Dissolved nitrogen concentration monitoring method, substrate cleaning method, and substrate cleaning apparatus

A method for cleaning a substrate by irradiating an ultrasonic wave onto a cleaning liquid disposed in a cleaning bath in which the substrate is dipped, the method comprising:

• introducing a cleaning liquid into the cleaning bath, the cleaning liquid including a mixture ratio of a first liquid and a second liquid, the first liquid comprising dissolved nitrogen gas;
• determining a measured amount of an increase of a dissolved oxygen concentration of the cleaning liquid resulting from an oxygen molecule generated from a water molecule as a result of a radical reaction caused by said irradiated ultrasonic wave;
• and
• adjusting the mixture ratio of the cleaning liquid with the dissolved nitrogen gas being introduced into the cleaning bath so as to achieve a target amount of increase of the dissolved oxygen concentration in a specific range.