Low-stress stencil masks using a doping method

Membrane stress control is one of the challenges for the commercial success of the stencil masks, such as electron projection lithography (EPL) and low energy electron-beam proximity projection lithography (LEEPL), since a stencil mask has perforation patterns in a membrane with image placement meeting stringent error budget. First, stress-induced distortions of stencil mask membranes were simulated by a finite element method (FEM). Second, we showed how the membrane stress varies with dopant concentration, using a pressure bulge method for stress measurements of die-size specimens. The results show doping SOI substrates provide a low-stress membrane. Third, correlation between the pressure bulge method and resonance frequency technique (RFT) was investigated and showed acceptable agreement. Fourth, stress distribution measurements were taken using, the RFT for a low-stress 200-mm EPL mask. Average stress value and cross-mask stress variations were 11.2 MPa and +/- 1.3 MPa respectively. Therefore, we revealed reliable stress distribution data across a 200-mm EPL mask and confirmed the doping method using SOI substrate is proper approach to fabricate a low-stress 200-mm stencil mask, with high uniformity of membrane stress, for EPL and LEEPL.