Owen Hildreth

Effect of catalyst shape and etchant composition on etching direction in metal-assisted chemical etching of silicon to fabricate 3D nanostructures.

Metal-assisted chemical etching (MaCE) of silicon in conjunction with shaped catalysts was used to fabricate 3D nanostructures such as sloping channels, cycloids, and spirals along with traditional vertical channels. The investigation used silver nanorods, nanodonuts along with electron beam lithography (EBL)-patterned gold nanodiscs, nanolines, squares, grids, and star-shaped catalysts to show how catalyst shape and line width directly influence etching direction. Feature sizes ranging from micrometers down to 25 nm were achieved with aspect ratios of at least 10:1 and wall roughness of 10 nm or less. This research demonstrates the potential of MaCE as a new, maskless nanofabrication technology.

Guided Three-Dimensional Catalyst Folding during Metal-Assisted Chemical Etching of Silicon

In recent years metal-assisted chemical etching (MaCE) of silicon, in which etching is confined to a small region surrounding metal catalyst templates, has emerged as a promising low cost alternative to commonly used three-dimensional (3D) fabrication techniques. We report a new methodology for controllable folding of 2D metal catalyst films into 3D structures using MaCE. This method takes advantage of selective patterning of the catalyst layer into regions with mismatched characteristic dimensions, resulting in uneven etching rates along the notched boundary lines that produce hinged 2D templates for 3D folding. We explore the dynamics of the folding process of the hinged templates, demonstrating that the folding action combines rotational and translational motion of the catalyst template, which yields topologically complex 3D nanostructures with intimately integrated metal and silicon features.

3D Spirals with Controlled Chirality Fabricated Using Metal-Assisted Chemical Etching of Silicon

The ability to fabricate 3D spiraling structures using metal-assisted chemical etching (MaCE) is one of the unique advantages of MaCE over traditional etching methods. However, control over the chirality of the spiraling structures has not been established. In this work, a systematic parametric study was undertaken for MaCE of star-shaped catalysts, examining the influence of arm shape, arm length, number of arms, center core diameter, and catalyst thickness on the rotation direction. This data was used to identify a set of geometric parameters that reliably induce rotation in a predefined direction such that large arrays of 3D spiraling structures can be fabricated with the same chirality. Electroless deposition into the MaCE template was used to examine the full etch path of the catalyst and an experimental fit was established to control rotation angle by adjusting the catalysts center core diameter. The ability to fabricate large arrays of 3D spiraling structures with predefined chirality could have important applications in photonics and optoelectronics.

Directed 2D‐to‐3D Pattern Transfer Method for Controlled Fabrication of Topologically Complex 3D Features in Silicon

A process that allows control over the 3D motion of catalyst nanostructures during metal-assisted chemical etching by their local pinning prior to etching is demonstrated. The pinning material acts as a fulcrum for rotation of the catalyst structures resulting in etching of silicon features with rotational geometry.

Printing Stretchable Spiral Interconnects Using Reactive Ink Chemistries

Stretchable electronics have important applications in health monitoring and integrated lab-on-a-chip devices. This paper discusses the performance of serpentine stretchable interconnects printed using self-reducing, silver reactive inks. It details process optimization, device fabrication, and device characterization, while demonstrating the potential applications for reactive inks and new design strategies in stretchable electronics. Devices were printed with an ethanol stabilized silver diamine reactive ink and cycled to stretch ratios of 140 and 160% over 1000 cycles with less than 2.5% variation in electrical resistance. Maximum deformation before failure was measured at 180% elongation. Additionally, interconnect deformation was compared to finite element analysis (FEA) simulations to show that FEA can be used to accurately model the deformation of low-strain printed interconnects. Overall, this paper demonstrates a simple and affordable route toward stretchable electrical interconnects.

Participation of focused ion beam implanted gallium ions in metal-assisted chemical etching of silicon

Metal-assisted chemical etching (MaCE) of silicon has proven to be a fast and effective method to fabricate 1D, 2D, and 3D micro- to nano-scale features in silicon. It has been shown that platinum catalysts deposited using focused ion beam (FIB) are a viable catalyst for MaCE; however, the feature fidelity of channels etched with FIB patterned catalysts are found to be significantly lower than catalysts formed using e-beam lithography. In this work we show that gallium (Ga+) ions implanted into the silicon during sample exposure result in significant etching in the irradiated regions as well as long-distance etching peripheral regions. The accelerating voltage, dose, and etching time were varied to show that the etch depth depends primarily on accelerating voltage and is largely independent of dose while the width of the peripheral region was found to scale with dose. The slope of the peripheral etching region was found to vary with both accelerating voltage and dose with three different etching times eva...

Wet chemical method to etch sophisticated nanostructures into silicon wafers using sub-25nm feature sizes and high aspect ratios

There are a number of emerging technologies such as metamaterials, photonic wave-guides, nano-imprint lithography (NIL), field emission devices and through silicon via (TSV), that require high resolution and high aspect ratio nanofabrication techniques for good performance. Unfortunately, current nanofabrication techniques, including photolithography and e-beam lithography, are limited to low aspect ratios on the order of 7:1 and cannot fabricate the high aspect ratio nanostructures needed for these emerging nanotechnologies. Deep reactive ion etching has traditionally been used to increase the aspect ratio nanostructures produced from traditional lithography techniques; however, the process is expensive, time consuming and cannot produce smooth sidewalls, lowering device performance. To overcome these obstacles our group has developed a new wet chemical nanofabrication technique that uses shaped catalysts to etch high aspect ratio nanostructures into silicon. The process is fast, does not require expensive equipment and has been used to produce features less than 25 nm wide, 25 µms long and microns deep in silicon using nanorod catalysts. 10 nm wide features were also fabricated using nano-donuts. This new, patented technique is compatible with existing silicon fabrication technologies and could be used for a wide variety of applications that require nanometer sized features and high aspect ratios.

Low-Temperature Drop-on-Demand Reactive Silver Inks for Solar Cell Front-Grid Metallization

Formation of high-conductivity metal contacts at low temperatures expands optoelectronic device opportunities to include thermally sensitive layers, while reducing expended thermal budget for fabrication. This includes high-efficiency silicon heterojunction solar cells with intrinsic amorphous silicon layers. Efficiencies of these cells are limited by series resistance; the primary cause of this is the relatively high resistivity of the low-temperature silver paste used to form front-grid metallization. In this paper, we report the formation of highly conductive features by drop-on-demand printing of reactive silver ink (RSI) at a low temperature of 78 °C, resulting in media resistivities of 3-5 μΩ·cm. When used as a front grid on a silicon heterojunction solar cell, RSI fingers give cell series resistance of 1.8 Ω·cm2 (without optimization of the process), which is impressively close to 1.1 Ω·cm2 for our commercially available screen-printed low-temperature silver paste metallization. We present here the promising first results of RSI as metallic finger for photovoltaics, which upon optimization of design parameters has the potential to outperform the screen-printed low-temperature silver paste counterpart.

Conformally coating vertically aligned carbon nanotube arrays using thermal decomposition of iron pentacarbonyl

Conformally coating vertically aligned carbon nanotubes (v-CNT) with metals or oxides can be difficult because standard line-of-sight deposition methods, such as dc sputter coating and electron-beam evaporation, are hindered by the low mean-free-path within the vertically aligned array. In this work, we present a facile method to conformally coat dense arrays of v-CNTs using thermal decomposition of iron pentacarbonyl at 205 °C and 30 mTorr. The resulting coatings were found to be uniform from top-to-bottom across an entire 1 × 1 cm2 array of v-CNTs. The thickness of the deposited coating was found to be 2–3 nm/cycle and the resulting film thickness were found to be 13 ± 3 nm after five cycles and 55 ± 5 nm after 20 cycles. This process demonstrates that metal organic chemical vapor deposition can be used to fabricate conformal coatings on v-CNTs.

A percolative approach to investigate electromigration failure in printed Ag structures

The ease of fabrication and wide application of printed microelectronics are driving advances in reactive inks. The long-term performance of structures printed using reactive ink is important for their application in microelectronics. In this study, silver lines are printed with low-temperature, self-reducing, silver-diamine based ink. The electromigration failure of the printed silver is first studied using Blacks equation. However, due to the porous nature of the printed Ag line, Blacks equation is not the best fit for predicting the lifetime, this is because Blacks equation does not take into account morphology-induced current crowding. We find that the resistivity of the printed Ag lines can be described (as a function of void fraction) by percolation theory. In addition, we also demonstrate that the failure lifetimes of the printed Ag can be predicted quite well by a percolative model of failure.