Bismuth Micro and Nano Hall Sensors on Photo-plastic Cantilevers for Scanning Hall Probe Microscopy

Conventional Hall devices are widely employed to measure magnetic fields produced by millimetre sized (or larger) current-caring conductors or permanent magnets with a standard active area of about (100 um)^2. Micro-Hall sensors have recently emerged as a powerful and sensitive tool for the investigation of local magnetic field properties of the micro and nano-structures. The conventional Hall effect devices are based on non-magnetic materials (typically semiconductors e.g: AlGaAs/GaAs and InSb or semimetals such as Bi). Bismuth is a semi-metal that presents a very long mean free path, a large spatial extension of the electron wave function, and a low electron density. This explains the amazing galvanomagnetic properties of bismuth nanostructures. Recently, Hall sensors having dimensions down to (100 nm)^2 have been fabricated. By mounting them on micro-scanning systems, a new magnetic imaging method, called Scanning Hall Probe Microscopy (SHPM), has been introduced for simultaneous magnetic and topography imaging. Miniaturized Hall sensors are employed to measure highly inhomogeneous magnetic fields such as those produced by ferromagnetic nanoparticles, vortices in superconductors, magnetic domains in ferromagnetic materials, magnetic force microscopy tips, and magnetic recording media. We have developed an alternative route to fabricate micro and nano Hall sensors on cantilevers. The approach was to incorporate bismuth Hall sensors of dimensions down to 100 nm on photo-plastic cantilevers made with SU-8, a material already used for the fabrication of atomic force microscopy (AFM) and near field scanning optical microscopy (NSOM). The smallest active area of the Hall sensors obtained by the focused Ion beam (FIB) is (60x60) nm^2. The electrical characteristics of our bismuth Hall sensors are comparable to the best results reported up to now. For instance, a device with an active area about (0.1x0.1) um^2 the current sensitivity, the Hall coefficient and the field resolution measured are 2 V/AT, 180x10^-9 m3/C and 120 uT/Hz^1/2, respectively.

Micro and nano tools for magnetic field imaging

Schahrazede-Lila Mouaziz

Scanning probe microscopy has become, nowadays, a branch of microscopy widely used to image surfaces of different specimens. Among the various applications, the magnetic imaging is one of the most exciting scanning probe microscopy techniques. It allows the study of topography of samples at the nanometer scale. The magnetic force microscopy is the best known and the most employed magnetic imaging technique. It has been successfully used for the imaging of various samples but with the major inescapable limitations that it is confined to the surface, non-quantitative and invasive. Techniques such as the scanning Hall probe microscopy (SHPM) and the magnetic resonance force microscopy (MRFM) have emerged, several years later, as promising approaches to overcome these limitations. In this thesis, we present the design, the fabrication and the characterization of, first, micro-Hall sensors on SU-8 cantilevers used for SHPM and, second, of ultrasensitive Si cantilevers used for MRFM experiments. A microfabrication technique has been developed to produce hybrid devices consisting of micro-nano Hall sensors integrated on SU-8 cantilevers. Two materials have been used to produce the Hall sensors. First, bismuth (Bi) was chosen for its low carrier charge density and its negligible surface charge depletion effects so that it should provide sensitive sensors. The second material tested was nickel (Ni), chosen for the dominant effect called the extraordinary Hall effect. The active areas of 5×5, 2×2 and 1×1 µm2 were realized and reproduced with the standard photolithography. Producing submicron Hall sensors required the use of the focused ion beam (FIB) that allowed a decrease of the size of the sensors down to 80×80 nm2. The delicate transferring of the nano sensors on the polymer cantilevers was successful. Bi micro-Hall sensors have shown a fragility making them delicate to handle. However, the Ni micro-Hall sensors had an excellent functionality. Their characterization has shown sensors with a low sensitivity compensated by the high applied current providing a high magnetic field resolution. The magnetic field images were carried out using the Ni micro-Hall sensors. They had a high stability during the imaging experiments. The spatial resolution was set by the sensors active area limited to the micro-meter scale. However, the magnetic field resolution is better than 1 T/√Hz a result comparable to the high resolution offered by the semiconductors. Single crystal silicon cantilevers have been fabricated for use in MRFM experiments. Also, cantilevers with integrated micro-magnets have been successfully produced. The developed microfabrication technique has allowed a high yield production (up to 70%). The softest fabricated cantilevers are 0.34 µm thick, 500 µm long and 10 µm wide. These cantilevers have been characterized under vacuum conditions (1×10-6 mbar). The resonance frequencies and the quality factors were measured using the frequency sweep or noise methods and both provided the same results. The Rayleigh method that we have adapted to the cantilevers geometry, made it possible to estimate the theoretical resonance frequencies with a good approximation and the calculated values fit well with the measured ones. All the cantilevers have a quality factor above 20000 and the force sensitivity achieved was between 10-16 and 10-17 N/√Hz A mathematical model has been developed based on the motion equation of a harmonic oscillator under the influence of a magnetic field. The model was studied to find the expression of the resonance frequency of cantilevers with integrated magnetic tips in the presence of a parallel or a perpendicular magnetic field. The shift in frequency due to the presence of the applied magnetic field depended on the magnetic tip magnetization. The measurement of the frequency shift should permit the calculation of the magnetization of these micro-magnets directly integrated on the cantilevers. The probes that we have realized represent micro and nano tools suitable for the magnetic field imaging that can be done with the SHPM and MRFM techniques.

EPFL2007

Highly sensitive vertical hall sensors in CMOS technology

Enrico Schurig

Vertical Hall sensors are capable of measuring surface-parallel components of the magnetic field. They allow therefore relatively easy conception of single-chip multi-axial magnetic sensors compared to solutions using horizontal Hall plates. The modern trend in the field of Hall sensors is to integrate them into electronic circuitry for signal processing. Before this thesis, highly sensitive vertical Hall sensors completely compatible to a technology adequate for co-integration of electronic circuitry were not available. This is the reason why we present in this thesis the developed knowledge that is necessary for the design and manufacture of highly sensitive vertical Hall sensors in CMOS technology. We first present the principles of the technology choice. A wide selection of CMOS processes is presently available, but in order to obtain optimal sensitivity we have to choose "the right" one. The performances can vary easily by 50% from one technology to another. The best choice are CMOS high-voltage technologies since they deliver n-diffusion layers with relatively low doping level and deep junction depth. We present a novel layout of vertical Hall sensors that has six contacts in the active sensor zone and is a development of the known layout that uses only four contacts. The additional contacts improve the sensitivity and reduce the systematic offset considerably. If we re-use layouts that were optimal for not-CMOS compatible technologies, we will see that the results are not satisfying. We have to develop our specific design parameters that are optimal for the chosen technology. The key for high sensitivities is the strong miniaturization of the devices down to technological limits given by the design rules, but sometimes even beyond that. Some of the design rules can be broken with benefit for the obtained sensor devices. In general, minimum contact sizes and contact distances are preferable as well as a minimum sensor thickness. The difficulty is however to find the absolute minimum before device degradation or even failure will occur. Our optimized sensors achieved voltage related sensitivities of about 0.04 V/(VT) and current related-sensitivities up to 400 V/(AT) that are comparable to existing CMOS compatible horizontal Hall plates. Such sensitivities were never reported for a standard CMOS process, without any additional pre- or post-processing steps. The miniaturization of the Hall devices has however also negative aspects e.g. an increase in sensor offset, noise and output non-linearity. Although Hall sensors have many advantages in comparison to other magnetic sensors (simple structure, low fabrication costs, very good linearity, robustness) they have the drawback of a big Abstract offset voltage that is often too big for many applications. This is why an enormous effort was made by researchers in the past to develop offset reduction methods for horizontal Hall plates. We state in this thesis that, in principle, the same techniques are applicable on vertical Hall sensors but for an optimal efficiency certain principles/parameters have to be respected. We reveal the relations of important parameters as the type of layout (number of sensor contacts), the use of single or coupled sensors, the type of spinning-current, bias level etc. In the optimal case we can obtain residual offset values lower than 200 μT for bias voltages up to 2 V. The spinning current method was originally developed for offset compensation, but it has other beneficial influences on the sensor output signal as well. It is capable to remove a large amount of flicker noise if the spinning is executes at adequately high clock frequencies. We show that the efficiency of flicker noise removal is dependent on the bias current through the sensor. We discovered another positive side effect of the spinning-current method: the elimination of the planar Hall effect. While the planar Hall sensitivity for our sensors is initially about 1% of the normal one at B = 2 T, we obtain with the compensation technique values of only 0.02%. After the presentation of general characteristics of the separate sensors, we present two microsystems based on the developed vertical Hall sensors. At first, we present a 2D magnetic microsystem which is well adapted for the construction of contactless angular encoders with a measurement range of 360°. We profit here especially from the very low offset (< 400 μT) obtained with the spinning current method, since offset is in general the main source of angular errors in such systems. Existing angular measurement systems need in general a special (offset) calibration of the sensor unit in order to achieve an accuracy of 2.5‰ full scale. We obtain such a precision directly with our system for a wide temperature range from -30 to 100 °C. With one specific sensor calibration for the compensation of residual offset, sensitivity mismatch and phase mismatch of the two axes, we attain a precision of about 0.5‰ FS over the same temperature range, a performance never before reported with a magnetic sensor system. The microsystem is therefore an excellent candidate for the majority of low-cost angular measurement applications. As a second example we presented the first fully CMOS integrated three-dimensional Hall probe, consisting of a combination of one Hall plate and four vertical Hall devices. The microsystem allows an application as a universal magnetic field probe in the wide field range from 0.1 mT to 20 T. The probe can be easily used for small volumes because of its small size and the fact that the signal processing circuitry already is directly integrated onto the chip. The precision as a 3D magnetometer is limited by the sensitivity mismatch of about 5% FS. The probe is an ideal candidate for low-cost teslameter applications with limited precision.

EPFL2005

MICRO-HALL SENSORS ON SU-8 CANTILEVERS FOR MAGNETIC IMAGINIG

Giovanni Boero, Chantal Imboden, Schahrazede-Lila Mouaziz

Micro-Hall sensors made of Bi have been previously fabricated and have shown interesting properties for magnetic field detection. In this project, new ideas to improve the performance of these Hall sensors and to explore ferromagnetic metals are developed. Ni and Bi protected with Al2O3 are used as materials to realize the sensing element. Different thicknesses between 10 and 100 nm of Ni and Bi are deposited by evaporation and structured with lift-off. The electrodes are made of Ti/Al. A Pt thin film is deposited between the sensor and the electrodes to improve electrical contact. Two SU-8 layers are structured with the standard photolithography process to build a cantilever and a carrier chip. Release of the sensor chip is performed with ICP SF6 plasma to etch the poly-Silicon layer.