7) Characterisation of FE thin films for NvFRAMs

Two key properties that affect reliability

control of microstructure; electrical properties

Why is microstructure so important in thin films

bulk is fine with 3-4% porosity/second phases; in thin films pores/second phases act as field concentrators and can cause electrical shorts,; this makes some bits unusable; thin film properties often dominated by contacts as SA/V ratio is larger

How do pores form in the sol-gel spin coating process

C in precursors combust using local O supply, in thick layers this can’t be replaced from air and reduces Pb2+ to metallic Pb crystals (typically in the centre); at higher temp (~500C) Pb reoxidise to PbO causing phase separation between Pb/PbO and other cations; inhomogeneous cation distribution leaves pores which can inhibit transformation of pyrochlore phase to perovskite; process dependent on layer thickness

Second phases in sol-gel spin coating

only pyrochlore phase, forms from nucleation at bottom Pt electrode; transitions to perovskite in columns; if stoiciometry is off can get remaining pyrochlore at surface which is very bad for contact with top electrode; excess PbO can limit this

Issues with MOCVD films

deposition of top electrode via sputtering can damage the surface producing an amorphous top layer producing a poor contact; post deposition anneal in O2 at 500C can restore crystalline microstructure

Orientation issues with sputtering

sputtered films show no strong crystal orientation causing problems with ferroelectric switching behaviour; coat substrate with seed compound which encourages directional crystal growth; for PZT coat with PbTiO3 which has a natural growth habit in the {001} plane regardless of the surface it grows on, PZT sputtered on top will then also grow in the {001} plane

What are the three main parameters for a NvFRAM thin film

hysterisis behaviour, fatigue properties, leakage behaviour

How is hysterisis affect by film thickness

thicker films behave more like bulk: more square with higher P_r; as film gets thinner loop tilts more and has lower P_r; thin dead layer (nonferroelectric) accounts for greater volume of thinner layers; effect seems to depend on how material grows in presence of electrode as dead layer seems larger on bottom electrode

Fatigue effects

largely dependent on electrode material; for PZT Pt causes very poor fatigue resistance compared to conducting oxides (IrO2)

Explain leakage current

when poling field is applied dipoles align and space charges are displaced the same way; when the field is removed the dipoles remain while the space charges are free to move and are repelled the other way by the depolarising field, they then screen the charge separation caused by the dipoles reducing the depolarising field and stabilising the polarisation; over time these charges can leak away through the electrodes reducing the screening and therefore increasing the depolarising field which starts to reverse dipoles and P_r slowly decreases

What causes leakage current

movement of O vacancies which occur during high T sintering of BT, MnO2 doping can suppress this as Mn4+ oxidise Ti3+ to 4+ which suppresses the conduction mechanism; porosity in thin films can concentrate fields increasing leakage current