Build-up and validation of a low-cost benchtop photolithography tool

Images of the solid-state photolithography tool

The central equipment piece for the lithography fabrication is an ultraviolet (UV) light source. Conventional UV-light sources are based on high-pressure mercury lamps. These lamps are requiring considerable maintenance but therefore provide a broad-band illumination with intensities that often changes in their lifetime. In this article we present a benchtop photolithography tool based on ninety-eight 375 nm light emitting diodes (LEDs).

In our model we choose a parallel series connection of three LEDs in series, together with a 47 Ω protective resistor. One series connection consists only of 2 LEDs and a 220 Ω protective resistor. Because of the arrangement of the LEDs on the circuit board a diffusor glass was used to homogenize the light intensity. For the production of the structures two different SU-8 photoresists were used.

The self-build light source was used to fabricate micro patterns with a thickness from ~10 μm to ~60 μm and a width of ~20 μm to ~200 μm. An exposure time of 20 s for a working distance of 1.5 cm between LED-head and wafer, produced patterns of good quality. In the experiments the effect that stripes can be better developed with an increasing distance between the stripes and a higher ratio of the width of stripes and the interspace between the stripes was observed.

The proposed light source is built with a total cost below 900€, requires a minimal amount of power (below 1% of the power a mercury lamp needs), is expected to last for more than 90 000 exposures (which is 50-100 times longer than a conventional mercury lamp), and is nearly maintenance-free. Therefore, this device is appealing for educational institutions which want to provide the possibility to students to work with photolithography, microfluidic teaching laboratories and microfluidic research and development laboratories but can ́t afford the expensive industrial solutions.

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