Principle and technology of ultrasonic cleaning of semiconductor silicon wafer

In order to clean silicon wafers in the semiconductor industry, it has always been a headache for semiconductor workers. The application of ultrasonic wave to clean the surface of silicon wafers is a commonly used method in the industry. The choice of ultrasonic cleaning power, frequency, size and cleaning agent temperature is closely related to the cleanliness of silicon wafers.

1 Principle of ultrasonic cleaning

Ultrasonic cleaning is the use of ultrasonic cavitation in the liquid, acceleration and direct flow effect on the liquid and dirt, so that the dirt layer is dispersed, emulsified, stripped to achieve the purpose of cleaning.

1.1 Cavitation

Cavitation is the transmission of ultrasonic waves to the liquid in the high-frequency transformation mode of compression force and pressure reduction interaction more than 20,000 times per second. When the pressure is reduced, the cavitation phenomenon occurs in the liquid, and when the compression force is used, the bubbles in the liquid produce a strong impact force when the pressure is crushed, thereby stripping the dirt on the surface of the cleaned object, so as to achieve the purpose of precise cleaning.

In the ultrasonic cleaning process, what can be seen by the naked eye is the air bubble, which has an inhibitory effect on the cavitation and reduces the cleaning efficiency. Only when the air bubbles in the liquid are completely removed, the vacuum nuclear group bubbles of cavitation can achieve the best effect.

1.2 Direct inflow action

The phenomenon that ultrasonic waves flow in the liquid along the direction of sound propagation is called direct flow. When the intensity of the sound wave is 0.5W/cm2, the direct flow can be seen by the naked eye, and the flow is perpendicular to the vibration plane, and the flow rate is about 10cm/s. Through this direct flow, the micro-oil dirt on the surface of the cleaned matter is stirred, and the cleaning liquid on the surface of the dirt is also convective, and the dissolved liquid of the dissolved dirt is mixed with the new liquid, which speeds up the dissolution rate and plays a great role in the cleaning of the dirt.

1.3 Acceleration

The acceleration caused by the push of liquid particles. For ultrasonic cleaning machines with higher frequencies, the cavitation effect is very insignificant, and the cleaning at this time mainly relies on the ultra-precision cleaning of the dirt by the acceleration of the impinging particles under the action of liquid particles.

2 Selection of ultrasonic cleaning power and frequency

2.1 Power selection

Ultrasonic cleaning effect is not necessarily proportional to (power × cleaning time), sometimes with small power, it takes a long time to remove dirt. If the power reaches a certain value, sometimes the dirt will be removed quickly. If the choice of power is too large, the cavitation strength will be greatly increased, the cleaning effect is improved, but at this time the more sophisticated parts also produce corrosion points, more than worth the loss, and the bottom of the cleaning cylinder vibration plate cavitation is serious, water point corrosion is also increased, in the use of trichloroethylene and other organic solvents, basically no problem, but the use of water or water-soluble cleaning solution, easy to be affected by water point corrosion, If the surface of the vibrating plate has been scarred, the cavitation corrosion under the strong power is more serious, so the ultrasonic power should be selected according to the actual use.

2.2 Frequency selection

The size of the ultrasonic frequency is a reflection of the ultrasonic energy, and the frequencies applicable to different sizes of dirt are not the same. Different frequencies target different particle sizes, as shown in the table below: 

Different frequencies have different advantages and disadvantages, and choosing the right frequency has a crucial impact on the cleanliness of the film cleaning. The selection basis is as follows: ① Low-frequency ultrasonic can clean very dirty silicon, has a strong cleaning force, but it is also easy to damage the silicon; High frequency ultrasound can clean small dirt, and very dirty silicon chips can not be washed off.

3 Select the cleaning liquid temperature and liquid level

3.1 Selection of cleaning fluid temperature

The most suitable cleaning temperature of water cleaning solution is 40-60 ° C, especially in cold weather if the low cavitation effect of cleaning solution temperature is poor, the cleaning effect is also poor. Therefore, some cleaning machines wrap heating wires around the outside of the cleaning cylinder for temperature control, and cavitation is easy to occur after the temperature rises, so the cleaning effect is better. When the temperature continues to rise, the gas pressure in the cavity increases, causing the impact sound pressure to decrease, reflecting the multiplication of these two factors.

3.2 Selection of cleaning liquid surface

Generally speaking, the liquid level of the cleaning liquid is more than 100mm above the surface of the vibrator. The liquid level of 300W and 24kHz is about 120mm; 600W, 24kHz liquid level about 150mm high. Because the single frequency cleaning machine is affected by the standing wave field, the amplitude at the node is small, and the amplitude at the wave amplitude is large, resulting in uneven cleaning. Therefore, the best choice of cleaning items should be placed at the amplitude.

4 Selection of cleaning agent

In the semiconductor ultrasonic cleaning process, the choice of cleaning agent is also crucial. The choice of cleaning agent not only affects the cleaning effect, but also involves many factors such as cost, environmental protection and safety. The following are some commonly used cleaning agents and their characteristics:

4.1 Water-based cleaning agent

Water based cleaning agent with water as the main component, adding appropriate amount of surfactants, detergent and other additives, has a good cleaning effect and environmental protection. It is suitable for the cleaning of most semiconductor devices, especially for the cleaning of surface residues. However, the cleaning effect of water-based cleaning agent on grease dirt is relatively poor, and it is easy to produce foam in the cleaning process, affecting the cleaning effect.

4.2 Organic solvent cleaning agent

Organic solvent cleaning agent with organic solvent as the main component, such as acetone, ethanol, trichloroethylene and so on. This kind of cleaning agent has a strong ability to dissolve grease and dirt, and the cleaning effect is good. However, the organic solvent cleaning agent is flammable and explosive, and it needs to pay attention to safety when using. In addition, the organic solvent cleaning agent is easy to volatilize, which has certain harm to the human body and the environment.

4.3 Alkaline cleaning agent

Alkaline cleaning agent with alkaline substances as the main components, such as sodium hydroxide, sodium carbonate and so on. This kind of cleaning agent has a strong cleaning ability on the metal surface oil, but the cleaning effect of semiconductor devices is relatively poor. Alkaline cleaning agents tend to corrode the surface of semiconductor devices, and caution is required when using them.

4.4 Acid cleaning agent

Acid cleaning agent with acidic substances as the main components, such as sulfuric acid, hydrochloric acid and so on. This kind of cleaning agent has a strong cleaning ability for metal oxides and other dirt, but it is also easy to corrode the surface of semiconductor devices. The use of acid cleaning agents requires strict control of concentration and temperature to avoid damage to the device.

When choosing a cleaning agent, it is necessary to comprehensively consider the material, type of dirt, cleaning requirements and other factors to be cleaned. At the same time, it is also necessary to pay attention to the safety, environmental protection and economic factors of the cleaning agent.

5 Optimization of cleaning process

In addition to selecting the appropriate ultrasonic cleaning parameters and cleaning agents, the cleaning effect can also be improved by optimizing the cleaning process. Here are some common optimization measures:

5.1 Preprocessing

Before ultrasonic cleaning, pretreatment of the cleaned material can remove most of the dirt and reduce the burden of ultrasonic cleaning. The pretreatment methods include wiping, soaking, spraying and so on.

5.2 Multi-Stage Cleaning

The multi-stage cleaning method can further improve the cleaning effect. Multistage cleaning usually includes rough washing, fine washing and rinsing steps. Rough washing is used to remove most of the dirt, fine washing is used to remove fine dirt, and rinsing is used to remove residual cleaning agent.

5.3 Recycling of the cleaning solution

Recycling the cleaning solution can save resources and reduce costs. However, it is necessary to pay attention to the concentration and cleanliness of the cleaning solution, and regularly replace or supplement the cleaning agent to ensure the cleaning effect.

5.4 Troubleshooting After cleaning

Proper post-treatment of the cleaned material after cleaning can further improve the cleaning effect. Post-treatment methods include drying, drying, vacuum drying, etc. During the drying process, pay attention to avoid thermal or electrostatic damage to semiconductor devices.

4 Conclusion

Since ultrasonic cleaning has been introduced into semiconductor cleaning, it has greatly improved the cleaning efficiency and cleaning effect. Ultrasonic cleaning machine has undergone several generations of evolution, the technology is more advanced, the effect is more significant.