G. Suchaneck

Domain populations in lead zirconate titanate thin films of different compositions via piezoresponse force microscopy

Pb (ZrxTi1−x)O3 (PZT) thin films with (111) texture were deposited onto commercially available Pt/Ti/SiO2/Si substrates via the sol–gel technique. Piezoforce microscopy (PFM) was then used to analyse the evolution of domain populations as a function of the Zr content x. Domain structures of virgin films, local piezoelectric properties of individual grains and piezoelectric histograms were studied in films with different compositions (x = 0.2–0.6), which cover both the tetragonal and rhombohedral sides of the phase diagram. In films with low Zr content mainly single-domain grains were observed. As the Zr content increased, a larger fraction of polydomain grains was found. The local piezoelectric response measured inside sufficiently big grains indicated that the strongest piezoelectric effect occurs in PZT30/70 (x = 0.3) films. This was attributed to two different effects: high out-of-plane polarization achieved due to the (111) texture and influence of the dielectric constant. In tetragonal films with their lower dielectric constants the electric field seen by a ferroelectric is higher as compared to other compositions, giving rise to an apparent increase of the effective piezoelectric response measured by PFM. The analysis of the domain images indicated that sol–gel derived PZT films are slightly self-polarized near the free surface. With increasing Zr/Ti ratio, the variation of domain populations resulted in reversing the sign of the average piezoelectric response at x≈0.3. It is demonstrated that PFM histograms are extremely sensitive to PZT composition and can be used as a signature of complex domain structures in ferroelectric thin films.

Ion-beam induced chemical and structural modification in polymers

In order to increase the sensitivity to moisture uptake of polyimide (PI) and polyethersulfone films applied in bimorphic humidity sensors 50, 130 and 180 keV boron ions with irradiation doses between 1013 and 1016 B+/cm2 were implanted. A complex investigation of the following features has been carried out: chemical changes in the surface regions by attenuated total reflection–FTIR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS); optical properties by spectroscopic ellipsometry; hardness and elastic modulus by depth-sensing low-load indentation technique; conductivity of modified polymer films. It could be shown, that the partial destruction of chemical bonding under ion bombardment leads to the creation of new amorphous and graphite-like structures, which increase the surface film conductivity by several orders of magnitude, and enhances the sensitivity of these polymer films to moisture uptake. The ion-beam irradiation destroys the anisotropic features of the refractive index of PI layers leading to its isotropization. Radiation-induced changes in the layer structure result in an increase of the hardness and elastic modulus of the modified layers up to ten and six times, respectively. The hardness and refractive index depth profiles were determined. The detectable effective modification depth estimated from the depth profiles is 250–300 nm at an ion energy of 50 keV and 400–450 nm at an ion energy of 180 keV.

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.

Low-temperature PECVD-deposited silicon nitride thin films for sensor applications

Abstract Polymer-like silicon-rich SiN x :H films suitable for transparent VIS/NIR/MIR-range optical coatings were deposited by PECVD at a substrate temperature of 80 and 150°C. Optical properties and film microstructure were investigated by transmission/reflection measurements and by FTIR. Air exposure for more than 1 year reveals no post-oxidation with time. The application as an antireflective coating for IR-sensor arrays is demonstrated.

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.

Influence of ion-beam induced chemical and structural modification in polymers on moisture uptake

Abstract Polyimide thin films are a promising material for microelectronics and aerospace applications. In particular, they are sensitive to moisture and gas uptake. This leads to film swelling which may be monitored by a corresponding change in piezoresistance caused by plate bending of a polymer–silicon double-layer or bimorphic sensor, respectively. However, the expansion of the polymer, induced by moisture or gas uptake, can usually be influenced by surface ion-beam modification. By this, the selectivity to a partial gas may be enhanced or decreased. In this work, the influence of ion-beam induced surface modification on both polymer structure and moisture uptake of polyimide and polyethersulfone is investigated. To modify the polymer layer surface boron ions were implanted with energies from 50 to 180 keV and irradiation doses between 1013 and 1016 B+/cm2. It could be shown that increase of irradiation dose leads partly to a destruction of the imide and aromatic groups. The aromatic structure is degraded by hydrogen abstraction. This corresponds to the creation of a new amorphous and graphite-like structure, which increases the modified surface film conductivity by several orders of magnitude, and which decreases the Freundlichs coefficient of the moisture-uptake behaviour.

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.

Investigation of the spatial polarization distribution of sputtered PZT thin films using limm

Abstract In this work, the laser-intensity-modulation method (LIMM), e.g. the determination of the spatial polarization profile from the pyroelectric current spectrum caused by the interaction of thermal waves generated by an intensity modulated laser and the unknown polarization distribution, is applied for investigation of sputtered PZT thin films on a thermally coupled bulk silicon substrate or a thermally isolating membrane, respectively. To investigate thin films with a thickness of about 1 μm the modulation frequency of the laser was extended up to 2 MHz. The reconstruction of the spatial polarization distribution requires to solve a Fredholms integral equation of the first kind which is a so called “ill-posed” problem. For deconvolution of the inverse LIMM problem several approaches like power series trial functions and Lagrange polynomials are used and compared with the original Fourier series approach for LIMM introduced by S.B. Lang. To obtain a non-oscillating solution of the inverse LIMM-prob...

Influence of Surface and Interface on PZT Film Optical Properties

We present our results of spectroellipsometric investigation of PbZr0.235Ti0.765O3 (PZT) films deposited onto Si/SiO2/adhesion-layer/(111)Pt substrate by rf sputtering. By means of spectroscopic ellipsometry the depth profile of refraction index was obtained. It exhibits a strong dependence of optical properties of the PZT film on the distance to its interfaces. The depth of these disturbed layers reaches the value of 200 to 250 nm that is much more than can be expected on the base of usual considerations. The refractive index of the middle undisturbed part of the PZT film approaches the value for PZT bulk.