MODELING AND EXPERIMENTAL STUDIES OF SCHOTTKY-CONTACTED COPLANAR WAVEGUIDE TRANSMISSION LINES ON SEMICONDUCTOR SUBSTRATES
by
MOHAMMED SAIFUL ISLAM, B.S.E.E., M.S.E.E.
DISSERTATION
Presented to the Faculty of the Graduate School of
The University of Texas at Austin
in Partial Fulfillment
of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
THE UNIVERSITY OF TEXAS AT AUSTIN
AUGUST 1994
Copyright
by
Mohammed Saiful Islam
1994
Supervisor : Dean P. Neikirk
An extensive study has been performed on Schottky-contacted coplanar waveguide (CPW) distributed transmission lines on lossy semiconductor substrates in this dissertation. The study includes the modeling of the planar line both from series and shunt circuit perspectives and rigorous experimental work to support the modeled results. In addition, it has been shown that the transmission line can successfully be used to implement either Schottky-controlled or optically-controlled phase shifters that can operate well into the millimeter wave regime (at least up to 40 GHz).
The model to calculate conductor losses due to the CPW electrodes is based on quasi-static conformal mapping of the CPW structure. The non-uniformity of the current distribution along the CPW conductors is transformed into a uniform distribution in the mapped domain, where the conductivity becomes irregular. The mapped conductor surfaces are then analyzed and a scaled surface impedance is calculated, from which the series impedance is evaluated. The modeled results show an accurate match with wide range of experimental results. Besides the conductor loss calculation, the lossy shunt circuit is modeled as well, which provides the physical interpretation of the actual propagation control mechanism of a Schottky-contacted CPW phase shifter. This model is also supported by various experimental results. In addition to these models, extensive experimental work has been performed to implement an optically-controlled phase shifter based on a semiconductor substrate. Improvements in phase shifter performance have been obtained with the use of an advanced processing technique for hybrid integration. This also allows the possibility of an integrated phase shifter with a semiconductor laser source on a single quartz substrate.
Chapter 1 Introduction ....................................................................................1
Chapter 2 Device Fabrication Process ...........................................................5
2.1 Introduction .......................................................................................5
2.2 Chlorobenzene Metal Lift Off ...........................................................6
2.3 Gold Electroplating ...........................................................................8
2.4 PREGOS ..........................................................................................11
2.5 Polyimide Metal Lift Off .................................................................14
2.6 Epitaxial Lift Off .............................................................................17
2.7 Summary .........................................................................................21
Chapter 3 Conductor Loss Calculation of Co-planar Waveguide ..............22
3.1 Introduction .....................................................................................22
3.2 Issues concerning conductor loss ....................................................23
3.3 Use of conformal mapping and scaled surface impedance .............24
3.4 Othe conductor loss calculations .....................................................31
3.5 Model calculation and comparison ..................................................33
3.6 Conductor loss calculations for high Tc superconductors ...............40
3.7 Summary .........................................................................................44
Chapter 4 Characterization of CPW : Shunt circuit model .......................45
4.1 Introduction .....................................................................................45
4.2 Basic quasi-static theory .................................................................46
4.3 Earlier explanations for changing device characteristics.................51
4.4 Simple circuit models ......................................................................53
4.5 Proposed Repi-C shunt circuit model ..............................................59
4.6 Low frequency results .....................................................................65
4.7 High frequency results .....................................................................80
4.8 Model results for MIS structures .....................................................89
4.9 Effect of backside sheet resistance ..................................................94
4.10 Summary .........................................................................................99
Chapter 5 Phase Shifter measurements .......................................................101
5.1 Introduction ...................................................................................101
5.2 Voltage-controlled phase shifters .................................................102
5.3 Optical control of phase shifting ..................................................116
5.4 Comparison with other phase shifters ..........................................129
5.5 LED-controlled mmWave 90o phase shifter .................................131
5.6 Pulse response measurements of the phase shifter ........................138
5.7 Summary .......................................................................................144
Chapter 6 Conclusions and Discussions.......................................................145
Appendix A Derivation of shunt impedance...................................................148
References ............................................................................................................150
Vita ........................................................................................................................164