Method for polymer-assisted chip transfer

One or more chips are transferred from one substrate to another by using one or more polymer layers to secure the one or more chips to an intermediate carrier substrate. While secured to the intermediate carrier substrate, the one or more chips may be transported or put through further processing or fabrication steps. To release the one or more chips, the adhesion strength of the one or more polymer layers is gradually reduced to minimize potential damage to the one or more chips.

TECHNICAL FIELD

The present disclosure relates to chip fabrication, and more specifically to transferring chips from one substrate to another during a fabrication process.

BACKGROUND

Semiconductor device fabrication often involve multiple steps that may require that at least part of the device be strongly secured at times, such as to a substrate. For example, when transporting a partially-fabricated semiconductor device, it is desirable that the device to be secured to the substrate strongly enough so that it does not shift or become detached from the substrate. Similarly, it is also desirable for the device to be secured during certain fabrication processes. One such process is laser lift-off (LLO), which is used to detach semiconductor devices from a substrate they are chemically adhered to. During LLO, a laser is focused on the interface of a substrate and base of the semiconductor device, often sapphire and gallium nitride, respectively. As the laser heats the interface, the bonds at the interface are broken, and the substrate can subsequently be removed. However, the devices need to be kept in place during the LLO process to avoid damaging the devices.

LLO may be part of a larger “chip transfer” process through which semiconductor device chips are removed from one substrate and placed onto another. Adhesive tapes are conventionally used for chip transfer processes and may be heat- or UV-release materials. However, such tapes are not suitable for ultra-fine-pitch interconnects (e.g., <10 μm pitch, <5 μm diameter, <5 μm height). Adhesive tapes are only able to provide a single level of adhesion, which is often too strong and prevents reliable chip release. Smaller devices especially may be damaged upon removal when the adhesion strength is too high. Alternatively, if the adhesion strength is too low, the devices may be disrupted during transportation or processing (such as LLO). Furthermore, adhesive tapes are highly flexible, which can cause chip misalignment during and after the transfer process.

SUMMARY

One or more chips are transferred from one substrate to another by using one or more adhesive layers to secure the one or more chips to an intermediate carrier substrate. While secured to the intermediate carrier substrate, the one or more chips may be transported or put through further processing or fabrication steps. To release the one or more chips, the adhesion strength of the one or more polymer layers is gradually reduced to minimize potential damage to the one or more chips.

In one embodiment, one or more chips are secured to a first substrate by depositing a first adhesive layer on the first substrate, depositing a second adhesive layer that is made of a different material than the first adhesive layer on the first substrate, and placing one or more chips on the first substrate through the first and second adhesive layers. The one or more chips are detached from the first substrate by removing the second adhesive layer from the first substrate, softening the first adhesive layer, and removing the one or more chips from the first substrate.

In another embodiment, one or more chips are secured to a first substrate by depositing a polymer layer on the first substrate, and placing one or more chips on the first substrate through the polymer layer, such that one or more portions of the polymer layer covering edges of the one or more chips. The polymer layer is then dried, such as by evaporation. The one or more chips are detached from the first substrate by removing the one or more portions of the polymer layer from the edges of the one or more chips, softening one or more remaining portions of the polymer layer, and removing the one or more chips from the first substrate.

In yet another embodiment, one or more chips are secured to a first substrate by depositing a first adhesive layer on the first substrate, placing one or more chips on the first substrate through the first adhesive layer, and depositing a second adhesive layer on the first substrate. The second adhesive layer is made of a different material than the first adhesive layer and comes into contact with a second substrate the one or more chips are attached to. The second adhesive layer covers edges of second substrate and some edges of the one or more chips but does not cover other portions of the one or more chips. The one or more chips are detached from the first substrate by removing the second adhesive layer from the first substrate, softening the first adhesive layer, and removing the one or more chips from the first substrate.

DETAILED DESCRIPTION

Generally disclosed herein are methods for transferring chips from one substrate to another using one or more adhesive layers to secure a chip to a surface of the substrate and reduce the adhesion strength of the one or more adhesive layers when removing the chip from the surface. By doing this, the disclosed transfer methods are better able to secure the chip to the surface strongly enough for transportation and processes such as LLO, while also limiting damage of the chip upon removal from the surface.

Herein, a “chip” refers to a semiconductor device such as, but not limited to, a micro-light-emitting diode (microLED). In one embodiment, the chip is specifically a gallium nitride (GaN) microLED.

The chip transfer methods are explained below for a single example chip being transferred from a “fabrication substrate” to a “target substrate” via a “carrier substrate.” The fabrication substrate is a substrate that the chip is attached to before it is transferred, such as a substrate the chip was fabricated on. Example fabrication substrates include sapphire and silicon. The target substrate is a substrate the chip is placed on after it is transferred. For example, the chip may be placed on a target substrate where it is part of a larger device, such as a microLED array. The carrier substrate is a hard material, which reduces the planar flexibility of the adhesive layer(s) used to adhere the chip to the carrier substrate. Thus, the combination of the hard carrier substrate and the adhesive layer(s) form a rigid adhesive surface. Example carrier substrates include glass and silicon. The fabrication and/or target substrates may not be involved in some embodiments of the disclosed methods. For example, one or more chips may have already been removed from their fabrication substrate and before being secured to the carrier substrate.

The adhesive materials discussed in conjunction with the disclosed methods include waxes, such as carnauba wax and bees wax, and polymer materials. The following polymer materials are specifically discussed: polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), polyvinyl pyrrolidone (PVP), and polyvinyl butyral (PVB). However, these adhesive materials are merely examples, as many different adhesive materials that meet the requirements described may be used in these methods. For example, the adhesive materials need not be limited to waxes and vinyl polymers.

Double Adhesive Layer System

FIGS. 1, 2A, and 2Billustrate a double adhesive layer chip transfer system, according to one embodiment.FIG. 1shows a cross-sectional view100of the chip110(on a fabrication substrate120) secured to a carrier substrate130via a double adhesive layer, according to one embodiment. A first adhesive layer140is on the surface of the carrier substrate130, while a second adhesive layer150is on top of the surface of the first adhesive layer140. The adhesion strength of the double adhesive layer system can be controlled by the thicknesses of the adhesive layers140and150. For example, the first adhesive layer140may be thinner than the second adhesive layer150, as shown inFIG. 1. Adhesive layer thicknesses are further discussed in conjunction withFIG. 2A.

FIG. 2Ais a flow chart of a method200for attaching the chip110to the carrier substrate130via a double adhesive layer, according to one embodiment. The first adhesive layer140is deposited202onto the carrier substrate130and the second adhesive layer150is deposited onto the first adhesive layer140. The adhesive layers140and150may be deposited202and204using standard thin film deposition techniques, such as spin-coating and dip-coating.

The first and second adhesive layers140and150are made of adhesive materials with different properties that allow them to be removed separately. Specifically, the first and second materials are soluble in different solvents and/or able to be dry etched by different chemical species. For example, the first material (of the first adhesive layer140) is soluble in a first solvent in which the second material (of the second adhesive layer150) is insoluble, and the second material may be soluble in a second solvent in which the first material is insoluble. At least one of the first and second materials is a polymer material. The other of the first and second materials may be a wax, such as carnauba or bees wax, or another polymer material. In one embodiment, the first material is a polymer material and the second material is a wax. Specifically, the second material may be carnauba wax. In another embodiment, the first material is PVAc and the second material is PVA. In yet another embodiment, the first material is PVP and the second material is PVB. Additionally or alternatively, the first and second materials have different glass transition temperatures, which allows the adhesion properties of the first and second adhesive layers140and150to be modified individually.

The first and second adhesive layers140and150may be of different thicknesses, so as to control the total level of adhesion and the reduced level of adhesion after the second adhesive layer150is removed. In one embodiment, the first adhesive layer140is approximately 1 micrometer thick, and the second adhesive layer150is 2-3 micrometers thick.

In some embodiments, the adhesive layers140and150are dried206, such as by evaporation, an electron beam, UV radiation, heat, or chemical drying agents.

The carrier substrate130is heated208to soften the adhesive layers140and150in preparation for chip placement210. The temperature for softening the adhesive layers140and150is dependent on the type of adhesive material being used. Typically, the temperature that the carrier substrate130is heated208at least to the glass transition temperatures of the adhesive layers140and150. For example, the carrier substrate130is heated to at least 85° C. if the adhesive layers140and150are PVAc and PVA, and the carrier substrate130is heated to at least 110-180° C. if the adhesive layers are PVP and PVB.

The chip110is weighted (e.g., with a few grams) and placed210on the heated carrier substrate130, through the second adhesive layer150and the first adhesive layer140. The chip110is fully secured to the carrier substrate130once the temperature returned to below the glass transition temperature of one or both of the adhesive materials, fixing the chip110in place and increasing the level of adhesion.

FIG. 2Bis a flow chart of a method250for releasing the chip110that is attached to the carrier substrate130via a double adhesive layer (made of adhesive layers140and150), according to one embodiment. Method250includes optional processing steps (indicated in dotted lines) through which the fabrication substrate120can be removed. These are an example of what processing may require a stronger level of adhesion. Alternatively or additionally, the carrier substrate130may be used to secure the chip110for transport or shipping.

While the chip110is strongly adhered to the carrier substrate130, the fabrication substrate120may be removed by performing252laser lift-off or a similar process to break any physical and/or chemical bonds between the chip110and the fabrication substrate120. Once those bonds are broken, the fabrication substrate120can be physically removed254from the chip110. The fabrication substrate120may be heated to aid in its removal254. For example, a sapphire fabrication substrate120is heated to 30° C. prior to removal254.

After the fabrication substrate120has been removed254, the level of adhesion between the chip110and the carrier substrate130is decreased by removing256second adhesive layer150from the carrier substrate130and the chip110. The chip110remains adhered to the carrier substrate130via the first adhesive layer140. In one embodiment, the second adhesive layer150is removed254by the rinsing carrier substrate130with a solvent in which the second adhesive layer150is soluble but the first adhesive layer140is not. Example solvents include water, butanone, ethanol, and isopropanol. Specifically, the carrier substrate130is rinsed with water if the second adhesive layer150is PVA (and the first adhesive layer140is not PVP), with butanone if the second adhesive layer150is PVAc, with water (if the first adhesive layer140is not PVA) or isopropanol if the second adhesive layer150is PVP, and the carrier substrate130is rinsed with ethanol if the second adhesive layer150is PVB.

The carrier substrate130is heated258to soften the first adhesive later140so that the chip110can be removed260from the carrier substrate260(e.g., so it can be placed on a target substrate). Again, the temperature that the carrier substrate130is heated to depends on the adhesive material being used. For example, the carrier substrate130is heated to at least 45° C. when the first adhesive layer140is PVAc, or to at least 110-180° C. when the first adhesive layer140is PVP. Alternatively or additionally, the first adhesive layer140can be dry etched to reduce its adhesion strength before removing260the chip110from the carrier substrate130.

Single Polymer Layer System

FIGS. 3, 4A, and 4Billustrate a single polymer layer chip transfer system.FIG. 3shows a cross-sectional view300of the chip110(on the fabrication substrate120) secured to the carrier substrate130via a single polymer layer340, according to one embodiment. The single polymer layer340is on the surface of the carrier substrate130and may also come into contact with the fabrication substrate120. In this embodiment, adhesion strength is controlled by the amount of the polymer layer340that is removed454, which is discussed in conjunction withFIG. 4B.

FIG. 4Ais a flow chart of a method400for securing the chip110to the carrier substrate130via a single polymer layer340, according to one embodiment. The carrier substrate130is prepared by depositing402the single polymer layer340on the carrier substrate130. The polymer layer340may be deposited202using standard thin film deposition techniques, such as spin-coating and dip-coating.

The polymer layer340is made of a polymer material that is able to be selectively removed by a laser, which is further described in conjunction withFIG. 4B. Examples of possible polymer materials include PVAc, PVA, PVP, and PVB. The thickness of the polymer layer340is such that the chip110can be fully embedded in the polymer layer340with the fabrication substrate120also coming into contact with the polymer layer340. For example, the thickness of the polymer layer340may be approximately equal to or a little less than a height of the chip110.

The chip110is placed404on the carrier substrate130, through the polymer layer340. After placement404, the sides of the chip110contact the polymer layer340. The sides of any morphology on the bottom of the chip110, such as ridges or prongs, also comes into contact with the polymer layer340.

In some embodiments, the polymer layer340is dried406, such as by evaporation, an electron beam, UV radiation, heat, or chemical drying agents, before or after chip110placement404. If the polymer layer340is dried before chip110placement404, the carrier substrate130may also be heated (similarly to step208of method200) before chip110placement404in order to soften the polymer layer340.

FIG. 4Bis a flow chart of a method450for releasing the chip110that is attached to the carrier substrate130via the single polymer layer340, according to one embodiment. Method450includes optional processing steps (indicated in dotted lines) through which the fabrication substrate120can be removed. These are an example of what processing steps may require a stronger level of adhesion. Alternatively or additionally, the carrier substrate130may be used to secure the chip110for transport or shipping.

While the chip110is strongly adhered to the carrier substrate130, the fabrication substrate120may be removed by performing452LLO or a similar process to break any physical and/or chemical bonds between the chip110and the fabrication substrate120.

To decrease the adhesion strength of the polymer layer340, portions of the polymer layer340covering the edges of the chip110are removed454from the carrier substrate130. The portions of the polymer layer340that are removed454include any portions of the polymer layer340contacting the edges of the fabrication substrate120. In some embodiments, portions of the polymer layer340contacting the underside of the fabrication substrate120may not be removed454. The removal454may be performed via a laser, such as a carbon dioxide laser.

The removal454of portions of the polymer layer340dislodges the fabrication substrate120so that it can be removed456from the chip110. The fabrication substrate120may be heated to aid in its removal456. For example, a sapphire fabrication substrate120is heated to 30° C. prior to removal456. In some embodiments, steps454and456may be reversed.

The carrier substrate130is heated458to soften the remaining portions of the polymer later340so that the chip110can be removed460from the carrier substrate130(e.g., so it can be placed on a target substrate). The temperature that the carrier substrate130is heated to depends on the polymer material being used. For example, the carrier substrate130is heated to at least 85° C. when the polymer layer340is PVA, at least 45° C. when the polymer layer340is PVAc, to at least 110-180° C. when the polymer layer340is PVP, or to at least 62-78° C. when the polymer layer340is PVB. Alternatively or additionally, the polymer layer340can be dry etched to reduce its adhesion strength before removing460the chip110from the carrier substrate130.

Edge Double Adhesive Layer System

FIGS. 5, 6A, and 6Billustrate an edge double adhesive layer chip transfer system, according to one embodiment.FIG. 5is a cross-sectional view500of the chip110(on a fabrication substrate120) secured to a carrier substrate130via an edge double adhesive layer, according to one embodiment. A base adhesive layer540is on the surface of the carrier substrate130. An edge adhesive layer550is the on the surface of the base adhesive layer540, but only around the sides and edges of the chip110. The adhesion strength of the edge double adhesive layer system can be controlled by the thicknesses of the adhesive layers540and550. For example, the base adhesive layer540may be thinner than the edge adhesive layer550, as shown inFIG. 5. Adhesive layer thicknesses are further discussed in conjunction withFIG. 6A.

FIG. 6Ais a flow chart of a method600for securing the chip110to the carrier substrate130via an edge double adhesive layer (made up of adhesive layers540and550), according to one embodiment. The base adhesive layer540is deposited602onto the carrier substrate130using standard thin film deposition techniques, such as spin-coating and dip-coating.

The base adhesive layer540and the edge adhesive layer550are made of adhesive materials with different properties such that they can be removed separately. The example combinations of materials discussed with respect to the double adhesive layer chip transfer system ofFIGS. 1, 2A, and 2Bmay also be used for the edge double adhesive layer chip transfer system discussed here. Specifically, at least one of the base and edge adhesive layers540and550is a polymer material, and, in some embodiments, the other of the base and edge adhesive layers540and550is a wax, such as carnauba wax or bees wax. In one embodiment, the base adhesive material comprises PVAc and the edge adhesive material is PVA. In another embodiment, the base adhesive material comprises PVP and the edge adhesive material is PVB. The thickness of the base adhesive layer540controls the reduced adhesion strength of the edge double adhesive layer system. Accordingly, the base adhesive layer540may be relatively thin to facilitate easy release of the chip110. In one embodiment, the base adhesive layer540is approximately 1 micrometer thick.

In one embodiment, the base adhesive layer540is separated into a first base adhesive layer and a second base adhesive layer. The first and second base adhesive layers may be similar to the first and second adhesive layers140and150discussed inFIGS. 1, 2A, and 2B. They may be made of different materials with different properties and may be softened and/or removed independently.

The base adhesive layer540is dried604, such as by evaporation, an electron beam, UV radiation, heat, or chemical drying agents.

The carrier substrate130is heated606to soften the base adhesive layer540in preparation for chip110placement210. The temperature required to soften the base adhesive layer540depends on the type of adhesive material being used. Typically, the carrier substrate130is heated606at least to the glass transition temperatures of the base adhesive layer540. For example, if the base adhesive layer540is PVAc, the carrier substrate130is heated to at least 85° C. prior to chip placement210. Alternatively, when the base adhesive layer540is PVP, the carrier substrate130is heated to or past 110-180° C.

The chip110is weighted (e.g., with a few grams) and placed608on the heated carrier substrate130, through the base adhesive layer540. In some embodiments, the base adhesive layer540is not dried604, the carrier substrate130is not heated606, and the chip110is not weighted during placement608.

After the chip110has been placed608, the edge adhesive layer550is deposited around the edges of the fabrication substrate120and coming into contact of edges of the chip110. In embodiments where multiple chips110are attached to the same fabrication substrate120, the edge adhesive layer550only comes into contact with the outermost edges of the multiple chips110. That is, the edge adhesive layer550does not come into contact with an edge of a chip110that is directly adjacent to an edge of another chip110. The edge adhesive layer550may be deposited using convention deposition techniques, or more targeted deposition techniques, such as injection.

FIG. 6Bis a flow chart of a method650for releasing a chip110that is attached to a carrier substrate130via an edge double adhesive layer (made up of adhesive layers540and550), according to one embodiment. Method650includes optional processing steps (indicated in dotted lines) through which the fabrication substrate120can be removed. These are an example of what processing may require a stronger level of adhesion. Alternatively or additionally, the carrier substrate130may be used to secure the chip110for transport or shipping.

While the chip110is strongly adhered to the carrier substrate130, the fabrication substrate120may be removed by performing652laser lift-off or a similar process to break any physical and/or chemical bonds between the chip110and the fabrication substrate120.

The level of adhesion between the chip110and the carrier substrate130is decreased by removing654the edge adhesive layer550from the carrier substrate130and the chip110. The chip110remains secured to the carrier substrate130via the base adhesive layer540. In one embodiment, the edge adhesive layer550is removed654by the rinsing the carrier substrate130with a solvent in which the edge adhesive layer550is soluble but the base adhesive layer540is not. Example solvents include water, butanone, ethanol, and isopropanol. Specifically, the carrier substrate130is rinsed with water if the edge adhesive layer550is PVA (and the base adhesive layer540is not PVP), with butanone if the edge adhesive layer550is PVAc, with water (if the base adhesive layer540is not PVA) or isopropanol if the edge adhesive layer550is PVP, and the carrier substrate130is rinsed with ethanol if the base adhesive layer550is PVB.

After the edge adhesive layer550is removed654, the fabrication substrate120, however, is no longer attached to the chip110(via chemical/physical bonds or via the edge adhesive layer550), so it can be removed656from the chip110. The fabrication substrate120may be heated to aid in its removal656. For example, a sapphire fabrication substrate120is heated to 30° C. prior to removal656.

In some embodiments, the removal654of the edge adhesive layer550and the removal656of the fabrication substrate120can be combined. Specifically, the solvent used to remove654the edge adhesive layer550is heated above 30° C. such that it simultaneously dissolves the edge adhesive layer550and weakens the interface between the chip110and the fabrication substrate120(e.g., by melting a gallium layer between a gallium nitride chip110and a sapphire fabrication substrate120after LLO). Accordingly, both the edge adhesive layer550and the fabrication substrate120are removed654and656after application of the heated solvent. Combining these steps may be advantageous to minimize the handling of the chips110and avoid extra precautions to preserve the quality of the chips110during these steps.

The carrier substrate130is heated658to soften the base adhesive later540so that the chip110can be removed660from the carrier substrate130(e.g., so it can be placed on a target substrate). Again, the temperature that the carrier substrate130is heated to depends on the adhesive material being used. For example, the carrier substrate130is heated to at least 45° C. when the base adhesive layer540is PVA, or to at least 110-180° C. when the base adhesive layer540is PVP. Alternatively or additionally, the base adhesive layer540can be dry etched to reduce its adhesion strength before removing660the chip110from the carrier substrate130.

ADDITIONAL CONSIDERATIONS