Precision machining of Silicon by using SPDT & MRF

Solutions for critical raw materials under Extreme Conditions

Neha Khatri 1,2*, Saurav Goel 3 , Vinod Karar 1,2

1 CSIR - Central Scientific Instruments Organisation, Chandigarh, 160030, India 2 Academy of Scientific & Innovative Research (AcSIR), CSIR-CSIO, Chandigarh, India

3 School of Aerospace, Transport and Manufacturing, Cranfield University, Bedfordshire, MK430AL, UK

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European Conference on nanofilms 2018

ECNF 2018 Conference at Cranfield, UK

(20th to 22nd March 2018)

Abstract submission is now open – just visit http://www.ecnf2018.org/

Broad themes:

• PROTECTIVE AND TRIBOLOGICAL COATINGS

• THIN FILMS FOR ENERGY CONVERSION, CATALYSIS AND RELATED PROCESSES

• FROM ATOMS TO SHEETS: GROWTH AND TRIBOLOGY OF THIN FILMS

• FUNCTIONALISATION AND CHARACTERISATION OF COATINGS AND NANOFILMS

• 2D MATERIALS: GRAPHENE AND BEYOND

• SENSORS AND INSTRUMENTS BASED ON NANOFILMS

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Graphene

Bulk

Thin film

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Critical raw materials and Cost Action CA15102

Silicon metal (Metallurgical grade silicon) is absolutely necessary to the production

of aluminium and is backbone to the manufacture of micro-processors and solar

cells. Currently, its supply is dominated by China and Brazil and alternatives are

being sought.

The intent of this work is not to develop a CRM substitute but to advance our

understanding on the CRMs that are classed as brittle materials

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• Manufacturing hard, brittle materials like silicon and germanium is

challenged by their low fracture toughness. Grinding followed by

polishing is the usual method deployed

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Motivation

Cutting fluids such as grinding sludge imposes serious threats to the environment

and sustainable manufacturing is increasingly emerging as a major challenge

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Introduction

• Silicon is ideal optical material used for IR applications and other high added

values product such as X ray optics as well as solar cells

Silicon IR lens for

thermal cameraUV MirrorsX-rays applications

The functional performance of these components largely depends the slope

errors present on the reflecting surface or in essence the quality of surface

produced by the manufacturing process

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• Submicron level of form accuracy & nanometre level of Surface

Roughness can be achieved by a hybrid fabrication process, i.e., by

combining two emerging technologies SPDT and MRF

Hypothesis formulated

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Introduction to Single point diamond turning

SPDT is featured with the

spectrum of operating

parameters, which needs fine

tuning to achieve the best

results.

This is practically not possible

due to tool offset errors, thermal

effects, spindle vibration and

centripetal distortions as well as

rapid tool wear during machining

of silicon.

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Introduction of Magnetorheological Finishing

Working Principle of Magneto Rheological Fluid in MRF

Schematic of MRF experimental setup

• The MRF process relies on a unique

"smart fluid", known as

Magnetorheological (MR) fluid

• MR-Fluids are suspensions of micron

sized magnetizable particles such as

carbonyl iron, dispersed in a non-

magnetic carrier medium like silicone

oil, mineral oil or water.

• MR Fluid are controllable smart

materials whose flow properties such

as viscosity, stiffness can be easily be

manipulated by applying external

magnetic field.

• In the absence of magnetic field, an

ideal MR fluid exhibits Newtonian

behaviour.

• Under the influence of magnetic field,

an MR Fluid transform from a fluid like

state to a semi-solid state by the

formation of chain clusters of magnetic

particles along the lines of magnetic

field.

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Results from SPDT of Si

(a) (b)Figure: (a) Surface Roughness (b) 3D Surface Morphology for the Best combination with Ra= 31.6 nm

• We used Taguchi Method and obtained the best surface whilst using three following

machining parameters:

Feed rate of 2.5 µm/rev,

Depth of cut of 1.5 µm

Spindle speed of 1500 rpm

The surface roughness achieved is 31.6 nm.

A phenomena known as ductile-regime machining was realised

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Results from MRF of machined Si wafers

• During MRF process, current, wheel speed and gap are three important factors affecting the

surface quality.

Current: 6A

Wheel speed: 400 RPM

Gap: 0.5mmAn increase in current leads to increase in

viscosity of MR fluid causing strong

bonding between the abrasives and

carbonyl iron particles, thus improving the

surface quality.

High wheel speed, leads to higher

centrifugal forces at periphery of ball end

tool, which enables the abrasive particles

to move outwards and improves the

efficiency of the process.

An increase in gap increases the number

of effective particle in contact with surface

to be more, leading to more temperature

at polishing spot, hence worsening the

surface roughness.

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Final results: SEM and AFM images

(a) (b) (c)

Figure: (a) Un-machined silicon substrate with initial Ra= 410 nm (b) Diamond turned silicon with Ra= 31.6 nm (c) MRF polished

silicon with Ra= 0.607 nm

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• The sequential approach of combining SPDT and MRF provides one of the

alternatives to slurry laden grinding and polishing processes towards

sustainable manufacturing to machine brittle materials like silicon down to sub-

nm surface roughness.

• As for SPDT, the results revealed that a low feed rate (2.5 µm/rev), low Depth of cut

(µm) and high cutting speed of 1500 RPM provides an optimum Ra of 31.6 nm

• For MRF process, it is concluded that smaller gap (0.5 mm), high wheel speed (400

RPM) and high current (6 A) provided the best Ra of 0.64 nm.

• The MRF process was able to deterministically remove the turning marks left over

from diamond turning. The diamond turning grooves are clearly visible in post-SPDT

surface, but after MRF these grooves were eliminated. The minimum surface

roughness achieved is 0.607 nm.

Conclusions

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QUESTIONS??