Study of BF 2 ion implantation into crystalline silicon: Influence of fluorine on boron diffusion
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Study of BF2 ion implantation into crystalline silicon : Influence of fluorine on boron diffusion Lilya Ihaddadene -Lecoq, Jerome Marcon and Kaouther Ketata Laboratory of Electronic Microtechnology and Instrumentation (LEMI) University of ROUEN, 76821 Mont saint Aignan, FRANCE.
ABSTRACT We have investigated and modeled the diffusion of boron implanted into crystalline silicon in the form of boron difluoride BF2+. Low energy BF2+ 1x1015 cm-2 implantations at 2.0keV were characterized using Secondary Ion Mass Spectrometry (SIMS) in order to measure dopant profiles. RTA was carried out at 950°C, 1000°C, 1050°C and 1100°C during 10s, 20s, 30s and 60s. The results show that concentration profiles for BF2+ implant are shallower than those for a direct B+ ion implantation. This could be attributed to the presence of fluorine which trap interstitial Si so that interstitial silicon supersaturation is low near the surface.
INTRODUCTION As the semiconductor industry continues to scale down silicon based microelectronics devices, the most challenge is to achieve shallower junction depths. Several experimental and theoretical investigations on the diffusion of fluorine and its effect on the boron diffusion and activation have been published in the literature [1-5]. It has been previously reported that the boron difluoride BF2+ yields shallower junctions than B when implanted into single crystal silicon at the same effective energy [6,7]. The implantation of the boron difluoride BF2+ has two major advantages : (1) the reduction of effective B implantation energy due to the mass ratio of boron to BF2+, and (2) the BF2+implanted silicon becomes amorphous [8]. Previous experiments have shown that fluorine can significantly inhibit boron TED (Transient Enhanced Diffusion) with improved short channel effects [9,10], enhance boron solubility and reduce sheet resistance [5]. Several previous studies within crystalline silicon have suggested the presence of fluorine as a sink for excess silicon interstitials [11] and described the ability of fluorine to bind with these silicon interstitials [5]. Therefore, in this paper we will present our work on the study of the diffusion of boron implanted into crystalline silicon in the form of boron difluoride BF2+. EXPERIMENTS In Czochralski n-type silicon wafers of 3-10 Ω cm, BF2+ was implanted at 2.0keV with a dose of 1.0x1015 cm-2. The rapid thermal annealing (RTA) was performed at different temperatures 950°C, 1000°C and 1050°C during 10s, 20s, 30s and 60s. The resulting samples were depth profiled in CAMECA IMS-4F SIMS analyser equipped with oxygen source at 3keV. RESULTS AND DISCUSSION Few studies have been devoted to accurately predict the as-implanted fluorine profiles for either fluorine implants [12] or BF2+ implants [13]. In [13], preliminary results of molecular dynamics
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(MD) simulation results were presented for BF2+ implants. In [12], fluorine implants were simulated with UT-MARLOWE’s “universal” model but this model is clearly inadequate for simulating the deeply
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