Article pubs.acs.org/ac
Effect of Cosputtering and Sample Rotation on Improving C60+ Depth Profiling of Materials Hua-Yang Liao,† Meng-Hung Tsai,‡ Hsun-Yun Chang,† Yun-Wen You,† Chih-Chieh Huang,‡ and Jing-Jong Shyue*,†,‡ †
Research Center for Applied Science, Academia Sinica, Tapei 115, Taiwan Department of Materials Science and Engineering, Nation Taiwan University, Taipei 106, Taiwan
‡
ABSTRACT: In the past decade, buckminsterfullerene (C60)based ion beams have been utilized in surface analysis instruments to expand their application to profiling organic materials. Although it had excellent performance for many organic and biological materials, its drawbacks, including carbon deposition, carbon penetration, continuous decay of the sputtering rate, and a rough sputtered surface, hindered its application. Cosputtering with C60+ and auxiliary Ar+ simultaneously and sample rotation during sputtering were proposed as methods to reduce the above-mentioned phenomena. However, the improvement from these methods has not been compared or studied under identical conditions; thus, the pros and cons of these methods are not yet known experimentally. In this work, a series of specimens including bulk materials and thin films were used to explore the differences between cosputtering and sample rotation on the analytical results. The results show that both of these methods can alleviate the problems associated with C60+ sputtering, but each method showed better improvement in different situations. The cosputtering technique better suppressed carbon deposition, and could be used to generally improve results, especially with continuous spectra acquisition during sputtering (e.g., dynamic secondary ion mass spectrometry (SIMS) depth profiling). In contrast, for the scheme of sputter-thenacquire (e.g., alternative X-ray photoelectron spectrometry or dual-beam static SIMS depth profiling), a better result was achieved by sample rotation because it resulted in a flatter sputtered surface. Therefore, depending on the analytical scheme, a different method should be used to optimize the experimental conditions.
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dramatically enhanced.22 In this case, the properties of the newly exposed surface and the ionization yield of the secondary ions changed significantly, causing the analysis results to be less representative of the original state. To further expand the application of C60+ sputtering, many studies have made efforts to resolve or suppress the abovementioned problems. The approaches in common use include sputtering at grazing angle,25−27 cooling specimens with liquid nitrogen,20,28 backfilling a trace of oxygen into the analytical chamber,29 rotating samples during the sputtering process,20,27,30 and adding a low-energy Ar+ stream as C60+ sputters concurrently.1,24,31−34 Since the chemical damage, carbon accumulation, and interfacial width of depth profiles decrease as the incident angle of C60+ beams increases, high incident angle such as 70° is often used. Backfilling oxygen as background gases and cooling the samples also can partially resolve the problem that resulted from C60+ sputtering, but the improvement is not greatly obvious and the apparatus needs to be established additionally. Therefore, this study focused on comparing the latter two methods which provide apparent
ecause the buckminsterfullerene (C60)-based ion source was applied to the field of surface analyses, including X-ray photoelectron spectrometry (XPS),1−3 secondary ion mass spectrometry (SIMS),4−11 and scanning probe microscopy (SPM),12,13 the sputter depth profiling of organic or biological materials has developed rapidly over the past few years. Originally designed as a surface smoothing tool,14 the gas cluster ion beam has also been used to remove structurally damaged surface layer of organic materials recently.15−19 The demerits and limitations of using the C60 ion as a sputter source were gradually observed and addressed as the technique developed.1,3,20−22 One of the most vital issues is the surface graphitization of some organic surfaces after long-term C60-ion sputtering.23 This phenomenon decreases the sputter yield, and the characteristic signals could disappear during depth profiling. Furthermore, the C60+ sputtering induced strong surface morphology;24 hence, each spatial location may need individual depth calibration.5 Additionally, the carbon deposition onto and penetration into the sputtered area were reported frequently.1,3,21 Unless the removal of materials and the graphitization/deposition/penetration processes can reach a steady state, the sputtering rate will not be constant during profiling and will present a fundamental problem for calibrating analysis depth. In some materials, a carbide species was also observed during the C60+ sputtering, and the oxygen uptake was © 2012 American Chemical Society
Received: July 23, 2012 Accepted: September 27, 2012 Published: September 27, 2012 9318
dx.doi.org/10.1021/ac3020824 | Anal. Chem. 2012, 84, 9318−9323
Analytical Chemistry
Article
best performance under specific experimental conditions. In general, cosputtering was more effective for suppressing carbon deposition and was suitable for general depth profiling, including continuous spectrum acquisition during sputtering. Using sample rotation, the smoothest surface and lowest interfacial width were observed. However, because the mechanical movement makes the condition unsteady, sample rotation is better suitable for a sputter-then-acquire scheme. These two methods can be used at the same time to further improve the results if continuous acquisition is not required.
improvements and can be operated at commercial instruments without additional accessories. Several reports indicate that rotating the sample during the C60+-sputtering process could suppress carbon deposition and penetration.20,27,30,35 For example, on a model, a multilayer sample composed of interlacing Irganox 1010 and 3114, a highquality depth profile could only be obtained with C60+ sputtering on a rotating sample.20 Additionally, the C60+sputtered specimens with and without rotation were compared, and the surface of the rotated sample was less rough. Similarly, molecular dynamic (MD) simulations also revealed that decreased surface roughness could be obtained if the sample was rotated during the C60+-sputtering process.30 A smoother surface and a decreased amount of carbon deposition are beneficial for the depth profile. Beside sample rotation, the cosputtering of a low-energy Ar+ stream with C60+ sputtering showed improved results for the generation of a smoother surface and suppression of carbon deposition compared to C60+ sputtering alone.1,24,31−34 With the steadier sputtering process, multilayer organic LED devices could be profiled by the cosputtering technique, yet similar results could not be obtained with single C60+ sputtering because of the decreased sputtering rate.32 This decreased sputtering rate also reflects the decaying intensity of the characteristic SIMS signals during prolonged C60+ sputtering, while a relatively stable SIMS signal could be obtained with the optimized cosputter of 10 kV, 10 nA C60+ and 0.2 kV, 300 nA Ar+.31,32 MD computer simulation also provided insight that the C60 bombardment creates an anisotropic roughened surface, while the Ar bombardment ameliorates it. 36,37 Furthermore, for lower Ar+ energy and current, the effect of cosputtering is less pronounced. On the other hand, higher Ar+ energy and current generate excessive damage to the remaining surface that cannot be removed by the C60+ beam effectively; hence, signal intensity decayed. Although the sample rotation and cosputtering techniques individually exhibited a better sputtering condition than C60+ sputtering alone, the difference between the techniques is unclear because they have not been compared experimentally under the same controlled conditions. Therefore, in this study, four sputtering methods, including C60+ sputtering, C60+−Ar+ cosputtering, C60+ sputtering with sample rotation, and C60+− Ar+ cosputtering with sample rotation, were compared for their effect on various model specimens. To compare the carbon deposition and penetration, a Si wafer, which is known to receive significant carbon deposition with C60+ sputtering,28,37 was utilized. To examine the sputter-induced surface roughness, bulk poly(methyl methacrylate) (PMMA), which is known to become extremely rough after C60+ sputtering,24 was selected as the model target. For the depth profiling experiments, the chemical composition of a common cyroprotectant, trehalose, films are known to alter scarcely under the C60+ bombardment. As a result, studies about the depth profile of trehalose films were reported extensively.31,38−40 Although different experimental parameters are used, hence the results cannot be compared directly, it can still serve as the model system. Trehalose films with two different thicknesses on silica substrates were used here, and both the alternative XPS depth profile and the continuous dynamic SIMS depth profile were obtained. It was found that both of the sample rotation and cosputtering techniques can generate improved results by alleviating carbon deposition, maintaining a stable sputtering rate, and leveling the sputtered surfaces, but each exhibited its
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EXPERIMENTAL SECTION XPS spectra were recorded on a PHI 5000 VersaProbe system (ULVAC-PHI, Chigasaki, Japan) using a microfocused (100 μm, 25 W) Al Kα X-ray with a photoelectron takeoff angle of 45°. The base pressure of the analytical chamber was 99%) with two different thicknesses were prepared by spin-coating 0.5 mL of 1.5 or 1 M aqueous solution onto the 1.5 cm ×1.5 cm silica surface at 2000 rpm for 45 s or 5000 rpm for 20 s, respectively. The thicknesses of the trehalose films and silica were then determined by ellipsometry (SpecEI-2000-Vis, Mikropack). The PMMA (Chimei CM205X, Tainan, Taiwan) sheets (1 mm in thickness) were wipecleaned with ethanol-soaked Kimwipes before any analysis.
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RESULTS AND DISCUSSION By bombarding surfaces with C60+ ion beams, carbon deposition and penetration will inevitably occur because C is 9319
dx.doi.org/10.1021/ac3020824 | Anal. Chem. 2012, 84, 9318−9323
Analytical Chemistry
Article
easily immobilized in the source as solid solutions or forms carbides. These implanted carbons enhance the C signal during subsequent spectra acquisition and make characterization more difficult. To explore how the sputtering parameters affect the C deposition, a silicon wafer was used. It was chosen as the sputter target because there is no intrinsic C before sputtering; hence, the observed C must be due to the carbon implantation. After 3 h of C60+ sputtering in an ultrahigh vacuum (UHV) environment, an equilibrium of removal/deposition is established, and the elemental composition of the sputtered surface was quantified by XPS as shown in Table 1. A significant Table 1. Surface Elemental Ratio for 3 h Sputtered Silicon Wafers sputter conditions atomic concn (%)
C60+ sputtering
C O Si
20.5
