The cleaning of silicon wafers is the final finish of processing silicon wafers, but in fact, it is involved in the grinding and cutting process of silicon wafers. After removing the light, the polished silicon wafers also need to be dewaxed and cleaned. The cleaned silicon wafer can better exert the device performance of the silicon wafer.

1. Impurities on the surface of silicon wafers

The surface of the silicon wafer is processed, and the impurities on it are mainly divided into three categories, namely organic impurities, particulate impurities and metal impurities, which will be introduced in detail below.

1.1 Organic impurities
The silicon wafer needs to be bonded and fixed to the crystal during cutting, and the silicon wafer will also be bonded during polishing. Therefore, the silicon wafer is contaminated by adhesives and wax impurities during these processes, which will be attractive during machining. lipid impurities.

1.2 Particulate impurities

For particulate impurities, they are usually introduced in grinding abrasives and air. Generally, mechanical scrubbing or ultrasonic waves are used to remove dirt with a particle size of ≥0.4µm, and megasonic waves are used to remove impurities with a particle size of ≥0.2µm.

1.3 Metal impurities
During the processing of silicon wafers, silicon wafers are always in an environment where metals exist, and metal impurities are inevitably introduced.

2. Cleaning of silicon wafers

Silicon wafer cleaning: Different cleaning methods are used according to the different types of impurities adsorbed on the surface of the silicon wafer.
Chemical method: use chemical reagents to corrode, dissolve, oxidize and complex impurities on the surface of silicon wafers, remove impurities on the surface of silicon wafers, usually use organic solvents such as alcohol and acetone to remove lipid impurities: use inorganic acids to remove salts in the polishing solution , weak bases, metal oxides and other impurities.
Physical method: mainly use ultrasonic and megasonic waves to remove particulate dirt.

In the ultrasonic transmission process, the sound wave is transmitted in a sinusoidal curve, one layer is strong and the other is weak. When the weak sound wave acts on the liquid, it will generate negative pressure on the liquid, causing many tiny bubbles in the liquid, and when the strong sound wave signal acts on the liquid. When a positive pressure is applied to the liquid, the small bubbles in the liquid will be crushed, and then a high temperature of hundreds of degrees Celsius and thousands of atmospheres will be generated in an instant. is peeled off, allowing the wafer to be cleaned.

In the actual production of silicon wafers, a combination of chemical cleaning and physical cleaning is usually used:
1 Use a strong oxidant to reduce the metal ions attached to the silicon surface to metal, dissolve in the cleaning solution or adsorb on the surface of the silicon wafer;
2 Replace the metal ions on the silicon surface with harmless small-diameter strong positive ions to dissolve them in the cleaning solution;

3. Use cleaning solution with ultrasonic cleaning to achieve the best cleaning effect;
   4 Ultrasonic cleaning and rinsing with deionized water to remove metal ions in solution and residual surface contamination.

The following takes the cleaning of silicon wafers after polishing as an example to introduce. The wafers that have been cleaned with wax have a crystal orientation of (111) and a diameter of 100mm.

(1) Preparations
1) Use protective equipment and acid and alkali-resistant gloves to prevent chemical damage to human hands and prevent impurities carried by human hands from contaminating silicon wafers: wear protective glasses to prevent chemicals from entering human eyes; ventilate the processing room to discharge the waste generated by cleaning silicon wafers. Toxic and harmful gases.
2) Connect the power, turn on the water and electricity switch, and debug the parameters of the equipment.
3) Check the cleaning machine, application machine and its accessories. The washing machine should be preheated in advance, and the drying machine should be properly equipped with hangers, and rinsed 3 to 5 times until the effluent quality meets the requirements (≥10M.cm).
4) Prepare the chemical reagents on duty and check whether the chemical reagents meet the requirements according to the regulations. Chemical reagents include HF, H2O2, HCl, NH4OH, etc.

(2) Wax removal for silicon wafers
1) Prepare cleaning solution. Silicon wafer dewaxing uses SC-1 solution to remove granular impurities on the surface of silicon wafers. The operator wears gloves and protective glasses during the preparation process. The number of chemicals used to prepare the cleaning solution is determined according to the number of cleaning containers and silicon wafers, and finally the chemicals are dissolved in pure water.
2) Heating. Put the silicon wafer on the flower basket with the lifting beam, put the flower basket into the quartz container containing the cleaning solution, heat and control the temperature at 80~85℃, and the time is 10~15min
3) Rinse, after the heating time is up, rinse with a large amount of pure water, put out the flower basket after cooling down, rinse in the PVC box, and continuously vibrate, change the water, and rinse until the water becomes neutral,
4) Repeat the above process. If inspection and sorting are required, put the rinsed silicon wafers into a special spin dryer, spin them with water for 2 minutes, and dry them for 1 minute. After drying, check.

(3) Cleaning
1) Soak in HF solution. Prepare HF solution according to the ratio of 1:5 (HF:H2O), first add water to the container and then pour HF slowly to prevent splashing. After soaking the silicon wafer for an appropriate time, rinse it with pure water until it becomes neutral.
2) Rinse with SC-1 solution. Rinse according to the procedure mentioned above.
3) Rinse with SC-2 liquid. The purpose of SC-2 liquid cleaning is to remove metal ions on the surface of silicon wafers. Clean according to the appropriate ratio, that is, HCl:H2O2:H2O=1:2:8. The conditions are the same as SC-1 Cleaning is the same, rinse with pure water until neutral, and then move the silicon wafer to the drying part.
4) Spin dry. Place the silicon wafer in the spin dryer in a balanced manner, cover with a sealing cap, rinse for 3 min, spin dry for 1 min, record the data and send it for inspection.

(4) End the work
After the work is over, clean all the tools used, put the tools back in their original places, turn off the wind, water, and power, and clean the room. Silicon wafer processing is a complex and repeatable process. The tumble grinding of crystalline silicon, crystal cutting, wafer grinding, wafer polishing, and wafer cleaning are all interrelated. Make sufficient preparations for equipment and reagents. The corresponding processing parameters must be strictly referenced during the process of processing. During the processing process, each link should be monitored from time to time. Once an error occurs in a link, it needs to be stopped immediately to check out the problem in the process. .

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