Pressure sensors use 3D MEMS technology

VTI Technologies in Finland is developing a series of sensing elements that is expected to bring new levels of performance to low power pressure sensors.

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 of performance to low power pressure sensors.

The company is introducing pressure sensing elements and devices mainly intended for a wide range of battery operated and handheld devices.

Pressure is a parameter that has an effect on our lives in numerous ways.

In the atmosphere, pressure tells us about the type of weather we can expect; in our blood, it can indicate our general state of health; in machines, it can indicate how well they are functioning and, in diving, it dictates how long you can safely remain underwater.

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.

VTI's technology allows the integration of tiny mechanical sensor elements and electronics in a small cost effective package.

The company has been a major player in MEMS sensors since its foundation in the early 1990s.

In 2005, it will launch 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 vehicle tyre pressure monitoring systems.

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

The sensor is very robust and can easily withstand a pressure significantly higher than ten times the measuring range.

The design of the sensor allows for optimised performance in different applications.

For high-resolution applications, the sensor has resolution that is better than 2Pa which corresponds to about 160mm resolution in altimeter applications, for example.

The rate at which the measurements are updated by reading the capacitance determines 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 3-4uA.

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 high volume applications.

The ASIC provides not only the serial interface to external equipment (SPI/I2C) but also carries out the overall control, processing, pressure calculations and temperature compensation.

All this is contained in a unit that is only 6.1mm in diameter and 1.7mm in height.

VTI Technologies' 3D-MEMS fabrication method is a powerful approach to sensor manufacturing.

It has made devices that have transformed automotive safety and stability systems, for example.

3D-MEMS allows true three-dimensional structures rather than thin films to be made on a silicon substrate.

This added dimension gives flexibility to the overall design such as in optimising the electrode insulation and connections to the outside world.

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 builds 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 the pressure value can be obtained.

In this approach, the relatively wide change in capacitance, typically between 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 onboard electronics.

It also means that the company can quickly respond to market or technology changes.

The company has an active development programme, which is aimed at providing devices and modules that can meet the diverse needs of a range of industrial and commercial products.

It is currently investing 17% of sales in research and development and in 2006 will complete the construction of new manufacturing facilities that will double its capacity.

The low power consumption and small design of the pressure sensor is one result of this investment.

It opens up the opportunity for affordable measurement devices for consumer and professional use.

Some potential applications include altimeter and pressure sensor devices for sports activities such as climbing, hiking, diving and in professional and medical applications where the added sensing provides additional intelligence and information to equipment.

VTI is already working with a number of companies on bringing the possibilities of this technology to practical use.

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