Arrays of Single Clamped Silicon Nanowires Electrostatically Actuated for Multiplexing Applications

Mechanical mass sensors based on silicon nanowires (SiNWs) or carbon nanotubes has been reported in several works. Multiplexing applications consisting on arrays of SiNWs independently addressable are still challenging. In this work a frequency-tuneable electrostatically-actuated array of SiNWs is presented. Electron beam lithography and dry etching have been used to pattern on purpose single clamped SiNWs on bare silicon wafers. Because of the high influence of the geometrical parameters on the mechanical resonant frequency and the feasibility to design and fabricate SiNWs with small variations in dimensionality, we present a device that consists on arrays of single clamped SiNWs of different dimensions where their vibration can be selected through the frequency of the excitation signal.

We prepared a customized setup for the mechanical characterization of the fabricated arrays of SiNWs. An arbitrary signal generator has been connected thought a feedthrough to a micromanipulator and a tungsten tip located inside a SEM vacuum chamber. The sample has been connected to the electrical ground. By applying an electrostatic field in close vicinity to the SiNWs we excited them at their resonant frequency and we detect their movement by SEM imaging.

The experimental results are in good agreement with the theoretical ones. The simplified figure of merit of the resonant frequency [ f₀ ≈ 0.16 t/l² (E/D)^1/2 ] for a 5.05 um length (l) and 140 nm diameter (t) SiNW; considering 120 GPa Young Modulus (E) for 140 nm thick Si and 2.329 g/cm³ density (D); gives a theoretical value of f₀ ≈ 6.3 MHz. Experimentally the f₀ measured for 5 different devices of the same dimensions are between 6.248 MHz < f₀ < 6.584 MHz. The width in frequency of each resonant SiNW is below 5 kHz that gives a quality factor (Q) of the resonators higher than 1200.