F4280 Technology of Thin Film Deposition
& Surface Treatments 8. Electrical Discharges
Lenka Zajíčková
Faculty of Science, Masaryk University, Brno & Central European Institute of Technology - CEITEC
lenkaz@physics.muni.cz
spring semester 2024
E3RMO UNIUER5ITV OF" TECHNOLOGV
UNI
F4280 Outline Lenka Zajickovä 2/25
• Electrical Discharges
8.1 Classification of Electrical Discharges
8.2 Low Pressure Capacitively Coupled Plasma
8.3 Low Pressure Inductively Coupled Plasma
8.4 Comparison of Plasma Sources (low p, high f)
8.5 Atmospheric Pressure DBDs (AP-DBDs)
8.6 Atmospheric Pressure Plasma Jets
F4280
Electrical Discharges
Lenka Zajíčková
3/25
Classification of Electrical Discharges
► according to pressure
► low pressure
► atmospheric pressure
importance of mean free path
A = 1/nga
ng is gas density, a is collisional cross section and its comparison to Debye length A,) and plasma reactor dimensions
► according to frequency
► d.c.
► low frequency (50 Hz, audio range, low radio frequency up to 1 MHz)
► high frequency (typically 13.56 MHz)
► microwave (typically 2.45 GHz)
=>- importance of electron and ion plasma frequencies
_  / e2ne
^pe,i — \ -
and their comparison to discharge frequency
F4280
Electrical Discharges
Lenka Zajíčková
4/25
Examples of Discharges
► d.c. - d.c. glow discharge (planar diode), d.c. magnetron, vacuum arc
► low frequency (50 Hz, audio range, low radio frequency up to 1 MHz) - low pressure glow discharge, atmospheric pressure dielectric barrier discharge, glide arc, plasma jet
► high frequency (typically 13.56 MHz) - radio frequency capacitively or inductively coupled discharges
► microwave (typically 2.45 GHz) - microwave resonator, surface wave discharge, atmospheric pressure plasma torch
Visualisation of a Townsend Avalanche
Electric field
! Key
Ionisation event
- Ionising electron path
- Liberated electron path
Anode
vv y	DC Voltage
	Source
■ Original ionisation event
Cathode
► (A) region of non-self-sustaining discharge
► (BC) Townsend discharge
► (CD) subnormal glow discharge
► (DE) normal glow discharge
► (EF) abnormal glow discharge
F4280
Electrical Discharges
Lenka Zajíčková
6/25
■
ow Pressure RF Discharges
radio frequency (13.56 MHz) ► capacitively coupled (CCP) discharge ► inductively coupled (ICP) discharge
gas mixture
KS
rf generator rVj- |_C
matching unit
plasma
sheath (dark space)
■   o • 'J
\\\ co» /,/
q    // gas mixture
blocking matching rf generator capacitor unit
GAS INLET
r—— Kl- f UVVtK —I
j tjjjjjjj! ,
™1 PLASMA J
1 r I
WAFERS
COIL VACUUM SYSTEM
Dielectric
zzzzzzzzzzzzzzzzzzz
Substrate
sustained by r.f. current and voltage coupled via capacitive plasma sheath
F4280
Electrical Discharges
Lenka Zajíčková
7/25
Atmospheric Pressure Discharges
► planar configurations - low frequency    few kHz) dielectric barrier discharge (DBD)
Example of DBD - volume DBD (parallel plate electrodes) 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
|jW or RF
Substrate
Tuner Dielectric
L. Bardos, H. Baránková, Thin Solid Films
F4280	Electrical Discharges			ka Zajickova 8/25
				
	-ow pressure uapacmveiy uoupie	a Kiasi	ma	
Radio frequency discharges (typically 13.56 MHz) sustained by r.f. current and voltage coupled via capacitive plasma sheath are capacitively coupled plasma (CCP).
CCPs belong to glow discharges
(discharges with high voltage cathode sheath - electrons originate by secondary emission from the cathode).
F4280
Electrical Discharges
ka Zajíčková
9/25
► 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:
sB
A
sA
q
sA
A
sB
where scaling exponent q = 1.25-4 depends on the sheath and plasma glow models (Lieberman 1989 J. Appl. Phys. 65 4186)
Easily measurable d.c. self bias is set up between RF electrode and the ground
^bias = — (^4b — ^4a)
It is negative in the usual case of smaller RF electrode, i-e. VsB > VsA.
A/ SHEATH
I
blocking capacitor
y) 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.
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 = 0.39 Vrf + 0J3Uhu
ias
600 r
^  500 \
0)
cd
o >
CD
CD
.c
400 :
300
200 r
100 r
60     80    100   120 140
power [W]
160
dc bias-voltage at RF electrode Uh varied with W and p
>
450 400 350
S) 300
250 200 150 100 50 0
CO
-I—'
o >
CO CD
Ar + HMDSO
0     20     40     60     80    100   120 140
power [W]
160
E. Kedronovä et al. Plasma Process. Polym. 12 (2015) 1231
^bias = 0.83 Vrf
where £ = AsA/AsB. For a highly asymmetric case AsA < AsB, the asymmetry term tends to -1.
horizontal reactor
with inner electrodes
proud plynu
with outer electrodes
vertical reactor with inner parallel-plate electrodes
(parallel-plate diode discharge) radial gas flow
elektrody
vzorky
prstenec —» | pro vstup plynů
T -
vakuová vývěva
inverse radiál gas flow (showerhead electrode)
vstup plynů elektroda       ,\ vzorky
reakční
p'yny
k vývěvě
k vývěvě
F4280
Electrical Discharges
ka Zajíčková
12/25
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 ^^^H
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)
F4280
Electrical Discharges
Lenka Zajíčková
13/25
Construction of ICP reactors
O cylindrical geometry
Dielectric ^ /
Substrate
Rf
Rf bias
O planar geometry
Dielectric
I
Substrate
TT
O helical resonator
F4280                                                         Electrical Discharges	Lenka Zajíčková                                 14 / 25	
		
8.4 Comparison of Plasma Sources (low p, h	igh f)	
plasma source	frequency	density [cm3]	el. temper.	mg. field
CCP	r.f.	109-10u	1-5	no
	r.f.	10n-1012 (< 1013)	2-7	optional
ECR	m.w.	1010-1012	2-7	875 G
helicon	r.f.	10n-1012 (10u-1014)	2-7	20-200 G
F4280
Electrical Discharges
Lenka Zajíčková
15/25
5 Atmospheric Pressure DBDs (AP-DBDs)
Plate-to-plate configuration
(a)
High-voltage pulsed DC or AC or RF
(c) Substrate
Up to ± 30kV
Streamers
Dielectric barrier
Plasma
—K-*-í-í-í-í-í-^-í-C-K    \    \ I
LÍÉÍÉÍÉÍÉÍÉÍÉÍÉÍÉÍÉÍÉÍÉÍiIJ
> > > i < < < < t > > v < < < < t t < f < < < < < < }>>>>>>>»»»>»
>   >   >   > 3
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.
F4280
Electrical Discharges
Lenka Zajíčková
16/25
Filamentary versus Homogeneous DBD
Two forms of dielectric barrier discharges (DBDs) with parallel plate electrodes:
al^fpil
----'-{
15^2 85
-Zif
silicone film
ceramics
H.V.
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»
F4280
Electrical Discharges
Lenka Zajíčková
17/25
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
F4280 Electrical Discharges Lenka Zajíčková 18/25
Hi
omogeneous DBD (APGD) in Ar/acetylene
filamentary DBD in Ar
filamentary DBD in Ar/CH4
homogeneous DBD in Ar/C2H2
						
	fiirwi'niftorťi'Ť					
						
(80 lis (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
F4280 Electrical Discharges Lenka Zajíčková 19/25
Why to Use Homogeneous DBD for Deposition?
to eliminate unwanted surface structures and non-uniformities
D. Trunec, Z. Navrátil, P. SťaheI et al. J. Phys. D: Appl. Phys. 37 (2004) 2112:
deposition in APTD (HMDSO/N2) and in filamentary discharge
20 p.m
(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 0 íl m
'20 urn
10 ľjiT-
Olini D urn
a deposition 2 deposition
areas
Discharge width (45 mm)
to modify temperature sensitive polymer substrates and polymer nanofibers
F4280
Electrical Discharges
ka Zajíčková
20/25
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*! 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
F4280
Electrical Discharges
Lenka Zajíčková
21 125
8.6 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
F4280
lectrical Discharges
ka Zajíčková
22/25
Arc-Based Atmospheric 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
A. Fridman, Plasma Chemistry, Cambridge University Press 2008
Gas Output
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
A _      ,      f = 50Hz
wSurfaceTreat   max ? = 500 w,
max. U = 10kV
typical operation conditions: 500 W, 10kV, (dry) air 11.8 slm
F4280	Electrical Discharges		Lenka Zajíčková                                 24 / 25
	asma 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.
APL 60 (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
F4280
Electrical Discharges
ka Zajíčková
25/25
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	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.