WO2016143804A1

WO2016143804A1 - Electrostatic comb actuator and variable shape mirror using the same

Info

Publication number: WO2016143804A1
Application number: PCT/JP2016/057258
Authority: WO
Inventors: Hiroyuki Ozaki; Kenji Tamamori
Original assignee: Canon Kabushiki Kaisha
Priority date: 2015-03-08
Filing date: 2016-03-02
Publication date: 2016-09-15

Abstract

An electrostatic comb actuator (101) includes fixed comb electrodes (108) extending from a support member (105), an elastic member (104) connecting a movable member (103) and the support member to each other, and movable comb electrodes (106) extending from the movable member substantially parallel to the fixed comb electrodes and engaging with the fixed comb electrodes with intervals therebetween. A gap (A) between the movable comb electrode positioned on the inner side and the fixed comb electrode is smaller than a gap (B) between the outermost movable comb electrode and the support member opposed thereto. A thickness (t), and the like of the outermost movable comb electrode is satisfied.

Description

DESCRIPTION

Title of Invention:

ELECTROSTATIC COMB ACTUATOR AND VARIABLE SHAPE MIRROR USING THE SAME

Technical Field

[0001]

The present invention relates to an electrostatic comb actuator, a variable shape mirror using the electrostatic comb actuator, and an apparatus using the variable shape mirror, such as an adaptive optics system.

Background Art

[0002]

A movable mirror and a variable shape mirror of a type to be displaced by an electrostatic attractive force are expected to be applied to various fields utilizing light. For example, the movable mirror and the variable shape mirror each can be utilized as an adaptive optics wavefront correction device to be installed in a fundus testing apparatus, an astronomical telescope, or the like. As a representative example of such a movable mirror whose reflective surface is displaced by an electrostatic attractive force, there is known a measure of enabling movement by using two parallel plate electrodes, but this parallel plate type has a disadvantage in that the moving amount is small. In contrast, . in recent years, a variable shape mirror that uses comb electrodes and can achieve a larger moving amount has been proposed. An example thereof is disclosed in PTL 1. As illustrated in FIG. 6, in this variable shape mirror 500, a support portion 530 that supports a comb electrode 520 on a movable side and a support portion 570 that supports a comb electrode 510 on a fixed side are respectively located on upper and lower sides in a direction perpendicular to . the drawing sheet. The movable comb electrode and the fixed comb electrode are opposed to each other, and are disposed so as to be alternately arrayed with a distance. With this, an electrode overlapping area larger than that in the parallel plate type can be achieved. Therefore, a larger electrostatic attractive force can be generated between the comb electrodes, and thus, a moving amount can be increased.

Citation List

Patent Literature

[0003]

PTL 1: U.S. Patent No. 6384952 B2

Summary of Invention

Technical Problem

[0004]

However, in the structure disclosed in PTL 1, the outermost comb electrode is disposed to be opposed to a comb electrode adjacent thereto on the inner side and to an outer wall 580 with different gaps formed therebetween. Therefore, when a drive voltage is increased so as to increase the moving amount, the electrostatic attractive force acting to the outermost comb electrode is not symmetric laterally (inward direction and outward direction) . Therefore, the electrostatic force acting toward the inwardly-adjacent comb electrode may become excessive as compared to the electrostatic force acting toward the outer wall to cause a phenomenon called pull-in. so that the outermost comb electrode collides with the inwardly-adjacent comb electrode. Therefore, in this structure, it is not easy to obtain a larger moving amount.

Solution to Problem

[0005]

The present invention has been accomplished in view of the above-mentioned problem, and is directed to providing a structure for an actuator including comb electrodes or for a variable shape mirror using the same, in which pull-in is less liable to occur even when a drive voltage is increased so as to increase a moving amount .

[0006]

An electrostatic comb actuator according to an aspect of the present invention has the following configuration. Specifically, the electrostatic comb actuator includes: a support member; a plurality of fixed comb electrodes supported by and extending from the support member; a movable member; an elastic member connecting the movable member and the support member to each other; and a plurality of movable comb electrodes provided to the movable member, extending from the movable member substantially parallel to the plurality of fixed comb electrodes, and engaging with the plurality of fixed comb electrodes with intervals therebetween, a surface of the movable member having the plurality of movable comb electrodes provided thereto and a surface of the support member having the plurality of fixed comb electrodes provided thereto being disposed substantially parallel to a movable direction of the movable member. Further, a gap (A) between corresponding one of the plurality of movable comb electrodes positioned on an inner side and corresponding one of the plurality of fixed comb electrodes is smaller than a gap (B) between an outermost movable comb electrode and the support member opposed thereto, and the following Relational Expression (1) is satisfied:

V²≤ ^{16gl + Et} - (Relational Expression 1)

243(3^ -^)^2 + ft '⁴

where t represents a thickness of the outermost movable comb electrode (length in the X direction), 1 represents a length of the outermost movable comb electrode (length in the Y direction), E represents a Young's modulus of the outermost movable comb electrode, εο represents a dielectric constant, gi represents the. gap (A) , g₂ represents the gap (B) , and V represents a drive voltage of the electrostatic comb actuator.

[0007]

Further, a variable shape mirror according to another aspect of the present invention includes: the electrostatic comb actuator and a mirror member, one surface of the mirror member being a reflective surface, in which the movable member of the electrostatic comb actuator is connected to a surface of the mirror member on a side opposite to the reflective surface.

[0008]

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings .

Advantageous Effects of Invention

[0009]

According to the present invention, there may be provided a technology for the electrostatic comb actuator having the comb electrode structure as described above, which enables suppression of the occurrence of the pull-in even when the electrostatic comb actuator is driven at a high drive voltage.

Brief Description of Drawings

[0010]

FIG. 1 is a top view of Embodiment 1 of the present invention.

FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D, FIG. 2E, FIG. 2F, FIG. 2G, FIG. 2H, FIG. 21 and FIG. 2J are sectional views of a manufacturing method of Embodiment 1.

FIG. 3A and FIG. 3B are sectional views for illustrating an operation of a variable shape mirror of Embodiment 1.

FIG. 4 is a graph for showing a relationship between the thickness of the outermost comb electrode and the drive voltage or the like of Embodiment 1.

FIG. 5 is a schematic view of an adaptive optics system and an ophthalmic apparatus using the same according to the present invention.

FIG. 6 is a sectional view for illustrating the related art.

Description of Embodiments

[0011]

Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings .

[0012]

In the following embodiments, an outermost movable, comb electrode is designed so as to satisfy at least Relational Expression (1). There are various structures that satisfy Relational Expression (1). For example, a length 1 of the outermost comb, electrode may be reduced, a thickness t of the outermost comb electrode may be increased, or a Young's modulus E of the outermost comb electrode may be increased. However, there are also other conditions (conditions that movable comb electrodes positioned on the inner side, V, gi, g₂, and the like are required to satisfy in order to obtain a sufficient drive force or facilitate the manufacture) , and hence the structure to be defined by Relational Expression (1) is limited. For example, in order to facilitate the manufacture, a gap (A) between the movable comb electrode positioned on the inner side and a fixed comb electrode is set smaller than a gap (B) between the outermost movable comb electrode and a support member opposed thereto. Other parts are designed similarly to the related art. In this manner, the outermost movable comb electrode often has a- higher rigidity than the inner comb electrode. When the gap (A) is set. smaller than the gap (B) , there is also obtained an effect of being capable of accurately and reliably manufacturing a periphery of a structure from the comb electrode through the support member to an elastic member.

[0013]

Now, the structure is described more specifically by means of embodiments, but as a matter of course, the present invention is not limited to those embodiments. Various modifications and changes may be made thereto without departing from the gist of the invention.

[0014]

(Embodiment 1) ·

With reference to FIG. 1, FIG. 2A to FIG. 2J, FIG. 3A, and FIG. 3B, a variable shape mirror 100 including an electrostatic comb actuator according to Embodiment 1 of the present invention is described. FIG. 1 is a top view of the variable shape mirror 100 of this embodiment. FIG. 2A to FIG. 2J are sectional views of a manufacturing method. FIG. 3A and FIG. 3B are each an A1-A2 sectional view of an electrode pair of a comb structure of the electrostatic comb actuator of the variable shape mirror 100 of this embodiment in two different states.

[0015]

An actuator 101 is formed by processing a first substrate 102. A movable member 103 is supported by a support member 105 with use of two or more elastic bodies (elastic member) 104. In this case, the elastic bodies 104 in a leaf spring form are provided at equal angular intervals (180° intervals in this case) around the movable member 103 having a sectional shape that is a rotation symmetric shape such as a square. With this, the vertical movement of the movable member 103 in a direction perpendicular to the drawing sheet of FIG. 1 is stably secured.

[0016]

A plurality of movable comb electrodes 106 are each extended from the movable member 103 in a direction parallel to the surface of the first substrate 102. A plurality of fixed comb electrodes 108, which are fixed to parts of the support member 105 insulated through intermediation of insulating portions 107 from parts of the support member 105 coupled with the outer ends of the elastic bodies 104, are extended in a direction parallel to the upper surface of the support member 105. The movable comb electrodes 106 and the fixed comb electrodes 108 are disposed so as to face each other, and are disposed so as to be alternately arrayed with intervals between the respective comb teeth, to thereby construct a comb electrode pair of the actuator. As described above, the actuator includes the plurality of fixed comb electrodes extending from the support member, the elastic member connecting the support member and the movable member to each other, and the plurality of movable comb electrodes extending from the movable member substantially parallel to the fixed comb electrodes and engaging with the fixed comb electrodes with intervals therebetween. Further, a surface of the movable member having the movable comb electrodes provided thereto and a surface of the support member having the fixed comb electrodes, provided thereto are disposed substantially parallel to the movable direction of the movable member. Further, as represented by the electric wiring of FIG. 1, the fixed comb electrodes 108 and the support member 105 opposed to the outermost movable comb electrode are configured to have the same electric potential.

[0017]

Further, the movable member 103 is bonded to a reflective member (mirror member) 109 with a gold bump 126 so that the mirror 100 is. deformed when the movable member 103 is moved.

[0018]

Next, a method of manufacturing a variable shape mirror is described. First, as illustrated in FIG. 2A, the first substrate 102 is prepared (S101) . The first substrate 102 is an SOI substrate including a handle layer (Si) 110, a BOX layer (Si0₂) 111, and an active layer (Si) 112.

[0019]

Next, as illustrated in FIG. 2B, a pattern of an insulating layer 113 is formed on both surfaces of the first substrate 102 (S102). In this step, silicon oxide (Si0₂) is formed as the insulating layer 113 by thermal oxidation, and then a resist pattern (not shown) is formed. The insulating layer 113 is etched with the resist pattern being used as a mask, to thereby form insulating layer patterns 113a and 113b.

[0020]

Next, as illustrated in FIG. 2C, a through electrode 114 is formed (S103) . A resist pattern (not shown) is formed on the back surface of the first substrate 102. The active layer (Si) 112 and the BOX layer (Si0₂) 111 are etched with the resist pattern being used as a mask, to thereby form a through hole. In addition, titanium (Ti) and gold (Au) , which are used as electrode materials, are laminated to form a film, and then a resist pattern (not shown) is formed. Gold (Au) and titanium (Ti) are etched with the resist pattern being used as a mask. In this way, the through electrode 114 is formed. The through electrode 114 is a contact hole (electrode) through which the handle layer and the active layer electrically communicate with each other.

[0021] Next, as illustrated in FIG. 2D, a pad 115 for bump bonding is formed (S104). Titanium (Ti) and gold

(Au) , which are used as pad materials, are laminated on the front surface of the first substrate 102 to form a film, and then a resist pattern (not shown) is formed. Gold (Au) and titanium (Ti) are etched with the resist pattern being used as a mask.

[0022]

Next, as illustrated in FIG. 2E, there is formed a mask to be used when the comb shape is formed (S105) . A resist pattern 116 is formed on the front surface of the first substrate 102, and the insulating layer 113b formed on the front surface of the first substrate 102 is etched.

[0023]

Next, as illustrated in FIG. 2F, a comb electrode region 123 (region including the movable comb electrodes 106 and the fixed comb electrodes 108) and an elastic body opening region 122 (region including an opening above the elastic body 104) are formed from the front surface of the first substrate 102 (S106) . In this step, the handle layer 110 (Si) is etched with the resist pattern 116 formed in Step S105 and the insulating layer 113b being used as masks. In order to form the desired comb shape by etching the handle layer (Si) 110, inductive coupled plasma-reactive ion etching (ICP-RIE) that enables etching in high sectional perpendicularity or the like is used. At this time, the comb electrode region 123 that is small in mask opening area and the elastic body opening region 122 that is large in mask opening area are simultaneously etched. Then, the support member 105 in the fixed region is separated from the outermost movable comb electrode 106 in the movable region and the movable member 103. In order to relatively easily and reliably perform such a manufacturing step, a gap (B) 125 between the outermost movable comb electrode 106 and the support member (outer wall) is set larger than a gap (A) 124 between the comb electrodes at the center

(see FIG . 1) .

[0024]

Next, as illustrated in FIG. 2G, level differences among the comb teeth are formed (S107). In order to form level differences among the fixed comb electrodes 108, the active layer (Si) 112 is etched with the insulating layer (Si0₂) 113a provided on the back surface being used as a mask. In addition, the BOX layer (Si0₂) 111 is etched with the active layer 112 being used as a mask. Further, silicon (Si) of the fixed comb electrode 108 is etched. Further, in order to form level differences on the movable comb electrode 106 side, the resist pattern 116 formed on the front surface and the resist pattern (not shown) formed on the back surface are removed, and then silicone (Si) of the movable comb electrodes 106 is etched with the insulating layer (Si0₂) 113b formed on the front surface being used as a mask.

[0025]

Next, as illustrated in FIG. 2H, the BOX layer (Si0₂) 111 is etched to release the movable comb electrodes 106 and the fixed comb electrodes 108 (S108) The etching of the BOX layer (Si0₂) 111 is performed with 0.5% hydrofluoric acid (HF) so that the BOX. layer (Si0₂) 111 is selectively subjected to wet etching.

[0026]

Next, as illustrated in FIG. 21, the first substrate 102, which is obtained through formation up to Step S108, is bonded to a second substrate 117 (S108). The second substrate 117 is an SOI substrate including a handle layer (Si) 118, a BOX layer (Si0₂) 119, and an active layer (Si) 120. Here, an insulating layer (not shown) of silicon oxide is formed on the front surface of the handle layer 118 in the second substrate 117 by thermal oxidation. Next, a resist pattern (not shown) is formed and a patterning (not shown) of the insulating layer is formed on the front surface of the handle layer (Si) 118 by wet etching. On the front surface of the active layer 120 in the second substrate 117, there is formed a pad portion 121 on which a gold (Au) bump as described below is formed. In order to form the pad portion 121, titanium (Ti) and gold (Au) are laminated to form a film, and then a resist pattern (not shown) is formed. Gold (Au) and titanium (Ti) are etched with the resist pattern being used as a mask. In addition, a gold (Au) bump 126 is formed on the pad portion 121. Next, the pad portion 115 in the first substrate 102 is accurately aligned to the gold (Au) bump 126 in the second substrate 117 for bump bonding. As a method of bonding, there can also be used fusion bonding, such as silicon-silicon (Si-Si) bonding, silicon oxide-silicon oxide (Si02-SiC>₂) bonding, and silicon-silicon oxide (Si-SiC>₂) bonding, or bonding by an adhesive.

[0027]

Next, as illustrated in FIG. 21 and FIG. 2J, the handle layer (Si layer) 118 and the BOX layer (Si0₂) 119 in the second substrate 117 are selectively etched

(S^'110) . Selective etching of the handle layer (Si) 118 can be performed when, for example, tetramethylammonium hydroxide solution (TMAH) or potassium hydroxide (KOH) is used as a chemical solution. The exposed BOX layer (Si0₂) 119 is selectively wet-etched. With this step, the active layer (Si) 120 is exposed so that the active layer

(Si) 120 becomes the reflective member, to thereby form the variable shape mirror 100. In this case, the movable member 103 of the actuator is connected to the surface of the mirror member 109 on the side opposite to the reflective surface.

[0028]

With reference to FIG. 3A and FIG. 3B, the operations of the actuator 101 and the variable shape mirror 100 are described. There is a level difference between the movable comb electrode 106 and the fixed comb electrode 108 in a direction perpendicular to the upper surface of the support member 105. That is, the movable and fixed comb electrodes have parts free from overlapping with each other in the direction perpendicular to the upper surface of the support member 105. This is because this embodiment employs a system (variable overlapping type) utilizing a phenomenon that a force acts in the overlapping direction for displacement when the comb electrodes are attracted to each other by the electrostatic attractive force. When the comb electrodes are entirely overlapped with each other in this phenomenon, no more displacement occurs. Therefore, it is necessary or preferred to reduce a part in which the comb electrodes overlap with each other at the initial position, and increase a part in which the comb electrodes overlap with each other when a voltage is applied.

[0029]

As illustrated in FIG. 1, with the insulating portion 107, the movable comb electrode 106 and the fixed comb electrode 108 are electrically insulated from each other. Therefore, by applying a voltage between the movable comb electrode 106 and the fixed comb electrode 108, the movable member 103 is displaced in the direction perpendicular to the upper surface of the support member 105 while the interval between both the electrodes 106 and 108 is maintained. An electrostatic attractive force Fz acting in the Z direction when a potential difference is given between the movable comb electrode 106 and the fixed comb electrode 108 is expressed by Relational Expression 2: Fz= [ (ε₀·Ν· S) / (2g) ] · (Vm-Vf ) ² (Relational Expression 2) where ο represents a vacuum dielectric constant, N represents the number of gaps between . the comb electrodes, S represents an overlapping area between the movable comb electrode and the fixed comb electrode (electrode area in which the electrostatic force is generated) , Vm represents a potential of the movable comb electrode, Vf represents a potential of the fixed comb electrode, and g represents a gap width between the comb electrodes.

[0030]

First, as illustrated in FIG. 3A as a state immediately after the voltage application, an electrostatic attractive force is generated by giving a potential difference between the movable comb electrode 106 and the fixed comb electrode 108 so that the comb electrodes are attracted to each other. With this, the movable comb electrode 106 and the fixed comb electrode 108 are attracted to each other, and the comb electrodes receive an electrostatic attractive force substantially equally on the right and left sides in the direction in which the comb electrodes face each other. Therefore, displacement occurs in the Z direction perpendicular to the reflective member 109.

[0031]

Subsequently, a balanced state as illustrated in FIG. 3B is obtained. That is, the movable comb electrode 106 is stopped at a position at which the restoring force of the elastic body 104 and the electrostatic attractive force that displaces the movable member 103 are balanced with each other. Then, when the potential difference between the movable comb electrode 106 and the fixed comb electrode 108 is set to 0 V, the movable comb electrode 106 returns to the initial position by the restoring force of the elastic body 104.

[0032]

When such an electrostatic comb actuator is driven, in the related-art electrostatic comb actuator, in order to increase the force to be generated, the movable comb electrode 106 and the fixed comb electrode 108 are formed thin to increase the density of the comb electrode portion. In such a case, the rigidity is reduced in the thickness direction of each of the movable comb electrode 106 and the fixed comb electrode 108 (X direction) . Further, as described in the step (S106) of the manufacturing method, when the comb electrode region 123 that is small in mask opening area and the elastic body opening region 122 that is large in mask opening area are simultaneously dry-etched, the following setting is necessary. That is, in order to suppress the pattern abnormality due to the loading effect and to separate the support member (fixed region) from the outermost movable comb electrode (movable region) , it is necessary to set the gap (B) between the outermost movable comb electrode and the support member (outer wall) larger than the gap (A) between the comb electrodes at the center. In this case, the outermost movable comb electrode 106 differs in the gap between the electrodes and the electrostatic force generated in the lateral direction. Therefore, the outermost movable comb electrode 106 is displaced as the drive voltage V is increased, and finally a phenomenon . called pull-in is liable to occur to cause contact to the inner fixed comb electrode 108.

[0033]

The above-mentioned loading effect is described. When the elastic body opening region 122, the gap (A) , and the gap (B) have the same opening area, the etching rate becomes uniform to. suppress the loading effect. However, as described above, it is necessary to decrease the gap (A) in order to increase the force to be generated. When the gap (A) is equal to the gap (B) , the opening of the elastic body opening region 122 is large, and hence the etching rate here is increased to easily cause pattern abnormality. Therefore, when the gap (A) is set smaller than the gap (B) as in this embodiment, the gradient of the etching rate can be reduced, and hence the pattern abnormality can be suppressed .

[0034]

In view of the circumstances above, in this embodiment, in order to solve the above-mentioned problem that the pull-in phenomenon is liable to occur, the rigidity of the outermost movable comb electrode 106 is set higher than that of the inner comb electrode so as to satisfy Relational Expression (1) obtained from the relationship between the electrostatic force to be generated and the gap between the electrodes that cause the pull-in. In this manner, the pull-in is prevented even with a large drive voltage, and a stable drive can be realized.

[0035]

Next, a method of calculating Relational Expression (1) is described. In general, a spring constant k of a cantilever spring is represented by Relational Expression (3) . Symbol E represents a Young's modulus of the movable comb electrode 106, w represents a width of the movable comb electrode 106, t represents a thickness of the movable comb electrode 106, and 1 represents a length of the movable comb electrode 106.

(Relational Expression 3)

k: spring constant, _. E: Young's modulus, w: width, t: thickness, 1: length [0036]

Electrostatic forces Fi and F₂ acting between the comb electrodes are represented by Relational Expressions (4) and (5). In this case, V represents a potential difference between the movable comb electrode and the fixed comb electrode, and a potential difference between the outermost movable comb electrode and the support member. Symbol x represents a displacement amount of the comb electrode in the X direction (the inward direction refers to a positive direction) , gi represents a gap between the movable comb electrode and the fixed comb electrode, and g₂ represents a gap between the comb electrode and the support member (outer wall) . Further, ε₀ represents a dielectric constant.

1

(Relational Expression 4)

(Relational Expression 5)

The electrostatic force F to be applied to the outermost movable comb electrode 106 in the inward direction (pull-in direction) is represented by Relational Expression (6).

(Relational Expression 6)

[0038]

In order to prevent the occurrence of the pull-in, it is necessary to satisfy Relational Expression (7) between the restoring force kx and the electrostatic force F to be applied to the outermost movable comb electrode 106 and also satisfy Relational Expression ( 8 ) ,

L·>R (Relational Expression 7)

X pul , ≤— (Relational Expression

Here, 1/3 in the right-hand side of Relational Expression (8) is based on the general recognition that the pull-in occurs when the displacement amount x is increased any more.

[0039]

When Relational Expression (7) and Relational Expression (8) are substituted into Relational Expre pression (9) is obtained.

V²≤

ne₀S(3g₂-_gl)(g₂+g,)

(Relational Expression 9)

[0040]

Further, when Relational Expression (3) and S=wl are substituted into Relational Expression (9), Relational Expression (1) can be obtained.

v²≤ (Relational Expression

243(3g₂ -_g{)(g_{2 +gl})_e

[0041]

As described above, Relational Expression (1) is satisfied to set the rigidity of the outermost movable comb electrode 106 (in other words, the Young's modulus E, the thickness t, or the like) to be higher than the rigidity of the inner comb electrode. In this manner, the pull-in can be prevented even with a large drive voltage, and a stable drive can be realized. When the same design as the related art is employed, the force to be generated is increased by increasing the number of comb teeth or the like, and hence the comb electrode tends to have a low-rigidity structure. However, when the outermost movable comb electrode 106 is designed so as to satisfy Relational Expression (1) in order to prevent the pull-in of the outermost movable comb electrode while increasing the force to be generated, the rigidity of the outermost movable comb electrode is often higher than that of the inner comb electrode.

[0042]

FIG. 4 is a graph for showing the relationship between the comb electrode thickness t, the drive voltage V, and the gap (A) 124 with respect to the inner fixed comb electrode 108 when the structural parameters (gi, g2, and the like) of the movable comb electrode 106 are input to Relational Expression (1). The input parameters at this time are as follows. The gap (B) 125 between the support member and the movable comb electrode, which is represented by g₂, is 12 μιη, the length 1 of the movable comb electrode is 500 μπι, the width w of the movable comb electrode is 200 μπι, the Young's modulus E of the movable comb electrode is 130 GPa, and the vacuum dielectric constant ε₀ is 8.85xl0^"12 F/m. [0043]

Further, the gap (A) 124, which is represented by gi, the thickness t of the outermost comb electrode, and the drive voltage V are determined based on the necessary force to be generated. When the drive at the maximum drive voltage 200 V is desired (typically, the drive voltage is 0 V or more and 200 V or less), the region satisfying Relational Expression (1) is a region A shown in FIG. 4. Therefore, when gi is 8 μιτι (represented by the solid curve) and t is 8 μπ\, the outermost movable comb, electrode does not satisfy Relational Expression (1), and hence the pull-in occurs Therefore, . the thickness t of the outermost movable comb electrode 106 is set to 11.4 μιη or more so that the rigidity of the outermost movable comb electrode 106 is set higher than that of the inner comb electrode In other words, at this time, the inner comb electrode has the related-art design (for example, the thickness of the inner comb electrode is 8 μπι) , and hence the rigidity of the outermost movable comb electrode is higher than that of the inner comb electrode. With this, the pull-in is prevented even with a large drive voltage, and a stable drive can be realized.

[0044]

Note that, in FIG. 1, there is illustrated a structure in which one actuator is connected to the reflective member 109 having a continuous, reflective surface, but this structure is merely an example. Increasing the number of actuators to be connected to the reflective member can realize a more complicated mirror surface shape at high accuracy. Further, there can also be employed a type in which one reflective member 109 is connected to each of a plurality of actuators 101 through intermediation of a connecting portion. With this, an optical path length of light to be reflected at each reflective member 109 can be changed, and hence the use as a wavefront correction device is possible.

[0045]

According to this embodiment, in the electrostatic comb actuator having the comb electrode structure as described above, the structure can be manufactured relatively easily, and the occurrence of the pull-in can be suppressed even with drive at a high drive voltage.

[0046]

(Embodiment 2: Ophthalmic Apparatus)

An adaptive optics system that uses the variable shape mirror described above as a wavefront correction device that compensates for an optical aberration is described with a scanning laser ophthalmoscope

(hereinafter described as "SLO apparatus") as an example. The SLO apparatus is an ophthalmic apparatus configured to irradiate a fundus with light so as to enable observation of a photoreceptor, a retinal nerve fiber layer, hemodynamics, or the like.

[0047]

FIG. 5 is an illustration of a schematic configuration of the SLO apparatus of this embodiment. Light emitted from a light source 301 travels through a single-mode optical fiber 302 and passes through a collimator 303 to become a collimated light, beam. The collimated light beam is transmitted through a beam splitter 304, which serves as a light splitting unit, as measurement light 305 to be guided to an adaptive optics system 320. The wavelength of the light source 301 for emitting, for example, laser light is not particularly limited, but particularly for fundus imaging, the wavelength of about 800 nm to about 1,500 nm (for example, wavelength of 850 nm or less) is suitably used for preventing dazzling of a subject and for maintaining the resolution. The adaptive optics system 320 includes a beam splitter 306 serving as a light splitting unit, a wavefront sensor (aberration measuring unit) 315, a variable shape mirror that forms a reflective optical modulator (wavefront correction device) 308, and reflective mirrors 307-1 to 307-4 for guiding the light to those members. The respective reflective mirrors 307 are placed so that at least the pupil of the eye to be inspected, the wavefront sensor 315, and the variable shape mirror 308 have an optically conjugate relationship.

[0048]

The light that has passed through the adaptive optics system 320 is scanned by a light scanning portion 309 one-dimensionally or two-dimensionally . The measurement light scanned by the light scanning portion 309 is radiated to an eye 311 to be inspected through eyepiece lenses 310-1 and 310-2. By adjusting the positions of the eyepiece lenses 310-1 and 310-2, optimum irradiation can be performed in accordance with the visibility of the eye 311 to be inspected. In this case, a lens is used in the eyepiece part, but a spherical mirror or the like may be used instead.

[0049]

The measurement light radiated to the eye .311 to be inspected is reflected or scattered by a fundus (retina) . The light reflected or scattered at the fundus of the eye 311 to be inspected travels, in an opposite direction, a passage similar to that during entrance, and is partially reflected by the beam splitter 306 to enter, the wavefront sensor 315. Thus, this partially reflected light is used to measure the wavefront of the light beam. As the wavefront sensor 315, a known Shack-Hartmann sensor can be used. The reflected or scattered light that has transmitted through the beam splitter 306 is partially reflected by the beam splitter 304 to be guided to a light intensity sensor 314 through a collimator 312 and an optical fiber 313. Light that has entered the light intensity sensor 314 is converted into an electrical signal to be processed into a fundus image by an · image processing unit 325.

[0050]

The wavefront sensor 315 is connected to an adaptive optics controller 316 serving as a control unit to transmit the wavefront of the received light beam to the adaptive optics controller 316. The adaptive optics controller 316 is connected to the variable shape mirror 308, and the variable shape mirror 308 is deformed into a shape instructed by the adaptive optics controller 316. The adaptive optics controller 316 calculates, based on the measurement result of the wavefront obtained from the wavefront sensor 315, a mirror shape that enables correction into a wavefront with no aberration. Then, in order to reproduce the shape in the variable shape mirror 308, a necessary application voltage difference for each of the comb electrodes is calculated and sent to the variable shape mirror 308. In the variable shape mirror 308, a potential difference sent from the adaptive optics controller 316 is applied between the movable comb electrode and the fixed comb electrode, to thereby deform the mirror surface into a predetermined shape.

[0051]

The measurement of the wavefront by the wavefront sensor 315, transmission of the wavefront to the adaptive optics controller 316, and instruction by the adaptive optics controller 316 to the variable shape mirror 308 for correction of the aberration as described above are repeatedly processed to be feedback controlled to constantly obtain an optimum wavefront. Note that, it is only necessary that the variable shape mirror that forms the reflective optical modulator is provided so as to correct a wavefront aberration of at least one of measurement light or return light.

[0052]

In the adaptive optics system according to this embodiment, the electrostatic comb actuator can be displaced in the two positive and negative directions in a direction perpendicular to the mirror surface. Therefore, the adaptive optics processing can be performed with substantially half of a drive amount of the variable shape mirror as compared to the related art .

[0053]

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

[0054]

This application claims the benefit of Japanese Patent Application No. 2015-045377, filed March 8, 2015, and Japanese Patent Application No. 2016-016751, filed January 30, 2016, which are hereby incorporated by reference herein in their entirety.

Reference Signs List

[0055]

100 variable shape mirror

101 electrostatic comb actuator

103 movable member

104 elastic body

105 support member

106 movable comb electrode

108 fixed comb electrode

Claims

[Claim 1]

An electrostatic comb actuator, comprising:

a support member;

a plurality of fixed comb electrodes supported by and extending from the support member;

a movable member;

an elastic member connecting the movable member and the support member to each other; and

a plurality of movable comb electrodes provided to the movable member, extending from the movable member substantially parallel to the plurality of fixed comb electrodes, and engaging with the plurality of fixed comb electrodes with intervals therebetween,

a surface of the movable member having the plurality of movable comb electrodes provided thereto and a surface of the support member having the plurality of fixed . comb electrodes provided thereto being disposed substantially parallel to a movable direction of the movable member,

wherein a gap (A) between corresponding one of the plurality of movable comb electrodes positioned on an inner side and corresponding one of the plurality of fixed comb electrodes is smaller than a gap (B) between an outermost movable comb electrode and the support member opposed thereto, and

wherein the following Relational Expression (1) is satisfied:

V² < ^16gl +gⁱ) ^Et (Relational Expression 1)

243(3*₂ -g,)(g₂

where t represents a thickness of the outermost movable comb electrode, 1 represents a length of the outermost movable comb electrode, E represents a Young's modulus of the outermost movable comb electrode, ε₀ represents a dielectric constant, gi represents the gap (A) , g₂ represents the gap (B) , and V represents a drive voltage of the electrostatic comb actuator.

[Claim 2]

The electrostatic comb actuator according to claim 1, wherein the outermost movable comb electrode has a higher rigidity than the plurality of fixed comb electrodes and movable comb electrodes positioned on the inner side.

[Claim 3]

The electrostatic comb actuator according to claim 1 or 2, wherein the plurality of fixed comb electrodes and the support member opposed to the outermost movable comb electrode are configured to have the same electrical potential.

[Claim 4]

The electrostatic comb actuator according to any one of claims 1 to 3, wherein the drive voltage is 0 V or more and 200 V or less.

[Claim 5]

A variable shape mirror, comprising:

the electrostatic comb actuator according to any one of claims 1 to 4; and

a mirror member, one surface of the mirror member being a reflective surface,

wherein the movable member of .the electrostatic comb actuator is connected to a surface of the mirror member on a side opposite to the reflective surface.

[Claim 6]

The variable shape mirror according to claim 5, wherein a plurality of the electrostatic comb actuators are connected to a plurality of the mirror members, respectively.

[Claim 7]

An ophthalmic apparatus- configured to obtain an image of an eye to be inspected, comprising:

a reflective optical modulator configured to correct a wavefront aberration of at least one of measurement light or return light;

an aberration measurement unit configured to measure an aberration caused at the eye to be inspected; and

a control unit configured to control the reflective optical modulator based on a result of the measurement by the aberration measurement unit,

the reflective optical modulator comprising the variable shape mirror according to claim 5 or 6.

[Claim 8]

An adaptive optics system configured to correct a wavefront aberration, comprising:

a reflective optical modulator configured to correct a wavefront aberration of incident light;

an aberration measurement unit configured to measure the wavefront aberration of the incident light; and

. the reflective optical modulator comprising the variable shape mirror according to claim 5 or 6.