Dual Strategy of Electroless Metal Deposition and Surface Silylation Toward Scalable Low-Temperature Hybrid Bonding for Advanced Packaging Applications

In extreme ultraviolet lithography (EUVL) systems, the collector mirror is a critical optical component that gathers and directs EUV light from the source toward the projection optics. Over time, the collector surface accumulates contaminant films — primarily tin (Sn) debris from the laser-produced plasma (LPP) source, along with other byproducts. These deposits reduce reflectivity, degrade throughput, and ultimately limit the uptime of the lithography tool. Therefore, in-situ cleaning methods that restore mirror performance without dismantling the system are essential.

Concept of Hydrogen Surface Wave Plasma (SWP) Cleaning
The hydrogen surface wave plasma cleaning method uses a low-pressure hydrogen plasma to remove contaminant layers from the collector. In this approach, a radio-frequency (RF) or microwave field sustains a surface wave along a dielectric tube or waveguide, generating a stable, large-volume plasma. Atomic hydrogen radicals (H*) formed in the plasma are the primary cleaning agents. These radicals chemically react with tin deposits to form volatile tin hydrides (SnH₄) that can be pumped away.

Advantages of the SWP Method
Hydrogen SWP offers several benefits over alternative plasma cleaning approaches:

• Low ion energy minimizes sputter damage to the underlying multilayer mirror coating.

• Uniform radical flux can be sustained over large, complex mirror geometries.

• Compatibility with in-situ operation, allowing cleaning without removing the collector from the EUV source chamber.

• High selectivity for tin over the Mo/Si multilayer stack, preserving optical performance.

Cleaning Mechanism
The cleaning proceeds through two main processes:

1. Chemical Reduction – Atomic hydrogen reacts with oxidized tin and tin layers to form volatile hydrides.

2. Physical Assistance – Low-energy ions and vacuum pumping assist in dislodging reaction products and residual particles.

Temperature plays a role in reaction efficiency; elevated mirror temperatures (typically 150–250°C) improve tin hydride desorption rates and overall cleaning speed.

Experimental Setup
The dissertation describes a system where the SWP source is positioned to generate plasma in proximity to the collector surface. Process parameters include:

• Pressure: A few Pascals of H₂.

• Microwave Power: Sufficient to sustain high-density plasma without damaging the optics.

• Cleaning Time: Dependent on deposit thickness and operating conditions.

Diagnostic tools such as optical emission spectroscopy (OES) are used to monitor plasma species and confirm the presence of atomic hydrogen.

Results and Performance
Hydrogen SWP cleaning successfully removed tin deposits from EUV collector samples while maintaining the reflectivity of the Mo/Si multilayer. The process achieved:

• High removal rates for tin films (order of nanometers per minute).

• Negligible reflectivity loss in the EUV wavelength range.

• Repeatability over multiple cleaning cycles, demonstrating process stability.

In some experiments, the SWP method was compared to downstream plasma cleaning and thermal hydrogen radical cleaning. SWP exhibited faster rates and better uniformity while minimizing collateral damage.

Challenges and Considerations
Despite its advantages, hydrogen SWP cleaning requires careful optimization:

• Over-cleaning risk – Prolonged exposure can lead to hydrogen-induced blistering or modification of the mirror substrate.

• Access geometry – The plasma source must be positioned to ensure full surface coverage.

• Plasma-surface interactions – Long-term effects on multilayer coatings require continued study.

Industrial Relevance
With EUV lithography being adopted for advanced semiconductor manufacturing nodes, uptime and cost-of-ownership are directly impacted by maintenance cycles. In-situ hydrogen SWP cleaning offers a way to extend collector life, reduce tool downtime, and maintain consistent exposure dose. This is particularly important for high-volume manufacturing (HVM) environments, where even minor throughput losses translate into significant production costs.

Conclusion
Hydrogen surface wave plasma cleaning presents a practical and effective method for restoring EUV collector performance. It combines chemical selectivity, gentle cleaning action, and operational compatibility with lithography tools. Continued optimization and long-term durability studies will help integrate this technique into standard EUV tool maintenance protocols.

You can see the full paper on IEEE Xplore here.

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