Producing Patterns on a Curve

Hank Hogan

In the past, researchers wanting to perform lithographic microfabrication on a nonflat lens or other optical surface have been thrown a curve — literally. Standard lithographic technology cannot do the job. Investigators, therefore, were restricted largely to planar surfaces, which can be less than ideal. Now Daniela Radtke and Uwe D. Zeitner of the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany, have demonstrated a laser-based lithography system that can create micro-optical elements on curved surfaces, using the method to fabricate a diffractive lens on a convex spherical substrate.

With the added tilt capability of the sample table and other enhancements, a modified commercial laser-based lithography system enables microfabrication on curved optical surfaces. Images courtesy of Daniela Radtke, Fraunhofer Institute for Applied Optics and Precision Engineering.

The scientists developed the system in conjunction with Heidelberg Instruments GmbH, also in Germany. The company, a maker of laser lithography equipment, provided crucial expertise. “For the purpose of modifying the system according to our needs, detailed knowledge about the lithography system was required, which can only be supplied by the manufacturer,” said Radtke, a graduate student.

In a standard setup, a laser is focused by a microscope objective onto a spot on a substrate that is mounted on a movable X-Y table. The spot traces out the desired path under computer control. In some cases, the intensity can be varied, leading to gray-scale exposure. After the photoresist is exposed, subsequent steps complete the process and produce a physical object.

Using a custom laser-lithography system, the researchers fabricated a diffractive element on a biconvex lens. This image, taken in differential contrast mode, shows part of the hybrid element.

The company modified one of its standard systems to meet the researchers’ specifications, adding two tilt frames to the substrate table, allowing for a maximum angle of ±10° in two directions. The group also incorporated tilt capability into the laser and beam-modulating optics plate, implemented an optical autofocus that could handle curved substrates and used interferometric control of the various components to ensure that the beam spot was positioned on curved substrates to better than 150-nm accuracy. In addition to hardware, the company developed the required controlling software.

To demonstrate the modified system’s capabilities, the investigators patterned a diffractive structure onto a standard biconvex spherical lens, with the goal of improving imaging performance and reducing chromatic aberration. In theory, the hybrid element would outperform the standard lens. Tests with transmission gratings ranging from one to 80 sinusoidal cycles per millimeter showed that to be the case.

However, the hybrid performance was not as good as theory predicted. The shortfall was a result mostly of nonuniformities in the manufacturing process and of the use of a wavelength range instead of a specific wavelength. The second issue cannot be helped, but Radtke noted that the first problem has some possible solutions. “The inhomogeneities in the lithography process could be reduced by applying special data preparation and sample treatment techniques,” she said.

With the system proven, the researchers are developing characterization methods for complex shapes and are seeking more applications. For its part, the company will build the system as a custom component for interested parties.

Optics Express, Feb. 5, 2007, pp. 1167-1174.