Abstract:
A MEMS device employed in an optical switch has a position sensor configured to determine mirror orientation in the device. The position sensor includes at least one light sensor located under an etch gap defining the mirror in the switch. Change of light intensity at each light sensor due to the change in separation corresponding to the etch gap during mirror motion is measured and related to the mirror deflection angle. Information about the angle may be used to provide feedback to the motion actuator, which then may be operated to orient the mirror more accurately.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to optical communication equipment and, more specifically, to micro-electromechanical devices for use in such equipment. 
   2. Description of the Related Art 
   Optical communication equipment often employs micro-electromechanical systems (MEMS). A typical MEMS system may include an array of micro-machined mirrors, each mirror individually movable in response to an electrical signal. Such an array may be employed in an optical cross-connect, in which each mirror in the array receives a beam of light, for example, from an input optical fiber. The beam is reflected from the mirror and can be redirected to a different location, e.g., at which is located an output optical fiber, by rotating the mirror. More details on the principle of operation and methods of manufacture of MEMS devices including mirror arrays may be found, for example, in commonly assigned U.S. Pat. No. 6,201,631, the teachings of which are incorporated herein by reference. 
   One problem with prior art MEMS devices is related to determining the actual position of each mirror given any particular input electrical signal, which is important for optimal operation of a MEMS device. With relatively thin springs supporting each mirror/gimbal, there is little space for implementing position sensors, e.g., a four-terminal piezo-voltage torsion sensor such as disclosed in U.S. Pat. No. 5,648,618, the teachings of which are incorporated herein by reference. 
   SUMMARY OF THE INVENTION 
   The problems in the prior art are addressed, in accordance with the principles of the invention, by a position sensor integrated into a MEMS device, which is configured to determine the position of a movable part of the MEMS device. The process of forming the movable part includes the step of etching a gap between structure corresponding to the movable part and structure corresponding to the rest of the MEMS device. During operation of the MEMS device, when the movable part is moved relative to the rest of the MEMS device, the separation between the movable part and the rest of the MEMS device, corresponding to the etched gap, changes. For example, for a MEMS device in which the movable part is a mirror rotatably coupled to a stationary part of the MEMS device, rotating the mirror with respect to the stationary part changes the separation between the mirror and the stationary part. 
   In accordance with the principles of the invention, the position sensor includes at least one light sensor located below the etched gap formed between the movable part and the rest of the MEMS device. When the movable part is moved relative to the rest of the MEMS device, the associated change in the separation results in a change in the amount of incident light that reaches the at least one light sensor. The resulting change in the electrical signal generated by the at least one light sensor provides information that can be used to determine the current position of the movable part with respect to the rest of the MEMS device. That information may be used to generate feedback signals for the motion actuator of the MEMS device to position the movable part more accurately. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-C  illustrate top and cross-sectional views of a representative MEMS array that may be used in an optical cross-connect; 
       FIGS. 2A-C  show top and cross-sectional views of a switch that may be part of the array of  FIG. 1  according to one embodiment of the invention; and 
       FIG. 3  illustrates response of a light sensor to mirror rotation in the switch of  FIG. 2  in one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
     FIG. 1A  shows a top view of a representative MEMS array  100  that may be used in an optical cross-connect. Array  100  comprises four two-axis switches  102 , each of which includes a movable mirror  104  and a movable gimbal  106 , both formed in an overlayer  122  of a wafer  120  using, e.g., reactive etching. Mirror  104  is defined by gaps  112   a-b  between the mirror and gimbal  106 . Similarly, gimbal  106  is defined by gaps  112   c-d  between the gimbal and the rest of overlayer  122 . Typically, wafer  120  is a silicon-on-insulator (SOI) wafer in which overlayer  122  comprises crystalline silicon. Gimbal  106  is supported on wafer  120  by a pair of springs  108   c-d , each connected between gimbal  106  and the rest of overlayer  122 . Mirror  104  is coupled to gimbal  106  by a pair of springs  108   a-b . In a representative embodiment shown in  FIG. 1 , each of springs  108   a-d  is a torsional rod that is about 2 μm wide. In a different embodiment, each of springs  108   a-d  may be of a different shape and/or have different dimensions. 
     FIG. 1B  shows a cross-sectional view of one switch  102  of array  100  along line EF in FIG.  1 A. Mirror  104  and gimbal  106  are supported above a cavity  110  defined in an insulating layer  124  and a substrate layer  126  of wafer  120 . A second wafer  130  includes electrodes  134   a-b  and  136   a-b  as well as electrical interconnections (not shown). Substrate layer  126  is attached to wafer  130  such that electrodes  134  and  136  are located beneath mirror  104  and gimbal  106 , respectively, in cavity  110 . 
     FIG. 1C  illustrates how mirror  104  can be rotated. More specifically, mirror  104  rotates about the axis defined by springs  108   a-b  (e.g., axis AB in  FIG. 1A ) in response to voltage applied to at least one of electrodes  134   a-b . In addition, mirror  104  rotates about the axis defined by springs  108   c-d  (e.g., axis CD in  FIG. 1A ) together with gimbal  106  when the gimbal rotates about that axis in response to voltage applied to at least one of electrodes  136   a-b . Changing the voltages applied to individual electrodes  134   a-b  and  136   a-b  can change the angles of rotation about the two axes (e.g., axes AB and CD in FIG.  1 A), thus enabling a cross-connecting function of array  100 . As already indicated above, detecting an instant position of each mirror  104  is important for enabling optimal operation of array  100 . 
     FIGS. 2A-C  show top and cross-sectional views of a switch  202  that may be part of an array analogous to array  100  according to one embodiment of the invention. Switch  202  is similar to switch  102 . However, instead of wafer  130 , switch  202  has wafer  230 . Wafer  230  has four electrodes  234   a-b  and  236   a-b  that are similar to electrodes  134   a-b  and  136   a-b , respectively. In addition, wafer  230  includes one or more light sensors  210 . In a representative embodiment shown in  FIG. 2 , wafer  230  has four sensors  210   a-d.    
     FIG. 2A  shows in dashed lines an exemplary layout of sensors  210   a-d  and electrodes  234   a-b  and  236   a-b . Each sensor  210   a-d  is positioned under gap  112   a-d . In a different embodiment, instead of being located under the corresponding gaps, each sensor  210  in switch  202  may be located beneath an aperture formed in the mirror and/or gimbal for that sensor. 
     FIGS. 2B-C  show cross-sectional views of switch  202  along line EF in FIG.  2 A. As illustratively shown in  FIG. 2B , switch  202  is illuminated by a source of light (not shown) such that sensors  210  are illuminated through the corresponding gaps. In one application, the wavelength of the illuminating light is different from that of an optical signal routed by switch  202 . In addition, sensor  210  may include a corresponding filter (not shown) to reduce interference from the optical signal. In another application, the illuminating light is the optical signal routed by switch  202 . 
     FIG. 2C  shows switch  202  when mirror  104  is tilted and gimbal  106  remains in a horizontal position. Due to the mirror tilt, the separation corresponding to each gap  112   a-b is widened, which allows more light to fall onto corresponding sensor  210   a-b . Similarly, when gimbal  106  is tilted with respect to substrate  126 , the separation corresponding to each gap  112   c-d  is widened, which allows more light to fall onto corresponding sensors  210   c-d . When both mirror  104  and gimbal  106  are tilted, the amount of light impinging on each sensor  210   c-d  depends on the tilt angle of gimbal  106  with respect to substrate  126 , whereas the amount of light impinging on each sensor  210   a-b  depends on the relative tilt angle of mirror  104  with respect to gimbal  106 . 
     FIG. 3  illustrates change in photocurrent generated, e.g., by sensor  210   a  in switch  202 , as a function of tilt angle in one embodiment of the invention. More specially,  FIG. 3  shows the change in photocurrent (Δi=i−i 0 ), when mirror  104  is tilted and gimbal  106  remains in the horizontal position as shown in  FIG. 2B , where i and i 0  represent the photocurrents in the tilted and non-tilted mirror positions, respectively. As seen in  FIG. 3 , the change in photocurrent is nonlinear and progressively increases with the tilt angle. For example, change of the angle from 1 degree to 2 degrees generates photocurrent increase of about 1.5%, whereas change of the angle from 2 to 3 degrees generates photocurrent increase of about 2.5%. Using  FIG. 3 , the tilt angle can be determined from the photocurrent change. In a different embodiment, where a light sensor is located beneath an aperture in mirror  104  or gimbal  106 , the photocurrent decreases when the mirror/gimbal is tilted, since the effective area of the aperture (i.e., normal to the incident light) decreases. 
   In one embodiment, the following illustrative procedure may be used to determine the orientation of mirror  104  in switch  202 . In a first step, the angle of rotation of gimbal  106  about axis CD ( FIG. 2A ) is determined using the photocurrent change at sensors  210   c  and/or  210   d . In a second step, the angle of rotation of mirror  104  relative to gimbal  106  about axis AB ( FIG. 2A ) is determined using the photocurrent change at sensors  210   a  and/or  210   b . In a third step, the orientation of mirror  104  in switch  202 , e.g., with respect to substrate  122 , is obtained using the angles determined in the first and second steps. 
   The following representative parameters may be implemented in switch  202  according to one embodiment of the invention: (1) about 5×10 −6  for the ratio of gaps area to the total switch area; (2) about 2.5 μA for the photocurrent generated using a 1-Watt light source; and (3) about 40 nA/degree for the sensor sensitivity around the tilt angle of 3 degrees. 
   The invention may provide one or more of the following advantages. A position sensor of the invention may be implemented in a switch configured with springs having a relatively narrow width. Information about the mirror position obtained with such sensor may be used to provide feedback to the actuating electrodes (e.g., electrodes  134  and  136 ). Such feedback may be used to provide shaped pulses to the actuating electrodes, where the voltage applied to each electrode is a function of mirror position. Shaped pulses may be used to increase the switching speed. In addition, the feedback may be used to stabilize the mirror in the unstable angle region close to or beyond snap-down. (As known in the art, when voltage applied to an actuating electrode approaches a critical value, the tilt angle of the mirror begins to increase rapidly and nonlinearly with the voltage. This behavior may cause a collision (i.e., snap-down) of the mirror against the electrode and/or wafer.) Stabilization of the mirror near snap-down may extended the available angular range. 
   While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Light sensors may be implemented on the same wafer as the actuating electrodes or on a different wafer. In the latter case, light sensors may be placed in the packaging adjacent to a chip having the switch where the bottom portion of the switch (e.g., wafer  130 ) has the corresponding apertures configured to expose the sensors to light. Various optical filters, lenses, and/or light-reflecting structures may be used in conjunction with light sensors as known in the art. Each light sensor may be differently shaped and be based on any suitable light-sensitive device, such as a photodiode, a phototransistor, a photogate, photo-conductor, a charge-coupled device, a charge-transfer device, or a charge-injection device. The light used for light-sensor illumination may be based on the optical signal applied to the switch or be generated by a different light source. Similarly, as used in this specification, the term “light” refers to any suitable electromagnetic radiation in any wavelength and is not necessarily limited to visible light. Also, a switch of the invention may be implemented in wafers different from SOI wafers. In a one-axis switch, as few as one sensor  210  may be used. Similarly, in a two-axis switch, as few as two sensors  210  may be used, one for each axis. Furthermore, the invention may be implemented in different MEMS devices for determining orientation/position of various movable parts, e.g., sliding (shutter) plates, in those devices. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims. 
   Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.