PERFORMANCE-MODELLIERUNG, THERMALUND STRUKTURMODELL FÜR DEN RF-PBN Frank Scholze1, N. Hamer2, H. Neumann1 1 IOM Leipzig 2 Sgenia S.L., Madrid XXIII. Erfahrungsaustausch Oberflächentechnologien mit Plasma- und Ionenstrahlprozessen
RFPBN Contents Qualification of a Radio Frequency Plasma Bridge Neutralizer (RFPBN) as engineering model (EM) Requirements Simulation Results 2
RFPBN 3 Requirements by means of Operation conditions Electrical parameter Working gas Lifetime assessment Thermal Number of cycles Temperature range Mechanical Frequency range Acceleration and amplitude maxima Performance model Thermal Model (ESATAN or compatibly) Mechanical Model (NASTRAN or compatibly) Prediction of Electron current Gas flow RF power Wall potential Steady-state Transient temperatures Heat flows Resonance frequency Vibration frequency Shock forces
Requirements Operation parameter Extracted electron current : 2 100 ma Used working gas : Xenon, (Krypton if possible) Lifetime : > 30,000 h Unspecified RF power range Frequency Power supplies 4
Requirements 5 Thermal cycle tests Number of cycles : 4 Dwell time (hot and cold) : 2 h Temperature rate of change : 1.0 K / min Stabilisation criteria <1.0 K / hr Temperature minimum : -40 C (nonoperation mode) Temperature maximum : 55 C (nonoperation mode) Temperature minimum : -30 C (operation mode) Temperature maximum : 50 C (operation mode) 1 cycle
Requirements 6 Mechanical stress tests Resonance search 5-2000 Hz 0.5 g, 2 octaves per minute Sinusoidal vibration 5-20 Hz 11 mm (0-peak), 2 octaves per minute 20-100 Hz 20 g, 2 octaves per minute Random vibration 20-60 Hz +9 db/octave 2 min/axis 60-400 Hz 0.5 g2 /Hz; 2 min/axis 400-2000 Hz -6 db/octave 2 min/axis Shock test 500 Hz 100 g all axes, not combined 1000 Hz 100 g 10,000 Hz 100 g f= 1 2π x x
RFPBN Design Based on a RFPBN breadboard model developed at IOM Consists of : Cylindrical RF discharge chamber with orifice for electron extraction Matching network Housing and interface plate 7
8 Performance Model
Performance Model Performance Model Flowchart Running the simulation in a loop : 0. Starting with input parameters and initial conditions 1. Energy balance equ.: calculation of the plasma density and electron temperature 2. Comparison : plasma density value from the last and new run 3. Matching network : calculation of the reflected, forwarded and absorbed RF power 4. New absorbed RF power is new input for the energy balance equation, loop is closed 9
Performance Model 10 Energy Balance Equation (Lieberman Model) Absorbed RF power by the plasma Pabs eu B n0 Aeff ET Inputs are : Geometry Gas and material specific properties Working parameter Energy losses by excitation, ionisation and recombination processes between neutrals, ions, electrons and the chamber wall in the plasma region Results are : Bohm velocity Effective loss area Total energy loss per ion electron pair Electron temperature Rate coefficient for ionization and excitation Neutral and plasma density Wall potential etc.
Performance Model 11 Transformer Model and Matching Network RFPBN / ICP is described as a equivalent circuit. Coil and plasma of the neutralizers form a transformer : Primary side L11: coil of the neutralizer with the number of turns N Secondary side L22 : plasma with the resistance RP, inductance LP and the number of turns 1 Mutual inductance L12 can be varied by the coupling factor k Transformation the RF generator impedance to the RFPBN impedance XRFPBN to minimize the reflected RF power Solving the Kirchhoff laws for each mesh Calculation of currents I and power in the mathematical complex space for the equivalent circuit
Performance Model Simulation Model Programmed in Scilab1 is: free and open source software for numerical computation for engineering and scientific applications released as open source under the CeCILL license (GPL compatible), and is available for download free of charge available under GNU/Linux, Mac OS X and Windows XP/Vista/7/8 Included : Energy balance equation Transformer model Matching network Output : Electron current with and without plasma bridge, plasma density, wall potential etc. Best value for matching network Graphics as function of RF power and gas flow 1 http://www.scilab.org/scilab/about 12
Performance Model 13 Simulated Electron Current Distinction between extracted electron current with and without plasma bridge For fix matching, electron current is nonlinear Lieberman model Lieberman model + matching network
Performance Model Orifice Diameter Electron current simulated as function of : Gas flow RF Power Current increases with power and gas flow Orifice diameter of 0.2 mm is too small for required amount of 100 ma 14
Performance Model Wall potential Measure of the erosion by plasma The smaller the better Erosion of Orifice (change of gas balance in the plasma chamber) Ion collector (deposition of sputtered material on the inner wall, shielding of the EM field) Plasma chamber wall (isolation strength between plasma and coil decreases) 15
16 Thermal Model SolidWorks is used for simulation Performend by
Thermal simulation 17 Map of temperatures at the highest point of the thermal cycle Temperature of the whole plasma chamber becomes quite close to the temperature of the plasma Coil temperature that is in contact with the plasma chamber is very close to the temperature of the plasma Heat is transmitted from the plasma chamber to the matching network through conduction through the coil and also through radiation Temperature in the matching network is very uniform Big gradient of temperature at the rear part of the plasma chamber
18 Mechanical Model SolidWorks is used for simulation Performend by
Mechanical Simulation Resonance frequency Mica plates show signicant resonances 19
Mechanical stress simulation Vibration simulation Maximum RMS displacements appear for 69 Hz and near 332Hz Corresponds to natural frequencies Shock test : maximum displacement appears on the mica sheets 20
21 Summary Performance Model was programmed in SciLab Electron current as function of the parameters and geometry was simulated Required electron current range is shown Estimation of the lifetime is difficult, criteria is unknown RFPBN was imported in SolidWorks FEM module Load of the mica plates is strongest, the resonance frequency is too small ( < 140Hz) Fixing of the housing must be strengthened (screws too small) Thermal load is without problem
22 Acknowledgement Ronny Woyciechowski (constructor, IOM) ESA for financing (contract 4000109932/14/NL/RA)
23 Thank you for your attention