T. Sandner

Self-polarization control of radio-frequency-sputtered lead zirconate titanate films

The phenomenon of self-polarization in sputtered lead zirconate titanate (PZT) thin films was analyzed. Vacancy formation enthalpies of PZT compounds were estimated for the first time considering a significant amount of covalent binding in PZT crystals. The mobility of vacancies was estimated by a ballistic migration process with a jump time inversely proportional to the phonon frequency. A value as low as 0.12 eV results for the oxygen vacancy enthalpy of migration, which is also responsible for fatigue in PZT capacitors in silicon microelectronic dynamic random access memories.

High Frequency LIMM - A Powerful Tool for Ferroelectric Thin Film Characterization

In this work, the laser intensity modulation method (LIMM) is applied to the investigation of sputtered self-polarized PZT thin films. A previous analytic solution of the LIMM Fredholm integral equation of the first kind by use of the Mellin transform is generalized and limitations of this approach are discussed. The numerically reconstruction of the pyroelectric coefficient profile is based on a eight-layer thermal model. The profile reconstruction was performed using MATLAB software containing algorithms for the inverse solution of the appropriate Fredholm integral equation and a Tikhonov regularization method for stable numerical solutions. Optimized algorithms for thermal parameter determination from the low frequency part of the pyroelectric current spectrum are presented. The impact of thermal parameters on the reconstructed profile was investigated. Monte-Carlo simulations were used for a comparison of different approaches for the regularization parameter estimation.

RF-sputtered PZT thin films for infrared sensor arrays

Abstract PZT thin films prepared by RF sputtering of a ceramic target of composition Pb(Zr0.25, Ti0.75)O3, show different textures with respect to sputtering conditions adopted. The films prepared were under high stress as shown by the stress measurements. PZT micro-structures with Pt electrodes sputtered on silicon wafers were investigated using the Raman peak of the single crystalline silicon. The Raman shift profiles were found to be dependent on the particular geometry of the investigated structures. Infrared sensor arrays described in this paper were fabricated with multitarget sputtered I μm PZT thin films. The array with 256 sensitive elements exhibits a noise equivalent power (NEP) of 0.42 nW at 20 Hz.

Characterization of RF-sputtered self-polarized PZT thin films for IR sensor arrays

Abstract In this work, a complex investigation of film composition, microstructure and physical properties of RF-sputtered self-polarized PZT thin films for IR sensor arrays was carried out. Hydrostatic stresses in Si substrates near edges of Pt/PZT microstructures were predicted theoretically by finite element calculations and measured by spatially resolved Raman spectroscopy. High hydrostatic stresses were obtained in patterned sensor pixels by Raman piezo-spectroscopy. The laser-intensity-modulation method was applied for the investigation of the self-polarization profile, whereas the depth profile of the refractive index was determined by means of spectroscopic ellipsometry. Polarization and refractive index profiles as well as interface stresses affect IR-radiation sensor performance. Thickness and area dependences of IR-radiation detector detectivity and noise equivalent temperature difference were calculated. The applications of self-polarized IR sensor arrays in presence detection and IR imaging are demonstrated.

Self-Polarized PZT Thin Films: Deposition, Characterization and Application

The aim of this work is the characterization of self-polarized PZT thin films. The laser-intensity-modulation method (LIMM) was applied for the investigation of the self-polarization profile. Pyroelectric coefficient profiles were reconstructed from the pyrocurrent spectrum measured up to frequencies of 2 MHz using MATLAB-software containing algorithms for the inverse solution of the appropriate Fredholm integral equation and a Tikhonov regularization method for stable numerical solutions. Measurement uncertainties and resolution of this measurement technique are discussed. The application of self-polarized IR sensor arrays in presence detection and IR imaging is demonstrated.

Properties of sputter and Sol-Gel deposited PZT thin films for sensor and actuator applications: Preparation, stress and space charge distribution, self poling

Abstract In this paper, properties of sputter– and sol–gel deposited PZT thin films are reviewed and compared. It is shown that film properties are influenced by type and parameter of fabrication technology. By means of determining both stress and space charge distribution in the PZT films, it can be seen that an oxygen vacancy leads to a strong self–polarization which is stable under sensor operation conditions and makes these films very advantageous for pyroelectric IR sensors.

Optical refraction index and polarization profile of ferroelectric thin films

Abstract Analytical calculations of polarization and optical refraction index in ferroelectric thin films are performed in the framework of thermodynamic theory. The solution of Euler-Lagrange equation with different extrapolation lengths δ1 and δ2 on different surfaces enables to obtain the thickness dependence of inhomogeneous polarization. The maximal polarization decreases with the decrease of film thickness. The dimensionless critical thickness l 1c of thickness induced ferroelectric phase transition was expressed via extrapolation lengths and was shown to decrease from l 1c= π (δ1 = δ2 = 0) to l c = 0 (δ1, δ2 → ∞). The thickness dependence of the optical refraction index was found to be proportional to squared polarization. Refraction index and pyroelectric coefficient profiles were measured respectively by ellipsometry and LIMM methods in PbZr0,235Ti0,765O3 (PZT) thin films deposited onto Si/SiO2/ adhesion layer/(111) Pt substrate by RF sputtering. The comparison of the theory with observed refraction index profiles in PZT films with 1780, 1000 and 470 nm as well as pyroelectric coefficient in the film 1000 nm had shown that the theory describes the main features of observed profiles and their thickness dependence.

Polarization profile of RF-sputtered self-polarized PZT thin films

Abstract In this work, the laser-intensity-modulation method (LIMM) is applied for investigation of self-polarized sputtered Pb(Ti1−xZrx)O3 thin films. By means of spectroscopic ellipsometry the depth profile of refractive index was obtained. C-V measurements on samples of various thickness were performed to determine the interface capacitance and the space charge density. The work function of Pb(Ti1−xZrx)O3 surfaces was estimated by a modified Anderson method in an electron beam setup using graphite as a reference. The experimental results are discussed in terms of space charge formation and oxygen vacancy drift in the interface layer during thin film deposition. The influence of dipoles formed by intrinsic point defects on self-polarization is considered.

Ellipsometry and LIMM investigations of the interaction between PZT thin films and platinum electrodes and air

Abstract In this work, we present new results of investigation of PbZr0.235Ti0.235O3 (PZT) films deposited onto Si/SiO2/ adhesion layer/(111) Pt substrate by RF sputtering. This paper continues and supplements our previous work reported in [1]. Both spectral ellipsometry and the laser-intensity-modulation method (LIMM) were used for thin film profiling. These two independent methods allow us to increase the accuracy of our investigation. Optical constants of PZT films, refraction index depth profile and polarization profile were determined.

Determination of polarization profiles inside ferroelectric thin films using the laser intensity modulation method

In this work, the laser intensity modulation method (LIMM) is applied to the investigation of the polarization distribution profile inside ferroelectric thin films. Here, a sinusoidal thermal wave is generated by a laser, thus causing a pyroelectric current. This current is influenced by the frequency and, hence, the penetration depth of the thermal wave inside the thin film as well as by the polarization state of this layer. The spatial polarization profile is then determined from the pyroelectric current spectrum by inverse solution of the appropriate FREDHOLM integral equation. Mathematically considered, this represents an ill-posed problem, which usually leads to numerically unstable solutions with an often severely disturbed waveform. Taking both profiles with larger gradients and superimposed noise at the pyroelectric current spectra into account, a TIKHONOV regularization method has to be employed to accomplish numerically stable and reliable results for the reconstructed polarization profiles. Based on the consideration of different typical polarization profiles, the influence of various regularization approaches was investigated, which determine the uncertainty of the reconstruction result. This work explains the effects of uncertainties of measurement due to data noise, non-optimal regularization parameters, material parameter variations and deviations of the thermal model and the influence of uncertainties due to non-optimal model assumptions. It will be shown that the lacking knowledge of precise thin film material parameters and noise inside the measuring setup represent the most decisive uncertainty sources for the LIMM method to determine polarization thickness profiles inside ferroelectric thin films.