Interface Structure of Photonic Multilayers Prepared by PECVD
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 Maurice Morton Institute of Polymer Science, The University of Akron, OH 44325 b Air Force Research Laboratory, Materials and Manufacturing Directorate, WPAFB, OH 54533 c Anteon Co. , Dayton, OH 45431 d NIST Center for Neutron Research, Gaithersburg, MD 20899 a
Research Interests in Photonics • Non-conventional, organic, multilayer thin films • Non-stacked filters • Weakest part of film failure is interface • Could smear interface to increase robustness • Non-inorganic films • Often inorganic l/4 stacks are deposited on plastics • Delamination problems arise • Ability to tailor refractive index • Many times, we require an index which is not readily available (1. 43) n = 1. 43 n = 1. 5 n = 1. 3
PECVD Schematic Roughing pump Molecular Drag Pump Butterfly Valve Sample Rotation Device Flow Controller Gate Valve Sample Pressure Transducer Flow Controller Precursor Gas Supply Flow Controller Reaction Zone RF Power Supply Precursor Bubbler UHP Argon
Multilayer Thin Films: 1/4 Wave Stack • Notch filter by Plasma Enhanced Chemical Vapor Deposition (PECVD) • Maximum interference occurs when thickness is l/4 n 0 l 0 Transmittance High Low High Increasing N Low ns Wavelength
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 n Bilayer of PP-(OFCB/B) n Multilayer of 5*PP-(B/OFCB) n Single layers of “copolymer” n PP-B PP-OFCB substrate
q=qz kf ki q q Thin film Substrate z x d Reflectivity, R X-ray or Neutron Specular Reflectivity Gives Averaged Structural Information 100 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 0 qc 0. 1 Dq 0. 2 0. 3 qz (Å-1) - qz = 4 psin(q)/l - Sensitive to the structure surface normal - Kiessig fringes, d=2 p/Dq - Roughness of interface - Scattering Length Density (SLD), (b/V) 0. 4
NR Gives SLD of PP-OFCB and Reveals Uniform (b/V)n of PP-OFCB Composition : CF 1. 8 Density, rb ≈ 1. 9 g/cm 3 s. Si. O 2 = 2 Å d. OFCB = 251 Å s. 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 Þ Preferential reactivity of CF species s. Si. O 2 = 2 Å d. OFCB = 251 Å s. OFCB = 4 Å
XR Confirms the Structure of PP-d. B Revealed by NR s. Si. O 2 = 3 Å dd. B = 249 Å sd. B = 5 Å Composition : CD 1. 23 Density, rb = 1. 16 g/cm 3
NR & XR Show Multilayer Structure in a Self-Consistent Manner XR NR s(OFCB/air) : s(d. B/OFCB) : s(OFCB/d. B) : 8Å 13 Å 20 Å d(d. B) : 101 ± 4 Å d(OFCB) : 136 ± 17 Å s(OFCB/air) : s(d. B/OFCB) : s(OFCB/d. B) : 6Å 16 Å 13 Å d(d. B) : 101 ± 2 Å d(OFCB) : 136 ± 17 Å
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 qx q qz b a PECVD film Substrate - Sensitive to in-plane correlation of interface structure - Liquid-like or not? - Periodic structures
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 n OFCB n Benzene
Kinetic roughening proposed n n n s 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 3 hrs PP-B in Toluene vapor Increase in thickness by ~2% PP-OFCB in THF vapor Increase in thickness by ~30%
Summary n PECVD creates well-defined, uniform, smooth films n XR and NR reveal structure of multilayer photonic film n Limited transient deposition behavior next to the substrate for OFCB and HMDS n Interfaces between PECVD layers can be sharp (~16 Å rms). n Air interface is very sharp ( < 8 Å rms). n 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 49620 -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-d. B and Reveals Uniform (b/V)n of PP-d. B s. Si. O 2 = 8 Å dd. B = 238 Å sd. B = 5 Å
Interface Width Revealed by both NR and XR for PP-(OFCB/d. B) Bilayer NR - s (OFCB / d. B interface): 16 Å XR - Transition region in OFCB - s (OFCB / d. B interface): 13 Å
Structure of HMDS
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