Abstract

Increasing the density of 3D memory in future technology nodes requires device concepts (3D-DRAM, 3D-NAND...) that can be integrated in three dimensions while maintaining controlled leakage, stable threshold voltage, sufficient drive current and manufacturable variability. Amorphous indium-gallium-zinc oxide (IGZO) is a promising semiconducting channel material for such architectures, but its electrical performance is strongly influenced by local composition, structural disorder, defects, interfaces, contacts and processing conditions. Since IGZO characterization is mainly limited to area-averaged electrical measurements, the physical mechanisms determining the IGZO resistivity profile from source to drain, remain poorly understood.

This PhD project will develop advanced atomic force microscopy (AFM)-based metrology methods (C-AFM, SSRM, KPFM and sMIM) to enable nm-scale analysis of IGZO-based 3D memory devices. with a particular focus on 3D-DRAM. Local electrical and surface-potential measurements will be combined with structural, chemical and device-level characterization to map conductivity, contact behavior, electrostatic potential and spatial non-uniformity and this at different levels of the device fabrication.

In parallel, the student will assess how these data relate to device characteristics and how they are impacted by process engineering, enabling more accurate descriptions of geometry, electrostatics, defect states, contacts, leakage, and variability.

A central objective is to create a predictive model linking AFM observables and other metrology results to measured electrical characteristics. This will allow us to identify performance-limiting mechanisms and propose metrology, process and architecture improvements to maximize the performance of IGZO channel-based   3D architectures.

Research objectives

• Develop reliable sample-preparation and AFM measurement workflows for blanket IGZO, IGZO-based stacks, cross-sections and three-dimensional device-relevant structures. Key objective is the establishment of a procedure to determine 2D/3D carrier distribution in IGZO with nm-scale spatial resolution.
• Establish correlative nanoscale maps of topography, current/resistance, surface potential and local electrical response using complementary AFM modes. Use the developed techniques to gain phenomenological insight in the impact of the fabrication process on the resistivity profile and carrier profile in IGZO. Conductive Atomic Force Microscopy (C-AFM) to probe local current transport, Scanning Spreading Resistance Microscopy (SSRM) to probe the impact of carrier distribution through resistance mapping, Kelvin Probe Force Microscopy (KPFM) to measure nanoscale surface potential variations, and Scanning Microwave Impedance Microscopy (sMIM) to characterize local conductivity and permittivity with nanometer-scale resolution.
• Transition from phenomenological insight into development of a predictive model which unravels the link between the fabrication process and electron density and/or oxygen vacancies in an IGZO layer, in particular near the contacts and at interfaces, and which also unravels the link with threshold voltage and performance variability of a corresponding FET.
• Validate your predictive model by feeding the predicted carrier profile and other metrology-based input into a TCAD device simulation tool providing the corresponding simulated electrical performance and by comparing this performance to the experimentally observed electrical performance.
• Develop fabrication processes, measurement strategies and device architecture options which are robust to IGZO material variability.
  
   

Proposed work program

Key technical questions

•  Which AFM observables are sufficiently quantitative and reproducible to constrain IGZO transport, contact and defect parameters?

•  How do local electrical non-uniformities propagate into device-to-device variability and three-dimensional array behavior?

•  Which combination of measurements best separates bulk, interface and contact contributions?

•  What level of model complexity is required to reproduce both quasi-static and transient device responses without over-parameterization?

•  Which process or architecture modifications deliver the greatest robustness to IGZO material variability?

Expected outcomes

• A validated AFM-based workflow for nanoscale electrical characterization of IGZO materials and 3Dmemory relevant structures.
• Correlative datasets and analysis routines connecting local morphology, potential and electrical response.

• A predictive model with documented assumptions and parameter ranges which links processing conditions (like temperature, presence of interfaces, ...) to resulting carrier densities in IGZO.

• A transferable metrology-to-simulation methodology for diagnosing process and device limitations.
• Peer-reviewed publications, conference contributions and recommendations for future test structures and process splits.

Candidate profile and project environment

The candidate should have a background in applied physics, physics engineering, nanoscience, materials science, electrical engineering, nanoelectronics or related discipline. Experience in semiconductor devices, scanning probe microscopy, numerical simulation or scientific programming is an advantage. The work requires a strong interest in combining experiments and modelling, rigorous data analysis, and collaboration across metrology, integration, device and simulation teams.