Chih-Cheng Lu

A Macroporous TiO2 Oxygen Sensor Fabricated Using Anodic Aluminium Oxide as an Etching Mask

An innovative fabrication method to produce a macroporous Si surface by employing an anodic aluminium oxide (AAO) nanopore array layer as an etching template is presented. Combining AAO with a reactive ion etching (RIE) processes, a homogeneous and macroporous silicon surface can be effectively configured by modulating AAO process parameters and alumina film thickness, thus hopefully replacing conventional photolithography and electrochemical etch methods. The hybrid process integration is considered fully CMOS compatible thanks to the low-temperature AAO and CMOS processes. The gas-sensing characteristics of 50 nm TiO2 nanofilms deposited on the macroporous surface are compared with those of conventional plain (or non-porous) nanofilms to verify reduced response noise and improved sensitivity as a result of their macroporosity. Our experimental results reveal that macroporous geometry of the TiO2 chemoresistive gas sensor demonstrates 2-fold higher (∼33%) improved sensitivity than a non-porous sensor at different levels of oxygen exposure. In addition, the macroporous device exhibits excellent discrimination capability and significantly lessened response noise at 500 °C. Experimental results indicate that the hybrid process of such miniature and macroporous devices are compatible as well as applicable to integrated next generation bio-chemical sensors.

High-sensitivity low-noise miniature fluxgate magnetometers using a flip chip conceptual design.

This paper presents a novel class of miniature fluxgate magnetometers fabricated on a print circuit board (PCB) substrate and electrically connected to each other similar to the current “flip chip” concept in semiconductor package. This sensor is soldered together by reversely flipping a 5 cm × 3 cm PCB substrate to the other identical one which includes dual magnetic cores, planar pick-up coils, and 3-D excitation coils constructed by planar Cu interconnections patterned on PCB substrates. Principles and analysis of the fluxgate sensor are introduced first, and followed by FEA electromagnetic modeling and simulation for the proposed sensor. Comprehensive characteristic experiments of the miniature fluxgate device exhibit favorable results in terms of sensitivity (or “responsivity” for magnetometers) and field noise spectrum. The sensor is driven and characterized by employing the improved second-harmonic detection technique that enables linear V-B correlation and responsivity verification. In addition, the double magnitude of responsivity measured under very low frequency (1 Hz) magnetic fields is experimentally demonstrated. As a result, the maximum responsivity of 593 V/T occurs at 50 kHz of excitation frequency with the second harmonic wave of excitation; however, the minimum magnetic field noise is found to be 0.05 nT/Hz1/2 at 1 Hz under the same excitation. In comparison with other miniature planar fluxgates published to date, the fluxgate magnetic sensor with flip chip configuration offers advances in both device functionality and fabrication simplicity. More importantly, the novel design can be further extended to a silicon-based micro-fluxgate chip manufactured by emerging CMOS-MEMS technologies, thus enriching its potential range of applications in modern engineering and the consumer electronics market.

Vector Magnetometer with Dual-Bridge GMR Sensors

A three-axis vector magnetometer based on two sets of dual-bridge GMR sensors and a fluxguide was investigated in this work. The sensors were mounted on a printed circuit board (PCB) with their sensing directions along the x- and y-axes respectively. A cylindrical ferrite fluxguide was put at the center of four sensors with the symmetric axis normal to the PCB plane. The fluxguide deflects the magnetic flux lines of an out-of-plane field to generate the in-plane components, making it possible to detect the z-axis field component with the x - y plane sensor pairs. When the device was driven by a peak-to-peak excitation voltage of 0.4 V, the demodulated dV/dB were found to be 230, 240, and 220 V/T, respectively, for the x, y, and z sensing axes. The field noise spectral densities at 1 Hz were found to be between 3 and 9 nT/√(Hz) for all three sensing axes. The proposed magnetometer design can be adapted to reduce the size and to lower the power consumption by integrating all the components on a chip.

A 3-Axis Miniature Magnetic Sensor Based on a Planar Fluxgate Magnetometer with an Orthogonal Fluxguide

A new class of tri-axial miniature magnetometer consisting of a planar fluxgate structure with an orthogonal ferromagnetic fluxguide centrally situated over the magnetic cores is presented. The magnetic sensor possesses a cruciform ferromagnetic core placed diagonally upon the square excitation coil under which two pairs of pick-up coils for in-plane field detection are allocated. Effective principles and analysis of the magnetometer for 3-D field vectors are described and verified by numerically electromagnetic simulation for the excitation and magnetization of the ferromagnetic cores. The sensor is operated by applying the second-harmonic detection technique that can verify V-B relationship and device responsivity. Experimental characterization of the miniature fluxgate device demonstrates satisfactory spatial magnetic field detection results in terms of responsivity and noise spectrum. As a result, at an excitation frequency of 50 kHz, a maximum in-plane responsivity of 122.4 V/T appears and a maximum out-of-plane responsivity of 11.6 V/T is obtained as well. The minimum field noise spectra are found to be 0.11 nT/√Hz and 6.29 nT/√Hz, respectively, in X- and Z-axis at 1 Hz under the same excitation frequency. Compared with the previous tri-axis fluxgate devices, this planar magnetic sensor with an orthogonal fluxguide provides beneficial enhancement in both sensory functionality and manufacturing simplicity. More importantly, this novel device concept is considered highly suitable for the extension to a silicon sensor made by the current CMOS-MEMS technologies, thus emphasizing its emerging applications of field detection in portable industrial electronics.

Tri-axis magnetometer with in-plane giant magnetoresistance sensors for compass application

A tri-axis magnetometer comprising three giant-magnetoresistance sensors and a cylindrical fluxguide are implemented for compass application. The sensors are mounted on a single printed circuit board (PCB) board with their sensing axes in a plane. A calibration process involving matrix manipulation was employed to make the device function as a tri-axis magnetometer with orthogonal sensing directions. The capability of the device for compass application was demonstrated by measuring its azimuth response to the geomagnetic field about different rotation axes. The proposed calibration technique can be used for the magnetometer system with either orthogonal or non-orthogonal sensor arrangement.

Design, Fabrication, and Characterization of a 3-D CMOS Fluxgate Magnetometer

A dual-core 3-D microfluxgate magnetometer fabricated by a simple and inexpensive fabrication process is described in this paper. The microfluxgate is able to operate along a nearly linear V-B relationship at the second harmonic frequency and features good characteristics of high sensitivity and low noise response. These characteristic results indicate a field-to-voltage transfer coefficient of 11 V/T measured at the second harmonic frequency, power consumption of 67.3 mW, and a field noise response less than 12 nT/√ Hz at 1 Hz. In brief, our proposed device not only enhances responsivity capability and linear V-B characteristics, but also is CMOS process compatible, which is considered both function-efficient and cost-effective.

Design of 3-D Magnetic Field Sensor With Single Bridge of Spin-Valve Giant Magnetoresistance Films

A 3-D magnetic field sensor for geomagnetic field measurement was designed, fabricated, and characterized in this paper. The device comprises spin-valve giant magnetoresistance sheet films of Si-SiO2/Ta/NiFe/CoFe/Cu/CoFe/IrMn/Ta to form a Wheatstone bridge using separate chips and a magnetic flux guide. The reduction of hysteresis and control of operation point were solved using the technique applying both the ac and dc magnetic biasing. The outputs from each axis were obtained using a computer-based driving and detection systems to retrieve the x-, y-, and z-field components. The sensitivities were found to be 0.25, 0.15, and 0.07 mV/V/Oe for x-, y-, and z-components, respectively. The proposed concept can be realized as a miniature 3-D magnetic field sensor of low cost and low power.

Reduction of Low-Frequency Noise in Tunneling-Magnetoresistance Sensors With a Modulated Magnetic Shielding

The high-noise level at low frequency in magnetic tunnel junction (MTJ) sensors is the key problem for their applications in measuring the low values of static magnetic field. In this paper, we investigated a shielding-type flux chopper to resolve the 1/f noise issue. By the using a cylindrical chopper with its axis perpendicular to the sensing direction of MTJ sensor, the noise at 1 Hz was reduced from 4 to 0.3 nT/√Hz by chopping the magnetic flux at 5 kHz. The reduction in noise can be qualitatively explained by the 1/f behavior and the chopping efficiency. The proposed method is promising in enhancing the detectivity of MTJ sensor for sensing the static geomagnetic field.

Enhancement in Sensitivity Using Multiple Harmonics for Miniature Fluxgates

The miniature fluxgates, or microfluxgates, are magnetic field sensors promising in applications of modern portable electronic devices, e.g., motion trackers and digital compasses. In comparison with the bulky traditional fluxgates, the major drawback arising from miniaturization is the reduced sensitivity and the enhanced noise level. In this work, we proposed the method to enhance the sensitivity of a miniature fluxgate by adding up the responses of four even harmonics. Numerical simulation shows that the harmonic spectrum of the induced voltage waveform consists of high-order even harmonics when the excitation current is strong enough to saturate the magnetic cores. Since the magnitude of high-order even harmonics are comparable to that of the second harmonic, it is possible to enhance the sensitivity by detecting several important harmonics. To implement the multiple-harmonic detection technique, the sensitivity and noise of a parallel-gating miniature fluxgate is investigated using the proposed method. With the reference waveform consisting of square waves at 2nd, 4th, 6th, and 8th harmonics of excitation, the sensitivity of the sensor is found to be enhanced by a factor of 2.6 in comparison with the case for a 2nd-harmonic reference alone. The field noise level is found to be reduced from 0.5 nT/√Hz to 0.2 nT/√Hz. The proposed method is promising in improving the sensitivity and reducing the field noise of a chip-scale fluxgate.

Odd-Harmonic Characteristics of the Field-Modulated GMR Magnetometer

Responsivity and field noise of a bridge-type GMR magnetometer modulated by a sinusoidal alternating magnetic field were investigated in this work. The odd harmonics of output voltage were found to be more sensitive to external dc fields in comparison with the even harmonics. By detecting the field-modulated output with a reference square wave at the 1st, 3rd, or 5th harmonic of modulation frequency, the sensor responses to external fields are bipolar, non-hysteretic, and quite linear with proper modulation amplitudes. The minimum field noises were 5, 2.5, and 5 nT/√Hz at 1 Hz for the 1st, 3rd, and 5th harmonics, respectively, with power consumption less than 10 mW. The proposed operation method is promising for application to electronic compasses.