High resolution electron beam lithography poses severe constraints on any suitable resist, namely the need to work with very thin layers in order to achieve highest resolutions, while at the same time possessing suitable resistance to plasma etching. Small molecular sizes are also an interesting avenue for reducing line edge roughness, but result in an increased threshold exposure dose. Several resists currently available cover the range from high resolution to high sensitivity. One interesting property demonstrated using the QSR-5TM resist is substrate conformability. This thermally evaporated resist has a controllable thickness down to 30 nm and surface roughness less than 2 nm and can be deposited onto very small surfaces. In this paper, we will present the results of patterning this resist in a configuration which may be suitable for very high speed field effect devices. A silicon nitride membrane 300 nm thick was prepared on a silicon substrate. QSR-5TM resist is then deposited in two steps, using a Joule effect thermal evaporator with the resist in a powdered state. After a deposition of 30 nm of resist, the substrate is flipped over and a second identical layer is deposited. The membrane is removed from vacuum during the reversal process. The lithography step follows the deposition step and is carried out using a field emission gun SEM converted to electron beam lithography operated at a beam energy of 20 keV. Test patterns with feature sizes ranging from 45 nm to 130 nm were successfully exposed. The advantage of this method is that perfect alignment between the patterns is obtained on both sides of the resist.
Proximity effects during electron beam exposure have been kept under control by using sophisticated correction algorithms and software, combined with a strategy which aims at increasing the electron beam energy to 50 keV and 100 keV. At these energies, the proximity effects are more uniform and provide a situation where they are easier to correct. However, as feature sizes shrink, and the pattern density increases, this task becomes extremely complex, since tolerances to pattern definition errors are becoming more restricted. An alternate approach is to move to lower electron energies where proximity effects become negligible. Several programs are underway to develop massively parallel electron beam (MPEB) writer systems that have greatly reduced energy in the ≤5keV regime. Selection of the electron beam energy becomes critical below 10 keV, since the tolerance window where proximity effects are indeed negligible is very small. A shot noise model has been elaborated providing minimum exposure doses required for resists at technology nodes of 45 nm and below. These doses increase rapidly with reducing linewidth and impose a minimum number of electron beams for MPEB systems in order to be able to pattern a surface corresponding to a standard full field 6 inch reticle in a reasonable time, and to directly pattern 300 mm wafers at rates of 5, 50 and 100 wafers per hour. An overall set of results is obtained indicating minimum number of electron beams and electron beam current.
A novel and effective approach to nano-fabrication lithography is the vapour deposition of the negative tone electron beam resists QSR-5 and QSR-15 (Quantiscript’s sterol based resist) onto a substrate. Vapour deposition is especially conducive for patterning thin delicate membranes (e.g. advanced masks for X-ray lithography - XRL, and Low Energy Electron Proximity Projection Lithography - LEEPL), that are susceptible to breakage during the spin coating process. With the capability for depositing highly uniform thin layers (<50nm) and a demonstrated resolution better than 60nm, QSR-5 and QSR-15 have potential for the fabrication of next generation lithography masks. Optimized for low energy electron exposure where proximity effects become negligible and thus well suited for 1X lithography mask patterning, QSR-5 and QSR-15 have shown exposure doses as low as 100μC/cm2 at 3KeV. In addition to this type of application, the versatility of QSR-5 and QSR-15 have also been demonstrated by the fabrication of a Fresnel zone plate lens on the tip of an optical fibre with the goal of improving the coupling of diode laser emission into the fiber. This application clearly shows the capabilities of this process for producing nano-scale patterns on very small area surfaces that are completely unsuitable for spin-coating of the resist. A second demonstration of the resist's capabilities is the patterning of optical diffractive elements directly on the facet of a semiconductor laser. This opens the way to direct patterning on laser diode facets in order to control the emission profile from the device. It has also proven capabilities in the manufacture of delicate photo masks. In their natural state, QSR-5 and QSR-15 are solids at room temperature and are sterol based heterocyclic compounds, with unsaturated bonding capable of cross linking. On their own merit, QSR-5 and QSR-15 are capable of cross linking under electron beam exposure and are comparable in certain properties to conventional spin-coated resists such as PMMA. When cross linked, their heterocyclic structure gives it excellent selective resistance to solvent based developers (such as alcohols and ketones) for pattern formation. They have also been shown to be highly resistant to etching solutions, even when working with thin high resolution layers on the order of 30 nm. They are highly stable and have a relatively long shelf life, greater than one year. Compared to conventional resists utilizing complex, toxic, and expensive resin systems, QSR-5 and QSR-15 are non-toxic and significantly cost effective. Before evaporation, the resists are in a powder state that allows for direct evaporation and sublimation onto a target substrate that contributes to film uniformity and capabilities for a very thin film; the powder state allows for a wide degree of adjustment in temperature of the vapour chamber, as a means to achieving the desired film thickness and uniformity.