A new VLSI memory cell using capacitance coupling (CC cell)

A new VLSI memory cell is proposed, which offers a small cell area, about 6F²(F is the feature size), readout signal gain, and high alpha-particle immunity. Since it employs capacitance coupling in a write operation, it requires only one bit line and is called a capacitance coupling (CC) cell. A CC cell consists of three transistors and a capacitor, which are integrated in a small area by sharing their nodes with one another. The charge is stored in a high-concentration p-type diffused layer (p⁺-layer) in a shallow n-type diffused layer (n-layer). The p⁺-layer potential controls the readout current which flows through the n-layer. To obtain both the p⁺-Iayer isolation and the large readout signal, trench isolation was employed. Experimental test devices having 0.7-µm deep n-layer, 0.2-µm deep p⁺-layer, and 1.5-µm deep trench isolation region were fabricated. They showed complete CC cell operation and "0"/"1" readout current ratio of more than 10.