Environmental Stress Testing of Wafer-Level Al-Al Thermocompression Bonds: Strength and Hermeticity

Hermeticity, reliability and strength of Al-Al thermocompression bonds realized by applying different bonding parameters have been investigated. Laminates of diameter 150 mm were realized by bonding wafers containing membrane structures to wafers with patterned bonding frames. The laminates were bonded applying a bond force of 36 or 60 kN at temperatures ranging from 300 to 400°C for 15, 30 or 60 minutes. The hermetic properties were estimated by membrane deflection measurements with white-light interferometry after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Bond strength was measured by shear test and pull tests. Laminates bonded applying a bond force of 60 kN at temperatures of 350 or 400°C resulted in hermetic bonds. No significant change in membrane deflection was observed after the steady-state life test or the thermal shock test. However, a gross leakage was observed in 1–11% of the dies after exposure to the moisture resistance test. The maximum leakage rate (MLR) estimated from membrane deflection measurements was below 10−11 mbar·l·s−1 for all laminates. The measured average bond strength of dies from selected laminates ranged from 28 to 190 MPa.acceptedVersio

Impact of SiO2 on Al–Al thermocompression wafer bonding

Al–Al thermocompression bonding suitable for wafer level sealing of MEMS devices has been investigated. This paper presents a comparison of thermocompression bonding of Al films deposited on Si with and without a thermal oxide (SiO2 film). Laminates of diameter 150 mm containing device sealing frames of width 200 µm were realized. The wafers were bonded by applying a bond force of 36 or 60 kN at bonding temperatures ranging from 300–550 °C for bonding times of 15, 30 or 60 min. The effects of these process variations on the quality of the bonded laminates have been studied. The bond quality was estimated by measurements of dicing yield, tensile strength, amount of cohesive fracture in Si and interfacial characterization. The mean bond strength of the tested structures ranged from 18–61 MPa. The laminates with an SiO2 film had higher dicing yield and bond strength than the laminates without SiO2 for a 400 °C bonding temperature. The bond strength increased with increasing bonding temperature and bond force. The laminates bonded for 30 and 60 min at 400 °C and 60 kN had similar bond strength and amount of cohesive fracture in the bulk silicon, while the laminates bonded for 15 min had significantly lower bond strength and amount of cohesive fracture in the bulk silicon.acceptedVersio

Al-Al thermocompression bonding for wafer-level MEMS sealing

Al–Al thermocompression bonding has been studied using test structures relevant for wafer level sealing of MEMS devices. Si wafers with protruding frame structures were bonded to planar Si wafers, both covered with a sputtered Al film of 1 μm thickness. The varied bonding process variables were the bonding temperature (400, 450 and 550 °C) and the bonding force (18, 36 and 60 kN). Frame widths 100 μm, 200 μm, with rounded or sharp frame corners were used. After bonding, laminates were diced into single chips and pull tested. The effect of process and design parameters was studied systematically with respect to dicing yield, bond strength and resulting fractured surfaces. The test structures showed an average strength of 20–50 MPa for bonding at or above 450 °C for all three bonding forces or bonding at 400 °C with 60 kN bond force. The current study indicates that strong AlAl thermocompression bonds can be achieved either at or above 450 °C bonding temperature for low (18 kN) and medium (36 kN) bond force or by high bond force (60 kN) at 400 °C. The results show that an increased bond force is required to compensate for a reduced bonding temperature for AlAl thermocompression bonding in the studied temperature regimeacceptedVersio

Bending machine for testing reliability of flexible electronics

A novel bending machine has been designed and tested. It enables flexible electronics to be subjected to repeated bending with constant radius and tension. In-situ electrical characterization can give accurate analysis of lifetime distributions if sufficiently many samples are ran to failure, allowing reliability prediction models to be developed. Four sets of test samples with different combinations of substrate, routing, interconnect technology and components were examined. A poor level of reliability was observed when using anisotropic conductive paste to form interconnects, whereas a significantly higher level of reliability was observed when using a bismuth-tin solder paste. The assembly of larger components resulted in shortened time to failure, whereas increasing the bending radius prolonged the observed lifetimes.acceptedVersio

3D Integration Technologies for Wireless Sensor Systems (e-CUBES)

The innovative approach presented here will realize so-called e-CUBES® (electronic cubes) [1], i.e. investigate and develop ultra small sensor cubes with dimensions of a few mm3 which are wirelessly communicating among each other. The fabrication of e-CUBES with their need for high-level miniaturization (see Fig. 1) can only be realized by system integration technologies, which use the third dimension: 3D System Integration. In general not only one 3D integration technology is suitable for the fabrication of the large variety of 3D integrated systems. Moreover, a single product may need several different technologies for a cost-effective fabrication. Wireless sensor systems represent an excellent example for the need of a suitable mixture. Consisting of MEMS, ASICs, memories, antennas, and power supply modules they can only be fabricated in a cost-effective way by application of 3D technologies, which are particularly optimized for the integration of the different sub-modules. The main objective is to achieve extreme miniaturization of wireless sensor nodes. In consequence, all wiring should be realized through the stacked devices. Wire bonds are replaced by through-Si vias combined with electrical and mechanical interconnections between the devices. The removal of long wire bonds is especially beneficial for sensors based on a capacitive sensing principle (rather than i.e. a piezo-resistive sensing principle), since parasitic effects will be reduced.3D Integration Technologies for Wireless Sensor Systems (e-CUBES

3D Integration Technologies for Wireless Sensor Systems (e-CUBES)

The innovative approach presented here will realize so-called e-CUBES® (electronic cubes) [1], i.e. investigate and develop ultra small sensor cubes with dimensions of a few mm3 which are wirelessly communicating among each other. The fabrication of e-CUBES with their need for high-level miniaturization (see Fig. 1) can only be realized by system integration technologies, which use the third dimension: 3D System Integration. In general not only one 3D integration technology is suitable for the fabrication of the large variety of 3D integrated systems. Moreover, a single product may need several different technologies for a cost-effective fabrication. Wireless sensor systems represent an excellent example for the need of a suitable mixture. Consisting of MEMS, ASICs, memories, antennas, and power supply modules they can only be fabricated in a cost-effective way by application of 3D technologies, which are particularly optimized for the integration of the different sub-modules. The main objective is to achieve extreme miniaturization of wireless sensor nodes. In consequence, all wiring should be realized through the stacked devices. Wire bonds are replaced by through-Si vias combined with electrical and mechanical interconnections between the devices. The removal of long wire bonds is especially beneficial for sensors based on a capacitive sensing principle (rather than i.e. a piezo-resistive sensing principle), since parasitic effects will be reduced.3D Integration Technologies for Wireless Sensor Systems (e-CUBES

Acoustic/Electronic stack design, interconnect, and assembly. Techniques available and under development

Acoustic/Electronic stack design, interconnect, and assemblyTechniques available and under developmen

Interconnects based on metal coated polymer spheres for improved reliability

3D packaging and heterogeneous integration has revealed new opportunities with regard to where instrumentation can be applied. Extreme miniaturization of systems makes it possible to include sensors and electronics in regions where this was not possible earlier due to size or weight limitations. However, some of these new application areas represent a challenging combination of extreme demands for reliability in rough environments. Interconnects are known to be prone to failures as a result of thermo-mechanical stress and require a specific focus in any instrumentation system. In this presentation we will give three examples of cases where traditional interconnect technologies are replaced with solutions based on metal coated polymer spheres (MPS) in order to improve the reliability. The idea behind the work is that the larger compliance of the polymer core of MPS compared to solid metal will reduce the level of stress imposed on the interconnect during thermal cycling or shock loading. The interconnect challenges in the three cases were to mount a MEMS device onto a PCB, to mount a silicon planar sensor onto an ASIC, and to mount a ceramic carrier onto a PCB. In the first example silver epoxy was replaced with an isotropic conductive adhesive filled with 4-30 µm sized MPS. In the second example microbumps were replaced with an anisotropic conductive film filled with 6 µm sized MPS, and in the last example lead free solid solder BGA balls were replaced with 310 µm sized MPS. Results indicating increased reliability for the devices, especially with regard to thermal cycling and rough mechanical treatment, will be shown. All three technologies based on MPS are expected to represent valuable alternatives within several 3D packaging solutions, although 3D packing in itself is not the primary scope for the interconnect technology development at present

TSV development for miniaturized MEMS acceleration switch

Fragile micromachined MEMS structures are usually protected by bonding a capping wafer to the device wafer itself. As opposed to using lateral interconnects at the interface between the cap wafer and the device wafer, the use of vertical through silicon vias (TSVs) significantly simplifies the mounting of the components and it also results in the smallest footprint. This paper presents the concept chosen for fabricating a miniaturized MEMS acceleration switch with TSVs through the SOI (silicon on insulator) device wafer, as well as the experimental results of the TSV process development that was done for this particular application. Especially challenging was the development of an etching process that can etch the thick buried oxide of the SOI wafer through high aspect ratio trenches