Ultrasonic cleaning method and ultrasonic cleaning apparatus

1. An ultrasonic cleaning method for cleaning an object in a liquid in which a gas is dissolved, the method comprising:

preparing the liquid in which the gas is dissolved;

stirring the liquid at a rotation speed of at least 1400 rpm while irradiating the liquid with the ultrasonic waves so as to realize a state where bubbles containing the gas dissolved in the liquid continue to be generated; and

cleaning the object in the state where the bubbles containing the gas continue to be generated and sonoluminescence occurs,

wherein the stirring of the liquid is performed using a stirring portion having a vane portion immersed in the liquid and a body portion, where the stirring portion is rotatable around a central axis of the body portion.

2. The ultrasonic cleaning method as recited in claim 1 , wherein the realizing the state where bubbles containing the gas continue to be generated is triggered by driving a stirring portion in the liquid.

3. The ultrasonic cleaning method as recited in claim 1 , wherein the gas is nitrogen and a dissolved gas concentration in the liquid is at least 5 ppm and less than 11 ppm.

4. The ultrasonic cleaning method as recited in claim 1 , wherein the ultrasonic waves have a frequency in a range of 20 kHz or more to 2 MHz or less.

5. The ultrasonic cleaning method as recited in claim 1 , wherein the ultrasonic waves have a frequency in a range of 400 kHz or more to 1 MHz or less.

6. The ultrasonic cleaning method as recited in claim 1 , wherein the ultrasonic waves have a frequency in a range of 400 kHz or more to 950 kHz.

7. The ultrasonic cleaning method as recited in claim 1 , wherein the ultrasonic waves have a frequency in a range of 400 kHz or more to 750 kHz.

8. The ultrasonic cleaning method as recited in claim 1 , wherein the gas is nitrogen and a dissolved gas concentration in the liquid is at least 6.0 ppm and less than 11.0 ppm.

9. The ultrasonic cleaning method as recited in claim 1 , wherein the gas is nitrogen and a dissolved gas concentration in the liquid is at least 7.8 ppm and less than 11.0 ppm.

10. The ultrasonic cleaning method as recited in claim 1 , wherein the gas is nitrogen and a dissolved gas concentration in the liquid is at least 7.8 ppm and less than 9.6 ppm.

11. The ultrasonic cleaning method as recited in claim 1 , wherein the object is a wafer comprising silicon.

12. The ultrasonic cleaning method as recited in claim 1 , wherein the object is a P-type silicon wafer.

13. The ultrasonic cleaning method as recited in claim 1 , wherein the liquid comprises water.

14. The ultrasonic cleaning method as recited in claim 2 , wherein the gas is nitrogen and a dissolved gas concentration in the liquid is at least 5 ppm and less than 11 ppm.

15. An ultrasonic cleaning method for cleaning a wafer in a liquid in which a gas is dissolved, the method comprising:

stirring a liquid in which the wafer is immersed, the liquid comprising a dissolved gas at a concentration in a range of more than 5 ppm to 11.0 ppm, at a rotation speed of at least 1400 rpm while irradiating the liquid with the ultrasonic waves so as to realize a state of sonoluminescence and continuous generation of bubbles comprising the gas dissolved in the liquid, thereby cleaning the wafer.

16. The ultrasonic cleaning method as recited in claim 15 , wherein the gas comprises at least 6.0 ppm of nitrogen.

17. The ultrasonic cleaning method as recited in claim 15 , wherein the gas comprises at least 7.8 ppm of nitrogen.

18. The ultrasonic cleaning method as recited in claim 15 , wherein the gas comprises at least 9.6 ppm of nitrogen.

19. The ultrasonic cleaning method as recited in claim 15 , wherein the wafer comprises silicon.