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New MEMS Oscillators

Quartz is under attack yet again. While some folks are bringing quartz into non-timing applications, others are trying to squeeze it out of its primary application: timing.

Sand 9 is the latest such company, having just debuted their basic platform. They point to some fundamental limitations of quartz as a material, limitations we’ve lived with for a long time. Issues they highlight in particular are vulnerabilities due to vibration and shock, degradation at high temperatures and frequencies, issues with rapid temperature dips, inconsistencies between suppliers, and something called “activity dips.”

These latter sound really obscure – and yet Sand 9 say that they constitute the cause of 0.5% of all cell phone failures. The problem occurs when secondary vibration modes move around due to stresses and temperature – and they move in a way that’s different from the fundamental. So those modes may actually cross the fundamental, causing what Sand 9 refer to as a “heart attack.”

Their solution to this is a silicon-based one – and they had to deal with the problem that silicon on its own has much worse temperature performance than quartz does (3000 ppm vs. 20 ppm). That’s because silicon softens as temperature goes up. But, conveniently, SiO2 gets stiffer with higher temperature – so they have brought the two together in a sandwich to counteract each other, giving stability of less than 200 ppm.

So they have a six-layer stack: on top is the inter-digitated transducer, which acts as a top electrode and gets the whole thing oscillating. This overlays a layer of AlN, which sits atop the bottom electrode. Below those are relatively thick layers: a sandwich of oxide/silicon/oxide that provides temperature stability.

From a product standpoint, they’ve announced two families and hinted at an upcoming third.

  • The simplest one, the MR family, is just a resonator targeted primarily at Internet of Things devices communicating via Bluetooth Smart.
  • The second is the TSMR family. The “TS” stands for “temperature sensing”; it has a built-in heater and temperature sensor for use in the factory in calibrating and dialing up compensation. The target is cell phones.
  • Hinted at is a future TSMO (O for Oscillator) family that will have a silicon cap with an integrated oscillator circuit. Also targeted at cell phones.

All of them are provided in wafer-level chip-scale packaging (WLSCP) for integration into systems-in-package (SiP) assemblies. They claim no activity dips or susceptibility to vibration (> 10-10/G) or shock (30,000 G) and excellent phase noise performance.

You can find out more in their announcement.

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