| For all the nanotechnology applications of NEMS devices, a key challenge is figuring out a fast, low-noise technique for translating small mechanical motion into detectable electronic signals to measure. The scanning tunneling microscope (STM) based on electron tunneling between a sharp tip and conducting sample is a promising method to measure the small displacements. For electron tunneling, the tunneling current is sensitive to the change in distance between the sharp tip and conducting sample, usually less than 1nm. However a limitation in scanning tunneling microscopy is the low temporal resolution because stray capacitance of readout circuitry between the amplifier and tunneling junction will cutoff the high frequency, usually at a few 10's of kHz. This limitation is far less than the fundamental limit of STM, IT/e, which is the electron tunneling rate. In a typical STM tunneling current 1nA, the electron tunnels at a rate of 5GHz. In this thesis, an STM-downmixing readout technique, which directly mixes high frequency signals in tunneling junction, is introduced and explored in order to overcome the detection bandwidth limitation of STM. With this technology we measure the high frequency vibrational modes (∼1 MHz) of MEMS doubly-clamped beams, well above the RC rolloff of our circuitry, and explore the effects of driving force, measuring position and STM parameters, such as DC tunneling current and DC bias voltage on the sensitivity of STM downmixing readout technique and the resonance frequencies of MEMS device. We anticipate that, with this technique, the GHz---and even higher frequency---information can be detected. |
