Abstract:
The present invention relates to optical MEMS components, and in particular, to a micromechanical optical switch. A removable layer is used during fabrication to define the gap between optical waveguides and a moveable element in the form of a mirror that is moved between states. This provides a high speed, low-power optical switch that is readily manufacturable.

Description:
RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 09/645,203 filed Aug. 25, 2000 U.S. Pat. No 6,411,754. The entire teachings of the above-referenced application are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Optical switches have been developed using guided wave devices or free-space mechanical devices. Guided wave devices use waveguides, whereas free-space devices use optical beams in free space with movable optical elements such as mirrors or lenses. 
     Guided wave devices typically divert light from one arm of the device into the other by changing the refractive index of one of the arms of the device. This is typically done using electrical, thermal, or some other actuating mechanism. 
     The free-space approach has an advantage over the guided-wave approach in some applications. It has very low cross talk because the waveguides are physically isolated from one another and coupling cannot occur. The principal source of cross talk in this approach is scattering off the movable optical element. In addition, free-space devices are wavelength-independent and often temperature-independent. 
     Existing designs employ mirrors positioned at the intersection of input fibers and output fibers. Due to the spreading of the light beam as it leaves the waveguide and travels toward the mirror large mirrors are used that require mounting and angular placement accuracy. There can be significant difficulty in actuating such a relatively large structure quickly and accurately at the switching speeds required for optical communication systems. 
     Thus, a need exists for an optical switch having the advantages of the free-space approach, without the disadvantage of existing designs. 
     SUMMARY OF THE INVENTION 
     The present invention relates generally to the field of optical MEMS (micro-electro-mechanical system) and more specifically to the use of fabrication techniques used in making micromechanical devices to fabricate high speed optical MEMS for optical communication networks. The method employs the use of a removable layer that is formed between an optical waveguide and a movable switch element that has been formed over the removable layer and the waveguide. The removable or sacrificial layer is preferably formed using a conformal material such as parylene (poly-para-xylene). 
     A preferred embodiment of the invention can use a silica substrate with optical waveguides formed therein as the initial structure in the manufacture of the optical MEMS. After formation of a trench in the substrate to define a gap between waveguide elements, a first mask is used to define the routing wire geometry on the upper surface of the substrate. The trench can have a width of about 3 to 20 μm. Subsequently the removable layer is formed followed by the use of a second mask for fabrication of a switch element layer. 
     After patterning of the switch element layer, a spacer layer, preferably a photoresist layer, is spun on the surface and patterned using a third mask. A metallization layer, preferably an evaporated layer of copper, is deposited and a further photoresist layer is formed using a two mask exposure sequence. This defines a mold for fabrication of a plating layer. In a preferred embodiment of the invention, nickel is electroplated into the mold to form an integral electrode structure. 
     The removable layer is then preferentially etched to release the switch element which has been fabricated with a spring that supports the switch relative to the substrate. The characteristics of the spring define the speed and pull-up voltage of the switch element. 
     The electrodes are used with an overlying actuating electrode structure to actuate movement of the switch element between states. A preferred embodiment of the invention uses a reflective element or mirror that is moved from a first position, in which light from a first waveguide is reflected by the mirror into a second waveguide, to a second position in which the mirror is translated vertically to permit light to pass through the gap on a linear optical path into a third optical fiber that is aligned along a single axis with the first fiber. The waveguides and/or the trench can be filled with air or in another embodiment can be filled with a fluid. Further details regarding the use index matching fluids are described in International Application No. PCT/US99/24591 filed on Oct. 20, 1999, the entire contents of which is incorporated herein by reference. 
     Another preferred embodiment of the invention involves the fabrication of an array of switches on a single substrate that can serve as a monolithic array, or alternatively, the substrate can be diced to provide separate switches or arrays of switches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1K illustrate a method of fabricating an optical switch in accordance with the invention. 
     FIG. 2 is a schematic cross-sectional view of a preferred embodiment of an optical switch in accordance with the invention. 
     FIG. 3 illustrates the switching time of a preferred embodiment of the invention. 
     FIG. 4 is a top view of an optical switch in accordance with the invention. 
     FIG. 5 is a top view of another preferred embodiment of the invention. 
     FIG. 6 is a top view of another preferred embodiment of the invention. 
     FIG. 7 is a top view of another preferred embodiment of the invention. 
     FIG. 8 is an array of optical switches made in accordance with the invention. 
    
    
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred method of fabricating a micromechanical optical switch is illustrated in the process sequence of Figures  1 A- 1 K. This method begins with a substrate  10 , such as silica wafer, in which a cavity or trench  12  has been formed by standard etching techniques. As described in detail below, the substrate can include one or more optical waveguides. A first mask pattern is formed for conductive routing lines by depositing a photoresist layer  14  and selectively removing portions thereof to define the metallization pattern  16 , 18 . 
     In Figure 1B, a conductor layer  20  is formed, preferably by evaporation of a metal. In a preferred embodiment, a plurality of layers including a titanium layer, a nickel layer and a gold layer are used to provide the desired electrical and mechanical properties. In an example, the titanium layer has a thickness of 500 Å, the nickel layer has a thickness of 1500 Å and the gold layer has a thickness of 3000 Å. A rinse can be used to remove excess metal and the photoresist and overlying metal is than removed to provide conductive routing lines  30  shown in FIG.  1 C. 
     A sacrificial layer  40  is then formed on the device, preferably a layer of parylene having a thickness in the range of 0.5 μm to 25 μm, depending on the width of the trench  12 . This is followed by a layer  42 , of a reflective material such as gold. In this particular example the parylene layer has a thickness of 3.5 μm and the gold layer is deposited in 0.21 μm steps for a total thickness of about 2.0 μm. 
     In FIG. 1E, the reflective layer  42  is patterned to form a reflector or mirror  50 . A photoresist (AZ9260) is spun, baked, exposed, and developed to define the mask pattern  55  and the exposed gold is removed with a Transene TFA etchant. Next, after removal of the mask, a directional (RIE) etch in an O 2  plasma is used to remove the parylene with the mirror  50  acting as a mask and leaving a residual layer  62  as seen in FIG.  1 F. 
     Another photoresist layer  70  is then deposited and patterned to define anchor positions  72  (FIG.  1 G). Layer  70  has a specified thickness to define a gap between the suspended structure that will support the mirror relative to the substrate. The gap is preferably in a range between 5 and 20 μm and in this particular example is about 15 μm. The size of the anchor openings can be measured to verify proper alignment and preferably each opening has an area in a range between 40 and 50 μm 2 . 
     FIG. 1H illustrates formation of a metal seed layer having a thickness in a range of 1000 to 50,000 Å. In this particular example, a copper layer having a thickness of 5000 Å is deposited by evaporation. 
     Another photoresist pattern  90  is formed as shown in FIG. 1I using a digitally controlled oven at 45° for 4 hours. A two step exposure sequence is used to minimize variations in thickness of the photoresist. Features  92  of the photoresist layer  90  are used to define electrodes in the suspended membrane that are used in actuating movement of the switch element. As shown in FIG. 1J, a nickel layer  100  is formed, preferably using an electroplating process in which three separate regions, the first region  102  being at the anchor, the second region at electrodes  104 ,  106  and the third region at the mirror  108 . In a preferred embodiment of the invention a nickel sulfate solution is used with a current of 17.5 mA at 45° C. with a plating time of 30 minutes to provide a 6 μm thick layer. 
     As shown in FIG. 1K, the photoresist  90 ,  92  is removed, the exposed copper is then etched using ammonium hydroxide and copper (II) sulphate to access the spacer material  70  which is then removed. Finally, a diclorobenzene etch is performed at 150° C. that removes the remaining parylene  61  to release the mirror structure  120 . 
     The above procedure can also be used in fabricating a mirror that can be displaced laterally in the trench using a different method of actuation such as electrostatic comb drive or thermal actuation which can be used to provide a bistable switch, for example. 
     Illustrated in the schematic cross-sectional view of FIG. 2 is an optical switch  200  in accordance with the invention. An overlying actuating electrode panel  202  having an actuating electrode  204  that is separated from the suspended membrane  208  by a gap  218  that is preferably about 50 μm. Spacers  206  can be made using an oxide to prevent shorting between electrodes  204  and  226  or the mirror surface  210 . 
     Note that optional pull down electrodes  216  can also be positioned in the gap  220  between the fiber cladding  224  and the membrane  208  that is preferably about 15 μm. The substrate has a thickness  222  that includes the cladding  224  surrounding the fiber core  212 . The fiber core is preferably about 6 μm. The mirror includes the switching element  214  that moves vertically within the trench  228 . The pitch of the membrane structure is in a range of 100 to 2000 μm, and is preferably about 500 μm. The upper panel  202  can be electrically connected to the lower substrate system using flip chip bonding or eutectic bonding. The driver circuit for the switch can be mounted on substrate or packaged separately. 
     FIG. 3 graphically illustrates the vertical mirror displacement as a function of time for three different pull-up voltages as a function of time. 
     FIGS. 4-7 illustrate preferred embodiments of the spring system that supports the membrane relative to the substrate. The spring is configured to provide a vertical displacement of between 20 and 30 μm. Generally, a higher spring constant in the range of 1.0 to 4.0 N/m along with a higher operating voltage in the range of 50-150V results in a faster response time. It is also desirable to minimize or eliminate rotation of the membrane during displacement. FIG. 4 illustrates a spring system  300  having four beams  302  extending from anchors  304  to a membrane connection  306 . This particular embodiment has a stiffness of 1.99 N/m, a rotation of 0.0025 degrees and a spacing of 10 μm. The spring system  320  of FIG. 5 has four spring elements  322  extending from anchors  324  to membrane connectors  326 . This embodiment has a higher stiffness at 4.1 N/m, a smaller rotation at 0.0015 degrees and a 10 μm spacing. The embodiment  340  of FIG. 6 has four spring elements  342  connected at anchors  344  and connected at  346 . The system has a stiffness of 2.8 N/m, a rotation of 0.002 degrees and a smaller spacing at 5 μm. The system  360  of FIG. 7 has stiffness of 4.49 N/m, no rotation and a 10 μm spacing. 
     FIG. 8 illustrates an array  400  of switches fabricated in accordance with the invention. The array can be 8×8, 32×32, 64×64 or any other desired configuration as needed for a particular application. In this particular embodiment an 8×8 having input fibers  402 , a diagonally positioned array of switch elements that either reflect light from the input fibers to output fibers  406 , or allow light to pass directly through the trench to output fibers  408 . The output fibers  406  can be orthogonally arranged relative to fibers  402 , or they can be arranged at some other oblique angle. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.