PATENT DOCUMENT

Publication Number: US-11404601-B2
Application Number: US-202016909672-A
Country: US
Kind Code: B2

Title: Conductive micro LED architecture for on-wafer testing

Abstract:
LED donor substrates and conductive architectures for on-wafer testing are described. In an embodiment, an array of LEDs is supported by an array of electrically conductive stabilization posts. The electrically conductive stabilization posts can be coupled with a test pad for on-wafer testing prior to transferring the LEDs to a receiving substrate.

Claims:
What is claimed is: 
     
       1. A donor substrate comprising:
 a carrier substrate; 
 a stabilization layer including a plurality of pillars formed of a non-metallic matrix material including an organic-based backbone or inorganic-based backbone; 
 a metallic liner layer over the plurality of pillars, wherein the metallic liner layer and the plurality of pillars form a plurality of stabilization posts; 
 an etch protection layer over the metallic liner layer; 
 a plurality of via openings extending through the etch protection layer; and 
 an array of LEDs supported by the array of stabilization posts, each LED including a bottom metal contact layer in direct contact with the metallic liner layer, wherein the plurality of stabilization posts protrudes through the plurality of via openings to contact the bottom metal contact layers of the array of LEDs, and wherein a top surface of the metallic liner layer for each stabilization post is in direct contact with a bottom surface of the bottom metal contact layer for each respective LED. 
 
     
     
       2. The donor substrate of  claim 1 , wherein the metallic liner layer includes a metal bonding layer and a conformal adhesion layer, wherein the metal bonding layer is in direct contact with the bottom metal contact layer. 
     
     
       3. The donor substrate of  claim 2 :
 wherein the bottom metal contact layer comprises Au; and 
 wherein the metal bonding layer comprises Au. 
 
     
     
       4. The donor substrate of  claim 1 , further comprising a sacrificial release layer between the array of LEDs and the stabilization layer, and the plurality of via openings extend through the sacrificial release layer, wherein the plurality of stabilization posts protrude through the plurality of via openings in the sacrificial release layer. 
     
     
       5. The donor substrate of  claim 1 , further comprising a metal adhesion layer between the etch protection layer and the metallic liner layer, and the plurality of via openings extend through the metal adhesion layer, wherein the plurality of stabilization posts protrude through the plurality of via openings in the metal adhesion layer. 
     
     
       6. The donor substrate of  claim 5 :
 wherein the etch protection layer comprises a material selected from the group consisting of Al 2 O 3 , amorphous silicon, HfO 2 , ZrO 2 , and Al x Hf y O z ; and 
 wherein the metal adhesion layer comprises a material selected from the group consisting of Ti, Cr, Ni, Mo, Ta, Nb, Pt, TiW, and NiCr alloy. 
 
     
     
       7. The donor substrate of  claim 1 , wherein the stabilization layer is electrically conductive. 
     
     
       8. The donor substrate of  claim 1 , further comprising an electrically conductive structure selected from the group consisting of particles, nanotubes and sheets dispersed in the non-metallic matrix material. 
     
     
       9. The donor substrate of  claim 1 , further comprising a test pad electrically coupled with a portion of the plurality of stabilization posts. 
     
     
       10. The donor substrate of  claim 1 , further comprising a local test pad electrically coupled with top conductive contacts of a group of test LEDs in the array of LEDs. 
     
     
       11. The donor substrate of  claim 1 , further comprising a plurality of groups of test LEDs, each group of test LEDs including a local test pad electrically coupled with top conductive contacts of the group of test LEDs. 
     
     
       12. The donor substrate of  claim 11 , further comprising:
 a sacrificial release layer between the array of LEDs and the stabilization layer; 
 a metal adhesion layer between the etch protection layer and the metallic liner layer; 
 wherein the plurality of via openings extend through the metal adhesion layer and the sacrificial release layer, wherein the plurality of stabilization posts protrude through the plurality of via openings in the metal adhesion layer and the sacrificial release layer; and 
 wherein the metallic liner layer includes a metal bonding layer and a conformal adhesion layer, wherein the metal bonding layer is in direct contact with the bottom metal contact layer.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Application No. 62/902,870 filed Sep. 19, 2019, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     Embodiments described herein relate to micro devices. More particularly embodiments relate to conductive stabilization structures for testing of micro devices on a donor substrate. 
     Background Information 
     State of the art displays for electronic devices such as wearable devices, portable electronics, desktop computers, and televisions are based on liquid crystal display (LCD) or organic light emitting diodes (OLED) technologies. More recently, it has been proposed to incorporate emissive inorganic semiconductor-based micro LEDs into high resolution displays, with the potential for energy efficiency and being less prone to lifetime degradation and sensitivity to moisture. 
     In one implementation, it has been proposed to transfer an array of inorganic semiconductor-based micro LEDs from a carrier substrate to a receiving (e.g. display) substrate using an array electrostatic transfer heads. For example, it has been proposed in U.S. Pat. No. 8,835,940 to stage an array of micro LEDs on an array of stabilization posts formed of an adhesive bonding material, such as a thermoset material. During the transfer process, it is described that the array of electrostatic transfer heads generate a sufficient pressure to overcome the adhesion strength between the adhesive bonding material and the micro LEDs. 
     SUMMARY 
     LED donor substrates and conductive architectures for on-wafer testing are described. In an embodiment a donor substrate includes a carrier substrate and a stabilization layer including a plurality of pillars formed of a non-metallic matrix material including an organic-based or inorganic-based backbone. A metallic liner layer is over the plurality of pillars, where the metallic liner layer and the plurality of pillars form a plurality of stabilization posts. An array of LEDs is supported by the array of stabilization posts, each LED including a bottom metal contact layer in direct contact with the metallic liner layer. 
     In an embodiment a donor substrate includes a carrier substrate, a bonding layer on the carrier substrate, and a metal adhesion layer on the bonding layer. A stabilization cavity layer is on the metal adhesion layer, with the stabilization cavity layer including an array of stabilization cavities. An array of via openings are through the stabilization cavity layer. An array of LED devices is at least partially contained within the array of stabilization cavities, each LED device including a p-n diode, a bottom contact structure, and a metal stabilization post. The metal stabilization posts each protrude from the bottom contact structure, extending through a corresponding via opening and in direct contact with the metal adhesion layer. 
     In an embodiment a donor substrate includes a carrier substrate and an array of LEDs supported on the carrier substrate with an array of stabilization posts. A (e.g. bottom electrode) test pad is on the carrier substrate and is electrically coupled to a portion of stabilization posts of the array of stabilization posts. In an embodiment a local (e.g. top electrode) test pad is electrically coupled with top conductive contacts of a group of test LEDs in the array of LEDs. 
     In an embodiment, a method of populating a display panel includes probing one or more test LEDs, which may be LEDs also eligible for transfer, within a first area of a donor substrate to determine operability of the one or more test LEDs. A first group of LEDs can then be picked up from the first area of the donor substrate and placed onto a receiving substrate depending upon the probing results. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional side view illustration of a donor substrate including a plurality of LEDs supported on a plurality of stabilization posts in accordance with an embodiment. 
         FIG. 2  is a process flow of a method of fabricating the donor substrate of  FIG. 1  in accordance with an embodiment. 
         FIGS. 3A-3M  are schematic cross-sectional side view illustrations of a method of fabricating the donor substrate of  FIG. 1  in accordance with an embodiment. 
         FIG. 4  is a schematic cross-sectional side view illustration of a donor substrate including an LED supported on a stabilization post within a staging cavity in accordance with an embodiment. 
         FIG. 5  is a schematic cross-sectional side view illustration of a donor substrate including an LED with an integrally formed stabilization post in accordance with an embodiment. 
         FIG. 6  is a process flow of a method of fabricating the donor substrate of  FIG. 5  in accordance with an embodiment. 
         FIG. 7  is a schematic top view illustration of a donor substrate structure for probing an LED for operability in accordance with an embodiment. 
         FIG. 8  is a schematic top view illustration of a donor substrate including a plurality of groups of test LEDs in accordance with an embodiment. 
         FIG. 9  is a process flow for a method of testing and transferring LEDs from a donor substrate in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe donor substrates and conductive micro LED architectures thereof that facilitate testing of the micro LEDs prior to transferring the micro LEDs from the donor substrate to a receiving substrate. In particular, embodiments describe electrically conductive stabilization structures that may function to both support and retain the micro LEDs on the donor substrate, while also providing electrical connection for testing of the micro LEDs to verify operability prior to being transferred to and integrated on a display substrate. 
     It has been observed that display panels fabricated using techniques involving micro LED transfer can be prone to the inclusion of defects with the integrated micro LED devices. Such defects can occur from a variety of sources related to the transfer process, integration process, or the micro LEDs themselves on the donor substrate. In this aspect, embodiments described herein allow for testing of the micro LEDs while on the donor substrate and prior to being transferred to a receiving substrate. 
     While embodiments are described with specific regard to micro LED devices comprising p-n diodes, 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 which are designed in such a way so as to perform in a controlled fashion a predetermined electronic function (e.g. diode, transistor, integrated circuit, display circuitry, sensor) or photonic function (LED, laser). Embodiments are also applicable to micro chips. 
     The terms “micro” device, “micro” mesa, “micro” chip, or “micro” LED device as used herein may refer to the descriptive size of certain devices, chips, or structures in accordance with embodiments of the invention. As used herein the term “micro device” specifically includes “micro LED device” and “micro chip”. As used herein, the terms “micro” devices or structures are meant to refer to the scale of 1 to 300 μm. In an embodiment, a single micro device or structure has a maximum dimension, for example length or width, of 1 to 300 μm, or 1 to 100 μm. In an embodiment, the top contact surface of each micro device, micro structure, or electrostatic transfer head has a maximum dimension of 1 to 300 μm, 1 to 100 μm, or more specifically 1 to 20 μm, or 1 to 10 μm. 
     In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the embodiments. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the embodiments. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “over”, “to”, “between”, “spanning” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “over”, “spanning” or “on” another layer or bonded “to” or in “contact” with another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     Referring now to  FIG. 1  a schematic cross-sectional side view illustration is provided of a donor substrate including a plurality of LEDs supported on a plurality of stabilization posts in accordance with an embodiment. As shown in  FIG. 1 , the donor substrate  100  can include a carrier substrate  200  and a stabilization layer  190 . The stabilization layer  190  includes a plurality of pillars  195  formed of a non-metallic matrix material including a carbon-based or oxygen-based backbone. A metallic liner layer  175  is over the plurality of pillars  195 . Together, the metallic liner layer  175  and the plurality of pillars  195  form a plurality of stabilization posts  156  which support an array of LEDs  150 , which may be micro LEDs. As shown, each LED  150  includes a bottom metal contact layer  118  in direct contact with the metallic liner layer  175 . 
     A variety of additional layers can be formed to facilitate the fabrication and transfer processes. For example, a sacrificial release layer  130  may be formed between the array of LEDs  150  and the stabilization layer  190 . The sacrificial release layer  130  may be removed, for example, by a vapor hydrofluoric acid (HF) etch prior to the transfer process to render the LEDs  150  poised for pick up. An etch protection layer  140  may be located over the metallic liner layer  175 . Furthermore, a metal adhesion layer  160  can be located between the etch protection layer  140  and the metallic liner layer  175 . For example, this can promote better adhesion to the etch protection layer  140  than would otherwise by achieved by the metallic liner layer  175 . The etch protection layer  140  may protect the metal adhesion layer  160  during removal of the sacrificial release layer  130 . As shown, the plurality of stabilization posts  156  each protrude through a via opening that is formed through the metal adhesion layer  160 , the etch protection layer  140 , and the sacrificial release layer  130 . Additionally, the metallic liner layer  175  can include a metal bonding layer  170  that is in direct contact with the bottom metal contact layer  118 , and a conformal adhesion layer  180 , which may be used to promote adhesion for the stabilization layer  190 . 
     The stabilization layer  190  in accordance with embodiments may be formed of a material suitable for substrate-substrate bonding, and capable of flowing into the via opening spaces to form the pillar  195  structures which can provide mechanical support for the stabilization posts  156 . Some exemplary materials include matrix materials with an organic-based (e.g. carbon-based) backbone such as benzocyclobutene (BCB), polyimide, etc. or matrix materials with an inorganic-based backbone. For example, an inorganic-based backbone may be a silicon-based backbone, or other inorganic such as boron, phosphorus, silicon-oxide, etc. Exemplary inorganic-based backbone materials include siloxanes such as polydimethylsiloxane (PDMS), spin on glass (SOG), etc. Such a base composition may be electrically insulative. In such a configuration, the metallic liner layer  175  provides electrical connection for testing of the LED  150 . In an embodiment, the stabilization layer may be electrically conductive. For example, this may be achieved by dispersing electrically conductive structures in the non-metallic material, including particles such as metal particles, carbon nanotubes, graphene sheets, etc. 
     Embodiments described herein are compatible with a variety of different LED configurations. In the exemplary embodiment illustrated in  FIG. 1 , the LED  150  includes a p-n diode  120  including a top doped layer  128  doped with a first dopant type (e.g. n-type), a bottom doped layer  124  doped with a second dopant type (e.g. p-type) opposite the first dopant type, and an active layer  126  therebetween. For example, the active layer may include one or more quantum well layers separated by barrier layers. 
     The LED  150  may include a top conductive contact  122  and bottom contact structure  110 . In an exemplary embodiment, the top conductive contact  122  is formed of a transparent material, such as a transparent conductive oxide (TCO) such as indium-tin-oxide. The bottom metal contact layer  118  may be a part of a more complex bottom contact structure  110 .  FIG. 1  illustrates an exemplary bottom contact structure  110 , though embodiments are not so limited, and may be compatible with a variety of alternative configurations of a bottom contact structure  110 . In the exemplary embodiment illustrated the bottom contact structure can include a conductive layer  112  on the bottom surface  105  of the p-n diode  120 . For example, the conductive layer  112  may be used to provide ohmic contact to the p-n diode  120 . In an embodiment, the conductive layer  112  is formed of a transparent conductive oxide (TCO) such as indium-tin-oxide (ITO). An insulating layer  114  such as an oxide (e.g. Al 2 O 3 ) or nitride (e.g. SiNx) can be formed over and around the conductive layer  112  and patterned to form an opening that exposes the conductive layer  112 . Insulating layer  114  may be formed of the same material as etch protection layer  140  to also protect against HF attack. A metal stack  116  is then formed over the insulating layer  114  and within the opening in the insulating layer to contact the conductive layer  112 . The metal stack  116  may include a number of combinations of layers such as a contact layer, mirror layer, barrier layer, and interface layer, though not all layers are required, and different layers may be included. For example, a metal stack  116  may include a first contact layer for ohmic contact, a mirror layer on the first contact layer, and a barrier layer on the mirror layer to prevent diffusion. Various adhesion layers may be formed between any of the layers within the layer stack. In an embodiment, contact layer is formed of a high work-function metal such as nickel. In an embodiment, a mirror layer such as silver or aluminum is formed over the contact layer to reflect the transmission of the visible wavelength emitted from the p-n diode  120 . In an embodiment, platinum is used as a diffusion barrier to bottom metal contact layer  118 . 
     Bottom metal contact layer  118  may be formed of a variety of materials that can be chosen for bonding to the receiving substrate and/or to achieve the requisite tensile strength or adhesion or surface tension with the stabilization posts. In an embodiment, the bottom metal contact layer  118  is formed of a conductive material (both pure metals and alloys) that can diffuse with a bonding layer (e.g. gold, indium, or tin) on a receiving substrate and is also amenable to forming a metal-to-metal joint with the metallic liner layer  175 . While embodiments are not limited to specific metals, exemplary materials for bottom metal contact layer  118  include gold and aluminum, as well as their alloys. 
     The metallic liner layer  175  can include a metal bonding layer  170  that is in direct contact with the bottom metal contact layer  118 , and a conformal adhesion layer  180 , which may be used to promote adhesion for the stabilization layer  190 . While embodiments are not limited to specific metals, exemplary materials for metal bonding layer  170  include gold and aluminum, as well as alloys in which elemental impurities can be added to tailor mechanical properties (e.g. yield strength, hardness, ductility) of the metal-to-metal joint. Exemplary elemental impurities that may be included are Co, Ni, Be, Al, Ca, Mo, Au. In an embodiment, a gold alloy material includes 0 to 5% by weight of impurity. The bottom metal contact layer  118  may additionally be formed of any of these materials. While embodiments are not limited to specific metallic materials, exemplary materials for conformal adhesion layer  180  include Ti, TiW, Cr and Ni. Conformal adhesion layer  180  may also be selected to control joint adhesion, and additionally the break point during the transfer process. Geometry of the stabilization posts  156  may also be varied to control adhesion. In an embodiment, each stabilization post  156  has a maximum contact surface  157  width of less than 0.5 μm. For example, the stabilization posts  156  and resultant metal-to-metal joints may be in the form of solid posts, annular rings, etc. The number of stabilization posts  156  and location can also be adjusted to control the pull force required for transfer. 
     In one aspect, the dimensions and compositions of the metal bonding layer  170  and the bottom metal contact layer  118  can be tailored to achieve a reliable and repeatable bond force between the layers (e.g. pull force required for transfer). 
       FIG. 2  is a process flow of a method of fabricating the donor substrate of  FIG. 1  in accordance with an embodiment.  FIGS. 3A-3M  are schematic cross-sectional side view illustrations of a method of fabricating the donor substrate of  FIG. 1  in accordance with an embodiment. In interest of clarity and conciseness,  FIGS. 2 and 3A-3M  are described concurrently so as to not unnecessarily obscure the embodiments. 
     Referring to  FIG. 3A , the processing sequence includes a p-n diode layer  104  formed on a growth substrate  102 . As shown, the p-n diode layer  104  includes a first doped layer  124  doped with a first dopant type (e.g. n-type), a second doped layer  128  doped with a second dopant type (e.g. p-type) opposite the first dopant type, and an active layer  126  between the doped layers  124 ,  128 . The active layer  126  may include one or more quantum well layers separated by barrier layers. In accordance with embodiments, the p-n diode layer  104  may be formed of III-V or II-VI inorganic semiconductor-based materials, and be designed for emission at a variety of primary wavelengths, such as red, green, blue, etc. 
     An array of laterally separate bottom contact structures  110  are then formed on the doped layer  124  as illustrated in  FIG. 3B . As previously described with regard to  FIG. 1 , the bottom contact structures  110  may include multiple layers, such as a conductive layer  112  for ohmic contact, a metal stack  116 , and a bottom metal contact layer  118 . Referring now to  FIG. 3C , the p-n diode layer  104  is etched to form an array of mesa structures that will become the p-n diodes  120 . In an embodiment, etching is performed using dry etching technique. Alternatively, the array of mesa structures can be etched prior to formation of the bottom contact structures  110 . 
     In an embodiment, a sacrificial release layer  130  is then formed over the plurality of micro LED mesa structures (p-n diodes  120 ) at operation  2010 . The sacrificial release layer  130  may be formed using a variety of configurations. For example, in the embodiment illustrated in  FIG. 3D , the sacrificial release layer  130  may be used as a planarization layer as well. In such an embodiment, the sacrificial release layer  130  may be deposited, followed by a planarization operation to form a planarized surface  131 . The sacrificial release layer  130  may completely cover the micro LED mesa structures and bottom contact structures  110 . Alternatively, the sacrificial release layer  130  may be a conformal layer, with more uniform thickness. Such a processing condition can be used to from the structures of  FIGS. 4-5  where the LEDs  150  will be at least partially contained within a staging cavity. 
     In an embodiment, sacrificial release layer  130  is formed of an oxide (e.g. SiO 2 ) or nitride (e.g. SiN x ), though other materials may be used which can be selectively removed with respect to the other layers. In the illustrated in embodiment, the sacrificial release layer  130  is thicker than the mesa structures forming the p-n diodes  120 . For example, the sacrificial release layer  130  may be 2-5 μm thick. In an embodiment that results in staging cavities, the sacrificial release layer  130  may be thinner than the mesa structures forming the p-n diodes  120 . In such an implementation the sacrificial release layer  130  is between approximately 0.1 and 2 μm thick, or more specifically approximately 0.2 μm thick. 
     Referring now to  FIG. 3E , at operation  2020  an etch protection layer  140  is formed on the sacrificial release layer  130 , for example, on planarized surface  131 . The etch protection layer  140  may be formed of a material which is resistant to the etchant used to remove the sacrificial release layer  130 . In this manner, the etch protection layer  140  can be used to protect underlying materials from the etchant. In an embodiment, etch protection layer  140  is formed of a material such as Al 2 O 3 , amorphous silicon, HfO 2 , ZrO 2 , and Al x Hf y O z . Still referring to  FIG. 3E , a metal adhesion layer  160  can then optionally be formed on the etch protection layer  140 . The optional metal adhesion layer  160  may be formed of a material suitable to improve adhesion of the metallic liner layer  175  that will be formed next. For example, the metal adhesion layer  160  can be formed of titanium (Ti), which may have better adhesion to the etch protection layer  140  than materials chosen for the metallic liner layer  175 . Other suitable materials for the metal adhesion layer  160  include Cr, Ni, Mo, Ta, Nb, Pt, TiW, and NiCr alloy. It has been observed that material used to form the metal adhesion layer  160  (e.g. Ti) can migrate when exposed to HF, and the etch protection layer  140  can protect the metal adhesion layer  160  during removal of the sacrificial release layer  130 . 
     In the illustrated embodiments, the etch protection layer  140  and metal adhesion layer  160  are continuous layers, blanket deposited over the underlying substrate structure. In an embodiment, the metal adhesion layer  160  is 1 nm to 500 nm thick. The metal adhesion layer  160  may be deposited using suitable techniques such as evaporation, sputtering, electroless plating and electroplating. In an embodiment, the etch protection layer  140  is 1 nm to 100 nm thick. The etch protection layer  140  may be deposited using a suitable technique such as chemical vapor deposition (CVD), or atomic layer deposition (ALD) for a higher quality protection layer. 
     Referring now to  FIG. 3F , at operation  2030  a plurality of via openings  152  are formed through the metal adhesion layer  160 , etch protection layer  140  and sacrificial release layer  130  to expose the plurality of bottom contact structures  110 , or more specifically the bottom surface  119  of the bottom contact structures  110  that corresponds to the bottom metal contact layer  118 . Each via opening  152  may have sidewalls  154  spanning the metal adhesion layer  160 , etch protection layer  140  and sacrificial release layer  130 . In accordance with embodiments, the width and shape of the via openings  152  determines the resultant contact area of the stabilization posts to be formed, and resultant adhesion strength for the LEDs. In an embodiment, the via openings  152  have a maximum width between 100 nm and 1,000 nm, such as approximately 200 nm-500 nm wide. 
     A metallic liner layer  175  is then formed over the underlaying structure and within the via openings  152  at operation  2040 . Specifically, the metallic liner layer  175  is formed over the optional metal adhesion layer  160  and within the via openings  152 , on the sidewalls  154  and bottom surface  119  of the bottom contact structures  110 . The metallic liner layer  175  may include one or more layers. In an embodiment the metallic liner layer  175  includes a metal bonding layer  170  and conformal adhesion layer  180 , which can be sequentially deposited. Referring now to  FIG. 3G , the metal bonding layer  170  can be a conformal layer deposited in direct contact with the underlying optional metal adhesion layer  160  and within the via openings  152 , in direct contact with the sidewalls  154  and bottom surface  119  of the bottom contact structure  110 . The top surface of the metal bonding layer  170  in contact with the bottom surface  119  of the bottom contact structure  110  forms the top contact surface  157  of the resultant stabilization posts  156 . A metal-to-metal joint is formed between the metal bonding layer  170  and the bottom metal contact layer  118 , which may be formed of similar materials. For example, the metal bonding layer  170  and the bottom metal contact layer  118  may each include a same base metal element, such as Au or Al. In an embodiment, each layer is formed of Au or an Au alloy. Element impurities can additionally be added to either layer to tailor mechanical properties (e.g. yield strength, hardness, ductility) of the metal-to-metal joint. Exemplary elemental impurities that may be included are Co, Ni, Be, Al, Ca, Mo, Au. In an embodiment, a gold alloy material includes 0 to 5% by weight of impurity. 
     In an embodiment, the metal bonding layer  170  is 1 nm to 250 nm thick. Such a thickness may allow for conformal deposition along the sidewalls  172 . The metal bonding layer  170  may be deposited using suitable techniques such as evaporation, sputtering, electroless plating and electroplating. Referring to  FIG. 3H , a conformal adhesion layer  180  may then be deposited as part of the metallic liner layer  175 . For example, the conformal adhesion layer  180  may be formed of a material with increased adhesion relative to the bonding layer  190  compared to the metal bonding layer  170 . Exemplary materials for the conformal adhesion layer  180  include Ti, TiW, Cr and Ni. In an embodiment, the conformal adhesion layer  180  is thinner than the metal bonding layer  170 . In an embodiment, the conformal adhesion layer  180  is 1 nm to 5 nm thick. As shown, the conformal adhesion layer  180  is formed along the sidewalls  172  of the metal bonding layer  170  within the via openings  152 . 
     Referring now to  FIG. 3I , at operation  2050  a stabilization layer  190  is formed on the metallic liner layer  175  and within volume remaining in the via openings  152 . As shown, the stabilization layer  190  may be formed in direct contact with the optional conformal adhesion layer  180  and in direct contact with sidewalls  182  of the conformal adhesion layer  180  within the via openings  152 . The volume of the stabilization layer  190  filling the remaining space within the via openings  152  may result in pillars  195 , which can include vertical or sloped sidewalls. In the embodiment illustrated in  FIG. 3I  the stabilization layer  190  covers the entire surface of the underlying structure. 
     The stabilization layer  190  in accordance with embodiments may be formed of a material suitable for substrate-substrate bonding, and capable of flowing into the via opening spaces to form the pillar  195  structures which can provide mechanical support for the stabilization posts  156 . Some exemplary materials include matrix materials with an organic-based backbone or inorganic-based backbone. Such a base composition may be electrically insulative. In such a configuration, the metallic liner layer  175  provides electrical connection for testing of the LED  150 . In an embodiment, the stabilization layer may be electrically conductive. For example, this may be achieved by dispersing electrically conductive materials in the non-metallic matrix, including electrically conductive structures, including particles such metal particles, carbon nanotubes, graphene sheets, etc. 
     Referring now to  FIG. 3J  a carrier substrate  200  can be bonded using the stabilization layer  190 . Carrier substrate  200  can be a variety of materials, such as a silicon, sapphire or gallium arsenide wafer, to provide mechanical support during subsequent handling and processing. The growth substrate  102  can then be removed as shown in  FIG. 3K  using a suitable technique such as laser lift off (LLO), or combination with grinding and polishing to expose top surfaces  121  of the p-n diodes  120 . Top conductive contacts  122  can then be formed on the top surfaces  121  of the p-n diodes  120 . Depending upon application, the top conductive contacts  122  can be reflective or transparent. Top conductive contacts  122  can include one or more layers, including thin metals layers and/or transparent conductive oxides (TCOs). In an embodiment, top conductive contacts  122  include ITO. 
     At this stage, the stabilized array of LEDs  150  are in condition for transportation or storage for future processing. Prior to transferring the LEDs  150  to a receiving substrate, for example, using a pick-and-place tool, the sacrificial release layer  130  can be removed as shown in  FIG. 3M . In an embodiment, removal of the sacrificial release layer  130  may include a vapor HF etch operation. Removal of the sacrificial release layer  130  may result in an open space  132  underneath the LEDs  150 , which are supported only by the stabilization posts  156 . 
     Referring now to  FIGS. 4-5  alternative stabilization structure arrangements are illustrated for the LEDS  150 .  FIG. 4  is a schematic cross-sectional side view illustration of a donor substrate including an LED  150  supported on a stabilization post  156  within a staging cavity  450  in accordance with an embodiment. Specifically, the stabilization structure illustrated in  FIG. 4  may be fabricated similarly as the previously described process, where after deposition of a conformal sacrificial release layer  130 , with uniform thickness, a stabilization cavity layer  400  is deposited, and planarized. For example, the stabilization cavity layer  400  may be formed of the same materials as the stabilization layer  190 . In such a configuration, the via openings  152  will additionally be formed through a thickness of the stabilization cavity layer  400 . As shown, each LED  150  may be at least partially contained within stabilization cavities  450  including sidewalls  410  that laterally surround a portion of the thickness of the LEDs  150 . 
       FIG. 5  is a schematic cross-sectional side view illustration of a donor substrate including an LED  150  with an integrally formed stabilization post  115  in accordance with an embodiment.  FIG. 6  is a process flow of a method of fabricating the donor substrate of  FIG. 5  in accordance with an embodiment. The process sequence may proceed similarly as the processing sequence up to  FIG. 3B . At operation  6010  a plurality of stabilization posts  115  can be plated onto the corresponding bottom contact structures  110 . Specifically, the stabilization posts  115  can be plated directly on the bottom contact layers  118 . The array of micro LED mesa structures forming p-n diodes  120  may then be etches at operation  6020 . A conformal sacrificial release layer  130  is formed at operation  6030 , followed by the formation of a stabilization cavity layer  400  at operation  6040 . This may include a polishing operation to planarize the bottom surface  401  of the stabilization cavity layer  400  and expose the stabilization posts  115 . A metal adhesion layer  160  may then be deposited at operation  6050  to promote adhesion to the stabilization posts with the bonding layer  590  which is formed at operation  6060 . Bonding layer  590  may be formed similarly and of the same materials as the stabilization layer  190  and stabilization cavity layer  400 . The carrier substrate  200  can then be bonded at operation  6070 , followed by removal of the growth substrate at operation  6080  as previously described. 
     In an embodiment, a donor substrate  100  includes a carrier substrate  200  and a bonding layer  590  on the carrier substrate  200 . A metal adhesion layer  160  is on the bonding layer  590 . A stabilization cavity layer  400  is on the metal adhesion layer  160 , with the stabilization cavity layer including an array of stabilization cavities  450 . An array of via openings extend through the stabilization cavity layer  400 , with an array of LED devices  150  being at least partially contained within the array of stabilization cavities  450 . As shown, each LED device  150  includes a p-n diode  120 , a bottom contact structure  110 , and a metal stabilization post  115  protruding from the bottom contact structure  110 , extending through a corresponding via opening  152  and in direct contact with the metal adhesion layer  160 . While the sacrificial release layer  130  has already been removed in the structure illustrated in  FIG. 5 , in an embodiment, the sacrificial release layer  130  is located between the array of LEDs  150  and the stabilization cavity layer  400 , where each via opening  152  extends through the sacrificial release layer  130 , and each corresponding metal stabilization post  115  extends through a corresponding via opening  152  in the sacrificial release layer  130 . In accordance with an embodiment, the metal adhesion layer  160  may also be electrically conductive to support on-wafer testing of the LEDs  150 . 
     In each of the above described structures, the arrays of LEDs  150  can be supported by an electrically conductive stabilization structure, that allows for both stabilization of the LEDs prior to being transferred, and on-wafer testing of the LEDs while still on the donor substrate  100  and prior to being transferred. This can facilitate the transfer of “known good” LEDs  150  to a receiving substrate. In some embodiments, clusters of test LEDs are provided so that good LED areas can be determined without testing each and every LEDs. In such a configuration, an inference is made for good donor wafer areas based on test data to determine a threshold evaluation for likelihood of known good LEDs being supported in a defined area. 
       FIG. 7  is a schematic top view illustration of a donor substrate structure for probing an LED for operability in accordance with an embodiment. The donor substrate  100  illustrated in  FIG. 7  can have one or more (e.g. bottom electrode) test pads  710  which are electrically connected to the stabilization posts of the LEDs  150 . For example, the test pads  710  can be connected to an electrically conductive layer such as the metal adhesion layer  160 , metallic liner layer  175 , conformal adhesion layer  180  or even an electrically conductive stabilization layer  190 . As shown, a probe  700 B can contact the test pad  710  while a probe tip  700 A can contact an individual LED  150 , such as top conductive contact  122 , with the probe tip  700 A at another potential or voltage level. Upon contact, an electrical circuit is completed, where an operable LED  150  will light up to indicate operability. In particular, luminance can be measured to determine whether a threshold value is obtained. Thus, each LED  150  in  FIG. 7  can be a test LED. 
       FIG. 8  is a schematic top view illustration of a donor substrate  100  including a plurality of groups of test LEDs  150 T in accordance with an embodiment. A noticeable difference in  FIG. 8  is the inclusion of permanent test LEDs  150 T. In such an embodiment, a conductor pattern  722  is formed over one or more test LEDs  150 T and in electrical connection with a local (e.g. top electrode) test pad  720 . Such a conductor pattern  722  is not formed over the LEDs  150  that are eligible for transfer. Similar to the description of  FIG. 7 , probes can be connected to test pad  710  (which can also be local), and local test pad  720 . A plurality of test pads  710  can be included for both  FIG. 7  and  FIG. 8 . Completion of the circuit then lights up the one or more test LEDs  150 T. The measured luminance can then be measured to determine likelihood of known good LEDs in a corresponding area of the donor substrate  100 . The illustrated example of  FIG. 8  shows four quadrants of corresponding areas, though this is exemplary, and any arrangement can be used. 
       FIG. 9  is a process flow for a method of testing and transferring LEDs from a donor substrate in accordance with an embodiment. At operation  9010  one or more test LEDs  150 / 150 T are probed within a first area of a donor substrate to determine operability of the one or more test LEDs. At operation  9020  a determination is made whether all of the test LEDs are operable. This determination may furthermore be a threshold luminance determination. If the LEDs pass the test, then a first group of LEDs  150  is picked up from the first area of the donor substrate  200  and then placed onto a receiving substrate. A determination is then made at operation  9040  as to whether another location should be tested. And the cycle repeated. It is not necessary however to perform the transfer sequence between probing operations. For example, all probing operations can be performed prior to the pick-and-place operation  9030 . The probing operation in accordance with embodiment may include probing a test pad  710  on the donor substrate while probing the one or more test LEDs. The test pad  710  in accordance with embodiments can be electrically coupled with one or more stabilization posts supporting the one or more test LEDs, and optionally the LEDs  150  that are eligible for pick-and-place if different from the test LEDs. 
     In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming a conductive micro LED architecture for on-wafer testing prior to transferring the micro LEDs from a donor substrate to a receiving substrate. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration.

Metadata:
Filing Date: 20200623
Publication Date: 20220802
Grant Date: 20220802
Priority Date: 20190919
Inventors: JOHN, RANJITH SAMUEL E.
ABRAHAMSEN, ADAM C.
CHAN, CLAYTON K.
RAJ, MADHAN M.
CHAN, MICHAEL Y.
JEEWAKHAN, NAZNEEN N.
YANG, Yu S.
Assignee: APPLE INC
CPC Classifications: [{"code": "H10H20/0364", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/857", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10H20/857", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10H20/0364", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/831", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10H20/01", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L2221/68354", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L25/0753", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01R31/2635", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68354", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68368", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L2221/68363", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L22/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68322", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L25/0753", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/6835", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R31/2831", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L22/32", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L22/14", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L22/32", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L21/6835", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2933/0066", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L22/32", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68363", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L33/0095", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L22/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L25/0753", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L21/6835", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L33/62", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L2221/68354", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 74879981