An easy and safe sample handling is a key issue to perform successful ex-periments. Several requirements have to be fulfilled for a well controlled sample cleaning, preparation, transfer and storage. All these different as-pects of sample handling have to be combined and realized by a possibly simple design of the sample holder.
3.5. SAMPLE HANDLING 25
Figure 3.10: Detailed view on the sampleholder parts and construction.
3.5.1 Sample Holder
The sample holder has to fulfill the following electrical requirements: The sample has to be mounted such that it is insulated from ground. Otherwise it would neither be possible to measure the ion current from sample to ground during sputtering nor to apply a high positive voltage with respect to ground for an effective electron bombardment during annealing. In Fig. 3.10 the parts of the sample holder are shown separately. The lower part of the sample holder contains the sample. It is electrically insulated from the upper part by an Al2O3 ring segment and Al2O3 washers that prevent the screws that connect the upper and the lower part from making short contacts. In the STM the bias voltage is applied to the sample by contacting the lower part of the sample holder with a Copper-Beryllium spring.
For a controlled annealing it is also inevitable to measure the temperature directly at the sample. For this reason two “type K” thermocouple wires are guided to the sample through small Al2O3 tubes and form small contact areas
manipulator head sample bias leaf spring sample holder
thermo couple connection
filament sample clamp mechanism
Figure 3.11: Head of the horizontal manipulator with and without the sam-pleholder attached.
on top of the sample holder.
Due to the high temperatures during annealing the material of the sample holder must have a high melting point. Molybdenum is the right material which is non-magnetic and combines a high melting point with a quite high thermal conductivity.
3.5.2 Sample Preparation
To clean a single crystal sample by the standard sputtering and annealing cycles it is clamped to the horizontal manipulator head as shown in Fig. 3.11.
The isolated lower part is contacted via a Molybdenum leaf spring, the upper part of the sample holder is grounded. In this configuration one can measure the ion current from the sample to the ground during sputtering.
By putting the sample holder in the manipulator also the temperature measurement is enabled: A pair of thermocouple wires from the manipulator side is pressed against the thermocouple wires on top of the sample holder with a spring. This leads to a direct thermocouple connection to the sample and a reliable measurement of the single crystal temperature. The filament for electron bombardment is located in the gap at the backside of the sam-pleholder (see Fig. 3.10) about 1 mm above the sample. This allows for an effective electron bombardment heating up to 800 K at voltages between 300 and 400 V and a filament current of 2.6 A.
Beside cleaning the sample there are various possibilities for further
sam-3.5. SAMPLE HANDLING 27 ple preparation. Two evaporators can be attached to the preparation cham-ber simultaneously for evaporating metals or molecules via Knudsen cell or electron bombardment. Because of the load-lock system with a sepa-rate turbo molecular pump and valves between evaporators and preparation chamber they can be easily exchanged without breaking the vacuum inside the preparation chamber. In addition a needle valve is attached to the vac-uum chamber, that allows for a well defined dosing of gases as well.
The horizonal manipulator is a flux cryostat suited for cooling with liquid Nitrogen and liquid Helium. Thus, the sample can be cooled down during preparation which offers the possibility to deposit single atoms or molecules and study different growth modes.
3.5.3 Sample Transfer
After cleaning and preparing the sample on the horizontal manipulator in the preparation chamber, it has to be transferred reliably to the STM. This is achieved by three transfer steps. In the first step the sample holder is picked up from the horizontal manipulator by the “wobble stick”. Its three pins fit in the holes at the front of the sample holder. The middle pin is rotatable and acts as a key that locks the sample holder and prevents it from falling off the “wobble stick”.
In the second step the sample holder is plugged to the vertical manipula-tor. After pulling out the “wobble stick” the transfer to the STM can start by moving down the vertical manipulator. Up to this step the transfer could still be controlled by eye. The sample has to be put into the STM with-out any visual control. To prevent any damages during this phase a simple mechanical sensor is implemented in the manipulator head and the applied forces are reduced and controlled.
Therefore, the manipulator head is designed such that it is pushed up elastically against three springs (Fig. 3.12) as soon as it is touching any rigid barrier like the STM in case of the sample transfer. By this an electrical contact opens indicating that the sample can be screwed in the receptacle.
Two small pins at the edge of the manipulator heads ending fit in gaps in the sample holder and make its rotation possible. For taking out the sample the pin on top of the sample holder has to be fastened to the manipulator. For this procedure slightly larger forces might be applied. In that case an upper position sensor closes an electrical contact before the springs are completely compressed.
cable connection
springs
position sensors
radiation shield holding mechanism
sample holder junction to vertical manipulator rod
Copper-Beryllium leaf spring
Figure 3.12: Head of the vertical manipulator with the sampleholder at-tached.
This mechanism also works for the transfer of the radiation shield. While the sample holder is plugged to the manipulator head the radiation shield is attached to the manipulator head and locked with the three pins by turning the manipulator about 60◦.