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Practical method for effective spot size characterization in spectroscopic ellipsometry/reflectometry setup

IP.com Disclosure Number: IPCOM000234146D
Publication Date: 2014-Jan-14
Document File: 3 page(s) / 125K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a practical method for effective spot size characterization in spectroscopic ellipsometry/reflectometry setup. The key idea is not to involve any type of optical modeling but to observe how the raw signal at different wavelengths (for instance intensity or polarization state of the reflected light) transforms while the measurement spot crosses a sharp thickness discontinuity at measurement box boundaries.

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Title

Practical method for effective spot size characterization in spectroscopic ellipsometry/reflectometry setup

Abstract

Disclosed is a practical method for effective spot size characterization in spectroscopic ellipsometry/reflectometry setup.

The key idea is not to involve any type of optical modeling but to observe how the raw signal at different wavelengths (for instance intensity or polarization state of the reflected light) transforms while the measurement spot crosses a sharp thickness discontinuity at measurement box boundaries.

Problem

In spectroscopic measurements of thin films and critical dimensions (CDs) by different measurement methods (e.g., ellipsometry, reflectometry (polarized or unpolarized) or a combination) the spot size of the probe beam on the sample surface (i.e. the measurement spot size) is a highly critical parameter. Currently, the measurement spot size of many production-grade optical tools, such as ellipsometers and reflectometers, is approximately ~50m in diameter. This is comparable to the size of a typical test box (the measurement box).

If the probe's spot size is larger than or equal to the measurement box size, then the signal received by tool's detector is a superposition (mixture) of light intensities originated from the test box as well as surrounding areas; mistakes in extracting physical parameters from optical modeling are then unavoidable. Thus, the spot size of the probe beam establishes the minimal feature that can be measured without degradation in measurement accuracy and use of sophisticated mixing algorithms. Therefore, it is important to have an accurate but practical method of monitoring measurement tools in terms of the associated spot size; however, this important quantity is often not easily measured to a high degree of accuracy.

Solution/Novel Contribution

The novel contribution is a practical method for effective spot size characterization in spectroscopic ellipsometry/reflectometry setup. It is based on scanning across the measurement box of a known size, but combines ordinary x- and y-directional scans into a single diagonal vector scan (Figure 1). The approach doesn't involve any optical modeling and numerical or analytical solution of ill-posed inverse problem and it is based on the analysis of variations of the raw spectra at selected wavelengths as the measurement spot crosses a sharp thickness discontinuity at measurement box boundaries. Corresponding profiles of some measurement quantities (for instance, intensity or polarization state of the reflected light) allow the users to accurately measure the effective spot size at different wavelengths, revealing hereby the aberration effects in the optical setup.


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Figure 1: Measurement of the tool's spot size by scanning diagonally the measurement box of a know...