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Claims  |
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What is claimed as new and desired to be secured by Leters Patent of the
United States is:
1. In a method of operating an atomic force microscope including a probe
including a probe tip mounted on one end of a lever arm and wherein the
probe tip is scanned across the surface of a sample and data resentative
of the surface of the sample is gathered in relation to the positioning of
the lever arm as the probe tip is scanned, the improvement comprising:
oscillating the probe tip at or near a resonant frequency of the probe or a
harmonic of said resonant frequency and with a free oscillation amplitude
A.sub.o sufficiently great so that the oscillating probe tip does not
stick to the surface of the sample when the oscillating probe tip contacts
the surface of the sample;
positioning the oscillating probe tip so that the oscillating probe tip
repeatedly taps the surface of the sample with the probe tip repeatedly
contacting and breaking contact with the surface of the sample without
sticking to the surface of the sample;
translating the oscillating probe tip across the surface of the sample with
the oscillating probe tip repeatedly tapping the surface of the sample so
that the oscillation amplitude of the probe tip is stably affected due to
changes in topography of the surface of the sample; and
producing signals indicative of variations in the topography of the surface
of the sample in relation to changes in the oscillation of the oscillating
probe tip upon repeated tapping of the oscillating probe tip against the
surface of the sample during translation of the oscillating probe tip
across the surface of the sample.
2. In an atomic force microscope (AFM) wherein a probe including a probe
tip mounted on a lever arm is scanned across the surface of a sample and
data reflecting the surface of the sample is gathered in relation to
positioning of the lever arm as the probe tip is scanned, the improvement
comprising:
first transducer means for oscillating the probe tip with a free
oscillation amplitude A.sub.o sufficiently great, so that the oscillating
probe tip does not stick to the surface of the sample'when the oscillating
probe tip contacts the surface of the sample;
second transducer means for positioning the oscillating probe tip so that
the oscillating probe tip repeatedly taps the surface of the sample,
thereby repeatedly contacting and breaking contact without sticking to the
surface of the sample, and and for translating the oscillating probe tip
across the surface of the sample with the oscillating probe tip repeatedly
tapping the surface of the sample such that the amplitude of oscillation
of the probe tip is affected by repeatedly tapping the sample surface; and
means for monitoring the oscillation of the probe tip during translating of
the oscillating probe tip to produce a signal indicative of variations in
the topography of the surface of the sample in relation to changes in the
oscillation of the oscillating probe tip during tapping against the
surface of the sample and translation across the surface of the sample.
3. The AFM according to claim 2, comprising:
a fluid in which said sample surface and said probe are immersed.
4. The AFM according to claims 2 or 3, wherein said scanning means
oscillates said probe at or near a resonant frequency of said probe.
5. The AFM according to claim 2, comprising:
means for oscillating said probe laterally adjacent a step in the sample;
and
means for detecting the amplitude of oscillation of the probe tip resulting
from the probe tip tapping the sidewall of the step.
6. The AFM according to claim 5, comprising:
means for moving said probe vertically adjacent said step and obtaining a
vertical profile of said step.
7. The AFM according to claim 5, wherein:
said second transducer means comprises means for scanning said laterally
oscillated probe between opposed steps of a trench; and
said monitoring means comprises means for determining a width of the trench
as a function of changes in the amplitude of oscillation of the probe tip
as the probe tip taps said opposed steps of the trench.
8. The AFM according to claim 2, comprising:
means for scanning said probe laterally across a trench while oscillating
said probe and measuring a maximum amplitude of oscillation of said probe
during said scanning; and
means for determining a width of said trench as a function of the measured
maximum amplitude of oscillation and the width of the probe tip.
9. The AFM according to claim 2, wherein:
said lever arm comprises a strain gauge or a piezoelectric gauge; and
said means for determining data values determines data values in relation
to an output of said stain gauge or piezoelectric gauge.
10. The AFM according to claim 2, wherein said monitoring means comprises:
means for determining data values as a function of changes in amplitude of
oscillation of the probe tip.
11. The AFM according to claim 2, comprising:
feedback means for producing a control signal to control a distance between
an end of said lever arm opposite said probe tip and said sample during
translating of the probe tip to maintain the amplitude of oscillation of
the probe tip essentially constant; and
said monitoring means comprising means for producing said signal indicative
of surface topography based on the control signal produced by said
feedback means to maintain the amplitude of oscillation of the probe.
12. The AFM according to claim 11, comprising:
output means for outputting said control signal as a function of position
of said probe tip during translating of said probe tip.
13. In a method of operating an atomic force microscope including a probe
including a probe tip mounted on one end of a lever arm and wherein the
probe tip is scanned across a sample surface including a surface fluid
layer, and data rpresentative of topography of the sample is gathered in
relation to the positioning of the lever arm as the probe tip is scanned,
the improvement comprising:
oscillating the probe tip at a free oscillation amplitude A.sub.0 equal to
or greater than 20 nanometers;
interacting the oscillating probe tip with the sample surface so that the
oscillating probe tip contacts and breaks contact with the sample surface
without sticking to the sample surface to produce changes in the
oscillation as a function of the topography of the sample; and
producing signals representative of the topography of the sample in
relation to the changes in the oscillation produced in said interacting
step.
14. The method according to claim 13, wherein the oscillating step
comprises oscillating the probe tip at a frequency of oscillation which is
at or near a resonant frequency of the probe.
15. The method according to claim 13, wherein:
said interacting step comprises controlling the distance between an
opposite end of the lever arm opposite the probe tip and the sample so
that the amplitude of oscillation of the probe tip is essentially constant
during scanning of the probe tip; and
said step of producing a signal comprises determining data values
representative of the topography of the sample in relation to a control
signal produced in said controlling step to maintain the amplitude of
oscillation of the probe tip essentially constant.
16. The method according to claim 13, wherein said step of producing
signals comprises:
determining data values representative of the topography of the sample in
relation to changes in the amplitude of oscillation of said probe tip
during scanning of said probe tip.
17. In a method of operating an atomic force microscope including a probe
including a probe tip mounted on one end of a lever arm and wherein the
probe tip is scanned across the surface of a sample and data
representative of the surface of the sample is gathered in relation to the
positioning of the lever arm as the probe tip is scanned, the improvement
comprising:
oscillating the probe to produce a free oscillation amplitude Ao of the
probe tip;
providing a body of fluid on the surface of the sample with a depth
sufficiently great to cover said oscillating probe;
positioning the oscillating probe tip in the body of fluid so that the
oscillating probe tip repeatedly taps the surface of the sample with the
probe tip repeatedly contacting and breaking contact with the surface of
the sample without sticking to the surface of the sample;
translating the oscillating probe tip in said body of fluid across the
surface of the sample with the oscillating probe tip repeatedly tapping
the surface of the sample so that the oscillation amplitude of the probe
tip is stably affected due to changes in topography of the surface of the
sample; and producing signals indicative of variations in the topography
of the surface of the sample in relation to changes in the oscillation of
the oscillating probe tip upon repeated tapping of the oscillating probe
tip against the surface of the sample during translation of the
oscillating probe tip across the surface of the sample.
18. The method according to claim 17, wherein the free amplitude A.sub.o is
selected to be greater than 10 nanometers.
19. The method according to claim 18, wherein the free amplitude A.sub.o is
selected to be greater than or equal to 20 nanometers.
20. The method according to claim 1 or 17, wherein said oscillating step
comprises:
oscillating said probe tip at a harmonic resonant frequency of said probe
of less than or equal to 2 MHz.
21. The method according to claims 1 or 17, wherein said step of producing
signals comprises:
determining data values representative of the surface of the sample as a
function of change in amplitude of oscillation of said probe tip.
22. The method according to claims 1 or 17, comprising:
controlling the distance between an opposite end of the lever arm opposite
the probe tip and the sample so that the amplitude of oscillation of the
probe tip is maintained essentially constant during said translating
steps; and
said signal producing step comprising determining data values
representative of the topography of the surface of the sample based on a
control signal produced in said controlling step to maintain the amplitude
of oscillation of the oscillating probe tip essentially constant.
23. The method according to claims 1 or 17, comprising:
a) oscillating said probe tip laterally adjacent a step in said sample; and
b) detecting the amplitude of oscillation of the probe tip as the probe tip
taps a sidewall of the step.
24. The method according to claim 23, comprising:
mowing said probe tip vertically adjacent said step and repeating said
steps a) and b) to obtain a vertical profile of said step. in said sample;
and
b) detecting the amplitude of oscillation of the probe tip as the probe tip
taps a sidewall of the step.
25. The method according to claim 23, comprising:
scanning said probe between opposed steps of a trench;
determining a width of the trench in relation to the amplitude of
oscillation of the probe tip as the probe tip taps the opposed steps of
the trench.
26. The method according to claims 1 or 17, comprising:
scanning said probe tip laterally across a trench while oscillating said
probe tip laterally and measuring a maximum amplitude of oscillation of
said probe tip during said scanning; and
determining a width of said trench as a function of the measured maximum
oscillation.
27. The method according to claim 1 or 17, comprising:
using a probe tip which includes a magnetic material; and
measuring a change in the amplitude of oscillation of the probe tip due to
a magnetic field at the surface of the sample to obtain a map of a
magnetic characteristic of the sample surface.
28. The method according to claims 1 or 17, comprising:
using a lever arm having a strain gauge or a piezoelectric gauge; and
detecting a deflection of said lever arm in relation to an output of said
strain gauge or said piezoelectric gauge. 45 |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to an ultra-low force atomic force microscope, and
particularly an improvement to the atomic force microscope described in
related commonly owned U.S. patent application Ser. No. 07/361,545 filed
Jun. 5, 1989.
Discussion of the Background
Atomic Force Microscopes (AFM's) are extremely high resolution surface
measuring instruments. Two types of AFM's have been made in the past, the
contact mode (repulsive mode) AFM and the non-contact (attractive mode)
AFM.
The contact mode AFM is described in detail in U.S. Pat. No. 4,935,634 by
Hansma et al, as shown in FIG. 2. This AFM operates by placing a sharp tip
attached to a bendable cantilever directly on a surface and then scanning
the surface laterally. The bending of the lever in response to surface
height variations is monitored by a detection system. Typically, the
height of the fixed end of the cantilever relative to the sample is
adjusted with feedback to maintain the bending at a predetermined amount
during lateral scanning. The adjustment amount versus lateral position
creates a map of the surface. The deflection detection system is typically
an optical beam system as described by Hansma et al. Using very small
microfabricated cantilevers and piezoelectric positioners as lateral
and'vertical scanners, AFM's can have resolution down to molecular level,
and may operate with controllable forces small enough to image biological
substances. Since AFM's are relatively simple, inexpensive devices
compared to other high resolution techniques and are extremely versatile,
they are becoming important tools in a wide variety of research and high
technology manufacturing applications. The contact mode AFM, in which the
tip is maintained in continuous contact with the sample, is currently the
most common type, and accounts for essentially all the AFM's sold
commercially to date.
The contact AFM has found many applications. However, for samples that are
very soft or interact strongly with the tip, such as photoresist, some
polymers, silicon oxides, many biological samples, and others, the contact
mode has drawbacks. As pointed out in Hansma et al, the tip may be
attracted to the surface by the thin liquid layer on all surfaces in
ambient conditions, thus increasing the force with which the tip presses
on the surface. The inventors and others have also observed that
electrostatic forces may attract the tip to the surface, particularly for
some tipsample combinations such as silicon nitride tips on silicon oxide
surfaces. When the tip is scanned laterally under such conditions, the
sample experiences both compressive and shearing forces. The lateral
shearing forces may make the measurement difficult and for soft samples
may damage the sample. Further, a stick-slip motion may cause poor
resolution and distorted images. Hansma et al's approach to this problem
was to immerse the tip, cantilever, and sample surface in liquid, thus
eliminating the surface layer forces, and for a polar liquid, the
electrostatic forces. This technique works very well, and has the further
advantage that it allows samples that are normally hydrated to be imaged
in their natural state. However for many samples and applications,
immersion in liquid is not of much use. Operating in liquid requires a
fluid cell and increases the complexity of using the AFM, and for
industrial samples such as photoresist and silicon wafers, immersion is
simply not practical.
The non-contact AFM, developed by Martin et al, J. Applied Physics, 61(10),
15 May, 1987, profiles the surface in a different fashion than the contact
AFM. In the non-contact AFM, the tip is scanned above the surface, and the
very weak Van der Waals attractive forces between the tip and sample are
sensed. Typically in non-contact AFM's, the cantilever is vibrated at a
small amplitude and brought near to the surface such that the force
gradient due to interaction between the tip and surface modifies the
spring constant of the lever and shifts its natural resonant frequency.
The shift in resonance will change the cantilever's response to the
vibration source in a detectable fashion. Thus the amount of change may be
used to track the surface typically by adjusting the probe surface
separation during lateral scanning to maintain a predetermined shift from
resonance. This AC technique provides greater sensitivity than simply
monitoring the DC cantilever deflection in the presence of the attractive
Van der Waals force due to the weak interaction between the tip and
surface. The frequency shift may be measured directly as proposed by
Albrecht et al, J. Applied Physics, 1991, or indirectly as was done
originally by Martin et al.
The indirect method uses a high Q cantilever, such that damping is small.
The amplitude versus frequency curve of a high Q lever is very steep
around the resonant frequency. Martin et al oscillated the lever near the
resonant frequency and brought the tip close to the surface. The Van der
Waals interaction with the surface shifts the resonance curve. This has
the effect of shifting the resonance closer or further to the frequency at
which the lever is oscillated, depending on which side of resonance the
oscillation is at. Thus, indirectly, the amplitude of oscillation will
either increase or decrease as a consequence of the resonance shift. The
amplitude change is measurable (AM type detection). This change in
amplitude close to the surface compared to the amplitude far away from the
surface (the free amplitude) can be used as a setpoint to allow surface
tracking. The direct method measures the frequency shift itself (FM type
detection). Both methods are bound by the same interaction constraints.
FIG. 5 illustrates this non-contact operation. The tip is driven at a known
amplitude and frequency of oscillation, which is typically near a
cantilever resonance. The amplitude of this oscillation is detected by a
deflection detector, which can be of various types described in the
references. When the tip is sufficiently far away from the surface, it
will oscillate at the free amplitude, A.sub.o, as shown in FIG. 5. As
shown in FIG. 5, when the tip is brought closer to the surface, the Van
der Waals interaction will shift the resonant oscillatory frequency
slightly. This shift causes either an increased or decreased amplitude,
A.sub.s, or the frequency shift may be measured directly. This modified
amplitude value may be used as a setpoint in the manner of other above
described SPM's, such that as the tip is scanned laterally, the tip height
may be adjusted with feedback to keep setpoint, A.sub.s, at a constant
value. Thus an image of the surface may be generated without surface
contact, and without electrical interaction as needed by a scanning
tunnelling microscope STM. The resonant shift may also be caused by other
force interactions, such as magnetic field interaction with a magnetic
tip. Thus this type of AFM may in theory be easily configured to map a
variety of parameters using the same or similar construction.
The Van der Waals force is very weak, and decreases rapidly with
separation, so the practical furthest distance for measurable interaction
is 10 nm above the surface, as shown in FIG. 1, taken from Sarid, Scanning
Force Microscopy, Oxford University Press, 1991. To shift the resonance of
the lever, the lever must oscillate within this envelope of measurable
force gradient. If just a small portion of the oscillation is within the
envelope, the resonance will not be appreciably affected. Thus the
oscillation amplitude must be small. A compendium of all noncontact AFM
research can be found in Scanning Force Microscopy by Sarid, above noted,
no researcher was able to operate a non-contact AFM with a free
oscillation amplitude of greater than 10 nm. This limitation as will be
shown limits the usefulness of the non-contact method.
Although developed at essentially the same time as the contact AFM, the
non-contact AFM has rarely been used outside the research environment due
to problems associated with the above constraints. The tip must be
operated with low oscillation amplitude very near the surface. These
operating conditions make the possibility very likely of the tip becoming
trapped in the surface fluid layer described by Hansma et al. This effect
is illustrated in FIG. 6, an amplitude versus displacement curve. A
cantilever with probe is oscillated at a free amplitude A.sub.o, and the
vertical position of the fixed end of the lever is varied from a height
where the probe is not affected by the surface to a point where the probe
is captured by the surface and oscillation ceases. The curve is typical
for oscillation amplitudes of 10 nm or less. Such curves have been
measured by the inventors, and were also described by Martin et al, and
also by Ducker et al, in "Force Measurement Using an AC Atomic Force
Microscope", J. of Applied Physics, 67(9), 1 May 1990. As the curve
clearly shows, when the tip is brought near the surface there is a narrow
region where the amplitude is affected by the Van der Waals interaction
before it becomes abruptly captured by the surface fluid layer, and
oscillation becomes very small. It is this narrow region in which the
non-contact AFM must operate. As a surface is scanned, any variations in
the surface topography may cause the tip to become captured if the
feedback cannot perfectly respond to the topography variations. If the tip
does become captured, the control system will lift the fixed end of the
lever until the tip breaks free, and then re-establish the setpoint. As
can be seen from FIG. 6, there is significant hysteresis in the withdraw
process, which will cause serious instability in the image data. Thus
non-contact microscopes must scan very slowly so the feedback loop has
sufficient time to prevent the tip becoming stuck to the surface.
Moreover, because the tip must be operated above the fluid layer, the
lateral resolution is inferior to the contact mode. Typically, the
non-contact AFM must operate with the tip 5-10 nm above the surface, which
limits the lateral resolution to 5-10 nm. Contact mode AFM's typically
have lateral resolution of better than 1 nm.
For measuring the frequency shift using amplitude detection, the
sensitivity depends on the cantilever having a very sharp resonance peak,
which in turn gives a very slow response time because undamped systems
require a long time to recover from a perturbation. Thus, sensitivity and
response time are inversely coupled. The high Q requirement also places
restrictions on the design of the lever to minimize the effect of air as a
damping agent. One could improve the time response by using cantilevers
which may be operated at a higher frequency, but such levers are stiffer
and therefore have reduced sensitivity to the Van der Waals interaction.
Thus it can be seen that high sensitivity and fast response are very
difficult to achieve with a non-contact AFM. Furthermore, the weak force
interaction places restrictions on the height at which the tip may be
operated and the amplitude of oscillation. The presence of the fluid layer
near this height makes capture of a lever with a small oscillation likely,
so slow time response is a serious stability problem. For these reasons,
despite their many potential advantages, non-contact AFM's have yet to be
successful commercially,
The non-contact AFM has been used successfully in the measurement of
magnetic fields on objects such as magnetic storage media. With a tip of,
or coated with, magnetic material, the force interaction between the tip
and magnetic sample is much stronger than the Van der Waals interactions,
and is longer range. Thus, the non-contact FM (also called magnetic force
microscope, MFM) may be operated without the need for ultra-high
sensitivity, as required for surface profiling. However since magnetic
fields are seldom continuous, some interaction is necessary to guide the
tip over the surface between magnetic regions. Rugar et al, (Magnetic
Force Microscopy, IBM Research Report, Almaden Research Center, Dec. 12,
1990) found that applying an electric field between the tip and sample
would produce a larger effect than the Van der Waals force, so the hard
disks could be scanned without the probe sticking to the surface. This
method limits the technique to conductive surfaces.
SUMMARY OF THE INVENTION
Accordingly, one object of this invention is to provide a novel AFM that
does not produce shear forces during scanning and does not have the
operability limitations of the noncontact AFM.
A further object of the invention is to provide a novel AFM and method to
profile surfaces, including soft or sticky surfaces, at high resolution
with high sensitivity and fast time response, thus overcoming the
drawbacks of prior art contact and non-contact mode AFM's.
It is a further object of this invention to provide an AFM that may map
magnetic or other force distributions while retaining the ability to track
topography without other force components.
These and other objects are achieved according to the present invention by
providing a new and improved AFM and method of operating an AFM, wherein
the probe is oscillated at or near resonance or a resonant harmonic to
strike the surface of the sample, so that the tip has minimal. lateral
motion while in contact with the surface, thus eliminating scraping and
tearing. The cantilever probe is oscillated at a large amplitude, greater
than 10 nm, preferably greater than 20 nm, and typically on the order of
100-200 nm, so that the energy in the lever is large enough, much higher
than that lost in each oscillation cycle due to, for example, damping upon
striking the same surface, so that the tip will not become stuck to the
surface. The oscillation amplitude is affected by the tip striking the
surface in a measurable fashion, and this limited amplitude is a direct
measure of the topography of the surface. Alternatively, a feedback
control can be employed to maintain the oscillation amplitude constant,
and then a feedback control signal can be used to measure surface
topography. The striking interaction is strong, so the sensitivity is
high. The resolution approaches the contact mode microscope because the
tip touches the surface. The technique can use high frequency jumps with
no loss in sensitivity since the measurement of the amplitude change does
not depend on frequency.
The invention may be employed in the measure of magnetic or other force
distributions in conjunction with the non-contact method, to track the
surface in regions where there is no other force.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes better
understood by reference to the following detailed description when
considered in connection with the accompanying drawings, wherein:
FIG. 1 is a graph showing the Van der Waals force as a function of height
above a surface, and where a non-contact mode microscope must operate;
FIG. 2 is a simplified functional block diagram of the probe positioning
apparatus of a prior art contact mode atomic force microscope;
FIG. 3 is a simplified functional block diagram of the probe positioning
apparatus of an atomic force microscope incorporating the present
invention;
FIG. 4 is a block diagram of another type of AFM which may use the present
invention;
FIG. 5 is an illustration of the operation of a vibrating lever brought
close to a surface in the prior art non-contact mode;
FIG. 6 is a graph of an amplitude rs. position curve that illustrates the
behavior of the probe oscillation in a prior art non-contact mode AFM as a
function of probe height above a surface;
FIG. 7 is an illustration of the operation of a vibrating lever brought
close to a surface in a preferred embodiment of the present invention;
FIG. 8 is a graph of an amplitude vs. position curve that illustrates the
behavior of the probe oscillation in a preferred embodiment of the present
invention as a function of probe height above a surface;
FIG. 9 is an illustration of how the present invention may be used to
achieve improved performance when measuring surfaces with steep walls and
trenches;
FIG. 10 is an illustration of an alternative approach where the present
invention may be used to achieve improved performance when measuring
surfaces with steep walls and trenches;
FIG. 11 is a graph of an amplitude-distance curve that illustrates the
behavior of the probe oscillation in a preferred embodiment of the present
invention as a function of probe height above a surface in a mode where
the probe oscillates within the surface fluid layer;
FIG. 12 is a graph of an amplitude-distance curve that stops before the
amplitude decreases below a predetermined point;
FIG. 13 is an illustration of how a probe is pulled into a steep wall in
the prior art contact AFM; and
FIG. 14a is an illustration of scanning a probe tip in a trench while
oscillating the probe tip and
FIG. 14b is a graph | | |
