Thin Film Strain Gauges Research Articles

Development of the invar36 thin film strain gauge sensor for strain measurement

This paper presents development of invar36 thin film strain gauges of various thicknesses ranging from 100 to 1400 Å for strain measurement. The strain gauges are deposited on microslides using the dc magnetron sputtering technique. Resistivity, temperature coefficient of resistance (TCR) and gauge factors of all gauges are measured and compared with each other. TCR is estimated by systematic annealing of gauges in vacuum and found as low as 190 ppm °C−1. A four-point bending setup is designed and fabricated to measure the gauge factors of all gauges. The gauge factor of relatively thinner strain gauge is found as high as 4.5 and for strain gauges with thickness greater than 500 Å gauge factor is found less than 2.5. The variations of gauge resistance with applied strain are studied in terms of linearity, hysteresis and repeatability. The developed strain gauges are connected in a full-bridge configuration and the output response to the applied strain is studied at different excitation voltages.

Underwater current leakage between encapsulated NiChrome tracks: Implications for strain-gauges and other implantable devices

We present the results of experiments aimed at identifying a suitable polymer for the encapsulation of thin-film strain gauges for underwater applications (with a view of using it in an instrumented bone fusion nail). The leakage currents across grooves cut (using a laser) in thin films of NiChrome over borosilicate glass were studied for encapsulated samples, immersed in water at 37°C. The selected encapsulants were five silicone rubbers (of both medical and engineering grades), produced by Nusil (MED-6015, MED4-4220, MED3-4013, CV14-2500 and EPM-2420) and Elast-Eon™2A, a co-polymer developed by Aortech. The effect of a primer, as well as that of a black dye mixed with the rubber, was also investigated.13% of samples exhibited slow current increases, peaking at 1–4nA, and 9% exhibited brief peaks up to 30nA (only one sample exhibited both). These were due to corrosion of the NiCr following ionic contamination. For the remaining 80%, the leakage current remained remarkably low (<1nA).Silicone rubber encapsulation appears as a realistic low-cost alternative to the hermetic packaging of thin-film strain-gauges. However, this is conditional to achieving a suitable degree of cleanliness of all surfaces prior to encapsulation. Cleaning and rinsing procedures should therefore be evaluated before opting for this method.

Large-Scale Sensing System Combining Large-Area Electronics and CMOS ICs for Structural-Health Monitoring

Early-stage damage detection for bridges requires continuously sensing strain over large portions of the structure, yet with centimeter-scale resolution. To achieve sensing on such a scale, this work presents a sensing sheet that combines CMOS ICs, for sensor control and readout, with large-area electronics (LAE), for many-channel distributed sensing and data aggregation. Bonded to a structure, the sheet thus enables strain sensing scalable to high spatial resolutions. In order to combine the two technologies in a correspondingly scalable manner, non-contact interfaces are used. Inductive and capacitive antennas are patterned on the LAE sheet and on the IC packages, so that system assembly is achieved via low-cost sheet lamination without metallurgical bonds. The LAE sheet integrates thin-film strain gauges, thin-film transistors, and long interconnects on a 50-μm-thick polyimide sheet, and the CMOS ICs integrate subsystems for sensor readout, control, and communication over the distributed sheet in a 130 nm process. Multi-channel strain readout is achieved with sensitivity of 18 μStrain RMS at a readout energy of 270 nJ/measurement, while the communication energy is 12.8 pJ/3.3 pJ per bit (Tx/Rx) over a distance of 7.5 m.

Fabrication and characterization of strain gauge integrated polymeric diaphragm pressure sensors

We demonstrated a simple fabrication of polymeric diaphragm pressure sensors by embedding a commercial thin-film strain gauge in a polymeric diaphragm. Two different polymer materials, polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET), were employed for fabrication. Performance of pressure sensors fabricated with two different polymers including sensitivity, linearity, and long-term stability was thoroughly compared using a characterization setup operating in either compressed air or water. Pressure sensitivities were similar for air and water environments regardless of the polymer material. However, the sensitivity and resolution of PET pressure sensors were about 2.5 times better than those of PDMS pressure sensors. Moreover, significant degradation of PDMS pressure sensors was observed over time while PET pressure sensors exhibited good stability in water. Overall, PET is better suited for making liquid pressure sensors than PDMS.

NiCr Thin Film Strain Gauges Fabricated on Glass Substrates

Abstract In order to investigate the strain gauge characteristics of NiCr thin films, 500 nm NiCr (80 wt.-% and 20 wt.-%, respectively) thin films were deposited on glass substrates by DC magnetron sputtering. After deposition, NiCr thin films were characterized by using X-Ray diffraction analysis, scanning electron microscope and four-point probe techniques inview of crystallization, phases, film structure and electrical resistivity. After characterization, NiCr thin films were shaped into strain gauges by photo lithography and wet etching techniques. Strain gauges were tested with different loads, and strain values were calculated by comparing the results with commercial NiCr strain gauges with the same surface area. Resistivity change vs. strain was plotted, and the gauge factor of fabricated thin film strain gauges was evaluated as 1.23.

Cr-N Strain-Gauge-Type Precision Displacement Sensor for Measuring Positions of Micro Stage

A strain-gauge-type precision displacement sensor, which is developed for a usage of micro-XY stage, is described in this paper. A thin-film strain-gauge element, which is made by Cr-N alloy, is directly fabricated on the base of the strain gauge. The direct fabrication and using the Cr-N element are expected to achieve higher sensitivity for displacement detection and better stability against the change of ambient temperature. In this study, several designs of the thin-film strain gauge, including both of two-gauge-type and four-gauge-type, are prepared to compare sensor performances such as sensitivity, stability and so on. The designed patterns of the strain-gauge element are directly fabricated on zirconia plates by using photolithography processes. The fabricated strain gauges are then evaluated as precision displacement sensors. At first, stability of the fabricated Cr-N strain-gauge-type displacement sensor was confirmed by comparing with the one made by a conventional strain gauge. Resolution of the fabricated Cr-N strain-gauge-type displacement sensors was then evaluated by comparing with a commercially-available laser displacement sensor, while giving sub-micrometer-order deformation to the strain-gauge-type displacement sensor. Details of the design, fabrication and evaluation results of the Cr-N strain-gauge-type displacement sensor are described.

A Novel Class of Strain Gauges Based on Layered Percolative Films of 2D Materials

Here we report on the fabrication and characterization of a novel type of strain gauge based on percolative networks of 2D materials. The high sensitivity of the percolative carrier transport to strain induced morphology changes was exploited in strain sensors that can be produced from a wide variety of materials. Highly reliable and sensitive graphene-based thin film strain gauges were produced from solution processed graphene flakes by spray deposition. Control of the gauge sensitivity could be exerted through deposition-induced changes to the film morphology. This exceptional property was explained through modeling of the strain induced changes to the flake-flake overlap for different percolation networks. The ability to directly deposit strain gauges on complex-shaped and transparent surfaces was presented. The demonstrated scalable fabrication, superior sensitivity over conventional sensors, and unique properties of the described strain gauges have the potential to improve existing technology and open up new fields of applications for strain sensors.

LTCC Substrates for High Performance Strain Gauges

Recent advances in the development of high gauge factor thin-films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion and a low modulus of elasticity for the application as support material for thin-film sensors. Constantan foil strain gauges were fabricated from this material by tape casting, pressure-assisted sintering and subsequent lamination of the metal foil on the planar ceramic substrates. The sensors were mounted on a strain gauge beam arrangement and load curves and creep behavior were evaluated. The accuracy of the assembled load cells correspond to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the LTCC substrates for fabrication of accurate strain gauges. To facilitate the deposition of thin film sensor structures onto the LTCC substrates, the pressure-assisted sintering technology has been refined. By the use of smooth setters instead of release tapes substrates with minimal surface roughness were fabricated. Metallic thin films deposited on these substrates exhibit low surface resistances comparable to thin films on commercial alumina thin-film substrates. The presented advances in material design and manufacturing technology are important to promote the development of high performance thin-film strain gauges.

Strain microgauge implementation on cylindrical metal substrates

This work mainly focuses on the development of germanium-based thin-film piezoresistive strain gauges on thin NiTi and stainless steel cylinders. These microgauges enable the real-time measurement of the cylinder's strain distribution from which its deflection status can be deduced, based on an analytical mechanical model of thin cantilever beams. The adaptation of thin cylinders to typical micromachining equipment requires several adjustments in preparation works. The fabrication process of germanium microgauges also has to be adjusted according to the particularities of metal substrates. A number of polycrystalline germanium strain microgauges of various dimensions are implemented on thin cylinders. The results of the fabrication process are presented. The existing constraints of the current process and the potential improvement measures are then discussed.

Automated micro-transfer printing with cantilevered stamps

This paper demonstrates the use of a flexible instrumented stamp to enable automated micro transfer printing as a route to large-area, deterministic assembly of microstructured device components or ‘inks’. The ability to instrument the stamp, a critical component for retrieval and placement of a micro device, to detect contact and monitor localized forces during critical events in the printing process not only allows for the development of a robust manufacturing process, but also for a unique vantage point from which to study fundamental issues and phenomena associated with adhesion and delamination of thin films from a variety of substrate materials. This paper presents basic design analysis on the requirements of the cantilevers for compatibility with a typical transfer printing environment. Off-the-shelf thin film strain gages are integrated with a thin elastomeric post as a preliminary prototype and the feasibility of transfer printing with it is demonstrated. Further, the set-up is calibrated to produce force signals for event detection and in situ diagnosis of the process.

Nanocomposite Thin Film Strain Gauges for Use in Harsh Environments

not Available.

Design and fabrication of a flexible three-axial tactile sensor array based on polyimide micromachining

This paper describes the design and fabrication of a flexible three-axial tactile sensor array using advanced polyimide micromachining technologies. The tactile sensor array is comprised of sixteen micro force sensors and it measures 13 mm × 18 mm. Each micro force sensor has a square membrane and four strain gauges, and its force capacity is 0.6 N in the three-axial directions. The optimal positions of the strain gauges are determined by the strain distribution obtained form finite element analysis (FEA). The normal and shear forces are detected by combining responses from four thin-film metal strain gauges embedded in a polyimide membrane. In order to acquire force signals from individual micro force sensors, we fabricated a PCB based on a multiplexer, operational amplifier and microprocessor with CAN network function. The sensor array is tested from the evaluation system with a three-component load cell. The developed sensor array can be applied in robots’ fingertips, as well as to other electronic applications with three-axial force measurement and flexibility keyword requirements.

Design and Characterization of Thin-Film System for Microsensors Embedding in Ti6Al4V Alloys

Reliable structures and systems are critical for numerous engineering applications. These systems are generally operated in harsh environments. Effective monitoring and diagnosis of these structures are of critical importance. To achieve this goal, thin-film micro/nano sensors can be embedded in metal structures to protect them from external harsh environments while providing measurements with desired special and temporal resolutions and accuracy, which cannot be achieved with traditional sensors. Targeting on new sensor applications in harsh environments, this paper presents experimental results on the design and characterization of a thin-film system, including diffusion barrier and insulation layers, for a successful sensor embedding in Ti6Al4V alloy through diffusion bonding. Based on the developed material system, an effective fabrication process of the Ti6Al4V-embedded thin-film PdCr strain gages was developed. The embedded sensors were validated by a functionality test and material characterization.

Effect of nitrogen doping on piezoresistive properties of a-Si x C y thin film strain gauges

In this study, the effect of nitrogen doping on piezoresistive properties of non-stoichiometric amorphous silicon carbide (a-Si x C y ) thin-film strain gauges was investigated. The films were prepared by plasma-enhanced chemical vapor deposition (PECVD) and the strain gauges were patterned by fabrication processes like conventional photolithography, metallization and reactive ion etching (RIE). The structure of the strain gauges consists of a a-Si x C y thin-film resistor with one Ti/Au electrical contact at each extreme of the resistor. In order to determine the piezoresistive properties, each strain gauge type was bonded near the clamped edge of a stainless steel cantilever beam and on the free edge calibrated weights were applied. The influence of the temperature on piezoresistive properties also was evaluated through the temperature coefficient of resistance (TCR) measurements from room temperature up to 250oC. It was observed that nitrogen doping increased the piezoresistive coefficient and TCR of a-Si x C y thin film strain gauges.

Sputter Deposition of Strain Gauges Using ITO/Ag

AbstractIn many industrial applications strain gauges are commonly used for measurements of applied forces or the loading status of work pieces. While commercial strain gauges using polyimide foils can cause errors due to influence of humidity, directly applied thin film strain gauges avoid this problems. Besides the improvement of the signal due to avoiding glue and polymer substrate, that can creep and swell, also the gauge factor can be further improved by using new sensor materials. Indium tin oxide (ITO) is a very promising material especially for high temperature application. This paper presents investigations of Ag doped ITO films. The sheet resistance of the films increased first with increase in the silver content up to 20 at.‐%. Further increase in silver led to a decrease in the sheet resistance reaching less than 10 Ω for more than 75 at.‐% silver content of the resulting films. The gauge factor showed a maximum of 6.5 at 20 at.‐% Ag content, while the temperature coefficient of resistance (TCR) showed zero crossing for approximately 40 at.‐% Ag content.

Development of Metal Embedded Microsensors by Diffusion Bonding and Testing in Milling Process

This paper presents a new method to embed microthin film sensors into metallic structures by diffusion bonding. The experiments were carried out using AISI 304 stainless steel substrates with a bonding temperature of 800°C and a pressure of 4 MPa, which the developed thin film system was able to sustain. The success of embedding was validated by sensor functionality tests and metallurgical characterization. This sensor embedding method can be extended to other engineering metallic materials. To demonstrate the applications of the embedded microsensors, a PdCr thin film strain gauge array that is suitable for in situ measuring of temperatures and strains in manufacturing processes was designed and fabricated for testing in the vertical end milling process. Time- and spatial-resolved signals were obtained during the milling process. The signals were decomposed to a static part, which apparently resulted from temperature changes, and the dynamic part, which resulted from dynamic strains induced by material removal during cutting. The milling tests demonstrated the ability of using this method to measure real-time temperatures and strains within the workpiece, which will be very valuable to the fundamental understanding of various manufacturing processes. It can also be used for bench marking numerical modeling and simulations.

Electromechanical properties of nichrome (80/20 wt-%) thin film fabricated by low energy ion sputtering

The electromechanical properties of nichrome (Ni–Cr 80/20 wt-%) used as a common material for application in thin film strain gauges have been studied. The composition and oxidation of Ni–Cr film fabricated by low energy ion beam sputtering has been studied by Auger electron spectroscopy. The microstructure of the film was determined by a TEM. The temperature coefficient of resistance has been determined by a Nanovolt/Microohm meter. The gauge factor has been determined by the cantilever method. Low stable temperature coefficient of resistance values of resistance stability and the desired sheet resistance have been obtained. It meets the main demanded properties for its application to strain sensitive film.

Embedding of micro thin film strain sensors in sapphire by diffusion bonding

Advanced ceramics have been increasingly used for various manufacturing processes. The current sensors used in ceramic tools are difficult to reliably provide thermomechanical measurements in or near the ceramic tool–workpiece interface. Thin film micro sensors could be embedded—thus avoiding direct contact with workpieces—at critical locations without interfering with normal manufacturing operation of the ceramic tool. However, little research has been conducted in embedding of thin film sensors in ceramic materials. To initiate the study on ceramic embedded thin film sensors, palladium-13 wt% chromium (PdCr) material was used to fabricate thin film strain gauges (TFSGs) on sapphire substrates and a diffusion bonding process was used to embed the TFSGs into sapphire. The interface between the sapphire and embedded sensors was characterized by high-resolution TEM (HRTEM). The results clearly indicated a good chemical bonding between the host sapphire and embedded thin films with no inter-diffusion. Moreover, TEM images showed there were local stresses at the interface due to the chemical bonding. The functionality of the embedded micro TFSGs into sapphire was successfully characterized. This study clearly demonstrates the feasibility of fabricating and embedding micro thin film sensors for potential use in ceramic tools for various applications.

STRAIN SENSITIVITY AND TEMPERATURE INFLUENCE OF NICHROME (80/20 wt.%) THIN FILM FABRICATED BY MAGNETRON SPUTTERING

The electromechanical properties of nichrome ( Ni – Cr 80/20 wt.%) used as a common material for application in thin film strain gauges have been studied. The surface topography and chemical composition of Ni – Cr thin films grown on the glass substrate by magnetron sputtering have been analyzed by atomic force microscope (AFM) and energy dispersive spectroscopy (EDS), respectively. The temperature coefficient of resistance (TCR) has been determined by a Nano-volt/Micro ohm meter. The gauge factor (FG) has been determined by the cantilever method. Low stable TCR values (22 ppm to 46 ppm in the 50–150°C temperature range) have been obtained. Resistance stability is achieved by rapid thermal annealing (RTA) at 300°C for 10 min combined with a 24 h thermal annealing (TA) at 150°C. The desired 45 Ω/m sheet resistance and a gauge factor of 2.6 have been attained for 40-nm-thickness films. The films have very small roughness of 2.1~4.4 nm.

Strain sensitivity enhancement of thin metal film strain gauges on polymer microscale structures

Arrays of identical polyimide cantilever beams have been fabricated with nickel-chrome strain gauges covering an increasing portion of the beam. In contrast to accepted beam models, testing showed that gauge factor increased 136% for polyimide beams as the strain gauge extended over a larger length of the beam. This effect is due to the high modulus of the thin film strain gauges relative to the polymer beams, a hypothesis supported by duplicate experiments with silicon nitride beams that showed a 65% reduction in gauge factor. Comparison with an analytical model and finite element analysis is also made, giving good agreement.