Fundamental Operation of Halo Well Profiles
Halo profiles are created by implanting extra dopants into the wells immediately after tip implantation. The implant is typically performed at an angle and energy high enough to ensure the implant dose is outside the final SDE profile. After spacer processing and S/D anneal, the resulting profile diffuses due to TED effects, resulting in a relatively flat profile over the dimensions of current device sizes. Figure 34 shows experimental results for the as implanted and final doping profile for a typical boron halo implant. The data includes the effect from damage generated by the SDE and S/D implants. As can be seen, the profile is quite flat over the characteristic channel length dimensions for today’s 0.25?m and 0.18?m technologies. However, even though the halo profile is relatively flat, it still causes an increase in well doping as the gate length is decreased. This is because the same halo implant dose is confined in a smaller area. For flat well devices, IOFF quickly decreases as the channel length is increased. This is due to the exponential relationship between the current and the potential barrier in the subthreshold region. For the halo cases, the leakage current does not decrease as quickly with size. In fact, for extremely strong halos, an increase in IOFF with increasing size can be seen. This can be explained by the change in the source-to-drain potential barrier for different size devices in the case of the halo well. For the strong halo devices, the threshold voltage is rapidly decreasing as the device size increases.
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