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BACKGROUND OF THE INVENTION
1. 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. 08/147,571 and
related U.S. Pat. Nos. 5,229,606 and 5,266,801.
2. 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 tip-sample 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 amplitudes 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 non-contact 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
noncontact 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 non-contact 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.
In further embodiments, the probe is oscillated and translated across the
surface of the sample at constant amplitude and changes in phase are
detected and corresponding output signals produced. In another variation,
a relative phase between a drive signal causing oscillation of the probe
and deflection of the probe is detected and the frequency of oscillation
of the probe varied so that the relative phase is kept essentially
constant during scanning. An output signal is then produced indicative of
variations in the frequency of oscillation of the probe as a function of
position during scanning. According to a further embodiment, oscillation
frequency is modulated by an amount .DELTA.f, changes in relative phase
.DELTA.p detected, and signals indicative of the slope .DELTA.p/.DELTA.f
output. In a further embodiment, data is obtained by operating at
different tapping amplitude setpoints and then compared to discriminate a
force dependent sample characteristic.
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 vs. 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. 9A is an illustration of how the present invention may be used to
achieve improved performance when measuring surfaces with steep walls and
trenches, and
FIG. 9B is a graph illustrating oscillation amplitude as a function of scan
position in the trench of FIG. 9A;
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 illustrating the
observed oscillation amplitude resulting from the scanning of FIG. 14a;
FIG. 15 is a block diagram of a further embodiment of the tapping AFM of
the present invention, in which probe tip oscillation phase is detected as
a function of lateral position during scanning;
FIG. 16 is a block diagram illustrating a further embodiment of a tapping
AFM according to the present invention, in which a change in resonant
frequency of the oscillating probe tip is detected during tapping at
constant amplitude; and
FIGS. 17a and 17c are graphs showing the effect of damping on oscillation
amplitude as a function of frequency, and FIGS. 17b and 17d are graphs
showing the effect of damping on the slope of the phase vs. frequency
curve.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention utilizes the inventors' discovery that if the probe
is oscillated at or near one of the resonant frequencies of the lever,
that in fact the probe tip has much less of a tendency to stick to the
surface because a resonant system tends to remain in stable oscillation
even if some damping exists. Thus the preferred embodiment of the present
invention utilizes a resonant oscillation of the cantilever at sufficient
oscillation amplitude to achieve the advantages described above without
the probe becoming stuck to the surface. This preferred embodiment also
provides many of the benefits of the non-contact AFM as described above.
Existing development of AFM's using oscillation of the probes has been
directed at avoiding surface contact, as described above, and as such is
limited in practicality despite the potential advantages of the technique.
For applications where the non-contact mode is desired, the inventors have
found that the amplitude-distance curve of FIG. 6 can of be of great aid
in establishing the setpoint for non-contact mode operation. Using the
amplitude-distance curve, one can optimize the operating frequency, free
amplitude and setpoint to achieve the most suitable operating
characteristics for a particular sample and cantilever combination. This
novel application of the curve was clearly not anticipated by Ducker or
Martin. Using the computational and display capabilities of typical
scanning probe microscopes, SPM's, the lever may be oscillated and the
lever vertical position varied, while the curve is displayed on a
terminal. The various parameters may be varied, such that desired
operating conditions may be determined, and used when the SPM is in the
imaging mode.
The AFM of the present invention does not avoid contact with the surface.
Thus, the invention is not limited in the amplitude of oscillation, and in
fact as will be shown, very large amplitudes compared to the non-contact
mode are advantageous. In FIG. 6, it is shown that for small oscillation
amplitudes as the tip is brought near the surface, it becomes trapped by
the fluid layer and oscillation ceases abruptly. If the oscillation
amplitude is larger, greater than 10 nm, preferably greater than 20 nm and
typically 100-200 nm, then the energy in the oscillation may be sufficient
in most cases to overcome the stickiness of the surface for a wide range
of vertical positions of the lever.
FIG. 7 shows that for a large free amplitude, A.sub.o, the lever may be
brought down to where the tip strikes the surface. The energy lost by
striking the surface and overcoming the fluid layer attraction limits the
oscillation to a reduced value, A, but does not stop the oscillation as
happens for low drive oscillation amplitudes. The difference in behavior
for higher amplitude oscillations is illustrated in FIG. 8 where the curve
of FIG. 6 is duplicated for a free amplitude, A.sub.o, of greater than 10
nm. As can be seen, there is wide range of limited amplitudes, with the
probe striking the surface, that could be used as an operating point for a
feedback loop. Abrupt capture of the probe does not take place, so stable
operation is possible. As the curve shows, the lever may be further
lowered such that oscillation is stopped. The withdraw characteristics are
similar to the low amplitude case in that the amplitude increases
gradually until a point is reached where the cantilever breaks free and
resumes oscillation at the free amplitude.
According to the present invention, the AFM is then operated at or near a
cantilever resonance with sufficient amplitude that upon the probe
striking the surface, the amplitude of oscillation of the probe is
affected and the probe does not stick to the surface. A preferred version
of this invention can be practiced on the AFM of FIG. 3. In FIG. 3, the
tip is driven at a known amplitude and frequency oscillation, which is
typically near a cantilever resonance. The amplitude, A.sub.o, of this
oscillation is detected by the deflection detector of FIG. 3, which is of
the type shown in FIG. 2 and described by Hansma et al. When the tip is
sufficiently far away from the surface, it will oscillate at the free
amplitude, A.sub.o, as shown in FIG. 7. The amplitude is measured in the
AFM of FIG. 3 as an RMS value of the AC deflection detector signal. As
shown in FIG. 7, when the tip is brought closer to the surface, striking
the surface will limit, typically due to damping, the oscillatory motion.
The amount of change is measurable as a decreased RMS value, A.sub.s. 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
cantilever height may be adjusted with feedback to keep the RMS setpoint,
A.sub.s, at a constant value. Alternatively, changes in the amplitude of
oscillation themselves can be used as a direct measure of surface
topography. Thus, an image of the surface may be generated. The preferred
embodiment uses a digital processor to provide the servo control by means
of feedback programs executed by the processor. An analog feedback system
is also possible. Strain gauges, such as resistive or piezo-resistive
strain gauges or piezoelectric elements built into the cantilever arm, may
be employed in place of the optical deflector detector shown.
As shown in FIG. 4, this version of the invention may also be implemented
with other types of AFM's. For instance, the Compact AFM, disclosed in
U.S. Pat. No. 5,189,906, describes an AFM where the probe is scanned
rather than the sample. This AFM has provision to attach the probe to a
separate positioner, which may be used to impart the oscillation, such
that a setpoint may be established for contact with the surface.
This preferred embodiment of the invention has several advantages. This AFM
can be operated with extremely light tapping forces. In general, the
inventors have found that even using relatively stiff levers, on the order
of 10's of newtons per meter in order to give high frequency oscillations
such as up to 2 MHz, the forces on the sample are still extremely light.
For instance, it is easy to establish a setpoint that is 10nm less than
the free oscillation amplitude, which may be on the order of 100 nm. Thus
the energy in the lever oscillation is much higher than that lost in each
cycle by striking the surface. A conservative estimate of the actual force
imparted to the surface is to assume the contact is inelastic and
therefore the bending of the lever due to surface contact is just the
amplitude gained in one cycle, approximately (A.sub.o -A.sub.s)/Q, where Q
is the quality factor of the lever. Typical silicon levers have Q's of 100
to 1000, so for a setpoint 10 nm below the free amplitude of 100 nm the
force per strike is 0.1 to 1 nanonewtons for a cantilever wi | | |
