Chiba University engineers measured the tiny recoil force from laser ablation to track drilling depth in silicon and detect the moment of full penetration in real time.

Every computer chip begins life as one of hundreds of copies printed on a thin disc of silicon called a wafer. At some point, those copies have to be separated. For decades, manufacturers have done this with diamond-edged blades that saw along the lines between chips. But as chips grow thinner and more delicate, mechanical sawing risks chipping, cracking and contaminating them.

Lasers offer a gentler alternative. Laser dicing uses focused pulses of light to cut wafers without physical contact, and it has been gaining ground for fragile materials. It comes with its own problem, however: knowing when to stop. Too few pulses leave a wafer partially cut. Too many can damage the material underneath or the tape that holds the wafer in place.

Modern wafers are typically 300 millimetres across and can be ground to a fraction of a millimetre thick before dicing, especially for chips destined for stacked memory or compact mobile devices. At those thicknesses, the margin for error is small. A cut that stops a few micrometres short leaves chips joined together, while one that goes too far can scorch the dicing tape or damage components on the other side.

Researchers at Chiba University in Japan have proposed a way to monitor the process in real time by measuring the faint kick each laser pulse delivers. The study is published in Optics and Laser Technology.

“By directly measuring the recoil force, we could detect the processing depth and sample penetration, allowing us to observe the laser processing conditions in real time,” said Professor Hirofumi Hidai of Chiba University’s Graduate School of Engineering, who led the work.

A push from vaporized material

When a short, intense laser pulse strikes a surface, it heats a tiny volume of material so fast that it vaporizes and blasts away, a process called ablation. As that material is ejected, it pushes back on the surface, in the same way that a rocket’s exhaust pushes the rocket forward. The result is a small reaction force known as recoil force.

The force from a single nanosecond pulse is minute, but it is measurable with a sufficiently sensitive sensor. Because it arises directly from the material being removed, it carries information about how much material each pulse is removing and from where.

Methods for monitoring cutting depth already exist, but the release notes that many are too slow or too fragile to be practical on fast-moving production lines. Recoil force offered an attractive alternative because it is a direct physical consequence of the ablation itself.

Drilling into silicon

The team drilled into silicon samples with a laser emitting pulses lasting 25 nanoseconds, or 25 billionths of a second. A sensitive load cell, a device that converts force into an electrical signal, recorded the recoil force produced by each pulse. The co-authors include Mutsumi Horikoshi of KISTLER Japan, part of a company that specializes in force and pressure sensors.

The measurements revealed clear patterns. Recoil force rose as pulse energy increased and fell when the laser was defocused, moved so that its focal point no longer sat on the surface. The researchers describe a mathematical relationship linking recoil force to processing conditions.

More importantly, the signal changed as the hole deepened. With each successive pulse drilling further into the wafer, the recoil force declined steadily and predictably. Then, at the moment the laser broke through the far side, the pattern shifted noticeably. That change, the authors say, can flag full penetration without the need to stop the process and inspect the wafer.

Using their model, the researchers estimated processing depth with a relative error of 24.3 percent.

Fewer defects, lower costs

A 24 percent error is not precise enough to replace other measurements outright, and the release does not report how many samples or trials the estimate is based on. The detection of full penetration may be the more immediately useful result, since it addresses the central question of when a cut is complete.

The potential payoff is significant. Semiconductor manufacturing is one of the most demanding industries in the world, and yield, the fraction of chips that come out working, has a large effect on cost. A monitoring system that prevents under- or over-processing could reduce waste at a step that every chip must pass through.

“By reducing under- or over-processing, we can minimize defective products, which is expected to lead to a stable supply and reduced costs for the electronic devices used in smartphones, automobiles, and medical equipment,” Hidai said.

Toward self-adjusting lasers

The researchers envision combining recoil force monitoring with laser systems that automatically adjust cutting conditions on the fly, increasing pulse energy if a cut is lagging or stopping the moment penetration is detected. That kind of closed-loop control is a growing goal across advanced manufacturing, where sensors feed information back to machines so they can correct themselves without human intervention.

That vision has yet to be tested. The study used drilling in silicon rather than full dicing along lines, and real production involves a wide variety of wafer materials, thicknesses and coatings, each of which may respond differently. Industrial laser dicing also includes techniques such as stealth dicing, in which a laser is focused inside the wafer to create an internal fracture line rather than ablating from the surface, which would not produce the same recoil signal.

Still, the approach offers a simple, direct way to sense what a laser is doing to a material as it happens. In an industry where fractions of a micrometre matter, that kind of real-time feedback could prove valuable.

---

Notes

1. Sato, M., Itoh, S., Horikoshi, M., Matsusaka, S., & Hidai, H. “Detection of processing depth and penetration based on recoil force induced by laser ablation.” Optics and Laser Technology 203 (2026). Published online 8 June 2026. DOI: 10.1016/j.optlastec.2026.115604.
2. Chiba University. “Monitoring laser ablation in real time: Detecting wafer penetration through recoil force.” EurekAlert!, 27 July 2026. https://www.eurekalert.org/news-releases/1137221.