Surface interactions of hydrocarbon films
radicals during the deposition of fluorocarbon and
Dongping Liu, and Ellen R. Fisher
Citation: Journal of Vacuum Science & Technology A 25, 1519 (2007); doi: 10.1116/1.2784717 View online: https://doi.org/10.1116/1.2784717 View Table of Contents: http://avs.scitation.org/toc/jva/25/6 Published by the American Vacuum Society
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Surface interactions of C3 radicals during the deposition of fluorocarbon and hydrocarbon films Dongping Liua兲 and Ellen R. Fisherb兲 Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
共Received 4 June 2007; accepted 22 August 2007; published 21 September 2007兲 The gas-phase density and surface interactions of the carbon trimer C3 have been examined in fluorocarbon and hydrocarbon plasmas. The 1⌸u- 1⌺+g fluorescence excitation spectra and relative gas-phase densities of C3 radicals have been collected using laser-induced fluorescence 共LIF兲 spectroscopy. The relative C3 density increases significantly with CH2F2 in the feed, indicating that C3 is primarily produced via decomposing CH2F2 and chemical reactions in the gas phase. In addition, the surface reactivity R of C3 has been measured during fluorocarbon and hydrocarbon film depositions using C3F8 / CH2F2 and CH4 / CH2F2 13.56 MHz rf plasmas. The C3 radicals were characterized using our LIF-based imaging of radicals interacting with surfaces technique. R values for C3 range from 0.10 to 0.38, depending on plasma conditions, but show no clear dependence on the gas mixture or the plasma conditions used. X-ray photoelectron spectroscopy measurements of the films deposited in these systems provide additional evidence that suggests that C3 carbon clusters may be contributing to the formation of more cross-linked films. © 2007 American Vacuum Society. 关DOI: 10.1116/1.2784717兴
INTRODUCTION Carbon clusters from the smallest C2 to the fullerenes are very interesting species for a variety of reasons including their chemical and astrophysical importance. They play a significant role in combustion and film deposition processes and in shaping the chemistry of stars and comets.1 The carbon trimer 共C3兲 has also been identified in a number of chemical vapor deposition 共CVD兲 systems and was found to be a dominant species in diamond depositing systems,2–4 expanding thermal plasmas,5 fluorocarbon 共FC兲 film deposition plasmas,6,7 and amorphous carbon film deposition systems generated by laser ablation of graphite.8,9 Although the role of C3 in film deposition processes is not well understood, it is thought to be a reactive species that may contribute to film growth.5 Fluorocarbon-based films deposited by hot filament CVD and plasma enhanced CVD 共PECVD兲 have many desirable properties, including low dielectric constants,10 11 12,13 biocompatibility, and superhydrophobicity. In these systems, a variety of reactive species have been purported to be film precursors. For example, it has been proposed that CF2 and gas-phase 共CF2兲n oligomeric species can be direct precursors for polytetrafluoroethylene-like films in FC deposition systems.14–16 Likewise, carbon clusters 共i.e., C2, C3, Cn兲 as well as CHx species have been proposed as film precursors in a variety of hydrocarbon plasmas.17–20 In general, film growth in many hydrocarbon plasmas is thought to be controlled by interactions of Cn and CHx species with radical sites on the surface of the growing film. a兲
Present address: School of Science, Dalian University, Dalian, People’s Republic of China 116600. b兲 Author to whom correspondence should be addressed; electronic mail:
[email protected] 1519
J. Vac. Sci. Technol. A 25„6…, Nov/Dec 2007
We have previously used our unique imaging of radicals interacting with surfaces 共IRIS兲 technique to measure CF2 surface reactivity during FC film deposition using a variety of FC precursors 共e.g., C2F6, hexafluoropropylene oxide, C3F8, and C4F8兲21–24 and CH surface reactivity during hydrocarbon 共HC兲 film deposition from CH4 plasmas.25 These two species exhibit very different reactivities. For example, CF2 has an extremely low surface reactivity 共R Ⰶ 1兲, and our IRIS studies revealed that energetic ions in the plasma lead to CF2 surface production during film deposition, resulting in extremely high scatter coefficients S, where R = 1 − S. In contrast, CH exhibits a very high surface reactivity 共R ⬃ 1兲 during PECVD of HC films, and, thus, is likely to play a significant role in HC film deposition.25 Here, we present IRIS surface reactivity data for C3 measured at depositing FC and HC film surfaces. Relative C3 density and surface reactivity values are measured as a function of deposition parameters, including gas composition and substrate bias. IRIS data are combined with x-ray photoelectron spectroscopy 共XPS兲 film composition measurements to provide insight into the role of C3 in both FC and HC film growth. EXPERIMENTAL DETAILS The IRIS technique has been described in detail previously.24,26 Briefly, IRIS uses molecular beam techniques and laser-induced fluorescence 共LIF兲 to measure the steady state surface reactivity of a single gas-phase species while the substrate is being processed by the full range of plasma species. Expansion of an inductively coupled rf plasma into a differentially pumped vacuum chamber generates a near effusive molecular beam consisting of virtually all species in the plasma. A tunable laser beam intersects the molecular beam downstream from the plasma source and excites the species of interest. LIF is generated at the intersection of the
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©2007 American Vacuum Society
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D. Liu and E. R. Fisher: Surface interactions of C3 radicals
molecular and laser beams and collected by an electronically gated, intensified charge coupled device 共ICCD兲 located directly above and perpendicular to the interaction region. The pixels of the ICCD were 4 ⫻ 4 binned to increase the signalto-noise ratio for all data presented here. In the present work, the sources of the molecular beam are 13.56 MHz rf plasmas created from either C3F8 / CH2F2 or CH4 / CH2F2 mixtures. For these experiments, the applied rf power and pressure in the plasma source chamber were maintained at 80 W and 100 mTorr, respectively. A wavelength range of 398.5– 409 nm is achieved by using a tunable XeCl pumped dye laser 共Exalite 404兲. The relative gas-phase density and surface reactivity data presented here were collected using the most intense line in the C3 spectrum at 405.0 nm, corresponding to the 共000-000兲 ⌸u-⌺+g vibrational transition.27–32 A lens is used to focus the laser radiation at the position of the molecular beam, yielding a well-defined laser beam ⬍1 mm wide. All density and reactivity experiments were performed at laser energies in the optical saturation regime such that the measurements are not affected by small fluctuations in laser power. Surface reactivity for the C3 molecule is determined using ICCD images collected with a room temperature Si substrate placed in the path of the plasma molecular beam and rotated clear of the molecular beam. For all experiments, the gate delay and gate width of the ICCD camera were set at 1.27 and 2.5 s, respectively. The gate width ensures that data are collected over the entire decay of the C3 fluorescence 共radiative lifetime of ⬃200± 10 ns兲.29 The laser-surface distance is maintained at ⬃3 mm. The resulting images are converted to one-dimensional cross sections using the geometry of the IRIS instrument. The cross sectional data are interpreted using a quantitative model of the experiment, which has been described in detail previously.26 The model calculates the spatial distribution of the radical number density in the molecular beam at the interaction region as well as the radical number density along the laser beam for molecules scattering from the substrate surface. Results are presented as a surface scattering coefficient S, which is essentially the ratio of the flux of scattered molecules to that of molecules in the incident molecular beam. We also define the surface reactivity R, where R = 1 − S. XPS analyses of deposited films were performed on a Physical Electronics PE5800 ESCA/AES system. Spectra were collected using a 7 mm monochromatic Al K␣ x-ray source 共1486.6 eV兲, hemispherical analyzer, and multichannel detector. A low energy 共⬃1 eV兲 electron neutralizer was used for charge neutralization. Survey spectra were collected with a pass energy of 93.90 eV. High resolution C1s spectra were acquired at an analyzer pass energy of 11.75 or 23.50 eV. Curve fitting was performed using Gaussian functions with the full widths at half maximum of ⬃2 eV, which is expected for plasma polymers. A photoelectron takeoff angle of 45° was used for all spectra. RESULTS AND DISCUSSION Figure 1 shows an experimental fluorescence excitation spectrum of the C3 1⌸u- 1⌺+g electronic transition using a J. Vac. Sci. Technol. A, Vol. 25, No. 6, Nov/Dec 2007
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FIG. 1. Experimental fluorescence excitation spectrum of C3 in a molecular beam formed from a 100% CH2F2 plasma. The spectrum was acquired with 0.05 nm steps with the dye laser. Assignments based on results from Ref. 28.
100% CH2F2 plasma as the molecular beam source. Note that the fluorescence from this molecule extends across a relatively wide wavelength range 共⬎10 nm兲, and that although not all are labeled in Fig. 1, all of the peaks in the spectrum can be attributed to C3 transitions.27–32 As noted in previous spectroscopic studies, C3 has a very complex spectrum with a variety of vibronic transitions.28 Figure 2 shows the relative density of C3 as a function of the fraction of CH2F2 in the gas feed of the plasma 共balance is C3F8兲. For these data, the laser was tuned to 405.0 nm, corresponding to the 共000-000兲 ⌸u-⌺+g vibrational transition. The C3 density in the plasma increases significantly with CH2F2 fraction, indicating that C3 is likely formed via the decomposition of CH2F2 and subsequent two- or three-body reactions, as is shown in the following reaction sequence, wherein M indicates a third body.
FIG. 2. Relative C3 LIF signal intensities 关using the 共000-000兲 vibrational band of the ⌸u-⌺+g transition at 405.00 nm兴 as a function of the CH2F2 / 共CH2F2 + C3F8兲 gas ratio. Error bars represent one standard deviation of the mean of at least three measurements. For some data points, the error bars are smaller than the size of the point.
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FIG. 3. Two-dimensional ICCD images of C3 LIF signals 共a兲 in the plasma molecular beam formed from a 13:87 C3F8 / CH2F2 plasma and 共b兲 with a 300 K Si substrate 共bias voltage of +200 V兲 rotated into the path of the molecular beam. The image shown in 共c兲 is the difference between the images in 共a兲 and 共b兲 and corresponds to signal from C3 radicals scattered from the surface. These images represent an area of 51.2⫻ 51.2 mm. The dashed lines indicate the locations of the molecular beam and the laser beam. All images are on the same intensity scale, with the highest signal of ⬃3500 counts for the image in 共a兲 共see Fig. 4兲.
CH2F2 + e− → CHF + HF + e−
共1兲
共Ref. 33兲, CHF + e− → C + HF + e−
共2兲
共Ref. 33兲, C + C + M → C2 + M
共3兲
共Refs. 9 and 34兲, C2 + C + M → C3 + M
共4兲
共Ref. 9兲. Note that reactions 共3兲 and 共4兲 could proceed without the third body M. C3 has also been observed using optical emission spectroscopy in plasmas formed from the hydrofluorocarbon C2H2F4.7 Interestingly, the C3 intensity is minimal in the 100% C3F8 system, indicating that C3 does not readily form from the decomposition of this fluorocarbon precursor. Moreover, the dramatic increase in the C3 intensity for the 100% CH2F2 system may suggest that the presence of C3F8 in the plasma actually hinders the formation of C3 from CH2F2. Figure 3 shows a typical set of two-dimensional ICCD images of C3 LIF signals using a C3F8 / CH2F2 plasma with a gas ratio of 13:87 共based on partial pressures in the system兲. For this data set, the substrate was biased 共+200 V兲 to prevent positive ions in the molecular beam from impinging on the substrate. This configuration allows determination of radical-surface interactions in the absence of significant ion bombardment. Signal from C3 in the incident molecular beam is shown in Fig. 3共a兲. In Fig. 3共b兲, a Si substrate was placed in the path of the molecular beam, and the image now includes LIF signal from both C3 in the incident molecular beam as well as C3 scattered from the substrate surface. The image in Fig. 3共c兲 is the difference between Fig. 3共b兲 and 3共a兲, indicating the signal resulting from desorbing C3. To quantify S共C3兲, one-dimensional cross sections of the beam and scatter images were generated by averaging a column 20 pixels wide 共an 8.0 mm swath兲 containing the LIF signal and plotting signal intensity as a function of distance along the laser axis, Fig. 4. Using our quantitative model of the JVST A - Vacuum, Surfaces, and Films
experiment that reproduces the scattering data in one dimension,35 we find S = 0.68 共R = 0.32兲 for this data set. S共C3兲 values were measured using CH2F2 / C3F8 and CH2F2 / CH4 plasmas as shown in Table I, with either a +200 V substrate bias or with a floating substrate 共0 V bias兲. As can be seen from Table I, S共C3兲 values were typically measured in the range of 0.62–0.90, which translate to R values ranging from 0.10 to 0.38, Table I. R共C3兲 values measured here show no clear dependence on the feed gas composition or on the substrate bias voltage. They are, however, significantly lower than C3 sticking coefficients measured on pyrolitic graphite at elevated substrate temperatures.36 These literature values, however, were found to be substrate tem-
FIG. 4. Cross-sectional data for the C3 LIF signals shown in Figs. 3共a兲 and 3共c兲. The dashed lines represent the simulated curves from the geometric model assuming S共C3兲 = 0.68.
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TABLE I. C3 scatter coefficients during deposition of FC and HC films. 共All measurements made with an applied rf power of 80 W for the plasma molecular beam source. Errors represent one standard deviation of the mean of at least three measurements.兲 Discharge gas
Pressure 共mTorr兲
Substrate bias 共V兲
S
R
CH2F2 CH2F2 C3F8 / CH2F2 C3F8 / CH2F2 C3F8 / CH2F2 C3F8 / CH2F2 C3F8 / CH2F2 C3F8 / CH2F2 CH4 / CH2F2 CH4 / CH2F2
100 100 13/ 87 13/ 87 30/ 70 30/ 70 50/ 50 50/ 50 10/ 90 10/ 90
0 +200 0 +200 0 +200 0 +200 0 +200
0.86± 0.07 0.90± 0.06 0.62± 0.11 0.68± 0.09 0.72± 0.07 0.88± 0.11 0.62± 0.11 0.67± 0.08 0.84± 0.11 0.87± 0.12
0.14± 0.07 0.10± 0.06 0.38± 0.11 0.32± 0.09 0.28± 0.07 0.12± 0.11 0.38± 0.11 0.33± 0.08 0.16± 0.11 0.13± 0.12 FIG. 5. High resolution XPS C1s spectra of films deposited in 共a兲 a 100% CH2F2 plasma and 共b兲 a 100% C3F8 plasma.
perature 共TS兲 dependent, with decreasing reactivity at higher substrate temperatures. The differences between our IRIS values and those measured previously by Philipps et al. could be explained by the differences in the substrate materials, the source of the C3 radicals, and the observation that in our system we are actively depositing an a-C : H film onto the substrate. Previous IRIS results in our laboratory have shown a similar dependence of radical surface reactivity on TS. The surface reactivity of OH radicals in a SiO2 deposition system decreases to essentially zero at TS ⬎ 300 K.35,37 In contrast, however, the surface reactivity of SiH radicals does not change with TS.26,38 Temperature dependence studies for IRIS measurements of C3 radicals are currently underway in our laboratory. The moderate reactivity values measured here nonetheless indicate that C3 radicals are relatively active at the substrate surface during film deposition. This is consistent with the results of van de Sanden and co-workers who found that C3 radicals were dominant contributors to a-C : H film deposition in a C2H2 expanding thermal plasma. Moreover, they concluded that C3 radicals were responsible for higher film quality with respect to density and hardness.5 In our system, it is possible that radical sites at the depositing film surface are being produced during plasma-surface interactions, perhaps via surface etching by F and H atoms in the plasma18 or by energetic ion bombardment. C3 may effectively contribute to the film growth if it adsorbs at these radical sites. One of the issues associated with IRIS measurements is that the presence of ions in our system often can complicate the interpretation of our data. Previous mass spectrometry measurements in our laboratory indicate that all ions in our fluorocarbon plasmas have similar energy distributions with mean ion energies ranging from ⬃30 to ⬃ 100 eV, depending on the rf power and gas pressure.39 In general, the major ionic constituents in C3F8, CH2F2, and CH4 plasmas are CF+y 共y = 1 – 3兲, CH2F+, and CxH+y 共x = 1 – 2, y = 2 – 4兲,40 respectively. In IRIS experiments, ion bombardment can contribute to the observed scatter coefficients in a variety of ways. As described previously,24 ions in the plasma can interact with J. Vac. Sci. Technol. A, Vol. 25, No. 6, Nov/Dec 2007
the surface and either undergo neutralization/desorption reactions; adsorp and either undergo neutralization/desorption reactions; adsorption/dissociation/desorption reactions; or neutralization/abstraction reactions. In addition, energetic ions can sputter fragments of the deposited film and, if the molecule of interest is produced, this will contribute to the observed scatter in our IRIS experiments. Thus, it is useful to perform IRIS experiments under conditions that will limit the effects of ions. Here, we used a +200 V substrate bias voltage to repel positive ions, thereby creating essentially ion-free conditions at the surface. We have explored the possibility of negative ions in these systems using our Hiden PSM mass spectrometer, but found that the negative ion density was below the detection limit of our mass spectrometer. Thus, we are relatively confident in this “ion-free” designation. Given that the scatter values measured with the biased substrates are within experimental error of those measured without a bias, Table I, energetic ions are clearly not responsible for the observed surface scatter of C3 radicals during film deposition. Moreover, this suggests that ion-surface interactions do not result in the production of C3 molecules. This may be related to the nature of the deposited material, as well as types of ions present in these systems. To further explore the nature of the films deposited in our plasmas, high resolution C1s XPS spectra of films deposited in 100% CH2F2 and 100% C3F8 plasmas are shown in Fig. 5. The spectrum for the film deposited in the CH2F2 plasma, Fig. 5共a兲, is dominated by the C – C / C – H peak, but also contains peaks attributable to CFx species. The film deposited in the 100% C3F8 plasma, Fig. 5共b兲, clearly contains a variety of CFx moieties and only a very small contribution from the C – C / C – H peak. In both cases, the films appear to be highly amorphous in nature. For fluorocarbon films, we can define the percentage cross-linking to be the sum of the cross-linking species in the film 共%CF + % C – CFx + C – H兲.13 Thus, the percentage cross-linking for the film represented by the XPS spectrum in Fig. 5共b兲 is ⬃54%. It is difficult to calculate a percentage cross-linking for the film represented
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by the spectrum in Fig. 5共a兲 as the C – C / C – H peak can contain both chain terminating groups as well as chain extending groups. Nonetheless, the presence of a significant contribution from the C – CFx peak in this spectrum 共⬃34% 兲 suggests that materials deposited under these conditions are also highly cross-linked. As C3 radicals are easily produced in CH2F2 plasmas, C3 or other carbon clusters adsorbing and reacting with film surface could easily contribute to the formation of a cross-linked film structure in these systems. Note, however, that the C3 surface reactivity is the lowest for the 100% CH2F2 system, Table I. C3 surface reactivity measurements were not performed in the 100% C3F8 system because the signal intensities were too low. Overall, our IRIS results, combined with the surface analysis data, indicate that C3 has a moderate reactivity at the surface of films deposited in all of our plasma systems and that the interaction of the C3 molecules may contribute to the production of highly cross-linked films. SUMMARY This work presents the first C3 data acquired during FC and HC film depositions on our IRIS apparatus. IRIS measurements reveal that C3 radicals are produced by the dissociation of CH2F2 and subsequent gas-phase chemical reactions. Under all conditions examined, C3 exhibits a moderate surface reactivity 共R = 0.10– 0.38兲 at the surface of the growing cross-linked, amorphous hydrofluorocarbon film. Our IRIS results, combined with surface analysis data, suggest that C3 radicals may contribute to the deposition of films with cross-linked structure. ACKNOWLEDGMENT Financial support for this work was provided by the National Science Foundation 共NSF-0613653兲. 1
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