1. Field of the Invention
The present invention relates to an optical tunable filter and a method of manufacturing such an optical tunable filter.
2. Description of the Related Art
There has been known an optical tunable filter for detecting an intensity distribution at each wavelength. The following optical tunable filters have been known in the art.
1) Filter Formed by Surface Micro-Machining
In a conventional optical tunable filter, the thickness of a variable gap is controlled only by the thickness of a sacrifice layer. When the thickness of a variable gap is controlled by the thickness of a sacrifice layer, the variable gap may have variations in thickness depending upon conditions for deposition of the sacrifice layer. Accordingly, uniform Coulomb forces are not produced between a thin film and a drive electrode. As a result, stable driving cannot be achieved.
For example, an optical tunable filter disclosed by JP-A No. 2002-174721 has a movable portion projecting on a surface of a substrate. This type of optical tunable filter has a large thickness.
2) Filter Using SOI Wafer
For example, U.S. Pat. No. 6,341,039 discloses that an SiO2 layer of a silicon-on-insulator (SOI) wafer is employed as a sacrifice layer to form a variable gap. According to this method, a variable gap can be formed with accuracy. However, an insulating structure is not provided between a drive electrode and a movable portion. Accordingly, the movable portion and the drive electrode are stuck on each other when a large electrostatic attraction is caused between the movable portion and the drive electrode.
3) Problems Common to Both Filters
In the above filters, a sacrifice layer is finally released to form a variable gap. In this case, a release hole is required to allow a liquid for release to be introduced into the sacrifice layer. Accordingly, an area to which Coulomb forces are applied is reduced to thereby increase a driving voltage. Further, in a case of a small variable gap, sticking is caused when a sacrifice layer is released. Specifically, a thin film and a drive electrode substrate are stuck on each other due to surface tension of a water. Thus, there has been desired a structure that does not need to release a sacrifice layer.
Further, because a portion of the movable portion through which light passes is made of silicon, a conventional optical tunable filter cannot be used for purposes other than wavelength separation of infrared light.