Micro light emitting diode

Embodiments generally relate to micro-device arrays. In some embodiments, an array comprises a substrate and a plurality of micro-devices. Each micro-device is suspended over a cavity in the substrate by at least one lateral hinge attached to a side post formed into the substrate. Each micro-device comprises a bonding layer; a metal contact; semiconductor device layers; and a buffer layer. The semiconductor device layers may comprise GaN-based LED layers; wherein the buffer layer comprises AlGaN; and wherein the substrate comprises (111) oriented Silicon. In other cases, the semiconductor device layers may comprise InGaAsP-based LED layers; wherein the buffer layer comprises InGaP; and wherein the substrate comprises GaAs.

FIELD OF INVENTION

This invention relates in general to the field of micro-devices, such as micro-LEDs, including arrays of such devices and methods of fabrication thereof.

BACKGROUND

Micro-LEDs (u-LEDs) are very small LEDs, in the range 1 um to 300 um, fabricated from epitaxially grown wafers. Typically, the materials needed for generating lights require dedicated equipment such as MOCVD or MBE. The materials are GaN, AlGaN, InGaN, GaAs, AlGaAs, InGaP, InGaAsP, etc. The substrates are typically Silicon, SiC, Sapphire, GaN, AlN, GaAs, InP, etc. After epitaxial growth, the wafers are processed using semiconductor fabrication process similar to, but not limited to, the CMOS processes. Using a series of lithographic processes, depositions and etching processes, LED devices are formed to facilitate efficient current injection into the active gain region to generate lights and efficient optical extraction of light from semiconductors. The fabrication techniques for making LED are well known art in the field. Typically, after the wafer process, the LED wafers are separated by dicing or breaking process into individual chips. The individual chips are assembled into other carriers designed for various applications.

In many applications such as displays, two dimensional arrays of LEDs are needed to be assembled. Often, the distances between LEDs are much larger than the size of the LEDs. For example, the size of a large display can be as large as 10-meter wide and the number of LEDs can be greater than 10 million. It would be too costly to use LEDs with conventional sizes in the order of 1 mm. For example, for typical high-resolution (HD) LED displays (1920×1080), there are ˜2 millions LEDs for each color. If each LED costs $0.05 (the material cost of 1 mm×1 mm LEDs), the cost of all LEDs for this HD display would be prohibitively $300,000, not including other costs. To reduce the cost of LEDs, the most effective way is to reduce its size. In order to make such high-definition LED displays into the affordable range, say $1,000, the cost of each LED must be reduced by 300 times or its area must be reduced by 300 times. This means the size of LED should be in the range of 50 um×50 um. Further cost reduction will require even smaller LEDs.

The challenge of making 50 um×50 um LEDs is not the lithography itself, but how to transfer the devices after lithography, from the native substrate in or on which they are fabricated to the destination display board (or other circuit or substrate), or more specifically:(1) How to pick up such small parts, if necessary distinguishing between selected parts and leaving unselected parts in their original positions(2) How to perform the pick-up economically and efficiently(3) How to position them into a precise array (if desired) at the destined location

Some current approaches use bonding metal to release micro-LEDs to be picked up by the pickup head via electrostatic force. In some cases, a compliance transfer head allows the pickup head to accommodate the non-ideal situation during the pickup. In other cases, post structures are built onto the bonding metal to reduce the surface area. In one case, micro-LEDs are released by physical breakage of shear release posts.

Major challenges of all these constructions lie in the release of selected LEDs to the pickup heads while keeping remaining LEDs in original position. Clearly, releasing by melting bonding material is not ideal. The release by physical breakage using shear force can be really problematic because such breakage is not well controlled.

Above 300 um, conventional methods of handling of LEDs can work reasonably well. For example, if the LEDs are made on a Sapphire substrate, breaking the substrate works for dimensions greater than 300 um but is problematic below that. Typically, LED substrates are thinned down to ˜100 um before cleaving or dicing. It is clear that when the edges of LEDs are less than 100 um in length, the LEDs are not even stable enough to place on a surface without supports. When the chip size becomes even smaller, the substrate has to be thinned further. The weight of each LED chip becomes so small that any small effect can be influential, for example Van der Waals force, internal stress between dissimilar materials, etc.

As noted above, some currently known approaches use bonding metal to release some micro-LEDs to be picked up by the pickup head via electrostatic force and the same bonding material is used to affix the released micro-LEDs onto the receiving substrate. In this process, the electrostatic force is competing against the surface tension of the melted bonding material. It has been recognized that the etching of bonding metal, typically done to remove metal between devices, could be very problematic because an uncontrollable amount of metal can be sputtered to the size wall of the device, causing shorting. It has also been recognized that the amount of electrostatic force depends on the contact area between the pickup head and the micro-LED. This may be addressed by using a compliance transfer head to allow the pickup head to accommodate non-ideal aspects of the situation, for example, warpage of substrates, non-parallelism between substrates, or even the presence of large particles dropped onto the surface. But, even with the compliant pickup heads, the release of micro-LED is still an issue.

Also as noted above, some current approaches build post structures onto the bonding metal to reduce the surface area. But, clearly, this process is quite delicate because the bonding metal, once melted, can move and enlarge its contact area with micro-LEDs. one other approach is to create a micro-LED structure which affix to a side wall in a selected area through undercut etch of sacrificial layer. These undercut micro-LED are released by physical breakage of attachments. The fabrication of shear release posts is very delicate, and the amount of force needed for breakage is not well controlled.

There is, therefore, a need for methods for precise and reliable transfer (pick up and release) of very small parts from the substrate on which they are formed to destination boards or substrates, and for designs for arrays of such parts that allow those methods to operate. Ideally such methods and designs would be compatible with and incorporated into conventional semiconductor manufacturing processes, making the whole operation very cost-effective.

SUMMARY

Embodiments generally relate to arrays of micro-devices designed and fabricated to facilitate handling, transfer between substrates, and test.

In some embodiments, an array comprises a substrate and a plurality of micro-devices, wherein each micro-device is suspended over a cavity in the substrate by at least one hinge attached to a side post formed into the substrate. Each micro-device comprises a bonding layer; a metal contact; semiconductor device layers; and a buffer layer.

In one of those embodiments, the active semiconductor layers comprise GaN-based LED layers; the buffer layer comprises AlGaN; and the substrate comprises (111) oriented Silicon. In another of those embodiments, the active semiconductor layers comprise InGaAsP-based LED layers; the buffer layer comprises InGaP; and the substrate comprises GaAs.

DETAILED DESCRIPTION

In some embodiments of this invention, a new micro-device structure, more specifically a micro-LED structure, is disclosed to overcome the handling problems discussed above, encountered in the prior arts. The application of the inventive ideas disclosed herein is not limited to micro-LEDs, although micro-LEDs are used as an example. Those familiar in the art will understand how to apply the principles to other kind of micro-devices, such as lasers, micro-sensors, small integrated circuit, or MEMS devices.

To make inorganic light-emitting diodes, typically, the materials are semiconductor crystals formed epitaxially on crystal substrates. For blue and green LEDs, the most popular substrates are Sapphire and SiC. For red LEDs, the most popular substrate is GaAs. However, in the present invention, the substrate for blue and green LEDs is chosen to be wet-etchable substrate such as Silicon. Although SiC can be wet etched, it requires very high temperature which is not compatible with semiconductor process. KOH can etch Silicon much faster than GaN.

On the other hand, growing GaN-based material on (111) oriented Silicon has been demonstrated with high quality. This is because (111) Si exhibits a 3-fold symmetry needed for as a template for hexagonal crystal structure of GaN.

Some approaches to microdevice fabrication and handling, prior to the present invention, involve the use of bonding metal to release some micro-LEDs to be picked up by the pickup head via electrostatic force.FIG. 1illustrates such an approach.FIG. 2illustrates another prior art approach, where a compliant transfer head allows the pickup head to accommodate non-ideal substrate flatness during device pickup. In some cases, post structures are built onto the bonding metal to reduce the surface area.FIG. 3shows one such case where u-LEDs are released by physical breakage of shear release posts.

FIGS. 4A through 4Dillustrate the formation of micro-LEDs according to some embodiments of the present invention.

FIG. 4Aillustrates the first step of a process carried out on the top surface of a silicon substrate (on which GaN-based LEDs are to be formed) according to one embodiment of the present invention. A buffer layer102is grown on top of 111-oriented Silicon substrate101, to greatly reduce the incidence of dislocations resulting from lattice mismatch between Si and Ga. The buffer layer102typically initiates with an AlN layer, to seal the silicon surface to avoid any exposure of Si to Ga. This is because Ga and Si could otherwise form an alloy, which would enter a liquid phase at typical epitaxial growth temperatures, such as above 800 C.

After buffer layer102is grown, n-layer103, which may include n-doped GaN or InGaN or AlGaN, is deposited, acting as a supply of electrons. Layer103may itself be made up of sub-layers. Next, light emitting layer104, typically a multiple quantum well structure (MQW), is grown. Finally, p-layer105, made up of p-doped GaN or AlGaN is deposited on top of MQW104, acting as a supply layer of holes. When an external bias voltage is applied, the electrons and holes supplied by103and105will flow into the MQW region104, where they combine to generate photons with the energy close to the bandgap of the MQW structure.

In other embodiments, in which red LEDs are fabricated, a layered structure like that shown inFIG. 4Ais grown, but with different materials for some of the layers. In these cases, substrate101is typically GaAs. While it is not essential, InGaP layer102is typically grown on this substrate to act as an etch stop. For example, HCl: 10 H3PO4: 1 H2O2can etch GaAs 100 times faster than InGaP. After InGaP etch stop layer102, a n-AlGaAs electron supply layer103is deposited, followed by the light generating layer104which can be AlInGaP or other appropriate material. The hole supply layer105such as p-AlGaAs is deposited on top of light generating layer104.

These layer structures are for illustration purposes. There are many other materials which can be used to form blue, green and red LEDs.

After the LED materials are deposited by either MOCVD or other epitaxial techniques to produce a structure as shown inFIG. 4A, the substrate wafers are processed using typical semiconductor processes. AsFIG. 4Bshows, p-metal contact110is selectively deposited on p-layer105, either by etching after uniform deposition or by a lift-off technique. Contact110is typically a p-metal stack, including many layers in order to achieve good electrical contact to the hole supply layer105, provide high reflectivity1to the light generated from the light-generating layer104, and act as a barrier material to prevent metal mixing with bonding layer116which will be deposited near the final step of the fabrication process (seeFIG. 6Ediscussed below).1An optical reflectivity of approximately 70% or greater is typically adequate. Reflectivities of 90% or greater may be preferable in some applications.

As shown inFIG. 4C, After the deposition of the p-metal stack, a confinement mesa etch is performed to selectively remove p-GaN105, MQW104and some n-GaN104, as shown inFIG. 4C, exposing the p-n junctions along mesa edge111of the micro-LEDs thus formed. Next, as shown inFIG. 4D, a thin dielectric layer112, comprising a material such as SiO2or SiN etc, is conformally deposited to protect the p-n junction on mesa111from exposure to subsequent processing steps. This will prevent unwanted leakage current from flowing through the etched p-n junction through surface contamination. After this point, the device area is electrically defined and the rest of the processing steps are carried out simply to form the mechanical features into the micro-LEDs so that they can be picked up individually or in small groups as desired, and transferred efficiently from the substrate on which they are formed to subsequent substrates or circuit boards.

FIGS. 5A through 5Cillustrate the formation of posts between micro-LEDs (already created using the process ofFIGS. 4A through 4D) according to one embodiment of the present invention.

FIG. 5Ashows the result of applying a standard dry etch technique such as RIE or CIBE to anisotropically etch deep trenches120through epitaxially grown layers102,103and into the substrate101. The trench depth122in substrate101may range from 1 um to 20 um. The trench (or via) width121may range from 1 um to 20 um. The precise side wall profile is not very important, but, in order not to waste device areas, it is preferred that the side wall be close to vertical, for example within 20 degrees of the vertical.

FIG. 5Bshows the substrate wafer after the next step of the post formation process, when post material123has been deposited and a planar top surface created. There are many way to accomplish this, depending in part on whether the material is inorganic (metallic or non-metallic) or organic. For example, if using metal as the post material, one can blanket-deposit a thin metal and then use metal plating to fill trenches120. The plating process is usually conformal. Optionally, a polishing step can be used to planarize the surface. Many metals, such as Al, Cu, Ni, etc. are easily plated using commercially available plating solutions. The choice of metals should facilitate the subsequent etching process.

Alternatively, the material forming the posts may include spin-on glass, polyimide or BCB insulating materials, that are spun onto the top surface of the structure. Such spin coatings have to be fully or partially cured to turn the material into solid form before further processing. Typically, the curing process will cause shrinkage of the spin-coating materials. Therefore, care must be taken during the process to avoid delamination within the trenches. The degree of curing should take into consideration of the etch rate of subsequent etching process.

FIG. 5Cshows the substrate wafer during the last step of the post formation process. Post material123has been etched from the top surface until the material remains only in the trenches. The top surface of each resulting post124is set to be close to the surface of the adjacent dielectric protection layer112. After the etching, if spin-on glass, polyimide or BCB are used, the post material should be cured further to strengthen the materials.

Posts are a very important feature of this invention. The posts need to extend into the substrate by a sufficiently large depth. The adhesion of post material to the substrate should be strong.

FIGS. 6A through 6Hillustrate the structural isolation, bonding-preparation, and suspension of micro-LEDs (already created using the process ofFIGS. 4A through 5C) according to one embodiment of the present invention.

FIG. 6Aillustrates a cross section view through part of an array, after the first step of the process of structural isolation has been carried out. This is a device isolation etch, performed to define the areas of the micro-LEDs on top of the substrate, which acts as an etch stop. The etchant is chosen to etch the post material minimally if at all. Although in the embodiment ofFIG. 6Aa post seems to be present between every pair of adjacent micro-LEDs, this is not necessarily the case.FIG. 6Aonly depicts one particular cross section through the substrate, where posts happen to be located. In other cross sections, there could be only isolation trenches between adjacent devices.

The isolation etch exposes more side wall areas113of n-GaN layer103and buffer layer102. Although these are not sensitive to contamination, there exists a potential issue of etching selectivity which will be described later. Therefore, a device protection layer114is isotropically deposited to cover the sidewalls of layers102,103, leaving the array structure shown inFIG. 6B. The benefits of this protection layer will be discussed later.

After device protection layer114is deposited, a dielectric spin-on filler125is deposited, to fill the trench spaces around the tops of the posts.FIG. 6Cillustrates the array after this deposition. The filler125can be just photoresist or a soft cured spin-on glass or BCB. An etch-back process may be used to remove the residual material left on the device surface. Typically, for organic materials such as photoresist and BCB, an Oxygen plasma is used to perform the etch.

FIG. 6Dillustrates the array after the formation of p-contact openings115, made by selectively etching parts of the dielectric layers114on top of p-metal stack110. The size of each opening115is purposely made much smaller than the top surface area of the corresponding p-metal stack110. Next, as illustrated inFIG. 6E, a bond metal stack layer116is selectively deposited over and around the top of each device, on top of dielectric layer114and through openings115to contacts110. The selective deposition may involve either a patterned etch process or a patterned photoresist liftoff process. In the embodiment shown in this Figure, in some areas, bonding layer116may run across trench filler125and extend over part of the top surface of each post124to allow hinges to be formed, as described below.

In other embodiments, not shown, the hinges may be formed before bonding layer116is deposited, using a different hinge material, and a separate masking step.

FIG. 6Fillustrates the array after the next step of the process, where after the deposition of bonding layer116, trench filler125has been dissolved by a solvent such as acetone. After the removal of trench filler125, each hinge130, roughly indicated by the dashed oval outline in the Figure, forms a bridge between the micro-LED and the post.FIG. 6Gshows the micro-device array after those parts of the dielectric protection layer114which were not covered by bonding layer116have been etched away by RIE. This etch exposes the bottom of the trenches.FIG. 6Hshows the micro-device array after the wafer has been immersed in a selective etch solution such as KOH. It is well known that KOH etches Silicon with a very strong dependence on crystal face orientation, with the etch rate of (111) KOH etches Silicon with a very strong dependence on crystal face orientation, with the etch rate of (111) surface being at least 100× slower than the rate for surfaces at other orientations. Therefore, it etches laterally far faster than vertically through the (111) oriented substrate of the shown embodiment. This unique feature allows very controllable creation of cavity127underneath the micro-LEDs. The etch rate of KOH through SiN or SiO2 is very slow. Therefore, the dielectric protection layer113will remain on the sidewall of the device. After the etch is complete, care must be taken in removing the etchant, to avoid device breakage.

While most of the above discussion of device fabrication fromFIG. 4Aonwards has used the example of a Silicon substrate, essentially the same processes apply to the case of red micro-LED fabrication, where substrate101is GaAs and n-layer103is GaAs and AlGaAs. To avoid the etch of n-layer103, the buffer layer (such as InGaP)102must serve as an etch stop. This is achieved by using H2O2based etchants to create the cavity underneath the micro-LEDs.

Care must be made after cavity127is created because the micro-LEDs now are suspended via the hinges130from posts124. After cavity etching, the substrate can be dried using the well-known critical point drying process, in which liquids self-remove with minimal disturbance to surrounding structures (in this case, the suspended micro-devices).

After the substrate is dried, the fabrication of the micro-LED array is completed. Although the description above has covered the use of typical or exemplary materials and fabrication steps, those who are familiar with the field know many other steps and materials can be used to produce similar micro-structures that will then be amenable to handling in ways corresponding to those to be described below.

Returning toFIG. 6H, the cross-section view depicted shows the essential elements of an array150according to one embodiment of the present invention. Array150comprises substrate101and a plurality of micro-devices140A,140B etc suspended over a cavity127in the substrate. Each device is suspended by one or more lateral hinges130, each hinge being attached to a side post124formed into and attached to substrate101. Each micro-device comprises a bonding layer115; a metal contact110; semiconductor device layers103,104,105; and buffer layer102.

FIGS. 4A through 6Hdepict cross sectional (side) views of the micro-array structures during their formation. The positioning of the posts relative to the micro-devices can vary for different embodiments depending on the design objectives.

FIG. 7Aillustrates a top down view of one possible layout of a micro-device array, in which (except for around the edges of the array) each micro-device140N is connected by four hinges130, one at each corner of the device, to four corresponding posts124. This design may result in weak posts, if the post dimensions are very small. The dashed line AA′ indicates the direction along which, if the device were “cut”, the cross-sectional view would appear roughly as shown inFIG. 6H.FIGS. 7B and 7Cshow top down views of alternative layouts using rectangular posts with different widths, corresponding to different trench widths121(seeFIG. 5A). Another design layout choice is whether the hinges can run only along one direction (as shown inFIGS. 7B and 7C) or two orthogonal directions (as shown inFIG. 7A).FIG. 7Dshows another layout in which one continuous post running parallel to and in between two rows of microdevices is shared by all of those micro-LEDs. Such layouts require separate masking steps and material for hinge formation, as noted above. In yet another layout, not shown, the posts can be positioned in a grid form for maximizing the strength of the posts and the fill factor of micro-LEDs.

FIGS. 7A through 7Donly illustrate embodiments where posts are laid out in arrangements with rectangularly symmetry. In other embodiments, post layouts may follow any of a variety of other symmetries, as long as they provide sufficient support to the suspended micro-LEDs. The cross section of the posts can be round or rectangular, or of any other practical shape.

The structural features of the arrays disclosed in this application allow micro-devices in the arrays to be individually transferred from the substrate on which they are formed to other substrates or destinations, with precision and reliability. In brief, in the case of a transfer, if a selected device is first “held” by a pickup head, for example by applying a voltage difference between the head and the top surface of the device, creating an electrostatic attraction therebetween. Then, a mechanical force is applied to lateral hinges connected to the device, by pushing the device towards the substrate; the hinges will break, leaving the previously suspended device free to be lifted away from the posts and the substrate by the pickup head. The pickup head may then be moved to a desired destination location, and if and when the force holding the device to the head is overcome by a competing force with respect to the destination site (involving applied voltage again, or a adhesive-coated surface, for example), the device will be released. In the interim period between picking up and releasing the device, it may be accessed for testing. The details of how such transfer may be executed and other benefits provided subsequent to the device transfer, for example device testing, will be disclosed in other related applications.

It should be noted that prior to the present invention, the size of the gap between a micro-LED and the corresponding substrate surface, created by etching of sacrificial layers, is very small, necessitating the use of compliant pickup heads to accommodate inevitable departures from perfectly flat surfaces. Such a pickup head would probably not be able to handle multiple devices. In the present invention, gap sizes may be much larger, 5 μm for example, allowing a single, non-compliant pickup head to be used. In addition, it is possible to break the hinge by pushing suspended devices via the pickup heads toward the substrate.

Now referring toFIG. 8A, a pickup substrate200having an array of pedestals are brought close to the micro-LED substrate. Each pickup pedestal201has one or more electrodes capable of supplying voltage. The surface is coated with dielectric layer with the thickness optimized for highest electrostatic force.

When the pickup heads are brought close to the micro-LEDs, charges are induced at the surface of the bonding layers. If a bipolar mode is used, both positive voltage and negative voltage are supplied to the electrodes embedded in the pickup pedestals, the net charge induced to the bonding layer surface is zero. This is a consideration when insulating posts are used. However, if the posts are made of conducting materials, such restriction is not required.

The amount of induced electrostatic charges is inversely proportional to the gap between the pedestal and the bonding layer. Only the micro-LEDs very close to the pedestals will experience significant electrostatic force. The distance range referred to by “very close” will depend on the applied voltage. For example, a range of up to approximately 1 um would correspond to a voltage of 25V, and up to approximately 3 um would correspond to a voltage of 100V. High voltages risk device damage, so a compromise must be reached to optimize pedestal height.

As shown inFIG. 8B, the pedestals are brought into contact with the micro-LEDs. It should be noted that the periodicity of the pedestals is not the same as the periodicity of the micro-LEDs. The distances between the pedestals are selected according to the applications.

Although inFIG. 8B, two pedestals are shown to be in contact with the micro-LEDs simultaneously, it is not practical to design a pickup process by assuming that such uniform contact can be achieved in a real process. In practice, all substrates might have warpage or non-ideal flatness of the surface. Typically, in the current state of the art, silicon wafers have warpage in the order of 5 um across 8″ wafers. So, while local flatness as shown inFIG. 8Bis possible, it is not reasonable to assume such flatness and parallelism can be achieved easily between two substrates.

To overcome this non-ideal flatness, the pickup technique disclosed herein allows the pickup substrate to be pushed against the micro-LED wafer as shown inFIG. 8C. By allowing the push into the micro-LEDs substrates, it accomplishes two things: (1) breaking the hinges which suspend the micro-LEDs, (2) allowing the contacts of all pedestals to the intended micro-LEDs. Now, one can appreciate the existence of the large cavity beneath the micro-LEDs which is made possible by this invention. In the prior art, the cavity was formed by a deposited sacrificial layer and the attachments of micro-LEDs to the supporting post is done by undercut etch of one of the dialectic layer.

As discussed above, the hinges can be made of different materials as the bonding layer if desired.

FIG. 8Ddepicts the removal of micro-LEDs by the pickup substrate when the substrate is lifted. The pickup is done by electrostatic force induced by the supplied voltage. Because there is no other competing force, the micro-LEDs are attached to the pedestals.

Before transferring the micro-LEDs to its destined board or substrate, a temporary receiving substrate may be used for an intermediate holder. There are many benefits for having such temporary holders.

First, it is beneficial to allow a visual inspection to be performed to see any physical damaged parts are present. If any part is damaged or even missing, a replacement action can be done prior to the final transfer.

Second, as described above, only p-metal contacts are made for the micro-LEDs. At this point, there is no clear way to perform any electrical test. However, any semiconductor process has some yield loss. Without testing, defective devices might be assembled into final product. So, it is beneficial to perform some screening tests of micro-LEDs. One approach is to perform a photoluminescence test (PL). This is an optical test which does not require electrical contact, but does require optical transparency. Referring toFIG. 8D, once the micro-LEDs are lifted from the original substrate, the bottom surfaces of the devices can be exposed to the excitation light (light with shorter wavelengths than the emitted light). Any micro-LEDs that do not emit light of the desired wavelength and desired intensity may be screened out i.e. discarded, if such a PL test can be performed.

Therefore, in the present invention, a temporary receiving substrate is introduced, to address the issue of defective devices by providing an effective method of identification and replacement.

As shown inFIGS. 9A and 9B, two possible temporary receiving substrates are illustrated, without losing generality of other possibilities. InFIG. 9A, the temporary receiving substrate300is coated by a soft material such partially cured photoresist or BCB301. When this type of material is not fully cured, the surfaces are still tacky. Once the micro-LEDs are placed on the tacky surface, the surface bonding can be sufficient to keep the devices from moving or falling off.

InFIG. 9B, a more reliable approach is accomplished by using interdigitated electrodes302covered by a thin dielectric coating303. It may be beneficial to apply voltages selectively, i.e. to only some rather than all electrodes302, so that electrostatic forces are created only in the areas of immediate interest for the device or devices to be picked up at a given time. It is quite easy to control the application of the voltage selectively via a control circuit. For example, if one device is identified as defective, the voltage applied to the electrodes underneath that device can be selectively removed, and the device can be easily removed by another special process such as using a single pickup head with an applied electrostatic force. After the removal of the defective devices, working devices can be aligned and placed into the vacated locations.

This replacement process is extremely important to make the high definition display to work. With more than millions of micro-LEDs in one single high definition display, even a yield of 99.999% will not be sufficient to make a defect less display. Therefore, although it appears to be an extra step toward the destined display board or substrate, this temporary receiving substrate is a very important step of the overall invention.

As shown inFIG. 9C, the pickup substrate200is positioned to the proximity of the temporary receiving substrate300. Although it is shown that all micro-LEDs land to the top surface of the temporary receiving substrate300, it might not be necessary to make physical contact. During the transfer steps, after the micro-LEDs are close to the temporary receiving substrate300, the voltages are selectively applied to the interdigitate electrodes. Thus, the electrostatic force will apply to the micro-LEDs from the bottom side. The, the voltages on the pickup substrate are removed, thus the electrostatic force from the top side is removed.

After the electrostatic force from the top is removed, the pickup substrate200is lifted and the micro-LEDs will be held down by the electrostatic force from the bottom as shown inFIG. 9D. As discussed above, this temporary transfer gives an opportunity to replace any defective devices, electrically or physically.

InFIG. 10A, the final receiving substrate400is positioned and aligned to the temporary receiving substrate300. On the receiving substrate400, many metal pads402are defined by lithographic process. Optionally between the substrate400and the metal pads401are insulating layer401if the substrate is conductive such Aluminum or other materials.

The metal pads402are for various purposes. For example, some pads are for blue LEDs, some pads for green LEDs, and some for red LEDs. Moreover, some pads can be used for control circuit to drive the LEDs. Moreover, some pads are used photodetectors. It is very clear that all these different devices might not have the same thickness. To transfer different types of devices to the same substrate will require some accommodations of tolerance and interference.

Prior to transfer the micro-LEDs to the receiving substrate400, the temperatures of both substrates increased by heaters attached to the holders of the substrates. Once the temperature of the temporary receiving substrate300is high enough, the bonding metal melts and balls up because of the surface tension of liquid metal surface. The surface energy between the bonding metal and the surround dielectric material should be chosen to be smaller than the surface energy of the bonding metal. This technique is quite well known in the industry for making ball grid array. This is also the reason why the opening of the p-metal contact is purposely made much smaller than the area of the p-contact stack to facility such form change. For example, if the micro-LED is 30 um×30 um, the p-contact opening is 5 um×5 um, and the bonding layer is 2 um thick, the height of the metal ball will be around 15 um which is large enough to accommodate the tolerance between various mechanical imprecision.

Now, the receiving substrate400is brought to touch the temporary receiving substrate300as shown inFIGS. 10B, 10C and 10D. As depicted inFIG. 10D, the alignment between the metal pad302and the micro-LEDs might not be perfect. This kind of misalignment can be caused by many practical reasons, such as lateral alignment, angular misalignment, different thermal expansion coefficients of different substrate materials, and etc. Also inFIG. 10D, it is shown the bonding metal is pressed and deformed because the gap difference between various location might be different.

But, all these imperfections due to mechanical tolerances are removed when the pickup head is lifted. The wetting of bonding metal to the metal pad402will maximize the surface area at the top and perform a self-aligned adjustment, as shown inFIG. 10E. The micro-LEDs will be precisely positioned based on the metal pads, rather than the pickup head. This is a very important feature of this invention.

As discussed above, replacement of micro-LEDs might be necessary. Having millions of micro-LEDs for one single display, a yield of 99.99% is not sufficient. Therefore, before the final assembly, the defective micro-LEDs need to be identified and replaced.

The defects of micro-LED can be mechanical or electrical. The mechanical defects can be determined using computer-added vision inspection. The electrical defect can be detected by photoluminescence as shown inFIG. 13in which the micro-LEDs120on the pickup substrate200are scanned under an excitation optical beam. When the excitation light800is absorbed by the LED materials, electrons and holes are generated and they drift to the active region. These electrons and holes recombine and emit light just similar to the light emission driven by electrical currents. A camera700is positioned on top of the micro-LEDs to capture the emitted light. Based on the intensity, the defective micro-LEDs can be identified. It is convenient to perform PL test on the pickup substrate. First, the focused excitation beam has a final size, such as 25 um diameter, which can be larger than the size of the micro-LEDs. In that case, it does not have the resolution to measure the responding emitting of each individual micro-LED. Even it could, precision alignment or registration can be done to isolated the response of each micro-LED. By measuring PL at the pickup substrate, the resolution is not an issue and the excitation beam can be made larger so that the registration of each micro-LED is not as critical. Second, once the micro-LEDs are picked up, the surface which is not coated by p-metal can be exposed to the excitation beam.

This unique feature should not be outlooked. Compared with prior art, the LEDs layers are transferred to a carrier substrate first through a bonding process after the p-metal is done. If the PL is used as contactless testing method, it will have to be done before or after the transfer.

For the prior art case where PL is done prior to the transfer, the PL system has to be precisely aligned to the p-metal and the excitation beam has to match the resolution of the micro-LED. This is possible but very challenging. For the case where PL is done after the transfer, the micro-LEDs are already bonded to the receiving substrate. So, even the defective micro-LEDs are identified, the replacement effort will be quite difficult.

Therefore, PL testing is not easy to carry out for micro-device arrays of the prior art. However, for micro-devices fabricated according to embodiments of the current invention, it is very straightforward.

Now, the advantages of having the temporary receiving substrate will become apparent. During the PL test, the camera can measure the intensity of the light emitted from micro-LEDs and perform visual inspection of the n-layer. Once the micro-LEDs are transferred to the temporary receiving substrate, another visual inspection can be performed on the p-layer side to ensure mechanical integrity of the micro-LEDs.

Once the defective micro-LEDs are identified, the replacements can be done by singular transfer to the temporary receiving substrate. As shown inFIG. 11A, a replacing pickup substrate500is brought to aligned to a defective micro-LED. This replacing pickup substrate can be very similar to the pickup substrate200, except it has fewer or only one pickup pedestal501. To ensure electrostatic force, the surface is coated with a dielectric layer502.

The replacement process is very similar to the transfer process. The replacing pickup substrate is lowered to make contact to the defective micro-LED as shown inFIG. 11B. Then, a voltage is applied to the replacing pickup pedestal to generate electrostatic force. The voltage on the temporary receiving substrate is momentarily turned off to remove its electrostatic force. The replacing pickup substrate is lifted along with the defective micro-LED as shown inFIG. 11C.

An alternative replacing pickup substrate600is shown inFIG. 12Ain which a metal pad603is deposited on the dielectric layer602on top of the pedestal601. After making contact to the defective micro-LED as shown inFIG. 12B, the pickup pedestal is heated up to above the melting point of the bonding material on top of micro-LED. The heating can be done either by electrical current or optical beam. The bonding material115of the defective micro-LED melts to wet to the metal pad603as shown inFIG. 12C. When lowering the temperature of the pedestal of the substrate600, the bonding material solidifies. The defective micro-LED is removed from the temporary receiving substrate when the replacing pickup substrate is lifted.

The advantage of using bonding material as pickup method for replacement is that it can be done more quickly. The replacing substrate can be just a metal probe. But, using one metal probe for one micro-LED might be too expensive. The metal probe can be made of Si MEMS process by which millions of metal probes can be made on one substrate which essentially look like what is depicted inFIG. 12A.

Once the defective micro-LED is removed from the temporary receiving substrate, a reverse process inFIG. 11can be used to insert a working micro-LED to the vacant position.

The alignment of these transfers will create position variations. However, the issue is addressed by this self-aligned process depicted previously inFIG. 10E.

Now, referring back toFIG. 10Ewhich only shows micro-LEDs after the first transfer and/or after replacement of defective micro-LEDs to the receiving substrate. In fact, for color displays, at least 3 colors of micro-LEDs are needed. With the bonding metal to accommodate as the vertical and horizontal tolerance, all transfers can be done to move micro-LEDs to their destined positions in sequence. Furthermore, at the last transfer, the receiving substrate will be pressed against a flat surface to squeeze bonding metal before cooling down. This final process will bring the bottom surfaces of all micro-LEDs and other devices to the same height, regardless their original layer thicknesses.

After the micro-LEDs are attached and fixed on the receiving substrate, a planarization process, such as BCB or molding compound, is used to fill the empty spaces between transferred devices as shown inFIG. 10F. It might be beneficial to have materials which is light absorbing to fill the spaces between micro-LEDs to avoid color bleach effects. After etching or polishing away the buffer layer or etch stop layer, the interconnect metal is deposited to contact the n-layer of the micro-LEDs. The interconnect metal will be connected to the driver circuit which will provide proper current to turn on the micro-LEDs.

In some embodiments, a receiving substrate, after undergoing the processes described above, will become the desired final display product. In other embodiments, particularly where very large displays are desired, it may be preferable to place micro-LEDs on multiple relatively small receiving substrates, making the necessary contacts to those individual devices, and then “tile” the substrates together into one assembly at the final installation site,

In conclusion, embodiments of the present invention allow for precise and reliable transfer of very small parts, such as micro-LEDs, from the substrate on which they are formed to destination boards or substrates, by disclosing detailed structural designs and associated fabrication methods for arrays of such parts, facilitating such transfer. Benefits include compatibility with conventional semiconductor manufacturing processes, enabling integrated, very cost-effective production.