Registration method in photolithography and equipment for carryi

Details Registration method in photolithography and equipment for carryi Registration method in photolithography and equipment for carryi

1988-12-22

1990-12-25

Wintercorn, Richard A.

Photocopying

Projection printing and copying cameras

Step and repeat

355 77, G03B 2742

Patent

active

049807188

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to the multiple exposure of optically sensitive coatings on a supporting substrate and has particular reference to the fabrication of microcircuits for semiconductor devices. In one aspect, the invention relates to an improved method of ensuring accurate registration of the multiple images required to be applied one after the other to the optically sensitive coatings applied sequentially to a substrate (e.g. a wafer of single crystal semiconductor material) to enable those coatings to be further processed (e.g. to form complex microcircuits for semiconductor chips). In another aspect, the invention relates to equipment (widely referred to as a "wafer stepper") to enable large numbers of such microcircuits to be fabricated side-by-side on a single wafer substrate.


DISCUSSION OF PRIOR ART

One of the key steps in the fabrication of microcircuits is lithography--the ability to define intricate patterns on the surface of a silicon wafer. Optical lithography, using wavelengths around 400 nm, is currently the dominant method of pattern replication and it is likely to remain so for some time.
The trend to smaller geometries for the microcircuits and the use of larger diameter wafers on which those microcircuits are to be produced has led to the use of direct step techniques using wafer steppers. These are essentially step-and-repeat cameras in which a mask pattern is imaged directly onto the coated surface of a wafer with a reduction ratio which might be 1Ox or 5x for example. By restricting the image field, usually to less than 20 mm square, projection lenses of high numerical aperture may be used The wafer is stepped progressively below the camera optics and the imaging of the required pattern is repeated on an xy stage mechanism to expose the entire usable surface of the wafer with pattern registration and focus corrections being made at each exposure site.
The most common source of illumination used with state of the art wafer steppers is a mercury vapour lamp On such wafer steppers, the intensity in the image plane is of the order of 100-1000 mW cm.sup.-2 leading to exposure times of 500-50 mS per site when used with positive optical resists. This means that the xy stage mechanism has to move, stop, settle, adjust registration and then remain stationary during the exposure before moving to the next site.
The use of a pulsed excimer laser source has been proposed A number of excimer lasers are available with wavelengths in the 160-350 nm range giving output pulses of about 10 ns duration at repetition rates in excess of 100 pulses per second The energy in each pulse of such a laser source is far in excess of the dose required to expose a conventional positive resist coating and hence a "flash-on-the -fly" system, with a continuously moving stage, has been proposed. This would improve throughput by a factor of 2 or 3 and the mechanical design of the xy stage mechanism would be easier: the problems of stop-start operation such as stick-slip and dynamic yaw would be removed Exposures are made when the scan carriage is travelling along a row of sites on the wafer and such exposures can all be made in the same direction of scan (i.e. with a flyback to the start of each new row) or with exposures made in opposite directions in each alternate row. Although the output from a wafer stepper would be reduced by unidirectional exposures, the mechanical repeatability of the xy stage mechanism would be more easily achieved.
The successful exposure of wafers requires not only good resolution of the fine geometries required for a microcircuit but also accurate registration of the new projected image with the existing wafer pattern at each exposure site. This invention relates to an improved dynamic registration method and suitable equipment for its implementation.


EXISTING REGISTRATION SYSTEMS USED WITH WAFER STEPPERS

There is no agreed standard on the type of alignment mark and mark detection technique for use in image registration on wafer steppers' there are as many systems as

REFERENCES:
patent: 4676649 (1987-06-01), Phillips

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