Tousimis SAMDRI-PVT-3B Critical Point Drying Apparatus

 

 

The Tousimis SAMDRI-PVT-3B is a critical point drying apparatus used after HF-release of a MEMS device to prevent stiction in the device.  The apparatus achieves this by avoiding the sudden change in densities caused by a fluid’s transition from liquid to gas during typical air-drying.  By using a critical point transition, the transition from liquid to vapor occurs at the same density.

 

Section One: Theory of Operation

 

The SAMDRI process begins with immersion of the device in an intermediate dehydrating fluid (100% methanol) in the chamber.  The process can then be divided into four stages:

 

1.      The device and the chamber are cooled and filled with liquid CO₂ and maintained at 08C 6 108C.

2.      The sample in the alcohol under pressure is purged of the alcohol with cold liquid CO₂ until all traces of methanol are removed.

3.      The chamber is heated to a temperature above the critical temperature of CO₂.  While the chamber volume is constant, there is an accompanying rise of pressure to above the critical pressure of CO₂, and the liquid phase is converted to the gaseous phase. 

4.      The gaseous CO₂ is exhausted at a temperature above 31.18C from the chamber.  The specimen is now critically dried and ready for manipulation.

 

This transition of the liquid CO₂ at its critical point avoids the distortion from evaporation boundaries and thermal expansion that leads to stiction problems.

 

Section Two: Sample Preparation

Super critical point drying is usually performed after release of bulk or surface micromachined devices to allow movement without stiction.  This release is usually performed by immersion of the device in a beaker of concentrated hydrofluoric acid for a specified period of time, followed by subsequent immersions in several beakers of DI water. 

 

The SAMDRI system requires that the device be completely free of water before introducing it to the chamber.  Water or oils remaining on the sample during critical point drying will not pass through the critical point with the transitional fluid.  Eventually, the water may evaporate from the device surface and cause damage or stiction problems.  To avoid this, the device should be subsequently placed in increasing concentrations of methanol, ending in a 100% methanol of the highest possible purity. 

 

Other precautions to take to insure dehydration of device:

1.      The methanol should fill the container containing the device in order to reduce exposure to atmospheric moisture outside the SAMDRI.

2.      The container should be tightly stoppered with a lid.

3.      The 100% fluid should be obtained from a newly-opened bottle of ultra-pure methanol or that it contains a dehydrant to insure no water contamination.

 

Section Three: General Information

1.      Instrument power must be on for at least 5 minutes before use.

2.      There should be at least 25lbs net weight of liquid CO₂ in the supply tank.  Note that the tank should come with 50lbs net weight of liquid CO₂ in the supply tank.  Each run takes 1-2lbs of liquid CO₂.  Low CO₂ levels could confuse the logic circuitry of the SAMDRI.

3.      Never overtighten any connectors unless you notice the escape of gas, and then only gently to stop escape of gas.

4.      If the unit has not been used for more than 2-3 days, make a dry run (CO₂ only) with the unit. 

5.      Time for purging may vary.  Keep in mind that it may take up to seven minutes to purge 10 ml of ethanol.

6.      Condensation may occur on the machine at certain points of the drying procedure.  It is necessary that the machine be wiped dry at these points.

 

Section Four: Device Overview

·        Liquid CO₂ tank connected through a filter to the SAMDRI unit:

 

 

·        Inlet and Exhaust Ports on side of SAMDRI unit:

 

·        Metering Valve Buttons:

Buttons are activated by pushing in.  Buttons will be highlighted when activated.  The metering valve buttons control the on-off activation of the corresponding valves.

 

·        Metering Valves:

Metering valves have been pre-set by the Super-user.  Valves should not be adjusted except by an experienced authorized user.  Metering valves are opened by turning counter-clockwise.  On-off activation of the valves is provided by metering valve buttons.

 

 

Section Five: Safety Precautions

1.      CAUTION – Use extreme care when handling METHANOL.  It is a FATAL poison.  Small internal doses, prolonged exposure of the skin to the liquid, or continued inhalation of the vapor can cause blindness.  Causes irritation to skin, eyes, and respiratory tract.  Affects the central nervous system, gastrointestinal tract, and liver. It is also FLAMMABLE as a liquid or a vapor.  The following chemical ratings are applicable to methanol:

·        Health Rating: 3 - Severe (Poison)

·        Flammability Rating: 3 - Severe (Flammable)

Further information is available by viewing the MSDS sheet on Methanol in the recording binder next to the machine in the MML cleanroom.

2.      CAUTION:  During the drying process, gaseous CARBON DIOXIDE could build up in the work area if not properly ventilated.  Insure that the room is well ventilated and that the cooling exhaust tube is exhausting out of the cleanroom.  CO₂ levels greater than 10% could cause fainting.  If you are feeling light-headed during operation of the unit, seek fresh air immediately.  Prolonged exposure to high levels of CO₂ could prove fatal.

3.      CAUTION:  Low liquid CO₂ levels in the tank could confuse the logic circuitry of the SAMDRI unit.  Check CO₂ level before each drying run to insure there is an adequate supply.  There must be at least 25lbs of liquid CO₂ in the tank.

4.      Safety valves and rupture disks have been engineered into the SAMDRI unit to prevent excessive pressure build-up.  Above 1300 psi, a safety valve will begin to release pressure from the chamber.  Also, a mechanical Automatic Pressure Control System will maintain the chamber pressure less than 1600 psi.  If the APCS should fail, a rupture disk will be activated at 2000 psi at 20°C.  In addition, safety thermostats should prevent the temperature from rising above 49°C.

 

Section Six: Approximate Timeline for Standard Drying

Time Start (mins.)	Time Stop (mins.)	Procedure
0	5	SAMDRI Unit Warm-up
5	6	COOL Procedure
6	8	FILL Procedure
8	18	PURGE Procedure
18	20	Repeat FILL Procedure (if necessary)
20	32	HEAT Procedure
32	44	BLEED Procedure
44	47	VENT Procedure
47	-	Drying Complete

 

Section Seven: Detailed Procedures

1.      Turn Toussimis unit on by pressing the Power button on the SAMDRI unit.  Power light and chamber light will go on.  Allow SAMDRI to warm up for five minutes before use. 

2.      Open sealed chamber on machine by loosening the 3-knurled nuts.  Transfer a sufficient amount of 100% pure methanol into chamber to cover the entire device.  Quickly transfer device from beaker to chamber, minimizing the time exposed to air. 

·        The chamber O-ring should lie completely in its groove.

·        Never grease or wet the O-ring with methanol. 

3.      Place the chamber lid down over the chamber and tighten the three knobs by hand with equal pressure

·        DO NOT OVERTIGHTEN!

4.      Make sure all buttons except for the power button are off.  Open the liquid CO₂ tank valve by turning counter-clockwise two full revolutions until you can verbally hear LCO₂ flowing into the machine.

5.      Begin COOL procedure.  Press ON COOL button.  The COOL meter valve should be set to 4 turns counter-clockwise.  Do not adjust the valve.  The temperature as read on the dial should drop from room temperature (about 30°C) to 0°C in 1 minute.  At this point, press COOL button OFF.  DO NOT CHANGE THE POSITION OF THE COOL METER VALVE.  The cooling function can be repeated as required independent of any of the other operations.

6.      Begin FILL procedure.  Press ON FILL button.  The FILL meter valve should be set to 4 turns counter-clockwise.  Do not adjust the valve.  Allow the chamber to fill with liquid CO₂ for 2 minutes.  A bubble should travel across the quartz viewing window indicating that the chamber is full.  After two minutes, press OFF FILL button.  DO NOT CHANGE THE POSITION OF THE FILL METER VALVE. 

·        During FILL and PURGE procedures, keep chamber temperature below 7°C (between +7°C and -30°C).  If temperature increases, press ON COOL button until chamber cools to within acceptable range.

7.      Begin PURGE procedure.  Place a polyethylene beaker at the end of the polyethylene exhaust tube connected to the purge-vent-bleed port to collect the ethanol.  Press ON PURGE button.  PURGE-VENT valve should be set at 2 turns counter-clockwise.  Do not adjust the valve.  FILL valve should already be set at 4 turns.  DO NOT CHANGE POSITION OF THE METER VALVES.  Allow PURGE to continue for 10 minutes.  At the end of the 10 minutes, check exhaust stream for methanol using the paper test described below.   

·        Characteristics visible to the operator during solvent change:

·        The methanol will appear as “oily currents” within the chamber.

·        The methanol and CO₂ will appear as a liquid or slurry when exiting the exhaust hose. 

·        Once thoroughly exchanged, the chamber liquid will be perfectly clear.  The exiting material will be cold and white. 

·        Test the exhaust for any alcohol by placing a single piece of paper in the exhaust stream.  Appearance of water marks on the paper indicate that there is still alcohol present in the exhaust.  Continue PURGE for 1 minute past the point where there are no water marks.  Note purge time on data sheet provided in the recording binder.

8.      End PURGE procedure.  Press off PURGE button.  DO NOT CHANGE ESTABLISHED SETTINGS FOR ALL METERING VALVES.

9.      Repeat FILL procedure to insure that the chamber is filled with liquid CO₂.

10.  Begin HEAT procedure.  Press HEAT button ON.  Pressure and Temperature will begin to escalate automatically.  Note the appropriate pressure and temperature in 1 minute increments on the appropriate data sheet in the recording binder.  The chamber pressure will increase to around 1300 psi, above the critical point of 1070 psi.  The chamber temperature will increase to between 31°C and 39°C.  Wait four minutes after the chamber temperature and pressure is above the critical points (31.1°C, 1070 psi).  After four minutes, end HEAT procedure by pressing OFF HEAT button. 

·        Critical point passage should take around 12 minutes.

·        During HEAT mode, the CO_{2 ­} may exhibit similar schlieren patterns (“oily currents”) to those in the PURGE mode.  This is caused by the variations in densities due to the heating.  This is not methanol in the chamber.

11.  Begin BLEED procedure.  Push ON BLEED button.  The BLEED valve should be set to achieve an exit flow rate of 7-8 SCFH measured by a flow meter attached to the exhaust tube at the beginning of the bleed procedure.  Confirm the BLEED valve setting by putting the flow meter into the polyethylene tube connected to the Purge-Vent-Bleed Port.  If the flow rate is different than 7-8 SCFH, please contact the Super-user.  The pressure should decrease at a rate of about 100 psi per minute.  Chamber temperature should remain constant.  When the chamber pressure reaches 350 psi, end BLEED procedure by pushing BLEED button OFF.

·        Decompression should take around 12 minutes.

12.  Begin VENT procedure.  Push ON VENT button.  The VENT-PURGE metering valve has already been set at 2 turns.  The pressure in the chamber should reach 0 psi in 3-5 minutes.  The flow meter at the start will be 15-20 SCFH.  When chamber appears to reach 0 psi, wait 2 more minutes.  End VENT procedure by pushing off VENT button. 

13.  The device has now been critically dried.  Open chamber by loosening the three nuts on the chamber top.  Quickly remove sample to a dehydrated clean environment to prevent further rehydration and stiction.

14.  Make certain the chamber is clean and the O-ring is in place and in good condition.  Replace lid and tighten by hand.  Close liquid CO₂ tank valve by turning clockwise until tight. 

15.  Begin Power down procedure.  Press COOL button ON.  Metering valve should already be set at 4 turns.  This will empty the lines of CO₂.  Turn OFF COOL button.  Do not adjust or close any of the metering valves.  Turn off POWER button. 

16.  Wipe unit down and cover.

 

Section Eight: Quick Procedures (Experienced Users Only)

1.      POWER on machine – (5-minute warm-up).

2.      Place sample in chamber with methanol.

3.      Open CO₂ tank.

4.      COOL – Press on COOL button, meter at 4 turns (1 minute).

5.      FILL – Press on FILL button, meter at 4 turns (2 minutes).

6.      Setup beaker for methanol-LCO₂ exhaust.

7.      PURGE – Press on PURGE button, meter at 2 turns (10 minutes).

8.      Re-FILL – Press on FILL button, meter at 4 turns (2 minutes).

9.      HEAT – Press on HEAT button, record T & P (12 minutes).

10.  BLEED – Press on BLEED button, meter set for 7-8 SCFH, record T & P (12 minutes).

11.  VENT – Press on VENT button, meter at 2 turns (3-5 minutes).

12.  Remove sample from chamber.

13.  Close CO₂ tank.

14.  Exhaust lines – Press on COOL button, meter at 4 turns (1-2 minutes).

15.  POWER off machine.

16.  Wipe and cover.