J. M. R. Weaver

Scanning thermal microscopy using batch fabricated thermocouple probes

We have developed scanning thermal microscopy probes for high resolution analysis of thermal properties in an atomic force microscope (AFM). Electron beam lithography and silicon micromachining have been used to batch fabricate Au/Pd thermocouples situated at the end of Si3N4 cantilevers. The cantilevers are patterned on the side of traditional style pyramidal AFM tips, giving a new shape of probe which is favorable for access to specimens containing significant topographic variation. Tip radius is approximately 50 nm and the probe has a macroscopic opening angle of 70°. The probes were scanned in the repulsive mode using a conventional AFM. Force feedback was optically employed to give topographic and thermal maps simultaneously by maintaining a constant force of approximately 5 nN. During initial scans using a photothermal test specimen, 80 nm period metal gratings were thermally resolved.

Fabrication of 3 nm wires using 100 keV electron beam lithography and poly(methyl methacrylate) resist

We report the fabrication of 3 nm NiCr wires on a solid silicon substrate. The process uses conventional 100 keV electron beam lithography and poly(methyl methacrylate) resist. The wires consist of short, continuous, lengths of metal that are attached at either end to 20 nm wide wires. Instead of exposing continuous lines in the resist, we blank the beam for several pixels to leave a gap. The resist in the gap is therefore exposed only by the secondary electrons from the neighboring regions that are directly exposed by the beam. The technique is repeatable and we demonstrate that it is possible to make 3 nm features on demand.

Highly localized thermal, mechanical, and spectroscopic characterization of polymers using miniaturized thermal probes

In this article, we demonstrate the versatility of use of cantilever-type resistive thermal probes. The probes used are of two kinds, Wollaston wire probes and batch-microfabricated probes. Both types of probe can be operated in two modes: a passive mode of operation whereby the probe acts as a temperature sensor, and an active mode whereby the probe acts also as a highly localized heat source. We present data that demonstrate the characterization of some composite polymeric samples. In particular, the combination of scanning thermal microscopy with localized thermomechanometry (or localized thermomechanical analysis, L-TMA) shows promise. Comparison with data from conventional bulk differential scanning calorimetry shows that inhomogeneities within materials that cannot be detected using conventional bulk thermal methods are revealed by L-TMA. We also describe a new mode of thermal imaging, scanning thermal expansion microscopy. Finally, we outline progress towards the development of localized Fourier tr...

SThM Temperature Mapping and Nonlinear Thermal Resistance Evolution With Bias on AlGaN/GaN HEMT Devices

Channel temperature has a strong impact on the performance of a microwave power transistor. In particular, it has a strong influence on the power gain, energetic efficiency, and reliability of the device. The thermal optimization of device geometry is therefore a key issue, together with precise measurements of temperature within the channel area. In this paper, we have used scanning thermal microscopy to perform temperature mapping, at variable dc bias points, on an AlGaN/GaN high-electron mobility transistor made on epilayers grown on silicon carbide substrate. We have analyzed the variation of the thermal resistance values, which are deduced from these measurements, with bias conditions VGS and VDS. The observed nonlinear behavior is found to be in excellent agreement with physical simulations, strongly pointing out the large variability of the extension of the dissipation area with the dc bias conditions

Parallel scanning near-field photolithography: The snomipede

The “Millipede”, developed by Binnig and co-workers (Bining, G. K.; et al. IBM J. Res. Devel. 2000, 44, 323.), elegantly solves the problem of the serial nature of scanning probe lithography processes, by deploying massive parallelism. Here we fuse the “Millipede” concept with scanning near-field photolithography to yield a “Snomipede” that is capable of executing parallel chemical transformations at high resolution over macroscopic areas. Our prototype has sixteen probes that are separately controllable using a methodology that is, in principle, scalable to much larger arrays. Light beams generated by a spatial modulator or a zone plate array are coupled to arrays of cantilever probes with hollow, pyramidal tips. We demonstrate selective photodeprotection of nitrophenylpropyloxycarbonyl-protected aminosiloxane monolayers on silicon dioxide and subsequent growth of nanostructured polymer brushes by atom-transfer radical polymerization, and the fabrication of 70 nm structures in photoresist by a Snomipede probe array immersed under water. Such approaches offer a powerful means of integrating the top-down and bottom-up fabrication paradigms, facilitating the reactive processing of materials at nanometer resolution over macroscopic areas.

Heat conduction nanocalorimeter for pl-scale single cell measurements

An ultrasensitive nanocalorimeter for use with pl-scale biological samples using silicon microfabrication technology has been developed in which a 720 pl reaction vessel, a calibration heater, and a thermoelectric transducer of 125 μK sensitivity were integrated into a single multilayer thin-film configuration. The resolution of the system ranged from 10 to 25 nW depending on the heat capacity, conductance and power density of the samples studied. The device has been used in heat conduction measurements of the energy released from the enzyme catalyzed hydrolysis of hydrogen peroxide using purified catalase, and for the determination of the catalase activity within a single mouse hepatocyte. The nanocalorimeter has the potential for integration in a high-density array format, where the change in temperature from ultralow volume cellular assays could be used as a generic analytical tool for high throughput screening of bioactive compounds.

Direct imprint of sub-10 nm features into metal using diamond and SiC stamps

We demonstrate the transfer of sub-10nm features into nickel using a hard stamp. Nanostructures were transferred directly from diamond and SiC in a single step by pressing the stamp into nickel at room temperature. The patterns were generated using ultrahigh resolution electron beam lithography. Patterns were transferred to the diamond and SiC using RIE etching with an O2 plasma used for the diamond and a SF6+O2 mixture used for the SiC. Hydrogen Silsesquioxane was used as a resist and served as a mask in the plasma etching.

Generic scanned-probe microscope sensors by combined micromachining and electron-beam lithography

We present a novel method for the fabrication of generic scanned-probe microscope probes by performing multiple level direct-write electron-beam lithography on the apex of micromachined atomic force microscope tips. Pattern transfer is by conventional etching or liftoff in a wide range of materials. Lithographic resolution is 50 nm or better. The substrates support the use of automatic alignment and allow for the fabrication of 50 probes/in2. The integration of a force-sensing cantilever permits simple height regulation of the probes during operation. The technology is illustrated by the fabrication of thermocouple and near-field optical probes.

The thermoelectric properties of Ge/SiGe modulation doped superlattices

The thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si1− y Ge y barriers are presented, which demonstrate enhancements in the thermoelectric figure of merit, ZT, and power factor at room temperature over bulk Ge, Si1− y Ge y , and Si/Ge superlattice materials. Mobility spectrum analysis along with low temperature measurements indicate that the high power factors are dominated by the high electrical conductivity from the modulation doping. Comparison of the results with modelling using the Boltzmann transport equation with scattering parameters obtained from Monte Carlo techniques indicates that a high threading dislocation density is also limiting the performance. The analysis suggests routes to higher thermoelectric performance at room temperature from Si-based materials that can be fabricated using micro- and nano-fabrication techniques.

A suspended membrane nanocalorimeter for ultralow volume bioanalysis

A nanocalorimetric suspended membrane sensor for pL volumes of aqueous media was fabricated by bulk silicon micromachining using anisotropic wet etching and photo and electron beam lithographic techniques. A high-temperature sensitivity of 125 microK and a rapid unfiltered time constant of 12 ms have been achieved by integrating a miniaturized reaction vessel of 0.7-nL volume on a 800-nm-thick and 300 x 300- microm2-large silicon nitride membrane, thermally insulated from the surrounding bulk silicon. The combination of a ten-junction gold and nickel thermoelectric sensor with an integrated ultralow noise preamplifier has enabled the resolution of 15-nW power in a single measurement, a result confirmed by electrical calibration. The combination of a high sensitivity and rapid response time is a consequence of miniaturization. The choice of gold and nickel as sensor material provided the maximum thermal sensitivity in the context of ease of fabrication and cost. The nanocalorimetric sensor has the potential for integration in an ultralow-volume high-density array format for the characterization of processes in which there is an exchange of heat.