IEEE SENSORS JOURNAL, VOL. 7, NO. 2, FEBRUARY 2007 219 A Novel Saw Device in CMOS: Design, Modeling, and Fabrication Onur Tigli, Student Member, IEEE, and Mona E. Zaghloul, Fellow, IEEE Abstract—The design, finite element modeling, fabrication, and characterization of a novel surface acoustic wave (SAW) delay line for bio/chemical and telecommunication applications in CMOS technology are introduced. A full modeling was carried out. The devices are designed in a standard semiconductor foundry 1.5-μm two-metal two-poly process. A unique maskless postprocessing se- quence is designed and completed. The three postprocessing steps are fully compatible with any standard integrated circuit tech- nology such as CMOS. This allows any signal control/processing circuitry to be easily integrated on the same chip. ZnO is used as the piezoelectric material for SAW generation. A thorough char- acterization and patterning optimization of the sputtered ZnO was carried out. The major novelties that are introduced in the SAW delay line features are the embedded heater elements for temperature control, compensation, and acoustic absorbers that are designed to eliminate edge reflections and minimize triple transit interference that is amplified by edge reflections. Both of these attributes are designed by using CMOS materials without disturbing SAW performance. Index Terms—Absorber, CMOS, finite element modeling, heater, surface acoustic wave (SAW). I. I NTRODUCTION S URFACE ACOUSTIC wave (SAW) devices are widely used as electronic filters, delay lines, and resonators in today’s communication systems. Although the telecommuni- cation industry is the largest user of these devices, SAW- based sensors have many attractive features to be explored for emerging technologies in automotive (torque, pressure), med- ical (biosensor), and commercial (vapor, gas, humidity) appli- cations [1]. They are small, inexpensive, can easily be designed for responding to various measurands, have wide dynamic range, and are passive devices which can also be deployed as wireless units. SAWs (both Rayleigh and pseudo-SAW) are generated at the free surface of a piezoelectric material. An application of a varying voltage to the metal interdigital transducer (IDT) gen- erates the acoustic wave on the input side. In the basic configu- ration, there is an input IDT and an output IDT [2]. The acoustic wave generated by the input IDT travels through the region called the delay line and reaches the output IDT, where the me- Manuscript received March 15, 2006; revised June 26, 2006 and July 11, 2006. This paper was supported by the National Science Foundation under Grant 0225431. The associate editor coordinating the review of this paper and approving it for publication was Prof. Fabien Josse. The authors are with the Department of Electrical and Computer Engi- neering, George Washington University, Washington, DC 20052 USA (e-mail: tigli@gwu.edu; zaghloul@gwu.edu). Color versions of Figs. 1, 2, 5–12, 14, and 15 are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2006.883771 Fig. 1. Basic principle of a SAW-based bio/chemical sensor employing the mass loading scheme. This architecture (i.e., IDTs on top of the piezoelectric material) is the most common architecture. chanical displacements due to the acoustic waves create a volt- age difference between the output IDT fingers. One of the most widely used and interesting sensing mechanisms that acoustic wave sensors employ is mass loading. Prominent applications are in film thickness monitoring, gas, liquid phase chemical sensing, and biosensing. The delay lines of SAW devices are coated with some bio/chemical coating, which selectively re- acts with the entity under analysis. This interaction produces a shift in the resonant frequency of the SAW device. By mea- suring this shift in frequency domain, a detailed analysis of the entity being sensed can be completed. Fig. 1 depicts the basic principle of SAW-based bio/chemical sensors employing the mass loading scheme. Using a combination of integrated circuit (IC) compatible technologies, such as Si micromachining, thin film deposi- tion, bio/chemical layer growth, integrated electronics, smart structures, and systems, can be realized [2]. Successful SAW- based sensors have been demonstrated in [2]–[4]. These include sensors to measure density, viscosity of liquids, to determine the properties of polymer films, to detect quantitatively the concentrations of volatile organic vapors, and both selective and sensitive detection of biochemically active compounds. Most of these devices are designed in full custom fabrication methods. Developing SAW devices on Si requires several fabrication steps, including deposition, etching, and patterning of various materials. Therefore, designing these devices in full custom fashion requires extra effort and increases the cost of produc- tion. There is a great interest in developing methods that would comply with current mass production standards in electronics without disturbing performance of well characterized SAW devices. The major challenge in this interest is to design a well- defined and characterized fabrication process (including the postprocessing steps) that would allow the designers to build SAW devices with comparable performance to the currently available off-the-shelf products. Therefore, it is of major inter- est to develop sensor systems employing standard IC technol- ogy such as CMOS processes. Considering the advantages that 1530-437X/$25.00 © 2006 IEEE