Development of novel SU-8 based nanoimprint lithography

There has been a considerable effort recently in the development of planar chiral metamaterials. Owing to the lack of inversion symmetry, these materials have been shown to display interesting physical properties such as negative index of refraction and giant optical activity. However, the biosensing capabilities of these chiral metamaterials have not been fully explored. Ultrasensitive detection and structural characterization of proteins adsorbed on chiral plasmonic substrates was demonstrated recently using UV-visible circular dichroism (CD) spectroscopy. Second harmonic generation microscopy is an extremely sensitive nonlinear optical probe to investigate the chirality of biomaterials. In this study, we characterize the chiral response of chiral plasmonic metamaterials using second harmonic generation microscopy and CD spectroscopy. These planar chiral metamaterials, fabricated by electron-beam lithography, consist of right-handed and left-handed gold gammadions of length 400 nm and thickness 100nm, deposited on a glass substrate and arranged in a square lattice with a periodicity of 800nm.

Low cost, rapid fabrication of durable molds of grating arrays for nanoimprint lithography

Nanoimprint lithography (NIL) technology has aroused great interests in both academia and industry due to its high resolution, low-cost, and high-volume nanopatterning capability. And as an expoxy resin-based negative amplified photoresist, SU-8 is an ideal candidate for NIL because of its low-glass-transition temperature, low-volume shrinkage coefficient, and good optical properties. In this reviewing paper, we highlight the major technical achievements in NIL on epoxy resin and its applications for bio- and nanophotonic structures. NIL was also applied for the duplication of imprint templates, originally fabricated by e-beam lithography (EBL) followed by reactive ion etch (RIE), using a SU-8/ SiO 2/PMMA tri-layer technique. And nanoimprint properties were systematically investigated for optimization. The developed nanoimprint process for different applications indicates promising industrial potentials in the next generation lithography resolution.

Optical chirality is highly demanded for biochemical sensing, spectral detection, and advanced imaging, however, conventional design schemes for chiral metamaterials require highly computational cost due to the trial-and-error strategy, and it is crucial to accelerate the design process particularly in comparably simple planar chiral metamaterials. Herein, we construct a bidirectional deep learning (BDL) network consists of spectra predicting network (SPN) and design predicting network (DPN) to accelerate the prediction of spectra and inverse design of chiroptical response of planar chiral metamaterials. It is shown that the proposed BDL network can accelerate the design process and exhibit high prediction accuracy. The average process of prediction only takes ∼15 ms, which is 1 in 40000 compared to finite-difference time-domain (FDTD). The mean-square error (MSE) loss of forward and inverse prediction reaches 0.0085 after 100 epochs. Over 95.2% of training samples have MSE ≤ 0.0042 and MSE ≤ 0.0044 for SPN and DPN, respectively; indicating that the BDL network is robust in the inverse deign without underfitting or overfitting for both SPN and DPN. Our founding shows great potentials in accelerating the on-demand design of planar chiral metamaterials.

Electron beam lithography: resolution limits and applications

Sub-micron width high aspect ratio beam/trench arrays are etched into silicon substrates using a Surface Technology Systems (STS) deep reactive ion etch (RIE) tool equipped with a time multiplexed plasma etch/passivation cycle scheme. The oxide mask is patterned by nanoimprint lithography and minimizes lateral trench etching by adjusting the significant etch parameters. High aspect ratio trench arrays 350nm wide with a 700nm period are etched to a depth of 10 μm with typical sidewall asperities on the order of 30nm. A dual etch process is used to reduce scalloping near the trench surface using HBr/Cl to etch the initial 500nm followed by the STS process using C4F8/SF6 chemistry. The dual etch process resulted in a reduction of sidewall asperities from 75nm to less than 25nm. In addition, the dual etch process reduced the trench array depth variation from a measured standard deviation of 0.7 to 0.1 representing significant improvement of etch repeatability across the wafer sample.

Electron beam (EB) lithography using polymethylmethacrylate (PMMA) andoxygen gas reactive ion etching (RIE) were used to fabricate fine patterns in adiamond mould. To prevent charge-up during EB lithography, thin conductivepolymer was spin-coated over the PMMA resist, yielding dented line patterns 2μm wide and 270 nm deep. The diamond mould was pressed into PMMAon a silicon substrate heated to 130, 150 and 170ºC at 43.6, 65.4 and87.2 MPa. All transferred PMMA convex line patterns were 2 μ mwide. Imprinted pattern depth increased with rising temperature andpressure. PMMA patterns on diamond were transferred by the diamondmould at 150ºC and 65.4 MPa, yielding convex line patterns 2 μm wide and 200 nm high. Direct aluminium and copper patterns wereobtained using the diamond mould at room temperature and 130.8 MPa. Thediamond mould is thus useful for replicating patterns on PMMA and metals.

We report the nanofabrication and characterization of x-ray transmission gratings with a high aspect ratio and a feature size of down to 65 nm. Two nanofabrication methods, the combination of electron beam and optical lithography and the combination of electron beam, x-ray, and optical lithography, are presented in detail. In the former approach, the proximity effect of electron beam lithography based on a thin membrane of low-z material was investigated, and the x-ray transmission gratings with a line density of up to 6666 lines/mm were demonstrated. In the latter approach, which is suitable for low volume production, we investigated the x-ray mask pattern correction during the electron beam lithography process and the diffraction effect between the mask and wafer during the x-ray lithography process, and we demonstrated the precise control ability of line width and vertical side-wall profile. A large number of x-ray transmission gratings with a line density of 5000 lines/mm and Au absorber thickness of up to 580 nm were fabricated. The optical characterization results of the fabricated x-ray transmission gratings were given, suggesting that these two reliable approaches also promote the development of x-ray diffractive optical elements.

This review concentrates on the applications of nanoimprint lithography (NIL) and hot embossing for the fabrications of nanolectronic devices, nanophotonic metamaterials and other nanostructures. Technical challenges and solutions in NIL such as nanofabrication of templates, removal of residual resist, pattern displacement in thermal NIL arising from thermal expansion are first discussed. In the nanofabrication of templates, dry etch in plasma for the formation of multi-step structures and ultra-sharp tip arrays in silicon, nanophotonic chiral structures with high aspect ratio in SiC are demonstrated. A bilayer technique for nondestructive removal of residual resist in thermal NIL is described. This process is successfully applied for the fabrication of T-shape gates and functional high electron mobility transistors. However, pattern displacement intrinsically existing in thermal NIL/hot embossing owing to different thermal expansions in the template and substrate, respectively, limits its further development and scale-up. Low temperature even room temperature NIL (RTNIL) was then proposed on HSQ, trying to eliminate the pattern distortion by avoiding a thermal loop in the imprint. But, considerable pressure needed in RTNIL turned the major attentions to the development of UV-curing NIL in UV-curable monomers at low temperature. A big variety of applications by low-temperature UV-curing NIL in SU-8 are described, including high-aspect-ratio phase gratings, tagging technology by nanobarcode for DNA sequencing, nanofluidic channels, nanophotonic metamaterials and biosensors. Hot embossing, as a parallel technique to NIL, was also developed, and its applications on ferroelectric polymers as well as metals are reviewed. Therefore, it is necessary to emphasize that this review is mainly attempted to review the applications of NIL/embossing instead of NIL technique advances.

The application of nanopatterned electrode materials is a promising method to improve the performance of thin-film optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics. Light coupling to active layers is enhanced by employing nanopatterns specifically tailored to the device structure. A range of different nanopatterns is typically evaluated during the development process. Fabrication of each of these nanopatterns using electron-beam lithography is time- and cost-intensive, particularly for larger-scale devices, due to the serial nature of electron beam writing. Here, we present a method to generate nanopatterns of varying depth with different nanostructure designs from a single one-dimensional grating template structure with fixed grating depth. We employ multiple subsequent steps of UV nanoimprint lithography, curing, and ion beam etching to fabricate greyscale two-dimensional nanopatterns. In this work, we present variable greyscale nanopatterning of the widely used electrode material indium tin oxide. We demonstrate the fabrication of periodic pillar-like nanostructures with different period lengths and heights in the two grating directions. The patterned films can be used either for immediate device fabrication or pattern reproduction by conventional nanoimprint lithography. Pattern reproduction is particularly interesting for the large-scale, cost-efficient fabrication of flexible optoelectronic devices.

Recent advances in microlithography have made the patterning and fabrication of microprocessors and memory chips with a critical feature size of less than 65 nm possible. Only nanoimprint lithography offers the advantages of being low-cost, high-throughput production methods suitable for the pattern transfer of structures of less than 30 nm over large areas. Nanowire has interesting prospects for future nanoelectronics applications. The aspect ratio of material fabricated by nanoimprint lithography is always low. Therefore nanowire has a large resistivity. The development of T molds with features of less than 30 nm and a high aspect ratio suitable for nanoimprint lithographies is anticipated. We can therefore anticipate the production of a T mold with features of 25 nm and high aspect ratio, suitable for use in nanoimprint lithography. In this study we discuss methods for the fabrication of just such a T mold using EBL and inductively coupled plasma reactive ion etching.

Square arrays of silicon dots with feature sizes from 30 to 400 nm and periodicities from 60 to 500 nm were obtained by standard lithography or nanoimprint and reactive ion etching. Co/Pt multilayers with perpendicular anisotropy were subsequently deposited by magnetron sputtering onto these prepatterned wafers. The deposition was performed either at normal or oblique incidence. The magnetic domain structures were characterized by magnetic force microscopy after zero field cooling (ZFC) from the (Co/Pt) Curie temperature or by alternative demagnetization (AD). It appears that the angle of deposition of the Pt influences the existence of direct exchange coupling between neighboring dots due to the formation of a magnetic layer on the sidewalls of the dots.

Summary form only given. Nanoscale metal-semiconductor-metal (MSM) photodetectors and MESFETs are key elements in ultra-fast optical and wireless communication systems. However, devices with sub-0.25 /spl mu/m feature sizes are typically fabricated using electron-beam lithography (EEL), which has high cost and low throughput. Nanoimprint lithography (NIL), on the other hand, has both low cost and high throughput, as well as sub-l0 nm resolution. We report the fabrication of MSM photodetectors on semi-insulating (SI) GaAs using NIL, and the examination of NIL on the quality of the nanoscale Schottky contact. We found that if the imprint pressure is 600 psi or lower, nanoscale GaAs MSM photodetectors fabricated using NIL are comparable to those fabricated by conventional EEL and photolithography, indicating no degradation to the photodetector Schottky contact induced by the imprint process.

Formation of high aspect ratio fused silica nanowalls by fluorine-based deep reactive ion etching

Nanophotonics is wide field covering many interesting applications branching from cutting edge science including plasmonics, metamaterials, cavity quantum electrodynamics in high-Q cavities all the way to applied sciences like silicon nanophotonics for on chip optical interconnections and single frequency semiconductor light sources. Most of the practical device demonstrations in these fields utilize nanopatterned surfaces. Applications require patterning of nanoscopic gratings, photonic crystals, waveguides and metal structures. There are many wonderful demonstrations of nanotechnology-based lasers and other photonic components. However, difficult questions related to fabrication need to be addressed before these components enter any market. Demonstrations in the scientific literature have relied heavily on the use of direct writing lithography methods, such as electron beam lithography or focused ion beam lithography. These methods, although excellent for scientific studies, cannot be scaled up to allow cost effective production of nanophotonics. Lithography solutions developed for integrated circuits can produce extremely narrow linewidths and deliver high precision but are difficult to transfer to photonics fabrication. There exist many alternative lithography methods, but their scale-up to cost effective volume production is challenging. Since the introduction of nanoimprint lithography (NIL) in 1995 (Chou et al. 1995), there has been widespread interest in the development of NIL for various applications. As early as in 2003 NIL had gained substantial support and was chosen as one of MIT's Technology Review’s “10 Emerging Technologies That Will Change the World” (Technology Review 2003). The selection was justified by the fact that NIL can bridge the gap between lab level nanotechnology research and production level manufacturing requirements. In this chapter, we briefly review the state-of-the-art lithography methods and introduce nanoimprint lithography (NIL), a very cost effective lithography method for nanophotonics applications. In section two we will introduce soft UV-NIL, an imprint method using soft and flexible stamps, as a method for patterning compound semiconductor optoelectronics. Finally, in section three, we highlight some NIL activities based on soft-UV-NIL.