Ji Hao Hoo

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 todays 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.

Programmable batch assembly of microparts with 100% yield

This paper demonstrates a novel method to perform micropart (370×370×150µm3) delivery to receptor sites (20×10 array) with a batch assembly process that leads to 100% yield within tens of seconds. The delivery mechanism is statistically characterized and a chemical kinetics inspired model previously proposed is extended. Based on this understanding, repeatable and programmable 100% yield assembly is achieved in open loop and feedback configurations.

Optimization of Angular Alignment in Self-Assembly of Thin Parts at an Air–Water Interface

This letter presents an analysis of self-assembly of thin disk-shaped parts (diameter: 2 mm; thickness: 100 μm) with the objective of optimizing their angular alignment. The assembly proceeds continuously on a substrate that is pulled up through an air-water interface where thin parts with magnetic markers are floating. Angular deviations from the assembly site are significantly reduced by repositioning magnets that guide the self-assembly process. Temporary Faraday waves aid one-to-one part-to-site registration. Ninety parts are assembled, row by row, in 1 min. The assembly rate scales with the width of the assembly substrate. Compared with that of our previous work, the assembly rate is increased threefold due to reduced part-to-part interactions.

3D Integration Using Self-Assembly at Air-Water-Solid Interface

This paper presents the first proof-of-concept 3D integration using fluidic self-assembly of chip-scale parts (2000 × 2000 ×100 μm3 ) at an air-water-solid interface. Four-layer 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 are achieved by solder reflow. Magnetic fields and temporary Faraday waves are introduced for one-to-one part-to-site registration in proper orientation. The alignment accuracy degrades with increasing number of layers due to weaker magnetic force. 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.

Wafer-level high density integration of surface mount technology components in through-silicon trenches

This paper reports a novel method to deliver and assemble standard 01005 format (0.016” × 0.008”, 0.4 mm × 0.2 mm) monolithic ceramic capacitors and thin-film resistors into through-wafer trenches, with a batch assembly process that can guarantee 100% assembly. This process is CMOS compatible and is competitive with capacitors and resistors fabricated through standard foundry processes.

Parallel Heterogeneous Integration of Chip-Scale Parts by Self-Assembly

This letter reports a novel methodology for the orientation-specific parallel heterogeneous integration of parts of various sizes. Assembly sites are designed to only attract specific parts from an unsorted pool using the combined effect of Faraday waves and magnetic forces, achieving one-to-one part-to-site registration. We demonstrate the assembly of two types of thin parts (2000 × 2000 × 100 and 4000 × 4000 × 100 μm3) onto the same substrate through a one-step process. Statistical analysis of the distributions of magnetic forces delimits the suitable range for the strength of Faraday waves to fix nonoptimal initial placements and orientations.

Programmable self-assembly for microsystem integration

We present microscale self-assembly as a suite of techniques that promotes the electronics industrys initiative towards functional diversification and function densification, and two specific programmable self-assembly projects that are driven by real-world limitations and aspirations of present assembly and packaging technologies. Towards said goals, we demonstrate that our processes can improve existing assembly and packaging techniques, and also enable possibilities restricted by current industry methodologies.

Effect of Fluid Viscosity on Dynamics of Self-Assembly at Air–Water–Solid Interface

This paper details the effect of fluid viscosity on previously presented self-assembly at an air-water-solid interface through experimental and analytical approaches. The assembly method is subdivided into three process steps (approach, rotation, and pull-up), and their viscosity dependence is investigated. The motion of a moving part is described using a derived modeling equation. The assembly proceeds row-by-row as an assembly substrate is pulled up through an air-water-solid interface. High yields at different viscosities are demonstrated using thin square parts (2000 × 2000 × 100 μm3). Near 100% yield can be achieved by adding visual feedback control for the application of Faraday waves. The combined effect of the application time and the fluid viscosity is analyzed.