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

Critical Point Dryer

Critical point dryer: positioned in cleanroom 4

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

In the critical point dryer, these structures are dried in CO$_{2}$. In the drying process, the sample is first put into Isopropanol, and loaded into the machine. In the process itself, 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”. In this way, there is never a liquid/gas interface, but instead the liquid is turned into a mixture of equal parts of gas and liquid. Because of this, the problem with surface tension, which destroys the fragile structures through capillary forces, is avoided.

Purpose	Drying of wafers or chips	To dry fragile structures (example membranes, grippers, cantilevers).
Process parameter range	Process Temperature	0oC to 45oC
	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)
	Substrate material allowed	Silicon wafers Quartz wafers Pyrex wafers InAlP GaAs
	Material allowed on the substrate	Silicon oxide Silicon nitride Poly Silicon SU-8

Comparison of samples dried in air and with Critical Point Dryer

This shows a comparison of samples that have been dried in the critical point dryer, and samples that have been dried in air. Below are pictures of two wafers that have been prepared in the same way- but dried with different methods after the final etching step. Cantilevers of SiN, up to 710 µm long and 100 µm wide, are fabricated in the etching step. The thickness of the cantilevers is about 0.5 µm

After drying wafers in air, the majority of the cantilevers are bended and sticking to the underlying surface. But after the wafers were dried in Critical point dryer, the cantilevers were free hanging.

Dried in air: most cantilevers are bended.

Dried in air: most cantilevers are bended.

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.