- Email: [email protected]
Cluster impacts on solids
Nuclear Instruments and Methods in Physics Research B 193 (2002) 227–239 www.elsevier.com/locate/nimb
Cluster impacts on solids D. Jacquet *, Y. Le Beyec Institut de Physique Nucl eaire, CNRS-IN2P3, 91406 Orsay, France
Abstract Polyatomic projectiles, which are now available in a large range of size and of incident energies, allow to deposit in solid samples high energy densities that induce large damage in the bulk of irradiated samples as well as very important emission of matter from the bombarded surfaces. Large secondary ion emission yields as well as total particle emission yields induced by Aun impacts were measured on various targets over a large range of incident energies covering the domain of energy deposited through atomic collisions and electronic excitations. An unexpected yield-energy dependence was observed with a strong maximum of the emission rate at moderate velocities (100–200 keV/atom) which may open promising developments in surface analysis. At higher velocity, it has been shown that the equilibrium charge state of the co-moving atoms of swift clusters in solid targets is significantly reduced by comparison with single atoms at the same velocity. This effect, due to the additional ionisation potential created by the proximity of the cluster constituents, has been investigated with carbon clusters of different size, in a velocity range between 600 keV up to 4 MeV/atom. The spatial distribution of cluster fragments exiting thin carbon targets were also measured with a multi-impact position sensitive detector. Comparison with theoretical simulations demonstrated that the in-target Coulomb explosion is strongly shielded. Large pulse height defects measured in silicon detectors are a direct consequence of the interaction of the spatially correlated atoms in the silicon material. Results on pulse height dependence on cluster size and energy will be presented for 1 and 2 MeV/atom carbon clusters. This set of results, obtained recently by the Orsay group (most of them in collaboration with other groups), illustrates different aspects of cluster collisions and/or transmission in solids. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 36.40.-c; 34.50.Dy; 61.80.Lj; 68.49.Sf; 34.50.Fa; 36.40.Fa; 36.40.Wa; 29.40.-n; 07.77.-n; 34.70.þe; 29.40.W
1. Introduction When a fast cluster projectile collides with a solid target the atomic constituents separate upon impact in the first layers and remain in close proximity for a certain distance. The types of clusters (size, atomic constituent, mass and shape), the incident velocity as well as the target material
*
Corresponding author. E-mail address: [email protected] (D. Jacquet).
and structure are important parameters that govern the physical effects occurring at the surface and in the bulk of a solid target. The simultaneous impact of several atoms on a small surface of a solid is a unique way to deposit large energy density in a material and to study the coherent (time and space) interactions of several ions traveling in a solid. The passage of swift light clusters through solids at relatively high velocity and the interaction of low velocity clusters with solid targets have now been studied for many years. However, it is only in
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 7 5 5 - 3
228
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
the last decade – due to advancements in cluster ion sources and acceleration techniques – that a variety of different clusters could be accelerated to several tens of MeV. Beams of multiply charged C60 at 40 MeV are a typical example. In the same period of time numerous and remarkable applications have been developed at a total energy of a few tens of keV and this is also an aspect which should not be ignored (although not discussed here) since small powerful commercial instruments are now used for surface analysis [1]. For swift projectiles of velocity larger than the Bohr velocity, most of the experiments on cluster solid interactions were focused on what happened to the projectile and its constituents while passing through solids as for example charge states of constituents, energy loss, projectile structures at the exit side of thin foils . . . . Hydrogen clusters þ þ and small molecular ions (Oþ 2 , N2 , CH , . . .) were used to first demonstrate evidence of ‘‘vicinage effects’’ in the energy loss of clusters [2–8] and in the lowering of charge state of ions issued from dimer molecules by comparison to single atoms at the same velocity [9–15]. Little was done on the important question of the deposit and relaxation of energy in a material with the consequence of ejection of particles and structure modification in the solid itself. The response of solid targets to impacts in terms of secondary emission was first investigated with dimers and trimers [16–19] and other small molecules at low energy [20–22]. An enhanced emission rate of particles during cluster bombardment had already been measured experimentally (see review papers and conference proceedings [23–26]) and small deformations or craters were observed after cluster impacts at the surface of targets [27]. In recent works at high velocity very important defects are visible at the surface as well as inside the material [28–31]. The permanent damages can help to investigate the trajectories of clusters in matter and the duration of collective effects in comparison with the estimated separation distance between constituents. The high velocity work also demonstrates the importance of the volumic energy density with respect to linear energy loss dE=dx [28].
In this paper we will focus on some of these experimental results obtained recently with gold clusters and carbon clusters produced with various electrostatic accelerators. The response of the material to the high energy density deposited by the spatially and temporally correlated atoms will be illustrated by the experimental results on secondary ion emission and total sputtering with Aun clusters over a very broad range of energy (from keV to MeV/atom). The large emission rate from insulating material and from metallic targets involves emission mechanisms in which an appreciable amount of the incoming energy is removed by the ejected particles. Light ions such as hydrogen are also emitted from surface contaminants with a large yield but for these particular ions the emission is very sensitive to the total ionization charge of the cluster atoms at the surface within a few atomic layers. The value of this yield can be used to determine the average charge of cluster constituents at the exit side of solid foils. Another more direct experimental procedure has however been developed with a position sensitive detector which measures the electrostatic deviation of charged ions exiting thin foils. Important charge suppression effects have been measured at energy above 1 MeV/atom with Cn clusters. The angular distributions of exiting cluster ions strongly depends on cluster size and target thickness and they have been compared to predictions of trajectory simulations which include potential screening and Coulomb repulsion between the comoving ions in the solid. The energy response of silicon barrier detectors was studied under impact of Carbon clusters Cn and C60 molecules. A very strong energy deficit was observed in comparison with impacts of single carbon atoms at the same velocity. The pulse height defect is a direct consequence of the proximity of the traveling ions in the Si material, integrated over the full range.
2. Cluster ion beam production Since the acceleration of hydrogen clusters, water clusters and small molecular ions in various
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
laboratories [32–34] there has been a great deal of efforts to produce cluster beams over a large scale of energy for a variety of purposes [25,35]. Mostly light atomic clusters were accelerated in Japan [35], Israel [36], China [37] and Switzerland [38] while light and heavy clusters are available now in Germany (Erlangen [39]), Denmark (Aarhus [40]) and France (Orsay and Lyon [41,42]). Beams of Cn , (eventually Aln , Nin , Agn ), Aun and C60 are accelerated at Orsay. Two procedures are used to produce clusters with electrostatic accelerators: (i) In Lyon and Orsay a Gold liquid metal ion source was installed to deliver Aun beams with a total energy of, respectively, 1.4 and 13 MeV/charge. In Lyon the maximum size of deflected clusters is n ¼ 13. In Orsay a Wien filter is used to separate the different cluster sizes at low energy before injection into the accelerator [43]. Aun beams with n up to 400 have been obtained in the zero angle beam line and 18 MeV Au100 have been magnetically deflected at 1.2°. (ii) Another procedure is to produce negative cluster ions in a conventional sputtering ion source and to use the two acceleration steps of a tandem accelerator. Positive cluster ions are then formed at the high terminal voltage in a gas cell containing a low pressure N2 gas. With this method the cluster identification is made by a magnetic separation coupled with a time of flight measurement plus an energy measurement with a surface barrier detector. The deflection angle is 1.2°. The Orsay cluster beam facility has been extensively used by numerous groups in the last few years.
3. Secondary emission from solids induced by Aun clusters The high energy density which is deposited in a solid during the interaction processes with the target atoms or with the electronic systems may dissipate through various exit channels. Emission of atomic ions or complex ions (molecules, clusters) is much less probable than the emission of neutral species but both can be used to study the projectile– solid interaction. As mentioned previously the re-
229
spective role and importance of the different modes of energy loss processes on secondary emission are not clearly identified at intermediate energy. Therefore the lack of consistent data covering a large range of velocity (and energy loss process) for a given target projectile combination was the basic reason to investigate the variation of emission yield over the largest range of energy. It was possible in our laboratory to explore the range of 5 keV/atom to about 5 MeV/atom with Aun clusters. The same experimental equipments and the same experimental procedures were used at the different beam facilities in both types of experiments on secondary ion emission as well as for total sputtering yield measurements. 3.1. Secondary ion emission Total sputtering as well as ion emission yields induced by cluster bombardment have shown to deviate from the sum of the yield of individual atoms. The yield is defined as the number of ejected atoms or ions of a given type divided by the number of impacting projectiles. The first observations of nonlinearity phenomena were made with dimer projectiles by Andersen and Bay [16] for neutral emission and by Wittmaack for secondary ion emission [19]. The nonlinear enhancement of yield beyond a direct proportionality to the number of constituents has since been well established in all regimes of velocities [23–26]. In a recent work of the Orsay group [44], thick organic layers of molecules as well as inorganic materials were used as solid targets. Time of flight mass spectrometry in the ion counting mode was used to identify the molecular ions from a phenylalanine target (C9 H11 O2 N) and cesium iodide cluster ions from CsI layers. The emission yield of various type of ions was measured as a function of Aun cluster velocity and size. Fig. 1 shows the variation with the projectile energy per atom of the emission yield of the intact molecular ion (M–H) emitted from organic layers of phenylalanine under impact of Aun (n ¼ 1–4) in the energy range from 5 keV to more than 5 MeV/atom. Similar shapes of yield variations with energy per atom were also obtained with other types of ions from organic and inorganic
230
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
Fig. 1. Secondary-ion emission yield of the negative molecular ion (M–H) , induced by Aun cluster (n ¼ 1–4) impact on a Phenylalanine target as a function of the projectile energy per atom. The solid lines are to guide the eyes. Calculations of nuclear ( ) and electronic ( ) energy losses for a single gold atom, performed with the TRIM code are also shown in the figure. (Results from [44].)
targets. Above 100 keV/atom the emission yields vary roughly as n2 with the number of cluster constituents, for n > 2. A larger dependence of secondary ion emission yield on n is observed between n ¼ 1 and n ¼ 2 at energy per atom below 500 keV/atom with the presence of a marked and unexpected peak for clusters around 40 keV/atom. A detailed analysis of the emission yield dependence on n can be found in [44]. Under single gold atom impact the emission yield variation is very different. It is very low until 1 MeV/atom and then increases rapidly with energy when the electronic energy loss processes become predominant. It is remarkable to note that above 1 MeV/atom the evolution of the yield curve with the projectile velocity is similar for single Au projectile and for Aun clusters. A variation of the calculated energy loss (nuclear and electronic) for a single gold atom (TRIM calculation [45]) is also shown in Fig. 1 for comparison with the experimental yield energy dependence. The observed experimental maxima at 50 keV/atom (see Fig. 1, Au4 ) are at energies per atom much lower than the peak corresponding to the maximum of the nuclear stopping of Au in organic material (380 keV/atom). It has been observed experimentally that the range of Aun clusters (n ¼ 1–3) implanted at 10–40
keV/atom in various targets (Si, Al, Cu) does not depend on cluster size and is equivalent to the range of Au atoms [46]. It is thus assumed that at higher energy per atom the nuclear stopping of a gold Aun cluster is n times the nuclear stopping of a single Au ion. Therefore the results of Fig. 1 show that the nuclear energy loss is not the only relevant parameter that controls ion emission from insulating solids. The energy density near the surface and the regime of velocity seem to be more important. Additional experiments with larger gold clusters should be performed in the future to further investigate the capabilities of such projectiles for applications such as providing a supplement to the bio-mass-spectrometry now done with laser desorption (MALDI) [47]. The dynamics of the energy deposit followed by the transfer of momentum to fragile molecular ions or cluster ions in the gas phase has been qualitatively described in both regimes of energy loss with keV and MeV atomic projectiles. Coherent ‘‘knock on’’ of a molecule by a number of recoiling atoms generated in a dense collision cascade, ‘‘explosion-like’’ or expansion of a highly energized region in the vicinity of the surface would push the surrounding molecule out. There is a numerous literature on the subject and most of it is recalled in the review by Demirev [48], Wien [49] and Reimann [50]. For cluster projectiles, especially in the energy range of several tens to hundreds of keV/atom there is a lack of predictable views and experiments are far ahead of any theoretical approach. It is likely that valuable information on the molecular ejection processes could be obtained from angular and velocity distributions and such experiments are in progress. 3.2. Total sputtering yield measurements of gold and silver targets The total sputtering from metallic targets bombarded with Aun (n ¼ 1–5) was already obtained with gold targets [51]. In experiments performed with Aun beams the total mass eroded from the Au film was measured with the quartz micro balance method [52]. The oscillating quartz was covered with a thick vapor deposited gold or
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
silver layer (1000 50 nm) and the sputtering yield directly determined from the variation of the quartz frequency during irradiation of a known dose of Aun projectiles. The whole technique requires a particular attention and a well controlled procedure to ensure measurement reproducibility. These measurements have been extended recently in Orsay and Lyon [53] to higher cluster sizes for gold and silver targets. As an example, Fig. 2 shows for the gold target, the variation of the sputtering yield per atom Y =n as a function of the energy per atom of Aun for n ¼ 1 to 13. The scale used in the figure immediately demonstrates the non-linear enhancement of the sputtering yield. The yield maxima for Au2 to Au7 projectiles occur at the same projectile velocity corresponding to about 200–250 keV/atom. For Au13 at 100 keV/ atom the total yield for the gold target is close to 15 000 atoms of gold ejected per impact. Such a high value is also reached with a silver target at a slightly smaller incident energy per atom. In addition to the very high sputtering yield values, which are the highest ever measured, the second
Fig. 2. Sputtering yield per atom Y =n induced by Aun cluster projectiles (n ¼ 1–13) on a gold target, shown as a function of the projectile energy per atom. The lines are to guide the eyes. (Results from [53].)
231
remarkable result of these series of experiments is the position of the yield maxima located at a velocity much lower than the velocity corresponding to the maxima of the nuclear energy loss for Aun on Au and Aun on Ag (respectively, located at 700 and 550 keV/atom). Atomic force microscopy (AFM) pictures of the gold surface were obtained after irradiation by 127 keV/atom Au11 with a dose of 1:6 1010 /cm2 . They show important surface deformations with the presence of craters of rather similar dimensions (several hundred Angstroms in diameter) and a density that agrees within 20% with the total dose. From the sputtering yield value (12 000 atoms/ impact in this case) and assuming a conical crater shape of diameter D and depth h one can calculate . The pictures tell that diameters that D h 93 A seem to be larger, but there is a rim at the crater side and the depths are difficult to measure. It is nevertheless obvious that craters are formed at the surface and the emission mechanism models will need to be able to explain their formation. Different analytical models are briefly discussed in [53] but their comparison with the large set of data presented in [53] is not straightforward. For instance, within the thermal spike model of Sigmund and Claussen [54], the experimental sputtering yields measured for the Aun projectiles could be reproduced only if one considers the initial cylindrical spike radius (related in this model to the straggling in the collision cascades) as a free parameter which increases with cluster size and energy. At this stage, predictions by the models are uncertain. More data on yields and data on velocity and angular distributions are needed with other target–projectile combinations to have a better understanding of the emission mechanisms and of the role of different parameters as the projectile-to-target mass ratio, the target sublimation energy, the projectile velocity and stopping power. Molecular dynamic simulations at these energies are not easy with such cluster sizes but the simulations performed at lower energies could nevertheless provide interesting information by using an approach different from the spike model [55–57]. In [56] the crater formation is visualized and the simulation shows, as a function of time (in
232
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
picoseconds) the evolution of the crater shape together with particle and cluster emission resulting from an initial phase explosion. For larger deposited energies this type of scenario seems still quite realistic. 3.3. Comments and prospects The very high yields obtained with heavy cluster projectiles raise the question of the fraction of incident energy which is removed trough particle ejection. A simple extrapolation from the data shown in Fig. 2 indicates that for massive clusters (n > 40) the total amount of energy released in one impact would be of the order of 30% of the incoming energy and one can further speculate on the observation of a saturation of the observed yield for heavier clusters or an eventual decrease of the kinetic energy or a modification of the mass distribution of the emitted species. As most of the sputtered material is emitted as neutrals difficult to detect, one has either to post-ionize these neutrals or to restrict the kinematical measurement to ionized species, assuming they are representative of the whole emission. For this purpose, in a first experiment realized with the 256 pixels detector briefly described later (and which allows to measure double differential cross-sections over angle and energy), the energy and angular distributions for ions emitted from a gold surface under Auþ 4 impact for three bombarding energies (800 keV, 4 MeV and 20 MeV) have been measured [58]. Preliminary analysis of the data concerning Au , Au 2 and Au3 ions indicate that, for the two lowest incident energies, the energy distributions of emitted gold ions are similar. Because the geometrical acceptance of our detector did not allow to detect ions with a radial energy greater than 16 eV, we have only analyzed the angular distributions of ions emitted with a total energy smaller than 16 eV: for the three bombarding energies, the emission of Au , Au 2 and Au 3 ions is perpendicular to the target surface and the angular distributions follow a cosine-type distribution. Further experiments are in progress with other types of targets such as CsI and lead targets. As shown in the previous section in the case of the
phenylalanine target, the emission yield for ðCsIÞn I secondary cluster ions is maximum when the Aun incident energy per atom is close to 50 keV/atom. Total sputtering yield measurements from CsI layers are in progress to check with the same target if for neutrals, the yield maxima occur at an energy per atom larger than the emission yield of cluster ions. This general behavior is of great interest for analytical applications since high ion emission yields occur at a low bombarding energy where damages in the solid are not yet important.
4. Carbon cluster projectiles through solids Secondary emission of atoms and ions induced by Cn clusters in the electronic energy loss regime may also result from energy transferred to the surface which ‘‘evaporates’’ atoms or molecules (as in the spike model) but it is probable that emission takes place from a certain depth below the surface. This is the case for example with C60 projectiles at 20 or 30 MeV which produce enormous craters in diameter and more than 150 A in depth) (500 A in organic material [30,31]. The range in solids of Au atoms from Aun clusters (at energies of a few 100 keV/atom) is only a few hundred Angstroms. With Cn clusters and C60 beams produced by the 15 MV Orsay–Tandem, we are in the electronic energy loss regime and the cluster constituents can up to travel long distances in solids (several 103 A 4 several 10 A in carbon foil). Energy loss measurements with carbon clusters were extensively discussed in several papers [59,60]. We report here on different interesting phenomena related to the charge states of carbon constituents in solids and exiting solids, to the cluster constituent spatial correlation and lateral velocity distribution. 4.1. Secondary emission of hydrogen ions and diminution of carbon constituent charge states in solids An interesting aspect of secondary ion emission with carbon clusters is the emission of hydrogen ions. It has been demonstrated that the emission
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
rate of hydrogen ions Hþ from a solid surface bombarded by multiply charged ions is strongly dependent on the charge state of the projectile in a large range of incident energies [61,62]. A power law Y q2:7 between the incident ion charge state q and the yield Y has been established for keV and MeV projectiles regardless of their masses and the solid target material (organic compound or hydrogen containing molecules adsorbed on metallic or carbon substrates). The average initial energies of Hþ are significantly higher than any other types of emitted secondaries (around 10 eV by comparison to a few eV for all other ions) and it was shown [61] that emission originates from the surface. A carbon cluster projectile is singly charged before hitting a target. It is therefore almost an assembly of neutral atoms that penetrates the solid and this is a unique feature of fast cluster projectiles. Upon impact each atom becomes ionized within a short distance as for example a single or carbon charge state is equilibrated within 10 A so at 1 MeV [63]. The ionization state is smaller for carbon cluster constituents (see later) but the total point charge in the first target layers is large compared to the approaching cluster charge before þ impact. With a C1þ 60 at 20 MeV the H emission is for example about 3000 times larger than the rate observed with a single carbon at the same velocity. This demonstrates that, in this velocity range, this is not the charge state approaching a surface that stimulates the hydrogen emission (and electron emission) by potential effects as predicted by the pre-impact model [64]. The transient local high density of charge at the surface followed by Coulomb explosion, hydrogen bonds breaking and fast Coulomb repulsion of Hþ ions seems to be a more appropriate picture. The particle-induced hydrogen ion emission is an important surface phenomena which is not yet completely understood in ion–solid collisions but which can be used to determine the charge state of fast ions at the entrance and exit sides of thin foils [61,65]. In the present case the method was applied to measure carbon charge states of clusters (C5 and C10 ) passing through thin carbon foils of more than 20 nm [66]. For example when a C5 cluster travels in a carbon foil the carbon atoms reach a certain ionization state. The carbon ions exiting
233
the foil with an average charge hqn i were further used to hit a second foil where they induced (as 5 independent projectiles) Hþ emission. The Hþ 2:7 emission yield, proportional to 5 hqn i , was then compared to the same measurements with single carbon atom having an average exiting charge state hq1 i at the same velocity. The ratio of these two charge state values can then be obtained from the corresponding Hþ yields ratio. A series of experiments was performed with different foil thicknesses. The main results are a lowering of the mean charge state per atom of the Cn constituents inside the solid by comparison to single atoms. For C5 at 2 MeV/atom passing in a 30 nm Carbon foil the diminution of charge is about 15%. It is only after a distance of 300 nm that the carbon atoms in the solid behave as independent atoms with respect to Hþ emission, i.e. that the Hþ emission with Cn is about equal n times that of C1 . 4.2. Reduced charge state of MeV carbon cluster atomic constituents and simulation of cluster correlations in matter A more direct method was used in [67] for the determination of average charge states of carbon atoms from Cn (n ¼ 3 to 10 and C60 ) at velocities corresponding to 0.5 MeV to 4 MeV/atom. Carbon foils of different thicknesses were used and the comparison with C1 atoms was made under the same experimental conditions. We briefly recall this method, which has been recently applied to the case of C8 and C60 cluster beams at 600 keV/ atom [68]. At the exit side of a carbon foil the carbon ions are electrostatically deflected by a set of two parallel and horizontal metallic plates placed at a distance of 5 mm from the carbon foil. Voltages of 3 to 15 kV can be applied onto the plates. A multi-pixel position sensitive detector consisting of two micro channel plates followed by 16 16 discrete anodes connected to a board of discriminators (256) and 32 independent Time Digitized Converters (8 stops channel each) was entirely built at the institute [69]. A careful calibration of the electrostatic deviation using different voltages was achieved with a 0:3 mm 0:3 mm collimated Cþ 1 beam.
234
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
The voltage value was set to ensure a complete detection of the charge distribution in the detector. Average charge state measurements and lateral width of spatial distributions of the exiting constituents were obtained with a good accuracy. Table 1 in Ref. [67] gives results for C1 , C3 , C5 and C8 projectiles at energy per atom between 1 and 4 MeV/atom. Fig. 3 illustrates the variation of hqn i=hq1 i as a function of the number n of atoms in the cluster for an incident energy of 2 MeV/atom, hq1 i is the average charge of a single carbon atom and hqn i the average charge of a cluster constituent. In this energy range hqn i is always smaller than hq1 i at the same velocity whatever the foil thickness although the ratio is close to one for the thickest foil (40 lg/cm2 ). The charge reduction in a 2.2 lg/cm2 carbon foil is as large as 30% for a 20 MeV C10 cluster and this reduction progressively vanishes with increasing target thickness. The charge reduction is much smaller at lower energy per atom. For example preliminary results obtained recently with C8 [68] seem to indicate that there is practically no reduction at 0.6 MeV/atom in a foil of 2.4 lg/cm2 . A C60 beam at the same velocity was also investigated. In the experiment with the C60 beam, the number of pixels that are hit simultaneously is important and the distribution is wide. Contrarily with what is observed for 20 MeV C10 , the charge reduction for C atoms
Fig. 3. Evolution with the number n of cluster constituents of the average charge state ratio hqn i=hq1 i (see text) measured at the exit of carbon foils of different thickness (2.2 and 40 lg/cm2 ) for 2 MeV/atom incident cluster energy. (Results from [67].)
from C60 would be only a few percent. A larger effect could have been expected for this large number of constituents. In thick foils the charge suppression observed with Cn vanishes as a result of the constituents becoming farther and farther apart and independent. A memory of the interactions between the co-moving atoms in the first part of the foil may however remain. Systematic measurements of the lateral spread of carbon ions exiting carbon foils were obtained with the multi-pixel detector. A simulation taking into account both the intracluster forces and the collisions between cluster constituents and target atoms was developed to reproduce the spatial distribution [70]. As the clusters penetrate the targets the ions suffer binary collisions with target atoms. The mutual interactions between cluster ions are described with a Molliere type potential taking into account the hard core repulsion and with a shielded Coulomb potential Vc ðrÞ ¼ ½qðvÞe 2 =r er=a (where a is the screening distance) to follow the Coulomb explosion contribution. Step by step calculations were performed. As a cluster travels forward by a lattice constant of length there is only a very small perturbation of the ion velocities and position due to intra-cluster forces. However over many lattice constants the Coulomb explosion has a noticeable effect. The algorithm used in the simulation provides intra-cluster correlations between ions at the exit side of thin and thick carbon foil targets. Bare Coulomb and shielded Coulomb with various screening length were applied in the calculation that also takes into account the shape of the cluster (linear or circular). In an annular structure the ions repelled from the center of the ring and the Coulomb energy is more effective than in a linear chain structure. The memory of the initial shape is lost much faster with non-linear clusters. Upon exit of the target, knowing the position and velocities of the constituents, unshielded Coulomb forces are applied to calculate the final distribution in the detector plane. For thin targets (2 lg/cm2 ) the lateral velocity is mainly due to Coulomb explosion behind the target while for thicker targets there is much more multiple scattering and the Coulomb effect behind the target becomes weak. A comparison with experimental spot sizes produced by
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
different incident clusters has been made. It is shown in the various simulations presented in Fig. 4 that for a rather thick target (40 lg/cm2 ), the Coulomb forces induce a significant increase of the distribution width on cluster size, which contrasts with the experimental observation of measured widths that do not depend on n. This comparison evidences very strong shielding. The screening length value is a sensitive parameter to fit the width of the calculated lateral velocity distribution to the experimental ones. This screening length of the Coulomb potential for 2 MeV carbon ions in a carbon target (40 lg/cm2 ) is found to be less than . 2.5 A The calculated constituent trajectories have been used to calculate the charge state suppression within a simple model of enhanced electron capture at the exit side of the target due to the presence of charged neighbors which contribute to bound more strongly the electrons to the cluster ions. The apparent additional potential DI for an
Fig. 4. Evolution with the cluster size of spatial distribution widths given by the trajectory simulation (see text) for 2 MeV/ atom Cn constituents exiting a 40 lg/cm2 carbon foil. Calculations are displayed for various values of the screening length as indicated in the legend. The widths of the lateral distributions predicted by simulations which include an in-target Coulomb explosion show a clear dependence upon cluster size which is not observed experimentally. (Results from [70].)
235
electron to escape reduces the velocity-dependent equilibrium charge q1 ðvÞ and the ion charge in the cluster becomes qi ¼ q1 ðvÞ BDI. DI can be approximated by taking a mean field with qi hqn i for all constituents of a Cn cluster, so that the ionization potential is proportional to (hqn iv), v being a geometrical contribution factor which depends on the respective positions of the exiting cluster ions. Then hqn i=q1 ðvÞ ¼ 1=ð1 þ BvÞ and the comparison with experimental data shown in [67] suggests that this approach of an additional potential is not unrealistic. Another less simple model [71] based on electron density variation in the vicinage potential of proximity atoms leads to an average number of bound electrons minimizing the total energy of bound electrons. The experimental variation with the cluster size of the ratio hqn i=q1 is also well fitted for thin targets. Vicinage effects in the charge state of swift Cn cluster constituents at 2 MeV/atom traversing thin foils were also recently studied in [72]. The potential energy of the electrons surrounding one constituent ion, moving in correlation with the other ions, comes from the interaction with their own nucleus plus the interaction potential with the (n 1) remaining ions which reduces this potential energy as compared to the case of an isolated ion. Following the time evolution of the inter-constituent distance given by the Coulomb explosion, the average charge ratio inside the foil and at the foil exit can be calculated. It is shown in [72] that the variation of the screening effect at the exit surface of thin foils is particularly important to reproduce the experimental charge ratio. A good agreement with our experimental values is obtained and information on the Cn molecular structure could be obtained. A phenomenological approach was proposed by Parilis. The basic idea is that the collision crosssections are larger for the front-runner atoms in the cluster than for the atoms behind [73].
5. Separation distance of fast carbon constituents in silicon material and reduced pulse height signals delivered by surface barrier silicon detectors The radial separation of the co-moving carbon atoms in the solid reduces the observed ‘‘collective
236
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
effects’’. Depending on the characteristic length scale of the involved processes, these collective effects may disappear at different levels of separation distance between constituents. Experiments on ion energy measurements were carried out with surface barrier detectors hit by carbon clusters and carbon atoms at the same velocity obtained by the fragmentation of carbon clusters Cn having passed through very thin foils. An important energy deficit (with respect to nC1 ) is measured and can reach 50% in the case of C60 at 20 MeV [74]. Systematic measurements of this energy deficit have been performed recently in Erlangen and Orsay [75] with Cn clusters (n 6 10) accelerated at 1 and 2 MeV/atom. The total pulse height or net recorded energy is: Eph ¼ Ei Dw DN Dr, where Ei is the incident energy, Dw is the total energy loss in the entrance gold window (and a possible dead layer underneath) of the detector, DN is the total nuclear energy loss and Dr the sum of all other phenomena which may modify the number of electron–hole pair production and collection. DN and Dw can be estimated by assuming that, within a few percent, the energy loss by a 1–2 MeV/atom Cn cluster is n-fold the energy loss by C1 at the same velocity. Therefore the energy difference between a Cn projectile and nC1 is due to the difference in the Dr of Cn and nC1 . The observed important deficit observed in direct impacts (without foil in front of the detector) is induced by the proximity of the co-moving atoms in the detector. All the energy is not converted or some processes dissipate or trap energy. At a given energy per atom the energy deficit DrðCn Þ nDrðC1 Þ increases with the number n of constituents as nðn 1Þ. Also for a given cluster size, the energy deficit increases linearly with the total deposited energy and reaches 0.8 MeV for C10 at 10 MeV. An experiment has been performed to measure the detector response as a function of the estimated average separation distance of the cluster atoms in the detector. For this purpose a rectangular surface barrier detector with a large active area was used. Additional gold layers were evaporated onto the entrance window in a stair-like structure consisting of 8 steps with increasing suc-
cessive thickness of 40 nm each. This detector was prepared by Voit in Erlangen. A C6 was precisely directed on these steps and the pulse height deficit (PHD) was measured for each step thickness. The cluster energy was 5.8 MeV when the cluster hit the thinnest step and was successively increased from step to step so that the energy after the passage of each step stays the same. The measured energy deficit DrðCn Þ nDrðC1 Þ remains constant as shown in Fig. 5. By multiple scattering in the gold layers the carbon atoms gradually separate and the separation is large (several tens of nm) for the largest thickness of 300 nm. Therefore the present results show that even after such a thickness of gold the cluster constituents at the entrance of the active Si area do not behave as n single atoms as they do when going through the thin foil upstream the detector. It is thus probable that trajectories must be hundreds of nm apart before the PHD vanishes. A model taking into account recombination of e–hole in a plasma column created by the passage of ions in Si was developed by Finch et al. [76]. As shown in [75], the results obtained with Cn projectiles agree qualitatively with the Finch model provided that the plasma column radius extends to hundreds of nanometers. The plasma columns associated with each carbon would then overlap in
Fig. 5. Pulse-height defect difference between intact cluster and independent constituents DrðCn Þ nDrðC1 Þ plotted as a function of the thickness dAu of the gold entrance window, measured for a C6 clusters with constant energy at the entrance of the active silicon area. (Results from [75].)
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
the case of Cn impacts and this would increase the rate of recombination.
6. Conclusion A number of unknown effects have been discovered with cluster projectiles at large velocity. The understanding of phenomena involved in the collision of clusters with solids is certainly in progress but several questions remain open. An assembly of fast co-moving atoms, which can become highly ionized in the first layers of solid targets, do not behave as individual atoms and surface induced as well as bulk-induced phenomena are not yet much predictable at large velocity. The ionization state of carbon constituents of Cn clusters is reduced by comparison with single atom and the charge reduction increases with n at a given energy per atom between 1 and 4 MeV/atom. Preliminary results show that this effect is not large with molecular projectiles as C60 at 0.6 MeV/atom. Unfortunately it is actually not possible to reach higher energy per atom for C60 projectiles. The lateral velocity distributions of C atoms have been reproduced in simulations using a simple algorithm that describes the microscopic correlations of fast ionic cluster constituent in carbon foils. It is shown that the strong pulse height reduction observed in the response of Silicon surface barrier detectors to MeV/atom Cn clusters is still related to the proximity of the traveling atoms in the solids all along their trajectory. Unless they are hundreds of nm apart, the effect is still observed. Secondary emission with cluster is another very interesting aspect of the cluster–solid interactions. The emission rate is very much enhanced with cluster projectiles but the emission yield is not directly related to the amount of energy deposited in the material. Larger dE=dx values due to larger velocities do not provide larger release of matter and there is a need for valuable predictions. At lower energy of impact, Molecular Dynamic (MD) simulations are already very useful to understand experimental results obtained with cluster projectiles. Very large size clusters of various
237
types, containing thousands of atoms at a few tens of keV are for example used in material surface processing applications that are guided by MD simulations. From the results discussed in this short review, the energy range of 50–300 keV/atom for Aun clusters appears to be very interesting since the secondary ion and neutral emission yields are surprisingly high in this domain of velocity. It would obviously be also very useful to perform MD simulations for the Aun impacts on metallic or organic targets in the overall energy range studied in this work. Additional data on energy and angular distributions will be soon available for these systems, which will bring more constraints for comparison with theoretical predictions. A continuation of these experiments will be pursued with larger size gold clusters. Applications with organic and bioorganic materials may already result from the present knowledge of secondary emissions induced in this intermediate velocity range.
Acknowledgements This paper reports on experimental results obtained at IPN-Orsay, IPN-Lyon and ErlangenUniversity in the last years, within long-standing collaborations. We are strongly indebted to our colleagues S. Bouneau, A. Brunelle, S. Della Negra, J. Depauw and M. Pautrat from Orsay, M. Fallavier and J.C. Poizat from IPN-Lyon, H.H. Andersen from NBI-Copenhagen, T. Tombrello and J. Hartman from Caltech-Pasadena and H. Voit and M. Seidl from Erlangen University. Fruitful discussions with E. Parilis are gratefully acknowledged. We also address special thanks to A. Schulz for his enthusiastic reading and comments.
References [1] A. Benninghoven, P. Bertrand, H. Migeon, H.W. Werner (Eds.), Proceedings of the 12th SIMS XII Conference, Elsevier, Amsterdam, 2000. [2] J.C. Poizat, J. Remillieux, Phys. Lett. 34A (1971) 53. [3] W. Brandt, A. Ratkowsky, R.H. Ritchie, Phys. Rev. Lett. 33 (1974) 1325.
238
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
[4] J.W. Tape, W.M. Gibson, J. Remillieux, R. Laubert, H.E. Wegner, Nucl. Instr. and Meth. 132 (1976) 75. [5] J.C. Eckardt, G. Lantschner, N.R. Arista, R.A. Baragiola, J. Phys. C 11 (1978) L851. [6] A.R. Nyaiesh, W. Steckelmacher, M.W. Lucas, J. Phys. C 11 (1978) 2917. [7] R. Laubert, IEEE Trans. Nucl. Sci. NS-26 (1979) 1020. [8] R. Levi-Setti, K. Lam, T.R. Fox, Nucl. Instr. and Meth. 194 (1982) 281. [9] D. Maor, P.J. Cooney, A. Faibis, E.P. Kanter, W. Koenig, B.J. Zabransky, Phys. Rev. A 32 (1985) 105. [10] A. Faibis, G. Goldring, M. Hass, R. Kaim, I. Plesser, Z. Vager, Nucl. Instr. and Meth. 194 (1982) 299. [11] A. Breskin, A. Faibis, G. Goldring, M. Hass, R. Kaim, Z. Vager, N. Zwang, Phys. Rev. Lett. 42 (1979) 369. [12] I. Plesser, E.P. Kanter, Z. Vager, Phys. Rev. A 29 (1984) 1103. [13] M.J. Gaillard, J.C. Poizat, A. Ratkowski, J. Remillieux, Nucl. Instr. and Meth. 132 (1976) 369. [14] B. Mazuy, A. Belkacem, M. Chevallier, M.J. Gaillard, J.C. Poizat, J. Remillieux, Nucl. Instr. and Meth. B 28 (1987) 497; Nucl. Instr. and Meth. B 33 (1988) 105. [15] M.F. Steuer, R.H. Ritchie, Nucl. Instr. and Meth. B 40/41 (1989) 372. [16] H.H. Andersen, H.L. Bay, J. Appl. Phys. 45 (1974) 953. [17] S.S. Johar, D.A. Thomson, Surf. Sci. 90 (1979) 319. [18] D.A. Thompson, S.S. Johar, Appl. Phys. Lett. 34 (1979) 342. [19] H.H. Andersen, in: Sput 92, The Royal Danish Academy of Sciences and Letters. Matematisk-Fysiske Meddelelser 43 (1993) 127. [20] K. Wittmaack, Surf. Sci. 90 (1979) 557. [21] J.P. Thomas, P.E. Filpus-Luyckx, M. Fallavier, E.A. Schweikert, Phys. Rev. Lett. 55 (1985) 103. [22] A.D. Appelhans, J.E. Delmore, Anal. Chem. 61 (1989) 1087. [23] P. Sigmund (Ed.), Fundamental Processes in Sputtering of Atoms and Molecules. Mat. Fys. Medd. Dan. Vid. (1993) 43. [24] Y. Le Beyec, Int. J. Mass Spectrom. Ion Processes 174 (1998) 101. [25] Proceedings of Conference on MeV, keV Ions and Cluster Interactions with Surfaces and Materials, J. Phys. C 2 (1989); Proceedings of Conferene on Polyatomic ion impact on solids and related phenomena, Nucl. Instr. and Meth. B 88 (1–2) (1994). [26] Proceedings of Conferences: Correlated Effects in Atomic and Cluster Ion Bombardment and Implantation, EMRS Conference, Proceedings in Nucl. Instr. and Meth. B 112 (1996); Nanometric Phenomena Induced by Laser, Ion and Cluster Beams, EMRS Conference, Nucl. Instr. and Meth. B 123 (1997). [27] K.L. Merkle, W. J€ager, Philos. Mag. 44 (1981) 741. [28] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135.
[29] A. Dunlop, G. Jaskierowicz, J. Jensen, S. Della-Negra, Nucl. Instr. and Meth. B 132 (1997) 93. [30] D.D.N. Barlo Daya, A. Hallen, J. Eriksson, J. Kopniczky, R. Papaleo, C.T. Reimann, P. Hakansson, B.U.R. Sundqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [31] C.H. Tomaschko, M. Schurr, R. Berger, G. SaemannIschenko, H. Voit, A. Brunelle, S. Della-Negra, Y. Le Beyec, Rapid Commun. Mass Spectrom. 9 (1995) 924. [32] R.J. Beuhler, L. Friedman, J. Appl. Phys. 48 (1977) 3928. [33] M. Fallavier, J. Kemmler, R. Kirsch, J.C. Poizat, J. Remillieux, J.P. Thomas, Phys. Rev. Lett. 65 (1990) 621. [34] M. Salhepour, M. Fistel, J. Hunt, Int. J. Mass Spectrom. Ion Processes 84 (1988) R7. [35] K. Narumi, K. Nakajima, K. Kimura, M. Mannami, Y. Saitoh, S. Yamamoto, Y. Aoki, H. Naramoto, Nucl. Instr. and Meth. B 135 (1998) 77. [36] D. Ben Hamu, A. Baer, H. Feldman, J. Levin, O. Heber, Z. Amitay, Z. Vager, D. Zajfman, Phys. Rev. A 56 (1997) 4786. [37] D. Shen, X. Lu, Z. Xia, X. Wang, Q. Zhao, Nucl. Instr. and Meth. B 129 (1997) 392. [38] M. D€ obeli, R.M. Ender, U.S. Fisher, M. Suter, H.A. Synal, P. Vetterli, Nucl. Instr. and Meth. B 94 (1994) 388. [39] C.H. Tomaschko, D. Brandl, R. K€ ugler, M. Schurr, H. Voit, Nucl. Instr. and Meth. B 103 (1995) 407. [40] P. Hvelplund, J.U. Andersen, K. Hansen, AIP Conf. Proc. 457 (1999) 220. [41] S. Della-Negra, A. Brunelle, Y. Le Beyec, J.M. Curaudeau, J.P. Mouffron, B. Waast, P. Hakansson, B.U.R. Sundquist, E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [42] M. Fallavier, Y. Champelovier, M. Ferrari, R. Kirsch, J.C. Poizat, J. Remillieux, J.P. Thomas, B. Canut, S.M.M. Ramos, P. Thevenard, S. Della-Negra, J.P. Mouffron, P. Nicol, Eur. Phys. J. D 9 (1999) 529. [43] P. Hakansson, S. Della-Negra, J.P. Mouffron, B. Waast, P.A. Sullivan, Nucl. Instr. and Meth. B 112 (1996) 39. [44] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 63 (2001) 022902. [45] TRIM code J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon Press, New York, 1985. [46] H.H. Andersen, A. Johansen, V.S. Touboltsev, Nucl. Instr. and Meth. B 164–165 (2000) 727. [47] M. Karas, F. Hillenkamp, Anal. Chem. 60 (1988) 2299. [48] P. Demirev, Mass Spectrom. Rev. 14 (1995) 279. [49] K. Wien, Radiat. Eff. Defects Solids 109 (1989) 137. [50] C. Reimann, in: P. Sigmund (Ed.), Fundamental Processes in Sputtering of Atoms and Molecules. Mat. Fys. Medd. Dan. Vid. (1993) 351. [51] H.H. Andersen, A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, J. Chaumont, H. Bernas, Phys. Rev. Lett. 80 (1998) 5433. [52] H.H. Andersen, H. Bay, Radiat. Eff. 13 (1972) 67. [53] S. Bouneau, A. Brunelle, S. Della-Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, H.H. Andersen, Phys. Rev. B 65 (2002) 144106.
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239 [54] P. Sigmund, C. Claussen, J. Appl. Phys. 52 (1981) 5433. [55] H.M. Urbassek, J. Michel, Nucl. Instr. and Meth. B 22 (1987) 480. [56] T.-J. Colla, R. Aderjan, R. Kissel, H. Urbasseck, Phys. Rev. B 62 (2000) 8487. [57] M.M. Jakas, E.M. Bringa, Phys. Rev. B 62 (2000) 824. [58] S. Bouneau, A. Brunelle, S. Della-Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, to be published. [59] N. Arista, Nucl. Instr. and Meth. B 164–165 (2000) 108. [60] J. Jensen, P. Sigmund, Phys. Rev. A 61 (2000) 032903. [61] S. Della-Negra, Y. Le Beyec, B. Monart, K. Standing, K. Wien, Phys. Rev. Lett. 58 (1987) 17. [62] M. Benguerba, S. Della-Negra, H. Joret, Y. Le Beyec, Int. J. Mass Spectrom. Ion Processes 107 (1991) R11. [63] G. Maynard, private communication. [64] E.F. da Silveira, M. Matos, Nucl. Instr. and Meth. B 99 (1995) 107. [65] S. Della-Negra, J. Depauw, H. Joret, Y. Le Beyec, I. Bitensky, G. Ben Assajag, R. Galera, K. Wien, in: K. Standing, W. Ens (Eds.), Nato Conference on Mechanisms for Producing Ions from Large Molecules, Minaki Proceedings, Plenum Press, New York, 1991, p. 19. [66] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Ch. Schoppmann, Nucl. Instr. and Meth B 125 (1997) 207.
239
[67] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 59 (1999) 4456. [68] S. Bouneau, A. Brunelle, S. Della-Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, to be published. [69] S. Della-Negra, D. Jacquet, P. Cohen, J. Depauw, R. Donnet, S. Du, J. Le Bris, R. Sellem, IPN Technical Report no. 72 1994–1995. [70] J.W. Hartman, T.A. Tombrello, S. Bouneau, S. DellaNegra, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 62 (2000) 043202. [71] Z.L. Miscovic, S.G. Davison, F.O. Goodman, Phys. Rev. A 61 (2000) 062901. [72] S. Heredia-Avalos, R. Garcia-Molina, N.R. Arista, Europhys. Lett. 54 (6) (2001) 729. [73] E. Parilis, Nucl. Instr. and Meth. B 193 (2002) 240. [74] K. Baudin, A. Brunelle, M. Chabot, S. Della Negra, J. Depauw, D. Gardes, P. Hakansson, Y. Le Beyec, A. Billebaud, M. Fallavier, J. Remillieux, J.C. Poizat, J.P. Thomas, Nucl. Instr. and Meth. B 94 (1994) 341. [75] M. Seidl, H. Voit, S. Bouneau, A. Brunelle, S. Della Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Nucl. Instr. and Meth. B 183 (2001) 502. [76] E.C. Finch, M. Asghar, M. Forte, Nucl. Instr. and Meth. 163 (1979) 467.
Cluster impacts on solids D. Jacquet *, Y. Le Beyec Institut de Physique Nucl eaire, CNRS-IN2P3, 91406 Orsay, France
Abstract Polyatomic projectiles, which are now available in a large range of size and of incident energies, allow to deposit in solid samples high energy densities that induce large damage in the bulk of irradiated samples as well as very important emission of matter from the bombarded surfaces. Large secondary ion emission yields as well as total particle emission yields induced by Aun impacts were measured on various targets over a large range of incident energies covering the domain of energy deposited through atomic collisions and electronic excitations. An unexpected yield-energy dependence was observed with a strong maximum of the emission rate at moderate velocities (100–200 keV/atom) which may open promising developments in surface analysis. At higher velocity, it has been shown that the equilibrium charge state of the co-moving atoms of swift clusters in solid targets is significantly reduced by comparison with single atoms at the same velocity. This effect, due to the additional ionisation potential created by the proximity of the cluster constituents, has been investigated with carbon clusters of different size, in a velocity range between 600 keV up to 4 MeV/atom. The spatial distribution of cluster fragments exiting thin carbon targets were also measured with a multi-impact position sensitive detector. Comparison with theoretical simulations demonstrated that the in-target Coulomb explosion is strongly shielded. Large pulse height defects measured in silicon detectors are a direct consequence of the interaction of the spatially correlated atoms in the silicon material. Results on pulse height dependence on cluster size and energy will be presented for 1 and 2 MeV/atom carbon clusters. This set of results, obtained recently by the Orsay group (most of them in collaboration with other groups), illustrates different aspects of cluster collisions and/or transmission in solids. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 79.20.Rf; 36.40.-c; 34.50.Dy; 61.80.Lj; 68.49.Sf; 34.50.Fa; 36.40.Fa; 36.40.Wa; 29.40.-n; 07.77.-n; 34.70.þe; 29.40.W
1. Introduction When a fast cluster projectile collides with a solid target the atomic constituents separate upon impact in the first layers and remain in close proximity for a certain distance. The types of clusters (size, atomic constituent, mass and shape), the incident velocity as well as the target material
*
Corresponding author. E-mail address: [email protected] (D. Jacquet).
and structure are important parameters that govern the physical effects occurring at the surface and in the bulk of a solid target. The simultaneous impact of several atoms on a small surface of a solid is a unique way to deposit large energy density in a material and to study the coherent (time and space) interactions of several ions traveling in a solid. The passage of swift light clusters through solids at relatively high velocity and the interaction of low velocity clusters with solid targets have now been studied for many years. However, it is only in
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 7 5 5 - 3
228
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
the last decade – due to advancements in cluster ion sources and acceleration techniques – that a variety of different clusters could be accelerated to several tens of MeV. Beams of multiply charged C60 at 40 MeV are a typical example. In the same period of time numerous and remarkable applications have been developed at a total energy of a few tens of keV and this is also an aspect which should not be ignored (although not discussed here) since small powerful commercial instruments are now used for surface analysis [1]. For swift projectiles of velocity larger than the Bohr velocity, most of the experiments on cluster solid interactions were focused on what happened to the projectile and its constituents while passing through solids as for example charge states of constituents, energy loss, projectile structures at the exit side of thin foils . . . . Hydrogen clusters þ þ and small molecular ions (Oþ 2 , N2 , CH , . . .) were used to first demonstrate evidence of ‘‘vicinage effects’’ in the energy loss of clusters [2–8] and in the lowering of charge state of ions issued from dimer molecules by comparison to single atoms at the same velocity [9–15]. Little was done on the important question of the deposit and relaxation of energy in a material with the consequence of ejection of particles and structure modification in the solid itself. The response of solid targets to impacts in terms of secondary emission was first investigated with dimers and trimers [16–19] and other small molecules at low energy [20–22]. An enhanced emission rate of particles during cluster bombardment had already been measured experimentally (see review papers and conference proceedings [23–26]) and small deformations or craters were observed after cluster impacts at the surface of targets [27]. In recent works at high velocity very important defects are visible at the surface as well as inside the material [28–31]. The permanent damages can help to investigate the trajectories of clusters in matter and the duration of collective effects in comparison with the estimated separation distance between constituents. The high velocity work also demonstrates the importance of the volumic energy density with respect to linear energy loss dE=dx [28].
In this paper we will focus on some of these experimental results obtained recently with gold clusters and carbon clusters produced with various electrostatic accelerators. The response of the material to the high energy density deposited by the spatially and temporally correlated atoms will be illustrated by the experimental results on secondary ion emission and total sputtering with Aun clusters over a very broad range of energy (from keV to MeV/atom). The large emission rate from insulating material and from metallic targets involves emission mechanisms in which an appreciable amount of the incoming energy is removed by the ejected particles. Light ions such as hydrogen are also emitted from surface contaminants with a large yield but for these particular ions the emission is very sensitive to the total ionization charge of the cluster atoms at the surface within a few atomic layers. The value of this yield can be used to determine the average charge of cluster constituents at the exit side of solid foils. Another more direct experimental procedure has however been developed with a position sensitive detector which measures the electrostatic deviation of charged ions exiting thin foils. Important charge suppression effects have been measured at energy above 1 MeV/atom with Cn clusters. The angular distributions of exiting cluster ions strongly depends on cluster size and target thickness and they have been compared to predictions of trajectory simulations which include potential screening and Coulomb repulsion between the comoving ions in the solid. The energy response of silicon barrier detectors was studied under impact of Carbon clusters Cn and C60 molecules. A very strong energy deficit was observed in comparison with impacts of single carbon atoms at the same velocity. The pulse height defect is a direct consequence of the proximity of the traveling ions in the Si material, integrated over the full range.
2. Cluster ion beam production Since the acceleration of hydrogen clusters, water clusters and small molecular ions in various
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
laboratories [32–34] there has been a great deal of efforts to produce cluster beams over a large scale of energy for a variety of purposes [25,35]. Mostly light atomic clusters were accelerated in Japan [35], Israel [36], China [37] and Switzerland [38] while light and heavy clusters are available now in Germany (Erlangen [39]), Denmark (Aarhus [40]) and France (Orsay and Lyon [41,42]). Beams of Cn , (eventually Aln , Nin , Agn ), Aun and C60 are accelerated at Orsay. Two procedures are used to produce clusters with electrostatic accelerators: (i) In Lyon and Orsay a Gold liquid metal ion source was installed to deliver Aun beams with a total energy of, respectively, 1.4 and 13 MeV/charge. In Lyon the maximum size of deflected clusters is n ¼ 13. In Orsay a Wien filter is used to separate the different cluster sizes at low energy before injection into the accelerator [43]. Aun beams with n up to 400 have been obtained in the zero angle beam line and 18 MeV Au100 have been magnetically deflected at 1.2°. (ii) Another procedure is to produce negative cluster ions in a conventional sputtering ion source and to use the two acceleration steps of a tandem accelerator. Positive cluster ions are then formed at the high terminal voltage in a gas cell containing a low pressure N2 gas. With this method the cluster identification is made by a magnetic separation coupled with a time of flight measurement plus an energy measurement with a surface barrier detector. The deflection angle is 1.2°. The Orsay cluster beam facility has been extensively used by numerous groups in the last few years.
3. Secondary emission from solids induced by Aun clusters The high energy density which is deposited in a solid during the interaction processes with the target atoms or with the electronic systems may dissipate through various exit channels. Emission of atomic ions or complex ions (molecules, clusters) is much less probable than the emission of neutral species but both can be used to study the projectile– solid interaction. As mentioned previously the re-
229
spective role and importance of the different modes of energy loss processes on secondary emission are not clearly identified at intermediate energy. Therefore the lack of consistent data covering a large range of velocity (and energy loss process) for a given target projectile combination was the basic reason to investigate the variation of emission yield over the largest range of energy. It was possible in our laboratory to explore the range of 5 keV/atom to about 5 MeV/atom with Aun clusters. The same experimental equipments and the same experimental procedures were used at the different beam facilities in both types of experiments on secondary ion emission as well as for total sputtering yield measurements. 3.1. Secondary ion emission Total sputtering as well as ion emission yields induced by cluster bombardment have shown to deviate from the sum of the yield of individual atoms. The yield is defined as the number of ejected atoms or ions of a given type divided by the number of impacting projectiles. The first observations of nonlinearity phenomena were made with dimer projectiles by Andersen and Bay [16] for neutral emission and by Wittmaack for secondary ion emission [19]. The nonlinear enhancement of yield beyond a direct proportionality to the number of constituents has since been well established in all regimes of velocities [23–26]. In a recent work of the Orsay group [44], thick organic layers of molecules as well as inorganic materials were used as solid targets. Time of flight mass spectrometry in the ion counting mode was used to identify the molecular ions from a phenylalanine target (C9 H11 O2 N) and cesium iodide cluster ions from CsI layers. The emission yield of various type of ions was measured as a function of Aun cluster velocity and size. Fig. 1 shows the variation with the projectile energy per atom of the emission yield of the intact molecular ion (M–H) emitted from organic layers of phenylalanine under impact of Aun (n ¼ 1–4) in the energy range from 5 keV to more than 5 MeV/atom. Similar shapes of yield variations with energy per atom were also obtained with other types of ions from organic and inorganic
230
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
Fig. 1. Secondary-ion emission yield of the negative molecular ion (M–H) , induced by Aun cluster (n ¼ 1–4) impact on a Phenylalanine target as a function of the projectile energy per atom. The solid lines are to guide the eyes. Calculations of nuclear ( ) and electronic ( ) energy losses for a single gold atom, performed with the TRIM code are also shown in the figure. (Results from [44].)
targets. Above 100 keV/atom the emission yields vary roughly as n2 with the number of cluster constituents, for n > 2. A larger dependence of secondary ion emission yield on n is observed between n ¼ 1 and n ¼ 2 at energy per atom below 500 keV/atom with the presence of a marked and unexpected peak for clusters around 40 keV/atom. A detailed analysis of the emission yield dependence on n can be found in [44]. Under single gold atom impact the emission yield variation is very different. It is very low until 1 MeV/atom and then increases rapidly with energy when the electronic energy loss processes become predominant. It is remarkable to note that above 1 MeV/atom the evolution of the yield curve with the projectile velocity is similar for single Au projectile and for Aun clusters. A variation of the calculated energy loss (nuclear and electronic) for a single gold atom (TRIM calculation [45]) is also shown in Fig. 1 for comparison with the experimental yield energy dependence. The observed experimental maxima at 50 keV/atom (see Fig. 1, Au4 ) are at energies per atom much lower than the peak corresponding to the maximum of the nuclear stopping of Au in organic material (380 keV/atom). It has been observed experimentally that the range of Aun clusters (n ¼ 1–3) implanted at 10–40
keV/atom in various targets (Si, Al, Cu) does not depend on cluster size and is equivalent to the range of Au atoms [46]. It is thus assumed that at higher energy per atom the nuclear stopping of a gold Aun cluster is n times the nuclear stopping of a single Au ion. Therefore the results of Fig. 1 show that the nuclear energy loss is not the only relevant parameter that controls ion emission from insulating solids. The energy density near the surface and the regime of velocity seem to be more important. Additional experiments with larger gold clusters should be performed in the future to further investigate the capabilities of such projectiles for applications such as providing a supplement to the bio-mass-spectrometry now done with laser desorption (MALDI) [47]. The dynamics of the energy deposit followed by the transfer of momentum to fragile molecular ions or cluster ions in the gas phase has been qualitatively described in both regimes of energy loss with keV and MeV atomic projectiles. Coherent ‘‘knock on’’ of a molecule by a number of recoiling atoms generated in a dense collision cascade, ‘‘explosion-like’’ or expansion of a highly energized region in the vicinity of the surface would push the surrounding molecule out. There is a numerous literature on the subject and most of it is recalled in the review by Demirev [48], Wien [49] and Reimann [50]. For cluster projectiles, especially in the energy range of several tens to hundreds of keV/atom there is a lack of predictable views and experiments are far ahead of any theoretical approach. It is likely that valuable information on the molecular ejection processes could be obtained from angular and velocity distributions and such experiments are in progress. 3.2. Total sputtering yield measurements of gold and silver targets The total sputtering from metallic targets bombarded with Aun (n ¼ 1–5) was already obtained with gold targets [51]. In experiments performed with Aun beams the total mass eroded from the Au film was measured with the quartz micro balance method [52]. The oscillating quartz was covered with a thick vapor deposited gold or
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
silver layer (1000 50 nm) and the sputtering yield directly determined from the variation of the quartz frequency during irradiation of a known dose of Aun projectiles. The whole technique requires a particular attention and a well controlled procedure to ensure measurement reproducibility. These measurements have been extended recently in Orsay and Lyon [53] to higher cluster sizes for gold and silver targets. As an example, Fig. 2 shows for the gold target, the variation of the sputtering yield per atom Y =n as a function of the energy per atom of Aun for n ¼ 1 to 13. The scale used in the figure immediately demonstrates the non-linear enhancement of the sputtering yield. The yield maxima for Au2 to Au7 projectiles occur at the same projectile velocity corresponding to about 200–250 keV/atom. For Au13 at 100 keV/ atom the total yield for the gold target is close to 15 000 atoms of gold ejected per impact. Such a high value is also reached with a silver target at a slightly smaller incident energy per atom. In addition to the very high sputtering yield values, which are the highest ever measured, the second
Fig. 2. Sputtering yield per atom Y =n induced by Aun cluster projectiles (n ¼ 1–13) on a gold target, shown as a function of the projectile energy per atom. The lines are to guide the eyes. (Results from [53].)
231
remarkable result of these series of experiments is the position of the yield maxima located at a velocity much lower than the velocity corresponding to the maxima of the nuclear energy loss for Aun on Au and Aun on Ag (respectively, located at 700 and 550 keV/atom). Atomic force microscopy (AFM) pictures of the gold surface were obtained after irradiation by 127 keV/atom Au11 with a dose of 1:6 1010 /cm2 . They show important surface deformations with the presence of craters of rather similar dimensions (several hundred Angstroms in diameter) and a density that agrees within 20% with the total dose. From the sputtering yield value (12 000 atoms/ impact in this case) and assuming a conical crater shape of diameter D and depth h one can calculate . The pictures tell that diameters that D h 93 A seem to be larger, but there is a rim at the crater side and the depths are difficult to measure. It is nevertheless obvious that craters are formed at the surface and the emission mechanism models will need to be able to explain their formation. Different analytical models are briefly discussed in [53] but their comparison with the large set of data presented in [53] is not straightforward. For instance, within the thermal spike model of Sigmund and Claussen [54], the experimental sputtering yields measured for the Aun projectiles could be reproduced only if one considers the initial cylindrical spike radius (related in this model to the straggling in the collision cascades) as a free parameter which increases with cluster size and energy. At this stage, predictions by the models are uncertain. More data on yields and data on velocity and angular distributions are needed with other target–projectile combinations to have a better understanding of the emission mechanisms and of the role of different parameters as the projectile-to-target mass ratio, the target sublimation energy, the projectile velocity and stopping power. Molecular dynamic simulations at these energies are not easy with such cluster sizes but the simulations performed at lower energies could nevertheless provide interesting information by using an approach different from the spike model [55–57]. In [56] the crater formation is visualized and the simulation shows, as a function of time (in
232
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
picoseconds) the evolution of the crater shape together with particle and cluster emission resulting from an initial phase explosion. For larger deposited energies this type of scenario seems still quite realistic. 3.3. Comments and prospects The very high yields obtained with heavy cluster projectiles raise the question of the fraction of incident energy which is removed trough particle ejection. A simple extrapolation from the data shown in Fig. 2 indicates that for massive clusters (n > 40) the total amount of energy released in one impact would be of the order of 30% of the incoming energy and one can further speculate on the observation of a saturation of the observed yield for heavier clusters or an eventual decrease of the kinetic energy or a modification of the mass distribution of the emitted species. As most of the sputtered material is emitted as neutrals difficult to detect, one has either to post-ionize these neutrals or to restrict the kinematical measurement to ionized species, assuming they are representative of the whole emission. For this purpose, in a first experiment realized with the 256 pixels detector briefly described later (and which allows to measure double differential cross-sections over angle and energy), the energy and angular distributions for ions emitted from a gold surface under Auþ 4 impact for three bombarding energies (800 keV, 4 MeV and 20 MeV) have been measured [58]. Preliminary analysis of the data concerning Au , Au 2 and Au3 ions indicate that, for the two lowest incident energies, the energy distributions of emitted gold ions are similar. Because the geometrical acceptance of our detector did not allow to detect ions with a radial energy greater than 16 eV, we have only analyzed the angular distributions of ions emitted with a total energy smaller than 16 eV: for the three bombarding energies, the emission of Au , Au 2 and Au 3 ions is perpendicular to the target surface and the angular distributions follow a cosine-type distribution. Further experiments are in progress with other types of targets such as CsI and lead targets. As shown in the previous section in the case of the
phenylalanine target, the emission yield for ðCsIÞn I secondary cluster ions is maximum when the Aun incident energy per atom is close to 50 keV/atom. Total sputtering yield measurements from CsI layers are in progress to check with the same target if for neutrals, the yield maxima occur at an energy per atom larger than the emission yield of cluster ions. This general behavior is of great interest for analytical applications since high ion emission yields occur at a low bombarding energy where damages in the solid are not yet important.
4. Carbon cluster projectiles through solids Secondary emission of atoms and ions induced by Cn clusters in the electronic energy loss regime may also result from energy transferred to the surface which ‘‘evaporates’’ atoms or molecules (as in the spike model) but it is probable that emission takes place from a certain depth below the surface. This is the case for example with C60 projectiles at 20 or 30 MeV which produce enormous craters in diameter and more than 150 A in depth) (500 A in organic material [30,31]. The range in solids of Au atoms from Aun clusters (at energies of a few 100 keV/atom) is only a few hundred Angstroms. With Cn clusters and C60 beams produced by the 15 MV Orsay–Tandem, we are in the electronic energy loss regime and the cluster constituents can up to travel long distances in solids (several 103 A 4 several 10 A in carbon foil). Energy loss measurements with carbon clusters were extensively discussed in several papers [59,60]. We report here on different interesting phenomena related to the charge states of carbon constituents in solids and exiting solids, to the cluster constituent spatial correlation and lateral velocity distribution. 4.1. Secondary emission of hydrogen ions and diminution of carbon constituent charge states in solids An interesting aspect of secondary ion emission with carbon clusters is the emission of hydrogen ions. It has been demonstrated that the emission
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
rate of hydrogen ions Hþ from a solid surface bombarded by multiply charged ions is strongly dependent on the charge state of the projectile in a large range of incident energies [61,62]. A power law Y q2:7 between the incident ion charge state q and the yield Y has been established for keV and MeV projectiles regardless of their masses and the solid target material (organic compound or hydrogen containing molecules adsorbed on metallic or carbon substrates). The average initial energies of Hþ are significantly higher than any other types of emitted secondaries (around 10 eV by comparison to a few eV for all other ions) and it was shown [61] that emission originates from the surface. A carbon cluster projectile is singly charged before hitting a target. It is therefore almost an assembly of neutral atoms that penetrates the solid and this is a unique feature of fast cluster projectiles. Upon impact each atom becomes ionized within a short distance as for example a single or carbon charge state is equilibrated within 10 A so at 1 MeV [63]. The ionization state is smaller for carbon cluster constituents (see later) but the total point charge in the first target layers is large compared to the approaching cluster charge before þ impact. With a C1þ 60 at 20 MeV the H emission is for example about 3000 times larger than the rate observed with a single carbon at the same velocity. This demonstrates that, in this velocity range, this is not the charge state approaching a surface that stimulates the hydrogen emission (and electron emission) by potential effects as predicted by the pre-impact model [64]. The transient local high density of charge at the surface followed by Coulomb explosion, hydrogen bonds breaking and fast Coulomb repulsion of Hþ ions seems to be a more appropriate picture. The particle-induced hydrogen ion emission is an important surface phenomena which is not yet completely understood in ion–solid collisions but which can be used to determine the charge state of fast ions at the entrance and exit sides of thin foils [61,65]. In the present case the method was applied to measure carbon charge states of clusters (C5 and C10 ) passing through thin carbon foils of more than 20 nm [66]. For example when a C5 cluster travels in a carbon foil the carbon atoms reach a certain ionization state. The carbon ions exiting
233
the foil with an average charge hqn i were further used to hit a second foil where they induced (as 5 independent projectiles) Hþ emission. The Hþ 2:7 emission yield, proportional to 5 hqn i , was then compared to the same measurements with single carbon atom having an average exiting charge state hq1 i at the same velocity. The ratio of these two charge state values can then be obtained from the corresponding Hþ yields ratio. A series of experiments was performed with different foil thicknesses. The main results are a lowering of the mean charge state per atom of the Cn constituents inside the solid by comparison to single atoms. For C5 at 2 MeV/atom passing in a 30 nm Carbon foil the diminution of charge is about 15%. It is only after a distance of 300 nm that the carbon atoms in the solid behave as independent atoms with respect to Hþ emission, i.e. that the Hþ emission with Cn is about equal n times that of C1 . 4.2. Reduced charge state of MeV carbon cluster atomic constituents and simulation of cluster correlations in matter A more direct method was used in [67] for the determination of average charge states of carbon atoms from Cn (n ¼ 3 to 10 and C60 ) at velocities corresponding to 0.5 MeV to 4 MeV/atom. Carbon foils of different thicknesses were used and the comparison with C1 atoms was made under the same experimental conditions. We briefly recall this method, which has been recently applied to the case of C8 and C60 cluster beams at 600 keV/ atom [68]. At the exit side of a carbon foil the carbon ions are electrostatically deflected by a set of two parallel and horizontal metallic plates placed at a distance of 5 mm from the carbon foil. Voltages of 3 to 15 kV can be applied onto the plates. A multi-pixel position sensitive detector consisting of two micro channel plates followed by 16 16 discrete anodes connected to a board of discriminators (256) and 32 independent Time Digitized Converters (8 stops channel each) was entirely built at the institute [69]. A careful calibration of the electrostatic deviation using different voltages was achieved with a 0:3 mm 0:3 mm collimated Cþ 1 beam.
234
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
The voltage value was set to ensure a complete detection of the charge distribution in the detector. Average charge state measurements and lateral width of spatial distributions of the exiting constituents were obtained with a good accuracy. Table 1 in Ref. [67] gives results for C1 , C3 , C5 and C8 projectiles at energy per atom between 1 and 4 MeV/atom. Fig. 3 illustrates the variation of hqn i=hq1 i as a function of the number n of atoms in the cluster for an incident energy of 2 MeV/atom, hq1 i is the average charge of a single carbon atom and hqn i the average charge of a cluster constituent. In this energy range hqn i is always smaller than hq1 i at the same velocity whatever the foil thickness although the ratio is close to one for the thickest foil (40 lg/cm2 ). The charge reduction in a 2.2 lg/cm2 carbon foil is as large as 30% for a 20 MeV C10 cluster and this reduction progressively vanishes with increasing target thickness. The charge reduction is much smaller at lower energy per atom. For example preliminary results obtained recently with C8 [68] seem to indicate that there is practically no reduction at 0.6 MeV/atom in a foil of 2.4 lg/cm2 . A C60 beam at the same velocity was also investigated. In the experiment with the C60 beam, the number of pixels that are hit simultaneously is important and the distribution is wide. Contrarily with what is observed for 20 MeV C10 , the charge reduction for C atoms
Fig. 3. Evolution with the number n of cluster constituents of the average charge state ratio hqn i=hq1 i (see text) measured at the exit of carbon foils of different thickness (2.2 and 40 lg/cm2 ) for 2 MeV/atom incident cluster energy. (Results from [67].)
from C60 would be only a few percent. A larger effect could have been expected for this large number of constituents. In thick foils the charge suppression observed with Cn vanishes as a result of the constituents becoming farther and farther apart and independent. A memory of the interactions between the co-moving atoms in the first part of the foil may however remain. Systematic measurements of the lateral spread of carbon ions exiting carbon foils were obtained with the multi-pixel detector. A simulation taking into account both the intracluster forces and the collisions between cluster constituents and target atoms was developed to reproduce the spatial distribution [70]. As the clusters penetrate the targets the ions suffer binary collisions with target atoms. The mutual interactions between cluster ions are described with a Molliere type potential taking into account the hard core repulsion and with a shielded Coulomb potential Vc ðrÞ ¼ ½qðvÞe 2 =r er=a (where a is the screening distance) to follow the Coulomb explosion contribution. Step by step calculations were performed. As a cluster travels forward by a lattice constant of length there is only a very small perturbation of the ion velocities and position due to intra-cluster forces. However over many lattice constants the Coulomb explosion has a noticeable effect. The algorithm used in the simulation provides intra-cluster correlations between ions at the exit side of thin and thick carbon foil targets. Bare Coulomb and shielded Coulomb with various screening length were applied in the calculation that also takes into account the shape of the cluster (linear or circular). In an annular structure the ions repelled from the center of the ring and the Coulomb energy is more effective than in a linear chain structure. The memory of the initial shape is lost much faster with non-linear clusters. Upon exit of the target, knowing the position and velocities of the constituents, unshielded Coulomb forces are applied to calculate the final distribution in the detector plane. For thin targets (2 lg/cm2 ) the lateral velocity is mainly due to Coulomb explosion behind the target while for thicker targets there is much more multiple scattering and the Coulomb effect behind the target becomes weak. A comparison with experimental spot sizes produced by
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
different incident clusters has been made. It is shown in the various simulations presented in Fig. 4 that for a rather thick target (40 lg/cm2 ), the Coulomb forces induce a significant increase of the distribution width on cluster size, which contrasts with the experimental observation of measured widths that do not depend on n. This comparison evidences very strong shielding. The screening length value is a sensitive parameter to fit the width of the calculated lateral velocity distribution to the experimental ones. This screening length of the Coulomb potential for 2 MeV carbon ions in a carbon target (40 lg/cm2 ) is found to be less than . 2.5 A The calculated constituent trajectories have been used to calculate the charge state suppression within a simple model of enhanced electron capture at the exit side of the target due to the presence of charged neighbors which contribute to bound more strongly the electrons to the cluster ions. The apparent additional potential DI for an
Fig. 4. Evolution with the cluster size of spatial distribution widths given by the trajectory simulation (see text) for 2 MeV/ atom Cn constituents exiting a 40 lg/cm2 carbon foil. Calculations are displayed for various values of the screening length as indicated in the legend. The widths of the lateral distributions predicted by simulations which include an in-target Coulomb explosion show a clear dependence upon cluster size which is not observed experimentally. (Results from [70].)
235
electron to escape reduces the velocity-dependent equilibrium charge q1 ðvÞ and the ion charge in the cluster becomes qi ¼ q1 ðvÞ BDI. DI can be approximated by taking a mean field with qi hqn i for all constituents of a Cn cluster, so that the ionization potential is proportional to (hqn iv), v being a geometrical contribution factor which depends on the respective positions of the exiting cluster ions. Then hqn i=q1 ðvÞ ¼ 1=ð1 þ BvÞ and the comparison with experimental data shown in [67] suggests that this approach of an additional potential is not unrealistic. Another less simple model [71] based on electron density variation in the vicinage potential of proximity atoms leads to an average number of bound electrons minimizing the total energy of bound electrons. The experimental variation with the cluster size of the ratio hqn i=q1 is also well fitted for thin targets. Vicinage effects in the charge state of swift Cn cluster constituents at 2 MeV/atom traversing thin foils were also recently studied in [72]. The potential energy of the electrons surrounding one constituent ion, moving in correlation with the other ions, comes from the interaction with their own nucleus plus the interaction potential with the (n 1) remaining ions which reduces this potential energy as compared to the case of an isolated ion. Following the time evolution of the inter-constituent distance given by the Coulomb explosion, the average charge ratio inside the foil and at the foil exit can be calculated. It is shown in [72] that the variation of the screening effect at the exit surface of thin foils is particularly important to reproduce the experimental charge ratio. A good agreement with our experimental values is obtained and information on the Cn molecular structure could be obtained. A phenomenological approach was proposed by Parilis. The basic idea is that the collision crosssections are larger for the front-runner atoms in the cluster than for the atoms behind [73].
5. Separation distance of fast carbon constituents in silicon material and reduced pulse height signals delivered by surface barrier silicon detectors The radial separation of the co-moving carbon atoms in the solid reduces the observed ‘‘collective
236
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
effects’’. Depending on the characteristic length scale of the involved processes, these collective effects may disappear at different levels of separation distance between constituents. Experiments on ion energy measurements were carried out with surface barrier detectors hit by carbon clusters and carbon atoms at the same velocity obtained by the fragmentation of carbon clusters Cn having passed through very thin foils. An important energy deficit (with respect to nC1 ) is measured and can reach 50% in the case of C60 at 20 MeV [74]. Systematic measurements of this energy deficit have been performed recently in Erlangen and Orsay [75] with Cn clusters (n 6 10) accelerated at 1 and 2 MeV/atom. The total pulse height or net recorded energy is: Eph ¼ Ei Dw DN Dr, where Ei is the incident energy, Dw is the total energy loss in the entrance gold window (and a possible dead layer underneath) of the detector, DN is the total nuclear energy loss and Dr the sum of all other phenomena which may modify the number of electron–hole pair production and collection. DN and Dw can be estimated by assuming that, within a few percent, the energy loss by a 1–2 MeV/atom Cn cluster is n-fold the energy loss by C1 at the same velocity. Therefore the energy difference between a Cn projectile and nC1 is due to the difference in the Dr of Cn and nC1 . The observed important deficit observed in direct impacts (without foil in front of the detector) is induced by the proximity of the co-moving atoms in the detector. All the energy is not converted or some processes dissipate or trap energy. At a given energy per atom the energy deficit DrðCn Þ nDrðC1 Þ increases with the number n of constituents as nðn 1Þ. Also for a given cluster size, the energy deficit increases linearly with the total deposited energy and reaches 0.8 MeV for C10 at 10 MeV. An experiment has been performed to measure the detector response as a function of the estimated average separation distance of the cluster atoms in the detector. For this purpose a rectangular surface barrier detector with a large active area was used. Additional gold layers were evaporated onto the entrance window in a stair-like structure consisting of 8 steps with increasing suc-
cessive thickness of 40 nm each. This detector was prepared by Voit in Erlangen. A C6 was precisely directed on these steps and the pulse height deficit (PHD) was measured for each step thickness. The cluster energy was 5.8 MeV when the cluster hit the thinnest step and was successively increased from step to step so that the energy after the passage of each step stays the same. The measured energy deficit DrðCn Þ nDrðC1 Þ remains constant as shown in Fig. 5. By multiple scattering in the gold layers the carbon atoms gradually separate and the separation is large (several tens of nm) for the largest thickness of 300 nm. Therefore the present results show that even after such a thickness of gold the cluster constituents at the entrance of the active Si area do not behave as n single atoms as they do when going through the thin foil upstream the detector. It is thus probable that trajectories must be hundreds of nm apart before the PHD vanishes. A model taking into account recombination of e–hole in a plasma column created by the passage of ions in Si was developed by Finch et al. [76]. As shown in [75], the results obtained with Cn projectiles agree qualitatively with the Finch model provided that the plasma column radius extends to hundreds of nanometers. The plasma columns associated with each carbon would then overlap in
Fig. 5. Pulse-height defect difference between intact cluster and independent constituents DrðCn Þ nDrðC1 Þ plotted as a function of the thickness dAu of the gold entrance window, measured for a C6 clusters with constant energy at the entrance of the active silicon area. (Results from [75].)
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
the case of Cn impacts and this would increase the rate of recombination.
6. Conclusion A number of unknown effects have been discovered with cluster projectiles at large velocity. The understanding of phenomena involved in the collision of clusters with solids is certainly in progress but several questions remain open. An assembly of fast co-moving atoms, which can become highly ionized in the first layers of solid targets, do not behave as individual atoms and surface induced as well as bulk-induced phenomena are not yet much predictable at large velocity. The ionization state of carbon constituents of Cn clusters is reduced by comparison with single atom and the charge reduction increases with n at a given energy per atom between 1 and 4 MeV/atom. Preliminary results show that this effect is not large with molecular projectiles as C60 at 0.6 MeV/atom. Unfortunately it is actually not possible to reach higher energy per atom for C60 projectiles. The lateral velocity distributions of C atoms have been reproduced in simulations using a simple algorithm that describes the microscopic correlations of fast ionic cluster constituent in carbon foils. It is shown that the strong pulse height reduction observed in the response of Silicon surface barrier detectors to MeV/atom Cn clusters is still related to the proximity of the traveling atoms in the solids all along their trajectory. Unless they are hundreds of nm apart, the effect is still observed. Secondary emission with cluster is another very interesting aspect of the cluster–solid interactions. The emission rate is very much enhanced with cluster projectiles but the emission yield is not directly related to the amount of energy deposited in the material. Larger dE=dx values due to larger velocities do not provide larger release of matter and there is a need for valuable predictions. At lower energy of impact, Molecular Dynamic (MD) simulations are already very useful to understand experimental results obtained with cluster projectiles. Very large size clusters of various
237
types, containing thousands of atoms at a few tens of keV are for example used in material surface processing applications that are guided by MD simulations. From the results discussed in this short review, the energy range of 50–300 keV/atom for Aun clusters appears to be very interesting since the secondary ion and neutral emission yields are surprisingly high in this domain of velocity. It would obviously be also very useful to perform MD simulations for the Aun impacts on metallic or organic targets in the overall energy range studied in this work. Additional data on energy and angular distributions will be soon available for these systems, which will bring more constraints for comparison with theoretical predictions. A continuation of these experiments will be pursued with larger size gold clusters. Applications with organic and bioorganic materials may already result from the present knowledge of secondary emissions induced in this intermediate velocity range.
Acknowledgements This paper reports on experimental results obtained at IPN-Orsay, IPN-Lyon and ErlangenUniversity in the last years, within long-standing collaborations. We are strongly indebted to our colleagues S. Bouneau, A. Brunelle, S. Della Negra, J. Depauw and M. Pautrat from Orsay, M. Fallavier and J.C. Poizat from IPN-Lyon, H.H. Andersen from NBI-Copenhagen, T. Tombrello and J. Hartman from Caltech-Pasadena and H. Voit and M. Seidl from Erlangen University. Fruitful discussions with E. Parilis are gratefully acknowledged. We also address special thanks to A. Schulz for his enthusiastic reading and comments.
References [1] A. Benninghoven, P. Bertrand, H. Migeon, H.W. Werner (Eds.), Proceedings of the 12th SIMS XII Conference, Elsevier, Amsterdam, 2000. [2] J.C. Poizat, J. Remillieux, Phys. Lett. 34A (1971) 53. [3] W. Brandt, A. Ratkowsky, R.H. Ritchie, Phys. Rev. Lett. 33 (1974) 1325.
238
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239
[4] J.W. Tape, W.M. Gibson, J. Remillieux, R. Laubert, H.E. Wegner, Nucl. Instr. and Meth. 132 (1976) 75. [5] J.C. Eckardt, G. Lantschner, N.R. Arista, R.A. Baragiola, J. Phys. C 11 (1978) L851. [6] A.R. Nyaiesh, W. Steckelmacher, M.W. Lucas, J. Phys. C 11 (1978) 2917. [7] R. Laubert, IEEE Trans. Nucl. Sci. NS-26 (1979) 1020. [8] R. Levi-Setti, K. Lam, T.R. Fox, Nucl. Instr. and Meth. 194 (1982) 281. [9] D. Maor, P.J. Cooney, A. Faibis, E.P. Kanter, W. Koenig, B.J. Zabransky, Phys. Rev. A 32 (1985) 105. [10] A. Faibis, G. Goldring, M. Hass, R. Kaim, I. Plesser, Z. Vager, Nucl. Instr. and Meth. 194 (1982) 299. [11] A. Breskin, A. Faibis, G. Goldring, M. Hass, R. Kaim, Z. Vager, N. Zwang, Phys. Rev. Lett. 42 (1979) 369. [12] I. Plesser, E.P. Kanter, Z. Vager, Phys. Rev. A 29 (1984) 1103. [13] M.J. Gaillard, J.C. Poizat, A. Ratkowski, J. Remillieux, Nucl. Instr. and Meth. 132 (1976) 369. [14] B. Mazuy, A. Belkacem, M. Chevallier, M.J. Gaillard, J.C. Poizat, J. Remillieux, Nucl. Instr. and Meth. B 28 (1987) 497; Nucl. Instr. and Meth. B 33 (1988) 105. [15] M.F. Steuer, R.H. Ritchie, Nucl. Instr. and Meth. B 40/41 (1989) 372. [16] H.H. Andersen, H.L. Bay, J. Appl. Phys. 45 (1974) 953. [17] S.S. Johar, D.A. Thomson, Surf. Sci. 90 (1979) 319. [18] D.A. Thompson, S.S. Johar, Appl. Phys. Lett. 34 (1979) 342. [19] H.H. Andersen, in: Sput 92, The Royal Danish Academy of Sciences and Letters. Matematisk-Fysiske Meddelelser 43 (1993) 127. [20] K. Wittmaack, Surf. Sci. 90 (1979) 557. [21] J.P. Thomas, P.E. Filpus-Luyckx, M. Fallavier, E.A. Schweikert, Phys. Rev. Lett. 55 (1985) 103. [22] A.D. Appelhans, J.E. Delmore, Anal. Chem. 61 (1989) 1087. [23] P. Sigmund (Ed.), Fundamental Processes in Sputtering of Atoms and Molecules. Mat. Fys. Medd. Dan. Vid. (1993) 43. [24] Y. Le Beyec, Int. J. Mass Spectrom. Ion Processes 174 (1998) 101. [25] Proceedings of Conference on MeV, keV Ions and Cluster Interactions with Surfaces and Materials, J. Phys. C 2 (1989); Proceedings of Conferene on Polyatomic ion impact on solids and related phenomena, Nucl. Instr. and Meth. B 88 (1–2) (1994). [26] Proceedings of Conferences: Correlated Effects in Atomic and Cluster Ion Bombardment and Implantation, EMRS Conference, Proceedings in Nucl. Instr. and Meth. B 112 (1996); Nanometric Phenomena Induced by Laser, Ion and Cluster Beams, EMRS Conference, Nucl. Instr. and Meth. B 123 (1997). [27] K.L. Merkle, W. J€ager, Philos. Mag. 44 (1981) 741. [28] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135.
[29] A. Dunlop, G. Jaskierowicz, J. Jensen, S. Della-Negra, Nucl. Instr. and Meth. B 132 (1997) 93. [30] D.D.N. Barlo Daya, A. Hallen, J. Eriksson, J. Kopniczky, R. Papaleo, C.T. Reimann, P. Hakansson, B.U.R. Sundqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instr. and Meth. B 106 (1995) 38. [31] C.H. Tomaschko, M. Schurr, R. Berger, G. SaemannIschenko, H. Voit, A. Brunelle, S. Della-Negra, Y. Le Beyec, Rapid Commun. Mass Spectrom. 9 (1995) 924. [32] R.J. Beuhler, L. Friedman, J. Appl. Phys. 48 (1977) 3928. [33] M. Fallavier, J. Kemmler, R. Kirsch, J.C. Poizat, J. Remillieux, J.P. Thomas, Phys. Rev. Lett. 65 (1990) 621. [34] M. Salhepour, M. Fistel, J. Hunt, Int. J. Mass Spectrom. Ion Processes 84 (1988) R7. [35] K. Narumi, K. Nakajima, K. Kimura, M. Mannami, Y. Saitoh, S. Yamamoto, Y. Aoki, H. Naramoto, Nucl. Instr. and Meth. B 135 (1998) 77. [36] D. Ben Hamu, A. Baer, H. Feldman, J. Levin, O. Heber, Z. Amitay, Z. Vager, D. Zajfman, Phys. Rev. A 56 (1997) 4786. [37] D. Shen, X. Lu, Z. Xia, X. Wang, Q. Zhao, Nucl. Instr. and Meth. B 129 (1997) 392. [38] M. D€ obeli, R.M. Ender, U.S. Fisher, M. Suter, H.A. Synal, P. Vetterli, Nucl. Instr. and Meth. B 94 (1994) 388. [39] C.H. Tomaschko, D. Brandl, R. K€ ugler, M. Schurr, H. Voit, Nucl. Instr. and Meth. B 103 (1995) 407. [40] P. Hvelplund, J.U. Andersen, K. Hansen, AIP Conf. Proc. 457 (1999) 220. [41] S. Della-Negra, A. Brunelle, Y. Le Beyec, J.M. Curaudeau, J.P. Mouffron, B. Waast, P. Hakansson, B.U.R. Sundquist, E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [42] M. Fallavier, Y. Champelovier, M. Ferrari, R. Kirsch, J.C. Poizat, J. Remillieux, J.P. Thomas, B. Canut, S.M.M. Ramos, P. Thevenard, S. Della-Negra, J.P. Mouffron, P. Nicol, Eur. Phys. J. D 9 (1999) 529. [43] P. Hakansson, S. Della-Negra, J.P. Mouffron, B. Waast, P.A. Sullivan, Nucl. Instr. and Meth. B 112 (1996) 39. [44] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 63 (2001) 022902. [45] TRIM code J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon Press, New York, 1985. [46] H.H. Andersen, A. Johansen, V.S. Touboltsev, Nucl. Instr. and Meth. B 164–165 (2000) 727. [47] M. Karas, F. Hillenkamp, Anal. Chem. 60 (1988) 2299. [48] P. Demirev, Mass Spectrom. Rev. 14 (1995) 279. [49] K. Wien, Radiat. Eff. Defects Solids 109 (1989) 137. [50] C. Reimann, in: P. Sigmund (Ed.), Fundamental Processes in Sputtering of Atoms and Molecules. Mat. Fys. Medd. Dan. Vid. (1993) 351. [51] H.H. Andersen, A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, J. Chaumont, H. Bernas, Phys. Rev. Lett. 80 (1998) 5433. [52] H.H. Andersen, H. Bay, Radiat. Eff. 13 (1972) 67. [53] S. Bouneau, A. Brunelle, S. Della-Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, H.H. Andersen, Phys. Rev. B 65 (2002) 144106.
D. Jacquet, Y. Le Beyec / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 227–239 [54] P. Sigmund, C. Claussen, J. Appl. Phys. 52 (1981) 5433. [55] H.M. Urbassek, J. Michel, Nucl. Instr. and Meth. B 22 (1987) 480. [56] T.-J. Colla, R. Aderjan, R. Kissel, H. Urbasseck, Phys. Rev. B 62 (2000) 8487. [57] M.M. Jakas, E.M. Bringa, Phys. Rev. B 62 (2000) 824. [58] S. Bouneau, A. Brunelle, S. Della-Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, to be published. [59] N. Arista, Nucl. Instr. and Meth. B 164–165 (2000) 108. [60] J. Jensen, P. Sigmund, Phys. Rev. A 61 (2000) 032903. [61] S. Della-Negra, Y. Le Beyec, B. Monart, K. Standing, K. Wien, Phys. Rev. Lett. 58 (1987) 17. [62] M. Benguerba, S. Della-Negra, H. Joret, Y. Le Beyec, Int. J. Mass Spectrom. Ion Processes 107 (1991) R11. [63] G. Maynard, private communication. [64] E.F. da Silveira, M. Matos, Nucl. Instr. and Meth. B 99 (1995) 107. [65] S. Della-Negra, J. Depauw, H. Joret, Y. Le Beyec, I. Bitensky, G. Ben Assajag, R. Galera, K. Wien, in: K. Standing, W. Ens (Eds.), Nato Conference on Mechanisms for Producing Ions from Large Molecules, Minaki Proceedings, Plenum Press, New York, 1991, p. 19. [66] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Ch. Schoppmann, Nucl. Instr. and Meth B 125 (1997) 207.
239
[67] A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 59 (1999) 4456. [68] S. Bouneau, A. Brunelle, S. Della-Negra, D. Jacquet, Y. Le Beyec, M. Pautrat, to be published. [69] S. Della-Negra, D. Jacquet, P. Cohen, J. Depauw, R. Donnet, S. Du, J. Le Bris, R. Sellem, IPN Technical Report no. 72 1994–1995. [70] J.W. Hartman, T.A. Tombrello, S. Bouneau, S. DellaNegra, D. Jacquet, Y. Le Beyec, M. Pautrat, Phys. Rev. A 62 (2000) 043202. [71] Z.L. Miscovic, S.G. Davison, F.O. Goodman, Phys. Rev. A 61 (2000) 062901. [72] S. Heredia-Avalos, R. Garcia-Molina, N.R. Arista, Europhys. Lett. 54 (6) (2001) 729. [73] E. Parilis, Nucl. Instr. and Meth. B 193 (2002) 240. [74] K. Baudin, A. Brunelle, M. Chabot, S. Della Negra, J. Depauw, D. Gardes, P. Hakansson, Y. Le Beyec, A. Billebaud, M. Fallavier, J. Remillieux, J.C. Poizat, J.P. Thomas, Nucl. Instr. and Meth. B 94 (1994) 341. [75] M. Seidl, H. Voit, S. Bouneau, A. Brunelle, S. Della Negra, J. Depauw, D. Jacquet, Y. Le Beyec, M. Pautrat, Nucl. Instr. and Meth. B 183 (2001) 502. [76] E.C. Finch, M. Asghar, M. Forte, Nucl. Instr. and Meth. 163 (1979) 467.