Electron Beam Lithography Patterning Research Articles

Diffractive optics for combined spatial- and mode- division demultiplexing of optical vortices: design, fabrication and optical characterization.

During the last decade, the orbital angular momentum (OAM) of light has attracted growing interest as a new degree of freedom for signal channel multiplexing in order to increase the information transmission capacity in today’s optical networks. Here we present the design, fabrication and characterization of phase-only diffractive optical elements (DOE) performing mode-division (de)multiplexing (MDM) and spatial-division (de)multiplexing (SDM) at the same time. Samples have been fabricated with high-resolution electron-beam lithography patterning a polymethylmethacrylate (PMMA) resist layer spun over a glass substrate. Different DOE designs are presented for the sorting of optical vortices differing in either OAM content or beam size in the optical regime, with different steering geometries in far-field. These novel DOE designs appear promising for telecom applications both in free-space and in multi-core fibers propagation.

Using low-contrast negative-tone PMMA at cryogenic temperatures for 3D electron beam lithography

We report on a 3D electron beam lithography (EBL) technique using polymethyl methacrylate (PMMA) in the negative-tone regime as a resist. First, we briefly demonstrate 3D EBL at room temperature. Then we concentrate on cryogenic temperatures where PMMA exhibits a low contrast, which allows for straightforward patterning of 3D nano- and microstructures. However, conventional EBL patterning at cryogenic temperatures is found to cause severe damage to the microstructures. Through an extensive study of lithography parameters, exposure techniques, and processing steps we deduce a hypothesis for the cryogenic PMMA’s structural evolution under electron beam irradiation that explains the damage. In accordance with this hypothesis, a two step lithography technique involving a wide-area pre-exposure dose slightly smaller than the onset dose is applied. It enables us to demonstrate a >95% process yield for the low-temperature fabrication of 3D microstructures.

Facile fabrication of high-resolution extreme ultraviolet interference lithography grating masks using footing strategy during electron beam writing

The authors demonstrate a method for facile fabrication of transmission grating masks used in extreme ultraviolet interference lithography (EUV-IL) based on a footing strategy during electron beam writing of mask gratings. By modifying the electron beam lithography pattern and dose distribution, a thin footing is generated between the mask grating lines. This thin footing layer effectively prohibits subsequent metal deposition using a standard electroplating setup in the grating region, while outside the grating region metal is electrodeposited freely, absorbing EUV radiation and serving as an EUV-IL mask grating central photon stop layer. The strategy the authors present greatly simplifies the mask fabrication process and increases the yield, as the crucial central stop region can be fabricated directly and without further overlay exposure steps either using electron beam or photolithography.

Nanolithography by scanning probes on calixarene molecular glass resist using mix-and-match lithography

Going “beyond the CMOS information-processing era,” taking advantage of quantum effects occurring at sub-10-nm level, requires novel device concepts and associated fabrication technologies able to produce promising features at acceptable cost levels. Herein, the challenge affecting the lithographic technologies comprises the marriage of down-scaling the device-relevant feature size towards single-nanometer resolution with a simultaneous increase of the throughput capabilities. Mix-and-match lithographic strategies are one promising path to break through this trade-off. Proof-of-concept combining electron beam lithography (EBL) with the outstanding capabilities of closed-loop electric field current-controlled scanning probe nanolithography (SPL) is demonstrated. This combination, whereby also extreme ultraviolet lithography (EUVL) is possible instead of EBL, enables more: improved patterning resolution and reproducibility in combination with excellent overlay and placement accuracy. Furthermore, the symbiosis between EBL (EUVL) and SPL expands the process window of EBL (EUVL) beyond the state of the art, allowing SPL-based pre- and post-patterning of EBL (EUVL) written features at critical dimension levels with scanning probe microscopy-based pattern overlay alignment capability. Moreover, we are able to modify the EBL (EUVL) pattern even after the development step. The ultra-high resolution mix-and-match lithography experiments are performed on the molecular glass resist calixarene using a Gaussian e-beam lithography system operating at 10 keV and a home-developed SPL setup.

Rapid and Low‐Cost Prototyping of 3D Nanostructures with Multi‐Layer Hydrogen Silsesquioxane Scaffolds

A layer-by-layer (LBL) method can generate or approximate any three-dimensional (3D) structure, and has been the approach for the manufacturing of complementary metal-oxide-semiconductor (CMOS) devices. However, its high cost precludes the fabrication of anything other than CMOS-compatible devices, and general 3D nanostructures have been difficult to prototype in academia and small businesses, due to the lack of expensive facility and state-of-the-art tools. It is proposed and demonstrated that a novel process that can rapidly fabricate high-resolution three-dimensional (3D) nanostructures at low cost, without requiring specialized equipment. An individual layer is realized through electron-beam lithography patterning of hydrogen silsesquioxane (HSQ) resist, followed by planarization via spinning SU-8 resist and etch-back. A 4-layer silicon inverse woodpile photonic crystal with a period of 650 nm and a 7-layer HSQ scaffold with a period of 300 nm are demonstrated. This process provides a versatile and accessible solution to the fabrication of highly complex 3D nanostructures.

Fabrication of Al/AlOx/Al Josephson junctions and superconducting quantum circuits by shadow evaporation and a dynamic oxidation process

Besides serving as promising candidates for realizing quantum computing, superconducting quantum circuits are one of a few macroscopic physical systems in which fundamental quantum phenomena can be directly demonstrated and tested, giving rise to a vast field of intensive research work both theoretically and experimentally. In this paper we report our work on the fabrication of superconducting quantum circuits, starting from its building blocks: Al/AlOx/Al Josephson junctions. By using electron beam lithography patterning and shadow evaporation, we have fabricated aluminum Josephson junctions with a controllable critical current density (jc) and wide range of junction sizes from 0.01 μm2 up to 1 μm2. We have carried out systematical studies on the oxidation process in fabricating Al/AlOx/Al Josephson junctions suitable for superconducting flux qubits. Furthermore, we have also fabricated superconducting quantum circuits such as superconducting flux qubits and charge-flux qubits.

PSF calibration patterns selection based on sensitivity analysis

Proximity effects in electron beam lithography impact feature dimensions, pattern fidelity and uniformity. These effects are addressed using a mathematical model representing the effect of the electron exposure and the subsequent effect of the resist. Therefore, one of the key steps of any proximity effect correction procedure is to determine properly the parameters of the model. However, the approach of extracting the parameters of a model based on measurements requires that the patterns measured are sensitive enough to the characteristics of the process that are described by the model. In this work it is presented a sensitivity analysis technique that allows the evaluation of the capability of a given test pattern to provide information over every parameter of a proposed model. Finally, a test pattern is presented in order to validate this approach. The proposed pattern is evaluated by the sensitivity analysis techniques described on this paper and then a calibration procedure is executed based on simulation results (with and without noise). Results shows accurate model calibration when the pattern set presents sensitivity to all its parameters.

Domain-wall controlled (Ga,Mn)As nanostructures for spintronic applications

Valence-band structure and magnetic properties, especially magnetic anisotropies, in the (Ga,Mn)As diluted ferromagnetic semiconductor are shortly discussed. Next, magneto-resistive, cross-like nanostructures fabricated by electron-beam lithography patterning and chemical etching from thin (Ga,Mn)As epitaxial layers are described. The nanostructures, composed of two perpendicular nanostripes crossing in the middle of their length, represent four-terminal devices, in which an electric current can be driven through any of the two nanostripes. In these devices, which make use of the patterning-induced magnetic anisotropy, a novel magneto-resistive memory effect related to a rearrangement of magnetic domain walls in the central part of the device, has been demonstrated. The effect consists in that the zero-field resistance of a nanostripe depends on the direction of previously applied magnetic field. The nanostructures can thus work as two-state devices providing basic elements of nonvolatile memory cells.

Investigation of a nanofabrication process to achieve high aspect-ratio nanostructures on a quartz substrate

This work investigates the development of a nanofabrication process to achieve high aspect-ratio nanostructures on quartz substrates using electron beam lithography (EBL) patterning and fluorinated plasma etching processes. An imaging layer of a poly(methyl methacrylate) bi-layer resist was spun coated on quartz substrate and exposed by an e-beam with the designed patterns of sub-100 nm feature sizes using a Raith-150 EBL patterning tool. Additive pattern transfer was employed by depositing a 40 nm thick Nichrome layer on the resist pattern using a metal evaporator which was later lifted off by soaking in acetone. Nichrome was employed as an etch mask and an Oxford Plasmalab 80Plus reactive ion etcher was used for the etching process. The etching process was carried out in a gas mixture of CHF3/Ar with a flow rate ratio of 50/30 sccm, pressure of 20 mTorr, radiofrequency power of 200 W and at room temperature. These etching process parameters were found to achieve a 10 nm min−1 etch rate and tall vertical side walls profile. An aspect-ratio of 10:1 was achieved on 60 nm feature size structures.

Large area single photon detectors based on parallel configuration NbN nanowires

The authors present superconducting single photon detectors (SSPDs) based on parallel nanostrips with an area up to 40 × 40 μm2. The SSPDs presented here are based on 100 nm wide ultrathin NbN nanostrips with a filling factor of 40%. The devices are fabricated by extending the standard electron beam lithography (EBL) patterning process to those densely structured large areas. By a thorough characterization it is shown that the electrical properties of the parallel SSPDs are comparable with those of smaller devices, as expected, proving in this way that the extended EBL process results in uniform nanostrips also in large area detectors. Furthermore, the estimated maximum count rate of the 40 × 40 μm2 parallel SSPDs was 33 MHz, showing that the parallel nanostrip configuration is much faster when compared with standard meandered serial SSPDs. The successful extension of parallel SSPDs to a large area coverage opens a new route to the use of such detectors also with multimode fibers.

Miniaturization of grayscale images

Printing of binary patterns onto substrates has been the strength of resist-based binary lithography that has advanced the semiconductor industry. When patterning grayscale structures however, grayscale-lithography processes that are often difficult to use are considered. Here, we describe a process using binary-lithography to create arbitrary grayscale patterns. Particularly, we demonstrate a novel algorithm for the miniaturization of grayscale images that preserves grayscale information when imaged in a scanning electron microscope (SEM). The brightness level of each pixel was adjusted by controlling the density of nanometer-scale pixel elements. Using 17 shades of gray, we demonstrate the electron-beam lithography patterning of an M.C. Escher mezzotint and the Lena image with pixel elements consisting of 10-nm-diameter nanoposts spaced by gaps as small as 10 nm. The patterned images were tens of microns in size and faithfully reproduce the original images under SEM inspection. The process described could find applications in the fabrication of deep sub-wavelength elements with gradually varying dimensions in nanophotonic devices, and in creating grayscale images as anti-counterfeit features on substrates.

Transport Comparison of Multiwall Carbon Nanotubes by Contacting Outer Shell and All Shells

Carbon nanotubes, particularly multiwall carbon nanotubes (MWCNTs) can serve as interconnects in nanoelectronic devices and integrated circuits because of their extremely large current-carrying capacity. Many experimental results about the transport properties of individual MWCNTs by contacting outer shell or all shells have been reported. In this work, a compatible method with integrated circuit manufacturing process was presented to compare the transport property of an individual multiwall carbon nanotube (MWCNT) by contacting outer shell only and all shells successively. First of the Ti/Au electrodes contacting outer shell only were fabricated onto the nanotube through the sequence of electron beam lithography (EBL) patterning, metal deposition and lift-off process. After the characterization of its transport property, focused ion beam (FIB) was used to drill holes through the same nanotube at the as-deposited electrodes. Then new contact to the holes and electrodes were made by ion-induced deposition of tungsten from W(CO)6 precursor gas. The transport results indicated that the new contact to all shells can clear up the intershell resistance and the electrical conductance of the tube can be improved about 8 times compared to that of by contacting outer shell only.

Orchestrated structure evolution: accelerating direct-write nanomanufacturing bycombining top-down patterning with bottom-up growth

Direct-write nanomanufacturing with scanning beams and probes is flexible and canproduce high quality products, but it is normally slow and expensive to rasterpoint-by-point over a pattern. We demonstrate the use of an accelerated direct-writenanomanufacturing method called ‘orchestrated structure evolution’ (OSE), where adirect-write tool patterns a small number of growth ‘seeds’ that subsequently growinto the final thin film pattern. Through control of seed size and spacing, it ispossible to vary the ratio of ‘top-down’ to ‘bottom-up’ character of the patterningprocesses, ranging from conventional top-down raster patterning to nearly purebottom-up space-filling via seed growth. Electron beam lithography (EBL) and copperelectrodeposition were used to demonstrate trade-offs between process time andproduct quality over nano- to microlength scales. OSE can reduce process times forhigh-cost EBL patterning by orders of magnitude, at the expense of longer (butinexpensive) copper electrodeposition processing times. We quantify the degradation ofpattern quality that accompanies fast OSE patterning by measuring deviationsfrom the desired patterned area and perimeter. We also show that the density ofOSE-induced grain boundaries depends upon the seed separation and size. As theseed size is reduced, the uniformity of an OSE film becomes more dependent ondetails of seed nucleation processes than normally seen for conventionally patternedfilms.

T1602-3-3 超微細塑性加工と異方性エッチングを利用した微細溝加工([T1602-3]マイクロナノ理工学:nmからmmまでの表面制御とその応用(3))

In this paper, a new nanofabrication process is proposed to fabricate nanostructures on silicon substrate. The process utilizes metal coating instead of using resist mask. The metal layer is patterned using nano plastic forming (NPF) followed by sputter etching, instead of ultraviolet lithography or electron beam lithography patterning. Then the pattern is transferred into the substrate by anisotropic chemical wet etching. Finally, the excess metal layer is removed to reveal nanostructure on the silicon substrate. The proposed process was experimentally studied and arrays of nano trenchs narrower than 20 nm in width were fabricated on silicon (110) wafer.

Batch fabrication of cantilever array aperture probes for scanning near-field optical microscopy

We have developed a novel batch fabrication process for cantilever array aperture probes used in scanning near-field optical microscopy (SNOM). The array probes, consisting of 16 parallel cantilevers with each tip having an identical aperture, are proposed to dramatically reduce the scanning time for imaging or increase the throughput of near-field optical microscopy and lithography. In this study, apertures are defined by direct-write electron beam lithography and subsequent pattern transfer by reactive ion etch. Better quality and reproducibility of apertures are demonstrated through optical throughput measurement of the probes. The fabrication of apertures having complex shapes is also demonstrated. The process includes such novel elements as the use of local oxidation of silicon and the use of an aluminium – compatible release etch. The process has demonstrated very high yield with a measured reproducibility of aperture diameter of 5 nm. Thermal oxide cantilevers allow the use of the probes at wavelengths as short as 250 nm.

Bistability of (Ga,Mn)As Ferromagnetic Nanostructures Due to the Domain Walls Switching

We designed and investigated four-arm nanostructures, composed of two perpendicularly crossed stripes, fabricated from ferromagnetic (Ga,Mn)As layer by means of electron-beam lithography patterning and chemical etching. The nanostructures exhibit a bistable resistance behavior resulting from two configurations of magnetic domain walls in the central part of the structures. We demonstrate a possibility of switching between two stable resistance states in zero magnetic field by applying a pulse of either weak magnetic field or electric current through the structure.

Soft Electron Beam Lithography (Soft-eBL) Patterning of 0- and 1-D Nanostructures of Magnetic and Ferroelectric Oxides

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Small angle X-ray scattering measurements of spatial dependent linewidth in dense nanoline gratings

Small angle X-ray scattering (SAXS) was used to characterize the cross section of nanoline gratings fabricated with electron beam lithography (EBL) patterning followed by anisotropic wet etching into a single crystal silicon substrate. SAXS results at normal incidence clearly bear the signature of positional dependent linewidth within the gratings; such non-uniformity is subsequently confirmed with scanning electron microscopy. The proximity effect of EBL is believed to be the cause of the spatial variations of linewidth. To quantitatively fit the SAXS results the linewidth near the periphery of the patterned field needs to be 80% greater than that in the central region, whereas the cross section of nanolines can be modeled as a simple rectangular shape, as expected from the anisotropic wet etching process.

Direct write electron beam patterning of DNA complex thin films

The authors report on the first use of direct write electron beam lithography (DW-EBL) patterning of DNA-based materials. Water insoluble and organic solvent soluble DNA:CTMA complexes were formed by reaction of DNA polymers with cationic surfactants and other molecular species. Thin films with thicknesses ranging from 85 to 300 nm were prepared by spin coating. DW-EBL was conducted using a Raith 150 system. The resulting exposed areas demonstrated either positive or negative resist properties depending on development solution. The characteristics of the DNA:CTMA material as a patternable electron sensitive resist medium are presented for different exposure conditions (10–30 kV), development conditions, structure sizes (100 nm to 20 μm), and structure complexities. Contrast values of ∼2 have been obtained in both positive and negative resist modes. Both simple (20 μm diameter circle and square) and complex (Fresnel lens) patterns with nanometer scale features (<100 nm) in DNA films are possible using this method.

The design of a reversible wafer holder for uniform photoresist coatings of deeply etched cavities

A wafer holder for photoresist coatings on wafers having deep 3D structures was designed and tested. Using a simple construction of a reversible three-point wafer holder and a two-step face down/face up spin coating procedure, we demonstrated uniform photoresist coatings in deeply etched cavities. The design can be implemented with minimum adaptation to any conventional spin coating systems. It is envisioned that this tool will be valuable for conventional photo or electron beam lithography patterning, especially in patterning small features situated deep within and especially near the sidewalls of etched cavities and recesses.