Sensitive, selective, and analytical improvements to a porous silicon gas sensor

Abstract

Conductometric porous silicon gas sensors consisting of a sensitive surface layer which is conducive to the rapid and reversible transduction of sub-ppm levels of analyte gas have been developed. Several new fabrication and testing methods allow the detection of a number of analytes including CO (<5 ppm), NO_x (<1 ppm), SO₂ (<1 ppm), and NH₃ (500 ppb). We outline a progression of fabrication techniques, including an HCl cleaning process, which allow the formation of much more efficient porous silicon sensors. Selectivity and enhanced sensitivity are developed using electroless metal deposition to form a gold or tin oxide nanostructured framework interacting with the nanopore-coated microporous surface. The ability to monitor sensor response in the presence of external noise sources is increased with the introduction of an FFT filtering technique. These studies present the first detection of CO with a porous silicon sensor as well as a considerable improvement in the sensitivity to NH₃. It is suggested that a diffusion-based model can be used to parameterize the response of the sensors. We demonstrate several applications of these sensors to the monitoring of gas mixtures as exemplified by the NH₃/NO_x system. The potential for these sensors in arrayed configurations with integrated CMOS devices is considered.

Introduction

Gas sensors are multifarious in modern industry. For each application, a different sensor configuration is usually needed to meet the criteria for a particular application. An arrayed device, capable of being inexpensively calibrated for a prescribed set of analyte gases, would present an ideal device for a diversity of applications. Efforts to form such a device utilizing a hybrid nanoporous/microporous silicon medium as the transduction site have produced effective individual gas sensors [1]. The present devices have been demonstrated not only to be uniquely low-cost and low-power devices, due to the formation of electrolessly deposited low resistance <20 Ω contacts [1], [2], but also are amenable to the creation of micro-fabricated arrays with integrated CMOS circuits. A novel process for coating the devices using the electroless deposition of metals provided enhanced sensitivity and selectivity to NO_x, CO, and NH₃. This process also allows the fabrication of the first room-temperature tin-oxide facilitated CO sensor, and represents the first porous silicon sensor to have sensitivity to CO.

At a recent NSF/NIST “Process Measurement and Control Workshop” several critical challenges for further sensor development were identified. Individual sensor systems should be produced cost effectively for any scale processes. Government and industry standards will require the operation of a device in a safely monitored and environmentally friendly manner. Key to developing these new capabilities must be the need for easier calibration models, methods to transfer calibration sensors with given performance properties and/or process parameters, calibration transfer among multiple ‘like’ sensors, and transitioning from laboratory development to plant operation.

Utilizing CMOS fabrication methods, we are developing a system to provide cost-effective monitoring of air quality and polluting gas emissions. We discuss the fabrication of porous silicon based sensors, which, while operating at room temperature, are selective to a wide variety of these polluting gases. To date, we have produced a highly sensitive, rapidly responding room temperature device based upon a porous silicon interface that is able to repeatedly detect gaseous, HCl, NH₃, SO₂, CO, and NO_x.