An alternating stack of insulating layers and sacrificial material layers is formed over a substrate, and memory stack structures are formed through the alternating stack. A backside trench is formed through the alternating stack, and backside recesses are formed by removing the sacrificial material layers. An undoped aluminum oxide backside blocking dielectric layer is formed in the backside recesses and on sidewalls the backside trench. A portion of the undoped aluminum oxide backside blocking dielectric layer located at an upper end of the backside trench is converted into a carbon-doped aluminum oxide layer. An electrically conductive material is deposited in the backside recesses and at peripheral regions of the backside trench. The electrically conductive material at the peripheral regions of the backside trench is removed by an etch process, with the carbon-doped aluminum oxide layer providing etch resistivity during the etch process.