Specific Process Knowledge/Wafer and sample drying/Critical Point Dryer

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Critical Point Dryer

Critical point dryer: positioned in cleanroom D-3.

The critical point dryer is used to dry fragile structures that may be damaged in a normal drying procedure. Fragile structures can be thin membranes, or free hanging structures like cantilevers and grippers.

The user manual(s), technical information and contact information can be found in LabManager:

Critical Point Dryer in LabManager

In the critical point dryer, samples are dried in supercritical CO$_{2}$. In the drying process, the sample is first put into Isopropanol for at least 1 hour, and then loaded into the machine. When the machine is started, the isopropanol is exchanged for liquid CO$_{2}$. When there is only CO$_{2}$ in the machine, the pressure and temperature in the chamber is raised, above the so called “critical point”. At this point, there is no longer a liquid/gas interfacen, but instead the CO$_{2}$ is in a supercritical state. Because of this, the problem with surface tension, which destroys the fragile structures through capillary forces, is avoided.

Equipment performance and process related parameters

Purpose	Drying of wafers or chips	To dry fragile structures (example membranes, grippers, cantilevers).
Process parameter range	Process Temperature	0 oC to 45 oC
	Process pressure	1 atm to (maximum) 95 atm (1400 PSI)
Substrates	Batch size	1 to 5 wafers per run. Sizes: 2”, 4" or 6" or Pieces (up to 10x10mm)
	Materials allowed	Please consult the Critical Point Dryer cross contamination sheet

Dried with Critical Point Dryer.

Dried with Critical Point Dryer. The cantilevers are free hanging, and have not bended. The dimensions of the cantilevers are 100 x 710 µm, and the thickness is about 0.5 µm.

Dried with Critical Point Dryer.

Dried with the Critical Point Dryer. The cantilevers structures are free hanging after the drying.

An acknowledgment goes to Tom Larsen, Nanoprobes, DTU Nanotech, who provided the pictures.