Photomask / Direct Write Lithography Documents

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Fabrication of subwavelength, binary, antireflection surface-relief structures in the near infrared

Subwavelength, binary surface-relief structures are artificial materials with an effective index of refraction that can be tailored by varying the duty cycle of the binary pattern. These structures have the significant advantage of requiring only a single lithography and etch step for fabrication. We demonstrate a specifically designed antireflection structure in a material system (GaAs) and at a wavelength (975 nm) directly integrable with GaAs-based vertical cavity surface-emitting lasers and which exhibits strong polarization-dependent properties. Fabrication is performed using electron beam lithography and reactive-ion-beam etching. The observed reflectivity is 2% for TE polarization and 23% for TM polarization, a difference in reflectivity of over a factor of 10 for the two polarizations.

Nanometer-scale resolution of calixarene negative resist in electron beam lithography

New nonpolymer materials, calixarene derivatives were tested as high-resolution negative resists for use in electron beam lithography. Arrays of 12-nm-diam dots with a 25 nm pitch were fabricated easily. The sensitivity of calixarene in terms of area dose ranged from 700 to 7000 µC/em2, and the required dose for dot fabrication was about 105 electrons/dot. The standard area dose for calixarene is almost 20 times higher than that for polymethyl methacrylate (PMMA), but the electron spot dose for dot fabrication by calixarene is almost the same as that for PMMA and other highly sensitive resists such as SAL (chemically amplified negative resist for electron beam made by Shipley). The electron spot dose for such extremely small dots does not seem to depend on standard area dose, but any resist tends to require the same dose under exposure in a 50 keV electron beam writing system. We propose a qualitative exposure model that suggests a tradeoff of dose and dot size. The calixarene seems to be promising material for nanofabrication.

Process optimization for production of sub-20 nm soft x-ray zone plates

We report here the optimization of processes for producing sub-20 nm soft x-ray zone plates, using a general purpose electron beam lithography system and commercial resist technologies. We have critically evaluated the failure point of the various process steps and where possible chosen alternate methods, materials, or otherwise modified the process. Advances have been made in most steps of the process, including the imaging resist, pattern conversion for electron beam exposure, and pattern transfer. Two phase shifting absorber materials, germanium and nickel with excellent quality using polymethyl methyl accrylate and zones as small as 20 nm have been fabricated in nickel using the calixarene resist. The total efficiency as well as the efficiency of different regions of the zone plates were measured. All zone plates have demonstrated good efficiencies, with nickel zone plates performing better than germanium zone plates.

Fabrication of 30 nm gate length electrically variable shallow-junction metal–oxide–semiconductor field-effect transistors using a calixarene resist

We have fabricated electrically variable shallow-junction metal–oxide–semiconductor field-effect transistors (EJ-MOSFETs) with an ultrafine gate for the first time. The gate length was reduced to 32 nm by using electron-beam lithography with a calixarene resist, which has an under 10 nm resolution with a sharp pattern edge. Moreover, normal transistor operation of 32 nm gate-length EJ-MOSFETs was confirmed.

Gate technology for 70 nm metal–oxide–semiconductor field-effect transistors with ultrathin (<2 nm) oxides

Results are described for a gate level technology module developed to produce metal–oxide–semiconductor transistors with physical gate lengths of 70 nm and below. Lithography is performed by direct write e-beam lithography (EBL) using a thermal field-emission EBL system in SAL 601 resist. Critical dimension (CD) control, as measured by several methods, is found to depend not only on dose control but also on writing parameters such as pixel spacing. The pattern transfer using a silicon dioxide hard mask is shown to exhibit a trade-off between anisotropy and selectivity. Transmission electron microscopy cross sections reveal that two atomic layers are removed even when the gate oxide stopping layer is completely intact. We report results for gate lengths down to 60 nm with edge roughness on the order of 5 nm, within the acceptable limits for threshold requirements, while stopping the etch process on oxides as thin as 1.2 nm.

C60-incorporated nanocomposite resist system for practical nanometer pattern fabrication

We propose a nanocomposite resist system that incorporates sub-nm size fullerene C60 molecules into a highly sensitive and moderately dry-etching resistant electron-beam positive resist, ZEP520. C60 incorporation leads to carbon reinforcement in the original resist material and enhances resist performance for nanometer pattern fabrication.

25 nm pitch GaInAs/InP buried structure: Improvement by calixarene as an electron beam resist and tertiarybutylphosphine as a P source in organometallic vapor phase epitaxy regrowth

To achieve a fine periodic semiconductor structure by electron beam (EB) lithography, calixarene was used as an EB resist. A 25 nm pitch InP pattern was formed successfully and 40 nm pitch InP structures were achieved with good reproducibility. A shorter developing time, precise stage motion, accurate control of the widths of lines and spaces, and slight O2 ashing were important to obtain a fine InP pattern by a two-step wet chemical etching process. Furthermore, the fabricated periodic InP pattern was buried in a GaInAs structure by organometallic vapor phase epitaxy. The introduction of tertiarybutylphosphine as the phosphorous source prevented the fine structure from deforming when the temperature was raised and a 25 nm pitch periodic structure was buried successfully.

Lithography for sub-60 nm resist nanostructures

As the semiconductor community continues to follow the Semiconductor Industry Association Roadmap, resist structures are being printed further into the nanometer domain. However, a persistent issue for successful sub-60 nm resist patterning is mechanical stability at high aspect ratios. The objective of this article is to understand what processing conditions facilitate processing resist nanostructures with useful aspect ratios for the fabrication of sub-60 nm transistors. We have found that, in aqueous based development and rinse, if the resist thickness is reduced, then the aspect ratio is sacrificed for the sake of resolution. The implication is that there is a resolution limit at which resist structures will have aspect ratios that are useful for device fabrication. We have also found that there are development effects that occur in the thick film regime that are not reproducible with thin films. The best resolution structures we have been able to print are lines of 28 nm in width using direct write electron-beam lithography on negative chemically amplified resists NEB-22 and NEB-31 (Sumitomo Chemical Inc.) with an aspect ratio of about 3. To put this result in perspective, this is about 40 molecules wide.

Double electron layer tunneling transistors by dual-side electron beam lithography

We describe the first demonstration of small-area double electron layer tunneling transistors (DELTTs) fabricated by dual-side electron beam lithography. The DELTT is a planar quantum device which operates by modulating the two-dimensional (2D)-to-2I) tunneling between two coupled quantum wells. The fabrication technique utilizes the epoxy-bond and stop-etch process to remove the substrate material which allows the backside gates to be placed in close proximity (less than 1 µm) to the frontside gates. The use of electron beam lithography provides precise alignment of the front and back features to each other, We have applied this technique to the fabrication of DELTTs on coupled AlGaAs/GaAs double quantum wells. Low temperature electrical characterization yields source-drain current voltage curves that exhibit negative differential resistance with peak-to-valley ratios of up to 8:1. The height and position of the resonant peak varies strongly with gate bias, demonstrating transistor action.

Nanometer-Scale Patterning of Polystyrene Resists in Low-Voltage Electron Beam Lithography

We studied nanometer-scale patterning using a polystyrene negative resist in electron beam lithography. We found that the use of a low-molecular-weight polystyrene enables 10-nm-level patterning at low-acceleration voltage. We also found that the spot dose of such ultrasmall patterns formed at a 5kV acceleration voltage was one-tenth of that formed at 50kV. Low-voltage electron beam lithography is a suitable technique for organic resist nanopatterning. The Charlesby theory can still be applied to nanodot formation, and we can therefore estimate the dot sensitivity for various polystyrene molecular weights. We suppose that an exposure model is based on polymer aggregation to explain the formation of a 10-nm-level pattern with a height of 40 nm can be formed by using a small molecule, not a large one.