Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2003-02-04
2003-12-16
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S285000, C359S311000
Reexamination Certificate
active
06665112
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical reticle substrate inspection apparatus that inspects a defect and a line width of a reticle substrate and a beam scanning method thereof, particularly, to an optical reticle substrate inspection apparatus for improving stability when a laser light source having a short wavelength is used and a beam scanning method thereof.
2. Description of the Related Art
In the optical reticle substrate inspection apparatus that inspects a defect and a line width of a reticle substrate, there exists an apparatus in which detection light is converged onto a substrate to detect the defect and the line width by transmission light or reflection light. In such a apparatus, a spot of a laser beam (a beam spot) is scanned in one direction parallel with a surface of the reticle substrate (hereinafter, this direction is referred to as a Y-axis direction), and a stage on which the reticle substrate is mounted is moved in an isokinetic manner in a direction parallel with the surface of the reticle substrate and a perpendicular direction to the Y-axis direction (hereinafter, this direction is referred to as an X-axis direction), and thus inspecting the defect and the like of the reticle substrate. As a scanning method of the beam spot, a method in which the beam spot is optically scanned is generally used in order to process inspection in a short time, and the beam scanning method using an acoustooptical element has been conventionally adopted.
In such a beam scanning method, aberration occurs to an angle of the scanning beam when the beam is converged by a normal cylindrical lens, and defocus occurs on the reticle substrate surface. For this reason, a method is adopted where the beam is converged using a lens effect by the acoustooptical element to reduce the defocus. A method converging the beam using the two acoustooptical elements is described in The U.S. Pat. No. 3,851,951, for example. And a substrate inspection apparatus that adopted the method is described in Japanese Patent Laid-Open (unexamined) No. Hei 6-294750.
FIG. 1
is a schematic view showing the conventional optical reticle substrate inspection apparatus.
In the optical reticle substrate inspection apparatus, a laser light source
1
, an acoustooptical element
2
and a group of cylindrical lenses
3
are arranged in this order in a rectilinear direction of a laser beam output from the laser light source
1
. The acoustooptical element
2
is arranged so as to scan the laser beam output from the laser light source
1
in a perpendicular direction to the straight line by frequency modulation, and the group of cylindrical lenses
3
is arranged so as to magnify the laser beam output from the acoustooptical element
2
only in the scanning direction by the acoustooptical element
2
.
An acoustooptical element
4
a
which gives a cylindrical lens effect to the laser beam output from the group of cylindrical lenses
3
is further provided to the optical reticle substrate inspection apparatus. A transducer
19
which oscillates ultrasonic to the acoustooptical element
4
a
is attached to one end portion of the acoustooptical element
4
a
. The acoustooptical element
4
a
is arranged such that a side where the transducer
19
is attached, that is, a side where the ultrasonic is input, is closer to the group of cylindrical lenses
3
. Furthermore, a group of the cylindrical convex lenses
18
that converges the laser beam output from the acoustooptical element
4
a
is arranged in the optical reticle substrate inspection apparatus. Moreover, a relay lens
24
which propagates the laser beam passed through the convergence spot to an optical system (not shown) in a post-step is arranged on a position apart from the convergence spot formed by the group of cylindrical convex lenses
18
. In addition, an objective lens (not shown) is arranged between the optical system and the reticle substrate being an object to be inspected, and a detector (not shown) that detects intensity and the like of the laser beam passed through the reticle substrate is further provided.
In the optical reticle substrate inspection apparatus constituted in this manner, scanning is performed in an angle made by an incident direction and an output direction to the laser beam output from the laser light source
1
by utilizing the frequency modulation by the first acoustooptical element
2
. An optical path of the laser beam transits from a path
12
to a path
13
due to this process. When the laser beam has its optical path on the path
12
, the laser beam is magnified by the group of cylindrical lenses
3
and output as a laser beam
30
with a width in the scanning direction.
Further, transducer
19
outputs a series of ultrasonic which is swept such that a wavelength lengthens in linear state as the passage of time, that is, the wavelength in a forefront portion
21
becomes shorter than that of an aftermost portion
20
. A plurality of parallel lines between an aftermost portion
20
and a forefront portion
21
in
FIG. 1
show that the wider the distance between the lines the longer the wavelength. The second acoustooptical element
4
a
, when the laser beam
30
is made incident thereto, outputs the laser beam
30
while converging it with functioning as a cylindrical convex lens by the ultrasonic oscillated from the transducer
19
. The laser beam
30
output from the acoustooptical element
4
a
is further converged by the group of cylindrical convex lenses
18
to form a convergence spot
22
. When the laser beam has its optical path on the path
13
, a convergence spot
23
is formed on a position off from the convergence spot
22
in the scanning direction. The laser beam
30
passed through the convergence spot
22
is made incident to the relay lens
24
while magnifying its width again. Then, the convergence spot
22
is imaged on the reticle substrate via the optical system and the objective lens, and the detector detects the intensity and the like of the transmission light.
Note that the same detection can be performed even if the acoustooptical element
4
a
is arranged such that the side where the ultrasonic is input is made far from the group of cylindrical lenses
3
and the ultrasonic is swept such that the wavelength becomes shorter in linear state as the passage of time.
However, in recent years, although higher resolving power has been demanded for an apparatus for inspecting the reticle substrate with demand for finer pattern, there is a problem that the conventional optical reticle substrate inspection apparatus cannot provide sufficient high resolving power.
As a method of obtaining the high resolving power, a method is considered where the convergence spot size on the reticle substrate surface is made small by shortening the wavelength of the detection light. Progress of shorter wavelength has been made also in the field of exposure due to advancement in technology. It is important that the wavelength of the detection light is shortened to make it close to that of an exposure light because the way how the defect appears changes depending on a detection wavelength. However, in the case of shortening the wavelength of the detection light, a usable material of the acoustooptical element is limited due to a problem of material absorption regarding the scanning method where convergence is performed by the conventional acoustooptical element. For example, since tellurium dioxide, which has been generally used as the acoustooptical element, does not transmit the detection light having the wavelength of 300 nm or less, the acoustooptical element made of quartz, which has a sonic speed of 5960 m/sec being about ten times faster than comparing to tellurium dioxide, needs to be used. However, there is a problem that a focal length lengthens when the acoustooptical element made of quartz is used, because a focal length of a cylindrical lens effect by the acoustooptical element is inversely proportional to the wavelength of light output from the light source a
Dickstein Shapiro Morin & Oshinsky LLP.
NEC Corporation
Stultz Jessica
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