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Thin films based on PbZr_1−xTi_xO₃and K_1−xNa_xNbO₃are increasingly being commercialized in piezoelectric MEMS due to the comparatively low drive voltages required relative to bulk actuators, as well as the facile approach to making sensor or actuator arrays. As these materials are incorporated into devices, it is critically important that they operate reliably over the lifetime of the system. This paper discusses some of the factors controlling the electrical and electromechanical reliability of lead zirconate titanate (PZT)-based piezoMEMS films. In particular, it will be shown the gradients in the Zr/Ti ratio through the depth of the films are useful in increasing the lifetime of the films under DC electrical stresses.

 

Lead zirconate titanate (PZT) films with high Nb concentrations (6–13 mol%) were grown by chemical solution deposition. In concentrations up to 8 mol% Nb, the films self-compensate the stoichiometry; single phase films were grown from precursor solutions with 10 mol% PbO excess. Higher Nb concentrations induced multi-phase films unless the amount of excess PbO in the precursor solution was reduced. Phase pure perovskite films were grown with 13 mol% excess Nb with the addition of 6 mol% PbO. Charge compensation was achieved by creating lead vacancies when decreasing excess PbO level; using Kroger-Vink notation, NbTi• are ionically compensated by VPb″ to maintain charge neutrality in heavily Nb-doped PZT films. With Nb doping, films showed suppressed {100} orientation, the Curie temperature decreased, and the maximum in the relative permittivity at the phase transition broadened. The dielectric and piezoelectric properties were dramatically degraded due to increased quantity of the non-polar pyrochlore phase in multi-phase films; εr reduced from 1360 ± 8 to 940 ± 6, and the remanent d33,f value decreased from 112 to 42 pm/V when increasing the Nb concentration from 6 to 13 mol%. Property deterioration was corrected by decreasing the PbO level to 6 mol%; phase pure perovskite films were attained. εr and the remanent d33,f increased to 1330 ± 9 and 106 ± 4 pm/V, respectively. There was no discernable difference in the level of self-imprint in phase pure PZT films with Nb doping. However, the magnitude of the internal field after thermal poling at 150 °C increased significantly; the level of imprint was 30 kV/cm and 11.5 kV/cm in phase pure 6 mol% and 13 mol% Nb-doped films, respectively. The absence of mobile VO••, coupled with the immobile VPb″ in 13 mol% Nb-doped PZT films, leads to lower internal field formation upon thermal poling. For 6 mol% Nb-doped PZT films, the internal field formation was primarily governed by (1) the alignment of (VPb″−VO•• )x and (2) the injection and subsequent electron trapping by Ti4+. For 13 mol% Nb-doped PZT films, hole migration between VPb″ controlled internal field formation upon thermal poling. 

Phase pure PbZr 0.52 Ti 0.48 O 3 (PZT) films with up to 13 mol. % Nb were prepared on Pt-coated Si substrates using chemical solution deposition; charge compensation for Nb was accomplished by reducing the concentration of lead in the film. For high Nb doping levels, (1) superoxidation of the PZT film surface makes the PZT/Pt interface more p-type and, hence reduces electron injection over the Schottky barrier, (2) the bulk charge transport mechanism changes from electron trapping by Ti 4+ to hole migration between lead vacancies, and (3) the ionic conductivity due to migration of oxygen vacancies decreases. For [Formula: see text] Nb, electrical degradation was controlled via field-induced accumulation of oxygen vacancies near the cathode, which, in turn, leads to Schottky barrier lowering and electron trapping by Ti 4+ . In phase pure 13 mol. % Nb doped PZT films, on the other hand, the increase in the leakage current during electrical degradation was dominated by hole migration between lead vacancies ([Formula: see text]. A much lower lifetime and drastic increase in the leakage current upon electrical degradation was observed in mixed phase PNZT films, which was attributed to (1) a more electrically conductive pyrochlore phase and (2) a high concentration of lead vacancies. 

Uniformly acceptor doped Pb(Zr 0.48 Ti 0.52 )O 3 (PZT) films with 2 mol. % Mg or Fe prepared by chemical solution deposition exhibited decreased dielectric constants and remanent polarizations relative to undoped PZT. For highly accelerated lifetime testing (HALT) at 200 °C and an electric field of 300 kV/cm in the field up direction, the HALT lifetimes (t 50 ) for undoped, Mg-doped, and Fe-doped PZT films were shortened from 2.81 ± 0.1 to 0.21 ± 0.1 and 0.54 ± 0.04 h, respectively. Through thermally stimulated depolarization current measurement, significant [Formula: see text] electromigration was found in homogeneously Mg-doped PZT thin films, a major factor in their short HALT lifetime. Because the concentration of oxygen vacancies increases with uniform acceptor doping, the lifetime decreases. In contrast, when a thin layer of Mg-doped or Fe-doped PZT was deposited on undoped PZT or Nb-doped PZT (PNZT), the HALT lifetimes were longer than those of pure PZT or PNZT films. This confirms prior work on PNZT films with a Mn-doped top layer, demonstrating that the HALT lifetime increases for composite films when a layer with multivalent acceptors is present near the negative electrode during HALT. In that case, the compensating electrons are trapped, presumably on the multivalent acceptors, thus increasing the lifetime. 

Charge transport mechanisms governing DC resistance degradation in ferroelectric films are influenced by defects, particularly oxygen vacancies. This paper demonstrates that oxygen vacancies migrate in lead zirconate titanate (PZT) films under a DC bias field and contribute to resistance degradation. Model PZT thin films were developed in which the concentration and distribution of oxygen vacancies were controlled via (a) changing the dopant type and concentration from 1%–4% Mn (acceptor) to 1%–4% Nb (donor) or (b) annealing undoped PZT films at varying partial pressures of PbO. The presence of associated (immobile) and dissociated (mobile) oxygen vacancies was distinguished by thermally stimulated depolarization current (TSDC) measurements. The impact of mobile oxygen vacancies on local defect chemistry and associated charge transport mechanisms was explored by electron energy loss spectroscopy (EELS). For Mn-doped PZT films, following resistance degradation, TSDC studies revealed only one depolarization peak with an activation energy of 0.6–0.8 eV; this peak was associated with ionic space charge presumably due to migration of oxygen vacancies. The magnitude of the depolarization current peak increased with increasing degradation times. A similar depolarization current peak attributed to the existence of mobile oxygen vacancies was also observed for undoped and Nb-doped PZT films; the magnitude of this peak decreased as the Nb or PbO contents in PZT films increased. An additional TSDC peak associated with polaron hopping between Ti3+ and Ti4+ was found in both Nb-doped PZT films and undoped PZT films annealed under low PbO partial pressure. Degraded Nb-doped samples exhibited a chemical shift in the TiL2,3 peak to lower energy losses and the appearance of shoulders on the t2g and eg peaks, implying a reduction of Ti cations in regions near the cathode.