The present invention provides a nonvolatile memory device having high reliability with novel sidewall spacer structures. The gate stack structure for use in a nonvolatile memory device comprises a semiconductor substrate, a gate stack formed on the semiconductor substrate. The gate stack has a sidewall and a top surface. A multi-layer sidewall spacer structure is formed on the sidewall of the gate stack. The multi-layer sidewall spacer structure includes a first oxide layer, a first nitride layer, a second oxide layer, and a second nitride layer that are sequentially stacked. With the present invention, even if the second nitride layer is perforated or damaged during the formation of contact holes, sidewalls of the gate stack of nonvolatile memory cell can be protected with the first nitride layer from mobile ions. Also, etching damage to source/drain regions or field regions can be reduced.