Radiation imagery chemistry: process – composition – or product th – Plural exposure steps
Reexamination Certificate
1999-08-23
2001-05-29
Letscher, Geraldine (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Plural exposure steps
C430S005000, C430S311000, C430S312000
Reexamination Certificate
active
06238850
ABSTRACT:
DESCRIPTION
Field of the Invention
The present invention broadly relates to the field of lithography; and more particularly, to a method of reducing comer rounding on very small images, i.e. images that are near the resolution limit of the optical system. The present invention also offers an alternative method of proximity correction especially useful for very small images.
Background of the Invention
In the field of integrated circuits (ICs), photolithography is used to transfer patterns, i.e. images, from a mask containing circuit-design information to thin films on the surface of a substrate, e.g. Si wafer. The pattern transfer is accomplished with a photoresist (an ultraviolet light-sensitive organic polymer). In a typical image transfer process, a substrate that is coated with a photoresist is illuminated through a mask and the mask pattern is transferred to the photoresist by chemical developers. Further pattern transfer is accomplished using a chemical etchant.
In current technologies, the masking process usually is repeated multiple times in the fabrication of an integrated circuit. For example, pattern transfer may be accomplished by either a 5X reduction step and repeat, or a 4X reduction step and scan exposure systems using short wavelength ultraviolet light.
It is well known in the field of microphotolithiography that very small images (on the order of 0.25 &mgr;m or below) print with rounded corners. In extreme cases, square vias print as round openings in the photoresist. For rectangular shapes, a prior art technique referred to as serifing can be used. In this prior art technique, if the desired image is a bar-like structure similar to the character “|”, then serifs are added to the mask image making it look similar to the letter “I”. This method reduces foreshortening, but there is still a degree of corner rounding present.
Another prior art technique for reducing corner rounding on a transferred image is to reduce the wavelength of the light used in the imaging process. Both of the above mentioned techniques have limits associated therewith. For example, reducing the wavelength of light used for imaging runs up against photoresist formulation and optical problems, including reduced DOF and limited equipment availability. Adding serifs can become data intensive, requiring very large data sets. Smaller serifs are also difficult to fabricate and inspect on a mask, for example placing four 0.1×0.1 &mgr;m serifs on the comers of a 0.25×0.25 &mgr;m via is very difficult to inspect with current equipment capabilities.
In view of the above mentioned drawbacks with prior art techniques, there remains a need for providing a new and improved photolithographic method which can be used in providing an image on a photoresist coated surface that has very sharp comers associated therewith. That is, a method is required which is capable of reducing comer rounding on images that are near the resolution limit of the optical system used for pattern transfer.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a photolithographic method which is capable of providing an image on a photoresist layer that has sharp comers, i.e. reduced comer rounding, associated therewith.
Another object of the present invention is to provide a method which is capable of proximity correction of very small images.
A further object of the present invention is to provide a method of forming transferred images that are near the resolution capabilities of the optical system.
A yet further object of the present invention is to provide a method of pattern transfer wherein conventional photoresist, i.e. photosensitive, materials and/or optical systems are used.
An added benefit of the present invention is the elimination of mask process induced comer distortion (i.e. comer rounding) on the resulting printed image.
These and other objects and advantages can be obtained in the present invention by utilizing a double exposure method wherein the masks used for pattern transfer are orientated in a fashion such that the latent image has edges at a substantial rotation relative to edges on the masks. Specifically, the method of the present invention, which provides an image in a photoresist (or photosensitive) layer using double exposure, comprises:
exposing a photoresist layer to a first mask having a first image, said first image having at least two edges;
exposing said photoresist layer to a second mask having a second image, said second image having at least two edges, the second image edges being substantially rotated with respect to the first image edges to produce a latent image in said photoresist layer having edges at a substantial angle relative to the first and second image edges; and
developing the photoresist layer to produce said image in said photoresist layer.
The pattern provided using the above described double exposure process may then be transferred to an underlying substrate by using conventional etching.
The present invention is capable of producing latent images that are shaped similar to a diamond, square or rectangle. The latent image produced may be isolated or nested in arrays or rows.
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Bula Orest
Cole Daniel
Conrad Edward Winston
Leipold William Charles
International Business Machines Corp.
Letscher Geraldine
Sabo, Esq. William D.
Scully Scott Murphy & Presser
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