|M.Sc Student||Ortenberg Lina|
|Subject||Characterization of the Stresses in Thin Silicon Nitride|
Films Deposited on GaAs Substrates
|Department||Department of Materials Science and Engineering||Supervisors||Dr. Dov Sherman|
|Professor Emeritus Yigal Komem|
|Full Thesis text|
Silicon nitride thin films (SiNx) are widely used in GaAs MMIC fabrication. The layers serve as dielectric films in MIM (metal-insulator-metal) capacitors, semiconductor surface passivation and mechanical protection. An important mechanical property of the silicon nitride is the residual stresses of the deposited film. It is well recognized that the stresses in the SiNx layer in GaAs-based device structures can affect the electrical performance. The most commonly used technique for SiNx deposition is plasma enhanced chemical vapor deposition (PECVD), using silane (SiH4) and ammonia (NH3) as the precursor species. The use of hydride source gases may result in hydrogen incorporation into the layer. Recently, the use of high-density plasma ICPECVD deposited SiNx was suggested. This technology enables to deposit SiNx films with NH3-free recipe. Different deposition process parameters can significantly affect the resulting SiNx layer physical properties, chemical composition and mechanical stresses.
The objective of this research was to evaluate the residual stresses of SiNx films deposited on GaAs substrate. The influence of SiNx film thickness and stochiometry (N/Si) ratio on the residual stresses of the films was studied. The affect of different SiNx deposition systems on the residual stresses was compared.
For the evaluation of the relationship between SiNx thickness and the residual stresses, 0.4-1.2 micron SiNx films were deposited in PECVD and in ICPECVD systems on semi-insulating GaAs substrates. Film thickness and refractive index were measured by ellipsometry with helium-neon laser. The residual stresses in the layers were determined by measuring the curvature changes using Stoney Equation. Tensile residual average stresses of 160-240 MPa were obtained for SiNx films deposited by PECVD and compressive average stresses of 420-350 MPa were measured in SiNx films deposited by ICPECVD.
The SiNx stochiometry influence on the residual stress was studied on ICPECVD SiNx films. The Si/N ratio was controlled by variation of the precursor gases. The chemical composition of the films was estimated using X-ray photoelectron spectrometer (XPS). The increase of N2/SiH4 gases ratio results in increase in N/Si ratio and decrease of the refractive of the films. The residual compressive stresses of 368-438 MPa were obtained and they turned to be less compressive with N/Si ratio increasing.
The deposition process parameters affect the resulting SiNx layer chemical composition and subsequently the mechanical stresses. Consequently, the residual stresses of SiNx films can be controlled by varying the parameters of pressure, RF power and gases flow.