  We developed a highly hydrogen-sensitive thermal desorption spectroscopy
(HHS-TDS) system to detect and quantitatively analyze low hydrogen
concentrations in thin films. The system was connected to an in situ
sample-transfer chamber system, manipulators, and an rf magnetron sputtering
thin-film deposition chamber under an ultra-high-vacuum (UHV) atmosphere of
~10^-8 Pa. The following key requirements were proposed in developing the
HHS-TDS: (i) a low hydrogen residual partial pressure, (ii) a low hydrogen
exhaust velocity, and (iii) minimization of hydrogen thermal desorption except
from the bulk region of the thin films. To satisfy these requirements,
appropriate materials and components were selected, and the system was
constructed to extract the maximum performance from each component.
Consequently, ~2000 times higher sensitivity to hydrogen than that of a
commercially available UHV-TDS system was achieved using H+-implanted Si
samples. Quantitative analysis of an amorphous oxide semiconductor InGaZnO4
thin film (1 cm x 1 cm x 1um thickness, hydrogen concentration of 4.5 x 10^17
atoms/cm3) was demonstrated using the HHS-TDS system. This concentration level
cannot be detected using UHV-TDS or secondary ion mass spectroscopy (SIMS)
systems. The hydrogen detection limit of the HHS-TDS system was estimated to be
~1 x 10^16 atoms/cm3, which implies ~2 orders of magnitude higher sensitivity
than that of SIMS and resonance nuclear reaction systems (~10^18 atoms/cm3).
