Quadruply self-aligned stacked high-capacitance RAM using Ta2O5high-density VLSI dynamic memory

A new one-transistor, one-capacitor RAM cell structure called a Quadruply Self-Aligned Stacked High Capacitance (QSA SHC) RAM is proposed as a basic cell for a future one-million-bit VLSI memory. This cell consists of a QSA MOSFET and a Ta
2
O
5
capacitor stacked on it. By this cell, the ultimate cell area
3F \times 2F
can be realized with sufficient operating margin. Here,
F
is the minimum feature size. The basic cell was fabricated and its operation was experimentally verified. The leakage current of Ta
2
O
5
film was small enough for the storage capacitor dielectric. Using a
3F \times 4F
cell and a
4F
pitch sense amplifier, a one-million-bit memory was designed with a 2-µm rule. A cell size of 6.5 × 8 µm
²
, and a chip size of 9.2 × 9.5 mm
²
were obtained. The access time, neglecting the RC time constant of the word line, was estimated to be about 170 ns. Based on this design, it is argued that a future one-million-bit memory can be realized by QSA SHC technology with a 2-1-µm process. The mask set of the 1-Mbit RAM was actually fabricated by an electron-beam mask maker. A photomicrograph of the 1-Mbit RAM chip patterned by the mask set is shown. This chip was patterned not to get an operating sample but to show an actual chip image of the future 1- Mbit RAM. The area of each circuit block including storage array can be seen in this chip image.