Freestanding nanostructured sensors fabricated by atomic layer deposition [electronic resource]
Keywords: Mechanical Engineering.

Sensor manufacturing has been revolutionized by the adoption of micro- and nanofabrication techniques developed in the pursuit of ever-smaller silicon electronics. With semiconductor scaling approaching atomic regimes, it has become possible to develop nanostructured sensors that leverage the unique material properties and design space accessible with atomically-thin transducers. However, the effective use of nanostructured materials requires substantial investments and a strong foundation in materials characterization, precision manufacturing at the atomic scale, and viable methods of integrating such materials into conventional high-volume wafer-scale fabrication. This thesis presents advancements in each of these areas towards the goal of realizing freestanding nanostructured sensors fabricated by atomic layer deposition (ALD). First, by combining vapor-phase etching common in micro electromechanical systems (MEMS) processing with atomically thin etch-stop layers formed by ALD, this work presents the first parallel method for preparing plan-view specimens for transmission electron microscopy. This advancement enables a platform for high-throughput electron microscopy supporting the development and characterization of nanostructured materials, while at the same time reducing preparation time and cost by more than an order of magnitude compared to state-of-the-art methods. Second, using this platform, the structural and electronic properties of sub-20 nm platinum layers fabricated by plasma-enhanced atomic layer deposition (PEALD) have been investigated. This works presents the first observations of abnormal grain growth, competing textures, and a crossover between large compressive (480 MPa) and tensile (230 MPa) stress during Volmer-Weber nucleation and growth of PEALD platinum on amorphous substrates. The properties that make platinum a desirable sensor material, including its large temperature coefficient of resistance and low 1/f noise, degrade below a critical cycle number corresponding to grain coalescence and high fractional substrate coverage during nucleation. Third, informed by this characterization, freestanding nanostructured sensors in the form of uncooled metal microbolometers are successfully demonstrated with atomically thin (~ 30 to 80 atom thick) thermistors having aspect ratios as large as 10,000:1. An increase in sensitivity by a factor of 15 to 75 MV/WA is achieved compared to previous uncooled metal microbolometers. The reduction in volume achieved with atomically-thin transducers and favorable scaling of thermal conductance enables thermistors to achieve sub-millisecond thermal time constants more than an order of magnitude faster than state-of-the-art thin film designs using vanadium oxide. However, degrading thermistor properties with decreasing ALD platinum cycle number negatively impact sensor performance including temperature resolution. This degradation presents design tradeoffs unique to atomically-thin transducers where interfacial effects such as thin film nucleation strongly govern the structural, chemical, and functional properties of materials.