Scanning probe microscope employing adjustable tilt and unitary head

This invention is a scanning probe microscope which uses three separate motorized legs to adjust the distance between the probe and sample and to adjust the tilt between the probe and the sample. The microscope is shown configured in various ways. One form is a scanner on a base in which the base contains the sample and legs. Another is a scanner which contains the legs and rests on the sample, or may also rest on a support that spans a larger sample allowing translation of the sample independent of the scanner. Another is a scanner which contains the legs and is mounted so that a sample holder sits on the legs. The latter configuration allows for easy access to the sample. One variation of this configuration has provision for the mounting of several samples which can be sequenced for probing automatically.

BACKGROUND OF THE INVENTION 
This invention relates to scanning probe microscopes and, more 
particularly, in a scanning probe microscope having a probe wherein the 
relationship between the probe and a sample to be scanned is defined by 
three legs, to the improvement to allow tilt between the probe and the 
sample to be adjusted comprising, each of the three legs including 
adjusting means for adjusting a length thereof; and, tilt control means 
attached to the adjusting means for independently adjusting the length of 
selected ones of the three legs. 
Scanning probe microscopes (SPMs) are instruments that provide high 
resolution information about the properties of surfaces. One common use of 
these devices is imaging, and some types of SPM have the capability of 
imaging individual atoms. Along with images, SPMs can be used to measure a 
variety of surface properties, over the range from a few angstroms to 
hundreds of microns. For many applications, SPMs can provide lateral and 
vertical resolution that is not obtainable from any other type of device. 
The first type of SPM developed was the scanning tunneling microscope 
(STM). The STM places a sharp, conducting tip near a surface. The surface 
is biased at a potential relative to the tip. When the tip is brought near 
the surface, a current will flow in the tip due to the tunneling effect. 
Tunneling will occur between the atom closest to the surface in the tip 
and the atoms on the surface. This current is a function of the distance 
between the tip and the surface, and typically the tip has to be within 20 
angstroms of the surface for measurable current to be present. An STM has 
a mechanism to scan the tip over the surface, typically in a raster 
pattern. While the tip is scanned over the surface, the tip is kept at a 
constant distance above surface features by means of a feedback loop 
employing the tunneling current and a vertical position controlling 
mechanism. The feedback loop adjusts the vertical position of the tip to 
keep the tunneling current, and thus the distance, constant. The vertical 
position of the tip is determined from the control signals applied to the 
vertical position controlling mechanism. The vertical position, as a 
function of horizontal scan position, produces a topographic map of the 
surface. STMs can easily image individual atoms, and can also be used for 
highly accurate surface measurements on larger scales, up to a few hundred 
microns. STMs also may be used for data other than topographic images. One 
alternative operation of an STM is to hold the tip stationary while 
varying the bias voltage applied to the sample and monitoring the 
tunneling current, thus measuring local current/voltage characteristics of 
the surface. STMs require a conducting sample surface for operation. 
Non-conducting surfaces may be coated with a thin conducting material such 
as gold or, in some cases, non-conducting materials a few atoms thick 
lying on a conducting surface may be imaged. 
Another SPM, the atomic force microscope (AFM), similarly scans a tip 
across a surface. The tip in this case is mounted on the free end of a 
lever or cantilever which is fixed at the other end. The tip is brought to 
a surface such that the force interaction of the tip with the surface 
causes the cantilever to deflect. An AFM may be operated such that the Van 
der Waals attractive force between the tip and surface are near 
equilibrium with the repulsive force, or at larger cantilever deflections 
where the repulsive force, dominates. A feedback loop employing the 
cantilever deflection information and the tip vertical position is used to 
adjust the vertical position of the tip as it is scanned. The feedback 
loop keeps the deflection, and thus the force, constant. The tip vertical 
position versus horizontal scan provides the topographic surface map. In 
this mode, the forces on the surface can be made very small so as not to 
deform biological molecules. AFMs can also be operated in a mode where the 
repulsive force deflects the cantilever as it scans the surface. The 
deflection of the tip as it is scanned provides topographic information 
about the surface. AFMs may also be operated in a non-contact mode where 
the cantilever is vibrated and the Van der Waals interaction between the 
tip and surface affects the vibration amplitude. AFMs have a means to 
detect the small movements of the cantilever. Several means for cantilever 
motion detection have been used with the most common method employing 
reflected light from the cantilever. The deflection of a light beam due to 
the cantilever motion may be detected, or the movement of the cantilever 
can be used to generate interference effects which can be used to derive 
the motion. Like an STM, AFMs can image individual atoms; but unlike an 
STM, AFMs can be used for non-conducting surfaces. AFMs may also be used 
for measurements such as surface stiffness. 
Other SPMs may use different probing mechanisms to measure properties of 
surfaces. Probing devices have been developed for such properties as 
electric field, magnetic field, photon excitation, capacitance, and ionic 
conductance. Whatever the probing mechanism, most SPMs have common 
characteristics, typically operating on an interaction between probe and 
surface that is confined to a very small lateral area and is extremely 
sensitive to vertical position. Most SPMs possess the ability to position 
a probe very accurately in three dimensions and use high performance 
feedback systems to control the motion of the probe relative to the 
surface. 
The positioning and scanning of the probe is usually accomplished with 
piezoelectric devices. These devices expand or contract when a voltage is 
applied to them and typically have sensitivities of a few angstroms to 
hundreds of angstroms per volt. Scanning is implemented in a variety of 
ways. Some SPMs hold the probe fixed, and attach the sample to the 
scanning mechanism while others scan the probe. Piezoelectric tubes have 
been found to be the best scanning mechanism for most applications. These 
tubes are capable of generating three dimensional scans. They are 
mechanically very stiff, have good frequency response for fast scans, and 
are relatively inexpensive to manufacture and assemble. Such scanners are 
used in a commercial STM sold by the assignee of this application, Digital 
Instruments, Inc., under the trademark NanoScope. These scanners are made 
in various lengths, the larger ones having larger scan ranges. 
As can be appreciated, SPMs are extremely useful research tools, allowing 
for information of higher resolution to be obtained more conveniently than 
previously possible. Some aspects of SPM performance require improvement, 
however, in order for SPMs to become more practical for applications 
requiring less operator interaction, accurate repeatable measurements for 
larger scale samples, and high throughput. 
In the scanning probe microscope, the piezoelectric scanners typically have 
ranges of a few microns, so the sample must be brought close to the probe 
with some kind of mechanical arrangement in order for the probing of the 
surface to occur. Presently, these arrangements include moving the sample 
straight toward the probe with a screw or piezoelectric inchworm, or 
tilting the scanner support to bring the probe toward the surface. A prior 
art scanning probe microscope, which is most representative of scanning 
tunneling microscopes, is illustrated in FIG. 1 where it is generally 
indicated as 10. In this device, a scanner 12 rests on two fixed supports 
14 and one movable support 16 attached to a base 18. The fixed supports 14 
can be hand adjusted while the movable support 16 is motor driven and 
allows for automatic final approach. The scanner 12 must be hand adjusted 
and leveled; so, the probe 20 must be placed very near the sample 22 by 
eye, usually using an optical microscope, before the automatic approach is 
engaged. This procedure is not difficult; but, requires an operator to 
prepare each new probe site by hand. Other prior art SPMs utilize systems 
that translate the scanner toward the sample with a motion parallel to its 
axis. These systems may be operated with less operator participation; but, 
have no flexibility to adjust for sample tilt. 
In many instances and for several reason, it would be useful to have the 
ability to control the tilt of the scanner with respect to the sample 
independent of positioning the probe vertically. One reason is related to 
the errors caused by non-linear behavior of the piezoelectric scanning 
elements. Piezoelectric non-linearity is a well known source of error in 
the art, and can affect SPM data in many ways. For large scans, one 
non-linear error is related to tilt between the probe and the sample. It 
is extremely difficult to mount a sample such that, on the scale of SPM 
measurements, there is not some tilt between the sample and probe. For 
large scans, the cumulative non-linearity errors due to the scanner make a 
tilted flat surface appear bowed. As one useful application of SPMs for 
larger scale samples is surface dimensional measurements, the distortion 
of a tilted sample is a serious problem. The tilt may be on only part of 
the sample, so having a flat sample holder will not solve this problem. 
What is needed is a scanner which minimizes this distortion by having the 
scanner able to be tilted with respect to the sample, thereby allowing 
compensation for an effect that otherwise decreases the utility of the 
instrument. 
On the other hand, in the scanning of surfaces which have very steep 
features, such as the surface of an integrated circuit, it is useful to 
have a known tilt between the probe and sample. Given a tapered probe 20, 
such as an etched tungsten probe in the case of an STM, the probe 20 will 
have some angle for its profile, as indicated by the arrows in FIG. 2. If 
the probe 20 is perpendicular to the bottom of a groove 23 as depicted in 
that figure, it can be seen that it is impossible to scan all the way to 
the edge of the groove 23 as the side of the probe 20 will hit the side of 
the groove 23 before the scanning point of the tip. Thus, in order to scan 
to the edge of the groove 23, one must tilt the scanner (and therefore the 
probe 20) with respect to the sample 22 by an angle which is greater than 
the tip profile angle as depicted in FIG. 3. A lesser tilt would, of 
course, improve the situation but not completely solve it. As shown, the 
tilting allows the tip of the probe 20 to travel down the sidewall and 
determine its profile. The scanner and probe 20 would be tilted in the 
opposite direction in order to image the other side of the groove 23. The 
images of the tilted surfaces could then be patched together with the 
computer to construct a proper image reflecting the true surface topology 
of the entire groove 23. A similar procedure could be used for any very 
steep feature, such as a step or bump. As will be seen, this unique method 
is possible with the present invention as described hereinafter. 
Not only would it be desirable to be able to tilt the scanner with respect 
to the sample in a controlled manner in order to remove tilt or create 
known tilts; but, it would be desirable also to be able to automatically 
approach the sample with the scanner in a straight line fashion over a 
long range so that there is no need to manually place the tip near the 
surface with a microscope or magnifier. Most desirable would be to have 
both of these abilities in a single device as it is not practical to 
approach a new sample or a new sample section automatically without some 
means to adjust the tilt. These abilities along with the ability to 
translate a large sample underneath the probe, or the ability to 
automatically sequence a series of samples to the probe would allow SPMs 
to be used for totally automatic inspection and characterization of either 
large area samples or multiple samples. Such capabilities would make SPMs 
much more useful for industrial applications such as imaging magnetic 
disks or integrated circuit wafers. 
Wherefore, it is an object of this invention to provide a scanning probe 
microscope head which has both vertical motion and tilt motion. 
It is another object of this invention to provide a scanning probe 
microscope head which can be used conveniently in SPMs that will have the 
capability for large samples, fully automated operation, and multiple 
samples. 
Other objects and benefits of the invention will become apparent from the 
detailed description which follows hereinafter when taken in conjunction 
with the drawing figures which accompany it. 
SUMMARY 
The foregoing objects have been achieved in a scanning probe microscope 
having a probe wherein the relationship between the probe and a sample to 
be scanned is defined by three legs, by the improvement of the present 
invention to allow tilt between the probe and the sample to be adjusted 
comprising, each of the three legs including adjusting means for adjusting 
a length thereof; and, tilt control means attached to the adjusting means 
for independently adjusting the length of selected ones of the three legs. 
In the preferred embodiment, each adjusting means comprises, an outer leg 
connected to the scanner; a threaded inner leg threadedly disposed within 
the outer leg, the inner leg having an outer end contacting a supported 
area adjacent a portion of the sample to be scanned; and, means for 
rotating the inner leg within the outer leg whereby the inner leg is 
threaded into and out of the outer leg to change a combined length of the 
inner leg and the outer leg. The preferred means for rotating the inner 
leg within the outer leg comprises a motor drive connected to the inner 
leg. The preferred motor drive comprises a DC motor with a reduction 
transmission connected between the DC motor and the inner leg. 
In one embodiment, the three legs, the adjusting means, and the tilt 
control means are located in a base with the legs facing upward and the 
piezoelectric scanner sits on the three legs. 
In another embodiment, the three legs, the adjusting means, and the tilt 
control means are disposed in combination with the piezoelectric scanner 
as part of a stand-alone head with the legs facing downward and the head 
sits on the three legs over (or on) a sample to be scanned. In one 
variation of this embodiment, there is a sample holding structure having 
an upper surface upon which the head sits, the upper surface having an 
opening therethrough through which the piezoelectric scanner can pass into 
an interior of the box to place the probe in contact with a surface of a 
sample disposed thereunder; and, sample holding and positioning means are 
disposed in the interior of the structure for holding a sample and for 
positioning selected areas of a surface of the sample under the probe of 
the scanner to be scanned thereby. In another variation of this embodiment 
there are, a sample holding member positioned over the stand-alone head 
and having a lower surface against which the head rests, the member having 
a plurality of openings therethrough through which the probe of the 
scanner can pass to place the probe in contact with a surface of a sample 
disposed within selected ones of the opening; a plurality of holding and 
positioning means removeably disposed in respective ones of the openings 
for holding individual samples and for positioning a surface of a sample 
held thereby over the probe of the scanner to be scanned thereby; and, 
indexing means for selectively positioning respective ones of the openings 
over the probe. 
Preferably in this latter variation, the sample holding member comprises a 
disk mounted for rotation about a shaft in a horizontal plane; the 
openings comprise a plurality of shouldered bores through the disk located 
at spaced scanning stations of the disk; and, the plurality of sample 
holding and positioning means comprises a plurality of disk-shaped inserts 
having a bottom surface for carrying a sample to be scanned whereby the 
inserts may be dropped into the bores from above to rest on shoulders of 
the bores. This latter variation may also include means for lowering the 
stand-alone head while the indexing means is selectively positioning a 
respective one of the openings over the probe and for raising the 
stand-alone head after the indexing means is through selectively 
positioning the respective one of the openings over the probe. This could, 
of course, also be accomplished in an inverted configuration wherein the 
head is above the samples. 
The probe can be fixed with the sample being mounted on a device wherein 
the orientation between the sample and the probe is determined by three 
legs on the device.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is depicted in its most basic form in FIG. 4 where it 
is incorporated into a scanning probe microscope 10'. As depicted therein, 
according to the improvement of this invention all three of the supports 
are moveable supports 16 which are independently movable by motor drives 
24 which are controlled by the control computer 26. In tested versions of 
this embodiment, the supports 16 were designed to have much longer travel, 
higher speed, and finer resolution than the single motorized support of 
the prior art microscope 10 of FIG. 1. These features allow for 
substantial increase in the utility of SPMs. It should also be noted 
initially that while the primary illustrations contained hereinafter have 
the sample fixed, within the scope of the invention the probe can also be 
fixed with the sample being mounted on a device wherein the orientation 
between the sample and the probe is determined by three legs on the 
device. 
In the basic embodiment of FIG. 4, the movable supports 16 are mounted in 
the base 18 with the scanner 12 resting on the supports 16. A more 
versatile (and preferred) configuration is shown in FIG. 5. This 
embodiment is functionally equivalent to the embodiment of FIG. 4; but, 
has the advantage that the head design can be used in many SPM 
configurations, as will be illustrated. This embodiment is a free-standing 
head generally indicated as 28. The piezoelectric tube scanner 12 is 
mounted perpendicularly downward in the center of a support structure 30 
(which may be, for example a cylindrical or triangular plate) which has 
three hollow legs 32 attached thereto and extending perpendicularly 
downward therefrom. While not completely necessary, it is preferred that 
the legs 32 be spaced radially at 120.degree. intervals about the scanner 
12. Threadedly disposed within each leg 32 is an inner leg 34 having a 
ball 36 on the bottom end thereof. The legs 32 could also be replaced by a 
solid structure such as a cylinder with threaded holes to receive the 
three inner legs 34 and a central bore for the scanner as depicted in FIG. 
7. The inner legs 34 are fine threaded screws (1/4-80 having been used in 
tested embodiments) which are rotated by individual small DC motors 38 
that drive individual 1000:1 transmissions 40 which, in turn, rotate the 
screws 34. The motors 38 can have optical encoders on them to monitor 
their rotation, if desired; but, this is not considered as necessary and 
is, therefore, not preferred. The motors 38 are connected through an 
appropriate interface for the particular implementation (not shown and as 
will be readily determined by those skilled in the art without undue 
experimentation) to tilt control logic 42 which is most likely contained 
within the control computer 26 which controls the entire microscope. A 
separate tilt controller could, of course, be employed if desired and more 
applicable in certain applications. The head 28 in this particular 
illustration rests on a base 44 which holds the sample 22. The base 44 
could be flat so that the head 28 could be moved around on it; or could 
have indexing marks (e.g., hole, groove, flat) to position the balls 36 to 
place the probe 20 over the sample 22 as shown in FIG. 5. The inventors 
herein have found that it may be useful to use magnetic balls or magnets 
behind ferromagnetic balls to hold the head 28 down snugly on the base 44. 
The DC motors 38 are energized by the tilt control logic 42 to rotate the 
threaded inner legs 34 and thereby move the legs 32 up and down which, in 
turn, moves the support structure 30 and scanner 12 up and down. When all 
of the legs 32 are driven simultaneously, the support structure 30 and 
scanner 12 move up and down without tilting. This type of motion would be 
used for approaching the tip of the probe 20 to the surface of a sample 
22. The motion can be quite large (several millimeters) so that the tip 
would not need to be placed near the sample 22 by an operator before 
automatic approach is started. 
The tilt of the head 28 is varied by not energizing the motors 38 equally. 
Given the configuration depicted in FIG. 5 (i.e. one leg 32 in front of 
the probe 20 on the left side as the figure is viewed and two legs 32 
spaced equally on either side of and behind the probe 20 on the right side 
as the figure is viewed), the scanner 12 can be tilted in the Y direction 
by raising/lowering the two right legs 32 an equal amount and/or 
lowering/raising the left leg 32. The scanner 12 can be tilted in X by a 
similar process, i.e., by raising/lowering the left leg 32 and one of the 
two right legs 32 an equal amount and/or lowering/raising the other right 
leg 32. The tilt can be monitored by the data taken from the scanning 
probe 20 and this data can be taken while the legs 32 are being raised and 
lowered so that the tilt can be set by the system even though the 
motorized screws do not have encoders. In this preferred approach, the 
feedback for the tilting comes from the scanning system itself by fitting 
to the plane of the vertical data instead of from positional readout 
devices on the motors 38. This preferred approach makes the scanning head 
28 simpler and less expensive. After the tilt of the head 28 is set to a 
particular value, the head 28 can then be raised and lowered for changing 
the sample 22 by driving all three legs 32 at the same rate and in the 
same direction. 
As thus described, the improved scan head 28 of FIG. 5 allows for long 
distance probe approach or removal without operator participation. At the 
same time, it also allows for compensation for probe/sample tilt, or for 
the addition of controlled tilt. These abilities allow for several new SPM 
configurations that will be capable of automatic operation with accuracy 
and high throughput for large samples, multiple samples, and special 
applications such as integrated circuits which have steep cliffs or 
trenches. These various uses for the free-standing, tiltable scan head 28 
of FIG. 5 will now be described in detail. 
FIG. 6 shows the scan head 28 resting directly on a large sample 22. The 
scan head 28 may be placed on an reasonably flat surface with the probe 20 
withdrawn above the bottom of the supports. The approach and leveling 
operations can be accomplished automatically, making this configuration 
extremely convenient to use for suitable applications. This configuration 
would be useful for verifying surface structure or finish on large objects 
that would not be damaged by supporting the scan head 28. 
FIG. 7 shows an extremely useful SPM configuration employing the 
free-standing, tiltable scan head 28. The legs 32 of the scan head 28 rest 
on a rigid structure 46. The structure 46 has an opening 48 in the top 
thereof located under the scan head 28 allowing the scan head 28 to lower 
the probe 22 into the structure 46. Within the structure 46 is a sample 
positioning system 50 that can translate a large sample 22 (or several 
separate samples) attached thereon in two horizontal axes on perpendicular 
shafts 52 by drive 54 under the control of sample positioning logic 56, 
allowing for rapid and automatic probing of any part of the sample 22. The 
positioning also could be done with a rotary stage. This would be useful 
for multiple samples which could be rotated into position under the scan 
head. Standard commercial computer-controlled positioning products, for 
optical and other applications, can be employed for the system 50 and have 
several inches of travel as well as resolution and repeatability of 1 
micron or less. Given a typical large scan head 28 that can cover up to 
100 microns square or more scan size, this system can probe any section of 
a large sample automatically. The inventors herein have tested this 
configuration with structures 46 made of aluminum, and also of ceramics. 
The structure 46 must be rigid and isolated from vibration to maintain the 
stability required between probe and sample. The inventors herein have 
demonstrated adequate stability for sample sizes of up to eight inches, 
which is adequate for integrated circuit wafers and most magnetic or 
optical storage media. The translation stage of the system 50 can be 
either x, y or r, .theta. oriented, depending on the application. The scan 
head 28 of this invention is critical to making a large sample system 
accurate and versatile as it provides the abilities to compensate for 
local sample tilt, or to tilt the probe 22 relative to the sample 20, 
allowing for accurate mapping of steep structures. In this regard, the 
tilt can be determined from the data gathered by fitting the vertical scan 
information to a plane and then calculating the tilt required to level the 
plane relative to the scanner axes. 
Another potentially useful SPM configuration as depicted in FIGS. 8 and 9 
employs the scan head 28 in an inverted orientation. A sample holding disk 
58 is disposed horizontally for indexed rotation around a support shaft 60 
by an indexing mechanism 62. The sample holding disk 58 has a plurality of 
shouldered bores 64 therein at sampling stations of the disk 58. This 
configuration facilitates the rapid changing of samples as an operator may 
attach the samples 22 to inserts 66 that may be dropped into the bores 64 
from above to rest on the shoulders 68 thereof supported by gravity 
without conflict with the scan head 28. This system could support 
continuous sample cycling as the samples 22 in the sample holding disk 58 
could be quickly changed without stopping the system. Preferably, the head 
28 is mounted on a raise and lower mechanism 70 that works in combination 
with the indexing mechanism 62 under the joint control of the control 
computer 26. The raise and lower mechanism 70, when engaged, pushes the 
legs against the sample holder, thus maintaining the tilting capability. 
To index the sample holding disk 58 to a new sample scanning position, the 
head 28 is dropped slightly by the raise and lower mechanism 70 and the 
sample holding disk 58 is rotated to the next position with a bore 64 
positioned under the probe 22. The head 28 is then raised by the raise and 
lower mechanism 70 until the balls 36 contact the bottom of the sample 
holding disk 58. The head 28 is then raised, lowered and tilted in the 
manner described above, as required to accomplish the scanning of the 
sample. As those skilled in the art will readily recognize and appreciate, 
this approach could also work well rotated 180.degree. to a "right side 
up" configuration and, in fact, such an orientation might be preferred in 
some instances as there would be no necessity of the positive upward force 
of the scanner mechanism against the sample mount.