  One of the main limitations in state-of-the art solid-state quantum
processors are qubit decoherence and relaxation due to noise in their local
environment. For the field to advance towards full fault-tolerant quantum
computing, a better understanding of the underlying microscopic noise sources
is therefore needed. Adsorbates on surfaces, impurities at interfaces and
material defects have been identified as sources of noise and dissipation in
solid-state quantum devices. Here, we use an ultra-high vacuum package to study
the impact of vacuum loading, UV-light exposure and ion irradiation treatments
on coherence and slow parameter fluctuations of flux tunable superconducting
transmon qubits. We analyse the effects of each of these surface treatments by
comparing averages over many individual qubits and measurements before and
after treatment. The treatments studied do not significantly impact the
relaxation rate $\Gamma_1$ and the echo dephasing rate $\Gamma_2^\textrm{e}$,
except for Ne ion bombardment which reduces $\Gamma_1$. In contrast, flux noise
parameters are improved by removing magnetic adsorbates from the chip surfaces
with UV-light and NH$_3$ treatments. Additionally, we demonstrate that SF$_6$
ion bombardment can be used to adjust qubit frequencies in-situ and post
fabrication without affecting qubit coherence at the sweet spot.
