[Zurück]

A. Bittner, H. Seidel, U. Schmid:
"High-Frequency Characterization of Porous Low-Temperature Cofired Ceramics Substrates";
Journal of the American Ceramic Society, 93 (2010), 11; S. 3778 - 3781.

In this work, the permittivity of porous low-temperature cofired
ceramics (LTCC) DP 951 is measured and evaluated. Porosification
is performed at originally dense LTCC substrates in the
fired state by a wet chemical etching procedure using hot phosphoric
acid. Choosing this approach, areas with tailored permittivity
can be generated in one single LTCC layer. The etch
time and the bath temperature precisely control the penetration,
and hence, the porosification depth. Therefore, the decrease in
dielectric constant of the LTCC substrate can be correlated to
the thickness of the porous layer. The dielectric constant is
measured using a ring resonator in the microstrip configuration.
From the resonances occurring in the transmission S-parameter
|S21| spectrum between 1 and 10 GHz, the relative dielectric
constant can be determined. Using 820-lm-thick substrates, a
relatively low reduction from er57.8 to 6.45 is achieved when a
porosification depth of about 35 lm is reached. Applying a fitting
procedure, the dielectric constant of the pure porous layer is
deduced to er52.3. Based on numerical simulations, the effective
dielectric constant for a 100-lm-thick glass-ceramic layer,
which is modified to a depth of 35 lm is calculated to 5.2,
whereas the thickness represents a lower limit for commercially
available tapes being typically implemented into the fabrication
process of monolithic LTCC systems with integrated metallization
planes. Compared with commercially available low-K LTCC
materials, this value is lower than all other commonly used tape
systems.

http://dx.doi.org/10.1111/j.1551-2916.2010.03928.x