Photomask plasma etching: A review

Photomask plasma etching was thoroughly reviewed over wide topics including history, equipment, etchant, absorbers, phase shifters, thermodynamics, and kinetics. Plasma etch obtained industrial applications for photomask fabrication in the 1990s and presently is a critical fabrication step for the “enabling” photomask industry. Among all types of photomasks (binary, embedded attenuated phase-shift mask, and alternating aperture phase-shift mask), chromium (Cr) containing material etching has been the basis and fundamental for all photomask etches. The main technological challenges for Cr etch occur on (isolated) dark features of a high load photomask due to the etch critical dimension (CD) bias dependence on the local loading. It determines the CD features on the Cr layer, phase shifter MoSi layer, and fused silica (quartz) layer. The CD deviation on pattern layers from the nominal value has been a challenge, especially for the Cr state-of-the-art 65 nm node photomasks even though data sizing exists. Inductively coupled plasma plus bias power using radio frequency wavelength is the dominant configuration of the photomask plasma etcher, with improved loading and CD mean-to-target potential. Thermodynamic applications (Gibbs energy minimization method) on photomask plasma etch provide a quick, easy, and low cost method to estimate the plasma etch feasibility and defect reduction at different plasma gas input conditions. Empirical relationships between operational parameters and etch properties significantly improve the only design of experiment procedure for etch process optimization.