Microelectromechanical system oscillators (MEMS oscillators) are devices that generate highly stable reference frequencies used to sequence electronic systems, manage data transfer, define radio frequencies, and measure elapsed 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. 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. 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.
… excerpt ends here. Continue reading the full article.
