Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2002-05-29
2004-06-29
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
Reexamination Certificate
active
06757110
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved lithography system and method. More specifically, this invention relates to a lithography system and method using catadioptric exposure optics that projects high precision images without image flip.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. The image refers to the original, or source, image being exposed. The projected image refers to the image which actually contacts the surface of the wafer. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
Exposure optics comprise refractive and/or reflective elements, i.e., lenses and/or mirrors. Currently, most exposure optics used for commercial manufacturing consist only of lenses. However, the use of catadioptric (i.e., a combination of refractive and reflective elements) exposure optics is increasing. The use of refractive and reflective elements allows for a greater number of lithographic variables to be controlled during manufacturing. The use of mirrors, however, can lead to image flip problems.
Image flip occurs when an image is reflected off of a mirror.
FIG. 1
shows an example of image flip. In this example, if one were to hold up plain English text to a mirror, one would notice that the text, viewed in the mirror, would appear to be written backwards. Thus, an image of the letter “F,” would be seen as “
” in the mirror. This shows that when an image is reflected off of a mirror, the projected image results in an incorrect image orientation, i.e., the image transfer produces image flip. Of course, if the image is reflected off of two mirrors, the image orientation of the projected image would be correct because the image is flipped twice. Thus, an image of the letter “F,” would be seen as “F” after the second reflection. Therefore, it can be seen that image flip results when an image is reflected an odd number of times. Conversely, it can be seen that image flip does not result when the image is reflected an even number of times.
Current lithographic systems typically include a reticle stage that is parallel to a wafer stage, such that the image from the reticle stage is projected downward onto the wafer stage. In addition, current lithographic systems typically include catadioptric exposure optics that require a magnifying mirror, such as a concave asphere. This mirror enhances the projected image and enables better exposure of the wafer. The parallel wafer and reticle stages together with the geometry of a magnifying mirror, however, makes it difficult for the catadioptric exposure optics to perform an even number of reflections.
To illustrate this point,
FIG. 2
shows a simplified example lithographic system
200
. System
200
shows a parallel reticle stage
202
and wafer stage
204
using catadioptric exposure optics
212
, having a first mirror
206
, a beam splitter
208
, a quarter wave plate
209
, and a magnifying mirror element group
210
. In this example system
200
, an image is projected from reticle stage
202
using P polarized light. This polarized light is reflected by first mirror
206
directly into magnifying mirror element group
210
. It should be noted that quarter wave plate
209
can rotate the polarization angle of the light. The reflected image from first mirror
206
passes through beam splitter
208
. This is due to the P polarization of the light being transmitted by beam splitter
208
. The reflected image from magnifying mirror element group
210
has its polarization angle rotated 90°. This light is reflected at the beam splitter surface onto wafer
204
. Thus, S polarization is not transmitted by beam splitter
208
. Subsequently, the image is reflected directly out of magnifying mirror element group
210
that contains quarter wave plate
209
. Besides flipping the image, magnifying mirror element group
210
also reverses the polarization of the image. Thus, the image reflected out of magnifying mirror element group
210
is then reflected by beam splitter
208
, since the image now has the opposite polarization as beam splitter
208
. The image is then projected onto parallel wafer stage
204
. Using this configuration, an odd number of reflections occur. As a result, image flip problems occur.
Several alternative lithographic system designs, however, have attempted to overcome the image flip obstacle. One such design is a centrally obscured optical system design.
FIG. 3
shows an example lithographic system
300
with a centrally obscured optical system design. System
300
shows a parallel reticle stage
302
and wafer stage
304
using catadioptric exposure optics
312
with a first mirror
306
and a magnifying mirror
308
. In this example system
300
, an image is projected from reticle stage
302
directly into magnifying mirror
308
. It should be noted that the image projected from reticle stage
302
passes through first mirror
306
. This is because first mirror
306
is polarized (in the same way as beam splitter
208
above). The image is then reflected directly out of magnifying mirror
308
and onto first mirror
306
. Besides flipping the image, magnifying mirror
308
also reverses the polarization of the image. The image is then reflected downwards by first mirror
306
, through a small hole
310
in magnifying mirror
308
and onto wafer stage
304
. In this configuration, magnifying mirror
308
is in the path of the projected reflection of first mirror
306
, which is why small hole
310
exists within magnifying mirror
308
. The projected reflection of first mirror
306
travels through small hole
310
in magnifying mirror
308
to reach wafer stage
304
. Using this configuration, an even number of reflections occur. Thus, there is no image flip problem. However, this configuration has its drawbacks. As the image is reflected by magnifying mirror
308
, some of the image information (namely the portion of the image that passes through small hole
310
in magnifying mirror
308
) is lost. This can produce aberrations or inconsistencies in the projected image.
Another lithographic system that has attempted to overcome the image flip obstacle is an off-axis design.
FIG. 4
shows an example lithographic system
400
with an off-axis design. System
400
shows a parallel reticle stage
402
and wafer stage
404
using catadioptric exposure optics
412
with a first mirror
406
and a magnifying mirror
Ivaldi Jorge
Sewell Harry
Shamaly John
ASML Holding N.V.
Collins Darryl J.
Sterne Kessler Goldstein & Fox P.L.L.C.
Sugarman Scott J.
LandOfFree
Catadioptric lithography system and method with reticle... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Catadioptric lithography system and method with reticle..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Catadioptric lithography system and method with reticle... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3343998