2-D Nano-Electro-Mechanical Systems for RF Systems
December 15, 2014, IEDM, San Francisco—Philip Feng from Case Western Reserve University presented results of experiments with NEMS devices as the frequency setting mechanism for RF oscillators. As many devices need an accurate clock, the NEMS devices offer high resolution and good sensitivity to tuning effects.
The march of miniaturization is pushing the limits for sensors of position, timing, and force. A force sensor can detect atto newtons per root hertz, a magnetometer operates in the zeta gauss to yotta gauss per root hertz, and displacement sensors are detecting femto meters. The advances and increasing commercialization of the various sensors calls for better timing circuits with circit Q of over 1 million and frequency*Q products of 1015.
A survey of existing detectors shows that many are using 2-D crystals for setting frequency. They identified the properties of note for 2-D crystals and found that the best solutions needed a large band gap, were configurable and had mechanical properties of much greater strength than 3-D crystals, which are brittle. The noise floor has to be determined by the material properties and thermo-mechanical nonlinearities and dynamic range have to be very stable.
A molybdenum sulfide resonator can be fabricated in a 10 micron process with a Q of over 1000 and resonant frequency in the 10’s of MHz. For configurations of less than 10 layers of the material, the devices act like membranes while over 100 layers acts like a plate. The different numbers of layers have distinct signatures. Even with the high Qs, the devices are tunable and have high dynamic range.
Another material of interest is black phosphorus. This material has anisotropic properties but needs new fabrication methods. Many other materials are possible for use as NEMS resonators and they all carry the important properties of geometry dependence rather than material characteristics. The alternatives to silicon all need less precise and lower cost processing than similar structures in silicon.
