Electron beam lithography: resolution limits and applications

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

Objective. To investigate the mechanical properties of injection-molded thermoplastic denture base resins. Material and methods. Four injection-molded thermoplastic resins (two polyamides, one polyethylene terephthalate, one polycarbonate) and, as a control, a conventional heat-polymerized polymethyl methacrylate (PMMA), were used in this study. The flexural strength at the proportional limit (FS-PL), the elastic modulus, and the Charpy impact strength of the denture base resins were measured according to International Organization for Standardization (ISO) 1567 and ISO 1567:1999/Amd 1:2003. Results. The descending order of the FS-PL was: conventional PMMA > polyethylene terephthalate, polycarbonate > two polyamides. The descending order of the elastic moduli was: conventional PMMA > polycarbonate > polyethylene terephthalate > two polyamides. The descending order of the Charpy impact strength was: polyamide (Nylon PACM12) > polycarbonate > polyamide (Nylon 12), polyethylene terephthalate > conventional PMMA. Conclusions. All of the injection-molded thermoplastic resins had significantly lower FS-PL, lower elastic moduli, and higher or similar impact strength compared to the conventional PMMA. The polyamide denture base resins had low FS-PL and low elastic moduli; one of them possessed very high impact strength, and the other had low impact strength. The polyethylene terephthalate denture base resin showed a moderately high FS-PL, moderate elastic modulus, and low impact strength. The polycarbonate denture base resin had a moderately high FS-PL, moderately high elastic modulus, and moderate impact strength.

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.

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.

To follow Moore’s law and the trend of devices to keep shrinking, the nanotechnology industry is challenged in finding a suitable technique for the mass production of integrated circuits with critical dimension in the sub-10 nm range. At the time this project started, EUV lithography, that employs light with a wavelength of 13.5 nm and thus requires the scanner to be kept under vacuum, was encountering difficulties in making its way into high volume manufacturing. Therefore different technologies were being explored as alternative to EUVL. The European project Single Nanometer Manufacturing for beyond CMOS devices (SNM) [1] aims at developing a manufacturing platform that routinely provides sub-10 nm resolution. To achieve that, fabrication processes based on Nano Imprint Lithography (NIL) are investigated. NIL is a low cost, high resolution and high throughput patterning technique, suitable for the mass production of devices. In NIL, a UV-transparent stamp is pressed on top of a substrate covered with polymer, called the NIL resist. The features on the stamp are imprinted in this polymer, that hardens under the exposure of UV light. The features on the polymer are then transferred into the underlying substrate using etching processes. To fabricate these stamps, we propose to use electron beam induced deposition (EBID)...

We present two kinds of novel nanoimprint lithography techniques based on SU-8 photoresist with single layer and tri-layer approaches. The imprint templates with high aspect ratio were first fabricated by electron beam lithography (EBL) and reactive ion etch (RIE), and then duplicated by the SU-8/SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /PMMA tri-layer technique we developed. Imprint properties such as pressure and temperature are determined to fabricate different nanostructures with various imprint depth. The developed processes, which provide us the capability to fabricate both imprinted nanostructures on resist and transferred nanopatterns on substrate with high volume and low cost, are further applied in producing high density gratings, planar chiral photonic metamaterials, and nanofluidics channels.

Exploring the resolution limit of electron-beam lithography is of great interest both scientifically and technologically. However, when electron-beam lithography approaches its resolution limit, imaging and metrology of the fabricated structures by using standard scanning electron microscopy become difficult. In this work, the authors adopted transmission-electron and atomic-force microscopies to improve the metrological accuracy and to analyze the resolution limit of electron-beam lithography. With these metrological methods, the authors found that sub-5 nm sparse features could be readily fabricated by electron-beam lithography, but dense 16 nm pitch structures were difficult to yield. Measurements of point- and line-spread functions suggested that the resolution in fabricating sub-10 nm half-pitch structures was primarily limited by the resist-development processes, meaning that the development rates depended on pattern density and/or length scale.

MOS-compatible fabrication and characterization of tunnel junctions in BESOI-material

Nanofabrication by electron beam lithography and its applications: A review

Nano-pattering process by low-voltage electron beam lithography based on microcolumn with beam energy of 500 eV has been developed. Low kV exposure provides the advantages of high sensitivity, reduced charging, and a lack of proximity and heating effects. However a low-voltage electron beam has very thin penetration range. At 500 V, the penetration range is less than 20 nm, while typical resist thickness is > 200 nm. A resist process with bilayer scheme, 17 nm-thick PMMA resist on 100 nm-thick SiO₂ layer, and wet etch method was demonstrated for 250 nm line patterns transfer to Si substrate. The process was applied to fabricate periodic grating patterns on a silicon substrate. The results of nano-pattern process by low energy microcolumn lithography will be discussed in detail.

Raster-Atomic force nanolithography: New insights towards the fabrication of 3D nanostructures on PMMA and Silicon Nitride

Fabrication of eight-step diffractive optical element for hologram-ROM

Visible light angular scatterometry is applied to characterize the geometry and physical properties of sub-100-nm-wide polymer gratings fabricated using nanoimprint lithography and electron beam lithography. Measurement sensitivities to small variations in linewidth and slope angle were evaluated theoretically, which suggests that TM polarized incident light offers improved sensitivity for the measurements of sub-45-nm critical dimensions (CDs). A variable angle scatterometer using a red laser is built, and measurement results of various polymethylmethacrylate (PMMA) gratings with sub-100-nm CDs reveals good accuracies and fit well to the scanning electron microscopy (SEM) measurements. In addition to geometry and dimension measurements, new functionality is implemented in the modeling to characerize polymer residue thickness, polymer flow dynamics, and evidence of stress in the nanoimprinted polymer gratings. Scatterometry is also applied to detect possible undercut line profiles resulting from electron beam lithography. The results promote using this low-cost and noninvasive technique to characterize polymer nanostructures as well as understand and control the underlying lithographic processes.

PURPOSEThis in-vitro study evaluated and compared the color stability of 3D-printed and conventional heat-polymerized acrylic resins following aging, mechanical brushing, and immersion in staining medium.MATERIALS AND METHODSForty disc-shaped specimens (10 mm in diameter and 3 mm thick) were prepared from two 3D-printed [DentaBASE (DB) and Denture 3D+ (D3D)] and one conventional polymethylmethacrylate (PMMA) denture materials. The specimens were thermo-cycled, subjected to mechanical brushing, and were immersed in either coffee, lemon juice, coke, or artificial saliva (AS) to simulate one and two years of oral use. Color measurements of the specimens were recorded by a spectrophotometer at baseline (T0), and after one (T1) and two years (T2) of simulation. The color changes (∆E) were determined and also quantified according to the National Bureau of Standards (NBS) units. Descriptive statistics, followed by factorial ANOVA and Bonferroni post-hoc test (α=.05), were applied for data analysis.RESULTSThe independent factors, namely material, staining medium, and immersion time, and interaction among these factors significantly influenced ∆E (P<.009). Irrespective of the materials, treatments, and time, the highest and the lowest mean ∆Es were observed for PMMA in lemon juice (4.58 ± 1.30) and DB in AS (0.41 ± 0.18), respectively. Regarding the material type, PMMA demonstrated the highest mean ∆E (2.31 ± 1.37), followed by D3D (1.67 ± 0.66), and DB (0.85 ± 0.52), and the difference in ∆E between the materials were statistically significant (P<.001). All the specimens demonstrated a decreased color changes at T2 compared to T1, and this difference in mean ∆E was statistically significant (P<.001).CONCLUSIONThe color changes of 3D-printed denture resins were low compared to conventional heat polymerized PMMA. All the tested materials, irrespective of the staining medium used, demonstrated a significant decrease in ∆E values over time.

Presented here is the novel use of thermoplastic siloxane copolymers as nanoimprint lithography (NIL) resists for 60 nm features. Two of the most critical steps of NIL are mold release and pattern transfer through dry etching. These require that the NIL resist have low surface energy and excellent dry‐etching resistance. Homopolymers traditionally used in NIL, such as polystyrene (PS) or poly(methyl methacrylate) (PMMA), generally cannot satisfy all these requirements as they exhibit polymer fracture and delamination during mold release and have poor etch resistance. A number of siloxane copolymers have been investigated for use as NIL resists, including poly(dimethylsiloxane)‐block‐polystyrene (PDMS‐b‐PS), poly(dimethylsiloxane)‐graft‐poly(methyl acrylate)‐co‐poly(isobornyl acrylate) (PDMS‐g‐PMA‐co‐PIA), and PDMS‐g‐PMMA. The presence of PDMS imparts the materials with many properties that are favorable for NIL, including low surface energy for easy mold release and high silicon content for chemical‐etch resistance—in particular, extremely low etch rates (comparable to PDMS) in oxygen plasma, to which organic polymers are quite susceptible. These properties give improved NIL results.