Scanning probe and an approach mechanism therefor

A method and apparatus for tunnelling microscopy utilising a tunnelling microscope head (11) having a tip (15) which is moveable by a driver (25) towards a sample (28) and is stopped in its advance as an electron flow between the tip and the sample is detected . The tip (15) is advanced towards the sample via a piezoelectric member (24) at a one voltage level, and on detection of the electron flow the voltage level of the piezoelectric member (24) is changed causing the piezoelectric member (24) to retract, stopping the advance of the tip (15) and stopping the driver (25) within the retraction distance.

FIELD OF INVENTION 
This invention relates to scanning probes and in particular to tunnelling 
microscopes and more particularly but not exclusively to scanning 
tunnelling microscopes. 
BACKGROUND OF INVENTION 
The tunnel effect is a sub atomic phenomenon in which particles, usually 
electrons, can pass from one material to another when the surfaces of the 
two materials are extremely close (in atomic terms). This effect is used 
for measuring distances between atoms 
A Scanning Probe is an instrument that produces magnified images of a 
surface by monitoring interactions between a sharp probe and the surface 
of the material to be examined that is the sample. Examples of probe 
microscopes include the Scanning Tunnelling Microscope (STM), Atomic Force 
Microscope (AFM) and the Lateral Force Microscope (LFM). Each of these 
probe techniques requires that the probe is brought into close proximity 
with the sample. Typical separations between the tip and sample are of the 
order of the dimensions of an atom for example 5 Angstroms (5.times.10-10 
m) . At these separations interactions between the probe and sample take 
place. By including these interactions into a feedback loop as the 
controlling factor a microscopic image of the surface can be obtained on 
scanning the probe relative to the sample by, for example, means of a 
piezoelectric tube scanner or orthogonally mounted piezo device. Such 
images include in the case of the STM a local density of States for 
conducting or semi-conducting materials, for the ATM, a contour map of the 
physical surface. For high resolution, high quality images the tip to 
sample separation must be kept as vibration free as possible. 
Before scanning with the probe in close proximity to the sample, the probe 
must first brought into the scanning position from an initial tip 
separation in the order of 3-5 mm (3-5.times.10-3 m) without contacting 
the sample and destroying the probe tip. An approach mechanism capable of 
moving the probe over a range of several millimeters with the capability 
of stopping within an atomic distance of the surface is therefor required. 
A number of ways of tackling this problem are currently used. The most 
common methods being those that use the principle of levers to reduce the 
rate of approach of the tip to the sample with respect to the approach 
drive mechanism. Another technique employs the special characteristics of 
piezoelectric materials to generate a crawling action to reduce the 
distance between tip and sample in which the separation is reduced by 
small increments before stopping when interaction with the sample is 
detected. Both of these techniques r and others, have the disadvantage 
that in practice the tip to sample control loop is in the order of 10 to 
20 cms. 
Further, since the tip is used for examining and measuring distances in the 
order of m.times.10-10 (Angstroms) and the mechanical loop between the 
microscope tip and the sample is in the order of 10 to 20 cms 
(m.times.10-2) then it will be seen that during the operation of for 
example, a tunnelling microscope it is necessary to keep vibration from 
any source to an absolute minimum. 
The present invention seeks to provide an approach mechanism suitable for a 
tunnelling microscope that overcomes the above problems. 
STATEMENTS OF INVENTION 
Accordingly there is provided a tip approach mechanism for a probe 
microscope having a tip which is moveable towards a sample, said tip being 
advanceable towards the surface via a piezoelectric member attached to a 
drive means and which retracts away from the tip when the tip reaches its 
operating position, the drive means stopping within the retraction 
distance. 
Preferably the microscope is a scanning tunnelling microscope wherein the 
mechanism is mounted in a body and the tip is mounted on a piezoelectric 
scanning tube which is in turn secured to the body . The tip may be held 
on the scanning tube by frictional engagement and may be connected to the 
piezoelectric member through a lost motion connection. 
Also according to the invention there is provided a probe microscope which 
includes a tip approach mechanism according to the present invention, and 
wherein the microscope has a voltage source for the piezoelectric member, 
said voltage source is controllably linked to a tip operation detect means 
so that the voltage source to the piezoelectric member is changed as 
electron flow between the tip and the sample is detected, causing the 
piezoelectric member to retract. 
When the piezoelectric member is attached to a screw threaded member and is 
advanced by rotation of the screw threaded member by a motor, the 
microscope may include a control means which switches off the motor on 
detection of the electron flow. 
Also according to the invention there is provided a method of tunnelling 
microscopy utilising a tunnelling microscope having a tip which is 
moveable by a drive means towards a sample to be examined and is stopped 
in its advance as an electron flow between the tip and the sample is 
detected, characterised in that the tip is advanced towards the sample 
through a piezoelectric member at a first voltage level, and on detection 
of said flow the voltage level of the piezoelectric member is changed to a 
second voltage level causing the member to retract, stopping the advance 
of the tip and stopping the drive means within the retraction distance.

DETAILED DESCRIPTION OF INVENTION 
With reference to FIGS. 1 and 2 of the drawings there is shown a scanning 
tunnelling microscope head (11) which in use may be secured to the flange 
(10) of a tube attached to an ultra high vacuum chamber. The head 
comprises a body (12) which is formed from a suitable metal e.g. stainless 
steel, bronze, tungsten steel. 
The body may be square, or round, and has a coaxial center bore (13) closed 
at one end and which is open at its underside to receive a sample holder 
(27). 
A tip (15) is mounted in a ti p assembly (16) which is fixed to the lower 
end of a cylindrical piezoelectric scanning tube (17). The tip assembly 
(16) comprises a tin mount (21) onto which the tip (15) is fixed by any 
suitable means e.g. adhesive, clamping , screw means etc. . . . The tip 
mount (21) is held in a plate (22) by a spring clip (23). Thus the tip 
mount is held in the assembly (16) by frictional engagement with the plate 
(22) such that the tip (15) can be moved relative to the plate (22) by the 
application of a load which overcomes the frictional engagement. The tip 
can be formed from any suitable electrically conductive metal such as 
tungsten, or platinum/iridium alloy for operation in vacuum, or gold or 
platinum/iridium alloy for operation in air. 
The scanning tube (17) may be made from a Standard Navy type piezoelectric 
material such as PZT 5A available from Morgan Matrix Ltd. The upper end of 
the scanning tube (17) is fixed to a shoulder (18) on the end face of the 
bore (13) by any suitable means such as adhesive. 
A piezoelectric element (24) which may be of the same material as the 
scanning tube (17) is fixed to one end of a screw threaded strut (25) 
arranged coaxially in the bore (13) and which passes through said closed 
end in alignment with the tip (15). The screw threaded strut (25) and 
element (24) pass through the center of the scanning tube (17) for 
abutment of the piezo element (24) against the rear end of the tip mount 
(21). The screw threaded strut (25) is rotatable by a motor (26) to 
advance or retract the strut (25) towards or away from the tip (15). 
The motor (26) may be an electric or hydraulic motor preferably a 10 volt 
DC electric motor, turning at two revolutions per minute and the pitch of 
the screw thread of the strut is 0.5 mm. As the motor (26) advances the 
screw threaded strut (25) with the voltage of the piezo electric element 
(24) set such that the element is at its fullest extension, the 
piezoelectric element (24) abuts the tip mount (21) and overcomes the 
friction between the tip mount (21) and the holder (22) and pushes the tip 
(15) downwards towards a sample (28). 
On detection of a tunnelling current the voltage to the piezoelectric 
element (24) is altered so as to reduce the length of the element by a 
distance `a` in FIG. 2. Simultaneously the electric motor is switched off, 
thus halting the advance of the tip (15) within a distance `b`, that is 
less than distance `a`. 
The tip (15) is now held within the tunnelling regime of between 5-20 
Angstrom is from the sample and yet not in contact with driven approach 
mechanism. The tip to sample mechanical loop is reduced to virtually twice 
the length of the scanning tube (17) i.e. about 2 cms and contains 
effectively no moving parts. 
A sample holder (27) and sample (28) are fixed to the underside of the body 
(12) by means of a pair of opposed electrically non-conductive slides (not 
shown) one on each side of the holder. The sample holder (27) is separated 
from the body (12) by means of an electrically insulating gasket (not 
shown) formed from ceramic material. 
In a scanning electron microscope the tip (15) and the sample (28) will be 
electrically connected to a detect means (31) which can detect electron 
flow between the tip (15) and a sample on the sample holder (27). The 
detect means (31) is connected to a microprocessor control means (32) 
which is in turn connected to the motor (26), the scanning tube 17, and 
the piezoelectric element (24). 
In a typical operation the head (11) is mounted in a tube (10) of a vacuum 
chamber with the sample holder (27) placed into position. 
The motor (26) will then operate to cause the tip (15) to advance slowly 
towards the sample. The piezoelectric element (24) will be held at a 
voltage of say +200 volts. As the tip (15) nears the sample to within a 
distance of 3-5..times.10-10 meters (3-5 Angstroms) the detect means (31) 
detects the flow of electrons. The control means (32) on receipt of a 
signal from the detect means (31) firstly changes the voltage to the 
piezoelectric element (24) from +200 volts to -200 volts and 
simultaneously switches off the motor (26). 
The change in voltage to the piezoelectric element should cause it to 
retract away from the back of the tip mount (16) by a distance `a` of 
between 4-20 microns and typically 15 microns, and the motor (26) should 
switch off within 1/10th of a second which gives a stopping distance `b` 
of 0.0016 mm that is 1.6 microns. 
It can be seen that the retraction of the piezoelectric element 24 is 
greater than the stopping distance required by the motor. Since the 
shortening or retraction of the piezoelectric element (24) is virtually 
instantaneous the tip (15) stops advancing instantly and the motor is 
switched off before the gap opened up between the element (22) and the tip 
(15) is closed. 
Since the sample holder (27) is fixed to the head (11) there is almost no 
mechanical loop between the tip (15) and the sample. This is a great 
improvement on the prior art and makes the use of the microscope less 
susceptible to vibrations. 
The piezoelectric element 24 is attached to the tip mount (16) via a lost 
motion connection 30 in the form of hooks which permit withdrawal of the 
tip (15) when the motor (26) is put into reverse. 
The microscope can be made to scan by the application of voltage to the 
piezoelectric scanning tube (17) through the control (32). The friction 
grip between the spring (23) and the tip mount (21) may result in the tip 
mount being held temporarily stationary and being pushed forward in large 
step movements as the friction load is overcome. In order to ameliorate 
this effect the control means (32) applies an oscillating voltage to the 
scanning tube (17) during the tip approach phase. This causes the tip 
mount (21) to tap against the end of the advancing screw strut (25) which 
nudges the tip mount forward in smaller more controlled increments. 
A tunnelling microscope head made according to the invention may be used in 
vacuum, air, or liquid as may be required.