title: Fabrication and integration of metallic nano and micro cones for on-chip electron field emitters
Creators: name: Roustaie, Farough
Publisher: name: UNSPECIFIED
Year: 2022

description: Nanotechnology enables a diversity of new effects compared to the classical physical properties of the material. The metallic wires with a dimension of less than 1 µm and a length between 10 µm to 50 µm exhibit a great aspect ratio. A high density of such wires particularly gives a great surface to volume ratio, which results in new mechanical, electrical, thermal, and chemical properties of the surfaces covered with them. These new physical and chemical effects enable a new level of more sensitive sensors like chemical, biological, gas flow, force, and inertial sensors. Also, low resistance micro switches, more efficient thermal interface materials, and room-temperature interconnects can be enabled with nanowired surfaces. The high aspect ratio of the wires enables them to be applied as a high-performance electron field emitter. For realizing these applications there is an obstacle to overcome: Vertical integration of the metallic nanowires into a 3D microsystem. This work introduces a technique for in-situ integration of nanowires into microsystems with a focus on an application in sensor technology and commercial and industry suitable fabrication. The objective of this work is to develop an on-chip electron field emitter, based on metallic nanowires, for measuring vacuum pressures less than 10^-12 mbar in cryogenic systems at temperatures below 6 K. A review of state-of-the-art technologies in vacuum measurement sets the basis for discussing possibilities to eliminate or minimize the problems of the field emitter based gauges. It is shown theoretically that using the metallic nanowires with high aspect ratio and sharp tips as an electron field emitter results in a great local electrical field enhancement, thus, a higher current emission. For fabricating the wires the so-called ion-track etch process is used. Such nanowires are also known as template grown wires because the nanowires are electrochemically synthesized in the pores of the ion-track etched template membranes. With this process, nanowires with a diameter from 30 nm to some µm and a length of 2 µm to 100 µm with different densities in the range of 10^4 cm^-2 to 10^9 cm^-2 can be realized. The development of a process for in-situ integration of the wires into a 10 mm x 30 mm surface (as 16x50 array of pads) and the developed devices and techniques are explained in detail. The process and the electrochemical deposition device are optimized to enable covering broad surfaces with nanowires. With the optimization, the nanowires can be integrated into the whole surface with 300 mm x300 mm dimensions and also industrial 12-inch wafers. Compared to cylindrical nanowires, conical structures show a much better thermomechanical performance. Therefore, the used ion-track templates are etched asymmetrically in an etching device, developed in this work including an electrical measurement process to control the apex angle of the conical pores. Theoretically, with the conical structure a stable current emission, sufficient for vacuum pressure measurement, with a significantly longer lifetime of the emitting cones is expected. These effects are experimentally explored with a large variety of samples. The field emission characteristics of the nanocones in a diode and triode setup are measured and described in detail. In a long term measurement a stable field emission current of 31 µA at an applied voltage of 290 V for 50 h and above 100 µA at an applied voltage of 338 V for 12.5 h shows the potential of this emitter structure for enough stable current emission for XHV vacuum pressure measurement. To complete the structure of an on-chip emitter, an extraction grid for applying the extraction voltage as well as transmission of the emitted current must be attached over the nanocones. A concept for an XHV suitable integration of the extraction grid is designed. This concept is pursued by the idea, which is to use nanowires as a hook and loop fastener. In this application, two surfaces are covered with nanowires. By pressing these surfaces on top of each other both surfaces are bonded at room temperature. This process takes place by entanglement and diffusion of the nanowires into each other. The development of this room temperature bonding technology and exploring the mechanical and electrical properties of the connections are discussed in detail. This new technology is presented for a heat-free bonding of semiconductors on pads down to 3 µm and pitches lower than 5 µm. Also, this technology has the potential of a wafer-wafer and die-wafer bonding in large scales and with temperatures far below 230 °C or at room temperature. In the context of this work, the company NanoWired GmbH was founded. This company transfers the developed technology to the market for applications in semiconductor, automotive, sensor, and light segments. Different conventional methods like glueing, welding, soldering, or screws can be substituted with this technology.