Investigation of the reflectivity uniformity for EUV lithography (2013)

EUV (Extreme Ultraviolet) lithography is the most promising technique to push forward the semiconductor industry. During the development of the EUV lithography, the non-uniform reflectivity of the EUV mask received considerable concern. This project is designed to investigate this problem. The reflectivity uniformity and its stability overtime were studied by experiments and numerical simulations. Among the eight mask samples, the 3 ? value of the peak reflectivity, on average, is 0.631% (absolute value), far exceeding the industrial requirement of 0.15% (absolute value). The stability of the reflectivity is also not reliable. The potential degrading factors, the carbonization and the thermally induced inter-diffusion were analyzed based on the theoretical models of K. van Maarten and U. Gosele, respectively. Based on the previous measurements and analysis, the reflectivity of the EUV mask is suggested to be measured regularly and compensated by adjusting other system parameters. It was proposed that the reflectivity uniformity can be indirectly predicted through measuring the uniformity of the multilayer period and the roughness. However, the variations of the roughness and the period cannot well explain the peak reflectivity variation based on the scattering theory. At last, an innovative mask metrology was proposed, which is capable to in-situ monitor the multilayer degrading (contamination and thermally induced period change) through monitoring the mask apodization over time and map the reflectivity through a &quot;self-reference&quot; method. A big advantage of this metrology is that no extra hardware is required. The photolithography and the EUV lithography system are briefly introduced in Chapter I. The relevant theories and metrology tools are outlined in chapter II and III, respectively. In chapter IV, the experimental and numerical results are analyzed and discussed. At last, the proposed mask metrology was described in Chapter V. Although some nice results were obtained and an innovative mask metrology was designed, more experimental investigations are still needed to understand the correlation between the peak reflectivity, the period and roughness, and the implementation and qualification of the proposed mask metrology should follow up later. EUV (Extreme Ultraviolet) lithography is the most promising technique to push forward the semiconductor industry. During the development of the EUV lithography, the non-uniform reflectivity of the EUV mask received considerable concern. This project is designed to investigate this problem. The reflectivity uniformity and its stability overtime were studied by experiments and numerical simulations. Among the eight mask samples, the 3 ? value of the peak reflectivity, on average, is 0.631% (absolute value), far exceeding the industrial requirement of 0.15% (absolute value). The stability of the reflectivity is also not reliable. The potential degrading factors, the carbonization and the thermally induced inter-diffusion were analyzed based on the theoretical models of K. van Maarten and U. Gosele, respectively. Based on the previous measurements and analysis, the reflectivity of the EUV mask is suggested to be measured regularly and compensated by adjusting other system parameters. It was proposed that the reflectivity uniformity can be indirectly predicted through measuring the uniformity of the multilayer period and the roughness. However, the variations of the roughness and the period cannot well explain the peak reflectivity variation based on the scattering theory. At last, an innovative mask metrology was proposed, which is capable to in-situ monitor the multilayer degrading (contamination and thermally induced period change) through monitoring the mask apodization over time and map the reflectivity through a &quot;self-reference&quot; method. A big advantage of this metrology is that no extra hardware is required. The photolithography and the EUV lithography system are briefly introduced in Chapter I. The relevant theories and metrology tools are outlined in chapter II and III, respectively. In chapter IV, the experimental and numerical results are analyzed and discussed. At last, the proposed mask metrology was described in Chapter V. Although some nice results were obtained and an innovative mask metrology was designed, more experimental investigations are still needed to understand the correlation between the peak reflectivity, the period and roughness, and the implementation and qualification of the proposed mask metrology should follow up later.