Introduction and Motivation

1
Optical Interconnects Optimal Etching
Conditions for the Fabrication of Channel
Waveguides
Navnit Agarwal, Shom Ponoth, Joel Plawsky,
Peter Persans Department of Electrical,
Computer and Systems Engineering, Department of
Chemical Engineering, Department of
Physics Rensselaer Polytechnic Institute, Troy NY
Ph [phone] Fax [phone] Email
agarwn_at_rpi.edu

• Introduction and Motivation
• Waveguided interconnects are one possible
  approach towards implementing optical
  interconnects for on-chip and off-chip
  applications
• Desirable optical properties include low losses
  and high index difference (between core and
  cladding) for increased scalability
• High index difference coupled with roughness at
  the core-cladding interface can increase
  scattering losses in the waveguide and impediment
  scaling
• Figure 1 shows the channel loss measured on
  polyimide channels of different widths. It is
  observed that the channel loss increases with
  decreasing channel width.
• A simple analysis of scattering from sidewalls
  predicts the scattering loss (a) proportional to
  the square of the intensity of the mode (f2) at
  the interface. Figure 2 confirms that the
  increased channel loss is from scattering at the
  channel interfaces
• Roughness is one of the consequences of plasma
  etching of channels. Our study focuses on the
  effect of plasma parameters on the evolution of
  roughness and strategies to minimize the same.
  Fluorinated polyimides (Ultradel series from
  AMOCO) were used as candidate materials in this
  study

Figure. 2
Figure. 1

• Samples A through E 40 mT, F 1000 mT, and G
  2000 mT plasma pressure
• At lower pressure Increasing roughness with
  increasing etch depth
• At higher pressure Almost no change in
  roughness with etch depth
• More oblique the incidence of the reactants with
  respect to the surface being etched, the smoother
  the etch
• Details available in 1. Based on our
  explanation, we estimate the transition region
  between a rough etching to a smooth etching
  regime to be 465 mT. Experimentally obtained
  data on the roughness produced on etching a fixed
  depth of film ( 750 nm) at different pressure
  conditions is shown in Figure 4.

Evolution of Surface Roughness of Polyimide on
Oxygen Plasma Etching
Reactant redistribution towards the valleys in
the feature results in increased roughness with
increasing etch depth
More oblique incidence causes preferential
etching at the peaks due to shadowing effects
Figure. 3
Figure. 4

• Channels etched under different pressure
  conditions
• Waveguides fabricated at lower pressure exhibited
  lower scattering loss. This can be explained
  based on the roughness evolution observed
  previously.
• With respect to the sidewall, the more oblique
  incidence occurs under the lower pressure etch
  conditions. At higher pressures, the undercutting
  results in an overhang of the hard mask above the
  patterned feature. This prevents the reactants
  normal to the substrate from impinging on the
  sidewalls and consequently a less smoothening
  etch. The presence of the overhang for high
  pressure etches was verified using SEM. Thus,
  etches performed at lower pressure produce lesser
  roughness at the sidewalls, resulting in lower
  scattering loss.

Effect of Plasma Etching on Sidewall Roughness
No overhang of SiNx hardmask during low pressure
etch
Overhang of SiNx hardmask on high pressure
etching
Figure. 5
Summary Plasma etching conditions and their
effect on interface roughness have been
investigated. Plasma pressure was found to be the
most important variable effecting interface
roughness. Based on our studies we propose low
pressure etching for waveguides in order to
minimize interface roughness and consequent
scattering loss.
1 Navnit Agarwal, Shom Ponoth, Joel Plawsky,
and P. D. Persans, Applied Physics Letters, vol
78, no 16, pp 2294-2296, 2001