Estimation of scattered particle exposure in ion beam aperture array lithography

This is the authors' abstract. We don't add key points for this paper.

In ion beam proximity lithography, ions that are incident on the nominally opaque regions of a stencil mask can scatter into the open windows and escape, exposing a wide area of the substrate. Since these ions can lose much of their initial energy in the mask, the scattered particle exposure is concentrated near the resist surface. The resulting loss of contrast can be mitigated to some extent by using aperture array lithography (AAL) where a mask of reduced density minimizes the number of windows from which a scattered ion can escape. Even so, the problem worsens as the pitch of an array, printed by multiple, offset exposures of the AAL mask, shrinks below about 250nm. The only solution is to increase the mask thickness, hence the window aspect ratio, to reduce the escape angles of the scattered particles. In this article, the authors characterize an effective background dose η in the first 75nm of poly(methylmethacrylate) resist for 30keV He+ ion exposures of 0.6μm thick masks with 45, 80, and 110nm circular windows on 150, 300, and 400nm pitches, respectively. They project that η would be (6.8±0.8)%(σ) of the primary ion dose for the printing of dense arrays, with period equal to twice the window diameter, over this range of feature sizes.