E-beam resists (electron beam resists) are designed for electron beam and deep UV applications for the fabrication of highly integrated circuits, mainly for mask fabrication. These resists generally allow the realisation of 100 to 500 nm structures on masks and wafers with film thickness values between 200 and 500 nm.
E-beam resist are also used in electron beam direct writing and multilayer processes. Thin layers of these resists < 100 nm are excellently suited for nanometre lithography. With an optimised process regime, even structures of less than 10 nm thickness can be realised at a film thickness of 50 nm.
They are characterised by a very good adhesion to silicon, glass and most metals. They are applied by spin coating between 6000 and 1000 rpm, and films with a thickness between 20 nm and 1.75 µm can be achieved. For special e-beam applications, film thickness values of even10 µm are possible (AR-P 6510).
Allresist offers several types of e-beam resists.
1) PMMA resists are composed of poly(meth)acrylates with different molecular masses (50K, 200K, 600K, and 950K) which are dissolved in chlorobenzene (AR-P 631 … 671) or the safer solvent ethyl lactate (AR-P 639 … 679) and 1-methoxy-2-propyl acetate (AR-P 6510). These resists work as positive tone resists. In comparison to polymer 950K, the sensitivity of polymer 50K is enhanced by 20 %. The glass temperature of PMMA layers is in the range of 105 °C, and polymers are thermostable up to a temperature of 230 °C.
2) Copolymer resists are composed of copolymers based on methyl methacrylate and methacrylic acid (PMMA/MA 33 %), which are dissolved in safer solvent 1-methoxy-2-propanole (AR-P 617). Copolymer resists also work as positive tone resists. Due to the chemical properties of methacrylic acid during the bake step at 200 °C in which dehydration leads to ring formation, copolymers possess a 3- to 4-fold higher sensitivity and a considerably higher contrast than PMMA resists. In addition, polymer layers are thermostable up to 240 °C, with a glass temperature of 150 °C.
As special resist for thick films of 5 – 250 µm suitable for LIGA techniques (synchrotron radiation) is AR-P 6510.
PMMA and copolymer layers are optically transparent above a wavelength of 260 nm. Since they resist however also absorbe at 248 nm, and a deep-UV exposure and patterning with these resists is thus possible, even though with lower sensitivity.
3.) CSAR 62 (AR-P 6200) is based on styrene acrylates and dissolved in the safer solvent anisole. Polymer layers are furthermore thermally stable up to 240 °C; the glass transition temperature is 148 °C. They are approximately twice as sensitive as PMMA resists and a bit less sensitive than the copolymer (AR-P 617). These differences may be exploited for two- and three-layer processes (e.g. T-gate manufacturing). CSAR 62 has a very high contrast (> 15) and good plasma etch stability.
4) Novolac-based e-beam resists (AR-P 7400, AR-N 7500…7700) are in general aqueous-alkaline developable. These resists are available as positive or negative tone electron beam resists. Positive-working resists contain naphthoquinone diazide and novolac (AR-P 7400). Negative resists contain in addition to the novolac organic or amine cross-linking agents and/or acid generators. As additional component, AR-N 7500 contains naphthoquinone diazide.
Novolac-based resists are approximately twice as stable in plasma etchings as compared to PMMA- and copolymer resists (AR-P 617); they are used for the fabrication of structures in electron beam lithographic processes and for mask production. A few e-beam resists also allow for mix-&-match processes, combining e-beam- and UV exposure (AR-P 7400, 7520 and 7700/30). The fine structures are written into the resist layer by e-beam lithography, immediately followed by UV exposure (i-line) of larger structures. Subsequently, resists are developed in one step according to the usual protocol.
A maximum resolution of < 10 nm can be realised for very thin films with AR-P 7400 and AR-N 7520. With AR-N 7520, even 6 nm lines could already be achieved, at an aspect ratio of 10.
Chemically enhanced e-beam resists are AR-N 7700, 7720 as well as AR-N 7700/30 and /-37. The latter resist has a very high sensitivity, is highly process- and storage stable, and allows also exposition in the deep UV and broad range UV spectrum. Resists of the AR-N 7700 series are high-resolution resists and reach a structural resolution of 50 – 100 nm with very good sensitivity. Resist AR-N 7720 is particularly well suited for three-dimensional structures, due to the low contrast which was specifically adjusted for this application.
An important addition to previous e-beam resists is Electra 92 (AR-PC 5090 and 5091). The conductive resist itself is not structurable, is however needed for the dissipation of electrical charges in order to realise accurate structural images on insulating substrates (glass, quartz, polymers).
Refractive indices for PMMAs are 1.48, for copolymers 1.49, for styrene acrylates 1,54 and for novolac-based e-beam resists 1.60 – 1.61.
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