ISSCC 2022: MEMS senses zeptograms
Author: EIS Release Date: Mar 7, 2022
Zeptogram’ caught the eye at ISSCC 2022 conference last week, as researchers from French lab CEA-Leti produced a mass sensor with 170zg resolution for weighing molecules.
Not only did they make one such sensitive weighing scale, but 1,024 of them in an array which can be scanned at high speed.
The weighing part had been revealed in scientific papers before, and what was discussed in detail at ISSCC was the multiplexed signal processing scheme invented to allow so many sensors to be packed into a 0.5 by 0.8mm patch.
Each sensing element is a 160nm think micromachined ribbon 300nm wide and ~10µm long suspended above the substrate (see diagram).
A choice of two capacitively-coupled electrodes drive the strip: a full-length one (connected to electrode VA,M1) drives the whole strip either one way or the other, and a half-length electrode (VA,M2) moves one-end-one-way and the-other-the-other-way (with a stationary node in the centre) – resonant frequency is ~28Mz or ~75MHz respectively.
All the 28MHz drive electrodes are wired together across the array, and the same with all the 75MHz drive electrodes.
ISSCC2022 12.7 Leti zeptogram memsThe other six electrodes are multiplexed per pixel. With the four VG,xx electrodes (which are each connected to the ribbon via a piezoelectric link) supplying bias voltages, and the two VSx electrodes providing output waveforms.
This passive MEMS chip is die-stacked on top of a 5V 130nm CMOS die that provides most of the electronics – the six (non-drive-electrode) connections of each pixel dropping through the top die on through-silicon vias.
Underneath, each pixel gets its own differential pair output amplifier and multiplexing switches.
Bias voltage waveforms are provided to each of the 32 columns of pixels in turn, to be picked up by the appropriate in-pixel switches, and the differential output pairs are sent via more switches along the 32 rows to per-row active-loads, which also have gain.
Signal levels are around 100mV for the electrostatic drive electrodes and 100μV coming off the piezo pick-ups.
Together, the local differential pair and the row-end active load provide 15dB of voltage gain and 2MHz unity-gain-bandwidth, while drawing <500μW from 3V.
The rest of the signal processing is off-chip, with all the pixels time-sharing one sophisticated digital phase-locked loop.
For each pixel, the PLL takes in the two amplified VSx signals, and establishes the exact full-length and half-length resonant frequencies by driving the capacitive drive electrodes in turn. This takes ~10ms per pixel.
The device has been designed for use inside the vacuum of a mass spectrometer to detect neutral particles. Measuring resonance at two frequencies allows the mass of a particle to be measured, along with its position on the ribbon.