PATENT DOCUMENT

Publication Number: US-9620478-B2
Application Number: US-201213372292-A
Country: US
Kind Code: B2

Title: Method of fabricating a micro device transfer head

Abstract:
A micro device transfer head and head array are disclosed. In an embodiment, the micro device transfer head includes a base substrate, a mesa structure with sidewalls, an electrode formed over the mesa structure, and a dielectric layer covering the electrode. A voltage can be applied to the micro device transfer head and head array to pick up a micro device from a carrier substrate and release the micro device onto a receiving substrate.

Claims:
What is claimed is: 
     
       1. A method of fabricating an array of electrostatic transfer heads comprising:
 forming an array of mesa structures on a base substrate, each mesa structure including sidewalls; 
 forming a passivation layer over the base substrate and the array of mesa structures, wherein forming the passivation over the base substrate and the array of mesa structures comprises a technique selected from the group consisting of conformal deposition of the passivation layer over the base substrate and the array of mesa structures and growing the passivation layer over the base substrate and the array of mesa structures; 
 forming a separate electrode directly on the passivation layer and over each corresponding mesa structure such that each electrode is electrically insulated from each corresponding mesa structure; and 
 depositing a dielectric layer over the array of mesa structures and each electrode. 
 
     
     
       2. The method of  claim 1 , wherein depositing the dielectric layer comprises atomic layer deposition. 
     
     
       3. The method of  claim 1 , wherein forming the separate electrodes directly on the passivation and over each corresponding mesa structure comprises forming a pair of electrodes over each corresponding mesa structure, wherein each electrode in each pair of electrodes are electrically insulated from each other, and depositing the dielectric layer over the array of mesa structures and each electrode comprises depositing the dielectric layer over each pair of electrodes. 
     
     
       4. The method of  claim 3 , further comprising forming a conductive ground plane over the dielectric layer and surrounding each of the mesa structures, wherein the conductive ground plane includes an array of openings surrounding the array of mesa structures, and each mesa structure protrudes through a corresponding opening in the conductive ground plane. 
     
     
       5. The method of  claim 1 , wherein forming the passivation over the base substrate and the array of mesa structures comprises conformal deposition of the passivation layer over the base substrate and the array of mesa structures. 
     
     
       6. The method of  claim 1 , wherein forming the passivation over the base substrate and the array of mesa structures comprises growing the passivation layer over the base substrate and the array of mesa structures. 
     
     
       7. The method of  claim 1 , wherein forming the array of mesa structures comprises etching an oxide layer formed over a semiconductor substrate. 
     
     
       8. The method of  claim 1 , wherein depositing the dielectric layer comprises depositing multiple dielectric layers. 
     
     
       9. The method of  claim 1 , wherein the array of mesa structures are integrally formed with the base substrate. 
     
     
       10. The method of  claim 1 , wherein each electrode comprises a material selected from the group consisting of platinum, titanium, vanadium, chromium, zirconium, niobium, molybdenum, ruthenium, rhodium, hafnium, tantalum, tungsten, rhenium, osmium, iridium and alloys thereof. 
     
     
       11. The method of  claim 1 , wherein each electrode comprises TiW. 
     
     
       12. The method of  claim 1 , wherein the dielectric layer comprises a dielectric material selected from the group consisting of Al 2 O 3  and Ta 2 O 5 . 
     
     
       13. A method of fabricating an array of electrostatic transfer heads comprising:
 forming an array of mesa structures on a base substrate, each mesa structure including sidewalls; 
 forming a pair of electrodes over each corresponding mesa structure, wherein the electrodes in each pair of electrodes are electrically insulated from each other; and 
 depositing a dielectric layer over the array of mesa structures and each electrode; 
 wherein a top surface of the dielectric layer over each mesa structure corresponds to a contact surface for a corresponding electrostatic transfer head in the array of electrostatic transfer heads. 
 
     
     
       14. The method of  claim 13 , further comprising forming a pair of electrode leads, wherein each electrode lead runs from a corresponding electrode over a top surface of the corresponding mesa structure along a sidewall of the corresponding mesa structure. 
     
     
       15. The method of  claim 13 , wherein the pair of electrodes are separated by 0.5 μm or less. 
     
     
       16. The method of  claim 13 , wherein forming the pair of electrodes over each corresponding mesa structure comprises a lift off technique. 
     
     
       17. The method of  claim 13 , wherein forming the pair of electrodes over each corresponding mesa structure comprises metal layer deposition and etching of the metal layer. 
     
     
       18. A method of fabricating an array of electrostatic transfer heads comprising:
 forming an array of mesa structures on a base substrate, each mesa structure including sidewalls; 
 forming a separate electrode over each corresponding mesa structure; 
 depositing a dielectric layer over the array of mesa structures and each electrode; and 
 forming a conductive ground plane over the dielectric layer and surrounding each of the mesa structures, wherein the conductive ground plane includes an array of openings surrounding the array of mesa structures, and each mesa structure protrudes through a corresponding opening in the conductive ground plane. 
 
     
     
       19. The method of  claim 18 , wherein the conductive ground plane has a same or lower melting temperature than the separate electrodes. 
     
     
       20. The method of  claim 18 , wherein the array of mesa structures are integrally formed with the base substrate.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/561,706 filed on Nov. 18, 2011, U.S. Provisional Patent Application Ser. No. 61/594,919 filed on Feb. 3, 2012 and U.S. Provisional Patent Application Ser. No. 61/597,109 filed on Feb. 9, 2012 the full disclosures of which are 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 one or more micro devices to a receiving substrate. 
     Background Information 
     Integration and packaging issues are one of the main obstacles for the commercialization of micro devices such as radio frequency (RF) microelectromechanical systems (MEMS) microswitches, light-emitting diode (LED) display systems, and MEMS or quartz-based oscillators. 
     Traditional technologies for transferring of devices include transfer by wafer bonding from a transfer wafer to a receiving wafer. One such implementation is “direct printing” involving one bonding step of an array of devices from a transfer wafer to a receiving wafer, followed by removal of the transfer wafer. Another such implementation is “transfer printing” involving two bonding/de-bonding steps. In transfer printing a transfer wafer may pick up an array of devices from a donor wafer, and then bond the array of devices to a receiving wafer, followed by removal of the transfer wafer. 
     Some printing process variations have been developed where a device can be selectively bonded and de-bonded during the transfer process. In both traditional and variations of the direct printing and transfer printing technologies, the transfer wafer is de-bonded from a device after bonding the device to the receiving wafer. In addition, the entire transfer wafer with the array of devices is involved in the transfer process. 
     SUMMARY OF THE INVENTION 
     A micro device transfer head and head array, and a method of transferring one or more micro devices to a receiving 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 integrated circuits (ICs), or a substrate with metal redistribution lines. 
     In an embodiment, a micro device transfer head includes a base substrate, a mesa structure including sidewalls, at least one is electrode formed over the mesa structure, and a dielectric layer covering the electrode. For example, the micro device transfer head can incorporate a monopolar or bipolar electrode structure. The mesa structure can be separately or integrally formed with the base substrate. The sidewalls can be tapered and protrude away from the base substrate to a top surface of the mesa structure, with the electrode formed on the top surface. 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 electrode and electrode leads may be covered with a deposited dielectric layer. Suitable materials for the dielectric layer include, but are not limited to, aluminum oxide (Al 2 O 3 ) and tantalum oxide (Ta 2 O 5 ). Since the dielectric layer is deposited, the electrode and electrode leads may be formed of a material which can withstand high deposition temperatures, including high melting temperature metals such as platinum and refractory metals or refractory metal alloys such as titanium tungsten (TiW). 
     In an embodiment, a method of transferring a micro device includes positioning a transfer head over a micro device connected to a carrier substrate. The micro device is contacted with the transfer head and a voltage is applied to an electrode in the transfer head to create a grip pressure on the micro device. The transfer head picks up the micro device and then releases the micro device onto a receiving substrate. The voltage can be applied to the electrode prior to, while or after contacting the micro device with the transfer head. The voltage can be a constant current voltage, or alternating current voltage. In an embodiment, an alternating current voltage is applied to a bipolar electrode structure. In an embodiment, an operation is additionally performed to create a phase change in a bonding layer connecting the micro device to the carrier substrate prior to or while picking up the micro device. 
     In an embodiment, the bonding layer is heated to create a phase change from solid to liquid in the bonding layer prior to or while picking up the micro device. Depending upon the operating conditions, a substantial portion of the bonding layer can be picked up and transferred with the micro device. 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 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 when contacting the receiving substrate and during the release operation onto the receiving substrate. In another embodiment, the portion of the bonding layer can be allowed to cool to a solid phase after being pick up. For example, the portion of the bonding layer can be in a solid phase prior to or during contacting the receiving substrate, and again melted to the liquid state during the release operation. A variety of temperature and material phase cycles can be performed in accordance with embodiments of the invention. 
     In an embodiment, a method of transferring an array of micro devices includes positioning an array of transfer heads over an array of micro devices. The array of micro devices is contacted with the array of transfer heads, and a voltage is selectively applied to a portion of the array of transfer heads. Selectively applying a voltage may include applying a voltage to all of the transfer heads in the array, or to a portion corresponding to less than all of the transfer heads in the array. The corresponding portion of the array of micro devices is then picked up with the portion of the array of transfer heads, and the portion of the array of micro devices is selectively released onto at least one receiving substrate. In an embodiment, the array of transfer heads may be rubbed on the array of micro devices while making contact in order to dislodge any particles which may be present on the contacting surface of either of the transfer heads or micro devices. In an embodiment, a phase change is created in an array of laterally separate locations of the bonding layer connecting the array of micro devices to the carrier substrate prior to picking up the array of micro devices. 
     In an embodiment, a method of fabricating a micro device transfer head array includes forming an array of mesa structures on a base substrate, with each mesa structure including sidewalls. A separate electrode is formed over each mesa structure, and a dielectric layer is deposited over the array of mesa structures and each electrode. In an embodiment, the dielectric layer is deposited with atomic layer deposition (ALD), and may be pin-hole free. The dielectric layer may include one or multiple dielectric layers. A conformal passivation layer may optionally be grown or deposited over the base substrate and the array of mesa structures prior to forming the separate electrode over each corresponding mesa structure. In an embodiment, a conductive ground plane is formed over the dielectric layer and surrounding each of the mesa structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view illustration of a monopolar micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 2  is an isometric view illustration of a monopolar micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 3  is a cross-sectional side view illustration of a bipolar micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 4  is an isometric view illustration of a bipolar micro device transfer head in accordance with an embodiment of the invention. 
         FIGS. 5-6  are top view illustrations of a bipolar micro device transfer head in accordance with an embodiment of the invention. 
         FIG. 7  is an isometric view illustration of a bipolar micro device transfer head including conductive vias in accordance with an embodiment of the invention. 
         FIG. 8  is an isometric view illustration of a bipolar micro device transfer head array in accordance with an embodiment of the invention. 
         FIG. 9  is an isometric view illustration of a bipolar micro device transfer head array including a conductive ground plane in accordance with an embodiment of the invention. 
         FIG. 10  is a cross-sectional side view illustration of a bipolar micro device transfer head array including a conductive ground plane in accordance with an embodiment of the invention. 
         FIG. 11  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. 12  is a schematic illustration of an alternating voltage applied across a bipolar electrode in accordance with an embodiment of the invention. 
         FIG. 13  is a schematic illustration of a constant voltage applied across a bipolar electrode in accordance with an embodiment of the invention. 
         FIG. 14  is a schematic illustration of a constant voltage applied to a monopolar electrode in accordance with an embodiment of the invention. 
         FIG. 15  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. 16  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. 17  is a cross-sectional side view illustration 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. 18  is a cross-sectional side view illustration 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. 19  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. 20  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. 21  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. 22  is a cross-sectional side view illustration of a micro device selectively released onto a receiving substrate in accordance with an embodiment of the invention. 
         FIG. 23  is a graphical illustration showing the pressure required to overcome the force of surface tension to pick up a micro device of various dimensions in accordance with an embodiment of the invention. 
         FIG. 24  is a graphical illustration of the relationship between surface tension and increasing gap distance created during a pick up operation in accordance with an embodiment of the invention. 
         FIG. 25  is a graphical illustration of the relationship between viscous force pressures and increasing gap distance created during a pick up operation at various pull rates in accordance with an embodiment of the invention. 
         FIG. 26  is a graphical illustration obtained by modeling analysis showing the grip pressure exerted by a transfer head on a micro device as the transfer head is withdrawn from the micro device 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. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention describe a micro device transfer head and head array, and method of transferring a micro device and an array of micro devices to a receiving substrate. 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 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 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. 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, 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 transfer heads. For example, the pre-fabricated micro devices may have a specific functionality such as, but not limited to, a 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 transfer heads matching 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 transfer head in the array of transfer heads may also be independently controllable, which enables selective pick up and release of the micro devices. 
     In one aspect, without being limited to a particular theory, embodiments of the invention describe micro device transfer heads and head arrays which operate in accordance with principles of electrostatic grippers, using the attraction of opposite charges to pick up micro devices. In accordance with embodiments of the present invention, a pull-in voltage is applied to a micro device transfer head in order to generate a grip force on a micro device and pick up the micro device. Grip force is proportional to charged plate area so is calculated as a pressure. According to ideal electrostatic theory, a non-conductive dielectric layer between a monopolar electrode and a conductive substrate yields a grip pressure in Pascal (Pa) in equation (1) of:
 
 P=[∈   o /2 ][V∈   r   /d]   2   (1)
 
where ∈ o =8.85.10 −12 , V=electrode-substrate voltage in volts (V), ∈ r =dielectric constant, and d=dielectric thickness in meters (m). With a bipolar gripper using two grip electrodes the voltage (V) in the above equation is half of the voltage between electrodes A and B, [V A −V B ]/2. The substrate potential is centered at the average potential, [V A =V B ]/2. This average is generally zero with V A =[−V B ].
 
     In another aspect, embodiments of the invention describe a bonding layer which can maintain a micro device on a carrier substrate during certain processing and handling operations, and upon undergoing a phase change provides a medium on which the micro device can be retained yet is also 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 device in place on a carrier substrate while also providing a medium from which the micro device is readily releasable. Without being limited to a particular theory, in determining the grip pressure which is necessary to pick up the micro device from the carrier substrate the grip pressure should exceed the forces holding the micro device to the carrier substrate, which may include but are not limited to, surface tension forces, capillary forces, viscous effects, elastic restoration forces, van-der-Waals forces, stiction and gravity. 
     In accordance with embodiments of the invention, when the dimensions of a micro device are reduced below a certain range, the surface tension forces of the liquid bonding layer holding the micro device to the carrier substrate may become dominant over other forces holding the micro device.  FIG. 23  is a graphical illustration of one embodiment obtained by modeling analysis showing the pressure required to overcome the force of surface tension to pick up a micro device of various dimensions, assuming a liquid indium (In) bonding layer with a surface tension of 560 mN/m at the melting temperature of 156.7° C. For example, referring to  FIG. 23  an exemplary 10 μm by 10 μm wide micro device is retained on a carrier substrate with a surface tension pressure of approximately 2.2 atmospheres (atm) with an indium bonding layer having a liquid surface tension of 560 mN/m at its melting temperature of 156.7° C. This is significantly larger than the pressure due to gravity, which is approximately 1.8×10 −6  atm for an exemplary 10 μm×10 μm wide×3 μm tall piece of gallium nitride (GaN). 
     Surface tension pressures and viscous effects may also be dynamic during the pick up operation.  FIG. 24  is a graphical illustration of one embodiment obtained by modeling analysis showing the relationship of surface tension and increasing gap distance created during the pick up operation of an exemplary 10 μm by 10 μm wide micro device retained on a carrier substrate with a molten indium (In) bonding layer. The gap distance referred to in  FIG. 24  is the distance between the bottom of the micro device and the carrier substrate, and starts at 2 μm corresponding to an un-molten thickness of the In bonding layer. As illustrated in  FIG. 24 , a surface tension pressure of 2.2 atm is initially overcome by the grip pressure at the beginning of the pick up operation. As the micro device is then lifted from the carrier substrate, the surface tension rapidly falls, with the pressure leveling out as the micro device is lifted further away from the carrier substrate. 
       FIG. 25  is a graphical illustration of one embodiment obtained by modeling analysis showing the relationship of viscous force pressures (atm) and increasing gap distance (μm) created during a pick up operation at various pull rates for an exemplary 10 μm by 10 μm micro device retained on a carrier substrate with a molten indium (In) bonding layer. The gap distance referred to in  FIG. 25  is the distance between the bottom of the micro device and the carrier substrate, and starts at 2 μm corresponding to an un-molten thickness of the In bonding layer. As illustrated, viscous force pressures are more apparent during faster lift speeds such as 1,000 mm/s than for slower lift speeds such as 0.1 mm/s. Yet, the pressures generated from the viscous effects using the exemplary lift speeds illustrated in  FIG. 25  are significantly less than the surface tension pressure generated and illustrated in  FIG. 24  which suggests that surface tension pressure is the dominant pressure which must be overcome by the grip pressure during the pick up operation. 
     If an air gap of size (g) is present between the dielectric layer of the micro device transfer head and a top conductive surface of the micro device then the grip pressure in equation (2) is:
 
 P=[∈   o /2 ][V∈   r /( d+∈   r   g )] 2   (2)
 
     It is contemplated that an air gap can be present due to a variety of sources including, but not limited to, particulate contamination, warpage, and misalignment of either surface of the transfer head or micro device, or the presence of an additional layer on the transfer head or micro device, such as a lip of a conformal dielectric barrier layer around the top conductive surface of a micro device. In a embodiment, a lip of a conformal dielectric barrier layer may create both an air gap where a contact opening is formed and increase the effective thickness of the dielectric layer of the transfer head where the lip is present. 
     As seen from equations (1) and (2) above, lower voltages may be utilized where no air gap is present between the micro device transfer head and micro device to be picked up. However, when an air gap is present this presents a series capacitance in which the air capacitance may compete with the dielectric layer capacitance. In order to compensate for the possibility of an air capacitance between any of an array of micro device transfer heads over a corresponding array of micro devices to be picked up, a higher operating voltage, higher dielectric constant for the dielectric material, or thinner dielectric material may be used to maximize the electric field. However, use of a higher electric field has limitations due to possible dielectric breakdown and arcing. 
       FIG. 26  is a graphical illustration of one embodiment obtained by modeling analysis showing the grip pressure exerted by a transfer head on a micro device as the transfer head is withdrawn from the top conductive surface of the micro device, corresponding to an increasing air gap size. The different lines correspond to different Ta 2 O 5  dielectric layer thicknesses between 0.5 μm and 2.0 μm on the transfer head, with the electric field being kept constant. As illustrated, no appreciable effect on grip pressure is observed at these conditions below air gap sizes of approximately 1 nm (0.001 μm), and even as high as 10 nm (0.01 μm) for some conditions. However, it is to be appreciated that the tolerable air gap can be increased or decreased by changing the conditions. Thus, in accordance with some embodiments of the invention a certain amount of air gap tolerance is possible during the pick up operation and actual contact with the micro device transfer head and the top conductive surface of the micro device may not be necessary. 
     Now assuming that the grip pressure required to pick up the micro device from the carrier substrate should exceed the sum of pressures retaining the micro device on the carrier substrate (as well as any pressure reduction due to air gap) it is possible to derive the interrelationship of operating voltage, dielectric constant and dielectric thickness of the dielectric material in the micro device transfer head by solving the grip pressure equations. For purposes of clarity, assuming that the air gap distance is zero, for a monopolar electrode this becomes:
 
sqrt( P* 2/∈ o )= V∈   r   /d   (3)
 
     Exemplary ranges of calculated dielectric thickness values are provided in Table 1 for desired grip pressures of 2 atm (202650 Pa) and 20 atm (2026500 Pa) for Al 2 O 3  and Ta 2 O 5  dielectric materials between operating voltages between 25 V and 300 V in order to illustrate the interdependence of grip pressure, voltage, dielectric constant and dielectric thickness in accordance with an embodiment of the invention. The dielectric constants provided are approximate, and it is understood that the values can vary depending upon manner of formation. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Dielectric 
                 Voltage 
                 Dielectric constant, ε   r     
                 Dielectric thickness,  
               
               
                 Material 
                 (V) 
                 (Hz-MHz range) 
                 d (microns) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 Grip pressure = 2 atm 
               
            
           
           
               
               
               
               
            
               
                 Al 2 O 3   
                 25 
                 9.8 
                 1.1 
               
               
                 Al 2 O 3   
                 100 
                 9.8 
                 4.6 
               
               
                 Al 2 O 3   
                 300 
                 9.8 
                 13.7 
               
               
                 Ta 2 O 5   
                 25 
                 25 
                 2.9 
               
               
                 Ta 2 O 5   
                 100 
                 25 
                 11.7 
               
               
                 Ta 2 O 5   
                 300 
                 25 
                 35.0 
               
            
           
           
               
            
               
                 Grip pressure = 20 atm 
               
            
           
           
               
               
               
               
            
               
                 Al 2 O 3   
                 25 
                 9.8 
                 0.4 
               
               
                 Al 2 O 3   
                 100 
                 9.8 
                 1.4 
               
               
                 Al 2 O 3   
                 300 
                 9.8 
                 4.3 
               
               
                 Ta 2 O 5   
                 25 
                 25 
                 0.9 
               
               
                 Ta 2 O 5   
                 100 
                 25 
                 3.7 
               
               
                 Ta 2 O 5   
                 300 
                 25 
                 11.1 
               
               
                   
               
            
           
         
       
     
     Since the grip pressure is proportional to the inverse square of the dielectric thickness, the calculated dielectric thicknesses in Table 1 represents the maximum thicknesses which can be formed to achieve the necessary grip pressure with the set operating voltage. Thicknesses lower than those provided in Table 1 may result in higher grip pressures at the set operating voltage, however lower thicknesses increase the applied electric field across the dielectric layer which requires that the dielectric material possess a dielectric strength sufficient to withstand the applied electric field without shorting. It is to be appreciated that the grip pressure, voltage, dielectric constant and dielectric thickness values provided in Table 1 are exemplary in nature, and provided in order to provide a foundation for working ranges of the micro device transfer head in accordance with embodiments of the invention. The relationship between grip pressure, voltage, dielectric constant and dielectric thickness values provided in Table 1 has been illustrated in accordance with ideal electrostatic theory, and embodiments of the invention are not limited by such. 
     Referring now to  FIG. 1 , a side view illustration is provided of a monopolar micro device transfer head and head array in accordance with an embodiment of the invention. As shown, each monopolar device transfer head  100  may include a base substrate  102 , a mesa structure  104  including a top surface  108  and sidewalls  106 , an optional passivation layer  110  formed over the mesa structure  104  and including a top surface  109  and sidewalls  107 , an electrode  116  formed over the mesa structure  104  (and optional passivation layer  110 ) and a dielectric layer  120  with a top surface  121  covering the electrode  116 . Base substrate  102  may be formed from a variety of materials such as silicon, ceramics and polymers which are capable of providing structural support. In an embodiment, base substrate has a conductivity between 10 3  and 10 18  ohm-cm. Base substrate  102  may additionally include wiring (not shown) to connect the micro device transfer heads  100  to the working electronics of an electrostatic gripper assembly. 
     Mesa structure  104  may be formed using suitable processing techniques, and may be formed from the same or different material than base substrate  102 . In one embodiment, mesa structure  104  is integrally formed with base substrate  102 , for example by using lithographic patterning and etching, or casting 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 over a semiconductor substrate, such as silicon. 
     In one aspect, the mesa structures  104  generate a profile which 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  121  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  110 ,  112 ,  120  may be formed over the mesa structure  104 , the width of the mesa structure may account for the thickness of the overlying layers so that the width across the top surface  121  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  FIG. 1 , 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  110  may then be optionally deposited or grown over the base substrate  102  and array of mesa structures  104 . Passivation layer  110  can be deposited by a variety of suitable techniques such as chemical vapor deposition (CVD), sputtering, or atomic layer deposition (ALD). In an embodiment, passivation layer  110  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 ). 
     A conductive layer  112  may then be deposited over the array of mesa structures  104  and optional passivation layer  110 , and patterned to form electrodes  116  and electrode leads  114 . For example, a lift off technique can be utilized to form the electrodes  116  and electrode leads  114  in which a resist layer is deposited and patterned over the substrate, followed by deposition of a metal layer, and lift off of the resist and portion of the metal layer on the resist leaving behind the desired pattern. Alternatively, metal layer deposition followed by patterning and etching can be performed to achieve the desired pattern. Electrode leads  114  may run from the electrode  116  over the top surface  108  of a mesa structure  104  (and top surface  109  of optional passivation layer  110 ) and along a sidewall  106  of the mesa structure  104  (and along a sidewall  107  of optional passivation layer  110 ). Conductive layer  112  used to form the electrodes  116  and electrode leads  114  may be a single layer or multiple layers. A variety of conductive materials including metals, metal alloys, refractory metals, and refractory metal alloys may be employed to form conductive layer  112 . In an embodiment, the conductive layer  112  has a thickness up to 5,000 angstroms (0.5 μm). In an embodiment, the conductive layer  112  includes a high melting temperature metal such as platinum or a refractory metal or refractory metal alloy. For example, conductive layer 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, conductive layer  112  is an approximately 500 angstrom (0.05 μm) thick titanium tungsten (TiW) refractory metal alloy. 
     A dielectric layer  120  is then deposited over the electrodes  116  and other exposed layers on the base substrate  102 . In an embodiment, the dielectric layer  120  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 may be a single layer or multiple layers. In an embodiment, the dielectric layer is 0.5 μm-2.0 μm thick, though 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 ). Referring back to Table 1 above, embodiments of Al 2 O 3  dielectric layers with applied electric fields (determined by dividing the voltage by dielectric thickness) of 22 V/μm to 71 V/μm and Ta 2 O 5  dielectric layers with applied electric fields of 9 V/μm to 28 V/μm were provided. In accordance with embodiments of the invention, the dielectric layer  120  possesses a dielectric strength greater than the applied electric field so as to avoid shorting of the transfer head during operation. Dielectric layer  120  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. Dielectric layer  120  may additionally be annealed following deposition. In one embodiment, the dielectric layer  120  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 than the calculated thicknesses provided in exemplary Table 1. 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  120 . Multiple layers of different dielectric materials may also be utilized to form dielectric layer  120 . In an embodiment, the underlying conductive layer  112  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. In an embodiment, following the deposition of dielectric layer  120  a thin coating (not illustrated) may be formed over the dielectric layer  120  to provide a specific stiction coefficient 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  121  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. 
       FIG. 2  is a close-up isometric view of electrode  116  and electrode lead  114  formed over an optional passivation layer  110  covering a mesa structure  104 . For purposes of clarity, the overlying dielectric layer  120  is not illustrated, and the optional passivation layer  110  and mesa structure  104  are illustrated as a single mesa structure/passivation layer  104 / 110 . In an exemplary embodiment, where the passivation layer  110  and dielectric layer  120  are both 0.5 μm thick, the top surface  108 / 109  of the mesa structure/passivation layer  104 / 110  onto which the 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, the electrode  116  covers the maximum amount of surface area of the top surface  108 / 109  of the mesa structure/passivation layer  104 / 110  as possible while remaining within patterning tolerances. Minimizing the amount of free space increases the capacitance, and resultant grip pressure which can be achieved by the micro device transfer head. While a certain amount of free space is illustrated on the top surface  108 / 109  of the mesa structure/passivation layer  104 / 110  in  FIG. 2 , the electrode  116  may cover the entire top surface  108 / 109 . The electrode  116  may also be slightly larger than the top surface  108 / 109 , and partially extend down the sidewalls  106 / 107  of the mesa structure/passivation layer  104 / 110  to ensure complete coverage of the top surface  108 / 109 . 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/passivation layer  104 / 110  in a 10 μm pitch. 
     Referring now to  FIG. 3 , a side view illustration is provided of a bipolar micro device transfer head  100  and head array in accordance with an embodiment of the invention. As shown, the bipolar device transfer head  100  may include a base substrate  102 , a mesa structure  104  including a top surface  108  and sidewalls  106 , passivation layer  110  including a top surface  109  and sidewalls  107 , a pair of electrodes  116 A,  116 B and electrode leads  114 A,  114 B formed over the mesa structure  104 , optional passivation layer  110  and a dielectric layer  120  covering the pair of electrodes  116 A,  116 B. 
       FIG. 4  is a close-up isometric view of electrodes  116 A,  116 B and electrode leads  114 A,  114 B formed over an optional passivation layer  110  covering a mesa structure  104 . For purposes of clarity, the overlying dielectric layer  120  is not illustrated, and the optional passivation layer  110  and mesa structure  104  are illustrated as a single mesa structure/passivation layer  104 / 110 .  FIG. 4  differs slightly from  FIG. 3  in that the electrode leads  114 A,  114 B are illustrated as running along a single sidewall rather than on opposite sidewalls of the mesa structure/passivation layer  104 / 110 . Electrode leads  114 A,  114 B may run along any suitable sidewall in accordance with embodiments of the invention. In an exemplary embodiment, where the top surface  108 / 109  of the mesa structure/passivation layer  104 / 110  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 / 109  of the mesa structure/passivation layer  104 / 110  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 slightly longer than the top surface  108 / 109  in one direction, and partially extend down the sidewalls of the mesa structure/passivation layer  104 / 110  to ensure maximum coverage of the top surface  108 / 109 . 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/passivation layer  104 / 110  in a 10 μm pitch. 
     Referring now to  FIGS. 5-6 , 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. 5 . However, embodiments of the invention are not so limited. In the embodiment illustrated in  FIG. 6 , each electrode  116  is formed on a separate mesa structure  104 A,  104 B separated by a trench  105 . An optional passivation layer  110  (not illustrated) can cover both mesa structures  104 A,  104 B. 
     Referring now to  FIG. 7 , an isometric view illustration of an alternative electrode lead configuration is provided 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 optional passivation layer  110  not illustrated) connecting the electrodes  116 A,  116 B to the respective electrode leads  114 A,  114 B. In such an embodiment, electrode leads  114 A,  114 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. 7  it is to be appreciated that the above described via or vias may also be integrated into monopolar electrode structures. 
     Referring now to  FIGS. 8-10 , an embodiment of the invention is illustrated in which a conductive ground plane is formed over the dielectric layer and surrounding the array of mesa structures.  FIG. 8  is an isometric view illustration of an array of micro device transfer heads  100  with a bipolar electrode configuration as previously described with regard to  FIG. 4 . For purposes of clarity, the optional underlying passivation layer and overlying dielectric layer have not been illustrated. Referring now to  FIGS. 9-10 , a conductive ground plane  130  is formed over the dielectric layer  120  and surrounding the array of mesa structures  104 . The presence of ground plane  130  may assist in the prevention of arcing between transfer heads  100 , particularly during the application of high voltages. Ground plane  130  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  130  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  120  after the formation of ground plane  130 . 
       FIG. 11  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  1110  a transfer head is positioned over a micro device connected to a carrier substrate. The transfer head may comprise a mesa structure, an electrode over the mesa structure, and a dielectric layer covering the electrode as described in the above embodiments. Thus, the transfer head may have a monopolor or bipolar electrode configuration, as well as any other structural variations as described in the above embodiments. The micro device is then contacted with the transfer head at operation  1120 . In an embodiment, the micro device is contacted with the dielectric layer  120  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  1130  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  1140 . The micro device is then released onto a receiving substrate at operation  1150 . 
     While operations  1110 - 1150  have been illustrated sequentially in  FIG. 11 , 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  1130  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. 
       FIG. 12  is a schematic illustration of an alternating voltage applied across a bipolar electrode with the transfer head in contact with a micro device in accordance with an embodiment of the invention. As illustrated, a separate alternating current (AC) voltage source may be applied to each electrode lead  114 A,  114 B with an alternating voltage applied across the pair of electrodes  116 A,  116 B so that at a particular point in time when a negative voltage is applied to electrode  116 A, a positive voltage is applied to electrode  116 B, and vice versa. Releasing the micro device from the transfer head may be accomplished with a variety of methods including turning off the voltage sources, lowering the voltage across the pair of electrodes, changing a waveform of the AC voltage, and grounding the voltage source.  FIG. 13  is a schematic illustration of a constant voltage applied to a bipolar electrode in accordance with an embodiment of the invention. In the particular embodiment illustrated, a negative voltage is applied to electrode  116 A while a positive voltage is applied to electrode  116 B.  FIG. 14  is a schematic illustration of a constant voltage applied to a monopolar electrode in accordance with an embodiment of the invention. Once the transfer head picks up the micro device illustrated in  FIG. 14 , the amount of time the transfer head can hold the micro device may be a function of the discharge rate of the dielectric layer since only single voltage is applied to electrode  116 . Releasing the micro device from the transfer head illustrated in  FIG. 14  may be accomplished by turning off the voltage source, grounding the voltage source, or reversing the polarity of the constant voltage. 
     In the particular embodiments illustrated in  FIGS. 12-14 , the micro devices  200  are those illustrated in  FIG. 27 , example  27 O. Though the micro devices illustrated in  FIGS. 12-14  may be from 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. For example, a micro LED device  200  may include a micro p-n diode  235 ,  250  and a metallization layer  220 , with the metallization layer between the micro p-n diode  235 ,  250  and a bonding layer  210  formed on a substrate  201 . In an embodiment, the micro p-n diode  250  includes a top n-doped layer  214 , one or more quantum well layers  216 , and a lower p-doped layer  218 . The micro p-n diodes can be fabricated with straight sidewalls or tapered sidewalls. In certain embodiments, the micro p-n diodes  250  possess outwardly tapered sidewalls  253  (from top to bottom). In certain embodiments, the micro p-n diodes  235  possess inwardly tapered sidewalls  253  (from top to bottom). The metallization layer  220  may include one or more layers. For example, the metallization layer  220  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  251  of the micro p-n diode  250  is wider than the top surface  252  of the micro p-n diode, and the sidewalls  253  are tapered outwardly from top to bottom. The top surface of the micro p-n diode  235  may be wider than the bottom surface of the p-n diode, or approximately the same width. In an embodiment, the bottom surface  251  of the micro p-n diode  250  is wider than the top surface  221  of the metallization layer  220 . 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  260  may optionally be formed over the micro p-n diode  235 ,  250  and other exposed surfaces. The conformal dielectric barrier layer  260  may be thinner than the micro p-n diode  235 ,  250 , metallization layer  220  and optionally the bonding layer  210  so that the conformal dielectric barrier layer  260  forms an outline of the topography it is formed on. In an embodiment, the micro p-n diode  235 ,  250  is several microns thick, such as 3 μm, the metallization layer  220  is 0.1 μm-2 μm thick, and the bonding layer  210  is 0.1 μm-2 μm thick. In an embodiment, the conformal dielectric barrier layer  260  is approximately 50-600 angstroms thick aluminum oxide (Al 2 O 3 ). Conformal dielectric barrier layer  260  may be deposited by a variety of suitable techniques such as, but not limited to, atomic layer deposition (ALD). The conformal dielectric barrier layer  260  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  260  may also protect the sidewalls  253 , quantum well layer  216  and bottom surface  251 , 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  260  can function as a physical barrier to wicking of the bonding layer material  210  up the sidewalls and quantum layer  216  of the micro p-n diodes U 15011250 . The conformal dielectric barrier layer  260  may also insulate the micro p-n diodes  250  once placed on a receiving substrate. In an embodiment, the conformal dielectric barrier layer  260  span sidewalls  253  of the micro p-n diode, and may cover a quantum well layer  216  in the micro p-n diode. The conformal dielectric barrier layer may also partially span the bottom surface  251  of the micro p-n diode, as well as span sidewalls of the metallization layer  220 . In some embodiments, the conformal dielectric barrier layer also spans sidewalls of a patterned bonding layer  210 . A contact opening  262  may be formed in the conformal dielectric barrier layer  260  exposing the top surface  252  of the micro p-n diode. 
     Referring to  FIG. 27 , the contact opening  262  may have a smaller width than the top surface  252  of the micro p-n diode and the conformal dielectric barrier layer  260  forms a lip around the edges of the top surface  252  of the micro p-n diode. Referring to  FIG. 28 , the contact opening  262  may have a slightly larger width than the top surface of the micro p-n diode. In such an embodiment, the contact opening  262  exposes the top surface  252  of the micro p-n diode and an upper portion of the sidewalls  253  of the micro p-n diode, while the conformal dielectric barrier layer  260  covers and insulates the quantum well layer(s)  216 . Referring to  FIG. 29 , the conformal dielectric layer  260  may have approximately the same width as the top surface of the micro p-n diode. The conformal dielectric layer  260  may also span along a bottom surface  251  of the micro p-n diodes illustrated in  FIGS. 27-29 . 
     In an embodiment, conformal dielectric barrier layer  260  is formed of the same material as dielectric layer  120  of the bonding head. Depending upon the particular micro LED device structure, the conformal dielectric barrier layer  260  may also span sidewalls of the bonding layer  210 , as well as the carrier substrate and posts, if present. Bonding layer  210  may be formed from a material which can maintain the micro LED device  200  on the carrier substrate  201  during certain processing and handling operations, and upon undergoing a phase change provide a medium on which the micro LED device  200  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  200  is readily releasable. In an embodiment, the bonding layer  210  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  201 , 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  202  on the substrate. 
     Solders may be suitable materials for bonding layer  210  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 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Liquidus  
                 Solidus  
               
               
                   
                 Chemical composition 
                 Temperature (° C.) 
                 Temperature (° C.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 100 In 
                 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 
               
               
                   
                 50In50Sn 
                 125 
                 118 
               
               
                   
                 52Sn48In 
                 131 
                 118 
               
               
                   
                 58Bi42Sn 
                 138 
                 138 
               
               
                   
                 97In3Ag 
                 143 
                 143 
               
               
                   
                 58Sn42In 
                 145 
                 118 
               
               
                   
                 99.3In0.7Ga 
                 150 
                 150 
               
               
                   
                 95In5Bi 
                 150 
                 125 
               
               
                   
                 99.4In0.6Ga 
                 152 
                 152 
               
               
                   
                 99.6In0.4Ga 
                 153 
                 153 
               
               
                   
                 99.5In0.5Ga 
                 154 
                 154 
               
               
                   
                 60Sn40Bi 
                 170 
                 138 
               
               
                   
                 100Sn 
                 232 
                 232 
               
               
                   
                 95Sn5Sb 
                 240 
                 235 
               
               
                   
                   
               
            
           
         
       
     
     An exemplary list thermoplastic polymers which may be utilized with embodiments of the invention are provided in Table 3. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 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. 15  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  1510  a transfer head is positioned over a micro device connected to a carrier substrate with a bonding layer. The 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  1520 . In an embodiment, the micro device is contacted with the dielectric layer  120  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  1525  an operation is performed to create a phase change in the bonding layer  210  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  1525  can be performed prior to operation  1520 . At operation  1530  a voltage is applied to the electrode to create a grip pressure on the micro device, and the micro device and a substantial portion of the bonding layer  210  are picked up with the transfer head at operation  1540 . For example, approximately half of the bonding layer  210  may be picked up with the micro device. In an alternative embodiment, none of the bonding layer  210  is picked up with the transfer head. At operation  1545  the micro device and portion of the bonding layer  210  are placed in contact with a receiving substrate. The micro device and portion of the bonding layer  210  are then released onto the receiving substrate at operation  1550 . 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  210  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 the contacting operation  1545  and during the release operation  1550 . 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 operation  1545 , and again melted to the liquid state prior to or during the release operation  1550 . A variety of temperature and material phase cycles can be performed in accordance with embodiments of the invention. 
       FIG. 16  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  1610  an array of transfer heads is positioned over an array of micro devices, with each transfer head having a mesa structure, an electrode over the mesa structure, and a dielectric layer covering the electrode. At operation  1620  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. 17  is a side view illustration of an array of micro device transfer heads  100  in contact with an array of micro LED devices  200  in accordance with an embodiment of the invention. As illustrated in  FIG. 17 , the pitch (P) of the array of transfer heads  100  matches the pitch of the micro LED devices  200 , 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  200  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  250  with straight sidewalls the top surface of the each micro LED device  200  has a width of approximately 8 μm. In such an exemplary embodiment, the width of the top surface  121  of a corresponding transfer head  100  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  200  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  200  has a width of approximately 3 μm. In such an exemplary embodiment, the width of the top surface  121  of a corresponding transfer head  100  is approximately 3 μm or smaller so as to avoid making inadvertent contact with an adjacent micro LED device  200 . However, embodiments of the invention are not limited to these specific dimensions, and may be any suitable dimension. 
       FIG. 18  is a side view illustration of an array of micro device transfer heads in contact with an array of micro LED devices  200  in accordance with an embodiment of the invention. In the embodiment illustrated in  FIG. 18 , 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. 16 , at operation  1630  a voltage is selectively applied to a portion of the array of transfer heads  100 . At operation  1640  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. 19  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  200  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. 20  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  200  in accordance with an embodiment of the invention. In a particular embodiment illustrated in  FIGS. 19-20 , the pick up operation includes picking up the micro p-n diode  250 , the metallization layer  220  and a portion of the conformal dielectric barrier layer  260  for the micro LED device  200 . In a particular embodiment illustrated in  FIGS. 19-20 , the pick up operation includes picking up a substantial portion of the bonding layer  210 . Accordingly, any of the embodiments described with regard to  FIGS. 16-22  may also be accompanied by controlling the temperature of the portion of the bonding layer  210  as described with regard to  FIG. 15 . For example, embodiments described with regard to  FIGS. 16-22  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  201  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  1650  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. Release may be accomplished by affecting the applied voltage with any of the manners described with regard to  FIGS. 12-14 . 
       FIG. 21  is a side view illustration of an array of micro device transfer heads holding a corresponding array of micro LED devices  200  over a receiving substrate  301  including a plurality of driver contacts  310 . The array of micro LED devices  200  may then be placed into contact with the receiving substrate and then selectively released.  FIG. 22  is a side view illustration of a single micro LED device  200  selectively released onto the receiving substrate  301  over a driver contact  310  in accordance with an embodiment of the invention. In another embodiment, more than one micro LED device  200  is released, or the entire array of micro LED devices  200  are released. 
     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.

Metadata:
Filing Date: 20120213
Publication Date: 20170411
Grant Date: 20170411
Priority Date: 20111118
Inventors: BIBL ANDREAS
HIGGINSON JOHN A.
LAW HUNG-FAI STEPHEN
HU HSIN-HUA
Assignee: APPLE INC
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Family ID: 47325228