Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2001-11-15
2004-07-13
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S747000
Reexamination Certificate
active
06762889
ABSTRACT:
BACKGROUND
Grating scale position measuring systems precisely measure the movement of an object by observing the light diffracted from a grating attached to the object.
FIG. 1
illustrates an example of a grating scale position measuring system
100
that measures the position of an object
110
. For the measurements, a reflection grating
120
is mounted on object
110
and illuminated with a collimated light beam
130
from a laser or other beam source
140
.
Grating
120
diffracts light beam
140
into a zero order maximum centered on a beam X
0
, a first order maxima centered on beams X
1
, second order maxima centered on beams X
2
, and higher order maxima (not shown). A lens
150
in system
100
receives the diffracted light and focuses light from the first order maxima X
1
onto an image plane
170
. A spatial filter
160
selectively transmits the light in the first order maxima and blocks the rest of the light diffracted from grating
120
.
On image plane
170
, light from the first order maximum forms a periodic intensity distribution
175
having a spatial period (or wavelength) that depends on the line spacing of gating
120
and the magnification of lens
150
. The location or phase of periodic intensity distribution
175
depends on the location of grating
120
. Accordingly, as object
110
and grating
120
move perpendicular to incident beam
140
, periodic intensity distribution
175
shifts on image plane
170
.
Detectors
180
measure light intensity at spatially separated locations along image plane
170
. Differences in the measured intensities at the spatially separated points indicate the location or phase of periodic intensity distribution
175
. Accordingly, movement or a phase change in periodic intensity distribution
175
indicates movement of object
110
. Detectors
180
measure the phase change of periodic intensity distribution
175
and thereby measure the movement of object
110
.
For precise measurements, detectors
180
require a sharp image on image plane
170
. In particular, using a spherical lens for lens
150
causes spherical aberrations that blur intensity distribution
175
making it difficult for detectors
180
to measure the phase of intensity distribution
175
. An aspheric lens can reduce spherical aberrations, but a standard off-the-shelf aspheric lens minimizes spherical aberrations if the object is at infinity. In system
100
, light from first order beams X
1
diverge from grating
120
so that the approximation of an object at infinity is inaccurate. Accordingly, even with an aspheric lens, aberrations can cause accuracy problems.
System
100
also has a drawback in that most applications of system
100
require a relatively large distance between lens
150
and image plane
170
. For example, when object
110
is a stage for a wafer in an integrated circuit fabrication device, the clearance between object
100
and lens
150
needs to be about 19 mm or more, which leads to an object distance of about 19 mm or more. Additionally, with a reasonable size grating (e.g., a 10 &mgr;m pitch), detectors
180
require a magnification of 9× or more of the grating pitch to allow measurement of the phase of periodic intensity distribution
175
. The clearance and magnification requirements result in a total optical path length of about 200 mm between the object and the image. A 200 mm long measuring device is often too large in space critical systems such as typical integrated circuit fabrication equipment.
Folding mirrors can fold the optical length inside a relatively compact package. One exemplary system employs seven folding mirrors to reduce size of the measurement device. However, the folding mirrors require alignment, which increases manufacturing costs. Additionally, the positions of folding mirrors are subject to drift during use of the measurement system, and periodic recalibration of the measurement system can be inconvenient or unacceptable in some applications.
In view of the drawback of existing grating scale position measuring systems, a system is desired that provides a compact device, does not require complicated mirror alignment, is not subject to measurement drift, and provides a light intensity distribution with a magnification and clarity that permits precise phase measurements.
SUMMARY
In accordance with an aspect of the invention, a grating scale position measuring system uses a telephoto lens that includes a pair of aspheric lenses positioned for finite conjugates. An additional magnifying system in the telephoto lens can magnify a periodic intensity distribution (i.e., the image) in the image plane to the size required for accurate phase measurements. The magnifying system can use spherical lenses because the aspheric lenses focus light to within a small aperture in the magnifying system, and the rays through the aperture are sufficiently paraxial to avoid introducing significant spherical aberrations.
One specific embodiment of the invention is a telephoto lens that includes a first aspheric lens and a second aspheric lens positioned to form a subsystem that operates at finite conjugates. In one particular configuration, the first aspheric lens is positioned so that an object is at a focal point of the first aspheric lens, and the second aspheric lens is positioned so that an image of the first aspheric lens is an object of the second aspheric lens. The aspheric lenses can be substantially identical and positioned so that the subsystem provides a real image with unit magnification. A magnifying system, that may include one or more negative lens, can magnify the image from the subsystem.
Another embodiment of the invention is a grating scale measurement system that includes a telephoto lens and a detector. The telephoto lens forms an image of a grating, and the detector measures movement of an intensity distribution that the telephoto lens forms in an image plane. The telephoto lens generally includes multiple aspheric lenses having configurations such as in the telephoto lenses described above.
REFERENCES:
patent: 5151820 (1992-09-01), Sillitto et al.
patent: 5204774 (1993-04-01), Owen et al.
patent: 5754278 (1998-05-01), Kurtz
patent: 6215755 (2001-04-01), Snyder et al.
patent: 6362924 (2002-03-01), Ohno
Agilent Technologie,s Inc.
Raizen Deborah
Sugarman Scott J.
LandOfFree
Compact telephoto lens for grating scale position measuring... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Compact telephoto lens for grating scale position measuring..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Compact telephoto lens for grating scale position measuring... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3207153