Crystallographic effects in micro/nanomachining of single-crystal calcium fluoride

Abstract

Single-crystal calcium fluoride (CaF2) is an excellent optical material in the infrared range and the ultraviolet range. It is an indispensable substrate material for the 193 and 157 nm wavelength laser optics for future large-scale semiconductor photolithography systems. Due to its delicate nature, for the most part the CaF2 elements have been fabricated using conventional pitch polishing combined with interferometry and local surface correction to form the desired flat, sphere or aspherical surface. In the present work, the feasibility of generating high quality optical surfaces on CaF2 by single-point diamond turning is examined. The development of this technology may provide the possibility of fabricating aspherical and diffractive optical components in an efficient way. The machining experiments described in this article were done on a high-stiffness, ultraprecision, numerically controlled diamond lathe with a sharply pointed single-crystal diamond tool. The scale of machining was varied from the micrometer level to the nanometer level. It was found that at the micrometer level, machining was significantly affected by the crystal orientation of the workpiece. The crystallographic anisotropy causes nonuniformity of the finished surface texture, microfracture topography and brittle–ductile transition boundary conditions. The results also indicate that by controlling the undeformed chip thickness below a critical value, namely, the minimum critical chip thickness (85 nm), a uniformly ductile-machined surface could be produced. Under the present experimental conditions, a surface with a maximum height (Ry) of 18.5 nm and arithmetical mean roughness (Ra) of 3.3 nm was obtained.

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

“…When rake angle is -20°, no cracks were observed on the cut surface, indicating that critical undeformed chip thickness is bigger than maximum undeformed chip thickness (200 nm) set in the present experiment. This result is similar to the single-point diamond turning results of CaF2 (Yan et al, 2004a(Yan et al, , 2004b, where a -20° rake angle tool led to a maximum undeformed chip thickness of ~1200 nm along specific directions. As known from previous studies, a suitable negative tool rake angle induces hydrostatic stress field in front of tool edge, which reduces brittle fractures.…”

“…No cracks occur on the surfaces for cutting directions of 90°, 210°, and 330°, indicating that the critical undeformed chip thickness in these directions is bigger than the maximum undeformed chip thickness (200 nm) used in this experiment. In diamond turning of CaF2 (111), the critical undeformed chip thickness in specific cutting directions reached 1200 nm [Yan et al, 2004b]. Komiya, Kimura, Nomura, Kubo and Yan, Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol.12, No.1 (2018) The strong anisotropy in brittle-to-ductile transition is caused by the orientation of slip planes in the CaF2 crystal.…”

“…In Komiya, Kimura, Nomura, Kubo and Yan, Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol.12, No.1 (2018) diamond turning of CaF2, surface microfracture is extremely easy to occur, and the cutting characteristics change strongly with the crystal planes of the workpiece due to its material anisotropy (Yan et al, 2004a). Ductile machining of CaF2 is only possible when no thermal shock occurs and the undeformed chip thickness is smaller than a critical value which ranges from a few tens to several hundreds of nanometers (Yan et al, 2004b).…”

“…In Komiya, Kimura, Nomura, Kubo and Yan, Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol.12, No.1 (2018) diamond turning of CaF2, surface microfracture is extremely easy to occur, and the cutting characteristics change strongly with the crystal planes of the workpiece due to its material anisotropy (Yan et al, 2004a). Ductile machining of CaF2 is only possible when no thermal shock occurs and the undeformed chip thickness is smaller than a critical value which ranges from a few tens to several hundreds of nanometers (Yan et al, 2004b).In this study, to fabricate a micro-flow cell which has a depth-varying rectangle cross section, fly cutting of CaF2 was conducted using a single-crystal diamond tool with straight edges. Because the transmittance of CaF2 depends on its surface roughness (Retherford et al, 2001), it is required that the surface roughness of the fabricated micro flow cell is in the nanometer level, without crack generation.…”

A method to include plastic anisotropy to orthogonal micromachining of fcc single crystals

“…Numerous studies on both single and polycrystal metals have elucidated the effect of the crystal orientation of the workpiece on the quality surface machined at small scales [6,7,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Understanding the difference in cutting forces for planning different orientations in the crystals has been a fundamental issue even though orthogonal planning is the simplest deformation case.…”