Negative-Ion Implantation
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ABSTRACT Negative-ion implantation is a promising technique for forthcoming ULSI (more than 256 M bits) fabrication and TFT (for color LCD) fabrication, since the surface charging voltage of insulated electrodes or insulators implanted by negative ions is found to saturate within so few as several volts, no breakdown of insulators would be expected without a charge neutralizer in these fabrication processes. Scatter-less negative-ion implantation into powders is also possible. For this purpose an if-plasma-sputter type heavy negative-ion source was developed, which can deliver several milliamperes of various kinds of negative ion currents such as boron, phosphor, silicon, carbon, copper, oxygen, etc. A medium current negative-ion implanter with a small version of this type of ion source has been developed.
INTRODUCTION Ion beams are effectively used for the surface modification and synthesis of materials under nonthermal-equilibrium conditions. All of their applications were performed using positive-ion beams because it has been believed that positive ions are more easily produced compared with negative ions and that the effects of negative ions on solid surfaces would be almost the same as those of positive ions. However, recently a new evolution in both heavy negative-ion production and their applications has occurred, and thus negative-ion beam technology for materials science has attracted a good deal of attention [1-3]. Regarding negative-ion production, especially heavy negative-ions investigations on negativeion production probabilities by secondary negative-ion emission by sputtering revealed various heavy negative ions could be produced in high yields [1-6]. In this new production process, negative ions are produced on a metal surface where an electron smoothly shifts from the Fermi level of the metal surface to the electron-affinity level of a sputtered atom. Negative-ion currents obtained by this process are comparable to positive-ion currents from conventional plasma type ion sources. An rf-plasma-sputter type heavy negative-ion source developed by the present author [2,3,7,8] delivers several milliamperes of negative-ion currents, which are enough for materials science applications. Fundamental data about negative-ion beam transport such as electron-detachment cross-sections and secondary-electron emission factors were also measured [1,9,10]. This made it possible to design a medium current negative-ion implanter. On the other hand, the negative-ion beam has various excitatory effects on solid surfaces. In the low kinetic energy range, a negative ion having an internal absorbable potential energy of electron affinity could be expected to have only a cooling effect on ion-beam deposition [3,11,12]. Therefore, the material synthesis mechanism using particles with a relatively large kinetic energy, i.e., kinetic bonding process [3,11], could be investigated. In the high kinetic energy range, some new ion-implantation techniques are expected. The surface charging voltage of insulated materials by negative
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