Swelling behaviour of poly(methylmeth­acrylate) thick resist layers in deep X-ray lithography

The understanding and optimization of lithographic conditions are particularly important for irradiation of thick resist layers in deep X-ray lithography. The causes and consequences of radiation-induced formation and accumulation of gaseous species inside the poly(methylmethacrylate) (PMMA) resist matrix are discussed. Thick PMMA layers were exposed to high-energy photons from a bending-magnet beamline at the Center for Advanced Microstructures and Devices (CAMD). The focus of this work was to explore the swelling behaviour of PMMA resist of thickness between 50 µm and 1.6 mm. The influence of various exposure parameters, such as photon energy, filtering conditions, electron current in the storage ring, scan length and scan speed, exposure atmosphere etc., on the swelling of the resist are discussed. The effect of pre-exposure annealing of the PMMA sheets and of temperature variation during irradiation was also investigated. Exposure with optimized X-ray spectral distribution, incident power density, absorbed dose and rate of energy deposition inside the resist allowed swelling of the resist to be minimized during exposure.