Oxidation

In the manufacture of integrated circuits, field oxide is used to isolate the devices from one another. In our CMOS process, field oxide is also used to define the source and drain regions for subsequent diffusion.

Gate oxide is much thinner and composed of better quality oxide than field oxide. The presence of gate oxide under the metal layer and above the substrate creates the MOS (metal-oxide-substrate) structure.

To achieve high-quality oxide growth (i.e. - a uniform film with good dielectric properties), "dry" oxidation alone is employed when gate oxide is grown. This process is slower than "wet" oxidation.

"Wet" oxidation is used to grow field oxide, since the quality of the dielectric properties of the field oxide aren't as critical as they are for the gate oxide. Also, the field oxide is much thicker than the gate oxide, and the rate of "wet" oxidation growth is faster than that of "dry" oxidation.

During this procedure, a layer of silicon dioxide is grown on the wafer surface by thermally oxidizing the silicon substate.

The following steps make up the oxidation process:

An RCA Clean should be performed immediately prior to any oxidation process.

During the RCA Clean, the oxidation furnace should be set-up.

Furnace Preparation

First, verify that the Gas Flow Control Panel is set in the "Idle" configuration.

The Gas Flow Control Panel controls gas flow to three furnaces - the anneal furnace, the boron furnace, and the oxidation furnace.

In the "Idle" configuration, nitrogen is flowing into all three furnaces in order to keep them clean and dry.

Next, turn on the oxidation furnace by adjusting the Oxidation Furnace Temperature Controller to 900 degrees C. This is accomplished by adjusting the furnace controller in the back of the furnace clean hood.

[Note: This step is only required for Field Oxidation!] Then, fill the Bubbler Flask with deionized water and turn the controller knob up to 50, then immediately back to 25.

This will start heating the water in the Bubbler Flask to a temperature between 90 and 100 degrees C.

Quartz Boat Removal

At the completion of the RCA Clean, remove the Quartz Boat from the Oxidation Furnace using the appropriate Quartz Push Rod and the Quartz Boat Loader.

Always wear chemically resistant poly gloves and/or high temperature gloves when handling quartzware.

DO NOT hold push rod in front of mark on rod. Keep the boat in the boat loader and set the boat loader down in front of furnace tubes, taking care that holder and boat remain in laminar flow.

First, remove the Quartz End Cap from the end of the Quartz Diffusion Tube.

The Quartz Push Rod has a white mark about halfway down the rod. To avoid introducing contamination into the furnace, Never touch the rod below the marking.

Loading Wafers Into Boat

Keep the Quartz Boat in the Quartz Boat Loader and very carefully set the Quartz Boat Loader down on the two parallel quartz rods in order to cool down the Quartz Boat. Take care that the Quartz Boat Loader and Quartz Boat remain in the laminar flow.

Now it is time to load the RCA-cleaned wafers into the Quartz Boat.

As you will remember, at the end of the RCA Clean process, the cleaned wafers are loaded into a black box which protects them as they are transported to other areas of the lab.

Using teflon or teflon-tipped tweezers, carefully load the wafers into the Quartz Boat.

Avoid passing your arms or head over cleaned wafers.

Loading Boat Into Oven

With boat loader, return wafers on boat to the oxidation furnace.

In order to prevent thermal stress which could cause the wafers to fracture, allow the wafers to sit just inside the mouth of the open furnace for two minutes.

Very slowly push the Quartz Boat into the mouth of the furnace using the Quartz Push Rod. Push the quartz rod just to the point where the white mark on the Quartz Push Rod is flush with the metal furnace door. This indicates that the center of the Quartz Boat is at the center of the furnace.

Now place the Quartz End Cap on the end of the Quartz Duffusion Tube.

Then shut oven door.

Field Oxidation Example

The oxidation furnace needs to be heated to the temperature at which the field oxidation will be performed.

In our example, the field oxidation is to be performed at 1100 degrees C so the furnace controller is set to ramp up to this temperature.

The process is:

a.  Load in N2      (Idle gas)
b.  Dry O2          5 min.
c.  Wet O2          120 min.
d.  Dry O2          5 min.
e.  Unload in N2    (Idle gas)

Wait until the temperature has stabalized (about 10 minutes).

Again, the reason we are raising the temperature slowly is to avoid thermally stressing and perhaps fracturing the wafers.

To begin dry oxidation, set the gas control panel to the "Dry Oxidation" configuration:

In this configuration, dry oxygen is flowing into the oxidation furnace.

Check the Flow Meter, and adjust the flow rate to 2.5 sccm (standard cubic cemtimeter minute).

When five minutes have elapsed, it is time to switch to wet oxidation.

As a safety precaution, first remove the cap from the Bubbler Flask.

Now set the gas control panel to the "Wet Oxidation" configuration.

In this configuration, oxygen gas is being bubbled up through the deionized water in the Bubbler Flask, generating a combination of oxygen and steam which is fed into the oxidation furnace.

When two hours have elapsed, switch back to dry oxidation by setting the gas control panel to the "Dry Oxidation" configuration.

Turn OFF HEATER to the wet oxidation Bubbler Flask.

When five minutes have elapsed, start nitrogen flow through the tube, i.e., return the gas delivery system to its "Idle" configuration.

Finally, set the furnace controller to ramp back down to 900 degrees C, lock in ramp mode and wait 10 minutes for it to cool to approximately 900 degrees C.

Allow wafers to cool completely in boat before removing and storing in N2 dry box.

Gate Oxidation Example

This process step will thermally oxidize the silicon substrate. The oxide grown will be a thin gate oxide, approximately 1000 A.

RCA clean device wafers and control wafers. During the cleaning, continue with the preparation of the oxidation furnace.

Verify that the gases flowing through the furnaces are in the IDLE configuration. In this configuration, nitrogen gas is flowing through all three furnaces.

The oxidation furnace needs to be heated to the temperature at which the gateoxidation will be performed.

In our example, the gate oxidation is to be performed at 1100 degrees C so the furnace controller is set to ramp up to this temperature.

The furnace will heat at 10 degrees/minute, thus it will take about 20 minutes for the furnace to go from 900 degrees C to 1100 degrees C.

At the completion of the RCA clean, remove the quartz boat from the furnace using the appropriate push rod and quartz boat loader.

Always wear poly gloves and/or high temperature gloves when handling quartzware. DO NOT hold push rod in front of mark on rod. Keep the boat in the boat loader and set the boat loader down in front of the furnace tubes, taking care that the boat remain in laminar flow.

Again, the reason we are raising the temperature slowly is to avoid thermally stressing and perhaps fracturing the wafers.

Place the RCA- cleaned wafers, still intheir carrier, into the carrier box, and transport them to the furnace loading station. Load wafers into quartz boat using Teflon or Teflon-tipped tweezers. Avoid passing arms or head over cleaned wafers.

With boat loader, return wafers on boat to the mouth of the oxidation furnace. Slowly push boat into the mouth of the furnace with push rod, to avoid wafer reacturing due to thermal stress.

The dry oxidation process is to be carried out at 1100 degrees C. The process is:

a.  Load in N2          (idle gas)
b.  Dry O2              45 minutes
c.  Unload in N2        (idle gas)

Ramp the furnace controller up to the oxidation temperature of 1100 degrees C. Wait until the temperature has stabilized (ramp mode is set at 15 degrees per minute).

Once the oxidation temperature has been reached, we will be performing 45 minutes of dry oxidation. This should give us about 1000A of oxide.

To begin dry oxidation, set the gas control panel to the "Dry Oxidation" configuration.

In this configuration, dry oxygen is flowing into the oxidation furnace.

Check the Flow Meter, and adjust the flow rate to 2.5 scfh.

When 45 minutes have elapsed, turn oxide furnace valve to N2. The gas delivery system is back to "Idle" configuration.

Finally, set the furnace controller to ramp back down to 900 degrees C.

Once again, this will take about 20 minutes.

Unloading Wafers

At this point, the oxidation has been completed.

Open the furnace door and use the Quartz Push Rod to pull the Quartz Wafer Boat to the front of the furnace.

Again, let the wafers sit for a couple of minutes in the furnace entrance before using the Quartz Boat Loader to remove them.

Allow wafers to cool completely in boat before placing them back into the black box.

With the Quartz Boat Loader, return the empty Quartz Boat to the Oxidation Furnace.

Slowly push the Quartz Boat into the furnace with Quartz Push Rod. Push the boat approximately 1/4 of the way into the furnace.

Loosely replace the Quartz End Cap and store the Boat Holder inside the metal furnace door.

Oxide Thickness Measurement

Once the wafers have cooled, the thickness of the oxidation layer can be measured using an ellipsometer.

First, the wafer is placed on the chuck. The stage is adjusted so that the beam is reflecting from the polarizer toward the analyzer for an accurate reading.

Wafer Storage

Upon completion of the field oxidation process, the cooled wafer should be stored in the Nitrogen Flow Storage Area (assuming that no other processes are to be performed during this session).

The Nitrogen Flow Storage Area is an enclosed cabinet which has nitrogen gas circulating through it.

The circulating gas prevents dust from settling on the wafer surface during storage.

Simulation

The Oxidation Thickness Calculator calculates the thickness of oxide layer given the type of oxidation, the initial thickness of the oxide layer, and the time and temperature at which the oxidation is performed. Once you have entered the appropriate values, press the execute button. The oxide thickness in micrometers will appear in the output field.

  • The Field Oxidation cross-section illustrates the device after the growth of the thick field oxide for use with P+ diffusion.
  • The Field Oxidation & Drive In cross-section illustrates the device after the growth of the thick field oxide for use with N+ diffusion.
  • The Gate Oxidation cross-section illustrates the device after the growth of the thin gate oxide


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