In an automated fabrication facility, the thin, fragile silicon wafers in which semiconductor circuits are formed must be transported to and from processing stations with a minimum of contact with other solid objects so as to minimize damage, contamination, and consequent lowering of product yield. This task has been undertaken for some time now, in IBM and elsewhere, by systems based on a lubricating film of air as a means for levitating and moving wafers. However, due to inherent motion instabilities and specific control needs, some solid contact is typically involved in effecting prescribed wafer motion. The need for solid contact control is greatly reduced by the air film system described in this paper. It is based on a surface configuration that combines two fluid mechanics phenomena to generate a supporting air film that provides and guides wafer motion. Wafer transportation and positioning are achieved with the air film operating in confiinction with special control device techniques.

Attraction Force Characteristics Engendered by Bounded, Radially Diverging Air FlowWhen axially directed air flow enters a parallel plate passage through a hole in one of the plates, the ensuing diverging radial flow is such that a depressed pressure region occurs to some extent over the inlet region of the passage. If the plate against which the inlet air stream impinges is allowed to move freely, it will, under proper flow and other conditions, assume a position of stable equilibrium reflecting a balance among plate weight, the momentum repelling force of the stream, and a net restraining attraction force due to the radial pressure distribution in the passage. This phenomenon, the "Bernoulli" or " axi-radial" effect, has long been of interest in areas such as gas film lubrication and radial diffusers, and it has been applied extensively in IBM systems for contactless transport and motion control of semiconductor wafers on an air film. A steady, laminar, incompressible flow analysis for a representative axisymmetric circular disk model is presented here. A one-dimensional approach, using the general energy equation in conjunction with a passage flow friction factor variation, is applied to obtain an approximate relationship for radial pressure distribution. The friction factor, embodying the influence of varying viscous and inertial forces, is postulated on the basis of specialized radial flow studies in the literature. By also applying the momentum balance condition, an approximate overall solution is obtained which, for arbitrary model dimensions, describes the relationship among equilibrium passage spacing, resultant reaction fluid force and free disk weight, and a flow Reynolds number. The analytical predictions are compared with results from model experiments, and generally favorable agreement is indicated.

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Thermal characteristics that are important to structural integrity are analyzed herein for a TTL, plastic-encapsulated package. By assuming that total module heat during operation is engendered at idealized junctions between lead wires and the chip surface. an analysis of its dissipation has been undertaken to determine internal steady-state temperature profiles and heat flux distribution. Based on junction heat sources of equal strength and on certain adiabatic assumptions, the multi-wire package has been modeled as a single-wire "composite" incorporating postulated heat dissipation mechanisms in representative plastic-to-wire and chip-to-lead frame thermal circuits. These circuits are treated, respectively, by axisymmetric and one-dimensional analyses. Instead of a partial differential equation approach in the former treatment, a less complicated method is devised which leads to characterization by a pair of linear ordinary differential equations. Their closed-form solution gives expressions for calculating two-dimensional temperature profiles and heat flux fractions. The resultant analyses are applied to a module containing 14 lead wires and operating at a given power level. The plastic and wire temperature profiles are seen to be nonlinear in the neighborhood of the chip surface and to coalesce axially into a common, essentially linear form in the outlying regions of the module. Constituent heat fluxes are also calculated for each thermal circuit, and some implications of the overall results to thermal stress arc qualitatively discussed. 292