Scanning Joule Expansion Microscopy Research Articles

Emerging multi-frequency surface strain force microscopy

During the past decade, Scanning Probe Microscopy (SPM) based surface strain detection techniques have been extensively used in the characterization of functional materials, structures, and devices. Here, we refer these techniques as Surface Strain Force Microscopy (SSFM), which mainly includes the Piezoresponse Force Microscopy, Atomic Force Acoustic Microscopy, Atomic Force Microscopy-Infrared spectroscopy (or photothermal induced resonance), Piezomagnetic Force Microscopy, and Scanning Joule Expansion Microscopy. The inception of SSFM opens up a pathway to study the nanoscale physical properties by using a sharp tip to detect the local field-induced surface strain. Through measuring the signals of the surface strain, multiple physical properties, such as the electromechanical, mechanical, photothermal, magnetic, thermoelastic properties, can be characterized with an unprecedented spatial resolution. In order to further develop and overcome the fundamental issues and limitations of the SSFM, the multi-frequency SPM technology has been introduced to the SSFM-based techniques, leading to the emerging of multi-frequency SSFM (MF-SSFM). As a technical breakthrough of the SSFM, MF-SSFM has demonstrated substantial improvements in both performance and capability, resulting in increased attentions and numerous developments in recent years. This Perspective is, therefore, aimed at providing a preliminary summary and systematic understanding for the emerging MF-SSFM technology. We will first introduce the basic principles of conventional SSFM and multi-frequency SPM techniques, followed by a detailed discussion about the existing MF-SSFM techniques. MF-SSFM will play an increasingly important role in future nanoscale characterization of the physical properties. As a result, many more advanced and complex MF-SSFM systems are expected in the coming years.

Characterisation of a micrometer-scale active plasmonic element by means of complementary computational and experimental methods.

In this article, we investigate an active plasmonic element which will act as the key building block for future photonic devices. This element operates by modulating optical constants in a localised fashion, thereby providing an external control over the strength of the electromagnetic near field above the element as well as its far-field response. A dual experimental approach is employed in tandem with computational methods to characterise the response of this system. First, an enhanced surface plasmon resonance experiment in a classical Kretschmann configuration is used to measure the changes in the reflectivity induced by an alternating electric current. A lock-in amplifier is used to extract the dynamic changes in the far-field reflectivity resulting from Joule heating. A clear modulation of the materials' optical constants can be inferred from the changed reflectivity, which is highly sensitive and dependent on the input current. The changed electrical permittivity of the active element is due to Joule heating. Second, the resulting expansion of the metallic element is measured using scanning Joule expansion microscopy. The localised temperature distribution, and hence information about the localisation of the modulation of the optical constants of the system, can be extracted using this technique. Both optical and thermal data are used to inform detailed finite element method simulations for verification and to predict system responses allowing for enhanced design choices to maximise modulation depth and localisation.

Conductive Filament Localization Within Crossbar Resistive Memories by Scanning Joule Expansion Microscopy

Control of filament growth within conductive bridge random access memories (CBRAM) is of crucial interest in order to ensure the reliability of such emerging devices. Here, we demonstrate that Scanning Joule Expansion Microscopy (SJEM) can be used to detect and precisely localize conductive filaments within operating crossbar CBRAM devices. Flexible memory devices based on Pd/Al2O3/Ag stacks are first elaborated at low temperature on polyimide substrate. These devices show set and reset operations at low voltage ( $4.3~\mu \text{m}$ is extracted at 50kHz and it is also demonstrated that the thermal expansion signal is proportional to the dissipated Joule power. We believe that the proposed procedure opens the way to reliability studies that can be applied to any family of memory device based on filamentary conduction.

Spontaneous current constriction in threshold switching devices

Threshold switching devices are of increasing importance for a number of applications including solid-state memories and neuromorphic circuits. Their non-linear characteristics are thought to be associated with a spontaneous (occurring without an apparent external stimulus) current flow constriction but the extent and the underlying mechanism are a subject of debate. Here we use Scanning Joule Expansion Microscopy to demonstrate that, in functional layers with thermally activated electrical conductivity, the current spontaneously and gradually constricts when a device is biased into the negative differential resistance region. We also show that the S-type negative differential resistance I–V characteristics are only a subset of possible solutions and it is possible to have multiple current density distributions corresponding to the same value of the device voltage. In materials with steep dependence of current on temperature the current constriction can occur in nanoscale devices, making this effect relevant for computing applications.

Thermal Engineering of GaN Semiconductor Devices

The development of gallium nitride (GaN) on a variety of substrates from Si to diamond has been used to create a variety of devices for both RF and power electronics technologies. In general, GaN devices can accommodate faster switching speeds, higher junction temperatures, and high voltages, allowing for devices operating at higher power with increased efficiency in smaller form factors. However, the lattice mismatch between c-axis hexagonal GaN and other non-native substrates requires the use of buffer layers in order to grow high quality, relaxed GaN required for advanced devices. Such layers can add to the thermal resistance which impacts to overall device thermal performance and thus, device reliability. In addition, these interfacial layers impact the growth and residual stress in the GaN epitaxial layers that can limit the overall voltage limits of lateral HEMT devices. In this work we use a variety of experimental techniques to measure the thermal boundary resistance (TBR) for GaN on Si, SiC, and diamond substrates. The impact of the structure of the interface on the overall TBR and thermal conductivity of the GaN layers grown by MOCVD will be discussed. In addition to the TBR, the overall stress in the devices caused by growth on the differing substrates, measured by Raman spectroscopy, will be presented. A unique superlattice structure used to reduce the stress in GaN layers on Si will also be highlighted. Finally, the electro-thermomechanical performance of GaN devices under DC and pulsed operation will be presented where the temperature of the devices was measured using Raman Spectroscopy while the mechanical deformation was measured by Scanning Joule Expansion Microscopy. Data will show that rapid heating and high tensile stresses near the drain side edge of the gate during pulsed operation is dependent on the bias conditions, the TBR, substrate material, and the mechanical properties of the device. Overall, challenges associated with the structure of GaN devices that must be overcome for effective thermally management will be presented.

Direct observation of resistive heating at graphene wrinkles and grain boundaries

We directly measure the nanometer-scale temperature rise at wrinkles and grain boundaries (GBs) in functioning graphene devices by scanning Joule expansion microscopy with ∼50 nm spatial and ∼0.2 K temperature resolution. We observe a small temperature increase at select wrinkles and a large (∼100 K) temperature increase at GBs between coalesced hexagonal grains. Comparisons of measurements with device simulations estimate the GB resistivity (8–150 Ω μm) among the lowest reported for graphene grown by chemical vapor deposition. An analytical model is developed, showing that GBs can experience highly localized resistive heating and temperature rise, most likely affecting the reliability of graphene devices. Our studies provide an unprecedented view of thermal effects surrounding nanoscale defects in nanomaterials such as graphene.

Heterogeneous nanometer-scale Joule and Peltier effects in sub-25 nm thin phase change memory devices

We measure heterogeneous power dissipation in phase change memory (PCM) films of 11 and 22 nm thin Ge2Sb2Te5 (GST) by scanning Joule expansion microscopy (SJEM), with sub-50 nm spatial and ∼0.2 K temperature resolution. The heterogeneous Joule and Peltier effects are explained using a finite element analysis (FEA) model with a mixture of hexagonal close-packed and face-centered cubic GST phases. Transfer length method measurements and effective media theory calculations yield the GST resistivity, GST-TiW contact resistivity, and crystal fraction of the GST films at different annealing temperatures. Further comparison of SJEM measurements and FEA modeling also predicts the thermopower of thin GST films. These measurements of nanometer-scale Joule, thermoelectric, and interface effects in PCM films could lead to energy-efficient designs of highly scaled PCM technology.

Thermomechanical Modeling of Scanning Joule Expansion Microscopy Imaging of Single-Walled Carbon Nanotube Devices

An analytical model, validated by experiments and finite element simulations, is developed to study the thermal imaging of single-walled carbon nanotube (SWNT) devices by scanning Joule expansion microscopy (SJEM). A simple scaling law for thermal expansion at low frequencies, which only depends on two nondimensional geometric parameters, is established. Such a scaling law provides a simple way to determine the surface temperature distribution and power dissipation per unit length in an SWNT from the measured thermal expansion in experiments. The results suggest the spatial resolution of the SJEM measurement is as good as ∼50 nm.

Direct observation of nanometer-scale Joule and Peltier effects in phase change memory devices

We measure power dissipation in phase change memory (PCM) devices by scanning Joule expansion microscopy (SJEM) with ∼50 nm spatial and 0.2 K temperature resolution. The temperature rise in the Ge2Sb2Te5 (GST) is dominated by Joule heating, but at the GST-TiW contacts it is a combination of Peltier and current crowding effects. Comparison of SJEM and electrical measurements with simulations of the PCM devices uncovers a thermopower of ∼350 μV K−1 and a contact resistance of ∼2.0 × 10−8 Ω m2 (to TiW) for 25 nm thick films of face centered-cubic crystalline GST. Knowledge of such nanometer-scale Joule, Peltier, and current crowding effects is essential for energy-efficient design of future PCM technology.

Quantitative Thermal Imaging of Single-Walled Carbon Nanotube Devices by Scanning Joule Expansion Microscopy

Electrical generation of heat in single-walled carbon nanotubes (SWNTs) and subsequent thermal transport into the surroundings can critically affect the design, operation, and reliability of electronic and optoelectronic devices based on these materials. Here we investigate such heat generation and transport characteristics in perfectly aligned, horizontal arrays of SWNTs integrated into transistor structures. We present quantitative assessments of local thermometry at individual SWNTs in these arrays, evaluated using scanning Joule expansion microscopy. Measurements at different applied voltages reveal electronic behaviors, including metallic and semiconducting responses, spatial variations in diameter or chirality, and localized defect sites. Analytical models, validated by measurements performed on different device structures at various conditions, enable accurate, quantitative extraction of temperature distributions at the level of individual SWNTs. Using current equipment, the spatial resolution and temperature precision are as good as ∼100 nm and ∼0.7 K, respectively.

Nanoscale thermally induced stress analysis by complementary Scanning Thermal Microscopy techniques

Thermally induced stress is determined using boundary values obtained by complementary Scanning Joule Expansion Microscopy and Scanning Thermal Microscopy. The stress function is then solved with the Finite Element Method. Surface stress analysis as well as stress analysis of multilayered structures are performed in order to demonstrate the application of the developed measurement technique to reliability investigations.

Finite element analyses assisted Scanning Joule Expansion Microscopy on interconnects for failure analysis and reliability investigations

Thermo-elastic properties on aluminum interconnects are characterized by Scanning Joule Expansion Microscopy and complementary finite element analyses. Temperature distributions can be obtained by the correlation of the three-dimensional thermal expansion vectors and the device structure. Vertical and lateral displacements give access to thermally induced strains, which is important for advanced failure analyses on degradation processes.

Joule expansion imaging techniques on microlectronic devices

We have studied the electrically induced off-plane surface displacement on two microelectronic devices using scanning Joule expansion microscopy (SJEM). We present the experimental method and surface displacement results. We show that they can be successfully compared with surface displacement images obtained using an optical interferometry method. We also present thermal images using scanning thermal microscopy (SThM) technique to underline that SJEM is more adapted to higher frequency measurements. The performances of the three methods are compared.

Size Effect on the Thermal Conductivity of Thin Metallic Films Investigated by Scanning Joule Expansion Microscopy

A technique to extract in-plane thermal conductivity of thin metallic films whose thickness is comparable to electron mean free path is described. Microscale constrictions were fabricated into gold films of thicknesses 43nm and 131nm. A sinusoidal voltage excitation across the constriction results in a local temperature rise. An existing technique known as scanning joule expansion microscopy, measures the corresponding periodic thermomechanical expansion with a 10nm resolution and determines the local temperature gradient near the constriction. A three-dimensional finite-element simulation of the frequency-domain heat transfer fits the in-plane thermal conductivity to the measured data, finding thermal conductivities of 82±7.7W∕mK for the 43nm film and 162±16.7W∕mK for the 131nm film, at a heating frequencies of 100kHz and 90kHz, respectively. These values are significantly smaller than the bulk thermal conductivity value of 318W∕mK for gold, showing the electron size effect due to the metal-dielectric interface and grain boundary scattering. The obtained values are close to the thermal conductivity values, which are derived from electrical conductivity measurements after using the Wiedemann–Franz law. Because the technique does not require suspended metal bridges, it captures true metal-dielectric interface scattering characteristics. The technique can be extended to other films that can carry current and result in Joule heating, such as doped single crystal or polycrystalline semiconductors.

Scanning Joule Expansion Microscopy of a Constriction in Thin Metallic Film

Scanning Joule Expansion Microscopy of a Constriction in Thin Metallic Film

Mapping nanometre-scale temperaturegradients in patterned cobalt-nickel silicidefilms

We have implemented a thermal mapping technique,scanning Joule expansion microscopy, with a high spatialresolution (about 20 nm) in order to investigate heat generationand dissipation in mesoscopic thin-film structures. Inparticular, we investigated narrow cobalt-nickel silicide lineshaving a granular morphology. We are able to directly visualizeheating of the silicide in between the grains due to a localincrease of the resistance, resulting in current crowding. Theincrease in resistance can be associated with a local reductionof the silicide thickness in between the larger grains. On theother hand, an enhanced silicide resistivity at grain boundariesmay also contribute to the resistance changes.

Thermal characteristics of submicron vias studied by scanning Joule expansion microscopy

Thermal characteristics of submicron vias strongly impact reliability of multilevel VLSI interconnects. The magnitude and spatial distribution of the temperature rise around a via are important to accurately estimate interconnect lifetime under electromigration (EM), which is temperature dependent. Localized temperature rise can cause stress gradients inside the via structures and can also lead to thermal failures under high current stress conditions, such as electrostatic discharge (ESD) events. This letter reports the first use of a novel thermometry technique, scanning Joule expansion microscopy, to study the steady state and dynamic thermal behavior of small geometry vias under sinusoidal and pulsed current stress. Measurement of the spatial distribution of temperature rise around a submicron via is reported with sub-0.1 /spl mu/m resolution, along with other thermal characteristics including the thermal time constant.

Study of the hot spot of an in-plane gate transistor by scanning Joule expansion microscopy

The local heat dissipation of an in-plane gate (IPG) transistor was investigated by means of a thermally modulated scanning Joule expansion microscope. The nanostructured sample was prepared by focused ion beam techniques. The temperature induced thermal expansion and the topographic information are measured simultaneously. The spatial resolution of the constructed microscope is below 50 nm. Heat spots of the semiconducting devices are visualized by heating them with modulated drain voltage. The heat spot position and the temperature amplitude of the heat spot on the IPG transistor depend on the gate voltage VG. With an increasing negative magnitude of VG the heat spot displaces in the direction from drain towards source. Simultaneously the temperature amplitude of the maximum of the heat spot and the thermal expansion integrated over the heated area decrease linearly.

Nanoscale Temperature Distributions Measured by Scanning Joule Expansion Microscopy

This paper introduces scanning Joule expansion microscopy (SJEM), which is a new thermal imaging technique with lateral resolution in the range of 10–50 nm. Based on the atomic force microscope (AFM), SJEM measures the thermal expansion of Joule-heated elements with a vertical resolution of 1 pm, and provides an expansion map of the scanned sample. Sunmicron metal interconnect lines as well as 50-nm-sized single grains of an indium tin oxide resistor were images using SJEM. Since the local expansion signal is a convolution of local material properties, sample height, and as temperature rise, extraction of the thermal image requires deconvolution. This was experimentally achieved by coating the sample with a uniformly thick polymer film, resulting in direct measurement of the sample temperature distribution. A detailed thermal analysis of the metal wire and the substrate showed that the predicted temperature distribution was in good agreement with the measurements of the polymer-coated sample. However, the frequency response of the expansion signal agreed with theoretical predictions only below 30 KHZ, suggesting that contilever dynamics may play a significant role at higher frequencies. The major advantage of SJEM over previously developed submicron thermal imaging techniques is that it eliminates the need to nanofabricate specialized probes and requires only a standard AFM and simple electronics.

Thermal microscopy and heat generation in electronic devices

This paper reports our progress on the development of thermal microscopy for studying heat generation in electronic devices and interconnects. The resolution of the scanning thermal microscope has been improved from about 500 nm achieved by the first wire thermocouple probes to about 25 nm for the more recent thin-film thermocouple probes. These have been used to measure the temperature distribution and hot spots of single transistors, short circuits in transistors created by electrostatic discharge failures, as well as novel devices such as vertical-cavity surface emitting lasers and magnetoresistive reading heads. Recently, a new technique called scanning Joule expansion microscopy has been developed to measure the temperature distribution of electrically heated samples with about 10 nm spatial resolution. The advantage of this technique is that it does not require fabrication of temperature-sensing probes and can use commercially-available cantilever probes that are employed in atomic force microscopes.