Solder bonding with a buffer layer for MOEMS packaging using induction heating

In this paper, the selective induction heating technology is applied to glass–glass and glass–silicon solder bonding for MOEMS (optical MEMS) packaging. The Ni bumping with a buffer layer is successful to release the thermal stress for avoiding delamination. The Au wetting layer must be thick enough to prevent from being solved entirely into Sn, and it will improve bonding strength. The bonding specimens are soaked into 25°C water and placed into 85°C/85% RH oven, respectively. No moisture penetrates into the cavity after 1 day in both test conditions. In the test condition of 125°C leakage-test liquid (Galden HS260), no bubble is observed. The lowest bonding strength is 3 MPa.     

A simple thermo-compression bonding setup for wire bonding interconnection in pressure sensor silicon chip packaging

In the process of piezo-resistive pressure sensor packaging, a simple thermo-compression bonding setup has been fabricated to achieve the wire bonding interconnection of a silicon chip with printed circuit board. An annealed gold wire is joined onto a pad surface with a needle-like chisel under a force of 0.5–1.5 N/point. The temperature of the substrate was maintained in the range of 150–200°C and the temperature of the chisel was fixed at around 150°C during wire bonding operation. The tensile strength of the wire bonding was measured with a bonding tester by the destructive-pulling experiment and was found to be at the average of 132 mN/mm². The microstructure of the bonding point was examined by scanning electron microscopy. The interface of the thermo-compression boning was shown to possess an acceptable level of reliability for a micro-electromechanical system (MEMS)-based device. The results showed that this setup can be easily operated for fabrication and is suitable for fabricating not only low-cost pressure sensors, but also other MEMS devices.

     

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).     

Effect of bonding temperature on hermetic seal and mechanical support of wafer-level Cu-to-Cu thermo-compression bonding for 3D integration

Hermetic seal and mechanical support of wafer-level Cu-to-Cu thermo-compression bonding with different bonding temperature are analyzed in this work. The investigation consists of two parts: hermetic seal study using helium bomb test and mechanical support study using four-point bending method. The wafer pairs are bonded at 250, 300 and 350 °C, respectively, under a bonding force of 5,500 N for a duration of 1 h in vacuum (~2.5 × 10^?4 mbar). The bonding medium consists of Cu (300 nm) bonding layer and Ti (50 nm) barrier layer. Excellent helium leak rate, which is smaller than the reject limit defined by MIL-STD-883E standard (method 1014.10), and outstanding interfacial adhesion energy are detected for all samples. The cavities sealed at 300 °C present an excellent reliability of temperature cycling test up to 500 cycles. Cu-to-Cu thermo-compression bonding at low temperature (≤300 °C) presents an attractive hermetic seal and a robust mechanical support for 3D integration application.     

Silicon–glass wafer bonding with silicon hydrophilic fusion bonding technology

Silicon–glass wafer bonding is realized with silicon hydrophilic fusion bonding technology. Tensile strength testing shows that the bonding strength is large enough for most applications of integrated circuits and transducers. The bonding strengths of 4 in. 525 μm thick #7740 glass–4 in. 525 μm thick silicon and of 1.5 in. 1000 μm thick #7740 glass–2 in. 380 μm thick silicon are larger than 9 MPa both with an annealing temperature of 450°C.     

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.

     

CO2-assisted thermal fusion bonding of heterogeneous materials by use of surface nano-pillars

Microchip components may involve different polymeric materials for integrated functions, and the thermal bonding of heterogeneous materials is a challenge. This study is devoted to the bonding of polycarbonate (PC) and polymethacrylate (PMMA) materials. For conventional thermal bonding of PC and PMMA, the temperature must be above 150 °C. CO₂ is used as a plasticizer to lower the bonding temperature to 90 °C. CO₂ also serves as a pressurizing agent to provide uniform bonding pressure. To further improve the bonding strength, surface nano-pillar structures are fabricated on PC before binding it with PMMA. Because of the nano-pillar structures, the contact areas significantly increased, and the structural inter-lock further increased the strength after bonding thereby yielding a bonding strength of 1.20 MPa.     

PMMA to Polystyrene bonding for polymer based microfluidic systems

A thermal bonding technique for Poly (methylmethacrylate) (PMMA) to Polystyrene (PS) is presented in this paper. The PMMA to PS bonding was achieved using a thermocompression method, and the bonding strength was carefully characterized. The bonding temperature ranged from 110 to 125 °C with a varying compression force, from 700 to 1,000 N (0.36–0.51 MPa). After the bonding process, two kinds of adhesion quantification methods were used to measure the bonding strength: the double cantilever beam method and the tensile stress method. The results show that the bonding strength increases with a rising bonding temperature and bonding force. The results also indicate that the bonding strength is independent of bonding time. A deep-UV surface treatment method was also provided in this paper to lower the bonding temperature and compression force. Finally, a PMMA to PS bonded microfluidic device was fabricated successfully.     

Water glass bonding

The author developed a low-temperature (80°C), low-external loads (electric field, magnetic field, load, etc.) bonding technology using water glass which is used in making molds (silica sand). The adhesive area ratio attained by this bonding technology was more than 95% and the bonding strength was about 290 kgf/cm². As this water glass bonding technology is applicable at comparatively low-temperatures, the residual stress of the bond is so small as to be able to bond eight 4-inch wafers together in the 3rd dimensional direction. In addition, as the thickness of a bonding layer is as small as several nanometers, the precision bonding of a tolerance of ±3 μm was also possible through alignment, and we could successfully fixed a cap with having a bonding seal of 0.32 mm wide in the vacuum. The leak rate was less than the detection limit of the He leak detector (1×10⁻¹⁰ Pa m³/s), showing excellent air-tightness. Using this bonding technology, the author made a self-package type IR microsensor on an experimental basis, and carried out an accelerated environmental test. As a result, its MTTF (mean time to failure) was estimated to be 6 years.     

Low-temperature quartz wafer bonding using hyperbranched polyurethane oligomers

In this paper, we introduce a new bonding technology for the assembly of micro- structured glass substrates for miniaturized chemical analysis. The protocol features a facile polymer chemistry method processing at lower temperatures (<100 °C). The method consisted of a proper cleaning of the two glass surfaces, followed by hydroxylization, aminosilylation and hyperbranched polyurethane oligomers (HPU) bridging on quartz wafer surfaces as the interlayer. Strong bonding with a shear force 4.5 MPa has been achieved. The present procedure avoids the possible micro-channel blockage and contamination by using conventional adhesives. Moreover, the microfluidic chips bonded by the above procedures are highly transparent therefore allowing for biochemical compositions to be easily characterized by UV–vis or IR spectroscopy.     

Bonding of planar poly (methyl methacrylate) (PMMA) nanofluidic channels using thermal assisted ultrasonic bonding method

Because of extremely small dimensions of nanochannels and low rigidity of polymer, most bonding techniques which are suitable for sealing polymer microscale channels (width and depth in tens to hundreds microns) are not competent for bonding of polymer nanochannels. In this study, a new thermal assisted ultrasonic bonding method for sealing poly (methyl methacrylate) (PMMA) nanochannels was presented. Substrates were preheated to 30–40°C lower than glass transition temperature (T _g ) of the material by hot plate. Then low amplitude ultrasonic vibration was employed to generate facial heat at the interface of the substrates. Influences of preheating temperature on bonding strength and dimension loss were studied. The nanochannels were successfully bonded with depth loss less than 5.3% (10.6 nm) and bonding strength of 0.21 J/cm² at the preheating temperature of 70°C. Thermal assisted ultrasonic bonding is proven to be competent for bonding of polymer nanochannels with high bonding strength, low dimension loss and short bonding time.     

Low temperature carbon nanotube and hexagonal diamond deposition with photo-enhanced chemical vapor deposition

Deposition of carbon nanotube and hexagonal diamond thin films at low substrate temperature with photo-enhanced chemical vapor deposition is described here. Extensive experimentation is conducted to optimize the catalyst layer utilized for deposition by varying Al/Ni/Al metal layer thicknesses on SiO₂ coated Si substrates. The coated substrates are annealed to transform the thin metal layers into nanoparticles. Suitable catalyst layer thicknesses obtained are 3/2/3, 5/1/5 and 5/3/5 nm for Al/Ni/Al sandwich metal layers. Suitable annealing conditions are in the range of 350–450 °C for substrate temperature and from 0.22 to 10 Torr for chamber pressure in ammonia ambient for 25 min. Carbon tetrachloride (CCl₄) is used as a carbon precursor in this work. Argon to CCl₄ flow ratio is varied in 1.5–19 range, chamber pressure is varied in 3–10 Torr range, and the substrate temperature is varied in 350–450 °C range. Carbon nanotubes (CNT) growth is observed at lower chamber pressure, lower partial pressure of CCl₄, lower substrate temperature and for thin Ni catalyst layer. The optimal CNT deposition condition observed is 5 Torr total chamber pressure, 9:1 partial pressure ratio of Ar to CCl₄, 400 °C substrate temperature and 5/1/5 nm thick Al/Ni/Al catalyst layers. The hexagonal diamond deposition is observed at a higher chamber pressure, higher partial pressure of CCl₄, higher substrate temperature and for a thicker Ni catalyst layer. The optimal condition for hexagonal diamond deposition observed is 10 Torr total chamber pressure, 7:3 partial pressure ratio of Ar to CCl₄, 450 °C substrate temperature and 5/3/5 nm thick Al/Ni/Al catalyst sandwich layers.     

Research on low-temperature anodic bonding using induction heating

A novel low-temperature anodic bonding process using induction heating is presented in this paper. Anodic bonding between silicon and glass (Pyrex 7740) has been achieved at temperature below 300 °C and almost bubble-free interfaces have been obtained. A 1 kW 400 kHz power supply is used to induce heat in graphite susceptors (simultaneously as the high-voltage electrodes of anodic bonding), which conduct heat to the bonding pair and permanently join the pair in 5 min. The results of pull tests indicate a bonding strength of above 5.0 MPa for induction heating, which is greater than the strength for resistive heating at the same temperature. The fracture mainly occurs inside the glass or across the interface other than in the interface when the bonding temperature is over 200 °C. Finally, the interfaces are examined and analyzed by scanning electron microscopy (SEM) and the bonding mechanisms are discussed.     

Fabrication of chamber for piezo inkjet printhead by SU-8 photolithography technology and bonding method

The chamber is an important part of the inkjet printhead. However, the present fabrication methods of chamber suffer from a low alignment resolution between nozzle plates and piezoelectric structure and residual SU-8 removing problems during chamber fabricating process. In this paper, a SU-8 chamber was fabricated by using ultraviolet (UV) photolithography and SU-8 thermal bonding method. By this method, the infilling problem of the chamber during thermal bonding process was solved, and low alignment resolution problem of conventional UV exposure system during assembly process was avoided. The thickness of the SU-8 nozzle plate was optimized, and the influence of bonding parameters on the deformation of chamber was analyzed. The simulation results show that the optimal thickness of the SU-8 nozzle plate is 40 μm and the optimal bonding parameters are bonding temperature of 50 °C, bonding pressure of 160 kPa and bonding time of 6 min. The tensile test results show the bonding strength of the SU-8 chamber is 2.1 MPa by using the optimized bonding parameter.     

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.     

Low-temperature anodic bonding of silicon to silicon wafers by means of intermediate glass layers

Silicon wafers have been anodically bonded to sputtered lithium borosilicate glass layers (Itb 1060) at temperatures as low as 150–180 °C and to sputtered Corning 7740 glass layers at 400 °C. Dependent on the thickness of the glass layer and the sputtering rate, the sputtered glass layers incorporate compressive stresses which cause the wafer to bow. As a result of this bowing, no anodic bond can be established especially along the edges of the silicon wafer. Successful anodic bonding not only requires plane surfaces, but also is determined very much by the alkali concentration in the glass layer. The concentration of alkali ions as measured by EDX and SNMS depends on both the sputtering rate and the oxygen fraction in the argon process gas. In Itb 1060 layers produced at a sputtering rate of 0.2 nm/s, and in Corning 7740 layers produced at sputtering rates of 0.03 and 0.5 nm/s, respectively, the concentration of alkali ions in the glass layers was sufficiently high, at oxygen partial pressures below 10^-4 Pa, to achieve anodic bonding. High-frequency ultrasonic microanalysis allowed the bonding area to be examined non-destructively. Tensile strengths between 4 and 14 MPa were measured in subsequent destructive tensile tests of single-bonded specimens.     

Aluminum–germanium wafer bonding of (AlGaIn)N thin-film light-emitting diodes

Eutectic aluminum–germanium wafer bonding was used to fabricate (AlGaIn)N thin-film light-emitting diodes (LEDs). Wafer bonding was carried out on 2″ wafer level at a bond temperature of 470 °C using patterned Al bond pads on the GaN-on-sapphire LED epiwafer and plain Ge substrates. The microstructure of the joint formation was characterized via cross-section analysis using scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy was used to investigate the bond interface. The shear strength was determined to be 1–2 kN/cm². The formation of a liquid Al–Ge phase is evident from cross-section analysis and optical microscopy. During solidification, Al and Ge are separated into distinct phases again, which is revealed by EDX. The obtained bond is not free of micro-voids, yet it is mechanically stable and suited for the fabrication of thin-film LEDs by removing the sapphire substrate via laser lift-off, which is also demonstrated.     

Taguchi optimization approach for the polypropylene fiber reinforced concrete strengthening with polymer after high temperature

In this study, the strengthening with polymer the polypropylene fiber reinforced concrete exposed to high temperature was examined. Taguchi L₉ (3³) orthogonal array was used for the design of experiments. Three different parameters were used in the study; polypropylene fiber percentage (0 %, 1 % and 2 %), high temperature degree (300 °C, 600 °C and 900 °C) and curing period (3, 7 and 28 days). Cube samples of 100x100x100 mm sizes were produced for the compressive strength and ultrasonic pulse velocity tests. The samples were removed from the water and dried at 105?±?5 °C, and then they were exposed to temperatures of 300 °C, 600 °C and 900 °C. Then, the polymerization of monomer and the vinyl acetate monomer impregnation on the samples were carried out. The compressive strength and ultrasonic pulse velocity tests were made. Taguchi analysis showed that the largest compressive strength and ultrasonic pulse velocity were obtained at a rate of 0 % from the samples with polypropylene fiber exposed to 600 °C and kept for 28 days as cure period. It was determined as the result of Anova analysis that high temperature had made biggest effect on the compressive strength and ultrasonic pulse velocity of the concrete reinforced with polymer.     

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.     

Bimodal variation of SST and related physical processes over the North Indian Ocean: special emphasis on satellite observations

Using sea surface temperature (SST) and wind speed retrieved by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), for the period of 1998–2003, we have studied the annual cycle of SST and confirmed the bimodal distribution of SST over the north Indian Ocean. Detailed analysis of SST revealed that the summer monsoon cooling (winter cooling) over the eastern Arabian Sea (Bay of Bengal) is more prominent than winter cooling (summer monsoon cooling). A sudden drop in surface short wave radiation by 57 W m^?2 (74 W m^?2) and rise in kinetic energy per unit mass by 24 J kg^?1 (26 J kg^?1) over the eastern Arabian Sea (Bay of Bengal) is observed in summer monsoon cooling period. The subsurface profiles of temperature and density for the spring warming and summer monsoon cooling phases are studied using the Arabian Sea Monsoon Experiment (ARMEX) data. These data indicate a shallow mixed layer during the spring warming and a deeper mixed layer during the summer monsoon cooling. Deepening of the mixed layer by 30 to 40 m with corresponding cooling of 2°C is found from warming to summer monsoon cooling in the eastern Arabian Sea. The depth of the 28°C isotherm in the eastern Arabian Sea during the spring warming is 80 m and during summer monsoon cooling it is about 60 m, while over the Bay of Bengal the 28°C isotherm is very shallow (35 m), even during the summer monsoon cooling. The time series of the isothermal layer depth and mixed layer depth during the warming phase revealed that the formation of the barrier layer in the spring warming phase and the absence of such layers during the summer cooling over the Arabian Sea. However, the barrier layer does exist over the Bay of Bengal with significant magnitude (20–25 m). The drop in the heat content with in first 50 m of the ocean from warming to the cooling phase is about 2.15 × 10⁸ J m^?2 over the Arabian Sea.