Fluidic Self-assembly Process Research Articles

Wave energy-assisted fluidic self-assembly of LED chips for display applications

Micro-light-emitting diode (micro-LED) displays have excellent image characteristics, particularly in terms of contrast ratio, response to electric field, and color expressions. However, these are expensive due to the price of the LED chips. This can be overcome by reducing the size of the LED chip, but the efficiency will decrease as a result. Moreover, the rapid and accurate arrangement of a few million chips with a size of ∼50 µm to form pixels on the substrate is a challenging task. In this study, fluidic self-assembly process was introduced. The self-assembly design and implementation were limited to building a micro-scale system. With geometric constraints, external forces may influence the outcome of a self-assembled product. In this case, wave energy was used as the external force to manipulate the LED chips on the substrate. Target-generated waveforms in the fluid were used to control the movement of the LED chips. The arrays of the LED chips were arranged on a fine metal mask, i.e. transfer cartridge. The chips were then transferred to a circuit-printed glass plate by face-to-face pressing under high temperature and high pressure. It was found that the wave energy-assisted self-assembly is applicable and beneficial to LED module fabrication.

A First Implementation of an Automated Reel‐to‐Reel Fluidic Self‐Assembly Machine

A first automated reel-to-reel fluidic selfassembly process for macroelectronic applications is reported. This system enables high-speed assembly of semiconductor dies (15 000 chips per hour using a 2.5 cm-wide web) over large-area substrates. The optimization of the system (>99% assembly yield) is based on identification, calculation, and optimization of the relevant forces. As an application, the production of a solid-state lighting panel is discussed, involving a novel approach to apply a conductive layer through lamination.

Fluidic Self-Assembly Using Molten Ga Bumps and Its Application to Resonant Tunneling Diodes

Fluidic self-assembly (FSA) using molten metal bumps is one of the most promising heterogeneous integration (HI) technologies, which enable us to integrate devices made of various materials on various substrates. We can fabricate the metal bumps using Ga having diameters of 24, 18, 12, and 8 µm with good yield. Using Ga has significant advantages; especially, it includes no toxic metals. These bumps were used for the FSA process of the metal dummy blocks having a diameter of 18 µm, and a good yield of 84% was obtained all over the substrate of about 1×1 cm2. Finally, we applied this method to the resonant tunneling diode (RTD) to verify good electrical, mechanical, and thermal contacts. The RTD device blocks having a diameter of 24 µm have been successfully assembled using the molten Ga bumps. This method is promising for high-performance RTD integration.

Orientation-controlled parallel assembly at the air–water interface

This paper presents an experimental and theoretical study with statistical analysis of a high-yield, orientation-specific fluidic self-assembly process on a preprogrammed template. We demonstrate self-assembly of thin (less than few hundred microns in thickness) parts, which is vital for many applications in miniaturized platforms but problematic for today's pick-and-place robots. The assembly proceeds row-by-row as the substrate is pulled up through an air–water interface. Experiments and analysis are presented with an emphasis on the combined effect of controlled surface waves and magnetic force. For various gap values between a magnet and Ni-patterned parts, magnetic force distributions are generated using Monte Carlo simulation and employed to predict assembly yield. An analysis of these distributions shows that a gradual decline in yield following the probability density function can be expected with degrading conditions. The experimentally determined critical magnetic force is in good agreement with a derived value from a model of competing forces acting on a part. A general set of design guidelines is also presented from the developed model and experimental data.

Orientation-specific fluidic self-assembly process based on a capillary effect

The investigation develops a novel design of a two-dimensional modified alignment mark of a tear-drop/elliptical hole with a tip angle of 60°. TDE-1 and TDE-2 pattern shapes are adopted to increase the recovery angle and reduce the energy barrier to uni-directional micropart alignment. The results of the experimental and surface energy models are compared, both qualitatively and quantitatively, to examine the feasibility of the new design patterns. Experimental results reveal that the micropart of the TDE patterns can be accurately aligned by rotation through 90° and a large capillary force. The acrylate adhesive force of the TDE-2 patterned micropart was estimated to be 41.2 ± 10 µN. Fluidic self-assembly (FSA) was performed in an aqueous environment, using a low-temperature solder or acrylate adhesive in part-substrate lubricant and template-assisted assembly. The standard deviation of the aligned angular orientation was 0.9° and the lateral accuracy was 15 µm; the re-aligned assembly yield was 100%. Micropart self-alignment with a unique in-plane orientation is achieved by exploiting shape recognition with the adhesive capillary effect.

Hydrodynamically Tunable Affinities for Fluidic Assembly

Most current micro- and nanoscale self-assembly methods rely on static, preprogrammed assembly affinities between the assembling components such as capillarity, DNA base pair matching, and geometric interactions. While these techniques have proven successful at creating relatively simple and regular structures, it is difficult to adapt these methods to enable dynamic reconfiguration of the structure or on-the-fly error correction. Here we demonstrate a technique to hydrodynamically tune affinities between assembling components by direct thermal modulation of the local viscosity field surrounding them. This approach is shown here for two-dimensional silicon elements of 500 microm length using a thermorheological fluid that undergoes reversible sol-gel transition on heating. Using this system, we demonstrate the ability to dynamically change the assembly point in a fluidic self-assembly process and selectively attract and reject elements from a larger structure. Although this technique is demonstrated here for a small number of passive mobile components around a fixed structure, it has the potential to overcome some of the limitations of current static affinity based self-assembly.

Increased robustness for fluidic self-assembly

Self-assembly methods have been developed at the micro- and nanoscale to create functional structures from subelements stochastically dispersed in a fluid. Self-assembly paradigms have limitations in terms of achievable complexity of the final structure, ability to perform error correction, and scalability. Fluidic self-assembly attempts to overcome these limitations by incorporating a controlled flow structure and/or complex geometric interactions to improve the assembly rate and the specificity of the final positioning. Since the initial position and orientation of a subelement in a stochastic system are indeterminate, the most robust of these schemes are those for which the dependence on the initial condition will be the weakest. In this paper we develop an analytical/numerical model for the fluid forces and torques on a two-dimensional subelement involved in a fluidic self-assembly process and describe the translational and rotational motions of the element due to these forces. We use this model to determine optimal subelement shapes and flow conditions that lead to successful assembly over the broadest range of initial conditions. We quantify the degree to which assembly has been successful by introducing two docking parameters that are descriptive of how close the final subelement position is to the ideal case. Robust self-assembly schemes were developed for the assembly of different tile shapes. This approach to evaluate a self-assembly process based on the final subelement position can be applied to other fluidic self-assembly techniques.

Numerical studies on micropart self-alignment using surface tension forces

The Fluidic Self-Alignment Approach provides an alternative means for fast, economic, and precise handling of thousands of micro-scale parts. The present study aims to examine the important parameters which govern the mechanisms of the fluidic self-assembly process by numerical simulations. A simplified 2D model system consists of a solid plate, a micro-scale liquid slug and a micropart. The computational model is based on first principle conservation equations and is constructed by the coupling of two-phase modeling, solid structure modeling, and fluid–structure coupling. A matching experimental system is set up for the micropart of aspect ratio from 3:1 to 10:1 to validate the 2D computational simulations. Simulations reveal that a high degree of hydrophilicity between the lubricant and the solid surfaces is required for the self-assembly of microparts. A lower lubricant height, a higher surface tension coefficient and a higher viscosity enforce the re-alignment/restoration process also. Characterization of the flow field inside lubricant slug also indicates that the asymmetry of the vortices/stress distribution at both ends of the lubricant meniscus is resulted as the micropart in a back-and-forth restoration process.

Direct integration of GaAs HEMTs on AlN ceramic substrates using fluidic self-assembly

Direct integration of AlGaAs/GaAs HEMTs on AlN ceramic substrates has been demonstrated based on the fluidic self-assembly (FSA) technology. The FSA is a unique technology to arrange small device blocks (around a few tens of microns) onto the other substrates. With this technology the core part of the HEMT having no pad can be mounted on the ceramic substrate. The HEMT core can then be connected electrically with the circuit on a ceramic substrate using a planar wiring process. This eliminates large stray capacitance and inductance in the conventional technology. It has been demonstrated that the good FET characteristics are obtained even after FSA process.

Fluidic Self-Assembly of Semiconductor Devices: A Promising New Method of Mass-Producing Flexible Circuitry

Small electronic chips have been successfully assembled in a rapid fashion using a technique called fluidic self-assembly (FSA). The FSA process is found to be sensitive to the size, shape and orientation of the devices flowed. Larger devices (up to 1000 µm on a side) will move faster than smaller devices on an inclined, submerged plane. Rectangular devices with large length-to-width ratios and similar total mass, however, will also move more slowly than square devices. Rectangular and asymmetrically shaped devices will also orient as the slide down a submerged plane, and this has important implications for filling yield during FSA assembly. Lastly, increasing the temperature of the fluid decreased the viscosity and resulted in higher device velocities. We propose a boundary layer lubrication model to explain the motion and behavior of devices during fluidic self-assembly which is consistent with all of the experimental observations.