What Are Plasma X-ray Sources?

Plasma x-ray sources are devices that generate x-ray or extreme ultraviolet (EUV) radiation by producing and sustaining a dense, highly ionized plasma at temperatures of millions of kelvin, conditions under which electron transitions in heavily stripped ions emit photons with wavelengths from roughly 0.1 to 100 nanometers. At these temperatures, atoms are stripped of many electrons and the remaining bound electrons occupy inner-shell configurations whose transitions release photons far more energetic than those produced by neutral or singly ionized atoms. Plasma x-ray sources differ from conventional x-ray tubes, which rely on electron bombardment of a solid metal target, by achieving higher peak brightness and narrow-band emission tailored to specific applications. They are used in semiconductor lithography, laboratory microscopy, materials characterization, and research on inertial confinement fusion.

Two principal methods create the dense hot plasma: focusing a high-power laser pulse onto a solid or gas target (laser-produced plasma, LPP), or driving a large current through a small-diameter gas or wire column to compress it magnetically (discharge-produced plasma, DPP). Both approaches appear in commercial and research settings, and their relative merits depend on target wavelength, repetition rate, and conversion efficiency requirements.

Laser-Produced Plasma Sources

Laser-produced plasma sources use a pulsed laser, typically a carbon dioxide or neodymium-doped glass laser operating at pulse energies from millijoules to tens of joules, to ablate and ionize a target material. Tin is the preferred target for EUV lithography sources because its 4d-to-4f and related electron transitions produce an intense unresolved transition array centered near 13.5 nm, the wavelength for which multi-layer molybdenum/silicon mirrors have peak reflectivity. Conversion efficiency from laser pulse energy to in-band EUV at 13.5 nm typically reaches 2 to 5 percent. By pulsing the laser at repetition rates of tens of kilohertz and collecting radiation with a large-aperture ellipsoidal mirror, LPP sources achieve the photon throughput required for high-volume semiconductor manufacturing. Research on laser plasma sources of soft x-rays and EUV for science and technology covers the scaling laws and optical design constraints that govern LPP source brightness.

Discharge-Produced Plasma and Z-Pinch Sources

Discharge-produced plasma sources generate x-ray emission by passing a fast, high-current pulse through a gas-filled capillary or a wire array. The magnetic field produced by the current exerts a compressive force on the current-carrying plasma column, a phenomenon called the Z-pinch, compressing it to millimeter or sub-millimeter diameters at densities and temperatures sufficient for x-ray emission. Xenon and tin vapor DPP sources operating at 13.5 nm have been developed commercially for EUV metrology and lithography mask inspection. An electrodeless variant, in which current is inductively coupled into the plasma without physical electrodes, avoids electrode erosion and debris contamination that would otherwise coat the collection optics. Z-pinch and discharge plasma sources for EUV applications describes the optical performance of compact soft x-ray microscopes built around these sources.

X-ray Laser Action in Dense Plasmas

A related category of plasma x-ray source exploits population inversion in a dense plasma column to produce amplified spontaneous emission at x-ray wavelengths, a true x-ray laser. In a neon-like or nickel-like ion scheme, a driving laser creates and rapidly heats a plasma column; collisional electron excitation populates an upper laser level faster than it can decay, establishing the population inversion needed for gain. Saturation lengths of a few centimeters are achievable at wavelengths from about 4 to 50 nm. Laser-produced high-Z plasma soft x-ray sources investigated by IEEE researchers show how higher atomic-number targets push emission to shorter wavelengths relevant to sub-nanometer imaging. X-ray lasers are of interest for coherent diffractive imaging of biological and materials samples.

Applications

Plasma x-ray sources have applications in a range of fields, including:

• Extreme ultraviolet lithography for semiconductor manufacturing at sub-10 nm feature sizes
• Laboratory x-ray microscopy for imaging biological cells and nanoscale material structures
• EUV mask inspection and metrology in semiconductor fabrication
• Inertial confinement fusion research, where x-ray emission diagnostics and hohlraum drive depend on plasma radiation
• Academic research in atomic physics, where plasma sources provide tabletop soft x-ray beams for spectroscopy