Semiconductor device and method for fabricating the same

1. A semiconductor device, comprising:

a magnetic tunneling junction (MTJ) on a substrate, wherein the MTJ comprises:

a bottom electrode layer, wherein the bottom electrode layer comprises a gradient concentration, the bottom electrode layer comprises tantalum nitride (TaN), a concentration of nitrogen increases from a bottom surface of the bottom electrode layer to a top surface of the bottom electrode layer, and a concentration of tantalum decreases from the bottom surface of the bottom electrode layer to the top surface of the bottom electrode layer;

a free layer on the bottom electrode layer; and

a top electrode layer on the free layer.

2. The semiconductor device of claim 1 , further comprising:

a pinned layer on the bottom electrode layer; and

a barrier layer on the pinned layer.

3. The semiconductor device of claim 1 , wherein a ratio of nitrogen to tantalum at a top surface of the bottom electrode layer is between 1.53 to 1.87.

4. The semiconductor device of claim 1 , wherein a ratio of nitrogen to tantalum at a bottom surface of the bottom electrode layer is between 0.18 to 0.22.

5. The semiconductor device of claim 1 , further comprising:

a metal interconnection under the MTJ;

an inter-metal dielectric (IMD) layer around the metal interconnection; and

an alignment mark adjacent to the metal interconnection.

6. The semiconductor device of claim 5 , wherein bottom surfaces of the metal interconnection and the alignment mark are coplanar.

7. The semiconductor device of claim 5 , wherein the alignment mark comprises:

a trench in the IMD layer; and

a spacer on sidewalls of the trench.