Thin Films Metrology

Advanced Model Based Analysis (MBA) is the most advanced Coherence Scanning Interferometry (CSI)-based thin film measurement technique available from ZYGO, and works by comparing a theoretical model of the sample film stack to an actual measurement signal as seen by the profiler. This patented technology simultaneously measures topography, thickness, and substrate topography for single layer films from 50 – 2000 nm in seconds. In addition to thin film characterization, MBA technology can be used to perform true topography measurements of dissimilar materials by adjusting for the phase change on reflection (PCOR) that occurs in these situations.

Challenges and Opportunities for CSI

CSI provides non-contact, areal surface topography maps across a wide range of surfaces. When those surfaces are transparent, a portion of the incoming light penetrates the surface, to possibly reflect from the next interface. For materials much thicker than the size of the CSI signal, it is relatively simple to separate the reflection from the top surface and the underlying substrate.

Typical CSI signals for (a) bare surface (no film); (b) thick film (well over 1 µm) with well separated signals from surface and substrate; and (c) sub-micron film with merged surface and substrate signals.

However, as the thickness of the transparent material is reduced below one micron, the signals begin to overlap. Traditional signal analysis methods fail in this regime, but there is a clear opportunity in decoding a signal which contains information about the properties of both the film, and the substrate it is deposited on.

Model-Based CSI

ZYGO’s model-based approach solves this problem. The user is asked to define the optical characteristics of their film stack (the index of refraction n and absorption coefficient k). Using system-level calibration information, the measurement is simulated to create a library of possible CSI signals for a range of thicknesses of the described film. Then, the measurement is compared to the signal library at each pixel to create a precise map of the film thickness, as well as the top and bottom surface topography.

Measurement flow for presented approach. Steps performed by the user are shown in green.

This method provides a measurement that maintains all the key advantages of non-contact, areal CSI metrology.

High throughput, with measurements taking less than 10 seconds for full-field acquisition, even for thickness search ranges over a micron.

Flexible and precise, supporting a wide range of magnifications, compatible with the full suite of ZYGO objective lenses. Film thickness values are stable at the nanometer level over 2 orders of magnitude of magnification.