Gate Oxidation

The gate oxide for MOSFETs should be very thin and have good electrical properties.

The field oxide used for dopant masking was grown using steam oxidation and thus is less dense (lower breakdown voltage), potentially has pinholes (can short gate to the channel), and has a large number of charges incorporated.

Gate oxide is grown using dry oxidation. Dry oxidation creates an oxide which is more dense with lower pinhole density and charge density. It also grows at a rate ~10x slower than steam oxidation, so thickness can be more easily controlled.

Equipment

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• Lindbergh-Tempress 8500 manual oxidation furnace chamber 8C.

Supplies

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• O₂

Operating parameters

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• Furnace temperature: 1000 °
• N₂ flow
  • standby: 110 ± 10
  • processing: 0
• O₂ flow
• standby: 0
• processing: 110 ± 10
• The oxidation will consist of one step:
  • dry oxidation (O₂ only) - use oxidation charts and Grove's model to determine time to grow 250 Å

Equipment/controls/tools locations

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• Temperature controller: on the side of the furnace
• Gas panel: third rack from the top in the back of the furnace
• Quartz handling: covered cart is to the left of the furnace, tongs are inside
• Boat: at the mouth of the furnace

Operating precautions

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High temperatures

Use the high temperature gloves when handling hot equipment.

Contamination issues

• Quartware is easily contaminated by alkali ions. This leads to premature quartz failure (breakage) due to devitrification as well as unstable MOSFET V_t. Once quartz is contaminated, little can be done to remove the contamination.
• Always wear latex gloves when working with the furnace.
• N₂ should always be flowing in standby to minimize contamination by backstreaming of air in the room into the hot chamber.

Operating procedure

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Load wafers into boat

1. Put the high temperature gloves on over your latex gloves.
2. Unload the quartz wafer boat from the mouth of the furnace 8C using the quartz tongs
3. Carefully transfer the furnace boat to the quartz rack on the cart. Leave the tongs under the boat.
4. Remove the high temp gloves.
5. Transfer the wafers that have just been RCA cleaned into the furnace boat using clean tweezers.
   • Be sure that the wafers are in the right set of slots – they should be perpendicular to the sides of the boat.
   • If the wafer is placed at an angle on the boat there is a good chance that it will break due to unintentional torque during unloading.
   • There are wafers already in the boat – these are guard wafers. They serve two purposes:
     • Protecting processed wafers from damage due to the pushrod.
     • Establishing the correct flow pattern around the boat – the first and last wafers will see the greatest variation in growth rate across the wafer.

Load boat into the furnace

1. Put on the high temp gloves again.
2. Using the quartz tongs, reload the wafer boat into the mouth of chamber 8C.
3. Remove the quartz fork and replace on the quartz rack on the cart.
4. Using the pushrod located directly next to chamber 8C, slowly push the boat into the center of the furnace.
   • The push should take at least 30 seconds from start to center.
   • The center of the furnace is located using the metal tape on the pushrod.
   • When the tape is lined up with the outside edge of the furnace vestibule, the wafer boat is in the center of the furnace.
   • Do not handle the pushrod past the metal tape. This is to prevent contamination.
   • Under no circumstance should the pushrod be inserted so far that the tape enters the mouth of the furnace. What is tape adhesive made of? Organics. What do organics do in furnaces? Contaminate!
5. Withdraw the quartz pushrod and let it cool for 20-30 seconds before placing back into the storage tube.
6. Cover the mouth of the furnace.

Start dry oxidation

1. Open the doors of the gas cabinet (at the back of the furnace) and press the O₂ button on the gas panel.
   • Start the timer
   • The green LED by O₂ will light and a click will be heard indicating the O₂ solenoid has opened
   • Check the flow by looking at the rotameter labeled O₂. It should be flowing 110 ± 10
   • If it is not correct, notify your instructor. The instructor will set the correct flow.
2. Quickly press the N₂ button.
   • The green LED by N2 will go out
   • A click will be heard indicating the N₂ solenoid has closed
   • The flow of the rotameter labeled N₂ should be 0. Notify your instructor if it is not.

Clean up

• Return the Teflon wafer boat to a quartz plate in the SC-1 fume hood.
• Straighten up the furnace area.

Unloading the boat from the furnace

1. After dry oxidation, press the N₂ button, then the O₂ button.
   • The green LED by the N₂ button will light, the green LED by the O₂ button will go out
   • N₂ is turned on before turning off the O₂
   • This keeps a constant flow of gas through the furnace chamber
   • The rotameter labeled N₂ should read 110 ± 10 and the rotameter labeled O₂ should read 0. If either one is not flowing correctly, notify your instructor.
2. Put the high temp gloves on over latex gloved hands.
3. Using the pushrod located next to chamber 8C withdraw the boat slowly to the mouth of the furnace.
   • Once again a slow pull is used to prevent uneven cooling of the wafer.
4. Once the boat has reached the mouth of chamber 8C, unhook the pushrod and allow to cool for 20 – 30 seconds before returning the pushrod to the storage tube.
5. Using the quartz tongs that are on the quartz rack on the cart, remove the wafer boat from the mouth of the furnace and transfer the boat to the quartz rack on the cart.
6. Remove the high temp gloves.

Unloading the wafers

1. Using clean tweezers, remove the wafer from the boat.
   • Pull the wafer with the tweezers straight up out of the boat.
2. Allow to the wafer to cool for 20 – 30 seconds to prevent melting of the wafer container.
3. Place the wafer into the wafer container and close the lid.
4. Put the high temp gloves on over latex gloved hands.
5. Transfer the boat back to the mouth of the chamber for storage.