Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-04-26
2001-03-13
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S290000, C359S295000, C359S223100, C359S230000, C438S052000, C335S222000, C345S084000
Reexamination Certificate
active
06201631
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical communication systems and more particularly to an optical device having a mirror array.
DESCRIPTION OF THE RELATED ART
Electro-optic devices often employ an array of micro-machined mirrors, each mirror being individually movable in response to an electrical signal. For example, the mirrors can each be cantilevered and moved by an electrostatic force. Typically, electro-optic mirror array devices can be used as, for example, optical cross connects in optical communication systems, visual presentations and displays. Generally, each mirror of the cross-connect is addressed by a number of electrical lead lines and receives a beam of light from, for example, an individual optical fiber in a fiber optic bundle. The beams of light reflected from the mirrors are individually directed to a prespecified location (for example, another fiber optic bundle) by individually moving the mirrors.
It is desirable to have a high density of optical transfer, meaning that mirrors having the desired diameter be spaced as close as possible. Spacing the mirrors as close as possible is problematic because each mirror typically requires a support structure. The electrical interconnection density can also present a serious bottleneck. As the number of mirrors in a tilting array increases, the number of electrical lead lines also increases, and these lead lines must be crowded into confined spaces. For example, a 256 mirror array chip (16×16 array) with four lead lines per mirror requires 1032 wirebond pads and electrical interconnections. The electrical leads must be adequately spaced to handle relatively high voltage (e.g., 100-150 volts). Hence there is a limit as to how small the leads can be made and how closely they can be spaced apart from each other. The routing of this number of electrical wires between the individual mirror elements, and routing from the chip center to the outer edge, forces the mirror spacing to be larger than desired and limits the useful size of the integrated array.
What is needed is an electro-optic device which includes a larger array of mirrors while not increasing the mirror spacing.
SUMMARY OF THE INVENTION
The present invention is directed to a process for fabricating an electro-optic device that includes a mirror array and a base substrate. The mirror array is a substrate having a plurality of spaced-apart mirrors thereon. The substrate also has cavities formed therein. The cavities are associated with and underlie individual mirrors. The cavities are configured to permit the overlying mirrors to rotate out of plane and thus be partially disposed in the cavity. The base substrate includes a plurality of electrodes as well as a plurality of electrical interconnections.
The mirror array is attached to the base substrate so that each mirror of the array as well as a cavity are supported above a set of electrodes. Each mirror of the mirror array rotates relative to the major plane of the substrate in response to an electrical signal. The application of an electrical potential to each mirror relative to at least one electrode of the set of electrodes causes the desired rotation (depending on the magnitude of the electrical potential) up to an angle of about 20 degrees.
The mirror array and cavity are defined in the substrate by providing a substrate in which two layers of semiconductor material are separated by a layer of oxidized semiconductor material. Such substrates are used because of the difference in etch resistance between the semiconductor material and the oxidized semiconductor layer. Specifically, the semiconductor material etches much faster in certain etch expedients (e.g. reactive ion etching (RIE)) than the oxidized semiconductor material. Similarly, the oxidized semiconductor material etches much faster in other etch expedients (e.g. fluorine-containing wet chemical etchants such as aqueous HF) than the semiconductor material. This difference in etch rates, termed etch selectivity, is exploited to fabricate the mirror array in the semiconductor substrate.
Examples of suitable substrates include silicon on insulator substrates. Silicon on insulator substrates (e.g. commercially available Simox substrates) are well known to one skilled in the art and are not discussed in detail herein. Silicon on insulator substrates typically have first and second silicon layers separated by a layer of silicon dioxide (SiO
2
). The silicon is either single crystalline silicon or polycrystalline silicon. Germanium compensated boron-doped silicon is also contemplated as suitable. With such substrates, the silicon left standing after patterning the mirrors therein is single crystal and stress free. The silicon does not curl, which is extremely important for mirror applications.
The substrates have a top layer of silicon that has a thickness from a few microns to about 50 microns. The intermediate layer, which acts as an etch stop for the silicon layers on either side, has a thickness in the range of several hundred nanometers (e.g. about 100 nm to about 500 nm). The base silicon layer has a thickness of several hundred microns (e.g., about 100 microns to about 500 microns).
A cavity is etched in the base silicon layer using a standard, anisotropic etching technique such as reactive ion etching (RIE). This etch automatically stops at the intermediate etch stop layer.
The mirror elements in the mirror array are then defined in the top silicon layer. The mirrors have torsional members, which permit the mirror elements to rotate in response to an actuating electrostatic force. In one embodiment, the mirror element is a mirror coupled to a gimbal via two torsional members. The torsional members are on opposing sides of the mirror and together define the mirror's axis of rotation. The gimbal, in turn is coupled to a support portion of the top silicon layer via two torsional members. The torsional members are on opposing sides of the gimbal and together define the gimbal's axis of rotation. The torsional members that couple the mirror to the gimbal are orthogonal to the torsional members that couple the gimbal to the substrate. The torsional members are also defined lithographically.
In a lithographic process for patterning a silicon layer, a layer of energy sensitive material is formed on the top silicon layer. An image of a pattern defining the mirror elements and torsional members is introduced into the energy sensitive material. The image is developed into a pattern. The pattern is then transferred into the underlying silicon layer using a conventional anisotropic etchant such as RIE with a chlorine etchant.
The lithographic techniques used to define the mirror elements and the torsional members are standard techniques well known to one skilled in the art. As such, these techniques are not discussed in detail herein.
The mirror element has a location in the top layer that corresponds to the location of a cavity in the base layer. The correspondence is such that, when the mirror element rotates out of the plane of the top layer, the mirror element is partially disposed in the cavity underlying the mirror element.
The intermediate etch stop layer underlying the mirror element and torsional members is then removed. A wet chemical etchant is contemplated as suitable for removing the etch stop layer. For the embodiment wherein the etch stop layer is SiO
2
, the wet chemical etchant is aqueous HF. After the etch stop layer is removed from underneath the mirror element, the mirror is free standing. In those embodiments that require the mirrors to have enhanced reflectivity, metal or dielectric materials are deposited on the mirrors. Because the mirror surface is exposed through the cavity in the base substrate layer, such materials are readily deposited on both sides of the mirror. Depositing such materials on both sides of the mirror reduces the stress and associated distortions that result when such materials are deposited on only one side of the layer.
A second substrate is provided that has met
Ben Loha
Botos Richard J.
Lucent Technologies - Inc.
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