Fabrication of 3D microstructures using grayscale lithography

Abstract

Following the demand for three-dimensional (3D) micromachined structures, additive and subtractive processes were developed for fabrication of real 3D shapes in metals, alloys and monocrystalline Si (c-Si). As a primary structuring step for well-defined 3D structuring of the photoresist, grayscale lithography by laser direct writing was used. For additive fabrication of 3D microstructures, structured photoresist was used as molds. They were sputtered and subsequently electroplated by a metal (Cu) and an alloy (NiCo). The derived electroplated structures were demolded from the photoresist using an organic stripper. These metal structures are satisfactory replicas of the photoresist pattern. For subtractive pattern transfer of 3D structures into c-Si, reactive ion etching (RIE) was used to transfer the 3D photoresist structure into c-Si with 1:1 pattern transferability. The process parameters of RIE were optimized to obtain a selectivity of 1 and an anisotropy factor close to 1. Whereas conventional X-ray lithography (LIGA) and nanoimprint lithography result in 2.5D patterns, these techniques allow the fabrication of almost any arbitrary 3D shapes with high accuracy. In many cases, 3D structures (‘free forms’) are required, e.g. for molding of optical components such as spheres (or aspheres), channels for lab-on-a-chip and pillars for biological applications. Moreover, 3D structures on Si could be used as optical gratings and sensors.

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“…The 3D convex micro-lens with concentric rings and concave tip was initially designed in a CAD layout as shown in Figure 1(a). Wherein, a specific GV was assigned to its each individual layer (the concept is explained in [6,7]) and the grayscale exposures were carried out. substrate.…”

“…The CAD layout geometry is shown in Figure 1 (a), wherein each color represents a specific layer and during the GSL exposure, each layer is exposed with a specific intensity corresponding to the required dose to clear for the needed geometry as shown in the schematic in Figure 1b DWL66FS from Heidelberg Instruments was used to make the grayscale exposures. The calibration curve correlating the exposure depth with the gray value (GV) of the DWL device was made for the coated AZ4562 with 3.8 µm thickness (explanation and procedure can be found in [6,7]). Following which the GSL exposures were made to expose the convex micro-lens having concentric rings on its surface with a concave depression at its tip.…”

“…Furthermore, the masked-based GSL has been reported for fabrication of 3D photoresist structures showing a gradient in their height; however, the requirement for the multiple masks and strict alignment needed to expose layouts makes this approach complicated, expensive and time consuming for the 3D exposures [1,3]. Besides, the use of GSL using direct writing laser (DWL) has been proven to be a promising technique for high throughput microfabrication of 3D microstructures at both micro-and nanoscale dimensions [1,[6][7][8]. The advantage of maskless exposure with the possibility of modulation of the laser exposure intensity (i.e.…”

“…efficacy of using GSL for the microfabrication of simple 3D geometries such as convex and concave micro-lenses, micropyramids etc. have been successfully reported wherein, the modulation of the laser intensity was gradual; thereby, resulting in smooth gradient surface profile of the exposed 3D microstructures [1,6,7]. However, the feasibility of pattern transferring of complex 3D geometries i.e.…”

“…Apart from this, such 3D microstructures with textured surface morphology can also be used as metrology test structures for the accurate estimation of the anisotropy factor (A f ) owing to the sharp transition of geometry on its surface. As reported in [6,7], A f is a critical parameter for the accurate 1:1 transfer of the pattern from the 3D structured photoresist to the underlying monocrystalline silicon wafer (c-Si) using RIE.…”