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 have been very effective, but they add cost and complexity to assays, provide only end-point interrogation, and often limit multiplexed detection. A move towards real-time, label free assays provides 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 frequency allows increased 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.
In this research, we have 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-chiposcillators at 850 MHz and 1.45 GHz have been demonstrated, and the integrated sensors have a mass sensitivity many times higher than that of a traditional QCM. 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.
The sensor platform has been 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.
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This paper reports first results of volatile organic compound (VOC) detection on a monolithically integrated film bulk acoustic resonator (FBAR) array on a silicon integrated circuit substrate. The combined sensor platform uses thin polymer layers as gas absorbers for individual FBAR functionalization, and frequency shifts are measured on-chip in response to changing VOC concentration. Integrating sensors, drive, and read- out functionality on a single CMOS die enables a robust, multiplex sensor platform and obviates external measurement equipment.
Johnston, M. L.; Kymissis, I.; Shepard, K. L., “FBAR-CMOS Oscillator Array for Mass-Sensing Applications,” Sensors Journal, IEEE , vol.10, no.6, pp.1042-1047, June 2010.
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Thin-film bulk acoustic resonators (FBAR) are an effective platform for sensitive biological and chemical detection, where their high operating frequencies make them many times more sensitive than a quartz crystal microbalance. Here, we present a monolithic, solidly mounted FBAR oscillator array on CMOS for mass-sensing applications. Through monolithic integration with CMOS drive circuitry, we aim to overcome the spatial and parasitic load limitations of externally coupled resonators to build dense sensor arrays without specialized fabrication techniques. The sensors in this work are constructed in a 6 4 array atop a 0.18μm CMOS active substrate, and mass sensitivity comparable to off-chip FBAR sensors is demonstrated.