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Gas Phase and Collision Processes

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Gas Phase and Collision Processes 1.1. Introduction Introduction In the previous chapter, we considered the behavior of neutral gas. Although cold plasmas involve ionized gases, the degree of ionization of a typical discharge is 10-4 Therefore plasma is typically at neutral ground state that can be described by the gas laws. Prime features of plasma is that ≈ 1018 electrons ions-pairs are produced per second. Collisions phenomena have therefore to be studied. ChapterChapter II: II: GasGas phase phase andand collisioncollision processesprocesses [1] Basic data of plasma physics. American Institute of Physics, New York. ISBN: 1-56396-296-X. [2] “Cold Plasma in Materials Fabrications”, A. Grill, IEEE Press, NY(1993). ISBN: 0-7803-1055-1. [3] “Film deposition by plasma techniques”, M.Konuma, Springer Verlag,(1992). ISBN: 3-540-54057-1. II.1 Plasma, S. Lucas II.2 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions Detailed modeling of hydrocarbon nanoparticle 2.1 Mean free path and reaction rate between two particles nucleation in C2H2 discharge K. De Bleecker, A. Bogaert, Phys. Rev. E 73, 026405(2006) Suppose a particle (a) traveling in a group of particles (b). For a given interaction between particle a and particles b, we have: RF // plate, 13.56 MHz, simulation 1 (II.1) λab = σ ab.N% b Thus, the collision frequency is ur <>V a ur (II.2) vVNab==<>a ..σ ab% ab λab That gives the reaction rate (R): ur 31− RV=<a >.σab . NNs%% a . b (cm . ) 31− (II.3) RkNN= .%%ab . (cm . s ) K: is the reaction rate constant (cm3 s-1). II.3 Plasma, S. Lucas II.4 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions 2.2 Type of collisions Elastic collisions θ M2 Particles have two types of energy: kinetic and potential M1 Kinetic: related to motion. An increase = increase of speed E4.M.M2122 Potential: related to electronic structure of the colliding ions, =.cos2 θ (II.4) atoms, molecules, …. An increase is manifested by E1 (M12 +M ) ionization and other excitation processes. An elastic collision is one in which there is an interchange of kinetic energy only. θ = 0 •There are conservation of momentum and kinetic energy of translation motion. If M1 = M2 ) E2/E1=1 •No excitation occurs Max energy is transferred (After collision, M is at rest, Example: billiard: only kinetic energy is exchanged 1 M2 is moving). An inelastic collision has no such restriction: there is If M << M (e.g. e → ion) ) K = 4.M / M ≈ 10-4. generally an increase in potential energy: 1 2 1 2 • Excitation • Ionization Electrons do not transfert kinetic energy in elastic mode II.5 Plasma, S. Lucas II.6 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions Inelastic collisions 2.3 Electron collisions ΔU θ M2 M1 ΔU M2 2 =.cosθ (II.5) E(M+M)112 é+ A → é+ A* é+ A2 → é+ A2* é + AB → é + AB* θ = 0 If M << M (e.g. e → ion) 1 2 Elastic collisions Inelastic collisions ΔU Excitation ≈1 Deviation E1 Ionization Electron attachement All the electron kinetic energy is transferred to and dissociation the heavier species in inelastic collision II.7 Plasma, S. Lucas II.8 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions 2.3 Electron collisions 2.3 Electron collisions é+ A é+ A* → Ionization Excitation é+ A2 → é+ A2* é + AB → é + AB* e + Ar → 2 é + Ar+ If de-excitation in 10-8 s: glow If longer: metastable will produce new ionizations. Energy requirements: K > Vi (Vi: Ioniz. potential of the atom/molecule) Ionization potential of atoms and molecules [2] Neutral Ion Ionization Potential (eV) Si Si+ 8.1 + CH4 CH4 13 + C2H2 C2H2 11.4 + H2 H2 15.4 + N2 N2 15.6 + O2 O2 12.2 + H2O H2O 12.6 + SiH4 SiH4 12.2 II.9 Plasma, S. Lucas II.10 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions 2.3 Electron collisions Ionization Electron impact treshold in C2H2 discharge Ionization potential of atoms [3] Atom 1 2 3 4 5 H 13.598 He 24.586 54.416 N 14.534 29.601 47.887 77.471 97.888 O 13.618 35.116 54.934 77.412 113.896 F 17.423 34.98 62.646 87.14 114.214 Cl 12.967 23.80 39.9 53.5 67.8 Ar 15.759 27.629 40.74 59.81 75.02 Detailed modeling of hydrocarbon nanoparticle 2 -17 2 π.a0 =8.82x10 cm nucleation in C2H2 discharge K. De Bleecker, A. Bogaert, Phys. Rev. E 73, 026405(2006) a0: radius of the first orbit of hydrogen II.11 Plasma, S. Lucas II.12 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions 2.3 Electron collisions 2.3 Electron collisions Electron attachment and dissociation Electron attachment and dissociation Cross section for dissociative capture and electron energy at the maximum [2] When an electron collides with a neutral gas molecule, it Molecule Ion W (eV) σ (10-17 cm2) can be captured to form negative ions. HI I- 0 2300 Negative ions return to neutral atom or molecule through I I- 0.3 300 dissociation, i.e. breaking apart if molecules. 2 HBr Br- 0.28 27 é+ AB → A + B- (Diss. Attachm. or capture) HCl Cl- 0.81 1.99 - + - O2 O 6.7 0.143 é+ A2 → A + A + é (Diss. Ionization) + - - é + AB → A + B + é (Ion pair formation) CO2 O 8.03 0.0482 - H2O H 8.6 0.13 é+ A → 2 A + é (Dissociation) 2 H H- 3.75 0.000016 é + AB → é+ A + B 2 Cross section for formation of neutral and positively charged Only if Eé > treshold value (H2:8.8, N2: 9.4 eV) fragments from CH4 by collision with 100 eV electrons [2] Exemple: Positive Ions Neutral fragments - -16 2 -16 2 é+ O2 → 2 O + é → O + O (a) Ion σ (x10 cm ) Species σ (x10 cm ) - é+ CF4 → CF3 + F + é → CF3 + F (b) H+ 0.04 H 2.4 H + 0.02 H 0.8 Reaction a: used for stripping photoresist in micro- 2 2 electronic processes. Atomic oxygen is produced, and reacts C+ 0.05 C - very easily with polymers to form CO, CO2 and H2O. CH+ 0.28 CH 0.1 + CH2 0.28 CH2 0.2 Reaction b: plasma etching of Si, SiO2, Si3N4. + CH3 1.5 CH3 1.2 + II.13 Plasma, S. Lucas CH4 II.141.8 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions 2.3 Electron collisions 2.3 Electron collisions Detailed modeling of hydrocarbon nanoparticle nucleation in C H discharge Electron attachment and dissociation 2 2 K. De Bleecker, A. Bogaert, Phys. Rev. E 73, 026405(2006) II.15 Plasma, S. Lucas II.16 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions 2.3 Electron collisions 2.3 Ion and neutral collisions Total cross-section 2.4.1 Ion - Neutral. + + B2 + A → B2 + A A+ + BC → A + BC+ A+ + BC → A + B+ + C 2.4.2 Ion - Ion. A+ + B- → AB + hv (Recombination) 2.4.3 Neutral-Neutral. A + B → A+ + B- (Rare in a cold plasma) 2.4.4 Penning effect. B* + A → A+ + B + e B* + A2 → 2A + B • 10 eV, RT, 5x10-3 Torr, purely elastic σ: 2x10-15 /cm2 Ex: Ne + e → Ne* + e + 18p (Torr) 20 3 1 Ne* + Ar → Ne + Ar + e N% = 9.6510 =1.6x10 at/cm λab ==2.1 cm T (°K) 2.σ .N% ab b This reaction is much more likely than Ne + é → Ne+ + é. )Ar acts as a catalyst. -16 -16 -17 • 100 ev: σelastic: 5x10 , σAr+:2x10 , σAr++:2x10 1 . λab ==6 cm During sputter deposition, the observed metal ions are 2.σ .N% ab b due to penning ionization induced by excited noble gas But: energy distribution atoms II.17 Plasma, S. Lucas II.18 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 2.2. Homogeneous Homogeneous reactions reactions 2.3 Ion and neutral collisions 2.3 Molecule-molecule collisions Detailed modeling of hydrocarbon nanoparticle nucleation in C2H2 discharge Detailed modeling of hydrocarbon nanoparticle nucleation in C2H2 discharge K. De Bleecker, A. Bogaert, Phys. Rev. E 73, 026405(2006) K. De Bleecker, A. Bogaert, Phys. Rev. E 73, 026405(2006) II.19 Plasma, S. Lucas II.20 Plasma, S. Lucas 2.2. Homogeneous Homogeneous reactions reactions 3.3. Heterogenous Heterogenous reactions reactions 2.3 Oxygen plasma case Interaction of a solid surface with atoms (A,B), monomer molecules (M), radicals (R) and polymers (P) formed in a plasma. 3.1 Adsorption Mg + S → Ms Rg + S → Rs 3.2 Recombination or compounds formation Deposition A on S + A → S + A2 Etching R on S + R1 → S + M RkN= . % Energy is released at the surface, Rate of surface recombination depends on the catalytic properties of the surface: σ(F + F → F2 on Teflon) ≈ 1/100 σ (F + F → F2 on Cu) 3.3 Polymerisation Rg + Rs → Ps Mg + Rs → Ps’ 3.4 Sputtering + + B on S + A → Surf + B + A Indices g and s = gas or solid phase II.21 Plasma, S. Lucas II.22 Plasma, S. Lucas.
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