Investigations of ductile-brittle transition and

associated machining damages in silicon wafer via

scratching process

Speaker: Chiraj Alreja

Venue : HSB 209 (Seminar Hall)

Date : 07th February, 2020 (Friday)

Time : 5.30 p.m.

Abstract

Machining especially ultra-precise machining (e.g. diamond turning), grinding, polishing of silicon wafers is critical with many applications in IR optics, MEMS, solar cells and electronic components which require smooth surface. The basic process involved in this is a series of overlapping scratches. However, processing of brittle silicon is challenging because of its low fracture toughness and poor machinability which results in brittle fracture and failure containing machining damages (phase transformations, residual stresses). But if the scale of machining goes down to nano where silicon being subjected to high hydrostatic pressures (around 12GPa) and is processed below the critical depth of cut then plastic deformation becomes favorable than fracture i.e. ductile regime machining dominate.

This research work experimentally, via unique rotational double-taper scratching set up, studies ductile-brittle transitions (critical depth of cut) in single crystal (100) p-type silicon. Scratches are induced using a conical diamond tool at room temperature where scratching speeds ranging between 0.1 m/s and 0.3 m/s. A well-defined way to determine critical depth of cut via linear crack density per unit crack length is proposed. For the first time, critical depth of cut has been comprehensively studied at all speed ranges which gives an inference that ultra-precise machining of silicon should be conducted at higher cutting speeds.

Higher temperature assisted processing of silicon, such as heat-assisted diamond turning, is often being considered to improve surface integrity. We also investigates the effect of temperatures (25-500 °C) on the phase transformations at scratching speed comparable to diamond turning process (1 m/s). Analytical pressure calculations show that at higher temperatures, phase transformations can happen in silicon at significantly lower pressures. This study is expected to help tune heat-assisted diamond turning conditions to improve surface formation.

About the speaker

Chirag enrolled at IITM in Jan 2015 as MS student. Learning in research and teaching interest him to convert from MS to Dual MS and PhD program of study at IITM. He completed his undergraduate in Mechanical engineering from Chitkara University, Punjab in 2013.