« PreviousContinue »
OPTICAL SWITCH WITH LOW-INERTIA MICROMIRROR
CROSS-REFERENCE TO RELATED APPLICATIONS
This U.S. patent application is being concurrently filed With U.S. patent application Ser. No. 09/765,520 entitled OPTICAL CROSS-CONNECT WITH MAGNETIC MICRO-ELECTRO-MECHANICAL ACTUATOR CELLS, by HichWa et al.; and U.S. patent application Ser. No. 09/764,919 entitled LOW INERTIA LATCHING MICROACTUAT OR by Fejerabend et al.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
REFERENCE TO MICROFICHE APPENDIX Not applicable.
BACKGROUND OF THE INVENTION
The invention relates generally to optical sWitches, and more specifically to a micro-electro-mechanical system (“MEMS”) optical sWitch With a mirror that rotates in the major plane of the device.
The use of optical signal transmission is rapidly groWing in the telecommunications (“telecom”) industry. In particular, optical transmission techniques are being used for local data transfer (“metro”), as Well as point-to-point “longhaul” transmissions. Similarly, the number of optical signals, or channels, carried on an optic fiber is groWing. The implementation of Wave-division multiplexing (“WDM”) has alloWed the number of channels carried on a fiber to increase from one to over 16, With further expansions planned.
Thus, the need for optical sWitching technology is expanding. In the case of WDM technology, an optical channel is removed from a multi-channel fiber With an optical bandpass filter, diffraction grating, or other Wavelength-selective device, and routed to one of perhaps several destinations. For example, a channel on a long-haul transmission line might be routed betWeen a local user for a period of time and then sWitched back onto the long-haul transmission line (“re-inserted”). This is knoWn as 1><2 sWitching because a single input is sWitched betWeen one of tWo possible outputs. As the complexity of optical netWorks groWs, the complexity of the desired sWitching matrices also groWs.
SWitching matrices are being developed for several different optical netWork applications. “Small fabric” applications have been developed using 1><2, 2><2, and 1><8 type sWitches. HoWever, there is a need for “medium fabric” applications that can provide 8><8 up to and beyond 32><32 type sWitching arrays, and even for “large fabric” sWitching arrays that can handle 1024><1024 or more sWitching applications. The sWitching arrays that alloW any input to be connected to any output are generally called “crossconnects”, but in some applications there may be limited sWitching of some ports.
Unfortunately, attempting to merely scale the techniques developed for small fabric applications may not meet system requirements, such as sWitching speed, sWitching array space limitations, and poWer limitations. In particular, it is often desirable to upgrade an optical netWork to handle more traffic by adding additional channels onto the installed fiber
base, and that the sWitching arrays be able to fit into the existing “footprint” alloWed for the sWitching matrix. In many cases, the footprint is actually a 3-dimensional restriction. Similar restrictions might apply to the available poWer, or alloWable poWer dissipation.
Various techniques have been developed to address the problems arising in the development of more complicated sWitching arrays. Several approaches have adapted photolithographic methods developed primarily for the field of semiconductor processing to the fabrication of optical sWitching arrays. In one approach, MEMS techniques are used to create a very small motive device (motor), such as an electrostatic comb drive, electrostatic scratch drive, magnetic drive, thermal drive, or the like, attached to an optical sWitching element, typically a mirror. The mirror is usually either fabricated in the major plane of the process Wafer and rotated to become perpendicular to a sWitchable light signal, or is fabricated perpendicular to the major plane of the Wafer. In the first instance, establishing and maintaining verticality of the mirror is very important to insure that the light signal is reflected to the desired output port. In the second instance, fabricating a mirror-smooth surface on a vertical plane of the Wafer can be difficult, as can be depositing a reflective metal layer on that surface.
Similar challenges arise from speed and poWer requirements. Generally, a higher sWitching speed for a given type of actuator requires greater poWer. MEMS devices are attractive in that their small size often results in loW poWer consumption, but this may also limit the inertia of the optical element that can be sWitched Within the required period. The inertia can be changed by reducing the mass of the optical element, but this may result in an optical element that is not sufficiently rigid to reliably perform the desired optical sWitching function.
Thus, a need exists for optical sWitches that rapidly change states With relatively loW poWer requirements. It is further desirable that such sWitches have a small size, but yet provide an optical element that achieves loW insertion loss.
BRIEF SUMMARY OF THE INVENTION
A (“MEMS”) optical sWitch is formed on an SOI Wafer having a silicon substrate separated from a single-crystal silicon superstrate by a thin layer of silicon dioxide. A base portion of a die cut from the Wafer is attached to a pivoting member formed in a layer of single-crystal silicon With a hinge formed of the layer of single-crystal silicon. The pivoting member rotates relative to the base portion about an axis essentially perpendicular to the major surface of the die.
A mirror attached to the pivoting member is formed from the layer of single-crystal silicon and has a mirror surface congruent With a major crystalline plane. A high-quality reflective coating of gold or other metal, or a dielectric stack can be deposited on the mirror surface because it is an open surface. A latching spring holds the pivoting member, and hence mirror, in one of tWo sWitch positions.
A magnetic drive is actuated With a simple pulse that both accelerates and decelerates the pivoting member as it sWitches betWeen states. The impedance of the magnetic drive can be measured to determine the position of the sWitch, or a separate sensing circuit can be integrated onto the MEMS die to determine sWitch position.
In a particular embodiment, the backside of the mirror is thinned to reduce the mass of the mirror and thus the inertia encountered When sWitching betWeen states. In a further embodiment, the backside of the mirror is patterned With reinforcing ribs to maintain a rigid mirror While reducing its
mass. In another embodiment, both sides of the mirror are reflective. Release of the relatively large mirror structure from the bonding layer is achieved by etching through the substrate to alloW etching of the bonding layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a simplified top vieW of a MEMS cell according to an embodiment of the present invention in a retracted state;
FIG. 1B is a simplified top vieW of the MEMS cell illustrated in FIG. 1A in an extended state;
FIG. 1C is a simplified cross section of a portion of a MEMS cell according to an embodiment of the present invention;
FIG. 1D is a simplified cross section of a portion of a MEMS cell With a tWo-sided mirror according to another embodiment of the present invention;
FIG. 1E is a simplified floW chart of a process for fabricating a MEMS cell according to an embodiment of the present invention;
FIG. 1F is a simplified graph of current and velocity versus time illustrating braking during sWitching of a MEMS cell according to an embodiment of the present invention;
FIG. 1G is a simplified floW chart of a method of operating a MEMS cell according to an embodiment of the present invention;
FIG. 2A is a simplified top vieW shoWing details of the spring structure illustrated in FIG. 1A;
FIG. 2B is a simplified chart illustrating the motion of the spring structure and pivoting member about tWo non-colocated centers;
FIG. 2C is a simplified top vieW of a spring pivot according to another embodiment of the present invention;
FIG. 3A is a simplified top vieW of a MEMS cell With sensing poles according to another embodiment of the present invention;
FIG. 3B is a simplified diagram of an actuating and measurement system according to an embodiment of the present invention;
FIG. 3C is a simplified floW diagram of a process of sensing the position of the movable element of a MEMS cell according to another embodiment of the present invention;
FIG. 4A is a simplified bottom vieW of a lightened and reinforced mirror according to an embodiment of the present invention; and
FIG. 4B is a simplified cross section of a MEMS cell With a lightened and reinforced mirror according to another embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
An optical sWitch With a relatively large mirror is fabricated using MEMS techniques. The optical sWitch includes a magnetic motor that raises and loWers the mirror in response to a control signal. The control signal can be a simple pulse of electric current at a relatively loW voltage, such as about 5 volts. The optical sWitch can be sWitched from either state to the opposite state With the same pulse, and the pulse duration can be chosen to decelerate the mirror assembly after sWitching to reduce ringing. The mirror can be a single-sided or double-sided mirror, and is typically formed on a major plane of a single-crystal silicon super
strate. Forming the mirror on a major surface of the singlecrystal produces a high-quality mirror, Which provides loW insertion loss When used in a sWitching application.
A silicon flexure pivot hinge on an end of an arm integrated With the mirror alloWs the mirror to rotate about an axis perpendicular to the major surface of the MEMS cell or die. The dies or strips of dice can be edge-mounted to a substrate serving as a miniture optical bench. The mirrors are raised and loWered into and out of the path of an optical beam to direct the beam in a selected fashion. The optical beam originates from an optical input fiber With fiber-end optics that convert the light signal carried on the fiber into a beam. Optics on the ends of the output fibers collect the light beam and focus it onto the end of the desired output fiber. This free-space optical beam approach provides loW inter-channel “cross-talk”.
II. A Magnetic MEMS Optical SWitching Cell
FIG. 1A is a simplified top vieW of a magnetic MEMS cell 10 in a first (retracted) position. The cell, or die, is about 2.3><3.1 mm. Generally speaking, several cells are fabricated on a silicon-on-insulator (SOI) substrate, and the individual cells are then cut out of the substrate. Features are defined in the overlying thin layer (~10—80 microns) of singlecrystal silicon (the “superstrate”) by photomasking and etching processes. The underlying oxide material is then removed from beneath at least the movable portions of the device using a selective etch process. In some instances, a strip of cells, i.e. a portion of a roW or column of cells on the substrate is cut out. The cell includes a base portion 12 and a pivoting member 14. The base portion is typically bonded to a carrier and the pivoting member moves relative to the base portion.
The pivoting member 14 is attached to the base portion 12 With a hinge 16 and essentially rotates about a hinge attachment post 18. The actual center of rotation of the pivoting member varies With the amount of rotation relative to the base. The true rotational center generally describes an arc as the pivoting member pivots, due to the offset betWeen the hinge attachment post 18 and the spring anchor point 48. Thus the motion of the movable element is approximately circular. The hinge is a narroW isthmus of single-crystal silicon that alloWs at least about thirty degrees of rotation. The hinge is a flexure pivot that provides stability to the pivoting member to maintain planarity With the base during rotation While avoiding “stiction” (sticking friction) that often plagues bearing structures included in MEMS devices. The hinge is also relatively stiff to tensile and compressive loads on the pivoting member. An attachment post (see FIG. 1C, ref. num. 52) underlying the hinge attachment post connects the pivoting member to the base portion.
The pivoting member 14 includes a magnetic tab 20 formed on an arm 22. The arm moves betWeen tWo poles 24, 26 When the die is toggled (compare to FIG. 1B) to raise and loWer a mirror 15 or other optical element. Current-carrying coils 27, 28 activate the magnetic circuit, Which is completed by the magnetic bridges 30, 32, 34 to pull the magnetic tab 20 into the gap 36 betWeen the poles to provide a loWer energy path for the flux betWeen the cores 35, 37. HoWever, the magnetic tab continues through the gap until the pivoting member achieves a stable position, i.e. latches. In this example, the pivoting member has tWo stable positions, Which Will be referred to as “retracted” (FIG. 1A) and “extended” (FIG. 1B).
Electric current is provided to the coils 27, 28 through metal traces 38, 40, 41 connecting the coils to edge connectors or bonding pads 42, 44. The traces and connectors/ pads can be made of any of several suitable conductive