  Precise measurements of the cosmic microwave background (CMB) are crucial in
cosmology, because any proposed model of the universe must account for the
features of this radiation. Of all CMB measurements that the scientific
community has not yet been able to perform, the CMB B-mode polarization is
probably the most challenging from the instrumental point of view. The
signature of primordial gravitational waves, which give rise to a B-type
polarization, is one of the goals in cosmology today and amongst the first
objectives in the field. For this purpose, high-performance low-temperature
bolometric cameras, made of thousands of pixels, are currently being developed
by many groups, which will improve the sensitivity to B-mode CMB polarization
by one or two orders of magnitude compared to the Planck satellite HFI
detectors. We present here a new bolometer structure that is able to increase
the pixel sensitivities and to simplify the fabrication procedure. This
innovative device replaces delicate membrane-based structures and eliminates
the mediation of phonons: the incoming energy is directly captured and measured
in the electron bath of an appropriate sensor and the thermal decoupling is
achieved via the intrinsic electron-phonon decoupling of the sensor at very low
temperature. Reported results come from a 204-pixel array of Nb$_{x}$Si$_{1-x}$
transition edge sensors with a meander structure fabricated on a 2-inch silicon
wafer using electron-beam co-evaporation and a cleanroom lithography process.
To validate the application of this device to CMB measurements, we have
performed an optical calibration of our sample in the focal plane of a dilution
cryostat test bench. We have demonstrated a light absorption close to 20% and
an NEP of about 7$\times10^{-16}$ W/$\sqrt{Hz}$, which is highly encouraging
given the scope for improvement in this type of detectors.
