Immersion lithography with numerical apertures above 2.0 using high index optical materials

The progress of optical lithography has approached the sub-30 nm regime using 193nm excimer lasers as the exposure sources. To increase the numerical aperture (NA) further, many issues, especially those related to materials, need to be addressed. In this paper, we present the analytical and experimental results of oblique two-beam lithography with sapphire (Al2O3) as the optical material. At 193nm, the index of sapphire is 1.92 while the typical index of a photoresist is near 1.70. Classical theory predicts that, ignoring the absorbance in the photoresist, once the NA is greater than the photoresist refractive index, no energy will be transmitted across the sapphire/photoresist boundary due to total internal reflection. However, it can be shown that the absorbance in the resist prevents a “critical angle” and total internal reflection will not occur. Photoresist exposure can result even when NA is greater than the photoresist refractive index. The image profile is strongly affected by the real and imaginary parts of the photoresist refractive index. Optimization of photoresist optical properties is necessary for good image profile. Lutetium aluminum garnet (Lu3Al5O12 or LuAG with an index 2.14 at 193 nm) is also investigated as an alternative lens material.