CMOS and RF-MEMS Integration

By Refugio Jones and Michael Hopkins The portable communications market has seen major changes in the last five years. Average mobile phone (wireless terminal) talk times are now up to four hours, while stand-by times are surpassing nearly three hundred hours. The demand by wireless terminal users to increase these times is driving operating voltages to new lows. The issue becomes more complex as services such as instant messaging, Simple Messaging (SMS), high-speed Internet access and e-mail are paired with on-board video/still camera and MP3 playback. Meanwhile, the mobile industry is driving towards third generation (3G) technologies such as WCDMA (UMTS), HSDPA and WLAN (VoIP), while GSM, EDGE and CDMA continue to support the greater part of the worlds mobile phone technologies. In order to provide these services and improve the quality of a mobile phone call, the once common 3.7 volt power supply (battery) in a mobile phone is moving to 2.7 volts and lower.

EGSM 900 RX EGSM 900 TX EGSM 1800 RX EGSM 1800 TX GSM 800 RX GSM 800 TX GSM 1900 RX GSM 1900 TX

GSM Transceiver

GSM DSP

CDMA Transceiver

GPS RX RX TX RX TX RX TX CDMA 800/900 DCS / PCS WCDMA 2100 TRIPLEXER

Figure 1The component complexity found in a typical mobile world phone.

The move to lower supply voltages is causing micro-electronic component makers to revise their product offerings. At the same time, Radio Frequency (RF) Micro Electro Mechanical Systems (MEMS) technologies are beginning to get the attention of systems vendors because of their potential to surpass the performance of semiconductors. RFMEMS devices such as tunable filters offer ultra-low insertion loss, high isolation and low Voltage Standing Wave Ratio (VSWR) performance. But there are many factors that limit the use of this technology in mobile phone applications. One such factor involves supplying control voltages to the micro machine elements. Considering that todays microelectronic systems are migrating to below three volts and will eventually go as low

CDMA DSP

HIGH-THROW T/R SWITCH

SAW & FBAR DUPLEXERS

POWER AMPLIFIERS

SAW & BPS FILTERS

BASE BAND CONTROL UNITS

ISOLATORS

DISCRETE ANTENNA TUNING

FRONT END RADIOS

as one volt leaves little room for wide spread adoption of these RF-MEMS components. This obstacle is rather difficult to surpass because RF-MEMS come in two varieties: low voltage / high current (electro-magnetic) or high voltage / no current (electro-static). The problem for mobile phone designers illustrated in Table 1 is in providing either the high currents required for one technology or the high voltages required for the other.

Electro-Magnetic Control Voltage 3V 5V Control Current ~ 100 mA ~ 125 mA

Electro-Static Control Voltage 30 V 50 V Control Current ~ 0.25 A ~ 0.50 A

Table 1 Comparison of control voltages and currents needed in RF-MEMS devices.

What is an RF-MEMS device? An RF-MEMS device is sub-millimeter mechanical system consisting of shunt or serial switching elements. The devices are specifically designed to act as and replace static radio frequency passive or active/passive components. For instance, a MEMS capacitor built from a cantilever (a suspended beam anchored at one or more points) is actuated (displaced) either electro-statically or electro-magnetically. A dielectric material rests under the cantilever and is comprised of an insulating material. These micro-elements can be fabricated in much the same way as a semiconductor where layers of materials are deposited, patterned, and etched into forming the basic architecture of the target devices whether they are capacitors, inductors, or switches. Standard semiconductor fabrication (FAB) plants can accommodate the manufacture of these devices in high volumes but, the process is non-trivial because of precise etching requirements. In keeping with the capacitor example, a typical dielectric such as Silicon Dioxide has to be layered in the right amounts in the right places for the structure to be useful. The process involves encasing the dielectric in materials that wash away during chemical etching. Although difficult at first, the process yields good results with repeated efforts and so almost any FAB can adapt their processing recipes to form these structures from layers of conductors and insulators.

Control and Operating Power Concerns RFMEMS components are actuated (opened or closed) in one of two ways, either electro-magnetically or electro-statically. Both methods are viable, but each has its drawback. Electro-magnetic MEMS devices draw current through its control line into a micrometer sized coil to generate a magnetic field that pulls or releases the cantilever. These devices often use a ferromagnetic plate in the construction of the beams so as to have a constant magnetic pole for latching the device. The added bulk of a hard magnet opposing an electromagnetic circuit can result in a larger than desired device. The electro-static version requires a high DC voltage to create a charged field much like a capacitor to deflect the cantilever similar to the one shown in Figure 2. In contrast to electro-magnetic devices, the electro-static versions have a thin layer of metal within the

beam structure, which makes them susceptible to damage from heat created by excess current.

< 750m

< 200m

Dielectric Plate

MEMS Deflection

Electrostatic Charges

Figure 2 Illustration of the electrostatic actuation of a doubly anchored RF-MEMS capacitor

Over the years, electro-static MEMS actuation or control voltages have dropped from 100 volts to less than 50 volts. This type of capacitor can translate the control voltage into mechanical movement and so a change in the actuation voltage can effectively change the capacitance of the device. This is done by altering the cantilevers electrostatic field, which in turn alters the distance between the capacitor plates. This variable feature is useful because a capacitor can be tuned to different values and when it is designed next to micro-scaled static components, other MEMS capacitors and MEMS switches; it becomes an electro-mechanically tunable circuit.

RF
Input

Conductors

Dielectric Material

MEMS Cantilever

DC
Control

Figure 3 Cross section circuit view of a doubly anchored RF-MEMS capacitor

Wireless terminals, however, do not have the high voltages (or currents) needed for MEMS actuation in order to take advantage of tunable RF-MEMS technology. Thus it becomes necessary for the MEMS developer to provide an intermediary step that enables the device to operate at much lower power. In an electro-static device, DC-to-DC voltage conversion can be used to do this task. Ultimately, increased integration allows for a voltage converter and logic controller to be designed within these high voltage devices to create a monolithic low voltage solution.

There are various DC-to-DC voltage conversion methods available to use including transformers and multistage amplifiers. Possible implementations of voltage conversion with RF-MEMS switches leverages the packaging techniques of Monolithic Microwave ICs (MMICs) and Multi-Chip Modules (MCMs) where multiple die are integrated into one package. Ultimately, the goal is to reach a monolithic or fully integrated solution. The illustration in Figure 4 shows the eventual path toward full integration. With FAB availability, there will be no problem in providing low voltage RFMEMS switches. Currently several semiconductor technology vendors in CMOS, SOI and GaAs are investigating this path toward integration.

MEMS actuation voltage generation and control Typically, 50 volts or more are needed for electro-static MEMS actuation. Such a high voltage cant be found in a mobile terminal. In a monolithically integrated system, these voltages would be generated and controlled on chip in order to fully leverage the advantages of MEMS designs. Integrating MEMS and CMOS technologies as shown in Figure 4, increases the flexibility of a tunable filter. The MEMS structures can be placed without consideration of external bond pads and allow for some interesting circuit designs. The more notable benefit is that the integration greatly decreases system or PCB complexity by containing the high actuation voltages to the internals of the integrated circuit.

Control > 50 VDC

RFIN

3V

RFIN
> 50 VDC

Tunable MEMS Filter

Digital Control

Digital Controller and HV-Charge Pump

Tunable MEMS Filter

> 50 VDC

Control > 50 VDC RFOUT High Voltage Version

RFOUT Low Voltage Version

To-date, a lot of attention has been placed into developing methods and circuits to generate high-voltage actuation voltages. These approaches include having separate MEMS & CMOS controller circuits. While good, these solutions draw too much power and potentially increase the costs related to PCB space, reflow and long term quality of the mobile phone.

In order to realize the path to MEMS integration, WiSpry, Inc has partnered with Jazz Semiconductor of Irvine, Calif. to develop the first integrated CMOS Controller / MEMS device. The resulting active CMOS controller is a multifunction IC with the capability to convert 2.7 V to the high voltages needed to control WiSprys electro-static RF-MEMS. The controller includes a charge pump, high voltage FET switches to multiplex or steer the high voltages from the charge pump to the MEMS, and a digital CMOS processor to manage it all. Getting RF-MEMS into a low power application such as a mobile terminal requires an ultra-low power consumption device so as to set the solution apart the dual chip MEMS plus controller approach. The WiSpry solution features an integrated MEMS/CMOS controller IC that minimally impacts battery life with power consumption in the low hundreds of micro-Watts range. The controller features an adjustable power profile that sets custom levels of power consumption, giving the controller the ability to consume less than 100 micro-Watts. Behind all of this is a variable efficiency on chip Charge Pump and switch multiplexer which will use more or less power depending on changing the output drive. This Charge Pump features a high efficiency current-reuse boost mode, enabling quick circuit start-up with reduced power consumption. Add to that a low power, efficient tracking / lock mode for output voltage regulation. The Charge Pump can source up to 60uA of load current, enabling the integration of large numbers of MEMS devices on a single chip. The circuit in Figure 5 shows an abbreviated view of the Charge Pump with a single stage amplifier. The solution creates an on-chip reference from which the charge pump generates a high voltage output which feeds into what is a FET switch multiplexer. The array shown is a simple 1:2 DeMux, which can easily be expanded to a 1:8, 1:16, or 1:32 configuration. The key aspect of the design is that it utilizes CMOS transistors to steer the high voltages to the MEMS beams. With the exception of an external high voltage monitor pin, which enables visibility into the part, all MEMS actuation voltages are contained safely on chip.

VDD VREF ~ 50V

Digital Control

Figure 5 Example of a switched output charge-pump circuit.

+ +

Z1

VGate VOUT-1 VOUT-2

Z2

The CMOS DSP is used to digitally control the output FET DeMux. A simple three wire control scheme like SPI generates all of the driver states for a 32 output device. Essentially the outputs of the DSP toggle the high voltage translator switches in a single or simultaneous output configuration. Physically, the active circuitry for the MEMS CMOS integrated controller resides beneath the MEMS switches. Jazz Semiconductor developed a process that allows this physical integration without noise and spurious coupling from the on-chip active circuitry. This coupling immunity allows for the construction of on chip RF structures, which previously could not be integrated into a cellular phone.

Full Integration in a Mobile Terminal The full integration of RF-MEMS in a mobile terminal results in a system that consumes low current in a small space without sacrificing functionality. Tunable MEMS stand to replace passive devices designed to filter, balance, drive, or switch discrete frequency bands with single chip solutions that cover multiple bands. The devices themselves will allow mobile terminal designers to integrate a wider band antenna to cover all the frequencies used in an optimized world phone as shown in Figure 6.

TUNABLE POWER AMPLIFIERS

TUNABLE RFMEMS DUPLEXER

MULTI-BAND MULTI-MODE SWITCH

SAW & BPS FILTERS

GSM Transceiver

GSM DSP

WORLD GSM RX WOLRD GSM TX

CDMA / GPS DSP

CDMA Transceiver

GPS RX

TX RX

WORLD CDMA / UMTS

Figure 6 The reduced complexity in a typical mobile world phone with RF-MEMS.

As shown above, a multi-band RF-MEMS network will convert the world phone shown in Figure 1, into a system with a reduced bill of materials. At the head-end, a multi-band matching network will fine tune and balance the line found between the transceivers and the antenna. That transmission line often suffers from mismatched impedances, which can negatively impact the efficiency of the antenna. Discrete matching circuits are often used to bring these differences as close as possible to 50 ohms, but often time the circuits are unable to match the loads for different frequency bands. This results in a higher VSWR performance and reduces the maximum efficiency of the antenna. An RF-MEMS

MULTI-BAND RF-MEMS TUNABLE MATCHING NETWORK

BASE BAND CONTROL UNITS

SPDT T/R SWITCH

FRONT END RADIOS

tunable matching network can dial-in coverage for both the up-link or down-link of a given frequency band with low insertion and return losses. When coupled with an RFMEMS band switch, these line losses can be kept to a minimum while maximizing performance. Essential passive devices such as the Band Pass Filters (BPF) and duplexers become replaceable with a few high quality factor RF-MEMS devices that cover multiple bands. Here, the tuning of the MEMS allows for band selection with in-band fine tuning along with excellent out of band rejection. RF-MEMS duplexers can help replace the isolators or circulators because they can be designed to switch off and isolate the power amplifiers from the antenna in case the transmission line to the antenna breaks. Ultimately, the most attractive potential use of RF-MEMS would be in the power amplifier itself. The power amplifier (PA) is the only active device between the transceiver and the antenna. A typical world phone can use seven or more PAs and easily limit the board space and battery life of the mobile terminal. A tunable PA could possibly yield higher efficiencies, but a band selectable amplifier could do the job of three or more such units. When viewed as such, the increased efficiency stemming from using RF-MEMS between the transceiver and antenna greatly improves the potential for this new and exciting technology in mobile terminals.

Summary The mobile phone or mobile terminal will likely reach a billion or more units sold per year in 2006 or 2007. With so many phones available, the market will pressure manufacturers to drive the cost down to a minimum while increasing the functionality and performance. A typical mobile phone contains dozens of passives for filtering, switching, and tuning the transmission lines to and from the antenna. These passives take up considerable board space and increase the overall cost of the phone. The implementation of fully integrated RF-MEMS with a CMOS controller could bring about a seed change in the way that mobile phones are designed. This integration has taken years to see the light of day, but soon the reality of low voltage RF-MEMS will be available. In short, devices for tuning third generation (3G) technologies such as WCDMA (UMTS), WLAN, HSDPA, GSM, EDGE, and CDMA will change the landscape of the mobile phone with small, low cost RF-MEMS ICs.