Methods of semiconductor device fabrication

Abstract

Aspects of the disclosure provide a semiconductor device and a method to manufacture the semiconductor device. The semiconductor device includes a string of transistors stacked in a vertical direction over a substrate of the semiconductor device having a channel structure extending in the vertical direction. The string of transistors includes first, second, and third substrings of transistors that are arranged along first, second, and third portions of the channel structure, respectively. Gate structures of transistors in the first, second, and third substring are separated by respective first, second, and third insulating layers and the second insulating layers have a higher etch rate than that of the third insulating layers.