Increasingly stringent requirements on the performance of diffractive optical elements (DOEs) used in wafer scanner
illumination systems are driving continuous improvements in their associated manufacturing processes. Specifically,
these processes are designed to improve the output pattern uniformity of off-axis illumination systems to minimize
degradation in the ultimate imaging performance of a lithographic tool. In this paper, we discuss performance
improvements in both photolithographic patterning and RIE etching of fused silica diffractive optical structures. In
summary, optimized photolithographic processes were developed to increase critical dimension uniformity and featuresize
linearity across the substrate. The photoresist film thickness was also optimized for integration with an improved
etch process. This etch process was itself optimized for pattern transfer fidelity, sidewall profile (wall angle, trench
bottom flatness), and across-wafer etch depth uniformity. Improvements observed with these processes on idealized test
structures (for ease of analysis) led to their implementation in product flows, with comparable increases in performance
and yield on customer designs.
We present advancements in the manufacture of high-performance diffractive optical elements (DOEs) used in
stepper/scanner off-axis illumination systems. These advancements have been made by employing high resolution
lithographic techniques, in combination with precision glass-etching capabilities. Enhanced performance of DOE designs
is demonstrated, including higher efficiency with improved uniformity for multi-pole illumination at the pupil plane,
while maintaining low on-axis intensity.
Theoretical predictions of the performance for several classes of DOE designs will be presented and compared with
This new process capability results in improved performance of current DOE designs, and enables greater customization
including control of the output spatial intensity distribution for future designs. These advancements will facilitate
continuous improvements in off-axis illumination optimization required by the end user to obtain larger effective
lithographic process windows.
We present designs of a diffractive polarizer having low zeroth-order reflectivity that is compact, potentially mass-producible and cost-effective, and compatible for high-power applications. It consists of subwavelength grating structures superimposed over a diffraction grating. Using rigorous coupled wave analysis, we optimized the parameters of multilevel grating structures to achieve antireflection for both incident TE and TM polarization states with one polarization passing in the zeroth order and the other into the first and higher orders. We focused on polarizer designs for the 1.31-µm wavelength range, and the theoretical values for zeroth-order reflection were calculated to be 0.01% for TE and 0.29% for TM modes. The zeroth-order transmission efficiencies were 97.2% for TE and 0.01% for TM modes. A prototype of one design was fabricated and tested to verify the functionality of the device, and the zeroth-order reflection was determined to be 1.2% and 3.5% for the two modes.