Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-01-04
2001-07-17
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S729000, C359S731000, C359S900000
Reexamination Certificate
active
06262826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to an optical system for use with short wavelength radiation in photolithography equipment.
2. Background of the Invention
Photolithography is a well known manufacturing process used to create devices upon substrates. The process typically involves exposing a patterned mask to collimated radiation, producing patterned radiation, which is passed through an optical reduction system. The reduced patterned radiation or mask image is projected onto a substrate coated with photoresist. Radiation exposure changes the properties of the photoresist allowing subsequent processing.
Photolithography machines or steppers, use two common methods of projecting a mask image onto a substrate, “step and repeat” and “step and scan”. The step and repeat method sequentially exposes portions of a substrate to a mask image. The step and repeat optical system has a projection field which is large enough to project the entire mask image onto the substrate. After each image exposure, the substrate is repositioned and the process is repeated.
In contrast, the step and scan method in contrast scans a mask image onto a substrate through a slit. Referring to
FIG. 1
, a ring field lithography system
100
for use in the step and scan method is shown. A moving mask
101
is illuminated by a radiation beam
103
, which reflects off of the mask
101
and is directed through a reduction ring field optical system
107
. Within the optical system
107
the image is reversed and a slit shaped reduced image beam
109
is projected onto a moving substrate
111
. The slit shaped reduced image beam
109
can only project a portion of the mask
101
, thus the image beam
109
must scan the complete mask
101
onto the substrate
111
. Because the mask
101
and substrate
111
move synchronously, a sharp image is scanned onto the substrate
111
. Once the complete mask
101
is scanned onto the substrate, the mask
101
and substrate
111
are repositioned and the process is repeated. The dimensions of the slit are typically described by a ring field radius and a ring field width.
As manufacturing methods improve, the minimum resolution dimension which can be achieved with reduced pattern radiation decreases allowing more electronic components to be fabricated within a given area of a substrate. The number of devices that can be fabricated within an area of substrate is known as device density. A common measure of device density is the amount of memory that can be fabricated on a single DRAM chip. The relationship between resolution dimension and DRAM memory size is shown in Table 1. With existing technology 0.25 &mgr;m resolution is possible.
TABLE 1
Resolution Dimension
DRAM Memory Size
0.30 &mgr;m
16-64 Megabytes
0.25 &mgr;m
64-256 Megabytes
0.18 &mgr;m
1 Gigabyte
0.13 &mgr;m
4 Gigabyte
0.10 &mgr;m
16 Gigabyte
0.07 &mgr;m
64 Gigabyte
One well-known means of improving the resolution dimension and increasing device density is to use shorter exposure wavelength radiation during photolithography processes. The relationship between resolution dimension and radiation wavelength is described in the formula: R=(K
1
&lgr;)/(NA), wherein R is the resolution dimension, K
1
is a process dependent constant (typically 0.7), &lgr; is the wavelength of the radiation, and NA is the numerical aperture of the optical system projecting the mask image. Either reducing, or increasing the NA will improve the resolution of the system.
Improving the resolution by increasing the numerical aperture (NA) has several drawbacks. The most prevalent drawback is the concomitant loss in depth of focus with increased NA. The relationship between NA and depth of focus is described in the formula: DOF=(K
2
&lgr;)/NA
2
, wherein DOF is depth of focus, and K
2
is a process dependent constant (typically close to unity). This simple relationship shows the inverse relationship between DOF and NA. For several reasons, including practical wafer flatness and scanning stage errors, a large depth of focus is on the order of ±1.0 micrometers is desirable.
Immediately, the shortcomings of resolution improvement via numerical aperture increase can be seen. As lithography technologies evolve toward shorter wavelengths, projection systems operate in different regions of wavelength-NA space. For EUV lithography at an operational wavelength of 13.4 nm, 0.1 &mgr;m resolution can be achieved with a projection system that has a numerical aperture of 0.10. This low numerical aperture affords a depth of focus of ±1 &mgr;m. In stark contrast, deep ultraviolet (DUV) lithography at 193 nm requires a projection system with a numerical aperture of 0.75 to achieve 0.18 &mgr;m features (assuming K
1
=0.7). At this NA, the depth of focus has been reduced to ±0.34 &mgr;m. In addition, fabrication and assembly tolerances make it impractical to build optical systems with such as large NA.
As is known in the art, short &lgr; radiation (less than about 193 nm) is not compatible with many refractive lens materials due to the intrinsic bulk absorption. To reduce the radiation absorption within an optical system reflective elements may be used in place of refractive optical elements. State of the art DUV systems use catadioptric optical systems which comprise refractive lenses and mirrors. The mirrors provide the bulk of the optical power and the lenses are used as correctors to reduce the field dependent aberrations.
To produce devices with smaller resolution dimensions and higher 15 device density than is possible with DUV systems, optical systems compatible with even shorter wavelength radiation are required. Extreme ultraviolet (EUV) radiation (&lgr; less than about 15 nm) cannot be focused refractively. However, EUV radiation can be focused reflectively using optical elements with multilayer coatings.
Early EUV lithographic projection optical systems focused on the development of optical systems that project two dimensional image formats. One example of a step and repeat optical system is disclosed in U.S. Pat. No. 5,063,586. The '586 patent discloses coaxial and tilted/decentered configurations with aspheric mirrors which project approximately a 10 mm×10 mm image field. The '586 patent system achieves an acceptable resolution of approximately 0.25 &mgr;m across the field, but suffers from unacceptably high distortion, on the order of 0.7 &mgr;m. The '586 patent optical system is impractical because the mask would have to pre-distorted in order to compensate for the distortion.
More advanced step and scan optical systems have been developed due to the unacceptable distortion of the large image fields of step and repeat optical systems. Step and scan systems have less distortion than step and repeat systems because distortion can be corrected over the narrow slit width in the direction of scan. Step and scan optical systems typically utilize ring fields. Referring to
FIG. 2
, in a step and scan optical system an image is projected by the inventive optical system onto the wafer through an arcuate ring field slit
201
which is geometrically described by a ring field radius
203
, a ring field width
205
and a length
207
. Ring field coverage is unlimited in azimuth.
One example of a step and scan optical system is disclosed in U.S. Pat. No. 5,315,629. Although the '629 patent optical system has low distortion, the ring field slit width
205
is only 0.5 mm at the wafer. High chief ray angles make it difficult to widen the ring field width
205
. The 0.5 mm width
205
of tie '629 patent limits the speed at which the wafer can be scanned, restricting throughput. The radius
203
of the '629 patent optical system ring field is 31.5 mm which does not cover a wide swath in the cross scan dimension, further reducing wafer throughput.
Another disadvantage of systems similar to the '629 patent optical system is that it may require the use of graded multilayer reflective coating optics as opposed to simpler uniform thickness mu
Phan James
The Regents of the University of California
Thompson Alan H.
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