Reuse and Characterization of Laser Lift-off (LLO) of Bonded Wafers for Advanced Packaging

Fusion and Hybrid bonding are essential steps increasing die to wafer (D2W), high-bandwidth memory (HBM), and artificial intelligence (AI) applications. Laser lift-off (LLO) uses a short pulsed IR laser beam to release permanently bonded Si carrier wafers without need of a glue layer or backside grinds processes. During the LLO process, the laser beam penetrates through the stacked layers inside dielectrics structures and could potentially damage the devices through mechanical stress from debonding and/or thermal conduction. To mitigate laser damage, optimization of an engineered stack is crucial for continuous use of the carrier wafer. Two thermal damage detection methods confirmed no damage on the semiconductor device layer after LLO processing. The characterization methods used a one second rapid thermal anneal (RTA) as reference with a known temperature to determine the thermal budget of post lift-off devices. The first method for detecting the 500°C-700°C thermal range is to find the phase change of metal silicide. Phase change is measured by X-ray diffraction (XRD).  The second method for 800°C-1000°C detection is to use tracer ion diffusion via time-of-flight secondary ion mass spectroscopy (ToF-SIMS).

The results of these thermal damage experiments were used to model a broader spectrum of stacked materials. The absorption through the stack and the resulting heat transfer was simulated. Since experimental measurements generally require orders of magnitude for longer timescales than the laser exposure, it is challenging to compare the energy spike in LLO simulation to a thermal budget based on RTA processing. If for a given stack it has been demonstrated experimentally that the heat energy from the laser is at an acceptable level, then it is reasonable to assume that other stacks that show equal or less heat energy in simulation are also acceptable. The temperature dynamics were understood by coupling the associated physics and confirmed with the experimental results.