Advances in MEMS technology have provided a solid foundation for the integration of gas sensors. Undoubtedly, MEMS-based design schemes will become one of the main development directions for gas sensors in the future.
Currently, MEMS gas sensors with single-crystal silicon as the substrate and non-silicon materials as the sensing layer are the most common on the market. Today, the editor of ICbuy.com will introduce the common types of MEMS gas sensors on the market.
1. MEMS conductivity-type gas sensor
The sensitive materials of MEMS conductivity-type gas sensors are metal oxide semiconductors or conductive polymers. The most commonly used metal oxide semiconductor is tin dioxide, followed by titanium dioxide, zinc oxide, etc. To improve the sensitivity and selectivity of gas sensors, catalysts, such as noble metals like platinum and palladium, or suitable metal oxides, are often added to the metal oxide.
When sensitive materials are exposed to the gas being tested, the gas reacts with them, causing changes in conductivity or resistivity. The resulting electrical signal is processed and output as a signal that can identify the gas composition or concentration.
MEMS (Metal Oxide Semiconductor) gas sensors use microelectronic technology to deposit a metal oxide sensitive layer on a silicon substrate. The resistor under the sensitive layer is used as a heater, and a diode is used as a temperature sensing element. Necessary signal circuits and readout circuits can also be integrated on the same silicon chip.
The fabrication process of MEMS micro gas sensors is shown in the figure. Its characteristic is that heating electrodes, insulating layers and test electrodes are stacked together one by one.
2. MEMS solid electrolyte gas sensor
Solid electrolyte gas sensors come in two types: current-type and voltage-type. Current-type sensors offer high sensitivity, a wide measurement range, and low temperature drift. However, their output current and sensitivity are closely related to the electrode size. Traditional sintered bulk devices are difficult to control in terms of electrode size, thus making it difficult to control the output current and sensitivity. Due to the high dimensional accuracy of devices fabricated using MEMS technology, MEMS solid electrolyte current-type gas sensors exhibit superior performance.
Currently, sensors based on the "sandwich" structure can achieve compatibility and processing with MEMS technology, solving problems such as poor process compatibility and complex device structure of traditional solid electrolyte gas sensors.
Advantages of MEMS gas sensors
(1) Miniaturization: MEMS devices are small in size, with the size of a single MEMS sensor typically measured in millimeters or even micrometers. They are lightweight and consume little power. At the same time, miniaturized mechanical components have advantages such as low inertia, high resonant frequency, and short response time. The higher surface area to volume ratio of MEMS can improve the sensitivity of surface sensors.
(2) Silicon-based processing technology is compatible with traditional IC manufacturing processes: Silicon has strength, hardness and Young's modulus comparable to iron, density similar to aluminum, and thermal conductivity close to molybdenum and tungsten. At the same time, it is largely compatible with silicon-based processing technology.
(3) Mass production: Taking a single 5mm×5mm MEMS sensor as an example, approximately 1,000 MEMS chips can be cut simultaneously on an 8-inch silicon wafer using silicon micromachining technology. Mass production can greatly reduce the production cost of a single MEMS.
(4) Integration: Generally speaking, a single MEMS often integrates an ASIC chip while encapsulating a mechanical sensor, controlling the MEMS chip and converting analog signals into digital outputs. At the same time, different packaging processes can integrate multiple sensors or actuators with different functions, different sensitivity directions or actuation directions into one, or form a micro-sensor array, micro-actuator array, or even integrate multiple functional devices together to form a complex microsystem.
(5) Multidisciplinary integration: MEMS involves multiple disciplines such as electronics, mechanics, materials, manufacturing, information and automatic control, physics, chemistry and biology, and integrates many cutting-edge achievements of today's scientific and technological development.
