Karsten Sager

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.).

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.

Influences of humidity and moisture on the long-term stability of piezoresistive pressure sensors

The protection of sensors and the stabilization of output signals in the presence of the disturbing influences of the environment (e.g. temperature, humidity and moisture, pollution) are today one of the predominant problems in sensor design and application. Especially in the field of miniaturized applications, a hermetic encapsulation of the active sensing device is impossible. This paper describes how unencapsulated silicon sensors are influenced by humidity and moisture of the ambient air. These influences lead to instabilities in the output signal. It has been found that changes in the ambient humidity induce measuring errors, that are not negligible, and long-term relaxation processes.

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.

Wie genau messen piezoresistive Sensoren

Der Aufsatz versucht Ansätze zur Beantwortung der Frage darzustellen, wie genau man beim gegenwärtigen Stand der mikromechanisch-mikroelektronischen Technologie mitpiezoresistiven Sensoren messen kann. Dabei ist von grundlegender Bedeutung, wie durch Maßnahmen der Primärund Sekundärpassivierung zufällige Prozesse infolge der jeweiligen Meßgröße sowie von Störgrößen wie Temperatur und Feuchte ausgeschlossen werden können. Um die meßtechnischen Eigenschaften unterschiedlicher Sensoren zu vergleichen, wurde anhand einfacher systemtheoretischer Betrachtungen ein Kennwert Reproduzierbarkeitsfehler eingeführt, der ein quantitatives Maß für die Genauigkeit des Sensors darstellt. Die Reproduzierbarkeit enthält dabei als Fehleranteile die Eigenstörungen sowie die durch die Meßgröße und durch Störgrößen hervorgerufenen stochastischen, nicht korrigierbaren Ausgangsspannungsanteile an piezoresistiven Sensoren. Bei der Untersuchung von zwei Varianten piezoresistiver Sensorchips mit unterschiedlichen halbleitertechnologisch realisierten Passivierungskonzepten wurde deutlich, daß die Genauigkeit mit zunehmendem Aufwand für die Primärund Sekundärpassivierung erheblich zu verbessern ist. So verringerte sich der Reproduzierbarkeitsfehler durch die Verwendung von Siliziumoxid j n i t r i d D o p p e l s c h i c h t e n zum Schutz der Widerstandsund der Leitbahnbereiche auf 160 fiVgegenüber 720 Ii V bei Sensoren mit einfacher primärpassivierender Siliziumoxidschicht.

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.

Humidity-dependent Mechanical Properties Of Polyimide Films And Their Use For Ic-comtatible Humidity Sensors

SUMMARY 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 wellknown temperature extension coefficient a humidity extension value of about 60 to 80 ppm / %RJ3 was observed. This physical effect can be used to create piezoresistive humidity sensors working similarly to bimetal elements influenced by temperature.