CMOScompatible optical waveguides for onchip and offchip IC interconnects

1
CMOS-compatible optical waveguides for on-chip
and off-chip IC interconnects
Principal investigator Peter Persans Co
investigator Joel Plawsky Senior
students/associates N. Agarwal, Dr. Y. Chen, G.
Dalakos, S. Ponoth
Goal develop and evaluate materials and
processes for fabrication of optical waveguides
and couplers for on-chip, chip to chip, and 3D
chip architectures
Motivation optical waveguides provide high
bandwidth, low dispersion, low cross-talk
alternatives to wire for on-chip, chip to chip,
chip to board, and through-chip 3D interconnects

• Important tasks
• Design new optical beam steering structures based
  on materials properties and optical beam
  propagation modeling
• Develop and model CMOS-compatible processes
• Characterize new materials properties (loss,
  adhesion, stability)
• Fabricate and test materials and structures for
  multi-scale applications

2
Scenarios for the use of optical waveguide
components

• device-to-device on chip
• vertical dimensions 3 ?m
• lateral dimensions 1 ?m
• length 1 mm
• semiconductor or other large ?n materials

• through wafer for 3D
• lateral dimensions 1 ?m
• length1 mm
• vertical length 10-100 ?m

• coupling from semiconductor waveguide components
• lateral dimensions 1-10 ?m
• length 0.1-20 mm

cladding
Si wafer
Si wafer
3
Scenarios for the use of optical waveguide
components

• Guidelines for waveguide design
• High index contrast ? better wavefunction
  confinement ? smaller size components, less
  crosstalk, smaller radius bends good
• High index contrast ? larger surface scattering
  bad
• Resonant structures (rings, multilayers) ?
  wavelength-selective transmission, reflection,
  coupling
• For short distances, coatings and multilayers may
  be used
• For small structures, care must be taken to match
  wavefunctions before, inside, and after the
  inserted component.

chip
substrate, w/metal and optical interconnects

• coupling from chip to chip with optics in an
  interconnect sub- or super- strate
• lateral dimensions 1-10?m
• length 20 mm

fiber

• coupling from chip to fiber
• fiber bonded to chip (eg V-groove)
• fiber bonded to interconnect superstrate
• lateral dimensions 10 ?m
• length 10 mm

4
Polymer and inorganic thin films design
flexibility

• Polymers
• low loss (500-1200 nm)
• spin-on deposition
• compatibility with all CMOS materials
• useful from micro to macro scale
• Specific Materials
• polyimides and fluorinated polyimides
• siloxane epoxies
• Processing
• direct exposure
• photoresist etch barriers
• photoablation, RIE, plasma, wet etch
• Issues
• thermal and optical stability
• adhesion
• etching
• IR absorption and scattering

• Inorganic components
• wide range of refractive index
• plasma or sputter deposition
• Materials
• Metals Al
• PECVD a-SiH, SiNx, Si3N4, SiOx
• Xerogels as cladding
• Processing
• (PE)CVD, evaporation deposition
• RIE, plasma or wet etch
• thermal
• Issues
• adhesion/stability
• absorption loss, band gap
• conformal coating. roughness
• microcrystallinity
• processing temperatures
• alignment

5
Polymer material options
- low loss at 1550 nm
6
Waveguide shaping by etch undercut - modeling
The undercut profile is modeled using 2-D
Fortran based finite element package, PLTMG. a)
Differential etching of sacrificial and working
layer. b) Photoresist lift-off can decrease
angle.
An example of profiles resulting from a silicon
nitride sacrificial layer and oxide working
layer We learn about position and slope of the
wedge.
(Ponoth et al.,Mat Res Soc Symp Proc, 597, 81
(2000).)
7
Waveguide shaping by etch undercut - experiments

• SEM of BOE etch undercut of SiO2 note cutback
  of edge

• Contact AFM ? determine slope and surface
  roughness

• Liftoff of etch mask can affect angle.
• Materials choice is important.
• Slopes from 5 to 50 degrees have been achieved

Experimental and predicted undercut angles using
PECVD nitrides and oxides
(Ponoth et al.,Mat Res Soc Symp Proc, 597, 81
(2000).)
8
Transferring etch undercut to polymer waveguides

• direct etch undercut using high pressure RIE and
  sacrificial layer

• transferring angle by low pressure
• directional RIE etch of overlying undercut oxide

• characterization of relative etch rates and
  roughness under different RIE etching conditions

(N. Agarwal)
9
Multilayer structures for beam-turning and guiding
omnidirectional reflection coatings and/or ARROW
layers can be used as mirrors or as waveguide
cladding . The structure above is a schematic
of a resonant reflection waveguide pair with
short coupling region. The spectra at top left
are the measured reflectivity for a
plasma-deposited Si/SiO2 multilayer stack for
varying incident angle. The yellow bar is the
predicted photonic bandgap for the structure
imaged to lower left.
(Dalakos et al., in prep.)
10
Trade-off between bend radius and interface
scattering

• surface and sidewall roughness causes scattering
  loss

• bend loss decreases as ?n and radius increase

from Kimerling, IFC Workshop, MIT, March 2001

• scattering
• proportional to ?n2
• proportional to ?2
• depends on correlation spectrum - sensitive to
  correlations of order ?/n
• increases with decreasing width w-3

One approach taper width of core before and
after bend.
11
Effects of RIE etching on polyimide waveguides
basic studies of the evolution of surface
roughness
40 mT
500 mT

• statistical description of roughness
• roughness w?d
• scaling exponent ?1
• correlation length 0.4 ?m

• high pressure plasma leads to smoother top surface

• top surface roughness increases with etch depth
• O2/HCF3 plasma etching

(Agarwal et al., App. Phys. Lett., 78 (2001).)
12
Effects of RIE etching on polymers sidewall
roughness and scattering loss

• loss is dominated by scattering
• loss increases as width decreases, consistent
  with computed field at surface

low pressure
high pressure

• sidewall roughness is lower for lower pressure
  RIE

(RIE modeling T. Cale et al., T. M. Lu et al. )
13
New materials xerogels for waveguides and
cladding
Xerogels porous silica made by sol-gel method.
Consists of packed clusters of silica spheres. n
1.458 - 0.458porosity (l589 nm)

• We have fabricated straight waveguides
• siloxane epoxy polymer core/xerogel substrate -
  
• SiO2/xerogel 0.8-9.5 dB/cm
• Issues adhesion, processing

Porosity (and index) are controlled using solvent
ratios
(Ponoth et al. Mat Res Soc Symp Proc,637, 2001)
14
New materials siloxane polymers for core and
cladding

• Si-O backbone
• thermoset at 160oC, no volume change on
  crosslinking
• stable against oxidation
• stable to 300oC, 1 hr
• can be photosensitized for UV crosslinking,
  laser-written low loss waveguides, gratings
• variable index of refraction 1.4-1.6 by changing
  ligands
• n 1.5 8.6x10-3?m2/?2 for one type
• further variation by formation of porous films
• good adhesion to Si, thermal SiO2, plasma SiO2,
  xerogel, self, Al, silicon nitride
• low loss in slab waveguides at 650, 830 nm (dB/cm)
• controllable OH, CH bond densities for low
  absorbance at 1550 nm (current absorbance for
  400-1600 nm

collaboration with J. Crivello and Polyset Inc.