Monitoring Conformality in ALD Manufacturing:Comparing Lateral and Vertical High Aspect Ratio Test Structures

Atomic Layer Deposition (ALD) technology enables manufacturing ofconformal thin films into such deep microscopic trenches and cavities thatthe film characterization becomes a true challenge. In ALD applicationsthese 3D microstructured substrates are typically vertically oriented highaspect ratio (HAR) structures. Monitoring and control of conformality reliespredominantly on cross-sectional sample preparation and SEM/TEMcharacterization. This approach has several challenges, e.g. need to breakthe wafer, seeing only thin slice, cleavage plane inaccuracy, multiplerepeated samples to get reliable data and long response times.A potential approach to circumvent the challenges is a MEMS-based allsilicon lateral high aspect ratio (LHAR) test structure, PillarHall® developedat VTT [1-2]. The LHAR test chip is IC cleanliness proven and thuspotentially compatible to any cleanroom environment. This study focusesto research questions: How reliable and accurate is LHAR test in 300 mmwafer manufacturing environment and, especially, how does it compare tovertical HAR structures.The LHAR Test Chip (LHAR3 -series, AR range 2:1 - 10000:1, 500nm gapheight) was employed for the first time on the carrier wafer in 300 mmwafer ALD process (Jusung Eureka 3000) in Fraunhofer IPMS. The ALDprocess was foundry’s default ZrO2/Al2O3 laminate process, 22 nm, carriedout in two process variation runs (A=optimized for 3D, B=planar) at sametemperature and cycle numbers. In the same run was employed LHAR andvertical trench test structures (AR 20:1). Conformality of both structureswere analyzed by SEM cross-sections, with appropriate samplepreparations.Findings show that conformality in LHAR is comparable to vertical HARwithin accuracy limits of step coverage metrology within the comparableAR range. Furthermore, in this study, higher aspect ratios in LHAR test chipshows significant differences between the process variations while in VHARthey are small. Therefore, even optical microscope metrics from LHARprovides fast relative insight to the process variations and can be utilized in monitoring. LHAR enables also access to gain more detailed compositionalinformation on the trench wall e.g. by ToF-SIMS, which is underexamination and a topic of further studies