EETimes.com – Startup builds MEMS in standard CMOS processes
LONDON — Baolab Microsystems SL (Barcelona, Spain) is a startup company pioneering the creation of microelectromechancial systems (MEMS) within the back-end-of-line structure of CMOS wafers.Whereas most MEMS make use of the mechanical properties of silicon, Baolab has chosen to construct NanoEMS using the interconnect metal of conventional CMOS processes in the hope of creating a low-cost addition to CMOS with superior potential for integration. The company, founded in July 2003, is applying its technology initially to the field of switching for miniature RF relays inside CMOS, enabling smaller mobile phones with improved battery life.
Baolab claims that the process it has developed is easier and quicker with fewer process steps than existing MEMS fabrication techniques that build the MEMS on the surface of the wafer. The in-process method can reduce the cost of MEMS by up to two thirds the company said. Typical MEMS structures are built in a separately optimized process semiconductor process and brought together with CMOS control circuitry as a two-chip solution or as a multichip component.
The Baolab NanoEMS technology uses the existing metal layers in a CMOS wafer to form the MEMS structure using standard mask techniques. The inter-metal dielectric (IMD) is then etched away through openings in the passivation layer using hydrogen fluoride vapour (vHF). The etching uses equipment that is already available for volume production and takes less than an hour, which is insignificant compared to the overall production time. The holes are then sealed and the chip packaged as required. As only standard CMOS processes are used, NanoEMS MEMS can be directly integrated with active circuitry as required.
As part of its development process Baolab has successfully created MEMS devices using standard 0.18-micron process on 200-mm diameter volume CMOS wafers with four or more metal layers, and has achieved minimum feature sizes down to 200 nanometers. This is an order of magnitude smaller than is currently possible with conventional MEMS devices, bringing NanoEMS into the realm of nanostructures, with the additional benefits of smaller sizes, lower power consumption and faster devices. The finished device can be packaged the same way as a conventional CMOS device.
“We have solved the challenge of building MEMS in a completely different way,” said Dave Doyle, Baolab’s CEO. “MEMS are regarded as fairly cost ineffective, with one or two notable exceptions, such as inkjet print heads and pressure sensors. They have to be either built on top of the wafer at a post production stage or into a recess in the wafer. By contrast, our new NanoEMS technology enables MEMS to be built using standard CMOS technologies during the normal flow of the CMOS lines.”
Doyle said Baolab plans to make a range of discrete MEMS including RF switches, digital compasses and accelerometers, along with solutions that combine several functions in one chip. The prototype stage has already proved the NanoEMS technology and evaluation samples will be available later this year. These are aimed at handset designers and manufacturers, and power amplifier and RF front-end module markets.
Building mechanical structures in metal does provide a number of challenges. For example, vHF is quite selective with respect to AlCu compared with the silicon dioxide dielectric. However, despite being a good etchant to free the BEOL it is not easy to control the zone that it etches. Baolab claimed to have solved this by a combination of design rules and a process tweak to change the refractive index of the silicon-nitride passivation layer.
Another challenge is that the metal layers are thin and not optimized for MEMS design. This could be addressed by adjustments to the CMOS process but Baolab wanted to have solutions that are foundry independent so the company has created designs that anticipate and allow for these issues. Similarly, sophisticated design work overcame the problems of metal creep by the aluminium along with limited voltage and current carrying capabilities.
Doyle said that larger Asian foundries tend to insist on allowing no changes to their processes, at least initially, while smaller specialist foundries that are looking for differentiation were prepared to consider allowing for thicker metal layers to support mechanical design.
Doyle declined to name Baolab’s foundry partners. “We are working with two of the largest foundries in the world. We’ve gone out of our way to make sure we are not impacting the CMOS process. We wanted something that was transferrable between foundries,” Doyle said. “We are also working with some smaller foundries, who can offer process flexibility,” he added. “By the end of 2010 we expect to be delivering engineering samples of a switch and control chip pair, and an integrated chip.”
Baolab was founded in 2003 by Josep Montanya i Silvestre, who serves the company as chief technology officer. The company has raised 6.5 million euro (about $10 million) including a 3 million euro Series A round from a Spanish venture capital fund nearly two years ago.