Masks for low energy electron proximity projection lithography (LEEPL) require thin membranes, which in turn make the development of low-distortion masks a critical issue for this technology. By using an evaporated resist, a flip side fabrication process is presented here in which mask patterning is carried out with the mask in the same orientation that it will have in the stepper. This new process reduces distortions of a typical LEEPL mask which usually requires patterning on the opposite side of the membrane causing a gravitational sag effects. In addition, an evaporated resist has significant advantages for mask fabrication as membrane distortion is reduced due to the absence of centrifugal force during the resist deposition process. Uniform heat distribution across the membrane during the etching process is also expected since the membrane can now be placed in direct contact with a cooled metal electrode, thereby improving the etch rate uniformity. Also, for large scale production, several mask replicates from the original mask must be made because they have limited lifetime when used in a stepper. Image placement distortion can be minimized and the yield can be improved in mask replication by using an evaporated resist.
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.