EUV Reflectometry

The routine availability of high-accuracy soft X-ray radiometry is a prerequisite for the development of extreme UV lithography (EUVL). In order to meet these demands, the PTB pursues the characterization and calibration of optical components and radiation detectors in the VUV and soft X-ray spectral regions as a major task. A large reflectometer (Fig. 1) enables the PTB to characterize EUVL optics up to 550 mm in diameter and 50 kg in mass, meeting the demands for the present designs of projection optics². The sample can be positioned with six axes, see Table 1. For the peak spectral reflectance of a mirror in the EUV region, an absolute uncertainty of u=0.10% is achieved (Table 2). The long-term reproducibility of the peak wavelength¹ is better than 1.1 pm with a short-term repeatability of 0.06 pm.

Fig.1 Mechanics of the EUV reflectometer.
The available motions for sample and detector are indicated.

Axis	Range	Accuracy
X	180 mm	10 μm
Y	310 mm	10 μm
Z	155 mm	10 μm
Φ	360˚	0.01˚
Tilt	20˚	0.01˚
Θ	125˚	0.01˚
2Θ	195˚	0.01˚
Det.-X	120 mm	10 μm
Det.-R	700 mm	100 μm
Ψ	180˚	0.01˚

Table 1 Compilation of sample and detector movements of the EUV reflectometer.

The measurements at PTB are carried out using highly polarized synchrotron radiation whereas EUVL uses EUV pulsed plasma sources emitting unpolarized radiation. For a full understanding and specification of the optics it is therefore essential to know the polarization properties of the optical components³.

Peak reflectance	Uncertainty contribution u/%	Peak wavelength	Uncertainty contribution u/pm
Stability of normalized intensity	0.02	Repeatability of wavelength	0.06
Inhomogeneity of the detector	0.04	Reproducibility of wavelength (reference to Be K-edge)	1.1
Higher diffraction orders	0.02	Kr resonance wavelength	1.6
Diffuse scattered light	0.08	sample temperature (ΔT = 5 K)	0.6
		incidence angle (Θ = 0.02˚ at Θ = 1.5˚)	0.1
Total uncertainty of peak reflectance	0.10	Total uncertainty of peak wavelength	2.0

Table 2 Compilation of uncertainty contributions for measured peak reflectance and peak wavelength.

References

¹F. Scholze, J. Tümmler and G. Ulm

High-accuracy radiometry in the EUV range at the PTB soft x-ray beamline

Metrologia 40, S224-S228 (2003)

²J. Tümmler, H. Blume, G. Brandt, J. Eden, B. Meyer, H. Scherr, F. Scholz, F. Scholze, G. Ulm

Characterization of the PTB EUV reflectometry facility for large EUVL optical components

Proc. SPIE 5037, 265-273 (2003)

³F. Scholze, C. Laubis, C. Buchholz, A. Fischer, S. Plöger, F. Scholz, G. Ulm

Polarization dependence of multilayer reflectance in the EUV spectral range

Proc. SPIE 6151, 615137 (2006)

last update: 2011-09-30