1. Field of the Invention
This invention is related to micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS).
2. Description of the Prior Art
The field of microelectromechanical systems (MEMS) has received increased attention in recent years in both the scientific and technological realms. Recent experimental efforts have expanded this field into the nanometer-scale regime (so-called nanoelectromechanical devices, or NEMS), increasing operating frequencies to the megahertz or even gigahertz range. At such frequencies a host of new applications become possible, from sensitive charge detection, to mass sensing, biological imaging, and quantum measurement.
Current methods for displacement detection in NEMS include magnetomotive, optical interferometric, and single electron transistor techniques. An alternate method utilizes piezoresistive strain sensors integrated directly into the device. Piezoresistors have previously been incorporated into microscale cantilevers and used in atomic force microscopy, data storage, and biosensing.
The piezoresistor containing devices achieve high strain sensitivity by using semiconductor-based piezoresistors, primarily doped Si or AlGaAs. Compared with other detection methods, piezoresistive systems have the advantages of being fully integrated sensors that operate from room temperature down to at least 4 K and do not require a magnetic field. Resonance detection of cantilevers up to 9 MHz has been achieved previously using doped Si piezoresistors. Generally, direct current (DC) biasing is used in the piezoresistive resonance detection of cantilevers.
However, it has proved difficult to achieve high quality results using the piezoresistive technique in NEMS due to the intrinsically high resistances (5-100 kΩ) of these devices, which leads to frequency-dependent signal attenuation at MHz frequencies if direct current detection is used. Such attenuation poses a significant limitation to this method as the resonance frequencies are increased to the high-frequency range and beyond.
One way of handling this frequency-dependent transmission loss is to transform the impedance of the NEMS device down to 50 Ohm by inserting appropriate circuitry before the transmission line. However, this may be difficult if the impedance mismatch ratio is greater than 100 and impractical if the same detection setup is to be used with many devices of different frequencies.