N. Agarwal

Optimized oxygen plasma etching of polyimide films for low loss optical waveguides

Sidewall roughness in waveguides is one of the consequences of a reactive ion etch (RIE) step. Sidewall roughness increases scattering losses and can impediment the scaling of waveguide dimensions to on-chip levels. This article presents results from our experiments on the study of optimal RIE conditions to minimize roughness on waveguide sidewalls. Roughness in planar polyimide films was studied for different plasma etch conditions to obtain an understanding of how roughness evolves. Pressure was found to be the dominant factor affecting interface roughness. RIE done at lower pressures was found to increase the roughness on the film surface. We propose that the roughness produced depends on the angle of incidence of the etchant species with respect to the surface the more obliquely the etchant species were incident on the surface, the lower was the roughness produced. Roughness produced on waveguide sidewalls was studied by measuring their propagation loss. For sidewalls, lower pressure RIE resulted in s...

Roughness evolution in polyimide films during plasma etching

We report an experimental study on the evolution of etch front roughness in fluorinated polyimide films in oxygen based plasmas. For standard low-pressure (40 mT) etching conditions, the root-mean-square roughness, w, of the polymer surface increases with the amount of material etched, d, as w=0.0265(d−116)β with β=1, independent of etch rate, rf power, and gas composition. The etched surfaces can be described by the statistics of self-affine surfaces with scaling exponent, α=0.6±0.1 and lateral correlation length, ξ, of ∼0.3 μm. A dramatic reduction in roughness is observed under higher pressure etching conditions of 1000–2000 mT.

Fabrication of controlled sidewall angles in thin films using isotropic etches

Angular structures in thin films have applications in microelectronics and optoelectronics. In this study we analyze isotropic etching of a two-layer system for the fabrication of angular sidewalls. Such a process is attractive because of its bulk processing capability. The isotropic etch process is demonstrated to provide good control of the sidewall angle. Theoretical analysis along with numerical simulation of the slope etching process is used to understand profile evolution. The numerical simulations showed that to obtain a flat angular sidewall face, reaction controlled etching is needed. Angular sidewalls are fabricated in plasma enhanced chemical vapor deposition (PECVD) silicon oxide with PECVD silicon nitride as the sacrificial layer using an isotropic, buffered hydrofluoric acid based, wet chemical etch. PECVD silicon nitride is shown to be a good material for use as the sacrificial layer because of its etch rate controllability through deposition conditions. Controlled angles ranging from 5° to...

Siloxane-based polymer epoxies for optical waveguides

We introduce a new class of siloxane-based epoxy polymers for thin film optical waveguide applications. The thickness of spun-on films can be controlled by varying either spin speed or viscosity using solvents. Cured films exhibit excellent adhesion to silicon oxide and Al and excellent thermal and chemical stability. Waveguides of ~2 micron thickness on silicon oxide exhibit <0.2dB/cm loss at 830 nm. We demonstrate the formation of a 45° vertical mirror using reactive ion etching slope transfer from photoresist to epoxy polymer film.

Optical Properties of a Polyimide for Waveguide Applications in On-Chip Interconnects

In this work. we report on characterization methods and the optical properties of a polyimide (Ultradel 9020D). The prism coupling technique was used to couple light into polyimide thin films. The scattered light from the surface was imaged to determine the optical loss at different wavelengths. The coupling angle was used to find the refractive index of the films. The stability of the polymer in terms of refractive index and absorption losses was studied by heat treatment under nitrogen at elevated temperatures. No significant change was observed in the properties of the film at annealing temperatures below 200°C. The bulk and surface losses were separated and it was found that the waveguide losses were dominated by bulk losses. Temperature stability of the polymer was evaluated by annealing the polymer at 125°C for up to 30 hours. No significant changes were seen in the optical losses for these times.

Plasma silicon oxide–silica xerogel based planar optical waveguides

In this study, silica xerogels are used as the cladding for high-refractive-index-contrast waveguide systems. Silicon oxide, due to its relatively low refractive index, is an extensively used cladding material. The lower-refractive-index silica xerogel films enable us, to report losses for planar-slab waveguide systems with silicon oxide as the core. A spin-on sol-gel process was used for the silica xerogel deposition. The silicon oxide core was deposited using plasma enhanced chemical vapor deposition (PECVD), with silane and nitrous oxide as the reactive gases. Slab waveguides systems with core thickness of 1 μm and refractive-index contrast (Δn) as high as 0.35 were fabricated. With regard to the PECVD process, a deposition temperature of 150 °C enabled a stable structure, however, unacceptably high optical losses of 7±1.01 dB/cm at a wavelength of 650 nm and 5.59±0.69 dB/cm at 830 nm were measured using a prism-coupler based setup. On increasing the deposition temperature of the silicon–oxide film to ...

Silicon CMOS BEOL Compatible Optical Waveguide Micro-mirrors

Optical waveguides are being explored for on-chip purposes to overcome the speed limitations of electrical interconnects. Passive optical components like waveguides and vertical outcouplers are important components in such schemes. In this study we fabricate planar waveguides with integrated vertical micro-mirrors using standard Back End of the Line silicon (BEOL) CMOS based processes. Around 1.6 μm of a hybrid alkoxy siloxane polymer with a refractive index of ∼ 1.50 at the intended wavelength of 830 nm is used as the core and plasma deposited silicon oxide with a refractive index of ∼ 1.46 is used as the cladding. The angular face in the polymer waveguide that would function as the mirror surface was fabricated by a pattern transfer method which involves transferring the angle in a template to the waveguide using anisotropic reactive ion etching. The sidewall angle realized in a positive resist on patterning was used as the angle template. Exposure and development conditions were adjusted for Shipley® S1813 photoresist to generate a sidewall angle of ∼ 65°. The anisotropic Reactive Ion Etching (RIE) was done using a CF4/O2 plasma chemistry. A gas composition of 50/50 CF4/O2 was chosen in order to minimize the etch related roughness of the polymer and the photoresist. The metallization of the mirror faces was done using a self-aligned maskless technique which ensures metal deposition only on the angular face and also eliminates a lithography step.

Optimal plasma etching for fabrication of channel waveguides

We report on a study of the evolution of the surface roughness of polymer films during plasma etching. Effect of plasma etching on sidewall roughness was also studied by fabricating different width waveguides and measuring their loss. The understanding of surface roughness evolution was used to explain the roughness effects observed in sidewalls and strategies to minimize the same have been developed. Fluorinated polyimides were used as candidate materials for this study. Roughness evolution on the top surface of the polyimide films was studied by exposing the film to an oxygen plasma under different conditions. After each run, the amount of material etched was measured and a non-contact AFM scan of the surface was obtained.