Sensors and Actuators B, 7 ( 1992) 661-664 661 Polymer membranes for modification of the selectivity of field-effect gas sensors Eva Hedborg, Anita Spetz, Fredrik Winquist and Ingemar Lundstriim Laboratory of Applied Physics, Linkiiping Institute of Technology, S-581 83 Linkiiping (Sweden) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Abstract Polymer membranes are used to increase the selectivity to certain gases of metal-silicon dioxide-semiconductor (MOS) structures. Other parameters which influence the selectivity of MOS structures are the type of gate metal, its microstructure (dense or porous) and the operating temperature of the device. Photoresists as membranes can be patterned by photolithographic methods. Membranes, 1-2 pm thick, of positive and negative photoresist are applied on MOS capacitors with 6 nm iridium as the gate metal, operated at 150 “C. The influence of the membranes on the response to three gases, hydrogen, ammonia and ethanol, has been investigated. The hydrogen response decreases by about half with the use of a photoresist membrane. The ammonia response shows a characteristic change in the kinetics, while the ethanol response almost disappears. Positive and negative resist influence the gas response in similar ways, in spite of their different molecular structures. 1. Introduction Detection of different gases is important in many fields, such as food technology, industrial processes, medical diagnosis and environmental protection. Thin-film metal-silicon dioxide-semiconductor (TMOS) structures have been investigated for their use as gas-sensitive devices [ 1, 21, and have found many applications [3,4]. Exposure of a TMOS capacitor to gas causes a shift of the capacitance- voltage characteristic along the voltage axis. This voltage shift is related to the amount of detectable molecules in the ambient. For thin porous metal films, charged species or strong dipoles located on the surface of the metal or on the insulator between the metal grains are detected through a capacitive coupling to the semiconductor surface. This capac- itive-coupling model was originally developed to explain the ammonia sensitivity of porous metal gates [5], but is valid for every gas that creates charges or dipoles on the sensor surface. Hydrogen and molecules that dissociate on the metal surface and deliver free hydrogen atoms to the metal contribute an additional sensing mechanism. The hydrogen atoms diffuse through the metal grains and build up a dipole layer of hydrogen atoms at the metal-silicon dioxide interface. This dipole layer at the metal-insulator interface also causes a voltage shift in the capacitance-voltage characteristics. 0925-4005/92/$5.00 The sensitivity and selectivity of gas sensors with catalytic metal gates depend on parameters such as the operating temperature of the sensor, the type of catalytic metal used for the gate and the microstructure of the metal gate [2, 61. Poly- mers on top of a thin metal gate can act as membranes, i.e., only certain gases can penetrate the membrane, while others are hindered by it. The polymer can also react with a certain gas and, for example, deliver reaction species which are detected. Gases that permeate the polymer mem- brane might change the work function properties of the polymer. This will also affect the capaci- tance-voltage characteristics of a TMOS capaci- tor with a polymer membrane on its gate. Other groups have used microelectronic fabrica- tion technology and photolithographic processes to pattern polymers for electrochemical sensors [7,8]. Positive and negative photoresist can be applied by spinning on the metal gates, and it is possible to pattern them by standard lithographic processes. The molecular structures of positive and negative resist are different, which implies the pos- sibility of different selectivity patterns. Thin iridium (Ir) films are used for this investi- gation due to their good long-term stability [9]. The effect of positive and negative photoresist as membranes on Ir TMOS structures has been inves- tigated for the response to hydrogen, ammonia and ethanol. @ 1992 - Elsevier Sequoia. All rights reserved