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1. Investigating the Effects on Adhesion of Surface Topography Across Length Scales

   Pradhan, Arushi (September 2025, University of Pittsburgh)

   The performance of manufactured devices and tools involving contact is governed by surface properties such as adhesion, friction, and wear—all strongly influenced by surface topography. For perfectly flat surfaces, adhesion can be described by an interaction potential, where the minimum energy required to separate the bodies is the intrinsic work of adhesion, and the range of interaction is the length scale over which the interaction operates. These parameters reflect interfacial interactions—such as covalent, van der Waals, or capillary forces. However, for most natural and engineered surfaces, the surface interaction is strongly modified by surface topography, which exists across a wide range of length scales from the atomic scale to the device scale. Significant prior work has established theoretical strategies to describe topography-dependent adhesion. Yet there is a significant gap in understanding of how to experimentally measure and analyze surfaces in order to apply this theory to describe and improve surfaces in real-world applications. The first part of this research addresses the characterization of surfaces with multi-scale topography. To date, the standard practice often reduces surface topography to a single scalar metric (e.g., Ra), often without specifying the size over which it was measured or computed. Even while it is widely recognized that these scalar metrics do not correlate with surface performance. To address this limitation in surface characterization, we conducted the Surface-Topography Challenge. In this international effort, 153 people from 64 different institutions characterized statistically equivalent “smooth” and “rough” samples, creating a total of 2088 measurements using diverse characterization methods. Significant variation was observed when length scale was ignored, and consensus was only established using scale-dependent parameters and by controlling for artifacts and extreme values. The findings from this investigation suggest three best practices for characterizing and specifying topography. In the second part of this research, we explore the relationship between surface topography and surface adhesion. Specifically, sinusoidal patterned samples with varying roughness were created using grayscale electron-beam lithography and adhesion was measured using 250-µm silicon spheres. Conventional roughness parameters failed to explain the observed adhesion trends. However, a simple numerical model incorporating AFM measurements of both the patterned surfaces and the spherical tips successfully captured the trends in adhesion. Additionally, experimental measurements showed wide variability in adhesion not captured by the model. Similar variability was also observed in adhesion experiments on silicon wafers using sharp AFM probes of three materials: DLC, CrN and diamond. The model was improved using 3D probe geometries derived from TEM images and surface roughness reconstructed from the inverse Fourier transform of the full power spectral density (PSD). Simulations that varied both the intrinsic work of adhesion and the interaction range revealed that both the average value and the deviation in adhesion are strongly influenced by nanoscale roughness. Significant errors in predicted adhesion were obtained when nanoscale roughness was not taken into account. 

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2. Velocity-weakening and -strengthening friction at single and multiasperity contacts with calcite single crystals

   https://doi.org/10.1073/pnas.2112505119  

   Fu, Binxin; Espinosa-Marzal, Rosa M. (May 2022, Proceedings of the National Academy of Sciences)

   Although earthquakes are one of the most notorious natural disasters, a full understanding of the underlying mechanisms is still lacking. Here, nanoscale friction measurements were performed by atomic force microscopy (AFM) on calcite single crystals with an oxidized silicon tip to investigate the influence of roughness, contact aging, and dry vs. aqueous environment. In dry environments, smooth and rough calcite surfaces yielding single- and multiasperity contacts, respectively, exhibit velocity-weakening ( β D ln V ) or neutral friction at slow sliding velocities and velocity-strengthening friction ( α D ln V ) at higher velocities, while the transition shifts to slower velocities with an increase in roughness. The origin of the velocity-weakening friction is determined to be contact aging resulting from atomic attrition of the crystalline surface. Friction measurements in aqueous environment show evidence of pressure solution at sufficiently slow sliding velocities, which not only significantly reduces friction on single-and multiasperity contacts but also, eliminates atomic attrition and thereby, velocity-weakening friction. Importantly, the friction scaling law evolves from logarithmic ( β D ln V ) into linear ( α P S V ), deviating from commonly accepted rate-and-state friction (RSF) laws; this behavior extends over a wider range of velocities with higher roughness. Above a transition velocity, the scaling law remains logarithmic ( α W ln V ). The friction rate parameters α D , β D , α P S , and α W decrease with load and depend on roughness in a nonmonotonic fashion, like the adhesion, suggesting the relevance of the contact area. The results also reveal that parameters and memory distance differ in dry and aqueous environments, with implications for the understanding of mechanisms underlying RSF laws and fault stability. 

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3. Photoresist characterization using a tabletop extreme ultraviolet source at 30 nm wavelength

   https://doi.org/10.1116/6.0004950  

   Flores, Ethan; Mohanty, Saurav; Mitchell, Richard; Tunell, Andrew; Kepenekci, Mehmet; Chang, Chih-Hao (December 2025, Journal of Vacuum Science & Technology B)

   In this paper, we investigate the viability of using ZEP520A, a commercially available electron-beam photoresist, as a positive resist for extreme ultraviolet (EUV) lithography at 30 nm wavelength. The resist characterization was performed using a tabletop, high-harmonic-generation EUV source driven by an ultrafast laser. The source produces an elliptical beam at focus, modeled as a Gaussian intensity profile to analyze the exposure dose distribution. A comprehensive protocol was developed, employing confocal microscopy to generate topography maps and extract critical dimensions of the patterned resist. AFM (atomic force microscopy) was subsequently used to measure postdevelopment resist thickness and refine estimates of the clearing dose and contrast, modeled with a binary resist approach. Experimental results show that ZEP520A exhibits a clearing dose of 35.4 mJ/cm2 and a contrast of 2.9 at 30 nm wavelength, supporting its application in EUV lithography. These findings demonstrate that ZEP520A can serve as an effective EUV resist and that a tabletop EUV source provides a practical platform for material characterization in next-generation lithography development. 

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4. Investigation of Protein/Lipid Interactions via Scanning Probe Acceleration Microscopy: Theory and Experiment

   https://doi.org/10.1115/DETC2012-70228  

   Legleiter, Justin; Burke, Kathleen A.; Yates, Elizabeth A. (August 2012, ASME 2012 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference)

   There is great interest in the application of proximal probe techniques to simultaneously image and measure mechancial properties of surfaces with nanoscale spatial resolution. There have been several innovations in generating time-resolved force interaction between the tip and surface while acquiring a tapping mode AFM image. These tip/sample forces contain information regarding mechanical properties of surfaces in an analogous fashion to a force curve experiment. Here, we demonstrate, via simulation, that the maximum and minimum tapping forces change with respect to the Young’s modulus and adhesiveness of a surface, but the roughness of the surfaces has no effect on the tapping forces. Using these changes in tapping forces, we determine the mechanical changes of a lipid membrane after exposure to a huntingtin exon1 (htt exon1) protein with an expanded polyglutamine (polyQ) domain. Expanded polyQ domains in htt is associated with Huntington’s disease, a genetic neurodegenerative disorder. The htt exon1 protein caused regions of increased surface roughness to appear in the lipid membrane, and these areas were associated with decreased elasticity and adhesion to the AFM probe. 

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5. Evaluating scanning electron microscopy for the measurement of small-scale topography

   https://doi.org/10.1088/2051-672X/ad49b9  

   Chadha, Vimanyu; Miller, Nathaniel; Ding, Ruikang; Beschorner, Kurt E; Jacobs, Tevis David (May 2024, Surface Topography: Metrology and Properties)

   Abstract For predicting surface performance, multiscale topography analysis consistently outperforms standard roughness metrics; however, surface-characterization tools limit the range of sizes that can be measured. Therefore, we evaluate the use of scanning electron microscopy (SEM) to systematically measure small-scale topography. While others have employed SEM for similar purposes, the novelty of this investigation lies in the development and validation of a simple, flexible procedure that can be applied to a wide range of materials and geometries. First, we established four different options that can be used for sample preparation, and we measured quantitative topography of each using the SEM. Then the power spectral density (PSD) was used to compare topography among the four preparations, and against other techniques. A statistical comparison of PSDs demonstrated that SEM topography measurements outperformed AFM measurements at scales below 100 nm and were statistically indistinguishable from (highly labor-intensive) TEM measurements down to 16 nm. The limitations of SEM-based topography are quantified and discussed. Overall, the results show a simple generalizable method for revealing small-scale topography. When combined with traditional stylus profilometry, this technique characterizes surface topography across almost seven orders of magnitude, from 1 cm down to 16 nm, facilitating the use of physical models to predict performance. 

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