AVS 46th International Symposium
    Plasma Science and Technology Division Thursday Sessions
       Session PS1-ThA

Paper PS1-ThA3
High Volume Self Aligned Contact Etch for SRAM

Thursday, October 28, 1999, 2:40 pm, Room 612

Session: High Fidelity Pattern Transfer
Presenter: U. Raghuram, Cypress Semiconductor
Authors: U. Raghuram, Cypress Semiconductor
J.E. Nulty, Cypress Semiconductor
Correspondent: Click to Email
Self-Aligned Contact (SAC) technology is used by Cypress Semiconductor in SRAM cells to form contacts which connect local interconnect and diffusion regions. SAC contacts allow designers to reduce the cell size by reducing the poly pitch. SAC technology is made feasible by the selective etch of the interconnect dielectric such as borophosphosilicate glass to the nitride etch-stop. This paper describes the key parameters for a manufacturable SAC etch, namely, gas chemistry, backside helium cooling, and total gas flow/ pressure. Polymer fill in the contacts provides the required selectivity to the etch-stop layer in SAC etch. The polymer profile is controlled by the sticking coefficient, which is controlled by the wafer temperature. Two common mechamisms for SAC etch are incomplete etch, 'grass', caused by excessive polymer deposition, and loss of selectivity, 'punch through', caused by insufficient polymer protection. Freon-23/ Freon 134a based chemistry provides self limiting nitride etch-stop performance. Uniform and constant wafer temperature is critical for process window in an etch controlled by polymer deposition. Using electrostatic chuck technology resulted in a wider process window with respect to backside helium control. The process runs at relatively higher pressures (50-100 mT) and total flows (70-150 sccm). Addition of argon allows the process to run at a moderate pressure and flow. It also improves the process uniformity. Backside helium setting of 8-12 Torr provides good wafer temperature control. The process was first developed for 0.5 micron design rules. The Cypress SAC process has been transferred to two different medium density plasma etcher platforms. This process has also been extended with minor modifications to etch SAC contacts for at least four SRAM technologies ranging from 0.5 to 0.21 micron design rules.