Understanding inhibitor-based area selective atomic layer deposition for dielectric-on-dielectric growth

Electronic device fabrication today is based on 'top-down' processes with multiple lithography and etching steps. As semiconductor dimensions shrink and adopt more complex architectures, these processes are reaching their limits. One promising solution is area-selective atomic layer deposition (AS-ALD), which combines a surface modification technique and atomic layer deposition (ALD) in a 'bottom-up' approach to nanopatterning. A common approach for achieving AS-ALD involves the use of densely-packed self-assembled monolayers (SAMs) as inhibiting layers against undesired ALD growth. Incorporating this technique into fabrication schemes requires an understanding of how SAMs interact with different surfaces and how this in turn affects the ability of the SAM to act as an effective inhibitor against ALD reactions. In the first part of this thesis, a comparative study of octadecylphosphonic acid (ODPA) SAM formation on four technologically relevant metals— Cu, Co, W and Ru—was carried out. Following SAM deposition, zinc oxide (ZnO) and aluminum oxide (Al2O3) model ALD processes were performed to assess the blocking ability of the SAM layer formed on each substrate. Surface characterization of these samples revealed that ODPA-treated Cu, Co and W inhibited the growth of ZnO and Al2O3 ALD to varying degrees, while no inhibition was observed on ODPA-treated Ru. Specifically, the ODPA-treated W prevented the growth of at least 17 nm of ZnO, compared to less than half this amount on Co and Cu. Similarly, 3-4 times more Al2O3 ALD was blocked using ODPA on W versus Cu and Co. The reaction mechanism of ODPA SAM formation and how surface properties, such as the roughness and the Lewis acid nature of the substrate, could impact that SAM's formation and in turn ALD inhibition is discussed. In the second part, a series of spectroscopic analyses were performed which provide details about the structure and coverage of the ODPA layer formed on each substrate. It was determined that the average tilt angle of the ODPA chains formed on Co, Cu and W are ~66° from the surface plane of the substrate, indicating that (on average) the molecules are oriented upright. Conversely, ODPA chains on the Ru substrate were found to be disordered. Further, it was determined that ODPA on Co and Cu formed a densely packed 'crystal-like' structure while on W and Ru, the ODPA molecules appeared to exhibit a more 'liquid-like' structure. The data indicate that although ODPA forms an incomplete layer on W, it still provides the best blocking against ZnO and Al2O3. The results highlight the importance of the chemical nature of the substrate surface in achieving SAM-based AS-ALD. Finally, since the overarching goal of this research is to achieve AS-ALD on metal/dielectric patterns (which mimic interconnect devices), the selective growth of ZnO and Al2O3 was demonstrated on Cu/SiO2 and Co/SiO2 2D-patterned substrates. Challenges in achieving SAM-assisted selective ALD and methods to improve the process on smaller feature sizes are also discussed

Keywords: Chemical Engineering.
Advisor: Bent Stacey
Jaramillo Thomas Francisco
Qin Jian (Professor of Chemical Engineering)