Nano-cantilevers flully integrated with CMOS for Ultrasensitive mass detection

This Ph.D. thesis deals with the development of mass sensitive silicon cantilever systems integrated with complementary metal oxide semiconductor (CMOS). The principle is based on the change in resonance frequency as mass adsorb onto the cantilever, which is measured using the signal amplification circuitry of the CMOS.

The cantilever is made to oscillate by using electrostatic actuation, and the resonance frequency is measured by capacitive readout as the cantilever oscillates in close proximity to a parallel electrode. The capacitive detection method necessitates CMOS integration due to the need to minimize the parasitic capacitance contribution, which otherwise would screen the resonance signal.

The CMOS integration is achieved by adopting a post-process scheme on standard CMOS. By reducing the dimensions of the cantilever to the nanometer regime, and consequently increasing the resonance frequency, an increased mass sensitivity is achieved. Hence, in order to define nanoresonators the use of lithography techniques such as electron beam lithography, scanning force microscopy based nanolithography, and direct write laser lithography have been evaluated.

Resonator structures comprising of cantilevers having widths of 400 nm, a thickness of 600 nm, and a length of 20 μm, having a resonance frequency of the order of 1.5 MHz have successfully been fabricated onto pre-processed standard CMOS. The developed post-process is directly transferable onto state-of-the-art CMOS.

The devices have been characterized in ambient air and vacuum conditions and the mass sensitivity has been determined by adding point masses onto the cantilever. The resulting mass sensitivity is of the order of attogram (10−21 kg), which is the same order of magnitude as a single medium size biomolecule such as heme.

Possible applications are found within the field of biosensors, in detection of hazardous or non-hazardous agents such as explosives, nerve gas, gas system diagnostics. Another possible application is characterization and calibration of advanced lithography systems such as atom- or molecular beam lithography systems.