Plasma and Dry Micro/Nanotechnologies 5. Electrical Discharges Lenka Zajíčková Faculty of Science, Masaryk University, Brno & Central European Institute of Technology - CEITEC lenkaz@physics.muni.cz spring semester 2023 Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 2/35 Classification of Electrical Discharges Various classification of discharges: ► according to pressure ► low pressure ► atmospheric pressure importance of mean free path A = 1/(ng- importance of electron and ion plasma frequencies _ / e2ne ^pe,i — \ - V £0"?e,i and their comparison to discharge frequency Plasma & Dry Technologies Classification of Electrical Discharges ka Zajíčková 3/35 ypes of Discharges ► d.c. - d.c. glow discharge or planar diode, d.c. magnetron, vacuum arc ► low frequency (50 Hz, audio range, low radio frequency up to 1 MHz) - low pressure planar diode, atmospheric pressure dielectric barrier discharge, glide arc, plasma jet ► high frequency (typically 13.56 MHz) - low pressure capacitively or inductively coupled discharges ► microwave (typically 2.45 GHz) - low pressure resonator, surface wave, atmospheric pressure plasma torch Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 4/35 radio frequency (13.56 MHz) capacitively coupled (CCP) discharge ► inductively coupled (ICP) discharge matching unit sustained by r.f. current and voltage coupled via capacitive plasma sheath Plasma & Dry Technologies lassification of Electrical Discharges ka Zajíčková 5/35 Atmospheric Pressure Discharges ► parallel plate electrodes - low frequency few kHz) dielectric barrier discharge (DBD) filamentary homogeneous ► plasma jets wide variety of frequencies (50 Hz gliding arc jet - 13.56 MHz plasma jet), configurations, working gases variations of gas temperature, active species, area Plasma & Dry Technologies classification of Electrical Discharges ka Zajíčková 6/35 ow pressure uapacmveiy uoupie Radio frequency discharges (typically 13.56 MHz) sustained by r.f. current and voltage coupled via capacitive plasma sheath are capacitively coupled plasma (CCP). ^ plasma sheath-(dark space) blocking capacitor blocking capacitor ^ RF CCPs belong to glow discharges (discharges with high voltage cathode sheath - electrons originate by secondary emission from the cathode). Plasma & Dry Technologies classification of Electrical Discharges ka Zajíčková 7/35 ► External electrical circuit usually contains „blocking" capacitor, i. e. dc current cannot flow Most CCPs are asymmetric AsA / AsB plasma acts as a voltage divider due to equal displacement currents through both the plasma sheaths: Vsb ( A^a q sA A sB An easily measurable d.c. self bias is set up between RF electrode and the ground ^bias = — (^4b — ^4a) which is negative in the usual case of smaller RF electrode, i.e. VsB > VsA. V PLASMA 1 SHEATH A J blocking capacitor ^ RF Ions are accelerated in high-voltage sheath at (smaller) RF electrode. Sheath voltage is proportional to RF voltage, i. e. RF power. If ions do not collide in the sheath (at low pressure of few Pa) they hit the surface with high energy of several 100 V. Plasma & Dry Technologies ^apacitiveiy uoupie lassification of Electrical Discharges asma - ka Zajíčková 8/35 Example for Ar + hexamethyldisiloxane (HMDSO) CCP at 13.56 MHz: Ar + HMDSO 1:1 mixture total flow rate 6 seem pressure p 7-30 Pa power l/l/ 5-150 W Sheath voltage at RF electrode VsB =0.39V0 + 0.73Ubias ► ► ► ► > (D 03 "5 > _l_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_ 0 20 40 60 80 100 120 140 160 dc bias-voltage at RF electrode Uh varied with W and p 450 _ 400 > 350 300 250 200 150 100 50 0 O) 03 o > 03 la Ar + HMDSO 0 20 40 60 80 100 power [W] 120 140 160 E. Kedronovä et al. Plasma Process. Polym. 12 (2015) 1231 power [W] Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 9/35 horizontal reactor with inner electrodes proud plynu with outer electrodes vertical reactor with inner parallel-plate electrodes (parallel-plate diode discharge) radial gas flow vzorky prstenec pro vstup plynů vákuová výveva inverse radial gas flow (showerhead electrode) vstup plynu elektróda Jj^ vzorky reakční r plyny k výveve k výveve Plasma & Dry Technologies lassification of Electrical Discharges ka Zajíčková 10/35 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 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 ^^^^M skin depth (collisionless) ^ 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) Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 11/35 Construction of ICP reactors O cylindrical geometry O planar geometry O helical resonator _ET short az as az az. T3 ZD ZD ZD 3D open Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 12/35 5.2 Cold Atmospheric Pressure Discharges Plate-to-plate configuration (a)_ Up to ± 30kV H ig h-voltage pulsed DC or AC or RF Dielectric barrier (c) Substrate Plasma jet configuration - — X TU uW or RF Streamers Plasma ——<:—t—c—T > i > > > s- a- ^ > > > ^^H- » > » » <<< 9 > > > > 9 Substrate At low p, the collision frequency is low electron energies remain high compared to ion neutral energies non-equilibrium (cold) plasma. At high p, the collision frequency is high plasma tends to equilibrate temperatures formation of streamers (fast-moving ionization fronts in the form of filaments) - precursors of sparks (hot plasmas) Suppression of sparks using: ► high-frequency AC fields or short-pulsed DC power ► dielectric barriers on AC electrodes ► high gas flow rates ► special electrode shapes with multiple structures suitable gas, e.g. He. Tuner Dielectric L. Bardos, H. Baran ková, Thin Solid Films Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 13/35 5.2 Atmospheric Pressure DBD (AP-DBD) Two forms of dielectric barrier discharges (DBDs) with parallel plate electrodes: al^fpil ----'-{ o, 15^2 85 -Zif silicone film ceramics H.V. TOT IT vinyl chloride ► filamentary ► homogeneous Stabilization of homogeneous DBDs requires suppression of filament formation. Important role of ► structure and material of electrodes e.g. M. Kogoma, S. Okazaki, JPD (1994) 27 1985 ► higher frequencies of power supply T. Nozaki et al., Plasma Process. Polym. (2008) 5 300) ► gas mixture (He, Ne, N2, Ar + NH3 etc.): ► homogeneous DBD in He, Ar/NH3 and N2 F. Massines et al. Surf. Coat. Technol. 174-175, 8 (2003); Plasma Phys. Controlled Fusion 47, B577 (2005). ► PECVD in HMDSO/N2 and HMDSO/N2/synthetic air mixtures D. Trunecetal. J. Phys. D: Appl. Phys. 37 (2004) 2112; J. Phys. D: Appl. Phys. 43 (2010) 225403 ► PECVD in Ar/C2H2 M. Elias etal. J. Appl. Phys. 117(10) (2015) 103301 metal mesh 0.033 32S» Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 14/35 H, omogeneous Dielectric Barrier Discharges Two different forms of homogeneous discharges were classified by Massines et al. Both start with Townsend breakdown initiating a Townsend discharge but ► in He, during the current increase, the discharge transits to a glow discharge (ne « 1011 cm-3) having a cathode fall and a positive column if gas gap is > 2 mm - atmospheric pressure glow discharge (APGD) ► in N2, the ionization level is too low (ne « 108 cm-3) to allow formation of cathode fall. Localization of the electrical field and the glow regime cannot be achieved - atm. pressure Townsend discharge (APTD). Visualisation of a Townsend Avalanche Electric field O lonisalion euenl | - Ionising electron path ' - Liberated electron path j i.----------------------------1 Anode y DC Voltage i Source Original ionization event Cathode ► (A) region of non-self-sustaining discharge ► (BC) Townsend discharge ► (CD) subnormal glow discharge ► (DE) normal glow discharge ► (EF) abnormal glow discharge Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 15/35 H, omogeneous DBD (APGD) in Ar/acetylene filamentary DBD in Ar filamentary DBD in Ar/CH4 homogeneous DBD in Ar/C2H2 a riVrrrt^iťié^ftVŤ (80 /is (one half-period) exposure time) ► difference caused by possibility of Penning ionization of C2H2 in Ar ► Ar 1 s5 metastable -11.55 eV, C2H2 ionization potential 11.40eV butCH4 12.61 eV M. Eliáš etai, J. Appl. Phys. 117(10) (2015) 103301 > > > 0.1 0.2 t(ms) DBD (a) DBD in pure Ar, (b) DBD in Ar/C (c) APGD in Ar/C2H2 Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 16/35 Why to Use Homogeneous DBD for Deposition? to eliminate unwanted surface structures and non-uniformities D. Trunec, Z. Navrátil, P. St'ahel et al. J. Phys. D: Appl. Phys. 37 (2004) 2112: deposition in APTD (HMDSO/N2) and in filamentary discharge 20 um (I M.m - 'o Mm 20 Mm H. Caquineauet. aid. Phys. D:Appl. Phys. 42(2009) 125201: Local increased of the deposition rate, "deposition spots", due to non-uniform power dissipation in micro-filaments: 298 n m 149 inn i) jim '20 urn 10 Mm flwlt'O Mm a deposition 2 deposition areas Discharge width (45 mm) to modify temperature sensitive polymer substrates and polymer nanofibers Plasma & Dry Technologies classification of Electrical Discharges ka Zajíčková 17/35 roblem of Film Uniformity Amospheric-pressure plasmas are characterized by high collision frequencies of particles (several orders of magnitude higher compared to low pressure) =>- Delivery of active species to the substrate is much more advection than diffusion-driven (opposed to low-pressure). High electron-neutral collision frequency =>- fast monomer conversion Basic gas delivery set-ups Gas flow Gas flew Gas flow r*l Pn&flH H H OH P A * * * * * * are modified for optimization of flow patterns by gas dynamics simulations P. Cools et ai, Plasma Process. Polym. H. Caquineau et al. J. Phys. D: Appl. 2015, 12, 1153-1163 Phys. 42(2009) 125201 grounded Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 18/35 Gas Dynamics; simulations in Our Set-up Solving the Navier-Stokes equations (laminar flow) in full 3D geometry for pure Ar (results are shown for 1550 seem): =>- Complex flow patterns inside the buffer chamber make the flow through the slit relatively even but better designs of the buffer chamber can be found! Plasma & Dry Technologies classification of Electrical Discharges ka Zajíčková 19/35 Variations of four different geometries tested gas inlet bottom ceramics [electrode underneath) V-UfJfJti electrodes plastic tube buffer chamber exit slit 0.02 0.02 -0.05 L. Zajíčková et al. Plasma Physics and Controlled Fusion 59(3) (2017) 034003 Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 20 / 35 Variations of four different geometries tested Velocity magnitude [m/s] and direction Velocity magnitude [m/s] and direction 1.4 1.2 1 0.8 0.6 0.4 0.2 ? -1 í í í í j 5 5 . V s \ \ \ \ \ \ \ \ \ "v. V \ \ \ \ \ \ \ \ -vv.v.N.\\\\\\ \\\\\\\\\ .'//////// '//////// f/s////ss - s / / / s s ^ - s / / S s 1 ■ s s " s / ■ / / ■ / / ViA " S / / / S -f S / / / / S / / / / / / / s / / / / / / Hill! i ii ^»^ ■. - ------ \\\\\\\^' \ \ \ \ \ \ \ \ ' i n \\ \\ n x-coordinate [cm] 0 x-coordinate [cm] U 1 0.8 0.6 0.4 0.2 Velocity magnitude [m/s] and direction Velocity magnitude [m/s] and direction 1.4 1.2 1 0.8 0.6 0.4 0.2 H 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 K-coordinate [cm] x-coordinate [cm] Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 21 / 35 Does It Work in Real Life? (case study for DBD co-polymerization of MA and C2H2 in Ar, no electrode movement) igas inlet plastic tube buffer chamber exit slit bottom ceramics [electrode underneath) LOfjpcr electrodes Interference colours are measured by imaging spectroscopy refractometry fitting of optical data provides spatially resolved film thickness 0 mm 5 1 10 15 20 25 5 10 15 20 254 Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 22/35 5.3 Atmospheric Pressure Plasma Jets ► operating in local thermal equilibrium (LTE) Te ~ Tn, ne > 1015 cm-3 - transferred arc (torch), plasmatron ► translational plasmas (non-LTE but with a significant heating of the background gas 7"n ~ several thousand Kelvin - gliding arc, expanding sparks, non-transferred arc ► non-LTE "cold" plasma jets 7"e > Tn, Tn = 300 - 1000K, ne < 1013 cm-3 Geometry 1 Gas O LU I Tube Tube -cÄo- T Geometry 2 Geometry 4 Geometry 7 Geometry 5 G.'i:. Geometry 3 Gas Geometry 6 Gas r-1 1 ) Gas J 9 Plasma Region / Gas flow Electrode 1 Electrode 2 Insulator or metal Insulator High Vollaye Source Microwave generator Q High Current Source | j Microwave resonator J. Winter at al. Plasma Sources Sei. Technol. 24 (2015) 064001 Plasma & Dry Technologies C lassification of Electrical Discharges Len ka Zajíčková 23 / 35 5.4 Arc- Based Atmosp heric Pressure Plasma Jets Plasma jets: ► operating in local thermal equilibrium (LTE) Te ~ Tn, ne > 1015 cm-3 - transferred arc (torch), plasmatron ► translational plasmas (non-LTE but with a significant heating of the background gas) 7"n ~ several thousand Kelvin - gliding arc, expanding sparks, non-transferred arc ► non-LTE "cold" plasma jets 7"e > Tn, Tn = 300 - 1000K, ne < 1013 cm"3 J. Winter at al. Plasma Sources Sci. Technol. 24 (2015) 064001 Industrial plasma jets based on arc: Sura Instruments, non-transferred arc (patent WO 2015/107059 A1), dc or low f, argon flow, precursors for deposition PlasmaTreat, non-transferred arc (DE10223865 A1, US2002179575, DE102008058783 A1), 1-100 kHz, airflow, plasma cleaning, activation, deposition ... and some others Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 24/35 A. Fridman, Plasma Chemistry, Cambridge University Press 2008 Gas Output V. Non-Equilibrium f State .Equilibrium estate Breakdown Gas Inlet The glide arc can be operated in the transitional regime (combines the benefits of both equilibrium and non-equilibrium discharges): ► the discharge starts thermal ► becomes non-thermal during the space-time evolution SurfaceTreat f = 50 Hz max. P = 500 W, max. U = 10kV typical operation conditions: 500 W, 10 kV, (dry) air 11.8 slm Plasma & Dry Technologies Classification of Electrical Discharges Lenka Zajíčková 25/35 .5 RF Plasma Jets "Cold" plasmas required for surface modification of thermosensitive materials (bonding, painting, printing) or plasma medicine/agriculture Non-LTE atmospheric pressure plasma jets need to prevent the transition to arc pulsed or high f discharges, a dielectric barrier at one or both the electrodes Earliest cold RF plasma jet Development of cold RF jets in Brno, Masaryk University proposed by Koinuma et al. APL60 (1992) 816 M. Klíma et al. Czech Patent PV147698 (1998), US6,525,481 (2003) J. Janča et al. Surf. Coat. Technol. 116-119 (1999) 547 asma )lassification of Electrical Discharges ka Zajíčková 26/35 In Brno, we developed a new type of RF plasma jet. Unlike other jets working with capacitive coupling (E component important) it should generate EM with both the components (E, H) high (according to preliminary EM field calculations Emax = 105 V/m, Hmax = 800 A/m). v e e e jj ' M L 1 8 1 1 -J I | I I I I I I I I I I II I I M I I <4 | • IL 551 n n RF plasma slit jet is successfully constructed with the width of 15 or 30 cm. typical operating conditions: 300-600 W, 50-100 slm of Ar, 0-4 slm of N2 The jet accommodates periodic structures consisting of varying combinations of inductors with specially designed geometry and winding - matching is an integral part.