Investigation of the Deposition of Microsphere Monolayers for Fabrication of Microlens Arrays

Abstract

Convective deposition of a monolayer of microspheres by drawing a meniscus of a suspension across a substrate is used to fabricate microlens arrays to enhance the photon extraction efficiency of light emitting diodes (LEDs). The self-assembly of a colloidal crystal within the blade-drawn thin film is dominated by capillary forces and the thickness of this crystal depends on many parameters, including the deposition rate and particle size. This study investigates these and other parameters such as angle and hydrophobicity of the deposition blade that have not previously been considered. Using a confocal laser scanning microscope, the local and long-range order of the deposited particles are evaluated by the radial distribution function, and the fraction of the number of nearest neighbors and local bond order, demonstrating the dependence of the microstructure on the deposition parameters. Our results suggest previous descriptions of the critical deposition parameters are inadequate for understanding how various processing conditions influence deposition. For instance, increasing the deposition blade angle from 20 degrees up to 90 degrees requires an increase in deposition rate to achieve a monolayer deposition. The microlens arrays were fabricated on LEDs where polystyrene and silica are coated in consecutive depositions. Heat is used to sacrifice the polystyrene layers to result in an ordered array of partially buried silica microspheres that act as lenses to scatter light from the device. Enhancement in light extraction efficiency of 2.66 times was demonstrated for InGaN-based light emitting diodes employing micron scale microlens arrays with 1 um diameter silica microspheres.

Continuous Convective-Sedimentation Assembly of Colloidal Microsphere Coatings for Biotechnology Applications

“…The menisci formed around these particles give rise to attractive capillary forces that pull adjacent particles together as the liquid evaporates, forming two-dimensional nuclei [25][26][27]. A liquid flux from the suspension bulk to the substrate-air-liquid contact line at the drying front offsets fluid loss due to evaporation, resulting in particle transport to the drying front as adjacent particles aggregate and subsequent proliferation of the coating [25,26,28]. A particle mass balance relates the coating growth rate, v c , to the fluid evaporation rate and particle volume fraction: (1) where β is an interaction parameter, j e is the evaporation rate, l is the drying length, φ is the volume fraction of the particles in suspension, h is the height of the deposited colloidal crystal array, and ε is the coating porosity [27,29].…”

“…The values of β vary between 0 ≤ β ≤ 1 and depend on particle-particle and particle-substrate interactions. For suspensions with low volume fractions and electrostatically stable particles, β → 1 [28]. Once v c is determined, the length of the thin film in which deposition occurs by convection can be calculated using a material flux balance: …”

“…The number and type of deposited particle layers are readily adjusted by altering the suspension volume fraction and coating knife speed, allowing for precise control in particle packing and coating thickness [8]. If the meniscus height is less than the particle diameter at the growth front, as in the case of faster knife speeds, the incoming particles form an open-packed structure [28]. Conversely, for slower knife speeds, if the meniscus height is greater than the particle diameter, multilayer deposition occurs [28].…”

Design and mechanism of cost-effective and highly efficient ultrathin (< 0.5 μm) GaAs solar cells employing nano/micro-hemisphere surface texturing

“…Moreover, the fabrication of 2-D closepacked microlens arrays with hemispherical shape employed for suppressing the total internal reflection and improving the light escape cone in light-emitting devices, has been developed by the rapid convective deposition method resulting in highly-dense arrays of hemispherical structures. [20][21][22] Similar fabrication methods are applicable for experimentally realizing the concept described in the current work.…”

“…Here it is worth noting that although the hemispherical nano/micro-pit array also provides an efficient light confinement design, 32 the fabrication of such structures is not that easy compared to the fabrication of the structure proposed in this study using the methods already developed. [20][21][22] Moreover, we just focus on the optimization of the array periodicity for the hemisphere surface texturing in this paper. Other structural parameters, i.e., various aspect ratios and shapes need to further optimized for realizing more efficient light absorption as recent works have also reported the fabrication of microlens-like structures with various aspect ratios 33 and shapes.…”