Characterization of EUV induced contamination on multilayer optics

Extreme ultraviolet lithography (EUVL) is a next generation photolithographic technique that uses 13.5 nm or Extreme UV radiation and multilayer coated reflective optics. The reflectance of these optical elements can be strongly reduced when, as a consequence of exposing the optics to EUV photons, a contamination layer is built up on the mirrors. Since this will reduce the throughput of EUV lithography machines, contamination monitoring is considered to be necessary. Direct observation of the EUV reflectance of the mirrors is hardly possible since the required accuracy can only be achieved in very sophisticated lab reflectometers.\ud This thesis describes experimental research on the topic of EUV induced contamination and its monitoring using alternative, in situ techniques. Occasional techniques for such a task have been mentioned, but no real investigations were carried out. This thesis reviews the suggested and new techniques and describes experimental work on the three most promising: laser-generated surface acoustic waves (LG-SAWs), secondary electron yield (SEY), and spectroscopic ellipsometry (SE). The goal was not only to develop an appropriate monitoring method, but also to get insight in the material properties, both mechanical and optical, of the contamination layer in order to predict the EUV reflectance loss and the possibilities to clean the optics

Carbon induced extreme ultraviolet (EUV) reflectance loss characterized using visible-light ellipsometry

Carbon deposition on extreme ultraviolet (EUV) optics was observed due to photon-induced dissociation of hydrocarbons in a EUV lithography environment. The reflectance loss of the multilayer mirror is determined by the carbon layer thickness and density. To study the influence of various forms of carbon, EUV-induced carbon, hot filament and e-beam evaporated carbon were deposited on EUV multilayer mirrors. Spectroscopic ellipsometry was used to determine the carbon layer thickness and the optical constants ranging from ultraviolet to near infrared. The carbon density (and thus reflectance loss) was determined from the optical constants using both Bruggeman's effective medium approximation and the Clausius–Mosotti equation. Both approaches result in a similar EUV reflectance loss, with an accuracy of about 4%. The application of this process to ultrathin carbon films is further discussed

Ellipsometry with an undetermined polarization state

We show that, under the right conditions, one can make highly accurate polarization-based measurements without knowing the absolute polarization state of the probing light field. It is shown that light, passed through a randomly varying birefringent material has a well-defined orbit on the Poincare sphere, which we term a generalized polarization state, that is preserved. Changes to the generalized polarization state can then be used in place of the absolute polarization states that make up the generalized state, to measure the change in polarization due to a sample under investigation. We illustrate the usefulness of this analysis approach by demonstrating fiber-based ellipsometry, where the polarization state of the probe light is unknown, and, yet, the ellipsometric angles of the investigated sample ($\Psi$ and $\Delta$) are obtained with an accuracy comparable to that of conventional ellipsometry instruments by measuring changes to the generalized polarization state.Comment: 6 pages, 4 figures, 1 tabl

Research progress of laser welding process dynamic monitoring technology based on plasma characteristics signal

During the high-power laser welding process, plasmas are induced by the evaporation of metal under laser radiation, which can affect the coupling of laser energy and the workpiece, and ultimately impact on the reliability of laser welding quality and process directly. The research of laser-induced plasma is a focus in high-power deep penetration welding field, which provides a promising research area for realizing the automation of welding process quality inspection. In recent years, the research of laser welding process dynamic monitoring technology based on plasma characteristics is mainly in two aspects, namely the research of plasma signal detection and the research of laser welding process modeling. The laser-induced plasma in the laser welding is introduced, and the related research of laser welding process dynamic monitoring technology based on plasma characteristics at home and abroad is analyzed. The current problems in the field are summarized, and the future development trend is put forward

Quality Monitoring for Laser Welding Based on High-Speed Photography and Support Vector Machine

In order to improve the prediction ability of welding quality during high-power disk laser welding, a new approach was proposed and applied in the classification of the dynamic features of metal vapor plume. Six features were extracted through the color image processing method. Three features, including the area of plume, number of spatters, and horizontal coordinate of plume centroid, were selected based on the classification accuracy rates and Pearson product-moment correlation coefficients. A support vector machine model was adopted to classify the welding quality status into two categories, good or poor. The results demonstrated that the support vector machine model established according to the selected features had satisfactory prediction and generalization ability. The classification accuracy rate was higher than 90%, and the model could be applied in the prediction of welding quality during high-power disk laser welding

Real-time Monitoring for Disk Laser Welding Based on Feature Selection and SVM

In order to automatically evaluate the welding quality during high-power disk laser welding, a real-time monitoring system was developed. The images of laser-induced metal vapor during welding were captured and fifteen features were extracted. A feature selection method based on a sequential forward floating selection algorithm was employed to identify the optimal feature subset, and a support vector machine (SVM) classifier was built to recognize the welding quality. The experiment results demonstrated that this method had satisfactory performance, and could be applied in laser welding monitoring applications

Fiber-based ellipsometry for in situ monitoring

A limitation of in situ ellipsometry is line-of-sight access. We show that accurate and precise values for Y and D can be obtained while using a polarization preserving fiber to deliver light to the sample

Characterization of EUV induced carbon films using laser-generated surface acoustic waves

The deposition of carbon layers on the surfaces of optics exposed to extreme ultraviolet (EUV) radiation has been observed in EUV lithography. It has become of critical importance to detect the presence of the carbon layer in the order of nanometer thickness due to carbon's extremely strong absorption of EUV radiation. Furthermore, the development of efficient cleaning strategies requires that the nature of these carbon layers is well understood. Here, we present experimental results on the detection and characterization of carbon layers, grown on Mo/Si EUV reflecting optics, by laser-generated surface acoustic waves (LG-SAW). It was found that SAW pulses with a frequency bandwidth of more than 220 MHz can be generated and detected for multilayer mirrors and LG-SAW is sensitive enough to detect EUV induced carbon layers less than 5 nm thick. It was inferred from the low Young's modulus (< 100 GPa) that the carbon layer induced by EUV illumination in these vacuum conditions is mechanically soft and polymeric in nature with a high percentage of hydrogen