Variable work function transistor high density mask ROM

Details Variable work function transistor high density mask ROM Variable work function transistor high density mask ROM

1996-12-17

1999-08-24

Chaudhuri, Olik

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

257401, 257407, 257413, H01L 2976, H01L 2994, H01L 31062, H01L 31113

Patent

active

059427868

ABSTRACT:
A mask ROM stores information by selecting the work function of the gates of each FET in an array of FETs. The polysilicon gates of some of the FETs are doped N-type and the gates of the other FETs are doped P-type to form gates having different work functions, thereby forming FETs having different threshold voltages. The ROM consists of a parallel array of buried N.sup.+ bit lines formed in the substrate, a gate oxide layer deposited over the bit lines and a layer of polysilicon deposited on the gate oxide. The polysilicon is blanket doped P-type and then an encoding mask is formed, with openings in the encoding mask exposing regions of the polysilicon to be formed into gates of FETs with low threshold voltages. Either arsenic or phosphorus is doped into the polysilicon through the mask openings. The mask is removed, a layer of conductive material such as tungsten silicide is deposited and the polysilicon and the conductive material are formed into word lines for the ROM. The word lines of the ROM serve as gates for the FETs and the bit lines serve as sources and drains for the FETs.

REFERENCES:
patent: 4559694 (1985-12-01), Yoh et al.
patent: 5055904 (1991-10-01), Minami et al.
patent: 5218232 (1993-06-01), Yuzurihara et al.
patent: 5260593 (1993-11-01), Lee
patent: 5536962 (1996-07-01), Pfiester

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