<title>Experimental validation of a thermal model used to predict the image placement error of a scanned EUVL reticle</title>

Abstract

Lithographic masks must maintain dimensional stability during exposure in a lithographic tool to minimize subsequent overlay errors. In extreme ultraviolet lithography (EUVL), multilayer coatings are deposited on a mask substrate to make the mask surface reflective at EUV wavelengths. About 40% of the incident EUV light is absorbed by the multilayer coating which leads to a temperature rise. The choice of mask substrate material and absorber affects the magnitude of thermal distortion. Finite element modeling has been used to investigate potential mask materials and to explore the efficiency ofvarious thermal management strategies. An experimental program was conducted to validate the thermal models used to predict the performance of EUV reticles. The experiments closely resembled actual conditions expected within the EUV tool. A reticle instrumented with temperature sensors was mounted on a scanning stage with an electrostatic chuck. An actively cooled isolation plate was mounted in front of the reticle for thermal management. Experimental power levels at the reticle corresponding to production throughput levels were utilized in the experiments. Both silicon and low expansion glass reticles were tested. Temperatures were measured at several locations on the reticle and tracked over time as the illuminated reticle was scanned. The experimental results coupled with the predictive modeling capability validates the assertion that the use of a low expansion glass will satisfy image placement error requirements down to the 3Onm lithographic node.

Comparison of EUV and optical device wafer heating

“…A contact conductance of 150 W/m 2 -K was originally estimated for clamping of the EUVL reticle to the flat chuck. 10 Because of the uncertainty of this contact condition, (influenced by the thermal conductivities of substrate and chuck, surface roughness, and interface pressure), a sensitivity study of the effect of contact conductance between the wafer and chuck was performed. In the thermal simulations, resist sensitivities of 10 mJ/cm 2 and 20 mJ/cm 2 were used for EUVL and 193-nm optical, respectively.…”

Thermomechanical modeling of the pin-chucked EUV reticle during exposure

“…In the past, thermal issues have been investigated only for the device wafer chucking. [1][2][3] Recently however, the research on thermal issues has spread to the reticle side of the manufacturing process. [4][5][6][7][8] One concern is that particles may become lodged between the chuck and reticle, inducing distortions once the reticle is chucked flat.…”

“…2,3 Thermomechanical analyses of electrostatic-chucked wafers have been previously investigated. [1][2][3][4] In this paper, the thermomechanical response of an EUV reticle during exposure has been determined, and preliminary design curves will be presented.…”