PATENT DOCUMENT

Publication Number: US-10183401-B2
Application Number: US-201615282225-A
Country: US
Kind Code: B2

Title: Mass transfer tool

Abstract:
Systems and methods for transferring a micro device from a carrier substrate are disclosed. In an embodiment, a mass transfer tool includes an articulating transfer head assembly, a carrier substrate holder, and an actuator assembly to adjust a spatial relationship between the articulating transfer head assembly and the carrier substrate holder. The articulating transfer head assembly may include an electrostatic voltage source connection and a substrate supporting an array of electrostatic transfer heads.

Claims:
What is claimed is: 
     
       1. A mass transfer tool comprising:
 a lower assembly including:
 a carrier substrate holder; and 
 a receiving substrate holder; 
 
 a stage located over the lower assembly; 
 an articulating transfer head assembly mounted on the stage; and 
 an actuator assembly to move the articulating transfer head assembly in six degrees of freedom. 
 
     
     
       2. The mass transfer tool of  claim 1 , wherein the stage is movable within an x-y plane. 
     
     
       3. The mass transfer tool of  claim 2 , wherein the lower assembly further comprises an upward-viewing imaging device. 
     
     
       4. The mass transfer tool of  claim 3 , wherein the upward-viewing imaging device is fixed in place relative to the carrier substrate holder. 
     
     
       5. The mass transfer tool of  claim 3 , wherein the upward-viewing imaging device comprises a digital camera. 
     
     
       6. The mass transfer tool of  claim 1 , further comprising a first position sensor fixed relative to the transfer head assembly to detect a position of the carrier substrate holder. 
     
     
       7. The mass transfer tool of  claim 6 , further comprising a second position sensor fixed relative to the carrier substrate holder to detect a position of the articulating transfer head assembly. 
     
     
       8. The mass transfer tool of  claim 1 , further comprising a flexure to dampen force when contacting the articulating transfer head assembly with a workpiece. 
     
     
       9. The mass transfer tool of  claim 8 , further comprising a position sensor fixed relative to the transfer head assembly to sense a deflection of the flexure. 
     
     
       10. The mass transfer tool of  claim 9 , wherein the position sensor comprises a spectral-interference laser displacement sensor. 
     
     
       11. The mass transfer tool of  claim 9 , wherein the position sensor is configured to detect a 50 nanometer deflection of the flexure. 
     
     
       12. The mass transfer tool of  claim 8 , wherein the flexure comprises an inner edge, an outer edge, and a plurality of slots between the inner edge and the outer edge. 
     
     
       13. The mass transfer tool of  claim 1 , further comprising a heater to heat a mounting surface of the articulating transfer head assembly. 
     
     
       14. The mass transfer tool of  claim 13 , further comprising a second heater coupled with the carrier substrate holder. 
     
     
       15. The mass transfer tool of  claim 13 , further comprising a third heater coupled with the receiving substrate holder.

Description:
RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 14/852,366, filed Sep. 11, 2015, which is a continuation of U.S. patent application Ser. No. 13/607,031, filed Sep. 7, 2012, now U.S. Pat. No. 9,162,880, the full disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to micro devices. More particularly, embodiments of the present invention relate to systems and methods for transferring a micro device from a carrier substrate. 
     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 diode (LED) display systems, and MEMS or quartz-based oscillators. 
     Traditional technologies for transferring of devices include transfer by wafer bonding from a transfer wafer to a receiving wafer. One such implementation is “direct printing” involving one bonding step of an array of devices from a transfer wafer to a receiving wafer, followed by removal of the transfer wafer. Another such implementation is “transfer printing” involving two bonding/de-bonding steps. In transfer printing a transfer wafer may pick up an array of devices from a donor wafer, and then bond the array of devices to a receiving wafer, followed by removal of the transfer wafer. 
     Some printing process variations have been developed where a device can be selectively bonded and de-bonded during the transfer process. 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. 
     SUMMARY OF THE DESCRIPTION 
     A mass transfer tool and methods of operating the mass transfer tool are disclosed. In an embodiment, a mass transfer tool includes an articulating transfer head assembly having an electrostatic voltage source connection and a substrate supporting an array of electrostatic transfer heads. The substrate can be releasably attachable to a mounting surface of the transfer head assembly and electrically connectable with the electrostatic voltage source connection. For example, the mounting surface can include a vacuum port coupled with a vacuum source to apply suction to the substrate. In an embodiment, the electrostatic voltage source connection can include a resilient conductor that presses against the substrate. The mass transfer tool can also include a carrier substrate holder, a receiving substrate holder, and an actuator assembly to adjust a spatial relationship between the articulating transfer head assembly and the carrier substrate holder or the receiving substrate holder. For example, the actuator assembly can adjust the spatial relationship in at least six degrees of freedom. More particularly, the actuator assembly can include a first actuator subassembly coupled with the articulating transfer head assembly to adjust an articulating transfer head assembly position in at least four degrees of freedom and a second actuator subassembly coupled with the carrier substrate holder to adjust a carrier substrate holder position in at least two degrees of freedom. The second actuator subassembly may also be coupled with the receiving substrate holder to adjust a receiving substrate holder position in at least two degrees of freedom. Alternatively, the receiving substrate holder may be coupled with a separate actuator subassembly. Even more particularly, the first actuator assembly can include a first flexure coupled with the articulating transfer head assembly to constrain movement of the articulating transfer head assembly in a direction orthogonal to a contact surface of the array of electrostatic transfer heads. The first actuator assembly can also include a second flexure coupled with the articulating transfer head assembly that includes a second flexing surface oriented substantially parallel to the first flexing surface. 
     In an embodiment, the mass transfer tool can include one or more heaters to heat the substrate, the carrier substrate holder, and the receiving substrate holder. For example, the articulating transfer head may include a heater, and the carrier and receiving substrate holder can each be coupled to a heater. 
     In an embodiment, the mass transfer tool can include a first position sensor fixed relative to the mounting surface to detect a position of a carrier substrate on the carrier substrate holder. The mass transfer tool can also include a second position sensor fixed relative to the carrier substrate holder to detect the articulating transfer head assembly position. Further still, the mass transfer tool can include a third position sensor coupled with the actuator assembly to detect a deflection of the first flexing surface. Each of these position sensors can be spectral-interference laser displacement meters. In yet another embodiment, the mass transfer tool can include a force gauge coupled with the carrier substrate holder to measure a force applied to the carrier substrate holder. 
     In an embodiment, the mass transfer tool can include one or more imaging devices, such as cameras. For example, a first imaging device fixed relative to the articulating transfer head assembly can have a first imaging plane. Further, the mass transfer tool can also include a second imaging device fixed relative to the carrier substrate holder that can have a second imaging plane. A fiducial mark may be located between the first imaging plane and the second imaging plane of the imaging devices. By way of example, the fiducial mark can be an asymmetric pattern that is part of a transparent plate. 
     In an embodiment, a method of operating the mass transfer tool includes establishing a frame of reference comprising an x-axis and an x-y plane, and aligning a substrate supporting an array of electrostatic transfer heads with the frame of reference. In an embodiment, the frame of reference is established by setting an x-y datum and setting a z-datum. The x-y datum may be set by aligning a first and second imaging devices to a fiducial mark between the first and second imaging devices. For example, the x-y datum may be set by aligning a first imaging device having a first imaging plane and a second imaging device having a second imaging plane with a fiducial mark located between the first imaging plane and the second imaging plane. By way of example, the imaging devices can be cameras. The first and second imaging planes can be parallel to the x-y plane. The z-datum may be set by sending a first and second coplanar surfaces that are between a first and second position sensors and parallel to an x-y plane having the x-y datum. For example, the z-datum may be set by sensing a first and second surface of a z-gauge with a first and second position sensor, respectively. The position sensors can both have sensing directions orthogonal to the x-y plane, and the first and second surfaces can be coplanar and parallel to the x-y plane. By way of example, the position sensors can be spectral-interference laser displacement sensors. The z-gauge can be releasably attached to the mounting surface of the mass transfer tool. For example, the z-datum can be distanced 100 micrometers or less from a contact surface of the array of electrostatic transfer heads. 
     In an embodiment, the substrate is aligned with the frame of reference by articulating and rotating the articulating transfer head assembly. The articulating transfer head assembly is articulated to align the array of electrostatic transfer heads parallel to the x-y plane. This articulating can include detecting a distance to each of four reference points on the substrate and moving the articulating transfer head assembly until the distance to each of the four reference points is equal. The reference points can be detected with a first position sensor, such as a spectral-interference laser displacement sensor, having a sensing direction orthogonal to the x-y plane. The articulating transfer head assembly may be rotated to align a reference line passing through a first reference mark and a second reference mark of the substrate parallel to the x-axis. This rotating can include detecting the first reference mark and the second reference mark with a first imaging device, such as a camera, having a first imaging plane parallel to the x-y plane. The first reference mark and second reference mark can be electrostatic transfer heads. 
     In an embodiment, establishing the frame of reference including heading the articulating transfer head assembly to a temperature used in an subsequent transfer operation. For example, the mounting surface of the articulating transfer head assembly is heated to a temperature range of about 100 to 350 degrees Celsius. In an embodiment, the substrate supporting the array of electrostatic transfer heads is heated to a temperature range of about 100 to 350 degrees Celsius when aligning the substrate with the frame of reference. 
     In an embodiment, a method of operating the mass transfer tool includes adjusting a spatial relationship between an articulating transfer head assembly and a carrier substrate holder with an actuator assembly. An array of micro devices on a carrier substrate on the carrier substrate holder is contacted by an array of electrostatic transfer heads coupled with the articulating transfer head assembly. Voltage is applied to the array of electrostatic transfer heads through an electrostatic voltage source connection of the articulating transfer head assembly and the array of micro devices is picked up from the carrier substrate. 
     Adjusting the spatial relationship can include determining an orientation of the carrier substrate on the carrier substrate holder and matching an orientation of the substrate coupled with the articulating transfer head assembly to that orientation. Determining the orientation of the carrier substrate can include detecting a distance to each of four reference points on the carrier substrate with a first position sensor, such as a spectral-interference laser displacement sensor, having a first sensing direction orthogonal to the x-y plane. Furthermore, a first imaging device, such as a camera, having a first imaging plane parallel to the x-y plane, can detect a first reference mark and a second reference mark on the carrier substrate. In an embodiment, the first imaging device can also detect the distance to each of four reference points on the carrier substrate by determining a focal length. Matching the orientations of the substrate and the carrier substrate includes rotating and articulating the articulating transfer head assembly. The articulating transfer head assembly is rotated to align a reference line passing through a first reference mark and a second reference mark of the substrate parallel to a line passing through a first reference mark and second reference mark of the carrier substrate. The rotating can include detecting the first and second reference marks with first and second imaging devices having respective first and second imaging planes that are parallel to the x-y plane. The articulating transfer head assembly is articulated to align the array of electrostatic transfer heads parallel to the carrier substrate. The articulating can include detecting a distance to each of four reference points on the carrier substrate with a first position sensor having a first sensing direction orthogonal to the x-y plane and moving the articulating transfer head assembly until the substrate is parallel to the carrier substrate. 
     In an embodiment, contacting the carrier substrate with the array of micro devices includes sensing contact. For example, contact can be sensed by sensing a deflection of a flexure coupled with the articulating transfer head assembly using a first position sensor, such as a spectral-interference laser displacement sensor. In an embodiment, the spectral-interface laser displacement sensor is capable of differentiating a 50 nanometer deflection of the flexure. Alternatively, contact can be sensed based on a change in a load applied to the carrier substrate holder, as measured by a force gauge coupled with the carrier substrate holder. In an embodiment, the force gauge can measure with at least a microgram resolution. 
     Picking up the micro devices can include sensing removal of the array of micro devices from the carrier substrate based on a deflection of a flexure coupled with the articulating transfer head assembly. The deflection can be sensed by a first position sensor, such as a spectral-interference laser displacement sensor coupled with the actuator assembly. Alternatively, removal of the array of micro devices from the carrier substrate can be sensed based on a change in a load applied to the carrier substrate holder as measured by a force gauge coupled with the carrier substrate holder. In an embodiment, the force gauge can measure with at least a microgram resolution. In an embodiment, after picking up the array of micro devices from the carrier substrate, attachment of the array of micro devices to the array of electrostatic transfer heads is confirmed, for example by optical inspection of the electrostatic transfer head or carrier substrate. 
     In an embodiment, a method of operating the mass transfer tool includes contacting a receiving substrate coupled with a receiving substrate holder using the array of micro devices. Contact can be sensed in several manners. For example, contact can be sensed by sensing a deflection of a flexure coupled with the articulating transfer head assembly using a first position sensor, such as a spectral-interference laser displacement sensor coupled with the actuator assembly. Alternatively, contact can be sensed based on a change in a load applied to the receiving substrate holder, as measured by a force gauge coupled with the receiving substrate holder. In an embodiment, the force gauge can measure with at least a microgram resolution. 
     In an embodiment, the voltage can be removed from the array of electrostatic transfer heads. The array of electrostatic transfer heads can then be removed from the array of micro devices on the receiving substrate. Removal of the electrostatic transfer heads from the micro devices can be sensed in several manners. For example, removal can be sensed by sensing a deflection of a flexure coupled with the articulating transfer head assembly using a first position sensor, such as a spectral-interference laser displacement sensor coupled with the actuator assembly. Alternatively, removal can be sensed based on a change in a load applied to the receiving substrate holder, as measured by a force gauge coupled with the receiving substrate holder. In an embodiment, the force gauge can measure with at least a microgram resolution. In an embodiment, after removing the electrostatic transfer heads from the micro devices, release of the array of micro devices from the array of electrostatic transfer heads is confirmed, for example by optical inspection of the electrostatic transfer head or receiving substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustration of a mass transfer tool in accordance with an embodiment of the invention. 
         FIG. 2  is a perspective view illustration of an upper assembly portion of a mass transfer tool having an articulating transfer head assembly in accordance with an embodiment of the invention. 
         FIG. 3  is a perspective view illustration of an upper assembly portion of a mass transfer tool having an articulating transfer head assembly in accordance with an embodiment of the invention. 
         FIG. 4  is a cross-sectional perspective view illustration taken about line A-A of  FIG. 2  of an upper assembly portion of a mass transfer tool having an articulating transfer head assembly in accordance with an embodiment of the invention. 
         FIG. 5  is a perspective view illustration of a flexure in accordance with an embodiment of the invention. 
         FIG. 6A  is a cross-sectional perspective view illustration of a portion of an articulating transfer head assembly of a mass transfer tool having an electrostatic voltage source connection in accordance with an embodiment of the invention. 
         FIG. 6B  is a schematic side view illustration of a substrate supporting an array of electrostatic transfer heads attached to the mounting surface and electrically connected with one or more electrostatic voltage source connections in accordance with an embodiment of the invention. 
         FIG. 7  is a perspective view illustration of a lower assembly portion of a mass transfer tool having a carrier substrate holder and a receiving substrate holder in accordance with an embodiment of the invention. 
         FIG. 8  is a cross-sectional perspective view illustration taken about line B-B of  FIG. 7  of a lower assembly portion of a mass transfer tool having a carrier substrate holder and a receiving substrate holder in accordance with an embodiment of the invention. 
         FIG. 9  is a perspective view illustration of an upper assembly portion of a mass transfer tool having a tripod actuator in accordance with an embodiment of the invention. 
         FIG. 10  is a perspective view illustration of a lower assembly portion of a mass transfer tool having sensors in accordance with an embodiment of the invention. 
         FIG. 11  is a side view schematic illustration of an upper assembly portion of a mass transfer tool having an articulating transfer head assembly in accordance with an embodiment of the invention. 
         FIG. 12A  is a flowchart illustrating a method of aligning a substrate supporting an array of electrostatic transfer heads with a frame of reference in accordance with an embodiment of the invention. 
         FIG. 12B  is a flowchart illustrating a method of establishing a frame of reference in accordance with an embodiment of the invention. 
         FIG. 12C  is a flowchart illustrating a method of operating a mass transfer tool to transfer an array of micro devices shown in accordance with an embodiment of the invention. 
         FIG. 13A  is a side view schematic illustration of a method of setting an x-y datum in accordance with an embodiment of the invention. 
         FIG. 13B  is a perspective view schematic illustration of a method of setting an x-y datum in accordance with an embodiment of the invention. 
         FIG. 14A  is a side view illustration of a method of setting a z-datum in accordance with an embodiment of the invention. 
         FIG. 14B  is a side view illustration of a method of setting a z-datum in accordance with an embodiment of the invention. 
         FIG. 15A  is a perspective view schematic illustration of a method of aligning a substrate with a frame of reference in accordance with an embodiment of the invention. 
         FIG. 15B  is a perspective view schematic illustration of a method of aligning a substrate with a frame of reference in accordance with an embodiment of the invention. 
         FIG. 17  is a flowchart illustrating a method of operating a mass transfer tool to pick up an array of micro devices shown in accordance with an embodiment of the invention. 
         FIG. 18  is a schematic illustration of an adjustment of a spatial relationship between an articulating transfer head assembly and a carrier substrate holder in accordance with an embodiment of the invention. 
         FIGS. 16A through 16C  are side view schematic illustrations of a method of matching an orientation and contacting a substrate to a carrier substrate using a mass transfer tool in accordance with an embodiment of the invention. 
         FIG. 19A  is a cross-sectional side view illustration of an array of electrostatic transfer heads positioned over an array of micro devices on a carrier substrate after the spatial relationship between the articulating transfer head assembly and the carrier substrate holder has been adjusted in accordance with an embodiment of the invention. 
         FIG. 19B  is a cross-sectional side view illustration of an array of electrostatic transfer heads in contact with an array of micro devices in accordance with an embodiment of the invention. 
         FIG. 19C  is a cross-sectional side view illustration of an array of electrostatic transfer heads picking up an array of micro devices in accordance with an embodiment of the invention. 
         FIG. 19D  is a cross-sectional side view illustration of an array of micro devices released onto a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 20  is a schematic illustration of an exemplary computer system  150  that may be used in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention describe systems and methods for transferring a micro device or an array of micro devices from a carrier substrate. In some embodiments, the micro devices or array of micro devices described herein may be any of the micro LED device structures illustrated and described in related U.S. patent application Ser. Nos. 13/372,222, 13/436,260, and 13/458,932. While some embodiments of the present invention are described with specific regard to micro LED devices, it is to be appreciated that embodiments of the invention are not so limited and that certain embodiments may also be applicable to other micro LED devices as well as other micro devices such as diodes, transistors, ICs, and MEMS. 
     In various embodiments, description is made with reference to the figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the present invention. In other instances, well-known processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the present invention. Reference throughout this specification to “one embodiment,” “an embodiment”, or the like, means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “one embodiment,” “an embodiment”, or the like, in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “micro” device or “micro” LED structure as used herein may refer to the descriptive size of certain devices or structures in accordance with embodiments of the invention. As used herein, the terms “micro” devices or structures are meant to refer to the scale of 1 to 100 μm. However, it is to be appreciated that embodiments of the present invention are not necessarily so limited, and that certain aspects of the embodiments may be applicable to larger, and possibly smaller size scales. In an embodiment, a single micro device in an array of micro devices, and a single electrostatic transfer head in an array of electrostatic transfer heads both have a maximum dimension, for example length or width, of 1 to 100 μm. In an embodiment, the top contact surface of each electrostatic transfer head has a maximum dimension of 1 to 100 μm, or more specifically 3 to 10 μm. In an embodiment a pitch of an array of micro devices, and a corresponding array of electrostatic transfer heads is (1 to 100 μm) by (1 to 100 μm), for example a 10 μm by 10 μm pitch or 5 μm by 5 μm pitch. 
     In one aspect, embodiments of the invention describe a manner for mass transfer of an array of pre-fabricated micro devices with an array of electrostatic transfer heads. For example, the pre-fabricated micro devices may have a specific functionality such as, but not limited to, an LED for light-emission, silicon IC for logic and memory, and gallium arsenide (GaAs) circuits for radio frequency (RF) communications. In some embodiments, arrays of micro LED devices which are poised for pick up have a micro sized pitch such as a 10 μm by 10 μm pitch or 5 μm by 5 μm pitch. At these densities a 6 inch substrate, for example, can accommodate approximately 165 million micro LED devices with a 10 μm by 10 μm pitch, or approximately 660 million micro LED devices with a 5 μm by 5 μm pitch. A mass transfer tool including an array of electrostatic transfer heads matching an integer multiple of the pitch of the corresponding array of micro LED devices can be used to pick up and transfer the array of micro LED devices to a receiving substrate. In this manner, it is possible to integrate and assemble micro LED devices into heterogeneously integrated systems, including substrates of any size ranging from micro displays to large area displays, and at high transfer rates. For example, a 1 cm by 1 cm array of electrostatic transfer heads can pick up and transfer more than 100,000 micro devices, with larger arrays of electrostatic transfer heads being capable of transferring more micro devices. 
     In another aspect, embodiments of the invention describe systems and methods that facilitate the mass transfer of micro devices using a substrate that supports the array of electrostatic transfer heads to apply an electrostatic force to the micro devices. In an embodiment, the substrate can be removed and reattached to the system, i.e., the substrate can be replaceable. Since the substrate is removable, the system and substrate can be more easily inspected, cleaned, and refurbished. Given that the substrate can have a life span corresponding to a rate of wear of the array of electrostatic transfer heads, the removal of the substrate allows for exchange of the used substrate before the array electrostatic transfer heads fail. Thus, a system with a replaceable substrate can improve system longevity and increase system reliability. 
     In another aspect, embodiments of the invention describe systems and methods of transferring an array of micro devices at elevated temperatures above room temperature. In some embodiments, pick up of the array of micro devices from a carrier substrate and/or placement of the array of micro devices on a receiving substrate may be performed at elevated temperatures, for example to create a phase change in a bonding layer connecting the array of micro devices to a carrier substrate or to create a phase change or alloy a bonding layer during placement of the array of micro devices on a receiving substrate. In some embodiments, where one or more operations during transfer from the carrier substrate to the receiving substrate are performed at an elevated temperature, the alignment operations of the mass transfer tool are also performed at the elevated operating temperature to compensate for shift in the transfer tool components due to thermal expansion. 
     In another aspect, embodiments of the invention describe systems and methods that facilitate the mass transfer of micro devices using an actuator assembly and various sensors that provide feedback related to the position of system components. The actuator assembly can include any number of actuators capable of generating motion of system components relative to one or more frames of reference and other system components. For example, the actuator assembly can move a substrate supporting an array of electrostatic transfer heads and an array of micro devices relative to one another in at least six degrees of freedom on a carrier substrate. Furthermore, the actuator assembly can be used for precise alignment and movement of system components. To enable precise motion control, various sensors can be used to provide feedback to a computer system or controller relating to the location and position of system components. For example, the sensors can detect locations of system components with resolutions of about 50 nanometers and actuators can be controlled, i.e., actuated, according to those locations. Thus, the combined actuator assembly and sensors enable spatial relationships of system components to be adjusted on a micrometer scale. As an example, the array of electrostatic transfer heads can be aligned parallel to the location of the array of micro devices within about one micrometer. Thus, it will be appreciated that a system with an actuator assembly and sensors as described below can precisely pick up and transfer the array of micro devices with high process throughput and yield. 
     In yet another aspect, embodiments of the invention describe systems and methods that facilitate the mass transfer of micro devices using sensors to sense the application or removal of contact between system components. For example, a position sensor or a force gauge can sense contact between an array of electrostatic transfer heads and an array of micro devices. Furthermore, actuators can be controlled, i.e., actuated, according to the contact. The position sensor or force gauge can similarly sense removal of the array of micro devices from a carrier substrate. Thus, it will be appreciated that a system with sensors to sense the application of contact between system components can precisely pick up and transfer the array of micro devices with high process throughput and yield. 
     Referring now to  FIG. 1 , a perspective view illustration of a mass transfer tool is shown in accordance with an embodiment of the invention. The mass transfer tool  100  operates according to the aspects described above. To do so, the mass 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 mass transfer tool  100  can include an upper assembly  102  having an articulating transfer head assembly  106  to receive a substrate containing an array of electrostatic transfer heads, as will be described further below. The articulating transfer head assembly  106  can include features that allow for the exchange of the substrate and for delivering a voltage to the electrostatic transfer heads to facilitate pick up of a micro device using an electrostatic force, as will be described further below. 
     The mass 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 supporting an array of micro devices. Furthermore, the receiving substrate holder  124  can be configured to hold a receiving 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, as will be described further below. 
     It will be appreciated that any reference to upper assembly  102  and lower assembly  104  is made for ease of description only, and that components and subassemblies of mass transfer tool  100  may be part of either or both of the upper assembly  102  and the lower assembly  104 . For example, both the upper assembly  102  and lower assembly  104  can include components of an actuator assembly  110 . The actuator assembly  110  moves various components of the mass transfer tool  100 , and more specifically, it can adjust spatial relationships between components in order to facilitate the transfer of micro devices using an array of electrostatic transfer heads on a substrate. For example, the articulating transfer head assembly  106  and the carrier substrate holder  108  and receiving substrate holder  124  can be adjusted such that the array of electrostatic transfer heads supported by the substrate attached to an articulating transfer head assembly conforms closely to a carrier substrate held by the carrier substrate holder or receiving substrate held by the receiving substrate holder. These types of adjustments require precise movements in multiple degrees of freedom. For example, the articulating transfer head assembly  106  can be moved by a tripod actuator  111  of the actuator assembly  110  having at least four degrees of freedom. Similarly, the carrier substrate holder  108  can be moved by an x-y stage  112  of the actuator assembly  110  having at least two degrees of freedom. Thus, the array of electrostatic transfer heads supported by the substrate attached to the articulating transfer head assembly and the array of micro devices supported by the carrier substrate held by the carrier substrate holder, as well as the receiving substrate held by the receiving substrate holder, can be precisely moved relative to each other with six degrees of freedom. It will be appreciated that the actuator assembly  110  is one of many possible configurations and it can include any number of components. For example, while the particular embodiment illustrated in  FIG. 1  illustrates an x-y stage  112  in the lower assembly  104  only, it is contemplated that the articulating transfer head assembly  106  in the upper assembly  102  can be mounted on an x-y stage in addition to, or alternative to x-y stage  112 . Thus, a variety of configurations are contemplated in accordance with embodiments of the invention which are capable of adjusting the spatial relationships between components in at least six degrees of freedom. 
     In addition to sharing components of the actuator assembly  110 , the upper assembly  102  and the lower assembly  104  can also include various sensors that are intended to sense spatial relationships, e.g., contact, between system components and to work together with the actuator assembly  110  to facilitate alignment of system components. For example, a downward-looking imaging device  126  and an upward-looking imaging device  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 (not shown) can be integrated within the mass transfer tool  100  and mounted relative to a carriage  120  and the articulating transfer head assembly  106  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 computer system  150  capable of receiving and processing inputs in order to control the system components accordingly. These and other sensors will be described in greater detail below. 
     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 , bracket  118 , and other structural connectors. Therefore, it will be appreciated that the mass 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. Having described the mass transfer tool  100  at a high level, the mass transfer tool  100  components and structure will now be addressed in more specific detail. 
     Referring now to  FIG. 2 , a bottom perspective view illustration of an upper assembly portion of a mass transfer tool including an articulating transfer head assembly is shown in accordance with an embodiment of the invention. As discussed above, the mass transfer tool  100  can include an articulating transfer head assembly  106  to receive a replaceable substrate  200  supporting an array of electrostatic transfer heads  204 . The substrate  200  is shown attached to the articulating transfer head assembly  106 . More particularly, the substrate  200  is attached to a mounting surface  202  of the articulating transfer head assembly  106 . The substrate  200  may be formed from a variety of materials such as silicon, ceramics, and polymers, which are capable of providing structural support. In an embodiment, the substrate  200  also includes wiring or vias that connect with the array of electrostatic transfer heads  204 . Each transfer head can further include a mesa structure, an electrode which may be formed over the mesa structure or integrally formed with the mesa structure, and a dielectric layer covering the electrode. The array of electrostatic transfer heads  204  can be formed with a pitch selected to match an integer multiple of a pitch of micro devices placed on a carrier substrate (not shown), as described below. In an embodiment, the array of electrostatic transfer heads  204  are any of the arrays of transfer heads described in related U.S. patent application Ser. Nos. 13/372,277, 13/466,966, 13/481,592, 13/543,675, and 13/543,684 each of which is incorporated herein by reference. The substrate  200  can also include one or more reference marks  206  to permit accurate positioning and alignment of the substrate  200 , as described below. 
     The articulating transfer head assembly can be encompassed by a housing  210 . Housing  210  can protect the articulating transfer head assembly  106  by separating it from external objects. Furthermore, housing  210  can support components of actuator assembly  110  components, such as a flexure and an axial actuator, which will be described further below. These actuator assembly  110  components can facilitate movement of the articulating transfer head assembly  106 . In at least one embodiment the housing  210  and the articulating transfer head assembly  106  can move relative to each other. In addition to being coupled with the articulating transfer head assembly  106  by various actuators, the housing  210  can also be coupled to a mounting plate  212 , which is either part of, or coupled to, other actuator assembly  110  components. For example, mounting plate  212  can be coupled to tripod  111  to permit actuation of the housing  210  and articulating transfer head assembly  106 . 
     As discussed above, the mass transfer tool  100  can include various sensors to measure and detect distances, and thus, to provide a control feedback that aids in the adjustment of the actuator assembly  110 .  FIG. 2  shows one such sensor included in the articulating transfer head assembly  106 , i.e., position sensor  208 . Position sensor  208  can have a position sensor lead  214  to communicate signals directly or indirectly with computer system  150 . The position sensor  208  can terminate in a distal end that is approximately coplanar with the mounting surface  202 . Furthermore, the distal end of the position sensor  208  can be fixed relative to the mounting surface  202 . As a result, the position sensor  208  can detect a distance to a surface relative to the mounting surface  202 . For example, when a spatial relationship between the articulating transfer head assembly  106  and the carrier substrate holder  108  is adjusted by the actuator assembly  110 , the position sensor  208  can provide feedback relating to a distance between the mounting surface  202  of the articulating transfer head assembly  106  and the carrier substrate holder  108 . This feedback may be informational, e.g., to provide a visual display of mass transfer tool status to a user, or it may be part of a positive feedback loop to control motion of the actuator assembly  110 . 
     In addition to sensing the spatial relationship between the articulating transfer head assembly  106  and the carrier substrate holder  108 , the position sensor  208  can be used to sense the spatial relationship between other components. For example, the position sensor  208  can be used to sense directly or indirectly a distance between the array of electrostatic transfer heads  204  supported by a substrate  200  attached to the mounting surface  202  and a carrier substrate held by the carrier substrate holder  108 . 
     In an embodiment, the position sensor  208  can include a spectral-interference laser displacement meter, such as a micro-head spectral-interference laser displacement meter manufactured by the Keyence Corporation of Osaka, Japan. The use of a spectral-interference laser displacement meter provides the advantage of absolute displacement measurement, without the necessity of meter calibration. Such capability can provide the benefit of increased efficiency in the applications described herein because there is less need for time consuming mass transfer tool alignment when, e.g., a substrate  200  is exchanged during operation. Furthermore, the spectral-interference laser displacement meter can sense multiple surfaces without the need for recalibration between sensing locations. Nevertheless, one skilled in the art will appreciate that position sensor  208  can include other types of sensors, including proximity sensors, optical sensors, and ultrasonic sensors. 
     Referring now to  FIG. 3 , a perspective view illustration of an upper assembly portion of a mass transfer tool having an articulating transfer head assembly is shown in accordance with an embodiment of the invention. In this illustration, substrate  200  is not attached to the mounting surface  202 . Thus, as described above, in at least one embodiment the substrate  200  can be releasably attached and detached from the mounting surface  202 . In an embodiment, the mounting surface  202  can include at least one vacuum port  302  coupled with a vacuum source (not shown) for drawing suction on an object placed against the mounting surface  202 . More particularly, when the substrate  200 , is positioned against the mounting surface  202 , suction is drawn through vacuum port  302  to create a negative pressure within one or more vacuum channels  304 . As shown, vacuum channels  304  can be formed in a pattern of intersecting lines to create a suction area. Thus, the substrate  200  is pushed against the mounting surface  202  by the pressure difference between the vacuum channels  304  and the surrounding atmosphere. As a result, the substrate  200  attaches to the mounting surface  202 . When the vacuum source is disconnected or the negative pressure in the vacuum channels  304  is insufficient to retain the substrate  200 , the attachment is released and the substrate  200  can be removed. 
     Although mounting surface  202  can be generally flat as shown in  FIG. 3 , it will be appreciated that the mounting surface  202  may instead have various contours. For example, in an embodiment, the mounting surface  202  can be wedge shaped or otherwise contoured to provide a reference feature that the substrate  200  can rest against. That is, in the case of a wedge shaped mounting surface  202  and a wedge shaped substrate  200 , the substrate  200  will be known to be oriented in the same angular orientation each time it is removed and installed within the wedge contour. 
     Referring now to  FIG. 4 , a cross-sectional perspective view illustration taken about line A-A of  FIG. 2  of a portion of an upper assembly portion of a mass transfer tool having an articulating transfer head assembly is shown in accordance with an embodiment of the invention. The substrate  200  is shown attached to the mounting surface  202  of the articulating transfer head assembly  106 . In an embodiment, the mounting surface  202  is thermally coupled with a heater  400 . For example, the heater  400  may include one or more heating elements  402 , such as heater rods, that generate heat in response to the application of electrical current. The heating elements  402  can increase in temperature to transfer heat to the substrate  200 . For example, heat can be conducted through a metal block (not shown). Alternatively, heat can be conveyed to the substrate  200  by convective or radiant heating across intervening air gaps. In one aspect, the heater  400  can be configured to raise the temperature of the mounting surface  202  to a range of about 50 to 500 degrees Celsius. More particularly, the heater  400  can be configured to raise the temperature of the mounting surface  202  to a range of about 100 to 350 degrees Celsius. It will be appreciated that other temperatures and temperature ranges may be contemplated within the scope of this disclosure. 
     It will be appreciated that the heating of the mounting surface  202  will result in transfer of heat to the array of electrostatic transfer heads  204  supported by the substrate  200 , and thus, heat can be delivered to an array of micro devices that the array of electrostatic transfer heads come into contact with. This heat can facilitate the removal of the micro devices from a carrier substrate and/or placement of the micro devices on a receiving substrate, as described further below. 
     As described above, the articulating transfer head assembly  106 , or a portion thereof, may be coupled to surrounding structures of the mass transfer tool  100 , such as housing  210 , through one or more components of actuator assembly  110 . For example, the articulating transfer head assembly  106  can be coupled to housing  210  of the mass transfer tool  100  by a flexure  404 . The flexure  404  may be fixed to the articulating transfer head assembly  106  along an inner edge of the flexure. Likewise, the flexure  404  may be fixed to a housing  210  of the mass transfer tool  100  either along an outer edge or through fastener holes. Thus, the articulating transfer head assembly  106  is able to move relative to the housing  210  through deflection of the flexure  404 . For example, plate  416 , which can define an upper surface of articulating transfer head assembly  106  can move relative to a position sensor  414  mounted on housing  210 . Furthermore, as explained below, movement of the transfer head assembly  106  can be constrained in a direction that is orthogonal to the mounting surface  202  since, in at least one embodiment, the mounting surface  202  can be parallel to the flexure surface. However, in some embodiments, the mounting surface  202  can be formed with a non-planar surface. Thus, in at least one embodiment, the flexure  404  can constrain movement of the transfer head assembly  106  in a direction that is orthogonal to an array of contact surfaces  205  (see  FIG. 6B ) of the array of electrostatic transfer heads  204  which may or may not be parallel to the mounting surface  202 . 
     In an embodiment, a second flexure  404 ′ can be used to further constrain movement of the articulating transfer head assembly  106  in the manner described below. The flexure  404 ′ can include a shape and configuration similar to the flexure  404 . Furthermore, the flexure  404 ′ can be coupled to a same or different structure as the flexure  404 . In an embodiment, the flexure surface of the flexure  404 ′ can be oriented substantially parallel to a flexure surface of the flexure  404 . As a result, the flexure  404  and the flexure  404 ′ can constrain movement of the mounting surface  202  in the same direction. 
     It will be appreciated that movement of the transfer head assembly  106  can be effected in at least two ways. First, if the articulating transfer head assembly  106  is driven such that the array of electrostatic transfer heads supported by an attached substrate  200  is driven into another object or surface, e.g., an array of micro devices supported on a carrier substrate, the reaction force from the impact will place a biasing load on the mounting surface  202  that can translate into deflection of the flexure  404 . Second, the articulating transfer head assembly  106  having the mounting surface  202  can be driven by another actuator component, such as an axial actuator  406  component of the actuator assembly  110 . In an embodiment, the axial actuator  406  can include a linear actuator. For example, the axial actuator  406  can include a voice coil actuator. In a voice coil actuator, an electrical current can be passed through a voice coil of the axial actuator  406  to generate a magnetic field that drives a permanent magnet such that the axial actuator extends linearly. It will be appreciated that other actuators, such as linear motors, hydraulic pistons, and other actuators that generate axial motion can be used. In one aspect, the flexure  404  constrains movement of the transfer head assembly  106  along a single direction, such that even actuators that impart significant lateral loads to the transfer head assembly can be used to perform the function of the axial actuator  406 . 
     Referring now to  FIG. 5 , a perspective view illustration of a flexure is shown in accordance with an embodiment of the invention. In this embodiment, the flexure  404  has a disc configuration, which includes outer edge  502 , inner edge  504 , and flexure surface  506  therebetween. Along flexure surface  506 , one or more slots  508  can be formed to increase the local flexibility of the slotted area. For example, several concentrically formed slots  508  arranged in an annular region between inner edge  504  and outer edge  502  can create flexing surface  510 . Flexing surface  510  can constrain flexure deflection in a single direction. More particularly, the flexing surface  510  can constrain motion of the inner edge  504  relative to the outer edge  502  in a direction along an axis that both the inner edge  504  and outer edge  502  are concentrically positioned around. 
     Referring now to  FIG. 6A , a cross-sectional view illustration of a portion of an articulating transfer head assembly portion of a mass transfer tool having an electrostatic voltage source connection is shown in accordance with an embodiment of the invention. The electrostatic voltage source connection  410  can be used to generate an electrostatic force with the array of electrostatic transfer heads  204 . In an embodiment, the electrostatic voltage source connection  410  can include a resilient conductor  602  having a contact  604 , a knee  606 , and a base clip  608 . The base clip  608  can attach to wiring of the mass transfer tool  100  to transfer a voltage to the contact  604 . The base clip  608  and the contact  604  can be placed at the terminal ends of the resilient conductor  602  and be separated by the knee  606 . The knee  606  can provide a flexing surface for the resilient conductor  602 . Due to the shape of the knee  606 , and the choice of materials used to construct the electrostatic voltage source connection  410 , the knee  606  can be resiliently compressed. In other words, the knee  606  can bend when the substrate  200  is attached to the mounting surface  202  and presses on the contact  604 . The substrate  200  can be attached to the mounting surface  202  by drawing suction on the substrate  200  through vacuum port  302  formed in the mounting surface  202 , as described above, and this suction can provide both the retention force applied to the substrate  200  as well as a compression load on the resilient conductor  602 . Thus, the knee  606  bends because the retention force applied to the substrate  200  is transmitted to the resilient conductor  602  at contact  604 , generating a load sufficient to cause material deflection and bending in knee  606 . Upon removal of the substrate  200 , the knee  606  can relax, allowing the contact  604  to extend beyond the mounting surface  202  away from the base clip  608 . Thus, the knee  606  stores energy to allow contact  604  to press against substrate  200  with sufficient force to maintain electrical contact during operation of mass transfer tool  100 . Resultantly, in at least one embodiment, the knee  606  can also function as an ejection mechanism for removing the substrate  200  when suction is removed by discontinuing vacuum through vacuum port  302 . 
     It will be appreciated that the knee  606  represents only one manner of providing a resilient structure to ensure that the resilient conductor  602  contacts the substrate  200  appropriately. Other potential shapes that provide a resilient structure include helical, arcuate, zigzag and other shapes conducive to elastic deformation of the overall structure. Furthermore, other structures, such as spring loaded contact pins, can be used in place of a resilient structure to ensure that adequate electrical contact is made between the electrostatic voltage source connection and the substrate. 
     In an embodiment, the electrostatic voltage source connection  410  can be formed in part or in whole from an electrically conductive material. For example, the electrostatic voltage source connection  410  can be formed from a beryllium copper alloy. The material may be stamped, bent, wound or otherwise processed to create a resilient structure for the resilient conductor  602 . 
       FIG. 6B  is a schematic side view illustration of a substrate  200  supporting an array of electrostatic transfer heads  204  attached to the mounting surface  202  and electrically connected with one or more electrostatic voltage source connections  410  in accordance with an embodiment of the invention. The voltage source connections  410  may be connected to a single voltage source or separate voltage sources. The one or more voltage source can apply a constant current voltage, or alternating current voltage. In an embodiment, an alternating current voltage is applied to a bipolar electrode structure in each electrostatic transfer head. As illustrated, the electrical coupling of the one or more electrostatic voltage source connections  410  to the array of electrostatic transfer heads  204  can be made, for example, through a via structure  207  or wiring that leads from the point of contact  604  through the substrate  200  to the array of electrostatic transfer heads  204 . Alternatively, electrical coupling can be made with wiring that leads from the point of contact  604  and over the substrate  200  to the array of electrostatic transfer heads  204 . 
     Referring now to  FIG. 7 , a top perspective view illustration of a lower assembly portion of a mass transfer tool having a carrier substrate holder and a receiving substrate holder is shown in accordance with an embodiment of the invention. The lower assembly  104  portion includes a carrier substrate holder  108  that is coupled with a carriage  120  of the mass transfer tool  100  and can be configured to hold a carrier substrate  706  supporting an array of micro devices. In one embodiment, the carrier substrate  706  rests within a recess of the carrier substrate holder  108 . For example, the carrier substrate holder  108  may include a counterbore formed in an upper surface, the counterbore having a profile that conforms with, and is slightly larger than, the profile of the carrier substrate  706 . 
     In an alternative embodiment, the carrier substrate  706  can be actively held within the carrier substrate holder  108 . For example, the carrier substrate  706  can rest on a holding surface, as described further below that includes a vacuum port coupled with a vacuum source. Suction can be applied to the carrier substrate  706  by the vacuum port when the carrier substrate  706  is placed over the holding surface. It will be appreciated that alternative methods of actively holding the carrier substrate  706  may be contemplated within the scope of this disclosure. For example, the carrier substrate holder  108  may include a chuck, such as a mechanical vise, having grippers that press against a surface of the carrier substrate  706  to retain the carrier substrate  706  within the carrier substrate holder  108 . Each of these alternative embodiments can serve a function of retaining and stabilizing the position of the carrier substrate  706  within the carrier substrate holder  108 . 
     In an embodiment, there is a force gauge  704  coupled with the carrier substrate holder  108 . For example, the carrier substrate holder  108  may be fastened to a plate of the force gauge  704  using various fasteners. As a result, the force gauge  704  can measure a force applied to the carrier substrate holder  108 . When no load is applied to the carrier substrate holder  108 , the force gauge  704  may measure the weight of the carrier substrate holder  108 . When a carrier substrate  706  is placed on the carrier substrate holder  108 , the force gauge  704  may then measure the cumulative weight of the carrier substrate  706  and the carrier substrate holder  108 . Furthermore, if an additional force were applied, such as if the array of electrostatic transfer heads  204  of the articulating transfer head assembly  106  were driven into the carrier substrate  706  by the axial actuator  406 , then the force gauge  704  may measure the cumulative weight and the force applied to the carrier substrate holder  108  by the articulating transfer head assembly  106 . It will be appreciated that force gauges of various specifications can be used within the scope of this disclosure, but in at least one embodiment, the force gauge  704  can measure with at least a microgram resolution. 
     It will be appreciated that the carrier substrate holder  108  can include additional features within the scope of this description. For example, the carrier substrate holder  108  may include jack screws (not shown) that can be adjusted to tilt a carrier substrate  706  held by the carrier substrate holder  108  or to otherwise adjust the carrier substrate orientation. This and other features will be contemplated by one skilled in the art within the scope of this disclosure. 
     Still referring to  FIG. 7 , the lower assembly  104  portion can include a receiving substrate holder  124  that is coupled with the carriage  120  of the mass transfer tool  100 . In an embodiment, a receiving substrate  714  rests within a recess of the receiving substrate holder  124 . For example, the receiving substrate holder  124  may include a counterbore formed in an upper surface, the counterbore having a profile that conforms with, and is slightly larger than, the profile of the receiving substrate  714 . 
     In an alternative embodiment, the receiving substrate  714  can be actively held within the receiving substrate holder  124 . For example, a holding surface, as described further below, may include a vacuum port coupled with a vacuum source. Suction can be applied to the receiving substrate  714  by the vacuum port when the receiving substrate  714  is placed over the holding surface. It will be appreciated that alternative methods of actively holding the receiving substrate may be contemplated within the scope of this disclosure. For example, the receiving substrate holder  124  may include a chuck, such as a mechanical vise, having grippers that press against a surface of the receiving substrate to retain the receiving substrate  714  within the receiving substrate holder  124 . Each of these alternative embodiments can serve a function of retaining and stabilizing the position of the receiving substrate  714  within the receiving substrate holder  124 . 
     In an embodiment, there is a force gauge  712  coupled with the receiving substrate holder  124 . For example, the receiving substrate holder  124  may be fastened to a plate of the force gauge  712  using various fasteners. As a result, the force gauge  712  can measure a force applied to the receiving substrate holder  124 . When no load is applied to the receiving substrate holder  124 , the force gauge  712  may measure the weight of the receiving substrate holder  124 . Thus, when a receiving substrate  714  is placed on the receiving substrate holder  124 , the force gauge  712  may then measure the cumulative weight of the receiving substrate  714  and the receiving substrate holder  124 . Furthermore, if an additional force were applied, such as if the array of electrostatic transfer heads  204  of the articulating transfer head assembly  106  were driven into the receiving substrate  714  by an axial actuator  406 , then the force gauge may measure the cumulative weight and the force applied to the receiving substrate holder  124  by the articulating transfer head assembly  106 . It will be appreciated that force gauges of various specifications can be used within the scope of this disclosure, but in at least one embodiment, the force gauge  712  can measure with at least a microgram resolution. 
     It will be appreciated that the receiving substrate holder  124  can include additional features within the scope of this description. For example, the receiving substrate holder  124  may include jack screws (not shown) that can be adjusted to tilt a receiving substrate  714  held by the receiving substrate holder  124  or to otherwise adjust the receiving substrate orientation. This and other features will be contemplated by one skilled in the art within the scope of this disclosure. 
     Referring now to  FIG. 8 , a cross-sectional perspective view illustration taken about line B-B of  FIG. 7  of a lower assembly portion of a mass transfer tool having a carrier substrate holder and a receiving substrate holder is shown in accordance with an embodiment of the invention. In an embodiment, the carrier substrate holder  108  receives the carrier substrate  706  on a holding surface  802 . The holding surface  802  can be a chamfer, or it may be another shape or combination of shapes. For example, the holding surface  802  can be a flat surface. Furthermore, as described above, the holding surface  802  can include a vacuum port (not shown) to apply a vacuum that actively holds the carrier substrate  706 . 
     When held by the carrier substrate holder  108 , the carrier substrate  706  can be apposed with a heat distribution plate  804 . The heat distribution plate  804  may be formed, for example, from a metal such as aluminum or silicon carbide, for thermal conduction. Thus, heat can be transferred readily from a heater  806  to the carrier substrate  706  through the heat distribution plate  804  to facilitate the transfer of micro devices from carrier substrate  706  to the array of electrostatic transfer heads  204 . The heater  806  can be a heating element having a number of different configurations. For example, the heater  806  can be an electric disc heater. Alternatively, the heater  806  can be a radiant heater. In an embodiment, heat transferred from the heater  806  to the carrier substrate  706  can increase the temperature of the carrier substrate  706  to a temperature range of about 100 to 200 degrees Celsius. In another embodiment, the temperature of the carrier substrate  706  can increase to a temperature range of about 140 to 180 degrees Celsius. In yet another embodiment, the temperature of the receiving substrate  714  can increase to a temperature of about 150 degrees Celsius. 
     In an embodiment, the receiving substrate holder  124  receives the receiving substrate  714  on a holding surface  810 . The holding surface  810  can be a chamfer, or it may be another shape or combination of shapes. For example, the holding surface  810  can be a flat surface. Furthermore, as described above, the holding surface  810  can include a vacuum port (not shown) to apply a vacuum that actively holds the carrier substrate  706 . 
     When held by the receiving substrate holder  124 , the receiving substrate  714  can be apposed with a heat distribution plate  812 . The heat distribution plate  812  may be formed, for example, from a metal such as aluminum or silicon carbide, which has good thermal conductivity properties. Thus, heat can be transferred readily from a heater  814  to the receiving substrate  714  to facilitate the transfer of micro devices from the array of electrostatic transfer heads  204  to the receiving substrate  714 . The heater  814  can be a heating element having a number of different configurations. For example, the heater  814  can be an electric disc heater. Alternatively, the heater  814  can be a radiant heater. In an embodiment, the heat transferred from the heater  814  to the receiving substrate  714  can increase the temperature of the receiving substrate  714  to a temperature range of between room temperature and about 250 degrees Celsius. In another embodiment, the temperature of the receiving substrate  714  can increase to a temperature range of about 100 to 200 degrees Celsius. In another embodiment, the temperature of the receiving substrate  714  can increase to a temperature of about 150 degrees Celsius. 
     In light of the description above, it will be apparent that in order to transfer an array of micro devices from a carrier substrate  706  held by the carrier substrate holder  108  to a receiving substrate  714  held by the receiving substrate holder  124  using an array of electrostatic transfer heads  204  supported by a substrate  200  attached to an articulating transfer head assembly  106 , the articulating transfer head assembly  106  can be moved relative to the substrate holders. More particularly, the relative motion and spatial relationship between various mass transfer tool  100  components can be adjusted to facilitate transfer of the array of micro devices. More specifically, a spatial relationship between articulating transfer head assembly  106  and carrier substrate holder  108  can be adjusted with six degrees of freedom, allowing for complete articulation in space between those components and any components fixed or coupled thereto. The spatial relationship adjustments can be made by various actuator assembly  110  components. 
     As discussed above, the lower assembly  104  can include actuator assembly  110  components coupled with the carrier substrate holder  108  to adjust the carrier substrate holder  108  relative to a reference point, such as gantry  113 , base  114 , or side beam  116 . For example, the lower assembly  104  can include actuator assembly  110  components having actuators that move linearly along two axes. These actuators can be an x-y stage  112  coupled with carriage  120 , such that a carrier substrate  200  held by a carrier substrate holder  108  mounted on the carriage  120  can be translated in a single plane with complete control in two degrees of freedom. The stages can thus impart translational motion to the carriage and components coupled with the carriage. Such an actuator would alone allow for adjustment between the array of electrostatic transfer heads  204  and the carrier substrate  706  in two degrees of freedom. 
     Referring now to  FIG. 9 , a perspective view illustration of an upper assembly portion of a mass transfer tool having a tripod actuator is shown in accordance with an embodiment of the invention. In an embodiment, the upper assembly  102  can include actuator assembly  110  components coupled with the articulating transfer head assembly  106  to adjust the articulating transfer head assembly  106  relative to a reference point, such as gantry  113 , base  114 , or side beam  116 . The upper assembly  102  portion can include one or more actuators that provide at least one axis of freedom. For example, the actuators may be part of a tripod actuator  111  having three linear actuators  902 . In the case of three linear actuators, each linear actuator  902  can be retracted or extended alone to cause a rotation of the mounting plate  212  about an axis relative to the base plate  906 . Similarly, all actuators  902  can be moved in tandem to cause linear motion of the mounting plate  212  along an axis orthogonal to the base plate  906 . 
     It will be appreciated that with additional axes of linear motion, the mounting plate  212  can be moved not only in a tip, tilt, extend, or retract direction relative to the base plate  906 . For example, in the case of a hexapod actuator with six linear actuators (not shown), rotational motion can be achieved to rotate the mounting plate  212  about an axis orthogonal to the base plate  906 . Of course, this rotational motion can also be achieved by the addition of a rotational actuator (not shown), such as a stepper motor, between the base plate  906  and another portion of the mass transfer tool  100 . Therefore, one skilled in the art will appreciate that any number of actuators can be added to increase the degrees of freedom between the mounting plate  212  and the base plate  906 . 
     The linear actuators  902  can include linear motors, hydraulic pistons, and other actuators that can generate linear motion. One skilled in the art will appreciate that an axis of freedom provides a degree of freedom, since movement of a point along the axis can be defined in terms of its position on the axis, i.e., its axial position has a single degree of freedom. The linear actuators  902  can also include an end to couple with a mounting plate  212  and an end to couple with a base plate  906 . The base plate  906  can be used to fix the tripod actuator  111  to a gantry  113  via bracket  118 , thereby creating a reference point for movement of the mounting plate  212 . The linear actuators  902  may be coupled with the plates through fasteners, hinges, or other linkages. Thus, actuation of the linear actuators  902  along their respective axes produces a change in a relative position between a point on the mounting plate  212  and a point on the base plate  906 . In this way, each linear actuator  902  provides at least one degree of freedom to the mounting plate  212 , relative to the base plate  906 . As described above, the articulating transfer head assembly  106  can be fixed to mounting plate  212 , and thus, tripod actuator  111  can be used to move the articulating transfer head assembly  106  relative to the base plate  906 , the gantry  113 , or other system components. 
     It will be appreciated that the actuator assembly  110  can include actuators different from those described above, within the scope of this disclosure. For example, the description above has referred to passive actuators, e.g., flexure  404 , as well as active actuators, e.g., tripod actuator  111 , axial actuator  406 , and x-y stage  112 . However, one skilled in the art will understand that other actuators can be included in the actuator assembly  110  to provide additional control over a spatial relationship between the articulating transfer head assembly  106  and the carrier substrate holder  108 . By way of example and not limitation, the actuator assembly  110  may include: electrical motors, pneumatic actuators, hydraulic pistons, relays, comb drives, piezoelectric actuators, and thermal bimorphs. 
     From the preceding description, it will be understood that movement between the system components, and more specifically, movement between the articulating transfer head assembly  106  and the carrier substrate holder  108  and receiving substrate holder  124  can be provided by actuator assembly  110 . The precise motion provided by the actuator assembly  110  can be controlled by a computer system  150  based on feedback inputs from various sensors throughout the mass transfer tool  100 . These various sensors and their mode of operation will be discussed further below. 
     Referring now to  FIG. 10 , a perspective view illustration of a lower assembly portion of a mass transfer tool having sensors is shown in accordance with an embodiment of the invention. The mass transfer tool can include one or more imaging devices  126 ,  128 . For example, an imaging device  126  can be coupled with upper assembly  102  and located near articulating transfer head assembly  106 . In an embodiment, imaging device  126  can be fixed relative to articulating transfer head assembly  106 . In addition, an imaging device  128  can be coupled to the lower assembly  104  and located near the carrier substrate holder  108 . In an embodiment, imaging device  128  can be fixed relative to the carrier substrate holder  108 . Thus, in an embodiment, movement of either imaging device to view a new location of interest results in a corresponding movement of the related articulating transfer head assembly  106  or carrier substrate holder  108 . In this way, relative motion between the articulating transfer head assembly  106  and carrier substrate holder  108  can be determined based on movements of imaging devices  126 ,  128 . 
     The imaging device  128  can include a camera having sufficient resolution and range of focus to view a single electrostatic transfer head in an array of electrostatic transfer heads  204  supported by substrate  200 . For example, the camera can have an image resolution allowing dimensions of less than one micrometer to be resolved. The imaging device  126  can include a camera having sufficient resolution and range of focus to view a single micro device supported by carrier substrate  706 . 
     In an alternative embodiment, there may be multiple imaging devices located on each of the upper and lower assemblies  102 ,  104 . For example, each subassembly can include a high magnification camera  126 ,  128  and a low magnification camera  1002 ,  1004 . By way of example and not limitation, the low magnification cameras  1002 ,  1004  may be used to provide feedback inputs to computer system  150  for controlling gross adjustments and movements of the actuator assembly  110 , while the high magnification cameras  126 ,  128  may be used to provide feedback inputs to computer system  150  for controlling fine adjustments and movements of the actuator assembly  110 . 
     It will be appreciated that imaging devices  126 ,  128  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 imaging devices, the mass transfer tool  100  may include capacitive proximity sensors for aligning the articulating transfer head assembly  106  and carrier substrate holder  108 . Other alternatives will be apparent from the following discussion regarding the functionality of the mass transfer tool  100 . 
     The imaging devices  126 ,  128  can facilitate identifying features and locations of interest on system components, e.g., reference marks  206  on substrate  200 , in order to generate data that can be used to establish reference points for the actuator assembly  110  and to control motion of the actuator assembly  110 . In an embodiment, to facilitate establishing a reference point between imaging devices  126 ,  128 , the mass transfer tool  100  can include an alignment tool  130 . In an embodiment, the alignment tool includes a fiducial mark  1006 . For example, the fiducial mark  1006  can be a part of a transparent plate  1008  (e.g., glass) that is supported by an alignment bracket  1010 . More particularly, plate  1008  having fiducial mark  1006  can be positioned between an upward-viewing imaging device  128  and a downward-viewing imaging device  126 . In an embodiment, the plate  1008  can be positioned between two planes, one plane approximately coinciding with an imaging plane of the upward-viewing imaging device  128  and another plane approximately coinciding with an imaging plane of the downward-viewing imaging device  126 . The imaging planes may be defined as being coplanar with a charge-coupled device (CCD) image sensor surface of a digital camera. Thus, the fiducial mark  1006  can be viewed by both the upward-viewing imaging device  128  and the downward-viewing imaging device  126  either simultaneously or at different times. 
     It will be appreciated that the fiducial mark  1006  can be formed using several different methods. For example, the fiducial mark  1006  can be printed on the plate  1008  using an ink or laser printing process. Alternatively, the fiducial mark  1006  can be etched into the plate  1008 , for example, using an acidic etchant such as a fluoride compound. Also, caustic or abrasive etchants can be used to form the fiducial mark  1006 . 
     In an embodiment, the fiducial mark  1006  includes an asymmetric pattern. For example, the fiducial mark  1006  may resemble the upper portion of the number “ 1 ”, in which there is a bend toward the left away from the upright portion of the numeral, but no bend toward the right. Alternatively, the fiducial mark  1006  could be a cross pattern in which the vertical and horizontal lines cross at a point other than the midline of the lines. Thus, when viewed under high magnification, the fiducial mark  1006  provides information related to its orientation. For example, if the bend of the numeral “ 1 ” is known to point toward the front of the mass transfer tool  100 , when viewed under a camera, the fiducial mark  1006  would provide information related to the orientation of the image relative to the mass transfer tool orientation. 
     While imaging sensors  126 ,  128  can facilitate the recognition of reference marks to establish reference frames and enable the movement of actuator assembly  110  in order to align components as will be described further below, it will be appreciated that additional position sensors can be included in the mass transfer tool  100  to provide feedback relating to the relative position of mass transfer tool components. One such position sensor  208  was already described above. In another embodiment, a position sensor  1011  can be mounted near the carrier substrate holder  108  to provide a feedback input that aids in the adjustment of the actuator assembly  110 . For example, the position sensor  1011  can terminate in a distal end that is approximately coplanar with the holding surface  802  ( FIG. 8 ) or a 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 . For example, the position sensor can provide feedback relating to the distance between a carrier substrate  706  held by the carrier substrate holder  108  and a substrate  200  attached to the articulating transfer head assembly  106  when those components are adjusted relative to each other. 
     In addition to detecting relative position of system components, various sensors of mass transfer tool can also be used to sense deflection and contact of system components. Referring to  FIG. 11 , a side view schematic illustration of an upper assembly portion of a mass transfer tool having an articulating transfer head assembly is shown in accordance with an embodiment of the invention. This view provides a schematic representation of the mass transfer tool portion previously shown in  FIG. 4  above. A substrate  200  is attached to a mounting surface  202  of an articulating transfer head assembly  106 . The articulating transfer head assembly  106  includes a plate  416 , coupled to a housing  210  of the mass transfer tool by one or more flexures  404 ,  404 ′. It will be appreciated that both portions of the articulating transfer head assembly  106  can move, but the flexures  404 ,  404 ′ isolate them such that contact with the substrate  200  will cause the moving plate  416  to move while the flexures  404 ,  404 ′ deflect and dampens any force that is transmitted to housing  210 . Thus, plate  416  can move relative to housing  210 . Furthermore, a position sensor  414  can be coupled with the housing  210 . The position sensor  414  can be fixed relative to the housing  210 , such that the detection beam senses the plate  416  and provides feedback related to the change in distance  1102  between the plate and the surrounding portion. When the distance  1102  detected by the position sensor  414  changes, it can be determined that the flexures  404 ,  404 ′ have deflected, indicating that contact between the substrate  200  and another structure, e.g., the carrier substrate  706  or receiving substrate  714 , has been made or removed. In an embodiment, the position sensor  414  can be a spectral-interference laser displacement sensor capable of detecting a 50 nanometer deflection of the flexure  404  or a 50 nanometer relative movement between plate  416  and housing  210 . 
     As described above, in an alternative embodiment, a force gauge  704  ( FIG. 7 ) coupled with the carrier substrate holder  108  can sense loads applied to the carrier substrate holder  108  and provide feedback related to those loads to control the actuator assembly  110 . For example, the load applied to the carrier substrate holder  108  will increase when the array of electrostatic transfer heads  204  contacts the carrier substrate  706 . This increase in load can be measured by the force gauge  704 , and the force gauge  704  can provide a feedback input to computer system  150  to control actuator assembly  110 . As described above, the force gauge  704  can, for example, be integrated with a carriage  120  that the carrier substrate holder  108  is mounted on. One skilled in the art will recognize that the force gauge  704  may be mounted in other manners and locations to sense when a load is applied to a carrier substrate  706  held by the carrier substrate holder  108 . Likewise, a force gauge  712  coupled with the receiving substrate holder  124  can sense loads applied to the receiving substrate holder  124  and provide feedback related to those loads to control the actuator assembly  110 . 
     Having described some aspects of the components and structure of a mass transfer tool  100 , some embodiments of methods of operating the mass transfer tool  100  will be described below. More specifically, a method of aligning mass transfer tool components and methods of transferring micro devices using the mass transfer tool are described. It will be appreciated that the following methods can be performed in combination and in any order within the scope of this description. Furthermore, not all operations need be performed. For example, the transferring method may occur each time a micro device is transferred, whereas the alignment method may be performed less frequently than that. 
     Furthermore, embodiments of the following methods of operating the mass transfer tool may be performed by processing logic that may include hardware (e.g. circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device) or a combination thereof. In one embodiment, the methods are performed by a mass transfer tool system including a mass transfer tool  100  and computer system  150 . Computer system  150  may be external to the mass transfer tool  100  or integrated into the mass transfer tool. 
     Referring now to  FIG. 17 , a flowchart illustrating a method of operating a mass transfer tool to pick up an array of micro devices is shown in accordance with an embodiment of the invention. For illustrational purposes, the following description of  FIG. 17  is also made with reference to the embodiments illustrated in  FIGS. 18-19D . At operation  1701  a spatial relationship between articulating transfer head assembly  106  coupled with the substrate  200  and carrier substrate holder  108  holding the carrier substrate  706  can be adjusted. The spatial relationship between the articulating transfer head assembly  106  and a carrier substrate holder  108  can be adjusted with actuator assembly  110 . The adjustment of spatial relationship can be effected by the actuation or movement of one or more actuators, such as linear or rotational actuators, that are coupled with the articulating transfer head assembly  106  and carrier substrate holder  108 . Furthermore, in this context a spatial relationship can refer not only to a location of the articulating transfer head assembly  106  relative to the carrier substrate holder  108 , but also to a position or orientation of the articulating transfer head assembly  106  relative to the carrier substrate holder  108 . More particularly, the spatial relationship can be defined by degrees of freedom between mass transfer tool components. 
       FIG. 18  is a schematic illustration of an adjustment of a spatial relationship between an articulating transfer head assembly and a carrier substrate holder shown in accordance with an embodiment of the invention. This illustrates an aspect of operation  1701  of  FIG. 17 . The articulating transfer head assembly  106  is shown as being movable in four degrees of freedom. More specifically, the articulating transfer head assembly  106  can move linearly on a z-axis  1802 , or it can rotate in x-rotation, y-rotation, or z-rotation, about the x-axis  1804 , y-axis  1806 , and z-axis  1802 . Similarly, the carrier substrate holder  108  is shown as being movable in two degrees of freedom. More specifically, the carrier substrate holder  108  can move linearly along an x′-axis  1808  and a y′-axis  1810 . In at least one embodiment, the x-axis can be aligned with the x′-axis and the y-axis can be aligned with the y′-axis. Thus, a spatial relationship between the articulating transfer head assembly  106  and the carrier substrate holder  108  is shown as having six degrees of freedom, given that the relative position between the articulating transfer head assembly  106  and the carrier substrate holder  108  can be described if the position of each component is known within the six degrees of freedom shown. Thus, this spatial relationship can be adjusted by moving either component in their respective degrees of freedom. For example, the carrier substrate holder  108  can be moved in one degree of freedom along the x′-axis  1808  by moving an x-y stage  112  that the carrier substrate holder  108  is attached to. In this manner, the spatial relationship can be adjusted by any of various actuators in an actuator assembly  110  of the mass transfer tool  100 . It will be appreciated that the adjustment will adjust a spatial relationship between the articulating transfer head assembly  106  and carrier substrate holder  108 , as well as components coupled thereto, such as substrate  200  and carrier substrate  706 . Furthermore, while the particular embodiment illustrated in  FIG. 18  illustrates four degrees of freedom of the articulating transfer head assembly  106  and two degrees of freedom of the carrier substrate holder  108 , it is understood that other configurations are contemplated in accordance with embodiments of the invention for adjusting the spatial relationships between components in at least six degrees of freedom. For example, the articulating transfer head assembly  106  can also be mounted on an x-y stage. 
       FIG. 19A  is a cross-sectional side view illustration of an array of electrostatic transfer heads  204  positioned over an array of micro devices  708  on a carrier substrate  706  after the spatial relationship between the articulating transfer head assembly and the carrier substrate holder has been adjusted, in accordance with an embodiment of the invention. Referring again to  FIG. 17 , at operation  1705  an array of electrostatic heads  204  supported by the substrate  200  is brought into contact with an array of micro devices on carrier substrate  706 .  FIG. 19B  is a cross-sectional side view illustration of an array of electrostatic transfer heads  204  in contact with an array of micro devices  708  in accordance with an embodiment of the invention. As illustrated, the pitch of the array of electrostatic transfer heads  204  is an integer multiple of the pitch of the array of micro devices  708 . Contact can be achieved by moving the articulating transfer head assembly  106  toward the carrier substrate holder  108  using one or more actuators of the actuator assembly  110 . It will be appreciated that due to the precise alignment of the substrate  200  and carrier substrate  706 , contact is made while the substrate  200  and carrier substrate  706  are oriented substantially parallel to each other. Thus, there is minimal side loading or flexing of the facing surfaces at the point of contact. This is beneficial because it can prevent mismatch between the array of electrostatic heads  204  and one or more micro devices disposed on the carrier substrate surface. It also reduces the risk of damage to the array of electrostatic heads  204  and the one or more micro devices. 
     Given the small size and structural characteristics of the array of electrostatic transfer heads  204  and micro devices, it may be important to accurately sense when contact has been made between the array of electrostatic transfer heads  204  and the array of micro devices on carrier substrate  706 . Contact can be controlled in numerous ways. For example, a drive to contact methodology can be used in which the articulating transfer head assembly  106  is moved toward the carrier substrate  706  by a predetermined distance based on the calculated positional differences between those components. In an embodiment, active feedback control can be used to sense when contact is made and to provide a related signal to control motion of the actuator assembly  110 . Various embodiments for sensing contact are described above, such as sensing a deflection of a flexure or a movement the articulating transfer head assembly, and sensing a change in load measured by a force gauge coupled with the carrier substrate holder. 
     At operation  1710  the array of electrostatic transfer heads  204  can be activated by applying a voltage to the array of electrostatic transfer heads  204  through the electrostatic voltage source connection  410 . The voltage can be applied to the electrodes of the electrostatic transfer heads  204  prior to, during, or after contacting the carrier substrate  706 . In an embodiment, the voltage can be a constant current or alternating current voltage. Application of voltage creates a grip pressure on the micro devices to enable a pick up of the micro devices. In an embodiment, sufficient grip pressures greater than 1 atmosphere (e.g. 2-20 atmospheres) for micro device pickup can be generated by applying an operating voltage between about 25 V and 300 V. 
     In an embodiment, heat may optionally be applied to the array of micro devices on the carrier substrate, for example, to create a phase change in a bonding layer holding the array of micro devices to the carrier substrate. For example, heat can be applied from a heater  400  in the articulating transfer head  106  and/or heater  806  connected with to the carrier substrate holder  108 . 
     Still referring to  FIG. 17 , at operation  1715 , the array of micro devices can be picked up from the carrier substrate  706 .  FIG. 19C  is a cross-sectional side view illustration of an array of electrostatic transfer heads picking up an array of micro devices  708  in accordance with an embodiment of the invention. In an embodiment, an actuator, such as an axial actuator  406  is used to move the articulating transfer head assembly  106  away from the carrier substrate  706 , thus causing the pick up of the array of micro devices that is gripped by the array of electrostatic transfer heads  204  attached to the mounting surface  202 . Alternatively, pick up can be achieved with a passive actuator, such as a flexure  404 , which applies a retraction force when another actuator, e.g., an axial actuator  406 , is de-energized. In such an embodiment, the de-energization removes an extension force being applied to the articulating transfer head assembly  106 , and thus, the articulating transfer head assembly  106  retracts from the carrier substrate  706  surface based on the inherent spring force of flexure  404 . 
     Pick up of the array of micro devices can be sensed in a manner similar to those described above for sensing the making of contact. In an embodiment, a position sensor  414  coupled with an actuator assembly  110  or a housing  210  can detect a deflection of a flexure  404  or a movement of articulating transfer head assembly  106 . In an embodiment, a change in load measured by a force gauge coupled with the carrier substrate holder can indicate pick up of the array of micro devices. 
       FIG. 19D  is a cross-sectional side view illustration of an array of micro devices  708  released onto a receiving substrate  714  in accordance with an embodiment of the invention. Prior to releasing the array of micro devices  708  the array of micro devices  708  may be lowered onto the receiving substrate  714  until they contact the receiving substrate. Contact between the receiving substrate  714  and the array of micro devices  708  can be made through actuation of the mass transfer tool. Furthermore, contact can be sensed using position sensors or force gauges, as described above with regard to operation  1705 . 
     Placement of the array of micro devices on a receiving substrate may be performed at an elevated temperature, for example to create a phase change in a bonding layer on the array of micro devices  708  and/or on the receiving substrate  706  or to create a phase change or alloy a bonding layer during placement of the array of micro devices on the receiving substrate. 
     After the array of micro devices is in contact with the receiving substrate  714 , the voltage applied to the array of electrostatic transfer heads  204  through the electrostatic voltage source connection  410  can be removed or altered resulting in the release the array of micro devices onto the receiving substrate  714 . 
     The articulating transfer head assembly  106  can then be moved away from the receiving substrate  714 . Removal of the array of electrostatic transfer heads from the array of micro devices can be sensed when moving away from the receiving substrate in a manner similar to those described above for sensing pick up. Furthermore, removal of the array of electrostatic transfer heads from the array of micro devices can be sensed using position sensors or force gauges, as described above with regard to operation  1715 . 
     In an embodiment, an upward-viewing imaging device  128  can be used to inspect an array of electrostatic transfer heads  204  for the presence of micro devices. More particularly, following pick up, the presence of the micro devices can be viewed by the upward-viewing imaging device  128  by moving the articulating transfer head assembly  106  and substrate  200  to a location over the imaging device  128  with an actuator assembly  110 . Viewing may be performed under both high and low magnifications, as is known in the art. 
       FIG. 12A  is a flowchart illustrating a method of aligning a substrate supporting an array of electrostatic transfer heads with a frame of reference in accordance with an embodiment of the invention. At operation  1201 , a frame of reference including an x-axis and an x-y plane is established. The frame of reference can be useful for enabling the alignment of various components within the mass transfer tool  100 . More specifically, the frame of reference allows for movement of system components relative to reference geometries and to each other. Movement of those components can be monitored and input to a computer system  150  that controls actuator assembly  110 . At operation  1220 , substrate  200  supporting an array of electrostatic transfer heads  204  is aligned with the established frame of reference using the various sensors of the mass transfer tool  100 . In an embodiment, the substrate  200  supporting the array of electrostatic transfer heads  204  is releasably attached to mounting surface  202  of the articulating transfer head assembly  106  and connected to one or more electrostatic voltage source connections  410  prior to aligning the substrate with the frame of reference. 
       FIG. 12B  is a flowchart illustrating a method of establishing a frame of reference in accordance with an embodiment of the invention. At operation  1203 , an x-y datum is set by aligning a first and second imaging devices to a fiducial mark between the first and second imaging devices. At operation  1205 , a z-datum is set by sending a first and second coplanar surfaces that are between a first and second position sensors and parallel to an x-y plane having the x-y datum. In an embodiment, the x-y datum and the z-datum are used to define the frame of reference. 
       FIG. 12C  is a flowchart illustrating a method of operating a mass transfer tool to transfer an array of micro devices shown in accordance with an embodiment of the invention. At operation  1230 , an orientation of a carrier substrate  706  on a carrier substrate holder  108  is determined using the various sensors of the mass transfer tool  100 . At operation  1240 , an orientation of a receiving substrate  714  on a receiving substrate holder  124  is determined. At operation  1245 , an orientation of the substrate  200  is matched to the carrier substrate  706 . At operation  1250 , the array of micro devices is picked up from the carrier substrate  706 . At operation  1255 , an orientation of the substrate  200  is matched to the receiving substrate  714 . At operation  1260 , the array of micro devices is released on the receiving substrate. It will be appreciated that the operations described in  FIG. 12C  can be sub-divided further or performed in another order. 
     The following description will provide additional details regarding the operations of the alignment process shown in  FIGS. 12A-12B  and the transfer process shown in  FIG. 12C . The description will occasionally refer back to  FIGS. 12A-12C  to clarify the specific operation for which additional detail is being provided. However, it will be appreciated that the following details can be applied to alternative methods of alignment and operation within the scope of this description and can be performed independently of the overall method of operations described above. 
     Referring to  FIG. 13A , a side view schematic illustration of a method of setting an x-y datum is shown in accordance with an embodiment of the invention. This illustration describes an aspect of operation  1203  of  FIG. 12B . The portion of the mass transfer tool shown includes a downward-looking imaging device  1302  and an upward-looking imaging device  1304 , both of which can include cameras, for example. A plate  1008  can be disposed between the upward-looking imaging device  1304  and the downward-looking imaging device  1302 . More particularly, the plate  1008  can be oriented between an imaging plane  1308  of the upward-looking imaging device  1304  and an imaging plane  1316  of the downward-looking imaging device  1302 . As described above, plate  1008  includes a fiducial mark  1006  and the upward-looking imaging device  1304  and the downward-looking imaging device  1302  can be moved to view the fiducial mark  1006  simultaneously by actuation of one or more actuators of an actuator assembly  110 . 
     When the upward-looking imaging device  1304  and the downward-looking imaging device  1302  view the fiducial mark  1006  simultaneously, and the fiducial mark  1006  is centered and focused within the respective images from the imaging devices, the imaging devices will be aligned. Thus, in that position, the fiducial mark  1006  becomes a reference point from which movement of either the upward-looking camera  1304  or the downward-looking camera  1302  can be compared to determine the relative position of the imaging devices in a plane parallel to the imaging planes. In an embodiment, when the upward-looking imaging device  1304  is fixed relative to the carrier substrate holder  108  and the downward-looking imaging device  1302  is fixed relative to the articulating transfer head assembly  106 , the fiducial mark  1006  becomes a reference point from which movement of the articulating transfer head assembly  106  or the carrier substrate holder  108  can be compared to determine the relative position of those components in an x-axis and y-axis direction. 
     Referring to  FIG. 13B , a perspective view schematic illustration of a method of setting an x-y datum is shown in accordance with an embodiment of the invention. This illustration also describes an aspect of operation  1203  of  FIG. 12B . An upward-looking imaging device  1304  and a downward-looking imaging device  1302  can be moved into position such that they both view a fiducial mark  1006  on plate  1008 , as described in regard to  FIG. 13A . In  FIG. 13B , the fiducial mark  1006  establishes an x-y datum  1320  when centered and focused on simultaneously by the imaging devices  1302 ,  1304 . Furthermore, an x-axis  1322  and a y-axis  1324  are determined to cross through the x-y datum  1320 . In an embodiment, the x-axis  1322  and y-axis  1324  correspond with axes of motion of an x-y stage that the upward-looking imaging device is coupled with. Furthermore, the x-axis  1322  and the y-axis  1324  define an x-y plane  1326 , which passes through the x-axis  1322 , y-axis  1324 , and x-y datum  1320 . Thus, a frame of reference having an x-axis  1322  and an x-y plane  1326  can be established according to the methods described above. As described above, while some embodiments are described and illustrated as including an x-y stage  112  in the lower assembly  104  only, it is contemplated that the articulating transfer head assembly  106  in the upper assembly  102  can be mounted on an x-y stage in addition to, or alternative to x-y stage  112 . In such an embodiment, the x-axis  1322  and y-axis  1324  may correspond with axes of motion of an x-y stage that the downward-looking imaging device is coupled with. 
     Referring to  FIG. 14A , a side view illustration of a method of setting a z-datum is shown in accordance with an embodiment of the invention. This illustration describes an aspect of operation  1205  of  FIG. 12B . A downward-looking position sensor  1402  is viewing in a downward direction  1404  toward the x-y plane  1326  of the frame of reference. Simultaneously, an upward-looking position sensor  1406  is viewing in an upward direction  1408  opposite to the downward direction. Thus, the directions of the upward and downward-looking position sensors  1402 ,  1406  are approximately parallel with each other and can be approximately orthogonal to the x-y plane  1326 . As described above, the position sensors can be spectral-interference laser displacement sensors capable of determining absolute distance to an object. 
     A gauge  1410  can be releasably attached to the mounting surface  202  of the articulating transfer head assembly  106  and positioned between the upward and downward-looking position sensors  1402 ,  1406 . The gauge  1410  can be referred to as a “z-gauge” because it is used to establish a z-datum in a frame of reference. Attachment of the z-gauge  1410  can be achieved in a manner similar to the attachment of a substrate  200  to the mounting surface  202 . For example, suction can be drawn on the z-gauge  1410  through a vacuum port of the mounting surface  202 . 
     As shown in  FIG. 14A , when the z-gauge  1410  is not parallel with the x-y plane  1326 , i.e., when the z-gauge  1410  is tilted within the frame of reference, the upward-looking position sensor  1406  can sense a distance to a first surface  1412  and the downward-looking position sensor  1402  can sense a distance to a second surface  1414 . These surfaces can be, for example, the base of two counterbores, formed in an outer surface of the z-gauge  1410 . The counterbored surfaces  1412 ,  1414  can be made coplanar with each other. In an embodiment, the surfaces are coplanar because the counterbores each extend through half of the thickness of the z-gauge  1410 . 
     In an embodiment, the first surface  1412  and the second surface  1414  are coplanar with a surface plane  1416 . For example, in an embodiment, the z-gauge  1410  can be formed from two silicon wafers having through holes. The two silicon wafers are bonded such that the openings to the through holes are adjacent to a surface of the other wafer. Thus, the base of the through-holes will be the surface of the other wafer, and since the wafers are apposed with each other, their bonding surfaces are coplanar. Therefore, the bases of the through holes are also coplanar and can be used as counterbores for the z-gauge  1410 . In another embodiment, the first and second surfaces  1412  and  1414  are not coplanar. For example, the surfaces  1412  and  1414  can be separated by a layer of uniform and known thickness. 
     When the z-gauge  1410  is not parallel with x-y plane  1326 , the distances to the first surface  1412  and the second surface  1414  from the respective upward-looking position sensor  1406  and downward-looking position sensor  1402  will be to different locations along a z-axis, since they exist at different points on a non-parallel plane. As a result, there is no common z-datum between the upward-looking position sensor  1406  and the downward looking position sensor  1402 . 
     Thus, either before or after installing the z-gauge  1410 , the mounting surface  202  of the articulating head assembly  106  can be made parallel to the x-y plane  1326  to facilitate establishing a z-datum of the frame of reference. To do so, the upward-looking position sensor  1406  can detect a distance to two or more points on the mounting surface  202 , the z-gauge  1410 , or any other structure that is known to be parallel to the mounting surface  202 . The articulating transfer head assembly  106  can then be tipped and tilted by the actuator assembly  110  until the distances to the various measured points are the same distance from the upward-looking position sensor  1406 . When this occurs, the mounting surface  202  can be orthogonal to the direction of detection of the upward-looking position sensor  1406 , and thus, the mounting surface  202  is approximately parallel to the x-y plane  1326 . Once the mounting surface is oriented parallel to the x-y plane  1326 , a z-datum can be established. 
     Referring to  FIG. 14B , a side view illustration of a portion of a mass transfer tool is shown in accordance with an embodiment of the invention. This illustration describes an aspect of operation  1205  of  FIG. 12B . Here, the surface plane is known to be parallel to the x-y plane  1326  since either mounting surface  202  or the z-gauge  1410  has been aligned parallel to the x-y plane  1326  by moving the articulating transfer head assembly  106  relative to the upward-looking position sensor  1406 , as described above. Thus, the distance to the first surface  1412  from the upward-looking position sensor  1406  and the distance to the second surface  1414  from the downward-looking position sensor  1402  can be registered as the known distance to the surface plane  1416  when the surface plane  1416  is parallel to the x-y plane  1326 . Given that the first and second surfaces  1412 ,  1414  are coplanar, the surface plane  1416  can be established as the z-datum  1420  in this orientation, and the distance to the z-datum  1420  can then be measured using either the upward-looking position sensor  1406  or the downward-looking position sensor  1402 . 
     Once the z-datum  1420  is established, along with the x-axis  1322  and x-y plane  1326 , a frame of reference is known for moving components of the mass transfer tool  100 . For example, the z-gauge  1410  can now be removed from the mounting surface and replaced by a substrate  200  supporting an array of transfer heads  204 . Substrate  200  can be attached to mounting surface  202  of the articulating transfer head assembly  106  using any of the manners described above. In an embodiment, a vacuum is used to hold the substrate  200  on the mounting surface  202  and to compress knee  606  of contact  604  to connect the substrate  200  with the one or more voltage source connections  410 . In an embodiment, the z-gauge  1410  can be formed such that the surface plane  1416  is within about 100 micrometers of a location that coincides with a contact surface  205  ( FIG. 6B ) of the array of electrostatic transfer heads  204  supported by substrate  200 . Thus, when the z-gauge  1410  is replaced by the substrate  200 , the z-datum  1420  is approximately coincident with the array of electrostatic transfer heads  204 , making subsequent adjustments using the upward and downward looking sensors  1402 ,  1406  substantially easier to perform. 
     Referring now to  FIG. 15A , a perspective view schematic illustration of a method of aligning a substrate with a frame of reference is shown in accordance with an embodiment of the invention. This illustrates an aspect of operation  1220  of  FIG. 12A . After a frame of reference is established, the substrate  200  can be aligned with the frame of reference. In an embodiment, the substrate  200  is aligned after establishing the frame of reference following by attaching the substrate  200  to a mounting surface  202  of the articulating transfer head assembly  106 . The frame of reference is known based on the methods described above. More particularly, the frame of reference is established based on the identification or assignment of an x-axis  1322 , a y-axis  1324 , and an x-y plane  1326 . Furthermore, a position of the substrate  200  relative to the frame of reference  1502  can be determined based on the various sensors of the mass transfer tool  100 . For example, an upward-looking imaging device  1406  can view a first alignment marker  1504  of the substrate  200 , and based on the position difference of the upward-looking imaging device  1406  while viewing the first alignment marker  1504  and while viewing a fiducial mark  1006  that coincides with an x-y datum  1320 , the relative position of the first alignment marker  1504  along the x-axis  1322  and y-axis  1324  can be determined. It will be appreciated that such position difference can be determined, for example, from data provided by an encoder of an actuator subassembly  110 . More particularly, an x-y stage  112  used to move the upward-looking imaging device  1406 , or a carriage  120  that the imaging device is coupled with, can include a rotary encoder to provide data related to the position of the x-y stage  112 , and hence the position of the upward-looking imaging device  1406 . Likewise, a position of a second alignment marker  1506  can be determined relative to the x-y datum  1320 . 
     Having identified at least two alignment markers  1504 ,  1506  on the substrate surface, an alignment axis  1508  can be calculated as running through the first alignment marker  1504  and the second alignment marker  1506 . Furthermore, a comparison can be made between the alignment axis  1508  and an axis of the frame of reference, e.g., the x-axis  1322 , to determine the orientation of the substrate  200  relative to the frame of reference  1502  about a z-axis  1510 . 
     It will be appreciated that the first alignment marker  1504  and the second alignment marker  1506  can be any known marker that is disposed on the substrate  200 . For example, in an embodiment, the alignment markers can be added to the substrate  200  using ink or laser printing, or even etching. Alternatively, the alignment markers can be two or more electrostatic transfer heads from an array of electrostatic transfer heads  204 . For example, two electrostatic transfer heads along an outer edge of the array  204  could create an alignment axis  1508  that coincides with, and allows referencing to, the outer edge of the array  204 . 
     An angular relation between the substrate  200  and the x-y plane  1326  can also be determined using an upward-looking position sensor  1406 . Two or more points on the substrate  200  surface can be detected by the upward-looking position sensor  1406 . For example, in an embodiment, four points on the substrate  200  surface can be detected by the upward-looking position sensor  1406 . The distances to the points can be used to calculate a plane passing through the points, i.e., a plane coinciding with the surface of the substrate  200 . This surface can be, for example, the contact surfaces  205  on the array of electrostatic transfer heads  204  such as the dielectric layer over the array of electrodes. Thus, a comparison can be made between the substrate surface and the x-y plane  1326  of the frame of reference  1502  to determine the orientation of the substrate  200  relative to the frame of reference  1502  about the x-axis  1322  and y-axis  1324 . 
     Referring to  FIG. 15B , a perspective view schematic illustration of a method of aligning a substrate with a frame of reference is shown in accordance with an embodiment of the invention. This illustrates an aspect of operation  1220  of  FIG. 12A . Based on the relationship between the frame of reference  1502  and the substrate  200 , the substrate  200  can be aligned with the frame of reference  1502 . More particularly, the articulating transfer head assembly  106  can be adjusted within several degrees of freedom in order to align the alignment axis  1508  of the substrate  200  parallel to the x-axis  1322  and in order to align the substrate surface parallel to the x-y plane  1326 . This can be achieved by tipping, tilting, and rotating the articulating transfer head assembly  106  using various actuators of the actuator assembly  110 . 
     In an alternative embodiment, alignment of the substrate  200  with the frame of reference  1502  can include viewing two or more points on the substrate  200 , e.g., alignment markers  1504 ,  1506 , with an imaging device to determine that the substrate  200  is parallel to an imaging plane of the imaging device. More specifically, an upward-viewing imaging device can view at least two points on the substrate  200  and the focal length for detecting those points can be determined. For example, a first electrostatic head, or an alignment marker  1504 , can be viewed and brought into focus by the imaging device. When the first electrostatic head is in focus, the image has a first focal length. The imaging device can then be used to view a second electrostatic head or alignment marker  1506  by moving imaging device to a new location in the same plane. If the electrostatic heads or alignment markers  1504 ,  1506  are the same distance from the imaging plane, then there will be no need to refocus, since the focal length will be the same for each. However, if the imaging device must refocus to bring the second electrostatic head into focus, then the focal lengths are different, and the substrate surface supporting the array of electrostatic heads is not parallel with the imaging plane. The articulating transfer head assembly  106  can therefore be tipped and tilted until the imaging device does not need to refocus when moving between locations to view the reference points on the substrate  200 . When this occurs refocusing is not required, and hence substrate  200 , is parallel to a corresponding plane of the frame of reference. 
     Having now described a method of aligning substrate  200  with the frame of reference  1502 , it will now be understood that the frame of reference is useful for enabling the alignment of various components within mass transfer tool  100 . More specifically, the frame of reference allows for the movement of system components relative to reference geometries and to each other. Movement of these components can be monitored and input to a computer system  150  that controls actuator assembly  110 . By way of example, after establishing a frame of reference, movement of the carrier substrate holder  108  mounted on a carriage  120  attached to an x-y stage  112  can be determined by an encoder of the x-y stage  112 . Thus, if the x-y stage  112  moves along a y-axis for 3 inches, then the encoder would determine that the carrier substrate holder  108  has changed positions in the y-axis direction by 3 inches. Similarly, a position change between the carrier substrate holder  108  and the articulating transfer head assembly  106  is known to be 3 inches in the y-axis direction if the articulating transfer head assembly  106  has remained stationary during the move. This is a basic demonstration of the importance of establishing a frame of reference, and indicates that the frame of reference can be used to determine relative positions between many different components throughout the system. As such, the following description will go into greater detail regarding aspects of the method shown in  FIG. 12C , which utilize the ability to move components relative to each other to transfer an array of micro devices from a carrier substrate to a receiving substrate. 
     Referring again to  FIG. 12C , at operation  1230  an orientation of a carrier substrate  706  can be determined. For example, the carrier substrate can be held on a holding surface  802  that applies vacuum to retain and stabilize the carrier substrate  706 . Alternatively, mechanical gripping mechanisms or friction fits can be used to retain the carrier substrate  706 . The carrier substrate  706  orientation can be determined using methods similar to those described above for determining the orientation of the substrate  200  attached to the mounting surface  202 . For example, a downward-viewing imaging device can view several markers on the carrier substrate  706  to determine the relative distance of the markers to the x-y datum  1320  of the frame of reference. This determination can be used to determine an orientation of the carrier substrate  706  about a z-axis of the frame of reference. Furthermore, a downward-viewing position sensor  1402  can be used to detect several points on a surface of the carrier substrate  706  in order to determine an orientation of the carrier substrate  706  about an x-axis and a y-axis of the frame of reference. For example, the downward-viewing position sensor  1402  can be used to detect four points on the surface of the carrier substrate  706 . Thus, an alignment axis and a surface orientation can be determined for the carrier substrate  706  to determine the relation of the carrier substrate  706  within the frame of reference. 
     At operation  1240  the orientation of the receiving substrate  714  can be determined in a manner similar to that described above with regard to operation  1230  of  FIG. 12C . For example, a receiving substrate  714  can be held on a receiving substrate holder in a manner similar to that described above with regard to the carrier substrate  706 . 
     After determining the orientation of the carrier substrate  706  and the receiving substrate  714 , transfer of the micro devices from the carrier substrate  706  to the receiving substrate  714  can be performed. In some embodiments, transfer of the array of micro devices from the carrier substrate to the receiving substrate may be performed at an elevated temperature, for example to create a phase change in a bonding layer connecting the array of micro devices to the carrier substrate, or to create a phase change or alloy a bonding layer when placing the array of micro devices on the receiving substrate. 
       FIGS. 16A through 16C  are side view schematic illustrations of a method of matching an orientation and contacting a substrate to a carrier substrate using a mass transfer tool in accordance with an embodiment of the invention. These illustrate additional aspects of operations  1240  and  1245  of  FIG. 12C . For illustrational purposes a difference in orientation of the carrier substrate  706  with regard to the substrate  200  and a holding surface of the carrier substrate holder  108  is exaggerated. 
     Referring now to  FIG. 16A , a substrate  200  is shown attached to the articulating transfer head assembly  106 . The orientation of the substrate  200  is shown already aligned with a frame of reference. More specifically, the substrate is shown with an alignment axis and a surface that are aligned with the x-axis and the x-y plane of the frame of reference, respectively. It will be appreciated that in at least one embodiment, the substrate can be aligned with other reference geometries of the frame of reference. 
     A carrier substrate  706  is shown mounted on a carrier substrate holder  108 . The orientation of the carrier substrate  706 , unlike the substrate  200 , is not aligned with the frame of reference. Thus, the orientations of the substrate  200  and the carrier substrate  706  are not aligned. However, the mismatch between these orientations can be determined. For example, the orientation of the substrate  200  and carrier substrate  706  is known based on the alignment with the frame of reference, as described above. Thus, a comparison can be performed to determine the offset in orientation between the carrier substrate  706  and the substrate  200 . 
     It will be appreciated that the orientation of the carrier substrate holder  108  can also be determined, rather than the orientation of the carrier substrate  706 . More specifically, surface points on the carrier substrate holder  108  that are approximately parallel to the surface of the carrier substrate  706  can be detected to define an orientation of the carrier substrate holder  108 . A comparison can then be performed to determine the offset in orientation between the carrier substrate holder  108  and the substrate  200 . 
     Referring to  FIG. 16B , the spatial relationship between the articulating transfer head assembly  106  and the carrier substrate holder  108  can be adjusted to align the substrate  200  and the carrier substrate  706 . This illustrates an aspect of operation  1245  of  FIG. 12C . More specifically, after determining the orientation of the carrier substrate  706  as shown in  FIG. 16A , the articulating transfer head assembly  106  can be moved by one or more actuators until the substrate  200  orientation is transformed to match the orientation of the carrier substrate  706 . Then, the substrate  200  and the carrier substrate  706  are in proximity with each other and their facing surfaces are parallel. 
     Referring to  FIG. 16C , an array of electrostatic transfer heads  204  supported by the substrate  200  is brought into contact with an array of micro devices on carrier substrate  706 . This illustrates an aspect of the pick up operation  1250  of  FIG. 12C . This can be achieved by moving the articulating transfer head assembly  106  toward the carrier substrate holder  108  using one or more actuators of the actuator assembly  110 . It will be appreciated that due to the precise alignment of the substrate  200  and carrier substrate  706 , contact is made while the substrate  200  and carrier substrate  706  are oriented substantially parallel to each other. Thus, there is minimal side loading or flexing of the facing surfaces at the point of contact. This is beneficial because it can prevent mismatch between the array of electrostatic heads  204  and one or more micro devices disposed on the carrier substrate surface. It also reduces the risk of damage to the array of electrostatic heads  204  and the one or more micro devices. Pick up operation  1250  can be performed in a variety of manners, and using a variety of sensors. For example, pick up operation  1250  can be performed similarly as the pick up operation describe above with regard to  FIG. 17 . 
     Referring again to  FIG. 12C , operations  1255  and  1260  can be performed to transfer the picked up micro devices to a receiving substrate  714 . These operations can be performed in a manner similar to operations  1245  and  1250  described above, as well as  FIG. 17  above. More specifically, at operation  1255 , a spatial relationship between articulating transfer head assembly  106  and receiving substrate holder  124  can be adjusted to bring the array of micro devices picked up by the array of electrostatic transfer heads  204  into proximity with a surface of the receiving substrate  714 . Furthermore, contact between the receiving substrate  714  and the array of micro devices can be made through further actuation of the mass transfer tool  100 . Contact between the array of micro devices  708  and the receiving substrate  714  can be sensed using position sensors, force gauges, and other sensors, in a manner similar to those described above. In some embodiments, placement of the array of micro devices on a receiving substrate may be performed at an elevated temperature, for example to create a phase change or alloy a bonding layer during placement of the array of micro devices on the receiving substrate. 
     At operation  1260 , when the array of micro devices is in contact with the receiving substrate  714 , the voltage applied to the array of electrostatic transfer heads  204  through the electrostatic voltage source connection  410  can be removed. Such removal may also remove the gripping pressure to release the array of micro devices onto the receiving substrate  714 . 
     After releasing the array of micro devices, the transfer of the micro devices from the carrier substrate  706  to the receiving substrate  714  is achieved. Subsequently, the articulating transfer head assembly  106  can be moved away from the receiving substrate  714 . Both the moving and sensing of removal the array of micro devices can be achieved in a manner similar to those described above. Additionally, the array of electrostatic transfer heads  204  can be inspected by an upward-viewing imaging device  128  to confirm the release of the array of micro device, a similar manner to that described above. 
     It will be appreciated that various components of the mass transfer tool can be heated during the operations described above. For example, in an embodiment, the substrate  200  supporting the array of electrostatic transfer heads  204  and/or mounting surface  202  can be heated to a temperature range of about 100 to 350 degrees Celsius during any of the operations  1201  through  1260 . For example, any of the sensing, alignment, and matching operation can be performed at an operating temperature used for transfer of the micro devices from the carrier substrate to the receiving substrate. In an embodiment, the operating temperature is an elevated temperature for creating a phase change or alloying of a bonding layer. In an embodiment, the mounting surface  202  is heated to a temperature range of about 100 to 350 degrees Celsius when setting the x-y datum and z-datum in operations  1203  and  1205 . In an embodiment, the mounting surface  202  and substrate  200  are heated to a temperature range of about 100 to 350 degrees Celsius when aligning the substrate with a frame of reference. In an embodiment, the mounting surface  202  and substrate  200  are heated to a temperature range of about 100 to 350 degrees Celsius when determining an orientation of the carrier or receiving substrate and matching the orientation of the substrate to the orientation of the carrier or receiving substrate. In an embodiment, the mounting surface  202  and substrate  200  are heated to a temperature range of about 100 to 350 degrees Celsius when contacting, picking up, or releasing the array of micro devices. In an embodiment, the carrier substrate is heated to a temperature range from room temperature to about 200 degrees Celsius when setting the x-y datum and x-datum in operations  1203  and  1205 . In an embodiment, the carrier or receiving substrate is heated to a temperature range from room temperature to about 200 degrees Celsius when determining an orientation of the carrier or receiving substrate and matching the orientation of the substrate to the orientation of the carrier or receiving substrate. In an embodiment, the carrier substrate  706  can be heated to a temperature range from room temperature to about 200 degrees Celsius while the array of micro devices on the carrier substrate  706  is contacted by the array of electrostatic transfer heads  204 . In an embodiment, the receiving substrate  714  can be heated to a temperature range of about 100 to 200 degrees Celsius while the receiving substrate  714  is contacted by the array of electrostatic transfer heads  204 . These are only examples and it will be appreciated that these or other components of the mass transfer tool  100  can be heated to these or different temperature ranges within the scope of the methods described above. 
     With reference now to  FIG. 20 , portions of embodiments of the invention are comprised of, or controlled by, non-transitory machine-readable and machine-executable instructions which reside, for example, in machine-usable media of a computer control system.  FIG. 20  is a schematic illustration of an exemplary computer system  150  that may be used in accordance with an embodiment of the invention. It is to be appreciated that computer system  150  is exemplary, and that embodiments of the invention can operate on or within, or be controlled by a number of different computer systems including general purpose networked computer systems, embedded computer systems, routers, switches, server devices, client devices, various intermediate devices/nodes, stand alone computer systems, and the like. 
     Computer system  150  of  FIG. 20  includes an address/data bus  2010  for communicating information, and a central processor unit  2001  coupled to bus  2010  for processing information and instructions. System  150  also includes data storage features such as a computer usable volatile memory  2002 , e.g. random access memory (RAM), coupled to bus  1210  for storing information and instructions for central processor unit  2001 , computer usable non-volatile memory  2003 , e.g. read only memory (ROM), coupled to bus  2010  for storing static information and instructions for the central processor unit  2001 , and a data storage device  2004  (e.g., a magnetic or optical disk and disk drive) coupled to bus  2010  for storing information and instructions. System  2012  of the present embodiment also includes an optional alphanumeric input device  1206  including alphanumeric and function keys coupled to bus  2010  for communicating information and command selections to central processor unit  2001 . System  150  also optionally includes an optional cursor control device  2007  coupled to bus  2010  for communicating user input information and command selections to central processor unit  2001 . System  2012  .of the present embodiment also includes an optional display device  2005  coupled to bus  210  for displaying information. 
     The data storage device  2004  may include a non-transitory machine-readable storage medium  2008  on which is stored one or more sets of instructions (e.g. software  2009 ) embodying any one or more of the methodologies or operations described herein. Software  2009  may also reside, completely or at least partially, within the volatile memory  2002 , non-volatile memory  2003 , and/or within processor  2001  during execution thereof by the computer system  150 , the volatile memory  2002 , non-volatile memory  2003 , and processor  2001  also constituting non-transitory machine-readable storage media. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.

Metadata:
Filing Date: 20160930
Publication Date: 20190122
Grant Date: 20190122
Priority Date: 20120907
Inventors: HIGGINSON, JOHN A.
BIBL, ANDREAS
ALBERTALLI, DAVID
Assignee: APPLE INC
CPC Classifications: [{"code": "H01L2224/75251", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/67132", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J15/0085", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02N13/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2224/7598", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2224/75753", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2224/75252", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L24/95", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/67259", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/67144", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/68", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/677", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "B65G49/07", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J9/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/95", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2224/75251", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2224/7598", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2224/75252", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/67144", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2224/75753", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/67259", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/67132", "inventive": true, "first": false, "tree": "[]"}, {"code": "B25J15/0085", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02N13/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/95", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 50233064