Karl F. Böhringer

Controlling Liquid Drops with Texture Ratchets

Controlled vibration selectively propels multiple microliter-sized drops along microstructured tracks, leading to simple microfluidic systems that rectify oscillations of the three-phase contact line into asymmetric pinning forces that propel each drop in the direction of higher pinning.

Microspring Characterization and Flip-Chip Assembly Reliability

Electronics packaging based on stress-engineered spring interconnects has the potential to enable integrated IC testing, fine pitch, and compliance not readily available with other technologies. We describe new spring contacts which simultaneously achieve low resistance ( <; 100 mΩ) and high compliance (>; 30 μm) in dense 2-D arrays (180 ~ 180-μm pitch). Mechanical characterization shows that individual springs operate at approximately 150-μN force. Electrical measurements and simulations imply that the interface contact resistance contribution to a single contact resistance is <; 40 mΩ . A daisy-chain test die consisting of 2844 contacts is assembled into flip-chip packages with 100% yield. Thermocycle and humidity testing suggest that packages with or without underfill can have stable resistance values and no glitches through over 1000 thermocycles or 6000 h of humidity. This paper suggests that integrated testing and packaging can be performed with the springs, enabling new capabilities for markets such as multichip modules.

Red-emitting silicon quantum dot phosphors in warm white LEDs with excellent color rendering

We demonstrate red-emitting silicon quantum dot (SiQD) phosphors as a low-cost and environment-friendly alternative to rare-earth element phosphors or CdSe quantum dots. After surface passivation, the SiQD-phosphors achieve high photoluminescence quantum yield = 51% with 365-nm excitation. The phosphors also have a peak photoluminescence wavelength at 630 nm and a full-width-at-half-maximum of 145 nm. The relatively broadband red emission is ideal for forming the basis of a warm white spectrum. With 365-nm or 405-nm LED pumping and the addition of green- and/or blue-emitting rare-earth element phosphors, warm white LEDs with color rendering index ~95 have been achieved.

A wireless intraocular pressure monitoring device with a solder-filled microchannel antenna

This paper presents the prototype of an intraocular pressure sensor as a major step toward building a device that can be permanently implanted during cataract surgery. The implantation will proceed through an incision of 2?3?mm using an injector, during which the complete device must be folded into a cross-section of 2?mm???1?mm. The device uses radio frequency (RF) for wireless power and data transfer. The prototype includes an antenna, an RF chip and a pressure sensor assembled on a printed circuit board with several circuit components used for testing and calibration. The antenna is fabricated and integrated with the circuit using a fabrication method employing solder-filled microchannels embedded in an elastomer. The monitoring device is powered at 2.716?GHz from a distance of 1?2?cm. The prototype has undergone electrical and mechanical tests for antenna and sensor performance. The flexible antenna can withstand a stress of 33.4 kPa without any electrical disconnection. It did not show a significant increase in electrical resistance after 50 bending cycles with a maximum applied stress of 116 kPa. Transmitted pressure data shows an averaged sensitivity of 16.66?Hz (mm-Hg)?1.

Surface passivation dependent photoluminescence from silicon quantum dot phosphors.

We demonstrate wavelength-tunable, air-stable and nontoxic phosphor materials based on silicon quantum dots (SiQDs). The phosphors, which are composed of micrometer-size silicon particles with attached SiQDs, are synthesized by an electrochemical etching method under ambient conditions. The photoluminescence (PL) peak wavelength can be controlled by the SiQD size due to quantum confinement effect, as well as the surface passivation chemistry of SiQDs. The red-emitting phosphors have PL quantum yield equal to 17%. The SiQD-phosphors can be embedded in polymers and efficiently excited by 405 nm light-emitting diodes for potential general lighting applications.

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.

Electrodeposition modeling and optimization to improve thin film patterning with orchestrated structure evolution.

Orchestrated structure evolution is an alternative nanomanufacturing approach that combines the advantages of top-down patterning and bottom-up self-organizing growth. It relies upon tool-directed patterning to create seed locations on a surface from which a subsequent deposition process produces the final, merged film. Despite its demonstrated ability to reduce patterning time by orders of magnitude, our prior reliance on mass transfer limited deposition and square seed arrays resulted in extraneous film growth along pattern edges, thereby limiting the pattern quality of the final film. Here, quality improvements are demonstrated by modeling and tuning the growth mechanism of the deposition step to include charge transfer effects. In addition, a seed positioning optimization technique derived from simulated annealing is introduced as a method for relocating the seeds to minimize film overgrowth at the pattern edges. These improvements enable OSE to maintain geometric quality while substantially reducing the time and cost compared to traditional direct-write manufacturing methods.

Converting Vertical Vibration of Anisotropic Ratchet Conveyors into Horizontal Droplet Motion

An anisotropic ratchet conveyor is an asymmetric, periodic, micropatterned surface that propels droplets when vibrated with a sinusoidal signal at certain frequencies and amplitudes. For each input frequency, there is a threshold amplitude beyond which the droplet starts to move. In this paper, we study the parameters that initiate droplet motion and the relationship between the input frequency and threshold amplitude among droplets with different volume, density, viscosity, and surface tension. Through this investigation we demonstrate how nondimensionalization reveals consistent behavior for droplets of different volumes. Finally, we propose a compact model that captures the essential features of the system to describe how a pure vertical vibration results in horizontal droplet motion. This model provides an intuitive understanding of the underlying physics and explains how the surface asymmetry is the key for lateral droplet motion.

Liquid droplet micro-bearings on directional circular surface ratchets

This paper presents de-ionized water droplets used as torque-generating micro-bearings between a glass plate and a micromachined Si substrate. The pattern on the Si substrate includes circular tracks, which allow droplet motion in a single direction. When vertical vibration is applied to the system, a rotation in the transverse plane is triggered. The system can be tailored to respond to a specific vibration frequency, from 36.5 to 83 Hz for droplet volumes from 13 to 1 μL. The system is tested for its frequency response at different droplet sizes and droplet counts. A figure of merit is determined to quantify the responsiveness of the system. The largest angular speed is recorded as 0.302 rad/s for a vibration at 61 Hz.

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.