NANOSTENCIL FABRICATION WITH DOUBLE EXPOSURE OPTICAL LITHOGRAPHY FOR SCALABLE RESIST-FREE PATTERNING OF METAL ON POLYMERS

Engineers require scalable processes for patterning nanoscale features on sensitive substrates to enable widespread manufacturability of advanced nanoelectronics. Nanostencils have shown promise, but prior work has relied on electron-beam (e-beam) and focused ion beam (FIB) processes. Nanostencils also frequently exhibit significant edge roughness. Here, we present a fabrication process for nanostencils using double exposure optical lithography and a novel capillary-driven lamination technique to reduce edge blurring, demonstrating sub-diffraction limit features of ~200nm on poly(methyl methacrylate) films. We demonstrate the utility of these stencils by generating metal patterns for use with the 3ω thermal conductivity measurement technique. We find a thermal conductivity of 0.24 Wm-1K-1 and an anisotropy ratio of 9.7. This work demonstrates that nanostencils can be used for scalable, resistfree patterning of nanoscale features on sensitive substrates. INTRODUCTION Traditional photolithography and patterning often requires exposure to caustic chemicals, high temperatures, and/or plasmas, which can damage non-traditional materials for heterogeneous integration applications, such as gate dielectrics [1], organic materials [2] (e.g. BEOL processes [3]), and 2D materials [4]. In particular, organic materials are commonly used in electronics packaging but are often thermally limiting, leading the research community to seek polymers with increased thermal conductivity. The 3ω method, a common thermal characterization technique, is well suited for studying polymers because it can be used on thin films and can distinguish between thermal conduction in multiple directions [5-6]. Although useful, the 3ω method has been difficult to implement with polymers due to the requirement of metal patterning and the incompatibility of many polymers with standard microfabrication techniques. Nanostencils have shown promise as a method of decoupling damaging fabrication processes from sensitive materials by evaporating metals and other materials through nanoscale apertures in contact with the surface [7-8]. Stencils with sub-micron features are generally fabricated using FIB or e-beam lithography, which are slow and costly, so it is valuable to improve manufacturability. Another key issue for nanostencils is that the gap between the substrate and membrane causes edge blurring. Ingle used a magnetic shadow mask and a magnet to pull the mask closer to the substrate to reduce the size of the penumbra [9]. Sidler et al. reported that compliant membranes showed reduced penumbra due to the ability of the membrane to follow the surface topography, including non-ideal roughness [10]. Here, we describe a scalable fabrication platform for nanostencil devices using double exposure optical lithography. We employ a novel capillary-driven lamination technique to bring the membranes into intimate contact with the substrates. We have used these stencils to fabricate platinum features as small as 200 nm on poly(methyl methacrylate) (PMMA) films without e-beam or FIB patterning, offering the first demonstration of manufacturing sub-diffraction limit stencils with double exposure optical lithography. We have also used the stencils to fabricate test structures for 3ω thermal conductivity measurements. Our measurements of a 170nm thin spin-cast PMMA sample show a through-plane thermal conductivity of 0.24 Wm-1K-1, and nearly an order of magnitude anisotropy. The strong anisotropy, favoring inplane thermal conduction, is due to the alignment of polymer chains during viscous shearing while spin coating. Kurabayashi et al. observed this phenomenon in spin-cast polyimide films [11]. This work extends prior work by Kurabayashi et al. by reporting a higher anisotropy ratio in a thinner film. Due to the high anisotropy ratio, heaters nearly an order of magnitude wider than the film thickness demonstrate significant sensitivity to anisotropy.