Critical Point Dryer

In the manufacturing of Microelectromechanic systems (MEMS) with micrometric dimensions, the drying process is a critical stage, this is due the working liquids (for example water, IPA, etc.) make that mobile parts of the devices, as cantilevers, get stuck to other device components and prohibit the movement. In order to avoid these adherence of mobile parts problems, the drying must be done through the critical point drying (CPD). The clean room at Microtechnologies Management in CIDESI Queretaro has an apparatus to perform the drying process CPD, which avoids the problems caused by surface tension forces and capilarity that could damage the manufactured structures, be our guest knowing more about this interesting apparatus!

Drying process of MEMS devices with the critical point dryer system located at the CIDESI Querearo clean room.


How is the structure release process by Critical Point Dryer that we use in MEMS manufacturing?

The drying processes that are used in the MEMS devices manufacture with geometry and dimensions that go from micrometer unities to hundreds of micrometers, in accordance with the design and application, they require to be suspended or floating at certain specific distances in respect to a substrate, edge or conditioner mechanism, generally require the use of the CPD technique.

During the manufacture, after the micromachining stage, drying is not recommended to be carried on air at room conditions when the device has mobile parts. The drying process with CPD uses liquid CO2 to displace a transference liquid, usually alcohol. Once alcohol is displaced by liquid CO2, it is taken to temperature and pressure conditions of its critical point (see phase diagram Fig. 1), where subsequently CO2 is evacuated so that finally, the CPD chamber comes back to room conditions. This way, a controlled drying process is done without damaging the manufactured devices.

Fig. 1. Phase diagram of carbon dioxide. the adventages of using CO2 in the CPD processes are the pressure and temperature conditions to reach its critical point.

The CPD apparatus is used in different projects of technologic development and sensors manufacturing of the Microtechnologies Management, see Fig. 2.

Fig. 2. The critical point dryer, located in CIDESI Queretaro clean room, is a Autosamdri Tousimis 815B – Serie B – Supercritical. This apparatus is attached to the net of National Laboratories CONACYT-SEDAM.

Fig. 3. In the image the CPD chamber filling with isopropanol is observed. Its function is to be an intermediary liquid or transference, which will be afterwards replaced by LCO2. The IPA must have a high purity with the objective of avoiding pollution of the microstructures at the moment of being released.

Stages of the critical point drying, CPD.

Next, the CPD process stages are described performed by the Tousimis apparatus.

  1. Openning of the CPD chamber (Fig. 4a) and filling  with Isopropanol (IPA) (Fig. 3).
  2. Transfer and introduction of the sample(s) to the processing chamber and a IPA filling is done again. Next closing the CPD chamber (Fig. 4b, 4c y 4d). The sample can be seen through the CPD peephole (Fig. 4 e).
  3. Cooling until reaching 0°C.
  4. Filling of the chamber with liquid CO2 Here the alcohol and liquid CO2 are mixed.
  5. Purge stage. The alcohol and liquid CO2 mixture is evicted and substituited with CO2 once more to assure the 100%.
  6. Heating. Heat is applied to the procesing chamber, rising the liquid CO2 temperature in order to reach the transition state of the critical point which is uppon 31°C and a pressure of 73.9bar.
  7. Stabilization stage it is mantained in this phase for some minutes assuring the critical point conditions.
  8. Decompression of the chamber, the gas is evicted of the CPD slowly.
  9. Venting Total ejection of the gaseous CO2 (Fig. 4 f).
  10. Extraction of the sample(s).

Fig. 4. In the image the main stages of the critical point drying, CPD, process are observed.

Applications where the CPD process intervenes.

The big diversity of the applications of this apparatus go from simple to complex structures, for example implemented in the air industry, automotive, electronic, chemical, pharmaceutical, plastics, food, farming, among others, depending on the project nature. In the Fig. 5 a researcher is observed inspecting by digital microscope a set of MEMS microstructures after the CPD process.

Fig. 5. A user is observed in the image visually inspecting the manufactured MEMS devices.

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