PATENT DOCUMENT

Publication Number: US-11948815-B2
Application Number: US-202117345253-A
Country: US
Kind Code: B2

Title: High density pick and sequential place transfer process and tool

Abstract:
Mass transfer tools and methods for high density transfer of arrays of micro devices are described. In an embodiment, a mass transfer tool includes a micro pick up array with an array of transfer heads arranged in clusters. The clusters of transfer heads can be used to pick up a high density group of micro devices followed by sequential placement onto a receiving substrate.

Claims:
What is claimed is: 
     
       1. A transfer process comprising:
 picking up a first group of light emitting diodes (LEDs) from a first donor substrate with a micro pick up array comprising a plurality of clusters of transfer heads, wherein adjacent transfer heads within a row of transfer heads within a cluster are separated by an intra-cluster spacing, and adjacent clusters within a row of clusters are separated by an inter-cluster spacing that is greater than the intra-cluster spacing; 
 positioning the micro pick up array over a first location of a display substrate; 
 placing a first LED of the first group of LEDs from each cluster onto the display substrate; 
 positioning the micro pick up array over a second location of the display substrate; and 
 placing a second LED of the first group of LEDs from each cluster onto the display substrate. 
 
     
     
       2. The transfer process of  claim 1 , further comprising continuing to position the micro pick up array over a new location and placing another LED of the first group of LEDs from each cluster onto the display substrate until all LEDs of the first group of LEDs held by the micro pick up array have been placed onto the display substrate. 
     
     
       3. The transfer process of  claim 1 , further comprising:
 translating the micro pick up array to a second donor substrate; 
 picking up a second group of LEDs from the second donor substrate with the micro pick up array comprising the plurality of clusters of transfer heads; 
 positioning the micro pick up array over a third location of the display substrate; 
 placing a first LED of the second group of LEDs from each cluster onto the display substrate; 
 positioning the micro pick up array over a fourth location of the display substrate; and 
 placing a second LED of the second group of LEDs from each cluster onto the display substrate. 
 
     
     
       4. The transfer process of  claim 3 , further comprising continuing to position the micro pick up array over a new location and placing another LED of the first group of LEDs from each cluster onto the display substrate until all LEDs of the first group of LEDs held by the micro pick up array have been placed onto the display substrate. 
     
     
       5. The transfer process of  claim 1 , wherein:
 the display substrate comprises a pixel area comprising a first array of subpixels immediately adjacent a second array of subpixels, and separated by an inter-subpixel pitch; and 
 the inter-cluster spacing is greater than the inter-subpixel pitch. 
 
     
     
       6. The transfer process of  claim 5 , wherein the row of transfer heads within a cluster fits within the inter-subpixel pitch. 
     
     
       7. The transfer process of  claim 1 , further comprising:
 picking up a second group of LEDs from the first donor substrate with a second micro pick up array comprising a second plurality of clusters of transfer heads; 
 positioning the second micro pick up array over a third location of the display substrate after placing the second LED of the first group of LEDs from each cluster onto the display substrate; 
 placing a first LED of the second group of LEDs from each cluster onto the display substrate; 
 positioning the second micro pick up array over a fourth location of the display substrate; and 
 placing a second LED of the second group of LEDs from each cluster onto the display substrate. 
 
     
     
       8. A transfer process comprising:
 picking up a first group of LEDs from a first donor substrate with a first articulating transfer head assembly; 
 picking up a second group of LEDs from the first donor substrate with a second articulating transfer head assembly, wherein the first group of LEDs is a different group of LEDs than the second group of LEDs, and the first articulating transfer head assembly is a different articulating transfer head assembly than the second articulating transfer head assembly; 
 translating the first articulating transfer head assembly and the second articulating transfer head assembly toward a display substrate; 
 positioning the first articulating transfer head assembly over the display substrate; 
 placing the first group of LEDs onto the display substrate; 
 positioning the second articulating transfer head assembly over the display substrate; and 
 placing the second group of LEDs onto the display substrate. 
 
     
     
       9. The transfer process of  claim 8 , further comprising:
 translating the first articulating transfer head assembly and the second articulating transfer head assembly over an inspection camera while translating the first articulating transfer head assembly and the second articulating transfer head assembly toward the display substrate. 
 
     
     
       10. The transfer process of  claim 9 , wherein:
 picking up the first group of LEDs from the first donor substrate comprises picking up the first group of LEDs with a first micro pick up array comprising a first plurality of clusters of transfer heads; and 
 picking up the second group of LEDs from the first donor substrate comprises picking up the second group of LEDs with a second micro pick up array comprising a second plurality of clusters of transfer heads. 
 
     
     
       11. The transfer process of  claim 8 , wherein the first articulating transfer head assembly and the second articulating transfer head assembly are attached to a same translation track and translating the first articulating transfer head assembly and the second articulating transfer head assembly toward the display substrate comprises translating the first articulating transfer head assembly and the second articulating transfer head assembly along the same translation track.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Application No. 63/051,125 filed Jul. 13, 2020 and U.S. Provisional Application No. 63/051,126 filed Jul. 13, 2020 each of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Field 
     Embodiments described herein relate to systems and methods for transferring micro devices. 
     Background Information 
     Integration and packaging issues are one of the main obstacles for the commercialization of micro devices such as radio frequency (RF) microelectromechanical systems (MEMS) microswitches, light-emitting diodes (LEDs), and MEMS or quartz-based oscillators. 
     Traditional technologies for transferring of devices such as “direct printing” and “transfer printing” include transfer by wafer bonding from a transfer wafer to a receiving wafer. In both traditional and variations of the direct printing and transfer printing technologies, the transfer wafer is de-bonded from a device after bonding the device to the receiving wafer. In addition, the entire transfer wafer with the array of devices is involved in the transfer process. 
     In one process variation a transfer tool including an array of electrostatic transfer heads is used to pick up and transfer an array of micro devices from a carrier (donor) substrate to a receiving substrate. In such an implementation, the transfer heads operate in accordance with principles of electrostatic grippers, using the attraction of opposite charges to pick up the micro devices. 
     In a particular implementation it has been suggested to use an array of electrostatic transfer heads to populate a display backplane with an array of micro LED devices, in which sequential pick and place transfer operations are performed to populate the display backplane with a plurality of different color-emitting micro LEDs from different donor substrates. 
     SUMMARY 
     Mass transfer tools and methods for high density transfer of arrays of micro devices are described. In accordance with embodiments a mass transfer tool (MTT) can include one or more articulating transfer head assemblies which carry a corresponding micro pick up array (MPA) to transfer an array of micro devices between a donor substrate and receiving substrate. The MAs may include an array of transfer heads, which may be arranged into a plurality of clusters. 
     In an embodiment, an MPA includes an array of transfer heads arranged in a plurality of clusters, with each cluster including a corresponding plurality of transfer heads. The clusters may optionally include rows of transfer heads. In an embodiment, adjacent transfer heads (e.g. within a row or column of transfer heads) within a cluster are separated by an intra-cluster spacing (Sh), and adjacent clusters (e.g. within a row or column of clusters) are separated by an inter-cluster spacing (Sc) that is greater than the intra-cluster spacing (Sh). The transfer heads can be designed for different modes of operation such as elastomeric contact surfaces for pick and place, vacuum, or operate in accordance with electrostatic principles. In an exemplary implementation each transfer head is an electrostatic transfer head. In an embodiment, each transfer head includes an elastomeric contact surface. 
     A variety of transfer head arrangements that may facilitate high density pick and place transfer process, as well as cluster arrangements. In an embodiment, an MPA includes a base substrate, and an array of polycrystalline compliant transfer heads on the base substrate. For example, the polycrystalline compliant transfer heads may be fabricated using an epitaxial growth and patterning in a layer-by-layer processing sequence to facilitate a vertically integrated spring structure. 
     In an embodiment an MPA includes a base substrate, an array of transfer heads over the base substrate, where each transfer head including a mesa structure. An electrically conductive layer may partially cover the mesa structure for each transfer head in the array of transfer heads to form an electrostatic shield. A first voltage source contact may be coupled with the electrically conductive layer, for example for grounding, and a second voltage source contact may be coupled with the array of mesa structures of the array of transfer heads, for example to provide an operating voltage to the array of transfer heads. Such a configuration with an electrostatic shield may be integrated with a monopolar transfer head configuration to facilitate further densification of the transfer heads and ability to hold the micro devices at a fixed potential and to shield the micro device from stray electric fields. 
     In an embodiment an MPA includes a base substrate and a base spring layer over the base substrate. The base spring layer may include a plurality of spring arms and a spring platform. An encapsulation membrane layer spans over the base spring layer, and a mesa structure protrude from the spring platform and through a corresponding opening in the encapsulation membrane layer such that the mesa structure is deflectable through the corresponding opening and toward the base substrate. In such a configuration the encapsulation membrane layer can function as an electrostatic shield may be integrated with a monopolar transfer head configuration to facilitate further densification of the transfer heads and ability to hold the micro devices at a fixed potential and to shield the micro device from stray electric fields. The transfer heads of such an MPA can also be polycrystalline compliant transfer heads fabricated using epitaxial growth and patterning in a layer-by-layer processing sequence. 
     In an embodiment, the MPA further includes a plurality of submesa interconnects, where each row of transfer heads spans over a pair of submesa interconnects, with each first mesa structure protruding from a first submesa interconnect and each second mesa structure protruding from a second submesa interconnect. The first and second submesa interconnects may be connected to different voltage sources to provide the bi-polar electrostatic gripping force. The transfer heads may also be monopolar or different multi-polar arrangements. 
     In an embodiment a transfer process includes picking up a first group of LEDs from a first donor substrate with an MPA including a plurality of clusters of transfer heads, positioning the MPA over a first location of a display substrate, placing a first LED of the first group of LEDs from each cluster onto the display substrate, positioning the MPA over a second location of the display substrate, and placing a second LED of the first group of LEDs from each cluster onto the display substrate. 
     In an embodiment, a transfer process includes picking up a first group of LEDs from a first donor substrate with a first articulating transfer head assembly, picking up a second group of LEDs from the first donor substrate with a second articulating transfer head assembly, translating the first and second articulating transfer head assemblies toward a display substrate, positioning the first articulating transfer head assembly over the display substrate, placing a first group of LEDs onto the display substrate, positioning the second articulating transfer head assembly over the display substrate, and placing the second group of LEDs onto the display substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic illustration of a mass transfer tool assembly in accordance with an embodiment. 
         FIG.  2    is an isometric view illustration of a micro pick up array and pivot mount mounted onto an articulating transfer head assembly in accordance with an embodiment. 
         FIG.  3 A  is a schematic plan view illustration of a micro pick up array including a plurality of clusters of transfer heads in accordance with an embodiment. 
         FIG.  3 B  is an isometric view illustration of a portion of a micro pick up array including a cluster of transfer heads in accordance with an embodiment. 
         FIG.  3 C  is a schematic cross-sectional side view taken along line X-X of  FIG.  3 B  in accordance with an embodiment. 
         FIG.  4 A  is a schematic top view illustration of a portion of a micro pick up array including a cluster of transfer heads with separate spring platforms in accordance with an embodiment. 
         FIG.  4 B  is a schematic cross-sectional side view taken along line X-X of  FIG.  4 A  in accordance with an embodiment. 
         FIG.  5    is a schematic plan view illustration of a portion of a micro pick up array including a cluster of transfer heads with shared submesas and polarities in accordance with an embodiment. 
         FIG.  6    is an isometric view illustration of a portion of a micro pick up array including a cluster of transfer heads with shared submesas and polarities in accordance with an embodiment. 
         FIG.  7 A  is an isometric view illustration of a portion of a micro pick up array including a cluster of monopolar transfer heads in accordance with an embodiment. 
         FIG.  7 B  is a schematic cross-sectional side view of the cluster of monopolar transfer heads of  FIG.  7 A  in accordance with an embodiment. 
         FIG.  8 A  is a schematic top view illustration of a portion of a micro pick up array including a cluster of monopolar transfer heads with separate spring platforms in accordance with an embodiment. 
         FIG.  8 B  is a schematic cross-sectional side view illustration of a monopolar transfer head with an electrostatic shield in accordance with an embodiment. 
         FIG.  9 A  is an isometric view of a cluster of polycrystalline compliant transfer heads in accordance with an embodiment. 
         FIG.  9 B  is a side view illustration of the cluster of polycrystalline compliant transfer heads of  FIG.  9 A  in accordance with an embodiment. 
         FIG.  10 A  is an isometric view of a cluster of polycrystalline compliant transfer heads in accordance with an embodiment. 
         FIG.  10 B  is a side view illustration of the cluster of polycrystalline compliant transfer heads of  FIG.  10 A  in accordance with an embodiment. 
         FIG.  11 A  is a schematic cross-sectional side view illustration of a partially processed micro pick up array including an array of polycrystalline compliant transfer heads in accordance with an embodiment. 
         FIG.  11 B  is a schematic cross-sectional side view illustration of a micro pick up array including an array of polycrystalline compliant transfer heads after removal of sacrificial layers in accordance with an embodiment. 
         FIG.  11 C  is a top-down view illustration of a micro pick up array including a cluster of compliant transfer heads in accordance with an embodiment. 
         FIG.  11 D  is a schematic cross-sectional side view illustration taken along line X-X of  FIG.  11 C  in accordance with an embodiment. 
         FIG.  12 A  is a schematic top view illustration of a donor wafer including an array of LEDs in accordance with an embodiment. 
         FIG.  12 B  is a schematic top view illustration of a donor wafer of  FIG.  12 A  after picking a plurality of clusters of LEDs with a micro pick up array including a plurality of clusters of transfer heads in accordance with an embodiment. 
         FIG.  13    is a schematic side view illustration of a micro pick up array holding a plurality of clusters of LEDs over a display substrate in accordance with an embodiment. 
         FIG.  14 A  is a schematic top view illustration of a micro pick up array including clusters of transfer heads holding clusters of LEDs over a display substrate in accordance with an embodiment. 
         FIG.  14 B  is a schematic cross-sectional side view illustration taken along line B-B of  FIG.  14 A  in accordance with an embodiment. 
         FIG.  14 C  is a schematic cross-sectional side view illustration taken along line C-C of  FIG.  14 A  in accordance with an embodiment. 
         FIGS.  15 A- 15 C  are plan view illustrations of a single pick and multiple place sequence for a plurality of first color-emitting LEDs in accordance with an embodiment. 
         FIGS.  15 D- 15 F  are plan view illustrations of a single pick and multiple place sequence for a plurality of second color-emitting LEDs in accordance with an embodiment. 
         FIGS.  15 G- 15 I  are plan view illustrations of a single pick and multiple place sequence for a plurality of third color-emitting LEDs in accordance with an embodiment. 
         FIG.  16    is a process flow for a sequence of transferring a group of LEDs with a micro pick up array comprising a plurality of clusters of transfer heads in accordance with an embodiment. 
         FIG.  17    is a process flow for a sequence of transferring multiple groups of LEDs with multiple articulating transfer head assemblies in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe systems and methods for high density transfer of arrays of micro devices from a donor substrate to a receiving substrate. For example, the arrays of micro devices may be micro LEDs. While some embodiments are described with specific regard to micro LEDs, the embodiments of the invention are not so limited and certain embodiments may also be applicable to other micro devices such as diodes, transistors, integrated circuit (IC) chips, MEMS, and bio-samples. 
     In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the embodiments. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the embodiments. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “over”, “to”, “between”, “spanning” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “over”, “spanning” or “on” another layer or bonded “to” or in “contact” with another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     The terms “micro” device or “micro” LED as used herein may refer to the descriptive size of certain devices or structures in accordance with embodiments. As used herein, the term “micro” is meant to refer to the scale of 1 to 300 μm. For example, each micro device may have a maximum length or width of 1 to 300 μm, 1 to 100 μm or less. In some embodiments, the micro LEDs may have a maximum length and width of 20 μm, 10 m, or 5 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 accordance with embodiments, a mass transfer tool (MTT) and method of operation are described that enable picking up a high density of micro devices from a donor substrate and sequentially placing groups of the micro devices onto one or more receiving substrates. In an embodiment, the MTT includes an articulating transfer head assembly that carries a micro pick up array (MPA) that, depending upon size of the MPA and specifications for the receiving substrate, may include thousands of individual transfer heads. 
     In an embodiment, an MPA includes an array of transfer heads arranged in a plurality of clusters, with each cluster including a corresponding plurality of transfer heads. In an exemplary implementation the transfer heads may be arranged in rows, and columns within a cluster. Adjacent transfer heads within a row of transfer heads within a cluster are separated by an intra-cluster spacing, and adjacent clusters within a row of clusters are separated by an inter-cluster spacing that is greater than the intra-cluster spacing. 
     In an embodiment, a transfer sequence includes picking up a first group of LEDs from a first donor substrate with an MPA including a plurality of clusters of transfer heads, positioning the MPA over a first location of a display substrate, placing a first LED of the first group of LEDs from each cluster onto the display substrate, positioning the MPA over a second location of the display substrate, and then placing a second LED of the first group of LEDs from each cluster onto the display substrate. This sequence can continue until all LEDs of the first group of LEDs are placed onto the display substrate. The sequence than then be repeated for a second group of LEDs from the same donor substrate (e.g. same color-emitting LEDs) or different donor substrate (e.g. different color-emitting LEDs, or same color-emitting LEDs to reduce donor associated defects). 
     In one aspect, the cluster pick and sequential place sequences, and MPAs fabricated to include such clusters of transfer heads, can provide a higher pick density compared to a transfer sequence in which every LED that is picked is then simultaneously placed. Furthermore, the sequential place operations can cover less distance, and resultingly time required for placement. Thus, overall throughput can be increased for the display assembly process and cost can be reduced. 
     A variety of MPA configurations can be used to perform the high density cluster pick and sequential place sequences. In some embodiments, the transfer heads are compliant transfer heads that are deflectable upon contact with a corresponding micro device (e.g. LED) or target substrate. This compliance can compensate for misalignment of the articulating transfer head assembly/MPA and target substrate, as well as for variations in height and contamination. Such compensation can result in reduced compressive forces applied to certain micro devices, leading to protection of the physical integrity of the micro devices and transfer head array. Furthermore, such compensation can facilitate application of uniform, or sufficient grip pressure with the transfer heads when operating in accordance with electrostatic principles. 
     In accordance with embodiments, dimensions of the clusters of transfer heads (and micro devices picked by the transfer heads), as well as the receiving substrate (e.g. display substrate) are designed so that a topography tolerance exists on the receiving substrate to receive all of the micro devices held by the transfer heads, including those not being placed. In an exemplary display panel fabrication sequence, this may include a display substrate topography being designed to accommodate the clusters of transfer heads, and corresponding clusters of LEDs held by the transfer heads, during the sequential placement of each LED from each cluster until all LEDs from the cluster have been placed onto the display substrate. In an embodiment, adjacent transfer heads within a row of transfer heads in a cluster are separated by an intra-cluster spacing, and adjacent clusters within a row of clusters (along same axis, or parallel with the row of transfer heads) are separated by an inter-cluster spacing that is greater than the intra-cluster spacing. Thus, the clusters are spaced out further than the transfer heads within the clusters. In order to accommodate the clusters of transfer heads, and LEDs held by the transfer heads, a cluster width of transfer heads within a cluster can fit within an inter-subpixel pitch between immediately adjacent first arrays of subpixels (e.g. first color-emitting subpixels) and second arrays of subpixels (e.g. second color-emitting subpixels). Consequently, the inter-cluster spacing will be greater than the inter-subpixel pitch. Such arrangements may negate the possibility of an LED already placed in a subpixel from interfering with subsequent placement of other color-emitting LEDs into another subpixel within the same pixel (or adjacent pixel), even when the other color-emitting LED is part of a cluster of other color-emitting LEDs held by an MPA. 
     The display substrate in accordance with embodiments may be designed to accommodate a high density of pixels, or pixels per inch (PPI). Consequently, increased PPI may correlate to a reduced (e.g. first-second) subpixel array spacing. In accordance with embodiments, each transfer head may be independently deflectable. For example, each transfer head may be supported by corresponding spring platform that is deflectable toward/into a cavity. Alternatively, a plurality of transfer heads (e.g. cluster) can be supported by a same spring platform. Consolidating multiple transfer heads onto a shared spring platform can reduce space required for multiple spring components and increase density. 
     In accordance with some embodiments, the transfer heads can be designed for different modes of operation. For example, the transfer heads can include elastomeric contact surfaces for pick and place, include vacuum holes, or operate in accordance with electrostatic principles in order to generate higher gripping pressure and reduced size. The transfer heads can include mesa structures to provide localized contact points for the transfer heads. The electrostatic transfer heads may be monopolar, or multi-polar (e.g. bi-polar, etc.). For example, multi-polar transfer heads may be utilized to mitigate against residual charge buildup or provide a charge differential where the target substrate (e.g. donor, receiving, display) is not maintained at a reference voltage. In one aspect, multi-polar transfer heads can include mesa structures extending from common submesa interconnects coupled to a same voltage source. Such an arrangement may facilitate further densification of the electrostatic transfer heads. 
     In another aspect, multiple articulating transfer head assemblies and corresponding MPAs can be utilized to increase transfer throughput and reduce assembly process cost. In an embodiment, a transfer sequence includes picking up a first group of LEDs from a first donor substrate with a first articulating transfer head assembly, picking up a second group of LEDs from the first donor substrate with a second articulating transfer head assembly, translating the first and second articulating transfer head assemblies (e.g. along a translation track) toward a display substrate, positioning the first articulating transfer head assembly over the display substrate, placing a first group of LEDs onto the display substrate, positioning the second articulating transfer head assembly over the display substrate, and placing the second group of LEDs onto the display substrate. 
       FIG.  1    is a schematic illustration of a mass transfer tool in accordance with an embodiment. Mass transfer tool  100  may include one or more articulating transfer head assemblies  200 , each for picking up an array of micro devices from a carrier (donor) substrate held by a carrier substrate stage  104  and for transferring and releasing the array of micro devices onto a receiving substrate held by a receiving substrate stage  106 . In an embodiment, an upward facing inspection camera  120  is located between the carrier substrate stage  104  and the receiving substrate stage  106 . In this manner, the underside of an articulating transfer head assembly  200  (e.g. a micro pick up array carrying a group of micro devices) may be inspected by the inspection camera while the articulating transfer head assembly  200  moves between the carrier substrate stage  104  and receiving substrate stage  106  to verify efficacy of the transfer operations. Operation of mass transfer tool  100  and articulating transfer head assembly  200  may be controlled at least in part by a computer  108 . 
     Referring to  FIG.  2   , a perspective view of an articulating transfer head assembly  200  is shown in accordance with an embodiment. An articulating transfer head assembly  200  may be used in the mass transfer tool  100  to transfer micro devices to or from a substrate, e.g., receiving substrate or donor substrate, using micro pick up array (MPA)  103  which is supported by a pivot mount assembly  300 . The pivot mount assembly  300  may include a support structure (e.g. base)  302 , a pivot platform  304 , and plurality of spring arms  306 , and the MPA  103  supporting an array of electrostatic transfer heads  115  is mounted on the pivot platform  304 . In an embodiment, the pivot mount assembly  300  may include a flex circuit  308  to communicate with a printed circuit board (PCB) that is located nearby within the articulating transfer head assembly  200  to reduce signal degradation by limiting a distance that signals must travel. 
     In an embodiment, the MPA  103  includes an array of electrostatic transfer heads  115 , where each transfer head operates in accordance with electrostatic principles to pick up and transfer a corresponding micro device. In an embodiment each transfer head has a localized contact surface characterized by a maximum dimension of 1-300 μm in both the x- and y-dimensions. In an embodiment, each transfer head contact surface has a maximum lateral dimension of 1 to 100 μm, or less. In some embodiments, each transfer head contact surface has a maximum length and width of 20 μm, 10 μm, or 5 μm. Similarly, each micro device, such as an LED or chip, may have a maximum lateral dimension of 1-300 μm, or 1-100 μm, such as 20 μm, 10 μm, or 5 μm. The articulating transfer head assembly  200  can include features that allow for the exchange of the MPA and for delivering voltage(s) to the transfer heads to facilitate pick up of a micro device using an electrostatic force. 
     Referring to both  FIGS.  1 - 2   , computer  108  may control the operation of articulating transfer head assembly  200  of the MTT  100 . For example, articulating transfer head assembly  200  may include an actuator assembly for adjusting the MPA  103  retained by the transfer head assembly with at least three degrees of freedom, e.g., tipping, tilting, and movement in a z direction, based on feedback signals received from various sensors of the MTT  100 . Computer  108  may also control movement of the articulating transfer head assembly  200  along translation track  110  (e.g. x direction) over the carrier substrate stage  104  and receiving substrate stage  106 . Additional actuators may be provided, e.g., between mass transfer tool  100  structural components and articulating transfer head assembly  200 , carrier substrate stage  104 , or receiving substrate stage  106 , to provide movement in the x, y, or z direction for one or more of those sub-assemblies. For example, a gantry may support articulating transfer head assembly  200  and move articulating transfer head assembly  200  along an upper beam, e.g., in a direction parallel to an axis of motion of translation track  110 . Thus, an array of transfer heads on MPA  103 , supported by transfer head assembly  200 , and a target substrate (e.g. supported by carrier substrate stage  104  or receiving substrate stage  106 ) may be precisely moved relative to each other within all three spatial dimensions. 
     The articulating transfer head assembly  200  in accordance with embodiments may provide for negligible lateral or vertical parasitic motion for small movements of MPA  103 , e.g., motion less than about 5 mrad about a neutral position. In an embodiment, the articulating transfer head assembly includes a tip-tilt assembly  210  and a piezoelectric stage assembly  250  mounted underneath the tip-tilt assembly  210 . Together the tip-tilt assembly  210  and the piezoelectric stage assembly  250  may provide six degrees of motion. Specifically, the tip-tilt assembly  210  may provide tip (θx) and tilt (θy), where the piezoelectric stage assembly  250  provides z motion, x motion, y motion, and rotation (θz). In the particular embodiment illustrated a mounting plate  280  is secured underneath the piezoelectric stage assembly  250 . The pivot mount assembly  300  may be mounted onto the mounting plate  280  using a variety of manners such as using tabs or lips to press the pivot mount assembly against the transfer head assembly  200 , bonding, vacuum, electrostatic clamping, or pogo pin array board. The MPA  103  can be mounted on the pivot platform  304  of the pivot mount assembly  300  using suitable techniques such as electrostatic clamps, vacuum, or mechanical clips. 
     Referring now to  FIG.  3 A , a schematic plan view illustration is provided of an MPA  103  including a plurality of clusters  310  of transfer heads  115  in accordance with an embodiment. In the illustrated embodiment, an MPA  103  includes an array of transfer heads  115  arranged in a plurality of clusters  310 , with each cluster  310  including a corresponding plurality of transfer heads  115 . The clusters  310  may optionally include rows of transfer heads  115 . In an embodiment, adjacent transfer heads  115  within a row of transfer heads within a cluster  310  are separated by an intra-cluster spacing (Sh), and adjacent clusters  310  within a row of clusters are separated by an inter-cluster spacing (Sc) that is greater than the intra-cluster spacing (Sh). Additionally, the clusters  310  of transfer heads may be arranged with an inter-cluster pitch (Pc), and transfer heads  115  within the clusters may be arranged with an intra-cluster pitch (Ph). Inter-cluster pitch (Pc) and intra-cluster pitch (Ph) are both illustrated as having the same dimensions in x-direction (e.g. row-wise) and y-direction (e.g. column-wise) though x-y dimensions may be different. Similarly, columnar spacing may be the same or different from row spacing for intra-cluster spacing (Sh) and inter-cluster spacing (Sc). 
     In the particular embodiment illustrated, a compliant bi-polar electrostatic transfer head  115  assembly is shown with the darker shading illustrating the electrical connection to a first voltage source (V A ), and the lighter shading illustrating the electrical connection to a second voltage source (V B ). As shown, the MPA  103  can include an array of compliant bipolar transfer heads  115  connected to an arrangement of trace interconnects  334 ,  336 , and bus interconnects  330 ,  332 . Bus interconnects  330 ,  332  may be formed around a periphery or outside a working area of the array of transfer heads  115 . In an embodiment, voltage contacts  338 ,  339  may make contact with bus interconnects  330 ,  332  in order to electrically connect the transfer heads  115  with working circuitry of a transfer head assembly. Where each transfer head  115  is operable as a bipolar transfer head, voltage sources V A  and V B  may simultaneously apply opposite voltages so that the opposing electrodes for each respective transfer head  115  has an opposite voltage. Furthermore, the transfer heads  115  may be deflectable toward/into cavities  315 . Each transfer head  115  may be deflectable into a separate cavity  315 , or a plurality (or cluster, or clusters) of transfer heads  115  can be deflectable toward/into a same cavity  315 . 
     While the particular embodiment illustrated in  FIG.  3 A  includes four transfer heads arranged in two rows within each cluster  310 , this is exemplary, and the clusters  310  may include another number of transfer heads, and may or may not be arranged in rows. Furthermore, it is not required for the transfer heads  115  to be compliant or have bi-polar arrangement. In the following description, additional figures and description are provided for transfer sequences utilizing an MPA  103  similar to that of  FIG.  3 A , however, it is understood this is exemplary and alternative transfer head  115  arrangements can be used. 
     Referring now to  FIGS.  3 B- 3 C ,  FIG.  3 B  is an isometric view illustration of a portion of an MPA including a cluster  310  of transfer heads  115  in accordance with an embodiment;  FIG.  3 C  is a schematic cross-sectional side view taken along line X-X of  FIG.  3 B  in accordance with an embodiment. As shown, each transfer head  115  may include a spring arm  340 ,  342  extending from a corresponding trace interconnect  334 ,  336  to a spring platform  327  that supports the corresponding plurality of transfer heads  115  for a corresponding cluster  310 . The spring platform  327  may be deflectable toward cavity  315  such that all transfer heads  115  for a cluster  310  are deflected together. In alternative embodiments, each transfer head  115  may be supported by a separate spring platform  327 , which can be separately deflectable toward the cavity  315 . A variety of spring structures and spring arm configurations can be used to achieve a specified compliance. 
     In the particular embodiment illustrated the spring platform  327  can be formed in part by interdigitated finger traces  344 ,  346  that extend from spring arms  340 ,  342  respectively. Spacing between finger traces  344 ,  346  may optionally be filled with a dielectric layer  360  material which can physically join the finger traces  344 ,  346  together and provide further robustness to the structure. In the bipolar configuration each transfer head can include a pair of mesa structures  354 ,  356 , which can optionally be separated by a dielectric joint  364 . In an embodiment, the dielectric joints  364  are parallel to each row of transfer heads. 
     Each cluster  310  can include a plurality of submesa interconnects  350 ,  352 , where each submesa interconnect  350 ,  352  spans underneath a corresponding mesa structure  354 ,  356  for a plurality of transfer heads  115  within a row of transfer heads. Specifically, the mesa structures  354 ,  356  may protrude from the submesa interconnects  350 ,  352 . As shown, a first plurality of first submesa interconnects  350  is coupled with a same first voltage source V A , and a second plurality of second submesa interconnects  352  is coupled with a same second voltage source V B . Furthermore, each cluster  310  can include a plurality of finger traces  344 ,  346  spanning underneath a corresponding mesa structure  354 ,  356  for a plurality of transfer heads within a row of transfer heads  115 . Specifically, the submesa interconnects  350 ,  352  may protrude from the finger traces  344 ,  346 . 
     Referring now to the schematic cross-sectional side view illustration of  FIG.  3 C , the MPA  103  may be formed with a silicon-on-insulator (SOI) type substrate stack. As shown, this may include a base substrate  320 , such as a silicon wafer. A cavity template layer  324  can optionally be located over the base substrate  320  for the formation of cavities  315 . Cavity template layer  324  may be separated from the base substrate  320  with a lower insulating layer  322 , such as an oxide (e.g. SiO 2 ). Alternatively, cavities  315  can be formed in the base substrate  320 . An upper insulating layer  326 , such as an oxide (e.g. SiO 2 ) can be formed on the cavity template layer  324 , and a device layer  328  on the upper insulating layer  326 . In accordance with embodiments, the device layer  328  and cavity template layer  324  may be formed of silicon. In an embodiment, the device layer  328  may be doped to improve conductivity. 
     The device layer  328 , and upper insulating layer  326 , may be patterned with multiple masks to form the bus interconnects  330 ,  332 , trace interconnects  334 ,  336 , spring arms  340 ,  342 , finger traces  344 ,  346 , submesa interconnects  350 ,  352 , and mesa structures  354 ,  356 . One or more top dielectric layers  360  may be formed over the patterned device layer  328 , and a top surface of the dielectric layer  360  on the top surfaces of the mesa structures  354 ,  356  can form the contact surfaces  363  for the transfer heads  115 . Dielectric joint  364  may optionally be formed of the same material as dielectric layer  360 , though different materials may be used. For example, dielectric layer  360  may be formed of a variety of oxides to provide a specified dielectric strength, and hence pick up pressure for the transfer heads  115 . 
     A variety of factors may be considered when designing a particular transfer head  115  assembly, including pixel density (PPI) of the display substrate to which the LEDs are transferred. An increased pixel density may affect the number of LEDs and transfer heads  115  that can fit within an inter-subpixel pitch. Furthermore, there may be minimum LED size requirements for system efficiency, and compliance tolerances for the compliant transfer heads particularly when multiple transfer heads share a common spring platform which can result in some amount of torque due to off-center loading. 
     Referring now to  FIGS.  4 A- 4 B ,  FIG.  4 A  is a schematic top view illustration of a portion of an MPA including a cluster  310  of transfer heads  115  with separate spring platforms  327  in accordance with an embodiment;  FIG.  4 B  is a schematic cross-sectional side view taken along line X-X of  FIG.  4 A  in accordance with an embodiment.  FIGS.  4 A- 4 B  are similar to the arrangement of  FIGS.  3 A- 3 B  with a double side clamped cantilever beam arrangement of spring arms  340 ,  342  and spring platform  327 . Likewise, the cluster  310  of  FIGS.  4 A- 4 B  can be arranged with the plan layout of  FIG.  3 A . Dimensions of the parts can be altered to achieve a specified stiffness and deflection limit. 
     Up until this point cluster  310  grouping has been described with 2×2 arrangements of transfer heads  115 . However, cluster  310  groupings can be adjustable in a variety of other patterns, including other row-column arrangements and non-row-column arrangements. 
       FIG.  5    is a schematic plan view illustration of a portion of a micro pick up array including a cluster of transfer heads with shared submesas and polarities in accordance with an embodiment.  FIG.  6    is an isometric view illustration of a portion of a micro pick up array including a cluster of transfer heads with shared submesas and polarities in accordance with an embodiment.  FIGS.  5 - 6    are substantially similar to the embodiments illustrated in  FIGS.  3 A- 3 B  with some differences. Firstly, the transfer head  115  grouping within a cluster  310  is a 3×3 arrangement with nine total transfer heads. This arrangement can be scaled, for example, 3×2, 4×4, etc. A second difference illustrated in  FIGS.  5 - 6    is that the mesa structures  354 ,  356  for transfer heads  115  in immediately adjacent rows or columns can be connected to a shared voltage source, V A  or V B . In this arrangement, interior mesa structures  354 ,  356  (e.g. those not along outer edge of the cluster  310 ) can be formed on shared submesa interconnects  350 ,  352  and/or finger traces  344 ,  346 , which can be interdigitated. Such an arrangement may facilitate densification of the transfer heads  115  within a cluster  310 . In the illustrated embodiment, polarity for the mesa structures  354 ,  356  switches for adjacent transfer heads  115  along a row or column. Adjacent submesa interconnects  350 ,  352  and/or finger traces  344 ,  346  can be separated by an insulating material  358  (e.g. oxide, etc.). This may be a similar material to dielectric joint  364 , for example. 
     In an embodiment, an MPA  103  includes a cluster  310  of a plurality of transfer heads  115  arranged in a plurality of rows of transfer heads  115 . Each transfer head  115  may be an electrostatic transfer head that includes a first mesa structure  354  and a second mesa structure  356 , and a plurality of submesa interconnects  350 ,  352 . Each row of transfer heads  115  may span over a pair of submesa interconnects  350 ,  352 , with each first mesa structure  354  protruding from a first submesa interconnect  350  and each second mesa structure  356  protruding from a second submesa interconnect  352 . 
     The plurality of submesa interconnects can include a first plurality of first submesa interconnects  350  coupled to a same first voltage source (VA), and a second plurality of second submesa interconnects  352  coupled with a same second voltage source (VB). In the embodiment illustrated in  FIG.  6   , the first mesa structures  354  for a first row of transfer heads  115  and the first mesa structures  354  for a second row of transfer heads can both protrude from a same first submesa interconnect  350 . Similarly, the second mesa structures  356  for the second row of transfer heads  115  and second mesa structures  356  for a third row of transfer heads  115  protrude from a same second submesa interconnect  352 . 
     While the above description with regard to  FIGS.  3 A- 6    has been with regard to multi-polar transfer heads, and bi-polar transfer heads in particular, embodiments are not so limited. For example, the transfer heads may be monopolar transfer heads. Alternatively, the transfer heads may be elastomeric stamps, including elastomeric contact surfaces. Utilization of transfer heads with single mesa structures can facilitate scaling down the size of the transfer heads, which can allow for higher density clusters, and also the transfer of smaller devices. 
       FIG.  7 A  is an isometric view illustration of a portion of a micro pick up array including a cluster  310  of monopolar transfer heads  115  in accordance with an embodiment.  FIG.  7 B  is a schematic cross-sectional side view of the cluster of monopolar transfer heads of  FIG.  7 A  in accordance with an embodiment.  FIGS.  7 A- 7 B  are similar to the arrangement of  FIGS.  3 A- 3 B  with a double side clamped cantilever beam arrangement of spring arms  341  and spring platform  327 . Likewise, the cluster  310  of  FIGS.  7 A- 7 B  can be arranged with the plan layout of  FIG.  3 A . Dimensions of the parts can be altered to achieve a specified stiffness and deflection limit. As shown a cluster of monopolar transfer heads can be supported by the same spring platform  327 . The spring arms  341  may be connected to the same voltage source for a monopolar configuration. 
     The device layer  328 , and upper insulating layer  326 , may be patterned with multiple masks to form the bus interconnects, trace interconnects  335 , spring arms  341 , spring platform  327 , optional submesa interconnects  351 , and mesa structures  355 . One or more top dielectric layers  360  may be formed over the patterned device layer  328 , and a top surface of the dielectric layer  360  on the top surfaces of the mesa structures  355  can form the contact surfaces  363  for the transfer heads  115 . For example, dielectric layer  360  may be formed of a variety of oxides to provide a specified dielectric strength, and hence pick up pressure for the transfer heads  115 . 
       FIG.  8 A  is a schematic top view illustration of a portion of a micro pick up array including a cluster  310  of monopolar transfer heads  115  with separate spring platforms  327  in accordance with an embodiment.  FIG.  8 A  is similar to the arrangement of  FIGS.  4 A- 4 B  with a double side clamped cantilever beam arrangement of spring arms  341  and spring platform  327 . 
     Referring now to  FIG.  8 B , a schematic cross-sectional side view illustration is provided of a monopolar transfer head  115  with an electrostatic shield  400  in accordance with an embodiment. The particular transfer head  115  of  FIG.  8 B  resembles that of  FIG.  8 A , though this is exemplary and an electrostatic shield  400  may be incorporated into a variety of monopolar transfer head configurations. 
     The electrostatic shield  400  includes an electrically conductive layer  402  and may be particularly suitable for monopolar transfer head configurations. For example, single pole clamping of a micro device, such as an LED, operates with a fixed potential on the micro device and a conductive path through the micro device. The electrostatic shield can function hold the micro device at a fixed potential, such as ground, and to shield the micro device from stray electric fields. The electrostatic shield  400  may additionally include apertures over the mesa structures  355 , which can function to control the electric field size that is generated. In operation, the electric field is generated by applying a voltage to the array of transfer heads  115  from a first voltage source, while holding the electrostatic shield  400  at a fixed potential with a second voltage source, which may be a ground connection. 
     The particular illustration in  FIG.  8 B  is a combination view of several different areas of the MPA  103 , including a central active area showing one of the array of transfer heads  115 , a ground shield contact area to the right side of the illustration, and an operating voltage contact area on the left side of the illustration. Similar to previous discussion, the MPA  103  can include a base substrate  320 , a lower insulating layer  322 , cavity template layer  324  and cavity  315 , an upper insulating layer  326  on the cavity template layer  324 , and a device layer  328  on the upper insulating layer  326 , and a dielectric layer  360  on the device layer  328 . A back side insulation layer  372 , such as an oxide, may be formed on the back side of the base substrate  320 . A contact template layer  374  can be formed over the back side insulation layer  372  and patterned to form openings  375 . A second back side insulation layer  376  may then be formed over the contact template layer  374 , and then be patterned to form openings through the back side insulation layer  372 . Back side conductive (e.g. metal) layers  382  can then be formed over the openings  375  through the back side insulation layers  372 ,  376  to form back side contacts which can be used, for example, to connect to a voltage source, such as ground, or a second voltage source (V B ) for supplying an operating voltage. 
     Prior to forming the back side contacts, the base substrate  320  can be patterned to form one or more plugs  380 , or another suitable vertical electrical connection. The plug  380  may be formed from the base substrate  320  (e.g. silicon), and may be electrically isolated from the base substrate  320 , for example, with sidewall insulation layers  383 ,  384  (e.g. oxides) and optional fill material  386 , such as polymer (e.g. epoxy), paste (e.g. glass), or gel (e.g. silicone) that can be applied in to the opening. The back side conductive layers  382  may be formed on and in electrical contact with the base substrate  320  and plug  380 . 
     Top side contacts may be similarly formed. For example, openings  395  can be formed through any of the lower insulating layer  322 , cavity template layer  324 , upper insulating layer  326 , device layer  328 , and dielectric layer  360  to expose the plug  380 . A top conductive contact layer  392  can then be formed in the opening  395  to contact the plug  380 . The top conductive contact layer  392  can also be formed through a device opening  366  in the dielectric layer  360  to contact the device layer  328  and complete the electrical path from the second voltage source (V B ) to the device layer  328  and complete the second voltage source contact  399 . 
     The first voltage source contact  399  is similarly connected. As shown, an opening  405  can be formed through the lower insulating layer  322 , cavity template layer  324 , upper insulating layer  326 , device layer  328 , and dielectric layer  360  to expose the base substrate  320 . The electrically conductive layer  402  is formed in the opening  405  to contact the base substrate  320 . The electrically conductive layer  402  is also patterned to form apertures  407  over the mesa structures  355 . For example, the apertures  407  may be circles that are fully enclosed by the electrically conductive layer  402 . The electrically conductive layer  402  contacts the base substrate  320  and can be electrically connected with the first voltage source, or ground, through the base substrate  320 . 
     In an embodiment, an MPA includes a base substrate  320 , an array of transfer heads  115  over the base substrate, with each transfer head  115  including a mesa structure  355 . An electrically conductive layer  402  spans over the mesa structure for each transfer head in the array of transfer heads to form the electrostatic shield  400 . For example, the electrically conductive layer  402  may be metal, polysilicon, etc. A first voltage source contact  397  is coupled with the electrically conductive layer  402 . For example, the first voltage source contact  397  can be coupled to a voltage source, including ground. The first voltage source contact  397  may also be electrically connected to the base substrate  320 . A second voltage source contact  399  is coupled with the array of mesa structures  355  of the array of transfer heads  115 . The second voltage source contact  399  may be coupled with a second voltage source (V B ) to provide the operating voltage for the transfer heads. The second voltage source contact  399  may include a plug that extends through the base substrate  320 , and is electrically isolated from the base substrate. 
     Referring now to  FIGS.  9 A- 9 B  and  FIGS.  10 A- 10 B  isometric and side view illustrations are provided of clusters  310  of polycrystalline compliant transfer heads  115  in accordance with embodiments. The MPAs and arrays of electrostatic transfer heads described up until this point can be fabricated using sequential wafer bonding and patterning of layers, and can also be fabricated using an additive approach of surface micromachining in which the layers are sequentially deposited or grown. The embodiments illustrated in  FIGS.  9 A- 9 B  and  FIGS.  10 A- 10 B  may leverage the additive surface micromachining approach for further size reduction, and also the formation of multiple level spring structures for the compliant transfer heads. Specifically, epitaxial layers, such as epitaxial polycrystalline silicon layers, can be sequentially deposited and patterned to bury the spring structures vertically, as opposed to laterally, resulting in space savings for equivalent spring action of a cantilever-type approach. 
     As shown, an MPA  103  can include a base substrate  320  and an array of polycrystalline compliant transfer heads  115  on the base substrate  320 . Each polycrystalline compliant transfer head  115  includes a spring platform  442  and a mesa structure  355  protruding from the spring platform  442 . For example, each polycrystalline compliant transfer head  115  may be a monopolar transfer head. Each polycrystalline compliant transfer head  115  further includes a plurality of anchor plugs  410  protruding from the base substrate  320  and connected to a base spring layer  420  at an opposite end. The anchor plugs  410  may optionally be the first epitaxially grown polycrystalline layer of the MPA, or a top surface of the base substrate  320  can include a blanket polycrystalline layer to which the anchor plugs  410  are attached. 
     The spring layer may include one or more base spring arms  422  connected to the anchor plugs  410  a corresponding vertical interconnect  430 . The plurality of vertical interconnects  430  may protrude from the base spring layer  420  and connected to an upper spring layer  440 , of which the spring platform  442  is a part. In the particular embodiment illustrated in  FIGS.  9 A- 9 B  the upper spring layer  440  is the spring platform  442 . In the particular embodiment illustrated in  FIGS.  10 A- 10 B  the upper spring layer  440  includes a plurality of upper spring arms  444  that connect the plurality of vertical interconnects  430  to the spring platform  442 . A dielectric layer  360  can be formed over the mesa structures  355  and upper spring layer  440  as previously described. In accordance with embodiments, each of the described layers above the base substrate  320 , other than the dielectric layer  360 , can be micromachined polycrystalline material, such as polycrystalline silicon. 
     In order to illustrate an exemplary fabrication sequence, a schematic cross-sectional side view illustration is provided in  FIG.  11 A  of a partially processed micro pick up array including an array of polycrystalline compliant transfer heads in accordance with an embodiment. The final structure after etch release of the sacrificial layers is illustrated in  FIG.  11 B . In an exemplary process flow, the process may begin with a silicon-on-insulator substrate. For example, this may include a base substrate  320 , back side insulation layer  372  (which can be the buried oxide layer of the SOI substrate, and a contact template layer  374  (which can be normal device layer of an SOI substrate). A second back side insulation layer  376  may then be formed over the contact template layer  374 , and then be patterned to form openings through the back side insulation layer  372 . Back side conductive (e.g. metal) layers  382  can then be formed over the openings  375  through the back side insulation layers  372 ,  376  to form back side contacts which can be used, for example, to connect to a voltage source for supplying an operating voltage. In this case the operating voltage is applied to the base substrate  320 , which transfers the operating voltage to the transfer heads  115  that will be formed. 
     The formation of the transfer heads can then begin with the formation of a lower insulating layer  322  such as an oxide (e.g. SiO 2 ), which can optionally be planarized and then patterned to form openings that will correspond to the anchor plugs  410 . An epitaxial layer is then formed over the lower insulating layer  322  and within the openings, and then patterned to form the base spring layer  420  and anchor plugs  410 . The epitaxial layer may be a polycrystalline layer, such as polycrystalline silicon. Thus, the anchor plugs  410  and base spring layer  420  may be a single layer. The polycrystalline structure may be at least partially attributed to being formed over an oxide layer (lower insulating layer  322 ). 
     The processing sequence is then repeated for the next epitaxial layer, where an upper insulating layer  326  is then formed over the lower insulating layer  322  and the base spring layer  420  and planarized, followed by patterning to form openings that will correspond to the vertical interconnects  430 . An epitaxial layer, which may be polycrystalline silicon, is then formed over the upper insulating layer  326  and within the openings, and then patterned to form the mesa structure  355  and upper spring layer  440  (including spring platform  442  and upper spring arms  444  if present). Thus, the anchor plugs vertical interconnects  430 , upper spring layer  440  and mesa structures  355  may be a single layer. A dielectric layer  360  can then be formed over the mesa structure  355  and upper spring layer  440  and patterned, resulting in the structure illustrated in  FIG.  11 A . The dielectric layer  360  may be formed of a material different from the lower insulating layer  322  and upper insulating layer  326  for their selective removal, as shown in  FIG.  11 B . For example, the dielectric layer  360  may be formed of aluminum oxide. 
     Referring to  FIGS.  11 C- 11 D ,  FIG.  11 C  is a top-down view illustration of a micro pick up array including a cluster  310  of compliant transfer heads  115  in accordance with an embodiment,  FIG.  11 D  is a schematic cross-sectional side view illustration taken along line X-X of  FIG.  11 C  in accordance with an embodiment. In particular, the compliant transfer heads  115  may be polycrystalline compliant transfer heads  115  fabricated using additive surface micromachining methods similar to those of  FIGS.  9 A- 11 B . Furthermore, the MPA may include an encapsulation membrane layer  540  over a base spring layer  420 , where the mesa structures  355  protrude through openings  555  in the encapsulation membrane layer  540 . The mesa structures  355  can be deflectable through the openings  555  toward the base substrate  320  and may do so without deflecting the encapsulation membrane layer  540 . Such a configuration may provide encapsulation for the MPA, protecting particles from cavities  315  into which the spring structures are deflectable. Additionally, the encapsulation membrane layer  540  can be held at a fixed potential, or grounded, similarly as the electrically conductive layer  402  previously described to provide an electrostatic shield and facilitate monopolar transfer head configurations. 
     As shown, an MPA  103  can include a base substrate  320  and a base spring layer  420  over the base substrate  320 . The base spring layer  420  can include a plurality of base spring arms  422  and a spring platform  442 . One or more mesa structures  355  can be formed on and protrude from the spring platform  442 , and an encapsulation membrane layer  540  spans over the base spring layer  420 . In an embodiment, the mesa structure(s)  355  protrudes through a corresponding opening  555  in the encapsulation membrane layer  540  and is deflectable through the corresponding opening  555  and toward the base substrate  320 . For example, each mesa structure  355  may protrude through a single corresponding opening  555 . Alternatively, a plurality of mesa structures  355  can protrude through a same opening  555 . The mesa structures  355  may be decoupled from the encapsulation membrane layer  540  such that the mesa structures can be defected toward the base substrate  320  (e.g. toward cavity  315 ) without deflecting the encapsulation membrane layer  540 . In an embodiment, an upper cavity  515  is between and separates the spring platform  442  and the encapsulation membrane layer  540 . 
     In the illustrated embodiment, back side conductive layers  382  may be provided to form voltage source contacts  391 ,  394 ,  393  to supply different potentials (e.g. voltages) to the MPA  103 . For example, the base substrate  320  may be coupled to a first voltage source contact  391 , while the base spring layer  420  is coupled to a second voltage source contact  394 . In operation, the second voltage source contact  394  may be connected with a second voltage source (V B ) to supply an operating voltage for the transfer heads, while the first voltage source contact  391  is connected with a first voltage source (V A ) to hold the encapsulation membrane layer  540  at a different potential, or ground. Furthermore, a region  325  of the bas substrate  320  underneath the spring portion of the base spring layer  420 , including the spring platform  442 , and base spring arms  422  can be connected to a third voltage source contact  393 , and third voltage source which can be ground, the same voltage source (V A ) as the remainder of the base substrate  320 , or a different voltage source. 
     In an embodiment, a plug  380  extends through the base substrate  320  to electrically connect the base spring layer  420  and the second voltage source contact  391 . Additionally, a plurality of anchor plugs  410  can connect the base spring layer  420  to the plurality of plugs  380  extending through the base substrate  320 . The plugs  380  may be formed from the original base substrate  320 , or alternatively can be deposited. In such an embodiment, the plugs  380  can be polycrystalline material (e.g. polysilicon). Furthermore, the base spring layer  420 , mesa structures  355 , and encapsulation membrane layer  540 , as well as the anchor plugs  410  and vertical interconnects  530  between the base spring layer  420  and encapsulation membrane layer  540  can be formed of polycrystalline material, such as polysilicon. 
     Formation of the transfer heads of  FIGS.  11 C- 11 D  may begin with a base substrate  320  (e.g. silicon wafer) that has been processed to include plugs  380 , fill material  386 , lower insulating layer  322  on a top side of the base substrate, and back side insulation layer  372  on a back side of the base substrate  320 . Contact hole etching can then be performed to form openings in the lower insulating layer  322  to expose the base substrate and plugs  380 . An epitaxial layer is then formed over the lower insulating layer  322  and within the openings, and then patterned to form the base spring layer  420  (including base spring arms  422 , spring platform  442 ), anchor plugs  410 , and base contacts  411 . The epitaxial layer may be a polycrystalline layer, such as polycrystalline silicon (polysilicon). Thus, the base spring layer  420 , anchor plugs  410  and base contacts  411  may be a single layer. Anchor plugs  410  can be formed on and in contact with the plugs  380  to transfer the operating voltage to the mesa structures  355  to be formed. Base contacts  411  can be formed on and in contact with the base substrate  320  for transfer of potential to the encapsulation membrane layer  540  to be formed. Base contacts  411  may additionally be used in a further processing operation for removal of the sacrificial layers to form cavities  315 ,  515 . 
     The processing sequence is then repeated for the next epitaxial layer, where an upper insulating layer  326  is then formed over the lower insulating layer  322  and the base spring layer  420  and planarized, followed by patterning to form openings that will be the mesa structures  355  and vertical interconnects  530 . An epitaxial layer, which may be polycrystalline silicon, is then formed over the upper insulating layer  326  and within the openings, and then patterned to form the mesa structures  355  and encapsulation membrane layer  540 . Dielectric layer  360  may then be formed over the mesa structures  355  and encapsulation membrane layer  540 . This may optionally include forming a back side dielectric layer  361  of the same or different material on the back side insulation layer  372 . For example, the dielectric layer  360  and back side dielectric layer  361  may be aluminum oxide. This may be followed by patterning openings in the back side dielectric layer  361  and back side insulation layer  372  to expose the base substrate  320  and plugs  380 , and deposition of back side conductive (e.g. metal) layers  382  to form voltage source contacts  391 ,  394 ,  393 . 
     At this point openings (e.g. holes)  545  may be formed through the dielectric layer  361  and encapsulation membrane layer  540  to expose the upper insulating layer  326 , which is also connected to the lower insulating layer  322 . Openings  555  can also be formed around the mesa structures  355  to decouple the mesa structures from the deposited encapsulation membrane layer  540 . An etch release operation, e.g. vapor hydrofluoric acid (HF), may then be performed to remove portions of the upper insulating layer  326  and lower insulating layer  322  to form cavity  315  and upper cavity  515 . In accordance with embodiments, cavity area may be contained by base contacts  411  and vertical interconnects  530  (e.g. walls). 
     Referring again to  FIG.  11 C , the white areas illustrate the openings  545  used for etch release of the spring structure, as wells as openings  555  around the mesa structures  355  and spring gaps (e.g. base spring arms  422 ). The lighter shaded area A may correspond to the spring area of the base spring layer  420 . Area B may correspond to an overlap of the released encapsulation membrane layer  540  confined by lateral edges of the upper cavity  515 . Area C may correspond to the anchored regions where both the encapsulation membrane layer  540  and base spring layer  420  are rigidly anchored to the base substrate  320 . 
     In the foregoing discussion various transfer head structures have been described, including elastomeric stamps, monopolar transfer heads, bi-polar transfer heads, etc. with particular arrangements in clusters. In particular various aspects of the embodiments facilitate being adopted in a cluster arrangement with a dense grouping of transfer heads. However, it is be appreciated that while the embodiments may be applicable to cluster arrangements, the described transfer head structures may be implemented in other arrangements to facilitate high density transfer sequences and are not limited to cluster arrangements. 
     Referring now to  FIGS.  12 A- 12 B , a donor substrate  401  including an array of LEDs  404  is illustrated in  FIG.  12 A , and  FIG.  12 B  is an illustration of the donor wafer of  FIG.  12 A  after picking a plurality of clusters of LEDs  404  with an MPA  103  including a plurality of clusters  310  of transfer heads  115  in accordance with an embodiment, where the white areas  406  illustrate missing LEDs  404  after being picked. In application, the contact surfaces  363  of the transfer heads  115  may have approximately a same size as the LEDs  404 , though this is not necessarily required. Arrangements of the transfer heads  115  and clusters  310  may be pitch-matched with integer multiples of the pitch between adjacent LEDs  404 . As shown, the inter-cluster pitch (Ph) between transfer heads  115  may be an integer multiple (in this case one) of the pitch between adjacent LEDs  404 . In the illustrated example, the inter-cluster pitch (Pc) between clusters  310  of transfer heads  115  is an integer multiple (in this case four) of the pitch between adjacent LEDs  404 . Inter-cluster pitch (Pc) and intra-cluster pitch (Ph) are both illustrated as having the same dimensions in x-direction (e.g. row-wise) and y-direction (e.g. column-wise) though x-y dimensions may be different. It is to be appreciated that the illustrations in  FIGS.  12 A- 12 B  are not to actual scale and are for illustrational purposes only. Furthermore, the size and spacing of the clusters  310  of transfer heads  115  is provided as a specific implementation of the embodiments, though embodiments are not limited to these specific arrangements. 
     Once the LEDs  404  have been picked from the donor substrate  401 , the articulating transfer head assembly  200  including the MPA  103  with transfer heads  115  holding the LEDs  404  can be translated toward and positioned over a receiving substrate. During translation, the articulating transfer head assembly  200  and MPA  103  may pass over an upward facing inspection camera  120  as described with regard to  FIG.  1    to inspect the bottom surface of the MPA  103  to verify the pick operation efficacy. 
       FIG.  13    is a schematic side view illustration of an MPA  103  holding a plurality of clusters of LEDs  404  over a display substrate  502  in accordance with an embodiment. For a color display panel, the display substrate  502  may include arrays of landing pads  510  (e.g. driver pads) to receive LEDs  404  for different color emission. A bonding material  504 , such as a solder material can be provided on each landing pad  510  to help receive and bond each LED  404 . Illustrated is a simplified arrangement for a red-green-blue (RBG) display with landing pads  510  to receive red-emitting, green-emitting, and blue-emitting LEDs. Selection of RGB display is exemplary, and embodiments are not limited to a specific color set or arrangement pattern. 
     Still referring to  FIG.  13   , a particular operation is illustrated in which a plurality of clusters of LEDs  404  are being placed, and bonded, to a plurality of corresponding landing pads  510  corresponding to red-emitting subpixels. As shown, the adjacent subpixels are separated by an inter-subpixel pitch (Sp), and a cluster width (Wc) of the cluster  310  of transfer heads  115 , and also for the corresponding LEDs  404  held by the cluster  310  of transfer heads  115  may fit within the inter-subpixel pitch (Sp). As shown, where transfer heads  115  are designed to be approximately the same size as the LEDs  404 , the cluster width (Wc) for the corresponding LED and transfer head clusters may be approximately the same. 
     As shown in  FIG.  13   , a pixel density (PPI) of the display substrate  502  may affect the number of LEDs and transfer heads  115  that can fit within an inter-subpixel pitch (Sp). Furthermore, inter-subpixel pitch (Sp) may not always be the same in x-y dimensions. For example, inter-subpixel pitch (Sp) may be greater row-wise, and tighter column-wise. Alternatively, intra-subpixel pad pitch for same color-emitting LEDs may be less than inter-subpixel pitch (Sp) between different color-emitting LEDs. For example, this may occur where redundant LEDs are placed within a same subpixel. Furthermore, fabrication of the landing pads  510  and underlying topography of the display substrate  502  may be designed so that the non-bonding transfer heads  115  and LEDs  404  in this particular transfer operation do not make contact with any underlying structures. 
       FIG.  14 A  is a schematic top view illustration of an MPA including clusters of transfer heads holding clusters of LEDs  404  over a display substrate that includes landing pads  510  connected to distribution lines  512  in accordance with an embodiment.  FIG.  14 B  is a schematic cross-sectional side view illustration taken along line B-B of  FIG.  14 A  in accordance with an embodiment.  FIG.  14 C  is a schematic cross-sectional side view illustration taken along line C-C of  FIG.  14 A  in accordance with an embodiment. In particular,  FIGS.  14 A- 14 C  illustrate an implementation of a 3×3 arrangement of transfer heads  115  within a cluster  310 , where inter-subpixel pitch (Sp) is different in x-y dimensions. As shown, a row of transfer heads and corresponding LEDs  404  within a cluster  310  fits within the inter-subpixel pitch (Sp) in a row of subpixels. Specifically, the cluster width (Wc) of the cluster  310  of transfer heads  115 , and also for the three corresponding LEDs  404  fit within the x-direction inter-subpixel pitch (Sp). However, an intra-subpixel pad pitch (Pp) between pads for same color-emitting LEDs within same subpixels may be less than the inter-subpixel pitch (Sp), and also may be less than the cluster width (Wc) of the cluster  310  of transfer heads  115 , and also for the corresponding LEDs  404 . In the illustrated embodiment, two rows of transfer heads  115 , and corresponding rows of LEDs  404 , may fit within the intra-subpixel pad pitch (Pp) between landing pads  510  to accommodate the transfer sequence for all of the LEDs  404 . 
     Referring now to  FIGS.  15 A- 15 I  plan view illustrations are provided for a process sequence for single pick and multiple placement of three different color-emitting LEDs in accordance with embodiments, for example, to populate an RGB display panel. As shown, the display substrate includes an array of landing pads  510  arranged in groups of pixels  520 , including subpixels  522 ,  524 ,  526 . In an exemplary RGB display panel, the subpixels  522 ,  524 ,  526  may accommodate blue-emitting LEDs, red-emitting LEDs, and green-emitting LEDs, respectfully, to form an RGB display. Furthermore, each subpixel may include a pair of landing pads  510  to accommodate placement of redundant LEDs.  FIG.  16    is a process flow for a sequence of transferring a group of LEDs with a micro pick up array comprising a plurality of clusters of transfer heads in accordance with an embodiment. In interest of clarity and conciseness the following description of the process flow of  FIG.  16    is made with regard to the sequence illustrated in  FIGS.  15 A- 15 I . 
     At operation  1610  a first group of blue-emitting LEDs  404 B is picked up from a first donor substrate with an MPA  103  including a plurality of clusters  310  of transfer heads  115 . The MPA  103  is then positioned over a first location of a display substrate at operation  1620 , followed by placement of a first blue-emitting LED  404 B of the first group of LEDs from each cluster  310  onto the display substrate at operation  1630 . In the particular embodiment illustrated, the lower left-hand blue-emitting LED  404 B is placed onto a landing pad  510 , though this is merely exemplary and any LED within the clusters can be placed over a corresponding landing pad  510 . 
     Referring to  FIG.  15 B  the articulating transfer head assembly and MPA  103  are then positioned over a second location of the display substrate at operation  1640 , and a second blue-emitting LED  404 B of the first group of LEDs from each cluster  310  is placed onto the display substrate at operation  1650 . In the particular embodiment illustrated, the upper left-hand blue-emitting LED  404 B is placed onto a landing pad  510 , though any remaining LEDs within the top two rows of the clusters can be placed over a corresponding landing pad  510 . 
     The MPA  103  can then continue to be positioned over a new location of the display substrate, and another blue-emitting LED  404 B of the first group of LEDs from each cluster  310  can be placed onto the display substrate until all blue-emitting LEDs  404 B of the first group of LEDs held by the MPA  103  have been placed onto the display substrate.  FIG.  15 C  is an exemplary illustration of a display substrate populated with blue-emitting LEDs  404 B of the first group of LEDs. While the process sequence begins with blue-emitting LEDs  404 B, this is also merely an illustrative example and embodiments are not so required. 
     The MPA  103  can then be translated to a second donor substrate, followed by picking up a second group of LEDs (e.g. red-emitting LEDs  404 R) from the second donor substrate with the MPA  103  comprising the plurality of clusters  310  of transfer heads  115 . Referring to  FIG.  15 D , the MPA  103  can then be positioned over a third location of the display substrate, followed by placement of a first red-emitting LED  404 R from the second group of LEDs from each cluster  310  onto the display substrate. Referring to  FIG.  15 E , the MPA  103  is then positioned over a fourth location of the display substrate followed by placement of a second red-emitting LED  404 R of the second group of LEDs from each cluster  310  onto the display substrate. The MPA  103  can then continue to be positioned over a new location of the display substrate, and another red-emitting LED  404 R of the second group of LEDs from each cluster  310  can be placed onto the display substrate until all red-emitting LEDs  404 R of the second group of LEDs held by the MPA  103  have been placed onto the display substrate.  FIG.  15 F  is an exemplary illustration of a display substrate populated with blue-emitting LEDs  404 B of the first group of LEDs and red-emitting LEDs  404 R of the second group of LEDs. Referring to  FIGS.  15 G- 15 I , the process sequence can then be repeated again to cluster pick and sequential place a through group of LEDs, such as green-emitting LEDs  404 G onto the display substrate. 
     Referring briefly back to  FIG.  1   , the cluster pick and sequential placement sequences can be performed with multiple articulating transfer head assemblies  200  and corresponding MPAs  103  in order to further increase assembly throughput. For example, each articulating transfer head assembly  200  and corresponding MPA  103  can pick up a corresponding group of LEDs from the same donor substrate, then both be translated toward the display substrate, followed by sequential placement of the first group of LEDs held by the first MPA  103  onto the display substrate, then sequential placement of the second group of LEDs held by the second MPA  103  onto the display substrate. 
       FIG.  17    is a process flow for a sequence of transferring multiple groups of LEDs with multiple articulating transfer head assemblies  200  in accordance with an embodiment. At operation  1710  a first group of LEDs is picked up from a first donor substrate with a first articulating transfer head assembly, followed by picking up a second group of LEDs from the same donor substrate with a second articulating transfer head assembly at operation  1720 . For example, this may include picking up the first group of LEDs with a first MPA  103  including a first plurality of clusters  310  of transfer heads  115 , and picking up the second group of LEDs with a second MPA  103  including a second plurality of clusters  310  of transfer heads  115 . The first and second articulating transfer head assemblies are then translated (e.g. along translation track  110 ) toward a display substrate at operation  1730 . This may include translating both the first and second articulating transfer head assemblies over an inspection camera  120 . 
     At operation  1740  the first articulating transfer head assembly, and corresponding MPA  103 , is positioned over the display substrate, followed by placing the first group LEDs onto the display substrate at operation  1750 . For example, this may be a sequential placement sequence as illustrated in  FIGS.  15 A- 15 C , for example. At operation  1760  the second articulating transfer head assembly, and corresponding MPA  103 , is positioned over the display substrate, followed by placing the second group of LEDs onto the display substrate at operation  1770 . For example, this may also be a sequential placement sequence as illustrated in  FIGS.  15 D- 15 F  or  FIGS.  15 G- 15 I , for example. The multiple articulating transfer head assembly transfer sequence can also be performed without the cluster pick and sequential placement operations. 
     In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for transferring an array of micro devices. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration. In particular, while the above embodiments have been specifically described with regard to LEDs, and more particularly to micro LEDs, the MTT  100  and sequences can also be applied to other applications to increase throughput for the population of devices, and specifically micro devices. Accordingly, the above descriptions and illustrations of LEDs and display substrates are generically applicable to other micro device applications and receiving substrates that can be populated using the MTT  100  and transfer sequences described.

Metadata:
Filing Date: 20210611
Publication Date: 20240402
Grant Date: 20240402
Priority Date: 20200713
Inventors: MANENS, ANTOINE
GOLDA, DARIUSZ
KIM, HYEUN-SU
Assignee: APPLE INC
CPC Classifications: [{"code": "H10H20/0364", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/857", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10H20/857", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/01", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/67144", "inventive": true, "first": true, "tree": "[]"}, {"code": "B65G47/90", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L25/0753", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L33/62", "inventive": true, "first": false, "tree": "[]"}, {"code": "B41F16/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2933/0066", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/67144", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L21/67144", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L25/0753", "inventive": false, "first": false, "tree": "[]"}, {"code": "B65G47/91", "inventive": true, "first": false, "tree": "[]"}, {"code": "B65G47/918", "inventive": true, "first": false, "tree": "[]"}, {"code": "B65G47/92", "inventive": true, "first": false, "tree": "[]"}, {"code": "B41F16/008", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L25/0753", "inventive": true, "first": false, "tree": "[]"}, {"code": "B41F16/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "B65G47/90", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 79173810