Quantitative Hydrocarbon Sensor for Ultra High Vacuum Applications David J. Davis, † Georgios Kyriakou, † Robert B. Grant, ‡ Mintcho S. Tikhov, † and Richard M. Lambert* ,† Chemistry Department, Cambridge UniVersity, Cambridge, CB2 1EW, England, and Lithography Subsystems, BOC EDWARDS, Manor Royal, Crawley, West Sussex, United Kingdom ReceiVed: September 30, 2006 A promising new quantitative sensing device intended for hydrocarbon detection under stringent technological conditions is described. It exhibits good sensitivity and reproducibility, as well as a useful degree of selectivity. The sensor is based on a novel approach that exploits the well-known properties of the oxygen/yttria-stabilized zirconia/platinum solid-state electrochemical system. Correlated spectroscopic and electrochemical measure- ments provide fundamental insight into the mode of action under potentiostatic conditions: the potential of the working electrode is set by the steady-state coverage of chemisorbed oxygen, which itself depends on the balance between rate of oxygen pumping to the Pt surface and the hydrocarbon impingement rate. Good quantitative agreement between theoretically predicted and measured hydrocarbon partial pressures is found. Nonlinearities that occur at sufficiently high hydrocarbon pressure are associated with the accumulation of small amounts of carbon at the surface of the sensing electrode. Introduction Semiconductor device fabrication is critically dependent on electron beam or photolithographic processing. The current state of the art involves usage of extreme ultraviolet (EUV) lithog- raphy, 1 which is envisaged to provide the basis for next- generation technology. In all these cases, but especially in the case of EUV lithography, which employs extremely costly optical stacks that incorporate multilayer mirrors, contamination of sensitive surfaces by adventitious hydrocarbon species (<10 -6 mbar) seriously damages system performance. Indeed, in the case of EUV lithography, hydrocarbon-induced loss of mirror reflectivity is the major obstacle to implementation of this technology. Accordingly, there is a need for compact, low cost, sensitive, and selective hydrocarbon sensors that could be distributed throughout large fabrication plants in order to protect key components by triggering appropriate countermeasures whenever an unforeseen incident leads to unacceptably high hydrocarbon partial pressures. Selectivity is an issue, because not all hydrocarbons are necessarily harmful. Generally speak- ing, whereas aromatics, unsaturates, and heavier hydrocarbons are harmful to varying degrees, shorter chain alkanes are considered benign. We have developed a novel approach to hydrocarbon sensing that is based on a new application of the robust and inexpensive oxygen/yttria-stabilized zirconia/platinum solid-state electro- chemical system, which is widely used in oxygen-sensing applications. The essential idea is that at temperatures around ∼800 K gaseous hydrocarbons react with adsorbed oxygen present at the sensing electrode surface leading to a steady- state oxygen coverage that determines the sensor voltage or current output, depending on the mode of operation (open circuit, pontentiometric, or amperometric). The use of Pt as an ap- propriate sensing electrode material for the range of species of interest has been demonstrated, 2,3 and it has also been shown that Pt/Au alloys should provide greatly enhanced selectivity so as to permit good discrimination between benign and harmful species. 4 Most recently we have demonstrated the validity of the proposed approach by constructing and operating a practical device under open circuit and potentiometric conditions. It was found that good, reproducible, sensitive, and fairly selective qualitatiVe sensing action could be achieved. The present paper extends the work to an investigation of quantitatiVe ampero- metric sensing action under potentiostatic conditions. A com- bination of in situ X-ray photoelectron spectroscopy (XPS) and electrochemical measurements was used to obtain fundamental insight into the mode of sensing action. Three different oxygen species were identified on/in the sensing electrode under working conditions, two of which are electroactive. The steady- state coverage of one of these species (oxygen adatoms on Pt) depends on the balance between rate of oxygen pumping to the Pt surface and the hydrocarbon impingement rate: this sets the potential of the sensing electrode and determines the steady- state oxygen ion current under potentiostatic conditions. Experimental The sensor has been described in detail elsewhere, 3 and Figure 1 shows a schematic diagram of the experimental setup. The sensing device consisted of an 8 mol % YSZ tube (FRIATEC AG), one end of which was closed; this was interfaced with a 7 cm 2 external Pt film that served as the working (sensing) electrode, and a 7 cm 2 internal Pt film that served as the counterelectrode. A small Au reference electrode was also located on the inner wall of the YSZ tube, all electrodes being deposited by DC sputtering. The outside of the YSZ tube was immersed in the UHV environment whose gas composition was to be analyzed while the inner (reference) compartment was maintained at atmospheric pressure by circulating air through it. Measurements were carried out in a UHV chamber operated at a base pressure of 3 × 10 -9 mbar. The system was equipped * To whom correspondence should be addressed. E-mail: rml1@cam.ac.uk. † Cambridge University. ‡ Lithography Subsystems. 1491 J. Phys. Chem. C 2007, 111, 1491-1495 10.1021/jp0664364 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006