U.S. patent application number 11/694795 was filed with the patent office on 2008-10-02 for adaptive light-path surface tilt sensor for machine vision inspection.
This patent application is currently assigned to MITUTOYO CORPORATION. Invention is credited to Joseph Daniel Tobiason.
Application Number | 20080239298 11/694795 |
Document ID | / |
Family ID | 39627786 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080239298 |
Kind Code |
A1 |
Tobiason; Joseph Daniel |
October 2, 2008 |
ADAPTIVE LIGHT-PATH SURFACE TILT SENSOR FOR MACHINE VISION
INSPECTION
Abstract
An adaptive light-path surface tilt sensing configuration is
provided that identifies when a ray bundle is projected along a
direction normal to a workpiece surface. As a result, the tilt of
the workpiece surface may be determined. The surface tilt sensor
may comprise an illumination and detector portion and an objective
lens. The illumination and detector portion may comprise a light
source, a collimating lens, a beamsplitter, a controllable ray
bundle position control portion, and a photodetector configuration.
These elements are configured to provide a ray bundle alignment
sensing arrangement that provides a signal indicating when a
projected ray bundle and a reflected ray bundle have the best
degree of alignment, in addition to other functions. The best
degree of alignment corresponds to a ray bundle that is projected
along the direction normal to the workpiece surface provides.
Inventors: |
Tobiason; Joseph Daniel;
(Woodinville, WA) |
Correspondence
Address: |
CHRISTENSEN, O'CONNOR, JOHNSON, KINDNESS, PLLC
1420 FIFTH AVENUE, SUITE 2800
SEATTLE
WA
98101-2347
US
|
Assignee: |
MITUTOYO CORPORATION
Kawasaki-shi
JP
|
Family ID: |
39627786 |
Appl. No.: |
11/694795 |
Filed: |
March 30, 2007 |
Current U.S.
Class: |
356/121 |
Current CPC
Class: |
G01B 11/26 20130101 |
Class at
Publication: |
356/121 |
International
Class: |
G01J 1/04 20060101
G01J001/04 |
Claims
1. An adaptive light-path surface tilt sensing configuration that
can be used to identify a ray bundle that is projected along a
direction normal to a point on a workpiece surface, the tilt
sensing configuration comprising: an objective lens; and an
illumination and detector portion including elements comprising a
light source, a collimating lens, a beamsplitter, a ray bundle
position control portion comprising an aperture, and a
photodetector configuration, wherein: a portion of the tilt sensing
configuration forms a ray bundle alignment sensing arrangement; and
the tilt sensing configuration is configured such that: an
illuminating beam from the light source is output via an
arrangement of the collimating lens, the beamsplitter arid the
aperture of the ray bundle position control portion to provide a
collimated projected ray bundle at a projected ray bundle position;
a portion of the objective lens inputs the projected ray bundle and
projects the projected ray bundle such that it is focused at a
point on the workpiece surface along a direction of incidence; the
objective lens inputs a reflected ray bundle from the workpiece
surface and outputs the reflected ray bundle to the ray bundle
alignment sensing arrangement; the ray bundle alignment sensing
arrangement outputs a measurement signal that is usable to
determine a degree of alignment between the projected ray bundle
and the reflected ray bundle, the degree of alignment between the
projected ray bundle and the reflected ray bundle corresponding to
a degree of alignment between the projected ray bundle and a
surface normal at the point on the workpiece surface; when the
measurement signal indicates that a projected ray bundle and a
reflected ray bundle are aligned, then that projected ray bundle is
identified as the ray bundle that is projected along a direction
normal to the point on the workpiece surface, and a corresponding
state of the ray bundle position control portion is indicative of
the workpiece surface tilt direction at the point on the workpiece
surface.
2. The surface tilt sensing configuration of claim 1, wherein the
ray bundle position control portion comprises at least one of a
controllable aperture position adjustment element and a ray bundle
direction control element.
3. The surface tilt sensing configuration of claim 1, wherein the
ray bundle position control portion comprises the controllable
aperture position adjustment element, which comprises one of (a) a
transmissive pixel array, (b) b a transmissive pixel array
including gray level control, (c) c a reflective pixel array, (d) d
a reflective pixel array including gray level control, and (e) e a
in micro-mirror array.
4. The surface tilt sensing configuration of claim 1, wherein the
portion of the tilt sensing configuration that forms the ray bundle
alignment sensing arrangement comprises the ray bundle position
control portion and the photodetector configuration, and the tilt
sensing configuration is further configured such that: the
objective lens inputs the reflected ray bundle from the workpiece
surface and outputs the reflected ray bundle to the ray bundle
position control portion, which spatially filters the reflected ray
bundle using the aperture to provide an amount of spatially
filtered measurement signal light, wherein the better the degree of
alignment between the projected ray bundle and the reflected ray
bundle, the greater the amount of spatially filtered measurement
signal light; and the amount of spatially filtered measurement
signal light is input to the photodetector configuration, which
outputs a measurement signal that has a value corresponding to the
amount of spatially filtered measurement signal light.
5. The surface tilt sensing configuration of claim 4, wherein the
photodetector configuration comprises a wavelength analyzer, and
the photodetector configuration furthermore outputs a measurement
signal that may be used to determine the dominant wavelengths
present in the spatially filtered measurement signal light.
6. The surface tilt sensing configuration of claim 5, wherein the
measurement signal that has a value corresponding to the amount of
spatially filtered measurement signal light and the measurement
signal that may be used to determine the dominant wavelengths
present in the spatially filtered measurement signal light, are the
same measurement signal.
7. The surface tilt sensing configuration of claim 1, wherein: the
ray bundle position control portion comprises a controllable
aperture position adjustment element that controls the position of
the aperture; the photodetector configuration comprises a
two-dimensional photodetector array; the illumination and detector
portion further comprises an alignment mapping that pairs each
position on the two-dimensional photodetector array to a
corresponding aperture position on the aperture position adjustment
element, under the condition that the projected and reflected ray
bundles are aligned for the paired positions; the portion of the
tilt sensing configuration that forms the ray bundle alignment
sensing arrangement comprises the aperture position adjustment
element, the two-dimensional photodetector array, and the alignment
mapping; and the tilt sensing configuration is further configured
such that: the objective lens inputs the reflected ray bundle from
the workpiece surface and outputs the reflected ray bundle to the
two-dimensional photodetector array; and the two-dimensional
photodetector array outputs a measurement signal that indicates a
spot position of the reflected ray bundle on the two-dimensional
photodetector array, wherein when the reflected ray bundle returns
to a spot position on the two-dimensional photodetector array that
maps to a current aperture position on the aperture position
adjustment element according to the alignment mapping, then the
projected ray bundle projected from the current aperture position
is identified as the ray bundle that is projected along a direction
normal to a point on a workpiece surface.
8. The surface tilt sensing configuration of claim 7, wherein: tile
illumination and detector portion further comprises a tilt mapping
that maps each respective aperture position to a corresponding
known surface tilt direction, under the condition that the
projected ray bundle and the reflected ray bundle are aligned when
the projected ray bundle projected from that respective aperture
position is projected to a surface having the corresponding known
tilt direction; and when the measurement signal indicates that a
current projected ray bundle and a reflected ray bundle are
aligned, the surface tilt direction at the point on the workpiece
surface is determined based on the current aperture position and
the tilt mapping.
9. The surface tilt sensing configuration of claim 1, wherein: the
photodetector configuration comprises a two-dimensional
photodetector array; the ray bundle position control portion
includes a ray bundle direction control element having respective
sets of ray bundle direction control components that determine
respective projected ray bundle positions; the aperture is located
between the beamsplitter and the light source; the illumination and
detector portion further comprises an alignment mapping that pairs
each position on the two-dimensional photodetector array to a
corresponding respective set of ray bundle direction control
components, under the condition that the projected and reflected
ray bundles are aligned for the paired positions and respective
sets of ray bundle direction control components; the portion of the
tilt sensing configuration that forms the ray bundle alignment
sensing arrangement comprises the ray bundle direction control
element, the two-dimensional photodetector array, and the alignment
mapping; and the tilt sensing configuration is further configured
such that: the objective lens inputs the reflected ray bundle from
the workpiece surface and outputs the reflected ray bundle to the
ray bundle direction control element and the two-dimensional
photodetector array; and the two-dimensional photodetector array
outputs a measurement signal that indicates a spot position of the
reflected ray bundle on the two-dimensional photodetector array,
wherein when the reflected ray bundle returns to a spot position on
the two-dimensional photodetector array that maps to a current set
of ray bundle direction control components according to the
alignment mapping, then the current projected ray bundle is
identified as the ray bundle that is projected along a direction
normal to a point on a workpiece surface.
10. The surface tilt sensing configuration of claim 9, wherein: the
illumination and detector portion further comprises a tilt mapping
that maps each respective set of ray bundle direction control
components to a corresponding known surface tilt direction, under
the condition that the projected ray bundle and the reflected ray
bundle are aligned when the projected ray bundle projected
according to that respective set of ray bundle direction control
components is projected to a surface having the corresponding known
tilt direction; and when the measurement signal indicates that a
current projected ray bundle and a reflected ray bundle are
aligned, the surface tilt direction at the point on the workpiece
surface is determined based on the current set of ray bundle
direction control components and the tilt mapping.
11. The surface tilt sensing configuration of claim 1, wherein the
surface tilt sensing configuration is integrated with a machine
vision inspection system all(l the objective lens comprises a lens
that is used for acquiring workpiece inspection images in the
machine vision inspection system.
12. A method for identifying that a ray bundle is projected along a
direction normal to a point on a workpiece surface, the method
comprising: providing a surface tilt sensing configuration
comprising an objective lens and an illumination and detector
portion including elements comprising a light source, a collimating
lens, a beamsplitter, a ray bundle position control portion
comprising an aperture, and a photodetector configuration, wherein
a portion of the tilt sensing configuration forms a ray bundle
alignment sensing arrangement; outputting an illuminating beam from
the light source via an arrangement of the collimating lens, the
beamsplitter and the aperture of the ray bundle position control
portion to provide a collimated projected ray bundle at a projected
ray bundle position; inputting the projected ray bundle to a
portion of the objective lens and using the objective lens to
project the projected ray bundle such that it is focused at a point
on the workpiece surface along a direction of incidence; inputting
a reflected ray bundle from the workpiece surface to the objective
lens and outputting the reflected ray bundle from the objective
lens to the ray bundle alignment sensing arrangement; using the ray
bundle alignment sensing arrangement to output a measurement signal
that is usable to determine a degree of alignment between the
projected ray bundle and the reflected ray bundle the degree of
alignment between the projected ray bundle and the reflected ray
bundle corresponding to a degree of alignment between the projected
ray bundle and a surface normal at the point on the workpiece
surface; and when the measurement signal indicates that a projected
ray bundle and a reflected ray bundle are aligned, then identifying
that projected ray bundle as the ray bundle projected along a
direction normal to the point on the workpiece surface, wherein a
corresponding state of the ray bundle position control portion is
indicative of the workpiece surface tilt direction at the point on
the workpiece surface.
13. The method of claim 12, wherein operating the ray bundle
position control portion comprises operating at least one of a
controllable aperture position adjustment element and a ray bundle
direction control element.
14. The method of claim 12, wherein: the portion of the tilt
sensing configuration that forms the ray bundle alignment sensing
arrangement comprises the ray bundle position control portion and
the photodetector configuration; outputting the reflected ray
bundle from the objective lens to the ray bundle alignment sensing
arrangement comprises outputting the reflected ray bundle to the
ray bundle position control portion; and using the ray bundle
alignment sensing arrangement to output a measurement signal that
is usable to determine the degree of alignment between the
projected ray bundle and the reflected ray bundle comprises: using
the aperture of the ray bundle position control portion to
spatially filter the reflected ray bundle to provide an amount of
spatially filtered measurement signal light, wherein the better the
degree of alignment between the projected ray bundle and the
reflected ray bundle, the greater the amount of spatially filtered
measurement signal light, inputting the amount of spatially
filtered measurement signal light to the photodetector
configuration, and outputting a measurement signal from the
photodetector configuration that has a value corresponding to the
amount of spatially filtered measurement signal light.
15. The method of claim 14, wherein the photodetector configuration
comprises a wavelength analyzer, and the method further comprises
outputting a measurement signal from the photodetector
configuration that may be used to determine the dominant
wavelengths present in the spatially filtered measurement signal
light.
16. The method of claim 15, wherein the method further comprises:
projecting a projected ray bundle along a direction normal to a
point on a workpiece surface that includes a material that forms a
thin film on the workpiece surface, and inputting the corresponding
spatially filtered measurement signal light to the photodetector
configuration; outputting a corresponding measurement signal from
the photodetector con figuration that may he used to determine the
dominant wavelengths present in the corresponding spatially
filtered measurement signal light; determining the dominant
wavelengths based on the corresponding measurement signal;
providing an index of refraction for the material that forms the
thin film; and determining the thickness of a thin film on the
workpiece surface based on the provided index of refraction and the
determined dominant wavelengths.
17. The method of claim 12, wherein: the ray bundle position
control portion comprises a controllable aperture position
adjustment element that controls the position of the aperture; the
photodetector configuration comprises a two-dimensional
photodetector array; the illumination and detector portion further
comprises an alignment mapping that pairs each position on the
two-dimensional photodetector array to a corresponding aperture
position on the aperture position adjustment element, under the
condition that the projected and reflected ray bundles are aligned
for the paired positions; the portion of the tilt sensing
configuration that forms the ray bundle alignment sensing
arrangement comprises the aperture position adjustment element, the
two-dimensional photodetector array, and the alignment mapping;
outputting the reflected ray bundle from the objective lens to the
ray bundle alignment sensing arrangement comprises outputting the
reflected ray bundle to the two-dimensional photodetector array;
and using the ray bundle alignment sensing arrangement to output a
measurement signal that is usable to determine the degree of
alignment between the projected ray bundle and the reflected ray
bundle comprises: operating the two-dimensional photodetector array
to output a measurement signal that indicates a spot position of
the reflected ray bundle on the two-dimensional photodetector
array, wherein when the reflected ray bundle returns to a spot
position on the two-dimensional photodetector array that maps to a
current aperture position on the aperture position adjustment
element according to the alignment mapping, then the projected ray
bundle projected from the current aperture position is identified
as the ray bundle projected along a direction normal to a point on
a workpiece surface.
18. The method of claim 17, wherein the illumination and detector
portion further comprises a tilt mapping that maps each respective
aperture position to a corresponding known surface tilt direction,
under the condition that the projected ray bundle and the reflected
ray bundle are aligned when the projected ray bundle projected from
that respective aperture position is projected to a surface having
the corresponding known tilt direction, and the method further
comprises: determining a workpiece surface tilt direction at the
point on the workpiece surface, when the measurement signal
indicates that a current projected ray bundle and a reflected ray
bundle are aligned, based on the current aperture position and the
tilt mapping.
19. The method of claim 12, wherein: the photodetector
configuration comprises a two-dimensional photodetector array; the
ray bundle position control portion includes a ray bundle direction
control element having respective sets of ray bundle direction
control components that determine respective projected ray bundle
positions; the aperture is located between the beamsplitter and the
light source; the illumination and detector portion further
comprises an alignment mapping that pairs each position on the
two-dimensional photodetector array to a corresponding respective
set of ray bundle direction control components, under the condition
that the projected and reflected ray bundles are aligned for the
paired positions and respective sets of ray bundle direction
control components; the portion of the tilt sensing configuration
that forms the ray bundle alignment sensing arrangement comprises
the ray bundle direction control element, the two-dimensional
photodetector array, and the alignment mapping; outputting the
reflected ray bundle from the objective lens to the ray bundle
alignment sensing arrangement comprises outputting the reflected
ray bundle to the ray bundle direction control element and the
two-dimensional photodetector array; and using the ray bundle
alignment sensing arrangement to output a measurement signal that
is usable to determine the degree of alignment between the
projected ray bundle and the reflected ray bundle comprises:
operating the two-dimensional photodetector array to output a
measurement signal that indicates a spot position of the reflected
ray bundle on the two-dimensional photodetector array, wherein when
the reflected ray bundle returns to a spot position on the
two-dimensional photodetector array that maps to a current set of
ray bundle direction control components according to the alignment
mapping, then the projected ray bundle corresponding to the current
set of ray bundle direction control components is identified as the
ray bundle that is projected along a direction normal to a point on
a workpiece surface.
20. The method of claim 19, wherein the illumination and detector
portion further comprises a tilt mapping that maps each respective
set of ray bundle direction control components to a corresponding
known surface tilt direction, under the condition that the
projected ray bundle and the reflected ray bundle are aligned when
the projected ray bundle projected according to that respective set
of ray bundle direction control components is projected to a
surface having the corresponding known surface tilt direction, and
the method further comprises: determining a workpiece surface tilt
direction at the point on the workpiece surface when the
measurement signal indicates that a current projected ray bundle
and a reflected ray bundle are aligned, based on the set of ray
bundle direction control components and the tilt mapping.
21. The method of claim 12, wherein providing the surface tilt
sensing configuration comprises providing the surface tilt sensing
configuration in a form that is integrated with a machine vision
inspection system, such that the objective lens comprises a lens
that is used for acquiring workpiece inspection images in the
machine vision inspection system.
22. The method of claim 12, wherein the method further comprises:
operating the ray bundle position control portion to provide a
plurality of respective aperture positions that output respective
projected ray bundles corresponding to respective reflected ray
bundles and respective measurement signals, including an aperture
position that outputs a projected ray bundle that is projected
along the direction normal to the workpiece surface; and when a
respective measurement signal indicates that a respective projected
ray bundle and a corresponding respective reflected ray bundle are
aligned, then identifying that respective projected ray bundle as
the ray bundle projected along a direction normal to a point on a
workpiece surface.
23. The method of claim 12, wherein the method further comprises:
operating the ray bundle position control portion to provide a
plurality of respective aperture positions that output respective
projected ray bundles projected at corresponding respective
directions of incidence to the workpiece surface in order to
provide corresponding respective measurement signals, including an
aperture position that outputs a projected ray bundle that is
projected along the direction normal to the workpiece surface; and
characterizing a surface finish of the workpiece surface based on
characterizing the values of the corresponding respective
measurement signals as a function of the corresponding respective
directions of incidence.
24. The surface tilt sensing configuration of claim 1, wherein: the
ray bundle position control portion includes a ray bundle direction
control element having respective sets of ray bundle direction
control components that determine respective projected ray bundle
positions; and the aperture is located between the beamsplitter and
the ray bundle direction control element.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to metrology systems, and
more particularly to an adaptive light-path surface tilt sensor
that may be utilized as part of a machine vision inspection
system.
BACKGROUND OF THE INVENTION
[0002] Precision machine vision inspection systems (or "vision
systems" for short) can be used to obtain precise dimensional
measurements of inspected objects and to inspect various other
object characteristics. Such systems may include a computer, a
camera and optical system, and a precision stage that is movable in
multiple directions so as to allow the camera to scan the features
of a workpiece that is being inspected. One exemplary prior art
system that is commercially available is the QUICK VISION.RTM.
series of PC-based vision systems and QVPAK.RTM. software available
from Mitutoyo America Corporation (MAC), located in Aurora, Ill.
The features and operation of the QUICK VISION.RTM. series of
vision systems and the QVPAK.RTM. software are generally described,
for example, in the QVPAK 3D CNC Vision Measuring Machine User's
Guide, published January 2003, and the QVPAK 3D CNC Vision
Measuring Machine Operation Guide, published September 1996, each
of which is hereby incorporated by reference in their entirety.
Such systems are known to incorporate various types of surface
height measurement sensors, either as built in features or as
accessories. However, efficient surface tilt measurement systems
that may be incorporated in such systems are not generally
known.
SUMMARY OF THE INVENTION
[0003] This summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This summary is not intended to identify
key features of the claimed subject matter, nor is it intended to
be used as an aid in determining the scope of the claimed subject
matter.
[0004] The present invention is directed toward an adaptive
light-path surface tilt sensing configuration or sensor that can be
used to identify when a ray bundle is projected along a direction
normal to a point on a workpiece surface. As a result, the tilt of
the workpiece surface may be determined. Other properties of the
surface may then be determined more reliably, if desired. Such a
sensor is of particular utility in a general purpose machine vision
inspection system for performing precision dimensional metrology
and other types of inspection.
[0005] In accordance with one aspect of the invention, the
essential elements of the adaptive light-path surface tilt sensor
may comprise an illumination and detector portion and an objective
lens. The illumination and detector portion may comprise a light
source, a collimating lens, a beamsplitter, a ray bundle position
control portion, and a photodetector configuration. The elements of
the illumination and detector portion are configured to provide a
ray bundle alignment sensing arrangement, in addition to other
functions. In various embodiments the ray illumination and detector
portion is configured to output an illuminating beam from the light
source through an arrangement of the beamsplitter, the ray bundle
position control portion, and the collimating lens, to provide a
collimated projected ray bundle at a projected ray bundle position.
In various respective embodiments, the beamsplitter, the ray bundle
position control portion, and the collimating lens may be arranged
in various respective orders along a beam path, as disclosed in
greater detail below. The projected ray bundle is input to the
objective lens, which focuses the projected ray bundle onto a
workpiece surface along a direction of incidence. A reflected ray
bundle from the workpiece surface is input by the objective lens
and transmitted to the ray bundle alignment sensing arrangement,
which outputs a measurement signal that is usable to determine the
degree of alignment between the projected ray bundle and the
reflected ray bundle. The ray bundle position control portion may
be controlled to provide a plurality of respective projected ray
bundles corresponding to respective measurement signals, including
a projected ray bundle that is projected along the direction normal
to the workpiece surface. A ray bundle that is projected along the
direction normal to the workpiece surface corresponds to a
respective measurement signal indicating the best degree of
alignment. In various embodiments, the ray bundle position control
portion may comprise a spatial light modulator. In various
embodiments, the spatial light modulator may comprise one of a
transmissive pixel array and a reflective micro-mirror array.
[0006] According to a further aspect of the invention, in one
embodiment, the ray bundle alignment sensing arrangement may
comprise the ray bundle position control portion and a
photodetector configuration, wherein the reflected ray bundle is
transmitted to the ray bundle position control portion, which
spatially filters the reflected ray bundle using an aperture, to
provide an amount of spatially filtered measurement signal light.
The amount of spatially filtered measurement signal light is input
to a photodetector configuration that outputs a measurement signal
that has a value corresponding to the amount of spatially filtered
measurement signal light. In such an embodiment, when the reflected
ray bundle returns along the path of the projected ray bundle it
may fully coincide with the aperture and the measurement signal
value may be maximized, corresponding to the ray bundle that is
projected along the direction normal to the workpiece surface.
[0007] According to a further aspect of the invention, in one
embodiment, the ray bundle alignment sensing arrangement may
comprise a two dimensional (2D) photodetector array, wherein the
reflected ray bundle is transmitted to the (2D) photodetector
array, which outputs a measurement signal that indicates a spot
position of the reflected ray bundle on the 2D array. An alignment
calibration or mapping is provided that pairs each 2D photodetector
spot position to a corresponding configuration characteristic of
the ray bundle position control portion (e.g. an 2D aperture
position provided by the ray bundle position control portion or a
set of control signal components used to provide a projected ray
bundle position), under the condition that the projected and
reflected ray bundles are normal to the workpiece surface for the
paired positions and configuration characteristics. The alignment
mapping may be determined based a calibration surface and
associated calibration procedure, for example. In such an
embodiment, when the reflected ray bundle returns to a spot
position on the 2D array that maps to the current configuration
characteristic of the ray bundle position control portion, this
condition indicates that the current ray bundle is projected along
the direction normal to the workpiece surface.
[0008] According to a further aspect of the invention, each
respective configuration characteristic of the ray bundle position
control portion may be mapped using a tilt mapping such that it
corresponds to a known respective surface tilt direction, under the
condition that the projected ray bundle and the reflected ray
bundle are aligned when a projected ray bundle projected according
to that respective configuration characteristic of the ray bundle
position control portion is projected to a surface having the
corresponding known tilt direction.
[0009] In accordance with another aspect of the invention, in
various embodiments, the illumination and detector portion of the
adaptive light-path surface tilt sensor includes a wavelength
analyzer that may determine the dominant wavelengths present in
measurement signal light that arises from a reflected ray bundle.
In one embodiment, a projected ray bundle at a normal incidence to
the workpiece surface is used to provide the reflected ray bundle
that provides the measurement signal light. The dominant
wavelengths and/or their relationship to one another may then be
used to determine the thickness of a thin film on the workpiece
surface, given a known index of refraction for the thin film.
[0010] In accordance with another aspect of the invention, in
various embodiments, the adaptive light-path surface tilt sensor
may be used to characterize the roughness of the workpiece surface.
The adaptive light-path surface tilt sensor is used to measure the
relative strength of a plurality of respective reflected ray
bundles corresponding to a plurality of respective directions of
incidence on the workpiece surface. The relative strengths of the
respective reflected ray bundles, as a function of their respective
directions of incidence, characterizes the surface roughness.
[0011] In accordance with another aspect of the invention, a method
is provided for identifying a ray bundle that is projected along a
direction normal to a workpiece surface in a machine vision
inspection system. In various embodiments, the method may comprise:
outputting an illuminating beam from a light source through an
arrangement of a beamsplitter, a ray bundle position control
portion, and a collimating lens, to provide a collimated projected
ray bundle at a projected ray bundle position; inputting the
collimated projected ray bundle to a portion of an objective lens;
projecting the projected ray bundle from the objective lens such
that it is focused on a workpiece surface along a direction of
incidence; inputting a reflected ray bundle from the workpiece
surface to the objective lens, and outputting the reflected ray
bundle to a ray bundle alignment sensing arrangement; using the ray
bundle alignment sensing arrangement to provide a measurement
signal that is usable to determine the degree of alignment between
the projected ray bundle and the reflected ray bundle; and
identifying a ray bundle corresponding to the respective
measurement signal that indicates the best degree of alignment as
the ray bundle that is projected along the direction normal to the
workpiece surface.
[0012] According to a further aspect of the invention, the
objective lens may comprise a lens that is used for acquiring
workpiece inspection images in a machine vision inspection system.
However, the adaptive light-path surface tilt sensor may also be
used as a stand alone surface tilt sensing system.
[0013] According to a further aspect of the invention, in various
exemplary embodiments the objective lens may comprise a
conventional lens, a liquid immersion lens, or a solid immersion
lens.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0015] FIG. 1 is a diagram of a first embodiment of an adaptive
light-path surface tilt sensor which may provide a ray bundle
projected along a direction normal to a workpiece surface,
including a first embodiment of an illumination and detector
portion;
[0016] FIG. 2 is a diagram of a second embodiment of an
illumination and detector portion of an adaptive light-path surface
tilt sensor which may provide a ray bundle projected along a
direction normal to workpiece surface;
[0017] FIG. 3 is a diagram of a third embodiment of an illumination
and detector portion of an adaptive light-path surface tilt sensor
which may provide a ray bundle projected along a direction normal
to workpiece surface;
[0018] FIG. 4 is a diagram of a fourth embodiment of an
illumination and detector portion of an adaptive light-path surface
tilt sensor which may provide a ray bundle projected along a
direction normal to workpiece surface;
[0019] FIG. 5 is a diagram of a fifth embodiment of an illumination
and detector portion of an adaptive light-path surface tilt sensor
which may provide a ray bundle projected along a direction normal
to workpiece surface;
[0020] FIG. 6 is a diagram of a sixth embodiment of an illumination
and detector portion of an adaptive light-path surface tilt sensor
which may provide a ray bundle projected along a direction normal
to workpiece surface; and
[0021] FIG. 7 is a flow diagram illustrating one exemplary
embodiment of a routine for identifying a ray bundle that is
projected along a direction normal to a workpiece surface according
to this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0022] FIG. 1 is a diagram of a first embodiment of an adaptive
light-path surface tilt sensing configuration 100 that may provide
a ray bundle projected along a direction normal to a workpiece
surface in accordance with the present invention. The essential
elements of the adaptive light-path surface tilt sensing
configuration 100 are an illumination and detector portion 110 and
an objective lens 170. As shown in FIG. 1, the illumination and
detector portion 110 may comprise a light source 112, a collimating
lens 114, a beamsplitter 116, a ray bundle position control portion
118 and a photodetector configuration 130. In the embodiment shown
in FIG. 1, the adaptive light-path surface tilt sensor 100 is
integrated into the imaging portion of machine vision inspection
machine, which provides a camera 150, a beamsplitter 160, and the
objective lens 170, as shown. The camera 150 and the beamsplitter
160 are not required in various embodiments that may provide a
stand-alone adaptive light-path surface tilt sensor according to
this invention.
[0023] In operation, the light source 112 outputs an illuminating
beam 113, which is at least approximately collimated by the
collimating lens 114 to provide a collimated illuminating beam
113', which may be used to provide various rays bundles that are
output from the illumination and detector portion 110 at ray bundle
positions that fall within outer ray limits 119A and 119B, as
described below. The collimated illuminating beam 113' is
transmitted through the beam splitter 116 and is input to the ray
bundle position control portion 118. In the embodiment shown in
FIG. 1, the ray bundle position control portion 118 may be
characterized as a controllable aperture position adjustment
element (e.g. an addressable spatial light modulator) that is
controlled to provide an aperture 120 that outputs or projects a
projected ray bundle 115 comprising a portion of light from the
collimated illuminating beam 113'. It should be appreciated that
the position of the aperture 120 may be controlled to vary along x'
and y' directions of the ray bundle position control portion 118,
to (x',y') positions other than the one illustrated in FIG. 1, in
order to control a projection position (x'',y'') of the projected
ray bundle 115 along the x'' and y'' directions within outer ray
limits 119A and 119B, as illustrated in FIG. 1. In the particular
embodiment shown in FIG. 1, the projected ray bundle 115 is then
reflected by the beamsplitting surface 160 and input to the
objective lens 170, which focuses it along a direction of incidence
to a workpiece surface 180. The workpiece surface 180 is tilted at
a tilt angle .theta., which may generally be characterized by two
tilt angle components (.theta.x in the x-z plane relative to the
x-axis and .theta.y in the y-z plane relative to the y-axis), at
the point of reflection, (e.g. the focus location) on the workpiece
surface 180. A reflected ray bundle 145 is reflected back from the
workpiece surface along a path that depends on the angle .theta.,
and then through the objective lens 170 to the beamsplitter surface
160, where it is reflected back to a ray bundle alignment sensing
arrangement of the illumination and detector portion 110. A portion
145' of the reflected ray bundle 145 may also be transmitted to the
camera 150, in some embodiments.
[0024] In the embodiment shown in FIG. 1, the ray bundle alignment
sensing arrangement of the illumination and detector portion 110
comprises the ray bundle position control portion 118 and the
photodetector configuration 130. In particular, the ray bundle
position control portion 118 spatially filters the reflected ray
bundle 145 using the aperture 120, which may transmit a spatially
filtered measurement signal light 117 if the reflected ray bundle
145 is at least partially aligned with the aperture 120. In such a
case, the spatially filtered measurement signal light 117 may be
reflected by the beamsplitter 116 and received by the photodetector
configuration 130, as illustrated. In the embodiment shown in FIG.
1, the photodetector configuration 130 comprises a focusing lens
132 and a photodetector 136. The photodetector 136 may output a
measurement signal that corresponds to the amount of spatially
filtered measurement signal light 117 it receives.
[0025] The amount of light received by the photodetector 136
depends upon how much spatially filtered measurement signal light
117 is transmitted by the aperture 120. If the reflected ray bundle
145 does not travel back along a path that at least partially
aligned with or overlapping the path of the projected ray bundle
115 at the aperture 120, it will be blocked by the ray bundle
position control portion 118 and the corresponding measurement
signal will have a minimum value. If the reflected ray bundle 145
is partially aligned with or overlapping the path of the projected
ray bundle 115 at the aperture 120, the measurement signal will be
greater than the minimum value, and indicative of the degree of
alignment or overlap. For the particular tilt angle .theta. and the
particular position of the aperture 120 illustrated in FIG. 1, the
projected ray bundle 115 is incident along the direction normal to
the workpiece surface 180. Therefore, the reflected ray bundle 145
is fully aligned with the projected ray bundle 115 and retraces its
path to approximately coincide with the aperture 120. Under this
condition, the amount of spatially filtered signal light 117 is
maximized, and the photodetector 136 outputs the maximum
measurement signal that may be obtained for the current arrangement
of the workpiece surface 180. Conversely, for the embodiment shown
in FIG. 1, the maximum measurement signal corresponds to the ray
bundle that is projected along the direction that is normal to the
workpiece surface 180.
[0026] As previously indicated, the ray bundle position control
portion 118 may be controlled to vary the position of the aperture
120, e.g. along x' and y' axes of the ray bundle position control
portion 118, in order to control a projection position (x'',y'') of
the projected ray bundle 115. In general, when the projected ray
bundle 115 is normal to the workpiece surface 180 such that the
reflected ray bundle 145 approximately coincides with the aperture
120, that aperture position along the direction of the x' axis
corresponds to the .theta.x tilt angle component, and that aperture
position along the direction of the y' axis corresponds to the
.theta.y tilt angle component. Accordingly, in various embodiments,
each respective aperture position may be mapped in a tilt mapping
such that it corresponds to a known respective surface tilt
direction, under the condition that the measurement signal of the
alignment sensing arrangement for that respective aperture position
indicates the best degree of alignment when a projected ray bundle
projected from that respective aperture position is projected to a
surface having the corresponding known tilt direction. The tilt
mapping may be stored in an associated signal processing and
control circuit of the illumination and detector portion 110, or a
host computer, or the like.
[0027] As previously indicated, in the embodiment shown in FIG. 1,
the ray bundle position control portion 118 may be characterized as
a controllable aperture position adjustment element. Any now-known
or later-developed type of controllable aperture position
adjustment element that can provide controllable light outputting
and light blocking functions in a desired pattern with a variable
position (e.g. by controllable transmission or reflection) may be
referred to as a spatial light modulator (SLM). In various
embodiments, the ray bundle position control portion 118 may
comprise a transmissive-type SLM. The SLM may comprise a light
transmitting and blocking element and associated control
electronics. In some embodiments, the control electronics may be
connected to a host computer (e.g. the host computer of a machine
vision inspection system) according to known techniques, and the
various surface tilt sensing methods disclosed herein may be
implemented in an interactive and/or automatically executed control
routine. In one embodiment, the transmissive-type SLM may comprise
an LCD pixel array that may generally be controlled by conventional
video signals, if desired, and that may provide an electronically
generated 8-bit gray-scale pattern that may transmit,
partially-transmit, or block light through any given pixel of the
array, depending on its gray-scale value. In one specific
embodiment, a micro-display graphics array and the associated
control electronics, available from CRL-Opto in Dunfermline,
Scotland, United Kingdom may be used as the ray bundle position
control portion 118.
[0028] In one embodiment, a SLM with controllable gray-scale is
used to provide a partially transmissive aperture 120, rather than
fully transmissive aperture 120, in order to prevent highly
reflective workpiece surfaces from saturating the photodetector
that is used to provide the measuring signal. A SLM with
controllable gray-scale may also be used to provide apodization,
for example the pixel gray scale levels in the aperture 120 may be
chosen to provide a Gaussian intensity profile, to limit
diffraction effects from the aperture 120 in the projected ray
bundle 115, or for other purposes.
[0029] Regarding one aspect of operating the adaptive light-path
surface tilt sensing configuration 100, as previously indicated, if
the reflected ray bundle 145 does not travel back along a path that
is at least partially aligned with or overlapping the path of the
projected ray bundle 115 at the aperture 120, it will be blocked by
the ray bundle position control portion 118 and no significant
measurement signal will result. Accordingly, a search routine may
generally be used to find an aperture position that provides a
projected ray bundle 115 that is incident along the direction
normal to the workpiece surface 180, such that the reflected ray
bundle 145 retraces its path to approximately coincide with the
aperture 120. In general, such a search routine may comprise
controlling the ray bundle position control portion 118 to provide
a plurality of respective aperture positions (x',y') and projected
ray bundle positions (x'',y'') that output respective projected ray
bundles 115 corresponding to respective measurement signals,
including an aperture position that outputs a ray bundle that is
projected along the direction normal to the workpiece surface. The
ray bundle that is projected along the direction normal to the
workpiece may be identified as the ray bundle corresponding to the
respective measurement signal that indicates the best degree of
alignment (e.g. for the embodiment shown in FIG. 1, the maximum
respective measurement signal). In one exemplary search routine,
the measurement signal may be sampled and stored while the ray
bundle position control portion 118 is controlled to provide each
possible aperture position for the aperture 120. The resulting
measurements are analyzed to identify the maximum signal and the
corresponding aperture position. In another exemplary search
routine, the size of the aperture 120 may be varied as well as it
position, to provide a coarse-to-fine search capability. For
example, an initial coarse aperture may be stepped in relatively
coarse steps to project a limited number of ray bundles that
together include all possible ray bundles. The corresponding
measurement signals may be analyzed to identify the maximum signal
and the corresponding coarse aperture position. A finer aperture
search may then be executed as outlined above, corresponding to the
coarse aperture area at the "maximum signal position", and so on,
until a finest desired aperture size has been used to identify an
aperture position that projects a ray bundle along the direction
normal to the workpiece surface 180. Of course, the foregoing
search procedures are exemplary only, and not limiting. Other, more
efficient, search procedures are possible. It should be appreciated
that an SLM (for example) and detector can be configured to alter
the aperture position and measure the resulting signal at a high
rate, e.g. 10 KHz, provided that the light source 112 is selected
and/or controlled to provide sufficient intensity in the operative
ray bundles 115 and 145. Therefore, the foregoing and/or other
search approaches may be executed at high speed in various
embodiments.
[0030] In some embodiments, the light source 112 may be a light
source having a controllable wavelength and/or polarization. The
wavelength and/or polarization may then be selected or varied to
provide the best measurement signal, depending on the
characteristics of the workpiece surface 180. In one example, a
wavelength may be adjusted to discriminate between the surfaces or
boundaries of various thin layers of material on a workpiece. In
another example, a polarization may be adjusted to at least
partially overcome measurement anomalies to a directional texture
or the like, on the workpiece surface 180.
[0031] FIG. 2 is a diagram of a second embodiment of an
illumination and detector portion 210, that may be used in an
adaptive light-path surface tilt sensing configuration according to
this invention (e.g. in place of the illumination and detector
portion 110 in the tilt sensing configuration 100). In operation,
the projected ray bundle 115 and the reflected ray bundle 145 may
be functionally equivalent to those previously described with
reference to FIG. 1. Several of the elements of the illumination
and detector portion 210 are also similar to those of the
illumination and detector portion 110 of FIG. 1, and similarly
numbered components may be similar or identical, except as
otherwise described below.
[0032] A first difference between the illumination and detector
portion 210 and the illumination and detector portion 110 is that
the ray bundle position control portion 118 is located in the
collimated illuminating beam 113' ahead of the beamsplitter 116,
such that only the projected ray bundle 115 is transmitted through
the beamsplitter 116. As a result, the reflected ray bundle 145
cannot be spatially filtered by the ray bundle position control
portion 118, before it is reflected at the beamsplitter 116 to
provide the measurement signal light 117 that is received by the
photodetector configuration 130. Therefore, a second difference is
that the illumination and detector portion 210 provides a different
type of ray bundle alignment sensing arrangement than the
illumination and detector portion 110, comprising the ray bundle
position control portion 118, a photodetector portion 230 that
comprises a 2D photodetector array 236, and an alignment mapping
described in greater detail below. The ray bundle alignment sensing
arrangement of the illumination and detector portion 210 may
operate at follows. The measurement signal light 117 that arises
from the reflected ray bundle 145 is received at a spot position
(x''', y''') on the 2D photodetector array 236, which outputs a
measurement signal that indicates the spot position (e.g. by
determining the x''' and y''' coordinates of the centroid of the
spot intensity distribution, for example). A predetermined
alignment calibration or mapping is provided for the illumination
and detector portion 210 (e.g. stored in an associated signal
processing and control circuit of the illumination and detector
portion 210 or a host computer) that pairs each 2D photodetector
spot position to a corresponding 2D aperture position, under the
condition that the projected and reflected ray bundles are normal
to the workpiece surface for the paired positions. The alignment
mapping may be determined based on a calibration surface and
associated calibration procedure, for example. In such an
embodiment, when a current measurement signal light 117 arising
from the reflected ray bundle 145 returns to a spot position (x''',
y''') on the 2D array 236 that maps to a current aperture position
(x', y') of the aperture 120 on the ray bundle position control
portion 118, this condition indicates that the projected ray bundle
115 and the reflected ray bundle 145 are aligned, and that
corresponding current projected ray bundle is projected along the
direction normal to the workpiece surface. When a current spot
position (x''', y''') approaches the alignment mapped position that
corresponds to the current aperture position (x', y'), the
projected ray bundle 115 and the reflected ray bundle 145 may be
approaching alignment. Thus, while previously outlined search
routines may be adapted for use with the illumination and detector
portion 210, in one embodiment of a search routine that is usable
in conjunction with the illumination and detector portion 210 the
relationship between the current spot position coordinates (x''',
y''') and the current aperture position coordinates (x', y') may be
analyzed to estimate a new aperture position that will improve the
alignment, thus speeding up the search.
[0033] FIG. 3 is a diagram of a third embodiment of an illumination
and detector portion 310, that may be used in an adaptive
light-path surface tilt sensing configuration according to this
invention (e.g. in place of the illumination and detector portion
110 in the tilt sensing configuration 100). In operation, the
projected ray bundle 115 and the reflected ray bundle 145 may be
functionally equivalent to those previously described with
reference to FIG. 1. Several of the elements of the illumination
and detector portion 310 are also similar to those of the
illumination and detector portion 110 of FIG. 1, and similarly
numbered components may be similar or identical, except as
otherwise described below.
[0034] The primary difference between the illumination and detector
portion 310 and the illumination and detector portion 110 is that a
ray bundle position control portion 318 may be characterized as a
reflective-type of aperture position adjustment element, whereas
the ray bundle position control portion 118 was a transmissive-type
aperture position adjustment element. Accordingly, an aperture 320
outputs the projected ray bundle 115 by reflection rather than by
direct transmission, and the illumination and detector portion 310
requires a slightly different layout. Otherwise, its operation is
analogous to that of the illumination and detector portion 110,
including the operation of its ray bundle alignment sensing
arrangement, wherein the aperture 320 and the photodetector
configuration 130 operate in substantially the same manner as
described for the aperture 120 and the photodetector configuration
130 of FIG. 1.
[0035] In various embodiments, the ray bundle position control
portion 318 may comprise a reflective-type SLM. The SLM may
comprise an array of light reflecting and blocking (or deflecting)
elements (e.g. a micro-mirror array) and associated control
electronics. In one specific embodiment the ray bundle position
control portion 318 may comprise a reflective pixel array including
gray level control, such as one of the liquid crystal on silicon
(LCOS) micro-display products, available from CRL-Opto in
Dunfermline, Scotland. In another specific embodiment, it may
comprise a digital light projector (DLP) micro-mirror product,
available from Texas Instruments DLP Products, Plano, Tex.
[0036] FIG. 4 is a diagram of a fourth embodiment of an
illumination and detector portion 410, that may be used in an
adaptive light-path surface tilt sensing configuration according to
this invention (e.g. in place of the illumination and detector
portion 110 in the tilt sensing configuration 100). In operation,
the projected ray bundle 115 and the reflected ray bundle 145 may
be functionally equivalent to those previously described with
reference to FIG. 1. Except for the photodetector configuration
430, the elements of the illumination and detector portion 410 are
similar to those of the illumination and detector portion 110 of
FIG. 1, and similarly numbered components may be similar or
identical, except as otherwise described below.
[0037] The primary difference between the illumination and detector
portion 410 and the illumination and detector portion 110 is that a
photodetector configuration 430 comprises a wavelength analyzer 438
(e.g. a spectrometer or spectrophotometer). With the exception of
the additional features offered by the wavelength analyzer 438, the
operation of the illumination and detector portion 410 is analogous
to that of the illumination and detector portion 110, including the
operation of its ray bundle alignment sensing arrangement, wherein
the aperture 120 and a measurement signal from a photodetector of
the photodetector configuration 430 operate in substantially the
same manner as described for the aperture 120 and the photodetector
configuration 130 of FIG. 1. For example, in one embodiment, for
the purposes of ray bundle alignment sensing, a measurement signal
indicative of the total amount of measuring signal light 117 may be
determined by integrating the intensity information obtained for
all wavelengths detected by a linear photodetector array of the
wavelength analyzer 438. In another embodiment, the photodetector
configuration 430 may include a beamsplitter and a separate
photodetector arrangement, in addition to the wavelength analyzer
438. The beam splitter may route a portion of the measuring signal
light 117 to the separate photodetector, which may provide the
measurement signal that is used for ray bundle alignment
sensing.
[0038] The wavelength analyzer 438 may determine the dominant
wavelengths present in the measuring signal light 117 (and their
relative intensities, in some embodiments). In one exemplary
method, a projected ray bundle 115 is optimized to be at normal
incidence to the workpiece surface and is then used to provide the
reflected ray bundle 145 that provides the measurement signal light
117. The dominant wavelengths and/or their relationship to one
another may then be used to determine the thickness of a thin film
on the workpiece surface 180, given a known index of refraction for
the thin film, according to known techniques. It will be
appreciated that the wavelength analyzer 428 must be designed to
input the measurement signal light 117 from any operative
reflection location on the beam splitting surface of the
beamsplitter 116. Thus, the wavelength analyzer 428 may generally
comprise a known configuration of lenses and/or curved mirrors, or
the like, in order to direct all operative paths of the measurement
signal light 117 to an entrance slit of the wavelength analyzer
438.
[0039] FIG. 5 is a diagram of a fifth embodiment of an illumination
and detector portion 510 that may be used in an adaptive light-path
surface tilt sensing configuration according to this invention
(e.g. in place of the illumination and detector portion 110 in the
tilt sensing configuration 100). In operation, the projected ray
bundle 115 and the reflected ray bundle 145 may be functionally
equivalent to those previously described with reference to FIG. 1.
Several of the elements of the illumination and detector portion
510 are also similar to those of the illumination and detector
portion 110 of FIG. 1, and similarly numbered components may be
similar or identical, except as otherwise described below.
[0040] The main difference between the illumination and detector
portion 510 and the illumination and detector portion 110 is the
arrangement of the beam splitter 116, the ray bundle position
control portion 118, and a collimation lens 614. In operation, the
light source 112 outputs a diverging illuminating beam 113 through
the beam splitter 116 to the ray bundle position control portion
118, where it is spatially filtered through the aperture 120, and a
diverging ray bundle is transmitted to the collimation lens 614.
The collimation lens 614 then projects the collimated projected ray
bundle 115 with a projected ray bundle position (x'',y''). In
general, the reflected ray bundle 145 returns to the collimating
lens 614, which transmits converging rays toward the ray bundle
position control portion 118. The ray bundle position control
portion 118 spatially filters the reflected ray bundle 145 using
the aperture 120, and may transmit converging spatially filtered
measurement signal light 517 if the reflected ray bundle 145 is at
least partially aligned with the aperture 120. In the embodiment
shown in FIG. 5, the converging spatially filtered measurement
signal light 517 is reflected from the beam splitter 116 and
converges to a focus or near focus at the photodetector 536 of the
photodetector configuration 530. The illumination and detector
portion 510 may otherwise operate in a manner similar to that
described for the illumination and detector portion 110 of FIG. 1.
It will be appreciated that in the embodiment shown in FIG. 5, the
ray bundle alignment sensing arrangement comprises the ray bundle
position control portion 118 and the photodetector configuration
530.
[0041] FIG. 6 is a diagram of a sixth embodiment of an illumination
and detector portion 610 that may be used in an adaptive light-path
surface tilt sensing configuration according to this invention
(e.g. in place of the illumination and detector portion 110 in the
tilt sensing configuration 100). In operation, the projected ray
bundle 115 and the reflected ray bundle 145 may be functionally
equivalent to those previously described with reference to FIG. 1.
The general operating principles of the illumination and detector
portion 610 are similar to those previously described for the
illumination and detector portions 110-510. Therefore, only certain
differences and features of the illumination and detector portion
610 are described below.
[0042] In operation, the light source 112 outputs a diverging
illuminating beam 113 through a converging lens 113, to be
reflected by the beam splitter 116 to the ray bundle position
control portion 618. In the embodiment shown in FIG. 6, the ray
bundle position control portion 618 comprises an aperture 618A and
a ray bundle direction control element 618B (e.g. a controllable
two-axis galvanometer mirror, or an adaptive optical element such
as a deformable mirror, or the like). The aperture 618A receives
the converging illuminating beam from the beam splitter 116 and may
control the size of the intermediate ray bundle 115'. In one
embodiment the aperture 618A may comprise a fixed aperture. In
another embodiment, the aperture 618A may comprise a SLM that can
provide at least some of the same features and benefits previously
outlined with reference to the use of SLMs, for example aperture
size control and/or apodization may be provided.
[0043] The ray bundle direction control element 618B is controlled
to deflect the intermediate ray bundle 115' to a selected direction
such that it emerges from the collimating lens 614 as a collimated
projected ray bundle 115, with a projected ray bundle position
(x'',y''). Accordingly, it will be understood that "x-y" control
signal components of the ray bundle direction control element 618B
are analogous to the (x',y') aperture position coordinates outlined
with reference to FIG. 1, and may be therefore be mapped to
corresponding workpiece surface tilt directions, and otherwise used
in an analogous manner.
[0044] In general, the reflected ray bundle 145 returns to the
collimating lens 614, which transmits converging rays to focus
approximately at the ray bundle direction control element 618B. The
ray bundle direction control element 618B does not move during a
measurement related to a particular projected ray bundle 115, and
deflects the converging reflected ray bundle 145 to become a
diverging reflected ray bundle that is spatially filtered by the
aperture 618A. The aperture 618A provides a diverging spatially
filtered measurement signal light 617 if the diverging reflected
ray bundle from the ray bundle direction control element 618B is at
least partially aligned with the aperture 618A. The diverging
spatially filtered measurement signal light 617 is received by the
photodetector 636 of the photodetector configuration 630. The
illumination and detector portion 610 may otherwise operate in a
manner analogous to that described for the illumination and
detector portion 110 of FIG. 1. It will be appreciated that in the
embodiment shown in FIG. 6, the ray bundle alignment sensing
arrangement comprises the ray bundle position control portion 618
and the photodetector configuration 630.
[0045] FIG. 7 is a flow diagram illustrating one exemplary
embodiment of a routine 700 for identifying a ray bundle that is
projected along a direction normal to a workpiece surface according
to this invention. The routine 700 begins, and at a block 705 an
illuminating beam is output from a light source through an
arrangement of a beamsplitter, a ray bundle position control
portion, and a collimating lens, to provide a collimated projected
ray bundle at a projected ray bundle position. In various
respective embodiments, the beamsplitter, the ray bundle position
control portion, and the collimating lens may be arranged in
various respective orders along a beam path, as outlined previously
herein. At a block 710, the collimated projected ray bundle is
input to a portion of an objective lens. At a block 715, the
projected ray bundle is projected from the objective lens such that
it is focused on a workpiece surface along a direction of
incidence. At a block 720, a reflected ray bundle from the
workpiece surface is input to the objective lens and is output from
the objective lens to a ray bundle alignment sensing arrangement.
At a block 725, the ray bundle alignment sensing arrangement is
used to provide a measurement signal that is usable to determine a
degree of alignment between the projected ray bundle and the
reflected ray bundle. At a block 730, a ray bundle corresponding to
the respective measurement signal that indicates the best degree of
alignment is identified as the ray bundle that is projected along
the direction normal to the workpiece surface, and the routine
ends.
[0046] It should be appreciated that, in various embodiments, the
ray bundle alignment sensing arrangement that outputs a measurement
signal in the operations at block 725 and the respective
measurement signal that indicates the best degree of alignment in
the operations at block 730, may take any of the compatible forms
outlined previously herein with reference to the operation of the
illumination and detector portions 110, 210, 310, 410, 510 and 610,
or any other similar form that is in accordance with the principles
of this invention as disclosed herein.
[0047] In various exemplary embodiments according to this
invention, the range of surface tilt measurement is limited to
correspond to the numerical aperture (NA) of the objective lens
(e.g. +/-44 degrees for NA=0.7). Using practical and economical
components, it is reasonable to provide an aperture 120 and/or a
projected ray bundle 115 with a dimension on the order of 100
microns, and a position adjustability on the order of one part in
1000 or better along the range of the x'' and y'' projected ray
bundle position axes. Accordingly, surface tilt angular measurement
resolution on the order of 0.001 times the angular measurement
range corresponding to the NA, or better (e.g. on the order of
0.044 degrees, or better, for NA=0.7) may be provided. More
generally, in various embodiments, a surface tilt measuring
configuration according to this invention may provide surface tilt
angular measurement resolutions of at least 0.1 degree, 0.05
degree, 0.01 degrees, and better, depending on the NA of the
objective lens in combination with other characteristics of the
configuration.
[0048] While the preferred embodiment of the invention has been
illustrated and described, numerous variations in the illustrated
and described arrangements of features and sequences of operations,
as well as additional types of surface inspection and measurement
that may be performed using such variations, will be apparent to
one skilled in the art based on this disclosure. Thus, it will be
appreciated that various changes can be made therein without
departing from the spirit and scope of the invention.
* * * * *