IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Plasma Science Tuesday Sessions
       Session PS2-TuM

Paper PS2-TuM11
Plasma Etching of Cesium Iodide@footnote 1@

Tuesday, October 30, 2001, 11:40 am, Room 104

Session: Emerging Applications of Plasmas
Presenter: X. Yang, Northeastern University
Authors: X. Yang, Northeastern University
J.A. Hopwood, Northeastern University
S. Tipnis, Radiation Monitoring Devices, Inc.
V. Nagarkar, Radiation Monitoring Devices, Inc.
V. Gaysinskiy, Radiation Monitoring Devices, Inc.
Correspondent: Click to Email
Scintillator films that convert an incident x-ray image into visible light play an important role in many 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 often the material of choice for scintillator films. For adequate detection of 8-70 keV x-rays, 30-200 µm thick CsI films are needed. Spreading of light in the scintillator volume, however, limits the resolution of the resulting images. To address this problem we are micromachining CsI screens to form a finely pixelated structure. When coated with a low refractive index material, each micro-pixel acts as an optical waveguide that minimizes the spread of scintillation light in the screen. The micromachining process uses a 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@super +@ sputtering alone. Without 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. Etching exhibits an Arrhenius-type behavior in which the etch rate increases from ~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 this CsI etch process, similar to alkali halide sputtering, is rate-limited by the thermal migration of ion-induced defects to the CsI surface. Additional results will support a more complete picture of the etching mechanisms. @FootnoteText@ @footnote 1@This work is supported by the NIH contract No. 2R44 CA76758-02.