Subsurface Imaging of Metal Lines Embedded in Dielectric with the Scanning Microwave Microscope

We demonstrate the ability of the scanning microwave microscope (SMM) to detect subsurface metal lines embedded in a dielectric film with sub-micrometer resolution. The SMM was used to image 1.2-μm-wide Al-Si-Cu metal lines encapsulated with either 800 nm or 2300 nm of plasma deposited silicon dioxide. Both the reflected microwave (S₁₁) amplitude and phase shifted near resonance frequency while the tip scans across these buried lines. The shallower line edge could be resolved within 900 nm ± 70 nm, while the deeper line was resolved within 1200 nm ± 260 nm. The spatial resolution obtained in this work is substantially better that the 50 μm previously reported in the literature. Our observations agree very well with calculated change in peak frequency and phase using a simple lumped element model for a SMM with a resonant transmission line. By conducting experiments at various eigenmodes, different contrast levels and signal-to-noise ratios have been compared. With detailed sensitivity studies, centered around 9.3 GHz, it has been revealed that the highest amplitude contrast is obtained when the probe microwave frequency matches the exact resonance frequency of the experimental setup. By RLC equivalent circuit modeling of the tip-sample system, two competing effects have been identified to account for the positive and negative S11 amplitude and phase contrasts, which can be leveraged to further improve the contrast and resolution.