Focused beam spectroscopic ellipsometry method and system

Abstract

A method and system for spectroscopic ellipsometry employing reflective optics to measure a small region of a sample by reflecting radiation (preferably broadband UV, visible, and near infrared radiation) from the region. The system preferably has an autofocus assembly and a processor programmed to determine from the measurements the thickness and / or complex refractive index of a thin film on the sample. Preferably, only reflective optics are employed along the optical path between the polarizer and analyzer, a sample beam reflects with low incidence angle from each component of the reflective optics, the beam is reflectively focused to a small, compact spot on the sample at a range of high incidence angles, and an incidence angle selection element is provided for selecting for measurement only radiation reflected from the sample at a single, selected angle (or narrow range of angles). The focusing mirror preferably has an elliptical shape to reduce off-axis aberrations in the focused beam. Some embodiments include both a spectrophotometer and an ellipsometer integrated together as a single instrument. In such instrument, the spectrophotometer and ellipsometer share a radiation source, and radiation from the source can be focused by either the spectrophotometer or the ellipsometer to the same focal point on a sample. Preferred embodiments of the ellipsometer employ a rotating, minimal-length Rochon prism as a polarizer, and include a spectrometer with an intensified photodiode array to measure reflected radiation from the sample, and a reference channel (in addition to a sample channel which detects radiation reflected from the sample).

Description

FIELD OF THE INVENTION [0001] The invention relates to methods and systems for obtaining ellipsometric and reflectance measurements of a small region of a sample over a range of UV (and preferably also visible) wavelengths, and optionally also for determining, from the measurements, the thickness and refractive index of a very thin film on the sample. The sample can be a semiconductor wafer having at least one thin layer over a silicon substrate. Preferred embodiments of the invention include both a spectrophotometer and an improved spectroscopic ellipsometer which share a common focal point on the sample and preferably a common radiation source. BACKGROUND OF THE INVENTION [0002] Among the well known nondestructive testing techniques are the techniques of spectroreflectometry and spectroscopic ellipsometry, which measure reflectance data by reflecting electromagnetic radiation from a sample. In spectroscopic ellipsometry, an incident radiation beam having a known polarization state...

AI Technical Summary

Benefits of technology

[0011] The spectroscopic ellipsometry method and apparatus of the invention employs reflective optics to measure a small (and preferably compact) region of a sample (e.g., a microscopically small, square-shaped spot on the sample) by reflecting broadband radiation having a range of UV (and preferably also visible and near infrared) wavelengths from the region. The method and apparatus of the invention optionally also determines from the measurements the thickness and / or complex refractive index of a thin film on the sample (such as a layer of a multiple layer stack over a silicon substrate of a semiconductor wafer). Preferred embodiments of the inventive ellipsometer employ only reflective optics (along the optical path between the polarizer and analyzer) to avoid aberration and other undesirable effects that would otherwise result from transmission of broadband ultraviolet (UV) radiation through transmissive optics, and direct the beam so that it reflects with low incidence angle from each such reflective optical component. Preferred embodiments of the inventive ellipsometer focus a beam having elongated cross-section from an elliptical focusing mirror to a small, compact spot on the sample at a range of high incidence angles. The elliptical shape of the mirror surface reduces off-axis aberrations such as “coma” in the focused beam. Use of a reflective focusing element (rather than a transmissive lens) eliminates chromatic aberration in the focused beam.

[0012] Preferred embodiments of the invention include a spectrophotometer and an improved spectroscopic ellipsometer integrated together as a single instrument. In such integrated instrument, the spectrophotometer and ellipsometer share a broadband radiation source, and radiation from the source can be focused by either the spectrophotometer or the ellipsometer to the same focal point on a sample. Some of these embodiments include means for operating a selected one of the spectrophotometer and the ellipsometer. Others of the embodiments include means for supplying a portion of the radiation from the source to each of the spectrophotometer and ellipsometer subsystems, thus enabling simultaneous operation of both subsystems to measure the same small sample region.

[0016] In some embodiments, the inventive ellipsometer includes a reference channel (in addition to a sample channel which detects radiation reflected from the sample). Illuminating radiation from the source is split into a sample beam and a reference beam, preferably by a bifurcated optical fiber. The sample beam reflects from the surface of a sample and is directed to the sample channel detector. The reference beam does not reflect from the sample, but is directed to the reference channel detector. By processing reference signals from the reference channel detector, as well as signals from the sample channel detector, the thickness of a very thin film on the sample (or the sample's refractive index) can be more accurately determined.

Problems solved by technology

However, it had not been known how to accomplish this using an ellipsometer with all-reflective optics (for use with broadband UV radiation).

The inventors have recognized that such transmissive optics are unsuitable for use with broadband radiation of ultraviolet (or UV to near infrared) wavelengths, and have also recognized that beams of such radiation incident on reflective ellipsometer components with high incidence angles undesirably undergo a large change in polarization upon reflection from each such reflective component.

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