Plasma-based illumination systems, such as laser-produced plasma (LPP) sources, laser-sustained plasma (LSP) sources, laser-driven light sources (LDLS), or discharge-produced plasma (DPP) sources, are often used to generate soft x-ray, extreme ultraviolet (EUV), and vacuum ultraviolet (VUV) wavelengths of illumination (e.g. wavelengths around 120 nm or shorter) for applications such as defect inspection, photolithography, or metrology. The illumination may be emitted by a plasma that is generated at or near a site where target material (e.g. xenon, tin, or lithium) is deposited and irradiated by an excitation source, such as a laser. Illumination emanating from the plasma may be collected via a reflective optic, such as a collector mirror (e.g. a near-normal incidence or grazing incidence mirror), and then directed and/or focused along an illumination delivery path.
During operation of the plasma-based illumination system, debris such as atomic vapor, micro-particles, or contaminants (e.g. hydrocarbons or organics) may be emitted from various sources including, but not limited to, the target material, plasma site, plasma-facing components, eroded surfaces in proximity of the target material or the plasma, a target-forming structure, and/or any other structure within a plasma-based illumination system. These debris can sometimes reach the reflective optic and degrade its performance or cause irreparable damage. Some methods of protecting the reflective optic include deflection of debris by magnetic fields, utilization of debris vanes or shields consisting of thin foil separators to allow soft x-ray, EUV, or VUV light to go through but capturing the atomic condensable vapor, and circulating gas to generate a gas buffer between the target and the collector. Each of the foregoing methods has some drawbacks, as outlined below.
Coils producing magnetic fields have significant design complexity, are expensive, and work well only for deflecting ions, while not effective for stopping neutrals (and neutral particles), which are produced when ions undergo charge exchange with the buffer gas. Debris vanes lead to transmission light loss due to occlusion, require complicated alignment procedure, and any material that ends up condensing on the surface of the debris vanes can be a subject of secondary erosion or sputtering due to high energy ions produced by the source. A gas buffer region located between the target and the collector slows down high-energy ions and neutrals but is not as effective at suppressing diffusion of atomic vapor and micro-particles.