Projects

Rotationally symmetric grating structures are used as diffractive optical elements (DOE) in a variety of applications in optical metrology, but also, for example, in laser beam shaping. An established method for manufacturing such structures is laser direct writing, in which the desired structures are written into a suitable mask material such as photoresist using a focused laser beam.

The smallest possible structure size is largely determined by the diffraction limit when focusing the laser beam. When using laser sources in the short-wave, visible wavelength range and a numerical aperture of NA = 0.6 (that is easy to control in practice), structure sizes in the order of 0.5 µm can be realized with reasonable process reliability. If smaller structures are to be manufactured, for example, the numerical aperture can be increased or the exposure wavelength reduced, resulting in a smaller focus diameter. However, objectives with a high numerical aperture have the disadvantage of a small working distance. Suitable short-wavelength laser sources are generally associated with higher costs; in addition, radiation protection requirements increase when working in the ultraviolet spectral range.

Another alternative is the use of highly nonlinear mask materials. However, these have a smaller process window than conventional materials, which increases the demands on the entire manufacturing process. In addition, with both approaches, the manufacturing time is generally longer due to the smaller step size during the writing process.

In this project, a process for the fabrication of rotationally symmetric structures was adapted and integrated into a polar coordinate laser direct writing system that addresses all these drawbacks. The scanning beam interference lithography enables the fabrication of structures with critical dimensions <0.5 µm using laser sources in the visible spectral range, while significantly reducing exposure time and providing high process stability.

In addition to the optics design for laser beam shaping, a system for stabilizing the interference fringe pattern was developed, which makes it possible to keep the position of the fringe pattern on the substrate stable down to a few nanometers. In this way, the high process stability and at the same time a very homogeneous optical phase function of the fabricated DOE could be realized. Thus, with the approach presented here, grating structures with a pattern width < 250 nm could be routinely fabricated at an exposure wavelength of 458 nm.