Sandia National Laboratories

Materials & Processing for Si Compatibility

Charles T. Sullivan

ctsulli@sandia.gov, 505/844-9254

Center for Compound Semiconductor Science and Technology

Microsystem Science, Technology and Components Center

Shawn Lin and Jim Fleming

Introductory overviewPhotonic Bandgap Materials

Optical Interconnects for High Performance Computing WorkshopOak Ridge 11/8-9/99

Sandia National Laboratories

Possible Options for On-Chip Waveguide Interconnects

• Hard dielectric waveguides

– low-loss optical fiber compatibility

» low index contrast N ~ 0.005

» = 0.1 dB/cm to < 0.01 dB/cm

» e.g., LPCVD-based buried BPSG/TEOS

– higher-index for higher-density routing

» high index contrast N > 0.1

» < 0.1 dB/cm ?

» e.g., LPCVD-based SiON/TEOS

• Polymeric waveguides

– low-temperature post-processing

» low index contrast N ~ 0.05-0.005

» ~ 0.1 dB/cm to 0.5 dB/cm, depending on » e.g., fluorinated acrylates or polyimides

Sandia National Laboratories

Possible Applications of PBG Materials

1) Passive devices - Infrared Mirrors - Thermal Emissivity Modification - Prisms - Optical Communications - Cavities - Spectroscopy - Military and Optical Communications - Waveguides - 90° bends possible in three dimensions 2) Active devices - Ultra-Fast Switches - Si Infrared LED’s - Si Infrared Lasers 3) Integrated devices - Photonic circuits

Potential Applications

Wavelength (µm)T

rans

mit

tanc

e (%

)

0 10 12 14 16 18 20

1000

100

10

1

The Photonic Bandgap is present at any angle of incidence

(a) = 0° (b) = 30° (c) = 40° (d) = 50° (e) = 60°

a b c d e

<011>

<100>

Wavelength (m)T

rans

mis

sion

Bandstop is LargelyIndependent of Angle

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Simulated Electric Field Patterns for90-degree Waveguide Bend

2D Square Lattice 3D Square Lattice

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90-degree Waveguide Bend at Millimeter-Wave Frequencies

<01>

<10>

Tra

nsm

issi

on E

ffic

ienc

y

Frequency (GHz)75 9080 1059585 110100

theory

experiment

1.2

1.0

0.8

0.6

0.4

0.2

0

Sandia National Laboratories

0 2 4 6 8 10 12 14 16

0.2

0.4

0.6

0.8

1.0

1.2

0

Wavelength of Mid-gap, (microns)

Min

imum

Fea

ture

Siz

e (m

icro

ns)

Highly Complex

More straight- forward, but limited by stress

Opt

ical

C

omm

un

icat

ion

s

Mil

itar

y A

ppli

cati

ons

The minimum feature size for a lattice with bandgap

of 1.5 micron is 0.18 microns!

The Microfabrication Challenge

Sandia National Laboratories

3D Silicon Photonic Crystal at Mid-IR Frequencies

Side View

Top View

poly-Si

Si substrate

Technology Challenges

- precise stacking;- smooth planarization;- large area uniformity.

4-layer crystal

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Mold Process Flow

4) Deposit thin SiN, CMP stopping layer.

5) Pattern the second level.

6) Fill gaps between patterned poly with oxide.

7) Chemical mechanically polish, stopping on SiN.

Mold Process Flow

1) Deposit oxide with thickness equal to that of the layer.2) Pattern and etch mold.

3) Fill mold with polysilicon. Poly thickness equal to that of the layer thickness. (Only works for designs where W ~T)

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The Simple Cubic StructureFabricated Using the Mold Process

0

20

40

60

80

100

120

4 6 8 10 12 14 16

Tra

nsm

itta

nce

(%

)

Wavelength ( m)

Simple Cubic

10L

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Fillet Flow Process

Fillet Process Flow

2) Pattern oxide with lines and spaces, 6500Å lines, 6500Å space. Etch oxide in HF, remove ~900Å isotropically.

1) Deposit SiN (first layer only), poly with layer thickness (2200Å) and SiN hard mask (500Å). Deposit 5000Å oxide sacrifical layer.

3) Deposit 1800Å polysilicon fillet layer

4) Form fillet using anisotropic RIE.

5 Remove sacrifical oxide in HF.

6) Etch SiN in hot phosphoric acid.

7) Use fillet as a mask for poly etch.

8) Fill spaces between lines with oxide.

9) CMP, stopping on the SiN.

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1.5µm Bandgap Fillet Structure

1.3 1.5 1.7

Wavelength (m)

1.4 1.6 1.8T

rans

mis

sion

Ef f

i ci e

ncy

100

20

bandgap

Measured Results

3L

4L

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Novel Structures Under Investigation

Sandia National Laboratories

Singlemode 3D Defect Cavity

1

2

3 4

56

7

6.4m(0.6)

9.2m (0.8 )

Defect Volume: 0.23

cavitymode

Modal Volume: 0.83

Higher-QResults