Taking the pressure

Advances in fabrication technology and design are now opening up new applications for pressure sensors. VTI Technologies in Finland is developing a series of sensing elements that is expected to bring new levels to performance to low pressure sensors. The company is introducing pressure sensing elements and devices mainly intended for a wide range of battery operated and hand-held devices.

Behind the development are advances in its own measurement techniques coupled with its ability to manufacture the devices using its own proprietary manufacturing technology called three dimensional microelectromechanical systems, 3D-MEMS. This technology allows the integration of tiny mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technologies.

Incorporation into next generation products

The company has been a major player in MEMS sensors since its foundation in the early 1990s. Last year, it launched a new sensor family that could be used in the next generation of consumer and industrial products from altimeters to wrist diving computers. It is based on the same technology that has allowed the development of sensors that have already been integrated into tyres for a more accurate pressure measurement for vehicles.

The low power absolute pressure sensor concept allows sensing elements and devices that can measure pressure in the range of barometric (100kPa) to 25 bar (2500kPa).

The sensor is robust and can easily withstand a pressure significantly higher than 10X the measuring range. The design of the sensor allows for optimized performance in different applications. For highresolution applications, the sensor has resolution better than 6Pa which corresponds to about 0,4m resolution for altimeter applications. The rate at which the measurements are updated by reading the capacitance determine both the resolution, the speed of operation, and the power consumption. The sensor can also be configured to different power modes. In the low power mode it is in standby between measurements and consumes only 3-4 microamps.

Separate control circuitry

The sensor has been integrated with the Application Specific Integrated Circuit (ASIC), within the device to create a package that is robust and suitable for highvolume applications. The ASIC does not only provide the serial interface to external equipment (SPI/12C) but also carries out the overall control, processing, pressure calculations and temperature compensation. All this is contained in a unit that is only 6.1 mm in diameter and 1.7 mm in height.

The company's 3D-MEMS technology is used to create sensors that rely on a change in capacitance for their operation. A capacitor is a device that can store electrical charge. The classical model of a capacitor is that of two electrodes or plates on which charges build up. At its simplest, the amount of charge that is built up on the capacitor plates can be used to determine changing pressure.

In VTI's pressure sensor, the pressure diaphragm is one plate of a capacitor that changes its value under pressure-induced displacement. In practice, two silicon wafers form the basic capacitor element with a membrane over which the force can be measured. The outer pressure exerts a force on the membrane causing it to bend towards the bottom electrode relative to a reference pressure inside the cavity between the wafers. The displacement of the membrane is detected as a change in capacitance between the membrane and bottom electrode. Using a simple mathematical calculation based on the fact that the force acting on the membrane is directly proportional to the inverse of capacitance one can get the pressure value.

Low noise

In this approach, the relatively wide change in capacitance, typically between 30% and 50%, makes measurement relatively easy and also results in low noise systems. The results are sensing elements that are small, highly accurate, and stable over a wide temperature range and have low power consumption.

One key factor with VTI's approach to its technology is that it has decided to separate the sensing elements and the ASIC, which provides the information processing and the electronic control. This gives a high-degree of flexibility in the range of products that the company can offer from individual sensing elements to complete modules with on-board electronics. It also means that the company can quickly respond to market or technology changes.

VTI Technologies Oy
Vantaa, Finland
vti.fi