Mask Aligner Lithography is a well-established back-end fabrication process in microlithography. Within the last few years, resolution enhancement techniques, like OPC or phase-shifting photomask [1], have been transferred and adapted from projection lithography to further develop the diffraction based pattern transfer of mask aligner lithography, especially concerning achievable resolution e.g. for the production of MEMS, sensor elements as well as micro-optics.
The application of the so-called Talbot effect also enables improved resolution and the generation of periodic pattern with sub-micron feature sizes utilizing large photomask-wafer-distances [2]. Further advanced techniques like double patterning or two-beam interference have already been used to push the resolution limit of diffractive mask aligner lithography down to a grating period of 145 nm [3].
The design of the photomask used for diffraction based proximity lithography has a strong impact on the pattern transfer. With advanced design algorithm sophisticated multi-level photomasks (fig. 1) can be computed in order to generate optimized photoresist profiles. We have demonstrated the successful fabrication of a high-resolution aperiodic pattern as well as of a blazed grating with an improved perpendicular back jump utilizing a multi-level photomask [4,5]. Besides using a sophisticated photomask, we have also developed a simple technology of a movable photomask holder to generate continuous photoresist profiles like blazed gratings [6]. This photomask displacement technology allows a high flexibility for the fabrication of a wide variety of patterns with a mask aligner. Therefore, multiple exposures can be combined with a lateral displacement of the photomask.

Weichelt_Fig2 (002)
Figure 1: Scanning-Electron Microscope pictures showing (a) a section of a 3-level phase-shifting photomask for the generation of an 800 nm period grating [7] and (b) a clip of 3-level amplitude and phase modeling photomask to fabricate an aperiodic L-shaped pattern of five lines with different lengths in photoresist, shown in (c) [4]. Both photomasks have been fabricated with e-beam lithography.(rights: IAP)

Nonetheless, the current technology using a mercury vapor lamp as a light source has several drawbacks. We extended the existing mask aligner illumination system by replacing the mercury vapor lamp with a solid-state laser [8]. Now a so-called "FLEX-PI" (Photomask Illumination) can be used applying the implemented galvanometer scanner. It allows a flexible choice of arbitrary angular spectrum distributions of the photomask illumination. Full-field mask aligner lithography is guaranteed by a rotating, holographic diffuser expanding the laser beam and minimizing undesired speckle effects.

Weichelt_Fig1 (002)
Figure 2: Examples of diffractive mask aligner lithography using a new illumination system based on a laser as exposure source: (a) A blazed grating in photoresist with an improved perpendicular back jump using an optimized 3-level photomask [5] and (b) F shaped pattern etched into silicon was generated with a customized angular distribution and a pinhole photomask [8].(rights: IAP)

[1] R. Voelkel, U. Vogler, A. Bramati, T. Weichelt, L. Stuerzebecher, U.D. Zeitner,
     K. Motzek, A. Erdmann, M. Hornung, R. Zoberbier, Advanced mask aligner
     lithography, Proc. SPIE 8326, Optical Microlithography XXV, 83261Y (2012).
[2] L. Stuerzebecher, T. Harzendorf, U. Vogler, U.D. Zeitner, R. Voelkel, Advanced
     mask aligner lithography, Fabrication of periodic patterns using pinhole array
     mask and Talbot effect, Opt. Express 18(19), 19485-19494 (2010).
[3] Y. Bourgin, D. Voigt, T. Käsebier, E.-B. Kley, U.D. Zeitner, Periodic sub-100nm
     structures fabricated by proximity i-line mask-aligner lithography (and self-
     aligned double pattering), Proc. SPIE Vol. 978014, (2016).
[4] T. Weichelt, U. Vogler, L. Stuerzebecher, R. Voelkel, U.D. Zeitner, Resolution
     enhancement for advanced mask aligner lithography using phase-shifting
     photomasks, Opt. Express 22(13) (2014).
[5] L. Stuerzebecher, F. Fuchs, T. Harzendorf, S. Meyer, U.D. Zeitner, Application
     of rigorously optimized phase masks for the fabrication of binary and blazed
     gratings with diffractive proximity lithography, Proc. SPIE 8974 (2014).
[6] T. Weichelt, R. Kinder, U.D. Zeitner, Photomask displacement technology for
     continuous profile generation by Mask Aligner Lithography, J. Opt. 18 (12)
     125401 (2016).
[7] L. Stuerzebecher, F. Fuchs, T. Harzendorf, U.D. Zeitner, Pulse compression
     grating fabrication by diffractive proximity photolithography, Opt. Lett.
     Vol. 39 (4), 1042-1045 (2014).
[8] T. Weichelt, Y. Bourgin and U.D. Zeitner, Mask aligner lithography using laser
     illumination for versatile pattern generation, Opt. Express 25(18), 20983-20992

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