Opportunities for MEMS-Based Timing Devices
Timing Devices, which are used to synchronize components, are an integral part of diverse devices that contain an IC or generate a radio signal, such as computer systems, consumer electronics (for example, mobile phones or other portable electronic devices), communications equipment, and other electronics devices or systems, including measurement equipment.
Typically, quartz crystals and quartz crystal oscillators (consisting of a quartz crystal resonator and an oscillation circuit) are to generate an output waveform at a specified frequency for timing. The crystal oscillator creates an electrical signal with a precise frequency. The frequency can be used to keep track of time, provide a clock signal for digital electronic circuits, or to stabilize frequencies for radio transmitters and receivers.
A quartz clock utilizes the piezoelectric property of the quartz crystal. When a quartz crystal vibrates, a difference in electric potential is produced between two of its faces. The crystal has a natural frequency of vibration that depends on its size and shape. If it is placed in an oscillating electric circuit having nearly the same frequency as the crystal, it is caused to vibrate at its natural frequency, and the frequency of the entire circuit becomes the same as the natural frequency of the crystal.
Quartz resonantors/oscillators have such advantages as high frequency stability, stability over temperature, the frequency dependence on temperature is low, and excellent processing ability. The use of photolithographic techniques are, moreover, enabling fabrication of smaller quartz crystals.
However, quartz crystal resonators/oscillators can have shortcomings, especially as electronics devices continue to become increasingly smaller. Quartz crystal resonators cannot be suitably or readily integrated onto silicon CMOS wafers, their cost can increase when their package volume decreases, and they are susceptible to performance degradation when subjected to severe levels of shock and vibration. Using mechanical processes to treat, cut, and shape the quartz crystal can be increasingly challenging with respect to, for example, producing high frequency crystal in smaller packages. Moreover, while photolithographic processes can enable production of smaller quartz crystals, MEMS (microelectromechanical systems) techniques can enable even smaller resonators to be created.
Moreover, MEMS resonators/oscillators (which essentially use a silicon mechanical vibrating beam for the resonator) can have other advantages over their quartz crystal counterparts, such as better shock and vibration characteristics, ability to be programmed to any frequency within a continuous frequency range (rather than requiring a separate quartz device for each frequency); the possibility of integrating the MEMS oscillator in a package or on a single chip with the silicon timing device could provide cost or form factor benefits; MEMS technology could be employed to fabricate multiple resonators on a single die to make system-on-a-chip timing chips; and the MEMS technology may facilitate building a range of different resonator shapes to achieve various properties, frequencies, or Q factors.
While MEMS resonators have had some performance challenges of their own in the past (such as limited temperature stability, thermal hysteresis, long-term stability, as well as the potential of contamination unless they are well-encapsulated), performance improvements have enabled MEMS-based timing devices to begin to make inroads against quartz crystal timing devices in, primarily, crystal oscillators with a MHz frequency output where the specifications for temperature stability can be easier to meet.
Moreover, there are opportunities for MEMS resonators/oscillators to begin to make inroads against quartz crystals in both lower performance applications for MEMS oscillators with a 32 kHz frequency output (such as, for example, standby clocking in cell phones) or higher performance applications (e.g., temperature compensated MHz frequency output oscillators for such applications as cell phones or GPS receivers).