Source/Drain Design - Lightly doped drain

Introduction: The extension close to the channel has shallower junction depth that minimizes short-channel effects. Sometimes, it is less heavily doped to diminish the lateral field for considering hot-carrier aging.

Source/Drain Design:
  For this purpose, it is referred to as lightly doped drain (LDD). The deeper junction depth away from the channel helps in minimizing the series resistance.

The gradient or sharpness of the source/drain profile is crucialfor minimizing series resistance as pointed out earlier. Fig. below serves as a reference to understand its origin.

The profile is never perfectly abrupt practically, and there is a region of accumulation layer (of n-type) before current starts spreading in bulk of the source/drain. This accumulation-layer resistance Rac is associated with the transition distance prior to doping attaining a critical level.

The development of the silicide contact technology, which began in early 1990s, was a milestone for source/drain design. In contrast to metal contact, silicide can be made to self-align to the gate, therebydiminishing the sheet-resistance component (Rsh) between contact and the channel.

In this manner,silicide becomes the metal contact as the contact resistance between metal and silicide is very insignificant. This self-aligned silicide process has been coined silicide which is described as follows.

After the gate definition, an insulator spacer is created on the gate sides. A metal layer for silicidation is uniformlydeposited and at this stage shortens the gate and source/drain. After thermal reaction occurs at low temperature (= 450°C), metal reacts with silicon on the sourceldrain regionto form silicide.

Its formation on the gate is optional and it depends on whether the gate is capped or not with insulation layer i.e. a part of the gate stack. Metal found over the spacer region and the field region (between transistors, i.e. not shown) remains as metal since no exposed silicon is available for reaction.

Then, metal is removed with a selective chemical that only etches metal without etching silicide, thereby eliminating the shorting paths. It is to be noted that silicide/silicon interface is somewhat recessed due to the intake of silicon during formation of silicide.

Examples of salicidesare , Nisi2, CoSi2, TiSi2and PtSi.

No comments:

Post a Comment