AN001 RF MEMS advantages over SOI

Application Note AN001

This document and the information included here are proprietary of DelfMEMS COMPANY. No use, disclosure or reproduction granted to any third parties without explicit written permission of DelfMEMS COMPANY

AN0001 DELFMEMS S.A.S. Hub Innovation, 11 rue de l'Harmonie 59650 Villeneuve d'Ascq, France. For

sales or technical support, contact DelfMEMS at (+33) 320 05 05 45 Page 1 of 6

RF MEMS advantages over SOI By: Igor Lalicevic, RF Director

Introduction

This application note outlines the benefits of RF MEMS switching technology over existing solid state switching solutions. The MEMS technology benefits are observed from switch figure of merit (FOM) perspective and through different aspects of mobile phone performance improvements that MEMS technology is capable of providing.

Special attention is placed on the next generation of RF switching requirements for the higher end of the LTE-A performance scale and a particular emphasis on the need for high performance, high quality and highly reliable RF switches where the advantages of RF MEMS over SOI are clearly visible.

Micro-Electro-Mechanical Systems (MEMS) technology is not new for the mobile phone. In fact, major OEMs & ODMs have become dominant buyers of MEMS products as mobile phones and tablets came to be the main drivers of MEMS market growth.

MEMS based microphones, accelerometers, gyroscopes and geomagnetic sensors are well recognized as key building blocks of a modern mobile phone architecture. Now, LTE technology with its ever increasing number of bands, especially at higher frequencies, and its need for low loss, high isolation and very linear RF components is setting the stage for the next round of MEMS technology, RF MEMS.

Extensive work has already been performed on RF MEMS solutions and it has been demonstrated that you can get a better antenna, filter and power amplifier by using RF MEMS. Along with applications such as tunable antennas and filters, RF MEMS are deemed to be an ideal choice to implement high performance RF switching.

RF Switching Technology Options

Transmit/receive and band select switching requirements have been defining RF switch technology choices since the early days of wireless telephone technology.

The requirements are determined by three major considerations; high isolation, low insertion loss and high linearity.

In addition to providing sufficient isolation between the transmitter and receiver, a switch needs low insertion loss to maintain high transmitter power efficiency and low receiver noise figure.

Multiple complexities result from the requirements to reduce intermodulation and cross modulation from the one or more receiver and transmitter paths. In this environment, for all RF components and particularly for the RF antenna switch, linearity performance is becoming a crucial specification.

RF MEMS advantages over SOI



The 3GPP standard is used by the industry to determine the degree of linearity required to avoid interference with other devices on the network. This is done by specifying the third order input intercept point (IIP3).

Each new generation of cellular network required progressively higher linearity. According to Intel Mobile Corporation (Table 1), the 2G requirement for switch linearity was an IIP3 of 55dBm, the 3G switch requirement was 65dBm while the LTE switch IIP3 requirement was set to 72dBm.

These performance requirements led to the selection of a GaAs pseudomorphic HEMT (pHEMT) as a 2G switching technology preferred choice, while increased 3G linearity network requests and the need for higher level of integration introduced solid state silicon-on-sapphire (SOS) and silicon-on-insulator (SOI) technologies for RF switch applications.

Recent advances in high-resistivity SOI technology has resulted in CMOS thin film SOI becoming the mainstream switching solution for 4G mobile phones along with its cost and performance benefits.

Today however, the LTE RF front end has become extremely complex and inefficient. 2G, as a first digital cellular standard used quad-band system solutions, while the 3G standard already supported up to 8 frequency bands. 4G, dominated by the need for global roaming and wider frequency bandwidth, introduces over 40 bands allocated to LTE applications.

On top of that, up-link carrier aggregation has established requirements for an extremely high linearity switch with IIP3 levels of 90dBm. There is no clear evidence that SOI today is capable providing switch components with an IIP3 levels above 75dBm. It has been claimed that IIP3 levels of 80-82dBm are achievable with SOI, but little or no public data is available supporting such claims and even this is 10dB short of the needed target for up-link carrier aggregation.

It is essential to start thinking about SOI switching technology limitations, which will have to be addressed with alternative solutions or a completely new technology that can meet existing challenges and provide the needed performance.

Table 1. (Network linearity requirements)

RF MEMS technology provides answers for these LTE-A challenges with the benefits of providing better call quality, longer battery life & enabling the 90dBm requirement for up-link carrier aggregation to improve smartphone performance.




Figure of Merit

In order to compare the performance of the different switch technologies, an industry standard figure of merit (FOM) has been established, which is defined as Ron*Coff, and is expressed in femtoseconds (fs).

Ron: ON Resistance (unit: Ω-mm) - Low Ron means less insertion loss

Coff: OFF Capacitance (unit: F/mm) - Low Coff means better isolation

Graph 1. (Switch Figure of Merit)

In Graph 1, DelfMEMS RF MEMS switching technology is compared to solid-state technologies. Its outstanding figure of merit provides a 10x improvement over the next best solution, which is today Peregrine SOI UltraCMOS technology.

Solid-state SOI switch designs are heavily dependent on the FET’s channel resistance when it is on (Ron), and the switch’s isolation cannot be better than the FET’s total capacitance when the switch is in off state (Coff). It follows that for a high throw count solid-state switch’s design, OFF state switch branches will become a dominant source of non-linearity, insertion loss and isolation degradation. Equally important are degradations that SOI technology will experience at higher frequency operation.

By contrast, DelfMEMS RF MEMS switches retain ultra-low insertion loss (0.35dB @ 3.5GHz), very high port-to-port isolation (35dB @ 3.5GHz) and ultra-high linearity (85dBm up to 98dBm) even for high multi-throw switch configurations and at higher frequency operation.

These parameters makes RF MEMS switches a perfect candidate for 4G and beyond and multi band environments where RF components have to demonstrate minimum power loss degradation at frequencies of up to and above 3.5GHz.




Graph 2 below compares insertion loss for RF MEMS and SOI switch technologies in a SP12T RF switch configuration.

Graph 2. (SP12T Switch Insertion Loss)

The benefits of RF MEMS switches for loss in high throw switch configurations over existing SOI switching solution at higher frequencies are clearly impressive.

The DelfMEMS switch can provide around 1dB of insertion loss reduction in the complex LTE RF front end environment. A simple calculation can translate this loss reduction into an impressive 17% longer talk time (battery life) gain, and 30% call quality improvement by way of increased data throughput. Detailed understanding of these calculations can be found in DelfMEMS Application Note AN0002 “RFFE system level design considerations for RF MEMS”.

DelfMEMS RF MEMS Switch Solution

The typical RF MEMS switch structure uses either a cantilever beam or a bridge and features highly conductive electrodes, which are electrostatically actuated in order to create an ohmic contact on a conducting line. The result is mechanical switching. These typical basic structures carry several serious issues such as stress on the anchors, a tendency for stiction, low switching speed, and metallic creep in the beam.

DelfMEMS RF MEMS switch design offers an innovative approach to address these problems, instead of just trying to reduce their effect. As a result, the switch simultaneously offers increased performance in terms of isolation and insertion loss along with its improvement in reliability. This




has been achieved through the development of a unique anchorless structure for mechanical RF switching.

Picture 1. (DelfMEMS unique mechanical solution)

DelfMEMS switch solution was from its very inception designed to address the question of RF MEMS reliability. It features a free moving flexible membrane, known as Free-Flex™, which carries the switch contact area and is held and positioned by two sets of pillars and stoppers. The membrane is always in a known controlled state as it is electrostatically actuated by two sets of electrodes. The electrostatically ON state is achieved by making physical contact between the membrane contact area and the transmission line and the similarly controlled OFF state is achieved by keeping a physical distance between the membrane contact area and the transmission line. This means that the switch contact area will either be attracted to the conductive line or repelled from it.

Complete control of the MEMS membrane allows for an increased gap between contact area and transmission line in the OFF state, which is directly linked to the switch isolation and allows for switch resetting in the unlikely event of stiction.

Moving from ON state to OFF state is made through an electrostatic active actuation, which de-couples between restoring forces, contact forces and the membrane mechanical properties. In addition, the effects of mechanical bounce are substantially reduced. Importantly this doesn’t depend only on the elastic restoration forces of the membrane. The advanced electrostatic actuation implementation results in a very short switching time, typically less than 3µs.




The ability to have a reduced gap between the membrane and the transmission line is a major advantage of the DelfMEMS switch structure. It ensures that an increased ON state contact force is achieved with reduced actuation voltages and consequently delivers ultra-low insertion losses. Due to the reduced gap, the maximum deflection of the membrane will be reduced as well, and as a result membrane mechanical stress and the creep effect decreases. The reduced gap combined with a unique metallurgy for the contacting surfaces results in a highly reliable RF MEMS switch.

Thanks to DelfMEMS original design approach, RF MEMS switches can be used effectively for the first time as an RF front end switching solution.

Conclusion

LTE technology with its global roaming needs, carrier aggregation, MIMO design approach and adoption of higher frequency bands have made RF front end architectures for high-end smartphones exceptionally complex and a key bottleneck in achieving the market’s needed RF performance.

Mobile handset battery life, call quality, data throughput and higher network linearity requirements are the most obvious victims of this excessive complexity.

An RF switch is first in line to encounter those problems. SOI switching technology is reaching its limitations and this fact is driving efforts to find solutions for 4G performance improvements in other RF front end areas.

Extensive work has already been performed to improve insertion loss, isolation and linearity of the existing RF front end system, but the results remain largely disappointing and no significant improvement has been demonstrated until now compared to 3G.

A number of companies are endeavoring to design tunable frequency component solutions that will reduce existing RF front end complexity, however this is fraught with difficulties. DelfMEMS simple yet innovative approach to the problem is to replace specific switching components using SOI technology with an enhanced solution utilizing RF MEMS technology. Furthermore, DelfMEMS switching solutions will allow system and architecture designers to simplify and reduce the number of needed components in RF front end reducing complexity. Overall, the enhancements that DelfMEMS switching solution can provide to LTE systems and beyond radically improve the major RF performance criteria of insertion loss, isolation and linearity.

Documents

AN001 RF MEMS advantages over SOI