Study of the Effects of a Two-Step Anneal on the End of Range Defects in Silicon

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Study of the Effects of a Two-Step Anneal on the End of Range Defects in Silicon Renata A. Camillo-Castilloa, Kevin. S. Jonesa, Mark E. Lawb, Leonard M. Rubinc a Department of Materials Science and Engineering, University of Florida, Gainesville, FL b Department of Electrical Engineering, University of Florida, Gainesville, FL c Axcelis Techologies, Beverly , MA ABSTRACT Transient enhanced diffusion (TED) is a challenge that the semi-conductor industry has been faced with for more than two decades. Numerous investigations have been conducted to better understand the mechanisms that govern this phenomenon, so that scale down can be acheived. {311} type defects and dislocation loops are known interstitial sources that drive TED and dopants such as B utilize these interstitials to diffuse throughout the Si lattice. It has been reported that a two-step anneal on Ge preamorphized Si with ultra-low energy B implants has resulted in shallower junction depths. This study examines whether the pre-anneal step has a measurable effect on the end of range defects. Si wafers were preamorphized with Ge at 10, 12, 15, 20 and 30keV at a dose of 1x1015cm-2 and subsequently implanted with 1x1015cm-2 1keV B. Furnace anneals were performed at 450, 550, 650 and 750oC; the samples were then subjected to a spike RTA at 950oC. The implant damage was analyzed using Quantitative Transmission Electron Microscopy (QTEM). At the low energy Ge preamorphization, little damage is observed. However at the higher energies the microstructure is populated with extended defects. The defects evolve into elongated loops as the preanneal temperature increases. Both the extended defect density and the trapped interstitial concentration peak at a preanneal temperature of 550oC, suggesting that this may be an optimal condition for trapping interstitials.

INTRODUCTION Post implantation thermal annealing is commonly used to repair the lattice damage from the ion implantation process as well as to electrically activate the implanted dopant species. During these thermal anneals B diffusion has been observed1 to be strongly enhanced; however the magnitude of this enhanced diffusion decreases with time and is known as transient enhanced diffusion (TED). TED arises from the coupling of the substitutional dopant with excess Si interstitials from the implantation process to form a mobile complex2. The increased interstitial concentration in the ion-implanted region shifts the local equilibrium between substitutional and mobile dopant2 resulting in the observed enhanced dopant diffusivity. Surface preamorphization is widely used prior to dopant implantation to achieve ultra shallow junction depths. Ge has gained much acceptance as an amorphization species because it has the advantage of inducing greater disorder in the crystal structure at lower doses than Si, owing largely to its bigger atomic structure. Complete removal of damage induced by Ge amorphization is possible and has been achieved upon annealing3,4. However, low energy Ge preamorphization alone has not bee