Soft photomask lithography and droplet spreading nano-imprinting for manufacturing patterned sapphire substrates

This paper proposes an innovative method for fabricating soft photomasks which can be used for contact type of photolithography. It utilizes a droplet spreading nano-imprinting technology to imprint a droplet of carbon-black photoresist (PR) on top of a glass substrate with a soft polydimethylsiloxane (PDMS) mold which contains patterned surface micro-cavities. By proper surface treatments on both surfaces of the PDMS mold and the glass substrate, it is possible to reversely fill the carbon-black resist into the surface micro-cavities of the mold and therefore result in a soft PDMS photomask. The experimental investigation is carried out with the application on photolithographic patterning on a 4″ sapphire substrate to achieve hexagonally arrayed PR micro-pillars with a feature size of around 1 to 2 μm. The soft PDMS photomask embedded with carbon-black resist is successfully produced. Excellent photolithography patterning results are also achieved on the 4″ sapphire wafer, which demonstrates the potential for manufacturing patterned sapphire substrates in the light-emitting-diode (LED) industry. Further developments and other possible applications of the proposed soft photomask lithography will be addressed.