Xiaorong Xiong

Controlled multibatch self-assembly of microdevices

A technique is described for assembly of multiple batches of micro components onto a single substrate. The substrate is prepared with hydrophobic alkanethiol-coated gold binding sites. To perform assembly, a hydrocarbon oil, which is applied to the substrate, wets exclusively the hydrophobic binding sites in water. Micro components are then added to the water, and assembled on the oil-wetted binding sites. Moreover, assembly can be controlled to take place on desired binding sites by using an electrochemical method to deactivate specific substrate binding sites. By repeatedly applying this technique, different batches of micro components can be sequentially assembled to a single substrate. As a post assembly procedure, electroplating is incorporated into the technique to establish electrical connections for assembled components. Important issues presented are: substrate fabrication techniques, electrochemical modulation by using a suitable alkanethiol (dodecanethiol), electroplating of tin and lead alloy and binding site design simulations. Finally, we demonstrate a two-batch assembly of silicon square parts, and establishing electrical connectivity for assembled surface-mount light emitting diodes (LEDs) by electroplating.

Geometric binding site design for surface-tension driven self-assembly

Surface-tension driven self-assembly techniques have been successfully employed to assemble and align micro parts on hydrophobic binding sites on a substrate. The driving force for assembly is provided by a liquid lubricant meniscus confined between two hydrophobic surfaces in an aqueous environment. Therefore, the hydrophobic pattern design becomes a critical issue for the self-assembly process. With an ideal design, the part can assemble in a unique position and orientation. In this paper, we study a series of geometric designs based on a first-order approximation energy model. An optimization method is developed to evaluate them, and a class of optimal designs is found consisting of asymmetric rings with additional geometric constraints.

Multi-batch micro-self-assembly via controlled capillary forces

Advances in silicon processing and microelectro-mechanical systems (MEMS) have made possible the production of very large numbers of very small components at very low cost in massively parallel batches. Assembly, in contrast, remains a mostly serial (i.e., non-batch) technique. We argue that massively parallel self-assembly of microparts will be a crucial enabling technology for future complex microsystems. As a specific approach, we present a technique for assembly of multiple batches of microparts based on capillary forces and controlled modulation of surface hydrophobicity. We derive a simplified model that gives rise to geometric algorithms for predicting assembly forces and for guiding the design optimization of self-assembling microparts. Promising initial results from theory and experiments and challenging open problems are presented to lay a foundation for general models and algorithms for self-assembly.

Towards optimal designs for self-alignment in surface tension driven micro-assembly

Fluidic self-assembly driven by surface tension force has demonstrated the capability of assembling micro parts to binding sites in parallel with high efficiency and accuracy. In this paper, we focus on the binding site design for this technique, as it is a critical factor not only for accurate assembly, but also to achieve unique alignment position and orientation. To find optimal designs, we use a first-order approximation model to evaluate a series of patterns including disks, rings and offset rings. From this analysis, optimal patterns assuring unique alignment are determined to be offset rings with specific geometric constraints. For comparison, experiments with different shapes are performed, and results matching with the simulations are observed.

Controlled Part-to-Substrate Micro-Assembly via Electrochemical Modulation of Surface Energy

A process designed for repeated parallel micro-assembly has been achieved by controlling the hydrophobicity of the binding sites between micro-parts and substrates. Active assembly sites consist of hydrophobic surfaces made of alkanethiol-coated gold, while inactive sites are clean, hydrophilic gold surfaces. Electrochemical reduction of the alkanethiolate monolayer on substrates with addressable binding sites returns the selected sites to their inactive, hydrophilic state. The parts are attracted, aligned, and anchored to the binding sites by a heat curable lubricant in an aqueous environment. The entire sequence is then repeated with a different set of active binding sites and parts. A two-batch assembly process is described in this paper. Principles for establishing electrical connections using electroplating are also demonstrated and discussed. This process forms the basis for a general technique to batch-assemble complex microstructures from simple components.

From micro-patterns to nano-structures by controllable colloidal aggregation at air-water interface

In this paper, we discuss a method to transform traditional-lithography micro patterns into nano-structures by self-assembly of nano-beads. In our approach, the destined substrate is prepared with hydrophilic micro patterns on hydrophobic background. Colloid with nano-beads wets exclusively the hydrophilic patterns due to interfacial forces when passing through air-water interface. After evaporation of water from the colloid, three-dimensional nano-bead structures are formed. A geometric model is proposed to describe this self-assembly process and its dependence on bead size, concentration, and pattern geometry, which can provide control over the aggregation of three-dimensional nano-structures.