Key Features

• Proven reliability - over 6,000 critical point dryer installations world-wide
• Simple robust construction - easy to maintain, many critical point dryer users carry out their own routine maintenance
• Horizontal chamber and large viewing window - gives unequalled visibility of the critical point drying process and an unsurpassed view of the fluid level in the chamber. Unlike many of the more complex critical point dryer designs it is much easier to see the phase change at the critical point.
• Large robust valves for ingress of CO2, draining of or fluids and venting of gas. Very durable. The rapid ingress of CO2 helps prevents pre-drying of samples.
• Safety - every unit critical point dryer is independently pressure tested to 2500psi. A certificate is issued. A pressure bursting disc is fitted to safeguard against misuse.
• Sample handling - optional sample holders for coverslips and TEM grids. Porous pots are available for very small samples.
• 3 year warranty

Critical Point Drying (CPD) is so named as it includes, as part of its process, the occurrence known as the continuity of state for which there is not apparent difference between the liquid and gas state of a medium, the surface tension between this interface reducing to zero. This occurs at a specific temperature and pressure with resulting density, and is known as the Critical Point. This condition of zero surface tension can be used to dry Biological Specimens, avoiding the damaging effects of surface tension.

In biological specimens we are mainly concerned with the removal of water. Unfortunately, the critical point of water of +347^oC and 3212 p.s.i. is inconvenient, and would cause heat damage to the specimen. The most common and convenient transitional medium for critical point drying is Carbon Dioxide CO₂, which has its critical point at 31^oC and 1072 p.s.i. However, it is not miscible with water, and therefore we have to involve a third medium, commonly Acetone, which is termed the intermediate fluid. We can now convert our transitional fluid, typically CO₂ from liquid to gas without surface tension at the critical point.

The K850 Critical Point Drier is designed for use with CO₂, having first replaced any water in the specimen by a series of dehydration, often in the same fluid such as Acetone, which will also be the intermediate fluid.

K850WM Critical Point Dryer

The K850WM is designed to critical point dry a complete 150mm (6") wafer. The K850WM has built-in heating and water cooling (see: E4860 heater / chiller) and will give temperature control of +5°C cooling and +35°C during heating. This ensures the critical point is accurately obtained, avoiding excess pressures or temperatures, or the need to rely on pressure relief valves to control pressure during the heating cycle. The K850WM has a vertical chamber which allows top loading of specimens. A viewing port is fitted in the top plate for sample observation. The K850WM sample exchange mechanism is simple to use and ensures the sample remains under liquid during loading.

Sample handling

100mm or 150mm diameter wafers are held in a PTFE holding tray. The tray including wafer is immersed in acetone in order to remove all moisture from the sample. After dehydration, the wafer and holder is transferred into the pre-cooled sample chamber using the wafer transfer device. On completion of the critical point drying process, the wafer is removed from the chamber using the transfer device prior to further processing.

Key Features & Benefits

• 170mm diameter chamber - optimised for wafer / MEMS drying
• Vertical chamber with top filling and bottom draining - ensures specimens do not become uncovered during drying
• Thermoelectric heating - accurate temperature control
• Fine control needle valve pressure let down - precise control
• Temperature monitoring and control with thermal cut-out protection
• Pressure monitoring with safety cut out for over pressure