Deposition of polymer thin films by PVD process Olivia Lehtimäki 9.3.2016

Polymer thin films Wet-coating techniques Microstructure control difficult Chemical vapor deposition (CVD) For polymers that are not soluble Physical vapor deposition (PVD) Controllable film morphology Multilayer structures? Image source: http://electronics.howstuffworks.com/oled1.htm How to deposit polymer films by PVD?

Pulsed laser deposition (PLD) A high-energy UV-laser ablates the polymer in a plume onto the substrate surface Polymers can degrade due to high energy laser Limited to only a few materials (PTFE, PMMA) [2]

Matrix-assisted pulsed laser deposition Deviation of PLD Target is in the form of solution Target is frozen Lower laser energy A more gentle mechanism for polymer deposition [5]

MAPLE Direct-write Patterned polymer films without lithography

INFORMATION SLIDES

Introduction Polymer thin films are important for optoelectronics, sensors and photovoltaics Traditional wet-coating techniques cannot produce films with controlled morphology These methods include spin-casting, ink-jet printing, drop-casting, spraying, Langmuir-Blodgett deposition CVD can be used for polymers that are insoluble in common solvents Specific reaction requirements needed to produce wanted polymers Methods need to be developed as the applications for organic thin films grow  PVD [1], [2], [3]

Pulsed laser deposition (PLD) Limited to only a few materials Polymer decomposition due to high energy Succesful for depositing addition polymers (PTFE, PMMA) A high-energy UV-laser ablates the polymer in a plume onto the substrate surface Target holder is rotated The film thickness control arises from the pulsed nature of the laser The theory of PLD reactions for polymers The UV laser converts the polymer to monomers and oligomers These recombine on the substrate surface The final film may not be chemically identical to the target material [2], [3], [4]

Matrix assisted pulsed laser evaporation A deviation of pulsed laser deposition (PLD) Same principle, difference in target and laser fluence The target material is solution which is frozen by liquid nitrogen Low concentration solution, highly volatile solvent Target holder is cryogenically cooled to keep the target frozen The polymer molecules evaporate with the solvent in a plume A pulsed laser source ablates the target The energy of the laser can be lower than for PLD as only the solvent needs to be evaporated The solution absorbs most of the energy and therefore protects the fragile polymer molecules Volatile solvent does not stick onto the substrate surface  pumped away [5]

Matrix assisted pulsed laser evaporation The target holder needs to be positioned on the bottom of the chamber Liquefied polymer solution can cause droplets in the vacuum Polymer/solvent mix important to be optimized Solvent volatile enough to be pumped away, and not to be incorporated in the film Solvent absorption coefficient in respect to the laser Solvent needs to be frozen by liquid nitrogen Polymer needs to be soluble in the solvent (concentrations 0.1-5 wt.%) Single target MAPLE deposition chamber [6]

Matrix assisted pulsed laser evaporation The method is still under investigation and most depositions are executed in a self-built apparatus The research is focusing on finding suitable deposition parameters (solvent material, laser energy) There can also be some difficulties in the process itself  Matrix needs to be carefully selected A chunk of the frozen target is ejected (1) The polymer chain is cut (3) The UV-laser energy can be enough to degrade some of the polymer chains [3], [4]

Variations of MAPLE MAPLE direct-write (MAPLE DW) Used to generate patterned polymer films without lithography Polymer is mixed with an UV-absorbent material, and a focused laser beam is directed through this mixture The laser vaporizes the matrix and releases ink on the substrate surface [5]

References R. Pate et. al., RIR-MAPLE deposition of conjugated polymers for application to optoelectronic devices, Applied Physics A 105 (2011) 555-563 D.B. Chrisey et. al., Laser Deposition of Polymer and Biomaterial Films, Chemical Reviews 103 (2003) 553-576 K.B. Shepard, R.D. Priestley, MAPLE Deposition of Macromolecules, Macromolecular Chemistry and Physics 214 (2013) 862-872 R. Eason, Pulsed Laser Deposition of Thin Films: Applications-Led Growth of Functional Materials, Wiley 2006, pp. 36-41 A. Piqué et. al., Laser processing of polymer thin films for chemical sensor applications, Surface and Coatings Technology 163-164 (2003) 293-299 J.A. Greer, Design challenges for matrix assisted pulsed laser evaporation and infrared laser evaporation equipment, Applied Physics A 105 (2011) 661-667