Characterization and Modeling of Wafer and Die Level Uniformity in Deep Reactive Ion Etching (DRIE)

Wafer and die level uniformity effects in Deep Reactive Ion Etching (DRIE) are quantitatively modeled and characterized. A two-level etching model has been developed to predict non-uniformities in high-speed rotating microstructures. The separation of wafer level and die level effects is achieved by sequentially etching wafers with uniformly distributed holes. The wafer level loadings range from 0.06% to 17.6%. Resulting wafer maps reflect the transition from an ion-limited region to a neutral-transport limited region. Additionally, long-range dielevel interactions are also evaluated. Resulting die-level etching non-uniformities have a comparable magnitude to wafer-level effects. A model taking into account both the diffusion and reaction rate of neutrals is applied to predict the etching of up to 21 dies. Agreement between measurement and prediction support the hypothesis that the depletion of radicals is the main cause of die-level etch variation. The characterization and prediction methods are applied to etching a micro-scale turbine engine. INTRODUCTION With etch depth variations on the order of 10 microns being regularly observed, DRIE nonuniformities are an extremely pertinent issue in MEMS fabrication. Our experiments show that the sources of non-uniformity in plasma etch can be sorted into three levels and characteristic lengths: wafer level, die level (millimeter scale), and feature level (micron scale). Extensive studies have described uniformity issues for plasma etching in integrated circuits, mainly focusing on feature-level effects. For MEMS, however, the die-level effects have a more prominent role due to large open area layouts. Etch depths for microengine turbine blades are plotted in figure 1. The peaks reflect a phenomenon known as RIE lag, while the sinusoidal variation results from a combination of global and die-to-die interactions. In this study, the wafer level uniformity maps are obtained by etching a series of masks under low aspect ratio conditions (<1.0). The loading densities ranged from 0.06% to 17.6%. Loading is defined as the ratio of etched to un-etched areas on the wafer surface. Then, masks are designed to capture the range and magnitude of die-level effects. Finally, a model based on neutral diffusion is used to simulate the etching for a twenty-one die layout. EXPERIMENT Sample preparation and measurement methods The experiments were conducted on a standard ASE etcher made by Surface Technology Mat. Res. Soc. Symp. Proc. Vol. 782 © 2004 Materials Research Society A10.2.1