Rolf H. Muller

Definitions and conventions in ellipsometry

Abstract Many parameters which appear in the theory of ellipsometry crucially depend on the choice of arbitrary conventions and definitions. This dependence is discussed for nine two-fold choices. On the basis of literature usage, preferences are expressed for one of each of the alternatives and for the nomenclature of the variables involved.

AM OPTICAL STUDY OF CATHODIC HYDROGEN EVOLUTION IN HIGH RATE ELECTROLYSIS

Hydrogen bubbles evolved cathodically under conditions encountered in electrochemical machining have been studied by s top-motion photography. Constant current densities up to 150 A / c m 2 and flow rates up to 2500 cm/sec have been employed with an exper imenta l flow channel of 0.5 mm gap width. The observed bubble size decreased strongly wi th increasing flow rate and increased with increasing current density. At flow rates above 800 cm/sec, the bubble size was always below 20~, the smallest d iameter resolved by the optical a r rangement used. Less gas was evolved in ni t ra te than in chloride electrolytes under otherwise identical conditions. The hydrogen bubbles were usual ly confirmed to a region near the cathode. Voltage oscillations and electric breakdown coincided with the appearance of a new type of bubble. In e lectrochemical machining, metals are dissolved anodically at current densities in the order of 100 A / c m 2 or higher. Hydrogen gas is formed cathodically and must be t ranspor ted away from the reaction zone by the electrolyte stream. This hydrogen may affect the electrolytic process in several ways: It may increase the ohmic resistance of the electrolyte, resul t ing in higher cell vol tage and different local current distr ibution. It may be oxidized at the anode and, thus, decrease the current efficiency of the meta l dissolution process. It may form a continuous gas blanket at the cathode and, thus, lead to sparking. It may accumulate in large bubbles, which extend over the ent ire in tere lect rode gap, and, thus, drast ically affect masst ransfer conditions at the anode. In order to obtain a be t ter under standing of the re la t ive importance of such phenomena, a photographic study of cathodically generated gas bubbles was initiated. During the course of this pursuit, somewhat re la ted work has appeared in the l i te ra ture (1, 2). In the present investigation, a more sophisticated optical a r rangement has been employed and a bet ter control and wider range of cri t ical variables, such as current density and flow velocity, have been employed in addition to the use of a wel l -def ined flow system. Experimental Technique The apparatus used has been described before (3). It consisted of a rec tangular flow channel cell of 8 mm width and 0.5 mm height. Its sidewalls were made of flat glass plates, which provided for the optical observation of the in tere lect rode gap. The total channel length was 8.5 cm; the center of the electrodes was positioned 1 cm from the downst ream end. The design of the flow cell provided for fu l ly developed veloci ty profiles at the electrodes. F low rates up to 2500 cm/sec were employed, corresponding to inlet pressures in the order of 10 atm. Since most of the pressure drop occurs in the ent rance length of the flow channel, the absolute pressure at the electrodes, even at the highest flow rate employed, was only about 2 atm. Al though under these conditions the effect of pressure on the gas vo lume has to be taken into account, at flow rates up to 1000 c m / *Elec t rochemica l Soc ie ty A c t i v e Member . 1 P r e s e n t addres s : U n i v e r s i t y of Ca l i fo rn ia , D e p a r t m e n t of E n g i nee r ing , Los AngeleS, Ca l i fo rn ia . sec, where most observations were made, the absolute pressure at the electrodes was only 1 to 1.3 arm, and its effect on bubble size was negligible wi th in the accuracy of the present measurements . The copper electrodes were 3.17 m m long in the flow direction and 0.53 mm wide. Before exper iments , the electrode surfaces were mechanical ly polished with 1~ diamond paste, cleaned wi th aqueous detergent and acetone, and degreased by hydrogen evolution in aqueous caustic. The optical a r rangement is shown schematical ly in Fig. 1. 2 A commerc ia l flash l ight source (A) of 0.5~ sec flash durat ion 3 was used for i l luminat ion of the gas bubbles generated in the flow cell (B) in t ransmi t ted l ight mode. A camera (H) wi th open shut ter and high speed film 4 was at tached to a microscope tube containing a 2x object ive (C) (f : 48 mm, N.A. ---0.08) and a 10x eyepiece (E). A small circular aper ture (D) of 1.6 mm diameter , located in the rear focal plane of the object ive (C) served as a te lecentr ic stop (4). The purpose of this device is to avoid variat ions in the apparent size of bubbles due to differing distances f rom the object ive and to increase the depth of field. Unfor tunately, the telecentr ic stop also reduces the speed and resolution of the objective, the la t te r being about 20~. In order to obtain a photographic distinction between the electrode surface and an adjacent, dense Des ign c o n s i d e r a t i o n s of the optical s y s t e m are d i scussed e lsew h e r e in more deta i l . 3 E G & G 594 microf lash . 4 K o d a k SO 340.

Present status of automatic ellipsometers

Abstract Operating principles and capabilities of presently-built automatic ellipsometers are reviwed. The instruments are classified according to their optical principles of operation and their capabilities are compared on the basis of uniform performance criteria. Possible future developments are indicated.

In Situ Scanning Tunneling Microscopy of Copper Deposition with Benzotriazole

In this paper, the submicron topography of Cu deposits is studied with scanning tunneling microscopy to investigate the effect of benzotriazole in the initial stages of deposition. The presence of benzotriazole results in a marked increase in over-potential for the deposition. It is found to eliminate the preferential growth of specific crystallographic planes and the formation of crystal facets. The number density of nuclei, determined from Fourier transforms of the surface profiles of 7 nm thick films, is found to increase with increasing overpotential of the deposition but is independent of the presence of the inhibitor.

Roughness Development in Metal Electrodeposition I . Experimental Results

The authors report copper electroforms of thicknesses of several hundred micrometers produced by deposition on a rotating cylinder at rates of up to 5.0 A/cm/sup 2/, corresponding to 110 pm/min. Digitalized surface profiles were analyzed by Fourier transform. Decline of roughness wavelength is observed with increasing current density at constant fraction of limiting current, in agreement with stability theory. Decline in roughness amplitude was also observed. This decline is due to an increase in the number density of nuclei. A comparison of the growth of features inside and outside the hydrodynamic boundary layer is presented.

High Rate Anodic Dissolution of Copper

The anodic dissolution of copper at current densities of 10–150 A/cm2 has been studied in neutral and solutions. An experimental flow system has been built which allows electrochemical measurements of the dissolution process to be performed under denned hydrodynamic conditions typical for the technical electrochemical machining operation. Design criteria for such a system have been analyzed. Copper dissolution proceeds in an active or a transpassive mode which differ in overvoltage by 10–20V. The onset of passivation seems to be due to the limiting mass transfer of dissolved metal from the electrode surface. The still higher dissolution rates in the passive state appear to be made possible by the removal of solid dissolution products. Etched, dull surfaces have resulted from active dissolution; pitted, bright surfaces from passive dissolution.

Roughness Development in Metal Electrodeposition II . Stability Theory

A stability analysis of metal electrodeposition on a smooth surface has been formulated and applied to copper deposition on a rotating cylinder

Fast, self-compensating spectral-scanning ellipsometer

The advantages of rapid spectral scanning have been combined with the inherent accuracy of the compensating ellipsometer operated in the polarizer‐compensator–sample‐analyzer configuration. Wavelength is scanned over the visible–UV (370–720 nm) at a maximum rate of 114 nm/s by rotating a continuously variable interference filter. A three‐reflection Fresnel rhomb serves as the achromatic quarter‐wave compensator. A microcomputer is used to collect spectroscopic measurements, perform instrument calibrations, digital filtering, and data interpretation.

Fast self‐compensating ellipsometer

A compensating automatic ellipsometer that employs Faraday rotators has been constructed. Speed of response and resolution superior to that of any similar instrument now in use have been achieved through novel Faraday solenoid designs, modulation with high frequency and large amplitude, and refinements in circuitry.

In Situ X‐Ray Diffraction of Surface Layers on Lithium in Nonaqueous Electrolyte

X-ray diffraction, combined with electrochemical techniques, has been used for the in situ identification of materials on electrode surfaces based on their crystal structure during the progress of electrochemical reactions. This approach avoids some long-recognized problems of ex situ measurements, which may not represent the nature of electrode materials because changes can occur upon transfer out of the electrochemical environment, when potential control is not possible. The combination of different in situ techniques has been reviewed by several authors. Previous studies with x-ray diffraction from electrodes includes work by Salkind et al. who obtained x-ray study of surface layers formed on lithium in nonaqueous electrolytes. The properties of these surface layers are important for the charge and discharge behavior of lithium electrodes in ambient-temperature batteries. Because of the high reactivity of lithium, use of in situ techniques for the characterization of surface layers is essential.