MEMS-Based Sensor Technology
   

MEMS-Based Sensor Technology


Micro-electromechanical systems (MEMS) are Freescale's enabling technology for acceleration and pressure sensors. MEMSbased sensor products provide an interface that can sense, process and/or control the surrounding environment.

Freescale's MEMS-based sensors are a class of devices that builds very small electrical and mechanical components on a single chip. MEMS-based sensors are a crucial component in automotive electronics, medical equipment, hard disk drives, computer peripherals, wireless devices and smart portable electronics such as cell phones and PDAs.

Benefits of MEMS:

  • Low Cost
  • Low Power
  • Miniaturization
  • High Performance
  • Integration

Freescale's Leadership Position

Freescale Semiconductor has been developing MEMS-based sensors for almost 30 years. The process technology used to manufacture the sensors can broadly be grouped into two: bulk micromaching and surface micromachining. Our organization produces acceleration sensors and pressure sensors. Freescale is a high volume manufacturer of MEMS-based sensors.

Applications

MEMS-based sensors are a crucial component in automotive electronics, medical equipment, smart portable electronics such as cell phones, PDAs, and hard disk drives, computer peripherals, and wireless devices. These sensors began in the automotive industry especially for crash detection in airbag systems. Throughout the 1990s to today, the airbag sensor market has proved to be a huge success using MEMS technology. MEMS-based sensors are now becoming pervasive in everything from inkjet cartridges to cell phones. Every major market has now embraced the technology.

HARMEMS Technology

Freescale's next-generation high aspect ratio micro-electromechanical systems (HARMEMS) technology is a proven technology for airbag sensing applications. The accelerometers have an advanced transducer design that enhances sensor offset performance. HARMEMS technology provides over-damped mechanical response and exceptional signal-to-noise ratio to address customer requirements. Since the airbag main ECU system is installed in the vehicle cabin, over-damped HARMEMS technology enables a high degree of immunity to high-frequency, high-amplitude parasitic vibrations. HARMEMS technology has also been introduced in dual-axis accelerometers used in electronic stability control (ESC) to measure the lateral acceleration of the vehicle.

HARMEMS Technology

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MEMS Surface Micromachining

In surface micromachining, the MEMS sensors are formed on top of the wafer using deposited thin film materials. These deposited materials consist of structural materials that are used in the formation of the sensors and sacrificial layers that are used to define gaps between the structural layers. Many of the surface micromachined sensors use the capacitive transduction method to convert the input mechanical signal to the equivalent electrical signal. In the capacitive transduction method, the sensor can be considered to be a mechanical capacitor in which one of the plates moves with respect to the applied physical stimulus. This changes the gap between the two electrodes with a corresponding change in the capacitance. This change in capacitance is the electrical equivalent of the input mechanical stimulus.

MEMS Surface Micromachining

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MEMS Bulk Micromachining

In bulk micromachining, the single crystal silicon is etched to form three-dimensional MEMS devices. This is a subtractive process in which the silicon in the wafer is specifically removed using anisotropic chemistries. Using this bulk micromachining method, sensors such as piezoresistive pressure sensors have been manufactured in high volume. In the simplest implementation, the silicon is selectively etched in certain areas to form a diaphragm. In an absolute pressure sensor, the silicon wafer is then bonded with another wafer (either of silicon or glass) to form a vacuum-sealed cavity below the diaphragm. The diaphragm then deflects in response to the applied pressure. The transduction mechanism that has been widely used is the piezoresistive effect. In piezoresistive materials, the change in the stress causes a strain and a corresponding change in the resistance. Thus, when implanted piezoresistors are formed at the maximum stress points of the diaphragm, the deflection under the applied pressure causes a change in the resistance. Typically, these piezoresistors are formed as a bridge network and the voltage applied between two terminals cause an output voltage to be measured between the other two terminals.

MEMS Bulk Micromachining

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