title: Depth profiling cross-linked poly(methyl methacrylate) films: a time-of-flight secondary ion mass spectrometry approach.
abstract: BACKGROUND
  In order to determine the degree of cross-linking on the surface and its variations in a nanometer-scale depth of organic materials, we developed an approach based on time-of-flight secondary ion mass spectrometry (TOF-SIMS), which provides rich chemical information in the form of fragment ions. TOF-SIMS is extremely surface-sensitive and capable of depth profiling with the use of a sputter ion beam to remove controllable amounts of substance.
  
  
  METHODS
  Poly(methyl methacrylate) (PMMA) films spin-coated on a Si substrate were cross-linked using a recently developed, surface sensitive, hyperthermal hydrogen projectile bombardment technique. The ion intensity ratio between two ubiquitous hydrocarbon ions, C6H-and C4H-, detected in TOF-SIMS, denoted as ρ, was used to assess the degree of cross-linking of the PMMA films. The cross-linking depth of the PMMA films was revealed by depth profiling ρ into the polymer films using a C60+sputter beam.
  
  
  RESULTS
  The control PMMA film spin-coated on a Si substrate was characterized by ρ = 32% on its surface when using a 25 keV Bi3+primary ion beam. This parameter on the PMMA films subjected to HHIC treatment for 10, 100 and 500 s increased to 45%, 56% and 65%, respectively. The depth profiles of ρ obtained using a 10 keV C60+ion beam resembled an exponential decay, from which the cross-linking depth was estimated to be 3, 15 and 39 nm, respectively, for the three cross-linked PMMA films.
  
  
  CONCLUSIONS
  We demonstrated that the ion intensity ratio of C6H-to C4H-detected in TOF-SIMS provides a unique and simple means to assess the degree of cross-linking of the surface of PMMA films cross-linked by the surface sensitive hyperthermal hydrogen projectile bombardment technique. With a C60+sputter beam, we were able to depth profile the PMMA films and determine cross-linking depths of the cross-linked polymer films at nanometer resolutions. Copyright © 2017 John Wiley & Sons, Ltd.
authors.affiliation: Surface Science Western, The University of Western Ontario, 999 Collip Circle, London, Ontario, N6G 0J3, Canada.|Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, N6A 3K7, Canada.|Advanced Mineral Technology Laboratory, 100 Collip Circle, London, Ontario, N6G 4X8, Canada.
authors.forename: Soheila
authors.initials: S
authors.lastname: Naderi-Gohar
authors.affiliation: Surface Science Western, The University of Western Ontario, 999 Collip Circle, London, Ontario, N6G 0J3, Canada.|Amec Foster-Wheeler, 700 University Avenue, Toronto, Ontario, M5G 1X6, Canada.
authors.forename: Kevin M H
authors.initials: KM
authors.lastname: Huang
authors.affiliation: Advanced Materials Laboratory, Xerox Research Centre of Canada, Mississauga, Ontario, L5K 2L1, Canada.|TE Connectivity, 306 Constitution Drive, Menlo Park, CA, 94025, USA.
authors.forename: Yiliang
authors.initials: Y
authors.lastname: Wu
authors.affiliation: Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu, Sichuan, 610207, China.
authors.forename: Woon Ming
authors.initials: WM
authors.lastname: Lau
authors.affiliation: Surface Science Western, The University of Western Ontario, 999 Collip Circle, London, Ontario, N6G 0J3, Canada.|Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, N6A 3K7, Canada.
authors.forename: Heng-Yong
authors.identifier: http://orcid.org/0000-0002-8287-5171
authors.initials: HY
authors.lastname: Nie