The rapid growth in nanotechnology coupled with the difficulties in fabricating structures below the 100 nm level has fueled an interest in the development of high-resolution lithographic technologies. Current microelectronics production technologies are essentially two-dimensional, and are well suited for the two-dimensional topologies prevalent in microelectronics. As semiconductor devices are scaled down in size, and coupled with the integration of moving parts on a chip, there is expected to be a rising demand for smaller microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) devices. High aspect ratio (height/width) three-dimensional microstructures with nanometer details are also of growing interest for optoelectronic devices, and biochips using micro- and nano- fluidic channels are considered to have potential in the biomedical field.
    Next-generation lithographies (NGLs) are actively being developed to take over from the highly successful optical lithography. As feature sizes shrink to well below 100 nm, diffraction limits imposed on the further development of optical lithography make it increasingly difficult to remain in step with Moore's law [1], although translating optical lithography to shorter wavelengths (e.g. extreme ultraviolet - EUV - lithography) alleviates such diffraction effects. Development of NGLs has focused on industrial production, and such masked lithographies (where primary radiation transmitting through large-area masks casts a pattern in a resist material) are considered the most efficient means of producing multiple and cheap components. More recently, however, direct-write technologies (where the primary radiation is focused to a small beam and scanned serially across a resist) have been considered as possible alternatives to masked technologies [2]. Although the relatively low fabrication speed of direct-write technologies has been previously considered too slow for mass production, these serial techniques may have some distinct advantages when used to write stamps or molds and combined with nanoimprinting and pattern transfer [3,4].

 

Direct write nano lithographic techniques e-beam/p-beam writing
The direct-write technologies currently employed for nanofabrication are, in general, based on charged particles that can be focused to nanodimensions, e.g. electrons (e-beam writing) and slow heavy ions (focused ion beam technology - FIB). Both these technologies have been highly successful, and e-beam writers and FIB instruments are available commercially.
    Recent developments in high-energy (MeV) proton focusing in which sub-100 nm levels have been demonstrated [5] have led to the emergence of a new direct-write nanofabrication technique: p-beam writing. Figures 1A, 1B and 1C show examples of high aspect ratio nano structures fabricated in CIBA using p-beam writing in SU-8, PMMA and HSQ resist respectively [6,7]. P-beam writing is essentially an analogue of e-beam writing, but which utilizes the high mass properties of protons compared with electrons. This new nano lithographic technique has been cited in Science "to be a powerful addition to the lithographic toolbag" [8].  In Japan p-beam writing has been put on the road map for super fine fabrication from 2010 onwards.
    The protons in p-beam writing as well as the electrons in e-beam writing will mainly lose their energy through multiple collisions with electrons in the resist material. Since protons are much heavier compared to electrons the protons will follow a practically straight path penetrating the resist material whereas the electrons in electrons in e-beam writing will undergo about 50% energy transfer giving rise to substantial beam spread in the resist material. At the same time in the case of p-beam writing the energy of the induced secondary electrons, also called delta rays, have relatively low energies (< 100 eV), whereas a substantial fraction of the delta rays in e-beam writing will gain high energies up to 10 keV or more. The energy required to alter resist material in such a way that it can be used for lithography is typically less than 10 eV. This tells us that the range of the delta ray in p-beam writing is only 1 or 2 nm whereas the delta rays in e-beam writing could be several microns causing unwanted resist exposure and therefore larger feature sizes [9]. 
    A proton beam with a fixed energy will have, through the nature of the interactions with the electrons in the resist martial a very well defined range in the resist material. This allows the fabrication of complex 3D nano structures as shown in figure 1D where we show a reproduction of the Parthenon in Athens scaled down by a factor of 1,000,0000. 
  
Next generation proton beam writing
The main reason why p-beam writers are not yet as popular as e-beam writers is the fact that focusing protons down is more difficult compared to focusing electrons. The best e-beam focusing systems can reach sub nm level. It must be noted that in e-beam writing the minimum feature size is not determined by the beam size but by forward scattering (or effective beam broadening) in the photoresist. The achievable minimum resolution is therefore limited by delta ray travel in the photo resist [9].
    Dr Jeroen van Kan in CIBA holds currently the best performance in the world for focusing proton beams down to 40 nm. Through this performance Dr Jeroen van Kan has obtained a grant from the US air force to develop a next generation lens system for proton beam writing. This new focusing system is currently under construction in CIBA and is expected to break the 10 nm barrier. The fact that more than 90% of the delta rays induced by the proton beam travel only 1-2 nm coupled to the fact that the proton beam maintains a practically straight path in the resist will allow the fabrication of sub 10 nm feature sizes in resist material. This will make p-beam writing a serious contender in nano fabrication.