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Atmospheric Pressure Plasma Technology For Ultra-Precision Engineering Of Optics For Applications In Aerospace, Defence And Science

New optical technologies increase the demands on the engineering specifications of optical surfaces, with manufacturing specifications of up to 1nm RMS form accuracy and 0.1nm RMS surface finish. To achieve these fabrication requirements novel ultra-precision methods must be developed. The proposed solution is microwave generated activate plasma figuring.

Student

Adam Bennett

Supervisor

Dr Renaud Jourdain

Project presentation

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Aim

Implement a microwave generated reactive plasma torch that will enable the
ultra-precision correction of surface errors on optics. The tool will be easily implemented into an existing manufacturing chain and will target 1mm3/min material removal rates.

Objectives

  1. Establish the design rules of atmospheric pressure microwave plasma systems;
  2. Commission and implement a microwave plasma torch;
  3. Prove the deterministic nature of the material removal function;
  4. Demonstrate the rapid ultra-precision surface correction capability of the microwave plasma torch, over the entire surface of an optical component;
  5. Optimise the technology for processing crystal quartz.

Applications

Applications

 

Background

New technologies in aerospace, defence and science applications increase the demands on the engineering specifications of optical surfaces. Optical surfaces are required to be manufactured to 1 nm RMS form accuracy and 0.1 nm RMS surface finish. The fabrication of these optical systems requires novel ultra-precision manufacturing technologies.

These papers focus on a bespoke Energy Beam (EB) based fabrication process. This EB process is created to achieve the technical specifications that are required for the next generation of ultra-precision photonic surfaces. The proposed EB is generated by electromagnetic waves. Electromagnetic energy transfers into a plasma jet at atmospheric pressure. Generation of electromagnetic waves by solid state technology yields ultra-precise frequency control of the electromagnetic waves coupling with the gas and hence a plasma jet is discharged with higher stability. One of the key features of this novel EB is its capability to process silicon based optical materials.

Characterisation of atmospheric pressure Coaxial Electrode Microwave Induced Plasma technology

1. Introduction

Microwave Induced Plasma (MIP)1 technology generates plasma through the inductive transfer of energy from standing waves within a resonator. One clear distinction that enables an observer to identify an MIP system from alternative microwave plasma technologies is the presence of a dielectric tube that is located within the resonant cavity. MIP systems have become the principal technology in the microwave plasma sector and the design typically employed for discharging atmospheric pressure plasma jets involves the use of a coaxial electrode to launch the microwaves into the gas, this design is called
Coaxial Electrode Microwave Induced Plasma (CE-MIP)2 technology.

Figure 1 Coaxial electrode enclosed within a fused quartz tube shows an example of the coaxial electrode arrangement. The holes in the fused quartz tube allow gas to flow into the quartz tube. Figure 2 Adtec Coaxial Electrode Microwave Induced Plasma torch shows the CE-MIP torch manufactured by Adtec.

Figure 1 Coaxial electrod
Figure 1 Coaxial electrode enclosed within a fused quartz tube


Figure 2 Adtec Coaxial Electrode
Figure 2 Adtec Coaxial Electrode Microwave Induced Plasma torch

This paper develops a novel characterisation process for the development of CE-MIP technology. Firstly, microwave spectrum analysis3, 4 was used to create a polar plot of the microwave energy being emitted from the coaxial electrode. The polar plot shows the microwave electromagnetic field emission about the coaxial electrode. This region is where the microwaves couple with the gas to generate the plasma jet. Then secondly an OES technique was used to create spatial maps of the photonic intensity distribution within the plasma jet when different additional gases were injected into it5.

2. Microwave Spectrum Analysis

2.1 Experimental Setup

Figure 3 Microwave polar plot
Figure 3 Microwave polar plot experimental setup

 

Figure 4 Microwave polar plot coaxial electrode position
Figure 4 Microwave polar plot coaxial electrode position (a) coaxial electrode starting position; (b) coaxial electrode finishing position

The previous experiments were conducted where the precision motion stage rotated the coaxial electrode through 90º pitch. To confirm that the electric field being emitted was axis symmetric, the above set of experiments where repeated where the setup was rotated through 90º yaw. The difference between pitch and yaw are shown in Figure 5 Roll, Pitch, Yaw.

Figure 5 Roll, Pitch, Yaw
Figure 5 Roll, Pitch, Yaw

Finally the polarisation of the Military Standard receiving antenna was considered. All of the previous experiments rotating the setup through pitch and yaw, where conducted with the Military Standard receiving antenna was orientated to receive the horizontal polarisation of the electric field. To ascertain the polarisation of the electric field, the pitch and yaw experiments were repeated with the Military Standard receiving antenna in the vertical polarization state, rotated 90º relative to the horizontal polarisation. This is detailed in Figure 6 Vertical & Horizontal Polarisation.

Figure 6 Vertical & Horizontal Polarisation of Antenna Electric Fields
Figure 6 Vertical & Horizontal Polarisation of Antenna Electric Fields

 

2.2 Results

2.2.1 Axis Symmetry

Figure 7 Pitch & Yaw
Figure 7 Pitch & Yaw

 

3. Optical Emission Spectroscopy

3.1 Experimental Setup

Figure 8 Optical Emission Spectroscopy experimental setup
Figure 8 Optical Emission Spectroscopy experimental setup

 

Figure 9 Adtec Coaxial Electrode Microwave
Figure 9 Adtec Coaxial Electrode Microwave Induced Plasma torch cross section

 

3.2 Results

3.2.1 Argon Plasma Jet with Argon Gas Injected Downstream

Figure 10
Figure 10 Comparison of argon plasma jet without and with argon gas injected downstream (a) main gas = 1L/min Ar; (b) main gas = 1L/min Ar + nozzle gas = 0.01L/min Ar

 

3.2.2 Argon Plasma Jet with Sulphur Hexafluoride Gas Injected Downstream

Figure 11
Figure 11 Comparison of argon plasma jet without and with sulphur hexafluoride gas injected downstreamLeft ) main gas = 1L/min Ar; Right ) main gas = 1L/min Ar + nozzle gas = 0.01L/min SF6

 

3.2.3 Argon Plasma Jet with Carbon Tetrafluoride Gas Injected Downstream

Figure 12
Figure 12 Comparison of argon plasma jet without and with carbon tetrafluoride gas injected downstreamLeft ) main gas = 1L/min Ar; Right ) main gas = 1L/min Ar + nozzle gas = 0.01L/min CF4

 

3.3 Analysis

3.3.1 Argon Plasma Jet Cross Section

Figure 15
Figure 15 Plasma jet analysis locus

Figure 14 Plasma jet cross section
Figure 14 Plasma jet cross section 1mm downstream from nozzle

 

3.3.2 Analysis of a Stationary Locus within the Plasma Jet  

Figure 15
Figure 15 Plasma jet analysis locus

 

Figure 16 Comparison of different secondary gases
Figure 16 Comparison of different secondary gases injected downstream of the argon plasma jet

 

Etching of optical surfaces using atmospheric pressure Surface Wave Launched Microwave Induced Plasma technology

Microwave Induced Plasma (MIP) technology has become the principal technology in the microwave plasma sector6. Typically a CE-MIP is used to couple microwaves into the gas, however the presence of reactive plasma interactions with the coaxial electrode’s surface typically results in electrode degradation. To avoid this degradation, a different plasma torch design that uses surface wave launching, which has previously been reported to discharge an argon plasma at atmospheric pressure, was chosen7.

Plasma Processing

Experimental Setup

A Surface Wave Launched Microwave Induced Plasma torch was installed into an existing plasma figuring machine, as shown in Figure 17 Microwave Induced Plasma System. The plasma figuring machine had a rapid figuring capability for 300mm diameter optics.

Figure 17 Microwave Induced Plasma System
Figure 17 Microwave Induced Plasma System

 

Plasma Jet Discharge

Figure 18 Plasma Jets
Figure 18 Plasma Jets (a) 25W 5L/min Ar; (b) 100W 5L/min Ar; (c) 200W 5L/min Ar; (d) 200W 5L/min Ar + 0.5L/min CF4

 

Interferometric Measurements

Surface characterisation was undertaken before and after each plasma processing stage, where the difference between the before and after measurements showed the surface change. Surface form characterisation was undertaken by using a Twyman-Green interferometer and surface roughness characterisation was performed by white-light interferometry with a Taylor Hobson CCI 6000 and a Nikon 50X magnification lens.  

Stationary Dwells

Material Removal

Figure 19 Stationary Dwell Power Investigation at 100W
Figure 19 Stationary Dwell Power Investigation at 100W, 5L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s

Figure 20 Stationary Dwell Power Investigation at 200W
Figure 20 Stationary Dwell Power Investigation at 200W, 5L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s

 

Figure 21 Stationary Dwell Power Investigation at 100W, 5L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s - Cross Section shows the cross section of the stationary dwell conducted at 100W in Figure 19 . The footprint is Gaussian and has a FWHM of 2.7mm

Figure 21 Stationary Dwell Power Investigation at 100W
Figure 21 Stationary Dwell Power Investigation at 100W, 5L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s - Cross Section

Figure 22 shows the effect on the maximum depth when the power is increased from 100W to 200W, at 10W intervals. The results of which are shown in .  

Table 1 Stationary Etch Dwell Power Investigation at 5L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s

Power (W)

Etch Depth (nm)

FWHM

(mm)

ΔFWHM

(%)

Sk2

100

74

2.7

1

0.0255

110

123

2.7

1

0.0255

120

161

2.7

1

0.0255

130

211

2.7

1

0.0255

140

265

2.8

1

0.0256

150

314

2.8

1

0.0256

160

367

2.8

1

0.0256

170

417

2.8

1

0.0256

180

483

2.9

1

0.0257

190

534

3.0

1

0.0258

200

578

3.1

1

0.0259

ΔFWHM = Difference between FWHM in x axis and FWHM in y axis

Sk2 = Pearson's Second Skewness Coefficient8 vector

FWHM values are calculated from theoretical Gaussian distribution

Figure 22 Stationary Etch Dwell Power Investigation
Figure 22 Stationary Etch Dwell Power Investigation at 5L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s

 

Single Trenches

Material Removal

Figure 23 Single Trench 150W
Figure 23 Single Trench 150W 5L/min Ar + 0.5L/min CF4 stand-off distance 10mm Speed 10mm/min

 

Figure 24 Single Trench 150W
Figure 24 Single Trench 150W 5L/min Ar + 0.5L/min CF4 stand-off distance 10mm Speed 10mm/min Top View

The FWHM of the trenches was 2mm.

 

Stationary Dwells

Material Deposition

 

Figure 25 Stationary Dwell Argon Gas Flow
Figure 25 Stationary Dwell Argon Gas Flow Investigation at 150W, 3L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s

 

Figure 26 Stationary Dwell Argon Gas Flow
Figure 26 Stationary Dwell Argon Gas Flow Investigation at 150W, 3L/min Ar + 0.5L/min CF4, stand-off distance 10mm, dwell time 10s - 3D View

Table 2 Stationary Deposition Dwell Argon Gas Flow Investigation at 150W, 0.5L/min CF4, stand-off distance 10mm, dwell time 10s

Argon Gas Flow (L/min)

Deposition Height (nm)

FWHM

(mm)

ΔFWHM

(%)

Sk2

3

371

3.65

9

1.5341

 

Optical Emission Spectroscopy

Experimental Setup

Figure 27 Spectroscopy Experimental Setup
Figure 27 Spectroscopy Experimental Setup

 

Results

Carbon Tetrafluoride Gas Flow Investigation

Figure 28
Figure 28 Plasma Map CF4 Gas Flow Investigation at 150W, 5L/min Ar + 0.4L/min CF4

 

Figure 29
Figure 29 Plasma Map CF4 Gas Flow Investigation at 150W, 5L/min Ar + 0.5L/min CF4

 

Figure 30
Figure 30 Plasma Map CF4 Gas Flow Investigation at 150W, 5L/min Ar + 0.6L/min CF4

 

Plasma Jet Stability

Figure 31 Stationary Temporal Investigation at 150W
Figure 31 Stationary Temporal Investigation at 150W, 5L/min Ar + 0.5L/min CF4

Figure 32 Stationary Temporal Investigation at 150W
Figure 32 Stationary Temporal Investigation at 150W, 5L/min Ar + 0.6L/min CF4

 

Carbon Tetrafluoride Distribution Within The Argon Plasma Jet

Figure 33 Argon Cross Sections
Figure 33 Argon Cross Sections

Figure 34 Carbon Tetrafluoride Cross Sections
Figure 34 Carbon Tetrafluoride Cross Sections

 

Power Investigation

Figure 35 Microwave Power Effect On Plasma Jet Discharge Photonic Intensity
Figure 35 Microwave Power Effect On Plasma Jet Discharge Photonic Intensity

Conclusion

A novel characterisation technique for a Coaxial Electrode Microwave Induced Plasma (CE-MIP) torch was developed using spectrum analysis and optical emission spectroscopy. A polar plot of the microwave energy emitted from the coaxial electrode was created and indicates that the plasma will be generated in one of two distinctive spatial locations. This result can indicate whether the plasma jet will operate in confined or non-confined mode.

The photonic intensity spatial maps generated show how the photonic energy within the plasma jet varies when different secondary gases are injected. The distribution of the energy was near Gaussian. Furthermore, the injection of 1% argon secondary gas into the surface layer region of the argon plasma jet resulted in an increase of the relative intensity of the near infrared photon emission by 9%.

A prototype Surface Wave Launched Microwave Induced Plasma system has been modified to discharge reactive atom plasma for the targeted ultra-precision material removal of optical surfaces. A Design Of Experiments has been conducted investigating the full range of all parameters and the result has been used to optimise the system for material removal of crystal quartz.

Further investigation into crystal quartz processing is underway.

Acknowledgements 

Funding: This work was supported by the UK EPSRC through the University Of Cambridge [EP/K503241/1]; and Cranfield University [EP/I033491/1]. The author would also like to thank Gooch & Housego for financial support.

Equipment: The author extends his thanks to ADTEC Europe for technical assistance and for the loan of the Coaxial Electrode Microwave Induced Plasma system.

  1. S. Moon et al., 2002, Characteristics of an atmospheric microwave-induced plasma generated in ambient air by an argon discharge excited in an open-ended dielectric discharge tube, AIP Journal: Physics of Plasmas, Volume 9, 4045
  2. K. Jankowski et E. Reszke, 2011, Microwave Induced Plasma Analytical Spectrometry, RSC Publishing, 1 – 36
  3. B. Smith et M. Carpentier, 1993, The Microwave Engineering Handbook: Microwave circuits, antennas and propagation, Volume 2, 4.14: Microwave measurements
  4. A. Balleri, 2017, Coordination of optimal guidance law and adaptive radiated waveform for interception and rendezvous problems, IET Radar, Sonar & Navigation, 
  5. C. Yubero et al., 2017, Gas temperature determination in an argon non-thermal plasma at atmospheric pressure from broadenings of atomic emission lines, Spectrochimica Acta Part B, Volume 129, 14 – 20
  6. K. Jankowski et E. Reszke, 2011, Microwave Induced Plasma Analytical Spectrometry, RSC Publishing, 1 – 36
  7. J. Amorim et al., 2015, Plasma Physics and Controlled Fusion, 57, 074001
  8. D. Doane, et E. Lori, 2011, Measuring Skewness: A Forgotten Statistic?, Journal of Statistics

 

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