Abstract: Determination of ion-implanted profiles using the MOS capacitance–voltage technique

Abstract

In this paper, the application of the MOS capacitance–voltage technique for the determination of ion-implanted impurity profiles is described experimentally and theoretically for moderate (1011–1012 ions/cm2) ion implants. The experimental vehicle was a large-geometry p-channel IGFET (250×250 μm) which was implanted with boron ions at an energy of 120 keV through the gate region (500 Å Al2O3 + 1000 Å SiO2). This results in a doping profile with a peak approximately 1700 Å beneath the Si–SiO2 interface and an effective Debye length the order of 500 Å. Since the application of a gate voltage results in a maximum surface depletion width which is less than the boron profile depth, a reverse bias was applied between the source-drain diffusions and the substrate in order to sweep out the inversion electrons that otherwise would shield the surface electric field. The resulting gate to source-drain capacitance curve in the surface depletion region was used to determine the implanted profile with the Kennedy–O’Brien majority carrier correction factor included. This was also compared to a theoretical Gaussian implant profile based on theoretical values for range and straggle. To examine the validity of the experimental results, a theoretical study was made using the following method: (1) Assume a Gaussian profile. (2) From the above, calculate the semiconductor capacitance. (3) Use the capacitance to calculate an ’’effective’’ profile. (4) Compare the ’’effective’’ profile with the Gaussian profile. This comparison showed that the measured profile can be determined with resolution substantially less than a Debye length. Based on this work, it is concluded that the substrate-biased MOS transistor capacitance of an ion-implanted depletion-mode IGFET is a practical method which can be used for process control of the peak of the implant to an accuracy which is better than a Debye length.