Self‐Assembly Process to Integrate and Connect Semiconductor Dies on Surfaces with Single‐Angular Orientation and Contact‐Pad Registration

Abstract

The construction of man-made artifacts such as cell phones and computers relies on robotic assembly lines that place, package, and interconnect a variety of devices that have macroscopic (> 1 mm) dimensions. The key to the realization of these systems is our ability to integrate/assemble components in 2D/3D as well as link/interconnect the components to transport materials, energy, and information. The majority of these systems that are on the market today are heterogeneous in nature. Heterogeneous systems can be characterized as systems that contain at least two separate parts that prohibit monolithic integration. Such systems are typically fabricated using robotic pick and place. The size of the existing systems could be reduced by orders of magnitudes if microscopic building blocks could be assembled and interconnected effectively. The difficulty is not the fabrication of smaller parts but the assembly and formation of interconnects. For components with dimensions less than 100 lm, adhesive capillary forces often dominate gravitational forces, making it difficult to release the components from a robotic manipulator. Micromanipulator-based assembly and wafer-to-wafer transfer methods work poorly on non-planar surfaces, in cavities, and in the fabrication of 3D systems. Serial processes, in general, are slow. Conventional robotic methods and assembly lines are challenged because of the difficulty in building machines that can economically manipulate parts in three dimensions that are only micrometers in size. At another extreme, nature forms materials, structures, and living systems by self-assembly on a molecular length scale. As a result, self-assembly-based fabrication strategies are widely recognized as inevitable tools in nanotechnology and an increasing number of studies are being carried out to “scale up” these concepts to close the assembly gap between nanoscopic and macroscopic systems. Recent demonstrations of processes that can assemble micrometerto millimeter-sized components include: shape-directed fluidic methods that position electronic devices on planar surfaces using shape recognition and gravitational forces, liquid-solder-based self-assemblies that use the surface tension between pairs of molten solder drops to assemble functional systems, capillaryforce-directed self-assembly that uses hydrophilic/hydrophobic surface patterns and photocurable polymers to integrate micro-optical components, micromirrors, and semiconductor chips on silicon substrates, and shape-and-solder-directed self-assembly that combines geometrical shape recognition with site-specific binding involving liquid solder to assemble and package heterogeneous microsystems. [15–17] While all of these methods share the common feature of providing parallel assembly on a large scale, process parameters and design rules remain not well characterized. Another great challenge is the realization of heterogeneous systems that contain many different parts and enable contact-pad registration. While current methods allow the positioning of a large number of identical components in a massive parallel manner, systems that consist of more than one repeating unit are difficult to build. Recent studies to overcome this problem include the activation of selected receptors to enable batch transfer on desired locations, and a sequential self-assembly process that uses geometrical shape recognition for component registration and surface tension, involving liquid solder to form mechanical and electrical connections. In both cases, batches of components are added sequentially to build the system, as opposed to adding all components at the same time. In this communication we present a study on controlling angular orientation and contact-pad registration during the selfassembly process. The presented process can be seen as an extension of prior work in the area of surface-tension-directed self-assembly involving liquid solder to form interconnects and is compatible with these processes. Angular-orientation control is important because dies, packaging, or optical elements need to be placed on a substrate with correct angular orientation to enable contact-pad registration or device operation. Angular-orientation control has been challenging in self-assembly. For example, Srinivasan et al. assembled silicon components and micromirrors onto a goldcoated silicon substrate using hydrophilic/hydrophobic interactions and a non-conducting adhesive lubrication layer. The assembly of 98 parts, 500 lm in size, was accomplished with 0.3° rotational precision. However, due to the squareshaped binding and receptor sites four stable angular orientations, 0, 90, 180, and 270°, were observed. While Liang et al. C O M M U N IC A IO N S