Chemically Assisted Ion Beam Etching of Tungsten using ClF 3
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CHEMICALLY ASSISTED ION BEAM ETCHING OF TUNGSTEN USING ClF 3 CHARLES GARNER Jet Propulsion Laboratory, California Institute of Technology Pasadena, CA 91109 A chemically assisted ion beam etching (CAIBE) technique is described which employs an ion beam from an electron bombardment ion source and a directed flux of ClF 3 neutrals. This technique enables the etching of tungsten foils and films in exce s of 40 imthick with good anisotropy and pattern definition over areas 5 mmV, and with a high degree of selectivity. (100) tungsten foils etched with this process exhibit preferred orientation etching, while polycrystalline tungsten films exhibit high etch rates approximately 80% that of (100) orientation tungsten. I.
INTRODUCTION
The patterning of refractory metal films such as tungsten has received increasing interest for its use in VLSI interconnects and in x-ray lithography masks, x-ray focusing elements and dichroic filters. The patterning processes have involved CF4 , SF6 , or CBrF 3 plasma and reactive ion etching of tungsten films of under 1 Pm thickness.[1-6] Another application for tungsten films is the dispenser cathode, which serves as an electron emitter in traveling wave tubes. The cathode, which dispenses a low work function oxide through holes in its surface, requires a controlled surface porosity patterned into the 25 im thick tungsten layer sheathing the cathode surface. Plasma and reactive ion etching techniques have rarely been used to pattern refractory metal films of this thickness because of low etch rates and masking problems. Recently, a new technique, chemically assisted ion beam etching (CAIBE), has been developed and used to pattern aluminum,[7] Mo, Ti and GaAs.[8] The etching mechanisms of CAIBE have been investigated thoroughly using Cl2 on Al and Cu,[9,10] and XeF 2 on tungsten.[9,11] This paper reports on the results of CAIBE to pattern (100) Wfoils and polycrystalline tungsten films. II.
EXPERIMENTAL
CAIBE of tungsten was performed using the apparatus depicted in Fig. 1. The vacuum tank was pumped by two diffusion pumps containing Fomblin-18 and sealed with viton O-rings. Tank pressure was monitored by a cold cathode. The tungsten targets were masked by electro-formed nickel meshes placed directly on the tungsten. Targets were bombarded by xenon ions extracted from the separate electron bombardment ion source. Xenon ions were used because of their efficient momentum transfer to tungsten. The ion source optics consisted of a two-grid accelerator system and were designed for minimum ion beam divergence angle which, under most operating conditions, was approximately 5 degrees.[12] Xenon ion energies ranged from 500 eV to 2000 eV, With current densities 10 cm downstream of the ion source optics of 1.0 mA/cm6 at 1500 eV ion energy. A directed flux of reactive, neutral halogen gas was provided by two 1.6-mm inner diameter tubes placed symmetrically as shown in Fig. 1 approximately 1.0 cm or less from the targets and 9.5 cm downstream of the ion source. Halogen gas flow was regulated by mic
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