Microelectromechanical System Oscillator

Microelectromechanical system oscillators (MEMS oscillators) Devices that generate highly stable reference frequencies (used to sequence electronic systems, manage data transfer, define radio frequencies, and measure elapsed time) to measure time. The core technologies used in MEMS oscillators have been in development since the mid-1960s, but have only been sufficiently advanced for commercial applications since 2006. MEMS oscillators incorporate MEMS resonators, which are microelectromechanical structures that define stable frequencies. MEMS clock generators are MEMS timing devices with multiple outputs for systems that need more than a single reference frequency. MEMS oscillators are a valid alternative to older, more established quartz crystal oscillators, offering better resilience against vibration and mechanical shock, and reliability with respect to temperature variation.

MEMS timing devices

Resonators

MEMS resonators are small electromechanical structures that vibrate at high frequencies. They are used for timing references, signal filtering, mass sensing, biological sensing, motion sensing, and other diverse applications. This article concerns their application in frequency and timing references.

For frequency and timing references, MEMS resonators are attached to electronic circuits, often called sustaining amplifiers, to drive them in continuous motion. In most cases these circuits are located near the resonators and in the same physical package. In addition to driving the resonators, these circuits produce output signals for downstream electronics.

Oscillators

By convention, the term oscillators usually denotes integrated circuits (ICs) that supply single output frequencies. MEMS oscillators include MEMS resonators, sustaining amps, and additional electronics to set or adjust their output frequencies. These circuits often include phase locked loops (PLLs) that produce selectable or programmable output frequencies from the upstream MEMS reference frequencies.

MEMS oscillators are commonly available as 4- or 6-pin ICs that conform to printed circuit board (PCB) solder footprints previously standardized for quartz crystal oscillators.

Clock generators

The term clock generator usually denotes a timing IC with multiple outputs. Following this custom, MEMS clock generators are multi-output MEMS timing devices. These are used to supply timing signals in complex electronic systems that require multiple frequencies or clock phases. For example, most computers require independent clocks for processor timing, disk I/O, serial I/O, video generation, Ethernet I/O, audio conversion, and other functions.

Clock generators are usually specialised for their applications, including the number and selection of frequencies, various auxiliary features, and package configurations. They often include multiple PLLs to generate multiple output frequencies or phases.

Real-time clocks

MEMS Real-time clocks (RTCs) are ICs that track time of day and date. They include MEMS resonators, sustaining amps, and registers that increment with time, for instance counting days, hours, minutes and seconds. They also include auxiliary functions like alarm outputs and battery management.

RTCs must run continuously in order to keep track of elapsed time. To do this they must sometimes run from small batteries and therefore must operate at very low power levels. They are generally moderate-sized ICs with up to 20 pins for power, battery backup, digital interface, and various other functions.

History of MEMS timing devices

First demonstration

Motivated by the shortcomings of quartz crystal oscillators, researchers have been developing the resonance properties of MEMS structures since 1965. However, until recently various accuracy, stability, and manufacturability issues related to sealing, packaging, and adjusting the resonator elements prevented cost-effective commercial manufacturing. Five technical challenges had to be overcome:
* First demonstrations
* Finding stable and predictable resonator materials,
* Developing sufficiently clean hermetic packaging technologies,
* Trimming and compensating the output frequencies, increasing the quality factor of the resonator elements, and
* Improving the signal integrity to meet various application requirements.

The first MEMS resonators were built with metallic resonator elements. These resonators were envisioned as audio filters and had moderate quality factors (Qs) of 500 and frequencies of 1 kHz to 100 kHz. Filtering applications, now for high frequency radio, are still important and are an active area for MEMS research and commercial products.

However, early MEMS resonators did not have sufficiently stable frequencies to be used for timing references or clock generation. The metallic resonator elements tended to shift frequency with time (they aged) and with use (they fatigued). Under temperature variation they tended to have large and not entirely predictable frequency shifts (they had large temperature sensitivity) and when they were temperature cycled they tended to return to different frequencies (they were hysteretic).

Material development

Work in the 1970s through the 1990s identified sufficiently stable resonator materials and associated fabrication techniques. In particular, single and polycrystalline silicon was found to be suitable for frequency references with effectively zero aging, fatigue and hysteresis, and with moderate temperature sensitivity.

Material development is still ongoing in MEMS resonator research. Significant effort has been invested in silicon-germanium (SiGe) for its low temperature fabrication and aluminium nitride (AlN) for its piezoelectric transduction. Work on micromachined quartz continues, while polycrystalline diamond has been used for high frequency resonators for its exceptional stiffness-to-mass ratio.

Packaging development

MEMS resonators require cavities in which they can move freely, and for frequency references these cavities must be evacuated. Early resonators were built on top of silicon wafers and tested in vacuum chambers, but individual resonator encapsulation was clearly needed.

The MEMS community had employed bonded cover techniques to enclose other MEMS components, for instance pressure sensors, accelerometers, and gyroscopes, and these techniques were adapted to resonators. In this approach, cover wafers were micromachined with small cavities and bonded to the resonator wafers, enclosing the resonators in small evacuated cavities. Initially these wafers were bonded with low melting temperature glass, called glass frit, but recently other bonding technologies including metallic compression and metallic amalgams, have replaced glass frit.

Thin film encapsulation techniques were developed to form enclosed cavities by building covers directly over the resonators in the fabrication process rather than bonding covers onto the resonators. These techniques had the advantage that they did not use as much die area for the sealing structure, they did not require preparation of second wafers to form the covers, and the resulting device wafers were thinner.

Frequency references generally require frequency stabilities of 100 parts per million (ppm) or better. However, the early cover and encapsulation technologies left significant amounts of contamination in the cavities. Because MEMS resonators are small, and particularly because they have small volume-to-surface area, they are especially sensitive to mass loading. Even single-atomic layers of contaminants like water or hydrocarbons can shift the resonator's frequencies out of specification.

When resonators are aged or temperature cycled, the contaminants can move in the chambers, and transfer onto or off of the resonators. The change in mass on the resonators can produce hysteresis of thousands of ppm, which is unacceptable for virtually all frequency reference applications.

Early covered resonators with glass frit seals were unstable because contaminants outgassed from the sealing material. To overcome this, getters were built into the cavities. Getters are materials that can absorb gas and contaminants after cavities are sealed. However, getters can also release contaminants and can be costly, so their use in this application is being discontinued in favor of cleaner cover bonding processes.

Likewise, thin film encapsulation can trap fabrication byproducts in the cavities. A high temperature thin film encapsulation based on epitaxial silicon deposition was developed to eliminate this. This epitaxial sealing (EpiSeal) process has been found to be exceptionally clean and produces the highest stability resonators.

Electronic frequency selection and trimming

In early MEMS resonator development, researchers tried to build resonators at the target application frequencies and to maintain those frequencies over temperature. Approaches to solving this problem included trimming and temperature compensating the MEMS resonators in ways analogous to those used for quartz crystal.

However, these techniques were found to be technically limiting and expensive. A more effective solution was to electronically shift the resonators’ frequencies to the oscillators’ output frequencies. This had the advantage that the resonators did not need to be individually trimmed; instead their frequencies could be measured and appropriate scaling coefficients recorded in the oscillator ICs. In addition, the resonators’ temperatures could be electronically measured, and the frequency scaling could be adjusted to compensate for the resonators’ frequency variation over temperature.

Improving signal integrity

Various applications require clocks with predefined signal and performance specifications. Of these, the key specifications are phase noise and frequency stability.

Phase noise has been optimized by raising the resonator's natural frequencies (f) and quality factors (Q). The Q specifies how long resonators continue to ring after drive to them is stopped, or equivalently when viewed as filters how narrow their pass-bands are. In particular, the Q times f, or Qf product, determines the near-carrier phase noise. Early MEMS resonators showed unacceptably low Qf products for reference. Significant theoretical work clarified the underlying physics while experimental work developed high Qf resonators. The presently available MEMS Qf performance is suitable for virtually all applications.

Resonator structural design, particularly in mode control, anchoring methods, narrow-gap transducers, linearity, and arrayed structures consumed significant research effort.

The required frequency accuracies range from relatively loose for processor clocking, typically 50 to 100 ppm, to precise for high speed data clocking, often 2.5 ppm and below. Research demonstrated MEMS resonators and oscillators could be built to well within these levels. Commercial products are now available to 0.5 ppm, which covers the majority of application requirements.

Finally, the frequency control electronics and associated support circuitry needed to be developed and optimized. Key areas were in temperature sensors and PLL design. Recent circuit developments have produced MEMS oscillators suitable for high speed serial applications with sub-picosecond integrated jitter.

Commercialization

The U.S. Defense Advanced Research Projects Agency (DARPA) funded a wide range of MEMS research that provided the base technologies for the developments described above. In 2001 and 2002 DARPA launched the Nano Mechanical Array Signal Processors (NMASP) and Harsh Environment Robust Micromechanical Technology (HERMIT) programs to specifically develop MEMS high stability resonator and packaging technologies. This work was fruitful and advanced the technology to a level at which venture capital funded startups could develop commercial products. These startups included Discera in 2001, SiTime in 2004, Silicon Clocks in 2006, and Harmonic Devices in 2006.

SiTime introduced the first production MEMS oscillator in 2006, followed by Discera in 2007. Harmonic Devices changed its focus to sensor products and was bought by Qualcomm in 2010. Silicon Clocks never introduced commercial products and was bought by Silicon Labs in 2010. Additional entrants have announced their intention to produce MEMS oscillators, including Sand 9 and VTI Technologies.

By sales volume, MEMS oscillator suppliers rank in descending order as SiTime and Discera. A number of quartz oscillator suppliers resell MEMS oscillators. SiTime announced it has cumulatively shipped 50 million units as of mid-2011. Others have not disclosed sales volumes.

Operation

One can think of MEMS resonators as small bells that ring at high frequencies. Small bells ring at higher frequencies than large bells, and since MEMS resonators are small they can ring at high frequencies. Common bells are meters down to centimeters across and ring at hundreds of hertz to kilohertz; MEMS resonators are a tenth of a millimeter across and ring at tens of kilohertz to hundreds of megahertz. MEMS resonators have operated at over a gigahertz.

Common bells are mechanically struck, while MEMS resonators are electrically driven. There are two base technologies used to build MEMS resonators that differ in how electrical drive and sense signals are transduced from the mechanical motion. These are electrostatic and piezoelectric. All commercial MEMS oscillators use electrostatic transduction while MEMS filters use piezoelectric transduction. Piezoelectric resonators have not shown sufficient frequency stability or quality factor (Q) for frequency reference applications.

Electronic sustaining amps drive the resonators in continuous oscillation. These amplifiers detect the resonator motion and drive additional energy into the resonators. They are carefully designed to maintain the resonators motion at appropriate amplitudes and to extract low noise output clock signals.

Additional circuits called fractional-n phase lock loops (frac-N PLLs) multiply the resonator's mechanical frequencies to the oscillator's output frequencies. These highly specialized PLLs set the output frequencies under control of digital state machines. The state machines are controlled by calibration and program data stored in non-volatile memory and adjust the PLL configurations to compensate for temperature variations.

The state machines can also be built to provide additional user functions, for instance spread-spectrum clocking and voltage controlled frequency trimming.

MEMS clock generators are built with MEMS oscillators at their core and include additional circuitry to supply the additional outputs. This additional circuitry is usually designed to provide the specific features required by the applications.

MEMS RTCs work like oscillators but are optimized for low power consumption and include auxiliary circuits to track the date and time. To operate at low power they are built with low frequency MEMS resonators. Care is taken in circuit design to minimize power consumption while providing the required timing accuracies.

Manufacturing

Resonators

Depending upon the type of resonator, the fabrication process is either done in a specialized MEMS fab or a CMOS foundry.

The manufacturing process varies with resonator and encapsulation design, but in general the resonator structures are lithographically patterned and plasma-etched in or on silicon wafers. All commercial MEMS oscillators are built from poly or single crystal silicon.

It is important in electrostatically transduced resonators to form narrow and well controlled drive and sense capacitor gaps. These can be either lateral for instance under the resonators, or vertical beside the resonators. Each option has its advantages and both are used commercially.

The resonators are encapsulated either by bonding cover wafers onto the resonator wafers or by depositing thin film encapsulation layers over the resonators. Here again, both methods are used commercially.

Bonded cover wafers must be attached with an adhesive. Two options are used, a glass frit bond ring or a metallic bond ring. The glass frit has been found to generate too much contamination, and thus drift, and is no longer commonly used.

For thin film encapsulation the resonators’ structures are covered with layers of oxide and silicon, then released by removing the surrounding oxide to form freestanding resonators, and finally sealed with an additional deposition.

Circuitry

The sustaining amps, PLLs, and auxiliary circuits are built with standard mixed-signal

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