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SiON Metrology with the CAMECA IMS Wf / SC Ultra

Depth profiling with sub-nm resolution and ppm sensitivity

SiON thin films are used as a substitute for SiO2 gate oxide films as a means to reduce gate leakage and dopant penetration through the oxide. Accurate knowledge of N in-depth profiles is crucial for process development and monitoring. A variety of process conditions allow the engineering of different N concentrations and distributions in SiON gate oxide. Associated metrology methods must be capable of delivering an accurate N dose value and N peak position wherever nitrogen is located across the layer. Hence the importance of ensuring adequate depth resolution (sub-nm) and sensitivity (0.001at% detection limit).

New SIMS protocol for SiON layer analysis

The new proposed SIMS protocol aims at minimizing ion yield variation between the SiON and Si matrices. It consists in sputtering with Cs primary ions and analyzing negative secondary ions in combination with O2 chamber backfilling. Primary beam parameters are 250eV impact energy and 48° incidence angle from sample normal. Oxygen flooding pressure setting is around 1e-7mbar. This low pressure does not prevent the analyst from monitoring the 16O- signal in order to determine the SiON/Si interface position.
Sputter yield variation knowledge between different matrices (Si and SiO2) is essential for accurate N distribution measurement, and sputter rate variation must be taken into account. For this purpose, the concept of Sputter Rate Variation function [1] is applied. The concentration calibration of the N profile is performed by ratioing the SiN- intensity to the Si- intensity measured in the Si substrate.

Towards accurate N depth profile in SiON

N depth profiling of  SiON/Si layerFigure 1 shows the N depth profile obtained in a relatively deep SiON structure with N enriched SiON/Si interface. The excellent sensitivity offered by the SiN- species gives access to a N detection limit in the range of 5e18at/cm3. O2 backfilling minimizes preferential sputtering effects and thus improves depth resolution. In order to achieve accurate N profile quantification, the sputter rate variation between SiON layer and Si substrate is corrected by applying the SRV curve (light green). This SR correction decreases the N dose value.
SIMS Nitrogen dose comparison with LEXES measurement shows a good agreement as shown in Table 1 below. 

Table 1: LEXES/SIMS comparison
 
LEXES dose
4.88e14at/cm2
SIMS dose
250eV Cs+ + O2 @1e-7mbar, RSF + SiN- normalization to SRV 
4.96e14at/cm2

 

N depth profiling of  SiON/Si layer with N enriched surface Figure 2: depth profiling of SiON/Si layer with N enriched surface under 250eV Cs primary beam sputterning in combination with O2 chamber backfilling @1e-7mbar.

Figure 2 on the right shows a N profile measured with the same protocol in a SiON structure with a N enriched surface. SIMS dose comparison with RBS is shown in Table 2 below. A good agreement between SIMS dose and RBS dose is demonstrated, giving confidence in the robustness of this new protocol for different SiON structures.
Table 2: RBS/SIMS comparison
RBS dose
1.99e15at/cm2
SIMS dose
250eV Cs+ + O2 @1e-7mbar, RSF + SiN- normalization to SRV
1.87e14at/cm2

[1] Quantitative Analysis of SiON Layers using ULE Cs+ and Negative Secondary Ions. A. Merkulov et al., Proceedings of the XVIIth International Conference on SIMS, Toronto, 2009.
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