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Integrated GaAs SAW Sensors
Acoustic Wave Chemical Detectors and Integrated Microelectronics

Fact Sheet

[GaAs and Quartz SAW Sensor] Figure 1: Comparison of GaAs and quartz SAW sensor response to perchloroethylene, a volatile organic compound. Similar responses indicate that GaAs SAW sensors have comparable sensitivity to quartz devices.
A novel integrated microsensor being developed at Sandia National Laboratories will enable increased sensitivity for trace detection and identification of a variety of chemical species. The sensor combines surface acoustic wave (SAW) technology with microelectronics to produce a monolithic, compact, low-power integrated circuit microsensor. The sensor is fabricated entirely using processing technology available in Sandia's Center for Compound Semiconductor Science and Technology (CCSST).

The combination of low power, compact size, high sensitivity and robustness makes integrated high frequency SAW sensors ideal candidates for applications requiring miniaturized and portable trace chemical detection, especially those requiring extended deployment in remote or inaccessible locations. Integrated SAW sensor arrays will allow both detection and identification of a vast number of chemical species. In the future, we expect these devices to be employed for:

[Photomicrograph] Figure 2: Photomicrograph of a monolithically integrated GaAs SAW sensor showing the SAW device and oscillator amplifier, This sensor operates at 470 MHz. Changes in the SAW velocity produce changes in the oscillator frequency.
Technical Approach
SAW devices consist of a piezoelectric substrate with two interdigitated transducers formed by photolithographic patterning of a thin metal layer. Application of an alternating voltage to the input transducer launches a surface acoustic wave that travels along the substrate surface before being converted back into an electrical signal by the output transducer. The velocity and attenuation of the propagating wave are very sensitive to the device's surface properties, such as absorbed mass and viscoelasticity. Quartz, the typical substrate for SAW sensors, is not a semiconductor while Silicon, the typical semiconductor used for microelectronics, is not piezoelectric and requires an additional piezoelectric film to enable SAW excitation. This makes gallium arsenide (GaAs) a unique material for use in integrated SAW microsensor applications. The piezoelectric properties of GaAs, which are similar to quartz, allow surface acoustic waves to be generated directly on a GaAs substrate. Chemical sensitivity of GaAs SAW sensors is comparable to that of quartz SAW sensors, as demonstrated in Figure 1. In addition, GaAs is a well-developed semiconductor device material for fabrication of high-frequency integrated microelectronics.

[Comparison of SAW Sensors] Figure 3: Comparison of 110 MHz and 380 MHz GaAs SAW sensors upon exposure to two concentrations of perchloroethylene. The higher frequency device is 10 times more sensitive yet occupies one-tenth the area of the lower frequency device.

The integrated GaAs SAW sensor is shown in Figure 2. It consists of a 470 MHz GaAs SAW device along with a multistage amplifier, forming a monolithic RF oscillator circuit. The amplifier circuit contains 4 gain stages and an impedance matching output stage encompassing 32 transistors. The transistors are fabricated using a standard 0.8 micron gate length metal-semiconductor field-effect transistor (MESFET) process developed in the CCSST at Sandia. Currently, 5 of these devices can be fabricated on 5 mm x 5 mm chip.

Monolithic integration of acoustic sensors and supporting microelectronics leads to a number of advantages including reduced power consumption, reduced size, simplicity of packaging, and more economical device fabrication. By eliminating the need for high frequency interconnections, integration is helping us move to higher frequency devices. As shown in Figure 3, increasing frequency provides enhanced sensitivity and smaller device size. The integrated SAW sensor is compatible with current state of the art SAW sensor coating technology developed for quartz sensors. Future designs of these devices will include:

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.
Steve Casalnuovo

(505) 844-6097

Ed Heller

(505) 844-1798

Greg Frye-Mason

(505) 844-0787

Last modified: August 23, 1999

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