Post Deposition Ultraviolet Treatment of Silicon Nitride Dielectric: Modeling and Experiment
- PDF / 103,724 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 73 Downloads / 168 Views
0910-A19-04
Post Deposition Ultraviolet Treatment of Silicon Nitride Dielectric: Modeling and Experiment Vladimir Zubkov, Mihaela Balseanu, Li-Qun Xia, and Hichem M'Saad Thin Films Group, Applied Materials Inc., 3050 Bowers Ave., Santa Clara, CA, 95054 ABSTRACT The use of high stress silicon nitride has been shown to improve transistor performance by increasing carrier mobility in the channel. Post-deposition ultraviolet (UV) treatment is one of the methods used to enhance the tensile stress of deposited PECVD silicon nitride films at low temperature. The increase in tensile stress is achieved by formation of strained Si-N bonds via breaking N-H and Si-H bonds in excited states and hydrogen removal from the film. The time-dependent DFT was applied to silicon nitride clusters, SixNyHz , for modeling the effect of electronic excitations caused by UV treatment on bond strengths. According to the model employed in the paper, the H abstraction from N-H is noticeably alleviated by UV cure. No such effect was found for the H abstraction from Si-H. The Si-N bonds are only marginally weakened in excited states. Experimentally, the Si-H and N-H content in the nitride film was monitored using transmission FT-IR. The N-H content decreases with increasing cure time only at a slightly higher rate than that of Si-H. This contradiction with DFT results suggests the existence of additional reactions taking place in the nitride film during UV cure. Possible reactions were suggested and modeled. INTRODUCTION Application of a uniaxial stress in the channel of NMOS (tensile stress) and PMOS (compressive stress) devices improves transistor performance [1-4]. The post-deposition ultraviolet treatment in the range λ=200-300 nm is one of the methods used to enhance the tensile stress of PECVD silicon nitride films [5]. It is usually assumed that UV treatment facilitates breaking of N-H and Si-H bonds with release of H atoms and formation of =N· and ≡ Si· radicals. Increase in tensile stress is achieved by formation of new Si-N bonds from these radicals. In this paper ab initio methods were used to clarify the impact of UV light on the dissociation of Si-H, N-H, and Si-N bonds. The UV cure brings about excited electronic states in the system. Ab initio method in the form of the time-dependent (TD) extension of density functional theory (DFT) was applied for modeling electronic excited states of two SixNyHz clusters of different structure. The impact of excitations on bond strengths is measured by comparison of increases in energies upon an initial small bond stretch in ground and excited states of clusters. This approach makes it possible to determine how the strength of the considered bond is affected by electronic excitations caused by UV treatment. However, it turned out that the predicted bond dissociations do not correlate completely with experimental results obtained by FTIR. This poor agreement with ab initio results suggests the existence of additional reactions involving H atoms in the nitride film during UV cure. DFT calculations
Data Loading...
