Method of forming isolation layer

A method of forming an isolation structure, comprising:

• providing a vertical structure that includes a source region, a channel region over the source region and a drain region over the channel region, the channel region and the drain region protruding from an isolation layer;
• forming a gate dielectric layer over the channel region and the drain region;
• forming one or more conductive layers over the gate dielectric layer;
• forming a first ILD layer over the one or more conductive layers, the first ILD layer including a top layer, a middle layer and a bottom layer;
• performing first CMP on the first ILD layer, and stopping the first CMP so that part of the middle layer remains;
• performing second CMP to remove the part of the middle layer and the bottom layer, and stopping the second CMP upon reaching an uppermost layer of the one or more conductive layers;
• and
• etching back the bottom layer to a level corresponding to the channel region,
• wherein the top layer is fully removed in the first CMP and the middle layer is fully removed in the second CMP.

A method of forming an isolation structure, comprising:

• providing a shallow trench isolation in a substrate;
• providing a vertical structure that includes a source region, a channel region over the source region and a drain region over the channel region, the channel region and the drain region protruding from an isolation layer, the isolation layer including an etch stop layer formed on the source region and the shallow trench isolation and a first ILD layer;
• forming a gate dielectric layer over the channel region and the drain region;
• forming one or more conductive layers over the gate dielectric layer;
• providing a second ILD layer over the one or more conductive layers;
• performing CMP on the second ILD layer, and stopping the CMP before reaching an uppermost layer of the one or more conductive layers disposed on an top of the vertical structure, thereby leaving a remaining layer of the second ILD layer to cover a highest portion of a structure disposed over a substrate;
• and
• etching back the second ILD layer with the remaining layer to a level corresponding to the channel region,
• wherein a thickness of the remaining layer of the second ILD layer from the highest portion is in a range from 10 nm to 100 nm.