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MEMS is the new analogue

Posted: 16 Oct 2008     Print Version  Bookmark and Share

Keywords:MEMS  RF design  frequency masks  IC manufacturing 

Out-of-the-box thinking is necessary if mobile phone design is to enable the ubiquitous services that future subscribers are going to expect. Many industry observers believe the next wave of enabling technology will come from MEMS design.

Indeed, MEMS devices have proven their usability in high-volume consumer market applications as diverse as microphones and gaming consoles. We are reaching a point where no system will be completed without the integration of MEMS functionality. In this way, MEMS is the new analogue, the element every system needs to make it functional, flexible and able to interface with the outside world.

While Moore's Law describes the progression of transistor density and computing power, the integration of MEMS will act as a multiplication factor and integrate most of the functions required in hybrid implementation directly on the chip.

Today's strongest trend in RF design is the drive towards configurable/band-free radio and antenna design. It is increasingly beneficial and ultimately necessary to make RF components digitally reconfigurable, so that frequencies and impedance levels can be accurately and digitally controlled, and continuously optimised for best system performance. Such a reconfigurable front-end is able to switch among frequencies and communication standards literally in a blink of an eye, while reusing the same signal path.

By combining MEMS technology with mainstream IC manufacturing processes to build a low-loss RF capacitor element with on-the-fly digital tunability and cost efficiency, WiSpry has enabled dynamic RF technology that, in turn, enables a true software-defined radio, where the RF front-end is digitally controlled by the base band and all the standard-specific functions are loaded as DSP programs. Once the front-end has become digitally tunable, much of the RF engineering work moves into the software domain, greatly reducing the number and cost of hardware redesigns and time spent manually tuning a circuit.

The programmable front-end may be used across multiple platforms and even provides some "future proofing," as new responses may be loaded into the platform's firmware.

Conflicting standards
Today, most wireless standards use two frequency masks for transmitting and receiving data—also called frequency duplexing—within the frequency band stipulated by the spectrum allocation plan. The regional differences in spectrum allocation and the rapid evolution and sheer number of competing wireless communication standards worldwide have led to a multiplicative effect on the number of frequencies a global cell phone platform must support. The drive to use the radio spectrum as efficiently as possible and to support new services in previously unused spectrum gaps is driving this trend.

However, the technical requirements that devices must fulfil to gain network entry have not changed. High-performance individual hardware solutions for the RF front-end are required to provide the necessary selectivity, linearity and isolation while minimising insertion loss and power use in the circuit.

With the first cell phones, radio designs were single-band, and subscribers were excited about placing a call away from their desk. RF designers had one frequency band with which their design had to work.

As technology quickly progressed, however, dual-band phones suddenly became a requirement to support an increasing number of users. As users started to travel with their phones, triple-, quad- and penta-band phone designs quickly became the norm, adding to the designer's headache.

MEMS is the new analogue; we are reaching a point where no system will be complete without the integration of MEMS functionality.

As bands have been added, the additive RF design approach has become progressively less tenable. The simplest problem is the expansion of size, cost and complexity.

Adding band coverage is a nearly linear progression. It is sublinear as, first, the switching solution has continually improved along with increases in the throw count. Second, technology enables the individual band elements to be smaller and less costly in each successive generation. And third, many of the individual elements are now combined into modules, reducing overhead but not solving the fundamental issue.

There is now a widespread realisation within the cell phone industry that simply continuing on this path is not feasible. Beyond the complexity, size and cost implications, the multi-chain approach imposes a fundamental performance limitation.

Each chain has different impedance characteristics for the band of interest. If each chain had an individual antenna, the overall chain could be optimised. However, individual antennas are neither space- nor cost-effective and can have significant cross coupling. Thus, the chains are forced to combine into a single path using switches and filters.

Even assuming perfect switches, it becomes increasingly difficult to maintain high performance for all bands as new bands are added, since compromises must be made in the shared circuitry.

At the same time, since every component in the chain has its specific fixed-frequency response, the band edge performance is typically suboptimal.

One chain approach
All of the above issues can be avoided if the RF front-end components are tunable. A single chain can then be optimised specifically for the channels currently in use.

Research into tunable front-end components has been progressing for a couple of decades, but only now are the requisite technologies maturing. The traditional challenges include size, cost, repeatability, reliability and performance. Each of these has been partly addressed in the past, but WiSpry is the first to bring to market a complete solution that suits high-volume production at a price point that is compatible with the cell phone industry.

By pioneering the integration of high-quality (Q) factor MEMS capacitor elements into a mainstream RF CMOS process technology, WiSpry brings together the benefits of a high-volume, low-cost process with the advantages of high-performance RF MEMS technology. Individual capacitor elements are integrated on-chip as tiny parallel plate capacitors with a digitally variable air gap. Individual shunt or series elements are combined into capacitance cells and then into arrays that can contain any combination of individual cells, resulting in a digital capacitor that is well behaved and free from higher modes, with capacitance ratios (max/min) greater than 10 and a Q-value well over 200 at 1GHz.

The manufacturing of this device benefits from the latest advancements in CMOS IC process technology. WiSpry is using a fabless model that integrates the programmable digital capacitor technology monolithically on mainstream 8-inch RF CMOS wafers that can be produced in extremely high volumes, reducing the size and cost concerns traditionally associated with high-performance MEMS technology.

The process flow also includes wafer-level encapsulation so that finished wafers from the foundry can be used directly in common, automated back-end processing such as bumping, thinning, dicing, packaging and test, making for a highly reliable end product that can be produced in a traditional RF IC manufacturing flow.

- Marten Seth
Director, Product Planning and Business Development

- Arthur Morris
Chief Technology Officer

WiSpry Inc.

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