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Optics characterization at IPHT: Measurement Techniques and Beyond


Optics characterization at IPHT: Measurement Techniques and Beyond

Christian M�hlig 

Laser based processing is one key technology for the convenience in our every day’s life. Maybe the most prominent but also less recognized is the optical lithography where state-of-the-art computer chips with feature sizes of 22nm are produced using intense, deep ultra-violet (DUV) laser light. Another important technology is laser processing (welding, cutting…) by means of multi-kW lasers, e.g. in the automotive industry.

In order to enable these technologies, highly sophisticated optical systems are designed. Due to their high costs, these systems needs to withstand the intense or high power laser light for very long operation times, e.g. many years in optical lithography, with barely any change in their performance. To secure that, it is required to investigate and understand possible effects of the intense laser light on the crucial optical properties.

At the Leibniz-Institute of Photonic Technology (IPHT) we use a combination of several laser based measurement techniques to investigate the impact of intense laser light on the optics’ absorption properties and to understand the underlying physical processes. The latter plays a key role in the further material and coating development.

In the talk I am going to firstly introduce the applied measurement techniques and will hereby focus on maybe not so common methods like direct absorption measurement, cavity ring-down and fs-pump-probe spectroscopy. Afterwards I will give some insights into selected investigations where the combination of the above techniques yields to a new or better understanding of the laser-matter-interaction (fused silica and CaF2 with 193nm laser light) or opens new ways to measure important optical parameters (photoluminescence quantum efficiency of silicon nanocrystals).     

Dr. Christian M�hlig

Leibniz-Institute of Photonic Technology (IPHT)

Department of Microscopy - Laser Diagnostics 

Albert-Einstein-Str. 9

07745 Jena / Germany 

Phone: +49 (0) 3641 206433

Fax:       +49 (0) 3641 206499

E-Mail: christian.muehlig@ipht-jena.de 

Short biography

Christian M�hlig studied Physics at the University of Jena (Germany) between 1990 and 1996 and received his Diploma in 1996. From 1994 until 1995 he studied at the University of Essex (Great Britain) where he received the MSc in “The Physics of Laser Communications” with distinction in 1995.

Since 1998 he has been a member of the research staff at Institute of Photonic Technology, Jena, Germany. In 2005 he received his PhD at the University of Jena (Germany) for his work on the ”Absorption of pulsed ArF laser irradiation in highly transparent optical materials”. In the same year he was awarded the IPHT-award for laser-based diagnostics of optical materials and coatings for the use in DUV laser lithography applications.

His main area of work covers the laser based characterization of optical materials and coatings by several techniques like direct absorption measurements, laser induced fluorescence and cavity-ring-down spectroscopy with the aim of understanding and modeling absorption phenomena under intense laser irradiation. In particular he is involved in the development and application of IPHT’s laser induced deflection (LID) technique to directly and absolutely measure low absorption in optical materials and coatings. 

      Main field of research:

  • Investigating the interaction between laser light and optics, especially for high-end or high-power laser applications. These applications range from laser-lithography in the deep UV range (manufacturing of high-end computer chips) to high-power laser material processing (e.g. welding in automotive industry). Optics include everything from the parts inside the lasers (focus on increasing laser efficiency and reliability) to the beam lines outside the laser and covers both, the optical materials (like many different glasses, crystals…) and the optical coatings (anti-reflecting, high-reflecting…).
  • Development of models that explain laser induced changes in the properties of optical materials, especially for high-power or high-intensity application.
  • Development and application (up to commercial systems) of laser based material characterization techniques like direct absorption measurement, cavity ring-down, laser induced fluorescence and Raman scattering.
  • Particular field of knowledge: behavior of high-end silica glass and CaF2 crystals in the deep UV range to be used for chip manufacturing optics.

      Main achievements:

  • Development of laser induced deflection technique (LID) for highly sensitive direct absorption measurements of optical materials and coatings. It is the first photo-thermal technique that is calibrated to absolute absorption data without knowledge of photo-thermal material properties. With respect to the IPHT slogan “From ideas to instruments” the technique has been developed to a customized product. There are now 3 patents on different measurement strategies for the LID technique. By applying the LID technique the current “world record” for low absorption in high-end fused silica has been measured (0.08 ppm/cm at wavelength 1030nm).
  • First experimental finding that the absorption of 193nm laser light in fused silica does not depend linear on intensity for common laser lithography applications. These findings have been supported by a new physical model for defect generation and annealing in fused silica at DUV laser wavelengths.
  • First establishment of a new short-time testing of fused silica for lithography applications which allows cutting so-called lifetime tests from several weeks to one day (patent).
  • Establishment of new physical models to explain the effect of particular impurities in CaF2 crystals on the performance under 193nm laser irradiation (several patents together with industry).
  • Proof of long-term degradation mechanisms in CaF2 under 193nm laser light using fs-pump-probe technique.

Lecture to be presented at Sapientia University Marosv�s�rhely (Tirgu-Mures), on 13 February, 2014, Thursday, by 13 p.m. 

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