I must also not forget Jaromir Hubalek, my first PhD companion, who helped me start research and introduced me to the methods used to fabricate thick film gas sensors. The aim of this doctoral thesis is to develop new sensors and sensor arrays that can improve the selectivity of metal oxide gas sensors and reduce their power consumption.
INTRODUCTION
In Chapter IV Fabrication of Thick Film Gas Sensors on Al2O3 Substrate, we present the manufacturing steps for manufacturing the main elements of the sensor substrate (electrodes, heater and temperature resistor). Then we continue with the introduction of the different pastes used for the active layers.
STATE OF THE ART OF SEMICONDUCTOR GAS SENSORS
Need of semiconductor gas sensors
Spectroscopic systems make a direct analysis of the molecular mass or vibrational spectrum of the target gas. The conductivity of the sensing material in semiconductor gas sensors changes in the presence of a specific gas.
Basic considerations
- Oxygen sensing (bulk type mechanism)
- Minority gases in air (surface type mechanism)
- Catalysts (basic assumptions)
- Metal diffusion inside the semiconductor bulk
- Superficial clustering
- Metal macro-agglomeration
- Metal ultra-dispersion
Similarly, the resistance can decrease due to the reduction in the surface potential barrier height and the depletion length. Currently, there is considerable interest in the effect of chemically equivalent foreign atoms on the surface reactivity of the metal oxides.
Sensor characteristics
- Operating temperature
- Conductance
- Calibration curve
- Base line
- Sensitivity
- Response and switch-on time
- Selectivity
- Stability
- Long-term effects
Unfortunately, this is probably one of the most difficult of all sensor operating parameters to measure. Slow changes in the properties of the bulk material or in the near-surface region are almost inevitable.
Active layer deposition technology
- Thin-film technology
- Thick-film technology
The screen is held at 0.5 mm from the substrate surface in the screen printing machine. The thickness of the tape is generally in the range 25 µm to 1 mm, but tapes as narrow as 5 µm can be produced.
Active layer materials
- Metal oxides
- SnO 2 based sensors
- WO 3 based sensors
We investigated pure and Pt-doped films and improved the sensitivity of the second type. Both sensors showed good detection reproducibility and stability at 1% H2 gas and improved thick film sensitivity.
Conclusions
For example, the sensitivity to acidic gases such as H2S or basic gases such as NH3 is increased when corresponding acidic or basic oxides are included to form adsorption sites for the gases [128].
THICK FILM TECHNOLOGY FOR GAS SENSOR FABRICATION
Basic assumptions
The next step in the process is to dry the film and remove the organic solvents from the paste. Different thick film manufacturers will have different design rules depending on the quality of the processing facilities they have available.
Screens
- Mesh materials
- Screen fabrication
Nylon is the most elastic of the three screen fabrics, which can be an advantage in certain circumstances. The photographic positive is exposed to the emulsion under ultraviolet light, washed and then gently pressed to the underside of the screen using a soft roller.
Deposition equipment
The print medium is applied to the top surface of the screen and the flexible rubber squeegee is passed over the stencil. The shape, material and pressure of the squeegee are all factors that determine the life of the screen and squeegee.
Monitoring of the film thickness
One of the most important aspects of thick film paints is the viscosity, which must be precisely controlled if the quality of the film deposition is to be high. For ideal (Newtonian) fluids, viscosity is independent of shear rate and varies only with temperature. For a more detailed measurement of the thickness of burned and dried films, a device called a surfometer can be used.
Annealing
- Drying process
- Firing process
The purpose of the drying phase is to remove the organic solvents and ensure that the printed film adheres to the substrate and is relatively immune to smudging. The high temperature bake cycle is designed to remove residual organic binders from the film, to develop the electrical properties of the ink and also to bond the ink to the substrate. A muffle tube traverses the length of the interior of the furnace and the belt is pulled through this tube.
Thick film materials
Some materials, such as copper and nickel, oxidize when baked in air and can only be processed in an inert atmosphere, such as nitrogen. One of the essential requirements of the dielectric structure is that it must be free of pinholes so that multiple prints can be used. Because other structures are often laid on top of the dielectric layer, the surface finish must be smooth.
Substrates and substrate properties
- Beryllia (BeO)
- Aluminium nitride (AIN)
- Porcelain enamelled steel substrates (PES)
- Glass-ceramic substrates
- Alumina (Al 2 O 3 )
Most of the substrates used in thick film technology are ceramic materials such as aluminum, beryllium, magnesia, zirconia. Typically the relative dielectric constant of low temperature dielectrics is between 3 and 8 and the TCE between 4 and 8 x 10-6 /K. The elastic moduli and mechanical strength of aluminum ceramics are high, which means that it is one of the strongest refractory oxides.
Optimising structures
- Meander
- Interdigitation
The total length, L, for a given total area is A / 2d, but what we need to optimize is L/d, or. Thus, for a typical minimum size of 0.3 mm, the general rule is similar to the one above: compared to a square inter-electrode space, the gain is of the order of 5A, where A is in mm2. These results highlight that optimizing structures have obvious advantages only when their total area is much larger than the square of the minimum feature size.
Conductor pastes
Some of the properties of the most commonly used conductive pastes are shown in Table III.4. One of the most important factors affecting the adhesion of the thick burn layer is the chemistry of the permanent binder. Glass intercalations extend from the substrate to the metal film, and sometimes to the surface of the conductor, to form a mechanical bond.
Active phase preparation concepts
- Organic vehicles
- Metals
- Metal oxides
- Glasses
The size and shape of metal particles strongly influence the properties of the fired films, e.g. The starting material, gas flow rate and reaction temperature are the most influential characteristics of the process. These conditions are prerequisites for the stability of the thick film layers at relatively high temperatures.
Bonding
The frequency range is between 15 and 60 kHz, depending on the material, the thickness of the wire and the type of bonder. It is therefore important that these residues form only a small part of the contact surface. In addition, the substrate (thickness) and the ductility of the materials involved play an important role.
Packaging and testing
The bonding parameters (ultrasound energy, time, pressure) must be controlled very carefully if the results are to be repeatable. A good contact between the sonotrode and the connecting partners is essential if there is to be no loss of ultrasound power in the bonding area. If the contact pressure is too high, the bond may be weakened; if it is too low, the surface of the sonotrode and contact area may be damaged by frictional heat dissipation.
Conclusions
Very thin wires of pure aluminum are too soft, so alloys with about 1 % silicon are generally used. Pure aluminum (Al 99.999) is used with wires of 100 µm diameter or more, because higher conductivity is preferable to mechanical strength.
Sensor fabrication
- Sensor substrate fabrication
- Fabrication of the heater/temperature sensor
- Fabrication of the pads
- Fabrication of the electrodes
- Preparation of lack matrix
- Powder preparation
The fabrication process is explained in detail, starting with the application of the measuring electrodes, heater, and temperature measuring resistance to the substrate, and a discussion of the pastes used for the active layer. The various steps that make up the sensor substrate fabrication process can be seen in Figure IV.2. The composition of the matrix ensures homogeneity and very good adhesion of the active layer to the ceramic substrate.
O 3 powder
O powder
Additives
This affects the sensitivity of the film as described in [139] where Bi2O3 is added. The amount of additives determines the sensitivity by isolating the active grain boundary or as a catalyst between the boundaries. As shown in reference [141], decreasing the amount of glass frits increases the porosity of the film.
Calcination and firing process
These materials typically increase film resistance by fitting the active grains together to form a compact adhesive film on the substrate, but lack catalytic properties. If the paste is more viscous, the layer does not flow after printing and the trace of the grid remains on the surface. The amorphous structure begins to form polycrystals in the triclinic phase between 400ºC and 600ºC.
Paste
The calcination lasts from 1 hour to 10 hours, and the firing process from 10 min to 1 hour. Drops of Lack Matrix 1 (LM1) were added and sequentially homogenized until the viscosity was good. The final step in the fabrication of the sensor is the firing of the active layer.
Doping
Catalytic filters
Bonding, packaging and testing
The graphic represents the electrical power that must be applied to the heater if we want to obtain the desired operating temperature. Power that must be supplied to the heating element to reach the desired operating temperature. It is clear that the power consumption of the sensors manufactured on alumina substrate is too high.
Physical characterization
- SnO 2 sensors
- XPS analysis
- WO 3 sensors with catalytic filters
- SEM analysis
- WO 3 sensors doped with Ag
- AFM results
- Raman measurements
An EDS analysis showed that the grains and the bottom of the voids contained tin (the equipment used was not suitable for detecting the presence of oxygen). To understand how loading affects the morphology of WO3 samples, AFM micrographs were recorded. The surface of the films is essentially inhomogeneous and consists of grains and voids.
Electrical characterization
- SnO 2 sensors
- Response to ethanol
- Response to ammonia
- Gas-kinetic interaction of NH 3 with Pt-doped SnO 2 surfaces
- WO 3 sensors
- WO 3 and SnO 2 sensors with surface adhesion promoters
- Response of WO 3 –based sensors to gases
- Response of SnO 2 –based sensors to gases
- Response to humidity
- WO 3 sensors with catalytic filters
Response of the 8 Pt (paste)-doped SnO2 sensors operating at 300ºC to successive injections of ethanol. Tungsten oxide sensors are highly selective for ammonia at low concentrations of the tested gases. Once again, sensitivity to benzene decreases as the sensor's operating temperature increases.
Conclusions
On the other hand, the Pt catalytic filter does not change the NO2 detection capability of WO3. In addition, the filter does not improve the response of the WO3 sensor to methane or carbon monoxide.
FABRICATION OF THICK FILM GAS SENSORS ON MICROHOTPLATE AND SILICON
- Substrate fabrication
- Micro-hot-plates fabricated at National Centre of Microelectronics
- SOI Micro-hot-plates fabricated at Catholic University of Louvain
- Active layers deposition
- Physical characterization
- SnO 2 layer based on nano-metric grains
- Doped SnO 2 / WO 3 active layers deposited on MHP
- Electrical characterization
- Pure SnO 2 /WO 3 micro-hotplate gas sensors
Table V.3 shows the working conditions in which the different photographs were taken. We also produced elemental maps of the samples to obtain the doping distribution in the active layer. The graphs below show the relationship between the resistance of the sensors at 10% R.H. this humidity level is taken as a reference value) and different humidity levels up to 85% R.H. The minimum values are in blue.
The first set consisted of the following SnO 2 micro-hotplate gas sensors
For each vapor or mixture and concentration measured, the ratio between the standard deviation of the response and its mean value (over five repeated measurements) was below 1.3×10-1. Furthermore, when the sensors were used at higher temperatures, the change in their resistance caused by a change in the moisture level was higher. The response to moisture can be considered moderate, compared to the sensitivities found for the various vapors studied.
The second set consisted of the following WO 3 micro-hotplate gas sensors
- Doped SnO 2 / WO 3 micro-hotplate gas sensors
Post-harvest ethylene gas control extends the life cycle of the fruit, allowing it to be kept for a much longer period of time. While cooling and humidity slow down, they don't stop the production of harmful ethylene gas. Ethylene gas and its removal are both important to give the consumer the best possible product.
The third set consisted of the following 1%wt. doped SnO 2 MHP sensors
Ethylene gas is also used in ripening plants to color fruit, which is then moved to a normal cold room with other products. While ethylene gas is used under controlled conditions as a ripening agent, even small amounts of ethylene gas during shipping and storage cause most fresh produce to spoil more quickly. The graph clearly shows that the Pt-doped sensors had the best response to ethanol (10 times higher than the undoped sensors).
The fourth set consisted of the following 1%wt. doped WO 3 MHP sensors
The response was minimal for the undoped layers, while the Pt doping catalyzes this reaction well. The change in sensor resistance is more than 78 times for the Pd-doped sensor when detecting ammonia and 127 times for the Au-doped sensor when detecting NO2. The humidity response for the tungsten oxide sensors was higher than in the case of tin oxide, but when compared to the sensor response on the tested species, it could be considered negligible.
The last set consisted of the following, multi-quantity doped WO 3 MHP sensors
- Pure SnO 2 / WO 3 SOI micro-hotplate gas sensors
- Applications with MHP arrays
- LDA used for discrimination and quantification of simple volatiles and binary mixtures with non-doped SnO 2 sensors
- PCA, MLP and Fuzzy ARTMAP used for discrimination and quantification of simple volatiles and binary mixtures with non-doped SnO 2 sensors
- PCA and Fuzzy ARTMAP used for discrimination and quantification of simple volatiles, binary mixtures and toxic gases with non-doped SnO 2 and WO 3 sensors
The data were centered because this is the usual pre-processing method when the initial variables are homogeneous (eg the responses of four tin oxide micro-sensors). This is in good agreement with the PCA results (see Figure V.40-c) where the clusters for these species almost overlap. b) Lower concentration of ammonia, intermediate concentrations of mixtures of acetone and ammonia, and lower and intermediate concentrations of mixtures of ammonia and ethanol. The results (discussed above) were similar when the tin oxide microarray responses were used to perform PCA.
