Every advanced packaging process engineer has encountered this problem before when trying to fabricate an optical transceiver.
A Photonic Integrated Circuit (PIC)-based device is being assembled and many critical stepshave been completed. Only one last step: the visual inspection. And then… it fails the test. Signal loss. Alignment drift. The checklist said “cleaned.” Yet, it still failed.
What are the Status quo solutions ?
CO₂ Dry ice, Manual Wet Cleaning and Plasma Cleaning. This blog is specifically about CO₂ Dry Ice Cleaning vs laser cleaning.
In today’s high-performance photonics packaging, it’s not the only the visible contaminants that cost you yield—it’s the invisible ones. Whether you’re aligning photonic facets in a co-packaged optics module, assembling a laser module into or trying For silicon photonics, CO₂ cleaning cannot offer this level of precision, because it was never built for that.As devices become smaller, stacked tighter, and more optically sensitive, CO₂ is no longer a fit-for-purpose solution.
Cleaning is needed at many critical steps during the manufacturing flow at the OSAT:
- Just before Wafer-level Testing for Known-Good Die (KGD) sorting : Contaminants can alter probing signals and lead to False positive dies → Lower KGD
- Before Fiber Attach Steps: Dicing and assembly steps will contaminate the optical coupling interface region where pristine interfaces are needed.
- If needed : Advanced Bonding Circuit when Heteregenous Integration of laser
- Die-level Testing for Final Product Circuit-level Testing : same problem as above for WLT.
- And many more… .
CO₂ Dry Ice Cleaning: Good for Particles. Challenging for precision.
CO₂ snow cleaning has earned its reputation as a dependable particle remover. It’s fast and dry. No solvents, no rinsing, no drying cycles. Perfect for dust, die saw debris, and some loose organics.
But here’s the catch: CO₂ works by force, not finesse. It blasts away what is weakly attached—but leaves behind what is strongly bonded. The very contaminants that matter most in PIC and HBM packaging—thin organic films, cured adhesives, flux residues—slip right through.
Mid-IR laser cleaning takes a different approach. It does not push—it targets. If the contaminant absorbs light at 2.8 µm, then it can be vaporized. That includes:
- Residual organic contaminants such as grease or isoproopanol near the fiber attach surface.
- Epoxies, polymers and other organic-based substances used on surfaces
- Alcohol (isopropanol)
Figure 1. CO₂ snow cleaning of dust, grease and isopropanol residue on a PIC vs laser cleaning
CO₂ Cleans Almost Everything Equally but NOT Effectively
CO₂ snow cleaning, like plasma cleaning, applies a blanket approach to cleaning. It treats every surface the same—regardless of what needs to be cleaned. Similarly:
- You end up cleaning zones that did not require it
- You may miss contaminants that cannot be targeted by CO2
- You introduce risks like of freezing damage or thermal shock to sensitive areas and coatings
Unlike CO₂ snow, mid-IR laser cleaning is inherently accurate at the µm-scale, material selective and spatially localized since it’s a laser pulse ~ a point ~ that is focused on the substrate. Selectivity matters as laser cleaning only interacts with materials that absorb light at 2.8 µm—meaning it ignores what shouldn’t be touched and targets only what should be removed.
Silicon Photonics: Cleaning Optical Interfaces Without Touching Them
In silicon photonics, it is all about optical performance, high-volume manufacturing and long-term reliability.
A speck of residue on optical couplers (edge, grating or evanescent) or fiber alignment surface may lead to:
- Insertion loss due to absorption by the contaminant
- Back-reflection from the contaminants
- Wasted alignment time
- Burning spot from 24/7 laser exposition during operation (Failure from optical damage at the position of the residue)
- Failed optical coupling
CO₂ cannot strip adhesives. But a 2.8 µm pulsed laser? It can remove contaminants around gratings, lenses, without damaging the substrate. The result: better coupling, fewer test failures, and less post-packaging rework.
Figure 2: CO₂ snow cleaning of dust, grease and isopropanol residue vs laser cleaning
Mid-IR Laser Cleaning: The Approach for Precision
Mid-IR Laser cleaning is different. It is a more precise, targeted solution built for the demands of modern PICs. Unlike traditional cleaning methods, Mid-IR laser cleaning is based on selectivity at the molecular level. Instead of treating every surface the same, it discriminates based on the optical absorption of the contaminant. This enables sub-micron targeting without physical contact—perfect for densely packed structures like waveguides in silicon photonics, co-packaged optics and, broadly speaking, any optical coupling in transceivers,. By tuning the laser to 2.8 µm, organics materials like epoxy, dust and other polymers absorb energy and are vaporized—without affecting the surrounding substrate. This leads to higher optical coupling yields, reduced rework, and fewer failures during final inspection.
Figure 3 : Optical transmission from UV to 3000nm for Epoxies (left) and different transparent substrates materials (right)
CO₂ vs. Mid-IR: Side-by-Side
| Critical Need | CO₂ Snow | Mid-IR Laser |
| Remove organics & residues | ❌ Not Always | ✅ Yes |
| Clean inside trenches / edge facets | ❌ Limited | ✅ Targeted |
| Consumables or gas logistics | ❌ Needs CO₂ supply | ✅ Energy only |
| Certainty in cleaning outcome | ❌ Visual only | ✅ Physics-driven (absorption) |
Table 1: Comparison between CO₂ snow cleaning and laser cleaning
Mid-IR Laser Cleaning: A Strategic Upgrade.
CO₂ snow has its place: bulk particle removal, non-critical clean steps, or post-CMP dusting. But in silicon photonics, it lacks precision.
The smart strategy: switch for a Mid-IR laser cleaning if precision is key to your process OR combine both technologies for an even lower risk of contamination:
- Use CO₂ for wide-surface particle removal.
- Deploy Mid-IR laser cleaning for high-value zones, sensitive interfaces, and final cleaning stages.
If you require more information on Femtum’s laser cleaning solution, please contact us to discuss your challenges or view Femtum.ai.