Size-selected cluster beam source based on radio ...

Size-selected cluster beam source based on radio frequency magnetron plasma sputtering and gas condensation S. Pratontep, S. J. Carroll, C. Xirouchaki, M. Streun, and R. E. Palmer Citation: Review of Scientific Instruments 76, 045103 (2005); doi: 10.1063/1.1869332 View online: http://dx.doi.org/10.1063/1.1869332 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/76/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A self-sputtering ion source: A new approach to quiescent metal ion beamsa) Rev. Sci. Instrum. 81, 02B306 (2010); 10.1063/1.3272797 Miniature quadrupole residual gas analyzer for process monitoring at milliTorr pressures J. Vac. Sci. Technol. A 16, 1157 (1998); 10.1116/1.581251 Compact sputter source for deposition of small size-selected clusters Rev. Sci. Instrum. 68, 3335 (1997); 10.1063/1.1148293 Gas condensation source for production and deposition of size-selected metal clusters Rev. Sci. Instrum. 68, 3327 (1997); 10.1063/1.1148292 The effect of using silicon based diffusion pump fluid on spectral quality in an electrospray ionization ion trap/time-of-flight mass spectrometer Rev. Sci. Instrum. 68, 3252 (1997); 10.1063/1.1148276

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REVIEW OF SCIENTIFIC INSTRUMENTS 76, 045103 共2005兲

Size-selected cluster beam source based on radio frequency magnetron plasma sputtering and gas condensation S. Pratontep,a兲 S. J. Carroll, C. Xirouchaki,b兲 M. Streun,c兲 and R. E. Palmer Nanoscale Physics Research Laboratory, School of Physics and Astronomy, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

共Received 12 December 2004; accepted 12 January 2005; published online 16 March 2005兲 We report on a source for producing size-selected nanoclusters based on the combination of radio frequency magnetron plasma sputtering and gas condensation. The use of plasma sputtering to vaporize a target is applicable to a large range of materials; Ag, Au, Cu, and Si have been attempted to date. The source, combined with a time-of-flight mass filter, can produce clusters in the size range from 2 up to at least 70 000 atoms, depending on the target material, with a constant mass 共M兲 resolution 共M / ⌬M ⬃ 25兲 at an intensity that produces atomic monolayer coverage in as little as a few minutes. The source is also attached to an ultrahigh vacuum analysis chamber, which allows in situ surface chemical and structural analysis. Examples of cluster deposition experiments with the source are also presented. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1869332兴

I. INTRODUCTION

Nanometer-sized particles exhibit properties that are not found in corresponding macroscopic systems. Much effort has recently been devoted to investigating their sizedependent properties, which offer “tunability” for creating materials; for example, in changes of the emission wavelengths of photoluminescent nanoparticles1 or the catalytic activity of metal clusters as a function of size;2 hence the desire to produce nanoclusters with well-defined size. The production of clusters by ion beam methods offers several advantages over other techniques, such as chemical preparation techniques3 and atomic vapor growth.4 Standard size selection techniques can be applied to cluster ion beams to reach an extremely high size resolution. The clusters produced are not covered by passivating materials, which allows direct study of their chemical activity. The deposition energy, which may influence the morphology of the clusters on a surface, can be controlled. The cluster beam may also be employed for surface patterning if the focused ion beam technique5 is incorporated. These features are not easily available with other techniques. Cluster ion beam methods include pulsed beam techniques, e.g., laser ablation6,7 and pulsed arc plasma,8 and continuous beam techniques, e.g., gas condensation9,10 and ion sputtering.11 The laser ablation technique has become the most popular, owing to its applicability to a wide range of materials, but the noncontinuous nature of the beam can prove undesirable in many applications. In the production of continuous beams, the gas condensation technique, which is often understood to involve evaporation of materials by heating and subsequent cluster a兲

Present address: National Nanotechnology Center, 111 Thailand Science Park, Paholyothin Rd., Klong Luang, Pathumtani 12120, Thailand. b兲 Author to whom correspondence should be addressed; electronic mail: [email protected] c兲 Present address: Zentralinstitut fuer Elektnonik, Forschungszentrum Juelich, 52425 Juelich, Germany. 0034-6748/2005/76共4兲/045103/9/$22.50

condensation by the introduction of an inert cooling gas, is the most commonly employed. However, the heating process limits its use to only materials with a sufficiently low melting point. Plasma sputtering, which is nowadays well established as a standard technique for thin film deposition, offers a method for vaporizing materials without involving the complications of the target heating process. The combination of plasma sputtering with gas condensation was reported in 1986.12 The technique has been improved and developed through the use of magnetron sputtering.13,14 Although gas condensation cluster sources have been growing in popularity, the performance and reproducibility of such a cluster source depends on many interrelated factors. To date a detailed account of this type of cluster source has not been reported. In the following sections, we report a detailed account of the design and performance of a size-selected cluster ion beam source based on the combined techniques of radio frequency 共rf兲 magnetron plasma sputtering and gas condensation. The factors that influence the cluster production process will be discussed. This cluster source has been employed in a number of studies on high energy deposition of size-selected clusters on surfaces,15–21 of which some examples will be given. II. THE CLUSTER SOURCE A. Overview

A schematic of the cluster source is shown in Fig. 1. The source consists of three differentially pumped sections: cluster production 共I兲, cluster beam formation 共II兲, and mass selection 共III兲. The cluster production takes place in the inner, liquid nitrogen cooled, chamber 关labeled 共b兲 in Fig. 1兴 in section I. A commercially available 2⬙ TORUS magnetron gun 共Kurt J. Lesker兲 has been employed for the sputtering process. The magnetron gun is mounted on a long axial

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FIG. 1. Schematic of the cluster source showing: 共a兲 magnetron axial mount; 共b兲 liquid nitrogen 共lN2兲 chamber; 共c兲 dark space shield; 共d兲 sputter target; 共e兲 adjustable nozzle; 共f兲 first skimmer; 共g兲 second skimmer; 共h兲 extraction lens; 共i兲 first einzel lens; 共j兲 deflection plates; 共k兲 second einzel lens; 共l兲 TOF mass filter acceleration region; 共m兲 TOF field-free region; 共n兲 TOF deceleration region; and 共o兲 third einzel lens to focus the cluster beam for deposition into a high vacuum chamber 共not shown兲.

mount 共a兲, enabling the distance between the front of the gun and the end of the chamber 共i.e., the cluster aggregation distance兲 to be varied by up to 300 mm. The sputter gas 共Ar兲 is injected from small orifices 共⬃0.1 mm in diameter兲 at the front of the magnetron gun. The Ar plasma is ignited by applying a rf 共13.56 MHz兲 high voltage 共Advanced Energy magnetron drive兲 to the sputter target 共d兲, e.g., 99.999% Ag 共Pi-Kem兲. The sputtering process creates a dense vapor of atomic ions and small clusters in front of the target. Cluster condensation from this vapor is induced by helium 共He兲 gas, injected from the back of the chamber. The Ar and He flow rates are regulated by mass flow controllers 共MKS Instruments兲. The gas lines can be pumped out via bypass valves for routine cleaning. The cluster and gas mixture leaves the inner chamber of section I via an adjustable nozzle 共e兲, with aperture diameter range of 0.5– 12.5 mm, allowing independent control of the gas flow rate and gas pressure. Two conical skimmers 关共f兲, 共g兲兴 共4 mm in diameter兲 are placed after the nozzle. Clusters and gases undergo supersonic expansion through the nozzle before entering section II via the first skimmer. The adjustable mounting of this skimmer allows its distance from the nozzle to be varied, ensuring extraction of the central portion of the supersonically expanding gas jet.10 Since a significant proportion of the clusters are ionized in the plasma, positive ions can be accelerated and focused into an ionized cluster beam by negative potentials applied to a set of ion optics, i.e., an extraction lens 共h兲, a pair of einzel lenses 关共i兲, 共k兲兴, and a set of deflection plates 共j兲, in the inner chamber in section II. The clusters enter this chamber via the second skimmer 共g兲. Small negative voltages are also applied to both the nozzle and skimmers. These three apertures practically serve as the first elements for forming the beam and enable the beam current to be increased by about 1 order of magnitude. The focused ion beam is then mass selected by a lateral time-of-flight 共TOF兲 mass selector22 in section III. An einzel lens 共o兲 placed after the TOF focuses the size-selected cluster beam into a high vacuum 共HV兲 chamber 共not shown in Fig. 1兲 with a base pressure of 10−8 mbar for cluster deposition. The cluster beam is detected by a simple Faraday cup setup, i.e., a circular aperture in front of a metal plate. A picoammeter 共Keithley 602 Electrometer兲 is used to measure the cluster current. The Faraday cap and the sample holder, which can accommodate up to four samples of 1.0 cm

⫻ 0.5 cm, are mounted onto a vertical linear drive. The impact energy of the clusters is determined by the negative bias voltage applied to the sample during deposition. During cluster production, a typical gas pressure in the condensation chamber is ⬃1 mbar, with a gas flow rate of a few hundreds sccm 共standard cubic centimeter兲. In order to achieve a HV 共10−7 mbar兲 level in the deposition chamber, a high throughput pump is installed in each section of the cluster source: 共I兲 1500 l / s Pfeiffer TMU-1600 turbomolecular pump; 共II兲 2400 l / s Varian VHS-6 diffusion pump; and 共III兲 700 l / s Edwards Diffstak diffusion pump. The base pressure of the source is ⬃10−8 mbar, while typical pressures during cluster production in the three sections are 10−3, 10−4, and 10−6 mbar, respectively. The HV deposition chamber, which is pumped by a 200 l / s Pfeiffer TMU-260 turbomolecular pump, is separated from the rest of the source by a gate valve, allowing a rapid sample exchange without the need to vent the source. The size-selected cluster beam can also be transported through the HV deposition chamber into an ultrahigh vacuum 共UHV兲 analysis system 共10−10 mbar兲, via an intermediate pumping stage equipped with a set of ion optics. The UHV chamber contains an extensive range of surface analysis techniques, i.e., low energy electron diffraction, scanning tunneling microscopy 共STM兲, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and high resolution electron energy loss spectroscopy, allowing in situ chemical and structural characterization of the cluster samples. B. Magnetron sputtering and gas condensation

The plasma sputtering technique is nowadays widespread in many applications and thus has been extensively studied.23 One of the advantages of sputtering over other techniques is that a significant proportion 共⬃30% 兲14 of sputtered material, including clusters from the condensation process, is readily ionized in the plasma. Thus a further ionization stage is not required. Due to the screening effect of plasma, its potential is always slightly more positive than the most positive confining surface 共usually at ground兲. A large electric field is therefore created between the plasma and the biased target, accelerating Ar ions to sputter material off the target surface 共e.g., Ag兲. This is the case of dc sputtering, in

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which a large negative voltage is applied to a conductive target. For rf sputtering, the target is electrically isolated from the supply. The high voltage rf signal 共13.56 MHz兲 that is coupled 共capacitively兲 to the target causes both attractive and repulsive phases to the plasma. Due to the greater mobility of electrons, more negative charge remains on the target. A negative self-biased voltage will develop on the target, creating a large electric field and inducing sputtering. rf sputtering has the advantage over dc sputtering of being also applicable to insulating materials. A magnetron setup, in which a strong magnet is positioned behind the target, is commonly used to enhance ionization and intensify the plasma; the presence of a magnetic field in the sputtering zone lengthens electron paths in front of the target. The sputtering process creates both atomic species and some small clusters, while larger clusters are formed by inducing He gas in the condensation chamber. However, this requires a high pressure level, which is found to pose problems to the sputtering operation. Conventional magnetron guns are designed to operate at 10−2 mbar, whereas our typical operating pressure is ⬃1 mbar. At such high pressure the plasma was unstable and was often found to cause sputtering on the inside of the magnetron gun. This inner part of a standard magnetron gun is protected from the plasma by a grounded shield, called the dark space shield 关labeled 共c兲 in Fig. 1兴. The minimum gap between a high potential surface and an anode 共ground兲 required for plasma ignition can be estimated by the mean free path of electrons for ionization collisions. For a collision cross section ␴c, the mean free path of the electron Le depends on the pressure P and the temperature T, following the expression23 Le =

k BT , ␴c P

共1兲

where kB is the Boltzmann constant. The estimate for our typical operating conditions is a few mm. However, in the original magnetron gun design, the gap between the dark space shield and the target was ⬃1 cm. The magnetron gun has therefore been modified with conductive shields to narrow the gap within a few mm, and thus prevent plasma ignition inside the magnetron gun and ensure a stable sputtering operation. C. Ion optics

Formation of a well-focused cluster beam is essential for both mass selection and deposition. The cluster source has been designed to detect only positively charged particles, thus the ion optics lenses are biased with negative voltages. The cluster condensates exit the condensation chamber via the nozzle 关共e兲 in Fig. 1兴, and after passing through the two skimmers 关共f兲, 共g兲兴 the cluster beam enters the main ion optics set. It is first accelerated by a high voltage 共2 kV兲 element, the extraction lens 共h兲, which is the first element after the second skimmer. The beam is then focused with a pair of einzel lenses 关共i兲, 共k兲兴 into the TOF mass selector. The middle element of each einzel lens is connected to a variable bias voltage. The other elements are biased to a common voltage 共typically 400– 750 V兲, which defines the cluster beam energy. A set of deflection plates 共j兲 in the middle of

the ion optics set allows fine adjustment of the beam trajectory into and through the TOF. The ion optics set was designed and optimized with the SIMION v6 ion trajectory modeling software. Due to the large amount of material sputtered from the target, after some period of operation, the nozzle, the skimmers, and the ion optics lenses are covered with powder material. This affects the focusing of the beam and reduces drastically the beam intensity. Most affected by this material deposition is the first skimmer, where a small wire grows out of its tip after a few hours of operation 共depending on the target material兲. However, this material is only loosely bound to the skimmer and can be removed by a standard vacuum manipulator. The second skimmer and the ion optics set have to be cleaned by venting the source. This imposes a limit on the duration of continuous operation of the source. D. Mass selector

The mass selection of the cluster beam is achieved by a lateral TOF mass filter. The basic idea of this mass selection technique is to use time-limited high voltage pulses to displace the cluster beam perpendicular to its original direction, rather than acceleration in the direction of the beam in the more conventional TOF mass filters. The main advantage of this technique is that the mass resolution is constant over the entire mass range, since the incoming beam has no perpendicular velocity distribution. Details of the mass selection setup are presented elsewhere.22 Briefly, as the ion beam enters the TOF mass filter, all the plates 共shown in Fig. 1兲 are biased to the same negative potential corresponding to the cluster beam energy. The top 共l兲 and bottom 共n兲 plates are connected to high frequency high voltage switches. A portion of the incoming beam experiences the first perpendicular acceleration pulse provided by the bottom plate, which raises its potential to zero for the length of the pulse. After the first pulse, all the plates are biased back to the beam potential, while the accelerated portion of the beam drifts upwards in the central field-free region 共m兲. A second pulse identical to the first is then applied to the top plate to stop the perpendicular movement of the beam. Since the time the ions need to travel from the acceleration region to deceleration region depends on their mass, the timing of the two pulses defines the mass of ions which are transmitted through the TOF; ions of different masses are dispersed into parallel beams with different displacements. A rectangular aperture is used to select a required mass. The mass resolution is given by the ratio of the lateral displacement and the exit aperture width, and it also depends on the focusing condition of the beam. A mass resolution achieved with this source is m / ⌬m = 25, with a high transmission of about 60%. III. PERFORMANCE OF THE SOURCE

The size range of the clusters that can be produced by the condensation process is determined by many interrelated parameters, of which the most influential are the Ar and He pressures. An improper choice of these parameters may lead to no cluster production, whereas their careful optimization can provide the cluster source with the versatility to produce

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TABLE I. Typical operating conditions and cluster sizes for different types of sputter target.

Material Ag Cu Au Si

Approximate maximum size detected 共atoms per cluster兲

Ar pressure 共mbar兲

Total pressure 共mbar兲

rf power 共W兲

Aggregation distance 共cm兲

⬃8000 ⬃60 000 ⬃100 ⬃20

0.3–0.5 0.3–0.5 0.3–0.5 0.3–0.5

0.5–1.2 0.5–1.2 0.5–1.2 1.0–2.0

30–70 30–70 30–70 80–150

18–23 18–23 ⬃20 14–17

a range of cluster sizes, from a few atoms to a few thousand of atoms per cluster, depending on the target material. Table I presents the typical operating conditions and cluster sizes detected for four different targets, Ag, Au, Cu, and Si, that have so far been employed, with the Ag target being the most extensively studied.

A. Argon and helium pressures

The Ar and He gas pressures are found to have the most critical effect on the cluster production. Ar is responsible for the sputtering process, while the cluster condensation process is effectively induced by He. Figure 2共a兲 shows a typical mass spectrum of small 共up to 20 atoms兲 ionized silver clus+ , produced by sputtering with 0.4 mbar of Ar 共no ters, AgN He兲. The cluster signal decreases with increasing cluster size, however, this decrease is not exponential, which would be expected for pure ion sputtering.11,24 This indicates that some cluster condensation is also induced by Ar. Figure 2共b兲 presents a typical mass spectrum of large silver clusters obtained after addition of He 共0.2 mbar兲 to the Ar 共0.4 mbar兲 in the condensation chamber. The cluster size is significantly

FIG. 2. Mass spectrum of silver clusters, AgN+ , produced with 共a兲 only Ar 共0.4 mbar兲 and 共b兲 both Ar and He 共0.4 mbar of Ar, total pressure 0.6 mbar兲.

increased, by about 1000 times. The size distribution is broad and follows a log-normal shape, as commonly observed in cluster condensation processes.25,26 The growth of clusters occurs in two stages: energetic atoms sputtered from the target are cooled by the He gas leading to the nucleation of small cluster “seeds;” the nucleation of these seeds is followed by the growth of the seeds into larger clusters.25 For the nucleation of small clusters, a three-body collision between two sputtered atoms and a cooling He atom is essential to remove excess kinetic energy from the sputtered atoms. Three-body collisions occur more frequently in a highly dense 共supersaturated兲 vapor than in normal gas conditions, requiring a high sputter rate and thus high Ar pressure. A minimum Ar pressure for initiating cluster production 共clusters containing more than three atoms兲 is 0.3 mbar. The growth of large clusters can occur via twobody collisions, such as cluster–cluster collisions27 and atomic vapor condensation onto the cluster seeds.28 The cluster condensation process is illustrated in Fig. 3, where the signal of Ag+3 clusters is monitored as a function of He 共or total兲 pressure. The Ag+3 signal initially increases with increasing the He gas pressure, indicating the growth of the cluster seeds. The sharp drop of the Ag+3 signal at the pressure of ⬃0.55 mbar coincides with the pressure regime in which larger clusters can be detected. Since He is primarily responsible for the cluster condensation process, the He partial pressure can be used to control the cluster size distribution. Figure 4 illustrates the variation + + − Ag5000 兲 induced by the He in the size of Ag clusters 共Ag200 pressure. The size distribution of large clusters becomes narrower and shifts towards a smaller size with increasing He pressure. This may at first seem counterintuitive. However,

FIG. 3. The onset of cluster nucleation as a function of He pressure. The sharp drop of the Ag+3 signal at the pressure of ⬃0.55 mbar is caused by the growth of these small cluster “seeds” into larger Ag clusters.

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FIG. 4. Variation of Ag cluster size distribution with He pressure. Each vertical line is a representation of a cluster mass spectrum. The mark on each line indicates the cluster size with maximum intensity, with the value given in pA. The circles represent the sizes at the half-maximum points 共see inset兲.

more He not only enhances the growth of large clusters from the cluster seeds; it also creates more nucleation of these seeds. When the latter process becomes dominant, the cluster size will be smaller, assuming that the amount of sputtered material does not change with the He pressure. B. Sputter power

The rf power applied to the sputter target is another parameter that was found to affect significantly the cluster production. Clusters can be detected only within an optimum range of rf power, depending on the target material. An insufficient rf power level results in a low self-bias on the target, leading to inadequate sputtering of the target, in which case large clusters cannot be detected. The cluster beam current is also strongly dependent on the self-bias voltage. On the other hand, when the rf power is too high the plasma becomes unstable, resulting in an intermittent cluster beam current. As a standard operating procedure, only Ar is first used to determine the rf power for which some small clusters can be produced; then He is gradually added to the Ar and the rf power is optimized to obtain a required cluster size. Once correct conditions are obtained, these can give reproducible results over many experimental runs, providing that other parameters remain unchanged. C. Gas flow rate

The gas flow rate can be varied by adjusting the size of the exit nozzle 关共e兲 in Fig. 1兴, while the gas pressure is kept constant. The effect of flow rate on the cluster size distribution has been investigated only within a small range owing to the limitations in the pumping speed of the turbomolecular pump. Figure 5 shows that the size distribution of Ag clusters shifts towards larger masses with increasing total flow rate from 79.1 to 89.3 sccm; the maximum detected cluster size rises from 15 to 25 atoms, respectively. For a total flow rate less than ⬃60 sscm, only Ag clusters containing up to three atoms can be detected. Although a higher gas flow rate may intuitively seem to lead to shorter aggregation time, the real flow pattern is very complex. Only a small fraction of the material sputtered from the target exits the condensation chamber; most material is precipitated on the chamber wall. As seen in Fig. 5共b兲, a higher cluster signal is obtained with

FIG. 5. Effect of total flow rate on the size distribution of Ag clusters: 共a兲 79.1 sccm and 共b兲 89.3 sccm. The total pressure of Ar and He in both cases was 0.5 mbar, with the same Ar partial pressure of 0.4 mbar.

increasing gas flow rate. This implies that material is more efficiently evacuated from the chamber with a higher gas flow. D. Aggregation distance

Similarly to the rf power, the variable aggregation distance from the front of the magnetron gun to the exit nozzle has to be within an optimum range for production of large clusters. The cluster beam current is also optimized in this range. For example, the peak of the size distribution of Ag clusters is shifted by ⬃20% towards larger masses when the aggregation distance is increased by 2 – 3 cm. It cannot be ruled out that some clusters are produced outside this optimum range of the aggregation distance but are beyond the detection limit of the beam current measurement by the simple Faraday cup, which is much less sensitive than a channeltron detector. However, there is a minimum aggregation distance of ⬃10 cm for stable plasma to be ignited. This is probably a characteristic of this setup in which the Ar gas is injected from small orifices at the front of the magnetron gun. At present, in order to vary the aggregation distance, the cluster source has to be vented. E. Choice of target

The maximum cluster size produced is strongly dependent on the target material used. Figure 6 shows the mass spectra of Cu, Au, and Si clusters at around the maximum obtainable size to date for each target 共see also Table I兲. Clusters containing more than 60 000 atoms were produced with the Cu target, whereas only clusters with up to 20 atoms were detected in the case of the Si target. The trend in this maximum cluster size is approximately in agreement with the sputter yield of the materials. The semiempirical yields29 in terms of the number of sputtered atoms per Ar ion 共at 500 eV兲 are 4 共Ag兲, 4 共Cu兲, 3 共Au兲, and 0.9 共Si兲. The sputter

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FIG. 7. The energy distribution of Ag+, Ag+7 , and Ag+2800 ions in the cluster beam, measured by the retarding potential method.

FIG. 6. Mass spectra at around maximum obtainable cluster size for Cu, Au, and Si clusters.

yield is therefore a useful qualitative indication of the cluster size obtainable with the source. In addition, the cluster size may be influenced by other physical properties, such as the ionization energy or the binding energy of the clusters. The stronger Cu–Cu bond compared to the Ag–Ag bond may explain the larger maximum cluster size achieved with the Cu target. However, other factors, as discussed in the following subsections, may also influence the cluster production process. F. Kinetic energy spread of the beam

Particles in the cluster beam may have some initial kinetic energy prior to ionization and cluster beam formation. This introduces some spread in the kinetic energy of the cluster beam, which would affect the deposition of clusters on surfaces, especially at low energy. In cluster beam formation, ionized particles with a certain energy distribution are accelerated by high voltage. The energy spread, however, remains in the beam and can be measured by applying a retarding potential to the beam. In the retarding potential

measurement, the beam current 共I兲 is measured as a function of the retarding potential 共V兲, i.e., from −100 to 0 V. The current decreases as the potential becomes closer to the voltage at which the ions are created. The energy distribution of the ions can be obtained by plotting dI / dV as a function of V. This would result in an approximately Gaussian profile with a peak around the initial voltage at which the ions are created. Figure 7 shows the energy distribution of Ag+, Ag+7 , and + Ag2800 ions, with standard deviations of 27, 8, and 12 eV, respectively. The narrower energy distributions of the clusters rather than the Ag ions correspond well to the cluster condensation process in which energetic sputtered Ag ions have to be thermalized by the cooling gas before the condensation can occur. As seen in Fig. 7, the peaks are not centered at zero. These offsets should reflect on the initial potential at which the ions are created, or in this case, most probably at which the collision amongs gas atoms and clusters in the beam ceases, i.e., in the region between the nozzle and the skimmers, where the beam is first accelerated. Thus the values of these offsets may not be absolute and may depend on the bias voltages applied on the nozzle and the skimmers. The level of the energy spread of a few eV in clusters may have some implications in the “soft landing” regime 共deposition energy ⬍0.1 eV per cluster atom兲14 in which the morphology of clusters is preserved upon deposition. For small clusters, methods for the suppression of this energy spread, such as using an inert gas buffer layer,30 would be required 共in the soft landing regime兲, whereas for large AgN clusters 共e.g., N ⬎ 100兲 the energy spread becomes insignificant. G. Reproducibility

The reliability of this type of cluster source is somewhat problematic. A number of factors that had been previously assumed to be trivial were found to influence the cluster production strongly. Consequently, the cluster size output from the source is sensitive to many other parameters, apart from those given in Table I. The cluster size distribution can be influenced by the deformation of the target surface. When a new smooth target is used, the source produces smaller clusters than when the target is eroded. The use of a magnet to enhance sputtering also causes the sputtering to be nonuniform across the target surface, resulting in a shallow trench on the surface. After the target has been in use for a period of time, the sputter rate

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increases, owing to a stronger magnetic field resulting from the target thinning in this trench. In general, sputtering and ionization are enhanced with a higher strength magnetic field.31 Consequently, the cluster size also depends on the type and strength of the magnet. A high-strength “unbalanced” magnet, which consists of a hollow cylindrical magnet with a small central piece, is used in this source. An “unbalanced” magnet setup should also produce a higher percentage of charged particles than a more conventional “balanced” setup.23 After prolonged use of the target, the cluster size and beam current become gradually smaller. This occurs when the trench deepens to approximately half the thickness of the target 共the full thickness is ⬃6.35 mm兲, which may cause distortion to the plasma and thus weaken sputtering. Other factors that can affect the cluster production are the set of voltages applied to the ion optic lenses, the liquid nitrogen cooling condition, and the alignment between the exit nozzle and the first skimmer. The optimum voltage settings for detection of large clusters are often different from those for atomic beams and small clusters; this sometimes makes the detection of large clusters difficult. The optimum settings also change with time, due to material precipitation on the lenses. The temperature of the condensation chamber is one parameter which has not been investigated systematically. However, no clusters 共not even dimers兲 can be detected without liquid nitrogen 共lN2兲 cooling. The lN2 is supplied to the chamber from a reservoir. Sufficient time 共⬃2 h兲 for the lN2 cooling is allowed before the gases are let in at the start of the operation, which should ensure a good thermal equilibrium at approximately lN2 temperature. Contaminants, such as water and carbon monoxide, have been detected at room temperature, but disappear almost completely from the beam when the chamber is cooled with lN2. In addition, a misalignment between the exit nozzle at the end of the condensation chamber and the first skimmer can cause the skimmer to accept the turbulent region of the supersonic expansion of the beam rather than the central molecular flow. This may inhibit the transmission of clusters of particular sizes. IV. DEPOSITION OF SIZE-SELECTED CLUSTERS A. Cluster beam intensity

A typical beam current for large size-selected clusters is a few tens of picoamperes, equivalent to ⬃6.0 ⫻ 107 clusters/ s. The beam may be focused to a spot of ⬍1 mm wide. However, for ease of analyzing cluster samples, a wider and more uniform beam is deposited in the HV chamber with an aperture of 3 mm in diameter placed in front of the sample. This gives a cluster flux density for deposition of ⬃1 ⫻ 109 clusters s−1 cm−2. For Ag clusters containing ⬃1000 atoms, an atomic monolayer coverage 共⬃1015 atoms/ cm2兲 on a surface can be achieved within ⬃15 min, while a monolayer coverage of clusters typically requires a few hours. For deposition in the UHV chamber, a larger aperture of 5 mm in diameter is used, which approximately reduces the flux density for uniform deposition by a half; hence, twice the deposition time is required for the same coverage as the HV deposition. The deposition energy is determined by the negative bias voltage 共up to 5 keV兲

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applied to the sample holder. A deposition with a uniform cluster density on the surface is currently limited to energies below 1.5 keV, due to the technical limitations of the ion optics. At higher energy, the beam is focused as it accelerates towards the sample holder. B. Examples of cluster beam deposition

Deposition of size-selected clusters on surfaces is required for technological applications. However, when clusters are deposited on a surface, significant changes in their morphology often occur, as a result of cluster diffusion and aggregation. Thus, it is crucial to preserve the selected size of gas-phase clusters on the surface. Recent research in our group has concentrated on exploring the deposition of sizeselected metal clusters on the model graphite substrate, via a combination of STM experiments and molecular dynamics 共MD兲 simulations. Through this study of cluster–surface interaction, two deposition energy regimes in which clusters can be immobilized against diffusion have been established: cluster implantation32 and the “pinning” of clusters on the surface.15 At high enough energies 共20 eV per cluster atom and above兲, the clusters can be implanted into the graphite surface and come to rest at the bottom of an open “well.” These nanometer-scale structures 共“cluster down a well”兲 have potential applications in model catalyst studies. The dependence of the implantation depth on the impact energy and size of the cluster has been investigated, and simple scaling relations have been established. For AgN clusters containing 20–200 atoms, the MD simulations revealed that the implantation depth scales linearly with the cluster size 共N兲 and impact energy 共E兲 as D = E / N2/3.32 Experimental studies of the implantation depth as a function of cluster impact energy have so far been conducted only for small clusters, e.g., Ag+7 , Au+7 , and Si+7 . The depth was found to scale linearly with the momentum of the cluster,16 which is in good agreement with the results of MD simulations.33 The different scaling behavior observed for the larger clusters is found to originate from a cluster velocity threshold below which interaction between graphite layers is involved during cluster implantation.34 Clusters with sufficient kinetic energy can be “pinned” to their individual points of impact on the graphite surface, via the creation of a defect in the topmost graphite layer as a result of cluster impact. Thus the natural tendency of the clusters to diffuse is suppressed. As predicted by MD simulations, and confirmed by STM experiments, the threshold energy required for cluster pinning, i.e., the pinning threshold, scales linearly with the cluster size for AgN clusters 共N = 50– 200兲, and is approximately 10.5 eV/ cluster atom.15 The pinning threshold corresponds to the collision energy where a surface carbon atom is displaced from its equilibrium configuration to an interstitial site, creating a point defect which binds the cluster strongly to the surface, more strongly than the simple cluster–surface interaction. Figure + 8共a兲 illustrates the result of depositing Ag147 clusters on graphite at an impact energy of 2.0 keV, i.e., above the pinning threshold. An array of clusters with similar sizes can be observed on the surface, indicating that no cluster aggregation occurs. The corresponding cluster diameter and height

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FIG. 8. Cluster deposition experiments: 共a兲 STM image 共tunneling conditions: 0.2 V and 0.8 nA兲 of Ag+147 clusters deposited at 2 keV on graphite; 共b兲 the corresponding diameter and height distributions; 共c兲 STM image 共tunneling conditions: 1.0 V and 0.03 nA兲 of Ag+2700 clusters deposited at 0.65 keV on a graphite surface predecorated with Ar+ defects; and 共d兲 the corresponding diameter and height distributions.

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distributions are presented in Fig. 8共b兲. The narrowness of the distributions is consistent with a 147± 7 atom Ag cluster of height 1–2 monolayers. The pinning of clusters enables the fabrication of well-defined nanoscale surface structures from size-selected clusters, which may serve as model catalysts and prototypical binding sites to immobilize biological molecules.19–21 The maximum impact energy of 5 keV, imposed by the present deposition system, limits the pinning process to, for example, Ag clusters with ⬃475 atoms. A complimentary method has thus been developed to fabricate arrays of larger clusters: the Ar+ beam has been employed to create defects on the graphite surface; subsequent deposition of sizeselected clusters at low energy allows cluster diffusion to transfer the clusters to the prefabricated binding sites where they are immobilized. Figure 8共c兲 shows a STM image of + clusters deposited in the UHV analysis chamber at an Ag2700 impact energy of 650 eV on a graphite surface predecorated with Ar+ defects.17 The cluster height distribution, shown in Fig. 8共d兲, indicates that individual clusters are immobilized by the prefabricated defects, although some aggregation into larger clusters may still occur as a result of the statistical nature of the deposition process. ACKNOWLEDGMENTS

This work was supported by the EPSRC and BNFL. S.P. and S.J.C. are grateful for studentship support from the University of Birmingham. S.P. also thanks the ORS award committee and the DPST, Thailand. 1

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