Extreme ultraviolet lithography (EUVL) was thoroughly reviewed over a broad range of topics, including history, tools, source, metrology, condenser and projection optics, resists, and masks. Since 1988, many studies on EUVL have been conducted in North America, Europe, and Japan, through state sponsored programs and industrial consortiums. To date, no “show stopper” has been identified, but challenges are present in almost all aspects of EUVL technology. Commercial alpha lithography step-and-scan tools are installed with full-field capability; however, EUVL power at intermediate focus (IF) has not yet met volume manufacturing requirements. Compared with the target of 180W IF power, current tools can supply only approximately 55–62W. EUV IF power has been improved gradually from xenon- to tin-discharge-produced plasma or laser-produced plasma. EUVL resist has improved significantly in the last few years, with 25nm 1:1 line/space resolution being produced with approximately 2.7nm (3σ) line edge roughness. A...

Extreme ultraviolet lithography: A review

This article reviews the progress in the development of practical table-top sources of soft x-ray laser radiation. The field is rapidly approaching the stage at which soft x-ray lasers sufficiently compact to fit onto a normal optical table will be routinely utilized in science and technology. This is the result of recent advances in the amplification of soft x-ray radiation in both compact laser-pumped and discharge-pumped devices. The use of excitation mechanisms that take full advantage of new ultrafast high power optical laser drivers and multiple pulse excitation schemes has resulted in the demonstration of saturated soft x-ray amplification at wavelengths as short as 14 nm using several Joule of laser-pump energy. Moreover, several schemes have demonstrated significant gain with only a fraction of a Joule of laser-pump energy. In addition, the demonstration of saturated table-top soft x-ray lasers pumped by very compact capillary discharges has shattered the notion that discharge-created plasmas are insufficiently uniform to allow for soft x-ray amplification, opening a route for the development of efficient, high average power soft x-ray lasers. Recently, a table-top capillary discharge laser operating at 46.9 nm has produced millijoule-level laser pulses at a repetition rate of several Hz, with a corresponding spatially coherent average power per unit bandwidth comparable to that of a beam line at a third generation synchrotron facility. This review summarizes fundamental and technical aspects of table-top soft x-ray lasers based on the generation of population inversions in plasmas, and discusses the present status of development of specific laser systems.

/pdf/table-top-soft-x-ray-lasers-1xt6argbcz.pdf

Table-top soft x-ray lasers

The phase-shifting point-diffraction interferometer (PS/PDI) was recently developed and implemented at Lawrence Berkeley National Laboratory to characterize extreme-ultraviolet (EUV) projection optical systems for lithography. Here we quantitatively characterize the accuracy and precision of the PS/PDI. Experimental measurements are compared with theoretical results. Two major classes of errors affect the accuracy of the interferometer: systematic effects arising from measurement geometry and systematic and random errors due to an imperfect reference wave. To characterize these effects, and hence to calibrate the interferometer, a null test is used. This null test also serves as a measure of the accuracy of the interferometer. We show the EUV PS/PDI, as currently implemented, to have a systematic error-limited reference-wave accuracy of 0.0028 waves (lambda/357 or 0.038 nm at lambda = 13.5 nm) within a numerical aperture of 0.082.

Extreme-ultraviolet phase-shifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy

The success of recent static printing experiments at Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS) using the EUV LLC Engineering Test Stand (ETS) Set-2 optic has demonstrated the utility of synchrotron-based EUV exposure stations. Although not viable light sources for commercial lithography, synchrotrons provide clean, convenient, and extremely flexible sources for developmental microfield lithography. The great flexibility of synchrotron-based illumination arises from the fact that such sources facilitate active coherence reduction, thus enabling the coherence function, or pupil fill, to be actively sculpted in real time.
As the commercialization of EUV progresses, the focus of developmental EUV lithography is shifting from low numerical aperture (NA) tools such as the 0.1-NA ETS to higher-NA tools such as the 0.3-NA Micro Exposure Tool (MET). To support printing with MET optics at the ALS, a new printing station has been developed, relying on a scanning illuminator to provide programmable coherence (pupil-fill) control. The illuminator is designed to operate up to a coherence factor (s) of 1 and support the full 200′600 design printed field of view. In addition to a new illuminator design, new focus sensing and dose-control systems have also been implemented. Here we describe the MET printing capabilities in detail and present preliminary printing results with the Sematech Set-2 MET optic.

Status of EUV micro-exposure capabilities at the ALS using the 0.3-NA MET optic

Undulator radiation, generated by relativistic electrons traversing a periodic magnet structure, can provide a continuously tunable source of very bright and partially coherent radiation in the extreme ultraviolet (EUV), soft X-ray (SXR), and X-ray regions of the electromagnetic spectrum. Typically, 1-10 W are radiated within a 1/N relative spectral bandwidth, where N is of order 100. Monochromators are frequently used to narrow the spectral bandwidth and increase the longitudinal coherence length, albeit with a more than proportionate loss of power. Pinhole spatial filtering is employed to provide spatially coherent radiation at a power level determined by the wavelength, electron beam, and undulator parameters. In this paper, experiments are described in which broadly tunable, spatially coherent power is generated at EUV and soft X-ray wavelengths extending from about 3 to 16 nm (80-430-eV photon energies). Spatially coherent power of order 10 /spl mu/W is achieved in a relative spectral bandwidth of 9/spl times/10/sup -4/, with 1.90-GeV electrons traversing an 8-cm period undulator of 55 periods. This radiation has been used in 13.4-nm interferometric tests that achieve an rms wavefront error (departure from sphericity) of /spl lambda//sub euv//330. These techniques scale in a straightforward manner to shorter soft X-ray wavelengths using 4-5-cm period undulators at 1.90 GeV and to X-ray wavelengths of order 0.1 nm using higher energy (6-8 GeV) electron beams at other facilities.

Tunable coherent radiation in the soft X-ray and extreme ultraviolet spectral regions

Techniques are described for at-wavelength interferometry of multilayer coated optics designed for use in extreme-ultraviolet lithography. Broadly tunable undulator radiation, which covers the spectral region from 45 to 400 A, is described. The coherent power available at these wavelengths is described, and several types of interferometer that might be suitable at these short wavelengths are also described.

/pdf/undulator-radiation-for-at-wavelength-interferometry-of-41aqc7edti.pdf

Undulator radiation for at-wavelength interferometry of optics for extreme-ultraviolet lithography.

The effects of mask topography and multilayer coating defect on areal images of reflective masks for soft x-ray projection lithography is studied using electromagnetic simulation. Masks made by depositing and patterning an absorber layer over the multilayer coating was found to be insensitive to variations in incidence angle and absorber layer edge profile. 100 nm of gold, germanium or carbon absorber is sufficient for high image contrast. Masks made by etching patterns into the multilayer stack is more sensitive to variations in incidence angle and edge profile. Defects on the substrate severely degrade the areal image. Different coverage profiles results in widely different areal image.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Effects of absorber topography and multilayer coating defects on reflective masks for soft x-ray/EUV projection lithography

Reflectivity of Mo/Si multilayer x-ray mirrors with molybdenum as the top layer decreased over time when stored in air due to oxidation of the top Mo layer to MoO3 and MoO2. High reflectivity can be maintained by having silicon as the top layer. In fact, reflectivity of Si-top multilayer coatings increase slightly during the first few days after deposition.

Top Layer Oxidation In Mo/SI Multilayer X-Ray Mirror

Self-aligning optical heterodyning is demonstrated with an acceptance angle as large as 40°. The receiver consists of a strontium barium niobate (SBN) crystal, a detector, and collecting lenses. The incoming beam interferes with the local oscillator to create a real-time grating in the SBN crystal, which diffracts and aligns the signal with the local oscillator. Heterodyne detection occurs as long as the written grating can diffract the input signal. All alignment requirements between the signal and local oscillator are automatically satisfied by the diffraction process, which permits the large acceptance angle. Insensitivity with respect to crystal orientation and background radiation has also been demonstrated.

Wide-angle self-aligning heterodyning with four-wave mixing.

: Considerations of the various issues relating to radiation sources for x-ray lithography are presented, focusing on two leading candidates-laser-produced plasma and synchrotron radiation sources. Issues to be addressed include power requirements of an x-ray source, deliverable power to wafer for both synchrotrons and laser plasma sources, and the requirement for collection solid angle and conversion efficiency for laser plasma sources. In our analysis, we attempt to bring out the relative merits of the sources and the advances necessary to make them viable for future x-ray lithographic systems.

Khanh Nguyen

Papers

Undulator radiation for at-wavelength interferometry of optics for extreme-ultraviolet lithography.

Effects of absorber topography and multilayer coating defects on reflective masks for soft x-ray/EUV projection lithography

Top Layer Oxidation In Mo/SI Multilayer X-Ray Mirror

Wide-angle self-aligning heterodyning with four-wave mixing.

Source Issues Relevant to X-Ray Lithography,