Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-01-18
2003-04-01
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06541166
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of semiconductor device manufacturing, and more specifically to a method and apparatus for lithographically printing tightly nested device features and isolated device features in integrated circuits.
BACKGROUND OF THE INVENTION
In the semiconductor industry, there is a continuing effort to produce devices having a higher density of device features. As a result, the printing of device feature patterns with extremely small spacings has been and continues to be required. However, the printing of such tightly nested features or contacts presents problems associated with the process window for the lithography process. The process window is a measure of the amount of process variation that can be tolerated while still maintaining the printed feature sizes within set tolerances. Common measurements of process window include depth of focus (DOF), exposure latitude (EL), and total process window (TW). TW is a measure of the area under the curve in a plot of DOF vs. EL.
When printing relatively isolated small contacts, a sufficient process window can be achieved through the use of attenuated phase-shifting masks along with a reduced partial coherence factor. Partial coherence factor is the ratio of the illumination pupil size to the imaging pupil size, and is a measure of the coherency of the imaging system. As the partial coherence factor approaches zero, the degree of coherence increases. As the coherency of an imaging system increases (and the partial coherence factor decreases), the interaction between neighboring features in an image increases. Thus, with an imaging system having a high degree of coherence, the way a feature is imaged, i.e., its size and shape when printed, depends on the other features surrounding it. With an imaging system having a low degree of coherence, the effect of the size and shape of each feature on its neighbor is much less than that of a coherent system.
In a photolithography process in which a mask pattern is imaged onto a semiconductor substrate, the degree of coherence has additional implications. An imaging system having a high degree of coherence allows stronger phase interactions to occur. In such a system, interaction will be greater between light passing through different points of the mask being imaged when projected onto the semiconductor substrate. Therefore, by carefully controlling the phase of the light passing through regions of the mask surrounding a feature, the way that feature will be imaged can be modified. This phase interaction will not occur with an imaging system having a low degree of coherence, and therefore changing the phase on regions of the mask surrounding the feature will not impact the way that feature is imaged. A mask which allows the phase of the light passing through various regions to be adjusted is called a phase-shifting mask. Thus, an imaging system having a high degree of coherence generally improves the performance of phase shifting masks used in printing relatively isolated device features.
However, tightly nested small contacts cannot be resolved using attenuated phase-shifting masks and reduced partial coherence factor alone.
FIGS. 1A and 1B
illustrate the problem of reduced process window associated with tightly spaced device features.
FIG. 1A
illustrates TW as a function of x pitch and y pitch, and
FIG. 1B
illustrates DOF at 10% EL as a function of x pitch and y pitch. The pitch is the distance from one feature to the next adjacent feature in either the x direction or y direction as shown in FIG.
3
A. In both
FIGS. 1A and 1B
, the partial coherence factor (&sgr;) is set to a relatively low value of 0.45.
FIGS. 1A and 1B
show that as the pitch is reduced in either the x direction or y direction, or both, the process window decreases rapidly. For example, in
FIG. 1A
, when the x pitch and y pitch are both a relatively large 740 nm, TW is about 28%-&mgr;m, which is acceptable for present-day processes. When the x pitch is reduced to 400 nm, leaving the y pitch at 740 nm, TW drops to about 14%-&mgr;m. Likewise, when the y pitch is reduced to 400 nm, leaving the x pitch at 740 nm, TW also drops to about 14%-&mgr;m. When both the x pitch and y pitch are reduced to 400 nm, TW drops even further to about 10%-&mgr;m, which is unacceptable for present-day processes.
This phenomenon is believed to be caused by diffraction of the light. As the pitch is reduced, the diffracted light is believed to spread wider at the pupil plane of the imaging lens. If the pitch becomes too small, a limit is reached where only one diffracted order falls within the lens pupil, resulting in no modulation of the light at the wafer plane and the mask pattern being completely unresolved. This limit or cutoff occurs when
pitch=&lgr;/(
NA
*(1+&sgr;))
where &lgr; is the imaging wavelength, NA is the numerical aperture (a measurement of the size of the imaging pupil), and &sgr; is the partial coherence factor.
A known solution to improving the process window in the printing of features having a tight pitch is to increase the partial coherence factor and thereby reduce the minimum resolvable pitch. In
FIGS. 2A and 2B
, the partial coherence factor (&sgr;) has been increased to 0.75.
FIG. 2A
illustrates TW as a function of x pitch and y pitch, and
FIG. 2B
illustrates DOF at 10% EL as a function of x pitch and y pitch.
FIGS. 2A and 2B
each show that when a contact is nested more tightly in any direction, i.e. as the pitch is reduced in any direction, the process window to print the contact remains acceptable. Note that there is no rapid decrease in process window as either the x pitch or y pitch, or both, decreases. However, at this relatively high partial coherence factor, the resolution in printing isolated small features suffers due to a reduction in phase interaction. Thus, when printing a mask pattern having both tightly nested and isolated device features, lithographers are often required to compromise between a sufficient process window for one and good imaging resolution for the other.
One solution to this dilemma was proposed in U.S. Pat. No. 5,424,154 to Borodovsky. Borodovsky discloses a method of improving lithographic resolution for isolated features on a mask which also contains tightly nested features. In this method, complementary or “dummy” features are added to isolated device features on a first mask so that the pitch of the isolated features is reduced to approximately the same as that of the tightly nested features. The dummy features are then obliterated by exposure to a second mask.
A similar solution was proposed in U.S. Pat. Nos. 5,242,770 and 5,447,810, both to Chen et al. Chen discloses methods of reducing proximity effects when printing both isolated features and tightly nested features on a mask. The process window of isolated features is improved by adding additional or “dummy” features adjacent to isolated features edges in the mask. The dummy features are the same transparency as the original feature and have dimensions less than the resolution of the exposure tool. Therefore, these dummy features are not transferred onto the photoresist layer.
Thus, in both the Borodovsky and Chen methods, dummy features must be added to the mask pattern to convert isolated features into tightly nested features. In addition, the Borodovsky method requires a second exposure to remove the dummy features created with the first exposure.
Another solution to the dilemma between sufficient process window and good imaging resolution was proposed in U.S. Pat. No. 5,563,012 to Neisser. Neisser describes methods of splitting a mask pattern having both isolated and tightly nested features into two or more modified or overlay masks. The mask features are divided among the two or more overlay masks such that each mask contains features having the same pitch. In a first embodiment, a mask pattern is split into two or more overlay masks, each having features which are relatively isolated, i.e., which have a relatively large pitch. The tight
Brunner Timothy A.
Culp James A.
Mansfield Scott M.
Wong Alfred K.
Pepper Margaret A.
Rosasco S.
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