An exit transmission window for mirrorless EUV and soft-x-ray lithography: hermetically separating a dense superfluorescent gain medium from the vacuum while transmitting the amplified beam

Abstract

Background: A mirrorless exposure architecture for extreme-ultraviolet (EUV) and soft-x-ray lithography places a population-inverted superfluorescent gain medium immediately above the wafer and amplifies a mask-patterned seed to resist-level dose without any projection mirror. That medium — a dense noble-gas or multiply-charged-ion plasma at ion densities of 10^12–10^19 cm^-3 — must be sealed inside a cell, yet the intense amplified burst has to exit into the wafer vacuum. The exit window therefore faces a unique, coupled set of requirements not met simultaneously by conventional EUV pellicles or soft-x-ray detector windows.

Aim: We define and analyze an exit transmission window that (i) transmits strongly absorbed EUV/soft-x-ray light with minimal loss, (ii) hermetically separates the dense internal medium from the external vacuum against their pressure difference, (iii) survives opposing extreme environments — plasma and hot gas on the medium side, hydrogen plasma and residual gas on the vacuum side — and (iv) rejects the heat of transmission for repeated exposure.

Approach: The window comprises a low-atomic-number transmission membrane (1–500 nm; e.g., carbon-nanotube nonwoven, few-layer graphene, SiN, polysilicon, Be, or B4C), a support grid that partitions it into sub-millimeter apertures to carry the pressure load, thin capping layers on one or both faces, and a frame with an ultrahigh-vacuum-compatible hermetic seal. Membrane and cap materials are selected per waveband, and grid geometry is set to keep its shadow off the wafer.

Results: Transmission computed from tabulated Henke optical constants shows that 20-nm solid films transmit, at 13.5 nm / 3.1 nm respectively: Be 97.2%/96.9%, polysilicon 96.7%/92.3%, B4C 90.9%/91.8%, and solid-density carbon 89.0%/90.9% — with porous CNT nonwovens exceeding the solid-carbon value, consistent with the demonstrated ~97–99% of full-field CNT pellicles at 13.5 nm. The internal pressure is at most ~40 kPa (10^19 cm^-3 at room temperature); a membrane burst-pressure model p_b = 2σ_eff·t/a, calibrated to demonstrated ultrathin-SiN window data (σ_eff ≈ 2.1 GPa), predicts burst pressures of ~84 kPa for a 5-nm membrane on 0.5-mm apertures and ~168 kPa for a 20-nm membrane on 1-mm apertures — 2–8× margins over the operating differential. An adiabatic single-shot heating estimate gives ~90 K per shot for Be and ~10^2 K for carbon at resist-level doses, within the demonstrated thermal tolerance of CNT pellicles under 500-W-class EUV exposure.

Conclusions: A physically realizable exit window built from established X-ray-window and pellicle technology makes the mirrorless superfluorescent exposure tool implementable for repeated operation. The component is disclosed in Japanese Patent Application No. 2026-099027 and supports the exposure architectures of Applications No. 2026-095217 and No. 2026-096037.