SiON thin films are used as a substitute for SiO2 gate oxide films to reduce the gate leakage and boron penetration through the oxide. A variety of process conditions are possible in order to engineer different N concentrations and distributions in the gate oxide. For process development, as well as for process control, accurate N in-depth profile monitoring is crucial. SIMS is the method of choice for this task.
New SIMS protocol to minimize matrix effects
A SIMS protocol for SiON metrology has been suggested by Kataoka et al. . The recommended sputtering conditions are 250eV O2+ at 70° incidence angle from normal. The goal of this new protocol is to minimize matrix effects between the SiON and Si matrices. This is achieved by monitoring the secondary ion intensity for SiO+, SiO2+, and Si2N+. For N analysis, matrix effects are cancelled by rationing Si2N+ to Si2O+. This normalization procedure is very similar to the quantification of implants using relative sensitivity factors (RSF). The SiON/Si interface position is determined from the dip of the SiO+ signal intensity.
Kataoka & al. demonstrated the accuracy of this approach by a direct comparison of SIMS data with Rutherford Forward Scattering Spectroscopy measurements. RFSS is generally acknowledged to be a quantitative method, but not practical for routine use in industry.
Figure 1: N depth profiling of 4nm SiON/Si. Comparison of SIMS data and Rutherford Forward Scattering Spectroscopy .
N depth profile comparison
A set of six Oxynitrides (samples 1-6), manufactured with different nitridation and Post Nitridation Annealing (PNA) process conditions was analysed with a CAMECA Quadrupole SIMS. After an in-situ cleaning step with argon, the nitridation was performed by means of a RTP process in NH3 atmosphere. The PNA-step was performed in steam. The SiON layer thickness of sample 1-6 was primarily determined by elipsometry and supported by TEM measurements.
Figure 2 shows a comparison of depth profiles resulting from various process conditions. Obviously the layer thickness, total nitrogen content and N distribution as well as the surface concentration differ greatly. It is worthwhile to notice that the measured peak concentration of sample #6 was 6E22 atoms/cm3, very close to the bulk concentration of Si3N4 (6.17E22 atoms/cm3).
Figure 2: SIMS profiles for a set of 6 thermally grown SiON thin films and nitridation at 900°C/1060°C with 10% NH3 in Ar.
 Y. Kataoka et al., SIMS XIV conference proceedings