Steven Noyce and Robert Davis, Physics

Introduction

Resonant cantilevers, or small vibrating beams, are used to detect small concentrations of chemicals. As molecules or atoms of the substance to be sensed adhere to the vibrating beam, the resonant frequency changes as a result of the change in mass. These sensors are built on the microscale to allow for mass parallelization. An array of cantilevers could each be coated with a different adhesion layer, making each beam sensitive to a unique substance. These sensors have previously been made of solid materials, but because the sensitivity is proportional to the surface area of the resonator, a porous cantilever could provide sensitivity improvements of tens of thousands of times. In addition, these porous cantilevers are capable of higher quality factors, leading to an additional increase in sensitivity as well as flexibility of use in a larger range of invironments. The reason that porous sensors are not generally made is that microfabrication techniques are not generally compatible with porous materials. We have developed a microfabrication method that allows for tunable porosity and used this method in creating and testing porous cantilever chemical sensors.

Methodology

The porous devices are made by first depositing a micro-patterned iron film onto a silicon substrate. Next, carbon nanotubes are grown from the iron catalyst, extruding the essentially two dimensional shape of the iron pattern. At this stage, the carbon nanotubes can be coated with several distinct materials. This study primarily used carbon as the filling material. As the interstices between the carbon nanotubes are filled, the structure becomes locked together, greatly increasing in strength. The time that the filling process is carried out acts as a control variable for the overall porosity of the resulting device, resulting in a wide range of possible porosities. After the cantilevers are made, they are subjected to resonance tests. The amplitude of the vibration is measured by reflecting a laser from the tip of the cantilever onto a photodiode. The cantilever is driven by means of a piezoelectric bimorph, and a computer controlled lock-in-amplifier allows for frequency control and automated measurement. Cantilevers of several geometries were exposed to various chemicals and pressures, with their resulting resonant frequency shifts determined under each set of conditions.

Results

The fabricated devices are extremely versatile, with even highly porous devices being mechanically robust. A wide range of porosities is possible, with achieved densities ranging from below 100 kg/m3 to above 1000 kg/m3. The resonant characteristics of the devices also showed promise, with the quality factor of some devices exceeding 1000 in air. Several chemicals were accurately sensed using the fabricated cantilevers, including acetone, isopropyl alcohol, cellulose, and water vapor.

Discussion

The device fabrication method has proven to be easily tunable, providing a wide range of flexibility in obtaining desired characteristics in the resulting device. The geometry, porosity, and base material can each be independently controlled over a large parameter space. The resonant characteristics of the resulting devices are favorable even before optimization. Initial use of the devices for chemical sensing shows a great deal of promise.

Conclusion

A flexible fabrication method has been refined and used to create porous resonant sensors. These sensors show favorable resonant characteristics, and initial sensing experiments show great promise for these devices. Further work is needed in developing and testing adhesion promoting coatings as well as parallelizable system control techniques such as dual capacitive drive and measurement.