These systems have a bottom resist that is not photosensitive but soluble in alkali. The polymers must be composed in such a way that the lacquer coat withstands a coating with a photoresist with the solvent PMA (PGMEA) without any problems. Otherwise there would be a mixing of both resist layers.
Spray coating is often used for the coating of complex topologies. There are various manufacturers of spray coating equipment, probably the best known are EV Group and Süss Microtec. Both producers use different strategies and devices for spraying, and therefore ready-to-use spray resists must be adjusted for the particular equipment.
The Resist Wiki article “Positive two-layer lift-off systems” describes the principle of these two-layer systems. During the application of lift-off structures, temperatures above 150 °C occur frequently, for example during sputtering. If the usual photo resists are used as top layer, the lift-off structure melts and thus becomes useless for the process.
Roland and Coopmans intensively studied the fundamentals of the top surface imaging technology which is based on a selective resist silylation process (DESIRE process). A positive photoresist specifically optimised for this purpose with considerably increased content of light-sensitive components (LSCs) is exposed image-wise.
In many cases, substrates meant to be structured may not be heated above 50 – 70 °C. These can be glass partitions which distort at higher temperatures and thus might lose their size accuracy. However, some organic polymers which are to be coated are temperature sensitive. Moreover, thermosensitive structures may already exist on the substrates.
Bottom resist AR-BR 5460 has already been used for a decade in combination with positive (e.g. AR-P 3510) or negative resists (e.g. AR-N 4340) for a large variety of lift-off applications.
Many applications demand coatings with excellent thermal stability. Structures of the temperature-stable negative resist SX AR-N 4340/6 are able to withstand temperatures of up to 350 °C with high shape accuracy
In addition to a structuring with photomasks which is frequently used in lithography, is it also possible to write the structures directly, i.e. without masks, with laser beams.
Resist AR-P 5900/4 is already well established on the market for many years. Structures of this positive resist are characterized by a considerably higher alkaline stability as compared to standard positive resists (e.g. AR-P 3510 or AZ resists).
In the CiS Institute for Microsensors, deep silicon etch grooves are structured for the fabrication of customer-specific components. For this purpose, a coating procedure via spray coating was developed.