A new versatile hybrid MEMS technology for high sensitivity, fluid proof sensing applications.

The field of micro electromechanical systems (MEMS) evolved from the microelectronic industry and the technologies developed to fabricate integrated circuits. As a result, MEMS are commonly fabricated on silicon wafers. The development of MEMS has been driven by three main merits: miniaturisation, microelectronics integration, and parallel fabrication with high precision. Many operational properties scale well with smaller sizes. In addition, integrated electronic circuitry allows embedding MEMS with computing or networking capabilities, while parallel manufacturing enables the fabrication of many identical devices on a single wafer, reducing the unit cost. The main MEMS application is transducers which transform signals from one form of energy to another, and can be used for perception (sensors) or to produce actions (actuators). Silicon and other microelectronic materials like metals have enabled very high-performance MEMS transducers because these materials can be used for a range of very effective actuating and sensing principles.More recently, polymers have started to be used in MEMS because of their unique electrical, physical and chemical properties which include biocompatibility, viscoelasticity and mechanical shock tolerance. They are also much softer than silicon or metals, and they can be processed using many techniques that allow unique low-cost, batch-style fabrication and packaging. However, polymers have relatively low glass-transition and melting temperatures. As a result, the advantages of polymers cannot easily be combined with high-performance actuating and sensing elements as these are based on silicon technology and require high-temperature fabrication steps. Here, a hybrid-MEMS fabrication process is used to address this issue. The developed devices are based on a trilayer structure, where a thick polymer core is sandwiched between two hard thin films, while electronic layers are embedded within in a fluid-compatible way.The objective of this thesis is to turn hybrid-MEMS into a benchmark MEMS technology. This involves many different aspects. First, sensing and actuation elements are integrated into trilayer devices, and their performance is analysed. Then, some more unique possibilities offered by hybrid-MEMS are exploited. A way to fabricate electronics on multiple layers is developed, which allows different electronic features to be integrated in parallel. In addition, three-dimensional bulk features fabricated at several levels are demonstrated. This is possible because the hybrid-MEMS process is based on the bonding of multiple wafers.All these new fabrication features are demonstrated in atomic force microscopy (AFM) applications. Self-actuated trilayer AFM cantilevers with sharp silicon tips are used to boost imaging speeds in the off-resonance tapping (ORT) mode, thanks to an order of magnitude faster surface probing rates. Furthermore, fully integrated ORT imaging is demonstrated with self-actuated piezoresistive cantilevers. Next, trilayer cantilevers with shielded conductive tips are introduced for electrical AFM modes. Finally, ongoing research projects are touched upon, including approaches to fabricate hybrid-MEMS with single crystal silicon piezoresistors for improved sensitivity, and an analysis of reproducibility of fabricated trilayer cantilevers.

Polarity-Controllable Devices and Circuits for Doping-Free 2D Electronics

Giovanni Vincenzo Resta

The growth of information technology has been sustained by the miniaturization of Complementary Metal-Oxide-Semiconductor (CMOS) Field-Effect Transistors (FETs), with the number of devices per unit area constantly increasing, as exemplified by Mooreâs law. Modern digital integrated circuits contain billions of transistors, fabricated with CMOS technology. As scaling of conventional silicon-based electronics is reaching its ultimate limit, considerable effort has been devoted to find new materials and new device concepts that could ultimately outperform standard silicon transistors. Among the materials that are studied for charge-based devices, two-dimensional materials of the Transition Metal Di-Chalcogenides (TMDCs) family show promising opportunities, thanks to their electrical and physical properties. In order to enable 2D electronics to be integrated in the Back-End-Of-the-Line (BEOL) with conventional CMOS it is essential to attain complementary operation of the transistors, in order to achieve low standby power consumption. In CMOS, ions are physically implanted in silicon to create unipolar devices with Ohmic contacts. However, this process requires high annealing temperatures, which are not compatible with the low-thermal budget requirements for monolithic 3D integration. For 2D materials, physical doping, through ion-implantation, has not proven to be successful due to extreme thinness of the 2D semiconductors. The possibility of introducing dopant atoms during growth of the 2D material has been reported, but does not allow for any control of the doping profile. Several chemical and molecular doping techniques have been developed, but are non-stable and non-CMOS compatible. A device concept that does not rely on any physical or chemical doping, and use instead un-doped materials, would be of great interest in this regard. In this thesis, we report the first experimental demonstration of a doping-free, polarity-controllable device fabricated on few-layer tungsten di-selenide. We show how modulation of the Schottky barriers at drain and source by a separate gate, named polarity gate, can enable the selection of the carriers injected in the channel, and achieve controllable polarity behaviour with ON/OFF current ratios > 1E6 for both electrons and holes conduction. Following this demonstration, we fabricate and characterize a variety of polarity-controllable logic gates such as Inverter, NAND, NOR, that are the building primitives used in the logic synthesis frameworks for conventional CMOS logic. Moreover, we experimentally show 2- and 3-Input XOR, and majority gates that, thanks to the polarity-controllable devices, can be realized with fewer transistors as compared to what is achievable in conventional CMOS. We demonstrate a complete standard cell library with the possibility of fabricating compact, highly-expressive logic gates that can be exploited to gain advantages at circuit level, using XOR and MAJ functions as logic primitives. Moreover, for the first time, we study scaling trends and evaluate the performances of polarity-controllable devices realized with undoped mono and bi-layer 2D materials. Using ballistic self-consistent quantum simulations combined with TCAD simulations, it is shown that, with the suitable channel material, such polarity-controllable technology can scale down to sub-10nm gate lengths, while showing performances comparable to the ones of unipolar, physically-doped 2D electronic devices.

EPFL2019

Design and fabrication of suspended-gate MOSFETs for MEMS resonator, switch and memory applications

Nicolas Abelé

Wireless communication systems and handset devices are showing a rapid growth in consumer and military applications. Applications using wireless communication standards such as personal connectivity devices (Bluetooth), mobile systems (GSM, UMTS, WCDMA) and wireless sensor network are the opportunities and challenges for the semi-conductor industry. The trend towards size and weight reduction, low power consumption and increased functionalities induces major technological issues. Today, the wireless circuit size is limited by the use of lots of external or "off-chip" components. Among them, quartz crystal, used as the time reference in any wireless systems, is the bottleneck of the miniaturization. Microelectromechanical systems (MEMS) is an emerging technology which has the capability of replacing the quartz. Based on similar technology than the Integrated Circuit (IC), MEMS are referred as electrostatically, thermally or piezoelectrically actuated mechanical structures. In this thesis, a new MEMS device based on the hybridization of a mechanical vibrating structure and a solid-sate MOS transistor has been developed. The Resonant Suspended-Gate MOSFET (RSG-MOSFET) device combines both advantages of a high mechanical quality factor and the transistor intrinsic gain. The physical mechanisms behind the actuation and the behavior of this device were deeply investigated and a quasi-static model was developed and validated, based on measured characteristics. Furthermore, the dynamic model of the RSG-MOSFET was created, taking into account the non-linear mechanical vibrations of the gate and the EKV model, used for MOSFET modeling. Two fabrication processes were successfully developed to demonstrate the proof of concept of such a device and to optimize the performances respectively. Aluminum-silicon (Al-Si1%) and pure silicon-based RSG-MOSFETs were successfully fabricated. DC and AC characterizations on both devices enabled to understand, extract and evaluate the mechanical and MOSFET effects. A specifically developed RF characterization methodology was used to measure the linear and non-linear behaviors of the resonator and to evaluate the influence of each polarization voltages on the signal response. RSG-MOSFET with resonant frequencies ranging from 5MHz to 90MHz and quality factor up to 1200 were measured. Since MEMS resonator quality factor is strongly degraded by air damping, a 0-level thin film vacuum packaging (10-7 mBar) process was developed, compatible with both AlSi-based and silicon-based RSG-MOSFET. The technology has the unique advantage of being done on already released structure and the room temperature process makes it suitable for above-IC integration. In parallel, a front-end compatible process was defined and major build blocks were developed in industrial environment at STMicroelectronics. This technology is based on the Silicon-On-Nothing technology, originally developed for advanced transistor, and therefore making the MEMS resonator process compatible with CMOS co-integration. DC characterizations of SG-MOSFET had shown interesting performances of this device for current switch and memory applications. Mechanical contact of the gate with the MOSFET channel induces a current switching slope greater than 0.8mV/decade, much better than the theoretical MOSFET limit of 60mV/decade. Maximum switch isolations of -37dB at 2 GHz and -27dB at 10GHz were measured on these devices. A novel MEMS-memory has been demonstrated, based on the direct charge injection to the storage media by the mechanical contact of the metal gate. Charge injection and retention mechanisms were investigated based on measured devices. Cycling study of up to 105 cycles were performed without noticing major degradations of the electrical behavior neither mechanical fatigue of the suspended gate. The measured retention time places this memory in between the DRAM and the FLASH memories. A scaling study has shown integration and compatibilities capabilities with existing CMOS.

EPFL2007

Stretchable and Highly Bendable Thin Film Transistors Based on Atomically Thin Chemical Vapor Deposited Molybdenum Disulfide

Yen-Cheng Kung

Two-dimensional (2D) materials have received tremendous research attention recently, as they possess peculiar physical properties in their monolayer and few-layer forms, which further lead to novel applications. A wide range of 2D materials covers insulators, metals, superconductors and semiconductors with various bandgap, giving a fresh platform to reinvent conventional electronic and optoelectronic devices with new advantages. Molybdenum disulfide (MoS2), belonging to the group of transition metal dichalcogenides (TMDCs), has been widely studied for its particular physical properties and potential applications as a leading example among 2D semiconductors. In this thesis, we focus on the applications ofMoS2 in microelectronic and micro-electromechanical devices. The MoS2 we studied is grown specifically by chemical vapor deposition, which offers the highest potential in large area growth. Utilizing the flexible nature such as high in-plane fracture strain and low bending stiffness of atomically thinMoS2, we developed stretchable thin film transistors (TFTs) on polydimethylsiloxane (PDMS) and highly bendable thin film transistors on polyimide. The fabrication process is ameliorated and designed to provide good chemical and temperature compatibility to the polymer substrates. Electrical performance of the flexible devices under different degrees of bending and stretching has been studied. The stretchableMoS2 TFTs could sustain up to 4% of uniaxial tensile strain maintaining device function, and the failure occurs at 5% of strain due to the fracture in gate dielectric layer. The highly bendableMoS2 TFTs with parylene-C gate dielectric layer exhibit tolerance to the smallest bending radius of 0.625 mm. Different encapsulation layers such as HfO2, Cytop and SU8 have been implemented on the bendable devices to improve the performance represented by the increase in field effect mobility, reduction in hysteresis and better endurance to mechanical deformation. The resulting flexibleMoS2 TFTs show field effect mobility of 21.4 cm^2V^-1s^-1 and a large on-off ratio of 106 in air. The SU8 encapsulation layer is further demonstrated as controllable n-type doping strategy with low temperature process, which is ideal for flexible MoS2 devices as it additionally exhibits good stability in air and water.

EPFL2018