Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-09-21
2001-06-05
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C355S053000
Reexamination Certificate
active
06242137
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus and method for uni-directionally improving the bandwidth of an asymmetrical optical mask system for use in photolithography operations. More specifically, the present invention relates to an apparatus and method for improving the bandwidth in the x-direction of an asymmetrical optical mask with a minimum sacrifice in the y-directional bandwidth, wherein the optical mask has a pattern that varies strongly in the x-direction, but weakly in the y-direction, thus occupying a cigar-shaped area in the transformed F
x
F
y
two-dimensional frequency domain (or frequency plan). The apparatus and method of the present invention are most advantageous for preparing semiconductor devices whose topography varies strongly in one direction but weakly in another direction. The present invention also allows the dimension of the semiconductor devices to be reduced, while still maintaining the desired resolution, and without incurring large capital investments.
BACKGROUND OF THE INVENTION
In photolithography operations which are important steps in the fabrication of semiconductor devices, optical masks are used typically to pattern a photoresist layer. With the design of semiconductor devices becoming increasingly more complicated in design and finer in dimensions, we are also seeing a trend of more radical departure in their layouts from the traditional, and more or less, symmetric, design. It has become not-all-too-uncommon to have semiconductor wafer layouts, and thus the associated optical masks, that involve strong variations in the first direction (say, x-direction), but relatively slow variations in the second direction (say, y-direction), creating an asymmetric optical mask system.
In a typical masked photo exposure operation, a coherent light (typically a KrF laser light passed through a condensing lens) is emitted upon the optical mask at an inclined angle. Two high frequency light beams and one direct light beam will be generated as a result of diffraction from the optical mask. The direct light beam is a relatively low frequency 0th-order diffracted light and the two relatively high frequency high beams are 1 st-order diffracted light beams. When the diffraction angle exceeds a certain value, the high frequency portion of the light beams may be diffracted outside the area of the objective lens disposed behind the optical mask, resulting in a substantial loss in the optical resolution, particularly in the x-direction, where variations are stronger. (It should be noted that designation of x- and y-directions are only arbitrary.) This can adversely affect the precision and sometimes the acceptability of the resultant semiconductor device.
In other words, because of the asymmetry in the variations of the pattern of the optical mask between the x- and y-directions, different bandwidths exist, and the pertinent bandwidths (i.e., the two-dimensional bandwidth distribution) occupy a two-dimensional cigar-shaped area in the transformed F
x
-F
y
frequency domain as shown in FIG.
1
. As a result, in order to improve semiconductor quality and maintain good production yield, it is important to research various ways that may reduce the asymmetry of the frequency domain.
SUMMARY OF THE INVENTION
The primary object of the present invention is to develop an apparatus and method that will improve the performance, i.e., overall bandwidth, of an optical mask system having a substantial degree of optical asymmetry. More specifically, the primary object of the present invention is to improve the bandwidth in the x-direction of an asymmetrical optical mask by sacrificing the y-directional bandwidth, wherein the optical mask system has a distinctive two-dimensional pattern that varies strongly in the x-direction, but only slowly in the y-direction, thus occupying (i.e., being represented by) a cigar-shaped area in the frequency domain (i.e., a graphical representation of the two-dimensional bandwidths), as shown in FIG.
1
. In
FIG. 1
, F
x
indicates the transformed frequency function in the x-direction, transformed F
y
indicates the frequency function in the y-direction, and the shaded cigar-shaped area indicates the asymmetrical two-dimensional bandwidths in the transformed F
x
- and F
y
-directions.
In the optical mask system disclosed in the present invention, a diffractional grating is placed before the optical mask. The diffractional grating is tilted at a predetermined angle &ohgr; wherein &ohgr; is about 30-60° from the x-axis (i.e., the axis corresponding to the axis of strong variations). The tilted diffractional grating has a frequency domain as shown in
FIG. 2
which comprises a main distribution, or cluster, as represented by a circle, at the center and two minor clusters (represented by two minor circles) diagonally opposing each other and at an angle &ohgr; relative to the x-axis. The signal, or the incident light, is convoluted between the tilted diffractional grating and the optical mask in the frequency plan, and the bandwidth of the convoluted spectrum will comprise the original cigar-shaped area, plus two identical clones respectively centered at the two minor circles.
The convoluted spectrum then passes through a pupil, which, in a preferred embodiment, comprises a pair of optical lenses. Because the bandwidth of the incident light is wider in the x-direction, the pupil does not affect the y-directional bandwidth. The filtered signal then passes through another diffractional grating tilted at a predetermined angle &phgr; wherein &phgr; is about 30-60° from the x-axis. Finally, a horizontal slit is provided to remove the extraneous frequency components that do not belong to the object spectrum. The result is an output image whose spectrum has a substantially broader bandwidth in the F
x
-direction.
REFERENCES:
patent: 5600743 (1997-02-01), Hilmer
patent: 6057065 (2000-05-01), Rolfson
Liauh W. Wayne
ProMos Technology, Inc
Rosasco S.
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