How to process P(VDF-TrFE) thin films for controlling short circuits in flexible non-volatile memories

Abstract

Severe electrical shorting is one of the biggest challenges for the development of flexible non-volatile ferroelectric memories with low operation voltage. In this work, short circuits in flexible ferroelectric capacitors based on copolymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], were explicitly investigated. It was found that processing conditions of P(VDF-TrFE) films have huge effects on electrical shorting conditions. Both deciding factors such as drying temperatures, solvent evaporation rates, and solution concentrations, and plausible factors such as post-drying thermal annealing, were clearly identified. These deciding factors were found to affect short circuits by altering the film porosity, surface roughness, and defect conditions in P(VDF-TrFE) thin films. The detailed correlation of these factors with film porosity were established and explained within the framework of liquid-liquid phase separation. The correlation with the surface roughness and defect conditions were explained with Marangoni convection. The net effects of these factors on electrical shorting conditions are rather complicated because a favorable factor with regards to film porosity may be detrimental in terms of surface roughness and/or defects. So these factors have to been carefully tailored to control short circuits and generate high yields in P(VDF-TrFE) thin film memory devices. Comparative study shows that drying at 80 °C in open air is the optimal condition for processing P(VDF-TrFE) thin films from PGMEA solution. Under this optimal drying condition, a yield of 76% was achieved from large-area devices of about 200 μm × 200 μm with thin P(VDF-TrFE) films of about 200 nm processed with roll-2-roll compatible techniques on plastic substrates. An effective two-step approach to manufacturing P(VDF-TrFE) layers for ferroelectric capacitors with low operation voltage but high yields, is presented. The key information gained from this work on ferroelectric capacitors could be generalized to another type of ferroelectric memories, namely ferroelectric field effect transistors. So it provides researchers and engineers a general guideline on how to control short circuits by processing P(VDF-TrFE) thin films for flexible non-volatile memories.