Cryogenic deep reactive ion etching of silicon micro and nanostructures

This thesis focuses on cryogenic deep reactive ion etching (DRIE) and presents how it can be applied to the fabrication of silicon micro- and nanostructures that have applications in microfluidics and micromechanics. The cryogenic DRIE process relies on inductively coupled SF6/O2 plasma at temperatures below -100 °C. Low etching temperatures can cause some photoresist materials to crack, but Al2O3 has been shown to be a very well-suited masking material for cryogenic etching. The anisotropy of the etching process is enhanced by a thin passivation layer on sidewalls that prevents lateral etching. The main parameters that are used to adjust the thickness of the passivation layer are the process temperature and the O2 flow. Under adequate conditions vertical sidewalls are obtained, whereas passivation layers that are too thin result in negatively tapered sidewall slopes. Under conditions where a passivation layer is not formed, at higher temperatures and/or without oxygen flow, the etching profiles are isotropic. On the other hand, too high oxygen flow results in over passivation. Under conditions where the sidewall is slightly over passivated, its slopes are positively tapered, while more pronounced over passivation results in the formation of black silicon (or silicon nanograss, silicon nanoturf or columnar microstructures). Typically, vertical sidewall profiles are desirable. However, this thesis shall also demonstrate the usefulness of under and over passivation regimes. Here, highly anisotropic etching conditions are utilized to create trenches with vertical sidewalls, fluidic channels with regular micropillar arrays, and high aspect ratio silicon nanopillars. An isotropic etching process is utilized during the release of aluminum heaters fabricated on top of perforated free-standing Al2O3 membranes and silicon dioxide coated thermal silicon actuators. The fabrication process of three-dimensional sharp electrospray ionization (ESI) tips takes advantage of etching conditions that result in negatively tapered sidewalls. A self-feeding ESI interface for mass spectrometry (MS) is fabricated by combining a lidless micropillar filled channel with a sharp tip. Two approaches to the fabrication of silicon nanopillars are presented, both of which are suitable for wafer-scale manufacturing. One method combines silica nanoparticles with a highly anisotropic DRIE step, while the other method relies on highly over passivating conditions in a maskless DRIE process. Due to a large surface area and efficient light absorption in UV-range, silicon nanopillar structured surfaces are utilized as sample plates in laser desorption/ionization (LDI) MS. The wetting of nanopillar structured silicon surfaces is also studied. Fluoropolymer coated nanopillar structured surfaces have a contact angle of more than 170° and are ultrahydrophobic, whereas oxidized nanostructured surfaces are completely wetting. The accurate patterning of both completely wetting and ultrahydrophobic areas side by side allows complex droplet shapes and droplet splitters to be tailored

Electronic cooling of a submicron-sized metallic beam

We demonstrate electronic cooling of a suspended AuPd island using superconductor-insulator-normal metal tunnel junctions. This was achieved by developing a simple fabrication method for reliably releasing narrow submicron sized metal beams. The process is based on reactive ion etching and uses a conducting substrate to avoid charge-up damage and is compatible with e.g. conventional e-beam lithography, shadow-angle metal deposition and oxide tunnel junctions. The devices function well and exhibit clear cooling; up to factor of two at sub-kelvin temperatures.Comment: 4 pages, 3 figure

Surface Assisted Laser Desorption/Ionization on Two-Layered Amorphous Silicon Coated Hybrid Nanostructures

Matrix-free laser desorption/ionization was studied on two-layered sample plates consisting of a substrate and a thin film coating. The effect of the substrate material was studied by depositing thin films of amorphous silicon on top of silicon, silica, polymeric photoresist SU-8, and an inorganic-organic hybrid. Des-arg9-bradykinin signal intensity was used to evaluate the sample plates. Silica and hybrid substrates were found to give superior signals compared with silicon and SU-8 because of thermal insulation and compatibility with amorphous silicon deposition process. The effect of surface topography was studied by growing amorphous silicon on hybrid micro- and nanostructures, as well as planar hybrid. Compared with planar sample plates, micro- and nanostructures gave weaker and stronger signals, respectively. Different coating materials were tested by growing different thin film coatings on the same substrate. Good signals were obtained from titania and amorphous silicon coated sample plates, but not from alumina coated, silicon nitride coated, or uncoated sample plates. Overall, the strongest signals were obtained from oxygen plasma treated and amorphous silicon coated inorganic-organic hybrid, which was tested for peptide-, protein-, and drug molecule analysis. Peptides and drugs were analyzed with little interference at low masses, subfemtomole detection levels were achieved for des-arg9-bradykinin, and the sample plates were also suitable for ionization of small proteins

Plasma etch characteristics of aluminum nitride mask layers grown by low-temperature plasma enhanced atomic layer deposition in SF6 based plasmas

The plasmaetch characteristics of aluminum nitride(AlN)deposited by low-temperature, 200 °C, plasma enhanced atomic layer deposition (PEALD) was investigated for reactive ion etch (RIE) and inductively coupled plasma-reactive ion etch (ICP-RIE) systems using various mixtures of SF6 and O2 under different etch conditions. During RIE, the film exhibits good mask properties with etch rates below 10r nm/min. For ICP-RIE processes, the film exhibits exceptionally low etch rates in the subnanometer region with lower platen power. The AlN film’s removal occurred through physical mechanisms; consequently, rf power and chamber pressure were the most significant parameters in PEALD AlN film removal because the film was inert to the SF+x and O+ chemistries. The etch experiments showed the film to be a resilient masking material. This makes it an attractive candidate for use as an etch mask in demanding SF6 based plasmaetch applications, such as through-wafer etching, or when oxide films are not suitable.Peer reviewe

Effective Passivation of Black Silicon Surfaces by Atomic Layer Deposition

The poor charge-carrier transport properties attributed to nanostructured surfaces have been so far more detrimental for final device operation than the gain obtained from the reduced reflectance. Here, we demonstrate results that simultaneously show a huge improvement in the light absorption and in the surface passivation by applying atomic layer coating on highly absorbing silicon nanostructures. The results advance the development of photovoltaic applications, including high-efficiency solar cells or any devices, that require high-sensitivity light response.Peer reviewe

Advances in Microfluidics and Lab-on-a-Chip Technologies

Advances in molecular biology are enabling rapid and efficient analyses for effective intervention in domains such as biology research, infectious disease management, food safety, and biodefense. The emergence of microfluidics and nanotechnologies has enabled both new capabilities and instrument sizes practical for point-of-care. It has also introduced new functionality, enhanced sensitivity, and reduced the time and cost involved in conventional molecular diagnostic techniques. This chapter reviews the application of microfluidics for molecular diagnostics methods such as nucleic acid amplification, next-generation sequencing, high resolution melting analysis, cytogenetics, protein detection and analysis, and cell sorting. We also review microfluidic sample preparation platforms applied to molecular diagnostics and targeted to sample-in, answer-out capabilities