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Quartz crystals clocking off?

Silicon based timing solutions set to give quartz crystals a run or their money

The exceptional mechanical and piezoelectric properties of quartz crystal oscillators have, for decades, ensured their success for frequency generation in a range of consumer, computing and communication applications.

Issues with reliability and manufacturing complexity, however, have threatened their position in the market and resulted in an increased demand for suitable alternatives.

MEMS based oscillators have emerged as a smaller, more cost effective and more reliable solution in recent years. But can silicon really take on quartz?

One company that thinks it can is California based IDT. In November 2011, the company announced what it believes are the first commercially available oscillators to incorporate piezoelectric microelectromechanical system (pMEMS) resonators.

The significance of this release, according to IDT's managing director of MEMS, Harmeet Bhugra, is that the devices combine the strong electromechanical coupling of piezoelectric quartz with the stability and low damping of single crystal silicon.

The pMEMS process works by layering aluminum-nitride piezoelectrics on top of a single crystal silicon MEMS resonator beam. The piezoelectric layer induces vibrations in the silicon beam and the bulk-acoustic waves are translated into electrical timing signals. As a result, IDT says native frequencies up to 1GHz can be achieved.

"The process is designed in a way that activity dips are eliminated; something not possible with quartz devices," claimed Bhugra. "The oscillators are inexpensive to manufacture and are naturally ruggedised for superior reliability." Measuring 0.56 x 0.43mm, they are also the smallest hermetically sealed wafer level packaged resonators available.

IDT believes the market for MEMS oscillators will be one of the fastest growing segments of the $5billion timing chip market over the next few years; and that is why it has been developing its pMEMS oscillators.

"MEMS is a natural step forward for us in further broadening our product offering," Bhugra noted. "The MEMS timing market is expected to grow rapidly over the next several years and we plan to garner a large share of it."

IDT, which already has a presence in the quartz, SAW and cmos timer chip market, says it is striving to become a one stop timing chip solution provider. But, with companies such as SiTime already claiming an 85% market share, it looks set to face serious competition.

SiTime announced in October 2011 what it claimed to be the highest performing MEMS oscillators in the timing industry. The SiT9121 and SiT9122 are designed to achieve 10ppm stability and have a jitter performance of just 500fs.

The release of these oscillators was followed by a series of Stratum 3 compliant silicon MEMS timing solutions, designed to replace legacy oven-and temperature controlled crystal oscillators (ocxo, tcxo).

The SiT5301 and SiT5302 have a frequency stability of ±0.1ppm over a temperature range of 0 to 70°C. According to SiTime's vp of marketing Piyish Sevalia, they are available in a 2520 compatible package, the industry's smallest ±100ppb solution.

"This represents a 60% reduction in size compared to any other Stratum 3 tcxos and a tenfold reduction compared to ocxos," Sevalia said. "The devices also have the widest frequency range of 1 to 220MHz with six digits of accuracy, allowing designers to program custom frequencies and develop systems with better performance and reliability."

While acknowledging that quartz suppliers have made improvements in stability, size and packaging in recent years, Sevalia believes MEMS based silicon timing solutions offer benefits not available from legacy quartz products.

"Quartz crystal based solutions have been the only timing solution for more than 60 years, but they have issues that people have to work around," he said. "In general, their performance, cost, reliability and availability are just not as good as silicon solutions and the electronics industry is beginning to recognise the silicon MEMS timing alternatives that are available.

"Silicon MEMS timing companies can leverage the immense infrastructure of the semiconductor packaging and manufacturing industries to offer timing products that have better features, higher performance, better reliability and lower cost trajectory than quartz crystal suppliers. We believe that this helps solve engineer's problems and helps them get their products to market more quickly and cost effectively."

This belief has been borne out by market research firm IHS iSuppli, which anticipates a high growth rate in the sector over the coming years.

"MEMS timing has been gaining market share against quartz oscillators and crystals, extending its reach from consumer devices to telecom infrastructure and hard disk drives," said Jérémie Bouchaud, IHS iSuppli's principal MEMS and sensors analyst. "Revenue will grow from $13.5million in 2010 to $205m in 2015."

The reason why silicon always wins, according to Bouchard and Sevalia, is that it offers designers improved cost and integration. However, Somerset based manufacturer IQD believes there will always be a need for quartz.

Neil Floodgate, IQD's vp of sales, doesn't see the technology taking off for the next 20 years. He maintained: "One of the many benefits of using quartz based timing products is that the technology is very well established, as opposed to the silicon based timing products which are still in their infancy. The silicon specifications are still in a state of flux as the silicon manufacturers continue to understand the limits of their technology.

"Technically, quartz offers a better phase noise and jitter specification and is able to achieve much tighter tolerances down to ±0.5ppm. Quartz also has a much smoother perturbation, which gives a much more repeatable change in frequency when operating temperature changes."

IQD believes the MEMS and silicon companies will only increase their share in the commodity sector over the coming years, whilst it anticipates that quartz crystals will move into a more high end market to keep pace.

The company recently announced a range of high specification AT cut crystals with frequency ranges of 10 to 42MHz at fundamental mode, 10 to 125MHz at third overtone and 70 to 175MHz at fifth overtone.

The crystals also offer aging as low as 0.1ppm per year and frequency tolerances at 25°C of as little as ±3ppm. "This compares to standard crystals, where typical aging figures would be 3ppm per year and ±10ppm frequency tolerance," commented Floodgate. "Temperature stabilities down to 4ppm at 0 to 70°C and 12ppm at -40 to 85°C can also be specified."

Floodgate maintained that, with the price of quartz based products steadily decreasing, there is now little price differential between quartz and silicon. He believes that MEMS devices still have to become more flexible to prove themselves fully.

"With frequencies up to 800MHz, quartz can cover a wide range of applications," Floodgate concluded. "Quartz is also flexible when discussing the customer's exact requirements and we can adjust the specification to the precise needs of the engineer. This is just not possible with silicon.

"It's fair to say that, at some stage, silicon MEMS will gain a larger share of the overall timing market, but we are confident that quartz based devices will be around for a long time."

Laura Hopperton

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