PATENT DOCUMENT

Publication Number: US-10150669-B2
Application Number: US-201715460915-A
Country: US
Kind Code: B2

Title: Micro pick up array pivot mount

Abstract:
Systems and methods for aligning a transfer head assembly with a substrate are disclosed. In an embodiment a pivot mount is used for generating a feedback signal in a closed-loop motion control system. In an embodiment, the pivot mount includes a plurality of spring arms, with each spring arm including a switch-back along an axial length of the spring arm such that a pair of first and second lengths of the spring arm are immediately adjacent the switch-back and are parallel to each other. A first strain sensing element is located at the first length, and a second strain sensing element is located at the second length.

Claims:
What is claimed is: 
     
       1. A pivot mount comprising:
 a pivot platform; 
 a base; 
 a plurality of spring arms, each spring arm fixed to the pivot platform at a corresponding inner root, and fixed to the base at a corresponding outer root; 
 wherein each spring arm includes a switch-back along an axial length of said each spring arm such that a pair of first and second lengths of said each spring arm immediately adjacent the switch-back are parallel to each other. 
 
     
     
       2. The pivot mount of  claim 1 , wherein the switch-back is an inner switch-back along an inner length of said each spring arm, and further comprising an outer switch-back along an outer length of said each spring arm. 
     
     
       3. The pivot mount of  claim 1 , wherein the plurality of spring arms comprises three or more spring arms. 
     
     
       4. The pivot mount of  claim 3 , wherein the switch-back is an inner switch-back along an inner length of said each spring arm, and further comprising an outer switch-back along an outer length of said each spring arm. 
     
     
       5. The pivot mount of  claim 1 , wherein the inner root is perpendicular to an inner length of said each spring arm extending from the pivot platform, and the outer root is perpendicular to an outer length of said each spring arm extending from the base. 
     
     
       6. The pivot mount of  claim 1 , wherein the pivot platform is movable relative to the base in a direction orthogonal to a contact surface of the pivot platform, wherein movement of the pivot platform in the direction orthogonal to the contact surface of the pivot platform causes a normal strain that is characterized as being parallel to the axial length of said each spring arm at the first and second lengths of said each spring arm. 
     
     
       7. The pivot mount of  claim 6 , wherein the normal strain comprises opposite signs on the first and second lengths of said each spring arm. 
     
     
       8. The pivot mount of  claim 7 , further comprising an electrostatic clamp contact on the pivot platform. 
     
     
       9. The pivot mount of  claim 8 , further comprising a first wiring that connects the electrostatic clamp contact to an electrical connection on the base. 
     
     
       10. The pivot mount of  claim 9 , wherein the first wiring runs along a first spring arm of the plurality of spring arms. 
     
     
       11. The pivot mount of  claim 9 , further comprising a voltage contact electrode on the pivot platform. 
     
     
       12. The pivot mount of  claim 11 , further comprising a second wiring that connects the voltage contact electrode to the electrical connection on the base. 
     
     
       13. The pivot mount of  claim 12 , wherein the second wiring runs along a second spring arm of the plurality of spring arms. 
     
     
       14. The pivot mount of  claim 12 , wherein the first wiring and the second wiring run along a same spring arm of the plurality of spring arms. 
     
     
       15. The pivot mount of  claim 12 , wherein the electrical connection is a flex circuit along an edge of the base.

Description:
RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 14/303,483, filed Jun. 12, 2014, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     The present invention relates to micro devices. More particularly embodiments relate to a micro pick up array pivot mount with integrated strain sensing elements for aligning an electrostatic transfer head array with a target substrate. 
     Background Information 
     The feasibility of commercializing miniature devices such as radio frequency (RF) microelectromechanical systems (MEMS) microswitches, light-emitting diode (LED) display systems, and MEMS or quartz-based oscillators is largely constrained by the difficulties and costs associated with manufacturing those devices. Miniaturized device manufacturing processes typically include processes in which miniaturized devices are transferred from one wafer to another. In one such implementation, a transfer wafer may pick up an array of miniaturized devices from a donor wafer and bond the miniaturized devices to a receiving wafer. Methods and apparatuses for aligning two flat surfaces in a parallel orientation have been described, and may be applied to miniaturized device transfer. 
     SUMMARY 
     A pivot mount and transfer tool are described. In an embodiment a pivot mount includes a pivot platform, a base, and a plurality of spring arms. Each spring arm is fixed to the pivot platform at a corresponding inner root, and fixed to the base at a corresponding outer root. Each spring arm also includes one or more switch-backs along an axial length of the spring arm such that a pair of first and second lengths of the spring arm immediately adjacent a switch-back are parallel to each other. A first strain sensing element may be located at the first length of the spring arm, and a second strain sensing element may be located at the second length of the spring arm. Likewise, a first reference gage may be located adjacent the first strain sensing element at the first length, and a second reference gage may be located adjacent the second strain sensing element at the second length. For example, the strain sensing elements may be strain gages that are bonded to the spring arm, deposited on the spring arm, or doped regions in the spring arm. In an embodiment, the plurality of spring arms includes three or more spring arms. In an embodiment, the one or more switch-backs includes an inner switch-back along an inner length of a spring arm, and an outer-switchback along an outer length of the spring arm. 
     In an embodiment, the inner root is perpendicular to an inner length of the spring arm extending from the pivot platform, and the outer root is perpendicular to an outer length of the spring arm extending from the base. In an embodiment, the pivot platform is movable relative to the base in a direction orthogonal to a contact surface of the pivot platform, and movement of the pivot platform in the direction orthogonal to the contact surface causes a normal strain at the surface of the spring arm that is characterized as being parallel to the axial length of the spring arm at the first and second lengths of the spring arm. In an embodiment, the normal strain at the surface of the spring arm is of opposite sign on the first and second lengths of the spring arm. 
     In an embodiment, a pivot mount includes a pivot platform with a plurality of compliant voltage contacts, a base, and a plurality of spring arms in which each spring arm is fixed to the pivot platform at a corresponding inner root and fixed to the base at a corresponding outer root. In an embodiment, each compliant voltage contact is at least partially formed by a channel extending through the pivot platform. The compliant voltage contacts may assume a variety of configurations including a winding contour and switch-back. In an embodiment, the pivot mount includes a clamping electrode on the pivot platform. Each compliant voltage contact may protrude from the pivot platform such that they are raised above the pivot platform and clamping electrode. 
     In an embodiment, any of the pivot mounts described above may be included in a transfer tool, including an articulating transfer head assembly, and a micro mick up array mounted onto the pivot platform of the pivot mount. The micro pick up array may include a plurality of electrostatic transfer heads. In an embodiment, the pivot platform includes a plurality of compliant voltage contacts as described above. The micro pick up array may include a plurality of voltage contacts arranged to mate with the plurality of compliant voltage contacts of the pivot platform. In an embodiment each electrostatic transfer head has a localized contact point characterize by a maximum dimension of 1-100 μm in both the x- and y-dimensions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is perspective view illustration of a mass transfer tool in accordance with an embodiment. 
         FIG. 2  is a perspective view illustration of a micro pick up array and pivot mount mounted onto a transfer head assembly in accordance with an embodiment. 
         FIG. 3  is an exploded cross-sectional side view illustration of a transfer head assembly, pivot mount, and micro pick up array in accordance with an embodiment. 
         FIG. 4  is a schematic top view illustration of a micro pick up array in accordance with an embodiment. 
         FIGS. 5A-5E  are cross-sectional side view illustrations of a method of forming a pivot mount including compliant voltage contacts in accordance with an embodiment. 
         FIGS. 6-7  are top view illustrations of various structural features of a pivot mount in accordance with an embodiment. 
         FIG. 8A  is a top view illustration of a pivot mount including electrical routing in accordance with an embodiment. 
         FIG. 8B  is a close up view illustration of Detail A in  FIG. 8A  in accordance with an embodiment. 
         FIG. 9A  is an illustration of strain components in a body. 
         FIG. 9B  is an illustration of strain components in a thin structure. 
         FIG. 10A  is an illustration of a spring arm under pure bending in accordance with an embodiment. 
         FIG. 10B  is an illustration of a spring arm under simultaneous bending and torsion in accordance with an embodiment. 
         FIG. 11A  is a perspective view illustration of a pivot platform of pivot mount  300  deflected with a uniform z displacement in accordance with an embodiment. 
         FIG. 11B  is a perspective view illustration of strain modeling for normal strain in the x direction for a pivot platform deflected with a uniform z displacement in accordance with an embodiment. 
         FIG. 11C  is a perspective view illustration of strain modeling for normal strain in the y direction for a pivot platform deflected with a uniform z displacement in accordance with an embodiment. 
         FIG. 11D  is a perspective view illustration of strain modeling for equivalent strain magnitude for a pivot platform deflected with a uniform z displacement in accordance with an embodiment. 
         FIG. 11E  is a perspective view illustration of strain modeling for surface shear strain for a pivot platform deflected with a uniform z displacement in accordance with an embodiment. 
         FIG. 11F  is a top view illustration of a pivot mount including eight correlated strain sensors in accordance with an embodiment. 
         FIG. 12  is top view illustration of a pivot mount in accordance with an embodiment. 
         FIG. 13A  is a schematic illustration of a control scheme for regulating a transfer head assembly in accordance with an embodiment. 
         FIG. 13B  is a schematic illustration of a method of generating a synthesized output signal in accordance with an embodiment. 
         FIG. 13C  is a schematic illustration of a method of generating a synthesized output signal in accordance with an embodiment. 
         FIGS. 13D-13E  are schematic illustrations of methods of generating synthesized output signals in accordance with embodiments. 
         FIG. 14  is a flowchart illustrating a method of aligning a micro pick up array relative to a target substrate in accordance with an embodiment. 
         FIG. 15  is a schematic illustration of a computer system in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe a pivot mount including a base, a pivot platform, and plurality of spring arms, each spring arm including a switch-back along an axial length of the spring arm such that a pair of first and second lengths of the spring arm immediately adjacent the switch-back are parallel to each other. A first strain sensing element is located at the first length, and a second strain sensing element is located at the second length of the spring arm. In this manner, when the pivot mount is moved in a direction orthogonal to a contact surface of the pivot platform normal strains of opposite sign are created at the surface of the spring arm on the first and second lengths of each spring arm. 
     The pivot mount can be coupled to an articulating head assembly of a mass transfer tool for accurate and repeatable alignment in 6 spatial degrees of freedom between the transfer tool and a target substrate. When accurately aligning two planar surfaces, lateral (x and y) and rotational (θz) alignments are relatively straightforward to achieve through use of a high-precision x-y stage and rotationally-positioned substrate chucks. The remaining three degrees of freedom, θx, θy, (or, “tilt” and “tip”) and z are difficult to independently control. Any changes to the tip and tilt angle necessarily change the distance z to any point not located at the center of rotation. While parallelism between two planes can be accomplished through use of a passive pivot mount, the pressure distribution between the two planar surfaces will not be centered or uniform unless the two surfaces were parallel to begin with. A transfer tool including a pivot mount in accordance with embodiments described herein may redistribute the pressure distribution to achieve a uniform pressure field. By placing strain sensing elements (strain gages) at high-strain locations on the pivot mount spring arms, a feedback signal of the position error can be generated and input to the transfer tool for operation in a closed-loop motion control system. Because strain is related to the stress state through Hooke&#39;s Law, both displacement and forces acting on the pivot mount can be known by measuring strain. 
     In one aspect, embodiments describe a pivot mount configuration that achieves a high strain sensing sensitivity and generates a feedback signal with a high signal to noise ratio. As a result the pivot mount can provide a position feedback signal with increased effective resolution to the transfer tool. By locating the strain sensing elements on opposite sides of switch-backs in an axial length of a spring arm, equal and opposite strain responses are measured. In this manner a strain signal for a given platform displacement may be effectively doubled. Such a configuration can also reduce noise for a given strain signal. Due to the differential sensing at the first and second lengths adjacent the switch-back the measured noise is effectively canceled. Accordingly, higher strain sensing sensitivity may be accomplished with a higher signal to noise ratio, and an increased effective resolution of the position feedback signal may be provided to the transfer tool. 
     In another aspect, embodiments describe pivot mount spring arm configurations that minimize the torsion applied to a spring arm at the roots where a spring arm is fixed to a pivot platform at one end and fixed to a base at another end. This creates a more uniform bending moment in the high strain regions of the spring arm with reduced strain variation and torsion in the spring arms, which allows the strain sensing elements to be located in the high strain regions near the roots. By comparison, in other configurations with spring arms that undergo both bending and torsional loading, the area of maximum strain may include both bending and torsion. Torsion in the spring arms is parasitic to surface strain sensing since it manifests as strain at the surface of the spring arm having components in both the x and y directions. Because the total strain energy distributed through the spring arms is constant for a given pivot platform displacement, the presence of strain components perpendicular to the strain sensing elements reduces the ratio of strain components that are aligned with the strain sensing elements. As a result, strain sensing elements located near areas of torsion may produce a lower effective feedback signal and sensitivity. In an embodiment, a pivot mount is arranged to create boundary conditions at the roots of the spring arms with a uniform bending moment, in which strain is substantially perpendicular to the roots and substantially parallel to strands in the strain sensing elements, which may be parallel to axial lengths of the spring arms in the high strain regions. Such a configuration directs substantially all of the strain energy from a given pivot platform displacement into strain components aligned with the strain sensing elements. As a result, higher strain may be measured and sense feedback signal strength may be increased for a given pivot platform displacement. Reduction of the torsional moment at the roots may additionally allow more freedom in stiffness requirements of the spring arms. In turn, reduced stiffness requirements allow for longer axial length of the spring arms in the same available real estate within the pivot mount, and consequently greater bending, resulting in increased normal strain at the surface of the spring arms and sense feedback signal strength. 
     In another aspect, reduction of the torsional moment applied to the spring arms at the roots may also increase the effectiveness of the reference gage(s) positioned adjacent the strain sensing elements. In an embodiment, each strain sensing element is located in a high strain region of a spring arm that sees only normal strain at the surface in the gage direction of the strain sensing element and sees no normal strain at the surface lateral to the gage direction. This allows the location of a reference strain gage adjacent to each strain sensing element with the result that the reference gages do not see strain caused by mechanical loading of the pivot platform. This in turn allows the reference gages to compensate for temperature variations in the system, and increase the signal to noise ratio. Since the strain sensing elements and reference gages are adjacent, they are exposed to the same temperature, meaning the thermal strain is identical in both a strain sensing element and a corresponding reference gage. Since the reference strain gages are not subjected to strain resulting from mechanical load, any strain signal they produce can be attributed to temperature (as noise), which is then subtracted as background noise from the strain measured by the adjacent strain sensing element. In an embodiment, strands in the reference gages are oriented perpendicular to strands in the strain sensing elements. In such a configuration, the normal strain at the surface of the spring arms is substantially parallel to the strands in the strain sensing elements, and perpendicular to the strands in the reference strain gages. Thus, by reducing the torsional moment and creating uniform bending moments in the spring arms in which normal strain at the surface of the spring arms is substantially perpendicular to the roots, the reference strain gages may be more accurate and a higher strain sensing sensitivity may be accomplished with a higher signal to noise ratio. 
     In another aspect, embodiments describe an arrangement of strain sensing elements into distributed, correlated sensors. In this manner, the loss of a strain sensing element or sensor does not prohibit use of the pivot mount, and the lifetime of the pivot mount use with a transfer tool can be extended. In an embodiment, each sensor includes one or more correlation sensors. For example, a correlated pair may each sense a same z-deflection. In another situation, a correlated pair may sense a same or equal but opposite θx, θy, (or, “tilt” and “tip”). In either situation, the loss of one of the correlated sensor may reduce the overall signal to noise ratio generated from the pivot platform, yet the remaining signal to noise ratio remains adequate for operation of the transfer tool. 
     In yet another aspect, embodiments describe a pivot mount with compliant voltage contacts, for providing a low contact resistance connections of the voltage contacts to a micro pick up array (MPA) that is mounted onto the pivot platform of the pivot mount. The compliant voltage contacts may protrude from the pivot platform such that they are elevated above the pivot platform, yet are compliant such that they exert a pressure upon the MPA contacts when the MPA is clamped onto the pivot mount pivot platform, for example, using an electrostatic clamp contact on the pivot mount platform. 
     Referring to  FIG. 1 , a perspective view of a mass transfer tool is shown. Mass transfer tool  100  may include a transfer head assembly  200  for picking up an array of micro devices from a carrier substrate held by a carrier substrate holder  104  and for transferring and releasing the array of micro devices onto a receiving substrate held by a receiving substrate holder  106 . Embodiments of mass transfer tool  100  are described in U.S. patent application Ser. No. 13/607,031, titled “Mass Transfer Tool”, filed on Sep. 7, 2012. Operation of mass transfer tool  100  and transfer head assembly  200  may be controlled at least in part by a computer  108 . Computer  108  may control the operation of transfer head assembly  200  based on feedback signals received from various sensors (e.g. strain sensing elements, reference gages) located on a pivot mount. For example, transfer head assembly  200  may include an actuator assembly for adjusting an associated MPA  103  with at least three degrees of freedom, e.g., tipping, tilting, and movement in a z direction, based on feedback signals received from sensors associated with a pivot mount that carries MPA  103 . Similarly, the carrier substrate holder  104  and receiving substrate holder  106  may be moved by an x-y stage  110  of mass transfer tool  100 , having at least two degrees of freedom, e.g., along orthogonal axes within a horizontal plane. Additional actuators may be provided, e.g., between mass transfer tool  100  structural components and transfer head assembly  200 , carrier substrate holder  104 , or receiving substrate holder  106 , to provide movement in the x, y, or z direction for one or more of those sub-assemblies. For example, a gantry  112  may support transfer head assembly  200  and move transfer head assembly  200  along an upper beam, e.g., in a direction parallel to an axis of motion of x-y stage  110 . Thus, an array of electrostatic transfer heads on MPA  103 , supported by transfer head assembly  200 , and an array of micro devices supported by a carrier substrate held by carrier substrate holder  104  may be precisely moved relative to each other within all three spatial dimensions. 
     Referring to  FIG. 2 , a perspective view of a transfer head assembly  200  is shown in accordance with an embodiment. A transfer head assembly  200  may be used in combination with mass transfer tool  100  to transfer micro devices to or from a substrate, e.g., receiving substrate or carrier substrate, using MPA  103  which is supported by a pivot mount  300 . More particularly, transfer head assembly  200  may provide for negligible lateral or vertical parasitic motion for small movements of MPA  103 , e.g., motion less than about 5 mrad about a neutral position. Accordingly, transfer head assembly  200  may be incorporated in mass transfer tool  100  to adjust an MPA  103  relative to mass transfer tool  100 . Thus, transfer head assembly  200  may be fixed to a chassis of mass transfer tool  100 , e.g., at a location along an upper beam or support. 
     As illustrated, the pivot mount  300  may include a base  302 , a pivot platform  304 , and plurality of spring arms  306 , and the MPA  103  supporting a transfer head array  115  is mounted on the pivot platform  304 . In an embodiment, the transfer head array  115  is an electrostatic transfer head array  115 , where each transfer head operates in accordance with electrostatic principles to pick up and transfer a corresponding micro device. In an embodiment each electrostatic transfer head has a localized contact point characterized by a maximum dimension of 1-100 μm in both the x- and y-dimensions. In an embodiment, the pivot mount  300  may communicate and send feedback signals to the mass transfer tool  100  through one or more electrical connections, such as a flex circuit  308 . As described below, feedback may include analog signals from strain sensing elements that are used in a control loop to regulate actuation and spatial orientation of the transfer head assembly  200 . In an embodiment, the feedback signals are sent to a position sensing module located near the pivot mount  300  to reduce signal degradation by limiting a distance that analog signals must travel from a strain sensing element to the position sensing module. In an embodiment, the position sensing module is located within the transfer head assembly  200 . 
     Referring now to  FIG. 3 , an exploded cross-sectional side view illustration is provided of a transfer head assembly  200 , pivot mount  300 , and MPA  103 . Generally, the pivot mount  300  is mounted onto the transfer head assembly  200 . This may be accomplished using a variety of manners such as using tabs or lips to press the pivot mount against the transfer head assembly  200 , bonding, vacuum, or electrostatic clamping. A deflection cavity  202  may be formed in the transfer head assembly  200  to allow a specified z-deflection distance of the pivot platform  304  along the z-axis. 
     As illustrated in  FIG. 3 , the pivot mount  300  may include channels  310  formed through a body of the pivot mount from a front surface  312  to back surface  314 . Channels  310  may be used for form a variety of compliant features of the pivot mount  300 , including defining the spring arms  306  and pivot platform  304 , as well as the compliant voltage contacts  316 , described in more detail in the following description. The compliant voltage contacts  316  may provide a low contact resistance connection to voltage contacts  120  of the MPA  103 . In the embodiment illustrated, the compliant voltage contacts  316  protrude from the pivot platform such that they are raised above the pivot platform. Upon clamping the MPA  103  onto the pivot platform of the pivot mount  300  with the opposing electrostatic clamp contacts  318 ,  122 , the compliant voltage contacts  316  exert a pressure upon the MPA contacts  120 . Additional features may be located on or in the pivot mount  300 . For example, strain sensing elements  320  (strain gages) and reference gages  340  may be located at high strain regions of the spring arms  306 , as described in further detail in the following description. 
     Referring now to  FIG. 4 , a schematic top view illustration of a MPA  103  is shown in accordance with an embodiment. In an embodiment, an area of the electrostatic clamp contact  122  on a back side of the MPA is larger than an area of the transfer head array  115  on the front surface of the MPA. In this manner, the alignment and planarity across the transfer heads in the transfer head array  115  can be regulated by alignment of the transfer head assembly. In such an embodiment, a plurality of voltage contacts  120  for supplying an operating voltage to the transfer head array  115  is located outside the periphery of the transfer head array  115 . 
     Referring now to  FIGS. 5A-5E , cross-sectional side view illustrations are shown for a method of forming a pivot mount  300  including compliant voltage contacts  316 . The processing sequence may begin with a commercially available silicon wafer  301  including a top oxide layer  330 , and bottom oxide layer  332  as illustrated in  FIG. 5A . While the following description is made with regard to a silicon wafer, embodiments are not so limited, and other suitable substrates can be used to form pivot mount  300 , such a silicon carbide, aluminum nitride, stainless steel, and aluminum, amongst others. In an embodiment illustrated in  FIG. 5B , the top oxide layer  330  is then removed, with bottom oxide layer  330  remaining. Referring to  FIG. 5C , the top and bottom surfaces of the wafer  301  may then be oxidized further resulting in a top oxide layer  334 , and bottom oxide layer  336  that is thicker than the previous bottom oxide layer  332  and thicker than the top oxide layer  334 . For example, this may be accomplished with a wet thermal oxidation operation. Following the formation of oxide layers  334 ,  336  various layers may be formed over the top oxide layer  334  to form the strain gages  320 , reference gages  340 , electrostatic clamp contact(s)  318 , and electrodes  317  for the compliant voltage contacts. In an embodiment, these various layers may be formed by one or more metal deposition processes. In an embodiment, the electrodes  317  for the compliant voltage contacts are thicker than other metallization layers used to form the strain gages  320 , reference gages  340 , and electrostatic clamp contact(s)  318 . Referring to  FIG. 5E , the bottom oxide layer  336  is removed and channels  310  are etched through the silicon wafer  301  and top oxide layer  334  to define the spring arms  306 , pivot platform  304 , and compliant voltage contacts  316 . As shown in  FIG. 5E , the contact surfaces including the electrodes  317  for the compliant voltage contacts  316  protrude from the pivot platform such that they are elevated above the surrounding pivot platform, including the strain gages  320 , reference gages  340 , and electrostatic clamp contact(s)  318 . This may be the result of releasing residual stress within the silicon wafer  301  during formation of the channels  310 . In an embodiment, the residual stress was created in the silicon wafer  301  during the oxidation and removal operation described and illustrated in  FIGS. 5A-5C . In accordance with embodiments of the invention, the channels  310  forming the compliant voltage contacts  316  may assume a variety of configurations such as switch-backs or a winding contour. In an embodiment, the channels forming the compliant voltage contacts  316  are made in a spiral configuration which can achieve a high amount of compliance within a small area. 
     Referring again to  FIG. 4 , the voltage contacts  120  of the MPA  103  align with the compliant voltage contacts  316  in the pivot platform  304  of the pivot mount  300 . Once the MPA is clamped onto the pivot mount pivot platform, for example, using an electrostatic clamp contact on the pivot mount platform, the compliant voltage contacts  316  exert a pressure upon the MPA voltage contacts  120  to achieve low contact resistance connections. 
       FIGS. 6-8B  illustrate various structural aspects of a pivot mount  300 . Referring to  FIG. 6 , in an embodiment pivot mount  300  includes a base  302 , a pivot platform  304 , and a plurality of spring arms  306 . Each spring arm  306  is fixed to the pivot platform  304  at a corresponding inner root  350 , and fixed to the base at a corresponding outer root  352 . Each spring arm  306  includes at least one switch-back along an axial length  354  of the spring arm such that a pair of lengths of the spring arm adjacent the switch-back are parallel to each other. In the embodiment illustrated in  FIG. 6 , each spring arm  306  includes an inner switch-back  356  along an inner length of the spring arm and an outer switch-back  358  along an outer length of the spring arm. In an embodiment, an inner length  370  of the spring arm extending from the pivot platform  304  (along the axial length  354  of the spring arm  306 ) is perpendicular to the inner root  350 . In an embodiment, an outer length  372  of the spring arm extending from the base  302  (along the axial length  354  of the spring arm  306 ) is perpendicular to the outer root  352 . 
     Referring now to  FIG. 7 , each switch-back along the axial length of the spring arm results in a parallel pair of lengths of the spring arm adjacent the switch-back. For example, a portion of the spring arm immediately adjacent the outer switch-back  358  includes a first length  360  and a second length  362  of the spring arm that are parallel to each other. Similarly, a portion of the spring arm immediately adjacent the inner switch-back  356  includes a first length  364  and a second length  366  of the spring arm that are parallel to each other. A first strain sensing element may be located at the first length of the spring arm adjacent a switch-back, and a second strain sensing element may be located at the second length of the spring arm adjacent the switch-back. Furthermore, first reference gage may be located adjacent the first strain sensing element at the first length, and a second reference gage may be located adjacent the second strain sensing element at the second length. In the particular embodiment illustrated in  FIG. 7 , a first strain sensing element  320 A is located at the first length of  360  the spring arm adjacent the outer switch-back  358 , and a second strain sensing element  320 B is located at the second length  362  of the spring arm adjacent the outer switch-back  358 . Furthermore, first reference gage  340 A is located adjacent the first strain sensing element  320 A at the first length  360 , and a second reference gage  340 B is located adjacent the second strain sensing element  320 B at the second length  362 . In the particular embodiment illustrated in  FIG. 7 , a first strain sensing element  320 A is located at the first length of  364  the spring arm adjacent the inner switch-back  356 , and a second strain sensing element  320 B is located at the second length  366  of the spring arm adjacent the inner switch-back  356 . Furthermore, first reference gage  340 A is located adjacent the first strain sensing element  320 A at the first length  364 , and a second reference gage  340 B is located adjacent the second strain sensing element  320 B at the second length  366 . 
     Referring now to both  FIGS. 6 and 7 , in an embodiment the first and second lengths  364 ,  366  of the spring arm (along the axial length  354  of the spring arm  306 ) adjacent the inner switch-back  356  are perpendicular to the inner root  350 . In an embodiment, the first and second lengths  360 ,  362  of the spring arm (along the axial length  354  of the spring arm  306 ) adjacent the outer switch-back  358  are perpendicular to the outer root  352 . 
       FIG. 8A  is a top view illustration of a pivot mount including electrical routing in accordance with an embodiment. As illustrated, wiring can be routed on the top surface of the pivot mount for operation of various components. In an embodiment wiring  380  is provided for operation of the strain sensing elements  320  and reference gages  340 . In an embodiment wiring  382  is provided for operation of the electrostatic clamp contacts  318 . In an embodiment wiring  384  is provided for operation of the compliant voltage contacts  316 . In the particular embodiment illustrated, the wiring  384  connects with the electrodes  317  for the compliant voltage contacts  316 , where the electrodes form a spiral pattern within the spiral channels  310  forming the compliant voltage contacts. Wiring  380 ,  382 , and  384  can run over one or more portions of the pivot mount including the base  302 , spring arms  306 , and pivot platform  304 . Wiring  380 ,  382 , and  384  may be formed using a suitable technique such s sputtering or e-beam evaporation, or may be a wire that is bonded to the pivot mount. 
     Wiring  380 ,  382 , and  384  may be routed to an electrical connection, such as a flex circuit  308 , at an edge of the base  302  of the pivot mount. For example, an operating voltage can be applied trough the flex circuit  308  to operate the electrostatic clamp contacts  318  to clamp the MPA onto the pivot mount  300 . Another operating voltage can be applied through the flex circuit  308  to operate the compliant voltage contacts  316  which transfer an operational voltage to the array of electrostatic transfer heads in order to provide a grip pressure to pick up micro devices. Additionally, the flex circuit  308  can transfer the feedback signals from the strain sensing elements  320  and reference gages  340  to a position sensing module or computer  108  to regulate actuation and spatial orientation of the transfer head assembly  200 . 
     Referring now to  FIG. 8B , an enlarged view of Detail A from  FIG. 8A  is illustrated. In the particular embodiment illustrated the strain sensing elements  320  and reference gages  340  along the first and second lengths  364 ,  366  of the spring arm adjacent the inner switch-back  356  are shown in more detail. In an embodiment, strain sensing elements  320  may be strain gages that measure deformation of spring arm  306 . The strain gages may exhibit an electrical resistance that varies with material deformation. More specifically, the strain gages may deform when spring arm  306  deforms. That is, the strain gage design may be selected based on environmental and operating conditions associated with the transfer of micro devices from a carrier substrate, to achieve the necessary accuracy, stability, cyclic endurance, etc. Accordingly, the strain gages may be formed from various materials and integrated with the spring arm in numerous ways to achieve this goal. Several such embodiments are described below. 
     A strain gage may be separately formed from spring arm  306  and attached thereto. In an embodiment, the strain gage includes an insulative flexible backing that supports a foil formed from polysilicon and electrically insulates the foil from spring arm  306 . The foil may be arranged in a serpentine pattern, for example. An example of an attachable strain gage is a Series 015DJ general purpose strain gage manufactured by Vishay Precision Group headquartered in Malvern, Pa. A strain gage that is separately formed from spring arm  306  may be attached to spring arm  306  using numerous processes. For example, the strain gage backing may be directly attached to spring arm  306  with an adhesive or other bonding operation. More specifically, strain gage backing may be fixed to a surface of spring arm  306  using solder, epoxy, or a combination of solder and a high-temperature epoxy. 
     In another embodiment, a strain gage may be formed on spring arm  306  in a desired pattern, such as a serpentine pattern. In an embodiment, a strain gage may be formed directly on spring arm  306  using a deposition process. For example, constantan copper-nickel traces may be sputtered directly on spring arm  306  in a serpentine pattern. The dimensions of a strand of a sputtered strain gage having a serpentine pattern may be about 8 micron width with about an 8 micron distance between strand lengths and may be deposited to a thickness of about 105 nanometers. 
     In another embodiment, the material of spring arm  306  may be modified to form an integrated strain gage. More specifically, spring arm  306  may be doped with a piezoresistive material to create a strain gage within spring arm  306 . As an example, the surface of spring arm  306  may be doped silicon. The doped material may be in a serpentine pattern, having dimensions that vary with an applied strain. Thus, the strain gage may be fully integrated and physically indistinct from the remainder of spring arm  306 . 
     During the transfer of micro devices from a carrier substrate, spring arm  306  and strain sensing elements  320  may be subjected to elevated temperatures, and thus, temperature compensation may be necessary. In an embodiment, strain sensing element  320  (strain gage) may be self-temperature compensated. More specifically, strain gage material may be chosen to limit temperature-induced apparent strain over the operating conditions of the transfer process. However, in an alternative embodiment, other manners for temperature compensation may be used. For example, temperature compensation may be achieved using a reference gage technique. 
     In an embodiment, strain sensing element  320  may be a strain gage on spring arm  306  having a pattern (e.g. serpentine) of lengthwise strands that align in a direction of anticipated normal strain at the surface of the spring arm. Still referring to  FIG. 8B , in an embodiment, a reference gage technique utilizes a reference gage  340  to compensate for strain sensing element  320 . More particularly, reference gage  340  may be located adjacent strain sensing element  320  in the same area of strain. While strands of strain sensing element  320  may align with the direction of applied strain, strands of reference gage  340  may extend perpendicular to the strands of strain sensing element  320  and to the direction of applied strain. Alternatively, reference gage  340  may be located in a non-strain area of the pivot mount  300 , apart from strain sensing element  320 , which is located in a high strain area of spring arm  306 . For example, reference gage  340  may be located on base  302  or pivot platform  304 . In each configuration, strain sensing element  320  detects a strain applied to spring arm  306  and reference gage  340  detects strain from thermal effects on the pivot mount  300 . Accordingly, a comparison of strain in the strain sensing element  320  and reference gage  340  may be used to determine, and compensate for, strain related to thermal expansion of spring arm  306 . 
     In particular, the strands  341  in the references gages  340  are oriented perpendicular to strands  321  in the strain sensing elements  320 . As will become more apparent in the following description, the normal strain at the surface that results at the first and second lengths  364 ,  366  of the spring arm during operation of the pivot mount is substantially parallel to the strands  321  in the strain sensing elements, and perpendicular to the strands  341  in the reference strain gages. Similar strain relationships are found at all of the inner switch-backs  356  and outer switch-backs  358 , wherein normal strain at the surface that occurs during operation of the pivot mount is substantially parallel to the strands in the strain sensing elements  320 . 
     Referring now to  FIG. 9A , strain at any point in a body may be described by nine strain components. These include three normal strains (εx, εy, εz) and six shear strain components (εxy, εxz, εyx, εyz, εzx, and εzy). Strain components in a thin structure are illustrated in  FIG. 9B . For a thin pivot mount structure shear strains on the surface (εzx and εzy) and out-of-plane normal strain (εz) are not significant. This idealization is known as plane stress. Accordingly, in an embodiment the strain gages (strain sensing elements and reference gages) on the surface of the pivot mount will measure components of the normal strains εx and εy. In an embodiment, the pivot mount includes regions of strain loaded only in either pure εx or pure εy and directs substantially all available strain into measurable strain. 
     Referring now to  FIGS. 10A-10B , the idealization of plane stress is illustrated as realized in accordance with embodiments.  FIG. 10A  is an illustration of a spring arm under pure bending in accordance with an embodiment. In such an embodiment, the spring arm undergoing pure bending may have a single normal strain component aligned with the spring arm axial length. A reference gage  340  may be oriented perpendicular to the spring arm axial length and not measure any strain due to bending.  FIG. 10B  is an illustration of a spring arm in both bending and torsion. In such a configuration, normal strain components and shear strain components are produced in multiple directions. In this case both the strain gage  320  and reference gage  340  may measure non-zero strain, which may reduce the ability of the reference gage  340  to compensate for temperature changes. 
     In order to illustrate strain confinement within the pivot mount, a pivot mount with a uniform z displacement of the pivot platform  304  is illustrated in  FIGS. 11A-11E  along with modeling data for strain fields located within the pivot mount. Referring to  FIG. 11A , a pivot platform  304  of pivot mount  300  is deflected with a uniform z displacement. Such deflection may be typical during a normal pick and place operation with the mass transfer tool, though the amount of deformation illustrated in  FIG. 11A  is exaggerated for illustrational purposes. In the particular embodiment illustrated in  FIG. 11A , the spring arm  306  along the first length  360  the spring arm adjacent the outer switch-back  358  and the first length  364  of the spring arm adjacent the inner switch-back  356  has a negative curvature and is in a condition of negative (compressive) normal strain at the surface. In the particular embodiment illustrated in  FIG. 11A , the spring arm  306  along the second length  362  the spring arm adjacent the outer switch-back  358  and the second length  366  of the spring arm adjacent the inner switch-back  356  has a positive curvature and is in a condition of positive (tensile) normal strain at the surface. 
     In accordance with embodiments of the invention, a pivot mount structure achieves a high strain sensing sensitivity and generates a feedback signal with a high signal to noise ratio by locating strain sensing elements on opposite sides of switch-backs in an axial length of a spring arm, where equal and opposite strain responses are measured. In this manner, strain signal for a given platform displacement may be effectively doubled, while also reducing noise for a given strain signal since the differential sensing can be used to effectively cancel the noise. 
     Referring to  FIG. 11B , modeling data is provided for the z displacement illustrated in  FIG. 11A  illustrating normal strain at the outer surface of the pivot mount in the x direction, εx. As illustrated, each spring arm  306  includes an outer switch-back  358  oriented in the y-direction, and an inner switch-back oriented in the x-direction. Of course, simply rotating the pivot mount reverses the orientations of the switch-backs in the x- and y-directions. Importantly, when in the condition of uniform z displacement, the high εx strain regions are located along the spring arms adjacent the inner switch-backs  356 , while minimal or no εx strain is located along the spring arms adjacent the outer switch-backs  358 . Some amount of localized strains are found at various locations within the pivot mount due to local stress concentrations, however these do not affect the strain measurement because the strain gages are located away from the localized strain regions  365 . As shown in the  FIG. 11B , the spring arm  306  along the first length  364  of the spring arm adjacent the inner switch-back  356  has a negative curvature and is in a condition of negative (compressive) normal strain at the surface, and the second length  366  of the spring arm adjacent the inner switch-back  356  has a positive curvature and is in a condition of positive (tensile) normal strain at the surface. Furthermore, the negative normal strain at first length  364  and positive normal strain at second length  366  are equal and opposite. 
     Referring to  FIG. 11C , modeling data is provided for the z displacement illustrated in  FIG. 11A  illustrating normal strain at the outer surface of the pivot mount in the y direction, εy. When in the condition of uniform z displacement, the high εy strain regions are located along the spring arms adjacent the outer switch-backs  358 , while minimal or no εy strain is located along the spring arms adjacent the inner switch-backs  356 . Some amount of localized strains are found at various locations within the pivot mount due to local stress concentrations, however these do not affect the strain measurement because the strain gages are located away from the localized strain regions  367 . As shown in the  FIG. 11C , the spring arm  306  along the first length  360  of the spring arm adjacent the outer switch-back  358  has a negative curvature and is in a condition of negative (compressive) normal strain, and the second length  362  of the spring arm adjacent the outer switch-back  358  has a positive curvature and is in a condition of positive (tensile) normal strain. Furthermore, the negative normal strain at first length  360  and positive normal strain at second length  362  are equal and opposite. 
       FIG. 11D  is an illustration of modeling data for equivalent strain magnitude at the outer surface of the pivot mount in both εx and εy for the z displacement illustrated in  FIG. 11A . As shown, substantially equal strain magnitudes are measured at the first and second lengths for both the inner and outer switchbacks.  FIG. 11E  is an illustration of modeling data for shear strain at the outer surface of the pivot mount for the z displacement illustrated in  FIG. 11A . As illustrated, there is substantially no measurable shear strain at the surface. Thus, the modeling data provided in  FIGS. 11A-11E  illustrates a pivot mount configuration with substantially uniform bending moments in the high strain regions of the spring arms. 
     Strain sensing elements  320  and reference gages  340  may be arranged into sensors so that the resulting sensor signals are correlated. A set of sensors is considered correlated, or dependent, if the signal of a missing or broken gage in the sensor may be approximated from the remaining set of signals. A minimum set of independent strain signals equal to the number of desired position measurements is required to calculate those measurements. Correlated strain signals in excess of the minimum required set may be included in the position calculation and used to improve the signal to noise ration of the measurement. If a strain gage ( 320 ,  340 ) or sensor failure occurs the calculation may be adjusted to maintain position output albeit with a reduced signal to noise ratio. In this way correlated signals provide redundancy as well as an improved signal to noise ratio. Referring to  FIG. 11F , a pivot mount including eight correlated strain sensors is illustrated in accordance with an embodiment of the invention. Specifically,  FIG. 11F  is an exemplary illustration similar to  FIG. 8A  described above, including 16 total strain sensing elements  320  (strain gages) and 16 total reference gages  340 . In such a configuration, a pair of strain sensing elements (strain gages) and references gauges on opposite sides of a switch-back may correspond to a single strain sensor. As previously described, these pairs of strain sensing elements  320  on opposite sides of a switch-back measure opposite strain types, of equal magnitude. Accordingly, in addition to the following discussion, these strain gages (as well as the corresponding reference gages  340 ) can also be considered correlated sensors. The strain sensors illustrated in  FIG. 11E  may be linearly dependent sets (correlated pairs) depending upon whether the pivot platform is rotated about the x-axis, rotated about the y-axis, or is subjected to a vertical displacement. Table I below describes certain correlated pairs of the exemplary embodiment. 
     
       
         
           
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Correlated pair strain sensors 
               
               
                   
               
             
            
               
                 Under rotation about the x axis 
               
               
                   
               
               
                 signal 1 = signal 2 
               
               
                 signal 3 = signal 4 
               
               
                  signal 5 = −signal 6 
               
               
                  signal 7 = −signal 8 
               
               
                   
               
               
                 Under rotation about the y-axis 
               
               
                   
               
               
                  signal 1 = −signal 2 
               
               
                  signal 3 = −signal 4 
               
               
                 signal 5 = signal 6 
               
               
                 signal 7 = signal 8 
               
               
                   
               
               
                 Under vertical displacement 
               
               
                   
               
               
                 signal 1 = signal 2 
               
               
                 signal 3 = signal 4 
               
               
                 signal 5 = signal 6 
               
               
                 signal 7 = signal 8 
               
               
                   
               
            
           
         
       
     
     In the above exemplary embodiment, several correlated pairs are described for an 8 channel (signal) operation, with each channel corresponding to a signal produced by a pair of strain gages and references gages adjacent a switch-back. Under normal operation, the feedback signal produced by the exemplary pivot mount operating under normal operation can be converted into a synthesized output signal by transformation matrix equation (1): 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             θ 
                             x 
                           
                         
                       
                       
                         
                           
                             θ 
                             y 
                           
                         
                       
                       
                         
                           Z 
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             1 
                           
                           
                             1 
                           
                           
                             
                               - 
                               1 
                             
                           
                           
                             
                               - 
                               1 
                             
                           
                           
                             1 
                           
                           
                             
                               - 
                               1 
                             
                           
                           
                             1 
                           
                           
                             
                               - 
                               1 
                             
                           
                         
                         
                           
                             1 
                           
                           
                             
                               - 
                               1 
                             
                           
                           
                             1 
                           
                           
                             
                               - 
                               1 
                             
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             
                               - 
                               1 
                             
                           
                           
                             
                               - 
                               1 
                             
                           
                         
                         
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                       
                       ] 
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           
                             
                               S 
                               1 
                             
                           
                         
                         
                           
                             
                               S 
                               2 
                             
                           
                         
                         
                           
                             
                               S 
                               3 
                             
                           
                         
                         
                           
                             
                               S 
                               4 
                             
                           
                         
                         
                           
                             
                               S 
                               5 
                             
                           
                         
                         
                           
                             
                               S 
                               6 
                             
                           
                         
                         
                           
                             
                               S 
                               7 
                             
                           
                         
                         
                           
                             
                               S 
                               8 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     While embodiments of pivot mounts have been described thus far in a square configuration, with switch-backs located along the x-direction or y-direction, embodiments are not so limited. Indeed, the strain sensing elements and reference gages can be located along a number of directions. A generalized transformation matrix for converting a pivot mount feedback signal to a synthesized output signal is represented in equation (2) for n strain signal inputs to 3 position measurement outputs (e.g. tilt, tip, z): 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             Out 
                             1 
                           
                         
                       
                       
                         
                           
                             Out 
                             2 
                           
                         
                       
                       
                         
                           
                             Out 
                             3 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               A 
                               1 
                             
                           
                           
                             
                               A 
                               2 
                             
                           
                           
                             
                               A 
                               3 
                             
                           
                           
                             … 
                           
                           
                             
                               A 
                               n 
                             
                           
                         
                         
                           
                             
                               B 
                               1 
                             
                           
                           
                             
                               B 
                               2 
                             
                           
                           
                             
                               B 
                               3 
                             
                           
                           
                             … 
                           
                           
                             
                               B 
                               n 
                             
                           
                         
                         
                           
                             
                               C 
                               1 
                             
                           
                           
                             
                               C 
                               2 
                             
                           
                           
                             
                               C 
                               3 
                             
                           
                           
                             … 
                           
                           
                             
                               C 
                               n 
                             
                           
                         
                       
                       ] 
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           
                             
                               S 
                               1 
                             
                           
                         
                         
                           
                             
                               S 
                               2 
                             
                           
                         
                         
                           
                             
                               S 
                               3 
                             
                           
                         
                         
                           
                             … 
                           
                         
                         
                           
                             
                               S 
                               n 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In an embodiment illustrated in  FIG. 12 , a pivot mount  300  includes a base  302 , pivot platform  304 , and spring arms  306 . Each spring arm  306  is fixed to the pivot platform  304  at a corresponding inner root  350 , and fixed to the base at a corresponding outer root  352 . Each spring arm  306  includes at least one switch-back along an axial length  354  of the spring arm such that a pair of lengths of the spring arm adjacent the switch-back are parallel to each other. In the embodiment illustrated in  FIG. 12 , each spring arm  306  includes an inner switch-back  356  along an inner length of the spring arm and an outer switch-back  358  along an outer length of the spring arm. In an embodiment, an inner length  370  of the spring arm extending from the pivot platform  304  (along the axial length  354  of the spring arm  306 ) is perpendicular to the inner root  350 . In an embodiment, an outer length  372  of the spring arm extending from the base  302  (along the axial length  354  of the spring arm  306 ) is perpendicular to the outer root  352 . The pivot mount illustrated in  FIG. 12  differs from other embodiments of pivot mounts described herein in that the spring arms  306  are arranged in a generally equilateral triangular configuration, rather than a generally square configuration. As a result, strain measured is not located within the only the εx and εy directions. Nevertheless, the same results of equal and opposite strain, uniform bending moments in the high strain regions, and distributed, correlated pairs is achieved. Accordingly, while embodiments of the invention have been described specific to creating and measuring strain in the εx and εy directions, embodiments are not so limited, and pivot mount feedback signals may be converted into a synthesized output signal for a variety of geometries. 
     In accordance with embodiments of the invention, the transfer head assembly  200  may adjust the orientation of the MPA  103  until a desired amount of and/or a desired distribution of pressure across pivot mount  300  is sensed by the pivot mount  300  strain sensing elements  320 . Thus, the transfer head array  115  on MPA  103  may be actively aligned with an array of micro devices on a mating substrate. For example, the spatial orientation representing alignment may be predetermined to include a plane passing through the transfer head array  115  being parallel to a plane passing through the array of micro devices. Alternatively, the spatial orientation representing alignment may include the planes not being parallel, but rather, being in some predetermined mutual orientation, such as angled such that only a portion of the transfer head array  115  make contact with respective micro devices when the arrays are brought together. More particularly, the spatial orientation representing alignment of the transfer head array  115  with the array of micro devices may be any predetermined spatial orientation. Such spatial orientation may be monitored, sensed, and measured to determine system characteristics such as distribution of pressure across pivot mount  300 . Thus, the measured system characteristics may be used as a proxy to represent alignment. Active alignment may increase the transfer rate of micro devices, since fine-alignment may be accomplished while picking up, and similarly while releasing, the micro devices. Furthermore, active alignment may be made on-the-fly without parasitic translation of the transfer head array  115  that may otherwise smear and damage the array of micro devices. Such on-the-fly adjustments may be useful when a donor substrate, e.g., carrier substrate, and/or a display substrate, e.g., receiving substrate, include surface irregularities and non-planar contours. 
     Referring to  FIG. 13A , a schematic illustration of a control scheme for regulating a transfer head assembly is shown in accordance with an embodiment. More particularly, the control loop may include multiple sub-loops that process a combination of position and strain inputs. The actuators of transfer head assembly may be driven by the sub-loops, first toward an initial desired location, and if contact between MPA  103  and a target substrate is sensed, then the initial desired location may be modified to move MPA  103  toward a desired stress state, e.g., to evenly distribute pressure across MPA  103  and/or to achieve a desired level of pressure at one or more locations on pivot mount  300  based on a deflection of the pivot mount  300  spring arms  306 . 
     A primary input  1302  may define a set of reference signals that correspond to an initial desired state of MPA  103 . More specifically, primary input  1802  may define a target spatial location of MPA  103  relative to an anticipated location of a micro device array or substrate surface. Primary input  1302  may be fed into one of several inner loops, each of which may correspond to an individual actuator. For example, x-actuator inner loop  1304  may correspond to a control loop for controlling an x-actuator of the transfer head assembly, and thus MPA  103 , to tip about a remote rotational center. Similarly, y-actuator inner loop  1306  may correspond to a control loop for controlling a y-actuator of the transfer head assembly, and thus MPA  103 , to tilt about the remote rotational center. Also, z-actuator inner loop  1308  may correspond to a control loop for controlling a z-actuator of the transfer head assembly and thus a location of MPA  103  along a z-axis. Therefore, the combination of inner loops allow for the control of actuators that adjust a tip, tilt, and z-spatial orientation of MPA  103 . 
     In an embodiment, inner loop control of transfer head assembly  200  actuators results in a primary output  1310 . More specifically, primary output  1310  may be an instantaneous geometric configuration of transfer head assembly  200  resulting from actuator movement. The geometric configuration may be inferred from data supplied by encoders or other sensors that track spatial position of individual transfer head assembly  200  components. That is, the geometric configuration may include a combination of individual geometric configurations such as a tip position, tilt position, and z-position. Primary output  1310  may also relate to a spatial position of MPA  103  as inferred from known physical dimensions of transfer head assembly  200  components. Alternatively, MPA  103  surface location may be sensed directly using, e.g., laser micrometers, accelerometers, etc., to provide spatial orientation feedback that may be included directly in primary output  1310 . Thus, a position of MPA  103  may be inferred or sensed to determine whether primary output  1310  has been achieved, i.e., equals the intended primary input  1302 . However, although MPA  103  may be driven toward a target substrate to achieve the positional command of primary input  1302 , in some cases, MPA  103  may contact the target substrate. Furthermore, once contact is detected, primary input  1302  may be modified by additional commands from several actuator outer loops, to achieve a neutral tip and tilt deformation of pivot mount  300  with a desired pressure distribution across pivot mount  300 . Accordingly, MPA array  103  may be driven to a tip deflection, tilt deflection, and z-compression target within an accuracy in the submicron range, e.g., on the order of less than about 250 nm. 
     After contact between a transfer head array  115  of MPA  103  and a micro device has been made, MPA  103  may be finely adjusted based on pressure feedback from the pivot mount  300 . More particularly, fine adjustment of MPA  103  may be enabled in response to system recognition of a contact disturbance  1312 . In an embodiment, enable logic is included to determine whether a contact disturbance  1312  is sensed prior to MPA  103  achieving the desired primary input  1302 , and if a contact disturbance  1312  is sensed, additional control loops may be closed to permit fine adjustment of the transfer head assembly  200 . More specifically, additional control loops may be closed to drive MPA  103  toward tip deflection, tilt deflection, and z-compression targets, rather than toward the initial positional target of primary input  1302 . 
     In an embodiment, a contact disturbance  1312  is sensed when, e.g., MPA  103  contacts a mating substrate out of alignment. For example, if MPA  103  and the mating substrate make contact in perfect alignment, the primary output  1810  may equal the primary input  1802  and micro devices may then be gripped by transfer head array  115  without requiring additional adjustment. However, if MPA  103  and the mating substrate are not perfectly aligned, displacement or strain measurements from each strain sensing element  320  on pivot mount  300  may be substantially different from each other and/or the desired level of pressure may not be achieved. That is, in an embodiment, an expected or desired tip, tilt, and compression state must be satisfied prior to initiating electrostatic gripping. If the desired state is not achieved, displacement or strain measurements may be fed as feedback signals  1314 . 
     In an embodiment, feedback signals  1314  correspond to analog signals from the strain sensing elements  320  and references gages  340 . In the exemplary embodiment above, feedback signals  1314  may include eight sensor signals from sixteen separate strain sensing elements  320  and sixteen reference gages  340 . The feedback signals  1314  may be conditioned by a signal conditioning and combination logic  1315  to transform the analog signals into a synthesized output signal representing a strain state of a respective strain sensing element. These synthesized output signals may furthermore be combined by signal conditioning and combination logic  1315  to synthesize one or more of a pivot mount  300  compression synthesized output signal, a pivot mount  300  tilt deflection synthesized output signal, and a pivot mount  300  tip deflection synthesized output signal represented by a transformation matrix equation, such as equation (1) or equation (2) described above. The synthesized output signals may be provided as inputs to dynamic control enable logic  1316 . More particularly, dynamic control enable logic  1316  may observe the one or more synthesized output signals to determine that a contact disturbance  1312  has occurred in one or more of a tip, tilt, or z-direction. For example, if a non-zero compression signal is synthesized by signal conditioning and combination logic  1315  that exceeds predetermined limits, dynamic control enable logic  1316  may recognize the contact disturbance  1312 . 
     In response to observing that a contact disturbance  1312  exists, dynamic control enable logic  1316  may close respective outer loops, each of which may be configured to provide output commands to modify the positional command of primary input  1302 . Thus, closing the outer loops may drive the actuators to achieve a desired state of pressure and orientation, rather than driving them to achieve an initial position command. For example, if dynamic control enable logic  1316  observes that a compression contact disturbance  1312  exists, z-actuator outer loop  1318  may be closed to respond to the contact disturbance  1312  by adjusting a z-actuator. Likewise, dynamic control enable logic  1316  may respond to tip deflection signals or tilt deflection signals by enabling x-actuator outer loop  1320  or y-actuator outer loop  1322 , respectively. 
     Deflection and compression feedback signals may be passed from signal conditioning and combination logic  1315  as synthesized output signals to respective outer loops for comparison with deflection command inputs  1340  provided to respective outer loops. In an embodiment, pivot mount  300  deflection command inputs  1340  may correspond to a desired pressure distribution across pivot mount  300  or MPA  103 . Thus, pivot mount  300  deflection command inputs  1340  may represent tip deflection, tilt deflection, and z-compression targets of pivot mount  300 . These targets may be compared to the synthesized output signals from signal conditioning and combination logic  1315 , which indicate an instantaneous pressure distribution across pivot mount  300 , to determine a difference. The difference, if any, may then be fed as an error signal to drive respective transfer head assembly  200  actuators. For example, if tipping of pivot mount  300  is sensed as a contact disturbance  1312  and dynamic control enable logic  1316  observes that the tipping exceeds an allowable amount, x-actuator outer loop  1320  may be closed and the tipping deflection signal may be compared with a pivot mount  300  tip deflection command  1340  to generate a motion control signal that will tip pivot mount  300  toward a desired stress state. The motion control signal may be fed to a servo filter and passed through inverse kinematics calculations to generate an outer loop command output  1330 . In an embodiment, the motion control signal may also be added with other transfer head assembly motion control signals at one or more of motion summation nodes  1350 . This may be the case, for example, when movement of multiple actuators is required to cause tipping. 
     In order to close the control loop, the outer loop command outputs  1330  may be combined with primary input  1302  and passed back into actuator inner loops. For example, a tipping outer loop command  1330  may be summed with primary input  1302  for an x-actuator and passed through x-actuator inner loop  1304 , thereby controlling an x-actuator in such a manner that pivot mount  300  tips toward a physical state of more even pressure distribution. Respective outer loop commands may be passed through to any actuator inner loop for which a contact disturbance  1312  was sensed. 
     The above control methodology may be performed and repeated until the transfer head assembly  200  is moved to a location at which pressure distribution across pivot mount  300 , and hence MPA  103 , is uniform and achieves a desired amount of pressure. Thus, transfer head assembly  200  may be controlled to bring an array of electrostatic transfer head array  115  on MPA  103  into contact with an array of micro devices on a mating substrate. Using the control system described above, if alignment between MPA  103  and the mating substrate is not initially perfect, which would be true of almost every transfer operation, pressure distribution control may be implemented to fine tune the alignment. The control methodology may be performed quickly, e.g., on the order of about 50 ms to sense a contact disturbance  1312 , enable the appropriate outer loop(s), and feed appropriate outer loop control commands to actuators, and thus, complete contact may be rapidly achieved between an electrostatic transfer head array  115  and an array of micro devices, enabling efficient transfer between a carrier substrate and a receiving substrate. 
     Referring now to  FIG. 13B , a schematic illustration is provided for a method of generating a synthesized output signal in an embodiment. As illustrated, feedback signals  1314  are received by a signal conditioning and combination logic  1315 , which combines the incoming feedback signals  1314  from the pivot mount  300  and generates synthesized output signals. In the simplest case, feedback signals received from the pivot mount (e.g. from sensors  1 - 8  described above with regard to  FIG. 11F ) are linearly combined by multiplication with a transformation matrix to form a set of output measurements (synthesized output signals). 
     Referring to the embodiment illustrated in  FIG. 13C , in a more complex implementation, correlated sets of strain sensors may be checked for signal quality. As illustrated, feedback signals  1314  are received by a signal conditioning and combination logic  1315 . At  1315 A, the feedback signals  1314  are checked to determine if they are within a predefined normal operating range. Sensors (including gages  320 ,  340 ) that are outside of the normal operating range are flagged as failed sensors. Failed sensor signals may then be rejected requiring a change in the transformation matrix. At  1315 B signals are checked for variation within the normal operating range. Sensors (including gages  320 ,  340 ) with variation that is greater or less than a normally operating sensor are flagged as failed sensors. Based on the sensors flagged as failed, a transformation matrix is selected that is able to synthesize the outputs from the remaining signals, and the transformation matrix is used to convert the resulting sensor signal vector into synthesized output signals (position measurement output) at  1315 C. In this way synthesized output signals are maintained at a reduced signal to noise ratio rather than sensor failure causing output failure. 
     Examples of generating a synthesized output signal utilizing the 8 channel embodiment of  FIG. 11F  and the transformation matrix equation (1) are provided in  FIGS. 13D-13E . It is to be appreciated that the following examples are provided for illustrational purposes, and that embodiments are not limited to the particular geometries or number of channels in the exemplary embodiments. Referring to  FIG. 13D , at  1315 A signal  2  is determined to read low (outside of the normal operation range) by the signal conditioning and combination logic  1315 , and is flagged as a failed sensor. All remaining sensors are determined to be operating within normal variation in the operating range at  1315 B. At  1315 C, based on sensor  2  as being flagged as failed, a transformation matrix is selected and used to convert the resulting sensor signal vector into synthesized output signals (position measurement output). Referring to  FIG. 13D , at  1315 A signal  2  is determined to read low (outside of the normal operation range) and signal  5  is determined to read high (outside of the normal operation range) by the signal conditioning and combination logic  1315 , and are flagged as failed sensors. At  1315 B, signal  7  is determined to have variation that is lower than normal variation by the signal conditioning and combination logic  1315 , and is flagged as a failed sensor. At  1315 C, based on sensors  2 ,  5 , and  7  as being flagged as failed, a transformation matrix is selected and used to convert the resulting sensor signal vector into synthesized output signals (position measurement output). 
     Referring to  FIG. 14 , a flowchart illustrating a method of aligning a MPA  103  coupled with a pivot mount  300  on a transfer head assembly  200  relative to a target substrate is shown in accordance with an embodiment. The method may be performed, e.g., during a pick-up or a placement operation as micro devices are transferred from a carrier substrate to a receiving substrate. At operation  1402 , mass transfer tool  100  moves transfer head assembly  200  along a z-axis toward a target substrate, e.g., carrier substrate held by carrier substrate holder  104  or receiving substrate held by receiving substrate holder  106 , according to primary input  1302 . More specifically, the MPA  103  and pivot mount  300  are moved toward the target substrate along the z-axis. Movement of MPA  103  along z-axis  510  may be achieved by actuating various actuators of mass transfer tool  100  or a substrate holder. 
     Initially, there may be no compressive loading applied to MPA  103  or pivot mount  300 . This initial state may correspond to a range of travel over which array of micro devices are physically separated from the electrostatic transfer head array. During this travel, MPA  103  and the target substrate may have misaligned surfaces, but there may be no indication of this misalignment since the pressure distribution state of pivot mount  300  may be uniform, i.e., all strain sensing elements may be outputting signals indicating zero strain. 
     At operations  1404  and  1406 , an electrostatic transfer head in the electrostatic transfer head array  115  may contact a micro device while other electrostatic transfer heads may remain separated from corresponding micro devices. That is, contact may be made while MPA  103  is misaligned with the target substrate. This positional misalignment may be sensed as uneven pressure distribution in pivot mount  300 . For example, a first strain output value from one strain sensing element  320  on pivot mount  300  and a different second strain output value from another strain sensing element  320  in pivot mount  300  may differ. The strain signals may be provided as feedback signals  1314  and conditioned and combined by into synthesized output signals (e.g. tip deflection, tilt deflection, and compression signals) by signal conditioning and combination logic  1315  indicating a contact disturbance  1312 . 
     Dynamic enable control logic  1316  may observe that the contact disturbance  1312  exists, and depending upon the level of contact disturbance  1312 , may activate actuator outer loops to determine driving signals for actuating various actuators of transfer head assembly  200  in order to adjust an orientation of MPA  103  such that pressure distribution across pivot mount  300  is uniform. For example, at operation  1408 , in response to the tip signal being recognized as a contact disturbance  1312  above a threshold, x-actuator outer loop  1320  may feed command signals  1330  to x-actuator inner loop  1304  in order to actuate an x-actuator to tip MPA  103  about remote rotational center. Similarly, at operation  1410 , in response to the tilt deflection signal being recognized as a contact disturbance  1312  above a threshold, y-actuator outer loop  1322  may feed command signals to y-actuator inner loop  1306  in order to actuate a y-actuator  708  to tile MPA  103  about remote rotational center. 
     At operation  1412 , in response to actuation of the x- and y-actuators based on the tip and tilt deflection signals MPA  103  may be rotated into alignment with the target substrate. Furthermore, with remote rotational center co-located with the contact surface of MPA  103 , the electrostatic transfer head array  115  may experience pure rotation about remote rotational center. Thus, as MPA  103  is aligned with the target substrate, the electrostatic transfer head array  115  may experience minimal parasitic lateral motion and micro devices may remain undamaged. 
     Actuation of transfer head assembly  200  according to synthesized output signals (tip, tilt, and z-compression signals) may continue until the electrostatic transfer head array  115  is in contact with micro devices on the target substrate. More particularly, actuation may continue until primary output  1310  is within the limits set by primary input  1302 , at which point actuation may be stopped. As discussed above, primary output  1310  may be a positional output that is modified to reach a desired pivot mount  300  state. For example, actuation of transfer head assembly  200  may continue until primary positional input is achieved and/or pressure distribution across pivot mount  300  is uniform. 
     After contact between the electrostatic transfer head array  115  and the micro devices is made, a voltage may be applied to the electrostatic transfer head array  115  to create a grip pressure on the array of micro devices. An electrostatic voltage may be applied to electrostatic transfer head array  115  compliant voltage contacts  316  and voltage contacts  120 . Additional electrical contacts and connectors may be integrated within transfer head assembly  200  and powered by voltage supplies based on control signals from computer  108 . For example, computer  108  may implement a control algorithm instructing that electrostatic transfer head array  115  be activated if a predefined deformation is simultaneously sensed by each displacement sensor on pivot mount  300  during a pick up process. As a result, the array of electrostatic transfer head array  115  may apply a gripping pressure to the array of micro devices after the entire array surface is in contact and uniform pressure is applied across the array. 
     After gripping the micro devices with electrostatic transfer head array  115 , the micro devices may be picked up from carrier substrate. During pick up, the electrostatic voltage supplied to the electrostatic transfer head array  115  may persist, and thus, the array of micro devices may be retained on the electrostatic transfer head array  115  and removed from the carrier substrate. 
     During the pick up operation, a heating element may direct heat toward pivot mount  300  and/or MPA  103 . Thus, the micro devices may be heated through contact with electrostatic transfer head array  115  on MPA  103  during pick up. For example, a heating element adjacent to pivot mount  300  may be resistively heated to transfer heat to MPA  103 , and thus, to the micro devices through the electrostatic transfer head array  115 . Heat transfer may occur before, during, and after picking up the array of micro devices from carrier substrate. 
     Although a pick up process is described in relation to  FIG. 14 , a similar methodology may be used to control the placement of micro devices onto a receiving substrate, such as a display substrate, held by receiving substrate holder  106 . For example, as the micro devices are gripped by the electrostatic transfer head array  115 , mass transfer tool  100  may move the MPA  103  over a receiving substrate, and align MPA with a target region of the receiving substrate. MPA  103  may be advanced toward, and aligned with, the receiving substrate using the control sequence described above until the array of micro devices held by the electrostatic transfer head array  115  are placed in uniform contact with the target region. Uniform contact may be inferred by sensing a strain state of pivot mount  300 . Subsequently, voltage may be removed from the electrostatic transfer head array  115  to release the micro devices onto the receiving substrate and complete the transfer operation. 
     Referring to  FIG. 15 , a schematic illustration of a computer system is shown that may be used in accordance with an embodiment. Portions of embodiments of the invention are comprised of or controlled by non-transitory machine-readable and machine-executable instructions that reside, for example, in machine-usable media of a computer  108 . Computer  108  is exemplary, and embodiments of the invention may operate on or within, or be controlled by a number of different computer systems including general purpose networked computer systems, embedded computer systems, routers, switches, server devices, client devices, various intermediate devices/nodes, stand-alone computer systems, and the like. Furthermore, although some components of a control system, e.g., signal conditioning and combination logic  1315  and dynamic control enable logic  1316 , have been broken out for discussion separately above, computer  108  may integrate those components directly or include additional components that fulfill similar functions. 
     Computer  108  of  FIG. 15  includes an address/data bus  1502  for communicating information, and a central processor  1504  coupled to bus  1502  for processing information and instructions. Computer  108  also includes data storage features such as a computer usable volatile memory, e.g. random access memory (RAM)  1506 , coupled to bus  1502  for storing information and instructions for central processor  1504 , computer usable non-volatile memory  1508 , e.g. read only memory (ROM), coupled to bus  1502  for storing static information and instructions for the central processor  1504 , and a data storage device  1510  (e.g., a magnetic or optical disk and disk drive) coupled to bus  1502  for storing information and instructions. Computer  108  of the present embodiment also includes an optional alphanumeric input device  1512  including alphanumeric and function keys coupled to bus  1502  for communicating information and command selections to central processor  1504 . Computer  108  also optionally includes an optional cursor control  1514  device coupled to bus  1502  for communicating user input information and command selections to central processor  1504 . Computer  108  of the present embodiment also includes an optional display device  1516  coupled to bus  1502  for displaying information. 
     The data storage device  1510  may include a non-transitory machine-readable storage medium  1518  on which is stored one or more sets of instructions (e.g. software  1520 ) embodying any one or more of the methodologies or operations described herein. For example, software  1520  may include instructions, which when executed by processor  1504 , cause computer  108  to control mass transfer tool  100  or remote center robot  500  according to the control scheme described above for aligning an MPA  103  with a target substrate. Software  1520  may also reside, completely or at least partially, within the volatile memory, non-volatile memory  1508 , and/or within processor  1504  during execution thereof by computer  108 , volatile memory  1506 , non-volatile memory  1508 , and processor  1504  also constituting non-transitory machine-readable storage media. 
     In utilizing the various aspects of this invention, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming a pivot mount with integrated strain sensing elements and/or compliant voltage contacts. Although the present invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as particularly graceful implementations of the claimed invention useful for illustrating the present invention.

Metadata:
Filing Date: 20170316
Publication Date: 20181211
Grant Date: 20181211
Priority Date: 20140612
Inventors: BATHURST, Stephen P.
PARKS, PAUL ARGUS
LIGHT, Nile Alexander
Assignee: APPLE INC
CPC Classifications: [{"code": "B25J7/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J7/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J15/0085", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J15/0085", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/6831", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02N13/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B2203/0163", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T74/20006", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81B2203/0163", "inventive": false, "first": false, "tree": "[]"}, {"code": "B25J19/02", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y10T74/20006", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01B7/18", "inventive": true, "first": true, "tree": "[]"}, {"code": "B25J15/0085", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B2203/0163", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/6831", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02N13/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "Y10T74/20006", "inventive": false, "first": false, "tree": "[]"}, {"code": "B25J7/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J19/02", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 53373615