Two-step Lithography Process Research Articles

Off-Stoichiometry Thiol-Ene (OSTE) Micro Mushroom Forest: A Superhydrophobic Substrate.

Superhydrophobic surfaces have been used in various fields of engineering due to their resistance to corrosion and fouling and their ability to control fluid movement. Traditionally, superhydrophobic surfaces are fabricated via chemical methods of changing the surface energy or mechanical methods of controlling the surface topology. Many of the conventional mechanical methods use a top-to-bottom scheme to control the surface topolopy. Here, we develop a novel fabrication method of superhydrophobic substrates using a bottom-to-top scheme via polymer OSTE, which is a prototyping polymer material developed for the fabrication of microchips due to its superior photocuring ability, mechanical properties, and surface modification ability. We fabricate a superhydrophobic substrate by OSTE-OSTE micro mushroom forest via a two-step lithography process. At first, we fabricate an OSTE pillar forest as the mushroom stems; then, we fabricate the mushroom heads via backside lithography with diffused UV light. Such topology and surface properties of OSTE render these structures superhydrophobic, with water droplets reaching a contact angle of 152.9 ± 0.2°, a sliding angle of 4.1°, and a contact angle hysteresis of less than 0.5°. These characteristics indicate the promising potential of this substrate for superhydrophobic applications.

Development of a simple two-step lithography fabrication process for resonant tunneling diode using air-bridge technology

This article reports on the development of a simple two-step lithography process for double barrier quantum well (DBQW) InGaAs/AlAs resonant tunneling diode (RTD) on a semi-insulating indium phosphide (InP) substrate using an air-bridge technology. This approach minimizes processing steps, and therefore the processing time as well as the required resources. It is particularly suited for material qualification of new epitaxial layer designs. A DC performance comparison between the proposed process and the conventional process shows approximately the same results. We expect that this novel technique will aid in the recent and continuing rapid advances in RTD technology.

Expanding chiral metamaterials for retrieving fingerprints via vibrational circular dichroism

Circular dichroism (CD) spectroscopy has been widely demonstrated for detecting chiral molecules. However, the determination of chiral mixtures with various concentrations and enantiomeric ratios can be a challenging task. To solve this problem, we report an enhanced vibrational circular dichroism (VCD) sensing platform based on plasmonic chiral metamaterials, which presents a 6-magnitude signal enhancement with a selectivity of chiral molecules. Guided by coupled-mode theory, we leverage both in-plane and out-of-plane symmetry-breaking structures for chiral metamaterial design enabled by a two-step lithography process, which increases the near-field coupling strengths and varies the ratio between absorption and radiation loss, resulting in improved chiral light-matter interaction and enhanced molecular VCD signals. Besides, we demonstrate the thin-film sensing process of BSA and β-lactoglobulin proteins, which contain secondary structures α-helix and β-sheet and achieve a limit of detection down to zeptomole level. Furthermore, we also, for the first time, explore the potential of enhanced VCD spectroscopy by demonstrating a selective sensing process of chiral mixtures, where the mixing ratio can be successfully differentiated with our proposed chiral metamaterials. Our findings improve the sensing signal of molecules and expand the extractable information, paving the way toward label-free, compact, small-volume chiral molecule detection for stereochemical and clinical diagnosis applications.

Graphene-on-plasmonic slot waveguide photodetector

Integrated silicon-based plasmonic photodetectors have attracted more and more attention recently, due to their potential applications in THz bandwidth. In this paper, by utilizing a simplified two-step lithography process, the photoconductive detector is fabricated with the core structure of plasmonic slot waveguide in two symmetric metallic slabs as microwave electrodes to integrate with mechanically exfoliated graphene. The 3-dB bandwidth of the graphene-on-plasmonic slot waveguide photodetector is exceeded 120 GHz under the theoretical calculation, and the measured bandwidth is still beyond 70 GHz due to the lack of larger-bandwidth equipment. High-speed data reception is demonstrated for 72 Gbit/s NRZ and 64 Gbit/s PAM-4 signals with a BER below 15% soft-decision FEC threshold. Based on the enhanced interactions of light and graphene induced by the subwavelength confinement from the plasmonic structure, the responsivity of 0.13 A/W is achieved under the condition of the 7-μm long detection zone and the 0.4-V bias voltage. With the advantages of ultra-compact, high-speed, large-bandwidth and compatible with the CMOS process, our photodetector has the potential to be used in high-speed optical interconnection, terahertz transmission and terahertz communication.

Temperature and Size Effect on the Electrical Properties of Monolayer Graphene based Interconnects for Next Generation MQCA based Nanoelectronics

Graphene interconnects have been projected to out-perform Copper interconnects in the next generation Magnetic Quantum-dot Cellular Automata (MQCA) based nano-electronic applications. In this paper a simple two-step lithography process for patterning CVD monolayer graphene on SiO2/Si substrate has been used that resulted in the current density of one order higher magnitude as compared to the state-of-the-art graphene-based interconnects. Electrical performances of the fabricated graphene interconnects were evaluated, and the impact of temperature and size on the current density and reliability was investigated. The maximum current density of 1.18 ×108 A/cm2 was observed for 0.3 μm graphene interconnect on SiO2/Si substrate, which is about two orders and one order higher than that of conventionally used copper interconnects and CVD grown graphene respectively, thus demonstrating huge potential in outperforming copper wires for on-chip clocking. The drop in current at 473 K as compared to room temperature was found to be nearly 30%, indicating a positive temperature coefficient of resistivity (TCR). TCR for all cases were studied and it was found that with decrease in width, the sensitivity of temperature also reduces. The effect of resistivity on the breakdown current density was analysed on the experimental data using Matlab and found to follow the power-law equations. The breakdown current density was found to have a reciprocal relationship to graphene interconnect resistivity suggesting Joule heating as the likely mechanism of breakdown.

Microfluidic to Nanofluidic Interface via a Thermally Decomposable Sacrificial Polymer

We present an original batch process to connect a bulk microfluidic channel to a surface nanofluidic channel for a wide variety of microfluidic and lab-on-chip applications. The approach relies on a two-step lithography process using a thermally decomposable polynorbornene resist unity to form the sacrificial channel layer. A high aspect ratio silicon via is deep reactive ion etched to the subsurface microchannel. Lithography of a high viscosity unity plug is followed by lithography of a low viscosity unity sacrificial layer to form the nanofluidic channel. A plasma-enhanced chemical vapor deposition nitride layer is deposited to encapsulate the nanochannel and the sacrificial polymer is decomposed by baking. This technique enables fabrication of complete off-chip to nanochannel fluidic systems. We emphasize the benefit of sacrificial polymers to form long 3-D nanofluidic channels. [2014-0288]

In situ electron-beam lithography of deterministic single-quantum-dot mesa-structures using low-temperature cathodoluminescence spectroscopy

We report on the deterministic fabrication of sub-μm mesa-structures containing single quantum dots (QDs) by in situ electron-beam lithography. The fabrication method is based on a two-step lithography process: After detecting the position and spectral features of single InGaAs QDs by cathodoluminescence (CL) spectroscopy, circular sub-μm mesa-structures are defined by high-resolution electron-beam lithography and subsequent etching. Micro-photoluminescence spectroscopy demonstrates the high optical quality of the single-QD mesa-structures with emission linewidths below 15 μeV and g(2)(0) = 0.04. Our lithography method has an alignment precision better than 100 nm which paves the way for a fully deterministic device technology using in situ CL lithography.

High-resolution laser lithography system based on two-dimensional acousto-optic deflection

We present an advanced high-resolution, compact laser lithography system for fast prototyping of complex integrated optics devices comprising microring resonators and photonic crystal structures. Precise and flexible structuring of photoresist patterns is achieved by combing three linear stages (xyz) for sample positioning and a two-dimensional acousto-optical deflector for laser beam steering and intensity control. A continuous wave diode laser operating at a wavelength of 375 nm is used to illuminate all types of photoresists including SU-8. Using a microscope objective with a numerical aperture of 1.40, structure widths of approximately 200 nm can be obtained. The write-field covered by acousto-optic deflection can be as large as 200x200 microm(2) when using an objective with a focal length of 4.5 mm. With a two-step lithography process, gaps as small as 150 nm between adjacent structures have been achieved, yielding superior photoresist masks for microring resonators with coupling ports.

Fabrication of ultra-short T gates by a two-step electron beam lithography process

This paper proposes a two-step electron beam lithography process, i.e., exposing feet and heads in two separate steps for ultra-short T gates, using a PMMA/UNIII resist stack separated by a thin lift off resist (LOR) layer. Results from both experiments and Monte Carlo simulations show that the two-step lithography process has advantages over the traditional one-step process of being able to pattern shorter feet with a much thicker top layer. The optimum thickness of the LOR for patterning T shape profiles in the PMMA/UVIII resist stack has been found to be about 20 nm.

High mobility electron gases and modulation-doped field effect transistors fabricated in Si/Si1−xGex by rapid thermal chemical vapor deposition

High mobility n-type Si0.62Ge0.38/strained Si modulation-doped structures have been fabricated on relaxed graded Si1−xGex buffers by rapid thermal chemical vapor deposition. The mobility and density of the two-dimensional electron gas was studied as a function of spacer thickness of the modulation-doped structure. Mobilities exceeding 40 000 cm2/V s at a density of 1.7×1012 cm−2 (10 K) and 15 000 cm2/V s at a density of 2.9×1012 cm−2 (77 K) were observed, giving a record low sheet resistivity of 140 Ω/⧠ at 77 K. Self-aligned depletion and enhancement mode modulation-doped field effect transistors were fabricated in similar structures using a novel two-step lithography process. Good saturation characteristics with low gate leakage currents were achieved, but the maximum room-temperature transconductance was limited by parasitic source and drain resistances.