Novel Ion Source for the Production of Extended Sheet Beams

Novel ion source for the production of extended ribbon beams Thomas N. Horsky*, Sami K. Hahto, and Tetsuro Yamamoto Nissin Ion Equipment USA, 34 Sullivan Road Suite 21, North Billerica, MA 01862, USA *Tom.Horsky@nissin-ion.com We describe a new ion source developed for a commercial high current ion implanter. The source generates a "ribbon beam" about 300 mm high and 25 mm wide, and produces ion species commonly used in silicon processing, such as 11B+, 75As+, 72Ge+, 12C+, and 31P+. We present mass spectra, lifetime data, and spatial beam profiles collected by a multi-pixel Faraday array, and discuss the design and operating principles of the ion source. The source incorporates an "ion pump" feature which increases its efficiency in generating certain molecular ions1. The ion source features dual electron guns at opposing ends of an elongated ionization chamber wherein a process gas is fed. Each electron gun is comprised of an indirectly-heated cathode (IHC), an anode, and a ground element held in contact with the ionization chamber. Each electron gun can produce several amperes of collimated electron current; the electron beams, which propagate along the length of the ionization chamber, are confined by a uniform magnetic field in which the source is immersed. In order to achieve good spatial uniformity of the extracted ion beam, the ion source incorporates five independent metering valve-regulated gas feeds which are distributed along the length of the ionization chamber, as indicated in Figures 1(b) and 1(c). By providing non-uniform gas delivery along the length of the plasma column, the uniformity of the plasma column may be adjusted. The plasma density along the column may be further adjusted by use of the magnetic coils which generate the confining magnetic field. We use a segmented coil structure which allows different coil currents, and hence different magnetic flux densities, to be generated along the length of the ion source. Figures 1(a)-1(c). (a): PH3 mass spectrum with and without the ion pump feature activated. (b): Rear perspective view of the ion source, showing mounting flange, multiple gas inlets, and feedthroughs for the dual electron guns. (c): Front perspective view of the ion source, showing plasma electrode and slot. Molecular ions are preferentially produced in the electron gun(s) when the anode voltage is high enough to form a local plasma discharge between an anode and its ground element. Figure 1(a) shows two phosphine mass spectra, in which the ratio of P2+ to P+ is increased by more than a factor of 3 by changing anode voltage from zero to 120 V. Extraction current was 60 mA in both cases, and beam energy was 40 keV. 1T. N. Horsky and S. K. Hahto, U.S. Patent 8,994,272 (2015).