Advances in high bandwidth nanomechanical force sensors with integrated actuation [electronic resource]
Keywords: Mechanical Engineering

Life is built upon mechanical forces, which play a central role in everything from cell division to embryonic development. Rather than acting as passive mechanical elements, cells and molecules sense and actively respond to mechanical loads. One example of cellular force sensing is mechanotransduction, the conversion of mechanical energy into an electrical signal, which underlies our senses of hearing, touch and balance. For example, the cochlear hair cells in your inner ear are exquisitely sensitive and fast, capable of sensing piconewton-scale forces at the microsecond-time scale. But in order to understand such fast mechanotransduction processes we must first be able to apply and measure small, fast forces. A variety of instruments have been developed for the precise measurement of atomic-scale forces and displacements in the past 25 years. The most commonly used techniques are atomic force microscopy, magnetic tweezers, and optical tweezers. Each provides a tradeoff in force, displacement and time resolution, but none of them are capable of applying and detecting forces fast enough for the study of cochlear hair cells. In order to address this technological gap we have developed microfabricated force probes for the application and measurement of forces at the piconewton- and microsecond-scale. In order to simultaneously achieve a high resonant frequency (20-400 kHz in air, 10-100 kHz in water), low spring constant (0.3-40 mN/m) and low minimum detectable force (1-100 pN), the probes are roughly 300 nm thick, 1 micron wide and 30-200 microns long. Force applied to the cantilever tip is transduced into a voltage by a piezoresistive silicon strain gauge that is embedded in the beam. Actuation is accomplished through a piezoelectric aluminum nitride film or a resistively heated aluminum film to enable high-speed operation without spurious resonant modes. The probes are mass produced on silicon wafers using conventional batch fabrication techniques, and their dimensions are individually adjusted lithographically to accommodate a wide range of desired force and time resolutions. Optics are not required for sensing or actuation so the probes can be integrated with any standard upright up inverted microscope. This thesis presents the design, fabrication and characterization of the force probes. Several enabling technologies and techniques were developed in the process. We will begin by discussing the mechanical, electrical and thermal design of the force probes with an emphasis on piezoresistor design. Numerical design optimization is utilized to satisfy the numerous design and performance constraints. Next, the seven- and nine-mask fabrication processes used to manufacture the thermally and piezoelectrically actuated probes will be presented. The sensing and actuation performance of the probes will be individually addressed before discussing their integration, particularly crosstalk compensation. Finally, preliminary data on the measurement of mammalian hair cell kinetics will be presented and possible future directions will be discussed. The improved design, fabrication and circuit methodologies described here enable numerous performance improvements over prior work. For example, piezoresistive cantilever force resolution is improved 10-20 fold over prior cantilevers of comparable thickness. Similarly, the crosstalk between the piezoelectric actuator and piezoresistor sensor are 10-fold smaller than the best results reported to date.