Uwe Leinhos

Photo-thermal measurement of absorptance losses, temperature induced wavefront deformation and compaction in DUV-optics

A measurement system for quantitative registration of transient and irreversible lens effects in DUV optics induced by absorbed UV laser radiation was developed. It is based upon a strongly improved Hartmann-Shack wavefront sensor with an extreme sensitivity of approximately lambda/10000 rms @ 193nm, accomplishing precise on-line monitoring of wavefront deformations of a collimated test laser beam transmitted through the laser-irradiated site of a sample. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of the test laser is distorted into a convex or concave lens, depending on sign and magnitude of index change and expansion. This transient wavefront distortion yields a quantitative measure of the absorption losses in the sample. In the case of fused silica, an additional permanent change indicates irreversible material compaction. Results for both fused silica and CaF(2) are presented and compared.

Changes in optical interference coatings exposed to 193-nm excimer laser radiation

The exposure of optical interference coatings to low-fluence DUV-radiation reveals changes of thin layer properties due to interactions between radiation field and thin film structure. An experimental set up for irradiating antireflective as well a high reflective coatings with 193nm excimer laser was used in order to study permanent cumulative changes in optical coatings at fluences ranging from 20mJ/cm2 with up to 240 106 laser pulses. The optical ex-situ monitoring of radiation induced modifications enabled the differentiation of coating specific and substrate inherent alteration effects. The identification of conditions as well as degradation processes during the exposure could be achieved for several types of DUV-coating materials. They were deposited with an ultra low loss evaporation process onto calcium fluoride and fused silica substrates. Fluoride coating included LaF3, Na3AlF6, MgF2, AlF3 oxide coatings consisted of SiO2 and Al2O3 exclusively.

Laser-induced damage tests under multiple wavelength irradiation of ATLID TXA optics for ESA-satellite mission EarthCare

A setup was developed at Laser-Laboratorium that allows determination of LIDT data under simultaneous 1064nm, 532nm and 355nm irradiation (3l), in order to investigate the influence of combined irradiation of specific elements with multiple wavelengths. Utilizing a fully characterized test laser beam and a well defined sample environment, 10.000-on- 1 LIDT data of high-reflecting mirrors (AOI 45°) in vacuum were determined both at multiple wavelengths (3l) and at 355nm alone. In addition, a test optics (HR355, 45°, ambient conditions) was irradiated for 150 million pulses at (3l) at a fixed fluence of 2 J/cm2 as a certification test.

Measurement of wavefront distortions in DUV optics due to lens heating

Lens heating due to absorbed UV laser radiation can diminish the achievable spatial resolution of the lithographic process in semiconductor wafer steppers. At the Laser Lab Göttingen a measurement system for quantitative registration of this thermal lens effect was developed. It is based upon a strongly improved Hartmann-Shack wavefront sensor with extreme sensitivity, accomplishing precise on-line monitoring of wavefront deformations of a collimated test laser beam transmitted through the laser-irradiated site of a sample. Caused by the temperature-dependent refractive index as well as thermal expansion, the formerly plane wavefront of the test laser is distorted to form a rotationally symmetric valley, being equivalent to a convex lens. The new sensor, which is capable to record relative changes in the range of λ/10000 (corresponding to deformations of < 100 pm), allows registration and precise characterization of induced wavefront distortions by real-time Zernike analysis. On the other hand, the photo-thermal technique can be employed for a rapid assessment of the material quality, since the extent of transient wavefront deformation is directly proportional to the absorption losses. When used in orthogonal test geometry on cuboid samples, quantitative determination of both surface and bulk contributions to the overall absorption can be obtained by comparison with thermal theory. Along with a description of the new technique we present photo-thermal measurements on various fused silica samples under 193 nm irradiation. The data are compared with theoretical results obtained from a semi-analytical solution of the heat diffusion equation. Excellent agreement is achieved regarding both shape and extent of the lens heating effect.

Interfacial absorption of DUV coatings

Complicated dielectric coatings consist of a large number of layers and thus have many interfaces, that may contribute to the total absorption of the coatings. We examined this contribution of the interfaces using two different approaches. For the determination of the absorption of the first interface between the substrate and the coatings we varied the thickness of dielectric single layers. For the examination of the influence of the interfaces within a dielectric stack, coatings consisting of (lambda) /2-layers were produced and their absorption was measured.

Testing of optical components for microlithography at 193-nm and 157-nm

Absorption loss in UV optics during 193nm and 157nm irradiation is investigated by employing a high-resolution calorimetric technique which allows determining both single and two-photon absorptance at low energy densities. UV calorimetry is also employed to investigate laser induced aging phenomena, e.g. color center formation in fused silica. A separation of transient and cumulative effects can be achieved, giving insight into the loss mechanisms. A strong wavelength dependence of the DUV and VUV absorption characteristics in CaF2 substrates is observed. In addition, Hartmann-Shack wavefront measurements are presented, which allow on-line monitoring of laser-induced compaction in fused silica.

Stability of optical interference coatings exposed to low-fluence 193-nm ArF radiation

We report on our investigations on the long-term behavior of optical coatings under 193 nm laser irradiation in dependence on coating materials, radiation conditions, and substrate properties. A wide variety of different highly reflective dielectric mirrors and antireflective coatings, deposited by an ultra low loss evaporation process onto calcium fluoride and fused silica, has been tested. Irradiation experiments with highly reflective coatings show that fluoride coatings exhibit nearly no changes of their optical function in air as well as in argon atmosphere due to low initial absorption levels. Temporal atmospheric contaminations can be removed by using appropriate irradiation conditions. Oxide layers tend to post-oxidize during 193 nm exposure in air and the DUV absorption level will be reduced. Effectively, reflectance of multilayer coatings on the basis of oxide materials can be improved through laser irradiation. Irradiation experiments with antireflective coatings point out the dominant role of bulk and surface properties of the substrate for prolonged laser irradiation. In addition, we present laser induced damage thresholds to demonstrate upper limits of laser radiation resistance that can be achieved nowadays with several types of coatings.

Quantitative absorption data from thermally induced wavefront distortions on UV, Vis, and NIR optics

A photothermal absorption measurement system was set up, deploying a Hartmann-Shack wavefront sensor with extreme sensitivity to accomplish spatially resolved monitoring of thermally induced wavefront distortions. Photothermal absorption measurements in the near-infrared and deep ultra-violet spectral range are performed for the characterization of optical materials, utilizing a Yb fiber laser (λ = 1070 nm) and an excimer laser (193nm, 248nm) to induce thermal load. Wavefront deformations as low as 50pm (rms) can be registered, allowing for a rapid assessment of material quality. Absolute calibration of the absorption data is achieved by comparison with a thermal calculation. The method accomplishes not only to measure absorptances of plane optical elements, but also wavefront deformations and focal shifts in lenses as well as in complex optical systems, such as e.g. F-Theta objectives used in industrial high power laser applications. Along with a description of the technique we present results from absorption measurements on coated and uncoated optics at various laser wavelengths ranging from deep UV to near IR.

Separation of different loss channels in DUV optical elements

A photo-thermal measurement device for quantitative determination of absorptance in DUV optics was developed. It is based upon a Hartmann-Shack wavefront sensor with extreme sensitivity, accomplishing spatially resolved monitoring of thermally induced wavefront distortions. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of a test laser is distorted into a convex or concave lens, depending on sign and magnitude of index change and expansion. Since the extent of deformation is directly proportional to the absorption loss, the parallelized photo-thermal technique can be employed for a rapid assessment of the material quality. Monitoring the fluence dependence of the thermal lens effect accomplishes evaluation of both single- and two-photon absorption coefficients. Moreover, a separation of surface and bulk absorptance can be achieved. Along with a description of the technique we present results from absorption measurements on fused silica and CaF2 under 193nm irradiation. The data are compared with theoretical results obtained from a solution of the heat diffusion equation.

Long-term laser irradiation tests of optical elements for ESA mission ADM-Aeolus

The European Space Agency ESA is running a series of earth observation missions. In order to perform global windprofile observation based on Doppler-LIDAR, the satellite ADM-Aelolus will be launched in April 2011 and injected into an orbit 400 km above Earths surface. ADM-Aeolus will be the first satellite ever that is equipped with a UV-laser (emitting at 355 nm) and a reflector telescope. At LLG, a setup was developed that allows monitoring transmission, reflection and fluorescence of laser-irradiated optical components, in order to assess their possible optical degradation due to radiation-induced contaminant deposition in orbit. For both a high-reflecting mirror and an anti-reflective coated window long-term irradiation tests (up to 500 million laser pulses) were performed at a base pressure < 10-9 mbar, using a XeF excimer laser (wavelength 351 nm, repetition rate 1kHz). At this, samples of polymers used inside the satellite (insulators for cabling, adhesives, etc.) were installed into the chamber, and the interaction of their degassing with the sample surfaces under laser irradiation was investigated. Various paramters were varied including pulse repetition rate, view factor and coatings. Optical degradation associated with contaminant adsorption was detected on the irradiated sample sites.