J. Phys. Chem. C 2007, 111, 1491-1495
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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 measurements 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) lithography,1 which is envisaged to provide the basis for nextgeneration 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 ( VWRo was imposed between the working and the reference electrode, oxygen was electropumped toward the working electrode; correspondingly, setting VWR < VWRo resulted in electropumping of oxygen away from the working electrode. In both cases the steady-state current, IWC, passed between the working and counter electrodes was a quantitative measure of the rate of oxygen pumping. Figure 2a shows O 1s XP spectra obtained from the surface of the (cleaned) working electrode as a function of the imposed value of VWR. Starting with VWR ) -0.4 V, it is apparent that stepwise decreases in potential resulted in progressive reduction in the amount of oxygen present on the working electrode. Spectrum a consists of a broad feature that can be fitted with the following components, as described in detail in our earlier paper:3 Component A centered at 529.5 eV, reported by others, has been assigned to backspillover “ionic” oxygen on the Pt surface,8 B centered at 530.4 eV, which corresponds to chemisorbed oxygen adatoms,9 C centered at 531.8 eV, which corresponds to platinum oxide.10 (Oxygen in the underlying YSZ is XPS invisible due to the thickness of the Pt film and the near-normal photoelectron detection geometry.) With decreasing VWR species A and B eventually disappeared, while C was essentially unaffected. This is fully consistent with the above assignmentssonly A and B should be electro-active, and as we shall see B is the key species that determines the value of VWR. The corresponding integrated intensities for A, B, and C are shown in Figure 2b. It is apparent that the coverage of Oa on Pt
Quantitative Hydrocarbon Sensor for UHV Applications (B) saturates at VWR ≈ -0.65 V at which point the amount of oxygen species (A) increases steeply. This is an important observation whose significance is discussed below. The sign of the steady-state current, IWC, reflects the direction of the oxygen anion motion within the YSZ: positive IWC values are used to denote oxygen moving toward the working electrode, and negative IWC values denote oxygen is moving away from the working electrode. In any particular case, imposing a certain value of VWR causes the system to respond by altering the concentration of electro-active oxygen species present on the working electrodesas the XPS data demonstrate directly. The steady-state oxygen coverage is determined by a balance between the rate of supply of electrochemically pumped oxygen and its rate of removal by catalytic reaction with oxidizable species adsorbed from the gas phase (background CO + H2; deliberately added hydrocarbons). Thus increasing the imposed value of VWR corresponds to a higher steady-state oxygen coverage which in turn calls for a higher steady-state current IWC which the potentiostat duly supplies. Figure 2b also shows how the steady-state current (IWC) changed with the imposed value of VWR. As the VWR increases from -1.10 to -0.55 V (became less negative), IWC increased very slowly: in this regime the coverage of Oa increased approximately linearly and saturated at VWR ≈ -0.65 V. The associated current is very small and corresponds to the rate of oxygen pumping that is required to maintain a particular steady oxygen coverage in the face of oxidation reactions (of CO, H2, and any other reducing gases) that act to deplete the oxygen coverage. At ∼-0.65 V, where Oa reaches its saturation coverage, several other effects appear. The amount of “ionic” oxygen on the Pt (species A) rises steeply as does the current IWC; at the same time gaseous oxygen began to be produced in the UHV chamber. This behavior may be understood as follows:8 electropumped oxygen ions spill over to the Pt surface producing an “ionic” species (A), which rapidly transforms to Oa (species B). Over the interval VWR -1.10 to -0.65 V, the coverage of Oa builds up on the Pt and the amount of A remains small. Once the Pt surface is saturated with Oa, oxygen starts to accumulate at the three-phase boundary and the amount of A detected by XPS rises sharply. This interfacial oxygen undergoes recombination and desorption to yield O2(g), and the cell current IWC rises steeply as a result. Notice from Figure 2b that the difference in overpotential between the clean and Oa-saturated Pt surface is ∼-0.35 V. This is close to the change in work function (∆φ ≈ 0.3 eV11) induced by saturating Pt(111) with Oa by adsorption of oxygen from the gas phase an observation that is in excellent agreement with the theoretical prediction made by Vayenas, namely, ∆VWR ) ∆φ.12 Amperometric Hydrocarbon Sensing under Potentiostatic Control. The principle we use is simple and appears not to have been used previously. The value of VWR is determined by the oxygen coverage. At the temperature of the measurement (883 K) chemisorbed oxygen reacts rapidly with hydrocarbon species adsorbed from the vacuum. Therefore, in a dynamic steady state in which oxygen is pumped to the surface of the working electrode where it reacts with the target molecules to be sensed, for a given imposed value of ∆VWR, the potentiostat supplies a constant current in order to maintain the oxygen coverage at the set level. As the partial pressure of the target molecules varies, the current supplied (IWC) varies in proportion, theoretically linearly, thus providing a quantitative measure of hydrocarbon pressure. For a given partial pressure, different hydrocarbons will induce different changes in IWC because, as we have shown and discussed elsewhere,2,3 their dissociative
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Figure 3. (a) IWC as a function of 1-butene pressure (a) VWR ) -0.45 V, (b) VWR ) -0.54 V, and (c) VWR ) -0.80 V. (b) Integrated O1s XPS intensities corresponding to data shown in Figure 3a.
sticking probabilities on Pt are different. This provides a basis for tuning the selectivity of the sensor, for example by using Pt/Au as the working electrode, as opposed to pure Ptsa concept we have previously validated.4 Figure 3a shows the steady state IWC response of the sensor to varying partial pressures of 1-butene for three different values of VWR corresponding to (a) VWR > open circuit value, VWRo, (anodic polarization); (b) VWR ) VWRo, and (c) VWR < VWRo (cathodic polarization). For low 1-butene partial pressures (
