Additive manufacturing of metals at small length scales – microstructure, properties and novel multi-metal electrochemical concepts

Abstract

Many emerging applications in microscale engineering demand the fabrication of threedimensional architectures in inorganic materials. Small-scale additive manufacturing (AM) aspires to provide access to these geometries with feature sizes in the micro- and submicrometer range. Yet, the synthesis of device-grade inorganic materials is still a challenge for AM, and the properties of additively manufactured materials are typically inferior to those of materials deposited via traditional, subtractive 2D fabrication routes – a major handicap for incorporating AM in advanced micro- and nanofabrication processes. Materials engineering is thus necessary to improve the quality of printed inorganic materials. This thesis revolves around the materials science of small-scale AM of metals, focusing on both, contemporary techniques and novel concepts introduced in this thesis. The work covers two major topics: first, it establishes a comprehensive overview of the microstructure and properties of metals synthesized by modern additive methods. Second, it explores new techniques that enable facile electrochemical AM of high-quality metals and unlock multi-metal printing of chemically architected geometries with spatially modulated properties at the submicron-scale. In combination, these studies present a further step towards the integration of AM into modern microfabrication routines. The first part of the thesis defines the state of the art of small-scale metal AM, providing a detailed literature review of current techniques and an experimental survey of their materials’ properties. Note that the thesis in general concentrates on the study and development of methods that enable direct additive deposition of metals. Thus, it considers indirect concepts based on the fabrication of organic templates by two-photon lithography in combination with subsequent metallization procedures in little detail only. Today, almost a dozen different methods are available for the direct deposition of metal 3D geometries with a resolution better than 10 μm. As these approaches build on different physico-chemical principles, their characteristics such as feature size, speed and complexity of printable geometries, as well as the synthesized metals and their microstructure, vary greatly. A discussion of the individual principles and capabilities puts the concepts in perspective to each other and projects their potentials. The thesis then presents an experimental study on the "quality" of metals deposited by these methods. In collaboration with most of the groups active in the field of small-scale metal AM, the thesis explores the microstructure and resulting mechanical properties of today’s materials. On one hand, we show that metals with a wide range of microstructures and elastic and plastic properties are synthesized. Especially electrochemical methods deposit dense and crystalline metals with excellent mechanical properties that compare well to those of thin-film nanocrystalline materials. On the other hand, the results reveal large variations in materials performance that can be related to the microstructure of the individual materials. Thus, the study provides practical guidelines for users of small-scale additive methods and establishes a baseline for the necessary optimization of printed metals. The second part of the thesis presents novel electrochemical AM methods that offer a spatial resolution 1 μm. First, two chapters introduce electrohydrodynamic redox printing (EHD-RP). This technique enables the direct, ink-free fabrication of polycrystalline multi-metal 3D structures with a resolution of 250 nm and a feature size of 100 nm. The electrochemical concept enables outstanding as-printed materials properties (for example a strength of copper that competes with highest values reported for nanocrystalline copper) printed at speeds that outperform current electrochemical techniques by one order of magnitude. Although neither its speed, its resolution nor its overall materials properties are unrivaled by competing methods, EHD-RP excels in an advantageous combination of these characteristics, readily permitting applications in microfabrication. Additionally, as a most unique feature, EHD-RP enables multi-metal printing with unprecedented detail. As shown, the additive control of the chemical architecture of metals with a chemical feature size <400 nm unlocks the synthesis of 3D bi-metal structures with spatially varying microstructure and thus locally modified properties. The last chapter discusses the potential of tip-induced deposition in the electrochemical scanning tunneling microscope (STM) for nanoscale AM. A strong confinement of deposition below the nanometer-sized probe enables 2D patterning of cobalt with a feature size of 50 nm. The potential for 3D printing is demonstrated, but reliable 3D fabrication is hampered by unsatisfactory stability of employed gold STM probes. Consequently, the chapter concludes with the development of more stable Pt and Pt-20at.%Ir probes, readying the designed setup for true 3D deposition. Nevertheless, a low deposition speed, a narrow processing window and a comparably complex instrumentation are identified as significant challenges for the proposed concept. In conclusion, the thesis experimentally identifies materials challenges for contemporary smallscale AM of metals but at the same time presents a potential solution by introducing EHD-RP – an electrochemical concept that offers ink-free printing of high-quality metals. Additionally, the demonstrated multi-metal printing with submicrometer resolution sketches a route towards the bottom-up fabrication of chemically designed 3D devices and materials with properties tuned at single-voxel level – staking out a niche for small-scale AM as an enabling technology for chemically architected, inorganic materials that could spark the development of novel materials for e.g. catalysis, active chemical devices, sensors, or metamaterials that combine architected geometry and chemistry.