Process development for gas sensors employing tungsten trioxide nanostructured layers

Objectives
This thesis focuses on the work done by the author within the Erasmus project
at the MINOS Research Group (directed by prof. Eduard Llobet Valero) of the
Departament d’Enginyeria Electronica Electrica i Automatica - DEEEA of the Uni-
versitat Rovira i Virgili of Tarragona (ES).
The main objectives of this project consisted in fabricating and characterizing
gas sensors employing nanostructured layers of tungsten oxide deposited onto a mi-
croelectronic substrate, which have been developed as well. This work has been
conducted at the clean room facility of the University, at the microscopy unit, at
the fabrication laboratory and at sensor characterization facility. Since many of the
steps involved in the sensor design were performed for the first time in the labs and
theauthorwasinchargetosetthem; thisthesisfocusesonthemajorattemptsdone
in order to obtain a working process to get a first sensor prototype.
Thethesisfirstdealswithsometheoreticalintroductionprinciples, thenexplains
the morphology of the sensor that we would like to obtain and the instrumentation
that is possible to use in the University. Finally, after the explanations of the at-
tempts done, the final working process is described and the sample devices obtained
are characterized.
v
Chapter 1
Introduction
Chemical sensors are devices that change one of their properties as a function
of the chemical environment. They detect and quantify chemicals that may be
present in gas, liquid, and solid phases. Traditional analytical instruments such as
mass spectrometry and chromatography are expensive, complex, and cumbersome.
In addition, most analysis require sample preparation, therefore on-line, real-time
analysisisdifficult. Solid-statechemicalsensorshavebeenwidelyused,buttheyalso
have limited capability and problems of selectivity, stability and accuracy. There
are numerous semiconductor devices which transduce these changes in an electric
response thus generating a signal related to the presence or concentration of the
substance to be measured. Semiconductor based chemical gas sensors are one type
of chemical sensors. Despite the variety of effects used and technological solutions,
semiconductor sensors have the following characteristics:
1. Small size that allows their use in portable equipment and power consumption
reduction.
2. Low cost due to the use of cheap mature silicon technology and due to the
subdivision of the non recurrent costs on a large number of devices.
3. Capability for integration in complex systems, like in arrays of sensors.
1
1.1. FUNDAMENTALS 2
However, sensors do have problems of reproducibility, stability and selectivity.
Newmetal-oxidesemiconductors,temperatureprogramming,filtersandmembranes,
and catalyst/promoter combinations are being investigated to improve selectivity.
1.1 Fundamentals
The detection of gases in air with semiconductor sensors is done primarily with
semiconducting metal oxides, even though studies in sensing hydrogen, mainly, have
been made using FETs (Field-Effect Transistors). In metal oxide semiconductors
suchasSnO
2
, achemicalreactionbetweenoxygenandthecombustiblegasoccursat
the surface of the solid, changing the resistance of the solid. To make the resistance
sensitive to such chemical activity, metal oxides with special forms or properties,
and special additives must be selected. Unlike other semiconductors which, under
long-term or cycled heating in air, undergo irreversible chemical transformations by
forming stable oxide layers, metal oxides bind oxygen on their surface in a reversible
way. Metal oxide sensors have been commercially available for many years. The
leading manufacturer is Figaro Engineering in Japan. The SnO
2
type of metal-oxide
sensor sold by Figaro is known as the “Taguchi sensor”, named after its developer.
Sensors made of TiO
2
and ZrO are widely used to control combustion processes.
Metal-oxide sensors operate as follow: when a combustible gas is detected in
the ambient atmosphere the resistance produced by the layer of powdered SnO
2
decreases. The most quoted model to explain the resistance change in a metal-oxide
semiconductor sensor is that, in air, oxygen adsorbs on the surface and dissociates
to form O
–
, where the electron is extracted from the semiconductor. This electron
extraction tends to increase the resistance (assuming n-type semiconductor). In the
presence of a combustible gas, i.e. hydrogen, the hydrogen reacts with the adsorbed
O
–
to form water and the electron is re-injected into the semiconductor, tending to
decrease the resistance. A competition results between oxygen removing electrons
and the combustible gas restoring these electrons. So, the steady-state value of the
1.2. SENSOR FEATURES 3
resistance depends on the concentration of the combustible gas. To illustrate we
could have the competing reactions:
O
2
+2e
−
→ 2O
−
(1.1)
H
2
+O
−
→ H
2
O+e
−
(1.2)
and the more H
2
present the lower the density of O
–
, the higher the electron
density in the semiconductor, and thus the lower the resistance.
Another model which could exist or coexist is that the combustible gas, if chem-
ically active, extracts a lattice oxygen from the metal oxide, leaving vacancies that
acts as donors. The oxygen from the air tends to re-oxidize the metal oxide, remov-
ing donor vacancies (and hence increasing the resistance) that depends only on the
concentration of combustible gas because the oxygen pressure is constant (as when
operating in air).
1.2 Sensor features
A number of behavioural trends have been established in metal oxide sensors.
Sensitivity in gases varies with the temperature. Maximum sensitivity occurs at dif-
ferenttemperatureswhenthegasesandoxidesaredifferent. Responsetimesdepend
highly on temperature (they are shorter at high temperatures), and the responses to
gases are non-linear as a function of concentration. Changes in the ambient temper-
ature cause changes in the sensor response. One method to attenuate the ambient
temperature variation is to control the voltage supply of the heater (which is one
of the essential sensor components) by means of an electronic circuit containing a
thermistor. Another common feature is that water vapour affects both the conduc-
tance in air and the sensitivity to other gases. This is why it is preferable to work in
a controlled atmosphere. Other possibilities are to search for materials that depend
less on the humidity or to compensate this dependence by characterising the sensor
1.2. SENSOR FEATURES 4
response to different humidity values. The objective of the characterization is to
establish the optimum operating conditions, and the reproducibility and reliability
of the sensor. This involves determining the range of the various operating param-
eters for different sensors and different operating conditions, and ascertaining the
stability of these parameters.
1.2.1 Sensor
For most semiconductor devices, there will be a high difference in the measured
characteristics. This is because both the chemical and the physical properties of the
semiconductor are affected by the presence of impurities. The surface reactions, on
which the sensors are totally dependent, are also affected by variations in surface
topography. The method used to fabricate the device can also affect the character-
istics. The situation can be further complicated when additives are introduced into
the oxide to improve some performance’s aspect ; both the amount of additive and
the method of its introduction can be critical.
1.2.2 Temperature of operation
Since the response of semiconductor gas sensors is highly dependent on temper-
ature, the study of the gas sensing properties of metal oxide gas sensors should be
carried out at different operating temperatures. The temperature range of interest
can be determined during the preliminary work and can be different for numerous
oxides and for many analyte gases. In some cases the conductances of some devices,
particularly at lower temperatures, take quite a long time to reach a steady state.
Hence, at any given temperature measurements must be made only after a specific
time, allowing the conductance to reach a steady value. It has also been suggested
that the sensor’s behavior can be somewhat dependent on the recent history of the
sensor. It is therefore advisable, at least in the early stages, to obtain data when
temperature is both increasing and decreasing over the range of interest. Semicon-
1.2. SENSOR FEATURES 5
ductor chemical sensors work at elevated temperatures, although there are sensors
that work at room temperature. In this project a semiconductor sensor made of
Silicon (Si) covered by a layer of Aluminium (Al) will be used. The Si-Al compound
melts around 570
◦
C, even though the Aluminium melts around 650
◦
C (the Silicon
goes up to 1400
◦
C). To prevent irreparable damages at the structure the sensor
should not operates at temperatures higher than 400
◦
C. A reasonable temperature
of operation can be from 150
◦
C (to have a response time in the order of minutes) up
to 300
◦
C. The maximum temperature is dictated by the need to reduce the stress
applied to the structure.
1.2.3 Analyte gases
The different gases and concentration ranges are determined by the particular
gas or application in which the sensor is to be used for and by the potentially
interfering gases likely to be present. It is therefore necessary to measure all these
gases at different temperatures and also to investigate the effects of mixtures of
gases, especially mixtures of the analyte gas with possible interfering gases. In
this context, H
2
O vapour, CO
2
and varying O
2
pressures are particularly important
because they are always present in normal atmospheres. Other impurity gases,
which may be present in minor concentrations in particular environments, may also
influence sensor response.
1.2.4 Sensor enclosure
The rate of access of the gas mixture to the sensor and the integrity of this gas
mixture strongly depends on the design of the enclosure and the material it is made
from. It is usually advantageous to minimize the volume of the enclosure, allowing
the gas to flow through the enclosure to the sensor, and to ensure that the material
which it is made of neither decomposes at the temperature which it is raised to by
thesensornorstronglyadsorbssomegases, sincebothadsorptionofanalytegasand