Fluidic Self-Assembly of Millimeter-Scale Thin Parts at Air-Water Interface
Park, Kwang Soon
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This dissertation focuses on a novel method to achieve high yield assembly of millimeter-scale thin silicon chips from an air-water interface. Surface functionalized silicon parts assemble in preprogrammed hydrophilic locations on an assembly substrate with self-alignment. The process and design factors is systematically analyzed and optimized using DOE (Design of Experiment) that leads to high yield. First, a novel fluidic self-assembly (FSA) at the air-water interface is introduced. An experimental and theoretical study of a high yield self-assembly process with a surface programmable template is undertaken and modeled. An analysis of the fluidic self-assembly method at an air-water interface is presented with an emphasis on the combined effect of a substrate tilting angle and part size. For 1 × 1, 3 × 3 and 5 × 5 mm2 parts with 100 µm thickness, the substrate tilting angles for effective assembly are experimentally determined and the surface tension induced torques are derived based on a newly developed model. The result indicates that there is a limit on the lateral size of the parts that can be assembled when only one substrate tilting angle is used. Based on the analysis, a novel method, which is capable of assembling parts of higher lateral dimensions using parametric changes in the substrate tilting angle, is proposed. Due to no control over the rotational angle, square parts are assembled in four possible orientations (0°, 90°, 180°, 270°) due to their square shape. The FSA method is improved to achieve orientation-specific self-assembly of millimeter-scale thin parts by adding magnetic materials. The effect of magnetic force is analyzed and a critical magnetic force that guarantees successful assembly is systematically derived. 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 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. Based on the analysis of the magnetic force, the parallel heterogeneous assembly system is developed. The assembly system selectively assembles appropriate parts at corresponding sites from a mixed collection of 2000×2000×100 and 4000×4000×100 µm3 parts using a single pull-up process. Statistical analysis of the distributions of magnetic forces delimits the suitable range for the strength of Faraday waves to fix non-optimal initial placements and orientations. Besides the square patterned parts, orientation-specific assembly of thin circular parts (diameter = 2 mm, thickness = 100 µm) is demonstrated. Components for optics (camera, microlens on photovoltaic cells), sensors, actuators and MEMS devices are often in circular shape because of simple and symmetric design. Thus, the assembly of thin circular parts in unique orientation is required to fully take advantage of 3D integration. Large deviation of rotation angle of parts is improved by optimizing magnet position through the analysis of in-plane magnetic fields, which induces the dominant force after the parts are placed in the trench. The effect of fluid viscosity on FSA is described through experimental and analytical approaches. The assembly method is classified and viscosity-dependent processes (approach, rotation) are investigated. The detailed effect of viscosity on assembly speed and yield is studied using a derived modeling equation and experimental results. The effect of the feedback control as well as its dependence on the fluid viscosity is analyzed. As extension of 2D self-assembly, the first proof-of-concept of 3D integration using the proposed FSA of chip-scale parts (100 µm thickness) is presented. 3D integration is achieved by assembling new parts over previously assembled parts. Assembly proceeds as an assembly substrate is pulled up through an air-water interface and electrical and mechanical bonding is achieved through a solder reflow process. The maximum number of layers that can be achieved is simulated and analyzed. Via resistance including the effect of degradation of solder over repeated reflow process is measured. Inspired by the analysis of the response of the parts to the magnetic field, a novel self-assembly of a 3D structure by controlling the folding angle through the analysis of the magnetic field is realized. The tendency of ferromagnetic thin Ni patterns to align along magnetic flux triggers self-folding after the capillary force is diminished. Two strategies to control the folding angle are presented with experimental results. As a proof of concept, a millimeter-scale bus (folding angle = 90°) and 3D pyramid shaped (folding angle > 90°) is assembled. As a potential application, a pyramid shaped 3D photovoltaic cell is proposed to increase light absorption by trapping the reflected light.
- Electrical engineering