Dual-RadioFrequency Capacitively-Coupled Plasma Reactors Tomás Oliveira Fartaria nº58595
Index Capacitive Reactors Dual Frequency Capacitively-Coupled reactors o Apparatus for improved etching uniformity So, why use dual frequencies? Etch rate Anisotropic etch Undesired effects Conclusions Bibliography
Capacitive reactors
Reactor apparatus
Etch profile with 3 electrode types Flat plate electrode Conical shape electrode Conically shape electrode with reverse cone in middle
Apparatus
So, why use dual frequencies? One frequency controls everything at same time Independent control over ion flux and bombarding energy is desired Solution: Use 2 frequencies to control each parameter!
Upper electrode 1. RF signal supplies power to electrons 2. Ions to heavy to move as fast as the signal 3. Mean value will arrise a low sheath potential 4. The increase of power raises density of plasma 5. More ions available, more flux
Lower electrode 1. High frequency will make appear a self-induced bias 2. Voltage bias will increase sheath s thickness 3. This increase will raise voltage drop and space to ions accelerate 4. Ions bombarding energies are controled and defined by the user
Etch rate Thickkness of material removed per time unit Important quantity to microchip industry Fine control over it s very desired and wanted Depends on ions flux and bombardment energy
Ion flux Г ion = v ion n ion Densities of 10 11 cm -3 and much more Controlled by upper electrode s frequency Raising the power, will raise flux How?
Ohmic heating Electrons heating Stochastic heating
Energy transfer 1. Energetic electrons colide elastically 2. Neutrals gain energy due to collision 3. If energy is high enough, electron separates from ion Electrons temperature much higher than ions
Ion bombardment energy Related to ion s velocity towards waffer Assuming collisionless Child-Langmuir sheath: V s = (9J i /(4ε 0 )) 2/3 (M/(2e)) 1/3 *x 4/3 v ion = 2e V s (x) M Depends on potential self-bias in the waffer
Ion energy distribution function Two regimes appear: o Low frequency signal o high frequency signal
Low frequency regime Ions take a small fraction of cycle Instant respond to V Sh Dependence on the phase Result -> IEDF broad and bimodal
High frequency regime Ions take many rf cycles to cross Respond to mean value of V Sh Result -> IEDF narrower
Tailored waveform
Anisotropic etch Mean free path For a sheath with: s = 5.4 mm, p=10 mtorr, V Sh = 300 V ion = 0.13 μs Mean free path range from 7 to 9mm Less collisions, less scatter, narrower IEDF!
Experimental results Anisotropic etch Isotropic etch
Simulations Numerical models are used due to complexity of analytical study Results close to experimental data......but still have some limitations
Results of PIC model Particle-in-cell Super-particles represent lots of particles Field Solver
Other method Monte Carlo Method Lots of repetions with arbitrary inicial parameters Probabilistic aproach Follows trajectories of particles in an average field
Undesired effects of high frequencies Skin effect charge travels in the sheath α = ω c Im κ p 1/2 δ 1 κ p ω pe 2 ω 2 (1 j υ m ω ) For υ m << ω: Collisionless skin deph For υ m >> ω: Collisional skin deph δ p = m e 2 μ 0 n s δ c = 2 ωμ 0 σ dc
Undesired effects of high frequencies Standing Wave Result of interference between the 2 RF signals applied Having 2 waves travelling in oposite directions: y 1 = y 0 sin(kx-ωt) y 2 = y 0 sin(kx+ωt) The resultant wave is: y= 2y 0 cos(ωt)sin(kx)
Conclusions To sum up a dual frequency reactor gives: Independet control over ion flux and bombardment energy; Easy apparatus Have big room for improvement: tailored wavefunction, electrodes shape Skin effect and standing waves problem can be solved
Bibliography http://paws.kettering.edu/~drussell/demos/superposition/superposition.html http://en.wikipedia.org/wiki/skin_effect http://en.wikipedia.org/wiki/standing_wave http://commons.wikimedia.org/wiki/file:etch_anisotropy.png Qin, X.V. et all, Tailored ion energy distribution at an rf-biased plasma electrode Salimian, S. et all, Dual-Frequency Capacitively-Coupled Plasma Reactors For Materials Processing, patent appl.no.:480,369 Kawamura, E. et all, Ion energy distributions in rf sheaths; review, analysis and simulation Boyle, P.C. et all, Independent control of ion current and ion impact energy onto electrodes in dual frequency plasma devices Lieberman, Lichtenberg, Electron heating of voltage-driven and matched dual frequency discharges Lieberman, Lichtenberg, Principles of Plasma Discharges and Materials Processing Latyshev, Yushkanov, Skin Effect in a Gaseous Plasma with a Collision Frequency Proportonal to the Electron Velocity Lieberman, et all, Standing wave and skin effects in large-area, high frequency capacitive discharges Chen, Chang, Lecture Notes on Principles of Plasma Processing