6) Deposition methods

Two categories of deposition

in situ and simultaneous crystallisation of the ceramic layer eg MOCVD, in situ sputtering and pulsed laser; films deposited as amorphous layers and undergo post deposition crystallisation eg sol gel spin coating and cold sputtering

Sol gel step 1) solution prep

metal organic compounds (alkoxides) containing the required ions are dissolved into a solvent (1-propanol) with stabilisers and water (enables gelation)

Sol gel step 2) Deposition and drying

small amount of sol dropped onto substrate (Si wafer with Pt electrode) and spun at high speeds to spread into a thin layer; as it spins it dries and begins to form a gel; the wet film is then dried at low T (200C) to removed the rest of the solvent leaving a dry amorphous oxide layer

Sol gel step 3) Crystallisation

amorphous layer annealed to crystallise into perovskite structure typically nucleates at electrode surface and grows up in columns; Conventional furnace annealing: film placed in preheated furnace (550-650C); Rapid thermal annealing: halogen lamp provides v high heating rate (50C/s) helps bypass intermediate steps that can degrade film quality

Sol gel step 4) Multi-layer build up and final anneal

each layer is very thin (50-100nm) so repeat process up to around 6 times to get desired thickness (200-300nm); a final anneal (eg 15 mins at 650C) to ensure complete crystallisation

How to get good quality from sol gel spin coating

want to avoid intermediate pyrochlore/fluorite phase which is non functional (it is not fully transformed perovskite); RTA is preferred as it heats faster helping to bypass the pyrochlore; this will often determine the PZT properties; Porosity: if layers are too thick (>100nm at once) C combustion can starve film of oxygen forming pores; Stoichiometry: lead is volatile at high T so often add extra Pb to compensate for losses during annealing

Describe the process of MOCVD

double/single alkoxide precursors evaporated and transported via inert carrier gas to the substrate; controlled vapour phase hydrolysis of the precursors; polycondensation of partially hydrolysed precursors onto substrate; in situ crystallisation of the perovskite by controlled heating of the substrate

Technical considerations for MOCVD

precursor chemistry: appropriate volatility and vapour pressure, for complex oxides mixing is essential; surface morphology: crystal facets are common on surface affecting flatness required in devices; interface integrity: top electrode deposition can damage the PZT creating thin amorphous layer on top, can fix with post depostion anneal; very high quality, reproducible films for commercial use

Sputtering (reactive in situ type)

Ar plasma generated using RF or DC power supply; Ar+ ions bombard target surface removing the target atoms; target atoms fly off and coat the substrate (O2 gas and heated substrate used here to get right oxygen stoichiometry) and crystallise in situ; if substrate is cold then post deposition anneal is required

Technical points for sputtering

rate of sputtering of metallic ions in target can affect rate of net deposition changing composition, change target composition to accommodate; Main issue: uneven coating (more at centre than at edges), can move or rotate substrate or make sure to have a target much bigger than the substrate to get more even coating

Why are pores back in these thin films

they concentrate fields due to low permittivity of air enabling a conducting path decreasing the quality