sábado, 29 de mayo de 2010

RF MEMS for Wireless Communication

RF MEMS for Wireless Communication
Abstract—This paper deals with a relatively new area of radio-frequency (RF) technology based on Microelectromechanical Systems (MEMS). RF MEMS provides a class of new devices and components which display superior high-frequency performance relative to conventional (usually semiconductor) devices, and which enable new system capabilities. In addition, MEMS devices are designed and fabricated by techniques similar to those of very large-scale integration, and can be manufactured by traditional batchprocessing methods. In this paper Micro-mechanical tunable capacitors are analyzed. Index Terms—RF, MEMS, Micromachined


HIS paper deals with another technology that has emerged in recent years with a comparable level of interest and more rapid development than RFIC's. The technology is the design and fabrication of Microelectromechanical Systems (MEMS) for RF circuits (RF MEMS). In some ways, MEMS represents the new revolution in microelectronics. It is similar to VLSI circuits in that it allows the execution of complex functions on a size scale orders of magnitude lower and at far less power than discrete circuits. However, MEMS enables this miniaturization on a class of sensors and transducers that traditionally were constructed on the model of a large, often bulky transducer or sensor coupled to a highly integrated VLSI readout circuit or processor.

Microelectromechanical systems (MEMS) allow a precise positioning and repositioning of suspended membranes and cantilevers that can be integrated with microwave circuits and thus offer a new approach for tuning of microwave circuits. One of the important applications of MEMS in RFcircuits and antennas is in the form of tunable capacitors. High Q-factor capacitors are needed in microwave communication systems to replace the semiconductor varactors. In MEMS, mechanical tuning avoids the high losses associated with semiconductors at high frequencies.

At the same time, MEMS leverages VLSI through the use of common design and batch processing methodologies and tools.

In RF MEMS, micromechanical tuning avoids the high resistive-capacitive losses associated with semiconductor varactors at high frequencies and the movement linearly of MEMS devices also allows the capacitors to tune linearly. Continuing progress in technology especially miniaturization of both IC, and sensors and actuators, is enabling more and more microsystems to be realized. To date, some efforts have been made to exploit the Microelectromechanical-systems technology for wireless RF applications. Tunable RF components such as voltage-controlled oscillators (VCO) and tunable filters have been developed using micromachining technology.


A. MEMS Processing: MEMS technology is based on a number of tools and methodologies, which are used to form small structures with dimensions in the micrometer scale (one millionth of a meter). Significant parts of the technology have been adopted from integrated circuit (IC) technology. For instance, almost all devices are built on wafers of silicon, like ICs. The structures are realized in thin films of materials, like ICs. They are patterned using photolithographic methods, like ICs. There are however several processes that are not derived from IC technology, and as the technology continues to grow the gap with IC technology also grows. There are three basic building blocks in MEMS technology, which are the ability to deposit thin films of material on a substrate (deposition process), to apply a patterned mask on top of the films by photolithographic imaging (lithography) and to etch the films selectively to the mask (etching process). A MEMS process is usually a structured sequence of these operations to form actual devices.

B. MEMS and Micromachining MEMS fabrication techniques empower conventional integrated circuit fabrication processes to produce three- dimensional (3-D) mechanical structures. Accordingly, there are three main approaches, namely bulk micromachining, surface micromachining and LIGA. [2] 1. Bulk Micromachining: In bulk micromachining, the 3-D structure is sculpted within the confines of a wafer by exploiting the anisotropic etching rates of different atomic crystallographic planes in the wafer. Alternatively, structures may be formed by the process of fusion bonding, which entails building up a structure by atomically bonding various wafers. Figure 1 shows a bulk micromachined structure.

Fig. 1 A bulk-micromachined structure. 2.

Surface Micromachining: In surface micromachining, the 3-D structure isbuilt up by the orchestrated addition and removal of a sequence of thin film layers to/from the wafer surface called structural and sacrificial layers, respectively. The success of this approach usually hinges on the ability to release/dissolve the sacrificial layers while preserving the integrity of the structural layers. Figure 2 depicts a surface-micromachined structure.

Fig. 2 A surface-micromachined structure

3. LIGA: LIGA is a German acronym consisting of the letters LI (Roentgen- Lithography, meaning X-ray lithography), G (Galvanik, meaning electrodeposition) and A (Abformung, meaning molding). Accordingly, in this technique thick photoresists are exposed to X-rays to produce molds that are subsequently used to form high-aspect ratio electroplated 3-D structures. Figure 3 depicts a junction of a CPW 6-dB coupler fabricated in a LIGA process

Fig. 3 Junction of a CPW 6 dB coupler
fabricated in a LIGA process.


The development to date tends to place them into different viewpoint. From the RF viewpoint, the MEMS devices are classes depending on whether one takes an RF or MEMS simply classified by the RF-circuit component they are contained in, be it reactive elements, switches, filters, or something else. From the MEMS viewpoint, there are three distinct classes depending on where and how the MEMS actuation is carried out relative to the RF circuit. The three classes are: 1) the MEMS structure is located outside the RF circuit, but actuates or controls other devices (usually micromechanical ones) in the circuit; 2) the MEMS structure is located inside the RF circuit and has the dual, but decoupled, roles of actuation and RF-circuit function; and 3) the MEMS structure is located inside the circuit where it has an RF function that is coupled to the actuation. We refer to each of these classes as: 1) RF extrinsic; 2) RF intrinsic; and 3) RF reactive. Each of the MEMS classes has produced convincing examples, e.g., the tunable micromachined transmission line in the RF-extrinsic class, shunt electrostatic microswitch and comb capacitors in the RF-intrinsic class, and capacitively coupled micromechanical resonator in the RF-reactive class.


There is a continuing demand to monolithically integrate complete receiver on a single chip. These receivers require voltage-controlled oscillators (VCOs) with low phase noise and RF filters with low insertion loss. High-Q tunable capacitors improve the phase noise performance of VCOs. [5] Micro-mechanical tunable capacitors have been used for getting high-Q for VCO applications [1-2]. Other strategies used to achieve wide tuning range include diode or MOS varactors or switched capacitor banks. Compared with solidstate varactors, MEMS tunable lower capacitors have advantages of lower loss, larger tuning range and higher linearity. Micromachining leads to reduced substrate coupling capacitance causing an increase in inductor self-resonance, resulting in an increased Q at higher frequencies. This allows usage of bigger inductors and smaller capacitors, leading to low power operation In this paper we propose oscillator which uses a comb drive actuator as the vibrating element. Displacement is sensed through a voltage divider network and feedback to achieve oscillation.

The basic principle involved in the operation of the oscillator can be explained with a series combination of MEMS varactor and a fixed capacitor. Fig (4) shows the series combination of the capacitors forming a voltage divider network [4]. The variable capacitors are mechanically coupled such that if the capacitance increases in one it decreases in the other. Due to some force if the plates of one varactor move closer (hence the plates of other move farther), the capacitance of that varactor increases and hence the voltage drop in corresponding fixed capacitor increases. This increase in voltage, if fedback properly to form a positive feedback, would generate oscillations.

Fig.4 Voltage divider network


This paper deals with recent developments in the field of RF MEMS. Broadly speaking, RF MEMS is a new class of passive devices (e.g., inductors) and circuit components (e.g., tunable capacitors) composed of or controlled by MEMS. Micro machined electromechanically tunable capacitors using comb drive mechanism have been mentioned. Mechanical tuning of capacitor facilitates wide tuning range for the oscillators.



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