Anisotropic etching of cesium iodide

Abstract

Summary form only given. Scintillator films that convert an incident X-ray image into visible light play an important role in both medical and non-medical imaging applications. Because of its superior light output (59,000 photons/MeV), high density (4.54 g/cc), high effective atomic number (52), and rugged nature, CsI(Tl) is the material of choice for many X-ray imaging applications. For adequate detection of the 8-70 keV X-rays that are typically used, 30-200 /spl mu/m thick CsI films are needed. Spreading of light in the scintillator volume, however, limits the spatial and contrast resolution of the resulting images. To address this problem we are micromachining CsI screens to form a finely pixilated structure. When coated with a low refractive index material, each micro-pixel acts as an optical waveguide and minimizes the spread of scintillation light in the screen. The micromachining process uses high-density inductively coupled plasma to etch CsI samples held by a heated, RF-biased chuck. Fluorine-containing gases such as CF/sub 4/ are found to enhance the etch rate by an order of magnitude compared to Ar/sup +/ sputtering alone. Without simultaneous inert-gas ion bombardment, however, the fluorine-based etch becomes self-limited within a few microns of depth. The formation of a thick passivation layer on the sidewalls of etched features is confirmed by SEM. EDS indicates the passivation layer has a high ratio of Cs to I. This evidence points to the ion-enhanced inhibitor mechanism. Etching exhibits an Arrhenius-type behavior in which the etch rate increases from /spl sim/40 nm/min. at 40 C to 380 nm/min. at 330 C. This temperature dependence corresponds to an activation energy of 0.13 eV. Similar activation energies have been reported for the electronic sputtering of other alkali halides. This suggests that the CsI etch process, similar to alkali halide sputtering, is rate-limited by the thermal migration of ion-induced crystal defects to the CsI surface. Additional experimental results will be presented to support a more complete picture of the etching mechanisms.