Monitoring of Dopant Activation in Sub-Surface P-Type Si Using the Surface Charge Profiling (SCP) Method
- PDF / 297,309 Bytes
- 5 Pages / 414.72 x 648 pts Page_size
- 53 Downloads / 203 Views
Engineering, The Pennsylvania State University, University Park, PA 16802, USA. *QC Solutions, Inc., 150-U New Boston St., Woburn, MA, 01801, USA. ABSTRACT In this study the SCP (Surface Charge Profiling) method, based on non-contact, smallsignal ac-SPV measurement is used to study thermal activation of boron in the near surface region of p-type Si wafers. Boron tends to form pairs with impurities such as hydrogen, iron and copper in the near surface region of Si substrates which render it inactive. During device processing, activation of boron may take place resulting in uncontrolled variations in active boron concentration in the near surface region. In this work, both boron doped, polished CZ wafers and wafers with boron doped epitaxial layers are studied. In the former case, the concentration of active boron in the near surface region was initially up to an order of magnitude less than the bulk concentration determined from four-probe measurements, but increased with the temperature of an anneal in ambient air and approached the bulk value. In contrast, the wafers with epitaxial layers showed no consistent variations of surface dopant concentration with temperature. These results confirmed previous findings that the near surface region of the polished wafers is contaminated with metals introduced during polishing operations. The SCP method was found to be very effective in monitoring variations in active boron concentration in the near-surface region. INTRODUCTION It is known that unlike n-type dopants in silicon, the p-type dopant, boron, can be rendered inactive by pairing with selected contaminants. This effect is particularly pronounced in the shallow region immediately adjacent to the Si surface, which can be penetrated more readily than the bulk of the material. Two types of interaction can be identified in this regard. The first involves penetration of the surface by hydrogen, which is likely to occur under a variety of process conditions, for instance, gas-phase etching. The second involves interaction of boron with metals such as copper originating from chemomechanical polishing [1]. A brief anneal is typically sufficient to break the boron-contaminant pair and render the boron electrically active. In the case of hydrogen, featuring a high diffusivity in silicon, this also results in hydrogen leaving the silicon. In the case of metals, however, the metals remain in the silicon, where they can subsequently re-pair with boron. These effects are well understood and can be used to determine the concentration of metallic impurities in the bulk of the Si wafer [2]. Until now, however, no convenient method that would follow boron deactivation/reactivation processes in the sub-surface region was available. The Surface Charge Profiling (SCP) method, a contactless ac-SPV method capable of measuring active doping concentration in the surface space-charge region, offers new possibilities in this regard. This work studies boron activation by hot-plate annealing in both p-type polished CZ wafers and wafers with p-ty
Data Loading...
