Nanostructured bottom electrodes for ferroelectric thin films

Abstract

Lead zirconate titanate (PZT) is a oxide, which posseses a perovskite type structure. The material is ferroelectric making it technically useful for a diverse range of applications from thermal imaging to non-volatile integrated memory devices. Thin films of PZT are commonly deposited onto platinised-silicon (Pt(111)/Ti/SíOX/Si) bottom electrodes by chemical solution deposition (CSD). Depending on the conditions used a transient intermetallic phase Pt3Pb can form in-situ with a (111) preferred orientation during processing, which reduces the lattice mismatch between the desired perovskite phase and bottom electrode. Thereby, making it easier for the perovskite phase to both nucleate and take on a preferred (111) orientation. In the work presented experiments were conducted on modifying the surface of platinised-silicon to achieve a better lattice-match to Pb(Zr0_3Ti0_7)O3 (PZT30/70) through coating, alloying or reacting the platinum with silver (Ag), gold (Au) or lead (Pb). Single layer and multi-layer PZT thin fihns were deposited by a CSD method onto these modified bottom electrodes and plain platinised-silicon. Two precursor sols were used one that had 10% excess Pb added (PZT30/70) and one that had no excess Pb added (StoichPZT30/70). In general, two drying regimes prior to crystallisation were employed for the precursor fihns. i. Dried at 200°C for 3min ii. Dried at 300°C for 5min Crystallisation of the dried films was conducted i general at 480°C. The primary result of the work, showed that the presence of a lattice-matched surface is highly beneficial for nucleation of perovskite PZT and the subsequent transformation from amorphous precursor into perovskite PZT at low crystallisation temperatures <500°C. All the modified bottom electrodes performed well when used i conjunction with fihns dried at 300°C for 5min, which otherwise would not transform on plain platinised-silicon. It was found that the Au and Ag modified platinised-silicon behaved i a similar manner to plain platinised-silicon with respect to in-situ Pt3Pb, except that the Ag modified platinised-silicon appeared to impede its formation and promote its oxidation. I general the perovskite phase was detected by x-ray diffraction earlier on Ag modified platinised-silicon than on either Au modified or plain platinisedsilicon. The PZT30/70 thin films dried at 200°C for 3min on Au and Ag modified Abstract bottom electrodes appeared to have a higher crystallinity than similar than similar films deposited onto plain platinised-silicon. This was not observed for the StoichPZT30/70 sol as the crystallinity of these films, dried at 200°C for 3mjn were similar i value with all being greater than comparable PZT30/70 sol thin films. It was also proposed that the transformation rate from pyrochlore to perovskite could be affected by precursor sol Pb content and its distribution within the film during crystallisation. This was illustrated when bottom electrodes containing Pt3Pb before the film deposition (ex-situ) were used. PZT30/70 films dried at 200°C for 3min, provide conditions for forming in-situ Pt3Pb and this produced poor quality perovskite PZT thin films on ex-situ Pt3Pb. Implying that i the concentration of Pb is too high at the film/electrode, not only is the preferred orientation of the perovskite thin film compromised but also the degree of transformation from an amorphous to the perovskite phase will be compromised. However, StoichPZT30/70 films dried at 300°C for 5min do not form in-situ Pt3Pb on crystallisation but When crystallised on ex-situ Pt3Pb they appear to transform rapidly into the perovskite phase with a preferred (111) orientation.