PATENT DOCUMENT

Publication Number: US-11480742-B1
Application Number: US-202017089509-A
Country: US
Kind Code: B1

Title: Micro device mass transfer tool

Abstract:
A micro device transfer tool and methods of operation are described. In an embodiment, the micro device transfer tool includes an articulating transfer head assembly capable of six degrees of motion. A miniatured camera assembly may be secured near the point of contact for the articulating transfer head assembly to aid in system alignment. In an embodiment, an encoder system is described for alignment of a micro pick up array and target substrate using complementary concentric grating patterns. In an embodiment a miniaturized position sensor design is described for sensing position of various system components during alignment or pick and place processes.

Claims:
What is claimed is: 
     
       1. A position sensor comprising:
 a light source; 
 a light source light guide including a light source light guide proximal end adjacent the light source and a light source light guide distal end adjacent a guide tip; 
 a first photodiode; 
 a first rigid light guide including a first rigid light guide proximal end adjacent the first photodiode and a first rigid light guide distal end adjacent the guide tip; 
 a second photodiode; and 
 a second rigid light guide including a second rigid light guide proximal end adjacent the second photodiode and a second rigid light guide distal end adjacent the guide tip. 
 
     
     
       2. The position sensor of  claim 1 , wherein the first photodiode and the second photodiode are mounted on laterally opposite sides of a circuit board. 
     
     
       3. The position sensor of  claim 2 , wherein the first photodiode and the second photodiode cover a same area on the circuit board. 
     
     
       4. The position sensor of  claim 3 , further comprising a plurality of thermally conductive vias in the circuit board extending between the first photodiode and the second photodiode. 
     
     
       5. The position sensor of  claim 2 , wherein the first rigid light guide distal end is located laterally between the light source light guide distal end and the second rigid light guide distal end. 
     
     
       6. The position sensor of  claim 5 , wherein the first rigid light guide distal end has a smaller area than the second rigid light guide distal end. 
     
     
       7. The position sensor of  claim 2 , wherein the light source is mounted on the circuit board. 
     
     
       8. The position sensor of  claim 2 , wherein the first and second photodiodes are mounted on a respective first and second thermoelectric coolers mounted on the circuit board. 
     
     
       9. The position sensor of  claim 2 , further comprising relay optics attached to the guide tip. 
     
     
       10. The position sensor of  claim 9 , wherein the relay optics include:
 a collimator assembly; and 
 a focus lens assembly distal to the collimator assembly. 
 
     
     
       11. The position sensor of  claim 10 , wherein the collimator assembly is fixed to the guide tip, and the focus lens assembly is releasably attachable to the collimator assembly. 
     
     
       12. The position sensor of  claim 11 , wherein the collimator assembly includes a collimator lens and a pupil stop. 
     
     
       13. The position sensor of  claim 12 , wherein the circuit board, the light source light guide, the first rigid light guide, and the second rigid light guide are contained in a housing. 
     
     
       14. The position sensor of  claim 1 , wherein:
 the first rigid light guide includes a single first core and a first cladding surrounding the single first core; and 
 the second rigid light guide includes a single second core and a second cladding surrounding the single second core. 
 
     
     
       15. The position sensor of  claim 1 , wherein:
 the first rigid light guide includes a first fiber bundle; and 
 the second rigid light guide includes a second fiber bundle. 
 
     
     
       16. A micro device transfer tool comprising:
 an articulating transfer head assembly including a mounting plate; 
 a pivot mount assembly that is releasably attachable with the mounting plate; 
 a plurality of position sensors arranged over the pivot mount assembly to measure displacement of the pivot mount assembly at a plurality of locations; and 
 a camera mounted inside the articulating transfer head assembly and extending though an opening in the mounting plate adjacent to the pivot mount assembly. 
 
     
     
       17. The micro device transfer tool of  claim 16 , wherein each position sensor comprises:
 a light source; 
 a light source light guide including a light source light guide proximal end adjacent the light source and a light source light guide distal end adjacent a guide tip; 
 a first photodiode; 
 a first rigid light guide including a first rigid light guide proximal end adjacent the first photodiode and a first rigid light guide distal end adjacent the guide tip; 
 a second photodiode; and 
 a second rigid light guide including a second rigid light guide proximal end adjacent the second photodiode and a second rigid light guide distal end adjacent the guide tip. 
 
     
     
       18. The micro device transfer tool of  claim 17 , wherein the first photodiode and the second photodiode are mounted on laterally opposite sides of a circuit board. 
     
     
       19. The micro device transfer tool of  claim 18 , wherein the first photodiode and the second photodiode cover a same area on the circuit board. 
     
     
       20. A position sensor comprising:
 a light source light guide including a light source light guide distal end adjacent a guide tip; 
 an inner fiber arrangement including an inner fiber arrangement distal end adjacent the guide tip; 
 an outer fiber arrangement including an outer fiber arrangement distal end adjacent the guide tip; 
 a collimator assembly fixed to the guide tip; and 
 a focus lens assembly releasably attachable to the collimator assembly. 
 
     
     
       21. The position sensor of  claim 20 , wherein the collimator assembly includes a collimator lens and a pupil stop.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 62/933,928 filed on Nov. 11, 2019 and U.S. Provisional Patent Application Ser. No. 62/933,941 filed on Nov. 11, 2019, the full disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     Embodiments described herein relate to systems and methods of micro device transfer, and related integrated sensor assemblies. 
     Background Information 
     Integration and packaging issues are one of the main obstacles for the commercialization of micro devices such as radio frequency (RF) microelectromechanical systems (MEMS) microswitches, light-emitting diodes (LEDs), and MEMS or quartz-based oscillators. 
     Traditional technologies for transferring of devices such as “direct printing” and “transfer printing” include transfer by wafer bonding from a transfer wafer to a receiving wafer. In both traditional and variations of the direct printing and transfer printing technologies, the transfer wafer is de-bonded from a device after bonding the device to the receiving wafer. In addition, the entire transfer wafer with the array of devices is involved in the transfer process. 
     In one process variation a transfer tool including an array of electrostatic transfer heads is used to pick up and transfer an array of micro devices from a carrier (donor) substrate to a receiving substrate. In such an implementation, the transfer heads operate in accordance with principles of electrostatic grippers, using the attraction of opposite charges to pick up the micro devices. 
     SUMMARY 
     Embodiments describe micro device transfer tools and subassemblies thereof to assist in alignment, including miniaturized cameras, encoder patterns of concentric gratings, and miniaturized position sensors. In an embodiment, a micro device transfer tool includes a carrier substrate holder, a receiving substrate holder, and an articulating transfer head assembly that is translatable along an x-translation direction between the carrier substrate holder and the receiving substrate holder. The articulating transfer head assembly may include a mounting plate and a camera pointing away from a bottom surface of the mounting plate. For example, the mounting plate may be used to secure a replaceable micro pick up array including an array of transfer heads used for the transfer of arrays of micro devices. In an embodiment, the camera is offset in a first direction relative to a center of the mounting plate. A second camera may also point away from the bottom surface of the mounting plate, and be offset in a second direction relative to the center of the mounting plate. In this manner, the miniaturized camera(s) may be located laterally adjacent to the MPA, and in the same frame of reference which allows for a more accurate system alignment with reduced thermal drift and geometric error. In a particular configuration, the first direction is parallel to the x-translation direction between the carrier substrate holder and the receiving substrate holder, and the second direction is orthogonal to the x-translation direction between the carrier substrate holder and the receiving substrate holder. 
     In an embodiment a micro device transfer tool includes a tip-tilt assembly, a piezoelectric stage assembly mounted underneath the tip-tilt assembly, a mounting plate underneath the piezoelectric stage assembly, and a camera mounted within the piezoelectric stage assembly and extending through an opening in a bottom surface of the mounting plate. The opening may be off-centered in the mounting plate as described above, for example in the first direction. In an embodiment, the camera is mounted in a z-flexure assembly of the piezoelectric stage assembly. For example, the piezoelectric stage assembly can include a plurality of stages nested inside one another, such as an x-stage assembly, a y-stage assembly, a θz stage (rotation) assembly. 
     A center-opening may also be formed trough a center of the mounting plate, and a kinetic sensor assembly may be mounted over the center-opening. For example, the kinetic sensor may be a position sensor to measure the position of a pivot mount assembly that is releasably attachable with the mounting plate such that the pivot mount assembly spans underneath the center-opening. In an embodiment, an exterior-most lens of the camera is located less than one inch (e.g. laterally) from a center of center-opening. In one implementation, the camera has a working distance of greater than 500 μm. An exterior-most lens of the camera may be less than 5 mm in diameter. Thus, the camera may be considered a miniaturized camera designed to focus in the same frame of reference of a micro pick up array held by the pivot mount assembly. The camera is optionally mounted on a z-adjustment stage, which can be motorized and independently movable relative to the piezoelectric stage assembly. The tip-tilt assembly, piezoelectric stage assembly, mounting plate, camera, etc. may be part of an articulating transfer head assembly that is translatable between a carrier substrate holder and a receiving substrate holder. In an embodiment, the camera is offset in a direction relative to a center of the mounting plate parallel to an x-translation direction between the carrier substrate holder and the receiving substrate holder. A second camera may also be mounted within the piezoelectric stage assembly and extend through a second opening in the bottom surface of the mounting plate, where the second camera is offset in a second direction relative to the center of the mounting plate orthogonal to the x-translation direction between the carrier substrate holder and the receiving substrate holder. 
     Either of the first or second cameras, or both can be used to align the micro device transfer tool with a target location. In an embodiment, an alignment method includes translating an articulating transfer head assembly over a place area of a target substrate. For example, the articulating transfer head assembly may include a micro pick up array (MPA) including an array of transfer heads, a first camera offset from a center of the MPA in a first direction and pointing away from the MPA, a second camera offset from the center of the MPA in a second direction, and pointing away from the MPA. At least one of the cameras can then be aligned with one or more alignment features of the target substrate, which in turn can align the array of transfer heads with a target array of placement locations (or pick up locations) on the target substrate. For example, where the target substrate is a donor substrate this method can be used to align the MPA with micro devices to be picked up. Where the target substrate is a receiving substrate, this method can be used to align the MPA with target locations to place an array of micro devices held by the array of transfer heads. 
     In accordance with embodiment, the alignment method may include translating the articulating transfer head assembly along an x-translation direction to the target substrate. Thus, the alignment method can be in-situ during a transfer process. In a particular implementation the first direction of the first camera offset is parallel to the x-translation direction, and the second direction of the second camera offset is orthogonal to the x-translation direction. In operation, aligning the at least one of the cameras with the one or more alignment features of the target substrate can include aligning the first camera with a corresponding first alignment feature while the second camera is not aligned with a corresponding alignment feature. For example, this may potentially occur near an edge of the placement area. In this manner, the x-y offset cameras can compensate for the one another when there is no available space for a corresponding alignment feature on the target substrate. 
     Alignment can also be achieved using encoder patterns. In an embodiment, micro device transfer tool alignment method includes translating a first concentric grating pattern coupled with a first substrate (e.g. MPA including an array of transfer heads) over a second concentric grating pattern coupled with a second substrate (e.g. target substrate such as donor substrate or display substrate), directing a light through the first concentric grating pattern toward the second concentric grating pattern, detecting reflected light with a detector array divided into multiple detection zones, and determining a relative x-y position of the first and second substrates. The alignment method may further include comparing intensity of light in each zone, and determining the relative x-y position based on the intensity of light in each zone. In an embodiment, the first concentric grating pattern includes a plurality of rings of first diameters, and the second concentric grating pattern includes a plurality of rings of second diameters offset from the first diameters. Where the second substrate is a display substrate, the second concentric grating pattern may optionally be superimposed over a plurality of subpixels on the display substrate. 
     The concentric circles on the second substrate may be permanent features. In an embodiment, a display includes a display substrate including a pixel area and a non-pixel area, a plurality of concentric grating patterns on the display substrate. In an embodiment, the plurality of concentric grating patterns is located in the non-pixel area. In an embodiment, the plurality of concentric grating patterns is superimposed over a plurality of subpixels in the pixel area. For example, the plurality of concentric grating patterns can be patterned to include a plurality of line openings directly over light emitting diode landing areas in the plurality of subpixels. 
     The various subassemblies and alignment methods described can be used for x-y spatial alignment as wells as alignment of level surfaces. For example, position sensors are described which may include various structuring features that can mitigate drift, including rigid light guides to mitigate drift due to flexing, and packaging considerations such as photodiodes being mounted on opposite sides of a circuit board to mitigate thermal drift. Furthermore, the rigid light guides and packaging solutions facilitate a miniaturized assembly that can be easily integrated in a variety of locations of the micro device transfer tool. 
     In an embodiment, a position sensor includes a light source, a light source light guide including a light source light guide proximal end adjacent the light source and a light source light guide distal end adjacent a guide tip, a first photodiode, a first rigid light guide including a first rigid light guide proximal end adjacent the first photodiode and a first rigid light guide distal end adjacent the guide tip, a second photodiode, and a second rigid light guide including a second rigid light guide proximal end adjacent the second photodiode and a second rigid light guide distal end adjacent the guide tip. In an embodiment, the first rigid light guide includes a single first core and a first cladding surrounding the single first core, and the second rigid light guide includes a single second core and a second cladding surrounding the single second core. In an embodiment, the first rigid light guide includes a first fiber bundle, and the second rigid light guide includes a second fiber bundle. In an embodiment, the first rigid light guide distal end is located laterally between the light source light guide distal end and the second rigid light guide distal end. The first rigid light guide distal end may have a smaller area than the second rigid light guide distal end in some configurations. 
     In an embodiment, the first photodiode and the second photodiode are mounted on laterally opposite sides of a circuit board. For example, the first photodiode and the second photodiode cover a same area on the circuit board. A plurality of thermally conductive vias may also be formed in the circuit board and extend between the first photodiode and the second photodiode. The light source may also be mounted on the circuit board, and the first and second photodiodes may be mounted on a respective first and second thermoelectric coolers mounted on the circuit board. The circuit board, the light source light guide, the first rigid light guide, and the second rigid light guide can be contained in a housing. 
     A relay optics can be attached to the guide tip. For example, the relay optics may include a collimator assembly, and a focus lens assembly distal to the collimator assembly. For example, the collimator assembly can include a collimator lens and a pupil stop. In an embodiment, the collimator assembly is fixed to the guide tip, while the focus lens assembly is releasably attachable to the collimator assembly. 
     In an embodiment, a position sensor includes a light source light guide including a light source light guide distal end adjacent a guide tip, an inner fiber arrangement including an inner fiber arrangement distal end adjacent the guide tip, an outer fiber arrangement including an outer fiber arrangement distal end adjacent the guide tip, a collimator assembly fixed to the guide tip, and a focus lens assembly releasably attachable to the collimator assembly, which can include a collimator lens and a pupil stop. 
     The position sensors in accordance with embodiments can be located in a variety of locations in the micro device transfer tool to support a variety of different processes. In a particular embodiment, the micro device transfer tool includes an articulating transfer head assembly including a mounting plate, a pivot mount assembly that is releasably attachable with the mounting plate, and a plurality of position sensors arranged over the pivot mount assembly to measure displacement of the pivot mount assembly at a plurality of locations. Generally, the pivot mount assembly may be a working substrate that can flex during the operation of the micro device transfer tool, and in particular during pick and place operations. Each position sensor can include a light source, a light source light guide including a light source light guide proximal end adjacent the light source and a light source light guide distal end adjacent a guide tip, a first photodiode, a first rigid light guide including a first rigid light guide proximal end adjacent the first photodiode and a first rigid light guide distal end adjacent the guide tip, a second photodiode, and a second rigid light guide including a second rigid light guide proximal end adjacent the second photodiode and a second rigid light guide distal end adjacent the guide tip. Thus, embodiments describe an articulating transfer head assembly that can include one or more light guide based position sensors behind a substrate that may flex during operation. In this manner the position sensors can detect distortions in the substrate, either due to temperature or deflection and adjust alignment of the articulating transfer head assembly. In an embodiment, the first photodiode and the second photodiode are mounted on laterally opposite sides of a circuit board. For example, the first photodiode and the second photodiode can cover a same area on the circuit board. A camera may additionally be mounted inside the articulating transfer head assembly and extend though an opening in the mounting plate adjacent to the pivot mount assembly to facilitate concurrent alignment processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a micro device transfer tool in accordance with an embodiment. 
         FIG. 2  is a bottom side perspective view of an articulating transfer head assembly in accordance with an embodiment. 
         FIG. 3A  is an exploded perspective view illustration of a pivot mount assembly and mounting plate in accordance with an embodiment. 
         FIG. 3B  is a schematic cross-sectional side view illustration of a camera mounted on a z-adjustment stage in accordance with an embodiment. 
         FIG. 4  is an exploded view of an articulating transfer head assembly in accordance with an embodiment. 
         FIG. 5  is an exploded view of a plurality of nestled stages in accordance with an embodiment. 
         FIG. 6A  is a cross-sectional plan view of a θx-linkage of an articulating transfer head assembly in accordance with an embodiment. 
         FIG. 6B  is a cross-sectional plan view of a θy-linkage of an articulating transfer head assembly in accordance with an embodiment. 
         FIG. 7  is a perspective view illustration of various cameras and sensors of a micro device transfer tool in accordance with an embodiment. 
         FIG. 8A  is a side view schematic illustration of a method of setting an x-y datum in accordance with an embodiment. 
         FIG. 8B  is a perspective view schematic illustration of a method of setting an x-y datum in accordance with an embodiment. 
         FIG. 8C  is a side view illustration of a method of setting a z-datum in accordance with an embodiment. 
         FIG. 9  is a schematic illustration of a micro device transfer tool with an x-axis translation track with a non-zero curvature in accordance with an embodiment. 
         FIG. 10A  is a schematic bottom view illustration of a bottom side of an articulating transfer head assembly including a pair of x-y offset cameras in accordance with an embodiment. 
         FIG. 10B  is a schematic top view illustration of a pair of corresponding x-y offset reference marks in a display area of a target substrate in accordance with an embodiment. 
         FIG. 10C  is a schematic top-down illustration of a method of aligning a pair of x-y offset cameras with a pair of x-y offset alignment features on a target substrate in accordance with an embodiment. 
         FIG. 10D  is a schematic top-down illustration of no corresponding x-offset alignment feature in accordance with an embodiment. 
         FIG. 10E  is a schematic top-down illustration of no corresponding alignment features in the target display area in accordance with an embodiment. 
         FIG. 10F  is a schematic top-down illustration of using an offset alignment feature from the target display area and an offset alignment feature from an adjacent target display area in accordance with an embodiment. 
         FIG. 11  is a schematic cross-sectional side view illustration of an encoder assembly in accordance with an embodiment. 
         FIG. 12  is a bottom side perspective view of an MPA with multiple encoder patterns in accordance with an embodiment. 
         FIG. 13 , a schematic top view illustration an exemplary moiré pattern superimposed over optics in accordance with an embodiment. 
         FIGS. 14A-14E  are illustrations of resultant moiré patterns indicative of x-y alignment and misalignment in accordance with embodiments. 
         FIG. 15  is a schematic top view illustration of a pair of moiré patterns indicative of angular misalignment in accordance with an embodiment. 
         FIG. 16A  is a schematic top view illustration of a concentric grating pattern superimposed over a plurality of subpixels in the pixel area of a display substrate in accordance with an embodiment. 
         FIG. 16B  is a schematic top view illustration of a complementary concentric grating pattern aligned over the concentric grating pattern of  FIG. 16A  in accordance with an embodiment. 
         FIG. 17A  is a schematic top view illustration of a concentric grating pattern superimposed over a plurality of subpixels in the pixel area of a display substrate in accordance with an embodiment. 
         FIG. 17B  is a schematic top view illustration of a complementary concentric grating pattern aligned over the concentric grating pattern of  FIG. 17A  in accordance with an embodiment. 
         FIG. 18  is an exploded cross-sectional side view illustration of an articulating transfer head assembly, pivot mount assembly, and MPA in accordance with an embodiment. 
         FIG. 19  is a schematic cross-sectional side view illustration of pivot mount assembly curvature in accordance with embodiments. 
         FIG. 20  is conceptual illustration of a method of determining a shape-fitting feedback signal in accordance with an embodiment. 
         FIG. 21  is a schematic cross-sectional side view illustration of a position sensor in accordance with an embodiment. 
         FIG. 22A  is schematic cross-sectional side view illustration of a circuit board of a position sensor in accordance in accordance with an embodiment. 
         FIG. 22B  is schematic top view illustration of a circuit board of a position sensor in accordance in accordance with an embodiment. 
         FIG. 23  is an isometric view illustration of a guide tip of a position sensor in accordance in accordance with an embodiment. 
         FIG. 24A  is a schematic bottom view illustration of a guide tip of a position sensor in accordance with an embodiment. 
         FIG. 24B  is a schematic cross-sectional view illustration of a right light guide in accordance with an embodiment. 
         FIG. 24C  is a schematic cross-sectional view illustration of a rigid light guide including a fiber bundle in accordance with an embodiment. 
         FIG. 25A  is a graphical illustration of a light intensity measured by the first and second rigid light guides as a function of displacement in accordance with an embodiment. 
         FIG. 25B  is a graphical illustration of a ratio of the light intensities measured by the first and second rigid light guides as a function of displacement in accordance with an embodiment. 
         FIG. 26  is a schematic side view illustration of a cone angle generated by position sensor in accordance in accordance with an embodiment. 
         FIG. 27  is a schematic cross-sectional side view illustration of relay optics attached to the guide tip of a position sensor in accordance in accordance with an embodiment. 
         FIG. 28  is a schematic cross-sectional side view illustration of a guide tip of a position sensor in accordance in accordance with an embodiment. 
         FIG. 29  is a schematic side view illustration of a cone angle generated by position sensor in accordance in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe systems and methods for transferring a micro device or an array of micro devices to or from a substrate. For example, the array of micro devices may be micro LED devices. 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, MEMS, and bio-samples. 
     In various embodiments, description is made with reference to 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, etc., in order to provide a thorough understanding of the embodiments. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the embodiments. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. 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”, “spanning” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One component “over”, “spanning” or “on” another component or bonded “to” or in “contact” with another component may be directly in contact with the other component or may have one or more intervening components. One component “between” components may be directly in contact with the competent or may have one or more intervening components. 
     In one aspect, embodiments describe a micro device transfer tool that includes an articulating transfer head assembly used to pick and place an array of micro devices. The articulating transfer head assembly in accordance with embodiments may provide six degrees of motion, including tip (θx angular motion), tilt (θy angular motion), z motion, x motion, y motion, and rotation (θz) during pick and place. The articulating transfer head assembly additionally can minimize parasitic lateral motions during motion at the plane of contact by establishing virtual axes of rotation in θx, θy at the interface of the target substrate and contact surface of a micro pick up array (MPA) attached to the articulating transfer head assembly. 
     In another aspect, embodiments describe a micro device transfer tool that includes one or more miniaturized cameras mounted within a piezoelectric stage assembly and extending through one or more openings in a bottom surface of a mounting plate. In this manner, the miniaturized camera(s) may be located laterally adjacent to the MPA, and in the same frame of reference which allows for a more accurate system alignment with reduced thermal drift and geometric error, which becomes particularly important with micro device transfer at elevated temperatures. As such, system alignment can be made with regard to the frame of reference of the MPA contact surface as opposed to an adjacent feature of the articulating transfer head assembly. This may be facilitated by an unconventional miniaturized camera design in which the optical lenses may have a diameter less than 5 μmm, for example, and a working distance of greater than 500 μm, for example. In an embodiment, the camera outermost focal lens is located (laterally) less than an inch from a center of the MPA. 
     In another aspect, embodiments describe an alignment method in which an articulating transfer head assembly can be aligned over a target substrate using one or more cameras mounting in the articulating transfer head assembly. In an embodiment, an alignment method includes translating an articulating transfer head assembly over a place area of a target substrate. The articulating transfer head assembly can include an MPA including an array of transfer heads, a first camera offset from a center of the MPA in a first direction (e.g. x-direction) and pointing away from the MPA (e.g. toward the target substrate), and a second camera offset from the center of the MPA in a second direction (e.g. y-direction) and pointing away from the MPA (e.g. toward the target substrate). At least one of the cameras can then be aligned with one or more alignment features of the target substrate. This may facilitate aligning the array of transfer heads with corresponding target locations in the place area on the target substrate using the cameras located in the same frame of reference as the MPA, which can allow for a more accurate in-situ system alignment with reduced thermal drift and geometric error. 
     In another aspect, embodiments describe an alignment method in which relative x-y position between substrates is determined with a pair of encoders with concentric grating patterns. For example, each concentric grating pattern can be characterized by line widths, spacing, and radii. Radius offset of matching rings can create a phase offset to enhance signals for detection zones. One pattern can be on a moving component (e.g. on the MPA) and the other pattern can be on a fixed target (e.g. on a display substrate, or donor substrate). In an embodiment, the pattern is translated along an x-axis translation track with the articulating transfer head assembly between substrate holders. The patterns are configured such that misalignment in x, y, or both creates a varying moiré pattern. Relative misalignment of the patterns results in a continuously varying moiré pattern that would have a corresponding intensity field across the pattern. This field can be divided into zones (e.g. quadrants) that is measured by an array of intensity signal integrating sensors (e.g. photodiodes or a CMOS image sensors) located in each of the zones. Differences among the sensors can be used to determine relative x and y position between the moving and fixed encoder patterns, and hence substrates. In accordance with embodiments, such an alignment technique can be used to establish alignment between the substrates (e.g. MPA and target substrate) in a single measurement, with every transfer event. Thus, the alignment is dynamic, and can occur continuously during a mass transfer sequence at high transfer rates. Furthermore, such an intensity-based measurement method is less computationally intensive than an intensive image analysis, which facilitates a reduction of measurement times to the order of 1 microsecond or faster (1 MHz), which can further speed up the mass transfer sequences while maintaining fine alignment. 
     In yet another aspect, embodiments describe a micro device transfer tool that provides a prescribed amount of compression with force feedback during contact. In an embodiment, a kinetic sensory assembly is included to measure displacement of the pivot mount assembly at a plurality of locations during the pick and place operations. The measured feedback is then provided to adjust various system components to maintain parallelism, pressure uniformity, and a prescribed amount of compression. In an embodiment, the kinetic sensor assembly includes a plurality of position sensors (e.g. displacement probes). The position sensors in accordance with embodiments may include various structural features that can mitigate drift, including rigid light guides to mitigate drift due to flexing, and packaging considerations such as photodiodes being mounted on opposite sides of a circuit board to mitigate thermal drift. Furthermore, the rigid light guides and packaging solutions facilitate a miniaturized assembly that can be easily integrated in a variety of locations of the micro device transfer tool. Accordingly, such position sensors are not limited to a kinetic sensor assembly and can be located at other various locations to support different functions such as upward or downward facing probes for setting a z-datum, leveling of the MPA, articulating transfer head assembly, and target substrate chuck leveling. Furthermore, the position sensors in accordance with embodiments may include relay optics that include a fixed collimator assembly, and replaceable focus lens assembly for multiple different applications across the micro device transfer tool that may utilize different working ranges or sensitivity. As used herein the term “position sensor” is intended to cover either or both measurements of an absolute distance (e.g. position), as well as a change in position (e.g. displacement). 
     Referring now to  FIG. 1 , a perspective view illustration of a micro device transfer tool  100  is shown in accordance with an embodiment of the invention. The micro device transfer tool  100  includes one or more assemblies having various components and sub-assemblies with functions that facilitate the mass transfer of micro devices using an array of electrostatic transfer heads. For example, the micro device transfer tool  100  can include an upper assembly  102  having an articulating transfer head assembly  200  to receive a micro pickup array (MPA) containing an array of transfer heads. In an embodiment, the MPA includes an array of micro electrostatic assemblies (MESAs), also referred to as electrostatic transfer heads, where each MESA operates in accordance with electrostatic principles to pick up and transfer a corresponding micro device. In an embodiment each MESA has a localized contact point characterized by a maximum dimension of 1-300 μm in both the x- and y-dimensions. In an embodiment, each MESA has a maximum lateral dimension of 1 to 100 μm, or less. In some embodiments, each MESA has a maximum length and width of 20 μm, 10 μm, or 5 am. Similarly, each micro device, such as a light emitting diode or chip, may have a maximum lateral dimension of 1-300 μm or 1-100 μm, such as 20 μm, 10 am, or 5 μm. The articulating transfer head assembly  200  can include features that allow for the exchange of the MPA and for delivering a voltage to the electrostatic transfer heads to facilitate pick up of a micro device using an electrostatic force. 
     The articulating transfer head assembly  200  in accordance with embodiments may provide for negligible lateral or vertical parasitic motion for small movements of MPA  103 , e.g., motion less than about 5 μmrad about a neutral position. In an embodiment, the articulating transfer head assembly includes a tip-tilt assembly  210  and a piezoelectric stage assembly  250  mounted underneath the tip-tilt assembly  210 . Together the tip-tilt assembly  210  and the piezoelectric stage assembly  250  may provide six degrees of motion. Specifically, the tip-tilt assembly  210  may provide tip (θx) and tilt (θy), where the piezoelectric stage assembly  250  provides z motion, x motion, y motion, and rotation (θz). 
     The micro device transfer tool  100  can also include a lower assembly  104  having a carrier substrate holder  108  and a receiving substrate holder  124 . The carrier substrate holder  108  can be configured to hold a carrier substrate (e.g. donor substrate) supporting an array of micro devices. Furthermore, the receiving substrate holder  124  can be configured to hold a receiving substrate (e.g. display substrate) for receiving the transferred micro devices. Thus, the array of micro devices can be transferred from the carrier substrate to the receiving substrate using the array of electrostatic transfer heads. It will be appreciated that any reference to upper assembly  102  and lower assembly  104  is made for ease of description only, and that some components and subassemblies of micro device transfer tool  100  may be transposed. 
     The various components and subassemblies can be coupled in various manners, e.g., through the use of a gantry  113 , base  114 , side beams  116 , carriage  118 , and other structural connectors. Therefore, it will be appreciated that the micro device transfer tool  100  shown in  FIG. 1  is not exhaustive of all components that can be part of a system in accordance with the scope of this invention, nor should the description be considered to be limiting in this regard. 
     The articulating transfer head assembly  200  may be fixed to a carriage  118  of the micro device transfer tool  100  e.g., at a location along a translation track and be moveable in the x, y, or z directions for translation between stages (e.g. for the substrate holders), as well as for tipping, tilting, rotating, and movement of the MPA  103  in a z direction based on feedback signals within the micro device transfer tool  100 . The carrier substrate holder  108  and receiving substrate holder  124  may also be movable in the x, y, or z directions with a stage  110 . While a single stage  110  is illustrated, it is to be appreciated that each subsystem (e.g. carrier substrate holder  108 , receiving substrate holder  124 , etc. can be located on a separate stage  110 ). 
     Operation of micro device transfer tool  100  and transfer head assembly  200  may be controlled at least in part by a host computer  109 . Host computer  109  may control the operation of articulating transfer head assembly  200  based on feedback signals received from various sensors, strain sensing elements, and image data. 
     The micro device transfer tool  100  may include various sensors (e.g. cameras, position sensors, etc.) that are intended to sense spatial relationships between system components and to work together to facilitate alignment of system components. For example, a downward-looking camera  126  or  290  (see  FIG. 2 ) and an upward-looking camera ( 140 , see  FIG. 7 ) or  128  can be aligned with one another using an alignment tool  130  in order to establish a frame of reference that components can be adjusted within. Similarly, position sensors ( 400 ,  FIG. 4 ) can be integrated within the micro device transfer tool  100  to further establish the frame of reference that components can be adjusted within. The various sensors can also be used to detect positions of components within the frame of reference and to provide feedback to a host computer  109  capable of receiving and processing inputs in order to control the system components accordingly. 
     Referring to  FIG. 2 , a perspective view of an articulating transfer head assembly  200  is shown in accordance with an embodiment. An articulating transfer head assembly  200  may be used in the micro device transfer tool  100  to transfer micro devices to or from a substrate, e.g., receiving substrate or donor substrate, using MPA  103  which is supported by a pivot mount assembly  300 . More particularly, articulating transfer head assembly  200  may provide for negligible lateral or vertical parasitic motion for small movements of MPA  103 , e.g., motion less than about 5 μmrad about a neutral position. 
     In the particular embodiment illustrated a mounting plate  280  is secured underneath the piezoelectric stage assembly  250 . The pivot mount assembly  300  may be mounted onto the mounting plate  280  using a variety of manners such as using tabs or lips to press the pivot mount against the articulating transfer head assembly  200 , bonding, vacuum, electrostatic clamping, or pogo pin array board. The pivot mount assembly  300  may include a support structure (e.g. base)  302 , a pivot platform  304 , and plurality of spring arms  306 , and the MPA  103  supporting an array of MESAs  115  is mounted on the pivot platform  304 . In an embodiment, the pivot mount assembly  300  may include a flex circuit  308  to communicate with a printed circuit board (PCB) that is located nearby within the articulating transfer head assembly  200  to reduce signal degradation by limiting a distance that signals must travel. 
     In accordance with embodiments, one or more cameras  290  can be mounted within the piezoelectric stage assembly  250  and extend through one or more corresponding openings  282  in a bottom surface  281  of the mounting plate  280 . As shown in  FIG. 3A , the openings  282  are off-centered in the mounting plate. In this manner, the miniaturized camera(s)  290  may be located laterally adjacent to the MPA  103  once mounted onto the pivot mount assembly  300 . In the particular embodiment illustrated the cameras  290  are positioned in x-y offset directions from the MPA  103  (e.g. from a center of the MPA  103 , or center of the mounting plate  280 ) and primary translation direction. For example, the cameras  290  can be located at a 3 o&#39;clock position located parallel to an x-axis translation direction (e.g. between the carrier substrate holder  108  and receiving substrate holder  124 ), and also a 6 o&#39;clock position (or 12 o&#39;clock position) being orthogonal to the 3 o&#39;clock position relative to a center of the MPA  103 , and mounting plate  280  center. It is to be appreciated that such positions are exemplary, and it is not required for the camera  290  positions to be parallel or orthogonal to the translation axis of the articulating transfer head assembly  200 . One or more position sensors  400  may also extend through the same or different openings  282 . Additionally, a slot opening  286  can be formed through the mounting plate  280  to accommodate the flex circuit  308 . In this manner, the flex circuit  308  can pass through a slot opening  286  and mate with a pogo pin array  380 , for example, on the back side of the mounting plate  280  (see  FIG. 4 ). Alternatively, the pogo pin array  380  can be mounted on the bottom (mounting) surface  281  of the mounting plate  280 . Also shown in  FIG. 3A  a center-opening  284  is formed through a center of the mounting plate  280 . The pivot mount assembly  300  may then be releasably attached to the mounting plate  280  such that the pivot mount assembly spans underneath the center-opening  284 , which can accommodate flexing of the pivot mount assembly  300  during pick and place operations. Furthermore, as shown in  FIG. 4 , a kinetic sensor assembly  295  can be mounted over the center-opening  284  to measure deflection of the pivot mount assembly  300 . 
     In some embodiments the camera(s)  290  can be mounted on/within a low coefficient of thermal expansion (CTE) structure. For example, the camera may be mounted on the mounting plate  280 , and within the piezoelectric stage assembly  250 . The mounting plate  280  may be formed of a low-CTE material, and may include one or more components. Exemplary low-CTE materials may be characterized by a CTE of less than 5 ppm/° C., or more particularly less than 2 ppm/° C. Exemplary materials include metals such as nickel-iron alloys (e.g. FeNi36), ceramics, etc. 
     In some embodiments the camera(s)  290  can be mounted on/within a z-adjustment stage.  FIG. 3B  is a schematic cross-sectional side view illustration of a camera  290  mounted on a z-adjustment stage  375  in accordance with an embodiment. The z-adjustment stage  375  in turn is mounted on the mounting plate  280  such that the camera  290  can extend through the opening  282 . The z-adjustment stage  375  may be a flexure stage for low backlash. For example, the z-adjustment stage  375  may include a body  372  (e.g. formed of the low-CTE material) including one or more living hinge flexures to provide low or no hysteresis and repeatable motion. An actuator  374  may be included to cause movement of the z-adjustment stage  375 . One or more position sensors  400  may also be optionally mounted on the z-adjustment stage  375 . 
     The z-adjustment stage  375  may allow independent movement of the camera  290  along the z-axis relative to the MPA  103  and mounting plate  280 . Such independent movement can allow the camera  290  to focus on the object plane, or frame of reference. In an embodiment the z-adjustment stage  375  can be motorized for automation. Such a configuration may be utilized to automatically move the camera  290  in coordination with a pick or place sequence to maintain focus during the sequence. 
     In accordance with embodiments, the camera(s)  290  may be located laterally adjacent to the MPA  103 , and in the same frame of reference which allows for a more accurate system alignment with reduced thermal drift and geometric error, which becomes particularly important with micro device transfer at elevated temperatures. As such, system alignment can be made with regard to the frame of reference of the MPA contact surface (e.g. localized contact surface of the MESAs) as opposed to an adjacent feature of the articulating transfer head assembly  200 . This may be facilitated by an unconventional miniaturized camera  290  design in which the exterior-most lens  299  of the camera  290  has a diameter less than 5 μmm, for example, and a working distance of greater than 500 μm, for example. In an embodiment, the camera outermost focal lens is located (laterally) less than an inch from a center of the center-opening  284 , and consequently the MPA  103 . 
     Camera(s)  290  in accordance with embodiments may be miniaturized systems including a light source  294 , such as a light emitting diode, sensors  292 , and objective lens assembly  298 . The exterior-most lens  299  of the objective lens assembly  298  may be the most distal lens located nearest the bottom lens flange  291  of the camera  290 . In operation the bottom lens flange  291  may be located above the MESAs on the MPA to avoid the potential for a collision incident, yet still close enough to focus on the frame of reference or object plane. Thus, automation of the camera  290  z-height can be used to maintain the camera in focus during a pick or place operation. In an embodiment, the camera  290  has a working distance of greater than 500 μm, for example, allowing the bottom lens flange  291  remain above the MESAs on the MPA while maintaining a minimum z-height difference from MPA to object plane less than the z-travel range of the piezoelectric stage assembly  250 . In an embodiment, the camera  290  has a numerical aperture (NA) of 0.25 or greater, allowing spatial resolutions ranging from 0.8 μm to 1.4 μm. In a further embodiment, the camera  290  has a numerical aperture of 0.45, allowing spatial resolutions of approximately 0.45 μm to 0.8 μm. Such spatial resolutions can be particularly useful for imaging microdevices with lateral dimensions as small as 1 μm. It is to be appreciated that further miniaturization of the camera  290  could potentially lead to a reduced working distance, along with associated changes to related parameters. 
     In an embodiment, articulating transfer head assembly  200  includes multiple linkages having independent ranges of motion. For example, articulating transfer head assembly  200  may include a θx-linkage  504  (for θx angular motion) coupled with carriage mount  206  and having links interconnected at joints such that the links exhibit kinematics in a first plane, e.g., movement of θx-linkage  504  may include a coupler link tipping relative to a ground link. Articulating transfer head assembly  200  may also include a θy-linkage  506  (for θy angular motion) coupled with θx-linkage  504  and having links that exhibit kinematics in a second plane different than the first plane, e.g., movement of θy-linkage  506  may include a coupler link tilting relative to a ground link. An x-actuator  554  (e.g. linear actuator) may impart a biasing load to θx-linkage  504  structure, while a y-actuator  558  may impart a biasing load to the θy-linkage  506  structure. 
     In the particular arrangement, the piezoelectric stage assembly  250  is coupled with the θy-linkage  506  to provide additional ranges of motion. For example, the piezoelectric stage assembly  250  may include a z-flexure assembly  285  (or z-stage) with a structure that lengthens and shortens along a z-axis  510 . In an embodiment, the one or more cameras  290  are mounted in the z-flexure assembly  285 . The piezoelectric stage assembly  250  can additionally include a plurality of nestled stages  275  that are nestled inside one another. Referring briefly to  FIG. 5 , this may include an x-stage assembly  270  with corresponding flexure and actuator  271  for motion along the x-axis  404 , a y-stage assembly  276  with corresponding flexure and actuator  277  for motion along the y-axis  402 , and θz stage assembly  260  with corresponding θz stage  262  and actuator  261  for rotational motion of the θz stage  262  about a z-axis  510 . Specifically, the x-stage assembly  270  and y-stage assembly  276  can cause motion of the θz stage  262  in the x, y directions. The z-flexure assembly  285  can be coupled with the θz stage  262 , such that motion of the θz stage  262  in either the x, y, or θz directions is transferred to the z-flexure assembly  285 , and hence the mounting plate  280 . As shown, the x-stage assembly  270  and y-stage assembly  276  are rim structures including bore holes for the actuators of each of the stages. In the particular arrangement illustrated the x-stage assembly  270  is nested inside the y-stage assembly  276 , and the θz stage  262  of the θz stage assembly  260  is nestled inside the x-stage assembly  270 . The θz stage  262  may optionally include a center-opening  264  and various other openings  266  to accommodate additional components (e.g. cameras, position sensors) or electrical connections. 
     Referring again to  FIG. 4 , the mounting plate  280  may be secured to an underside of the piezoelectric stage assembly  250 , such as to an underside of the z-flexure assembly  285 . A kinetic sensor assembly  295  can be mounted over the center-opening  284  to measure deflection of the pivot mount assembly  300 . In an embodiment, the kinetic sensor assembly  295  includes a plurality of position sensors  400 , such as light guide displacement sensors, laser displacement sensors, etc. One or more position sensor(s)  400  may also be secured through opening(s)  282 , for example, to be positioned adjacent a camera  290 . 
     In an embodiment, θx-linkage  504 , θy-linkage  506 , and piezoelectric stage assembly  250  are structurally connected such that kinematics of each linkage is related but independent. Referring now to the schematic illustrations in  FIGS. 6A-6B  in combination with  FIG. 4 , the θx-linkage  504  includes a ground link  602 , which may be fixed relative to the carriage mount, and a left link arm  604  connected to the ground link  602  with an upper left joint  606 . For example, this may be a living hinge flexure that allows the θx-linkage left link arm  604  to pivot relative to the θx-linkage ground link  602 . θx-linkage  504  may also include θx-linkage right link arm  608  located opposite z-axis  510  from θx-linkage left link arm  604 . More particularly, θx-linkage right link arm  608  may pivot about an θx-linkage upper right joint  610 , which may be located opposite the x-z plane, i.e., the plane encompassing x-axis  404  and z-axis  510 , from θx-linkage upper left joint  606 . θx-linkage  504  may also include θx-linkage coupler  612  connected with one or both of θx-linkage left link arm  604  and θx-linkage right link arm  608 . For example, θx-linkage coupler  612  may connect with θx-linkage left link arm  604  at θx-linkage lower left joint  614  and θx-linkage coupler  612  may connect with θx-linkage right link arm  608  at θx-linkage lower right joint  616 . Thus, θx-linkage  504  may include a four-bar linkage having a ground link, two link arms, and a coupler link, interconnected by four joints  606 ,  610 ,  614 ,  616 . 
     In an embodiment, θy-linkage  506  includes at least one link arm connected to θy-linkage  506  ground link  702 . For example, a θy-linkage left link arm  704  may connect to θy-linkage ground link  702  at a θy-linkage upper left joint  706 . θy-linkage upper left joint  706  may include a living hinge flexure that allows θy-linkage left link arm  704  to pivot relative to θy-linkage ground link  702 . θy-linkage  506  may also include θy-linkage right link arm  708  located opposite z-axis  510  from θy-linkage left link arm  704 . More particularly, θy-linkage right link arm  708  may pivot about a θy-linkage upper right joint  710 , which is located opposite the y-z plane, encompassing y-axis  402  and z-axis  510 , from θy-linkage upper right joint  710 . θy-linkage  506  may also include θy-linkage coupler  712  connected with one or both of θy-linkage left link arm  704  and θy-linkage right link arm  708 . For example, θy-linkage coupler  712  may connect with θy-linkage left link arm  704  at θy-linkage lower left joint  714  and θy-linkage coupler  712  may connect with θy-linkage right link arm  708  at θy-linkage lower right joint  716 . Thus, in an embodiment, θy-linkage  506  includes a linkage structure similar to θx-linkage  504 , having a ground link, two link arms, and a coupler link, interconnected by four joints  706 ,  710 ,  714 ,  716 . 
     The θx-linkage coupler  612  of θx-linkage  504  may carry a ground link  702  of θy-linkage  506 , and a θy-linkage coupler  712  of θy-linkage  506  may carry a second end of piezoelectric stage assembly  250  such that movement of any of θx-linkage  504 , θy-linkage  506 , and piezoelectric stage assembly  250  results in a relative movement between carriage mount  206  and mounting plate  280 . Rather than being fixed relative to carriage mount  206 , θy-linkage ground link  702  may be moveable relative to carriage mount  206 . More particularly, θy-linkage ground link  702  may be fixed relative to θx-linkage coupler  612 . Accordingly, when θx-linkage  504  is moved by actuating θx-linkage  504  link arms, θx-linkage coupler  612  moves relative to carriage mount  206  and since θy-linkage ground link  702  is carried on θx-linkage coupler  612 , so may θy-linkage ground link  702  move relative to carriage mount  206 . 
     In an embodiment, the interrelated movement of θx-linkage  504 , θy-linkage  506 , and piezoelectric stage assembly  250  may nonetheless be independent, such that movement of any one of the components provides pure motion of mounting plate  280 . For example, movement of θx-linkage  504  may cause mounting plate  280  to tip relative to carriage mount  206 , movement of θy-linkage  506  may cause mounting plate  280  to tilt relative to carriage mount  206 , and movement of z-flexure assembly  285  of the piezoelectric stage assembly  250  may cause mounting plate  280  to extend away from or retract toward carriage mount  206 . Likewise, movement of the θz stage assembly  260  will rotate the mounting plate  280  about the z-axis relative to the carriage mount  206 , and actuation of the x-stage assembly  270  or y-stage assembly  276  may move the mounting plate in the x or y direction, respectively, relative to the carriage mount  206 . Such relative movement between mounting plate  280  and carriage mount  206  may be independently constrained within the ranges of motion of each linkage without motion in one reference frame causing motion in another. For example, tipping of mounting plate  280  within a first plane caused by movement of θx-linkage  504  may not include any parasitic tilting in a second plane. Thus, decoupling the linkages may provide pure motion of mounting plate  280  about different rotational centers and in different axial directions. 
     The linkages may further be aligned with planes that are substantially orthogonal, i.e., θx-linkage  504  and θy-linkage  506  may be substantially orthogonal to each other. For example, θx-linkage  504  may act within a y-z plane encompassing a y-axis  402  and z-axis  510 , while θy-linkage  506  may act within an x-z plane encompassing an x-axis  404  and z-axis  510 . 
     In an embodiment, θx-linkage  504  and θy-linkage  506  are geometrically symmetric. For example, the links of θx-linkage  504  and θy-linkage  506  may have substantially equal lengths. More particularly, corresponding links of each linkage may or may not have substantially equal lengths. For example, link arms of each linkage may have a same first length and coupler links of each linkage may have a same second length, but the first length may or may not be equal to the second length. Equal length links may provide for a similar thermal response of θx-linkage  504  and θy-linkage  506  to environmental changes. For example, if temperatures local to the articulating transfer head assembly  200  change due to heating provided during use, each of the links may undergo similar thermal expansion. 
     θx-linkage  504  and θy-linkage  506  may have linkage angles that are symmetric about a given reference geometry, and may be structurally symmetric. For example, an angle between link arms of each linkage and z-axis  510  may be equal, or approximately equal, and the linkages may have a same composite stiffness. Therefore, linkage response to actuator inputs may be similar. For example, given that θx-linkage  504  and θy-linkage  506  may have similar geometric angles, actuation of respective link arms by respective actuators  554 ,  558  may cause similar angular changes of the link arms relative to z-axis  510 . More particularly, an x-actuator  554  coupled with an θx-linkage  504  link arm and a y-actuator  558  coupled with a θy-linkage  506  link arm may be moved similar amounts to cause similar tipping or tilting responses in θx-linkage  504  and θy-linkage  506 , respectively. 
     In addition to exhibiting symmetries about respective planes passing through z-axis  510 , θx-linkage  504  and θy-linkage  506  may also be symmetric relative to each other. That is, upper joints  606 ,  610  of θx-linkage  504  may be coplanar within an upper plane relative to upper joints  706 ,  710  of θy-linkage  506 . Similarly, lower joints  614 ,  616  of θx-linkage  504  may be coplanar within a lower plane relative to lower joints  714 ,  716  of θy-linkage  506 . 
     In an embodiment, θx-linkage  504  includes a remote tipping center  622  located at an intersection of θx-linkage left axis  618  and θx-linkage right axis  620 . Remote tipping center  622  may be a virtual center of rotation with a spatial position that remains constant as θx-linkage  504  link arms change orientation. For example, in an embodiment, as θx-linkage left link arm  604  pivots about θx-linkage upper left joint  606 , a corresponding motion of θx-linkage right link arm  608  may cause the angles between θx-linkage left axis  618 , θx-linkage right axis  620 , and z-axis  510  to change. However, the location of remote tipping center  622  may remain stationary despite the change in linkage geometry. More particularly, as θx-linkage  504  geometry changes, θx-linkage coupler  612  may rotate about remote tipping center  622 . Thus, a point on an object that is fixed relative to θx-linkage coupler  612  and located at remote tipping center  622  may experience pure rotation, unaccompanied by translational parasitic motion, when θx-linkage  504  geometry is varied. 
     Similarly, θy-linkage  506  may include a remote tilting center  722  located at an intersection of θy-linkage left axis  718  and θy-linkage right axis  720 . Remote tilting center  722  may be a virtual center of rotation with a spatial position that remains constant as θy-linkage  506  link arms change orientation. For example, in an embodiment, as θy-linkage left link arm  704  pivots about θy-linkage upper left joint  706 , a corresponding motion of θy-linkage right link arm  708  may cause the angles between θy-linkage left axis  718 , θy-linkage right axis  720 , and z-axis  510  to change. However, the location of remote tilting center  722  may remain stationary despite the change in linkage geometry. More particularly, as θy-linkage  506  geometry changes, θy-linkage coupler  712  may rotate about remote tilting center  722 . Thus, a point on an object that is fixed relative to θy-linkage coupler  712  and located at remote tilting center  722  may experience pure rotation, unaccompanied by translational parasitic motion, when θy-linkage  506  geometry is varied. 
     The remote tipping center  622  and remote tilting center  722  may be co-incident, at the same location, and the phenomenon of pure rotation about the remote tipping and tilting centers  622 ,  722  may be exploited by locating transfer elements for which parasitic motion is undesirable at the remote tipping and tilting centers  622 ,  722 . For example, the MESAs on an MPA  103  may be co-located with the remote tipping and tilting centers  622 ,  722  so that the MESAs experience pure tipping and tilting and remain in the same lateral location, rather than shifting under parasitic translation and potentially smearing a corresponding micro device on a target substrate surface. 
     In the following discussion related to  FIGS. 7-10F  various alignment methods and structures are described for aligning the articulating transfer head assembly. Generally, this can include system alignment in which a frame of reference is established. For example, this may include establishing a reference plane with an x-y datum, as well as a z-datum. In an embodiment, the frame of reference is to correspond to the MESA contact surfaces of the MPA  103 . Furthermore, the co-incident remote tipping center  622  and remote tilting center  722  may be located at a center of the frame of reference at a center of the MPA  103 . 
     Referring now to  FIG. 7 , a perspective view illustration is provided of a portion of a micro device transfer tool  100  including one or more cameras  290 ,  128 . For example, a camera  290  can be coupled with upper assembly  102  and within the piezoelectric stage assembly  250  as previously described. In an embodiment, camera  128  can be coupled to the lower assembly  104  and located near the carrier substrate holder  108  or donor substrate holder. In an embodiment, camera  128  can be fixed relative to the stage  110 , or more particularly to the carrier substrate holder  108  and the receiving substrate holder  124 . Thus, in an embodiment, movement of either camera to view a new location of interest results in a corresponding movement of the related articulating transfer head assembly  200  or target substrate holder  108 ,  124 . In this way, relative motion between the articulating transfer head assembly  200  and target substrate holder  108 ,  124  can be determined based on movements of cameras  290 ,  128 . 
     The camera  128  can include a camera having sufficient resolution and range of focus to view a single MESA structure. For example, the camera can have an image resolution allowing dimensions of less than one micrometer to be resolved. The camera  290  can include a camera having sufficient resolution and range of focus to view a single micro device supported by target substrate (e.g. carrier substrate, receiving substrate). 
     In an alternative embodiment, there may be multiple cameras located on each of the upper and lower assemblies  102 ,  104 . For example, each subassembly can include high magnification and low magnification cameras. By way of example and not limitation, the low magnification cameras  126 ,  140  may be used to provide feedback inputs to host computer  109  for controlling gross adjustments and movements of the system while the high magnification cameras  290 ,  128  may be used to provide feedback inputs to host computer  109  for controlling fine adjustments and movements of the system. 
     It will be appreciated that cameras represent only one alternative for providing feedback related to the position of the upper and lower subassemblies  102 ,  104  or components attached thereto. Other devices can be contemplated within the scope of this disclosure. For example, rather than utilizing cameras, the micro device transfer tool  100  may include position (proximity) sensors  400 , such as capacitive sensors, laser sensors, or position sensors. 
     In an embodiment, to facilitate establishing a reference point between cameras  290 ,  128 , the micro device transfer tool  100  can include an alignment tool  130 . In an embodiment, the alignment tool includes a fiducial mark  136 . For example, the fiducial mark  136  can be a part of a transparent plate  138  (e.g., glass) that is supported by an alignment bracket  135 . More particularly, plate  138  having fiducial mark  136  can be positioned between an upward-viewing camera  128  and a downward-viewing camera  290 . In an embodiment, the plate  138  can be positioned between two planes, one plane approximately coinciding with an imaging plane of the upward-viewing camera  128  and another plane approximately coinciding with an imaging plane of the downward-viewing camera  290 . Thus, the fiducial mark  136  can be viewed by both the upward-viewing camera  128  and the downward-viewing camera  290  either simultaneously or at different times. 
     It will be appreciated that the fiducial mark  136  can be formed using several different methods. For example, the fiducial mark  136  can be printed on the plate  138  using an ink or laser printing process. Alternatively, the fiducial mark  136  can be etched into the plate  138 . 
     While cameras  290 ,  128  can facilitate the recognition of reference marks to establish reference frames it will be appreciated that additional position sensors can be included in the micro device transfer tool  100  to provide feedback relating to the relative position of micro device transfer tool components. In an embodiment, a position sensor  400  can be mounted near the carrier substrate holder  108  to provide a feedback input that aids in the adjustment of the actuator assembly for stage  110 . For example, the position sensor  400  can terminate in a distal end that is approximately coplanar with the holding surface of the carrier substrate holder  108 . Thus, the position sensor can detect a distance to a surface relative to the carrier substrate holder  108  or the receiving substrate holder  124 . For example, the position sensor can provide feedback relating to the distance between a target substrate held by the substrate holder  108 ,  124  and an MPA  103  attached to the articulating transfer head assembly  200  when those components are adjusted relative to each other. 
     Referring to  FIGS. 8A-8B  schematic side view and perspective view illustrations are provided of a method of setting an x-y datum in accordance with an embodiment of the invention. As shown, plate  138  including fiducial mark  136  is oriented between an imaging plane  808  of the upward-looking camera  128  and an imaging plane  816  of the downward-looking camera  290 . When the upward-looking camera  128  and the downward-looking camera  290  view the fiducial mark  136  simultaneously, and the fiducial mark  136  is centered and focused within the respective images from the cameras, the cameras will be aligned. As shown in  FIG. 8B , the fiducial mark  136  establishes an x-y datum  820  when centered and focused on simultaneously by the cameras  290 ,  128 . Furthermore, an x-axis  822  and a y-axis  824  are determined to cross through the x-y datum  820 . In an embodiment, the x-axis  822  and y-axis  824  correspond with axes of motion of stage  110  that the upward-looking camera  128  is coupled with. Furthermore, the x-axis  822  and the y-axis  824  define an x-y plane  826 , which passes through the x-axis  822 , y-axis  824 , and x-y datum  820 . Thus, a frame of reference having an x-axis  822  and an x-y plane  826  can be established according to the methods described above. 
     Thus, in this position, the fiducial mark  136  becomes a reference point from which movement of either the upward-looking camera  128  or the downward-looking camera  290  can be compared to determine the relative position of the cameras in a plane parallel to the imaging planes. In an embodiment, when the upward-looking camera  128  is fixed relative to a target substrate holder and the downward-looking camera  290  is fixed relative to the articulating transfer head assembly  200 , the fiducial mark  136  becomes a reference point from which movement of the articulating transfer head assembly  200  or the target substrate holder can be compared to determine the relative position of those components in an x-axis and y-axis direction. 
     Following establishment of the x-y datum, a z-datum can be established for the frame of reference. Once the z-datum  920  is established, along with the x-axis  822  and x-y plane  826 , a frame of reference is known for moving components of the micro device transfer tool  100 . Prior to doing so, the bottom (mounting) surface  281  of the mounting plate  280  can be made parallel to the x-y plane  826 . To do so, the upward-looking position sensor  400  can detect a distance to two or more points on the bottom surface  281 , the z-gauge  910 , or any other structure that is known to be parallel to the bottom surface  281 . The articulating transfer head assembly  200  can then be tipped and tilted and translated until the distances to the various measured points are the same distance from the upward-looking position sensor  400 . When this occurs, the bottom surface  281  can be orthogonal to the direction of detection of the upward-looking position sensor  400 , and thus, the bottom surface  281  is approximately parallel to the x-y plane  826 . Once the bottom surface  281  is oriented parallel to the x-y plane  826 , a z-datum can be established. 
     Referring to  FIG. 8C , a side view illustration of a method of setting a z-datum is shown in accordance with an embodiment. A downward-looking position sensor  400  is viewing in a downward direction  904  toward the x-y plane  826  of the frame of reference. Simultaneously, an upward-looking position sensor  400  is viewing in an upward direction  908  opposite to the downward direction. Thus, the directions of the upward and downward-looking position sensors  400  are approximately parallel with each other and can be approximately orthogonal to the x-y plane  826 . The position sensors can be a variety of sensors capable of determining absolute distance to an object, such as a spectral-interference laser displacement sensor or light guide displacement sensor. 
     A z-gauge  910  can be releasably attached to the mounting plate  280  of the articulating transfer head assembly  200  and positioned between the upward and downward-looking position sensors  400 . For example, the z-gauge  910  can be a permanent fixture that is rotated into place during alignment, or releasably attached. The z-gauge  910  can be referred to as a “z-gauge” because it is used to establish a z-datum in a frame of reference. In an embodiment, in order to establish the z-datum, the upward-looking position sensor  400  senses a distance to a first surface  912  and the downward-looking position sensor  400  can sense a distance to a second surface  914  of the z-gauge  910 . These surfaces can be, for example, the base of two counterbores, formed in an outer surface of the z-gauge  910 . The counterbored first and second surfaces  912 ,  914  can be made coplanar with each other and a surface plane  916 . For example, in an embodiment, the z-gauge  910  can be formed from two silicon wafers having through holes. When aligned, given that the first and second surfaces  912 ,  914  are coplanar, the surface plane  916  can be established as the z-datum  920  in this orientation, and the distance to the z-datum  920  can then be measured using either the upward-looking position sensor  400  or the downward-looking position sensor  400 . In an embodiment, the z-gauge  910  can be formed such that the surface plane  916  is within about 100 micrometers of a location that coincides with a contact surface of the MESAs  115 . 
     In the foregoing discussion with regard to  FIGS. 7-8C  structures and methods have haven described and illustrated for establishing a frame of reference which can be used for operation of the micro device transfer tool  100 , as well as for alignment of additional components, such as the MPA  103 . Furthermore, the MESAs on an MPA  103  may be co-located with the remote tipping and tilting centers  622 ,  722  within the frame of reference so that the MESAs experience pure tipping and tilting and remain in the same lateral location, rather than shifting under parasitic translation and potentially smearing a corresponding micro device on a target substrate surface. It will be appreciated that various components of the micro device transfer tool can be heated during the pick and place sequences. For example, in an embodiment, the pivot mount assembly  300  supporting the MPA  103  and/or mounting plate  280  can be heated to a temperature range of about 100 to 350 degrees Celsius during any of the pick and place operations or alignment operations. 
     Referring now to  FIG. 9 , the articulating transfer head assembly  200  including the downward facing camera(s)  290  can be translated, for example along an x-axis track, between carrier (donor) substrate holder  108  and receiving (display) substrate holder  124  during a pick and place sequence. The x-axis can have a curvature that is non-zero, on the order of arc seconds. The source of this curvature can be both static (e.g. from manufacturing imperfections) and dynamic (such as those from thermal expansion). It has been observed that any curvature introduces a rotational or yaw component to the articulating transfer head assembly  200  as it moves from one position to another position. In particular,  FIG. 9  illustrates an initial MPA location MPA (Xi, Yi) and initial camera location CAM (Xi, Yi). Upon translation to a final MPA location MPA (Xf, Yf) and final camera location (Xf, Yf) a yaw angle (γ) is introduced which results in a change in offset between the MPA and camera with respect to the frame of reference (corresponding to the global x-y coordinate system). This change in offset is a position error with x and y components, error x (Ex) and error y (Ey), respectively. As can be surmised from the interposed positions at the final location, a larger fixed distance between the MPA and camera (CAM) results in a larger position offset after the introduction of the yaw angle. 
     Error can be calculated as follows, given an x component of offset from camera to MPA (D x ) and a y component of offset from camera to MPA (D y ) represented by:
 
( D   x )=CAM Xi −MPA Xi   (1)
 
( D   y )=CAM Yi −MPA Yi   (2)
 
with magnitude of offset from camera to MPA represented by the length of the hypotenuse of the right triangle whose orthogonal sides are components D x  and D y :
 
 D =sqrt( D   x   2   +D   y   2 )  (3)
 
With this basis, the error x (E x ) and error y (E y ) can be calculated as follows:
 
     
       
         
           
             
               
                 
                   
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     However, error due to change in curvature caused by dynamic sources cannot be compensated without remapping the frame of reference. Such frequent remapping may be unsuitable for mass transfer sequences at high transfer rates. In one aspect of the embodiments described herein, it has been observed that the magnitude of x-y error is directly proportional to the distance (in x and y) between the MPA  103  and the downward facing camera  290 . Hence, reducing the fixed distance between the MPA and camera (CAM) results in a reduction of error. To put into perspective, mass transfer sequences of micro devices of maximum dimensions on the order of several to tens of microns may necessitate a reduction of such error to an order of nanometers as opposed to microns. In accordance with embodiments the downward facing camera  290  is mounted inside the piezoelectric stage assembly  250  and extends through an opening in a bottom surface of the mounting plate  280  in order to locate the downward facing camera  290  as close as practical to the MPA  103  center. Thus, the camera  290  is located within the articulating transfer head assembly  200 , which may have six degrees of motion. This may be facilitated by an unconventional miniaturized camera design in which the optical lenses may have a diameter less than 5 μmm, for example, and a working distance of greater than 500 μm, for example. In an embodiment, the camera outermost focal lens is located (laterally) less than an inch from a center of the MPA. 
     In accordance with embodiments, location of the camera may additionally be facilitated with a low-CTE (coefficient of thermal expansion) structure to which both the MPA  103  and camera  290  are mounted. This reduces dynamic offset error due to thermal expansion. Further, such a mount is impractical for conventional microscopes due to size. 
     In an embodiment, the camera  290  is used to find and maintain alignment to a target substrate during pick and place. In one embodiment, the camera  290  can be mounted on its own z-stage, which would allow the camera to move independently of the MPA  103  during pick and place. Such an arrangement can allow the camera  290  to maintain focus at a working distance even as the MPA is moved from a distance from the target substrate to contact with the substrate (for pick and place) and then moved back to a distance from the substrate. 
     As previously indicated, once the frame of reference is established various system components can be aligned with the frame of reference, and to one another. For example, the upward facing cameras can be used to align an MPA  103  to a reference location within the frame of reference. Additionally, downward facing cameras and position sensors can be used to align target substrate (e.g. donor substrates, or receiving substrates) with a reference location within the frame of reference. Encoders may also be utilized to align substrates to one another, such as aligning an MPA  103  with a donor substrate or a receiving substrate (e.g. display substrate). 
     In one aspect, an embodiment of an MPA alignment encoder facilitates positional alignment between an MPA and a target substrate on a scale ranging from one nanometer to one micron. Positional alignment may refer to relative positioning along axes of a plane. By positionally aligning an MPA and target substrate, a reference location may be established with the MPA alignment encoder, and a distance between the MESAs and a target location on a target substrate may be calculated. Thus, the MESAs be moved into alignment with the target location, e.g., a micro device. 
     In accordance with embodiments the camera(s)  290  can be used to align the articulating transfer head assembly  200  and MPA  103  with a target substrate. In an embodiment, a micro device transfer tool  100  includes a carrier substrate holder  108 , a receiving substrate holder  124 , and an articulating transfer head assembly  200  that is translatable along an x-translation direction between the carrier substrate holder  108  and the receiving substrate holder  124 . The articulating transfer head assembly  200  may include a mounting plate  280 , and one or more cameras pointing away form the bottom surface  281  of the mounting plate  280 . For example, the one or more cameras  290  can be mounted within the piezoelectric stage assembly  250  and extend through one or more openings  282  in the bottom surface  281  of the mounting plate  280  as previously described. Each camera  290  may be secured at a location offset in a direction relative to a center of the mounting plate  280 , for example. 
       FIG. 10A  is an exemplary schematic bottom view illustration of a bottom side of mounting plate  280  of the articulating transfer head assembly  200 . As shown a pair of x-y offset cameras  290  may be located along x,y axes, with the x-axis being parallel to an x-translation direction between a carrier substrate stage and donor substrate stage (illustrated as the arrow between  FIGS. 10A-10B ).  FIG. 10B  is a schematic top view illustration of a pair of corresponding x-y offset alignment features  1152  in a display area  1150  of a target substrate offset from a place area  1155 . In accordance with embodiments, the alignment features  1152  are offset from the place area  1155  with the same x,y offset as the cameras  290  are offset from the MPA  103  active area. As illustrated in  FIG. 10C , the MPA  103  can then be aligned with the target place area  1155  by aligning the one or both cameras  290  with one or both of the alignment features  1152 . 
     In the particular embodiments illustrated, the cameras  290  are illustrated as being in a direction (e.g. x-direction, or y-direction) relative to a center  296  of the mounting plate  280  and/or MPA  103 . For example, the x-direction may be parallel to, or along the same axis as the x-translation direction between the carrier substrate stage and donor substrate stage. Alternatively, the cameras  290  may be located at other pre-determined x,y offset locations including both x,y components. Likewise, the corresponding alignment features  1152  can be located at corresponding locations that are offset from the center  1156  of a place area  1155  on the target substrate, which may include an array of such place areas  1155 , centers  1156 , and corresponding alignment features  1152 . 
     In an embodiment, an alignment method for the MTT  100  includes translating an articulating transfer head assembly  200  over a place area  1155  of a target substrate. The articulating transfer head assembly  200  can include an MPA  103  including an array of transfer heads (e.g. MESAs  115 ), a first camera  290  offset from a center  296  of the MPA  103  in a first direction (e.g. x-direction) and pointing away from the MPA  103  (e.g. toward the target substrate), and a second camera  290  offset from the center  296  of the MPA  103  in a second direction (e.g. y-direction) and pointing away from the MPA  103  (e.g. toward the target substrate). At least one of the cameras  290  can then be aligned with one or more alignment features  1152  of the target substrate. This may facilitate aligning the array of transfer heads (e.g. MESAs  115 ) with corresponding target locations in the place area  1155  on the target substrate. 
     In accordance with embodiments when the MPA  103  active area is positioned at a place area  1155  there can be at least one alignment feature  1152  at a substantially equal offset position that is the same offset position of at least one of the cameras  290 . For example, this may be at an x-offset position, y-offset position, or some combination of x-y offset. In the particular embodiment illustrated in  FIG. 10C  the pair of cameras  290  is aligned with corresponding pair of alignment features  1152  in the same display area  1150 . However, this may not always be possible to have multiple alignment feature  1152  offsets at a given place area  1155  due do display area  1150  constraints. Thus, including more than one possible offset position of an alignment feature  1152  for each place area  1155  helps ensure that at least one of the cameras  290  would have an alignment feature  1152  in view. For example, when placing at the extreme x edge of a display area  1150  it may not be possible to have an alignment feature also offset in x, but still possible to have an alignment feature that is offset in y, and vice versa. It is also possible that corresponding alignment features may be within the same display area  1150  as the place area  1155 , or the alignment feature may be within a neighboring display area  1150 . 
       FIG. 10D  is a schematic top-down illustration of no corresponding x-offset alignment feature in accordance with an embodiment. In this circumstance, only one of the cameras  290  may be used for alignment. Similarly, only one of the cameras  290  would be used for alignment if there were no corresponding y-offset alignment feature available.  FIG. 10E  is a schematic top-down illustration of no corresponding alignment features in the target display area  1150  in accordance with an embodiment. In this circumstance, only one of the alignment cameras  290  is aligned with an alignment feature  1152  from an adjacent display area  1150 .  FIG. 10F  is a schematic top-down illustration of using an offset alignment feature from the target display area and an offset alignment feature from an adjacent target display area in accordance with an embodiment. In this circumstance, one or both cameras may be used for alignment. 
     In another aspect, an embodiment of an MPA alignment encoder facilitates rotational alignment between an MPA and a target substrate. Rotational alignment may refer to relative positioning about an axis orthogonal to the MPA and target substrate. In an embodiment, the rotational alignment may be determined with reference to a plurality of encoder pattern pairs in separate regions of the MPA and target substrate. Distances between the pairs may be measured and used to calculate angular alignment between the MPA and the target substrate. By establishing an estimate of angular alignment, the MPA may be rotated into alignment with the target substrate. 
     Referring now to  FIG. 11 , a schematic cross-sectional side view illustration is provided of an encoder assembly in accordance with an embodiment. In the particular embodiment illustrated, the encoder assembly is configured for alignment of the MPA  103  with a target substrate  1100 , such as a display substrate or donor substrate. As previously described a pivot mount assembly  300  can be secured to a mounting plate  280  of the articulating transfer head assembly  200 , and spanning across a center-opening  284 . A piezoelectric actuator element  340  (e.g. plate) may optionally be secured on a back side of the pivot mount assembly  300  to control bending, for example, which may occur during thermal profiles. An MPA  103  can be mounted on the front side of the pivot mount assembly  300 . As shown, the MPA  103  can support an array of MESAs  115 . 
     Optics of the encoder assemblies can include one or more light sources  1106 , detector arrays  1102 , relay optics  1104 , and beam splitters  1108 . In operation, light  1101  is directed from a light source  1106  toward a beam splitter  1108  which directs the light through an encoder pattern  160  coupled to the MPA  103 , and toward an encoder pattern  1110  coupled to the target substrate  1100 , where the light is reflected back toward the MPA  103  (and optionally back through the encoder pattern  160 ), through the beam splitter  1108  and relay optics  1104  to the detector array  1102 . Light  1101  may be collimated in some embodiments, or may have a slight focus to reduce diffraction ringing. Different types of encoder patterns  160 ,  1110  may be utilized, such as transmissive-type and reflective-type encoder patterns. In an embodiment, encoder pattern  160  is a transmissive-type which includes opaque line/ring patterns over a transparent substrate, while encoder pattern  1110  also includes light absorbing, anti-reflective line/ring patterns over a reflective substrate. Alternatively, encoder pattern can include reflective line/ring patterns over a light absorbing, anti-reflective substrate layer. 
     Referring now to  FIG. 12 , a bottom side perspective view illustration is provided of an MPA with one or more encoders in accordance with an embodiment. As shown, the MPA  103  includes an array of MESAs  115  over a central portion of a base substrate  164 . One or more encoder patterns  160  with concentric grating patterns are located over the base substrate  164  at known distances form the MESAs  115 . For example, encoder patterns  160  may be positioned corners or quadrants of micro MPA  103 , or in a direct x-axis translation track in the micro transfer tool assembly, such as between the carrier substrate holder and receiving substrate holder when the MPA  103  is secured to the articulating transfer head assembly  200 . Accordingly, once the position of encoder pattern(s)  160  are identified, the known distance to MESAs  115  may be used to calculate a position of each MESA. 
     Each encoder pattern  160  may be a concentric grating pattern including a plurality of rings  162  of first diameters. For example, each ring  162  may have a same line width, and be separated from a second ring by a same spacing, which may be the same as the line width. Each encoder pattern  1110  may be similarly arranged with a plurality of rings  1162  of same line width, and spacing. In an embodiment, the diameters of the rings  1162  are offset from the diameters of the rings  162 , such that they are all slightly larger or smaller, which facilitates creation of the moiré patterns. For example, the offset from the diameters of the rings  1162  may be one quarter of the line width offset from the diameters of the rings  162 . 
     In an embodiment, a micro device transfer tool alignment method includes translating a first concentric grating pattern coupled with a first substrate (e.g. MPA) over a second concentric grating pattern coupled with a second substrate (e.g. target substrate, donor substrate, display substrate), directing light  1101  through a first concentric grating pattern (e.g. encoder pattern  160 ) toward the second concentric grating pattern (e.g. encoder pattern  1110 ), detecting reflected light with a detector array  1102  divided into multiple detection zones, and determining a relative x-y position of the first and second substrates. 
     Referring to  FIG. 13 , a schematic top view illustration is provided of a resultant moiré pattern of light after passing through encoder patterns  160 ,  1110  toward optics  1104 . As shown, the optics  1104  may be a bundle of fibers  1121  used for collection optics, which can then be routed to a corresponding photodiode array such as an array of CMOS image sensors or photodiodes. In the particular embodiment illustrates four zones, or quadrants are shown, though there may be more or less. In accordance with embodiments, the detector array  1102  measures the intensity of incident light in each of the zones. A determination of the relative x-y position is then made based on the intensity of light in each zone. 
     Referring now to  FIGS. 14A-14E  various moiré patterns are provided to illustrate how x-y alignment and misalignment can be determined in accordance with embodiments. Specifically,  FIG. 14A  is an illustration of aligned substrates, where the two complimentary gratings are aligned.  FIG. 14B  is an illustration of pure positive x-misalignment,  FIG. 14C  is an illustration of pure positive y-misalignment,  FIG. 14D  is an illustration of equal positive and an y misalignment (quadrant  1 ), and  FIG. 14E  is an illustration of negative x, positive y misalignment (quadrant  2 ). 
     In some embodiments a plurality of complementary encoder patterns  160 ,  1110  can be used to measure in-situ angular displacement. Referring now to  FIG. 15 , a schematic top view illustration of a pair of moiré patterns is provided in accordance with an embodiment. As shown, the primary complementary pair of encoder patterns  160 ,  1110  on the left are aligned for both x and y. However, the secondary pattern for the complementary pair of encoder patterns  160 ,  1110  on the right side are offset with an angular misalignment. Thus, in such an embodiment, the articulating transfer head assembly  200  can be adjusted to align both complementary pairs of encoder patterns to maintain alignment during the pick and place sequences. 
     In yet another embodiment, the plurality of complementary encoder patterns  160 ,  1110  can be used to measure thermally induced error. For example, this could potentially occur during a thermal ramp cycle. Such thermally induced error would however be a “scale error” in which the pitch of the MESAs would differ from a corresponding pitch (e.g. micro devices) on a target substrate. Such a scale error can be corrected by adjusting the temperature of any of the MPA, donor substrate, or receiving substrate, all of which have independent temperature control and heater elements. This scale error can be used as historical system data for inputting parameters during micro device transfer. 
     In accordance with embodiments, the relative position signals (x, y, angular displacement) can be fed back into a closed-loop position servo system to maintain a relative position between the target and positioned concentric grating patterns. Determination of intensity can be image-based or non-image based. An image-based implementation may analyze the moiré fringes to determine relative positions. A non-imaged based implementation may measure integrated intensity over an area, and may possibly be performed at a faster rate. Furthermore, a fourth parameter, scale error, can also be fed into a heater control feedback loop to maintain matching pitches among the MPA and target substrates. 
     Referring now to  FIGS. 16A and 17A , different concentric grating patterns are illustrated as being superimposed over a plurality of subpixels in a pixel area  1124  of a display substrate in accordance with embodiments. As shown, each subpixel  1125  includes a landing area  1120  which can receive a pair of redundant micro LEDs  1122 . Each landing area  1120  may have a conductive landing pad area for each corresponding micro LED  1122  (which may not yet be placed). A variety of conductive or insulating layers may be located in portions of the pixel areas  1124  between the landing areas  1120 . As previously described, the encoder patterns  1110  include a plurality of concentric rings (e.g. circles)  1162  defined by line widths and spacings. As shown, each ring  1162  may include a line break  1163  so as to not interfere with optics inside the landing areas  1120 . In the embodiment illustrated in  FIG. 16A , the line breaks  1163  are patterned to match the landing areas  1120 . In the embodiment illustrated in  FIG. 17A  the line breaks  1163  are made along entire display rows of subpixels. It is to be appreciated that these two configurations are exemplary, and embodiments are not limited to these two particular configurations.  FIGS. 16B and 17B  are schematic top view illustrations after the complementary encoder (grating) pattern  160  is aligned over the encoder pattern  1110  (e.g. grating pattern). As shown, characteristic moiré patterns are created despite the line breaks. 
     In an embodiment, a display includes a target (display) substrate  1100  including a pixel area and a non-pixel area. One or more concentric grating patterns (encoder pattern  1110 ) is on the target substrate  1100 . The concentric grating pattern can be located in the pixel area or the non-pixel area. When located in the non-pixel area, the concentric grating pattern can be superimposed over a plurality of subpixels in the pixel area. In some embodiments, the concentric grating pattern is patterned to include a plurality of line openings directly over the LED landing areas  1120  in the plurality of subpixels. 
     Referring now to  FIG. 18 , an exploded cross-sectional side view illustration is provided of an articulating transfer head assembly  200 , pivot mount assembly  300 , and MPA  103  in accordance with an embodiment. Generally, the pivot mount assembly  300  is mounted onto the articulating transfer head assembly  200 . This may be accomplished using a variety of manners such as using tabs or lips to press the pivot mount assembly against the articulating transfer head assembly  200 , bonding, vacuum, or electrostatic clamping. Center-opening  284  may function as a deflection cavity to allow a specified z-deflection distance of the pivot platform  304  along the z-axis. 
     The pivot mount assembly  300  may include a base substrate  311 . In an embodiment, the base substrate  311  is made from silicon using micro electrical mechanical systems (MEMS) fabrication techniques. Channels  310  may be formed through a body of the base substrate  311  from a front side  312  to back side  314 . Channels  310  may be used to form a variety of compliant features of the pivot mount assembly  300 , including defining the spring arms  306  and pivot platform  304 , as well as compliant voltage contacts  316 . The compliant voltage contacts  316  may provide a low contact resistance connection to voltage contacts  120  of the MPA  103 . In the embodiments illustrated, the compliant voltage contacts  316  protrude from the pivot platform  304  such that they are raised above the pivot platform  304 . Electrodes  318 ,  122  may be utilized to provide a voltage to electrostatically clamp the MPA  103  onto the pivot platform  304 . In accordance with embodiments, the clamping pressure is sufficient to withstand thermal distortions of the structure, and physical distortions induced by a piezoelectric actuator element  340  as well as any external factors. Upon clamping the MPA  103  onto the pivot platform of the pivot mount assembly  300  with the opposing electrodes  318 ,  122 , the compliant voltage contacts  316  exert a pressure upon the MPA voltage contacts  120 . The electrodes  318  and compliant voltage contacts  316  may each include a metallic layer on the front side  312  of the base substrate  311 . In an embodiment, the metallic layer of the electrodes  318  is covered by a dielectric material layer, while the metallic layer of the compliant voltage contacts  316  is exposed. Additional features may be located on the base substrate  311 . For example, one or more heater elements  320  (e.g. thin film heater elements) may be located on the front side  312  of the base substrate  311 . During micro device transfer, the heater elements  320  may maintain the pivot mount assembly  300  and MPA  103  at an elevated temperature relative to a target substrate. Wiring layers  322  may connect the one or more heater elements  320 , and metallic layers of the electrodes  318  and compliant voltage contacts  316  to the flex circuit  308  on the front side  312  of the base substrate  311 . 
     In the illustrated embodiment, the pivot mount assembly  300  additionally includes a piezoelectric actuator element  340  bonded to the back side  314  of the base substrate  311  to control a curvature of the base substrate  311 , and resultantly, a curvature of the MPA  103  and the contact surfaces of the array of MESAs  115 . In an embodiment, the piezoelectric actuator element  340  is bonded with metal-metal bonding or an adhesive (e.g. a thermoset material) to the pivot platform  304 . The piezoelectric actuator element  340  may be electrically connected with the pivot mount assembly  300  using various techniques, including wire bonds  342  or bottom contacts  348  on the bottom side of the piezoelectric actuator element  340 . In either configuration, the wire bonds  342  or bottom contacts  348  may be connected to electrical wiring  344  on the back side  314  of the base substrate  311 . The electrical wiring  344  may additionally be electrically connected to the flex circuit  308  through additional wire bonding, or vias formed through the base substrate  311 . 
     The surface to which the piezoelectric actuator element  340  is bonded may be opposite the side used for clamping the MPA  103 . In an embodiment, the piezoelectric actuator element  340  material is lead zirconate titanate (PZT). In an embodiment, the piezoelectric material is PZT Type 5A. The bonded structure of the base substrate  311  and piezoelectric actuator element  340  together comprise a 2-layer monomorph bender in which the active layer is the piezoelectric actuator element  340  and the passive layer is the pivot platform  304  of the base substrate  311 . In an embodiment, the piezoelectric actuator element  340  is poled normal to the base substrate  311  such that the d 33  direction is in the “Z” direction of the structure. Because the piezoelectric actuator element  340  is bonded to the pivot platform  304 , forming a monomorph bending structure, changes in voltage across the piezoelectric actuator element  340  will result in out-of-plane bending of the structure. If the piezoelectric actuator element  340  is substantially symmetric with the pivot platform  304 , the out-of-plane bending will be substantially symmetric to the pivot platform (centered on the structure—the center of curvature coincides with the z-axis of the structure). 
     During micro device transfer, the MPA  103  may be electrostatically clamped to the pivot mount assembly  300 . The clamping pressure between the MPA  103  and pivot mount assembly  300  may be between 1 and 10 atmospheres in some embodiments. It has been demonstrated that a clamping pressure of this magnitude range can be sufficient to maintain shape conformity of the MPA  103  to the pivot mount assembly  300 . Hence, changes in shape in the pivot mount assembly  300  will correspond directly to a change in curvature in the MPA  103 . In other words, by actively controlling the shape of the pivot mount assembly  300  it is possible to correspondingly control the shape of the MPA  103 , and hence the curvature along the contact surfaces of the array of MESAs  115 . 
     In accordance with embodiments, the shape control elements are incorporated into the articulating transfer head assembly  200  and pivot mount assembly  300 . This approach may allow for active shape control of the MPA  103  without requiring new features and added cost in the MPA  103 , supporting a strategy in which the MPA  103  is a consumable element in a micro device transfer manufacturing process. 
     Still referring to  FIG. 18 , in an embodiment, a plurality of contactless position sensors (e.g. displacement sensors)  400  may be aimed at a face of the monomorph bending structure, which includes the base substrate  311  and the piezoelectric actuator element  340 . In accordance with embodiments, the position sensors  400  are any of the position sensors (e.g. displacement sensors)  400  described and illustrated with regard to  FIGS. 21-29 . Other exemplary position sensors may include the SI-F Series spectral interference laser displacement sensor from Keyence Corporation of Japan. Other types of appropriate displacement sensors include a Shack-Hartmann wavefront sensor, capacitive sensor, and inductive sensor. 
     In an embodiment, a position sensor  400  may be placed at a fixed reference position relative to a pivot mount assembly  300  bending structure, for example in the kinetic sensor assembly  295 . The position sensor may be aimed to measure the z position of a point on the pivot mount assembly  300  bending structure. The point may be located at the axis of symmetry (e.g. along x, y, z axes) of the pivot mount assembly  300  bending structure, or may be located at a known position off-axis. In such an arrangement the position sensor will measure a change in z position at the point that is proportional to the change in the curvature of the pivot mount assembly  300  bending structure. While such an arrangement can produce a signal corresponding to the change in curvature, additional reference information about the initial shape of the pivot mount assembly  300  bending structure is utilized by an active shape control algorithm to report the measured surface profile/curvature, such as a radius of curvature, of the deformed structure. 
     In another embodiment, two or more position sensors  400  may be placed at fixed reference positions relative to a pivot mount assembly  300  bending structure. The fixed reference positions may be common, such as along a same radius from the axis of symmetry (along the z-axis) of the pivot platform  304 . One or more of the position sensors  400  may be located near the axis of symmetry. The position sensors  400  may be aimed to measure the z positions of two or more different points on the pivot mount assembly  300  bending structure. The points may each be located at two different radii from the axis of symmetry of the pivot mount assembly  300  bending structure. In such an arrangement the position sensors may each measure a change in the z position at each of the points that is proportional to the change in the radius of curvature of the pivot mount assembly  300  bending structure. In accordance with embodiments, when the pivot mount assembly  300  bending structure is substantially flat (having zero curvature) the two or more position sensors  400  will record the same distance. This may correspond to calibrated values of the position sensors so that their measurements are referenced to a truly flat state. As the curvature of the pivot mount assembly  300  bending structure changes, the position sensors  400  will record different positions. In an embodiment, using these displacement values at each of the points and knowing the position of each of the points relative to the axis of symmetry, it is possible to calculate the equation of a sphere whose center lies on the axis of symmetry and whose surface corresponds to the surface of the pivot mount assembly  300  bending structure, and, by extension, the face of the MPA  103  (e.g. the contact surface  117  of the MESAs  115 ). From this equation, the radius of curvature of the face of the MPA  103  (e.g. contact surface  117 ) can be known. 
     In an embodiment, one or more mirror patterns  360  are formed on the back side of the piezoelectric actuator element  340 , and vertically aligned with the one or more position sensors  400 . In this manner, the reflective mirror pattern  360  can aid signal integrity. In an embodiment, the one or more mirror patterns  360  are formed of a reflective material (e.g. gold), and may have an average surface roughness, Ra, of 2,000 Angstroms or less. The one or more mirror patterns  360  may be formed on the back side of the base substrate  311  in addition to, or alternatively to the back side of the one or more piezoelectric actuator elements  340 . 
     In an embodiment, three or more position sensors  400  are arranged at a common radius from the axis of symmetry of the pivot platform  304  and an additional position sensor  400  is located near the axis of symmetry. Using this arrangement information about the rotation of pivot mount assembly  300  about the x axis, rotation of the pivot mount assembly  300  about the y axis, displacement of the pivot mount assembly  300  in the z direction, and curvature of the pivot mount assembly  300  are obtained. Incorporation of a larger array of position sensors  400  may be deployed to obtain more complex topography information. 
     During a transfer sequence in accordance with embodiments there may be large thermal differentials at the contact interface between the MPA  103  and the target substrate (e.g. donor substrate or receiving substrate). For example, pre-contact thermal differentials can be on the order of 200° C. between the MPA  103  and the target substrate. This can result in a thermal gradient from the front side of the MPA  103  to the back side of the pivot mount assembly  300  at the time of initial contact. This gradient can result in a thermal-strain-induced curvature of the MPA  103 , which would otherwise be flat when the thermal gradient is zero. As illustrated in  FIG. 19 , such curvature may be positive (concave up) or negative (concave down), depending on the conditions. Out-of-plane curvature in the MPA  103  may be on the order of microns deviation from flat in some embodiments. 
     In accordance with embodiments, the change in curvature of the pivot mount assembly  300  may be measured directly during pick and place operations and simultaneously correlated to the changing temperature gradient during the transfer process. Results from experimentation, analysis, and simulation demonstrate that the expected shape change in the MPA  103  and pivot mount assembly  300  structure may be dominated by plate bending. This is because the magnitude of the thermal gradient in the MPA and pivot mount assembly structure can be greatest in the Z dimension. Further, results show that the most common manifestation of the shape change may be out-of-plane bending that is substantially constant in radius (spherical) and rotationally symmetric about the center of the pivot mount assembly  300  pivot platform  304  (along the z-axis). 
     A pick and place operation in accordance with embodiments may include a temperature profile of the pivot mount assembly in accordance with embodiments. Different elevated temperatures may be associated with the MPA contacting an array of micro devices on a donor substrate, translating the MPA from a location over the donor substrate to a location over a receiving substrate, contacting the MPA with the receiving substrate, and removing the MPA from the receiving substrate and translating back toward a position over the donor substrate. Thermal disturbances and physical strain inducing moments (e.g. during contact, or disengaging) may occur at various periods during the transfer sequence resulting in a change in curvature of the MPA, and correspondingly the curvature of the pivot mount assembly  300 . In accordance with embodiments, the bending assembly may have a corresponding unique curvature with each temperature or strain inducing moment, and the active shape control system addresses this by countering the thermally induced and physically induced strain. 
       FIG. 20  is conceptual illustration of a method of determining a shape-fitting feedback signal in accordance with an embodiment. In the particular embodiment illustrated, four position (e.g. displacement) sensors  400  are arranged to measures a distance in the z direction to either of the base substrate  311  or piezoelectric actuator element  340 . As shown, position sensors  1 - 3  may be at (x, y) positions not in a straight line, for example a circle. In an embodiment, the position sensors  1 - 3  are equally spaced along a circle concentric to the center axis of the piezoelectric actuator element  340 . The fourth position sensor may be at an (x, y) position that is not along the radius of the circle. For example, the fourth position sensor may be located a known distance from the center axis. In accordance with embodiments, multiple position sensors  400  measure a distance to the target at each sensor location, resulting in the array of points (x 1 , y 1 , z 1 ), (x 2 , y 2 , z 2 ), (x 3 , y 3 , z 3 ), (x 4 , y 4 , z 4 ) which are input into a shape fitting algorithm. The shape fitting algorithm can use various methods for defining the shape. 
     In an embodiment, the shape filling algorithm can calculate an equation of a sphere defined by the four points, and output a measured radius of curvature based on a best-fit sphere calculation. This measured radius of curvature (i.e. measured MPA shape) can be compared against a reference (e.g. desired MPA radius of curvature) with the active shape control algorithm which outputs an error signal to an amplifier that then applies one or more gain values to the error signal and adjusts its output voltage to the piezoelectric actuator element  340  in response to the error signal. In accordance with embodiments, this may cause the piezoelectric actuator element  340  to flatten the measured radius of curvature. 
     In an embodiment, the shape filling algorithm can calculate an equation of a plane defined by the first three points, and additionally calculate an offset of the distance between the plane and the fourth point (x 4 , y 4 , z 4 ). The control loop in accordance with embodiments, functions to reduce the offset, with the flat state corresponding to a zero offset value. The measured offset (i.e. measured MPA shape) can then be compared against a reference (e.g. zero value corresponding to a flat state) with the active shape control algorithm, which outputs an error signal to the amplifier that then applies one or more gain values to the error signal and adjusts its output voltage to the piezoelectric actuator element  340  in response to the error signal (e.g. to drive the offset to zero). 
     Referring now to  FIGS. 21-29 , in accordance with some embodiments, the position sensors  400  may be formed from an arrangement of light guides. In some embodiment, the position sensors  400  may be displacement sensors used for measurement of absolution distance in various process control and/or calibration functions. In some aspects, the position sensors  400  may be packaged in a manner to fit within limited available space within the micro device transfer tool  100  where other sensors would be too large. The position sensors  400  may additionally include various structural features that can mitigate drift, including rigid light guides to mitigate drift due to flexing, and packaging considerations such as photodiodes being mounted on opposite sides of a circuit board to mitigate thermal drift. Furthermore, integration of photodiodes for intensity measurements in accordance with embodiments may further facilitate the ability for higher speed measurements, such as on the order of 10-300 kHz or even as high as the MHz range. This may be higher than available with commercially available laser displacement sensors that use image sensors or linear sensor arrays, which take more time to read and result in lower measurement speed, such as on the order of 1 kHz or lower. 
       FIG. 21  is a schematic cross-sectional side view illustration of a position sensor  400  in accordance with embodiments. In an embodiment, a position sensor  400  includes a light source  426 , such a light emitting diode (LED), and a light source light guide  416  (which may optionally be a fiber) including a proximal end adjacent the light source  426  and a distal end adjacent a guide tip  452 . The position sensor  400  additionally includes a first photodiode  424 , and a first rigid light guide  414  including a proximal end adjacent the first photodiode  424  and a distal end adjacent the guide tip  452 , as well as a second photodiode  422 , and a second rigid light guide  412  including a proximal end adjacent the second photodiode  422  and a distal end adjacent the guide tip  452 . In this arrangement, the distal ends of the light source light guide  416 , first rigid light guide  414  and the second rigid light guide  412  are all substantially coplanar at the guide tip  452 . Additionally, the rigid light guides  414 ,  412  may be entirely rigid between their proximal and distal ends. The light source light guide  416  may optionally be rigid, or may optionally be flexible (e.g. flexible optical fiber). 
     In an embodiment, the first photodiode  424  and the second photodiode  422  may be mounted on laterally opposite sides  434 ,  432  of a circuit board  430 . It has been observed that the photodiodes themselves can function as heat generators, leading to thermal drift. In accordance with embodiments, potential thermal gradients caused by operation of the photodiodes can be mitigated by locating the first and second photodiodes  424 ,  422  on laterally opposite sides of the circuit board  430  so that generated heat is distributed to both photodiodes. Furthermore, the first and second photodiodes  424 ,  422  may cover a same area on the circuit board  430 . 
       FIGS. 22A-22B  are schematic cross-sectional side view and top view illustrations of a circuit board  430  in accordance with an embodiment. A shown, a plurality of thermally conductive vias  436 , such as metal (e.g. copper) vias, can be located in the circuit board  430  extending between the first and second photodiodes  424 ,  422 . Such thermally conductive vias  436  can help to distribute heat that may be unevenly generated by first and second photodiodes  424 ,  422  due to different light intensities absorbed by the respective photodiodes during use, as will be explained in further detail below. 
     The circuit boards  430  may have additional configurations to control or distribute heat. As shown in  FIG. 21 , the first and second photodiodes  424 ,  422  may optionally each be mounted on a respective thermoelectric cooler  423  on the circuit board  430  to act as a sink for heat generated by the first and second photodiodes  424 ,  422 , or alternatively to provide uniform temperature substrates (upon which the photodiodes are mounted) during the thermal temperature cycles of the micro device transfer tool. In this manner, the photodiodes exhibit less thermal drift during operation of the tool. 
     Until this point discussion of thermal matching structures has been made with regard to the first and second photodiodes  424 ,  422 . In accordance with embodiments, the light source  426  may or may not be contained within the housing  450  of the position sensor  400 . For example, the light source  426  may be located elsewhere within the micro device transfer tool, or also located on the circuit board  430 . In an embodiment, a dummy light source  427  is also located on the opposite side of the circuit board as the light source  426  for thermal matching purposes, and may have the same area, etc. as the light source  426 , similarly described above with regard to the photodiodes. This may allow for more equal thermal distribution from the light source  426  toward the photodiodes. The light source  426  and dummy light source  427  may also be optionally mounted on a thermal electric cooler  423 . Additionally, thermally conductive vias  436  can be located in the circuit board  430  beneath the light source  426 , and the dummy light source  427  may also be operational, similar to the light source  426 , only not connected to an optical fiber so that a same amount of heat is generated. 
     Referring now to  FIG. 22B , thermally conductive (e.g. metal) patterns  438  (e.g. strips) may be deposited along edges of the circuit board  430  that are secured to the housing  450 . This may mitigate the formation of any thermal hot spots on the circuit board  430  adjacent the first or second photodiodes  424 ,  422  that could result from conduction of heat from the housing  450  to the circuit board  430 . The housing  450  may be formed of an insulating material as well as a thermally conductive material (e.g. metal) to promote even thermal spreading across the position sensor  400 . 
     Additional components may be mounted on the circuit board  430 , such as one or more thermistors  440  to measure temperature of the photodiodes, which can be used in an algorithm to compensate thermal drift, or to operate the thermal electric coolers  423 . Other components  442  may also be present such as, but not limited to, any of a signal conditioner, amplifier, passive device, etc. A flex circuit  454  may be connected to the circuit board  430 , for example at a proximal end, for connection to additional controlling electronics of the micro device transfer tool. 
       FIG. 23  is an isometric view illustration of a guide tip  452  of a position sensor in accordance in accordance with an embodiment. In particular,  FIG. 23  illustrates housing(s)  450  surrounding the respective optical fibers/light guides in which the distal ends  417 ,  415 ,  413  of the light source light guide  416 , first rigid light guide  414  and the second rigid light guide  412 , respectively, are all substantially coplanar with one another at the guide tip  452 . The distal end of the housing  450  may also be coplanar, as illustrated. As shown, in an embodiment, the first rigid light guide  414  (and distal end  415 ) are located laterally between the light source light guide  416  (and distal end  417 ) and the second rigid light guide  412  (and distal end  413 ). For example, such an arrangement may be a result of light guide bundling, where discrete light guides, with specified bending angles are integrated.  FIG. 24A  is a schematic bottom view illustration of a guide tip  452  of a position sensor in accordance with an embodiment. In an embodiment, the first rigid light guide  414  distal end  415  has a smaller area than the second rigid light guide  412  distal end  413 . Relative areas may result in relative intensities of light collected by the respective photodiodes  424 ,  422 . 
       FIG. 24B  is a schematic cross-sectional view illustration of a right light guide in accordance with an embodiment. For example, the rigid light guide may correspond to the first rigid light guide  414 , or second rigid light guide  412  each of which may have a single core  461  (e.g. extruded glass piece that is transmissive to light) and cladding  462 . For example, the core  461  and cladding  462  may both be formed of a glass material, but with different refractive index. The light source light guide  416  may be similarly fabricated with a fiber core  461  and cladding  462 . 
     Referring now to  FIG. 24C  a schematic cross-sectional side view illustration is provided of a rigid light guide fiber bundle  460  in accordance with an embodiment. For example, the rigid light guide fiber bundle  460  may be characteristic of either the first and second rigid light guides  414 ,  412 , and optionally the light source light guide  416 . As shown, the rigid fiber bundle  460  can include a plurality of fibers, each including a fiber core  461  and cladding  462 . For example, the fiber core  461  and fiber cladding  462  may both be formed of a glass material, but with a different refractive index. In the illustrated embodiment the fiber claddings  462  are fused together. Thus, each fiber bundle can be fabricated by drawing each fiber, bringing them together to bind/fuse the fiber bundles, followed by imparting any necessary bends followed by cooling, cutting, and polishing. Each fiber bundle may be a close-packed structure. 
     In accordance with embodiments the optical fibers may be fused together into rigid light guide fiber bundles as illustrated in  FIG. 24C  for at least two practical purposes, including packaging fine-sized optical fibers together into a rigid and sturdy structure, as well as for the thermal management issues related to the photodiodes. For example, the fibers may be tens of microns or less in diameter, such as less than 10 μm in diameter. Furthermore, in a specific application, the resulting small size of the position sensors  400  allows for multiple position sensors to be packages behind the pivot mount assembly, for example as shown in  FIG. 20 . 
     Referring now to  FIGS. 25A-25B  graphical illustrations are provided as a method of operating the position sensors  400  in accordance with an embodiment.  FIG. 25A  is a graphical illustration of a light intensity measured by the first and second rigid light guides as a function of displacement in accordance with an embodiment.  FIG. 25B  is a graphical illustration of a ratio of the light intensities measured by the first and second rigid light guides as a function of displacement in accordance with an embodiment. As shown, the first rigid light guide  414  and corresponding first photodiode  424  receive a first intensity of light (Rx 1 ) at a certain displacement of the target (e.g. pivot mount assembly) from the position sensor  400 . Likewise, the second rigid light guide  412  and corresponding second photodiode  424  receive a second intensity of light (Rx 2 ) the same displacement. In accordance with embodiments the ratio of Rx 2 Rx 1  may be calculated to determine a specified displacement with improved accuracy. As shown in  FIG. 24A  and  FIG. 25A , the relative areas of the rigid light guides may be attributed to the relative intensities at a distance from the light source light guide  416  distal end  417 . 
       FIG. 26  is a schematic side view illustration of a cone angle  419  generated by position sensor  400  in accordance in accordance with an embodiment. In accordance with embodiments, the cone angle  419  may be related to the numerical aperture of the optics of the position sensor  400 . In the embodiment illustrated in  FIG. 26 , the linear, side-by-side, relationship of the light source light guide  416 , first rigid light guide  414 , and second rigid light guide  412  are illustrated. Furthermore, since light is captured in in single x-direction, the area of the second rigid light guide  412  may be increased relative to the first rigid light guide  414 . 
     Referring now to  FIG. 27 , a schematic cross-sectional side view illustration is provided of relay optics  475  attached to the guide tip  452  of a position sensor  400  in accordance in accordance with an embodiment. For example, the relay optics  475  may control the numerical aperture, working distance, and sensitivity of the position sensor  400 . For example, a smaller numerical aperture may be selected for a higher range and lower sensitivity (resolution), while a larger numerical aperture may be selected for a lower range and greater sensitivity (resolution). In accordance with embodiments, similar position sensors with different relay optics  475  may be incorporated throughout the micro device transfer tool, allowing for design uniformity across the tool with flexibility in use. 
     As illustrated, the relay optics  475  can include a collimator assembly  470  and a focus lens assembly  480  distal to the collimator assembly  470 . In an embodiment, the collimator assembly  470  is fixed (e.g. permanently) to the guide tip  452 , and the focus lens assembly  480  is releasably attachable to the collimator assembly. For example, this may be accomplished by a pair of male-female threads  476 ,  484  on the collimator assembly  470  and focus lens assembly  480 , respective, or vice versa. In an embodiment the collimator assembly includes a collimator lens  472  contained inside a housing  471  and a pupil stop  474 . Pupil stop may optionally be forms as part of the focus lens assembly  480 . In an embodiment the focus lens assembly  480  includes a housing  481  and focus lens  482 . Focus lens  482  can determine the working length of the relay optics  475 . Light exits and enters the relay optics  475  through a distal end  486  of the focus lens assembly  480 . 
     In the above description, position sensors  400  are describes as including rigid light guides  414 ,  412  arranged with photodiodes for light sensing. Such rigid light guides can contribute to size reduction, reduced measurement times, and reduced drift. However, embodiments described herein may also be practiced with position sensors  400  that do not include rigid light guides. For example, relay optics  475  with a replaceable focus lens assembly  480  can be integrated with light guides that are not rigid. 
     Referring now to  FIGS. 28-29 ,  FIG. 28  is a schematic cross-sectional view illustration of a guide tip of a position sensor in accordance in accordance with another embodiment, while  FIG. 29  is a schematic side view illustration of a characteristic cone angle  419  generated by such a position sensor in accordance in accordance with an embodiment. The principle of operation of the position sensor of  FIGS. 28-29  remains the same, though with an alternative fiber arrangement. In such an embodiment, the arrangement of optical fibers includes a central fiber  496 , a first concentric ring of inner fibers  494  arranged around the central fiber  496 , and a second concentric ring of outer fibers  492  arranged around the first concentric ring of inner fibers  494 . The concentric rings of fibers may be arranged in close-packed bundle. Similar to the rigid light guide fiber bundle embodiments, proximal ends of the central fiber  496 , first concentric ring of inner fibers  494  and second concentric ring of outer fibers  492  can be arranged adjacent to a light source, first photodiode, and second photodiode, respectively. Similarly, measured light intensity or power by the first and second photodiodes will be proportional to the distance between the fibers and a target substrate. 
     In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for assembling and operating a micro device transfer tool. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration.

Metadata:
Filing Date: 20201104
Publication Date: 20221025
Grant Date: 20221025
Priority Date: 20191111
Inventors: Czarnota, Patrick J.
PARKS, PAUL A.
MA, EDMUND L.
WANG, WEI
Assignee: APPLE INC
CPC Classifications: [{"code": "H02N2/0095", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/4271", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01D5/30", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/4206", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01D5/39", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01D5/39", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/4206", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/426", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01D5/30", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L31/0232", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/4271", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F77/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/6831", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/681", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/68", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/67144", "inventive": true, "first": false, "tree": "[]"}, {"code": "B65G47/90", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J15/0085", "inventive": false, "first": false, "tree": "[]"}, {"code": "B25J9/0015", "inventive": false, "first": false, "tree": "[]"}, {"code": "B25J7/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/426", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/4206", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/06", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 83695636