Gerald Gerlach

Review of micromachined thermopiles for infrared detection

During the last few years, thermopiles have come increasingly under the spotlight of commercial infrared sensing. This growing interest has motivated us to write an overview of micromachined thermopiles. The first part deals with the Seebeck effect and discusses the most important physical parameters with their interactions. We also describe the main noise sources and give a derivation of the figures of merit and their relevance for thermopile detectors. In the second part, a number of material systems, techniques and micromachined structures are discussed on the basis of different examples. We explain the motivation behind miniaturized thermopile detectors and give a functional explanation of physical interrelations. Finally, different applications are presented and discussed in terms of their future potential.

Hydrogel Sensors and Actuators

General Properties of Hydrogels.- Synthesis of Hydrogels.- Swelling-Related Processes in Hydrogels.- Modelling and Simulation of the Chemo-Electro-Mechanical Behaviour.- Hydrogels for Chemical Sensors.- Hydrogels for Biosensors.- Hydrogels for Actuators.- Polymer Hydrogels to Enable New Medical Therapies.

Humidity-dependent mechanical properties of polyimide films and their use for IC-compatible humidity sensors

Abstract Thin polyimide layers have been investigated to determine their volume expansion as a result of humidity absorption and desorption, respectively. Furthermore, we have used the experiments in order to clarify the physical and chemical processes of polyimide films in a wet environment as well as their humido-mechanical properties. It can be shown that the humidity-induced material extension is fairly linear in a wide range of humidity. The observed material behaviour strongly depends on the conditions of the polyimide fabrication process. Analogous to the well-known temperature extension coefficient, a humidity extension value of about 60 to 80 ppm/%RH is observed. This physical effect can be used to create piezoresistive humidity sensors working similarly to bimetal elements influenced by temperature.

A piezoresistive humidity sensor

Abstract This paper deals with a new type of humidity sensor. Whereas conventional humidity sensors utilize the change of the specific resistivity or the dielectric properties here the humidity induced volume change of a polyimide layer leads to a deformation of a silicon membrane. The bending stresses will be transformed into an output voltage by a piezoresistive Wheatstone bridge. This principle offers many advantages: (i) a separation of electrical transducing elements (piezoresistors) from measured humidity value, (ii) an advanced electrical long-term stability owing to qualified passivation concepts for the piezoresistors, (iii) the well-known opportunities to miniaturize humidity sensors using semiconductor technologies and micromechanics, (iv) use of only conventional IC processes. The fabrication process of the developed humidity sensor is nearly like that of a common pressure sensor. The first fabricated and tested piezoresistive humidity sensors showed a comparable behaviour to capacitive thin-film humidity sensors, that means fast transient response (T90≈20–25 s) and high accuracy (down to 1% r.h.).

Hydrogel-based piezoresistive pH sensors: investigations using FT-IR attenuated total reflection spectroscopic imaging.

The strong swelling ability of the pH-responsive poly(acrylic acid)/poly(vinyl alcohol) (PAA/PVA) hydrogel makes the development of a new type of sensor possible, which combines piezoresistive-responsive elements as mechanoelectrical transducers and the phase transition behavior of hydrogels as a chemomechanical transducer. The sensor consists of a pH-responsive PAA/PVA hydrogel and a standard pressure sensor chip. However, a time-dependent sensor output voltage mirrors only the physical swelling process of the hydrogel but not the corresponding chemical reactions. Therefore, an investigation of the swelling behavior of this hydrogel is essential for the optimization of sensor design. In this work, Fourier transform infrared (FT-IR) spectroscopic imaging was used to study the swelling of the hydrogel under in situ conditions. In particular, laterally and time-resolved FT-IR images were obtained in the attenuated total reflection mode and the entire data set of more than 80,000 FT-IR spectra was evaluated by principal component analysis (PCA). The first and third principal components (PCs) indicate the swelling process. Molecular changes within the carboxyl groups were observed in the second and fourth PC and identified as key processes for the swelling behavior. It was found that time-dependent molecular changes are similar to the electrical sensor output signal. The results of the FT-IR spectroscopic images render an improved chemical sensor possible and demonstrate that in situ FT-IR imaging is a powerful method for the characterization of molecular processes within chemical-sensitive materials.

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.

Fabrication of a 3D differential-capacitive acceleration sensor by UV-LIGA

Abstract A novel three-dimensional (3D) acceleration sensor has been fabricated by combining the low-cost UV-LIGA surface microstructuring process with a sacrificial layer technique. It detects the triaxial accelerations using three independent, yet on a common substrate-integrated, sensor elements. Each element is configured as a differential capacitor with its movable seismic mass as the middle electrode. The fabrication is a simple planar batch procedure comprising only a few processing steps. The entire structures are first grown electrochemically within the UV-patterned thick AZ4562 photoresist on an electroplating base that is composed of rigid (Cu) and sacrificial (Ti) layers. Movable Ni-parts are then obtained by removing the underneath Ti sacrificial layer using wet etching. Sensor structures up to 30 μm with an aspect ratio of about 10:1 can be reliably manufactured. It is thought that this fabrication approach can be widely applied to economically realise other micromechanical components with oscillating structures.

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.

A humidity sensor of a new type

Abstract The recently most talked about and used humidity sensors are capacitive sensors with a thin polymer film as the sensitive layer. Despite the many advantages, these sensors have shown some reliability problems caused by special effects of the polymer (e.g. swelling) and the direct moisturising action on the electrical transducing elements. To overcome these disadvantages we have developed a new type of humidity sensor, a piezoresistive transducer, fabricated with integrated circuit technology. In this paper we describe the sensor design, methods of analytical simulation of the sensor behaviour and measured characteristics of fabricated sensors. The tested elements showed a fast transient response to humidity and a very good reproducibility.

A resonant poliyimide-based humidity sensor

A recently introduced type of humidity sensor uses the humidity dependent change of mechanical properties of polymer layers as sensing effect. To determine whether the mass increase of the resonating system or the swelling of the polymer material are the main causes of the humidity sensitivity, finite-element simulations are carried out and compared with measured results. The comparison of the results shows that, despite former assumptions, the swelling of the layer caused by sorption of water molecules mainly determines the humidity sensitivity of this type of sensor.