Patent ID: 12199058

While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments or implementations have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of an invention as defined by the appended claims.

DETAILED DESCRIPTION

Many micro devices, including light emitting diodes (LEDs), Organic LEDs, sensors, solid state devices, integrated circuits, MEMS (micro-electro-mechanical systems) and other electronic components, are typically fabricated in batches, often on planar substrates. To form an operational system, micro devices from at least one donor substrate need to be selectively transferred to a receiver substrate.

Substrate and Transfer Structure:

FIG.1shows a donor substrate100and receiver substrate200, before the transfer process begins. Micro devices102a,102b,102cbegin in an array attached to donor substrate100. The receiver substrate consists of an array of landing areas202a,202b,202cwhere the micro devices will sit. The landing areas202a,202b,202ceach include at least one force modulation element204a,204b,204cand at least a contact pad206a,206b,206c. The force modulation element and contact pads can be different as shown inFIG.1Aor can be the same structure as shown inFIG.1B. The micro devices102may be coupled or connected to a circuit or a potential on the receiver substrate200through contact pads206a,206b,206c. The force modulation elements204a,204b,204ccreate a transfer force to hold the micro device102a,102b,102cselectively on the receiver substrate200and separate them from the donor substrate100. The donor substrate100is the substrate upon which micro devices102are manufactured or grown or another temporary substrate onto which they have been transferred. Micro devices102can be any micro device that is typically manufactured in planar batches including LEDs, OLEDs, sensors, solid state devices, integrated circuit, MEMS, and other electronic components. Donor substrate100is chosen according to the manufacturing process for a particular type of micro device102. For example, in the case of conventional GaN LEDs, donor substrate100is typically sapphire. Generally, when growing GaN LEDs, the atomic distance of donor substrate100should match that of the material being grown in order to avoid defects in the film. Each micro device102is attached to donor substrate100by a force, FD, determined by the manufacturing process and the nature of the micro devices102. FD will be substantially the same for each micro device102. Receiver substrate200can be any more desirable location for micro devices102. It can be, for example, a printed circuit board (PCB), a thin film transistor backplane, an integrated circuit substrate, or, in the case of optical micro devices102such as LEDs, a component of a display, for example a driving circuitry backplane. The landing area on the receiver substrate as shown inFIG.1Brefers to the location where micro device sits on the receiver substrate and may consist of at least one contact pad101aand at least one force modulation element101b. Although in some of the figures the landing area may be the same size as the contact pads202, the contact pads202can be smaller than the landing area. Contact pads202are the locations where micro devices may be coupled or directly connected to the receiver substrate200. In this description, landing area and contact pads are used interchangeably.

The goal in selective transfer is to transfer some, selected micro devices102, from donor substrate100to receiver substrate200. For example, the transfer of micro devices102aand102bonto contact pads206aand206bwithout transferring micro device102cwill be described.

Transfer Process

Following steps describe a method of transferring selected micro devices in an array of micro devices each of which is bonded to a donor substrate with a donor force to contact pads in an array on a receiver substrate:a. aligning the donor substrate and the receiver substrate so that each of the selected micro devices are in line with a contact pad on the receiver substrate;b. moving the donor substrate and the receiver substrate together until each of the selected micro devices are in contact with or proximity with at least one contact pad on the receiver substrate;c. generating a receiver force that acts to hold the selected micro devices to their contact pads;d. moving the donor substrate and the receiver substrate apart leaving the selected micro devices on the receiver substrate while other non-selected micro devices from donor substrate stays on donor substrate despite possible contact with or proximity contact with the system substrate during steps b and c.

If the donor force is too strong for receiver force to overcome for transferring the micro device to the receiver substrate, the donor force for micro devices is weakened to assist micro device transfer. In addition, if the receiver force is applied globally or selective receiver force is not enough to transfer the micro devices selectively, the donor force for the selected micro devices is weakened selectively to improve selectivity in micro device transfer.

FIGS.2A-2Cshow exemplary flowcharts of selective transfer methods1000A-1000C.FIG.1shows a donor substrate100and a receiver substrate200suitable for performing any of methods1000. Method1000A will be described with reference toFIGS.3A-3E. Methods1000B and1000C are analogous variations of method1000A. One can use the combination of methods1000A-1000C to further enhance the transfer process.

At1002A donor substrate100and receiver substrate200are aligned so that selected micro devices102a,102bare in line with corresponding contact pads202a,202b, as shown inFIG.3A. Micro device102cis not to be transferred so, although shown as aligned, it may or may not align with contact pad202c.

At1004A, donor substrate100and receiver substrate200are moved together until the selected micro devices102a,102bare positioned within a defined distance of contact pads202a,202b, as shown inFIG.3B. The defined distance may correspond to full or partial contact but is not limited thereto. In other words, it may not be strictly necessary that selected micro devices102a,102bactually touch corresponding contact pads202a,202b, but must be near enough so that the forces described below can be manipulated.

At1006A, forces between selected micro devices102, donor substrate100and receiver substrate200(and contact pads202) are modulated so as to create a net force towards receiver substrate200for selected micro devices and a net force towards donor substrate100(or zero net force) for other micro devices102c.

Consider the forces acting one of the selected micro devices102. There is a pre-existing force holding it to donor substrate100, FD. There is also a force generated between micro device102and receiver substrate200, FR, acting to pull or hold micro device102towards receiver substrate200and cause a transfer. For any given micro device102, when the substrates are moved apart, if FR exceeds FD the micro device102will go with receiver substrate200, while if FD exceeds FR the micro device102will stay with donor substrate100. There are several ways to generate FR that will be described in later sections. However, once FR has been generated, there are at least four (4) possible ways to modulate FR and FD to achieve transfer of selected micro devices.1. Weaken FD to be less than FR on micro devices selected for transfer2. Strengthen FR to be greater than FD on micro devices selected for transfer.3. Weaken FR to be less than FD on micro devices NOT selected for transfer4. Strengthen FD to be greater than FR on micro devices NOT selected for transfer

Different combinations and arrangements of the above are also possible. Using combinations may, in some cases, be desirable. For example, if the required change in FD or FR is very high, one can use a combination of modulation of FD and FR to achieve the desired net forces for the selected and the non-selected micro devices. Preferably, FR can be generated selectively and therefore act only on selected micro devices102a,102b, as shown inFIG.3C-1. FR can also be generated globally and apply across all of receiver substrate200and therefore act on micro devices102a,102b,102c, as shown inFIG.3C-2here donor forces may selectively get weakened. The landing area on the receiver substrate may include a force modulation element to cause FR force modulation, fully or partially. Methods for selective and global generation of FR will be described below, including adhesive, mechanical and electrostatic and magnetic techniques. Additionally, examples of force modulation elements in landing area are described below. However, one of skill in the art knows that different variations of the force modulation elements that are not listed here are possible. Moreover, it should be understood that the shapes and structures of the contact pads and the force modulation elements are used for explanation and are not limited to the ones used in this description.

In one embodiment, donor force FD is selectively weakened for selected micro devices102a,102b, so that FD′ is less than FR, as shown inFIG.3D. This may be done, for example, using laser lift off techniques, lapping or wet/dry etching. In some cases, it may be desirable to use selective and global generation of FR simultaneously. For example, it may be infeasible to generate a selective FR of sufficient magnitude to overcome FD′ alone. In that case, the global component of FR should preferably remain small, ideally less than FD′, while the sum of the global and the selective components of FR is greater than FD′, but less than FD.

It should also be noted that activities performed during steps1002A-1006A can sometimes be interspersed with one another. For example, selective or global weakening of FD could take place before the substrates are brought together.

At1008A, donor substrate100and receiver substrate200are moved apart, leaving selected micro devices102a,102battached to corresponding contact pads202a,202b, as shown inFIG.3E. Once donor substrate100is separated from receiver substrate200, further processing steps can be taken. For example, donor substrate100and receiver substrate200can be re-aligned and steps1002A to1008A can be repeated in order to transfer a different set of micro devices102to a different set of contact pads202. Additional layers can also be deposited on top of or in between micro devices102, for example, during the manufacture of a LED display, transparent electrode layers, fillers, planarization layers and other optical layers can be deposited.

FIG.2Bshows method1000B; an alternative embodiment of method1000A.

At1002B, the force between micro devices102a,102band donor substrate100are modulated globally (for all devices in an area of donor substrate) or selectively (for selected micro devices102a,102bonly) so as to weaken donor force, FD.

At1004B donor substrate100and receiver substrate200are aligned so that selected micro devices102a,102bare in line with corresponding contact pads202a,202b.

At1006B, donor substrate100and receiver substrate200are moved together until the selected micro devices102a,102btouch contact pads202a,202b. It may not be strictly necessary that selected micro devices102a,102bactually touch corresponding contact pads202a,202b, but must be near enough so that the forces described below can be manipulated.

At1008B, if needed the forces between selected micro devices102and receiver substrate200(and contact pads202) are modulated so as to create a net force towards receiver substrate200for selected micro devices and a net force towards donor substrate100(or zero net force) for other micro devices102c.

At1010B, donor substrate100and receiver substrate200are moved apart, leaving selected micro devices102a,102battached to corresponding contact pads202a,202b.

At1012B, optional post processing is applied to selected micro devices102a,102b. Once donor substrate100is separated from receiver substrate200, further processing steps can be taken. Additional layers can be deposited on top of or in between micro devices102, for example, during the manufacture of a LED display, transparent electrode layers, fillers, planarization layers and other optical layers can be deposited. Step1012B is optional and may be applied at the conclusion of method1000A or1000C as well.

FIG.2Cshows method1000C; an alternative embodiment of method1000A.

At1002C, contact pads202a,202bcorresponding to selected micro devices102a,102bare treated to create extra force upon contact. For example, an adhesive layer may be applied, as described in greater detail below.

At1004C donor substrate100and receiver substrate200are aligned so that selected micro devices102a,102bare in line with corresponding contact pads202a,202b.

At1006C, donor substrate100and receiver substrate200are moved together until the selected micro devices102a,102btouch contact pads202a,202b.

At1008C, if needed the forces between selected micro devices102and donor substrate100are modulated so as to create a net force towards receiver substrate200for selected micro devices and a net force towards donor substrate100(or zero net force) for other micro devices102c.

At1010B, donor substrate100and receiver substrate200are moved apart, leaving selected micro devices102a,102battached to corresponding contact pads202a,202b.

Multiple Applications

Any of the methods1000A,1000B,1000C can be applied multiple times to the same receiver substrate200, using different or the same donor substrates100or the same donor substrate100using different receiver substrates200. For example, consider the case of assembling a display from LEDs. Each pixel may comprise red, green and blue LEDs in a cluster. However, manufacturing LEDs is more easily done in batches of a single colour and on substrates that are not always suitable for incorporation into a display. Accordingly, the LEDs must be removed from the donor100substrate, possibly where they are grown, and placed on a receiver substrate, which may be the backplane of a display, in RGB clusters. In case, the color This is simplest when the pitch of the array of pixels can be set to match the pitch of the array of LEDs on the donor substrate.

When this is not possible, the pitches of each array can be set proportionally.FIGS.4A and4Bshow arrangements where the pitch of the LEDs on the donor substrate is one seventh the pitch of the contact pads on the receiver substrate.

In general, however, matching the pitch of an array of pixels to the donor substrate is likely to be infeasible. For example, one generally tries to manufacture LEDs with the smallest possible pitch on the donor substrate to maximize yield, but the pitch of the pixels and the array of contact pads on the receiver substrate is designed based on desired product specifications such as size and resolution of a display. In this case, one may not be able to transfer all the LEDs in one step and repetition of any of the methods1000A,1000B,1000C will be necessary. Accordingly, it may be possible to design the donor substrate and the receiver substrate contact pad array so that a portion of each pixel can be populated during each repetition of any of methods1000A,1000B,1000C as shown inFIG.4CAt I, receiver substrate and donor substrate are not aligned. At II, all red LEDs are transferred. At III, all green LEDs are transferred. At IV, all blue LEDs are transferred. Repositioning of donor substrate and receivers substrate is required between each transfer step.

Those of skill in the art will now understand that that additional variations and combinations of methods1000A,1000B and1000C are also possible. Specific techniques and considerations are described below that will apply to any of methods1000, alone or in combination.

Use of Heat for Force Modulation

Selective and global heating can be used in multiple ways to assist in method1000A. For example, heat can be used in step1008A to weaken FD or after step1008A to create a permanent bond between micro devices102and contact pads202. In one embodiment, heat can be generated using resistive elements incorporated into donor substrate100and/or receiver substrate200.

FIG.5Ashows selective and global heating elements incorporated into substrates. Selective heating elements300and global heating element302may be incorporated into donor substrate100while selective heating elements304and global heating element306may be incorporated into receiver substrate200. In another embodiment, selective heating can be achieved using a patterned global heater, shown in plan view inFIG.5B.

FD can be weakened by applying heat to the interface between a micro device102and donor substrate100. Preferably, selective heating elements300are sufficient to heat the interface past a threshold temperature where micro devices102will detach. However, when this is not feasible, global heater302can be used to raise the temperature to a point below the threshold while selective heaters300raise the temperature further, only for selected micro devices102a,102babove the threshold. An environmental heat source, e.g. a hot room, can substitute for the global heater.

Heat can also be used to create a permanent bond between micro devices102and contact pads202. In this case, contact pads202should be constructed of a material that will cure when heated, creating a permanent bond. Preferably, selective heating elements304are sufficient to heat contact pads202past a threshold temperature to cause curing. However, when this is not feasible, global heater306can be used to raise the temperature to a point below the threshold for curing while selective heaters304raise the temperature for selected contact pads202a,202babove the threshold. An environmental heat source, e.g. a hot room, can substitute for the global heater. Pressure may also be applied to aid in permanent bonding.

Other variations are possible. In some cases, it may be feasible for micro devices102or contact pads202to themselves act as the resistive elements in selective heaters300,304. Heat can also be applied in a selective manner using lasers. In the case of lasers, it is likely that at least one of the donor substrate100and the receiver substrate200will have to be constructed of material that is at least semi-transparent to the laser being used. As shown inFIG.5C, in one case, shadow mask can be used to selectively block the laser from the non-selected devices. Here, the shadow mask501is aligned with the receiver substrate or donor substrate depending on direction of laser. Then laser can cover the either substrate partially or fully. In case of partial coverage, raster scan or step-and-repeat may be used to cover the entire intended area on the substrate. To further improve the heat transfer from the laser, a layer with higher laser absorption rate can be added to the force modulation element. It is possible to use the contact pad as the force modulation element in the receiver substrate.

Adhesive Force Modulation

In another embodiment of selective transfer, FR is generated by adhesive. Here, the FR is modulated either by selective application of adhesive to the landing area on the receiver substrate (or selected micro devices) or by selective curing of an adhesive layer. This method can be used in combination with weakening the donor force selectively or globally and is compatible with any of the methods1000A,1000B, and1000C or any combination of them. Although, the following description is based on1000A similar approaches can be used for1000B,1000C and the combination of the methods. In addition, the order of donor force weakening step1110can be changed in reference to other steps without affecting the results.

FIG.6Ashows a flowchart of method1100, a modified version of method1000specific to the use of adhesive to generate FR.FIG.6Bshows donor substrate100and receiver substrate200setup to perform method1100. Donor substrate100is shown in cross section and receiver substrate200is shown in cross section and plan view. Donor substrate100has an array of micro devices102attached. Donor force FD acts to hold micro devices102to donor substrate100.

Receiver substrate200has an array of contact pads212attached. AlthoughFIG.6Bshows the force modulation element500connected to the contact pads212, they can be physically separated.

As shown inFIG.6B, contact pads212a,212bare surrounded by a ring of adhesive500. Adhesive500has been applied selectively to contact pads212where transfer of a micro device is desired so that when donor substrate100and receiver substrate200are moved together, micro devices102a,102bwill make contact with adhesive500as well as contact pads212a,212b.

Method1100will be explained with reference toFIGS.6B-6F. At1102, adhesive is selectively applied as shown inFIG.6B.

At1104donor substrate100and receiver substrate200are aligned so that selected micro devices102a,102bare in line with corresponding selected contact pads212a,212b, as shown inFIG.6C.

At1106, donor substrate100and receiver substrate200are moved together until selected micro devices102a,102bare in contact with corresponding selected contact pads212a,212band adhesive500, as shown inFIG.6D.

At1108, receiver force, FR, is generated, as shown inFIG.6E. FR is generated by adhesion between micro devices102a,102b, adhesive500and at least one of contact pads212a,212band receiver substrate200. FR acts to hold selected micro devices102to corresponding selected contact pads212. Preferably, FR can be generated selectively by applying adhesive500selectively, as shown.

At1110, donor force FD is selectively (or globally) weakened for selected micro devices102a,102b, so that FD′ is less than FR, as shown inFIG.6F. The may be done, for example, using laser lift off techniques, lapping or wet/dry etching. In another case, donor force FD can be weakened for all the micro devices. In this case, force modulation is done by selective adhesive application to the selected force element on the receiver substrate. The order of FD and FR modulation can be changed. This step may be eliminated if the adhesive force modulation is selective and FR is larger than FD.

At1112, donor substrate100and receiver substrate200are moved apart, leaving selected micro devices102a,102battached to corresponding selected contact pads212a,212b, as shown inFIG.6G. Once donor substrate100is separated from receiver substrate200, further processing steps can be taken. For example, donor substrate100and receiver substrate200can be re-aligned and steps can be repeated in order to transfer a different set of micro devices102to contact pads212. Additional layers can also be deposited on top of or in between micro devices102, for example, during the manufacture of a LED display, a transparent electrode layers, fillers, planarization layers and other optical layers can be deposited.

One possible additional step, at1114, is curing adhesive500. Curing may create a permanent bond between micro devices102and contact pads212. In another embodiment, curing takes place as part of step1108and is part of generating FR. If several sets of selected micro devices102are to be transferred to a common receiver substrate200curing may be done after all the transfers are complete or after each set is transferred.

Adhesive500can be applied in many ways. For example, adhesive500can be applied to any or all of micro devices102, contact pads212or receiver substrate200. It will often be desirable that an electrical coupling exist between a micro device102and its corresponding contact pad202. In this case, the adhesive may be selected for its conductivity. However, suitable conductive adhesives are not always available. In any case, but especially when a conductive adhesive is not available, adhesives can be applied near contact pads or may cover only a portion of the contact pad.FIG.7Ashows some other possible arrangements of adhesive on receiver substrate200, (I) including four corners, (II) opposite sides, (III) center and (IV) one side geometries.

In another embodiment, one or more cut-outs can be provided for the adhesive500.FIG.7Bshows a contact pad212with a cut out (I) before and (II) after application of an adhesive.

The adhesive500can be stamped, printed or patterned onto the contact pads212, micro devices102or receiver substrate200by any normal lithography techniques. For example,FIG.8shows a stamping process that can be used to apply adhesive500to, for example, contact pads212. Selectivity in generating FR can be achieved by selecting which contact pads212will receive adhesive500. An analogous procedure can be used to apply adhesive to micro devices102or receiver substrate200. At (I), a stamp with a profile matching the desired distribution of adhesive500is wet. At (II), the stamp is brought into contact with the receiver substrate200and selected micro devices102. At (III), receiver substrate is now wet with adhesive and ready to receive transfer of selected micro devices102. Depending on the needs of the process, stamps with reverse profiles can also be used. In another embodiment, both the micro devices102and contact pads212may be wet with adhesive.

Adhesive500may be selected so that it will cure when heat is applied. Any of the techniques described with regard to heating can be suitably applied by one of skill in the art, according to the needs of a specific application.

Mechanical Force Modulation

In another embodiment of selective transfer, FR is generated by mechanical force. Here, the FR is modulated by application of mechanical forces between the landing area on the receiver substrate and the micro device. This method can be used in combination with weakening the donor force selectively or globally and is compatible with any of the methods1000A,1000B, and1000C or any combination of them. Although, the following description is based on1000A similar approaches can be used for1000B,1000C and the combination of the methods. In addition, the order of donor force weakening step1210can be changed in reference to other steps without affecting the results.

In one example, differential thermal expansion or pressure force can be used to achieve a friction fit that will hold micro devices102to contact pads202.

FIG.9shows a flowchart of method1200, a modified version of method1000A suitable for mechanical generation of FR.FIG.10shows a donor substrate100and a receiver substrate200setup to perform method1200. Donor substrate100is shown in cross section and receiver substrate200is shown in cross section and plan view. Donor substrate100has an array of micro devices102attached. Donor force FD acts to hold micro devices102to donor substrate100. Micro devices102and donor substrate100are shown as connected to ground244.

Receiver substrate200has an array of contact pads232attached. In the embodiment shown, the array of contact pads232is of the same pitch as the array of micro devices102; i.e., there is one micro device102for each contact pad232. As discussed above, this need not be true, although it is preferable that the pitch of the array of contact pads232and the pitch of the array of micro devices102be proportional as this facilitates the transfer of multiple devices simultaneously.

Method1200will be described with reference toFIGS.11A-11E. At1202the substrates are prepared for mechanical force modulation. In case of a mechanical grip, the grip is opened by different means. In one example heat is applied to force modulation element222which can be the same a contact pad on the landing area. Here, mechanical grip and contact pads are used interchangeably. However, it is obvious to one of skill in the art that the mechanical grip and contact pad can be different. It is possible to integrate the mechanical grip in the micro devices as well. The heat can be applied globally or selectively using heaters304causing the grip to open, as shown by the double arrows inFIG.11A. Note that contact pads222are constructed with a central depression224and peripheral walls226. It should also be noted that a combination of selective heaters304and global heater306or a combination of selective heaters304and an environmental heat source or external heat source in combination or alone could also be used.

At1204, donor substrate100and receiver substrate are aligned so that selected micro devices102a,102bare in line with corresponding contact pads222a,222b, as shown inFIG.11B.

At1206, donor substrate100and receiver substrate200are moved together until the selected micro devices102a,102bfit into the space defined by the peripheral walls of corresponding mechanical grip as shown inFIG.11B. As noted above, each contact pad222is constructed with a central depression224and peripheral walls226. These features of contact pads222are sized so as to fit snugly around a micro device102. The material of the mechanical grips is chosen, in part, due to thermal properties; specifically so that the mechanical grips have a higher coefficient of thermal expansion than micro devices102. Accordingly, when heat is applied to the mechanical grips they expand more than a micro device102would expand at the same temperature so that the central depression and peripheral walls will be able to accommodate a micro device102with a gap228. The expanded size of mechanical grip allows micro devices102to fit easily.

At1208, a receiver force, FR, is generated. FR is generated by selectively cooling contact pads222corresponding to selected micro devices102, causing peripheral walls226to contract around selected micro devices102, closing gap228and exerting a compressive force on micro device102, holding it in place, as shown inFIG.11C. Selectivity can be achieved by selectively turning off selective heaters304.

At1210, donor force FD is selectively (or globally) weakened for selected micro devices102a,102b, so that FD′ is less than FR, as shown inFIG.11D. This may be done, for example, using laser lift off techniques, lapping or wet/dry etching. In some embodiments FD is weaker than FR, in which case selective weakening of FD is not required. This step may be eliminated if the mechanical force modulation is selective and the FR is larger than FD.

At1212, donor substrate100and receiver substrate200are moved apart, leaving selected micro devices102a,102battached to corresponding contact pads222a,222b, as shown inFIG.11E. Once donor substrate100is separated from receiver substrate200, further processing steps can be taken. For example, donor substrate100and receiver substrate200can be re-aligned and steps can be repeated in order to transfer a different set of micro devices102and to contact pads222. Additional layers can also be deposited on top of or in between micro devices102, for example, during the manufacture of a LED display, transparent electrode layers, fillers, planarization layers and other optical layers can be deposited.

Electrostatic Force Modulation

In another embodiment of selective transfer, FR is generated by an electrostatic force or magnetic force. In case of magnetic force a current passes through a conductive layer instead of charging a conductive layer for electrostatic force. Although the structures here are used to describe the electrostatic force similar structures can be used for magnetic force. Here, the FR is modulated by application of selective electrostatic forces between the landing area on the receiver substrate and the micro device. This method can be used in combination with weakening the donor force selectively or globally and is compatible with any of the methods1000A,1000B, and1000C or any combination of them. Although, the following description is based on1000A similar approaches can be used for1000B,1000C and the combination of the methods. In addition, the order of donor force weakening step1410can be changed in reference to other steps without affecting the results.

In another embodiment of selective transfer, FR is generated by an electrostatic force. Here, the FR is modulated by application of selective electrostatic forces between the landing area on the receiver substrate and the micro device. This method can be used in combination with weakening the donor force selectively or globally and is compatible with any of the methods1000A,1000B, and1000C or any combination of them. Although, the following description is based on1000A similar approaches can be used for1000B,1000C and the combination of the methods. In addition, the order of donor force weakening step1410can be changed in reference to other steps without affecting the results.

FIG.12Ashows a flowchart of method1300, a modified version of method1000suitable for electrostatic generation of FR.FIG.12Bshows a donor substrate100and a receiver substrate200setup to perform method1300. Donor substrate100is shown in cross section and receiver substrate200is shown in cross section and in plan view. Donor substrate100has an array of micro devices102attached. Donor force FD acts to hold micro devices102to donor substrate100. Micro devices102and donor substrate100are shown as connected to ground244.

The landing area on the receiver substrate200has at least a contact pad232attached and a force modulation element234.

Contact pads232are surrounded by a ring of conductor/dielectric bi-layer composite, hereinafter called an electrostatic layer234. The shape and location of force modulation element234can be changed in the landing area and in relation to the contact pad. Electrostatic layer234has a dielectric portion236and a conductive portion238. Dielectric portion236comprises a material selected, in part, for its dielectric properties, including dielectric constant, dielectric leakage and breakdown voltage. The dielectric portion can also be part of the micro device or a combination of the receiver substrate and the micro device. Suitable materials may include SiN, SiON, SiO, HfO and various polymers. Conductive portion238is selected, in part, for its conductive properties. There are many suitable single metals, bi-layers and tri-layers that can be suitable including Ag, Au and Ti/Au. Each conductive portion238is coupled to a voltage source240, via a switch242. Note that although conductive portions238are shown as connected in parallel to a single voltage source240via simple switches242, this is to be understood as an illustrative example. Conductive portions238might be connected to one voltage source240in parallel. Different subsets of conductive portions238may be connected to different voltage sources. Simple switches242can be replaced with more complex arrangements. The desired functionality is the ability to selectively connect a voltage source240, having a potential different than that of the micro devices102, to selected conductive portions238when needed to cause an electrostatic attraction between the selected conductive portions238and corresponding selected micro devices102.

Method1300will be explained in conjunction withFIGS.13A-13E. At1302, donor substrate100and receiver substrate are aligned so that selected micro devices102a,102bare in line with corresponding contact pads232a,232b, as shown inFIG.13A.

At1304, donor substrate100and receiver substrate200are moved together until the micro devices102come into contact with contact pads232, as shown isFIG.13B.

At1306, a receiver force, FR, is generated, as shown inFIG.13C. FR is generated by closing switches242a,242bthat connect conductive portions238of electrostatic layers234to voltage source240creating charged conductive portions238at the potential of voltage source240. Selected micro devices102a,102b, being at a different potential, e.g. ground potential (or other relative potential), will be electrostatically attracted to conductive portions238. The electrostatic charge can be generated by different potential levels. For example, for a 300 nm dielectric, to get a proper grip on a micro device, a voltage difference between 20V to 50V may need to be applied to the electrostatic force element. However, this voltage can be modified depending on the device, gap size, and the dielectric constant.

At1308, donor force FD is selectively weakened for selected micro devices102a,102b, so that FD′ is less than FR, as shown inFIG.13D. This may be done, for example, using laser lift off techniques, lapping or wet/dry etching.

At1310, donor substrate100and receiver substrate200are moved apart, leaving selected micro devices102a,102battached to corresponding contact pads232a,232b, as shown inFIG.13E. Once donor substrate100is separated from receiver substrate200, further processing steps can be taken and the ground244may be removed. For example, donor substrate100and receiver substrate200can be re-aligned and steps can be repeated in order to transfer a different set of micro devices102and to contact pads232. Additional layers can also be deposited on top of or in between micro devices102, for example, during the manufacture of a LED display, transparent electrode layers, fillers, planarization layers and other optical layers can be deposited. It should be noted that FR will cease to operate if the connection to voltage source240is removed. Accordingly, further processing steps to create a permanent bond between micro devices102and contact pads232are desirable. Curing contact pads232, as described above, is a suitable further processing step that will create such a bond and enable further working or transporting receiver substrate200.

In other embodiments, electrostatic layer234can take on other configurations.FIG.14shows some alternative placements for electrostatic layer234. Possible alternative placements of electrostatic layer234relative to each contact pad232include: (A) four corners, (B) opposite sides, (C) center and (D) one side. Those of skill in the art will now be able to design a configuration suitable to particular applications.

In other embodiments, the geometry of contact pads232, electrostatic layer234and micro devices102can be changed to varying effect.FIG.15illustrates some possible alternative geometries.FIG.15Ashows an embodiment where electrostatic layer234extends above the top of contact pad232to form a hollow240and micro device102has a mesa242that will fit within hollow240.FIG.15Bshows an embodiment where electrostatic layer234extends above the top of contact pad232to form a hollow240and micro device102has an extension244attached to it that will fit within hollow240. Extension240may be made of the same material as contact pad232so that later curing will fuse extension244and contact pad232. Sloping geometries, as shown inFIG.15E, are also possible. Geometries with mesa242or extension244can help guide micro devices102into contact pads232and insure a proper fit and prevent tilting of micro devices102when detaching from donor substrate100. Preferably, the geometry of micro devices102and contact pads232are chosen to match so as to maximize the electrostatic force.

FIG.15Cshows an embodiment where electrostatic layer234forms a hollow240, but conductive portion238remains in the same plane as contact pad232.FIG.15Dshows an embodiment where electrostatic layer234forms a hollow240, but also overlaps with contact pad232and conductive portion238is in a different plane than contact pad232, allowing the fine tuning of the electrostatic force.

Transfer of Micro Devices of Different Heights

In another embodiment of selective transfer, the force on the donor substrate is modulated to push the device toward the receiver substrate. In one example, after removing the donor force other forces such as electrostatic forces can be used to push the device toward the receiver substrate. In another case, a sacrificial layer can be used to create a push force in presence of heat or light sources. To selectively create the push force, a shadow mask can be used for applying a light source (e.g. laser) to the selected micro devices. In addition, the FR can be generated by one of aforementioned methods (e.g., mechanical, heating, adhesive, electrostatic). For example, the FR can be modulated by application of selective electrostatic forces between landing area on the receiver substrate and the micro device. This method is compatible with any of the methods1000A,1000B, and1000C or any combination of them. Although, the following description is based on1000A, similar approaches can be used for1000B,1000C and the combination of the methods. In addition, the order of donor force modulation step1410can be changed in reference to other steps without affecting the results. However, the most reliable results can be achieved by applying the FR first and then applying the push force to the micro device.

FIG.16shows a flowchart of method1400based on electrostatic FR. However, other FR forces can be applied as well. Method1400is a modified version of method1300and is particularly suited to simultaneous transfer of micro devices102of different heights. At1402, donor substrate100and receiver substrate are aligned so that selected micro devices102a,102bare in line with corresponding contact pads232a,232b, as shown inFIG.17A. Note that micro device102ais of a different height than micro device102b.

At1404, donor substrate100and receiver substrate200are moved together until the micro devices102are close enough for electrostatic FR to act on micro devices102. Donor substrate100and receiver substrate200may be held so that no micro devices102make contact with contact pads232or, as shown inFIG.17B, substrates100,200may stop approaching when some micro devices102make contact with contact pads232.

At1406, a receiver force, FR, is generated, as shown inFIG.17C. FR is generated by closing switches242a,242bthat connect conductive portions238of electrostatic layers234to voltage source240creating charged conductive portions238at the potential of voltage source240. Selected micro devices102a,102b, being at a different potential, e.g. ground potential, will be electrostatically attracted to conductive portions238.

At1408, donor force FD is selectively weakened for selected micro devices102a,102b, so that FD′ is less than FR. This may be done, for example, using laser lift off techniques, lapping or wet/dry etching. At this point, micro devices102a,102bwill detach from donor substrate100. Micro device102bwill jump the gap to their corresponding contact pads232a,232bon receiver substrate200.

At1410, donor substrate100and receiver substrate200are moved apart, leaving selected micro devices102a,102battached to corresponding contact pads232a,232b, as shown inFIG.17E. Once donor substrate100is separated from receiver substrate200, further processing steps can be taken. For example, donor substrate100and receiver substrate200can be re-aligned and steps can be repeated in order to transfer a different set of micro devices102and to contact pads232. Additional layers can also be deposited on top of or in between micro devices102, for example, during the manufacture of a LED display, transparent electrode layers, fillers, planarization layers and other optical layers can be deposited. It should be noted that FR will cease to operate if the connection to voltage source240is removed. Accordingly, further processing steps to create a permanent bond between micro devices102and contact pads232are desirable. Curing contact pads232, as described above, is a suitable further processing step that will create such a bond and enable further working or transporting receiver substrate200.

One application of this method is development of displays based on micro-LED devices. An LED display consists of RGB (or other pixel patterning) pixels made of individual color LEDs (such as red, green or blue or any other color). The LEDs are manufactured separately and then transferred to a backplane. The backplane circuit actively or passively drives these LEDs. In the Active form each sub-pixel is driven by a transistor circuit by either controlling the current, the ON time, or both. In the Passive form, each sub-pixel can be addressed by selecting the respective row and column and is driven by an external driving force.

The LEDs conventionally are manufactured in the form of single color LEDs on a wafer and patterned to individual micro-devices by different process such as etching. As the pitch of the LEDs on their substrate is different from their pitch on a display, a method is required to selectively transfer them from their substrate to the backplane. The LEDs' pitch on their substrate is the minimum possible to increase the LED manufacturing yield on a wafer, while the LED pitch on the backplane is dictated by the display size and resolution. According to methods implemented here, one can modulate the force between the LED substrate and the micro-LEDs and uses any of the technique presented here to increase the force between selected LED and backplane substrate. In one case, the force for LED wafer is modulated first. In this case, the force between LED devices and substrate is reduced either by laser, backplane etching, or other methods. The process can selectively weaken the connection force between selected LEDs for transfer and the LED substrate or it can be applied to all the devices to reduce the connection force of all the LED devices to the LED substrate. In one embodiment, this is accomplished by transferring all LEDs from their native substrate to a temporary substrate. Here, the temporary substrate is attached to the LEDs from the top side, and then the first substrate is removed either by polishing and/or etching or laser lift off. The force between the temporary substrate and the LED devices is weaker than the force that the system substrate can selectively apply to the LEDs. To achieve that a buffer layer may be deposited on the temporary substrate first. This buffer layer can be a polyamide layer. If the buffer layer is not conductive, to enable testing the devices after transfer to the temporary and system substrate, an electrode before or after the buffer layer will be deposited and patterned. If the electrode is deposited before the buffer layer, the buffer layer maybe patterned to create an opening for contact.

In another method, the LED connection-force modulation happens after the LED substrate and the backplane substrate are in contact and the system substrate forces to LED are selectively modulated by the aforementioned methods presented here. The LED substrate force modulation can be done prior to the backplane substrate force modulation as well.

As the force holding the LEDs to the backplane substrate after transfer is temporary in most of the aforementioned methods, a post processing step may be needed to increase the connection reliability to the backplane substrate. In one embodiment, high temperature (and/or pressure can be used). Here, a flat surface is used to apply pressure to the LEDs while the temperature is increased. The pressure increases gradually to avoid cracking or dislocation of the LED devices. In addition, the selective force of the backplane substrate can stay active during this process to assist the bonding.

In one case, the two connections required for the LED are on the transfer side and the LED is in full contact with the backplane after the transfer process. In another case, a top electrode will be deposited and patterned if needed. In one case, a polarization layer can be used before depositing the electrode. For example a layer of polyamide can be coated on the backplane substrate. After the deposition, the layer can be patterned to create an opening for connecting the top electrode layer to system substrate contacts. The contacts can be separated for each LED or shared. In addition, optical enhancement layers can be deposited as well before or after top electrode deposition.

Testing Process

Identifying defective micro devices and also characterizing the micro devices after being transferred is an essential part of developing a high yield system since it can enable the use of repair and compensation techniques.

In one embodiment shown inFIG.18, the receiver substrate is put in test mode during a transfer process. If needed, the donor substrate may be biased for test mode. If the micro device is an optoelectronic device, a sensor1810(or sensor array) is used to extract the optical characteristics of the transferred devices. Here, the receiver substrate is biased so that only the selected device1802is activated through selected contact pads1804. Also, unselected devices1806stay deactivated and unselected pads1808stay inactive to prevent any interference. For connectivity testing, the micro device is biased to be active (for the LED case, it emits light). If a micro device is not active, the device can be flagged as defective. In another test, the micro device is biased to be inactive (for the LED case, it does not emit light). If a micro device is active, the device can be flagged as defective.FIG.19shows an example of a pixel biasing condition for activating or deactivating a micro device. Here, the micro device1906is coupled1908to a bias voltage1910(supply voltage) to become activated. For deactivating the micro device1906, it is disconnected from the voltages. Here, the donor substrate1900can be biased for enabling the test. In another case, the micro devices are tested during post processing. While a surface is used to apply pressure to the devices to create permanent bonding, the circuit is biased to activate the micro devices. The surface can be conductive so that it can act as another electrode of the micro devices (if needed). The pressure can be adjusted if a device is not active to improve any malfunction in the connection to the receiver substrate. Similar testing can be performed to test for open defective devices. For performance testing, the micro device is biased with different levels and its performance (for the LED case, its output light and color point) is measured.

In one case, the defective devices are replaced or fixed before applying any post processing to permanently bond the device into receiver substrate. Here, the defective devices can be removed before replacing it with a working device. In another embodiment, the landing area on the receiver substrate corresponding to the micro devices comprises at least a contact pad and at least a force modulation element.

It should be understood that various embodiments in accordance with and as variations of the above are contemplated.

In another embodiment, the net transfer forces are modulated by weakening the donor force using laser lift off. In another embodiment, the net transfer forces are modulated by weakening the donor force using selectively heating the area of the donor substrate near each of the selected micro devices. In another embodiment, the net transfer forces are modulated by selectively applying adhesive layer to the micro devices. In another embodiment, a molding device is used to apply the adhesive layer selectively. In another embodiment, printing is used to apply the adhesive layer selectively. In another embodiment, a post process is performed on the receiver substrate so that the contact pads permanently bond with the selected micro devices. In another embodiment, the post process comprises heating the receiver substrate. In another embodiment, the heating is done by passing a current through the contact pads. In another embodiment, the method is repeated using at least one additional set of selected micro devices and corresponding contact pads. In another embodiment, the contact pads are located inside an indentation in the receiver substrate and each selected micro device fits into one such indentation. In another embodiment, the pitch of the array of micro devices is the same as the pitch of the array of contact pads. In another embodiment, the pitch of the array of micro devices is proportional to the pitch of the array of contact pads. In another embodiment, each of the selected micro devices comprises a protrusion and the contact pads comprise a depression sized to match the protrusion on each micro device. In another embodiment, the net transfer forces are modulated by generating electrostatic attraction between the selected micro devices and the receiver substrate. In another embodiment, the electrostatic forces are applied to the entire array of micro devices on the donor substrate by a force element on the receiver substrate or behind the receiver substrate. In another embodiment, the electrostatic forces are generated selectively by the force modulation element of the landing area. In another embodiment, the force modulation element of the landing area on the receiver substrate comprises a conductive element near each contact pad, each conductive element capable of being linked to a voltage source in order to sustain an electrostatic charge. In another embodiment, each conductive element comprises one or more sub-elements. In another embodiment, the sub-elements are distributed around the contact pad. In another embodiment, each conductive element surrounds a contact pad. In another embodiment, the force modulation element of the landing area on the receiver substrate comprises a conductive layer and a dielectric layer throughout a substantial portion of the landing area, the conductive layer capable of being linked to a voltage source in order to sustain an electrostatic charge. In another embodiment, the donor substrate and the receiver substrate are brought close together, but the selected micro devices and the contact pads do not touch until after the net transfer forces are modulated whereupon the selected micro devices move across the small gap to the contact pads. In another embodiment, the height of the selected micro devices differ. In another embodiment, the contact pads are concave. In another embodiment, the force modulation element of the receiver substrate generates a mechanical clamping force. In another embodiment, the mechanical force modulation element forms part of at least one contact pad. In another embodiment, the mechanical force modulation elements are separate from the contact pad. In another embodiment, the mechanical force modulation is created by thermal expansion or compression of at least one of the force modulation element or micro device. In another embodiment, each contact pad has a concave portion and each selected micro device is inserted into a concave portion of a contact pad.

In another embodiment, the receiver substrate is heated before the donor substrate and the receiver substrate are moved together so that the concave portion of the contact pads expands to be larger than a selected micro device and the receiver substrate is cooled before the donor substrate and the receiver substrate are moved apart so that the concave portion of the contact pads contracts around the selected micro devices and provides the receiver force via mechanical clamping of the selected micro devices.

In another embodiment, the force modulation element in the landing area of the receiver substrate is an adhesive layer positioned between the selected micro devices and the receiver substrate. In another embodiment, the adhesive layer is conductive. In another embodiment, a portion of each of the contact pads on the receiver substrate is coated with an adhesive layer. In another embodiment, a portion of each of the selected micro devices is coated with an adhesive layer. In another embodiment, a portion of the area near the contact pads is coated with an adhesive layer.

In another embodiment, the net transfer force is modulated both on the donor substrate with at least one of the aforementioned methods and on the receiver substrate with at least one of the described methods.

While particular implementations and applications of the present disclosure have been illustrated and described, it is to be understood that the present disclosure is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of an invention as defined in the appended claims.