High-resolution etching of nanophotonic cavities using a chromium hard mask

Optical microcavities can greatly enhance light-matter interactions and have a wide range of applications such as lasers, cavity quantum electrodynamics, and nonlinear photonics [1, 2, 3]. The enhancement of the decay rate of an emitter in the cavity can be quantified by the Purcell factor and is proportional to the quality factor, Q, divided by the mode volume, V. Conventional nanophotonic cavities have mode volumes greater than the diffraction-limit, (λ/2/n)³, and large device footprints. However, recent theoretical works have shown that dielectric discontinuities can be exploited to achieve mode volumes below the diffraction limit [1]. Recently, a powerful technique was demonstrated for designing dielectric bowtie nanocavities with a high Q/V-ratio and a small device footprint using topology optimization (TO) [4]. The deep sub-wavelength mode volume in these nanocavities depends on the critical dimension of the bowtie, thus limited by the fabrication constraints [5].

In this study, we demonstrate high-resolution etching of topology-optimized bowtie nanocavity (Fig 1b) with critical dimensions of 10 nm. Nanocavities were etched using 65 nm CSAR resist as a soft mask and 30 nm chromium as a hard mask. Thin resists are used to achieve high-resolution features with electron-beam writing [6]. However, using thin resists as a soft mask (Fig 1a) has the disadvantage of low selectivity during plasma etching [6]. We etch the nanocavities using the recently introduced fluorocarbon-free CORE (Clear-Oxidize-Remove-Etch) process which uses a sequence of SF6 and O2 plasma to directionally etch silicon at the nanoscale [7]. Thin soft masks require long cycles to etch due to their limited selectivity, which results in sidewall roughness and causing over-etching of small features (Figs 1c-e). Soft masks can also cause mask retraction and lateral mask etching, which limits the fabrication of small features such as the bowtie (Fig 1f).

Hard masks have high resistivity to plasma etching, which allows for etching of silicon with short etching cycles [8]. Polysilicon is used as an intermediate mask to transfer the pattern from CSAR resist into chromium with high selectivity and high fidelity (Fig 2a). In a two-step process consisting of an oxygen-rich step and a fluorine-rich step, chromium is etched with 60 cycles [9]. A chromium hard mask allows silicon to be etched with short cycles (smaller scallops), producing vertical and smooth sidewalls, which are important features for the performance of nanocavities and their optical performance (Figs 2c-f). We use 25 cycles to etch a 220 nm silicon device layer. Chromium can be stripped from the silicon device layer using oxygen plasma in a barrel asher [8] or by wet chemistry. Our work shows that hard-mask processes have significant advantages for fabricating bowtie nanocavities with high resolution and, more generally, for achieving features with nanometer-scale critical dimensions.