Method and apparatus for measuring registration between layers of a semiconductor wafer

DETAILED DESCRIPTION OF THE INVENTION An automated system for measuring registration between layers of a semiconductor wafer is illustrated in FIGS.
1 and 2.
The major elements of the system, including a wafer handler, an optical system and a computer system, are mounted in a cabinet 10.
A cassette wafer holder 12 containing wafers to be measured is mounted on the system.
A wafer transport pick (not shown) removes a wafer 16 from the cassette 12 and places it on a prealigner 14.
The prealigner 14 rotates the wafer to a predetermined orientation by sensing the wafer flat.
The wafer transport pick then transfers the wafer 16 from the prealigner 14 to a measurement stage 18.
A suitable wafer handler is a Model CKG1 or CKG3 available from FSI.
The stage 18 is movable in three dimensions for positioning selected registration patterns relative to the optical system as described hereinafter.
The optical system includes a microscope 20 and a video camera 22 positioned above the wafer 16.
The microscope 20 typically carries several objectives ranging in power from 2.
5X to 200X magnification.
The wafer 16 is positioned on the stage 18 in a horizontal orientation.
The microscope 20 and the camera 22 have a vertical optical axis.
The stage 18 is moved until the registration patterns to be measured are located directly under the microscope 20.
The microscope turret rotates to the desired objective, and a focused image of the registration patterns is recorded by the camera 22.
In an example of the optical system, the microscope 20 is a type Zeiss Axiotron, and the camera 22 is a Dage MT168series camera.
Electrical signals representative of the image are supplied to an image processor 28 and a computer 30.
Coupled to the computer 30 are an image monitor 32 for display of the image recorded by the camera 22, a text screen 34 and a keyboard 36 which constitute an input terminal for entering operator commands and a disk drive 38 for storing system software and data.
A suitable image processor 28 is available from Recognition Technology, Inc