Low residue, high temperature resistant dry adhesive and method of use

Abstract

A dry adhesive microfiber array includes a plurality of fibers having tips adapted to contact a surface, where a dry adhesive is capable of adhering to the surface at an elevated temperature. The bonding strength of the dry adhesive remains constant or increases as the temperature of the substrate / dry adhesive / carrier increases. The dry adhesive may be peeled off without leaving a residue on the surface of the substrate. In addition, the influence of temperature on the bonding strength of the dry adhesive is reversible.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 434,377, filed on Dec. 21, 2022, under 35 U.S.C.§119, which is incorporated herein by reference.

[0003] Statement Regarding Federally Sponsored Research

[0004] Not applicable. Background Art

[0005] The present invention generally relates to dry adhesives. More specifically, the present invention relates to dry adhesives comprising arrays in micron - and / or nano - scale fiber arrays, which can be used to temporarily attach silicon wafers, glass, metals, and other high - temperature - resistant substrates to other substrates (referred to as carriers) without damaging either substrate or leaving undesirable residues on either substrate.

[0006] Semiconductor and glass manufacturing involve several high - temperature processing steps at temperatures above 230°C. These processes can last for several minutes or longer, even over an hour. Such processes include, but are not limited to, solder reflow, tempering, lamination, layer deposition, and other high - temperature processes. During these processes, a substrate (glass / silicon / polysilicon / metal, etc.) is applied to a carrier via a removable adhesive, the substrate undergoes one or more high - temperature processes, and then the substrate is removed from the carrier. These processes are often automated and require high throughput to be cost - effective. Thus, after the substrate is removed from the carrier at the end of the high - temperature process, the substrate often is automatically transferred to another automated process (either at an elevated temperature or not), and the carrier will have a new substrate applied to it such that the process can be repeated on the new substrate. During these processes, it is crucial that the substrate remains firmly attached to the carrier at high temperatures and can be removed from the carrier without damaging the carrier or the substrate and leaving a minimal amount of residue on the substrate.

[0007] Current solutions for high - temperature bonding of silicon wafers to carriers often rely on the use of liquid adhesives that cross - link before high - temperature processing to form a strong bond. At the end of the high - temperature process, the liquid adhesive can be removed from the carrier using a strong solvent or a chemical etching process. These processes can be time - consuming and expensive, may rely on materials that pose safety or environmental concerns, and may result in reduced process throughput. Thus, there has long been a need in the industry for high - temperature, low - residue adhesives that provide reversible, repeatable bonding and can be removed without the use of solvents or etching processes.

[0008] The industrial standard for removable adhesives is Adhesive tape The adhesive tape is composed of a polyamide carrier and a silicone pressure sensitive adhesive (PSA). Although the selected PSA can withstand temperatures above 230 °C for a short period of time, due to rapid oxidation and pyrolysis, the PSA is unstable at these temperatures for more than one hour. Oxidation and pyrolysis cause the PSA layer to deteriorate and subsequently leave residues on the substrate. In addition, highly viscoelastic materials such as PSA will suffer from a decrease in adhesion as the temperature increases.

[0009] Therefore, developing a dry adhesive that overcomes these limitations associated with high temperatures would be advantageous by providing an adhesion that does not decrease with temperature, where the adhesive does not degrade significantly after long-term exposure at elevated temperatures, thus allowing the adhesive to be removed from the substrate in a residue-free manner. Summary of the Invention

[0010] One embodiment of the present invention is a dry adhesive having an array of fibers that can adhere to a smooth flat substrate or a patterned substrate, such as the surface of a silicon wafer. In one embodiment, the dry adhesive includes an array of microscale and / or nanoscale fibers extending from a backing, where the fibers have enlarged formed tips. The adhesive is described as "dry" because it does not rely on a pressure sensitive adhesive, liquid, or glue for adhesion. Rather, the structure of the fibers in the array is responsible for adhesion. When the tip contacts the surface of the wafer, the tip provides the adhesive force. Removal can be accomplished by peeling the dry adhesive from the substrate or moving the substrate in a direction parallel to the surface of the dry adhesive. In addition, the dry adhesive can be produced from materials that are stable at high temperatures, such as liquid silicone rubber (LSR). Therefore, the adhesive does not degrade highly at high temperatures, its adhesion increases with temperature, and it can be removed without leaving residues on the adhesive surface. Since the dry adhesive is made entirely of high-temperature resistant materials, a PSA layer is not required, thus eliminating the instability factor of the finished product.

[0011] The dry adhesive can be formed into a film, tape, or directly fabricated onto the surface of a carrier. In addition to physically removing the wafer from the carrier, removing the dry adhesive does not require chemicals or complex processing steps. Since the dry adhesive is not composed of complex multi-component liquid adhesives, the amount of residue left on the surface of the substrate after removal (including at elevated temperatures) is significantly reduced. Brief Description of the Drawings

[0012] Figure 1A-1B is an image showing the structure of a dry adhesive according to one embodiment.

[0013] Figure 1C A dry adhesive showing an object adhered to a fiber is shown.

[0014] Figure 2 It is a graph showing the adhesive force as a function of temperature.

[0015] Figure 3 It is a graph showing the shear force as a function of temperature.

[0016] Figure 4 It is a graph showing the shear force as a function of contact time and temperature.

[0017] Figure 5 It is a graph showing the shear force when the temperature changes from low to high.

[0018] Figure 6 It is a graph comparing the shear force of PSA and dry adhesive as a function of temperature. Detailed Description

[0019] In one exemplary embodiment as shown in Figure 1A-1B , the dry adhesive microfiber array 100 includes a plurality of fibers 101, and the plurality of fibers 101 are adhered to a backing layer, a carrier or a substrate 102. In one embodiment, the fibers 101 are adhered to the backing layer, the carrier or the substrate 102 at a substantially perpendicular angle at the proximal end (see Figure 1A ). In this embodiment, each fiber includes a stem portion 103 and a tip 104, and the tip 104 can be enlarged (i.e., the radius of the tip 104 is greater than the radius of the stem portion 103). In one embodiment, the tip 104 is a mushroom-shaped tip 104 having a flat surface at the distal end of the fiber 101. The stem portion 103 and the tip 104 are symmetric about the axis of symmetry, such that the radius a of the stem portion 103 (up to the connection point 105 with the tip 104) is constant along the length of the stem portion 103. However, in an alternative embodiment, the radius of the stem portion 103 can vary along its length, including an embodiment in which the radius of the stem portion 103 near the backing layer 102 is enlarged. In this exemplary embodiment, the tip 104 is also symmetric and fixed in the radial direction so as to increase the contact with a surface (such as a semiconductor device, a silicon wafer, a chip, a die, a semiconductor package or other similar devices). In Figure 1B , a top view of the tip 104 is shown. In one embodiment, the surface of the tip 104 and the cross-section of the stem portion 103 are circular. However, in other embodiments, an oval or elliptical shape and / or cross-section can be used for the stem portion 103 or the tip 104. The side shape on the bottom side of the tip 104 is linear, but alternatively, it can be convex or concave with respect to the axial direction of the stem portion and the surface of the tip.

[0020] In an alternative embodiment, the dry adhesive 100 can include a film or tape having fibers 101 on opposite sides, similar to a double-sided tape. In such a configuration, the tape or dry adhesive 100 can be placed on a carrier, and then the semiconductor device can be placed on top of the tape 100, as Figure 1C shown. During removal, the manufacturer can choose to remove the carrier from the device or the device from the carrier. For example, if a wafer is to be transferred to a different carrier for subsequent processing steps, the wafer and the tape 100 can be removed from the carrier and placed on the surface of the different carrier. Since the dry adhesive fiber array 100 does not lose its adhesiveness when removed, it will adhere to the different carrier. By fixing the dry adhesive 100 to the wafer, the handling steps involving the device side of the wafer are reduced.

[0021] As is known in the art, during the bonding process, multiple fibers 101 of the dry adhesive 100 conform, bond, or otherwise attach to the surface of the device. More specifically, the tips 104 of the fibers 101 contact the surface of the device and provide the adhesive force. The bonding strength of the dry adhesive 100 can be customized according to specific processing steps. Using a lower bonding strength reduces the probability of damaging the device when removing the dry adhesive 100. The bonding strength can be adjusted by changing the parameters of the fiber design, which include fiber length, fiber radius, backing layer thickness, tip diameter, tip height, the angle between the surface of the tip and the side of the tip, fiber density, and material selection. In an exemplary embodiment, the fibers 101 are constructed from liquid silicone rubber in a molding process known to those skilled in the art, where the liquid silicone rubber is injected into a mold and cured into a solid form. In this exemplary embodiment, the dry adhesive 100 can have fibers 101 with a rod radius of 4 μm, a tip radius of 8 μm, and a length of 20 μm. In other embodiments of the present invention, for example, the dry adhesive 100 can have fibers 101 with a rod radius between 5 μm and 100 μm, a tip radius between 6 μm and 200 μm, and a fiber length between 5 μm and 200 μm. The liquid silicone rubber can be platinum-cured silicone rubber, such as Shinetsu KEG 2000-40, Shinetsu KE 1950-50, Elastosil series LR 3043 / 50, or Elkem Silbione LSR 4340.

[0022] In other embodiments, the dry adhesive 100 is made of liquid silicone rubber, thus exhibiting very good chemical resistance to most acids, alkalis, inorganic chemicals, organic chemicals, and solvents. In alternative embodiments, the shaft 103 of the fiber 101 may be made of a first material, and the tip 104 may be constructed of a second material. For example, the shaft 103 may be made of high-temperature silicone to maintain its tensile strength, while the tip 104 may be made of a typical silicone that exhibits reliable adhesion within a temperature range, which will be discussed below.

[0023] In certain exemplary embodiments, the high-temperature resistant dry adhesive 100 is constructed of an array of micro- and / or nanostructures having enlarged tips 104 and / or enlarged shaft bases 103, as discussed above. The enlarged tip 104 may include a mushroom shape, where the tip 104 has a thickness and a radius larger than that of the shaft 103. In other embodiments of the present invention, the dry adhesive 100 with enhanced adhesion may be constructed of a high-temperature resistant resin from other patterned structures known to enhance or modify adhesion, the patterned structures including: solid prismatic shapes with uniform cross-sections; prismatic shapes with non-uniform cross-sections; enlarged prismatic tip shapes; spatula tip shapes; mushroom tip shapes; concave tip shapes; micro-patterned features recessed into the surface of the component; and other similar shapes. In many of these examples, the shape of the fiber 101 and / or the tip 104 increases the contact surface area between the dry adhesive 100 and the component to be adhered. Other fiber properties may also be altered to adjust the bonding strength.

[0024] Temperature can also affect the adhesion properties of the dry adhesive 100. Figure 2 Shows the adhesion force of the dry adhesive 100 at various temperatures in the range of 20 °C to 225 °C. Figure 2 The y-axis shown represents the normal force in Newtons per square centimeter, and the x-axis represents the surface temperature of the dry adhesive 100. Figure 2 Shows the single measurement value at each temperature. As Figure 2 shown, the adhesion force in the normal direction increases with increasing temperature after showing a slight decrease at 70 °C. Different from the results of the dry adhesive 100 shown in Figure 2 , liquid adhesives and pressure-sensitive adhesives generally exhibit an inverse relationship with temperature.

[0025] Figure 3 Shows the shear force of the dry adhesive 100 at various temperatures in the range of 20 °C to 300 °C. Figure 3 The y-axis shown represents the shear force in Newtons per square centimeter, and the x-axis represents the surface temperature of the dry adhesive 100. Figure 3Shows the average of five measured values at each temperature and the standard deviation of these measured values. As Figure 3 shown, the shear force, like the adhesion force in the normal direction, shows an overall increasing trend with increasing temperature.

[0026] In some cases, the increase in the adhesion and shear of the fiber array 100 with increasing temperature is due to the formation of hydrogen bonds with the substrate at the tip 104. It is known that platinum-cured silicone generates hydroxyl groups at elevated temperatures. However, in the absence of contact with a hydrophilic substrate, the hydroxyl groups tend to migrate into the bulk of the silicone. When the fiber 101 contacts a hydrophilic surface (such as glass, silicon, and other surfaces that can form hydrogen bonds), hydroxyl groups are generated and remain at the surface at a higher rate with increasing temperature. The increase in the number of hydroxyl groups increases the number of hydrogen bonds, thereby increasing the adhesion. Once the surface is separated from the silicone microfiber and both the substrate and the microfiber array 100 are cooled to room temperature, the hydroxyl groups disappear and the adhesion returns to a lower value at room temperature.

[0027] Typical pressure-sensitive adhesives (PSAs) are viscoelastic materials, and their tackiness mainly stems from their viscosity properties. As the temperature increases, the viscosity of the PSA decreases, resulting in a decrease in the normal degree, shear degree, and peel adhesion. For example, 3M published the results of a 180-degree peel experiment for one of its high-temperature tapes (3M Transfer Adhesive Tape 9082) as a function of temperature. It was reported that the 180-degree peel at 72°F was approximately 5 pounds per inch and gradually decreased at higher temperatures. The reported 180-degree peel results at higher temperatures were as low as approximately 2 pounds per inch, indicating a significant decrease in adhesion. All four tested 3M high-temperature PSAs showed a similar trend, that is, they exhibited lower peel resistance with increasing temperature.

[0028] Due to temperature-related material degradation and mainly due to the decrease in intermolecular attraction caused by high thermal fluctuations, it is expected that soft materials (such as the soft materials used to construct the dry adhesive 100) will perform poorly at high temperatures. Generally, the stronger the intermolecular attraction between the surface molecules of relatively contacting surfaces, the closer the distance between the molecules. At higher temperatures, the thermal fluctuations of the surface molecules result in a larger average separation distance (compared to absolute zero where the surface molecules are immobile), and thus a weaker bond between the relative surfaces due to the larger average separation. However, the structure of the dry adhesive 100 allows for reliable adhesion at elevated temperatures.

[0029] Figure 4 Shows the effect of the contact time between the dry adhesive 100 and the substrate on the shear force as a function of temperature. Figure 4The y-axis shown represents the shear force in Newtons per square centimeter, and the x-axis represents the surface temperature of the dry adhesive 100, where each column represents the contact time at elevated temperatures in the range from 1 minute to 60 minutes. The data shows that after long-term contact, the rate of change of shear of the dry adhesive 100 increases with increasing temperature. Additionally, for all temperatures tested, the shear increases with contact time. In all test cases, the dry adhesive 100 samples can be removed from the heated substrate without any visible damage to the substrate or the dry adhesive 100, and no visible residue is left on the substrate itself when the dry adhesive 100 returns to ambient temperature. Additionally, the adhesion of the dry adhesive 100 not only varies with temperature but is also reversible. This effect allows the dry adhesive 100 to exhibit high adhesion at high temperatures while still allowing removal without residues. Thus, the dry adhesive 100 can be cooled and returned to its cold-temperature adhesion level, allowing for easy removal of the dry adhesive 100.

[0030] Typical dry adhesives are constructed of soft elastomers. Accordingly, it is expected that the adhesive properties of the dry adhesive exhibit behavior similar to that of soft materials, that is, it is expected that the adhesion of the dry adhesive decreases with increasing temperature, as with typical soft materials, due to the reduced intermolecular attraction resulting from thermal fluctuations. In contrast, the dry adhesive 100 of the present disclosure exhibits robust adhesion even though it is composed of a soft elastomer.

[0031] Figure 5 Shows the shear force in Newtons per square centimeter of the dry adhesive 100 when the temperature is cycled between 35°C and 235°C. Figure 5 Shows the average of five measurements at each temperature and the standard deviation of these measurements. The relative increase in shear at higher temperatures compared to lower temperatures indicates that the dry adhesive is a reusable and reversible adhesive. Such data also shows that the degradation of the material due to exposure to high temperatures or the peel process is small. This observation is confirmed by visual inspection of the dry adhesive 100 before and after undergoing temperature cycling. No broken fibers 101 may be observed, nor any discoloration of the dry adhesive material, nor any visible residue left on the test surface.

[0032] Figure 6Shows the shear force in Newtons per square centimeter of dry adhesive 100 as a function of temperature compared to a silicone-based high-temperature pressure-sensitive tape (Kapton tape), where the shear force of the silicone-based high-temperature pressure-sensitive tape (Kapton tape) decreases from slightly above 8 N / cm2 at 150 °C to approximately 4 N / cm2 at 300 °C. In contrast, the adhesion of dry adhesive 100 is from approximately 4 N / cm2 at 150 °C to approximately 6 N / cm2 at 300 °C. Figure 6 Shows the average of five measurements at each temperature and the standard deviation of these measurements. The measurement results indicate that the Kapton tape loses shear performance with temperature, and after 250 °C, dry adhesive 100 provides a higher shear force.

[0033] The increasing shear force and normal force with temperature can be utilized to minimize the possibility of adhesion loss between the substrate and the carrier at elevated temperatures.

[0034] Dry adhesive 100 provides unique advantages over existing bonding and peeling mechanisms. For example, the dry adhesive 100 of the present invention does not lose adhesion at elevated temperatures, thereby ensuring that the substrate adheres firmly to the carrier. This is contrary to PSA, where elevated temperatures significantly reduce adhesion. Additionally, dry adhesive 100 does not degrade because it is made of a high-temperature-stable silicone. Therefore, even after long-term exposure to high temperatures, dry adhesive 100 can be removed from the substrate without residues, which will increase the processing throughput, eliminate additional cleaning steps, and enable high yields. Furthermore, dry adhesive 100 can be reused multiple times in multiple heating cycles without loss of performance, thereby minimizing the amount of material required to operate the process within extended cycles, saving time, and providing a more sustainable solution than disposable adhesives.

[0035] Although the present invention describes embodiments of a high-temperature-resistant dry adhesive produced using liquid silicone rubber, other embodiments of the present invention may be produced from other resins known to those skilled in the art to be capable of forming different micron-scale and / or nanoscale structures and being capable of withstanding high temperatures. These include, but are not limited to: compression-molded silicone, cast silicone, fluorinated elastomer compounds, perfluorinated elastomer compounds, chlorosulfonated polyethylene rubber, hydrogenated acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer, and polytetrafluoroethylene.

[0036] Features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in the manner of performing the disclosed functions or for obtaining the disclosed results by methods or processes, may be used, where appropriate, alone or in any combination of such features, to implement the invention in different forms. Specifically, one or more features in any one of the embodiments described herein may be combined with one or more features in any other embodiment described herein.

[0037] Protection may also be sought for any feature disclosed in any one or more of the published documents, which are mentioned in a combined manner with the present disclosure and / or incorporated herein by reference in a combined manner with the present disclosure.

Claims

1. A dry adhesive, comprising: Multiple fibers, including a rod portion and a tip provided at a distal end of the fiber; A backing layer, wherein a proximal end of the fiber is adhered to a first surface of the backing layer, Wherein the dry adhesive maintains adhesiveness within a temperature range.

2. The dry adhesive according to claim 1, wherein the temperature range includes a temperature higher than 230 °C.

3. The dry adhesive according to claim 1, wherein the multiple fibers comprise silicone rubber.

4. The dry adhesive according to claim 1, further comprising: Additional fibers provided on a second surface of the backing layer, wherein the second surface is opposite to the first surface.

5. The dry adhesive according to claim 1, wherein the rod portion comprises a first polymer and the tip comprises a second polymer.

6. The dry adhesive according to claim 5, wherein the first polymer comprises high-temperature silicone.

7. A method of bonding a device to a carrier, comprising: Providing a dry adhesive including multiple fibers, the multiple fibers including a rod portion and a tip, wherein the tip is provided at a distal end of the fiber; Bringing the dry adhesive into contact with the device; and Subjecting the device and the dry adhesive to a maximum temperature of at least 225 °C, wherein when subjected to the maximum temperature, the adhesive force between the dry adhesive and the device remains constant or increases.

8. The method according to claim 7, further comprising: Cooling the device and the dry adhesive to a temperature lower than the maximum temperature; And Removing the dry adhesive from the device.

9. The method according to claim 7, wherein the device includes a silicon wafer, a silicon carbide wafer, a semiconductor device, glass, or a computer processor.

10. The method according to claim 7, wherein the dry adhesive and the device are subjected to the maximum temperature for a period of at least 1 minute.

11. The method according to claim 7, wherein the dry adhesive and the device are subjected to the maximum temperature for a period of at least 60 minutes.

12. The method according to claim 7, further comprising cycling the temperature between a minimum temperature and the maximum temperature.

13. The method according to claim 7, wherein the adhesive force is at least 5 N / cm2 at a temperature of 250 °C.

14. A method of bonding a hydrophilic material to a substrate, comprising: Providing a dry adhesive on a surface of the substrate, the dry adhesive comprising: Multiple fibers, including a rod portion and a tip provided at a distal end of the fiber; and A backing layer, wherein a proximal end of the fiber is adhered to a first surface of the backing layer, Fixing the hydrophilic material to the dry adhesive at a first temperature; and Heating the dry adhesive to a second temperature.

15. The method according to claim 14, wherein the second temperature is at least 230 degrees Celsius.

16. The method according to claim 14, further comprising: Cooling the dry adhesive to a temperature lower than the second temperature; And Removing the hydrophilic material from the dry adhesive.

17. The method according to claim 14, wherein hydroxyl groups are present on the tip at the second temperature.

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