Common-mode-rejection demodulation lock-in technique for high-resolution characterization of ion implantation in silicon wafers

Abstract

In this article, we present the use of frequency-scan and lock-in common-mode-rejection demodulation (CMRD) laser photothermal radiometry to the study of B+, P+, and As+, ion implanted silicon wafers, with and without surface-grown oxides. The implantation energy of the wafers was 100 keV in all the wafers and doses ranged between 1×1011–1×1013 ions/cm2. The CMRD technique is a new demodulation method that was tested after a theoretical study and its implementation in hardened Zr–2.5Nb samples. This technique is applied to silicon ion-implantation monitoring and we report a superior signal resolution in dose range where the conventional frequency scans essentially overlapped: B+ implants in the dose range 1×1012–1×1013 ions/cm2, and P+ implants in the 1×1011–1013 ions/cm2 range. In all other cases where conventional frequency scans could resolve implantation doses, CMRD did not present any significant resolution advantages. It was further established that the pulse separation increment δΔ is the critical CMRD wave form parameter, which controls dose resolution through substantial signal background and noise suppression. The dose resolution improvements afforded by the CMRD technique may be important toward better control of the ion-implantation process in electronic devices, in a dose range which has traditionally been difficult to monitor optically owing to the effects introduced by the early stages of the amorphization process in the implanted layer.