Pedro J. García-Ramírez

Resonant Magnetic Field Sensors Based On MEMS Technology.

Microelectromechanical systems (MEMS) technology allows the integration of magnetic field sensors with electronic components, which presents important advantages such as small size, light weight, minimum power consumption, low cost, better sensitivity and high resolution. We present a discussion and review of resonant magnetic field sensors based on MEMS technology. In practice, these sensors exploit the Lorentz force in order to detect external magnetic fields through the displacement of resonant structures, which are measured with optical, capacitive, and piezoresistive sensing techniques. From these, the optical sensing presents immunity to electromagnetic interference (EMI) and reduces the read-out electronic complexity. Moreover, piezoresistive sensing requires an easy fabrication process as well as a standard packaging. A description of the operation mechanisms, advantages and drawbacks of each sensor is considered. MEMS magnetic field sensors are a potential alternative for numerous applications, including the automotive industry, military, medical, telecommunications, oceanographic, spatial, and environment science. In addition, future markets will need the development of several sensors on a single chip for measuring different parameters such as the magnetic field, pressure, temperature and acceleration.

A resonant magnetic field microsensor with high quality factor at atmospheric pressure

A resonant magnetic field microsensor with a high quality factor at atmospheric pressure has been designed, fabricated and tested. This microsensor does not require vacuum packaging to operate efficiently and presents a compact and simple geometrical configuration of silicon. This geometry permits us to decrease the size of the structure and facilities its fabrication and operation. It is constructed of a seesaw plate (400 × 150 × 15 µm3), two torsional beams (60 × 40 × 15 µm3), four flexural beams (130 × 12 × 15 µm3) and a Wheatstone bridge with four p-type piezoresistors. The resonant device exploits the Lorentz force principle and operates at its first resonant frequency (136.52 kHz). A sinusoidal excitation current of 22.0 mA with a frequency of 136.52 kHz and magnetic fields from 1 to 400 G are considered. The mechanical response of the microsensor is modeled with the finite element method (FEM). The structure of the microsensor registered a maximum von Mises stress of 53.8 MPa between the flexural and the torsional beams. Additionally, a maximum deflection (372.5 nm) is obtained at the extreme end of the plate. The proposed microsensor has the maximum magnetic sensitivity of 40.3 µV G−1 (magnetic fields <70 G), theoretical root-mean square (rms) noise voltage of 57.48 nV Hz−1/2, theoretical resolution of 1.43 mG Hz−1/2 and power consumption less than 10.0 mW.

Analytical Modeling for the Bending Resonant Frequency of Sensors Based on Micro and Nanoresonators With Complex Structural Geometry

Micro- and nanoresonator sensors have important applications such as in chemical and biological sensing, environmental control, monitoring of viscosity and magnetic fields, and inertial forces detection. However, most of these resonators are designed as complex structures that complicate the estimation of their resonant frequencies (generally of the bending or torsional mode). In this paper, we present an analytical model to estimate the resonant frequency of the first bending mode of micro- and nanoresonators based on a beam system under different load types. This system is constructed of beams with different cross sections joined through a series-parallel arrangement. The analytical model is derived using the Rayleigh and Macaulay methods, as well as the Euler-Bernoulli beam theory. In addition, we determined the deflection function of the beam system, which can be used to establish its bending structural response under several load types. We applied the model to both a silicon microresonator (with a thickness of 5 μ m) for an experimental magnetic field sensor developed in our laboratory and for a polycrystalline silicon nanoresonator (with a thickness of 160 nm) of a mass sensor reported in the literature. The results of our analytical model have a comparable agreement with those obtained from the finite-element models (FEMs) and with the experimental measurements. Our analytical model can be useful in the mechanical design of micro- and nanoresonators with complex structural configurations.

Signal Conditioning System With a 4–20 mA Output for a Resonant Magnetic Field Sensor Based on MEMS Technology

Several resonant magnetic field sensors based on microelectromechanical systems (MEMS) technology use piezoresistive detection techniques to convert the magnetic field signal into an electrical signal. We present a signal conditioning system implemented in a printed circuit board (PCB) for a resonant magnetic field sensor based on MEMS technology. This sensor is formed by a resonant structure of thin silicon beams (5 μm thick), an aluminum loop (1 μm thick), and a Wheatstone bridge with four p-type piezoresistors. The Wheatstone bridge is biased with an alternating voltage of 2 Vpp at 1 kHz and the aluminum loop is supplied using an alternating current with a root-mean-square (RMS) value of 20 mA. This current is applied to the resonant frequency of the sensor (14.38 kHz) through an oscillator that has a frequency stability of ± 100 ppm at atmospheric temperature and a resolution of 1 Hz. The proposed system obtains the sensors electrical response in voltage or current mode, which presents an approximately linear behavior for a range of magnetic field density from -150 to +150 μT. This system minimizes the offset of the sensors electrical response and allows the detection of the polarity and magnitude of the magnetic field density. A virtual instrument is designed using Lab VIEW software to visualize the 4-20 mA output of the sensor. The designed system can help the development of portable measurement equipment to detect (at pressure atmospheric) low magnetic field densities with a sensitivity and resolution of 4 V · T-1 and 1 μT, respectively.

Respiratory Magnetogram Detected with a MEMS Device

Magnetic fields generated by the brain or the heart are very useful in clinical diagnostics. Therefore, magnetic signals produced by other organs are also of considerable interest. Here we show first evidence that thoracic muscles can produce a strong magnetic flux density during respiratory activity, that we name respiratory magnetogram. We used a small magnetometer based on microelectromechanical systems (MEMS), which was positioned inside the open thoracic cage of anaesthetized and ventilated rats. With this new MEMS sensor of about 20 nT resolution, we recorded a strong and rhythmic respiratory magnetogram of about 600 nT.

Development of Resonant Magnetic Field Microsensors: Challenges and Future Applications

Microelectromechanical Systems (MEMS) integrate electrical and mechanical components with feature sizes in the micrometer-scale, which can be fabricated using integrated circuit batch-processing technologies (Gad-el-Hak, 2001). The development of devices using MEMS has important advantages such as small size, light weight, low-power consumption, high sensitivity and high resolution (Herrera-May et al., 2009a). MEMS have allowed the development of several microdevices such as accelerometers (L. Li et al., 2011), gyroscopes (Che et al., 2010), micromirrors (Y. Li et al., 2011), and pressure sensors (Mian & Law, 2010). Recently, some researchers (Mohammad et al., 2010, 2011a, 2011b; Wang et al., 2011) have integrated acceleration, pressure or temperature sensors using MEMS. A potential market for MEMS will include magnetic field microsensors for applications such as automotive industry, telecommunications, medical and military instruments, and consumer electronics products (Lenz & Edelstein, 2006). The most sensitive magnetic field sensor is the Superconducting Quantum Interference Device (SQUID), which has a resolution on the order of several femptoteslas (JosephsFranks et al., 2003). It operates at low temperature based on two effects: flux quantization and Josephson effects. This sensor needs a sophisticated infrastructure that increases its size and cost, which limits its commercial applications. Hall effect sensors have a low cost, small size, and a power consumption from 100 to 200 mW. They are fabricated on standard Complementary Metal-Oxide Semiconductor (CMOS) technology and can measure either constant or varying magnetic field between temperature ranges from -100 to + 100 oC (Ripka & Tipek, 2007). Nevertheless, Hall effect sensors have a low resolution from 1 to 100 mT and require temperature compensation circuits (Popovic, 2004). Fluxgate sensors can measure static or low frequency magnetic field with a resolution of 100 pT (Ripka & Tipek, 2007). They have a size of several millimeters and a power

Design, Fabrication, and Characterization of a Resonant Magnetic Field Sensor Based on MEMS Technology

We present the design, fabrication, and characterization of a resonant magnetic field sensor based on Microelectromechanical systems (MEMS). This sensor exploits the Lorentz force principle and uses a Wheatstone bridge of p-type piezoresistors. Its resonant structure is integrated with silicon beams (15 μm wide and 5 μm thick) and an aluminum loop (9 μm wide and 1 μm thick). The sensor operates in its first flexural resonant frequency (22.99 kHz) and has a linear response, a high resolution (43 nT), a sensitivity of 1.94 V·T−1, a quality factor of 96.6 at atmospheric pressure, and power consumption close to 16 mW.

Experimental and Simulation Results of Magnetic Modulation of Gate Oxide Tunneling Current in Nanoscaled MOS Transistors

An experimental-simulation methodology to explore the spatially nonhomogeneous properties of the tunneling current in nanoscaled MOSFET is introduced. The magnetic field B is introduced into the Schrödinger-Poisson system, which allows simulating the effect of the B field on the gate oxide tunneling current and be compared with experimental data. We found out that sweeping the B field from negative to positive values is equivalent to scan or map the tunneling mechanism along the channel from source to drain. The proposed methodology is useful for studying nonhomogeneous space distributed conductive properties, and it was validated with a 28-nm n-type Si MOSFET.