CMOS Clean

Introduction

CMOS clean process is a standard set of wafer cleaning steps needed to be performed before high-temperature processing steps (oxidation, diffusion, CVD) of silicon wafers in semiconductor manufacturing. It is performed in the CMOS Clean Station in the IEN facility. It involves the following steps :

Removal of the organic contaminants (Piranha Clean)
Removal of ionic contamination (SC-II Clean)
Removal of thin oxide layer (BOE Oxide Strip)

The first step (Piranha Clean) is performed with a sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) mixture solution at 120C in the Piranha tank. This treatment removes the organic residuals and part of the metallic contaminants on silicon wafer surfaces. The second step is performed in a 1:1:6 solution of hydrogen chloride (HCl) + hydrogen peroxide (H2O2) + water (H2O) at 70C in the SC-II tank. This treatment effectively removes the remaining traces of metallic (ionic) contaminants.

The last step is performed in a buffered oxide etch (BOE) solution at room temperature in the BOE tank. This treatment further removes all the contaminants by stripping the native oxide layer off silicon wafer surfaces. Silicon wafers are rinsed with DI water repeatedly between each step to prevent contamination. Finally, silicon wafers are rinsed and spin dried in the Semitool Spin Rinse Dryer to finish the entire clean process.

Important Notes

CMOS clean process is a metal-free process. NO metal containing equipment or containers are allowed in the entire process including: metal tweezers, pyrex glassware (sodium rich), or any item that has been used in a metal containing environment.
CMOS clean process can ONLY be performed in the CMOS Clean Station. A training session is required for the operation of the station. Only CMOS clean dedicated wafer handles, blue Teflon wafer boats, Teflon volumetric cylinders or beakers can be used in the CMOS Clean Station.
Only CMOS compatible substrates can be cleaned with the CMOS clean process. Metal containing substrates or substrates that have gone through any metal containing process should NOT be cleaned with the CMOS clean process.
Personal protective equipment (PPE) must be worn during the entire clean process. Please follow the proper gowning and de-gowning procedures.
NO polymer (photo-resist) stripping is allowed in the CMOS Clean Station.
Chemicals in the CMOS Clean Station are changed weekly. Report any contamination to cleanroom staff.
Do NOT run any other processes in the CMOS Clean Station including the Solvent Clean process.
Rinse any empty bottles 3 times, mark the bottles as "Rinsed" and send them to storage area.

Procedure

	Log into the CMOS Clean Station and Semitool Spin Rinse Dryer through the access controller on the wall.
	Power on the controllers for the Piranha tank and the SC-II tank. Check the settings on the controller: 120C for 10 min on the Piranha tank, and 70C for 10 min on SC-II tank.
	Check the liquid levels in the Piranha tank and BOE tank DO NOT use it when the liquid level is lower than 50% or higher than 75% of the total tank depth. Report to cleanroom staff if there is a problem.
	Check the scheduler of the SC-II tank to see if any user has used it within the last 3 hours. If yes, you can use it If no, you need to drain the solution inside and refill the tank with fresh chemicals.
	Start the temperature ramping for the Piranha tank through the controller. Make sure the lid is on the tank.
	Put on personal protective equipment.
	Refill the SC-II tank if needed. First, drain the solution in the tank through the controller. Then stop the draining, and fill the tank with 1 gallon of DI water, 900mL of H2O2 and 900mL of HCl using a Teflon volumetric beaker. Cover the tank with the lid and rinse the beaker with DI water 3 times.
	Start the temperature ramping for the SC-II tank through the controller.
	Load wafers into a Teflon wafer boat with the vacuum picker attached to the station. Make sure the handle fits tightly on the boat.
	Measure 100 mL of H2O2 in the Teflon volumetric cylinder or beaker
	Wait until the Piranha tank reaches 120C Then remove the lid and submerge the wafer boat inside the Piranha tank.
	Gently pour the 100 mL of H2O2 into the Piranha tank. Rinse the used container with DI water for 3 times. Start the timer on the Piranha tank controller.
	After the timer finishes and alarms, remove the wafer boat from the Piranha tank and immediately put it into the dump rinser. Start the rinser. Cover the rinser with the lid to prevent spilling. Ensure the cycle number is greater than or equal to 5.
	When the rinsing cycle finishes, check the temperature on the SC-II tank controller to make sure it reaches 70C. Remove the wafer boat from the dump rinser and submerge the wafer boat into the SC-II tank. Start the timer on the SC-II tank controller
	After the timer finishes and alarms, remove the wafer boat from the SC-II tank and immediately put it in the dump rinser. Start the rinser. Cover the rinser with the lid to prevent spilling. Ensure the cycle number is greater than or equal to 5
	When the rinsing cycle finishes, remove the wafer boat from the dump rinser and submerge the wafer boat into the BOE tank. Start a personal timer.
	After 30 seconds, remove the wafer boat from the BOE solution. While holding the boat over the solution, watch the wafer surface to see whether it is hydrophilic or hydrophobic. If the wafer surface is still hydrophilic, do an extra 30 sec clean.
	When the BOE clean is done, remove the wafer boat from the BOE tank, immediately put it in the dump rinser. Start the rinser. Cover the rinser with the lid to prevent spilling. Make sure the cycle number is greater than or equal to 5.
	When the rinsing cycle finishes, remove the wafer boat from the dump rinser and then load it into the Semitool Spin Rinse Dryer. Make sure the "H" side of the boat faces inside the chamber. Start the tool.
	Remove the wafer boat from Semitool Spin Rinse Dryer when the process is done.
	Remove all personal protection equipment.
	Clean up the CMOS Clean Station with texwipes. Put all the lids on the tanks, and make sure there are no spills left on the bench.
	Power off all the controllers.
	Log off the CMOS Clean Station and the Semitool Spin Rinse Dryer through the access controller on the wall.