1. Field of the Invention
The present invention relates to a deformable mirror apparatus.
2. Related Art
In general, the eyeground observation apparatus is an apparatus which irradiates a retina of an eye to be examined with illumination light and receives and detects light of an image of the retina to be examined through a retina image forming optical system by using an image pickup device (for example, a CCD camera) to observe the retina of the eye. For detecting and preventing a disease concerning eyes, it is desirable that the detection accuracy and resolution are high as far as possible. Since an eyeball is not an ideal lens having no aberration, however, the eyeball has wavefront aberration which becomes a factor of lowering the detection accuracy and resolution.
Therefore, a deformable mirror which can vary its surface shape on the basis of information supplied from a controller is provided between the image pickup device which detects the image of the retina and the retina of the eye to be examined. The image of the retina of the eye to be examined reflected by the deformable mirror is led to a wavefront sensor (for example, the Shack Hartmann sensor) to detect the wavefront aberration. On the basis of the detected wavefront aberration, a control device orders the deformable mirror to deform so as to reduce or eliminate the wavefront aberration. As a result of the order, an image having no wavefront aberration is obtained from the image pickup device.
A deformable mirror having a shape changed by electrostatic sucking force is known (see, for example, FIG. 2 in JP-A 2-101402 (KOKAI)). The deformable mirror shown in FIG. 2 in JP-A 2-101402 (KOKAI) has a configuration obtained by forming a fixed electrode film 12 on an insulative substrate 11, forming a spacer 18 having an opening in the center on the fixed electrode film 12, laminating a reflection film 17, a movable electrode film 16 and a SiO2 insulation film 14 on the spacer 18 so as to cover the opening, and forming a silicon substrate 13 having an opening in the center on the laminated film. Therefore, the laminated film formed of the reflection film 17, the movable electrode film 16 and the SiO2 insulation film 14 is constituted as a membrane part having a peripheral part fixed by the spacer 18 and the silicon substrate 13 and a central part deformed by electrostatic force between the fixed electrode 12 and the movable electrode film 16.
In the membrane part, “generated force (load)—deflection characteristics” obtained when a predetermined voltage is applied between the fixed electrode and the movable electrode vary and have anisotropy (non-uniformity) in the plane, under the influence of thermal stress of a film forming the membrane part and a casing part which supports the membrane part (generated by differences in thermal expansion coefficient) and intrinsic stress existing within the thin film of the membrane part (generated from the film structure).
The degree of influence of the boundary conditions (conditions for fixing the membrane part) exerted upon the “generated force (load)—deflection characteristics” is also very large. As a result of composite factors: thermal stress, genuine stress and boundary conditions (conditions for fixing the membrane part), the magnitude of the internal stress on the membrane part (residual stress) changes and the “generated force (load)—deflection characteristics” are influenced.
In the conventional deformable mirror apparatus having small internal stress (residual stress), the deflection shape of the membrane part caused by “generated force (load)—deflection characteristics” becomes gentle and it is difficult to deform the membrane part to a complicated shape (high-order shape) in some cases.