MICROWAVE GENERATED PLASMA FIGURING FOR ULTRA PRECISION ENGINEERING OF

OPTICSAdam Bennett1; Dr Renaud Jourdain2; Dr Paul Kirby3; Dr Peter MacKay4;

Professor Paul Shore5; Professor John Nicholls6; Paul Morantz7

1 PhD Researcher, Centre for Doctoral Training in Ultra Precision2 Academic Fellow, Cranfield University3 Academic Fellow, Cranfield University4 Principal Technologist, Gooch & Housego5 Director of Engineering Measurement, NPL; Director, Loxham Precision Ltd;Principal Investigator, Centre for Innovative Manufacturing in Ultra Precision6 Professor of Coatings Technology, Cranfield University7 Director of Centre for Innovative Manufacturing in Ultra Precision

Content

Aim & Applications

Ultra Precision Surface Processing Route for Large Optics

Current Plasma Figuring Machines at Cranfield University

Prototype Microwave Plasma System

Optical Emission Spectroscopy Results

EPSRC Centre for Doctoral Training in Ultra Precision

EPSRC Centre for Innovative Manufacturing in Ultra Precision

• Only UK University with an Airport• Only UK 100% Post Graduate

Institution• 1,559 Staff• 4,639 Students• Strong links between Teaching,

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• Awards 10% of the UK Science & Engineering Doctorates

Aim

Focusing Lens Optic 700mm processing <30nm RMS roughness

Microwave Plasma Energy Beam100mm by 100mm Optics

Crystal Quartz

Material Removal Rate of 5mm3/min Form accuracy < 10nm RMS Surface texture < 1nm RMS

Acousto-Optics: Beam Deflectors, Frequency Shifters, Modulators, Q Switches

II) Acousto-Optic, http://goochandhousego.com/capabilities/acousto-optic-capabilities, 21/04/2015

I) I)II)

Applications

I) T. Fellers, M. Davidson, 2009, Acousto-Optic Tunable Filters, Report for the National High Magnetic Field Laboratory, USA

Fusion Energy Research: Using Metre Scale Optics to Focus High Power Laser Beams

III) Cold Fusion, http://phys.org/news/2013-08-laser-fusion-yields-energy.html, 21/04/2015

Focusing Lens Optic 700mm processing <30nm RMS roughness

Other Applications

Surface roughness < 1 nm RMS

10 nm form accuracy

Metre scaleoptical component

BoX (Ultra precision grinding)

1 mm form accuracy 1 μm form accuracy

Stage 2

Polishing process

Stage 1

Grinding process

Stage 3

Plasmaprocess

Helios1200 (Plasma figuring machine)

Polishing machine(Robot)

300nm form accuracy

Ultra Precision Surface Processing Route for Large Optics

Plasma Figuring Machines at Cranfield UniversityRAP Plasma Figuring Machine

300mm x 300mm optical substrate processing capability

Helios Plasma Figuring Machine1200mm x 1200mm optical substrate processing capability

Both plasma figuringmachines currently employ anInductively Coupled Plasmatorch

Atmospheric Pressure Plasma Figuring

Experimental SetupPrototype Microwave Plasma System

• Microwave power• Gas flow rate

• Type of gas• Quartz tube design• Nozzle design

Main plasma torch parameters investigated:

Rectangular Tuner

Frequency (GHz)

Pow

er (W

)

2.45GHz ± 10Hz

Microwave Propagation Through The Rectangular Tuner

Maxwell’s equations explain the electromagnetic model of microwave propagation through a medium. The general forms of the time varying differential equations are given as:

Where is the electric field (V/m); is the magnetic field (A/m); is the electric flux density (Coulombs/m2); is the magnetic flux density (Wb/m2); is the imaginary magnetic current density (V/m2); is the electric current density (A/m2); and is the electric charge density (Coulombs/m3).

Resonance Frequency of the Rectangular Tuner

The resonance frequency of a rectangular cavity is derived by imposing boundary conditions upon the electromagnetic field expressions, thus giving:

And as the wavenumber is defined as:

Then this yields:

Where kmnl is the wavenumber (m-1); m, n, l, are the mode numbers; a, b, d, are the corresponding dimensions (m); c is the speed of light in a vacuum (m/s); µr is the relative permeability (H/m); and, εr is the relative permittivity (F/m).

The result is the same whether operating in TM Mode or TE Mode: advantage of a rectangular cavity.

Microwave Power Absorption by Carrier Gas

Electrons gain energy from the varying electric field and from elastic collisions with other electrons.

The energy gained by the electrons is then dissipated through elastic and inelastic collisions with ionic and neutral species: thus causing further excitation and ionization.

The mean power absorbed by an electron from the field is given as:

Where e is the charge on the electron (C); m is the mass of the electron (kg); EMAX is the maximum field strength (N/C); v is the collision frequency between the electron and the gas atoms (Hz); and, w is the field frequency (Hz).

Microwave power is transferred into the surface layers of the plasma discharge:

Where is the resistivity of the gas (Ωm); is the angular frequency of the current (radians/s); is the permeability of free space x relative magnetic permeability of the gas (H/m); is the permittivity of free space x relative permittivity of the gas (F/m).

Microwave Plasma Energy Transfer

The Microwave Energy is transferred via the microwave electrical field into the electron energy states within the gas atoms. The electronic energy is then transferred to neutral species by inelastic and ionizing collisions.

This Micro Wave Plasma system has very low gas temperatures, Tg, compared to its electron temperatures, Te, and this relationship is directly proportional to the electron mean free path, the mass of the gas and the electric field strength applied:

Where mg is the mass of a neutral particle (kg); me is the mass of an electron (kg); kB is the Boltzmann constant (m2 kg s-2 K-1); E is the electric field strength (N/C); e is the elementary charge on an electron (C); and, λe is the electron mean free path (m).

Microwave Plasma Energy Transfer

Dissociation

Analytical CharacterisationOptical Emission Spectroscopy

The Adtec plasma torch was set to a fixed position within the processing chamber of a Plasma Figuring machine.

An optical fibre and collimating lens were mounted onto a precision motion stage.

The end of the diagnostic tool was moved in two dimensions: horizontal and vertical.

The diagnostic tool was an Ocean Optics HR4000 Spectrometer.

The resolution of the spectrometer was 240pm.

For each experiment, the plasma jet was scanned over a 2mm x 5mm area, at 100µm intervals. At each interval the intensity of all the wavelengths between 400nm and 850nm were recorded. The data was then processed using a bespoke Matlab routine and 3D maps of the plasma jet were created.

Optical Emission Spectroscopy

5mm

2mm

100µm intervals

Lens Connected To Spectrometer

• Frequency = 2.45GHz• Power = 15W• Main Gas Flow = 4L/min• Nozzle Gas Flow = 0.5L/min

Frequency (GHz)

Pow

er (W

)

2.45GHz ± 10Hz

Microwave Energy Transmitted Into Main Gas Flow

Optical Emission Spectroscopy Results

Argon Helium Argon + Argon Argon + SF6 Argon + CF4

2mm 2mm 2mm 2mm 2mm

5mm

5mm

5mm

5mm

5mm

Summary

A Prototype Microwave Plasma Energy Beam was characterised byusing an Optical Emission Spectroscopy (OES) technique.

Secondary gases were injected into a Bespoke Nozzle and their effectson the plasma discharge were analysed.

Further Work

A New Prototype Microwave Plasma System is being created that willoperate at up to 300W.

The Optical Emission Spectroscopy (OES) technique developed willunderpin the characterisation of the new system.

This system will be targeted at Plasma Figuring optics in 2 hours, whichwill have a Form Accuracy of less than 10nm RMS and a Surface Finishof less than 1nm RMS.

Acknowledgements

Cranfield University EPSRC Centre for Innovative Manufacturing in Ultra PrecisionEP/I033491/1: Plasma Figuring Laboratory

University Of Cambridge EPSRC Centre for Doctoral Training in Ultra PrecisionEP/K503241/1: Project Funding

Gooch & Housego: Project Funding

ADTEC: Technical Support & Pioneering Microwave Plasma System Loan

Defence Academy: Military Grade Microwave Characterisation

Adam Bennett

Doctoral Research Scientist

EPSRC Centre for Doctoral Training in Ultra Precisionhttp://www.CDT‐UP.Eng.Cam.ac.uk/People/Adam‐Bennett

EPSRC Centre for Innovative Manufacturing in Ultra Precisionhttp://www.UltraPrecision.org

https://www.LinkedIn.com/in/AdamDMBennett