PMMA is a high resolution positive tone photoresist. It costs approximately $0.75 per milliliter and remains stable for many years.
HSQ is a high resolution (sub 10 nm) negative tone resist. It costs approximately $5.25 per milliliter and has a short shelf-life of 6 months. The maximum thickness for HSQ is approximately 180 nm.
PHOST is a low resolution negative tone resist. PHOST resist is not sold commercially, but can be prepared in the lab.