Concentrations of Implanted Species

The steady-state compositions are the same as those predicted by (12.10), i.e. N/JNb = r(Y- 1). Note that for r = 2, it takes about twice as much sputtering to reach the steady-state composition. 

This section will examine the influence of sputtering yield, Y, and the sputtering parameter r, on the buildup of the concentration of implanted species. Specific examples will include implanting Pt and Si into Si, Pt and PtSi. 

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Gas bubble formation and blistering effects have been widely observed in high-dose implantations of inert-gas ions. Backscattering measurements of depth distributions often show very low concentrations of implanted species in the nearsurface region. This indicates that the inert-gas atoms can escape from the material even without sputtering. In these cases, the simple model described in the previous sections does not apply. 

II.2a) Reactive primary ion bombardment Yield enhancement is brought about by direct implantation of a reactive species in the sample by using an O2+ (or 0") or a Cs+ primary beam. Liebl [5] has described the build-up in the surface concentration of implanted species during sputter erosion. After sputtering to a depth of approximately Rp + 2ARp, where Rp is the projected range and p the standard deviation, the sputter rate of previously implanted primary ions will exactly balance the implantation rate. In this steady state situation, an equilibrium surface composition is achieved and the secondary ion yields stabilize. Figure 5 shows that upon commencement of bombard-... 

Quantitative chemical information is somewhat easier to come by from an in vivo experiment. Mini- or microelectrodes should be calibrated with standard solutions of well-behaved electroactive species before and, if possible, after implantation and experimentation. o-Dianisidine (3,3 -dimethoxyben-zidine) in 0.1 M H2SO4 was found very satisfactory. After implantation (and, in the chronic experiments, recovery), data can be obtained using chronoam-perometry. Using the calibration data as a standard it is possible to monitor the concentration of all species that are oxidizable at the applied potential. 

It should be noted that even at a low energy of implanted species (100 keV) the size of nanopores that are formed in the implanted layer turns out to be enough to make the insertion of large molecules possible (for instance, the dicarbollyl complex of cobalt readily diffuses into polyethylene implanted with 150-keV ions [75]). In the case of energetic ions (with energies of several hundreds of MeV), the pore size increases and the implanted polymer can be doped with fullerenes [61]. Thus, the concentration of C o molecules that difhise into polyimide implanted with 500-MeV ions from toluene solution amounts to as much as 1.8 x 10 fullerene molecules per track (the fullerene concentration was evaluated by a neutron depth profiling technique using Li ions, known to form the insoluble adduct with Cfio as the tracer [61]). 

Dose levels of implanted species can be well controlled to ensure that peak concentrations remain below 1 atomic %. As covered in Section 3.3.2, additional matrix effects can be introduced if the concentration lies above this limit particularly if chemically reactive elements are introduced... 

To obtain the peak atomic concentration resulting from this peak number of implanted ions requires knowing N, the atomic density of the substrate. The general relation for the concentration of the implanted species at the peak of the distribution is given by equation 13 ... 

In contrast to other analytical methods, ion-selective electrodes respond to an ion activity, not concentration, which makes them especially attractive for clinical applications as health disorders are usually correlated to ion activity. While most ISEs are used in vitro, the possibility to perform measurements in vivo and continuously with implanted sensors could arm a physician with a valuable diagnostic tool. In-vivo detection is still a challenge, as sensors must meet two strict requirements first, minimally perturb the in-vivo environment, which could be problematic due to injuries and inflammation often created by an implanted sensor and also due to leaching of sensing materials second, the sensor must not be susceptible to this environment, and effects of protein adsorption, cell adhesion, and extraction of lipophilic species on a sensor response must be diminished [13], Nevertheless, direct electrolyte measurements in situ in rabbit muscles and in a porcine beating heart were successfully performed with microfabricated sensor arrays [18],... 

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