To date, very little has been written about the influence of the substrate
layer on the overall sensor impedance of single- and multilayer planar
sensors (e.g., metal-oxide sensors). However, the substrate is an elementary
part of the sensor element. Through the selection of a substrate, the sensor
performance can be manipulated. The current contribution reports on the
substrate influence in multilayer metal-oxide chemical sensors. Measurements
of the impedance are used to discuss the sensor performance with quartz
substrates, (laboratory) glass substrates and substrates covered by silicon-dioxide insulating layers. Numerical experiments based on previous
measurement results show that inexpensive glass substrates contribute up to
97 % to the overall sensor responses. With an isolating layer of 200 nm
SiO
A metal-oxide sensor consists of a substrate (typically aluminum oxide,
which, however, cannot be used for thin-film sensors owing to its high
surface roughness), electrodes (typically aluminum or gold) and an active
layer (differs in composition by application, mostly SnO
We have investigated three different types of sensor structures, called types
A, B and C. All of them consist of planar interdigital electrodes (IDE) made
of 200 nm ion-beam-deposited aluminum and respectively featuring 55 and 56
electrode fingers with a finger width of 15
The 80 nm thick TiO
The metal oxides were made by ion-beam deposition of the metals (titanium
respectively tin) and thermal oxidation immediately afterwards. The
oxidation was performed in a glass furnace which was heated to
500
Comparison between sensor types.
Schematic cross section of the sensor structure (type B).
Schematic detail of the busbar area (blue: Al) and covering metal
oxides (olive green: TiO
The thermal postprocessing could lead to a doping of the active layers with
aluminium from the electrodes due to the absence of a diffusion barrier
layer. Furthermore, a diffusion of titanium to the tin-oxide layer (and
reverse) could not be prevented. But as similar measurement results have
been obtained with thermally oxidized Ti and with reactively ion-beam-sputtered TiO
As to the surface roughness of the metal oxides, it is caused completely by the thermal treatment of the sensors. Straight after the sputtering process the roughness was negligible and increased by the thermal oxidization.
Schematic drawing of the test bed used to characterize the sensors.
One-dimensional approximation of basis cell of an IDE-based gas sensor (type B) (Fischerauer et al., 2011).
To characterize the sensors, they were mounted on the lower part of a DIL-14
metal case (with six grounded pins) and placed inside a measurement chamber,
which in turn was placed in a temperature-controlled oven (Fig. 3). The
chamber itself consisted of stainless steel and contained a base out of Macor
(Corning Inc.) which holds the metal case and contacts the pins of the sensor
package. The atmosphere in the measurement chamber could be controlled via
custom-specific gas-mixing equipment involving mass-flow controllers. The
chamber was first heated to 270
The sensor response to concentration steps in hydrogen or H
To model the sensor element, a one-dimensional approximation with a geometry comprising two parallel tracks was used as described in Fischerauer et al. (2011) and illustrated in Fig. 4.
The overall impedance
Nyquist plot of the measured impedance of an unbonded sensor in its
test cell at various temperatures. The frequency was swept from 10.3 Hz to
10 kHz. For the temperature of 25
It turned out, however, that the test environment also contributed to the
device response. To include this influence (as a basis for an eventual
de-embedding procedure), Eq. (1) is modified to
Nyquist plot of the measured impedance of of a type-A sensor element
without active layers (pure IDE on glass substrate). Again, the frequency was
swept from 10.3 Hz to 10 kHz. At the peak temperature of 247
Comparison of type-B sensor impedance (element without any active
layer)
The measurement results will be presented in three steps. First, in Sect. 4.1, the characteristics of the bare substrate will be discussed. Next, Sect. 4.2 focusses on the influence of humidity on the sensor signal. Finally, the key aspect of Sect. 4.3 is the influence of insulating layers or substrates on the sensor response.
To estimate the influence of the test environment, unbonded sensors were characterized. In this manner, all aspects of the test environment (contact resistances, cable capacitances, etc.) can be quantified for later de-embedding of measurement data obtained with sensors.
Time series of the inflexion frequency
As one can see in Fig. 5, even at elevated temperatures, the insulation of
the chamber and the package is very high. As expected, it does not respond to
H
Measured type-B sensor impedance
As revealed by Figs. 5 and 6 , the substrate (glass) conductivity of a type-A sensor is about 100 times higher than that of the measurement environment at the same temperatures. Figure 7 demonstrates that the type-B substrate (insulated glass) leads to a slightly higher shunt conductance (parallel to the coated IDT) than the test bed alone. This means that the test bed influence can be neglected for “high-conductivity” substrates as in type-A sensors, but must be taken into account with “low-conductivity” substrates as in type-B sensors.
Apart from this, the results clearly show that a layer thickness of about
200 nm (atomic force microscopy (AFM) measurements yielded a layer
thickness, for the given sensor, of 175
To estimate the magnitude of the influence, we performed a numerical
experiment based on measurement data for the test environment with a type-A
substrate (yielding
As introduced in Fischerauer and Fischerauer (2011), the inflexion frequency in the Nyquist plot of the sensor impedance (cf. Fig. 5) strongly depends on temperature. Because the dependency is non-linear, this could be used to determine the temperature of the sensor element (cf. Fig. 8). The outliers in this plot are due to noise which perturbs the reconstruction algorithm for the inflexion frequency.
Figure 9 shows measurement results obtained for type-B sensors in various
atmospheres (N
The assertation that humidity does not change the sensor response is
corroborated by the fact that the
To estimate the influence of the substrate, different sensors were characterized. Figure 10 shows the results for a type-B sensor (insulated glass substrate) as compared to a type-C sensor (quartz substrate). Although the impedance level of the former is lower by about 30 %, it still is in the same range as that of the latter. The variation visible in the figure is caused by noise, not by drift. No recognizable drift could be identified during the illustrated detail, because the repeatedly measured spectra did not “move” in a specific direction.
Measured impedances of a type-B sensor (squares) and a type-C sensor
(circles). The dark symbols refer to measurement in pure N
Thus, even a thin layer of SiO
Besides this, it can also be concluded from the curves in Fig. 10 that the
response of the type-B and type-C sensors to hydrogen is similar. The
respective reactances decrease by about 2 and 1 % in the two cases. We
would not claim that type-B sensors systematically respond to H
We assume that the influence of the more conductive substrates (type-A) are
caused by the ionic conductivity of the glass at elevated temperatures. For
type-B sensors the amount of insulating SiO
We have demonstrated experimentally that the substrate plays an essential
role for the overall performance of planar H
Numerical experiments showed that high-conductivity substrates like glass can
dominate the overall sensor response (impedance change) by 97 % at a
frequency of 10.3 Hz. But a thin layer of SiO
Thin-film gas sensors do not need special properties of the substrate (like crystallinity) except the compatibility with thin-film technology and mechanical stability. Hence, there is no need to use expensive substrates such as quartz. By depositing silicon dioxide on top of the substrate, even inexpensive substrates like glass can be made to work.
Sensors with both conducting substrate plus insulating thin film and
insulating substrate respond to H
The authors gratefully acknowledge support by the German Research Foundation (DFG) under contract number Fi 956/4-1. Edited by: M. J. da Silva Reviewed by: two anonymous referees