Low Temperature RF MEMS Inductors Using Porous Anodic Alumina

In today’s communication devices, the need for high performance inductors is increasing as they are extensively used in RF integrated circuits (RFICs). This need is even more pronounced for variable inductors as they are widely required in tunable filters, voltage controlled amplifiers (VCO) and low noise amplifiers (LNA). For RFICs, the main tuning elements are solid state varactors that are used in conjunction with invariable inductors. However, they have limited linearity, high resistive losses, and low self resonant frequencies. This emphasizes the need for developing another tuning element that can be fabricated monolithically with ICs and can offer high range of tuning. Due to the ease of CMOS integration and low cost silicon based IC fabrication, the inductors currently used are a major source of energy loss, therefore driving the overall quality factor and performance of the chip down. During the last decade there has been an increase in research in RF MicroelectroMechanical Systems (RF MEMS) to develop high quality on chip tunable RF components. MEMS capacitors were initially proposed to substitute the existing varactors, however they can not be easily integrated on top of CMOS circuits. RF MEMS variable inductors have recently attracted attention as a better alternative. The research presented here explores using porous anodic alumina (PAA) in CMOS and MEMS fabrication. Due to its low cost and low temperature processing, PAA is an excellent candidate for silicon system integration. At first, PAA is explored as an isolation layer between the inductor and the lossy silicon substrate. Simulations show that although the dielectric constant of the PAA is tunable, the stress produced by the required thicker layers is problematic. Nevertheless, the use of PAA as a MEMS material shows much more promise. Tunable RF MEMS inductors based on bimorph sandwich layer of aluminum PAA and aluminum are fabricated and tested. A tuning range of 31% is achieved for an inductance variation of 5.8 nH to 7.6 nH at 3 GHz. To further improve the Q, bimorph layers of gold and PAA are fabricated on Alumina substrates. A lower tuning range is produced; however the quality factor performance is greatly improved. A peak Q of over 30 with a demonstrated 3% tuning range is presented. Depending on the need for either high performance or tunability, two types of tunable RF MEMS inductors are presented. Although PAA shows promise as a mechanical material for MEMS, the processing parameters (mainly stress and loss tangent) need to be improved if used as an isolation layer. To our knowledge, this is the first time this material has been proposed and successfully used as a structural material for MEMS devices and CMOS processes

Transposeurs intégrés ultra large bande continûment accordable de 1 à 20 GHz, utilisant les technologie de silicium micro-usiné dans un perspective de consommation ultra faible (quelques mW)

Le sujet de cette thèse porte sur la réalisation d'un bloc de transposition de fréquence de 1 à 20GHz à base de composants MEMS. Cette thèse s'est traduite par la conception et la réalisation d'un nouveau type de capacité MEMS RF variable, qui se base sur des structures rotatives de type gyroscope pour l'actionnement, et sur une variation de surface pour la variation de capacité. Comparée à différentes architectures publiées à ce jour, cette structure a l'avantage d'avoir la partie actionnement (la partie MEMS) et la partie RF (la capacité) isolées électriquement, ce qui permet d'éviter le phénomène d'auto actionnement avec la puissance du signal RF traversant. Un autre avantage de la structure développée est la possibilité d'avoir simultanément 8 capacités variables sur une puce unique, avec un seul système d'actionnement. La fabrication de ces puces nécessite l'utilisation d'un wafer SOI pour la partie MEMS et d'un wafer en verre pour la partie RF, ce qui offre la possibilité d'une mise en boitier du MEMS directement pendant le procédé de fabrication. Ces travaux ont également porté sur l'étude du phénomène de pullin dans le cadre des peignes interdigités incurvés (curved combdrive), laissant apparaître les paramètres physiques critiques lors du dimensionnement. Cette étude paramétrique a été utilisée pour améliorer la structure d'actionnement en utilisant des peignes interdigités à largeur de doigt et à gap variable, pour repousser ce phénomène de pullin en dehors de la plage utile d'actionnement. Cette nouvelle capacité variable a ensuite été intégrée dans un système simple d'oscillateur accordable sur alumine pour valider ses performances RF et pourra être associée à un mélangeur pour réaliser le bloc complet de transposition de fréquenceThis thesis deals with the realisation of a frequency transposition block from 1 to 20 GHz based on MEMS components. It results in the design and fabrication of a new kind of tuneable RF MEMS capacitor based on a rotational gyroscope structure for the actuation part and on a surface variation for the capacitance change. Compared to other architectures published, this structure presents the advantage to have an actuation part (the MEMS part) and a RF part (the capacitor) that are electrically separated in order to avoid the phenomenon of self-actuation with RF signal crossing power. Another advantage of this structure is the possibility to simultaneously tune 8 different capacitors on a single chip, with only one actuation system. The fabrication of the chips requires the use of a SOI wafer for the MEMS part and a glass wafer for the RF part, which offers on chip packaging opportunity. This work also focused on the study of the pull-in effect in the case of curved comb-drives, highlighting the most critical physical parameters for the design. This parametric study has been used to improve the actuation structure and more particularly the topology of the curved comb-drives by variation of the finger width and gap. These modifications were done in order to push the pull-in effect out of the actuation operating range. This new tuneable capacitor has been integrated into a simple VCO circuit on alumina to validate the RF performances and could be associated to a RF mixer in order to realize the full frequency transposition blockPARIS-EST-Université (770839901) / SudocSudocFranceF