Microstructure and Activity of Thin Films for a Microelectronic Gas Sensor
- PDF / 4,586,886 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 86 Downloads / 179 Views
irreversible changes in film morphology which may alter the sensing characteristics of the device. A Pt/Ti film can be modified by heating in an oxygen atmosphere. The oxygen can partially oxidize the sensing film to form Pt/TiOx, which has different electronic characteristics than the metallic Pt/Ti film. With further oxidation, the film will become Pt/TiO 2 , a metalinsulator mixture. Platinum has been known as a catalytic material for a long time. It has been widely used in catalytic converters to oxidize uncombusted hydrocarbons, hydrogen, and carbon monoxide to carbon dioxide and water. On the thin film gas sensors, the Pt atoms can act as catalytic sites. The device, is capable of measuring small changes in temperature, which might result from catalytic combustion of gases. As a calorimeter, the change in temperature of the sensor's surface can be related to the concentration of a certain gas in the sensors immediate environment. This calorimetric effect may be used as method for enhancing the sensors selectivity to combustible gases. EXPERIMENT Device Fabrication The thin film sensor fabrication process involves several lithographic and micromachining steps. 3 The active surface of the sensor is a thin film which may be composed of metal or semiconducting materials. A 65 A Ti layer is deposited on all devices as an adhesion layer. Three different sensing films were characterized. The first set contained only the Ti sensing films. The second set had a Pt (75 A) layer and the third set had a layer of Au (50 A). All reported film thicknesses are monitored during the deposition process. The sensing films were thermally deposited on a 1.3 gim thick membrane containing three layers of silicon dioxide (SiO 2 ), silicon nitride (Si 3N 4 ), and SiO 2 . The membrane is suspended from a 500 gim thick outer rim of silicon. On top of the active sensing film, electrical leads of 400 A Ti under 1200 A Pt are deposited to define the four-point probe electrodes. Underneath the membrane is a meandering temperature sensing resistor (TSR) and a boron-doped silicon heater, capable of rapidly raising the sensing film temperature to well over 1000 *C. The dielectric membrane is stress compensated to make it resistant to thermal shock and it provides a high degree of thermal isolation between the sensing film and the silicon rim. Figure 1 shows a schematic cross section and a magnified top view of the sensor.
Heater Bonding Pads-.
Sensing
Ventconts dcCadcec
• Iurn t/ 300 - 500
Electrodes Eetoe
Hester ta
Silicon
&TSR
k
Cross Section Diagram (not to scale)
Figure 1. Top view and cross section diagram of the microelectronic gas sensor. Flow System and Experiments The sensing experiments were performed in a flow system with mass flow controllers to maintain accurate flow control over a variety of gases. A manifold is used for adding different compositions and concentrations of gases to an oxygen/nitrogen carrier stream. The
560
gases enter a quartz tube in a conventional tube furnace. The sensor has electrical connection
Data Loading...
