Isolation layers for reducing leakages between contacts

A method comprising:

• forming a first source/drain region, wherein the first source/drain region is aside of a gate stack;
• forming a contact etch stop layer over a part of the first source/drain region;
• forming an inter-layer dielectric over the contact etch stop layer;
• forming a first silicide region over the first source/drain region;
• forming a first source/drain contact plug over and contacting the first silicide region;
• and
• forming a first isolation layer, wherein the first isolation layer encircles the first source/drain contact plug, wherein in a top view of the first source/drain contact plug, the first source/drain contact plug is elongated, and the first isolation layer comprises:
• an end portion at an end of the first source/drain contact plug;
• and
• a middle portion between opposing ends of the first source/drain contact plug, wherein a first end-portion thickness of the end portion is greater than a first middle-portion thickness of the middle portion.

A method comprising:

• forming gate spacers over a semiconductor region;
• forming a gate stack over the semiconductor region, wherein the gate spacers are on opposite sides of the gate stack;
• forming a hard mask comprising a first portion over one of the gate spacers, and a second portion between the gate spacers;
• forming a silicide region;
• forming a source/drain contact plug overlying and contacting the silicide region, wherein the source/drain contact plug is aside of the gate stack;
• and
• forming an isolation layer, wherein the isolation layer is spaced apart from the silicide region, wherein after both of the source/drain contact plug and the isolation layer are formed, the isolation layer encircles the source/drain contact plug, and wherein in a top-view of the isolation layer, the isolation layer has a non-uniform thickness.