resist 170, the resist 170 is removed by a photolithography process (see FIG. 6C).
(4) The etching time is controlled by dry etching to stop the etching just before the lower n-type ZnO thin film 140 (see FIG. 6D).
(5) Al electrodes 181 and 182 are formed, and an n-type ZnO layer (low resistance layer) 140-ZnO layer (piezoelectric conductive film) 162-n-type ZnO layer (low resistance layer) 172 constitutes a piezoelectric element. (See FIG. 6 (e)).

In the case of a transmission type duplexer, the light transmitted through the translucent metal films 132 and 134 and the sapphire substrate 115 is reflected by the translucent metal film 132 and reflected by the surface of the translucent metal film 134. It is only necessary that the transmitted light passing through the sapphire substrate 115 interferes.
The interference condition corresponds to the film thickness, and as shown in FIG. 4B of the interference due to the thin film, the interference between the transmitted light is added and the brightness becomes bright when the refractive index is 1 and 2d = mλ. Since m = 0, 1, 2, 3 ---, considering the first order interference, m = 1. Assuming that the thickness of the Si ₃ N ₄ layer is sufficiently smaller than the thickness of the Ge layer, the thickness d of the sacrificial layer where the Ge is removed is d = λ / 2. Considering the secondary interference, since m = 2, d = λ. If the wavelength of the incident light is 0.5 μm, d = 1.25 μm when m = 1, and 0.5 μm when m = 1. As the wavelength to be demultiplexed is changed, the thickness d of the sacrificial layer is changed according to the interference equation by the input voltage.

<Fabrication of optical switch>
A manufacturing process of an optical switch using a cantilever piezoelectric element utilizing the characteristics of highly transparent ZnO will be described with reference to the schematic diagram of FIG.
(1) Referring to FIG. 7A, a part of the substrate 110 after the completion of the thin film deposition process described in FIG. 1 is exposed by the etching process, and the Si substrate surface is exposed.

(2) A part of the Ge thin film 126 as a sacrificial layer is removed by etching to form a cantilever structure. Here, the substrate is vertically set in a 30% hydrogen peroxide solution heated to 60-70 ° C. and immersed to the height of the etched portion to remove a part of the Ge thin film 126 (see FIG. 7B).

(3) In order to manufacture a piezoelectric element with a ZnO layer, after applying a resist 175, a part of the resist 175 is removed by a photolithography process (see FIG. 7C).
(4) The etching time is controlled by dry etching to stop the etching just before the lower n-type ZnO thin film 145 (see FIG. 7D).

(5) Al electrodes 181 and 182 are formed (see FIG. 7E). By applying a voltage to the Al electrodes 181 and 182, the ZnO thin film 156 forming the piezoelectric element is deformed around the cantilever portion. This deformation serves as an optical switch that can switch the optical path.

(6) The deposition process of SiO ₂ 194 and Si ₃ N ₄ 192 is repeated to form a light introduction path 512 corresponding to the number of optical switch circuits, and then formed into a desired shape by etching (see FIG. 7 (f)). . The optical switch shown in FIG. 7F is a configuration example of three circuits. Light is incident parallel to the thin film from the ZnO thin film side, and a voltage is applied to the piezoelectric element to deform it, thereby determining the optical axis to be introduced into the three-circuit optical waveguide.

  As described above, in the present invention, germanium (Ge) is used as a sacrificial layer, and the sacrificial layer of Ge is etched with an etchant of hydrogen peroxide to produce a cantilever piezoelectric element. Using a ZnO cantilever piezoelectric element having such a structure, an optical demultiplexer and an optical switch were constructed by taking advantage of the highly transparent characteristics of ZnO. A duplexer or optical switch with this configuration has a high response speed, and it is based on high-density wavelength division multiplexing (DWDM) that can expand transmission capacity by multiplexing a plurality of optical signals having different wavelengths on one fiber. Ideal for optical communications.

It is a schematic diagram explaining the preparation process of the thin film deposition for a piezoelectric element. It is a schematic diagram explaining the manufacturing process and its structure of a reflection type duplexer. It is a figure for demonstrating total reflection. It is a figure for demonstrating the interference by reflection / transmission of light. It is a schematic diagram explaining the manufacturing process and its structure of a photodetector built-in duplexer. It is a schematic diagram explaining the manufacturing process and structure of a transmission type duplexer. It is a schematic diagram explaining the manufacturing process and structure of an optical switch.

Claims (7)

Forming a germanium (Ge) layer on a silicon (Si) substrate;
Forming a silicon oxide (SiO ₂ ) film on the Ge layer;
After forming a piezoelectric element layer of zinc oxide (ZnO) on the SiO ₂ film,
Etching the Ge layer with hydrogen peroxide solution, leaving a portion of the Ge layer, making a cantilever structure,
An optical element forming method using a cantilever structure zinc oxide piezoelectric element, wherein an electrode is formed on the piezoelectric element layer to form an optical element. An optical element formed by the optical element forming method according to claim 1,
The optical element is driven by the voltage applied to the electrode of the piezoelectric element layer of ZnO, and the reflected light that is reflected after being interfered with incident light is output at the portion where the Ge layer is removed. An optical element which is a duplexer. 3. The optical element according to claim 2, wherein the optical element is a light detection element having a pn junction formed on a part of a Si substrate, and detects light that is not reflected by a duplexer. An optical element formed by the optical element forming method according to claim 1,
The optical element is an optical switch in which the conduction path of output light is switched when the piezoelectric element layer of ZnO is driven by a voltage applied to the electrode and the optical path with respect to incident light is changed. Optical element. 5. The optical element according to claim 4, wherein a part of the conduction path is formed on the substrate. In the optical element formation method according to claim 1,
The substrate is a transparent substrate in the visible light region, a semi-transparent metal film is formed on the substrate, a germanium (Ge) layer is formed, and then a semi-transparent metal film similar to the semi-transparent metal film is formed. And forming the silicon oxide (SiO ₂ ) film on the translucent metal film. An optical element formed by the optical element forming method according to claim 6,
The optical element is a duplexer in which incident light to the piezoelectric element layer is output as transmitted light that passes through the substrate after receiving interference at a portion where the Ge layer is removed. Optical element.

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