image: Figure 1. Mechanism of photothermal-enhanced defect inspection.
Credit: Prof. Jinlong Zhu et al.
Breaking the limit of optical defect inspection
As semiconductor manufacturing advances toward sub-10nm nodes, identifying deep-subwavelength defects in dense nanopatterns has become a "needle in a haystack" challenge. Traditional Bright-Field Inspection (BFI) faces a fundamental bottleneck: the weak scattering signals from tiny defects are completely overwhelmed by the intense background noise generated by dense nanostructures. This low signal-to-noise ratio (SNR) often renders critical defects "invisible" under conventional optical systems.
To overcome this impasse, the joint research team formed by Prof. Shiyuan Liu and Prof. Jinlong Zhu from Huazhong University of Science and Technology (HUST) proposed SPM². Unlike traditional static imaging, SPM² introduces an active "time-domain" dimension to optical inspection. By applying a localized pump beam to induce a controlled temperature rise, the team leveraged the high thermo-optic coefficient of silicon to modulate the refractive index of the structures. This localized heating triggers a precise resonance redshift, allowing the system to "tune" the background scattering and isolate the defect signatures.
High contrast and microsecond response
The core innovation of SPM² lies in its role as a "physical signal amplifier." When the nanostructure is heated, the resonance shift causes a drastic nonlinear change in the scattering field. This mechanism significantly enhances the perturbations caused by defects while suppressing static background intensity. Numerical simulations reveal that for a break defect, only a fraction of the probe wavelength, the contrast can be enhanced by nearly an order of magnitude (e.g., from 0.12 to 1.02) compared to standard BFI. Consistent enhancement has been observed in experimental inspection, where defects that were "invisible" in the pump-off state emerged as intense signals under pump excitation, yielding a five-fold increase in the SNR.
The system also demonstrates exceptional temporal efficiency. Theoretical analyses show that the photothermal excitation and dissipation processes reach a steady state within microseconds (approx. 4.6 μs for heating and 7 μs for cooling). Such a high-speed duty cycle ensures that the technology can maintain the high-throughput requirements of industrial-grade wafer inspection. Furthermore, the peak temperature remains well below the damage threshold of silicon, ensuring a completely non-destructive and reversible inspection process. Experimental verification confirms that the optical response before and after photothermal excitation is fully reversible, with no observed morphological changes to the nanostructures.
Broad applications
A key advantage of the SPM² framework is its compatibility with existing optical architectures. It requires no fundamental changes to the core imaging optics; instead, it can be implemented by simply integrating an external pump light module. This feature drastically reduces the cost and complexity of hardware upgrades for the current inspection system.
The impact of this work extends beyond simple wafer inspection.
The SPM² principle is universally applicable to any system exhibiting optical resonance, providing a versatile platform for the dynamic control of photonic devices and the inspection of buried defects in 3D structures.
Journal
Light: Advanced Manufacturing
Article Title
Spatiotemporal photothermal modulation microscopy (SPM²) for high-sensitivity deep-subwavelength defect inspection