Specific Process Knowledge/Wafer and sample drying/Critical Point Dryer
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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 COFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _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 COFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} . When there is only COFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _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 |
0Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rm{^o}} C temperature to 45Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rm{^o}} C |
| Process pressure |
1 atm to (maximum) 95 atm (1400 PSI) | |
| Piston Force |
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| Substrates | Batch size |
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| Substrate material allowed |
| |
| Material allowed on the substrate |
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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.