Development of a Monolithic Fiber-Based Electric Field Sensor

This project has been done on the development of a monolithic fiber-based electric field sensor, with a focus on establishing a reliable, cost-effective, and scalable production cycle to be implemented initially at a pilot and consequently at an industrial scale. The small scale and the low cost of these electrically passive sensors would enable their application where applying an external electric current is either forbidden (e.g. by the ATEX directive), or would distort the electric field and thus limit the possibility of monitoring sensitive assets. This electric field sensor works on the principles of electrostatic induction and translates the electric field to the displacement of an array of mirrors and holes suspended over a second array of mirrors. By shining the rays of light onto the suspended array, it would be possible to monitor the variations of the reflected light due to the displacement of the suspended array over the second array which is deposited on a transparent substrate. At the first phase of the project, by utilizing the COMSOL Multiphysics simulation software, the optimal parameters of the electric sensor were simulated. Based on the results of the simulations, a general process flow was drafted and received the approval from the experts at the EPFL Center of Micro/Nano Technology (CMi). Based on the finite element simulations, in order to maximize the displacement of the sensor along the electric field, it has been shown that reducing the thickness of the sensor would significantly increase the longitudinal displacement of the sensor. Additionally, in order to further decrease the stiffness of the springs connecting the suspended mass to the frames, the structural material of the sensor was selected to be Copper, which also has a high electrical permittivity and low fabrication cost. Next, the micro fabrications based on the optimized design and the process flow were carried out at the CMi ISO 5 cleanroom facilities. In order to adapt the process flows to the capabilities and the availability of the machines, several process flows have been tested and the result of which have all been documented in this report. Moreover, regarding the limited range of the available choices for having a process flow which is compatible with the CMi facilities, the recommendations for further simplifying the process and decreasing the final production costs are mentioned in this report.

Nano-electromechanical systems based on ultra-thin semiconductors

Sajedeh Manzeli

Nowadays, the interest in 2D materials has gone far beyond graphene. Specially, monolayers of transition metal dichalcogenides (TMDs) offer a broad spectrum of electronic and optical properties, and show the potential to revolutionize the electronics industry. The promising electronic properties of 2D semiconductors combined with their mechanical strength and flexibility, makes them ideal candidates for nanoelectromechanical systems (NEMS). This thesis focuses on realizing NEMS based on graphene and MoS2, which is one of the most appealing TMDs. First, we present the realization of graphene NEMS by fabricating single and bilayer graphene transistors featuring a doubly clamped suspended channel. The electromechanical response of monolayer graphene nanoribbons show a strain-induced increase in their electrical resistance, making it possible to estimate an upper limit for the piezoresistive gauge factor. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. Our numerical simulations indicate that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of the two graphene layers with respect to each other. Our results report on the rare observation of room temperature electronic interference in bilayer graphene. Next, we investigate the static electromechanical response of atomically thin MoS2. MoS2 exhibits high youngâs modulus and fracture strength. Furthermore, the bandgap of MoS2 is highly strain-tunable. This coupling between electrical and mechanical properties makes MoS2 a promising material for NEMS. Here we incorporate monolayer, bilayer and trilayer MoS2 in a suspended membrane configuration with the electrodes acting as mechanical clamps. Strain-induced bandgap tuning is detected via electrical conductivity measurements and the emergence of the piezoresistive effect in MoS2 is demonstrated. We observe a reversible bandgap modulation in atomically thin MoS2 membranes with a thickness dependent modulation rate. Finite element method (FEM) simulations are used to obtain the spatially varying bandgap profile on the membrane and to quantify the rate of bandgap change. The piezoresistive gauge factor is calculated for single layer, bilayer and trilayer MoS2. Our results reveal that monolayer and bilayer MoS2 show a piezoresistive effect which is comparable to the state-of-the-art silicon strain sensors and two orders of magnitude higher than in graphene. Finally, we present the investigation of MoS2 NEMS resonators working in the VHF range and featuring piezoresistive transduction. The atomic thickness of monolayer MoS2 places it as a promising candidate for miniaturization of electromechanical devices to the limits of vertical scaling. While the small mass of MoS2 leads to an increased resonant frequency and a higher mass sensitivity, the presence of piezoresistivity offers a transduction mechanism in addition to the traditional capacitive transduction. Operating in the tension-dominated regime, monolayer MoS2 NEMS resonators not only allow tunability of the resonant frequency using an external voltage, but also show the strain-induced enhancement of their dynamic range. Furthermore, the resonators are driven into the nonlinear regime allowing the study of nonlinear effects. This work sheds light on the potential of TMD based NEMS as ultra-low power switches, sensors and resonators for applications in RF range.

EPFL2018

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

Magneto-thermal finite element modeling of 2nd generation HTS for FCL design purposes

Bertrand Dutoit, Francesco Grilli, François Roy, Frédéric Sirois

Coated conductors are very promising for the design of novel and e±cient Fault Current Limiter (FCL). However, before considering using them in a power grid, their thermal and electromagnetic behaviors in the presence of over-critical currents need to be investigated in details. In this context, we performed ¯nite element magneto-thermal modeling of coated conductors under over-critical current on several geometries. Accordingly, we have investigated the substrate electrical connectivity and thermal properties on the HTS-FCL behavior. All simulations were performed using in COMSOL Multiphysics, a commercial finite element package, which has a built-in coupling between the thermal and electrical equations, allowing us to compute both quantities simultaneously during the solving process. Our simulations allowed us to formulate thresholds for the current density usable in coated HTS as well as limitation capability of a device made of these new conductors.