Robust shadow-mask evaporation via lithographically controlled undercut

Abstract

Fabrication of nanometer size photoresist wire patterns with a silver nanocrystal shadowmask Suspended shadow-mask evaporation is a simple, robust technique for fabricating Josephson-junction structures using scanning electron-beam lithography. The basic process entails the fabrication of an undercut structure in a resist bilayer to form a suspended "bridge," followed by two angle evaporations of superconducting material with a brief oxidation step in between. The result is two overlapping wires separated by a thin layer of oxide. Josephson junctions with sub-50-nm diameters are of particular interest in quantum computing research. Unfortunately, standard shadow-mask fabrication techniques are highly variable at linewidths below 100 nm, due to the difficulty of simultaneously fabricating a narrow line and a large undercut region. While most previous processes used poly͑methylmethacrylate͒ ͑PMMA͒ for the top ͑imaging͒layer and either lower-molecular-weight PMMA or a PMMA/methacrylic acid copolymer for the bottom ͑support͒ layer, the authors' process uses a PMMA/poly͑methylglutarimide͒ ͑PMGI͒ bilayer. The advantage of using PMGI as the support layer is that it develops in aqueous base solutions, while PMMA is insensitive to aqueous solutions and only develops in certain organic solvents. This allows the two layers to be developed independently, ensuring that the imaging layer is not biased during the development of the support layer and allowing the process to achieve the full resolution of the PMMA imaging layer, which can be extremely high. Additionally, the extent of the undercut in the support layer can be precisely controlled by defining it lithographically, rather than simply varying the PMGI development time as in previous processes. Although PMGI is sold as a "liftoff resist" and widely assumed to be electron insensitive, their experiments have shown that this is not the case. Instead, when dilute developer and low electron doses are used, PMGI behaves very much like a conventional photoresist. By exploiting this behavior, as well as its high electron sensitivity with respect to PMMA, the authors were able to define undercuts by defining low-dose regions adjacent to their features, exposing the underlying PMGI separately. In this manner, it is possible to create well-controlled undercut regions as large as 600 nm. Extensive modeling of both the exposure and development processes was used to verify their results. By using a Monte Carlo simulation of electron scattering to simulate the electron exposure and mass-transfer relationships to simulate the process of developing the undercut region, the authors were able to produce a model that closely matches experimental results. With the process fully characterized, it is possible to produce nearly any linewidth/undercut combination, limited only by PMMA resolution and the mechanical stability of large overhang structures. This robustness, combined with the high resolution of the PMMA imaging layer, will allow the reliable fabrication of many interesting devices ...

Limiting factors in sub-10nm scanning-electron-beam lithography

“…Unlike PMMA and most other resists, HSQ has a highly nonlinear development rate, as demonstrated by Yang et al 14 The reason for this self-limiting behavior is unknown, but we hypothesize that it is due to a combination of negative charge buildup on the resist surface during development and/or cross-linked material in the developing resist. When combined with the fact that mass transfer issues are known to slow down development rates for small, deep features ͑such as the gaps in our dense HSQ gratings͒ by limiting the flow of developer into the reaction site and reaction product out of the reaction site, 15 it seems plausible that, in gaps below a certain threshold width, undeveloped HSQ cannot dissolve quickly enough to create a gap visible to SEM/TEM imaging before the reaction self-limits. If this is the case, the final resolution of HSQ-based lithography is limited by the resist itself, not the exposure tool.…”

High resolution electron beam lithography of PMGI using solvent developers

“…While it is generally believed that PMGI will not be dissolved by solvent developers [3,5], in this paper we will show that, at higher dose (>500 lC/cm 2 for 30 kV), the common solvent developers for PMMA can actually be used to develop PMGI with a performance similar to that of PMMA. PMGI displays several advantages over PMMA: resistant to general solvents, less susceptible to intermix with other polymers or resists when stacked [2], slightly more resistant to acids, slower dry etch rate (by 30% in CHF 3 plasma), and higher thermal stability with a glass transition temperature of 189°C (vs. 105°C for PMMA).…”

“…PMGI (polydimethyl glutarimide) has also shown to be sensitive to e-beam exposure [2,3] and 150 nm wide lines have been achieved by EBL using (low concentration) aqueous base developer such as TMAH. PMGI is more sensitive than PMMA but with a much lower contrast.…”

Three-dimensional nanofabrication using hydrogen silsesquioxane/poly(methylmethacrylate) bilayer resists

“…10,11 PMMA and poly(methylglutarimide) (PMGI) bilayer have also been used to pattern controlled 3D undercut structures. 12 However, these methods had restrictions on the final structures because all structures must be derived from only undercut structures.…”

“…10,11,12 Using a hydrogen silsesquioxane (HSQ) and poly(methylmethacrylate) (PMMA) bilayer resist stack, 3D nanostructures have been fabricated by patterning the top HSQ layer and reactive-ion etching of the bottom PMMA layer. 10,11 PMMA and poly(methylglutarimide) (PMGI) bilayer have also been used to pattern controlled 3D undercut structures.…”