Jiwang Yan

On the ductile machining of silicon for micro electro-mechanical systems (MEMS), opto-electronic and optical applications

The role of hydrostatic pressure in the ductile machining of silicon is demonstrated experimentally using a single crystal diamond tool with a large negative rake (40°) and a high side cutting edge angle (SCEA) ( 88°) and undeformed chip thickness in the nanometric range (50 nm) using an ultraprecision machine tool and a special machining stage inside a high external hydrostatic pressure (400 MPa) apparatus.

Ductile regime turning at large tool feed

Ductile regime turning is a new technology for obtaining a crack-free surface on brittle material. However, the fundamental obstacle for industrial application of this technology is diamond tool wear. This problem is difficult to solve for existing methods of turning with roundnosed tools due to limitation on tool feed. In this paper, ductile regime turning using the straight-nosed diamond tool is proposed. This method enables thinning of undeformed chip thickness in the nanometric range and at the same time provides significant cutting width ensuring plain strain conditions. Adopting a small cutting edge angle enables ductile regime turning at a large tool feed up to a few tens of micrometers. Single crystal silicon is machined and chip morphology and machined surface texture are examined for clarifying the brittle‐ductile transition mechanism. Ductile surface with nanometric roughness is obtained and generation of plastically deformed continuous chips is confirmed. # 2002 Elsevier Science B.V. All rights reserved.

Micro grooving on single-crystal germanium for infrared Fresnel lenses

Single-crystal germanium is an excellent optical material in the infrared wavelength range. The development of germanium Fresnel lenses not only improves the optical imaging quality but also enables the miniaturization of optical systems. In the present work, we developed a ductile-mode micro grooving process for fabricating Fresnel lenses on germanium. We used a sharply pointed diamond tool to generate the micro Fresnel structures under three-axis ultraprecision numerical control. By adopting a small angle between the cutting edge and the tangent of the objective surface, this method enables the uniform thinning of the undeformed chip thickness to the nanometric range, and thus provides complete ductile regime machining of brittle materials. Under the present conditions, a Fresnel lens which has a form error of 0.5 µm and surface roughness of 20–50 nm Ry (peak-to-valley) was fabricated successfully during a single tool pass.

Laser micro-Raman spectroscopy of single-point diamond machined silicon substrates

Laser micro-Raman spectroscopy was used to examine the silicon substrates machined by single-point diamond turning at machining scales ranging from 10 to 1000 nm under plane strain conditions. The results showed that the subsurface layer was partially transformed to amorphous, the extent of amorphization depending strongly on the undeformed chip thickness. The intensities of the crystalline phase and the amorphous phase show opposite tendencies with respect to the undeformed chip thickness. In brittle regime machining, Raman spectra differ depending on the test locations. The intensity of the amorphous phase reaches maximum near the ductile–brittle transition boundary. In ductile regime machining, the intensity of the amorphous phase decreased sharply as the undeformed chip thickness decreased. This work provides technological insights into the possibility of direct manufacturing of subsurface damage-free optical and optoelectronic products of silicon by ductile machining without the need for or with a de...

Nanoindentation tests on diamond-machined silicon wafers

Nanoindentation tests were performed on ultraprecision diamond-turned silicon wafers and the results were compared with those of pristine silicon wafers. Remarkable differences were found between the two kinds of test results in terms of load-displacement characteristics and indent topologies. The machining-induced amorphous layer was found to have significantly higher microplasticity and lower hardness than pristine silicon. When machining silicon in the ductile mode, we are in essence always machining amorphous silicon left behind by the preceding tool pass; thus, it is the amorphous phase that dominates the machining performance. This work indicated the feasibility of detecting the presence and the mechanical properties of the machining-induced amorphous layers by nanoindentation.

Nanoindentation of polysilicon and single crystal silicon: Molecular dynamics simulation and experimental validation

This paper presents novel advances in the deformation behaviour of polycrystalline and single crystal silicon using molecular dynamics (MD) simulation and validation of the same via nanoindentation experiments. In order to unravel the mechanism of deformation, four simulations were performed: indentation of a polycrystalline silicon substrate with a (i) Berkovich pyramidal and a (ii) spherical (arc) indenter, and (iii and iv) indentation of a single crystal silicon substrate with these two indenters. The simulation results reveal that high pressure phase transformation (HPPT) in silicon (Si-I to Si-II phase transformation) occurred in all cases; however, its extent and the manner in which it occurred differed significantly between polycrystalline silicon and single crystal silicon, and was the main driver of differences in the nanoindentation deformation behaviour between these two types of silicon. Interestingly, in polycrystalline silicon, the HPPT was observed to occur more preferentially along the grain boundaries than across the grain boundaries. An automated dislocation extraction algorithm (DXA) revealed no dislocations in the deformation zone, suggesting that HPPT is the primary mechanism in inducing plasticity in silicon.

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

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 mic...

Effects of tool edge radius on ductile machining of silicon: an investigation by FEM

The submicron-level orthogonal cutting process of silicon has been investigated by the finite element approach, and the effects of tool edge radius on cutting force, cutting stress, temperature and chip formation were investigated. The results indicate that increasing the tool edge radius causes a significant increase in thrust force and a decrease in chip thickness. A hydrostatic pressure (~15 GPa) is generated in the cutting region, which is sufficiently high to cause phase transformations in silicon. The volume of the material under high pressure increases with the edge radius. Temperature rise occurs intensively near the tool–chip interface while the highest cutting temperature (~300 °C) is far lower than the necessary temperature for activating dislocations in silicon. As the edge radius is beyond a critical value (~200 nm), the primary high-temperature zone shifts from the rake face side to the flank face side, causing a transition in the tool wear pattern from crater wear to flank wear. The simulation results from the present study could successfully explain existing experimental phenomena, and are helpful for optimizing tool geometry design in silicon machining.

Transmission electron microscopic observation of nanoindentations made on ductile-machined silicon wafers

Nanoindentation tests were performed on a ductile-machined silicon wafer with a Berkovich diamond indenter, and the resulting indents were examined with a transmission electron microscope. It was found that the machining-induced subsurface amorphous layer undergoes significant plastic flow, and the microstructure of the indent depends on the indentation load. At a small load (∼20mN), most of the indented region remains to be amorphous with minor crystalline nuclei; while under a large load (∼50mN), the amorphous phase undergoes intensive recrystallization. The understanding and utilization of this phenomenon might be useful for improving the microscopic surface properties of silicon parts produced by a ductile machining process.

Response of machining-damaged single-crystalline silicon wafers to nanosecond pulsed laser irradiation

Ultraprecision diamond-machined silicon wafers have been irradiated by a nanosecond pulsed Nd:YAG laser. The results indicate that at specific laser energy intensity levels, the machining-induced subsurface damage layer of silicon has been reconstructed to a perfect single-crystalline structure identical to the bulk. Laser irradiation causes two effects on silicon: one is the epitaxial regrowth of the near-surface amorphous layer, and the other is the complete removal of the dislocations from the crystalline layer. It is the dislocation-free crystalline region that serves as the seed layer to recrystallize the amorphous layer, enabling excellent crystalline perfection. These findings may offer practical alternatives to current chemo-mechanical processing methods for silicon wafers.