N-Type Diffusion

This module describes the solid-source method for N+ dopant diffusion. The n-type (phosphorus) diffusion, will dope the source/drain regions in the p-well region. All source/drain regions are defined by gaps in the field oxide. Methods for evaluation of the diffused layer will be performed later.

N-Type (Phosphorus) Diffusion

The phosphorus source is a ceramic wafer of LnP5O14. LnP5O14 is phosphorus pentoxide (P2O5) mixed with a rare earth oxide (Ln2O3) . The source is oxidized in a 1000 degree C, 25% oxygen atmosphere, resulting in a surface layer of phosphorus pentoxide (P2O5) glass.

During the infinite-source diffusion (or "pre-deposition") step, this glass slowly evaporates from the LnP5O14 wafer and condenses on the silicon wafer. The thin layer of P2O5 provides elemental phosphorus for the diffusion into the silicon via the reaction:

2 P2O5 + 5 Si -> 4 P + 5 SiO2

After the desired amount of dopant is deposited on the silicon wafer, the dopant source is removed, and the dopant is diffused deeper into the silicon wafer during a high-temperature limited-source diffusion (or "drive-in") step. The drive-in step can also be done concurrently with the field/gate oxidation step. In the N+ diffusion process, drive-in occurs during the gate oxidation step.

The following steps make up the N+ diffusion process:

Oxidize Phosphorus Wafers RCA Clean and Check Furnace Load Wafers and Sources Into Boat Load Boat Into Oven Pre-Deposition Example Unload Wafers Wafer Storage Simulation

Oxidize Boron or Phosphorus Wafers

Before using the LnP5O14 source wafers the first time, it is necesary to oxidize them. (This is not necessary for every use, as the sources are compromised if oxidized too frequently. Previously oxidized wafers are stored in the boron furnace, which should be idling at 600 degrees C).

The instructor will have prepared the souce wafers, if required, using the following procedure.

Oxidize LnP5O14 wafers at degrees C in % O2 for approximately hours. After oxidation, the LnP5O14 wafers need to be stabalized in N2 in the furnace for at least minutes. The source wafers should be stored at the mouth of the furnace to avoid hydration.

RCA Clean and Furnace Check

An RCA Clean should be performed immediately prior to the diffusion process step.

Verify that that the Phosphorus Furnace temperature is set to either 850 degrees C.

And 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.

Load Wafers Into Boat

At completion of RCA clean, load wafers into boat with sources.

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

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

The wafers should be inserted such that the device side of each is facing an P2O5 source wafer. There are two wafer slots between sources, allowing for a device wafer facing both sides.

Avoid passing your arms or head over cleaned wafers.

Load Boat Into Oven

Return quartz boat, which now contains the wafers, to the Phosphorus furnace using the quartz boat loader.

Load the boat into the mouth of the boron furnace.

Pre-Deposition Example

The phosphorus furnace needs to be heated to the temperature at which the diffusion will be performed. For our example, turn the furnace controller up to the deposition temperature of 850 degrees C. Wait until the temperature has stabilized (about 10-15 minutes).

Begin nitrogen flow through the tube by setting the gas flow rate to 3000 sccm (standard cubic centimeter per min)

Wafers should be loaded in the wafer boat, between each solid source of the Phosphorus and placed in the mouth of the furnace tube.

Using the push rod, push the wafers into the center of the tube using a continuous movement until you reach the center zone.

Maintain wafers in the center of the tube at 850 degrees C for 2 hours.

When 2 hours have elapsed, phosphorus deposition is completed.

Using the pushrod return wafers back to the mouth of the furnace using a continuous movement.

Finally, set the furnace controller to ramp back down to 600 degrees C. Wait for the furnace to reach 600 degrees. This will take about 10-11 minutes.

Allow the wafers to cool down in the mouth of the tube for at least five minutes before removing.

Unload Wafers

Remove the boat from the furnace and allow the wafers to continue to cool before removing them from the wafer carrier.

The wafers are now covered with phosphosilicate glass, which served to dope the exposed regions and the polysilicon. Other areas were masked from diffusion by the field oxide.

When wafers are cool, place them in the RCA wafer carrier and dip them back into the HF bath for 30 seconds and rinse for 5 minutes. This is to remove the outer phosphorus film from surface that is no longer needed.

Note: The sheet resistance will decrease when the Phosphorus is removed from the surface because the phosphorus doping tends to decrease conductivity (and therefore increase resistivity). When this phosphorus is removed, the conductivity increases and resistivity (as well as sheet resistance) decreases.

Wafer Storage

Upon completion of the diffusion 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

Use the Diffusion Calculator to find the junction depth after and/or either a single pre-deposition and/or drive-in or a series of drive-ins given the surface concentration Cs (usually the solid solubility of the dopant), the time and temperature of the pre-deposition or drive-in and the substrate doping (Csub).

The Phosphorus Predeposition cross-section shows the device after the phosphorus deposition step.

Phosphorus Predepositon Cross-SectionStep 8. This cross-section shows the device following the phosphorus deposition which creates the source and drain regions in the NMOS device.