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Diffusion Furnaces

Manufacturer Model Description Serial Number
Lindbergh/Tempress 8500 Dual Stack Furnace Bank
 

INTRODUCTION TO THE DIFFUSION FURNACES

In general, the furnaces in the cleanroom are kept at a standby temperature of  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.  

MANUAL 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. The furnaces are divided into manual and automated sides so that hands on experience and real-world operation are both covered.

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 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 Digital Temperature Comtrollers (DTC) by pressing:
  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. Carefully, use the lifting fork to remove the boat from the chamber.
  5. 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.
  6. Load the boat. Remember, it's hot!

    7. 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.
     

  7. Reload the boat into the chamber with the lifting fork.
  8. 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.
  9. Allow the pull rod to cool for several seconds before returning it to the quartz storage tube.
  10. Switch gases as the "recipe" dictates. Hopefully, the computer can be used for this.

MANUAL 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. 30 second pushrod insertion. Count to 30 while inserting the long pushrod into the chamber.
  2. 30 second slow pull. Pull the boat to the mouth of the furnace using the long pull rod for that furnace.

    3. 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. Use the lifting fork to re move the boat from the furnace chamber and place the boat on the quartz disc.
  5. Unload the boat. Remember, it's hot!

    6. 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?
     

  6. Place the boat back into the mouth of the chamber using the lifting fork.

AUTOMATED FURNACE LOADING

The Lindbergh/Tempress 8500 furnace is equipped with both a Tempress 261 Digital Temperature Controller (DTC) and a Tempress 263 Digital Process Controller (DPC).  The DPC allows for full automation of the diffusion process:

The DTC and DPC combination will provide more repeatable results for the process.
 
  1. Remove boat from the cantilever with the lifting fork
  2. Load boat
  3. Replace boat onto the cantilever
  4. Press
  5. Turn on the proper gasses when the DPC prompts you
  6. When the recipe is complete, the cantilever will withdraw fully
  7. Remove the wafer boat with the lifting fork
  8. Unload boat
  9. Replace boat on cantilever

A word about the furnace procedures: the process just described is not the ideal situation. The furnace chambers which were donated by Intel are a flange type which does not facilitate the use of a white elephant, as is normally the case. As the lab progresses, an improved loading/unloading procedure will be implemented. 

ECE 344 home page.

Written by Dane Sievers - U of Illinois ECE Dept. - dsievers@eceuil.ece.uiuc.edu
and was inspired by Mike Fitzsimmons.  E-mail comments and suggestions to ece344@uiuc.edu or use the FEEDBACK FORM.
I like to edit this file with the command: vuepad ece344/equipment/Furnaces/diffusion/Instructions.html &

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