Thermal stress relaxation of plasma enhanced chemical vapour deposition silicon nitride
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Thermal stress relaxation of plasma enhanced chemical vapour deposition silicon nitride
P. Morin, E. Martinez, F. Wacquant and J. L. Regolini ST Microelectronics, 850 rue Jean Monnet, F-38926 Crolles Cedex [email protected] ABSTRACT Mechanical and thermal properties of silicon nitride films deposited by different plasma process type have been studied. The initial mechanical stress, thickness and hydrogen content have been evaluated respectively by wafer curvature measurements, ellipsometry and Fourier Transform Infra Red spectrometry. These nitrides presented as-deposited stress values ranging from compressive to tensile. High temperature Rapid Thermal Anneal (RTA) at 1100°C or longer thermal treatments at medium temperature, from 700°C to 850°C were carried out on these materials. The evolution of their properties along the different anneals have been measured and compared to the behaviour of high temperature thermal nitride. One can observe that these stoechiometric plasma nitrides have shifted to an equilibrium tensile stress around 1100-1200 Mpa when submitted to the RTA, independently of their initial stress values. Results are interpreted in terms of H desorption and Si-N bond formation. Chemical reaction Si2-N-H + 2 NH 2 Si-N + NH3 appears to be the best candidate to figure out the phenomena. INTRODUCTION Silicon nitride is extensively used in CMOS VLSI, as hard mask, sidewall spacers or etch stop layer. Plasma Enhanced Chemical Vapour Deposition (PECVD) processes provide low temperature nitrides films that fit the advanced CMOS front end of line requirements. These nitride films are also used as stressor liners to generate strained silicon channel and enhanced carrier mobility [1] especially because their mechanical stress can be tuned by deposition process parameters, from highly compressive to highly tensile values. However, these layers can drastically evolve during the subsequent CMOS thermal budgets. In particular, a general trend of PECVD nitride layers is an increase to more tensile stress when annealed [2-4]. These authors suggest that this irreversible stress increase is due to H release and Si-N bonding reorganisation. On the contrary, in [5], it is proposed that the stress change is generated by micro voids shrinkage. In this paper, we have studied several PECVD nitride films deposited on silicon wafers with different kind of reactors. They were subjected to different anneal, to study the evolution of the material properties under thermal treatment. Dynamic behaviour of stress and hydrogen content are presented and compared. EXPERIMENTS PECVD silicon nitride layers were deposited on (100) 200mm silicon wafers, either in a “Cxz” Centura chamber from Applied Materials or in a “Sequel” reactor from Novellus. Both processes use silane amonia chemistry, and samples with thickness ranging between 1300A and 2400A have been prepared. For comparison purpose, stoechiometric Low Pressure CVD
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(LPCVD) reference films were processed in furnace, using di-chloro silane and amonia precursors
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