M. Akizuki

Surface processing by gas cluster ion beams at the atomic (molecular) level

Gas cluster ion beam techniques have been developed for atomic and molecular level surface modification processing. Shallow implantation, high yield sputtering, surface smoothing, and low damage surface cleaning have been demonstrated experimentally. This article reports recent results concerning surface treatments that are distinctly different from those produced by conventional monomer ion irradiation. Possible applications of gas cluster ion beam processing to new areas of surface modifications are suggested.

Sputtering of elemental metals by Ar cluster ions

Abstract Energetic cluster bombardment effects have been examined for various materials. The sputtering yields of various materials with Ar cluster ions are two orders of magnitude higher than those with Ar monomer ions. The sputtering yield by cluster ion bombardment is proportional to the reciprocal of the sublimation energy of the target atoms. A dramatic reduction of Cu contamination on silicon surfaces has been obtained with Ar cluster ion bombardment at low ion dose. Low damage surface processing can be achieved, because the energy of each constituent atom is very low. This feature is quite suitable for low damage processing of electronic materials.

Indium oxide film formation by O2 cluster ion-assisted deposition

Abstract It has been difficult to deposit high quality indium tin oxide (ITO) films on to substrates at low temperature (below 100 °C) or subjected to post-deposition annealing. We have developed a new process based on O2 cluster ion assisted deposition of metallic indium and carried out deposition of high quality indium oxide films. At optimized indium deposition rate, a film with resistivity of 4.7×10−4 Ω cm and transparency of 80% for the wavelength of 550 nm, has been deposited at room temperature on to soda lime slide glass substrates, A wide process window of indium deposition rate has also been observed. This indicates that a reproducible and stable process is obtained with this new technique.

SiO2 film formation at room temperature by gas cluster ion beam oxidation

Abstract High quality SiO 2 film of thicknesses less than 11 nm were formed on Si substrate surfaces at room temperature by irradiation with O 2 and CO 2 cluster ions. The acceleration voltages were in the range of 5–10 kV. The minimum cluster size was 250 molecules, thus the maximum energy per molecule was less than 40 eV. These SiO 2 films which were formed had a stoichiometric composition, smooth interfaces with Si substrates and low carbon content. The results indicate a high chemical reactivity of gas cluster ions. Thus, the gas cluster ion beam technique has a potential for the formation of various kinds of films at low temperatures.

Low-Temperature Oxidation of Silicon by O2 Cluster Ion Beams.

High-quality SiO 2 films up to 11 nm thick were grown on Si substrate surfaces at room temperature by O 2 cluster ion irradiation. No damage was observed after N 2 annealing at 400°C for 30 min in the I-V characteristics of the thermally grown gate oxides irradiated with 5 keV Ar cluster ions. These results indicate that O 2 cluster ions strongly enhanced the oxidation with low irradiation damage.

Sputtering effect of gas cluster ion beams

Abstract Sputtering effects of CO2 cluster ion beams have been studied in comparison with the effect of monomer ions. A surface smoothing effect was obtained for Au, Pt, Cu, poly-Si, SiO2 and Si3N4 films, Si substrates and Soda-lime glasses. The energy dependence of smoothing and sputtered depth was different, and a higher ratio between reduction of surface roughness and sputtered depth was obtained at lower cluster energy. Surface smoothing can be achieved by cluster ion irradiation even at 5 keV. Cluster ion irradiation-induced sputtering produced very flat surfaces due to a different sputtering behaviour from monomer ions, which is expressed as lateral sputtering.

LOW-DAMAGE SURFACE TREATMENT BY GAS CLUSTER-ION BEAMS

Low-damage irradiation effects of gas cluster-ion beams have been studied at acceleration voltages below 20 kV. The surfaces of targets have been smoothened significantly by CO2-cluster-ion irradiation at normal incidence. Si substrate surfaces have been cleaned and exhibited low damage after CO2- and Ar-cluster-ion irradiation at low doses. In the case of CO2-cluster-ion irradiation, SiO2 film of about 5.5-nm thickness have grown on Si substrate at room temperature. A damaged layer of less than 2.5-nm thickness has been formed underneath the SiO2 film.

Low-temperature formation of perovskite PbTiO3 films by O2 cluster ion-assisted deposition

Abstract The formation of PbTiO 3 films on Pt films using O 2 cluster ions, Pb and Ti(i-OC 3 H 7 ) 4 , was examined. Perovskite-PbTiO 3 films could be formed at a substrate temperature of 400 °C, which was about 100 °C lower than that which is possible with CVD and sol-gel methods. An acceleration energy of 1 keV was sufficient to form the perovskite structure. These films were obtained owing to a high oxidant flux to the surface and high-density energy deposition during growth. Another feature of the O 2 cluster ion assisted deposition was that no pyrochlore structure was observed at lower substrate temperatures. This might be due to surface reactions in the high vacuum (3 × 10 −5 Torr).

SPUTTERING WITH GAS CLUSTER-ION BEAMS

A high-current (~100 nA) cluster-ion-beam equipment with a new mass filter has been developed to study the energetic cluster-bombardment effects on solid surfaces. A dramatic reduction of Cu concentration on silicon surfaces has been achieved by 20-keV Ar cluster (N~3000) ion bombardment. The removal rate of Cu with cluster ions is two orders of magnitude higher than that with monomer ions. A significantly higher sputtering yield is expected for cluster-ion irradiation. An energetic cluster-ion beam is quite suitable for removal of metal.

Cluster ion assisted thin film formation

A new thin films formation technique using gas-cluster ion beams is demonstrated. Oxygen cluster ion beams are utilized to grow oxide films. We have found that high quality SiO2 films of up to 11 nm thick can be formed on Si substrate surfaces at room temperature by direct oxidation using O2 and CO2 cluster ion beams at acceleration voltages of less than 10 keV. O2 cluster ions were also used to irradiate Si substrates covered with SiO2 films, during the evaporation of Pb atoms. Increasing the acceleration voltages, the O2 cluster ion bombardment enhanced the oxidation and surface smoothing. At the acceleration voltages above 5 kV, the polycrystalline lead oxide films were oriented with their (111) direction parallel to the surface. A significant smoothing effect was observed, even if the acceleration voltage was as low as 1 kV. An average surface roughness of 0.9 nm was obtained at 7 kV. These results indicate that O2 cluster ions strongly enhanced the oxidation and the smoothness without causing irradia...