Resist profile control in immersion lithography using scatterometry measurements

Control of critical dimension (CD) and resist profile is increasingly important in low-k₁ lithography, and becomes more difficult in thin resist processing due to the chemical interaction occurring at the resist surfaces. Implementation of immersion lithography will make the control even more challenging since more sources of chemical interaction can play a role, e.g. leaching of photo active material from the resist into the water, or diffusion of the water into the resist. Moreover, the contact of the liquid in the scanner showerhead with the wafer surface is a dynamic and local interaction, which needs to be understood and quantified, since variations in soak time are a possible source of intra-field and across wafer CD-variations. In this paper we developed a methodology to understand and to quantify the impact of immersion scanner soak on resist profile control and CD-control. The methodology is on the one hand based on the simulation of the showerhead movements over the wafer during the immersion lithography process, where for a particular location on the wafer the soak time is calculated by accumulating the interaction time every time the showerhead is passing that particular location. On the other hand the methodology quantifies experimentally how much resist profile change and CD-variation is caused by a particular pre- and post-soak time, by testing the process in a virtual immersion set-up and measuring the CD-response with high-precision scatterometry. In this way, we were able to predict CD-variations related to immersion soak. Using the initial resist and topcoat processes, we recently experimentally verified on the ASML XT:1250Di immersion tool at IMEC that these soak related CD-variations exist. The effects are small, but in line with the soak time simulations and the CD-response obtained on the virtual immersion set-up. This demonstrates that the methodology described above could be very useful to select materials for lithography processes and to set specifications for allowed CD-variations in line with to the over-all allowed CD-budget.