AFFORDABLE PHOTOLITHOGRAPHY

Despite the wide range of applications of photolithography, from integrated circuits (IC) to microelectromechanical systems (MEMS), the high cost of such a tool limits its accessibility. This study describes the design, construction, characterization, and optimization of an affordable photolithography system for the Nanotechnology Lab at Wentworth Institute of Technology. Starting from scratch, an entire process of photolithography from chemical deposition to exposure and development was developed for under $4000. The light source, an array of LEDs with a wavelength of 400 nm, was determined to produce a circular beam with maximum diameter of ~5 cm and a maximum output power of approximately 25 mW/cm2. The characterized source was then used to expose a ~30 µm layer of negative photoresist SU-8 on a silicon wafer for 7 minutes, which corresponds to a dosage of 10 J/cm2, to fabricate a set of inertial focusing devices. The created structures fell within 5% error in comparison to the mask. Further characterization of the source showed that the output optical power decayed over time, resulting in an inaccurate dose if the control relies solely on exposure time. To compensate for decay and other unexpected voltage fluctuations, a feedback control, a feature unique to our design, was implemented to regulate the delivered dose to the photoresist. This was achieved by monitoring the dose in real time using a photodiode, processing the information using an Arduino microcontroller, and powering off the light source when the desired dose is reached. A preliminary test revealed that the feedback control is capable of reducing the discrepancy between the desired and executed dose from 10% to 5%. The design presented above provides a critical addition to the Nanotechnology Lab at Wentworth Institute of Technology, which allows the Wentworth community and beyond the access to the standard nano-/micro-fabrication techniques and therefore opens up excellent opportunities of new course development and undergraduate research.