Wafer bonding is a critical process step in advanced semiconductor manufacturing. It is used to join two wafers face-to-face to create stacked structures for 3D integration, MEMS packaging, power devices, and image sensors. The bond interface must be continuous, void-free, and mechanically robust across the full 300 mm wafer diameter. Voids at the bond interface — regions where the two surfaces have failed to make intimate contact — are among the most damaging defects in bonded wafer pairs. They locally disrupt the electrical, thermal, and mechanical continuity of the stack, causing device failure, delamination under thermal cycling, or catastrophic yield loss during downstream thinning and dicing. Detecting, mapping, and characterising voids across the full wafer area is therefore an essential quality control step in any bonding process, whether direct fusion bonding, oxide-to-oxide bonding, or hybrid bonding. The ability to measure void size, shape, depth, and distribution inline, without destructive cross-sectioning, is exactly the capability that lineWLI brings to this critical inspection challenge.
The original data is under NDA, so we replicated the use case using two glass slides with a water film in between. The water film naturally has inclusions in the form of air bubbles. While this may be a nuisance in other scenarios, it exactly replicated the desired effect. Silicon is not transparent in the visible range as is glass. However, the wavelength range of lineWLI is not limited to the visible range. With the right optics and InGaAs-detectors, lineWLI can see through silicon and characterize bonding layers between silicon wafers.
The first measurements provided an overview scan with a scale of 200 µm per pixel. In a void the index of refraction changes abruptly from the value of the film – here 1.33 – to 1 for air or vacuum within the void. The separation of the two glass slides does not change at the void – film interface. As a result, there is a large change in the measured optical path distance that shows up as a large step of 33% of the original film thickness.
LineWLI with microscope optics reveals further details. Profile A-A reveals the combined surface roughness of the interfaces as a variation of the film thickness. Likewise, there’s a tiny scratch visible at between x=800µm and x=900µm.
