Plasma Physics 2 Lesson 05: High frequency discharges Vylepsit … 1 Lecture series contents 1.Townsend breakdown theory, Paschen‘s law 2.Glow discharge 3.Electric arc at low and high pressures 4.Magnetized low-pressure plasmas and their role in material deposition methods. 5.Brief introduction to high-frequency discharges 6.Streamer breakdown theory, corona discharge, spark discharge 7.Barrier discharges 8.Leader discharge mechanism, ionization and discharges in planetary atmospherres 9.Discharges in liquids, complex and quantum plasmas 10.Thermonuclear fusion, Lawson criterion, magnetic confinement systems, plasma heating and intertial confinement fusion. Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 2 High frequency discharges •enable plasma discharge even for non-conductive electrodes • • •electron plasma frequency • • • • •numerically • • • • • • • • • • • • • • •varieties: - MHz = RF plasma, ohřev plazmy posuvným proudem - capacitively coupled plasma – collisionall and collisionless heating, can run at very low pressures - inductively coupled plasma – heating on the plasma surface or in plasma volume - GHz = microwave discharges, surface wave heating or volume heating • electrons do have enough time to react to the external electric field! Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic Text Description automatically generated with medium confidence 2a High frequency discharges Chart Description automatically generated A picture containing clock, watch Description automatically generated A picture containing text Description automatically generated A picture containing text, watch, clipart Description automatically generated Logo Description automatically generated with low confidence •enable plasma discharge even for non-conductive electrodes • • •electron plasma frequency • • • • •numerically • • • • • • • • • • • • • • Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic HIGH FREQUENCY BREAKDOWN • •At about 100 kHz, the time that the electron needs to transfer between the electrodes starts to be comparable with the period of the external electric field. The electron avalanche does not reach the electrode and it reverses direction! • •electron heating in the external field is given as: • • • • • • • • • • • • • • •The optimum power is reached when: 3 High frequency breakdown Mean electron energy External electric field Low frequencies Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 3a High frequency breakdown •At about 100 kHz, the time that the electron needs to transfer between the electrodes starts to be comparable with the period of the external electric field. The electron avalanche does not reach the electrode and it reverses direction! • •electron heating in the external field is given as: • • • • • • • • • • • • • • • •The optimum power Mean electron energy External electric field Mean electron energy (prior to multiplication) External E field nízké frekvence High frequency Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 3a CCP vs ICP vs MW plasma Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic DIFFUSION THEORY FOR HF BREAKDOWN • 5 High Frequency breakdown •The Paschen curve for RF breakdown has two minima: 1) Classical DC-like: Electron attains some „ideal“ energy when traversing between electrodes 2) Resonance capture of electrons: electrons bounce off a plasma sheath that is approaching them. Drift-diffusion branch – secondary emission affects the breakdown voltage multi-pactor branch – electrons interact more with walls than with themselves Emission-free branch, breakdown voltage does not depend on the electrode distance Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic CAPACITIVELY COUPLED PLASMA ANATOMY • 3a CCP plasma anatomy Near-electrode electron heating (stochastic) Bulk plasma heating (ohmic) Typical frequencies: 10 MHz – 100 MHz (13.56 MHz ”standard”) Typical pressure: 1 Pa – 1000 Pa Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 6 CCP plasma anatomy •Simples case are two planar electrodes, covered with dielectric. Potential and density evolution during phase as follows: Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 8 CCP plasma – self bias voltage •In CCP plasmas, we see emergence of the so-called self-bias DC voltage. • •empirically, we see: where A1, A2 are electrode surface areas • • • •An approximate derivation can be made as follows – we assume that the ion current on both electrodes has to be the same: • • • • • •And if we further assume that the ion densities in front of both electrodes are the same we obtain • • • • • • •And ultimately The smaller electrode has to have higher negative voltage to draw the same ion current => it acts as a cathode and it can be used e.g. for sputtering of material This derivation is still extremely simplistic, will be done more properly in Plasma Physics 3 Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic INDUCTIVELY COUPLED PLASMA ANATOMY • 3a ICP plasma anatomy Localized power deposition Typical frequencies: 10 MHz – 100 MHz (13.56 MHz ”standard”) Typical pressure: 1 Pa – 1000 Pa Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 9 ICP plasma anatomy •Plasma is generated by electromagnetic induction – The RF coil creates H field, which creates poloidal E field in the volume •This wireless transfer of energy makes it possible to run the system at very high temperatures, because the walls can be made of ceramics with high thermal resistivity. Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic MICROWAVE PLASMA ANATOMY • 3a MW plasma anatomy Typical frequencies: 100 MHz – 10 GHz MHz (2.45 GHz ”standard”) Gas pressure: 100 Pa – 1 atm Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 10 MW plasma anatomy •There are various setups for MW plasma generation – surfatron, surfaguide, coaxial line, …. Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic APPLICATIONS OF CCP, ICP, MW • 10 CCP and ICP applications – semiconductor etching Etch Overview RPI SCOREC - Plasma Etch Modeling Introduction to Plasma Etching - Oxford Instruments Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 10 CCP application – gridded ion thruster Gridded ion thruster - Wikipedia Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 10 ICP application – DC torch analogue Inductively coupled plasma - Wikipedia Inductively Coupled Plasma (ICP) Spectrometry - Centre for Advanced Coating Technologies •At atm. pressure and high power levels, so-called ICP torches can generate fully ionized plasmas. Support similar power ranges (5 kW – 1 MW) •Advantage over DC torches: no contact with metal Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 10 MW application – DC torch analogue •MW plasma is also being used as an analogue to DC torches •As opposed to DC or ICP torches, MW plasma typically operates at lower powers (tens of kW) •Microwave plasma can also be non-LTE (plasma does not have to be so conductive to enable MW absorption) => lower plasma densities achievable Atmospheric Microwave Plasma Technology – High Temperature and Power-to-X Applications - Muegge Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic 10 MW application – Plasmaline for solar cells •At atm. pressure and high power levels, so-called ICP torches can generate fully ionized plasmas. •Advantage over DC torches: no contact with metal Duo-Plasmaline | Institute of Interfacial Process Engineering and Plasma Technology | University of Stuttgart Duo-Plasmaline for Green Technology: Always a step ahead in productivity and throughput - Muegge Pozor je to pro verejnost … jednoduseji, schemata nazorna, ne moc analyz, jen par rovnic Takeaways •High frequency breakdown • •Different layouts for wave-driven discharges • •CCP, ICP, MW – different power absorption mechanisms • •Applications