A method of forming a device includes the following steps. A multi-layer stack is formed, wherein the multi-layer stack includes a plurality of dielectric layers and a plurality of first sacrificial layers stacked alternately. A first trench is formed in the multi-layer stack. A memory material layer is formed on a sidewall of the first trench. A channel layer is conformally on the sidewall of the first trench and over the memory material layer. A plurality of conductive pillars are formed in the first trench.