Millisecond Duration Annealing of Boron Implants in Silicon
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MILLISECOND DURATION ANNEALING OF BORON IMPLANTS IN SILICON D.A.Smith. R.A.McMahon, H.Ahmed, D.J.Godfrey*, Microelectronics Research Laboratory, Department of Physics, University of Cambridge CB4 4FW, U.K. * G.E.C. Research Limited , Hirst Research Centre, East Lane, Wembley, Middlesex, HA9 7PP, U.K. ABSTRACT A dual electron beam machine has been used to anneal boron implanted layers in order to study the diffusion and activation behaviour over a wide range of doses. The annealed implants have been characterized by spreading resistance profiling and secondary ion mass spectroscopy (SIMS). Carrier concentration profiles show that millisecond duration anneals can activate boron implants. A boron dose of 1E16 ions/cm 2 was annealed to give a sheet resistance of 30 Wsq with 40% of the implant activated. The SIMS technique showed there were no significant
differences between the atomic profiles of the as-implanted samples and specimens subjected to a
millisecond anneal or to a low temperature 8500 C rapid isothermal anneal for lOs. Introduction
The need to fabricate shallow, highly conducting n+ and p+ source/drain regions for fine geometry CMOS technologies has led to the development of several techniques for optimising the annealing of dopants. In particular, rapid thermal annealing (RTA) using lamps, scanned electron beams and other methods has been widely reported [1,2]. These methods can be categorized as isothermal because the wafer is heated so that its temperature is uniform throughout its volume [3]. We report in this paper on a non-isothermal annealing method which enables very short temperature cycles to be achieved, compared to those in RTA methods. Heating cycles of this kind have been obtained in a dual electron beam system. One electron beam was raster scanned over the back surface of the wafer or chip to be annealed, giving isothermal background heating, the other provided localised surface heating. A background temperature of 850 0 C was chosen to limit solid state diffusion, whilst preventing any macroscopic slip or lattice strain resulting from the high thermal gradients during the surface heating [4]. A second electron beam of energy 20keV and a spot size of 70 i.tm was rapidly scanned at a frequency of 100kHz, by a triangular driving signal, to form a line. The line beam was then swept across the wafer, 0
heating the wafer to a peak temperature of about 1200 C for a few milliseconds. Cooling was by conduction to the bulk [5]. This paper presents results on the annealing of boron implants, since achieving high activation whilst restricting diffusion is a particular problem for boron as opposed to arsenic. The annealing behaviour of boron implants over a wide range of doses has been studied for these short timescales with particular emphasis on activation, diffusion and crystal
regrowth.
Exvtrimental The starting material for these experiments was (100) Czochralski grown 60CI cm n-type silicon wafers. The wafers were implanted with boron at an energy of 25 keV, at doses of 1E13 to 1E16 ions/cm 2
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