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 b Air Force Research Laboratory, Materials and Manufacturing Directorate, WPAFB, OH c Anteon Co., Dayton, OH d 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 UHP Argon Sample Rotation Device Precursor Bubbler Precursor Gas Supply Reaction Zone

High Low nsns n0n0 0 Notch filter by Plasma Enhanced Chemical Vapor Deposition (PECVD) Maximum interference occurs when thickness is l/4 Wavelength Transmittance Increasing N 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-OFCB PP-B

X-ray or Neutron Specular Reflectivity Gives Averaged Structural Information - q z = 4  sin(  / - Sensitive to the structure surface normal - Kiessig fringes, d=2  /  q - Roughness of interface - Scattering Length Density (SLD), (b/V) q z (Å -1 ) Reflectivity, R qcqc qq kiki kfkf q=q z d Thin film Substrate   x z

NR Gives SLD of PP-OFCB and Reveals Uniform (b/V) n of PP-OFCB Composition : CF 1.8 Density,  b ≈ 1.9 g/cm 3  SiO 2 = 2 Å d OFCB = 251 Å  OFCB = 4 Å

XR Shows Low SLD Transition Region in PP-OFCB Single Layer Transition region between substrate and OFCB film (~ 6Å) with composition of CF  Preferential reactivity of CF species  SiO 2 = 2 Å d OFCB = 251 Å  OFCB = 4 Å

XR Confirms the Structure of PP-dB Revealed by NR Composition : CD 1.23 Density,  b = 1.16 g/cm 3  SiO 2 = 3 Å d dB = 249 Å  dB = 5 Å

NR & XR Show Multilayer Structure in a Self-Consistent Manner  (OFCB/air) : 8 Å  (dB/OFCB) : 13 Å  (OFCB/dB) : 20 Å d(dB) : 101 ± 4 Å d(OFCB) : 136 ± 17 Å  (OFCB/air) : 6 Å  (dB/OFCB) : 16 Å  (OFCB/dB) : 13 Å d(dB) : 101 ± 2 Å d(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 Substrate PECVD film qqzqz qxqx  

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 (F ). - Use of Advanced Photon Source supported by the U.S. DOE, Office of Science, Office of Basic Energy Science, under Contract No. W ENG Dr. Michael Silverstein and Dr. Takao Usami for helpful discussions. - Experimental assistance from Bulent Akgun.

NR Gives SLD of PP-dB and Reveals Uniform (b/V) n of PP-dB  SiO 2 = 8 Å d dB = 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