Anisotropic high aspect ratio etch for perfluorcyclobutyl polymers with stress relief technique

Abstract

The authors have developed an anisotropic, high aspect ratio (18:1) etch for perfluorocyclobutyl (PFCB) polymers with trenches as narrow as 800nm using a CO∕O2 etch chemistry in an inductively coupled plasma reactive ion etcher. Anisotropy is achieved by carbon sidewall passivation. The motivation for this etch development is to use the air trenches as very compact waveguide splitters [S. Kim et al., Opt. Eng. 45, 054602 (2006)]. The authors report a new trench widening mechanism due to tensile stress of the PFCB films and a method of avoiding this widening through the use of additional stress relief trenches on both sides of the desired trench.

Compact and high efficiency polymer air-trench waveguide bends and splitters

“…This stress tends to pull apart the edges of the trench to create a wider trench than would otherwise result if the film were unstressed. We have reported a method of avoiding this stress-induced broadening by etching narrow stress-relieving trenches on both sides of the main trench to take up the stress and protect the main middle trench from widening 8 We proceeded to fabricate splitters using this etch and the fabrication process mention in Ref. 3.…”

Ultracompact AWG Using Air-Trench Bends With Perfluorocyclobutyl Polymer Waveguides

“…Film cracking is normally not a problem when processing the core layer to form single mode waveguides and multimode interference (MMI) slab regions. However, as discussed in [19], fabrication processes that occur on top of the complete film stack (i.e., after the overclad has been applied on top of the waveguides that have been etched in the core layer) often end up promoting the development of cracks owing to stress in the film stack. As pointed out in [19], this stress forms during thermal curing of the PFCB layers because of the coefficient of thermal expansion mismatch between PFCB and silicon.…”

“…However, as discussed in [19], fabrication processes that occur on top of the complete film stack (i.e., after the overclad has been applied on top of the waveguides that have been etched in the core layer) often end up promoting the development of cracks owing to stress in the film stack. As pointed out in [19], this stress forms during thermal curing of the PFCB layers because of the coefficient of thermal expansion mismatch between PFCB and silicon. As shown in Table II, the polyimide substrate has a coefficient of thermal expansion (CTE) close to that of PFCB, which greatly diminishes the formation film stress during the curing process.…”

Science and technology of perfluorocyclobutyl aryl ether polymers

“…87,88 Recently, they have developed an anisotropic high aspect ratio (18:1) etch for PFCB aryl ether polymers with trenchs as narrow as 800 nm; thereby allowing their air-trench bends to reduce the area required by a factor of 20, as compared with conventional arrayed waveguide gratings. 89,90 Jiang et al 91,92 from the Communications Research Center in Canada, used PFCB aryl ether polymers as core and cladding materials to fabricate compact wide-band wavelength division multiplexers, based on arrayed waveguide grating structures, for coarse wavelength division multiplexing (Fig. 9).…”