SELECTIVE TRANSFER OF MICRO DEVICES
What is disclosed is a method of selectively transferring micro devices from a donor substrate to contact pads on a receiver substrate. Micro devices being attached to a donor substrate with a donor force. The donor substrate and receiver substrate are aligned and brought together so that selected micro devices meet corresponding contact pads. A receiver force is generated to hold selected micro devices to the contact pads on the receiver substrate. The donor force is weakened and the substrates are moved apart leaving selected micro devices on the receiver substrate. Several methods of generating the receiver force are disclosed, including adhesive, mechanical and electrostatic techniques.
This application is a continuation-in-part of U.S. application Ser. No. 15/002,662 filed Jan. 21, 2016, which is hereby incorporated by reference in its entirety. This application claims priority to Canadian Application No. 2,921,737 filed Feb. 25, 2016, and Canadian Application No. 2,936,523, filed Jul. 19, 2016, each of which is hereby incorporated by reference herein in its entirety.
FIELD OF THE INVENTIONThe present disclosure relates to device integration into system or receiver substrates. More specifically, the present disclosure relates to selective transfer of micro devices from a donor substrate to a receiver substrate.
BRIEF SUMMARYAccording to one aspect there is provided, 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, the method comprising: 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 (in case contact pad does not pre-exist, other markers in the receiver substrate can be used for alignment); moving the donor substrate and the receiver substrate together until each of the selected micro devices are in contact or proximity with a respective contact pad on the receiver substrate; generating a receiver force that acts to hold the selected micro devices to their contact pads while not affecting other micro devices in contact with or proximity contact with the receiver substrate; and moving the donor substrate and the receiver substrate apart leaving the selected micro devices on the receiver substrate.
Some embodiments further comprise weakening the donor force bonding the micro devices to the donor substrate to assist micro device transfer.
In some embodiments, the donor force for the selected micro devices is weakened to improve selectivity in micro device transfer. In some embodiments, the receiver force is generated selectively to improve selectivity in micro device transfer. Some embodiments further comprise weakening the donor force using laser lift off. Some embodiments further comprise weakening the donor force by heating an area of the donor substrate. Some embodiments further comprise modulating the force by magnetic field. Some embodiments further comprise modulating the receiver force by heating the receiver substrate.
In some embodiments the heating is performed by passing a current through the contact pads. In some embodiments the receiver force is generated by mechanical grip. Some embodiments further comprise performing an operation on the receiver substrate so that the contact pads permanently bond with the selected micro devices.
In some embodiments the receiver force is generated by electrostatic attraction between the selected micro devices and the receiver substrate. In some embodiments the receiver force is generated by an adhesive layer positioned between the selected micro devices and the receiver substrate. Some embodiments further comprise removing the donor force; and applying a push force to selected micro devices to move the devices toward the receiver substrate.
In some embodiments the push force is created by a sacrificial layer deposited between the selected micro device and the donor substrate.
According to another aspect there is provided a receiver substrate structure comprising: an array of landing areas for holding micro devices from a donor substrate selectively, each landing area comprising: at least one contact pad for coupling or connecting a micro device to at least one circuit or a potential in the receiver substrate; and at least one force modulation element for creating a receiver force for holding a micro device on the receiver substrate. For clarity, the area where the micro device sits on the receiver substrate is called the landing area. The contact pad can pre-exist on the receiver substrate or be deposited after the micro device is transferred to the receiver substrate.
In some embodiments the force modulation element is an electrostatic structure. In some embodiments the force modulation element is a mechanical grip. In some embodiments, for each landing area, a same element acts as the force modulation element and the contact pad.
The foregoing and additional aspects and embodiments of the present disclosure will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next.
The foregoing and other advantages of the disclosure will become apparent upon reading the following detailed description and upon reference to the drawings.
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 DESCRIPTIONIn one aspect of the invention is transferring a micro device where the method comprising: positioning a donor substrate comprising the micro device proximal to a receiver substrate, wherein the micro device is affixed to the donor substrate by a donor force; and transferring the micro device from the donor substrate to the receiver substrate responsive to selectively reducing the donor force affixing the micro device to the donor substrate.
In one case, the donor force is reduced by physically shielding the micro device from the donor force.
Alternatively, the selectively reducing the donor force comprises changing the bias condition of donor force.
In another method, selectively reducing the donor force comprises selectively applying a form of light to the micro device using a shadow mask.
In another method, reducing the donor force comprises changing a distance between the micro device and a source of the donor force.
Another aspect of the invention is a method of transferring a micro device, the method comprising: positioning a donor substrate comprising the micro device proximal to a receiver substrate, wherein the receiver substrate comprises a force modulator element; and transferring the micro device from the donor substrate to the receiver substrate responsive to selectively reducing the distance between the micro device and the force modulator element.
In one case, reducing the distance between the micro device and the force modulator element comprises moving the micro device toward the force modulator element using a membrane.
In another case, reducing the distance is done mechanically moving the device closer to the receiver substrate.
Alternatively, a sacrificial layer can be used that changes volume under some triggers such as temperature, light, or voltage potential and so moving the micro device closer to the system substrate.
Another aspect of the invention is a method of transferring a micro device, the method comprising: positioning a donor substrate comprising the micro device proximal to a receiver substrate, wherein the receiver substrate comprises a force modulator element creating transfer force for transferring the selected micro devices; and reducing the effect of said force generated by the force modulator element on unwanted micro devices.
In one aspect of the invention, selectively reducing the effect of a force generated by the force modulator element comprises generating a reverse polarity of force surrounding the force modulating element.
Another aspect of the invention is a method of transferring micro devices, the method comprising: positioning a donor substrate comprising micro devices proximal to a receiver substrate, wherein the receiver substrate comprises a current curable bonding layer; and transferring the micro devices by applying current to the bonding layer of selected micro devices.
In one case, the current is applied using a circuit in the receiver substrate.
In one case, the circuit in the receiver substrate is shared with a circuit associated the driving or controlling the selected micro devices.
In another case, the bonding layer comprises one or more contact pads and selectively curing a portion of the bonding layer comprises curing a portion of the bonding layer between one or more contact pads.
In another aspect of the invention, a receiver substrate used to receive a micro device from a donor substrate, the receiver substrate comprising, an array of one or more pad structures, wherein each pad structure comprises a conductive layer and a dielectric layer
In one case, the dielectric layer can be modulated to be a conductive layer.
In another case, the dielectric layer is modulated to the conductive layer during operation to couple a circuit in the receiver substrate to the micro device.
In another case, the dielectric layer is modulated to the conductive layer using a doped layer.
In another alternative case, the dielectric layer is modulated to the conductive layer using a laser to induce dielectric breakdown.
In another alternative case, the dielectric layer creates electrostatic force that attracts the micro device from the donor substrate.
Another aspect of the invention is a method of transferring a micro device, the method comprising: positioning a donor substrate comprising a micro device proximal to a receiver substrate, wherein the receiver substrate comprises a pad structure having a dielectric layer and a conductive layer; transferring the micro device from the donor substrate to the pad structure; and coupling transferred micro devices and the conductive layer by removing the dielectric layer.
In one case, the dielectric layer is removed by the means of mechanical force.
In another case, the dielectric layer is removed by the means of thermal force.
Another aspect of the invention is a method of using a deformable layer on top of the receiver pads or landing area to adjust for different height in the micro devices.
In one aspect, the deformable layer is an either conductive layer, high resistive layer, or a dielectric layer.
The conformal layer can be used in combination with any of the transfer methods.
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:
The goal in selective transfer is to transfer some, selected micro devices 102, from donor substrate 100 to receiver substrate 200. For example, the transfer of micro devices 102a and 102b onto contact pads 206a and 206b without transferring micro device 102c will be described.
Transfer ProcessThe 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; (in case the contact pad does not pre-exist on the receiver substrate, the alignment can be also done using other marks; or device can be aligned to transfer force modulation element).
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 receiver substrate during steps b and c.
In some cases, the contact pad can be deposited after the device is transferred to the receiver substrate. 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.
At 1002A donor substrate 100 and receiver substrate 200 are aligned so that selected micro devices 102a, 102b are in line with corresponding contact pads 202a, 202b, as shown in
At 1004A, donor substrate 100 and receiver substrate 200 are moved together until the selected micro devices 102a, 102b are positioned within a defined distance of contact pads 202a, 202b, as shown in
At 1006A, forces between selected micro devices 102, donor substrate 100 and receiver substrate 200 (and contact pads 202) are modulated so as to create a net force towards receiver substrate 200 for selected micro devices and a net force towards donor substrate 100 (or zero net force) for other micro devices 102c.
Consider the forces acting one of the selected micro devices 102. There is a pre-existing force holding it to donor substrate 100, FD. There is also a force generated between micro device 102 and receiver substrate 200, FR, acting to pull or hold micro device 102 towards receiver substrate 200 and cause a transfer. For any given micro device 102, when the substrates are moved apart, if FR exceeds FD the micro device 102 will go with receiver substrate 200, while if FD exceeds FR the micro device 102 will stay with donor substrate 100. 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 transfer.
2. 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 transfer.
4. 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 devices 102a, 102b, as shown in
In one embodiment, donor force FD is selectively weakened for selected micro devices 102a, 102b, so that FD′ is less than FR, as shown in
It should also be noted that activities performed during steps 1002A-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.
At 1008A, donor substrate 100 and receiver substrate 200 are moved apart, leaving selected micro devices 102a, 102b attached to corresponding contact pads 202a, 202b, as shown in
At 1002B, the force between micro devices 102a, 102b and donor substrate 100 are modulated globally (for all devices in an area of donor substrate) or selectively (for selected micro devices 102a, 102b only) so as to weaken donor force, FD.
At 1004B donor substrate 100 and receiver substrate 200 are aligned so that selected micro devices 102a, 102b are in line with corresponding contact pads 202a, 202b.
At 1006B, donor substrate 100 and receiver substrate 200 are moved together until the selected micro devices 102a, 102b touch contact pads 202a, 202b. It may not be strictly necessary that selected micro devices 102a, 102b actually touch corresponding contact pads 202a, 202b, but must be near enough so that the forces described below can be manipulated.
At 1008B, if needed the forces between selected micro devices 102 and receiver substrate 200 (and contact pads 202) are modulated so as to create a net force towards receiver substrate 200 for selected micro devices and a net force towards donor substrate 100 (or zero net force) for other micro devices 102c.
At 1010B, donor substrate 100 and receiver substrate 200 are moved apart, leaving selected micro devices 102a, 102b attached to corresponding contact pads 202a, 202b.
At 1012B, optional post processing is applied to selected micro devices 102a, 102b. Once donor substrate 100 is separated from receiver substrate 200, further processing steps can be taken. Additional layers can be deposited on top of or in between micro devices 102, for example, during the manufacture of a LED display, transparent electrode layers, fillers, planarization layers and other optical layers can be deposited. Step 1012B is optional and may be applied at the conclusion of method 1000A or 1000C as well.
At 1002C, contact pads 202a, 202b corresponding to selected micro devices 102a, 102b are treated to create extra force upon contact. For example, an adhesive layer may be applied, as described in greater detail below.
At 1004C donor substrate 100 and receiver substrate 200 are aligned so that selected micro devices 102a, 102b are in line with corresponding contact pads 202a, 202b.
At 1006C, donor substrate 100 and receiver substrate 200 are moved together until the selected micro devices 102a, 102b touch contact pads 202a, 202b.
At 1008C, if needed the forces between selected micro devices 102 and donor substrate 100 are modulated so as to create a net force towards receiver substrate 200 for selected micro devices and a net force towards donor substrate 100 (or zero net force) for other micro devices 102c.
At 1010B, donor substrate 100 and receiver substrate 200 are moved apart, leaving selected micro devices 102a, 102b attached to corresponding contact pads 202a, 202b.
Multiple ApplicationsAny of the methods 1000A, 1000B, 1000C can be applied multiple times to the same receiver substrate 200, using different or the same donor substrates 100 or the same donor substrate 100 using different receiver substrates 200. 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 donor 100 substrate, 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.
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 methods 1000A, 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 methods 1000A, 1000B, 1000C as shown in
Those of skill in the art will now understand that that additional variations and combinations of methods 1000A, 1000B and 1000C are also possible. Specific techniques and considerations are described below that will apply to any of methods 1000, alone or in combination.
Use of Heat for Force ModulationSelective and global heating can be used in multiple ways to assist in method 1000A. For example, heat can be used in step 1008A to weaken FD or after step 1008A to create a permanent bond between micro devices 102 and contact pads 202. In one embodiment, heat can be generated using resistive elements incorporated into donor substrate 100 and/or receiver substrate 200.
FD can be weakened by applying heat to the interface between a micro device 102 and donor substrate 100. Preferably, selective heating elements 300 are sufficient to heat the interface past a threshold temperature where micro devices 102 will detach. However, when this is not feasible, global heater 302 can be used to raise the temperature to a point below the threshold while selective heaters 300 raise the temperature further, only for selected micro devices 102a, 102b above 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 devices 102 and contact pads 202. In this case, contact pads 202 should be constructed of a material that will cure when heated, creating a permanent bond. Preferably, selective heating elements 304 are sufficient to heat contact pads 202 past a threshold temperature to cause curing. However, when this is not feasible, global heater 306 can be used to raise the temperature to a point below the threshold for curing while selective heaters 304 raise the temperature for selected contact pads 202a, 202b above 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 devices 102 or contact pads 202 to themselves act as the resistive elements in selective heaters 300, 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 substrate 100 and the receiver substrate 200 will have to be constructed of material that is at least semi-transparent to the laser being used. As shown in
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 methods 1000A, 1000B, and 1000C or any combination of them. Although, the following description is based on 1000A similar approaches can be used for 1000B, 1000C and the combination of the methods. In addition, the order of donor force weakening step 1110 can be changed in reference to other steps without affecting the results.
Receiver substrate 200 has an array of contact pads 212 attached. Although
As shown in
Method 1100 will be explained with reference to
At 1104 donor substrate 100 and receiver substrate 200 are aligned so that selected micro devices 102a, 102b are in line with corresponding selected contact pads 212a, 212b, as shown in
At 1106, donor substrate 100 and receiver substrate 200 are moved together until selected micro devices 102a, 102b are in contact with corresponding selected contact pads 212a, 212b and adhesive 500, as shown in
At 1108, receiver force, FR, is generated, as shown in
At 1110, donor force FD is selectively (or globally) weakened for selected micro devices 102a, 102b, so that FD′ is less than FR, as shown in
At 1112, donor substrate 100 and receiver substrate 200 are moved apart, leaving selected micro devices 102a, 102b attached to corresponding selected contact pads 212a, 212b, as shown in
One possible additional step, at 1114, is curing adhesive 500. Curing may create a permanent bond between micro devices 102 and contact pads 212. In another embodiment, curing takes place as part of step 1108 and is part of generating FR. If several sets of selected micro devices 102 are to be transferred to a common receiver substrate 200 curing may be done after all the transfers are complete or after each set is transferred.
Adhesive 500 can be applied in many ways. For example, adhesive 500 can be applied to any or all of micro devices 102, contact pads 212 or receiver substrate 200. It will often be desirable that an electrical coupling exist between a micro device 102 and its corresponding contact pad 202. 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.
In another embodiment, one or more cut-outs can be provided for the adhesive 500.
The adhesive 500 can be stamped, printed or patterned onto the contact pads 212, micro devices 102 or receiver substrate 200 by any normal lithography techniques. For example,
Adhesive 500 may 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 ModulationIn 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 methods 1000A, 1000B, and 1000C or any combination of them. Although, the following description is based on 1000A similar approaches can be used for 1000B, 1000C and the combination of the methods. In addition, the order of donor force weakening step 1210 can 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 devices 102 to contact pads 202. In another example, thermal and pressure can be applied to create a bonding and this bonding can act as FR as well.
Receiver substrate 200 has an array of contact pads 232 attached. In the embodiment shown, the array of contact pads 232 is of the same pitch as the array of micro devices 102; i.e. there is one micro device 102 for each contact pad 232. As discussed above, this need not be true, although it is preferable that the pitch of the array of contact pads 232 and the pitch of the array of micro devices 102 be proportional as this facilitates the transfer of multiple devices simultaneously.
Method 1200 will be described with reference to
At 1204, donor substrate 100 and receiver substrate are aligned so that selected micro devices 102a, 102b are in line with corresponding contact pads 222a, 222b, as shown in
At 1206, donor substrate 100 and receiver substrate 200 are moved together until the selected micro devices 102a, 102b fit into the space defined by the peripheral walls of corresponding mechanical grip as shown in
At 1208, a receiver force, FR, is generated. FR is generated by selectively cooling contact pads 222 corresponding to selected micro devices 102, causing peripheral walls 226 to contract around selected micro devices 102, closing gap 228 and exerting a compressive force on micro device 102, holding it in place, as shown in
At 1210, donor force FD is selectively (or globally) weakened for selected micro devices 102a, 102b, so that FD′ is less than FR, as shown in
At 1212, donor substrate 100 and receiver substrate 200 are moved apart, leaving selected micro devices 102a, 102b attached to corresponding contact pads 222a, 222b, as shown in
In another embodiment of selective transfer, FR is generated by an electrostatic force or magnetic force. Although the structures here are used to describe the electrostatic force similar structures can be used for magnetic force. In embodiments where magnetic force is used instead of electrostatic force, a current pass through a conductive layer instead of charging a conductive layer for electrostatic force.
Using electrostatic force for selective transfer, 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 methods 1000A, 1000B, and 1000C or any combination of them. Although, the following description is based on 1000A similar approaches can be used for 1000B, 1000C and the combination of the methods. In addition, the order of donor force weakening step 1410 can be changed in reference to other steps without affecting the results.
The landing area on the receiver substrate 200 has at least a contact pad 232 attached and a force modulation element 234.
Contact pads 232 are surrounded by a ring of conductor/dielectric bi-layer composite, hereinafter called an electrostatic layer 234. The shape and location of force modulation element 234 can be changed in the landing area and in relation to the contact pad. Electrostatic layer 234 has a dielectric portion 236 and a conductive portion 238. Dielectric portion 236 comprises 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 portion 238 is selected, in part, for its conductive properties. There are many suitable single metals, bi-layers and tri-layers that can be suitable for use as a conductive portion 238 including Ag, Au and Ti/Au. Each conductive portion 238 is coupled to a voltage source 240, via a switch 242. Note that although conductive portions 238 are shown as connected in parallel to a single voltage source 240 via simple switches 242, this is to be understood as an illustrative example. Conductive portions 238 might be connected to one voltage source 240 in parallel. Different subsets of conductive portions 238 may be connected to different voltage sources. Simple switches 242 can be replaced with more complex arrangements. The desired functionality is the ability to selectively connect a voltage source 240, having a potential different than that of the micro devices 102, to selected conductive portions 238 when needed to cause an electrostatic attraction between the selected conductive portions 238 and corresponding selected micro devices 102.
Method 1300 will be explained in conjunction with
At 1304, donor substrate 100 and receiver substrate 200 are moved together until the micro devices 102 come into contact with contact pads 232, as shown is
At 1306, a receiver force, FR, is generated, as shown in
At 1308, donor force FD is selectively weakened for selected micro devices 102a, 102b, so that FD′ is less than FR, as shown in
At 1310, donor substrate 100 and receiver substrate 200 are moved apart, leaving selected micro devices 102a, 102b attached to corresponding contact pads 232a, 232b, as shown in
In other embodiments, electrostatic layer 234 can take on other configurations.
In other embodiments, the geometry of contact pads 232, electrostatic layer 234 and micro devices 102 can be changed to varying effect.
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 methods 1000A, 1000B, and 1000C or any combination of them. Although, the following description is based on 1000A, similar approaches can be used for 1000B, 1000C and the combination of the methods. In addition, the order of donor force modulation step 1410 can 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.
At 1404, donor substrate 100 and receiver substrate 200 are moved together until the micro devices 102 are close enough for electrostatic FR to act on micro devices 102. Donor substrate 100 and receiver substrate 200 may be held so that no micro devices 102 make contact with contact pads 232 or, as shown in
At 1406, a receiver force, FR, is generated, as shown in
At 1408, donor force FD is selectively weakened for selected micro devices 102a, 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 devices 102a, 102b will detach from donor substrate 100. Micro device 102b will jump the gap to their corresponding contact pads 232a, 232b on receiver substrate 200.
At 1410, donor substrate 100 and receiver substrate 200 are moved apart, leaving selected micro devices 102a, 102b attached to corresponding contact pads 232a, 232b, as shown in
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 processes 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 receiver 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 receiver 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 receiver 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 receiver 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 ProcessIdentifying 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 in
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 differs. 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 element is 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.
In one embodiment, the force on the donor substrate is modulated by selectively lifting off the micro devices. In one case, a shadow mask is used to block the laser from the unwanted devices. In one case as shown in
In another embodiment shown in
In another embodiment, the donor substrate 2204 is using either electrostatic or electromagnetic force 2202 to hold an array of devices 2206. After picking the array of micro devices 2206 from the original substrate, the force for holding the selected micro devices 2206-a on the donor substrate is reduced (or the force for the unselected device 2206-b is increased). As a result, the transfer force 2210 from receiver substrate 2208 acts more effectively on selected devices 2206-a. The selected micro devices 2206-a are moved into receiver substrate 2208 while the remaining devices 2206-b on the donor substrate 2204 can be used to populate the rest of receiver substrate 2208 or another receiver substrate. In case of using electrostatic force for holding the array of micro devices 2206, the force can be changed by either manipulating the voltage or by changing dielectric characteristic. In case of manipulating the voltage, the device 2206 may need to be biased. As a result, either the micro devices 2206 are biased after being in contact with the receiver substrate 2208 contact pads (it can be similar to the contact pads described in the landing area or a different pad) or there is a contact pads on the donor substrate that bias the micro devices.
In one embodiment, distance between selected micro devices 2510-a and receiver substrate is reduced compared to the distance between unselected micro devices 2510-b and receiver substrate. Here, the devices 2510 can move forward or backward 2506 by using proper structure 2502 in donor substrate. In one case shown in
In another embodiment demonstrated in
In one embodiment, the transfer force of receiver substrate is confined by using another adjacent force. For the example shown in
In another embodiment, the force of receiver substrate is confined by using different dielectric layer. As shown in
In one embodiment, the donor substrate or receiver substrate has sensing devices. As the micro devices are being integrated into the receiver substrate, the sensing device can test the functionality of each micro device. This information can be used to repair the faulty device if needed. Also, this information can be used to control the transfer faulty devices by increasing the donor force or reducing the transfer for such faulty devices. In case of emissive device, the sensing element is a photo-sensor that can detect the output of emissive micro devices. In this case, the micro device is biased during the transfer to emit (or be off). The output is measured by sensing element and so it is used to identify if the device is normal, always ON, or always OFF, or other stages of operation. If sensing device is located on the donor substrate, the testing can be done during the transferring the device to donor substrate as well. The sensing device can be part of donor or receiver substrate or extra element added to the substrate.
In the embodiments associated with
In all embodiment referred in these applications, more than one force modulation element can be used for each micro device. In case of electrostatic force for example, one can use two electrodes for different polarity. In one case, the bias voltage of electrodes can be DC in another case the bias voltage for the electrodes can be AC.
Referring to
In another embodiment shown in
In all the embodiment, planarization layer can be deposited between structure on receiver substrate and force modulation elements. This can improve the surface profile and so make the transfer easier.
Dual Function Pads for Selective Transfer.In another aspect of the present invention, a deformable bonding layer is positioned between the pads in receiver substrate and micro devices. The bonding layer can be deformed to accommodate different height in micro devices. In addition, the bonding layer can cover most of the landing area and, therefore, relax alignment accuracy between donor substrate and receiver substrate. In one embodiment a deformable bonding layer between receiver substrate and micro devices is used to accommodate the height difference in micro devices. In one aspect of the invention, the bonding layer is current curable.
In one embodiment of the invention, a transfer mechanism is used for holding devices from donor substrate to receiver substrate where the bonding layer is cured selectively by applying current to the bonding layer. Here, either the current is applied selectively or the bonding layer is applied selectively or both.
In one aspect of the invention, the current is applied by circuit in the receiver substrate. In another aspect of the invention, the circuit applying current is partially or fully shared with the circuit driving the micro devices in operation mode. where operation mode can be different for different type of micro devices. For example, for micro LED devices, operation mode is where the device generates lights based on driving circuit output. In another example, for micro sensor device, the operation mode is where device generate a signal (e.g. charge current, voltage, impedance, etc) based on a stimulating input signals.
In another aspect of the invention, the current is applied selectively by the donor substrate. Here the donor substrate can control the current of each individual micro devices or groups of micro devices.
In one aspect of the invention, the curing current flows through micro device and receiver substrate. In another aspect of the invention, the curing current flows between to contact in the receiver substrate.
In one embodiment shown in
Still referring to
Referring to
In another embodiment, the micro devices 3303 are transferred to the receiver substrate 3300 using other selective transfer techniques (e.g. substrate assisted transfers or pick-and-place techniques). In this case, the bonding layer 3305 can be also part of the transfer mechanism. Here, during transfer, the bonding layer 3305 can be dielectric and so that a voltage on pad 3301 can act as receiver electrostatic force. After transfer, the current flowing through the bonding layer can cure the bonding layer 3305 and turn it to a conductive layer. As shown in
Referring to
Alternatively, as shown in
In one embodiment shown in
In another embodiment, referring to
Referring to
Alternatively, referring to
The next step, block 4302, is preparation of micro devices for transfer process. This process may be a combination of several steps. In this step, bonding between the micro devices and native substrate may be weaken by any appropriate method. In addition, micro devices may be transferred to the temporary substrate.
The next step, block 4304, is the formation of dual function bonding or dielectric layer on top of the contact pad on the receiver substrate by various methods. Etching step may also be employed to form dielectric layer on top of the contact pad.
The next step, block 4306, is the transfer process in which micro devices are transferred from the donor substrate to the acceptor/receiver one with the electrostatic or other type of forces. Here, the dielectric (resistive) layer act as normal dielectric (or high resistive layer) and create electrostatic force (or thermal force) in combination with the contact pad.
After the micro device transfer step, referring to block 4308, several post processing steps such as cleaning, planarization, formation of additional layer, etc. may be employed on the receiver substrate.
After that the dielectric property is changed to couple the contact pads to the micro device at step 4310.
Further post processing steps 4312 such as deposition of electrode, light confinement, and other process can be performed.
Referring to
In an embodiment, attraction force between the landing pad and micro device is an electrostatic force. Referring to
Dielectric layer 4408 between the contact pad and micro device is insulator and prevents electrical biasing of the micro device during normal operation. Referring to process 4310, one can modulate the dielectric properties of this layer to make it conductive in order to electrically connect the micro device to the contact pad for operation biasing.
In one embodiment, one can partially dope the dielectric layer during process 4304 and modulate its dielectric properties to conductive in process 4310.
Referring to
In any of the above embodiment, a blocking layer can be used on micro devices or receiver substrate to prevent migration of unwanted materials to the micro devices or receiver substrate.
Referring to
Referring to
In another embodiment, referring to the process flow steps shown in
In another embodiment, prior to the micro device transfer, side of the micro device (e.g. either top or bottom side), which requires to be connected to the contact pad is doped first and then the micro device is transferred to the receiver substrate.
Referring to
Referring to
In an embodiment, another method to change dielectric layer to conductive layer is to expose this layer to the laser beam.
Laser beam exposure induces breakdown of the dielectric layer (i.e. insulator) and shorting between the contact pad and device electrode. One can also use laser as a heating source to melt the dielectric layer. In another embodiment, laser beam may be used to heat the metal contact and melt the contact allowing diffusion of metal ions into the dielectric layer or promoting reaction of metal with the dielectric layer. At least one contact, either micro device electrode or receiving substrate contact pad requires to be transparent allowing a penetration of laser into the dielectric layer.
Referring to
In an embodiment, mechanical or thermal force may be used to remove partially or fully the dielectric layer 4818 to provide electrical connection between the contact pad 4816 and micro device electrode 4802.
In an embodiment, soft material may be used to form a dielectric layer for landing pad. The soft dielectric layer can be made from organic or inorganic materials such as polymers and polyimide, etc. Here the term “soft materials” refers to any materials that can be easily modified or deformed by a mechanical or thermal force such as gels and polymers. Depending on the material specific characteristics, variety of methods such as thermal evaporation, spin coating, inkjet printing, stamping, spray coating, etc. can be used to form the dielectric layer.
In another embodiment, after the micro device transfer step, one can put thermal force on the dielectric layer, referring again to block 4908 in
Referring to process 4904, for formation of dual functional dielectric layer from soft material, different methods can be used to pattern the dielectric layer. In one case, dielectric layer is formed on the entire surface of the receiver substrate and then patterning steps are employed. In another case, referring to
In another embodiment, stamping technique can be used to pattern the dielectric layer.
Referring to
In another embodiment, referring to
In an embodiment, soft dielectric layer can be formed directly on the micro devices rather than receiver substrate. Here, micro devices may be on the growth substrate or on the carrier one. Dielectric layer is formed on the surface of the micro device, which requires to be attached to the contact pad. Methods such is immersion, spray coating or spin coating may be used to form this layer. The next step is the transfer of the device to the receiver substrate.
Following the transfer, disclosed methods are used to remove or modify the dielectric layer. In an embodiment, mechanical or thermal force can be used to partially remove the dielectric layer. Here, soft material, which includes metal nanoparticles, can be used as the dual functional dielectric layer. Metal nanoparticles such as gold or silver are first dispersed in the dielectric material or solvent. Referring to
After the micro device transfer process, referring to
In an embodiment, referring to
In an embodiment, referring to
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.
Claims
1. A method of transferring a micro device, the method comprising:
- positioning a donor substrate comprising the micro device proximal to a receiver substrate, wherein the micro device is affixed to the donor substrate by a donor force; and
- transferring the micro device from the donor substrate to the receiver substrate responsive to selectively reducing the donor force affixing the micro device to the donor substrate.
2. The method of claim 1, wherein selectively reducing the donor force comprises physically shielding the micro device from the donor force.
3. The method of claim 1, wherein selectively reducing the donor force comprises changing the bias condition of donor force.
4. The method of claim 1, wherein selectively reducing the donor force comprises selectively applying a form of light to the micro device using a shadow mask.
5. The method of claim 1, wherein selectively reducing the donor force comprises changing a distance between the micro device and a source of the donor force.
6. A method of transferring a micro device, the method comprising:
- positioning a donor substrate comprising the micro device proximal to a receiver substrate, wherein the receiver substrate comprises a force modulator element; and
- transferring the micro device from the donor substrate to the receiver substrate responsive to selectively reducing the distance between the micro device and the force modulator element.
7. The method of claim 6, wherein reducing the distance between the micro device and the force modulator element comprises moving the micro device toward the force modulator element using a membrane.
8. A method of transferring a micro device, the method comprising:
- positioning a donor substrate comprising the micro device proximal to a receiver substrate, wherein the receiver substrate comprises a force modulator element creating transfer force for transferring the selected micro devices; and
- reducing the effect of said force generated by the force modulator element on unwanted micro devices.
9. The method of claim 8, wherein selectively reducing the effect of a force generated by the force modulator element comprises generating a reverse polarity of force surrounding the force modulating element.
10. A method of transferring micro devices, the method comprising:
- positioning a donor substrate comprising micro devices proximal to a receiver substrate, wherein the receiver substrate comprises a current curable bonding layer; and
- transferring the micro devices by applying current to the bonding layer of selected micro devices.
11. The method of claim 10, wherein the current is applied using a circuit in the receiver substrate.
12. The method of claim 10, wherein the circuit in the receiver substrate is shared with a circuit associated the driving or controlling the selected micro devices.
13. The method of claim 10, wherein the bonding layer comprises one or more contact pads and selectively curing a portion of the bonding layer comprises curing a portion of the bonding layer between one or more contact pads.
14. A receiver substrate used to receive a micro device from a donor substrate, the receiver substrate comprising:
- an array of one or more pad structures, wherein each pad structure comprises a conductive layer and a dielectric layer.
15. The receiver substrate of claim 14, wherein the dielectric layer can be modulated to be a conductive layer.
16. The receiver substrate of claim 15, wherein the dielectric layer is modulated to the conductive layer during operation to couple a circuit in the receiver substrate to the micro device.
17. The receiver substrate of claim 15, wherein the dielectric layer is modulated to the conductive layer using a doped layer.
18. The receiver substrate of claim 15, wherein the dielectric layer is modulated to the conductive layer using a laser to induce dielectric breakdown.
19. The receiver substrate of claim 14, wherein the dielectric layer creates electrostatic force that attracts the micro device from the donor substrate.
20. A method of transferring a micro device, the method comprising:
- positioning a donor substrate comprising a micro device proximal to a receiver substrate, wherein the receiver substrate comprises a pad structure having a dielectric layer and a conductive layer;
- transferring the micro device from the donor substrate to the pad structure; and
- coupling the first micro device and the conductive layer by removing the dielectric layer.
21. The method of claim 20, wherein the dielectric layer is removed by the means of mechanical force.
22. The method of claim 20, wherein the dielectric layer is removed by the means of thermal force.
23. The method of using a deformable layer on top of the receiver pad or landing or the micro device to adjust for different height in the micro devices.
24. The method of claim 23, where the deformable layer consists of either conductive layer, high resistive layer, or a dielectric layer.
Type: Application
Filed: Feb 24, 2017
Publication Date: Jul 27, 2017
Inventor: Gholamreza Chaji (Waterloo)
Application Number: 15/442,293