Bottom lateral expansion of contact plugs through implantation

A structure comprising:

• a gate stack;
• a gate spacer contacting a sidewall of the gate stack;
• a source/drain region aside of the gate spacer;
• a silicide region over and contacting the source/drain region;
• a first inter-layer dielectric;
• a first contact plug over and contacting the silicide region, with the first contact plug being in the first inter-layer dielectric;
• an etch stop layer over and contacting the first contact plug;
• a second inter-layer dielectric over and contacting the etch stop layer;
• and
• a second contact plug comprising:
• a first portion in the second inter-layer dielectric;
• a second portion in the etch stop layer, wherein at least a lower part of the second portion extends laterally beyond edges of the first portion, wherein the lower part of the second portion has a curved surface;
• and
• a third portion extending into the first contact plug, wherein the third portion extends laterally beyond edges of the second portion.

A structure comprising:

• a first dielectric layer;
• a first contact plug in the first dielectric layer, wherein upper portions of the first contact plug and the first dielectric layer comprise a dopant having first dopant concentrations, and lower portions of the first contact plug and the first dielectric layer are free from the dopant, and wherein the dopant extends deeper into the first dielectric layer than into the first contact plug;
• a second dielectric layer over the first contact plug;
• and
• a second contact plug comprising:
• a first portion penetrating through the second dielectric layer;
• and
• a second portion in the first contact plug, wherein the second portion is laterally wider than the first portion, and wherein the first portion and the second portion are continuously connected without distinguishable interface in between.