J. Phys. Chem. C 2007, 111, 9953-9960
9953
XPS and QCM Studies of Hydrocarbon and Fluorocarbon Polymer Films Bombarded by 1-20 keV C60 Ions Igor L. Bolotin, Stephanie H. Tetzler, and Luke Hanley* Department of Chemistry, UniVersity of Illinois at Chicago (m/c 111), 845 West Taylor Street, 4500 SES, Chicago, Illinois 60607-7061 ReceiVed: March 5, 2007; In Final Form: May 4, 2007
Poly(methyl methacrylate) or PMMA, Teflon AF1600, and poly(3-hexylthiophene) films are studied after bombardment with different fluences of C60 ions with energies of 1-20 keV by a quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS). C 1s XP spectra show little or no change in the film chemistry upon ion bombardment up to 1013-1014 ions/cm2. This result supports prior observations from secondary ion yields that at least some organic films have no apparent static limit in secondary ion mass spectrometry (SIMS) using C60 ion projectiles until the film is sputtered away. Changes in C 1s XP spectra observed at the highest ion energies and fluences >1015 ions/cm2 are explained predominantly by differential charging effects. Some changes in film composition are also observed, but their extent varies with the polymer studied. Measurements for the total sputter yield of films acquired using a QCM show that each C60 cluster leads to an efficient emission of ∼105 amu per C60 ion at 20 keV impact. The energy dependence for the total sputtering yield changes from a power law (with n ∼ 1.4) to linear after 5-10 keV. The sputtering yield at >10 keV of polymer films shows total enhancement of ∼5 times higher compared with gold targets.
I. Introduction Cluster ion beams such as C60+ and SF5+ accelerated to kiloelectronvolt kinetic energies confine their modifications to the topmost layer of a surface upon impact, making them versatile sources for analysis of organic films by secondary ion mass spectrometry (SIMS) when compared to atomic ion projectiles.1-12 Specifically, C60+ cluster ion beams have been shown to display significant yield enhancements during SIMS characterization of organic, polymeric, and biological films.7,10,13-20 C60+ ion beams also show promise for SIMS depth profiling of such molecular solids.9,16,17,21,22 Molecular dynamics (MD) simulations of C60+ impact with kinetic energies up to 20 keV on various substrates show that a crater forms upon ion impact, with the impact energy initially deposited into the near-surface region.18,19,23-27 It has been argued that the ability of cluster projectiles to sputter surfaces both more efficiently and with less damage to the substrate than atomic projectiles allows the use of cluster projectiles for low damage surface cleaning.25,28,29 The rapid accumulation of chemical damage created by a kiloelectronvolt incident atomic ion beam creates a static limit to the ion fluence, greatly reducing the amount of pristine material available for detection. Studies of ion beam effects on organic surfaces are sometimes performed at relatively high ion fluence so that the overlap of ion tracks or craters dominates, obscuring single ion sputtering events. Under such high fluence conditions (known as dynamic SIMS), the overall chemical modification of the organic film is the result of many possible chemical modifications induced by multiple cluster ion impacts within a given surface area. However, at least some organic films display no apparent static limit in C60+ SIMS with little or no change evident in * Corresponding author. E-mail:
[email protected].
secondary ion yields until the film is completely sputtered away.7,9,10,17,20-22,30 The aim of the present paper is to report experiments examining the modification and sputtering of polymers under kiloelectronvolt C60 ion bombardment. This paper investigates the energy dependence of the total sputter yield and the effects on several polymer films induced by kiloelectronvolt C60+,2+ ion bombardment. X-ray photoelectron spectroscopy (XPS) experiments are conducted to investigate C60 ion modification of spin-coated hydrocarbon and fluorocarbon polymer thin films: poly(methyl methacrylate) or PMMA, poly(3-hexylthiophene), and Teflon AF1600. Conditions for the static regime are extracted from separate measurements for the total sputter yield of these polymer films using a quartz crystal microbalance (QCM). It was shown previously that the QCM can be used to study the etch rates of organic films by ion beams.5,17,31 Some of the QCM results were presented previously.32 PMMA, Teflon AF1600, and poly(3-hexylthiophene) are chosen for study here because of their different properties. PMMA, shown in Figure 1a, is widely studied in cluster SIMS1,32 and has previously been shown to allow depth profiling by both SF5+ and C60+ cluster ion beams.5,17,30,33 Fluorocarbons are examined since prior work examined SIMS7 and depth profiling21 of Teflon by C60 ion beams. Teflon AF1600, shown in Figure 1b, is an amorphous fluorocarbon copolymer composed of 35% tetrafluoroethylene or TFE and 65% 2,2bistrifluoromethyl-4,5-difluoro-1,3-dioxole or PDD (Figure 1b). Teflon AF1600 has the advantage over regular Teflon of solubility in certain perfluorinated solvents, permitting the preparation of films via spin-coating. Poly(3-hexylthiophene) is a conductive polymer which is examined here to allow comparison of possible charging effects during bombardment and XPS analysis, since PMMA and Teflon AF1600 both are insulators.
10.1021/jp0718000 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007
9954 J. Phys. Chem. C, Vol. 111, No. 27, 2007
Figure 1. Chemical structures of the repeat units of (a) PMMA or poly(methyl methacrylate) and (b) Teflon AF1600. Teflon AF1600 has a 2:1 ratio of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole or PDD to tetrafluoroethylene or TFE. Each polymer has four chemically distinct carbons which show separate XPS peak positions. PMMA has two chemically distinct oxygen atoms which show separate XPS peak positions.
II. Experimental Details Details of the instrumentation used for cluster ion bombardment and photoelectron analysis have been described previously.5,32,34 Briefly, the system consists of a differentially pumped, Colutron ion source attached to a preparation chamber which is kept at a base pressure of 5 × 10-9 Torr. C60+,2+ ions are formed in an electron impact ion source, accelerated by 1-10 keV, mass separated by a Wien filter, bent 3° to remove fast neutrals, and guided by a series of dc ion optical lenses to impact the substrate at a normal angle. The electron impact ion source utilizes a cylindrical reservoir into which C60 powder is loaded. Evaporation of the C60 occurs when the cylinder is heated by electron impact, thereby injecting C60 vapor into the ionization region. The principal components of the primary beam consist of C60+ and C602+, and when the Wien filter selects C602+, the absolute energy of the beam is doubled, to a maximum of 20 keV. The data presented below indicate no difference in sputtering between C60+ and C602+ at equivalent impact energies, so the projectile ions are generically referred to below as C60 ions. A Faraday cup is used to obtain the true value of the total current and reduce the influence of secondary electrons. The primary ion current exhibits a slow drift in value during a given measurement and is therefore measured before and after bombardment to provide an approximation of the average current during sputtering. Typical average ion currents are between 50 and 500 pA over a 4 mm2 spot (the aperture area of the Faraday cup), and ion fluences are quoted using these values. Of course, the ion fluences outside this spot area are lower for any given analysis. The bombardment time varies between 30 min and a few hours. Bombardment and XPS analyses are completed in different vacuum chambers which are directly connected by a gate valve, allowing determination of the bombardment spot size via XPS mapping of the Si 2p core level from the substrate after high ion fluences. These measurements show that the crater formed in the PMMA film is several millimeters wide. The amount of material removed from the film during bombardment is determined using a quartz crystal microbalance (QCM; Model 934-000, 6 MHz, 14 mm diameter, Sigma Instruments with SQC 222 controller). The gold-coated QCM is mounted on the manipulator located in the preparation chamber and is fixed to the opposite side relative to the sample holder. This geometry allows use of the same film sputtering conditions for an independent sample and the QCM crystal through a simple manipulator rotation, with the only geometric limitation the requirement of an impact angle of 85° for the QCM rather than 90°. Any slow drift in the QCM frequency due to a variation in C60 ion current, crystal temperature, or other effects is compensated for by analysis of several
Bolotin et al. periods with the beam switched on and off. Differences in the QCM data for the on and off periods are statistically evaluated. The response for a rigid film on a quartz crystal to sputtering by an ion beam follows the modified Sauerbrey equation,35 in which the change in frequency is inversely proportional to the mass change and sputtering yield defined in terms of mass loss per incident ion.32 The frequency may also shift due to variation in the elastic properties of the investigated polymer layers on the surface, the temperature, and the pressure, but these effects are considered insignificant at low ion fluence. A frequency increase of 1 Hz corresponds to a mass depletion of ∼3.7 ng for this QCM. Here 0.5-3% solutions of PMMA (Aldrich, average Mw ) 15 000) in toluene, 1% solutions of poly(3-hexylthiophene) (Sigma-Aldrich) in chloroform, and 3% solutions of Teflon AF1600 (DuPont) in perfluoro(2-butyltetrahydrofuran) (Fluorinert-75 or C8F16O, Oakwood), are used for spin-coating at 3000 rpm for 30 s onto two types of substrates. Si wafers (Atomergic Chemical Corp., Si(100) p-type, boron doped) are hydrogen terminated with a minimum oxide by HF etching36 and used for photoelectron spectroscopy. Samples for quartz crystal microbalance (QCM) analysis are prepared by spin-coating the solution onto the gold sensor crystals. The QCM samples are coated as thick as possible while maintaining a stable QCM frequency in a vacuum. After coating, these samples are annealed on a hot plate at 120 °C for 20 min. The absolute thicknesses of organic films are perhaps best determined by X-ray reflectivity measurements,37 but these were deemed not feasible for this work. However, the PMMA and Teflon AF1600 film thicknesses are estimated at ∼200 nm and ∼3 µm, respectively, based upon comparison with other spin-coating experiments and estimates from sputtering and QCM measurements. XPS analysis is performed with a high-resolution monochromatic Al KR X-ray source (15 keV, 25 mA emission current, VSW MX10 700 mm Rowland circle monochromator) and a 150 mm concentric hemispherical analyzer with a multichannel detector (VSW Class 150) operated in constant energy analyzer mode. The photoemission angle is normal to the surface and the pass energy is 22 eV. The base pressure of the XPS chamber is 1 × 10-9 Torr. Samples are transferred from the ion bombardment preparation chamber into the analytical chamber without atmospheric exposure. In the course of the investigation of all samples, survey spectra and high-resolution 1s core level spectra of C, F, and O, as well as 2p spectra of Si, are recorded at different sample positions that correspond to native and C60 ion-bombarded areas. No electron flood gun is used to reduce sample charging, except as explicitly noted. Rather, charge compensation is accomplished by referencing to specific core levels in the same or a distinct sample, as discussed further below. All spectra are recorded after one scan to minimize the possibility of X-ray induced charging or damage effects and also because a 3 µm are cast onto a Si substrate, but the Si 2p substrate peak does not appear even after several hours of ion bombardment and the crater cannot be directly obtained by XPS mapping. This is not surprising since the total sputtering depth through Teflon AF1600 is estimated as 10 keV the energy dependence of the sputtering yield is closer to linear. This change in energy dependence is consistent with higher energy C60 ions penetrating more deeply into the film, which would lead to a decreased efficiency for sputtering. However, secondary ion yields generally increase at higher C60 ion energies.20 This is close to the energy dependence for the various spike models (Ym ∝ E) (see discussion in ref 32). Wucher and co-workers have found that particle emission by polyatomic projectiles cannot be interpreted in terms of a thermal spike sputter model and is instead characterized by a spike emission model consistent with a free expansion of a supercritically heated subsurface volume.57,58
Polymer Films Bombarded by 1-20 keV C60 Ions
J. Phys. Chem. C, Vol. 111, No. 27, 2007 9959
QCM measurements can be used to approximate the nonoverlapping crater or static regime. Under a simplified approximation that each impacting C60 ion forms a semispherical crater with radius rcr, the results for sputtering yields can be used to estimate the overlapping crater regime as
( )
1 1 2π F 1 ) ≈ Acr πr 2 π 3 Ym cr
2/3
(2)
where Acr is the “surface area” of the crater resulting from a single C60 impact and F is the density of the film. Under this simple approximation, the total sputtering yield of ∼8 × 104 amu of the Teflon AF1600 film (assuming F ) 1.7 g/cm3) at 20 keV C60 impact leads to a nonoverlapping crater regime of
