G. Gerlach

The measurement of dissolved and gaseous carbon dioxide concentration

In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. Electrochemical CO2 sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO2 sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO2 measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO2. In particular the pH value and the composition of the analyte matrix exert important influence on the results of the measurements.

Design studies on piezoresistive humidity sensors

Abstract The present paper summarizes a detailed design study of a novel piezoresistive humidity sensor, the operation of which relies on the humidity-induced swelling of thin polyimide layers. The purpose of this study was to gain a better understanding of the general metrological behavior of the sensors, with a systematic optimization of the device structure in mind. The design study is based on newly developed analytical models for the offset and full-scale output parameters of the sensor.

Characterization of ion-beam modified polyimide layers

Thin chemically modified polyimide films are widely used as functional layers for new microelectronic sensors. Modification of the chemistry of these polymers can lead to different mechanical, optical and electrical properties. Ion implantation is a preferred method to modify polyimide structures. In this work the ion-induced changes of chemical structures of three polyimides were analyzed by attenuated total reflection. Fourier transform infrared spectroscopy (ATR-FTIR); X-ray photoelectron spectroscopy (XPS); Raman spectroscopy; and spectroscopic ellipsometry and atomic force microscopy (AFM). The results indicate that during the implantation process the imide structures were partly destroyed. Carbon-rich, graphite-similar and amorphous structures were formed in the surface-near area of the polyimide layers. The changes in molecular structures especially depend on the dose of implanted boron ions.

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.

Swelling behavior of thin anisotropic polymer layers

The swelling of thin anisotropic aromatic polymer films in microelectronic devices and micromechanical systems can lead to significant reliability problems. Polymer swelling is caused by the reversible sorption of gas molecules such as water or volatile organic compounds in the polymer. In this paper, we investigate the relationship between the number of sorbed molecules and the resulting anisotropic volume expansion, with particular focus on in-plane-stress and out-of-plane expansion. Based on analytical and phenomenological studies of the polymer-gas interactions we have developed a model describing the microphysical mechanisms of sorption and swelling. The model is valid for a variety of gases and polymers, the chemical and structural properties of which are available in the literature or may be measured directly.

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.

The metrological behaviour of bimorphic piezoresistive humidity sensors

The paper describes some design aspects of newly developed bimorphic piezoresistive humidity sensors. Their operation relies on the bimorphic bending of a polymer-coated diaphragm as a result of the humidity-induced swelling of the polymer. The mechanical bending is evaluated by means of piezoresistors located on the diaphragm, but physically separated from the polymer. This physical separation between the mechanical and the electrical transducer is expected to improve the reliability and long-term stability of the sensor devices. In order to evaluate the benefits and limitations of the piezoresistive humidity sensor, we have developed empirical models based on consistent material parameters to describe the sensor behaviour. Transfer functions correlating the sensor output voltage to the relative humidity have been developed assuming a zero-humidity offset voltage and a linear relationship between the voltage and the humidity. The transfer functions were validated by measurements on prototypes. The measurement accuracy and long-term stability of the sensors were found to be limited by the temperature sensitivity of the polymers response function and by an initial voltage drift due to the relaxation of technologically induced mechanical stress. These undesired effects can, however, be reduced by means of electronic temperature compensation and controlled artificial ageing.

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.