Gold metallizations for eutectic bonding of silicon wafers

Gold eutectic bonding of silicon wafers is a good candidate for wafer level vacuum packaging of vibrating MEMS: in this paper we investigated several e-beam evaporated metallizations stacks including a titanium adhesion layer, an optional diffusion barrier (Ni or Pt) and a gold film for eutectic bonding on Si and SiO₂/Si wafers. Interdiffusion in the multilayers for annealing temperatures (380–430°C) larger than the Au–Si eutectic temperature (363°C) and times corresponding to a bonding process was characterized by RBS, roughness and resistivity measurements. Au/Pt/Ti and Au/Ti/SiO₂ were found to have the best characteristics for bonding. This was confirmed by bonding experiments.     

Gold metallizations for eutectic bonding of silicon wafers

Gold eutectic bonding of silicon wafers is a good candidate for wafer level vacuum packaging of vibrating MEMS: in this paper we investigated several e-beam evaporated metallizations stacks including a titanium adhesion layer, an optional diffusion barrier (Ni or Pt) and a gold film for eutectic bonding on Si and SiO₂/Si wafers. Interdiffusion in the multilayers for annealing temperatures (380–430°C) larger than the Au–Si eutectic temperature (363°C) and times corresponding to a bonding process was characterized by RBS, roughness and resistivity measurements. Au/Pt/Ti and Au/Ti/SiO₂ were found to have the best characteristics for bonding. This was confirmed by bonding experiments.

     

Electrical and morphological characterization of platinum thin-films with various adhesion layers for high temperature applications

In this paper we report on the morphological and electrical properties of platinum (Pt) thin-films with Titanium (Ti) and, alternatively, Titanium dioxide (TiO₂) as adhesion layers for high temperature applications. All films were sputter deposited on silicon substrates and afterwards annealed in air up to 800 °C. The results show that Ti diffuses into Pt grain boundaries forming oxide precipitates (TiO_x) in the Pt grain boundaries. The resistivity of Pt/Ti thin-films increased continuously with annealing temperature up to 500 °C and decreases again continuously above 500 °C. In contrast, TiO₂ demonstrates a dense stable oxide layer after annealing. Pt/TiO₂ thin-films show a continuous decrease in the sheet resistance with increasing the annealing temperature. Accordingly, TiO₂ thin-film is the preferable adhesive layer for Pt over Ti thin-films for high temperature applications.

     

Manufacture of microfluidic glass chips by deep plasma etching, femtosecond laser ablation, and anodic bonding

Two dry subtractive techniques for the fabrication of microchannels in borosilicate glass were investigated, plasma etching and laser ablation. Inductively coupled plasma reactive ion etching was carried out in a fluorine plasma (C₄F₈/O₂) using an electroplated Ni mask. Depth up to 100 μm with a profile angle of 83°–88° and a smooth bottom of the etched structure (Ra below 3 nm) were achieved at an etch rate of 0.9 μm/min. An ultrashort pulse Ti:sapphire laser operating at the wavelength of 800 nm and 5 kHz repetition rate was used for micromachining. Channels of 100 μm width and 140 μm height with a profile angle of 80–85° were obtained in 3 min using an average power of 160 mW and a pulse duration of 120 fs. A novel process for glass–glass anodic bonding using a conductive interlayer of Si/Al/Si has been developed to seal microfluidic components with good optical transparency using a relatively low temperature (350°C).     

固体电解质硫化氢气体传感器的特性研究

采用溶胶-凝胶法制备NASICON(钠离子导体)固体电解质及镍/钛复合氧化物材料。并以NASICON为离子导电层,镍/钛复合氧化物为敏感电极制作固体电解质硫化氢气体传感器。在260-380℃温度范围内,以镍/钛复合氧化物为敏感电极制作的器件对1.10-6～100.10-6硫化氢具有良好的敏感特性。在320℃时器件的灵敏度(斜率)为-72.4mV/decade。并且器件具有良好的选择性、抗湿性及响应恢复特性。器件对5.10-6,50.10-6硫化氢的响应时间为10s,4s和20s,40s。最后对器件的敏感机理做了分析。     

Studies on contact resistance in graphene based devices

Contact resistance is an important limiting factor for the on-state current of graphene based devices. In this paper, both transmission line method and four-probe method are applied to measure the contact resistance in graphene-metal (Cr/Au and Ti/Au) interface. The calculated contact resistivity values by both methods are concentrated at 10⁴ Ωμm². These two methods are compared and four-probe method showed higher stability. At last, the graphene-Ti/Au devices are annealed at 400 °C with argon and hydrogen gas flow. After annealing, the contact resistivity values are reduced to 10³ Ωμm².     

Investigation of the phase equilibria in Ti-Ni-Hf system using diffusion triples and equilibrated alloys

In present work, the phase equilibrium relations in the Ti-Ni-Hf ternary system, which are of great importance for the design of Ti-Ni based high temperature shape memory alloys, were investigated using diffusion triples and sixteen key equilibrated alloys. Based on the experimental results from electron-probe microscopy analysis (EPMA) and X-ray diffraction (XRD) techniques, two isothermal sections were constructed, which consist of 13 and 12 three-phase regions at 900 °C and 800 °C, respectively. Hf can substitute for Ti in TiNi and Ti₂Ni phases increasing from 30, 62 at% at 800 °C to 36, 64 at% at 900 °C, respectively. The Hf₇Ni₁₀ and Hf₉Ni₁₁ phases show wide ternary composition ranges, while the solubility of Ti in HfNi₅, Hf₂Ni₇, and HfNi phases are relatively limited. A new ternary phase of τ was detected for the first time, and the stoichiometry of τ phase is close to Ni:(Hf,Ti) = 11:14, with Ti substituting for Hf from ～5 at% to ～22 at%. The single-phase region of the τ phase became narrow as the decreasing of annealing temperature. Based on comparison of phase relations at 900 °C and 800 °C, it is speculated there is an invariant reaction TiNi + τ → HfNi + Ti₂Ni at between 900 °C and 800 °C.     

Low temperature transient liquid phase bonding of Au/Sn and Cu/Sn electroplated material systems for MEMS wafer-level packaging

This paper presents the development of a low temperature transient liquid phase bonding process for 8″ wafer-level packaging of micro-electro-mechanical systems. Cu/Sn and Au/Sn material systems have been investigated under varying bonding temperatures from 240 to 280 °C and different dwell times from 8 to 30 min. The used bond frame had a width of 80 μm and lateral dimensions of 1.5 mm × 1.55 mm. The sealing frame of the cap wafer consisted of Au and Cu, respectively, and Sn. The MEMS wafer only holds the parent metal of Au or Cu. High quality bonds were confirmed by shear tests, cleavage analysis, polished cross-section analysis using optical and electron microscope, energy dispersive X-ray spectroscopy and pressure cocker test. The samples showed high shear strength (>80 MPa), nearly perfect bond regions and no main failure mode in the cleavage analyses. Non-corroded Cu test structures confirmed the hermeticity.     

Too hot to carry on? Disinclination to persist at a task in a warm office environment

We investigated the effect of an elevated ambient temperature on performance in a persistence task. The task involved the coding of incorrect symbols and participants were free to decide how long to spend performing this task. Applying a between-subject design, we tested 125 students in an office-like environment in one of the three temperature conditions. The comfort condition (Predicted Mean Vote [PMV] = 0.01) featured an average air temperature of 24 °C. The elevated ambient temperature condition was 28 °C (PMV = 1.17). Condition three employed an airstream of approximately 0.8 m/s, intended to compensate for performance decrements at the elevated air temperature (28 °C, PMV = 0.13), according to Fanger’s thermal comfort equation. Participants in the warm condition were significantly less persistent compared with participants in the control and compensation conditions. As predicted by the thermal comfort equation, the airstream seemed to compensate for the higher temperature. Participants’ persistence in the compensation and comfort conditions did not differ.

Practitioner Summary: A laboratory experiment involving a simulated office environment and three ambient temperature conditions (24 °C, 28 °C and 28 °C plus airstream) showed that persistence at a task is significantly impaired at 28 °C. An airstream of 0.8 m/s at 28 °C compensated for the disinclination to persist with the task.     

Multitemporal analysis of land surface temperature using NOAA-AVHRR: preliminary relationships between climatic anomalies and forest fires

Advanced Very High Resolution Radiometer (AVHRR) National Oceanic and Atmospheric Administration (NOAA)-14 imagery was used to analyse changes in land surface temperature in an area of Central Mexico during the course of each dry season (November–April) for the period 1996–2000. Daily surface temperature was obtained by the split-window method and cloud-free monthly composites were subsequently built. This value was related to maximum air temperatures recorded at meteorological stations and to forest fires detected from night-time images. During 1996–1997 and 1997–1998 (El Niño) dry seasons, monthly surface temperature ranged from 35°C to 46°C and from 33°C to 51°C, respectively; during 1998–1999 (La Niña) and 1999–2000 it was lower, ranging from 28°C to 47°C, and from 28°C to 41°C, respectively. At the end of El Niño, land surface temperatures higher than 50°C were registered, and 730 forest fires were detected, suggesting that this temperature increment also contributed to the vulnerability of vegetation to fire. It is concluded that land surface temperature during the first four months of the dry season can be used as a variable for modelling the probability of forest fire occurrence, in combination with other environmental variables. Similarities between land surface temperature and maximum air temperature suggest the potential use of NOAA-AVHRR imagery for evaluating El Niño/La Niña effects on the continental surface.     

Vertically aligned multi-walled CNT arrays coated by gold nanoparticles for surface-enhanced Raman scattering

A unique substrate for surface-enhanced Raman scattering on vertical multi-walled carbon nanotube (MWCNT) arrays coated by Au nanoparticles was reported. The vertically aligned MWCNT arrays were prepared by thermal chemical vapor deposition at temperature of 720 °C, and then coated by gold nanoparticles by sputtering. The possible mechanisms for the SERS sensitivity were discussed. Raman spectroscopy experiments for detecting Rhodamine6G were carried on and some obvious Raman peaks were observed and analyzed.     

Effects of wearing aircrew protective clothing on physiological and cognitive responses under various ambient conditions

Heat stress can be a significant problem for pilots wearing protective clothing during flights, because they provide extra insulation which prevents evaporative heat loss. Heat stress can influence human cognitive activity, which might be critical in the flying situation, requiring efficient and error-free performance. This study investigated the effect of wearing protective clothing under various ambient conditions on physiological and cognitive performance. On several occasions, eight subjects were exposed for 3 h to three different environmental conditions; 0°C at 80% RH, 23°C at 63% RH and 40°C at 19% RH. The subjects were equipped with thermistors, dressed as they normally do for flights (including helmet, two layers of underwear and an uninsulated survival suit). During three separate exposures the subjects carried out two cognitive performance tests (Vigilance test and DG test). Performance was scored as correct, incorrect, missed reaction and reaction time. Skin temperature, deep body temperature, heart rate, oxygen consumption, temperature and humidity inside the clothing, sweat loss, subjective sensation of temperature and thermal comfort were measured. Rises in rectal temperature, skin temperature, heart rate and body water loss indicated a high level of heat stress in the 40°C ambient temperature condition in comparison with 0°C and 23°C. Performance of the DG test was unaffected by ambient temperature. However, the number of incorrect reactions in the Vigilance test was significantly higher at 40°C than at 23°C (p = 0.006) or 0°C (p = 0.03). The effect on Vigilance performance correlated with changes in deep-body temperature, and this is in accordance with earlier studies that have demonstrated that cognitive performance is virtually unaffected unless environmental conditions are sufficient to change deep body temperature.     

MEMS-based microthruster with integrated platinum thin film resistance temperature detector (RTD), heater meander and thermal insulation for operation up to 1,000°C

We have fabricated microthruster chip pairs—one chip with microthruster structures such as injection capillaries, combustion chamber and converging/diverging nozzle machined using the deep reactive ion etching process, the other chip with sputtered platinum (Pt) thin film devices such as resistance temperature detectors (RTDs) and a heater. To our knowledge, this is the first microelectromechanical systems-based microthruster with fully integrated temperature sensors. The effects of anneal up to 1,050°C on the surface morphology of Pt thin films with varied geometry as well as with/without PECVD-SiO₂ coating were investigated in air and N₂ and results will also be presented. It was observed that by reducing the lateral scale of thin films the morphology change can be suppressed and their adhesion on the substrate can be enhanced. Chemical analysis with X-ray photoelectron spectroscopy showed that no diffusion took place between neighboring layers during annealing up to 1?h at 1,050°C in air. Electrical characterization of sensors was carried out between room temperature and 1,000°C with a ramp of ±5?Kmin^?1 in air and N₂. In N₂, the temperature-resistance characteristics of sensors had stabilized to a large extent after the first heating. After stabilization the sensors underwent up to eight further temperature cycles. The maximum drift of the sensor signal was observed for temperatures above 950°C and was less than 8.5?K in N₂. To reduce the loss of combustion heat, chip material around microthruster structures was partially removed with laser ablation. The effects of thermal insulation were investigated with microthruster chip pairs which were clamped together mechanically. The heater was operated with up to 20?W and the temperature distribution in the chip pairs with/without thermal insulation was monitored with seven integrated RTDs. The experiments showed that a thermal insulation allows the maximum temperature as well as the temperature gradient within the microthruster chip pairs to be increased.     

Au–Sn flip-chip solder bump for microelectronic and optoelectronic applications

As an alternative to the time-consuming solder pre-forms and pastes currently used, a co-electroplating method of eutectic Au–Sn alloy was used in this study. Using a co-electroplating process, it was possible to plate the Au–Sn solder directly onto a wafer at or near the eutectic composition from a single solution. Two distinct phases, Au₅Sn (ζ-phase) and AuSn (δ-phase), were deposited at a composition of 30 at.%Sn. The Au–Sn flip-chip joints were formed at 300 and 400°C without using any flux. In the case where the samples were reflowed at 300°C, only an (Au,Ni)₃Sn₂ IMC layer formed at the interface between the Au–Sn solder and Ni UBM. On the other hand, two IMC layers, (Au,Ni)₃Sn₂ and (Au,Ni)₃Sn, were found at the interfaces of the samples reflowed at 400°C. As the reflow time increased, the thickness of the (Au,Ni)₃Sn₂ and (Au,Ni)₃Sn IMC layers formed at the interface increased and the eutectic lamellae in the bulk solder coarsened.     

Tuning of the optical properties of the gold nanocluster ensemble formed in polytetrafluoroethylene film

Gold-filled polytetrafluoroethylene films with various gold concentrations were deposited in vacuum. Gold (Au) nanocluster size is increased with Au concentration elevation. Films were heated up to 300°C. Optical spectra were recorded during heating. The changes in Au plasmon band wavelength and shape during heating are not linearly related with Au concentration and heating temperature. This is caused by different thermal behaviour of the complex processes, which are taking place in each of the two materials presented in the film. The final Au cluster size and optical properties of the whole ensemble can be purposefully produced by varying Au concentration and annealing temperature of the film.     

Experimental study and thermodynamic assessment of the Cu–Ni–Ti system

The Cu–Ni–Ti ternary system has been systematically investigated combining experimental measurements with thermodynamic modeling. With selected equilibrated alloys, the equilibrium phase relations in the Cu–Ni–Ti system at 850 °C were obtained by means of SEM/EDS (Scanning Electron Microscopy/Energy Dispersive Spectrum), EPMA (Electron Probe Micro-Analysis) and XRD (X-ray Diffractometry). Phase transformation temperatures were measured by DSC (Differential Scanning Calorimetry) analysis in order to construct various vertical sections in the Cu–Ni–Ti system. The liquidus projection of the ternary system was determined by the identifying primary crystallization phases in the as-cast alloys and from the liquidus temperatures obtained from the DSC analyses. Based on the available data of the binary systems Cu–Ni, Cu–Ti, Ni–Ti and the ternary system Cu–Ni–Ti from the literature and the present work, thermodynamic modeling of the Cu–Ni–Ti ternary system was performed using the CALculation of PHAse Diagram (CALPHAD) approach. A new set of self-consistent thermodynamic parameters for the Cu–Ni–Ti ternary system was obtained with an overall good agreement between experimental and calculated results.     

A microfluidic gel valve device using reversible sol–gel transition of methyl cellulose for biomedical application

We have fabricated a microfluidic gel valve device that used reversible sol–gel transition of methyl cellulose (MC). A microheater and a microtemperature sensor were implemented in each microchannel in the gel valve device. Before evaluating the performance of the gel valve device, various properties of the MC solution were investigated using viscometer, spectrophotometer, and NMR. Gelation temperature was increased as the MC concentration was increased. Clear gel, an intermediate state between clear sol and turbid gel, was found at the temperature range from 30–40°C to 50–60°C. Temperature at each microchannel of the device was measured and the effect of the temperature difference on the valve operation was elucidated. In order to have normal operation of the gel valve, it was important to keep the temperature of the heated microchannel around 60°C while keeping the temperature of the flowing microchannel below 35°C. The temperature difference between two microchannels was about 23 K when fan forced cooling (FFC) method was used. For normal performance of the gel valve device, a temporary pause of fluid flow for at least 5 s was required to complete the local gelation in the microchannel. Stable gel valve performance was obtained at the flow rates larger than 5 μl/min. The gel valve device showed no leakage up to 2.07×10⁴ Pa.     

Micro-hotplate based temperature stabilization system for CMOS SAW resonators

Due to the sensitivity of the piezoelectric layer in surface acoustic wave (SAW) resonators to temperature, a method of achieving device stability as a function of temperature is required. This work presents two methods of temperature control for CMOS SAW resonators using embedded polysilicon heaters. The first approach employs the oven control temperature stabilization scheme. Using this approach, the device’s temperature is elevated using on-chip heaters to T_max = 42°C to maintain constant device temperature. Both DC and RF measurements of the heater together with the resonator were conducted. Experimental results have indicated that the TCF of the CMOS SAW resonator of −97.2 ppm/°C has been reduced to −23.19 ppm/°C when heated to 42°C. The second scheme uses a feedback control circuit to switch the on-chip heaters on and off depending on the ambient temperature. This method provided reduction of the TCF from −165.38 ppm/°C, to −93.33 ppm/°C. Comparison of both methods was also provided.