MEMS switch technology extended

Electrical test and measurement systems, defence systems applications, and healthcare equipment are some areas that can reach previously unattainable levels of performance and form factor, all enabled by MEMS switch technology.

When compared to traditional electromechanical relays, the MEMS switch technology of Analog Devices (ADI) enables a huge leap forward in RF and DC switch performance, reliability, and in miniaturisation.

Contemporary switching technologies all have drawbacks with no one technology ideal. Relay drawbacks include narrow bandwidths, limited actuation lifetimes, limited number of channels, and large package sizes. MEMS technology has always had the potential to deliver RF switch performance with orders of magnitude improvements in reliability in a small form factor, compared to relays.

Unique challenges

Unlike many other functions for which MEMS devices have been commercialised for many years, RF switches present unique challenges. Achieving wide bandwidths, high reliability, and commercial producibility in a tiny RF MEMS switch has been an elusive goal since the technology was first realised. However, according to sources at ADI, the path is now open for MEMS to deliver performance unachievable by any other technology.

ADI has MEMS history: the first MEMS accelerometer product successfully developed, manufactured, and commercialised was ADI’s ADXL50 accelerometer, which was released as long ago as 1991.

Central to ADI MEMS switch technology is the concept of an electrostatically actuated, micromachined cantilever beam switching element. It can be thought of as a micrometre scale mechanical relay, with metal-to-metal contacts that are actuated via electrostatics.

The switch is constructed on a high resistivity silicon wafer, which has a thick dielectric layer deposited on top to provide superior electrical isolation from the substrate below. A standard back-end CMOS interconnect process is used to realise interconnections to the MEMS switch. Low resistivity metal and polysilicon are used to make an electrical connection to the MEMS switch, this embedded into the dielectric layer. To wire bond pads elsewhere on the die, metal vias are used to provide a connection to the switch input, output, and the gate electrode. The cantilever MEMS switch itself is surface micromachined using a sacrificial layer to create the air gaps under the cantilever beam.

It has been obvious for decades that the microelectromechanical systems (MEMS) switch could potentially replace PIN diode, mechanical, FET, and other types of switches in a broad swath of RF and microwave applications. A MEMS switch is smaller and lighter than any other switch technology; has very little insertion loss; provides very high isolation; can operate well into the millimetre-wave region; has exceptionally low intermodulation distortion; and can handle reasonable amounts of RF power.

So why didn’t RF MEMS switches take the world by storm years ago? The answer lies in the obstacles that have defied developers’ efforts to tame them, primarily suitability for low-cost mass production and demonstrated reliability over billions of switching cycles.

The latest Analog Devices MEMS SP4T switches, the ADGM1004 and ADSM1304, operate from DC to 13 and 14 GHz, respectively. Both devices are about 95% smaller, 10X more reliable, 30X faster, and consume 10X less power than a typical RF switch. They are available in quantity, have demonstrated reliability over more than 1 billion switching cycles, and use advanced packaging techniques to ensure high resistance to electrostatic discharge, the latter being one of the major traditional challenges.