Specs STM LT chamber

The Specs STM LT (Low Temperature) UHV chamber houses an STM (Specs GmbH Aarhus 150 STM) and LEED optics (see figure 3.6 for a scheme of the chamber). The machine consists of one single chamber, equipped with a turbo/rotary pump system, an ion pump and a titanium sublimation pump, along with an ion gauge for pressure measurements. A gate valve can separate the turbo pump from the chamber: this is mainly to switch the turbo pump off during STM acquisition, while the pumping is assured by the non-mechanical ion pump. The base pressure normally reached during operation is 10−10_{mbar. The dosing section, used for introducing the source for the} molecular sublimation in vacuum, is pumped with another turbo/rotary set and is separated from the main chamber with a manual gate valve. The instrument is endowed with a sample manipulator, used for the crystal cleaning procedure and for the storage of up to five different samples, and two transfer arms, one long, used for the dosing process and to perform LEED experiments, and one short, used to move the samples between the long arm, the manipulator and the STM. The manipulator can be heated resistively and, if necessary, with the aid of electron beams, in order to induce sample annealing. A thermocouple is attached to the manipulator. The chamber is provided with a sputter gun, used for sputtering the crystal with Ar+_{. In the proximity of the} sputter gun, there is a leak valve, connected to a gas line that brings argon into the chamber. An additional leak valve is used for letting other gases (e.g. oxygen) into the chamber at the desired pressure.

The STM is positioned on a stage that is held with springs, in order to provide a damping system against vibrations during STM operation. However, the stage can be locked to a fixed position for some operations, for instance during sample transfers, with a specific horizontal linear drive. The STM operations are driven with the built-in Specs GmbH software. The complex STM electronic controls are in connection with the chamber and are interfaced with the computer. The software shows the formation of a STM image in real time, and allows some tip-conditioning activities to be performed, basically applying a user-defined voltage to the tip for a short time, in order to improve the tip resolution.

The extremity of an STM apparatus, consisting in the unit that allows the scanning and the electron tunneling to take place, is termedSTM head.

Figure 3.7a, taken from reference [12], represents a picture of the UHV flange onto which the STM stage is mounted. The stage is held with springs, that function as an anti-vibrational system, during normal operation. When the STM is unused, the STM stage can be locked with a horizontal linear drive: this fixes the position of the STM stage and allows operations such as the transfer of the sample. In the drawing of figure 3.7c, also taken from reference [12], the sectioned STM head is shown. It is visible that the rod that sustains the STM tip has a cylindrical shape, with its vertical movement

Figure 3.6: Schematic representation of the Specs STM chamber. Diagram inspired by reference [11].

controlled by a dedicated motor. In this system, the sample faces downwards, and is kept tight on the sample-holder with the aid of specific copper clips. When the clips become loose due to the continuous use, a significant noise is detected in STM investigations, and they thus need replacing.

Regarding the vertical upwards movement of the motor rod towards the sample, this is regulated by a specific mechanism called inchworm. This mechanism was de-

signed at the University of Aarhus [13, 14] and was scaled up commercially by Specs GmbH. The motor used for the coarse approach consists in a hollow cylinder made of piezoelectric material that envelops the rod. Three specific electrodes compose the inchworm motor: two, placed at the top and and the bottom, are designed to hold accurately the motor rod and serve as clamps. The central tube can also be expanded and contracted. Applying a defined software-controlled sequence of voltages the rod can be moved upwards. In particular, the sequence "upper clamping, lower unclamp- ing, central contraction, lower clamping, upper unclamping, center expansion" induces the rod to proceed upwards [15]. The opposite sequence drives the rod in the oppos- ite direction. Also the speed of this coarse movement is controlled by the software. A typical rate for a coarse tip approach towards the sample is 1 mm/min. The fine (x,y,z) movement of the tip, used during the STM scanning, is controlled by a different

piezotube, which is placed on top of the motor road and below the tip (see diagram in figure 3.7b).

The complex electronic controls required for the functioning of this STM are guar- anteed via feedthroughs that are placed at the external side of the UHV flange. The electrical connections are then established with wires that run in vacuum to the STM head. Three separate feedthroughs are connected to the STM UHV flange. A preamp- lifier is used to amplify the tunneling current and to transform it into a voltage. Addi- tional pieces of equipment, though unused for the production of the results presented in this thesis, include a Variable Temperature Controller (VTC20), that allows STM investigations to be performed in the range 90-400 K, and Monitor-Digipot controller, that permits to manually control the position of the head in the x,y directions.

Figure 3.7: a) Picture of an Aarhus type Specs 150 STM, showing the STM stage and the UHV flange onto which it is mounted. Picture taken from reference [12]. b) Schematic representa- tion of the Specs STM head, with a focus on the piezotubes that drive the movement of the probe. The scheme is adapted from reference [15]. c) Scheme, reproduced from reference [12], representing the sectional view of a Specs STM head.