PATENT DOCUMENT

Publication Number: US-9484237-B2
Application Number: US-201514814307-A
Country: US
Kind Code: B2

Title: Mass transfer system

Abstract:
Micro pick up arrays for transferring micro devices from a carrier substrate are disclosed. In an embodiment, a micro pick up array includes a compliant contact for delivering an operating voltage from a voltage source to an array of electrostatic transfer heads. In an embodiment, the compliant contact is moveable relative to a base substrate of the micro pick up array.

Claims:
What is claimed is: 
     
       1. A system comprising:
 a transfer head assembly including an operating voltage contact and a clamping voltage contact; and 
 a micro pick up array including a base substrate, a plug formed through the base substrate, and an array of electrostatic transfer heads electrically coupled with the plug; 
 wherein the operating voltage contact is alignable with the plug and the clamping voltage contact is alignable with a backside of the micro pick up array opposite the array of electrostatic transfer heads on a frontside of the micro pick up array. 
 
     
     
       2. The system of  claim 1 , wherein a gap separates the plug from the base substrate, and the array of electrostatic transfer heads are electrically coupled with the plug. 
     
     
       3. The system of  claim 1 , wherein the transfer head assembly comprises a plurality of operating voltage contacts, and the micro pick up array comprises a plurality of plugs, wherein the plurality of operating voltage contacts are alignable with the plurality of plugs. 
     
     
       4. The system of  claim 1 , wherein each electrostatic transfer head comprises: a mesa structure having an electrode surface, and a dielectric layer covering the electrode surface. 
     
     
       5. The system of  claim 2 , wherein the gap is filled with a dielectric material. 
     
     
       6. The system of  claim 2 , wherein the plug is movable relative to the base substrate.

Description:
RELATED APPLICATIONS 
     This application is a divisional of co-pending U.S. patent application Ser. No. 13/909,892, filed on Jun. 4, 2013, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to micro devices. More particularly embodiments of the present invention relate to micro pick up arrays having compliant contacts. 
     2. 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 diode (LED) display systems, and MEMS or quartz-based oscillators. 
     Traditional technologies for transferring devices include, e.g., “transfer printing”, which involves using a transfer wafer to pick up an array of devices from a donor wafer. The array of devices are then bonded to a receiving wafer before removing the transfer wafer. Some transfer printing process variations have been developed to selectively bond and de-bond a device during the transfer process. In both traditional and variations of the 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. 
     More recently it has been proposed to transfer a semiconductor die from a host substrate to a target substrate using elastomeric stamps in which a stamp surface adheres to a semiconductor die surface via van der Waals forces. 
     SUMMARY OF THE INVENTION 
     Micro pick up arrays for transferring micro devices from a carrier substrate are disclosed. In an embodiment, a micro pick up array includes a base substrate having a via, a flexible membrane over the via, and a plug supported by the flexible membrane and moveable relative to the base substrate within the via. The flexible membrane may include a silicon layer and be deflectable such that the plug is moveable by not more than 5 μm along an axis orthogonal to the flexible membrane, relative to the base substrate. A gap may separate the plug from the base substrate. In an embodiment, a breakdown voltage of the gap may be greater than 100 volts at ambient pressure. For example, the gap may separate the plug from the base substrate by more than 10 μm to achieve the breakdown voltage. 
     In an embodiment, an array of electrostatic transfer heads may be electrically coupled with the plug. The electrostatic transfer heads may be deflectable into a cavity in the base substrate. Each electrostatic transfer head may include a mesa structure with an electrode surface covered by a dielectric layer. Each electrostatic transfer head may also include a second electrode surface covered by the dielectric layer adjacent the electrode surface. An electrode interconnect may electrically couple the electrode surface with the plug. Likewise, a second electrode interconnect may electrically couple the second electrode surface with a second plug. For example, the electrode interconnect may couple with a topside contact on the plug. The topside contact may contact the plug over a contact area that is coplanar with a topside plug area and is less than two-thirds of the topside plug area. A contact pad may be on the plug opposite the topside contact and may be electrically coupled with the topside contact through the plug. An electrical resistance across the plug between the contact pad and topside contact may be in a range between 1 and 100 kiloohms. 
     In an embodiment, a method of forming a micro pick up array includes etching a top silicon layer of a silicon-on-insulator (SOI) stack to form an array of electrodes and etching through a bulk silicon substrate of the SOI stack to a buried oxide layer of the SOI stack to form a gap separating a plug and a base substrate of the bulk silicon substrate. The plug may be moveable relative to the base substrate. The base substrate may also be etched to form one or more cavities directly underneath the array of electrodes such that one or more electrodes is deflectable into the one or more cavities. The method of forming the micro pick up array may also include etching the top silicon layer to form an electrode interconnect, forming a dielectric layer over the array of electrodes, and forming a topside contact on the bulk silicon substrate. Forming the dielectric layer may include thermal oxidation of the array of electrodes. Alternatively, forming the dielectric layer may include blanket depositing the dielectric layer using atomic layer deposition or depositing the dielectric layer using chemical vapor deposition. The method of forming the micro pick up array may also include etching through the dielectric layer, the electrode interconnect, and the buried oxide layer to expose the plug of the bulk silicon substrate. The topside contact may be formed on the exposed area of the plug. The topside contact may be electrically coupled with the array of electrodes through the electrode interconnect. The method of forming the micro pick up array may also include etching through a backside oxide layer of the SOI stack to expose the plug of the bulk silicon substrate and forming a contact pad on the plug of the bulk silicon substrate opposite the topside contact. The contact pad may be electrically coupled with the topside contact through the plug. 
     In an embodiment, a system includes a transfer head assembly and a micro pick up array. The transfer head assembly may include one or more operating voltage contacts and a clamping voltage contact. The micro pick up array may include a base substrate, one or more compliant contacts formed through the base substrate, and an array of electrostatic transfer heads on a frontside of the micro pick up array electrically coupled with the one or more compliant contact. The one or more operating voltage contacts may be alignable with the one or more compliant contacts and the clamping voltage contact may be alignable with a backside of the micro pick up array opposite the array of electrostatic transfer heads. Accordingly, when a clamping voltage is applied to the clamping voltage contact the micro pick up array is retained against the transfer head assembly and the plug moves relative to the base substrate. 
     In an embodiment, the micro pick up array may also include a via in the base substrate, a flexible membrane over the via, and a plug supported within the via by the flexible membrane. A gap may separate the plug from the base substrate and the plug may be movable relative to the base substrate. The array of electrostatic transfer heads may be electrically coupled with the plug. Furthermore, each electrostatic transfer head may include a mesa structure having an electrode surface, and a dielectric layer covering the electrode surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustration of a transfer head assembly holding a micro pick up array with a compliant contact in accordance with an embodiment of the invention. 
         FIG. 2A  is a plan view illustration of a micro pick up array having an array of monopolar electrostatic transfer heads in accordance with an embodiment of the invention. 
         FIG. 2B  is a plan view illustration of a micro pick up array having an array of bipolar electrostatic transfer heads in accordance with an embodiment of the invention. 
         FIG. 3  is a combination cross-sectional side view illustration taken along lines A-A, B-B, and C-C of  FIG. 2B  illustrating a micro pick up array having an array of electrostatic transfer heads electrically coupled with a compliant contact in accordance with an embodiment of the invention. 
         FIG. 4A  is a cross-sectional side view illustration taken along a portion of line B-B or C-C of  FIG. 2B  illustrating a compliant contact in accordance with an embodiment of the invention. 
         FIG. 4B  is a cross-sectional side view illustration taken along a portion of line B-B or C-C of  FIG. 2B  illustrating a compliant contact with a dielectric-filled gap in accordance with an embodiment of the invention. 
         FIG. 5  is a perspective view illustration of a topside portion of a micro pick up array having a compliant contact in accordance with an embodiment of the invention. 
         FIG. 6A  is a cross-sectional side view illustration of a moveable portion of a micro pick up array having a compliant contact supported by a flexible membrane in accordance with an embodiment of the invention. 
         FIG. 6B  is a cross-sectional side view illustration of a moveable portion of a micro pick up array having a load applied to a compliant contact supported by a flexible membrane in opposition to a clamping force applied to a clamping area of the micro pick up array in accordance with an embodiment of the invention. 
         FIGS. 7-24  illustrate a method of forming a micro pick up array having an array of electrostatic transfer heads electrically coupled with a compliant contact in accordance with an embodiment of the invention. 
         FIG. 25  is a cross-sectional side view illustration of a system having a micro pick up array and a transfer head assembly in accordance with an embodiment of the invention. 
         FIG. 26  is a schematic top view illustration of contacts between a micro pick up array and a transfer head assembly in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention describe apparatuses and methods for transferring a micro device or an array of micro devices. For example, the micro device or array of micro devices may be any of the micro LED device or micro chip structures illustrated and described in related U.S. patent application Ser. Nos. 13/372,222, 13/436,260, 13/458,932, and 13/711,554. While some embodiments of the present invention are described with specific regard to micro LED devices, the embodiments of the invention are not so limited and certain embodiments may also be applicable to other micro LED devices and micro devices such as diodes, transistors, integrated circuit (IC) chips, and MEMS. 
     In various embodiments, description is made with reference to the figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions, and processes, in order to provide a thorough understanding of the present invention. In other instances, well-known processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the present invention. Reference throughout this specification to “one embodiment,” “an embodiment”, or the like, means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “one embodiment,” “an embodiment”, or the like, in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “over”, “to”, “between”, and “on” as used herein may refer to a relative position of one layer or component with respect to other layers or components. One layer “over” or “on” another layer or bonded “to” another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     Without being limited to a particular theory, embodiments of the invention describe a micro pick up array supporting an array of electrostatic transfer heads which operate in accordance with principles of electrostatic grippers, using the attraction of opposite charges to pick up micro devices. In accordance with embodiments of the present invention, a pull-in voltage is applied to an electrostatic transfer head in order to generate a grip pressure on a micro device. The terms “micro” device or “micro” LED devices as used herein may refer to the descriptive size of certain devices or structures in accordance with embodiments of the invention, such as on a scale of 1 to 100 μm. However, embodiments of the present invention are not necessarily so limited, and certain aspects of the embodiments may be applicable to larger, and possibly smaller size scales. In an embodiment, a single micro device in an array of micro devices, and a single electrostatic transfer head in an array of electrostatic transfer heads both have a maximum dimension, e.g., a length or a width of a contact surface, of 1 to 100 μm. In an embodiment, a pitch of an array of micro devices, and a pitch of a corresponding array of electrostatic transfer heads is (1 to 100 μm) by (1 to 100 μm). At these densities a 6 inch carrier 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 transfer tool including a micro pick up array and 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, transfer, and bond 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, transfer, and bond more than 100,000 micro devices per transfer operation, with larger arrays of electrostatic transfer heads being capable of transferring more micro devices. 
     In one aspect, embodiments of the invention describe a micro pick up array having an array of electrostatic transfer heads and one or more compliant contacts. The array of electrostatic transfer heads may be supported by a base substrate having a via. The compliant contact may include a flexible membrane over the via and supporting a plug within the via. The back side of the plug can be physically coupled with a transfer head assembly that can be used to position the micro pick up array including the array of electrostatic transfer heads. When a clamping force is applied to a clamping area on a backside of the micro pick up array, an operating voltage contact of the transfer head assembly may apply an opposing reactive load to the plug, causing the flexible membrane to deflect. Deflection of the flexible membrane may result in the base substrate moving around the plug to produce relative movement between the plug and the base substrate. Thus, while the micro pick up array is secured to the transfer head assembly by the clamping force, the reactive load creates compressive loading and pressure between the operating voltage contact of the transfer head assembly and the plug, such that a uniform electrical contact is provided therebetween. 
     In another aspect, embodiments of the invention describe a manner of forming a micro pick up array having an array of electrostatic transfer heads and one or more compliant contacts from a commercially available silicon-on-insulator (SOI) stack. Embodiments of the invention describe forming portions of the micro pick up array, e.g., an array of electrodes, an electrode interconnect, and one or more compliant contacts, etc., from the SOI stack using semiconductor device fabrication processes. 
     In another aspect, embodiments of the invention describe applying a voltage through the compliant contacts to the electrostatic transfer heads to create a gripping pressure between the array of electrostatic transfer heads and an array of micro devices. More specifically, an electrostatic charge may be generated at the array of electrostatic transfer heads to grip the array of micro devices during transfer. Furthermore, the electrostatic charge may be maintained by a voltage delivered to the electrostatic transfer heads through a plug of a compliant contact. Since the electrode circuit may operate under electrostatic conditions during most of the transfer operation, the plug may be considered to transfer an electrostatic voltage, rather than an electrical current. Thus, the pick up and placement of micro devices may be relatively insensitive to a response time of the electrode circuit and/or the plug. As a result, in an embodiment, an electrical resistance across the plug may be in a range higher than 1 to 1,000 ohms without compromising pick up and placement. 
     Referring now to  FIG. 1 , a perspective view of a transfer head assembly holding a micro pick up array with a compliant contact is illustrated in accordance with an embodiment of the invention. Transfer head assembly  102  may be a component of a larger system, such as a mass transfer tool used to transfer micro devices from a carrier substrate to a receiving substrate using micro pick up array  104 . Transfer head assembly  102  may retain micro pick up array  104  in numerous manners, including clips, vacuum ports, and by clamping one or more clamping areas on a backside surface of micro pick up array  104  with an electrostatic gripping pressure. For example, in an embodiment, transfer head assembly  102  may include an electrostatic clamping contact that can receive an electrostatic voltage from a voltage source. The clamping contact may physically appose a clamping pad or clamping area on a backside surface of micro pick up array  104 . Thus, micro pick up array  104  may be gripped and retained against the transfer head assembly  102  by the clamping contact. 
     In addition to delivering an electrostatic voltage to the clamping contact on the transfer head assembly  102  to grip the micro pick up array  104 , the transfer head assembly  102  may deliver one or more electrostatic transfer head operation voltages to from voltage sources  106 ,  206  to voltage interconnects  108  of micro pick up array  104 . Voltage interconnects  108  may be compliant contacts. In addition to being compliant contacts, voltage interconnects  108  may also relay electrostatic voltage through micro pick up array  104  into electrode interconnects  112  toward an array of electrostatic transfer heads  114 . Thus, micro pick up array  104  may include compliant contacts that are both compliant and able to transfer electrostatic voltage through micro pick up array  104 . 
     Referring now to  FIG. 2A , a plan view illustration of a micro pick up array having an array of monopolar electrostatic transfer heads is illustrated in accordance with an embodiment of the invention. The micro pick up array  104  may include a plurality of electrostatic transfer heads  114  formed in an array on a front side surface. Each electrostatic transfer head  114  may be electrically coupled with an electrode interconnect  112  running over the front side surface and placed in electrical connection with a voltage interconnect  108 . The voltage interconnect  108  may include numerous structures, which are described further below and allow for the transfer of voltage from a back side surface of the micro pick up array  104  to the front side surface. For example, in an embodiment, voltage interconnect  108  includes a compliant contact having a plug and a flexible membrane. Thus, when micro pick up array  104  is electrically coupled with voltage source  106 , a voltage can be transferred to electrode surface  202  on electrostatic transfer head  114 . 
     In the embodiment illustrated in  FIG. 2A , the voltage interconnect  108  on the left side of the illustration may be connected to voltage source  106  denoted V A , and the voltage interconnect  108  on the right side of the illustration may be connected to a voltage source  206  denoted V B . Alternatively, the voltage interconnect  108  on the right side of the illustration may connect to the voltage source  106  denoted V A . Where each transfer head is operable as a monopolar transfer head, voltage sources  106  denoted V A  and  206  denoted V B  may simultaneously apply the same voltage so that each electrode surface  202  has the same voltage. However, as described below, this arrangement for monopolar electrostatic transfer heads  114  is not limiting. 
     Referring now to  FIG. 2B , a plan view illustration of a micro pick up array  104  having an array of bipolar electrostatic transfer heads  114  is illustrated in accordance with an embodiment of the invention. As in  FIG. 2A , each electrostatic transfer head  114  may be electrically coupled with voltage interconnects  108  through electrode interconnects  112 . The voltage interconnects  108  may include a compliant contact, as in  FIG. 2A . However, in the embodiment illustrated in  FIG. 2B , each electrostatic transfer head  114  is bipolar, and includes electrode surface  202  and second electrode surface  204 . Thus, in an embodiment, the upper and lower electrode interconnects  112  in the illustration may be connected to a voltage source  106  denoted V A , and the middle electrode interconnect  112  in the illustration may be connected to a second voltage source  206  denoted V B . Where each electrostatic transfer head  114  is operable as a bipolar transfer head, voltage source  106  denoted V A  may simultaneously apply a voltage to electrode surface  202  that is opposite to a voltage applied to second electrode surface  204  by second voltage source  206  denoted V B . Thus, each electrostatic transfer head  114  may include a pair of oppositely charged electrodes, leading to enhanced gripping pressures on corresponding micro devices. For example, gripping pressures between each bipolar electrostatic transfer head  114  and a corresponding micro device can be about 20 atm or higher. 
     The monopolar and bipolar electrostatic transfer head configurations may be interchangeable in various embodiments of micro pick up array  104 . Indeed, micro pick up array  104  may include alternative patterns for the array of electrostatic transfer heads  114 , electrode interconnects  112 , etc., depending on the available space on transfer head assembly  102 , the micro device pattern on the carrier substrate, the bonding pattern on the receiving substrate, and other features incorporated in micro pick up array  104 . For example, micro pick up array  104  may optionally include features such as flexible cantilever beams  210  that suspend electrostatic transfer heads  114  over one or more cavities  212  underneath the array of electrostatic transfer heads  114 . Electrode interconnects  112  may be routed over or within flexible cantilever beams  210  over cavities  212 . 
     Although the description below is made in relation to a bipolar electrode configuration, the description is also applicable to other electrode configurations, e.g., monopolar electrode configurations. Furthermore, although the description below is made in relation to micro pick up array  104  incorporating cavities  212 , such features are not required. The compliant contacts described below may be incorporated into a variety of micro pick up array designs and are not limited to the specific micro pick up array embodiments described and illustrated herein. 
     Referring now to  FIG. 3 , a combination cross-sectional side view illustration is taken along lines A-A, B-B, and C-C of  FIG. 2B  illustrating a micro pick up array having an array of electrostatic transfer heads electrically coupled with a pair of compliant contacts in accordance with an embodiment of the invention. The combination views do not precisely represent the sizes or locations of the features of micro pick up array  104 , but rather, are intended to combine features into a single view for ease of description. For example, while the combination cross-sectional side view illustrations show voltage interconnect  108  of  FIG. 2B  having plug  304 , contact pad  306 , and topside contact  307  electrically connected with only one electrode surface  202  through electrode interconnect  112 , it is clear from  FIG. 2B  and the accompanying description that voltage interconnect  108  may be electrically connected with several electrode surfaces  202  through one or more electrode interconnects  112 . 
     In an embodiment, the cross-section taken along line A-A corresponds to a portion of micro pick up array  104  that includes a bipolar electrostatic transfer head  114 . The bipolar electrostatic transfer head  114  includes electrode surface  202  and second electrode surface  204 , both over a top surface of mesa structures  311 . A dielectric layer  312  may cover electrode surface  202  and second electrode surface  204 , and may also cover a side surface of mesa structures  311  laterally between the pair of mesa structures  311  for the pair of electrodes in a bipolar electrostatic transfer head  114 . Thus, the top surface of dielectric layer  312  over electrode surface  202  and second electrode surface  204  is offset from, e.g., above electrode interconnect  112 , and provides a raised contact point for pressing against a micro device on a carrier substrate or receiving substrate. 
     In an embodiment, dielectric layer  312  and buried oxide layer  314  surround and separate mesa structures  311  and electrode interconnect  112  of individual electrode circuits from each other and from other portions of micro pick up array  104  to isolate a desired pathway between voltage sources  106 ,  206  and respective electrostatic transfer heads  114 , and to prevent shorting between electrode surfaces  202 ,  204 , electrode interconnects  112 , and voltage interconnects  108  that are maintained at different electrical potentials. 
     The embodiment illustrated in  FIG. 3  includes electrostatic transfer head  114  supported above cavity  212  by a flexible cantilever beam  210 , such that electrostatic transfer head  114  is deflectable into cavity  212 . In other embodiments, cavity  212  is not present. 
     Referring now to  FIG. 4A , a cross-sectional side view illustration taken along a portion of line B-B or C-C of  FIG. 2B  illustrates a compliant contact in accordance with an embodiment of the invention. More specifically, the cross-section taken along lines B-B or C-C corresponds to a portion of micro pick up array  104  that includes voltage interconnect  108  having a compliant contact. Thus, voltage interconnect  108  transfers voltage from a voltage source  106  or  206  to electrode interconnect  112 , but may also be moveable relative to other portions of micro pick up array  104 , e.g., base substrate  214  or electrostatic transfer head  114 . In an embodiment, base substrate  214  includes via  402  extending from a backside surface of micro pick up array  104  to buried oxide layer  314 . Via  402  may have numerous cross-sectional shapes, for example, via  402  may be cylindrical and have a circular cross-section. Alternatively, the cross-section of via  402  may be rectangular, rectangular with rounded corners, oval, etc. 
     In an embodiment, via  402  is partially filled by plug  304 , which extends through via  402  from buried oxide layer  314  and is laterally separated from the surrounding base substrate  214  by gap  308 . Plug  304  may be formed separately or simultaneously with via  402 . For example, in an embodiment, plug  304  may be deposited onto a backside surface of buried oxide layer  314  through via  402 . In an alternative embodiment, gap  308  may be formed by etching through a bulk silicon substrate, and thus, plug  304  is defined by the removal of material occupying gap  308 . Regardless of the method used to form via  402  and plug  304 , gap  308  may surround the periphery of plug  304 , resulting in plug  304  being coupled with base substrate  214  by flexible membrane  310 . As illustrated in  FIG. 4A , the width of flexible membrane  410  may be represented by the gap  308  surrounding the periphery of plug  304 . For example, where via  402  and plug  304  are circular, the width of flexible membrane  410  may be the difference in the radii of the via  402  and plug  304 . 
     Since gap  308  may extend around the periphery of plug  304 , it may provide a dielectric barrier between plug  304  and base substrate  214 . More particularly, gap  308  may prevent discharge from plug  304  to base substrate  214  when a voltage is applied to plug  304  from voltage source  106  or  206 . To function as a dielectric barrier, gap  308  may be shaped and sized depending on the operating voltage of micro pick up array  104 . For example, in some embodiments, micro pick up array  104  operates with an electrostatic voltage of between about 100 to 150 volts applied through contact pad  306  and plug  304  to electrostatic transfer heads  114 . Accordingly, gap  308  may be an air-filled space around plug  304  with a breakdown voltage of at least 100 volts at ambient pressure. In an embodiment, assuming that the breakdown voltage of air is about 327 volts at standard atmospheric pressure across a gap distance of 7.5 μm, gap  308  distance may be maintained higher than about 10 μm to prevent discharge across gap  308 . In an embodiment, the minimum distance across gap  308  may be between about 10 and 300 μm or more to prevent breakdown at normal operating conditions. More specifically, the minimum distance across gap  308  may be chosen to be about 20 μm. 
     Plug  304  may be concentrically located within via  402  such that gap  308  is uniformly distributed around plug  304  periphery. Alternatively, plug  304  may be configured within via  402  such that gap  308  distance between plug  304  and base substrate  214  varies. For example, via  402  may be shaped differently from plug  304 , or plug  304  may be eccentrically located within via  402 , such that gap  308  distance varies. Nonetheless, a minimum distance across the gap  308  may be controlled to achieve the required breakdown voltage and to accommodate the operating voltage delivered through plug  304 . 
     Referring now to  FIG. 4B , a cross-sectional side view illustration taken along a portion of line B-B or C-C of  FIG. 2B  illustrates a compliant contact with a dielectric-filled gap in accordance with an embodiment of the invention. In alternative embodiments, the breakdown voltage of the gap  308  may be controlled by introducing a suitable dielectric substance into gap  308 . For example, gap  308  may be filled with a fluid that deforms under shear stress. For example, gap  308  may be filled with a liquid dielectric  406 , such as a silicone oil, that does not impede relative movement between plug  304  and base substrate  214 , but which also has a higher dielectric constant than air and allows for the distance across gap  308  to be narrowed, as compared to gap  308  filled with air, while still maintaining the requisite breakdown voltage of gap  308 . The gap can be filled with liquid dielectric  406  by, for example, dispensing liquid dielectric  406  into gap  308  using an air-powered fluid dispenser, a syringe, or another type of dispenser that can inject controlled volumes of fluid into small areas. Depending on the viscosity of liquid dielectric  406  that is inserted into gap  308 , there may be a need to retain liquid dielectric  406 . For example, in the case where surface tension alone is unable to keep liquid dielectric  406  from flowing out of gap  308 , a seal  408  may be formed over or within gap  308  to prevent liquid dielectric  406  from leaving gap  308 . In an embodiment, seal  308  may include a flexible adhesive material, such as a silicone polymer, deposited as a thin layer within gap  308  to bond with base substrate  214  and plug  304  while retaining liquid dielectric  406 . Seal  408  may be thin and flexible so as not to impede relative movement between plug  304  and base substrate  214 . 
     In an alternative embodiment, non-liquid dielectrics, such as solid or gaseous dielectric materials may be introduced into and sealed within gap  308 . For example, gap  308  may be at least partially filled with a solid dielectric including polymers such as acrylic, polyimide, or epoxies. The polymer dielectric may be introduced into gap  308  using an ink-jetting process. 
     In  FIGS. 4A-4B , flexible membrane  310  may permit relative movement between plug  304  and base substrate  214 . In an embodiment, flexible membrane  310  may be sized to flex when opposing loads are applied to plug  304  and base substrate  214 . The physical dimensions and material properties of top silicon layer  404  and gap  308  may be the leading contributors to the overall stiffness and flexibility of flexible membrane  310 . In an embodiment, an overall thickness of flexible membrane  310  includes portions of top silicon layer  404 , buried oxide layer  314 , and dielectric layer  312  that are located over via  402 . In an embodiment, the width of flexible membrane  310  may be between about 10 to 50 times the overall thickness of flexible membrane  310 . For example, where the width of the flexible membrane is 10 times the overall thickness of flexible membrane  310 , the thickness is about 5 μm while, as described above, flexible membrane  310  width may be about 50 μm. 
     Referring now to  FIG. 5 , a perspective view illustration of a topside portion of a micro pick up array having a compliant contact is illustrated in accordance with an embodiment of the invention. In an embodiment, electrode interconnect  112  includes an electrode trace, wire, or other connector electrically connected with topside contact  307 . For example, electrode interconnect  112  may run over buried oxide layer  314  and base substrate  214  from mesa structure  311  to topside contact  307 . A path of electrode interconnect  112  may vary depending on the topside geometry of micro pick up array  104 , taking into account features such as flexible cantilever beams  210  supporting electrostatic transfer heads  114 . Therefore, electrode interconnect  112  pattern may include various bends, curves, etc. Furthermore, dielectric layer  312  may cover electrode interconnect  112 . In contrast, rather than being covered by dielectric layer  312 , topside contact  307  may instead extend through dielectric layer  312 , electrode interconnect  112 , and buried oxide layer  314 , to a topside plug area  504 . 
     Topside plug area  504  is represented with hidden lines to illustrate that it may be supported by flexible membrane  310  and under buried oxide layer  314 . Topside plug area  504  may correspond to a portion of plug  304  that apposes buried oxide layer  314 . Thus, topside contact  307  may contact topside plug area  504  over contact area  506 . Contact area  506  may be proportionally less than topside plug area  504  because contact area  506  may be no larger than plug  304  width and because minimizing contact area  506  mitigates the risk of buried oxide layer  314  delaminating from topside plug area  504 . In an embodiment, contact area  506  may be less than about half of topside plug area  504 . For example, contact area  506  may have an effective diameter of between about 50 to 100 μm while topside plug area  504  may have an effective diameter of between about 300 to 500 μm. However, other contact area  506  and topside plug area  504  dimensions may be used to similarly minimize the ratio between contact area  506  and topside plug area  504 , and to provide a strong interface between topside plug area  504  and buried oxide layer  314 . 
     Topside contact  307  may also transfer voltage. In an embodiment, topside contact  307  provides an electrical pathway from plug  304  to electrode interconnect  112  through buried oxide layer  314  without considerably compromising the function of flexible membrane  310 . To provide this pathway, topside contact  307  may be formed from various conductive materials, such as gold, NiCr, Cr, TiW, Ti, Al, alloys thereof or polysilicon, that provide for electrical conductivity between plug  304  and electrode interconnect  112 . 
     As described above with regard to the structures shown along lines A-A and B-B of  FIG. 3 , voltage interconnect  108  may include contact pad  306  on a backside surface of plug  304 . Contact pad  306  may be electrically coupled with a corresponding operating voltage contact of transfer head assembly  102  to transfer voltage from voltage source  106  or  206 . Thus, voltage may be delivered through contact pad  306  into plug  304 , and toward topside contact  307  on topside plug area  504 . Topside contact  307  may further be electrically coupled with electrode interconnect  112 , and resultantly, voltage may be delivered from voltage source  106  or  206  through plug  304  and electrode interconnect  112  to electrode surface  202 . Furthermore, voltage source  106  or  206  may transfer voltage to second electrode surface  204  of a bipolar electrostatic transfer head  114  in a similar manner using corresponding structures shown along lines A-A and C-C of  FIG. 3 . 
     Operation of micro pick up array  104  may include the application and removal of voltage to and from the array of electrostatic transfer heads  114 . For example, voltage may be applied to electrostatic transfer heads  114  through plug  304  to grip micro devices and the voltage may be removed from electrostatic transfer heads  114  to release micro devices. This application and removal may be accompanied by a spike in electrical current as charge is generated or dissipated in the array of electrostatic transfer heads  114 . However, during steady state operation of the array of electrostatic transfer heads  114 , minimal or no current is required to be delivered through plug  304  since the charge can be maintained with minimal power draw from voltage source  106  or  206 . Therefore, electrical resistance across plug  304  between contact pad  306  and topside contact  307  may be less than about 25 kiloohms without degrading the RC time constant of an electrode circuit to a point that micro pick up array  104  is unable to transfer micro devices in the manner described below. More specifically, since the pick up and placement of micro devices occurs over relatively long periods of time, e.g., seconds, as compared to the response time of the electrode circuit, e.g., microseconds, resistance across plug  304  may be increased without disrupting the ability to pick up or place the micro devices. For example, electrical resistance across plug  304  between contact pad  306  and topside contact  307  may be in a range higher than 1 to 1,000 ohms. In an embodiment, electrical resistance across plug  304  may be in the megaohm range without compromising the transfer of micro devices as described in the following description. More specifically, in an embodiment, plug  304  has a nominal resistance value in a range of about 1 to 100 kiloohms. 
     Referring now to  FIG. 6A , a cross-sectional side view illustration of a moveable portion of a micro pick up array having a compliant contact supported by a flexible membrane is illustrated in accordance with an embodiment of the invention. Prior to attaching micro pick up array  104  to transfer head assembly  102 , i.e., when no external loads are being applied to micro pick up array  104 , flexible membrane  310  may have sufficient resilience to flatten across gap  308  and bring plug  304  into alignment with base substrate  214  relative to axis  302 . 
     Referring now to  FIG. 6B , a cross-sectional side view illustration of a moveable portion of a micro pick up array having a load applied to a compliant contact supported by a flexible membrane in opposition to a clamping force applied to a clamping area of the micro pick up array is illustrated in accordance with an embodiment of the invention. When micro pick up array  104  is clamped to transfer head assembly  102 , e.g., by applying an electrostatic clamping load  601  to pull a clamping area over base substrate  214  toward a clamping contact of transfer head assembly  102 , reactive load  602  may be applied to plug  304  by an operating voltage contact of transfer head assembly  102 . This reactive load may be applied, for example, due to a mismatch in position between a surface of the clamping contact and a surface of the operating voltage contact. More specifically, the operating voltage contact may extend further from transfer head assembly  102  than the clamping contact. Accordingly, the operating voltage contact touches contact pad  306  before the clamping contact touches the clamping area over base substrate  214  and flexible membrane  310  encounters a bending moment that causes it to deflect. This deflection permits plug  304 , which floats within via  402 , to move relative to base substrate  214 . As flexible membrane  310  deflects and plug  304  moves, both base substrate  214  and plug  304  remain in contact with the clamping contact and operating voltage contact of transfer head assembly  102 , respectively. More specifically, flexible membrane  310  accommodates relative movement between base substrate  214  and plug  304  to allow micro pick up array  104  to be secured to transfer head assembly  102  while establishing an electrical connection between plug  304  and voltage sources  106 ,  206 . 
     The deflection of flexible membrane  310 , and thus the movement of base substrate  214  relative to plug  304 , depends on numerous characteristics of the micro pick up array  104 , and each of these characteristics may be modifiable to adjust the degree of movement between base substrate  214  and plug  304  that results from, e.g., various offsets between surfaces of a clamping contact and an operating voltage contact of the transfer head assembly  102 . Without exhaustively listing these variables, some of the micro pick up array  104  characteristics that may be modified are width of flexible membrane  310  and stiffness of top silicon layer  404  ( FIG. 4 ). An example of the impact of just these two variables is provided through a model in which top silicon layer  404  within flexible membrane  310  has a thickness of 5 μm. In a first instance, where flexible membrane  310  is modeled with a gap  308  width of 50 μm and top silicon layer  404  has a stiffness of 233 mN/μm, movement of base substrate  214  relative to plug  304  is estimated to be about 0.4 μm when a clamping load  601  and reactive load  602  correspond to a 300 MPa pressure applied to plug  304 . Alternatively, when the same pressure is applied to a plug  304  with a flexible membrane  310  having a gap  308  width of 100 μm and a top silicon layer  404  with a stiffness of 34 mN/μm, movement of plug  304  relative to base substrate  214  is estimated to be about 1.1 μm. In either of these alternatives, via  402  may have a diameter of about 2000 μm and a depth of about 600 μm. These estimates show not only that movement of plug  304  is affected by factors that are both external and internal to micro pick up array  104 , e.g., gap width (internal factor) and loading pressure (external factor), but also illustrates that these factors are controllable through micro pick up array  104  design to tune movement of plug  304  relative to base substrate  214  under the expected operating conditions of micro pick up array  104 . 
     Referring now to  FIG. 25 , a cross-sectional side view of a system having a micro pick up array and a transfer head assembly is shown in accordance with an embodiment of the invention. Micro pick up array  104  may be physically and electrically coupled with transfer head assembly  102 . More specifically, base substrate  214  of micro pick up array  104 , or more particularly backside dielectric layer  1402  over base substrate  214 , may be physically secured to a clamping contact  2504  of transfer head assembly  102 . Contact pad  306  of micro pick up array  104  may also be electrically coupled with an operating voltage contact  2502  of transfer head assembly  102 . 
     Transfer head assembly may include one or more clamping contact  2504 . In an embodiment, clamping contact  2504  is electrically coupled with a clamping voltage source  2506  to supply an electrostatic voltage to clamping contact  2504 . Clamping contact  2504  may include a conductive electrode, optionally covered by a thin dielectric layer. Thus, by aligning the energized clamping contact  2504  with a backside of base substrate  214 , an electrostatic voltage may be supplied to clamping contact  2504  that exerts clamping load  601  on base substrate  214 . Clamping load  601  may pull in on base substrate  214  to physically secure micro pick up array  104  to transfer head assembly  102 . 
     Transfer head assembly may also include one or more operating voltage contacts  2502 . In an embodiment, an operating voltage contact  2502  is aligned with a contact pad  306  prior to securing micro pick up array  104  to transfer head assembly  102 . Operating voltage  2502  may include a bare conductor, such as a metallic pin. In accordance with embodiments of the invention, as base substrate  214  is attracted toward clamping contact  2504 , operating voltage contacts  2502  and exerts a reactive load  602  upon contact pad  306 . Reactive load  602  may deflect flexible membrane  310 , causing plug  304  to move relative to base substrate  214  and create a residual compressive load between operating voltage contact  2502  and clamping pad  306 . This residual compressive load may persist while micro pick up array  104  is secured to transfer head assembly  102 . Furthermore, the residual compressive load may result in a firm pressure between the contacting surfaces that creates a uniform surface interface and a robust electrical contact. Therefore, the flexibility of flexible membrane  310  allows for an electrostatic voltage to be reliably supplied from voltage sources  106 ,  206  through one or more operating voltage contacts  2502  into one or more contact pads  306 . 
     In accordance with some embodiments of the invention, the top contact surfaces of the electrostatic transfer heads  114  protrude further away from the micro pick up array than the surfaces adjacent the deflected compliant contacts. In this manner the deflected compliant contacts do not interfere with operation of the transfer head assembly. For example, in the exemplary embodiments described above the plug moves 0.4 μm-1.1 μm relative to the base substrate when deflected. As will be described in further detail below, the height of the electrostatic transfer heads may be greater than the range of deflection of the compliant contacts. In an embodiment, the height of the mesa structures defining electrode surfaces  202 ,  204  (see  FIG. 12 ) rising above the silicon interconnects  112  is greater than range of relative movement between the plug and base substrate. 
       FIG. 26  is a schematic top view illustration of contacts between a micro pick up array and a transfer head assembly in accordance with an embodiment of the invention. In one embodiment, the contact area of the one or more clamping contacts  2504  on the transfer head assembly may be larger than the area  115  on the micro pick up array containing the array of transfer heads  114 . Thus, the contact area of the clamping contact(s)  2504  may be around the area  115  containing the array of transfer heads  114 . In this manner, the alignment and planarity across the array of transfer heads  114  can be regulated by the alignment of the transfer head assembly. In such an embodiment, a plurality of compliant contacts, referenced by the plugs  304  in  FIG. 26 , are outside the periphery of the areas  2504 ,  115 . In the particular embodiment illustrated, compliant contacts are positioned on four sides of the area  115  including the array of transfer heads  114 . 
     Referring now to  FIG. 7-24 , a method of forming a micro pick up array having an array of electrostatic transfer heads electrically coupled with one or more compliant contacts is illustrated in accordance with an embodiment of the invention. The processing sequence may begin with a commercially available SOI stack  702 , as illustrated in  FIG. 7 . The SOI stack  702  may include bulk silicon substrate  704 , top silicon layer  404 , buried oxide layer  314  between bulk silicon substrate  704  and the top silicon layer  404 , and backside oxide layer  706 . In an embodiment, bulk silicon substrate  704  is a silicon ( 100 ) handle wafer having a thickness of 500 μm+/−50 μm, buried oxide layer  314  is 1 μm+/−0.1 μm thick, and top silicon layer  404  is 7-20 μm+/−0.5 μm thick. The top silicon layer  404  may also be doped to improve conductivity. For example, a phosphorous dopant concentration of approximately 10 17  cm −3  yields a resistivity of less than 0.1 ohm-centimeter. In an embodiment, the backside oxide layer  706  is a thermal oxide having a thickness up to about 2 μm thick, which is the approximate upper limit for thermal oxidation of silicon. 
     Referring to  FIG. 8 , a mask layer  802  may be formed over the top silicon layer  404 . Mask layer  802  may be deposited, or alternatively thermally grown from the top silicon layer  404 . In an embodiment, mask layer  802  is a thermally growth SiO 2  layer having a thickness of approximately 0.1 μm. In an embodiment, where mask layer  802  is thermally growth SiO 2 , the mask layer  802  has a thickness which is significantly less than the thickness of buried oxide layer  314 . This helps maintain structural stability for the partially patterned SOI stack  702  during removal of the patterned mask layer  802 . 
     Referring to  FIG. 9 , the mask layer  802  is then patterned to form an array of islands  902  which will correspond to the mesa structures  311  of electrostatic transfer heads  114 . In an embodiment, mask layer  802  is a thermally grown SiO 2  layer, and islands  902  are formed by applying a positive photoresist, exposing, and removing undeveloped areas of the photoresist with a potassium hydroxide (KOH) developer solution. The mask layer  802  is then dry etched, stopping on top silicon layer  404 , to form islands  902  using a suitable technique such as ion milling, plasma etching, reactive ion etching (RIE). 
     The array of islands  902  correspond to mesa structures  311  of electrostatic transfer heads  114  and are sized accordingly. In an embodiment, a length and a width of islands  902  correspond to electrode surfaces  202 ,  204  of electrostatic transfer heads  114  that are between about 1 to 100 μm. For example, an island  902  may have length and width dimensions of 10 μm by 10 μm corresponding to an electrode surface  202  having length and width dimensions of 10 μm by 10 μm, or a length and width dimensions of 2.5 μm by 2.5 μm corresponding to an electrode surface  202  having length and width dimensions of 2.5 μm by 2.5 μm. However, these dimensions are exemplary, and other dimensions are envisioned in accordance with embodiments of the invention. As a contact surface of electrostatic transfer head  114  varies, e.g., between about 1 and 100 μm in length and/or width, dimensions of islands  902  may be varied accordingly. Islands  902  may be sized and located according to whether micro pick up array  104  includes monopolar or bipolar electrodes. Thus, in the case of a monopolar design, only a single island  902  is required over each electrostatic transfer head  114 . In the embodiment shown in  FIG. 9 , two islands  902  are placed over an electrostatic transfer head  114 , corresponding to a bipolar electrode design. 
     Referring to  FIGS. 10-13 , the mesa structures  311  and electrode interconnects  112  are patterned in a multi-part etching sequence. First, as illustrated in  FIG. 10 , the top silicon layer  404  between islands  902  is etched through to form trench  1002 . In an embodiment, this may be accomplished using a thin patterned positive photoresist and DRIE etching through top silicon layer  404  to buried oxide layer  314 . The patterned positive photoresist can be removed, resulting in the structure illustrated in  FIG. 10 . Second, as illustrated in  FIG. 11 , the top silicon layer  404  is partially etched, defining the mesa structures  311  and the electrode interconnects  112 . In an embodiment, this may be accomplished with a thin patterned positive photoresist or with a thermal oxide mask followed by DRIE etching, e.g., to remove approximately 5 μm of a 7-10 μm thick top silicon layer  404  in a timed etch, resulting in the structure illustrated in  FIG. 11 . Thus, the thickness after etching of top silicon layer  404  defining electrode interconnects  112  may be about 5 μm in an embodiment in which approximately 5 μm of a 10 μm thick top silicon layer  404  is removed by DRIE etching. This is consistent with the 5 μm thick top silicon layer  404  within flexible membrane  310  described above. Alternatively, the thickness of top silicon layer  404  within electrode interconnects  112  may be about 3 μm in an embodiment in which approximately 5 μm of a 7 μm thick top silicon layer  404  is removed by DRIE etching. Accordingly, the thickness of top silicon layer  404  within electrode interconnects  112  may be equal to the thickness of top silicon layer  404  within flexible membrane  310 . However, the thickness of top silicon layer  404  within flexible membrane  310  need not be the same as the thickness of top silicon layer  404  defining electrode interconnects  112 , but may instead be thinner or thicker. After DRIE etching, a buffered oxide etch having hydrofluoric acid and a buffering agent is used to remove the islands  902  without removing a substantial thickness of the buried oxide layer  314 , thereby revealing electrode surfaces  202 ,  204  and resulting in the structure illustrated in  FIG. 12 . Next, a patterned positive photoresist, e.g., having a thickness of about 12 to 15 μm, may be followed by DRIE etching of previously etched areas of electrode interconnect  112  to form electrode traces, resulting in the structure illustrated in  FIG. 13 . 
     Referring to  FIG. 14 , a dielectric layer  312  is formed over top silicon layer  404  in order to passivate the mesa structures  311  and electrode interconnect  112  and a backside dielectric layer  1402  is formed. Atomic layer deposition, thermal oxidation, or chemical vapor deposition may be used to form a dielectric layer  312  over mesa structures  311  and electrode interconnect  112 , as well as within trench  1002 , and dielectric layer  1402  on the back surface of the bulk silicon substrate  704 . Optionally, an insulating layer  1404  may be deposited over dielectric layer  312  using, for example, blanket atomic layer deposition. In an embodiment, insulating layer  1404  includes Al 2 O 3 . Thus, the structure illustrated in  FIG. 14 , having dielectric layer  312  and backside dielectric layer  1402 , is reached. 
     Dielectric layer  312  may be formed from various materials, including SiO 2 , Al 2 O 3 , HfO 2 , or SiN x . In accordance with embodiments of the invention the gripping pressure generated by the array of electrostatic transfer heads  114  on the array of micro devices is proportional to the dielectric constant of dielectric layer  312 , and thus, the choice of dielectric material may be chosen to balance gripping pressure with manufacturability. In an embodiment, dielectric layer  312  is formed from Al 2 O 3  having a thickness of about 5,000 angstroms and a dielectric constant of about 9. 
     Referring to  FIG. 15 , a spring pre-release for forming cavity  212  is created around portions of electrode interconnect  112  in a multi-etch sequence. First, a patterned positive photoresist is applied and followed by RIE etching of dielectric layer  312  to buried oxide layer  314 . Optional insulating layer  1404  is not shown in  FIG. 15 , but in a case where insulating layer  1404  is included, RIE etching may also be used to etch through insulating layer  1404  to dielectric layer  312 . Second, RIE etching of buried oxide layer  314  to bulk silicon substrate  704  is performed, resulting in the structure illustrated in  FIG. 15 . 
     Referring to  FIGS. 16-18 , contact area  506  is exposed on topside plug area  504  through a multi-etch sequence. First, the previously applied patterned positive photoresist can be removed before a new patterned positive photoresist is applied and RIE etching of dielectric layer  312  to top silicon layer  404  is performed, resulting in the structure illustrated in  FIG. 16 . Second, the patterned positive photoresist can be removed before a second patterned positive photoresist is applied and DRIE etching of top silicon layer  404  to buried oxide layer  314  is performed, resulting in the structure illustrated in  FIG. 17 . Third, RIE etching of buried oxide layer  314  to bulk silicon substrate  704  is performed, resulting in the structure illustrated in  FIG. 18 , having contact area  506  exposed on a topside of bulk silicon substrate  704 . 
     Referring to  FIG. 19 , topside contact  307  is formed through the opening and in electrical contact with contact area  506 . The patterned positive photoresist can be removed before a patterned negative lift-off photoresist is applied and 500-1,000 angstroms TiW and 1,000-5,000 angstroms Au is sputtered to create topside contacts  307 , resulting in the structure illustrated in  FIG. 19 . 
     Referring to  FIGS. 20-21 , a backside surface of bulk silicon substrate  704  may be exposed through backside dielectric layer  1402  and backside oxide layer  706 . First, a patterned positive photoresist is applied and RIE etching of backside dielectric layer  1402  to backside oxide layer  706  is performed, resulting in the structure illustrated in  FIG. 20 . Second, RIE etching of backside dielectric layer  1402  to bulk silicon substrate  704  is performed, resulting in the structure illustrated in  FIG. 21 . 
     Referring to  FIG. 22 , one or more contact pad  306  may be formed on a backside surface of bulk silicon substrate  704 . About 500 to 1,000 angstroms TiW and 1,000 to 5,000 angstroms Au may be sputtered to create contact pads  306 , resulting in the structure illustrated in  FIG. 22 . 
     Referring to  FIG. 23 , a gap  308  separating a plug  304  and a base substrate  214  of bulk silicon substrate  704  may be formed around contact pads  306 . A patterned positive photoresist is applied and DRIE etching of bulk silicon substrate  704  to buried oxide layer  314  is performed, resulting in the structure illustrated in  FIG. 23 . The arrangement of plug  304  and base substrate  214  is described above. Plug  304  and base substrate  214  may be considered as portions of bulk silicon substrate  704  that are defined during the formation of micro pick up array  104  and therefore become individual features of micro pick up array  104 . 
     Referring to  FIG. 24 , one or more cavities  212  may optionally be etched in bulk silicon substrate  704  underneath the array of electrostatic transfer heads  114  such that the array of electrostatic transfers heads are deflectable into the one or more cavities  212 . In an embodiment, a separate cavity  212  is formed underneath each electrostatic transfer head  114 . In an embodiment, a single cavity  212  is formed underneath the array of silicon electrodes in electrical communication with the electrode interconnects  112 . In an embodiment, cavity  212  is formed with a timed release etch into the bulk silicon substrate  704  and undercuts the electrode interconnect  112  and mesa structures. For example, etching may be performed with a fluorine based chemistry such as XeF 2  or SF 6 . During etching, the backside of SOI stack  702  may be protected with dicing tape. 
     Following the optional formation of the one or more cavities  212 , the SOI substrate may be diced, for example using laser dicing, to form one or more micro pick up arrays  104  having compliant contacts interconnected with electrostatic transfer heads  114  through electrode interconnects  112 . Furthermore, the micro pick up array  104  may include one or more contact pads  306  that electrically connect electrostatic transfer heads  114  with working circuitry or voltage sources  106 ,  206  of transfer head assembly  102 . 
     Referring again to  FIG. 25 , the system having micro pick up array  104  physically and electrically coupled with transfer head assembly  102  may be positioned over an array of micro devices  2510  on a carrier substrate  2508 . More specifically, the system may be moved relative to both carrier substrate  2508  and a receiving substrate while supplying an electrostatic voltage to electrostatic transfer heads  114  as needed in order to grip, transfer, and release micro devices  2510  from carrier substrate  2508  to the receiving substrate. As an example, the receiving substrate may be, but is not limited to, a display substrate, a lighting substrate, a substrate with functional devices such as transistors or ICs, or a substrate with metal redistribution lines. During movement between carrier substrate  2508  and the receiving substrate, the array of micro devices  2510  may be retained by the array of electrostatic transfer heads  114  using a persistent electrostatic gripping pressure maintained by a transfer of voltage to electrostatic transfer heads  114 . Alternatively, voltage application may be discontinued during movement between carrier substrate  2508  and the receiving substrate, and the array of micro devices  2510  may still be retained against the array of electrostatic transfer heads  114  by non-electrostatic forces, such as van der Waals forces. The array of micro devices  2510  may be released onto receiving substrate following transfer from carrier substrate  2508 , for example, by discontinuing the voltage supply to electrostatic transfer heads  114 . 
     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: 20150730
Publication Date: 20161101
Grant Date: 20161101
Priority Date: 20130604
Inventors: BIBL ANDREAS
GOLDA DARIUSZ
Assignee: APPLE INC
CPC Classifications: [{"code": "H01L2224/75725", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2924/12041", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2924/1461", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2924/12042", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2924/12041", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2924/1461", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/76877", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68322", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68354", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68368", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68368", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2924/12042", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68381", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68322", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68381", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/76802", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/76898", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68354", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68363", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/76898", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/6833", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/6835", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/6835", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68322", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68363", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68368", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/76877", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2924/12042", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/76898", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/6835", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2924/00", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2924/1461", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/6833", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68381", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/76802", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2924/12041", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68354", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 51033520