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MEMS/cmos process to replace quartz oscillators?

Almost every electronic product needs a clock to provide its 'heartbeat' and, traditionally, this heartbeat has been supplied by a quartz oscillator.

While there is nothing wrong with the basic principle of quartz oscillation, there are many elements of the production process which complicate things.

Initially, the quartz needs to be cut, lapped and polished before further operations create a crystal with the required performance. Because of this complexity, yield is a problem. And then, once the crystal is created, it has to be housed in a hermetically sealed ceramic package, along with a silicon amplifier. Only then can the device be tested to see if it meets requirements.

So it's no surprise to find that companies have been looking for other ways to address the timing requirements of electronic systems. One of those is Silicon Laboratories.

Mike Petrowski, general manager of the company's timing products division, believes quartz has had its day. "It only works on one frequency," he said, "it has long lead times and one of the biggest headaches is the supply chain." He also noted that quartz is a mature technology – almost 100 years old – which is susceptible to shock and vibration, as well as being a relatively high cost solution.

In fact, the company has been working on alternatives to quartz since 2004, when it introduced crystal oscillators (XOs) which brought mixed signal technology to bear on the issue. The result was devices which allowed almost any frequency to be generated from one crystal based reference frequency.

While the approach – called DSPLL – removed the need for unique crystals to generate specific frequencies and simplified the supply chain, the process still created multi-die products. Meanwhile, the industry was beginning to look at MEMS technology as a potential replacement for quartz, with companies such as Discera, SiTime and Epson undertaking some of the early work. Also making inroads into the technology was IDT with its pMEMS based products.

An acquisition made in 2010 brought Silicon Labs into the race. It bought Silicon Clocks, an early stage company creating MEMS based timing technology. Its CMEMS – standing for cmos plus MEMS – technology allowed MEMS resonators and other sensor structures to be created directly on standard cmos wafers. The approach did away with the need for special semiconductor processing for the MEMS elements and addressed performance, integration and size issues. It also addressed parasitics and packaging issues.

"We made the acquisition with the goal of being a one stop timing shop," Petrowski claimed.

Now, Silicon Labs has launched CMEMS based devices. One of the advantages is a shorter manufacturing process. "It's just like making a chip," Petrowski continued. "We can make these parts in less than two weeks, compared with 12 weeks for quartz."

The Si50x range is said by Silicon Labs to feature the most highly integrated MEMS based oscillators currently available. The range has been designed to replace general purpose crystal oscillators in cost sensitive, low power and high volume industrial, embedded and consumer electronics applications.

"The Si50x family introduces an important technological step forward," said Petrowski, "combining all the manufacturing advantages of a single die MEMS based solution, while retaining some of the best characteristics of general purpose crystal oscillators and improving on reliability and lead times."

The manufacturing process starts with standard passivated cmos wafers. Polycrystalline SiGe and pure germanium are then surface micromachined to create integrated MEMS devices on top of the cmos circuitry and interconnects. With the MEMS structure in place on the cmos wafer, a full oscillator system is created. The devices can then be encapsulated in a vacuum. The result is timing devices which are guaranteed to have a frequency stability of ±20ppm over 10 years, along with high shock and vibration resistance.

Four devices in new range
Four devices have been introduced, between them covering any frequency between 32kHz and 100MHz. With low power, low jitter options available, the devices also feature customisable drive strength to reduce emi and trace impedance matching.

One of the issues which CMEMS technology addresses is that of temperature drift. Previously, MEMS based resonators have seen their frequencies drift by up within the range from -30 to 40ppm/°C. Petrowski claims the CMEMS based parts will show temperature related frequency drift of ±2ppm/°C. "One of the technology's key features is passive temperature compensation," he noted, "and this comes as a result of the resonator being made from SiGe and SiO2.

"These materials have different temperature performance," he explained. "SiGe gets harder as the temperature drops, while SiO2 gets softer. The interaction between the two materials means frequency drift due to temperature is minimised."

In terms of construction, the resonator is made from SiGe, but features SiO2 slits (see fig 3). Silicon Labs says this composite material approach provides passive compensation, while the supporting cmos system compensates for remaining frequency drift over the operating life of the product. "Typically," said Petrowski, "Si50x devices show a frequency error of less than 7ppm across the industrial temperature range."

The four devices launched initially are: the Si501, a single frequency device with output enable (OE) functionality; the Si502, a dual frequency oscillator with OE and frequency select (FS) functionality; the Si503, quad frequency oscillator with FS technology; and the Si504, a fully programmable oscillator said to support all potential configurations. The parts are available in four pin DFN packages measuring 2 x 2.5mm, 2.5 x 3.2mm and 3.2 x 5mm.

Flat and revolutionary
MEMS timing pioneer SiTime has just launched its TempFlat MEMS technology. It says TempFlat MEMS is a revolutionary breakthrough that eliminates temperature compensation, resulting in dramatically higher performance, smaller size, lower power and cost.

"SiTime was founded with the vision of revolutionising the timing market with game changing MEMS and analogue technology," said Rajesh Vashist, SiTime's ceo. "In 2011, SiTime demonstrated 100ppb stability with our high performance temperature compensation. With TempFlat MEMS – which performs 30 times better than SiTime's prior resonators – we can get to 5ppb oscillators for Stratum 3E timing in basestations, small cells and optical networking. SiTime can now service all segments of the electronics industry."

Executive vice president of marketing, Piyush Sevalia, added: "TempFlat MEMS dramatically improves frequency stability over temperature. From the user and application point of view, TempFlat enables high precision operation with significant benefits over quartz in areas such as stability, power consumption, size and performance. For example, the TempFlat MEMS resonators in SiTime's SiT15xx family of 32kHz oscillators are up to 85% smaller and consume 50% less power. TempFlat MEMS also gives SiTime a roadmap to 5ppb OCXOs for high performance telecom, optical and networking equipment.

"TempFlat technology is proprietary and cannot be disclosed," he said, "but TempFlat MEMS and programmable analogue are bringing new features and benefits that were not possible with quartz and this 'siliconisation' is changing the landscape of frequency control components."

Graham Pitcher

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