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Abstract
We report the transfer printing of GaN-based microscale vertical-type light-emitting diodes (μ-VLEDs) using a functional layer and a biomimetic stamp. An oxide-based functional layer is inserted onto the structure of a μ-VLED and used to separate the chip from the μ-VLED wafer by absorbing the pulse of a UV pulse laser during pick-up of the transfer printing process. Polydimethylsiloxane (PDMS)-based biomimetic stamps have been fabricated to mimic the gecko lizard cilia for improved adhesion and repeatability. The biomimetic stamp has an adhesion force of 25.6 N/cm2, which is 12 times the adhesion of a flat stamp; an adhesion force of 10 N/cm2 or more was maintained after 100,000 repeated adhesion tests. A flexible 10 × 10 prototype array on a polyimide substrate was fabricated, and its bending test results indicated that the strain effect on the forward voltage and the output power was less than 1%. The stable bending test results of the prototype indicate that μ-VLEDs using biomimetic stamps allow the necessary stability for practical transfer printing.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultra-small pixels have become the focus of display research to expand applications of near-to-eye displays such as smart phones, virtual reality devices, and wearable displays [1]. An inorganic light-emitting diode (LED) can provide the light source of the display by considering properties such as brightness, lifetime, and efficiency. In addition, LEDs are suitable for nanoscale-resolution when applied in semiconductor processes [1–3]. However, their assembly processing yield for ultra-small pixel applications encounters the problems of low quality thin film growth, low extraction efficiency, and low thermal stability. Of particular importance is solving the transfer problem that pick-and-place ultra-small pixels from a wafer to a target substrate. The microscale LED (μ-LED) chip is difficult to wafer saw and wire bond directly owing to the small chip size requirements; thus, the μ-LED must be transferred to the target substrate without being packaged. Furthermore, Since the number of chips increases as the chip becomes smaller,the conventional transfer method of moving chips one by one is very inefficient. Several approaches have been evaluated to replace the conventional transfer methods for μ-LED chips. Jiang et al. proposed a transfer method in which the LED matrix array is directly bonded with a complementary metal-oxide semiconductor (CMOS) driver substrate using indium bumps [4]. Rogers et al. (UIUC (USA)) and other groups picked up μ-LED chips from a wafer using the Van der Waals forces of an elastic polymer material to transfer the thin GaN chips onto a flexible substrate [5–11]. Bibl et al. transferred μ-LED chips to a stamp using an electrostatic method [12–14]. Ahn et al. picked up and placed μ-LED chips for an overlay-aligned roll-transfer printing process [15].
We present a transfer method that improves the stability and efficiency of the transfer process by using a biomimetic stamp and vertical-type μ-LED (μ-VLED) with a functional layer. The μ-VLED chips are flat, with the n-contact and p-contact at opposite sides of a chip, and their step-free structure facilitates stable contact with the stamp. A functional layer is used as a sacrificial layer to separate the μ-VLED chip from the wafer when picking up the chip. The functional layer is deposited between an intermediate a sub-substrate (used for inversion of epitaxy for vertical structure) and the p-type GaN layer of the μ-VLED chip. The functional layer bandgap of 3.5 eV absorbs light energy with wavelengths lower than 350 nm depending on the relationship between wavelength and band gap energy. An excimer pulsed laser with a wavelength of 248 nm irradiates the functional layer and fractures it owing to excessive energy absorption, which causes the μ-VLED chip to separate from the wafer. Finally, a polydimethylsiloxane (PDMS)-based stamp is used to move the chips. A stamp printing method capable of transferring multiple chips at a time is commonly used in the transfer of μ-LEDs; however, twisting or falling of chips may occur owing to weak adhesion between the stamp and the LEDs. To solve this problem, biomimetic studies are being carried out to hold chips stable through improved adhesion of the stamp. We used biomimetic stamps designed to mimic the cilia of a gecko lizard’s foot [16,17]. A stamp having a conventional micro ciliary structure has increased contact area via the tilting of the cilium with a high-aspect ratio [18–20]. However, the movement of chips that may occur owing to the tilting of cilia can cause failures during transfer. Therefore, a new structure should be evaluated for this purpose. We propose using biomimetic stamps that improve the adhesion using a sucker disk and column structure that easily removes the air that interferes with the adhesion between the chip and the stamp. Finally, a flexible 10 × 10 prototype array on a polyimide (PI) substrate was fabricated to verify the feasibility of transfer using the biomimetic stamp and μ-VLED combination.
2. Experimentals
First, metal-organic chemical vapor deposition (MOCVD) was used to grow the LED structure (u-GaN/n-GaN/MQWs/p-GaN) on a sapphire substrate. The LED wafer was cleaned using a solution of H2SO4 and H2O2 (2:1). A reflector (Ni 10 Å / Ag 3000 Å) layer and the blocking layer (SiO2) were then formed. The blocking layer not only protects from undercut during the wet-etching process but also protects the reflector layer from damage due to the etching solution. The wafer was annealed for ohmic contacts with p-type GaN and reflector at 500 °C for 1 minute in N2 atmosphere using rapid thermal annealing (RTA). The LED wafer and temporary substrate are eutectic-bonded using Au-based metals. The temporary substrate is composed of two layers. The first is a functional layer and the second is an Au-based bonding metal. The functional layer is oxide-based, which absorbs the light energy from the excimer pulsed laser and is used to separate the chip and substrate. The LED wafer and the temporary substrate were bonded in a vacuum atmosphere at a pressure of 6 kgf/cm2 at 350°C for 30 min. The sapphire grown on GaN was removed using a laser. After removal of the sapphire, the top layer of the sample became a µ-GaN layer. The entire surface of the µ-GaN layer was etched using inductively coupled plasma (ICP) in a Cl2/BCl3 gas atmosphere until the n-GaN was exposed. For the subsequent step, an isolation pattern is needed to define the chips; it was etched using ICP until the bonding metal was exposed. Finally, the exposed metals were removed using wet etchants until the functional layer was exposed.
The biomimetic stamp mimics the foot structure of a gecko lizard. Silicon on Insulator (SOI) wafers were used as master molds for replication. Using Si molds, the mold for the sucker disk of the bump was first fabricated on the SOI wafer. The mask used to form the pattern made from photo-resist using photolithography. The SOI wafer was etched using deep reactive ion etching in a C4F8 and SF6 atmosphere [21]. The SOI wafer was additionally bonded and was formed into a pattern for the column mold by the same method used for the sucker disk pattern. A 10:1 solution of PDMS oligomer and curing agent was poured into the fabricated SOI master mold with a degassing process. The PDMS was then cured in a 70°C oven for 1 h [22]. The manufacture of the biomimetic stamp was finally completed after detaching the SOI master mold.
3. Results and discussions
We report on GaN-based μ-VLEDs of 70 × 70 μm2 area emitting approximately 450 nm with a sapphire substrate. Figure 1 presents schematics and a SEM image of the fabrication process for a μ-VLED. First, the reflector and the blocking layer are fromed. The blocking layer is applied for protecting reflector during a full-isolation process. The LED wafer is bonded with the sub-substrate (sapphire) coated with the functional layer using a bonding metal, and the sapphire wafer is removed through a laser lift-off (LLO) process. The epitaxial layer and bonding metals are then etched until the functional layer is exposed. Details are discussed in section 2. The fabricated μ-VLED wafer and a chip are shown in Fig. 1(f).
Fig. 1 (a) LED wafer grown on sapphire using metal-organic chemical vapor deposition. (b) A blocking layer was formed on the wafer, and reflective films were deposited. (c) The bonding metal was deposited on a sub-substrate coated with the functional layer and the LED wafer material, and both wafers were bonded under pressure and heat. (d) The sapphire substrate of the GaN-grown wafer was removed using the laser lift-off process. (e) The full-isolation line is etched to define the chips, and the functional layer is exposed between the chips. (f) A SEM image of μ-VLED wafer having a vertical structure and a chip.
Figure 2 presents images related to the stamp used in this paper. Gecko lizards have special feet that can adhere well to slippery surfaces, such as glass walls (Fig. 2(a)). We fabricated a biomimetic stamp by exploiting the cilia of the gecko lizard, which produce a high adhesion force. The bump on the biomimetic stamp consists of a sucker disk that acts as a sucker disk mounted on a column (Fig. 2(b)). The diameter of the sucker disk is 15 μm, while the upper diameter of the column (immediately below the sucker disk) is 12 μm, the lower diameter is 14 μm, and its height is 20 μm (Fig. 2(c)). The bump size is designed so that the μ-VLED chips will not tilt when placed, and thus will uniformly make contact with the entire stamp surface. The bumps are octagonal-arrayed to create a formation in which many bumps contact each μ-VLED chip. The protruding columns have the effect of smoothly discharging air bubbles when the chip and stamp are in contact. In addition, sucker disks with a diameter larger than the column diameter increase the contact area and improve adhesion. This structure not only improves the adhesion force but also increases the number of transfer operations because the decrease in adhesion force is low. The upper image of Fig. 2(d) shows the tool used to measure the adhesion force, and Fig. 2(d) describes its use. After samples (2.5 × 2.5 cm2) and GaN templates are attached, a pressure of 10 N/cm2 is applied, and when detaching at 200 mm/min, the necessary force is detected by the sensor. Figure 2(e) shows results of the repeated adhesion/detachment tests of biomimetic and flat stamps performed 100,000 cycles. We used a GaN template for adhesion testing of the biomimetic stamp. The initial adhesion force of the biomimetic stamp was 25.6 N/cm2 compared to a flat stamp of 2.1 N/cm2. From the adhesion force tests based on 100,000 cycles, the residual adhesion force of the biomimetic stamp and the flat stamp was determined to be 10.1 N/cm2 and 1.2 N/cm2, respectively. These results verify that the formation of a structure mimicking the cilia of the foot of a gecko improved the adhesion force by approximately 12 times that of a flat stamp. Because the chips to be picked up are completely separated from the wafer, the normal flat stamp holds the chips with very weak adhesion forces, which can lead to problems during transfer. However, biomimetic stamps hold the chips with strong adhesion forces, resulting in the reliable transfer of chips without movement of the chips during the process. Therefore, the biomimetic stamp has a superior adhesion force to the flat stamp even after 100,000 adhesion tests, ensuring higher reliability for up to 100,000 transfers or more.
Fig. 2 (a) The foot and cilia of a gecko lizard [22]. (b) SEM image of octagonal-arrayed biomimetic bumps. (c) SEM image of biomimetic bumps. (d) Measurement tool and schematic for measuring adhesion force. (e) Adhesion force graph for repeated adhesion/detachment tests (100,000 cycles) of biomimetic stamp and flat stamp.
Figure 2 presents images related to the stamp used in this paper. Gecko lizards have special feet that can adhere well to slippery surfaces, such as glass walls (Fig. 2(a)). We fabricated a biomimetic stamp by exploiting the cilia of the gecko lizard, which produce a high adhesion force. The bump on the biomimetic stamp consists of a sucker disk that acts as a sucker disk mounted on a column (Fig. 2(b)). The diameter of the sucker disk is 15 μm, while the upper diameter of the column (immediately below the sucker disk) is 12 μm, the lower diameter is 14 μm, and its height is 20 μm (Fig. 2(c)). The bump size is designed so that the μ-VLED chips will not tilt when placed, and thus will uniformly make contact with the entire stamp surface. The bumps are octagonal-arrayed to create a formation in which many bumps contact each μ-VLED chip. The protruding columns have the effect of smoothly discharging air bubbles when the chip and stamp are in contact. In addition, sucker disks with a diameter larger than the column diameter increase the contact area and improve adhesion. This structure not only improves the adhesion force but also increases the number of transfer operations because the decrease in adhesion force is low. The upper image of Fig. 2(d) shows the tool used to measure the adhesion force, and Fig. 2(d) describes its use. After samples (2.5 × 2.5 cm2) and GaN templates are attached, a pressure of 10 N/cm2 is applied, and when detaching at 200 mm/min, the necessary force is detected by the sensor. Figure 2(e) shows results of the repeated adhesion/detachment tests of biomimetic and flat stamps performed 100,000 cycles. We used a GaN template for adhesion testing of the biomimetic stamp. The initial adhesion force of the biomimetic stamp was 25.6 N/cm2 compared to a flat stamp of 2.1 N/cm2. From the adhesion force tests based on 100,000 cycles, the residual adhesion force of the biomimetic stamp and the flat stamp was determined to be 10.1 N/cm2 and 1.2 N/cm2, respectively. These results verify that the formation of a structure mimicking the cilia of the foot of a gecko improved the adhesion force by approximately 12 times that of a flat stamp. Because the chips to be picked up are completely separated from the wafer, the normal flat stamp holds the chips with very weak adhesion forces, which can lead to problems during transfer. However, biomimetic stamps hold the chips with strong adhesion forces, resulting in the reliable transfer of chips without movement of the chips during the process. Therefore, the biomimetic stamp has a superior adhesion force to the flat stamp even after 100,000 adhesion tests, ensuring higher reliability for up to 100,000 transfers or more.
Figures 3(a) and 3(b) present the schematic and an optical image of attaching a μ-VLED wafer and biomimetic stamp. The biomimetic stamp with the soft characteristics of PDMS is deformed to accommodate the steps of the chip. Figures 3(c) and 3(d) are enlarged images of the front and back of an attached chip. Note that all the bumps of the biomimetic stamp adhere to the entire area of the LED wafer using the octagonal-array pattern.
Fig. 3 (a) Cross-sectional schematic of the attachment of the biomimetic stamp and μ-VLED wafer. (b) Optical image showing the attachment between the biomimetic stamp and the LED wafer. Images (c) and (d) present enlarged views of the front and back images of a chip, respectively.
After attaching the biomimetic stamp and the μ-VLED wafer, chips are picked up using an excimer pulsed laser. Figure 4(a) shows a schematic diagram that describes the transfer process using an excimer pulsed laser. The laser, which emits 248 nm UV pulses using KrF gas, is used to separate the chip from the wafer by inducing fracturing of the functional layer. Because sufficient power is required to break the functional layer, the excimer pulsed laser must irradiate with high power of 40 W over a pulse area of 75 × 75 um2 emitting pulses at 50 Hz. The pulse emitted from the UV pulse laser passes through the sapphire and reaches the inserted functional layer, which absorbs the energy of the pulse and fractures. The fracture of the functional layer leads to the separation of the chip and the LED wafer. The biomimetic stamp then reacquires its initial shape by releasing the tension caused by the attached LED. Note that the high energy injection of the pulse can actually fracture the chip, but it is mitigated by the buffering action of the bumps. Figure 4(b) shows a SEM image of the μ-VLED wafer after the 10 × 10 array is picked up, and Fig. 4(c) presents a SEM image of the 10 × 10 array on the stamp. Figure 4(d) shows that the functional layer is fractured in the shape of the laser pulse irradiation area, revealing the underlying sapphire. The chips that were not exposed to the pulse remained intact (Fig. 4(e)). Irradiated chips were then picked up and attached to the biomimetic stamp without misalignment (Fig. 4(f)).
Fig. 4 (a) Schematic of the transfer of the μ-VLED wafer to the biomimetic stamp. The inset on the left side shows that the functional layer absorbs the light energy of the excimer pulsed laser, and the right inset shows that the chip is picked up with the biomimetic stamp by the separation of the μ-VLED wafer and chip as the functional layer is fractured. SEM images of (b) the μ-VLED wafer and (c) the biomimetic stamp after chips are picked up from the μ-VLED LED wafer with the biomimetic stamp. (d) SEM image of the portion where the chip was picked up after transfer. (e) The SEM image of the remaining layers on the μ-VLED wafer. (f) SEM image of picked up μ-VLED chip attached to the biomimetic stamp.
The pick-and-place of the μ-VLED chips is illustrated in Fig. 5. The array picked up with the biomimetic stamp was then imprinted on the target substrate. The array was placed on a biomimetic stamp through the following process; first attaching a biomimetic stamp and a μ-VLED wafer, then separating the chips from the wafer using a UV pulse laser and detaching the μ-VLED wafer (Figs. 5(a)-5(c)). The tops of the chips on the biomimetic stamp are exposed Au bonding metal. The PI target substrate was prepared using an e-beam evaporator (Au-In layers (Ti 1000 Å / Au 1000 Å / In 20000 Å / Au 500 Å)). After the Au layer of the μ-VLED array on the stamp and the Au-In layer on the PI target substrate were attached, the μ-VLED array was imprinted under pressure and heat using manufactured tool (Fig. 5(g)). The pressure and heat for printing were as follows. The initial hot-plate temperature was 100 °C at a pressure of 1 kgf/cm2, the hot-plate temperature was then raised to 200 °C and was maintained for 10 minutes, and then the sample was naturally cooled to room temperature. The unloading temperature was 100 °C.”. The indium in the Au-In layers diffused into the Au layer of the chips under pressure and heat, and the chips and the Au-In layers on the PI substrate became eutectic-bonded [23]. After the biomimetic stamp was detached, the μ-VLEDs array was placed on the PI substrate.
Fig. 5 Schematic illustrations and images corresponding to the pick-and-place of the μ-VLED array using the biomimetic stamps: (a) μ-VLED wafer attaches to the biomimetic stamp. (b) Pick-up of the array from the μLED wafer to the biomimetic stamp by UV pulse laser. (c) μ-VLED wafer detaches from the biomimetic stamp. (d) μ-VLED array attaches to the PI substrate. (d) printing the μ-VLED array to the PI substrate. (f) μ-VLED array on the PI substrate. (g) Manufactured tool for printing (upper stage: loading the μ-VLED array (on the biomimetic stamp), x, y stage: placing the PI substrate, microscopy: for alignment of the upper and lower stages, z axis controller: controlling pressure through sensor, hot-plate: for heating.)
For fabrication of the single chip and 10 × 10 μ-VLED array prototypes, the array on the transferred PI substrate was passivated and connected to an n- and p-pad. The structure of the prototype single chip and 10 × 10 μ-VLED array is shown in Fig. 6(a). UV-sensitive imide-based polymer (Su08) was used to form a passivation layer that separated the p- and n-contacts. Then, Cr/Au was deposited as an n-pad metal using an e-beam evaporator. The prototype 10 × 10 μ-VLED array emits uniformly and is highly flexible owing to the PI substrate (Fig. 6(b)). Figures 6(c) and 6(d) show the electrical and optical properties of the prototype single chip and 10 × 10 μ-VLED array. The I-V characteristic indicates that the prototype single chip and 10 × 10 μ-VLED array exhibit similar tendencies. The contact resistance increased slightly owing to the absence of the n-ohmic contact layer, which was not formed in order to achieve high external extraction efficiency. The output power of the prototype single chip and 10 × 10 μ-VLED array also exhibit similar tendencies. Their output power does not increase linearly owing to self-heating caused by the high input current [24,25]. However, there is no problem present for the μ-VLED characteristics due to the low input current. The graph insert in Fig. 6(d) shows that the emitted dominant wavelength is at 445 nm by electroluminescence (EL) measurement. Figure 6(e) shows the electrical and optical properties of the prototype 10 × 10 μ-VLED array during bending tests (3000 cycles) with a bending radius of 2 cm while injecting 100 μA. The forward voltage strain effect on the prototype array was less than 0.1 V and the strain of the output power was measured to be less than 0.4 μW/cm2. As shown by these results, flexible prototype arrays fabricated using biomimetic stamps and μ-VLEDs have been successfully implemented.
Fig. 6 (a) Structure of the prototype single chip and 10 × 10 μ-VLED array. (b) Image of the illuminated flexible prototype 10 × 10 μ-VLED array with bending, and the inset is an enlargement of the illuminated array. (c) Input current density-voltage (I - V) characteristics of the prototype single chip and 10 × 10 μ-VLED array. (d) Output power-input current (Po - I) characteristics of the prototype single chip and 10 × 10 μ-VLED array (inset: EL spectra at 0.1 mA). (e) Electrical and optical properties of the prototype 10 × 10 μ-VLED array during bending tests (3000 cycles) with a bending radius of 2 cm while injecting 100 μA.
4. Summary
We have developed a transfer method that improves the stability and efficiency of the stamp printing process using a biomimetic stamp and μ-VLED with an inserted sacrificial functional layer. The functional layer was fractured by high pulse energy absorption to separate the μ-VLED chip and the sapphire substrate. The biomimetic stamp was fabricated by mimicking the cilia structure of a gecko lizard. The adhesion force of the biomimetic stamp was 25.6 N/cm2, which was 12 times higher than that of a flat stamp (2.1 N/cm2). After transferring 100,000 cycles, the adhesion force of the biomimetic stamp was approximately 10 N/cm2, indicating that the proposed method can be used in the practical application for moving LED chips. A flexible 10 × 10 prototype LED array on a PI substrate was fabricated using the biomimetic stamp and a μ-VLED transfer strategy and verified by bend testing. After the 3000-cycle bending test of the flexible prototype array, the strain effects on the forward voltage was less than 0.1 V and the output power was 0.4 μW/cm2. These results demonstrate the improved stability of the stamp printing method created by the biomimetic stamp and μ-VLED combination.
Funding
The R & D Program of the Ministry of Trade, Industry and Energy (MOTIE), Korea (Grant 10070201) and The International R&D Collaboration of MOTIE, Korea (Grant P019800005).
Acknowledgements
This work was supported in part by the R & D Program of the Ministry of Trade, Industry and Energy (MOTIE), Korea, under Grant number 10070201, the International R&D Collaboration of MOTIE, Korea, under Grant number P019800005, and the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology under Grant number NRF-2018R1D1A1B07051009.
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