Nanotech group tackles ‘grand challenges’

Imec, an international consortium focused on micro and nano technology, has a large global staff, and that’s a good thing because it has a lot on its plate.

Headquartered in Leuven, Belgium, Imec has additional offices in the Netherlands, the U.S., China, Taiwan and Japan. Its staff includes 1,400 people plus another 500 guest researchers and residents from companies that work with Imec on various projects. Research areas include smart electronics, health-care technology, sustainable energy and transportation safety.

CMOS below 22nm

Key issues on Imec’s agenda include research into the production of sub-22nm CMOS (complementary metal-oxide semiconductor) nodes. (The 22nm node, which refers to the distance between chip features, was introduced this year by semiconductor companies.) Researchers are investigating materials, transistor architectures, tool and process steps, integration options and process technology platforms that can be used in high-volume integrated-circuit manufacturing.

Today, Imec is focusing mainly on the 14nm node, which is scheduled to enter production in 2015, according to Rudi Cartuyvels, the consortium’s senior vice president of smart systems and energy technologies.

Electronic devices for life-science applications are a key part of Imec’s portfolio. Shown is Imec’s 3-D brain probe for in-vivo applications. All images courtesy Imec.

In the process of scaling to 22nm and smaller nodes, Cartuyvels describes Imec’s task as developing potential technology solutions for its industrial partners. “Our role is to select options that will work for manufacturing. Then our partners will make their own selections of what they will implement in their fabrication facilities,” he explained.

Nearly all of the leading companies in the semiconductor industry have representatives onsite at Imec. By working together in the early stages of technology development, these companies share development costs, noted An Steegen, Imec’s senior vice president of process technology.

Just as important, they share ideas with other key industry players. “We tackle the grand challenges,” said Aaron Thean, director of Imec’s logic program. “Without collaboration upfront, it is difficult to overcome them.”

One semiconductor technology of interest to Imec is FinFET, short for fin field-effect transistor. The semiconductor industry is accustomed to building transistors on a flat silicon surface. With FinFET, silicon devices are no longer flat. Instead, they include tiny “fins” that protrude from the surface.

What’s the advantage of this configuration? Consider Intel’s version of FinFET, called Tri-Gate, where the fins are 3-D channels through which electricity flows. The flow is surrounded on three sides by gates, a configuration that allows the gates to achieve tighter control than gates on just one side of the flow, which is the case with conventional planar transistors. As a result, transistors can operate at lower voltages and experience less leakage, improving performance and energy efficiency.

According to Cartuyvels, Imec started working on FinFET about 8 years ago, pioneering the technology with its partners. Intel is now using FinFET technology in 22nm chips, he added, and other manufacturers may follow to meet the demands of smaller nodes.

When it comes to lithography processes for producing 22nm-node and smaller semiconductors, Imec is developing both argon fluoride (ArF) and extreme ultraviolet (EUV) options. For ArF, new techniques are needed “to get to the next level of printability and scalability,” said Steegen. Next-generation EUV lithography components, which Imec has been working on for years, include tooling, masks and resists.

Imec won’t be picking a winner in the ArF vs. EUV competition. “We are responsible for delivering solutions for both,” said Steegen, adding that Imec’s manufacturing partners will choose ArF or EUV based on factors such as cost and throughput.

MEMS-CMOS combo

Another recent Imec breakthrough was the fabrication of a polysilicon-germanium-based piezoresistive pressure sensor directly above a 0.13µm, copper-backend CMOS-processed substrate. This is the first time a poly-SiGe MEMS device has been processed on top of such a 0.13µm CMOS, according to Cartuyvels.

In the past, Imec had integrated poly-SiGe MEMS above aluminum-backend CMOS layers made with 0.18µm and older process technologies. In producing its latest MEMS-CMOS combination, Imec replaced aluminum with copper because of copper’s lower resistivity and superior reliability.

Cross-section SEM photo of the integrated pressure sensor. At the bottom, the two Cu metal lines of the CMOS circuit are visible. Seen above are the MEMS layers (the poly-SiGe membrane and piezoresistors, the oxide sealing layer and the metal interconnects).

Compared to alternative approaches for combining MEMS and CMOS, Imec’s technique allows MEMS fabrication to be separated from CMOS production. In addition, this “MEMS-last” approach to CMOS-MEMS integration results in smaller die areas and doesn’t require any changes to standard foundry CMOS processes, according to Imec.

Imec’s new MEMS-CMOS combination includes a micromachined sensor with a poly-SiGe membrane and four poly-SiGe piezoresistors, along with an instrumentation amplifier with copper interconnects in two metal layers, oxide dielectric and tungsten-filled vias. Imec also included a passivation layer to protect the electronic circuit during the aggressive etch and deposition steps used to fabricate the MEMS device. In this fabrication process, MEMS and sacrificial materials are deposited, then the sacrificial materials are removed, leaving the free-standing poly-SiGe MEMS structure.

Polycrystalline SiGe was chosen for the MEMS material because it provides the desired mechanical properties but doesn’t require extremely high processing temperatures. “Polysilicon is a very good material, but the deposition temperature of a polysilicon process on top of CMOS is way too high,” said Cartuyvels. To allow above-CMOS integration, the maximum sensor processing temperature was kept below 455°C.

MEMS-CMOS integration offers superior performance. According to Imec, the sensitivity of the poly-SiGe piezoresistive sensor alone is approximately 2.5 mV/V/bar, while the sensitivity of the integrated sensor-CMOS amplifier combination is about 158 mV/V/bar—nearly 64 times higher than the standalone sensor. “Bringing the MEMS very close to the electronics reduces the parasitics of the interconnect, which gives you much better sensitivity from a system-level perspective,” Cartuyvels explained.

At present, Imec is developing prototypes for customers interested in using the poly-SiGe MEMS device for sensing tire pressure. Instead of a battery that would need to be replaced periodically, Imec would like to power the sensor with an “energy harvester” capable of converting vibration energy produced by a moving vehicle into electrical energy, Cartuyvels said.

Solving medical problems

New electronics technology also figures prominently in Imec’s life-sciences work. The consortium combines biological and semiconductor knowledge to create problem-solving electronic devices. Examples under development include chips that interface with living tissue to measure how the tissue reacts to drug treatment, as well as smart probes with embedded electronics that provide neurosurgeons with accurate readouts of electrical signals in the brain.

An Imec development project with potentially lifesaving impact involves nanoelectronic tools for detecting cancer cells in blood at a very early stage of the disease.

The challenge, Cartuyvels said, is to find “one cell out of a billion in a 5ml blood sample. Today, the tools to do that analysis are very slow. We are developing tools that speed up the process significantly.” µ