Focused Ion Beam-Induced Carbon Deposition
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FOCUSED ION BEAM-INDUCED CARBON DEPOSITION
L.R. HARRIOTT AND M.J. VASILE AT&T Bell Laboratories Murray Hill,
New Jersey 07974
ABSTRACT A process for repair of micron and submicron sized transparent defects on photomasks is described. Opaque films are deposited at the intersection of the flux of organic monomers from a gas jet and a 20 keV Ga ion beam. Focused ion beam-induced deposition differs from other ion-induced, electron beam and laser processes due to the very high ion current density and the We have sputtering of the material as it is being deposited. explored the deposition-sputtering rate competition for several precursor materials as a function of monomer flux and ion beam dose rate. Our results suggest a model for deposition which requires polymerization of the precursor through carbon-carbon double bonds to favor deposition over sputtering by creating high molecular weight material at the target. INTRODUCTION Defect repair for lithographic masks is currently the largest commercial application for focused ion beams[l-6]. Masks used for photolithography are usually composed of a glass or quartz substrate with patterned Cr as a light absorber. Defects in the form of excess Cr (opaque defects) can be removed by physical sputtering with a finely focused ion beam as illustrated in Fig. la. The ion beam is usually Ga focused to a submicron spot at 10-30 keV beam energy. In mask repair the ion beam is first used to form an image of the defect area in order to locate it precisely (defect locations are only approximately known as a result of the optical inspection process). The ion beam is scanned over the defect based on information from the ionoimage. Typical ion dopes required for removal of the (850 A) Cr film are 5x101 ions/cm2. This results in repair times of the order of seconds per square micron for available ion beam systems. Repair of clear defects or missing Cr features can also be accomplished by sputtering. Refracting structures such as prisms, gratings, or lenses are machined into the glass surface with the focused ion beam. When the mask is used in a projection printing system, light will be directed away from the entrance pupil of the imaging lens and the area of the mask will print dark. Clear repair by this procedure is effective, although slow (tens of seconds per square micron) and not reversible. Figure lb illustrates the alternative approach of ion induced deposition for clear defect repair. A precursor gas is introduced into the vacuum chamber through a needle-sized tube (~100 jum dia.) in close proximity to where the ion beam strikes the target. Under the right conditions, a deposit is formed at the substrate only within the area irradiated by the ion beam. The primary difference between focused ion beam induced deposition and other (electron or laser beam) techniques is the sputtering caused by the ion beam. During the irradiation of the deposition precursor adsorbed at the substrate, the ion beam
Mat. Res. Soc. Symp. Proc. Vol. 131. 91989 Materials Research Society
544
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