Rapid synthesis of ZnO dandelion-like nanostructures and their applications in humidity sensing and photocatalysis

In this study, ZnO dandelion-like nanostructures were rapidly synthesized on Si substrates using a two-step thermal oxidation approach. The ZnO nanostructures were grown at various thermal oxidation temperatures ranging from 400 °C to 700 °C. These nanostructures were then applied to humidity sensing and photocatalysis. The ratio of measured resistances in the humidity sensors for relative humidity (RH) levels of 11% and 95% at room temperature (RT) were found to rise from 10² to 10⁵ times for humidity sensors constructed with the nanostructures grown at temperatures from 400 °C to 700 °C, respectively, and sensor response time decreased from 15 s to 5 s. These results show that the proposed ZnO dandelion-like nanomaterial shows promise as a candidate for fabricating high-performance humidity sensors when the nanostructures are grown at 700 °C. In addition, the photocatalytic effect of the nanostructures was tested with a decomposition of methyl orange (MO) dye under UV illumination. Experimental results show that the ZnO dandelion-like nanomaterial grown at a thermal oxidation temperature of 700 °C exhibits an excellent photocatalytic effect, which degrades to almost 90% of the MO activity over 120 min.     

Study on the thermal stability of Ga-doped ZnO thin film: A transparent conductive layer for dye-sensitized TiO2 nanoparticles based solar cells

Generally, optoelectronic devices are fabricated at a high temperature. So the stability of properties for transparent conductive oxide (TCO) films at such a high temperature must be excellent. In the paper, we investigated the thermal stability of Ga-doped ZnO (GZO) transparent conductive films which were heated in air at a high temperature up to 500 °C for 30 min. After heating in air at 500 °C for 30 min, the lowest sheet resistance value for the GZO film grown at 300 °C increased from 5.5 Ω/sq to 8.3 Ω/sq, which is lower than 10 Ω/sq. The average transmittance in the visible light of all the GZO films is over 90%, and the highest transmittance is as high as 96%, which is not influenced by heating. However, the transmittance in the near-infrared (NIR) region for the GZO film grown at 350 °C increases significantly after heating. And the grain size of the GZO film grown at 350 °C after annealing at 500 °C for 30 min is the biggest. Then dye-sensitized TiO2 NPs based solar cells were fabricated on the GZO film grown at 350 °C (which exhibits the highest transmittance in NIR region after heating at 500 °C for 30 min) and 300 °C (which exhibits the lowest sheet resistance after heating at 500 °C for 30 min). The dye-sensitized solar cell (DSSC) fabricated on the GZO film grown at 350 °C exhibits superior conversion efficiency. Therefore, transparent conductive glass applying in DSSCs must have a low sheet resistance, a high transmittance in the ultraviolet–visible–infrared region and an excellent surface microstructure.     

Junction temperature and reliability of high-power flip-chip light emitting diodes

In this work, infrared micro-imaging, emission microscope measurements are performed on the chip surface of flip-chip light emitting diodes (FCLEDs). The temperature deviation on the chip surface increases from 19 to 146 °C when the injection current changes from 20 to 2000 mA. When the structure of FCLED is optimized, the temperature deviation becomes smaller. And the thermal resistance is achieved to as low as 10.4 °C/W. The finite element method calculation based on the model of steady-state current field and temperature field is carried out to investigate the effects of current spreading on thermal performance of FCLED.     

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.     

Effect of annealing temperature on optical and electrical properties of lead sulfide thin films

Lead sulfide (PbS) thin films with 150 nm thickness were prepared onto ultra-clean quartz substrate by the RF-sputtering deposition method. Deposited thin films of PbS were annealed at different temperatures 100 °C, 150 °C, 200 °C, 250 °C and 300 °C. X-ray diffraction pattern of thin films revealed that thin films crystallized at 150 °C. Crystalline thin films had cubic phase and rock salt structure. The average crystallite size of crystalline thin films was 22 nm, 28 nm and 29 nm for 150 °C, 200 °C and 250 °C respectively. From 150 °C to 250 °C increase in annealing temperature leads to increase in crystallite arrangement. FESEM images of thin films revealed that crystallite arrangement improved by increasing annealing temperature up to 250 °C. Increase in DC electrical conductivity by increasing temperature confirmed the semiconductor nature of crystalline thin films. Increase in dark current by increasing annealing temperature showed the effect of crystallite arrangement on carrier transport. Photosensitivity decreased by increasing annealing temperature for crystalline thin films that it was explained at the base of thermal quenching of photoconductivity and adsorption of oxygen at the surface of thin films that leads to the formation of PbO at higher temperatures.     

Optimization and validation of thermal management for a RF front-end SiP based on rigid-flex substrate

This paper mainly presents a new 3D stacking RF System-in-Package (SiP) structure based on rigid-flex substrate for a micro base station, with 33 active chips integrated in a small package of 5cm × 5.5cm × 0.8cm. Total power consumption adds up to 20.1 Watt. To address thermal management and testability difficulties of this RF SiP, a thermal test package is designed with the same package structure and assembly flow, only replacing active chips with thermal test dies (TTDs). Optimization and validation of thermal management for the thermal test package is conducted. Effects of the structure, chip power distribution, and ambient temperature aspects on the thermal performance are studied. Thermal vias designed in the organic substrate provide a direct heat dissipation path from TTDs to the top heatsink, which minimizes junction temperature gap of the top substrate from 31.2 °C to 5.3 °C, and enables junction temperatures of all the chips on the face to face structure to be well below 82 °C. Chip power distribution optimization indicates placing high power RF parts on the top rigid substrate is a reasonable choice. The ambient temperature optimizes with forced air convection and cold-plate cooling method, both of which are effective methods to improve thermal performances especially for this micro base station application where environment temperature may reach more than 75 °C. The thermal management validation is performed with a thermal test vehicle. Junction temperatures are compared between finite-volume-method (FVM) simulation and thermal measurement under the natural convection condition. The accordance of simulation and measurement validates this thermal test method. Junction temperatures of typical RF chips are all below 80 °C, which shows the effectiveness of thermal management of this RF SiP.     

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.     

Failure and stress analysis of through-aluminum-nitride-via substrates during thermal reliability tests for high power LED applications

The objective of this study is to evaluate the reliability of through-aluminum-nitride-via (TAV) substrate by comparing those experimental results with the finite element simulation associated with measurements of aluminum nitride (AlN) strength and the thermal deformation of Cu/AlN bi-material plate. Two reliability tests for high-power LED (Light emitting diode) applications are used in this study: one is a thermal shock test from − 40 °C to 125 °C, the other is a pressure cook test. Also, the strength of AlN material is measured by using three-point bending test and point load test. The reliability results show that TAV substrates with thicker Cu films have delamination and cracks after the thermal shock test, but there are no failure being found after the pressure cook test. The determined strengths of AlN material are 350 MPa and 650 MPa from three-point bending test and point load test, respectively. The measurement of thermal deformation shows that the bi-material plate has residual-stress change after the solder reflow process, also indicating that a linear finite element model with the stress-free temperature at 80 °C can reasonably represent the stress state of the thermal shock test from − 40 °C to 125 °C without considering Cu nonlinear effect. The further results of the finite element simulation associated with strength data of AlN material have successfully described those of the reliability test.     

Investigation on thermal fatigue of SnAgCu,Sn100C,and SnPbAg solder joints in varying temperature environments

Thermal cycling tests have been performed for a range of electronic components intended for avionic applications, assembled with SAC305, SN100C and SnPbAg solder alloys. Two temperature profiles have been used, the first ranging between −20 °C and +80 °C (TC1), and the second between −55 °C and +125 °C (TC2). High level of detail is provided for the solder alloy composition and the component package dimensions, and statistical analysis, partially supported by FE modeling, is reported. The test results confirm the feasibility of SAC305 as a replacement for SnPbAg under relatively benign thermomechanical loads. Furthermore, the test results serve as a starting point for estimation of damage accumulation in a critical solder joint in field conditions, with increased accuracy by avoiding data reduction. A computationally efficient method that was earlier introduced by the authors and tested on relatively mild temperature environments has been significantly improved to become applicable on extended temperature range, and it has been applied to a PBGA256 component with SAC305 solder in TC1 conditions. The method, which utilizes interpolated response surfaces generated by finite element modeling, extends the range of techniques that can be employed in the design phase to predict thermal fatigue of solder joints under field temperature conditions.     

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.     

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 annealing on the microstructure and bonding interface properties of Ag–2Pd alloy wire

This study investigated the mechanical and electrical properties of Ag–2Pd wire after thermal annealing. The thermal stability of the tested wire was examined by separately imposing static annealing at 275 °C, 325 °C and 375 °C in a vacuum environment. It was found that annealing the Ag–2Pd wire at 275 °C promoted the formation of a fully annealed structure with equiaxed grains. Annealing Ag–2Pd wire had a shorter heat affect zone (HAZ) length than those of conventional wire, and offered outstanding mechanical properties. A long-term electrical test found Ag₃(Pd)Al and Ag₂(Pd)Al compounds between the Ag–Pd ball and Al pad. These results confirmed the high-reliability properties of annealed Ag–2Pd wires for the wire bonding process.     

Power cycle reliability of Cu nanoparticle joints with mismatched coefficients of thermal expansion

The power cycle reliability of Cu nanoparticle joints between Al₂O₃ heater chips and different heat sinks (Cu-40 wt.%Mo, Al-45 wt.%SiC and pure Cu) was studied to explore the effect of varying the mismatch in the coefficient of thermal expansion (CTE) between the heater chip and the heat sink from 4.9 to 10.3 ppm/K. These joints were prepared under a hydrogen atmosphere by thermal treatment at 250, 300 and 350 °C using a pressure of 1 MPa, and all remained intact after 3000 cycles of 65/200 °C and 65/250 °C when the CTE mismatch was less than 7.3 ppm/K, despite vertical cracks forming in the sintered Cu. When the CTE mismatch was 10.3 ppm/K, the Cu nanoparticle joint created at 300 °C endured the power cycle tests, but the joint created at 250 °C broke by lateral cracks in the sintered Cu after 1000 cycles of 65/200 °C. The Cu nanoparticle joint created at 350 °C also broke by vertical cracks in the heater chip after 1000 cycles of 65/250 °C, suggesting that although sintered Cu can be strengthened to tolerate the stress by increasing the joint temperature, this eventually causes the weak and brittle chip to fracture through accumulated stress. The calculation results of stresses on the heater chip showed that the stress can be higher than the strength of Al₂O₃ when the CTE mismatch is 10.3 ppm/K and the Young's modulus of the sintered Cu is higher than 20 GPa, suggesting that the heater chip can be broken.     

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.     

CMOS micro-heater design for direct integration of carbon nanotubes

Carbon nanotubes (CNTs) are nanomaterials that exhibit many remarkable electrical, mechanical and thermal properties, which can be exploited in various smart sensing applications by integrating them in standard CMOS processes. However, such integration technique is challenging since CMOS does not tolerate high temperatures required for local CNT synthesis. This work involves designing power efficient CMOS micro-heaters that can generate CNT growth temperature (~ 900 °C) in a post-CMOS CNT fabrication step, while maintaining CMOS compatible temperature (< 300 °C) in the microsystem. One suitable metal interconnect layer and a polysilicon layer available in AMS 0.18 μm CMOS technology have been used to design and simulate the micro-heaters. This paper proposes and compares six different optimal micro-heater designs alongside their thermal and thermomechanical analysis using multiphysics simulation software, ANSYS. Feasibility of implementing the designed micro-heaters in a real chip is discussed based on the analysis. Required CMOS post processing steps for the designed micro-heaters are also discussed. The promising results are expected to lead the way for successful implementation of carbon nanotube based sensors in a commercial CMOS process.     

Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Taguchi robust design to optimize synthesis of lead oxalate nano-disks

Disk-shaped lead oxalate nanoparticles were synthesized via sol-precipitation in aqueous media without any surfactant, template or catalyst. Significance of reaction conditions such as: lead and oxalate concentrations, flow rate of reagent addition and temperature of reactor on diameter of synthesized lead oxalate disk-shaped particles were investigated and optimized. The participation of the studied variables in the particle size control of lead oxalate was quantitatively evaluated by analysis of variance. The results showed that lead oxalate nano-disks can be synthesized via controlling effective procedure parameters. Under optimum conditions, disk shaped nano-sized lead oxalate particles with two dimensions (95 nm diameter and 35 nm thickness) were synthesized. The structure and morphology of the lead oxalate nano-disks obtained under optimum conditions of synthesis process were characterized by scanning electron microscopy, X-ray diffraction, infrared spectroscopy and thermal analysis techniques. The Thermal analysis of the nanoparticles obtained under optimum conditions indicates that the main thermal degradation of the nano-disks occurs in the temperature range of 310–380 °C; while, submicron particles decomposed exothermally in more wider temperature range of 320–430 °C.     

Preparation and photocatalytic performance of Ag/ZnO nano-composites

Ag/ZnO nano-composites sized between 20 and 50 nm were prepared by the coordination homogeneous co-precipitation method. Thermogravimetry/differential thermal analysis (TG-DTA), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) were used to characterize the microstructure and morphology of the precursor and the products obtained at various temperatures. The research on the growth dynamics of product Ag/ZnO showed that the dynamic growth index was 1.32, indicating that the rate of the grain growth was quick at 300–600 °C. The results of the photocatalytic degradation of methyl orange (MO) in aqueous solution indicated that the Ag/ZnO photocatalyst prepared by the coordination homogeneous co-precipitation method exhibited better photocatalytic performance than that prepared by the photoreduction method, especially the photocatalyst calcined at 300 °C, and the photocatalytic performance decreased when the calcining temperature increased from 300 to 700 °C.     

High temperature induced failure in Ti/Al/Ni/Au Ohmic contacts on AlGaN/GaN heterostructure

Ti/Al/Ni/Au (200/1200/500/2000 Å) Ohmic contact on AlGaN/GaN was prepared and it was subjected to thermal aging experiments. Thermal processing at 400 and 500 °C did not change the contact resistance significantly, while high temperature storage at 600 °C resulted in a surge in the contact resistance. The Al–Au alloy in the contact metal is believed to re-melt because its lowest melting temperature is 525 °C. The liquid of Al–Au alloy is observed to diffuse to the AlGaN surface and consume some AlGaN layer. In addition, voids are found to be produced during thermal process, which can reduce the effective contact area and thus lead to higher contact resistance. The TEM and EDX results of Ohmic contact’s cross sectional images provide evidence for this proposed mechanism.     

Experimental investigation of temperature and relative humidity effects on resonance frequency and quality factor of CMOS-MEMS paddle resonator

This paper reports an experimental approach to analyse the performance of an externally actuated CMOS-MEMS paddle resonator with proof mass. The surface morphology test of the device is performed with the help of field emission scanning electron microscopy (FESEM), before and after the reliability tests. The effects of temperature variation on the resonance frequency response of the fabricated CMOS-MEMS resonator is analysed under the variation of temperature from 25 °C to 80 °C inside a custom made environmental chamber at a constant relative humidity (32%RH). In the next step, the variation in the quality factor of the MEMS resonator is studied under the effect of varying temperature. Finally, the resonance frequency behavior is analysed under the variation of relative humidity from 32%RH to 90%RH at a constant temperature of 25 °C. The device is found to be eroded and there are some wastes of humidity on it. A total change of 6.9 Hz in resonance frequency is recorded from 25 °C to 80 °C. The drop in the resonance frequency of the MEMS device is found to be 137 MHz/°C with the rise in temperature. Under the temperature variation from 25 °C to 80 °C, the quality factor is found to be nonlinear. A total change of 1.3 Hz in the resonance frequency is observed from 32%RH to 90%RH. The resonance frequency is found to be − 21.8 MHz/RH% with an increasing humidity level.