Master thesis, Rune Hviid: Hafnium Oxide Upgrades pH Sensing InAs Nanowire FET Devices

The primary insulation material in semiconductor is SiO₂ made by heat-treatment of Si. This material is becoming obsolete and new oxides with a higher dielectric constant are needed. HfO₂ is the most promising replacement due to its low reactivity and low leak currents when interfaced with Si. In this thesis I explore mechanical, chemical and electrical properties of HfO₂ when incorporated in microelectronic fabrication of InAs nanowire Lable-Free Bio-FET sensors.
I introduce the new material, HfO_2, into microelectronic device fabrication and determined the conditions for which lithographic patterns can be defined. HfO₂ can be deposited on any surface by a process of Atomic Layer Deposition (ALD) that exposes a surface to separate reactive gasses in a controlled amount of time, thus depositing a single monolayer of HfO₂ at a time. Next, I define capacitor devices with HfO₂ as the dielectric to determine the relative permittivity and breakthrough voltage of amorphous HfO₂. Both values were found to be highly dependant on smoothness of the electrode/oxide interface, and HfO₂ here found to grow in islands which might have caused tunneling currents through thin oxides.
Due to surface reconstruction of InAs nanowires, the density of electrons is highest at the surface which makes their conductance very sensitive to charges near the surface, and the nanowires easy to contact electrically with UV and E-beam lithographic. I determine the optimal fabrication procedure for making InAs NW FET devices with contact resistance of a few kΩ. Then I incorporated HfO₂ into InAs nanowire FET devices as the insulating material for electrode and nanowires to prevented short circuit when the device is under water. This role can not be filled by SiO₂ since it is permeably to protons and will cause current leaks to the gate. I used the fabricated devies to measure pH on different aqueous solutions, which appeared as clear reproducible steps in current stabilization within a few seconds.