Metal deposition ellipsometry

Until quite recently the very initial stages of metal deposition were difficult to characterize in detail by structure- and morphology-sensitive techniques. As a consequence and for practical purposes - multilayers were more useful for applications than monolayers - the main interest was focussed onto thick deposits. Optical and electron microscopy, ellipsometry and specular or diffuse reflectance spectroscopy were the classic tools, by which the emerging shape of the deposit was monitored [4-7],... 

Ellipsometry at noble metal electrode/solution interfaces has been used to test theoretically predicted microscopic parameters of the interface [937]. Investigated systems include numerous oxide layer systems [934-943], metal deposition processes [934], adsorption processes [934, 944] and polymer films on electrodes [945-947]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [948]. Film thickness as a function of the state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [949]. The deposition and electroreduction of Mn02 films has been studied [950] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by an increase in thickness, which started at the ox-ide/solution interface. 

Reflection spectroscopy, Raman spectroscopy, and ellipsometry complement the various electrochemical methods to study metal deposition. The optical methods can be used for a direct monitoring of the deposition process. The great advantage of optical spectroscopy... 

The SiC Schottky diodes and capacitors that have been processed by the authors were processed on either 6H or 4H substrates (n-type, about 1 x 10 cm ) with a 5-10- m n-type epilayer (2-6 x lO cm" ) [123, 124]. A thermal oxide was grown and holes were etched for the metal contacts. In the case of the Schottky sensors, the SiC surface was exposed to ozone for 10 minutes before deposition of the contact metal. This ozone treatment produces a native silicon dioxide of 10 1 A, as measured by ellipsometry [74, 75]. The MISiC-FET sensors (Figure 2.9) were processed on 4H-SiC, as previously described [125]. The catalytic metal contacts consisted of 10-nm TaSiyiOO-nm Pt, porous Pt, or porous Ir deposited by sputtering or by e-gun. 

Another real-time study of the reaction of M-FA films with H2S utilized ellip-sometry to monitor changes in film thickness concurrent with metal sulfide formation (53). The reactions appeared to reach equilibrium within the same period of time (within 2 h), with a change per monolayer of 0.2 nm for CdBe and 0.9 nm for both CuBe and ZnBe. Their ellipsometry results, in agreement with Peng et al. (66), also show a dependence of the reaction rate on the H2S pressure and the surface pressure at which the films were deposited. 

Ellipsometry is suitable for the measurement of films whose thickness is much less than the wavelength of light. The thickness measurement of films having hundreds of molecular layers is best carried out using a stylus device such as the Talystep. Here a groove is scribed in the film and the stylus measures the depth of this groove. In the case of very soft materials, the film can be coated with a metallic layer having a thickness of about 20 nm which is deposited in vacuo after the line has been scribed. 

Armelao et al. (2005) fabricated LaCoOs thin films by the combination of chemical vapor deposition (CVD) and sol-gel methods. Two sequences were adopted to prepare the target film (i) sol-gel of Co-O on CVD La-O (ii) CVD of Co-O on sol-gel La-O. Losurdo et al. (2005) further investigated the spectroscopic properties of these films by ellipsometry in the near-IR and UV range. The former film has a larger crystallite size, a lower refractive index, and a higher extinction coefficient. It also presents a semiconductor-to-metal transition at a temperature of 530 K. Contrarily, the latter film has a smaller crystallite size, a higher refractive index, a lower extinction coefficient and a semiconductor behavior. 

Ellipsometry can measure films from subnanometer to a few micrometers, depending on material properties and wavelength of the light source. It has been widely used for thin film measurement in various applications, from biology to semiconductor, and from solid/solid to solid/liquid interfaces [24,25]. Ellipsometer with electrochemical cell for in situ thin film analysis is available from J.A. Woollam Co., Inc. and has been used in the research on electrochemical deposition [26]. However, in situ measurement of anodic films is more challenging because the films are usually metal complexes with unknown optical properties and difficult to verify with other ex situ techniques. 

Zerbino et al. used in-situ ellipsometry to study the initial stages of deposition of Re, comparing the effect of the substrate (Au and Pt). It was concluded that, on Au, a monolayer of Re was formed in parallel with hydrogen evolution. On Pt, a monolayer could be deposited in the region of formation of adsorbed atomic hydrogen (at +0.1 V vs. RHE). When the potential was shifted to -0.1 V vs. RHE, layers of metallic Re as thick as 5-30 nm were deposited. 

As described in the article on the theory of surface plasmon resonance, surface plasmons create a surface-bound evanescent electromagnetic wave which propagates along the surface of an active medium (usually a thin metallic film), with the electric field intensity maximized at this surface and diminishing exponentially on both sides of the interface. As a consequence of this property, the phenomenon has been utilized extensively in studies of surfaces and of thin dielectric films deposited on the active medium. Although numerous other optical techniques have also been applied to such systems (e.g. ellipsometry, interferometry, spectrophotometry, and microscopy the surface plasmon resonance (SPR) method has some important advantages over all other optical techniques, as follows. The method utilizes a relatively simple optical system, it has a superior sensitivity, and the complete system of measurement is located on the side of the apparatus that is remote from the sample, and thus there is no optical interference from the bulk medium. Furthermore, the surfaces of the sample need no extra treatment to increase... 

Thin layers of electrochemically deposited metals and thin polymer layers deposited on electrode surfaces can be conveniently studied by ellipsometry combined with other electrochemical experiments. Electrocrystallization of nickel was studied by Abyaneh, Visscher, and Barendrecht with ellipsometry and simultaneous amperometric measurements. The initial changes in A and ij/ showed nonlinear variations with the deposition time (Fig. 12), which is apparently abnormal, indicating a marked deviation of the optical properties of the deposited film from the bulk metal properties. The observed trend was explained by theoretical calculations using equations of effective medium theory (see Section IV.4 for effective medium theory) for hemispherical growth of the nucleation centers. The observed ellipsometry data clearly demonstrate that in the initial stage of nonuniform deposition the measured parameters, ij/ in particular, can change in a... 

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