Bo Chang

Capillary-driven self-assembly of microchips on oleophilic/oleophobic patterned surface using adhesive droplet in ambient air

This letter describes a capillary-driven self-assembly technique using oleophilic/oleophobic patterned surface and adhesive in ambient air environment. We use a topographical microstructure of porous ormocer functionalized with a fluorinated trichlorosilane for the oleophobic area and gold patterns for the oleophilic area. The resulted oleophilic/oleophobic patterns show significant wettability contrast for adhesive (Delo 18507), with a contact angle of 119° on oleophobic part and 53° on the oleophilic part. Self-alignment of SU-8 microchips on the oleophilic/oleophobic patterns has been demonstrated. The results provide a promising solution for self-alignment of microparts using commercial adhesives in ambient air environment.

Microassembly system with controlled environment

Environmental conditions such as temperature and humidity can influence the adhesion forces, material and microtribological properties of microparts. To have a good understanding of microassembly, it is essential to study the influences of environmental conditions on the microassembly process and system. This paper presents our research in environmentally controlled microassembly. We have developed a microassembly system with controlled environment, which consists of an environmental control system and a microassembly platform. The environmental control system controls the environmental conditions including temperature and humidity in a closed chamber, providing a clean and vibration isolated environment for microassembly. The temperature can be controlled in the range of -10-40°C and relative humidity in the range of 5-80% RH. The microassembly platform, which is installed in the environmental control system, contains microrobotic instruments such as microgrippers, micromanipulator, positioning stages, microdispenser, ultraviolet light source, microscopes, etc. Experiments on the influences of environmental conditions on microassembly instruments and pick-and-place operations are systematically performed. The results are analyzed and presented.

Self-alignment in the stacking of microchips with mist-induced water droplets

This paper reports a novel and versatile water droplet self-alignment technique where the water is delivered in mist form onto the assembly site. The droplet forming process has been carefully investigated using machine vision, where each individual droplet on the microchip surface can be identified and the volume per surface area can be calibrated at a specific time. The result reveals that the volume of water droplets on the assembly surface grows linearly as a function of time. Self-alignment based on the mist-induced droplets has been studied, where a robotic microgripper is used to deliver the microchips on the assembly site. The paper also investigates the maximum tolerance of the initial placement error in stacking SU-8 chips 200 × 200 × 70 µm in size, and the possibility of stacking two SU-8 chips of different dimensions using the proposed self-alignment technique. Moreover, self-alignment of chips on hydrophilic/hydrophobic patterns covered by mist-induced water droplets has been studied. The experimental results indicate that this novel self-alignment technique is very promising. Furthermore, a statistical model has been used to validate the experimental results.

Self-alignment of RFID dies on four-pad patterns with water droplet for sparse self-assembly

This paper reports an in-depth study of a water-droplet-assisted self-alignment technique that self-aligns radio frequency identification (RFID) dies on four-pad patterns. The segmented structure of four hydrophilic pads on a hydrophobic substrate brings freedom to the design of the electrical functionality and the surface functionality. The paper investigates the influence of the key parameters that may affect the self-alignment in theory and experiment. The theoretical model justifies that RFID dies can be reliably aligned on the segmented four-pad pattern even when the initial placement error is as large as 50% of the size of the die and the gap between the four pads is about 10% of the size of the die. A method has been introduced to estimate the sufficient droplet volume for self-alignment. A series of experiments have been carried out to verify the results of the model. The experiments indicate that the self-alignment between the 730 × 730 µm RFID dies and the pattern occurs reliably when the releasing bias between the RFID die and antenna is less than 400 µm for patterns with 50 and 100 µm gaps, and successful self-alignment is possible even with greater bias of 500 µm.

Surface-tension driven self-assembly of microchips on hydrophobic receptor sites with water using forced wetting

This letter reports water droplet self-alignment methods for self-assembly of microchips on hydrophobic receptor sites in ambient air environment. It is an open question if lyophobic receptor site of the self-alignment medium can be used for self-assembly. We investigate this question using both numerical simulation and experimental studies on hydrophobic receptor sites (advancing contact angle of 118°) with superhydrophobic substrate (contact angle of 180°). We demonstrate that self-alignment is possible using two forced wetting methods: (a) introducing an excessive amount of water and (b) applying external pressure. The results suggest that surface-tension driven self-alignment can be applied in a wider combination of materials and mediums.

Surface Tension-Driven Self-Alignment of Microchips on Low-Precision Receptors

Surface tension-driven self-alignment is reported to be very accurate when the chip and receptor site are well-defined. This paper investigates surface tension-driven self-alignment of microchips on low-precision receptors through experimental studies and theoretical analysis to understand the relation between alignment accuracy and the precision of the receptor edges. Three different types of low-precision receptors have been designed and fabricated to study the alignment accuracy: 1) receptors with a single triangular defect pointing outward or inward with amplitude of 10, 30, and 60 μm and corresponding width of 20, 60, and 120 μm; 2) receptors with constant triangular edge jaggedness with amplitude of 2, 4, 5, and 15 μm; and 3) receptors with random triangular edge jaggedness with amplitude of 2, 4, and 8 μm. The DI water is used as the self-alignment medium. The alignment accuracy has been closely measured with an environmental scanning electron microscope. Numerical simulators, e.g., surface evolver have been used to analyze the results. The experimental results show that low-precision receptors impair alignment accuracy in a much lesser degree than the scale of the defects on a receptor.

Surface-Tension-Driven Self-Alignment of Microchips on Black-Silicon-Based Hybrid Template in Ambient Air

In this paper, we demonstrate self-alignment of microchips on a simple-to-fabricate hybrid template with both water and UV-curing adhesive (EPO-TEK UVO-114). The hybrid template contains receptor sites with solid edges for droplet confinement and large wetting contrast between the receptor sites and the substrate for microchip manipulation. Nanostructured black silicon surface functionalized with fluoropolymer is used as a substrate, while protruded silicon dioxide patterns covered with fluoropolymer serve as receptor sites. A simple and fast process consisting only of one pass of photolithography, cryogenic deep reactive-ion etching (RIE), and RIE steps is used to fabricate the hybrid template. The self-assembly tests are carried out in a hybrid microassembly setup. Dummy microchips of sizes 200 μm × 200 μm × 50 μm are self-aligned on 200 μm × 200 μm receptor sites in ambient air with both water and adhesive.

6 DOF dexterous microgripper for inspection of microparts

This paper presents a novel 6 DOF (degree-of-freedom) piezoelectric micro gripping/handling system for automatic dexterous manipulation and inspection of microoptoelectronic components. The purpose of the system is to pick-and-place and align microparts having size of 300 to 400 mum square. The alignment task includes both translational alignment for field-of-view and focusing, and rotational alignment for uniform focusing. All six actuation axes are controlled based on strain gauge sensor feedback. A network-based control system is used for automatic control of the gripper. The control system includes three hierarchical layers: actuator control layer, motion planning layer, and mission layer. Visual servoing is applied in automatic handling. The performance of the system is demonstrated in a fully automated inspection task

Hybrid micro assembly of microchips on segmented patterns

this paper reports a novel process that positions RFID chips on segmented artificial antenna pattern using hybrid assembly technology. A droplet of water is firstly dispensed to an artificial pattern composed of four pads. Then a 730×730µm RFID chip with four bumps of 20µm in height is released onto the artificial pattern using a robotic microgripper. Finally the RFID chip is self-aligned with the outer edge of the pattern due to the surface tension of water. The artificial antenna pattern is designed to have four pads on the substrate to mimic the shape of the real antenna with four electric or mechanical connections. The pattern is hydrophilic, while the rest of the substrate is hydrophobic. A series of experiments have been carried out to understand the influences of the hydrophobic gap to the result of self-alignment, as well as to clarify the influence of dispensing position and releasing bias to the result of self-alignment. The result indicates that RFID chips can be reliably assembled on the segmented pattern with 50 µm and 100 µm gaps, and the assembly is also possible with larger gaps. There is also no obvious influence towards the assembly results by choosing dispensing water to the releasing position or dispensing water to the center of the antenna. Furthermore, the experimental results show that the self alignment between the RFID chips and the pattern occurs reliably when the releasing bias between the RFID and antenna is less than 300µm for patterns with 50 µm and 100 µm gaps.

Hybrid microassembly for massively parallel assembly of microchips with water mist

This paper proposes a hybrid microassembly technique for massively parallel assembly of 200μm × 200μm × 30μm SU-8 chips. The hybrid microassembly technique combines the robotic pick-and-place technique and water mist induced self-assembly technique. The robotic handling tool is used to place microchips roughly on chips of the same size at a fast speed, and then water mist composed of microscopic droplets is delivered to achieve high accuracy and massively parallel alignments. The results indicate the hybrid assembly technique is promising for assembly of microchips. A 200μm × 200μm × 70μm SU-8 chip is assembled on the top of another SU-8 chip of the same size; where the success rate can reach 80% with bias as high as 130μm in x- and y directions. We have demonstrated that the proposed technique in assembly a matrix of 30 200μm × 200μm SU-8 chips. The results also show that alignment can reach sub micrometer accuracy.