JP2002267958A - Optical modulating device and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the stability and signal responsiveness of a beam and to lower the driving voltage in an optical modulating device which performs optical modulation by bending the beam with an electrostatic force. SOLUTION: One end 106 of the beam 101 is fixed to a substrate 102 and an other end opposite to the end 106 can be displaced along the length of the beam and held on the substrate 102 with a beam presser 107 so that it can not be displaced at right angle to the substrate surface. The gap between the beam 101 and an electrode 103 for driving the beam is preferably not parallel to the beam 101.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator, and more particularly, to optical switching (optical modulation) by displacing a beam by electrostatic force to change the direction of reflection of an incident light beam by a light reflecting surface of the beam. The present invention relates to a light modulation device that performs the operation. The light modulation device of the present invention can be widely applied to image devices such as an optical writing device in an electrophotographic process and video devices such as a projector.

[0002]

2. Description of the Related Art A cantilever is bent using electrostatic force,
2. Description of the Related Art There is known an optical modulation device that changes the reflection direction of incident light and performs optical modulation. An optical modulator that modulates light by bending a beam fixed at both ends with electrostatic force is also known (Japanese Patent Laid-Open No. 2000-2842).
reference).

[0003]

An optical modulator using a cantilever has the following problems. First, when a beam is formed of a thin film, residual stress is generated, but in the case of a cantilever,
The beam is deformed by the residual stress. In addition, since the residual stress is alleviated with the passage of time, the deformation state of the cantilever changes with time. For these reasons, cantilevers have poor stability.
The cantilever freely vibrates when the electrostatic force is released and the state of bending is restored by the spring property, but due to this free vibration, the signal response is poor, and the natural frequency of the cantilever is low. Response speed cannot be increased because of low speed.

Further, in an optical modulator using a beam fixed at both ends, the stability of the beam is improved and the signal responsiveness is also improved, as compared with a cantilever beam. However, a beam having both ends fixed requires a large amount of energy for deformation because stress is generated by deformation, and a high drive voltage is required to increase the amount of deformation of the beam in order to increase the direction of light reflection. There is a problem.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide an optical modulation device which has good beam stability and responsiveness and can be driven at a low voltage, and a low-cost manufacturing method thereof.
Other purposes will be described in connection with embodiments described later.

[0006]

According to a first aspect of the present invention, there is provided an optical modulator comprising a substrate, an electrode formed on the substrate, and a gap formed between the electrode and the electrode. A beam having a light reflecting surface, and a beam retainer, wherein a first end of the beam is fixed to the substrate and a second end of the beam facing the first end of the beam. The part is held on the substrate by the beam retainer, and the beam retainer is a second part of the beam in a direction perpendicular to the substrate.
Constraining the displacement of the end of the beam, allowing the displacement of the second end of the beam in the direction connecting the first and second ends of the beam,
Light modulation is performed by applying a voltage between the electrode and the beam, deforming the beam by electrostatic force, and changing the direction of reflection of the incident light beam by the light reflecting surface.

Another feature of the light modulation device of the present invention is that
According to another aspect of the present invention, the gap is a gap that is not parallel to the beam.

Another feature of the light modulator of the present invention is that the beam is made of a single-crystal silicon film, a polycrystalline silicon film, or a silicon nitride film.

Still another feature of the optical modulator of the present invention is that
According to a sixth aspect of the present invention, the substrate is made of optically transparent optical glass or single crystal silicon.

The method of manufacturing a light modulator according to the present invention is characterized in that a concave portion is formed in a substrate, a sacrificial layer is formed in the substrate, and the substrate is planarized.
8. An optical modulator according to claim 1, wherein a beam fixed at both ends is formed on the substrate, and the sacrificial layer is removed.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1A and 1B are a schematic sectional view and a schematic plan view, respectively, for explaining the configuration of an embodiment of the light modulator of the present invention. FIG. 1A shows a cross-sectional structure in which the light modulation device is vertically cut along a central horizontal line in FIG. 1B.

In FIG. 1, reference numeral 101 denotes a beam having a light reflecting surface. One end (hereinafter, referred to as a fixed end) 106 of the beam 101 is fixed to the substrate 102, and the other end (hereinafter, referred to as a non-fixed end) opposed thereto is a beam holder 10.
7 and is held on the substrate 102. This beam holder 10
In the direction perpendicular to the upper surface of the substrate 102, the displacement of the non-fixed end of the beam 101 is restricted, but the direction connecting the fixed end and the non-fixed end of the beam 101, that is, the length direction of the beam 101 (in the drawing) , Left and right directions), the displacement of the non-fixed end of the beam 101 is allowed. That is, the beam retainer 107 is
Is a means for holding the non-fixed end to the substrate 102 with one degree of freedom.

The recess formed in the electrode 102 has a gap 1
An electrode 103 opposing the beam 101 is formed through the electrode 05. This electrode 103 is an electrode for driving the beam 101. 104 is a protective film for protecting the electrode 103. In this embodiment, the electrode 103 is formed so as to protrude to the upper surface of the substrate 102, and the pad opening 108 for applying a driving voltage to the electrode 103 from the outside is formed in the protective film 1.
04 is formed. As the protective film 104, an oxide film formed by a vacuum film forming method is generally used. Protective film 104
Has the function of preventing the electrode 103 and the beam 101 from contacting each other and causing a short circuit.

The beam 101 is formed of a thin film of single crystal silicon, polycrystal silicon, or silicon nitride. Since the beam 101 formed of single crystal silicon has few defects and a long life, it is advantageous in improving the reliability of the light modulation device. The beam 101 made of polycrystalline silicon can be formed by a method such as CVD, which is advantageous in reducing the cost of the light modulation device. Beam 10 formed of a thin film of silicon nitride
No. 1 is advantageous in increasing the speed of the optical modulator because the switching response speed can be increased because the Young's modulus of the silicon nitride thin film is large.

On the upper surface of the beam 101, a light reflecting surface is formed to reflect an incident light beam. This light reflecting surface is generally formed of a metal thin film, but may be formed of a multilayer film of a dielectric material.

Further, the beam 101 has a role as another electrode for generating an electrostatic force for bending the beam. This electrode may be formed independently, but when the light reflecting surface of the beam 101 is formed of a metal thin film, the metal thin film can be used as an electrode. When the beam 101 is formed of single crystal or polycrystalline silicon, the resistance of the single crystal silicon or polycrystal silicon is reduced by impurities,
It is also possible to function as an electrode. Although not shown, an electrode for externally applying a drive voltage to the beam 101 is also provided.

As the electrode 103 for driving the beam 101, a thin film of a metal or a metal compound such as Al, Cr, Ti, and TiN is generally used. When the substrate 102 is formed of optical glass, a transparent conductive film (I
When TO) is used, the state of the beam 101 can be observed from the back side of the substrate 102, which is advantageous at the time of inspection. When the substrate 102 is single-crystal silicon, the electrode 1 is formed by an impurity diffusion method.
03 can be formed, and a wiring matrix can be formed by combining diffusion methods, which is advantageous for forming a complicated and large number of wirings. Further, part or all of a driver circuit can be formed in the substrate 102. This is advantageous for reducing external wiring and avoiding signal delay.

The light modulation operation of the light modulation device will be described with reference to FIG. When no driving voltage is applied between the beam 101 and the electrode 103 and no electrostatic force acts on the beam 101, FIG.
As shown in (a), the beam 101 is in a plane state parallel to the substrate 102. Therefore, the incident light beam is specularly reflected by the light reflecting surface of the beam 101, and the reflected light beam travels in the direction indicated by the arrow in the figure. When a driving voltage is applied between the beam 101 and the electrode 103, an electrostatic force acts on the beam 101, and as shown in FIG. The direction of the reflected light is disturbed. When viewed from the reflection direction in the state of FIG. 2A, it looks bright due to specular reflection in the state of FIG. 2A (on state), but looks dark in the state of FIG. 2B because the reflection direction is disturbed. (Off state). The light modulation is performed in this manner.

Since the beam 101 in this light modulation device has a configuration in which only one end is fixed and the other end is held with one degree of freedom, the beam 101 is smaller than a light modulation device using a cantilever beam or a beam fixed at both ends. There are advantages such as:

First, the cantilever has poor signal responsiveness because free vibration is generated when the beam is restored from the bent state by the spring property. When the beam is formed of a thin film, a residual stress is generated in the beam, but the cantilever is deformed by the residual stress. In addition, since the residual stress is gradually reduced with the passage of time, the deformation state of the cantilever changes with time. Thus, the cantilever has poor stability.

On the other hand, in the light modulation device of the present invention, the non-fixed end of the beam 101 is allowed to be displaced in the length direction of the beam 101, but the vertical movement is restricted by the beam retainer 107. When the electrostatic force is released, free vibration hardly occurs. Further, even if there is residual stress in the beam 101, the vertical position of the non-fixed end is restrained by the beam retainer 107, so that the beam 101 is less likely to be deformed by residual stress than a cantilever beam, and its change with time. Also less. From the above, beam 1
01 has better signal response and stability than cantilever,
According to the present invention, compared to a light modulator using a cantilever,
An optical modulator having excellent responsiveness and stability can be realized.

Further, the light modulation device of the present invention can lower the driving voltage as compared with a device using fixed beams at both ends. That is, in the case of a beam fixed at both ends, since both ends are fixed, it is necessary to extend the beam to bend. Since a large electrostatic force is required to extend and bend the beam in this way, it is difficult to reduce the driving voltage. In particular, when the bending amount of the beam is large, a high driving voltage is required.

On the other hand, in the light modulation device of the present invention, the non-fixed end of the beam 101 is held so as to be movable in the length direction, and when the beam 101 is bent, the non-fixed end moves together with the bending. Therefore, the extension of the beam 101 is not required.
Therefore, the beam 101 can be bent with a small electrostatic force, and the driving voltage can be reduced.

FIGS. 3A and 3B are schematic cross-sectional views for explaining the configuration of another embodiment of the light modulation device according to the present invention. FIG. This shows a state where the voltage is applied.

In this embodiment, as shown in FIG. 3A, the gap 105 formed between the beam 101 and the electrode 103 is substantially the same as the gap non-parallel to the beam 101. Thus, the shape of the concave portion of the substrate 102 is changed. The electrode 103 is formed over the entire length of the gap 105.

According to such a configuration, the driving voltage required for deforming the beam 101 can be further reduced.
That is, since the electrostatic force acting on the beam 101 is inversely proportional to the square of the distance between the beam 101 and the electrode 103, when a driving voltage is applied, the beam 101 starts to deform from the narrow portion of the gap 105 where the strong electrostatic force acts. . Since the distance is small, the deformation can be started with a low starting voltage. Then, as the distance between the beam 101 and the electrode 103 is gradually reduced in other portions of the gap in accordance with the deformation of the beam 101, the deformation of the beam 101 is advanced with a low driving voltage, and finally, as shown in FIG. Beam 101 can be flexed as described above.

Next, an embodiment of the manufacturing method of the present invention for manufacturing the light modulator of the present invention as described above will be described. 4 to 11 are views for explaining typical steps, in which (a) is a schematic sectional view,
(B) is a schematic plan view.

<< FIG. 4 >> Reference numeral 402 denotes a silicon substrate on which an oxide film is formed, and corresponds to the substrate 102 of FIG. 1 or FIG. A concave portion 405 corresponding to the gap 105 in FIG. 1 or FIG. 3 is formed in the silicon substrate 402 by photolithography and dry etching. By using a photomask formed with an area gradation pattern or a thermal deformation method of a resist material, a recess 405 for the non-parallel void 105 as shown in FIG. 3 can be formed. The size of the recess 405 is, for example, 20 μm in width.
m and a depth of 2.4 μm.

<< FIG. 5 >> In the concave portion 405, a TiN thin film 403 is formed to a thickness of, for example, 0.01 μm by a sputtering method using Ti as a target. This TiN thin film 403 is
Patterning is performed to a width of, for example, 20 μm by photolithography and dry etching. The patterned TiN thin film 403 corresponds to the electrode 103 in FIG. 1 or FIG. Note that a part of the TiN thin film 403 serving as an electrode is patterned so as to protrude from the concave portion 405 to the substrate surface for connection to the outside.

<< FIG. 6 >> An oxide film 404 is formed by a plasma CVD method so as to cover the TiN thin film 403 and fill the concave portion 405. Next, the oxide film 404 is planarized by polishing or dry etching. The oxide film 404 after the planarization becomes the protective film 104 in FIG. 1 or FIG. 3 and also becomes a sacrificial layer for forming the void 105.

<< FIG. 7 >> On the planarized oxide film 404,
A silicon nitride film 401 serving as a beam material is formed to a thickness of, for example, 0.04 μm by a thermal CVD method. Next, the silicon nitride film 401 is patterned into a beam shape by photolithography and dry etching. Its shape and dimensions are, for example, 20 μm in width and 27 μm in length.
It is. This patterned silicon nitride film 401
Corresponds to the beam 101 in FIG. 1 or FIG. 3, but at this stage, it is in a state of a beam fixed at both ends.

<< FIG. 8 >> An oxide film 407 is formed by plasma CVD so as to cover the patterned silicon nitride film 401.
The film is formed to a thickness of, for example, 0.3 μm by the above method.

<< FIG. 9 >> The beam holder 107 shown in FIG. 1 or FIG.
After fixing the beam holding portion through the oxide film 404 and the oxide film 407 at a position corresponding to the above, the silicon nitride film 408 is formed to a thickness of, for example, 0.1 μm by a thermal CVD method.
Is formed over the entire surface. Then, the silicon nitride film 408 is patterned into the shape of the beam retainer 107 by photolithography and dry etching.

<< FIG. 10 >> The oxide film 404 (sacrifice layer) below the oxide film 407 and the silicon nitride film 401 is removed by etching. At the end of this step, a gap 410 corresponding to the gap 105 of FIG. 1 or FIG. 3 is formed, a beam holder 107 is formed by the silicon nitride film 408, one end is fixed by the silicon nitride film 401, and the other end is fixed. The beam 101 is formed so as to be movable only in the length direction.

<< FIG. 11 >> On the silicon nitride film 401 constituting the beam 104, an Al thin film as a light reflecting surface is formed to a thickness of, for example, 1 μm by sputtering. Also,
Oxide film 404 corresponding to protective film 104 in FIG. 1 or FIG.
Next, a pad opening 406 for external connection of the TiN film 403, that is, the electrode 103 of FIG. 1 or FIG. 3 is formed. Thus, the light modulator of the present invention as shown in FIG. 1 or FIG. 3 is completed.

The above-described light modulation device of the present invention is used as one element, and a plurality of elements are arranged one-dimensionally or two-dimensionally on a substrate, so that light modulation capable of linear or planar light modulation is possible. An apparatus is also included in the present invention. It is clear that such a light modulation device can be easily realized based on the above description, and a description of a specific example will be omitted.

[0038]

As described in detail above, claim 1 is as follows.
According to the inventions described in (1) to (7), it is possible to prevent deformation of the beam due to free vibration or residual stress of the beam such as a cantilever and its change with time, so that light modulation operation excellent in responsiveness and stability becomes possible. Further, since the beam does not need to be stretched when the beam is bent, the driving voltage can be reduced. According to the second aspect of the present invention, the drive voltage can be further reduced by making the gap non-parallel. According to the third aspect of the present invention, the defect of the beam can be reduced and its life can be prolonged, so that the reliability of the light modulation device can be improved. According to the fourth aspect of the present invention, a method such as CVD can be used for forming the beam, which is advantageous in reducing the cost of the light modulation device. According to the fifth aspect of the present invention, since the response speed of the beam can be increased, a high-speed light modulation device can be realized. Claim 6
According to the described invention, the state of the beam can be observed from the back side of the substrate, so that the inspection of the light modulation device becomes easy. According to the seventh aspect of the present invention, in addition to the electrodes, a part of or all of the driving wiring and the driving circuit can be easily formed on the substrate. This is advantageous for avoiding delay. According to the invention described in claim 8,
The light modulation device of the present invention can be manufactured in a small number of steps, and so on.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view and a schematic plan view for explaining the configuration of an embodiment of a light modulation device according to the present invention.

FIG. 2 is a schematic sectional view for explaining an operation of the light modulation device shown in FIG.

FIG. 3 is a schematic sectional view illustrating the configuration and operation of another embodiment of the light modulation device according to the present invention.

FIG. 4 is a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device of the present invention.

5A and 5B are a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device according to the present invention.

6A and 6B are a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device of the present invention.

FIGS. 7A and 7B are a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device of the present invention.

8A and 8B are a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device of the present invention.

9A and 9B are a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device of the present invention.

10A and 10B are a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device according to the present invention.

11A and 11B are a schematic cross-sectional view and a schematic plan view for explaining a manufacturing process of the light modulation device of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 101 Beam 102 Substrate 103 Electrode 104 Protective film 105 Void 106 Fixed end 107 Beam holding

Claims (8)

[Claims] 1. A substrate, an electrode formed on the substrate,
A beam having a light reflecting surface, facing the electrode via a gap, and a beam retainer; a first end of the beam is fixed to the substrate; and a first end of the beam is fixed to the substrate. An opposing second end is held on the substrate by the beam retainer, the beam retainer restrains displacement of a second end of the beam in a direction perpendicular to the substrate, In the direction connecting the first and second ends, displacement of the second end of the beam is allowed, a voltage is applied between the electrode and the beam, the beam is deformed by electrostatic force, and the light reflection is performed. A light modulation device for performing light modulation by changing the direction of reflection of an incident light beam by a surface. 2. The light modulation device according to claim 1, wherein the gap is a gap that is not parallel to the beam. 3. The light modulation device according to claim 1, wherein the beam is made of a single crystal silicon film. 4. The light modulation device according to claim 1, wherein the beam is made of a polycrystalline silicon film. 5. The light modulation device according to claim 1, wherein the beam is made of a silicon nitride film. 6. The light modulation device according to claim 1, wherein the substrate is made of a light-transmitting optical glass. 7. The light modulation device according to claim 1, wherein the substrate is made of single crystal silicon. 8. The method for manufacturing a light modulation device according to claim 1, wherein a concave portion is formed in the substrate, a sacrificial layer is formed on the substrate, and the substrate is planarized. Forming a beam fixed at both ends on the substrate, and removing the sacrificial layer.