Alternative Spin-On-Glass (SoG) material characterization for Deep-Submicron (<0.35 um) soft reflow fabrication process

Currently a submicron soft reflow process developed in University of Manchester uses silicon nitride (Si₃N₄) as the hard mask layer to support the T-Gate structure of pHEMTs in order to make it mechanically stable. However, an alternative material known as Spin-on-Glass (SoG) is introduced to replace Si₃N₄, offering shorter processing time and consequently, a much simpler and more cost-effective alternative to the e-beam lithography of nanometre-scale gate length transistors. The SoG deposition through plasma-enhanced chemical vapour deposition process requires only 30 minutes to complete, as opposed to the one-day process of depositing Si₃N₄. In this study, the SoG material used is Silicafilm, and the minimum deposition thickness achieved is 138 nm, enabling 150-nm gate length devices to be fabricated. The SoG is also successfully etched at very low power and pressure (20 W and < 25 mTorr respectively), eliminating the detrimental effect of high power plasma etching to the 2DEG carriers. In addition to that, no film cracks observed even by using a single coating and a single baking temperature of 200 °C. All these results indicate the potential of SoG as a suitable hard mask layer alternative to the silicon nitride for the use soft reflow fabrication process.