Pushing lithography sources to ever-shorter wavelengths is only one of the many challenges that face the semiconductor industry as it works to bring extreme ultraviolet (EUV) lithography to fruition. The availability of affordable metrology equipment fitted with EUV sources is nearly as critical as the availability of an effective exposure source. Today, two approaches show promise for economically generating EUV/x-ray laser light: electrical discharge in plasma and high-harmonic generation (HHG). Championed by Jorge Rocca at Colorado State University (Fort Collins, CO), electrical discharge in plasma generates a relatively high photon flux but at a low repetition rate and at a higher wavelength than the target 13.2 nm. Recently, Margaret Murnane, Henry Kapteyn, and collaborators at the University of Colorado (Boulder, CO) demonstrated an HHG alternative, EUV source that offers high temporal and spatial coherence at high repetition rates and at the target wavelength for next-generation lithography.
The tabletop UC light source incorporates a femtosecond titanium-doped sapphire (Ti:sapphire) ultrafast pump laser source, some directional optics, an argon-filled hollow fiber x-ray cell, and an x-ray-sensitive, thinned CCD camera (Andor Inc.; Belfast, Ireland). Operating at 760 nm, the Ti:sapphire pump source generates high repetition rate (5 kHz), 25-fs pulses with energies of 0.8 mJ (see figure). Those pulses are focused into a 10-cm long, 150-mm-diameter hollow-core fiber cell filled with argon gas. The intense laser literally rips electrons from the gas atoms, then accelerates them away from the ion. When the field reverses a fraction of a femtosecond later, some electrons are slammed into the parent ions, releasing their energy as EUV photons. This process was first observed nearly 15 years ago, but work to date demonstrated only limited spatial coherence.
According to Murnane, because the EUV conversion takes place in a gas-filled hollow fiber instead of a gas jet, the generated EUV beams are spatially coherent or "laser-like." The hollow fiber guides the laser beam and converts the perfect spatial mode of the laser into an equivalent mode at the EUV wavelengths. The fiber also allows the UC team to make the generation process more efficienteach harmonic contains around 10-5 of the original pump power. "The fiber allows us to ensure that the harmonic waves are generated in phase with those of the laser by providing a long interaction length and controlling the pressure of the gas," Murnane says. "Unless this happens, harmonics might be generated in the middle of the fiber but might interfere destructively with harmonics generated in the end of the fiber so that no strong EUV beam would emerge. We've always been able to see hundreds of harmonics but haven't been able to phase-match in all of those energies." She adds that all the harmonics are radiated at about the same energy, unlike solid nonlinear optical crystals such as potassium dihydrogen phosphate, which yield harmonics of widely varying energy.
Figure 1. Maintaining 30 Torr within the hollow fiber enables wider phase matching between the pump beam and the x-ray emission, leading to shorter wavelengths. (Colorado State University)
Applications for short-wavelength light sources are growing as more scientists turn their attention to nanoscale interactions. "There is a lot of metrology work to be done for next-generation lithography, and we hope to find some moderately sized EUV sources," says David Attwood with the EUV LLC (Livermore, CA) and the University of California Berkeley. "HHG EUV has very high coherence properties and very short pulse rates for dynamic studies like watching molecules unfold during chemical reactions, but whether it has enough photon flux for metrology applications is something we'll have to wait and see."
While electrical-discharge EUV lasers can produce a few hundred nanosecond pulses at 10 Hz, the repetition rate of HHG EUV sources can go to 10 kHz and higher. "The EUV beams can be used for nano- and coherent imaging, element-specific spectroscopies, biological and material microscopies, and nanomachining, as well as ultra-high time resolution visualization on femtosecond timescales," Murnane says. "Some of these applications have already been demonstrated."