Ultrahigh Resolution Electron Beam Lithography Research Articles

Surface-enhanced Raman spectroscopy detection of protein-ligand binding using D-glucose and glucose binding protein on nanostructured plasmonic substrates

Conjugated nano-biological architectures interfacing solid nano-structured surfaces with biological polymers have gained significant attention due to their potential biosensing and biocatalytic applications. However, efficient characterization of such integrated systems remains a challenge. We describe surface enhanced Raman spectroscopy (SERS) detection of complex of D-glucose with glucose binding protein (GBP) immobilized on substrates. Substrates comprised of dense Ag nanostructure arrays on Ni-coated fused silica wafers were fabricated employing ultrahigh resolution electron beam lithography. Glucose-bound and glucose-free histidine-tagged GBP was immobilized on the substrates and probed using SERS while the samples were kept in solution, and the observed Raman spectra were recorded. Three substrate designs were tested for SERS detection of the protein-ligand binding. SERS spectra of immobilized glucose-free and glucose-bound GBP exhibited pronounced differences in their Raman signatures, demonstrating the potential of SERS as a sensitive method for the detection of protein-ligand molecular recognition on a solid surface. However, morphology of the nano-patterned plasmonic structures was found to influence the SERS signatures significantly. In order to interpret the findings, simulations of electric field around the nano-structured substrates were performed. An interplay of two factors, the availability of space between Ag features where the GBP could bind to Ni, and the effectiveness of the electromagnetic enhancement of the Raman scattering in “hot spots” between these features, was concluded to determine the observed trends.

Self-assembled monolayers of poly(ethylene glycol) siloxane as a resist for ultrahigh-resolution electron beam lithography on silicon oxide

Self-assembled monolayers of 2-[methoxypoly(ethyleneoxy)propyl]-trimethoxysilane (Si-PEG) reduce the nonspecific binding between silicon oxide surfaces and a variety of biomolecules. The film can be patterned by electron beam lithography at 30nm resolution. Optimal electron beam lithography exposure conditions are 4nC∕cm at 75keV. Exposed regions of the PEG film become negatively charged and less resistant to biomolecule binding, which leads to selective adhesion of biomolecules. The patterned film acts as a template for biomolecule attachment, while the intact PEG background is strongly resistant to nonspecific binding. Binding selectivities of up to 26:1 were observed for patterning cowpea mosaic virus, Salmonella phage P22 tailspike protein and poly(lysine) at 30nm linewidths.

Influence of hydrogen silsesquioxane resist exposure temperature on ultrahigh resolution electron beam lithography

Performance of hydrogen silsesquioxane (HSQ) resist material with respect to the temperature during electron beam exposure was investigated. Electron beam exposure at elevated temperatures up to 90 °C shows sensitivity rise and slight contrast (γ) degradation compared to lower temperature cases. Ultrahigh resolution structures formed at elevated temperatures manifest better uniformity together with aspect ratio improvement and less linewidth broadening with overdose. Potential mechanisms for observed phenomena are proposed.

Clusters of interacting single domain Co nanomagnets for multistate perpendicular magnetic media applications

In this work we develop prototype elements for multistate (beyond binary) perpendicular data storage using interacting nanomagnet clusters. This experimental work confirms earlier theoretical work that predicted multiple discrete values of stable remanent magnetization for such clusters. The fabrication scheme is based on ultrahigh resolution electron beam lithography performed on a thin suspended silicon nitride membrane to reduce the secondary backscattered electrons from the substrate. A Co nanomagnet cluster array is deposited into the nanotemplate via pulse-reverse electrodeposition to create nanomagnets with the favored uniaxial perpendicular anisotropy. Magnetic force microscopy (MFM) measurements show the perpendicular magnetization of individual Co nanomagnets and the combined multiconfiguration behavior of a nanomagnet cluster. In concept, the discrete values of net remanent magnetization of the cluster, which represent distinct information states, can be “programmed” by a uniform applied field.

Direct imprint of sub-10nm features into metal using diamond and SiC stamps

We demonstrate the transfer of sub-10nm features into nickel using a hard stamp. Nanostructures were transferred directly from diamond and SiC in a single step by pressing the stamp into nickel at room temperature. The patterns were generated using ultrahigh resolution electron beam lithography. Patterns were transferred to the diamond and SiC using RIE etching with an O2 plasma used for the diamond and a SF6+O2 mixture used for the SiC. Hydrogen Silsesquioxane was used as a resist and served as a mask in the plasma etching.

Nanolithography using ultrasonically assisted development of calixarene negative electron beam resist

Negative resist image distortion is caused by resist swelling during development and is a major problem in ultrahigh resolution electron beam lithography. This problem has been overcome through the use of ultrasonically assisted development. In addition, exposure dose latitude is increased by 50% compared to conventional dip development, due to the improvement in contrast. These advantages have been exploited in order to realize ∼6 nm wide lines in calixarene resist.

A study of the effect of ultrasonic agitation during development of poly(methylmethacrylate) for ultrahigh resolution electron-beam lithography

Experiments on poly(methylmethacrylate) electron resist have been performed in order to understand how ultrasonic agitation during development affects sensitivity, contrast, and resolution. A modified JEOL 4000 EX electron microscope with a beam diameter of 1 nm was used for writing the resist patterns. First, the development/exposure characteristics of the ultrasonic development process were measured using large area (2×2 μm) patterns. With a 5 s develop in a mix of 3:7 cellosolve:methanol, it was found that ultrasonic agitation increases the effective sensitivity of the resist by roughly 20% but resist contrast does not change significantly (5.7±0.5). It was also found that the sensitivity increases approximately linearly with accelerating voltage from 75 to 400 kV but contrast does not change with voltage significantly. Resolution was explored by writing fine lines at progressively increasing doses in 60 nm of poly(methylmethacrylate) and examining these lines with high resolution scanning electron microscopy. Complete development to the substrate was verified by wet etching a thin chromium layer underneath the resist. The narrowest lines that developed to the substrate had a width of about 10 nm although at this width considerable ‘‘bridging’’ of the lines was observed. We were unable with the ultrasonic agitation conditions used in our experiments to reduce the degree of bridging or the minimum linewidth.

RIE of sub-50 nm high aspect-ratio pillars, ridges, and trenches in silicon and silicon-germanium

Sub-50 nm high aspect-ratio pillars, ridges, and trenches have been patterned in Si and SiGe using reactive ion etching (RIE) with Cr masks defined by ultra-high resolution electron beam lithography and a lift-off process. Using an optimized mixture of Cl 2 and SiCl 4 gases, sub-50 nm features with aspect ratios greater than 10 were readily and consistently achieved. For achieving similar nanostructures in SiGe, an identical gas mixture at lower pressures is required.

Sub-50 nm high aspect-ratio silicon pillars, ridges, and trenches fabricated using ultrahigh resolution electron beam lithography and reactive ion etching

We present the fabrication of sub-50 nm Si pillars, ridges, and trenches with aspect ratios greater than 10 using ultrahigh resolution electron beam lithography and chlorine based reactive ion etching. These nanoscale Si features can be further reduced to 10 nm using an additional HF wet etch. No photoluminescence was observed from arrays of 10 nm Si structures passivated with HF.

An error measure for dose correction in e-beam nanolithography

In this paper we address the problem of dose correction in ultrahigh resolution electron beam lithography. Here, the emphasis is on providing precise resist development profiles. This contrasts with the case of standard near-micron e-beam lithography in which computational efficiency and an ability to handle large data files are the major goals. The approach employed here is one of non-linear optimization. Rather than using the conventional quadratic cost function for dose optimization, the specifics of the resist development curve (i.e., saturation behavior, minimum critical dose, and gamma) are incorporated as inequalities. Results (both experimental and theoretical) are given for half-micron lines. Preliminary results on subhalf-micron lines are also given.

Novel properties of a 0.1- mu m-long split-gate MODFET

A MODFET with two 30-nm-long gates (separated by 40 nm) has been fabricated using ultrahigh-resolution electron-beam lithography. The proximity of the two gate fingers along with the ability to independently bias them results in the following features: (a) tunability of the threshold voltage, (b) enhancement of the transconductance, especially at low current levels, (c) reduction in short-channel effects, and (d) high-voltage gain and cutoff frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Observation of electron resonant tunneling in a lateral dual-gate resonant tunneling field-effect transistor

A new lateral resonant tunneling field-effect transistor (LARTFET) has been fabricated using molecular beam epitaxy and ultrahigh-resolution electron beam lithography. The LARTFET has two 80-nm-long gate electrodes separated by 100 nm. The dual gates create double potential barriers in the channel and a quantum well in between. Conductance oscillations are observed, which, for the first time, indicate electron resonant tunneling through the energy states in a lateral double-barrier quantum well formed electrostatically. Furthermore, after illumination, two additional negative transconductance peaks are observed. These additional peaks may be related to electron resonant tunneling through the donor-related deep levels in silicon-doped Al0.35Ga0.65As .

Lithography issues in fabricating high-performance sub-100-nm channel metal–oxide semiconductor field effect transistors

Specific issues related to achieving high device performance in extremely small channel length field effect transistor circuits are discussed. Ultrahigh resolution electron beam lithography is needed to obtain the ultrashort channels which are key for fast devices, and for the important gate area, double‐layer poly(methylmethacrylate) for lift‐off of a metal reactive ion etching mask has proven to be a suitable technique. Good level to level overlay and dimensional control in the contact process allow miniaturization of all device elements, in particular, reduction of the distance between source/drain contacts and optimization of the contact size and geometry. These are key factors in minimizing parasitic effects. The design considerations are discussed and the fabrication techniques are described which resulted in devices with a maximum measured transconductance of 910 mS/mm at 77 K for a 70‐nm gate device, exhibiting the clearest evidence so far for electron velocity overshoot, and unloaded ring oscillators with 13‐ps switching time per stage.

Ultra-high resolution electron-beam lithography with a research scanning electron microscope

An electron microscope of the type normally used for analysis has been employed as an experimental beam writing instrument. With this instrument less than 10 nm size structures have been fabricated in a variety of materials by electron beam lithography. The important parameters that determine the resolution limit are: electron beam probe size, electron beam energy, characteristics of the beam sensitive material and the nature of the substrate. These parameters were studied with a Philips 400 T TEM/STEM/SEM modified for external beam control and interfaced to a DEC MINC 11-23 computer for pattern generation. With this microscope's ability to image thick samples by secondary and backscattered electron detection conventional type lithography was done on semiconductor substrates. The general lithographic technique is described, for example, in reference 2. In addition fabrication techniques on thin films were studied. The effect of the resolution determining parameters and several examples of lithographic results are described in this paper.