Vertically tapered layers for optical applications fabricated using resist reflow

Abstract

This paper reports on the IC-compatible fabrication of vertically tapered optical layers for use in linear variable optical filters (LVOF). The taper angle is fully defined by a mask design. Only one masked lithography step is required for defining strips in a photoresist with trenches etched therein of a density varying along the length of the strip. In a subsequent reflow, this patterned photoresist is planarized, resulting in a strip with a local thickness defined by the initial layer thickness and the trench density at that position before reflow. Hence a taper can be flexibly programmed by the mask design to be from 0.001• to 0.1 • , which enables the simultaneous fabrication of tapered layers of different taper angles. The 3D pattern of resist structures is subsequently transferred into Si or SiO 2 by appropriate etching. Complete LVOF fabrication involves CMOS-compatible deposition of a lower dielectric mirror using a stack of dielectrics on the wafer, tapered layer formation and deposition of the top dielectric mirror. Design principle, processing and simulation results plus experimental validation of the technique on the profile in the resist and after transfer of the taper into Si and SiO 2 are presented.

Medical apps in need of optical microspectrometers

“…The incident light spot should be imaged over the entire detector array, resulting is a poor optical throughput in general purpose, wideband applications, as compared to gratingbased microspectrometer systems. However, many promising application involve spectral analysis in a narrow band (Emadi et al 2009) around an absorption line or luminescence line, which would favor the LVOF concept with typically 40-80 channels within this band. Particularly promising are applications in which several absorption lines need to be analyzed simultaneously, since multiple LVOFs can be fabricated simultaneously on a chip using a specially designed mask and reflow to result in tapered layers with different taper angles.…”

“…CMOS-compatible LVOF fabrication is based on the reflow of a specially patterned layer of resist (Emadi et al 2009). Figure 2 shows the basic process sequence for fabrication.…”

Design and implementation of a sub-nm resolution microspectrometer based on a Linear-Variable Optical Filter

“…The process starts by deposition of the lower dielectric mirror stack and the oxide layer that results in the cavity layer (layers 1-8 in Table 1). Photoresist is spin coated as the next step and lithography is applied to define the strip-like structure in the resist layer to be reflowed [8,10]. A series of trenches of constant width and with variable spatial frequency or trenches of variable width and constant pitch are etched over the length of the strip of resist to vary the effective amount of resist per unit area.…”

“…This is the main technological challenge in the fabrication of an ICcompatible LVOF. A process based on reflow of a patterned layer of photoresist has been developed for the fabrication of such tapered layers and is presented in [10].…”

Linear variable optical filter-based ultraviolet microspectrometer

“…The thickness profile of the tapered resist structures were characterized by a stylus profilometer [19]. The tapered resist pattern was transferred into a SiO 2 cavity layer by a dry-etching process that was optimized for minimum surface roughness.…”