Subsurface scanning probe microscopy for reference overlay metrology

Recent developments in the semiconductor industry have yielded an increase in density in transistor layout. Therefore, an accurate metrology tool is required to measure the reduced overlay and EPE budgets. Subsurface scanning probe microscopy (SSPM) is a non-destructive metrology method for imaging buried structures through opaque layers at a small scale. In this work, an SSPM technique is used to showcase feasibility for acting as a reference metrology mode focusing on accuracy and precision of extracted overlay between two consecutive layers. A 3σ value of 0.13 nm was obtained over 10 static repeats on 1 target with a measured mean shift between the M1 and M2 markers of 0.26 nm. Additionally, a 3σ value of <1.2 nm was obtained on 5 different targets within the same field. The mode was also assessed to be capable of resolving grating structures with a pitch of 100 nm buried at a depth greater than 100 nm under opaque layers. Although improvements to the technology have to be developed to be able to use SSPM as a reference metrology tool for overlay application, these results show excellent potential for such purposes.