Negative Ion Beam Deposition Research Articles

Negative-ion source applications (invited)

In this paper heavy negative-ion sources which we developed and their applications for materials science are reviewed. Heavy negative ions can be effectively produced by the ejection of a sputtered atom through the optimally cesiated surface of target with a low work function. Then, enough continuous negative-ion currents for materials-science applications can be obtained. We developed several kinds of sputter-type heavy negative-ion sources such as neutral- and ionized-alkaline metal bombardment-type heavy negative-ion source and rf-plasma sputter type. In the case where a negative ion is irradiated on a material surface, surface charging seldom takes place because incoming negative charge of the negative ion is well balanced with outgoing negative charge of the released secondary electron. In the negative-ion implantation into an insulator or insulated conductive material, high precision implantation processing with charge-up free properties can be achieved. Negative-ion implantation technique, therefore, can be applied to the following novel material processing systems: the surface modification of micrometer-sized powders, the nanoparticle formation in an insulator for the quantum devices, and the nerve cell growth manipulation by precise control of the biocompatibility of polymer surface. When a negative ion with low kinetic energy approaches the solid surface, the kinetic energy causes the interatomic bonding (kinetic bonding), and formation of a metastable material is promoted. Carbon films with high constituent of sp3 bonding, therefore, can be formed by carbon negative-ion beam deposition.

Dependence of carbon interatomic bonds on incident ion energy in carbon negative ion beam deposited films

Atomic bonds of carbon thin films prepared by negative-ion beam deposition were investigated with respect to the dependence on incident ion energy. Mass-separated carbon negative ions of C − and C 2 − were deposited on Si(100) and Si(111) at a low ion energy of 50–400 eV with a current density of about 20 μA/cm 2 under 1 × 10 −6 Pa for 5 h after removing surface oxide layer on the substrate by heating up to 900 °C for 15 min. Deposited films were found from their broad Raman band in 1400–1600 cm −1 to be diamond-like carbon films. From decomposed XPS C lS spectra into three bonds of C C (sp 3 bonds), C C (sp 2) and O, it was found that the fraction of sp 3 bonds increased with an increase in ion energy, and that it decreased gradually with the further increase in ion energy for both ion species. It is considered that the kinetic energy of ions contributes to form the metastable bond like sp 3 bond but that the excessive kinetic energy results damage in the films. As a result, the sp 3 bond fraction of more than 80 % was obtained at ion energy of 75 eV per a carbon atom for both C − and C 2 − depositions.

CN molecular negative-ion beam deposition and ion energy dependence of atomic bonds between carbon and nitrogen in the films

The atomic bonds in carbon nitride films prepared by CN molecular negative-ion beam deposition were investigated with respect to the dependence on the incident ion energy. Mass-separated CN negative ions were deposited on n-Si(1 0 0) at a low ion energy in a gas pressure of 5.0 × 10 −5 Pa. Raman spectra showed a broad Raman band similar to that of diamond-like carbon and a small band corresponding to CN triple bonds near 2220 cm −1. The maximum nitrogen content was obtained at an ion energy of 75 eV, where the N/C ratio was 0.6, as found from RBS analysis. Detailed XPS spectra of N 1s were measured to investigate inter-atomic bonds between C and N. The amount of C–N single bonds was increased with an increase in ion energy up to 75 eV. The amount of C–C single bonds showed a similar dependence upon the ion energy as the C–N single bonds.

Extraction of molecular negative-ion beams of CN from radio frequency plasma-sputter-type heavy negative ion source for negative-ion beam deposition

A high current CN negative-ion beam was obtained from a radio frequency plasma-sputter-type heavy negative ion source in a target-gas mode operation and CN negative-ion beam deposition was investigated. CN negative ions of 0.88 mA were safely obtained by using a carbon sputtering target and nitrogen gas instead of cyanogen for ionization. Even in this target-gas mode operation of the source with N2 gas, the work function of the sputtering target surface was effectively decreased by introducing cesium vapor so that the production of CN negative ions was remarkably enhanced. In a CN negative-ion beam deposition on silicon substrate, the nitrogen concentration in deposited layers depended on ion energy and the maximum ratio (N/C) of 0.3 was obtained at an energy of 70 eV. From a Raman spectra with these results, it was found that CN-deposited films were a diamondlike carbon film including nitrogen atoms.

Negative-ion sources for modification of materials (invited)

The properties of negative ions, such as charging–free ion implantation and new materials syntheses by pure kinetic bonding reaction, have been shown to be promising in terms of their interaction with material surfaces. However, high-current or high-brightness negative-ion sources are required for these purposes. Several kinds of sputter-type negative-ion sources have been developed for negative-ion implantation and deposition in order to obtain high-current heavy negative ions. Recently, a microwave discharge oxygen negative-ion source for negative-ion beam deposition and a surface plasma type hydrogen negative-ion source for projection ion-beam lithography have been investigated. In this article, these negative-ion sources for modification of materials are reviewed.

Carbon Nitride Film Formation by Low Energy Positive and Negative Ion Beam Deposition

AbstractUsing a newly developed ion beam apparatus, PANDA (Positive And Negative ions Deposition Apparatus), carbon nitride films were prepared by simultaneous deposition of mass-analyzed low energy positive and negative ions such as C2-, N+, under ultra high vacuum conditions, in the order of 10−6 Pa on silicon wafer. The ion energy was varied from 50 to 400 eV. The film properties as a function of their beam energy were evaluated by Rutherford Backscattering Spectrometry (RBS), Fourier Transform Infrared spectroscopy (FTIR) and Raman scattering. From the results, it is suggested that the C-N triple bond contents in films depends on nitrogen ion energy.

A heavy negative ion sputter source: Production mechanism of negative ions and their applications

Heavy negative ions can be produced by sputtering a metal surface at high efficiency by means of a secondary ion emission process. The most important factor for obtaining high negative ion production efficiencies is to maintain approximately one-half monolayer of Cs on the sputter target which produces a minimum in the surface work function. In a dynamic situation in which the surface is being bombarded by heavy ions, the cesium coverage is determined by a balance between the incoming and outgoing cesium fluxes. The supply of additional cesium to the surface and control of the target temperature are then essential for high negative ion production efficiency. The Neutral and Ionized Alkaline metal Bombardment type heavy Negative Ion Source (NIABNIS) in which a continuous negative ion current of a few hundred μA for several elements can be extracted, can be directly used for applications in materials science, such as negative ion beam deposition and negative ion implantation.

Negative ion source (NIABNIS) and preparation of transparent carbon films by negative carbon ion beam deposition

A high current heavy negative ion source of the neutral and ionized alkaline metal bombardment type (NIABNIS) has been newly developed for negative ion beam applications such as deposition and implantation. This source is a sputter type, and the sputtering target is placed on the extractor of a cesium plasma ion source in order to obtain a large current of cesium ions bombarding the target simultaneously with a controlled amount of neutral cesium atoms. The work function of the sputtering target surface in operation is kept as low as possible so that a high efficiency of the negative ion production is realized. Mass-separated ion beams of 0.74 mA for C −, 200 μA for C 2 −, 320 μA for Cu −, and 170 μA for Si − were obtained. A negative ion beam deposition system with the NIABNIS, in which divergent effects of the ion beam were effectively suppressed, has also been developed. A decelerated C − ion current of several tens of μA in the range of a final energy of 25–100 eV, and about 100 μA in the range higher than 100 eV was obtained. By using C − and C 2 − ion beams with an energy of 10–1000 eV, transparent carbon films of several thousands of Å in thickness were deposited.

Neutral and ionized alkaline metal bombardment type heavy negative ion source (NIABNIS)

A high current NIABNIS has been developed for negative ion beam applications such as negative ion beam deposition and negative ion implantation. In the NIABNIS, a sputter cone for negative ion production is placed on a cesium ion extractor of a cesium plasma ion source in order that a large amount of cesium ions together with controlled neutral cesium particles are simulteneously bombarded on the sputter cone surface. The temperature of the sputter cone can be controlled. Thus, the optimal surface for the negative ion production with the minimum work function can be obtained. Ion beams of 0.74 mA for C −, 320 μA for Cu −, and 170 μA for Si − were produced, and are adequate for negative ion beam applications. A mass-separated negative ion beam deposition system with the NIABNIS was developed and a transparent carbon film of several thousands of Å in thickness was deposited by using a C − ion beam with an energy of 10–1000 eV. A negative ion implanter with the NIABNIS in which an ion beam was accelerated to an energy of 5–60 keV was also developed and experiments such as destructive cross-section of negative ion beams with gases and SiC formation by C − ion implantation into Si substrates were performed.