Ion-induced surface activation, chemical sputtering, and hydrogen release during plasma-assisted hydrocarbon film growth

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JOURNAL OF APPLIED PHYSICS 97, 094904 2005 Ion-induced surface activation, chemical sputtering, and hydrogen release during plasma-assisted hydrocarbon film growth C. Hopf and W. Jacob a Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching, Germany A. von Keudell b Arbeitsgruppe Reaktive Plasmen, Institut für Experimentalphysik II, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany Received 22 June 2004; accepted 9 February 2005; published online 19 April 2005 Synergisms between different species emerging from hydrocarbon plasmas can enhance the chemisorption of radicals at the surface of a growing film. It has been shown that the rate of 3 chemisorption can be increased by a simultaneously incident flux of ions or H atoms; the latter species cause the formation of surface dangling bonds, which serve as preferred adsorption sites for incoming 3. These synergisms can, however, be counteracted by erosion processes due to the same species. The interplay between the enhancement of film growth by ion/h atom assisted chemisorption and simultaneous erosion processes is investigated in a particle-beam experiment. An a-c:h film is exposed to three individually controllable quantified particle beams ions, 3, and atomic hydrogen. The data can be consistently explained if we include two effects counteracting ion- or H-induced chemisorption: i recombination of neighboring dangling bonds and ii the occurrence of chemical sputtering in case of an intense H flux. Finally, the balance between H incorporation and ion-induced hydrogen release is investigated. The latter process can be compensated by incident thermal hydrogen atoms as long as the ion range inside the film does not exceed the range of the hydrogen atoms. 2005 American Institute of Physics. DOI: 10.1063/1.1883729 I. INTRODUCTION Deposition of thin films in a low-temperature plasma is an important technology for the preparation of materials. Applications range from bio-compatible coatings and wearresistant thin films to electronic materials. For all these applications a precursor gas is dissociated and ionized and radicals as well as ions impinge on a plasma-exposed surface and cause film formation. The identification and quantification of the heterogeneous reactions at the growing film surface in these systems is challenging due to potential synergisms or anti-synergisms among the reacting species. Such synergistic surface reactions are addressed in this article with the deposition of amorphous hydrogenated carbon films a-c:h being our system of interest. This thin film material is used for a multitude of industrial applications, mostly as protective coatings on such devices as hard disks, 1 modern diesel-injection systems, 2 and coronary stents. 3,4 Experiments with reactive plasmas make an identification of elementary reaction mechanisms extremely difficult if not impossible as the number of species impinging simultaneously on the growing film surface is too large; furthermore, the individual fluxes cannot be controlled separately and even quantitative measurement of the fluxes of reactive species is not at all trivial. Therefore, the experiments reported in this article were performed with quantified particle beams of atomic hydrogen, ions, and 3 radicals. a Author to whom correspondence should be addressed; electronic mail: wolfgang.jacob@mpg.ipp.de b Electronic mail: achim.vonkeudell@rub.de This approach allows us to study the interaction of one, two, or three of the species with the surface in great detail, at the expense, however, of restricting ourselves to the species which are accessible to our method of preparation. 5,6 In case of the deposition of a-c:h films two synergistic processes have recently been identified: i Ion-induced chemisorption of 3 : 7,8 By displacing bonded hydrogen at the surface, ions create dangling bonds which serve as chemisorption sites for incident 3 radicals. An increase of the 3 sticking probability by two orders of magnitude was observed. ii Chemical sputtering: 9 Impinging ions break C C bonds in the film within their penetration range. Additionally incident hydrogen atoms, which are known to penetrate the film up to about two nanometers, 10,11 passivate the broken bonds. Repeated bond breaking and passivation leads to the formation of volatile hydrocarbon species at and below the surface which diffuse to the surface and leave the film. Chemical sputtering is a highly effective erosion mechanism which has to be considered whenever reactive ions 12 or a combination of reactive neutrals and energetic ions interact with the film. These ion-induced processes can be quantitatively understood on the basis of TRIM.SP calculations. TRIM.SP 13 is a Monte Carlo code based on the binary collision approximation. It has proved successful in computing physical sputtering 14 yields, implantation depth profiles, and displacement yields. Chemical reactions are, however, not included in the program. Nevertheless, we have successfully simulated reaction rates of the synergistic ion-neutral interactions by using TRIM.SP results as parameters in analytical models 0021-8979/2005/97 9 /094904/6/$22.50 97, 094904-1 2005 American Institute of Physics

094904-2 Hopf, Jacob, and von Keudell J. Appl. Phys. 97, 094904 2005 which describe the chemical part of the interaction. 8,9 Ionenhanced chemisorption as well as chemical sputtering were quantitatively well described and modeled as single synergistic processes. In a plasma system, however, radicals, ions and H atoms bombard the surfaces simultaneously. The question arises, whether the modeling of mechanisms i and ii can be directly transferred to the more realistic experiment of letting all three particle beams interact simultaneously. In addition to having strong impact on growth or erosion rates, hydrogen and ions both have influence on the properties of the growing film. Hardness, friction, and the refractive index of the films are all strongly linked with the hydrogen content. 15 The latter can be increased by incorporation of hydrogen atoms into the growing film. Ion bombardment, on the other hand, leads to preferential displacement of bonded H and, consequently, hydrogen depletion of the film. 16 The balance between incorporation of hydrogen and ion-induced hydrogen depletion is discussed at the end of this article. II. EXPERIMENT Ions are produced in a commercial Colutron G2D iongun system 17 which delivers mass-selected ions at low to medium high energies. For practical experiments energies between 20 and 1000 ev are accessible at sufficient ion flux densities 5 10 12 cm 2 s 1. The radicals hydrogen as well as 3 are produced via dissociation of precursor molecules in a resistively heated tungsten capillary. To produce atomic hydrogen the precursor is H 2 and the resulting flux density is typically 10 15 cm 2 s 1. For 3 production azomethane N 2 3 2 is dissociated yielding a methyl radical flux of 10 14 cm 2 s 1 and molecular nitrogen as nonreactive side product. The three particle-beam sources are mounted to a UHV chamber where the beams merge in one point on the substrate. The substrate is a hard, about 30-nm-thick a-c:h film deposited on a silicon substrate before the experiment. The Si substrate allows one to reflect a laser beam for ellipsometry. This in situ measurement technique is suitable to determine growth rates with high precision even for small thickness changes of a few monolayers. Additionally, it provides the complex index of refraction which can be used as a good overall figure of merit for the film properties. The sample is slightly heated by thermal radiation from the hot capillaries of the radical sources. All experiments described herein are conducted at the resulting temperature of 340 K. A detailed description of the experiment was recently published. 6 Examples of raw ellipsometry data from ionenhanced growth experiments can be found in earlier publications. 7,8 III. ION-INDUCED 3 EMISORPTION To start, we briefly review the ion-energy dependence of ion-induced 3 chemisorption considering an experiment with He + ions and methyl radicals which was previously published. 8 The flux densities of the incident species were held constant at j ion 1.8 10 13 cm 2 s 1 and j 3 2 10 14 cm 2 s 1 while the ion energy was varied. The growth FIG. 1. a Dependence of the growth yield due to 200 ev He + / 3 on the ion flux density data points. Y from Eq. 3 and Y from Eq. 6 are shown as lines; b Nonreduced and recombination-reduced dangling-bond coverages according to Eqs. 2 and 5. yield Y is defined as the number of carbon atoms incorporated per incoming ion. Within the ion energy range between 50 and 800 ev the yield is generally above 0.1. This translates into sticking probabilities of methyl radicals s =Y j ion / j 3 10 2, which is by two orders of magnitude larger than the sticking probabilities observed without ion bombardment. 18 20 The enhancement is believed to be due to the creation of dangling bonds at the surface by displacement of bonded hydrogen. Methyl radicals are known to chemisorb at existing dangling bonds with a unity probability p 3 add, 21 meaning that the macroscopic sticking probability s is given by the dangling bond coverage db, i.e., the fraction of surface sites which are not occupied. Therefore, in the case of a methyl radical flux density which by far exceeds the ion-induced dangling-bond creation rate db,asinour experiment, the observed growth rate is expected to be proportional to db. This hypothesis is supported by TRIM.SP calculations of the yield Y db of ion-induced dangling-bond creation with db =Y db j ion. As demonstrated earlier, 8 it agrees reasonably well with the data; more important than the good agreement between the absolute values of data and calculation is the fact that the observed energy dependence is well reproduced. The decrease of the growth yield with energy is a result of the reduced displacement yield at the surface as the ions penetrate deeper into the film. In this experiment the steady-state dangling bond coverage of the surface has a maximum at ion energies well below 100 ev. With the given fluxes it cannot be increased beyond this value. To examine the question as to what limits the maximum possible enhancement of 3 sticking, the ratio of the fluxes of ions and methyl radicals was varied. Figure 1 a shows the growth yield, in this case as a function of the He + flux density at constant ion energy of 200 ev. The flux density of the methyl beam was again 2 10 14 cm 2 s 1. Within the experimental errors the yield is constant up to an ion flux density of 10 13 cm 2 s 1. Above this flux it decreases rapidly.

094904-3 Hopf, Jacob, and von Keudell J. Appl. Phys. 97, 094904 2005 FIG. 2. Visualization of the restricted area around a dangling bond for the simple case of a hexagonal arrangement of surface sites: a If a new dangling bond is created within the restricted area gray and open circles of dangling bond 1, e.g., in position 2, 1 and 2 will recombine. In contrast, a new dangling bond 3 will survive. b The restricted areas of two dangling bonds 1 and 2 can overlap, the sites marked with a cross belong to both restricted areas. Up to now we have used the simplifying assumption that the gross growth rate is directly proportional to the rate of dangling-bond creation. This is, however, only true for a sufficiently large methyl flux compared to the ion flux, or more precisely j 3 p 3 add j ion Y db. If this condition is violated a most simple model has to at least consider the creation of dangling bonds by ions and their destruction by chemisorption of 3. The rate equation for the dangling-bond coverage db reads hexagonal arrangement of surface sites. In this case the restricted area gray and open circles of a dangling bond 1 is A=7 surface sites. If the dangling bond coverage is low, db 1, we can neglect the cases where two restricted areas overlap as in Fig. 2 b. Then, for a given db the coverage of surface sites where new dangling bonds can be created is 1 A db and the source term in Eq. 1 becomes j ion Y db 1 A db. Additionally, if a new db is created within the A 1 surface sites around an existing db the old db will be destroyed due to recombination. We take this into account by the additional loss term j ion Y db A 1 db. Finally, with B =2A 1 Eq. 1 is modified to d n db 0 dt = j ion Y db 1 B db j 3 p 3 add db, 4 where distinguishes the quantities from the corresponding ones in Eqs. 1 3 where db recombination is not included. In analogy with Eq. 2 we obtain db = j ion Y db Bj ion Y db + j 3 p 3 add 5 n 0 d db dt = j ion Y db 1 db j 3 p 3 add db, 1 and for the growth yield accordingly where n 0 is the areal density of surface sites. Under stationary conditions the derivative on the left side vanishes and we obtain j ion Y db db =. 2 j ion Y db + j 3 p 3 add Finally, the growth yield Y = j 3 p 3 add db / j ion becomes j 3 p 3 add Y db Y =. 3 j ion Y db + j 3 p 3 add Again Y db is estimated by TRIM.SP yielding Y db =0.285 for He + ions at 200 ev. As chemisorption probability p 3 add =1 is used as introduced above. Y from Eq. 3 is shown as a solid line in Fig. 1 a. The model predicts a nearly constant yield over the whole range of ion fluxes shown. Obviously, the 3 flux density is not yet limiting the growth rate. The model cannot explain the experimentally observed strong decrease above j ion =10 13 cm 2 s 1. The discrepancy can be resolved if one considers the maximum dangling bond db coverage that can exist on an a-c:h surface. The surface of the growing film is very rich in hydrogen with little cross-linking between the carbon atoms. Consequently, rotation and changes of binding angles of surface atoms or groups are not yet very much restricted. Therefore, we may assume that two dangling bonds which are close to each other can recombine. We include this into our model by assuming that each dangling bond occupies a certain number of surface sites where no other dangling bond can be created, as otherwise the two would simply recombine. We call these surface sites restricted area and denote their number by A. The situation is schematically depicted in Fig. 2 a for the simple case of a Y = j 3 p 3 add db /j ion. 6 With B=13 Eq. 6 yields the dashed line in Fig. 1 a. It reproduces the decrease above j ion =10 13 cm 2 s 1 very convincingly. The dangling bond coverages db and db are shown in Fig. 1 b. B=13 means that, on the average, the restricted area of a dangling bond is A=7 surface sites. This number appears reasonable as, e.g., in the case of a strictly hexagonal arrangement of the surface sites as in Fig. 2 each site has six next neighbors and consequently A=7. For our amorphous material there will only be a mean number of next neighbors and, hence, A does not necessarily have to be an integer. One might be tempted to rather describe the danglingbond recombination by a loss term in Eq. 4 which is proportional to db and a rate coefficient k rec. Concerning the 2 detailed mechanism, this treatment implies that dangling bonds are mobile and can thus find each other. Both formulations yield very similar results. In any case the dynamics of dangling-bond recombination will strongly depend on temperature. Future experiments exploiting the temperature dependence will provide more detailed insight. In order to check the db range in which Eq. 4 can be applied we originally demanded db 1 we have calculated the evolution of the dangling-bond coverage with ion fluence in the absence of chemisorbing species j 3 =0 both using Eq. 4 and with a simple Monte Carlo MC simulation. For simplicity we assume ions with Y db =1 and look at a fixed number M of surface sites within an area G. If N is the total number of incident ions, then the ion flux density is given by j ion = 1/G dn/dt and the areal density of sites reads n 0 =M /G. Thus, we can rewrite Eq. 4 in the form

094904-4 Hopf, Jacob, and von Keudell J. Appl. Phys. 97, 094904 2005 FIG. 3. Dangling-bond coverage as a function of the number of ions impinging on a total of 10 000 surface sites calculated analytically without db recombination dotted, with db combination dashed, and calculated by a Monte Carlo simulation solid. For simplicity it is assumed that each ion produces one db, i.e., Y db =1. d db dn = 1 M B M db, 7 which, under the boundary condition db N=0 =0, has the solution N db N = B 1 1 exp B. 8 M Both the reduced db coverage db N with B=13 and the coverage established without db recombination, db N calculated with B=1, are shown in Fig. 3 as dashed and dotted lines, respectively. The number of sites was set to M = 10 000. Without recombination the coverage reaches 100% after the impact of about 30 000 ions. With db recombination Eq. 8 predicts saturation at 1/B 7.7%. In the MC calculation ions hit at random positions on a hexagonal 100 100 checkerboard like that in Fig. 2 and create dangling bonds with Y db =1. A newly created db and an existing one are both cancelled if they are next neighbors, e.g., db 2 and 1 in Fig. 2 a. Periodic boundary conditions are applied to avoid artifacts at the edges of the board. The restricted area A is seven surface sites in this case and, hence, B=13. The coverage db N calculated with this method is plotted as solid line. The agreement between analytical and MC calculation is excellent up to db =0.07. In Fig. 1 b db stays below 4% in the whole depicted range. Thus, the simple analytical treatment is justified. The MC simulation, however, saturates at the somewhat higher value db 0.105 compared to the analytical calculation because it treats the overlap of restricted areas correctly. Summarizing, we may conclude that the dangling-bond coverage of a growing a-c:h film is limited to far below 100%. For large ion fluxes ion bombardment can consequently increase the sticking probability of 3 to not more than s max = p 3 add db,max 0.1. IV. ION-INDUCED 3 EMISORPTION VERSUS EMICAL SPUTTERING Our particle-beam experiment is a very simplified model system for a deposition plasma. The situation, however, becomes more realistic if we add the atomic hydrogen beam as FIG. 4. Comparison between the dependences of the chemical sputtering yield due to 200 ev H 2 + /H solid squares, right hand scale and the growth yield due to 200 ev H 2 + / 3 /H open circles, left hand scale on the ratio R of the flux densities of H and H 2 +. Dashed line: Growth yield according to Eq. 10 ; solid line: Model of the flux dependence of chemical sputtering, Eq. 11 ; the dotted line indicates the zero of the right hand scale. a third beam. A 4 plasma, for example, has a high density of hydrogen molecules which is simply a result of the fact that less than one H atom ends up in the film per incorporated C while the source gas has four H per C. Consequently, atomic hydrogen and hydrogen ions play important roles among the reactive species, too. As ion species H + 2 was used in this experiment. The open data points in Fig. 4 show the growth yield observed in an experiment with all three beams as a function of the ratio R of the flux densities of atomic hydrogen and ions. The data points belong to the left hand scale and negative yields mean erosion. The H + 2 ion and the methyl flux densities were held constant at j ion =2.8 10 13 cm 2 s 1 and j 3 =2.2 10 14 cm 2 s 1, while the hydrogen flux density was varied. The deposition rate decreases with increasing hydrogen atom flux. In the case of all three beams we must at least consider the following processes for the dangling-bond balance: i Ions create dangling bonds by H displacement at the surface with a yield Y db, ii atomic hydrogen creates dangling bonds by abstraction of surface-bonded hydrogen with a probability p H abs, iii atomic hydrogen chemisorbs at dangling bonds with the probability p H add and iv methyl chemisorbs at dangling bonds with the probability p 3 add. For small coverages db, i.e., if dangling-bond recombination is negligible, the balance equation reads d db 0=n 0 dt = j ion Y db 1 db + j H p H abs 1 db j H p H add db j 3 p 3 add db. With Y 1 = 1 / j ion = j 3 p 3 add db / j ion yield Y 1 = j 3 p 3 add j ion 9 we obtain as growth j ion Y db + j H p abs j ion Y db + j H p ads + j H p H add + j 3 p add 3. 10

094904-5 Hopf, Jacob, and von Keudell J. Appl. Phys. 97, 094904 2005 Meier, Preuss, and Dose 21 have formulated a rateequation model for the simultaneous interaction of methyl radicals and atomic hydrogen. Their set of equations also contains the parameters p H abs, p H add, and p 3 add for which they give probability distributions calculated by means of Bayesian probability theory. However, their model considers a larger number 3 - and H-induced processes and makes a distinction between two kinds of saturated surface sites. Therefore, the parameters for our simpler model will differ somewhat from Meier s expectation values which guide our choice. We choose p 3 add =1 and p H H add =0.22. The value p abs =0.0045 is chosen such, that for j ion =0 the rate from Eq. 10 equals the rate measured for 3 /H exposure. The yield Y db =0.15 was chosen to reproduce the rate at j H =0. The model is shown as a dashed line in Fig. 4. It is in contradiction with the data almost constant over the depicted range. A variation of the model parameters within reasonable limits can by no means reproduce the data. Equations 9 and 10 only take care of the danglingbond balance at the surface. However, there is another mechanism active when ions and hydrogen atoms interact simultaneously with the film chemical sputtering. The dependence of the chemical sputtering rate on the ratio of the ion and hydrogen flux densities was investigated for 200 ev Ar + ions. 9 If we neglect physical sputtering the fluxdependent yield can be analytically described by Y eros = Y chem R/ R + S, 11 where R= j H / j ion is the flux ratio, S is a parameter describing the efficiencies of H incorporation and release, and Y chem is the maximum chemical sputtering yield, i.e., the yield for R. The same model can also be applied to the H + 2 case. The solid squares in Fig. 4 show the erosion yield eroded C per incident ion due to 200 ev H + 2 ions plus atomic hydrogen for two different flux ratios. The data points belong to the right hand scale. Negative yields again mean erosion. As physical sputtering is negligible in this case, we know that the yield at R=0 is zero. A good fit the solid line in Fig. 4 is obtained with Y chem =0.48 and S=130. The two ordinates in Fig. 4, which have the same scale, were shifted such that zero chemical erosion right scale and thin dotted line is on the same level as the growth yield left scale at R= j H =0 where no chemical sputtering can occur. Apparently the solid line also describes the R dependence of the growth data almost perfectly. This leads to the conclusion that the observed growth rate is a linear superposition of gross growth and chemical sputtering R 2 = 1 j ion R + S Y chem. 12 Obviously at least in the observed parameter regime chemical sputtering has no significant influence on the dangling-bond balance at the surface. V. FILM PROPERTIES Besides playing a role in the deposition or erosion rates, ions have influence on the film properties. 15,22 24 It is known FIG. 5. a Energy dependence of the refractive index of films grown due to H + 2 / 3 solid symbols, H + 2 / 3 /H open symbols, and 3 /H triangle. The dashed lines are intended to guide the eye. b Displaced hydrogen atoms per nm depth interval and incoming H + ion calculated with TRIM.SP for 5, 25, 50, 100, 200, and 400 ev H + energy. The corresponding H + 2 energies E H + 2 =2E H + are indicated in the figure. The shaded area symbolizes the range of H atoms. that a-c:h films contain less hydrogen when deposited at higher ion energy. A good parameter to characterize film properties is the refractive index. There exists a correlation between real and imaginary part of the refractive index, n and, density, hardness, and the hydrogen content of the films; all of these quantities increase if the hydrogen content decreases. 15 Figure 5 a shows as solid symbols the influence of ion energy on the real part n of the refractive index in case of simultaneous H 2 + / 3 bombardment. The flux densities were j 3 =2.2 10 14 cm 2 s 1 and j ion 6 10 12 cm 2 s 1. Overall, a strong increase of n with energy is found, in agreement with plasma experiments. At very low ion energies the films have a refractive index below 1.7 which is typical of a comparably soft film. The open symbols in Fig. 5 a show the refractive index if the hydrogen beam is on in addition to H 2 + and 3. There is no significant difference between the cases with and without the H beam at 100 ev and above. Below that energy the refractive index with atomic hydrogen bombardment is significantly lower. The triangular data point close to the left scale corresponds to 0 ev ion energy and was determined in growth experiments with 3 and H only. The dotted lines are intended to guide the eye.

094904-6 Hopf, Jacob, and von Keudell J. Appl. Phys. 97, 094904 2005 An explanation is provided by a comparison of the ranges of H and H 2 +. Figure 5 b shows the hydrogen displacement yields as a function of depth below the film surface. It was calculated by TRIM.SP for H + ions at energies of 5, 25, 50, 100, 200, and 400 ev. It is assumed that a H 2 + ion behaves like two H + ions at half the energy, thus the corresponding H 2 + energies are twice as high. Atomic hydrogen is known to penetrate the film to a maximum of about 2 nm, 10,11 which is symbolized by the shaded area in Fig. 5 b. Obviously up to 50 ev H 2 + almost all displacement is caused within these first 2 nm. The loss of bonded hydrogen due to displacement can, therefore, be compensated by the incident thermal hydrogen atoms. In contrast, at higher energies a growing part of the displacement events is induced beyond the H range. Hence, it is irreversible and causes an increase of the refractive index. VI. SUMMARY AND CONCLUSIONS The different processes we have discussed in this article can be summarized in terms of a three layer model, according to the processes that occur within them we distinguish three different active depth intervals: i The very surface. The dangling bond coverage at the very surface of the film determines the rate of 3 incorporation. It results from all process that can either produce or destroy dbs, namely ion-induced dangling-bond creation, db creation via abstraction of surface bonded H, e.g., due to incident H or 3, and the saturation of dbs due to the chemisorption of reactive species. We have found indication that the db coverage is ultimately limited by the recombination of neighboring dangling bonds. Extrapolation of our model predicts that this limit is around 10%, hence, preventing further ion-induced enhancement of radical sticking. For j 3 j ion the maximal 3 sticking probability is s 10 1. It is still appreciably higher than the maximum sticking probability for 3 /H which is about 0.04. 20 A significant increase of the sticking probability can only be obtained for species with a very low sticking probability on nonactivated surfaces. For methyl radicals with s 10 4 an enhancement by orders of magnitude is possible. For other species, such as C 2 H, it has been found that the surface loss probabilities are of the order of one. 25 Assuming that s is of the same order as, no significant increase of the sticking probability is therefore anticipated for C 2 H. The ratio of the fluxes of highly reactive to low-reactive carbon-carrying species will determine whether in a specific deposition plasma ioninduced radical chemisorption is an important contribution to the total growth rate or not. ii The overlap of H and ion ranges. It is given by the shorter of the two ranges, usually the range of thermal hydrogen atoms which is 2 nm. Within this depth interval chemical sputtering was identified as a process competing with growth in the presence of ion bombardment and hydrogen atoms. The net growth rate is the difference between the gross growth and chemical sputtering rates. In hydrocarbon deposition plasmas there is generally atomic hydrogen present. Its flux to the surface depends on many factors, mainly the type of source gas, the residence time of the molecules in the reactor, the absorbed power, and the reactor geometry. Besides chemical sputtering, the ions also modify this region by depleting it from hydrogen. This reduction of the hydrogen content within this depth interval is, however, reversible if a sufficiently large flux of hydrogen atoms is incident on the surface. iii The ion range beyond the range of H. The final film properties are mainly determined in this region as ioninduced hydrogen depletion is irreversible here. Light ions are especially effective for film modification due to their larger penetration range and more effective collisional energy transfer to bonded hydrogen. Generally, the resulting hydrogen content and with it the film properties are a function of the balance between incorporation and ioninduced depletion of hydrogen. The primary incorporation of hydrogen depends on the H/C ratio of the source gas and saturates at high H/C ratios of 1. 15,20 1 J. Robertson, Thin Solid Films 383, 81 2001. 2 R. Gåhlin, M. Larsson, and P. Hedenqvist, Wear 249, 302 2001. 3 I. De Scheerder, M. Szilard, Y. M. Huang, X. B. Ping, E. Verbeken, D. Neerinck, E. Demeyere, W. Coppens, and F. V. de Werf, J. Invasive Cardiol. 12, 389 2000. 4 U. Sigwart, S. Prasad, P. Radke, and I. Nadra, J. Invasive Cardiol. 13, 141 2001. 5 T. Schwarz-Selinger, M. Meier, C. Hopf, A. von Keudell, and W. Jacob, Vacuum 71, 361 2003. 6 W. Jacob, C. Hopf, A. von Keudell, M. Meier, and T. Schwarz-Selinger, Rev. Sci. 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