Tube Furnace SOP

Parameters

Temperature

1100°C

Total Time

1 Hour

Pre-doping time

5 min

Diffusion time

30 min

Purpose

The primary use of the tube furnace is for diffusion of dopant atoms into silicon. This introduces charge carriers, increasing the conductivity of the diffused region and making it either N or P type. Diffusion of a solid into another solid is quite slow, so high temperatures and long times are needed, hence the high-temperature tube furnace.

Many types of dopant sources are used, including solid, liquid, and glass. Our process uses spin on glass containing phosphorus for N doping and boron for P doping.

Tools

  1. Tube Furnace (ThermoFisher Manual) with fused silica tube

  2. Fused silica rod

  3. Fused silica microscope slide

Materials

  1. Chip with glass containing phosphorus or boron.

Procedure

Pre-heating the Furnace

  1. Turn on the furnace.

  2. Press and hold “set/ent” for three seconds until “mode res” is showing. Press the up arrow twice until mode “LCL” is selected. Press “enter” once. Now the furnace will hold the temperature shown. Use the arrow keys to adjust and press “enter” once to set the setpoint.

  3. Wait ~15 min for the furnace to come to temperature.

  4. If the furnace stops and holds at a lower temperature than your setpoint, the high temperature alarm may be too low. Read the instructions on page 6-7 to fix this.

Inserting chips

  1. Put the fused silica microscope slide in the entrance of the tube. Do not use a regular borosilicate microscope slide, or it will melt inside the tube. The edge of a fused silica slide is pure white, while borosilicate is a little bit green.

A chip permanently fused to the tube furnace walls when improper slide was used 🔥

  1. Place your chips on the slide.

  2. Use the glass rod to push the slide into the tube, being careful to not scratch the inside of the tube too much. In order to minimize thermal stress, insert the chip in steps:

    1. Push the chip to the entrance of the white insulation. Wait 30 seconds.

    2. Push the chip to the other side of the white insulation. Wait 30 seconds.

    3. Push the chip an additional 3 inches. Wait 30 seconds.

    4. Push the chip to the center of the tube. Start the timer. Do not count the steps in the total diffusion time.

  3. Place the rod aside.

Removing chips

  1. Use the glass rod to push the slide to the other end of the tube.

    1. Repeat the stepping process in reverse. 30 seconds each at: 3" from the inside of the insulation, at the inside of the insulation, and at the outside of the insulation.

    2. Push the slide all the way to the end until it is reachable.

  2. Wait a few minutes for the slide to cool down.

  3. Use metal tweezers to take the slide out. Don't use the orange ones because they don't close all the way.

  4. Put the slide on a heat resistant surface and wait 5 minutes before handling the chip.

Safety

The furnace is obviously very hot. Keep away from any hot parts of the furnace. The rod will heat up in the few seconds that it is inside the tube, so when removing it, only hold it by the end.

The rod and tube also channel heat out of their ends via internal reflection of radiation.

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