pellicle
A thin plastic membrane used as a beam splitter or protective cover (for example, to protect a photomask).
pellicle: the ultra-thin membrane that bends light without breaking it
A pellicle is a microscopically thin membrane, typically 0.5 to 2 micrometers thick, stretched across an aperture or mounted at a shallow angle to divide or protect optical paths. In semiconductor lithography, a pellicle acts as a protective barrier mounted just above a photomask, keeping dust and debris from settling on the critical patterned surface during exposure. In beam-splitting applications, a pellicle coating or thinned polymer film reflects a controlled fraction of incident light while transmitting the rest, functioning with minimal thickness and thus minimal wavefront distortion.
The material is usually polyester, polycarbonate, or specialized fluoropolymers chosen for transparency across the wavelength range in use, chemical resistance to processing solvents and etchants, and minimal birefringence. Thickness and coating composition determine the reflectivity and transmission ratio. In deep ultraviolet (DUV) lithography at 248 nm or 193 nm, pellicles must be manufactured from materials like calcium fluoride or specialized thin-film coatings, as many polymers absorb or scatter at these wavelengths. Extreme ultraviolet (EUV) lithography at 13.5 nm relies on multilayer reflective pellicles rather than transmission membranes.
Pellicles are held under tension in a frame, mounted typically 5 to 10 mm above the mask surface in lithography. They must remain flat and parallel to avoid introducing phase aberrations into the exposure pattern. Contamination, tears, and material creep under tension are the primary failure modes. Any particulates that contact the pellicle surface can shadow or scatter light onto the mask below. Over time, pellicles degrade under UV exposure and chemical attack, requiring periodic replacement in high-volume fabs.
The name derives from the Latin pellicula, meaning small skin, capturing the essence of a structure that is transparent yet protective, present yet nearly invisible to the optical system. In confocal microscopy and laser scanning systems, anti-reflective or dichroic pellicles perform beam combining or beam rejection with acceptance angles of 45 degrees or wider. In spectroscopy, very thin pellicles eliminate multiple reflections that would otherwise compromise transmission measurements.