Page 32 - Chip Scale Review_May June_2021-digital

By means of high-energy sputtering
or thermal vapor deposition, glass
can be covered with a broad range
of metals. The metallization can
be precisely str uctured with f ine
lithographic techniques to provide
high-quality guiding lines for direct
current (DC), high-frequency (HF),
and microwave signals up to 50GHz.
In this manner, glass provides a high
integration potential by combining
electrical and optical functionalities in
one single EOCB substrate (Figure 3).
Finally, compared to metals, glass
has superior properties concerning a
low outgassing rate and permeability
for atmospheric gases, and therefore,
has successf ully been used as a
material for ultra-high vacuum (UHV)
2
Figure 2: Optical waveguides buried in ion-exchange process into a 400x300mm panel.
chambers for cooling and trapping of
increase the surface strength, panel- with a proper choice of the profile neutral atoms and ions, as well as for
size glasses benefit from the industrial of the waveguide’s refractive index, passive, high-vacuum spectroscopy
developments for touch displays. we are able to process single-mode cells for hot atomic vapors. The
Thin glass is an excellent material waveguides for the wavelengths down combination of these unique features
for integration of in-plane optical to 830nm. of glass is a basis for the concept of
waveguides (Figure 2), e.g., by ion- I n c o nt r a s t t o waveg u id e s o n integrated vapor cells presented in the
exchange (IoX) techniques [7]. Unlike s e m i c o n d u c t or pl at for m s , IoX- following section.
Si, the IoX-waveguides are low loss waveguides are low-contrast, weakly-
(<0.1dB/cm) in a broad spectral range guiding waveguides, which do not Integrated vapor cells
covering telecom down to visible allow for realization of curvature radii Glass vacuum cells can be considered
optical waveleng ths. In order to lower than a few millimeters. On the as a hermetic and optically transparent
optically address the single quantum other hand, a waveguide diameter of a package for atomic qubits. Such
emitters, the light intensity pattern few microns perfectly matches the core cells are commonly used in atomic
has to be well defined on a sub-micron diameter of optical fibers and thereby s p e c t r o s c opy. T hey a r e u s u a l ly
scale. The waveguides, therefore, ensures a low-loss interconnection made by means of glass blowing
have to necessarily be single mode in w i t hou t t h e n e e d of b e a m - s i z e techniques, have a volume of several
the entire spectral range. Currently, converting structures. cubic centimeters, and the atomic
vapor is excited by a free-propagating,
collimated laser beam.
MEMS technology contributed to
the miniaturization of glass vacuum
c el l s a nd t he s c a l a bi l it y of t he
manufacturing process [8]. The next
natural step is wafer- or panel-level
integration of miniaturized cells into
more complex hybrid quantum systems.
One challenging task here is hermetic
sealing of glass under vacuum. A
proven technique, anodic bonding,
is performed at high temperatures
above 300°C and can thereby damage
the optical IoX-waveguides. We are
working on more promising approaches
such as laser welding or soldering,
where glass is heated rapidly and
locally, only.
Another challenge currently targeted
at Fraunhofer IZM is the ability to
bend planar waveguides into the third
dimension in order to realize optical
Figure 3: Electro-optic circuit board.

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