Fabrication and Measurement of a Suspended Nanochannel Microbridge Resonator Monolithically Integrated with CMOS Readout Circuitry

Abstract

We present the fabrication and characterization of a suspended microbridge resonator with an embedded nanochannel. The suspended microbridge resonator is electrostatically actuated, capacitively sensed, and monolithically integrated with complementary metal-oxide-semiconductor (CMOS) readout circuitry. The device is fabricated using the back end of line (BEOL) layers of the AMS 0.35 μm commercial CMOS technology, interconnecting two metal layers with a contact layer. The fabricated device has a 6 fL capacity and has one of the smallest embedded channels so far. It is able to attain a mass sensitivity of 25 ag/Hz using a fully integrable electrical transduction.

Keywords: CMOS-NEMS; NEMS; nanochannel; resonators; suspended channel.

Figure 1

Conceptual sketch of a suspended nanochannel resonator (SNR) formed by an in-plane microbridge resonator and a nanochannel embedded inside.

Figure 2

Illustrations of the suspended nanochannel microbridge resonator at different stages of the process to empty the nanochannel and release the bridge. For the sake of clarity, the size proportions of the different elements of the suspended nanochannel microbridge resonator (access holes, electrodes-resonator gaps, bridge nanochannel aperture, etc.) are modified compared to the actual device. (a) Simplified 3D illustration of the suspended nanochannel bridge resonator once empty and released. The dashed lines indicate the four different cross-sections. (b) Cross-section D-D’ of the fabricated suspended nanochannel bridge resonator showing all the layers of the AMS 0.35 μm commercial CMOS technology. PROT1 and PROT2 correspond to passivation layers; MET1, MET2, MET3, and MET4 to metal layers; VIA1, VIA2, and VIA3 to contact layers; FOX, ILDFOX, IMD1, IMD2, and IMD3 correspond to insulator layers. The evolution of the cross-section D-D’ is not shown, since it remains unchanged after the completion of all the carried out processes. (c) Cross-sections A-A’, B-B’, and C-C’ showing the bridge with the built-in nanochannel and the access holes at different stages of the emptying and releasing fabrication process.

Figure 3

Scanning electron microscope (SEM) images after focused ion beam (FIB) milling showing suspended nanochannel microbridge resonators fabricated using different layers and presenting nanochannels of different widths. (a) MET3-VIA3-MET4 nanochannel of 450 nm width, the minimum width allowed by the technology. (b) MET2-VIA2-MET3 nanochannel of 3 μm width.

Figure 4

Optical image of the suspended nanochannel microbridge resonator and its adjacent CMOS readout circuitry at some intermediate step in the emptying and releasing process.

Figure 5

SEM images of a fabricated suspended microbridge resonator 10 μm long and 1.45 μm wide, with a built-in nanochannel of 450 nm, before and after some FIB cuts. (a) General view of the suspended nanochannel bridge resonator before making any FIB cut. (b) SEM image showing the interior of the nanochannel after a first FIB cut. We can easily appreciate that the nanochannel is empty for a quarter of its length. (c) SEM image showing the interior of the nanochannel after a second FIB cut. The nanochannel is empty for almost half its length. (d) SEM image showing the interior of the nanochannel after a third FIB cut. We see that the nanochannel is empty for more than half its length. Thus, it will be completely empty since the BHF bath etches silicon dioxide from both extremes. (e) SEM image showing the interior of the nanochannel after a fourth FIB cut. From the sequence of FIB cuts, we see that the nanochannel is completely empty from access hole to access hole. (f) SEM image showing a zoom in of the nanochannel. The cross-section image shows that the nanochannel has a width in the base of 470 nm and a height of 865 nm.

Figure 6

Experimental frequency response in air conditions of the capacitively detected suspended nanochannel microbridge resonator with monolithically integrated CMOS readout circuitry for different resonator DC voltages. The device has undergone the following wet etchings: 18 min + 18 min + 18 min + 10 min. (a) Magnitude frequency response. (b) Phase frequency response.