Sudharshanan Raghunathan

Experimental measurements of telecentricity errors in high-numerical-aperture extreme ultraviolet mask images

Nontelecentric illumination in extreme ultraviolet (EUV) lithography leads to pattern shifts through focus called telecentricity errors. As the industry moves toward finer pitch structures and higher numerical apertures (NA) to improve resolution, the effects of telecentricity errors become more significant. These telecentricity errors are dependent on pattern pitch, pattern type, lens aberrations, mask stack, to name a few. In this paper, a novel technique to measure telecentricity errors using EUV mask images from an actinic mask inspection tool, called the SEMATECH High NA Actinic Reticle Review Project (SHARP) is presented. SHARP is SEMATECHs second generation actinic mask imaging tool developed by Lawrence Berkeley National Laboratory. The SHARP can image masks at different numerical aperture settings, even beyond the currently available scanner NA of 0.33 (high-NA EUV) and also has a set of programmable illuminator choices. A tuned multilayer EUV mask blank was fabricated with test structures optim...

Extreme ultraviolet lithography resist-based aberration metrology

Abstract. Extreme ultraviolet lithography (EUVL) at 13.5 nm is currently the most promising technology for advanced integrated circuit manufacturing nodes. Since the wavelength for EUVL is an order of magnitude smaller than current optical lithography systems (193 nm), aberration tolerances on lens manufacturing must be tightened to avoid image distortion and contrast loss as they scale with wavelength. Therefore, understanding the aberrations of an EUVL system both in idle and production conditions is paramount. This study aims to assess a photoresist-based aberration metrology technique for capturing pupil information of EUVL systems that can be implemented during full system use. Several datasets have been collected on a full-field EUVL system. Various one-dimensional and two-dimensional binary structures were imaged and used for pupil wave front extraction in conjunction with computational modeling and simulations. Results show a successful extraction of a stable aberration signature over several measurements, showing the method to be sensitive to subnanometer levels of intentional aberration change through lens manipulation.

Mask 3D effects and compensation for high NA EUV lithography

Mask shadow compensation for EUV lithography has typically been performed using simple rule-based schemes during optical proximity correction (OPC). However, as feature sizes decrease, the required corrections get more complex as they become dependent on both feature size and type. Thus, OPC models that account for these 3D mask effects are becoming essential. These models become even more important for higher numerical aperture EUV systems due to larger angles of incidence on the mask and tighter process budgets for CD and overlay. This paper will focus on estimating these 3D mask effects and evaluate the extendibility of current available OPC models for some specific higher numerical aperture EUV systems. It is concluded that the current available 3D mask models are capturing the primary effects and it is believed that with further refinement they are likely extendable to meet the needs of future high-NA tools. Additionally, a combination of thinner mask absorber, tighter scanner focus control and/or larger optical magnification will likely be required to print sub-30nm pitch structures with higher numerical aperture EUV systems.

EUVL resist-based aberration metrology

Extreme Ultraviolet Lithography (EUVL) at 13.5 nm is currently the most promising technology for advanced integrated circuit (IC) manufacturing nodes. Since the wavelength for EUVL is an order of magnitude smaller than current optical lithography systems (193 nm), wavelength scaled tolerances on lens manufacturing must be tightened to avoid image distortion and contrast loss as these scale with wavelength. Therefore understanding the aberrations of an EUVL system both in idle and production conditions is paramount. This study aims to assess a photoresist based technique for capturing pupil information of EUVL systems that can be implemented during full system use. Several data sets have been collected on an ASML EUV Alpha-Demo Tool (ADT) using the latest Center for Nanoscale Science and Engineering (CNSE) baseline resist Shin-Etsu SEVR139. Various one-dimensional and two-dimensional binary structures were imaged and used for pupil extraction in conjunction with computational modeling and simulations. Results show a stable extracted aberration signature over several measurements. Results also show that the method is sensitive to sub-nm levels of aberration change.

A method of image-based aberration metrology for EUVL tools

We present an approach to image-based EUV aberration metrology using binary mask targets and iterative model-based solutions to extract both the amplitude and phase components of the aberrated pupil function. The approach is enabled through previously developed modeling, fitting, and extraction algorithms. We examine the flexibility and criticality of the method using two experimental case studies. The first extracts the pupil phase behavior from an ASML NXE:3100 exposure system and shows primary aberration sensitivity below 0.2 mλ. The second experiment extracts both components of the pupil function from the SHARP EUV microscope.

Alternative materials for high numerical aperture extreme ultraviolet lithography mask stacks

In this paper we compare the imaging performance of several options currently under consideration for use in 0.33 and higher numerical aperture (NA) extreme ultraviolet (EUV) mask stacks, Mo/Si ML reflective coatings with 40 bilayers, Ru/Si multilayer (ML) reflective coatings with 20 bilayers, and a new thinner Ni-based absorber layer on each of these mask stacks. The use of a Ru/Si ML coating with its shallower effective reflectance plane and a 2x thinner Ni-based absorber is expected to significantly reduce both shadow bias requirements and mask telecentricity errors. The conclusions of the paper are supported with the results of both experimental measurements and rigorous simulations.

Characterization of a 0.25NA full-field EUV exposure tool

The performance of a 0.25NA full-field EUV exposure tool is characterized in terms of CD uniformity, focus and overlay control, as well as dose uniformity. In addition to the characterization of the scanner, we explore the use of scatterometry techniques for the measurements of extremely fine resolution features, with critical dimensions below 40 nm. The stability of the scanner performance over an extended period of time is assessed.