Why silicon surfaces slip more easily under pressure
Surfaces can start slipping more easily when pressed harder – a surprising discovery by researchers at the University of Amsterdam and ARCNL. Their work reveals how microscopic contact points behave like a crowd crossing at a red light, offering new insights for precision positioning in semiconductor manufacturing.
In their experiment, Liang Peng, Thibault Roch, Daniel Bonn and Bart Weber pressed a smooth silicon surface against a rough one. The researchers, from the University of Amsterdam and the Advanced Research Center for Nanolithography, then explored how the friction behaved when the strength with which the two surfaces are pressed together was varied. Does it get harder to slide the two surfaces along one another when one presses harder? And, importantly: why?
Understanding the why
It turned out that the amount of friction depends on a very interesting underlying process. At low applied force, only one tiny contact point – a so-called “asperity” – bears the load, and it needs to be pushed hard before it slips. However, as the force perpendicular to the interface increases, many asperities come into contact. The team discovered that in this situation, once a few asperities start slipping, others are triggered to follow—just like the first bold pedestrians prompt a crowd to cross.
As a result, and perhaps counterintuitively, the surface starts sliding more easily, and the relative resistance to motion – the so-called static friction coefficient – decreases. Using a simple mathematical model to support their experiments, the researchers were able to show that the crowd-like behavior of the asperities explains why static friction weakens at higher loads.
Application in semiconductor industry
In the semiconductor industry, the construction of electronic devices often requires clamping curved surfaces to a flat table. This results in an interface that is right at the boundary of slipping and not slipping. The new research explains how the onset of sliding is influenced by the scale of the contact, which is important to know when accurately constructing devices using all sorts of materials.
ARCNL group leader Bart Weber explains: “In real life, there is no such thing as perfectly flat and parallel surfaces. This means that when you clamp a ‘flat’ object onto a ‘flat’ table, small misalignments cause the object to slip and slide over the table. However, the relative motion is hindered by friction. The onset of frictional slip is a complex process, especially for larger interfaces that involve many microscopic contact points.
Our new paper illustrates how the interaction between contact points influences the friction during the onset of sliding. These insights can support our ability to understand and manipulate precision positioning, which is important in the semiconductor industry, for example for the production of computer chips with small feature sizes.”
Publication
The decrease of static friction coefficient with interface growth from single to multi-asperity contact, Liang Peng, Thibault Roch, Daniel Bonn, and Bart Weber. Physical Review Letters 134 (2025) 176202.
