We report boron penetration and thermal instability of p+ polycrystalline-Si (poly-Si)/ZrO2 (100 Å)/SiO2 (∼7 Å)/n-Si metal-oxide-semiconductor (MOS) structures. The flatband voltage shift (ΔVFB) of the p+ poly-Si/ZrO2/SiO2/n-Si MOS capacitor as determined by capacitance–voltage measurement was ∼0.18 V, corresponding to a p-type dopant level of 1.1×1012 B ions/cm2 as the activation temperature increased from 800 to 850 °C. Additional ΔVFB of ∼0.24 V was measured after the anneal from 850 to 900 °C. Noticeable boron penetration into the n-type Si channel as observed by secondary ion mass spectroscopy also confirmed the VFB instability with activation annealing above 850 °C. An abnormal decrease of accumulation capacitance was also found after anneal at 900 °C due to an excessive leakage current which was attributed to the formation of ZrSix nodules at the poly-Si/ZrO2 interface. We observed 4–5 orders of magnitude lower leakage current from the small-size capacitors (≪50×50 μm2) up to the activation anneal of 850 °C for 30 min, while the formation of interfacial ZrSi- x nodules at 900 °C cannot be avoidable even at 0.6-μm-wide gate lines. © 2002 American Institute of Physics.