Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-09-27
2002-08-27
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S022000
Reexamination Certificate
active
06440616
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-274701, filed Sep. 28, 1999; and No. 11-375472, filed Dec. 28, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a focus-monitoring mask and a method for focus monitoring, which are suited for use in setting a focus condition of a projection aligner on the occasion of manufacturing a semiconductor device or a liquid crystal display device, etc.
As a minimum line width of ULSI pattern is approaching the resolution limit in optical microlithography, it is now becoming very difficult to sufficiently secure a process margin such as exposure latitude and the depth of focus. Therefore, it is now required to develop a technique for monitoring the dose of exposure and the focus of exposure light for the purpose of effectively utilizing a small margin available as well as for the purpose of preventing the deterioration of yield.
According to the conventional method of monitoring the focus, the focus is determined in such a way that an exposure is performed onto a QC mask having a rhomboid mark as shown in
FIG. 1A
while changing the defocus thereof, and when the length L of pattern of the rhomboid mark (
FIG. 1B
) that has been transferred on the surface of wafer becomes the largest, the focus point giving the largest length L is assumed as being a best focus value. In this case, the relationship between the length L of the pattern and the defocus can be represented as shown in
FIG. 2
(Japanese Patent Unexamined Publication H10-335208).
When the pattern is transferred with this best focus, very fine details of the rhomboid mark can be resolved. Whereas as the focus goes out of this best focus, such fine details can no more be resolved. Therefore, the length L of pattern of the rhomboid mark transferred on the surface of wafer would become the largest at this best focus point, the length L of the pattern symmetrically decreasing gradually on both sides of the best focus point, i.e. plus and minus defocus sides. This method of utilizing the mark can be applied to a situation wherein an exposure is performed while changing the defocus of mark so as to determine the best focus before each lot is placed into processing steps thereof.
This conventional method however is accompanied with the following problems. Namely, if the condition of focus for each lot being subjected to an exposure under the same exposure condition is to be managed by making use of the rhomboid mark, there will be raised the problems that (1) the direction of defocus cannot be determined, and (2) the focusing is influenced by the fluctuation in quantity of exposure, if the best focus is to be determined only through the monitoring of the length L of the projected pattern of the rhomboid mark.
As an alternative method for focus monitoring, there has been proposed a method wherein the magnitude of fluctuation of focus is detected, without being influenced by the quantity of exposure, as a magnitude of dislocation of pattern (U.S. Pat. No. 5,300,786). The sensitivity of detecting a focus by making use of a mark according to this method depends largely on the configuration of light source (coherence factor &sgr;), so that there is a problem that even though it may be possible to obtain a sufficient sensitivity under an exposure condition where the value of &sgr; is relatively low, it is impossible to obtain a sufficient sensitivity under an exposure condition where the value of &sgr; is relatively large or under an annular exposure condition as is conventionally employed.
As another conventional method of monitoring the focus, a method of employing a QC mask having a mark as shown in
FIG. 3A
is known (Japanese Patent Unexamined Publication H11-102061). This mark is featured in that a central light-shielding portion
20
is disposed inside the peripheral light-shielding portion
10
. Each of four sides of the central light-shielding portion
20
is formed into a saw-tooth configuration consisting of a plurality of fine wedge-like fine projections
22
and
24
, the saw-tooth configurations formed on opposite sides (right and left sides, of top and bottom sides) are made asymmetrical with each other. In this case, an exposure to this mask is performed while changing the defocus, and when the magnitude of dislocation (L=S
2
−S
1
) of pattern of the pattern (
FIG. 3B
) that has been transferred on the surface of wafer becomes the minimum, the focus point giving this minimum value is assumed as being a best focus value.
The relationship between the magnitude of dislocation L of pattern and the defocus in this case can be represented as shown in FIG.
4
. The magnitude of dislocation L changes almost symmetrically with respect to the defocus. However, when the focus is to be monitored by means of lotlink after arranging the aforementioned pattern over the dicing region of device mask, the exposure on the occasion of processing each lot is performed with a constant value without changing the set value of focus, so that the value of focus is determined through the conversion of the magnitude of dislocation L of the mark. In this case however, there is a problem that it is difficult to know the sign (+ or −) of the focus.
Relative to this, there has been also proposed a modified method in Japanese Patent Unexamined Publication H11-102061 (Watanabe et al) wherein the substrate for transferring a mark is modified as shown in FIG.
5
. Namely, the substrate is partitioned as shown in
FIG. 5
into regions
30
and regions
40
, the elevations of which are differentiated from the other. By constructing the substrate in this manner, a difference or step is caused to generate between the magnitude of dislocation in X direction (&sgr;Sx) and the magnitude of dislocation in Y direction (&sgr;Sy) in the transferred pattern of mask as shown in FIG.
6
. Therefore, when this difference between this couple of dislocations is plotted relative to the defocus (FIG.
7
), the magnitude of dislocation of focus can be monitored together with the sign thereof. Since it is possible in this method to monitor the magnitude of dislocation of focus together with the sign thereof, it is possible to control the focus by means of lotlink. However, in the prevailing technique of today where CMP process is frequently employed, the method of forming a step in the substrate does not seem to be practical.
As explained above, the method of employing a rhomboid mark for monitoring the focus is certainly effective in detecting the magnitude of defocus from the best focus, but is defective in that the direction of defocus cannot be determined. Further, a method of detecting a magnitude of fluctuation of focus as a magnitude of defocus of pattern is accompanied with a problem that it is impossible to obtain a sufficient sensitivity under an exposure condition where the value of &sgr; is relatively large or under an annular exposure condition.
Furthermore, the aforementioned method of forming a difference in level in the substrate for the purpose of measuring the deviation of focus together with the sign thereof is defective in that the steps of process would be increased, and is not practical since CMP process is employed in the manufacturing process.
BRIEF SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a focus-monitoring mask which is capable of very precisely and easily detecting the magnitude as well as direction of defocus from the best focus, thereby making it possible to improve the exposure precision.
Another object of the present invention is to provide a method for focus monitoring which is capable of very precisely and easily detecting the magnitude as well as direction of defocus from the best focus, thereby making it possible to improve the exposure precision.
Namely, according to the present invention, there is provided a focus-monitoring mask which is
Fujisawa Tadahito
Inoue Soichi
Izuha Kyoko
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Rosasco S.
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