3D-MEMS devices open new sensing applications

Ulf Meriheina, Business Development Manager at VTI Technologies in Finland, predicts a bright future for 3D-MEMS sensing technologies in both industrial and consumer sectors.

Over the next few years, technological and price barriers will fall away and create new applications for sensors in a wide range of consumer and industrial sectors.

The growth in sensing technology is being fuelled by the maturity of computing and telecommunications markets which has brought about increasingly small, competitively priced products and services, that are, literally, within the reach of everyone's pockets.

As information systems make the transition from fixed locations to being mobile, this opens up new ways to control machines and the environment.

Sensing technology is a way of bringing a new level of intelligence to the environments that humans inhabit.

"It offers the chance to bring a more human dimension to technology", Ulf Meriheina, Business Development Manager at VTI Technologies in Finland, explains.

"The 21st century is the era where sensing is growing and coming of age".

"VTI is a company that is helping develop the technology that will underpin this maturity".

As well as advanced applications in high performance instruments for distance and positions measurement, VTI sees the growth in a host of new applications in sports and fitness, medical technology, terminals and moving machines.

Increasingly humans want information that helps them improve the lives they lead.

"Through sensing technology it will be possible to control machines in novel ways, to provide better information about how effective we are in carrying out activities and to gain better information about where we are in the world through affordable, small, battery powered devices", explains Meriheina.

In medicine, for example, sensing technology adds additional intelligence to life-saving implants and will open up new types of monitoring activities that will support independent living.

New approaches to the interface between humans and their mobile gadgets - phones, PDAs, could change the way we use these devices.

In sports, sensors can not only tell us how effective we are in exercising, but the right way to do it.

The list of potential applications in which sensing could be used is virtually endless.

To exploit these opportunities, information systems will need to sense and act as well as compute.

As important, the technology that underpins sensing needs to be small, robust, accurate and inexpensive.

This is the driving force for the development of three dimensional microelectromechanical systems (3D-MEMS) at VTI Technologies.

In essence, basic MEMS technology allows the integration of tiny mechanical elements, sensors and actuators, and electronics on a common silicon substrate through microfabrication technology.

While the electronics are made using traditional integrated circuit processes such as CMOS, bipolar or BiCMOS, the micromechanical components are produced using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.

MEMS technology, in general, is seen as an important enabling technology.

Today, many countries and academic institutions worldwide have active research programmes to develop an enormous range of devices in fields as diverse as medicine, navigation to biotechnology and control.

Equipment already on the market using MEMS devices range from automobiles and telecomms switching to printers and inertial guidance systems.

VTI is aware that although its own 3D-MEMS devices will be a relatively small fraction of the cost, size and weight within applications, they will be key to their operation, reliability and affordability.

The 3D-MEMS devices, and the smart products they make possible, are likely to be the performance differentiator for a wide variety of commercial products.

There are three basic building blocks in MEMS technology, which are the ability to deposit thin films of material on a substrate, to apply a patterned mask on top of the films by photolithographic imaging, and to etch the films selectively to the mask.

A MEMS process is usually a structured sequence of these operations to form actual devices.

With its own brand of this technology, 3D-MEMS, VTI Technologies has a powerful approach to sensor manufacturing.

It already makes devices that have transformed automotive safety and stability systems, for example.

Now it 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 more than double its capacity.

One key factor with VTI's approach to its technology is that it has decided to separate the sensing elements and the ASICs (application specific integrated circuits) that provide 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.

VTI's 3D-MEMS expertise is built around devices that measure basic parameters - pressure, acceleration, inclination - that can also be translated into other parameters such as position, movement and vibration.

Between them they open up an enormous range of sensing applications that cover automotive, industrial, consumer, medical and sports industries.

Using its technology, the company is able to make true three-dimensional structures rather than thin films on a silicon substrate, which is typical of other MEMS approaches.

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 simply a device that can store electrical charge.

The classical model of a capacitor is that of two plates with a sandwich of another material - a dielectric - in between.

This material can be air or a gas.

At its simplest, the amount of charge that is built up on the capacitor plates can be used to determine changing pressure, acceleration and inclination.

In VTI's sensing technology, the capacitor elements are made from single crystal silicon.

In practise, the design creates an element that is a double capacitor.

A small slice of silicon, which provides a moving mass, moves in a gap between two outer plates or surfaces.

The proof mass builds up charge on each of its surfaces to create two capacitors with the outer surfaces.

Depending on the movements of the silicon mass, the charge between the two capacitors will be different and the way in which the elements have been set up, this can be used to determine a variety of parameters simply through the deflection of the silicon proof mass.

In this approach, the relatively wide change in capacitance, typically between 30 and 50%, makes measurement relatively easy and also results in low noise.

The results are sensing elements that are small, highly accurate and stable over a wide temperature range and have low power consumption.

The 3D-MEMS technology is a powerful and flexible approach to sensor manufacturing.

For VTI it provides almost unlimited opportunities to create solutions that begin with providing basic sensing elements that can be integrated to a specific application to modules that can be regarded as plug and play.

The technology has already proven its power in automotive sensing applications where it is used in the form of accelerometers with electronic stability control and electrically controlled suspension systems.

Tyre pressure measurement is another major application area using VTI technology where the sensing element is integrated within the tyre itself.

The company has now embarked up developing the applications in collaboration with customers outside automotive industry.

The automotive sector has been an excellent proving ground for 3D-MEMS technology because of the stringent performance and quality criteria expected in this industry.

Today, it is not a matter of, if, but when, these new applications become more widespread.

VTI is launching a number of new sensor products and is increasing its manufacturing capacity to meet future demand for its 3D-MEMS-based solutions.

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