Rapid Thermal Process-Induced Defects : Gettering of Internal Contaminants
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RAPID THERMAL PROCESS-INDUCED DEFECTS: GETTERING OF INTERNAL CONTAMINANTS PAUL SIFFERT BOUCHAIB HARTITI, WOLFGANG EICHHAMMER, JEAN-CLAUDE MULLER and Centre de Recherches Nucl~aires (IN2P3), Laboratoire PHASE (UPR du CNRS n0292), B.P. 20, F-67037 STRASBOURG CEDEX, France ABSTRACT We show inthis study that RTP-induced defects analysed by Deep Level Transient Spectroscopy (DLTS) are related to residual impurities present in as-grown silicon wafers. For one particular material an0activation of a specific residual metallic impurity was observed inthe temperature range 800 - 1000 C. This impurity can be returned to an electrically inactive precipitated form by classical thermal annealing (CTA) with a slow cooling rate or neutralized by means of low-energy hydrogen ion implantation. INTRODUCTION Inrecent years, rapid thermal processes (RTP) have been widely studied with the aim to replace conventional thermal processes. As RTP inherently include quenching steps at the end of the thermal cycle, the determination of the origin of RTP-related defects insilicon is of great importance for many device applications. Metallic impurities are known (1] as major contaminants which degrade the performances of silicon devices. They may be present in the as-grown material or unintentionally introduced during high-temperature processing steps. They induce deep levels which act as efficient recombination centers. The removal of these metallic impurities by a gettering process becomes necessary for most applications as they control the device performance. In our study, we present results showing that the association of RTP, which electrically activates defects, and of Deep Level Transient Spectroscopy (DLTS), which can detect the associated levels at concentrations as low as 10-4 of the dopant level, is an efficient means to reveal residual metallic impurities present incommercially available silicon. We have, inparticular, shown in two previous works [2, 3] that residual or externally introduced metallic impurities show the tendency to redistribute inhomogeneously by gettering during the short processing time. This result was obtained by following the defect level by serial sectioning insteps of about 25 lim over the whole thickness of the samples. By this way, we were able to separate the levels of activated impurities from levels which are due to unintentional contamination of the surfaces during the thermal processing step. EXPERIMENTAL PROCEDURE The samples used inthis study and inthe two preceding ones (2,3] were commercial float-zone (FZ) 380 gm thick, n-type silicon wafers from Wacker with a resistivity of 1-5 Qcm. All the samples, similarly prepared as before [2, 3], were mechanically damaged on the backside by scratching the whole surface with a diamond scriber, then RCA cleaned and introduced into the RTP furnace. Two temperature cycles of 800 0C and 10000C for 10 s were used, with the cooling rate adjusted at 80°C/s. For the DLTS analysis, Schottky diodes were made by evaporation of 1 mme gold dots on the surfaces. In order to
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