PATENT DOCUMENT

Publication Number: US-10022859-B2
Application Number: US-201615054977-A
Country: US
Kind Code: B2

Title: Mass transfer tool manipulator assembly

Abstract:
Systems and methods for transferring a micro device from a carrier substrate are disclosed. In an embodiment, a mass transfer tool manipulator assembly allows active alignment between an array of electrostatic transfer heads on a micro pick up array and an array of micro devices on a carrier substrate. Displacement of a compliant element of the mass transfer tool manipulator assembly may be sensed to control alignment between the array of electrostatic transfer heads and the array of micro devices.

Claims:
What is claimed is: 
     
       1. A method comprising:
 moving a mass transfer tool manipulator assembly toward a carrier substrate; 
 contacting an array of micro devices on the carrier substrate with an array of electrostatic transfer heads coupled with a pivot platform of the mass transfer tool manipulator assembly; 
 sensing deformation of a compliant element coupled with the pivot platform; 
 stopping relative movement between the mass transfer tool manipulator assembly and the carrier substrate; 
 applying a voltage to the array of electrostatic transfer heads to create a grip pressure on the array of micro devices; and 
 picking up the array of micro devices from the carrier substrate. 
 
     
     
       2. The method of  claim 1 , wherein sensing deformation comprises sensing strain in a displacement sensor integrated with the compliant element. 
     
     
       3. The method of  claim 1 , further comprising adjusting a position of a base coupled with the compliant element after sensing deformation and before stopping relative movement. 
     
     
       4. The method of  claim 1 , wherein adjusting the position comprises actuating an actuator assembly of the mass transfer tool manipulator assembly, the actuator assembly coupled to the base, to further align the base to a plane of the carrier substrate by tipping or tilting the base after sensing deformation. 
     
     
       5. The method of  claim 1 , further comprising applying heat to the array of electrostatic transfer heads while picking up the array of micro devices. 
     
     
       6. A method comprising:
 moving a mass transfer tool manipulator assembly toward a receiving substrate; 
 contacting the receiving substrate with an array of micro devices carried by an array of electrostatic transfer heads coupled with a pivot platform of the mass transfer tool manipulator assembly; 
 sensing deformation of a compliant element coupled with the pivot platform; 
 stopping relative movement between the mass transfer tool manipulator assembly and the receiving substrate; 
 removing a voltage from the array of electrostatic transfer heads; and 
 releasing the array of micro devices onto the receiving substrate. 
 
     
     
       7. The method of  claim 6 , wherein sensing deformation comprises sensing strain in a displacement sensor integrated with the compliant element. 
     
     
       8. The method of  claim 6 , further comprising adjusting a position of a base coupled with the compliant element after sensing deformation and before stopping relative movement. 
     
     
       9. The method of  claim 8 , wherein adjusting the position comprises actuating an actuator assembly of the mass transfer tool manipulator assembly, the actuator assembly coupled to the base, to further align the base to a plane of the receiving substrate by tipping or tilting the base after sensing deformation. 
     
     
       10. The method of  claim 6 , further comprising applying heat to the array of electrostatic transfer heads before removing the voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/776,158, filed Feb. 25, 2013, and related to U.S. patent application Ser. No. 13/776,188, filed Feb. 25, 2013, now U.S. Pat. No. 9,095,980, the full disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     The present invention relates to micro devices. More particularly, embodiments of the present invention relate to systems and methods for transferring a micro device from a carrier substrate. 
     Background Information 
     The feasibility of commercializing miniaturized 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. Manufacturing processes typically include wafer based processing and transferring techniques. 
     Device transferring processes include transfer from a transfer wafer to a receiving wafer. One such implementation is “direct printing” involving one bonding step of an array of devices from a transfer wafer to a receiving wafer, followed by removal of the transfer wafer. Another such implementation is “transfer printing” involving two bonding/de-bonding steps. In transfer printing a transfer wafer may pick up an array of devices from a donor wafer and bond the devices to a receiving wafer. Following transfer, the transfer wafer may be removed using techniques that include laser lift-off (LLO), grinding or polishing, and etching. 
     Gimbal mechanisms have been used in wafer polishing equipment to facilitate evenly polishing a wafer. For example, passive gimbal mechanisms in polishing equipment facilitate alignment of wafers with a polishing pad. 
     SUMMARY OF THE DESCRIPTION 
     A mass transfer tool manipulator assembly and methods of using the mass transfer tool manipulator assembly to transfer an array of micro devices from a carrier substrate are disclosed. In an embodiment, the mass transfer tool manipulator assembly includes a housing, a tip-tilt-z flexure, an actuator assembly, and a micro pick up array mount. A micro pick up array may be provided separately from the mass transfer tool manipulator assembly or integrally formed with the mass transfer tool manipulator assembly. The tip-tilt-z flexure may include a top flexure component joined with the housing and connected with a bottom flexure component by a flexible coupling. For example, the top flexure component and bottom flexure component may be flanges connected by the flexible coupling. The actuator assembly may be operably coupled with the bottom flexure component such that actuation of the actuator assembly moves the bottom flexure component relative to the top flexure component. For example, in an embodiment, the mass transfer tool manipulator assembly includes a distribution plate coupling the actuator assembly with the bottom flexure component. The micro pick up array mount may also be coupled with the bottom flexure component. Furthermore, the micro pick up array mount may include a pivot platform coupled with a compliant element, such as a beam. A displacement sensor may be integrated with the compliant element. In an embodiment, a micro pick up array having a substrate supporting an electrostatic transfer head may be joinable with the pivot platform. 
     In an embodiment, the micro pick up array mount may further include a base laterally around the pivot platform with the compliant element between the pivot platform and the base and coupled with the pivot platform and base at pivots. For example, the compliant element may be coupled with the base at an outer pivot on a base edge, and coupled with the pivot platform at an inner pivot on a pivot platform edge that is orthogonal to the base edge. The compliant element may also be coupled with the pivot platform at a second inner pivot across the pivot platform from the inner pivot and coupled with the base at a second outer pivot across the pivot platform from the outer pivot. In an embodiment, the micro pick up array mount may include a second compliant element coupled with the base by the second outer pivot on a second base edge and coupled with the pivot platform by the second inner pivot on a second pivot platform edge. Furthermore, a second displacement sensor may be integrated with the second compliant element. 
     In an embodiment, the displacement sensor may be a strain gauge attached to a high strain region of the compliant element near the inner pivot or the outer pivot. For example, the strain gauge may be bonded to the high strain region. Alternatively, the strain gauge may be deposited on the high strain region. Furthermore, the strain gauge may be formed by doping the high strain region. In an embodiment, the micro pick up array mount may include a reference strain gauge adjacent to the displacement sensor on the compliant element. The displacement sensor and the reference strain gauge may provide adjacent legs in a half Wheatstone bridge. 
     In an embodiment, the micro pick up array mount may include various contacts and electrical connections. For example, the micro pick up array mount may include a displacement sensor contact on the base in electrical connection with the displacement sensor. In an embodiment, the mass transfer tool manipulator assembly may include a position sensing module in electrical connection with the displacement sensor through the displacement sensor contact. For example, the displacement sensor contact may be in electrical connection with the position sensing module through a flex circuit or a spring contact. In an embodiment, the micro pick up array mount may include a base operating voltage contact on the base in electrical connection with a pivot platform operating voltage contact on the pivot platform. Furthermore, the micro pick up array mount may include a base clamp contact on the base in electrical connection with a clamp electrode at a bonding site on the pivot platform. In an embodiment, the micro pick up array mount may include a bonding site on the pivot platform that includes a metal such as gold, copper, or aluminum. 
     In an embodiment, the micro pick up array mount may also include a temperature sensor and a heating element on the pivot platform. The heating element may include a resistance alloy or a surface-mount technology resistor, for example. Furthermore, the mass transfer tool manipulator assembly may include an insulation plate between the heating element and the position sensing module. The base of the micro pick up array mount may be coupled with the insulation plate and the insulation plate may further be coupled with the distribution plate. 
     In an embodiment, a method includes moving a mass transfer tool manipulator assembly toward a carrier substrate and contacting an array of micro devices on the carrier substrate with an array of electrostatic transfer heads coupled with a pivot platform of the mass transfer tool manipulator assembly. The method may also include sensing deformation of a compliant element coupled with the pivot platform. For example, sensing deformation may include sensing strain in a displacement sensor integrated with the compliant element. In an embodiment, the method further includes adjusting a position of a base coupled with the compliant element after sensing deformation and before stopping relative movement between the mass transfer tool manipulator assembly and the carrier substrate. For example, adjusting the position may include actuating an actuator assembly coupled to the base to further align the base to a plane of the carrier substrate by tipping or tilting the base. The method may also include applying a voltage to the array of electrostatic transfer heads to create a grip pressure on the array of micro devices and picking up the array of micro devices from the carrier substrate. In an embodiment, the method includes applying heat to the array of electrostatic transfer heads while picking up the array of micro devices. 
     In an embodiment, a method includes moving a mass transfer tool manipulator assembly toward a receiving substrate and contacting the receiving substrate with an array of micro devices carried by an array of electrostatic transfer heads coupled with a pivot platform of the mass transfer tool manipulator assembly. The method may also include sensing deformation of a compliant element coupled with the pivot platform. For example, sensing deformation may include sensing strain in a displacement sensor integrated with the compliant element. In an embodiment, the method further includes adjusting a position of a base coupled with the compliant element after sensing deformation and before stopping relative movement between the mass transfer tool manipulator assembly and the receiving substrate. For example, adjusting the position may include actuating an actuator assembly coupled with the base to further align the base to a plane of the receiving substrate by tipping or tilting the base. The method may also include removing a voltage from the array of electrostatic transfer heads and releasing the array of micro devices onto the receiving substrate. In an embodiment, the method includes applying heat to the array of electrostatic transfer heads before removing the voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustration of a mass transfer tool in accordance with an embodiment of the invention. 
         FIG. 2  is a perspective view illustration of a mass transfer tool manipulator assembly holding a micro pick up array in accordance with an embodiment of the invention. 
         FIG. 3  is a cross-sectional perspective view illustration of a mass transfer tool manipulator assembly, taken about section line A-A of  FIG. 2 , in accordance with an embodiment of the invention. 
         FIG. 4A  is a side view illustration of an actuator assembly having an actuator and a flexure attachment in accordance with an embodiment of the invention. 
         FIG. 4B  is a perspective view of a tip-tilt-z flexure of a mass transfer tool manipulator assembly in accordance with an embodiment of the invention. 
         FIG. 5A  is a perspective view of a micro pick up array mount having a displacement sensor integrated with a compliant element in accordance with an embodiment of the invention. 
         FIG. 5B  is a plan view of a displacement sensor integrated with a compliant element of a micro pick up array mount, taken from detail X of  FIG. 5A , in accordance with an embodiment of the invention. 
         FIG. 6  is a perspective view of a micro pick up array mount having a heating element on a pivot platform in accordance with an embodiment of the invention. 
         FIG. 7  is a side view of a micro pick up array having a substrate supporting an array of electrostatic transfer heads in accordance with an embodiment of the invention. 
         FIG. 8  is a side view illustration of a micro pick up array mount joined with a micro pick up array in accordance with an embodiment of the invention. 
         FIG. 9  is a perspective view of a micro pick up array mount having a displacement sensor integrated with a compliant element and an array of electrostatic transfer heads on a pivot platform in accordance with an embodiment of the invention. 
         FIG. 10  is a perspective view of a micro pick up array mount having a heating element on a pivot platform in accordance with an embodiment of the invention. 
         FIG. 11  is a cross-sectional side view illustration of a micro pick up array mount in electrical connection with a spring contact, taken about section line B-B of  FIG. 9 , in accordance with an embodiment of the invention. 
         FIG. 12  is a perspective view illustration of a micro pick up array mount having a flexible region in accordance with an embodiment of the invention. 
         FIG. 13  is a side view illustration of a mass transfer tool manipulator assembly holding a micro pick up array and interconnected with a control system in accordance with an embodiment of the invention. 
         FIG. 14  is a schematic illustration of a control loop to regulate a mass transfer tool manipulator assembly in accordance with an embodiment of the invention. 
         FIG. 15  is a flowchart illustrating a method of picking up an array of micro devices from a carrier substrate in accordance with an embodiment of the invention. 
         FIG. 16  is a schematic illustration of a mass transfer tool manipulator assembly moving toward a carrier substrate in accordance with an embodiment of the invention. 
         FIG. 17  is a schematic illustration of an array of electrostatic transfer heads coupled with a mass transfer tool manipulator assembly contacting an array of micro devices on a carrier substrate in accordance with an embodiment of the invention. 
         FIG. 18  is a schematic illustration of a mass transfer tool manipulator assembly adjusting a position of a micro pick up array mount in accordance with an embodiment of the invention. 
         FIG. 19  is a schematic illustration of a mass transfer tool manipulator assembly picking up an array of micro devices from a carrier substrate in accordance with an embodiment of the invention. 
         FIG. 20  is a flowchart illustrating a method of placing an array of micro devices on a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 21  is a schematic illustration of a mass transfer tool manipulator assembly moving toward a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 22  is a schematic illustration of an array of micro devices carried by an array of electrostatic transfer heads coupled with a mass transfer tool manipulator assembly contacting a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 23  is a schematic illustration of a mass transfer tool manipulator assembly adjusting a position of a micro pick up array mount in accordance with an embodiment of the invention. 
         FIG. 24  is a schematic illustration of a mass transfer tool manipulator assembly releasing an array of micro devices onto a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 25  is a schematic illustration of computer system that may be used in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention describe systems and methods for transferring a micro device or an array of micro devices from a carrier substrate. For example, the micro devices or array of micro devices may be any of the micro LED device structures illustrated and described in related U.S. patent application Ser. Nos. 13/372,222, 13/436,260, 13/458,932, and 13/625,825. While some embodiments of the present invention are described with specific regard to micro LED devices, the embodiments of the invention are not so limited and certain embodiments may also be applicable to other micro LED devices and micro devices such as diodes, transistors, integrated circuit (IC) chips, and MEMS. 
     In various embodiments, description is made with reference to the figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions, and processes, in order to provide a thorough understanding of the present invention. In other instances, well-known processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the present invention. Reference throughout this specification to “one embodiment,” “an embodiment”, or the like, means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “one embodiment,” “an embodiment”, or the like, in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “over”, “to”, “between”, and “on” as used herein may refer to a relative position of one layer or component with respect to other layers or components. One layer “over” or “on” another layer or bonded “to” another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     The terms “micro” device or “micro” LED structure as used herein may refer to the descriptive size of certain devices or structures in accordance with embodiments of the invention. As used herein, the terms “micro” devices or structures are meant to refer to the scale of 1 to 100 μm. However, embodiments of the present invention are not necessarily so limited, and certain aspects of the embodiments may be applicable to larger and possibly smaller size scales. In an embodiment, a single micro device in an array of micro devices, and a single electrostatic transfer head in an array of electrostatic transfer heads both have a maximum dimension, for example length or width, of 1 to 100 μm. In an embodiment, the top contact surface of each micro device or electrostatic transfer head has a maximum dimension of 1 to 100 μm. In an embodiment, the top contact surface of each micro device or electrostatic transfer head has a maximum dimension of 3 to 20 μm. In an embodiment, a pitch of an array of micro devices, and a pitch of a corresponding array of electrostatic transfer heads, may be (1 to 100 μm) by (1 to 100 μm), for example, a 20 μm by 20 μm or a 5 μm by 5 μm pitch. In one aspect, without being limited to a particular theory, embodiments of the invention describe micro device transfer heads and head arrays which operate in accordance with principles of electrostatic grippers, using the attraction of opposite charges to pick up micro devices. In accordance with embodiments of the present invention, a pull-in voltage may be applied to a micro device transfer head in order to generate a grip pressure on a micro device and pick up the micro device. 
     In an aspect, embodiments of the invention describe systems and methods for the mass transfer of micro devices using a mass transfer tool manipulator assembly having a feedback mechanism for regulating alignment of an array of electrostatic transfer heads with an array of micro devices on a carrier substrate. In an embodiment, a mass transfer tool manipulator assembly includes a tip-tilt-z flexure, an actuator assembly, and a micro pick up array mount having one or more displacement sensors integrated with one or more compliant elements. For example, the displacement sensors may be strain gauges attached to high strain regions of the compliant elements. In this manner, the displacement sensors may be used to sense deformation of the compliant elements when an array of electrostatic transfer heads contact an array of micro devices. In an embodiment, based on feedback from the displacement sensor(s), the actuator assembly of the mass transfer tool manipulator assembly may adjust a spatial orientation of the micro pick up array mount to change a center of pressure on the micro pick up array mount. Thus, the mass transfer tool manipulator assembly may facilitate active alignment of an array of electrostatic transfer heads mounted on the micro pick up array mount with an array of micro devices based on a closed feedback loop. 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. 
     In another aspect, embodiments of the invention describe systems and methods for the mass transfer of micro devices using a tip-tilt-z flexure coupled with an actuator assembly of a mass transfer tool manipulator assembly. In an embodiment, the tip-tilt-z flexure imparts a reactive load on the actuator assembly to smooth motion of a micro pick up array mount during adjustment by the actuator assembly. In an embodiment, the tip-tilt-z flexure imparts a restorative load on the micro pick up array mount to pick up an array of micro devices from a carrier substrate. Thus, the mass transfer tool manipulator assembly may facilitate contact with, and pick up, of an array of micro devices using an array of electrostatic transfer heads without damaging the micro devices or the electrostatic transfer heads. 
     In another aspect, embodiments of the invention describe a manner for the mass transfer of an array of pre-fabricated micro devices with an array of electrostatic transfer heads. For example, the pre-fabricated micro devices may have a specific functionality such as, but not limited to, a LED for light-emission, silicon IC for logic and memory, and gallium arsenide (GaAs) circuits for radio frequency (RF) communications. In some embodiments, arrays of micro LED devices which are poised for pick up are described as having a 20 μm by 20 μm pitch, or 5 μm by 5 μm pitch. At these densities a 6 inch substrate, for example, may accommodate approximately 165 million micro LED devices with a 10 μm by 10 μm pitch, or approximately 660 million micro LED devices with a 5 μm by 5 μm pitch. A mass transfer tool manipulator assembly including an array of electrostatic transfer heads matching an integer multiple of the pitch of the corresponding array of micro LED devices may be used to pick up and transfer the array of micro LED devices to a receiving substrate. In this manner, micro LED devices may be integrated and assembled into heterogeneously integrated systems, including substrates of any size ranging from micro displays to large area displays, and at high transfer rates. For example, a 1 cm by 1 cm array of electrostatic transfer heads may pick up and transfer more than 100,000 micro devices, with larger arrays of electrostatic transfer heads being capable of transferring more micro devices. 
     Referring to  FIG. 1 , a perspective view illustration of a mass transfer tool is shown in accordance with an embodiment of the invention. As illustrated, mass transfer tool  100  may include a mass transfer tool manipulator assembly  102  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 . Operation of mass transfer tool  100  and mass transfer tool manipulator assembly  102  may be controlled at least in part by a computer system  108 . In an embodiment, computer system  108  may control the operation of mass transfer tool manipulator assembly  102  based on feedback signals received from various sensors on a micro pick up array mount coupled with the mass transfer tool manipulator assembly  102  as described in further detail below. 
     In an embodiment, components and subassemblies of mass transfer tool  100  and mass transfer tool manipulator assembly  102  may be moved relative to each other. For example, mass transfer tool  100  and mass transfer tool manipulator assembly  102  may adjust spatial relationships between components in order to facilitate transferring an array of micro devices with an array of electrostatic transfer heads. Such adjustments may require precise movements in multiple degrees of freedom. For example, mass transfer tool manipulator assembly  102  may include an actuator assembly for adjusting a micro pick up array mount with at least three degrees of freedom, e.g., tipping, tilting, and movement in a z direction. Similarly, the carrier substrate holder  104  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. Thus, in an embodiment, an array of electrostatic transfer heads supported by mass transfer tool manipulator assembly  102  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 with five degrees of freedom. However, mass transfer tool  100  and mass transfer tool manipulator assembly  102  may include additional actuators that provide more degrees of freedom between the array of micro devices and the array of electrostatic transfer heads, or between other components of the system. For example, mass transfer tool manipulator assembly  102  may be mounted on an x-y stage that moves relative to x-y stage  110 , establishing an additional two degrees of freedom between the array of electrostatic transfer heads supported by mass transfer tool manipulator assembly  102  and the array of micro devices supported by the carrier substrate held by carrier substrate holder  104 . 
     Referring to  FIG. 2 , a perspective view illustration of a mass transfer tool manipulator assembly holding a micro pick up array is shown in accordance with an embodiment of the invention.  FIG. 2  presents an overview of the structural components of an embodiment of mass transfer tool manipulator assembly  102 . Mass transfer tool manipulator assembly  102  may include a housing  210  coupled with a mass transfer tool mount  200  of mass transfer tool  100 . Housing  210  may have a columnar construction coupled with a tip-tilt-z flexure  230 . An actuator assembly  220  may be wholly or partially contained within housing  210 , and furthermore, actuator assembly  220  may be coupled with tip-tilt-z flexure  230  through a distribution plate  240 . Distribution plate  240  may also be coupled with a micro pick up array mount  250 . In an embodiment, micro pick up array mount  250  may be coupled with distribution plate  240  through insulation plate  260 , e.g., by retaining micro pick up array mount  250  directly on insulation plate  260 . In an embodiment, micro pick up array mount  250  may be joined with an intermediate component, e.g., retainer plate  270 , which is held against insulation plate  260  by a retaining ring  280 . Furthermore, a micro pick up array  290  supporting an array of electrostatic transfer heads may be integrated with micro pick up array mount  250 . 
     Referring to  FIG. 3 , a cross-sectional perspective view illustration of a mass transfer tool manipulator assembly, taken about section line A-A of  FIG. 2 , is shown in accordance with an embodiment of the invention.  FIG. 3  presents more detail of the mechanical interaction between structural components of an embodiment of mass transfer tool manipulator assembly  102 . For example, actuator assembly  220  may include one or more actuators  310  having a first actuator attachment  312  that may be fixedly coupled with housing  210  and/or mass transfer tool mount  200 . Actuator  310  may further include second actuator attachment  314  moveable relative to first actuator attachment  312 . As described above, second actuator attachment  314  may further be fastened with distribution plate  240 . Thus, actuation of actuator  310  may cause relative movement between distribution plate  240  and housing  210 . 
     Actuation of actuator  310  may therefore have at least two results. First, since micro pick up array mount  250  may be directly or indirectly coupled with distribution plate  240 , actuation of actuator  310  may change a spatial relationship between micro pick up array mount  250 , or micro pick up array  290  joined with micro pick up array mount  250 , and housing  210 . Second, since distribution plate  240  and housing  210  may be coupled with opposite ends of tip-tilt-z flexure  230 , actuation of actuator  310  may apply tensile, compressive, and/or torsional loads to tip-tilt-z flexure  230  as distribution plate  240  moves relative to housing  210 . 
     In an embodiment, insulation plate  260  may be used to thermally isolate micro pick up array mount  250  from other components of mass transfer tool manipulator assembly  102 . For example, insulation plate  260  may be placed between micro pick up array mount  250  and actuator assembly  220 , or other components of mass transfer tool manipulator assembly  102 . Furthermore, contact between insulation plate  260  and micro pick up array mount  250  or other components of mass transfer tool manipulator assembly  102  may be minimized by limiting contact area between the components. For example, insulation plate  260  may be coupled with distribution plate  240  using insulating posts connected to the components with fasteners, rather than coupling the components using a conductive coupling, such as a welded seam. 
     In an embodiment, insulation plate  260  may be formed from a material exhibiting low thermal conductivity, e.g., thermal conductivity below about 1.5 W/m*° C. when heated to 200 degrees Celsius. For example, insulation plate  260  may be formed from an opaque fused quartz material, or another material having insulating properties. In an embodiment, insulation plate  260  is formed from a high purity opaque fused quartz material containing uniformly distributed microscopic bubbles of less than about 20 micron, e.g., “Pyro-LD80” manufactured by Pyromatics Corp. headquartered in Mentor, Ohio. Thus, insulation plate  260  may function as a thermal barrier to thermally isolate components of mass transfer tool manipulator assembly  102  such as actuators  310  (e.g. piezoelectric actuators) and sensing module  316  from a heating element used to heat the micro pick up array  290  supporting the array of electrostatic transfer heads as described in further detail below. 
     In an embodiment, retainer plate  270  and micro pick up array mount  250  may be formed from materials have similar thermal expansion coefficients. For example, micro pick up array mount  250  may be formed from silicon and retainer plate  270  may be formed from a controlled-expansion nickel alloy, e.g., low expansion “Alloy 39”. Alloy 39 is a controlled-expansion alloy that in an embodiment includes a chemical composition of 0.05 C, 0.40 Mn, 0.25 Si, 39.00 Ni, Bal. Fe. By comparison, Alloy 39 exhibits a coefficient of thermal expansion of about 2 (×10-6/° C.) near 25° C., while silicon exhibits a linear coefficient of thermal expansion of about 3 (×10-6/° C.) near the same temperature. Thus, micro pick up array mount  250  and retainer plate  270  need not have identical thermal expansion characteristics, but those components may expand and contract within the same order of magnitude when subjected to changing temperatures. 
     In an embodiment, retaining ring  280  may be fastened to insulation plate  260 , or directly to distribution plate  240 , using clips, threaded fasteners, or other known fastening mechanisms. Furthermore, retaining ring  280  may include one or more tabs or lips that press against micro pick up array  290  or retainer plate  270  to clamp retainer plate  270  against insulation plate  260  and couple micro pick up array mount  250  with distribution plate  240 . Other manners of retaining micro pick up array mount  250  may be used. For example, retainer plate  270  may be bonded directly to insulation plate  260  using known adhesive or thermal bonding techniques, e.g., welding or soldering. 
     Referring to  FIG. 4A , a side view illustration of an actuator assembly having an actuator and a flexure attachment is shown in accordance with an embodiment of the invention. In an embodiment, actuator assembly  220  includes at least one actuator  310  that creates motion between first actuator attachment  312  and second actuator attachment  314 . For example, actuator assembly  220  may include three linear actuators that each move first actuator attachment  312  relative to second actuator attachment  314  in a single linear direction. Thus, actuator assembly  220  may create a total of at least two degrees of freedom between mass transfer tool mount  200  coupled with first actuator attachment  312  and distribution plate  240  coupled with second actuator attachment  314 . More particularly, actuator assembly  220  may tip and tilt distribution plate  240  relative to mass transfer tool mount  200 . The quantity and type of actuator  310 , may be varied in actuator assembly  220  to change the degrees of freedom and/or range of motion between mass transfer tool mount  200  and distribution plate  240 , e.g., actuator  310  may be a rotary actuator instead of a linear actuator. Accordingly, in an embodiment, actuator assembly  220  may provide a third degree of freedom in a z direction by extending each of three linear actuators simultaneously. However, in another embodiment, additional degrees of freedom may be provided by actuators external to mass transfer tool manipulator assembly  102 , such as by a single linear actuator of mass transfer tool  100  that may move mass transfer tool mount  200  in a z direction. Similarly, as described above, x-y stage  110  may provide additional degrees of freedom between components of mass transfer tool  100  and mass transfer tool manipulator assembly  102 . Thus, in an embodiment, actuation of distribution plate  240  may not depend solely on movement of actuator assembly  220 , but it may also depend on external actuators. 
     In an embodiment, actuator  310  may be a piezoelectric actuator. Although other linear actuators may be used, e.g., hydraulic, pneumatic, or electromechanical actuators, a piezoelectric actuator may exhibit fine positioning resolution through relatively short movements when controlled by signals communicated through actuator lead  404 . In an embodiment, actuator  310  may be a piezoelectric actuator with a range of motion of about 30 microns. 
     In an embodiment, first actuator attachment  312  may include first flexure attachment  402 . First flexure attachment  402  may include one or more flexure relief  406 . Flexure relief  406  may be configured to provide flexibility to first flexure attachment  402  in directions other than the direction of motion of actuator  310 . For example, flexure relief  406  may include a channel machined in first flexure attachment  402  to provide flexibility in a direction orthogonal to the length of actuator  310 . Furthermore, first flexure attachment  402  may provide movement without hysteresis to counteract any backlash that may be present in actuator  310 . Actuator  310  and first flexure attachment  402  may be coupled with a coupling shaft  408  having ends that engage bores formed in actuator  310  and first flexure attachment  402 . Coupling shaft  408  may be allowed to float within the bores, or be rigidly fixed therein using known bonding and clamping methods. 
     Referring to  FIG. 4B , a perspective view of a tip-tilt-z flexure of a mass transfer tool manipulator assembly is shown in accordance with an embodiment of the invention. Tip-tilt-z flexure  230  may include a top flexure component  410  and a bottom flexure component  412 . In an embodiment, top flexure component  410  and bottom flexure component  412  are connected by a flexible coupling  414 . Flexible coupling  414  may have numerous configurations, for example, flexible coupling  414  may include a beam coupling or a helical coupling having one or more radial slot  416  through a portion of a sidewall of tip-tilt-z flexure  230 . In an embodiment, the radial slots  416  may be separated from each other by one or more partition  418 . Alternatively, radial slot  416  may be a single helically formed slot through tip-tilt-z flexure  230 . 
     Flexible coupling  414  may be configured to allow top flexure component  410  and bottom flexure component  412  to move relative to each other along a z axis  420  and about a tip axis  422  and a tilt axis  424 . Resultantly, when top flexure component  410  couples with mass transfer tool mount  200  through a rigid housing  210 , and bottom flexure component  412  couples with actuator assembly  220  through a rigid distribution plate  240 , motion between top flexure component  410  and bottom flexure component  412  mirrors the motion between mass transfer tool mount  200  and distribution plate  240 . Thus, tip-tilt-z flexure  230  allows actuator assembly  220  to adjust distribution plate  240 , as well as micro pick up array mount  250  and/or micro pick up array  290  coupled with distribution plate  240 , relative to mass transfer tool mount  200 . 
     In addition to allowing the actuation of micro pick up array mount  250  and/or micro pick up array  290  coupled with distribution plate  240 , tip-tilt-z flexure  230  may facilitate such actuation in numerous ways. For example, a stiffness of flexible coupling  414  of tip-tilt-z flexure  230  may be tuned to permit micro pick up array mount  250  to deform when contacting a micro device on a carrier substrate. Also, the stiffness of flexible coupling  414  of tip-tilt-z flexure  230  may be tuned to smooth movement of actuator assembly  220 . Furthermore, the stiffness of flexible coupling  414  of tip-tilt-z flexure  230  may be tuned to provide a pick up force that retracts a micro device gripped by an electrostatic transfer head  703  from a carrier substrate. 
     In an embodiment, flexible coupling  414  may be stiffer than the compliant element of micro pick up array mount  250  described in further detail below. Matching stiffness between flexible coupling  414  and the compliant element in this way may permit the compliant element to deform as needed when an array of electrostatic transfer heads contacts an array of micro devices. That is, rather than having the contact load absorbed by flexible coupling  414 , the contact load may instead be absorbed by a compliant element. Furthermore, the compliant element may deform under such load and the deformation may be sensed by displacement sensor  518  integrated with the compliant element and used as feedback to adjust actuator assembly  220 . 
     In an embodiment, flexible coupling  414  may provide a reactive load to distribution plate  240  as actuator assembly  220  moves distribution plate  240 . For example, in the case of tilting distribution plate  240  by actuator assembly  220  having three actuators, the kinematics of each actuator may be slightly mismatched, resulting in unwanted jerkiness or torsion, e.g., yawing, of distribution plate  240 . The stiffness of flexible coupling  414  may be tuned to counteract this kinematic mismatch and resist unwanted movement. For example, in an embodiment, flexible coupling  414  having beam coupling as described above, i.e., having partitions  418  between radial slots  416 , a torsional stiffness of flexible coupling  414  may be sufficiently high to prevent rotation about z axis  420  and thereby limit motion of distribution plate  240  entirely to tipping and tilting about tip axis  422  and tilt axis  424 . 
     In an embodiment, flexible coupling  414  may be expanded in length under a tensile load applied by actuator assembly  220 , but the work exerted on flexible coupling  414  may result in potential energy being stored to cause a restorative load after deactivation of actuator assembly  220 . In other words, flexible coupling  414  may act as a tension spring to pull on distribution plate  240 , and micro pick up array mount  250  coupled with distribution plate  240 , after removing the biasing load of actuator assembly  220 . In the case where the array of electrostatic transfer heads  703  electrostatically grip an array of micro devices attached to a carrier substrate, the restorative load generated by flexible coupling  414  may be greater than the load required to pick up the array of micro devices from the carrier substrate, i.e., the breaking pressure. For example, the breaking pressure may be expected to be about two atmospheres in an embodiment, and thus, flexible coupling  414  may be tuned to generate a restorative load equivalent to a pressure higher than two atmospheres when extended. Thus, after the array of electrostatic transfer heads have been made to grip an array of micro devices, actuator assembly  220  may be deactivated and the pick up pressure may be provided by restorative loading from flexible coupling  414 . 
     In an embodiment, micro pick up array mount  250  includes sensors that provide feedback signals to a position sensing module  316  and/or computer system  108  through one or more electrical connections, such as flex circuit  318 . As described below, feedback may include analog signals from displacement sensors that are used in a control loop to regulate actuation of actuator  310 , and therefore, spatial orientation of micro pick up array mount  250 . Position sensing module  316  may be located nearby micro pick up array mount  250  to reduce signal degradation by limiting a distance that analog signals must travel from a displacement sensor to position sensing module  316 . Position sensing module  316  may also be located on an opposite side of insulation plate  260  to reduce heat transfer from micro pick up array mount  250  to position sensing module  316  and actuators  310 . Maintaining thermal isolation between position sensing module  316  and micro pick up array mount  250  may reduce signal distortion caused by heat effects on position sensing module  316 . Maintaining thermal isolation between actuators  310 , such as piezoelectric actuators, and micro pick up array mount  250  may protect against thermal drift of the actuators  310  and consequently the ability of the mass transfer tool manipulator assembly  102  to accurately adjust the spatial orientation of the micro pick up array mount  250  supporting the array of micro devices. 
       FIGS. 5A-6  and  FIGS. 8-12  illustrate alternative embodiments of a micro pick up array mount  250  that may be coupled with distribution plate  240  to allow the spatial orientation of micro pick up array mount  250  to be adjusted when actuator assembly  220  adjusts distribution plate  240 . Each of the embodiments enable a spatial orientation of an electrostatic transfer head to be adjusted through articulation of micro pick up array mount  250  or a micro pick up array  290 . In an embodiment, micro pick up array mount  250  may include any of the auto-aligning structures illustrated and described in related U.S. application Ser. Nos. 13/715,557 and 13/715,591, which are hereby incorporated by reference. 
     Referring to  FIG. 5A , a perspective view of a micro pick up array mount having a displacement sensor integrated with a compliant element is shown in accordance with an embodiment of the invention. For the purpose of reference, the illustrated view may be referred to as a “front side” or “front face” of micro pick up array mount  250 . In an embodiment, micro pick up array mount  250  includes base  502  and pivot platform  504 . In an embodiment, base  502  surrounds all or a part of pivot platform  504 . For example, base  502  may extend laterally around pivot platform  504 . In an alternative embodiment, base  502  does not surround pivot platform  504 . Base  502  and pivot platform  504  may be interconnected by one or more compliant elements. For example, in the illustrated embodiment, a compliant element may be represented by beam  506 . Beam  506  may connect with base  502  and pivot platform  504  at one or more pivot locations, such as inner pivot  508 ,  514  and outer pivot  510 ,  516 . In an embodiment, inner pivots  508 ,  514  and outer pivots  510 ,  516  may be located on edges of base  502  and pivot platform  504  that are orthogonal to each other. 
     In accordance with embodiments of the invention, micro pick up array mount  250  may be formed from one or more portions or parts. For example, in an embodiment, base  502 , pivot platform  504 , and one or more compliant elements (e.g. beam  506 ) may be formed from a silicon wafer to produce distinct regions. More specifically, known processes, such as deep etching, laser cutting, etc. may be used to form channels  522 . In at least one embodiment, channels  522  may therefore define the structure of micro pick up array mount  250  by providing separations between, e.g., base  502 , beam  506 , and pivot platform  504  regions. For example, channels  522  may create a separation of about one hundred microns between base  502  and beam  506 , as well as between beam  506  and pivot platform  504 . Materials other than silicon may be utilized for the micro pick up array mount  250 , based on the ability of a material to deflect under applied load, thermal stability, and minimal spring mass. For example, beside silicon, suitable material choices for forming a micro pick up array mount  250  may include, but are not limited to, silicon carbide, aluminum nitride, stainless steel, and aluminum. 
     Beam  506  may extend from inner pivot  508  to outer pivot  510  laterally around pivot platform  504 . More particularly, beam  506  may conform to base  502  and pivot platform  504  by fitting between those components and at least partially filling a void between those components. In an embodiment, the lateral extension of beam  506  provides a lever arm that allows for bending and torsion in beam  506 , inner pivots  508 ,  514 , and outer pivots  510 ,  516 , when forces are applied to pivot platform  504  or to a micro pick up array  290  mounted on pivot platform  504 . More specifically, when a force is applied to pivot platform  504 , such as when an electrostatic transfer head on a mounted micro pick up array  290  contacts a micro device on a carrier substrate, pivot platform  504  may deflect relative to base  502 . This deflection may be accompanied by the development of one or more high strain areas, as represented by dotted line region Detail X, near outer pivot  510 . Similar strain regions may develop near inner pivots  508 ,  514  and outer pivot  516  depending on the location that force is applied to pivot platform  504 . 
     In an embodiment, beam  506  stiffness may be selected to facilitate both pick up and placement of a micro device from a carrier substrate or a receiving substrate. For example, beam  506  stiffness may be tuned to ensure that electrostatic transfer heads on pivot platform  504  are not damaged after contacting micro devices on a carrier substrate, or after micro devices gripped by electrostatic transfer heads contact a receiving substrate. That is, beam  506  stiffness may permit beam deformation sufficient to allow pivot platform  504  to deflect through a contact range. For example, in an embodiment, pivot platform  504  may be expected to deflect upward at least thirty microns when electrostatic transfer heads contact an array of micro devices with a load less than the load required to damage electrostatic transfer heads. 
     In addition, beam  506  stiffness may be tuned to prevent plastic deformation of beam  506  during pick up of a micro device from a carrier substrate. For example, when an electrostatic transfer head grips a micro device on a carrier substrate, retraction of the mass transfer tool manipulator assembly  102  may move base  502  upward relative to pivot platform  504  associated with the electrostatic transfer head. In essence, micro pick up array mount  250  acts like a tension spring pulling the array of micro devices gripped by the array of electrostatic transfer heads. In an embodiment, beam  506  stiffness allows such movement without causing plastic deformation in beam  506 . For example, in an embodiment in which an expected amount of about two atmospheres of pressure is required to lift a micro device from a carrier substrate, beam  506  resists at least two atmosphere of pressure applied to pivot platform  504  prior to being plastically deformed. 
     In an embodiment, one or more displacement sensors  418  may be integrated with beam  506  at or near a high strain area. Displacement sensors  418  may be capable of sensing beam  506  displacement resulting from loads applied to portions of micro pick up array mount  250 , such as pivot platform  504 . For example, displacement sensors  418  may detect movement of beam  506  directly, or it may detect internal deformation to infer movement of beam  506 . 
     Referring to  FIG. 5B , a plan view of a displacement sensor integrated with a compliant element of a micro pick up array mount, taken from Detail X of  FIG. 5A , is shown in accordance with an embodiment of the invention. In an embodiment, displacement sensor  518  may be a strain gauge that measures deformation of beam  506 . The strain gauge may exhibit an electrical resistance that varies with material deformation. More specifically, the strain gauge may be configured to deform when beam  506  deforms. That is, the strain gauge 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 gauge may be formed from various materials and integrated with beam  506  in numerous ways to achieve this goal. Several such embodiments are described below. 
     A strain gauge may be separately formed from beam  506  and attached thereto. In an embodiment, the strain gauge includes an insulative flexible backing that supports a foil formed from polysilicon and electrically insulates the foil from beam  506 . The foil may be arranged in a serpentine pattern, for example. An example of an attachable strain gauge is a Series 015DJ general purpose strain gauge manufactured by Vishay Precision Group headquartered in Malvern, Pa. A strain gauge that is separately formed from beam  506  may be attached to beam  506  using numerous processes. For example, the strain gauge backing may be directly attached to beam  506  with an adhesive or other bonding operation. More specifically, strain gauge backing may be fixed to a surface of beam  506  using solder, epoxy, or a combination of solder and a high-temperature epoxy. 
     In another embodiment, a strain gauge may be formed on beam  506  in a desired pattern, such as a serpentine pattern. In an embodiment, a strain gauge may be formed directly on beam  506  using a deposition process. For example, constantan copper-nickel traces may be sputtered directly on beam  506  in a serpentine pattern. The dimensions of a strand of a sputtered strain gauge 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 beam  506  may be modified to form an integrated strain gauge. More specifically, beam  506  may be doped with a piezoresistive material to create a strain gauge within beam  506 . As an example, the surface of beam  506  may be doped with silicon. The doped material may be in a serpentine pattern, having dimensions that vary with an applied strain. Thus, the strain gauge may be fully integrated and physically indistinct from the remainder of beam  506 . 
     In an embodiment, displacement sensor  518  may be a strain gauge on beam  506  having a pattern (e.g. serpentine) of lengthwise strands that align in a direction of anticipated strain. For example, beam  506  may be expected to see compressive or tensile loads in a high strain area that aligns with channels  522  and thus the lengthwise strands of displacement sensor  518  may be parallel with channels  522 . However, in an embodiment of micro pick up array mount  250  having compliant elements that see primary strain planes in other directions, displacement sensor  518  may be oriented to detect such strains. 
     During the transfer of micro devices from a carrier substrate, beam  506  and displacement sensor  518  may be subjected to elevated temperatures, and thus, temperature compensation may be necessary. In an embodiment, displacement sensor may be self-temperature compensated. More specifically, strain gauge 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 dummy gauge technique. 
     Still referring to  FIG. 5B , in an embodiment, a dummy gauge technique utilizes a reference strain gauge  520  to compensate for displacement sensor  518 . More particularly, reference strain gauge  520  may be located near displacement sensor  518  in the same area of strain. While strands of displacement sensor  518  may align with the direction of applied strain, strands of reference strain gauge  520  may extend orthogonally to the strands of displacement sensor  518  and to the direction of applied strain. Alternatively, reference strain gauge  520  may be located in a non-strain area of micro pick up array mount  250 , apart from displacement sensor  518 , which is located in a high strain area of beam  506 . For example, reference strain gauge  520  may be located on base  502  or pivot platform  504 . Therefore, displacement sensor  518  may be configured to detect strain applied to beam  506  and reference strain gauge may be configured to detect strain from thermal effects on micro pick up array mount  250 . Accordingly, a comparison of strain in both strain gauges may be used to determine, and compensate for, strain related to thermal expansion of beam  506 . 
     Referring again to  FIG. 5A , in an embodiment, reference strain gauge  520  and displacement sensor  518  may be wired into adjacent legs of a half Wheatstone bridge to cancel out temperature effects between displacement sensor  518  and reference strain gauge  520 . Each displacement sensor  518  and reference strain gauge  520  may form a half Wheatstone bridge to sense strain in high strain areas near inner pivots  508 ,  514  or outer pivots  510 ,  516 . However, each inner pivot  508 ,  514  and outer pivot  510 ,  516  may include a second high strain area, opposite from the first high strain area, and near a second lateral edge of the pivot defined by channels  522 . Another displacement sensor  518 , or a pair of displacement sensor  518  and reference strain gauge  520 , may be located in this second high strain area to sense deformation. Furthermore, both pairs of displacement sensor  518  and reference strain gauge  520  may be wired together in a full Wheatstone bridge that may be monitored to determine the material strain near the inner pivots  508 ,  514  and outer pivots  510 ,  516 . As described below, monitoring these strain signals may be used to infer the pressure applied to pivot platform  504 . Furthermore, strain signals may be used by a control algorithm to determine the required tip, tilt, and z (orthogonal to pivot platform face) movement required to evenly distribute pressure across pivot platform  504 . 
     Other types of sensors may be used to sense deformation or displacement in a compliant element of micro pick up array mount  250 . For example, different strain gauge types, including capacitive strain gauges and strain gauges that utilize fiber optic sensing may be used to sense beam  506  deformation. Alternatively, displacement of either a compliant element, or another component of micro pick up array mount  250 , such as pivot platform  504 , may be measured directly. In an embodiment, laser interferometers may be used to sense displacement of a compliant element or pivot platform  504 . In another embodiment, capacitive displacement sensors may be used to sense displacement of a compliant element or pivot platform  504 . Thus, numerous manners may be selected to measure and provide feedback related to displacement of a pivot platform  504  or compliant element. In an embodiment, selection may be guided by trade-offs such as cost, required accuracy, and environmental considerations. For example, the ability to compensate for thermal effects on a displacement sensor  518  may be one selection criteria. 
     In an embodiment, micro pick up array mount  250  includes one or more pivot platform operational voltage contacts  530  on pivot platform  504 . Pivot platform operational voltage contacts  530  may function to transfer an operating voltage to an array of electrostatic transfer heads on micro pick up array  290  when operably connected with micro pick up array mount  250 . In an embodiment, pivot platform operational voltage contacts  530  may be formed using a suitable technique such as, but not limited to, sputtering or electron beam evaporation of a conductive material (e.g., metal) onto a surface of pivot platform  504 . 
     In an embodiment, micro pick up array mount  250  may include one or more bonding sites for mounting micro pick up array  290 . In an embodiment, a bonding site includes one or more clamp electrodes  540  located on pivot platform  504 . More particularly, clamp electrodes  540  may be located on the same surface of pivot platform  504  as pivot platform operating voltage contact  530 . Clamp electrodes  540  may be constructed to secure or clamp micro pick up array  290  using electrostatic principles. For example, clamp electrodes  540  may include one or more conductive pads covered by a dielectric layer. In accordance with the principles of electrostatic grippers, when the conductive pads are maintained at a voltage and placed adjacent to metal or semiconductor film clamp areas on micro pick up array  290 , an electrostatic force clamps micro pick up array  290  to micro pick up array mount  250 . Here, the term adjacent may refer to the conductive pads being separated from the clamp areas only by a thin dielectric layer. 
     The components on the front face of micro pick up array mount  250  may be placed in electrical connection with other components of mass transfer tool  100  and mass transfer tool manipulator assembly  102  through various leads. For example, front flex circuit  550  may extend from external components of mass transfer tool  100  and mass transfer tool manipulator assembly  102  to electrically connect with front flex circuit connector  552  on a face or edge of base  502 . Front flex circuit  550  may be, for example, a multi-conductor ribbon cable and front flex circuit connector  552  may be a mating connector. Furthermore, front flex circuit connector  552  may include terminal contacts from which various traces originate and extend to components on the front face of micro pick up array mount  250 . 
     As an example, displacement sensor  518  may be electrically connected with front flex circuit connector  552  through one or more displacement sensor trace  554 . More particularly, displacement sensor  518  may be electrically connected with two traces, an input and an output trace ( FIG. 5B ), that connect with separate terminal contacts of mating connector. The one or more traces are graphically depicted as a single line in  FIG. 5A , and furthermore, traces are either omitted or shown with broken lines to indicate that the number of actual leads has been portrayed schematically for the sake of brevity in illustration. 
     Similarly, reference strain gauge  520  may be electrically connected with front flex circuit connector  552  through one or more reference strain gauge trace  556 . Pivot platform operating voltage contact  530  may be electrically connected with front flex circuit connector  552  through one or more operating voltage trace  558 . Clamp electrode  540  may be electrically connected with front flex circuit connector  552  through one or more clamp electrode trace  560 . In an embodiment, traces may be formed directly on micro pick up array mount  250  using a suitable technique such as sputtering or e-beam evaporation. In an alternative embodiment, traces may be a wire separate from, or bonded to a surface of, micro pick up array mount  250 . 
     Referring to  FIG. 6 , a perspective view of a micro pick up array mount having a heating element on a pivot platform in accordance with an embodiment of the invention. For the purpose of reference, the illustrated view may be referred to as a “back side” or “back face” of micro pick up array mount  250 . Micro pick up array mount  250  may include one or more heating elements  602  over a back side of pivot platform  504  of micro pick up array mount  250 . In an embodiment, heating element  602  may be formed from a resistance alloy, such as a nickel-chromium alloy sputtered on micro pick up array mount  250 . Thus, heating element  602  may undergo Joule heating as electrical current passes through it. Therefore, heat may be transferred from heating element  602  to micro pick up array mount  250  and/or micro pick up array  290  joined therewith. In an alternative embodiment, heating element  602  may be a surface mounted resistor based on surface-mount technology that dissipates heat at a rate depending on a current applied to the resistor. In an embodiment, micro pick up array mount may be heated by an external heating component, such as an infrared heating source directed toward pivot platform  504 . 
     In an embodiment, micro pick up array mount  250  includes one or more temperature sensors  610  to sense the temperature of micro pick up array mount  250  or nearby structures, e.g., a micro pick up array  290 . For example, temperature sensor  610  may be located on a back side of pivot platform  504  to measure the temperature of the pivot platform  504 . For example, temperature sensor  610  may be located in a center of pivot platform  504 , a corner of pivot platform  504 , or on base  502  or beam  506 . Temperature sensor  610  may be a thermistor, thermocouple, or other type of temperature sensor. Furthermore, temperature sensor  610  may be potted or otherwise adhered or mechanically fixed to pivot platform  504 . 
     In accordance with embodiments of the invention, heating element  602  and/or temperature sensor  610  may be located on a front or back side of micro pick up array mount  250 . The choice of location may be driven by considerations such as available space and whether the heating element  602  and temperature sensor  610  will interfere with other functions. For example, the components may be placed to avoid disrupting electrical charge in clamp electrode  540  of micro pick up array mount  250  or electrostatic transfer heads of micro pick up array  290 . Furthermore, the components may be placed to avoid interfering with bonding of micro pick up array  290  to micro pick up array mount  250 . Temperature sensor  610  may be placed to closely approximate the peak temperature of micro pickup array  290 . Temperature offsets may be employed as necessary to achieve this approximation. 
     The components on the back face of micro pick up array mount  250  may be placed in electrical connection with other components of mass transfer tool  100  and mass transfer tool manipulator assembly  102  through various leads. For example, back flex circuit  620  may extend from external components of mass transfer tool  100  and mass transfer tool manipulator assembly  102  to electrically connect with back flex circuit connector  630  mounted on a face or edge of base  502 . Back flex circuit  620  may be, for example, a multi-conductor ribbon cable and back flex circuit connector  630  may be a mating connector. Furthermore, back flex circuit connector  630  may include terminal contacts from which various traces originate and extend to components on the back face of micro pick up array mount  250 . As such, heating element  602  may be electrically connected with back flex circuit connector  630  through one or more heating trace  640 . Temperature sensor  610  may be electrically connected with back flex circuit connector  630  through one or more temperature sensor trace  642 . In an embodiment, traces may be formed directly on micro pick up array mount  250  using a suitable technique such as sputtering or e-beam evaporation. In an alternative embodiment, traces may be a wire separate from, or bonded to a surface of, micro pick up array mount  250 . 
     Referring to  FIG. 7 , a micro pick up array having a substrate supporting an array of electrostatic transfer heads is shown in accordance with an embodiment of the invention. Micro pick up array  290  may include a base substrate  702 , formed from one or more of silicon, ceramics, and polymers, supporting an array of electrostatic transfer heads  703 . Each electrostatic transfer head  703  may include a mesa structure  704  including a top surface  708 , which may support an electrode  712 . However, electrode  712  is illustrative, and in another embodiment, mesa structure  704  may be wholly or partially conductive, such that electrode  712  may be unnecessary. A dielectric layer  716  covers top surface  708  of each mesa structure  704  and electrode  712 , if present. A top contact surface  718  of each electrostatic transfer head  703  has a maximum dimension, for example a length or width of 1 to 100 μm, which may correspond to the size of a micro device to be picked up. 
     Mesa structure  704  protrudes away from base substrate  702  so as to provide a localized contact point of the top contact surface  718  to pick up a specific micro device during a pick up operation. In an embodiment, mesa structure  704  has a height of approximately 1 μm to 5 μm, or more specifically approximately 2 μm. In an embodiment, mesa structure  704  may have top surface  708  with surface area between 1 to 10,000 square micrometers. Mesa structure  704  may be formed in a variety of geometries, e.g., square, rectangular, circular, oval, etc., while maintaining this general surface area range. The height, width, and planarity of the array of mesa structures on base substrate  702  are chosen so that each electrostatic transfer head  703  can make contact with a corresponding micro device during a pick up operation, and so that an electrostatic transfer head  703  does not inadvertently make contact with a micro device adjacent to an intended corresponding micro device during the pick up operation. 
     Still referring to  FIG. 7 , electrode lead  714  may place electrode  712  or mesa structure  704  in electrical connection with a terminal of operating voltage via  720  and with substrate operating voltage contact  722 . Thus, an operating voltage may be transferred from substrate operating voltage contact  722  of micro pick up array  290  to an array of electrostatic transfer heads  703  through operating voltage via  720 . Operating voltage via  720  may be formed in numerous manners. For example, operating voltage via  720  may be formed by drilling or etching a hole through base substrate  702 , passivating the hole with an insulator, and forming a conductive material (e.g., metal) into the passivated hole to form operating voltage via  720  using a suitable technique such as sputtering, e-beam evaporation, electroplating, or electroless deposition. 
     Micro pick up array  290  may include one or more substrate clamp contacts  724  formed on a back side of micro pick up array  290 . In one embodiment, substrate clamp contact  724  includes a conductive pad, such as a metal or semiconductor film. The conductive pad may be electrically isolated from the other active regions of the micro pick up array  290 . For example, insulating layers may be formed under, over, and around the conductive pads. In another embodiment, substrate clamp contact  724  may be integrally formed with micro pick up array  290 , for example by forming micro pick up array  290  and substrate clamp contact  724  from bulk silicon, and electrically isolating substrate clamp contact  724  from the other active regions of micro pick up array  290 . 
     Referring to  FIG. 8 , a cross-sectional side view illustration of a micro pick up array mount joined with a micro pick up array is shown in accordance with an embodiment of the invention. Micro pick up array  290  and micro pick up array mount  250  may be physically and operably joined. As described above, in accordance with the principles of electrostatic grippers and using the attraction of opposite charges, substrate clamp contact  724  of micro pick up array  290  may be aligned with, and electrostatically retained by, clamp electrode  540  on micro pick up array mount  250 . More specifically, upon applying an electrostatic voltage to clamp electrode  540  through clamp electrode trace  560 , an electrostatic gripping pressure will be applied to substrate clamp electrode  540 , causing micro pick up array  290  to physically join with micro pick up array mount  250 . Furthermore, one or more substrate operating voltage contacts  722  of micro pick up array  290  may be aligned with, and placed adjacent to, pivot platform operating voltage contacts  530 . Thus, a voltage applied to pivot platform operating voltage contact  530  through operating voltage trace  558  may be transferred through substrate operating voltage contact  722  and operating voltage via  720  to one or more electrostatic transfer heads  703 . Thus, micro pick up array mount  250  and micro pick up array  290  may be electrically connected to enable micro pick up array  290  to generate an electrostatic gripping force on an array of micro devices. 
     Heat may be delivered from micro pick up array mount  250  to micro pick up array  290  and/or to an array of micro devices gripped by micro pick up array  290  when those components are physically joined. More specifically, heating element  602  on micro pick up array mount  250  may be resistively heated by delivering electrical current through heating trace  640 . Thus, heat may be transferred from heating element  602  through pivot platform  504  to micro pick up array  290 . Furthermore, the heat delivered to micro pick up array  290  may dissipate through the array of electrostatic transfer heads  703  into an array of micro devices gripped by the array of electrostatic transfer heads  703 . 
     The embodiments described above with regard to  FIGS. 5A-8  thus far have characterized a configuration of micro pick up array mount  250  that may be reversibly paired with micro pick up array  290 . However, such a configuration is intended to be illustrative and not exhaustive. For example, an alternative embodiment of micro pick up array mount  250  may include different modes of electrical connection with components of mass transfer tool  100  or mass transfer tool manipulator assembly  102 . Furthermore, electrostatic transfer head  703  and/or micro pick up array  290  may be alternatively joined with micro pick up array mount  250  in different manners. Additionally, the design of a compliant element in micro pick up array mount  250  may be changed within the scope of the invention. The following  FIGS. 9-12  illustrate several alternative embodiments in accordance with such variations. 
     Referring to  FIG. 9 , a perspective view of a micro pick up array mount having a displacement sensor integrated with a compliant element, and an array of electrostatic transfer heads on a pivot platform is shown in accordance with an embodiment of the invention. Most components of the embodiment of micro pick up array mount  250  shown in  FIG. 9  are the same or similar to those shown in  FIG. 5A . However at least two substantial differences are described below. First, electrical connections between the components on the front face of micro pick up array mount  250  are achieved differently. Second, rather than utilize a separate micro pick up array  290 , the array of electrostatic transfer heads  703  are directly integrated with micro pick up array mount  250 . 
     In an embodiment, component traces may terminate at voltage landing pads on base  502  to make electrical connections. For example, displacement sensor trace  554  may interconnect displacement sensor  518  with displacement sensor landing pad  902 . Similarly, reference strain gauge trace  556  may interconnect reference strain gauge  520  with reference strain gauge landing pad  904 . Furthermore, operating voltage trace  558  may interconnect electrostatic transfer head  703  formed on pivot platform  504  with base operating voltage landing pad  906 . The landing pads may be located on via structures that pass through base  502  from a front side to a back side of micro pick up array mount  250 . Landing pads may be formed using processes similar to those used to form traces, e.g., using sputtering processes. 
     In an embodiment, an array of electrostatic transfer heads are supported directly by pivot platform  504 . The structure and formation of the array of electrostatic transfer heads  703  may be the same or similar to that described above with respect to  FIG. 7 . For example, each electrostatic transfer head  703  may include mesa structure  704  with top surface  708  covered by dielectric layer  716  and optionally supporting electrode  712 . However, the array of electrostatic transfer heads are located on a surface of pivot platform  504  instead of micro pick up array  290  surface. Furthermore, operating voltage traces  458  may replace electrode leads  714 . 
     Referring to  FIG. 10 , a perspective view of a micro pick up array mount having a heating element on a pivot platform is shown in accordance with an embodiment of the invention. In an embodiment, one or more contacts may be located on base  502  and placed in electrical connection with components of micro pick up array mount  250 . Some of the base  502  contacts may be placed in electrical connection with components on the front side of micro pick up array mount  250 . For example, displacement sensor contact  1002  may be located at a terminal of displacement sensor via ( FIG. 11 ) in electrical connection with displacement sensor landing pad  902 . Similarly, reference strain gauge contact  1004  may be located at a terminal of a via (not shown) in electrical connection with reference strain gauge landing pad  904 . Furthermore, base operating voltage contact  1006  may be located at a terminal of base operating voltage via ( FIG. 11 ) in electrical connection with base operating voltage landing pad  906 . Others of the base  502  contacts may be placed in electrical connection with components on the back side of micro pick up array mount  250 . For example, heating contact  1008  may be placed in electrical connection with heating element  602  through heating trace  640 . Similarly, temperatures sensor contact  1010  may be placed in electrical connection with temperature sensor  610  through temperature sensor trace  642 . 
     Referring to  FIG. 11 , a cross-sectional side view illustration of a micro pick up array mount in electrical connection with a spring contact, taken about section line B-B of  FIG. 9 , is shown in accordance with an embodiment of the invention. One or more of the contacts, e.g., displacement sensor contact  1002  or base operating voltage contact  1006 , may be pressed against spring contact  1106 . Spring contact  1106  may further be connected with components of mass transfer tool  100  or mass transfer tool manipulating assembly  102  through electrical connections such as wiring leads and/or contact boards (not shown). Thus, numerous manners are available to electrically connect components on micro pick up array mount  250  and components of mass transfer tool  100  or mass transfer tool manipulator assembly  102 . 
     Referring to  FIG. 12 , a perspective view illustration of a micro pick up array mount having a flexible region is shown in accordance with an embodiment of the invention. Most components of the embodiment of micro pick up array mount  250  shown in  FIG. 12  are the same or similar to those shown in  FIG. 5A . However at least two differences are described below. First, in an embodiment, micro pick up array mount  250  illustrated in  FIG. 12  may be permanently joined with micro pick up array  290 . Second, in an embodiment, micro pick up array mount  250  illustrated in  FIG. 12  includes a compliant element without a beam  506 . 
     In an embodiment, micro pick up array mount  250  and micro pick up array  290  may be joined using one or more bonding pads  1202  to replace clamp electrode  540 . Bonding pad  1202  may be formed of a variety of materials including polymers, solders, metals, and other adhesives to facilitate the formation of a permanent bond with another structure. In an embodiment, bonding pads  1202  may include gold, copper, or aluminum to facilitate thermocompression bonding with an adjacent structure. However, thermocompression bonding represents only one manner of forming a permanent bond between structures, and bonding pad  1202  may include other materials that facilitate the formation of a bond between the micro pick up array mount  250  and another part or structure with other bonding mechanisms. For example, direct bonding, adhesive bonding, reactive bonding, soldering, etc., may be used at numerous bonding sites having various shapes and sizes. 
     To facilitate permanent bonding between micro pick up array  290  and micro pick up array mount  250 , substrate clamp contact  724  on micro pick up array  290  may be formed of a metallic material that facilitates a thermocompression bond with bonding pad  1202 , for example, both bonding pad  1202  and substrate clamp contact  724  may be formed from gold. Prior to permanently bonding micro pick up array mount  250  and micro pick up array  290 , pivot platform operating voltage contact  530  and substrate operating voltage contact  722  may be aligned to allow the components to be operably joined. After aligning the components, a permanent thermocompression bond may be formed to permanently join micro pick up array mount  250  with micro pick up array  290 . 
     In an embodiment, the compliant element of micro pick up array mount  250  includes a singular surface not having beam  506 . More specifically, a compliant element may be located between pivot platform  504  and base  502 , without being separated by channels  522 . For example, a compliant element may include flexible region  1204  delineated by a dotted line, which exists between pivot platform  504  and base  502 . Flexible region  1204  may be integrally formed with pivot platform  504  and base  502 , but may have different stiffness from those components. Alternatively, the difference in stiffness may be due to varied structural characteristics, such as through forming flexible region  1204  with a thinner cross-section or a flexible form, e.g., as in the case of a bellows. The reduced stiffness of flexible region  1204  may permit flexible region  1204  to flex and allow relative movement between pivot platform  504  and base  502 . Thus, one or more displacement sensor  518  may be integrated with flexible region  1204  to sense deformation of flexible region  1204 . In an embodiment, electrical leads may be directly routed across flexible region  1204  of micro pick up array mount  250 . For example, operating voltage trace  558  may cross directly through flexible region  1204 , as opposed to being routed around channels  522  as shown in the embodiment of  FIG. 5A . 
     Having described several of the individual components of mass transfer tool manipulator assembly  102 , attention shall now be turned to the overall function and control of the mass transfer tool manipulator assembly  102 . Referring to  FIG. 13 , a side view illustration of a mass transfer tool manipulator assembly holding a micro pick up array and interconnected with a control system is shown in accordance with an embodiment of the invention. The illustrated system may be used to perform methods including the transfer of micro devices from carrier substrate to receiving substrate. More specifically, the system may be used to actively control the spatial relationship between an array of electrostatic transfer heads  703  coupled with micro pick up array mount  250  and an array of micro devices on a carrier substrate or a receiving substrate. Furthermore, the system may be used to control an electrostatic gripping force between the array of electrostatic transfer heads  703  and the array of micro devices. In addition, the system may be used to control heat delivered to the array of electrostatic transfer heads  703 , e.g., while the array of electrostatic transfer heads  703  contacts the array of micro devices. Furthermore, the system may be used to control retention of an array of electrostatic transfer heads  703  against micro pick up array mount  250 . 
     In an embodiment, actuation of actuator assembly  220 , under the control of computer system  108 , affects motion of micro pick up array  290 . For example, computer system  108  may be connected with an actuator power supply  1302  directly or through intermediate controllers to provide control signals that cause actuator power supply  1302  to regulate movement of one or more actuator  310 , e.g., piezoelectric actuators, to move distribution plate  240  coupled with micro pick up array mount  250 . Micro pick up array mount  250  may retain micro pick up array  290 . Such regulation may be based on signals delivered from actuator power supply  1302  to actuator assembly  220  through actuator lead  404 . 
     In an embodiment, activating an array of electrostatic transfer heads provides for electrostatic gripping of an array of micro devices. For example, computer system  108  may be connected with an operating voltage supply  1304  directly or through intermediate controllers to provide control signals that cause operating voltage supply  1304  to deliver an electrostatic voltage to electrostatic transfer heads through operating voltage lead  1306 . Operating voltage lead  1306  may be integrated within, e.g., front flex circuit  550  or back flex circuit  620 , to deliver operating voltage as described above. 
     In an embodiment, heating of an array of electrostatic transfer heads may be controlled by delivering power to heating element  602 . For example, computer system  108  may be connected with heating voltage supply  1308  directly or through intermediate controllers to provide control signals that cause heating voltage supply  1308  to deliver power to heating element  602  through heating voltage lead  1310 . Heating voltage lead  1310  may be integrated within, e.g., front flex circuit  550  or back flex circuit  620 , to deliver heating power as described above. 
     In an embodiment, micro pick up array  290  having an array of electrostatic transfer heads may be retained against micro pick up array mount  250  by delivering an electrostatic voltage to clamp electrode  540 . For example, computer system  108  may be connected with a clamping voltage supply  1312  directly or through intermediate controllers to provide control signals that cause clamping voltage supply  1312  to deliver an electrostatic voltage to clamp electrode  540  through clamping voltage lead  1314 . Clamping voltage lead  1314  may be integrated within, e.g., front flex circuit  550  or back flex circuit  620 , to deliver clamping voltage as described above. 
     Control of the motion, electrostatic gripping, and heating functions of mass transfer tool manipulator assembly  102  may be based on feedback delivered from sensors associated with micro pickup array mount. For example, temperature data may be provided from temperature sensor  610  to computer system  108  through, e.g., back flex circuit  620 . Similarly, position-related data may be delivered from one or more displacement sensor  518  to computer system  108  through, e.g., front flex circuit  550 . 
     In an embodiment, position-related data from displacement sensor  518  may be input to, and transformed by, position sensing module  316  prior to being delivered to computer system  108 . For example, position sensing module  316  or another component may apply an excitation voltage to one or more displacement sensor  518 , e.g., strain gauges, and an analog output voltage from displacement sensor  518  may be monitored by position sensing module  316 . The analog output voltages from the one or more displacement sensors may then undergo analog-to-digital processing by position sensing module  316 , and the resulting digital signals may be input to computer system  108 , or further processed through logical operations, to facilitate performance of a control algorithm for controlling motion of mass transfer tool manipulator assembly  102 . 
     Referring to  FIG. 14 , a schematic illustration of a control loop to regulate a mass transfer tool manipulator assembly is shown in accordance with an embodiment of the invention. In an embodiment, the control loop may be closed to achieve the goal of evenly distributing pressure across micro pick up array mount  250 . In other words, the control loop may regulate mass transfer tool manipulator assembly  102  to change the center of pressure on micro pick up array mount  250  to a desired location, e.g., to center pressure applied to pivot platform  504  and evenly distribute pressure throughout the compliant element(s) surrounding pivot platform  504 . Thus, setpoint  1402  may define a set of reference signals that correspond to each displacement sensor  518  sensing the same deformation in a respective beam  506 . Displacement measurements from each displacement sensor  518  may be input to position sensing module  316  as feedback related to a current state of pressure distribution across micro pick up array mount  250 . Position sensing module  316  may perform analog-to-digital signal processing and calculate, or deliver processed signals to computer system  108  for calculation of, e.g., an error signal. Based on the error signal, computer system  108  may use a control algorithm to determine appropriate control signals to actuate actuator assembly  220  to achieve even distribution of pressure across micro pick up array mount  250 . These control signals may be delivered directly to actuator assembly  220 , or they may be modified, e.g., by increasing control signal power, with amplifier  1404 . Furthermore, the control signals may be fed directly to actuator assembly  220  or to actuator power supply  1302  for driving actuator assembly  220 . Displacement measurements from each displacement sensor  518  may continue to be monitored and fed into a control algorithm to continue to adjust actuator assembly  220  until output  1406  equals setpoint  1402 , i.e., until pressure evenly distributes across micro pick up array mount  250 . This basic control loop model will be described further below in relation to embodiments of methods for using mass transfer tool manipulator assembly  102  to pick up and place an array of micro devices. 
     In the following description, reference is made to  FIGS. 15-24  when describing manners of operating a mass transfer tool manipulator assembly to transfer an array of micro devices in accordance with embodiments of the invention. It is to be appreciated that the schematic illustrations provided in  FIGS. 16-19  and  FIGS. 21-24  are simplified two dimensional illustrations. For example, deflection of compliant elements such as schematic beams  1606 ,  1608  and actuation of the mass transfer tool manipulator assembly  102  with a pair of schematic actuators  1602 ,  1604  is illustrated and described in two dimensions. It is to be appreciated however, that deflection and actuation of the mass transfer tool manipulator assembly  102  in accordance with embodiments of the invention is not so limited. For example, as described above, various actuators may be used to provide additional degrees of freedom, and these degrees of freedom may not be fully represented by the two-dimensional depiction of  FIGS. 16-19  and  FIGS. 21-24 . More particularly, as shown in  FIG. 4A , the actuator assembly  220  may include more than two actuators, e.g., three actuators  310 . In such a case, pivot platform  504  may be tilted or tipped in a third dimension about an axis running across the page surface, which is not represented by  FIGS. 16-19  and  FIGS. 21-24 . 
     Referring to  FIG. 15 , a flowchart illustrating a method of picking up a micro device from a carrier substrate is shown in accordance with an embodiment of the invention. For illustrational purposes, the following description of  FIG. 15  makes reference to the embodiments illustrated in  FIGS. 16-19 . At operation  1501 , mass transfer tool manipulator assembly  102  moves toward carrier substrate. Referring to  FIG. 16 , a schematic illustration of a mass transfer tool manipulator assembly moving toward a carrier substrate  1601  is shown in accordance with an embodiment of the invention. Movement of manipulator assembly, and more specifically pivot platform  504 , may be achieved by actuation of various actuators of mass transfer tool  100  or by actuating both first schematic actuator  1602  and second schematic actuator  1604  to extend in length. Electrostatic transfer heads  703  are schematically represented as being mounted on pivot platform  504 , although electrostatic transfer heads  703  may instead be mounted on micro pick up array  290  retained against pivot platform  504 . As shown, pivot platform  504  may be undeflected relative to base  502 , and thus, both first schematic beam  1606  and second schematic beam  1608  may be undisplaced or undeformed. In this initial state, there may be a gap between array of electrostatic transfer heads  703  and array of micro devices  1610  on carrier substrate  1601 , e.g., this snapshot may be prior to contacting array of micro devices  1610  with array of electrostatic transfer heads  703 . Here, the illustrated exaggeration of the gap indicates that pivot platform  504  and carrier substrate  1601  may be misaligned with each other. 
     Referring again to  FIG. 15 , at operation  1505  array of micro devices  1610  on carrier substrate  1601  may be contacted with array of electrostatic transfer heads  703  coupled with pivot platform  504  of mass transfer tool manipulator assembly  102 . Referring to  FIG. 17 , a schematic illustration of an electrostatic transfer head coupled with a mass transfer tool manipulator assembly contacting a micro device on a carrier substrate is shown in accordance with an embodiment of the invention. In an embodiment, as pivot platform  504  approaches carrier substrate  1601  out of alignment, an electrostatic transfer head  703  nearest first schematic beam  1606  may contact a micro device  1610  before contacting a micro device  1610  with an electrostatic transfer head  703  nearest second schematic beam  1608 . Thus, first schematic beam  1606  may deform, while second schematic beam  1608  may not. 
     Referring again to  FIG. 15 , at operation  1510  deformation of a compliant element coupled with pivot platform  504  may be sensed. Referring again to  FIG. 17 , in an embodiment, as first schematic beam  1606  deforms, displacement sensor  518  (see  FIG. 5A ) generates a displacement signal associated with first schematic beam  1606 . The displacement signal may be monitored and/or measured, e.g., by position sensing module  316 . For example, the displacement signal may be fed back to position sensing module  316  to determine that deformation of first schematic beam  1606  has occurred, and to calculate an error signal indicating the presence of an uneven pressure distribution across pivot platform  504 . 
     Referring to  FIG. 18 , a schematic illustration of a mass transfer tool manipulator assembly adjusting a position of a micro pick up array mount is shown in accordance with an embodiment of the invention. After sensing deformation in first schematic beam  1606  and calculating an error signal from the measured data, a control signal may be delivered from computer system  108  to actuator assembly  220 , causing second schematic actuator  1604  to extend while maintaining first schematic actuator  1602  length. More specifically, second schematic actuator  1604  may be extended to adjust the spatial orientation of pivot platform  504  until the nearby electrostatic transfer head  703  contacts a micro device  1610 , e.g., once pivot platform  504  aligns with carrier substrate  1601 . Furthermore, adjustment may be based on continued feedback signals from displacement sensors associated with first schematic beam  1606  and second schematic beam  1608 . That is, adjustment may continue until the measured deformation in first schematic beam  1606  and second schematic beam  1608  is approximately equal. At this point, the pressure distribution across pivot platform  504  in the illustrated plane may be even. 
     Referring again to  FIG. 15 , at operation  1515  relative movement between the mass transfer tool manipulator assembly  102  and carrier substrate  1601  stops. Referring again to  FIG. 17 , once pressure is evenly distributed across pivot platform  504 , actuation of actuator assembly  220  according to control signals may be ceased. At this point, output  1406  of the control loop may equal setpoint  1402 . That is, the error signal may be zero or within a predefined range, indicating that the deformation sensed by each displacement sensor  518  is approximately equal. This deformation value may be further defined through the control loop to achieve a desired pressure between array of electrostatic transfer heads  703  and array of micro devices  1610 . For example, sufficient pressure may be applied to ensure secure contact while avoiding damage to electrostatic transfer heads  703  and micro devices  1610  from excessive pressure application. 
     Referring again to  FIG. 15 , at operation  1520  a voltage may be applied to array of electrostatic transfer heads to create a grip pressure on array of micro devices. As shown in  FIG. 18 , with array of electrostatic transfer heads  703  placed in contact with array of micro devices  1610 , an electrostatic voltage may be applied to electrostatic transfer heads  703  through various contacts and connectors, e.g., operating voltage lead  1306 , operating voltage trace  558 , operating voltage via  720 , etc., of the mass transfer tool manipulator assembly  102 , micro pick up array mount  250 , and micro pick up array  290 . More specifically, voltage may be transmitted from operating voltage supply  1304  to array of electrostatic transfer heads  703  based on control signals from computer system  108 . For example, the control signals may be based on a control algorithm instructing that electrostatic transfer heads be activated if a predefined deformation is simultaneously sensed by each displacement sensor  518  during a pick up process. As a result, array of electrostatic transfer heads applies a gripping pressure to array of micro devices  1610 . 
     Referring again to  FIG. 15 , at operation  1525  array of micro device  1610  may be picked up from carrier substrate  1601 . Referring to  FIG. 19 , a schematic illustration of a mass transfer tool manipulator assembly picking up a micro device from a carrier substrate is shown in accordance with an embodiment of the invention. First schematic actuator  1602  and second schematic actuator  1604  may be controlled by computer system  108  to retract pivot platform  504  from carrier substrate  1601 . During retraction, first schematic beam  1606  and second schematic beam  1608  may return toward an undeformed state, as the beams release stored energy and spring back to an initial configuration. Simultaneously, displacement sensors associated with the beams may transmit signals to position sensing module  316  that indicate the beams are not deformed. However, at this stage a control algorithm may instruct that pivot platform  504  be retracted further to clear the array of micro devices  1610  for transfer to a receiving substrate. This retraction may be achieved through actuation of actuator assembly  220 , or in another embodiment, through actuation of various actuators of mass transfer tool  100 . Furthermore, in an embodiment, retraction may be achieved by deactivating actuator assembly  220  and allowing the inherent stiffness of flexible coupling  414  of tip-tilt-z flexure  230  to restore tip-tilt-z flexure  230  to an initial state, which causes retraction of micro pick up array mount  250 . During pick up, the electrostatic voltage supplied to the array of electrostatic transfer heads may persist, and thus, array of micro devices  1610  may be retained on the electrostatic transfer heads  703  and removed from carrier substrate  1601 . 
     During the pick up operation described with respect to  FIG. 15 , heating element  602  on micro pick up array mount  250  may be heated. For example, heating element  602  may be resistively heated to transfer heat to micro pick up array  290  and to micro devices in contact with electrostatic transfer heads. Heat transfer may occur before, during, and after picking up the array of micro devices  1610  from carrier substrate  1601 . 
     Referring to  FIG. 20 , a flowchart illustrating a method of placing a micro device on a receiving substrate is shown in accordance with an embodiment of the invention. For illustrational purposes, the following description of  FIG. 20  makes reference to the embodiments illustrated in  FIGS. 21-24 . At operation  2001 , mass transfer tool manipulator assembly  102  moves toward a receiving substrate. Referring to  FIG. 21 , a schematic illustration of a mass transfer tool manipulator assembly moving toward a receiving substrate is shown in accordance with an embodiment of the invention. Movement of manipulator assembly, and more specifically pivot platform  504 , may be achieved by actuation of various actuators of mass transfer tool  100  or by actuating both first schematic actuator  1602  and second schematic actuator  1604  to extend in length. As shown, pivot platform  504  may be undeflected relative to base  502 , and thus, both first schematic beam  1606  and second schematic beam  1608  may be undisplaced or undeformed. In this initial state, there may be a gap between array of micro devices  1610  gripped by array of electrostatic transfer heads  703  and receiving substrate  2101 , e.g., this snapshot may be prior to contacting receiving substrate  2101  with array of micro devices  1610 . Here, the illustrated exaggeration in the gap indicates that pivot platform  504  and receiving substrate  2101  may be misaligned with each other. 
     Referring again to  FIG. 20 , at operation  2005  receiving substrate  2101  is contacted with array of micro devices carried by array of electrostatic transfer heads coupled with pivot platform of mass transfer tool manipulator assembly. Referring to  FIG. 22 , a schematic illustration of a micro device carried by an electrostatic transfer head coupled with a mass transfer tool manipulator assembly contacting a receiving substrate is shown in accordance with an embodiment of the invention. In an embodiment, as pivot platform  504  approaches receiving substrate  2101  out of alignment, a micro device  1610  gripped by an electrostatic transfer head  703  nearest first schematic beam  1606  may contact receiving substrate  2101  before receiving substrate  2101  contacts a micro device gripped by an electrostatic transfer head nearest second schematic beam  1608 . Thus, first schematic beam  1606  may deform, while second schematic beam  1608  may not. 
     Referring again to  FIG. 20 , at operation  2010  deformation of a compliant element coupled with pivot platform  504  may be sensed. Referring again to  FIG. 22 , in an embodiment, as first schematic beam  1606  deforms, displacement sensor  518  associated with first schematic beam  1606  generates a displacement signal. The displacement signal may be monitored and/or measured, e.g., by position sensing module  316 . For example, the displacement signal may be fed back to position sensing module  316  to determine that deformation of first schematic beam  1606  has occurred, and to calculate an error signal indicating the presence of an uneven pressure distribution across pivot platform  504 . 
     Referring to  FIG. 23 , a schematic illustration of a mass transfer tool manipulator assembly adjusting a position of a micro pick up array mount is shown in accordance with an embodiment of the invention. After sensing deformation in first schematic beam  1606  and calculating an error signal from the measured data, a control signal may be delivered from computer system  108  to actuator assembly  220 , causing second schematic actuator  1604  to extend while maintaining first schematic actuator  1602  length. More specifically, second schematic actuator  1604  may be extended to adjust the spatial orientation of pivot platform  504  until the nearby electrostatic transfer head  703  contacts a micro device  1610 , e.g., once pivot platform  504  aligns with receiving substrate  2101 . Furthermore, adjustment may be based on continued feedback signals from displacement sensors associated with first schematic beam  1606  and second schematic beam  1608 . That is, adjustment may continue until the measured deformation in first schematic beam  1606  and second schematic beam  1608  is approximately equal. At this point, the pressure distribution across pivot platform  504  in the illustrated plane may be even. 
     Referring again to  FIG. 20 , at operation  2015  relative movement between mass transfer tool manipulator assembly  102  and receiving substrate  2101  may be stopped. Referring again to  FIG. 23 , once pressure is evenly distributed across pivot platform  504 , actuation of actuator assembly  220  according to control signals may be ceased. At this point, output  1406  of the control loop may equal setpoint  1402 . That is, the error signal may be zero or within a predefined range, indicating that the deformation sensed by each displacement sensor  518  is approximately the same. This deformation value may be further defined through the control loop to achieve a desired pressure between array of micro devices  1610  and receiving substrate  2101 . For example, sufficient pressure may be applied to ensure secure contact while avoiding damage to micro devices from excessive pressure application. 
     Referring again to  FIG. 20 , at operation  2020  a voltage is removed from array of electrostatic transfer heads. As shown in  FIG. 23 , with array of micro devices  1610  placed in contact with receiving substrate  2101 , an electrostatic voltage may be removed from electrostatic transfer heads  703 . More specifically, operating voltage transmitted from operating voltage supply  1304  to array of electrostatic transfer heads  703  may be discontinued based on control signals from computer system  108 . For example, the control signals may be based on a control algorithm instructing that electrostatic transfer heads  703  be deactivated if a predefined deformation is sensed in each displacement sensor  518  simultaneously during a placement operation. As a result, array of micro devices  1610  are released from array of electrostatic transfer heads  703 . 
     Referring again to  FIG. 20 , at operation  2025  array of micro devices  1610  may be released onto receiving substrate  2101 . Referring to  FIG. 24 , a schematic illustration of a mass transfer tool manipulator assembly releasing a micro device onto a receiving substrate is shown in accordance with an embodiment of the invention. First schematic actuator  1602  and second schematic actuator  1604  may be controlled by computer system  108  to retract pivot platform  504  from receiving substrate  2101 . During retraction, first schematic beam  1606  and second schematic beam  1608  may return toward an undeformed state, as the beams release stored energy and spring back to an initial configuration. Simultaneously, displacement sensors associated with the beams may transmit signals to position sensing module  316  that indicate no deformation of the beams. However, at this stage a control algorithm may instruct that pivot platform  504  be retracted further to clear the pivot platform  504  and to begin another pick up operation. This retraction may be achieved through actuation of actuator assembly  220 , or in another embodiment, through actuation of various actuators of mass transfer tool  100 . Furthermore, in an embodiment, retraction may be achieved by deactivating actuator assembly  220  and allowing the inherent stiffness of flexible coupling  414  of tip-tilt-z flexure  230  to restore tip-tilt-z flexure  230  to an initial state, which causes retraction of micro pick up array mount  250 . 
     During the placement operation described with respect to  FIG. 20 , heat may be applied to the array of micro devices  1610 . For example, heating element  602  may be resistively heated as described above to transfer heat through micro pick up array mount  250  into the array of electrostatic transfer heads that grip micro devices  1610 . Maintaining an elevated temperature of micro pick up array mount  250  in this manner may avoid some problems that arise from temperature variations in an operating environment. Micro devices  1610  may be heated continuously throughout the placement operation. However, more particularly, micro devices  1610  may be heated after deflection of compliant element is sensed and/or after micro devices  1610  are in contact with receiving substrate  2101 . In an embodiment, each electrostatic transfer head  703  in the array is heated uniformly, e.g., to a temperature of 50 degrees Celsius, 180 degrees Celsius, 200 degrees Celsius, or even up to 350 degrees Celsius. These temperatures may cause melting or diffusion between micro devices  1610  and receiving substrate  2101  to bond the micro devices to the receiving substrate. 
     Referring to  FIG. 25 , a schematic illustration of a computer system that may be used is shown in accordance with an embodiment of the invention. Portions of embodiments of the invention are comprised of or controlled by non-transitory machine-readable and machine-executable instructions which reside, for example, in machine-usable media of a computer system  108 . Computer system  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., amplifier  1404  and position sensing module  316 , have been broken out for discussion separately above, computer system  108  may integrate those components directly or include additional components that fulfill similar functions. 
     Computer system  108  of  FIG. 25  includes an address/data bus  2502  for communicating information, and a central processor  2504  unit  2504  coupled to bus  2502  for processing information and instructions. Computer system  108  also includes data storage features such as a computer usable volatile memory  2506 , e.g. random access memory (RAM), coupled to bus  2502  for storing information and instructions for central processor  2504  unit, computer usable non-volatile memory  2508 , e.g. read only memory (ROM), coupled to bus  2502  for storing static information and instructions for the central processor  2504  unit, and a data storage device  2510  (e.g., a magnetic or optical disk and disk drive) coupled to bus  2502  for storing information and instructions. Computer system  108  of the present embodiment also includes an optional alphanumeric input device  2512  including alphanumeric and function keys coupled to bus  2502  for communicating information and command selections to central processor  2504  unit. Computer system  108  also optionally includes an optional cursor control device  2514  coupled to bus  2502  for communicating user input information and command selections to central processor  2504  unit. Computer system  108  of the present embodiment also includes an optional display device  2516  coupled to bus  2502  for displaying information. 
     The data storage device  2510  may include a non-transitory machine-readable storage medium  2518  on which is stored one or more sets of instructions (e.g. software  2520 ) embodying any one or more of the methodologies or operations described herein. Software  2520  may also reside, completely or at least partially, within the volatile memory  2506 , non-volatile memory  2508 , and/or within processor  2504  during execution thereof by the computer system  108 , the volatile memory  2506 , non-volatile memory  2508 , and processor  2504  also constituting non-transitory machine-readable storage media. 
     As used above, “coupling”, “fastening”, “joining”, “retaining”, etc., of one component against or with another may be accomplished using various well-known methods, such as bolting, pinning, clamping, thermal or adhesive bonding, etc. The use of such terms is not intended to be limiting, and indeed, it is contemplated that such methods may be interchangeable in alternative embodiments within the scope of the invention. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.

Metadata:
Filing Date: 20160226
Publication Date: 20180717
Grant Date: 20180717
Priority Date: 20130225
Inventors: GOLDA, DARIUSZ
HIGGINSON, JOHN A.
BIBL, ANDREAS
PARKS, PAUL ARGUS
BATHURST, Stephen Paul
Assignee: APPLE INC
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Family ID: 51388330