Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-07-09
2001-07-10
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06258489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of semiconductor fabrication and more particularly to a mask used in the photolithography process during semiconductor fabrication.
2. Description of the Related Art
In the semiconductor industry, photolithography is used to transfer patterns corresponding to a circuit layout from a mask to a semiconductor wafer to form a semiconductor device. The layout, and hence the patterns on the masks, are made to conform to dimensional “design rules,” which are determined by photolithographic and semiconductor processing parameters and circuit design criteria. Adhering to these design rules ensures that patterns on the mask transfer properly to the semiconductor wafer and ensures circuit functionality. One important design rule that determines the overall size and density of the device is the critical dimension, which is defined as the smallest width of a line or the smallest space between two lines.
Once the layout of the circuit has been created, an exposure tool is used to irradiate a layer of photoresist on the semiconductor wafer. An important limiting characteristic of the tool is its resolution. The resolution of an exposure tool is defined as the minimum feature that the exposure tool can repeatedly expose onto the wafer. As the critical dimensions of a mask layout approach the resolution limit of the lithography system, proximity effects begin to influence the manner in which features on a mask transfer to the resist layer such that the masked and actual layout patterns begin to differ. Proximity effects are known to result from optical diffraction in the projection system. This diffraction causes adjacent features to interact with each other in such a way as to produce pattern-dependent variations. One specific variation occurs when features are designed to have the same dimension but are placed in different proximity to other features in a layout. The difference in proximity causes features such as contact holes which are in close proximity to other features to print differently from features which are relatively isolated from other features. This variance between isolated features and closely-packed features is detrimental to process margin/latitude. As used herein, an isolated feature refers to a feature that is not appreciably affected during the photolighography process by proximity effects from other features, while a closely-packed feature refers to a feature that is appreciably affected during the photolithograph process by proximity effects from other features.
The proximity effects described above are not always detrimental. Diffracted exposure radiation from neighboring closely-packed features can actually interact to reduce sidelobe effects, thereby increasing depth of focus and hence process margin/latitude. Accordingly, and perhaps counter-intuitively, it is possible to achieve a smaller feature size and/or improved printing characteristics such as edge sharpness for closely-packed features than is possible for isolated features.
What is needed is a method and/or mask for, that can uniformly print both isolated and densely-packed features and that can improve printing of all features, whether closely-packed or isolated.
SUMMARY OF THE INVENTION
The present invention overcomes to a great extent the aforementioned problems by placing dummy features adjacent to features to be printed (referred to herein as printable features) on a photolithography mask. The dummy features are smaller than the resolution (which is approximately equal to the critical dimension) of the photolithography system so that the resist is not fully developed in the areas of the dummy features. Thus, the dummy features will not be etched into the surface below. However, the proximity effects caused by the dummy features will act to suppress sidelobes and/or increase the depth of focus. Dummy features may be provided adjacent to both isolated and closely-packed printable features, thereby equalizing the proximity effects acting on both types features so that isolated printable features will print approximately the same as densely-packed printable features and both will be enhanced.
In one embodiment, a mask is formed with dummy features in the form of dummy contact openings provided around printable contact openings. The dummy contact openings are essentially contact openings with an island disposed therein. Various parameters associated with the dummy contact opening, including the width of the island, the spacing between the edges of the island and the inner edges of the opening, and the spacing between the closest edges of the dummy contact opening and the printable contact opening, may be adjusted to maximize depth of focus as well as the uniformity of the isolated and densely-packed contact openings.
REFERENCES:
patent: 5468577 (1995-11-01), Bae
patent: 5707765 (1998-01-01), Chen
patent: 5795682 (1998-08-01), Garza
patent: 5821014 (1998-10-01), Chen et al.
patent: 6048647 (2000-04-01), Miyazaki et al.
Agarwal Vishnu K.
Stanton William
Dickstein , Shapiro, Morin & Oshinsky, LLP
Micro)n Technology, Inc.
Rosasco S.
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