Quantitative tomography with subsurface scanning ultrasound resonance force microscopy

Extracting quantitative information about dimensions and material properties of buried structures is continuing to be an important but difficult task in metrology. Examples of questions asking for this capability include critical dimension metrology of fins such as the profile (bottom width, top width, height) or the presence and extent of voids. In recent years TNO has demonstrated the concept of using Atomic Force Microscopy (AFM) in combination with ultrasound to image buried structures based on their (visco-)elasticity in a technique called Subsurface Scanning Ultrasound Resonance Force Microscopy (SSURFM). We have successfully imaged structures less than 10nm wide, as well as structures buried up to a micrometer deep. However, extracting quantitative information from this data is not trivial, as the induced stress field in the sample depends on many parameters in a non-linear way: experimental parameters such as applied force, tip size and tip shape, and geometry and material properties of the buried structures themselves. Therefore, measurements based on this technique have a point spread function which varies in a complicated way with the sample properties that need to be measured. However, a solid understanding of the physics and mechanics involved, and the modeling of the expected structures and their response to externally applied stress, enable quantitative measurements. We specifically show our progress on characterizing a sample comprising fins from a 7nm node manufacturing test run.