Single aperture optical system for photolithography systems

Details Single aperture optical system for photolithography systems Single aperture optical system for photolithography systems

2002-06-06

2004-06-29

Mathews, Alan (Department: 2851)

Photocopying

Projection printing and copying cameras

Illumination systems or details

C355S053000

Reexamination Certificate

active

06757052

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an exposure apparatus and, more particularly, to a projection exposure apparatus featuring an adjustable aperture-free optical illumination system for exposing a semiconductor wafer, an in particular, a photoresist layer on a semiconductor wafer.
2. Description of the Related Art
When manufacturing a semiconductor integrated circuit, printed circuit board, or liquid crystal substrate, a predetermined pattern is formed on a photo-resist film deposited on a substrate using a photolithographic technique, i.e., by light exposure from a projection exposure apparatus. Such a projection exposure apparatus typically has a light source that illuminates a reticule (mask) having an opaque circuit pattern, and when an illumination light passes through the reticule, an image of the reticule pattern is formed on the photo-resist, thereby exposing the photo-resist material. As the resolution of the patterns increases with higher-density integration, however, such conventional lithographic techniques can produce missing or broken lines in the resulting patterns.
FIG. 1
illustrates a conventional projection exposure apparatus. Exposure light L
1
emitted from an illumination optical system
2
illuminates a reticule R, held by a reticule holder
3
. When the exposure light L
1
passes through the reticule pattern PA of the reticule R, a pattern image of the reticule pattern PA is projected onto wafer W. In the illumination optical system of the projection exposure apparatus, exposure light L
1
emitted from a light source
10
of the illumination optical system
2
is collected by an elliptical mirror
11
. After reflection by mirror
12
, the intensity distribution of the exposure light L
1
is made uniform using a fly-eye lens
14
and is directed to a rotational aperture plate
20
, which is positioned in the focal plane behind the fly-eye lens
14
.
Rotational aperture plate
20
further includes a plurality of selectable aperture stops (
21
-
26
), each one being a different type or having a unique size, that are concentrically formed at predetermined intervals around the circumference of rotational aperture plate
20
as shown in
FIG. 2. A
motor
30
provides rotational control for moving a selected aperture stop (
21
-
26
) into the axis of light beam L
1
to provide a predetermined configuration to the light beam L
1
for illuminating reticule R. Disadvantageously, such a rotational aperture exposure system typically must be made large to accommodate the size of rotational aperture plate
20
and motor
30
.
Further, having a light beam that has a cone-shaped dispersion characteristic passing through a central portion of a reticule R provides an illumination pattern that extends to areas beyond the edges of the reticule R.
FIG. 3
illustrates one such dispersed light beam passing through reticule R of FIG.
1
. An optimized beam portion
6
would preferably be collimated so as to be directed parallel to the optical axis to an edge
7
of reticule R and thence to a target on wafer W, such that dispersion is restricted by a shadowing effect of an opaque edge of reticule R. Conical dispersion back toward the center of the optical axis would further illuminate the center of wafer W without detriment. In conventional projection apparatus, however, a typical beam from a single light source
11
of
FIG. 1
is not directed parallel to the optical axis at the edges of the reticule R, as with beam
6
, but is instead directed along the central axis
8
of the reticule R, with a conical dispersion that illuminates wafer W at point
9
, which is outside the desired illumination area. Any photoresist in such an area would be exposed and subsequently processed with undesirable consequences to the targeted area/circuitry.
Another disadvantage with this type of conventional aperture wheel is that a particular one of the available apertures is selected to optimally match the exposure characteristics of the aperture to a desired pattern to be masked. For example, a thin conductive line would typically be exposed using a “line” aperture, while a pad or a thick line could be formed using a larger area aperture, such as a “flash,” a “circular,” or a “rectangular” aperture. While a thin line aperture can be used to create a large area pad using repeated passes, each one having a slight offset, misalignment between passes can leave undesirable trace defects where the exposure passes did not overlap sufficiently.
A more significant disadvantage of such a system is a lack of precision in the rotational stops and control of the rotational drive device, i.e., motor
30
. Depending on the particular motor control techniques being employed, the plate can be rotated only to within a tolerance of the slippage or step of the motor, which can vary over time and with environmental conditions. Slow response properties and degraded positional-detection accuracy of a control and motion sensing mechanism can also detrimentally affect the precision of the aperture alignment. Further, properties of the attachment mechanism, such as bearings and gears, can produce unwanted motion and/or displacement in any of the X, Y, Z directions, producing undesired variations in a finished pattern.
In still another conventional approach using an optical aperture rather than an aperture wheel, an annular intensity profile is created and varied by using diverging and counter-diverging elements that are movable relative to each other in the illumination path, preferably under control of a computer. As shown in
FIG. 4
, an apparatus that provides such a beam variation comprises a pair of refractive elements
38
and
39
with confronting conical surfaces.
FIG. 5
illustrates an exploded view of the conical refracting apparatus
25
shown in FIG.
4
.
In the operation of the apparatus shown in
FIG. 4
, a concave conical surface of a first element diverges the illumination into an annular configuration having a radius that increases with distance from the first element. A complementary convex conical surface of a second element counters the divergence caused by the first element and restores the original directional orientation to the illumination, thereby fixing the radius of the annular intensity profile. The first and second elements are movable relative to each other, such that the distance between them is variable. The radius of the annular divergence of the illumination is a function of the distance between the first and second elements.
Disadvantageously, the complementary concave and convex conical elements are difficult to manufacture, and thus expensive, and the apparatus only provides for an annular intensity profile, rather than the multitude of aforementioned desired geometric shapes. In contrast, embodiments according to the present invention produce illumination profiles using simple planar surfaces which can be created with greater ease and precision of manufacturing while also providing for the creation and variation of both dipole and quadrapole intensity profiles.
SUMMARY OF THE INVENTION
A first embodiment of an exposure apparatus for creating a variable light intensity profile in a photolithographic process according to the present invention preferably comprises at least one complementary pair of transparent elements, wherein a first transparent element has an adjacent pair of first planar interface surfaces for dividing an incident light beam and refracting the portions of the divided beam away from an incident optical axis and a second transparent element has an adjacent and complementary pair of second planar interface surfaces for restoring the original directionality to the beam portions via refraction at the second planar interface surface, producing a bipolar beam.
The second planar interface surface is preferably parallel to and orthogonally displaced from the first planar interface surface. The degree of dispersion of the light beam portions is directly proportional to the displacement between the opposing refractive surfaces.

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