An investigation to develop a better understanding of the fundamentals behind the operation of spray-etching processes revealed several interesting phenomena that explain some of the difficulties encountered during operation. Higher spray pressures are required for the top sides than for the bottom sides in order to achieve equal amounts of etching on each side of the boards being processed. This is due to additional mass-transfer limitations that are created when puddles of liquid collect on the top side of the boards. Another phenomenon observed was an increase in the etch angle, resulting in more vertical sidewalls, as the extent of etch undercut increases. Based on these experimental observations, a dynamic model for the spray-etching process is developed.
Experimental and modeling work has been performed to examine the effects of reactor pressure, etchant gas flow rate, and wafer location on the uniformity of plasma etching silicon using CF4/O2 in a parallel-plate-radial flow reactor. Intrawafer etch rates were measured at 12 points across a 3 in. wafer at pressures between 125 and 200 mtorr, 70~ and gas residence times between 1.28 and 2.14s. Depending on the operating conditions, an edge-to-center decrease in etch rate of 5-25% was observed. A combined reactor/reaction model is able to predict this degree of nonuniformity. Uniformity was improved by increasing the reactor pressure and decreasing the flow rate of the etchant gas. Etch uniformity was also found to be a function of wafer location within the reactor. Data are presented which show the influences of process parameters on both etch rate magnitude and uniformity.Intrawafer nonuniform etching in plasma and reactive ion etching systems is a major problem in current IC processing technology. The result of intrawafer nonuniformities is a direct decrease in circuit yields or overdesign to compensate for etch rate variations. In particular, the plasma etching of silicon, silicon dioxide, and aluminum in a parallel plate radial flow system often displays an edge-to-center decrease in etch rate, a phenomenon sometimes referred to as the "bullseye effect" (1, 2). Clearly, the inability to etch consistent circuits on each chip of a wafer will continue unless the causes of the bullseye effect are better understood and corrections are made.Several papers have been published which discuss the problem of etch nonuniformities. Stenger et al.(1) studied the interwafer nonuniformities of NF3 etching silicon in a radial flow reactor. The etch rates for this system were found to be more uniform at lower flow rates holding pressure, feed gas composition, and temperature constant.Since they evaluated etch rates by measuring the decrease in wafer mass after etching, intrawafer nonuniformities could not be examined. Dalvie et al.(3) have developed a model of CF4 etching silicon in a radial flow reactor. Their model assumes uniformly distributed silicon on the lower electrode, low CF4 dissociation rates, and radially constant electron density. The effects of pressure, flow rate, and discharge power on etch rate were simulated. They found that average etch rates increased as the flow rate decreased or the reactor pressure increased. The primary effect of increasing the discharge power in their model was to increase the average electron density, which resulted in higher etch rates. Their model also predicted intrawafer as well as interwafer etch nonuniformities; however, no experimental data were offered for comparison. Alkire and Economou (4) examined the nonuniform stripping of photoresist with 02 in a barrel reactor, which is a common application of barrel etchers. The transport of etching species to the wafer in barrel reactors relies predominantly on diffusion, with little convective influences. This results in a depletion ...
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