Keith H. Jackson

System integration and performance of the EUV engineering test stand

The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k1 of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.

Characterization of an EUV Schwarzschild objective using phase-shifting point diffraction interferometry

We report wavefront measurement of a multilayer-coated, reflective optical system at 13.4-nm wavelength performed using a novel phase-shifting point-diffraction interferometer. Successful interferometric measurements of a 10x Schwarzschild objective designed for extreme ultraviolet projection lithography with 0.1-micrometer resolution demonstrate high- precision with sub-nanometer resolution. The goal of the interferometry is to achieve wavefront measurement accuracy beyond lambda/50 rms at EUV wavelengths. Preliminary measurements are discussed and the paths toward achieving the target accuracy are identified.

Masks for high aspect ratio x-ray lithography

The requirements for deep x-ray lithography (DXRL) masks are reviewed and a recently developed cost effective mask fabrication process is described. The review includes a summary of tabulated properties for materials used in the fabrication of DXRL masks. X-ray transparency and mask contrast are calculated for material combinations using simulations of exposure at the Advanced Light Source (ALS) at Berkeley, and compared to the requirements for standard x-ray lithography (XRL) mask technology. Guided by the requirements, a cost-effective fabrication process for manufacturing high contrast masks for DXRL has been developed. Thick absorber patterns () on a thin silicon wafer (m) were made using contact printing in thick positive (Hoechst 4620) and negative (OCG 7020) photoresist and subsequent gold electrodeposition. Gold was deposited using a commercially available gold sulphite bath with low current density and good agitation. The resultant gold films were fine-grained and stress-free. Replication of such masks into thick acrylic sheets was performed at the ALS.

Static microfield printing at the advanced light source with the ETS Set-2 optic

While interferometry is routinely used for the characterization and alignment of lithographic optics, the ultimate performance metric for these optics is printing in photoresist. The comparison of lithographic imaging with that predicted from wavefront performance is also useful for verifying and improving the predictive power of wavefront metrology. To address these issues, static, small-field printing capabilities have been added to the EUV phase- shifting point diffraction interferometry implemented at the Advanced Light Source at Lawrence Berkeley National Laboratory. The combined system remains extremely flexible in that switching between interferometry and imaging modes can be accomplished in approximately two weeks.

Extreme ultraviolet lithography capabilities at the advanced light source using a 0.3-NA optic

Extreme ultraviolet lithography is a leading candidate for volume production of nanoelectronics at the 32-nm node and beyond. In order to ensure adequate maturity of the technology by the start date for the 32-nm node, advanced development tools are required today with numerical apertures of 0.25 or larger. In order to meet these development needs, a microexposure tool based on SEMATECHs 0.3-numerical aperture microfield optic has been developed and implemented at Lawrence Berkeley National Laboratory, Berkeley, CA. Here we describe the Berkeley exposure tool in detail, discuss its characterization, and summarize printing results obtained over the past year. Limited by the availability of ultrahigh resolution chemically amplified resists, present resolving capabilities limits are approximately 32 nm for equal lines and spaces and 28 nm for semi-isolated lines.

Adding static printing capabilities to the EUV phase-shifting point diffraction interferometer

While interferometry is routinely used for the characterization and alignment of lithographic optics, the ultimate performance metric for these optics is printing in photoresist. Direct comparison of imaging and wavefront performance is also useful for verifying and improving the predictive power of wavefront metrology under actual printing conditions. To address these issues, static, small-field printing capabilities are being added to the extreme ultraviolet (EUV) phase-shifting point diffraction interferometer (PS/PDI) implemented at the Advanced Light Source at Lawrence Berkeley National Laboratory. This Sub- field Exposure Station (SES) will enable the earliest possible imaging characterization of the upcoming Engineering Test Stand (ETS) Set-2 projection optics. Relevant printing studies with the ETS projection optics require illumination partial coherence with (sigma) of approximately 0.7. This (sigma) value is very different from the coherent illumination requirements of the EUV PS/PDI and the coherence properties naturally provided by synchrotron undulator beamline illumination. Adding printing capabilities to the PS/PDI experimental system thus necessitates the development of an alternative illumination system capable of destroying the inherent coherence of the beamline. The SES is being implemented with two independent illuminators: the first is based on a novel EUV diffuser currently under development and the second is based on a scanning mirror design. Here we describe the design and implementation of the new SES, including a discussion of the illuminators and the fabrication of the EUV diffuser.

At-wavelength characterization of the extreme ultraviolet Engineering Test Stand Set-2 optic

At-wavelength interferometric characterization of a new 4x-reduction lithographic-quality extreme ultraviolet (EUV) optical system is described. This state-of-the-art projection optic was fabricated for installation in the EUV lithography Engineering Test Stand (ETS) and is referred to as the ETS Set-2 optic. EUV characterization of the Set-2 optic is performed using the EUV phase-shifting point diffraction interferometer (PS/PDI) installed on an undulator beamline at Lawrence Berkeley National Laboratorys Advanced Light Source. This is the same interferometer previously used for the at-wavelength characterization and alignment of the ETS Set-1 optic. In addition to the PS/PDI-based full-field wavefront characterization, we also present wavefront measurements performed with lateral shearing interferometry, the chromatic dependence of the wavefront error, and the system-level pupil-dependent spectral-bandpass characteristics of the optic; the latter two properties are only measurable using at-wavelength interferometry.

EUV Interferometry of the 0.3 NA MET Optic

A new generation of 0.3 numerical aperture prototype EUV optical systems is now being produced to provide an opportunity for early learning at 20-nm feature size. Achieving diffraction limited performance from these two-mirror, annular projection optics poses a challenge for every aspect of the fabrication process, including final alignment and interferometric qualification. A new phase-shifting point diffraction interferometer will be used at Lawrence Berkeley National Laboratory for the measurement and alignment of the MET optic at EUV wavelengths. Using the previous generation of prototype EUV optical systems developed for lithography research, with numerical apertures up to 0.1, EUV interferometers have demonstrated RMS accuracy levels in the 40-70 pm range. Relative to the previous generation of prototype EUV optics, the threefold increase to 0.3 NA in the image-side numerical aperture presents several challenges for the extension of ultra-high-accuracy.

High flux undulator beamline optics for EUV interferometry and photoemission microscopy

A long undulator installed at a low emittance storage ring, generates quasi-monochromatic beams of high brightness and partial coherence properties; however, this also raises concerns regarding high heat loads on beam line components. There have been intensive research efforts to develop beam line optics to exploit brightness and coherence properties from undulators. These components must withstand high heat loads produced by intense synchrotron radiation beams impinging on their surface which could degrade beam line performance. The effects of high flux undulator radiation on beam line optics for EUV interferometry and photoemission microscopy will be discussed. Specifically, beam line schematics, design considerations of indirectly side cooled mirror and grating assemblies developed at the Center for X- Ray Optics and recent data of performance under undulator radiation load from beam line BL12.0 being commissioned at the ALS will be presented in this study.

Preparations for extreme ultraviolet interferometry of the 0.3 numerical aperture Micro Exposure Tool optic

An at-wavelength interferometer is being created for the measurement and alignment of the 0.3 numerical aperture Micro Exposure Tool (MET) projection optic at extreme ultraviolet (EUV) wavelengths. The prototype MET system promises to provide early learning from EUV lithographic imaging down to 20 nm feature size. The threefold increase to 0.3 NA in the image-side numerical aperture presents several challenges for the extension of ultrahigh-accuracy interferometry, including pinhole fabrication and the calibration and removal of systematic error sources.