Woman using iPhone 4

MEMS devices build high for new applications

The iPhone 4 sees the first three-axis gyroscope in a consumer application as manufacturers pile MEMS high.

Just ahead of Apple's launch of the iPhone 4, Carmelo Papa, general manager of STMicroelectronics' industrial and multi-segment sector, was bullish about a new market for the company. He declared that this year would be the dawn of the 'era of the gyroscope'.

A few days later, Apple CEO Steve Jobs was demonstrating what the combination of an accelerometer and three-axis gyroscope could do in a mobile handset. ST made its own video for the company's Field Trip, a series of presentations to financial analysts, to show how the gyro and accelerometer combination could be used to navigate through the streets of Venice, using dead reckoning where the signals from GPS satellites cannot be easily seen.

As ST is the only vendor claiming to make an integrated three-axis gyroscope, this is the one that is suspected to be inside the iPhone 4. Teardown experts such as Chipworks and TechInsights believe die markings confirm ST as the manufacturer.

Benedetto Vigna, head of ST's MEMS division, claims that the consumer market for gyros will be three times bigger than that for similar sensors in automotive - where they are more widely used today for stabilisation control - by 2014.

Although the initial demonstrations revolve around games, Vigna says dead-reckoning calculations make it possible to pinpoint a user's location inside buildings such as shops and museums, narrowing location-based information to a rack of clothes or a museum exhibit.

While it does not need all three axes, optical image stabilisation is another use for cameras and phones. Papa says applications for the MEMS gyro can go further, including vibration control in washing machines. 'The potential market is huge,' he says.

According to Vigna, the decision to make a three-axis gyro was taken quite late in the day in an indication of how integration and fast turnaround are becoming crucial in getting MEMS into high-volume consumer designs. 'By the end of last year, we understood that the market was willing to move faster,' he says, explaining that the company had been working on a two-axis design.

'On 21 October, we put the first transistors on the layout. And you are now holding the product,' he says as he hands out tiny 4mmx4mm packages. 'Because of the nimbleness of the team we now have a three-axis gyroscope.'

ST's aim is to integrate the gyro and accelerometer and other sensors with analogue processing using a stack of chips. The MEMS can potentially all go on the same die, with the analogue and ultimately microcontroller going underneath in a multichip package.

Consumer gyroscopes

In contrast to ST, Freescale is in no hurry to develop consumer-level gyroscopes. ST's Papa claims his company will see $100m in revenue from its gyroscope MEMS products by the end of this year, with practically zero in sales at the start of this year. Stephane Gervais-Ducouret, director of global marketing for sensors in the consumer market segment of Freescale, concedes a few million will sell this year but is not expecting such as quick take-off. Freescale is looking to 2012 as the time 'there will be a booming in terms of the market for gyroscopes,' he adds.

'We have been focusing so far on gyroscopes for automotive,' says Gervais-Ducouret, adding that power consumption is an issue for this kind of sensor.

Typically, automotive-grade gyroscopes consume more than 10mA per channel but have the benefit of running from a bigger power source. 'If you look at the mobile phone, you are looking for less than 1mA,' he says. ST's current gyro consumes around 2mA per channel.

Where Freescale has put its effort is into integration. '[ST's offering] is not integrated all in one. It is not integrated to this level,' says Gervais-Ducouret, referring to the inclusion of a microcontroller along with the MEMS-based sensors and analogue conditioning electronics. 'The only thing not on the same piece of silicon are the MEMS-based sensors.'

By moving sensor processing into a local microcontroller, Gervais-Ducouret says it's possible to achieve power savings compared to running those same algorithms on a host applications processor. 'It offers customisable power management - you can change the sampling frequency and have auto-wake-up and auto-sleep.'

Once the processing is sitting inside a dedicated microcontroller, it's possible to watch for wake-up gestures without forcing the more power-hungry applications processor to run. This can be extended to applications such as pedometers that use readings from the accelerometers to count steps. 'It can run all day without waking up the whole phone,' he claims.

For the first product, which incorporates accelerometers, Freescale has developed a number of packaged routines that will detect different types of user interaction. The idea is that the company can provide these as canned functions that handset and gadget makers can easily roll into their designs.

Where Freescale and ST see multichip integration as the way forward, Belgian research institute IMEC favours monolithic integration, at least for the types of device that the institute is investigating.

For its CMore technology, IMEC programme manager Stephane Donnay says the institute is looking mainly at devices that need not just a few connections to the processing silicon - the case with accelerometers or gyroscopes - but hundreds or thousands. 'We can have distributed and massively parallel connections between MEMS and ICs. It is the only technical solution for arrays of MEMS such as micromirrors,' he says, referring to the technology used in digital projectors. 'The real killer application is big arrays. It's the only way of doing it.'

Other applications, says Donnay, are arrays of sensors, which can act as detectors for a range of chemicals, active gratings for smart lenses or advanced ultrasound transducers and arrays of loudspeakers.

Because both CMOS and MEMS go on the same wafer, IMEC has to deal with limitations of the substrate, which is based on silicon germanium. 'We use poly-silicon germanium rather than aluminium because it's very flexible and modular. But it does not have good reflective properties; you need to use coatings. However, silicon germanium is interesting because we can play with doping to tune the resistivity and its thermal coefficient,' Donnay claims.

In contrast to Freescale and ST, IMEC is not looking at huge wafer volume, perhaps a few hundred wafers a year, using high-voltage processed CMOS wafers from Taiwanese foundry TSMC. Vigna argues that manufacturers need to run a lot of wafers to iron the kinks out of MEMS designs, which is why the consumer market is so important to ST rather than traditional industrial. However, healthcare, which is where IMEC and ST are looking to a future, larger market for MEMS and processing stacks, could be even bigger.

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