FinFET structure and method for manufacturing thereof

A FinFET structure, comprising:

• a substrate;
• an insulating layer over the substrate, wherein a first portion of the insulating layer comprises a first dopant concentration of a dopant, and the first portion of the insulating layer interfaces with the substrate;
• a fin protruding from the substrate, comprising a first portion and a second portion below the first portion, wherein a first section of the second portion of the fin has a highest dopant concentration of the dopant among the fin, the first portion of the fin is above a top surface of the insulating layer, the second portion of the fin is at a position below the top surface of the insulating layer, the first section of the second portion of the fin comprising a second dopant concentration of the dopant, the first portion of the fin comprises a dopant concentration less than the first dopant concentration, and an upper boundary of the first portion of the insulating layer is below the first section of the second portion of the fin;
• and
• a gate over the fin.

A FinFET structure, comprising:

• a substrate;
• a fin protruding from the substrate, comprising:
• a first portion, wherein the first portion comprises a first dopant concentration of a dopant;
• a second portion below the first portion, wherein the second portion comprises a second dopant concentration of a dopant, wherein the second dopant concentration is greater than the first dopant concentration;
• and
• a third portion below the second portion, wherein the third portion comprises a third dopant concentration of a dopant;
• a gate over the fin, wherein the second portion of the fin is below a bottom surface of the gate;
• and
• an insulating layer over the substrate and proximal to the second portion of the fin, wherein the insulating layer comprises:
• a first part, comprising a fourth dopant concentration of the dopant, wherein an upper boundary of the first part of the insulating layer is below the second portion of the fin, wherein the fourth dopant concentration is greater than the first dopant concentration, and a lower boundary of the first part of the insulating layer coincides a lower boundary of the insulating layer;
• and
• a second part adjacent to the first part, comprising a fifth dopant concentration of the dopant, wherein the fourth dopant concentration is greater than the fifth dopant concentration.