In order to extract ever more performance from semiconductor devices on the same device area, the semiconductor industry is moving towards device structures with increasingly complex material combinations and 3D geometries. To ensure cost effective fabrication of next generation devices, metrology solutions are needed that tackle the specific challenges that come from these developments such as 3 dimensional imaging of structures and imaging of deeply buried structures under arbitrary, complex layers. Compared to existing metrology solutions for high end manufacturing, ultrasonic inspection techniques have advantages: they are unaffected by optically opaque layers, the acoustic wavelength (60nm @ 100GHz in SiO2) can be smaller than optical wavelengths and the measurement depth can be larger. However, traditional acoustic microscopy tops out at a few GHz due to manufacturing tolerances and the required liquid couplant. We propose to combine very high frequency ultrasound with scanning probe microscopy. By locating the transducer above the cantilever tip, it guides sound into the sample with a dry tip-sample contact. This allows for very high acoustic frequencies and a resolution of O(wavelength).
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.
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