Andreas Schroth

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 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.

Simulation of a complex sensor system using coupled simulation programs

Abstract The increasing complexity of microsystems, which include coupled subsystems based on very different physical principles such as electronics, mechanics, optics or fluids, demands new modelling and simualtion strategies. In this paper a new simulation strategy is introduced, which allows the coupling effects of such subsystems to be included. This strategy is based on the modelling of subsystems in different, appropriate simulation programs, which are coupled to simulate the behaviour of the whole system. The approach is demonstrated for the coupling of a finite-element method (FEM) and a network simulation tool, with the example of a mechano-electrical system of a resonant-beam force sensor.

Ambient humidity and moisture — a decisive failure source in piezoresistive sensors

Abstract Piezoresistive sensors have found widespread applications (e.g. in the car industry, chemical processes, industrial measurement techniques, etc.). In many cases a hermetic encapsulation of the sensor elements is impossible, so that humidity, moisture or other forms of pollution in disturbing quantities can attack the sensor chip besides the well-known influence of the ambient temperature. In this paper we present the results of metrological investigations that deal with the influence of humidity and moisture on the stability of the output voltage of piezoresistive sensors. The main conclusion is that the influence of humidity and moisture on the sensor behaviour cannot be neglected. The forming of condensed water on the sensor surface causes changes in the sensor offset voltage up to the nominal output voltage during time ranges of 0.1–0.5 h. Disturbing processes induced by condensation did not cause any irreversible change in the zero offset voltage of the sensors during the investigation time range. The observed humidity-induced instabilities of sensor output voltage are caused by very complex physical and chemical mechanisms, which are described in this paper.

Simulation of a humidity-sensitive double-layer system

Abstract Humidity-dependent swelling of polyimide layers on silicon membranes is described. A simulation strategy using the finite element method (FEM) to predict the resultant deformation of the double-layer system is obtained. A humidity extension coefficient α ϕ is introduced and determined by means of X-ray bending measurement and backward simulation. The FEM is used to investigate the influence of possible technological tolerances of layer thickness, coefficient α ϕ and chip-clamping on the behaviour of the system. For convenience an analytical equation for temperature- or humidity-induced deformation of bimorphs is presented, using the concentrated-element theory. Finally, a comparison of the results simulated by FEM shows a satisfactory agreement with experimental investigations.

Transient measurement of surface deflection for beams and membranes in micromechanical devices

This paper deals with the application of laser- interferometric vibration measurement for experimental characterization of beams and membranes in micromechanical devices. Such small structures are used in many sensor and actuator applications, where they represent the functional key elements. Due to the down-scaled geometrical size and to the fabrication process, the behavior is strongly influenced by many interactions and cross-coupling effects, which are extremely difficult to describe by theoretical models. For demonstration, two different examples are examined: a piezoelectric driven micropump and a 2D-scanning mirror device. The measured data can be compared to the simulated behavior of the structures, and contains important information for the optimum design of the devices. The two main conclusions are, that firstly certain effects of these devices can not be described by theoretical models alone, but have to be combined with experimental measurements, and secondly, that the deflection curvature of the structures must be determined by scanning rather than single-point measurements.

Micropump based on temperature dependence of liquid viscosity

A new micropump principle without mechanical valves is proposed. Flow rectification is achieved by a pair of dynamic valves, the pressure drop through each of which can be individually adjusted by controlling the liquid temperature in the valve channel, thus changing its viscosity. This method has a potential for miniaturization of complex liquid handling systems, since it allows bi- directional liquid transfer with a single micropump, by applying appropriate activation sequences for the valves and the pressure source. The necessary specification and the possible performance are predicted through FEM analysis of thermal and flow systems. By a preliminary experiment using a prototype pump structure fabricated with silicon based technology, the basic function of the valve elements has been confirmed.

Deposition of PZT thin films by pulsed laser ablation for MEMS application

Deposition of PZT with UV laser ablation for realization of thin film sensors and actuators. Deposition rate of more than 3 micron/hour was attained by pulsed KrF excimer laser deposition, which is fairly better than those obtained by other methods like sputtering and sol-gel process. Perovskite phase was obtained at room temperature deposition with fast atom beam (FAB) treatment and annealing. Cohesion between substrate and PZT were also improved with FAB irradiation during deposition. Smart MEMS is now a subject of interest in the field of micro optical device, micro pump, AFM cantilever devices etc. It can be fabricated by deposition of PZT thin films and micromachining. The reported thin films have been mostly prepared by sol-gel process and sputtering process. However PZT film of more than 1 micron thickness is difficult to obtain because of low deposition rate for sputtering process. However PZT film of more than 1 micron thickness is difficult to obtain because of low deposition rate for sputtering and accumulated thermal stress in sol-gel process. This is the reason why we applied excimer laser ablation for thin film deposition. Deposition was done in an ultra high vacuum chamber with 6 targets and a 3 inches wafer size substrate which can be heated up to 1273 K. Targets and substrate were rotated during deposition for improvement of thickness uniformity of deposited layer. Effects of deposition parameters such as LASER wave length, power density and radiation conditions were investigated on deposition rate and film properties. Effect of annealing was also investigated with XRD analysis. Relative dielectric constant is measured for thin films, and the micromachined cantilever beams were fabricated for measuring the deflection of the beams.

Humidity-Induced Volume-Expansion of Polyimide Films

Thin polyimide layers were investigated to determine their volume expansion as a result of humidity absorption and desorption, respectively. Furthermore, we used the experiments in order to clarify the physical and chemical processes of polyimide films in wet environment as well as their humido-mechanical properties. It could 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 90 ppm/%RH was observed. This physical effect can be used to create piezoresistive humidity sensors working similarly to bimetal elements influenced by temperature.