Ion Implantation and Synthesis of Materials - Studium

12.5 High-Dose Ion Implantation 169to be related to the Ar energy. However, the steady-state surface composition wasquite independent of the Ar energy.The majority of sputtered atoms emerge only from the outermost few atomiclayers. Therefore, with preferential sputtering, a compositional change is onlyexpected in the outermost few atomic layers versus a compositional change in adepth comparable to the ion range. The observed thickness of the altered layerrequires some atomic mixing or interdiffusion that can propagate the compositionalchange from the surface to the deeper region. Either the atoms move inwardto dilute the surface enrichment, or they move outward to replenish the depletionof preferentially sputtered atoms at the surface, eventually changing thecomposition over the whole layer.12.5 High-Dose Ion ImplantationIn high-dose implantation, the atomic mixing, sputtering, and chemical effectsdetermine the state of the material after implantation. In general, the maximumconcentration attainable by ion implantation is given by the reciprocal of thesputtering yield. This occurs because of the receding of the sample surface (due tosputter erosion) or, equivalently, because of the sputter removal of the implantedspecies. This maximum concentration is obtained after the sputter removal of alayer of thickness comparable to the ion range, R P (more exactly, R P + ∆R P ).However, more careful consideration should be given if there is preferentialsputtering between atoms of the host material and those of the implanted species.There is also an interesting variation when ion species A is implanted into acompound, AB. In this analysis it will be assumed that atomic mixing is veryefficient, so that the implanted species spreads out rather uniformly over aneffective width R P , after an initial amount of implantation, say 10 16 atoms cm −2 .The shape of the profile is assumed to remain approximately unchanged, with itsamplitude increasing with further implantation. This model is illustrated inFig. 12.8.The conservation of atoms requiresdNARP= Ji− JdtA(12.17)where N A is the atomic concentration of the implanted species, J i is the flux ofincident ions (of species A), and J A is the flux of the sputtered A atoms. Therelationship between J A , J B , and J i is the same as given by (12.6) and (12.7),namely J B /J A = r(N B /N A ) and (J A + J B ) = YJ i . The sputtering yield Y isapproximated as a constant throughout the implantation process, and the variable xis defined as x = N A /N B . Then, (12.6) and (12.7) give