Scanning tunneling microscope having proper servo control function

A scanning tunneling microscope includes a piezoelectric driver expanding and contracting according to a voltage applied thereto to adjust the distance between a sample and a probe. A servo circuit outputs a servo voltage to control the expansion and contraction of the piezoelectric driver to keep a tunnel current flowing between the sample and the probe at a constant value. A correction voltage generating circuit generates a given correction voltage to correct a voltage to be applied to the piezoelectric driver. An adding circuit adds the servo voltage and the correction voltage together and supplies an added output to the piezoelectric driver. A control circuit controls the correction voltage according to the servo voltage to set the added output to a given reference voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a scanning tunneling microscope which can be used 
to observe the surface of a sample in the unit of atom size. 
2. Description of the Related Art 
Recently, a scanning tunneling microscope (STM) which can be used to 
observe the surface of a sample in the unit of atom size has been 
developed. 
It is generally well known in the art that when a metal probe having a 
sharp tip end is set as close as approx. 1 nm to the surface of an 
electrically conductive sample and a preset voltage is applied between the 
probe and the sample, a tunnel current flows between the probe and the 
sample. The tunnel current is largely dependent on a distance between the 
probe and the sample. The STM utilizes the property of the tunnel current 
to observe the surface of the sample. That is, when the probe is mounted 
on the actuator which can be moved in a 3-dimensional direction and it is 
scanned while the tunnel current is kept constant, the probe will move 
along the irregularity of the surface of the sample with a preset distance 
kept therebetween. Thus, variation of the surface state of the sample can 
be observed in the unit of atom size as an image by outputting the 
position of the probe as a 3-dimensional image. 
In general, a servo circuit is used to adjust the distance between the 
probe and the sample in the STM. The servo circuit detects a tunnel 
current flowing between the probe and the sample and controls the driving 
operation of the actuator to keep the distance between the probe and the 
sample at a constant value based on the detected tunnel current. 
In the conventional STM, adjustment of the distance between the probe and 
the sample is controlled only by use of the above servo circuit. 
Therefore, when the surface condition of the probe is bad, desired control 
cannot be effected. That is, when the sample has a slanted surface, 
undulated surface or a surface having holes formed therein and if the 
servo output is displayed on a CRT as an output indicating the surface 
condition of the sample based on the detected tunnel current, the CRT 
image plane cannot be effectively used and the STM image cannot be 
displayed within the image plane. In this case, the distance between the 
probe and the sample cannot be controlled by the servo system so that the 
STM image obtained can be displayed only with a low resolution in a 
vertical direction and the dynamic range thereof may be deviated from the 
central position. 
Further, as described before, the STM is a microscope having a super high 
resolution and can be used to observe the surface configuration and 
surface properties of the sample in the unit of atom size. Therefore, in a 
case where a desired portion of the sample is observed, it is necessary to 
first observe a wide range (several .mu.m) previously set to include the 
desired portion and then observe the desired portion. 
Further, when an STM image (3-dimensional image) of the wide scanning range 
is observed, it is sometimes required to enlarge part of the image for 
more specific observation. 
In order to meet the above requirements, in the prior art, an image 
obtained by wide range scanning with a preset resolution is displayed, 
then the scanning range is changed to a narrow scanning range and the 
probe is upwardly moved away from the sample surface by such a distance 
that the tunnel current cannot flow. Then, the scanning center is set on 
the position at which a portion to be enlarged lies by an X-Y rough moving 
mechanism using a pulse motor and the like and the lifted probe is set 
closer to the sample surface so as to be set within a tunnel current 
region, and then the scanning operation is effected again to display an 
image. 
However, in the above method, the precision of the rough moving mechanism 
for moving the sample influences the reliability of the STM image. 
Actually, the precision of the rough moving mechanism is extremely lower 
than the resolution of the STM. According to the method in which the probe 
is separated from the sample after the wide range scanning operation is 
effected, and then moved by use of the rough moving mechanism and set 
closer to the sample again to display an STM image, the desired position 
cannot always be correctly set because of the low resolution and precision 
of the rough moving mechanism. Therefore, an enlarged image at exactly the 
desired position cannot always be obtained. 
SUMMARY OF THE INVENTION 
Accordingly, a first object of this invention is to provide a scanning type 
tunnel microscope in which a servo system for controlling the distance 
between the probe and the sample can always be set in a proper condition 
irrespective of the surface condition of the sample. 
A second object of this invention is to provide a scanning type tunnel 
microscope capable of setting the starting position of the scanning 
operation for a desired scanning range to a desired position after the 
wide range scanning operation is effected without using a rough moving 
mechanism necessary for movement of the probe in a vertical direction so 
as to always correctly set the desired position and maintain the 
reliability of an enlarged image. 
In order to attain the first object, a scanning tunneling microscope of 
this invention comprises: 
a piezoelectric driver capable of expanding and contracting to adjust a 
distance between a probe and a sample according to a voltage applied 
thereto, the distance including a distance at which a tunnel current can 
flow between the probe and the sample; 
a servo circuit for outputting a servo voltage for controlling expansion 
and contraction of the piezoelectric driver to keep the tunnel current at 
a constant value; 
correction voltage generating means for generating a given correction 
voltage to correct a voltage to be supplied to the piezoelectric driver; 
adding means for adding the servo voltage output from the servo circuit and 
the correction voltage supplied from the correction voltage generating 
means to each other to supply an added output to the piezoelectric driver; 
and 
control means for controlling the correction voltage supplied from the 
correction voltage generating means based on the servo voltage output from 
the servo circuit so as to keep the added output from the adding means at 
a given reference voltage. 
In order to attain the second object, the scanning tunneling microscope of 
this invention further includes a scanning circuit for scanning the probe 
along the sample, and the scanning circuit includes means for causing the 
probe to scan at least a first scanning range corresponding to a portion 
of the sample to be observed and a second scanning range including the 
first scanning range and setting the size of said first scanning range; 
and adding means for adding output data from the setting means and data 
relating to a reference position of the first scanning range, 
corresponding to a reference position of the second scanning range, to 
each other in a digital manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
There will now be described embodiments of this invention with reference to 
the accompanying drawings. 
FIG. 1 shows the construction of an STM (scanning tunneling microscope) 
according to this invention. 
In FIG. 1, 1 denotes a 8-bit CPU (central processing unit) controller for 
controlling the whole portion of the STM. The 8-bit CPU controller 1 is 
connected to an interface controller 2 and to a host computer 100 via an 
interface (GPIB). 
The 8-bit CPU controller 1 is connected to a Y-stage moving pulse motor 
driver (P.M.D) 3, an X-stage moving P.M.D 4, an STM scanning circuit 5, a 
Z-stage moving P.M.D 6, a 12-bit A/D converter 7 for subjecting a 
detection signal of tunnel current or Z electrode voltage signal to the 
A/D (analog/digital) conversion, a 10-bit D/A converter 8 for bias voltage 
application, and a 16-bit D/A converter 31 for Z voltage addition via 
respective 8-bit data buses. 
The 16-bit D/A converter 31 converts binary data supplied from the 8-bit 
CPU controller 1 via the 8-bit data bus into an analog signal (ZD/A 
output). 
The Y-stage moving P.M.D 3 drives a Y-stage moving P.M. 9 according to a 
driving signal (pulse data) from the 8-bit CPU controller 1 and moves the 
Y stage 10 in a Y direction (a direction perpendicular to the paper in the 
drawing). 
The X-stage moving P.M.D 4 drives an X-stage moving P.M. 11 according to a 
driving signal from the 8-bit CPU controller 1 and moves the X stage 12 in 
an X direction (a horizontal direction in the drawing). 
The Z-stage moving P.M.D 6 drives a Z-stage moving P.M. 13 according to a 
driving signal from the 8-bit CPU controller 1 and moves the Z stage 14 in 
a Z direction (a vertical direction in the drawing). 
A tube scanner (piezoelectric driver) 15 constituting an actuator which can 
be moved in a 3-dimensional direction is mounted on the bottom surface of 
the Z stage 14, and a tunnel probe 16 used as a metal probe having a sharp 
tip end is supported on the bottom surface of the tube scanner 15. The 
tunnel probe 16 is mounted to be supplied with a bias voltage (V) by means 
of the 10-bit D/A converter 8. 
On the other hand, a sample 17 is disposed on the top surface of the X 
stage 12 which faces the Z stage 14. A tunnel current (I) flows in the 
sample 17 when a preset bias voltage is applied thereto with the tunnel 
probe 16 set as close as approx. 1 nm to the surface of the sample. The 
tunnel current flowing in the sample 17 is supplied to a servo circuit 19, 
12-bit A/D converter 7 and approach detection/instantaneous contraction 
circuit 32 via a tunnel current amplifying pre-amplifier 18. 
The servo circuit 19 creates such a Z electrode voltage signal (Z servo 
voltage) for keeping the distance between the tunnel probe 16 and the 
sample 17 constant based on a detection signal of the tunnel current 
supplied via the pre-amplifier 18, and outputs the same to the 12-bit A/D 
converter 7 or Z voltage adder (addition circuit) 33. The servo circuit 19 
is constituted by a PI control circuit for creating a Z servo voltage (for 
example, -10 V to +10 V) and an analog switching circuit for selecting the 
destination to which the Z servo voltage is supplied. 
The adder 33 adds a ZD/A output from the 16-bit D/A converter 31 and a Z 
servo voltage output from the servo circuit 19 together and outputs the 
addition result to the approach detection/instantaneous contraction 
circuit 32. 
The approach detection/instantaneous contraction circuit 32 selects one of 
an addition output from the adder 33 and a maximum contraction voltage 
which is previously determined to cause the tube scanner 15 to be in a 
maximum contracted state and outputs the selected signal to a Z electrode 
applying high voltage amplifier (H.V.Z) 20. The approach 
detection/instantaneous contraction circuit 32 is constituted by an analog 
switching circuit 132 (FIG. 2) for selecting one signal and a flip-flop 
circuit 133 (FIG. 2) for instantaneously setting the analog switching 
circuit to the maximum contraction voltage position when it is detected 
that a tunnel current is produced from the pre-amplifier 18 when the 
tunnel probe 16 has approached the surface of the sample 17. 
The H.V.Z 20 amplifies an output of the approach detection/instantaneous 
contraction circuit 32 by ten times, for example, and applies the 
amplified output to the tube scanner 15. As a result, the tube scanner 15 
is expanded or contracted to change the distance between the tunnel probe 
16 and the sample 17. In this case, the length of the tube scanner 15 is 
set as a reference length when a signal (Vz) applied to the Z electrode is 
0 V, and the tube scanner 15 is contracted by 1 .mu.m by application of 
-100 V and expanded by 1 .mu.m by application of +100 V. 
As shown in FIG. 2, the 12-bit A/S converter 7 includes an A/D converter 7a 
for converting an input signal to digital (binary) data and outputting the 
converted data to the CPU controller 1 and a real time line memory circuit 
7b for storing the digital data converted by the A/D converter, and the 
digital data is displayed on a monitor TV 34 as a real time cross-section 
image. 
The STM scanning circuit 5 effects the count up/down operation according to 
a scanning starting signal supplied from the 8-bit CPU controller 1 via 
the 8-bit data bus to create a scanning signal in the X direction and a 
scanning signal in the Y direction. 
An X scanning 16-bit D/A converter 21 to which the scanning signal in the X 
direction is supplied creates an analog voltage signal (X application 
voltage) corresponding to an input and outputs the same to a -X electrode 
application high voltage amplifier (H.V.-X) 24 via a +X electrode 
application high voltage amplifier (H.V.+X) 22 and an inverter circuit 23. 
When the +X application voltage and -X application voltage are applied to 
the tube scanner 15 via the H.V.+X 22 and H.V.-X 24, respectively, the 
tube scanner 15 is deformed so as to cause the tip end of the tunnel probe 
16 to scan the surface of the sample 17 in the X direction. 
A Y scanning 16-bit D/A converter 25 to which the scanning signal in the Y 
direction is supplied creates an analog voltage signal (Y application 
voltage) corresponding to an input and outputs the same to a -Y electrode 
application high voltage amplifier (H.V.-Y) 28 via a +Y electrode 
application high voltage amplifier (H.V. +Y) 26 and an inverter circuit 
27. When the +Y application voltage and -Y application voltage are applied 
to the tube scanner 15 via the H.V.+Y 26 and H.V.-Y 28, respectively, the 
tube scanner 15 is deformed so as to cause the tip end of the tunnel probe 
16 to scan the surface of the sample 17 in the Y direction. 
FIG. 2 shows a scanning system in the Z direction which is taken out as a 
distance adjusting circuit in the first embodiment of this invention. 
In FIG. 2, 1a denotes a CPU serving as a control circuit constituting the 
8-bit CPU controller 1. 7a denotes an A/D converter, 7b denotes a real 
time line memory circuit, and the A/D converter 7a and real time line 
memory circuit 7b are combined to constitute the 12-bit A/D converter 7. 
Next, the automatic approaching method by use of the above construction is 
explained. 
First, a ZD/A output of the 16-bit D/A converter 31 is set to 0 V by the 
control operation of the CPU 1a. The analog switching circuit of the servo 
circuit 19 is turned off to prevent a Z servo voltage from being output to 
the adder 33. Further the approach detection/instantaneous contraction 
circuit 32 is set to select an addition output from the adder 33. As a 
result, a voltage (Vz) applied to the tube scanner 15 via the H.V.Z 20 is 
set to 0 V. Thus, as shown in FIG. 3A, the tube scanner 15 is set to the 
reference length. 
If the Z stage moving P.M.D 6 is controlled by the CPU 1a in this 
condition, the P.M. 13 is driven to move the Z stage in the downward 
direction. Then, the tube scanner 13 is lowered by the movement of the Z 
stage 14, thus setting the tunnel probe 16 close to the sample 17. In this 
case, as shown in FIG. 3B, the tunnel probe 16 is gradually moved 
according to the resolution (for example, 0.1 .mu.m) of the P.M. 13. 
Assume that a tunnel current caused by application of a bias voltage is 
detected at the approaching time. Then, the downward movement of the Z 
stage 14 by the P.M. 13 is stopped and the analog switching circuit in the 
approach detection/instantaneous contraction circuit 32 is switched to a 
position of maximum contraction voltage (-10 V). Therefore, a voltage 
(Vz=-100 V) which is amplified by 10 times by means of the H.V.Z 20 is 
applied to the tube scanner 15. As a result, as shown in FIG. 3C, the tube 
scanner 15 is instantaneously contracted. However, only the Z stage 14 is 
stopped in position where it is lowered by .DELTA. because of the 
resolution of the P.M. 13. 
Further, at this time, the analog switching circuit of the Z servo circuit 
19 is turned on to permit a Z servo voltage to be applied to the adder 33. 
Then, a ZD/A output of the 16-bit D/A converter 31 is set to -10 V by 
control of the CPU 1a. Therefore, the tube scanner 15 is applied with a 
voltage Vz (-10 V) which is obtained by amplifying an addition output of a 
ZD/A output (-10 V) of the 16-bit D/A converter 31 and a Z servo voltage 
(+9 V) from the servo circuit 19 by 10 times by means of the H.V.Z 20. As 
a result, as shown in FIG. 3D, the tube scanner 15 is expanded nearly to 
the reference length and thus set into the tunnel region. However, the 
length of the tube scanner 15 is shorter than the reference length by 0 to 
0.1 .mu.m because of the resolution of the P.M. 13. 
After this, the ZD/A output of the 16-bit D/A converter 31 is controlled by 
the CPU 1a while the servo voltage from the servo circuit 19 is being read 
by the CPU 1a via the A/D converter 7. Then, when the servo voltage is set 
to 0 (reference voltage), the ZD/A output of the 16-bit D/A converter 31 
is fixed. As a result, as shown in FIG. 3E, a state in which a voltage Vz 
(-10 V) obtained by amplifying an addition output of the ZD/A output (-1 
V) from the 16-bit D/A converter 31 and the Z servo voltage (0 V) from the 
servo circuit 19 by 10 times by means of the H.V.Z 20 is applied is 
maintained. 
When the Z servo voltage is 0 V, irregularity data with 0 set as its center 
can be derived by scanning if the input dynamic range of the A/D converter 
7a is set from -10 V to +10 V. In this case, an image with the central 
horizontal line set as a reference is displayed as a real time 
cross-section image on the monitor TV 34. 
As described above, in this invention, the tunnel probe 16 is set closer to 
the sample 17 by using a rough moving mechanism such as the P.M. 13 which 
effects the stepwise movement and the tube scanner 15 is instantaneously 
contracted immediately after it is set into the tunnel region and the 
rough movement is interrupted. At this time, application of the Z servo 
voltage is started to expand the tube scanner 15 to substantially the 
reference length, thus setting up the servo state. Next, deviation of the 
application voltage Vz from the reference voltage due to the resolution of 
the rough movement in the above state can be corrected (subjected to the Z 
servo voltage adjustment) by adjusting the ZD/A output. 
Further, according to this method, the STM operation can be started at a 
high speed in comparison with a case wherein the approaching operation is 
effected by repeatedly effecting the rough and fine movements. 
FIG. 4 shows a scanning system in the Z direction taken as distance 
adjusting means in a second embodiment of this invention. 
In FIG. 4, 7a denotes an A/D converter, 7b denotes a real time line memory 
circuit and 7c denotes an amplification switching circuit acting as a 
variable amplifier, and the A/D converter 7a, real time line memory 
circuit 7b and amplification switching circuit 7c are combined to 
constitute a 12-bit A/D converter 7. 
As shown in FIG. 5, for example, the amplification switching circuit 7c is 
constructed mainly by an analog multiplexer 71 for selecting various 
resistors and an inverting type amplifier 72 and amplifies a Z servo 
voltage supplied from the servo circuit 15 by 2.sup.n (n=0 to 4). 
Selection by the analog multiplexer 71 is effected according to a 
selection signal from the CPU 1a. 
Next, a method of amplifying and displaying a real time cross-section image 
in the above construction is explained. 
In a case where an irregular portion to be observed is previously 
determined, the tunnel probe 16 is first approached to the portion by the 
above method and then scanned by one line in the X and Y directions. A 
real time cross-section image obtained at this time is displayed on the 
monitor TV 34 and therefore whether the target irregular portion can be 
correctly approached or not can be determined by observing the real time 
cross-section image. 
When the real time cross-section image displayed on the monitor TV 34 shows 
that variation in the irregularity is small as shown in FIG. 6, the 
amplification factor of the amplification switching circuit 7c is changed 
by the CPU 1a. After this, a one-line scanning operation is effected 
again. As a result, as shown in FIG. 7, a real time cross-section image in 
which the target irregular portion is enlarged in the Z direction 
according to the amplification factor of the amplification switching 
circuit 7c is displayed on the monitor TV 34. That is, when the 
irregularity is small, for example, when variation in the Z servo voltage 
is small, the servo voltage is amplified with .+-.0 V set as the center of 
the variation and then supplied to the A/D converter 7a . Thus, it is 
possible to easily determine whether the target irregular portion is 
correctly approached or not. 
In a case where the amplification factor of the amplification switching 
circuit 7c is set to x16, irregularity data having substantially the sam 
resolution as data which is derived by an A/D converter having a 
resolution of 16 bits can be obtained. 
In this way, data of high resolution can be obtained from the sample even 
if the irregularity thereof is small by correcting the offset component to 
set the Z servo voltage to 0 V according to the ZD/A output without 
amplifying the offset component to cause a saturated state. 
Next, with the above construction, there is explained a case where the 
scanning range for a slanted sample 17 is narrowed and the scanning 
operation is effected again after the scanning center position is moved. 
For example, assume that, as shown in FIG. 8, the tunnel probe 16 which is 
set under the servo condition is moved in the right direction in the 
drawing by 3.75 .mu.m from the central position of of an area of 10 .mu.mo 
on the slanted sample 17 and then the operation of scanning an area of 2.5 
.mu.mu is effected again with the above set position used as a scanning 
center position. In this case, the scanning counter circuit 5 is 
controlled by the 8-bit CPU controller 1 so as to output scanning signals 
in the X and Y directions which respectively correspond to a deviation 
amount (3.75 .mu.m) from the center of the area of 10 .mu.m.quadrature. 
and the range (2.5 .mu.m) of rescanning. Then the tube scanner 15 is 
deformed by application of the voltages corresponding to the scanning 
signals so that the tunnel probe 16 can be moved while it is kept under 
the servo condition. 
At this time, since the sample 17 is slanted, the tube scanner 15 is 
contracted by .DELTA.Z as shown in FIGS. 9A and 9B. That is, a state in 
which the ZD/A output of the 16-bit D/A converter 31 is set at -1 V and 
the servo voltage of the servo circuit 19 is set at 0 V and as a result 
the voltage Vz (-10 V) amplified by 10 times by the H.V.Z 20 is applied to 
the tube scanner 15 (refer to FIG. 9A) is changed into a state in which 
the Z servo voltage is set at -4 V and as a result the voltage Vz applied 
to the tube scanner 15 is set to -50 V (refer to FIG. 9B). 
In this state, as described in the automatic approaching method, the ZD/A 
output of the 16-bit D/A converter 31 is changed in a negative direction 
to change the Z servo voltage towards 0 V. Then, when the Z servo voltage 
is set at 0 , the ZD/A output of the 16-bit D/A converter 31 is fixed. 
That is, as shown in FIG. 9C, the Z servo voltage is changed from -4 V to 
0 V and the ZD/A output is changed from -1 V to -5 V, but the same voltage 
Vz -50 V is applied to the tube scanner 15. 
As a result, even if the irregularity is small as shown by a in FIG. 10, 
data can be taken by the A/D converter 7a after the servo voltage is 
amplified with .+-.0 V set as the center of the variation. Thus, as shown 
by b in FIG. 10, data (real time cross-section image) of high resolution 
can be obtained for the range of 2.5 .mu.m.quadrature.. 
FIG. 11 shows a scanning system in the Z direction as a distance adjusting 
circuit in a third embodiment of this invention. In this embodiment, the 
STM is used as an STS for measuring the physical property of the surface 
(electron state density of the sample surface) of the sample 17 by 
modulating the distance between the tunnel probe 16 and the sample 17 and 
deriving a tunnel current. 
That is, in the case of this distance adjusting circuit, the 8-bit CPU 
controller 1 includes a Z modulation pattern data generator lb as a 
modulation pattern generating circuit in addition to the CPU la used as a 
control circuit. Further, the 12-bit A/D converter 7 includes an A/D 
converter 7a , real time line memory circuit 7b, amplification switching 
circuit 7c and signal selection circuit 7d. 
The Z modulation pattern data generator 1b includes an up/down counter 
circuit having a counting range of, for example, "0000.sub.H " to 
"000F.sub.H ", "001F.sub.H " and "003F.sub.H " and changes the up/down 
counting range according to the modulation range of the tunnel probe 16. 
Output binary data of the counter circuit is supplied to the 16-bit D/A 
converter 31 to finely modulate the ZD/A output of the 16-bit D/A 
converter 31. The start and interruption of the operation and change of 
the counting range in the Z modulation pattern data generator 1b can be 
controlled by signals from the CPU 1a. 
The signal selection circuit 7d selects one of the Z servo signal 
(irregularity data) and a detection signal (converted into voltage) of 
tunnel current according to a signal from the CPU 1a and outputs the same 
to the amplification switching circuit 7c. 
Further, a switching circuit only for PI control/I control and an I control 
gain switching circuit are additionally provided in the servo circuit 19. 
Next, a method for measuring the electron state density of the sample 
surface with the above construction is explained 
First, the servo circuit 19 is switched to the I control circuit under the 
control of the CPU 1a and the (integration control) gain of the I control 
circuit is set to sufficiently follow the irregularity of the surface of 
the sample 17. 
Then, the Z modulation pattern data generator 1b is controlled to be 
operated at a frequency relatively higher than roughness at which the 
image varies, that is, the space frequency to be followed by the I 
control, thus making it possible to modulate the ZD/A output of the 16-bit 
D/A converter 31 at a relatively high speed in comparison with the time 
constant of the servo circuit 19. 
After this, the Z servo voltage from the servo circuit 19 and the ZD/A 
output from the 16-bit D/A converter 31 are added together by an adder 33. 
Then, the added output of the adder 33 is amplified by the H.V.Z 20 and 
applied to the tube scanner 15. 
In this state, the X-Y scanning operation is effected. Then, the tip end of 
the tunnel probe 16 is moved on the surface of the sample 17 along a locus 
shown in FIG. 12. 
At this time, a signal to be taken by the A/D converter 7a is switched from 
the Z servo voltage to the detection signal of tunnel current by 
controlling the signal selection circuit 7d. The electron state density of 
the sample surface can be measured by sampling signal data at points 
indicated in FIG. 13. 
The tip end modulation sequence is well known in the art as a method for 
measuring the electron state density of the sample surface and its 
operation can be adjusted programmably by use of the distance adjusting 
circuit of this invention. 
Thus, it can be used as an STS by deriving a detection signal of tunnel 
current in accordance with modulation of the ZD/A output of the 16-bit D/A 
converter 31. 
As described above, a servo control suitable for the surface condition of 
the sample can be effected by operating the distance adjusting circuit in 
accordance with the surface condition obtained by the tunnel current. 
That is, deviation of an application voltage from the reference voltage in 
the Z direction caused by the resolution of the rough moving mechanism 
when the tunnel probe is approached to the surface of the sample can be 
corrected by applying a voltage which causes the servo voltage to be set 
to 0 V. As a result, the operation for the approach of the tunnel probe 
can be simplified and the distance between the tunnel probe and the sample 
can be servo-controlled in accordance with the surface condition of the 
sample. Therefore, since irregularity data with 0 V set as its center can 
be derived in the scanning operation, reduction in the resolution of the 
real time cross-section image and deviation of the dynamic range from its 
center can be prevented. 
Further, since irregularity data with 0 V set as its center can be derived, 
variation in the small irregularity of the sample surface can be enlarged 
and displayed by amplifying the irregularity data. 
In addition, since the distance between the tunnel probe and the sample can 
be servo-controlled in accordance with the surface condition of the 
sample, it can be used as an STS for measuring the electron state density 
of the sample surface by modulating the distance between the tunnel probe 
and the sample 
In the above embodiment, the probe is mounted so as to be supported by the 
piezoelectric driver, but this is not limitative and it is possible to 
support the sample. 
Next, a fourth embodiment of this invention which is improved over the STM 
scanning circuit 5 is explained with reference to FIG. 14. 
In FIG. 14, portions which are the same as those of FIG. 1 are denoted by 
the same reference numerals. Therefore, the explanation therefor is 
omitted, but in this embodiment, a Z electrode voltage signal (Z servo 
voltage) from a servo circuit 19 is directly supplied to a 12-bit A/D 
converter 7 and a Z electrode applying high voltage amplifier (H.V.Z.) 20. 
The Z servo voltage is applied to the tube scanner 15 via the H.V.Z. 20 to 
expand and contract the tube scanner 15 so as to keep the distance between 
the probe 16 and the surface of a sample 17 constant. 
In this embodiment, the STM scanning circuit 5 is constituted mainly by a 
bit shift circuit and a digital adder. That is, the STM scanning circuit 5 
includes a scanning counter circuit (scanning 16-bit output counter) 51 
for effecting the counting up/down operation in response to a scanning 
signal supplied from an 8-bit CPU controller 1 via a CPU data bus, a bit 
shift circuit (scanning range switching bit shifter) 52 for shifting an 
output of the scanning counter circuit 51 to the LSB (Least Significant 
Bit) side by a desired bit number according to a shift number signal 
(range setting data) supplied from the 8-bit CPU controller 1, latch 
circuits (scanning offset 16-bit data latch circuits) 53 and 54 for 
latching scanning offset values (reference position data) supplied from 
the 8-bit CPU controller 1, a digital adder 55 for creating a scanning 
signal in the X direction, for example, by adding an output of the latch 
circuit 53 and an output of the bit shift circuit 52 together, and a 
digital adder 56 for creating a scanning signal in the Y direction, for 
example, by adding an output of the latch circuit 54 and an output of the 
bit shift circuit 52 together. 
An output of the digital adder 55 is supplied to an X-scanning 16-bit D/A 
converter 21. The X-scanning 16-bit D/A converter 21 creates an analog 
voltage output (X application voltage) corresponding to an input and 
outputs the same to a -X electrode applying high voltage amplifier 
(H.V.-X) 24 via a +X electrode applying high voltage amplifier (H.V.+X) 22 
and an inverter circuit 23. When the +X application voltage and -X 
application voltage are applied to the tube scanner 15 via the H.V.+X 22 
and H.V.-X 24, respectively, the tube scanner 15 is deformed by 
application of the voltages so that the tip end of the probe 16 can scan a 
desired scanning area on the surface of the sample 17 in the X direction 
according to the scanning starting position corresponding to the reference 
position data 
An output of the digital adder 56 is supplied to a Y-scanning 16-bit D/A 
converter 25. The Y-scanning 16-bit D/A converter 25 creates an analog 
voltage output (Y application voltage) corresponding to an input and 
outputs the same to a -Y electrode applying high voltage amplifier 
(H.V.-Y) 28 via a +Y electrode applying high voltage amplifier (H.V.+Y) 26 
and an inverter circuit 27. When the +Y application voltage and -Y 
application voltage are applied to the tube scanner 15 via the H.V.+Y 26 
and H.V.-Y 28, respectively, the tube scanner 15 is deformed by 
application of the voltages so that the tip end of the probe 16 can scan a 
desired scanning area on the surface of the sample 17 in the Y direction 
according to the scanning starting position corresponding to the reference 
position data 
In this way, in this invention, the scanning range and the scanning 
starting position thereof can be changed without separating the probe 16 
from the sample 17 or using a digital addition output of a rough moving 
mechanism by using an output of the bit shifter to which range setting 
data is input and the reference position data as a scanning signal. 
FIG. 15 shows the construction of the STM scanning circuit 5 in more 
detail. For convenience, only a scanning system in the X direction is 
shown and explained in this example. 
An X scanning counter circuit 51a repeatedly effects the count up/down 
operation between "0000.sub.H " and "FFFF.sub.H " according to the 
scanning starting signal from the 8-bit CPU controller 1 by a number of 
times equal to the number of scanning lines and stops the counting 
operation when "0000.sub.H " is reached. 
An X scanning count bit shift circuit 52a shifts a 16-bit count value 
supplied from the X scanning counter circuit 51a to the LSB side by a bit 
number corresponding to a shift number signal from the 8-bit CPU 
controller 1 and outputs data (output count value) having "0" set into a 
vacant bit or bits of the MSB side to the X count value digital adder 55. 
That is, in the X scanning count bit shift circuit 52a, in a case where a 
count value from the X scanning counter circuit 51a is "FFFF.sub.H ", for 
example, "7FFF.sub.H " is output as an output count value when a 1-bit 
shift is set and "3FFF H " is output as an output count value when a 2-bit 
shift is set. Thus, the output count value from the X scanning count bit 
shift circuit 52a is set to 1/2, 1/4, 1/8, --of a count value supplied 
from the X scanning counter circuit 51a by changing the setting of the bit 
shift number. Therefore, when the 1-bit shift is set, the X scanning count 
value becomes equal to a value obtained by repeatedly effecting the 
counting operation between "7FFF.sub.H " and "0FFF.sub.H " by a number of 
times corresponding to a scanning line number in the output of the digital 
adder 55. 
The X count value digital adder 55 is constituted by an upper digit side 
digital adder 55a and a lower digit side digital adder 55b, and divides an 
X scanning offset value set into a scanning offset 16-bit data latch 
circuit 53 by the 8-bit CPU controller 1 and an output count value 
supplied from the X scanning count bit shift circuit 52a into the upper 
digit portion and lower digit portion and add them together in respective 
digit sides. That is, in the upper digit side digital adder 55a, the upper 
8 bits of the offset value supplied from an upper digit side latch circuit 
53a of the latch circuit 53 and the upper 8 bits of an output count value 
supplied from the X scanning count bit shift circuit 52a are added 
together. Likewise, in the lower digit side digital adder 55b, the lower 8 
bits of the offset value supplied from a lower digit side latch circuit 
53b of the latch circuit 53 and the lower 8 bits of an output count value 
supplied from the X scanning count bit shift circuit 52a are added 
together. Therefore, in a case where the bit shift number of the X 
scanning count bit shift circuit 52a is set at "1", an X scanning count 
which is an output of the digital adder 55 becomes equal to a value 
obtained by repeatedly effecting the counting operation between 
"4000.sub.H " and "BFFF.sub.H " by a number of times corresponding to a 
scanning line number when "4000.sub.H " is latched in the latch circuit 53 
as an offset value, for example. 
The X scanning 16-bit D/A converter 21 to which an output of the digital 
adder 55 is supplied outputs an X application voltage of "0" V when it is 
supplied with "0000 H " as an input, for example, and outputs an X 
application voltage of "10" V when it is supplied with "FFFF.sub.H ". 
The construction described above is for a system for moving the probe 16 in 
the X direction, but a scanning system in the Y direction for moving the 
probe in the Y direction has substantially the same construction. However, 
in the case of the Y-direction scanning system, a Y scanning counter 
circuit effects the counting up/down operation between "0000.sub.H " and 
"FFFF.sub.H " only once according to the scanning starting signal from the 
8-bit CPU controller 1. That is, a Y scanning clock at the counting-up 
time has a frequency obtained by dividing the frequency of an X scanning 
clock (scanning line number .times.2) and is changed to have the same 
frequency as the X scanning clock when all the lines are scanned in the X 
direction. Then, a counting-down operation is started and stopped when 
"0000.sub.H " is reached. 
The scanning count bit shift circuit converts an output count value to 
1/2.sup.n times by shifting a 16-bit count value supplied from the Y 
scanning counter circuit by n bits. In this case, since the scanning range 
is square, the value of n (bit shift number) is set to the same value for 
the X and Y scanning. 
Next, the operation with the above construction is explained. In this case, 
an STM having the maximum scanning range of 10 .mu.m .times.10 .mu.m is 
used. First, a wide scanning operation for an area of 10 .mu.m.quadrature. 
is effected and a scanning operation for a scanning area of 2.5 
.mu.m.quadrature. with a desired point set as its reference after an STM 
image thereof is observed. 
For example, assume now that a scanning starting signal indicating the wide 
range scanning for an area of 10 .mu.m.quadrature. is supplied from the 
8-bit CPU controller 1 to the STM scanning circuit 5 via the CPU data bus 
while the probe 16 is servo-controlled on the scanning starting position 
(reference position) on the sample 17. In this case, suppose that "0" is 
previously set in the bit shift circuit 52 as a bit shift number according 
to a shift number signal from the 8-bit CPU controller 1, and "0000.sub.H 
" is set in the latch circuits 53 and 54 as X- and Y-scanning offset 
values, respectively. 
Then, an X scanning count value obtained by repeatedly effecting the 
counting up/down operation between "0000.sub.H " and "FFFF.sub.H " by a 
number of times corresponding to a scanning line number, for example, 512 
time, according to the scanning starting signal is output from the STM 
scanning circuit 5 to the X-scanning 16-bit D/A converter 21 and a Y 
scanning count value obtained by effecting the counting up/down operation 
between "0000.sub.H " and "FFFF.sub.H " by one time is output to the 
Y-scanning 16-bit D/A converter 25. 
Therefore, an X application voltage (0 to 10 V) corresponding to an X 
scanning count value supplied from the STM scanning circuit 5 is output 
from the X-scanning 16-bit D/A converter 21 and is applied to the tube 
scanner 15 as an X-direction scanning signal. Likewise, a Y application 
voltage (0 to 10 V) corresponding to a Y scanning count value supplied 
from the STM scanning circuit 5 is output from the Y-scanning 16-bit D/A 
converter 25 and is applied to the tube scanner 15 as a Y-direction 
scanning signal. 
As a result, the tube scanner 15 is deformed to move the tip end thereof in 
a range of 0 to 10 .mu.m in the X direction and is moved in the Y 
direction in a range of 0 to 10 .mu.m in each movement in the X direction. 
Then, as shown in FIG. 16, the probe 16 scans an area A of 10 
.mu.m.quadrature. on the sample 17. 
Data obtained in the scanning operation is sampled at 512 points which is 
the same in number as the scanning lines, for example, and an STM image 
thus obtained is displayed on the top-view display by means of a host 
computer 100, thus making it possible to observe the contour or surface 
state of the sample 17 in the wide scanning area of 10 .mu.m.quadrature.. 
In this case, the STM image is displayed in a 2-dimensional (X, Y) manner 
and the Z direction is displayed by variation in brightness (luminance). 
Assume that, when the STM image is observed, it is desired to enlarge and 
observe an image in an area of 2.5 .mu.m.quadrature. having a position 
(desired point) which is set as a reference point and is separated from 
the reference position of a wide scanning area of 10 .mu.m.quadrature. by 
4000.sub.H (2.5 .mu.m) in the X direction and by 8000.sub.H (5 .mu.m) in 
the Y direction. In this case, a scanning starting signal defining that 
the scanning range is 2.5 .mu.m.times.2.5 .mu.m is output from the 8-bit 
CPU controller 1 to the STM scanning circuit 5. That is, the bit shift 
number of the bit shift circuit 52 is set to "2" by a shift number signal 
from the 8-bit CPU controller 1. At this time, "4000.sub.H " is set into 
the latch circuit 53 as the X-scanning offset value and "8000.sub.H " is 
set into the latch circuit 54 as the Y-scanning offset value. 
Then, in the STM circuit 5, the counting up/down operation between 
"0000.sub.H " and "FFFF.sub.H " is repeatedly effected by a number of 
times corresponding to the scanning line number, for example, 512 times by 
the scanning counter circuit 51 (more precisely, the X scanning counter 
circuit 51a) according to the scanning starting signal and the counting 
result or X scanning count value is output to the bit shift circuit 52 
(more precisely, X scanning count bit shift circuit 52a ). 
The X scanning count value is bit-shifted by the bit shift circuit 52 
according to the bit shift number "2" set by the shift number signal and 
then output to the digital adder 55. 
An output from the bit shift circuit 52 or an output count value in the X 
direction is added together with X-scanning offset value "4000.sub.H " set 
in the latch circuit 53 and then output to the X scanning 16-bit D/A 
converter 21. 
On the other hand, a Y scanning count value obtained by effecting a count 
up/down operation between "0000.sub.H " to "FFFF.sub.H " by one time 
according to the above scanning starting signal is output from the 
scanning counter circuit 51 (more precisely, a Y scanning counter circuit 
not shown in the drawing) to the bit shift circuit 52 (more precisely, a Y 
scanning count bit shift circuit shot shown in the drawing). 
Then, in the bit shift circuit 52, the Y scanning count value is 
bit-shifted and is output to the digital adder 56. 
An output count value in the Y direction is added together with Y direction 
offset value "8000.sub.H " set in the latch circuit 54 by the digital 
adder 56 and then output to the Y-direction 16-bit D/A converter 25. 
Therefore, an X application voltage (2.5 to 5.0 V) corresponding to an X 
scanning count value supplied from the digital adder 55 is output from the 
X-scanning 16-bit D/A converter 21 and is applied to the tube scanner 15 
as an X-direction scanning signal. Likewise, a Y application voltage (5.0 
to 7.5 V) corresponding to a Y scanning count value supplied from the 
digital adder 56 is output from the Y-scanning 16-bit D/A converter 25 and 
is applied to the tube scanner 15 as a Y-direction scanning signal. 
As a result, the tube scanner 15 is deformed to move the tip end thereof in 
a range of 2.5 .mu.m to 5.0 .mu.m in the X direction and is moved in a 
range of 5.0 .mu.m to 7.5 .mu.m in the Y direction for each movement in 
the X-direction. Thus, as shown in FIG. 16, the range (hatched portion) B 
of 2.5 .mu.mo of the sample 17 is scanned by the probe 16. 
In this case, an STM image of partly high resolution can be obtained by 
sampling the scanning data at points of the same number as the scanning 
line number or 512 points, for example, that is, at 512 points which is 
the same sampling number as that used in the case of scanning the wide 
range of 10 .mu.m.quadrature.. 
As described above, the probe can be moved without being separated from the 
sample and part of the STM image obtained in the wide range scanning 
operation can be enlarged and displayed by re-scanning an area 
corresponding to the part of the STM image. 
That is, a shift number signal for setting the size of a desired scanning 
range after the wide range scanning operation is completed is input to the 
bit shifter and an addition output obtained by adding an output of the bit 
shifter and offset values of the X and Y directions of the desired 
scanning range with respect to the reference position of the wide scanning 
range in a digital manner is used as a scanning signal so that the 
scanning range and the scanning starting position can be changed without 
separating the probe from the sample and using a rough moving mechanism. 
Therefore, part of the STM image obtained in the wide range scanning 
operation can be enlarged and displayed by re-scanning an area 
corresponding to the part of the STM image without causing scars on the 
sample by the probe at the approaching time and receiving an influence of 
an error caused by the precision of the rough moving mechanism. As a 
result, an STM image obtained may have a higher resolution in comparison 
with an image which is enlarged and displayed by the image processing, 
making it possible to precisely reproduce variation in the actual shape 
and reduce the load and time for effecting the re-scanning operation. 
Some embodiments of this invention are explained above, but this invention 
is not limited to these embodiments and can be variously modified without 
changing the technical scope of this invention. 
As described above, according to this invention, a scanning type tunnel 
microscope can be provided in which, since the distance adjusting means is 
operated according to the surface condition obtained by the tunnel 
current, a servo system for controlling the distance between the probe and 
the sample can always be properly operated irrespective of the surface 
condition of the sample. 
Further, according to this invention, a scanning type tunnel microscope can 
be provided in which, since a digital addition output of an output of the 
bit shifter and reference position data is used as a scanning signal, a 
scanning starting position for a desired scanning range in a desired 
position set after the wide range scanning operation is completed can be 
set without using a rough moving mechanism, thereby always correctly 
setting the position and maintaining the reliability of an enlarged image. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and representative devices shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.