- freely available
Materials 2019, 12(15), 2410; https://doi.org/10.3390/ma12152410
1.1. Micro-Hotplate Fabrication
1.2. Sensing Film Deposition
2. SMO Gas Sensor
2.1. Gas Sensing Mechanisms
2.2. SMO Sensor
- The main concerns with the mechanical stability of SMO sensors is due to the need of high temperature operation of the metal oxide film. Regularly heating a device to temperatures between 250 C and 500 C from room temperature and then cooling them back down results in added mechanical stresses and poor stability in all involved layers. Allowing for a reduction in the operating temperature to levels below 100 C would lead to an improved stability, reliability, and power consumption.
- In order to operate at high temperatures, a microheater must be integrated underneath the sensing layer. Due to this requirement, thermal isolation must be provided from the surrounding devices, complicating the fabrication process and demanding a MEMS suspended membrane. Furthermore, the temperature provided has a large impact on the sensing response, while knowing the exact microheater behavior is not always possible, especially since the properties of the microheater materials change with time under operation. These changes can be brought up by the induced thermal stresses, thermo-migration, or electro-migration .
- The SMO’s selectivity is another concern. This is currently being addressed by introducing a sensor array, where multiple sensors are individually engineered to increase their selectivity towards a particular gas [50,51,52,53,54,55]. By combining many sensors, each with a prevalent response towards a particular gas, the collected data set can be post-processed to better pin-point which gas or gases are adsorbed at the surface . The requirement of added post-processing makes efficient CMOS integration even more essential, since integration with CMOS electronics would allow the sensor to operate at increased speeds while reducing the power and signal losses readily associated with long interconnect lines.
- There are several research groups looking into the processes taking place during the SMO sensors’ operation; however, a full understanding is as of yet not available. Until recently, it was thought that sensing was only due to a redox surface reaction with adsorbed oxygen. However, it was not long ago shown that even when oxygen is not present, a thin accumulation layer can form at the SMO film’s surface. This layer is formed due to the direct adsorption of gas molecules by the surface oxygen vacancies and results in a change in the film’s resistivity. Many studies also show that adding a dopant to an SMO film can improve the sensitivity or selectivity towards desired gases. Modeling all the simultaneously-occurring phenomena, including dopant influences, is not currently available. Such a predictable model would be very beneficial towards developing a technology computer aided design (TCAD) environment for the design and optimization of SMO sensors.
- Due to the nature of the sensing mechanism, gas molecules can remain adsorbed to the surface even after a sensing cycle has already concluded. In industrial applications, annealing to higher temperatures in clean air or vacuum (above 500 C) promotes the removal of adsorbed species and surface contaminants; however, this is not feasible in portable electronics . Ideally, the simple cooling to room temperature should result in the desorption of all species on the surface, but this is not the case and a removal procedure must be incorporated in the portable device. Removing the previous species is essential in order to ensure that all initially available surface adsorption sites are once again ready for the next measurement cycle.
- As can be expected, the surface of the sensitive SMO film must be exposed to the ambient in order to interact with the target gas molecules. Therefore, its deposition must be performed at the end of the CMOS front end of line sequence. This means that the deposition must proceed at low temperatures, not exceeding the typical back end of line (BEOL) fabrication temperature of about 400 C.
- A microheater element is the essential component used to heat the SMO layer locally to quite high temperatures to ensure enough energy is reached to initiate gas sensing. Here we concentrate on several modeling and simulation aspects for the membrane which is required to house the microheater/sensor element as well as the microheater itself.
2.3. Choice of Sensing Film
3. Modeling the SMO Sensor Structure
- Wet chemical etching: After a 150 min KOH bath with a concentration of 30% at 70 C the right size of hole was generated and the suspended membrane was released. The etch rate for the silicon wafer depends on the crystallographic orientation which, under the processing setup used, was found to be 13.3 nm/s, 24.2 nm/s, 0.1 nm/s and 23.9 nm/s for directions <100>, <110>, <111>, and <311>, respectively. Although the final structure, shown in Figure 4a appears to be very smooth and clean, with no unwanted lateral etching, the wet chemical etching step can be very corrosive to FEOL devices and surrounding features. Therefore, as an additional alternative analysis using plasma etching was carried out for the same structure. Plasma etching is a commonly used process in CMOS fabrication, which is much less corrosive than a wet chemical bath.
- Dry plasma etching: The simulation for plasma etching involves a stochastic approach for particles which represent the molecules, atoms, and ions, all commonly found in a plasma etch chamber. While not all particles which are found in the chamber contribute to the etch rate, the ones which do are simulated using Monte Carlo ray tracing. The particles can be neutral or charged, representing the chemical and physical accelerated ion etch components, respectively. Because the physical etching component etches layers indiscriminately, the chemical etch is the main contributor, since it can be more selective. However, the negative aspect is the resulting lateral etch, since chemical etching is non-directional. In Figure 4b the increased amount of lateral etching is evident. This simulation setup involved an SF plasma chemistry with a surface fluorine flux of 1 × 10 cm s. The required plasma etch time was found to be much shorter than the wet chemical bath, as a 300 s etch was long enough to fully expose the membrane, as shown in Figure 4b.
3.2. Mechanical Stability
3.3. Electro-Thermo-Mechanical Analysis
3.3.1. Electro-Thermal Behavior
3.3.2. Thermo-Mechanical Behavior
4. Modeling the Thermal Response
4.1. Heater Materials
4.2. Heater Designs
4.3. Heat Loss Mechanisms
4.4. Transient Response
5. Modeling the SMO Sensing Mechanism
5.1. SMO Conductivity
5.2. Surface Reactions
- In an inert environment (e.g., N) the surface energy bands are flat and no depletion or accumulation region is present. The number of charges at the surface is the same as that found in the bulk.
- When oxygen is present in the environment, the oxygen vacancies found on the surface of the SMO film are filled by adsorbed O or O and one or two charges are trapped, respectively. The bulk donates one or two electrons to the adsorbed oxygen and a depletion region is formed, resulting in energy band bending, depicted in Figure 18a.
- When CO gas is in the ambient together with O gas the oxygen will be adsorped on the surface and subsequently removed by CO to form CO. The surface will, thereby continuously re-oxidize, leading to a reduction in the depletion region, which depends on the amount of CO found. This is depicted in Figure 18b.
- If only a target gas is present without any oxygen, an oxygen vacancy at the surface of the SMO film can react with a CO molecule, ultimately reducing the surface. In this interaction, CO donates an electron on the surface, forming an accumulation region, depicted in Figure 18c.
5.2.1. Mass Action Law
5.2.2. Power Law Response
5.2.3. Langmuir Adsorption Model
Conflicts of Interest
|BEOL||back end of line|
|CAD||computer aided design|
|CMOS||complementary metal oxide semiconductor|
|CTE||coefficient of thermal expansion|
|CVD||chemical vapor deposition|
|DRIE||deep reactive ion etching|
|FEM||finite element method|
|FEOL||front end of line|
|IoE||internet of everything|
|IoT||internet of things|
|ITO||indium tin oxide|
|RAM||random access memory|
|SMO||semiconductor metal oxide|
|TCAD||technology computer aided design|
|TCR||temperature coefficient of resistance|
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|Parameter||SMO||Catalyic Pellistor||Piezo- Electric||Electro- Chemical||Thermal Pellistor||Photo- Ionization||Infrared Adsorption|
|Thermal Parameter||Electrical Equivalent|
|Temperature (K)||Voltage (V)|
|Specific heat (J kg K)||Permittivity (F m)|
|Thermal resistivity (K m W)||Electrical resistivity ( m)|
|Thermal resistance (K W)||Resistance (V A)|
|Heat flow (W)||Current (A)|
|Heat (J = W s)||Charge (C = A s)|
|Thermal conductivity (W K m)||Electrical conductivity (S m)|
|Capacitance (J K)||Capacitance (F)|
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