Plasma and Dry Micro/Nanotechnologies 1. Introduction to Plasma Processing
Lenka Zajíčková
Faculty of Science, Masaryk University, Brno & Central European Institute of Technology - CEITEC
lenkaz@physics.muni.cz
spring semester 2023
Plasma & Dry Technologies
. Introduction to Plasma Processing
ka Zajíčková
2/19
o 1. Introduction to Plasma Processing
1.1 How to Create Plasma?
1.2 Fundamental Plasma Parameters
1.3 Conditions for Plasma as Ionized Gas
1.5 Plasma Sheath
1.6 Overview of Plasma Processing Methods
► What is plasma? The 4th state of matter: the gas containing electrons and ions, fulfilling some special conditions    ionization processes, ionization degree
► Fundamental plasma parameters: electron temperature and density. What about parameters of other particles?
► Quantities and terms important for plasma physics: Debye length, plasma frequency, cyclotron frequency, Larmor radius.
► Plasma interacting with solid matter - plasma sheath (Boltzmann relation for electron density, Bohm velocity), plasma potential, floating potential.
► Why plasma in material processing?
► Many existing methods: plasma treatment, magnetron sputter-deposition, plasma enhanced CVD, plasma polymerization, plasma synthesis of nanoparticles, plasma etching.
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1.1 How to Create Plasma?
Plasma is 4th state of matter (created from neutral gas by ionization, i. e. generation of electron-ion pairs):
♦ j
energy
Adding sufficient energy to molecular gas leads to the dissociation of molecules into atoms due to collisions of the particles having energies higher than bond energy.
If the particles have even higher energie, the collisions leads to ionization (electrons are set free from the atom).
creation of plasma as quasineutral system of electrons, ions and neutrals.
plasma ionization degree a =
a « 1 - fully ionized plasma a < 1 - weakly ionized plasma
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► Increase of temperature    The system is in thermodynamic equilibrium. Electron temperature 7"e and degree of ionization ai = /7i/(/7i + ng) are binded by Saha equation - not usual for laboratory plasma but can be often found in nature (space plasma).
S Systém je v termodynamické rovnováze - tj. popsán jedním parametrem = teplotou T
S Jestliže uvažujeme systém N slabě interagujících částic, který je uzavřený (nevyměňuje si částice s okolím), pak je průměrná hodnota počtu částic ve stavech s energií E{ dán Boltzmanovým vztahem (faktorem)
Ei
Nt = C exp —
kTJ'
kde C je normalizační konstanta určená ze vztahu   ^    ^    -ex^^ EJkT)
Výše jsme předpokládali, že počet stavuje pro každou skupinu stavů o energii E{ stejný. Pokud musíme vzít do úvahy statistickou váhu stavu g{ /   Et \
Nt = C Qi exp -
kTJ'
S Pro plazma v termodynamické rovnováze je (elektronová) teplota a stupeň ionizace jsou svázány Sáhovou rovnicí. 2
a
1 — a
Eioniz.\
= -Cexp[-^r)
i
n
S Laboratorní plazma není obvykle v termodynamické rovnováze, v přírodě je to častější (astrofyzikální plazma). ^^^^^^^^m
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1. Introduction to Plasma Processing
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Several Methods for Plasma Generation
► Using additional ionization processes    the ionization degree is increased above its equilibrium value. When the source of additional ionization is switched off the plasma fades out due to recombination.
photoionization - ionization potential of e. g. oxygen atom is 13.6eV    photon with 91 nm (vacuum UV)
photon
ejected eteclmw
gaseous electrical discharges - el.
field accelerates free electrons to energies sufficient for ionization
Anode
Positive column with filiations
Negative glow Cathode
glow
Faraday dark space
Cathode layer
atfrn
resulting ion
example: d.c. glow discharge - laboratory plasma
example: Earth ionosphere - natural photoionized plasma
Plasma & Dry Technologies 1. Introduction to Plasma Processing Lenka Zajíčková 6/19
Where to find plasma?
glow discharge for etching &
thin film deposition
(CCP) n =1014-1016 m-3 T =1-2 eV
plasma torch for waste treatment ne=1021-1024 m-3 Te=0.5-2 eV (LTE)
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1. Introduction to Plasma Processing
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25
Solid Si at room temperature
3 De
20
High pressure arcs
Laser		
plasma		Focus
Stwck tubes
Treta pinches
Fusion reactor
electron temperature Te in eV, 1 eV = 11 600 K
Outside thermodynamic equilibrium other temperatures discussed: ions (7"i), neutrals (7"n)
electron concentration ne = n{
magnetic field B
Other essential physical quantities are derived from Te, ne, B\
Debye length AD =
plasma frequency ujp « upe
cyclotron frequency uc = qB/m Larmor radius rc = v±/uc thermal velocity etc.
e2r?e e0me
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Introduction to Plasma Processing
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Natural length scale in plasma is Debye Natural frequency (time) scale in plasma
length is plasma frequency
V nee2 J
1/2 / 2\1/2
meso
► Ionized gas is the plasma namely if ne = ri[ on the scales of L > Ad-
► Plasma contains many interacting charged particles, condition: neAD > 1
► Plasma exhibits collective behavior of electrons that is not much disturbed by electron-neutral collisions (collision frequency i/en), conditions: a;pe/(27r) > v{
en
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1. Introduction to Plasma Processing
Lenka Zajíčková
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Plasma Conditions - Collective Behaviour
Plasma contains many interacting charged particles. Condition:   ne\D ^> 1.
Plasma exhibits collective behavior of electrons
(plasma frequency)
pe
Hee2
1/2
that is not much disturbed by electron-neutral collisions: a;pe/(27r) > */en
ID D
C
U U
A plasma oscillation: displaced electrons oscillate around fixed ions. The wave does not necessarily propagate.
podle Chen & Chang 2003
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Introduction to Plasma Processing
Lenka Zajíčková
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Low temperature plasma of gaseous discharges provides unique environment for material processing:
► hot electrons (Te few eV, 1 eV = 11 600 K)
dissociation of molecules into reactive species
e~ + AB —> A + B + e"
► positive ions that can be accelerated to hundreds of eV near solid surface
sputtering of targets, implantation, modification of surfaces and growing films
► cold neutral gas
=>- highly energetic process can be kept in a vessel, heat sensitive materials can be treated (e.g. polymers)
Quasineutrality ne « n{ is fulfilled on the scale L > AD, i. e. on the dimensions larger than Debye length
but this is violated in regions adjacent to walls and other solid objects in contact with plasma - plasma sheath.
Plasma sheath regions are very important for plasma processing. Plasma potential is always the most positive potential    electrons are repelled by a Coulomb barrier, ions accelerated towards solid surfaces.
1/2
-d
t)
d
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1.5 Plasma Sheath for Low Voltage Drop
Charge densities and potential in bulk plasma, presheath and sheath adjacent to the wall or electrode
sheath
Relations valid for
► low sheath voltage (at floating or grounded walls)
► weakly ionized plasmas 7"e ~ feweV, 71 ^ 0
Densities of electrons and positive ions are expressed as
eV
— n*ekTe
A?i = A?s    1 -
2eV
1/2
where vs is ion velocity at the sheath edge, approximated by so called Bohm velocity uB
vs> uB =
kTe M
Charge density at the sheath edge is
ns « 0.5n0.
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. Introduction to Plasma Processing
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1.5 Plasma Sheath at Floating Wa
Charge densities and potential in bulk plasma, presheath and sheath adjacent to the wall or electrode
Electron and ion fluxes
sheath
1 SkTe eV
re    -na\ -e^e   r\ nsuB
4      V 7T/T?
have to equal at the floating wall
i/        i/        - kTe i f2lim\
afloat — ^plasma — ~- m
2e
KIT
For a typical low pressure discharge:
► Te = 2eV, ne = 108 cm"3
► in argon
floating potential is approx. 57"e = 10 V sheath thickness is approx. 5AD = 0.37 mm.
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. Introduction to Plasma Processing
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1.5 Plasma Sheath for High Voltage Drop (Applied Voltage)
High-voltage sheath (a voltage is applied) can be approximated by a model with Child-Langmuir sheath:
Sheath is artificially divided into Debye sheath which contains electrons and high-voltage Child-Langmuir sheath which has ions only.
Then, current density y, voltage drop V0 and sheath thickness d are related by the Child-Langmuir Law of Space-Charge-Limited Diodes
1/2 e0V*/2
d2
following previous example with assumption V0 = 400 V d = 30AD, total sheath thickness 35AD; i.e. about 1 cm
i
Chrid-Lanepnur
PLASMA
An exact calculation for a plane sheath shows that C-L scaling is not followed unless the sheath is very thick (notice log-log scale)
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1. Introduction to Plasma Processing
Lenka Zajíčková
15/19
Table 4-2  Secondary Electron Coeficients 7( for Argon Ion Impact
Ion Energy
	10 eV	100 eV	1000eV
Mo	0.122	0.115	C.118
W	0.096	0.095	0.099
Si (100)	0.024	0.027	0.039
Ni (111)	0.034	0.036	0.07
Ge (111)	0.032	0.037	0.047
high-energy ion bombardment at the cathode
Plasma
Quasi-Neutral Transition Region
CATHODE
PLASMA POTENTIAL
ANODE
Positive
Self-limiting process a^e : 1) electron move away from the plasma Region —> 2) the plasma results in more positive
—» 3) it hinders the escape of the negative electrons
I APPLIED I POTENTIAL
CATHODE SHEATH
A. LARGE ANODE
2.0
Secondary emission     y o ratio 6
el. bombardment significant at the anode and walls
u(0)>|
V4 rrijUiO)2 = eV(0)
therefore,
V(0) = miU(0)2/2e
= (nV2e) (kiyrrii)
= kTe/2e
MO
1000
1500
2000
Incident energy E. eV
Secondary emission coefficient« of different metals as a function of the energy of incident electrons (Hemenway et al. 1967)
Plasma & Dry Technologies
1. Introduction to Plasma Processing
Lenka Zajíčková
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1.6 Overview of Plasma Processing Methods
Plasma etching - irreplaceable
anisotropic dry etching: combination of chemistry and effect of ions (reactive ion etching)
® I cm
O Material atom O Activated material atom Vdal i le product
*. V.*. '« *■*» -m VaT.V.'. . *# ****** *•*■*. VV**■»*•**** ■. *»*, '« *■*.** '«*»*» . V ÍVW>> .tV-****: V***'* ** *•*• ** ** ***■ ** ****** ******** 1 ************** *»***•*. ******** ***.
Vť*W*V*V**-W#>*^^
Plasma treatment
dry modification of the top surface layer (no material added)
► roughness
► surface chemistry
► dangling bonds
in Ar, 02, NH3 ... discharges
Plasma synthesis - high purity
plasma in liquids
► plasma synthesis of nanoparticles
e.g. iron oxide superparamagnetic NPs (minimum toxic effects for cells)
Plasma deposition of thin films
see next slide
Plasma & Dry Technologies
1. Introduction to Plasma Processing
Lenka Zajíčková
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Plasma Deposition
Plasma deposition of thin films
► plasma enhanced chemical vapor deposition (PECVD)
Si(OC, 1,1-,\ + e -+ Ki{pC,U,)y(OU)+ QMi + c~ O, + tf -* 20 + (~
O + SHOC^J^OH)^ Si {OCH J,(OH), + Otf-O
iiiilJ^llll^flSliii
,\he;ith
feature
Silicon
► physical vapor deposition (PVD) - dc diode sputtering, magnetron sputtering
Ground shield -
1
Anode
Substrate
8L «
Plasma
\
Cathode (target)
OOOOOOOOOOOOO
waier cooling
T
Plasma & Dry Technologies
1. Introduction to Plasma Processing
Lenka Zajíčková
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Applications of Plasma Treatment and Deposition
Material surface can be plasma treated or plasma coated with a thin film thickness of the plasma modified layer ranges from few nm to tens of /xm.
► hydrophilic surfaces for improved painting, printing, lacquering
► surfaces for improved adhesion of coatings or strength of adhesive bonds
► hydrophobic surfaces for nonadhesive, self-cleaning or antifouling applications
► thin films for electronic applications (a-Si:H, Si-based dielectric films)
► thin films for optical applications (low and high refractive index oxides)
► hard and tribological coatings (metal nitrides, metal carbides, diamond like carbon)
► barrier coatings (a-C:H, organosilicon plasma polymers)
► bioapplications such as biosensors, drug immobilization, tissue engineering (surface functionalization, plasma polymers)
^^^^^^
Plasma & Dry Technologies
. Introduction to Plasma Processing
ka Zajíčková
19/19
Unique Features of Plasma Technologies:
► dry process (gas phase), i.e. with low consumption of chemicals,
► offering replacement of toxic and explosive reactants, i.e. environmentally and user friendly
► irreplaceable for anisotropic etching required in microelectronics or MEMS applications
► preparation of new materials that cannot be obtained by pure chemical methods How it can be used?
► in vacuum reactor (at low pressure) - excellent control over the process
► at atmospheric pressure with no need of vessel (except because of safety reasons in case of toxic chemicals)