Fraunhofer Institute for Applied Optics and Precision Engineering IOFThe photolithographic process describes the partial exposure of a light-sensitive material (resist) through a mask with the aim of transferring the structure true to size. After the subsequent development step, the structured photoresist in turn serves as a mask for processing optical materials, e.g. etching, lift-off. In addition, the resulting surface profile itself can also be used for optical functionality. An inherent advantage is the very high lateral precision and high reproducibility under the constant process conditions.
Generation of Microoptical Structures by Photolithography
Technical Equipment
In the Fraunhofer IOF cleanroom, mask-aligner devices and direct-writing projection exposure devices are available for photolithography and address structure sizes from approx. 1 µm. The process chain is designed for substrates up to Ø 300 mm or 9“ x 9”, especially for optical materials and their requirements. While exposure with the mask aligner exposes the entire substrate at once and can therefore generate a high throughput, the direct writing process with the help of a digital reticle enables very flexible processing, in particular the realization of freely configurable surface profiles (lens profiles) or the lithographic structuring of special substrates and non-planar surfaces.
Photoresist Reflow for Microlenses
The lithographic structuring of thick photoresist layers is the starting point for the production of microlens profile shapes using thermal melting. For this purpose, a photoresist layer with a precisely set layer thickness is binary structured using mask lithography.
Tempering on a hotplate or in an oven reduces the viscosity of the photoresist and rounds the edges. Photoresist columns become spherical caps, i.e. circular microlenses, whereas linear bars are transformed into cylindrical lenses.
The outline and position of the microlenses is specified with high precision by the photomask. Special temperature processes also make it possible to set a low asphericity. This process is advantageous in terms of uniformity across the wafer substrate and surface quality (roughness).
An area fill factor of up to approx. 90% is possible in a hexagonal arrangement. The process always produces convex lens profiles, which can, however, be inverted accordingly via molding processes.
5-axis direct-write lithography tool
Gray Scale Photolithography for Micro Freeforms
Direct-writing lithography with a digital reticle makes it possible to introduce different UV light doses into the resist in a spatially resolved manner, resulting in a surface profile in the development step. This process is very flexible and enables not only virtually any microlens shape (including concave and/or stronger asphericity) but also other optical profile shapes (list not exhaustive):
Examples of structuring:
- Microlensarray with quasi 100% fill factor
- Microlenses with integrated spacer
- Micro Fresnel lenses
- Arrays of off-axis lens segments
- Deterministic diffusers / tailored light diffusers
- Diffractive optical elements (e.g. correction of spherical / chromatic aberrations)
- Microprism and micromirror arrangements
- Retroreflectors (e.g. truncated cornercubes)
- (Blazed) gratings (period >~ 5 µm)
The addressable structure depth range is approx. 500 nm - 80 µm. The transfer into optical materials is done via replication by molding into UV-curable highly transparent polymer or plasma etch transfer processes. We work together with partner companies to produce Ni shims and sleeves for replication processes.
Structuring of opaque Panels and Functional Layers
High-quality apertures are necessary for the optical function or intermediate apertures are also helpful. In addition to the optical density (O.D.), it is also crucial to introduce low back reflection into the system, which is why black layers usually need to be structured. Depending on the application wavelength and requirements, spectrally adapted aperture layers (UV-SWIR) can be structured. In the visible range, it is also possible to structure pigment-colored photoresist directly. With the aid of the mask aligner, several aperture layers can be lithographically structured with a high degree of accuracy. By bonding several wafers, aperture layers can be successively stacked, for example to suppress optical crosstalk between neighboring channels.
The same principle also enables the structuring of optical filter layers, e.g. (RGB) or conductive transparent layers, e.g. ITO.
- »Wafer-level hybrid integration of complex micro-optical modules.«
(P. Dannberg, F. Wippermann)
(Micromachines, 5(2), Page 325-340) - »Imaging Grating Elements for Compact Spectrometers«
(R. Steinkopf, R. Jende, R. Leitel)
(Annual Report 2017, Page 22-23) - »High dynamic grayscale lithography with an LED-based micro-image stepper«
(H.-C. Eckstein, U.D. Zeitner, R. Leitel )
(Optical Microlithography XXIX. Proceedings - »Direct write grayscale lithography for arbitrary shaped micro-optical surfaces«
(H.-C. Eckstein)
(20th Microoptics Conference (MOC), Fukuoka, Japan, 2015, Page 62-68) - »High dynamic grayscale lithography with an LED-based micro-image stepper«
(H.-C. Eckstein, U.D. Zeitner, R. Leitel)
(Optical Microlithography XXIX (Vol. 9780), Page 62-68) - »Sub-aperture EUV collector with dual-wavelength spectral purity filter«
(T. Feigl, M. Perske, U. Zeitner, R. Leitel)
(Extreme Ultraviolet (EUV) Lithography VI, 94220E, 16 March 2015) - »Evaluation of lenslet fabrication technologies for micro-optical array projectors.«
(R. Leitel)
(Optical Fabrication, Testing, and Metrology VI. Volume 10692, 2018) - »Grayscale lithography system enables high-volume production of freeform micro-optics«
(A. Thoss, D. Mitin, R. Leitel)
(Laser Focus World 2021, September, Page 39-42) - »Thin film black coatings based on interference and absorption«
(P. Munzert, S. Schwinde, R. Leitel)
(Advances in Optical Thin Films VIII, Volume 13020, Page 14-20)
