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SIMS has been utilized for depth profiling of dopant distribution in ULSI devices. However, it is difficult to acquire accurate depth profiles of ultra-shallow junctions in advanced devices even if conventional ion beams of a few hundred eV are used, because of the degradation of the depth resolution caused by atomic mixing. One of the solutions to improve the depth resolution is cluster ion bombardment. Since the energy of the constituent atoms of a cluster ion that includes many atoms is low, the use of cluster ions is expected to reduce atomic mixing. Therefore, we have developed a prototype cluster ion source using the metal cluster complex of Ir4(CO)12 [1,2]. The ion source emits a uniform-size cluster ion beam of Ir4(CO)7+ (molecular weight: 964.9), which has 18 atoms including heavy atoms of iridium.
Boron delta-doped silicon samples were analyzed using the cluster ion with oxygen flooding, and the depth resolution that depends on primary ion conditions was investigated. The cluster ion bombardment provides better depth resolution than that by oxygen ion bombardment at comparable impact energies and angles, because atomic mixing is suppressed. The depth resolution under the cluster ion bombardment decreased monotonously from 10 keV to 5 keV. The depth resolution was also affected by surface roughness. We found that surface roughness was suppressed at small incident angles. The best depth resolution of ~0.9 nm was obtained under the cluster ion bombardment of 5 keV, 45 with oxygen flooding [3].
However, at less than 5 keV, the depth resolution did not improve with decreasing energy even though surface roughness was suppressed. One possible reason for the limit of depth resolution is carbon on the sputtered surface, which was originated from the cluster ion. The cover-layer of carbon may suppress the surface oxide growth that buffers atomic mixing, and therefore, it can degrade depth resolution. To improve depth resolution, it will be necessary to eliminate carbon from the cluster ion.
[1] T. Fujimoto et al., Surf. Interface Anal. 37 (2005) 164.
[2] Y. Fujiwara et al., J. Appl. Phys. 100 (2006) 043305.
[3] M. Tomita et al., Appl. Phys. Lett. 89 (2006) 053123.
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