Brent A. Nelson

Direct deposition of continuous metal nanostructures by thermal dip-pen nanolithography

We describe the deposition of continuous metal nanostructures onto glass and silicon using a heated atomic force microscope cantilever. Like a miniature soldering iron, the cantilever tip is coated with indium metal, which can be deposited onto a surface forming lines of a width less than 80 nm. Deposition is controlled using a heater integrated into the cantilever. When the cantilever is unheated, no metal is deposited from the tip, allowing the writing to be registered to existing features on the surface. We demonstrate direct-write circuit repair by writing an electrical connection between two metal electrodes separated by a submicron gap.

Electrical, Thermal, and Mechanical Characterization of Silicon Microcantilever Heaters

Silicon atomic force microscope (AFM) cantilevers having integrated solid-state heaters were originally developed for application to data storage, but have since been applied to metrology, thermophysical property measurements, and nanoscale manufacturing. These applications beyond data storage have strict requirements for mechanical characterization and precise temperature calibration of the cantilever. This paper describes detailed mechanical, electrical, and thermal characterization and calibration of AFM cantilevers having integrated solid-state heaters. Analysis of the cantilever response to electrical excitation in both time and frequency domains aids in resolving heat transfer mechanisms in the cantilever. Raman spectroscopy provides local temperature measurement along the cantilever with resolution near 1 mum and 5degC and also provides local surface stress measurements. Observation of the cantilever mechanical thermal noise spectrum at room temperature and while heated provides insight into cantilever mechanical behavior and compares well with finite-element analysis. The characterization and calibration methodology reported here expands the use of heated AFM cantilevers, particularly the uses for nanomanufacturing and sensing

Shape recovery of nanoscale imprints in a thermoset "shape memory" polymer

This letter reports temperature-dependent recovery of atomic force microscope tip-formed indentations in a thermoset shape memory polymer. The indentations are made at both room temperature and 69°C, and then recovered at temperatures between 40°C and 70°C. The shape recovery is more complete for higher anneal temperatures, and is relatively independent of time for 102–104s. The experiments show shape memory in the 1–100nm size scale.

Nanoscale deposition of solid inks via thermal dip pen nanolithography

We demonstrate that nanolithography can be performed using a heated atomic force microscope (AFM) cantilever tip to control the deposition of a solid organic “ink.” The ink, octadecylphosphonic acid (OPA), has a melting temperature near 100°C and can self-assemble on mica. Postdeposition analysis shows that deposition occurs only when the cantilever tip is heated above OPA’s melting temperature, that the deposited structure does not spread significantly while cooling, and that a cool tip coated with OPA does not contaminate the substrate during subsequent imaging. Single lines were written with a width of 100nm. This approach greatly expands the potential of dip pen nanolithography, allowing local control of deposition and deposition of materials typically immobile at room temperature, while avoiding potential problems arising from inadvertent deposition and postdeposition diffusion.

Measuring material softening with nanoscale spatial resolution using heated silicon probes

This article describes the use of heated silicon atomic force microscopy probes to perform local thermal analysis (LTA) of a thin film of polystyrene. The experiments measure film softening behavior with 100 nm spatial resolution, whereas previous research on LTA used probes that had a resolution near 10 microm, which was too large to investigate some types of features. This article demonstrates four methods by which heated silicon probes can perform thermal analysis with nanoscale spatial resolution. The polystyrene softening temperature measured from nanoscale LTA techniques is 120 degrees C, compared to 100 degrees C, measured with bulk ellipsometry. The discrepancy is attributed to the thermal contact resistance at the end of the silicon probe tip, on the order of 10(7)K/W, which modulates heat flow between the tip and sample and governs the fundamental limits of this technique. The use of a silicon probe for LTA enables bulk fabrication, parallelization for high-throughput analysis, and fabrication of a sharp tip capable of nanoscale spatial resolution.

MODELING AND SIMULATION OF THE INTERFACE TEMPERATURE BETWEEN A HEATED SILICON TIP AND A SUBSTRATE

This article presents an analytical model and finite difference simulations that predict the interface temperature between a heated atomic force microscope (AFM) tip and a substrate. The thermal resistances for the tip, interfacial contact between the tip and substrate, and spreading into the substrate are all considered. The thermal properties and geometry of the tip closest to the apex govern heat transport through the entire tip. The models thus r uire boundary-constricted thermal conductivity in the tip and a separate thermal resistance to account for the geometry at the tip apex. The tip-substrate interface temperature depends upon the contact impedance, contact force, and ambient environment thermal conductivity. For a silicon tip, the combined thermal resistance of the substrate and contact is on the order of 107–108 K/W and dominates the heat transfer. The model identifies dimensionless parameters that govern the tip-substrate interface temperature, which can inform cantilever design and application development.

A study of biologically-inspired design as a context for enhancing student innovation

This article describes an investigation of the use of biologically-inspired design as a context from which to teach innovative design. The research compared ideation behavior among mechanical engineering students from a capstone design class to mechanical engineering students who had taken a semester-long course specifically focused on biologicallyinspired design. Both groups of students were presented with the same design challenge, and pre-established metrics were used to characterize the novelty and variety of the resultant designs generated by the students. The designs from the biologically-inspired design students had an average novelty score 80% higher than those from the control group of capstone students, and the result was statistically-significant. The biologically-inspired design students also had a 37% higher average variety score, although a small sample size led to a high variance and prevented statistical significance. The increased scores for novelty and variety imply a greater tendency toward innovative design among the biologically-inspired design students. The source of greater innovation is unclear but may be due to improved analogical reasoning capabilities among the biologically-inspired design students.

Applications of Heated Atomic Force Microscope Cantilevers

Heated AFM cantilevers have been used for thermal property measurement, microsystems actuation, and thermal processing, but applications using these capabilities have only begun to realize their full potential. Many physical, chemical, and biological phenomena depend upon temperature, and the most interesting measurements are likely yet to be demonstrated. For example, few precision force measurements have been made with heated AFM cantilevers, even though they are outstanding force transducers. Additionally, no investigations that we are aware of have explored the effects of heated probes as highly localized heat sources in biological or biochemical systems.

Using a competency-based instructional approach in thermodynamics

Many engineering classes are highly sequential, causing students that fail to grasp initial topics to struggle as courses progress. Despite instructor exhortations to master fundamental subjects, students often continue to struggle instead of investing the time to review. Because students do not proactively re-learn initial content, a competency-based approach was implemented in a highly sequential thermodynamics course. In the approach, students scoring below 80% on the first exam were required to pass an online review quiz in order to take subsequent exams. Only 3 out of 135 students were unable to take the second exam, but all were able to pass by the time of the third exam. Significant increases (p<;0.01) were achieved in both the average course grade and average grade on the third (final) exam as compared to the previous offering of the course, with the average grade on the final exam rising from 66.5% to 74.0% and the overall course grade rising from 76.2% to 83.2%. The competency-based structure forced students to review fundamental material that is necessary both later within a course and in subsequent courses, and seems to particularly benefit the poorer performing students. This may have impacts on student retention and persistence.

Transport in Thermal Dip Pen Nanolithography

The manufacture of nanoscale devices is at present constrained by the resolution limits of optical lithography and the high cost of electron beam lithography. Furthermore, traditional silicon fabrication techniques are quite limited in materials compatibility and are not well-suited for the manufacture of organic and biological devices. One nanomanufacturing technique that could overcome these drawbacks is dip pen nanolithography (DPN), in which a chemical-coated atomic force microscope (AFM) tip deposits molecular ‘inks’ onto a substrate [1]. DPN has shown resolution as good as 5 nm [2] and has been performed with a large number of molecules, but has limitations. For molecules to ink the surface they must be mobile at room temperature, limiting the inks that can be used, and since the inks must be mobile in ambient conditions, there is no way to stop the deposition while the tip is in contact with the substrate. In-situ imaging of deposited molecules therefore causes contamination of the deposited features.Copyright