This module describes the solid-source method for dopant diffusion. In this case, a p-type (boron) diffusion , will dope the source/drain regions which are defined by gaps in the field oxide. Methods for evaluation of the diffused layer will be performed later. The boron source is a ceramic wafer of boron nitride (BN), which has been oxidized in a 1000 degree C, 25% oxygen atmosphere, resulting in a surface layer of B2O3 glass.
During the infinite-source diffusion (or "pre-deposition") step, this glass slowly evaporates from the BN wafer and condenses on the silicon wafer. The thin layer of B2O3 provides elemental boron for the diffusion into the silicon via the reaction:
2 B2O3 + 3 Si -> 4 B + 3 SiO2
After the desired amount of boron is deposited on the silicon wafer, the BN source is removed, and the boron is diffused deeper into the silicon wafer during a high-temperature limited-source diffusion (or "drive-in") step. In the PMOS process, drive-in will occur during the subsequent gate oxidation step.
The following steps make up the diffusion process:
Before using the BN 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 BN wafers at 900 degress C in 25% O2 for approximatedly six hours. This is not necessary for every use, as the sources are compromised if oxidized too frequently. After oxidation, the BN wafers need to be stabalized in N2 in the furnace for at least 30 minutes. The source wafers should be stored at the mouth of the furnace to avoid hydration.
An RCA Clean should be performed immediately prior to the diffusion process step.
Verify that that the Boron Furnace temperation is set to 600 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.
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 a BN 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.
Return quartz boat, which now contains the wafers, to the boron furnace using the quartz boat loader.
Load the boat into the mouth of the boron furnace.
The boron 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 900 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 1000 sccm (standard cubic centimeter per min)
Wafers should be loaded in teh wafer boat, between each solid source of boron and placed in the mouth of the furnace tube.
Using the push rod, push the waters 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 900 degrees C for 30 minutes.
When 30 minutes have elapsed, boron 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.
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 borosilicate glass, which served to dope the exposed source/drain 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 15 seconds and rinse for 2 minutes. This is to remove outer boron film from surface that is no longer needed.
Note: The sheet resistance will increase when the Boron is removed from the surface because the boron doping tends to increase conductivity (and therefore reduce resistivity). When this boron is removed, the conductivity decreases and resistivity (as well as sheet resistance) goes up.
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.
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).