Ion beam lithography

The ion beam lithography, ion beam lithography often abbreviated, is in the semiconductor industry to a process for forming a patterned thin layer, which is used as a sacrificial layer for subsequent deposition, Etch and implantation processes ( cf. photolithography ). The method together with the very similar, the group of electron beam lithography Teilchenstrahllithografien

Variants

Generally, two variants are distinguished, the ion-beam lithography using a mask and using a focused ion beam. Both based on the structure of an ion- sensitive " photoresist layer " by a chemical reaction of the ions used (mostly protons or helium ions) with the lacquer layer.

In the use of a focused ion beam, the structure is introduced directly into the coat. To a very narrow ion beam is initially generated and focused on the wafer by a corresponding focusing unit. Via corresponding (electrical) deflection units of the ion beam can be performed over the entire wafer similarly to an electron beam. In this way the structures can be gradually "written" in the paint.

In the second variant the structure from an existing shading is similarly transferred to the layer as in modern photolithography process. It is therefore also called masked ion -beam lithography (English, MIBL ). Here too, there are similar as in the photo lithography, different sub- processes: flood exposure (masked flood beam) in the step-and- repeat method and projection exposure (ion projectionlithography, IPL).

As a mask used for example free-standing structures of silicon (English stencil mask ) that vs. the ion beam locally. the layer to be patterned shade. In the absorption of ions through the mask there is a boots mask that can be felt in an unwanted deflection of the ion beam and hence degrades the quality of the image. This can be prevented by a thin conductive layer, for example by graphite (similar to some Probenpräperationstechniken in scanning electron microscopy) on the mask. A disadvantage of the use of ion beams is occurring sputtering effect, which means that the free-standing mask structures gradually lose stability.

Pros and Cons

The main advantages of ion beam lithography are the low scattering of the ions ( in comparison with electron beam lithography ) in the resist film due to the significantly higher mass. Therefore, the proximity effect can be largely ignored. The input energy is much higher than that of the electron beam lithography, which is reflected by a higher sensitivity of the paint. However, there is a maximum depth to which the ions penetrate into the substrate / layer, it is for ions having energies less than 1 MeV, with a maximum of 500 nm Thus, only very thin layers can be patterned.

History and applications

Currently ( 2011), the technique is considered one of several alternative lithography process, which could replace the current methods of photolithography for the fabrication of structures below 20 nm in the future, see next-generation lithography. Currently, the ion beam lithography rate is still in the research and development stage, which means it is not yet used in industrial practice.

By the use of heavier ions, such as boron or arsenic may be ion beam lithography for the maskless ( no structured layer on the wafer ), ion implantation, for example, for local doping of the wafer with impurities, can be used.