Thick Film Materials Research Articles

Dielectric Properties of UV Cured Thick Film Polymer Networks through High Power Xenon Flash Lamp Curing

ABSTRACTHigh-energy flash cure lamps process thick film materials (<10 um) over large areas (<100 cm2) within milliseconds and are capable to deliver higher energy and power densities (20 J/cm2 and 20 kW/cm2) allowing for a more complete curing and elimination of flaws that would exist in conventional treatment. Click reactions are especially attractive for patterned devices as they have minimal shape change during curing and have a more predictable structure compared to free radical acrylate polymerization. Pentaerythritol tetrakis(3-mercaptopropionate) and 2,4,6-Triallyloxy-1,3,5-triazine were combined at 3:4 by weight and then spin coated on copper foil substrates. The solutions were processed both thermally and with exposure to a xenon flash bulb. Thermal treatment consisted of heating the sample at 80°C on a hot plate over night. Flash curing was accomplished using a Novacentrix Pulseforge 1300 system. The flash lamp curing fluence and intensities were varied to determine their effects on degree of cross-linking, dielectric constant, breakdown field and energy storage. The degree of cross-linking was determined through comparative FTIR studies. Dielectric constant was measured using an Agilent 4294a impedance analyzer from 100 Hz-100 MHz with a two terminal setup. Breakdown strength and energy density measurements were taken using Radiant Technology's Precision Ferroelectric tester with a 10 kV source. The printed films averaged 1-3 microns thick as observed by an SEM cross section measurement. It was found that dielectric constant varies with both treatment intensity and fluence. Energy densities were calculated using the ideal capacitor equation and ranged from 1.5-4.8 J/cm3.

Thick-film Load-sensing bridges – Effect of Temperature and Mechanical Boundary Conditions

Although inorganic thick-film materials are usually very stable, load sensors (i.e. force, torque and pressure) based on the piezoresistive effect of thick-film resistors often exhibit some drift, with major contributions stemming from a) temperature changes or gradients, coupled with the temperature coefficient of resistance (TCR) of the sensing bridge resistors, and b) ill- defined mechanical boundary conditions, due to insufficient decoupling, plastic deformation used materials, friction or loading errors. These problems also potentially affect test samples for measuring piezoresistive response, a.k.a. the gauge factors. Here, we examine and discuss effects on the output signal of simple cantilever force sensors of two disturbances: parasitic loads and temperature gradients, and discuss / demonstrate mitigation options, and a system to reliably measure gauge factor.

Planar platform for temperature dependent four-wire impedance spectroscopy—A novel tool to characterize functional materials

Electroceramic functional materials often need to be characterized in a sensor-like setup close to application. We present a novel sensor transducer that allows four-wire impedance spectroscopy of planar thick-film materials. Fast and easy temperature adjustment is possible by an integrated heater element. The tool can be manufactured either with Au or with Pt electrodes. The measurement of the transducers idle capacitance and calibration measurements with water covering allowed us to define a geometry factor, which is useful to determine specific materials parameters directly from the four-wire measurements. Characterization of three different kinds of materials (an n-type semiconductor, an oxygen ion-conductor and a sodium ion-conductor material) demonstrates the applicability of planar four-wire impedance spectroscopy as a powerful tool to investigate functional materials close to application.

Long-term mechanical reliability of ceramic thick-film circuits and mechanical sensors under static load

In this work, we study the long-term mechanical strength under static load – i.e. static fatigue performance or resistance to subcritical crack growth – of ceramic thick-film circuits, namely how the static fatigue of the substrate is affected by the presence of thick-film compositions on the surface subjected to tensile stress. Three substrate materials were compared: standard 96% alumina (Al2O3) and two grades of high-strength zirconia-toughened alumina (ZTA). The tested thick-film compositions included Ag- and Au-based conductors, a multilayer dielectric, resistors, and an overglaze, alone or in combination. The tests were carried out by applying a nominally constant load on cantilevers, at room temperature and in nominally 100% humidity, with stress data extracted according to log-normal and Weibull statistics. In the blank state, both ZTA grades exhibit higher short-term strength than 96% Al2O3, as well as much higher resistance to static fatigue. However, many thick-film compositions are found to degrade the static fatigue performance, with higher-strength ZTA being in general more affected. This implies that thick-film materials used in circuits under high mechanical stress, such as force and pressure sensors and devices operating in harsh environments, must be carefully chosen and placed in order to ensure reliable long-term operation.

Comparison of two experimental methods for the mechanical characterization of thin or thick films from the study of micromachined circular diaphragms

The purpose of this study was to compare two experimental methods and evaluate the effectiveness of a set of analytical models in order to measure the initial stress and the Young's modulus value of thin and thick film materials. Two types of experiments were performed on micromachined circular diaphragms: bulge testing and vibrometry. The range of validity and accuracy of the analytical models with respect to the vibration of the diaphragms was discussed from the finite element simulations. It was shown that the a/t ratio should be considered carefully to determine the value of the Young's modulus by vibrometry with an acceptable error. A relative error of approximately ±10% on E was obtained for a/t ≤ 750. For 750 ≤ a/t ≤ 1000, the value of the dimensionless parameter k must also be considered. It has been shown that the residual stress value can be obtained with an accuracy of 10% or less, given that k > 12. As an illustration, experimental methods and models were applied to the characterization of a thick electroplated gold film and a sputter-deposited Inconel thin film. Circular structures were defined by vertical sidewalls etched on the back of a Si wafer using the deep reactive ion etching technique. In addition to analytical models, parametric finite element simulations and a design optimization technique were used to determine the material's mechanical properties. The static deflections of the diaphragms were measured as a function of the applied pressure. The resonant frequencies and mode shapes of the vibrating structures were observed under vacuum by white-light interferometric microscopy. For gold, it was found that E = (53 ± 20) GPa and σ(0) = (180 ± 10) MPa. For Inconel, it was found that E = (157 ± 14) GPa and σ(0) = (172 ± 5) MPa.

New conductive thick-film paste based on silver nanopowder for high power and high temperature applications

A new thick-film material for screen-printing technology, based on nanoscale silver powders with the particle size distribution 5–55 nm is presented. Silver nanopowder used for paste preparation was elaborated by the authors. The compatibility of investigated paste was proven with alumina, silicon, Kapton foil and glass. The main advantage of this paste is sinterability at much lower temperatures (around 300 °C) compared to pastes obtained from micro-powders (650–850 °C). The thicknesses of obtained layers are 2–3 μm. The elaborated layers are dense and well sintered, exhibit good adhesion to all above mentioned substrates and low resistivity as well as very good resistance to high power and elevated temperatures. The results of loading the layers deposited on alumina substrates with high current and exposed to high temperature are presented as well.

Low-firing thick-film piezoresistive sensors for medical instruments

In this work, a low-firing thick-film materials system allowing fabrication of piezoresistive sensors on surgical alloys is presented in detail, with application to a force-sensing surgical instrument. The system comprises a series of individual thick-film dielectric, conductor, resistive and overglaze compositions based on a lead borosilicate glass matrix. The moderate achieved firing temperature, around 625°C, greatly increases compatibility with metallic substrates, allowing the use of high-strength medical alloys with low thermal degradation. Specific fillers for the dielectric layers increase adhesion on steel substrates and allow thermal matching to austenitic and ferritic/martensitic steels, as well as titanium alloys, and preliminary work demonstrating reactive stabilisation of the dielectric layer to achieve an even wider process window is also shown. Finally, the functionality of this materials system is successfully demonstrated here by implementing it into a previously developed ligament-balancing force sensor for total knee arthroplasty (TKA) [1].

Microelectronic materials and structures characterization by impedance spectroscopy

The impedance spectroscopy relies on the measurement of a linear electrical response of an investigated material to a low electromagnetic signal in a wide range of frequency. Further analysis of this response is applied to reveal some information about physicochemical properties of the material. Impedance spectroscopy is a useful, non-destructive tool for the analysis of many material properties. It is also commonly applied for the determination of the electrical parameters of complex electronic and microelectronic structures. This kind of knowledge enables better understanding of the mechanisms of operation of the microelectronic devices under investigation. Fundamental issues are discussed, including the proper interpretation of the experimental results and the construction of the equivalent circuits, which shall consist of the elements representing the impact of the particular mechanisms of electrical conductivity and polarization on the electrical parameters eventually observed in macroscale. The considerations are illustrated by results of the author’s research on thin and thick film structures and materials. Some examples of the impedances spectra analysis and the construction of the equivalent models are presented for various structures, including the, metal–SiO 2–GaAs, metal–TiVPd–Si and thick film humidity and gas sensors.

How Silver Powder Metallurgy Affects the Physical Properties of Low Temperature Firing Silver Conductor

With the implementation of RoHS (the Restriction of Hazardous Substance) Directive banning the use of Lead, Cadmium, Mercury and Hexavalent Chromium, hybrid microelectronic manufacturers are globally embracing the lead free movement. These manufacturers must not only understand the implications of their material choice but must be aware of the interaction between lead free solder alloys and their RoHS compliant thick film materials. It is commonly known that lead free solder alloys process at much higher reflow temperatures than lead containing solder which can directly impact the fired film leach resistance and the loss of adhesion. There are also other concerns; lead free solders alloys generally require a different organic flux system to promote wetting and reflow, but this change may cause a fired film conductor to leach more easily than the flux used in the lead containing solders. The use of lead free solders such as SAC305, SAC405 or 95/5 on a low firing (550–570 °C) pure silver conductor has the tendency of leaching the fired film more readily than conductors containing small amounts of palladium or platinum. Many of these situations provide new challenges for the hybrid circuit manufacturer. There is little information available regarding the effects of the lead free solders on low firing silver thick film conductors. This paper discusses the results of a newly developed Pb and Cd free silver thick film conductor paste with a modified silver powder metallurgy to improve the leach resistance, solder acceptance and adhesion using lead free solder. In addition, the pure silver conductor was fired on top of a low temperature dielectric paste. This conductor was evaluated by comparing lead free solder alloys to traditional tin-lead-silver solder alloys. This study included evaluations based on SEM photos, solderability, leach resistance, and initial and long term adhesions. Results are published describing the difference in behavior between the different solder alloys in conjunction with the different silver powder metallurgy.

RF Sensitivity of Multi Chip Package Development by Using Embedded Resistor in Package Substrate

Recently, mobile age is blooming and makes change of 21st century. Especially multi chip package – MCP- is located center position of this flash era. Moreover, radio frequency –RF- sensitivity determines the product quality and life, therefore we try to develop the RF sensitivity of MCP in wireless circumstance. In this study, MCP comprised NAND Flash memory, Mobile DRAM, and a substrate, we focused on the substrate, changed substrate design and performance by using resistor embedded into 1 layer of substrate. The purpose of adaption of embedded resistor is in order to control the slew rate of signal channel of DRAM. NiCr thin film was used for the resistor in substrate. NiCr material presents superior temperature coefficient of resistance –TCR- to thick film material. NiCr coated on Cu foil and this foil laminated by conventional substrate manufacturing method. Embedded resistor applied on signal line as 3 kinds; 1. Signal and signal strobe line by 50 Ohm, 2.Signal strobe line by 50 Ohm, and 3. Signal and signal strobe line by 20 Ohm. In case of 3, result showed 0.5 dBm increment of RF sensitivity in full strength mode, also we got the same performance at half strength mode. Embedded resistor formed substrate did not affect the reliability of MCP, from the results of reliability test; pre-conditioning test, thermo-cyclic, temperature humidity, HAST, un-Bias HAST, and hot temperature storage test. This study showed signal waveform of DRAM is significantly effective in RF sensitivity for MCP. Passive component, resistor, made change of slew rate and revealed increase of sensitivity level finally.

Long-Term Mechanical Reliability of Ceramic Thick-Film Circuits and Mechanical Sensors Under Static Load

In this work, we study the long-term static fatigue (constant load, room temperature, ∼100% humidity) performance of ceramic thick-film circuits, as a function of substrate and thick-film compositions, comparing standard 96% alumina with high-strength zirconia-toughened alumina (ZTA). Blank ZTA exhibits higher short-term strength and resistance to static fatigue than alumina. However, many thick-film compositions degrade the static fatigue behaviour, with ZTA being in general more affected. This implies that choice of thick-film materials is important for mechanical reliability.

Selectivity Modification of SnO2‐Based Materials for Gas Sensor Arrays

AbstractAn enhancement of selectivity of sensor materials, based on nanocrystalline SnO2 is reported. Selectivity toward target gases such as CO, NO2, NH3, H2S and acetone vapor, could be achieved by selection of catalytic cluster distributed over the surface of thick film material. Presented results allow us to propose application of obtained materials in “electronic nose” sensor systems.

Analysis of Adhesion and Fracture Energy of Nano-Particle Silver in Electronics Packaging Applications

Nano-particle silver (NPS) conductors are increasingly being investigated for package level electronics applications. Unlike traditional thick film materials and conductive inks, nano-particle conductors often do not incorporate compounds to promote interfacial adhesion such as binders used in thick films and polymer adhesives used in conductive inks as these adhesion promoters can degrade the electrical performance. The NPS is concerned with low adhesion to most of processed polymer surface such as liquid crystal polymer (LCP), polyimide, and benzocyclobutene (BCB). Moreover, the adhesion mechanism of NPS has not been identified yet. Thus, as a first step to identify NPS adhesion mechanism and thus, to improve NPS adhesion, quantitative measurement of the adhesion strength of NPS is necessary. Since conventional adhesion test methods are not directly applicable to thin (~ 2 ¿m) NPS film adhesion test, a new adhesion test method is developed in this paper to estimate the adhesion strength of NPS films. The newly developed adhesion test method is called modified button shear test (MBST) because it modifies the conventional button shear test and integrates the generally known die shear test. The MBST is used for measuring not only interfacial bond strength, but also interfacial fracture energy. The interfacial bond strength in tension and the interfacial fracture energy of NPS with LCP substrate measured by MBST are 24.4 MPa and 17.2 J/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. The MBST is generic in nature and can be extended to other thin films adhesion test for measuring interfacial bond strength and interfacial fracture energy.

Thick-film multi-DOF force/torque sensor for wrist rehabilitation

A complete six degree of freedom (6 DOF) force/torque sensor has been designed and fabricated, adapted to wrist rehabilitation applications, with the focus laid on simple, straightforward manufacturing processes. This paper details the mechanical design, 3D modeling, manufacture and characterisation of the sensor. Compared to previous work [1], this design has the advantage of simple, fully planar machining, and the load-sensing elements all lie on the same plane, making the device compatible with single-film deposition or a foil bonding process. The sensor was machined from steel, and the piezoresistive load-sensing bridges were deposited using thick-film technology. We used commercial thick-film materials in the work described here. A new lead-free materials system compatible with low processing temperatures (<700 °C) will shortly replace the commercial one and is expected to eventually also be adaptable to aluminium substrates. Such a sensor could find applications in multi-DOF therapeutic robotic systems as well as in isometric motor training.

High-performance piezoelectric thick film based energy harvesting micro-generators for MEMS

Energy harvesting, known also as energy scavenging, covers a great body of technologies and devices that transform low grade energy sources such as solar energy, environmental vibrations, thermal energy, human motion into usable electrical energy. In this paper vibrations are used as energy source and are transformed by the energy harvesting micro-generator into usable electrical signal. The micro-generator comprises a silicon cantilever with integrated InSensor® TF2100 PZT thick film deposited using screen-printing. The output power versus frequency and electrical load has been investigated. Furthermore, devices based on modified, pressure treated thick film materials have been tested and compared with the commercial InSensor® TF2100 PZT thick films. It has been found that the structures based on the pressure treated materials exhibit superior properties in terms of energy output.

Low-firing thick-film piezoresistive sensors for medical instruments

In this work, a low-firing thick-film materials system allowing fabrication of piezoresistive sensors on surgical alloys is presented in detail, with application to a force-sensing surgical instrument. The system comprises a series of individual thick-film dielectric, conductor, resistive and overglaze compositions based on a lead borosilicate glass matrix. The moderate achieved firing temperature, around 625∘C, greatly increases compatibility with metallic substrates, allowing the use of high-strength medical alloys with low thermal degradation. Specific fillers for the dielectric layers increase adhesion on steel substrates and allow thermal matching to austenitic and ferritic/martensitic steels, as well as titanium alloys. The functionality of this materials system is successfully demonstrated here by implementing it into a previously developed ligament-balancing force sensor for total knee arthroplasty (TKA).

‘Lead Free’ thick film thermistors: a study of variation in glass frit concentration and organics composition

There is a need to develop environment friendly electronic materials due to worldwide constraint for the use of hazardous chemicals such as lead, cadmium which are the main constituents of the electronic components. Considering such need, the work on lead free thick film thermistors has been initiated. Thick film materials have been proved to possess economical processing and functional advantage over other technologies and quick turnaround production of hybrid microcircuits. Thick film thermistors are different from the conventional thermistors (bulk) not only by the preparation process but also in composition, transport properties etc. In this paper, we have specifically focused on the influence of glass frit content and organic composition on the properties of lead free spinel based NTC thermistors. We noted that the glass frit concentration is responsible for the change in physical as well as electrical properties of the thick film thermistor. However, the type of organic vehicle (i.e., composition) did not show any adverse effect on microstructure and electrical properties of the thermistor.

Laser transfer processing for the integration of thin and thick film ferroelectrics

Laser transfer processing (LTP) offers the potential to overcome the problems of integrating ferroelectric thin and thick film materials with polymers and other technologically useful substrate materials that cannot sustain the high process temperatures, 600–1,000 °C, required for normal film deposition. The LTP technique involves the fabrication of a ceramic film on a high-temperature substrate material such as sapphire, and subsequent release by application of pulsed ultra-violet laser radiation. Here, the LTP technique is reviewed in the context of ferroelectric thin and thick films, and current developments are presented. Micro- and nanostructural features of the films before and after transfer to a second substrate are revealed using scanning and transmission electron microscopy. The consequences of laser-generated structural changes on ferroelectric properties are illustrated, and measures to mitigate the effects of an amorphous damage-layer are discussed.

Thick-film multi-DOF force / torque sensor for wrist rehabilitation

A complete six–degree-of-freedom (6-DOF) force / torque sensor has been designed and fabricated for wrist rehabilitation applications, with the focus laid on simple, straightforward manufacturing processes. This paper details the mechanical design, 3D-modeling, manufacture and characterisation of the sensor.Compared to previous work, this design has the advantage of simple, fully planar machining, and the load-sensing elements all lie on the same plane, making the device compatible with single-film deposition or a foil bonding process. The sensor was machined from steel, and the piezoresistive load-sensing bridges were deposited using thick-film technology. We used commercial thickfilm materials in a first time. A new lead-free materials system compatible with low processing temperatures (<700°C) will shortly replace the commercial one and is expected to eventually also be adaptable to aluminium substrates.

On the processing conditions and interfacial chemistry of composite PZT thick films on platinised silicon substrates

Planar piezoelectric actuators have been fabricated in a range of complex geometries with PZT thick films on platinised silicon substrates. Spiral actuators have been fabricated where the spiral shape allows long beams to be contained compactly in order to aid deflection. The stabilisation of PZT thick films derived from composite slurries derived from powder sol dispersions has been achieved with a YSZ barrier layer. Interfacial formation of Pt x Pb intermetallic and PbSiO 3 at the substrate–electrode interface occurs at low and high oxygen partial pressures respectively. Densification of the thick film material, assisted by infiltration of sol, lowers interfacial pO 2 through inhibiting oxygen diffusion. Under extreme circumstances, re-oxidation of the intermetallic is not possible and it remains even after sintering. Densification of thick film materials proceeds more readily under an atmosphere of high oxygen partial pressure although in the absence of a suitable barrier layer, film delamination is promoted. Delamination was found to occur at the interface between electrode and substrate and not between film and electrode.