Method for forming semiconductor device

A method of forming a semiconductor device, comprising:

• providing a substrate having a transistor, wherein the transistor comprises a gate structure and a source/drain;
• forming a first dielectric layer covering the substrate;
• forming a first contact plug in the first dielectric layer, wherein the first contact plug is electrically connected to the source/drain;
• forming a second dielectric layer on the first dielectric layer;
• forming a mask layer on the second dielectric layer, wherein the mask layer comprises a multilayer structure having a first layer and a second layer stacked on the first layer;
• patterning the second layer to form a slot-cut pattern;
• forming a sacrificial mask layer on the slot-cut pattern;
• forming two contact slot patterns in the first layer and the sacrificial mask layer, wherein one of the contact slot patterns penetrates the sacrificial mask layer and the first layer, and another one of the contact slot patterns penetrates the sacrificial mask layer and partially penetrates the slot cut pattern;
• removing the sacrificial layer and the second layer;
• and
• forming a first contact slot in the second dielectric layer through the contact slot pattern in the first layer.