P was implanted into 76‐mm‐diam (111) Si wafers at an energy of 40 keV with a dose of 2.54×1016/cm2. An increase in extended defects was found at a high insertion speed of wafer loading into a furnace at 1000–1100 °C in a N2ambient. The increase was due to nonuniformity in the temperature history and resultant thermal stress in the wafer, though no slip line was observed. The temperature nonuniformity was monitored by thermal‐oxide thickness nonuniformity for heating in a dry‐O2ambient. A low insertion speed resulted in uniform heating and reduced extended defects.