STM Technical Aspects

4. Technical Aspects

Demands

Controlling the tip-sample distance from a few mm down to 0.01Å
Exact lateral positioning
Stabilized tip-sample distance
Sharp tip
Measuring a current in the range of 0.01nA-50nA

4.1. Positioning

The large distance range the tip has to be controlled on makes it necessary to use two positioners: a coarse and a fine positioner. The fine positioner is also used as a scanner. Every fine positioner/scanner is made out of a piezocrystal or piezoceramic material.

Piezoelectricity

Piezoelectric Effect: Applying stress on piezo materials causes electric polarisation
Reverse Piezoelectric Effect: An electric field applied to a piezo material causes mechanical strain
Piezocrystals: quartz, BaTiO₃ Piezoceramics: Pb(Ti,Zr)O₃ (PZT)

In the following the directions x,y and z are labeled as 1,2 and 3, respectively. Applying a voltage V to a rectangular piece (thickness z) of piezoelectric material as shown above yields an electric field of
formula

The strain is generated, which can be described by a tensor with xx- and zz-components
formula

The piezoelectric constants are defined as the ratios of the strain component over the component of the electric field intensity. Thus we get
formula

and

.
Typical values for the piezoelectric components of PZT are
d₃₁=-1 to -3 Å/V
d₃₃=2 to 6 Å/V
Example:

EBL#1(Stavely Sensors)
d₃₁=-1.27Å/V (tranversal)	d₃₃=2.95Å/V (longitudial)

Fine Positioner (Scanner)

Tripod

The tip is controlled by three orthogonal piezo bars

Bimorph

A Bimorph is made out of two thin plates of piezocrystals which are stacked in such a way that a voltage applied on the plates causes stress of opposite sign which generates a deflection (principle is similar to the bimetallic thermometer).
By combining several bimorphs a three-dimensional scanner can be constructed.

Tube Scanner

A cylinder, made out of piezoceramics, is covered inside and outside with metal, which acts as electrodes. The outer electrode is divided into four sectors. A motion in z-direction (along the longitudinal axis) can be achieved by applying a voltage between the inner and the outer electrode. A deflection in xy-direction is generated by a different bias voltage applied to the two opposite electrodes.

Coarse Positioner

Mechanical Positioner

The tip approaches the sample with the help of springs, demagnifier and microscrews.

Louse

The sample is mounted on a piezoelectric plate with three electrically isolated feet, which electrostatically can be attached to a ground plate. Thus by expanding and contracting the piezoelctric plate and clamping and detaching the feet the louse can move step by step towards the tip.

Beetle

The Beetle consists of a plate with three piezoelectric tubes, standing on a cylindric ramp. The sample is mounted in the middle of the ramp whereas the tip with the fine positioner is mounted on the plate. By applying a voltage on the piezoelectric feet, the plate is rotated. Reducing the voltage quickly causes the feet to slip on the ramp due to the inertia of the plate. Thus the tip can be moved towards the sample or away (stick/slip motion).

4.2. Electronic Circuit

In case of measuring in the constant current mode a feedback circuit has to be build up to control the z-piezo.

The tunneling current (0.01mA-50mA) is converted into a voltage by a current amplifer. To get a linear response with respect to the tunneling gap (the current is exponentially dependant on the tip-sample distanance) the signal is processed by a logarithmic amplifier. The output of the logarthmic amplifier is compared with a predetermined voltage which is used as a reference current. The error signal is passed to feedback electronics, which applies a voltage to the z piezo to keep the difference between the current set point and the tunneling current small.
Care has to be taken to keep the noise signal ratio on a low level. Also the response time of the feedback has to be minimized without loosing accuracy.

4.3. Vibration Isolation

The tip-sample distance must be kept constant within 0.01Å to get good atomic resolution. Therefore it is absolutly necessary to reduce inner vibrations and to isolate the system from external vibrations.
Environmental vibations are caused by:

Vibration of the building 15-20 Hz
Runnning people 2-4Hz
Vacuum pumps
Sound

	Regarding the transfer function (ratio of amplitudes) of a damped harmonic oscillator with a sinusodial external force, it is clear that the resonant freqency must be much lower than the external frequency to be eliminated.
Damping can be done by Suspension with springs (including additional eddy current dampers) Stacked plate systems Pneumatic systems

4.4. Tip

The tip is the trickiest part in the STM experiment. It needs a small curvature to resolve coarse structures.

For atomic resoltion a minitip with a one atomic end is necessary.

Tips typically are made out of tungsten, platinum or a Pt-Ir wire.
A sharp tip can be produced by:

Cutting and grinding

Electrochemical etching

The tungsten wire is put into a solution of NaOH and kept on a positve potential towards a counter electrode. The etching process takes place predominately on the surface of the solution. When the neck is thin enough the wire fractures due to its weight. Thus actually two tips are produced. The tip has to be cleaned with deionized water and pure ethanol or methanol.
Most often the tip is covered with an oxide layer and contaminations from the etchant and is also not sharp enough. Thus other tratments to the tip, like annealing or field evapouration are necessary.

It is also possible to do tip-sharpening during tunneling.

Sudden rise of the bias voltage to about -7V (at the sample) for 2-4 scan lines.
By this treatment some W atoms may walk to the tip apex due to the nonuniform electric field and form a nanotip.
Controlled collision on Si surface.
The tip may pick up a Si-cluster which forms a monoatomic apex with a p_zlike dangling bond.