A Micro Electro Mechanical System (MEMS) for mass detection is presented. It has been developed by the monolithic integration of the mechanical transducer with the CMOS control circuit. The sensor transducer consists on an array of four resonating cantilevers; oscillation is achieved by electrostatic excitation. The independent control on each cantilever of the arrays allows multiple sensing on a single device. The microresonators are fabricated on polysilicon in a compatibilized process with the front-end CMOS circuitry. The readout of the cantilevers oscillation is achieved by a current amplifier. Expected Mass resolution in air is 80 ag/Hz.
In this work, we present a non-linear electromechanical model of an electrostatically excited cantilever that can be used to perform system level electrical simulations. This model is implemented by using an analog hardware description language (VHDL-AMS) that allows its use in a common IC CAD environment like CADENCE. Small-signal and large-signal simulations are performed and the results are compared with a simple linear model (RLC//C) showing the benefits of this model. This model is validated by its fit with the experimental results obtained from a monolithic sub-micrometer cantilever based sensor
A detailed description of a read-out amplifier for high frequency MEMS resonators is done. Both read-out requirements and circuit architecture are presented. The architecture of the system is mainly based on three blocks: a trans-impedance amplifier, followed by a three-stage voltage-to-voltage amplifier, and finally by an output buffer amplifier. Physical design is based on AMS 0,35 μm technology. Also, simulation and fabrication results are presented and analyzed. Simulation results show an AC transimpedance gain of 70 dBΩ and a cut-off frequency of 400 MHz, for a band-pass bandwidth over 350 MHz. The fabricated amplifier has an input noise current spectral density of 11 pA/(Hz)1/2, a power dissipation of 200 mW, and occupies an active area of 600 μm * 450μm. Integration of read-out circuit with MEMS resonator has been designed and implemented, by properly connection of MEMS signals to the amplifier, in order to enable characterization of a set of MEMS resonators. Integration analysis will allow future extraction of electrical parameters of the resonator.
Nanolithography based on atomic force microscopy is a widely used techniques for the prototyping of nanostructures. This technique has attracted great attention due to its simplicity, versatility and precise control. Oxidation is performed at normal atmosphere where the meniscus connecting tip and surface plays a key role. The present study describes the electrical conductivity of this nanometer-size meniscus. By acquiring force vs distance curves, we determine the relationship between the tip-surface separation and electrical current. It is observed an increase of the electrical current at small finite separations (< 2 nm) due to a change in water meniscus properties, and a decrease of electrical current when the meniscus is elongated.
The objective of this paper is to present the compatibilization between a standard CMOS on bulk silicon process and the fabrication of nanoelectromechanical systems using Silicon On Insulator (SOI) wafers as substrate. This compatibilization is required as first step to fabricate a very high sensitive mass sensor based on a resonant cantilever with nanometer dimensions using the crystal silicon COI layer as the structural layer. The cantilever is driven electrostatically to its resonance frequency by an electrode placed parallel to the cantilever. A capacitive readout is performed. To achieve very high resolution, very small dimensions of the cantilever (nanometer range) are needed. For this reason, the control and excitation circuitry has to be integrated on the same substrate than the cantilever.
Prior to the development of this sensor, it is necessary to develop a substrate able to be used first to integrate a standard CMOS circuit and afterwards to fabricate the nano-resonator. Starting from a SOI wafer and using very simple processes, the SOI silicon layer is removed, except from the areas in which nano-structures will be fabricated; obtaining a silicon substrate with islands with a SOI structure. The CMOS circuitry will be integrated on the bulk silicon region, while the remainder SOI region will be used for the nanoresonator. The silicon oxide of this SOI region is used as insulator; and as sacrificial layer, etched to release the cantilever from the substrate. To assure the cover of the different CMOS layers over the step of the islands, it is essential to avoid very sharp steps.
KEYWORDS: Sensors, Transducers, Field effect transistors, Microelectromechanical systems, Ferroelectric materials, Diodes, Capacitance, Voltage controlled current source, Bridges, Control systems
The aim of this paper is to report the experimental setup designed, developed and tested in order to achieve the first vibrating mode of a lateral cantilever with mechanical excitation. The on-plane oscillating cantilever is the basis of a proposed mass sensor with an expected resolution in the atto-gram scale. In a first system design, the cantilever is driven electrostatically by an electrode, which is placed parallel to the cantilever. The cantilever is driven to its first resonant mode applying an AC voltage between the cantilever and a driver. Also, a DC voltage is applied to increase the system response. The signal read-out of the transducer is the capacitive current of the cantilever-driver system. The mass sensor proposed, based on this cantilever-driver structure (CDS), is integrated with a CMOS circuitry in order to minimize the parasitic capacitances, that in this case take special relevance because of the low level output current coming from the transducer. Moreover, the electrostatic excitation introduces a parasitic current that overlaps the current due to the resonance. The mechanical excitation is an alternative excitation method which aim is to eliminate the excitation current. Here we describe the experimental facilities developed to achieve mechanical excitation and report preliminary results obtained by this excitation technique. The results are complemented with dynamic simulations of an equivalent system model that are in accordance with the experimental values.
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