Proton beam writing (or p-beam writing) is a direct-write

lithography Lithography () is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone ( lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by the German ...

process that uses a focused beam of high energy (

MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. ...

)

protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron m ...

to pattern resist material at nanodimensions. The process, although similar in many ways to direct writing using

electrons The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...

, nevertheless offers some interesting and unique advantages.

Protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron m ...

, which are approximately 1800 times more massive than

, have deeper penetration in materials and travel in an almost straight path. This feature allows the fabrication of three-dimensional, high aspect ratio structures with vertical, smooth sidewalls and low line-edge roughness. Calculations have also indicated that p-beam writing exhibits minimal proximity effects (unwanted exposure due to secondary electrons), since the secondary electrons induced in proton/electron collisions have low energy. A further advantage stems from the ability of protons to displace atoms while traversing material, thereby increasing localized damage especially at the end of range. P-beam writing produces resistive patterns at depth in

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

, allowing patterning of selective regions with different optical properties as well as the removal of undamaged regions via electrochemical etching. The primary mechanisms for producing structures in resist materials is, in general, bond scissioning in positive resists such as

PMMA PMMA may refer to: * para-Methoxymethamphetamine ''para''-Methoxy-''N''-methylamphetamine (also known as PMMA, Red Mitsubishi), chemically known as methyl-MA, 4-methoxy-''N''-methylamphetamine, 4-MMA) or (4-PMDA, as listed to its original phy ...

(polymethylmethacrylate), or cross-linking in negative resists such as SU-8. In positive resists the regions damaged by protons are removed by chemical development to produce structures, whereas in negative resists the development procedures remove the undamaged resist leaving the cross-linked structures behind. In e-beam writing, the primary and secondary electrons create the scissioning or cross-linking, whereas in p-beam writing the damage is caused by short range proton-induced

secondary electron Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation (the ''primary'' radiation). This radiation can be in the form of ions, electrons, or photons with suffici ...

s. The proton fluence required for exposure varies from 30-150 nCmm⁻² depending on the resist material, and is around 80-100 times less than that required by e-beam writing. ''Remark'': The unit of the fluence in proton beam writing is usually given in "charge/area". It can be converted into "particles/area" by dividing "charge/area" by the charge of a proton, Q = 1,602·10⁻¹⁹C. P-beam writing is a new technology of great potential, and both current experimental data and theoretical predictions indicate that sub-10 nm 3D structuring is feasible. However, the lack of a user friendly commercial instrument with a small footprint is currently holding back the potentially wide range of application fields in which p-beam writing could make a substantial impact. Hopefully, this will be addressed in the near future.

References

{{DEFAULTSORT:Proton Beam Writing Lithography