Effects of Methyl Silsesquioxane Electron-Beam Curing on Device Characteristics of Logic and Four-Transistor Static Random-Access Memory

Abstract

The as-spun methyl silsesquioxane (MSQ) film cured by an electron beam (e-beam) did not show water absorption after a five-day exposure to ambient air. MSQ was applied to the triple-level metal (TLM) Logic and double-level metal (DLM) four-transistor (4-T) static random-access memory (SRAM) as intermetal dielectric by means of the non-etchback process. When MSQ treatment conditions were properly controlled, the top layer of the as-spun films was cured by e-beam exposure while the bottom layer of the film was thermally cured for a short period of time. The as-cured MSQ offered good surface planarity. In addition, neither via poisoning, bowing nor cracking was observed. The results showed that, by the application of the e-beam cured MSQ in the fabrication of interconnect structures, the cache time of DLM 4-T SRAM could be improved to 10 ns compared with 11.5 ns for the SRAM fabricated using the conventional furnace cure spin-on-glass (SOG) process (400°C annealing for one hour). E-beam exposure has little effect on n-channel metal-oxide-semiconductor (NMOS) device characteristics, such as saturation current, threshold voltage and channel length. In contrast, e-beam exposure has a significant effect on p-channel MOS (PMOS) device characteristics, resulting in a shift of the threshold voltage as well as an increase in the channel length. It is notable that the e-beam exposure almost did not affect NMOS device characteristics in the 4-T SRAM, since the polysilicon load resistor could serve as a shield against electron bombardment. In such a case, the resistance of the polysilicon load resistor was significantly decreased.