Patent Publication Number: US-9895902-B2

Title: Compliant micro device transfer head

Description:
RELATED APPLICATIONS 
     The present application is a continuation of co-pending U.S. patent application Ser. No. 15/157,247, filed May 17, 2016, which is a continuation of U.S. patent application Ser. No. 14/723,231 filed May 27, 2015, now U.S. Pat. No. 9,370,864, which is a continuation of U.S. patent application Ser. No. 13/466,966 filed May 8, 2012, now U.S. Pat. No. 9,105,492, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     The present invention relates to micro devices. More particularly, embodiments of the present invention relate to a micro device transfer head and a method of transferring an array of micro devices to a different substrate. 
     Background Information 
     Integration and packaging issues are one of the main obstacles for the commercialization of micro devices such as integration of radio frequency (RF) microelectromechanical systems (MEMS) microswitches, light-emitting diode (LED) integration onto image display systems, and MEMS or quartz-based oscillators. 
     Traditional technologies for transferring of devices include transfer by wafer bonding from a transfer wafer to a receiving wafer. One such implementation is “direct printing” involving one bonding step of an array of devices from a transfer wafer to a receiving wafer, followed by removal of the transfer wafer. Another such implementation is “transfer printing” involving two bonding/debonding steps. In transfer printing a transfer wafer may pick up an array of devices from a donor wafer, and then bond the array of devices to a receiving wafer, followed by removal of the transfer wafer. 
     Some printing process variations have been developed where a device can be selectively bonded and debonded during the transfer process. Still, in both traditional and variations of the direct printing and transfer printing technologies, the transfer wafer must be debonded from a device after bonding the device to the receiving wafer. In addition, the entire transfer wafer with the array of devices is involved in the transfer process. 
     SUMMARY OF THE INVENTION 
     A compliant micro device transfer head and method of transferring an array of micro devices to a different substrate are disclosed. For 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 a substrate with metal redistribution lines. 
     In an embodiment, a micro device transfer head includes a base substrate and a spring member. The spring member includes a spring anchor coupled to the base substrate and a spring portion deflectable into a space between the spring portion and the base substrate. The spring portion also comprises an electrode. A dielectric layer covers a top surface of the electrode. The spring portion may further comprise a mesa structure that protrudes away from the base substrate, where the mesa structure has tapered sidewalls and the electrode is formed on a top surface of the mesa structure. The mesa structure can be separately or integrally formed with the spring portion. 
     An electrode lead may extend from the electrode in order to make contact with wiring in the base substrate and connect the micro device transfer head to the working electronics of an electrostatic gripper assembly. The electrode leads can run from the electrode on the top surface of the mesa structure and along a sidewall of the mesa structure. The electrode lead can alternatively run underneath the mesa structure and connect to a via running through the mesa structure to the electrode. The spring portion may additionally comprise a second electrode and electrode lead. 
     In an embodiment, the micro device transfer head comprises a sensor to measure an amount of deflection of the spring portion. The sensor may be coupled to the spring member or formed within the spring member. The sensor may comprise two electrodes, one formed on the bottom surface of the spring member and a second formed directly beneath the first electrode within the space underlying the spring portion of the spring member. The sensor may measure strain or capacitance to determine the amount of deflection of the spring portion. The amount of deflection measured by the sensor may indicate, for example, whether the transfer head has made contact with a micro device, or whether contamination exists between the surfaces of the micro device and the transfer head. In an embodiment, the sensor is configured to measure a resonant frequency of the spring portion in order to determine whether a micro device has been picked up by the transfer head. 
     The space underlying the spring portion may be a cavity in the surface of the base substrate. Alternatively, the spring portion may be elevated above the base substrate by the spring anchor. The spring member may be a spring arm having a first end coupled to the base substrate or spring anchor, and a second end suspended above the cavity, wherein the spring anchor comprises the first end and the spring portion comprises the second end. A mesa structure may be formed on the second end of the spring arm. The spring member may comprise multiple spring arms. Alternatively, the spring portion may also completely cover the cavity. The mesa structure may be formed on the top surface of the spring portion, over a center of the cavity. 
     In an embodiment, a method for selective transfer of micro devices includes bringing an array of compliant micro device transfer heads, each comprising a deflection sensor, into contact with an array of micro devices. The amount of deflection of each transfer head may then be measured, and each transfer head may be selectively activated based on the amount of deflection detected by a sensor in the transfer head, such that only those transfer heads whose deflection indicates contact with the surface of a micro device are activated in order to pick up the corresponding micro device. 
     In an embodiment, a method for selective pick up of an array of micro devices includes an array of micro device transfer heads where each transfer head includes an electrode on a backside of the spring portion and a corresponding electrode at the bottom of a cavity, opposite the backside electrode. One of the backside or opposing electrodes may be covered by a dielectric layer to prevent shorting. When a transfer head is depressed, a voltage may be applied across the two electrodes to lock the transfer head in the depressed position. To enable selective transfer, the transfer heads in an array may first be depressed and locked in the depressed position. The voltage may then be selectively removed from a portion of the transfer heads, releasing the selected transfer heads from the depressed position so that they are poised to pick up micro devices. The transfer head array may be positioned above an array of micro devices on a carrier substrate, and brought into contact so that only the selectively released transfer heads contact and pick up a corresponding portion of micro devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view and isometric illustration of a bipolar cantilever micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 2  is an isometric illustration of a monopolar cantilever micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 3  is an isometric illustration of a bipolar cantilever micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 4  is an isometric illustration of a cantilever bipolar micro device transfer head including conductive vias in accordance with an embodiment of the invention. 
         FIGS. 5A-B  are top-down illustrations of a bipolar cantilever micro device transfer head in accordance with an embodiment of the invention. 
         FIGS. 6A-D  are cross-sectional side view illustrations of sensor components of a cantilever micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 7  is an isometric illustration of a bipolar cantilever micro device transfer head array in accordance with an embodiment of the invention. 
         FIG. 8  is an isometric illustration of a bipolar cantilever micro device transfer head array including a conductive ground plane in accordance with an embodiment of the invention. 
         FIG. 9  is a cross-sectional side view illustration of a bipolar cantilever micro device transfer head array including a conductive ground plane in accordance with an embodiment of the invention. 
         FIG. 10  is an isometric illustration of a bipolar micro device transfer head comprising multiple spring arms in accordance with an embodiment of the invention. 
         FIG. 11  is a cross-sectional side view illustration of a bipolar membrane micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 12  is an overhead isometric illustration of a membrane micro device transfer head in accordance with an embodiment of the invention. 
         FIGS. 13A-E  are cross-sectional side view illustrations of bipolar membrane micro device transfer heads according to an embodiment of the invention. 
         FIGS. 14A-E  are cross-sectional side view illustrations of a method for forming a bipolar membrane micro device transfer head according to an embodiment of the invention. 
         FIGS. 15A-K  are cross-sectional side view illustrations of a method for forming a bipolar membrane micro device transfer head according to an embodiment of the invention. 
         FIGS. 16A-D  are cross-sectional side view illustrations of an elevated micro device transfer heads in accordance with an embodiment of the invention. 
         FIG. 17  is a flow chart illustrating a method of picking up and transferring a micro device from a carrier substrate to a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 18  is a flow chart illustrating a method of picking up and transferring an array of micro devices from a carrier substrate to at least one receiving substrate in accordance with an embodiment of the invention. 
         FIG. 19  is a flow chart illustrating a method of picking up and transferring an array of micro devices from a carrier substrate to at least one receiving substrate in accordance with an embodiment of the invention. 
         FIG. 20  is a flow chart illustrating a method of picking up and transferring a selected portion of an array of micro devices from a carrier substrate to at least one receiving substrate in accordance with an embodiment of the invention. 
         FIG. 21  is a flow chart illustrating a method of picking up and transferring a portion of an array of micro devices from a carrier substrate to at least one receiving substrate based on information from one or more sensors in each of the micro device transfer heads in accordance with an embodiment of the invention. 
         FIGS. 22A-B  are cross-sectional side view illustrations of an array of micro device transfer heads in contact with an array of micro LED devices in accordance with an embodiment of the invention. 
         FIG. 23  is a cross-sectional side view illustration of an array of micro device transfer heads picking up an array of micro LED devices in accordance with an embodiment of the invention. 
         FIG. 24  is a cross-sectional side view illustration of an array of micro device transfer heads picking up a portion of an array of micro LED devices in accordance with an embodiment of the invention. 
         FIG. 25  is a cross-sectional side view illustration of an array of micro device transfer heads with an array of micro LED devices positioned over a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 26  is a cross-sectional side view illustration of an array of micro devices released onto a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 27  is a cross-sectional side view illustration of a variety of micro LED structures including contact openings with a smaller width than the top surface of the micro p-n diode in accordance with an embodiment of the invention. 
         FIG. 28  is a cross-sectional side view illustration of a variety of micro LED structures including contact openings with a larger width than the top surface of the micro p-n diode in accordance with an embodiment of the invention. 
         FIG. 29  is a cross-sectional side view illustration of a variety of micro LED structures including contact openings with the same width as the top surface of the micro p-n diode in accordance with an embodiment of the invention. 
         FIG. 30  is a cross-sectional side view illustration of an array of micro device transfer heads illustrating varying degrees of deflection of the spring portion of a transfer head during a pick up operation. 
         FIG. 31  is a cross-sectional side view illustration of an array of micro device transfer heads where a portion of the transfer heads have been locked in the depressed position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention describe a compliant micro device transfer head and head array, and a method of transferring a micro device and an array of micro devices from a carrier substrate to a receiving substrate. 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 integrated circuits (ICs), or a substrate with metal redistribution lines. In some embodiments, the micro devices and array of micro devices described herein may be a micro LED device, such as the structures illustrated in  FIGS. 26-28  and those described in related U.S. patent application Ser. No. 13/372,222, which is incorporated herein by reference. While some embodiments of the present invention are described with specific regard to micro LEDs, it is to be appreciated that embodiments of the invention are not so limited and that certain embodiments may also be applicable to other micro devices such as diodes, transistors, integrated circuits (ICs), and MEMS. 
     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 present invention. In other instances, well-known semiconductor 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 “in 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 with respect to other layers. One layer “over” or “on” another layer or bonded “to” another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     The terms “micro” device or “micro” LED structure as used herein may refer to the descriptive size of certain devices or structures in accordance with embodiments of the invention. As used herein, the terms “micro” devices or structures are meant to refer to the scale of 1 to 100 μm. However, 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 one aspect, embodiments of the invention describe a manner for mass transfer of an array of pre-fabricated micro devices with an array of compliant transfer heads. For example, the pre-fabricated micro devices may have a specific functionality such as, but not limited to, an LED for light-emission, silicon IC for logic and memory, and gallium arsenide (GaAs) circuits for radio frequency (RF) communications. In some embodiments, arrays of micro LED devices which are poised for pick up are described as having a 10 μm by 10 μm pitch, or 5 μm by 5 μm pitch. At these densities a 6 inch substrate, for example, can accommodate approximately 165 million micro LED devices with a 10 μm by 10 μm pitch, or approximately 660 million micro LED devices with a 5 μm by 5 μm pitch. A transfer tool including an array of compliant transfer heads matching an integer multiple of the pitch of the corresponding array of micro LED devices can be used to pick up and transfer the array of micro LED devices to a receiving substrate. In this manner, it is possible to integrate and assemble micro LED devices into heterogeneously integrated systems, including substrates of any size ranging from micro displays to large area displays, and at high transfer rates. For example, a 1 cm by 1 cm array of micro device transfer heads can pick up and transfer more than 100,000 micro devices, with larger arrays of micro device transfer heads being capable of transferring more micro devices. Each compliant transfer head in the array of compliant transfer heads may also be independently controllable, which enables selective pick up and release of the micro devices. 
     In one aspect, embodiments of the invention describe a compliant micro device transfer head and a method of transfer in which an array of the micro device transfer heads enable improved contact with an array of micro devices as compared to an array of incompliant transfer heads. An array of compliant micro device transfer heads, wherein each transfer head includes a spring member, is lowered onto an array of micro devices until the transfer heads make contact with the micro devices. The spring member components of the compliant transfer heads can compensate for variations in height of the micro devices or for particulate contamination on top of a micro device. For example, without the spring members it is possible that an array of transfer heads would not make contact with each and every micro device in the array. An irregular micro device height or a particle on a top surface of a single micro device could prevent the remainder of the transfer heads from making contact with the remainder of the micro devices in the array. As a result, an air gap could be formed between those transfer heads and micro devices. With such an air gap, it is possible that the target applied voltage would not create a sufficient electrostatic force to overcome the air gap, resulting in an incomplete pick-up process. In accordance with embodiments of the invention, the spring members associated with taller or contaminated micro devices may deflect more than spring members associated with shorter micro devices on a single transfer substrate. In this manner, the spring members can also compensate for variations in height of the micro devices, assisting each compliant transfer head to make contact with each micro device, and ensure that each intended micro device is picked up. 
     In one aspect, the compliant micro device transfer head structure includes a sensor to monitor an amount of deflection of the spring member when the transfer head is brought into contact with a micro device. The sensor may be used for a variety of reasons. In one application, the sensor can be used to determine if contact has been made with a respective micro device. In another application, the sensor can be used to detect an irregularly shaped or contaminated micro device. In this manner, it may be determined whether to proceed to attempt to pick up the irregular or contaminated micro device. Additionally, it may be determined whether to apply a cleaning operation to the transfer head array or micro device array prior to reattempting a pick up operation. In another application, the sensor can be used to detect whether a micro device is attached to the transfer head, and has successfully been picked up. 
     In another aspect, a method for selective transfer of micro devices includes bringing an array of compliant micro device transfer heads, each comprising a deflection sensor, into contact with an array of micro devices. The amount of deflection of each transfer head may be measured by the deflection sensor to determine whether the transfer head has contacted a micro device, to indicate the presence of contamination or irregularities on the surface of the micro device, or indicate the absence of a micro device. As such, each transfer head may be selectively activated based on the amount of deflection detected by the deflection sensor, so that only those transfer heads whose deflection indicate contact with the surface of a micro device are activated to pick up the corresponding micro device. 
     In another aspect, a method for selective transfer of micro devices includes depressing the compliant transfer heads in an array, locking the transfer heads in the depressed position, and then selectively releasing a portion of the transfer heads from the depressed position so that each released transfer head may contact and pick up a corresponding micro device in an array of micro devices. 
     Referring now to  FIG. 1 , an isometric view of a compliant transfer head  100  with a monopolar electrode and a corresponding cross-sectional side view of a compliant transfer head array are illustrated in accordance with an embodiment of the invention.  FIG. 2  is a close-up isometric view of the spring member  110  shown in  FIG. 1 . The spring member feature of the micro device transfer head disclosed herein may be executed using a variety of structures that enable deflection of the transfer head. Exemplary embodiments include a cantilever beam (see, e.g.,  FIG. 1 ), multiple spring arms (see, e.g.,  FIG. 10 ), a membrane (see, e.g.,  FIG. 11 ), and elevated platforms (see, e.g.,  FIGS. 16A-D ). Other structures may be possible to enable a compliant transfer head. Additional features of a particular embodiment of a transfer head may be determined by the structure of the spring member, such as the addition of a mesa structure, the placement of electrode leads, and the type and location of deflection sensors. Accordingly, though features—such as the materials and characteristics of the base substrate, spring member, electrode(s), and dielectric layer—are described with reference to the cantilever spring member structure shown in  FIGS. 1 and 2 , it is to be understood that certain features are equally applicable to other spring member structure embodiments subsequently described. 
     Each transfer head may include a base substrate  102 , a spring member  110  comprising a spring anchor  120  coupled to the base substrate  102  and a spring portion  122  comprising electrode  116 , and a dielectric layer  113  covering the top surface of the electrode. The spring portion  122  is deflectable into a space  112  between the spring portion  122  and the base substrate  102 . The dielectric layer  113  is not shown in the isometric view illustrations in  FIGS. 1-2  so that the underlying elements may be illustrated. Spring portion  122  may include a spring arm  124  and a mesa  104  including a top surface  108  and tapered sidewalls  106 . 
     Base substrate  102  may be formed from a variety of materials such as silicon, ceramics and polymers that are capable of providing structural support. In an embodiment, base substrate  102  has a conductivity between 10 3  and 10 18  ohm-cm. Base substrate  102  may additionally include interconnect  130  to connect the micro device transfer head  100  to the working electronics of an electrostatic gripper assembly via electrode lead  114 . 
     Referring again to  FIG. 1 , spring portion  122  of spring member  110  is deflectable into a space  112  separating spring portion  122  from the base substrate  102 . In an embodiment, one end of spring member  110  comprises the spring anchor  120 , by which spring member  110  is coupled to base substrate  102 , and the other end comprises the spring portion  122  suspended above space  112 . In an embodiment, spring portion  122  comprises spring arm  124 , mesa structure  104 , electrode  116 , and electrode lead  114 . Spring arm  124  is formed from a material having an elastic modulus that enables deflection of spring portion  122  into space  112  over the working temperature range of the micro device transfer process. In an embodiment, spring arm  124  is formed from the same or different material as base substrate  102 , for example, semiconductor materials such as silicon or dielectric materials such as silicon dioxide and silicon nitride. In an embodiment, spring arm  124  is integrally formed from base substrate  102 , such as, during the etching of space  112 . In another embodiment, spring arm  124  is formed from a layer of material deposited, grown, or bonded onto base substrate  102 . 
     In an embodiment, the material and dimensions of spring arm  124  are selected to enable spring portion  122  to deflect approximately 0.5 μm into space  112  when the top surface of transfer head  100  is subjected to up to 10 atm of pressure at operating temperatures up to 350° C. Referring to  FIG. 2 , spring arm  124  has a thickness T, width W, and length L, according to an embodiment of the invention. In an embodiment, spring arm  124  is formed from silicon and has a thickness T of up to 1 μm. The thickness T of spring arm  124  may be greater or less than 1 μm, depending on the elastic modulus of the material from which it is formed. In an embodiment, the width W of spring arm  124  is sufficient to accommodate additional spring portion and transfer head elements, such as electrode  116  and mesa structure  104 . In an embodiment, the width W of spring arm  124  may correspond to the size of the micro device to be picked up. For example, where a micro device is 3-5 μm wide, the width of the spring arm may also be 3-5 μm, and where a micro device is 8-10 μm wide, the width of the spring arm may also be 8-10 μm. The length L of spring arm  124  is long enough to enable deflection of spring portion  122  given the modulus of the material from which spring arm  124  is formed, but less than the pitch of the transfer heads  100  in the transfer head array. In an embodiment, the length L of spring arm  124  may be from 8 to 30 μm. 
     Spring portion  122  of the cantilever spring member  110  shown in  FIGS. 1 and 2  further includes mesa structure  104  protruding away from base substrate  102 . Mesa structure  104  has tapered sidewalls  106  and top surface  108 . Mesa structure  104  may be formed using any suitable processing technique, and may be formed from the same or different material than spring arm  124 . In one embodiment, mesa structure  104  is integrally formed with spring arm  124 , for example by using casting or lithographic patterning and etching techniques. In an embodiment, anisotropic etching techniques can be utilized to form tapered sidewalls  106  for mesa structure  104 . In another embodiment, mesa structure  104  may be deposited or grown, and patterned on top of the base substrate  102 . In an embodiment, mesa structure  104  is a patterned oxide layer, such as silicon dioxide, formed on a silicon spring arm  124 . 
     In one aspect, the mesa structures  104  generate a profile that protrudes away from the base substrate so as to provide a localized contact point to pick up a specific micro device during a pick up operation. In an embodiment, mesa structures  104  have a height of approximately 1 μm to 5 μm, or more specifically approximately 2 μm. Specific dimensions of the mesa structures  104  may depend upon the specific dimensions of the micro devices to be picked up, as well as the thickness of any layers formed over the mesa structures. In an embodiment, the height, width, and planarity of the array of mesa structures  104  on the base substrate  102  are uniform across the base substrate so that each micro device transfer head  100  is capable of making contact with each corresponding micro device during the pick up operation. In an embodiment, the width across the top surface  126  of each micro device transfer head is slightly larger, approximately the same, or less than the width of the top surface of the each micro device in the corresponding micro device array so that a transfer head does not inadvertently make contact with a micro device adjacent to the intended corresponding micro device during the pick up operation. As described in further detail below, since additional layers may be formed over the mesa structure  104  (e.g. passivation layer  111 , electrode  116 , and dielectric layer  113 ) the width of the mesa structure may account for the thickness of the overlying layers so that the width across the top surface  126  of each micro device transfer head is slightly larger, approximately the same, or less than the width of the top surface of the each micro device in the corresponding micro device array. 
     Still referring to  FIGS. 1 and 2 , mesa structure  104  has a top surface  108 , which may be planar, and sidewalls  106 . In an embodiment, sidewalls  106  may be tapered up to 10 degrees, for example. Tapering the sidewalls  106  may be beneficial in forming the electrodes  116  and electrode leads  114  as described further below. A passivation layer  111  may cover the base substrate  102  and array of spring arms  124  and mesa structures  104 . In an embodiment, the passivation layer may be 0.5 μm-2.0 μm thick oxide such as, but not limited to, silicon oxide (SiO 2 ), aluminum oxide (Al 2 O 3 ) or tantalum oxide (Ta 2 O 5 ). 
     Spring member  110  further comprises electrode  116  and electrode lead  114 , according to an embodiment. In an embodiment, electrode  116  is formed on the top surface  108  of mesa structure  104 . In an exemplary embodiment, the top surface  108  of the mesa structure  104  onto which electrode  116  is formed is approximately 7 μm×7 μm in order to achieve a 8 μm×8 μm top surface of the transfer head  100 . In accordance with an embodiment, electrode  116  covers the maximum amount of surface area of the top surface  108  of the mesa structure  104  as possible while remaining within patterning tolerances. Minimizing the amount of free space increases the capacitance and resultant grip pressure that can be achieved by the micro device transfer head. While a certain amount of free space is illustrated on the top surface  108  of the mesa structure  104  in  FIGS. 1 and 2 , electrode  116  may cover the entire top surface  108 . The electrode  116  may also be slightly larger than the top surface  108 , and partially or fully extend down the sidewalls  106  of the mesa structure  104  to ensure complete coverage of the top surface  108 . It is to be appreciated that the mesa array may have a variety of different pitches, and that embodiments of the invention are not limited to the exemplary 7 μm×7 μm top surface of the mesa structure  104  in a 10 μm pitch. 
     Electrode lead  114  may run from electrode  116  over the top surface  108  of mesa structure  104 , down sidewall  106  of the mesa structure, along the top surface of spring arm  124 , and over spring anchor  120 . In an embodiment, electrode lead  114  connects to interconnect  130  in base substrate  102 , which may run through the base substrate to a back side of the base substrate. 
     A variety of conductive materials including metals, metal alloys, refractory metals, and refractory metal alloys may be employed to form electrode  116  and electrode lead  114 . In an embodiment, electrode  116  has a thickness up to 5,000 Å (0.5 μm). In an embodiment, the electrode  116  includes a high melting temperature metal such as platinum or a refractory metal or refractory metal alloy. For example, an electrode may include platinum, titanium, vanadium, chromium, zirconium, niobium, molybdenum, ruthenium, rhodium, hafnium, tantalum, tungsten, rhenium, osmium, iridium and alloys thereof. Refractory metals and refractory metal alloys generally exhibit higher resistance to heat and wear than other metals. In an embodiment, electrodes  116  are each an approximately 500 Å (0.05 μm) thick layer of titanium tungsten (TiW) refractory metal alloy. 
     In an embodiment, a dielectric layer  113  covers electrode  116 . The dielectric layer  113  may also cover other exposed layers on transfer head  100  and base substrate  102 . In an embodiment, the dielectric layer  113  has a suitable thickness and dielectric constant for achieving the required grip pressure of the micro device transfer head  100 , and sufficient dielectric strength to not break down at the operating voltage. The dielectric layer  113  may be a single layer or multiple layers. In an embodiment, the dielectric layer is 0.5 μm-2.0 μm thick, though the thickness may be more or less depending upon the specific topography of the transfer head  100  and underlying mesa structure  104 . Suitable dielectric materials may include, but are not limited to, aluminum oxide (Al 2 O 3 ) and tantalum oxide (Ta 2 O 5 ). In accordance with embodiments of the invention, the dielectric layer  113  possesses a dielectric strength greater than the applied electric field so as to avoid shorting of the transfer head during operation. The dielectric layer  113  can be deposited by a variety of suitable techniques such as chemical vapor deposition (CVD), atomic layer deposition (ALD) and physical vapor deposition (PVD) such as sputtering. The dielectric layer  113  may additionally be annealed following deposition. In one embodiment, the dielectric layer  113  possesses a dielectric strength of at least 400 V/μm. Such a high dielectric strength can allow for the use of a thinner dielectric layer. Techniques such as ALD can be utilized to deposit uniform, conformal, dense, and/or pin-hole free dielectric layers with good dielectric strength. Multiple layers can also be utilized to achieve such a pin-hole free dielectric layer. Multiple layers of different dielectric materials may also be utilized to form dielectric layer  113 . In an embodiment, the underlying electrode  116  includes platinum or a refractory metal or refractory metal alloy possessing a melting temperature above the deposition temperature of the dielectric layer material(s) so as to not be a limiting factor in selecting the deposition temperature of the dielectric layer  113 . In an embodiment, following the deposition of the dielectric layer  113 , a thin coating (not illustrated) may be formed over the dielectric layer  113  to provide a specific stiction coefficient, so as to add lateral friction and keep the micro devices from being knocked off the transfer head during the pick up operation. In such an embodiment, the additional thin coating replaces top surface  126  as the contacting surface, and this surface retains the dimensional array requirements described herein. Furthermore, the additional coating can affect the dielectric properties of the micro device transfer head which may affect the operability of the micro device transfer head. In an embodiment, the additional coating thickness can be minimal (e.g. below 10 nm) so as to have little to no appreciable effect on the grip pressure. 
     Spring portion  122  is deflectable into the space  112  between spring portion  122  and base substrate  102 . In an embodiment, space  112  is a cavity in the surface of base substrate  102 . In another embodiment, spring portion  122  is elevated above base substrate  102  to create space  112 . In an embodiment, space  112  extends underneath the spring arm  124  of spring portion  122 . Space  112  may also comprise an undercut portion beneath the top surface of base substrate  102 . The dimensions of space  112  are selected to enable deflection of spring portion  122  into space  112 , as discussed above with respect to spring arm  124 . 
       FIG. 3  is a close-up isometric view of a spring member  110  having a bipolar electrode, according to an embodiment of the invention. In an embodiment, electrodes  116 A and  116 B cover mesa structure  104 . For purposes of clarity, the overlying dielectric layer is not illustrated. In an embodiment, electrodes  116 A and  116 B are formed over a passivation layer (not shown) that covers mesa structure  104 . In an exemplary embodiment, where the top surface  108  of the mesa structure  104  is approximately 7 μm×7 μm corresponding to a mesa array with a 10 μm pitch, the electrodes may cover the maximum amount of the surface area of the top surface  108  of the mesa structure  104  as possible while still providing separation between electrodes  116 A,  116 B. The minimum amount of separation distance may be balanced by considerations for maximizing surface area, while avoiding overlapping electric fields from the electrodes. For example, the electrodes  116 A,  116 B may be separated by 0.5 μm or less, and the minimum separation distance may be limited by the height of the electrodes. In an embodiment, the electrodes are longer than the top surface  108  in one direction, and partially or fully extend down the sidewalls  106  of the mesa structure  104  to ensure maximum coverage of the top surface  108 . It is to be appreciated that the mesa array may have a variety of different pitches, and that embodiments of the invention are not limited to the exemplary 7 μm×7 μm top surface of the mesa structure  104  in a 10 μm pitch. 
     Electrode leads  114 A and  114 B connect to electrodes  116 A and  116 B, respectively, on the top surface  108  of mesa structure  104 . Electrode leads  114  may run down a single inclined sidewall  106  of mesa structure  104  and along the top surface of spring arm  124  to spring anchor  120 . The incline of sidewall  106  aids in the deposition and etching of metal to form electrode leads  114 . In an embodiment, electrode leads  114 A and  114 B are each located in proximity to the edges of spring arm  124  so as to permit the formation of a spring arm sensor (not shown) between the electrode leads, on the top surface of spring arm  124 . Electrode leads  114 A,  114 B may be formed of the same or different conductive material as electrodes  116 A,  116 B. 
     Referring now to  FIG. 4 , an isometric view is provided of a spring member  110  having a bipolar electrode with an alternative electrode lead configuration in accordance with an embodiment of the invention. In such an embodiment the electrode leads  114 A,  114 B run underneath a portion of the mesa structure  104 , and conductive vias  117 A,  117 B run through the mesa structure  104  (and an optional passivation layer not illustrated) connecting the electrodes  116 A,  116 B to the respective electrode leads  114 A,  114 B. In such an embodiment, conductive vias  117 A,  117 B may be formed prior to formation of mesa structure  104 , and may be formed of the same or different conductive material as electrode leads  114 A,  114 B and electrodes  116 A,  116 B. While vias  117 A,  117 B are illustrated with regard to a bipolar electrode structure in  FIG. 4 , it is to be appreciated that the above described via or vias may also be integrated into monopolar electrode structures. 
     Referring now to  FIGS. 5A-B , top view illustrations of electrodes  116 A,  116 B of a bipolar micro device transfer head are provided in accordance with embodiments of the invention. Thus far, mesa structure  104  has been described as a single mesa structure as shown in  FIG. 5A . However, embodiments of the invention are not so limited. In the embodiment illustrated in  FIG. 5B , each electrode  116  is formed on a separate mesa structure  104 A,  104 B separated by a trench  105 . An optional passivation layer (not illustrated) may cover both mesa structures  104 A,  104 B. 
       FIGS. 6A-D  each illustrate an embodiment of a micro device transfer head  100  incorporating one or more sensors. Sensors can serve a variety of purposes during operation of the transfer head. For example, where a sensor is used to measure an amount of deflection of the transfer head, this information can be used to determine if (1) contact has been made with a micro device to be picked up, (2) contamination is present on the micro device, or alternatively the micro device has been damaged or deformed, or (3) whether a micro device has been picked up. 
       FIGS. 6A-B  illustrate cross sectional side views of a transfer head comprising a strain sensor  128 A/ 128 B, according to an embodiment of the invention. In an embodiment, strain sensor  128 A/ 128 B is a strain gauge capable of measuring the amount of deflection of spring portion  122  into space  112 . When a transfer head contacts the surface of a micro device during a pick up operation, it may deflect some amount in response to the contact pressure. By measuring the amount of deflection of a spring portion  122  and comparing it to the amount of deflection known to indicate clean contact with a micro device surface, strain sensor  128 A/ 128 B can indicate whether transfer head  100  has contacted the top surface of a micro device in an array and as such is ready to execute a pick up operation. Detection of too little deflection may indicate that a micro device is absent from that position in the array, while detection of too much deflection may indicate separation or incomplete contact between the surface of the micro device and the surface of the transfer head due to either the presence of contamination particles or an otherwise damaged or deformed micro device. In both cases, a voltage may not be applied to the transfer head so as not to attempt to pick up the absent or damaged micro device. In the case where contamination is detected, a cleaning operation may be applied to the transfer head, micro device, or their respective array prior to reattempting the pick up operation. 
     In another embodiment, strain sensor  128 A/ 128 B is capable of measuring the resonant frequency of spring portion  122 . A spring arm  124  bearing the weight of transfer head elements such as mesa structure  104 , electrodes (not shown) and a dielectric layer (not shown), will have a natural resonant frequency. Upon picking up a micro device on the surface of the transfer head, the resonant frequency will change due to the additional weight of the micro device. In an embodiment, strain sensor  128 A/ 128 B can detect a change in the resonant frequency of spring portion  122 , which indicates that a micro device has been successfully picked up by the transfer head. 
     In an embodiment, sensors  128 A/B can be formed directly on or in base substrate  102 . In an embodiment, sensors  128 A/B can be formed on or in a spring layer  132  formed over substrate  102 . For example, spring layer  132  is silicon, in which case a passivation layer (not shown) is formed between strain sensor  128 A and the interface of spring anchor  120  and spring portion  122  in order to isolate the sensor. In another embodiment, spring layer  132  is an oxide or nitride layer. Referring to  FIG. 6A , in an embodiment, strain sensor  128 A is formed on spring layer  132  over the interface of spring anchor  120  and spring portion  122 . When spring portion  122  deflects into space  112 , strain along spring arm  124  is not uniform; spring arm  124  experiences the maximum amount of strain at the interface of spring portion  122  and spring anchor  120 . In an embodiment, strain sensor  128 A spans the interface of spring anchor  120  and spring portion  122 , so as to be subject to the maximum amount of stress associated with the deflection of spring portion  122 . 
     In an embodiment, strain sensor  128 A comprises a piezoelectric material. A piezoelectric material accumulates charge in response to an applied mechanical stress. The accumulation of charge along strained surfaces of a piezoelectric sensor can generate a measurable voltage related to the amount of strain. As such, as spring portion  122  deflects into space  112 , the voltage between the upper and lower surface of the strain sensor increases as the strain at the interface of spring anchor  120  and spring portion  122  increases, enabling calculation of the amount of deflection of spring portion  122 . Piezoelectric materials include, for example, crystalline materials such as quartz and ceramic materials such as lead zirconate titanate (PZT). 
     In another embodiment, strain sensor  128 A comprises a piezoresistive material. The electrical resistivity of a piezoresistive material changes in response to an applied mechanical stress. As such, strain sensor  128 A may be subject to an electrical current, so that when spring portion  122  deflects into space  112 , the electrical resistivity of strain sensor  128 A increases as the strain at the interface of spring anchor  120  and spring portion  122  increases, causing a measurable increase in the voltage across the sensor. The amount of deflection can be calculated from the changes in voltage. Piezoresistive materials include, for example, polycrystalline silicon, amorphous silicon, monocrystalline silicon, or germanium. 
     In an embodiment, strain sensor  128 B is formed within the surface of base substrate  102  at the interface of spring anchor  120  and spring portion  122 , as shown in  FIG. 6B . In an embodiment, spring anchor  120  and spring arm  124  of spring portion  122  are formed from silicon. In an embodiment, portion of the interface of spring anchor  120  and spring portion  122  is doped to form a piezoresistive strain sensor  128 B. For example, the silicon surface may be doped with boron for a p-type material or arsenic for an n-type material. The changing mobility of charge carriers when the doped sensor region is strained gives rise to the piezoresistive effects. 
     In another aspect, strain sensor  128 A,  128 B is used to measure the resonant frequency of spring portion  122 . In an embodiment, spring portion  122  oscillates at a resonant frequency determined in part by the weight of the elements forming spring portion  122 . The oscillation results in a correspondingly oscillating amount of strain at the interface of spring anchor  120  and spring portion  122 . After a micro device has been picked up by the transfer head, the additional weight of the micro device will change the resonant frequency of spring portion  122 , resulting in changes in the oscillating strain at the interface of spring anchor  120  and spring portion  122  that can be measured by strain sensor  128 . In this manner, strain sensor  128 A,  128 B may be used to determine if a transfer head has successfully picked up a micro device during a pickup operation. 
     Referring to  FIG. 6C , opposing electrodes are formed on each of spring portion  122  and bulk substrate  102 , according to an embodiment of the invention. In an embodiment, the bottom surface of spring arm  124  facing space  112  comprises a backside electrode  134 . In an embodiment, backside electrode  134  is positioned on the bottom surface of spring arm  124  opposite the mesa structure  104  formed on the top surface. In an embodiment, opposing electrode  138  is formed on base substrate  102 , directly opposite backside electrode  134  within space  112 . In an embodiment, dielectric layer  136  covers opposing electrode  138 . In another embodiment, dielectric layer  136  covers backside electrode  134 . 
     In an embodiment, electrodes  134  and  138  function as a capacitive sensor. The capacitance between two parallel conductors increases as the distance between the conductors decreases. In an embodiment, a voltage is applied across electrodes  134  and  138 . As the spring portion  122  is depressed within space  112  toward base substrate  102 , the distance between electrodes  134  and  138  decreases, causing the capacitance between them to increase. In this manner, the amount of deflection of spring portion  122  can be calculated from changes in the capacitance between electrodes  134  and  138  across dielectric layer  136  and space  112 . Dielectric  136  prevents shorting between the electrodes when spring portion  122  is fully depressed within space  112 . The opposing electrodes  134 ,  138  may be formed from any suitable conductive material, such as those discussed above with respect to electrodes  116 . 
     In another application, electrodes  134  and  138  may be used to measure the resonant frequency of spring portion  122 . As discussed above, in an embodiment, spring portion  122  oscillates at a resonant frequency determined in part by the weight of elements forming spring portion  122 . The oscillation may result in a correspondingly oscillating capacitance between electrodes  134  and  138 . After a micro device has been picked up by the transfer head, the additional weight of the micro device will change the resonant frequency of spring portion  122 , resulting in changes in the oscillating capacitance as measured by electrodes  134  and  138 . In this manner, electrodes  134  and  138  may be used to determine if a transfer head has successfully picked up a micro device during a pickup operation. 
     In yet another application of the structure illustrated in  FIG. 6C , the electrodes  134  and  138 , together with dielectric  136 , are capable of locking spring portion  122  in a fully depressed position. In an embodiment, prior to the pickup operation, the array of transfer heads may be depressed to the point that backside electrode  134  contacts the surface of dielectric  136 . A voltage may then be applied between opposing electrode  138  and backside electrode  134 , across dielectric  136 , locking spring portion  122  in the depressed position. In the depressed position, the transfer heads may be “deflected” so that the topography is reduced and the transfer heads are not in position for pickup. The voltage across the dielectric may then be selectively removed for select transfer heads, which allows the spring arm to be released and return to the undeflected, neutral position. This position may correspond to a “selected” position, which has a higher topography and “selected” transfer heads are in position for pick up of the micro device. 
     In an embodiment illustrated in  FIG. 6D , transfer head  100  comprises both a strain sensor  128 A and electrodes  134 ,  138 . The strain sensor  128 A and the electrodes  134 ,  138  may have different functions. For example, strain sensor  128 A may measure deflection while electrodes  134 ,  138  measure the resonant frequency of spring portion  122 , or vice versa. In another embodiment (not shown), a transfer head  100  comprises both a strain sensor  128 B, formed within spring anchor  120  and spring portion  122 , and electrodes  134 ,  138 . 
     Referring now to  FIGS. 7-9 , an embodiment of the invention is illustrated in which a conductive ground plane is formed over the dielectric layer and surrounding the array of transfer heads.  FIG. 7  is an isometric view illustration of an array of compliant micro device transfer heads  100  with a bipolar electrode configuration as previously described with regard to  FIG. 3 . For purposes of clarity, the optional underlying passivation layer and overlying dielectric layer have not been illustrated. Referring now to  FIGS. 8-9 , a conductive ground plane  140  is formed over the dielectric layer  113  and surrounding the array of transfer heads  100 . The presence of ground plane  140  may assist in the prevention of arcing between transfer heads  100 , particularly during the application of high voltages. Ground plane  140  may be formed of a conductive material which may be the same as, or different as the conductive material used to form the electrodes, or vias. Ground plane  140  may also be formed of a conductive material having a lower melting temperature than the conductive material used to form the electrodes since it is not necessary to deposit a dielectric layer of comparable quality (e.g. dielectric strength) to dielectric layer  113  after the formation of ground plane  140 . 
       FIG. 10  is an isometric view of a spring member structure where the spring portion comprises multiple spring arms, according to an embodiment of the invention. In an embodiment, spring member  110  comprises spring portion  122  and multiple spring anchors  120 A-D. In an embodiment, spring portion  122  comprises mesa structure  104  formed on spring platform  144 , four spring arms  124 A-D, two electrodes  116 A-B forming a bipolar electrode, and two electrode leads  114 A-B. Spring platform  144  provides a structural base for the formation of mesa structure  104  and additional elements of the transfer head (e.g., electrodes and dielectric layer). 
     In an embodiment, multiple spring arms  124 A-D enable top surface  108  and the additional device components formed thereon to remain level when spring portion  122  is deflected into underlying space  112 . A level top surface of the transfer head may improve contact with the top surface of a micro device during a pickup operation. In an embodiment, each spring arm  124  extends from a corner of spring platform  144  and runs parallel to the edge of spring platform  144  before attaching to the base substrate at a spring anchor  120 . By running the length of one edge of spring platform  144 , spring arms  124 A-D have sufficient length to enable a desired degree of deflection of spring portion  122  into underlying space  112 . In an embodiment, spring arms  124  have a thickness T less than their width W to ensure that the spring portion  122  deflects downward into underlying space  112  in response to pressure applied to top surface  108 , while experiencing minimal torsional/lateral deformation. The specific dimensions of spring arms  124  depend on the modulus of the material from which they are formed. Spring arms  124  may be formed from any of the materials discussed above with respect to spring arm  124  in  FIGS. 1-2 . 
     Additionally, mesa structure  104  may have the characteristics discussed above with respect to a mesa structure formed on a cantilever structure spring portion. In an embodiment, mesa structure  104  is formed integrally with spring platform  144 . In another embodiment, mesa structure  104  is formed over spring platform  144 . 
     Still referring to the embodiment illustrated in  FIG. 10 , electrode leads  114 A,  114 B each run from a respective electrode  116 A,  116 B, down a tilted sidewall  106  of mesa structure  104  and along a respective spring arm  124 A,  124 B to spring anchors  120 A,  120 B. In an embodiment, electrode leads  114  connect the micro device transfer head to the working electronics of an electrostatic gripper assembly via interconnects in the base substrate. It is to be understood that other electrode and electrode lead configurations may be used in conjunction with a spring member having multiple spring arms, such as, a monopolar electrode ( FIG. 2 ) and electrode lead vias ( FIG. 4 ), as discussed above with respect to a spring member having a single-arm cantilever structure. 
     Referring now to  FIG. 11 , a side view illustration is provided of a compliant micro device transfer head  200  having a spring member with a membrane structure and a bipolar electrode, along with a corresponding transfer head array, according to an embodiment of the invention. As shown, the bipolar device transfer head  200  may include a base substrate  202 , a spring member comprising spring anchor  220  coupled to base substrate  202  and a spring portion  222  comprising electrodes  216 A/ 216 B, and a dielectric layer  213  covering the top surface of electrodes  216 A/ 216 B. The spring portion  222  is deflectable into a space  212  between the spring portion  222  and the base substrate  202 . In an embodiment of the invention, spring portion  222  additionally comprises spring layer  266 , and mesa structure  204  having top surface  208  and tapered sidewalls  206 . 
     Base substrate  202  may be formed from a variety of materials such as silicon, ceramics and polymers that are capable of providing structural support, as described above with respect to base substrate  102 . Base substrate  202  may additionally include interconnect  230  to connect the micro device transfer head  200  to the working electronics of an electrostatic gripper assembly via electrode lead  214 A or  214 B. 
     A top-down view of spring member  210  having a membrane structure is illustrated in  FIG. 12 , according to an embodiment of the invention. In an embodiment, the spring anchor  220  comprises the full perimeter of spring portion  222 , at the interface of spring portion  222  and base substrate  202 . In an embodiment, spring portion  222  comprises a mesa structure  204  that is centrally positioned with respect to spring anchor  220 . Other elements of spring member  210  have been omitted from  FIG. 12  for clarity. 
     Referring back to  FIG. 11 , spring portion  222  comprises spring layer  266 , mesa structure  204 , electrodes  216 A,  216 B, and electrode leads  214 A,  214 B. Spring layer  266  is formed from a material having an elastic modulus that enables deflection of spring portion  222  into space  212  over the working temperature range of the micro device transfer process. In an embodiment, spring layer  266  is formed from the same or different material as base substrate  202 , for example, semiconductor materials such as silicon or dielectric materials such as silicon dioxide and silicon nitride. In an embodiment, spring layer  266  is integrally formed from base substrate  202 , such as, during the etching of space  212 . In another embodiment, spring layer  266  is formed from a layer of material bonded onto the surface of base substrate  202 . An optional passivation layer (not shown) may be formed over spring layer  266  in order to isolate spring layer  266  from electrodes  216 . In an embodiment, spring layer  266  is from 0.5 μm to 2 μm thick. 
     In an embodiment, electrodes  216 A,  216 B are formed over spring layer  266  and over the top surface  208  of mesa structure  204 . Electrode leads  214 A,  214 B may run from electrodes  216 A,  216 B along the top surface  209  of spring layer  266 , and over spring anchor  220 . In an embodiment, electrode leads  214 A,  214 B connect to interconnect  230  in base substrate  202 . The materials and dimensions of electrodes  216 A,  216 B and electrode leads  214 A,  214 B may be the same as described above with respect to electrodes  116  and electrode leads  114 . 
     Dielectric layer  213  is formed over the surface. In an embodiment, the dielectric layer  213  has a suitable thickness and dielectric constant for achieving the required grip pressure of the micro device transfer head, and sufficient dielectric strength to not break down at the operating voltage. The dielectric layer  213  may be a single layer or multiple layers, and may be the same or different material as the optional passivation layer. Suitable dielectric materials may include, but are not limited to, aluminum oxide (Al 2 O 3 ) and tantalum oxide (Ta 2 O 5 ), as described above with respect to dielectric layer  113 . In an embodiment, dielectric layer  213  is from 0.5 to 2 μm thick. In an embodiment, top surface  226  of dielectric layer  213  over the mesa structure  204  corresponds to the top surface of the compliant micro device transfer head  200 . 
     In an embodiment space  212  is a cavity in the surface of base substrate  202 . In an embodiment, spring portion  222  completely covers space  212 . Spring portion  222  is deflectable into space  212 . The depth of space  212  is determined by the amount of deflection desired for spring portion  222 , while the width of space  212  is determined by the pitch of the transfer head array, as discussed above with respect to space  112 . The width of space  212  is less than the pitch of the transfer heads, but greater than the top surface  226  of each transfer head  200 . 
     In an embodiment, transfer head  200  further comprises backside electrode  234  and opposing electrode  238 . In an embodiment, backside electrode  234  is formed on the lower surface of spring portion  222 , underneath mesa structure  204 . In an embodiment, opposing electrode  238  is formed within space  212  opposite backside electrode  234 . In an embodiment, dielectric layer  236  covers opposing electrode  238 . In another embodiment, dielectric layer covers backside electrode  234 . Electrodes  234  and  238  may be operated so as to sense deflection of spring portion  222 , to monitor the resonant frequency of spring portion  222 , and/or to lock spring portion  222  in the deflected position, as described above with respect to backside electrode  134 , opposing electrode  138 , and dielectric  136  in  FIG. 6C . 
       FIGS. 13A-E  illustrate cross-sectional views of additional embodiments of spring members having a membrane structure. In  FIG. 13A , the layers of material forming spring portion  222  of spring member  210  are shaped to form a mesa structure  204 , according to an embodiment of the invention. In  FIG. 13B , spring portion  222  further comprises grooves  215 , according to an embodiment of the invention. Grooves  215  may reduce the pressure required to deflect spring portion  222  into space  212 . In  FIG. 13C , mesa structure  204  is formed over spring layer  266 , according to an embodiment of the invention. In an embodiment, electrodes  216 A,  216 B are formed over mesa structure  204 , and dielectric layer  213  covers electrodes  216 A,  216 B. 
     In  FIG. 13D , the compliant transfer head comprises strain sensor  228 A. In an embodiment, strain sensor  228 A is formed on dielectric layer  213 , over the interface of spring anchor  220  and spring portion  222 . When the spring portion  222  deflects into space  212  during a pick up operation, spring portion  222  and dielectric layer  213  deflect, straining strain sensor  228 A. As such, the degree of deflection of membrane spring member  210  during a pick up operation can be measured. In addition, strain sensor  228 A can be used to detect changes in the resonant frequency of spring portion  222  that indicate a micro device has been picked up by the transfer head. Strain sensor  228 A may be formed from a piezoelectric or piezoresistive material, as described above with respect to strain sensor  128 A in  FIG. 6A . 
     In  FIG. 13E , the compliant transfer head comprises strain sensor  228 B. In an embodiment, strain sensor  228 B is formed within a silicon spring layer  266 , spanning the interface of spring anchor  220  and spring portion  222 . When spring portion  222  deflects into space  212  during a pick up operation, the spring layer  266  portion of spring portion  222  deflects, straining strain sensor  228 B. As such, the degree of deflection of spring member  210  during a pick up operation can be measured. In addition, as discussed above with respect strain sensors  128 A,  128 B, and  228 A, strain sensor  228 B can be used to detect changes in the resonant frequency of spring portion  222  that indicate a micro device has been picked up by the transfer head. In an embodiment, strain sensor  228 B is formed from a piezoresistive material, as described above with respect to strain sensor  128 B in  FIG. 6B . In an embodiment where spring layer  266  is silicon, strain sensor  228 B may be formed by doping a portion of spring layer  266 . 
       FIGS. 14A-E  illustrate a method for forming a micro device transfer head, according to an embodiment of the invention. In an embodiment, a base substrate  1402  having active zones  1454  is provided, as shown in  FIG. 14A . A dielectric layer  1456  comprising interconnects  1452  and buried electrode  1438  is formed over base substrate  1402 . Interconnects  1452  and electrode  1438  each connect to active zones  1454  in base substrate  1402 . A metal bump  1450  is formed over each interconnect  1452 . In an embodiment, base substrate  1402  has the characteristics discussed above with respect to base substrate  202 . In an embodiment, base substrate  1402  is silicon. Active zones  1454  may be n-type or p-type doped. Interconnects  1452  and buried electrode  1438  may be any suitable conductive material, such as Al or Cu. Metal bumps  1450  may be any suitable conductive material, such as Cu or Au. In an embodiment, metal bumps  1450  are 2 μm thick. 
     Next, a handle substrate  1460  having an oxide layer  1462  and a spring layer  1466  is provided, as shown in  FIG. 14B . An SOI substrate may be used, wherein handle substrate  1460  is a silicon wafer, oxide layer  1462  is silicon oxide, and spring layer  1466  is silicon. In an embodiment, oxide layer  1462  is approximately 2 μm thick. In an embodiment, spring layer  1466  is 0.5 μm to 1 μm thick. In an embodiment, the spring layer  1466  is coupled to metal bumps  1450  via metal pads  1464 . In an embodiment, metal pads  1464  are Au and up to 1 μm thick. In an embodiment, backside electrode  1434  is formed on the surface of spring layer  1466  between metal pads  1464 . Backside electrode  1434  may be any suitable conductive material, as discussed above with respect to backside electrode  234  in  FIG. 11 . 
     Spring layer  1466  is bound to metal bumps  1450  via metal pads  1464 , creating space  1412  between the surfaces of spring layer  1466  and dielectric layer  1456 , according to an embodiment. In an embodiment, the subsequently formed spring portion comprising spring layer  1466  will be deflectable into space  1412 . In an embodiment, space  1412  is approximately 2 μm thick, corresponding to the 2 μm thickness of metal bumps  1450 . In an embodiment, backside electrode  1434  is aligned over buried electrode  1438 . 
     In an embodiment, handle substrate  1460  is then removed. Handle substrate  1460  may be removed by any appropriate method, such as chemical-mechanical polishing (CMP) or wet etch. In an embodiment, oxide layer  1462  is patterned to form mesa structure  1404  on the surface of spring layer  1466 , as shown in  FIG. 14C . Oxide layer  1462  may be patterned by any appropriate method known in the art. In an embodiment, oxide layer  1462  is completely removed from the surfaces of spring layer  1466  that are adjacent to mesa structure  1404 . A passivation layer  1411  may then be formed over the top surface of spring layer  1466  and mesa structure  1404 , as shown in  FIG. 14D . Passivation layer  1411  electrically isolates metal electrodes  1416  from spring layer  1466  to prevent shorting. In an embodiment, passivation layer  1411  is 500 Å thick. Passivation layer  1411  may be any suitable insulating dielectric material, such as Al 2 O 3  or Ta 2 O 5 . In another embodiment, oxide layer  1462  is etched to leave a thin portion of oxide material covering the surface of spring layer  1466  between mesa structures  1404 . In such a case, passivation layer  1411  may be omitted. 
     A layer of metal is then blanket deposited over the surface of passivation layer  1411  and patterned to form electrodes  1416 A,  1416 B and signal lines  1417 . Electrodes  1416  and signal lines  1417  may be formed from a conductive material such as those discussed above with respect to electrodes  116 A,  116 B. 
     Dielectric layer  1413  is then blanket deposited over the surface, as shown in  FIG. 14D . In an embodiment, the dielectric layer  1413  has a suitable thickness and dielectric constant for achieving the required grip pressure of the micro device transfer head, and sufficient dielectric strength to not break down at the operating voltage. The dielectric layer  1413  may be a single layer or multiple layers, and may be the same or different material as passivation layer  1411 . Suitable dielectric materials may include, but are not limited to, aluminum oxide (Al 2 O 3 ) and tantalum oxide (Ta 2 O 5 ), as described above with respect to dielectric layer  113 ,  213 . Dielectric layer  1413  is from 0.5 μm to 2 μm thick. In an embodiment, dielectric layer  1413  is a 0.5 μm thick layer of Al 2 O 3 . In an embodiment, dielectric layer  1413  is deposited by atomic layer deposition (ALD). 
       FIG. 14D  illustrates an embodiment of a micro device transfer head having a spring member with a membrane structure. The structure in  FIG. 14D  may be further processed to form a transfer head having other spring member structures. For example, a portion of spring layer  1466 , passivation layer  1411 , and dielectric layer  1413  are patterned to define a spring member with a multi spring-arm structure, as illustrated in cross-section by  FIG. 14E . Spring layer  1466 , passivation layer  1411 , and dielectric layer  1413  may also be patterned to form a cantilever structure, such as that described above with respect to  FIGS. 1-2 . 
     In the embodiments shown in each of  FIGS. 14D and 14E , backside electrode  1434  and buried electrode  1438  form a capacitive sensor as described above with respect to backside electrode  134  and opposing electrode  138  in  FIG. 6C , and backside electrode  234  and opposing electrode  238  in  FIG. 11 . In another embodiment, backside electrode  1434  and buried electrode  1438  enable the locking of the spring portion in the deflected position, as described above with respect to backside electrode  134  and opposing electrode  138  in  FIG. 6C , and backside electrode  234  and opposing electrode  238  in  FIG. 11 . 
       FIGS. 15A-K  illustrate a cross sectional view of a method for forming a micro device transfer head having spring member with a membrane structure, according to an embodiment of the invention. Handle substrate  1560  is provided, as shown in  FIG. 15A . Handle substrate  1560  may be formed from a variety of materials such as silicon, ceramics and polymers that are capable of providing structural support for subsequent formation of device layers. In an embodiment, handle substrate  1560  is a silicon wafer. 
     Next, mesa cavities  1561  are patterned in the surface of handle substrate  1560 , according to an embodiment of the invention shown in  FIG. 15B . Mesa cavities may be formed by any suitable process, such as photolithography and etching. In an embodiment, patterning layer  1562  is formed over one surface of handle substrate  1560  for the patterning of mesa cavities  1561 . In an embodiment, patterning layer  1562  is photoresist. In another embodiment, patterning layer  1562  is a hardmask material, such as silicon oxide or silicon nitride. In an embodiment, mesa cavities  1561  are spaced at intervals corresponding to the pitch of the micro device transfer head array, for example 5 μm to 10 μm, corresponding to an integer multiple of the array of micro devices to be picked up. The dimensions of each mesa cavity  1561  are determined by the desired dimensions of the top surface of the transfer head after the addition of additional device components, such as the electrodes and the dielectric layer, as described above with respect to  FIGS. 1-2 and 11 . In an embodiment, mesa cavities  1561  are 7 μm×7 μm wide and 2 μm deep. After the etching of mesa cavities  1561 , patterning layer  1562  may removed as shown in  FIG. 15C . 
     A spring layer  1566  is then formed over the surface of patterned handle substrate  1560 , according to an embodiment of the invention shown in  FIG. 15D . Spring layer  1566  may be any material suitable to form the structural basis of a membrane spring member in a micro device transfer head. The material and dimensions of spring layer  1566  are selected to enable the spring portion of the subsequently formed transfer head to deflect a desired amount under the operating conditions of the transfer process, as discussed above with respect to spring layer  266 . Spring layer  1566  may be an oxide or nitride layer. In an embodiment, spring layer  1566  is a grown thermal oxide on the surface of silicon handle substrate  1560 . In another embodiment, spring layer  1566  is formed by plasma enhanced chemical vapor deposition (PECVD). In an embodiment, spring layer  1566  is 0.5 μm to 3 μm thick. 
     In an embodiment, grooves  1515  are etched into the surface of spring layer  1566 . Grooves  1515  may reduce the pressure required to deflect the transfer head spring portion that is subsequently formed comprising spring layer  1566 . In another embodiment, after the formation of spring layer  1566 , the remaining volume of each mesa cavity  1561  is filled with material, such as oxide or nitride (not shown), to be planar with the lower surfaces of spring layer  1566 . 
     A base substrate  1502  having pits  1519  is then provided, as shown in  FIG. 15E . Base substrate  1502  may be any of the materials described above with respect to base substrate  202 . In an embodiment, pits  1519  are each at least 20 μm wide and 2 μm deep. In an embodiment, the pitch of pits  1519  matches the pitch of mesa cavities  1561 . 
     Handle substrate  1560  having spring layer  1566  thereon is then coupled to base substrate  1502 , as shown in  FIG. 15F . Spring layer  1566  and base substrate  1502  may be coupled by any suitable process, such as wafer bonding. In an embodiment, the mesa cavities  1561  in spring layer  1566  align with pits  1519  to enclose spaces  1512 . Handle substrate  1560  is then removed, leaving spring layer  1566  bonded to the surface of base substrate  1502  as shown in  FIG. 15G . Handle substrate  1560  may be removed by any suitable process or processes, such as CMP and wet etch. The removal of handle substrate  1560  leaves a membrane of spring layer  1566  having a mesa structure  1504  covering each space  1512 . 
     Metal layer  1563  is then formed over the surface of spring layer  1566 , as shown in  FIG. 15H . Metal layer  1563  may be formed from any suitable metal or layer of metals that adheres well to the underlying spring layer  1566 , as discussed above with respect to materials for electrodes  116 ,  216 . In an embodiment, metal layer  1563  is formed by sputter deposition. Metal layer  1563  is up to 0.5 μm thick. In an embodiment, metal layer  1563  is a 500 Å thick layer of TiW. 
     Metal layer  1563  is then patterned to form electrodes, according to an embodiment of the invention. Metal layer  1563  may be patterned by forming mask  1565  on the surface of metal layer  1563 , as shown in  FIG. 15I . Metal layer  1563  is then etched to form electrodes  1516 A,  1516 B and electrode leads  1514 A,  1514 B, as shown in  FIG. 15J . 
     Next, dielectric layer  1513  is formed over spring layer  1566  and electrodes  1516 . Dielectric layer  1513  has the properties described above with respect to dielectric layer  113 ,  213 . In an embodiment, dielectric layer  1513  is a 0.5 μm thick layer of Al 2 O 3 . In an embodiment, dielectric layer  1513  is deposited by atomic layer deposition (ALD). 
       FIGS. 16A-D  illustrate cross-sectional views of micro device transfer head structures wherein the spring portion is elevated above the surface of the base substrate. In an embodiment, spring portion  1622  comprises spring layer  1624  to create a spring member  1610  with a cantilever structure. Spring layer  1624  is elevated above the top surface of base substrate  1602  to create space  1612 , as shown in  FIG. 16A . Spring portion  1622  is deflectable into space  1612 . Spring portion  1622  further comprises one or more electrodes  1616 . Dielectric layer  1613  covers spring member  1610 . 
     In another embodiment, spring portion  1622  is elevated above the top surface of base substrate  1602  by spring anchors  1620 A and  1620 B, as shown in  FIG. 16B . Spring portion  1622  additionally comprises electrodes  1616 A and  1616 B. Dielectric layer  1613  is formed over the top surface of spring member  1610 . Space  1612  is formed between spring portion  1622  and base substrate  1602 . Spring portion  1622  is deflectable into space  1612 . 
     In another embodiment, spring portion  1622  further comprises mesa structure  1604  on spring layer  1624 . An embodiment of a spring member  1610  with a cantilever structure, wherein the spring portion comprises a mesa structure  1604  is shown in  FIG. 16C . An embodiment of a spring member  1610  with a table structure, wherein the spring portion comprises a mesa structure  1604  is shown in  FIG. 16D . 
       FIG. 17  is a flow chart illustrating a method of picking up and transferring a micro device from a carrier substrate to a receiving substrate in accordance with an embodiment of the invention. At operation  1710  a compliant transfer head is positioned over a micro device connected to a carrier substrate. The compliant transfer head may comprise a base substrate, a spring member including a spring anchor coupled to the base substrate and a spring portion comprising an electrode where the spring portion is deflectable into a space between the spring portion and the base substrate, and a dielectric layer covering the top surface of the electrode as described in the above embodiments. The transfer head may have a monopolar or bipolar electrode configuration and a cantilever or membrane spring member structure, as well as any other structural variations as described in the above embodiments. The micro device is contacted with the compliant transfer head at operation  1720 . In an embodiment, the micro device is contacted with the dielectric layer of the transfer head. In an alternative embodiment, the transfer head is positioned over the micro device with a suitable air gap separating them which does not significantly affect the grip pressure, for example, 1 nm (0.001 μm) or 10 nm (0.01 μm). At operation  1730  a voltage is applied to the electrode to create a grip pressure on the micro device, and the micro device is picked up with the transfer head at operation  1740 . The micro device is then released onto a receiving substrate at operation  1750 . 
     While operations  1710 - 1750  have been illustrated sequentially in  FIG. 17 , it is to be appreciated that embodiments are not so limited and that additional operations may be performed and certain operations may be performed in a different sequence. For example, in one embodiment, after contacting the micro device with the transfer head, the transfer head is rubbed across a top surface of the micro device in order to dislodge any particles which may be present on the contacting surface of either of the transfer head or micro device. In another embodiment, an operation is performed to create a phase change in the bonding layer connecting the micro device to the carrier substrate prior to or while picking up the micro device. If a portion of the bonding layer is picked up with the micro device, additional operations can be performed to control the phase of the portion of the bonding layer during subsequent processing. 
     Operation  1730  of applying the voltage to the electrode to create a grip pressure on the micro device can be performed in various orders. For example, the voltage can be applied prior to contacting the micro device with the transfer head, while contacting the micro device with the transfer head, or after contacting the micro device with the transfer head. The voltage may also be applied prior to, while, or after creating the phase change in the bonding layer. 
     The micro device may be any of the micro LED device structures illustrated in  FIGS. 27-29 , and those described in related U.S. patent application Ser. No. 13/372,222. For example, referencing  FIG. 27 , a micro LED device  300  may include a micro p-n diode  335 ,  350  and a metallization layer  320 , with the metallization layer between the micro p-n diode  335 ,  350  and a bonding layer  310  formed on a substrate  301 . In an embodiment, the micro p-n diode  335 ,  350  includes a top n-doped layer  314 , one or more quantum well layers  316 , and a lower p-doped layer  318 . The micro p-n diodes can be fabricated with straight sidewalls or tapered sidewalls. In certain embodiments, the micro p-n diodes  350  possess outwardly tapered sidewalls  353  (from top to bottom). In certain embodiments, the micro p-n diodes  335  possess inwardly tapered sidewalls  353  (from top to bottom). The metallization layer  320  may include one or more layers. For example, the metallization layer  320  may include an electrode layer and a barrier layer between the electrode layer and the bonding layer. The micro p-n diode and metallization layer may each have a top surface, a bottom surface and sidewalls. In an embodiment, the bottom surface  351  of the micro p-n diode  350  is wider than the top surface  352  of the micro p-n diode, and the sidewalls  353  are tapered outwardly from top to bottom. The top surface of the micro p-n diode  335  may be wider than the bottom surface of the p-n diode, or approximately the same width. In an embodiment, the bottom surface  351  of the micro p-n diode  350  is wider than the top surface  321  of the metallization layer  320 . The bottom surface of the micro p-n diode may also be wider than the top surface of the metallization layer, or approximately the same width as the top surface of the metallization layer. 
     A conformal dielectric barrier layer  360  may optionally be formed over the micro p-n diode  335 ,  350  and other exposed surfaces. The conformal dielectric barrier layer  360  may be thinner than the micro p-n diode  335 ,  350  metallization layer  320  and optionally the bonding layer  310  so that the conformal dielectric barrier layer  360  forms an outline of the topography it is formed on. In an embodiment, the micro p-n diode  335 ,  350  is several microns thick, such as 3 μm, the metallization layer  320  is 0.1 μm-2 μm thick, and the bonding layer  310  is 0.1 μm-2 μm thick. In an embodiment, the conformal dielectric barrier layer  360  is approximately 50-600 angstroms thick aluminum oxide (Al 2 O 3 ). Conformal dielectric barrier layer  360  may be deposited by a variety of suitable techniques such as, but not limited to, atomic layer deposition (ALD). The conformal dielectric barrier layer  360  may protect against charge arcing between adjacent micro p-n diodes during the pick up process, and thereby protect against adjacent micro p-n diodes from sticking together during the pick up process. The conformal dielectric barrier layer  360  may also protect the sidewalls  353 , quantum well layer  316  and bottom surface  351 , of the micro p-n diodes from contamination which could affect the integrity of the micro p-n diodes. For example, the conformal dielectric barrier layer  360  can function as a physical barrier to wicking of the bonding layer material  310  up the sidewalls and quantum layer  316  of the micro p-n diodes  350 . The conformal dielectric barrier layer  360  may also insulate the micro p-n diodes  350  once placed on a receiving substrate. In an embodiment, the conformal dielectric barrier layer  360  span sidewalls  353  of the micro p-n diode, and may cover a quantum well layer  316  in the micro p-n diode. The conformal dielectric barrier layer may also partially span the bottom surface  351  of the micro p-n diode, as well as span sidewalls of the metallization layer  320 . In some embodiments, the conformal dielectric barrier layer also spans sidewalls of a patterned bonding layer  310 . A contact opening  362  may be formed in the conformal dielectric barrier layer  360  exposing the top surface  352  of the micro p-n diode. 
     Referring to  FIG. 27 , the contact opening  362  may have a smaller width than the top surface  352  of the micro p-n diode and the conformal dielectric barrier layer  360  forms a lip around the edges of the top surface  352  of the micro p-n diode. Referring to  FIG. 28 , the contact opening  362  may have a slightly larger width than the top surface of the micro p-n diode. In such an embodiment, the contact opening  362  exposes the top surface  352  of the micro p-n diode and an upper portion of the sidewalls  353  of the micro p-n diode, while the conformal dielectric barrier layer  360  covers and insulates the quantum well layer(s)  316 . Referring to  FIG. 29 , the conformal dielectric layer  360  may have approximately the same width as the top surface of the micro p-n diode. The conformal dielectric layer  360  may also span along a bottom surface  351  of the micro p-n diodes illustrated in  FIGS. 27-29 . 
     In an embodiment, conformal dielectric barrier layer  360  is formed of the same material as dielectric layer  113 ,  213  of the compliant transfer head. Depending upon the particular micro LED device structure, the conformal dielectric barrier layer  360  may also span sidewalls of the bonding layer  310 , as well as the carrier substrate and posts, if present. Bonding layer  310  may be formed from a material which can maintain the micro LED device  300  on the carrier substrate  301  during certain processing and handling operations, and upon undergoing a phase change provide a medium on which the micro LED device  300  can be retained yet also be readily releasable from during a pick up operation. For example, the bonding layer may be remeltable or reflowable such that the bonding layer undergoes a phase change from solid to liquid state prior to or during the pick up operation. In the liquid state the bonding layer may retain the micro LED device in place on the carrier substrate while also providing a medium from which the micro LED device  300  is readily releasable. In an embodiment, the bonding layer  310  has a liquidus temperature or melting temperature below approximately 350° C., or more specifically below approximately 200° C. At such temperatures the bonding layer may undergo a phase change without substantially affecting the other components of the micro LED device. For example, the bonding layer may be formed of a metal or metal alloy, or a thermoplastic polymer which is removable. For example, the bonding layer may include indium, tin or a thermoplastic polymer such as polyethylene or polypropylene. In an embodiment, the bonding layer may be conductive. For example, where the bonding layer undergoes a phase change from solid to liquid in response to a change in temperature a portion of the bonding layer may remain on the micro LED device during the pick up operation. In such an embodiment, it may be beneficial that the bonding layer is formed of a conductive material so that it does not adversely affect the micro LED device when it is subsequently transferred to a receiving substrate. In this case, the portion of conductive bonding layer remaining on the micro LED device during the transfer may aid in bonding the micro LED device to a conductive pad on a receiving substrate. In a specific embodiment, the bonding layer may be formed of indium, which has a melting temperature of 156.7° C. The bonding layer may be laterally continuous across the substrate  301 , or may also be formed in laterally separate locations. For example, a laterally separate location of the bonding layer may have a width which is less than or approximately the same width as the bottom surface of the micro p-n diode or metallization layer. In some embodiments, the micro p-n diodes may optionally be formed on posts  302  on the substrate. 
     Solders may be suitable materials for bonding layer  310  since many are generally ductile materials in their solid state and exhibit favorable wetting with semiconductor and metal surfaces. A typical alloy melts not a single temperature, but over a temperature range. Thus, solder alloys are often characterized by a liquidus temperature corresponding to the lowest temperature at which the alloy remains liquid, and a solidus temperature corresponding to the highest temperature at which the alloy remains solid. An exemplary list of low melting solder materials which may be utilized with embodiments of the invention are provided in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Chemical composition 
                 Liquidus 
                 Solidus 
               
               
                 (weight %) 
                 Temperature (° C.) 
                 Temperature (° C.) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 100In 
                 156.7 
                 156.7 
               
               
                 66.3In33.7Bi 
                 72 
                 72 
               
               
                 51In32.5Bi16.5Sn 
                 60 
                 60 
               
               
                 57Bi26In17Sn 
                 79 
                 79 
               
               
                 54.02Bi29.68In16.3Sn 
                 81 
                 81 
               
               
                 67Bi33In 
                 109 
                 109 
               
               
                 90In10Sn 
                 151 
                 143 
               
               
                 48In52Sn 
                 118 
                 118 
               
               
                 50In50Sn 
                 125 
                 118 
               
               
                 52Sn48In 
                 131 
                 118 
               
               
                 58Sn42In 
                 145 
                 118 
               
               
                 97In3Ag 
                 143 
                 143 
               
               
                 94.5In5.5Ag 
                 200 
                 — 
               
               
                 99.5In0.5Au 
                 200 
                 — 
               
               
                 95In5Bi 
                 150 
                 125 
               
               
                 99.3In0.7Ga 
                 150 
                 150 
               
               
                 99.4In0.6Ga 
                 152 
                 152 
               
               
                 99.6In0.4Ga 
                 153 
                 153 
               
               
                 99.5In0.5Ga 
                 154 
                 154 
               
               
                 58Bi42Sn 
                 138 
                 138 
               
               
                 60Sn40Bi 
                 170 
                 138 
               
               
                 100Sn 
                 232 
                 232 
               
               
                 95Sn5Sb 
                 240 
                 235 
               
               
                 100Ga 
                 30 
                 30 
               
               
                 99In1Cu 
                 200 
                 — 
               
               
                 98In2Cu 
                 182 
                 — 
               
               
                 96In4Cu 
                 253 
                 — 
               
               
                 74In26Cd 
                 123 
                 123 
               
               
                 70In30Pb 
                 175 
                 165 
               
               
                 60In40Pb 
                 181 
                 173 
               
               
                 50In50Pb 
                 210 
                 184 
               
               
                 40In60Pb 
                 231 
                 197 
               
               
                 55.5Bi44.5Pb 
                 124 
                 124 
               
               
                 58Bi42Pb 
                 126 
                 124 
               
               
                 45.5Bi54.5Pb 
                 160 
                 122 
               
               
                 60Bi40Cd 
                 144 
                 144 
               
               
                 67.8Sn32.2Cd 
                 177 
                 177 
               
               
                 45Sn55Pb 
                 227 
                 183 
               
               
                 63Sn37Pb 
                 183 
                 183 
               
               
                 62Sn38Pb 
                 183 
                 183 
               
               
                 65Sn35Pb 
                 184 
                 183 
               
               
                 70Sn30Pb 
                 186 
                 183 
               
               
                 60Sn40Pb 
                 191 
                 183 
               
               
                 75Sn25Pb 
                 192 
                 183 
               
               
                 80Sn20Pb 
                 199 
                 183 
               
               
                 85Sn15Pb 
                 205 
                 183 
               
               
                 90Sn10Pb 
                 213 
                 183 
               
               
                 91Sn9Zn 
                 199 
                 199 
               
               
                 90Sn10Au 
                 217 
                 217 
               
               
                 99Sn1Cu 
                 227 
                 227 
               
               
                 99.3Sn0.7Cu 
                 227 
                 227 
               
               
                   
               
            
           
         
       
     
     An exemplary list thermoplastic polymers which may be utilized with embodiments of the invention are provided in Table 2. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Polymer 
                 Melting Temperature (° C.) 
               
               
                   
               
             
            
               
                 Acrylic (PMMA) 
                 130-140 
               
               
                 Polyoxymethylene (POM or Acetal) 
                 166 
               
               
                 Polybutylene terephthalate (PBT) 
                 160 
               
               
                 Polycaprolactone (PCL) 
                  62 
               
               
                 Polyethylene terephthalate (PET) 
                 260 
               
               
                 Polycarbonate (PC) 
                 267 
               
               
                 Polyester 
                 260 
               
               
                 Polyethylene (PE) 
                 105-130 
               
               
                 Polyetheretherketone (PEEK) 
                 343 
               
               
                 Polylactic acid (PLA) 
                 50-80 
               
               
                 Polypropylene (PP) 
                 160 
               
               
                 Polystyrene (PS) 
                 240 
               
               
                 Polyvinylidene chloride (PVDC) 
                 185 
               
               
                   
               
            
           
         
       
     
       FIG. 18  is a flow chart illustrating a method of picking up and transferring a micro device from a carrier substrate to a receiving substrate in accordance with an embodiment of the invention. At operation  1810  a compliant transfer head is positioned over a micro device connected to a carrier substrate with a bonding layer. The compliant transfer head may be any transfer head described herein. The micro device may be any of the micro LED device structures illustrated in  FIGS. 27-29  and those described in related U.S. Provisional Application No. 61/561,706 and U.S. Provisional Application No. 61/594,919. The micro device is then contacted with the transfer head at operation  1820 . In an embodiment, the micro device is contacted with the dielectric layer  113 ,  213  of the transfer head. In an alternative embodiment, the transfer head is positioned over the micro device with a suitable air gap separating them which does not significantly affect the grip pressure, for example, 1 nm (0.001 μm) or 10 nm (0.01 μm). At operation  1825  an operation is performed to create a phase change in the bonding layer  310  from solid to liquid state. For example, the operation may include heating an In bonding layer at or above the melting temperature of 156.7° C. In another embodiment, operation  1825  can be performed prior to operation  1820 . At operation  1830  the micro device is picked up with the compliant transfer head. For example, a voltage can be applied to an electrode to create a grip pressure on the micro device. A substantial portion of the bonding layer  310  may also be picked up with the transfer head at operation  1840 . For example, approximately half of the bonding layer  310  may be picked up with the micro device. In an alternative embodiment, none of the bonding layer  310  is picked up with the transfer head. The micro device, and optionally a portion of the bonding layer  310 , is placed in contact with a receiving substrate. The micro device is then released onto the receiving substrate at operation  1850 . In accordance with an embodiment of the invention, a variety of operations can be performed to control the phase of the portion of the bonding layer when picking up, transferring, contacting the receiving substrate, and releasing the micro device and portion of the bonding layer  310  on the receiving substrate. For example, the portion of the bonding layer which is picked up with the micro device can be maintained in the liquid state during contacting the receiving substrate and during the release operation  1850 . In another embodiment, the portion of the bonding layer can be allowed to cool to a solid phase after being picked up. For example, the portion of the bonding layer can be in a solid phase during contacting the receiving substrate, and again melted to the liquid state prior to or during the release operation  1850 . A variety of temperature and material phase cycles can be performed in accordance with embodiments of the invention. 
       FIG. 19  is a flow chart illustrating a method of picking up and transferring an array of micro devices from a carrier substrate to at least one receiving substrate in accordance with an embodiment of the invention. At operation  1910  an array of compliant transfer heads is positioned over an array of micro devices. The compliant transfer heads may be any transfer head described herein. At operation  1920  the array of micro devices are contacted with the array of transfer heads. In an alternative embodiment, the array of transfer heads is positioned over the array of micro devices with a suitable air gap separating them which does not significantly affect the grip pressure, for example, 1 nm (0.001 μm) or 10 nm (0.01 μm).  FIG. 22A  is a side view illustration of an array of micro device transfer heads  200  in contact with an array of micro LED devices  300  in accordance with an embodiment of the invention. As illustrated in  FIG. 22A , the pitch (P) of the array of transfer heads  200  matches the pitch of the micro LED devices  300 , with the pitch (P) of the array of transfer heads being the sum of the spacing (S) between transfer heads and width (W) of a transfer head. 
     In one embodiment, the array of micro LED devices  300  have a pitch of 10 μm, with each micro LED device having a spacing of 2 μm and a maximum width of 8 μm. In an exemplary embodiment, assuming a micro p-n diode  350  with straight sidewalls the top surface of the each micro LED device  300  has a width of approximately 8 μm. In such an exemplary embodiment, the width of the top surface  226  of a corresponding transfer head  200  is approximately 8 μm or smaller so as to avoid making inadvertent contact with an adjacent micro LED device. In another embodiment, the array of micro LED devices  300  may have a pitch of 5 μm, with each micro LED device having a spacing of 2 μm and a maximum width of 3 μm. In an exemplary embodiment, the top surface of the each micro LED device  300  has a width of approximately 3 μm. In such an exemplary embodiment, the width of the top surface  226  of a corresponding transfer head  200  is approximately 3 μm or smaller so as to avoid making inadvertent contact with an adjacent micro LED device  300 . However, embodiments of the invention are not limited to these specific dimensions, and may be any suitable dimension. 
       FIG. 22B  is a side view illustration of an array of micro device transfer heads in contact with an array of micro LED devices  300  in accordance with an embodiment of the invention. In the embodiment illustrated in  FIG. 22B , the pitch (P) of the transfer heads is an integer multiple of the pitch of the array of micro devices. In the particular embodiment illustrated, the pitch (P) of the transfer heads is 3 times the pitch of the array of micro LED devices. In such an embodiment, having a larger transfer head pitch may protect against arcing between transfer heads. 
     Referring again to  FIG. 19 , at operation  1930  a voltage is selectively applied to a portion of the array of transfer heads  200 . Thus, each transfer head  200  may be independently operated. At operation  1940  a corresponding portion of the array of micro devices is picked up with the portion of the array of transfer heads to which the voltage was selectively applied. In one embodiment, selectively applying a voltage to a portion of the array of transfer heads means applying a voltage to every transfer head in the array of transfer heads.  FIG. 23  is a side view illustration of every transfer head in an array of micro device transfer heads picking up an array of micro LED devices  300  in accordance with an embodiment of the invention. In another embodiment, selectively applying a voltage to a portion of the array of transfer heads means applying a voltage to less than every transfer head (e.g. a subset of transfer heads) in the array of transfer heads.  FIG. 24  is a side view illustration of a subset of the array of micro device transfer heads picking up a portion of an array of micro LED devices  300  in accordance with an embodiment of the invention. In a particular embodiment illustrated in  FIGS. 23-24 , the pick up operation includes picking up the micro p-n diode  350 , the metallization layer  320  and a portion of the conformal dielectric barrier layer  360  for the micro LED device  300 . In a particular embodiment illustrated in  FIGS. 23-24 , the pick up operation includes picking up a substantial portion of the bonding layer  310 . Accordingly, any of the embodiments described with regard to  FIGS. 19 and 22A-24  may also be accompanied by controlling the temperature of the portion of the bonding layer  310  as described with regard to  FIG. 18 . For example, embodiments described with regard to  FIGS. 19 and 22A-24  may include performing an operation to create a phase change from solid to liquid state in a plurality of locations of the bonding layer connecting the array of micro devices to the carrier substrate  301  prior to picking up the array of micro devices. In an embodiment, the plurality of locations of the bonding layer can be regions of the same bonding layer. In an embodiment, the plurality of locations of the bonding layer can be laterally separate locations of the bonding layer. 
     At operation  1950  the portion of the array of micro devices is then released onto at least one receiving substrate. Thus, the array of micro LEDs can all be released onto a single receiving substrate, or selectively released onto multiple substrates. For 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. 
       FIG. 25  is a side view illustration of an array of micro device transfer heads holding a corresponding array of micro LED devices  300  over a receiving substrate  401  including a plurality of driver contacts  410 . The array of micro LED devices  300  may then be placed into contact with the receiving substrate and then selectively released.  FIG. 26  is a side view illustration of the entire array of micro LED devices  300  released onto the receiving substrate  401  over a driver contact  410  in accordance with an embodiment of the invention. In another embodiment, a subset of the array of micro LED devices  300  is selectively released. 
     In the particular embodiments illustrated in  FIGS. 22A-26 , the micro devices  300  are those illustrated in  FIG. 27 , example  270 . However, the micro devices illustrated in  FIGS. 22A-26  may be from any of the micro LED device structures illustrated in  FIGS. 27-29 , and those described in related U.S. patent application Ser. No. 13/372,222. 
       FIG. 20  is a flow chart illustrating a method for picking up and transferring an array of micro devices from a carrier substrate to at least one receiving substrate in accordance with an embodiment of the invention. At operation  2010  an array of compliant transfer heads is positioned over an array of micro devices. The compliant transfer heads may be any transfer head described herein. At operation  2020  the array of micro devices are contacted with the array of transfer heads. 
     At operation  2030  the sensor element is used to measure the degree of deflection of the spring portion of each transfer head. The sensor may measure that the deflection of the spring portion is within an expected range, or alternatively that the deflection exceeds or falls below the expected amount.  FIG. 30  illustrates a cross-sectional view of an array of transfer heads  200 A- 200 D in contact over an array of micro devices  300 A,  300 B,  300 D. In an embodiment, transfer head  200 A is in contact with the surface of a micro device  300 A, causing the spring portion to deflect an amount D into space  212 . In another embodiment, transfer head  200 B is in contact with contamination particle  400  on the surface of micro devices  300 B, causing the spring portion to deflect an amount D+X into space  212 . In another embodiment, there is no micro device in the array position corresponding to transfer head  200 C, so that transfer head  200 C has not deflected any amount into space  212 . In yet another embodiment, the surface of micro device  300 D is irregular or damaged, such that deflection of transfer head  200 D is outside the expected range. 
     At operation  2040 , a voltage is selectively applied to those transfer heads that have deflected within the target range identified as indicating good contact for micro device pick up. In an embodiment, the pull in voltage is not applied to transfer heads that have deflected to a degree greater than the target amount of deflection, to transfer head that have deflected less than the target amount of deflection, or to both. In an embodiment, the pull in voltage is applied to all transfer heads in the array. At operation  2050 , a portion of micro devices is picked up corresponding to the selectively activated portion of micro device transfer heads. At operation  2060  the portion of the array of micro devices is then released onto at least one receiving substrate. 
       FIG. 21  is a flow chart illustrating a method of picking up and transferring an array of micro devices from a carrier substrate to at least one receiving substrate in accordance with an embodiment of the invention. Each transfer head in the array has a base substrate, a spring member including a spring anchor coupled to the base substrate and a spring portion comprising an electrode where the spring portion is deflectable into a space between the spring portion and the base substrate, and a dielectric layer covering the top surface of the electrode as described in the above embodiments. At operation  2110  each transfer head in an array of micro device transfer heads is fully depressed. An array of transfer heads may be depressed by, for example, positioning the transfer head array above a flat surface, contacting the array with the flat surface with sufficient pressure to depress each transfer head until the backside electrode  134 ,  234  of each transfer head contacts the dielectric layer  136 ,  236  covering the opposing electrode  138 ,  238  on the base substrate  102 ,  202 . 
     At operation  2120 , each transfer head is locked in the depressed position by applying a voltage across each set of electrodes to lock each transfer head in the depressed position. Where a flat surface has been used to depress the array of transfer heads, the transfer head may then be removed from the flat surface. At operation  2130 , the locking voltage is removed from a portion of the transfer heads in order to release them from the depressed position. The selectively released transfer heads are then poised to pick up micro devices.  FIG. 31  illustrates a cross-sectional view of an array of micro device transfer heads  200 , where a portion of the transfer heads  200 A,  200 D are locked in the depressed position, and a portion of the transfer heads  200 B,  200 C have been selectively released from the depressed position. 
     At operation  2140 , the selectively released array of transfer heads is positioned over an array of micro devices. At operation  2150  the array of micro devices are contacted with the array of selectively-released transfer heads. At this operation, only those transfer heads that have been selectively released from the depressed position contact the corresponding micro device in the micro device array. Those transfer heads that remain locked in the depressed position do not contact the surface of a corresponding micro device. In an alternative embodiment, the selectively-released array of transfer heads is positioned over the array of micro devices with a suitable air gap separating them which does not significantly affect the grip pressure between the selectively-released transfer heads and the corresponding portion of micro devices. The arrays may be separated by an air gap distance of, for example, 1 nm (0.001 μm) or 10 nm (0.01 μm). 
     A voltage may then be applied to the array of transfer heads  200  at operation  2160 . In an embodiment, the pull in voltage is applied to all transfer heads in the array. In another embodiment, the pull in voltage is applied only to transfer heads that have been selectively released from the depressed position. At operation  2170  a corresponding portion of the array of micro devices is picked up with the portion of the array of transfer heads that have been selectively released from the depressed position. At operation  2180  the portion of the array of micro devices is then released onto at least one receiving substrate. 
     In utilizing the various aspects of this invention, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming a micro device transfer head and head array, and for transferring a micro device and micro device array. Although the present invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as particularly graceful implementations of the claimed invention useful for illustrating the present invention.