Cluster Beam Source Research Articles

Cluster beams for metallization of microstructured surfaces

Cluster beams from a nozzle source are characterized by a high directionality (point source) and a high intensity. These properties have been used for thin film metallization of flat and microstructured surfaces. The paper describes the cluster beam source developed for metals like silver and magnesium. At present, the final collimated cluster beam covers a total angle of 7°, which for a source distance of 1 m gives a beam diameter of 120 mm. The deposition rate at that distance is up to 10 nm/s, and the total beam flux is 20% of the metal flux out of the nozzle. The metallic films obtained with this new deposition technique are a close image of the respective substrate structure. Good metallization of substrates has been observed up to cluster incidence angles of 80°, and the negligible thermal divergence of the cluster beam allows metal deposition, e.g. into contact holes.

Cluster beam deposition: Optimization of the cluster beam source

Cluster beams from supersonic nozzle expansions of a vapor-inert gas mixture are being used for thin film formation of metals or refractory compounds. The cluster beam source is optimized with respect to enlarging the deposition area, given by the width of the cluster beam, and lowering the effective pumping speed for the carrier gas argon. With silver as a representative material for our source, the cluster beam diameter measured at 0.78 m downstream of the nozzle was 94 mm, corresponding to a beam angle of 6.8°, or 0.011 sr, and the deposition rate was up to 14 nm/s. The effective pumping speed of the nozzle discharge chamber could be lowered to 0.1 m3/s at 25.5 Pa. The cluster beam used for thin film formation contained up to 20% of the total silver flux out of the nozzle.

Laser-driven flow reactor as a cluster beam source

A novel technique for the production of expansion-cooled cluster beams from materials with low vapor pressure is presented. The clusters are produced in a flow reactor from gas phase reactants by aggregation of CO2-laser-induced decomposition products. By introducing a conical nozzle into the reaction zone, they are extracted into a molecular beam apparatus and analyzed with a time-of-flight mass spectrometer. Depending on the type of CO2-laser employed, the source can be operated in the pulsed or continuous mode. The generation of carbon and silicon clusters is demonstrated by decomposing gaseous C2H2 and SiH4, respectively. The laser-driven cluster course is also employed to generate fullerenes and nanosized silicon particles.

Structural and magnetic evolution in granular FeAg alloys produced by the cluster beam technique

Using cluster beam sources, we have obtained FeAg granular films, which display giant magnetoresistance, GMR, without any heat treatment. The experimental results of small angle X-ray scattering and high-resolution transmission electron microscopy indicate that a few nanometer Fe clusters are embeded in Ag matrices with 10 nm order geometrical and chemical fluctuations. The extended X-ray absorption fine structure measurement indicates that the short range structure of Fe clusters is bcc but distorted in the as-deposited state. The magnetoresistance (MR) does not saturate at 140 kOe: conduction-electrons suffer spin-disorder scattering even in high fields, because small Fe clusters in Ag matrices reveal a spin-glass character and Fe atoms at the interface a superparamagnetic. After annealing above 570 K, MR saturates at high fields, which is ascribed to the ferromagnetism of grown Fe clusters. The concentration dependence of electrical resistivity and MR can be interpreted by geometrical and magnetic percolation of nanometric Fe clusters.

Characteristics of polyimide film deposited by ionized cluster beam

Ionized cluster beam deposition (ICBD) has been employed to fabricate high-purity polyimide (PI) films. Pyromellitic dianhydride and oxydianiline were deposited using dual ionized cluster beam (ICB) sources. X-ray photo-electron spectroscopy shows that the chemical properties of the surface are very sensitive to the ICBD conditions such as cluster ion acceleration voltage and ionization voltage. At optimum ICBD conditions, the PI films have maximum imidization, and negligible impurities (∼ 1% isoimide). A metal-insulator-semiconductor device has been fabricated using PI as an insulator to investigate the interfacial properties. From C− V characteristics, the PI film deposited with ICB is found to have a fairly low interface trap density. The conduction mechanism in ion implanted PI film is described in terms of the conducting grain model. The main factor determining the conductivity in PI is found to be closely related to the deposited damage energy induced by ion irradiation.

Properties of ferroelectric PbTiO3 films grown in an ionized cluster beam system

Two ionized cluster beam (ICB) sources were used to deposit PbTiO3 films 300 nm thick onto Pt-metallized, oxidized Si wafers 100 mm in diameter. Pb was deposited from one ICB source and Ti from the other. The deposition was carried out in a mixture of O2 and 6% O3 at a total pressure of 5×10−5 Torr. No substrate heating was used during deposition. However, thermal radiation from the ICB sources caused the substrate temperature to reach approximately 100 °C soon after the start of deposition. The as-deposited films were amorphous according to x-ray diffraction patterns. Annealing the films at 550 °C in 1 atm pressure O2 for 1 h led to the formation of a mixture of a perovskite-type phase and possibly some pyrochlore- and PbO-type structures. However, an anneal at 750 °C for 1 h in 1 atm pressure O2 produced the perovskite phase PbTiO3 with no other detectable phases. It was found that a slightly Pb-rich or stoichiometric Pb/Ti ratio in as-deposited films gave the best ferroelectric film properties. A Pb/Ti ratio much higher than stoichiometric (Pb/Ti ratio≳2) led to the formation of pits and pinholes in the films after annealing at 750 °C, whereas a Pb/Ti ratio lower than stoichiometric did not produce the perovskite phase after annealing at 750 °C. The pits and pinholes associated with Pb/Ti ratio larger than stoichiometric were probably caused by the evaporation of excess PbO during annealing. Films were deposited with and without the use of ionization and acceleration to 2 keV. In both cases, the as-deposited films were amorphous. There were no substantial differences between annealed films deposited with or without ionization and acceleration of the Pb and Ti beams. Ferroelectric domains in the annealed films have been observed by transmission electron microscopy. Ferroelectric switching (hysteresis loops) was recorded in annealed PbTiO3 films with stoichiometric Pb/Ti ratios after the films were capped with Pt to form small capacitors.

Supersonic cluster beam source using a differential cryopumping system

We have developed a supersonic cluster beam source for the use of an electron-cluster crossed-beam experiment to study the electronic excitation processes in rare gas clusters. A cryopump is used for the differential pumping system instead of a diffusion pump, which is used in conventional cluster beam sources. The performance of this differential pumping system is examined. The size distribution in the cluster beam is estimated by measuring the kinetic energy and velocity distribution as a function of the stagnation pressure.

Electron binding and stability of excess-electron alkali halide clusters: Localization and surface states.

Metal-excess alkali halide clusters have been investigated experimentally using a cluster-beam source comprised of laser ablation of alkali metal and reaction of helium-seeded halogen gas. Metal-excess neutrals and charged clusters, ${\mathit{M}}_{\mathit{n}}$${\mathit{X}}_{\mathit{m}}^{(+)}$ where M=Na or Li and X=F, Cl, or Br, have been produced, with the neutral series, ${\mathit{M}}_{\mathit{n}}$${\mathit{X}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$, displaying a wide range of excess-electron-dominated behavior. An extensive investigation of the abundances and electronic properties of ${\mathrm{Na}}_{\mathit{n}}$${\mathrm{F}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$ (n100) is described. A simple quantitative model of electron localization is proposed to account for the observed patterns of abundances and electron binding energies. The model correctly predicts that (i) for the singular values of n that correspond to a filled cubic lattice, the excess electron is very weakly bound, and that (ii) clusters that can form a cubic lattice with a single anion vacancy have the electron localized in the vacancy, stabilizing the neutral cluster against atom loss.

Seeded supersonic alkali cluster beam source with refilling system

A supersonic sodium cluster beam source with a refilling system is described in detail. Due to the external refilling system a new loading of the oven, which is designed for high-temperature application at 800 °C, is accomplished very fast and interrupts the measuring process only for 1–2 h. Additional purification of the alkali metal in the reservoir allows the usage of standard quality sodium samples.

Properties of large area ErBa2Cu3O7−x thin films deposited by ionized cluster beams

ErBa2Cu3O7−x films have been produced by simultaneous deposition of Er, Ba, and Cu from three ionized cluster beam (ICB) sources at acceleration voltages of 0.3–0.5 kV. Combining ozone oxidation with ICB deposition at 650 °C eliminated any need of post anneal processing. The substates were rotated at 10 rotations per minute during the deposition which took place at a rate of about 3 to 4 nm. ErBa2Cu3O7−x films with areas up to 70 mm in diameter have been made by ICB deposition. These films, 100‐nm thick, were deposited on SrTiO3(100) substrates at 650 °C in a mixture of 6 at. % O3 in O2 at a total pressure of 4×10−4 Torr. They had Tc ranging from 84.3 to 86.8 K over a 70 mm diameter and Jc above 106 A/cm2 at 77 K. Another set of three samples, deposited within a 50 mm diameter, was examined by magnetization measurements. These samples had Jc ranging from 8.2×106 to 1.1×107 A/cm2 at 4.2 K and from 2.4×105 to 5.1×105 A/cm2 at 70 K. X‐ray diffraction measurements of the three samples showed preferential c‐axis orientation normal to the substrate surface. Rocking curves showed small variation in grain misorientation with sample position relative to the center of the substrate holder. Scanning electron micrographs (SEM) of the three samples also show some texture dependence on sample position. For the three samples, there is a correlation between SEM texture, full width at half‐maximum of rocking curves and Jc versus temperature curves.

Measurements of kinetic energy release following the unimolecular and collision-induced dissociation of argon cluster ions, Ar+n, for n in the range 2–60

A double-focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision-induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar+5–Ar+60, and for the CID studies the range was Ar+2–Ar+30. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid (<10−7 s) loss of two Ar atoms within the confines of a collision cell; (iii) sequential loss of two Ar atoms on a time scale >10−7 s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n>4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the experimental CID results in cluster ions beyond Ar+15.

Characteristics of Palladium Thin Films Deposited by the Ionized Cluster Beam Technique

ABSTRACTAn ionized cluster beam (ICB) source was used to deposit Pd onto oxidized silicon substrates. The ICB source was operated in both the neutral mode (no ionization and no acceleration) and in the ICB mode with ionization and acceleration voltages at 3 kV and 6 k.V. Also, substrate temperatures were varied between 100°C and 400°C. The Pd film thicknesses were generally between 1, 200Å and 1, 800Å, with one film thickness about 500Å. The films were examined by transmission electron microscopy (TEM), transmission electron diffraction (TED), and x-ray diffraction (XRD). Grain size measurements by TEM and XRD showed that ionization and acceleration of Pd resulted in a slight increase in grain size compared to films deposited without ionization or acceleration at any substrate temperature. However, the grain size increased significantly as the substrate temperature rose. XRD showed that all ICB deposited Pd films have significant (111) texturing as determined by comparison to XRD data for Pd powder. For Pd films deposited at 400°C, almost all grains were oriented with the (111) planes parallel to the substrate surface. The electrical conductivity of all Pd films was comparable to that of bulk Pd.

Formation of Aluminum Films Using a High Rate ICB Source

ABSTRACTAn ionized cluster beam (ICB) source has been developed for high rate deposition, and its possible application to ultra large scale integrated circuit (ULSI) metallization was investigated. Aluminum films were deposited onto oxidized silicon wafers using the ICB source. It was shown that an electrical resistivity was almost the same as the value for bulk aluminum, and the surface morphology of deposited films was improved by controlling the ionization and acceleration conditions of the cluster beam. It was confirmed that the ICB method showed an excellent coverage profile in contact holes when compared with the conventional sputtering method. Using the directivity of cluster beams, contact holes of 1.5 in aspect ratio were successfully metallized. From these results, it became evident that the ICB source is a favorable method for ULSI metallization.

XPS and FTIR Study of the Thin Polyimide Films Fabricated by Ionized Cluster Beam Deposition

ABSTRACTThe ionized cluster beam deposition (ICBD) technique has been employed to fabricate high-purity polyimide (PI) films. The pyromellitic dianhydride (PMDA) and oxydianiline (ODA) were deposited using dual ionized cluster beam (ICB) sources. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoemission spectroscopy (XPS) studies show that the bulk and surface chemical properties are very sensitive to the ICBD conditions such as cluster ion acceleration voltage and ionization voltage. At optimum ICBD conditions, the PI films have a maximum imidization and negligible impurities(∼;1% isoimide) probably due to the high surface migration energy and surface cleaning effect.

High Damage Threshold Optical Films for Excimer Lasers

AbstractHigh damage threshold coating for high energy KrF excimer laser has been developed by the multiple ion beam deposition system, which contains a couple of the ionized cluster beam (ICB) sources and the ionized gas beam source. The damage threshold of low refractive SiO2, high refractive A12O3 and SiO2/A12O3 multilayer coatings is found to be more than 105 shots at 8J/cm2 laser energy. Oxidation is enhanced by ion bombardment during the film growth.The refractive indices of SiO2 and A1203 films were 1.46 and 1.62 at the ion current density of 0.8μA/cm2. The film density of SiOx approaches to the bulk SiO2 of 2.3Og/cm3 with increasing ion current density. The stress-free SiO2 film can be obtained at the ion current density of around 0.5 μA/cm2.

Formation of negative cluster ions from (CO2)m in collision with high-Rydberg atoms

Negative ions of CO2 clusters (CO2)m produced by electron transfer from highly excited Rydberg atoms were investigated by mass spectrometry. The relative intensities of (CO2)−n (n≥3) (size distribution) showed a characteristic structure with magic numbers at 9, 14, and 16. The dependence of the intensities of (CO2)−n (n=7–20) on the stagnation pressure of the cluster beam source was measured. The threshold pressure at which a cluster anion of a given size started to appear increased irregularly with the size. These observations suggest that evaporation of CO2 molecules occurs by dissipation of the excess energy after electron attachment and that the number of evaporating molecules m–n depends on the size of the resulting ion. The evaporating molecules are fewer at n=9, 14, and 16 than those at adjacent sizes. The cluster ions of these sizes are shown to have higher dissociation energies than other ions, and the magic numbers observed in the mass spectrum are ascribed to the difference in the stability of the ions.

Surface roughness of aluminum thin films deposited by effusive and ionized cluster beams

The surface roughness of aluminum thin films is of importance in a number of microelectronic device applications as well as in other technologies. A question one needs to address is how thin film surface roughness depends on substrate temperature and film thickness for various deposition methods. We have used an effusive source and an ionized cluster beam (ICB) source in the same apparatus to deposit pure aluminum films. A study of the surface roughness of these films shows that for a fixed substrate temperature T s (1) the roughness average R a is a power function of the film thickness t such that R a = At n where A and n are constants, (2) A and n do not depend on the deposition method except at T s = 200 °C, and (3) for ICB deposition, A and n are not dependent on acceleration voltages between 0 and 6 kV. These results apply for oxidized silicon substrates with T s between 70 and 400 °C and for aluminum thicknesses less than 800 nm. The initial substrate R a was less than 1 nm. At T s between 70 and 100 °C, R a = 1.5 × 10 −3 t 1.5, at T s = 200 °C, R a = 3.5 × 10 −2 t 1.08 for ICB deposition, and R a = 0.36 t 0.61 for physical vapor deposition, at T s = 300 °C, R a = 1.0 × 10 −2 t 1.30, and at T s = 400 °C, R a = 6.53 × 10 −3 t 1.38, where R a and t are expressed in nanometers.

Direct injection supersonic cluster beam source for FT-ICR studies of clusters

A miniaturized pulsed supersonic beam source has been developed using laser vaporization of a computer-controlled target disk, producing intense beams of cluster ions with excellent repeatability and control. Due to its small size and narrow pulse width, the entire source is adequately pumped by a single 170 ℓ /s turbopump. The resultant vacuum quality permits this source to be attached to a Fourier transform ion cyclotron resonance apparatus (FT-ICR) such that the supersonic cluster ion beam is directly injected. The result is a powerful but simple FT-ICR instrument of wide applicability. The new source is suited as well for a variety of other uses such as molecular beam epitaxy.

Time-of-flight mass spectroscopy of ionized cluster beam in film deposition conditions

A time-of-flight mass spectroscopy technique was developed that enables continuous monitoring of ion mass distribution in the output of a typical ionized cluster beam (ICB) source with minimum modification of the setup and negligible disturbance of film growth. The low mass resolution of this spectrometer is sufficient for cluster size determination and is counterbalanced by a number of advantages promising a more general applicability of the technique. This spectrometer has been used to study germanium cluster formation in a commercially available ICB source. No clusters larger than a few atoms were observed during several deposition runs, under conditions relevant to practical applications. It is difficult to reconcile these findings with the original hypothesis of homogeneous nucleation and growth of large clusters by free jet expansion of metal vapor. Since the precise temperature gradient in the crucible could not be controlled in this type of source, it cannot be excluded that large clusters may be formed under very particular conditions. If this is the case, vapor condensation on the nozzle’s walls must play the key role in the formation of clusters, of germanium at least, making ICB less attractive as a semiconductor thin film growth technique.

Aluminum surface mobility on silicon nitride and on several silicon oxides

This report concerns the measurement of Al atom surface mobility on various substrates during physical vapor and neutral cluster deposition. One objective is to determine Al surface mobility dependence on substrate type and temperature. Another objective is to compare Al diffusion distances produced by an effusive source and a cluster beam source. The experiment is done with a simple wire mask held on the substrates during Al deposition. During deposition, the substrates were held at constant temperatures between 80 and 400 °C. After the wire is removed, the shadowed areas are examined by optical and electron microscopy, mechanical profilometer, and Auger electron line scans. Measurements show that Al diffusion distances in the shadow zones are between 8 and 16 μm on oxidized Si films on Si substrates. The diffusion distance of Al on silicon nitride ranges from 20 to 25 μm. The diffusion distances of Al on all substrates can be understood in terms of the mean diffusion distance X=(2Dsτs)1/2, where Ds is the surface diffusion coefficient and τs is the mean time to desorption after Al atoms are deposited. These results show that on the micrometer scale the diffusion distance of Al atoms, while temperature and substrate dependent, is independent of the deposition method.