Uv-led Lithography for 3-d High Aspect Ratio Microstructure Patterning

This paper presents a UV lithography method that utilizes a UV-LED (ultraviolet light-emitting diode) array as a micropatterning light source for high aspect ratio polymer microstructure fabrication. The sidewalls of commercial 5-mm-diamater UV-LEDs were coated with an opaque polymer to suppress the side propagation of UV light and to enhance UV collimation, resulting in a maximum flare angle of 15°. The UV-LEDs were then assembled into a 10x10 array to form the light source. Although the flare angle of most conventional UV lithography tools is 2~3°, resulting in a slightly better light collimation quality, the polymer structures microfabricated with the UV-LED approach showed similar reproduction fidelity and similar exposure times to patterns fabricated using conventional lithography. The UV-LED approach has the potential for cost reduction as well as simplicity of initial system setup and management. This latter feature is of particular importance in advanced, inclined three-dimensional (3-D) lithography schemes to create complex 3-D structures. The UV-LED system has been demonstrated to perform satisfactorily in these advanced 3-D lithography schemes. INTRODUCTION UV lithography systems based on mercury lamp sources have traditionally been used in micro/nano patterning [1]. Modifications of these basic systems, such as deep UV lithography [2] and addition of UV filters [3], can further enhance fine resolution patterning. In MEMS, mercury-lamp-based UV lithography has been further extended to encompass unconventional methods including thermal reflow processes [4], multidirectional UV lithography [5], diffuser lithography [6], and timed-development and-thermal-reflow process [7]. Recent advances in light emitting diode technology have resulted in LEDs that emit in the near ultraviolet. Such UV-LEDs have been used not only in conventional applications such as counterfeit detection (bills, credit cards, documents, etc.) and sterilization, but also as a microfabrication tool. Examples include a single UV-LED for direct write lithography [8], and an array of LEDs in which the spatially nonuniform emission of the LEDs was exploited for microlens array fabrication [9]. Since the UV-LED has the advantages of simple operation and relatively low cost, yet produces high quality near-monochromatic light, it shows great potential for use in microfabrication. Table 1 compares the performance of UV-LEDs with a mercury lamp for several key parameters of interest for UV light source designers. It can be seen that the UV-LED shows performance very comparable to the conventional light source. In this study, the efficacy of a 100-element array of modified UV-LEDs as a light source for conventional and advanced (3-D) UV lithography is assessed. The modification consisted of suppressing the high flare angle light of each UV-LED by means of an opaque blocking layer. The intensity distribution and attenuation as a function of distance of the array are measured. The performance of the UV-LED source is also compared to the performance of a conventional mercury-lamp-based lithography system. SYSTEM SET-UP AND TEST A 5mm through hole LED (RL5-UV0315-380, Superbrightleds, Inc) is adopted for the UV-LED exposure system. This ultraviolet LED has a single wavelength peak centered at 380 nm with a maximum radiant power of 30 mW. It has a clear epoxy lens situated on top of a transparent cylindrical package, resulting in a viewing angle of 15 degrees. In order to suppress the significant light leakage through the sidewall of the cylindrical package, the sidewall was coated with an opaque epoxy band to block any UV light emitted through the sidewall. Figure 1 shows a notional ray trace of light emitted from the LED and traversing to a photoresist-coated mask substrate. The photoresist (e.g., SU-8) in Figure 1 is illuminated using backside exposure (although note that the UV-LED technique is not restricted to backside exposure). After the UV light emerges from the LED lens, it has a 15° viewing angle, which is equivalent to a 7.5° half-flare angle incident upon the glass photomask. Application of Snell‟s law results in a final refracted angle at the photoresist layer Table1: Comparison between UV-LED[10] and Mercury lamp[11]. UV LED (100 LEDs, RL5-UV0315-380) Mercury lamp (6286 Mercury Arc Lamp) Lifetime Approximately 2,000 hours @ 30mA / 23°C Approximately 1,000 hours @ 6A Radiation Power 8 mW/cm 2 @ 12W (30 mW/LED ) ~20 mW/cm 2 @ 350W Wavelength characteristics Single peak (380 nm) Multiple peaks (365 nm, 405 nm, 436 nm) Ancillary systems (power supply, mirrors, etc.) Simple Complex Figure 1: Schematic diagram of UV-LED lithography. 9780964002494/HH2012/$25©2012TRF 481 Solid-State Sensors, Actuators, and Microsystems Workshop Hilton Head Island, South Carolina, June 3-7, 2012 of 4.43° where the refractive indices of air, glass, and SU-8 are assumed as 1, 1.3, and 1.69 respectively [5]. Since the SU-8 exhibits generally a negatively sloped sidewall (undercut) in top exposure and vice versa in backside exposure caused by diffraction and attenuation, the 4.43° of the refracted angle enhance the sidewall profile. Figure 2 shows the profile of the emitted light from a single UV-LED with and without coating the sidewall with opaque material (left), as well as typical SU-8 exposure and development results from each LED (right). The emitted light from a single LED is mostly propagated in the forward direction through the top mounted clear lens. However, there is a considerable amount of leakage light as shown in Figure 2(a). Leakage light results in non-uniform sidewalls and multiple patterning of the fabricated microstructures as shown in Figure 2(b). Figure 2(c) shows the UV-LED with opaque sidewalls and suppressed leakage. As illustrated in Figure 2(d), the corresponding fabricated microstructures exhibit straight sidewalls. To expose larger areas, a 10x10 element LED array is constructed. This array is realized on a custom printed circuit board (PCB) where the 30 μm thick double sided copper layers were