Scanning capacitance microscopy and - spectroscopy on SiO2 films with embedded Ge and Si nanoclusters

Embedded nanoclusters in SiO2 films have attracted increasing interest for applications in memory devices. Charge storage effects are easily observed in MOS structures with nanoclusters, but the nature of the trap states is still unclear. In particular the implantation damage when the clusters are formed by ion beam synthesis might play a role. Spectroscopic and spatial information is required in order to examine the trapping sites responsible for the charge storage. Scanning probe techniques which detect electrical properties seem to be suitable to study the local trapping properties of implanted layers.
Scanning capacitance microscopy (SCM) and scanning capacitance spectroscopy (SCS) were developed recently to study the charging phenoma and trapping states in insulating films. Inspite of the unambiguous impact of trapped oxide charges on local dC/dV(V) curves the utilization of SCS data for a quantitative determination of local electrical properties of insulating films has not been used hitherto, due to experimental issues such as reproducibility, tip-sample effects and accuracy. We will demonstrate the suitability of the SCM / SCS method for the estimation of oxide trap densities and decay times of localized charges as well as for the implementation of local electrical stress in SiO2 films.
Nanocluster formation was accomplished by the implantation of Ge or Si into 20 nm SiO2 on (100) oriented p-type silicon and a subsequent annealing in N2 ambient at temperatures between 950°C and 1150°C, respectively. Applying a dc bias of up to +/- 10 V to the conductive tip for a certain time, a local charge injection either from the tip or the substrate into the SiO2 was performed. SCM images visualize the localized charge trapped in the injection spot area (Fig. 1).
In addition, at each spot the dC/dV signal was acquired by sweeping the dc bias between +5 V and -5 V. The shift of the peak position along the voltage axis is a measure of the trapped oxide charge (Fig. 2). Trap concentrations are derived for a set of different samples from SCS measurements and compared to results from high frequency capacitance voltage measurements on gated structures. Hysteresis effects will be discussed. Dependencies of the trap density on implantation dose and energy were detected by SCS. The decay of localized charge clusters was monitored on a timescale from a few minutes up to 14 days. At last, via tip biasing sequences local electrical stress was applied to the films. Local degradation and charge trapping is obvious from a comparison of SCS curves before and after the implementation of several stress routines.