In the world of semiconductor manufacturing, from front-end fabs to back-end OSATs, cleanliness isn’t optional—it’s mission-critical. For decades, plasma cleaning has been one of the most used methods for removing contaminants in semiconductor fabs. It’s shown to be reliable and embedded in workflows from wafer fabrication to advanced packaging. But here’s the uncomfortable truth: plasma cleaning was built for yesterday’s challenges. It works well in most general cases, but this method is being challenged by advanced applications where surface sensitivity and residue avoidance is critical, especially with more complex silicon photonics (Co-Packaged Optics (CPO) / Photonics Integrated Circuits (PIC) and High Bandwidth Memory (HBM) stacks. Blanket plasma treatments are starting to damage critical surfaces more than they clean. Sensitive materials—especially polymers and photonic structures—are suffering from over-etch, or plasma-induced defects, where a trace of surface alteration can jeopardize yield.

Plasma Cleaning – The current method but with Limitations

Plasma systems are widely adopted in the industry because they are applicable to a broad range of materials, they can process large batches, they are inherently dry, necessitate vacuum and they can operate in cleanrooms. All these features make them a suitable choice for many use cases, especially when surface activation is needed prior to a bonding step. However, plasma cleaning requires the component to be positioned in a large reactive gas chamber where cleaning happens at the top surface of the samples everywhere in the chamber, indiscriminately. Today, plasma cleaning faces many fundamental challenges for high-volume manufacturing such as:

• Hard-to-impossible cleaning use cases (Critical factor): See silicon photonics example below (Figure 1). Epoxy and fingerprints on silicon photonics are next to impossible to clean using plasma.
• Non-selective treatment: There are risks of over-cleaning sensitive areas. It may damage the surface on some components and not clean enough on others.
• Potential substrate surface modification: Plasma cleaning can alter surfaces of certain materials.
• Plasma density is not uniform in the chamber.
• Chemical reaction needs maintenance: and requires recurrent maintenance of the system.

Figure 1: Plasma Cleaning of Photonics Die – Removal of Epoxy Droplet

Enter Mid-IR Laser Cleaning: A Precision-First Alternative 

Mid-IR laser cleaning works differently, especially at 2.8µm. Short-pulsed lasers are ideal for high-precision applications: they spatio-temporally limit heat diffusion in the substrate resulting in a very small or quasi-absent heat-affected zone. Furthermore, the laser’s wavelength precisely targets specific contaminants through their distinct linear optical absorption. Organic contaminants — such as epoxy (see Fig. 2a below) — and particles absorb laser energy strongly at 2.8 µm, enabling efficient, localized, and highly selective ablation compared to VIS-NIR wavelengths. Notably, most adhesives used in silicon photonics manufacturing and assembly are organic materials, which are also among the most common sources of contamination. Once vaporized, the residues are sucked in by a fume extractor (vacuum). The full process leaves the underlying substrate pristine given it does not absorb at that wavelength.

Figure 2 : Optical transmission from UV to 3000nm for Epoxies (left) and different transparent substrates materials (right)

Here’s what makes it different:

• Selective absorption – 2.8 μm light is absorbed by organic contaminants while being transmitted through typical semiconductor substrate materials.
• Dry and Zero contact – The laser avoids damage, stress and solvents (chemicals).
• Fully automatable process – Laser Cleaning can be integrated into fully automated manufacturing process
• High throughput when using ‘Seek and Destroy’ feature – Localize the contaminant with image vision, then laser-clean only the spots that matter. This approach significantly accelerates the process time compared to blanket treatment.  

Figure 3: Laser Cleaning of Grating Coupler

Where Laser Cleans Better: Silicon Photonics and HBM

Silicon Photonics (CPO / PIC) are where mid-IR lasers shine. These devices are incredibly sensitive to contamination, and even more sensitive to cleaning damage. In some cases, plasma cleaning may roughen optical coatings and surfaces. A laser pulse is tuned to vaporize only the residue on the critical optical surface—without ever touching the surface itself.

Laser cleaning presents significant potential in Advanced Packaging, particularly for High-Bandwidth Memory (HBM). This is especially true in scenarios where organic residue and particles between Thru Glass Vias (TGV), Thru Silicon Vias (TSV), or bonding sites could lead to failures. Achieving ultraclean surfaces is paramount for ensuring optimal yield, reliability, and performance.

In summary, here are the main differences between plasma and laser cleaning:

	Plasma Cleaning	Laser Cleaning
Precision	❌ Non-selective	✅ Targeted, sub-micron
Environment	❌ Gas-based, waste-producing	✅ Dry, no chemicals
Integration	❌ Large footprint, vacuum	✅ Compact, open-air
Yield Impact	❌ Moderate improvement	✅ Maximized from selective cleaning
Maintenance	❌ Frequent	✅ Minimal, contactless

But… Is laser cleaning going to kill plasma cleaning?

In short, not exactly. While mid-IR laser cleaning surpasses plasma in precision, it is not a complete replacement. 

If your fabs already have plasma cleaning, you would want to add a tool and have the complete solution: plasma for uniform, batch cleaning or surface activation and mid-IR lasers for higher precision cleaning on contaminants that cannot be removed any other way. 

Of course, mid-IR laser cleaning requires upfront investment, along with careful tuning and process development. But when it comes to sensitive substrates, silicon photonics, or advanced packaging, it’s often what separates average fabs and OSATs from true industry leaders.

If you require more information on Femtum’s laser cleaning solution, please contact us to discuss your challenges or view Femtum.ai.