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.     

MEMS based low cost piezoresistive microcantilever force sensor and sensor module

In the present work, we report fabrication and characterization of a low-cost MEMS based piezoresistive micro-force sensor with SU-8 tip using laboratory made silicon-on-insulator (SOI) substrate. To prepare SOI wafer, silicon film (0.8 µm thick) was deposited on an oxidized silicon wafer using RF magnetron sputtering technique. The films were deposited in argon (Ar) ambient without external substrate heating. The material characteristics of the sputtered deposited silicon film and silicon film annealed at different temperatures (400–1050 °C) were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The residual stress of the films was measured as a function of annealing temperature. The stress of the as-deposited films was observed to be compressive and annealing the film above 1050 °C resulted in a tensile stress. The stress of the film decreased gradually with increase in annealing temperature. The fabricated cantilevers were 130 μm in length, 40 μm wide and 1.0 μm thick. A series of force–displacement curves were obtained using fabricated microcantilever with commercial AFM setup and the data were analyzed to get the spring constant and the sensitivity of the fabricated microcantilever. The measured spring constant and sensitivity of the sensor was 0.1488 N/m and 2.7 mV/N. The microcantilever force sensor was integrated with an electronic module that detects the change in resistance of the sensor with respect to the applied force and displays it on the computer screen.     

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.     

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.     

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.     

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.     

Strain-insensitive and high temperature fiber sensor based on a Mach–Zehnder modal interferometer

In this paper, a strain insensitive high temperature fiber sensor based on the modal interferometer is proposed. It is composed of a piece of small-core photosensitive fiber (SCPSF) which is spliced between two pieces of single mode fiber (SMF). Compared to other high temperature fiber sensor based on the modal interferometer, the sensor owns the highest temperature sensitivity of 106.64 pm/°C from 200 °C to 1000 °C. The temperature to strain cross sensitivity of the sensor is low and only 0.00675 °C/με. The reasons for realizing the high temperature sensitivity is also discussed.     

Nb-doped TiO2 sol–gel films for CO sensing applications

Nb doped titania (TiO₂:Nb) multilayered films (1–10 layers) with anatase structure were obtained by the low-cost sol–gel and dipping method on microscope glass substrates, followed by thermal treatment at 450 °C for 1 h. After each layer deposition, an intermediate annealing step was performed at 300 °C for 30 min. Doping TiO₂ sol–gel films with a low amount of Nb (0.8 at%) allows obtaining an improved CO sensor able to operate under environmental atmosphere (air). It was found that the sensor sensitivity is less dependent on the film thickness but is significantly influenced by Nb doping at the optimal working temperature of 400 °C. Good recovery characteristics were obtained for a wide CO detection range, between 0 and 2000 ppm. The gas-sensing behavior of the films was correlated with the structural, chemical and morphological properties of the multi-layered structures.     

Dual stage modeling of moisture absorption and desorption in epoxy mold compounds

A dual stage diffusion model is developed in this paper for both absorption and desorption processes. Both stages in moisture absorption and desorption, i.e., Fickian and non-Fickian process, are described mathematically using a combination of Fickian terms. Absorption and desorption tests are also conducted on six distinct commercial epoxy mold compounds (EMCs) used in electronic packaging. For absorption, the samples are subjected to 85 °C/85% relative humidity and 60 °C/85% relative humidity soaking, respectively. Desorption conditions are above glass transition temperature at 140 °C and 160 °C. The dual stage models generate reasonable results for the diffusive properties and display outstanding experimental fits. All six compounds show strong non-Fickian diffusion behaviors, which are further verified by the experiments with different thicknesses. For absorption, while Fickian diffusion is dominant in the beginning of process, non-Fickian mechanism plays a large role with time increasing. Saturated moisture concentration associated with Fickian-stage diffusion appears to be independent of temperature under the tested conditions. For desorption, higher temperature leads to less percentage of the permanent residual moisture content in most compounds. At 160 °C, 90% of the initial moisture for all samples is diffused out within 24 h, following an approximate modified Fickian diffusion process. The dual stage model developed in this paper provides a mathematical formulation for modeling anomalous moisture diffusion behavior using commercial finite element analysis software.     

Corrosion behavior of crystalline silicon solar cells

Corrosion behavior of crystalline silicon (C-Si) solar cells was investigated. For this purpose, three groups of cells were conducted with three kinds of aging test which cells setting in indoor environment (25 °C, 45% RH, 0– 2 months), cells immersing in moisture atmosphere (25 °C, 85% RH, 0– 240 h) and cells immersing in acetic acid atmosphere (25 °C, 85% RH, 0– 240 h). Subsequently the microstructure characteristic of the alumina paste layer (APL), rear electrode and soldered connection of cells and the corrosion production during aging test were analyzed and compared. The results show that the smooth oxide coating and looser structure were found in the APL of cell after aging test. In addition the discoloration and elements diffusion were found on the rear electrode. And the corrosion region expanded gradually from the edge to the center of soldered connection along the interface between Ag electrode and Sn₃₇Pb alloy. Thereafter, a model was put forward to try to explain the degradation mechanism of traditional photovoltaic (PV) modules during damp-heat (DH) test based on corrosion behavior of C-Si cells in this experiment.     

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.     

Thermal stability of organic transistors with short channel length on ultrathin foils

We demonstrate an effective design for fabrication of short channel organic transistors (<3 μm channel length) on ultrathin 1 μm thick substrates that exhibit excellent thermal stability. For short channel transistors, we demonstrate durability up to 170 °C, with a theoretical cutoff frequency above 100 kHz, and stable performance in cyclic heating tests up to 120 °C. We fabricate inverter circuits to investigate their behavior upon heating and show that inverter gain can be improved by 150%. Device performance and topology changes were systematically analyzed after annealing steps to gain better understanding on the mechanism behind the performance change. This report on the thermal stability of short channel transistors on ultrathin films shows good durability at elevated temperatures and paves the way for high frequency imperceptible electronics.     

Empirical prediction model for Li/SOCl2 cells based on the accelerated degradation test

The empirical prediction model of residual capacity (Cap) for D-size Li/SOCl₂ cells has been developed and validated based on the accelerated degradation test (ADT) data. In this experiment, a series of constant storage temperatures (25 °C, 55 °C, 70 °C, and 85 °C) was selected and the residual capacity of each cell was monitored continuously during the aging test. The model was established by fitting twice. Firstly, time dependence of Cap (t, T) was investigated. Secondly, the generalized model of residual capacity was built. The prediction model, as a function of storage time and temperature, can precisely predict the value of residual capacity. The generalized empirical model of Cap, involving two aging processes, is valid for the degradation condition of temperatures from 25 °C to 70 °C. The first aging process completed rapidly within 7 days. The second aging process was accelerated by temperature with time^1/2 kinetics. For the cells stored at 85 °C, another failure mechanism may exist based on the departure of linear fitting coefficients.     

A fast moisture sensitivity level qualification method

In this paper, a fast moisture sensitivity level (MSL) qualification method and a fast moisture characterization method are discussed. The fast moisture characterization uses a stepwise method to obtain more reliable and more material moisture properties. The established relationships for moisture diffusion coefficients and moisture saturation levels with respect to the temperature and relative humidity can be used to predict moisture properties in the MSL range. Fast moisture sensitivity level qualification is accomplished with the aid of simulation combined with the characterized moisture diffusion properties. Moisture absorption processes at different conditions are simulated using a 3D model at conditions according to the moisture sensitivity test levels. Simulation of weight change at different condition and simulation of local moisture concentration are performed and compared between different conditions. Simulations show that at 696 h preconditioning time at 30 °C/60%RH for MSL level 2a can be decreased to 42 h at 85 °C/85%RH. Time required for package reliability and moisture sensitivity analysis is largely shortened.     

Studies on various chip-on-film (COF) packages using ultra fine pitch two-metal layer flexible printed circuits (two-metal layer FPCs)

Various fine pitch chip-on-film (COF) packages assembled by (1) anisotropic conductive film (ACF), (2) nonconductive film (NCF), and (3) AuSn metallurgical bonding methods using fine pitch flexible printed circuits (FPCs) with two-metal layers were investigated in terms of electrical characteristics, flip chip joint properties, peel adhesion strength, heat dissipation capability, and reliability. Two-metal layer FPCs and display driver IC (DDI) chips with 35 μm, 25 μm, and 20 μm pitch were prepared. All the COF packages using two-metal layer FPCs assembled by three bonding methods showed stable flip chip joint shapes, stable bump contact resistances below 5 mΩ, good adhesion strength of more than 600 gf/cm, and enhanced heat dissipation capability compared to a conventional COF package using one-metal layer FPCs. A high temperature/humidity test (85 °C/85% RH, 1000 h) and thermal cycling test (T/C test, ?40 °C to + 125 °C, 1000 cycles) were conducted to verify the reliability of the various COF packages using two-metal layer FPCs. All the COF packages showed excellent high temperature/humidity and T/C reliability, however, electrically shorted joints were observed during reliability tests only at the ACF joints with 20 μm pitch. Therefore, for less than 20 μm pitch COF packages, NCF adhesive bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for over 25 μm pitch COF applications. Furthermore, we were also able to demonstrate double-side COF using two-metal layer FPCs.     

Young's modulus of a sintered Cu joint and its influence on thermal stress

Al₂O₃ chips and pure Cu plates were joined by Cu nanoparticles at 250 °C and 350 °C, and the Young's moduli of the sintered Cu were evaluated by nanoindentation tests. The average Young's moduli were 52.7 ± 19.8 GPa and 76.5 ± 29.7 GPa at 250 °C and 350 °C, respectively, indicating that the sintered structures were strengthened at higher temperatures. The calculation results indicated that the joint at 350 °C has a high Young's modulus, but make the stress higher than the chip strength, resulting in breakage of the chip during 65/250 °C power cycling.     

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.     

Moisture absorption by molding compounds under extreme conditions: Impact on accelerated reliability tests

The Highly Accelerated Stress Test (HAST) [130 °C/85%RH or 110 °C/85%RH with applied bias voltage] or Autoclave test [121 °C/100%RH] are well-accepted tests during qualification and reliability testing of automotive microelectronics. These tests are often preferred since results can be obtained relatively fast due to the high acceleration. On the other hand, there is a risk of accelerating non-relevant failure modes e.g. due to changes in material behavior under test conditions, which will not occur during application. However, predicting what happens under test conditions is not straightforward since one needs to understand material behavior under such extreme conditions. Especially the amount of water absorption, which is highly relevant for test result interpretation, is challenging to obtain at temperatures above 100 °C, since here we deal with pressurized conditions.In this paper a novel, and straightforward, method is proposed to determine the moisture absorption under pressurized conditions. To verify the method, data obtained under pressurized conditions is compared to behavior under standard conditions. Under autoclave conditions it is shown that the moisture uptake at higher temperatures is much higher than predicted by extrapolation. The reported data can also be used to predict more reliably the moisture saturation level, and rate of moisture saturation during HAST.     

A study on MIS Schottky diode based hydrogen sensor using La2O3 as gate insulator

In this work, a Metal–Insulator–Semiconductor (MIS) based Schottky-diode hydrogen sensor was fabricated with La₂O₃ as a gate insulator. The electrical properties (current–voltage characteristics, change in barrier height and sensitivity) and hydrogen sensing performance (dynamic response and response time) were examined from 25 °C to 300 °C and towards H₂ with different concentrations. The conduction mechanisms were explained in terms of Fowler–Nordheim tunneling (below 120 °C) and the Poole–Frenkel effect at temperatures (above 120 °C). The results show that at an operating temperature of 260 °C, the sensitivity of the device can reach a maximum value of 4.6 with respect to 10,000-ppm hydrogen gas and its response time was 20 s.     

Temperature dependent resistivity study on zinc oxide and the role of defects

The temperature dependent (30–550 °C) resistivity of zinc oxide (ZnO) has been studied by the standard four probe resistivity method. The room-temperature resistivity of the sample is measured as 0.75 M Ωm. Resistivity versus temperature plot of the sample shows normal NTCR (negative temperature coefficient of resistance) behavior up to 300 °C. However, a crossover from NTCR to a PTCR (positive temperature coefficient of resistance) behavior is observed at ～300 °C. The origin of the PTCR behavior is explained with the defects present in the ZnO annealed up to 550 °C. Temperature dependent S-parameter (positron annihilation line-shape parameter) indicates the formation of oxygen vacancy like defects in this temperature region. At the PTCR region, the activation energy for the electron conduction is calculated ～2.6 eV. This value is very close to the theoretically predicted defect level energy of 2.0 eV for oxygen vacancies present in ZnO.