Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-04-26
2004-10-05
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S022000, C430S030000, C716S030000, C716S030000, C716S030000
Reexamination Certificate
active
06800401
ABSTRACT:
NOTICE OF COPYRIGHT
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
BACKGROUND OF THE INVENTION
Lithographic processes are known for semiconductor fabrication which employ an actinic light source focused through a mask onto a substrate, or resist, through a lens. The mask has a predetermined pattern which selectively passes the actinic light source to produce a desired image on the resist. However, the light is subject to interference, such as diffraction, as it passes from the mask to the resist surface. The diffraction pattern contains all the information about the predetermined pattern but passes through a lens of finite size and some of the information contained within the diffraction pattern is filtered out. Accordingly, the image on the resist can become slightly distorted from the predetermined image on the mask. In semiconductor fabrication, however, it is desirable to minimize such distortion to enable a smaller minimum feature size on the substrate. Since the minimum feature size defines the density of the electronic elements on the substrate, a smaller minimum feature size allows more elements to be placed per unit area on the substrate or for the same density to make a electronic element faster.
FIG. 1
shows a prior art conventional and phase-shift mask for use in a lithographic process. Referring to the mask cross-sectional views in
FIG. 1
, a conventional mask
2
has opaque chrome
8
or other light blocking regions, which define a predetermined pattern to be formed. Actinic light
6
is directed at the mask
2
to form an image on a wafer (not shown) or other substrate according to the predetermined pattern. A phase-shift mask
4
further includes a shifter layer
9
according to the predetermined pattern. The shifter layer
9
shifts the phase of the actinic light
6
180 degrees to produce the light amplitude shown in graph
3
. The light amplitude results in a light amplitude distribution shown in graph
5
, which results in the light intensity distribution shown in graph
7
.
Phase-shift masks may be employed to reduce the distortion caused from loss in information due to diffraction by creating destructive interference on the resist plane between the light of opposite phases. However, the actinic light
6
source generates an electric field between light beams of different phases of zero magnitude and elimination of zero-frequency light, or zero-order light, between the beams of different phases by balancing the opposing electric fields reduces phase error between the diffracted beams and allows a smaller minimum feature size.
It would be beneficial, therefore, to provide a system and method for providing a photolithographic phase-shift mask which balances the energy between electric fields of opposed phase to substantially reduce or eliminate zero-frequency energy to produce a strong phase-shifted, dual beam to reduce diffraction and reduce the minimum feature size on a substrate to which the dual beam is directed.
SUMMARY OF THE INVENTION
A system and method of strong phase-shifting a beam from an actinic light source in a lithographic process includes focusing a beam from an electromagnetic beam source onto a mask adapted to selectively phase-shift at least a portion of the beam according to a predetermined pattern. The beam is passed from the actinic light source through the mask producing a phase-shifted beam, and the phase-shifted beam is directed at a substrate such as a semiconductor wafer adapted to be selectively etched according to the predetermined pattern. The strong phase-shift serves to substantially eliminate zero-order light in the phase-shifted beam. Strong phase-shift mask techniques, through a two electromagnetic beam interference imaging process, are known in the art of microlithography to form imaging results for an isolated primary feature of a size well below the limit of conventional prior art imaging.
The use of sub-resolution assist features in the field of microlithography is known to provide optical proximity compensation, reduce the mask error enhancement factor (MEEF), minimize the effect of aberrations and boost isolated line performance with off-axis illumination. Assist features such as scattering bars are opaque or semitransparent features that are offset from primary features in the bright field and anti-scattering bars are their dark field analogues. They were first used as an optical proximity correction technique; or, if phase-shifted, as a weak phase-shift mask technique. Later, it was shown that they could reduce MEEF and aberrations. Finally, they have been shown to boost the performance of isolated line features using off-axis illumination. The invention as defined by the present claims is based on the use of phase-shifted assist features to improve the imaging capability of an isolated, or primary feature by balancing the opposing electric fields of the primary and assist features to minimize or eliminate the electric field at the zero frequency of the primary features. In the art, when the electric field is eliminated at zero frequency, the imaging is said to be strong phase-shifted and the image is constructed using a two-beam imaging technique. Two-beam imaging is better than conventional imaging because it restricts interference angles needed to reconstruct the image in a way where their phase relationship is maintained to improve resistance to change in focus and exposure, as well as, to provide improved performance in the presence of other aberrations. Most phase-shifting techniques used for imaging isolated features are not strong. This is because properly balanced features would be large enough to print an unwanted pattern in the resist, and as a result, the microlithography community did not actively pursue this technique.
The invention as defined by the present claims provides a method to design and fabricate a strong phase-shift mask for use in the imaging of a photoresist material during the fabrication of semiconductor devices. This invention is not limited only to the fabrication of semiconductors, but also extends to the manufacture of other elements that use the microlithography imaging technique. The method has special application to isolated or semi-isolated clear field features placed on a dark background. The method works by determining the layout and fabrication requirements of a photomask such that as actinic energy is passed through the mask forming a diffraction pattern, the electric fields that form of opposite phase are equal, and thus balanced in strength with respect to their average integrated amplitudes. By balancing the energy between electric fields of opposing phase, this method eliminates zero-frequency energy and makes a strong phase-shifted, two-beam imaging system. Thus, it works much in the fashion of a strong phase-shift mask but is not limited to the normal methods of making a strong phase-shift mask and can use most any phase-shifter technique that is known in the imaging arts. The invention provides a method for layout and fabrication of a strong phase-shift feature that takes into account the final size of the feature, the ability of a photoresist not to image assist features, the capability of the projection printer, and the phase-shift mask's topographical modification of the electric field.
The method as disclosed herein is not limited to making isolated or semi-isolated clear features on a dark background, but alternatively can be used for other feature density as long as the electric field between regions of opposing phase can be balanced to give the desired result. Examples of using this type of imaging technique would be the fabrication of discrete semiconductor devices such as is common, but not restricted to gallium-arsenic technology and to
Hamilton Brook Smith & Reynolds P.C.
Huff Mark F.
Petersen Advanced Lithography, Inc.
Sagar Kripa
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