Silicon stencil masks for lithography below 0.25 μm by ion-projection exposure

Large-area silicon stencil masks for demagnifying ion-projection lithography have been fabricated and characterized. The masks were made by patterning silicon membranes 120 mm in diameter and 2.5 μm in thickness. The membranes, prepared on 150 mm host wafers using a p/n-junction electrochemical etch stop, were designed with low tensile stress (10 MPa) to obtain minimum pattern distortion. Mask blanks were made by field bonding the membrane-containing host wafers to pyrex rings. The blanks were patterned by e-beam lithography over a 60×60-mm field in a conventional novolak resist. The resist patterns were transferred into the silicon membranes in a magnetically enhanced reactive ion etcher using molecular bromine. The intermediate pattern-transfer layer was low-stress chemical vapor deposition silicon dioxide. Mask openings as small as 0.5 μm were achieved. The sidewalls of the openings were smooth and had positive tapers no more than 3° from vertical. The linewidth uniformity across the mask was <±10%, and linewidth bias was small, nearly constant, and uniform across the mask. Mask pattern distortion measurements indicated a pattern-placement deviation of less than 0.2 μm (3σ).