Probe interaction with semiconductor surfaces in viscous media for enabling 3D nanoelectronics device characterization

Important breakthroughs in nanoelectronics device manufacturing are directly linked to disruptive innovations in the materials characterization domain as well. Scanning probe microscopy (SPM) is one of these enabling techniques since already more than three decades because it can measure a broad range of physical properties (e.g. topography, electrical, and magnetic signals) at unrivaled spatial resolution. Today, it is a workhorse for semiconductor device analysis and is being used in all major semiconductor fabrication facilities around the world. In SPM, a small tip is brought into contact with the substrate surface and is scanned across it while measuring certain signals. Imec especially focused on developing dedicated electrical SPM methods which allow for example to characterize the doping profile of most advanced device architectures with nanometer spatial resolution in a quantitative manner. Scientific research has been the critical enabler for establishing these methods (e.g. imec’s invention of scanning spreading resistance microscopy - SSRM), probes (inhouse development and fabrication of conducting diamond tips), and quantification schemes (design and make of doping calibration staircase structures). The transition from planar transistor concepts to three-dimensional devices architectures such as FinFETs represented another game changer which was addressed by the development of electrical SPM nanotomography. This method uses a slice-and-view approach, termed scalpel AFM, whereby a scanning tip is alternatively removing material on the nanometer scale and performing concurrent electrical measurements creating tomograms of e.g. 3D transistors and nanoscopic metallic filaments of advanced memory structures. This new 3D metrology concept has attracted lots of attention from internal and external customers, as well as from tool manufacturers. Despite these achievements, the wide spread application of electrical SPM nanotomography is still hampered by the rapid tip degradation/bluntening (resulting in low resolution) and the high debris accumulation around the scan site (limiting scalpel depth and measurement quality) which is a direct result of the physical materials removal.

This project aims at addressing the challenges of nanoscopic materials removal in scalpel based SPM by studying the fundamental interactions of a scanning tip and the substrate surface in a liquid environment. The liquid environment serves as a facilitator to transport the scan debris away from the scan site and should reduce the tip wear by lubrication the tip-substrate contact. Although this principle works well in the macroscopic world where for example milling processes are commonly executed in cutting fluids, it remains to be shown how this principle can be successfully applied on the nanometer scale. First feasibility experiments have shown promising results but indicate also that a good theoretical understanding of the local tip-substrate interaction (atomic interactions) in the liquid is essential to establish a well controlled scalpel based removal process. There are existing models which describe the tip-substrate interaction in ambient environment which can be used as a starting point in this work. An experimental study explores the parameter space and boundary conditions of the removal process in suitable fluid media (e.g. oil – must be electrically insulating) with different viscosities (high vs. low) using diamond tips. The main substrate material to be studied is silicon but emerging SiGe and III-V materials will also be investigated. The experimental results will serve as input for molecular dynamics (MD) simulations which should lead to the development of a model for the scalpel-based tip-substrate interaction in liquids. This fundamental  insight is then used to demonstrate liquid-enabled 3D SPM tomography measurements on state-of-the-art device structures (e.g. FinFETs, nanowire transistors, memory) with ultra-high spatial resolution (≤1 nm) and low wear (no significant resolution loss for entire tomogram). The impact of this research work will be furhter increased by applying the gained knowedge also on the recently demonstrated reverse tip sample SPM (RTS SPM) technique. Imec's world leading expertise in electrical SPM and diamond tips forms the backbone of this research aspect.