APPENDIX F

THE FURNACES

In general the furnaces in the cleanroom are kept at a standby temperature of about 600°C. A higher temperature standby condition is not utilized because the life of the furnace core windings decreases rapidly at elevated temperatures. The furnaces are not completely shut off when not in use because the cycling of temperature from room temperature, about 30°C to above 1000°C devitrifies (crystallizes) the quartz tubes. The term "quartz" is a common misnomer implying crystallinity. The material is actually anything, but crystalline. Although a thin surface layer of crystal is beneficial, only ultra-high purity and totally amorphous SiO2 has the properties needed in a diffusion furnace tube. Extreme temperatures and thermal gradients are experienced by the furnace tubes. They also provide a barrier to contaminants and are themselves "clean."

The ECE 344 furnaces can provide pyrogenic steam (hydrogen burned is in the tube) when rapid oxidation is required. Proper interlocks and sensors make the process relatively safe. Silicon is not the only material that gets oxidized in the furnaces. The boron source wafers, actually boron nitride, are oxidized to form a B2O3 layer prior the predeposition run (usually by a TA). It is the oxide which has a significant vapor pressure at the diffusion temperatures. B2O3 reacts with silicon to form SiO2 with an extremely high concentration of boron. The wafers used for phosphorus predeposition are made with SiP2O7 on a fine SiO2 lattice The SiP2O7 decomposes at difusion temperatures to form P2O5 which vaporizes and reacts with silicon. The student is encouraged to balance the predeposition reactions (or to read section 6.14 of Anner) since they are basic to understanding how you make ICs and are, therefore, excellent lab final material.

F.1 FURNACE LOADING

The ECE 344 diffusion furnaces are very much like those described in Appendix H of Anner's Planar Processing Primer. Refer to it for construction details. A home-made computer control system has been added to the ECE 344 furnaces which is intended to automate the calibration, temperature settings and gas flows. The following instructions are for manual operation, however.

CAUTION: Quartz hot enough to severely burn does not necessarily look any different than cool quartz. Quartz is also extremely expensive and can be ruined by contamination. The 4" furnace tubes cost over $500, the white elephants are nearly $200, the boats over $100. The worst thing a student can do is to knowingly spread contamination from, say a small piece of burned glove on a boat, to the other quartzware and, consequently, to the wafers of innocent classmates. IF YOU SUSPECT ACCIDENTAL CONTAMINATION, NOTIFY THE INSTRUCTOR IMMEDIATELY! Do not worry about your letter grade.

  1. Verify that the correct temperature is set on the middle potentiometer for the furnace you need.
  2. Check that the gas panel power is ON and that it is in the MANUAL mode.
  3. Check that only nitrogen is flowing in the furnace.
  4. Put on the high temperature gloves over your PVC gloves. Open the scavenger hood door and carefully remove the end cap. Place it on the stainless steel counter in front of the opposite bank of furnaces so it's well out of the way. Be quick, it's very hot.
  5. Put the appropriate "white elephant" on the furnace tube. Your instructor will demonstrate how the tapered joints do NOT need to be forced on in order to be secure.
  6. Take off the high temperature gloves.
  7. Carefully, use the short pull rod to pull the furnace boat into the white elephant just past the tapered joint. Do not pull any farther than necessary.
  8. Put the high temperature gloves back on and slowly move the white elephant to the stainless steel counter in front of a quartz disc. Access to the boat will be maximized if the white elephant is placed diagonally on the narrow counter.
  9. Remove the high temperature gloves and place them out of the way.
  10. Use the short pull rod to drag (slowly) the boat to the end of the white elephant so it's hanging out slightly.
  11. Use the lifting fork to move the boat to the quartz disc. Do not bother to remove the lifting fork from the boat's lifting tube.
  12. Load the boat. Remember, it's hot!

    If doing a predep, the wafer should face the nearest source wafer. Otherwise it should not matter, but note which way it is loaded as a matter of good scientific practice. In the case of predeps, use the diagram below to determine the boat position of your wafer for the electronic logsheet entry. It will help if you observe the dimensions of the boat now since you will have to hook it at a considerable distance without damaging the wafers later. Dummy wafers are used in all the boats not only for protection from the pull rods, but because the first and last wafers experience different gas flow conditions.

  13. Reload the boat into the white elephant with the lifting fork. It must be clear of the tapered joint.
  14. Use the high temperature gloves to put the white elephant back onto the furnace tube.
  15. Remove the high temperature gloves.
  16. Push the boat just past the end of the tapered joint with the short pull rod.
  17. Carefully, remove the white elephant from the furnace tube. Use the high temperature gloves!
  18. Without the high temperature gloves, use the appropriate long pull rod to slowly push the boat until the tape mark is flush with the scavenger hood face. Each furnace has its own long pullrod.
  19. Allow the pull rod to cool for several seconds before returning it to the quartz storage tube.
  20. Use the high temperature gloves to replace the end cap on the furnace. Always back the end cap off slightly from full on so it won't get stuck.
  21. Close the scavenger hood door.
  22. Switch gases as the "recipe" dictates. Hopefully, the computer can be used for this.

F.2 FURNACE UNLOADING

The unloading procedure is basically the same with the obvious differences that the long pull rod must be used to retrieve the boat from the center of the furnace and wafers will be removed from the boat.

  1. Put on the high temperature gloves over your PVC gloves. Open the scavenger hood door and carefully remove the end cap. Place it on the stainless steel counter in front of the opposite bank of furnaces so it's well out of the way.
  2. 30 second slow pull. Pull the boat to the mouth of the furnace using the long pull rod for that furnace.

    Depth perception helps a great deal in hooking the boat when it's in the middle of the furnace. Remember, the boat was left where the tape lined up with the face plate. It may help to gently touch the boat without lifting the pullrod in order to calibrate the depth. It also helps to use the end of the furnace tube as a fulcrum and pivot the hook upward as you hook the boat. Avoid touching the wafers with the rod.

    Pulling too fast will result in an abnormally high sheet resistance because a significant number of atoms will be frozen off lattice sites, making them inactive.

  3. Allow the pull rod to cool for several seconds before returning it to the quartz storage tube.
  4. Put the appropriate "white elephant" on the furnace tube.
  5. Take off the high temperature glove.
  6. Carefully, use the short pull rod to pull the furnace boat into the white elephant just past the tapered joint. Do not pull any farther than necessary.
  7. Put the high temperature gloves back on and slowly move the white elephant to the stainless steel counter in front of a quartz disc. Access to the boat will be maximized if the white elephant is placed diagonally on the narrow counter.
  8. Remove the high temperature gloves and place them out of the way.
  9. Use the short pull rod to drag (slowly) the boat to the end of the white elephant so it's hanging out slightly.
  10. Use the lifting fork to move the boat to the quartz disc. Leave the fork in the boat.
  11. Unload the boat. Remember, it's hot!

    Hold the wafers in air for 10 seconds or so to cool before placing them into the plastic wafer carriers. Although wafers cool very fast, the quartz boat will retain heat and keep wafers hot for a relatively long time. What implications does this have on the "real" diffusion time?

  12. Reload the boat into the white elephant with the lifting fork. It must be clear of the tapered joint.
  13. Use the high temperature glove to put the white elephant onto the furnace tube.
  14. Remove the high temperature gloves. Push the boat just past the end of the tapered joint with the short pull rod.
  15. Carefully, remove the white elephant from the furnace tube. Use the high temperature gloves!
  16. Use the high temperature gloves to replace the end cap on the furnace. Always back the end cap off slightly from full on so it won't get stuck.
  17. Close the scavenger hood door.
  18. Switch gases as the "recipe" dictates. Hopefully, the computer can be used for this.

ECE 344 home page.

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Written by Mike Fitzsimmons - U of Illinois ECE Dept. - mikef@uiuc.edu
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