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PMOS and NMOS Process Monitors for Fast In-Test High Volume Manufacturing Process

IP.com Disclosure Number: IPCOM000146524D
Publication Date: 2007-Feb-15
Document File: 4 page(s) / 57K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method that uses NMOS and PMOS ring oscillators as the main building blocks of the process monitor PROCMON. Based on speed, the disclosed method determines whether changes in the process are related to the NMOS or PMOS transistors. Benefits include better characterizations of speed paths.

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PMOS and NMOS Process Monitors for Fast In-Test High Volume Manufacturing Process

Disclosed is a method that uses NMOS and PMOS ring oscillators as the main building blocks of the process monitor PROCMON. Based on speed, the disclosed method determines whether changes in the process are related to the NMOS or PMOS transistors. Benefits include better characterizations of speed paths.

Background

Typical ring oscillators used to date have been built with the standard CMOS logic. The disadvantage of the CMOS logic is that the process change in the NMOS transistor (and its subsequent impact on the ring oscillator speed) indirectly changes the PMOS (NMOS) transistor impact on the overall ring oscillator speed. In CMOS technology, the ring oscillator speed is always controlled by both transistors (PMOS and NMOS) no matter which process changes. As a result, the CMOS ring oscillator does not provide sufficient separation of the NMOS from the PMOS process, and vice versa.

General Description

In the disclosed method, the NMOS and PMOS ring oscillators are built only with transistors of the same polarity Figure 1 shows the schematic of the PMOS oscillator, and Figure 2 shows the schematic of the NMOS oscillator. The NMOS can be used for monitoring NMOS-only process variations, and is totally independent of the PMOS process changes. The PMOS can be used for monitoring PMOS-only process variations, and is totally independent of the NMOS process changes.

The PMOS and NMOS ring oscillators consist of the following four sub circuits:

·        PMOS/NMOS ring oscillator

·        Ring oscillator output pull up & enable and delayed enable logic

·        Differential amplifier 

·        CMOS buffer with pull up

PMOS/NMOS Ring Oscillator:

The PMOS/NMOS ring oscillator consists of seven stages, with four transistors per stage. The top transistor of each PMOS ring oscillator’s stage is the switch; the bottom three transistors are the load. The top 3 transistors of each NMOS RO stage are the load, the bottom transistor is the switch. The output of the seventh stage is fed back into the input of the first stage, causing the ring oscillator to oscillate. The number of stages and transistors per stage may vary based on specific requirements and needs.

Ring Oscillator Output Pull Up & Enable and Delayed Enable Logic: (PMOS)

The ring oscillator is disabled when the enable pulse is high. The enable pulse is fed into the gate of the first load transistor of each, except the first and seventh stage, thereby biasing them OFF. The source of the second to sixth stage switch transistors is almost at VCC potential when the enable pulse is high.  The gate of the 1st load transistor of the 1st stage is connected permanently to ground thus biasing the transistor to ON state. The source of this transistor will be almost at ground potential. This configuration assures t...