Surface tension directed fluidic self-assembly of semiconductor chips across length scales and material boundaries

This publication provides an overview and discusses some challenges of surface tension directed fluidic self-assembly of semiconductor chips which are transported in a liquid medium. The discussion is limited to surface tension directed self-assembly where the capture, alignment, and electrical connection process is driven by the surface free energy of molten solder bumps where the authors have made a contribution. The general context is to develop a massively parallel and scalable assembly process to overcome some of the limitations of current robotic pick and place and serial wire bonding concepts. The following parts will be discussed: (2) Single-step assembly of LED arrays containing a repetition of a single component type; (3) Multi-step assembly of more than one component type adding a sequence and geometrical shape confinement to the basic concept to build more complex structures; demonstrators contain (3.1) self-packaging surface mount devices, and (3.2) multi-chip assemblies with unique angular orientation. Subsequently, measures are discussed (4) to enable the assembly of microscopic chips (10 μm–1 mm); a different transport method is introduced; demonstrators include the assembly of photovoltaic modules containing microscopic silicon tiles. Finally, (5) the extension to enable large area assembly is presented; a first reel-to-reel assembly machine is realized; the machine is applied to the field of solid state lighting and the emerging field of stretchable electronics which requires the assembly and electrical connection of semiconductor devices over exceedingly large area substrates

Advanced instrumented stamps for micro transfer printing and novel application areas

Transfer printing refers to a set of techniques for deterministic assembly of functional micro/nano scale devices into two and three dimensional spatial arrangements. It provides a versatile route for realizing multifunctional heterogeneously integrated systems such as flexible electronics, biocompatible sensing and therapeutic devices, transparent and curved optoelectronic systems etc. Micro-transfer printing is an automated process that implements deterministic micro scale assembly using a molded viscoelastic stamp typically made out of PDMS. The process relies upon the control of adhesion and fracture at the interfaces between the stamp and the devices being assembled to pick up and release them. A widely exploited strategy to achieve variable adhesion from the stamp is to use the rate dependent effects of the viscoelastic stamp material. It is a very versatile process and has been used in the realization of many novel heterogeneously integrated systems. The process has been implemented industrially to assemble ultra-high concentration photovoltaic panels. This body of work presents the development of new stamp technologies to address the challenges associated with increasing parallelism and shortcomings associated with fixed geometry stamps. Starting from the concept of an active composite material with distributed sensing, actuation and compliance tuning, new stamp architectures are developed. These novel stamps replace the compliance of a bulk PDMS stamp with active functional structures with tunable stiffness; without effecting the ability of the stamps to be used for transfer printing. The new stamp architecture enables active monitoring and control of the micro transfer printing process. Using instrumentation to sense deflections/forces at each post allows detection, measurement and compensation of misalignments between the stamp and donor/receiving substrates. Furthermore this information is used to detect pick up and printing errors at individual posts, allowing for error handling to increase process robustness. Moreover the ability to selectively actuate allows to engage/disengage individual posts. This enables new transfer printing modes such as collect and place. Finally results of pilot experiments conducted to test the feasibility of using micro transfer printing in novel application areas are presented