Of hard and soft light

How exposure shapes semiconductor structures

The effect of hard and soft light is well known from photography. With hard light, the surroundings can be illuminated with clearly defined, sharp contours in the shadows cast. Soft light diffuses the image and blurs outlines. The decisive factor for the respective effect is the size of the light source in comparison to the illuminated object: a relatively small light source leads to hard shadow formation, an extended light source to softer,

Structures down to the nanoscale

Playing with sharpness and blurriness not only allows impressive design options for visual art. Sharp contours are also desired when it comes to
creating defined structures for semiconductor technology on very small length scales. However, the diffraction of light at very small structures leads to the blurring of clear patterns, which always happens when the orders of magnitude are in the range of the optical wavelength and thus on the nanoscale.

These diffractive effects dictate the minimum resolution that can be achieved when exposed to a given wavelength or color of light. Real hard light would be ideal for achieving very fine nanostructures or very densely packed patterns. Especially since light affects not only the contours, but also the properties of the exposed materials, for example in photolithography, an important method for manufacturing semiconductor components.

It all depends on the paint

Photoresists are used in semiconductor technology to create nanopatterns. The solubility of photoresists depends on how they are exposed: there are resists that harden or chemically cross-link when exposed to light, and there are resists that soften when exposed to light of a certain wavelength. Hardening paints are suitable for protecting materials, while softening paints are used, for example, to define areas in which further process steps are to take place.

The sophisticated sequence of exposures with both types of photoresist is the basis for the production of nanostructures that process electrical or optical signals and can be used as integrated components (chips) for electronics and optics. For so-called integrated optics, i.e. chip-based optics, as we are developing them in our working group at the Kirchhoff Institute for Physics at Heidelberg University, nanostructures smaller than one micrometer are usually aimed for. For comparison: A hair has a diameter of about 50 micrometres.


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