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FBAR-CMOS Sensor Platform

Traditional chemical and biological assays rely on secondary reporters for detection of binding events, as with the use of fluorescent reporters for microarrays or colorimetric enzyme labels for immunoassays. These techniques add cost and complexity to assays, provide only end-point interrogation, and often limit multiplexed detection. A move towards real-time, label-free assays would confer many advantages. We are working towards this goal using piezoelectric resonant sensors on CMOS.

A thin-film bulk acoustic resonator (FBAR) can be employed as the micron-scale equivalent of a quartz crystal microbalance (QCM); mass attaches to the surface of a piezoelectric crystal, causing the resonance frequency to decrease slightly. Whereas a quartz crystal sensor operates in the megahertz regime, FBAR structures resonate in the low gigahertz regime. Their small size allows array integration of sensors, similar to a microarray, and the increased resonance frequency allows for increased detection sensitivity. Both of these features make FBARs ideal for direct CMOS integration, where sensors can be built in dense arrays and used without bulky external measurement equipment.

We previously fabricated FBAR structures monolithically on a custom CMOS substrate. The resonators are solidly mounted, and mechanical isolation is achieved with a multi-layer acoustic reflector. Monolithic fabrication enables an array of integrated resonators, and the underlying CMOS circuitry forms an independent FBAR-CMOS oscillator around each device. The CMOS substrate also contains a dedicated digital frequency counter for each oscillator, enabling parallel on-chip frequency measurement of all sites. On-chip oscillators from 850 MHz to 1.45 GHz have been demonstrated, and the integrated sensors have a mass sensitivity hundreds of times higher than that of a traditional QCM.

The sensor platform has been successfully applied to volatile organic compound (VOC) quantification, where a semi-selective polymer layer absorbs low concentrations of VOC vapors, causing a frequency shift in the underlying resonator. This interaction is reversible, allowing vapor concentration to be quantified continuously and in real time. Future work will extend this technology to broader chemical and biological sensing applications.

In addition to sensing, this methodology may find significant utility in RF applications, where it enables monolithic integration of high-Q elements directly on a standard CMOS substrate.



Figure 1. FBAR-CMOS sensor platform at decreasing zoom, including monolithic FBAR resonator on CMOS surface (A), array of 24
resonators on custom 0.18µm CMOS chip (B), and functional hand-held prototype system with sensor array at center 

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