Enhanced printed temperature sensors on flexible substrate

In this paper we present the development of enhanced printed temperature sensors on large area flexible substrates. The process flow is a fully screen printed technology that uses exclusively solution-processed materials. These Screen printed temperature sensors are based on resistive pastes integrated in a Wheatstone bridge circuit. Substrate is a commercial Poly Ethylene Naphtalate (PEN) with a thickness of 125 µm. Functional temperature sensors are demonstrated and characterized with good electrical properties, showing a good sensitivity of 0.06 V/°C at V_in=4.8 V. This sensitivity is enhanced by the annealing and the O₂ plasma treatment. Based on this temperature sensor, we have developed a demonstrator for human body temperature detection.     

Corrosion testing of anisotropic conductive adhesive interconnections on FR4, liquid crystal polymer and polyimide substrates

Anisotropic conductive adhesive films (ACF) have been widely studied for numerous applications. However, their resistance to corrosion in highly corrosive environments has been studied only very little. This study investigated the reliability and behaviour of ACFs in corrosive salt spray environment. ACF was used to attach flip chip (FC) components on FR4, liquid crystal polymer (LCP) and polyimide (PI) substrates and the FC packages were subjected to a salt spray test lasting 3000 h. The FC packages had daisy chain structures which were measured continuously in real time during testing. After testing cross sections of the tested packages were examined using an optical microscope and a scanning electron microscope (SEM). Most components failed during the test and the results showed significant differences between the various substrate materials. The LCP substrate performed considerably better than the other substrates and the PI substrate proved to have the poorest reliability. Corrosion of the pads on the substrates as well as open joints was seen in all substrate materials. The corrosion behaviour as well as the differences between the substrates showed that the substrate structure and material are critical factors in corrosive environments and should be carefully considered. The reliability of the ACF FC package with the LCP substrate was found to be good, as the test was very severe and no failures occurred during the first 625 h of testing and only 20% failed during the first 1000 h.     

Accurate operation of a CMOS integrated temperature sensor

A high-accuracy temperature sensor is designed by applying the temperature characteristics of substrate bipolar transistor in CMOS technology. Initial accuracy of the temperature sensor can be improved by chopper amplifiers and dynamic element matching. Using these two methods, the circuit realization of reference voltage is also described. Simulation results show that the inaccuracy is within×0.4 °C from ?40 to +100 °C. Experimental results, obtained from circuits fabricated in 0.5 μm CMOS process, indicate that the sensor is inaccurate within×0.7 °C from ?40 to +100 °C. The power dissipation is 0.35 mW and the chip area is 889 μm×620 μm. Compared with previously reported work, the temperature sensor in the paper has lower inaccuracy without calibration.     

CMOS temperature sensor with ring oscillator for mobile DRAM self-refresh control

This paper presents novel low-cost CMOS temperature sensor for controlling the self-refresh period of a mobile DRAM. In the proposed temperature sensor, the temperature dependency of poly resistance is used to generate a temperature-dependent bias current, and a ring oscillator driven by this bias current is employed to obtain the digital code pertaining to on-chip temperature. This method is highly area-efficient, simple and easy for IC implementation as compared to traditional temperature sensors based on bandgap reference. The proposed CMOS temperature sensor was fabricated with an 80 nm 3-metal DRAM process, which occupies extremely small silicon area of only about 0.016 mm² with under 1 μW power consumption for providing 0.7 °C effective resolution at 1 sample/s processing rate. This result indicates that as much as 73% area reduction was obtained with improved resolution as compared to the conventional temperature sensor in mobile DRAM.     

Design and implementation of micro-power temperature to duty cycle converter using differential temperature sensing

The CMOS based temperature detection circuit has been developed in a standard 180 nm CMOS technology. The proposed temperature sensor senses the temperature in terms of the duty cycle in the temperature range of −30 °C to +70 °C. The circuit is divided into three parts, the sensor core, the subtractor and the pulse width modulator. The sensor core consists of two individual circuits which generates voltages proportional (PTAT) and complementary (CTAT) to the absolute temperature. The mean temperature inaccuracy (°C) of PTAT generator is −0.15 °C to +0.35 °C. Similarly, CTAT generator has mean temperature accuracy of ±1 °C. To increase thermal responsivity, the CTAT voltage is subtracted from the PTAT voltage. The resultant voltage has the thermal responsivity of 6.18 mV/°C with the temperature inaccuracy of ±1.3 °C. A simple pulse width modulator (PWM) has been used to express the temperature in terms of the duty cycle. The measured temperature inaccuracy in the duty cycle is less than ±1.5 °C obtained after performing a single point calibration. The operating voltage of the proposed architecture is 1.80±10% V, with the maximum power consumption of 7.2 μW.     

A low power MEMS gas sensor based on nanocrystalline ZnO thin films for sensing methane

Nanocrystalline ZnO based sensor using micromachined silicon substrate has been reported for efficient detection of methane as opposed to conventional SnO₂ based micromachined sensors for its higher compatibility to silicon IC technology and greater response. A suitably designed nickel microheater has been fabricated on to the micromachined Si platform. The optimum temperature for highest response magnitude and lowest response time were found to be 250 °C although relatively high (76.6%) response is obtained even at as low as 150 °C. Our study showed quite high response magnitude (87.3%), appreciably fast response time (8.3 s) and recovery time (17.8 s) to 1.0% methane at 250 °C. The sensor showed appreciably fast response (14.3 s) and recovery time (28.7 s) at 150 °C. The power consumption at an operating temperature of 250 °C was 120 mW and at 150 °C is only ～70 mW. Moreover, this type of sensor was found to give fairly appreciable response for lower methane concentrations (0.01%) also. For higher methane concentrations (>0.5%) response is detectable even at 100 °C where the power consumption is only ～40 mW.     

End of life and acceleration modelling for power diodes under high temperature reverse bias stress

This work is motivated by the growing importance of lifetime modelling in power electronics. Strongly accelerated High Temperature Reverse Bias (HTRB) testing of power diodes at different stress conditions is performed until alterations and fatigue mechanisms become evident. Two categories of effects can be separated: Drifting breakdown voltage and hard failures with complete loss of blocking capability. Nevertheless the overall stress duration needed to provoke destructive failures is very high with test durations > 2500 h even at almost 230 °C and 100% rated voltage. For both mechanisms the temperature and voltage acceleration is evaluated. Especially temperature acceleration is significant in the regime of testing between 200 °C and 230 °C and an activation energy E_a in the regime > 1 eV can be deduced which is higher compared to values commonly reported in the literature. Failure analysis shows that both package and also chip related effects could contribute to the observed hard failures in HTRB stress under extreme conditions.     

Effect of the substrate temperature on the electrical and structural properties of spray-deposited SnO2:F thin films

SnO₂:F thin films were prepared by the spray pyrolysis (SP) technique at substrate temperature in the range 360–480 °C. The effect of varying the substrate temperature on the electrical and structural properties of the films was investigated by studying the I–V characteristics, the X-ray diffraction patterns (XRD), and the scanning electron microscope images (SEM). The I–V characteristics of the films were improved by increasing the substrate temperature, i.e. the resistivity of the films had decreased from 98 to 0.22 Ω cm. The X-ray diffraction patterns taken at 400 and 480 °C showed that the films are polycrystalline and two directions of crystal growth appeared in the difractogram of the film deposited at the lower substrate temperature, which correspond to the reflections from the (1 1 0) and (2 0 0) planes. With the increase in the substrate temperature a new direction of crystal growth appeared, which corresponds to the reflection from the (1 0 1) plane. Also the (1 1 0) and (2 0 0) lines were slightly grown at the higher substrate temperature, which means the crystal growth was enhanced and the grain size had increased. The SEM images confirmed these results and showed larger grains and more crystallization for the higher substrate temperature too.     

Fabrication of nano-sized resist patterns on flexible plastic film using thermal curing nano-imprint lithography

Due to polymer’s excellent flexibility, transparency, reliability and light weight, it is a good candidate material for substrate of devices including organic electronic devices, biomedical devices, and flexible displays (LCD and OLED). In order to build such devices on polymer, nano- to micron-sized patterning must be accomplished. Since polymer materials reacts with organic solvents or developer solutions which are inevitably used in photolithography and cannot bear high temperature (∼140 °C) process for photoresist baking, conventional photolithography cannot be used to polymer substrate. In this research, monomer based thermal curing imprinting lithography was used to make as small as 100 nm dense line and space patterns on flexible PET (polyethylene-terephthalate) film. Compared to hot embossing lithography, monomer based thermal curing imprint lithography uses monomer based imprint resin which consists of base monomer and thermal initiator. Since it is liquid phase at room temperature and polymerization can be initiated at 85 °C, which is much lower than glass temperature of polymer resin, the pattern transfer can be done at much lower temperature and pressure. Hence, patterns as small as 100 nm were successfully fabricated on flexible PET film substrate by monomer based thermal curing imprinting lithography at 85 °C and 5 atm without any noticeable degradation of PET substrate.     

Dislocations in Si generated by fatigue at room temperature

Dislocations in silicon can be generated in many ways, and they often induce a leakage current at the p–n junction and give rise to data retention failures of the semiconductor devices. In this study, it was found that dislocations could be generated in silicon even at room temperature by fatigue. The dislocations generated in a semiconductor device were investigated by transmission electron microscopy. They formed a cluster 3 μm in diameter, which emerged from the interface between the silicon substrate and a tungsten stud. Most of the dislocations were lying on the (1 1 1) planes. It was discovered that cyclic deformation of the device by ultrasonic vibration during the cleaning process generated these dislocations.     

The hybridization and optimization of complementary DNA molecules on organic field-effect transistors

A DNA sensor based on pentacene thin film transistors (TFTs) is an excellent candidate for disposable sensor applications on account of their low-cost fabrication process and fast detection. We fabricated pentacene TFTs for the optimization of DNA hybridization. Different temperature and hybridization time were introduced during the DNA hybridization process in order to improve the hybridization efficiency of DNA molecules on the pentacene substrate. In this case, the optimized process conditions (45 °C and a hybridization of 30 min) for the DNA hybridization on the pentacene film were obtained, which are essential to enhance the sensitivity of OTFT-based DNA sensors.     

Determination of safe reliability region over temperature and current density for through wafer vias

Circular and slot backside vias are stressed over current and temperature and the resulting failure times are fitted to Black's equation. Contour plots of the FIT rate are generated and the reliability of circular and slot vias are compared. It is demonstrated that in most cases the FIT rate of the circular via is statistically significantly lower than that of the slot via. However, both types are easily able to meet a goal of 100 FITs in 10 years at T = 125 °C and J = 0.25 × 10⁶ A/cm². The contour map of the FIT rate defines the region where the via can operate reliably. By use of the 95% upper confidence bound, the region of safe operation is reduced in size, adding a layer of margin to the prediction of via reliability. The approach described here provides a “reliability map” for designers allowing trade-offs between temperature current to be made when designing for high reliability.     

Effect of substrate temperature on the properties of ZnO thin films prepared by spray pyrolysis

Sprayed ZnO films were grown on glass at different substrate temperatures from 200 °C to 500 °C and their structural, optical and electrical properties were investigated. All films are polycrystalline with hexagonal wurtzite structure. ZnO films at substrate temperatures above 400 °C appear to be better crystalized with (002) plane as preferential orientation. Optical transmission spectrum shows that ZnO films have high transmission (above 80%) in visible region for substrate temperatures above 400 °C. Photoluminescence spectra at room temperature show an ultraviolet emission and two visible emissions at 2.82 eV and 2.37 eV. The resistivity of ZnO films increases with increasing substrate temperatures (above 400 °C). The ZnO film deposited at 400 °C shows highest figure of merit.     

Study on temperature and current sensors based on optical driving optical fiber transmission

A sensing measurement scheme based on the optical fiber coupling optical driving converting probe is proposed. Using the optical feedback stabilized 5 mW LD as illuminant, the optical driving distance is 100 m. The power of designed low-consumption temperature-sensitive sensor probe is small to 2.2 mW. The design idea of this new sensor and the key technology of realizing optical driving are introduced in this paper. Experimental results show that the temperature sensor with high resolution can resist high-voltage and electromagnetic interference, and has high sensitivity and precision with 1000 Hz/°C.     

Consideration of temperature and current stress testing on flip chip solder interconnects

This work discusses the experimental set-up and data interpretation for high temperature and current stress tests of flip chip solder joints using the four-point Kelvin measurement technique. The single solder joint resistance responses are measured at four different four-point Kelvin structure locations in a flip chip package. Various temperatures (i.e., 125–165 °C) and electric current (i.e., 0.6–1.0 A) test conditions are applied to investigate the solder joint resistance degradation behavior and its failure processes. Failure criterion of 20% and 50% joint resistance increases, corresponding to solder and interfacial voiding, are employed to evaluate the solder joint electromigration reliability. The absolute resistance value is substantially affected by the geometrical layout of the metal lines in the four-point Kelvin structure, and this is confirmed by finite element simulation.Different current flow directions and strengths yielded different joint resistance responses. The anode joint, where electrons flow from the die to the substrate, usually measured an earlier resistance increase than the cathode joint, where electrons flow in the opposite direction. The change in measured joint resistances can be related to solder and interfacial voiding in the solder joint except for ±1 A current load, where resistance drop mainly attributed to the broken substrate Cu metallization as a result of “hot-spot” phenomenon. The solder joint temperature increases above the oven ambient temperature by ～25 °C, ～40 °C and ～65 °C for 0.6 A, 0.8 A and 1.0 A stress current, respectively. It is found that two-parameter log-normal distribution gives a better lifetime data fitting than the two-parameter Weibull distribution. Regardless of failure criterion used, the anode joint test cells usually calculated a shorter solder joint mean life with a lower standard variation of 0.3–0.6, as compared to the cathode joint test cells with a higher standard variation of 0.8–1.2. For a typical flip chip solder joint construction, electromigration reliability is mainly determined by the under bump metallization consumption and dissolution, with intermetallic compound formation near the die side of an anode joint.     

Effect of elevated temperature on PCB responses and solder interconnect reliability under vibration loading

In this paper, vibration tests are conducted to investigate the influence of temperature on PCB responses. A set of combined tests of temperature and vibration is designed to evaluate solder interconnect reliability at 25 °C, 65 °C and 105 °C. Results indicate that temperature significantly affects PCB responses, which leads to remarkable differences in vibration loading intensity. The PCB eigenfrequency shifts from 290 Hz to 276 Hz with an increase of test temperature from 25 °C to 105 °C, during which the peak strain amplitude is almost the same.Vibration reliability of solder interconnects is greatly improved with temperature rise from 25 °C to 105 °C. Mean time to failure (MTTF) of solder joint at 65 °C and 105 °C is increased by 70% and 174% respectively compared to that of solder joint at 25 °C. Temperature dominates crack propagation path of solder joint during vibration test. Crack propagation path is changed from the area between intermetallic compound (IMC) layer and Cu pad to the bulk solder with temperature increase.     

Effect of silver additive on electrical conductivity and methane sensitivity of SnO2

A composite powder of tin oxide (SnO₂) and silver (Ag) clusters was prepared by a simple and cost effective method of reducing their aqueous mixture with sodium borohydride (NaBH₄). Gas sensors based on the composite were made by powder pressing procedure and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrical conductivity and gas sensing behavior of the sensors for methane (CH₄) gas were studied as a function of Ag concentration (0.3, 0.5, 0.8 and 1.5 wt%). The Ag additive is found to improve sensor response and widen its working temperature range with notable sensor response. The best sensor response was achieved by the sensor with 0.5 wt% Ag. The enhanced response was proved to be due to both electrical and chemical mechanisms.     

Determination of the average channel temperature of GaN MOSHFETs under continuous wave and periodic-pulsed RF operational conditions

We present a method to determine the average device channel temperature of AlGaN/GaN metal–oxide–semiconductor heterostructure field effect transistors (MOSHFETs) in the time domain under continuous wave (CW) and periodic-pulsed RF (radiation frequency) operational conditions. The temporal profiles of microwave output power densities of GaN MOSHFETs were measured at 2 GHz under such conditions and used for determination of the average channel temperature. The measurement technique in this work is also being utilized to determine the thermal time constant of the devices. Analytical temporal solutions of temperature profile in MOSHFETs are provided to support the method. The analytical solutions can also apply to generic field effect transistors (FETs) with an arbitrary form of time-dependent heat input at the top surface of the wafer. It is found that the average channel temperature of GaN MOSHFETs on a 300 μm sapphire substrate with the output power of 10 W/mm can be over 400 °C in the CW mode while the average channel temperature of GaN MOSHFETs on a SiC substrate with the same thickness only reaches 50 °C under the same condition. The highest average channel temperature in a pulsed RF mode will vary with respect to the duty cycle of the pulse and type of the substrate.     

Dual functional performance of fiber Bragg gratings coated with metals using flash evaporation technique

Metallic and other type of coatings on fiber Bragg grating (FBG) sensors alter their sensitivity with thermal and mechanical stress while protecting the fragile optical fiber in harsh sensing surroundings. The behavior of the coated materials is unique in their response to thermal and mechanical stress depending on the thickness and the mode of coating. The thermal stress during the coating affects the temperature sensitivity of FBG sensors. We have explored the thermal response of FBGs coated with Al and Pb to an average thickness of 80 nm using flash evaporation technique where the FBG sensor is mounted in a region at room temperature in an evacuated chamber having a pressure of 10^?6 Torr which will minimize any thermal stress during the coating process. The coating thickness is chosen in the nanometer region with the aim to study thermal behavior of nanocoatings and their effect on FBG sensitivity. The sensitivity of FBGs is evaluated from the wavelengths recorded using an optical sensing interrogator sm 130 (Micron Optics) from room temperature to 300 °C both during heating and cooling. It is observed that the sensitivity of the metal coated fibers is better than the reference FBG with no coating for the entire range of temperature. For a coating thickness of 80 nm, Al coated FBG is more sensitive than the one coated with Pb up to 170 °C and it reverses at higher temperatures. This point is identified as a reversible phase transition in Pb monolayers as the 2-dimensional aspects of the metal layers are dominant in the nanocoatings of Pb. On cooling, the phase transition reverses and the FBGs return to the original state and for repeated cycles of heating and cooling the same pattern is observed. Thus the FBG functions as a sensor of the phase transitions of the coatings also.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.