Initial Fabrication and Characterization of Chemically-Etched Silicon Slits for KOSMOS

TL;DR

This paper presents a novel nanofabrication method for creating reflective silicon slits for the KOSMOS spectrograph, demonstrating smoother edges and comparable width uniformity to traditional machined slits, with implications for improved spectrograph performance.

Contribution

It introduces a chemically-etched silicon slit fabrication technique and compares its properties to traditional wire EDM machined slits, highlighting advantages in surface roughness and potential for enhanced spectrograph use.

Findings

Chemically-etched slits have smoother surfaces than machined slits.

Etched slits show comparable width uniformity to machined slits.

Reflectivity, not roughness, dominates scattering in the slits.

Abstract

KOSMOS is a low-resolution, long-slit, optical spectrograph that has been upgraded at the University of Washington for its move from Kitt Peak National Observatory's Mayall 4m telescope to the Apache Point Observatory's ARC 3.5m telescope. One of the additions to KOSMOS is a slitviewer, which requires the fabrication of reflective slits, as KOSMOS previously used matte slits machined via wire EDM. We explore a novel method of slit fabrication using nanofabrication methods and compare the slit edge roughness, width uniformity, and the resulting scattering of the new fabricated slits to the original slits. We find the kerf surface of the chemically-etched reflective silicon slits are generally smoother than the machined matte slits, with an upper limit average roughness of 0.42 $\pm$ 0.03 $\mu$m versus 1.06 $\pm$ 0.04 $\mu$m respectively. The etched slits have width standard deviations of 6 $\pm$ 3 $\mu$m versus 10 $\pm$ 6 $\mu$m, respectively. The scattering for the chemically-etched slits is higher than that of the machined slits, showing that the reflectivity is the major contributor to scattering, not the roughness. This scattering, however, can be effectively reduced to zero with proper background subtraction. As slit widths increase, scattering increases for both types of slits, as expected. Future work will consist of testing and comparing the throughput and spectrophotometric data quality of these nanofabricated slits to the machined slits with on-sky data, in addition to making the etched slits more robust against breakage and finalizing the slit manufacturing process.