Thin Film Strain Research Articles

Piezo Electric Properties of Epitaxial SiGe

Piezo resistivity of B- and P-doped epitaxial Si1-xGex (x=10-30%) is investigated to assess its application potential for thin film strain sensors. The gauge factor (GF) is calculated based on the change in resistivity to the externally induced compressive uniaxial strain along the current flow direction. In the case of B-doped SiGe, the resistivity decreases under the induced compressive strain which may be related to hole mobility enhancement, while no influence on the resistivity of the P-doped SiGe. A significant increase in the GF is observed by lowering B concentration. At the same B concentration, slightly higher GF is observed for higher Ge content. Moreover, the GF is slightly improved by lowering the SiGe growth temperature. The slight improvement may be related to improved crystallinity. These results suggest that the low-doped p-type SiGe deposited at low temperature has reasonable GF and can potentially be applied in strain sensors.

High-temperature thin-film strain sensors with low temperature coefficient of resistance and high sensitivity via direct ink writing

High-temperature thin-film strain sensors are advanced technological devices for monitoring stress and strain in extreme environments, but the coupling of temperature and strain at high temperature is a challenge for their use. Here, this issue is addressed by creating a composite ink that combines Pb2Ru2O6 and TiB2 using polysilazane (PSZ) as a binder. After direct writing and annealing the PSZ/Pb2Ru2O6/TiB2 film at 800 °C in air, the resulting thin film exhibits a low temperature coefficient of resistance (TCR) of only 281 ppm/°C over a wide temperature range from 100 °C to 700 °C, while also demonstrating high sensitivity with a gauge factor approaching 19.8. This exceptional performance is attributed to the intrinsic properties of Pb2Ru2O6, which has positive TCR at high temperature, and TiB2, which has negative TCR at high temperature. Combining these materials reduces the overall TCR of the film. Tests showed that the PSZ/Pb2Ru2O6/TiB2 film maintains stable strain responses and significant signal output even under varying temperature. These findings provide valuable insights for developing high-temperature strain sensors with low TCR and high sensitivity, highlighting their potential for applications in high-temperature strain measurements.

Motion Sensing by a Highly Sensitive Nanogold Strain Sensor in a Biomimetic 3D Environment.

Recent advancements in flexible electronics have highlighted their potential in biomedical applications, primarily due to their human-friendly nature. This study introduces a new flexible electronic system designed for motion sensing in a biomimetic three-dimensional (3D) environment. The system features a self-healing gel matrix (chitosan-based hydrogel) that effectively mimics the dynamics of the extracellular matrix (ECM), and is integrated with a highly sensitive thin-film resistive strain sensor, which is fabricated by incorporating a cross-linked gold nanoparticle (GNP) thin film as the active conductive layer onto a biocompatible microphase-separated polyurethane (PU) substrate through a clean, rapid, and high-precision contact printing method. The GNP-PU strain sensor demonstrates high sensitivity (a gauge factor of ∼50), good stability, and waterproofing properties. The feasibility of detecting small motion was evaluated by sensing the beating of human induced pluripotent stem cell (hiPSC)-derived cardiomyocyte spheroids embedded in the gel matrix. The integration of these components exemplifies a proof-of-concept for using flexible electronics comprising self-healing hydrogel and thin-film nanogold in cardiac sensing and offers promising insights into the development of next-generation biomimetic flexible electronic devices.

Low-cost two-dimensional digital image correlation system

A low-cost digital image correlation (DIC) system for measuring two-dimensional displacement fields is described. Using an off-the-shelf camera, lens, and open-source DIC analysis software, displacement fields in aluminum and steel dog-bone tensile specimens were measured. Correction for out-of-plane motion is used to obtain strains from the displacement fields. Proposed DIC strain measurement system is validated using conventional thin-film strain gauges for strains below 1%. Average strain measurements from the DIC system were found to be within 1% of the measurement obtained using strain gauges. For strains between 1% and 10%, strain histories from the DIC systems displayed excellent correlation with strain measurements obtained using an extensometer.

Unraveling the Intrinsic Thermal Behavior of Freestanding Ionomer Films Depending on Thickness from Commercial Membrane to Nanofilm.

Proton exchange membrane fuel cells (PEMFCs) for automotive applications are required to achieve mechanical reliability at various temperatures ranging from subfreezing to 80 °C. The thermal behavior of the electrode should be considered at the initial design stage to design a robust automotive fuel cell electrode. Recently, a behavior different from that of the bulk state has been reported for ionomers with a few nanometers of thickness. Therefore, the intrinsic thermal behavior of ionomer films with thicknesses from micrometers to nanometers is quantitatively investigated in this study. By introducing the fabrication of a pseudo-freestanding Nafion thin film and in-plane thermal strain measurement method on the water surface, the thermal expansion of the freestanding Nafion thin film was successfully measured with minimizing substrate constraints. Thermal strain measurement and X-ray scattering studies revealed that the weakening of intermolecular interaction within the hydrophobic and hydrophilic domains in the Nafion thin film caused thermal expansion, and well-structured hydrophobic domains could suppress thermal expansion. The thermal expansion behavior with different heat treatments provides evidence of the thin-film-to-bulk transition of the fully hydrated Nafion film. Intrinsic thermal behavior without substrate interactions can facilitate an understanding of the thermal behavior of electrodes and provide insight into designing a robust PEMFC in temperature-varying environments.

Study on Electrical and Mechanical Properties of Double-End Supported Elastic Substrate Prepared by Wet Etching Process.

Preparing elastic substrates as a carrier for dual-end supported nickel chromium thin film strain sensors is crucial. Wet etching is a vital microfabrication process widely used in producing microelectronic components for various applications. This article combines lithography and wet etching methods to microprocess the external dimensions and rectangular grooves of 304 stainless steel substrates. The single-factor variable method was used to explore the influence mechanism of FeCl3, HCl, HNO3, and temperature on the etching rate, etching factor, and etching surface roughness. The optimal etching parameter combination was summarized: an FeCl3 concentration of 350 g/L, HCl concentration of 150 mL/L, HNO3 concentration of 100 mL/L, and temperature of 40 °C. In addition, by comparing the surface morphology, microstructure, and chemical and mechanical properties of a 304 stainless steel substrate before and after etching treatment, it can be seen that the height difference of the substrate surface before and after etching is between 160 μm and -70 μm, which is basically consistent with the initial design of 0.2 mm. The results of an XPS analysis and Raman spectroscopy analysis both indicate that the surface C content increases after etching, and the corrosion resistance of the surface after etching decreases. The nano-hardness after etching increased by 26.4% compared to before, and the ζ value decreased by 7%. The combined XPS and Raman results indicate that the changes in surface mechanical properties of 304 stainless steel substrates after etching are mainly caused by the formation of micro-nanostructures, grain boundary density, and dislocations after wet etching. Compared with the initial rectangular substrate, the strain of the I-shaped substrate after wet etching increased by 3.5-4 times. The results of this study provide the preliminary process parameters for the wet etching of a 304 stainless steel substrate of a strain measuring force sensor and have certain guiding significance for the realization of simple steps and low cost of 304 stainless steel substrate micro-nano-processing.

A Novel Semiconductor Piezoresistive Thin-Film Strain Gauge With High Sensitivity

A Novel Semiconductor Piezoresistive Thin-Film Strain Gauge With High Sensitivity

High-Temperature PdCr Thin-Film Strain Gauge With High Gauge Factor Based on Cavity Structure

High-Temperature PdCr Thin-Film Strain Gauge With High Gauge Factor Based on Cavity Structure

Temperature self-compensation thin film strain gauges based on nano-SiO2/AgNP composites

Nano-SiO2/AgNP temperature self-compensation thin film strain gauges for micro-strain by direct ink writing technology.

Directly Deposited Thin-Film Strain Gauges for Force Measurement at Guide Carriages

Directly Deposited Thin-Film Strain Gauges for Force Measurement at Guide Carriages

Measurements of Polymer-Derived Ceramic Thin-Film Strain Gauges at High Temperatures

Measurements of Polymer-Derived Ceramic Thin-Film Strain Gauges at High Temperatures

Structural Design and Optimization of the Milling Force Measurement Tool System Embedded with Thin-Film Strain Sensors.

A milling force measurement tool system is designed with an elastic beam structure, which is divided into a two-end ring hoop compression sensor mode and a two-end square hoop compression sensor mode to improve the strain sensitivity. A simplified mechanical model of the elastic beam is established, and the relationship between the strain and force of the elastic beam under the action of three cutting force components is investigated, which can act a guide for subsequent milling force measurement tool system calibration tests. Thin-film strain sensors occupy a central position in the milling force measurement tool system, which consists of a substrate, transition layer, insulating layer and resistance grid layer. The resistance grid layer has a particularly significant effect on the thin-film strain sensor's performance. In order to further improve the sensitivity of thin-film strain sensors, the shapes of the substrate, the transition layer, the insulating layer and the resistance grid layer are optimized and studied. A new thin-film strain sensor is designed with a resistance grid beam constructed from an insulating layer and a resistive grid layer double-end-supported on the transition layer. The flow of the wet-etching process of thin-film strain sensors is studied and samples are obtained. The surface microforms of the sensor samples are observed by extended depth-of-field microscopy, confocal microscopy and atomic force microscopy. It can be seen that the boundary of the resistance grid layer pattern is tidy and has high dimensional accuracy, thus enabling the basic achievement of the expected effect of the design. The electrical performance of the samples is tested on an experimental platform that we built, and the results show that the resistive sensitivity coefficient of the samples is increased by about 20%, to 51.2%, compared with that of the flat thin-film strain sensor, which fulfils the design's requirements.

High-quality VO2 films synthesized on polymer substrates using room-temperature pulsed laser deposition and annealing

With the growing trend towards portable and adaptable devices in various fields, the demand for flexible electronics and coatings utilizing phase transition materials like VO2 as a functional layer is rising. However, depositing high-quality crystalline VO2 films directly onto polymer substrates remains challenging due to the thermosensitivity of such materials being barely compatible with usual VO2 deposition methods. In this study, we present a simple method for directly growing VO2 on Kapton substrates. VOx films were first deposited onto the Kapton substrate at room temperature using pulsed laser deposition and subsequently annealed in an oxygen atmosphere at 390 °C. The electrical and optical properties of the VO2/Kapton samples with different film thicknesses were investigated. The 200 nm-thick vanadium dioxide films exhibit the best resistivity contrast with 3.1 orders of magnitude (OOM), while the 50 nm-thick films show the best transmittance change of 54 % at 2500 nm. The switching performances are comparable or even superior to those achieved previously using more complicated synthesis processes such as film transfer, introduction of buffer layers, and ink injection. Additionally, by coupling W elemental doping and thin film strain, we successfully tuned the VO2 transition temperature to room temperature with a dopant concentration of only 1.1 at.% while maintaining a high electrical contrast (2 OOM). The impressive electrical and optical transition performance of the VO2/Kapton samples, combined with their tunable transition temperature, makes them promising candidates for flexible electronic devices and thermochromic smart coatings.

Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films.

The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it isunambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low-spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it isinferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. Thestudy highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.

Study on the Fabrication and Characteristics of Tantalum-Nitride (TaN) Thin-Film Strain Gauges on the Silicon Substrate for Silicon-Based Heterogeneous Integration Application

Study on the Fabrication and Characteristics of Tantalum-Nitride (TaN) Thin-Film Strain Gauges on the Silicon Substrate for Silicon-Based Heterogeneous Integration Application

Piezoresistive failure behaviors and mechanisms of the ITO thin films at high temperatures

ITO is drawing enormous attention in fabrication of thin film strain gauges (TFSGs) due to its excellent oxidation resistance, high melting point and great piezoresistivity. However, the operating temperature of ITO TFSGs is far lower than its melting point. In this work, 8.9 μm ITO thin films and TFSGs were prepared using magnetron sputtering to reveal their piezoresistive failure behaviors and mechanisms at various high temperatures. Results show that surface bulges and significant thickness reduction of the ITO thin films appear at 1275 °C due to their repaid sublimation. The Hall mobility of the ITO thin films decreases sharply due to the formation of the surface bulges and significant thickness reduction at 1275 °C. The piezoresistive failure behaviors of the ITO TFSGs mainly result in their significant resistance drift and the decline of the gauge factor (GF) at 1275 °C. The drift rate of the ITO TFSG at 1275 °C is nearly 7 times higher than that at 1250 °C. The GF of the ITO TFSGs drops from 4.67 to 1.77 when the testing temperature rises from 1250 °C to 1275 °C. These can be attributed to the sharp decrease of the Hall mobility of the ITO thin films.

High-Tolerance Thin-Film Solder Joints for Electrical Interconnection in Harsh Environments

This paper presents a pioneering use of thin-film solder joint in the electrical interconnection of high-temperature thin-film sensors, which aims to improve the low tolerance of traditional soldered balls in extreme environments. The structure of the thin-film solder joint, composed of silver foil, solder joint paste, and metal wire, has a negligible mass compared to traditional soldered balls. As a result, the inertia force on the solder joint/substrate interface during vibration and force impact is greatly reduced, improving the reliability of the solder joint under thermal-mechanical coupling impacts. The thin-film solder joint has been verified using both metal (AgPd paste) and ceramic (TiB2/SiCNO paste) solders, and can withstand extreme environments such as thermal shock and high "g" loading. The designed thin-film solder joint can withstand impact energy of up to 3 J at 1000 °C without failure, which is 6 times higher than that of traditional soldered balls (with a statistical failure rate of up to 67% under 0.5 J impact). The thin-film solder joint was successfully used for electrical interconnection of high-temperature thin-film strain gauges, demonstrating excellent reliability under thermal-mechanical coupling impacts at 1000 °C. Our high-tolerance thin-film solder joints show promising potential for use in electrical interconnection in harsh environments.

Ultrafast high-temperature sintering of polymer-derived ceramic nanocomposites for high-temperature thin-film sensors

Polymer-derived ceramic nanocomposites (PDC-NCs) are promising materials for high-temperature thin-film sensor (TFS) applications. However, owing to their inherent high-resistance characteristics and the high-temperature furnace sintering process, it is difficult to integrate PDC-NCs with high-conductive and low high-temperature tolerant alloy/metal components. In this study, an ultrafast high-temperature sintering (UHS) method was employed to prepare PDC-NC TFSs in minutes. The TiB2/SiCNO film sintered by the UHS technique shows high-sensitive microstructure and properties to sintering parameters. The UHS technique allows rapid sintering toward the desired phase, controls the grain size and microstructure, promotes the precipitation of free carbon in the SiCNO matrix within minutes, and suppresses high-temperature oxygen pollution. For example, TiB2/SiCNO films show high conductivity (18.1 S cm−1 at room temperature) and high-temperature stability (up to 800 ℃). In comparison, these properties are orders of magnitude better than those of samples sintered in the furnace. High-quality films sintered using ultrafast heating and cooling rates can also withstand thermal shocks of up to 1000 ℃. With the UHS technique, high-quality and high-conductivity films can be prepared using an ultrafast sintering rate to prevent substrate thermal damage. The TiB2/SiCNO thin-film strain gauge with high sensitivity (GF = 8.6) and low thermal noise was fabricated on alloy substrates, and its high-temperature (25–800 ℃) strain sensing performance was verified. This method offers a new route for co-sintering devices with alloy/metal substrates that suffer from low high-temperature tolerance and regulating the microstructure and electrical performance of these devices.

A Bilayer Thin-Film Strain Gauge With Temperature Self-Compensation

A Karma–CuNi bilayer strain gauge with a low-temperature coefficient of resistance (TCR) is designed for temperature self-compensation. The impact of heat treatment and structural parameters on its performance is investigated. The TCR of the bilayer thin-film strain gauge can be adjusted to near 0 ppm/°C by controlling the thickness ratio and the annealing temperature. Besides, the Karma–CuNi thin films had a gauge factor (GF) of 2.2. Finally, the Karma–CuNi bilayer thin-film strain gauge was fabricated on the metal substrates and bone-like metal elastomer. An experimental setup based on bending beam theory was developed to determine the GF of the strain gauges fabricated on metal substrate. It shows promising applications for in situ strain or torque measurement of some metal component when high sensitivity and high spatial resolution is required.

A thin film force sensor on AISI 5140 steel surface with multilayer SI3N4/AL2O3 film as insulation structure

To develop a thin film force sensor on AISI 5140 steel surface, alternating stack thin films of silicon nitride and alumina were sputtered on the 5140 steel surface as the insulating structure before NiCr thin film electrodes were deposited. The insulation was investigated by measuring the resistance between different electrodes. The sample cross-section was examined and a nano-scratch test was carried out. Finally, a Ni80Cr20 thin film strain gauge was formed on the surface of a well-insulated sample with a hard masque, and its force (0 ∼ 250 kN and 0 ∼ 500 N) and temperature (25 °C ∼ 200 °C) responses were investigated. The results show that the alternating four-layer Si3N4/Al2O3 thin film structure can provide insulation close to 1.2 GΩ with a thickness of 2360 nm. The insulation layers have fine adhesion to 5140 steel, and there is no significant inter-layer diffusion which indicates the integrity of the multi-layer structure. The resistance strain coefficient of the sensor is 1.57 ∼ 1.67, and the resistance temperature coefficient is 190.3 ppm/ °C.