High-speed tapping-mode atomic force microscopy using a Q-controlled regular cantilever acting as the actuator: proof-of-principle experiments.

We present the proof-of-principle experiments of a high-speed actuation method to be used in tapping-mode atomic force microscopes (AFM). In this method, we do not employ a piezotube actuator to move the tip or the sample as in conventional AFM systems, but, we utilize a Q-controlled eigenmode of a cantilever to perform the fast actuation. We show that the actuation speed can be increased even with a regular cantilever.

Note: Seesaw actuation of atomic force microscope probes for improved imaging bandwidth and displacement range.

The authors describe a method of actuation for atomic force microscope (AFM) probes to improve imaging speed and displacement range simultaneously. Unlike conventional piezoelectric tube actuation, the proposed method involves a lever and fulcrum "seesaw" like actuation mechanism that uses a small, fast piezoelectric transducer. The lever arm of the seesaw mechanism increases the apparent displacement range by an adjustable gain factor, overcoming the standard tradeoff between imaging speed and displacement range. Experimental characterization of a cantilever holder implementing the method is provided together with comparative line scans obtained with contact mode imaging. An imaging bandwidth of 30 kHz in air with the current setup was demonstrated.

Optimizing the driving scheme of a self-actuated atomic force microscope probe for high-speed applications.

We investigate the optimum driving scheme of a dynamic atomic force microscope with a self-actuated probe for high-speed applications by performing numerical simulations. We compare the recently developed methods such as Q-control, dynamic PID control, and modified Q-control methods to the standard tapping mode by considering scan speed and peak transient forces. In addition, the effects of driving frequency and set-point amplitude on the maximum achievable scan speed for the same probe-sample system are discussed. We find that the scan speed can be increased significantly at the expense of increased peak transient forces.

Combined quantitative ultrasonic and time-resolved interaction force AFM imaging.

The authors describe a method where quantitative ultrasonic atomic force microscopy (UAFM) is achieved during time-resolved interaction force (TRIF) imaging in intermittent contact mode. The method uses a calibration procedure for quantitative UAFM. It improves elasticity measurements of stiff regions of surfaces while retaining the capabilities of the TRIF mode for topography, adhesion, dissipation, and elasticity measurements on soft regions of sample surfaces. This combination is especially advantageous when measuring and imaging samples with broad stiffness range in a nondestructive manner. The experiments utilize an active AFM probe with high bandwidth and the UAFM calibration is performed by measuring the magnitude of the time-resolved UAFM signal at a judiciously chosen frequency for different contact stiffness values during individual taps. Improved sensitivity to stiff surface elasticity is demonstrated on a special sample. The results show that combining UAFM with TRIF provides 2.5 GPa (5%) standard deviation on the silicon surface reduced Young's modulus, representing 5× improvement over using only TRIF mode imaging.

Athermalization in atomic force microscope based force spectroscopy using matched microstructure coupling.

The authors describe a method for athermalization in atomic force microscope (AFM) based force spectroscopy applications using microstructures that thermomechanically match the AFM probes. The method uses a setup where the AFM probe is coupled with the matched structure and the displacements of both structures are read out simultaneously. The matched structure displaces with the AFM probe as temperature changes, thus the force applied to the sample can be kept constant without the need for a separate feedback loop for thermal drift compensation, and the differential signal can be used to cancel the shift in zero-force level of the AFM.

A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea.

A miniature differential microphone is described having a low-noise floor. The sensitivity of a differential microphone suffers as the distance between the two pressure sensing locations decreases, resulting in an increase in the input sound pressure-referred noise floor. In the microphone described here, both the diaphragm thermal noise and the electronic noise are minimized by a combination of novel diaphragm design and the use of low-noise optical sensing that has been integrated into the microphone package. The differential microphone diaphragm measures 1 x 2 mm(2) and is fabricated out of polycrystalline silicon. The diaphragm design is based on the coupled directionally sensitive ears of the fly Ormia ochracea. The sound pressure input-referred noise floor of this miniature differential microphone has been measured to be less than 36 dBA.

A Charge-Based Low-Power High-SNR Capacitive Sensing Interface Circuit.

This paper describes a low-power approach to capacitive sensing that achieves a high signal-to-noise ratio. The circuit is composed of a capacitive feedback charge amplifier and a charge adaptation circuit. Without the adaptation circuit, the charge amplifier only consumes 1 µW to achieve the audio band SNR of 69.34dB. An adaptation scheme using Fowler-Nordheim tunneling and channel hot electron injection mechanisms to stabilize the DC output voltage is demonstrated. This scheme provides a very low frequency pole at 0.2Hz. The measured noise spectrums show that this slow-time scale adaptation does not degrade the circuit performance. The DC path can also be provided by a large feedback resistance without causing extra power consumption. A charge amplifier with a MOS-bipolar pseudo-resistor feedback scheme is interfaced with a capacitive micromachined ultrasonic transducer to demonstrate the feasibility of this approach for ultrasound applications.

Quantitative mechanical characterization of materials at the nanoscale through direct measurement of time-resolved tip-sample interaction forces.

We introduce a new method for material characterization at the nanoscale using a recently developed atomic force microscope (AFM) probe. The FIRAT (force sensing integrated readout and active tip) probe is integrated into a commercial AFM system to obtain time-resolved interaction forces (TRIFs) between the probe tip and sample at speeds suitable for nondestructive and fast imaging of material properties. We present a basic interaction model to extract the material elasticity and surface energy. Numerical simulations are performed and compared to the experimental results for three different polymers and a silicon sample. We find that our interaction model does not completely explain the observed long-range surface forces, but it agrees fairly well with the measurements during the tip-sample contact.

Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays.

In this paper, we report on the characterization of 1-D arrays of capacitive micromachined ultrasonic transducers (cMUT). A 275- x 5600-micron 1-D CMUT array element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver, the transducer has a 0.28-fm/square root of Hz displacement sensitivity, and, as a transmitter, it produces 5 kPa/V of output pressure at the transducer surface at 3 MHz with a DC bias of 35 V. The transducer has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging. The radiation pattern of isolated single elements, as well as those of array elements are measured, and two major sources of acoustical cross talk are identified. A weakly dispersive non-leaky interface wave (Stoneley wave) is observed to be propagating at the silicon substrate-fluid interface at a speed close to the speed of sound in the fluid. This wave causes internal reflections, spurious resonance, and radiation from the edges of the silicon substrate. The large lateral component of the particle velocity generated by the membranes at the edge of the cMUT array elements is found to be the source of this interface wave. Lowest order Lamb waves in the silicon substrate are also found to contribute to the cross talk between elements. These waves are excited at the edges of individual vibrating membranes, where they are anchored to the substrate, and result in a narrowing of the beam profile of the array elements. Several methods, such as trench isolation and wafer thinning, are proposed and implemented to modify the acoustical cross coupling between array elements.

Simulation and experimental characterization of a 2-D capacitive micromachined ultrasonic transducer array element.

In this letter, a 400-mumx400-mum 2-D capacitive micromachined ultrasonic transducer (cMUT) array element is experimentally characterised, and the results are found to be in good agreement with theoretical predictions. As a receiver, the transducer has a 1.8x10(-7) nm/ radical(Hz) displacement sensitivity, and, as a transmitter, it produces 16.4 kPa/V of output pressure at the transducer surface at 3 MHz. The transducer also has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging.

A Lamb wave lens for acoustic microscopy.

In a conventional scanning acoustic microscope the excited leaky modes contribute significantly to the high contrast obtained in the images. However, all such modes exist simultaneously, and the interpretation of the images is not straightforward, especially in layered media. A lens geometry is proposed that can be used with acoustic microscopes to image layered solid structures. This lens can focus the acoustic waves in only one of the Lamb wave modes of the layered solid with a high efficiency. V(Z) curves obtained from this lens are more sensitive to material properties compared to that obtained from conventional lens. Measuring the return signal as a function of frequency results in another characteristic curve, V(f). The Lamb wave lens and the associated characterization methods for the layered structures are described. The results presented show that the Lamb wave lens is at least an order of magnitude more sensitive than the conventional lens and can easily differentiate between a good bond and disbond in a layered structure.