 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to scanning probe microscopes and more
particularly to an apparatus and method for producing a measurement of the
surface representative of a parameter of the surface other than topography
or for performing a task on the surface. Scanning probe microscopes such
as a scanning tunneling microscope or an atomic force microscope operate
by scanning a probe over a surface in which the probe is very close to the
surface, lightly contacts the surface, or taps on the surface.
In a scanning tunneling microscope, the tip is at a distance of just a few
atoms from the surface in order for a tunneling current to flow between
the probe tip and the surface. The tunneling current is either measured to
represent the distance between the probe and the surface or more generally
used in a feedback system, which regulates the vertical height of the
probe, to keep the current and therefore the distance of the probe from
the surface constant. The feedback signal therefore is a measurement of
the topography of the surface. In an atomic force microscope the tip may
be mounted on a bendable arm and therefore small deflections of the arm
are measured in order to detect the profile of the surface under study.
Alternately, a feedback system may be used to maintain the probe force
constant on the surface and with the feedback signal representing the
topography of the surface.
Both types of microscopes described above are variations of a general
device referred to as a scanning probe microscope. Originally, scanning
probe microscopes only used the two types of interactions described above,
which are specifically the tunneling current or the contact force with the
atomic force microscope. These types of interactions were used to adjust
the height of the probe to trace the topography of the surface.
There have been a number of recent developments which include the use of
other types of interactions between the probe and the surface so as to
attempt to form different types of measurements or images of the surface.
For example, it may be desirable to produce images of a surface
representative of parameter such as Van der Waals forces, magnetic forces,
electric forces, ionic conductance, electro-chemical activity and light
intensity, wavelength or polarization. Since these new types of
interactions measure parameters of the surface other than the topography
of the surface, it is difficult to measure these new types of interactions
while at the same time measuring topography.
The prior art scanning probe microscopes which have tracked the surface of
a sample with a probe tip by sensing some parameter have included, as
indicated above, tunneling current, contact force, Van der Waal attractive
force, magnetic force, electro-static force, ionic conduction,
electro-chemical activity and light intensity, wavelength or polarization.
Some of these parameters such as tunneling current and contact force are
generally easy to sense and are representative of the topography of the
surface.
Others of these parameters, such as the magnetic force, are more difficult
to detect or may not be directly related to the topography of the surface.
This causes any measurement signals, such as feedback signals, responsive
to these parameters, to either be marginal or unstable and not useful as a
position signal. For example, some of these parameters are not continuous
across the surface, i.e. the magnetic force may vary over a surface,
disappearing or changing direction from down to up. Therefore, any
position signal from these parameters is not stable and so the probe is
not able to track the surface using these other parameters.
There are times when it is desirable to provide an image of the surface,
representative of parameters other than topography and therefore, it would
be advantageous when measuring these parameters, other than topography,
which may be weak or discontinuous to not rely on these interactions for
position information to control the height of the probe over the sample
surface. It would therefore be advantageous for the measurement of such
other parameters to move the probe a known distance away from the surface
at all points along its contour while measuring these other parameters.
For example, such a fixed separation is useful when measuring
electro-chemical currents on a fluid covered surface. In this case, the
desired spacing is too large to use tunneling currents to control the
probe height and even if the spacing were reduced, the electro-chemical
and tunneling currents would be combined so as to confuse the position
control system.
One important example of a desirable parameter for measurement, other than
topography, is the measurement of magnetic fields at a sample surface. One
prior art attempt as suggested by Rugar and Wickramasinghe, Appl. Phys.
Lett. 52, Jan. 18, 1988, p244, included vibrating a magnetic probe or tip
above the surface and detecting the change in the frequency of vibration
due to the sample. The sample caused both magnetic forces and Van der Waal
attraction of the probe and so the feedback data contained both magnetic
and probe height information. For many samples, these forces are extremely
weak and give a poor feedback signal which causes the probe to hit and
stick to the sample or drift away from the surface. As a result, the
technique as suggested by Rugar and Wickramasinghe has not found
widespread use.
Another technique for measuring the magnetic fields is that suggested by
Moreland and Rice, which uses a tunneling microscope with a flexible
magnetic probe or tip supported on a cantilever. The feedback signal is a
tunneling current which is used to keep the tip just above the surface.
Magnetic attraction pulls the flexible tip toward the surface and the
position control system then lifts the tip back into position by bending
the cantilever. Thus, the magnetic field patterns appear to be raised and
lowered regions of the surface. Unfortunately, this mixing of position and
magnetic data is a disadvantage to the Moreland and Rice system since
inaccuracies are introduced. In addition, the sample must be electrically
conducting to obtain a tunneling current and this is a disadvantage for
many important magnetic media such as magnetic tape or magneto-optical
disks which are not conducting.
As can be seen from the above discussion, in general it would be desirable
to be able to scan a probe relative to a surface at a known height to
measure a parameter other than topography. In addition, scanning probe
systems have the capability to modify or construct surface features on a
very fine scale. Typically, such functions may eliminate or are not
compatible with a position feedback signal. In the prior art, the scanning
probe measurements other than topography, have been carried out or tasks
performed while simultaneously sensing the height of the probe. Two prior
art patents, which are directed to improvements in scanning which may be
used in the present invention, are the Elings and Gurley U.S. Pat. No.
4,889,988 and the Elings and Maivald U.S. Pat. No. 4,954,704.
The Elings and Gurley patent is directed to the use of digitally controlled
motion of the probe or sample in scanning probe microscopes and teaches
the use of digitally stored position data to better control the scanning
motion of the scanning probe microscope. This patent is directed to the
measurement of topography and not to the measurement of sample surface
properties other than topography. However the present invention may use
the digitally controlled motion of the Elings and Gurley patent for the
scanning probe of the present invention and the teachings of this patent
is therefore incorporated into the present application.
The Elings and Maivald patent teaches a method of rapid scanning in which
stored digital topographical data is used to control the return motion of
a probe so that it can move rapidly above the sample surface without any
risk or damage but still allow the probe to be quickly positioned for the
next scan. However, this patent does not have any indication of the
measurement of properties of the sample other than topography during the
return scan or any subsequent scan. This patent may also be used in the
present inventions to control the scanning and its teaching are
incorporated into the present application.
Both the Elings and Gurley and Elings and Maivald patents, although
directed to improvements in scanning and therefore useful in the present
invention, do not anticipate the measurement of properties of the sample
surface other than topography or performance of a task at the sample
surface, which is the focus of the present invention.
SUMMARY OF THE INVENTION
The present invention is directed to a two-phase scanning apparatus and
method. The invention first scans the surface topography of a sample using
an accurate position sensor such as a tunneling current or an atomic force
probe to store a precise representation of the topography of the surface
in memory. The present invention then secondly, uses this stored
topographical information to accurately position the probe with respect to
the surface while another measurement is taken of the surface other than
topography or some task is performed. The topographical sensing function
followed by the controlled position measurement or task function may be
performed on a point by point, line by line or raster image basis.
In the second phase of the operation of the present invention, there is not
necessarily any direct position feedback since the probe may be positioned
entirely by using the stored data. In the second phase, the probe
positioning apparatus uses the stored data representative of the surface
to position the probe as desired for the selected measurement or task. The
preferred probe position during the second phase of the operation depends
on the measurement or task so that the probe may be in contact with the
surface, or above the surface by an optimum distance dependent upon the
measurement or task, or even alternately lifting above and then contacting
the surface or any other type of movement relative to the surface.
The present invention may therefore use the stored topographical data to
carry out other measurements or activities with the scanning probe
instrument to provide for a wide range of applications. For example, the
present invention may provide for the measurement of magnetic fields at
the surface of magnetic recording media, the measurement of the
temperature or electric fields at the surface of an operating integrated
circuit, the micromachining of quantum well transistors, manipulation of
strands of DNA identified by attached molecules, electro-chemical mapping
of biological cell memories, or other types of measurements or activities
which can be enhanced by the prior accurate topographical measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a prior art structure suggested by rugar for the
measurement of magnetic fields at a sample surface;
FIG. 2 is a prior art structure suggested by Moreland and Rice, also for
detecting magnetic fields at a sample surface;
FIG. 3 illustrates in general, the present invention for detecting a
parameter of a sample surface other than topography or performing a task
at the sample surface;
FIG. 4 is a first embodiment of the present invention for measuring
magnetic fields at the surface of a sample;
FIG. 5 is a third embodiment of the present invention for measuring
electric fields at the surface of a sample; and
FIG. 6 is a fourth embodiment of the present invention for measuring
electro-chemical currents at the surface of a sample.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1 and 2 are representative of prior art devices for providing the
measurement of magnetic fields at the surface of the sample. For example,
as shown in FIG. 1, in a prior art device, as suggested by Rugar, a
magnetic tip 10 supported at the end of a cantilever arm 12 is vibrated
above a surface of a sample 14. The vibration of the magnetic tip and the
X, Y and Z positioning is provided by a position control 16 which is
connected to an X Y positioner 18 and to the magnetic tip 10 through the
cantilever arm 12. A detector 20 detects the change in frequency in
vibration of the magnetic tip 10 due to various forces at the surface of
the sample 14.
A feedback signal is provided from the detector 20 to a signal processor 22
so as to provide an output signal from the signal processor, theoretically
representative of the magnetic field. The signal processor 22 is coupled
to the position control 16 to control the position of the tip 10 relative
to the sample 14. Unfortunately, the sample would normally cause a number
of forces to act on the magnetic tip 10, including both magnetic fields
and Van der Waal attraction forces and so the feedback signal would
contain both magnetic and probe height information. In addition, for many
samples, these forces are extremely weak giving a poor feedback signal
which causes the probe to hit and stick to the sample surface or drift
away from the surface. The various difficulties with this type of device
has limited the use of the Rugar device for measuring magnetic fields or
any other parameters of the sample surface.
FIG. 2 illustrates another magnetic field measurement technique as
suggested by Moreland and Rice. In the Moreland and Rice device, a
tunneling microscope includes a flexible magnetic tip 50 positioned above
the surface of a sample 60. A feedback signal, which is a tunneling
current produced by a current detector 62, is applied to a signal
processor 52 which in turn controls a position control 54 to keep the tip
50 just above the surface. The tip is supported above the surface of the
sample 60 at the end of a cantilever 56 and as magnetic attraction pulls
the tip 50 towards the surface, the position control 54 then lifts the tip
back into position by bending the cantilever. The position control 54 also
controls an X Y positioner 58.
Unfortunately, with the Moreland and Rice device, the magnetic field
patterns appear to be raised and lowered regions of the surface, so that
there is a mixing of the topography and magnetic data. This of course is a
disadvantage since the different types of data cannot be separated and a
true image of the magnetic fields is not possible. In addition, the sample
surface must be electrically conducting in order to obtain a tunneling
current. This is a disadvantage for many important magnetic media such as
magnetic tape or magneto-optic disks, which are not conducting. Therefore,
the Moreland and Rice structure has not found wide us for measuring
magnetic fields.
FIGS. 3 through 6 illustrate, first in generalized form and then with
specific embodiments, the apparatus and method of the present invention.
In the descriptions of FIGS. 3 through 6 of the present invention, the
same reference characters are used to describe elements, which provide
either a similar or the same function. In addition, although reference
sometimes is made to an atomic force microscope and at other times to a
scanning tunneling microscope, it is understood that in using applicant's
invention one or the other of these forms of microscopes may be used. In
general, the topographical information is produced using a scanning probe
microscope which could be one or the other of the above.
In addition, in the various described embodiments, a separate X Y
positioner is shown to move the sample relative to a probe tip. In turn,
the probe tip is shown to be moved in the Z direction relative to the
sample. It is to be appreciated that in place of the sample being moved in
the X Y directions, the probe tip could be moved in the X Y directions in
addition to the height or Z direction movement of the probe tip relative
to the sample surface. Therefore, although the position control is shown
only to control the Z or height movement of the probe tip relative to the
sample, all of the movements can be combined in the probe tip and with the
sample maintained stationary. Alternately, the probe tip could remain
stationary and with the sample moved, both in the X Y directions and in
the Z direction. For simplicity of description, the various movements are
as shown in the Figures but as indicated above, the invention is not to be
limited to these specific structures.
FIG. 3 illustrates in generalized form the apparatus and method of the
present invention. As can be seen in FIG. 3, the present invention is a
two-phase scanning instrument which first scans the surface topography of
a sample using an accurate position sensor such as an atomic force or a
tunneling current probe to store a precise representation of the
topography of the surface in memory. The stored topographical information
is then used to accurately position the probe with respect to the surface
during a second scan while another measurement other than topography or a
separate task is performed.
The present invention may provide for this two-phase operation either on a
point by point, line by line or raster image basis because the probe may
be positioned during the second phase of the operation by using only the
stored data. It is not necessary to have any direct position feedback
during the second phase, but it is to be appreciated that if such feedback
would be desirable, it may be used. Normally, however, in the second phase
of the operation, the probe position control uses the data stored in
memory which describes the topography of the surface to position the probe
as desired for the measurement other than topography or the performance of
some other task. The preferred probe position relative to the surface
would depend on the measurement or task, but it is to be appreciated that
the probe may be in contact with the surface, or above the surface by an
optimum distance, or even alternately lifting above and then contacting
the surface or moving in other ways.
As shown in FIG. 3, a probe 100 is illustrated either in contact with or
just above the surface of a sample 102. If the probe is detecting
topographical information on the basis of an atomic force microscope, then
the probe would normally be just in contact with the surface of the sample
102. However, if the probe is detecting the topography of the surface of
the sample 102 using a scanning tunneling microscope, then the probe would
be positioned just above the surface at a distance of a few atoms.
Depending upon the type of scanning probe microscope that is used, a
detector 104 detects the topography of the surface of the sample 102 and
provides a signal which is applied to a signal processor 106.
In a normal scanning probe microscope, a signal processor provides signals
to a position control 108 which may control the Z position of the probe
100 shown at the end of a cantilever 110 and which also may provide for
the X Y position between the probe 100 and the sample using an X Y
positioner 112. As indicated above, the probe 100 could alternately be
controlled in the X Y position, thereby eliminating the need to move the
sample 102. Similarly, the sample 102 could be not only moved in the X Y
direction but also in the Z direction and with the probe tip 100
maintained stationary.
The output from the signal processor 106, representative of the Z position
of the probe 100 or the sample 102 as well as the X Y position is then
stored in a memory 114. The information thereby stored in the memory 114
is essentially the topography of the surface of the sample 102. This
stored topography data may then be provided at a later time as a signal
representative of the topography of the surface of the sample 102.
Specifically, the stored information representative of the topography of
the sample 102 may be supplied during a second scan of the surface of the
sample 102 as an input signal to the signal processor 106. The stored
topographical signal can thereby control the position control 108 to have
the probe tip in a desired X Y Z position relative to the now known
topography of the surface so as to provide for some measurement or perform
some task other than topography.
The probe tip 100 may incorporate other features so that the detector 104
may now be used to carry out other measurements with the scanning probe
instrument or the probe tip may perform other activities. For example,
during the second scan, the following measurements or activities may be
performed; the measurement of magnetic or electric fields, the measurement
of surface temperature; various electro-chemical measurements; the
micromachining of the surface; the manipulation of strands of DNA. All of
these other measurements or activities can occur with great reliability,
since during the second scan of the surface, the topography of the surface
had been accurately mapped so that the probe position may either be in
contact with the surface or above the surface by an optimum or constant
distance or any other desired movement relative to the surface so as to
produce the most reliable further measurement or activity.
As indicated above, FIGS. 4-6 illustrate three specific embodiments of the
invention, but it is to be appreciated that many other types of
measurements or activities may be performed other than the specific
embodiments illustrated in this application.
FIG. 4 illustrates a specific embodiment of the invention for the
measurement of magnetic fields wherein the sample 102 may be a magnetic
digital storage medium such as a hard disk. In the embodiment of FIG. 4,
the probe 100 would incorporate a magnetic tip and preferably, the first
scan for topography would be provided by an atomic force microscope (AFM).
As indicated above, the present invention provides for at least two scans
of the surface of the sample. In the embodiment of FIG. 4, first there is
the AFM scan to measure and store the topography of the sample 102 in the
memory 114. In a second scan, the magnetic tip 100 is maintained a
specific distance above the surface of the sample 102 to measure the
magnetic forces in the sample.
As shown in FIG. 4, and specifically as shown in the second scan, magnetic
forces 120 are localized at a particular portion of the sample 102. During
the second scan, the deflection of the cantilever 110, as provided by the
magnetic forces 120 operating on the tip 100, may be detected by the
detector 104 to produce an output signal representative of the magnetic
field image. This is shown by signal 122 in FIG. 4. The deflection of the
cantilever 110 can be measured directly by the detector 104. Alternately,
another technique which may be used is to vibrate the cantilever near
resonance and with the resulting amplitude of vibration measured by the
detector 104 to detect the magnetic field. Also the topography could be
measured by tapping an oscillating tip on the surface of the sample as
described in a copending application entitled An Ultra Low Force Atomic
Force Microscope by the same inventors. The topography could also be
measured in the non-contact AFM mode according to Rugar, although this
method is, at present, somewhat unstable.
Most importantly, however, the present invention provides for the probe 100
to be guided over the surface of the sample 102 reliably and accurately
using the stored topographical information from the memory 114
irrespective of the strength, direction or discontinuities of the magnetic
forces. The present invention also works for either conducting or
non-conducting samples since if the sample is not non-conducting an atomic
force microscope would be used to detect topography and if the sample is
conducting then either an atomic force or a scanning tunneling microscope
could be used.
The use of a vibrating or oscillating cantilever can also provide for the
detection of a magnetic field gradient. Specifically, a constant magnetic
field causes the center of oscillation to shift while a magnetic field
gradient changes the effective spring constant and shifts the resonant
frequency of the cantilever and probe tip. The oscillation may therefore
may be driven just off resonance and the change in detected resonant
frequency due to the magnetic field gradient may be observed as changes in
the amplitude of oscillation. Since the probe tip 100 during the second
scan is not being used for height measurement and is not in contact with
the surface of the sample, the embodiment of FIG. 4 provides for a pure
measurement of the magnetic force or force gradient as a function of the X
Y position of the probe tip 100. Both attractive and repulsive magnetic
force can be measured since the tip is off the surface and can respond in
either direction.
FIG. 5 illustrates another embodiment of the invention which is the
measurement of electric field strength of a sample. This type of
measurement may also be used for testing integrated circuits. Typically,
for the measurement of electric field the tip 100 would be a conducting
tip and again the first scan could either be using an atomic force
microscope or a scanning tunneling microscope and with the information
representative of the topographic image stored in the memory 114. In the
second scan, the stored data representative of the topography is used to
control the scanning of the conductive tip 100 above the surface of the
sample 102.
A voltage source 126 provides for a voltage differential between the tip
100 and the sample 102 to produce an electric field between the tip 100
and the sample 102. This field may be between the surface and the tip, or
between the tip and structures below the surface, such as gates in an
integrated circuit. This electric field between the tip and the sample
results in an attractive force between the tip and the sample which can be
measured as the tip is scanned above the sample surface. The attractive
force may be varied by varying the voltage between the tip and the sample.
In addition, the voltage source 126 may include an oscillating component
so that the detector 104 may include filters and phase sensitive detectors
to measure the electric field above the sample. Typically, the frequency
of oscillation would be chosen to be near the resonant frequency of the
probe/cantilever structure 110 so as to enhance the sensitivity.
Again, the stored topographic information in the memory 114 which is used
to control the scanning of the probe tip 100 permits the probe to be
maintained at a constant height above the surface of the sample 102 and
avoids mixing topographic data with electric field data. One example of
the use of the device as shown in the embodiment in FIG. 5 would be the
scanning of an integrated circuit to measure the voltage of an operating
circuit at various places. The embodiment of FIG. 5 will work even when
there is a thin layer of insulating material, such as silicon dioxide,
over the circuit which would prevent contact measurements of the circuit
voltage. The output signal from the detector 104 may be representative of
the electric field image as shown in graph 128.
FIG. 6 shows yet another embodiment of the invention for the measurement of
electric-chemical signals at the surface of the sample 102. In FIG. 6 a
conducting tip 100 may be used. In a first scan, using either an atomic
force or scanning tunneling microscope, the accurate topographical image
of the surface of the sample 102 is stored in the memory 114 using the
information from the signal processor 106 and the position control 108.
The stored topographical information is then used in a second scan of the
surface of the sample 102 to hold the tip 100 an optimum height above the
surface for electro-chemical measurement. If the conducting tip were held
too close to the surface, tunneling currents or contact currents would
interfere with the electro-chemical measurement. Therefore, the use of the
stored topographical data allows for an accurate measurement of the
desired electro-chemical signals without interference from effects due to
the height of the probe tip 100 above the surface. The output signal from
the detector 104 representative of the electro chemical image may be as
shown in graph 130.
With all of the embodiments of the invention, the second measurement of the
parameter other than topography or the performance of a desired task may
be made either on a point by point, line by line or whole raster image.
Therefore, there is an alternation between the first scan and the second
scan which can be chosen again either on a point by point, line by line or
whole raster image by whole raster image. For example, if we chose to
alternate scan lines then for every other scan line the device functions
as a normal scanning probe microscope and during the alternate scan the
device detects the parameter other than topography or performs the desired
task. The stored signal is then used to control the height or Z position
of the probe relative to the surface for all of the various X Y positions.
If it is desired to measure the parameter other than topography or perform
a desired task on a point by point basis then again first the height may
be measured in the first scan at a given point and then a desired height
for the probe maintained in accordance with the previously stored height.
If a whole image is used, then typically a scanning pattern is formed by a
series of sweeping X axis motions with small Y increments to form a raster
which covers the surface area to be measured. For each Y increment the
scanning probe microscope scans along the X axis and the height of the
sample surface at each point is stored for later use in positioning the
probe in the second scan.
The scanning, either point by point, line by line or raster by raster,
produces two separate images. One image is of the sample surface
topography and the other image is of the other parameter other than
topography which has been measured. Separating the two kinds of
information provides for an important advantage of the present invention.
Other aspects of the invention allow for different types of measurement
than is possible with the prior art.
For example, it is possible to measure both the magnetic fields and the
magnetic field gradient. Since there is no contact with the surface during
the second scan, the direct deflection of the cantilever by the magnetic
force is proportional to the magnetic field strength alone. On the other
hand, the resonant frequency of oscillation of a cantilever and magnetic
probe tip is shifted by the magnetic field gradient. The field gradients
produce a force which changes with distance adding to the spring constant
of the cantilever.
Therefore, if desired, both the magnetic field and the magnetic field
gradient can be measured by using a sequence of two scans. One scan for
topography and then a second scan with an oscillating drive in which the
average deflection and the cantilever resonant frequency are observed to
obtain both the magnetic field and field gradient. Alternately, a third
scan may be provided since the optimum detection height may be different
for the magnetic field and the magnetic field gradient
It can be seen therefore that the present invention has great versatility
in providing for many different measurements at the surface of the sample
and with the measurements clearly separated from forces which relate to
topography. The present invention provides for the scanning operation to
be in two phases and with a first scan to obtain and store topographical
information and a second scan to carry out other measurements or
interactions with the surface while the probe height is controlled using
the stored topographical information.
In addition, the two phase operation could be used to modify or construct
on the surface. In the second place, the tip could be very accurately used
to mark the surface with a known depth or force relative to the surface,
or to travel at a fixed height above the surface, to apply accurate
electric fields between the tip and surface. Such techniques have been
shown to have potential to construct or modify on a very fine scale. These
techniques can benefit from the present invention.
Although the invention has been described with reference to particular
embodiments, it is to be appreciated that various adaptations and
modifications may be made and the invention is only to be limited by the
appended claims.
* * * * *
|
|
 |
|
 |
Description |
|
