AVS 59th Annual International Symposium and Exhibition
    Plasma Science and Technology Thursday Sessions
       Session PS2-ThM

Paper PS2-ThM9
Theoretical Calculation of Neutralization Efficiency of Positive and Negative Chlorine Ions with Consideration of Excited States

Thursday, November 1, 2012, 10:40 am, Room 25

Session: Low Damage Processing
Presenter: S. Ohtsuka, Mizuho Information & Research Institute, Inc., Japan
Authors: S. Ohtsuka, Mizuho Information & Research Institute, Inc., Japan
N. Watanabe, Mizuho Information & Research Institute, Inc., Japan
T. Kubota, Tohoku University, Japan
T. Iwasaki, Mizuho Information & Research Institute, Inc., Japan
Y. Iriye, Mizuho Information & Research Institute, Inc., Japan
K. Ono, Mizuho Information & Research Institute, Inc., Japan
S. Samukawa, Tohoku University, Japan
Correspondent: Click to Email
We investigated the generation mechanism of neutral particles in high efficiency neutral beam source developed by Samukawa et al [1], by collision of positive and negative chlorine ions against graphite surface. It is already known experimentally that neutralization efficiency of negative ion (Cl-) is much higher than that of positive ion (Cl2+) [2] and that realized high efficiency and low energy neutral beam by usage of negative ions generated in pulse-time-modulated plasma. However, the mechanism has not been clarified. Recently we investigated the neutralization mechanism by using numerical simulations based on quantum mechanics [3-5] and succeeded in explaining higher neutralization efficiency of negative ions than positive ions. In this study, to obtain more reliable neutralization efficiency, we introduced consideration of excited states. A unit cell with a graphite surface and a chlorine particle (Cl, Cl+, Cl-, Cl2, or Cl2+) was used for the calculation. During a collision of the particle against the graphite, time evolution of wave functions of ion and graphite electrons was calculated by solving TD-Kohn-Sham equation [3, 4]. The probability of neutralization of an ion was estimated by calculating electron occupation numbers of each electron on each occupied and unoccupied molecular orbital. As a result, neutralization efficiencies of Cl- and Cl2+ ions were calculated as 87% and 77%, respectively. This result is consistent with our previous result that negative ions have higher neutralization efficiency due to resonant neutralization mechanism than that of positive ions with Auger neutralization mechanism. It was also found that electrons of the neutralized Cl2+ occupied some unoccupied orbitals of the grounded Cl2, namely Cl2+ became excited Cl2* by the collision. On the other hand, electrons of the neutralized Cl- did not occupy unoccupied orbitals of the grounded Cl, namely Cl- became grounded Cl by the collision. Since excited Cl2* are likely to be re-ionized to Cl2+ by the following collisions, so we consider the re-ionization process should be also an important factor to estimate the neutralization probability of Cl2+ ion. A part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO).[1] S. Samukawa et al, Jpn. J. Appl. Phys. 40, L779 (2001).[2] S. Samukawa, Jpn. J. Appl. Phys. 45, 2395 (2006).[3] N. Watanabe et al., Phys. Rev. E 65, 036705 (2002).[4] http://www.mizuho-ir.co.jp/solution/research/semiconductor/nano[5] T. Kubota et al., J. Phys. D: Appl. Phys. 45, 095202 (2012).