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| Inventor(s): |
Binnig; Gerd , Richterswil, Switzerland
Rohrer; Heinrich , Richterswil, Switzerland
|
| Applicant(s): |
International Business Machines
Corporation, Armonk, NY
News,
Profiles, Stocks and More about this company
|
| Issued/Filed Dates: |
Aug. 10, 1982 /
Sept. 12, 1980
|
| Application Number: |
US1980000186923
|
| IPC Class: |
G01N 23/00;
|
| Class: |
Current: 250/306; 250/423.F; 505/848;
Original: 250/306; 250/423.F;
|
| Field of
Search: |
250/306,307,441,457,423 F
|
| Priority Number(s): |
|
| Legal
Status: |
| Gazette date |
Code |
Description (remarks) List all
possible codes for US |
| Aug. 10, 1982 |
A |
Patent
|
| Sept. 12, 1980 |
AE |
Application data
|
| Sept. 12, 1980 |
AS02 |
Assignment of
assignor's interest (INTERNATIONAL BUSINESS MACHINES
CORPORATION, ARMONK, N.Y. 10504 A CORP. OF N.Y. * BINNIG GERD
: 19800902; ROHRER HEINRICH : 19800902) |
| Sept. 20, 1979 |
AA |
Priority
|
|
| Abstract: |
The vacuum tunnel effect is utilized to form a scanning
tunneling microscope. In an ultra-high vacuum at cryogenic
temperature, a fine tip is raster scanned across the surface of a
conducting sample at a distance of a few Angstroms. The vertical
separation between the tip and sample surface is automatically
controlled so as to maintain constant a measured variable which is
proportional to the tunnel resistance, such as tunneling current.
The position of the tip with respect to the surface is controlled
preferably by piezo electric drive means acting in three coordinate
directions. The spatial coordinates of the scanning tip are
graphically displayed. This is conveniently done by displaying the
drive currents or voltages of piezo electric drives.
|
| Attorney, Agent, or
Firm: |
Drumheller; Ronald L.;
|
| Primary/Assistant
Examiners: |
Anderson; Bruce C.;
|
U.S. References: |
none
|
CLAIMS:
[Hide claims]: |
Having thus described our invention, what we claim as new,
and desire to secure by Letters Patent is:
1. Apparatus for
investigating surface structures utilizing the vacuum tunnel effect,
comprising:
- an ultra-high vacuum chamber which can be cooled down to a
temperature close to absolute zero;
- a fine conducting tip in said chamber;
- means for positioning a conductive sample surface in said
chamber sufficiently close to said conducting tip that tunnelling
current flows between said tip and the sample surface;
- means for scanning said tip across the sample surface at a
tunneling distance while tunnelling current flows between said tip
and the sample surface;
- means for measuring a tunnel effect variable between said tip
and the sample surface while said tip is scanned across the
surface;
- means for automatically controlling the separation distance
between said tip and the sample surface in response to the
measured tunnel effect variable such that the measured tunnel
effect variable remains substantially constant during scanning;
and
- means for graphically displaying the spatial coordinates of
said scanning tip to produce a topological map of said surface.
2. Apparatus as
defined in claim
1 wherein said means for positioning comprises a piezo electric
drive means acting in the direction along which the separation
distance between said tip and said sample surface is controlled,
said direction being the z direction.
3. Apparatus as defined in claim
2 wherein said piezo electric z direction drive means comprises
a piece (38) of piezo ceramic having hind legs, a body, fore legs,
and a head, a first electrode (42) covering said hind legs, a second
electrode (43) covering said body, a third electrode (44) covering
said fore legs, a fourth electrode (45) covering said head, said
hind and fore legs being guided along parallel notches in a guide
member (37), coarse adjustment in the z direction being achieved by
applying drive currents of different phases to said first, second
and third electrodes, whereby said hind legs and fore legs
alternately loosen and clamp to said guide member and said body
contracts and elongates thereby causing worm-like movement of said
piece along said guide member, fine adjustment in the z direction
being achieved by applying a drive current to said fourth electrode,
said head carrying said tip.
4. Apparatus as defined in claim
2 wherein said means for scanning comprises piezo electric drive
means acting in directions x and y, both being perpendicular to the
z direction.
5. Apparatus
as defined in claim
4 wherein said piezo electric x and y direction drive means
comprises an L-shaped piece (32) of piezo ceramic having a first
electrode (33) on one leg thereof and a second electrode (34) on the
other leg thereof, one end of said piece being fixed while the other
end thereof is free to move in the x and y directions by elongation
or contraction of the legs in proportion to drive currents applied
to said first and second electrodes.
6. Apparatus as defined in claim
4 wherein said ultra-high vacuum chamber contains a hovering
support (52) which carries said tip, said z direction piezo electric
drive means, said x and y direction piezo electric drive means and
the sample to be scanned, said support being caused to hover due to
the magnetic interaction of a plurality of permanent magnets carried
by said support with a superconductive tray (59), said
superconductive tray being positioned within an inner cooling jacket
(49) for liquid helium and an outer cooling jacket (46) for liquid
nitrogen, said inner and outer jackets and superconductive tray also
being within said vacuum chamber.
7. Apparatus as defined in claim
4 wherein said means for graphically displaying the spatial
coordinates comprises means for graphically displaying the drive
current or voltage of the z direction piezo electric drive means as
a function of the drive currents or voltages of the x and y
direction piezo electric drive means.
8. Apparatus as defined in claim
2 wherein said means for automatically controlling the
separation distance between said tip and the sample surface
comprises a feedback system controlling said z direction piezo
electric drive means in response to the measured tunnel effect
variable.
9. Apparatus as
defined in claim
1 wherein said measured tunnel effect variable is tunneling
current.
10. Apparatus as
defined in claim
1 wherein said measured tunnel effect variable is tunnel
resistance derived as the ratio of tunneling voltage to tunnelling
current.
11. Apparatus as
defined in claim
1 wherein said measured tunnel effect variable is tunneling
voltage.
12. Apparatus as
defined in claim
1 wherein the separation distance between said tip and the
sample surface is modulated about a mean value, said mean value
being automatically controlled in response to the mean value of the
measured tunnel effect variable such that the mean value of the
measured tunnel effect variable remains constant during scanning, a
phase and frequency selective amplifier monitoring the derivative of
tunneling current with respect to said distance modulation for
changes in surface work function.
|
| Background/Summary: |
DESCRIPTION
This
invention relates to apparatus for investigation of surface
structures utilizing the vacuum tunnel effect. An ultra-high vacuum
chamber is cooled down to a cryogenic temperature in the vicinity of
absolute zero. A conductive sample is placed in this UHV chamber and
serves as a base electrode with respect to a fine conductive tip
that serves as a scanning electrode. The scanning electrode is
poised above the base electrode at a distance of only a few
Angstroms.
A well known method for
investigation of surface structures is by visual inspection with the
human eye. However, there are natural boundaries for optical
resolution with the naked eye. Optical instruments can be used to
further improve optical resolution. However, even with the best
optical instruments, limits are reached which are imposed by the
nature of light.
Resolution can be
further improved using apparatus operating with radiation of
effective wave-length which is shorter than visible light, such as
the electron microscope. However, more complicated apparatus is
needed because an electron microscope operates in a vacuum and the
results of the inspection must be made visible on a screen or
photosensitive layer. In comparison with optical microscopes,
lateral resolution is improved remarkably. However, vertical
resolution again soon reaches a limit.
Apparatus for investigation of surface
structures operates either with electromagnetic radiation or with a
corpuscular radiation interacting with the surface of the sample.
Strictly speaking, instruments capable of resolving fields in the
atomic or molecular range do not image a surface in the sense of
producing an image for visual inspection. However, such instruments
do give information sufficient to allow conclusions to be made about
the structure and composition of the surface of the sample. For
example apparatus exists for observation of selective diffraction of
low energy electrons at a surface (LEED). Another apparatus uses
secondary ion mass spectroscopy (SIMS).
The term microscopy is used where a
surface is imaged with radiation of the same energy. Where radiation
of different voltages of frequencies is used, i.e., with varying
energy, the term spectroscopy is generally used. Dual purpose
instruments are usually called microscopes even if they allow
spectroscopic investigations as well.
All these known instruments require that
the surface investigations be made in a good vacuum of e.g.,
10-10 Torr. Temperatures should be as low as possible in
the cryogenic range. The particles used are free particles moving in
a high vacuum under the influence of applied fields. These particles
need to be freed previously, of course, by some cathode or ion
source.
In a atomic system or in a solid
body, if charged particles are subjected to an interaction composed
of a long range repelling component and a short range attractive
component, then the resulting force builds a potential wall or a
barrier. According to classical conceptions such a barrier can be
crossed only by particles having energy greater than the barrier.
There are nevertheless always a finite number of particles by a
potential barrier which are capable of crossing the potential
barrier even though they do not have sufficient energy. In a sense,
they undercross the potential barrier via a tunnel. This so-called
tunnel effect can be explained only by wave mechanics. Atomic
particles have a two-fold nature in that only part of their
properties can be explained by particle mechanics, another part of
their properties being interpreted only by the wave concept. The
tunnel effect is a wave property comparable in a sense with the wave
matching phenomenon at an interface between different media.
According to the tunnel effect there
exists a calculable probability that a finite number of electrons
bound by a potential can cross the tunnel barrier even at low
voltage differences. A tunnel barrier may be provided by a thin
layer in a solid body. A high vacuum may also represent a tunnel
barrier when the high vacuum distance to be crossed is between a few
and several hundred Angstroms. Some bound electrons are capable of
tunneling through such distances. In experiments with vacuum tunnel
barriers a very weak tunneling current has flowed from a fine
conductive tip to a flat counter electrode when the tip is posed
above the counter electrode within a small distance. However, known
experiments have needed expensive apparatus and have required a lot
of time due to considerable technical difficulties. Many hours have
been needed to get a single measuring point. A series of
measurements has required several days.
Experiments with field electron emission
also have been carried out wherein a fine tip serves as an electron
source or as a so-called cold cathode. Here the tunnel effect is
utilized only to free electrons from the metal of the tip into the
vacuum. Under the influence of a strong electrical field, electrons
are freed from the emitting tip and are accelerated towards and kind
of imaged upon a screen or photosensitive layer. The distances
traveled in the vacuum by the electrons are considerably longer than
the required short distance within which the vacuum tunnel effect is
possible with bound electrons.
The
object of this invention is to provide a new instrument for
investigation of surface structures of highest resolution which
utilizes the vacuum tunnel effect. Therefore, the apparatus is
operating only with electrons bound by a potential. Information
about the surface of the sample being investigated preferably should
be available in a relatively short time.
These objects are met by the scanning
tunneling microscope described herein.
In order to implement such an
instrument, considerable technical difficulties must be overcome.
The apparatus must operate in a ultra-high vacuum of better than
10-10 Torr. Furthermore, the temperature should be as
close to absolute zero as possible. This means cryogenic
temperatures lower than liquid helium temperature of 4.2 K. The
operating temperature should be lower than 1 K. and preferably lower
than 0.3 K. Under these extreme conditions, position adjusting
drives should still operate and have a sensitivity on the order of
Angstroms. The drives should also be capable of being positioned
accurately and reproducibly. A vertical drive is especially
difficult to implement. On the one hand it must move in a relatively
coarse fashion over a distance in the range of millimeters at the
beginning of an investigation, when the apparatus is loaded with a
sample to be investigated. But during the actual investigation it
must be capable of operating very finely with an accuracy on the
order of fractions of an Angstrom. Special attention should be
directed towards obtaining absolute freedom from vibrations. Thermal
fluctuations, which normally produce variations on the order of
magnitude of Angstroms, and hence in the order of magnitude of the
operating range of the instrument, have already been substantially
removed by the extreme cooling down. However, every sound pulse, no
matter how small, will generate a disturbing elastic wave within the
material. Therefore, an optimal suspension or support of the
essential parts of the instrument is very important.
|
| Drawing
Descriptions: |
The present invention for
investigation of surface structures will now be described in more
detail with the aid of the drawings in which:
FIG. 1 shows schematically a block
diagram of the essential parts of the apparatus for investigation of
surface structures according to the invention.
FIG. 2 is used to illustrate the tunnel
effect through a barrier between two metals.
FIG. 3 shows graphically the strong
dependence of vacuum tunnel effect tunneling current upon barrier
dimensions.
FIG. 4 illustrates a scan in
the lateral direction while controlling the tip distance above the
sample surface during the scan.
FIG. 5
illustrates repeated lateral scanning of the probed surface with
parallel raster scan lines.
FIG. 6 shows
schematically a practical 3-dimensional plot of the investigation
results either as values of the tip position or as the proportional
piezo voltages of the piezo drives.
FIG.
7 illustrates the principle of an additional variation or modulation
in the vertical distance of the scanning tip during a scan to get
additional information, such as possible change in the work function
of the sample surface.
FIG. 8 shows
schematically a practical plot of the additional information
obtained in accordance with the principle described in connection
with FIG. 7.
FIG. 9 shows a comparative
review of resolution limits of the human eye, of several microscopes
and of the scanning tunneling microscope according to the invention.
FIG. 10 shows a possible arrangement of
the piezo drives in the lateral directions.
FIG. 11 shows a vertical piezo drive
suitable for coarse adjustment as well as for fine adjustment.
FIG. 11.1 schematically shows drive
signal pulses used for coarse adjustment of the piezo drive
illustrated in FIG. 11.
FIG. 12 shows a
perspective view of a section of the trough-like guide member of the
vertical piezo drive shown in FIG. 11.
FIG. 13 shows a vertical section of the
inner parts of the apparatus and illustrates a vibration free
suspension.
FIG. 14 shows a vertical
section of the high vacuum chamber and the inner parts of the
apparatus.
|
| Description of
Preferred Embodiments: |
The block diagram of FIG. 1
shows the essential parts of the new instrument for investigation of
surface structures. An ultra-high vacuum chamber 1 comprises means
for generating a high vacuum better than 10-10 Torr. This
means is generally shown as vacuum pump 2, even though in reality it
may include several pumps or other equipment. A cryogenic source 3
cools the ultra-high vacuum chamber 1. Any suitable device may serve
as a cryogenic source. In order to reach temperatures as low as
possible preferably either a liquid helium or a dilution
refrigerator is used operating with both helium isotopes
3 He and 4 He. An outer liquid nitrogen jacket
provides pre-cooling. A sample 4 acts as a base electrode above
which a tip 5 is poised at only a short distance away. Both
electrodes are drawn schematically in exaggerated size. Relative to
each other they can be moved in three dimensions. Symbolically, this
is shown by three axes crossing rectangularly and designated by x, y
and z. The electrodes are further provided with three piezo drives
6, 7 and 8. Piezo drives 6 and 7 operate in lateral dimensions x and
y. For example, they may act on sample 4 and move it relative to tip
5. Alternatively, sample 4 may be fixed, and both lateral drives 6
and 7 act on tip 5. Vertical drive 8 adjusts the relative position
of the electrodes in the z dimension.
Apparatus for supplying energy and
coolants, and means for control of the instrument and for analyzing
and indicating the investigation results, are all located outside of
the ultra-high vacuum chamber 1. Some of the essential parts of this
apparatus are shown in FIG. 1. Measuring equipment 9 is part of the
electronic control means and is connected to the electrodes, i.e.,
to sample 4 and to tip 5, as well as to piezo drives 6, 7 and 8. A
control means 10 is connected to the measuring equipment 9 and acts
upon the vertical drive 8. Measuring equipment 9 is connected to
analyzing means 11 which in turn is connected e.g., to a plotter 12
and to a viewing screen 13.
The
mechanical dimensions of the electrodes, sample and tip, as well as
their possible ranges of adjustment are extraordinarily tiny because
of the delicate nature of the vacuum tunnel effect. Electronic
control equipment needs to be able to operate very precisely and the
measuring equipment must be extremely sensitive. In a vacuum, the
barrier to be crossed by bound electrons by tunneling lies in the
order of magnitude of about 10 A (1 nm) to 100 A (10 nm). The tip
electrode is moved above the sample at a distance from the sample of
about the same order of magnitude. It may not strike against the
sample and thus make conductive contact. It also may not get so far
away from the surface of the sample that pure tunneling currents are
no longer possible. The current density of the tunneling currents is
on the order of 100 A cm-2. However, due to the
extraordinarily small dimensions, the tuneling currents which flow
in reality are only about 10-10 A. The tip's radius of
curvature in this case is in the order of magnitude of 1000 A (100
nm). By calculation it can be shown that about half of the tunneling
current flows through a central region between the tip and sample
having a diameter of only about 100 A (10 nm). In the outer
direction, the current density decreases rapidly. Thus, the
tunneling microscope has a kind of "focus" with a radius of about 50
A (5 nm). In the lateral dimension the tip therefore should be moved
across the sample surface in steps which are not greater than 50 A
(5 nm).
Individual single steps may be
shorter because overlapping measuring points still contain
analyzable information.
The vertical
drive should have higher accuracy because tunneling current varies
very strongly with the z dimension. The tunneling probability, and
hence, the tunneling current depends exponentially upon the
electrode distance. Therefore, the accuracy should be at least 0.1 A
(0.01 in order to guarantee a vertical resolution in the order of
magnitude of 1 A (0.1 nm).
The drive
means should be not only adjustable over small distances in all
dimensions, but set positions should also be definite and
reproducible. The drives should also be reliable in ultra-high
vacuum, and should be operable at temperatures close to absolute
zero. Drive means meeting such conditions are for instance piezo
drives. In three coordinates the exact position of tip 5 is known
from the values set, i.e., from the piezo voltages.
In physical experiments one can measure
tunneling current at a defined applied voltage, wherefrom one can
derive the tunnel resistance of the vacuum tunnel barrier for a
certain combination of electrodes as a function of the spatial
coordinates. The new instrument for investigation of surface
structures not only delivers data about individual measuring points
but also gives information about a whole area of a sample surface
within a short time period. It operates much like a scanning
microscope. The sample surface is investigated in raster lines one
after the other, and the whole image is composed of the scanning
lines of the scanning pattern. During the scan the drive means of a
first lateral dimension is operating for a raster line, while the
drive means of the transverse other lateral dimension is kept fixed.
After a lateral shift of about a line width, the next line is
scanned by the first drive means and so forth.
While scanning with a tip poised above a
sample surface a certain danger exists of making inadvertent contact
between the tip and sample because roughness of the surface may be
on the order of magnitude of the vacuum tunnel barrier and therefore
also of the vertical distance of the tip above the sample. Such
involuntary contact should be avoided. This instrument avoids such
danger automatically while operating in the scanning tunneling
microscope mode. The scan operation is defined in the lateral
dimensions. However, the tip's vertical distance is variable. The
measuring method itself, or the control method, automatically keeps
a correct distance between the tip and the sample surface. By
continuously measuring tunnel effect parameters, such as tunneling
current and applied tunneling voltage, the tunnel resistance can be
determined at all times as the ratio thereof. The operation can be
conducted such that the voltage applied to the electrodes is kept
constant by fine adjustment of the tip's vertical distance above the
sample. Alternatively, the tunneling current can be controlled to a
desired value at all times. With suitable control equipment the tip
to surface distance also can be controlled directly to produce a
constant tunnel resistance. Other electrical parameters proportional
to the tunnel resistance may be used instead for control of tip to
surface distance. Applied voltage may possibly be used to control
the vertical distance of the instrument if tunneling current is kept
constant. In every case the instrument is controlled during scan in
a lateral dimension according to an electrical variable which is
proportional to the tunnel resistance. A tunnel effect variable is
measured and kept constant by fine adjustment of the vertical drive
in the z dimension by means of a closed loop control system.
The scanning pattern is preferably a
line raster whereby the area is scanned in a first lateral dimension
(x) in straight parallel lines, one after the other. The second
lateral dimension (y) is the other scanning parameter. The vertical
distance (dimension z) is controlled with a feedback system in
accordance with the measured variable which is proportional to the
tunnel resistance. Since the position of a piezo drive is
proportional to the piezo voltage, or to the respective drive
current, the drive currents of the three piezo electrical drive
means represent values equivalent to the position of the tip in each
dimension. Generally, the coordinates are cartesian coordinates with
three orthogonal axes. However, curved scanning is measured and kept
constant by fine adjustment of lines are also permitted, if they are
reproducible.
Data analysis is made as a
three dimensional representation. Both lateral dimensions can easily
be shown on a plotter 12 or on a viewing screen of screen device 13.
A suitable representation must be chosen, however, for the third
dimension. One possibility is to represent the measuring values as a
set or family of curves x(z) as a function of the parameter y.
Another possibility is to show the z values as brightness steps at
point x,y. A corresponding graphical representation may comprise
points with different areas of other symbols. When the
representation occurs on a screen, the brightness of the cathode ray
tube may be controlled according to the values of the third
dimension.
The tip passes across the
surface of the sample at a vertical distance such that during scan
the tunnel resistance of the vacuum tunnel barrier is controlled to
a constant value. The thickness of the vacuum tunnel barrier remains
constant if the work function of the sample material is locally
constant. This means that during the scanning movement, the tip is
following all unevenness and roughness of the sample surface at a
constant distance therefrom. The drive current of the vertical drive
means thus is a true image of the surface structure. The image
produced by the scanning tunneling microscope is an extremely
enlarged image of the sample surface, provided the sample's work
function is constant.
This new
instrument also can provide valuable surface information when the
work function of the sample is not locally constant. Work function
variation may be caused e.g., by enclosures in the sample material,
by oxide layers, by adsorbants or by other disturbances of the
material. The measuring method can be improved in various ways. For
example, a superimposed mechanical modulation of the tip's distance
in combination with a suitable frequency or phase dependent
measuring method will detect variations in the tunneling current
with respect to the z dimension. Conclusions may be drawn from such
tunneling current variation as to variations of the work function of
the sample.
Normally, conduction
electrons are able to leave a metal if they fullfill the work
function .PHI., which is on the order of several electron volts,
e.g., of about 5 eV. In the energy diagram shown in FIG. 2, the
electrons of a first metal are bound in a first potential well 14,
and there they occupy the lowest energy states or levels, as
indicated by hatching. A second metal is characterized by a second
potential well 15 separated from the first potential well 14 by a
potential barrier 16. Generally, this potential barrier 16 cannot be
crossed by the electrons. Electrons are able to leave the first
metal and enter into a vacuum when their energy E is raised to the
value of the upper limit of the bound energy states plus the work
function .PHI., the so-called emission edge 17. Electrons having at
least such energy are emitted from the metal, and can move freely in
the vacuum as free electrons. They are then able to reach the second
metal at any time and can later occupy the lower energy states
indicated by the hatching at the bottom of the second potential well
15. According to the classical particle concept, no electrons of the
first metal can reach the second metal without energy being supplied
to it equal to at least the amount of the work function .PHI., the
barrier thickness d and atomic natural constants, a current of bound
electrons of the first metal can reach the second metal without
energy sufficient to overstep the potential barrier 16 and hence,
the emission edge 17. Some of this so-called tunneling current
already flows when a small energy difference or potential difference
exists between the first metal and the second metal. In FIG. 2 the
first potential well 14 appears a little bit raised in comparison
with the second potential well 15. This can be caused by a small
electrical voltage occurring between both metals. An energy
difference of only 1 meV corresponds to a thermal stimulation of
11.6K. Arrow 18 symbolizes the tunneling current flowing from the
first to the second metal. To avoid thermal stimulation of
conduction electrons, the temperature should be as low as possible
and close to absolute zero.
Tunneling
probability depends very strongly upon the barrier thickness. As
represented graphically in FIG. 3, tunneling current I versus the
barrier thickness d is an exponential function. In the related
formula all atomic natural constants are combined and represented by
constant a. The exponent is the negative product of the square root
of the work function .PHI., the barrier thickness d and the constant
a. This formula is valid also for a vacuum tunnel barrier. It
follows that with a locally constant work function .PHI., the signal
of the scanning tunneling microscope is a true image of the sample
surface structure.
FIG. 4 shows
schematically and very much enlarged the scanning of a sample 4 in a
lateral dimension by a tip 5 poised above the surface of the sample
by a distance corresponding to the vacuum tunnel barrier. Tunneling
current is flowing through the vacuum between the electrodes. The
direction of tip movement 19 may correspond to the x dimension for
example. The vertical distance of tip 5 above the sample surface may
be on the order of about 20 A (2 nm). Unevenness and roughness of
the sample surface has a magnitude of about the same order. If the
vertical distance between the tip and the structure of the sample
surface were not automatically controlled during scanning movement,
a step of only one atomic layer would already severely affect
results. Control of the vertical position of the tip is indicated in
the figure by the double-headed arrow 20. During scan in direction
19, the tip 5 is continuously controlled by piezo drive 8 according
to electrical parameters measured. The drive current or piezo
voltage of the vertical drive means is measured as the independent
variable. This signal is used or analyzed to represent the sample
surface structure being investigated.
After completing the scanning of a
raster line in a first lateral dimension (x), the tip 5 is shifted
in the transverse second lateral dimension (y) by about the
thickness of a raster line. Subsequently, another raster line is
scanned parallel to that first line. By repeated lateral scanning of
parallel raster lines, the whole sample surface will be scanned line
by line. In FIG. 5 both electrodes 4 and 5 are schematically shown
much enlarged. At the sample surface the dashed line indicates the
path of the shadow of tip 5 over the surface of the sample 4. The
dotted line indicates the path of the tip itself at a distance over
the surface determined by the vacuum tunnel barrier thickness. The
axis system x,y,z indicates the coordinates of the dimensions. For
example, scanning may occur in the x dimension. Between repeated
scans, the tip is shifted in the y dimension about the thickness of
a raster line in each case. Fine adjustment by the vertical drive
means automatically controls the z dimension position of the tip so
as to maintain a constant tunnel resistance. The results of the
scanning tunneling microscope scan are displayed by plotting the
drive currents or piezo voltages as a set of curves representing
three dimensions. These measured currents or voltages correspond to
the position dimensions of the fine tip 5 in three coordinate
directions. For example, FIG. 6 shows the z direction piezo voltage
Vz as a function of the x direction piezo voltage Vx for each of a
family of different y direction piezo voltages Vy. If the work
function .PHI. of the sample material is constant over the scanned
region, the vacuum tunnel barrier is also constant so that the
tunnel resistance may be kept constant too. When the work function
is constant, the scanning tip follows the sample surface at a
constant vertical distance. The family of curves of FIG. 6 is then a
true enlarged image of the sample surface structure.
When there is a local variation in the
work function .PHI. of the sample, the vacuum tunnel barrier
thickness varies even though the tunnel resistance is held constant.
Therefore, the vertical position coordinate of the scanning tip does
not exactly correspond to the structure of the sample surface.
Changes in work function of the sample during the scan can be
detected by modifying the measuring method and analysis. Similar to
FIG. 4, FIG. 7 illustrates scanning of the surface of sample 4 by
tip 5 in a lateral dimension. Similar designations have the same
meaning. Here the sample 4 has a relatively smooth surface. However,
it contains an inhomogeneity 21 at which the work function .PHI.
changes. When tip 5 moving in direction 19 reaches inhomogeneity 21,
the tunnel barrier thickness changes even though the tunnel
resistance is kept constant by the automatic control system. The
dashed line marks the position of the scanning tip during the scan
and shows the change in level when it passes inhomogeneity 21. To
get additional information about possible change in work function
.PHI., a superimposed periodic movement in the vertical direction is
applied to the scanning tip 5. This distance variation or modulation
of the scanning tip occurs e.g., at 100 Hz. From FIG. 3 it can be
seen that the tunneling current I has an exponential dependence upon
barrier thickness d and that the square root of the work function
.PHI. appears in the exponent.
A change
in barrier thickness caused by a change in the work function can be
detected from the change in tunneling current during variation of
the distance between the tip and measured surface. The z coordinate
is modulated about a mean value corresponding to a constant tunnel
resistance, which is determined by the automatic control system. The
modulation amplitude is constant or at least known. The derivative
of the tunneling current I with respect to the vertical deflection
in the z direction has as a factor the square root of the work
function.
Since other variables in this
derivative can be regarded as constant at a measuring point in a
first approximation, the variation of tunneling current with the z
distance variation dI/dz is essentially proportional to the square
root of the work function .sqroot..PHI.. The signal generated by the
additional modulation can be filtered out and analyzed separately
using a phase and frequency selective amplifier, sometimes called a
block-in amplifier. It may be advisable to mathematically square
these measuring values because the square of dI/dz is practically
proportional to the work function .PHI.. FIG. 8 illustrates a
possible way of displaying these results. On a second plotter or a
second viewing screen the work function .PHI. is plotted as a
function of x direction piezo voltages Vx for each of a family of
different y direction piezo voltages Vy. In FIG. 7 the large
double-headed arrow 20 indicates the z position variation of tip 5
necessary to maintain constant tunnel resistance, while the
additional variation in the vertical distance of the tip 5 is
indicated by the small double-headed arrow 22.
This new scanning tunneling microscope
exhibits an extraordinarily good resolving power. FIG. 9 compares
the resolution limits of some kinds of microscopes with the human
eye. Lateral resolution is indicated along the abscissa and is in
the range of 109 A through 1 A (108 nm through
0.1 nm). The ordinate corresponds to vertical resolution in the
range of about 109 A through 10-2 A
(108 nm through 10-3 nm). The resolution
limits 23 of the human eye are shown lying in the range of about
109 A through 106 A of lateral resolution and
109 A through 107 A of vertical resolution.
Roughly three power ranges of microscopes may be defined. Between
about 107 A through 104 A lateral and
106 A through 10 A vertical there are located different
kinds of optical microscopes 24 through 28. The most unfavorable
instruments are the simple low power optical microscopes 24 having a
numeric aperture of about 0.1. High power optical microscopes 25
with numeric aperture of about 0.4 show a better lateral resolution.
The better vertical resolution is shown by instruments such as
multiple-beam interferometers 26, differential interference
microscopes 27 or phase-contrast microscopes 28. Electron
microscopes are located between about 105 A through 10 A
lateral and 10 6 A through 102 A vertical. The
electron microscope 29 covers the largest range of lateral
resolution as yet available. The scanning electron microscope 30 is
better with respect to vertical resolution. The scanning tunneling
microscope 31 according to this invention covers practically the
whole range of 108 A through 102 A lateral and
107 A through 10-1 A vertical. It should be
noted especially that vertical resolution such as this has not been
achieved yet by any other instrument.
Piezo electric drive means can be
fabricated from disks of piezo ceramic. Such disks are covered on
both sides with metallic coatings to form electrodes. Each disk is
cut such that an applied positive electrical field causes an
elongation in the direction of the thickness dimension and a
contraction in all other directions. Depending upon the design, the
metal layer on one side may be divided into separate electrodes
while the metal layer on the other side may be continuous and
function as a common ground electrode. Drive currents may be applied
to the electrodes by sliding contacts. FIG. 10 shows an example of a
lateral drive means. An L-shaped piece 32 with rectangular legs is
cut from a ceramic disk. The upper surface is provided with two
electrodes 33, 34. One end 35 of the device is immobile. Drive
currents applied to the first electrode 33 cause a proportional
elongation or contraction of the associated leg in the x dimension.
This causes also a shift of the other leg parallel to itself. Drive
currents applied to the other electrode 34 cause a proportional
elongation or contraction of the other leg in the y dimension. Using
both of these lateral drive means, the free end 36 of the other leg
may be positioned with respect to both the x and y coordinates.
Sample 4 may be attached to free end 36, for example, while the
scanning tip 5 is attached to a z direction drive. The vertical z
direction drive means in this case has a fixed position in other
directions. However, it is also possible to mount the z direction
drive means onto the free end 36. In this case, sample 4 could be
fixed in position while tip 5 is attached to the free end of the z
direction drive means and scans in all three coordinate directions.
FIGS. 11, 11.1 and 12 illustrate a z
direction adjusting means. This device can be adjusted in a
relatively coarse manner over a vertical distance of several
millimeters. This is necessary to be able to load the instrument
with a sample at the beginning of an investigation. This vertical
distance drive also can be adjusted very precisely during actual
scan of the sample surface. A piezo ceramic piece 38 moves in the z
direction in a worm-like manner along a trough-like guide member 37.
The edges 39 of the guide member 37 are parallel to each other.
Guide member 37 is somewhat flexible because of the longitudinal
notch 40. Thus, at every temperature and even when submerged in
liquid helium, edges 39 slidably support the legs of the shaped
piece 38. In plan view shaped piece 38 appears essentially H-shaped.
Two pairs of legs are guided by notches in edges 39 of guide member
37. The body of the shaped piece 38, corresponding to the crossbeam
of the H, moves longitudinally along the middle of guide member 37.
Head 41 is an extension of the H crossbeam and carries scanning tip
5.
The lower side of shaped piece 38 is
covered with a continuous metallization acting as a common
electrode. Drive electrodes 42, 43 and 44 cover corresponding parts
of the shaped piece 38 and serve to produce worm-like movement when
4-phase drive currents are applied. The hind legs carry the drive
electrode 42, the body is covered with the drive electrode 43 and
the fore legs are carrying the drive electrode 44. The head 41 bears
a fine adjustment electrode 45. The front end carries the scanning
tip 5. The drive signal waveforms comprise pulses of about 1000 V
with a repetition rate of 100 Hz. The waveforms are illustrated in
FIG. 11.1 where subscript numbers correspond to the respective drive
electrodes. Drive signals V42 and V44 applied
to drive electrodes 42 and 44 respectively are opposite in phase.
They cause a pair of legs to contract and therefore becomes loose
within the guide notches while the other pair of legs expands and
therefore becomes tight within the guide notches. The drive signal
applied to drive electrode 43 of the body causes alternate
contraction or elongation of the body. It overlaps either the
leading edge or the trailing edge of drive pulses applied to a pair
of legs. Depending upon the phase relationship of the series of
drive signals the drive member 38 moves in worm-like fashion in the
z direction in one or in the other direction.
When coarse adjustment is finished in
single steps of 10 A to 30 A over an adjustment range on the order
of millimeters, the periodic drive signals V42,
V43 and V44 are switched off and the legs of
the shaped piece 38 are clamped firmly in the guide notches. Now
fine adjustment occurs by applying analog drive currents to drive
electrode 45. The trough-like guide member 37 may be made of
stainless steel. Both edges 39 may be fabricated of ceramic
material. As can be seen from the perspective view of FIG. 12 edges
39 may be attached to the guide member 37 by screws.
The inner parts of the apparatus are
well cooled and well suspended in an ultra-high vacuum chamber. A
schematic view is shown in FIG. 13. For example, an outer cooling
jacket 46 may be provided which may be filled with liquid nitrogen
to an upper level via an inlet 47. Gaseous nitrogen exits via a
larger outlet 48. This first cryostat may also include additional
equipment which is not shown. An inner cooling jacket 49 contains
liquid helium. Preferably this cooling vessel is designed as a
dilution refrigerator operating with both helium isotopes
3 He and 4 He. However, only a thin liquid
helium inlet 50 and a thicker gaseous helium outlet 51 is shown for
simplicity. Additional optional components associated with operation
and control of this second cryostat are omitted. A hovering support
52 suspends the sample to be investigated, the scanning tip with its
drive means and the first stages of electronic equipment for
analysis such as measuring amplifiers, for example.
Two contradictory requirements must be
fulfilled for operation of the instrument. On the one hand heat must
be removed from the inner parts to achieve a very low temperature.
This is possible only by convection, which suggests a dense
packaging of metallic parts. On the other hand, the arrangement 52
on the hovering support must be suspended completely free of
vibration. This forbids a dense packaging of mechanical parts. FIG.
13 illustrates a possible solution.
Coarse adjustment mechanism 53 serves
both purposes. Inside of cooling jacket 49 a pot 54 is movable in
the vertical direction and is positioned by three metal bellows 55.
Metal bellows 55 are filled with sufficient pressure to lift pot 54
using preferably a gas which stays gaseous at an operating
temperature below 1 K. The helium isotope 3 He has proven
to be suitable at such temperatures. Helium pipes 56 are connected
from level control means (not shown) to the metal bellows 55. A
large helical spring 57 is positioned between pot 54 and inner
cooling jacket 49. Since the apparatus is arranged in a vacuum,
outer air pressure is so low that metal bellows 55 might not
collapse when internal pressure is reduced except for the additional
return force supplied by helical spring 57.
Several columns 58 within pot 54 carry a
calotteshaped superconducting tray 59 of niobium. Each column 58
comprises a series of oscillating members of different eigen
frequency. The eigen frequencies of the oscillating members are
mistuned with respect to each other and chosen such that no sound
pulse will be transferred to the inner parts of the apparatus, e.g.,
from an operating turbopump. The different masses of the oscillating
members comprise metal blocks 60, which are very heat conducting,
e.g., made of copper. Each of the metal blocks 60 carry feet which
mate with corresponding feet on adjacent metal blocks 60 to provide
heat conducting contact when they are brought into contact during
operation of the coarse adjustment mechanism 53. This quickly drains
heat from the apparatus when it is being cooled down. Each column 58
contains a row of spiral springs 61 which connect individual
oscillating members to each other, each oscillating member
comprising a spring and metal block. The top and bottom of each
column 58 are acoustically mismatched. They are suspended, for
example, in glass wadding to minimize transfer of sound.
The hovering support of the arrangement
52 has a lower surface with a shape that matches the calotte-shaped
superconductive tray 59. The lower surface contains a large number
of permanent magnets. When the tray 59 has become superconducting at
a low temperature, arrangement 52 hovers above the tray. Every
virtual movement of the hovering support generates strong currents
within the superconductor, which generates a magnetic counter field
that keeps the arrangement 52 hovering. The equilibrium state is
stable due to the form of a calotte. Disturbing sound pulses are not
transferred through the intermediate space because it is a vacuum.
The described coarse adjustment
mechanism 53 fulfills both basic requirements described above. For
cooling down the apparatus, metal blocks 60 of the columns 58 are
brought into heat conducting contact with each other. During
operation of the scanning tunneling microscope, the columns 58 are
elongated so that superconducting tray 59 is lifted to a position
where arrangement 52 hovers. Excessive movement of the coarse
adjustment mechanism 53 is limited by stops. When mechanism 53 is in
a low position or tray 59 is not superconducting, arrangement 52 is
prevented from dropping also by stops.
FIG. 14 is a cross-sectional view of the
ultra-high vacuum chamber. Similar reference numbers again refer to
similar parts. Ultra-high vacuum chamber 1 is a cylindrical steel
vessel. An intermediate flange 62 allows separation of the bell jar
upper part. The inner parts of the apparatus are arranged on an
annular girder 63 located in the lower part near to the upper edge
thereof. The upper part is essentially a vacuum bell jar with
observation windows and with closable stubs. The lower part is
firmly mounted. It contains a number of feed-throughs for supply
lines and a port connected to a pumping means located in the
direction of arrow 2. In the upper part of the ultra-high vacuum
chamber 1 there is sufficient space also to provide other surface
structure investigation instruments in addition to the scanning
tunneling microscope. Thus, the same sample may be simultaneously
investigated in different ways. For example, another instrument
which also needs ultra-high vacuum and low temperatures is the
spectroscope which analyzes scattered or diffracted electrons,
X-rays or corpuscular radiation.
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References: |
none
(No patents reference this one)
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| Other Abstract
Info: |
CHEMABS 095(10)089950S
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| Other
References: |
- "The Topografiner: An Instrument for Measuring Surface
Microtopography", Young et al., Review of Sci. Ins., vol. 43, No.
7, Jul. 1972, pp. 999-1011.
- "Field-Emission Microscopy from Glass-coated Tips", Rihon,
Phys. Stat. Sol(a), vol. 54, No. 1, Jul. 1979, pp. 189-194.
- "Field Emission Ultramicrometer", Young, Rev. of Sci. Ins.,
vol. 37, No. 3, Mar. 1966, pp. 275-278.
- "Vacuum--Tunneling Spectroscopy," Plummer et al., Physics
Today, vol. 28, No. 4, Apr. 1975, pp. 63-71.
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