High fill-factor microlens array mold insert fabrication using a thermal reflow process

Abstract

An extreme high fill-factor microlens array mold insert in photoresist fabrication using a thermal reflow process is presented. The experimental results proved that a square microlens array could be produced without a peripheral gap. A square microlens array with an extreme high fill-factor (almost 100%) was successfully fabricated. In this experiment, square photoresist columns were formed on a silicon substrate using a lithographic process. The square pattern was laid out in an ortho-square on a polyethylene terephthalate (PET) based mask. Precise temperature and time control was used during the thermal reflow process. The square microlens array was formed from the uniformly flowing melted photoresist. The photoresist column surface transforms into a spherical profile due to the surface tension effect. The error was within ±8% between the fabricated microlens characteristics and the theoretical model used to predict the photoresist column thickness and actual thickness. This model is feasible for fabricating various sized high fill-factor square microlens arrays.

Thermal Annealing in Hydrogen for 3-D Profile Transformation on Silicon-on-Insulator and Sidewall Roughness Reduction

“…Hydrogen-induced surface migration not only changes the surface morphology but also affects the global profile if the surface migration length is comparable to or larger than structural dimensions. This effect is similar to the reflow process in glasses [15] or polymers [16], but unlike the reflow process, this mechanism only depends on surface-atom movement and the crystalline structure is preserved. Thermal annealing in hydrogen ambient has been reported to produce round corners [17] and various voids [18], [19] in bulk silicon.…”

“…Although such structures have been demonstrated by a reflow technique, however, they are limited to noncrystalline materials such as glass [15] or polymers [16]. Exploiting the hydrogen-enhanced surface diffusion, we can make various round structures in monocrystalline silicon by controlling the process parameters.…”

Fabrication of Microlens Arrays with Well‐controlled Curvature by Liquid Trapping and Electrohydrodynamic Deformation in Microholes

“…Laser ablation, ion-beam milling, and other material microremoving processes have produced optics structures with aspheric surface shapes, [ 3 , 4 ] but are obviously unsuitable for mass application due to the poor cost-effectiveness or the poor surface smoothness which results from the cascaded material removal. Thermal refl ow techniques [5][6][7] and the hot embossing method, [ 8 , 9 ] based on interfacial-tension-induced deformation of the photoresist, solgel glass, or other thermoplastic polymers, have been used to fabricate MLAs of high surface smoothness economically. But these thermal approaches can only be used to fabricate microlenses with convex surfaces and are faced with a diffi culty in controlling the lens surface geometry or focal length.…”

Design and Fabrication of Multi-Focusing Microlens Array with Different Numerical Apertures by using Thermal Reflow Method

“…There are several methods for fabricating MLAs, such as ink-jet printing [14], lithographic patterning methods using an electron beam [15], a proton beam [16] or an ion beam [17], and thermal reflow methods [18][19][20][21][22][23]. In thermal reflow methods, photoresist (PR) cylinder structures made by photolithography are melted by thermal energy above the glass transition temperature of the PR.…”