U.S. patent application number 10/888013 was filed with the patent office on 2005-01-27 for simultaneously achieving circular symmetry and diminishing effects of optical defects and deviations during real time use of optical devices.
This patent application is currently assigned to SYMMETRITECH LTD.. Invention is credited to Yanowitz, Shimon.
Application Number | 20050018289 10/888013 |
Document ID | / |
Family ID | 11072073 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050018289 |
Kind Code |
A1 |
Yanowitz, Shimon |
January 27, 2005 |
Simultaneously achieving circular symmetry and diminishing effects
of optical defects and deviations during real time use of optical
devices
Abstract
A method for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time use of optical devices, and, a corresponding device and system
for implementing the method thereof. The method features rotating
an entire optical device, rotating at least one optical part of an
entire optical device such as an optical assembly or an optical
element during real time viewing or projecting by the optical
device, in order to spread and blur the optical defects and
deviations present in the at least one optical part of the optical
device. In a first embodiment of the method, an optical rotation
device is activated and controlled for rotating at least one
optical part of an optical device during real time use of a viewing
or projecting optical device. In a second embodiment of the method,
there is included a step for aligning the optical axis of the at
least one optical part of the optical device with respect to the
rotation axis. In a first, simple, yet practical, embodiment of an
optical rotation device for rotating the at least one optical part
of the optical device, there are provided means and mechanisms for
manual alignment during real time use of an optical device,
whereas, in a second, more advanced, embodiment of an optical
rotation device for effecting the rotation of the optical part of
the optical device, there are provided means and mechanisms for
highly accurate and automatic aligning of the optical axis of the
optical part of the optical device with the rotation axis, thereby
simultaneously achieving a high level of circular symmetry with
respect to the optical part of the optical device, and significant
diminishment of optical defects and deviations in at least one
optical part of the optical device.
Inventors: |
Yanowitz, Shimon; (Haifa,
IL) |
Correspondence
Address: |
DR. MARK FRIEDMAN LTD.
C/o Bill Polkinghorn
Discovery Dispatch
9003 Florin Way
Upper Marlboro
MD
20772
US
|
Assignee: |
SYMMETRITECH LTD.
|
Family ID: |
11072073 |
Appl. No.: |
10/888013 |
Filed: |
July 12, 2004 |
Current U.S.
Class: |
359/462 |
Current CPC
Class: |
G03F 7/70258 20130101;
G03F 7/70825 20130101; G02B 26/0875 20130101; G02B 27/0025
20130101; G02B 26/0883 20130101 |
Class at
Publication: |
359/462 |
International
Class: |
G02B 027/22 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 26, 1999 |
WO |
PCT/IL99/00565 |
Oct 26, 1998 |
IL |
126771 |
Claims
What is claimed is:
1. A method for diminishing effects of optical defects and
deviations during real time use of an optical device, the optical
device including a camera, comprising the steps of: (a) including
at least one additional camera in the optical device; (b)
positioning the camera and each of said at least one additional
camera, such that the camera and each of said at least one
additional camera faces a different direction spaced apart at
equally spaced angular intervals; (c) including a rotation variant
optical element in the optical device corresponding to each said at
least one additional camera, said rotation variant optical element
is selected from the group consisting of a part mirror and a beam
splitter; (d) positioning each said rotation variant optical
element for diverting images toward each corresponding said at
least one additional camera; (e) recording a first image of an
object by the camera, and recording another image of said object by
said each of said at least one additional camera; and (f)
processing said first image and said at least one additional image
of said object by cancelling out distortion errors of said cameras,
thereby obtaining image data corresponding to original orientation
and magnitude of said object.
2. The method of claim 1, whereby the optical device is a folded
optical device selected from the group consisting of a folded
optical device for viewing and a folded optical device for
projecting.
3. An optical rotation device for simultaneously achieving circular
symmetry and diminishing effects of optical defects and deviations
during real time use of an optical device, comprising: (a) a column
for containing at least one optical part of the optical device; (b)
a mount for holding said column, said mount including a sleeve; (c)
a rotation mechanism for enabling rotation of said mount; (d) a
rotation mechanism housing for housing said rotation mechanism; (e)
a motor for actuating rotation of said mount; (f) a transmission
for enabling said motor to effect rotation of said mount; and (g)
an adjustment mechanism for adjusting a position of said column
relative to said mount.
4. The optical rotation device of claim 3, wherein said adjustment
mechanism features two sets of at least two screws for horizontally
adjusting said position of said column along x-axis direction and
along y-axis direction.
5. An optical rotation device for simultaneously achieving circular
symmetry and diminishing effects of optical defects and deviations
during real time use of an optical device, comprising: (a) a column
for containing the at least one optical part of the optical device;
(b) a mount for holding said column, said mount including a sleeve;
(c) a ring for providing slight freedom of movement required to
align said column with respect to said mount; (d) a main rotation
mechanism for enabling rotation of said mount; (e) a main rotation
mechanism housing for housing said main rotation mechanism; (f) a
motor for actuating rotation of said mount; (g) a transmission for
enabling said motor to effect rotation of said mount; (h) two
self-aligned rotation mechanisms positioned at either side of said
main rotation mechanism; (i) pre-loaded flexures for mounting,
holding, and moving said two self-aligned rotation mechanisms; and
(j) two sets of actuators for actuating said pre-loaded
flexures.
6. The optical rotation device of claim 5, wherein said ring is
selected from the group consisting of metallic flexure and elastic
material.
7. The optical rotation device of claim 5, wherein said actuators
are piezo-electric transducers.
8. A system for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time viewing by an optical device, comprising: (a) said optical
rotation device of claim 5; (b) an electronic control unit for
activating actuator mechanisms, thereby changing positions of said
actuators, said actuator mechanisms include piezo-electric
transducers; (c) a camera for recording images viewed by the
optical device; (d) a digital frame grabber for capturing
electronic images of said camera; and (e) a computer for
controlling said electronic control unit.
9. A system for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time projecting by an optical device, comprising: (a) said optical
rotation device of claim 5; (b) an electronic control unit for
activating actuator mechanisms, thereby changing positions of said
actuators, said actuator mechanisms include piezo-electric
transducers; (c) a light source for projecting an image through the
optical device (d) a beam splitter placed in front of optics of the
projecting optical device; (e) a camera for viewing images
projected by the optical device; (f) a digital frame gabber for
capturing electronic images of said camera; and (g) a computer for
controlling said electronic control unit.
10. The optical rotation device of claim 3, whereby said at least
one optical part of the optical device is selected from the group
consisting of the optical device in its entirety, at least one
optical assembly of the optical device, and at least one optical
element of the optical device.
11. The optical rotation device of claim 3, whereby said at least
one optical part of the optical device exhibits a property selected
from the group consisting of rotation invariance and rotation
variance.
12. The optical rotation device of claim 10, wherein said optical
element is selected from the group consisting of a window, a lens,
a mirror, and a prism, wherein said lens includes a convex lens and
a concave lens, said mirror includes a flat mirror, a part-mirror,
and a parabolic mirror, and said prism includes a beam splitter and
a dove prism.
13. The optical rotation device of claims 3, wherein said motor the
optical device functions as a rotor.
14. The optical rotation device of claim 3, whereby rotating said
at least one optical part of the optical device is effected
according to two rotation parameters, said two rotation parameters
are rotation mode and rotation speed, said rotation mode is
selected from the group consisting of discontinuous rotation and
continuous rotation.
15. The optical rotation device of claim 5, whereby said at least
one optical part of the optical device is selected from the group
consisting of the optical device in its entirety, at least one
optical assembly of the optical device, and at least one optical
element of the optical device.
16. The optical rotation device of claim 5, whereby said at least
one optical part of the optical device exhibits a property selected
from the group consisting of rotation invariance and rotation
variance.
17. The optical rotation device of claim 15, wherein said optical
element is selected from the group consisting of a window, a lens,
a mirror, and a prism, wherein said lens includes a convex lens and
a concave lens, said mirror includes a flat mirror, a part mirror,
and a parabolic mirror, and said prism includes a beam splitter and
a dove prism.
18. The optical rotation device of claim 5, wherein said motor the
optical device functions as a rotor.
19. The optical rotation device of claim 5, whereby rotating said
at least one optical part of the optical device is effected
according to two rotation parameters, said two rotation parameters
are rotation mode and rotation speed, said rotation mode is
selected from the group consisting of discontinuous and continuous
rotation.
20. A system for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time viewing by an optical device, comprising: (a) said optical
rotation device of claim 3; and (b) a camera for recording images
viewed by the optical device.
21. A system for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviation during real
time projecting by an optical device, comprising: (a) said optical
rotation device of claim 3; and (b) a light source for projecting
an image through the optical device.
22. The system of claim 8, whereby said computer features a
software program for analyzing said captured electronic images for
sharpness, for effecting said changing said positions of said
actuators of said optical rotation device until a sharpest said
electronic image is obtained, and for controlling speed of said
motor of said optical rotation device.
23. The system of claim 9, whereby said computer features a
software program for analyzing said captured electronics images for
sharpness, for effecting said changing said positions of said
actuators of said optical rotation device until a sharpest said
electronic image is obtained, and for controlling speed of said
motor of said optical rotation device.
Description
FIELD AND BACKGROUND OF THE INVENTION
[0001] The present invention relates to optical systems, devices,
tools, and instruments, and to other systems having optical
components and, more particularly, to a method for simultaneously
achieving circular symmetry and diminishing effects of optical
defects and deviations during real time use of optical devices,
and, a corresponding device and system for implementing the method
thereof.
[0002] In contrast to common recreational and educational uses of
optical viewing or projection devices and systems, the field of
semiconductor device fabrication requires the technology of design,
manufacture, and implementation of such devices and systems to be
pushed to the utmost limit. Here, semiconductor devices are
fabricated on silicon wafers, where a single wafer, capable of
containing multiple semiconductor devices, is made up of a multiple
of overlaid layers, in sequence, one on top of the other.
Photolithography is an initial stage in the process of repetitively
producing a single layer, involving the use of a stepper machine
for optically projecting a patterned slide or mask onto a light
sensitive layer or coating known as photo-resist, previously
deposited onto the silicon wafer. The exposed photo-resist layer is
then developed, leaving a patterned layer of photo-resist on the
wafer, matching the pattern of the mask. Following completion of
each layer, photo-resist can again be deposited on the wafer for
forming another layer, and so on.
[0003] The continuously increasing technological requirements of
complexity and speed of operation of semiconductor devices imply
that wafer patterns must contain extremely fine features, on the
order of a fraction of a micron wide. The design rule, or width of
the finest pattern on a wafer, has significantly declined, by about
a factor of 10, during the past decade. Today's fastest devices
typically feature a 0.25 micron design rule, however, new devices
are currently being developed using half this size. This translates
to requiring a stepper featuring an optical system to achieve a
high level of optical quality such that geometrical distortion and
resolution of an image are each significantly less than the design
rule.
[0004] Another implication of current semiconductor technology is
that features printed on one layer of a wafer must be well aligned
with other features, existing underneath in preceding layers, so as
to minimize alignment error between layers, commonly known as
overlay or misregistration error. Maximum allowable overlay error,
known as overlay budget, is about one-third of the design rule.
[0005] An overlay, or registration, metrology tool operates in
conjunction with the stepper, by using a microscope for viewing
patterns created by the stepper. By using the overlay metrology
tool for viewing patterns of different layers and a computer for
image analysis, one is able to measure the overlay error between
layers. Such measurements are used for calibrating, testing, and
adjusting the stepper in order to minimize the overlay error. An
overlay metrology tool, however, inherently introduces its own
error into the overlay measurement. This error has two components,
known as the accuracy error, and the repeatability or
reproducibility error. Accuracy error, also referred to as Tool
Induced Shift (TIS), directly arises from distortions and
aberrations in the optics of the overlay metrology tool.
Repeatability error may arise from several factors, optics being
one of them. Since the overlay metrology tool is used to monitor
and control the stepper, tolerances placed on the total error are
significantly tighter. The stepper must produce an overlay better
than or within the overlay budget, however, the overlay metrology
tool must in turn, produce a total error less than about one tenth
of that. This tight margin of total error, especially the TIS
component, translates to extremely strict requirements on the
optical quality of an overlay metrology tool.
[0006] A critical dimension (CD) metrology tool is another
metrology tool used for calibrating, testing, and adjusting the
stepper, which is used for measuring the width of the finest lines
produced by the stepper. Currently, line width tools typically
feature an electron microscope, rather than conventional optical
viewing systems, but the latter are still used. Other types of
metrology tools featuring an optical system may also be used for
making critical measurements of wafer fabrication processes.
[0007] Thus, the constant drive to increase complexity and speed of
semiconductor devices invokes the tightest possible constraints and
tolerances on the optics and quality of steppers and metrology
tools used in wafer fabrication processes. These characteristics
determine and limit the achievable complexity and speed of next
generation semiconductor devices.
[0008] Another field where optical quality of optical viewing
devices and systems is of extreme importance is aerial or satellite
photography. Although features and objects, such as landscape and
buildings, viewed by such optical equipment are relatively large,
the large distances from which they are viewed result in minute
details appearing in a viewed image, which can be thought of as
scaling to similar conditions and dimensions involved in
micro-lithography.
[0009] With respect to understanding the present invention, the
following terminology and definitions are provided here, referred
to and used hereinafter. An optical system refers to any system
including at least one optical device, along with any number of
other devices, mechanisms, units, and/or components enabling
functional and cooperative operation of the optical device and the
system. An optical device refers to a device, such as a tool,
instrument, or piece of equipment, featuring at least one optical
assembly, and at least one peripheral structure and/or at least one
peripheral mechanism, positioned and/or functioning along an
optical path of the at least one optical assembly for enabling
viewing or projecting by the optical device.
[0010] An optical assembly features at least one optical element,
and at least one peripheral structure and/or peripheral mechanism
positioned and/or functioning for holding, moving, or changing the
direction or orientation of the at least one optical element. An
optical element is ordinarily considered as a piece of material,
such as uncoated or coated glass or plastic, specially shaped to
affect light rays in a specific way, including refraction,
reflection, transmission, absorption, diffraction, and
scattering.
[0011] Exemplary optical elements are a lens, a window or flat, a
reflector or mirror, and a prism. Special types of optical
elements, featuring a specially configured optical element or a
combination of optical elements, include a curved mirror such as a
parabolic mirror, a part-mirror, and a beam splitter, also known as
a cube. A part-mirror functions by partly enabling reflection and
transmission. A beam splitter features two prisms geometrically
configured for splitting a beam. An optical assembly featuring at
least one lens and/or at least one mirror, is commonly referred to
as a lens assembly. An optical assembly, such as that featured in a
particle beam microscope in general, and an electron microscope in
particular, can also be of a non-conventional form such as an
electric field, a magnetic field, or an electromagnetic field,
serving as a lens for affecting not light in the classical form,
but rather in the form of charged particles.
[0012] A peripheral structure refers to a structure peripherally
positioned and functioning for holding, moving, and/or changing the
direction or orientation of at least part of an optical device,
such as a mount, frame, cell, tube, column, barrel, turret,
eyepiece, and nosepiece. A peripheral mechanism refers to a
mechanism peripherally positioned and functioning for enabling
operation of at least part of an optical device, such as a source
for providing electromagnetic radiation such as light or a particle
beam for viewing or projecting an image. An optional peripheral
mechanism is positioned and functioning for enabling optional
operation of at least part of an optical device, such as a detector
for detecting pixel intensities, for example, a camera for
recording an image.
[0013] A microscope is an example of an optical device used for
viewing and featuring the above described optical components. FIG.
1 is a schematic diagram illustrating optical components of a
typical light microscope. Microscope 10 includes a plurality of
optical or lens assemblies, including a condenser 12, an objective
14, and a tube-lens 16, where each lens assembly includes a
plurality of optical elements or lenses 18, 20, and 22,
respectively. Microscope 10 also includes another optical element,
mirror 24. Light source 26 is an exemplary peripheral mechanism,
whereas camera 28 is an exemplary optional peripheral mechanism of
microscope 10 used for recording an image of a viewed object
30.
[0014] FIG. 2 is a schematic diagram illustrating a partial cross
section of a typical microscope objective 32. As illustrated,
cement 34 is used to secure an optical element, glass lens 36, in a
peripheral structure, lens holder or lens cell 38. Any number of
various lens cells are then assembled into some sort of peripheral
structure, such as a tube 40, for holding and ultimately orienting
the lens cells, forming objective 32. Mechanical axis 42 of lens
cell 38, coinciding with the mechanical axis of tube 40, is shown
as a reference.
[0015] Property and characteristics of circular symmetry or
rotation invariance. Certain optical elements feature the property
of having an optical axis, where an optical axis refers to an axis
of symmetry of the optical element, whereby the optical element can
be freely rotated with respect to the optical axis without
affecting or changing the optical behavior of light or radiation
interacting with or passing through the optical element. This
property is referred to as circular symmetry or rotation
invariance, and an optical element featuring this property is
circularly symmetric or rotation invariant. An optical element is
rotation invariant with respect to its optical axis, which is its
axis of rotational symmetry.
[0016] Exemplary optical elements having an optical axis, and
therefore, being circularly symmetric or rotation invariant,
include lenses and mirrors. Optical elements such as a window and a
flat mirror have more than one optical axis because they are flat,
whereby they can be rotated with respect to any axis perpendicular
to the plane of the optical element. Exemplary optical elements not
having an optical axis, and therefore, not being rotation
invariant, include a prism and a beam splitter. Such optical
elements cannot be rotated without affecting or changing the
optical behavior of light or radiation interacting with or passing
through the optical element.
[0017] A single lens is one of the simplest types of optical
elements in an optical assembly, which in turn, is used for forming
an optical device. FIG. 3 is a schematic diagram illustrating two
basic lenses, a convex lens 44 and a concave lens 46, commonly used
in optical devices. Assume that each lens is perfect, having no
defect such as a blemish, imperfection, or form error. An optical
device, featuring a single lens 44 or 46, is extremely performance
limited and of poor quality. The optical device would exhibit
spherical aberration, chromatic aberration, pincushion or barrel
distortion, and field curvature. Nevertheless, such an optical
device features circular symmetry as a result of the presence of
optical axis 48, about which either lens 44 or lens 46 is rotation
invariant. Moreover, optical aberrations and distortions produced
by either lens are also circularly symmetric.
[0018] Design, manufacture, and assembly of optical devices and
systems. An ideal optical device or system, used for viewing or
projecting a planar field, equally treats each point in the
respective field of view or field of projection. All points in the
respective field of view or field of projection are simultaneously
focused and equally magnified, with no image distortion. In
reality, such is not the case. With regard to interaction of light
with matter, the laws of physics imply that, even if all basic
components, such as lenses and mirrors, of an optical device or
system are perfectly designed, manufactured, and assembled, the
optical device or system as a whole would still deviate from ideal
desired optical behavior. Dispersion of a light beam as it
interacts with, and passes through, a perfectly designed,
manufactured, and assembled optical device such as a lens assembly
featuring a glass lens in a lens holder, is just one example of
such deviant behavior.
[0019] In principle, as complexity of design of an optical device
or system, and its number of optical elements increase, approach
toward ideal desired optical behavior increases. This scenario is
analogous to the well known mathematical technique of improving
accuracy of approximating the exact form of a function by using a
polynomial function with an increasing number of coefficients,
where the coefficients of the function are analogous to optical
elements of an optical device. The problem with designing a more
complex optical device, with the goal of achieving a better
theoretical optical behavior, is that as the number of optical
elements featured in an optical device grows, so does the
probability for introduction of additional optical defects and
deviations into the optical device, originating from the
manufacturing and assembly of additional optical elements.
[0020] Aside from applying theoretical principles and laws of
optics, which are part of the process of designing an optical
device, practical skills and art are used in manufacturing and
assembling complex optical devices and systems, with the objective
of minimizing the effects of optical defects and deviations
introduced during the manufacture and assembly of a given complex
optical device. No matter how much care and cost go into
manufacturing individual optical elements of an optical device,
such as glass lenses and mirrors, there are always found varying
degrees of optical defects such as impurities, imperfections,
and/or blemishes. Moreover, every polished glass surface has some
degree of deviation from the required shape, known as form errors.
In addition, individual lenses are usually covered with a coating
material, such as anti-reflective coating, which introduces
additional form errors and blemishes.
[0021] Following the manufacture of optical elements, there is the
very elaborate and skillful task of assembling them into the final
optical device. Even if all the individual optical elements have
passed various quality control criteria, deviations may still be
introduced into the optical device during assembly of the optical
elements.
[0022] FIG. 4 is a schematic diagram illustrating various common
optical deviations existing following assembly of an optical
element such as a glass lens into a lens cell, in relation to the
mechanical axis of symmetry 50 of the lens cell. In (A), as a
reference, lens 52 is perfectly positioned in a lens cell 54, in
relation to mechanical axis of symmetry 50 of lens cell 54, such
that optical axis 50 of lens 52 is exactly aligned and coincides
with mechanical axis of symmetry 50. In (B), lens 56 is
horizontally displaced with respect to mechanical axis of symmetry
50 of lens cell 58, such that optical axis 60 of lens 56 is
misaligned with respect to mechanical axis of symmetry 50. In FIG.
2C, lens 62 is angularly displaced with respect to mechanical axis
of symmetry 50 of lens cell 64, such that optical axis 66 of lens
62 is misaligned with respect to mechanical axis of symmetry 50. In
FIG. 2D, lens 68 is a first lens of a compound lens called a
doublet, and is both horizontally and angularly displaced with
respect to the second lens 70 of the doublet and with respect to
mechanical axis of symmetry 50 of lens cell 72, such that optical
axis 74 of lens 68 is misaligned with respect to mechanical axis of
symmetry 50 of lens cell 72. In FIG. 4, lens 52 is rotation
invariant, whereas, lenses 56, 62, and 68, are not rotation
invariant, with respect to each of their respective lens cells.
[0023] FIG. 5 is a schematic diagram of an optical assembly 96
illustrating various common optical deviations existing following
assembly of a plurality of lens cells, with each lens cell holding
a lens, into a peripheral structure such as a tube or similar
multiple lens cell holder, in relation to the common mechanical
axis of symmetry 76 of tube 78. As a reference, lens cell 80,
holding lens 82, is perfectly aligned with and coincides with
common mechanical axis of symmetry 76 of tube 78, such that optical
axis 76 of lens 82 is also exactly aligned and coincides with
mechanical axis of symmetry 76 of lens cell 80, and with common
mechanical axis of symmetry 76 of tube 78. Lens cell 84, holding
lens 86, is horizontally displaced with common mechanical axis of
symmetry 76 of tube 78, such that optical axis 88 of lens 86 is
misaligned with respect to common mechanical axis of symmetry 76 of
tube 78, even though optical axis 88 is aligned and coincides with
mechanical axis of symmetry 88 of lens cell 84. Lens cell 90,
holding lens 92, is angularly displaced with common mechanical axis
of symmetry 76 of tube 78, such that optical axis 94 of lens 92 is
misaligned with respect to common mechanical axis of symmetry 76 of
tube 78, even though optical axis 94 is aligned and coincides with
mechanical axis of symmetry 94 of lens cell 90.
[0024] In FIG. 5, each lens 82, 86, and 92, is rotation invariant
or circularly symmetric with respect to each mechanical axis of
symmetry 76, 88, and 94, respectively, but only lens 82 is rotation
invariant with respect to tube 78, or with respect to optical
assembly 96. Thus, as an entirety, optical assembly 96 is not
rotation invariant or circularly symmetric with respect to common
mechanical axis of symmetry 76. Any deviation existing following
positioning a cell in tube 78 introduces further deviation in the
final position of the respective lens, in additional to any
deviation already incurred following mounting the respective lens
into the cell as illustrated in FIG. 4. The presence of optical
deviations described and shown in FIGS. 4 and 5 significantly
influence any alignment process which is ordinarily performed on an
optical device in order to achieve proper operation and performance
of the optical device during real time viewing or projecting.
[0025] The effects of these optical defects and deviations in an
optical device must be measured and monitored during quality
control processes in order minimize the rejection rate of finished
products, and more importantly, to prevent the release of an
optical device failing required quality levels, specifications, and
tolerances. Performing quality control inspection and testing on an
optical device typically requires elaborate and time-consuming
procedures, and even with sophisticated testing equipment and
instrumentation, it is impossible to detect all defects.
Manufacturing costs and time involved in rejecting finished
products failing specifications are extremely high. Alternatively,
instead of rejecting finished products failing the specs, a
situation can arise where a manufacturer of an optical device
decides to use sub-quality components, or widen the passing range
of one or more quality control specifications, in order to maintain
continuous operation of a production line.
[0026] The above described optical defects and deviations cause an
optical device to deviate from its designed optical behavior,
resulting in the occurrence of various aberrations and
disturbances, such as coma and astigmatism, during real time
application of the optical device. A common practice by a
manufacturer of an optical device is to reduce the level of
aberrations in the optical device by stopping it down using means
of an aperture. However, this practice has the negative effect of
reducing optical resolution of the optical device, as optical
resolution is proportional to the numerical aperture (NA) of the
optical device, decreasing with physical size of the aperture.
Stopping down a lens assembly may be quite acceptable for
recreational or educational applications of optical devices,
however, typically it is highly undesirable for applications in
leading edge technologies such as semiconductor fabrication.
[0027] In addition to the presence of optical defects and
deviations following the cycle of design, manufacture, and
assembly, of an optical device, shipment of the optical device is
another way for a variety of things to go wrong. During shipment,
the optical device may be exposed to mechanical shock, severe
pressure change if shipped by air, and severe temperature changes.
Often, characteristics and operation of an optical device,
initially meeting all manufacturer quality control specifications,
considerably change to the extent of being out-of-spec upon
reaching the final destination of an end-user. Thus, even following
a comprehensive, costly, and quality controlled cycle of design,
manufacture, and assembly, an optical device may still feature
defects and/or deviations at the time of application by an
end-user.
[0028] In practice, an optical device, as previously described,
typically includes several optical assemblies, such as lens
assemblies, where one of the lens assemblies may be an objective,
with each lens assembly featuring a plurality of optical elements
such as lenses and/or mirrors, where each element has a varying
extent of surface concavity and/or convexity, as shown in FIG. 3.
Each lens and mirror may have a different radius of curvature, and
may feature different glass and/or coating materials, for the
purpose of attempting to correct and compensate for the presence of
various degrees of optical aberrations, distortions, and field
curvature of the optical device.
[0029] FIG. 6 is a schematic diagram of an optical assembly 100
illustrating an example of a plurality of lenses 102, 104, 106, and
108, having a common optical axis 110 of circular symmetry
perfectly aligned and coinciding with common mechanical axis 110.
Moreover, in the event that entire optical assembly 100 is rotated
about common mechanical axis 110, during rotation, optical axis 110
is aligned with and coincides with rotation axis 110. Thus,
according to the shown configuration of the lenses in optical
assembly 100, entire optical assembly 100 is rotation invariant or
circularly symmetric with respect to common mechanical axis
110.
[0030] If all the lenses and/or mirrors of an optical device, in
general, and of an optical assembly or lens assembly, in
particular, for example, shown in the objective of FIG. 6, were
perfectly designed, manufactured, and assembled, the optical
device, in general, and the optical assembly or lens assembly, in
particular, would feature the property of circular symmetry and be
rotation invariant. The entire optical device could be freely
rotated about its optical axis of symmetry without any effect on
the behavior of light or radiation interacting with the optical
device. Unfortunately, this is never the case, since the components
of an optical device, in general, and of an optical assembly or
lens assembly, including optical elements such as lenses and/or
mirrors, in particular, can never be perfectly manufactured, and
assembled, for producing a perfectly functioning optical device.
Moreover, the presence of optical defects and/or deviations from
ideal symmetry is likely to increase with increasing sophistication
in the design of an optical device, as an increasing number of
optical elements, peripheral structures, and peripheral mechanisms
inherently leads to the presence of additional optical defects and
deviations in the optical device.
[0031] There are three major contributing factors preventing the
achievement of circular symmetry or rotation invariance of an
optical device. The first factor relates to raw materials used for
manufacturing the optical elements of an optical device. Glass or
plastic, from which an optical element, capable of featuring
circular symmetry, such as a lens, a mirror, and a window, is made,
always has some level of impurity or blemish in it. Impurities or
blemishes are randomly scattered throughout the raw material,
preventing the manufacture of a uniform glass or plastic, and hence
preventing the achievement of circular symmetry of the optical
element, and consequently, of the optical device.
[0032] The second factor relates to the manufacturing process of
the optical element, for instance polishing a precursor of a lens,
a window, or a mirror, and optional coating of a precursor of a
lens, a window, or a mirror. In a process of making a lens, a flat
piece of glass is polished down in order to form the necessary
curved surfaces of the final, usable, lens. However, such polished
surfaces usually feature random locations of irregularities. In
addition, in a polished lens, there always exist form errors or
shape irregularities, including symmetric and/or asymmetric form
errors. The common practice of plating an optical element with a
coating material, such as anti-reflective coating, introduces yet
additional blemishes and form errors into the optical element. The
presence of these form errors and irregularities prevent the
polished lens, and consequently, the optical device, from featuring
a high degree of circular symmetry.
[0033] The third factor relates to aligning the optical elements as
part of properly assembling an optical device. During assembly of
an optical device, all the individual optical elements, capable of
and preferably being circularly symmetric, need to be fully aligned
with respect to the common axis of symmetry, or optical axis, of
the optical device. However, as described and shown in FIGS. 2 and
3, some combination of horizontal and/or angular deviations are
inevitably present in the assembled optical device, thereby
preventing the entire optical device, and not only the separate
optical elements, from featuring circular symmetry.
[0034] In addition to the presence of optical defects and
deviations in optical assemblies and/or optical elements, factors
relating to peripheral mechanisms of an optical device or system
can significantly influence overall performance of the optical
device or system. For example, optical devices and systems
including a peripheral mechanism such as a radiation source, in
general, or a light source, in particular, may be subject to
further non-uniformities introduced by the source. In general, it
is desirable, but practically impossible, to obtain a source that
produces radiation of uniform intensity over the entire field of
view, or field of projection.
[0035] Optical viewing devices and systems including an optional
peripheral mechanism, such as a CCD camera, may be subject to
further deviations introduced by the camera. CCD cameras contain
both an optical image-sensing element as well as electronic
circuitry for processing an image, both of which may introduce
deviations to a recorded image.
[0036] Principle of exposure/integration time. A viewing or
projection system may include one or more devices for detecting and
recording images of an object. During normal operation, there is
always a finite or non-zero amount of time during which a viewing
or projection system is required to detect and record an image.
When projecting an image, for example, the electromagnetic
radiation or particle beam source passing through an image slide
remains activated for a finite period of time. Similarly, when
viewing an image or a series of instantaneously generated images,
for example, by a camera, during the short time period of recording
a picture of the series of instantaneously generated images, the
recorded image is being formed for as long as the aperture of the
camera remains open. This time period is commonly referred to as
the exposure time. In viewing and projecting, the formed image is
actually an integral of all the instantaneously generated images
which are viewed by the optics during the exposure.
[0037] In the case of using an electronic charge coupled device
(CCD) camera, the exposure time is also referred to as the
integration time. Integration occurring during exposure time is
usually considered in the art as very undesirable. When taking a
picture, for example, too long of an exposure time, or,
equivalently, too slow of an aperture speed, can cause blurring or
smearing in images of an object. Thus, in addition to the presence
of optical defects and deviations in optical elements, integration
time is another factor relating to optical deviations and overall
performance of an optical device or system including imaging
equipment.
[0038] There is a large variety of prior art methods, devices, and
systems for eliminating, diminishing, or at least, compensating
for, the effects of optical defects and deviations in optical
devices and in systems including optical elements. In general, each
of these involves translational and/or rotational movement of
optical elements and/or optical assemblies with a goal of properly
aligning or centering all such components of an optical device
relative to the optical axis, while dealing with the effects of
optical defects and deviations in the optical device.
[0039] In a disclosure involving rotating optical elements, but
specifically relating to spatial filtering, Riggs, in U.S. Pat. No.
3,620,591 describes a method and apparatus for optically processing
seismic or other data by spatial filtering in order to discriminate
against optical noise and enhance recoverable information. Optical
elements acting on the seismic signals are mounted in special
assemblies and are rotated at different angular velocities with
respect to one another. A series of partial exposures is made of
the output information at selected time intervals and added to give
a composite exposure.
[0040] In the Riggs disclosure, separate optical elements are
mounted and separately rotated with different velocities, which
places substantial mechanical complexity on such an apparatus. For
optical devices comprising a large number of optical elements,
especially elements which need to be tightly packed, for example,
doublets and triplets wherein elements tightly touch each other, if
not glued together, the disclosed method becomes impracticable.
Accordingly, the method lends itself to very simple optical devices
and systems, including only a few sparse elements, such as the
Fourier-Transform spatial filtering projection system used by
Riggs, which includes only four optical elements, each in its own
optical assembly. Moreover, the fact that individual optical
elements need to be separately handled, prohibits the use of
proven, inexpensive and readily available mass produced
off-the-shelf sealed optical assemblies such as objectives, which
ordinarily cannot be taken apart for facilitating approach to the
individual optical elements. The method thus forces the design,
manufacturing, and assembly of totally new and unique optical
assemblies, again placing substantial constraints on the
practicality of the method.
[0041] Additional limitations apparent in the Riggs disclosure are
that the method uses a series of separate exposures at time and
rotation intervals, which is limited in its effect compared to a
single prolonged exposure which better `smears` and diminishes the
effect of optical disturbances, and there is not provided a means
for aligning the mechanical axis of rotation of the individual lens
mounts with the optical axis of the lens being rotated.
[0042] A general significant limitation of other prior art methods,
is that they deal with effects of optical defects and deviations at
the time of manufacture, assembly, and/or set-up of an optical
device, for example, by translating, rotating, and aligning optical
elements and/or assemblies of the optical device to an optimum
configuration and performance level, immediately followed by
permanently fixing, such as by cementing, the optical elements
and/or assemblies, prior to release of the optical device to an
end-user, or prior to use of the optical device by the end-user. In
essentially all of these devices and methods, no optical assembly
or optical element is rotated during regular real time use of the
optical device.
[0043] In particular, some practices of manufacturing and
assembling optical devices attempt to minimize effects of
orientational deviations of lenses in their lens cells, as shown in
FIG. 5, by procedures such as rotating one or more lens cells 84 or
90, about the common mechanical axis of symmetry 76, while slightly
adjusting the orientation of the lens it holds, which is initially
misaligned with respect to axis 76, by means of micro-manipulators,
such that each lens axis of symmetry, optical axis 88 and 94,
respectively, becomes aligned and coincides with the common
mechanical axis of symmetry 76 of lens assembly 96. This procedure
is commonly performed individually for all lenses of an optical
assembly. Despite such advanced optical device assembly procedures,
orientational deviations of lenses in lens cells can be minimized,
but not entirely eliminated.
[0044] Other practices of manufacturing and assembling optical
devices attempt to minimize effects of deviations of lenses in the
final assembled optical device, such as the lens assembly shown in
FIG. 5. One such procedure involves designating one or more lenses
in a lens assembly to remain capable of slight movement, such as
that caused by the force of adjustment screws, after the lens
assembly has been assembled. By slightly rotating and/or
translating these movable lenses with respect to the other optical
elements of the lens assembly, an attempt is made to reduce the
amount of optical disturbances of the entire optical assembly or
device. This procedure is very limited, since simple movements of
one optical element usually cannot compensate for complex
disturbances originating in other optical elements of the same
optical device.
[0045] In U.S. Pat. No. 5,852,518, Hatasawa et al. describe a
projection optical unit for projecting a pattern on a mask, and a
method for adjusting image formation of the projection unit,
including relatively rotating, at least two optical elements or
lenses previously and intentionally subjected to `astigmatic
surface processing`, in order to correct characteristic asymmetric
aberration of the overall projection optical system. Although not
specifically stated, it is implied and understood by those skilled
in the art, that rotating the lenses is done only during a one-time
setup procedure, either during assembly, or after shipment,
unpacking and installation, prior to commencing regular real time
use of the projection unit. The lenses are rotated according to an
empirical orientation that optimizes overall behavior of the unit,
and are then left fixed in place. Any additional rotation would
degrade the performance of the projection unit. Nothing is rotated
during actual real time use of the projection unit.
[0046] In the disclosure of Hatasawa et al., rather than
alleviating asymmetry of the unit, an attempt is made to compensate
for asymmetric disturbances by introducing yet additional
asymmetric disturbances. The method only has benefits if some
systematic and well-behaved distortion is present, which can be
corrected by applying a reciprocal distortion. This clearly is not
the case for the occurrence of general disturbances in optical
systems, which, as previously explained, are mostly random in
nature. The method is limited by not providing means for correcting
various types of optical disturbances, and may even lead to
introduction of new systematic disturbances. Moreover, this method
can be only applied by the manufacturer of the actual projection
unit, for incorporating the distorted elements into the design,
production and assembly. This necessitates detailed knowledge of
the design of the specific unit and is custom tailored to that
unit.
[0047] In each of the following disclosures, rotation of one or
more optical elements or optical assemblies, is performed
exclusively during manufacturing, assembly, or under test
conditions, for improving the quality of the particular optical
device, prior to actual real time use of an optical device. In U.S.
Pat. No. 5,835,208, Hollmann et al. discloses an apparatus and
method for non-contact measuring wedge and centering errors in
optical elements under test, including a lens holder rotatably
supported on an air bearing having an axis such that the lens
holder may be rotated about the axis. In U.S. Pat. No. 3,782,829,
Herriott describes a lens alignment apparatus and method, including
rotation of a lens holder about a selected optical axis until
alignment is achieved, followed by permanently fixing or grounding
the lens. In U.S. Pat. No. 3,762,821, Bruning et al. discloses a
lens assembly apparatus and method for centering and aligning a
lens element along a predetermined axis, including means of tilting
the lens element about axes by using a combination of linear
actuators. In U.S. Pat. No. 3,544,796, Baker discloses a lens
centering apparatus including a rotatable lens holder in which the
lens to be tested is permanently mounted.
[0048] In U.S. Pat. No. 2,352,179, Bosley describes a device for
centering a lens on a rotating support in alignment with the axis
of rotation of the support. Again, rotating the lens support and
the lens is performed during assembly, for improving optical
quality, prior to actual application of an optical device. Included
here is also the commonly known and used procedure for aligning the
optical axis of a single lens to the axis of rotation. This
procedure pertains to lens elements, and is only used during
assembly of lens elements. When the device is fully assembled,
everything is glued or otherwise fixed in place and is expected not
to rotate or even move at all.
[0049] The following additional references relate to devices and
methods employed exclusively for aligning optical devices,
assemblies, and elements. In U.S. Pat. No. 5,400,133, Hinton et al.
disclose a raster output scanner (ROS) system including a mechanism
for adjusting and aligning the optical center line of scanning
beams. A lens barrel has eccentric rings mounted along its
circumference which, when rotated, effect movement of the lens
barrel so as to change the center line of the collimated beam
output. In U.S. Pat. No. 5,233,197, Bowman et al. describe a
fluorescent emission imaging microscope including a galvanometer
rotatable mirror placed in the sample image path, and includes an
automatic objective lens piezoelectric translator for directing
fine movements to achieve proper alignment. In this disclosure, the
mirror is rotated for the purpose of focusing, and not for
correcting optical defects and/or deviations. In U.S. Pat. No.
3,533,700, Alexander discloses a laser projection device involving
coordinated orientation of at least two laser beams, including
methods of optical alignment. In U.S. Pat. No. 5,453,606, issued to
Hojo, an apparatus is disclosed for adjusting the optical axis of
an optical system during the assembly process of a lens assembly,
including automatic two-dimensional adjustment of the lenses,
followed by fixing of the lenses to a lens frame.
[0050] As described above, even after applying such devices and
methods for diminishing effects of optical defects and deviations,
there still remains high likelihood of the presence of optical
defects and deviations of the optical device by the time an
end-user includes the optical device in an application. In
addition, following repetitive or modified use of an initially
optimally configured and performing optical device, optical defects
and deviations are expected to appear, thereby limiting further
application of the optical device.
[0051] To one of ordinary skill in the art, there is thus a need
for, and it would be highly advantageous to have a method for
simultaneously achieving circular symmetry and diminishing effects
of optical defects and deviations during real time use of optical
devices, and, a corresponding device and system for implementing
the method thereof.
SUMMARY OF THE INVENTION
[0052] The present invention relates to a method for simultaneously
achieving circular symmetry and diminishing effects of optical
defects and deviations during real time use of optical devices,
and, a corresponding device and system for implementing the method
thereof.
[0053] It is therefore an object of the present invention to
provide a method for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time viewing and projecting by optical devices.
[0054] It is another object of the present invention to provide a
corresponding device for implementing the method of simultaneously
achieving circular symmetry and diminishing effects of optical
defects and deviations during real time viewing and projecting by
optical devices.
[0055] It is another object of the present invention to provide a
corresponding system for implementing the method of simultaneously
achieving circular symmetry and diminishing effects of optical
defects and deviations during real time viewing and projecting by
optical devices.
[0056] Thus, according to the present invention, there is provided
a method for diminishing effects of optical defects and deviations
during real time use of an optical device, comprising the steps of:
(a) providing an optical rotation device for rotating at least one
optical part of the optical device during real time use of the
optical device; and (b) rotating the at least one optical part of
the optical device about a rotation axis during real time use of
the optical device, by activating and controlling the optical
rotation device, thereby spreading and blurring about the rotation
axis the optical defects and the deviations present in the at least
one optical part of the optical device.
[0057] According to another aspect of the present invention, there
is provided a method for simultaneously achieving circular symmetry
and diminishing effects of optical defects and deviations during
real time use of an optical device, comprising the steps of: (a)
providing an optical rotation device for rotating at least one
optical part of the optical device during real time use of the
optical device; (b) aligning an optical axis of the at least one
optical part of the optical device with a rotation axis of the at
least one optical part of the optical device, causing the at least
one optical part of the optical device to be circularly symmetric
with respect to the rotation axis; and (c) rotating the at least
one optical part of the optical device about the rotation axis
during real time use of the optical device, by activating and
controlling the optical rotation device, thereby circularly
symmetrically spreading and blurring about the rotation axis the
optical defects and the deviations present in the at least one
optical part of the optical device.
[0058] According to another aspect of the present invention, there
is provided a method for aligning the optical axis of at least one
optical part of an optical device with a rotation axis of an
optical rotation device used for rotating the at least one optical
part of the optical device, comprising: (a) holding the at least
one optical part by a peripheral structure of the at least one
optical part, at two or more points along the peripheral structure,
wherein points of projection on the optical axis of the two or more
points are separated by corresponding distances along the optical
axis; and (b) moving the peripheral structure held by the two or
more points, such that each of the points of projection on the
optical axis is moved towards the rotation axis, such that the
optical axis of the at least one optical part of the optical device
becomes aligned and coincident with the rotation axis of the
optical rotation device.
[0059] According to another aspect of the present invention, there
is provided a method for diminishing effects of optical defects and
deviations during real time use of an optical device, the optical
device including a light source, comprising the steps of: (a)
including at least one rotation variant optical element in the
optical device, such that the light source generates light rays
passing through the at least one rotation variant optical element;
(b) providing an optical rotation device for rotating the at least
one rotation variant optical element during real time use of the
optical device; and (c) rotating the at least one rotation variant
optical element about a rotation axis during real time use of the
optical device, by activating and controlling the optical rotation
device, thereby spreading and blurring about the rotation axis the
optical defects and the deviations present in the light rays of the
light source passing through the at least one rotation variant
optical element.
[0060] According to another aspect of the present invention, there
is provided a method for diminishing effects of optical defects and
deviations during real time use of an optical device, the optical
device including a camera, comprising the steps of: (a) including
at least one additional camera in the optical device; (b)
positioning the camera and each of the at least one additional
camera, such that the camera and each of the at least one
additional camera faces a different direction spaced apart at
equally spaced angular intervals; (c) including a rotation variant
optical element in the optical device corresponding to each at
least one additional camera, the rotation variant optical element
is selected from the group consisting of a part mirror and a beam
splitter; and (d) positioning each rotation variant optical element
for diverting images toward each corresponding at least one
additional camera.
[0061] According to another aspect of the present invention, there
is provided an optical rotation device for simultaneously achieving
circular symmetry and diminishing effects of optical defects and
deviations during real time use of an optical device, comprising:
(a) a column for containing at least one optical part of the
optical device; (b) a sleeve functioning as a mount for holding the
column; (c) a rotation mechanism for enabling rotation of the
sleeve; (d) a rotation mechanism housing for housing the rotation
mechanism; (e) a motor for actuating rotation of the sleeve; (f) a
transmission for enabling the motor to effect rotation of the
sleeve; and (g) an adjustment mechanism for adjusting a position of
the column relative to the sleeve.
[0062] According to another aspect of the present invention, there
is provided an optical rotation device for simultaneously achieving
circular symmetry and diminishing effects of optical defects and
deviations during real time use of an optical device, comprising:
(a) a column for containing the at least one optical part of the
optical device; (b) a sleeve functioning as a mount for holding the
column; (c) a ring for providing slight freedom of movement
required to align the column with respect to the sleeve; (d) a main
rotation mechanism for enabling rotation of the sleeve; (e) a main
rotation mechanism housing for housing the main rotation mechanism;
(f) a motor for actuating rotation of the sleeve; (g) a
transmission for enabling the motor to effect rotation of the
sleeve; (h) two self-aligned rotation mechanisms positioned at
either side of the main rotation mechanism; (i) pre-loaded flexures
for mounting, holding, and moving the two self-aligned rotation
mechanisms; and (j) two sets of actuators for actuating the
pre-loaded flexures.
[0063] According to another aspect of the present invention, there
is provided a system for simultaneously achieving circular symmetry
and diminishing effects of optical defects and deviations during
real time viewing by an optical device, comprising: (a) the
described optical rotation device; (b) an electronic control unit
for activating actuator mechanisms, thereby changing positions of
the actuators, the actuator mechanisms include piezo-electric
transducers; (c) a camera for recording images viewed by the
optical device; (d) a digital frame grabber for capturing
electronic images of the camera; and (e) a computer for controlling
the electronic control unit.
[0064] According to another aspect of the present invention, there
is provided a system for simultaneously achieving circular symmetry
and diminishing effects of optical defects and deviations during
real time projecting by an optical device, comprising: (a) the
described optical rotation device; (b) an electronic control unit
for activating actuator mechanisms, thereby changing positions of
the actuators, the actuator mechanisms include piezo-electric
transducers; (c) a beam splitter placed in front of optics of the
projecting optical device; (d) a camera for viewing images
projected by the optical device; (e) a digital frame grabber for
capturing electronic images of the camera; and (f) a computer for
controlling the electronic control unit.
[0065] The method for simultaneously achieving circular symmetry
and diminishing effects of optical defects and deviations during
real time use of optical devices, and, a corresponding device and
system for implementing the method thereof, according to the
present invention, provide the following advantages and benefits
for real time using optical devices and systems. Improvement in the
quality of an optical device or system, through reduction of
asymmetrical aberrations, such as coma, reduction of distortion and
astigmatism, and reduction in the effect of blemishes and
impurities. This allows for better performance and tighter
specifications, raising the value of the device or system. Due to
elimination or reduction of said effects of optical defects and
deviations of an optical device or system, the optical device or
system more closely approaches its theoretical design model,
becoming more predictable, allowing for designing more
cost-effective optical solutions. Allows increasing the numerical
aperture of the optics of an optical device or system, thus
increasing optical resolution and ability to view or project finer
patterns, resulting in further improving specifications and value
of the optical device or system. Improves accuracy and reduces tool
induced shift (TIS) in optical measurement systems such as overlay
metrology tools. Improves long-term stability and reliability of
optical devices and systems.
[0066] Additionally, with respect to manufacturing optical devices
and systems, implementation of the present invention reduces
production time and effort required to achieve tight design
specifications of an optical device or system, resulting in lower
production costs and shorter delivery times. Enables acceptance of
components and elements of an optical device or system that
otherwise would be rejected during testing processes, resulting in
reduced manufacturing costs and time. Removes the risk of optical
device or system failure due to effects of shipment and/or
environmental conditions.
[0067] Implementation of the method of the present invention
involves performing or completing selected tasks or steps manually,
automatically, or a combination thereof. Moreover, according to
actual instrumentation and equipment of a given preferred
embodiment of the device or system, several selected steps of the
present invention could be implemented by hardware or by software
on any operating system of any firmware or a combination thereof.
For example, as hardware, selected steps of the invention could be
implemented as a chip or a circuit. As software, selected steps of
the invention could be implemented as a plurality of software
instructions being executed by a computer using any suitable
operating system. In any case, selected steps of the method of the
invention could be described as being performed by a data
processor, such as a computing platform for executing a plurality
of instructions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] The invention is herein described, by way of example only,
with reference to the accompanying drawings, wherein:
[0069] FIG. 1 is a schematic diagram illustrating optical
components of a typical light microscope;
[0070] FIG. 2 is a schematic diagram illustrating a partial cross
section of a microscope objective;
[0071] FIG. 3 is a schematic diagram illustrating two basic lenses
used in optical devices;
[0072] FIG. 4 is a schematic diagram illustrating various common
optical deviations existing following assembly of an optical
element such as a glass lens into a lens cell;
[0073] FIG. 5 is a schematic diagram of an optical assembly
illustrating various common optical deviations existing following
assembly of a plurality of lens cells into a tube;
[0074] FIG. 6 is a schematic diagram of an optical assembly
illustrating an example of a plurality of lenses having a common
optical axis of circular symmetry perfectly aligned and coinciding
with a rotation axis;
[0075] FIG. 7 is a schematic diagram illustrating misalignment of
an effective optical axis of an optical device, such as a
multi-lens optical assembly, with respect to a rotation axis;
[0076] FIG. 8 is a schematic diagram illustrating three dimensional
alignment of optical axes of at least a part of an optical device
with respect to a rotation axis;
[0077] FIG. 9 is a schematic diagram illustrating application of
the method of the present invention to an optical device having a
folded optical axis;
[0078] FIG. 10 is a schematic diagram illustrating rotation of a
rotation variant optical element, such as a dove prism, in an
exemplary optical device, a metallurgic microscope;
[0079] FIGS. 11A-11B are schematic diagrams illustrating the method
for diminishing the effects of optical deviations introduced by a
camera into an optical device used for real time viewing;
[0080] FIG. 12 is a schematic diagram illustrating a first
preferred embodiment of the optical rotation device, used for
implementing the method for simultaneously achieving circular
symmetry and diminishing effects of optical defects and deviations
during real time viewing or projecting by optical devices;
[0081] FIG. 13 is a schematic diagram illustrating a second
preferred embodiment of the optical rotation device, used for
implementing the method for simultaneously achieving circular
symmetry and diminishing effects of optical defects and deviations
during real time viewing or projecting by optical devices;
[0082] FIG. 14 is a schematic diagram illustrating a preferred
embodiment of the system, used for implementing the method and
device for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time viewing by optical devices; and
[0083] FIG. 15 is a schematic diagram illustrating a preferred
embodiment of the system, used for implementing the method and
device for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time projecting by optical devices.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0084] The present invention introduces a unique method for
simultaneously achieving circular symmetry and diminishing effects
of optical defects and deviations during real time use of optical
devices, and, a corresponding device and system for implementing
the method thereof. The method features rotating an entire optical
device, or rotating at least one optical part of an entire optical
device such as an optical assembly or an optical element during
real time use of the optical device.
[0085] Steps, components, operation, and implementation of a method
for simultaneously achieving circular symmetry and diminishing
effects of optical defects and deviations during real time use of
optical devices, and, a corresponding device and system for
implementing the method thereof, according to the present invention
are better understood with reference to the drawings and the
accompanying description. It is to be understood that the invention
is not limited in its application to the details of construction,
arrangement, and composition of the components set forth in the
following description and drawings. The invention is capable of
other embodiments or of being practiced or carried out in various
ways. Also, it is to be understood that the phraseology and
terminology employed herein is for the purpose of description and
should not be regarded as limiting.
[0086] As described above, for purposes of understanding the
present invention, an optical system refers to any system including
at least one optical device, along with any number of other
devices, mechanisms, units, and/or components enabling functional
and cooperative operation of the optical device and the system. An
optical device refers to a device, such as a tool, instrument, or
piece of equipment, featuring at least one optical assembly, and at
least one peripheral structure and/or at least one peripheral
mechanism, positioned and/or functioning along an optical path of
the at least one optical assembly for enabling viewing or
projecting by the optical device.
[0087] An optical assembly features at least one optical element,
and at least one peripheral structure or peripheral mechanism
positioned and/or functioning for holding, moving, or changing the
direction or orientation of the at least one optical element. An
optical element is ordinarily considered as a piece of material,
such as uncoated or coated glass or plastic, specially shaped to
affect light rays in a specific way, including refraction,
reflection, transmission, absorption, diffraction, and
scattering.
[0088] A peripheral structure refers to a structure peripherally
positioned and functioning for holding, moving, and/or changing the
direction or orientation of at least part of an optical device
and/or an optical assembly, such as a mount, frame, cell, tube,
column, barrel, turret, eyepiece, and nosepiece. A peripheral
mechanism refers to a mechanism peripherally positioned and
functioning for enabling operation of an optical device and/or an
optical assembly, such as a source for providing electromagnetic
radiation such as light or a particle beam for viewing or
projecting an image. An optional peripheral mechanism is positioned
and functioning for enabling optional operation of an optical
device and/or an optical assembly, such as a detector for detecting
pixel intensities, for example, a camera for recording an
image.
[0089] Based on this terminology, the simplest optical device can
be considered as featuring a single optical assembly and a single
peripheral structure and/or peripheral mechanism, where the single
optical assembly features a single optical element and a single
peripheral structure or mechanism. An optical part of an optical
device is considered any part or extent of the optical device being
or including at least one optical element. Accordingly, an optical
device features at least one optical part, along with at least one
peripheral structure and/or peripheral mechanism. A peripheral
structure and/or a peripheral mechanism are not considered an
optical part of an optical device by themselves, but may be
included in an optical part of the optical device. Thus, the most
basic optical parts of the simplest optical device are a single
optical assembly, and, in the case of the single optical assembly
featuring a single optical element, the most basic optical part of
the optical device is the single optical element.
[0090] Certain optical elements feature the property of having an
optical axis, where an optical axis refers to an axis of symmetry
of the optical element, whereby the optical element can be freely
rotated with respect to the optical axis without affecting or
changing the optical behavior of light or radiation interacting
with the optical element. This property is referred to as circular
symmetry or rotation invariance, and an optical element featuring
this property is circularly symmetric or rotation invariant.
[0091] An optical element is rotation invariant with respect to its
optical axis, which is its axis of rotational symmetry. This
implies that if the element is rotated, there exists an externally
induced rotation axis, and there is rotation invariance provided
that the optical axis coincides with the rotation axis. Fulfillment
of this condition is shown in FIG. 6, where an entire optical
assembly 100 is rotation invariant or circularly symmetric with
respect to common mechanical axis 110, by virtue of common optical
axis 110 perfectly aligned and coinciding with rotation axis 110.
If this condition is not satisfied during the rotation, there will
be some extent of variance in the optical performance of the
optical element, such that an image passing through it will
slightly vary, and, in the case of using a camera for recording the
image, during the exposure time, the recorded image will become
slightly blurred, depending on how much deviation exists between
the optical axis and the rotation axis. In reality, if an image is
only slightly blurred, there may still be a benefit in rotating an
optical assembly or element, due to the diminished effect of
optical defects, which may outweigh the slight blurring of the
image.
[0092] Two preferred embodiments of the method of the present
invention are herein described. The second preferred embodiment
differs from the first preferred embodiment of the method by
including a step for simultaneously achieving circular symmetry of
the at least one optical part of the optical device by aligning the
optical axis of the at least one optical part of the optical device
with the rotation axis, such that the optical axis of the at least
one optical part is aligned with the rotation axis during rotation
by the optical rotation device, during real time viewing or
projecting by the optical device.
[0093] The first preferred embodiment of the method is for
diminishing effects of optical defects and deviations during real
time viewing or projecting by an optical device of the present
invention, and is herein described. In Step 1, there is providing
an optical rotation device for rotating at least one optical part
of the optical device during real time viewing or projecting by the
optical device. In particular, there is providing an optical
rotation device for rotating the optical device in its entirety,
or, rotating at least one optical assembly of the optical device,
or, rotating at least one optical element of the optical device,
during real time viewing or projecting by the optical device.
[0094] In Step 2, there is rotating the at least one optical part
of the optical device during real time viewing or projecting by the
optical device, by activating and controlling the indicated optical
rotation device, thereby spreading and blurring about the rotation
axis optical defects and deviations present in the at least one
optical part of the optical device. In particular, there is
rotating the optical device in its entirety, or, rotating at least
one optical assembly of the optical device, or, rotating at least
one optical element of the optical device, during real time viewing
or projecting by the optical device, by activating and operating
the indicated optical rotation device.
[0095] In this first embodiment of the method, Step 2 of rotating
at least one optical part of the optical device causes spreading
and blurring, about the rotation axis of the at least one optical
part of the optical device, of optical defects and deviations
present in the at least one optical part of the optical device.
This results in diminishing the effects of optical defects and
deviations during real time viewing or projecting by the rotated at
least one optical part of the optical device, while an image that
is either viewed or projected by the optical device remains
intact.
[0096] As previously discussed and illustrated in FIG. 5, an
optical device, such as a lens assembly, can feature at least one
optical element having its optical axis perfectly aligned and
coinciding with the common mechanical axis of the tube of the lens
assembly, while simultaneously featuring at least one optical
element having its optical axis misaligned with the common
mechanical axis of the tube of the lens assembly. With respect to
the configuration of the entire optical device, the at least one
optical element having its optical axis misaligned with the common
mechanical axis of the optical device causes the entire optical
device to be less than fully or perfectly aligned. Accordingly, a
misaligned optical device is characterized by at least one optical
part of an optical device having an optical axis misaligned or
deviating to a certain extent from the common mechanical axis of
symmetry of a peripheral structure such as a tube of the optical
device.
[0097] In addition to misalignment of at least one optical part of
an optical device with respect to the mechanical axis of the
optical device, mechanical components enabling rotation, such as
mechanical bearings, of a rotation mechanism used for rotating the
at least one optical part of the optical device, always have some
deviation, known as `run-out`, between the rotation axis and the
common mechanical axis of the rotation mechanism. For example,
run-out error could be caused by asymmetry of one or more
mechanical components, such as mechanical bearings, of the rotation
mechanism. This situation is illustrated in FIG. 7, where the
`effective` optical axis 112 of all four lenses in optical assembly
114 is misaligned from common mechanical axis 116 of tube 118, and
is misaligned with respect to rotation axis 120 of mechanical
bearings 122. It is noteworthy that in optical assembly 114, each
of the four lenses is not aligned with respect its own lens cell,
but is aligned with respect to the three other lenses, such that
there exists an effective optical axis 112.
[0098] In order to simultaneously achieve circular symmetry and
diminish the effects of optical defects and deviations during real
time viewing or projecting by rotating at least one optical part of
an optical device according to the above method, there needs to be
a step for aligning the at least one optical part of the optical
device with respect to appropriate components of the rotation
mechanism, such that the optical axis of the at least one optical
part of the optical device is aligned and coincides as best as
possible with the rotation axis during rotation.
[0099] FIG. 8 is a schematic diagram illustrating three dimensional
alignment of optical axes of at least a part of an optical device
with respect to a rotation axis. The rotation axis 124 coincides
with the z-axis. The optical axis 126 is misaligned with respect to
rotation axis 124. In order to align optical axis 126 with rotation
axis 124, optical axis 126 is held by at least two points, 128 and
130, which lie on opposite sides of origin 132 of rotation axis
124. These holding points are moved in space by at least two
respective vectors 134 and 136. Vectors 134 and 136 are shown along
with their respective x-axis and y-axis components. Altogether,
four controls, corresponding to four degrees of freedom, are
ordinarily needed for performing the alignment, two controls for
the x-axis and two controls for the y-axis. In practice, the
magnitude of the x-axis and y-axis vector components are on the
order of microns, since the present art of optics manufacturing is
capable of this level of accuracy. The z-axis separation between
the two points 128 and 130, however, corresponding to the length of
the at least one part of the optical device, is typically on the
order of at least centimeters. Effectively, this means that points
128 and 130 need not be moved along rotation and z-axis 124.
[0100] Therefore, with respect to the method of the present
invention, an additional step is included for aligning the optical
axis of the at least one optical part of the optical device with
the rotation axis of the optical rotation device in order to
achieve circular symmetry of the at least one optical part of the
optical device simultaneous to diminishing effects of optical
defects and deviations of the at least one optical part of the
optical device. Thus, the second preferred embodiment of the method
is for simultaneously achieving circular symmetry and diminishing
effects of optical defects and deviations during real time viewing
or projecting by an optical device of the present invention is
herein described.
[0101] In Step 1, there is providing an optical rotation device for
rotating at least one optical part of the optical device about the
optical axis of the at least one optical part during real time
viewing or projecting by the optical device. In particular, there
is providing an optical rotation device for rotating the optical
device in its entirety about the optical axis of the optical
device, or, rotating at least one optical assembly of the optical
device about the optical axis of the at least one optical assembly,
or, rotating at least one optical element of the optical device
about the optical axis of the at least one optical element, during
real time viewing or projecting by the optical device.
[0102] In Step 2, there is aligning the optical axis of the at
least one optical part of the optical device with the rotation axis
of the optical rotation device, causing the at least one optical
part of the optical device to be circularly symmetric with respect
to the rotation axis, during real time viewing or projecting by the
optical device. In particular, there is aligning the optical axis
of the optical device in its entirety with the rotation axis of the
optical rotation device, or, aligning the optical axis of the at
least one optical assembly with the rotation axis of the optical
rotation device, or, aligning the optical axis of the at least one
optical element with the rotation axis of the optical rotation
device, during real time viewing or projecting by the optical
device.
[0103] In Step 3, there is rotating the at least one optical part
of the optical device during real time viewing or projecting by the
optical device, by activating and controlling the indicated optical
rotation device, thereby circularly symmetrically spreading and
blurring about the rotation axis optical defects and deviations
present in the at least one optical part of the optical device. In
particular, there is rotating the optical device in its entirety,
or, rotating at least one optical assembly of the optical device,
or, rotating at least one optical element of the optical device,
during real time viewing or projecting by the optical device, by
activating and controlling the indicated optical rotation
device.
[0104] In this second embodiment of the method, Step 2 of aligning
the optical axis of the at least one optical part of the optical
device, achieves circular symmetry of the at least one optical part
of the optical device, according to the alignment procedure
described and illustrated in FIG. 8. Combined with Step 3 of
rotating the at least one optical part of the optical device,
causes circularly symmetrical spreading and blurring about the
rotation axis of optical defects and deviations present in the at
least one optical part of the optical device. This results in
additionally diminishing the effects of optical defects and
deviations present in the at least one optical part of the optical
device, compared to rotating without aligning the at least one
optical part of the optical device, during real time viewing or
projecting, while an image that is either viewed or projected by
the optical device remains intact.
[0105] In the second embodiment of the method, Step 2 of aligning
the optical axis of the at least one optical part of the optical
device with the rotation axis of the optical rotation device for
achieving circular symmetry, can be performed either before,
during, or, both before and during, rotation of the at least one
part of the optical device by the optical rotation device, during
real time viewing or projecting by the optical device. Aligning the
optical axis with the rotation axis during rotation gives the
advantage of enabling real time dynamic correction to alignment
errors as they are detected, whereas aligning the optical axis with
the rotation axis before rotation, limits the capability of
correcting alignment errors which may arise during rotation of the
optical part of the optical device.
[0106] In each embodiment of the method, there are two primary
rotation parameters for effecting the step of rotating by an
optical rotation device. The first primary rotation parameter is
rotation mode, of discontinuously or continuously rotating the at
least one part of an optical device by an optical rotation device
during real time viewing or projecting by the optical device. The
second primary rotation parameter is rotation speed or frequency,
of rotating the at least one part of an optical device by an
optical rotation device during real time viewing or projecting by
the optical device.
[0107] Actual extent or degree of simultaneously achieving circular
symmetry and diminishing effects of optical defects and deviations
during real time viewing or projecting by an optical device depend
upon particular ways of implementing the method of the present
invention, especially with regard to activating and controlling a
given optical rotation device according to the rotation parameters
of discontinuous or continuous rotation mode, and rotation speed.
Moreover, level of sophistication and cost of mechanical and
electrical mechanisms and components used for rotating and aligning
the at least one part of the optical device also influence the
final results of achieving circular symmetry and diminishing
effects of optical defects and deviations during viewing or
projecting.
[0108] General steps of rotating the at least one optical part
according to the discontinuous rotation mode include (i) rotating
the at least one optical part of the optical device through a full
circle, or 360 degrees, with a whole number, N, equal to or greater
than two, of stops, at unequally or equally spaced angular
intervals, where at every stop a new image is either viewed or
projected, thus forming N independent viewed or projected images,
(ii) performing an image analysis on each of the N independent
viewed or projected images, thereby generating N independent image
analysis results, and (iii) numerically processing the N
independent image analysis results according to an algorithm such
as averaging, to produce a single combined viewed or projected
image analysis result. As indicated, the discontinuous rotation
mode is applicable to both viewing and projecting optical
devices.
[0109] By having two stops, preferably at 0 degrees and at 180
degrees, the rotated optical part of the optical device
sequentially faces two opposite directions, thereby causing defects
and deviations of the optical part to also face in opposite
directions. The stopping procedure produces opposite effects in two
independently measured and analyzed images, such that averaging of
the analyzed images, to a good approximation, diminishes the
effects of the optical defects and deviations present in the
rotated optical part of the optical device. As the number, N, of
rotational stops increases, such as by sequentially stopping at
every 90 degrees through the circle of rotation, accuracy of
averaging the larger number of independently measured and analyzed
images increases. The actual number of stops is ordinarily set
according to practical considerations with regard to a particular
optical rotation device used for effecting the stopped rotations,
and by the time required for completing sequences of image
measurement, analysis, and numerical processing of a given set of
image analysis data.
[0110] In the discontinuous mode of rotating at least one part of
an optical device during viewing or projecting, the condition of
having perfect alignment of the optical axis of the at least one
part of the optical device with respect to the rotation axis for
achieving circular symmetry of the at least one optical part, can
be relaxed. A viewed or projected image might slightly move within
the field of view or projection, between measurements, because of
slight misalignment. However, this is of no particular concern in
this mode of operation, because every measurement involves a
different image. The optical device pauses before viewing or
projecting each image and there is no blurring due to integration
during the exposure time. The level of accuracy of optical and
rotational alignment in this case has to be such that the target
being focused upon remains in the field of view of a camera during
rotation of the optical part.
[0111] Implementing the first embodiment of the method using a
discontinuous rotation mode, does not offer all the benefits of
full circular symmetry, however, it does offer some benefits, at a
far greater simplicity. Accordingly, there is no requirement for
perfectly aligning optical and rotation axes. For a viewing or
projecting optical device including a camera for recording images,
there is no need to synchronize rotation speed of the rotated at
least one optical part and the exposure time of the camera. In
addition, much less care needs to be taken in assuring the
stability of the at least one optical part of the optical device,
or of the optical rotation device. These milder conditions of
implementing the method, result in application of simpler
mechanical and electrical structures and mechanisms, to be
described and illustrated below for the different preferred
embodiments of the device and system of the present invention.
[0112] A practical example of using the discontinuous rotation mode
for implementing the method of the present invention, is the case
of using an overlay metrology tool for viewing patterns, as
described above, by measuring magnitude and direction of an
overlay, or misregistration, error between two pattern layers. Such
measurements are used for calibrating, testing, and adjusting a
stepper in order to minimize the overlay error. An overlay
metrology tool, however, inherently introduces its own error into
the overlay measurement, accuracy error, or Tool Induced Shift
(TIS), directly arising from distortions and aberrations in the
optics of the overlay metrology tool, resulting in image
distortion. Since an overlay metrology tool is used to monitor and
control a stepper, an overlay metrology tool must produce a total
error significantly less than that of a stepper. This tight margin
of error, especially the TIS component, translates to extremely
strict requirements on the optical quality of an overlay metrology
tool, and therefore presents a strong need for diminishing the
effects of optical defects and deviations during viewing by an
overlay metrology tool.
[0113] The misregistration is a planar vector with x and y
components. To a first approximation, after recording a first
misregistration measurement, if at least one optical part of an
overlay metrology tool is discontinuously rotated, for example, by
180 degrees, followed by recording a second misregistration
measurement, the error vector points to the opposite direction. The
average of the two misregistration measurements cancel out the
error, thus giving the true misregistration or overlay value.
[0114] A better approximation of the true misregistration value is
achieved by discontinuously rotating the at least one optical part
of an overlay metrology tool, 90 degrees at a time, making four
independent measurements, analyzing the four independent
measurements, and then numerically processing, such as by
averaging, the four independent analysis results, to produce a
single combined result of the misregistration value. For an overlay
metrology tool, this rotation mode equally affects the x and y
components of each measurement.
[0115] Regarding the rotation parameter of speed or frequency, in
principle, a particular speed of rotating the at least one optical
part of an optical device is set in relation to the rotation mode
being either discontinuous or continuous rotation. As described
above, during discontinuous rotation, involving stopping the
rotating optical part of the optical device a number of times
during each rotation, for recording that number of independent
images during each rotation, rotation speed is not important, and
preferably is of small magnitude, thereby minimizing
electromechanical requirements of the optical rotation device for
effecting the rotations. In contrast, however, for implementing
either embodiment of the method of the present invention according
to continuous rotation, setting an appropriate rotation speed can
be very relevant to optimizing the results of achieving circular
symmetry and diminishing the effects of optical defects and
deviations of the at least one optical part of the optical
device.
[0116] An optical device used for viewing typically includes an
optional peripheral mechanism such as a detector for detecting
pixel intensities of an image, for example, a camera for recording
a viewed or projected image of the pixel intensities. An optical
device used for projecting typically includes a peripheral
mechanism such as a radiation or light source for enabling
projection of an image onto another object. An important
operational parameter of such optical devices relating to setting
an appropriate rotation speed for the mode of continuous rotation,
is the exposure time, such as the exposure time of a camera used
for viewing an image, or the exposure time of a light source used
for projecting an image, and, in the case of using an electronic
CCD camera, integration time of the CCD. In general, rotation speed
of the rotating at least one optical part of the optical device is
either asynchronous or synchronous with the exposure time of an
appropriate viewing or projecting mechanism. It is noteworthy that
when the at least one optical part of the optical device is rotated
according to a continuous mode, asynchronously or synchronously,
there exists a gyro effect which serves to stabilize the rotating
components, including the at least one optical part of the optical
device, the optical device, and the optical rotation device.
[0117] For rotation speed asynchronous with the exposure time of a
viewing or projecting mechanism, there is a number of rotations
during the exposure time. The number of rotations can correspond to
a fraction of a single rotation, a single rotation, and a plurality
of rotations, where the plurality of rotations may be a non-whole
number. In order to achieve a relatively even spreading and
blurring of optical defects and deviations present in the rotated
at least one optical part of the optical device, it is preferred
that the optical rotation device effect a large number of rotations
during the exposure or integration time.
[0118] For example, for a case where a plurality, but not a whole
number, of rotations are performed, for instance 10.5 rotations are
completed by the optical part of the optical device. The initial 10
rotations cause spreading and blurring of optical defects and
deviations over a full 360 degree circle, and are thus perfect. The
final 0.5 rotation fails to spread and blur the optical defects
over a full circle. However, since the duration of the final 0.5
rotation is only 0.5/10.5 of the entire integration period, its
negative effect is relatively insignificant. This result is in
strong contrast to a case where a single 0.5 rotation is performed
during 100 percent of the integration time. By this analysis, it is
clear that the larger the number of rotations completed during the
exposure or integration time, the need decreases for highly
controlling the exact speed of rotation. Thus, asynchronously
rotating to an extent of 100.5 rotations per exposure or
integration time, is significantly better than 10.5 rotations per
exposure or integration time.
[0119] For rotation speed synchronous with the exposure time of a
viewing or projecting mechanism, a constant angular rotation speed
is used such that an exact whole number of rotations are completed
during the exposure time of recording an image. Synchronization of
the rotation speed with the exposure time causes optical defects
and deviations of the at least one optical part of the optical
device to be perfectly spread and blurred over a full 360 degrees
circle, thereby achieving circular symmetry with respect to optical
defects and deviations of the at least one optical part of the
optical device during real time viewing or projecting. Although any
whole number of rotations can be completed by the optical rotation
device during the exposure time, preferably, one exact rotation is
completed during the exposure time of recording each image, thereby
limiting the speed as much as possible for purposes of stability of
the at least one optical part of the optical device being rotated,
of the optical device, and of the optical rotation device.
[0120] The method of the present invention is applicable to
rotation invariant optical devices as well as to rotation variant
optical devices, where, in a particular rotation variant optical
device, there is present at least one optical part exhibiting
rotation invariance. Inclusion of at least one rotation invariant
optical part of an optical device is relevant with respect to
applying the present invention to optical folding and folded
optical devices. Optical devices used for real time viewing or
projecting can be folded over one or more times, where folding
refers to changing the direction of the path of light, and
therefore also changing the direction of the optical axis. Folding
of an optical device is commonly achieved by using some combination
of mirrors, prisms, and/or beam splitters.
[0121] An example of a folded optical device is a telescopic
periscope, which is actually a folded terrestrial telescope.
Referring to FIG. 9, in (A), a terrestrial telescope 140 features a
plurality of lens assemblies 142, 144, and 146, stacked in a
peripheral structure such as a column, in a configuration such that
the entire optical device is rotation invariant with respect to
optical axis 148. In (B), a telescopic periscope 150, includes two
prisms 152 and 154 which fold the optical axis 156, thereby
breaking the single column configuration of telescope 140. However,
as a result of the presence of rotation variant prisms 152 and 154,
telescopic periscope 150 is rotation variant and as an entirety
cannot be symmetrically rotated about optical axis 156.
Nevertheless, in order to implement the method of the present
invention for diminishing optical defects and deviations which may
be present in an optical device having rotation variant features
like the telescopic periscope, at least one optical part of the
optical device featuring rotation invariance can be rotated about
an appropriate optical axis. In the case of telescopic periscope
150, at least one optical part 158, 160, and 162, can be rotated by
an optical rotation device.
[0122] In addition to the presence of optical defects and
deviations in optical assemblies and/or optical elements, factors
relating to peripheral mechanisms of an optical device or system
can significantly influence overall performance of the optical
device or system. For example, optical devices and systems
employing a radiation source, such as a light source, may be
subject to further non-uniformities introduced by the source. In
general, it is desirable, but practically impossible, to obtain a
source that produces radiation of uniform intensity over the entire
field of view, or field of projection.
[0123] FIG. 10 is a schematic diagram illustrating rotation of a
rotation variant optical element, such as a dove prism, in an
exemplary optical device, a metallurgic microscope, for rotating
light rays of a light source without having to rotate light source,
in order to diminish, by averaging out, non-uniformities present in
the light rays produced by the light source.
[0124] For optical device 170 schematically shown in (A) of FIG.
10, light rays originating from light source 172 pass through an
aperture 174 and a condenser 176, and are folded downwards by means
of a part mirror or a beam splitter 178, from which the light rays
pass through an objective 180 and illuminate a viewed object 182.
In (B), a rotation variant dove prism 184 is placed in the
illumination path. By rotating dove prism 184, the light rays can
be rotated without having to rotate light source 172.
[0125] Such a method based on rotating a rotation variant optical
element replaces the need for rotating a light source as part of an
optical device. Typically, the light source of an optical device
may be bulky and not convenient to rotate. Accordingly, another
embodiment of the method of the present invention is herein
described. This method is for diminishing effects of optical
defects and deviations during real time use of an optical device,
where the optical device includes a light source.
[0126] In Step 1, there is inclusion of at least one rotation
variant optical element in the optical device, such that the light
source generates light rays passing through the at least one
rotation variant optical element.
[0127] In Step 2, there is providing an optical rotation device for
rotating the at least one rotation variant optical element during
real time use of the optical device.
[0128] In Step 3, there is rotating the at least one rotation
variant optical element about a rotation axis during real time use
of the optical device, by activating and controlling the optical
rotation device, thereby spreading and blurring about the rotation
axis any optical defects and deviations present in the light rays
of the light source passing through the at least one rotation
variant optical element.
[0129] In particular, the at least one rotation variant optical
element is a prism, preferably, a dove prism. Moreover, Step 3 of
rotating the at least one rotation variant optical element of the
optical device is effected according to the same two rotation
parameters previously described, rotation mode and rotation speed,
where the rotation mode can be either a discontinuous mode of
rotation or a continuous mode of rotation.
[0130] For achieving even better results, the method can further
include a step for aligning a position of at least one of the at
least one rotation variant optical element with respect to the
rotation axis, such that a high level of uniformity is achieved
among the light rays of the light source, thereby diminishing the
optical defects and deviations present in the light rays of the
light source passing through the at least one rotation variant
optical element. In particular, the step of aligning the position
of each of the at least one rotation variant optical element with
the rotation axis is temporally performed before the rotation,
during the rotation, or, before and during the rotation, of the at
least one rotation variant optical element.
[0131] Optical viewing devices and systems including an optional
peripheral mechanism such as a CCD camera may be subject to further
deviations introduced by the camera. CCD cameras contain both an
optical image sensing element as well as electronic circuitry for
processing images, both of which may introduce some deviations to
recorded images.
[0132] Cameras are rotation variant peripheral mechanisms of an
optical device, and cannot be freely rotated. In fact, cameras
ordinarily have to be kept in as rigid as possible alignment with a
viewed object, in order to keep the image of the object intact
while recording it, to prevent blurring of the image. A method that
can be used to diminish the effects of optical deviations
introduced by a camera into an optical device used for real time
viewing is described in FIGS. 11A-11B.
[0133] In FIG. 11A, in an optical device 190, aside from first
camera 192, at least one additional camera 194 is included, and
positioned such that each of the at least one additional cameras
faces a different direction, preferably spaced at equally spaced
angular intervals. As an example, when one additional camera 194 is
included in optical device 190, additional camera 194 is preferably
positioned in an opposite direction, 180 degrees, with respect to
first camera 192, such that additional camera 194 records an
up-side-down image with respect to first camera 192. If two
additional cameras are used, each is preferably positioned at
120-degree intervals with respect to the first camera. A
corresponding number of additional part mirrors or beam splitters
196 are included in optical device 190, for diverting the image
towards each respective camera. Additional cameras may be used,
each additional camera requiring an additional part mirror or beam
splitter.
[0134] Referring now to FIG. 11B, suppose it is desired to measure
the magnitude and angle, or x and y vector components, similar to
an overlay or misregistration measurement, of some real object 198.
In the case of including one additional camera 194 to the optical
device, object 198 is viewed by two identical cameras, 192 and 194,
with opposite orientations. The first camera 192 records an image
I, and a second camera 194 records image K. The two images are
equally distorted by the error vector 204. However, by negating the
vector K, for example, by negating both its x and y components, and
averaging the result, the result equals the average of I and of the
inverse of K, (I, -K). Technically, this procedure is the same as
separately averaging the x and the y components, whereby the error
cancels out and the original orientation and magnitude are
restored.
[0135] Two preferred embodiments of the optical rotation device
used for implementing the above described embodiments of the method
for simultaneously achieving circular symmetry and diminishing
effects of optical defects and deviations during real time viewing
or projecting by optical devices are herein described. The first
embodiment of the optical rotation device of the present invention
requires manual adjustment to align the optical axis of the at
least one optical part of the optical device with respect to the
rotation axis, whereas the second embodiment of the optical
rotation device includes a plurality of mechanisms for
automatically, highly accurately aligning the optical axis of the
at least one optical part of the optical device with respect to the
rotation axis. The second embodiment of the optical rotation device
enables achieving a very high level of circular symmetry of the at
least one optical part of the optical device during rotation, in
contrast to the first embodiment which enables approaching a
practical or working level of circular symmetry during rotation of
the at least one optical part of the optical device. Either
embodiment of the optical rotation device is successful for
diminishing the effects of optical defects and deviations during
real time use of viewing and projecting optical devices.
[0136] FIG. 12 is a schematic diagram illustrating a first
preferred embodiment of the optical rotation device of the present
invention. In FIG. 12, optical rotation device 210 includes (a) a
column or casing 212, for containing the at least one optical part
of the optical device, (b) a sleeve 214, functioning as a mount for
holding column or casing 212 inside a bearing or other rotation
mechanism, (c) a bearing or other rotation mechanism 216, for
enabling rotation of sleeve 214, column 212, and the at least one
optical part of the optical device, (d) a bearing or rotation
mechanism housing 218, (e) a motor 220, preferably electric, but
could be any type of electromechanical or even battery powered
motor, for actuating rotation of sleeve 214 through a transmission,
(f) a transmission 222, for enabling motor 220 to cause rotation of
sleeve 214, and (g) two sets of adjustment screws 224 and 226, for
adjusting the position of column 212 relative to sleeve 214.
[0137] Each set of adjustment screws 224 and 226, includes four
screws for adjusting the position of column 212 relative to sleeve
214. Two screws are for adjusting column 212 along the x-axis, and
two screws (not shown) are for adjusting column 212 along the
y-axis. For each set of adjustment screws 224 and 226, three or
more, preferably four, screws can be employed for adjusting the
position of column 212 relative to sleeve 214. Moreover, other
means or mechanisms of adjustment can be used instead of, or in
addition to, adjustment screws 224 and 226. The rotation axis of
optical rotation device 210 is established according to the mutual
configuration of sleeve 214 mounted inside rotation mechanism 216,
which in turn is housed by rotation mechanism housing 218.
[0138] FIG. 13 is a schematic diagram illustrating the second
preferred embodiment of the optical rotation device, used for
implementing the method for simultaneously achieving circular
symmetry and diminishing effects of optical defects and deviations
during real time viewing or projecting by optical devices. In FIG.
13, optical rotation device 230 includes (a) a column or casing
232, for containing the at least one optical part of the optical
device, (b) a sleeve 234, functioning as a mount for holding column
or casing 232, which is connected by means of (c) a ring 236, of
metallic flexure, elastic material, or by any other means capable
of providing the slight freedom of movement required to align
column 232 with respect to sleeve 234. Sleeve 234 is mounted in (d)
a main bearing or rotation mechanism 238, shown with (e) a main
bearing or main rotation mechanism housing 240, which forms the
rotation axis of optical rotation device 230.
[0139] Sleeve 234 is rotated by (f) a motor 242, preferably
electrical, but could be any type of electromechanical or even
battery powered motor, for actuating rotation of sleeve 234 through
(g) a transmission 244, for enabling motor 242 to cause rotation of
sleeve 234. Other methods of actuating the rotation of the at least
one optical part of the optical device, such as hydraulic or
pneumatic actuation, can be included in optical rotation device
230. Column 232 is also attached to (h) two self-aligned bearings
or rotation mechanisms 246 and 248, at either side of the main
bearing or main rotation mechanism 238. Rotation mechanisms 246 and
248 are mounted in (i) pre-loaded flexures 250 and 252, both of
which are capable of moving in the x and y directions. Flexures 250
and 252 are actuated by (j) two sets of actuators 254 and 256,
where the actuators are preferably piezo-electric transducers, but
can be any other type of accurate actuator or transducer mechanism
for actuating flexures 250 and 252. Each set of actuators 254 and
256 features two actuator, preferably, transducer, mechanisms. One
set of actuator mechanism is for the x-axis, and one set of
actuator mechanisms is for the y-axis (not shown). Each set of
actuators can feature more than two actuator mechanisms.
[0140] The piezo-electric transducers are devices that change their
length in relation to the electric voltage applied to them. These
devices are capable of moving the optical axis in extremely fine
sub-micron resolutions. This is by far finer than any
production-set tolerance can achieve. In similar manner, this
second embodiment of the optical rotation device of the present
invention offers a much greater degree of accuracy than is
available by using the first preferred embodiment of the optical
rotation device, described and shown in FIG. 12.
[0141] In addition to restoring the circular symmetry, this
embodiment has another advantages over conventional optical
devices. As previously discussed, any optical device is prone to
drifts over time and to change during shipment, which inadvertently
affects its performance. With conventional factory-sealed systems,
such changes might be impossible to control. This embodiment of the
optical rotation device offers the capability to calibrate and
adjust the optical device anytime such an adjustment is
required.
[0142] Two preferred embodiments of the system for implementing the
above described embodiments of the method and optical rotation
device, for simultaneously achieving circular symmetry and
diminishing effects of optical defects and deviations during real
time viewing or projecting by optical devices are herein described.
The first embodiment is of a system applicable to optical devices
used for viewing, and the second embodiment is of a system
applicable to optical devices used for projecting.
[0143] In order to carry out the fine alignment of the optical axis
with the piezo-electric devices, a special computerized control
system is required. FIG. 14 is a schematic diagram illustrating a
preferred embodiment of the system of the present invention, used
for implementing the method and device for simultaneously achieving
circular symmetry and diminishing effects of optical defects and
deviations during real time viewing by optical devices.
[0144] In FIG. 14, system 260 includes an electronic control unit
262. The piezo-electric transducers are activated by electronic
control unit 262. The optical device is positioned to focus on some
pattern 264. An electronic camera 266 is mounted on the optical
device. The electronic image of the camera is captured by a digital
frame grabber 268. Frame grabber 268 is connected to a computer
270. A software program that runs on the computer analyzes the
digital image for sharpness. The same computer and software also
control electronic control unit 262, which actuates the
piezo-electric transducers, thus forming a closed-loop control
system. Control unit 262 changes the positions of the piezo
transducers, until the sharpest image is obtained. Similar criteria
can also be used for changing the positions of the piezo-electric
transducers. Computer 270 also controls the speed of the motor by
means of an electronic motor-control unit 272. The speed of the
motor is synchronized with the exposure time of camera 266.
[0145] In the event that the optical device is used for viewing, it
already has a camera and a frame grabber, without which no
automated viewing can take place. If, on the other hand, the
optical device is used for projection, it might not employ a camera
and a frame grabber, or it might employ a camera and a frame
grabber, but not through the main optics. In this case, the
designer might elect to add these components and to use the optical
device as a viewing optical device just for the purpose of
calibration and alignment. Another option is to place the camera
and frame grabber such that they view the projected image.
[0146] This solution is depicted in FIG. 15, which is a schematic
diagram illustrating another preferred embodiment of the system,
used for implementing the method and device for simultaneously
achieving circular symmetry and diminishing effects of optical
defects and deviations during real time projecting by optical
devices. In system 280 of FIG. 15, a beam-splitter 282 is placed at
the front of the optics such that the projected image 284 is also
viewed by camera 286.
[0147] While the invention has been described in conjunction with
specific embodiments thereof, it is evident that many alternatives,
modifications and variations will be apparent to those skilled in
the art. Accordingly, it is intended to embrace all such
alternatives, modifications and variations that fall within the
spirit and broad scope of the appended claims.
* * * * *