A new stack capacitor for polysilicon active matrix arrays (TFTs)

A novel stack capacitor structure is proposed for dynamic data storage in circuits using self-aligned polysilicon thin-film transistor technology. Unlike the conventional planar thick-oxide capacitor formed between the gate-polysilicon storage node and the metal ground line, the stack capacitor is fabricated by adding an island-polysilicon plate underneath the gate-polysilicon storage node, thus creating a sandwiched stack capacitor structure. Owing to the high capacitance per unit area of the thin oxide capacitor, significant savings in chip area can be achieved. The stack capacitor structure has been successfully applied to active matrix arrays using self-aligned polysilicon thin-film transistors as the control switches. Improved data retention and decreased feed-through voltage, characteristic of higher storage capacitance, are demonstrated. The stack capacitor also features high immunity to crosstalk due to its unique surrounding ground plate which serves to screen each individual storage node. The capacitor therefore appears quite attractive for circuit applications which require large (>0.1-pF) capacitance.<>