  Most of the current research in superconducting radio frequency (SRF)
cavities is focused on ways to reduce the construction and operating cost of
SRF based accelerators as well as on the development of new or improved cavity
processing techniques. The increase in quality factors is the result of the
reduction of the surface resistance of the materials. A recent test on a 1.5
GHz single cell cavity made from ingot niobium of medium purity and heat
treated at 1400 C in a ultra-high vacuum induction furnace resulted in a
residual resistance of about 1nanoohm and a quality factor at 2.0 K increasing
with field up to 5x10^10 at a peak magnetic field of 90 mT. In this
contribution, we present some results on the investigation of the origin of the
extended Q0-increase, obtained by multiple HF rinses, oxypolishing and heat
treatment of all Nb cavities.
