Precise Polymer Micro-Optical Systems
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Precise Polymer Micro-Optical Systems
Andreas Braeuer, Peter Dannberg, Gunnar Mann, and Michael Popall
Introduction The demand for precise micro-optical elements and subsystems is increasing drastically, influenced mainly by the exploding information and communications markets. For many applications, technologies capable of high-volume production of these elements are needed. Favored methods are replication technologies such as injection molding, hot embossing, or UV molding. The challenge for materials scientists is to synthesize materials that can be formed into micro-optical structures and that can resist extreme environmental conditions during operation and stocking (e.g., temperature changes from 45C to 125C). Additionally, typical properties the materials must possess include high optical transparency in the visible and near-IR ranges; an rms wavefront aberration of less than 1/20 of the wavelength of operation; a homogeneous and reproducible (and even tunable) refractive index; low, reproducible shrinkage; and suitability for patterning of submicrometer structures. Polymers exhibiting these properties have been developed in recent years that allow for industrial application in the near-IR spectral region. In this article, we describe advances in the UV-molding process and its combination with a refined photoresist reflow technology for master-tool fabrication. It is shown that this molding technology enables the generation of thin polymeric films carrying micro-optical elements on top of substrates like glass, silicon, and III–V semiconductors by means of a process performed in a modified contact mask aligner. Double-sided replication, resulting in micro-optical subsystems fabricated on the wafer scale, is demon-
MRS BULLETIN/JULY 2001
strated as well. The optical elements are characterized by high precision and stability, temperature stability and precise pitch (lateral period of the optical elements), refractive-index homogeneity, and uniformity across the wafer, and they fulfill additional requirements (coating, separating, cleaning, mounting, etc.) for practical applications.
Ultraviolet-Curing Materials: Organic–Inorganic Copolymers Organic–inorganic copolymers can be processed like conventional UV-curing polymers, with the inorganic component leading to improved optical, mechanical, and thermal properties. The resins used in the technology are based on multifunctional methacrylate alkoxysilanes.1 They show low polymerization shrinkage (2% linear), and they can be mixed to tune the refractive index to values between 1.46 and 1.57. Their inorganic network (–Si–O–Si–) improves the optical homogeneity of the resins. Furthermore, the hardened material shows low water absorption (2%), very good thermal stability, higher abrasion resistance than conventional polymers, a tunable refractive index, and very good adhesion to silicon and oxide substrates. For the micro-optics technology described in the next section, it is important that the material can be applied without solvent and hardened
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