This is the Home page for the research on RF MEMS at the Department
of Informatics, University of Oslo.
What is MEMS?
MEMS at the Nanoelectronics group
Research focus: RF MEMS
Course: INF5490 RF MEMS
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Imagine thousands of tiny machines and sensors fabricated in microscale for just a few cents per unit. This opens a new world of exciting applications!
Micromotor from Sandia
MicroElectroMechanical Systems, MEMS, are systems based on a range of techologies whereby tiny mechanical elements, both sensors and actuators, can be implemented as well as mechanical couterparts of electrical circuit elements, such as micromechanical switches, resonators, mixers and filters with high performance. It turns out that these elements have excellent system properties.The elements are often interfaced to microelectronical driving or sensing components (ICs) by appropriate packaging or on the same silicon wafer. The semiconductor silicon is not only good for making electronics but its material properties are extremely good.
Most MEMS components are implemented by using processes resembling the ones used for production of micro chips (VLSI circuits). In the earlier days of the MEMS development diffusions and etchings into bulk wafers were primarily used (”bulk micromachining”). Later on, ”surface micromaching” has been developed, a technique which has given the field a real boost. That type of process can be compared to baking a cream cake by stacking various layers. The ”cream” in the cake resembles what is called ”sacrificial layers” which separate other ”structural layers” when building up the unit. The sacrificial layers are spacers which later on are removed, causing the structural layers to be released. Thereby mechanical elements such as beams, diaphragms or disks are free to move as intended. The advantage of using techniques from IC processing is not only to be able to implement the microstructures, but also to allow thousands or millions of equal elements to be fabricated at the same time to a low cost (batch processing).
RF MEMS switch (C.L. Goldsmith et al.)
In this micro world the designer has to cope with effects and forces of quite other dimensions than in the macroscopic world, such as atomic forces and surface effects. However, a completely new degree of freeedom is also given to the system designer. In addition to what can be achieved by using only electronical components, another exciting range of effects are available! The designer can utilize mechanical or other physical properties in the materials and select the ones which are most adequate for his application. A whole bunch of physical principles exist when selecting the one to be used for instance in an actual detector design. Typically, a capacitor or resistor value can change when an elastic microbeam or thin membrane is deflected. The change of mechanical stress in a structure can be the result when the MEMS experience an accelleration or by an applied pressure load (sensing). Microelements can be forced to move using electrostatic activation (actuation). Thereby micro motors, movable micromirrors or steerable gratings can be implemented, or small fluidic particles can be forced through different temperature zones etc. To implement complete systems it is essential that the physical effects on the micro elements, such as strain and stress, can be converted to electrical currents and potential differences which further on can be handled by microelectronics in more or less integrated ways (e.g. integrated sensors).
MEMS have been used in accelerators detecting when an airbag should be released, in video projectors having a million individually controlled micromirrors, in gyroscopes, pressure gauges (for instance in the tires of cars) or as micro optical systems for fiber optical communication. Very exciting is MEMS used for fast electrophoresis in DNA amplification and separation, various systems for biological analysis or other biomedical applications, microrobotics, micro tweezers and neural probes. It seems to be a considerable potential for using microsystems within areas such as medicin, car industry, space technology, within communication, security and in a lot of the components surrounding us in our daily life.
Today there is an increasing activity in developing MEMS processes, design tools and applications. Great expectations exist as to the importance of the field in the future. In the same way as microelectronics and PCs have revolutionized our daily life and reached a widespread use, it seems likely, according to the most enthusiastic researchers, that microsystems might be the next wave. Numerous types of units based on MEMS technology could be produced in large quantities and spread around for diverse applications which directly or indirectly could detect or help us control our physical environment. Some people say that this field will have a huge and penetrating impact on the development in our society.
The MEMS field is by its nature a mixture of quite diverse disciplines such as physics, chemistry, mathematics and informatics, where material technology, electronics, modeling and CAD tools should be emphasized. The research field is quite diverse, comprising fields as developing new fabrication and processing techniques, investigating new physical principles and structures, ASIC (Application Specific Integrated Circuits) for MEMS, design tools, applications etc. A continuing development towards miniaturization (nanotechnology) and thereby a denser integration, is a strong driving force.
MEMS inductor (J.-B. Yoon et al.)
MEMS tunable capacitor (A. Dec & K. Suyama)
Design of electronical systems containing mechanical parts is a relatively new activity at the NANO group.The MEMS (MicroElectroMechanical Systems) field has reached a growing attention, not only internationally, but also in Norway. An advanced MEMS laboratory (MiNaLab) has been established at SINTEF and UiO in a building next to IFI. The Norwegian Research Council has been engaged by sponsoring this initiative. The NANO group with its strong expertise and experience in designing analog and digital systems seems to have a lot to contribute to developing MEMS systems in a broader context.
My personal interests in this area is more specifically directed towards design activities where the MEMS units are used in a larger system context. Central to this is interfacing the mechanical to the electrical world and investigating the possibility of implementing systems containing both MEMS components and surrounding microelectronics integrated on-chip. This will comprise activities towards design methods, modeling and analysis, and designing and implementing actual systems. Due to the very broad MEMS research field I have restricted my activities to cover systems which are central to and can be used in high frequency radio systems for wireless transmission, RF MEMS (Radio Transmission MEMS). This is a very interesting and exciting field in fast development. As a background to the field, intended for Master and Ph.D. students, a course in RF MEMS has been developed (INF5490 RF MEMS) and taught for the first time in the spring 2005.
MEMS technology can be used to implement high quality switches, varactors (variable reactors), inductors, resonators, filters and phase shifters. Among the broad range of applications the MEMS technology gives a unique possibility to implement micromechanical resonatores and filters with high performance regarding selectivity and Q-factors. When combining these mechanical structures with microelectronics, central parts in wireless systems, RF systems (Radio Frequency systems) can be implemented. Examples can be various types of oscillators, VCOs (Voltage Controlled Oscillators), mixers and sharp filters. The MEMS structures can thereby replace traditional costly and large off-chip discrete components by making possible integrated solutions that can be batch processed. Vibrating MEMS resonators and filters that have been implemented so far are based on mechanical vibrations in lateral or vertical directions on Silicon wafers. Different types of beams, comb structures and disks can be used.
RF MEMS filter (A.-C. Wong, H. Ding, and C.T.-C. Nguyen)
In recent years I have expanded my research area in VLSI and electronic system design by investigating new possibilites for including micromechanical components, MEMS (Microelectromechanical systems). After a sabattical at SINTEF MicroNanoLaboratory (2003-2004), I focused my research area to RF MEMS and the possibility of integrating electronics (CMOS circuits) and MEMS on the same chip. Thereby system designers could benefit from the high performance and flexibility given by introducing micromechanical components into the systems. RF MEMS can replace the bulky, off-chip components that are necessary in todays RF tranceivers to obtain sufficient performance. Thus, oscillators, mixers and RF filters are very important units that can take advantage from being implemented by their mechanical counterparts.
The publications from me and my students in the last 5 years focus on designing mechanically movable structures (e.g. resonators, filters, mixers) using a promising concept of making MEMS out of ordinary CMOS processes: CMOS-MEMS. By using the CMOS multilayer metal-dielectric stack, laterally movable MEMS structures can be made. A maskless post-processing of the CMOS chips releases the mechanical parts, whereas the pure electronics is shielded. Even if the MEMS parts would not have as good performance separately as units made in a specialized MEMS process, the overall system performance could be very good due to the low parasitics, impedances and short wires between the mechanical parts and the neighbouring electronics.
The post-processing method which has been used in our prototypes and publications is developed at Carnegie Mellon University (CMU), USA. Their “ASIMPS procedure” was offered by the European Si broker, CMP, and our group was one of the first in Europe to use this option. The ASIMPS process offered to Europe was based on the 0.25 µm technology from STMicroelectronics. Our contact with CMU and Professor Gary K. Fedder´s group has been very valuable, and one of our PhD students has recently stayed at his group for 9 months. We are also pushing the limits by designing CMOS-MEMS in 90 nm CMOS technology. Two of our PhD students have recently designed central parts in a Delta Sigma A/D converter where MEMS structures implement the frequency variable oscillator. Even if real vibrating resonators and mechanical filters have been the main focus of my research, my students have also designed varactors and inertial sensors by using the same technology.
I am currently working on combining MEMS with circuitry in ultra low power, robust and radiation tolerant CMOS for future space applications. The MEMS parts will typically implement sensors, actuators, or central parts in the radio transmission system in future pico-satellites. A new PhD student has started to look at this option. Finite-Element simulations of the electromechanical structures are performed by using the CoventorWare CAD tool, and complete CMOS-MEMS systems are simulated in Cadence. Real prototypes are made.
Oddvar Søråsen , Professor, Room 5412 Ole-Johan Dahls Building (22 85 24 56)
Jan Erik Ramstad
Srinivasa Reddy Kuppi Reddi
Oddvar Søråsen , Professor, Room 5412 Ole-Johan Dahls Building (22 85 24 56)
Updated: March 16. 2011