dreds of volts across a thin, metalsemiconductor-metal sandwich. Such devices were prone to drift induced by irreversible electrochemical reactions at the semiconductor-metal interface and were not easily accessible to ambi ent gas exposure. By using a microfabricated surface conductivity mea surement cell having the semiconduc tor film coated onto an interdigitated electrode structure (Figure 3), these problems can be prevented. Because the "fingers" of the interdigital elec trode are very long relative to their close spacing, the electrode array has a very high ratio of electrode perimeter distance to interelectrode distance, and measurable ohmic currents can often be obtained from typical organic semiconductors with low applied bias voltages (e.g., 1 V). The electrodes are frequently made of gold to form an ohmic contact and occupy a few square millimeters of area or less. Metal-substituted phthalocyanine films, which are less than 1 μηι thick, are interesting chemiresistor coatings because they respond to a variety of vapors in a sensitive and often revers ible fashion (15). Furthermore, the central metal atom has a significant effect on the selectivity exhibited to various vapors. The mechanism of op eration is not well understood, but it is believed to involve the formation of a weak charge transfer complex between
the vapor and semiconductor, which alters the number of charge carriers and hence the measured resistance. Chemiresistors are attractive because of their very low cost and size and their high sensitivity (ppm detection limits are common) to a number of or ganic and inorganic vapors. Thin-film tin oxide gas sensors. Tin oxide and other metal oxide semi conductors are sensitive to low con centrations of vapors (16). In opera tion a sintered block of the semicon ductor is heated to several hundred degrees centigrade and its electrical conductivity is monitored. Reducing or oxidizing gases that interact with adsorbed oxygen on the hot surface can cause a dramatic change in the de vice conductivity. Tin oxide is the basis for the Taguchi gas sensor man ufactured by Figaro Engineering in Japan, which is used extensively for explosive vapor monitors, combustion hazard alarms, and breath alcohol me ters. Recently, this technology has been applied to microfabricate a sensor (17) (shown in Figure 3). A thin film of tin oxide is formed over a set of lithograph ically defined electrodes deposited on a thermally conductive substrate. The reverse side of the substrate has elec trodes with a resistive film deposited over them. This film can be used as a heater for the device. Thus, the com-
SYSTEMS F0« RESEARCH up to 10,000,000 watts of peak power From deep UV to infrared _ 10 nanoseconds to 20 milliseconds Are you doing research on the following? • Specialized Photography • Photochemistry • Photobiology
• Fluorescence Lifetimes • E.S.R. Spectrometry We welcome inquiries for custom flashiubes and custom pulsed haht systems. X E N O Nc o r p o r a t i o n 66 Industrial Way, Wilmington, MA 01887 (617) 658-8940 TWX: 710-347-0630 CIRCLE 240 ON READER SERVICE CARD
94 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
XE-001
plete sensor-heater package can be made quite small, which is of great value in conserving heater power con sumption. Many organic vapors are detectable at the 1-10-ppm level. The primary disadvantage of this technol ogy is the relatively poor selectivity exhibited for specific vapors. Numer ous partially successful attempts have been made to modify the selectivity of the tin oxide with empirically derived additions of noble metals or other metal oxides. Indeed such modifica tions have formed the basis of dozens of patents (primarily from Japan) fo cused toward specific detection of ethanol, methane, carbon monoxide, and other gases of interest. More details of the operational characteristics of metal oxide semiconductor gas sensors can be found in References 18 and 19. Microdielectrometer. The microdielectrometer is a direct descendant of the charge flow transistor microsensor first described by Senturia and co workers in 1977 (20). The device, presently manufactured by Micromet, consists of a planar interdigital microelectrode array, one side of which is attached as a floating gate to an onchip FET charge amplifier (Figure 3). The other side of the microelectrode array is driven with a sinusoidal volt age. By measuring the amplitude and phase differences of the signal applied to the driven gate and the signal pro duced by the floating gate, it is possi ble to determine the complex imped ance (i.e., the resistance and capaci tance) of the medium in contact with the electrode. The integration of the FET amplifier and microelectrode permits extremely weak currents to be measured easily. Thus, the device can provide dielectric information over a frequency range not readily accessible by any other technique. Microdielectrometers are capable of monitoring the cure of epoxy resins and have an on-chip temperature sensor to permit use at elevated cure temperatures (21 ). The device can also be used to monitor the impedance of thin films coated onto the microelectrode. Mea surements of the ac conductivity of coatings that are poor insulators (e.g., most undoped organic semiconduc tors) can be obtained quite readily. Thus, chemical microsensors can be made with this device if one coats it with a material that undergoes a change in either conductivity or di electric constant when it is exposed to the species of interest. Microdielectrometers coated with aluminum oxide have been studied as humidity sensors (22). Other Microsensor Approaches Surface acoustic wave sensors. Chemical microsensors using the sur face acoustic wave (SAW) phenome-
