Highly stable HoScO3 with different thicknesses as the Cu diffusion barrier on Si is prepared and characterized. The ultrathin (3 nm) HoScO3 barrier scheme is stable after 50 h annealing at 400 degrees C, while its electrical resistivity shows no noticeable increase after annealing at 600 degrees C for 1 h. Both the 5 and 10 nm thick barrier schemes are stable up to 700 degrees C according to the resistivity result. Transmission electron microscopy and Auger electron spectroscopy reveal that the ultrathin amorphous barrier is uniformly present at the interface after annealing at high temperatures. The results indicate that the 3 nm thick HoScO3 is an effective diffusion barrier for copper metallization.