Transfer printing based microassembly and colloidal quantum dot film integration

Micro / nanoscale manufacturing requires unique approaches to accommodate the immensely different characteristics of the miniscule objects due to their high surface area to volume ratio when compared with macroscale objects. Therefore, surface forces are much more dominating than body forces, which causes the significant difficulty of miniscule object manipulation. Because of this challenge, monolithic microfabrication relying on photolithography has been the primary method to manufacture micro / nanoscale structures and devices in place of microassembly. However, by virtue of the two-dimensional (2D) nature of photolithography, formation of complex 3D shape architectures via monolithic microfabrication is inherently limited, which would otherwise enable improvements in performance and novel functionalities of devices. Furthermore, monolithic microfabrication is compatible only with materials which survive in a wet condition during photolithography. Delicate nanomaterials such as colloidal quantum dots cannot be processed via monolithic microfabrication. In this context, transfer printing has emerged as a method to transfer heterogeneous material pieces from their mother substrates to a foreign substrate utilizing a polymeric stamp in a dry condition. In this thesis, advanced modes of transfer printing are studied and optimized to enable a 3D microassembly called ‘micro-Lego’ and a novel strategy of quantum dot film integration. Micro-Lego involves transfer printing for material piece pick-and-place and thermal joining for irreversible permanent bonding of placed material pieces. A microtip elastomeric stamp is designed to advance transfer printing and thermal joining processes are optimized to ensure subsequent material bonding. The mechanical joining strength between material pieces assembled by micro-Lego are characterized by means of blister tests and the nanoindentation. Moreover, the electrical contact between two conducting materials formed by micro-Lego are examined. Lastly, inspired from the subtractive transfer printing technique, protocols of quantum dot film patterning using polymeric stamps made of a shape memory polymer as well as a photoresist are established for the convenient integration of quantum dots in various geometries and configurations as desired. Transfer printing-based micro / nanoscale manufacturing presented in this thesis opens up new pathways to manufacture not only complex 3D functional micro devices but also high resolution nano devices for unparalleled performance or for an unusual functionality, which are unattainable through monolithic microfabrication

Solvent-Free Patterning of Colloidal Quantum Dot Films Utilizing Shape Memory Polymers

Colloidal quantum dots (QDs) with properties that can be tuned by size, shape, and composition are promising for the next generation of photonic and electronic devices. However, utilization of these materials in such devices is hindered by the limited compatibility of established semiconductor processing techniques. In this context, patterning of QD films formed from colloidal solutions is a critical challenge and alternative methods are currently being developed for the broader adoption of colloidal QDs in functional devices. Here, we present a solvent-free approach to patterning QD films by utilizing a shape memory polymer (SMP). The high pull-off force of the SMP below glass transition temperature (Tg) in conjunction with the conformal contact at elevated temperatures (above Tg) enables large-area, rate-independent, fine patterning while preserving desired properties of QDs

3D SU-8 structures assembled via micro-Lego

Deterministic multi-layer micro-scale assembly of SU-8 polymer objects as well as the cohesively joined SU-8 interface strength have been investigated. The process of SU-8 photoresist (PR) into retrievable individual micro-scale objects format called 'ink' is reviewed and the construction of three-dimensional (3D) unique micro-structures via micro-Lego, which refers to the transfer printing of the ink onto a target site in conjunction with permanent joining has been demonstrated. Owing to the additive 3D manufacturing nature of the micro-Lego, the demonstrative micro-scale architectures are unattainable through other means of micro-manufacturing including photolithography. The mechanical integrity of the SU-8 and SU-8 interface formed by micro-Lego is inspected by probing SU-8 cantilever specimens using a nanoindenter and the results are examined by finite element analysis (FEA). The analysis reveals that the joining strength is ∼ 0.988 J/m1

Micro-masonry of MEMS sensors and actuators

Micro-masonry is a route to microassembly that involves elastomeric-stamp-based micromanipulation and direct bonding. This paper presents the assembly of MEMS mechanical sensors and actuators using micro-masonry, demonstrating its capability of constructing 3-D microdevices that are impossible or difficult to realize with monolithic microfabrication. Microfabrication processes for retrievable MEMS components (e.g., combs, spacers, and flexure beams) are developed. As micromanipulation tools, microtipped elastomeric stamps with reversible dry adhesion are also designed and fabricated to pick up and deterministically place those components. After the manipulation, the components are permanently bonded together via rapid thermal annealing without using any additional intermediate layers. The assembled MEMS device is modeled and analyzed in consideration of the microassembly misalignment. The sensing and actuating capabilities of the assembled MEMS devices are experimentally characterized. © 1992-2012 IEEE

Electrical Contact at the Interface between Silicon and Transfer-Printed Gold Films by Eutectic Joining

This paper presents the electrical and morphological properties at the interface between a metal (Au) and a semiconductor (Si) formed by a novel transfer-printing technology. This work shows that a transfer-printed thin (hundreds of nanometers) Au film forms excellent electrical contact on a Si substrate when appropriate thermal treatment is applied. The successful electrical contact is attributed to eutectic joining, which allows for the right amount of atomic level mass transport between Au and Si. The outcomes suggest that transfer-printing-based micromanufacturing can realize not only strong mechanical bonding but also high-quality electrical contact via eutectic joining

Solvent-Free Patterning of Colloidal Quantum Dot Films Utilizing Shape Memory Polymers

Colloidal quantum dots (QDs) with properties that can be tuned by size, shape, and composition are promising for the next generation of photonic and electronic devices. However, utilization of these materials in such devices is hindered by the limited compatibility of established semiconductor processing techniques. In this context, patterning of QD films formed from colloidal solutions is a critical challenge and alternative methods are currently being developed for the broader adoption of colloidal QDs in functional devices. Here, we present a solvent-free approach to patterning QD films by utilizing a shape memory polymer (SMP). The high pull-off force of the SMP below glass transition temperature (Tg) in conjunction with the conformal contact at elevated temperatures (above Tg) enables large-area, rate-independent, fine patterning while preserving desired properties of QDs

High-reflectivity, broadband monolithic silicon photonic crystal mirrors on two-axis MEMS scanner by transfer-printing

We demonstrate a two-axis electrostatic MEMS scanner integrated with high-reflectivity monolithic silicon photonic crystal (PC) mirrors by transfer printing. The PC mirrors show low polarization dependence and reflectivity over 85% in the wavelength range of 1490nm∼1505nm and above 90% over the wavelength band of 1550∼1570nm. The integration of nanophotonic devices on a MEMS platform with transfer printing enables novel devices with more flexible design and new functionality. © 2013 IEEE.1

A Simple Fabrication Process Based on Micro-masonry for the Realization of Nanoplate Resonators with Integrated Actuation and Detection Schemes

International audienceIn this work, we use the micro-masonry technique to fabricate nanoplate resonators with integrated electrostatic actuation and capacitive detection in a few steps. Our approach is an alternative solution to the current fabrication methods used to create membranes and plates that usually rely on the selective etching of a sacrificial layer. Highly doped silicon plates were transfer printed using microtip elastomeric stamps onto insulated bases displaying cavities in order to form suspended structures with airtight gaps. By post-processing adequate interconnections, the fabricated resonators were actuated and their resonant frequency measured in a fully integrated manner. The tested nanoplate devices behave as predicted by theory and offer quality factors of more than 30 in air

Micro-masonry for the simple fabrication of nanoplate resonators with integrated electrostatic transduction transduction

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