Tunnel junctions with AI-N barriers fabricated by microwave-excited plasma were studied. When the Al thickness, nitridation time, and annealing temperature were 1 nm (0.8 nm), 50 s (35 s), and $280^{circ}C$ ($300^{circ}C$), TMR ratio and resistance-area product (RA) were 49% (34%) and $3 ${ imes}$ 10^4$</TEX> $Omega$$mum^2$ ($1.5 ${ imes}$ 10^4$</TEX> $Omega$$mum^2$), respectively. In order to clarify the annealing temperature dependence of TMR ratio, the local transport properties were measured for Ta 5 nm/Cu 20 nm/Ta 5 nm$29_{76}$ $Fe_{24}$ 2 nm/Cu 5 nm/M $n_{75}$$Ir_{25}$ 10 nm/ $Co_{71}$ $Co_{29}$ 4nm/Al-N junction with Al thickness of 0.8 nm and nitridation time of 35s at various temperatures. The increase of TMR ratio after annealing at $300^{circ}C$, where the TMR ratio of the corresponding MTJ had the maximum value of 34%, can be well explained by the enhancement of the average barrier height ($Phi_{ave}$) and the reduction of its fluctuation. After further annealing at $340^{circ}C$, the leakage current was observed and the TMR ratio decreaseded