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

This page is under construction and the values and data below are not all correct right now./ KN

Critical Point Dryer

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	Room temperature to 500${\rm {^{o}}}$C
	Process pressure	~5$\cdot$10-4mbar - 2000mbar
	Piston Force	Depending on the area, for 4" wafers 200-20000N.
Substrates	Batch size	One 4" or 6" wafer per run Pieces are only allowed with speciel permission
	Substrate material allowed	Silicon wafers Quartz wafers Pyrex wafers
	Material allowed on the substrate	Silicon oxide Silicon (oxy)nitride Poly Silicon Photoresist PMMA TOPAS SU-8 Metals: Au, Sn, Ag, Al, Ti.

Process parameter range Process Temperature 0 oC temperature 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.

Pic 1 Dried in air: most cantilevers are bended.

Pic 2 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.

Pic 3 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.