Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-08-31
2001-05-29
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06238824
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of semiconductor fabrication, more particularly, to a reticle for use in a photolithography process during semiconductor fabrication, and a method for designing such a reticle.
BACKGROUND OF THE INVENTION
In the manufacture of semiconductor chip devices, photolithographic processes are often used to pattern various layers on a wafer in order to produce circuit features (e.g., transistors, polygates and interconnects) positioned as specified in a circuit feature layout. Typically, a circuit feature layout calls for different circuit features to be provided in different layers and/or through different combinations of layers of the chip devices. Examples of circuit feature layouts which include such circuit features positioned in such relationships are numerous. An example of a device having such a circuit feature layout is one which includes multiple layers, at least one of the layers having one or more conductive paths (e.g., digitlines), and at least one interconnect passing through two or more of the layers but not making contact with the conductive path(s). In manufacturing such a device, it is common to form the one or more conductive paths by patterning a conductive layer in one photolithographic patterning step, and subsequently (e.g., after applying one or more layers over the conductive layer) to form the interconnect(s) using a separate photolithographic patterning step.
In each of such photolithographic processes, a layer of photo resist is deposited on the layer being patterned, and the resist is then exposed using an exposure tool and a template. These templates are known in the art as reticles or masks. For purposes of the present application, the term reticle includes both reticles and masks. During the exposure process, the reticle is typically placed over the resist, and then a form of radiant energy such as ultraviolet light is directed toward the reticle to selectively expose the resist in a desired pattern. A preferred device for creating such exposure is known as a stepper.
In performing such photolithographic processes on a device which is being manufactured, it is necessary to align each reticle relative to the device under fabrication. There are a number of ways of providing such alignment, one common way being to provide for accurate alignment of the device relative to the stepper, as well as accurate alignment of each reticle relative to the stepper. One common way of providing such accurate alignment is by providing alignment marks on the device, which can be aligned with corresponding marks on the stepper. Other alignment techniques could be used, e.g., positioning one or more edges of the device in contact with a mating surface in the stepper, registering a notch in the edge of the device with an engaging structure in the stepper, registering a hole in the device with an engaging structure in the stepper, etc. Likewise, each reticle is accurately aligned with the stepper using a suitable alignment technique. In practice, such alignment can only be guaranteed within certain limits. Accordingly, different reticles used in different steps in the manufacture of a semiconductor device can be misaligned up to a maximum amount referred to herein as the alignment budget therefore the alignment errors will produce defective product.
One type of reticle which is commonly used in photolithographic processes is referred to as a binary reticle. A binary reticle includes reticle features, namely transparent features (areas through which exposure passes) and opaque features (areas which block exposure). The design of the reticle features is typically shown in a two-dimensional reticle layout, although the reticle itself typically includes two or more layers (e.g., a transparent layer and a patterned opaque layer). In use, radiant energy is directed toward the binary reticle, and the radiant energy is blocked by the opaque areas but passes through the transparent areas to pattern-wise expose the resist. After pattern-wise exposure, the resist is developed to remove either the exposed portions of the resist (a positive resist) or the unexposed portions of the resist (a negative resist), thereby forming a patterned resist on the layer being patterned. The patterned resist is then used to protect a corresponding pattern of underlying areas on the layer during subsequent fabrication processes, such as deposition, etching or ion implantation processes. Thus, the patterned resist prevents or substantially prevents the effects of the fabrication process(es) from being produced in the layer in areas of the layer which lie beneath portions of the resist which have not been removed. The reticle is designed so as to enable exposing the resist in a pattern which corresponds to the feature or features which are desired to be formed.
There are a number of effects caused by diffraction of exposure which tend to distort the patterns formed in a resist, i.e., which cause the pattern formed in a resist to differ from the pattern formed in the reticle.
Due to limitations imposed by the wavelength of light used to transfer the pattern, resolution degrades at the edges of the patterns of the reticle. Such degradation is caused by diffraction of the exposure such that it is spread outside the transparent areas. Phase shift masks (PSMs) have been used to counteract these diffraction effects and to improve the resolution and depth of images projected onto a target (i.e., the resist covered wafer). There are a variety of PSMs. One kind of PSM includes a phase shifting layer having areas which allow close to 100% of the exposure to pass through, but phase shifted 180 degrees relative to exposure passing through a transparent layer. Attenuated PSMs utilize partially transmissive regions which pass a portion of the exposure, e.g., about three to eight percent, out of phase with exposure through transparent areas. Typically, the shift in phase is 180 degrees, such that the portion of exposure passing through the partially transmissive regions destructively interferes with exposure which is spread outside the transparent areas by diffraction. Phase shift masks can thereby increase image contrast and resolution without reducing wavelength or increasing numerical aperture. These masks can also improve depth of focus and process latitude for a given feature size. Designs of such reticles typically are represented using one or more two-dimensional reticle layouts including appropriate reticle features, e.g., selected from among transparent features, opaque features, phase shifting features and phase shifting attenuating features.
There has been an ongoing need to increase the density of features contained in semiconductor devices, by making the features smaller and/or reducing the amount of space between features. Advances in feature density have required that reticles include correspondingly smaller and/or more densely packed features. The extent to which features printed by photolithographic methods can be reduced in size is limited by the resolution limit of the exposure tool. The resolution limit of an exposure tool is defined as the minimum feature dimension that the exposure tool can repeatedly expose onto the resist, and is a function of the wavelength of exposure emitted by the stepper, the aperture through which exposure is emitted, the depth of focus and other factors. Thus, reticle design is limited in that the gaps between respective features on the reticle (i.e., transparent regions, opaque regions and/or phase shifted regions) must be large enough for the circuit features to be correctly printed.
The critical dimension (CD) of a circuit pattern is defined as the smallest width of a line in the pattern, or the smallest space between lines in the pattern. The CD thus directly affects the size and density of the design. As the density of features in a pattern is increased, the CD of the design approaches the resolution limit of the stepper. As the CD of a circuit layout approaches the resolution limit of the step
Futrell John R.
Pierrat Christophe
Stanton William
Dickstein , Shapiro, Morin & Oshinsky, LLP
Micro)n Technology, Inc.
Rosasco S.
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