Title: Caliper method, system, and apparatus
Abstract: A caliper atomic force microscope (AFM) comprises two AFM probes (each comprised of an oscillator and an attached tip) that operate on a sample in a coordinated manner. The coordinated operation of the AFM probes may be spatially or temporally coordinated. The result of the coordinated operation may be an image of the sample or a dimensional measurement of an unknown sample. The probes of the caliper AFM may be tilted, or the tips may be tilted at a non-orthogonal angle with respect to the probes, so as to enable the tips to access vertical sample surfaces or to enable the tips to touch each other. The tip shapes may include conical, boot-shaped, cylindrical, or spherical and materials from which the tips are fabricated may include silicon or carbon nanotubes. The oscillators may be beveled to allow the tips to operate in close proximity or in contact without interference of the oscillators. The disclosure of the present invention is discussed in terms of an atomic force (van der Waalls) interaction. Other interaction forces are contemplated, such as electrostatic force, magnetic force, and tunneling current. The caliper AFM may be calibrated with the help of a sample with known dimensions or by touching the probe tips. The tip-to-tip calibration enables absolute measurements without the need for a reference artifact, and it enables in-line calibration that may be performed during the measurement process.
Patent Number: 6,955,078 Issued on 10/18/2005 to Mancevski, et al.
| Inventors:
|
Mancevski; Vladimir (Austin, TX);
McClure; Paul (Austin, TX)
|
| Assignee:
|
Xidex Corporation (Austin, TX)
|
| Appl. No.:
|
115274 |
| Filed:
|
April 1, 2002 |
| Current U.S. Class: |
73/105 |
| Intern'l Class: |
G01B 021/30; G01B 011/30 |
| Field of Search: |
73/105,179
250/306-307
|
References Cited [Referenced By]
U.S. Patent Documents
| 5047633 | Sep., 1991 | Finlan et al.
| |
| 5239863 | Aug., 1993 | Kado et al.
| |
| 5382795 | Jan., 1995 | Bayer et al.
| |
| 5461907 | Oct., 1995 | Tench et al.
| |
| 5540958 | Jul., 1996 | Bothra et al.
| |
| 6002131 | Dec., 1999 | Manalis et al.
| |
| 6545273 | Apr., 2003 | Singh et al.
| |
| 6612160 | Sep., 2003 | Massie et al.
| |
| 6862921 | Mar., 2005 | Chand et al.
| |
| 6884999 | Apr., 2005 | Yedur et al.
| |
| 2003/0033863 | Feb., 2003 | Ashby et al.
| |
| 2003/0200798 | Oct., 2003 | Lendig et al.
| |
Other References
U.S. Appl. No. 60/242,650.
SPIE 2002 Presentation by Vladimir Mancevski, "Dual-Probe Caliper CD-AFM." pp.
1-17 by Jul. 2002.
Manuscript discussing content of SPIE 2002 Presentation by Vladimir Mancevski,
"Development of a dual-probe Caliper CD-AFM for near model-independent nanometrology."
p pages by Jul. 2002.
|
Primary Examiner: Noland; Thomas P.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the following U.S. Provisional Applications,
all of which are hereby incorporated by reference:
| |
| COMMONLY OWNED AND PREVIOUSLY FILED |
| U.S. PROVISIONAL PATENT APPLICATIONS |
| Atty. Dkt. # |
Ser. No. |
Title |
Filing Date |
| |
| 500929.000024 |
60/280,193 |
Caliper AFM for near- |
Mar. 30, 2001 |
| |
|
model-independent |
| |
|
nanometrology |
| 500929.000030 |
60/287,822 |
Multiple head caliper |
May 1, 2001 |
| |
|
atomic force microscope |
| |
The benefit of 35 U.S.C. § 120 is claimed for all of the above referenced
commonly owned applications. The contents of the applications referenced in the
table above are not necessarily identical to the contents of this application.
All references cited hereafter are incorporated by reference to the maximum extent
allowable by law. To the extent a reference may not be fully incorporated herein,
it is incorporated by reference for background purposes and indicative of the knowledge
of one of ordinary skill in the art.
Claims
1. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip are moved in a coordinated way with respect to each other in more than one
direction; and
wherein the first tip and the second tip can measure a test sample.
2. The caliper AFM of claim 1, wherein at least one of the tips is moved in a
coordinated way relative to the test sample.
3. The caliper AFM of claim 1, wherein the first tip moves, the second tip moves,
and the test sample is stationary.
4. The caliper AFM of claim 1, wherein the first tip moves, the second tip moves,
and the test sample moves.
5. The caliper AFM of claim 1, wherein the first tip moves in at least one direction.
6. The caliper AFM of claim 1, wherein the shape of the first tip is boot, cylindrical,
carbon nanotube, or spherical, and wherein the shape of the second tip is boot,
cylindrical, carbon nanotube, or spherical.
7. The caliper AFM of claim 1,
wherein the first AFM probe includes a first multiresonant oscillator on which
the first tip is mounted; and
wherein the second AFM probe includes a second multiresonant oscillator on which
the second tip is mounted.
8. A caliper AFM system, comprising a plurality of caliper AFMs of claim 1.
9. The caliper AFM system of claim 8, wherein the plurality of caliper AFMs are
configured to measure the test sample in a coordinated manner.
10. The caliper AFM of claim 1, wherein the directions are orthogonal to each other.
11. The caliper AFM of claim 1, wherein the first tip moves in first direction,
and the second tip moves in second direction.
12. The caliper AFM of claim 1, wherein at least one of the tips extends from
its AFM probe at a non-orthogonal angle.
13. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip can be coordinated with respect to each other in more than one direction; and
wherein one of the tips is stationary.
14. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip can measure a test sample in a coordinated manner; and
wherein the first AFM probe is configured to tilt on a first axis to enable the
first tip to move close to the second tip.
15. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip can measure a test sample in a coordinated manner; and
wherein the first AFM probe is not configured to tilt, and the first tip is tilted
to enable the first tip to move close to the second tip.
16. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip can measure a test sample in a coordinated manner;
wherein the first AFM probe includes a first oscillator on which the first tip
is mounted; and
wherein the first oscillator is beveled to allow the first tip to move close
to the second tip without contacting the second AFM probe.
17. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip are moved in a coordinated way with respect to each other in at least one direction;
wherein said one direction lies in a plane parallel to the sample plane; and
wherein the first tip and the second tip can measure a test sample.
18. The caliper AFM of claim 17, wherein the first tip is stationary.
19. The caliper AFM of claim 17, wherein the shape of the first tip is conical,
pyramidal, boot, cylindrical, carbon nanotube, or spherical, and wherein the shape
of the second tip is conical, pyramidal, boot, cylindrical, carbon nanotube, or spherical.
20. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip are independently moved in a coordinated way with respect to a test sample
in more than one direction; and
wherein the first tip and the second tip can measure the test sample.
21. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip are moved in a coordinated way with respect to each other in more than one
direction;
wherein the caliper AFM is configured such that the first tip and the second
tip are independently moved in a coordinated way with respect to a test sample
in more than one direction; and
wherein the first tip and the second tip can measure the test sample.
22. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip are moved in a coordinated way with respect to each other in a first direction;
wherein the caliper AFM is configured such that the first tip and the second
tip are independently moved in coordinated way with respect to a test sample in
a second direction; and
wherein the first tip and the second tip can measure the test sample.
23. The caliper AFM of claim 22, wherein at least one of the tips is stationary,
and the test sample can move.
24. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip is moved in a coordinated
way with respect to a test sample in more than one direction;
wherein the caliper AFM is configured such that the second tip is moved in a
coordinated way with respect the first tip in more than one direction; and
wherein the first tip and the second tip can measure the test sample.
25. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip are moved in a coordinated way with respect to each other in at least one direction;
and
wherein the shape of the first tip is carbon nanotube or spherical, and wherein
the shape of the second tip is carbon nanotube or spherical.
26. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the caliper AFM is configured such that the first tip and the second
tip are moved in a coordinated way with respect to each other in at least one direction;
wherein the shape of the first tip is conical, pyramidal, boot, cylindrical,
carbon nanotube, or spherical, and wherein the shape of the second tip is conical,
pyramidal, boot, cylindrical, carbon nanotube, or spherical; and
wherein the shape of the first tip is same as the shape the second tip.
27. A caliper AFM comprising:
a first atomic force microscope (AFM) probe having a first tip;
a second AFM probe having a second tip;
wherein the apexes of the tips are the closest points between the fist and the
second probe; and
wherein the caliper AFM is configured such that the first tip and the second
tip are moved in a coordinated way with respect to each other in at least one direction.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to atomic force microscopes (AFMs) and
in particular to a Caliper AFM for nanometer-scale length measurements.
2. Description of Related Art
Conventional atomic force microscope (AFM) probes have been developed
to perform nanometer-scale measurements on test surfaces, but while those solved
many problems, significant problems remained. For example, calibration of such
probes is difficult. Great effort must be expended to characterize probe shape
in the presence of tip wear, estimate the tip-surface force profile in the presence
of surface contamination and variable material composition, and calibrate the scanning
stages and other electro-optical sensors and actuators in the presence of manufacturing
defects and environmental noise.
In other countries, the only similar caliper work that we are aware of is at
the
Physikalische Technische Bundesantstalt (PTB), the counterpart of NIST in Germany.
This work involves caliper type measurement on a much larger scale. Two opposed
scanning tunneling probes are being used to measure the lengths of gauge blocks.
One alternative way of obtaining CD measurements with substantial probe modeling
is to section the wafer and obtain a cross-section SEM image. However, even on
thinned sections the edge resolution of SEMs is limited by the beam-sample interaction.
A second and related approach is to manufacture thinned wafer sections that can
be measured in a transmission electron microscope. The edge resolution of this
approach is extremely good. However, the scale calibration of TEM relies on comparison
of the measured images with calibrated artifacts, such as atomic lattice spacings,
under nearly identical imaging conditions and is difficult to carry out with high
precision. Furthermore, both of these techniques are destructive and involve off-line
measurements in vacuum.
BRIEF SUMMARY OF THE INVENTION
In accordance with an embodiment of the invention, a caliper atomic force microscope
(AFM) includes a first AFM probe having a first tip, and a second AFM probe having
a second tip. The caliper AFM is configured such that the first tip and the second
tip can measure a test sample in a coordinated manner.
Optionally, the caliper AFM may be configured so that the first tip can
move and the second tip can move. The moves may be coordinated. The moves may be
relative to a test sample. The coordination may be touch-probing. The move of the
first tip may occur relative to the test sample. The first tip may move, the second
tip may move, and the test sample may be stationary. The first tip may move, the
second tip may stationary, and the test sample may move. The first tip may be stationary,
the second tip may be stationary, and the test sample may move. The first tip and
the second tip may be stationary relative to each other, and the test sample may
move. The first tip and the second tip may be stationary relative to each other,
and the test sample may be stationary. The first tip may move, the second tip may
move, and the test sample may move. The first tip may move in only one dimension.
The first AFM probe may be configured to tilt on a first axis to enable the first
tip to move close to the second tip. The shape of the first tip may be boot-shaped,
cylindrical shaped, carbon nanotube, or spherical, and the shape of the second
tip may be boot-shaped, cylindrical shaped, carbon nanotube, or spherical. The
first AFM probe may not be configured to tilt, and the first tip may be tilted
to enable the first tip to move close to the second tip. The first AFM probe may
include a first oscillator on which the first tip is mounted and the first oscillator
may be beveled to allow the first tip to move close to the second tip without contacting
the second AFM probe. The first AFM probe may include a first multiresonant oscillator
on which the first tip is mounted; and the second AFM probe may include a second
multiresonant oscillator on which the second tip is mounted. A caliper AFM system,
may comprise a plurality of caliper AFMs. The plurality of caliper AFMs comprising
a caliper AFM system may be configured to measure the test sample in a coordinated manner.
According to another embodiment of the invention, a method of test sample
measurement using a caliper that has a first tip of a first AFM probe and a second
tip of a second AFM probe includes positioning the first tip on a first surface
of the test sample, positioning the second tip on a second surface of the test
sample, creating a first data set by measuring the test sample with the first tip,
and creating a second data set by measuring the test sample with the second tip
and the first tip and the second tip are operably coordinated.
Optionally, the creating a first data set may include creating a first
data set by measuring the test sample with the first tip in reference to the second
tip and the creating a second data set may include creating a second data set by
measuring the test sample with the second tip in reference to the first tip. The
first data set may include a first relatively referenced data set and the second
data set may include a second relatively referenced data set. The first relatively
referenced data set and the second relatively referenced data set may be the same.
The creating a first data set may include creating a first data set by measuring
the test sample with the first tip in reference to a global coordinate system,
and the creating a second data set may include creating a second data set by measuring
the test sample with the second tip in reference to the first tip. The first data
set may include a first globally referenced data set and the second data set may
include a second relatively referenced data set. The creating a first data set
may include creating a first data set by measuring the test sample with the first
tip in reference to a global coordinate system and the creating a second data set
may include creating a second data set by measuring the test sample with the second
tip in reference to the global coordinate system. The first data set may include
a first globally referenced data set and the second data set may include a second
globally referenced data set. The method of test sample measurement may include
a plurality of measurements. The plurality of measurements may be at least part
of a transverse scan. The plurality of measurements may be at least part of a longitudinal
scan. The tips may not touch the test sample between measurings nor during measurings.
The tips may touch the test sample between measurings and during measurings. The
tips may not touch the test sample between measurings; and the tips may touch the
test sample during measurings. Measuring the test sample with the first tip may
occur at approximately the same time as the measuring the test sample with the
second tip. Measuring the test sample with the first tip may not occur at approximately
the same time as the measuring the test sample with the second tip. The first tip
and the second tip may be positioned so that at least one of their coordinates
is approximately equal.
According to another embodiment of the invention, a method of calibrating
a caliper that has a first tip of a first AFM probe and a second tip of a second
AFM probe includes taking a measurement of an artifact having a known dimension;
and adjusting the caliper based on the difference between the measurement and the
known dimension.
Optionally, the taking the measurement may include taking a measurement
of an artifact having a known dimension while the tips are in contact with the
artifact. The taking the measurement may include taking a measurement of an artifact
having a known dimension, using the extreme lateral points of the tips, while the
tips are in contact with the artifact. The taking the measurement may include taking
a measurement of an artifact having a known dimension, using the extreme vertical
points of the tips, while the tips are in contact with the artifact. The taking
the measurement may include taking a measurement of an artifact having a known
dimension, while the tips are not in contact with the artifact. The taking the
measurement may include taking a measurement of an artifact having a known dimension,
using the extreme lateral points of the tips, while the tips are not in contact
with the artifact. The taking the measurement may include taking a measurement
of an artifact having a known dimension, using the extreme vertical points of the
tips, while the tips are not in contact with the artifact. The first tip may have
a first apex, the second tip may a second apex, and the taking the measurement
may include characterizing the apexes using an artifact having a known dimension.
According to another embodiment of the invention, a method of calibrating
a caliper that has a first tip of a first AFM probe and a second tip of a second
AFM probe includes positioning the tips such that they are at a known tip-to-tip
distance for which they have an known interaction, measuring a measured interaction
of the tips; and adjusting the caliper based on the difference between known interaction
and measured interaction.
Optionally, the positioning may include positioning the tips such that
their extreme lateral points are at a known tip-to-tip distance for which they
have a known interaction; and the measuring may include measuring a measured interaction
of the extreme lateral points of the tips. The measuring may include characterizing
the each apex with the other apex.
According to another embodiment of the invention, a method of calibrating
a caliper AFM that has a first tip of a first AFM probe and a second tip of a second
AFM probe, wherein the first AFM probe can move, and wherein the second AFM probe
can move, such that the first tip and the second tip coordinate in a caliper manner,
includes at least one calibrating the tips, and at least one measuring the test
sample with the caliper AFM. The calibrating a caliper AFM may include in-line
calibration. The calibrating a caliper AFM may include controlling the operation
of the method by a logic circuit. The controlling the operation of the method by
a logic circuit may include controlling the operation of the method by a logic
circuit responsively to the results of past operations of the method.
According to another embodiment of the invention, a method of traceably
measuring a test sample using a caliper that has a first tip of a first AFM probe
and a second tip of a second AFM probe, includes traceably calibrating the caliper
AFM, traceably positioning the first tip on a first surface of a test sample, traceably
positioning the second tip on a second surface of the test sample, creating a first
traceable data set by measuring the test sample with the first tip, and creating
a second traceable data set by measuring the test sample with the second tip.
Optionally, the traceably calibrating may include taking a measurement
of an artifact having a traceable known dimension, and adjusting the caliper based
on the difference between the measurement and the known dimension. The traceably
calibrating may include positioning the tips such that they are at a known tip-to-tip
distance for which they have a known interaction, measuring a measured interaction
of the tips, and adjusting the caliper based on the difference between known interaction
and measured interaction.
In accordance with yet another embodiment of the invention, a measuring system
includes a first AFM probe having a first tip and a first reflective surface. The
system also includes a control circuit adapted to cause the first AFM probe to
move the first tip relative to a test sample and a collimated laser source configured
to produce a collimated laser, wherein the collimated laser is directed so that
the first reflective surface falls within the collimated laser, whereby the first
reflective surface produces a first reflection. The system would further include
a first mirror, configured to receive the first reflection, whereby the first mirror
produces a first further reflection, a position sensitive detector (PSD) configured
to receive the first further reflection, and a measurement circuit adapted to ascertain,
based on the PSD's output, a first point on the test sample.
The system might also include a second AFM probe with a tip and reflective surface
and a second mirror, wherein the control circuit is adapted to cause the second
AFM probe to move the second tip relative to the test sample, the collimated laser
is directed so that the second reflective surface falls within the collimated laser,
whereby the second reflective surface produces a second reflection, wherein the
second mirror is configured to receive the second reflection, whereby the second
mirror produces a second further reflection, wherein the PSD is configured to receive
the second further reflection, and wherein the measurement circuit is adapted to
ascertain, based on the PSD's output, a second point on the test sample. The first
reflective surface might have a first fiducial and the second reflective surface
might have a second fiducial. The first AFM probe might include a first cantilever,
to which the first tip is attached, and the first reflective surface might include
all of at least one face of the first cantilever, wherein the first AFM probe includes
a second cantilever, to which the second tip is attached, and the second reflective
surface includes all of at least one face of the second cantilever. The PSD's output
includes an AC signal and a DC signal, wherein the measurement circuit is configured
to simultaneously process the AC signal and the DC signal in ascertaining the first
point on the test sample and the second point on the test sample. The first AFM
probe might have a first mounting chip with a bevel and a first cantilever with
a first end and a second end, wherein the first end is attached to the first mounting
chip and the first tip is attached to the second end, wherein the first reflective
surface is located on the bevel. The first AFM probe might have a first mounting
chip and a first cantilever having a first end and a second end, wherein the first
end is attached to the first mounting chip, and wherein the first tip is attached
to the second end, wherein the first reflective surface comprises a fiducial located
on the first mounting chip.
In accordance with a still further embodiment of the present invention, a method
of measuring a test sample includes ascertaining a first plurality of points on
a first sidewall of the test sample with a first AFM probe, wherein the first plurality
of points is included in a first line roughness (LR).
The first LR of the method might include a transverse LR or a longitudinal LR.
If the LR is measured along an edge of the first sidewall, it would be a line edge
roughness (LER). Sidewall roughness (SWR) could be measured by combining LR measurements.
The method might include ascertaining a second plurality of points on a second
sidewall of the test sample with a second AFM probe, wherein the second plurality
of points is included in a second LR, the first AFM probe and the second AFM probe
coordinate in a caliper manner, the second LR is positionally approximately opposite
the first LR, and a linewidth roughness (LWR), comprising the first LR and the
second LR, is measured. Three-dimensional linewidth roughness (3DLWR) could be
made by combining LWR measurements.
In accordance with another embodiment of the invention, a method of measuring
a test sample includes measuring a line roughness (LR) using a probe and determining
spatial frequency data based on the LR using Fourier Transform (FT).
The LR might comprise fractals or wavelets. The probe might be an AFM, SEM, or
reflectometry probe.
In accordance with another embodiment of the invention, a MEMS caliper apparatus
includes a first AFM oscillator having a first tip, a second AFM oscillator having
a second tip, wherein the first AFM oscillator and the second AFM oscillator are
configured so as to be able to move with respect to each other in three dimensions,
a control circuit that controls the movement of the first AFM oscillator and the
movement of the second AFM oscillator, and a sensing circuit that measures a test
sample based on the first AFM oscillator's output and the second AFM oscillator's output.
The control circuit and the sensing circuit may be integrated. The apparatus
may include a substrate, an arm, from which the first AFM oscillator and the second
AFM oscillator extend, a flex link, wherein the arm is attached to the substrate
by the flex link, which allows three degrees of freedom, and wherein the substrate,
the flex link, the arm, the first AFM oscillator, the second AFM oscillator, the
first tip, and the second tip are included in a single measuring structure. The
first and second tips might each include a vertical tip having a first end and
a horizontal tip attached to the first end, opposite which the horizontal tip has
an apex, wherein the apexes can touch. The first tip may have a first apex, wherein
the first tip is tilted such that the part of the first tip closest to the second
tip is the first apex, the second tip has a second apex, the second tip is tilted
such that the part of the second tip closest to the first tip is the second apex,
and the apexes can touch.
In accordance with another embodiment of the invention, a method of detecting
extreme lateral tip points includes bringing a first tip and a second tip into
lateral proximity, wherein the first tip has a first extreme lateral point and
is included in a first AFM probe, and wherein the second tip has a second extreme
lateral point and is included in a second AFM probe; scanning in two dimensions
of a plane approximately orthogonal to an imaginary line between the first tip
and the second tip; and wherein maximum tip distance indicates that the first extreme
lateral point is touching the second extreme lateral point.
BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings form part of the present specification and are included
to further demonstrate certain aspects of the present invention. The figures are
not necessarily drawn to scale. The invention may be better understood by reference
to one or more of these drawings in combination with the detailed description of
specific embodiments presented herein.
FIG. 1 shows a boot-shaped probe making a conventional linewidth measurement,
in accordance with prior art.
FIG. 2-A shows a calibration of a caliper AFM, in accordance with an embodiment
of the present invention. [FIG. 2-B shows a calibration of a prior art AFM]
FIGS. 3-A and 3-B show scan paths of caliper AFM tips, in accordance
with an embodiment of the present invention.
FIG. 4 shows a caliper AFM system, in accordance with an embodiment of the present invention.
FIG. 5 shows two probes coordinating as a caliper, in accordance with an embodiment
of the present invention.
FIG. 6 shows a caliper AFM configuration with scanned probes and a stationary
test sample, in accordance with an embodiment of the present invention.
FIG. 7 shows a caliper AFM configuration with stationary probes and a scanned
test sample, in accordance with an embodiment of the present invention.
FIG. 8 shows a tilted probe in accordance with an embodiment of the present invention.
FIG. 9 shows two tilting probes coordinating as a caliper, in accordance with
an embodiment of the present invention.
FIG. 10 shows a mechanism for tilting AFM probe, in accordance with an embodiment
of the present invention.
FIG. 11—Measurement using a caliper AFM
FIG. 12—An illustration showing paths of transverse and longitudinal scans
FIG. 13—Line roughness (LR) measurements along a transverse and
a longitudinal path on the surface of a feature
FIG. 14—Illustration of a Y raster scan
FIG. 15 shows a logic of utilizing a reflective fiducial mark for sensing the
AC and DC position of a caliper AFM probe, in accordance with an embodiment of
the present invention.
FIG. 16 shows use of a reflective fiducial mark for navigating the caliper AFM
probe to a target location on a wafer, in accordance with an embodiment of the
present invention.
FIG. 17 Non contact mode measurement
FIG. 18—Extreme lateral and vertical points of left probe tip (Illustrated
in two dimensions)
FIG. 19—Illustration showing how sharper tips can shrink the required
arcs to arbitrarily small sizes in the case of a Caliper AFM
FIGS. 20-A and 20-B show a reference linewidth measurement and a linewidth
measurement using a caliper AFM, in accordance with an embodiment of the present invention.
FIG. 21—Calibration of a caliper AFM using an artifact having known dimensions
FIG. 22—Calibration of a caliper AFM using the lateral extreme
points of the tips
FIG. 23—Calibration of a caliper AFM using the vertical extreme
vertical points of the tips
FIG. 24—Non-contact mode calibration of a caliper AFM using an
artifact having known dimensions
FIG. 25—Example of calibration with extreme lateral points at a
known tip-to-tip distance
FIG. 26—Contact mode calibration of a caliper AFM
FIG. 27—Example illustrating characterization of left tip by right tip.
FIG. 28—Example illustrating characterization of right tip by left tip.
FIG. 29—Line roughness (LR) measurement along an arbitrary path
on the surface of a feature
FIG. 30—Sidewall roughness (SWR) measurement made in a region of
interest on the feature surface
FIG. 31—Line roughness measurements made to determine linewidth roughness
FIG. 32—Line edge roughness (LER) measurement made along a path
that traverses the edge of a feature
FIG. 33—Three-dimensional linewidth roughness (3DLWR) measurement
FIG. 34 shows a spatial spectrum of probe data, in accordance with an embodiment
of the present invention.
FIGS. 35-A and 35-B show the position of the test sample as it is scanned
by a caliper AFM with stationary probes and scanned test sample, in accordance
with an embodiment of the present invention.
FIG. 36 shows the scanned path of the caliper AFM with stationary probes and
scanned test sample, in accordance with an embodiment of the present invention.
FIGS. 37-A and 37-B show two possible layouts for multiple-head caliper
AFM systems, in accordance with an embodiment of the present invention.
FIG. 38 shows a MEMS caliper embodiment of the present invention.
FIG. 39 shows a top view of MEMS cantilevers with probe tip structures attached
FIG. 40 shows a top view of MEMS slanted cantilevers with probe tip structures attached
FIG. 41 shows an end view of MEMS probe tip structures with carbon nanotubes attached
FIG. 42 shows an end view of MEMS probe tip structures with tips mounted near
inner sides of cantilevers
FIG. 43 shows an end view of MEMS probe tip structures slanted inward to facilitate
imaging of a feature
FIG. 44 shows an end view of MEMS probe tip structures with inverted umbrella
shaped probe tips
FIG. 45 shows an end view of MEMS probe tip structures with pointed shapes pointing
inward toward the feature
FIG. 46 shows an top view of MEMS probe tip structures with pointed shapes pointing
inward toward the feature
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Throughout this description, primed reference characters correspond to
unprimed reference characters. For example, reference character
1′
would correspond to the reference character
1.
The Problem Solved by the Present Invention
The main disadvantage of scanning-probe measurement tools is their dependence
on a priori knowledge of the probe's shape and its interaction with the test sample
to reconstruct the dimensions of the test sample from raw measurement data. In
the case of a conventional atomic force microscope (AFM), great effort must be
expended to characterize probe tip shape in the presence of tip wear, estimate
the tip-surface force profile in the presence of surface contamination and variable
material composition, and calibrate the scanning stages and other electro-optical
sensors and actuators in the presence of manufacturing defects and environmental noise.
Model dependence refers to reliance on such a priori information about operation
of a measurement tool that must be removed from raw data to extract a quantity
of interest. Calibration is the process of determining the required a priori information
by measuring some known quantity and adjusting the output of the measurement tool
so that it provides the correct reading. The larger, more complex, and more variable
its model dependence, the more difficult it is to calibrate a measurement tool.
There are two approaches for dealing with model dependence. One approach is
to accept the conventional architecture of the measurement tool for what it is
and attempt to determine all of the a priori information required to calibrate
it. The alternative approach adopted in the present invention rejects conventional
AFM design in favor of an entirely new dual-scanning-probe caliper architecture
that is virtually model-independent. Taken together, removal of the three major
sources of uncertainty in scanning probe tools provides an exciting opportunity
to demonstrate a revolutionary new breed of AFM metrology tool that paves the way
for scanning probe measurements that are both precise (i.e., highly repeatable)
and accurate (i.e., traceable to reference artifacts).
The semiconductor industry represents an important example illustrating the utility
of the present invention. Critical dimension metrology of features involved in
semiconductor manufacturing relies on extensive modeling of measurement tool-induced
uncertainties, and on the repeatability of measurements to keep the manufacturing
process under control. This conventional approach is rapidly nearing the end of
its usefulness as feature sizes shrink to the level where measurement tool uncertainties
dominate the measurement process. Therefore, a new approach for length metrology
is required.
The Problem of Tip Shape Uncertainty
Recording the motion of the tip of a scanning probe measurement tool with
respect to a sample surface provides an image of the surface that is convolved
with the shape of the probe tip. The shape of the tip must be deconvolved from
the scanned image to generate the true topography the sample. For pitch and height
measurements this may not be a critical limitation, since two respective points
on the feature surface can be measured by the same point on the tip. Pitch and
height measurements can therefore be recovered with acceptable uncertainty even
in the presence of poor knowledge about the tip shape. For feature width measurements,
however, uncertainty in the tip shape translates directly into measurement uncertainty
whenever different points on a single probe tip are used to make measurements at.
Conventional single-tip scanning probe tools are inherently limited by the need
to perform frequent, extensive characterization of tip shape.
Referring to FIG. 1 The problem of tip shape uncertainty is illustrated
by comparing a linewidth measurement made with a conventional AFM having a single
boot-shaped tip
100 and a linewidth measurement with a dual-probe caliper
AFM. The linewidth measurement w
boot obtained with a single boot-shaped
tip is:
wboot=(
y-x)-
pw-sf (1)
where x and y are the respective left and right stage positions as recorded
by the sensing system of the scanning stage, pw
R and pw
L are
the respective right and left probe widths, pw is the total probe width, sf
R
and sf
L are the respective right and left tip-surface distances,
and sf is the total combined tip-surface distance of the measurement. As can be
seen from Equation 1, the linewidth w
boot depends on the probe width
pw and the tip-surface distance sf. Assuming that the mode of scanning is contact,
the linewidth W
boot is still a function of the probe width pw, as can
be seen from Equation 2.
Referring to FIG. 2, The present invention solves this problem by using
dual scanning probes,
200 and
201, that operate in coordinated fashion
as a caliper, instead of reliance on a single probe tip. This allows different
regions on the surface of a convex or concave feature to be measured by points
on the surfaces of two different probe tips. The equivalent linewidth measurement
w
caliper obtained by the caliper AFM with two tips would be:
where x and y are the respective left and right stage positions as recorded
by the sensing system of the scanning stage, and x
R and y
R are
the respective reference stage positions as recorded by the sensing system of the
scanning stage at the moment when the two tips are touching each other to establish
a known reference. In this illustration, the known reference is a zero reference.
The effective probe width pw and tip-surface distance sf are defined the same way
as for the single probe measurement.
The left side of a convex feature can be measured by a left caliper tip and the
right side of a convex feature can be measured by a right caliper tip. In the case
of a concave feature, the situation is reversed with the left side of the feature
being measured by a right caliper tip and the right side of the feature being measured
by a left caliper tip. A zero reference point can be established by bringing the
left and right caliper tips together into a known reference configuration, for
example, with the tips touching at their respective lateral extreme points. This
zero referencing procedure provides the required calibration for measurements of
extension, such as line width and hole diameter. Alternatively, a reference artifact
with a finite, known dimension can be used to calibrate the measurement tool. Use
of dual scanning probes that operate together as a caliper virtually eliminates
model-dependent uncertainties associated with probe tip shape for nanometer-scale
measurements of extension, such as line width and hole diameter.
The Problem of Tip-To-Surface Interaction Uncertainty
Another major source of modeling uncertainty for a conventional scanning
probe tool is the tip-surface interaction. Tapping mode and non-contact mode of
operation are the two methods most commonly used with conventional, single-probe
AFMs. Tapping mode is the more stable of the two and is nearly independent of surface
contamination effects. However, it relies on nonconservative energy dissipation
of the tip in the atomic force region of the surface to maintain the tip-surface
gap constant. This mode of sensing requires an amplitude of oscillation of the
probe of 10s of nanometers and hence a comparable uncertainty in its position.
Non-contact mode depends on the long-range conservative surface forces that can
strongly depend on surface contamination and can vary with the material composition
of the sample. This dependence makes the non-contact mode of operation also prone
to variations and therefore greater uncertainties. Therefore, there is a modeling
related uncertainty associated with both the short range, nonconservative forces
(e.g., dissipation upon impact, surface contamination) and the long range conservative
forces (e.g., van der Waals profile). Contact mode relies on short-range (sub-nanometer)
attractive surface forces and requires no modeling of the tip-surface attractive
interaction. However, scanning in contact mode can rapidly degrade both the tip
and the sample.
Referring to FIGS. 3-A and
3-B, the present invention solves this
problem by combined use of two sensing modes; non-contact mode scanning for local
navigation of the probes with respect to the sample alternating with independent
touch-probing of the two scanning probe tips with respect to the sample to obtain
the required dimensional measurement. Taking measurements during touch-probing,
while the tip is momentarily in contact with the sample surface, removes model-dependent
uncertainties associated with tip-surface interaction from the measurement data.
In general the tip-to-tip distance sf′ during a non-contact type interaction
is different from the tip-surface distance sf Nevertheless, the two distances cancel
each other to minimize the effect of the tip-surface distance on the linewidth
measurement (Equation 6), and, in the preferred way, it is null for a touch-probe
contact mode of operation (Equation 7). As can be seen from Equation 3, the probe
width pw does not enter into the computation of the linewidth w
caliper,
and the effect of the tip-surface distance is minimized even for non-contact mode
of operation. For touch-probe contact mode of operation, the linewidth w
caliper
is presented by Equation 4.
The Problem of Mechanical Loop Uncertainty
Another source of modeling uncertainties in scanning probe tools is associated
with mechanical loops in the system. The tip-sample loop via the structure is an
illustration of one such mechanical loop, where the mechanical chain extends from
the tip to the sample through the oscillator, the base of the oscillator, the clamping
fixtures, the scanning stage, and the tool frame. Each of the mechanical structures
in the chain of the loop is subject to vibration, thermal expansion, and stress,
as sources of uncertainty. Uncertainties due to mechanical loops can be reduced
by proper design, selection of temperature-stable materials, and use of components
with similar mechanical properties, but nevertheless impose severe limitations
the achievable measurement precision and accuracy when nanometer-scale feature
dimensions are involved.
Referring to FIG. 4, the present invention solves this problem of reducing
measurement uncertainty introduced by mechanical loops by using a sensing system
in conjunction with use of dual scanning probes that makes a measurement involving
the shortest possible mechanical loop from the right caliper tip to the left caliper
tip. One way to accomplish this is to measure the left-scan-stage-to-right-scan
stage relative distance directly. Each scanning stage can be calibrated accurately
with existing methods that are traceable (i.e., relatable to an absolute length
standard, such as a particular wavelength of light). The stage position can be
measured precisely (i.e., with high repeatability) with the help of interferometric
sensors or capacitive sensors that have been calibrated with an interferometer.
Two such calibrated scanning stages will determine the relative stage-to-stage
distance,
400, and therefore the relative tip-to-tip distance. The relative
tip-to-tip distance provides the measurement of extension, such as line width and
hole diameter. Alternatively, the relative probe-to-probe distance, and therefore
the relative tip-to-tip distance, can be directly measured by using fiducial dots,
401 and
402, on the surfaces of the probes as reflective markers
and a single continuous position sensitive detector,
403, configured electronically
to measure (through reflected light beams) the fiducials' positions and their relative
distance. The relative fiducial-to-fiducial distance and its projection on the
aperture of the continuous position sensitive detector provides a scaled measurement
of extension, such as line width and hole diameter. This procedure is equivalent
to directly measuring the respective difference terms, (y-x), and (y
R-x
R)
in Equation 4 rather than measuring x and y separately, taking the difference to
get (y-x), measuring x
R and y
R separately, and taking the
difference to get (y
R-x
R). The preferred procedure results
in an overall reduction of measurement uncertainty because it enables four typically
large errors associated with conventional stage-based measurements x, y, x
R
and y
R to be replaced with two relatively small errors associated
with direct measurement of the difference terms, (y-x), and (y
R-x
R).
Description of the Caliper AFM
In the basic embodiment of the present invention the Caliper AFM, illustrated
in FIG. 5, comprises two atomic force microscope (AFM) probes that operate on a
test sample in a coordinated manner. The result of operating on a test sample can,
for example, be to generate an image of the test sample, to make a dimensional
measurement of an unknown test sample, or to calibrate the caliper AFM based on
measurement of a sample with known dimensions.
The functioning of a single AFM probe is well known to those skilled in the art.
Coordination means that the probes are capable of working together in a common
operation, for example, to produce an image or a measurement of the test sample.
An example of coordination is the imaging of a resist line on a semiconductor circuit
where each probe images one side of the resist line and where the combination of
the two images will produce the image of the entire resist line. Another example
is the measurement of a polysilicon line on a semiconductor circuit where each
probe measures the distance between a common reference and a respective side of
a polysilicon line, where the combination of the two distances will produce the
linewidth of the polysilicon line. The details of imaging a resist line and measurement
of a polysilicon line, are known to those skilled in the art. Other examples of
coordinating the operation of the probes of a caliper AFM include coordination
for the purpose of measuring a line edge roughness spectrum or a line width roughness
spectrum, coordination for the purpose of executing maneuvers necessary for transverse
or longitudinal scanning, coordination for the purpose of achieving clearance between
the probe structures and coordination for the purpose of achieving clearance between
both probes and the sample.
Parallel operation on an array of AFMs, each of them operating independently
(i.e., not working together) and not towards a common operation, is not an example
of coordination of AFM probes as described in the disclosed invention. The operation
of micro-sized or nano-sized tweezers that work together towards a common operation
of grasping an object is also not an example of coordination of AFM probes, as
described in the disclosed invention, because the tweezers' probes do not have
AFM capabilities and cannot produce an image nor a measurement.
Another characteristic of the basic embodiment of the Caliper AFM is that
the probes are capable of working together in a common operation in more than one
manner. For example, the operation of each probe can be spatially or temporally
coordinated. In a spatially coordinated operation the probes can operate on the
same test sample feature or they can operate on different sample features somewhat
removed from each other but still within the range of motion of both the probes.
In a spatially coordinated operation on the same test sample feature the probes
can be aligned in any two of the tool-fixed XYZ coordinates or they can be aligned
in any one of the XYZ coordinates, or not be aligned at all. In temporally coordinated
operation, the probes can operate simultaneously, with some time lag, or one after
another. In one representative example, two spatially and temporally coordinated
probes can image a polysilicon line on a semiconductor circuit in such a manner
that that the probes are aligned in the Y raster direction and their height is
kept the same in the Z direction while the probes simultaneously scan in the X
direction and obtain an image in the XZ plane. Several alternative caliper AFM
embodiments that take advantage of different spatial and temporal coordination
are described later in this disclosure.
In another example of a common operation in more than one manner, the probes
and
the sample stage can be arranged so that, during an operation, the probes and the
sample stage are either fixed, stationary (e.g., temporarily fixed), or moving
(e.g., scanning), in any combination with respect to each other and in respect
to any degree of freedom of motion. In one example, illustrated in FIG. 6, the
two probes are moving over a fixed sample stage, and in another example, illustrated
in FIG. 7, the stationary probes are above the moving stage. In this disclosure
the first arrangement is described as scanned-head Caliper AFM, and the second
arrangement is described as the fixed-head Caliper AFM with scanned sample stage.
Several specific alternative Caliper AFM embodiments that take advantage of different
probes and sample stage arrangements will be described later in this disclosure.
Although the disclosure of the present invention is discussed in terms of
an atomic force (e.g., the van der Waals force), other interaction forces commonly
used in scanning probe microscopy may also be applicable. Examples of such other
interactions include electrostatic force, magnetic force, and tunneling current.
Use of Tilted Probes
Tilted probes for use in a Caliper AFM are described. Use of tilted probes
raises issues related to clearance and positioning of the oscillators to allow
their tips to be in proximity of each other or touch each other as the oscillators
are tilted and brought laterally next to each other. The issues related to probe
tilting are discussed for the more restrictive case of tip-to-tip touching.
It is known to those skilled in the art that each AFM probe consists of an oscillator
that acts as a force sensor, a tip whose apex is engaged in interacting with the
surface, and an oscillator mounting chip that carries the oscillator and facilitates
its attachment to a motion stage. Conventional AFMs and their conventional oscillator
mounting chips, oscillators, and tips have been designed to operate on a relatively
flat horizontal surface and are not best suited for use as a Caliper AFM as described
in the disclosed invention.
A conventional vertically oriented AFM probe with sharp conical tip could not
interact
with (for example, image or measure) a vertical or re-entrant sidewall of a sample
with its apex. A conventional AFM operating over a vertical surface will completely
miss any near vertical sidewall surfaces in which the half-cone angle of the tip
is larger than the slope of the sidewall. For a vertical sidewall the half-cone
angle of the tip is always larger than the slope of the sidewall. Reentrant sidewalls
are also inaccessible by vertically oriented probes with sharp conical tips
In another embodiment of the Caliper AFM at least one probe is tilted around
an
axis to enable better access of the tip to a vertical or re-entrant sidewall of
a sample, as illustrated in FIG.
8. It is implied that the tip apex, and
not the entire tip, needs to access the sample. Tilting the probe in more than
one axis may enable better access of the tip to the sample. Tilting of both probes
may be desirable.
Similarly, two conventional vertically oriented probes with sharp conical
tips could not image or measure the same nanometer sized sample feature at about
the same time. It is implied that the tip apexes, and not the entire tips, are
engaged in operating on the sample. The width of the oscillators would prevent
the probes from working in proximity to each other since the oscillators would
collide before the tips are in sufficient proximity to operate on the same nanometer
sized feature at about the same time. In practice, the sample features in the current
generation of semiconductor circuits are less than 250 nm wide, requiring that
the tips also be less than 250 nm apart.
In another embodiment of the Caliper AFM at least one probe is tilted around
an
axis to enable better access of the two tips to the same nanometer sized sample
feature at about the same time. Tilting the probe in more than one axis may enable
better access of the two tips to the sample. Tilting of both probes may be desirable.
In an extreme case, two conventional vertically oriented probes with sharp conical
tips could not touch each other to implement a zero-reference calibration procedure
that is described elsewhere in this disclosure of the invention. However, the width
of the oscillators would prevent the probes from touch each other since the oscillators
would collide before the tips are in sufficient proximity to touch.
In another embodiment of the Caliper AFM at least one probe is tilted around
an
axis to enable the two tips to touch each other. Tilting the probe in more than
one axis may enables better access of the two tips to the sample. Tilting of both
probes may be desirable.
Oscillator Clearance
For the best result, three tilts may be combined to enable the required clearance,
as illustrated in FIG.
9. Lowering of the oscillator tip down with respect
to its base (Tilt
2) is typical with commercial AFMs and is intended to
provide clearance for clamping of the base of the oscillat
