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ECE 344 Equipment Instructions for:

The Autosequencing Evaporator

Manufacturer: Cooke

Model: 301-Auto

Description: This is the evaporator with a 12 inch bell jar that has electrical switches on the control panel to operate the valves.

Introduction.

Emergency Procedure.

Start Up Procedure.

Operating Instructions - prepare to pump down.

Shut Down Procedure.

Related files:

INTRODUCTION TO THE COOKE CVE 301

The Cooke CVE 301 is a low cost ($12K) one button pump down vacuum evaporator. Although it can pump down from a standby state with the push of a button, you will be operating the system in manual mode. This means you must have an understanding of how the system works.

The basic concept of a diffusion pump is to remove the atmosphere that diffuses into jets of boiling hot oil directed away from the chamber. The momentum transfer from the diffusion pump oil molecules to the atmospheric particles carries the undesireable atmosphere away so it cannot contaminate the film to be deposited. The pumping mechanism is not unlike the cause of the wind that one feels at the bottom of a waterfall on a calm day.

Complications arise from the fact that no diffusion pump oil known is capable of pumping from atmospheric pressure to more than eight orders of magnitude below. The problem is oxidation of the pump oil. It proceeds at an unacceptable rate at pressures above about 10-4 atmospheres. Therefore a mechanical pump must be employed to keep the pressure at either end of the diffusion pump below 100 microns when the diffusion pump oil is hot. Your responsibility will be to make sure the valves are sequenced properly to keep the oil from "cracking" (a term for rapid oxidation of pump oil).

The special property that distinguishes diffusion pump oil from other oils is that it can be recycled within the diffusion pump. See figure 1. Water cooled side walls of the pump body condense virtually all the diffusion pump oil. The oil is then re-boiled (high vapor pressure) at the base of the diffusion pump while the mechanical pump removes other particles swept down by the oil jets.

Diffusion pump oil does have a measurable vapor pressure of its own, even at the cooling water temperature. The price goes up as that vapor pressure goes down. To keep diffusion pump oil from diffusing up into the chamber, first a water cooled (chevron) baffle and then a liquid nitrogen cooled "trap" are used to contain the oil. Both are "optically dense" meaning that light or particles with a mean free path greater than the system dimensions must collide with at least one surface. The cool surfaces will condense the majority of diffusion pump oil. The 77 degree Kelvin walls of the liquid nitrogen trap also enhance pumping speed by literally freezing out some of the atmosphere and compacting the rest (PV=nRT), a phenomenon called cryopumping.

Thin film deposition can be done by many methods. You will use the simplest. Once the atmosphere is sufficiently removed for a clean deposition, aluminum will be boiled by an electrically heated filament coating everything within sight of it. Aluminum has the useful property of clinging to the relatively inexpensive tungsten filaments when it melts rather than falling through.

Figure 1
Diffusion pump cross section

EMERGENCY PROCEDURE:

A popular topic for lab final questions tests your response to high (>80 microns) pressure in a hot diffusion pump. If the foreline pressure ever does go too high, you should:

  1. Close the Hi-Vac valve.
  2. Turn off the Diffusion pump heater.
  3. Make sure the mechanical pump is pumping through the foreline valve.
    1. Mechanical pump should be running.
    2. Roughing valve should be closed (for at least 5 sec. before c.).
    3. Foreline valve should be Open.
  4. Check for leaks if system is still not recovering. Pumping out the bell jar helps Hi-Vac leaks.

Normally your instructor will warm up the system before lab periods requiring it.

If the CVE 301 is busy or otherwise unavailable, use the older CVE 300. Instructions for that system follow at the end of this appendix.

Figure 2
Control Panel of CVE-301

OPERATION OF THE COOKE 301

Start Up Procedure.

(To be done by TAs only)*

  1. Turn on the following utilities:* 
       COOLING WATER - ON
   LINE AIR PRESSURE - ON ( > 80 PSI)
   NITROGEN - ON (minimum pressure which will vent the bell jar in
   one vent cycle)
  2. Put the panel switches in the following positions:* 
       THERMOCOUPLE GAUGE - TC1 (LEFT)
   COLD CATHODE GAUGE - OFF (DOWN)
   VENT VALVE - CLOSED (DOWN)
   HI VAC VALVE - CLOSED (DOWN)
   ROUGHING VALVE  - CLOSED (DOWN)
   FORELINE VALVE - CLOSED(DOWN)
   MECHANICAL PUMP - ON (UP)
   DIFFUSION PUMP - OFF (DOWN)
   AUTO/MANUAL - MANUAL (RIGHT)
   MECHANICAL PUMP VENT (INSIDE CABINET) - CLOSED
  3. Turn on the power strip. The mechanical pump should start.*
  4. Press CONTROL POWER ON. The MANUAL light should come ON.*
  5. OPEN the FORELINE VALVE (UP). Note: The system will not go into manual unless all the valves are momentarily closed.*
  6. When TC1 reads < 50 microns, you may turn on the diffusion pump heater as long as TC1 stays below 100 microns. Turn off as necessary. Tend the unit long enough to be sure it will stay below 100 (at least 5 minutes!). Note the time when the diffusion pump was turned on.*
  7. About 5 minutes after the diffusion pump is turned on, activate the cold cathode gauge. If it reads > .01 torr, turn it off and try later. Cold Cathode gauges cannot reliably initiate ionization when the pressure is too low. The power supply can be damaged by excessive current when the pressure is too high, however.*
  8. Switch to AUTO if it will be used in the automatic mode (usually not). Since this vents the system, it is best to wait until load time.*

* To be done by the instructor before class.

Prepare to Pump Down

The diffusion pump must be given at least 20 minutes to warm up before attempting to pump down (step 7 below).

Do not touch anything with ungloved hands which must go into the vacuum. Water, finger grease, and similar contaminants will severely slow down the pumping speed and decrease the MTBF (Mean Time Between Failures). It is good practice not to even touch anything which will touch anything which will touch anything ... which goes into the vacuum system.

  1. Raise the bell jar carefully. If you can't raise the bell jar, cycle the vent switch if necessary until the bell jar can be raised. If in AUTO, switch to MANUAL, close (turn DOWN) all the valve switches, and open (flip UP) the foreline and vent switches.
  2. Carefully lift the metal chimney straight up and place it on the glass plate by the asher. Be careful that any microscope slides do not slide off.
  3. Load the filament(s) with three 0.025" diameter 3cm long aluminum wires cleaned with IPA and a kimwipe. By assuming that the entire volume of aluminum will be deposited uniformly in all directions, it is possible for you to calculate the thickness of the aluminum on your wafer if you measure the distance from the filament to your wafer.

    NOTE1: If a filament is broken or severely deteriorated, have your instructor look at it. If you plan to use both filaments in series, they should look nearly identical. If one seems more used than the other, then they should only be used one at a time.

    NOTE2: Always compare the configuration of the copper bars behind the door below the variac with the drawing below to see which filament(s) are active. Ask your instructor if you need a different configuration. Normally the filament on the front left (F1) will be used.

    NOTE3: Bent pieces of quartz are provided to prevent the aluminum from evaporating down into the diffusion pump. A third piece can also be used to prevent both filaments from evaporating through a metal mask and generating double images. This is desireable for the dry oxide experiment.

  4. Carefully load wafers into all six positions. It should go without saying that the thickest film will be on the wafers directly above an active loaded filament(s) so that is where your wafer should be. Use "dummy" wafers as necessary to fill all the other holes so the bell jar is not coated. This can be done concurrently with the previous step.
  5. Return the chimney assembly back to it's position. Carefully guide the hole in the wafer plate over the vent tube.
  6. If there are no uncoated areas left on the slides, replace them with an IPA and kimwiped slide and discard the old one(s).
  7. Make sure there is no path for aluminum from a filament to the bell jar. Monitor slides should be placed over the small holes near the filaments. These get visibly coated during the evaporation.

    It is strongly recommended that you also place slides such that every silicon wafer has part of a slide over it. Otherwise, the wafers may move and possibly break during venting.

  8. Wipe down (with IPA on a kimwipe) as many surfaces as you can except for the bell jar gasket and surfaces inside the chimney/wafer holder assembly. The bell jar gasket has high vacuum grease on it which should never be removed. The inside of the chimney has layers of aluminum which flake off and get into the rest of the system when disturbed. The instructors will take care of excessive aluminum deposits.

    Generally, time invested in cleaning is repaid with interest by a fast pump down.

  9. Exercising extreme care, slowly lower the $425 bell jar into position.

Automatic Pump Down

Skip to next section if Manual Pump Down is desired.
  1. While pressing the bell jar down onto the base plate press START. You may take your hand away as soon as the mechanical pump begins to pull a vacuum on the jar.
  2. Pour in a thermos full of liquid nitrogen. Use only light finger pressure to close the liquid nitrogen tank valve.
  3. As the system pumps down, monitor TC1. If it goes above 100 microns, tell your instructor. Occasionally check TC2 to monitor the bell jar pressure. If using an external cold cathode gauge controller, put it into the START mode until TC2 reads <20 microns.

Manual Pump Down

Skip to Evaporation if Automatic Pump Down was used. Manual pumpdown can be dangerous to the system, but if done properly, it will be better for both the pump and the thin film. The operator can make sure the hot diffusion pump oil is never exposed to as much pressure as it often is during the automatic pump down sequence. The film quality can be better because the bell jar does not have to be evacuated as much by the mechanical pump before switching to the diffusion pump. Therefore, "backstreaming" of mechanical pump oil into the bell jar and onto the wafer is minimized. Cold traps or oil-free pumps are sometimes used to keep oil out of the bell jar.

The key concept to keep in mind during pump down is that the diffusion pump oil must never be exposed to more than 100 microns of pressure. For this reason, leave the thermocouple switch on TC-1 except for short periods to monitor TC-2. Always be ready to answer the question: What would you do if TC-1 suddenly went above 100 microns?

  1. If not already in Manual mode, switch to Manual.
  2. Close the foreline valve by flipping the switch downward.
  3. While pressing the bell jar down onto the base plate, open the roughing valve by flipping its switch upward. You may take your hand away as soon as the mechanical pump begins to pull a vacuum on the jar.
  4. Pour in a thermos full of liquid nitrogen. Use only light finger pressure to close the liquid nitrogen tank valve.
  5. WHILE TC-2 measures >80 microns, DO 
       Monitor TC-1, checking TC-2 only occaisionally.
   IF TC-1 > 80 microns THEN
      Close roughing valve
      Wait 5 seconds
      Open Foreline valve until TC-1 < 20 microns.
      Close Foreline valve
      Re-open roughing valve
   END IF
  6. When TC-2 = 80 microns, close the roughing valve
  7. Wait 5 seconds.
  8. Open the Foreline valve.
  9. Wait at least 10 seconds for the Foreline pressure to "blank off" (reach its ultimate pressure).
  10. If using an external cold cathode gauge controller, switch it to START now.
  11. Open the Hi Vac valve for a couple of seconds and re-close it to keep the foreline pressure below 100 microns. Observe how high the foreline pressure got.
  12. After the foreline pressure recovers, open the Hi Vac valve again. This time only close it if the foreline pressure seems as though it will exceed 100 microns. Repeat as necessary until the Hi Vac valve can be safely left open.
  13. An external cold cathode gauge controller can now be safely turned to the appropriate scale to monitor the pump down.

Evaporation and Vent

  1. When the cold cathode gauge reads less than 1 X 10-5 torr you may perform the evaporation, but the lower the pressure, the better the film will be.

    NOTE: If using an external gauge controller, return it to START before evaporating.

    1. Rotate the variac control all the way to zero (CCW).
    2. Turn on the power supply switch.
    3. Rotate the variac slowly (take ~10sec.) clockwise (CW) to obtain and maintain a current of 60 AMPs for 5 seconds after the aluminum begins to darken the viewing slide. Then quickly return the variac to zero. Excessive durations will deposit tungsten from the filament itself as well as other materials nearby.
    4. Turn off the power supply switch.
  2. Wait about 20 seconds to allow the filament to cool.
  3. If in Auto Mode, Press STOP.
  4. If in Manual Mode, Close the Hi Vac valve and then open the vent valve.
  5. Wait for 20 more seconds after the system has vented to allow further cooling in the nitrogen before exposing the hot surfaces to oxygen. Why?
  6. Carefully, raise the bell jar and remove your wafers. If you can't raise the bell jar, it's probably because the vent cycle was incomplete. The following sub-steps are for additional venting if necessary:
    1. Switch to MANUAL mode.
    2. Close (flip DOWN) all the valve switches.
    3. Open (flip UP) the foreline and vent valve switches.
    4. Cycle the vent switch if necessary until the bell jar can be raised.
    5. Close the vent valve and return to AUTO. An automatic vent cycle will start, but don't worry about it.
  7. Wipe out the system as in step 5. Keeping a vacuum system scrupulously clean is so important that the instructors will deduct lab performance points from persons ignoring this step.
  8. Lower the bell jar even more carefully than before. Do not get careless with equipment as you get more "used to it."

Standby (To be done by TAs)*

  1. Switch to MANUAL mode.*
  2. Close all the valves.*
  3. While pressing down on the bell jar, open the Roughing valve.*
  4. Wait 30 seconds, then close the roughing valve.*
  5. Wait 5 seconds, then open the foreline valve.*

Shut Down Procedure.

(To be done by the TA at the end of the day)*

Overnight, the system should be left OFF as far as utilities go. The bell jar and diffusion pump body should be left under vacuum; however, the mechanical pump must never be left OFF with vacuum on one side of its seals and atmospheric pressure on the other. Therefore, the following procedure must be executed by the instructor before leaving the lab for the day.

  1. Switch OFF (DOWN) the diffusion pump heater. It is strongly recommended that this be done immediately after the last students to use the system have vented (STOPPED). It will take about an hour for the diffusion pump oil to cool.*
  2. Place the panel switches in the following positions (if they are not there already).* 
       THERMOCOUPLE GAUGE - TC1 (LEFT)
   COLD CATHODE GAUGE - OFF (DOWN)
   VENT VALVE - CLOSED (DOWN)
   HI VAC VALVE - CLOSED (DOWN)
   ROUGHING VALVE - CLOSED (DOWN)
   FORELINE VALVE - OPEN (UP)
   MECHANICAL PUMP - ON (UP)
   DIFFUSION PUMP - OFF (DOWN)
   AUTO/MANUAL - AUTO (LEFT)
  3. Switch to manual.* 
       AUTO/MANUAL - MANUAL (RIGHT)
  4. Close the foreline valve.* 
       FORELINE VALVE - CLOSED (DOWN)
  5. Open the roughing valve.* 
       ROUGHING VALVE - OPEN (UP)
  6. Monitor TC2. Rough out the bell jar for 30 seconds or until TC2 reaches 80 microns. Whichever is less. Return to monitoring TC1.*
  7. Close the roughing valve.* 
       ROUGHING VALVE - CLOSED (DOWN)
  8. Wait 2 seconds.*
  9. Open the foreline valve.* 
       FORELINE VALVE - OPEN (UP)
  10. Wait until ALL parts of the diffusion pump body (behind the main access door) are cool enough that you could rest your hand on them indefinitely.*
  11. When the diffusion pump is cool, close the foreline valve.*
  12. Turn off and vent the mechanical pump. Use the manual vent valve behind the access door.*
  13. Turn off the main power switch on the outlet box on the floor. (I disagree with the microprocessor's program. A power glitch would open the foreline valve.)*
  14. Turn off the other utilities.* 
       COOLING WATER - OFF (unless Liquid Nitrogen was used within
   the last hour) 
   LINE AIR PRESSURE - OFF
   NITROGEN - OFF

* To be done by TAs after class only.

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Warning! This is the archived 1999 Fabweb site! Here is the latest site