X. F. Ang

Self-assembled monolayers for reduced temperature direct metal thermocompression bonding

A reduction in the bonding temperature required for direct gold-gold (Au–Au) thermocompression bonding is observed by coating a monolayer of dodecanethiol on gold surfaces prior to bonding. The results show that Au–Au bonded joints of 26.9g per bump strength can be achieved at a temperature as low as 160°C. The temperature drop becomes more apparent when both coated and blank gold samples are exposed in air over some time before bonding. The authors propose that self-assembled monolayers passivate metal surface by obviating the adsorption of surface contaminants, in particular, carbon and oxygen but get desorbed just before bonding takes place.

Critical temperatures in thermocompression gold stud bonding.

A study on temperature dependence in gold-gold (Au–Au) thermocompression bonding was performed. Gold studs were bonded to two kinds of surfaces—cofired gold on alumina and electroless nickel covered with immersion Au on silicon. A critical bonding temperature was observed for both substrates. No bonding occurs when the temperature is below this threshold value, whereas bond strength increases with bonding temperature beyond the threshold. This critical temperature can be related to the activation of organic films on the bonding surfaces. Under similar bonding conditions, the critical temperature is lower for a harder substrate than for a softer substrate, primarily because of larger interfacial shear stresses. This is supported by the observation on the interfacial shear stress distribution at the bonding interface based on finite element simulation models of substrates with different hardness.

Effect of chain length on low temperature gold-gold bonding by self assembled monolayers.

The tensile strength of thermocompression gold joints formed with prior surface coatings of alkanethiol self-assembled monolayers (SAMs) depends on the chain length (n) of the SAM. Enhancement of bond strength is most significant at n=6 while no improvement can be achieved using octadecanethiol (n=18). These contrasting behaviors can be interpreted as a consequence of two dominant roles of alkanethiols that govern the bonding phenomenon, namely, the passivation of gold surfaces and the ease of mechanical and/or thermal displacement.

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.

Advanced high density interconnect materials and techniques

The trend in micro/nanosystems is to be lighter, smaller and cheaper. At the same time there is a prodigious push for increasing functionalities. Such demands can only be fulfilled by progressively higher density integrated devices and circuits. With 2D IC reaching its physical limitation soon, 3D IC has attracted tremendous attentions and interests worldwide. For either 2D or 3D IC, the interconnection and packaging will be one of the major challenges for the development and commercialization of micro/nanosystems. Flip chip at chip level and wafer-level have the advantages of having the lowest possible inductance per lead, highest frequency response speed as well as the lowest cross talk and simultaneous switching noise. Challenges arise when interconnection method of flip chip is gradually growing into the mainstream in the integration and packaging industry where the issue of size becomes increasingly critical for interconnection and pitches. Therefore, the development of new interconnection materials and techniques is necessary to meet the ever-stringent requirements of mechanical, thermal and electrical properties of interconnection when the interconnection dimension and pitch size are reduced to very fine scales. Furthermore, such advanced interconnection materials and techniques can also be used to stack 3D IC. In this paper, the development of novel lead-free solder nanocomposites, room to low temperature Cu-Cu and Au-Au bonding, and carbon nanotube interconnection techniques will be reported. The developed micro/nanointerconnection techniques can be easily adopted by the industry to realize high density and multifunctional integration and packaging.

Direct metal to metal bonding for microsystems interconnections and integration

In microsystems integration, microsystem devices with different functions are needed to be electrically connected. To meet higher component needs, more functions are configured within each device which leads to a significant potential in microsystems integration, industrial interests have grown exponentially in search of complementing integration and low temperature direct interconnection bonding technology to bridge this bottleneck. This area is especially profound in flip-chip thermocompression technology and chip design for high-density interconnection, since conventional soldering process may not be able to address the generic limitation of relatively high process temperature, solder bump geometry, under-bump-metallization (UBM) schemes and intermetallic effects. This underlying problem necessitates a low temperature direct metal bonding technique for joining multifunctional microsystems which offers more reliable and higher density interconnections than soldering and the wire bonding techniques. In this paper, Au-Au bonding was identified as potential technique to integrate microsystems. Bonding parameters and bond quality were deliberated in multifactorial experiments to determine optimum loading and temperature. Bonding mechanisms and reliability were established with tensile and shear test evaluation. The results show that Au-Au bonding can be achieved at temperatures only above a threshold value. Results for Au-Au bonding exhibit a critical temperature beyond which no bonding can take place. Above the critical temperature, tensile strength of the Au-Au joint reaches a maximum with increase in bonding pressure

Pressure dependence in peripheral bonding in gold-gold thermocompression

A study is performed to evaluate the peripheral bonding exhibited during bond formation for both gold studs and plated gold bump joints. Temperature and pressure are two key parameters determining the quality of thermocompression metal joints. While it has been reported that temperature plays a significant role in inducing the onset of bonding, bonding pressure is required to provide sufficient amount of interfacial deformation to reduce asperities and mechanically remove barrier films. This paper focuses on analyzing the role of bonding pressure in peripheral bond formation found in gold-gold joints despite differences in bump geometries. Using a finite element model of a single metal-metal joint representing gold stud and plated bump respectively, applying higher pressure to the model showed higher shear stress at the bonding interface and this stress remains highest at the periphery. It represents the inwardly radial progression of bonding from the perimeter, consistent with experimental observations for both gold studs and plated bumps. These results can be combined into a compression bonding model where the normal pressure applied during bonding induces an interfacial shear stress which translates into a corresponding lateral strain (equivalent to sliding or displacement) between the two contacting surfaces that are allowed to slide, producing bonds. The highest amount of interfacial shear stress occurs at the outermost region of the bump and when this exceeds a critical value, bonding will initiate from these regions and extend inwards. These findings could facilitate the optimization of thermocompression parameters, especially crucial for fragile components and fine geometries.

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

Low Temperature Direct Metal Bonding by Self Assembled Monolayers

Elevated bonding temperature for interconnection deteriorates the reliability of both the device and the interconnect; hence the imperative for developing low temperature bonding methods. This study investigates the feasibility of using self-assembled monolayers (SAMs) to assist direct gold-gold bonding. This involves a simple molecular self-assembly process whereby a monolayer of alkyl chains with a sulfur end group is attached to the gold surface prior to thermocompression bonding. Using this method, we have achieved gold to gold bonding at a bonding temperature below 100°C, a significant reduction compared to the conventional bonding temperatures of above 150 °C. We attribute this temperature reduction to two properties of SAMs - (1) surface passivation of the Au surface that precludes adsorption of surface contaminants, and (2) The easy displacement of SAMs through thermal desorption just before bonding occurs. This SAMs-assisted bonding mechanism is supported by X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) results.

Studies of Temperature and Pressure Dependence in Thermocompression Gold Joints

Low temperature interconnection is a critical component of 3D integration and packaging technology. In this study, we investigate the characteristics of thermocompression metal bonding using gold stud bumps formed on Si die in the temperature range of 100-300 °C and the pressure range of 200–600 g/bump. We observed a critical bonding temperature below which bonding did not occur and above which shear strength improves linearly with bonding temperature. This critical temperature can be interpreted to be the onset of the break-up of organic barrier films while the linear rise in shear strength can be attributed to the increase in the true bonded area. Above this critical temperature, the tensile strength of the Au-Au bond exhibits a maximum with increasing bonding pressure. This can be related to the pressure dependence of the interfacial stress distribution and its effect on unbonded radius, r. SEM fractographs of the failed surfaces suggest a combination of cohesive and adhesive failures along the bonded interface.Copyright