Residual pressure effects on SEM/SE YBCO image brightness versus temperature

It has been reported on several occasions that, during investigations in a scanning electron microscope (SEM), a reduction of secondary electron (SE) image brightness takes place on cooling high-Tc superconductors from room temperature to liquid nitrogen temperature. The SE image signal shows a continuous and initially slow decrease, then, when the specimen temperature is close to that of liquid nitrogen, a sudden drop (by about 15%) takes place. The effect appears to be reversible. Several hypotheses have been put forward to explain this effect; however, none is entirely satisfactory. Recent experimental results indicate that the effect is related to the residual atmosphere in the specimen chamber. To check this idea, a mass spectrometer was incorporated in an SEM. Combined mass spectroscopy and X-ray spectroscopy have proved that, during cooling, several gases condense on the specimen and form a spurious thin film on its surface, which reduces the SE emission. Subsequent specimen heating produces evaporation of the film, thus restoring the initial brightness of the signal.