Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-05-01
2002-01-01
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C716S030000, C716S030000
Reexamination Certificate
active
06335130
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates optical lithography and more particularly to the design layout and fabrication of transparent or semitransparent phase-shifting masks or reticles that can be used in the manufacture of semiconductor devices.
BACKGROUND OF THE INVENTION
In lithography, an exposure energy, such as ultraviolet light, is passed through a mask (or reticle) and onto a target such as a silicon wafer. The reticle typically may contain opaque and transparent regions formed in a predetermined pattern. The exposure energy exposes the reticle pattern on a layer of resist formed on the target. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. This forms a resist mask. A mask typically may comprise a transparent plate such as fused silica having opaque (chrome) elements on the plate used to define a pattern. A radiation source illuminates the mask according to well-known methods. The radiation transmitted through the mask and exposure tool projection optics forms a diffraction limited latent image of the mask features on the photoresist. The resist mask can then be used in subsequent fabrication processes. In semiconductor manufacturing, such a resist mask can be used in deposition, etching, or ion implantation processes, to form integrated circuits having very small features.
As semiconductor manufacturing advances to ultra-large scale integration (ULSI), the devices on semiconductor wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges, of various features.
As the nominal minimum feature sizes continue to decrease, control of the variability of these feature sizes becomes more critical. For example, the sensitivity of given critical dimensions of patterned features to exposure tool and mask manufacturing imperfections as well as resist and thin films process variability is becoming more significant. In order to continue to develop manufacturable processes in light of the limited ability to reduce the variability of exposure tool and mask manufacturing parameters, it is desirable to reduce the sensitivity of critical dimensions of patterned features to these parameters.
As feature sizes decrease, semiconductor devices are typically less expensive to manufacture and have higher performance. In order to produce smaller feature sizes, an exposure tool having adequate resolution and depth of focus at least as deep as the thickness of the photoresist layer is desired. For exposure tools that use conventional or oblique illumination, better resolution can be achieved by lowering the wavelength of the exposing radiation or by increasing the numerical aperture of the exposure tool, but the smaller resolution gained by increasing the numerical aperture is typically at the expense of a decrease in the depth of focus for minimally resolved features. This constraint presents a difficult problem in reducing the patterning resolution for a given radiation wavelength.
A reduction projection exposure method that features mass-producibility and excellent resolution has been used widely for forming patterned features. According to this method, the resolution varies in proportion to the exposure wavelength and varies in inverse proportion to the numerical aperture (NA) of the projection optical system. The NA is a measure of a lens' capability to collect diffracted light from a mask and project it onto the wafer. The resolution limit R (nm) in a photolithography technique using a reduction exposure method is described by the following equation:
R=K
1
&lgr;/(
NA
)
Where:
&lgr; is the wavelength (nm) of the exposure light;
NA is the numerical aperture of the lens; and
K
1
is a constant dependent on a type of resist.
So far, increases in the resolution limit have been achieved by increasing the numerical aperture (high NA). This method, however, is approaching its limit due to, a decrease in the depth of focus, difficulty in the design of lenses, and complexity in the lens fabrication technology itself. In recent years, therefore, attention has been given to an approach for shortening the wavelength of the exposure light in order to form finer patterns to support an increase in the integration density of LSIs. For example, a 1-Gbit DRAM requires a 0.2-micrometer pattern while a 4-Gbit DRAM requires a 0.1-micrometer pattern. In order to realize these patterns, exposure light having shorter wavelengths must be used.
However, because of increased semiconductor device complexity that results in increased pattern complexity, and increased pattern packing density on the mask, distance between any two opaque areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes so that the space between the two opaque areas is not resolved, therefore, making diffraction a severe limiting factor for optical photolithography.
A conventional method of dealing with diffraction effects in optical photolithography is achieved by using a phase shift mask, which replaces the previously discussed mask. Generally, with light being thought of as a wave, phase shifting is a change in timing of a shift in waveform of a regular sinusoidal pattern of light waves that propagate through a transparent material.
Typically, phase-shifting is achieved by passing light through areas of a transparent material of either differing thickness or through materials with different refractive indexes, or both, thereby changing the phase or the periodic pattern of the light wave. Phase shift masks reduce diffraction effects by combining both diffracted light and phase shifted diffracted light so that constructive and destructive interference takes place favorably. On the average, a minimum width of a pattern resolved by using a phase shifting mask is about half the width of a pattern resolved by using an ordinary mask.
There are several different types of phase shift structures. These types include: alternating aperture phase shift structures, subresolution phase shift structures, rim phase shift structures, and chromeless phase shift structures. “Alternating Phase Shifting” is a spatial frequency reduction concept characterized by a pattern of features alternately covered by a phase shifting layer. “Subresolution Phase Shifting” promotes edge intensity cut off by placing a subresolution feature adjacent to a primary image and covering it with a phase shifting layer. “Rim Phase Shifting” overhangs a phase shifter over a chrome mask pattern.
In general, these phase shift structures are constructed on reticles (or masks) having three distinct layers of material. An opaque layer is patterned to form light blocking areas that allow none of the exposure light to pass through. A transparent layer, typically the substrate, is patterned with light transmissive areas, which allow close to 100% of the exposure light to pass through. A phase shift layer is patterned with phase shift areas which allow close to 100% of the exposure light to pass through but phase shifted by 180° (&pgr;). The transmissive and phase shifting areas are situated such that exposure light diffracted through each area is canceled out in a darkened area therebetween. This creates the pattern of dark and bright areas, which can be used to clearly delineate features. These features are typically defined by the opaque layer (i.e., opaque features) or by openings in the opaque layer (i.e., clear features).
For semiconductor manufacture, alternating aperture phase shift reticles may typically be used where there are a number of pairs of closely packed opaque features. Howev
Chen J. Fung
Petersen John S.
ASML Masktools Netherlands B.V.
Rosasco S.
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