Session: 14-06-01: Applied mechanics and materials in micro- and nanosystems

Paper Number: 172530

Probing the Onset of Plasticity in Hybrid Bonding Copper via Multimodal Atomic Force Microscopy 

The advancement of semiconductor technology cannot continue solely under the paradigms of Moore’s and More Moore’s laws, as the dimensions of transistors approach the physical limits. Nonetheless, ongoing developments in fields such as artificial intelligence, large-scale computing, big data, and robotics are driving the need for improvements in power, performance, area, and cost (PPAC). In this context, various advanced packaging techniques (an umbrella term for various architectural accelerators) have emerged as potential solutions to sustain miniaturization trends and meet PPAC requirements. One promising approach is hybrid bonding technology, which enables direct metal-metal (copper-copper) and dielectric-dielectric connections with interconnect pitches <10 μm, without the need for wires or bumps in the circuit. The merits of hybrid bonding hinge on the successful integration of two critical steps: chemical-mechanical polishing followed by annealing and diffusion bonding, both of which involve intricate nanoscale mechanical and thermomechanical processes. As such, to understand and improve this technology, it is crucial to characterize the elastic and plastic properties of bonding-ready copper at the nanoscale, which differ from those measured at the macro-, meso- and micro-scale due to the emergence of size and processing effects.

Fundamental micro- and nano-scale plasticity studies in copper have primarily been conducted using nanoindentation, miniaturized uniaxial testing, and forward and inverse computational methods. Noticeably, there is a lack of studies focusing on atomic force microscopy (AFM) for incipient-plasticity characterization of materials specific to the semiconductor industry. This contrasts with the continued downward trend in size-scaling, and the critical need for application-relevant, robust metrology, as identified in the International Roadmap for Devices and Systems.

We address existing gaps by presenting a multimodal AFM-based metrology framework designed to characterize the nanoscale elastic and plastic response of copper at room temperature within actual hybrid bonding patterns, prior to the bonding process. Specifically, we employ contact resonance (CR-AFM) and single-step and multi-step AFM-indentation together with the topographic capabilities of AFM to measure local moduli, characterize incipient plasticity, and derive indentation stress-strain curves at room temperature. Our multimodal AFM approach, featuring tip radii less than 40 nm, enables probing the mechanics of nanoscale volumes on precise nanostructural surface features (such as grain boundaries) with sub-nm resolution, and without additional sample preparation. Through these measurements, we elucidate the mechanisms of early plasticity and determine the elastoplastic constitutive response of the copper pads, including elastic modulus, yield stress, and strain-hardening slope. In addition to providing lenghtscale-relevant metrology, our approach offers a pathway to leveraging an industry-standard instrument towards the characterization of thermomechanical properties relevant to semiconductor metallization. This work also sets the stage for future nanoscale metrology that combines topographic, mechanical, electrical, and thermal capabilities.

Presenting Author: Nicolas Alderete NIST

Presenting Author Biography: Nicolas Alderete is a postdoctoral associate in the Material Measurement Laboratory at the National Institute of Standards and Technology. As part of the Nanomechanical Properties Group, he is focusing on experimental analysis and computational modeling of micro- and nanoindentation experiments towards scale-relevant metrology for the semiconductor industry. Before joining NIST he obtained a Ph.D. in Theoretical and Applied Mechanics from Northwestern University, where he focused on experimental and computational research on shape-morphing programmable metamaterials and natural phononic materials.

Authors:

Nicolas Alderete NIST
Gheorghe Stan NIST
Cristian Ciobanu Colorado School of Mines
Paresh Daharwal Intel Corporation