Fengzhou Fang

Diamond Cutting of Silicon with Nanometric Finish

Abstract Micro-cutting is a viable alternative to grinding and polishing techniques in the fabrication of high quality brittle materials. Using different diamond tools with rake angles of 0 degree and −25 degree at different cutting speeds, taper cutting experiments were carried out with increasing depth of cut on silicon. The grooves formed were analyzed by Scanning Electron Microscopy and Optical Measurement Inspection System. Topographic details were studied using Atomic Force Microscope. For turned silicon surfaces with roughness value of Ra=23.8 nm and Rmax=140 nm, mirror surfaces of 1 nm roughness were achieved repeatedly by micro-cutting. The integrity of the resulting surfaces were then studied in detail by using Atomic Force Microscopy and Scanning Electron Microscopy. The results prove that the micro-cutting mechanism, proposed in theory, is justified in actual material removal in the range of nanometers.

Topological Photonic Phase in Chiral Hyperbolic Metamaterials

Recently, the possibility of achieving one-way backscatter immune transportation of light by mimicking the topological properties of certain solid state systems, such as topological insulators, has received much attention. Thus far, however, demonstrations of nontrivial topology in photonics have relied on photonic crystals with precisely engineered lattice structures, periodic on the scale of the operational wavelength and composed of finely tuned, complex materials. Here we propose a novel effective medium approach towards achieving topologically protected photonic surface states robust against disorder on all length scales and for a wide range of material parameters. Remarkably, the nontrivial topology of our metamaterial design results from the Berry curvature arising from the transversality of electromagnetic waves in a homogeneous medium. Our investigation therefore acts to bridge the gap between the advancing field of topological band theory and classical optical phenomena such as the spin Hall effect of light. The effective medium route to topological phases will pave the way for highly compact one-way transportation of electromagnetic waves in integrated photonic circuits.

On minimum exit-burr in micro cutting

Advances in product miniaturization lead to size and weight reduction which can substantially increase the convenience and value of many products. Micro machining with mechanical cutting tools is capable of producing high profile accuracy, surface finish quality and sub-surface integrity at reasonably low cost. It is the primary choice amongst various manufacturing processes in fabricating micro components. However, in micro cutting, the main problem is the formation of burrs along the feature edges. This is because the relatively large size of burrs would more severely affect the functionality of the micro components. To reduce the burrs, an extensive study has been carried out both theoretically and experimentally. Nano indentation is employed to analyze the plastic and elastic deformation on the nanometre scale and the results applied to aid the interpretation of cutting experiments. Although the burrs were not eliminated completely, it is shown that they can be minimized to less than 25 nm in height.

Cylindrical coordinate machining of optical freeform surfaces

The cylindrical coordinate machining method (CCM) is systematically studied in generating optical freeform surfaces, in which the feature points are fitted to typical Non-Uniform Rational B-Splines (NURBS). The given points have the mapping coordinates in the variable space using the point inversion technique, while the other points have their NURBS coordinates due to the interpolation technique. The derivation and mathematical features are obtained using the fitting formula. The compensation and optimized values for tool geometry are studied using a proposed sectional curve method for fabricating designed surfaces. Typical freeform surfaces fabricated by the CCM method are presented.

Ultra-Precision Cutting for ZKN7 Glass

Abstract There is an immense need to obtain nanometric surface finish on optical glasses owing to the advantage of improved performance of the components. Thus, the use of ultra-precision machining becomes critical. According to the hypothesis of ductile machining, all materials, regardless of their hardness and brittleness, will undergo a transition from brittle to ductile machining region below a critical undeformed chip thickness. Below this threshold, the energy required to propagate cracks is believed to be larger than the energy required for plastic deformation, so that plastic deformation is the predominant mechanism of material removal in machining these materials. However, the mere use of ultra-precision machining for this brittle material would not yield mirror surface finish. Therefore, experiments were conducted using diamond cutting for machining ZKN7 glass. Based on the experimental work, a new strategy was proposed for obtaining nanometric surface finish and the cutting mechanism was studied in detail. A new method for measuring the sharpness of the cutting tools was introduced using the confocal laser scanning microscopy. Also, in this paper an alternative approach was recommended for the fracture toughness measurement of brittle materials.

Tool geometry study in micromachining

With the trend towards miniaturization, micromachining becomes more and more important in fabricating micro parts. Investigations have shown that unpredictable tool life and premature tool failure present a serious concern in micromachining. To further develop the process, a systematic study of various types of tool geometry has been carried out. The tool failure modes and the ways in which tool life can be improved were also studied. Experimental results show that the tool tip rigidity of the semi circle-based (D-type) end-mills is much higher than that of the two-flute (commercial type) end-mills, and the machining quality with the D-type tools is better than that of the triangle-based (Δ-type) end-mills. The tool breakage of end-milling operations simulated using real tool geometry is in good agreement with experimental data. Both the experimental study and FEM analysis have shown that the D-type end-mills are more suitable for micromachining.

Experimental investigation of superfocusing of plasmonic lens with chirped circular nanoslits

A plasmonic lens with metallic chirped circular nanoslits corrugated on Au film supported on quartz substrate for the purpose of superfocusing was put forth and fabricated by means of focused ion beam direct milling technique. Topography of the lens was imaged using an atomic force microscope. After that a near-field scanning optical microscope was employed for optical characterization of focusing performance of the lens. Our experimental results verify the focusing performance and further demonstrate that they are in agreement with the theoretical calculation results. Focusing performance is significantly improved in comparison to that of the non-chirped lens. The lenses are possible to be used for the applications of bioimaging, detection, and inspection in submicron scale resolution.

Ultra-precision machining of sinusoidal surfaces using the cylindrical coordinate method

Micro- and ultra-precision machining is an effective approach to achieving a nanometric surface finish, which is important for the sinusoidal surface as the calibrator of multi-axis precision machines. A cylindrical coordinate micromachining method is studied for the ultra-precision cutting of sinusoidal surfaces in this paper. The three-axis (X, Z, C) turning machine is used, whose NC path is generated in a spiral curve considering the workpiece surface and tool geometry. The avoiding interference technique is proposed, including the optimal selection of tool geometry and simulation of the cutting process. The tool geometry is optimized by using a sectional curve method. The cutting simulation is implemented using a constrained B-spline path fitting and a pipe model of the cutting face. With the method developed, sinusoidal surfaces with a nanometric finish of 5.54 nm in Ra are achieved effectively.

Study on nanometric cutting of germanium by molecular dynamics simulation

Three-dimensional molecular dynamics simulations are conducted to study the nanometric cutting of germanium. The phenomena of extrusion, ploughing, and stagnation region are observed from the material flow. The uncut thickness which is defined as the depth from bottom of the tool to the stagnation region is in proportion to the undeformed chip thickness on the scale of our simulation and is almost independent of the machined crystal plane. The cutting resistance on (111) face is greater than that on (010) face due to anisotropy of germanium. During nanometric cutting, both phase transformation from diamond cubic structure to β-Sn phase and direct amorphization of germanium occur. The machined surface presents amorphous structure.

Fabrication of micro DOE using micro tools shaped with focused ion beam

A novel method is proposed to fabricate micro Diffractive Optical Elements (DOE) using micro cutting tools shaped with focused ion beam (FIB) milling. Micro tools with nanometric cutting edges and complicated shapes are fabricated by controlling the tool facets orientation relative to the FIB. The tool edge radius of less than 30 nm is achieved for the nano removal of the work materials. Semi-circular micro tools and DOE-shaped micro tools are developed to fabricate micro-DOE and sinusoidal modulation templates. Experiments show that the proposed method can be a high efficient way in fabricating micro-DOE with nanoscale surface finishes.