Harnessing water’s stickiness for adhesion control
Printing computer chips with nanoscale features requires extremely precise positioning. Electric charges and water on surfaces can impact this precision, but these effects have not been fully understood. Now, ARCNL postdoctoral researcher Tunç Çiftçi and his colleagues have studied this phenomenon and uncovered a way to use it as a tool for adhesion control and precision positioning, rather than an obstacle to overcome.
Microscopic surface roughness
In semiconductor equipment, the silicon wafers on which computer chips are printed need to be clamped in place with high precision.
When two surfaces touch, the contact is not perfect. Even a shiny surface has microscopic roughness, which on the atomic scale seems like a mountain range. So, contact between two surfaces is really two atomic-scale mountain ranges touching each other, making contact only at the tips of mountain peaks.
In the field of tribology – the study of friction, wear and lubrication – these mountain peaks are called “asperities.” The way that the asperities of a silicon wafer and a positioner contact each other influences the positioning of the wafer, and thus the development and measurement of nanoscale features in a chip. This is especially noticeable as chip features become nanometer scale.
Single-asperity aspirations
Electric charges on interacting surfaces may also play a role in the contact forces exchanged, especially at the nanometer scale. Characterizing the effect of these charges has proven difficult in the past, because two surfaces touching means two sets of mountain ranges, both with varying heights. This makes it a “multi-asperity” problem.
Tunç Çiftçi, postdoctoral researcher at ARCNL, realized that he could change this into a simpler, “single-asperity” problem using atomic force microscopy (AFM): “These atomic-scale mountain peaks look a lot like AFM probes.” The microscopic probes of an AFM physically interact with a sample’s surface in order to create nanoscale images.
Treating the probe of an atomic force microscope as a singular asperity allowed Tunç and his colleagues, Jon Cottom, Rachid Hahury, Emilia Olsson and Bart Weber, to study how two rough surfaces interact on the nano scale. Specifically, they looked at what happened when they applied an electric potential difference between the AFM probe and the sample surface, and were surprised by what they found.
Water as a positioning guide
The applied charge difference between the probe and the surface attracted water in the environment to the area, polarizing the molecules – much like a magnet attracts and polarizes iron shavings. The polarization also makes the water electrically “sticky,” an effect called adhesion. What was surprising, however, was that even after the applied charge was removed, the water adhesion remained even an hour later.
This unexpectedly long-lasting adhesion can be useful for precision positioning. Water would usually be expected to pull surfaces together in unwanted ways, but by controlling where water adhesion occurs, water can solve its own problem. “Think of the electrically enhanced water molecules as construction workers guiding a crane,” explains Tunç. “They organize themselves at the contact points and help align the surfaces precisely.”
The study, published in the journal Tribology Letters, shows that applying a charge difference between surface contact points can help enhance their water adhesion. The technique is reversible and non-invasive, making use of humidity in the air. In the long term, adhesion control could enable the production of higher-quality computer chips.
Contact
For more information about this work, contact Dr. Tunç Çiftçi (Email: T.Ciftci@arcnl.nl).
Reference
Tunç Çiftçi, Jonathon Cottom, Rachid Hahury, Emilia Olsson & Bart Weber, Adhesion Control Through Electric Field-Induced Water Adsorption at Oxidized Silicon Interfaces, Tribology Letters 74, 4 (2026).
