A direct positive patterning of PPA layers is possible by electron bombardment. Similar to the irradiation of normally used e-beam resists like e.g. CSAR 62 or PMMA, the electron beam causes a fragmentation of the polymer chains.
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.
Among the first resists used (since about 1980) and still applied in many cases are short-chain (50.000 g/mol (50k)) as well as long-chain (950.000 g/mol (950k)) poly(methyl methacrylate) (PMMA resist, sensitivity at 100 keV approximately 250 – 500 µC/cm2).
Significant components of electron beam lithography systems are the electron source, the electro-optical system and the focusing system (deflection or respectively projection unit). Hot cathodes are used for equipment with lower resolution, devices with higher resolution, however, preferably need thermic field emission sources.
Alongside the occuring elastic scattering effects at a collision with other electrons or atoms, incoming primary electrons can also be scattered inelastically when entering or traversing resist layers.
There are both mask-based and maskless writing procedures in electron beam lithography. More up-to-date systems use defined beams with, by the application of masks, selectively adjusted geometrical cross sections or respectively profiles, which are deflected upon the various positions (vector scan mode).
A further variant for the structure of three-layer systems uses 950k PMMA as bottom resist, AR-P 617 as middle layer and CSAR 62 as top resist. There is no mixing during the coating, thus ensuring a defined layer buildup. AR-P 617 can be coated on all PMMA’s (50k – 950k) without any problems, besides, the coating with AR-P 6200 (CSAR 62) is successful as well since anisole does not dissolve AR-P 617.
T-gate structures are often required for the fabrication of electronic components (MEMS, HEMTs). Corresponding nanostructures can be realized via e-beam lithography in multi-layer processes. Generally, resist layers with different sensitivities like e.g. PMMAs with varying molecular weight distributions are coated on top of each other, irradiated with electrons and then developed in one step.
Polymer resists (layer builders)
Other resist components (adhesion promoter, tenside, solvent, colorant)
Process information such as: Cleaning of substrates, adhesive strength, dilution of resists, yellow light, softbake, rehydration, exposure and storage
Process procedures such as: Lift-off procedures, wet-chemical etching, dry-chemical etching, UV-curing, lithographic procedures and stabilization/curing of resist layers
See Photo resists: General
Electron beam lithography is a special procedure for the structuring of electron beam sensitive resist layers in order to produce microelectronic circuits and photomasks used in photolithography.