Electrostatically actuated encased cantilevers

Viscous damping severely limits the performance of resonator based sensing in liquids. We present encased cantilevers that overcome this limitation with a transparent and hydrophobic encasement built around the resonator. Only a few micrometers of the cantilever probe protrude from the encasement and water does not enter the encasement. This maintains high Q-factors and reduces the thermo-mechanical noise levels by over one order of magnitude and reaches minimal detectable forces of 12 fN/•Hz in liquids. These probes expand the frontiers of cantilever based sensing. We discuss their design and fabrication with special focus on squeeze film damping and demonstrate their successful application for quantitative mass sensing of single nanoparticles and gentle Atomic Force Microscopy imaging of soft matter in liquids.

Journal of Applied Physics, 2013

Review of Scientific Instruments, 2003

Quicker imaging times for tapping mode atomic force microscopy in liquid could provide a real-time imaging tool for studying dynamic phenomena in physiological conditions. We demonstrate faster imaging speed using microcantilevers with integrated piezoelectric actuators. The exposed electric components of the cantilever necessitate an insulation scheme for use in liquid; three coating schemes have been tested. Preliminary tapping mode images have been taken using the insulated microactuator to simultaneously vibrate and actuate the cantilever over topographical features in liquid, including a high speed image of steps on a mica surface in water and an image of two e coli bacteria taken in saline solution at 75.5 m/s, a threefold improvement in bandwidth versus conventional piezotube actuators.

Nanotechnology, 2009

In this paper, we present a detailed investigation into the suitability of atomic force microscopy (AFM) cantilevers with integrated deflection sensor and micro-actuator for imaging of soft biological samples in fluid. The Si cantilevers are actuated using a micro-heater at the bottom end of the cantilever. Sensing is achieved through p-doped resistors connected in a Wheatstone bridge. We investigated the influence of the water on the cantilever dynamics, the actuation and the sensing mechanisms, as well as the crosstalk between sensing and actuation. Successful imaging of yeast cells in water using the integrated sensor and actuator shows the potential of the combination of this actuation and sensing method. This constitutes a major step towards the automation and miniaturization required to establish AFM in routine biomedical diagnostics and in vivo applications.

Ultramicroscopy, 2004

This article summarizes improvements to the speed, simplicity and versatility of tapping mode atomic force microscopy (AFM). Improvements are enabled by a piezoelectric microcantilever with a sharp silicon tip and a thin, low-stress zinc oxide (ZnO) film to both actuate and sense deflection. First, we demonstrate self-sensing tapping mode without laser detection. Similar previous work has been limited by unoptimized probe tips, cantilever thicknesses, and stress in the piezoelectric films. Tests indicate self-sensing amplitude resolution is as good or better than optical detection, with double the sensitivity, using the same type of cantilever. Second, we demonstrate self-oscillating tapping mode AFM. The cantilever's integrated piezoelectric film serves as the frequency-determining component of an oscillator circuit. The circuit oscillates the cantilever near its resonant frequency by applying positive feedback to the film. We present images and force-distance curves using both self-sensing and self-oscillating techniques. Finally, highspeed tapping mode imaging in liquid, where electric components of the cantilever require insulation, is demonstrated.

Journal of Applied Physics, 2009

Piezoresistive cantilevers fabricated from doped silicon or metal films are commonly used for force, topography, and chemical sensing at the micro-and macroscales. Proper design is required to optimize the achievable resolution by maximizing sensitivity while simultaneously minimizing the integrated noise over the bandwidth of interest. Existing analytical design methods are insufficient for modeling complex dopant profiles, design constraints, and nonlinear phenomena such as damping in fluid. Here we present an optimization method based on an analytical piezoresistive cantilever model. We use an existing iterative optimizer to minimimize a performance goal, such as minimum detectable force. The design tool is available as open source software. Optimal cantilever design and performance are found to strongly depend on the measurement bandwidth and the constraints applied. We discuss results for silicon piezoresistors fabricated by epitaxy and diffusion, but the method can be applied to any dopant profile or material which can be modeled in a similar fashion or extended to other microelectromechanical systems.

Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266), 2002

Actuation of microcantilevers in fluids has applications such as atomic force microscopy, fluid mixing and mechanical property characterization in microchannels. In this paper of a microfluidic actuator, which utilizes acoustic radiation pressure (ARP) to exert DC and AC forces on a microcantilever is discussed. An atomic force microscope (AFM) cantilever has been used to demonstrate the capabilities of this actuator. The method allows the application of a localized force at a desired location on a cantilever with a 5MHz bandwidth. The localized nature of the applied force allows the characterization of flexural and torsional oscillation modes of the AFM cantilevers. Tapping mode atomic force microscopy was also carried out using the ARP actuator.

Review of Scientific Instruments, 2002

We show that standard silicon nitride cantilevers can be used for tapping mode atomic force microscopy (AFM) in air, provided that the energy of the oscillating cantilever is sufficiently high to overcome the adhesion of the water layer. The same cantilevers are successfully used for tapping mode AFhif in liquid. Acoustic modes in the liquid excite the canti1eve.r. On soft samples, e.g., biological material, this tapping mode AFM is much more gentle than the regular contact mode AFM. Not only is the destructive infuence of the lateral forces minimized, but more important, the intrinsic viscoelastic properties of the sample itself are effectively used to "harden" the soft sample.

Applied Physics Letters, 2002

We report on the evidence for the cantilever-sample ͑CS͒ capacitive force contribution to the piezoelectric force microscopy ͑PFM͒. In addition, we present that positioning of the tip near the edge of the sample surface can significantly reduce this spurious contribution for any combinations of tip cantilever and film. As proof of both the existence of CS interaction and its reduction, the domains formed by the application of voltage pulses through the tip are observed by PFM at two different positions, i.e., sample center and edge. In accordance with the model that a piezoresponse consists of a piezoelectric vibration of the film and an electrostatic force induced vibration of cantilever, the domain contrasts are characterized by dot structure in the amplitude and negligible contrast in the phase images when the tip is placed in the center of the sample surface. However, reducing the CS interaction by placing the tip near the sample edge yields domain contrasts showing ring structure in the amplitude and a clear 180°phase shift in the phase images. Accompanying resolution enhancement in phase images results in smaller size of domains ͑bits͒ produced by identical voltage pulses as is evidenced from bit size estimation. Additional evidence for reduction of CS interaction is obtained from piezoresponse hysteresis measurement.

Journal of Applied Physics, 1996

An atomic force microscope ͑AFM͒ design providing a focused spot of order 7 m in diameter was used to analyze the motion of vibrating cantilevers in liquid. Picking an operating frequency for tapping mode AFM operation in liquid is complex because there is typically a large number of sharp peaks in the response spectrum of cantilever slope amplitude versus drive frequency. The response spectrum was found to be a product of the cantilever's broad thermal noise spectrum and an underlying fluid drive spectrum containing the sharp peaks. The geometrical shape of transverse cantilever motion was qualitatively independent of the fluid drive spectrum and could be approximately reproduced by a simple theoretical model. The measurements performed give new insights into the behavior of cantilevers during tapping mode AFM operation in liquid.