Large-scale fabrication of nanopatterned sapphire substrates by annealing of patterned Al thin films by soft UV-nanoimprint lithography
© Cui et al.; licensee Springer. 2013
Received: 4 July 2013
Accepted: 10 October 2013
Published: 11 November 2013
Large-scale nanopatterned sapphire substrates were fabricated by annealing of patterned Al thin films. Patterned Al thin films were obtained by soft UV-nanoimprint lithography and reactive ion etching. The soft mold with 550-nm-wide lines separated by 250-nm space was composed of the toluene-diluted polydimethylsiloxane (PDMS) layer supported by the soft PDMS. Patterned Al thin films were subsequently subjected to dual-stage annealing due to the melting temperature of Al thin films (660°C). The first comprised a low-temperature oxidation anneal at 450°C for 24 h. This was followed by a high-temperature annealing in the range of 1,000°C and 1,200°C for 1 h to induce growth of the underlying sapphire single crystal to consume the oxide layer. The SEM results indicate that the patterns were retained on sapphire substrates after high-temperature annealing at less than 1,200°C. Finally, large-scale nanopatterned sapphire substrates were successfully fabricated by annealing of patterned Al thin films for 24 h at 450°C and 1 h at 1,000°C by soft UV-nanoimprint lithography.
KeywordsAl Annealing Patterned sapphire substrates UV-nanoimprint lithography Reactive ion etching
High output power GaN-based light-emitting diodes (LEDs) attract much attention because of their various applications in traffic signals, full-color displays, backlight in liquid crystal displays, solid-state lighting, and so forth . At present, because of the difficulty of obtaining high-quality and reasonable-cost GaN substrates, sapphire is most commonly used as the substrate for LEDs due to its high-temperature stability and physical robustness. However, owing to the large lattice mismatch and thermal expansion between the epitaxial GaN film and the underneath sapphire substrate, high threading dislocation densities with the order of 109 to 1010 cm−2 and deterioration of the electrical and optical properties, therefore, lead to poorer internal quantum efficiency (ηint) and reliability [2, 3]. On the other hand, the refractive index of nitride films (n = 2.5) is higher than that of sapphire substrates (n = 1.78) and air (n = 1). The critical angle of the escape cone is about 23°, which indicates that only about 4 % of the generated light in the active layer can be extracted from the surface and mostly absorbed by the electrode at each reflection and gradually disappears due to total internal reflection, and is then converted to heat .
Many different growth approaches have been proposed to improve the performances of epitaxial GaN films; the epitaxial lateral overgrowth (ELOG) technique is known to significantly reduce threading dislocations effectively [5, 6]. However, this approach is a time-consuming process and often requires a two-step growth procedure and introduces uninterrupted dopants or contaminations. Recently, it has been reported that one can not only reduce the threading dislocation density in GaN films but also enhance the light extraction efficiency by using a patterned sapphire substrate (PSS) [7, 8]. However, the dimension of PSS with grooves or other patterns is usually in micron-scale range. Theoretical and experimental studies indicate that a further reduction in defect density is possible if the dimension of the lateral overgrowth patterns is extended to nanoscale range [9–11].
Many articles reported that sapphire substrates are nanopatterned by dry etching and wet etching. It is known that sapphire is chemically inert and highly resistive to acids at room temperature. Thus, it is extremely difficult to etch sapphire substrates using a chemical solution at room temperature. Compared with wet etching, dry etching can provide us an anisotropic profile and a reasonably fast etching rate , but dry-etched substrates will be inevitably damaged, and the device performance is compromised . To resolve the problem in dry and wet etching processes, Cui et al.  have reported the effect of exposure parameters and annealing on the structure and morphological properties of nanopatterned sapphire substrates prepared by solid-state reaction and e-beam lithography. However, e-beam lithography is not a cost-effective solution due to expensive equipment and low efficiency for the fabrication of large-area patterns. UV-nanoimprint lithography (UV-NIL) has been gaining attention in the semiconductor industry as one of the candidates for the next-generation manufacturing technology of low cost, wide distribution, and high patterning resolution [15, 16]. Moreover, UV-NIL using soft polydimethylsiloxane (PDMS) mold has advantages over conventional methods for patterning of imprinted area, surface roughness, and curvature of substrate . Therefore, in this study, large-scale nanopatterned sapphire substrates (NPSS) were fabricated by dual-stage annealing of patterned Al thin films prepared by soft UV-NIL and reactive ion etching (RIE).
High-purity Al thin films were deposited on sapphire (0001) substrates by direct current (DC) sputtering in a JGP-450a magnetron sputtering system. Prior to deposition, the sapphire substrates were ultrasonically cleaned with acetone for 10 min and alcohol for another 10 min, rinsed with deionized water, and then dried withN2. A 99.999 % pure Al target of 2-in. diameter was used, and the plasma of Ar (99.999 %) was used for sputtering. The distance between the target and substrate was 70 mm. The base pressure was less than 8 × 10−5 Pa. Deposition was carried out at a working pressure of 0.2 Pa after presputtering with Ar for 10 min. When the chamber pressure was stabilized, the DC generator was set to 60 W. The deposition rate utilized was 18 nm/min.
After the deposition of Al thin films, the 220-nm-thick UV-curable resin AMONIL-MMS4 (AMO GmbH, Aachen, Germany) was spin-coated at a speed of 3,000 rpm for 30 s onto 150-nm-thick Al thin films. At 100°C, the AMONIL-MMS4 was prebaked on a hot plate. The UV-NIL was performed on an EVG620 (EVG Group, Schärding, Austria). The nanoimprint pressure is 3 × 104 Pa, and the hold time of UV exposure is 90 s. The residual polymer layer was then removed by RIE (CRIE-100, AST, Hsinchu County, Taiwan). The O2 gas flow rate, working pressure, radio-frequency (RF) power, DC bias voltage, and etch time were maintained at 200 sccm, 13 Pa, 50 W, −200 V, and 120 s, respectively. The patterns were subsequently transferred into Al thin films by RIE. The BCl3 and Cl2 gas flow rates, working pressure, RF power, DC bias voltage, and etch time were maintained at 100 and 25 sccm, 1 Pa, 600 W, −200 V, and 90 s, respectively.
The nanopatterned Al thin films were subsequently subjected to dual-stage annealing. Our experimental results reveal that the hillock formation on Al thin films was minimized with an oxidation anneal at 450°C . Therefore, the first comprised an oxidation anneal, where the annealing temperature was 450°C for 24 h. The temperature ramp rate was 10°C/min. This was followed by a high-temperature annealing in the range of 1,000°C to 1,200°C for 1 h. The temperature ramp rate was 10°C/min up to 800°C and then 5°C/min thereafter. All annealing treatments were carried out in air in a box furnace with the substrates contained in a high-purity alumina crucible. In this study, the surface morphology was examined using an atomic force microscope (AFM; Veeco DID3100, Plainview, NY, USA) and scanning electron microscope (SEM; Hitachi S-4700, Tokyo, Japan).
Results and discussion
Therefore, it is believed that the above process has potential for the large-scale fabrication of NPSS for high output power GaN-based light-emitting diodes.
In this study, large-scale NPSS were fabricated by dual-stage annealing of patterned Al thin films prepared by soft UV-NIL and RIE. The soft mold with 550-nm-wide lines separated by 250-nm space was composed of the toluene-diluted PDMS layer supported by the soft PDMS. The nanoimprint pressure is 3 × 104 Pa, and the hold time of UV exposure is 90 s. Patterned Al thin films were subsequently subjected to dual-stage annealing. The first comprised a low-temperature oxidation anneal, where the annealing temperature was 450°C for 24 h. This was followed by a high-temperature annealing in the range of 1, 000°C to 1,200°C for 1 h to induce growth of the underlying sapphire single crystal to consume the oxide layer. The SEM results indicate that the patterns were retained on sapphire substrates after high-temperature annealing at less than 1,200°C. Finally, large-scale nanopatterned sapphire substrates were successfully fabricated by annealing of patterned Al thin films for 24 h at 450°C and 1 h at 1,000°C by soft UV-nanoimprint lithography. It is believed that the above process has potential for the large-scale fabrication of NPSS for high output power GaN-based light-emitting diodes.
This project was supported by the National Natural Science Foundation of China (grant no.50902028), the Natural Science Foundation of Guangdong Province (grant no. 9451805707003351), the Weapon & Equipment Pre-research Foundation of General Armament Department (grant no. 9140A12050213HT01175), the Basic Research Plan Program of Shenzhen City in 2012 (grant no. JCYJ20120613134210982), and the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (grant no. HIT.NSFIR.2011123).
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