Chemical Vapour Deposited Multilayer Silicon Nitride Films as Alkali Diffusion Barriers

Silicon nitride (SiNx) is used as a diffusion barrier for alkali such as potassium hydroxide (KOH), which is often used to etch silicon. Films generally fail at pinholes present in them; while one way to prevent failure is to carefully control process parameters to deposit films with fewer pinholes, the other, less characterized method, is to deposit multiple thinner layers to achieve the same final thickness. Multilayer films deposited by chemical vapour deposition techniques are known to yield fewer pinholes, thought to be due to the misalignment of pinholes in adjacent layers. In this work, we characterize the multilayer SiNx films based on their ability to act as diffusion barriers to KOH. Here, we measure performance of these films as diffusion barriers as a function of the number of layers and thickness of each layer. We show that the ability of the SiNx multilayer structure to protect the substrate is a critically dependent on the individual layer characteristics and the deposit parameters.