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We demonstrate that polarization-sensitive optical coherence tomography (PS-OCT) is suitable to map the stress distribution within materials in a contactless and non-destructive way. In contrast to transmission photoelasticity measurements the samples do not have to be transparent but can be of scattering nature. Denoising and analysis of fringe patterns in single PS-OCT retardation images are demonstrated to deliver the basis for a quantitative whole-field evaluation of the internal stress state of samples under investigation., 10 pages, 6 figures; Copyright: Blackwell Publishing Ltd 2008; The definitive version is available at: www.blackwell-synergy.com

In a close collaboration the team of Micromotion, micro resist technology, BESSY, and LSU-CAMD have successfully mastered the challenges of LIGA production of ultra-precision microparts for Micro Harmonic Drive® gears. The complementary expertise ranging from design and application know-how to process research and technical support resulted in high quality LIGA microparts and superior Micro Harmonic Drive® gears taking advantages of the free 2D design capability and material choice. It is also an excellent example that through contributions from partners with different background and expertise LIGA precision parts can be produced with high quality and yield and for a market competitive price.

First promising investigations of SU-8 removal experiments with a novel plasma etching technique are presented. The basic idea of this technique is to separate the highly effective generation of chemical radicals (e.g. oxygen radicals) using a traveling wave reactor (TWR) microwave source with water cooled plasma zone from the chemical reaction with the resist polymer. The etching tool operates in a remote and downstream mode with very high radical density allowing precise thermal management of the substrates on the chuck giving controlled process conditions without deviation in temperature, and generally preventing ion bombardment, at least resulting in gentle processing without jeopardizing the integrity of the metal structures. Very good removal of SU-8 with very few residues and very high etching rates up to 10 μm per minute are observed in first experiments which are offering chances to get even more than 20 μm per minute. The etching process is isotropic, and the rate stays stable during the whole removing process even for very thick films of 1 mm and more. First application examples of SU-8 removal are demonstrating the great potential of the presented microwave plasma based technique not only for the cleaning of metallic microparts but also for other more sensitive materials which is demonstrated by SU-8 removal from graphite X-ray mask substrates.

Optical coherence tomography (OCT) is a technique for contactless and non-destructive imaging of internal structures within semi-transparent materials. When OCT is performed polarisation-sensitively (PS-OCT), additional information about internal anisotropies and strains is obtained. Originally developed for the biomedical sector, OCT increasingly attracts interest for material characterisation. In this work, we apply PS-OCT imaging for non-biological samples using a novel transversal ultra-high resolution PS-OCT set-up. We demonstrate the advantages of this technique for structural analysis and strain mapping for different types of samples: we evaluate photoresist moulds for the fabrication of micro-electromechanical parts (MEMS), and we investigate the glass-fibre composite outer shell of helicopter rotor blades where defects have formed.

Mittels optischer Kohärenztomografie (OCT) können in kontaktfreier Weise Querschnittsbilder von semitransparenten Proben aufgenommen werden. In diesem Artikel werden instrumentelle Erweiterungen der OCT vorgestellt, welche in der zerstörungsfreien Materialcharakterisierung zur Anwendung kommen können. Messungen an diversen Materialproben, wie z. B. an Fotolackschichten für die Fabrikation von mikromechanischen Teilen, wurden mit verschiedenen OCT-Aufbauten durchgeführt, um die individuellen Vorteile der ultrahochauflösenden OCT, der schnellen Fourier-Domain-OCT und der polarisationssensitiven OCT aufzuzeigen.

A case study of use of negative type SU-8 X-ray sensitive resist for fabrication of advanced, highly precise, ultra tall direct LIGA mechanical microparts is presented in this paper. Using direct LIGA technique, ∼1 mm tall highly precise metallic gear wheels are being fabricated, previously using PMMA based process. Starting from a non-optimized non-satisfying SU-8 process, significant process parameters for process optimization were identified using statistical design of experiment. By varying the significant process parameters, SU-8 process was further optimized with respect to critical aspect of sidewall bow and tilt of metallic structures. After the optimization, metallic parts fabricated using SU-8 process showed comparable quality as those fabricated using PMMA based process.

LIGA is the basic idea of promising developments for the manufacturing of microelectromechanical system parts containing high aspect ratio microstructures. Aim of the work is a brief discussion of the starting-points for inline process inspection within a direct LIGA technology using deep X-ray lithography for the production of micromechanical gear wheels with critical dimensions of ∼35 μm width at ∼1 mm height as well as to show methodic and technical measuring possibilities. Firstly, results of the determination of residual solvent content distribution within ultra-thick SU-8 films are shown obtained from refracted near field optical measurements. Furthermore, the capability of X-ray computer tomographic imaging is discussed and measurements for the determination of the three-dimensional shape of high aspect ratio microstructures are practically demonstrated with microscopic and interferometric optical methods. Finally, first results demonstrate the potential of the optical coherence tomography for several further important measurement tasks, among others, e.g. for the imaging of the distribution of mechanical stress at the resist–substrate interface. The results show that much information which is essential in the LIGA process can be achieved with recently available measurement methods. However, further development of non-destructive measurement techniques would be desirable for an effective inline process control of mass production of micromechanical parts.

A novel crosslinkable, conductive, highly transparent composite material based on a photoresist and an ionic liquid (the names of the composites are not announced here due to the current procedure of patenting) is presented. The composite possesses a good and stable ionic conductivity (up to 10 mS cm−1 at room temperature) over a wide frequency bandwidth (1 kHz – 1 MHz) and is optically transparent (transmission value of 90 % for a 170 µm thick film). In addition, an approach for the patterning of the composite material by conventional photolithography with a good spatial resolution (line width of 20 – 30 µm) is introduced. The unique properties of the material are utilized for time- and cost-saving direct manufacturing of electrically conductive, highly transparent microcomponents.

We describe a new line of SUEX thick epoxy resist sheets which are pre-cut to substrate dimensions and can be made to thicknesses of 100µm to more than 1mm. The sheets need only to be laminated to a substrate surface and are ready to use in minutes. We will describe the preliminary results of our developmental antimony-free thick dry film epoxy sheets (TDFS) for consideration as a next generation multi-purpose MEMS material. SUEX sheets show higher transparency than other currently available epoxy resist films making them highly suited for thick resist applications. The initial results show excellent uniformity, resolution and side wall profiles.

Optical coherence tomography (OCT), originally developed and so far nearly exclusively used for biomedical applications (e.g., [1,2]), is a contact-free, non-destructive technique based on low-coherence interferometry to image structures within translucent and turbid materials. Commonly, cross-sectional reflectivity images with a depth-resolution determined by the coherence length of the near-infrared light source are obtained. When OCT is performed in a polarization sensitive way (PS-OCT), additional information about birefringence within a material is obtained by mapping the retardation between ordinary and extraordinary rays [3]. Because birefringence is induced when strain occurs, PS-OCT provides depth resolved information about the internal stress within a sample.

Optical coherence tomography (OCT) is a contactless and non-invasive technique nearly exclusively applied for bio-medical imaging of tissues. Besides the internal structure, additionally strains within the sample can be mapped when OCT is performed in a polarization sensitive (PS) way. In this work, we demonstrate the benefits of PS-OCT imaging for non-biological applications. We have developed the OCT technique beyond the state-of-the-art: based on transversal ultra-high resolution (UHR-)OCT, where an axial resolution below 2 μm within materials is obtained using a femtosecond laser as light source, we have modified the setup for polarization sensitive measurements (transversal UHR-PS-OCT). We perform structural analysis and strain mapping for different types of samples: for a highly strained elastomer specimen we demonstrate the necessity of UHR-imaging. Furthermore, we investigate epoxy waveguide structures, photoresist moulds for the fabrication of micro-electromechanical parts (MEMS), and the glass-fibre composite outer shell of helicopter rotor blades where cracks are present. For these examples, transversal scanning UHR-PS-OCT is shown to provide important information about the structural properties and the strain distribution within the samples.

Optical coherence tomography (OCT), so far mainly used in the biomedical field, has a high potential as non-destructive and contactless technique for material characterization and analysis. For these applications, OCT systems with ultra-high resolution in the micrometer range and capable of high imaging speeds are required. In this work, we combine ultra-high resolution imaging using a femtosecond Ti:sapphire laser as light source with the concepts of transversal OCT. Based on acquisition by heterodyne detection via acousto-optic modulators (AOMs), and by using an xy-galvano scanner unit we are able to obtain en-face scans with sizes as large as 3 x 3 mm2 within a few seconds. The ultra-high resolution of our OCT system of 2.95 mm axially and 4 mm laterally, both in air, is shown to be essential for imaging of different compounds and fibre materials. We demonstrate the benefits of en-face scanning OCT for various applications in material investigation where in-plane information is of interest which can hardly be obtained by cross-sectional OCT.

Poly-methylmethacrylate (PMMA), a positive resist, is the most commonly used resist for deep X-ray lithography (DXRL)/LIGA technology. Although PMMA offers superior quality with respect to accuracy and sidewall roughness but it is also extremely insensitive. In this paper, we present our research results on SU-8 as negative resist for deep X-ray lithography. The results show that SU-8 is over two order of magnitude more sensitive to X-ray radiation than PMMA and the accuracy of the SU-8 microstructures fabricated by deep X-ray lithography is superior to UV-lithography and comparable to PMMA structures. The good pattern quality together with the high sensitivity offers rapid prototyping and direct LIGA capability. Moreover, the combinational use of UV and X-ray lithography as well as the use of positive and negative resists made it possible to fabricate complex multi-level 3D microstructures. The new process can be used to fabricate complex multi-level 3D structures for MEMS, MOEMS, Bio-MEMS or other micro-devices.

The occurrence of internal stress of ultrathick photoresists, like SU-8 is a well known problem in lithographic processes. We investigated chemically modified SU-8-like photoresists to extend the processing latitude by reducing the internal stress of the resist images. Firstly, the composition of the polymeric binder and secondly that of the photoacidgenerator (PAG) was changed. The influence of these two variations on the stress behavior, the process conditions and the lithographic performance was studied in resist layers of 250micrometers and of 65-140 micrometers thickness, respectively. The chemical modification resulted in a drastic reduction of the internal stress occurring during the post exposure bake and by an additional hardbake. In comparison to SU-8, stress values of the modified resists reduced by 70% were achieved. With optimized process conditions for each test resist the improvement of stress behavior was linked with a lithographic performance yielding high-quality patterns with high resolution and a good aspect ratio.

Optical coherence tomography (OCT) and its extension, polarization-sensitive (PS-)OCT, are techniques for contactless and nondestructive imaging of internal structures. In this work, we apply PS-OCT for material characterization. We use a transversal scanning, ultra-high resolution (UHR-)PS-OCT setup providing cross-sectional as well as inplane information about the internal microstructure, the birefringence and the orientation of the optical axis within the material. We perform structural analysis and strain-mapping for different samples: we show the necessity of UHR imaging for a highly strained elastomer sample, and we discuss the effect of large birefringence on the PS-OCT images. Furthermore, we investigate high-aspect ratio photoresist moulds for the production of microelectromechanical parts (MEMS), demonstrating that transversal UHR-PSOCT is a promising tool for non-destructive strain-mapping.