Zachary James Davis

Fabrication and characterization of nanoresonating devices for mass detection

We report on a novel fabrication process and preliminary characterization of a nanomechanical resonating device, which is to be used for mass detection. The fabrication of the device is based on laser lithography on Al coated SiO2/p++Si/SiO2/Si structures, followed by dry and wet etching. We have fabricated highly doped polysilicon free-hanging cantilevers and anchored drivers for lateral cantilever vibration, where the motion of the cantilever is parallel to the substrate. The cantilevers are actuated electrically by applying an ac voltage between the cantilever and driver. The laterally vibrating cantilever structures are approximately 30–50 μm in length, 1.8 μm in height, and 500 nm in width. The characterization of the resonators was performed by direct observation of the cantilever through an optical microscope. An electrical measuring technique is also presented and discussed. Typical values of resonant frequency and quality factor, at 1 atm, are approximately 500 kHz and 50, respectively. Moreover,...

Electromechanical model of a resonating nano-cantilever-based sensor for high-resolution and high-sensitivity mass detection

A simple linear electromechanical model for an electrostatically driven resonating cantilever is derived. The model has been developed in order to determine dynamic quantities such as the capacitive current flowing through the cantilever-driver system at the resonance frequency, and it allows us to calculate static magnitudes such as position and voltage of collapse or the voltage versus deflection characteristic. The model is used to demonstrate the theoretical sensitivity on the attogram scale of a mass sensor based on a nanometre-scale cantilever, and to analyse the effect of an extra feedback loop in the control circuit to increase the Q factor.

AFM lithography of aluminum for fabrication of nanomechanical systems

Nanolithography by local anodic oxidation of surfaces using atomic force microscopy (AFM) has proven to be more reproducible when using dynamic, non-contact mode. Hereby, the tip/sample interaction forces are reduced dramatically compared to contact mode, and thus tip wear is greatly reduced. Anodic oxidation of Al can be used for fabricating nanomechanical systems, by using the Al oxide as a highly selective dry etching mask. In our experiments, areas as large as 2 micro m x 3 micro m have been oxidized repeatedly without any sign of tip-wear. Furthermore, line widths down to 10nm have been routinely obtained, by optimization of AFM parameters, such as tip/sample distance, voltage and scan speed. Finally, AFM oxidation experiments have been performed on CMOS processed chips, demonstrating the first steps of fabricating fully functional nanomechanical devices.

Design, fabrication, and characterization of a submicroelectromechanical resonator with monolithically integrated CMOS readout circuit

In this paper, we report on the main aspects of the design, fabrication, and performance of a microelectromechanical system constituted by a mechanical submicrometer scale resonator (cantilever) and the readout circuitry used for monitoring its oscillation through the detection of the capacitive current. The CMOS circuitry is monolithically integrated with the mechanical resonator by a technology that allows the combination of standard CMOS processes and novel nanofabrication methods. The integrated system constitutes an example of a submicroelectromechanical system to be used as a cantilever-based mass sensor with both a high sensitivity and a high spatial resolution (on the order of 10/sup -18/ g and 300 nm, respectively). Experimental results on the electrical characterization of the resonance curve of the cantilever through the integrated CMOS readout circuit are shown.

Aluminum nanocantilevers for high sensitivity mass sensors

We have fabricated Al nano-cantilevers using a very simple one mask contact UV lithography technique with lateral dimensions under 500 nm and vertical dimensions of approximately 100 nm. These devices are demonstrated as highly sensitive mass sensors by measuring their dynamic properties. Furthermore, it is shown that Al has a potential higher sensitivity than Si based dynamic sensors. Initial testing of these devices has been conducted using a novel scanning electron microscope setup were the devices were tested under high vacuum conditions. The Q-factor was measured to approximately 200 and the mass sensitivity was measured to 2 attograms/Hz by depositing electron beam induced carbon at the end of the nano-cantilever.

Micro-differential thermal analysis detection of adsorbed explosive molecules using microfabricated bridges

Although micromechanical sensors enable chemical vapor sensing with unprecedented sensitivity using variations in mass and stress, obtaining chemical selectivity using the micromechanical response still remains as a crucial challenge. Chemoselectivity in vapor detection using immobilized selective layers that rely on weak chemical interactions provides only partial selectivity. Here we show that the very low thermal mass of micromechanical sensors can be used to produce unique responses that can be used for achieving chemical selectivity without losing sensitivity or reversibility. We demonstrate that this method is capable of differentiating explosive vapors from nonexplosives and is additionally capable of differentiating individual explosive vapors such as trinitrotoluene, pentaerythritol tetranitrate, and cyclotrimethylenetrinitromine. This method, based on a microfabricated bridge with a programmable heating rate, produces unique and reproducible thermal response patterns within 50 ms that are characteristic to classes of adsorbed explosive molecules. We demonstrate that this micro-differential thermal analysis technique can selectively detect explosives, providing a method for fast direct detection with a limit of detection of 600x10(-12) g.

Monolithic integration of mass sensing nano-cantilevers with CMOS circuitry

Miniaturization of cantilever dimensions will increase both the mass and spatial resolution of a resonating cantilever-based mass sensor, which monitors the mass change of the cantilever by measuring its resonant frequency shift. A fabrication method for nanometer-sized cantilevers with electrostatic excitation and integrated capacitive readout is introduced. The dynamic behavior of the nanometer-sized cantilever is characterized at atmospheric pressure using optical microscopy and in vacuum using scanning electron microscopy (SEM). A monolithic integration method for combining the nano-cantilevers with CMOS circuitry is described in detail. The circuitry is used to enhance the capacitive readout. The fabrication results, showing integrated nano-cantilevers with a CMOS analog amplification circuit, are presented along with preliminary electrical characterization of the device.

Detection of adsorbed explosive molecules using thermal response of suspended microfabricated bridges

Here we present a thermophysical technique that is capable of differentiating vapor phase adsorbed explosives from nonexplosives and is additionally capable of differentiating individual species of common explosive vapors. This technique utilizes pairs of suspended microfabricated silicon bridges that can be heated in a controlled fashion. The differential thermal response of the bridges with and without adsorbed explosive vapor shows unique and reproducible characteristics depending on the nature of the adsorbed explosives. The tunable heating rate method described here is capable of providing unique signals for subnanogram quantities of adsorbed explosives within 50 ms.

Combined laser and atomic force microscope lithography on aluminum: Mask fabrication for nanoelectromechanical systems

A direct-write laser system and an atomic force microscope (AFM) are combined to modify thin layers of aluminum on an oxidized silicon substrate, in order to fabricate conducting and robust etch masks with submicron features. These masks are very well suited for the production of nanoelectromechanical systems (NEMS) by reactive ion etching. In particular, the laser-modified areas can be subsequently locally oxidized by AFM and the oxidized regions can be selectively removed by chemical etching. This provides a straightforward means to define the overall conducting structure of a device by laser writing, and to perform submicron modifications by AFM oxidation. The mask fabrication for a nanoscale suspended resonator bridge is used to illustrate the advantages of this combined technique for NEMS.

Ultrasensitive bulk disk microresonator-based sensor for distributed mass sensing

In the framework of the development of an ultrasensitive microfabricated mass sensor for distributed mass sensing applications we present a bulk resonator-based mass sensor. The two devices presented are based on a polysilicon disk resonating at 132 and 66 MHz, respectively, actuated electrostatically in a wine-glass mode. By using bulk mode resonators it has been possible to reduce the thickness of the sensor layer without affecting the resonance frequency, reaching an extremely high distributed mass to a frequency shift sensitivity of 11.3 kHz µm2 fg−1 and a markedly small mass resolution of 8.7 pg cm−2 in air and at room temperature.