This work investigates the integration of very thin sputtered Ta and reactively sputtered TaN barriers with Cu and a low-dielectric-constant (low-K) layer of poly(arylene ether) (PAE-2). It is found that Cu readily penetrates into PAE-2 and degrades its dielectric strength in metal-insulator semiconductor capacitors of Cu/PAE-2/Si structure at temperatures as low as 200 degrees C. Very thin Ta and TaN films of 25 nm thickness sandwiched between Cu and the low-K dielectric served as effective barriers during a 30 min thermal annealing at temperatures up to 400 and 450 degrees C, respectively. We propose a failure mechanism of outgassing induced gaseous stress of PAE-2 under the Ta film to explain its premature barrier degradation. The TaN barrier did not suffer from this gaseous stress problem because of its stronger adhesion to PAE-2 than that of Ta to PAE-2, leading to a better long-term reliability.