Ion-implanted thin polycrystalline-silicon high-value resistors for high-density poly-load static RAM applications

An application of ion-implanted polycrystalline-silicon resistors for a 4- and 16-Mbit MOS static RAM cell is discussed. 4M and 16M static RAM´s are designed with assumed minimum feature sizes of 0.5 and 0.25 µm of double-level polycrystalline-silicon process and power supply voltages of 3.0 and 1.5 V, respectively. The load current of the memory cell is kept at about 30 pA per cell, which is as low as that of state-of-the-art 64K static RAM´s. LPCVD polycrystalline-silicon film of about 25-nm thickness is used to etch fine resistor patterns of 0.25 ∼ 0.5 µm feature sizes. Sheet resistances of 24 and 12 GΩ/ for 4M and 16M static RAM cells are controlled by arsenic implantation of about 4 × 10¹³and 3 × 10¹³cm^-2at 10 keV, respectively. The ion-implanted polycrystalline-silicon resistors show nearly ideal linear current-voltage characteristics. Grain sizes of the polycrystalline-silicon films calculated from the current-voltage characteristics are in good agreement with those measured by transmission electromicrographs. It was shown that 4M and 16M static RAM´s would be realized by combining the scaling-down of state-of-the-art 64K RAM cell of the double-level polycrystalline-silicon process and the thin ion-implanted polycrystalline-silicon resistors.