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-chip oscillators 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.
- M. L. Johnston, H. Edrees, I. Kymissis, and K. L. Shepard, "Integrated VOC Vapor Sensing on FBAR-CMOS Array," The 25th International Conference on Micro Electro Mechanical Systems (MEMS 2012), pp. 846-849 (2012).
- M. L. Johnston, I. Kymissis, and K. L. Shepard, "FBAR-CMOS Oscillator Array for Mass-Sensing Applications," Sensors Journal, IEEE, vol. 10, no. 6, pp. 1042–1047 (2010).
- M. L. Johnston, I. Kymissis, and K. L. Shepard, "An array of monolithic FBAR-CMOS oscillators for mass-Sensing applications," Proc. of 15th International Conference on Solid-State Sensors, Actuators & Microsystems (Transducers 2009), pp. 1626-1629 (2009).