The cold fracture stage: this unit has two separate functions:

1. To allow cryo-fracturing of the sample at a precisely controlled temperature
2. To allow the user to load and store up to 2 samples in a cold ‘parking area’

The fracture stage unit is LN2 cooled, and can be maintained at -180°C. The whole unit moves in and out to allow samples to be parked in the ‘parking area’, picked up for placement in the fracture unit, or transferred (following fracture) into the sample analysis chamber.

ToF-SIMS researchers have found empirically that water-containing samples should be prepared by fracturing at between -105°C to -110°C. If the temperature is too cold then subsequent surface spectra are dominated by water molecules. Too high a temperature appears to allow excessive mobility of surface species leading to smearing of surface features. Thus, accurate control of the sample temperature during fracture is essential.

During fracturing a large pressure burst is observed. Condensation of this water back onto the sample will tend to obscure the freshly fractured surface. To minimise this, both the ‘knife’ and the surrounding material of the fracture stage are held at -180°C during fracture. Also, the inside of the preparation chamber is lined with a cryo shield.

The fracture tool itself is specifically for the preparation of fresh surfaces on biological materials. It too is actively cooled with LN2. The sample usually comprises a layer of biological/soft sample material sandwiched between two flat plates (Si wafers, copper plates, etc), and it is this ‘sandwich’ that is cleaved apart by the fracture tool. The fracture tool is not strictly a knife. The tool is really a ‘finger’ that can be hooked under the top plate in order to pull it free, thereby causing fracture.

Once the sample has been fractured, the sample stub is picked up again on the transfer rod and transferred through the analytical chamber gate valve and onto the cold stage.

The ‘Transfer Rod’ allows the transfer of the sample stub from the fast entry probe onto the transfer rod.

The fast entry probe is then withdrawn and the entry gate valve is then closed. At this point the sample stub is in the preparation chamber on the end of the transfer rod. The sample is subsequently transferred to the fractured stage parking facility or fracture facility, or onto the cold stage in the analytical chamber. The preparation chamber is separated from the analytical chamber by a gate valve.

There is no active cooling at the end of the transfer rod; a thermal insulating break minimizes the heat flow during the transfer stage.

The sample probe assembly allows the analyst to immerse the sample stub into the coolant (nitrogen, propane, ethane etc).

Sample preparation using appropriate materials and techniques results in vitreous quenching of the samples: there is no gross disruption of cell material by water crystal formation during cooling.

After reaching the temperature of the coolant, the probe is removed and the cylindrical cup is lowered over the sample stub. The low volume, combined with evolving coolant gas, ensures that a moisture-free atmosphere is preserved around the sample for the short time it takes to transfer the assembly over to the fast entry lock.

The cylindrical cup mates with a receiving cup on the fast entry lock and a vacuum seal is made with a spring-energised lip seal. A valve to the sorption pump is then opened. Volume sharing (5cc : 5litres) alone produces an instantaneous reduction of pressure to 1mbar, and then sorption pumping in the next minute results in a pressure of ~10^-3 mbar.

To allow the analyst to position the fracturing tool precisely, a high resolution CCTV camera is mounted on an optically flat glass window on the preparation chamber. The zoom camera covers a field of view from >3mm to ~450µm. At high magnification, the lateral resolution is ~2-3µm.

Sample illumination is achieved using a halogen lamp with an integral dichroic reflector assembly mounted externally on the preparation chamber (not shown).

Modules (1)-(4) are mounted on a sample chamber which features a turbo molecular pump and an inner cryo shield assembly (shown in blue below). The cryo shield assembly is LN2 cooled and has various openings into which the cold fracture modules are inserted. By maintaining the cryo shield at liquid nitrogen temperature, researchers have observed a significant reduction in the magnitude of the pressure bursts during fracturing of biological material. This results in cleaner, fractured surfaces.

The sample stub designed by Kore has powerful features such as on-stub thermometry and on-stub heating in an OHFC copper block. It has a sample mounting surface area of 22mm x 22mm. In order to accommodate heating and thermometry the stub is 16.5mm deep. Naturally, the stub has been designed to fit both the fracture stage and the analytical cold stage designed by Kore.

Kore has designed a number of high-precision sample stages for operation in UHV environments. For matching the cryo preparation system, we have designed an X, Y, Z stage with ±12 mm of motion in all directions. The stage fits on to a 6″ flange and is designed to maintain samples cold for surface analysis of otherwise volatile materials. Using on-stub thermometry and control of the LN2 flow, the sample can be held at any temperature from −180°C to room temperature to within ±1°C. The X, Y and Z motions are achieved through manual, rotary motion, and an option exists to equip the drives with digital micrometer readouts for greater positioning precision.