Patent ID: 9691884
Date: 2017-06-27
CPC Classifications: H01L

Claim:
1. A method of making a monolithic three dimensional NAND string, comprising: forming a stack of alternating first material layers and second material layers over a major surface of a substrate, wherein each of the second material layers comprises a layer of a first silicon oxide material between two layers of a second silicon oxide material different from the first silicon oxide material, and the first material layers comprise a material that is different than the first silicon oxide material and the second silicon oxide material; etching the stack to form a front side opening in the stack; forming a cover layer of an insulating material over the sidewall of the front side opening in the stack; forming at least a portion of a memory film over a sidewall of the cover layer within the front side opening; forming a semiconductor channel in the front side opening such that at least a portion of the memory film is located between the semiconductor channel and the sidewall of the front side opening; etching through the stack to form a back side opening in the stack; forming first backside recesses by removing first material layers and portions of the cover layer at each level of the first material layers selective to the second material layers by introducing an etchant through the back side opening; forming electrically conductive layers in the first back side recesses; removing each layer of the first silicon oxide material selective to remaining discrete segments of the cover layer and the layers of the second silicon oxide material to form second back side recesses after formation of the electrically conductive layers in the first back side recesses; and depositing an additional insulating material over a sidewall of the back side opening to enclose the second back side recesses each of which is present between respective two layers of second silicon oxide material, wherein an air gap is formed between the respective two layers of second silicon oxide material in each of the second back side recesses, and a vertical sidewall of a respective one of the remaining discrete segments of the cover layer is physically exposed to the air gap after depositing the additional insulating material over the sidewall of the back side opening.