Brett J. Rolfson

Effects of reticle birefringence on 193nm lithography

Unpolarized light has traditionally been used for photolithography. However, polarized light can improve contrast and exposure latitudes at high numerical aperture (NA), especially for immersion lithography with an NA > 1.0. As polarized light passes through a reticle, any birefringence (BR) in the reticle material can cause a change in the orientation or degree of polarization, reducing the contrast in the final resist image. This paper shows the effects of reticle BR on dry and immersion imaging for 193nm lithography. The BR magnitude and orientation of the fast axis were mapped across several unpatterned mask blanks, covering a range of BR from 0 to 10 nm/cm. These reticles were printed with a series of open areas surrounded by test structures. The BR was measured again on the patterned reticles, and several locations were selected to cover a range of magnitudes at different orientations of the fast axis. Dry and immersion imaging were evaluated, looking at BR effects on dense lines and contact structures. Mask error enhancement factor (MEEF), line edge roughness (LER), and dose and focus latitudes were studied on line/space patterns. Dose and focus latitudes and 2-D effects were studied on contact patterns. Based upon these results, the effect of reticle BR on CD is minimal, even for BR values up to 10 nm/cm.

Process margin improvement using custom transmission EAPSM reticles

Low k1 lithography poses a number of challenges to the process development engineer. Although polarization and immersion lithography will allow us to create processes at lower k1 than previous paradigms allowed, the lithographer will quickly be looking for Resolution Enhancement Techniques (RET) to push to the ultra-low k1 regime, or to extend older generation tools and avoid the aforementioned expensive options. Reticle transmission is a RET that can enable a low k1 process by increasing image contrast. With work performed in conjunction with our MP Mask facility, we have been able to obtain custom-transmission EAPSM reticles. Reticle transmission optimization can be carried out through simulation. Optimum transmission varies depending on optical parameters and feature size. Moreover, when working with 2D patterns, reticle transmission can be optimized for weaker features, without significantly sacrificing image contrast on primary features. Process improvement by optimizing reticle transmission will be explored for a variety of device types using both 248nm and 193nm lithography. Simulation, custom-transmission reticle fabrication, and empirical wafer results will be presented.