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. 2015 Mar 25;15(4):7206-27.
doi: 10.3390/s150407206.

Performance and stress analysis of metal oxide films for CMOS-integrated gas sensors

Lado Filipovic  1 , Siegfried Selberherr  2
Affiliations

Performance and stress analysis of metal oxide films for CMOS-integrated gas sensors

Lado Filipovic et al. Sensors (Basel). .

Abstract

The integration of gas sensor components into smart phones, tablets and wrist watches will revolutionize the environmental health and safety industry by providing individuals the ability to detect harmful chemicals and pollutants in the environment using always-on hand-held or wearable devices. Metal oxide gas sensors rely on changes in their electrical conductance due to the interaction of the oxide with a surrounding gas. These sensors have been extensively studied in the hopes that they will provide full gas sensing functionality with CMOS integrability. The performance of several metal oxide materials, such as tin oxide (SnO2), zinc oxide (ZnO), indium oxide (In2O3) and indium-tin-oxide (ITO), are studied for the detection of various harmful or toxic cases. Due to the need for these films to be heated to temperatures between 250°C and 550°C during operation in order to increase their sensing functionality, a considerable degradation of the film can result. The stress generation during thin film deposition and the thermo-mechanical stress that arises during post-deposition cooling is analyzed through simulations. A tin oxide thin film is deposited using the efficient and economical spray pyrolysis technique, which involves three steps: the atomization of the precursor solution, the transport of the aerosol droplets towards the wafer and the decomposition of the precursor at or near the substrate resulting in film growth. The details of this technique and a simulation methodology are presented. The dependence of the deposition technique on the sensor performance is also discussed.

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Figures

Figure 1
Figure 1
Sensor array with interface electronics blocks, which include the amplifiers, multiplexer (MUX), low-pass filter (LPF), analog to digital converter (ADC), microcontroller (μC) and display and/or memory.
Figure 2
Figure 2
Setup of the integrated gas sensor as a system-on-chip. TSV, through-silicon via.
Figure 3
Figure 3
Schematic representation of the band bending effect caused by oxygen adsorption and subsequent introduction of a reducing gas.
Figure 4
Figure 4
Gas sensing function and conduction mechanism for a porous metal oxide, where the oxygen and reducing gas can penetrate to interact with each grain.
Figure 5
Figure 5
Gas sensing function for a compact tin oxide film. The reaction occurs only at the top surface of the deposited tin oxide. The symbol R refers to a reducing gas.
Figure 6
Figure 6
ZnO thin film sensor response dependence on the concentrations of various reacting gases. The temperatures used during measurement correspond to the optimal temperature of operation for the detection of the particular gas.
Figure 7
Figure 7
In2O3 thin film sensor response dependence on the concentrations of various reacting gases. The temperatures used during measurement correspond to the optimal temperature of operation for the detection of the particular gas.
Figure 8
Figure 8
ITO thin film sensor response dependence on the concentrations of various reacting gases. The temperatures used during measurement correspond to the optimal temperature of operation for the detection of the particular gas.
Figure 9
Figure 9
SnO2 thin film sensor response dependence on the concentrations of various reacting gases. The temperatures used during measurement correspond to the optimal temperature of operation for the detection of the particular gas.
Figure 10
Figure 10
Steps during film formation using the Volmer–Weber growth mode. (a) 1-nucleation; (b) 2-impingement; (c) 3-coalescence; (d) 4a-columnar thickening; (e) 4b-polycrystalline thickening.
Figure 11
Figure 11
Stress evolution during the growth of metal and metal oxide films.
Figure 12
Figure 12
From Position 1 to Position 2, two islands impinge, resulting in a grain boundary with height z0.
Figure 13
Figure 13
Relationship between the surface stress f (J/m2) and contact angle θ (°) during the deposition of the metal oxides ZnO, In2O3, ITO and SnO2.
Figure 14
Figure 14
Thermo-mechanical stress for the metal oxide materials introduced in this study and its dependence on the process temperature. A linear dependence on temperature is evident. The metal oxide with the highest CTE (ITO) experiences the largest thermo-mechanical stress. Similarly, the metal oxide with the lowest CTE (ZnO) experiences the lowest stress.
Figure 15
Figure 15
Schematic of the experimental spray pyrolysis deposition process as set up for SnO2 deposition.
Figure 16
Figure 16
Deposited SnO2 thickness versus deposition time. The inset shows the dependence of temperature and solution aging on the deposited thickness. A half-day aged solution is used in the given experiment.
Figure 17
Figure 17
Top view of the electrode locations on the substrate. The sensing area (SnO2) is deposited on top of the electrodes. The sensing area is 100 μm × 100 μm.
Figure 18
Figure 18
Geometry of the deposited SnO2 on the aluminum electrodes and silicon oxide passivation layers.
Figure 19
Figure 19
SnO2 thin film sensor response dependence on the concentration of hydrogen in the ambient gas. The 50-nm thin film is deposited using a spray pyrolysis burst of 30 s, while the wafer is heated to 400 °C. The sensing measurements are performed at 350 °C.
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