Improved Rinse Quench for a more Uniform Etch of Thermal Oxide in Buffered Oxide Etch (BOE)
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ABSTRACT An improved method for quenching a Buffered Oxide Etch (BOE) process in a DI water overflow rinse tank is shown. Many different factors were investigated in several preliminary experiments. Most of these factors were thought to influence etch uniformity yet proved not to be significant. The transfer process from chemical to rinse was identified as contributing negatively to etch uniformity. A chemical carryover layer formed during the transfer process was identified as the most significant factor. The hydrodynamics of liquids are outlined to help explain how the carryover layer is formed and shaped. Several sensitivity analysis experiments show transfer speeds to significantly affect carryover layer thickness and uniformity. Slow transfer speeds are determined to produce consistent and improved uniformity. Results of a follow up experiment quantify the uniformity improvement. INTRODUCTION Buffered Oxide Etch (BOE/BHF) is a solution of ammonium fluoride (NH 4F) and hydrofluoric acid (HF) which is widely used in semiconductor fabrication to etch or remove thick films of silicon dioxide (Si0 2). The buffering agent of ammonium fluoride (NH 4F) is added to prevent depletion of fluoride ions and thus maintain stable etching characteristics, it also increases the etch rate [1]. Because of its rapid etch rate BOE can easily over etch SiO 2 making it very difficult to control its etch uniformity. As the uniformity of the average BOE etch has been adequate for some of current process technology, little effort had been made to improve upon it. Still the next generation of submicron processing will reach dimensions where the current BOE etch process may be inadequate and further process improvement will be necessary. Thus the objective of this experiment was to improve BOE etch uniformity for more demanding submicron process'. Several exploratory, or screening experiments showed most of the factors under investigation to have little statistical influence on etch uniformity. Yet experiments with rinsing identified much of the nonuniformity to occur during wafer transfer and initial rinse. Given this knowledge and previous research results the thickness and uniformity of a chemical carryover layer was determined to be the most significant factor of etch uniformity [2,3]. When a wafer is extracted from liquid a residual of the liquid adheres to the surface, if it's hydrophilic, and is carried into the rinse [2,3]. This residual liquid or carryover volume can be nonuniform such as small droplets, or it can form a fairly uniform layer called a carryover layer. The balance of gravitational, viscous, and surface tension forces acting on a carryover layer influence its form and thickness [2] (see Figure 1). The viscous or shear force of the liquid, Fv, resists gravity, Fo, and surface tension, Fs, to hold the chemical layer on the wafer surface.
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Mat. Res. Soc. Symp. Proc. Vol. 477 @1997 Materials Research Society
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Figure 1. Balance of Forces Acting on a Carryover Layer. Threeforcesform andshape the carryoverl
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