Ground plane (both top and bottom)
4.4 Discussion
The properties of BST and metal films were characterised experimentally using square varactors as described in appendix A. For the lowpass filter, the EM full wave simulation of the filter, using Sonnet, was compared with the experimental frequency sweep to extract the permittivity of the fabricated BST thin film. To compare the device as accurately as possible, the loss from the metal and BST film was carefully considered in the simulation. The conductivity of the metal was set to 64.3 % of its ideal case, according to appendix A. The thickness of top silver layer was chosen as 400 nm and the bottom metal was 250 nm platinum, which was a little different with the Pt/Au/Pt multilayer in practice. The size of the capacitor was set to 4 µm × 4 µm as it was closer to its actual size. The loss tangent of BST film was set to 0.1 and thickness to 400 nm. The extracted permittivity of BST thin film varies from 485 to 295 with 0 - 15 V bias. The EM full wave simulation using the above parameters compares well the experiments results despite small difference in S11 as shown in Fig.4- 11.
-50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 0 5 10 15 20 25 30 35 40 45 50 S 1 1 a nd S 21 (dB) Frequency (GHz) 0 V 15 V εr = 485 εr = 295 S11 S21
Fig.4- 11 The comparison between simulation and measurement of the lowpass filter. The dashed lines are the simulation results and the solid lines are the measurement results.
As discussed in appendix A, the BST loss is dominant in the total loss of a varactor. However, the situation is different for the lowpass filters, where the metal loss is significantly aggravated as a result of current crowding in the narrow lines, which also leads to a degraded power handling capability. A large current density concentrated in the meander lines and the narrow shunt lines in the bottom layer is shown in Fig.4- 12.
Fig.4- 12 Current crowding in the parallel plate capacitor area of the filter
The losses from different parts of the filter were investigated by full wave EM simulation. The simulated performance of the lowpass filter considering separated losses from top metal layer, which was 400 nm silver, bottom metal layer, which was 250 nm platinum, and BST film with tanδ = 0.1 respectively was shown in Fig.4- 13. The conductivity of metal was set to 64.3 % of its ideal value. The lossy filter, which considered all the above losses, and the lossless case were also shown in Fig.4- 13. The loss from silicon substrate and radiation loss was small in this case and not shown here. It can be seen that either the loss from top or bottom metal layer is comparable to or even worse than that of BST film, which is a result of the severe current crowding and the very small thickness (smaller than skin depth). The large current concentration in this small device (1.8 mm × 0.9 mm) makes metal loss dominant in the total loss. This also leaves room for improvement - thicker metal will ensure lower insertion loss.
The transmission zero at 42 GHz is caused by the resonance of the parallel plate capacitor and the inductance of the 40 µm wide shunt line to ground. The other resonance in lossless situation is caused by the resonance of ground plane at certain frequency and will not appear with loss. -80 -70 -60 -50 -40 -30 -20 -10 0 0 5 10 15 20 25 30 35 40 45 50 S11 and S21 (dB ) Frequency (GHz) lossless lossy Ag400 BST01 Pt250 S11 S21
Fig.4- 13 The losses from top metal layer (400 nm thick silver), bottom metal layer (250 nm thick platinum), BST film (loss tangent of 0.1) compared with lossy (including all the above losses) and lossless situations. Solid lines represent S21 and dot lines S11.
Except using thicker metals, the insertion loss of the filter can be also improved by modifying the filter layout. In the new layout, as shown in Fig.4- 14, the shunt lines in the bottom layer were moved to the upper layer where silver is used instead of platinum and the narrow central lines (i.e. 5 µm width lines) were removed. As silver has much higher conductivity than platinum and the top metal can be made thicker, the metal loss was expected to be reduced. In addition, mitered corners were used in the meander lines in order to reduce current reflection and hence achieved a much more uniform current distribution. The improvement in the simulated performance of the lowpass filter is shown in Fig.4- 15. The insertion loss is about 1 dB better at 25 GHz using the new layout. However, experimental verification of the filter in new layout is not available.
(a)
(b)
Fig.4- 14 Layout of the modified lowpass filter (a) and enlarged view of the parallel plate capacitor area (b). -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 0 5 10 15 20 25 30 35 40 S11 an d S 21 (d B) Frequency (GHz) new layout S11 new layout S21 old layout S11 old layout S21
Fig.4- 15 The insertion loss improvement of the lowpass filter in the new layout. The Solid lines represent the filter in the modified layout while the dashed lines represent the old layout.
4.5 Summary
In this chapter a K-band BST lowpass filter on a high resistivity silicon substrate was demonstrated. The 10 dB cut-off frequency of the filter was tuned 32.1 % from 18.52 GHz at zero bias to 24.47 GHz with 15 V bias. Several lowpass filters covering a wide frequency operating range from 4 GHz to 25 GHz were demonstrated as well. The loss from metal and BST film were separated and analysed. Although the BST loss was more important in the varactors, the metal loss became dominant in the filters as a result of severe current crowding. A modified layout of the lowpass filter aiming at reducing the conductor loss was also suggested.
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