High aspect-ratio MEMS devices for the next generation of THz/MHz passive components

Abstract

The realization of efficient passive devices directly on chip represents one of the most intriguing challenges in IC fabrication processes. The performance of such devices are intrinsically determined by physical parameters that cannot be easily scaled, making the on-chip integration of such components a complex task to solve. To pursue the envisioned scaling towards the realization of more compact and fast electronic devices, it is then clear that new, engineered materials and innovative device architectures have to be explored. The aim of this dissertation is to provide the technological means to implement the fabrication of two particular type of passive components, integrated Terahertz (THz) planar antennas and solid state supercapacitors. The first part of this thesis is dedicated to the realization of an efficient THz planar antenna, directly on chip. The realization of more efficient sensing devices working in the THz range of frequencies open the possibilities to exploit the enormous bandwith available for communication purposes. Moreover, an extremely large amount of physical phenomena can be explored by using this radiation, such as for cosmic back-ground radiation and molecular spectroscopy. However the sensing devices employed for these applications suffer from poor performance due to limited radiation efficiency and the lack of efficient power sources. These topics are discussed in Chapter 2. As most relevant scientific contribution of this work, a new engineered anisotropic material, called Artificial Dielectric Layer (ADL) working in the THz frequencies is introduced in Chapter 3. By embedding a periodic array of squared metal patches into a conventional dielectric host matrix, we demonstrate that it is possible to obtain a new material with high and anisotropic dielectric constant. When placed on top a conventional planar antenna, this material enhances the front-to-back ratio of more than 10 dB, with almost no power dispersed in surface waves. Due to scaling of the ADL parameters with the frequency, PCB technologies cannot be used to fabricate this component (and the antenna itself). For this reason we decided to employ integrated circuits fabrication technologies. This choice permits the possibility to use the large variety of materials available in a integrated circuits (IC) laboratory for the fabrication. The use of different materials, with tunable mechanical properties, is fundamental to the realization of the antenna/ADL structure. This device basically consists of a large suspended membrane in which both the antenna and the ADL are integrated. Due to the high aspect ratio of the device, the stress of the deposited layers is tuned to achieve only minor buckling effects. Numerical simulations of ADL mechanical properties have been performed, showing that is possible to predict the deformation of the membrane by creating an effective layer that resembles the metal/oxide composite structure. With this technique, the computational time required to simulate the post-releasing buckling has been drastically reduced, while preserving a very good accuracy in the deformations calculation. In Chapter 4 the measurements of the ADL properties are reported. By measuring the antenna radiation pattern and the relative gain, the effectiveness of this material is demonstrated. As independent technique to confirm the obtained results, the ADL optical properties have also been investigated also by using a Time Domain Spectroscopy system. In Chapter 5 an alternative version of the ADL is proposed. As dielectric host material a layer of Silicon Carbide is used instead of the conventional silicon oxide. This choice allows a consistent reduction in the ADL final thickness, with obvious benefit for manufacturability. In the first part of this thesis, it has been shown that a new, composite high aspect-ratio metamaterial for THz applications can be realized by using conventional IC technologies. The potential of this approach can be exploited also in the fabrication of other passive devices. In the second part of this thesis it is indeed shown that high aspect-ratio composite nanostructures can be used as to build a new generation of fully solid state supercapacitors. In Chapter 6 the concept of supercapacitor is introduced and the applications in the IC industry discussed. Nowadays these components are widely used in System in Package integration, as decoupling filters. Moreover, with the increasing demand of large power supply directly on chip, as for harvester and mobile systems, there is a growing interest in efficient and good performing integrated supercapacitors. In Chapter 7 a new type of nanostructured supercapacitor is discussed. This device is based on the use of metallic carbon nanotubes (CNTs) as high aspect-ratio nanoelectrodes. By coating these nanostructures with a dielectric and conductive layer, both deposited by Atomic Layer Deposition (ALD), the electrode surface area is greatly enhanced, leading to very large capacitance values. The capacitor performance is evaluated by electrical impedance measurements. By fitting the obtained data with an analytical device model, the relevant capacitor parameters are extracted. As major scientific result, the presented devices retail their capacitance in a wide range of frequencies (1 Hz - 1 MHz), while similar devices presented by other groups fails already at 10 KHz. To evaluate the coating of the CNTs by the ALD layers, high-resolution TEM analysis of the coated structures is performed. Moreover, the diffusion of the gas ALD precursors into the porous CNTs bundles is studied in detail, demonstrating that the coating efficiency decreases drastically after a depth of 400 nm. The obtained capacitance values are in a very good agreement with this theoretical model. Finally, the dielectric breakdown of the devices is studied, showing that the premature breaking at 1.1 Volt can be reasonably accountable to defects in the dielectric coating on the CNTs.