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Digital, analog, and RF applications are requiring ever-greater performance from oscillator technology—specifically in terms of size, cost, and frequency. Skyrocketing data rates, military precision, and millimeter-wave technologies are at the forefront of these demanding applications. Fortunately, a consistently scaling technology method may be able to satisfy these demands. For example, complementary-metal-oxide-semiconductor (CMOS) processes may be reaching a performance level that enables microelectromechanical-systems (MEMS) technology to replace the comparatively bulky crystals in the latest oscillators (Figs. 1 and 2).

Such a move would drastically change the industry, as quartz-crystal technology for precision frequency control and timing has dominated the market since 1917 (when the first crystal-controlled oscillator was patented).  The first quartz-crystal clock was developed in 1928. In the 1950s, atomic clocks were used, although their materials are generally too large and costly for use in most modern electronic applications. In the early 2000s, other technologies finally began catching up to the resonate performance of quartz crystals.

Quartz-crystal resonators operate according to the principle dictating that elastic materials have resonant frequencies of vibration. The resonant frequency of a material depends upon its size, elasticity, speed of sound, and shape. As long as they are properly cut and mounted, quartz crystals will physically distort in the presence of an electric field when a voltage is applied to an electrode near the crystal. This electrostriction, or inverse piezoelectricity effect, causes a resistive, capacitive, and inductive (RLC) resonant-circuit-like response at a very precise resonant frequency. Crystal resonators can be manufactured to resonant frequencies ranging from a few kilohertz to several hundred megahertz.

The manufacturing process for performance crystals requires specialized equipment and hermetic sealing. Often, it also demands temperature-control technologies in complex packages that can support these options. Such specialized manufacturing stages are required to mitigate the limitations of quartz-crystal technology. The higher the frequency of resonance, the more susceptible this technology is to interference from parasitics. After all, any additional capacitance or inductance will cause shifting in the resonant frequency. Furthermore, a higher frequency of operation tends to lead to sensitivities and loss of performance at temperature and vibration variations. Compensating for these limitations requires ever-greater steps to control the temperature and buoy the resonating element to provide protection from vibrations.

With the telecommunications, test and measurement, military, and space markets demanding ever-smaller footprints for oscillator technologies, development has been increasing for technologies without these limitations. Surface-acoustic-wave (SAW) and bulk-acoustic-wave (BAW) resonators have been used for oscillation technologies. Yet they suffer many of the same size, cost, and reliability issues as quartz-crystal technologies. Over the past few years, other methods have finally surfaced at companies like Silicon Labs, Toshiba, Vectron, SiTime,  Synergy, Sand9, Micrel, Discera, and others. They are producing MEMS-based oscillators in very small and flat packages (Fig. 3). To create the final oscillator device, the earlier versions of this technology still rely on a MEMS resonator chip packaged with another IC in a multichip module (MCM).

MEMS MCM specialized packages require fabrication facilities with unique capabilities. Unfortunately, relying on non-mainstream technology tends to drive up costs while slowing iterative improvements. Additionally, strain and rapid temperature fluctuations are still significant performance degraders for MEMS MCM technology, which has only just reached quartz-crystal-oscillator capabilities (Figs. 4). A logical next step is to integrate all of the oscillator elements onto a single die that can be manufactured in a standard process. CMOS technology nodes, for instance, continue to increase in performance and capabilities. As a result, many technologies are moving to CMOS to enable the development of miniature and cost-effective solutions.

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