Measurement of initial absorption of fused silica at 193nm using laser induced deflection technique (LID)

The ongoing development in microlithography towards further miniaturization of structures creates a strong demand for lens material with nearly ideal optical properties. Beside the highly demanding requirements on homogeneity and stress induced birefringence (SIB), low absorption is a key factor. Even a small absorption is associated with a temperature increase and results in thermally induced local variations of refractive index and SIB. This could affect the achievable resolution of the lithographic process. The total absorption of the material is composed of initial absorption and of absorption induced during irradiation. Thus, the optimization of both improves the lifetime of the material. In principal, it is possible to measure transmission and scattering with a suitable spectrometer assembly and calculate absorption from them. However, owing to the influence of sample surfaces and errors of measurement, these methods usually do not provide satisfactory results for highly light-transmissive fused silica. Therefore, it is most desirable to find a technique that is capable of directly measuring absorption coefficients in the range of (1...10)•10-4 cm-1 (base 10) directly. We report our first results for fused silica achieved with the LID technique. Besides a fused silica grade designed for 193 nm applications, grades with higher absorption at 193 nm were measured to test the LID technique. A special focus was set on the possibility of measuring initial absorption without the influence of degradation effects.

[1]  Detlev Ristau,et al.  Calorimetric measurement of optical absorption and transmissivity with sub-ppm sensitivity , 1996, Optical Systems Design.

[2]  Bodo Kuehn,et al.  KrF laser-induced absorption in synthetic fused silica , 1997, Laser Damage.

[3]  W. Triebel,et al.  Application of the laser induced deflection (LID) technique for low absorption measurements in bulk materials and coatings , 2005, SPIE Optical Systems Design.

[4]  Klaus R. Mann,et al.  High-resolution calorimetric absorption measurements on optical components for excimer lasers , 1997, Laser Damage.

[5]  Andreas Tünnermann,et al.  Bulk scattering properties of synthetic fused silica at 193 nm. , 2006, Optics express.

[6]  Klaus R. Mann,et al.  Surface and bulk absorption in CaF2 at 193 and 157 nm , 2005 .

[7]  Klaus R. Mann,et al.  Optical metrology in the VUV and EUV spectral range , 2002, SPIE Optics + Photonics.

[8]  Sergey A. Kuchinsky,et al.  Scattering losses in fused silica and CaF2 for DUV applications , 2003, SPIE Advanced Lithography.

[9]  Norbert Kaiser,et al.  Investigation of the absorption induced damage in ultraviolet dielectric thin films , 1997 .

[10]  W. Triebel,et al.  Novel method to measure bulk absorption in optically transparent materials , 2000 .

[11]  K O Greulich,et al.  Transient 210-nm absorption in fused silica induced by high-power UV laser irradiation. , 1991, Optics letters.

[12]  K. Mann,et al.  Characterization of absorption losses in deep UV optical materials , 2007, SPIE Laser Damage.