Pulsed Laser Deposition; laser ablation Final apresentation for TPPM Diogo Canavarro, 56112 MEFT
Summary What is PLD? What is the purpose of PLD? How PLD works? Experimental Setup Processes in PLD The physical phenomena behind the processes Examples of practical results Advantages/Disadvantages of PLD Future Directions References Acknowledgements
What is PLD? Pulsed Laser Deposition (PLD) is a thin film deposition, son of Physical Vapor Deposition (PVD) There are several techniques for PVD: Evaporative deposition, Electron beam physical vapor, Sputter deposition, Cathodic Arc Deposition and PLD Discovered in the early 60 s, encouraged by technical realization of the first laser (Light Amplification by Stimulated Emission of Radiation) by Maiman First results weren t so good: the properties of deposition films inferior to those obtained by other techniques
What is PLD? But the improvement of lasers resulted in an improvement of PLD results! Big breakthrought in 1987 when Dijkkamp and Venkatesan were able to laser deposit a thin film of YBa 2 Cu 3 O 7, a high temperature superconductive material However, we can say, in a general conception, that PLD isn t better or worst that other techniques. In fact, PLD is just another way to achieve the same objective
What is the purpose of PLD? The main objective is to produce thin films, semiconductors or supercondutors, used in every kind of applications: photovoltaic cells, nanoelectronics, etc. Transfer atoms from a target to a vapor (or plasma) to a substract (velocities typically ~ 10 6 cms -1 )
What is the purpose of PLD? After an atom is on surface it difusses according to: D = D 0 exp(-ε D /k B T) ε D : Activation energy for difussion (~2, 3 ev) k B T : Energy of atomic species Want sufficient diffusion for atoms to find best sites. Either use energetic atoms or heat the substract.
What is the purpose of PLD? So as we can see what makes the difference between the different deposition techniques is HOW we extract the atoms from the source/target Evaporative deposition The material to be deposited is heated to a high vapor pressure by electrically resistive heating in "low" vacuum.
What is the purpose of PLD? Electron beam physical vapor The material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum.
What is the purpose of PLD? Sputter Deposition A glow plasma discharge (usually localized around the "target" by a magnet) bombards the material sputtering some away as a vapor.
What is the purpose of PLD? Cathodic Arc Deposition A high power arc directed at the target material blasts away some into a vapor.
How PLD works? Experimental Setup Target: Almost anything! (metals, semiconductors...) Substrate: Small area ( ~ 1cm 2 ) Laser: Typically excimer (UV, few ns pulse) Deposition rate: High (~100s Å/min) Vacuum: Atmospheres to ultrahigh vacuum
How PLD works? Experimental Setup PLD @ IST Lisbon caracteristics Work frequency: 13.56 MHz Pressure in vaccum: 10-7 mbar Laser Energy: 200 mj Laser pulse: 3 ns 150 µs Laser frequency: 10 Hz (maximum) Laser wavelength: 1064 nm; 523 nm; 266 nm
How PLD works? Experimental Setup
How PLD works? Experimental Setup The plume in PLD. What it is and how appears?
How PLD works? Processes in PLD Laser pulse Target
How PLD works? Processes in PLD Target The laser pulse produces an electronic excitation on target!
How PLD works? Processes in PLD Target lattice Energy relaxation to lattice (~1 ps)
How PLD works? Processes in PLD Target lattice Heat diffusion (over microseconds)
How PLD works? Processes in PLD lattice Melting (tens of ns), Evaporization, Plasma formation (µs) and Resolidification
How PLD works? Processes in PLD lattice If laser pulse is long (ns) or repetition rate is high, laser may continue interactions
How PLD works? The physical phenomena behind the processes 1. Absorption of laser pulse in materials Metals, absorption depths ~ 10 nm, depends on λ 2. Relaxation of energy (~ 1 ps) Electron-phonon interaction
How PLD works? The physical phenomena behind the processes 3. Heat transfering, melting and evaporation When electrons and lattice at thermal equilibrium (long pulses) we use heat conduction equation (or heat diffusion model)
How PLD works? The physical phenomena behind the processes 4. Plasma Creation Threshold intensity: P laser typically ~ 10 8 Wcm -2 Governed by Saha equation:
How PLD works? The physical phenomena behind the processes 5. Absorption of light by plasma, ionization Inverse of Bremsstrahlung! 6. Interaction of target and ablated species with plasma 7. Cooling between pulses Resolidification between pulses
Examples of practical results Combinatorial PLD thin film 200 nm Nb, BaTiO 3, SrTiO 3, film
Examples of practical results YBCO Superconductors Threinch YBCO films on LaAlO 3
Examples of practical results Isotope Enrichment Example: B 10 /B 11 Plasma centrifuge by toroidal and axial magnetic fields of 0.6MG!
Examples of practical results Nanoelectronics
Advantages/Disadvantages of PLD Advantages: Versatile: many materials can be deposited in a wide variety of gases over a broad range of gas pressures High energy of ablated species (internal and kinetic) Low temperature process (sensitive materials) Combinatorial thin film grow Fast: high quality samples can be grown reliably in 10 or 15 minutes Exact transfer of complicated materials (YBCO) Atoms arrive in bunches, allowing for much more controlled deposition Greater control of growth (e.g., by varying laser parameters)
Advantages/Disadvantages of PLD Disadvantages Uneven coverage Particulate concentration Not well suited for largscale film growth Mechanisms and dependence on parameters not well understood
Future Directions PLD with ultrafast pulses (< 1 ps) A new research area If the pulse width < electron latticrelaxation time, heat diffusion and melting significantly reduced! Means cleaner holes and cleaner ablation Direct conversion of solid to vapor, less plasma formation Reactive chemistry: energetic ions, ionized nitrogen, high charge states Leads to less target damage (cleaner holes), and smoother films (less particulates)
Future Directions PLD with ultrafast pulses (< 1 ps) τ > 50 ps: Conventional melting, boiling and fracture threshold fluence for ablation scales as τ 1/2 τ < 10 ps: Electrons photoionized, collisional and multiphoton ionization. Plasma formation with no melting. Deviation from τ 1/2 scaling
Future Directions MAPLE deposition System MAPLE: Matrix-Assisted Pulsed Laser Evaporation Polymer/solvent mix frozen inside deposition chamber Use chilled target stage with liquid nitrogen Frozen mixture acts as target for an incident laser Small section of frozen target will evaporate in a very short period of time, throwing mixture into vapor phase Polymer material will deposit on the substrate Solvent will be pumped away This process has been used to successfuly deposit a wide of polymer and other materials
References Pulsed Laser Deposition of Thin Films, Chrisey and Hubler (Wiley, New York, 1994) Pulsed Laser Vaporization and Deposition, Wilmott and Huber, Reviews of Modern Physics, Vol. 72, 315 (2000) "Transient States of Matter during Short Pulse Laser Ablation", K. Sokolowski-Tinten et al., Phys. Rev. Lett., 81, pp. 224-227 "Isotope Enrichment in Laser-Ablation Plumes and Commensurately Deposited Thin Films", P. P. Pronko, et al. Phys Rev. Lett., 83, pp. 2596-2599
Acknowledgement Special thanks to professor Reinhard Schwarz and professor Rachid Ayouchi for all information about PLD system, in particular for PLD @ IST