Gerd Harnisch

Electrostatic chucks for lithography applications

Abstract High precision electrostatic chucks with diameters up to 12 inches are being developed at IOF for electron/ion-beam lithography applications. For optimal performance, selection of the appropriate chuck dielectric is crucial. We have tested various materials, including sapphire, quartz and glass-ceramics with respect to chucking force under vacuum conditions. Differences in electrostatic force of more than an order of magnitude were observed and are attributed to Coulomb and Johnsen–Rahbek behaviour. For the former, reasonable agreement with theoretical calculations was obtained when taking the corresponding dielectric constants and a finite gap between wafer and support into account. Time constants for chucking and dechucking were determined for the latter.

Handling of micro-optical components for microassembly

Technical challenges and directions for microoptical and optoelectronical packaging include the development of special handling tools for manual and automated assembly stations. These tools not only have to be precise, but also reliable in a mass production environment. The design process for handling devices therefore is a major issue and an approach to it will be presented. Several examples illustrate how the proposed design algorithm was used to solve assembly tasks like fiber handling and microlens- positioning.

Low temperature GRISM direct bonding

For spectroscopy in space, GRISM elements –obtained by patterning gratings on a prism surface – are gaining increasing interest. Originally developed as dispersive elements for insertion into an imaging light path without deflecting the beam, they are progressively found in sophisticated multi stage dispersion optics. We report on GRISM manufacturing by joining the individual functional elements –prisms and gratings – to suitable components. Fused silica was used as glass material and the gratings were realized by e-beam lithography und dry etching. Alignment of the grating dispersion direction to the prism angle was realized by passive adjustment. Materials adapted bonds of high transmission, stiffness and strength were obtained at temperatures of about 200°C in vacuum by hydrophilic direct bonding. Examples for bonding uncoated as well as coated fused silica surfaces are given. The results illustrate the great potential of hydrophilic glass direct bonding for manufacturing transmission optics to be used under highly demanding environmental conditions, as typical in space.

Slit manufacturing and integration for the Sentinel-4 NIR and UV-VIS spectrometers

The sentinel–4 spectrometer´s slits are the key components of the ultraviolet–visible (UV–VIS) and the near infrared (NIR) channels for earth observation, with absolute slit width accuracy and variation required as < 0.1 ?m, respectively, and slit planarity < 0.4 ?m peak to valley (P-V). Adapted lithographic structuring techniques as developed for the dry- and wet etching of silicon-on-insulator (SOI) wafers combined with special integration devices for accurate alignment as well as precision optical polishing of the mounting planes of the slit holders together with spring elements can fulfil these requirements. Protected aluminum coating ensures a light tight optical density at wavelengths between 200 nm and 1200 nm, electrical grounding, and chemical protection.

Grism manufacturing by low temperature mineral bonding

By uniting a grating with a prism to a GRISM compound, the optical characteristics of diffractive and refractive elements can be favorably combined to achieve outstanding spectral resolution features. Ruling the grating structure into the prism surface is common for wavelengths around 1 μm and beyond, while adhesive bonding of two separate parts is generally used for shorter wavelengths and finer structures. We report on a manufacturing approach for joining the corresponding glass elements by the technology of hydrophilic direct bonding. This allows to manufacture the individual parts separately and subsequently combine them quasimonolithically by generating stiff and durable bonds of vanishing thickness, high strength and excellent transmission. With this approach for GRISM bonding, standard direct-write- or mask-lithography equipment may be used for the fabrication of the grating structure and the drawbacks of adhesive bonding (thermal mismatch, creep, aging) are avoided. The technology of hydrophilic bonding originates from “classical” optical contacting [1], but has been much improved and perfected during the last decades in the context of 3-dimensinal stacking Si-wafers for microelectronic applications [2]. It provides joins through covalent bonds of the Si-O-Si type at the nanometer scale, i.e. the elementary bond type in many minerals and glasses. The mineral nature of the bond is perfectly adapted to most optical materials and the extremely thin bonding layers generated with this technology are well suited for transmission optics. Creeping under mechanical load, as commonly observed with adhesive bonding, is not an issue. With respect to diffusion bonding, which operates at rather high temperatures close to the glass transition or crystal melting point, hydrophilic bonding is a low temperature process that needs only moderate heating. This facilitates provision of handling and alignment means for the individual parts during the set-up stages and greatly eases joining optical materials of different thermal expansion. The technology has been successfully used in the past for bonding various glasses as well as crystalline optical materials [3, 4]. Here we will focus on bonding prisms elements and binary gratings of fused silica with and without coatings at the bonding interface. Further, preliminary results on bonding prism-grating-prism (PGP) combinations will be presented.

Multi-resolution waveguide image slicer for the PEPSI instrument

A waveguide image slicer with resolutions up to 270.000 (planned: 300.000) for the fiber fed PEPSI echelle spectrograph at the LBT and single waveguide thicknesses of down to 70 μm has been manufactured and tested. The waveguides were macroscopically prepared, stacked up to an order of seven and thinned back to square stack cross sections. A high filling ratio was achieved by realizing homogenous adhesive gaps of 3.6 μm, using index matching adhesives for TIR within the waveguides. The image slicer stacks are used in immersion mode and are miniaturized to enable implementation in a set of 2x8. The overall efficiency is between 92 % and 96 %.

Adjustment robot for fiber optics assemblies

Precise adjustment of the optical components may be achieved by stepped transfer of momentum via special stroke actuators (impulse hammers), which act onto a pre-stressed fiber- optical component. The motion of the component is controlled by a computer and a measurement device. The present paper discusses theory and experiment of this adjustment method, in particular motion behavior of pushed components under the influence of applied momentum, pre-stressing and frictional forces. Additionally it describes generically the wide application range of this adjustment method. In particular the article describes an innovative, automatic adjustment machine (robot) for the alignment of a single- mode fiber assembly, which was developed by the German Fraunhofer-Institute for Applied Optics and Precision Engineering (IOF) in collaboration with Agilent Technologies Inc., a global technology leader in communications, electronics and life sciences. The achieved adjustment accuracy for the fiber optical assembly is in a low micron range for the focusing motion and in a sub-micron for entering of the optics.