Critical point drying
| Critical point drying | |
|---|---|
 | |
| Technology Details | |
| Other Names | CPD |
| Technology | Release |
| Equipment | Tousimis 915B Critical Point Dryer |
Critical point drying Critical point drying make use of liquid CO2 under high pressure and lower temperatures to remove any water from around the MEMS device to reduce or eliminate stiction.
Contents
Equipment
Tousimis 915B critical point dryer
The Tousimis 915B cricital point dryer (CPD 1) is a 6" automated, critical point dryer used to dry samples that have had a final rinse in Methanol. It consists of a high pressure chamber that is connected to two CO2 tanks, one for cooling the chamber and one for filling the chamber with high pressure, liquid CO2. The tool runs an automated fill, purge (replacing final rinse material with CO2,) heating to critical point and then bleading/venting process. The 6" chamber has spacers for substrates ranging from 6" wafers down to small pieces.
Method of operation
Final rinse and removal of water
- First the substrate is soaked in methanol to remove the water or other aqueous solutions.
- Depending on how much distance between the edge of the substrate and the bulk region, you will need to give time to allow the water to diffuse out from under the free standing structure. **The time it will take is proportional to the square of the distance and can take anywhere from an hour to 48 hours.
- Typically it helps to change the methanol part way through the diffusion process, though make sure that the device does not dry.
- Once the device has soaked and water removed from the structure, Methanol is filled in the CPD chamber and the substrate is placed inside.
Critical point drying process
The purpose of critical point drying is to eliminate surface tension/stiction associated with the drying of a liquid by avoiding the phase transition boundary from liquid to gas. The process goes "around" the high-temperature/pressure side of the gas/liquid transition point by pushing the liquid past the critical point and converting the liquid to a supercritical fluid and then converting that fluid to gas.
- The CPD is a 4-step process (see diagram below, green and blue arrow indicate the sample is still in methanol.)
- Cool and fill: sample, still in methanol, is cooled and then exposed to a high pressure liquid CO2 (6°C, 850PSI in the case of the LNF CPD tools)
- Purge: methanol is then purged and displaced with liquid CO2
- Heat: chamber is then heated and kept under high pressure until the CO2 is pushed past the critical point (>31°C, >1072PSI)
- Bleed and vent: Pressure is released and you are left with a dried product with no surface tension.
Parameters
Final rinse soaking time
It is important that the samples be soaked for a long enough time that all the water has been removed from around the structures. Remaining water in the sytem will cause stiction.
Purging Time
The purge step is when the CPD replaces the final rinse methanol with the liquid CO2. If the purge is not long enough then the sample will come out with residual methanol leading to possible stiction.
Pressure and Temperature
The tool is automated in it's operation so it should reach critical point. However, if there is an issue with sealing or heating, then the tool will not reach critical point (>31°C, >1072PSI) which will cause damage or stiction upon venting. Operation of the tool requires the user to verify the sealing before the tool gets too far along in the process.
See also
Further reading
- Overview of Critical Point Drying: Wikipedia article on Supercritical Drying
