Interface Structure of Photonic Multilayers Prepared

by PECVD

Hyeonjae Kim a, Mark D. Foster a, Hao Jiang b,c, Scott Tullis b, Timothy J. Bunning b,

Charles F. Majkrzak d

a Maurice Morton Institute of Polymer Science, The University of Akron, Akron, OH 44325b Air Force Research Laboratory, Materials and Manufacturing Directorate, WPAFB, OH 54533c Anteon Co., Dayton, OH 45431d NIST Center for Neutron Research, Gaithersburg, MD 20899

Research Interests in Photonics

•Non-inorganic films•Often inorganic l/4 stacks are deposited on plastics•Delamination problems arise

•Non-conventional, organic, multilayer thin films•Non-stacked filters

•Weakest part of film failure is interface

•Could smear interface to increase robustness

•Ability to tailor refractive index•Many times, we require an index which is not readily available (1.43)

n = 1.5

n = 1.3

n = 1.43

PECVD Schematic

Molecular Drag Pump

Gate Valve

Butterfly Valve

Pressure Transducer

Roughing pump

Sample

RF Power Supply

Flow Controller

Flow Controller

Flow Controller

UHP Argon

Sample RotationDevice

Precursor Bubbler

Precursor Gas Supply

Reaction Zone

High

High

High

Low

Low

Low

ns

n0

0

4

4

40tn f

• Notch filter by Plasma Enhanced Chemical Vapor Deposition (PECVD)

• Maximum interference occurs when thickness is l/4

Wavelength

Tra

nsm

ittan

ce

Increasing N

decreasesnn lh )(

Multilayer Thin Films: 1/4 Wave Stack

Objective

Structure ?

Thickness of each layer Interface width (roughness) Composition Density Crosslink density

Structure of PECVD Multilayers

Three monomers considered

* Benzene (B) : high refractive index, 1.61

* Octafluorocyclobutane (OFCB) : low refractive index, 1.40

* Hexamethyldisiloxane (HMDS) : 1.45

Films studied

Single layer films of single monomer Bilayer of PP-(OFCB/B) Multilayer of 5*PP-(B/OFCB) Single layers of “copolymer” substrate

PP-OFCBPP-B

X-ray or Neutron Specular Reflectivity Gives Averaged Structural Information

- qz = 4sin(/- Sensitive to the structure surface normal- Kiessig fringes, d=2/q- Roughness of interface - Scattering Length Density (SLD), (b/V)

0 0.1 0.2 0.3 0.4qz (Å-1)

Ref

lect

ivity

, R

100

10-1

10-2

10-3

10-4

10-5

10-6

qc

qki kf

q=qz

dThin film

Substrate

x

z

NR Gives SLD of PP-OFCB andReveals Uniform (b/V)n of PP-OFCB

Composition : CF1.8

Density, b ≈ 1.9 g/cm3

SiO2 = 2 Å

dOFCB = 251 Å

OFCB = 4 Å

XR Shows Low SLD Transition Regionin PP-OFCB Single Layer

Transition regionbetween substrate andOFCB film (~ 6Å) with composition of CF

Preferential reactivity of CF species

SiO2 = 2 Å

dOFCB = 251 Å

OFCB = 4 Å

XR Confirms the Structure of PP-dB Revealed by NR

Composition : CD1.23

Density, b = 1.16 g/cm3

SiO2 = 3 Å

ddB = 249 Å

dB = 5 Å

NR & XR Show Multilayer Structurein a Self-Consistent Manner

(OFCB/air) : 8 Å(dB/OFCB) : 13 Å (OFCB/dB) : 20 Å

d(dB) : 101 ± 4 Åd(OFCB) : 136 ± 17 Å

(OFCB/air) : 6 Å(dB/OFCB) : 16 Å (OFCB/dB) : 13 Å

d(dB) : 101 ± 2 Åd(OFCB) : 136 ± 17 Å

NR XR

Structure of B-OFCB Copolymer Films

- Uniform films can be made with B-OFCB copolymer

- Transition region varies in B-OFCB copolymer

Structure of OFCB-HMDS Copolymer Films

- Uniform, smooth films

- Transition region for both precursors

Off-specular X-ray Scattering Probes Interface Lateral Structure

Transverse scans from PS brushes

- Sensitive to in-plane correlation of interface structure- Liquid-like or not?- Periodic structures

SubstratePECVD film

q qz

qx

Surface Fluctuations Highly Suppressed On Single Layer

- Behavior different from that of tethered brush

- Surface fluctuations more suppressed on PECVD film

Long wavelength fluctuations do not grow with thickness

OFCB Benzene

Kinetic roughening proposed

results from competition between

film deposition and surface relaxation.

Deposition rates: PP-B : 55Å/min

PP-OFCB : 12Å/min

PP-OFCB surface relaxes more during deposition

=> Longer wavelengths not as strongly suppressed => Faster increase in roughness with thickness.

Swelling in solvent vapor for over 3hrs

PP-B in Toluene vapor PP-OFCB in THF vapor

Increase in thickness by

~2%

Increase in thickness by

~30%

Summary

PECVD creates well-defined, uniform, smooth films

XR and NR reveal structure of multilayer photonic film

Limited transient deposition behavior next to the substrate for OFCB and HMDS

Interfaces between PECVD layers can be sharp (~16 Å rms).

Air interface is very sharp ( < 8 Å rms).

Surface roughness may be dictated by kinetic roughening

Future work

- Better understand connection between surface roughness and deposition rate

- Study variations in crosslink density with depth

- Characterize structure of films with other types of interfaces

Acknowledgements

- Funded by the Collaborative Center for

Polymer Photonics (F49620-02-1-0428).

- Use of Advanced Photon Source supported by the U.S.

DOE, Office of Science, Office of Basic Energy Science,

under Contract No. W-31-109-ENG-38.

- Dr. Michael Silverstein and Dr. Takao Usami for

helpful discussions.

- Experimental assistance from Bulent Akgun.

NR Gives SLD of PP-dB andReveals Uniform (b/V)n of PP-dB

SiO2 = 8 Å

ddB = 238 Å

dB = 5 Å

Interface Width Revealed by both NR and XR for PP-(OFCB/dB) Bilayer

NR

- (OFCB / dB interface): 16 Å - Transition region in OFCB- (OFCB / dB interface): 13 Å

XR

Structure of HMDS

Me 3Si SiMe 3O