Patent Description:
These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings.

The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures.

The present disclosure relates to a system and method for transferring large number of micro objects (e.g., particles, chiplets, mini/micro-LED dies) from a donor substrate to another substrate in parallel while maintaining high position registration of the individual micro objects. Prior art methods and systems are disclosed in <CIT> and <CIT>. The method and system allows selectively transferring of micro objects from a transfer substrate and selectively place the micro objects to the destination or target substrate. This method and system can be used for assembling microLED displays and similar devices.

MicroLED is emerging as next generation display technology because of its potential to be a thinner, brighter, lighter, and low power display. MicroLED displays are made with arrays of microscopic LEDs each forming a pixel. Both OLED displays and microLED displays offer greatly reduced energy requirements compared to conventional LCD systems. Unlike OLED, microLED is based on conventional GaN LED technology, which offers higher total brightness than OLED produces, as well as higher efficiency in terms of light emitted per unit of power. It also does not suffer from the shorter lifetimes of OLED.

A single <NUM> television utilizing microLED has ~<NUM> million small LED subpixels that then need to be assembled. Mass transfer of chiplets is one technology that may be used for microLED manufacturing. The display assembly process which relies on mass transfer technology is one of the bottlenecks in the micro LED manufacturing process. Display typically requires a very high pixel yield which is higher than a single epitaxy wafer can achieved. Thus, assembly techniques should provide a way to avoid transferring non-functional LEDs to the display substrates. For example, defective micro LEDs on wafers and donor carriers could be eliminated and replaced with good ones during the mass transfer process in an efficient way, leading to overall yield improvement with low manufacturing cost.

In this disclosure, a mass-transfer method and system are described that can exploit such kind of selectively transfer head to assemble microLED displays of high pixel yield in an efficient way. The system and method supports known good die (KGD) chip transfer and parallel pixel repairing during the mass transfer process.

Being able to selectively transfer chiplets in an arbitrary pattern on demand is useful to facilitate the effective transfer process, pixel repair, hole/vacancy refill for microLED display manufacturing, which will lead to high process yield. An elastomer stamp has been used to deterministically transfer microscale LED chips for this type of application. However, an elastomer stamp has fixed pattern and cannot transfer arbitrary pattern of chiplets. Inevitably, some subset of the chiplets will be defective, and therefore it becomes difficult to replace a select few of them using such a stamp.

In <FIG>, block diagrams show an example of an assembly process that can be achieved using devices, systems, and methods according to an example embodiment. In <FIG>, a donor wafer/substrate <NUM> is shown that includes an array of chiplets <NUM> that may have been grown or placed on the substrate <NUM>. The shaded chiplets in the array <NUM> have been identified as defective, and when the chiplets are transferred to a target substrate <NUM>, only a subset 101a of the chiplet array are transferred, namely the good chiplets that are not shaded. This may be achieved with a transfer substrate <NUM> as shown in <FIG> that can selectively pick up just the subset 101a from the donor substrate <NUM> once they are identified. As shown in <FIG>, the transfer substrate <NUM> subsequently picks up a second set of chiplets <NUM> (e.g., from a different donor substrate). The locations of the chiplets within the set <NUM> correspond to the locations of the defective chiplets on the first donor substrate <NUM>. The transfer substrate <NUM> moves this set <NUM> to the target substrate <NUM>, resulting in a full set <NUM> of operational chiplets being located on the target substrate <NUM>.

The transfer substrate with a set of transfer elements (e.g., transfer pixels) can selectively hold a subset of micro objects. Thus, even when all of the transfer elements are in contact with an array of micro objects that is greater than the subset, only the subset will be adhere and be transferred, and the objects outside the subset will be left behind or otherwise unaffected. Similarly, the transfer substrate may be able to selectively release a subset of micro objects that are currently attached to the substrate, such that only the subset is transferred to a target even all of the transfer elements are currently holding a micro object. This process is repeatable and reversible, such that no permanent bonding is need to affect the selective holding or releasing of the objects.

In <FIG>, a side view illustrates details of an apparatus <NUM> according to an example embodiment. The apparatus includes a transfer substrate <NUM> with two or more transfer elements <NUM>. The transfer elements <NUM> can selectively made to change stiffness, which can be expressed as the Young's modulus of the material from which the elements are made. The Young's modulus is a measure of stress (force per unit area) divided by strain (proportional deformation) in a material in the linear elasticity regime. Generally, materials with higher Young's modulus (lower strain for a stress σ) is stiffer than a material with lower Young's modules (higher strain for the same σ). Other measures may also be used to represent stiffness of a material, such as storage modulus, which also accounts for dynamic performance of the material. Some measures may be used to represent stiffness of a part, such as a spring constant, that may be functionally equivalent in defining performance of the part. However the stiffness is defined, the transfer elements <NUM> have a change in stiffness in response to temperature that can be utilized in device transfer as described below.

Each of the transfer elements <NUM> includes an adhesion element <NUM> having a higher Young's modulus > <NUM> MPa at a lower temperature and a lower Young's modulus < <NUM> MPa at a higher temperature. Each of the transfer elements <NUM> also includes a thermal element <NUM> operable to change a temperature of the adhesion element <NUM> in response to an input, e.g., via inputs <NUM>. A controller <NUM> is coupled to provide the inputs <NUM> to the thermal elements <NUM>, thereby causing a subset of the transfer elements <NUM> to selectably pick up and hold objects <NUM> to and (optionally) release the objects <NUM> from the transfer substrate <NUM>. In particular, the objects <NUM> will not stick to the transfer elements <NUM> at the lower temperature but will stick at the higher temperature. To increase the reliability of the adhesion, the transfer elements may be cooled before attempting to pull the objects <NUM> away from transfer substrate <NUM>. Note that the change in temperature may affect other properties of the adhesion element <NUM> that can assist in selective adhesion and release of objects <NUM>, such as tackiness, stickiness, porosity, fluid content, density, etc..

The apparatus <NUM> may be part of a micro-transfer system <NUM>, which is a system used to transfer micro-objects (e.g., <NUM> to <NUM>) from the transfer substrate <NUM> to a target substrate <NUM>. The adhesion element <NUM> may be formed of a multi-polymer that contains stearyl acrylate-based (SA). In such a case, a difference between the higher and lower temperatures may be less than <NUM> (or in other cases less than <NUM>) in order to adjust the tackiness of the adhesion element <NUM> such that there is a marked difference in surface adhesion and Young's modulus, e.g., from < 1MPa at the higher temperature to > 6MPa at the higher temperature. The controller <NUM> in such as a system may be coupled to actuators that induce relative motion between substrates to facilitate object transfer as described herein.

The thermal element <NUM> may include one or both of a heating element and a cooling element. The inputs <NUM> may include electrical signals and/or laser light. The inputs <NUM> may be configured (e.g., using a matrix circuit) such that there are fewer lines going to the controller <NUM> than the total number of transfer elements <NUM>. The transfer elements <NUM> may further include a thermal insulator <NUM> between the adhesion element <NUM> and the transfer substrate <NUM>. The insulator <NUM> helps prevent heat transfer to the substrate <NUM>, thereby decreasing the amount of energy needed to affect temperature change at the adhesion element <NUM> and decrease response time.

Generally, the transfer elements <NUM> form an intermediate transfer surface whose compliance can be modulated (e.g., have a sharp rigid-to-soft transition) as a function of temperature. Such a surface can be used to pick up and release groups of micro-objects in a controlled and selectable manner. According to the invention, each transfer element <NUM> has lateral dimensions W from one or more micrometers to several hundreds of micrometers to pick up micro objects of like dimensions. Each transfer element <NUM> may have a total thickness T from less than one micron to several hundred microns. The pitch of the transfer array may vary from several microns to several millimeters. In some embodiment, the thermal elements <NUM> and insulating layers <NUM> are continuous layers that are not physically isolated from one another. As such, the transfer element "pixel" is the region where the heating/cooling elements can be individually addressed and controlled. The substrate <NUM> material may include but is not limited to glass, quartz, silicon, polymer and silicon carbide (SiC). The substrate <NUM> may have a thickness ranges from several tens of microns to several millimeters and lateral dimensions from several millimeters to one meter.

As noted above, the transfer substrate <NUM> can be part of an automated system used to build devices from micro-components. For example, microLEDs can be assembled into displays using the transfer substrate <NUM>. The controller <NUM> (which may include multiple processors and apparatuses) can also be used to control other devices to perform this automated assembly. By way of example, a robotic arm <NUM> and conveyer <NUM> are shown that can perform operations on wafers <NUM>, such as flipping, bonding, selective removal and addition of micro objects, etc. The configuration and operation of these and other automated devices for purposes of micro device assembly is well-established in the art.

There are many possible variations on the structure of the transfer substrate <NUM> shown in in <FIG>. Also, there are different materials that may be used for the transfer elements. A more complete description of these alternate embodiments are shown and described in commonly-owned <CIT>, the content of which is hereby incorporated by reference. Many of these embodiments may be applicable to the production of a micro-LED device as described in greater detail below.

In <FIG>, a diagram illustrates high-level steps of a micro-LED device (e.g., display) manufacturing process according to an example embodiment. The first step <NUM> involves transferring (all or a subset, e.g. KGD) of the microLED chips from the epitaxy wafer where the chips were fabricated to a transition substrate. The transition substrate has an adhesive surface to temporarily hold the chips.

High-yield coupons are created <NUM> by selectively transferring a subset of chips from the transition substrate. This may involve repairing the coupon pixels through at least one of the two following activities: (<NUM>) selectively remove a subset of chips from the coupon substrate; (<NUM>) fill the vacancies with chips from a transition substrate or another coupon substrate. Step <NUM> may be repeated until the pixel yield of the coupon achieves some requirement, e.g., a threshold of <NUM>% known good pixels. The chips are then transferred <NUM> from the coupon substrate to a final substrate, where electrical interconnects are made to the backplane.

In <FIG>, a diagram shows more detailed processes involved in step <NUM> shown in <FIG>. Note that some of these processes may be optional, and the full listing of process steps is provided for purposes of illustration and not limitation. An epitaxy wafer <NUM> is formed <NUM>, which includes a plurality of microLED chips <NUM>. As indicated by the shaded rectangles on the wafer <NUM>, some of the chips <NUM> may be determined defective, e.g., via visual inspection (e.g., microscope), electroluminescence and/or photoluminescence measurement. All or a subset of the micro-LED chips <NUM> are transferred <NUM> from the epitaxy wafer <NUM> (where they are fabricated) to a carrier substrate <NUM> (also referred to herein as a carrier wafer) which has an adhesive coating <NUM>. In one embodiment, this transferring involves flipping the epitaxy wafer <NUM> and pressing to the adhesive coating <NUM> of the carrier substrate <NUM>. The epitaxy wafer <NUM> is then released (shown in step <NUM>) using a laser, e.g., by shining laser light through the epitaxy wafer <NUM> and/or the carrier substrate <NUM>.

The adhesive coating <NUM> includes but not limited to ultraviolet (UV) releasable adhesive and thermal releasable adhesive. In these cases, the adhesive loses adhesion when treated with heating or UV illumination. As shown in <NUM>, the carrier substrate <NUM> is flipped and placed in contact with a coupon substrate <NUM> which is a substrate with a mild adhesion force that holds the chips <NUM> to facilitate a repairing process. The coupon substrate <NUM> (which may include glass, silicon, quartz, etc.) is coated with at least a soft adhesion layer <NUM> of material such as PDMS and/or silicone gel. The soft adhesion layer <NUM> may also be referred to as a mild adhesion layer and/or a weak adhesion layer. As seen in <NUM>, the carrier substrate <NUM> has been thermally released.

Generally, the soft adhesion layer <NUM> has sufficient adhesion to hold the chips <NUM> in place but allows for selective removal of the chips using a transfer substrate <NUM> such as shown in the apparatus <NUM> of <FIG>. For example, a material with tensile strength between <NUM>-<NUM> MPa may be used for this removal and other soft adhesion layers described herein. The ultimate tensile strength selected for the soft adhesion material <NUM> may be dependent on the tensile strength of the transfer elements in their holding and release states. Generally, an adhesion force exerted by the soft adhesion layers <NUM> is less that the holding force of the transfer elements (when signals are applied to cause the transfer element to be in a holding state) and release force of the transfer elements (when signals are applied to cause the transfer element to be in a release state). For example, if the release/hold Young's moduli of the transfer element ranges from <NUM> MPa to <NUM> MPa, the Young's modulus of the soft adhesion layer <NUM> may be selected to be around <NUM> MPa.

In some embodiments, the operations shown in <FIG> may further comprise a process to adjust the pitches of chips <NUM> in the array, as seen in step <NUM>. This may involve removing a subset of the chips from the coupon substrate <NUM> such that the remaining chips satisfy the new pitch specification. Alternatively, as shown in <FIG>, the subset of chips <NUM> can be transferred to a second coupon substrate <NUM>. The second coupon substrate <NUM> has a material layer <NUM> with similar properties as material layer <NUM>. The transfer process, e.g., removing a subset of chips <NUM> from one coupon substrate <NUM> to a second coupon substrate <NUM> can be accomplished using a transfer substrate <NUM> such as shown in the apparatus <NUM> of <FIG>.

Note that in <FIG>, a failed/defective microLED <NUM> is shown transferred to the second coupon substrate <NUM>. In other embodiments, the transfer from the first coupon substrate <NUM> to the second coupon substrate <NUM> may exclude any defective microLEDS. This may be accompanied by a change in the pitch of the transferred microLEDs <NUM> or not. Even if defective microLEDs are excluded from the transfer to the second coupon substrate <NUM>, this does not preclude further removal and adding of microLEDS from/to the second coupon substrate <NUM> in later operations, as described below.

In <FIG>, a diagram shows more detailed processes involved in step <NUM> shown in <FIG>. This process is shown using the first coupon substrate <NUM>, although could be performed on second coupon substrate <NUM>. Generally as indicated by step <NUM>, a subset of defective chips is identified (e.g., shaded chips <NUM> shown in <FIG>) and lifted off the coupon substrate <NUM> using a transfer substrate <NUM>. This technique may also be used to remove chips that are mis-positioned or were not removed in the pitch-adjustment process shown in <FIG>.

The removal of a subset of chips <NUM> from the coupon substrate <NUM> may create vacancies, e.g., where properly placed but defective chips were place. These vacancies are filled with new chips <NUM> to form high yield coupons, as indicated in step <NUM>. This may involve using a transfer substrate <NUM> to selectively remove chips <NUM> from another coupon or similar donor substrate (e.g., substrate of glass, silicon, quartz or the like that is coated with PDMS, silicone gel or the like). The pattern of the removed <NUM> chips (which is set by applying appropriate activation signals to the transfer elements <NUM> of transfer substrate <NUM>) may correspond to the pattern of the vacancies on the coupon substrate <NUM>. The removed chips <NUM> are then placed on the coupon substrate <NUM> by the transfer substrate <NUM>, where they are then release by selective application (or removal) of signals to the corresponding transfer elements <NUM>.

In some embodiments, if more chips <NUM> are accidently or intentionally removed from the donor substrate by the transfer substrate <NUM> than are needed for the coupon substrate <NUM>, only those chips <NUM> needed to fill the vacancies can be selectively released, while other chips remain attached to the transfer substrate <NUM>. This can be achieved by the selective application/removal of signals to those transfer elements <NUM> such that chips that should not be transferred are held onto the transfer substrate <NUM>, while chips that should be transferred will released by sending different signals to the affected transfer elements <NUM>. After this occurs, the new chips <NUM> may be tested on the coupon substrate <NUM> together with the old chips <NUM> to ensure the coupon yield is sufficiently high.

In <FIG>, a diagram shows more detailed processes involved in step <NUM> shown in <FIG>. As seen in step <NUM>, the high-yield pixel coupon substrate <NUM> from <FIG> (which may alternatively be high-yield coupon substrate <NUM>) is flipped and the microLED chips <NUM> are then aligned with and pressed against a backplane substrate <NUM>, which is part of a display assembly. The backplane substrate <NUM> includes surface electrodes <NUM> that interface with corresponding electrodes on the microLED chips <NUM>. The surface electrodes <NUM> are coupled to other components (e.g., electrical traces, passive or active electrical components such as resisters, capacitors, thin-film transistors, etc.). These other components (not shown) may be on the same surface as the electrodes <NUM> and/or on another surface (e.g., opposing surface) and/or internally embedded within layers of the backplane substrate <NUM>.

In one embodiment, the process further involves a permanent bonding process (which may be performed as part of step <NUM>) where the microLED chips <NUM> are bonded to the electrodes <NUM> on the backplane <NUM> using soldering, indium, low temperature metaling alloys, etc. During this bonding, the chips may be sandwiched between the coupon substrate <NUM> and the backplane <NUM>. In some embodiments, after the bonds are formed, the coupon substrate <NUM> can be removed (e.g., peeled off), leaving the microLED chips seated on and electrically connected to the backplane as shown in step <NUM>. In some embodiments as shown in step <NUM>, a front cover <NUM> can put on top of the stack of the backplane <NUM> and microLED chips <NUM>.

In <FIG>, a flowchart shows a method according to an example embodiment. The method involves transferring <NUM> microLED chips from an epitaxy wafer to a first coupon substrate. The first coupon substrate has a first, soft adhesive layer that temporarily holds the microLED chips. Using a first transfer substrate, a subset of the microLED chips are transferred <NUM> from the first coupon substrate to a second coupon substrate having a second, soft adhesive layer that temporarily holds the subset of microLED chips. A pattern of microLED chips are transferred <NUM> from another substrate to the second coupon substrate via a second transfer substrate to fill vacancies in the subset of microLED chips. Note that the second transfer substrate may be the same as the first transfer substrate. The vacancies may be due to a failed transfer from the first to second coupons, e.g., one or more microLEDs did not get picked up by or would not release from the first transfer substrate. The vacancies may also be due to failed microLEDs that were first selectively removed from the second coupon substrate. The first and second transfer substrates are operable to hold and release pluralities of micro objects, and may be the same transfer substrate.

In <FIG>, a flowchart shows a method of transferring <NUM> the microLED chips from the carrier substrate to the first coupon substrate according to an example embodiment. The method involves coating <NUM> an adhesive on a carrier substrate. The epitaxy wafer is moved <NUM> (e.g., flipped) to contact the carrier-substrate such that the microLED chips are bonded to the adhesive. A substrate of the epitaxy layer is thermally or optically released <NUM> from the microLED chips, e.g., by shining laser light through one of the carrier substrate and epitaxy wafer. The carrier substrate is moved <NUM> to contact the first coupon substrate such that the microLED chips adhere to the first, soft adhesive layer. The microLED chips are transferred <NUM> from the carrier substrate to the first coupon substrate. The transfer <NUM> may occur by application of ultraviolet light, heat, or mechanical force (e.g., shock and/or vibration).

Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term "about. " Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. <NUM> to <NUM> includes <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>) and any range within that range.

The various embodiments described above may be implemented using circuitry, firmware, and/or software modules that interact to provide particular results. One of skill in the arts can readily implement such described functionality, either at a modular level or as a whole, using knowledge generally known in the art. For example, the flowcharts and control diagrams illustrated herein may be used to create computer-readable instructions/code for execution by a processor. Such instructions may be stored on a non-transitory computer-readable medium and transferred to the processor for execution as is known in the art. The structures and procedures shown above are only a representative example of embodiments that can be used to provide the functions described hereinabove.

Claim 1:
A method comprising:
transferring microLED chips (<NUM>) from an epitaxy wafer (<NUM>) to a first coupon substrate (<NUM>), the first coupon substrate having a first, soft adhesive layer that temporarily holds the microLED chips;
transferring via one or more transfer substrates, a subset (101a) of the microLED chips from the first coupon substrate to a second coupon substrate (<NUM>) having a second, soft adhesive layer that temporarily holds the subset of microLED chips; and
transferring a pattern of microLED chips from another substrate to the second coupon substrate via the one or more transfer substrates to fill vacancies in the subset of microLED chips,
wherein the first and second transfer substrates have a plurality of transfer elements (<NUM>),
each transfer element having lateral dimensions (W) from one or more micrometers to several hundreds of micrometers to pick up micro-objects of like dimensions, wherein the plurality of transfer elements are individually actuated to simultaneously hold and release pluralities of the microLED chips.