Electrostatic chucking for extreme ultraviolet lithography: Simulations and experiments

Abstract

The purpose of this research is to assess the effectiveness of electrostatic chucks in reducing low-spatial frequency mask ͑or reticle͒ flatness variations and to validate finite element ͑FE͒ models of the chuck-mask interaction. The flatness of a sample extreme ultraviolet lithography reticle and an electrostatic pin chuck were measured using a Zygo interferometer. The measured flatness data were entered into the FE models, and electrostatic chucking was simulated by applying an area-weighted average pressure on the reticle. The shape of the mask when clamped by the electrostatic chuck was then predicted using the FE model. To validate these predictions, experiments were conducted in which the previously measured reticle was electrostatically clamped using the pin chuck. These experiments were conducted in a vacuum chamber to minimize the effects of humidity. Interferometric plots of the chucked reticle surface were obtained and compared with the FE predictions. It was found that the measured and predicted shapes were in good agreement. Results from the experiments and simulations will be used to establish specifications for the chuck design and to identify the range of flatness variations that can be accommodated with electrostatic chucking.

Analytical treatment of the deformation behavior of EUVL masks during electrostatic chucking

“…If we plot c T x , c T z and c p (see Fig. 2), two observations can be made: first, high spatial frequencies will be damped by the material and are not expected to be transmitted to the front-side (see (6) and (7)); second, for givenσ bs zz = p, the corresponding back-side amplitudeū bs z decreases as k increases (see (5)). …”

“…One method to predict for the mask deformation is based on the finite element analysis. [4][5][6] While providing the presumably most accurate results, these simulations are computationally expensive -especially when a dense mesh is needed to resolve small features of the mask and chuck non-flatness measurement. In this article we propose a new analytical method to compute the mask OPD and IPD based on the solution of the mechanical equilibrium equations in the context of linear elasticity.…”

Status of EUVL reticle chucking

“…Previous studies have shown the necessity of performing all experiments in a vacuum environment. 3 A Zygo GPI system was positioned vertically over the chamber to enable measurements of the mask nonflatness through the top viewport. Figure 6(a) is a photograph of this viewport, which uses a fused silica flat with a diameter of 8.0 in.…”

Assessing the mask clamping ability of a low thermal expansion material chuck

“…8͒. 2 To simulate chucking of the individual masks, interferometric data from the two masks and the chuck were used as input into the models. Contact friction between the backside of the mask and the surface of the chuck has been previously measured for typical EUVL materials and was assumed to be 0.2 in all of the models.…”