Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-04-16
2001-04-17
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06218057
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention is directed to lithographic processes for device and mask fabrication and, in particular, to resolution enhancement techniques for such processes.
2. Art Background
In lithographic processes for device fabrication, radiation is typically projected onto a patterned mask (also referred to as a reticle) and the radiation transmitted through the mask is further transmitted onto an energy sensitive material formed on a substrate. Transmitting the radiation through a patterned mask patterns the radiation itself and an image of the pattern is introduced into the energy sensitive material when the energy sensitive resist material is exposed to the patterned radiation. The image is then developed in the energy sensitive resist material and transferred into the underlying substrate. An integrated circuit device is fabricated using a series of such exposures to pattern different layers of material formed on a semiconductor substrate.
An integrated circuit device consists of a very large number of individual devices and interconnections therefore. Configuration and dimensions vary among the individual devices. The pattern density, (i.e. the number of pattern features per unit area of the pattern) also varies. The patterns that define integrated circuit devices are therefore extremely complex and non-uniform.
As the complexity and density of the patterns increase, so does the need to increase the accuracy of the lithographic tools that are used to create the patterns. The accuracy of lithographic tools is described in terms of pattern resolution. The better the resolution, the closer the correspondence between the mask patterns and the pattern that is created by the tool. A number of techniques have been used to enhance the pattern resolution provided by lithographic tools. The most prevalent technique is the use of shorter wavelength radiation. However, this technique is no longer viable when exposure wavelengths are in the deep ultraviolet (e.g., 248 nm, 193 nm and 157 nm) range. Using wavelengths below 193 nm to improve resolution is presently not economically and technologically feasible because the materials used for lenses in optical lithography cameras absorb this shorter wavelength radiation.
Resolution enhancement techniques (RET) other than simply using shorter wavelength radiation have been proposed. These techniques use exotic illumination from the condenser (e.g. quadrupole illumination), pupil filters, phase masks, optical proximity correction, and combinations of these techniques to obtain greater resolution from an existing camera. However, such techniques typically improve resolution only for some of the individual features of a pattern. The features for which resolution is improved are identified as the critical features. The resolution of many other features is either not improved or actually degraded by such resolution enhancement techniques. Thus, current RETs require a compromise between resolution enhancement for the critical features and resolution degradation for the non-critical features. Such compromises usually require sub-optimal illumination of the critical features in order to avoid significant degradation in the illumination of the non-critical features.
Resolution enhancement techniques have been proposed to customize mask feature illumination in projection lithography for the various different features in the mask. One such technique is described in Matsumoto, K., et al., “Innovative Image Formation: Coherency Controlled Imaging,”
SPIE
, Vol. 2197, p. 844 (1994). That technique employs an additional mask and additional lens to customize the radiation incident on each feature of the mask. A similar system is described in Kamon, K., “Proposal of a Next-Generation Super Resolution Technique,” Jpn.
J. Appl. Phys
., Vol. 33, Part 1, No. 12B, p. 6848 (1994).
In the resolution enhancement techniques described in Matsumoto et al. and Kamon et al., the first mask has features that are identical to the features on the second mask. The features on the first mask diffract the radiation incident on the mask, and the diffracted radiation illuminates the identical feature on a second mask. For example, radiation transmitted through a grating pattern on the first mask is projected onto an identical grating pattern on the second mask. Similarly, radiation transmitted through an isolated line on the first mask is projected onto an identical isolated line on the second mask. When the diffracted energy from the first mask illuminates the identical feature on the second mask, the resulting image is often superior to an image obtained from quadrupole illumination of the pattern. Therefore, this resolution enhancement technique provides an improvement in aerial image contrast (i.e. the image in the focus plane of the projection lens) over conventional off-axis illumination using the quadrupole system.
The above-described resolution enhancement technique provides customized illumination for more features than quadrupole illumination. However, the above-described technique does not improve the resolution of all features in the pattern. Furthermore, the two-mask system is costly and complex. Specifically, the system requires two precisely patterned masks instead of one. The corresponding features on the first and second masks must match precisely. The alignment of the first and second masks is also critical. Furthermore, the technique is limited because the features on the first mask are illuminated uniformly. Thus, the problems associated with non-customized illumination of a patterned mask are not eliminated by this system, but simply stepped further back into the optics of the system. Therefore, resolution enhancement techniques that improve the resolution of all features and are cheaper and easier to implement are sought.
SUMMARY OF THE INVENTION
The present invention is directed to a lithographic process with sub-wavelength resolution. The lithographic process is used for device fabrication or for mask fabrication. In the present invention, an image of a pattern is introduced into an energy sensitive resist material by projecting radiation through a first mask feature which defines an image of a first pattern. The radiation is light of a particular wavelength (e.g. 248 nm, 193 nm, etc.). Radiation also includes electron beam or ion beam radiation. That first pattern is then developed and, in certain embodiments, transferred into the underlying substrate. This first pattern is referred to as an intermediate feature. The intermediate feature has dimensions that are typically, greater than or approximately equal to the wavelength of the radiation that is used to transfer the image of the first mask feature into the energy sensitive resist material. It is advantageous for the intermediate feature to have dimensions greater than or equal to the wavelength of the exposing radiation because the images from which the features are created are easier to resolve than images with sub-wavelength size dimensions. Also, creating images of features having dimensions greater than or equal to the wavelength of exposing radiation requires less proximity effect correction and suffers from fewer adverse diffraction effects compared to creating images of features with sub-wavelength dimensions. Adverse diffraction effects include a loss of contrast, a loss of intensity, adverse proximity effects, etc.
A layer of energy sensitive material is then formed over substrate having the intermediate feature. An image of a pattern is then introduced into the energy sensitive resist material by projecting radiation through a second mask feature which defines a second image. The radiation used for the second exposure does not have to have the same wavelength as the radiation used for the first exposure. This second image is developed into a second pattern, which is referred to as a second intermediate feature. The second intermediate feature has dimensions that are greater than or approximately equal to the wavelength of the radiation that
Cirelli Raymond Andrew
Nalamasu Omkaram
Pau Stanley
Watson George Patrick
Botos Richard J.
Lucent Technologies - Inc.
Rosasco S.
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