Jochen Hetzler

Characterizing lateral resolution of interferometers: the Height Transfer Function (HTF)

The lateral resolution of an interferometer is limited mainly by the design of the optical arrangement as well as the size of the beam stop. For its characterization the MTF1,2 is not very useful. The height of a structure normal to the surface under test is transferred into a phase of a reflected wavefront. Since imaging mechanisms for intensity and phase are very different, we propose a Height Transfer Function (HTF) to describe the lateral resolution of interferometers. The HTF shows the quotient of the reconstructed and the original height of a sine-modulated surface structure as a function of the spatial frequency. The HTF can be measured with a test sample of varying periodical surface profiles and spacings. Simulations can be made using a combination of geometrical ray tracing and Fourier transformation techniques. Two different layouts of null systems for the test of an asphere are compared. A device to measure the HTF is shown along with results for a variety of different interferometers.

On the benefit of high resolution and low aberrations for in-die mask registration metrology

With the introduction of complex lithography schemes like double and multi – patterning and new design principles like gridded designs with cut masks the requirements for mask to mask overlay have increased dramatically. Still, there are some good news too for the mask industry since more mask are needed and qualified. Although always confronted with throughput demands, latest writing tool developments are able to keep pace with ever increasing pattern placement specs not only for global signatures but for in-die features within the active area. Placement specs less than 3nm (max. 3 Sigma) are expected and needed in all cases in order to keep the mask contribution to the overall overlay budget at an accepted level. The qualification of these masks relies on high precision metrology tools which have to fulfill stringent metrology as well as resolution constrains at the same time. Furthermore, multi-patterning and gridded designs with pinhole type cut masks are drivers for a paradigm shift in registration metrology from classical registration crosses to in-die registration metrology on production features. These requirements result in several challenges for registration metrology tools. The resolution of the system must be sufficiently high to resolve small production features. At the same time tighter repeatability is required. Furthermore, tool induced shift (TIS) limit the accuracy of in-die measurements. This paper discusses and demonstrates the importance of low illumination wavelength together with low aberrations for best contrast imaging for in-die registration metrology. Typical effects like tool induced shift are analyzed and evaluated using the ZEISS PROVE® registration metrology tool. Additionally, we will address performance gains when going to higher resolution. The direct impact on repeatability for small features by registration measurements will be discussed as well.

Analysis of the uniqueness of an inverse grating characterization method

We propose a method for the characterization of one- and two-dimensional diffraction gratings by means of the measurement of diffraction efficiencies. The method is based on the comparison of measured and calculated efficiencies. For the numerical calculation we use the Rigorous Coupled Wave Analysis RCWA and an optimization algorithm to determine the grating shape that fits best to the measured data. We analyzed in which cases the method is able to determine the grating shape without ambiguity and which measurement parameters should be used. By systematically analyzing a given inverse problem, we try to derive the theoretical limits of the method.