A rapid thermal annealing (RTA) technique has been employed to process lead zirconate titanate (PZT) films prepared by reactive magnetron sputtering. The films were fabricated by dc sputtering a multielement metal target in an oxygen ambient at a substrate temperature of 200 °C. A subsequent postdeposition RTA at 600 °C for 5 s crystallizes the films into a perovskite‐type structure through various intermediate phases. Due to the short postdeposition processing times inherent in the RTA method, the initial nature of the as‐grown films has a critical influence on the crystallization kinetics. The reaction sequence in the formation of perovskite PZT from the films deposited at low substrate temperatures by the sputtering technique has been evaluated, and various key factors influencing the crystallization of PZT have been identified. As‐grown films are constituted of polycrystalline orthorhombic lead oxide in an amorphous matrix of titania and zirconia. During annealing lead oxide transforms into a cubic ph...

Crystallization of sputtered lead zirconate titanate films by rapid thermal processing

We report the thickness and annealing temperature dependence of the structural, morphological, compositional, and electrical properties including ferroelectric characteristics of Pb(Zr0.52Ti0.48)O3 thin films deposited by a sol-gel method. The thickness and annealing temperature of the films were in the range of 0.12–0.36 μm and 520–670 °C, respectively. The crystalline structure and growth behavior of the films have been studied by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. It is demonstrated that the weak ac electric field dependence of the permittivity of the films in terms of the Rayleigh law can be explained. The coercive field obtained from a sweep up and down schedule of the capacitance-voltage curves is asymmetrical and shows different behavior according to the annealing temperature and thickness. We also show that the activation energy coefficient, γ, obtained from frequency dependent polarization-electric field hysteresis loops is related to the annealing tempe...

Dielectric and ferroelectric response as a function of annealing temperature and film thickness of sol-gel deposited Pb(Zr0.52Ti0.48)O3 thin film

Polarization of ferroelectrics decreases as the number of polarization reversals increase. This phenomenon is known as fatigue. Understanding fatigue mechanisms is important for improving the reliability of ferroelectric thin films. A quantitative fatigue model is proposed for lead zirconate titanate (PZT) thin film capacitors on the basis of effective one-directional movement of defects, defect intrapment at electrode-PZT interfaces (and/or at grain and domain boundaries) and the resultant polarization loss at those surfaces under an alternating pulse. An asymmetric polarization may occur on alternating polarity because of asymmetry in electrode-ferroelectric interfaces, and/or non-uniform defect distribution in the bulk. The internal field difference owing to the asymmetric polarization causes an effective one-directional movement of defects such as oxygen vacancies, lead vacancies, mobile impurity ions, etc. These defects can be trapped at electrode-PZT interfaces, grain boundaries, and/or domain boundaries because of their lower potential energies at those sites than those in PZT bulk. This entrapment in turn results in the loss of polarization. In accordance with this fatigue source, a fatigue equation is derived which consists of three constants: initial saturation polarization, piling constant, and decay constant. The predicted values from the model agree very well with the experimental data.

Fatigue modeling of lead zirconate titanate thin films

Temperature and voltage dependence of fatigue parameters were studied for PZT thin film capacitors on the basis of our fatigue model. It was found that the coefficient of dielectric viscosity of PZT and initial internal field difference of PZT capacitors play a key role in determining fatigue rate. Jump distance was calculated from voltage dependence of decay constant, and it turned out to be essentially the same as the lattice parameter. This result indicates that vacancies (oxygen vacancies, mainly) move parallel to the polarization direction during fatigue. Activation energy was calculated from the temperature dependence of the decay constant. It was confirmed that activation energy for domain wall movement determines fatigue rate at various temperatures. This fatigue model can be used to predict the fatigue behavior and can be extended to develop an accelerated test for fatigue.

Fatigue Parameters of Lead Zirconate Titanate Thin Films

After a brief review of the experimental data concerning phase transitions in ferroelectric thin films, a theoretical treatment based on the Landau-Devonshire expansion is presented. A surface is characterised by a length δ, the negative of the logarithmic derivative of the order parameter at the surface. For negative δ, the critical temperature Tc of a film is increased above the bulk value, while for positive δ it is decreased. A careful discussion of depolarisation effects is presented. It is shown that these reduce Tc below the value found if depolarisation is neglected. Various possible extensions of the theory are pointed out.

Phase Transitions in Thin Films

Pulse switching and hysteresis measurements are commonly employed to characterize thin‐film ferroelectric materials. Capacitance–voltage (C–V) techniques have found wide applications in semiconductor technology to determine interface trap charge in silicon metal–oxide–semiconductor devices. C–V measurements made on potassium nitrate (KNO3 ) thin‐film memory devices at high frequency (1 MHz) with slow ramp rates (200–2000 mV/s) are reported here for the first time. The C–V plots are integrated to determine the quantity of displaced charge during switching. These data are compared with the corresponding results from pulse and hysteresis measurements. The effect of ramp rate on the C–V measurements suggests mobile charge (ion) migration under bias. The fatigue problem in the device structures is explained by means of buildup of charge at KNO3 /contact interfaces during continuous switching.

A new technique for characterization of thin-film ferroelectric memory devices

Electrical properties of ferroelectric thin film KNO3 memory devices

Several fabrication and processing parameters such as deposition rate, thickness of the sample, annealing time and temperature, and electrode materials were varied in Au–KNO3 –Au thin‐film ferroelectric memory devices to optimize the electrical properties of these devices. The deposition rate was varied from 2.5 to 10 nm/min in steps of 2.5 nm/min. The samples deposited at a rate of 5 nm/min yielded better electrical results compared to samples deposited at other rates. The electrical testing involved pulse testing and capacitance–voltage measurements. The switching time of these devices decreased with a decrease in the thickness of KNO3 as expected. However, the switching thresholds were sharper in the 200‐nm‐thick KNO3, as compared to 160‐ and 120‐nm‐thick KNO3 samples. The samples were annealed in forming gas at 6, 10, 17, 21, 24, and 44 h and no significant changes were observed in the switching time and polarization. Instead of gold electrodes, indium–tin electrodes gave better performance but they were not easily repeatable.

Dependence of the electrical properties of KNO3 memory devices on fabrication and processing parameters

The terminal fatigue characteristics of vacuum deposited phase-III potassium nitrate (KNO/sub 3/-III) ferroelectric memory devices are reported. It is concluded that the decrease in the polarization values after several million read/write cycles is probably due to the interfacial degradation at the gold/KNO/sub 3/ interface observed by Auger electron-spectroscopy techniques. >

Fatigue mechanisms in thin film potassium nitrate memory devices

We employed the multiple angle incident ellipsometer to study the growth mechanism and optical properties of semi-insulating polycrystalline silicon (SIPOS) films deposited by the low-pressure chemical vapor deposition (LPCVD) technique. A significant difference of the imaginary part of the refractive index between film at the edge and those in the central region of the wafer was observed. Our analyses showed that it was consistent with the N20 depletion model. This N20 depletion phenomenon was confirmed by Auger analysis. Moreover, we found that spatial N20 depletion at the edge of the wafer was greatly influenced by the flow rate of Sill4 to N20 gases. Excessively high N20 flow rate suppressed the silicon microcrystal formation, resulting in a thinner SIPOS film.

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G. Rohrer

Papers

A new technique for characterization of thin-film ferroelectric memory devices

Electrical properties of ferroelectric thin film KNO3 memory devices

Dependence of the electrical properties of KNO3 memory devices on fabrication and processing parameters

Fatigue mechanisms in thin film potassium nitrate memory devices

Characterization of Semi-Insulating Polycrystalline Silicon Prepared by Low Pressure Chemical Vapor Deposition