The wafer-level production of Fused Silica microlens arrays is limited by systematic process non-uniformities. The common molten resist-reflow process with subsequent dry-etching allows for manufacturing of microlens arrays on 200 mm wafers. A thorough process review yielded one free parameter. By switching from the geometrical lens description via radius of curvature and conic constant to a functional assessment via the optical design figure of merit we can describe microlens via their optical quality for the intended application with one parameter for a wide variety of cases. Leveraging these points we show improvements on the uniformity of microlens arrays by a factor of 2 for Fused Silica microlens arrays bigger than 100 mm by 100 mm.
We present an aspheric collimating lens for mid-infrared (4-14 μm) quantum cascade lasers. The lenses were
etched into silicon by an inductively coupled plasma reactive ion etching system on wafer level. The high
refractive index of silicon reduces the height of the lens profile resulting in a simple element working at high
numerical aperture (up to 0.82). Wafer level processes enable the fabrication of about 5000 lenses in parallel.
Such cost-effective collimating lens is a step towards the adoption of quantum cascade lasers for all its potential
applications.
Micro-optics is an indispensable key enabling technology (KET) for many applications today. The important role of
micro-optical components is based on three different motivations: miniaturization, high functionality and packaging
aspects. It is obvious that miniaturized systems require micro-optics for light focusing, light shaping and imaging. More
important for industrial applications is the high functionality of micro-optics that allows combining these different
functions in one element. In DUV Lithography Steppers and Scanners an extremely precise beam shaping of the Excimer
laser profile is required. High-precision diffractive optical elements are well suited for this task. For Wafer-Level
Cameras (WLC) and fiber optical systems the packaging aspects are more important. Wafer-Level Micro-Optics
technology allows manufacturing and packaging some thousands of sub-components in parallel. We report on the state
of the art in wafer-based manufacturing, testing and packaging.
A wide range of lasers from the UV to the IR are selected based on their optical power and spectral characteristics to
match the particular absorption behavior for the material to be processed. Periodic microlens arrays are often used as
multi-aperture integrators to transform the Gaussian or non-uniform beam profile into a homogenized intensity profile
either in 1-D or 2-D distribution. Each microlens element samples the input inhomogeneous beam and spreads it over a
given angular distribution. Incoherent beams that are either temporally or spatially incoherent can produce very uniform
intensity profiles. However, coherent beams will experience interference effects in the recombination of the beams
generated by each individual microlens element. For many applications, for example pulsed laser sources, it is not
possible to use a rotating or moving element, such as a rotating diffuser, to circumvent the interferences resulting from
the beam coherence. Micro-optical elements comprised of a randomly varying component can be used to help smooth
out the interference effects within the far-field intensity profile.
Refractive, diffractive and reflective micro-optical elements for laser beam shaping and homogenizing have been manufactured and tested. The presented multifunctional optical elements are used for shaping arbitrary laser beam profiles into a variety of geometries like, a homogeneous spot array or line pattern, a laser light sheet or flat-top intensity profiles. The resulting profiles are strongly influenced by the beam properties of the laser and by diffraction and interference effects at the micro-optical elements. We present general design rules for beam shaping and homogenizing. We demonstrate the application of such multifunctional micro-optical elements for a variety of applications from micro-laser machining to laser diagnostic systems.
Wafer-based manufacturing of Micro-Optics is based on standard technologies from Semiconductor Industry, like resist coating, lithography, reactive ion etching, deposition, sputtering, and lift-off. These well-established technologies allow the manufacturing of almost any Micro-Optics' structure shape. The excellence of the Micro-Optics component depends much on the proper choice of the manufacturing equipment and the process control. As all processes are standard Semiconductor technology, the quality is merely a question of the budget and the optimization effort. For characterization and testing, the current situation is different. Neither the test equipment from Semiconductor industry nor the test equipment from classical optics manufacturing is suitable to for Micro-Optics. Most of test instruments Micro-Optics industry is using today have been developed by research institutes or by the manufacturing companies
themselves. As Micro-Optics is still a niche market, all instruments are built in small series. This lack of suitable test equipment is a major problem for the Micro-Optics industry today. All process optimization in manufacturing is closely related to the capability to measure the quality of the products. We report on the state of the art in wafer-based manufacturing and summarize the standard characterization tools for Micro-Optics.
The metrology of refractive microlens arrays is analyzed using Twyman-Green, Mach-Zehnder, and white light interferometers. The advantages and limitations of each are discussed in their application to the measurement of spherical and aspherical microlens arrays.
We report on the fabrication of aspherical refractive microlens arrays on 8'' fused silica and silicon wafers at Suss Neuchatel, Switzerland. Refractive, plano-convex microlenses are fabricated by using photolithography, a reflow or melting resist technique and reactive ion etching. Diffraction-limited optical performance of the microlenses is achieved for refractive microlenses from 100 microns to 1.5 mm diameter and 2 to 50 microns sag. Aspherical lens profiles (aspherical constant from k equals -0.5 to -5.2) are obtained by varying the etch parameters during the reactive ion etching transfer. Microlens arrays in fused silica and silicon are fabricated for high-efficient fiber coupling and telecommunication. Densely packed arrays of cylindrical lenses (packing density > 98%, parabolic profile) are fabricated for flattop illumination at UV-wavelengths. Excellent array uniformity of is required for microlenses used within Microlens Projection Lithography systems.
We report on the fabrication of high quality microlens arrays on 4', 6' and 8'-fused silica wafers. Refractive, plano-convex microlenses are fabricated by using photolithography; a reflow or melting resist technique and reactive ion etching. A diffraction-limited optical performance (p-v wave aberrations of < (lambda) /8, Strehl ratio $GTR 0.97) is achieved. Aspherical lens profiles are obtained by varying the etch parameters during the reactive ion etching transfer. The microlens arrays are used for Microlens Projection Lithography (MPL) and within UV-light illumination systems. Microlens Projection Lithography is an innovative technique using KARL SUSS Mask Aligners equipped with an ultra-flat microlens-based projection system. The projection system consists of 500.000 identical micro-objectives side- by-side. Each micro-objective consists of 3 to 4 microlenses. A fully symmetrical optical design eliminates coma, distortion and lateral color. The lens system is frontal- and backside telecentric to provide a unit magnification (+1) over the whole depth of focus. Each micro- objective images a small part of the photomask pattern onto the wafer. The partial images from different channels overlap consistently and form a complete aerial image of the photomask. Microlens Projection Lithography provides an increased depth of focus ($GTR 50 microns) at a larger working distance ($GTR 1 mm)than standard proximity printing. Microlens Projection Lithography allows photolithography on curved on non-planar substrates, in V-grooves, holes, etc. using a KARL SUSS Mask Aligner.
Microlens arrays made in photoresist can be transferred into fused silica substrates by reactive ion etching herby, the etch rates of resist and silica differ by a factor of up to 3 depending on the oxygen content of the reacting gases in the etching machine. The resulting lenses are tested for the surface quality with the help of a Mach- Zehnder interference microscope. Merit functions such as point spread function and modulation transfer function can be calculated from the measured wave aberration data.
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