Belgian semiconductor research lab IMEC is renowned for developing advanced CMOS processes, collaborating with firms including Intel and Samsung.
A few years ago it branched out to develop sensor technology as well, in a programme called CMORE.
The results are world-class technology demonstrators, and a toolbox of technologies including CMOS, silicon-photonics, MEMS, image sensors and packaging, as well as design, testing and reliability.
Companies can come to IMEC to have sensing products developed, and even have them made in the on-site 200mm chip fab.
As one of its capability demonstrations, IMEC recently unveiled a poly-SiGe piezoresistive pressure sensor built on top of 0.13µm copper interconnect CMOS.
Building micromachines over CMOS saves space compared with adjacent construction, but the high temperatures needed to make conventional poly-Si MEMS will ruin underlying CMOS.
Moving parts can also be constructed from the CMOS metallisation layers, but the metal is not as reliable as polysilicon, and there are other issues.
“The MEMS-last approach is the most interesting approach for CMOS MEMS monolithic integration as it leads to smaller die areas and enables integrating the MEMS without introducing any changes in standard foundry CMOS processes,” said IMEC.
Polycrystalline SiGe is emerging as a promising MEMS structural material since it provides reasonable mechanical properties at lower temperatures than poly-Si.
With poly-SiGe, MEMS fabrication can be completely decoupled from the CMOS fabrication, offering a more generic and flexible technology for above-CMOS integration.
IMEC has already proved the potential of poly-SiGe for MEMS above-aluminium-backend CMOS integration.
“However, aggressive interconnect scaling has led to the replacement of the traditional aluminium metallisation by copper, due to its lower resistivity and improved reliability. Our results now broaden the applications of poly-SiGe to the integration of MEMS with the advanced CMOS technology nodes,” said IMEC.
“This latest demonstration represents not only the first integrated poly-SiGe pressure sensor directly fabricated above its readout circuit, but also the first time that a poly-SiGe MEMS device is processed on top of copper backend CMOS,” said IMEC
The sensor has a poly-SiGe membrane, four poly-SiGe piezoresistors, and an instrumentation amplifier, all fabricated in IMEC’s 0.13µm standard CMOS technology, with two layers of copper interconnect, oxide dielectric and tungsten-filled vias.
To prevent damaging the CMOS, a passivation layer was included to protect the underlying transistors from MEMS etch and deposition, and the maximum processing temperature of the complete sensor was kept below 455ºC - including fabricating the poly-SiGe piezoresistors.
“The CMOS showed no significant deterioration after the MEMS processing,” said IMEC.
With a 250x250µm sensing membrane, the raw sensor showed 2.5mV/V/bar sensitivity, rising 64x to 158mV/V/bar with the underlying amplifier in-circuit.
The lab also develops applications for its CMORE sensing technology.
Characteristic of its clean-sheet approach is a wireless tyre pressure monitor programme.
Now a legal requirement for new cars in the USA, built-in tyre pressure monitoring is also likely to become mandatory in Japan and Europe.
Existing solutions are battery-powered, requiring new batteries every five years or so, and weighing enough to affect wheel balance.
IMEC set itself the task of designing sensor package that could transmit tyre pressure to the car’s computer, powered entirely by locally-harvested energy.
It added the transmission of a tyre serial number at the request of tyre makers, and three-axis acceleration data - All in a package that could be permanently bonded to the inside of a tyre.
Using commercial off-the-shelf components would require 7mW, claims IMEC, of which 15% would be consumed by the pressure sensor, compared with 82% for the wireless transmitter.
Using in-house wireless protocols - low-power wireless is another speciality - the lab is proposing a 100µW design with a micromachined generator to powering it, now all in proof-of-concept form.
The power budget is: pressure monitor 39µW, 315/434MHz transmitter 27µW, acceleration sensor 12µW, microcontroller 21µW, and ADC 1µW.
“The mass works out to under 10g so you don’t need to align [balance] the tyre,” said IMEC engineer Rob van Schaijk.
Mechanics for the generator are a micromachined mass and cantilever, with a piezo capacitor on the cantilever to convert flexing into electrical power.
Many attempts at mechanical energy harvesting require a high-Q mechanical structure to resonate at a particular vibration frequency available in the system concerned - for example, 50Hz harvesters that attach to electric motor housings for telemetry.
Tyres have no accurately-defined mechanical resonant frequencies, so IMEC employs a high-Q resonator (fr=929Hz) that is flicked into motion by individual road shocks.
Once in motion, energy can be extracted until oscillation dies away - A 120gravity shock produces a +/7Vac decaying transient from the prototype generator.
In practice, these shocks are frequent, says van Schaijk: “There are large shocks available inside a tyre. Up to 100µW at 100km/h is feasible.”
Soft x-ray sensing
Away from mechanical sensing, IMEC has designed and is manufacturing extreme ultra-violet (EUV - ‘soft’ x-ray) radiation sensors for Dutch photo-lithography equipment firm ASML.
ASML is developing EUV production chip scanners for future-generation ICs that will have features as small as 14 or 11nm - Its pre-production NXE:3100 EUV lithography tool is actually installed at IMEC.
Wafers and beams have to be aligned within the tool with extraordinary accuracy under harsh radiation.
Towards this, two of the IMEC sensors calibrate, align, and focus the tool’s lens system, and a third one monitors the EUV dose.
“These are now being integrated in ASML’s in the field, improving the tools’ overlay and critical dimension tool performance,” said IMEC.
“Next to these EUV sensors, CMORE is developing several other customer chip solutions. They include technology from IMEC’s strategic areas such as bio-sensing, energy- and power management, and specialty imaging and photolithography among others,” it added.
See: Imec says high speed SiGe:C devices are ready for volume