Ion beam lithography is used to create fine nanostructures on a surface, such as circuit boards. It can be used to directly write on the material, rather than using a photomask, as in photolithography.
This is essential for creating single runs, prototypes, modifications or repairs, where the cost of a high-definition photomask can run well above $10,000.
Characteristics of Created Structures
Three-dimensional, high aspect ratio structures with smooth vertical walls of less than 10 nm resolution can be produced in a resist layer above a substrate material. This can be induced to become more or less soluble upon exposure to an ion beam before immersion in a solvent to remove the more soluble area.
Advantages of Ion Beam Lithography
No requirement of additional resists
Unlike electron beam lithography, where both the primary and secondary electrons induce cross-linking or scissoring of the resist layer, ion beam lithography relies on short range secondary electrons. Thus, there is no requirement of specially designed resists used in electron beam lithography, which leads to heavily scattered or diffracted primary or secondary electrons.
Ion beam lithography offers higher resolution than photolithography or electron beam lithography, as the ions used in this technique are far heavier than photons or electrons. Ion beam possesses a smaller wavelength, and thus produces very little diffraction or scattering of the particles. Ions also follow a straighter path through material than electrons, while any secondary electrons produced are of lower energy due to the lower speed of the ion.
Ion beam lithography opens some additional avenues not available in electron beam lithography, such as the ability to locally mill away atoms by physical sputtering, or locally deposit material. Ion-induced mixing can also directly modify material.
Disadvantages of Ion Beam Lithography
Ion beam lithography requires the use of thinner resist layers than in electron beam lithography, since the penetration depth of ions is lower than electrons. Use of a resist bilayer that is first irradiated with gallium ions to create a mask pattern is employed to circumvent this limitation. Reactive ion etching, which uses a plasma to remove deposited material, is then used to transfer the image of the gallium resist layer to the secondary layer.
This allows deep 3D structures to be created, while also improving write speed and with very low requirement of ion beam density. Optical or electron beam lithography is still generally much quicker than ion beam lithography, as they can be done in parallel, with several beams creating a pattern on a single substrate.
One ion source is electron bombardment, which irradiates a gas to ionize it. Ions produced in this way tend to be of a small but low energy range. The currently favored method of ion is a liquid metal ion source. Usually, a thin needle is wetted by a thin film of heated liquid metal.
Gallium is frequently employed for this purpose, and an electric field is applied to shape the liquid metal into a Taylor cone. Strengthening the electric field creates a sharper tip to the cone, leading to ion production by field evaporation.