Conference papers

Laser focus sensors (LFS)^1 attached to a scanning nano-positioning and measuring machine (NPMM) enable near diffraction limit resolution with very large measuring areas up to 200 x 200 mm^1. Further extensions are planned to address wafer sizes of 8 inch and beyond. Thus, they are preferably suited for micro-metrology on large wafers. On the other hand, the minimum lateral features in state-of-the-art semiconductor industry are as small as a few nanometer and therefore far beyond the resolution limits of classical optics. New techniques such as OCD or ODP^3,4 a.k.a. as scatterometry have helped to overcome these constraints considerably. However, scatterometry relies on regular patterns and therefore, the measurements have to be performed on special reference gratings or boxes rather than in-die. Consequently, there is a gap between measurement and the actual structure of interest which becomes more and more an issues with shrinking feature sizes. On the other hand, near-field approaches would also allow to extent the resolution limit greatly^5 but they require very challenging controls to keep the working distance small enough to stay within the near field zone. Therefore, the feasibility and the limits of a LFS scanner system have been investigated theoretically. Based on simulations of laser focus sensor scanning across simple topographies, it was found that there is potential to overcome the diffraction limitations to some extent by means of vicinity interference effects caused by the optical interaction of adjacent topography features. We think that it might be well possible to reconstruct the diffracting profile by means of rigorous diffraction simulation based on a thorough model of the laser focus sensor optics in combination with topography diffraction^6 in a similar way as applied in OCD. The difference lies in the kind of signal itself which has to be modeled. While standard OCD is based on spectra, LFS utilizes height scan signals. Simulation results are presented for different types of topographies (dense vs. sparse, regular vs. single) with lateral features near and beyond the classical resolution limit. Moreover, the influence of topography height on the detectability is investigated. To this end, several sensor principles and polarization setups are considered such as a dual color pin hole sensor and a Foucault knife sensor. It is shown that resolution beyond the Abbe or Rayleigh limit is possible even with "classical" optical setups when combining measurements with sophisticated profile retrieval techniques and some a-priori knowledge. Finally, measurement uncertainties are derived based on perturbation simulations according to the method presented in 7.

A novel approach of measuring small volumes is made in our project. The goal is to utilise the Nano Measuring Machine NMM 1 in combination with a laser focus sensor to determine the volume of small water droplets. The droplet surface is scanned by the focus sensor with picometre resolution. Based on the shape of the scan lines, the actual volume of the droplet can be calculated. However, due to the small distance between the water surface and the glass carrier, the laser focus sensor signal was distorted by undesired reflections from the glass carrier the droplets are deposited on. To reduce these undesired reflections, the BK7 carrier was anti-reflection coated. When a SiO2 layer of appropriate thickness is deposited on the carrier, the reflectance at the water-glass interface can be considerably lowered. This drastically improves the sensor signal, thus enables the correct measurement of the droplets' shapes and hence the calculation of their volume.

A small multiple fixed-point cell (SMFPC) was designed to be used as in situ calibration reference of the internal temperature sensor of a dry block calibrator, which would allow its traceable calibration to the International Temperature Scale of 1990 (ITS-90) in the operating range of the block calibrator from 70 ˚C to 430 ˚C. The ITS-90 knows in this temperature range, three fixed-point materials (FPM) indium, tin and zinc, with their respective fixed-point temperatures ([theta]FP), In ([theta]FP = 156.5985 ˚C), Sn ([theta]FP = 231.928 ˚C) and Zn ([theta]FP = 419.527 ˚C). All of these FPM are contained in the SMFPC in a separate chamber, respectively. This paper shows the result of temperature measurements carried out in the cell within a period of 16 months. The test setup used here has thermal properties similar to the dry block calibrator. The aim was to verify the metrological properties and functionality of the SMFPC for the proposed application.