Optical: systems and elements – Diffusing of incident light
Reexamination Certificate
2001-06-05
2003-10-28
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Diffusing of incident light
C359S296000, C359S871000
Reexamination Certificate
active
06639724
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to an optical device and, more specifically, to a mirror having a barrier layer located therein or an array of mirrors having barrier layers located therein, and a method of manufacture therefor.
BACKGROUND OF THE INVENTION
Optical communication systems typically include a variety of optical devices, for example, light sources, photo detectors, switches, cross connects, attenuators, modulators, mirrors, amplifiers, and filters. The optical devices transmit optical signals in the optical communications systems. Some optical devices are coupled to electro-mechanical structures, such as thermal actuators, forming an electro-mechanical optical device. The term electro-mechanical structure, as used herein, refers to a structure that moves mechanically under the control of an electrical signal.
Some electro-mechanical structures move the optical devices from a predetermined first position to a predetermined second position. Cowan, William D., et al., “Vertical Thermal Actuators for Micro-Opto-Electro-Mechanical Systems,” SPIE, Vol. 3226, pp. 137-146 (1997), describes one such electro-mechanical structure useful for moving optical devices in such a manner.
These micro-electro-mechanical systems (MEMS) optical devices often employ a periodic 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, piezoelectric, magnetic, or thermal actuation. See articles by L. Y. Lin, et al., IEEE Photonics Technology Lett. Vol. 10, p. 525, 1998, R. A. Miller, et al. Optical Engineering Vol. 36, p. 1399, 1997, and by J. W. Judy et al., Sensors and Actuators, Vol.A53, p.392, 1996, which are incorporated herein by reference.
The mirrors used in these optical devices are typically made up of a material which reflects light with high reflectivity at a desired operating wavelength of the light, for example an operating wavelength ranging from about 1000 nm to about 1600 nm for SiO
2
optical fiber-based telecommunication systems. Some examples of such reflective materials are gold, silver, rhodium, platinum, copper, aluminum and their alloys. These reflective metal films typically have a thickness ranging from about 20 nm to about 2000 nm, and are deposited on a movable membrane substrate such as a polysilicon or silica substrate. At least one adhesion-promoting bond layer is desirably added between the reflective metal film and the substrate in order to prevent the reflective metal film from getting peeled off. Examples of such adhesion-promoting bond layers include titanium, zirconium, hafnium, chromium and tantalum.
A typical MEMS mirror comprises a metal-coated silicon mirror movably coupled to a surrounding silicon frame via a gimbal. Two torsional members on opposite sides of the mirror connect the mirror to the gimbal, and on opposite sides of the mirror, defining the mirror's axis of rotation. The gimbal, in turn, is coupled to the surrounding silicon frame via two torsional members defining a second axis of rotation orthogonal to that of the mirror. Using the typical MEMS mirror, the light beam can be reflected and steered in any direction.
Commonly, electrodes are disposed in a cavity underlying the mirror and the gimbal. Voltages applied between the mirror and an underlying electrode, and between the gimbal and an electrode, electrostatically control the orientation of the mirror. Alternatively, an electrical current can control the position of the mirror magnetically, thermally or piezoelectrically.
Turning to Prior Art
FIGS. 1 and 2
, illustrated is a typical MEMS mirror device and its application.
FIG. 1
illustrates a prior art optical MEMS mirror device
100
. The device
100
comprises a mirror
110
coupled to a gimbal
120
on a polysilicon frame
130
. The components are fabricated on a substrate (not shown) by micromachining processes such as multilayer deposition and selective etching. After etching, the mirror
110
, the gimbal
120
and the polysilicon frame
130
, are raised above the substrate by upward bending lift arms
140
, typically using a release process. The mirror
110
in the example illustrated in
FIG. 1
, is double-gimbal cantilevered and attached onto the polysilicon frame
130
by springs
150
. The mirror
110
can be tilted to any desired orientation for optical signal routing via electrostatic or other actuation, using electrical voltage or current supplied from outside. Typically, the mirror
110
includes a light-reflecting mirror surface
160
coated over a polysilicon membrane
170
, which is typically of circular shape. The light-reflecting mirror surface
160
is generally deposited by known thin film deposition methods, such as evaporation, sputtering, ion-beam, electrochemical or electroless deposition, or chemical vapor deposition.
Turning briefly to Prior Art
FIG. 2
, illustrated is an important application of the mirror
110
illustrated in FIG.
1
.
FIG. 2
illustrates an optical cross connect system
200
for optical signal routing, including an array of mirrors
210
. The optical cross connect system
200
shown in
FIG. 2
includes an optical input fiber
220
, an optical output fiber
230
and the array of MEMS mirrors
210
, including a primary mirror
212
and an auxiliary mirror
215
. As is illustrated, an optical signal from the input fiber
220
is incident on the primary mirror
212
. The primary mirror
212
, with the aid of the auxiliary mirror
215
, is electrically controlled to reflect the incident optical signal to the optical output fiber
230
. In alternative schemes, the input fibers and the output fibers are in separate arrays, and a pair of MEMS mirror arrays are used to perform the cross connect function.
The tilting of each mirror is controlled by applying specific electric fields to one or more of the electrodes beneath the mirror. Undesirable variations in the gap spacing between the mirror layer and the electrode layer, symmetric or nonsymmetric, may alter the electric field for the applied field, which affects the degree of electrostatic actuation and hence the degree of mirror tilting. This, in turn, alters the path or coherency of light signals reaching the receiving fibers, thus increasing the signal loss during beam steering.
An array of such MEMS mirrors is essentially composed of two layers: a mirror layer comprising the array of mirror elements movably coupled to a surrounding frame, and an actuator layer comprising the electrodes and conductive paths needed for electrical control of the mirrors. One approach to fabricating the array is to fabricate the actuator layer and the mirror layer as successive layers on the same workpiece and then to lift up the mirror layer above the actuator layer using vertical thermal actuators or using stresses in thin films.
An alternative approach is to fabricate the mirror layer on one substrate, the actuator layer on a separate substrate and then to assemble the mating parts with accurate alignment and spacing. A two-part assembly process is described in U.S. Pat. No. 5,629,790 issued to Neukermans et al. on May 13, 1997, which is incorporated herein by reference. Such two-part assembly processes generally provide a more robust structure, greater packing density of the movable mirrors, and permits larger mirror sizes and rotation angles, in addition to being easily scalable for larger arrays using silicon fabrication processes. The movable membrane in such a MEMS device is preferably made of single crystal silicon, and is typically only several micrometers thick. Such a thin silicon membrane is made, for example, by using the well-known silicon-on-insulator (SOI) fabrication process. The SOI process allows a convenient way of fabricating a thin silicon membrane, and the presence of a buried oxide layer is useful as an etch-stop barrier in photolithographical fabrication of the mirror, gimbal and spring/torsion bar structures. Selected patterned areas of the SOI
Bower John Eric
Gross Michal E.
Jin Sung-ho
Mavoori Hareesh
Ramirez Ainissa
Lucent Technologies - Inc.
Nguyen Thong
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