Methods of forming an SRAM

Details Methods of forming an SRAM Methods of forming an SRAM

1999-11-04

2001-03-20

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

Having insulated gate

C438S237000, C438S380000, C438S381000, C438S382000, C438S554000, C438S555000, C438S556000, C438S564000, C438S979000

Reexamination Certificate

active

06204110

ABSTRACT:

TECHNICAL FIELD
This invention relates generally to semiconductor processing methods of forming resistors from semiconductive material, and to static random access memory (SRAM) cells incorporating resistors, and to other integrated circuitry incorporating resistors.
BACKGROUND OF THE INVENTION
One of the common elements required in electrical circuit devices is the pull-up or pull-down device from an active device to one of the power supply buses, typically referred to as Vcc. The pull-up is simple if used to construct a circuit using discrete components in that all that is required is selecting a resistor of the desired resistance and tolerance, connecting it between an active device such as an open collector transistor and Vcc, and the transistor's output would be pulled up to Vcc once the transistor is forward biased. With the advent of the integrated circuitry, however, fabricating a resistance onto a wafer substrate, such as silicon or gallium arsenide, takes special consideration, particularly when resistivity and tolerances play an important part in circuit operation.
For example, as SRAMs have evolved from the 4 Kb memory arrays to more densely packed array sizes, tolerances of pull-up resistances had to be tightly controlled. In order to minimize standby current, many fabrication processes adopted use an active device as the pull-up. In CMOS fabrication, it is common to see a PMOS transistor acting as the current path between a memory cell access transistor and the power supply bus. In this manner, the PMOS transistor can be gated “on” only when the desired line is to be pulled up to Vcc and turned off otherwise. This in essence eliminated leakage current and minimizes standby current for the SRAM device as a whole.
The main drawback to using an active device for a pull-up device is the amount of space required to fabricate the device. Now that the SRAM generation has crown to the 1 Mb array size, die space is a critical factor to consider. Technology has basically pushed all types of integrated circuits to be more densely packed, and pull-ups are a common element in many circuit designs.


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