Manufacturing and measurement of freeform optics

Multicrystalline silicon for solar cells

Single crystalline silicon was plunge-cut using diamond tools at a low speed. Cross-sectional transmission electron microscopy and laser micro-Raman spectroscopy were used to examine the subsurface structure of the machined sample. The results showed that the thickness of the machining-induced amorphous layer strongly depends on the tool rake angle and depth of cut, and fluctuates synchronously with surface waviness. Dislocation activity was observed below the amorphous layers in all instances, where the dislocation density depended on the cutting conditions. The machining pressure was estimated from the micro-cutting forces, and a subsurface damage model was proposed by considering the phase transformation and dislocation behavior of silicon under high-pressure conditions.

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Fundamental investigation of subsurface damage in single crystalline silicon caused by diamond machining

Synthesis of Silicon Nanowires with Wurtzite Crystalline Structure by Using Standard Chemical Vapor Deposition

Group 14 elements (C, Si, and Ge) exist as various stable and metastable allotropes, some of which have been widely applied in industry. The discovery of new allotropes of these elements has long attracted considerable attention; however, the search is far from complete. Here we computationally discovered a tetragonal allotrope (12 atoms/cell, named T12) commonly found in C, Si, and Ge through a particle swarm structural search. The T12 structure employs sp3 bonding and contains extended helical six-membered rings interconnected by pairs of five- and seven-membered rings. This arrangement results in favorable thermodynamic conditions compared with most other experimentally or theoretically known sp3 species of group 14 elements. The T12 polymorph naturally accounts for the experimental d spacings and Raman spectra of synthesized metastable Ge and Si-XIII phases with long-puzzling unknown structures, respectively. We rationalized an alternative experimental route for the synthesis of the T12 phase via deco...

Tetragonal allotrope of group 14 elements.

Raman spectroscopy was used for analysis of phase transformations and residual stress in machined silicon. Wear debris from dicing of silicon was scanned with a Raman spectrometer. Recorded spectra manifest the presence of amorphous Si, hexagonal phase (Si-IV), bc8 phase (Si-III) and pristine Si-I under residual stress. On surfaces of diced wafers as well as lapped silicon wafers, the r8 phase (Si-XII) was detected in addition to the above phases. The composition of phases in diced cross sections of silicon wafers differs dramatically between high and low speed cuts. The quantification of these phases was attempted by curve fitting each spectrum with corresponding peaks of each phase. Subsequently, relative intensity maps of specific phases were generated. Thus, Raman spectroscopy studies of machined surfaces demonstrated metallization of Si under a variety of machining conditions including lapping, grinding, scratching, dicing and slicing. All metastable phases of silicon disappear after etching and polishing of respective wafers. No evidence of phase transformations was found on a quartz-damaged silicon wafer surface. Residual stress having a characteristic distribution was observed in this case.

http://iopscience.iop.org/article/10.1088/0268-1242/14/10/310/pdf

C Baek

Papers

Raman microspectroscopy study of processing-induced phase transformations and residual stress in silicon