Electron beam lithography limitations

Electron Beam Lithography. LB PMMA films with thicknesses of 6.3 nm (7 layers) are sufficient for patterning a Cr film suitable for photomask fabrication. For ultrathin PMMA films the resolution (see Fig. 1) is limited by the smallest spot diameter available on MEBES I (1/8 pm). However, it is not possible to obtain this resolution if a thicker resist (>100 nm) is used under the same exposure and development conditions, which demonstrates that ultrathin resists are able to minimize the proximity effect. Also, since the radius of gyration of 188,100 Mw PMMA is about 10 nm in the bulk, and the thickness of the 7 layer film (6.3 nm) is less than 10 nm, it is reasonable to assume there must be an alteration of chain configuration in the ultrathin films. This will be particularly true when the post-deposition baking temperature of the multilayer films is less than the glass transition temperature (115°C), as is the case for the present experiments. In such a case, interdiffusion of PMMA chains between the deposited layers may not result in chain configurations characteristic of the bulk. 

Ultimately, resolution in electron beam lithography is set by the range over which the primary electrons interact with the resist. That is by the distance over which the low energy secondary electrons are created (the resist is exposed mainly by secondaries) and by the range of the secondaries in the resist. For thin resists, and thin substrates (thin compared with the primary electron penetration) this resolution limit has been measured to be about 12 nm (43). 

Conventional patterning techniques, for example, photolithography and electron beam lithography, are frequently used for fabrication of patterned substrates owing to their capability to produce nanometer features with remarkable perfection. However, the slow process of electron beam lithography has limited most of its application fabricating high-end devices.69,70... 

I—Current Limit of Resolution for Electron-Beam Lithography... 

Acrylate resists such as polymethylmethacrylate (PMMA) have been used extensively in electron beam lithography because of their excellent resolution and contrast, despite their limited dry etch resistance and low sensitivity (1). Copolymers of PMMA, containing chemical groups more sensitive to radiation induced degradation, have also been studied and have shown up to a four-fold improvement in sensitivity (2). One approach has been to form a crosslinked gel, in-situ on the wafer, which contains radiation sensitive crosslinks and leads to improved sensitivity and improved contrast during development (3-7). 

Vieu C, Carcenac F, Pepin A. Electron beam lithography Resolution limits and applications Appl Surf Sci 2000 164 111-7(7). 

The practice of using a beam of electrons to generate patterns on a surface is known as electron beam lithography. The primary advantage of this technique is that it is one of the ways to beat the diffraction limit of light and make features in the sub-micrometer regime. Beam widths may be on the order of nanometer as of the year 2005. This form of lithography... 

Scattering angular-limited projection electron beam lithography... 

---