F. Campabadal

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.

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.

Resonators with integrated CMOS circuitry for mass sensing applications, fabricated by electron beam lithography

A resonator system has been fabricated directly on a pre-processed CMOS chip. The system is to be used for high sensitivity mass sensing applications in air and vacuum. The resonator system, corresponding of a cantilever and structures for electrostatic actuation and capacitive read-out, have been defined by electron beam lithography on top of a charge and radiation sensitive CMOS layer in predefined areas as a post-process step. This has been accomplished without affecting the electronic properties of the pre-processed CMOS circuits. The subsequent etching steps to fully release the cantilevers have been obtained without stiction of the cantilevers to the substrate. Cantilevers are driven at their mechanical resonance in a lateral mode, and the frequency is monitored by capacitive read-out on the chip. CMOS integration enables signal detection directly on the chip, which radically decreases the parasitic capacitances. Consequently, low-noise electrical measurements with a very high mass sensitivity are obtained. Fabricated resonator systems were characterized to have resonance frequencies of approximately 1.49 MHz, which is in good agreement with a theoretical estimation of 1.41 MHz. The theoretical mass resolution, partial derivativem/partial derivativef, is approximately 17 ag Hz(-1) using a Young modulus value of 160 GPa. (Less)

Wafer level packaging of silicon pressure sensors

In this paper, a new pre-packaging technique for silicon pressure sensors on the wafer level is presented. It is based on the use of UV photopatternable silicone which is deposited over the whole wafer by means of a novel device suitable for low-viscosity material coating and mask alignment. The process consists of the exposure of the deposited layer to UV light leading to cross-linking of the polymer according to the pattern of a chrome mask, which leaves the membrane area as well as the bonding pads free from silicone. After development and dicing, the chips are packaged and conventional wire bonding is performed. The area surrounding the UV photopatternable silicone pattern can then be filled with soft silicone gel for protection of the electrically active components against aggressive media. Results are presented for silicon piezoresistive pressure sensors on which 1.5-mm high octagonal wall structures of silicone elastomer have been patterned. Despite the thickness of the deposited layer, good resolution and aspect ratio are obtained, and the temperature dependence of the packaged pressure sensors is not influenced by the polymer layers.

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.

Fabrication of cantilever based mass sensors integrated with CMOS using direct write laser lithography on resist

A CMOS compatible direct write laser lithography technique has been developed for cantilever fabrication on pre-fabricated standard CMOS. We have developed cantilever based sensors for mass measurements in vacuum and air. The cantilever is actuated into lateral vibration by electrostatic excitation and the resonant frequency is detected by capacitive readout. The device is integrated on standard CMOS circuitry. In the work a new direct write laser lithography (DWL) technique is introduced. This laser lithography technique is based on direct laser writing on substrates coated with a resist bi-layer consisting of poly(methyl methacrylate) (PMMA) on lift-off resist (LOR). Laser writing evaporates the PMMA, exposing the LOR. A resist solvent is used to transfer the pattern down to the substrate. Metal lift-off followed by reactive ion etching is used for patterning the structural poly-Si layer in the CMOS. The developed laser lithography technique is compatible with resist exposure techniques such as electron beam lithography. We demonstrate the fabrication of sub-micrometre wide suspended cantilevers as well as metal lift-off with feature line widths down to approximately 500 nm.

Photolithographic packaging of silicon pressure sensors

Abstract The packaging technique presented here provides direct interaction between the sensing element and the physical or chemical variable to be measured as well as hermetic insulation and mechanical protection of the silicon sensor and its electrically active components. The sensors are embedded in a mechanically decoupling shell of silicone elastomer with low Youngs modulus, leaving the sensing area free from covering. The process can be considered as a synthesis of conventional soft-mounting techniques and photolithographically controlled partial encapsulation. It has been applied to package piezoresistive silicon pressure sensors for use in humid environments and the results obtained show the suitability of the concept. Due to its modularity and unlike application-specific packaging methods, it can potentially be applied to a variety of microelectronic sensors.

Novel packaging technique and its application to a wet/wet differential pressure silicon sensor

The packaging technique presented provides direct interaction between the sensing element and the physical or chemical variable to be measured as well as hermetic insulation and mechanical protection of the silicon sensor and its electrically active components. Unlike application specific packaging methods, it therefore can potentially be applied to a variety of microelectronic sensors.

Nanocantilever based mass sensor integrated with CMOS circuitry

We have demonstrated the successful integration of a cantilever based mass detector with standard CMOS circuitry. The purpose of the circuitry is to facilitate the readout of the cantilevers deflection in order to measure resonant frequency shifts of the cantilever. The principle and design of the mass detector are presented showing that miniaturization of such cantilever based resonant devices leads to highly sensitive mass sensors, which have the potential to detect single molecules. The design of the readout circuitry used for the first electrical characterization of an integrated cantilever is described in detail. The integration of the cantilever is a post processing module and the full process sequence is discussed. One of the main challenges during the fabrication of the cantilevers is sticktion of the cantilever to the bottom substrate after underclothing. Two dry release techniques were used to solve the problem, namely freeze-drying and resist-assisted release. The fabrication results of cantilevers defined by laser and E-beam lithography are shown. Finally, an AFM based characterization setup is presented and the electrical characterization of a laser-defined cantilever fully integrated with CMOS circuitry is demonstrated. The electrical characterization of the device shows that the resonant behavior of the cantilever depends on the applied voltages, which corresponds to theory.

The smart-orifice meter: a mini head meter for volume flow measurement

Abstract Differential pressure based flow meters generally consist of a flow restriction element which generates a differential pressure and a pressure transducer, externally piped to the restriction, which measures the flow related differential pressure. The smart-orifice mini head meter presented takes advantage of silicon technology by incorporating a differential pressure microsensor. In contrast to conventional head meters, it represents a single compact and economic device for general flow meter applications, in particular where small size is of concern. Computational fluid dynamics analyses were applied to develop a non-standard orifice design and prototypes of the smart-orifice were fabricated. The performance of the mini head meter in water flow measurement was determined in a computer supported test bench facility. It was compared to the results predicted by the simulation, as well as to a conventional head meter arrangement with externally mounted pressure transducer, including measurements with water at elevated temperature and different absolute line pressures. The results are very promising and verify the competitiveness of the smart-orifice as a mini head meter.