C.3 Acoustically Optimized Matrix Varactor Design
D.1.2 Influence of the Processing Routine on Electrical Properties
In regard to custom made bulk ceramics, the impact of the manufacturing process on the electrical properties of the obtained pellets is evaluated. From a large variety of manufac-turing parameters of the ceramic powder as well as of the varactor pellets, the sintering temperature and duration is evaluated more closely. The reason for this is the grain size effect [97], which is known to have significant influence on the electrical behavior of a technical ceramic. Especially, the effect of giant grain growth and the creation of liquid phases in the ceramic is a source for large amorphous crystal domains, which significantly increase the permittivity, tunability but also dielectric losses of the ceramic. The suitability of this effect for the manufacturing of highly tunable bulk ceramic varactors is investigated.
As a base material, custom made Ba0,7Sr0,3TiO3co-doped with 0.5 % manganese is set up.
For sintering, three one-step and one two-step profile are applied. The exact profiles are 1250◦C for 10 h plus 1290◦C for 1 h, 1290◦C for 1 h, 1330◦C for 1 h and 1360◦C for 1 h. The ceramic texture of the obtained ceramic sintered at at 1290◦C for 1 h and at 1330◦C for 1 h are depicted in figure D.4.
Figure D.4: SEM cross-sectional image of Ba0,7Sr0,3TiO3co-doped with 0.5 % manganese sintered 1290◦C for 1 h (left) and sintered at 1330◦C for 1 h (right). The images show the loss of grain texture in the pellet sintered at a higher temperature (right). Giant grain growth and liquid phases are the consequence.
A significant loss of grain texture and the introduction of giant grains and liquid phases are the consequence of the increased sintering temperature of 1330◦C. A silver metalization is applied to all pellets in a screen-printing process and the pellets are characterized at 50◦C with biasing voltages up to 1.1 kV. In figure D.5 the characterization results of the Ba0,7Sr0,3TiO3 sintered at 1250◦C are depicted. At 13.56 MHz, the varactor shows a capacitive tunability of maximum 17.2 %, reducing the capacitance from 1.7 nF to 1.4 nF.
The quality factor of 340 in unbiased state decreases by 35 % to 222 in biased state. The acoustic resonances of the pellet are measured at 7.5 MHz, 12.5 MHz and 17.3 MHz with periodic repetition in biased state, resulting in a static spacing of∼5 MHz. Intermediate resonances occur at 9.8 MHz, 14.8 MHz and 19.7 MHz with a static periodic spacing of 5 MHz.
For the pellet sintered at 1290◦C for 1 h an unbiased capacitance of 1.5 nF and a tunability of maximum 15.8 %, reducing the capacitance to 1.3 nF in biased state at 13.56 MHz. A corresponding Q-factor of 278 is obtained in unbiased state, decreasing to 225 in biased state by 19 %. Compared to the varactor sintered at 1250◦C, these values are consistent. In regard to acoustics, characterization data shows a significantly changed behavior. Acoustic activity of the pellet is measured at 7 MHz, 12 MHz and 16.8 MHz in biased state, resulting in a static periodical spacing of∼5 MHz. The resonances are much broader compared to the pellet sintered at a lower temperature and intermediate resonances have completely vanished. A detailed evaluation of this effect is presented in section 5.1.1.
At sintering temperatures above 1330◦C, giant grain growth and liquid phased are intro-duced in the ceramic texture. As a result, the electrical behavior of the pellet drastically changes. At 13.56 MHz, an increased capacitance of 5.3 nF is measured, decreasing by 31 % to 3.6 nF in biased state. An unbiased quality factor of 540 is obtained. Compared to the
5 10 13.56 15 20 25 0
0.5 1 1.5 2
Frequency / MHz
Capacitance/nF
0 250 500 750 1000
Q-factor
0 0.2 0.4 0.6 0.8 1
ESR/Ω
Capacitance 0 V Capacitance 1.1 kV
Q-factor 0 V Q-factor 1.1 kV
ESR 0 V ESR 1.1 kV
Figure D.5: C, Q and ESR of a BST based bulk ceramic disk varactor sintered at 1250◦C for 10 h. Measurement temperature 50◦C.
5 10 13.56 15 20 25
0 0.5 1 1.5 2
Frequency / MHz
Capacitance/nF
0 250 500 750 1000
Q-factor
0 0.2 0.4 0.6 0.8 1
ESR/Ω
Capacitance 0 V Capacitance 1.1 kV
Q-factor 0 V Q-factor 1.1 kV
ESR 0 V ESR 1.1 kV
Figure D.6: C, Q and ESR of a BST based bulk ceramic disk varactor sintered at 1290◦C for 1 h. Measurement temperature 50◦C.
5 10 13.56 15 20 25 0
2.5 5 7.5 10
Frequency / MHz
Capacitance/nF
0 250 500 750 1000
Q-factor
0 1 2 3 4
ESR/Ω
Capacitance 0 V Capacitance 1.1 kV
Q-factor 0 V Q-factor 1.1 kV
ESR 0 V ESR 1.1 kV
Figure D.7: C, Q and ESR of a BST based bulk ceramic disk varactor sintered at 1330◦C for 1 h. Measurement temperature 50◦C.
same base material sintered at 1290◦C, this means a decrease in ESR of 86 % from 0.028Ω to 0.004Ω. However in biased state, a significant decrease in Q-factor by 75 % to 134 is obtained, compared to 225 for the pellet sintered at 1290◦C. The acoustic resonances of the pellet are measured at 9.4 MHz, 16 MHz and 22.4 MHz, resulting in a static spacing of
∼7.4 MHz. Intermediate resonances are not measured.
For a sintering temperature of 1360◦C, the ceramic texture is indistinguishable from the one at 1330◦C. Consistently, the electrical behavior is well comparable to the one of pellet sintered at 1330◦C. The varactor introduces an unbiased capacitance of 5.6 nF and a tun-ability of maximum 37 %, reducing the capacitance to 3.6 nF in biased state at 13.56 MHz. A corresponding Q-factor of 660 is obtained in unbiased state, significantly decreasing to 56 in biased state by 91 %. Acoustic activity of the pellet is measured at 9.4 MHz, 15.8 MHz and 21.9 MHz in biased state, resulting in a static periodical spacing of∼5.4 MHz.
Overall, a significant impact of sintering time on the electrical behavior is observed. For the intended application in combination with the presented manganese co-doped base material Ba0,7Sr0,3TiO3, a sintering time of 1290◦C for 1 h appears to be advantageous compared to the others. The pellets introduce an acceptable high Q-factor in unbiased state with only a slight decrease in biased state. The acoustical behavior of the pellet is significantly improved compared to pellets sintered at 1250◦C. For pellets sintered at temperatures>1330◦C, giant grain growth and liquid phases in the ceramic texture are observed, as proclaimed. The intended significant increase in permittivity and tunability as well as reduced dielectric zero-bias losses are also obtained with this set up. In unbiased state, the pellets show Q-factors larger than any other material evaluated within the scope of this work. In biased state, however, the structure is prone to acoustic resonance excitation, as the scattering boundaries between grains are removed by giant grains and liquid phases. As a result,
5 10 13.56 15 20 25 0
2.5 5 7.5 10
Frequency / MHz
Capacitance/nF
0 250 500 750 1000
Q-factor
0 1 2 3 4
ESR/Ω
Capacitance 0 V Capacitance 1.1 kV
Q-factor 0 V Q-factor 1.1 kV
ESR 0 V ESR 1.1 kV
Figure D.8: C, Q and ESR of a BST based bulk ceramic disk varactor sintered at 1360◦C for 1 h. Measurement temperature 50◦C.
the Q-factor significantly deteriorates in biased state, rendering them unsuitable for the intended application.
In figure D.9, the characterization results of a custom made Ba0,7Sr0,3TiO3based ceramic are presented. The ceramic is 0.5 % manganese co-doped and sintered at 1315◦C for 1 h.
A relative density of 92.6 % is obtained with ad50diameter of 2 µm. The ceramic texture contains isolated giant grains, indicating a too high sintering temperature. The pellets are metalized by hand with a stencil. A commercially available silver paste is utilized. The texture of the material is depicted in figure D.10.
At 13.56 MHz, the varactors show an unbiased capacitance of 2.3 nF, decreasing by maxi-mum 17 % in biased state to 1.9 nF. A corresponding Q-factor of 237 is obtained in unbiased state, which decreases by 16 % to 198 in biased state. The acoustical activity impacts the capacitance trace of the measurement. Resonances are obtained at 9.7 MHz, 16.1 MHz and 22.5 MHz with a characteristic gap of 6.4 MHz. Intermediate resonances are not observed.