Microelectromechanical system oscillator

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. 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 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. 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 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 resonator's 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 CMOS processes fabricated in CMOS foundries. Integrated MEMS oscillators with CMOS circuits on the same IC die have been demonstrated but to date this homogeneous integration is not commercially viable. Instead, it is advantageous to produce the MEMS resonators and CMOS circuitry on separate die and combine them at the packaging stage. Combining multiple die in a single package in this way is called heterogeneous integration or simply die stacking. Packaging The completed MEMS devices, enclosed in small chip-level vacuum chambers, are diced from their silicon wafers, and the resonator die are stacked on CMOS die and molded into plastic packages to form oscillators. MEMS oscillators are packaged in the same factories and with the same equipment and materials that are used for standard IC packaging. This is a significant contributor to their cost-effectiveness and reliability as compared to quartz oscillators, which are assembled with specialized ceramic packages in custom-built factories. Package dimensions and pad shapes match those of standard quartz oscillator packages so the MEMS oscillators can be soldered directly on PCBs designed for quartz without requiring board modification or re-design. Testing and calibration Production tests check and calibrate the MEMS resonators and CMOS ICs to verify they are performing to specification and trim their frequencies. In addition, many MEMS oscillators have programmable output frequencies that can be configured at test time. Of course the various types of oscillators are configured from specialized CMOS and MEMS die. For instance, low power and high performance oscillators are not built with the same die. In addition, high precision oscillators often require more careful calibration than lower precision oscillators. MEMS oscillators are tested much like standard ICs. Like packaging, this is done in standard IC factories with standard IC test equipment. Using standard IC packaging and test facilities (called subcons in the IC industry) gives MEMS oscillators production scalability. These facilities are capable of large production volumes, often hundreds of millions of ICs per day. This capacity is shared across many IC companies, so ramping production volumes of specific ICs, or in this case specific MEMS oscillators, is a function of allocating standard production equipment. Conversely, quartz oscillator factories are single-function in nature, so that ramping production requires installing custom equipment, which is more costly and time-consuming than allocating standard equipment. ==Comparing MEMS and quartz oscillators==

Quartz oscillators are sold in much larger quantities than MEMS oscillators, and are widely used and understood by electronics engineers. Therefore, quartz oscillators provide the baseline from which MEMS oscillators are compared. Recent advances have enabled MEMS-based timing devices to offer performance levels similar, and sometimes superior, to quartz devices. MEMS oscillator signal quality as measured by phase noise is now sufficient for most applications. Devices with phase noise of at a 10 kHz offset for a 19.2 MHz carrier frequency are now available, a level that is generally only needed for radio frequency (RF) applications. MEMS oscillators are now available with integrated jitter under 1.0 picosecond, measured from 12 kHz to 20 MHz, a level that is normally required for high speed serial data links, such as SONET and SyncE, and some instrumentation applications. Short term stability, startup time, and power consumption, are similar to those of quartz. In some cases, MEMS oscillators show lower power consumption than that of quartz. High precision MEMS temperature-compensated oscillators (TCXOs) have recently been announced with ±0.1 ppm frequency stability over temperature. This exceeds the performance of all but the very high-end quartz TCXOs and oven-controlled oscillators (OCXOs). MEMS TCXOs are now available with output frequencies over 100 MHz, a capability that only a few specialized quartz oscillators (e.g., inverted mesa,) can provide. In RTC applications MEMS oscillators are performing slightly better than the best quartz tuning forks in terms of frequency stability over temperature and solder-down shift, while quartz is still superior for the lowest power applications. Manufacturing and stocking quartz oscillators to the wide variety of specifications that users require is difficult. Various applications require oscillators with specific frequencies, accuracy levels, signal quality levels, package sizes, supply voltages, and special features. The combination of these leads to a proliferation of part numbers which makes stocking impractical and can lead to long production lead times. MEMS oscillator suppliers solve the diversity problem by leveraging circuit technology. While quartz oscillators are usually built with the quartz crystals driven at the desired output frequencies, MEMS oscillators commonly drive the resonators at one frequency and multiply this to the designed output frequency. In this way, the hundreds of standard application frequencies and the occasional custom frequency can be provided without redesigning the MEMS resonators or circuits. There are, of course, differences in the resonator, circuits, or calibration required for different categories of parts, but within these categories the frequency translation parameters can often be programmed into the MEMS oscillators late in the production process. Because the components are not differentiated until late in the process the lead times can be short, typically a few weeks. Technologically, quartz oscillators can be made with circuit-centric programmable architectures like those used in MEMS, but historically only a minority have been built this way. MEMS oscillators are also significantly immune to shock and vibration and have shown production quality levels higher than those associated with quartz. Quartz oscillators are secure in specific applications where suitable MEMS oscillators have not been introduced. One of those applications, for instance, is voltage-controlled TCXOs (VCTCXOs) for cell phone handsets. This application requires a very specific set of capabilities for which quartz products are highly optimized. Quartz oscillators are superior in the extreme high ends of the performance range. These include OCXOs that can maintain stabilities within a few parts per billion (ppb), and surface acoustic wave (SAW) oscillators that can deliver jitter under 100 femtoseconds at high frequencies. Until recently, MEMS oscillators did not compete in the TCXO product range, but new product introductions have brought MEMS oscillators into that market. Quartz is still dominant in clock generator applications. These applications require highly specialized output combinations and custom packages. The supply chain for these products is specialized and does not include a MEMS oscillator supplier. ==Typical applications==

MEMS oscillators are replacing quartz oscillators in a variety of applications such as computing, consumer, networking, communications, automotive and industrial systems. Programmable MEMS oscillators can be used in most applications where fixed-frequency quartz oscillators are used, such as PCI-Express, SATA, SAS, PCI, USB, Gigabit Ethernet, MPEG video, and cable modems. MEMS clock generators are useful in complex systems that require multiple frequencies, such as data servers and telecom switches. MEMS real-time clocks are used in systems that require precise time measurements. Smart meters for gas and electricity are an example that is consuming significant quantities of these devices. The "X" in the names of oscillator types originally denoted "crystal". Some manufacturers have adopted this convention to include MEMS oscillators. Others are substituting "M" for "X" (as in "VCMO" versus "VCXO") to differentiate MEMS-based oscillators from quartz-based oscillators. ==Limitations==

MEMS oscillators may be detrimentally affected by helium. In 2018, a helium leak from a hospital MRI machine caused widespread failure of nearby iPhones due to their MEMS oscillators. A helium concentration of as little as 2% has been shown to cause complete failure of a MEMS oscillator. ==See also==