Sandy Peterhänsel

Development of functional sub-100 nm structures with 3D two-photon polymerization technique and optical methods for characterization

Investigations of two-photon polymerization (TPP) with sub-100 nm in the structuring resolution are presented by using photosensitive sol-gel material. The high photosensitivity of this material allows for TPP using a large variety in laser pulse durations covering a range between sub-10 fs and ≈140 fs. In this study, the authors demonstrate TPP structuring to obtain sub-100 nm in resolution by different approaches, namely, by adding a cross-linker to the material and polymerization with sub-10 fs short pulses. Additionally, a simulation and model based characterization method for periodic sub-100 nm structures was implemented and applied in an experimental white light interference Fourier-Scatterometry setup.

Phase errors in high line density CGH used for aspheric testing: beyond scalar approximation

One common way to measure asphere and freeform surfaces is the interferometric Null test, where a computer generated hologram (CGH) is placed in the object path of the interferometer. If undetected phase errors are present in the CGH, the measurement will show systematic errors. Therefore the absolute phase of this element has to be known. This phase is often calculated using scalar diffraction theory. In this paper we discuss the limitations of this theory for the prediction of the absolute phase generated by different implementations of CGH. Furthermore, for regions where scalar approximation is no longer valid, rigorous simulations are performed to identify phase sensitive structure parameters and evaluate fabrication tolerances for typical gratings.

Depth sensitive Fourier-Scatterometry for the characterization of sub-100 nm periodic structures

Recently Fourier-Scatterometry has become of increasing interest for quantitative wafer metrology. But also in other fields the fast and precise optical characterization of periodical gratings of sub 100 nm size is of great interest. We present the application of Fourier-Scatterometry, extended by the use of the coherent properties of white light for the characterization of sub-wavelength periodic gratings of photosensitive material structured by two-photon polymerization. First a simulation-based sensitivity comparison of Fourier-Scatterometry at one fixed wavelength, Fourier-Scatterometry using a white light light source and also additionally using a reference-branch for white-light-interference has been carried out. The investigated structures include gratings produced by two-photon polymerization of photosensitive material and typical semiconductor test gratings. The simulations were performed using the rigorous-coupled-waveanalysis included in our software package MicroSim. A sensitivity comparison between both methods is presented for the mentioned structure types. We also show our experimental implementation of the measurement setup using a whitelight- laser and a modified microscope with a high-NA (NA: 0.95) objective as well as a Linnik-type reference branch for the phase sensitive measurements. First measurements for the investigation of the performance of this measurement setup are presented for comparison with the simulation results.

Detection of overlay error in double patterning gratings using phase-structured illumination.

With the help of simulations we study the benefits of using coherent, phase-structured illumination to detect the overlay error in resist gratings fabricated by double patterning. Evaluating the intensity and phase distribution along the focused spot of a high numerical aperture microscope, the capability of detecting magnitude and direction of overlay errors in the range of a few nanometers is investigated for a wide range of gratings. Furthermore, two measurement approaches are presented and tested for their reliability in the presence of white Gaussian noise.

Solving the inverse grating problem with the naked eye

We make use of the color sensitivity of the naked human eye to solve the inverse grating problem. We conduct color-matching experiments between simulated colors and the color of the zero diffraction order, and show that human color vision may reveal structure dimensions at an accuracy in the order of ten nanometers, which is comparable to the precision of destructive methods such as scanning electron microscopy. Our results suggest that for a wide range of structures, the color observation may help to get quick, but still accurate, results, without any sophisticated instrumentation.

Human color vision provides nanoscale accuracy in thin-film thickness characterization

We study how accurately a naked human eye can determine the thickness of thin films from the observed color. Our approach is based on a color-matching experiment between thin-film samples and a simulated color field shown on an LCD monitor. We found that the human color observation provides an extremely accurate evaluation of the film thickness, and is comparable to sophisticated instrumental methods. The remaining color differences for the matched color pairs are close to the literature value for the smallest visually perceivable color difference.

Limits of diffractometric reconstruction of line gratings when using scalar diffraction theory

We study errors that occur in geometry and phase reconstruction when using scalar diffraction theory in line gratings with periods below 10 μm. The application of those gratings in so-called computer-generated holograms in high-precision interferometric testing of aspheres and free-form surfaces imposes high demands on the generated phase, leading to error budgets in the range of λ/100. Using rigorous simulations as references, we identify the limits where scalar diffraction theory fails to accurately describe grating geometries and identify the significant error mechanisms.

Measurement of asymmetric side wall angles by coherent scanning Fourier scatterometry

We propose a measurement technique which enables the precise determination of side wall angles (SWAs) with absolute values below 1°. Our simulations show that a differentiation between asymmetric SWAs is also possible. The grating structure under investigation has a grating period on the order of a few micrometers. Each grating line consists of a fine sub-grating with 40 nm period and 20 nm critical dimension. Our approach is based on coherent high-NA Fourier scatterometry, extended by a lateral scan over the sample. Additionally, a 180°-shearing element allows for coherent superposition of the higher diffraction orders.

Robust determination of asymmetric side wall angles by means of coherent scanning Fourier scatterometry

Optical metrology of grating parameters with small scattering volumes, such as side wall angles (SWAs), is an indispensable prerequisite for accurate process control in modern semiconductor lithography. However, current scatterometric technologies suffer from low sensitivity towards SWA and hence, large measurement uncertainties. In order to overcome this deficit, we propose an interferometric sensor design which enables the precise determination of asymmetric SWAs with values that deviate by less than 1° from the ideal 90°. Our measurement technique is based on coherent scanning Fourier scatterometry, extended by a reference arm in Mach-Zehnder/Linnik configuration, a spatially-structured aperture stop in the object arm, and a self-referencing shearing element in front of the detector. We demonstrate the validity and advantages of our approach by presenting rigorous simulations of an exemplary silicon line grating with a grating period of 800 nm. Each grating line consists of a fine sub-grating with 40 nm pitch and 20 nm critical dimension. A variation of the major grating parameters height and critical dimension highlights the robustness of the method. Although our simulation study focuses on the determination of asymmetric SWAs, it should be noted that the presented technique features high sensitivity towards all kinds of structural asymmetries, such as floor tilt or asymmetric bottom roundings.

Phase-structured illumination as a tool to detect nanometer asymmetries

Abstract. Phase-structured illumination is investigated as a possible extension of scatterometric measurement methods for silicon line gratings. This is done by means of rigorous simulations. Special emphasis is put on the capability of this approach to detect nanoscale fabrication asymmetries such as sidewall angles, bottom rounding, and floor tilt. The studied setup features a focused spot (numerical aperture=0.7), i.e., scanned over the sample, while analyzing the phase distribution in the image plane. This phase distribution can be accessed via holographic imaging. The results are compared to conventional nonstructured illumination. It is shown that by employing phase structuring, the resulting phase changes are larger, even if only symmetric deviations are considered. For asymmetric deviations, phase-structured illumination provides much higher sensitivity and better capability to detect the sign of the asymmetry.