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Sample mounting

General

In general, suitable samples are flat plates of max. 10×10 mm² surface area (depending on the sample holder used). The materials to be analyzed must be UHV compatible, electrically conductive and if possible homogeneous. Some other types of samples can be studied only if deposited on flat conductive substrates, e.g. evaporated or sputtered thin films, powders pressesd into indium foil etc.

Several sample holders are available.

The base plate, which is the same for all sample holders, is grounded, but sample can be mounted on two insulated supports with electrical contacts, each of them tightened by two molybdenum M1.2 nuts. It allows sample to be heated and/or grounded or biased.

The thermocouple (K-type, chromel-alumel) can touch the sample from the backside or be spot-welded to the sample..

The design can be further developed depending on sample properties and user requirements (cooling, heating, thermocouple reading...).



Sample holder selection table


Sample requirements Plain mounting Crystals Silicon wafer Pocket with clamps Sapphire plates STM type
number of samples on one holder up to 12 1 1 1 1 1
can be grounded yes yes yes yes yes yes
can be biased no yes yes yes no yes
annealing up to 400 °C (700 K) no yes yes yes yes yes
annealing up to 1000 °C (1300 K) no yes yes yes no yes
measurement during annealing  ≤400 °C (700 K) no yes no yes yes yes
measurement during annealing  ≥400 °C (700 K) no no no no no yes
maximum annealing current 20 A 10 A 20 A 5 A 5 A

Sample mounting

  1. Mount the sample(s) on the selected holder.
  2. Check that all nuts are properly tightened. Do not exaggerate with the force in order not to break the fragile ceramic insulators.
  3. Measure the distances in red circles in the above drawing for both heating pins and both thermocouple pins. The heating pins can be a bit higher than the indicated 3.5 mm, 3.6-3.7 mm might still be fine.
  4. Check electrical connections (if installed):
    • sample to ground: infinity resistance, depending on the cleanliness of the ceramic insulators, practically several kΩs
    • heating element contacts: low resistance, <1Ω for metals, higher for Si wafers
    • thermocouple contacts: low resistance, several Ωs at maximum
    • sample to thermocouple: low resistance, several Ωs at maximum
  5. Blow away any dust from the sample surface.
  6. Proceed with sample insertion.

Plain mounting

If the samples do not need to be annealed it is possible to attach them directly to the base plate, using double-sided copper or carbon tape or custom clips fixed in the base plate threaded holes. The glue on the double-sided tapes is not well conductive which may lead to charging of the samples during photoemission if not sufficiently pressed on the tape.

Up to 12 samples can fit on one base plate. Bear in mind that such small samples might be difficult to correctly align with respect to the photon beam and energy analyzer.

The samples can cover up to the area marked in red in the left and middle picture. In the right picture you can see hypothetically mounted 11 round samples of 6 mm diameter.





Metal crystals

Polished metal single crystals are usually mounted on Ta heating wires (diameter 0.25 or 0.4 mm) annealed by passing current.

Round shape and 5-10 mm diameter is optimal, the thickness in the range 1-2 mm. The smaller the crystal the higher annealing temperatures can be easily reached. The annealing limit for small and thin crystals (5 mm diameter) is about 1100 °C/1400 K, larger crystals (10 mm diameter) will hardly reach 700 °C/1000 K.

The crystal should have 2 slits (0.3-0.5 mm width). If not it can be spot-welded directly to the heating wires (only some materials). Spot-welding improves the heating contact and provides easier annealing and cooling.

The thermocouple can touch the crystal from the backside like a spring or be spot-welded to the crystal (only some materials). Spot-welding improves the thermal contact and provide more precise temperature reading. The error can be up to 100 K/°C.

For annealing usually the 70 V×24 A DC power supply is sufficient. Maximum annealing current is 20 A. The centre of the sample is theoretically at half of the annealing voltage so it is difficult to acquire spectra during annealing.

In the following pictures Cu (8 mm diameter × 2 mm thickness) and Ir (5 mm diameter × 1 mm thickness) crystals are shown.






Silicon wafers

For silicon wafers or similar samples heated by current passing through we clamp the sample directly to the electrical contacts.

It requires self-supporting sample of rectangular shape approximately 10 mm long and 3-10 mm wide. The best width is 5-7 mm. The suggested thickness is about 0.5 mm.

The electrical resistance should be in the range 0.1-1000 Ω depending on required temperature (in the case of silicon it means that it must be sufficiently doped). If the resistance is too high it is impossible to anneal the sample.

For annealing we usually use the 70 V×24 A power supply..

The centre of the sample is theoretically at half of the annealing voltage so it is difficult to acquire spectra during annealing.

The temperatures up to about 1100 °C/1400 K can be reached. Maximum annealing current is 10 A.

In the figure there is the sample holder with Si wafer 6×10 mm2 and the thermocouple mounted with Ta spacer in order to avoid migration of Ni on the sample.




Pocket with clamps

Samples of irregular shape, with low conductivity or with other properties preventing them from mounting like crystals or silicon wafers can be mounted to the "metallic pocket", usually spot-welded from Ta wires and sheets.

The sample is simply clamped by the pre-stressed sheet so thermal contact with pocket is not always excellent and uniform. It can also cause difference between thermocouple reading and the real temperature of the sample surface. The error can be up to 100 K/°C.

Sample size is up to 10×10 mm2 and the maximum annealing temperature is about 700 °C/1000 K. Maximum annealing current is 20 A.
 
The centre of the sample is theoretically at half of the annealing voltage so it is difficult to acquire spectra during annealing.





Sapphire plates

The sample on this holder is annealed by a Ta spiral closed between two insulating plates made of sapphire.

The sample of maximum 10×10 mm2 size is clamped by two wires that are fixed by screws to the base plate. Therefore the sample is permanently grounded and the spectra can be measured during annealing.

The thermocouple is touching the sample surface which makes the temperature reading more accurate than on the pocket sample holder. However, the error can be up to 50 K/°C so you can consider spot-welding of the thermocouple to the sample if your material permits to.

Maximum temperature is about is about 500 °C/800 K. Maximum annealing current is 5 A.




STM type

The design of this sample holder is coming from the STM aparatus at the Charles University in Prague.

The sample on this holder is annealed by a molybdenum block containing a heating spiral made of very thin tantalum wire, which allows reaching high temperatures (up to 1100 °C/1400 K) using quite low currents in comparison to the other sample holders (maximum annealing current is 5 A). Therefore, in many cases the spectra can be measured during annealing although the sample is not permanently grounded.

The clamp can be modified according to the sample size and shape and it can be grounded to the base plate if requested. The maximum sample dimensions are 10×10 mm2.

The thermocouple is touching the rear surface of the sample. As it is not spot-welded, the error can be up to 100 K/°C.






Last Updated on Thursday, 25 March 2021 09:19