Plasma-Induced Deactivation Of P, B, Sb By Low-Energy (<30 eV) Ion Bombardment During Low-Temperature Silicon Epitaxy

Abstract

We have experimentally shown for the first time that threshold energies of plasma-induced deactivation for phosphorus, boron and antimony in silicon epitaxy by using a low-energy ion bomibardment process [l-41. The deactivation energy of phosphorus, boron and antimony at a growing silicon film surface is -13 eV, -5 eV and -10 eV respectively as shown in Table 1. Since the deactivation energy of boron is extremely small (< 5 eV), ion bombardment energy must be precisely controlled to be lower than 5 eV in order to make the activation ratio of dopants 100 %. The experimental results of plasma-induced deactivation energy of dopants will be crucial value for plasma processing, especially for low temperature processing using ion bombardment processes. Figure 1 schematically shows an rfdc coupled mode bas sputtering system [l] used in silicon epitaxy. The kinetic energies of ions incident on the target and the substrate were independentlly determined by two external dc voltages applied to the sputtering target and the substrate, respecitiiely. Prior to the film deposition, in situ surface cleaning [1,3] under the optimized condition was carried out to remove physically adsorbed molecules onto the wafer surface during the air exposure in a clean room. The deposition rate was controlled to be 10 ndmin in this work. The sputtering target material was phosphorus (2-3x10‘’ cmS), boron ( 6 ~ 1 0 ’ ~ cmS) antimony ( 3 ~ 1 0 ’ ~ cmS) doped silicon. The crystallinity of grown silicon films was evaluated by reflective high energy electron diffraction (RHEED) analysis. The resistivrty of the in situ doped epitaxial silicon film was measured by a four-point probe method. Figure 2 shows the resistivity of a silicon film deposited using the antimony doped silicoin target as a function of the ion bombardment energy. The substrate temperature was kept at 3150’C during the deposition. Normalized ion flux which is defined as the number of bombarding argon ions per each deposited Si atom is the same in all points. In region (I), ion bombardment energy is not enough to enhance silicon film growth. Crystal structure of the grown film is polycrystal or aimorphous. Increase of resistivlty is caused by this degraded crystallinity. This situation is schemalically illustrated in Fig. 3(1). In region (II), antimony impunty was fully activated and perfect single cirystal was achieved. Kikuchi lines were observed in the RHEED photograph. It indicates that the crystallinlty of the as-deposited silicon epitaxial layer is high quality single crystal. This is also illustrated in Fig. 3(11). In region (Ill), antimony impurity was not fully activated in the as-deposited silicon epitaxial layer. However, the RHEED photograph showed Kikuchi lines. This clearly shows that antimony is unstable at growing silicon surface and easily displaced from lattice site by ion bombardment higher than -10 eV. This situation is illustrated in Fig. 3(111). In this article, deactivation energy of dopant at the growing film surface is determined by the threshold energy separating region (11) from region (111). Therefore, the deactivation energy of antimony is -10 eV. Figure 4 and Figure 5 summarizes the results of phosphorus dopoed and boron dopoed silicon epitaxy at 350°C using argon ion bombardment respectively. The substrate temperature was 3!50°C. The vertical axis represents the normalized ion flux, while the horizontal axis represents the ion bombardment energy. The boundary line separating region (11) from region (111) is -5 eV for boron and -13 eV for phosphorus. Single crystal film with completely activated boron can be realized in extremely small region denoted as region (11). The ion bombardment energy must be less than -5 eV to cbtain 100% boron activation ratios. Compared with antimony doped or phosphorus doped silicon epitaxy, it is extremely difficult to achieve highquality silicon film with 100% boron activation ratios. In conclusion, we have shown the threshold energies of plasma-induced deactivation for phosphorus, boron and antimony in silicon epitaxy. In order to to achieve perfect dotpant activation ratio, plasma parameter especially ion bombardment energy must be precisely controled to be less than the deactivation energy of dopant. This discovery will be a critical issue in plasma processing and guide us to realize high quality film in silicon epitaxy using a low-energy ion bombardment.