Laser Assisted Chemical Vapor Deposition of Tin-Oxide Gas Sensors Using Tin Tetrabromide As a Precursor

The modem production processes used to produce today\u27s goods and services have created many pollution problems. The increase in industrialization makes it absolutely necessary to constantly monitor and control air pollution in the environment, factories, laboratories, and domestic situations. Several types of gases are used in different areas of industrial development. For example, some gases are used as raw materials in production processes, while others are by-products of a reaction process. In either case to prevent human exposure to toxic or flammable gas highly sensitive gas detectors must be developed. These gas detectors should be able to continuously monitor the concentration of particular gases in the environment in a quantitative and selective fashion. In order to perform effectively, gas sensors must operate under certain constraints and meet certain requirements. First a sensor must be chemically selective, which means it is able to give a distinct response for distinct gases. Sensor response must also be reversible, in that the sensor returns to the original state once the interacting gas species is removed. The response should also be fast, so that high concentrations of dangerous gas do not build up before detection. Other key factors in sensor design are durability, simple to operate, portability (small size), and simple to fabricate. The types of sensors that appear to show the most promise are solid-state sensors. Solid-state sensors undergo changes in electrical parameters, i.e. a change in electrical conductivity, induced by adsorption or reaction of gases on the solid surface. Solid-state gas sensors tend to be small in size and easy to fabricate offering a considerable decrease in production cost. The primary goal of this work is to optimize fabrication and characterization techniques for the development of a thin film solid-state tin-oxide gas sensor. Though gas sensors have been developed using other materials tin-oxide is an attractive material because it is inexpensive, not very reactive, and has electrical properties that make it ideal for sensor development. Tin-oxide has been used to create bulk thick film sensors for a number of years, however interest in its used as a thin film sensors has just recently become of great interest. The use of tin oxide as a thin film sensor material offers more efficient sensing, because of lower power requirements for sensor operation. The nature of the thin film also provides greater gas sensitivity in the sensors. Thin film based tin-oxide gas sensors allow for the use of a broad range of fabrication techniques such as reactive sputtering and chemical vapor deposition (CVD), which allow the production of smaller sensing devices. Recently laser assisted chemical vapor deposition (LCVD) has proven to be a useful technique in the fabrication of microelectronic devices7 • Our sensor fabrication technique uses the combined photothermal and photolytic properties of laser radiation to assist in the photochemical formation and deposition of tin-oxide on the surface of various quartz substrates. The crystal structure of the substrate has been shown to influence the crystal growth of the material being deposited or grown on the substrate surface. The nature of the substrate was varied in our experiments to determine the effective of the substrates crystal characteristics on tin-oxide film growth