MEMS pressure sensors
What are MEMS pressure sensors?
Microelectromechanical systems (MEMS) devices combine small mechanical and electronic components on a silicon chip.
The fabrication techniques used for creating transistors, interconnect and other components on an integrated circuit (IC) can also be used to construct mechanical components such as springs, deformable membranes, vibrating structures, valves, gears and levers.
This technology can be used to make a variety of sensors including several types of pressure sensor. It enables the combination of accurate sensors, powerful processing and wireless communication (for example, Wi-Fi or Bluetooth) on a single IC.
Large numbers of devices can be made at the same time so they benefit from the same scaling advantages and cost efficiencies as traditional ICs.
Function
Several types of pressure sensor can be built using MEMS techniques. Here we will discuss two of the most common: piezoresistive and capacitive. In both of these, a flexible layer is created which acts as a diaphragm that deflects under pressure but different methods are used to measure the displacement.
MEMS capacitive pressure sensors
To create a capacitive sensor, conducting layers are deposited on the diaphragm and the bottom of a cavity to create a capacitor. The capacitance is typically a few picofarads.
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A cross section of a MEMs capacitive pressure sensor |
Deformation of the diaphragm changes the spacing between the conductors and hence changes the capacitance (see right). The change can be measured by including the sensor in a tuned circuit, which changes its frequency with changing pressure.
The sensor can be used with electronic components on the chip to create an oscillator, which generates the output signal. Because of the difficulty of fabricating large inductances on silicon, this will usually be based on an RC circuit.
This approach is well suited for wireless readout because it generates a high frequency signal that can be detected with a suitable external antenna.
Alternatively, the capacitance can be measured more directly by measuring the time taken to charge the capacitor from a current source. This can be compared with a reference capacitor to account for manufacturing tolerance and to reduce thermal effects.
In both cases, the proximity of the electronics and the sensor element minimises errors caused by stray capacitance and noise.
For more information on capacitive pressure sensors head to chapter 6.2.
MEMS piezoresistive strain gauge sensors
Piezoresistive strain gauge sensors were the first successful MEMS pressure sensors and are widely used in applications such as automotive, medical and household appliances.
Conductive sensing elements are fabricated directly on to the diaphragm. Changes in the resistance of these conductors provide a measure of the applied pressure. The change in resistance is proportional to the strain, which is the relative change in length of the conductor.
The resistors are connected in a Wheatstone bridge network, which allows very accurate measurement of changes in resistance. The piezoresistive elements can be arranged so that they experience opposite strain (half are stretched and the other half are compressed) to maximise the output signal for a given pressure (see diagrams below).
Two ways in which piezoresistive elements might be arranged
An excitation voltage Vex is applied and the output voltage is proportional to the change in resistance:
For more information on piezoresistive strain gauge sensors head to chapter 6.3.
Other MEMS pressure sensors
There are other ways of making MEMS pressure sensors that can be used.
For example, a mechanical structure can be created with a resonant frequency that is a function of applied pressure (like tuning a piano string). A signal is applied to cause the structure to vibrate and the change in resonant frequency is measured. Such devices can be very accurate but are difficult to manufacture and are sensitive to other environmental factors, such as temperature, that also change the resonant frequency.
A surface acoustic wave (SAW) sensor works by sending vibrations through a thin film of piezoelectric material. The waves are picked up by another transducer and converted back to an electrical signal. The changes in the amplitude or phase of the acoustic signal caused by deformation of the surface can be measured to give an indication of pressure.
Construction
MEMS sensors can be used to measure physical parameters such as acceleration, temperature and pressure. Electronic components can be constructed on the same chip to measure the output of the sensors, perform signal processing and provide wireless communication.
Alternatively, the sensor and the electronics can be on separate devices connected together in a single multi-chip package.
Fabrication
The techniques for constructing MEMS are based on those used for semiconductor manufacturing.
Manufacturing starts with a wafer of high-purity silicon. A combination of lithographic patterning with photoresist, etching and deposition of materials is used to build up multi-layer structures to create the components and the connections between them.
Mechanical components can be made by removing surrounding material to create a structure that is free to move. This technique is used to make devices such as accelerometers, inkjet nozzles and even complete “lab on a chip” systems.
Finally, the wafer is cut into individual die, which can be less than a millimetre to several millimetres in size so there can be thousands per wafer. These are then packaged and connecting wires attached. The final cost can be from 10s of pence to a few pounds.
A single wafer can be used to create a variety of different chips at the same time, spreading the manufacturing costs across several products or customers. This also enables relatively low cost semi-customised sensors where specific parameters of a standard device can be customised for a particular application.
The semiconductor material normally used is silicon. This may be combined with other materials for particular applications. For example, for high-speed, low-power electronics, the silicon structures may be constructed on an insulating material such as sapphire or silicon dioxide to create silicon on insulator (SoI) devices.
Silicon is not suitable for very high temperature pressure sensors because its mechanical and electrical properties degrade above about 500ºC. For high temperature applications, the sensor may be constructed from silicon carbide (SiC). This has greater stiffness and fracture strength and also resists wear, oxidation and corrosion better than silicon. This makes it a better material for producing stable pressure sensors for harsh environments.
Packaging
The packaging of the sensor needs to be designed to cope with the environment where the device will be used. A particular challenge for pressure sensors is providing sufficient environmental exposure to allow the external pressure to be measured while also giving adequate protection from magnetic fields, temperature, shock, liquids and gases.
An important aspect of the packaging process for pressure sensors is obtaining a good seal, particularly for absolute pressure sensors, which need to maintain a vacuum cavity below the sensor to achieve long-term stability.
The pressure sensor is often bonded onto a Pyrex glass substrate because its thermal properties are a very close match to silicon.
Applications
Pressure sensors have long been used in medicine, in non-invasive applications such as controlling the air pressure in respiratory equipment and measuring blood pressure. More recently, the miniaturisation provided by MEMS devices has enabled use in more invasive applications such as catheter tip sensors, as well as for implantable devices monitoring properties such as blood pressure and heart rate.
For medical applications there is a challenge in making the standard package (which is made from rigid material with sharp edges) compatible with the biological environment. This can be achieved by enclosing the device in biocompatible plastic or wire.
The small size, low power consumption and long-term stability of MEMS devices also makes them well suited to markets such as aerospace where long life and reliability are important. They are used in a variety of applications including cabin pressure monitoring, engine control, and instruments such as altimeters and barometers.
Advantages and disadvantages
Because of their small size and close integration with the electronics, MEMS sensors can be very low power. In some cases, they can be powered by a small battery that lasts for several years. Some can even operate without a battery, either using energy harvested from the environment or provided by the external device that reads the sensor data.
The capacitive sensor has the advantages of lower power consumption, greater sensitivity and temperature independence.
The main advantages of piezoresistive sensors are high linearity and stability.
MEMS sensors have the advantage of very small size. This means they can respond rapidly to small changes in pressure. It also enables them to be used in new application areas such as implantable medical devices.
Want to learn more about the other core technologies used in pressure sensors? Click the links below to jump to the section you're interested in.
Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.
