Journal of Sol-Gel Science and Technology 13, 707–712 (1998) c  1998 Kluwer Academic Publishers. Manufactured in The Netherlands. Optical Viscometry of Spinning Sol Coatings F. HOROWITZ AND A.F. MICHELS Instituto de Fisica, UFRGS, Campus do Vale CP15051, 91501-970 Porto Alegre, RS, Brasil flavio@if.ufrgs.br E.M. YEATMAN Department of Electrical and Electronic Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BT, UK Abstract. Optical interferometric monitoring of spin coating (optospinography) has allowed close observation of the temporal evolution of a thin silicate sol film (typically at 2000 rpm, 100 Hz data acquisition). The kinematic viscosity data obtained, using a simple analytical model, are validated with those from a mineral oil standard, with agreement well within the experimental uncertainties. For spin coating in open air, the influence of variations in refractive index, rheological properties and air flow are discussed. Inflections in the temporal evolution of the optical thickness of silicate sol films are analyzed, which indicate the usefulness of optospinography, particularly when applied in the proximity of the rotation axis and evaporation is minimized, to monitor time variations in the kinematic viscosity of these sols during spin coating. Keywords: spin coating, kinematic viscosity, optical viscometry, monitoring of spin coating 1. Introduction Sol-gel coatings, due to their wide flexibility in compo- sition and microstructure, have increasingly been used in magnetic, opto- and microelectronics applications. The viscosity of the coatings during formation is a key parameter for process control and reproducibility. In addition, valuable information on the solution structure of a sol, and on its subsequent transition to a gel, is pro- vided by determination of its viscosity dependence on time [1, 2]. When a sol is spin cast in open air, the optospinog- raphy technique, where an interferometric representa- tion is obtained for the temporal variation of the optical thickness of the film during the process, has allowed us to distinguish four main stages of evolution: (I) pre- spinning oscillations in the sol, (II) ultrafast convective mass flow, (III) convection-evaporation, and (IV) lim- ited evaporation [3], as can be seen in Fig. 1. We have recently proposed use of this technique for the measure- ment of viscosities during spin coating, and considered its potential and validity for spinning sols [4]. In this study, we examine temporal variation of the optical thickness, and its relationship with kinematic viscosity of the film of silicate sol. 2. Experimental Analysis is made of a HeNe laser beam reflected from the liquid film on a silicon wafer or glass substrate, spinning at speeds in the range of 700–3000 rpm, in the experimental setup described in [3]. The ratio be- tween the reflected and reference signals is computed after processing by an A/D converter, and readings are taken every 10 ms. Alignment of the system is critical, to ensure that the illuminated spot remains at the center of the substrate at all spinning times, which are typi- cally up to 60 s. Conversion from optical to physical thickness was performed by index measurement using a commercial Abbe refractometer. For three different sets of process parameters, Fig. 2 shows the temporal evolution of the sol optical thickness, as inferred from the optospinogram data [3].