The evolution of IBM CMOS DRAM technology

The development of DRAM at IBM produced many novel processes and sophisticated analysis methods. Improvements in lithography and innovative process features reduced the cell size by a factor of 18.8 in the time between the 4Mb and 256Mb generations. The original substrate plate trench cell used in the 4Mb chip is still the basis of the 256Mb technology being developed today. This paper describes some of the more important and interesting innovations introduced in IBM CMOS DRAMs. Among them, shallow-trench isolation, I-line and deep-UV (DUV) lithography, titanium salicidation, tungsten stud contacts, retrograde n-well, and planarized back-end-of-line (BEOL) technology are core elements of current state-of-the-art logic technology described in other papers in this issue. The DRAM specific features described are borderless contacts, the trench capacitor, trench-isolated cell devices, and the “strap.” Finally, the methods for study and control of leakage mechanisms which degrade DRAM retention time are described.