The thickness uniformity of photoresist films deposited by spin coating critically influences the resolution of photolithography. This thickness uniformity depends on uniform evaporation from the film during drying. Simple scaling arguments demonstrate that, if the mass transfer coefficient at the surface of the wafer does not vary with radial position, then the dry coated resist film thickness will be independent of radial position. A model is presented for the compressible, laminar, steady-state, axisymmetric air flow in a spin coating apparatus for 6-in.-diam wafers. Flow fields computed by a finite-element–Newton method are used to evaluate the radial profile of the mass transfer coefficient at the surface of the rotating wafer, and to calculate the trajectories of particles that are generated as photoresist is flung from the edge of the spinning wafer. At a spin speed of 2000 revolutions/min and exhaust flow rate of 100 ℓ/min through the coater, the calculations predict that the mass transfer coefficient should be independent of radius. Comparison with film contours measured from experiments at these conditions indicates radial nonuniformities in the film thickness and suggests the importance of hydrodynamic instabilities in the gas on the uniformity of the coating.
Flow visualization in the gas above the surface of a rotating wafer in an industrial spin coating apparatus demonstrates the presence of an Ekman-like flow instability that adversely affects the uniformity of the dried film. Experiments performed with a 6-in.-diam wafer and typical operating conditions show 6–8 spiral vortices around the wafer oriented with negative angle, as is indicative of the type II flow instability predicted by linear stability analysis. The critical Reynolds number for onset is in reasonable agreement with the theory.
In this paper we present a new non-contact, non-destructive method for real-time evaluation of both surface and interface metallic contamination from resists. The method allows for independent testing at the completion of a coat/ash or ash/removal/clean steps in processing. Using a standard 1.2i resist in both as-received and intentionally doped versions wherein alkaline and transition metals such as K, Fe, Cr, and Cu were added to the initial solution, we demonstrate that at both these crucial steps in resist processing important electrical and material parameters such as bulk Fe concentratior, minority carrier diffusion length, relative surface recombination velocity, and surface charge can be detected directly on-a processed test wafer with no electrical test structures using a combination of low and high injection level surface photovoltage.. Since most production fabs only test for residual particles in-line, or rely on expensive, time-consuming analytical techniques such as AAS, DLTS, or TXRF to evaluate contaminant metals, this approach offers a faster and a more economical way to control this problem.
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