Proposed By: Assist Prof Dr Özlem Aydin Çivi and Assist Prof Dr Simsek Demir. This proposal is intended for: "Nanotechnologies, intelligent materials and new production processes" area of the European Commission's 6th Framework Programme in reference to "Key Action 4 - Essential Technologies and Infrastructures (4.8 Microelectronics – optoelectronics)" IST 2001 Action Line.
CMOS, RF MEMS and MMIC technologies are mature for different applications. In this action, we propose integration of these different technologies to have a full system on a single chip. MEMS devices have proven their usefulness in sensors, micromachines, and control components and now are candidates to take their place in the world of microwave applications with their reduced cost, improved performance, and miniaturized dimensions feasible for batch fabrication. The novel structures of these systems are widening the scopes of the currently used microwave devices, [1-2]. MEMS components seem to take the place of off-chip components in wireless communications as well as they are used in signal routing, phase shifting, timedelaying in phased array radar and antenna applications in microwave systems. Some of the RF-MEMS components designed are micromechanical filters with high Q and voltage controlled resonance frequency , voltage controlled capacitors , some passive RF circuit components , switches , phase shifters and microwave delay circuits . The wide application ranges of CMOS circuits are well known to the engineering community. MEMS structures are produced in similar processes, therefore both structures can be produced in the same process or these structures can be integrated after production such as using flip-chip techniques. GaAs MMIC structures are currently the only IC (Integrated Circuits) structures for microwave applications (for C band and above; for L and S bands CMOS RF circuits can still be used). Integration of MMIC circuits with MEMS components will also enable system implementation on chip level.
Recently, research on RF-MEMS devices has been started in METU EE Department as a joint study of recognized RF and MEMS groups. Some components like switches, phase shifters, matching circuits are designed and manufactured using the MEMS facilities in METU. Our department also possesses extensive experience on CMOS design and fabrication processes.
The first GaAs MMIC design, and postproduction measurements in Turkey are realized in our department in collaboration with another university. Since then a number of research thesis and project have been completed including circuit design and transistor modeling studies.
Current Studies on RF MEMS structures
Over the RF MEMS components designed, switches have an important role since they are not only used as discrete components, but also used as a part of other components, such as phase-shifters. RF MEMS switches are superior to their rivals, pin diodes, in terms of performance. RF MEMS switches have lower power dissipation and loss, higher on/off impedance ratio and isolation, wider band of operation frequency. Figure of merits in MEMS switch design are on/off capacitance ratio, actuation voltage, and switching time. Current efforts are on reducing the actuation voltage, switching time, and area of the switches. There are two ways to reduce the actuation voltage of a switch: reducing the spring constant of the structure and increasing the capacitance area, which is the source of the electrostatic force. The former increases release time of the switch significantly, while the latter increases the total area which can not be undertaken when large number of switches are considered. Therefore, there is a trade-off between actuation voltage and the switching time. Several RF-MEMS switches are designed in METU. Fabrication process of the devices is started and will be followed by the measurements, . Especially for phased array antennas, controllable components are vital. Phase and amplitude of the antenna elements are two of the required crucial controls. In the literature, there are a number of phase shifters manufactured using MEMS technology. A monolithic phased array is implemented using RF MEMS technology, . The phased array is composed of a linear array of four patch antennas and a new phase shifter design, monolithically integrated into a glass substrate. We have developed a triple stub matching circuit. The stub lengths are electrically adjustable and therefore impedance matching is achieved for any impedance depending on changing conditions such as frequency of operation. Furthermore, this triple stub matching circuit is utilized in the design of adjustable power divider. There are no adjustable power dividers for RF applications present in the literature, . Most of the research has been done so far is concentrated on RF-MEMS devices, but the ultimate aim is to apply RF-MEMS to system levels to overcome the limitations exhibited by integrated RF devices so to increase the system performance to a level that cannot be achievable otherwise.
Description of the Action
It is possible to implement different circuits/components with different technologies. In order to achieve the ultimate performance, the optimum technology should be chosen for the corresponding circuit/component. For example, CMOS technology can be used for digital control units and the same technology can also be used for RF circuits (e.g. for L band) where high performance is not required. On the other hand, MMIC technology can be utilized for high performance RF circuits whereas some specific components together with the antenna elements can be implemented by MEMS technology. In system level, a number of circuits should be integrated to perform a certain task. The best way of system implementation would be the integration of the system on a single chip. Certain improvements on system performance will be the additional benefits to mass production advantages.
In the proposed action, we want to implement different parts of a system with the optimum technologies and later on, these parts will be integrated on a single chip. An example system can be the Collision Avoidance Radar, which is utilized in the automotive industry. In such a radar the RF signal can be generated by an oscillator on a GaAs MMIC. A phased array antenna together with its Beam Forming Network (BFN) implemented by MEMS technology can transmit this signal. The received signal can be amplified and down converted to an intermediate frequency by an MMIC. CMOS processing circuits can handle the information on the received and down converted signal. All these parts can be produced separately and integrated using flip chip technology, resulting in a system in a single chip (system-in-a-chip). System-in-a-chip will have high performance because losses will be less due to reduced connectors and cables. It will also be small and light weight. System-in-a-chip will be low cost because for each part most appropriate technology will be used and post-production labor requirement will be less. System-in-a-chip best fits to mass production.