The Effect of Photoresist Outgassing on Boron Clustering and Diffusion in Low Energy BF 2 + Ion Implantation
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The Effect of Photoresist Outgassing on Boron Clustering and Diffusion in Low Energy BF2+ Ion Implantation Peter Kopalidis and Serguei Kondratenko Axcelis Technologies 108 Cherry Hill Drive, Beverly, MA 01915, USA ABSTRACT During high current ion implantation into photoresist-covered substrates, evolution of gaseous by-products of photoresist breakdown occurs that can affect the dose control of the process as well as diffusion and activation of the implanted dopants in silicon. The dosimetry effects are well understood and accounted for in modern ion implanter design. In this work, we report on the effect of photoresist outgassing on the distribution of boron concentration in silicon during the subsequent annealing process, boron electrical activation and sheet resistance measurements. Experiments were performed on monocrystalline and pre-amorphized silicon and oxide covered wafers that help elucidate the mechanism of boron accumulation and diffusion. Higher carbon concentration on the wafer surface due to photoresist outgassing results in boron clustering and reduction of transient enhanced diffusion, leading to higher sheet resistance at low anneal temperature. Implantation through a thin oxide layer can be used to reduce carbon surface contamination and minimize the effects on the boron profiles and sheet resistance. INTRODUCTION As the dimensions of integrated circuits continue to shrink, low energy ion implantation is increasingly used in mainstream production of ICs. This brings to light new effects in the formation of ultra-shallow junctions, relating to dopant diffusion and activation. Boron and BF2+ ion implantation is widely used in the formation of p-type shallow junctions and the diffusion and activation characteristics of boron in silicon have been studied extensively [1,2]. Boron exhibits strong transient enhanced diffusion (TED) effects that depend on the distribution of damage in the silicon crystal, caused by the implantation process [3]. In particular, the presence of interstitial Si atoms in the crystal lattice can significantly increase the diffusion coefficient of boron. In this paper, we discuss the effect of photoresist outgassing during low energy BF2+ ion implantation, on the boron concentration profile in silicon, diffusion and clustering phenomena. Typical photoresist materials used in IC manufacturing tend to undergo chemical bond breaking during ion implantation and release gaseous byproducts such as H2, CO2, CO, CH2, CH4 and C2H2 [4]. These byproducts can become adsorbed on the silicon surface and driven into the silicon lattice by the energetic ion beam, thereby altering the chemical composition at the top surface layer of silicon. This can influence the diffusion of boron during subsequent high temperature annealing and affect the junction depth and sheet resistance. In particular, the presence of carbon can significantly reduce the transient enhanced diffusion of boron by trapping Si interstitials generated by the implant [5,6].
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EXPERIMENTAL In a first set of experi
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