MOSFET: Threshold Adjustment by Ion Implantation
- The most valuable tool for controlling threshold voltage is ion implantation.
- Since very precise quantities of impurity can be introduced by this method, it is possible to maintain close control of VT.
- For example, Fig. given below illustrates a boron implantation through the gate oxide of a p-channel device such that the implanted peak occurs just below the Si surface.
- The negatively charged boron acceptors serve to reduce the effects of the positive depletion charge Qd As a result, VT becomes less negative.
- Similarly, a shallow boron implant into the p-type substrate of an n-channel transistor can make VT positive, as required for an enhancement device.
- If the implantation is performed at higher energy, or into the bare Si instead of through an oxide layer, the impurity distribution lies deeper below the surface.
- In such cases the essentially gaussian impurity concentration profile cannot be approximated by a spike at the Si surface. Therefore, effects of distributed charge on the Qd term must be considered.
- Calculations of the effects on VT in this case are more complicated, and the shift of threshold voltage with implantation dose is often obtained empirically instead.
- The implantation energy required for shallow VT adjustment implants is low (50-100 keV), and relatively low doses are needed.
- A typical VT adjustment requires only about 10 s of implantation for each wafer, and therefore this procedure is compatible with large-scale production requirements.
- If the implantation is continued to higher doses, VT can be moved past zero to the depletion-mode condition shown in figure given below.
- This capability provides considerable flexibility to the integrated-circuit designer, by allowing enhancementand depletion-mode devices to be incorporated on the same chip.
- For example, a depletion-mode transistor can be used instead of a resistor as a load element for the enhancement device.
- Thus an array of MOS transistors can be fabricated in an IC layout, with some adjusted by implantation to have the desired enhancement mode VT and others implanted to become depletion loads.
As mentioned, VT control is important not only in the MOSFETs but also in the isolation or field regions. In addition to using a thick field oxide, we can
do a channel stop implant (so called because it stops turning on a parasitic channel in the isolation regions) selectively in the isolation regions under the field oxide.
- Generally, a B channel stop implant is used for n-channel devices.