Estimation of scattered particle exposure in ion beam aperture array lithography

Abstract

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 ͑AAL͒ 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 250 nm. 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 in the first 75 nm of poly͑methylmethacrylate͒ resist for 30 keV He + ion exposures of 0.6 m thick masks with 45, 80, and 110 nm circular windows on 150, 300, and 400 nm pitches, respectively. They project that would be ͑6.8± 0.8͒ % ͑͒ 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.

Controlled deterministic implantation by nanostencil lithography at the limit of ion-aperture straggling

“…Localized doping of silicon with B, P and As has been achieved down to 50 nm in size and analysis of the implanted regions showed that ion straggling in the substrate was the factor limiting the spatial precision of the process [36] and not the penumbral blurring as in the case of evaporation. Further, 30 keV He ion projection lithography [37] has also been demonstrated and the spatial precision was limited by ion straggle in the nanostencil itself. In this case the forward scattered beam amounted to 7% of the ions reaching the substrate for dense arrays with feature sizes 45-110 nm.…”

Direct Patterning of Conductive Polymer Domains for Photovoltaic Devices

“…For example, ion beams effect the same cross-linking reaction as electronsbut at much lower doses (see Supporting Information). Proximity ion beam lithography systems can provide high throughput (on the order of square meters per hour), − offering a route to low-cost nanopatterning of polymer semiconductors for a variety of applications.…”

Neutral particle lithography: a simple solution to charge-related artefacts in ion beam proximity printing

“…Figure 11 shows how this inclination results in offset exposures on the wafer. We have shown that dense arrays of 45 nm features can be printed with an effective background dose of only 14% of the primary ion dose [36]. This contrast should prove adequate for a robust exposure process.…”

“…The finest features have been fabricated by magnetically enhanced reactive ion etching in a molecular bromine plasma [33]. The particles 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 (figure 10) [35,36]. Since these particles can lose much of their initial energy in the mask, the scattered particle exposure is concentrated near the resist surface.…”

“…Lithographic particles can (a) pass through the stencil openings, (b) be stopped within the thickness of the mask, (c) scatter out into the stencil opening and contribute to the exposure or (d) scatter out into the opening and be recaptured within the mask. Reprinted with permission from[36]. Schematic representation of AAL.…”