CMOSCompatible Waveguides for Optical Interconnects

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CMOS-Compatible Waveguides for Optical
Interconnects
SEMICONDUCTOR SUPPLIERS
Navnit Agarwal, Shom Ponoth, George Dalakos,
Yuchuan Chen, Joel Plawsky, Peter Persans
Goal to design, develop, and evaluate
structures, materials, and processes that will
enable the implementation of optical waveguiding
for on-chip and chip to chip interconnects.
Interconnect Focus Center
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Outline

• overview of waveguide considerations
• scaling considerations
• index difference and roughness/pitch/bend rad
• modeling of vertical mirrors
• alignment loss, in 3D, bend coupling and tapers
• control of interface roughness (scattering)

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Overview of Waveguide Design Issues
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Overview of waveguide issues
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Design and fabrication considerations
Design of optical interconnects
Special considerations for CMOS-manufacturing

• wavelength of operation - 830 nm
• straight-guide loss of lt 0.5 dB/cm
• loss per element lt 0.5 dB
• waveguide core dimensions of lt 2 microns at
  lowest level
• waveguide length 2 cm
• waveguide spacing microns
• out-coupling to optical fiber/free space

• low temperature (lt350oC) deposition and
  processing
• short-term thermal stability to gt400oC
• long-term thermal stability at lt150oC
• low water uptake/humidity sensitivity
• adhesion compatibility to SiO2, metal
• avoid impurity sources
• etch by standard wet, plasma, or RIE methods

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Polymer and low temperature inorganic thin films
design flexibility

• Polymers
• Advantages
• spin-on deposition
• compatibility with CMOS materials
• useful from micro to macro scale
• Characteristics
• low loss for ?lt1200 nm
• Processing
• direct exposure
• photoresist etch barriers
• photoablation
• RIE, plasma
• Issues
• thermal and optical stability
• adhesion
• etching
• IR absorption and scattering
• refractive index

• Inorganic components
• Advantages
• wide range of refractive index
• plasma or sputter deposition
• Processing
• (PE)CVD, evaporation deposition
• RIE, plasma or wet etch
• thermal
• Issues
• compatibility
• adhesion/stability
• absorption loss, band gap
• conformal coating. roughness
• microcrystallinity and scattering loss
• processing temperatures
• alignment

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Scaling
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Waveguide Size Scaling
In order to fabricate waveguides with small
radius bends and low crosstalk, it is necessary
to have large index difference (?n) between the
core and cladding of the waveguide. Increasing ?n
leads to increased scattering from surface
roughness.
Computation of multimode transmission of 90o
radius bend as a function of index difference
with radius as a parameter.
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Consequences of Scaling
Roughness Statistics
Waveguide geometry and material
Experimental data on U9020D polymer measured
using a prism coupler at two different wavelengths
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Scaling Trade-off between scattering loss and
waveguide pitch

• An example
• Choosing a line pitch d of lt 4 microns assuming
  d2a) ? ?n gt 0.2
• Taking ?n gt 0.2 with constraint of loss lt 0.5
  dB/cm ? roughness must be lt 5 nm

• The minimum allowable pitch d decreases with ?n.
• For fixed waveguide width, scattering loss ??n2,
  (except at large ?n.
• For fixed waveguide width, scattering loss
  roughness2. Roughness lt10 nm is required for
  usable pitch.
• When the allowable index (determined by
  crosstalk) for a given pitch is considered, the
  scattering loss is an even stronger function of
  ?n.

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Control of Interface Roughness
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Evolution of Surface Roughness with Plasma Etching
power spectrum of height-height correlation
For standard RIE conditions, the surface
roughness and the lateral correlation length grow
linearly with etch depth.
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Roughness evolution is determined by etch
conditions

• Low pressure etch large mean free path (l)
• High pressure etch small mean free path (l)
• More oblique the incidence of the reactants with
  respect to the surface being etched, the smoother
  the etch.

Experimental data on the roughness produced on
etching a fixed depth of film ( 750nm) at
different pressures. Top surface scattering loss
decreases with increasing pressure
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Roughening Mechanisms in Sidewalls

• At low pressures, reactive species normally
  incident w.r.t. the substrate
• Thus, for low pressures, at the sidewalls, the
  reacting species are more obliquely incident and
  the etch is smoother (based on the conclusions
  derived earlier)
• At high pressures, there are more reactive
  species that are obliquely incident w.r.t the
  substrate. These species more normally incident
  on the sidewall
• The species normally incident on the substrate,
  which would be obliquely incident w.r.t. the
  sidewalls are prevented from hitting the
  sidewalls by the hard mask overhang which exists
  because of undercutting at high pressure etch.
• Thus, for high pressures, at the sidewalls, the
  reacting species are more normally incident and
  the etch is rougher (based on the conclusions
  derived earlier)

Low Pressure
High Pressure
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Roughness control in PECVD films

• Deposition of inorganic thin films on
  temperature-sensitive substrates requires
  low-temperature processing
• Deposition at low temperatures leads to rougher
  films as seen in the film growth exponent, b, as
  a function of temperature
• RMS roughness, w, increases with time, t - w
  tb
• Ion bombardment modification during film growth
  yields smoother films at low-temperatures

Growth exponent, ??, plotted as a function of
temperature revised with results from this study
(from Dalakos et al., 2002) Note that the low
temperature cathodic film falls well below anodic
films.
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Optical Modeling
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Modeling of vertical bends
Waveguide microstructures are modeled using
BeamProp andFullWave or FEMLab. Left TIR
waveguide bend. Right Metal mirror
bend. Conclusions Low loss 90o
waveguide-to-waveguide bends can be achieved with
either high ?n alone or low ?n with a metal
mirror.
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Mirror efficiency sensitivity to angle and offset
variation of mirror angle alters efficiency for
right angle coupling

• variation of offset and overhang (misalignment)
  alters efficiency
• overhanglt 1 micron is acceptable
• offset should be lt 0.3 micron

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Modeling of HDMI-Optical structures
Width of beam at 1st level Si

• Simulation done using FullWAVE
• TE polarization
• Core Polyset Polymer
• Cladding Silicon Oxide, l830 nm.

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2001-2002 Publications

• PECVD Silicon Oxide-Aerogel and Polymer-Aerogel
  Optical Waveguides, S. Ponoth et al., MRS Proc.
  637 ,125 (2001).
• Roughness evolution in polyimide films due to
  plasma etching, N. Agarwal et al., Appl. Phys.
  Lett. 78 2294 (2001).
• Optimal Plasma Etching for Fabrication of
  Channel Waveguides, N. Agarwal, S. Ponoth, J.
  Plawsky, P. D. Persans, Conference Proceedings
  2001 IEEE LEOS 14th Annual Meeting, p. 578.
• "Porous Materials as Low-k Dielectrics for
  Electronic and Optical Interconnects", J.L.
  Plawsky, A. Jain, S. Ponoth, S. Rogojevic, N.
  Agarwal, W.N. Gill, P. Persans, Thin Solid Films
  398, 513 (2001).
• Dielectric Materials in Optical Waveguide and
  Packaging Applications, P. D. Persans, N.
  Agarwal, S. Ponoth, F. Huang, J. Plawsky in,
  Interlayer Dielectrics for Semiconductor
  Technologies, Ed. Shyam P. Murarka, Moshe
  Eizenberg, and Ashok K. Sinha (Academic Press)
  (in press).
• Surface roughness evolution of PECVD cathodic
  and anodic a-SiH, G. T. Dalakos, J. L. Plawsky,
  and P. D. Persans, MRS Proc. 715 (2002) (in
  press).
• Optical Waveguides with Embedded Air-gap Cladding
  Integrated with a Sea-of-Leads Wafer-level
  Package, T. V. Mule, M. Bakir, J. Jayachandran,
  R. Villalez, H. Reed, N. Agarwal, S. Ponoth, J,
  Plawsky, P Persans, P. Kohl, K. Martin, E.
  Glytsis, T. Gaylord, J. Meindl, Proc. IITC 2002
  (in press).
• Optimized Oxygen Plasma Etching of Polyimide
  Films for Low Loss Optical Waveguides, N.
  Agarwal, S. Ponoth, J. L. Plawsky, P. D. Persans,
  J Vac Sci Tech A (in press).