PATENT DOCUMENT

Publication Number: US-9425151-B2
Application Number: US-201414307325-A
Country: US
Kind Code: B2

Title: Compliant electrostatic transfer head with spring support layer

Abstract:
A compliant electrostatic transfer head and method of forming a compliant electrostatic transfer head are described. In an embodiment, a compliant electrostatic transfer head includes a cavity in a base substrate, a spring support layer on the base substrate, and a patterned device layer on the spring support layer. The spring support layer includes a spring support layer beam profile that extends over and is deflectable toward the cavity, and the patterned device layer includes an electrode beam profile that is supported by the spring support layer beam profile and extends over and is deflectable toward the cavity.

Claims:
What is claimed is: 
     
       1. A compliant electrostatic transfer head structure comprising:
 a base substrate; 
 a cavity completely in the base substrate; 
 a spring support layer on the base substrate, wherein the spring support layer includes a spring support layer beam profile that extends over the cavity from a first side of the cavity to a second side of the cavity and is deflectable toward the cavity; 
 a first insulating layer on the spring support layer; and 
 a patterned device layer on the spring support layer, wherein the patterned device layer includes an electrode beam profile that is supported by the spring support layer beam profile and extends over the cavity from the first side of the cavity to the second side of the cavity and is deflectable toward the cavity. 
 
     
     
       2. The compliant electrostatic transfer head structure of  claim 1 , wherein the first insulating layer electrically insulates the patterned device layer from the spring support layer. 
     
     
       3. The compliant electrostatic transfer head structure of  claim 1 , wherein the electrode beam profile of the patterned device layer comprises a pair of electrodes, and the pair of electrodes includes a first electrode lead integrally formed with a first mesa structure protruding above the first electrode lead, and a second electrode lead integrally formed with a second mesa structure protruding above the second electrode lead. 
     
     
       4. The compliant electrostatic transfer head structure of  claim 3 , further comprising a second insulating layer between the base substrate and the spring support layer, wherein the second insulating layer electrically insulates the spring support layer from the base substrate. 
     
     
       5. The compliant electrostatic transfer head structure of  claim 4 , wherein the second insulating layer spans along a top surface of the base substrate, sidewalls of the cavity, and a bottom surface of the cavity. 
     
     
       6. The compliant electrostatic transfer head structure of  claim 3 , wherein the patterned device layer further comprises a first trace interconnect integrally formed with the first electrode, and a second trace interconnect integrally formed with the second electrode. 
     
     
       7. The compliant electrostatic transfer head structure of  claim 6 , wherein the patterned device layer is formed of silicon. 
     
     
       8. The compliant electrostatic transfer head structure of  claim 6 , further comprising:
 a first via in the base substrate and a first plug within the first via, the first plug electrically coupled to the first trace interconnect and the first electrode lead; and 
 a second via in the base substrate and a second plug within the second via, the second plug electrically coupled to the second trace interconnect and the second electrode lead. 
 
     
     
       9. The compliant electrostatic transfer head structure of  claim 6 , wherein the electrode beam profile extends between the first and second trace interconnects. 
     
     
       10. The compliant electrostatic transfer head structure of  claim 9 , wherein the spring support layer beam profile extends between the first and second trace interconnects, and the spring support layer beam profile is wider than the electrode beam profile. 
     
     
       11. The compliant electrostatic transfer head structure  claim 9 , wherein the each of the first and second electrodes comprises a double bend. 
     
     
       12. The compliant electrostatic transfer head structure of  claim 11 , wherein the electrode beam profile is characterized by an S-shape configuration. 
     
     
       13. The compliant electrostatic transfer head structure  claim 3 , wherein the first and second mesa structures are separated by trench characterized by a width of 1.0 μm or less. 
     
     
       14. The compliant electrostatic transfer head structure of  claim 13 , wherein the trench is filled with a dielectric material. 
     
     
       15. The compliant electrostatic transfer head structure of  claim 1 , further comprising a second insulating layer directly on and spanning along a top surface of the base substrate, sidewalls of the cavity, and a bottom surface of the cavity, wherein the second insulating layer electrically insulates the spring support layer from the base substrate. 
     
     
       16. The compliant electrostatic transfer head of  claim 15 , wherein the electrode beam profile of the patterned device layer comprises a pair of electrodes, and the pair of electrodes includes a first electrode lead integrally formed with a first mesa structure protruding above the first electrode lead, and a second electrode lead integrally formed with a second mesa structure protruding above the second electrode lead. 
     
     
       17. The compliant electrostatic transfer head of  claim 16 , wherein the patterned device layer further comprises a first trace interconnect integrally formed with the first electrode lead, and a second trace interconnect integrally formed with the second electrode lead. 
     
     
       18. The compliant electrostatic transfer head of  claim 15 , wherein the base substrate comprises silicon, and the second insulating layer comprises silicon oxide. 
     
     
       19. The compliant electrostatic transfer head of  claim 18 , wherein the spring layer comprises silicon. 
     
     
       20. The compliant electrostatic transfer head of  claim 18 , wherein the first insulating layer comprises silicon oxide, and the patterned device layer comprises silicon.

Description:
BACKGROUND 
     1. Field 
     The present invention relates to micro devices. More particularly embodiments relate to a compliant electrostatic transfer head array and a method of transferring micro devices to a receiving substrate. 
     2. Background Information 
     Integration and packaging issues are one of the main obstacles for the commercialization of micro devices such as radio frequency (RF) microelectromechanical systems (MEMS) microswitches, light-emitting diode (LED) display systems, and MEMS or quartz-based oscillators. Traditional technologies for transferring of devices include transfer by wafer bonding from a transfer wafer to a receiving wafer. Such implementations include “direct printing” and “transfer printing” involving wafer bonding/de-bonding steps in which a 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. 
     Other technologies for transferring of devices include transfer printing with elastomeric stamps. In one such implementation an array of elastomeric stamps matching the pitch of devices on a source wafer are brought into intimate contact with the surface of the devices on the source wafer and bonded with van der Walls interaction. The array of devices can then be picked up from the source wafer, transferred to a receiving substrate, and released onto the receiving substrate. 
     In another implementation, the technology for transferring of devices is enabled by an array of electrostatic transfer heads as described in U.S. Pat. No. 8,415,767. As described, an array of electrostatic transfer heads may be formed from a silicon-on-insulator (SOI) substrate. Furthermore, the array of electrostatic transfer heads may be made compliant such that each silicon electrode is deflectable into a cavity between a base silicon substrate and the silicon electrode. In this manner, each compliant electrostatic transfer head can compensate for variations in height of the devices during the transfer process. 
     SUMMARY 
     A compliant electrostatic transfer head, method of forming a compliant electrostatic transfer head are described, 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 compliant electrostatic transfer head includes a cavity in a base substrate, a spring support layer on the base substrate, and a patterned device layer on the spring support layer. A first insulating layer may be formed on the spring support layer, wherein the first insulating layer electrically insulates the patterned device layer from the spring support layer. The spring support layer includes a spring support layer beam profile that extends over and is deflectable toward the cavity, and the patterned device layer includes an electrode beam profile that is supported by the spring support layer beam profile and extends over and is deflectable toward the cavity. In some embodiments, any or all of the base substrate, spring support layer, and patterned device layer are formed of silicon. In an embodiment, a second insulating layer is formed between the base substrate and the spring support layer, where the second insulating layer electrically insulates the spring support layer from the base substrate. For example, the second insulating may span along a top surface of the base substrate, sidewalls of the cavity, and a bottom surface of the cavity. 
     In an embodiment, the compliant electrostatic transfer head includes a bipolar electrode configuration. For example, the electrode beam profile of the patterned device layer includes a pair of electrodes, and the pair of electrodes includes a first electrode lead integrally formed with a first mesa structure protruding above the first electrode lead, and a second electrode lead integrally formed with a second mesa structure protruding above the second electrode lead. The patterned device layer may further include a first trace interconnect integrally formed with the first electrode, and a second trace interconnect integrally formed with the second electrode. In an embodiment, the electrode beam profile extends between the first and second trace interconnects. Likewise, the spring support layer beam profile may extend between the first and second trace interconnects. The spring support layer beam profile may be wider than the electrode beam profile. In an embodiment each of the first and second electrodes includes a double bend. For example, a double bend may be in the form of an S-shape configuration. 
     In an embodiment, a first via is formed in the base substrate and a first plug is formed within the first via, the first plug electrically coupled to the first trace interconnect and the first electrode lead. In an embodiment, a second via is formed in the base substrate and a second plug is formed within the second via, the second plug electrically coupled to the second trace interconnect and the second electrode lead. 
     The compliant electrostatic transfer heads in accordance with embodiments may be substantially smaller than conventional transfer heads. For example, a first top surface of the first mesa structure may have a maximum width and length of less than 50 microns in both x and y dimensions, respectively, and a second top surface of the second mesa structure may have a maximum width and length of less than 50 microns in both x and y dimensions, respectively. More specifically, the first top surface of the first mesa structure may have a maximum width of 10 μm or less and length 4.5 μm or less, and the second top surface of the second mesa structure may have a maximum width of 10 μm or less and length 4.5 μm or less. In an embodiment, the first and second mesa structures are separated by a trench characterized by a width of 1.0 μm or less. The trench may be filled with one or more dielectric materials. 
     In an embodiment, a method of forming a compliant electrostatic transfer head includes bonding a wafer stack including a spring support layer and device layer to a base substrate that includes a cavity formed in the base substrate. The patterned device layer is then patterned to include an electrode beam profile above the cavity, and the spring support layer is patterned to include a spring support layer beam profile underneath and supporting the electrode beam profile. Patterning the spring support layer may include etching a beam profile opening through the spring support layer to expose the cavity. The spring support layer may completely cover the cavity prior to patterning the spring support layer to include the spring support layer beam profile. Patterning the device layer may include forming a pair of electrodes over the cavity, with each electrode including an electrode lead and a mesa structure. Such a configuration is exemplary of a bipolar compliant electrostatic transfer head configuration. 
     Bonding the wafer stack to the base substrate includes bonding an insulating layer formed on a top surface of the base substrate to the spring support layer in an embodiment. For example, this bonding may be fusion bonding. In an embodiment, the insulating layer is formed on a top surface of the base substrate, sidewalls of the cavity, and a bottom surface of the cavity prior to bonding to the wafer stack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan view illustration of a micro pick up array including an array of bipolar compliant electrostatic transfer heads in accordance with an embodiment. 
         FIG. 1B  is a plan view illustration of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes with double bends and a mesa joint supported by a spring support layer in accordance with an embodiment. 
         FIG. 1C  is a cross-sectional side view illustration taken along transverse line C-C of the bipolar compliant electrostatic transfer head illustrated in  FIG. 1B  in accordance with an embodiment. 
         FIG. 1D  is a cross-sectional side view illustration taken along longitudinal line D-D of the bipolar compliant electrostatic transfer head illustrated in  FIG. 1B  in accordance with an embodiment. 
         FIG. 2  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with cantilever beam and continuous joint supported by a spring support layer in accordance with an embodiment. 
         FIG. 3  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with cantilever beam and mesa joint supported by a spring support layer in accordance with an embodiment. 
         FIG. 4  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with double sided clamped beam and continuous joint supported by a spring support layer in accordance with an embodiment. 
         FIG. 5  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes with double bends and a mesa joint supported by a spring support layer in accordance with an embodiment. 
         FIG. 6  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes with single bends and a mesa joint supported by a spring support layer in accordance with an embodiment. 
         FIG. 7  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes with double bends and a mesa joint in accordance with an embodiment. 
         FIG. 8  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped supported beam and pair of silicon electrodes supported by a spring support layer in accordance with an embodiment. 
         FIG. 9  is a combination cross-sectional side view illustration taken along lines V-V, W-W, X-X, Y-Y, and Z-Z from  FIG. 1A  in accordance with an embodiment. 
         FIGS. 10-12  are cross-sectional side view illustrations of a method of forming a base substrate with one or more cavities in an embodiment. 
         FIG. 13  is a cross-sectional side view illustration of a double SOI stacked wafer in accordance with an embodiment. 
         FIGS. 14-36  are cross-sectional side view illustration of a method of bonding a double SOI stacked wafer to a base substrate with one or more cavities and patterning the bonded structure to form a bipolar compliant electrostatic transfer head array supported by a spring support layer in accordance with an embodiment. 
         FIG. 37  is a flow chart illustrating a method of picking up and transferring an array of micro devices from a carrier substrate to a receiving substrate in accordance with an embodiment. 
         FIG. 38  is a cross-sectional side view illustration of an array of bipolar compliant electrostatic transfer heads positioned over an array of micro devices on a carrier substrate in accordance with an embodiment. 
         FIG. 39  is a cross-sectional side view illustration of an array of bipolar compliant electrostatic transfer heads in contact with an array of micro devices in accordance with an embodiment. 
         FIG. 40  is a cross-sectional side view illustration of an array of bipolar compliant electrostatic transfer heads picking up an array of micro devices in accordance with an embodiment. 
         FIG. 41  is a cross-sectional side view illustration of contacting a receiving substrate with an array of micro devices held by an array of bipolar compliant electrostatic transfer heads in accordance with an embodiment. 
         FIG. 42  is a cross-sectional side view illustration of an array of micro devices released onto a receiving substrate in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe a compliant electrostatic transfer head and micro pick up array including a spring support layer, and method of transferring a micro device and an array of micro devices to a receiving substrate. In an embodiment, a compliant electrostatic transfer head includes a cavity in a base substrate, a spring support layer on the base substrate, and a patterned device layer on the spring support layer. The spring support layer includes a spring support layer beam profile that extends over and is deflectable toward the cavity, and the patterned device layer includes an electrode beam profile that is supported by the spring support layer beam profile and extends over and is deflectable toward the cavity. In an embodiment the compliant electrostatic transfer head includes a bipolar electrode configuration. For example, the electrode beam profile of the patterned device layer may include a pair of electrodes, and the pair of electrodes includes a first electrode lead integrally formed with a first mesa structure protruding above the first electrode lead, and a second electrode lead integrally formed with a second mesa structure protruding above the second electrode lead. Embodiments are not limited to bipolar electrode configurations. For example, embodiments may be directed toward monopolar electrode configurations and multiple electrode configurations including more than two electrodes. 
     In accordance with some embodiments fabrication of a micro pick up array includes forming an array of cavities within a base substrate, bonding an SOI stack to the patterned base substrate, and then patterning an array of silicon electrodes above the array of cavities. In this manner, the dimensions of the cavities toward which the silicon electrodes deflect is precisely controlled at an initial fabrication stage, and an etch release of the beam profiles of the spring support layer can be performed at a terminal stage in the fabrication process, thereby preserving the integrity of the silicon electrodes. 
     Without being limited to a particular theory, embodiments describe electrostatic transfer heads and micro pick up 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, a pull-in voltage is applied to an electrostatic transfer head in order to generate a grip pressure on a micro device and pick up the micro device. For example, the electrostatic transfer head may include a bipolar electrode configuration. The compliant electrostatic transfer head and head arrays in accordance with embodiments may be used to transfer micro devices such as, but not limited to, diodes, LEDs, transistors, MEMS, silicon integrated circuits (ICs) for logic or memory, and gallium arsenide (GaAs) circuits for radio frequency (RF) communications from a carrier substrate to a receiving substrate such as, 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. 
     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. 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. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “over”, “to”, “spanning”, “between” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “over”, “spanning” or “on” another layer or bonded “to” 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. 
     In one aspect, embodiments of the invention describe a micro pick up array including an array of compliant electrostatic transfer heads, and method of operation in which the array of compliant electrostatic transfer heads enables improved contact with an array of micro devices as compared to an array of non-compliant transfer heads. In application, as a micro pick up array is lowered onto an array of micro devices, each compliant electrostatic transfer head is independently deflectable toward a base substrate, e.g. toward a cavity in the base substrate. In this manner, each compliant electrostatic transfer head can compensate for variations in height of the micro devices, impurities (e.g. particles) on the micro devices, or surface profile variations of the carrier substrate such as surface waviness. Such compensation can result in reduced compressive forces applied to certain micro devices, leading to protection of the physical integrity of the micro devices and transfer head array. Such compensation can also assist each compliant electrostatic transfer head to make contact with each micro device, and ensure that each intended micro device is picked up. Without the compliant nature of the compliant electrostatic transfer heads an irregular micro device height, wavy carrier substrate, 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 grip pressure to overcome the air gap, resulting in an incomplete pick-up process. 
     In another aspect, embodiments describe a micro pick up array including a spring support layer beneath a device layer including the silicon electrodes. The spring support layer beam profiles of the spring support layer can function to stabilize the electrode beam profiles of the silicon electrodes, particularly in a bipolar electrode design in which a dielectric layer supports the two silicon electrode mesa structures. In such a configuration, the spring support layer may relieve bending stress that may result due to loading the two silicon electrode mesa structures, particularly when the silicon electrodes deflect. In addition to adding physical integrity, the inclusion of a spring support layer can function to decouple the mechanical requirements of the device layer and allows an additional degree of freedom for tuning the mechanical spring function of a compliant electrostatic transfer head such that a particular mechanical spring function can be achieved irrespective of the electrode design or pattern in the device layer. 
     In another aspect, embodiments describe a manner of forming an array of compliant electrostatic transfer heads from commercially available silicon and silicon-on-insulator (SOI) substrates. In an embodiment, one or more cavities are formed in a base substrate prior to bonding an SOI substrate stack to the patterned base substrate including the one or more cavities. In this manner, the cavities with precisely controlled depths and dimensions can be fabricated prior to patterning the spring support layer and device layer. This allows for a spring release etch operation of the spring support layer beam profiles to be performed at or near a terminal end of the processing sequence, and preserves the structural and electrical integrity of the compliant electrostatic transfer heads. Additionally, this may ensure a uniform profile of the cavity, or arrays of cavities beneath the arrays of compliant electrostatic transfer heads. 
     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. As used herein, the terms “micro” devices or structures are meant to refer to the scale of 1 to 300 μm, for example, each micro device or electrostatic transfer head including a maximum length or width of a contact surface or mesa structure of 1 to 300 μm. For example, each electrostatic transfer head may include a pair of silicon electrodes, with each silicon electrode including a mesa structure with a maximum width or length of 1 to 300 μm, 1 to 100 μm, or more specifically 1 to 10 μm. In an exemplary embodiment, an electrostatic transfer head has a contact surface of approximately 10 μm by 10 μm. In an embodiment, a bipolar electrostatic transfer head includes a pair of mesa structure of approximately 4.5 μm (width) by 10 μm (length) separated by a 1 μm gap. In another exemplary embodiment, a bipolar electrostatic transfer head having a contact surface of approximately 5 μm by 5 μm includes a pair of mesa structure of approximately 2.25 μm (width) by 5 μm (length) separated by a 0.5 μm gap. However, it is to be appreciated that embodiments are not necessarily so limited, and that certain aspects of the embodiments may be applicable to larger, and possibly smaller size scales. 
     In some exemplary embodiments, arrays of micro devices which are poised for pick up are described as having a size of 10 μm (in x and/or y dimensions), or size of 5 μm (in x and/or y dimensions). However, it is to be appreciated that embodiments are not necessarily so limited, and that certain aspects of the embodiments may be applicable to larger, and possibly smaller size scales as described above with regard to the electrostatic transfer heads. A transfer tool including an array of compliant electrostatic transfer heads matching an integer multiple of a pitch of the corresponding array of micro devices on a carrier substrate can be used to pick up and transfer the array of micro devices to a receiving substrate. In this manner, it is possible to integrate and assemble micro 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 compliant electrostatic transfer heads can pick up and transfer more than 100,000 micro devices, with larger arrays of compliant electrostatic transfer heads being capable of transferring more micro devices. 
     Referring now to  FIG. 1A , a plan view illustration is provided for a micro pick up array including an array of bipolar compliant electrostatic transfer heads, and includes views at different depths. In the particular embodiment illustrated, the shaded area illustrates an arrangement of silicon electrodes and silicon interconnects as viewed from the top surface of the bipolar compliant electrostatic transfer head array. The darker shading illustrates a top side via connection as viewed from the top side surface of the bipolar compliant electrostatic transfer head array. Exemplary locations of cavities  136  are illustrated as dotted lines underneath the silicon electrodes. In this manner, the plan view illustration provides detail regarding structures at various depths from a top side of the SOI wafer stack. It is to be appreciated that while  FIG. 1A  illustrates a bipolar electrode configuration, that embodiments are not limited to bipolar electrode configurations, and embodiments are also applicable to other electrode configurations including monopolar electrode configurations or electrode configurations including more than two electrodes. 
     As illustrated, the micro pick up array  100  includes an array of compliant electrostatic transfer heads  102  connected to an arrangement of silicon trace interconnects  104 , and bus interconnects  106 . As illustrated, bus interconnects  106  may be formed around a periphery or outside a working area of the micro pick up array including the array of compliant electrostatic transfer heads  102 . In an embodiment, each compliant electrostatic transfer head  102  includes a pair of silicon electrodes  110 , with each silicon electrode  110  including a mesa structure  112  and an electrode lead  114  connected to a silicon interconnect  104 . As illustrated, each compliant electrostatic transfer head  102  is in the form of a double sided clamped beam profile clamped at opposite sides to silicon trace interconnects  104 . As illustrated, a silicon electrode double sided clamped beam may include a pair of silicon electrode leads  114  each with a double bend  115 , and pair mesa structures  112  separated by a mesa dielectric joint  119  that extends in a transverse width of the double sided clamped beam parallel to the pair of silicon interconnects  104 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a transverse width of the double sided clamped beam between the pair of mesa structures  112 . In the particular embodiment illustrated, the beam is in an S-shape configuration, though a variety of other configurations are contemplated. In the embodiment illustrated, the array of mesa structure  112  pairs in the micro pick up array  100  are arranged with approximately the same pitch as the micro devices to be picked up, and placed, for example, corresponding to a pixel pitch on a display substrate for exemplary micro LED devices. 
     In an embodiment, a plurality of vias  120  are formed through the micro pick up array SOI stack to provide a backside electrical contact to interconnects  106  in order to electrically connect the silicon electrodes  110  with working circuitry of a transfer head assembly. In the embodiment illustrated in  FIG. 1A , the interconnect  106  on the left side of the illustration may be connected to a first voltage source V A , and the interconnect  106  on the right side of the illustration may be connected to a second voltage source V B . Where each compliant electrostatic transfer head  102  is operable as a bipolar electrostatic transfer head, voltage sources V A  and V B  may simultaneously apply opposite voltages so that each of the silicon electrodes  110  in a respective compliant electrostatic transfer head  102  has an opposite voltage. 
       FIG. 1B  is a close-up plan view illustration of a single bipolar compliant electrostatic transfer head  102  of  FIG. 1A . As illustrated, the double sided clamped beam includes a pair of silicon electrode leads  114  with double bends  115  and a mesa joint  119  between mesa structures  112 , all supported by a spring support layer  150 . The silicon electrodes form an electrode beam profile, and the spring support layer  150  forms a spring support layer beam profile underneath and supporting the electrode beam profile, where the spring support layer beam profile is wider than the supported electrode beam profile. A cavity  136  is formed within the base substrate  136  and the electrode beam profile and spring support layer beam profile are deflectable toward the cavity  136 . In an embodiment, a separate cavity  136  is formed underneath each compliant electrostatic transfer head  102 . In an embodiment, a single cavity  136  spans underneath multiple compliant electrostatic transfer heads  102 . 
       FIG. 1C  is a cross-sectional side view illustration taken along transverse line C-C of the bipolar compliant electrostatic transfer head illustrated in  FIG. 1B  in accordance with an embodiment. In the embodiment illustrated in  FIG. 1C , each silicon electrode  110  in a bipolar electrode configuration extends from a separate silicon interconnect  104 .  FIG. 1D  is a cross-sectional side view illustration taken along longitudinal line D-D of the bipolar compliant electrostatic transfer head illustrated in  FIG. 1B  in accordance with an embodiment. As illustrated in  FIGS. 1C-1D , both the silicon electrode mesa structures  112  and leads  114  extend over and are deflectable toward a cavity  136  between the base substrate  130  and the silicon electrode  110 . In an embodiment, a separate cavity  136  is formed underneath each bipolar silicon electrode  110  in the micro pick up array and between two separate silicon interconnects  104 . In an embodiment, a single cavity  136  is formed underneath an array of bipolar silicon electrodes  110  and between two separate silicon interconnects  104 . Referring again to  FIG. 1A , a single or multiple separate cavities  136  can be formed between arrays of silicon interconnects  104 . In an embodiment, cavities  136  are the same cavity. Trenches  116  may also be formed in the patterned silicon layer defining the silicon electrodes  110  and silicon interconnects  104 ,  106  as described in more detail in the following description. 
       FIGS. 2-8  illustrate various modifications of bipolar compliant electrostatic transfer heads spanning between silicon interconnects  104  in accordance with embodiments. While  FIGS. 2-8  are illustrated separately from the detailed processing sequences illustrated in  FIGS. 10-36 , it is to be appreciated that many of the various modifications described with respect to  FIGS. 2-8  can be implemented into the processing sequences. Similar to  FIG. 1A , for clarity purposes, only a single bipolar compliant electrostatic transfer head  102  is illustrated in  FIGS. 2-8  as spanning between two silicon trance interconnects  104 , though an array of bipolar electrostatic transfer heads may span between the silicon interconnects  104  in accordance with embodiments. Also, similar to the single bipolar compliant electrostatic transfer head described with regard to  FIGS. 1B-1D ,  FIGS. 2-8  each illustrate a pair of silicon electrodes that form an electrode beam profile and spring support layer beam profile that is wider than and supports the electrode beam profile. 
       FIG. 2  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with cantilever beam and continuous joint in accordance with an embodiment. As illustrated, a silicon electrode cantilever beam may include a pair of silicon electrode leads  114  extending from two silicon interconnects  104 , and pair mesa structures  112  separated by a continuous dielectric joint  119  which extends in a longitudinal length of the cantilever beam parallel to the pair of silicon interconnects  104 , all supported by a spring support layer  150 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a longitudinal length of the cantilever beam along both the pair silicon electrode leads  114  and pair of mesa structures  112 . As illustrated, the silicon electrode leads  114  may include a bend  115  (illustrated as a 90 degree bend). 
       FIG. 3  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with cantilever beam and mesa joint in accordance with an embodiment. As illustrated, a silicon electrode cantilever beam may include a pair of silicon electrode leads  114  extending from two silicon interconnects  104 , and pair mesa structures  112  separated by a mesa dielectric joint  119  which extends in a longitudinal length of the cantilever beam parallel to the pair of silicon interconnects  104 , all supported by a spring support layer  150 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a longitudinal length of the cantilever beam along the pair of mesa structures  112 . As illustrated, the pair of silicon electrode leads  114  are physically separated by patterning and may include a bend  115  (illustrated as a 90 degree bend). 
       FIG. 4  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with double sided clamped beam and continuous joint in accordance with an embodiment. As illustrated, a silicon electrode double sided clamped beam may include a pair of bent silicon electrode leads  114  extending from two silicon interconnects  104 , and pair mesa structures  112  separated by a continuous dielectric joint  119  which extends in a longitudinal length of the cantilever beam parallel to the pair of silicon interconnects  104 , all supported by a spring support layer  150 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a longitudinal length of the double sided clamped beam along both the pair silicon electrode leads  114  and pair of mesa structures  112 . As illustrated, the silicon electrode leads  114  may each include bends  115  (illustrated as 90 degree bends) at proximal and distal locations where the electrode leads extend from the silicon interconnects  104 . 
       FIG. 5  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes with double bends and a mesa joint in accordance with an embodiment. As illustrated, a silicon electrode double sided clamped beam may include a pair of silicon electrode leads  114  extending from two silicon interconnects  104 , each lead  114  with a double bend  115 , and pair mesa structures  112  separated by a mesa dielectric joint  119  which extends in a transverse width of the double sided clamped beam parallel to the pair of silicon interconnects  104 , all supported by a spring support layer  150 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a transverse width of the cantilever beam between the pair of mesa structures  112 , and the pair of silicon electrode leads  114  are physically separated by patterning. In the embodiment illustrated, each electrode lead  114  is split, so that the beam configuration assumes an 8-shape configuration with the silicon electrode leads  114 . 
       FIG. 6  is a plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes with single bends and a mesa joint in accordance with an embodiment. As illustrated, a silicon electrode double sided clamped beam may include a pair of silicon electrode leads  114  extending from two silicon interconnects  104 , each lead  114  with a single bend  115 , and pair mesa structures  112  separated by a mesa dielectric joint  119  extends in a transverse width of the double sided clamped beam perpendicular to the pair of silicon interconnects  104 , all supported by a spring support layer  150 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a transverse width of the double sided clamped beam between the pair of mesa structures  112 , and the pair of silicon electrode leads  114  are physically separated by patterning. 
       FIG. 7  is plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes with double bends and a mesa joint in accordance with an embodiment. As illustrated, a silicon electrode double sided clamped beam may include a pair of silicon electrode leads  114  each with a double bend  115 , and pair mesa structures  112  separated by a mesa dielectric joint  119  which extends in a transverse width of the double sided clamped beam parallel to the pair of silicon interconnects  104 , all supported by a spring support layer  150 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a transverse width of the double sided clamped beam between the pair of mesa structures  112 . In the particular embodiment illustrated in  FIG. 7 , the beam is in a W-shape configuration. 
       FIG. 8  is plan view illustration and cross-sectional side view illustration taken along line A-A of a bipolar compliant electrostatic transfer head with a double sided clamped beam including a pair of silicon electrodes and a mesa joint in accordance with an embodiment. As illustrated, a silicon electrode double sided clamped beam may include a pair of silicon electrode leads  114  and a pair mesa structures  112  separated by a mesa dielectric joint  119  which extends in a transverse width of the double sided clamped beam parallel to the pair of silicon interconnects  104 , all supported by a spring support layer  150 . In such an embodiment, the dielectric joint  119  electrically insulates the pair of silicon electrodes in the bipolar electrode configuration along a transverse width of the double sided clamped beam between the pair of mesa structures  112 . 
     Referring now to  FIG. 9  a combination cross-sectional side view illustration is provided taken along lines V-V, W-W, X-X, Y-Y, and Z-Z from  FIG. 1A  in accordance with an embodiment. The combination view is not a representation of the precise relative locations for all of the different features illustrated, rather the combination view combines specific features at different locations previously identified in  FIG. 1A  in order to more easily represent the particular variations in processing sequences. For example, while the combination cross-sectional side view illustration shows one via  120  corresponding to one silicon electrode  110 , it is clear from  FIG. 1A  that one via  120  may be electrically connected with a plurality of silicon electrodes  110  along one or more interconnects  104 . As illustrated, lines W-W and Y-Y are along vias  120 . As illustrated, lines V-V and Z-Z are along one or more trenches  116  defining the silicon electrodes  110  and silicon interconnects  104 ,  106 . As illustrated, line X-X is across a bipolar compliant electrostatic transfer head including a pair of silicon electrodes  110 . Referring again to  FIG. 1A , one or more cavities  136  may be formed around and beneath all silicon electrodes  110 , and between interconnects  104 ,  106 . 
     A silicon electrode  110  includes a mesa structure  112  and an electrode lead  114 , where the mesa structure  112  is an elevated portion of the silicon electrode  110 . In an embodiment, the mesa structures  112  may be separated by a trench with a width of 1 μm or less. A first dielectric layer  118  may cover a top and side surfaces of the pair of silicon electrodes  110  and interconnects  104 ,  106 . The first dielectric layer  118  may also cover a side surface of the mesa structures  112  within the trench laterally between the pair of mesa structure  112  for the pair of silicon electrodes  110  in a bipolar compliant electrostatic transfer head  102 . As illustrated, the first dielectric layer  118  may form an dielectric joint  119  that fills the trench laterally between the pair of mesa structures  112 . Since the dielectric joint  119  connects the silicon electrodes  110 , the bipolar electrode assembly illustrated in  FIG. 9A  is characterized as a supported beam structure spanning between silicon interconnects, in which the joined supported beam structure is deflectable toward cavity  136 . 
     The bipolar compliant electrostatic transfer head includes a base substrate  130 , a spring layer  150  on the base substrate  130 , and a patterned device layer  140  on the spring support layer  150 . The patterned device layer includes the pair of silicon electrodes  110  that is deflectable toward the base substrate  130 . For example, the pair of silicon electrodes  110  is deflectable toward the cavity  136  in the base substrate. Each silicon electrode  110  includes an electrode lead  114  that is integrally formed with a mesa structure  112  that protrudes above the corresponding electrode lead  114 . In an embodiment, each mesa structure  112  is approximately 15 μm tall, corresponding to the thickness of device layer  140  after the formation of mesa etch masks  144  described in further detail below, and the electrode leads  114  are approximately 5 μm thick. These dimensions are exemplary, and other dimensions are contemplated. A first insulating layer  124  is located on the spring support layer  150  to electrically insulate the patterned device layer  140  from the spring support layer  150 . A second insulating layer  152  is located on the base substrate  130  to electrically insulate the spring support layer  150  from the base substrate  130 . The silicon electrodes  110  form an electrode beam profile, and the spring support layer  150  forms a spring support layer beam profile underneath and supporting the electrode beam profile, where the spring support layer beam profile is wider than the supported electrode beam profile. Together, both the spring support layer beam profile and electrode beam profile are deflectable toward the cavity  136 . 
     A via opening  120 D may extend through the base substrate  130  from a backside of the base substrate. In the particular embodiment illustrated, via opening  120 D terminates at a bottom surface of a second insulating layer  152  between the base substrate  130  and the spring support layer  150 , and beneath where interconnect  106  is located. A via plug  135  is formed within the via opening  120 D. With such a via plug configuration the via plug  135  is electrically isolated from the base substrate  130 . 
     A top side via opening  120 B may be formed over the backside via opening  120 D. In the embodiment illustrated the top side via opening  120 B is filled with top conductive contact  123 . In the particular embodiment illustrated, top side via opening  120 B is formed through the patterned device layer  140 , spring support layer  150 , and insulating layers  124 ,  152  in order for top conductive contact  123  to provide an electrical connection to plug  135 . Collectively, openings  120 A,  120 B,  120 C,  120 D, conductive contacts  122 ,  123 , and via plug  135  are referred to herein as via  120 . In an embodiment, in addition to being formed within top side via openings  120 B, top side conductive contact  123  is also formed on an exposed top surface of the silicon interconnect  106 . In this manner, partially forming conductive contacts  123  over the top surface of the silicon interconnects  106  can provide greater surface area for ohmic contact with the silicon interconnects  106 . 
     In an embodiment, via plug  135  is formed from the base substrate  130 , and provides for an electrical connection with top conductive contact  123 . In this manner, a first via plug  135  is electrically coupled to a first bus interconnect  106 , and a second via plug  135  is electrically coupled to a second bus interconnect  106 . In an embodiment, vias  120  contact one or more bus interconnects  106  in the patterned device layer  140 . In other embodiments, vias  120  may contact other features or interconnects in the patterned silicon layer  140 . Via  120  along line W-W may be electrically connected to a first bus interconnect  106  which is connected to a first voltage source V A , and via  120  along line Y-Y may be electrically connected to a second bus interconnect  106  which is connected to a second voltage source V B . 
     Still referring to  FIG. 9 , in an embodiment the first dielectric layer  118  is formed on top and side surfaces of the patterned device layer  140 . The first dielectric layer  118  may additionally be formed on side surfaces of the via openings  120 B. In this manner, the first dielectric layer  118  electrically insulates the top conductive contact  123 . In an embodiment, the first dielectric layer  118  functions to provide the desired dielectric constant and/or dielectric breakdown strength, and resultant pick-up pressure of the compliant electrostatic transfer head. In an embodiment, first dielectric layer  118  is atomic layer deposition (ALD) SiO 2 , Al 2 O 3 , Ta 2 O 5 , HfO 2 , Si 3 N 4 , or RuO 2 . 
     In another embodiment, an optional second dielectric layer  126  is formed over the first dielectric layer  118  to provide the desired capacitance. In such an optional arrangement, the first dielectric layer can provide alternative or additional functions, such as an etch protection layer. In such an embodiment, first dielectric layer  118  is formed of a nitride material. In an embodiment, the second dielectric layer  126  has a higher dielectric constant and/or dielectric breakdown strength than the first dielectric layer  118 . In an embodiment, first dielectric layer  118  is a deposited silicon nitride (SiN x ), and second dielectric layer  126  is atomic layer deposition (ALD) SiO 2 , Al 2 O 3 , Ta 2 O 5 , HfO 2 , Si 3 N 4 , or RuO 2 . 
       FIGS. 10-36  illustrate a method of forming a micro pick up array including an array of bipolar compliant electrostatic transfer head in accordance with an embodiment. Similar to  FIG. 9 ,  FIGS. 10-36  are combination cross-sectional side view illustrations taken along lines V-V, W-W, X-X, Y-Y, and Z-Z from  FIG. 1A . Initially, the process may begin with preparing a base substrate and double SOI stacked wafer.  FIGS. 10-12  are cross-sectional side view illustrations of a method of forming a base substrate with one or more cavities in an embodiment. Referring to  FIG. 10 , base substrate  130  may be formed of any suitable material for providing a supporting structure. Where via plugs  135  are to be formed from the base substrate  130 , the base substrate is formed of a material capable of transferring charge. In an embodiment, base substrate is formed of a semiconductor wafer, such as a single crystalline, or polycrystalline silicon substrate. In an embodiment, base substrate  130  is a double side polished (DSP) silicon wafer. For example, base substrate may be a DSP ( 100 ) wafer having any suitable thickness, such as 500 μm+/−50 μm. 
     One or more cavities  136  are then formed within the base substrate  130  as illustrated in  FIG. 11 . Any suitable method may be used to form the one or more cavities  136 , such as etching. In an embodiment, a silicon substrate  130  is etched using a timed dry reactive ion etching (DRIE). In an embodiment cavities are approximately 2 μm deep. Depth of cavities may be determined such that sufficient room is allowed for deflection of the compliant electrostatic transfer heads toward the cavity. Each bipolar compliant electrostatic transfer head may be deflectable toward a corresponding cavity, or a plurality of bipolar compliant electrostatic transfer heads may be deflectable toward a same cavity. The number, size, and shape of cavities  136  may be dependent upon particular design. For example, in one configuration a cavity  136  has a width of approximately 10 μm to 50 μm, and a length to support one or more bipolar compliant electrostatic transfer heads. For example, a length of 10 μm to 50 μm may support one bipolar compliant electrostatic transfer heads, with a larger length supporting more bipolar compliant electrostatic transfer heads. Following the formation of cavities  136 , the substrate  130  is cleaned, and a thermal oxide may be growth on the top and bottom surfaces. For example, a thermally grown first insulating layer  152  and first back side passivation layer  132  are grown on the top and bottom surfaces, respectively, to a specified thickness such as 0.5 μm, for example. In an embodiment, first insulating layer  152  is formed on the top surface  131  of base substrate  130 , sidewalls  137  of cavity  136 , and the bottom surface  139  of the cavity  136 . In an embodiment, first insulating layer  152  is a continuous layer, with uniform thickness and composition. For example, where base substrate  130  is formed of silicon, first insulating layer  152  is formed of SiO 2 . 
     Referring to  FIG. 13 , in an embodiment, a double SOI wafer configuration is prepared for bonding to the patterned base substrate  130 . The double SOI wafer configuration may include spring support layer  150  and device layer  140  grown on a handle substrate  142  to specified thicknesses for achieving specific spring characteristics and electrode configurations. In an exemplary embodiment, the double SOI wafer stack includes a 2 μm backside oxide (SiO 2 ) layer  143 , a 500 μm silicon handle substrate  142 , a 1 μm thick etch stop layer  141  (buried oxide, SiO 2 ), a 15 μm thick device layer  140  (silicon), a 1 μm thick second insulating layer  124  (buried oxide, SiO 2 ), and a 3 μm thick spring support layer  150  (silicon). 
     In an embodiment the double SOI wafer configuration of  FIG. 13  is then bonded to the patterned base substrate as illustrated in  FIG. 14 . In this manner, the cavities  136  can be pre-patterned prior to patterning the spring support layer  150  and device layer  140 . This may protect the integrity of the final spring support layer  150  and final device layer  140  by not requiring an under-etch after their formation to form the cavities  136 . As illustrated in  FIGS. 15-16 , the spring support layer  150  may be fusion bonded to the insulating layer  152  to form a Si—SiO 2  fusion bond. Following wafer bonding, the oxide layer  143  is removed, for example using reactive ion etching (RIE) or grinding, followed by thinning of the handle substrate  142  using an etching or grinding technique. The final portion of the thinned handle substrate  142  may then be removed, using DRIE etching, for example, stopping on the etch stop layer  141 . 
     Following removal of the thinned handle substrate  142 , the etch stop layer  141  is removed as illustrated in  FIG. 18 , exposing device layer  140 . In one embodiment, the etch stop layer  141  is removed using a wet etching technique, such as a buffer oxide etch (BOE) chemistry. A BOE chemistry may be more selective than a DRIE technique, for example, allowing for a resultant uniform thickness of the device layer  140 , from which the mesa structures  112  will be formed. In this manner, a controlled and uniform height of the compliant electrostatic transfer heads is achieved. 
     Referring to  FIG. 19 , a mesa etch mask  144  is formed on the device layer  140 . In an embodiment, mesa etch mask  144  is formed by thermal oxidation (SiO 2 ) of the device layer  140 , followed by reactive ion etching (RIE) stopping on the underlying device layer  140 . In an embodiment, the mesa etch mask  144  is approximately 0.3 μm thick. Exemplary RIE etching chemistries may include fluorinated chemistries such as CHF 3 , CF 4 . Other suitable etching techniques of the thermal oxide include ion milling, plasma etching, reactive ion beam etching (RIBE), electron cyclotron resonance (ECR), or inductively coupled plasma (ICP). Following the formation of the mesa etch mask  144 , any remaining patterned positive photoresist used may be removed using O 2  ashing flowing by piranha etch. An additional patterned positive photoresist may then be formed, with openings between the mesa etch mask  144  pairs, followed by DRIE etching of the device layer  140  to form trenches  117  between the mesa structures  112  of the silicon electrodes to be formed, stopping on the first insulting layer  124 . In an embodiment, DRIE etching is performed using a fluorine based chemistry such as SF 6  or C 4 F 8 . Following the formation of trenches  117 , any remaining patterned positive photoresist used may be removed using O 2  ashing flowing by piranha etch resulting in the structure illustrated in  FIG. 20 . 
     Referring to  FIG. 21 , the mesa etch masks  144  may remain, while the remainder of the device layer  140  is thinned down using a timed etch to achieve a resultant thickness of device layer  140  that will correspond to a thickness of the electrode leads  114  and interconnects  104 ,  106 . O 2  plasma etching may then be performed to remove DRIE residue followed by a BOE etch for removal of the mesa etch masks  144 , resulting in the structure illustrated in  FIG. 22A . In an embodiment, a thickness or height (EL H ) of the thinned device layer  140  corresponding to the electrode leads  114  and interconnects  104 ,  106  is approximately 5 μm. In an embodiment, a height of the mesa structures  112  (M H ) is approximately 15 μm. In an embodiment, a membrane is formed over the cavity  136  at this stage that includes spring support layer  150 , second insulating layer  124 , and patterned device layer  140 . In an embodiment, the total silicon thickness of the membrane at locations other than where the mesa structures  112  or trenches  117  are formed is approximately 8 μm (3 μm spring support layer  150 , 5 μm patterned device layer  140 ). 
       FIG. 22B  is a schematic top view illustration of an exemplary mesa structure configuration in a bipolar compliant electrostatic transfer head in accordance with an embodiment. In the particular configuration illustrated, each bipolar compliant electrostatic transfer head has an approximate square contact surface. As such a mesa length (M L ) is approximately equal to the sum of two mesa widths (M W ) and a trench  117  width (T W ). The dimensions of the mesa structures  112  illustrated in  FIG. 22B  are approximately the same as the mesa etch masks  144  used to form the mesa structures  112 . By way of example, for an exemplary 10 μm×10 μm electrostatic transfer head, each mesa structure  112  includes M W ×M L  dimensions of approximately 4.5 μm×10 μm, and a T W  of approximately 1 μm. By way of example, for an exemplary 5 μm×5 μm electrostatic transfer head, each mesa structure  112  includes M W ×M L  dimensions of approximately 2.25 μm×5 μm, and a T W  of approximately 0.5 μm. It is to be appreciated that these dimensions are exemplary, and that both larger and smaller dimensions are contemplated in accordance with embodiments. 
     Referring now to  FIG. 23  the device layer  140  is patterned. Specifically, the device layer is etched to form silicon interconnect  104 ,  106  and silicon electrode lead  114  profiles. As illustrated, beam profile openings  145  correspond to the electrode lead  114  patterns, and particularly the electrode beam profiles illustrated in  FIG. 1A . Trenches  116  correspond to trenches  116  that partially define the silicon electrodes  110  and silicon interconnects  104 ,  106 . Via openings  120 B correspond to openings in the device layer  140  for providing an electrical connection to plug  135 , yet to be formed. Via opening  120 B is a portion of a collection of features referred to herein collective as via  120 . 
     Following the patterning of trenches  116 , via openings  120 B, and beam profile opening  145  in the device layer  140 , the trenches  116 , via openings  120 B, and beam profile opening  145  are etched though the first insulating layer  124  using a suitable technique, such as RIE using a fluorine based chemistry such as CF 4  or CHF 3 . In the particular embodiment illustrated in  FIG. 24 , beam profile opening  145  through the device layer  140  are wider than the beam profile opening  145  through the first insulating layer  124 . This may be attributed to using separate masks for patterning of the device layer  140  and first insulating layer  124 . As illustrated in  FIG. 23 , the first insulating layer  124  is not etched underneath trenches  117 . 
     Referring now to  FIGS. 25-26 , following etching of the first insulating layer  124 , openings  120 B are etched through the spring support layer  150  using a suitable etching techniques such as DRIE (e.g. SF 6  chemistry) followed by etching through the second insulating layer  152  using a suitable etching technique such as RIE (e.g. CF 4  or CHF 3  chemistry), stopping on the base substrate  130 . In an embodiment, the same etching mask is used for etching through both the spring support layer  150  and the second insulating layer  152 . 
     Referring now to  FIGS. 27-28  a first dielectric layer  118  and optionally a second dielectric layer  126  are formed over the patterned device layer  140 . Formation of dielectric layers  118 ,  126  may also simultaneously form backside passivation layers  134 ,  138 , respectively. Depending upon the particular configuration, first dielectric layer  118  can perform a variety of functions. In one application, first dielectric layer can be used to provide the desired dielectric constant and/or dielectric breakdown strength, and resultant pick-up pressure of the electrostatic transfer head. In an embodiment, first dielectric layer  118  is atomic layer deposition (ALD) SiO 2 , Al 2 O 3 , Ta 2 O 5 , HfO 2 , Si 3 N 4 , or RuO 2 . In an embodiment, first dielectric layer  118  is an approximately 5,000 angstrom thick ALD Al 2 O 3  layer. In an embodiment, first dielectric layer  118  fills trench  117  between mesa structures  119  and provides a dielectric joint  119  between and connecting the pair of silicon electrodes  110 . Such a dielectric joint  119  may provide additional mechanical stability to the electrode design. 
     In an embodiment, first dielectric layer  118  can be used as an etch protection layer when a second dielectric layer  126  is formed over the first dielectric layer. While a second dielectric layer  126  is illustrated and described, it is understood that the second dielectric layer  126  is optional. In an embodiment, first dielectric layer is a nitride layer. For example, first dielectric layer  118  may be an approximately 500 angstrom thick SiN x  layer. In such an embodiment, second dielectric layer  126  is deposited over the first dielectric layer to provide the desired dielectric constant and/or dielectric breakdown strength, and resultant pick-up pressure of the electrostatic transfer head. In an embodiment, second dielectric layer  126  is atomic layer deposition (ALD) SiO 2 , Al 2 O 3 , Ta 2 O 5 , HfO 2 , Si 3 N 4 , or RuO 2 . In an embodiment, second dielectric layer  126  is an approximately 5,000 angstrom thick ALD Al 2 O 3  layer. In an embodiment, a combination of the first and second dielectric layers  118 ,  126  fill trench  117  and form dielectric joint  119 . 
     In accordance with embodiments, the formation of first dielectric layer  118  may also simultaneously form second back side passivation layer  134 , characterized by the same composition and thickness as the first dielectric layer  118 . Likewise, the optional formation of second dielectric layer  126  may also simultaneously form optional third back side passivation layer  138 , characterized by the same composition and thickness as the second dielectric layer  126 . 
     Whether a first dielectric layer  118 , or first and second dielectric layers  118 ,  126  are formed, the dielectric layer(s) may be formed over the patterned device layer  140 , and within the trenches  116 ,  117 , via openings  120 B, and beam profile opening  145 . In this manner, the dielectric layer(s) provide electrical insulation. For example, the dielectric layer(s) may provide electrical insulation within the via opening  120 B. Referring now to  FIGS. 29-30  via openings  120 B, contact openings  120 C, and beam profile opening  145  are formed through the first dielectric layer  118 , and optional second dielectric layer  126 , stopping on the base substrate  130  (for via opening  120 B), patterned device layer  140  (for via opening  120 C), and spring support layer  150  (for beam profile opening  145 ). Referring briefly back to  FIG. 1A , in an embodiment, contact openings  120 C are located above bus interconnects  106 . In the particular embodiment illustrated in  FIGS. 29-30  including a SiN x  first dielectric layer  118  and Al 2 O 3  second dielectric layer  126 , the second dielectric layer  126  is etched within via openings  120 B, contact openings  120 C, and beam profile openings  145  using a suitable etching chemistry such as Cl 2  RIE, stopping on the first dielectric layer  118 . In such an embodiment, the underlying SiN x  first dielectric layer  118  may protect the underlying silicon layers  130 ,  140 ,  150  against the potential for forming volatile chlorides, and preserve the integrity of the silicon layers. Following etching of the second dielectric layer  126 , a short O 2  plasma clean may be performed, followed by etching of the first dielectric layer  118  using a suitable etching chemistry such as a fluorine based RIE (e.g. CHF 3 , CF 4 ), stopping on the underlying silicon layers  130 ,  140 ,  150 . 
     In an alternative embodiment, a second dielectric layer  126  is not formed. In such an embodiment, the first dielectric layer  118  may be formed of a material such as (ALD) SiO 2 , Al 2 O 3 , Ta 2 O 5 , HfO 2 , Si 3 N 4 , or RuO 2 . For example, first dielectric layer  118  may be ALD Al 2 O 3 . In such an embodiment, via openings  120 B, contact openings  120 C, and beam profile openings  145  are etched in the first dielectric layer  118  to expose the underlying base substrate  130 , patterned device layer  140 , and spring support layer  150  using a suitable etching chemistry such as a fluorine based RIE (e.g. CHF 3 , CF 4 ), stopping on the underlying silicon layers  130 ,  140 ,  150 . Following etching of the second dielectric layer(s) an O 2  plasma and solvent wet clean may be performed to remove any residues and photoresist used for patterning. 
     Referring now to  FIG. 31 , a top conductive contact  123  is formed within each via opening  120 B to make electrical contact with the base substrate  130 . In the particular embodiment illustrated, top conductive contact  123  also spans along a top surface of the dielectric layer(s) and is formed within contact opening  120 C and on bus electrode  106 . In this manner, each top conductive contact  123  provides an electric path from a bus electrode  106  to the base substrate  130 . In an embodiment, top conductive contacts  123  include a TiW and Au stack. In an embodiment, top conductive contacts  123  are formed of about 500 to 1,000 angstroms TiW followed by 1,000 to 5,000 angstroms Au. Top conductive contacts may be formed by any suitable method such as sputtering. 
       FIGS. 32-35  illustrate a manner of forming via plugs in the base substrate  130 , in accordance with an embodiment. As illustrated in  FIG. 32 , via openings  120 A are formed through the back side passivation layers  138 ,  134  using a suitable technique such as ion milling or RIE, stopping on the back side passivation layer  132 . As illustrated in  FIG. 33 , via opening  120 A is etched through back side passivation layer  132  using a suitable technique such as a BOE or RIE to contact the base substrate  130 . Following etching through the passivation layers to contact the base substrate  130 , the back side of the SOI stack is O 2  plasma and solvent cleaned to remove any photoresist, and the base substrate  130  may be pre-cleaned with an Ar plasma clean. Referring now to  FIG. 34  back side conductive contacts  122  are formed on the exposed base substrate  130  within via openings  120 A. In an embodiment, back side conductive contacts  122  include a TiW and Au stack. In an embodiment, back side conductive contacts  122  are formed of about 500 to 1,000 angstroms TiW followed by 1,000 to 5,000 angstroms Au. Back side conductive contacts may be formed by any suitable method such as sputtering. 
     Following the formation of back side conductive contacts  122 , the back side of the SOI stack is O 2  plasma and solvent cleaned to remove any photoresist, and the base substrate  130  is etched to form via openings  120 D through the base substrate  130 . In the particular embodiment illustrated in  FIG. 35 , via openings  120 D terminate at a bottom surface of a second insulating layer  152  between the base substrate  130  and the spring support layer  150 , and beneath where bus interconnect  106  is located. As a result a via plug  135  is formed within the via opening  120 D. With such a via plug configuration, via plug  135  is electrically isolated from the base substrate  130 . In an embodiment, via openings  120 D are etched using a suitable etching technique such as DRIE with a fluorine based chemistry such as SF 6 . Following the formation of via plugs  135 , the back side of the SOI stack is O 2  plasma and solvent cleaned to remove any photoresist. 
     Referring now to  FIG. 36 , a spring release etch operation is performed in accordance with embodiments. Up until this point the spring support layer  150  entirely covers each cavity  136 . Referring to  FIG. 36 , the beam profile opening  145  are now etched through the spring support layer  150  to expose the one or more cavities  136  and release the electrode beam profiles of the silicon electrodes and the spring support layer beam profiles of the spring support layer  150 . This results in the spring support layer  150  spring support layer beam profile underneath and supporting the electrode beam profile over the cavity  136 . In an embodiment, the spring release etch operation is performed using a time DRIE technique using a fluorine based chemistry such as SF 6  or C 4 F 8 . In an embodiment, the spring release etch operation is performed using a gas phase XeF 2  etch. Following the spring release etch operation the SOI stack is O 2  plasma and solvent cleaned to remove any photoresist. The SOI stack may then be diced if multiple micro pick up arrays are to be singulated from the same SOI stack. 
       FIG. 37  is a flow chart illustrating a method of picking up and transferring an array of micro devices from a carrier substrate to a receiving substrate in accordance with an embodiment. At operation  3710  a micro pick up array including an array of bipolar compliant electrostatic transfer heads is positioned over an array of micro devices on a carrier substrate.  FIG. 38  is a cross-sectional side view illustration of an array of bipolar compliant electrostatic transfer heads  102  positioned over an array of micro devices on a carrier substrate  200  in accordance with an embodiment. At operation  3720  the array of micro devices are contacted with the array of bipolar compliant electrostatic transfer heads. In an alternative embodiment, the array of bipolar compliant electrostatic 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 to 10 nm.  FIG. 39  is a cross-sectional side view illustration of an array of bipolar compliant electrostatic transfer heads  102  in contact with an array of micro devices  202  in accordance with an embodiment. As illustrated, the pitch of the array of bipolar compliant electrostatic transfer heads  102  is an integer multiple of the pitch of the array of micro devices  202 . At operation  3730  a voltage is applied to the array of bipolar compliant electrostatic transfer heads  102 . The voltage may be applied from the working circuitry within a transfer head assembly  160  in electrical connection with the array of bipolar compliant electrostatic transfer heads through vias  120 . At operation  3740  the array of micro devices is picked up with the array of bipolar compliant electrostatic transfer heads.  FIG. 40  is a cross-sectional side view illustration of an array of bipolar compliant electrostatic transfer heads  102  picking up an array of micro devices  202  in accordance with an embodiment. At operation  3750  the array of bipolar compliant electrostatic transfer heads contacts the receiving substrate with the array of micro devices  202 .  FIG. 41  is a cross-sectional side view illustration of contacting a receiving substrate with an array of micro devices held by an array of bipolar compliant electrostatic transfer heads in accordance with an embodiment. At operation  3760  the array of micro devices is then released onto 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 ICs, or a substrate with metal redistribution lines.  FIG. 42  is a cross-sectional side view illustration of an array of micro devices  202  released onto a receiving substrate  300  in accordance with an embodiment. 
     While operations  3710 - 3760  have been illustrated sequentially in  FIG. 37 , 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, an operation is 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. For example, the bonding layer may have a liquidus temperature less than 350° C., or more specifically less than 200° C. The bonding layer may be formed of a material which provides adhesion to the carrier substrate, yet also a medium from which the micro device is readily releasable. In an embodiment, the bonding layer is a material such as indium or an indium alloy. 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. For example, heat can be applied to the bonding layer from a heat source located within the transfer head assembly  160 , carrier substrate  200 , and/or receiving substrate  300 . 
     Furthermore, operation  3730  of applying the voltage to create a grip pressure on the micro devices can be performed in various orders. For example, the voltage can be applied prior to contacting the array of micro devices with the array of compliant electrostatic transfer heads, while contacting the micro devices with the array of bipolar compliant electrostatic transfer heads, or after contacting the micro devices with the array of bipolar compliant electrostatic transfer heads. The voltage may also be applied prior to, while, or after creating a phase change in the bonding layer. 
     Where the bipolar compliant electrostatic transfer heads  102  include bipolar silicon electrodes, an alternating voltage is applied across the pair of silicon electrodes in each compliant electrostatic transfer head  102  so that at a particular point when a negative voltage is applied to one silicon electrode, a positive voltage is applied to the other silicon electrode in the pair, and vice versa to create the pickup pressure. Releasing the micro devices from the compliant electrostatic transfer heads  102  may be accomplished with a varied of methods including turning off the voltage sources, lowering the voltage across the pair of silicon electrodes, changing a waveform of the AC voltage, and grounding the voltage sources. Release may also be accomplished by discharge associated with placing the micro devices on the 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 bipolar compliant electrostatic 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: 20140617
Publication Date: 20160823
Grant Date: 20160823
Priority Date: 20140617
Inventors: GOLDA DARIUSZ
BATHURST STEPHEN P.
HIGGINSON JOHN A.
BIBL ANDREAS
BIRKMEYER JEFFREY
Assignee: APPLE INC
CPC Classifications: [{"code": "H01L24/75", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L24/95", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L23/5384", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L21/76898", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/76898", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/95", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/95", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81C99/002", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L24/75", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L23/5384", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 54836796