Subsurface Imaging of Ungrounded Metal Lines Embedded in Dielectric with the Scanning Microwave Microscope
Lin You, Jungjoon Ahn, Yaw S. Obeng, Joseph J. Kopanski
We demonstrate the ability of the scanning microwave microscope (SMM) to measure the subsurface location of ungrounded. 1.2-μm wide metal lines embedded in a dielectric film. The SMM was used to image Al-Si-Cu metal lines in a test chip that were encapsulated with approximately 600 nm or 2200 nm of plasma deposited TEOS (in PECVD SiOx bulk) with additional dielectric beneath the lines. Both the peak frequency (resulting in a change in S11 reflected amplitude when imaged at fixed frequency) and the S11 phase of the reflected SMM response changed between when the tip was over a region without a buried metal line and when it was over a region with a buried metal line. The shallower line edge could be resolved within about 900 nm ± 70 nm and the deeper line within 1200 nm ± 260 nm. We calculate the expected change in peak frequency and phase of the SMM response from the change in the tip capacitance using the lumped element model for a SMM with a resonant transmission line. The SMM technique shows some promise for a subsurface imaging technique with depth and resolution appropriate for metrology problems in integrated circuit technology, solar cells, batteries, and biological applications.