Tousimis Critical Point Dryer Operation

Roger Robbins                                                                                                                                                                      5/30/2014

Purpose

This Appendix describes a new method of mounting samples in the Tousimis Supercritical Automegasamdri Model 815B Series C Critical Point Dryer and a new cleaning procedure to prevent sample contamination from accumulated particles in the chamber.

The purpose of a Critical Point Dryer is to dry objects without causing harm to the object by liquid surface tension and capillary forces as the liquid evaporates.  The CPD tool accomplishes this by changing the temperature and pressure in such a manner as to avoid the liquid-gas phase boundary during the drying process.  That is the process flushes the sample’s protective alcohol bath with liquid carbon dioxide and takes conditions in the sample chamber around the carbon dioxide phase diagram’s critical point so that the liquid goes through its supercritical phase, which is identical to the vapor phase in density and ends up in the gas phase without ever creating a liquid-gas surface so that surface tension and capillary forces remain zero through the drying transition and delicate device structures are preserved in their native form.  Sample preparation for this process requires that after creating the delicate structures in a liquid etchant, the sample is rinsed and immersed in Isopropyl Alcohol and then inserted into an alcohol bath in the tool without ever having the surface dry in air.

In light of the recent bout with particle contamination in the Tousimis CPD tool, we have finally found the hiding place of particles and arrived at a simple procedure for cleaning them out and maintaining a clean chamber on a consistent basis.  The answer to maintaining a clean chamber lies in a consistent routine of wiping particles out of the chamber after every run.  However the trick we discovered lies in the type of wipes used to clean the chamber.  The normal wipes consist of a weave of fiber having a filament diameter of about 18 microns.  This wipe will not remove the particles that interfere with the cleanliness of samples.  The new wipes we have purchased are constructed of 30% Nylon and 70% Polyester with a fiber diameter of 5 microns.  The small diameter fiber will pick up particles as small as a half micron and up to macroscopic size particles.  The wiping technique required to clean these particles relies on wiping in one direction only and not backing up with the wipe which will redeposit some of the particles.  The new wipes are expensive and reserved for this tool only.  They can be found in a closed container near the tool.

We have also designed and made a new Delrin Universal Sample holder that fills chamber volume and reduces the amount of IPA required and also reduces the purge time required to remove IPA.

New CPD Process Procedure

Tool Prep

  1. Turn on the Tousimis Critical Point Dryer (CPD) tool power (may already be on) and switch on the chiller power at least 30 minutes before you need to start the CPD run.
  2. Switch on the CPD condenser Power and the chamber power at least 5 minutes before you start the run.
  3. Remove the nuts and washers from the lid and remove the lid to the CPD process chamber. (Set the lid upside down on a handy table top (do not scratch the lid sealing surface!).
  4. NOTE:  Hovering over the open chamber has been shown to add particles to the chamber that land on substrates.  Please minimize hovering…
  5. Insure that the pre-cleaned Volume-Filler Universal Sample Holder is in the CPD chamber
  6. Press the “Vent” key and then fill the chamber with pure IPA to a level that will just cover any sample that you will place on the Universal Sample Holder.
  7. Replace the lid on the chamber (temporarily – no need to screw the nuts onto the studs).

Sample Prep

  1. Process your sample(s) through the structure release chemistry (or through the e-beam resist develop process).  Never let your sample dry in air…
  2. Rinse the sample(s) and completely immerse in IPA in a transportable container (beaker, etc.).

Load Sample in CPD Chamber

  1. Remove the CPD lid and quickly immerse your sample(s) upside down in the chamber IPA – arranging it or them appropriately on the Universal Sample Holder.  Never allow HF or other acid to enter the CPD chamber!
  2. Set the splash shield on the Universal Sample Holder and align it to the two LCO2 input holes at the top of the chamber wall.
  3. Reattach the lid and firmly tighten the nuts on the washers in a “star” tightening pattern using the special open-end wrench.

Start CPD Process

  1. Set the Purge time knob to an appropriate value to ensure that all the IPA will flush out during the purge time. (~5 – 10 min)
  2. Press the “Cool” button to start the chamber chilling process (~2.5 minutes to attain at least a temperature of 8 C or less).
  3. When the temperature reaches the proper value, press the “Fill” button to start the automatic CPD run.

Remove Sample

  1. At the end of the run when the pressure reaches 0 psi, remove the lid nuts and washers, lift the lid off and retrieve the samples. (Always set the stainless steel lid upside down on a table to preserve the smooth sealing surface.)
  2. Remove samples.

Cleanup Chamber

  1. Wipe down the lid, chamber, Universal Sample Holder, and splash shield with the special micro-fiber lint-free cloth dampened with IPA.   Fold it several times and wipe multiple times in one direction only – do not swish the wipe around in all directions.  Particles will scrape off the wipe if it goes backwards after collecting them in the forward direction.
    1. Cover the studs on the lower chamber rim with the Copper shields to avoid contaminating the chamber with the grease on the threads during wiping.
    2. Use the special micro fiber lint-free wipes (Teknipure TC3MBWS-99)  reserved for this tool
    3. Minimize hovering over the open chamber
  2. Replace the lid, nuts, and washers on the chamber to keep it clean and unscratched.  (nuts loose)
  3. If no one is using the tool later in the day, please turn off the chiller, condenser and chamber power via switches on the instrument panel.