RF MEMS in mobile phones

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     RF MEMS in mobile phones
     The need for multiband, multimode band switching at low insertion loss
     while maintaining excellent linearity in mobile phones is driving the need for
     RF MEMS-based switches. Since this switching problem gets even more acute
     as new complex waveforms, such as WiMAX are added to this mix, this article
     looks at the current state of development in RF MEMS switches and discusses
     its impact on 3G cellular phones.

     By Refugio Jones and Mark Chapman

     C    arriers are launching third generation or
          3G wireless networks globally. These
     newer 3G standards provide a variety of
                                                                                                           strict performance limits of one standard or
                                                                                                           the other. For instance, a T/R switch is used
                                                                                                           with the time division family of standards
     services, including data and on-demand                                                                (TDMA & GSM), which are half-duplex in
     video. But as wireless networks advance,                                                              nature. Code division standards allow for
     so too are challenges for mobile phone                                                                full-duplex use of the frequency band and
     designers. In addition, recent developments                                                           use a dual or multichannel duplexer device.
     in wireless communications have resulted in                                                           Multiband phones can use a combination of
     hand-held cellular phones that can use up to                                                          all three of these devices to provide for full
     seven different wireless standards or bands                                                           world phone services.
     including DCS, PCS, GSM, EGSM, CDMA,                                                                      Typical antenna switches have to pass fre-
     WCDMA, GPS and Wi-Fi. Each standard                         Tx                             Rx         quencies up to 5 GHz and are supported with
     has its own unique characteristics and con-                                                           silicon-on-insulator (SOI) devices, compound
     straints and brings with it its own specific                                                           semiconductors such as gallium arsenide
     challenges. RF micro electromechanical                                                                (GaAs), or PIN diodes. These switches con-
     systems (MEMS) may help engineers design                                                              sume little power (15 µW), have good isolation
     phones that meet the challenges of inte-                          Mobile Phone Chipset
                                                                                                           (up to 35 dB depending on packaging) and low
     grating multiple bands while maintaining                                                              insertion loss (0.8 dB). GaAs and PIN diode
     long battery life and progressively reducing                                                          solutions have been the perennial favorites
     the overall size of the handset, adding new                                                           because of power handling and flexibility. SOI
     capabilities, while keeping these devices small                                                       provides an entry point for new technologies
     and affordable.                                      Figure 1. Basic mobile phone block diagram.      to participate in this competitive marketplace,
        About 75% of the 100 or so components in         use a transmit/receive (T/R) switch or a band     but SOI vendors are finding that surpassing
     a mobile phone are “passive” elements such          switch, and/or duplexers at the point where the   the 4 GHz mark is difficult unless new design
     as inductors or variable capacitors. MEMS           phone’s antenna interfaces with the cellular      techniques are implemented. Finally, standard
     versions of these components promise to make        phone’s chipset as shown in Figure 1. The use     CMOS continues to fall behind on servicing
     phones more reliable and power efficient.            of one or any combination of these depends        this market, but smaller die geometries are
                                                         on the number of different bands employed         making it possible for manufacturers to provide
     Cellular phones of today                            by each of the major cellular systems opera-      high-frequency products capable of passing
       Most cellular phones on the market today          tors. Each type of these devices supports the     up to 2.5 GHz.

              Standard                Transmission Frequency              Reception Frequency           Full-Duplex     Half-Duplex       Max. Output
                                            Band (MHz)                        Band (MHz)                                                    Power
               GSM 900                           890 – 915                      935 – 960                                     √                2W
              EGSM 900                           880 – 915                      925 – 960                                     √                2W
              DCS 1800                          1710 – 1785                    1805 – 1880                  √                                  2W
              PCS 1900                          1850 – 1910                    1930 – 1990                  √                                  2W
            UMTSW-CDMA                          1920 – 1980                    2110 – 2170                  √                                  1W
                CDMA                             824 – 849                      869 – 894                   √                                 1.5 W
                 GPS                          1227.6 & 1575.42                                                                √                2W
                 Wi-Fi                        2.4 GHz & 5 GHz                                                                 √                1W
               WI-MAX                    Varies: 2.4 – 5.9 GHz                                              √                                  2W
     Table 1. Worldwide wireless standards.

20                                                                                                            September 2005
RF Design   21
                   SP8T RF –MEMS
                     Band Switch                    Duplexers
                                                                                          Rx CDMA                In detail, RF MEMS switch devices re-
                                                                                          Tx                  semble a mechanical relay, but the geometries
                                                                                          Rx                  are typically in the submillimeter or hundreds
                                                                                          Tx WCDMA
                                                                                                              of micrometers in size. The scales of size make
                                                                                          Rx                  these devices attractive because they make
                                                                                          Tx DCS
                                                                                                              it possible to have switching solutions that
                                                                                          Tx PCS              can ideally take up 1 mm2 or less of space. In
                                                                                          Rx                  addition, the switches can be altered to create
                                                                                          Tx GSM              a variety of micro applications such as delay
                                                                                          Rx                  lines and switched capacitor networks.
                                                                                          Tx EGSM
                                                                                                                 In theory, RF MEMS technology is capable
                                                                                          Rx GPS              of surpassing the performance of high-speed
                                                                                          Rx Wi-Fi
                                                                                                              semiconductors with devices that can route
                                                                                          Tx                  and control well up to 50 GHz signals. The
                                                          T/R                                                 reality is that there are many factors that have
                                                        Switches                                              limited the viability of RF MEMS in mobile
                        Decoder &
                       Control Block                                                                          phone applications. Such factors include fabri-
                                                                                                              cation processes, packaging (hermetic isolation
                                                                                                              and parasitics), control voltages, long-term
                                                                                                              switching life cycles (contact point stiction),
     Figure 2. Example of a multiband switch for various mobile phone standards.
                                                                                                              switching speed, RoHS compliance (reflow
     T/R and band switching                               transistor electronics (cost, manufacturability,    temperatures), and manufacturing costs.
        An alternative approach to consider for           reliability and performance).                          An RF MEMS switch is not much different
     T/R switching comes in the form of RF                                                                    than an optical MEMS switch, but because
     MEMS switches. These switches can provide            A brief background                                  of the power-handling requirements of RF,
     low power consumption, lower insertion loss,            MEMS technology dates back to the 1970s          slightly different design techniques have to
     higher isolation, and excellent linearity as         when micromachines first began to see use in         be employed to limit and reduce the impact
     shown in Table 2 in a side-by-side comparison        the automotive industry as pressure sensors.        of current and the resultant heat on the contact
     to GaAs and SOI technologies.                        Further developments in the automotive              points of the switch. A typical switch is built
        The T/R switch had traditionally been the         industry led to the creation and implementa-        up with a cantilever (a suspended beam
     focus of many RF MEMS developers because             tion of MEMS accelerometers for collision           anchored at one point) and is actuated either
     of the highly compelling benefits of space and        airbags. Today, MEMS-based gyroscopes               electrostatically or electromagnetically. A
     power savings. The slowed rate of introduction       are being implemented to help fine tune              contact head rests at the “floating” end of
                                                                                                                    each cantilever and is comprised of
      In the electronics and high-technology industries,                                                            conducting metals not typically used
                                                                                                                    in semiconductor fabrication.
     the MEMS switch was seen as a hopeful entry into                                                               Problems and solutions
       the optical switching market, which significantly                                                                Fabrication: RF MEMS devices
                                                                                                                   are fabricated as layered structures on
                   slowed in the year 2000.                                                                        top of a variety of substrates. The manu-
                                                                                                                   facturing options include semiconductor
     of RF MEMS switches and the cost-competi-            location finding in automotive GPS systems.          processing, which tends to be the most popular
     tive nature of T/R switching has caused the             In the electronics and high-technology           method on either silicon or ceramic substrate
     market to move beyond the price range of             industries, the MEMS switch was seen as a           wafers. The ceramic substrates method can be
     RF MEMS. But a new switching application             hopeful entry into the optical switching mar-       more expensive, but allows for easy packag-
     has emerged in the form of band switching.           ket, which significantly slowed in the year          ing by hermetically bonding the lids on to
     Today, the demand for worldwide compat-              2000. The collapse of this main market driver       the RF MEMS wafer and sawing/singulating
     ibility of mobile phones has forced phone            caused MEMS switch development to stall.            the finished components. MEMS switches
     developers to implement triple and quad-band         But the continual growth of the mobile phone        can be built from almost any semiconductor
     and multimode solutions. Switching between           market along with the evolving problems of          technology including polysilicon and GaAs.
     up to eight different wireless bands compli-         multiband/multimode phones has spurred              This makes for a truly agnostic technology,
     cates the possibility of developing a single         a renewed interest in MEMS for switching            which when given time and resources can be
     all-encompassing world phone. Figure 2               and other capabilities.                             adapted to almost any fabrication process.
     illustrates the potential need for a switch that
     functions as a superset to the T/R switch.             Technology          Insertion         Isolation        Linearity        Return          Power
         RF MEMS can potentially provide a solid                                Loss (dB)           (dB)            (dBm)          Loss (dB)        (mW)
     replacement for existing solid-state switches,
                                                           CMOS                    0.00                0               0                0             5
     but the devices have been on the verge of
     breaking into the high-tech electronics indus-        SOI                     1.50               20               30              20             15
     try for more than 20 years. The main barriers         GaAs                    0.90               35               48              35             35
     for development fall into the similar categories
                                                           RF-MEMS                 0.45               40               70              40            <1.0
     that were faced when the integrated circuit
     was on the verge of competing with discrete          Table 2. Typical specifications for a single-pole, double-throw RF switch at 6 GHz.

22                                                                                                                   September 2005
RF Design   23
         RF-MEMS                                                               Die over-                                       <500 µm
         Cantilever                                                            Molding

      Semiconductor                                                             Gold (Au)
      Substrate                                                                 Contacts
                                                                                                      <500 µm                                      Conductor

     Figure 3. Cut-away view of RF MEMS level die sealing.

     The main concern in manufacturing is the                 Hermetically sealed ceramic
     flatness of the substrate so that the photoli-         packaging is typically used be- Electrostatic
     thography steps used in the process result in         cause it can protect the ac-           Charges                                Conductor
     consistently flat layers to build up each struc-       tuators, but the packaging costs
     ture. Standard silicon tends to be an excellent       are far greater than the typical Figure 4. Illustration of the electrostatic actuation of RF MEMS
     choice for this because the wafer-polishing           plastic molding used to house
     step is part of the process. Ceramic wafers           GaAs switches and standard semiconductor hermetic seal, which protects the RF MEMS
     are not normally polished or smoothed and so          components. Ceramic packaging differs in and eliminates the need for the more expensive
     adding this step to the process adds to the over-     price from plastic packaging by as much as ceramic packaging. This method is perhaps
     all cost of the manufacturing, which in turn          five times due to materials and assembly costs. the most promising because it could enable
     makes RF MEMS on ceramics somewhat cost               On the other hand, standard plastic costs less, manufacturers to house RF MEMS in read-
     prohibitive for mobile phone applications.            but it does not offer sufficient environmental ily available plastic packages in a variety of
         Solid-state circuits and RF MEMS are              isolation to be contamination free. Typical standard surface-mount configurations.
     similar in that they have various layers of           ceramic packages resemble those used in SAW           Control and operating power concerns:
     conducting and insulating materials. Alumi-           oscillators and filters. They are comprised of Typical RF MEMS devices are mechani-
     num (Al) and now copper (Cu) have become              a ceramic base with vias and a ceramic lid. cally actuated (displaced) in one of two ways,
     the primary conducting materials found in             The lid alone typically comprises the majority either electromagnetically or electrostati-
     silicon processing. RF MEMS developers use            of the cost of this choice of packaging.          cally. Electrostatic actuation is probably the
     these conducting materials to create the metal           However, lower cost and newer packaging simplest method of mechanical switching,
     layers needed to create the electrostatic or          alternatives exist, which can enable RF– but it requires higher voltages to create the
     electromagnetic fields to actuate or move the          MEMS to solve problems in mobile phones. forces that move the cantilever. This method
     cantilever beams. But the switch contact points       These methods include die level sealing or involves creating a charged field much like
     have to be made with a highly conductive              overmolding and near hermetic chip scale a capacitor that deflects the cantilever in Figure
     material in order to complete the circuit path        packaging. There are various methods used 4. The problem is that the when compared to a
     needed in a micromechanical switch. So far,           to seal a die. Methods include growing a layer capacitor, a MEMS switch needs much larger
     the material of choice for conduction is pure         of semiconductor insulator or depositing a thin voltages to create the needed electric field.
     gold (Au). The majority of standard silicon           film over the device as shown in Figure 3.            Over the years, the actuation voltages have
     fabs do not have Au processes and this limits            The overmold can be grown using semi- dropped from 100 volts to about 50 volts
     development. Pure-play MEMS and specialty             conductor processes or deposited as a thin and lower. In addition, the structures cannot
     fabs use Au in their processes and thus open          film. In either case, the process results in a handle high currents above 500 mA because
     up the possibilities for device development.
     Other fabrication vendors use GaAs, which
     normally include gold implantation in their
     process, but a limiting factor of this technology            RF                                                         3V         RF
                                                                          > 50 V
     is the inability to integrate the high-voltage
     control circuitry needed to actuate the RF
     MEMS switches. However, new developments
                                                                                                                                          > 50 V
     in GaAs fabrication are showing promising
     possibilities for full integration of control and
     switching functions.                                                                                                 MEMS
          Packaging concerns: Package cost and                                                    RF
                                                                                                         Control         Controller                    RF
     size are the leading considerations in T/R                                                                           Block
     switch development. The high demand for T/R
     switches has driven the cost to the tens of cents,                                                                                   > 50 V
     which makes it difficult to provide RF MEMS
     for this type of application. At the same time, the
     choice of packaging is constrained by the fact                        Control:
     that RF MEMS switches are moving devices                     RF                                                                    RF
     that have to operate in a clean environment. A                       > 50 V
     clean environment is needed because the me-
     chanical construction and minute dimensions                    High Voltage Version                                  Low Voltage Version
     of the switches makes them vulnerable to con-
     tamination from various environmental sources
     such as dust, moisture and gas vapors.                Figure 5. Conversion of a high-voltage RF MEMS switch into a low-voltage device.

24                                                                                                                  September 2005
RF Design   25
                                                  of the heat created by the resultant power.       nanoseconds in order to make for a seam-
                                                  Thus, the actuation voltage has to be cur-        less transition between transmit and receive.
                                                  rent-limited (dc) in order to avoid damage to     The mobile phone user should not notice
                                                  the actuator.                                     any difference in the quality of the call when
                                                      Mobile phones do not have these types         talking or listening. RF MEMS devices switch
                                                  of voltages available and so it becomes nec-      in the tens of microseconds, which makes them
                                                  essary for the MEMS developer to provide          far too slow for the T/R switch application.
                                                  an intermediary step that enables an RF               On the other hand, as a band switch, an
                                                  MEMS switch to operate at much lower              RF MEMS device has both the qualities of
                                                  voltages. Dc-dc voltage conversion can be         switching speed and hot switching lifespan
                                                  used to do this task. With more integration,      to make it an appropriate fit for the applica-
                                                  a voltage converter and logic controller can      tion. Band switches need not switch as fast
                                                  be integrated with a high-voltage RF MEMS         or as many times as a T/R switch. Once a
                                                  device to create a low-voltage solution as        band is selected, the switch stays connected
                                                  shown in Figure 5.                                to the corresponding throw until another
                                                      There are various dc-dc voltage conversion    band is detected or required. In a typical
                                                  methods available including transformers          multiband application a band switch is used
                                                  and multistage amplifiers. CMOS fabrica-           along with duplexers and T/R switches.
                                                  tion along with MEMS is achievable, but               The band switch serves as a routing de-
                                                  MEMS require a good contact material for          vice that enables other components to do the
                                                  the actuators such as gold (Au) or gold alloys.   needed functions of filtering and bandpassing.
                                                  Gold is normally deposited on the surface         Since a band switch is an additional compo-
                                                  of die as die-attach material, but implanting     nent in the signal path, it must have as little
                                                  gold in between semiconductor layers is           impact on the overall performance of each
                                                  complicated and forces changes into the           band and this makes RF MEMS switches
                                                  typical fab process flow. The Au process           ideal candidates because of the low power
                                                  hurdle and the availability of high-voltage       consumption, small overall size, low insertion
                                                  transistors for the voltage conversion por-       loss, and high isolation as previously shown
                                                  tion of the circuit complicate integration        in Table 2.
                                                  into an RF MEMS switch. These factors                 In conclusion, the availability, cost and
                                                  have limited the integration that has been        performance of RF MEMS switches for
                                                  achieved to date.                                 mobile phones will continue to improve.
                                                      Current implementations of voltage conver-    First, applications have evolved to such a point
                                                  sion next to RF MEMS switches leverages           as to mandate an alternative to semiconduc-
                                                  the packaging techniques of monolithic mi-        tor solutions. Second, new fabrication and
                                                  crowave ICs (MMICs) and multichip modules         packaging techniques will enable the mass
                                                  (MCMs) where multiple die are integrated into     production of the devices and accelerate their
                                                  one package. Ultimately, the goal is to reach     market acceptance. The fabrication techniques
                                                  a monolithic or fully integrated switch solu-     that have evolved to support low-cost solu-
                                                  tion. The illustration in Figure 5 shows the      tions complete a virtuous cycle of product
                                                  eventual path toward full integration. With       definition where volume drives cost and
                                                  fab availability there will be no problem in      cost drives applications. RFD
                                                  providing low-voltage RF MEMS switches.
                                                  Currently, several semiconductor technol-
                                                  ogy vendors in CMOS, SOI and GaAs are                    ABOUT THE AUTHORS
                                                  investigating this path toward integration.
                                                                                                       Refugio Jones is a product manager at
                                                       Long-term switching life-cycles and
                                                                                                       WiSpry Inc., developer of low-cost,
                                                  switching speed: A standard T/R switch
                                                                                                       high-performance RF-MEMS tunable
                                                  sees in the order of hundreds of millions
                                                                                                       radio frequency (RF) components and
                                                  of switching cycles in its lifetime of use,
                                                                                                       modules for the wireless industry. He
                                                  which can typically run from two to four
                                                                                                       is a 10-year veteran of the analog and
                                                  years. The conditions can vary from low
                                                                                                       linear semiconductor industry and holds
                                                  current to high current hot switching in low
                                                                                                       a B.S.E.E. degree from the University
                                                  voltages. An RF MEMS switch can typically
                                                                                                       of Arizona in Tucson, AZ.
                                                  actuate up to tens of billions of times under
                                                  no load and current conditions before the            Mark Chapman is vice president of
                                                  switch begins to near the useful end of              business development for WiSpry Inc.
                                                  its lifetime. But when hot switched, the             His past experience includes serving
                                                  lifetimes can drop to the hundreds of millions       as the president and CEO of Ditrans
                                                  of cycles. This puts RF MEMS switches                Corp., and executive level positions
                                                  at the edge of usefulness in T/R switch              for Comarco Wireless Technologies
                                                  applications.                                        and Rockwell Semiconductor (now
                                                      In addition, the switching speed of a            Conexant).
                                                  T/R switch needs to be in the tens of

     Circle 17 or visit

26                                                                                                      September 2005

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