Concentrations in High-Dose Ion Implantation

Consider the implantation of atomic species A into the alloy AB. Having an AB alloy to begin with sets a different boundary condition at the steady state. Instead of JA = Ji as given in (12.8), the new condition is 

This gives the steady-state surface composition. 

When Y 1, (12.26) reduces to NA/NB = r, which is the same result as inert-gas ion sputtering of alloy AB. On a physical basis, this is because, with high sputtering yield Y, very few of the implanted atoms are retained in the material. 

For the example of implantation into PtSi, the steady-state surface composition is given by (12.26), i.e., NSl/Nn = r(Y + )/(Y- 1), with r = 1/2. The surface composition is therefore dependent upon the total sputtering yield, Y. This dependence is plotted in Fig. 12.10. For Y 3, the implanted PtSi sample becomes depleted of Si, because Y is sufficiently large that not enough implanted Si atoms stay in the sample to overcome the preferential sputtering of Si. For Y= 3, the Si-implanted PtSi sample remains PtSi. For Y 3, more implanted Si atoms stay in, and the sample becomes Si enriched. 

Since the majority of sputtered atoms have relatively low energies and emerge from the first few atomic layers near the surface, the probability of sputtering is very sensitive to surface conditions. A thin layer of surface contaminants or oxide 

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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. 

In ion implantation the gaseous dopant is ionized and accelerated into the wafer. SIMS has been used extensively to obtain dopant concentration depth profiles because it is the most sensitive of the surface analytical techniques and because sputtering is an intrinsic part of the dynamic SIMS process (40). AES, combined with ion sputtering, has also been used to obtain depth profiles for high dose implants (41). 

An excellent way to create standards is ion implantation of the elements of interest into the matrix. This works exceptionally well for semiconductors since one can usually start with high-purity single-crystal materials that represent the matrix of interest. Also the use of Eq. (4.8) is well suited for this purpose since ion implanters usually quote doses in atoms per square centimeter. However, Eq. (4.5) serves just as well by converting the matrix concentration to atoms per cubic centimeter. In this procedure, the implant profile is sputtered through, the implant element secondary ions and the matrix element secondary ions are each summed, and the depth of the sputter profile is determined, usually by using a stylus profilome-ter. The sensitivity factor is then calculated from... 

During ion implantation in Si, the vacancy concentration can be very high. Using SRIM, calculate the vacancy concentration at the damage peak for 40 keV B implanted to a dose of 1 x 1014 B cm-2. The sample will be heated to 1,000°C for 10 s to activate the B. What enhancement is expected in the diffusivity due to radiation-enhanced diffusion ... 

Fig. 12.4. Schematic view of the development of the concentration profile of ions implanted from low (L) to high (H) doses. The projected range, RP, in this example is 100 nm (from Hubler, NRL Memorandum Report 5928, 1987)...

A major driving force in the development of the ion beam mixing process is the process s ability to produce ion-modified materials with higher solute concentrations at lower irradiation doses than can be achieved with conventional high-dose implantation techniques. A case-in-point is the formation of Au-Cu alloys on Cu substrates (Fig. 13.2). 

The implantation of MeV Si ions into a Si substrate can also suppress boron-enhanced diffusion normally associated with a high boron concentration layer (Shao et al. 2003). Junction depths of 20 nm were achieved in samples implanted with 0.5 keV B ion at a dose of 1015 cm-2 following a 1,000°C thermal anneal. 

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