Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


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Victor Boureau phD defense

PhD thesis entitled : Strains induced during FDSOI transistors manufacturing : a study by dark-field electron holography

The thesis will be held Tuesday, April 5th 10:30 a.m. in CEMES conference room

 

Jury members

 
- M. Olivier Thomas, professeur, IM2NP, Marseille (rapporteur).
- M. Philippe Vennéguès, ingénieur de recherche, CRHEA-CNRS, Nice (rapporteur).
- M. David Cooper, chercheur, CEA-LETI, Grenoble (examinateur).
- M. Jean-Michel Hartmann, directeur de recherche, CEA-LETI, Grenoble (examinateur).
- Mme Chantal Fontaine, directrice de recherche, LAAS-CNRS, Toulouse (examinatrice).
- M. Alain Claverie, directeur de recherche, CEMES-CNRS, Toulouse (directeur de thèse).
- M. Daniel Benoit, ingénieur-docteur, STMicroelectronics, Crolles (co-directeur de thèse).
- M. Martin Hÿtch, directeur de recherche, CEMES-CNRS, Toulouse (co-encadrant de thèse).
 
 

Abstract

After being considered harmful for a long time, stress became one of the principal means to improve metal-oxide-semiconductor (MOS) device performance. Indeed, the generated strains significantly increase carrier mobility in silicon. Within this context, I used dark-field electron holography (DFEH) to study the crystalline strains generated by some key steps of the manufacturing process of latest generation of planar transistors, fully depleted as produced on silicon on insulator substrates (FD- SOI). DFEH is a transmission electron microscopy (TEM) technique, recently invented at CEMES, which allows crystalline strain to be mapped with nanometric resolution and an accuracy of 10-4 over micrometric fields of view. I developed and used finite element models in order to understand, then reproduce, my experimental results and thus identify the mechanical phenomena involved during different processing steps.

After proving that DFEH is suitable for strain fields mapping in FDSOI MOS structures (Si surface layer disorientated in respect of the reference substrate), I have been interested in the conversion process of thin Si films into SiGe, by a method known as "germanium condensation". I showed that this technique enables pseudomorphous thin SiGe films (SGOI) of variable composition to be obtained. The out-of-plane strain measured by DFEH emphasises the two mechanisms affecting the Ge redistribution (diffusion and injection), whose relative importance depends on the temperature of the process. Moreover, I showed that these thin SGOI films, initially stressed, relax strongly during the etching carried out to manufacture co-integrated SOI/SGOI substrates. I could identify that this effect, initially observed by electrical measurements and known as "SA/SB" effect, can only be explained by a degradation of the mechanical characteristics of the SiGe/SiO2 interface.

I have also been interested in some of the key steps of the transistor manufacturing suspected to modify the structural strain state, such as the grid stack and sources/drains processes, as well as salicidation necessary to form the contacts. I was able to explain how and why these steps impact the final strain state of the transistor channel and thus its performance.

In a separate development, I have shown how DFEH can be used to measure doping concentrations while preserving a nanometric resolution, and discuss its limits. I studied in particular the (favourable) case of boron doping in silicon and, after electrical measurements coupling, I calculated the coefficient connecting the measured strains to the boron substitution concentrations.

Finally, I compared and discussed the differences between information obtained by DFEH and high resolution X-rays diffraction. An appendix completes this work and discusses the optical and optimal use conditions of Schottky field emission sources equipping a TEM, in particular the contribution of side- emission lobes on the degree of coherence of the probe.