Optimization of Low Stress PECVD Silicon Nitride for GaAs Manufacturing

It is well recognized that the stress of the SiNx layer in GaAs-based device structures can impact the electrical performance and lead to degradation. For GaAs MESFET and HEMT devices, it has been demonstrated not only the magnitude of the stress but also the stress state, compressive or tensile, can affect the performance [1]. Stressinduced failure via microvoid formation in SiNx MIM capacitors has also been reported [2]. Therefore, the capability of tailoring the magnitude and state of the SiNx stress required for a specific device structure is very important. For SiNx, a common technique to control the stress in a conventional 13.56 MHz parallel plate PECVD reactor is through the addition of low frequency power. At 13.56 MHz, SiNx films prepared from standard gas mixtures of SiH4, NH3, and N2 are typically tensile in nature. The added low frequency (< 1 MHz) component results in high energy ion bombardment of the growing SiNx film. This results in a change of the stress state from tensile to compressive [3, 4]. At Unaxis, a He dilution method has been developed as a simpler alternative technique to control the stress of PECVD SiNx. As illustrated in Figure 1, the addition of He to the standard gas mixture of SiH4, NH3, and N2, enables stress control from about 300 MPa, tensile through zero to about –300 MPa, compressive. Plasma-induced damage during the SiNx deposition process, resulting in physical and electronic degradation of GaAs devices is a very important issue [5–7]. Without the requirement of a low frequency power source, the possibility of damage is reduced with the He dilution method. The RF power density at 13.56 MHz is very low and typically less than 50 mW/cm. A designed experiment (DOE) was implemented to characterize and optimize a low stress SiNx process based on the He dilution method on a Unaxis PECVD production platform developed for high volume GaAs manufacturing. To understand the mechanism involved in this technique for stress control, optical spectroscopic analysis of different He/N2 plasmas in the PECVD reactor has been performed.