C. C. Wong

Low Temperature Copper-Copper Thermocompression Bonding

Successful direct copper thermocompression bonding was demonstrated at room temperature under ambient environment, yielding shear strength of 21 MPa. Studies on the effect of bonding temperature on the copper joint shear strength revealed a unique phenomenon at the low temperature regime (~80degC -140degC) whereby bond integrity degrades with increasing temperature. Beyond 140degC, direct relationship between temperature and joint shear strength was observed. A hypothesis on the bonding mechanism between copper surfaces is proposed to explain the anomalous bonding behaviour with temperature.

Detachment Dynamics of Cancer Cells

Cell adhesion and detachment are crucial components in cancer spreading, often leading to recurrence and patient death [1]. Probing the mechanical behavior at the whole cell level while the cell is undergoing spreading and detachment during would enhance our understanding on cancer metastasis. However, these processes are not well understood in a quantitative sense, especially for the cancer cells [2]. In this article, we propose a biohybrid micro-device for the investigation of cellular attachment and detachment dynamics. This device comprises of silicon nanowires as electromechanical strain sensors, embedded in a suspended doubly-clamped silicon dioxide (SiO2) microbridge (Fig. 1A & Fig. 2A) for breast cancer (MCF-7) cells seeding and attachment (Fig. 1B).

Stability of Self-Assembled Monolayers on Gold for MEMS/NEMS Applications

Stability of self-assembled monolayers on gold under various environmental conditions is a crucial component in many biological, chemical and mechanical surface-functionalizations. In this study, we investigate the effects of relative humidity, ambient conditions (air, nitrogen-purged) and temperature on the structural stability of alkanethiols on gold at different chain length using contact angle measurements and time-of-flight secondary ions mass spectroscopy (TOF-SIMS). The ability of self-assembled monolayers functioning under these conditions is critical in protecting gold metal surfaces especially, from surface contamination. This in turn, affects the bonding conditions required in wafer level bonding process which is a key fabrication step in microelectromechanical (MEMs) and nanoelectromechanical (NEMs) systems. Such findings are particularly important in bioMEMs or bioNEMs since gold is one of the most common microfabrication material used in MEMs drug delivery devices due to its superior biocompatibility and reduced biofouling.Copyright

Factors Affecting the Mechanical Properties of Cu/Electroless Ni-P/Sn-3.5Ag Solder Joints

This work investigates the factors that affect the mechanical properties of Cu/electroless Ni-P/Sn-3.5Ag solder joints. For the investigation, solder joints were tensile tested after solid-state aging at different temperatures for various durations. Several factors, such as the growth of interfacial compounds (IFCs), Ni 3 Sn 4 morphology, the accumulation of spalled Ni 3 Sn 4 intermetallic particles at the solder/Ni 3 Sn 4 interface, and the formation of Kirkendall voids at the Ni 3 P/Cu interface, are found to deteriorate the mechanical properties of the joints. Among all these factors, the formation of a layer of Kirkendall voids at the Ni 3 P/Cu interface, which is a result of Cu diffusion from the interface, causes the most severe decrease in tensile strength with a brittle fracture at the Ni 3 P/Cu interface. This layer of Kirkendall voids remains the main cause of brittle failure even after the transformation of the Ni 3 P layer into a Ni-Sn-P layer.

Electric Current Induced Brittle Failure of Eutectic Lead and Lead-free Solder Joints with Electroless Ni-P Metallization

The mechanical properties of thermally-aged and electric current-stressed eutectic lead (Sn-37Pb) and lead-free (Sn-3.5Ag) solder joints with electroless Ni-P metallization were investigated using tensile testing. Multi-layered test samples, electroless Ni-P/solder/electroless Ni-P, having two electroless Ni-P/solder interfaces were prepared. Tensile testing results showed that for both types of solder, high density electric current causes the brittle failure of solder joint. The eutectic lead solder joint was found to be more prone to current induced brittle failure compared to the lead-free solder joint. In the eutectic lead solder joint, brittle failure always occurred at the cathode side electroless Ni-P/Sn-37Pb interface (where electrons flowed from Ni-P to solder), whereas no such polarity effect was observed in the case of lead-free solder joint.

Optimum pressure in low temperature direct metal bonding

The influence of bonding pressure during low temperature direct metal bond formation between gold studs and electroplated copper joints respectively was evaluated. The findings obtained showed an optimum pressure where bond strength peaks before degrading at further increase in bond pressure. Furthermore, it was found that the increase in bond strength does not correspond to a relative dilation of the bonded area. Both observations deviate from the common understanding that higher bond strength can be achieved with higher bonding pressure and corresponding increase in bonded area. Such contradiction is possibly related to the interfacial deformation phenomenon between locally bonded sites at the interface rather than the bulk deformation behaviour of the bump when bonding pressure exceeds a critical value.

Organic Monolayers for Low Temperature Copper – Gold Bonding

In this study low temperature copper-gold bonding in ambient is demonstrated successfully with the help of organic monolayers. Further investigation is made to evaluate the influence of chain length of these monolayers to alleviate bonding temperature. We found that a single coat of organic monolayer on copper showed superior bond strength as compared to a double-coating on both copper and gold surfaces respectively. Despite effective surface passivation against oxide formation (Cu) and organic contaminant from ambient (Au) by long chain organic monolayers, when both surfaces are coated with the longest monolayer, weaker bond strength was observed. The results suggest that the ease of removal of organic monolayers from Cu and Au surface respectively prior to bonding plays a dominant role in enabling low temperature Cu-Au bonding.Copyright

Extracting Piezoresistance in SiNWs using Thermal Induced Buckling of Micro-bridges

The recent reports on giant piezoresistance effect in highly resistive silicon nanowires (SiNWs) have offer greater sensitivity in stress measurements. Despite enhanced sensitivity, the piezoresistance of highly conductive silicon are preferred as they are less prone to thermal noises and hence better accuracy. Here we report a thermal induced buckle micro-bridge technique to accurately characterize the temperature dependent piezoresistivity effect in SiNWs. Phosphorus doped SiNWs with 50 nm width, 95 nm thickness and 100 μm length were encapsulated within SiO 2 micro-bridges. The electrical measurement of both reference SiNWs and SiNWs at micro-bridges was carried out, followed by the optical profiling of the micro-bridges with embedded SiNWs. N -type SiNWs with doping of 1×10 20 ion/cm 3 exhibit a strong dependence on temperature with a piezoresistive coefficient that decreases by 22.5 % between 25 o C to 60 o C; whereas its bulk counterpart is independent of temperature across this range. The results demonstrated that thermal noises may be more detrimental to nano-scale electromechanical sensors than its bulk counterparts.

Organic Monolayers for Room Temperature Copper Bonding

Wafer level bonding process is a key fabrication step in several integration systems including microelectromechanical (MEMs) and nanoelectromechanical (NEMs). Often, harsh bonding conditions used result in large thermomechanical stresses built-up which leads to undesired degradation in device performance. Our recent study revealed the capability of nanostructured organic coatings (NSOCs) in reducing the bonding temperature needed to bond copper surfaces from 300°C to 60°C. In this study, room temperature copper bonding is demonstrated successfully with the help of the organic layers. Further investigation is made to evaluate the influence of thickness of NSOCs to alleviate bonding temperature. We found that all NSOCs (1, 2, 3) showed superior bond strength (>25MPa) as compared to that of the uncoated copper (<23MPa) at bonding temperatures from 25°C to 80°C. Since it is imperative that surface oxide has to be removed for bonding to take place, the enhancement exhibited is attributed to an effective surface passivation by the organic layer. It is postulated that this ultrathin layer, which behaves as a milder layer as compared to the bulk oxide layer, can be easily displaced for bond formation.Copyright

Evolution of Microstructure and Mechanical Properties of Eutectic Sn-Pb Solder Joint Aged under Thermal Gradient

Thermal gradients exist in solder joints used in high power device packages due to high device temperature and low substrate temperature. This paper presents the effect of thermal gradient on the microstructure and mechanical properties of eutectic Sn-37Pb solder joint. The solder joint comprises a multilayered Cu/electroless Ni-P/Sn-37Pb/electroless Ni-P/Cu material system. Thermal gradients ranging from 1.3 times 103 to 1.5 times 103degC/cm were imposed on the solder joint by heating its one end and cooling the other. The thermal gradient in the solder joint has been found to influence the size distribution of Sn-rich and Pb-rich phases, solder composition, and fracture behavior. An explanation of these thermal gradient induced phenomena is provided.