Advanced Lateral High Aspect Ratio Test Structures for Conformality Characterization by Optical Microscopy

Atomic Layer Deposition (ALD) is a key technology in 3D microelectronics enabling conformal coatings into deep microscopic trenches and high aspect ratio cavities. However‚ conformality characterization in 3D trench walls is challenging and requires sample preparation. The MEMS-based all-silicon Lateral High Aspect Ratio (LHAR) test structure‚ developed at VTT‚ named PillarHall® [1-3]‚ provides a fast and accurate substrate and characterization concept for the thin film conformality analysis [4‚5]. The most important outcome from the PillarHall® characterization is the film saturation profile‚ which provides valuable data for reaction kinetics modelling and quantifying conformality. Since the LHAR enables utilization of planar metrology instruments‚ multiple approaches are compatible.

In this study‚ we focus on characterization methods suitable to extract the saturation profile and show the advantage of simple optical microscopy image analysis. Studied material was prototypical ALD Al2O3 (40-50 nm thick layers) made in two separate research facilities. The study consisted of SEM/EDX planar view‚ micro-spot reflectometry and ellipsometry and optical microscopy image analyses. These experiments were carried out by two research labs and supported by leading edge optical metrology tool vendors‚ Semilab Ltd‚ JA Woollam and Filmetrics.

We also introduce advanced LHAR 4th generation design‚ which enables characterization of the penetration depth profile with distinct advantages. Namely‚ new pillar design enables employment of optical line scanners up to 50 µm spot sizes. Furthermore‚ internal distance indicators support positioning the characterization tools more accurately. These features are illustrated in Supplementary Material Fig S1.

The results show that the gray-scale optical microscopy image analysis with the thickness determination in opening area gives similar results as the reflectometry or ellipsometry line scans‚ within the accuracy limits. Optical thickness/λ of the film is a limitation for the image analysis‚ but designing the experiments properly‚ the grayscale optical micrograph can be a powerful‚ widely compatible and easy method for conformality analysis.
Support from Semilab Ltd is gratefully acknowledged.

References

[1] F. Gao et al.‚ J. Vac. Sci. Technol. A Vacuum‚ Surfaces‚ Film.33‚ 010601 (2015).

[2] R. Puurunen and F. Gao‚ 2016. doi: 10.1109/BALD.2016.7886526

[3] M. Mattinen et al.‚ Langmuir‚ 32‚ 10559 (2016).

[4] M. Ylilammi et al.‚ J. Appl. Phys.‚ 123‚ 205301 (2018).
[5] V. Cremers et al.‚ Applied Physics Reviews‚ in press (2019).