7.3 Plasma Sources according to pressure • low pressure • atmospheric pressure according to excitation frequency • d.c. (d.c. glow discharge or planar diode, thermionically supported diode, d.c. magnetron, vacuum arc) • audio frequencies (50 Hz up to 30 kHz) • low radio frequencies (30 kHz – 1 MHz) • high radio frequencies (usually 1 – 60 MHz, typically 13.56MHz, 27.12MHz) • microwave frequency (typically 2.45 GHz) s n/1importance of mean free path: n – density, s  collision frequency , ,0 2 , ie e ipe m ne   importance of electron and ion plasma frequencies: Classification of plasma sources: Laboratory plasma sources (DPE MU Brno) microwave torch 2.45 GHz, 106 Pa microwave resonator discharge (AsTeX) 2.45 GHz,  102 Pa 13.45 MHz,  101 Pa capacitively coupled RF discharge 13.45 MHz,  101 Pa inductively coupled RF discharge  102 kHz,  106 Pa coplanar barrier discharge at atmospheric pressure 7.3.1 Typical Low Pressure, High Frequency Plasma Sources  radio frequency capacitively coupled plasma (CCP)  r.f. inductively coupled plasma (ICP)  plasma sustained by electromagnetic wave: electron cyclotron resonance, helicon discharge, surface wave discharge Principle of r.f. CCP discharges discharge is sustained by r.f. current and voltage coupled via capacitive plasma sheath electrical circuit usually contains „blocking“ capacitor  dc current cannot flow in the circuit q B A sA sB A A V V        voltage divider action due to displacement current in plasma sheaths Construction of CCP reactors  horizontal reactor gas flow pump rf electrode grounded electrode - with outer electrodes - with inner electrodes  vertical reactor with inner parallel electrodes (parallel-plate diode discharge) withradialgasflow withinverseradialflow– showerheadelectrode Construction of CCP reactors Reactive Ion Etcher (RIE) R.F. Diode GEC (Gaseous Electronics Conference) reference cell DPE MU Brno UCP Processing Ltd. (Balzers) Principle of ICP discharges r.f. antenna in the form of coil attached to dielectric window – electromagnet creating rf mg field – induction of rf el field  non-capacitive coupling is a key point for low voltages (typically 20- 30 V) in sheaths at electrodes and reactor walls  Farraday shielding is used to surpress capacitive coupling (high voltage on the coil) Energy of electrical field is transferred to the electrons in thin „skin“ layer.  non-collisional processes – electrons „collide“ with induced oscilating el. field  energy is dissipated by collisional (ohmic) processes BE  rot skin depth (collisionless) Construction of ICP reactors  cylindrical geometry  planar geometry  helical resonator Principle of ECR discharges elektrony rotují kolem siločár magnetického pole s elektronovou cyklotronovou frekvencí, která je v rezonanci s frekvencí elmag pole, a takto získávají energii meBc /  f = 2,45 GHz  B = 875 G Princip helikonového výboje (helicon wave source) Pomocí speciální rf antény (13,56 MHz) se generuje helikonová vlna, která postupuje podél vnějšího mg pole. Tato vlna způsobuje velmi efektivní ionizaci => vysoké hustoty plazmatu. Mg pole má tři funkce (a) zvětšuje tloušťku skinové vrstvy, (b) pomáhá udržet delší dobu elektrony, (c) je dalším volným parametrem pro zlepšení uniformity hustoty 2 2 ck k c p z    kkz /cos   je úhel mezi směrem šíření (vlnovým vektorem k) a vnějším mg. polem B0 )( )( tkzmi erBB     Konstrukce helikonových zdrojů podle konstrukce antény je možné generovat určitý mód helikonové vlny díky vysoké hustotě generovaného plazmatu se používají v „downstream“ konfiguraci M0RI PMT komerční zdroj Comparison of (high f, low p) plasma sources plasma source frequency density [cm-3] el. temper. [eV] mg. field CCP r.f. 109-1011 1-5 no ICP r.f. 1011-1012 (  1013 ) 2-7 optional ECR m.w. 1010-1012 2-7 875 G helicon r.f. 1011-1012 (1011-1014) 2-7 20-200 G