Ion beam lithography

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The ion beam lithography , ion beam lithography often abbreviated as is, in the semiconductor technology , a method of forming a patterned thin layer serving as sacrificial layer is used for subsequent deposition, and Ätzungs- implantation processes (see FIG. Photolithography ). Together with the very similar electron beam lithography, the process forms the group of particle beam lithographies .

variants

A distinction is generally made between two variants, ion beam lithography using a mask and that using a focused ion beam . Both are based on the structuring of an ion-sensitive " photoresist layer " through a chemical reaction of the ions used (mainly protons or helium ions ) with the lacquer layer.

When using a focused ion beam, the structure is introduced directly into the paint layer. For this purpose, a very narrow ion beam is first generated by a corresponding focusing unit and focused on the wafer. Using appropriate (electrical) deflection units, the ion beam can be guided over the entire wafer in a similar way to an electron beam. In this way, the structures are gradually “written” into the paint.

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

Free-standing structures made of silicon (English. Stencil mask ), for example, serve as a mask , which the ion beam against. shade the layer to be structured. When ions are absorbed by the mask, the mask is charged, which can become noticeable in an undesired deflection of the ion beam and therefore deteriorates the quality of the image. This can be prevented by a thin conductive layer, for example by graphite (similar to some sample preparation techniques in scanning electron microscopy ) on the mask. The disadvantage of using ion beams is the sputtering effect that occurs , which means that the free-standing mask structures gradually lose their stability.

Advantages and disadvantages

The main advantages of ion beam lithography are the low scattering of the ions (compared to electron beam lithography) in the lacquer layer due to the significantly higher mass. Therefore, the proximity effect can largely be neglected. The energy introduced is also much higher than with electron beam lithography, which is shown by the higher sensitivity of the paint. However, there is a maximum depth to which the ions can penetrate into the substrate / layer; for ions with energies below 1 MeV, it is a maximum of 500 nm. Therefore, only very thin layers can be structured.

History and areas of application

Currently (as of 2011), the technology is traded as one of several alternative lithography processes that could replace the current photolithography processes for the production of structures below 20 nm in the future, cf. Next generation lithography . Ion beam lithography is currently still in the research and development stage, which means that it is not yet being used in industrial practice.

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

literature

  • John Melngailis: Applications of Ion Microbeams Lithography and Direct Processing . In: John N. Helbert (Ed.): Handbook of VLSI microlithography: principles, technology, and applications . William Andrew, 2001, ISBN 978-0-8155-1444-2 , pp. 790-855 .