Thermal emissivity of carbon coated p-doped silicon stencil masks for ion projection lithography

The determination and optimization of the thermal emissivity is an essential topic for ion projection lithography (IPL), one of the most promising candidates for the next generation lithography. In order to determine the thermal emissivity of stencil masks, infrared transmittance and reflectance spectra have been measured in the spectral region 100–6000 cm−1. The infrared absorption could be mainly described by a simple Drude model. The high quality of the silicon masks became evident in a Fano resonance of the optical phonons with the intervalence-band absorption and the free-carrier continuum. The obtained values of 0.5 to 0.6 for the emissivities have been used to model the thermomechanical response of the IPL exposure station. Radiation cooling is sufficient to cool the stencil mask to room temperature at the desired ion fluxes which cause heat generation rates of 8.5–10 mW cm−2. At high irradiation intensities, the membrane distortions have to be minimized instead of optimizing the uniformity of the temperature distribution. For 7 mW cm−2, temperature variations of 3.5 K are needed to keep mask distortions below 10 nm.