SIMS depth profiling of deuterium labeled polymers in polymer multilayers
Introduction
Depth profiling of tracer-labeled polymers in polymer films using well-established techniques such as neutron reflectometry (NR), forward recoil spectrometry (FRES), and secondary ion mass spectrometry (SIMS) has been the subject of numerous investigations in recent years [1]. The film thicknesses and geometry required for analysis using these techniques are ideal for probing various phenomena related to physical confinement of polymer chains near surfaces and interfaces [2]. Although FRES has been used extensively for depth profiling of deuterium labeled polymers, it suffers from poor depth resolution (≈80 nm) with the so-called high energy (≈2.7 MeV) technique [1], [3]. Improvements in depth resolution (≈30 nm) have been made with the development of the low energy (≈1.3 MeV) and ToF (≈2.0 MeV) FRES techniques, but convolution resulting from secondary scattering below the surface is inherent to this method [3]. Deuterium (D) has a much higher scattering length density for coherent scattering of neutrons than protium (H), and therefore, NR (specular scattering) can be used for depth profiling [1]. Although it has superior depth resolution (≈1 nm), inversion of the profile to real-space is model dependent, requiring a priori knowledge of the profile functionality [4]. This often requires a depth profiling technique, such as FRES or SIMS, to be used in conjunction with NR [1]. Because of its superb depth resolution for depth profiling of polymers (≈10 nm) and ability to provide real-space depth profiles directly, SIMS has become a prominent method for analysis of thin polymer films and multilayers [5], [6]. It should be noted that much better depth resolution (<10 nm) is possible with SIMS analysis of highly structured materials using very low impact energies [7]. With polymers, problems associated with charge neutralization arise with low impact energies [6], and the process by which the films are assembled makes it extremely difficult to obtain effective depth resolutions below 5 nm [8].
At heterogeneous polymer/polymer interfaces, however, SIMS analysis can become difficult due to problems associated with differing matrix ion yields and sputtering rates between the two immiscible polymers. These problems exist due to extremely complex molecular interactions that are strongly dependent on the primary ion species and chemical environment [9], [10], [11]. By systematically investigating depth profiles through these polymer/polymer interfaces, one can obtain a better understanding of the parameters involved in accurate SIMS analysis of polymer films and multilayers. Several model polymer bilayer systems have been employed in this investigation, including atactic polystyrene (PS) and syndiotactic poly(methyl methacrylate) (PMMA), PS and atactic poly(2-vinylpyridine) (P2VP), and atactic poly(cyclohexyl methacrylate) (PCHMA) and PMMA. In all three cases atactic deuterated polystyrene (dPS), which was imbedded in PS or PCHMA, was the tracer polymer. Using a CAMECA IMS-6f magnetic sector spectrometer, SIMS depth profiles of deuterated polymers have been obtained under varied analysis conditions using Cs+ and O2+ primary ion bombardment. Matrix ion yields (YM), which are the detected intensities (counts/s) of atomic 12C, are shown for all three systems, with particular attention to the heterogeneous interface. Sputtering rates (SR) for PS and PMMA films have been quantified for both Cs+ and O2+ bombardment and referenced to intrinsic (100) Si (SR/SR,Si).
Section snippets
Materials and sample preparation
The polymers used in this investigation were purchased from Polymer Source and Scientific Polymer Products. Low-doped silicon wafers (100) were purchased from Wafer World and cleaned using previously outlined procedures [12]. First, the wafers were cut into 2 cm × 2 cm squares and soaked in a hydrogen peroxide/sulfuric acid solution at approximately 100 °C for 30 min and subsequently washed with deionized (DI) water. Next, they were soaked in 10% (v/v) hydrofluoric acid for 1 min and again washed with
Results and discussion
Much of the physically relevant information obtainable from a depth profile is gathered at or near the surface or heterogeneous interface, and as such, these are the most important regions for which a detailed understanding of the conditions for optimal SIMS analysis are required. Here, attention will be focused on the heterogeneous interface, as this is the most challenging region. Both Cs+ and O2+ were used to evaluate YM for the three types of bilayers, as shown in Fig. 2. For the profiles
Conclusions
The utility of SIMS for analyzing three different polymer/polymer bilayer systems (PS/PMMA, PCHMA/PMMA, and PS/P2VP) has been demonstrated. The sputtering rates for PS and PMMA using 25 nA (5.5 keV impact) O2+ and 10 nA (6.0 keV impact) Cs+ have been measured. It has been clearly established that there are profound effects due to sample preparation and history, chemical composition of the samples, and type of primary ion bombardment, which result in changes in the sputtering rates and secondary ion
Acknowledgements
This work was supported by the National Science Foundation (DMR-0071743) and the U.S. Department of Energy (DE-FG02-98ER45737).
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