Patent Publication Number: US-2023146362-A1

Title: Wire bonded common electrical connection in a piezoelectric micro-electro-mechanical system scanning mirror assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 17/146,339 filed on Jan. 11, 2021, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     In traditional piezoelectric micro-electro-mechanical system (MEMS) scanning mirror systems, a metal frame acts as the common electrical connection for all of the piezoelectric elements. Accordingly, an electrical connection must also be made between the metal frame and the substrate, which serves as the electrical and mechanical interconnect for the device. To accommodate this electrical connection, a conductive adhesive is used to mechanically and electrically connect the frame to the substrate. However, combining both electrical and mechanical function into one adhesive bond affects the mechanical performance of the bond. Moreover, because the consistency of conductive and structural adhesives may vary at each attachment point, variations in the mechanical strength of the bonds cause efficiency issues in the system. Alternatively, using conductive adhesives alone suffers from marginal mechanical strength of the bond causing the system components to fail. 
     BRIEF SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     Aspects of the present invention relate to scanning mirror systems. More particularly, aspects of the present invention relate to creating electrical connections between the actuator frame of a piezoelectric MEMS scanning mirror system and the substrate separate from the structural adhesive creating the mechanical bond between the actuator frame and the substrate. To do so, a structural bond (with no conducive properties) is used to attach the actuator frame to the substrate. After the bond is fully formed, separate electric connections can be created in one of two ways. In one aspect, the actuator frame may be coated with a coating that enables a surface of the actuator frame to be wire bondable. A wire bond can then be created between the actuator frame and substrate. In another aspect, a trace of conductive material is deposited on the outside edge of the mechanical bond between the actuator frame and the substrate and a final protection layer may be applied over the conductive trace to protect the trace from mechanical or environmental damage. The result is a system with maximum mechanical performance and increased efficiency. 
     These and other aspects of the invention will become apparent to one of ordinary skill in the art upon a reading of the following description, drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described in detail herein with reference to the attached drawing figures, wherein: 
         FIG.  1    shows a schematic view of an example display device that includes a MEMS bonded by structural adhesive interfaces, in accordance with aspects of the present disclosure; 
         FIG.  2    shows an example scanning mirror system for the example display device of  FIG.  1   ; 
         FIG.  3    shows an exploded view of the scanning mirror system of  FIG.  2   . 
         FIG.  4    shows a side view of the scanning mirror system of  FIG.  2   . 
         FIG.  5    is a side elevation of a scanning mirror system, in accordance with aspects of the present disclosure; 
         FIG.  6    is a top plan view of a scanning mirror system, in accordance with aspects of the present disclosure; 
         FIG.  7    is a side elevation illustrating movement of a scanning mirror system, in accordance with aspects of the present disclosure; 
         FIG.  8    is a side elevation of an exemplary wire bonded common electrical connection in a scanning mirror system, in accordance with aspects of the present disclosure; 
         FIG.  9    depicts a flow diagram of an exemplary method for wire bonding a common electrical connection in a scanning mirror system, in accordance with aspects of the present disclosure; 
         FIG.  10    is a side elevation of an exemplary conductive bridge in a scanning mirror system, in accordance with aspects of the present disclosure;; 
         FIG.  11    shows an example method for creating a conductive bridge in a scanning mirror system, in accordance with aspects of the present disclosure; 
         FIG.  12    shows a side perspective view of a computing device in the form of a head mounted display (HMD) device that includes the example display device of  FIG.  1   ; and 
         FIG.  13    is a block diagram that illustrates an exemplary computing device. 
     
    
    
     DETAILED DESCRIPTION 
     The subject matter of the present disclosure is described with specificity herein to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. Moreover, although the terms “step” and/or “block” may be used herein to connote different elements of methods employed, the terms should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     As noted in the Background, in traditional piezoelectric MEMS scanning mirror systems, a metal actuator frame (not compatible with wire bonding) acts as the common electrical connection for all of the piezoelectric elements (i.e., the actuators). Since the substrate serves as the electrical and mechanical interconnect for the device, an electrical connection must also be made between the metal actuator frame and the substrate. In previous devices, a conductive adhesive was typically used to mechanically and electrically connect the actuator frame to the substrate. However, conductive adhesives are not as mechanically strong as adhesives that are dedicated for structural purposes. The decrease in mechanical strength introduced reliability issues into the device. 
     In an attempt to prevent these reliability issues, the conductive and structural adhesives were mixed (e.g., ten percent conductive and ninety percent structural). However, the adhesives did not mix uniformly throughout the bond and variations in the mechanical strength and conductive properties resulted. For example, the conductive adhesive may be covered by the structural adhesive and conductive issues occur. Similarly, the structural adhesive may be covered or diluted at a place where the structural bond occurs, and structural issues may occur. 
     Ultimately, each iteration of the device suffered efficiency issues. As the efficiency decreases, the device ceases to function properly. In particular, additional modes of motion are created because the resonant frequency shifts due to the structural weakening of the bond. The wire bonds attached to the mirror provide a feedback element so the angle the central mirror is moving can be determined. As the bond fails, the required voltage must increase to move the anchor portions of the mirror at a greater angle to achieve the desired effect in the mirror. Eventually, the system is no longer able to supply enough voltage to move the mirror to the required angle. Even if the voltage supply is able to be increased, the actuator will eventually fail as well. 
     Aspects of the present invention relate to creating electrical connections between the actuator frame of a piezoelectric MEMS scanning mirror system and the substrate separate from the structural adhesive creating the mechanical bond between the actuator frame and the substrate. Initially, a structural bond (with no conducive properties) is used to attach the actuator frame to the substrate. After the bond is fully formed, separate electric connections can be created in one of two ways. In one aspect, the actuator frame may be coated with a coating that enables a surface of the actuator frame to be wire bondable. A wire bond can then be created between the actuator frame and substrate. In another aspect, a trace of conductive material is deposited on the outside edge of the mechanical bond between the actuator frame and the substrate and a final protection layer may be applied over the conductive trace to protect the trace from mechanical or environmental damage. Both the wire bond and the trace of conductive material can be utilized in one device for redundancy. Similarly, more than one wire bond or more than one trace of conductive material can be utilized in one device for redundancy. The result is a system with maximum mechanical performance and increased efficiency over previous devices. 
     Accordingly, one aspect of the present disclosure is directed to a wire bonded common electrical connection in a scanning mirror system. The system comprises an actuator frame consisting of a frame material coated with a coating that enables a surface of the actuator frame to be wire bondable. The system also comprises at least one actuator having a top electrode on a top surface of the actuator and a bottom electrode on a bottom surface of the actuator. The at least one actuator may be attached to a top surface of the actuator frame with a partially conductive adhesive. The system further comprises a mirror extending across a gap in a central mounting member of the actuator frame. Anchor portions of the mirror are attached to the top surface of the actuator frame with a structural adhesive. The system also comprises a substrate connected to a bottom surface of the actuator frame with a structural adhesive. An electrical pad on the substrate enables an electrical connection between the top electrode on the top surface of the at least one actuator and the substrate, the mirror and the substrate, and the top surface of the actuator frame to the substrate. 
     In another aspect, the present disclosure is directed to a wire bonded common electrical connection in a scanning mirror system. The system comprises an actuator frame coated with a coating that enables a surface of the actuator frame to be wire bondable. The system also comprises at least one actuator attached to a top surface of the actuator frame with a partially conductive adhesive. The system further comprises a mirror extending across a gap in central mounting member of the actuator frame. Anchor portions of the mirror are attached to the top surface of the actuator frame with a structural adhesive. The system also comprises a substrate connected to a bottom surface of the actuator frame with a structural adhesive having no conductive properties, an electrical pad on the substrate enabling an electrical connection between a top surface of the actuator frame and the substrate. 
     In yet another aspect, the present disclosure is directed to a conductive bridge in a piezoelectric MEMS scanning mirror system. The system comprises an actuator frame consisting of a frame material. The system also comprises at least one actuator having a top electrode on a top surface of the actuator and a bottom electrode on a bottom surface of the actuator. The at least one actuator may be attached to a top surface of the actuator frame with a partially conductive adhesive. The system further comprises a mirror extending across a gap in a central mounting member of the actuator frame. Anchor portions of the mirror are attached to the top surface of the actuator frame with a structural adhesive. The system also comprises a substrate connected to a bottom surface of the actuator frame with a structural adhesive having no conductive properties. The system further comprises a trace of conductive adhesive enabling an electrical connection between the top surface of the actuator frame and the substrate. 
       FIG.  1    schematically shows an example display device  100  in communication with a video source  102 . Display device  100  includes a controller  104  operatively coupled to a scanning mirror system  106  and to a light source  108 . Controller  104  is configured to control light source  108  to emit light based on video image data received from video source  102 . Light source  108  may include any suitable light-emitting element(s), such as one or more lasers, and may output light in any suitable wavelength ranges, such as red, green, and blue. In other examples, light source  108  may output substantially monochromatic light, or other wavelength bands than red/green/blue. 
     Scanning mirror system  106  comprises one or more scanning mirrors  110  controllable to vary an angle at which light from the light source is reflected to thereby scan an image. As mentioned above, the scanning mirror system  106  may include a single mirror driven in both horizontal and vertical directions, or two mirrors separately driven in horizontal and vertical directions. Light reflected by scanning mirror system  106  is directed toward an output  112  for display of a scanned image. Output  112  may take any suitable form, such as projection optics, waveguide optics, etc. As examples, display device  100  may be configured as a virtual reality head-mounted display (HMD) device with output  110  configured as an opaque surface, or as an augmented reality HMD device with the output configured as a see-through structure that allows virtual imagery to be combined with a view of the surrounding real-world environment. Display device  100  also may assume other suitable forms, such as that of a head-up display, mobile device screen, monitor, or television, as examples. 
     Scanning mirror system  106  further includes an electromechanical actuator system  114  comprising actuator(s)  116  to effect movement of the scanning mirror(s)  110 . Various type of actuators may be used to control a MEMS mirror system. 
     As illustrated in  FIG.  1   , one or more of the scanning mirror(s)  110  and the electromechanical actuator system  114  are bonded to an actuator frame  118  by a structural adhesive interface  120 . In one example, the structural adhesive interface  120  includes both a non-conductive structural adhesive portion  122  and a conductive structural adhesive portion  124  arranged such that the non-conductive structural adhesive portion  122  at least partially surrounds and encompasses the conductive structural adhesive portion  124 . The controller  104  may be configured to drive the actuator(s)  116  of the electromechanical actuator system  114  via electricity conducted through the actuator frame  118  and the conductive structural adhesive portion  124  to the actuator(s)  116 . 
       FIGS.  2 - 4    show a specific example of a scanning mirror system  200  that includes a scanning mirror assembly  206  comprising an example of an actuator frame  210 . Scanning mirror assembly  206  comprises a MEMS mirror  218  attached to the actuator frame  210  via a first flexure  222  and a second flexure  224 . First and second flexures  222  and  224  may provide respective pivots via which mirror  218  can rotate and thereby change its angular orientation to vary the angle at which light from a light source is reflected. Mirror  218  may scan in a horizontal or vertical direction, depending upon an orientation in which scanning mirror system  200  is incorporated into a display device. 
     The first flexure  222  of scanning mirror assembly  206  is connected to a first anchor portion  228 . This first anchor portion is affixed to a first moveable member  232  of the actuator frame  210  by a structural adhesive interface. In a similar manner, the second flexure  224  of scanning mirror assembly  206  is connected to a second anchor portion  238  that is affixed to a second moveable member  242  of the actuator frame  210  by a structural adhesive interface. As described in more detail below, actuators affixed to the moveable members are controlled to cause corresponding movement in the mirror  218 . 
     In this example, the scanning mirror system  200  includes an electromechanical actuator system comprising a first actuator pair affixed to the first moveable member  232  adjacent to the first flexure  222 , and a second actuator pair affixed to the second moveable member  242  adjacent to the second flexure  224 . The first actuator pair comprises first actuator  244  and second actuator  246 , and the second actuator pair comprises third actuator  248  and fourth actuator  250 . 
     The actuators may be controlled to cause a desired oscillation in the mirror  218 . For example, the actuators may comprise a lead zirconate titanate (PZT) material or piezoelectric ceramic material that changes dimension based upon an applied voltage. For example, upon receiving an electrical signal having a first polarity (e.g., positive), actuators  244  and  248  may apply a contractive force to portions of moveable members  232  and  242 , respectively, underlying these actuators. On the other hand, upon receiving the electric signal having the first polarity, actuators  246  and  250  may apply a dilative force to portions of moveable members  232  and  242 , respectively, underlying these actuators. An electrical signal having a second, different polarity (e.g., negative) may cause actuators  244  and  248  to apply a dilative force to the respective underlaying portions of the moveable members, and may cause actuators  246  and  250  to apply a contractive force to the respective underlaying portions of the moveable members  232  and  242 . The magnitude of force applied by actuators  244 ,  246 ,  248  and  250  may be controlled by controlling the magnitude of an electrical signal applied to the actuators. As discussed above, the electrical signal applied to the actuators may be conducted through the conductive structural adhesive portion  124  of the structural adhesive interface  120  bonding each actuator to respective underlaying portions of the moveable members  232  and  242 . In this manner, the electrical signal may be applied to the conductive material of the actuator frame  210  and conducted to each of the actuators  244 ,  246 ,  248  and  250  via the conductive structural adhesive portion  124 . 
     In other examples the electromechanical actuator system of scanning mirror system  200  may utilize any suitable type of actuators. For example, each actuator may comprise a magnetic actuator, wherein a magnetic force between magnetic elements can be varied via electrical signal. In other examples, each actuator may comprise an electrostatic actuator, where an electric field between electrodes can be varied to adjust contractive or dilative forces. As a further example, each electromechanical actuator may utilize one or more bimetallic strips, where differing coefficients of thermal expansion of different materials can be leveraged to vary the applied forces. It will also be appreciated that actuator(s) of an electromechanical actuator system may be arranged at other suitable locations in a scanning mirror system. 
     In this example, a central mounting member  256  of the actuator frame  210  is affixed to an underlying substrate  260  via a spacer  262 . In some examples the actuator frame  210  may comprise a metallic material, such as steel, and the substrate  260  may comprise a PCB, ceramic material, or any other suitable material. The spacer  262  may comprise any suitable metallic material, such as steel. The central mounting member  256  may be bonded to the spacer  262 , and the spacer bonded to the substrate  260  via the structural adhesive interface  120 , such that an electric signal may be conducted from the substrate  260  to the actuators  244 ,  246 ,  248  and  250 . 
     In the example of  FIGS.  2 - 4   , spacer  262  elevates the actuator frame  210  above the substrate  260  to thereby enable movement of the moveable members  232  and  242 . More particularly and as shown in  FIG.  4   , spacer  262  creates gaps  266  and  268  between moveable members  232  and  242 , respectively, and the underlying substrate  260 . In this manner, the first moveable member  232  and the second moveable member  242  float above the substrate  260 , and thereby may cause movement of the mirror  218  via flexures  222  and  224 . In one example, each of the gaps  266  and  268  may be approximately 0.4 mm to 0.6 mm, or more specifically 0.5 mm, to enable y-axis movement of the first moveable member  232  and second moveable member  242  relative to the substrate  260 . In other examples, any other suitable gap distances may be utilized to accommodate different scanning mirror system configurations and desired mirror movements. In the present example, the spacer  262  has substantially the same shape as the mounting member  256  of the actuator frame  210 . In other examples, the spacer  262  may have a shape different from the mounting member  256 . As shown in  FIG.  3   , the actuators  244 ,  246 ,  248  and  250  are bonded to respective underlaying portions of the moveable members  232  and  242  of the actuator frame  210  by partially conductive adhesive  274 . 
     As illustrated in  FIG.  3   , the actuator frame  210  comprises a first hinge  270  that connects a central portion  272  of the mounting member  256  with a central portion  276  of the first moveable member  232 . In this example, the first hinge  270  is located substantially equidistant from the opposing ends  234  and  236  of the first moveable member  232 . Similarly, the central portion  276  of the first moveable member  232  is located substantially midway between the opposing ends  234  and  236  of the first moveable member. In this example, both central portion  272  and central portion  276  comprise an aperture. It will be appreciated that in other examples, the mounting member  256  and first moveable member  232  may have different configurations that include one or more apertures of different shapes, sizes, and/or locations, or configurations that include no apertures. 
     In a similar manner, actuator frame  210  comprises a second hinge  280  that connects central portion  272  of the mounting member  256  with a central portion  282  of the second moveable member  242 . As with the first hinge  270 , the second hinge  280  is located substantially equidistant from the opposing ends  278  and  286  of the second moveable member  242 . The central portion  282  of the second moveable member  242  is also located substantially midway between the opposing ends  278  and  286  of the second moveable member. In this example, both central portion  272  and central portion  282  comprise an aperture. As with the first moveable member  232 , in other examples the mounting member  256  and second moveable member  242  may have different configurations that include apertures of different shapes, sizes, and/or locations, or configurations that include no apertures. In some examples, actuator frame  210  may be formed from micromachined silicon dies. 
     As illustrated in  FIG.  3   , each of the actuators  244 ,  246 ,  248 , and  250  may be bonded to respective underlaying portions of the moveable members  232  and  242  of the actuator frame  210  by partially conductive adhesive  274 . In one example, the anchor portion  228  and  238  connected to the mirror  218  may also be bonded to underlaying portions of the moveable members  232  and  242  by structural adhesive. 
     Referring now to  FIG.  5   , a side elevation of a scanning mirror system  200  is illustrated, in accordance with aspects hereof. Some components referenced in  FIGS.  2 - 4    are not depicted in  FIGS.  5 - 8  and  10    to more clearly emphasize particular aspects of the present disclosure. Although each of  FIGS.  2 - 8  and  10    depict certain components, it will be appreciated that in various aspects of the present disclosure, the scanning mirror system may have different configurations that include one or more components described herein in different shapes, sizes, and/or locations (e.g., a different shaped actuator frame), or different configurations that include or exclude some of these components. As illustrated, the bottom surface of an actuator frame  210  is attached to a substrate  260  with a structural adhesive. Importantly, the structural adhesive has no conductive properties which enables maximum mechanical performance of the bond between the actuator frame  210  and the substrate  260  to be obtained. In aspects, the actuator frame  210  is made of a frame material that has a thermal expansion matched to a mirror. For example, the frame material may be alloy  42 . The actuator frame  210  is also coated with a coating (e.g., electroless nickel immersion gold coating or electroless nickel electroless palladium immersion gold coating). The coating enables a surface of the actuator frame  210  to be compatible with wire bonding that would not otherwise be wire bondable. 
     The scanning mirror system  200  also includes at least one actuator  248 ,  250 . The actuator(s)  248 ,  250  has a top electrode on the top surface and a bottom electrode on the bottom surface and is attached to a top surface of the actuator frame  210  with a partially conductive adhesive  274 . In this way, the bottom electrode of the actuator  248 ,  250  is shorted to the top surface of the actuator frame  210 . 
     In the scanning mirror system  200 , the electrical connections and routing occur in the substrate  260 . The top electrode of the actuator is wire bonded (not shown) to an electrical pad (not shown) on the substrate  260 . In aspects, four wire bonds enable the electrical connection between the top surface of each of four actuators and the substrate  260 . 
     A mirror extends across a gap (not shown in  FIG.  1   ) in a central mounting member of the actuator frame  210 . Anchor portions  228 ,  238  of the mirror are attached to the top surface of the actuator frame  210  with a structural adhesive  590 . The mirror is wire bonded (not shown) to an electrical pad (not shown) on the substrate  260 . In some aspects, four wire bonds enable the electrical connection between the mirror and the substrate  260 . 
       FIG.  6    is a side elevation of a scanning mirror system  200 , in accordance with aspects hereof. As illustrated, the scanning mirror system  200  includes four actuators  244 ,  246 ,  248 ,  250 . As described above, each of the actuators  244 ,  246 ,  248 ,  250  has a top electrode on the top surface and a bottom electrode on the bottom surface and is attached to a top surface of the actuator frame  210  with a partially conductive adhesive (not shown in  FIG.  2   ). The top electrode of each of the actuators  244 ,  246 ,  248 ,  250  is wire bonded (not shown) to an electrical pad (not shown) on the substrate  260 . The wire bonds enable the electrical connection between the top surface of each of the actuators  244 ,  246 ,  248 ,  250  and the substrate  260 . 
     As described above, a mirror  218  extends across an aperture  292  in a middle of the actuator frame  210 . Anchor portions  228 ,  238  of the mirror  218  are attached to the top surface of the actuator frame  210  with a structural adhesive (not shown in  FIG.  2   ) and are connected to the mirror  218  by first flexure and second flexure  224 . The mirror is also wire bonded (not shown) to an electrical pad (not shown) on the substrate  260 . In some aspects, four wire bonds enable the electrical connection between the mirror and the substrate  260 . 
       FIG.  7    depicts a side elevation illustrating movement of a scanning mirror system  200 , in accordance with aspects hereof. The movement of the scanning mirror system  200  in FIG.  7  is exaggerated for illustrative purposes. For clarity, the piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. Conversely, when an electric field is applied to these materials, the materials become stressed and can shrink or expand. 
     In the context of the scanning mirror system  200  described herein, when an electric field is applied across an actuator  248 ,  250  in the Z direction, the actuators  248 ,  250  attempt to shrink or expand in the X and Y direction. Since the actuators  248 ,  250  are constrained by the actuator frame  210  (i.e., the actuator frame attempts to keep the actuator  248 ,  250  the same size), the actuators  248 ,  250  curve. In the case where an actuator on one side of the actuator frame  210  attempts to shrink (e.g., the actuator  248 ) and an actuator on the other side of the actuator frame  210  attempts to expand (e.g., the actuator  250 ), the actuators  248 ,  250  curve in opposite directions, causing second moveable member  242  of the actuator frame  210  to tilt slightly. Since the anchor portion  238  of the mirror  218  is attached to the second moveable member  242 , the tilting of the anchor portion  238  causes movement of the mirror  218  via the flexure(s) (not shown in  FIG.  7   ) at a much higher degree. 
     Referring now to  FIG.  8   , a side elevation of an exemplary wire bonded common electrical connection in a scanning mirror system  200  is illustrated, in accordance with aspects hereof. As illustrated, the scanning mirror system  200  includes a wire bond  892  between the common trace  894  on the substrate  260  to the surface of the actuator frame  896 . 
     As illustrated, the bottom surface of an actuator frame  210  is attached to a substrate  260  with a purely structural adhesive  590 . As emphasized above, the structural adhesive  590  has no conductive properties which enables maximum mechanical performance of the bond between the actuator frame  210  and the substrate  260  to be obtained. The actuator frame  210  is coated with a coating (e.g., electroless nickel immersion gold coating or electroless nickel electroless palladium immersion gold coating). The coating enables a surface of the actuator frame  896  to be compatible with wire bonding that would not otherwise be wire bondable. The wire bond  892  applied at the surface of the actuator frame  896  completes an electrical connection between the actuator frame  210  and the substrate  260  (by way of an electrical pad  894  on the substrate  260 ). In some aspects the wire bond  892  is a gold wire and the coating on the surface of the actuator frame  896  and the trace on the electric pad  894  on the substrate  260  comprise a gold surface finish (e.g., electroless nickel immersion gold or electroless nickel electroless palladium immersion gold). In some aspects, multiple wire bonds can be applied for in different locations for redundancy. In some aspects, the wire bonded common electrical connection can be combined with the conductive bridge described with respect to  FIG.  10    for redundancy. 
       FIG.  9    depicts a flow chart illustrating a method  900  of providing a wire bonded common electrical connection in a scanning mirror system. Although the method is presented in a particular order, it should be appreciated that the method can be performed in a variety of orders. As illustrated, at step  902 , a structural adhesive is dispensed on the substrate and the actuator frame (that has been coated with a coating compatible with wire bonds) is placed on the substrate. At step  904 , the system is placed in an oven and cured. A partially conductive adhesive is dispensed on the actuator frame, at step  906 , and the actuators are placed on the actuator frame. At step  908 , the system is placed in an oven and cured. A structural adhesive is dispensed on the actuator frame, at step  910 , and the mirror is placed on the actuator frame via the anchor portions of the mirror. At step  912 , the system is placed in an oven and cured. A wire bond is created, at step  914 , between the surface of the actuator frame and an electrical pad on the substrate. 
     Turning now to  FIG.  10   , a side elevation of an exemplary conductive bridge in a scanning mirror system  200  is illustrated, in accordance with aspects hereof. As illustrated, the bottom surface of an actuator frame  210  is attached to a substrate  260  with a purely structural adhesive  590 . As mentioned above, the structural adhesive  590  has no conductive properties which enables maximum mechanical performance of the bond between the actuator frame  210  and the substrate  260  to be obtained. To create the electrical connection between the actuator frame and the substrate, a conductive adhesive  998  is dispensed at the edge of the actuator frame, down the side of the structural bond, and to an electrical pad on the substrate  260 . In some aspects, a protection layer is applied over the conductive adhesive  998  to protect the conductive bridge from mechanical or environmental damage. The conductive bridge can be applied in multiple locations between the actuator frame and the substrate for redundancy. In some aspects, the conductive bridge can be combined with the wire bonded electrical connection described with respect to  FIG.  8    for redundancy. 
       FIG.  11    depicts a flow chart illustrating a method  1100  of providing conductive bridge in a piezoelectric MEMS scanning mirror system. Although the method is presented in a particular order, it should be appreciated that the method can be performed in a variety of orders. As illustrated, at step  1102 , a structural adhesive is dispensed on the substrate and the actuator frame is placed on the substrate. The system is placed in an oven and cured, at step  1104 . A partially conductive adhesive is dispensed on the actuator frame, at step  1106 , and the actuators are placed on the actuator frame. At step  1108 , a conductive adhesive is dispensed at the edge of the actuator frame, down the side of the structural bond, and to an electrical pad on the substrate, forming the conductive bridge. The system is placed in an oven and cured, at step  1110 . A structural adhesive is dispensed on the actuator frame, at step  1112 , and the mirror is placed on the actuator frame via the anchor portions of the mirror. At step  1114 , the system is placed in an oven and cured. A protection layer is applied over the conductive adhesive, at step  1116 , to protect the conductive bridge from mechanical or environmental damage. 
       FIG.  12    shows an example head mounted display (HMD) device  1200  that may include the example display device  100  illustrated in  FIG.  1    and the example scanning mirror system  200  illustrated in  FIGS.  3 - 5   . The HMD device  1200  may be worn by a user according to an example of the present disclosure. In other examples, an HMD device may take other suitable forms in which an at least partially see-through display is supported in front of a viewer&#39;s eye or eyes in an augmented reality HMD device configuration. 
     In the example of  FIG.  12   , the HMD device  1200  includes a frame  1202  that wraps around the head of the user to position a display device  1204  close to the user&#39;s eyes. The display device  1204 , may, for example, take the form of the example display device  100  illustrated in  FIG.  1    and described above. The frame supports additional components of the HMD device  1200 , such as, for example, a processor  1206  and input devices  1208 . The processor  806  includes logic and associated computer memory configured to provide image signals to the display device  1204 , to receive sensory signals from input devices  1208 , and to enact various control processes described herein. The processor  1206  may take the form of the controller  104  illustrated in  FIG.  1   . 
     The input devices  1208  may include various sensors and related systems to provide information to the processor  1206 . Such sensors may include, but are not limited to, an inertial measurement unit (IMU)  1208 A, one or more outward facing image sensors  1208 B, and one or more inward facing image sensors  1208 C. The one or more inward facing image sensors  1208 B may be configured to acquire image data in the form of gaze tracking data from a wearer&#39;s eyes. 
     The one or more outward facing image sensors  1208 B may be configured to capture and/or measure physical environment attributes of the physical environment in which the HMD device  1200  is located. In one example, outward facing image sensors  1208   b  may include a visible-light camera configured to collect a visible-light image of a physical space. Further, the one or more outward facing image sensors  1208 B may include a depth camera configured to collect a depth image of a physical space. More particularly, in one example the depth camera is an infrared time-of-flight depth camera. In another example, the depth camera is an infrared structured light depth camera. 
     Data from the outward facing image sensors  1208 B may be used by the processor  1206  to generate and/or update a three-dimensional (3D) model of the physical space. Data from the outward facing image sensors  1208 B may be used by the processor  1206  to identify surfaces of the physical space and/or measure one or more surface parameters of the physical space. The processor  1206  may execute instructions to generate/update virtual scenes displayed on display device  1204  and identify surfaces of the physical space in any suitable manner. 
     In augmented reality configurations of HMD device  1200 , the position and/or orientation of the HMD device  1200  relative to the physical environment may be assessed so that augmented-reality images may be accurately displayed in desired real-world locations with desired orientations. As noted above, the processor  806  may execute instructions to generate a 3D model of the physical environment including surface reconstruction information that may be used to identify surfaces in the physical space. In both augmented reality and non-augmented reality configurations of HMD device  1200 , the IMU  1208 A of HMD device  1200  may be configured to provide position and/or orientation data of the HMD device  1200  to the processor  1206 . 
     In some embodiments, the methods and processes described herein may be tied to a computing system of one or more computing devices. In particular, such methods and processes may be implemented as a computer-application program or service, an application-programming interface (API), a library, and/or other computer-program product. 
     With reference to  FIG.  13   , computing device  1300  includes a bus  1310  that directly or indirectly couples the following devices: memory  1312 , one or more processors  1314 , one or more presentation components  1316 , one or more input/output (I/O) ports  1318 , one or more I/O components  1320 , and an illustrative power supply  1322 . Bus  1310  represents what may be one or more busses (such as an address bus, data bus, or combination thereof). Although the various blocks of  FIG.  13    are shown with lines for the sake of clarity, in reality, these blocks represent logical, not necessarily actual, components. For example, one may consider a presentation component such as a display device to be an I/O component. Also, processors have memory. The inventors hereof recognize that such is the nature of the art and reiterate that the diagram of  FIG.  13    is merely illustrative of an exemplary computing device that can be used in connection with one or more aspects of the present technology. Distinction is not made between such categories as “workstation,” “server,” “laptop,” “handheld device,” etc., as all are contemplated within the scope of  FIG.  13    and with reference to “computing device.” 
     Computing device  1300  typically includes a variety of computer-readable media. Computer-readable media can be any available media that can be accessed by computing device  1300  and includes both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, computer-readable media may comprise computer-storage media and communication media. 
     Computer-storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVDs) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by computing device  1300 . Computer storage media does not comprise signals per se. 
     Communication media typically embodies computer-readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media, such as a wired network or direct-wired connection, and wireless media, such as acoustic, RF, infrared, and other wireless media. Combinations of any of the above should also be included within the scope of computer-readable media. 
     Memory  1312  includes computer storage media in the form of volatile and/or nonvolatile memory. The memory may be removable, non-removable, or a combination thereof. Exemplary hardware devices include solid-state memory, hard drives, optical-disc drives, etc. Computing device  800  includes one or more processors  1314  that read data from various entities such as memory  1312  or I/O components  1320 . Presentation component(s)  1316  presents data indications to a user or other device. Exemplary presentation components include a display device, speaker, printing component, vibrating component, and the like. 
     The I/O ports  1318  allow computing device  1300  to be logically coupled to other devices, including I/O components  1320 , some of which may be built in. Illustrative components include a microphone, joystick, game pad, satellite dish, scanner, printer, wireless device, etc. 
     The I/O components  1320  may provide a natural user interface (NUI) that processes air gestures, voice, or other physiological inputs generated by a user. In some instances, inputs may be transmitted to an appropriate network element for further processing. An NUI may implement any combination of speech recognition, touch and stylus recognition, facial recognition, biometric recognition, gesture recognition both on screen and adjacent to the screen, air gestures, head and eye tracking, and touch recognition associated with displays on the computing device  1300 . The computing device  1300  may be equipped with depth cameras, such as stereoscopic camera systems, infrared camera systems, RGB camera systems, and combinations of these, for gesture detection and recognition. Additionally, the computing device  1300  may be equipped with accelerometers or gyroscopes that enable detection of motion. The output of the accelerometers or gyroscopes may be provided to the display of the computing device  1300  to render immersive augmented reality or virtual reality. 
     Some aspects of computing device  1300  may include one or more radio(s)  1324  (or similar wireless communication components). The radio  1324  transmits and receives radio or wireless communications. The computing device  1300  may be a wireless terminal adapted to receive communications and media over various wireless networks. Computing device  1300  may communicate via wireless protocols, such as code division multiple access (“CDMA”), global system for mobiles (“GSM”), or time division multiple access (“TDMA”), as well as others, to communicate with other devices. The radio communications may be a short-range connection, a long-range connection, or a combination of both a short-range and a long-range wireless telecommunications connection. When we refer to “short” and “long” types of connections, we do not mean to refer to the spatial relation between two devices. Instead, we are generally referring to short range and long range as different categories, or types, of connections (i.e., a primary connection and a secondary connection). A short-range connection may include, by way of example and not limitation, a Wi-Fi® connection to a device (e.g., mobile hotspot) that provides access to a wireless communications network, such as a WLAN connection using the 802.11 protocol; a Bluetooth connection to another computing device is a second example of a short-range connection, or a near-field communication connection. A long-range connection may include a connection using, by way of example and not limitation, one or more of CDMA, GPRS, GSM, TDMA, and 802.16 protocols. 
     It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     While specific elements and steps are discussed in connection to one another, it is understood that any element and/or steps provided herein is contemplated as being combinable with any other elements and/or steps regardless of explicit provision of the same while still being within the scope provided herein. Since many possible embodiments may be made of the disclosure without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.