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Ellipsometry with Pulsed Tunable Laser Sources

IP.com Disclosure Number: IPCOM000086579D
Original Publication Date: 1976-Sep-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 73K

Publishing Venue

IBM

Related People

Dill, FH: AUTHOR [+3]

Abstract

Hauge and Dill [1] have described an automated Fourier ellipsometer which uses a narrow-band CW laser source. Laser sources are desirable for ellipsometry because of their high spectral radiance. There are, however, applications for automated ellipsometers (e.g., for identification of chemical residues on semiconductor surfaces) where broad-band tunability of the light source is required. In the past, the lack of tunable CW laser sources has resulted in the use of CW incoherent sources for such ellipsometers,[2] at great sacrifice in spectral radiance. In recent years, pulsed dye lasers have been developed with sufficient tunability (including tunability well into the UV with frequency-doubling crystals) for the above-mentioned applications, but the low pulse repetition rate (

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Ellipsometry with Pulsed Tunable Laser Sources

Hauge and Dill [1] have described an automated Fourier ellipsometer which uses a narrow-band CW laser source. Laser sources are desirable for ellipsometry because of their high spectral radiance. There are, however, applications for automated ellipsometers (e.g., for identification of chemical residues on semiconductor surfaces) where broad-band tunability of the light source is required. In the past, the lack of tunable CW laser sources has resulted in the use of CW incoherent sources for such ellipsometers,[2] at great sacrifice in spectral radiance. In recent years, pulsed dye lasers have been developed with sufficient tunability (including tunability well into the UV with frequency-doubling crystals) for the above-mentioned applications, but the low pulse repetition rate (</approx. 30 per second) sets an upper limit on the data acquisition rate far below that which is commonly attained with CW sources in automated Fourier ellipsometry (e.g., 1800 per second in the ellipsometric thickness analyzer (ETA) ellipsometer [1]). New techniques are thus required to take advantage of the very high spectral radiance of these lasers. For example, the number of photons available in a single one microsecond pulse from the CHROMATIX* CMX-4 tunable dye laser is about the same as would be available for detection in about one hour of data acquisition on the ETA ellipsometer, if ETA were equipped with a typical high-brightness CW incoherent source emitting into a five meV bandwidth in the visible range.

This article describes novel means of data acquisition in automated ellipsometers with pulsed laser sources. By taking maximum advantage of the large number of photons per pulse available from pulsed dye laser sources, the disadvantage of low-pulse repetition rate is overcome.

Fig. 1 shows an arrangement, using beam splitters and multiple detectors, which allows complete determination of the polarization state of light reflected from a sample directly in terms of measured Stokes parameters [3]. An optical pulse from the laser 10 is split into five parts with beam splitters 12, 14, 16 and 18, with one part going to a reference detector 20. The other four parts are first polarized and then focused onto the sample 22. The angular spread of the four beams can easily be kept negligibly small (</approx. 2 degrees). The reflected beams are then analyzed and detected by photomultipliers 24, 26, 28 and 30, as shown in the diagram and the table below. Analysis of Reflected Beams

Stokes Parameter

Beam Optical Elements in Beam Path Measured 1 Detector only S(0) 2 Analyzer (0 degrees w.r.t. plane of incidence) S(1) followed by detector. 3 Analyzer (45 degrees w.r.t. plane of incidence) S(2) followed by detector. 4 Achromatic compensator (45 degrees) followed by S(3) analyz...