Ultraprecision cutting of single-crystal calcium fluoride for fabricating micro flow cells

Single-crystal infrared (IR) spectroscopy is a promising method for protein structure analysis, where a protein crystal sample is fixed in a micro flow cell. Single-crystal calcium fluoride (CaF2) is expected as the flow cell substrate material for its excellent optical property. However, CaF2 is a highly brittle material having strong anisotropy, thus is extremely difficult to machine. Up to date, there is no available literature on fabrication of CaF2 flow cells. In this study, micro flow cells of single-crystal CaF2 were fabricated by ultraprecision cutting technology. Fly cutting was conducted using a single-crystal diamond tool having straight edges to generate a depth-varying rectangle cross section for the flow cell. The effects of cutting direction, workpiece orientation, undeformed chip thickness and tool rake angle on cutting behavior were investigated. Based on experiments and analysis, optimal conditions for ductile machining of micro grooves in CaF2 were identified. As a result, a 10 μm deep CaF2 micro flow cell with surface roughness of 2.4 nmRa was successfully fabricated. Using the fabricated flow cell, IR spectroscopic analysis of a protein single crystal at room temperature was succeeded. This study demonstrated the effectiveness of ultraprecision cutting technology in CaF2 micro flow cell fabrication, which contributes to the IR analysis of protein, and in turn, the advance of life science.