U.S. patent application number 11/192808 was filed with the patent office on 2005-12-01 for system for sensing a sample.
Invention is credited to Eaton, Steven G., McWaid, Thomas, Panagas, Peter, Samsavar, Amin, Wheeler, William R..
Application Number | 20050262931 11/192808 |
Document ID | / |
Family ID | 23217939 |
Filed Date | 2005-12-01 |
United States Patent
Application |
20050262931 |
Kind Code |
A1 |
McWaid, Thomas ; et
al. |
December 1, 2005 |
System for sensing a sample
Abstract
A profiler or scanning probe microscope may be scanned across a
sample surface with a distance between them controlled to allow the
sensing tip to contact the surface intermittently in order to find
and measure features of interest. The distance is controlled so
that when the sensing tip is raised or lowered to touch the sample
surface, there is no lateral relative motion between the tip and
the sample. This prevents tip damage. Prior knowledge of the height
distribution of the sample surface may be provided or measured and
used for positioning the sensing tip initially or in controlling
the separation to avoid lateral contact between the tip and the
sample. The process may also be performed in two parts: a fast find
mode to find the features and a subsequent measurement mode to
measure the features. A quick step mode may also be performed by
choosing steps of lateral relative motion to be smaller than 100
nanometers to reduce probability of tip damage. In this mode, after
each vertical step to increase the separation between the tip and
the sample, it is detected as to whether the tip and the sample are
in contact. If they are still in contact after the vertical step,
one or more vertical steps are taken to increase the separation,
and no vertical step to reduce the separation is taken and no
lateral relative motion is caused until it is determined that the
tip and the sample are no longer in contact.
Inventors: |
McWaid, Thomas; (Fremont,
CA) ; Panagas, Peter; (Santa Clara, CA) ;
Eaton, Steven G.; (Sunnyvale, CA) ; Samsavar,
Amin; (Saratoga, CA) ; Wheeler, William R.;
(Saratoga, CA) |
Correspondence
Address: |
PARSONS HSUE & DE RUNTZ LLP
595 MARKET STREET
SUITE 1900
SAN FRANCISCO
CA
94105
US
|
Family ID: |
23217939 |
Appl. No.: |
11/192808 |
Filed: |
July 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11192808 |
Jul 28, 2005 |
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10729609 |
Dec 5, 2003 |
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6931917 |
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10729609 |
Dec 5, 2003 |
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10330901 |
Dec 26, 2002 |
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10330901 |
Dec 26, 2002 |
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09313962 |
May 18, 1999 |
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6520005 |
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09313962 |
May 18, 1999 |
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08730641 |
Oct 11, 1996 |
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5948972 |
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08730641 |
Oct 11, 1996 |
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08598848 |
Feb 9, 1996 |
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08598848 |
Feb 9, 1996 |
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08362818 |
Dec 22, 1994 |
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5705741 |
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Current U.S.
Class: |
73/105 |
Current CPC
Class: |
Y10S 977/851 20130101;
G01B 7/34 20130101; G01Q 60/34 20130101; G01B 3/008 20130101; G01Q
10/06 20130101 |
Class at
Publication: |
073/105 |
International
Class: |
G01N 013/10 |
Claims
1-82. (canceled)
83. A method apparatus for sensing a high aspect ratio feature on a
surface of a sample, employing a sensor assembly comprising a probe
and at least a first and a second sensing tips on the common probe
with known spatial relationship to each other, comprising: causing
the first sensing tip to scan across the surface with the first tip
in contact with the surface until the feature is found; and
scanning the second sensing tip across the surface with the second
tip in intermittent contact with the surface to measure the
feature.
84. The method of claim 83, the second sensing tip comprising a
nanotube; wherein said scanning scans the surface so that the tip
of the nanotube is in intermittent contact with the surface to
measure the feature.
85. The method of claim 84, said nanotube extending beyond the
first sensing tip, so that during the scan of the first sensing tip
across the surface, the nanotube is buckled.
86. A method for sensing a high aspect ratio feature on a surface
of a sample employing a sensing tip of a profiler or scanning probe
microscope, said method comprising the steps of: providing height
information of the surface within a target area of the surface;
positioning the tip above one location of the surface in the target
area; scanning the sensing tip across the surface; and controlling
a distance between the surface and the tip using said information
during the scanning so that the tip contacts the surface
intermittently and so that the tip does not contact the surface
laterally when the tip is scanned across the surface; wherein said
controlling controls said distance so that the tip is scanned
without substantially moving the tip and the surface laterally
relative to each other when the tip and surface are in contact,
wherein the tip does not penetrate said feature when scanning
across the surface.
87. A method for sensing a surface of a sample employing a probe
with a sensing tip, said method comprising the steps of: (a)
positioning the sensing tip above one location of the surface of
the sample; (b) reducing a distance between the surface and a
sensing tip without substantially moving the tip and the surface
laterally relative to each other until the tip touches the surface;
(c) measuring data related to a height of the sample surface with
the tip in contact with the sample at a contact point; (d)
increasing a distance between the tip and the contact point of the
surface without substantially moving the tip and the surface
laterally relative to each other until such distance is
substantially equal to a predetermined value; (e) causing lateral
relative motion between the sensing tip and the surface and
positioning the tip so that the tip is above a location adjacent to
and spaced apart from said one location; and (f) repeating steps
(b) through (e) at a plurality of locations of the surface to
obtain an image of the surface; wherein said lateral relative
motion in step (e) is over a lateral distance less than about 100
nm.
88. The method of claim 87, wherein step (a) positions the sensing
tip without adequate prior information concerning extent of height
variations of the surface to avoid lateral contact between the tip
and the surface when the tip is scanned across the surface.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 08/730,641, filed Oct. 11, 1996, entitled "Dual Stage
Instrument for Scanning a Specimen" which, in turn, is a
continuation-in-part of application Ser. No. 08/598,848, filed Feb.
9, 1996.
BACKGROUND OF THE INVENTION
[0002] Stylus profilers are used for obtaining surface profiles of
samples. The stylus of the profiler follows the surface under a
small contact force, and the resulting motions of the stylus are
measured with a sensor assembly. The sensor assembly includes a
stylus, a mechanical linkage (usually a stylus arm) connecting the
stylus to a flexure pivot, and a transducer. When the stylus is
scanned across the surface of the sample, the force exerted by the
sample surface on the stylus causes a rotation of the stylus arm
about the flexure pivot. The vertical displacement of the stylus is
converted by the transducer into an electrical signal which
indicates the profile of the sample surface.
[0003] Advanced profilers also include a force control mechanism,
such as an electromagnetic actuator, for maintaining a constant
contact force between the stylus and the sample surface as the
stylus is scanned across the surface. To maintain a constant
contact force between the stylus and the sample surface, the spring
action of the flexure pivot is calibrated and the force control
magnetic actuator is controlled to counteract the change in the
force applied by the flexure spring on the stylus caused by
rotation of the stylus arm. Thus, a constant force is exerted by
the stylus against the sample surface, as the stylus is scanned
across the surface. As an example of a profiler which has been used
in the semiconductor and disk drive industries, please see U.S.
Pat. No. 5,705,741 and U.S. Pat. No. 5,309,755; both patents are
incorporated herein in their entirety by reference.
[0004] As the semiconductor industry progresses to smaller
dimensions with each new generation of products, there is an
increasing need for scanning instruments that can measure
sub-micrometer scale surface features. While the depths or vertical
dimensions (dimensions normal to the plane of the wafer surface) of
the features such as trenches, or via holes, in semiconductor
wafers, commonly exceed one micrometer, the lateral dimensions
(dimensions in the plane of the wafer surface) have been
continually reduced. At the current state of the art, the lateral
dimensions of features such as trenches are less than 0.5
micrometer. With the continual reduction of the lateral dimensions
of features such as trenches and via holes in the surface of
semiconductor wafers, the ratio of depth to the lateral dimension
of such features, also known as the aspect-ratio, is continually
increased.
[0005] In order to measure such high aspect-ratio features, a very
sharp, thin but long (high aspect-ratio) stylus must be used.
However, a sharp, thin but long stylus is fragile and may easily
break, especially when subjected to lateral forces (forces in
directions in, or parallel to, the plane of the sample surface).
Thus, when a high aspect-ratio stylus contacts a steep feature,
such as the side wall of a trench or via hole, the contact force
has a relatively large lateral component and a relatively small
vertical component. Stylus profilers, such as the profilers
described in the two patents referenced above, are designed such
that motion of the stylus is constrained to one degree of freedom,
namely, rotation about the flexure pivot. This degree of freedom is
substantially normal to the sample surface. The stylus arm is
relatively stiff in all other degrees of freedom. Consequently, the
lateral forces generated when the high aspect-ratio stylus
encounters a steep wall can easily break the stylus and damage the
sample being measured.
[0006] The stylus arm in a profiler has a single degree of freedom,
which comprises rotations about a pivot. The stylus or sensing tip
travels along a path normal to a radial line passing through the
center of rotation at the pivot and the tip. Since the sensing or
stylus tip must be located "below" or at a lower elevation than the
pivot to ensure that the tip and not the body of the sensor
assembly contacts the sample, the motion of the stylus or sensing
tip is not truly normal to the plane of the sample surface, but is
in the shape of an arc. While the main direction of travel of the
tip is downwards, it nevertheless also travels in the lateral
direction in the plane of the sample surface. This lateral motion
is also known as parasitic motion of the sensing tip. The parasitic
motion of the sensing tip may hamper or even preclude the sensor
assembly from measuring relatively deep and narrow features.
[0007] It is therefore desirable to provide an improved surface
measurement system which overcomes the above drawbacks.
SUMMARY OF THE INVENTION
[0008] The above-described difficulties can be overcome by allowing
the sensing tip of the profiler to contact the sample surface
without substantially rotating the stylus arm about the pivot.
Instead, a distance between the sample and the sensing tip of the
profiler is reduced until the tip touches the sample, without
moving the tip and the sample laterally relative to each other. By
avoiding lateral relative motion between the tip and the sample
before the tip touches the surface, the above-described problems
are avoided. When such a scanning process is used, thin and long
(high aspect-ratio) styli can be used to penetrate high
aspect-ratio features for measurement. Data related to the height
of the sample may then be measured with the tip stationary and in
contact with the sample. After the measurement, the tip and the
sample are separated and moved laterally relative to each other to
measure the sample surface at a different location.
[0009] With minor modifications, the above-described scanning
process may also be applied to other scanning instruments, such as
the scanning probe microscope, which includes atomic force
microscopes and scanning tunneling microscopes.
[0010] As described above, the feature on the sample surface may be
found and measured by repeatedly causing the sensing tip (of the
profiler or scanning probe microscope, for example) and a sample to
repeatedly contact at different locations of the sample surface. In
this process, the sensing tip and the sample are brought together
substantially without lateral relative motion between them until
they contact, separated again substantially without lateral
relative motion between them, and moved laterally relative to each
other until the tip is at a location above a different portion of
the sample. This process is repeated at different locations of the
sample. If the separation between the tip and the sample during
such lateral motion is less than the change in height of the sample
surface, the lateral motion will cause the sensing tip to contact
the sample surface laterally, thereby causing damage to the sensing
tip. To reduce the probability of such damage, the separation may
be increased to a large value before lateral relative motion is
initiated. If no knowledge of the height variation or distribution
of the sample surface is available, such value should be large
enough that it exceeds any probable height variations of the sample
surface one may encounter. The resulting process can be quite time
consuming, especially if the sample surface is to be measured at
many different locations. This difficulty can be avoided by
separating the tip and the surface by just enough to avoid such
lateral contact.
[0011] A number of techniques may be employed to assure that the
sensing tip and the sample are separated by an adequate distance so
that the sensing tip will not contact the sample surface during the
subsequent lateral relative motion. In the preferred embodiments,
if certain height information is provided concerning the sample
surface or a portion thereof (such as within a target area), then
the sensing tip may be positioned at or close to the portion of the
sample having the highest elevation. If the height information of
the sample surface or a portion thereof is not readily available,
such information can be acquired quickly by actually measuring the
height of the surface at several sampling locations. Yet another
technique that can be employed is to actually measure data related
to the height of the sample when the tip and the sample come into
contact as the tip is scanned across the sample and use such
measured data to predict an elevation of the next location of the
sample to be measured, so that the separation between the tip and
the sample can be set to be higher than such predicted elevation.
These are, of course, only some examples of the techniques that can
be used to implement the above general concept.
[0012] In some applications, it may be desirable to first find the
feature quickly, and then take an appropriate amount of time to
actually measure the feature. In this instance, the distance
between the sensing tip and the sample is controlled so that the
distance between the tip and the sample is periodically increased
and then decreased as the tip scans across the sample surface until
the tip either touches the surface or until either the tip or the
surface has traveled, or the two together have traveled in
aggregate, by a preset distance without causing the tip and the
surface to contact. In other words, the sensing tip does not
completely penetrate the feature when scanning across the
surface.
[0013] For some applications, to save time, even without any prior
knowledge concerning the topology of the sample surface, the sample
surface can be quickly scanned and measured without incurring undue
risk in breaking the sensing tip. This involves determining whether
the tip and the surface remain in contact after the distance
between them is increased to a predetermined value, before lateral
relative motion between them is initiated or continued. If the tip
and the surface remain in contact after they are being separated
from each other by a predetermined distance, the distance between
them is further increased until they are no longer in contact
before moving the tip and the surface laterally with respect to
each other.
[0014] A sensor assembly having a sensing probe may be used in any
one of the above-described processes for sensing the sample. The
sensor assembly includes a base portion and a moveable sensing tip
connected to the base portion. When the tip contacts the sample,
the tip may move relative to the base portion of the sensor
assembly. A moving stage is used to cause vertical relative motion
between the sensor assembly and the sample. Thus, when the moving
stage causes a distance between the sensor assembly and a sample to
be reduced until the sensing tip contacts the sample, the actual
change in distance between the sensing tip and the sample is given
by a combination of the relative motion between the sensor assembly
and the sample, and of the relative motion between the sensing tip
and the base portion. By taking into account both motions, a more
accurate measure of data related to the height of the sample can be
obtained. In different embodiments, the sensor assembly may be that
of a profiler, an atomic force microscope or other types of
scanning probe microscopes.
[0015] One way to increase measurement speed while avoiding
significant lateral forces between the sensing tip and the sample
is to separate the process into two parts: an initial fast find
mode to find the feature of interest, and after finding the
feature, a second measurement mode to measure the feature.
[0016] As one possible embodiment of the invention to implement the
above two part process, the sensing tip is scanned across the
sample surface with the tip in contact with the surface until the
feature is found. Scanning the sensing tip across the surface with
the tip in contact with the surface speeds up the scanning process.
After the feature is found, the sensing tip is scanned across the
feature with the tip in intermittent contact with the sample
surface to measure the feature. In this manner, the time required
to find and measure the feature is reduced without undue risk of
large lateral forces between the sensing tip and the sample
surface. In the preferred embodiment, two different styli with
known tip offsets are used in this process. The first stylus is
used to scan while in contact with the surface to find the feature.
The second stylus is then used to measure the feature.
[0017] In yet another aspect of the invention, a sensor assembly
having a sensing probe with a sensing tip is employed. When the
sensing tip of the probe is used for sensing a sample, the vertical
distance moved by the sensor assembly (or by both the assembly and
the surface in aggregate) until the tip contacts the surface within
a feature of interest may be taken as the depth of the feature. To
determine that the tip has contacted the surface, the sensor
assembly is driven towards the surface until the distance moved by
the tip relative to the assembly exceeds a threshold, at which
point the vertical relative motion between the tip and the surface
is stopped and the vertical distance moved by the assembly (or the
sum of the distances moved by both the assembly and surface) is
noted to indicate the depth of the feature. To yield a more
accurate measure of such depth, the motion of the probe tip
relative to the assembly is taken into account in calculating such
depth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1A is a schematic view of a prior art stylus sensor
assembly, similar to that of U.S. Pat. No. 5,309,755.
[0019] FIG. 1B is a schematic view of how a high aspect-ratio
sensing tip may be destroyed by large lateral forces generated when
the sensing tip contacts a side wall in a conventional profiling
scheme.
[0020] FIG. 1C is a schematic view of a sensing tip of the stylus
sensor assembly and the high aspect-ratio feature illustrating the
arcuate motion of the stylus, or its parasitic motion, which
prevents the tip from reaching the bottom of high aspect-ratio
features in a conventional profiling scheme.
[0021] FIG. 1D is a schematic view illustrating how the probe of
the stylus sensor assembly may be lowered without rotation in order
to reach the bottom of high aspect-ratio features.
[0022] FIG. 2A is a schematic view of a profiler which includes the
stylus sensor assembly of FIG. 1A controlled by a digital signal
processor and mounted to both a Z stage and an XY stage to
illustrate the preferred embodiment of this invention.
[0023] FIG. 2B is a schematic view of an atomic force microscope
(AFM) with an XYZ stage for moving the sample to illustrate this
invention.
[0024] FIG. 3 is a schematic view of the scanning path of the
sensing tip in the profiler of FIG. 2A to illustrate one embodiment
of the invention.
[0025] FIG. 4A is a schematic view of the scanning path of the
sensing tip in the profiler of FIG. 2A or the AFM in FIG. 2B to
illustrate a fast find mode of this invention.
[0026] FIG. 4B is a schematic view of the scanning path of the
sensing tip in the profiler of FIG. 2A or in the AFM in FIG. 2B to
illustrate another embodiment of the fast find mode of this
invention.
[0027] FIG. 5 is a schematic view of the scanning path of the
sensing tip in the profiler or in the AFM of FIGS. 2A and 2B along
an inclined surface of the sample with a feature to illustrate
another aspect of the invention in the fast find mode.
[0028] FIG. 6 is a schematic view of the scan path of the sensing
tip in the profiler or in the AFM in FIGS. 2A and 2B to illustrate
the fast profiling mode, where the tip is positioned using
knowledge of the surface height distribution of the surface to be
scanned.
[0029] FIG. 7A is a schematic view of the scan path of the sensing
tip in the profiler or in the AFM of FIGS. 2A and 2B in a quick
step mode of this invention where no surface height distribution
information is utilized.
[0030] FIG. 7B is a schematic view of a scan path of the sensing
tip in the profiler or in the AFM of FIGS. 2A and 2B in a quick
step mode of this invention, illustrating how the sensing tip
senses an inclined side wall.
[0031] FIGS. 8A-8C are cross-sectional views of a surface in
intermittent search paths employing the sensing tip of FIG. 2A or
2B to illustrate another embodiment of the invention. This
embodiment is taken from parent application Ser. No.
08/730,641.
[0032] FIG. 9A is a schematic view of a stylus arm and two sensing
tips to illustrate another embodiment of the invention.
[0033] FIG. 9B is a simplified perspective view of a portion of a
stylus sensor assembly with two probes, each probe having a sensing
tip to illustrate another embodiment of the invention.
[0034] For simplicity in description, identical components are
identified with the same numerals in this application.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] FIG. 1A is a simplified schematic view of a stylus sensor
assembly similar to the one of U.S. Pat. No. 5,309,755 useful for
illustrating this invention. As shown in FIG. 1A, stylus sensor
assembly 10 includes a base portion which includes the support body
12 and a transducer support 14 rigidly attached together. The
support body 12 has a pivot 16 therein equipped with a spring so
that the pivot 16 is a flexure pivot. Connected rotatably to the
flexure pivot 16 is one end of a stylus arm 18 having a sensing tip
or stylus 20 at or near the other end of the stylus arm. The stylus
arm 18 is connected to a vane 22 on the other side of the pivot 16.
Therefore, when the stylus 20 contacts a surface and is caused to
move up or down, causing the stylus arm 18 to also rotate, vane 22
will rotate as well, thereby causing a change in the capacitance
between the two capacitor plates 24 which are supported by the
transducer support 14. The change in capacitance is fed to a
digital signal processor (shown in FIG. 2A) which computes the
distance rotated by stylus 20 from the change in capacitance.
[0036] The stylus arm 18 is biased downwards so that stylus 20
applies a force against the surface that is being profiled. This
biasing is accomplished by means of a force coil 26 through which a
current is passed. The amount of current passed through the force
coil is such that stylus 20 applies a predetermined force against a
surface during profiling. When the tip is rotated up or down by the
sample, the flexure spring is stretched or compressed, where such
stretching or compression applies a variable force in addition to
the force applied by the current in the force coil. As a further
improvement as set forth in U.S. Pat. No. 5,705,741, the effect of
the flexure spring when the tip is rotated about pivot 16 is
calibrated so that the amount of current applied to the force coil
26 is altered as a function of the displacement of stylus 20, to
thereby substantially cancel out the effect of the flexure spring
on the force applied by the tip 20 to the sample. In this manner,
the force applied by stylus 20 to the sample surface remains
constant despite rotation and displacement of the stylus.
[0037] FIG. 1B illustrates a possible scenario where a high
aspect-ratio stylus 20 of the stylus sensor assembly 10 is used to
profile a high aspect-ratio feature, such as a deep trench or via
hole. As shown in FIG. 1B, stylus or sensing tip 20 scans the
surface 40 of a sample, by causing lateral relative motion between
the stylus 20 and surface 40, shown along the X axis in FIG. 1B.
When the sensing tip 20 enters a high aspect-ratio feature, such as
a trench or via hole 42, large lateral forces (forces in directions
substantially along or parallel to the sample surface 40, such as
along the X axis) will be generated, which may cause a portion of
the stylus or sensing tip 20 to break off when it contacts the side
wall of the feature.
[0038] As noted above, stylus 20 is rotated about pivot 16 by the
sample surface 40 when it is scanned across the sample surface.
Therefore, stylus 20 travels along a path 44 which is curved or
arcuate in shape. For high aspect-ratio features, such arcuate
motion of the stylus or sensing tip 20 may make it impossible for
the sensing tip to reach the bottom of the trench or hole 42.
[0039] This invention is based on the recognition that, in order to
avoid the problems described above in reference to FIGS. 1B, 1C,
instead of rotating stylus or sensing tip 20 about pivot 16 in
order to measure the bottom of high aspect-ratio features, the
stylus 20 is simply lowered vertically into the feature 42 without
rotating the stylus 20 or stylus arm 18 about pivot 16 as
illustrated in FIG. 1D. As shown in FIG. 1D, if the stylus 20 is
simply lowered into the feature 42 without moving the tip and the
sample laterally relative to each other and without substantially
rotating the arm about the pivot, then it is possible for stylus or
sensing tip 20 to reach and measure the bottom of feature 42.
[0040] In order to lower the stylus 20 into the feature, stylus
sensor assembly 10 is mounted onto a Z stage 62 and an XY stage 64
as illustrated in FIG. 2A. FIG. 2A is a schematic view of a
profiler which includes the stylus sensor assembly 10 of FIG. 1A
controlled by a digital signal processor 52 and mounted to both a Z
stage and an XY stage to illustrate the preferred embodiment of
this invention. As shown in FIG. 2A, the transducer support 14 of
the base portion of stylus sensor assembly 10 is attached to the Z
stage 62 which, in turn, is attached to the XY stage 64. Therefore,
the stylus 20 may be lowered into the feature 42 of FIG. 1D by
means of Z stage 62, by lowering the entire stylus sensor assembly
10. In this manner, the above-described problems associated with
the operation in FIGS. 1B, 1C can be avoided. Z stage 62 raises or
lowers the stylus sensor assembly 10 without moving the stylus 20
laterally relative to the sample surface 40. The XY stage 64 causes
lateral relative motion, for example, in the X direction, between
the stylus 20 and the sample surface 40; the X direction is
substantially parallel to the sample surface 40.
[0041] FIG. 3 is a schematic view of a scan path of the stylus tip
20 of FIG. 2A for finding and measuring a feature in the surface to
illustrate one embodiment of the invention. Thus, stylus sensor
assembly 10 is positioned so that stylus 20 is at position 72 above
a location 74 of the sample surface 40. Stylus 20 is then lowered
by means of Z stage 62 by lowering the entire stylus sensor
assembly until it is determined that stylus 20 touches or is in
contact with surface 40 at location 74. Stylus 20 is then raised,
again by means of Z stage 62 by raising the entire stylus sensor
assembly 10 until the stylus 20 is again at the starting point 72.
The XY stage 64 then causes lateral relative motion between the
sample surface and the stylus 20, by moving the stylus sensor
assembly 10 and the Z stage 62 along the X direction by a
predetermined step size dx to point 76 which is above another
location 78 of the sample surface 40, where location 78 is spaced
apart from but adjacent to location 74. The Z stage 62 is again
used to lower stylus 20 by lowering the stylus sensor assembly 10
until it is determined that stylus 20 is in contact with the
surface 40 at location 78. Stylus 20 (and the stylus sensor
assembly) is then raised by means of Z stage 62 back to point 76
and lateral relative motion between the sample surface and stylus
20 is again caused by XY stage 64. The lateral motion step size dx
may be in a range of about 1 nm to 50 mm.
[0042] It is noted from the above description that when the
distance separating the stylus or sensing tip 20 and the sample
surface 40 is increased or decreased, there is substantially no
lateral relative motion between the sample surface and the stylus.
Points 72, 76 . . . to which the stylus 20 is raised after contact
with the sample surface are at a distance of Z2 above sample
surface 40. In the embodiment of FIG. 3, after each time the stylus
contacts the surface 40, the stylus is raised by the same amount Z2
above the location of the sample surface it was in contact with
prior to the raising. Therefore, as will be shown below, if this
distance Z2 is greater than the depth of any feature that the
stylus may encounter during the scanning motion of the stylus
across sample surface 40, then the stylus would not come into
lateral contact with any side walls of the sample surface 40 to
cause the type of problems described above in reference to FIGS.
1B, 1C.
[0043] Thus, the above process of lowering, raising and lateral
movement in reference to FIG. 3 is repeated until the stylus is at
point 82 above location 84 within a feature 42a of the sample
surface. The Z stage 62 lowers the stylus sensor assembly and the
stylus until the stylus contacts the sample surface at location 84
within the feature 42a. Then when the Z stage 62 raises the stylus
by distance Z2, since Z2 is greater than the depth Z of any feature
on the sample surface 40, including the depth of feature 42a, after
the Z stage 62 raises the stylus to point 86, point 86 will be at
an elevation higher than sample surface 40. Therefore, when the XY
stage 64 causes lateral relative motion between the sample surface
40 and stylus 20 along the X axis, stylus 20 will not come into
contact with any side walls of feature 42a. Thus, the scanning
process continues until the stylus reaches point 88 where it is
above location 90 on surface 40 outside feature 42a. The Z stage 62
lowers stylus 20 by a short distance when it comes into contact
with surface 40 at location 90. Then again, Z stage 62 raises the
stylus to point 92 by a distance of Z2 above location 90 before the
XY stage 64 again causes lateral relative motion between the sample
surface 40 and stylus 20, to repeat and continue the
above-described intermittent contact scanning across surface 40.
Obviously, in all of the embodiments of this application, instead
of moving the stylus sensor assembly of the profiler (or of AFM,
SPM) to cause relative motion between the sensing tip and the
sample surface, such relative motion can be caused by moving the
sample instead or a combination of motions of the sample and of the
sensing tip; all such variations are within the scope of the
invention.
[0044] In reference to FIG. 2A, when stylus 20 comes into contact
with surface 40, surface 40 will cause stylus arm 18 and stylus 20
to be rotated about pivot 16 while Z stage 62 continues to lower
stylus sensor assembly 10 towards the surface 40, while system 60
is determining whether the tip 20 has contacted the sample surface.
In order to be certain that stylus 20 has indeed come into contact
with surface 40, it is preferable for a threshold distance to be
set, and it is determined that stylus 20 has come into contact with
surface 40 only after the distance rotated by stylus 20 about pivot
16 has exceeded the threshold distance. Such threshold distance may
be a parameter that can be set through the digital signal processor
52. Thus, when the change in capacitance across capacitance plates
24 caused by rotation of stylus arm 18 and vane 22 indicates that a
distance rotated by stylus 20 is equal to or has exceeded the
threshold distance, the digital signal processor 52 will send a
signal along one of the lines 54 to Z stage 62 to cause the Z stage
to stop lowering the stylus sensor assembly 10.
[0045] Since the stylus sensor assembly of the profiler continues
to be lowered towards the sample after the tip 20 touches the
sample and the lowering motion is stopped only after the tip has
been rotated by a threshold distance, the stylus sensor assembly
has been lowered by a distance greater than the distance traveled
by the sensing tip, by the distance rotated by the tip. In order to
measure the height of surface 40, the Z stage 62 records the
distance that the stylus sensor assembly 10 has been lowered until
it is determined that stylus 20 has come into contact with surface
40. If such distance is taken as the distance between the starting
point of stylus 20 and the end point in the lowering process, such
as the distance between point 72 and 74, such distance may actually
be greater than the distance traveled by stylus 20, by the distance
that is rotated by stylus 20, after the tip contacts the surface
but before the Z stage 62 stops lowering stylus sensor assembly 10.
The DSP 52 sends a signal along one of the lines 54 to the
controller (not shown) of the Z stage 62 to indicate the actual
distance rotated by stylus 20 before motion of the Z stage is
stopped and the controller of the Z stage will then subtract such
distance from the distance that the Z stage 62 has moved stylus
sensor assembly 10, to obtain a more accurate measure of the actual
distance traveled by stylus 20. This will give a more accurate
measure of data related to the height of sample surface 40 and of
the profile of any features of interest in the sample surface.
[0046] The methods of this invention described herein for finding
and measuring surface features may be carried out by means of SPMs
instead of profilers. In such event, instead of raising or lowering
a stylus sensor assembly, one would raise or lower a SPM sensor
assembly. Where an AFM is used instead of a profiler stylus sensor
assembly as the sensing probe, a threshold distance can also be
defined to determine when the probe tip has come into contact with
the sample surface. FIG. 2B is a schematic view of an AFM with an
XYZ stage 91 for moving the sample to illustrate this invention. As
shown in FIG. 2B, the amount of bending of the cantilever arm 92 is
monitored in a conventional manner (such as by sensing the strain
in the arm 92, or by the amount of bending of the arm), and
controller 94 causes the probe or the arm to be lifted or lowered
to maintain a constant strain or amount of bending in the arm. If
the tip 92a of arm 92 has come into contact with the sample
surface, this will cause a strain and bending of the arm. In other
words, when the sensing probe carrying the sensing tip or stylus
approaches the sample surface, if the tip comes into contact with
the surface, the continued motion of the probe towards the surface
will cause the arm to bend and strain to develop in the arm, where
the strain developed and the amount of bending will correspond to
the distance moved by the probe and sensor assembly after the tip
is in contact with the surface. Thus a threshold distance may be
set which corresponds to a value for the strain or bending of the
arm. By sensing whether the strain or bending of the arm has
exceeded such value, it is possible to determine whether the tip
92a has come into contact with sample surface. The same process can
be performed for other SPMs such as the scanning tunneling
microscope, by setting a threshold value for the current between
the tip and the sample, for example.
[0047] Measurements of surface profiles and depths of features
using the SPM can be improved in accuracy in the same manner as
that described above. Where the sensing tip of a AFM is used in the
depth measurement, for example as shown in FIG. 2B, the probe 92
and tip 92a are caused to approach the surface until the tip 92a
touches the surface, and the probe is driven further towards the
surface until it is determined that the strain in the probe or
amount of bending of the probe has reached a certain threshold. The
distance moved by the tip relative to the SPM probe positioner 96
(which forms a base portion of the AFM sensor assembly, not
completely shown in FIG. 2B) when the sensor assembly is driven
towards the surface should be taken into account (e.g. by
subtracting such distance from the depth measurement) when
calculating the depth of a feature.
[0048] While the process or mode of operation illustrated in FIG. 3
may be advantageous for some applications, it may be time consuming
and cumbersome for other applications, since the stylus must be
raised to a distance greater than the expected height variation or
height distribution of sample surface 40. This is true especially
where the sample surface 40 is inclined or tilted. In such
circumstance, or in others where one is unsure of the amplitude of
height variations of the sample surface, in order to avoid lateral
contact between the stylus and any side walls, the stylus must be
raised to relatively large heights above the sample surface 40.
This may be time consuming and cumbersome.
[0049] In order to avoid having to raise the stylus by distances
much larger than the actual height variations of the sample
surface, it will be useful to have some prior knowledge of the
height distribution of the sample surface (e.g. within a target
area) before scanning starts. For example, if the portion or point
of the highest elevation of the sample surface is known, the stylus
20 or 20' may be positioned at a point directly above or close to
such highest point or portion before scanning starts. Then such
starting point and the distance by which the tip is subsequently
raised above the prior point of contact with the sample surface can
be much reduced. In one embodiment, such distance can be in a range
of about 100 to 500 nanometers. Then the above-described procedure
in reference to FIG. 3 may be carried out without the risk of the
stylus coming into lateral contact with a side wall of the sample
surface, where Z2 can be reduced to the sum of the expected feature
depth and a shorter distance such as one in a range of about 100 to
500 nanometers.
[0050] When prior knowledge of the sample surface 40 is not
available before scanning starts, it may be a simple and fast
procedure to obtain such height distribution information by
carrying out the process as illustrated in FIG. 3, with Z2 at a
large value, but only at a few sampling locations of surface 40,
such as 3 to 25 locations. Since the heights of only a few
locations of the sample surface 40 are measured, this process will
not take an inordinate amount of time even when a large value of Z2
is used. Typically, the user is able to position the stylus sensor
assembly over the general area of a feature of interest, such as at
a point that is above a surface location within about 1 or 2
microns from the feature of interest. Therefore, a target area of
several microns by several microns (e.g. 2 by 2 microns) may be
defined, and the several sampling locations chosen within the
target area. The above-described process in reference to FIG. 3 may
be carried out only at such locations in such small target area to
find out the height distribution over such area. After the height
distribution of the target area is known, then the stylus may be
positioned at a point which is above a portion of the sample
surface which is at or close to the point of highest elevation in
the distribution. It should be noted that, even if the stylus is
not placed immediately above the point of the highest elevation of
the target area, as long as the distance by which the stylus is
raised after contacting such point causes the stylus to be higher
in elevation than any portion of the sample surface within the
target area, the stylus will not come into lateral contact with any
portion of the sample surface to damage the stylus in the
subsequent lateral relative motion between the tip and the surface.
This allows the user a higher tolerance in positioning the tip.
[0051] The total time required for finding and measuring features
of interest on a sample surface can be further reduced by
separating the process into two parts: a first fast find mode in
which the stylus does not necessarily completely penetrate the
features of interest so as to first find the features, and a second
measuring mode for measuring the features found. This permits the
stylus to be raised or lowered by short distances during the fast
find mode in order to find the feature, and a subsequent
measurement mode in which the stylus penetrates the features by
touching the sample surface at one or more locations within a
feature of interest for measuring data related to the height of the
feature and of the surrounding sample surface. This is illustrated
in reference to FIGS. 4A, 4B.
[0052] FIG. 4A is a schematic view of a scan path illustrating a
fast find mode for finding a feature. In the embodiment of FIG. 4A,
it is assumed that information concerning height variations of
sample surface 40 is available or has been measured, so that such
information can be used to position the sensing tip of a stylus
sensor assembly or SPM above the sample surface 40. Such
positioning is particularly simple where the sample surface is
characterized by two discreet levels or heights as illustrated in
FIG. 4A Thus, outside of the feature 42a, the sample surface is at
a particular height, and within the feature, the sample surface has
two side walls 42a(s) and a bottom portion 42a(b) at a depth Z
below the top surface. In such event, as long as the starting
position of the sensing tip is above the top surface by a certain
distance, and the sensing tip is raised to the same elevation as
the starting position above the sample surface each time the
sensing tip is raised and separated from the sample surface, the
sensing tip will not come into lateral contact with the sample
surface such as side wall 42a(s).
[0053] To speed up the process of scanning in the fast find mode,
the Z stage 62 lowers the sensing probe of the stylus sensor
assembly or of the SPM until either the sensing tip of the probe
touches the surface 40 of the sample or has been lowered by a
preset distance. In one example, the sensing tip is placed a short
distance (e.g. approximately 100-500 nanometers) above the top
(upper) surface of the sample at the starting point. In a process
similar to that described above in reference to FIG. 3, the sensing
tip is lowered until it is determined that the sensing tip has come
into contact with the sample surface without any substantial
lateral relative motion between the sensing tip and the sample
surface, raised again after such contact also without any
substantial lateral relative motion therebetween to a point, moved
laterally across the surface of the sample and the process repeated
over a target area of the sample surface, similar to the process of
FIG. 3.
[0054] The sample surface 40 has two discrete levels: a top level
of surface portion 40t and the level of the bottom surface 42a(b)
within the feature 42a. In this example, the sensing tip is raised
by a short distance in a range of about 100-500 nanometers. In one
embodiment, the tip is raised to about 300 nanometers above the top
surface after each contact with the top sample surface 40t, except
when the sensing tip is directly above bottom surface 42a(b) of the
feature 42a, at position 104. When the sensing tip is lowered into
the hole or trench 42a, the lowering of the sensing probe is
stopped after the probe has been lowered by a preset distance even
though it is determined that the sensing tip has not come into
contact with the sample surface. This is determined by the fact
that the tip has not been caused to rotate by at least the
threshold distance as described above, for example. In the same
example, such preset distance may be 450 nanometers. In the
embodiment of FIG. 4A, where the sample surface is at two discreet
levels, this means that the sensing tip will be lowered to a
position 106 approximately 150 nanometers below the top discreet
level of the sample surface 40t at which point the lowering motion
of the sensing probe would be stopped. The sensing tip is then
again raised by the Z stage 62. In the embodiment of FIG. 4A, it is
raised by the same distance each time, whether or not the sensing
tip has contacted the sample surface, or in other words, by 300
nanometers. This means that when the sensing tip is scanning
immediately above feature 42a, the sensing tip will be raised to
only 150 nanometers above the top level of the sample surface 40t,
to position 108. This process is repeated until the sensing tip is
at position 110 at which point the XY stage 64 moves the sensing
probe along the X axis to a position 112. Since points 110 and 112
are still approximately 150 nanometers above the top surface 40t of
the sample (FIG. 4A not drawn to scale), the sensing tip will not
come into contact with the sample during this lateral relative
motion. The sensing tip may then again be lowered by the Z stage
which lowers the entire sensing probe until the tip touches the
surface at position 114. The tip is then raised to position 116 and
the process is then repeated across the remainder of the sample
surface within the target area.
[0055] The location of the feature can then be determined by
recording the XY positions of the points at which the sensing tip
was lowered by the preset distance but did not come into contact
with the sample surface. After such process, the system is operated
in a measurement mode in which the probe is lowered into feature
42a until it is determined that the sensing tip has come into
contact with the bottom 42a(b) of the feature of 42a, in order to
measure data related to the height or depth of the bottom surface
of the feature. Since features of interest are typically small, the
measurement mode is normally not very time consuming.
[0056] FIG. 4B is a schematic view of a scan path and of a sample
surface similar to those illustrated in FIG. 4A, but where the
sensing tip and the sensing probe are raised by a greater distance
when they are immediately above the feature than when they are not.
In the embodiment of FIG. 4B, for example, the sensing tip is
raised to substantially the same elevation throughout the scan
across the surface in the target area. Thus, as in the example of
FIG. 4A, when the sensing tip and probe are above the sample
surface but not above the feature 42a, the sensing tip is raised to
approximately 300 nanometers about the top surface of the sample.
Where the probe is immediately above feature 42a, the probe is
raised so that the sensing tip is retrieved to the same elevation
as the starting point 102. If the sensing tip is allowed to be
lowered into the feature beneath the top surface of the sample by
150 nanometers (or lowering the tip by a total of 450 nm) to point
122 at which point the Z stage stops lowering the probe and sensing
tip, then the Z stage would raise the probe and sensing tip by such
preset distance which is 450 nanometers so that at point 124, the
sensing tip is at the same elevation as the starting position 102.
Aside from such difference, the scanning mode in FIG. 4B is
substantially the same as that illustrated above in reference to
FIG. 4A.
[0057] The scanning process described above in reference to FIGS.
4A and 4B is quite effective where the sample surface is level and
the Z stage 62 raises and lowers the sensing probe and tip in
directions substantially normal to the sample surface. In other
words, the Z direction of motion of the Z stage 62 is substantially
perpendicular to the sample surface 40. Where this assumption is
not true, as would be the case where the sample surface is tilted
with respect to the Z direction of motion of the Z stage, or where
the sample surface has a portion that is inclined and has a slope,
the above-described schemes may still cause the sensing tip to come
into lateral contact with the sample surface, which may result in
tip damage. Applicants have developed a technique to avoid such
contact as described above in reference to FIG. 5.
[0058] As shown in FIG. 5, the sample surface is inclined with a
certain slope. Thus, if the scanning process described above in
reference to FIGS. 4A, 4B are employed, when the sensing tip is in
or above the feature 42b and scanning along the dotted line 132,
the sensing tip may come into contact with the side wall 42b(s) of
the feature 42b, thereby causing tip damage. The above scenario can
be avoided by actually measuring the slope of the top surface 40t'
during the fast find mode and use such data to predict the
elevation of the next location of the sample surface to be sampled.
Thus, from data related to the height of the top surface 40t' at
the various locations of the surface taken before the sensing tip
reaches the feature 42b, a slope of the top surface 40t may be
derived by means of the digital signal processor 52. Such slope may
then be used to predict the elevation of another portion of the top
sample surface 40t' yet to be sampled by the sensing probe.
Therefore, even though the sensing tip does not come into contact
with the sample surface when it is immediately above feature 42b,
such slope information is used and extrapolated so that the
scanning path follows the general slope of the top surface 40t and
so that when the scanning probe reaches the vicinity of side wall
42b(s), the sensing tip will be at an elevation above the side wall
and not come into contact with it, as shown by the solid line scan
path in FIG. 5. Where the top surface 40t' of the sample is
relatively flat, its slope can be calculated readily. Where the top
surface 40t' of the sample is not flat, the elevation of the next
location of the sample surface to be sampled may be predicted by a
curve fitting process which is known to those skilled in the art
and will not be elaborated herein for that reason.
[0059] In some applications, it may be desirable to use information
concerning height distribution of the surface that is provided or
obtained as described above, and then simply find and measure
features of interest within a target area, without separating the
process into a finding process and a measuring process. This is
illustrated in FIG. 6. Thus, as in the case of FIG. 4A, height
information of the surface 40 is employed to position the sensing
tip at a location 102 above the sample surface, where one is
certain that the starting position of the sensing tip is at an
elevation higher than any part of the target area of the sample to
be measured. One can then be certain that the sensing tip will not
come into lateral contact with the sample surface to cause tip
damage. In the scanning process across the sample surface, the
separation between the sensing tip and the sample surface is
controlled so that the sensing tip comes into contact
intermittently with the sample surface, where the sensing tip is
lowered to contact the sample surface, and raised to the same
elevation as the starting position 102 after each contact with the
sample surface, without causing relative lateral motion between the
sensing tip and the sample. Lateral relative motion is caused only
when the sensing tip has been raised to the same elevation as that
of the starting point 102. In other words, lateral motion is caused
only when the sensing tip is at an elevation higher than all points
in the target area of the sample surface so that the sensing tip
will not come into lateral contact with the sample surface while
lateral relative motion is caused between the sensing tip and the
sample.
[0060] Thus, from the above several embodiments, it will be seen
that a useful method for sensing a feature on the surface of a
sample employing a sensing tip has been described. The sensing tip
is positioned above one location of the surface. A distance between
the surface and the sensing tip is reduced without substantially
moving the tip and the surface laterally relative to each other. In
the embodiment of FIG. 6, for example, the distance is reduced
Until the tip touches the surface. In the embodiments of FIGS. 4A,
4B and 5, the distance is reduced until the tip touches the surface
or until the tip or the surface has traveled, or the tip and the
surface together have traveled in aggregate, by a preset distance
without the tip contacting the surface. A distance or separation
between the tip and the surface is then increased without
substantially moving the tip and the surface laterally relative to
each other until such distance is substantially equal to a
predetermined value. The predetermined value is such that after
such separation has been increased to the predetermined value, the
tip is higher in elevation than another location of the surface
adjacent to and spaced apart from the one location. (This
predetermined value may be one that is calculated and predicted
from a measurement of the sample surface, such as that illustrated
above in reference to FIG. 5 where the slope of the top surface 40t
is measured. Alternatively, such predetermined value may be arrived
at by using knowledge of the height distribution of the sample
surface, such as by measuring at a few sampling locations the
height distribution of the sample surface within a target area as
described above.) Lateral relative motion is then caused between
the sensing tip only after the tip is raised by a distance equal to
such predetermined value from the surface of the sample and the tip
is positioned so that it is above another location. The steps
immediately described above of reducing the distance, increasing
the distance and causing lateral relative motion are then repeated
at a plurality of locations of the surface to find or measure the
feature. In the embodiments of FIG. 4A, 4B, during the fast find
mode, the height of the sample surface is not measured. In the
embodiment of FIGS. 5 and 6, on the other hand, data related to the
height of the sample surface is measured. During such measurements,
it is preferable for the sensing tip to be stationary and in
contact with the sample. In the preferred embodiment, the
predetermined value for judging how far to raise the tip may be not
more than about 1 micron.
[0061] For many applications, it is desirable to quickly scan the
surface to find and measure any features of interest without undue
risk of damage to the tip and of obtaining false data. This is
known as the quick step mode as illustrated in FIGS. 7A, 7B. In
this mode, the sensing tip of a stylus sensor assembly (of a
profiler or SPM) is placed at a small distance above one location
of the sample, such as one between 100 and 500 nanometers. In one
embodiment, the tip is raised to around 300 nanometers above the
sample. The sensing tip is then scanned across the surface of the
sample with the distance separating the sample and the tip
controlled so that the sensing tip contacts the sample
intermittently as it is scanned across the sample surface 40. After
each contact with the sample, the sensing tip is raised to a small
distance above the surface of the sample preferably without any
substantial lateral relative motion between them, before lateral
relative motion is caused between the sample and the tip. Such
small distances may be the same during each raise and may be
substantially the same as the initial separation, namely, about 300
nanometers. This mode of scanning runs the risk of the sensing tip
contacting a side wall such as 42a(s) at location 152 as shown in
FIG. 7A but if the step size of the lateral relative motion is
chosen to be small, the sensing tip or stylus of the stylus sensor
assembly or SPM may be able to stand the stress generated by the
side wall impact at location 152. Thus, the mode of operation in
the quick step mode of FIG. 7A is similar to that shown in FIG. 3,
except that the vertical distances raised by the Z stage 62 may not
be adequate to avoid lateral contact with the side wall. The quick
step mode of FIG. 7A, however, differs from that in FIG. 3 in that
it is further determined whether the sensing tip and the surface
are in contact after a vertical step has been taken to increase the
distance between the sensing tip and the sample.
[0062] After a vertical step has been completed, the desired
lateral relative motion is made. The tip is then made to approach
the surface. If the tip rises relative to the sensor assembly as
soon as the approach begins, no gap between the tip and the surface
had resulted from the initial vertical step. This can be determined
since the movement of the tip is monitored. Alternatively, the
amount of movement of the tip relative to the sensor assembly can
be monitored when the tip approaches the surface to determine
whether the distance moved by the tip relative to the assembly has
exceeded the set threshold as described above. In either case,
another vertical step to increase the vertical separation between
the sensing tip and the sample is carried out, at which point it is
again determined whether the tip and the surface are in contact. In
reference to FIG. 7A, for example, when the sensing tip contacts
the side wall 42a(s) at location 152, the sensing tip will attempt
to move laterally but will stay in contact with the side wall at
location 152. The sensing system will then determine whether the
tip and the surface are still in contact. Since the tip and the
surface at location 152 are still in contact at such moment, any
downward motion of the sensor assembly will cause the stylus to
move relative to the assembly by the same amount and the downward
motion is terminated. The sensor assembly is instead raised by
another vertical step to location 154. At such location, the
measurement system determines that the sensing tip and the sample
are still in contact. The sensing tip is therefore again raised
vertically to position 156 in contact with the side wall. This
process is then repeated until the sensing tip reaches the position
162 after the vertical step, at which point the measuring system
determines that the sensing tip and surface 40 are no longer in
contact. At this point, the sensing tip is again lowered to come
into contact with sample surface 40 at location 164 and raised
again to position 162 before it is moved laterally as before to
perform intermittent contact and measurement of the top surface 40t
of the sample. In order to reduce the probability of tip damage,
the steps of lateral relative motion may be over a lateral distance
less than about 100 nanometers. FIG. 7B illustrates essentially the
same process as in FIG. 7A, but as applied to a sample surface with
an inclined surface portion.
[0063] In the conventional mode of operation of the stylus sensor
assembly of FIG. 1A, a desirable tracking force is applied by
applying an appropriate current through the force coil 26 so that
stylus 20 applies a desired force against the sample surface that
is being scanned. In the different modes of operation described
above for this invention, however, the stylus or sensing tip of the
stylus sensor assembly would start at a position not in contact
with the sample surface. A preset value of a desired tracking force
is set at the DSP 52. To improve the stability of scanning, an
initial current is applied to the force coil 26 in FIG. 2A to
compress the spring in flexure pivot 16 and to hold the stylus 20
at a steady position while the stylus sensor assembly 10 is being
lowered and raised by the Z stage 62 or transported laterally by
means of stage 64. When the stylus 20 touches the sample surface, a
force will develop between the tip and the sample. Such force may
increase as the tip is rotated above pivot 16 due to the action of
the spring. If the force exerted by the spring caused by its
stretching or compression is precalibrated as described in U.S.
Pat. No. 5,705,741, then the amount of force between the tip and
the sample can be found by the amount of rotation of the stylus 20
about pivot 16. DSP 52 compares such force to a preset value stored
at the DSP. When the force between the tip and the sample reaches
the preset value, DSP 52 applies a control signal to a power supply
(not shown) to change the amount of current applied to the force
coil 26, so as to maintain the force between the tip and the sample
substantially at the preset value when the tip is rotated further
by the sample until the distance that the tip is raised is
substantially equal to the threshold value as explained above. The
threshold value may be set to a distance of not more than about 500
nanometers. In this manner, the threshold value for determining
whether the probe tip has come into contact with the sample surface
can be set by setting a value for a corresponding force between the
tip and the sample at the DSP 52. Similarly, a threshold value for
determining whether the probe tip has come into contact with the
sample surface can be set in the case of AFM or other SPM by
setting a value for a corresponding force between the tip and the
sample at the controller 94.
[0064] FIGS. 8A-8C together with their accompanying description
below are taken from the parent application.
[0065] In order to measure the profile or geometry of a surface, in
reference to FIG. 8A, system 20 lifts the probe tip by a
predetermined distance h from the surface, record the lateral
distance .delta.x traveled by the tip before it is lowered again to
touch the surface and record the distance by which the probe tip
has been lowered before it touches the surface again. Preferably,
the tip is again lifted from such point of contact by the distance
h, moved laterally by distance .delta.x, lowered again to touch the
surface, and the distance that the tip is lowered again recorded.
This process is then repeated until the scan across the target area
is completed. A record of such distance .delta.x and the distances
that the tip is repeatedly lowered before it touches the surface in
the intermittent contact mode throughout the scan will give an
indication of the geometry or profile of the surface.
[0066] In the embodiment of FIG. 8A, the probe tip is lifted after
it is lowered to touch the surface 200, without dragging the probe
tip along the surface. In other words, the probe tip is caused to
gently tap surface 200 before it is lifted and the probe tip is not
moved laterally across the surface while it is in contact with the
surface. In some applications, it may be desirable to drag the
probe tip along the surface after the tip is lowered to touch the
surface, in an embodiment illustrated in FIG. 8B. After the probe
tip has been dragged along the surface 200 for a predetermined
distance, the probe tip is again lifted by a predetermined
distance, such as h, moved laterally by a predetermined distance,
and then again lowered to touch the surface 200. After the tip
touches the surface, the tip is again dragged along the surface for
a predetermined distance and the above-described process repeated
until a scan across the entire target area is completed as before.
In the operational mode of FIG. 8B, in addition to recording the
quantities h, .delta.x and the distances by which the tip is
repeatedly lowered before it touches the surface in the
intermittent contact mode throughout the scan, system 20 also
records the change in height of the probe tip when the tip is
dragged along the surface 200. Such information, in conjunction
with h, .delta.x, and the distances by which the tip is lowered
before it touches the surface, will give an indication of the
geometry or profile of the surface when system 20 is operated in
the mode indicated in FIG. 8B.
[0067] Yet another operational mode of system 20 in the
intermittent contact mode is illustrated in FIG. 8C. Such mode is
similar to that in FIG. 8A, where in the operational modes of both
FIGS. 8A and 8C, the probe tip is not moved laterally to drag the
tip across the surface after the tip is lowered to touch the
surface, but is lifted to a predetermined height h. However,
instead of moving the probe tip up and down and laterally along
substantially straight lines as in FIG. 8A, the tip in FIG. 8C is
moved along a more or less sinusoidal path across surface 200 until
it scans across the target area. Such and other variations are
within the scope of the invention.
[0068] Where intermittent contact mode is employed, the values of
.delta.x and height h employed in reference to FIGS. 8A-8C are
chosen so that it is unlikely for the probe tip to "jump over"
bumps or valleys on a surface to be sampled. A suitable range for h
may be 10-1,000 Angstroms, and a suitable value for .delta.x may be
a fraction of the expected size of the feature or object.
[0069] In yet another mode of operation of the apparatuses in FIGS.
2A, 2B, the sensing tip may be scanned across a sample surface with
the tip in contact with the surface until the feature is found.
Then, the same or a different sensing tip may be used to scan the
surface with the tip in intermittent contact with the surface to
measure the feature. This is illustrated in FIG. 9A. As shown in
FIG. 9A, a scanning probe 300 includes a stylus arm 318 and a
sensing tip 320 attached to the stylus arm at or near one end of
the arm. At the tip of sensing tip 320 is a flexible sensing tip
322 such as a nanotube. Thus, the spatial relationship between
sensing tip 320 and flexible tip 322 is known, so that if a feature
of interest is located by means of the sensing tip, the other
sensing tip may be accurately positioned above the feature found
and used to measure the feature without again having to find the
feature. Thus, in one embodiment, arm 318 may be scanned across a
sample with sensing tip 320 in contact with the surface to find the
feature. After the feature has been found, the flexible tip 322 may
be used in an intermittent contact or contact mode to measure the
feature. Since the flexible tip in the form of a nanotube is long
and thin, it is particularly suitable for measuring high-aspect
ratio features. When sensing tip 320 is used to measure or find a
feature in a contact mode, the flexible tip simply buckles so that
sensing tip 320 may be used during the contact mode scan as if the
flexible tip is not present. Nanotubes are very flexible and will
not significantly affect the operation of the sensing tip 320 in
contact mode. After the feature has been found, sensing tip 320 is
withdrawn from the surface so that the flexible tip such as a
nanotube will snap back to the original geometry as shown in FIG.
9A and will be in a position to be used for sensing and measuring
the feature found.
[0070] FIG. 9B illustrates an alternative embodiment to that of
FIG. 9A. Instead of mounting two sensing tips on the same common
probe, two separate probes 318a and 318b may be used and two
different sensing tips 330 and 332 mounted respectively onto probes
318a, 318b may be used where the spatial relationship between the
two sensing tips is known. Therefore, one sensing tip, such as
sensing tip 330 may be used in a contact or intermittent contact
mode to find the feature. Since the spatial relationship between
the two tips is known, sensing tip 332 may then be readily
positioned accurately above the feature found for measuring the
feature in a contact or intermittent contact mode. The roles of the
two tips may of course be reversed so that sensing tip 332 may be
used for finding the feature and sensing tip 330 may be used for
measuring the feature.
[0071] While the invention has been described above by reference to
various embodiments, it will be understood that changes and
modifications may be made without departing from the scope of the
invention, which is to be defined only by the appended claims and
their equivalents.
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